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+--Forum: Cabbages and Kings
+---Topic: immune system evolution started by rafe gutman
Posted by: rafe gutman on Dec. 17 2002,18:45
i'd like to use this thread to collect references to articles and research relevant to the evolution of the immune system. i imagine that i'll be the only one contributing to this, but others are certainly welcome. i included several references in my posts in an < ISCID thread > on the topic, and i'll probably copy my posts from there to here.
there are also some good references < here >.
Posted by: rafe gutman on Dec. 17 2002,18:58
from the < ISCID > thread:
martin poenie: Again Inlay refers to ITAMs in a tunicate gene. This is a reference to a protein called A74 that has no similarity to any other known protein. So what are ITAMs? ITAMs are an arrangement of two tyrosines in a peptide with a certain spacing between them. Tyrosines are widely used in cell signaling. When phosphorylated, they can become ligands for proteins with SH2 domains. What makes ITAMs special is that there are proteins such as ZAP70 that contain dual SH2 domains and bind to the ITAM phosphorylated tyrosines as a unit. At present, no such protein has been identified in tunicates. The ITAM motif is based on four residues. It is possible that these are ordinary SH2 binding sites and that no dual SH2 domain-containing will be found. Until and unless one does find such a protein this is not a compelling argument. If proteins with these dual SH2 domains are found in tunicates, then it becomes a good argument.
rafe: i'm not an expert on signal transduction, but it seems pretty clear that A74 is involved in immune-related signal transduction, and that tyrosine phosphorylation of the ITAM is part of it. as for the SH2-containing proteins, i should point out that homologues for syk and ZAP70 have been discovered in organisms as distant as hydra , and as similar (to tunicates) as sea urchin . additionally, a CD45 homologue, which can augment signaling through ITAMs, was found in hagfish . so while we don't have a tunicate syk-family homologue, many of the other pieces are already in place. it's not unreasonable to think that the downstream targets of A74 will be identified soon, i guess we can reserve judgement til then. do you think it will be homologous to syk/ZAP70?
 Steele RE, Stover NA, Sakaguchi M.
Appearance and disappearance of Syk family protein-tyrosine kinase genes during
Gene. 1999 Oct 18;239(1):91-7.
 Sakuma M, Onodera H, Suyemitsu T, Yamasu K.
The protein tyrosine kinases of the sea urchin Anthocidaris crassispina.
Zoolog Sci. 1997 Dec;14(6):941-6.
Nagata T, Suzuki T, Ohta Y, Flajnik MF, Kasahara M.
The leukocyte common antigen (CD45) of the Pacific hagfish, Eptatretus stoutii:
implications for the primordial function of CD45.
Immunogenetics. 2002 Jul;54(4):286-91.
i should also note, in reference to dr. poenie's argument, that the genome of a tunicate has been sequenced and dozens of zap-70/syk homologues have been found. i'm not sure what the best way to search through it is, but i entered "syk" into the < search engine > for the tunicate genome and found a bunch of hits. here's < one >
Posted by: niiicholas on Dec. 17 2002,20:27
I would just like to say that I think the name Ciona intestinalis sounds like a disease rather than a tunicate.
(or, maybe, the scientist who named it thought it resembled a bit of intestine)
< Ciona genome homepage >
Posted by: rafe gutman on Dec. 17 2002,22:17
although it doesn't reference anything, i came across a post of mine where i present a model for the origin of the adaptive immune system. since it's original material (the wording, that is, the model itself has been around for awhile), i thought i'd post it here as well:
here's a run down of the general details of this model
1. the RAG genes and RSSs were originally a transposon, and the RAG proteins had transposase activity.
2. a gene existed in the ancestor to the jawed vertebrates that encoded a receptor with at least one immunoglobulin domain
3. this gene was only expressed in a somatic lineage, like hemocytes
4. the receptor could activate a pathway somehow involved in immune-related function
5. the RAG transposon integrated into the receptor near the binding site-coding region.
6. once inserted, the transposase
would be under the receptor's transcriptional control.
7. the RAG's ability to reassemble the receptor gene once activated was sloppy, and made slight sequence changes upon reassembly.
8. those changes were beneficial to the host
and the evidence supporting it:
1. there seems to be ample evidence suggesting that the RAG genes were once transposases, do you dispute this? the fact that they have transposase activity (in vitro) basically says it all.
2. the immunoglobulin domain is well-represented in nearly all organisms (even prokaryotes, i think). because its structure is so stable, a large portion of cell-surface molecules possess it, even some related to immunity (outside of the jawed vertebrates). however, no direct homologue to the antibody proteins have been found. so here is a prediction of the model: a non-rearranging receptor with an immunoglobulin domain existed in the ancestor to the jawed vertebrates. of course, this ancestral gene may no longer exist, but if it is ever discovered, that would fill in a major hole.
3. a lot of genes have tissue specific expression. as for the extant antibody genes, they are heavily regulated by enhancers (because of the need to prevent premature recombination, or recombination on both alleles). there's no reason to think that the enhancer wasn't there in the ancestral receptor (such as, if it was located between the V and J segments).
4. see 2.
5. this is the only step that appeals to chance. as yersinia said, there could have been many integrations by transposons (our genome is littered with remnants of retrotransposons). all it takes is one to be beneficial, and it will become fixed into the population.
6. a common molecular technique is transgenics, where whole genes are inserted into the chromosome randomly. one of the main problems with this approach is the lack of consistency in transgene expression. this is because the transgene becomes subject to the local transcription control. unless the transgene has a really powerful promoter, it's uncertain whether or not it will be expressed correctly.
7. the current mechanism of RAG-mediated recombination is sloppy. one of the reason is through the generation of hairpin loops at the two exposed ends. this is intrinsic to the activity of the RAG and is common to transposases (in fact, this is one of the observations that led scientists into thinking the RAGs were originally transposons).
8. here the evidence may never be to your satisfaction, but it's very reasonable to suppose that a high rate of mutation would benefit immune receptors. why do you think new flu vaccines come out every season? because the influenza's receptors can mutate very rapidly to avoid host recognition. furthermore, they don't need to mutate drastically and change the total structure of their receptors, only just enough to hamper the host's receptor's binding ability. if during tthe lifetime of the first RAG-integrated organism, one of the thousands to millions of hemocytes alive at the time form a receptor beneficial to the host, this will increase the host's chances for survival and the nature will have something to select upon.
maybe this requires more evidence to convince you, but is it really that unreasonable?
Posted by: rafe gutman on Dec. 17 2002,22:24
here's a link to a recent article on the discovery of lymphocytes in lamprey:
< http://sciencenow.sciencemag.org/cgi/content/full/2002/1003/3 >
another post i found on ISCID:
a couple of interesting papers on the immune system have popped up recently:
1. Mayer WE, Uinuk-Ool T, Tichy H, Gartland LA, Klein J, Cooper MD.
< Isolation and characterization of lymphocyte-like cells from a lamprey >.
Proc Natl Acad Sci U S A. 2002 Oct 29;99(22):14350-5.
this paper presents evidence for the existence of lymphocytes (cells intimately involved in the adaptive immune response) in lampreys, which don't have an adaptive immune system. previously, one could have argued that rearranging antigen receptors and lymphocytes were irreducible. now it doesn't look like that is the case.
2. Uinuk-Ool T, Mayer WE, Sato A, Dongak R, Cooper MD, Klein J.
< Lamprey lymphocyte-like cells express homologs of genes involved in
immunologically relevant activities of mammalian lymphocytes. >
Proc Natl Acad Sci U S A. 2002 Oct 29;99(22):14356-61.
this article, the complement to the previous article, shows that the lamprey lymphocytes express genes similar to ones expressed in mammalian lymphocytes. these genes are also involved in the adaptive immune response in mammals, so it's strange that they are present in lamprey. incidently, both these articles are freely accessible online.
3. Dodds AW.
Which came first, the lectin/classical pathway or the alternative pathway of
Immunobiology. 2002 Sep;205(4-5):340-54.
this article provides a model for the evolution of the complement system. if paul wants to call the author's model "storytelling", he's free to do so, but now it's not just some random internet poster's idea.
i won't quote his model (it spans 2 pages), but he does say this about the model:
"the scheme outlined in figures 5 and 6 involves a stepwise increase in effectiveness of the system, each step giving benefit to the species involved."
since the time this thread began, probably a dozen articles have been published that support the notion that the immune system evolved. has ID advanced in any way since then?
Posted by: rafe gutman on Dec. 17 2002,22:32
here's an extremely important article from nature immunology:
Nat Immunol 2002 Dec;3(12):1200-7
Identification of diversified genes that contain immunoglobulin-like variable regions in a protochordate.
Cannon JP, Haire RN, Litman GW.
 Immunology Program, H. Lee Moffitt Cancer Center and Research Institute, 12902 Magnolia Avenue, Tampa, FL 33612, USA.  All Children's Hospital, 801 Sixth Street South, St. Petersburg, FL 33701, USA.
The evolutionary origin of adaptive immune receptors is not understood below the phylogenetic level of the jawed vertebrates. We describe here a strategy for the selective cloning of cDNAs encoding secreted or transmembrane proteins that uses a bacterial plasmid (Amptrap) with a defective beta-lactamase gene. This method requires knowledge of only a single target motif that corresponds to as few as three amino acids; it was validated with major histocompatibility complex genes from a cartilaginous fish. Using this approach, we identified families of genes encoding secreted proteins with two diversified immunoglobulin-like variable (V) domains and a chitin-binding domain in amphioxus, a protochordate. Thus, multigenic families encoding diversified V regions exist in a species lacking an adaptive immune response.
< pubmed link >
< link to article >
incidently, i did a blast search of one of these genes on the recently-sequenced ciona intestinalis (tunicate) genome, and got a < hit >.
Posted by: rafe gutman on Dec. 17 2002,22:38
last one for now:
< pubmed link >
Immunobiology 2002 Sep;205(4-5):467-75
The biological functions of MBL-associated serine proteases (MASPs).
Hajela K, Kojima M, Ambrus G, Wong KH, Moffatt BE, Ferluga J, Hajela S, Gal P, Sim RB.
Department of Biochemistry, University of Oxford, UK.
The Mannose-binding lectin-associated serine proteases (MASPs) have been the subject of intensive research particularly over the past 10 years. First one, then two, and currently 3 MASPs have been characterized. Initially it was thought likely that the MBL + MASPs system would resemble very closely the C1 complex of the complement classical pathway, and that MASP1 and MASP2 would have similar activities to their classical pathway homologues C1r and C1s. MASP2 does certainly have similar activities to C1s, but MASP1 does not have the activities of either C1r or C1s. MASP1 has been thought to act on the complement system by cleaving C3 directly, but work with recombinant and purified native MASP1 shows that direct C3 cleavage by this protease is very slow, and may not be biologically significant. MASP1 and MASP2 appear not to have such a narrow specificity as C1r and C1s, and may have significant substrates other than complement proteins. As an example, MASP1 does cleave fibrinogen, releasing fibrinopeptide B (a chemotactic factor) and also cleaves and activates plasma transglutaminase (Factor XIII). These reactions are also relevant to defence against microorganisms, and may represent a biologically significant action of MASP1.
this is a review article that mentions an interesting finding, that a serine protease exists that has substrates in two functionally different pathways, the blood-clotting system and the complement system. i looked up the reference, but the article it mentions hasn't been published yet. i guess we'll have to wait til then to see the details.
Posted by: rafe gutman on Dec. 18 2002,18:24
here's a good recent discussion of the implications of the finding of a proto-immunoglobulin in amphioxus, branchiostoma floridae:
(in nature immunology)
< The origins of the adaptive immune system: whatever next? >
The discovery of a V-like Ig multigene family in the protochordate amphioxus provides new insights into the evolution of the adaptive immune response.
Diversity is at the same time the essential product of evolution and the essential substrate on which it must act. Although diversity can be the result of many different pressures and mechanisms, it is particularly evident and rapidly evolving in the responses of hosts to pathogens and parasites. Of the many defense systems described from the simplest single-celled bacteria to the most complicated plants and animals, none has been more intensively studied than the mammalian adaptive immune system. It has only recently become apparent that the adaptive immune system arose in the jawed vertebrates, but little is known about the deeper origins of this system or the relationship with other defense systems in nonvertebrate organisms. In this issue of Nature Immunology, Litman and colleagues describe a set of newly identified sequences from the protochordate amphioxus (Fig. 1), which make up a diversified multigene family and could hold some clues to the emergence of the adaptive immune system.
for the rest of the article, follow the link at the top.
Posted by: charlie d on Jan. 07 2003,10:37
Interesting < new article > on the origin of NK cells, a possible "bridge" between innate and adaptive immunity.
Urochordates and the origin of natural killer cells: Identification of a CD94/NKR-P1-related receptor in blood cells of Botryllus
Konstantin Khalturin *, Matthias Becker *, Baruch Rinkevich , and Thomas C. G. Bosch *
*Zoological Institute, Christian Albrechts University Kiel, Olshausenstrasse 40, 24098 Kiel, Germany; and National Institute of Oceanography, Tel Shikmona, P.O. Box 8030, Haifa 31080, Israel
Published online before print January 7, 2003
Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0234104100
Transplantation immunity based on the recognition of MHC molecules is well described in vertebrates. Vertebrates, however, do not undergo transplantation reaction naturally. The phylogenetically closest group in which transplantation reactions can occur is the Urochordata. Therefore, these animals occupy a key position for understanding the evolution of the vertebrate immune system. When screening for genes differentially expressed during allorecognition in Botryllus schlosseri, we isolated a gene coding for a type II transmembrane protein with a C-type lectin-binding domain and close similarity to vertebrates CD94 and NKR-P1. Here we show that the gene, BsCD94-1, is differentially regulated during allorecognition and that a subpopulation of blood cells carries the corresponding receptor on its cell surface. Southern blot analysis with DNA from individual colonies and intronless BsCD94-1 probe reveal variation between individuals at the genomic level. CD94 in vertebrates is one of the markers for natural killer cells and binds to MHC class I molecules. Natural killer cells play a major role in recognition and elimination of allogeneic cells. Their evolutionary origin, however, remained unknown. The results presented here indicate that the elaboration of the vertebrate immune system may have its roots in an ancestral population of cells in the urochordate blood.
Posted by: rafe gutman on Mar. 21 2003,16:20
it's been known for several years now that the RAG proteins can mediate transposition in vitro, providing pretty strong evidence for the theory that the RAG genes evolved from an ancient transposase (though not the only evidence). skeptics of the theory claim that the ability to catalyze a reaction in vitro does not necessarily mean that it does so in vivo, or more to the point, that it did so. there is some basis to this argument. additionally, it has been theorized that RAG-mediated transposition in vivo could be a source of mutation. the < paper > below not only demonstrates that RAG proteins can act as transposases, but also that the activity is possibly tumorigenic:
In vivo transposition mediated by V(D)J recombinase in human T lymphocytes
Terri L. Messier, J.Patrick O’Neill, Sai-Mei Hou, Janice A. Nicklas and Barry A. Finette
The rearrangement of immunoglobulin (Ig) and T-cell receptor (TCR) genes in lymphocytes by V(D)J recombinase is essential for immunological diversity in humans. These DNA rearrangements involve cleavage by the RAG1 and RAG2 (RAG1/2) recombinase enzymes at recombination signal sequences (RSS). This reaction generates two products, cleaved signal ends and coding ends. Coding ends are ligated by non-homologous end-joining proteins to form a functional Ig or TCR gene product, while the signal ends form a signal joint. In vitro studies have demonstrated that RAG1/2 are capable of mediating the transposition of cleaved signal ends into non-specific sites of a target DNA molecule. However, to date, in vivo transposition of signal ends has not been demonstrated. We present evidence of in vivo inter-chromosomal transposition in humans mediated by V(D)J recombinase. T-cell isolates were shown to contain TCR(alpha) signal ends from chromosome 14 inserted into the X-linked hypo xanthine–guanine phosphoribosyl transferase locus, resulting in gene inactivation. These findings implicate V(D)J recombinase-mediated transposition as a mutagenic mechanism capable of deleterious genetic rearrangements in humans.
Posted by: niiicholas on Mar. 31 2003,22:46
Since this came up again with the discussion with Nelson Alonso on the ISCID thread, I just came across a review of a recent workshop on evolutionary immunology. I quote the bit about the origin of recombining receptors:
Dev Comp Immunol 2003 Apr;27(4):263-71
< Workshop report: evolutionary immunobiology--new approaches, new paradigms. >
Du Pasquier L, Smith LC.
3. The boundary between invertebrates and vertebrates
Those working on innate immunity and who are impressed by the efficiency of these defense mechanisms frequently ask, "why did vertebrates acquire an adaptive immune system that was based on somatic rearrangement?" The multiplicity and diversity of the innate systems described above may provide the beginnings of an answer to this question. If an animal like Drosophila, which lives for a relatively short time, requires many germ line encoded innate mechanisms to survive, how many mechanisms will be required for the survival of long lived species? At some point during metazoan evolution, an increase in the number of genes devoted to innate immunity would have been required. Rather than an amplification of genetic material devoted to immune defenses becoming detrimental, the introduction of a somatic mechanism to generate a repertoire of recognition structures resulted in an economy or at least a status quo in the amount of the genome that was devoted to immune responsiveness. At the same time a somatic mechanism providing in fact a `kit' to generate one's repertoire by oneself would probably be favored by species producing few offspring where the value of a single individual would be very important. Therefore it would not be that surprising that phyla other than vertebrates have invented (by means that could be different from those of the vertebrates) a somatic way of increasing the diversity of their receptors (e.g. see the above mentioned possibilities of alternate RNA processing and splicing). Today, one of the striking paradoxes of the evolution of the combinatorial immune system is that RAG homologues appear to be totally lacking in the lower deuterostomes and in all non-deuterostome metazoans. This suggests a rapid evolutionary emergence and phylogenetic restriction for the capacity to generate specific inducible immunity in higher vertebrates. This implies that the introduction of the rearranging machinery occurred in a gene encoding a receptor that was already committed to expression in a lymphocyte-like cell, and perhaps involved in innate immune defense mechanisms. Therefore, to understand this major evolutionary step, a search is under way for the origin of the lymphocyte lineage and for an Ig superfamily (IgSf) gene target which may be similar to a T-cell receptor or an immunoglobulin gene. The animals used in these searches include modern members of classes that preceded the gnathostomes in evolution.
Michele Anderson (Pasadena) presented information on transcription factors involved in hematopoiesis and the definition of lymphoid lineages. If diversity was a master theme during this workshop, conservation of patterns was another and not only for the Toll cascade. A good example is the conservation of patterns (in structure, function and pathways) of transcription factors. Anderson discussed her investigations of skates, lampreys, amphioxus and sea urchins, and searches for homologues of transcription factors important in hematopoiesis. She has been monitoring the Ikaros/Aiolos/Helios family, PU.1, and GATA-1/2/3 families of transcription factors, all of which include essential or important factors in lymphoid development. In the clear nose skate (Raja eglanteria, a species that is also suitable for developmental studies), the existence of EBF, GATA3, members of the Ikaros family, and PU.1, are all consistent with the finding of Ig and TCR genes expressed at this level of vertebrate phylogeny. Expression of the transcription factor, Spi-C, appeared to be specific to the spleen and therefore, may be involved with B cell development. It was assumed that multiple members of each transcription factor family found in the elasmobranchs arose by gene duplication from a less complex population of progenitor genes. In invertebrates the number of homologues found so far fits with this hypothesis: fewer in the invertebrates and increased gene family sizes in higher vertebrates. For example, PU.1 and Spi-C are not found in the Drosophila genome, while one member of the PU.1 family is found in the lamprey, even though no classical adaptive immune system has been characterized in this species. Based on the sequence of the lamprey GATA gene, it appears to encode a pre-duplication molecule with GATA 2/3 features. The existence of PU.1 and GATA-1/2/3 family transcription factors in the lamprey, a jawless vertebrate, suggests that the stage was set for lineage-specific expression of certain antigen receptors just prior to the introduction of RAG-1 and RAG-2 in the earliest jawed vertebrates. Anderson stressed the importance of the cis-acting elements that control the expression of the duplicated GATA genes and orient them in different patterns of expression. She also warned us about terminology in this field and about the difficulty of identifying orthologues. The workshop participants discussed and agreed upon the need for a strong gene ontology if highly diverse systems are to be compared fruitfully.
Still within the context of transcription factors, Jack Marchalonis (Tucson) described the lamprey vitamin D receptor (VDR), a ligand-specific transcription factor that is a member of the Nuclear Receptor Superfamily (NRS), which has been cloned from `protospleen' cDNA of the larval stage. In mammals, the VDR is a transcription factor that mediates the actions of its ligand, vitamin D, and can promote monocyte/macrophage differentiation and inhibit proliferation and cytokine production by activated T lymphocytes. These functions make the VDR relevant and interesting for innate immunity. Like VDRs of jawed vertebrates, the lamprey molecule consists of two C4-type zinc fingers, and both DNA-binding segments and ligand-binding segments show significant conservation across species. Cladograms of the NRS family, using the ecdysone receptor of insects as an outgroup (a distantly related member of this gene family) indicated that the lamprey VDR clustered unambiguously with the other vertebrate VDRs and constituted the basal member of the group.
Marchalonis (Tucson) also commented on the complete characterization of the RAG1/RAG2 gene cluster in the sandbar shark and the evolutionary and functional implications of this analysis. The ancestors of the sharks were apparently the first vertebrates in phylogeny to have acquired RAG genes which have been highly conserved during evolution both in terms of sequence and gene organization. In common with other vertebrate species, the shark RAG2 coding region lacks introns and is closely linked in opposite orientation to the RAG1 gene. The intergenic region (~10 kb) is comparable in length to that of tetrapods. About 40% of the spacer consists of SINE and LINE fragments homologous to those found in other sharks, reptiles and birds.
(the bit about the regulation of lamprey receptors is key...)
In the conclusion the old theme of change-of-function is emphasized:
At the heart of it, we are attempting to understand the dynamics of complex systems by comparative methods. For example, are there common threads in the metabolic machinery from one system to another? Clearly there must be, as one of the themes of this conference was the adaptation of pre-existing molecules and metabolic pathways to new functions. Likewise, are there common features of the immune (or other metabolic) system that can be extracted and used to understand more fully the evolution of immunity and organisms? This field has become known as phylogenomics. Further, the response of an organism to environmental change is equivalent in concept to immune responses to invading organisms. Does it really matter whether the environmental challenge is a pathogen or an elevation in temperature? The organism will respond to environmental changes that alter metabolic functions. The application of genomic tools to the study of ecosystems, ecogenomics, is an emerging discipline dedicated to this end and owes much to developments in immunology. We are very familiar with the common features in ecosystems of primary producers, herbivores, carnivores and decomposers which circumscribes the flow of energy and mass through the system. In phylogenetics, there are accepted hierarchies in vertebrate groups, and some invertebrates as well, which are assumed to reflect the ancestor–descendent relationships (i.e. flow of genetic material over time). However, similar unifying principles and classification schemes are completely lacking in studies of cellular mechanisms except where individual components are concerned (e.g. the TCA cycle). Hence, the concepts developed in a systems approach to immune function can cut across a wide range of biological studies and mathematical tools are needed to exploit these data.
...this last bit seems to be saying that understanding the evolution of a complex system is much easy when you have a good phylogenetic understanding of the system (as with the immune system) than when you don't (e.g. most of Behe's IC systems, which originated in single-celled organisms).
Posted by: niiicholas on Mar. 31 2003,22:49
In the "here is part of what non-rearranging receptors do" category:
Vet Immunol Immunopathol 2003 Jan 10;91(1):1-12
< TOLL-like receptors linking innate and adaptive immune response. >
Werling D, Jungi TW.
Institute of Veterinary Virology, University of Berne, Langgass-Str. 122, CH-3012 Bern, Switzerland.
Invading pathogens are controlled by the innate and adaptive arms of the immune system. Adaptive immunity, which is mediated by B and T lymphocytes, recognises pathogens by rearranged high affinity receptors. However, the establishment of adaptive immunity is often not rapid enough to eradicate microorganisms as it involves cell proliferation, gene activation and protein synthesis. More rapid defense mechanisms are provided by innate immunity, which recognises invading pathogens by germ-line-encoded pattern recognition receptors (PRR). Recent evidence shows that this recognition can mainly be attributed to the family of TOLL-like receptors (TLR). Binding of pathogen-associated molecular patterns (PAMP) to TLR induces the production of reactive oxygen and nitrogen intermediates (ROI and RNI), pro-inflammatory cytokines, and up-regulates expression of co-stimulatory molecules, subsequently initiating the adaptive immunity. In this review, we will summarize the discovery and the critical roles of the TLR family in host defense, briefly allude to signaling mechanisms mediating the response to TLR ligands, and will provide an update on current knowledge regarding the ligand specificity of these receptors and their role in immunity of domestic animals, particularly cattle.
Posted by: rafe gutman on April 05 2003,16:47
this is more of a reminder to myself to read this article, since it is now freely available online:
< http://genomebiology.com/2000/1/2/REVIEWS/1014 >
From lymphocytes to sharks: V(D)J recombinase moves to the germline.
Genome Biol. 2000;1(2):REVIEWS1014. Review.
The antigen-receptor genes of vertebrates are rearranged by a specialized somatic recombination mechanism in developing lymphocytes - and, unexpectedly, also in the germline of cartilaginous fishes. The recombination system that carries out these DNA rearrangements may thus be a significant evolutionary force, perhaps not limited to rearrangements at antigen-receptor loci.
Posted by: rafe gutman on April 07 2003,19:43
here's a group that has found functional homologues for some human TCR beta V pseudogenes in chimpanzees. check it out
< http://www.jimmunol.org/cgi/content/abstract/170/8/4161?etoc >
Analysis of the TCR(beta) Variable Gene Repertoire in Chimpanzees: Identification of Functional Homologs to Human Pseudogenes
Dirk Meyer-Olson*, Kristen W. Brady*, Jason T. Blackard, Todd M. Allen*, Sabina Islam*, Naglaa H. Shoukry, Kelly Hartman*, Christopher M. Walker and Spyros A. Kalams2
The Journal of Immunology, 2003, 170: 4161-4169.
Chimpanzees are used for a variety of disease models such as hepatitis C virus (HCV) infection, where Ag-specific T cells are thought to be critical for resolution of infection. The variable segments of the TCR(alpha/beta) genes are polymorphic and contain putative binding sites for MHC class I and II molecules. In this study, we performed a comprehensive analysis of genes that comprise the TCR variable gene (TCRBV) repertoire of the common chimpanzee Pan troglodytes. We identified 42 P. troglodytes TCRBV sequences representative of 25 known human TCRBV families. BV5, BV6, and BV7 are multigene TCRBV families in humans and homologs of most family members were found in the chimpanzee TCRBV repertoire. Some of the chimpanzee TCRBV sequences were identical with their human counterparts at the amino acid level. Notably four successfully rearranged TCRBV sequences in the chimpanzees corresponded to human pseudogenes. One of these TCR sequences was used by a cell line directed against a viral CTL epitope in an HCV-infected animal indicating the functionality of this V region in the context of immune defense against pathogens. These data indicate that some TCRBV genes maintained in the chimpanzee have been lost in humans within a brief evolutionary time frame despite remarkable conservation of the chimpanzee and human TCRBV repertoires. Our results predict that the diversity of TCR clonotypes responding to pathogens like HCV will be very similar in both species and will facilitate a molecular dissection of the immune response in chimpanzee models of human diseases.
Posted by: niiicholas on April 10 2003,23:56
A few key papers on the origin of the innate immune system:
Immunopharmacology 1999 May;42(1-3):107-20
< Complement systems in invertebrates. The ancient alternative and lectin pathways. >
Smith LC, Azumi K, Nonaka M.
Department of Biological Sciences and Institute of Biomedical Sciences Graduate Program in Genetics, George Washington University, Washington, DC 20052, USA.
The complement system in higher vertebrates is composed of about thirty proteins that function in three activation cascades and converge in a single terminal pathway. It is believed that these cascades, as they function in the higher vertebrates, evolved from a few ancestral genes through a combination of gene duplications and divergences plus pathway duplication (perhaps as a result of genome duplication). Evidence of this evolutionary history is based on sequence analysis of complement components from animals in the vertebrate lineage. There are fewer components and reduced or absent pathways in lower vertebrates compared to mammals. Modern examples of the putatively ancestral complement system have been identified in sea urchins and tunicates, members of the echinoderm phylum and the protochordate subphylum, which are sister groups to the vertebrates. Thus far, this simpler system is composed of homologues of C3, factor B, and mannose binding protein associated serine protease suggesting the presence of simpler alternative and lectin pathways. Additional components are predicted to be present. A complete analysis of this invertebrate defense system, which evolved before the invention of rearranging genes, will provide keys to the primitive beginnings of innate immunity in the deuterostome lineage of animals.
Immunobiology 2002 Sep;205(4-5):340-54
< Which came first, the lectin/classical pathway or the alternative pathway of complement? >
Department of Biochemistry, University of Oxford, UK. email@example.com
It is a widely accepted canon of immunology that the alternative pathway is more primitive and hence older in evolutionary terms than the lectin/classical pathway. This idea has been reinforced by the discovery of "C3" and "factor B" proteins in invertebrate species. However, it is clear that the gene duplications which gave rise to C3/C4/C5 and factor B/C2 occurred in the vertebrate lineage. Hence, the naming of the invertebrate proteins may be based on preconceptions rather than on solid structural or functional evidence. Lectins and associated MASP/C1r/C1s-like proteins have been found in invertebrates, while factor D, the defining component of an alternative pathway, has so far been found only in the bony fish and higher species. It is a principle of Darwinian evolution that complex systems develop through small sequential steps. It is possible to imagine such a series of steps for the evolution of a lectin pathway, involving as it does recognition of non-self. It is difficult to see how the alternative pathway, which lacks a recognition molecule, could have evolved without the prior development of control proteins to protect self from attack.
Posted by: niiicholas on April 12 2003,18:33
A highly entertaining new ISCID thread on the
< Intelligent Design of Immunity >
Posted by: niiicholas on April 13 2003,17:51
I'm going to post some posts here for safekeeping on this thread:
< http://www.iscid.org/ubbcgi....#000054 >
Nelson, what is this babble about an immune system producing "not enough specific antibodies to make a difference"?
Way back on the other thread, < charlie pointed out to you >:
Antibodies do not necessarily have single specificities. Indeed, the vast majority of our circulating immunoglobulins (the so-called "natural" antibodies) are low affinity, broad specificity antibodies directed towards common antigens (bacterial wall moieties, for instance).
Antibodies become highly specific and gain high affinity only late during an antigen-specific immune response, through a process of mutation/selection called affinity maturation. This however has nothing to do with the VDJ recombination process we are discussing here, which takes place, irrespective of antigen, during B cell differentiation in the bone marrow.
"Naive", newly generated B cells, as they emerge from the bone marrow, carry antibodies that are mostly of low affinity. For insatnce, the antibodies produced early during an immune response (which reflect the naive repertoire) bind antigen with a Kd in the 10^-5-10^-6 M range - compared that with the high affinity, "matured" antibodies of late immune responses, which have a Kd of 10^-8-10^-9 M. Most antibodies in the primary repertoire are also not very specific – in fact, polyspecific antibodies abound (which goes along with their low affinity for antigen). As an aside, the vast majority of antibodies in the primary repertoire do not in fact recognize anything at all, and the B cells that make them die after a while without ever seeing any "action" (one of the drawbacks of the darwinian approach of the adaptive immune system – high, widespread wastefulness for rare but exceptional returns).
As for innate immunity receptors, again you are mistaken. While some of them do indeed have broad spectrum, many have quite subtle specificities, for instance TLR4 binds very specifically to the lipid A moiety of the very large bacterial lipolysaccharide (LPS) molecules. Their binding constants also actually compare quite well with those of most primary response antibodies (in the 10^-6-10^-7 M range).
The fundamental difference between adaptive and innate immunity receptors is in fact neither in their affinity nor in their specificity, but in their logic. The adaptive immune system, using VDJ recombination, can generate an almost infinite variety of specificities, and thanks to clonal selection can pick any extremely rare, low affinity antibody molecule and turn it into close to a “magic bullet” (this however has again nothing to do with VDJ recombination). The innate immune system, on the other hand, can count on only a limited array of receptors, which must focus on a few abundant antigens (sometimes classes of antigens) commonly found on pathogens (often, like LPS, molecules that we ourselves do not produce); moreover, the binding of the ligand has to be good to start with, because these antigens cannot undergo mutation and selection processes.
So we have the following facts:
1) Many innate receptors have similar specificity to "naive" (first-generation lymphocyte) recombinant antibodies. Most of the antibodies in your and my blood, right now, are therefore "not specific enough to make a difference". According to you.
2) Specificity is not produced just by having cells that mysteriously produce lots of specific antibodies, it is produced by the selective replication of those very few lymphocytes that happen to match whatever the antigen is. Further somatic mutation and selection is what produces many copies of the very few antibody phenotypes that are "specific enough to make a difference".
3) Therefore at no point are huge numbers of diverse, "single specificity" antibodies produced.
So speaking crudely, phylogenetically, we have this sequence of organisms:
(a) invertebrates, with many non-rearranging receptors of moderate specificity (similar to the specificity of "naive" antibodies)
(b) cartilagenous fish, which add diverse rearranging receptors of moderate specificity, genes in VDJ VDJ VDJ arrangement
© "lower" vertebrates, which have diverse rearranging receptors of moderate specificity in a VVVV DDDD JJJJ-type arrangement
(d) mammals, like lower vertebrates except that a few of the rearranging receptors, which happen to match the antigen, get replicated and gradually improve from moderate specificity to high specificity via somatic mutation & selection.
(charlie can refine the above if I garbled things)
And yet, Nelson, you've been proclaiming for endless pages that there is some sort of requirement somewhere to produce large numbers high-specificity antibodies with different specificities.
Please, Nelson, can you help us out here?
It can't, anymore that a human system can get a V1 fused to a V2 in its Ig clusters. V segments have RSSs only on their 3', and they all have the same spacers (for steric reasons related to RAG structure, recombination only occurs between RSSs that vave different sized spacers), so that recombination cannot occur. What can occur is a V from one shark cluster recombining with a D in another.
Thanks, that's what I was trying to explain when I wrote,
But it wouldn't make a difference anyway, because RAG figures out where to cut based on RSS (IS in above diagram), and these sequences would get copied along with the V, D or J segments as they get duplicated [...]
Yet another supposed reason for the unevolvability of VDJ recombination hits the dust!! It's like shooting skeet.
All Nelson has left is that:
(1) the exact non-rearranging ancestor receptor has not yet been identified, and (2) that he thinks a transposon insertion, a well-known natural event that is happening all the time (it causes some cancers for instance) is "non-Darwinian" and for some mysterious reason therefore an intelligent intervention.
Regarding (1), even though (a) Ig domains are common, (b) we know that non-rearranging receptors would work because we have a bunch of them, and © due to selection for immune system diversity sequence homology will decay very fast.
To emphasize the Ig domain point:
...all those circles are Ig(-like) domains.
Regarding (2), < there are a multitude of transposon types and events >; there is no need to postulate intelligent intervention in order to explain a transposon inserting into a non-rearranging receptor. Plus, there is the published literature and experiments which the immunologists view as having tested and strengthened the hypothesis.
Posted by: rafe gutman on April 18 2003,17:33
Comparative analyses of immunoglobulin genes: surprises and portents.
Nat Rev Immunol. 2002 Sep;2(9):688-98. Review.
< abstract >:
The study of immunoglobulin genes in non-mouse and non-human models has shown that different vertebrate groups have evolved distinct methods of generating antibody diversity. By contrast, the development of T cells in the thymus is quite similar in all of the species that have been examined. The three mechanisms by which B cells uniquely modify their immunoglobulin genes -- somatic hypermutation, gene conversion and class switching -- are increasingly believed to share some fundamental mechanisms, which studies in different vertebrate groups have helped (and will continue to help) to resolve. When these mechanisms are better understood, we should be able to look to the constitutive pathways from which they have evolved and perhaps determine whether the rearrangement of variable, diversity and joining antibody gene segments -- V(D)J recombination -- was superimposed on an existing adaptive immune system.
Posted by: niiicholas on July 25 2003,08:59
Not moving the goalposts quite yet. From DBB:
In this chapter I have looked at three features of the immune system - clonal selection, antibody diversity, and the complement system - and demonstrated that each individually poses massive challenges to a putative step-by-step evolution. [Niic, for the sake of argument, I'll grant you this one]. But showing that the parts can't be built step by step only tells part of the story, because the parts interact with each other...an animal that has a clonal selection system won't get much benefit out of it if there is no way to generate antibody diversity. A large repertoire of antibodies won't do much good if there is no system to kill invaders. A system to kill invaders won't do much good if there's no way to identify them. At each step we are stopped not only by local system problems, but also by requirements of the integrated system.
Um, jon_e, haven't you gathered (from this thread and from Inlay's FAQ), that Behe's three systems are in fact separable, because they are built on each other in a phylogenetic sequence? To wit:
(1) Invertebrates (echinoderms, tunicates, etc.) and lower vertebrates (hagfish and lampreys) have just the complement system
(2) Cartilagenous fish have the complement system + rearranging antibody diversity
(3) Tetrapods (well, most of them) have the complement system + rearranging antibody diversity + clonal selection (IIRC)
So in fact you don't need all three systems together. Rather, the systems were added one-by-one, each building on the previous. Once again, Behe is contradicted by very basic immunological facts.
Now, let me move from Behe to one of your earlier sources, the recent (but out-of-date) collection of articles by a number of experts in the field, Origin and Evolution of the Vertebrate Immune System
(Current Topics in Microbiology and Immunology, 248). "New Approaches Towards an Understanding of Deuterostome Immunity" J. P. Rast et al
Both of these attributes, the tendency towards mechanistic novelty and a high rate of sequence evolution, may emerge from the dynamic nature of host-pathogen interactions and thus be a universal characteristic of immune systems.
Universal, Niic, universal.
Now, I'm not sure if Rast et al are talking about the same thing as Behe exactly, but it would seem to me that the points they are making are very similar. In order for an immune system of any type to work properly, several features need to be present all at once. Why else would the authors be proposing universality for "the tendency towards mechanistic novelty and a high rate of sequence evolution?"
Unfortunately, jon_e, you're reading something into the quote that just ain't there. They're not talking about parts of a system reacting within an organism, they're talking about the diversity of one basic component, the receptor, and how natural selection favors receptor diversity in evolution.
What they are talking about is the fact that it is beneficial for the organism to have diverse antigen receptors, because then more kinds of potential invaders can be bound and destroyed. Therefore, there is selection for duplication and diversification of receptor genes. E.g., humans have many slightly different homologs of both non-rearranging and rearranging receptor genes. The pattern of diversity is also found in organisms that only have non-rearranging receptors -- they apparently get all the diversity they need by encoding diversity in the germline. The somatic rearrangement that occurs in vertebrates is simply an enhancement to this diversity.
It is, in fact, a classic case of positive selection:
Immunol Rev. 2002 Dec;190:161-8.
< Natural selection and the diversification of vertebrate immune effectors >.
The molecules of the vertebrate immune system provide some of the best documented examples of natural selection acting at the molecular level. The major histocompatibility complex (MHC) molecules are a family of highly polymorphic loci whose products present peptides to T cells. Four distinct lines of evidence support the hypothesis that the natural selection acts to maintain MHC polymorphism: (1) evidence from the unusual allelic frequency distribution seen at MHC loci; (2) evidence from the pattern of nucleotide substitution at MHC loci, which shows an enhanced rate of nonsynonymous (amino acid-altering) substitution in the codons encoding the peptide-binding region of the molecules; (3) the existence of long-lasting polymorphisms at certain MHC loci; and (4) the fact that introns at MHC loci are homogenized by recombination and subsequent genetic drift. Certain other immune system gene families provide evidence that natural selection has acted to create diversity among family members. Examples include molecules of the specific immune system (such as immunoglobulin V region genes) and molecules of the innate immune system (such as defensins).
Another reason for this diversity is that the invading pathogens are continually evolving *away* from being recognized by current receptors, so there is continual selection on the organism for new receptor genes to evolve to "track" the diseases.
So now we can see that your excited conclusion:
It's interesting to me that those few words seem to sum up a major hurdle for the standard Darwinistic explanation of things - if I understand them correctly, they are suggesting that every creature that has, or has ever had, an immune system, has the ability to respond very quickly to the presence of enemy organisms, creating new features or substances on-the-fly so to speak, and to generate a high rate of sequence evolution (are they refering to a high rate of somatic mutations?) in response to the threat.
...was based on a misconception. The "response" "creating new features or substances" is an *evolutionary* response that occurs in populations, not individual organisms. The exception is rearranging receptors+clonal selection, which *does* allow "within organism" evolution of a sort, but which is not found invertebrates etc.
As I keep saying over and over, "the immune system" as a whole is big and ill-defined.
Now, do you mean by this that features of the immune system are difficult to explain or describe?
Or what? Your comment seems to contradict your basic claim that Dembski and Behe are all wet regarding current research and literature. What is it, exactly, that you mean by your comment?
Jon_e, look. What I was trying to do was distinguish between one of Behe's specific IC system, receptor rearrangement, which is reasonably well-defined with three basic molecular components, vs. the whole immune system, which is not similarly well-defined. You were continually confusing the rearranging receptors with the immune system as a whole, and I was trying to get you clear that these are not equivalent things.
I was also pointing out that in general "the immune system" doesn't fit the IC box very well at all. Most of the vertebrate system is lacking in other animals, many of the "parts" have immunity and non-immunity-related functions, etc. You still haven't told me whether or not you think that skin is part of the immune system. Various cells are definitely a part of the immune system, but cells are a long ways from Behe's "molecular machine" paradigm for defining a system. We could add mucous membranes and a human prediliction for cleanliness if we wanted to. And then we have organismal variations. A sponge with just a few primitive non-rearranging receptors has an immune system of a sort. This is the problem with defining "the" immune system.
But the definition of "immune system" is really a side-show, you're not going to find the salvation of Behe and Dembski there.
Or do you mean, It's difficult to cram the immune system into the restrictive, overly reductionistic and simplistic non-explanations of classical Darwinian theory?
You want a "non-explanation", here's one: "Poof". That's all the IDists have. We evolutionists, on the other hand, have oodles of literature. My only beef at the moment is that Behe and Dembski are misleading you all by saying that this literature doesn't exist.
Based on what you've learned in this thread, jon_e, what do you think of Behe's statement from Darwin's Black Box, p. 138?
"We can look high or we can look low, in books or in journals, but the result is the same. The scientific literature has no answers to the question of the origin of the immune system."
Is this an accurate or misleading statement, jon_e?
Posted by: rafe gutman on July 29 2003,19:04
Sci Prog 2001;84(Pt 2):125-36
The serpins: nature's molecular mousetraps.
Huntington JA, Carrell RW.
University of Cambridge, Department of Haematology, Wellcome Trust Centre for Molecular Mechanisms in
Disease, Wellcome Trust/MRC Building, Hills Road, Cambridge CB2 2XY, UK.
A special family of inhibitors, known as the serpins, has evolved an extraordinary mechanism to enable the
control of the proteolytic pathways essential to life. The serpins undergo a profound change in conformation to
entrap their target protease in an irreversible complex. The solving of the structure of this complex now
completes a video depiction of the changes involved. The serpin, just like a mousetrap, is seen to change with
a spring-like movement from an initial metastable state to a final hyperstable form. The structure shows how
this conformational shift not only inhibits the protease but also destroys it. A bonus from these structural
insights is the realisation that a number of diseases, as diverse as thrombosis, cirrhosis and dementia, all share
a common mechanism arising from similar mutations of different serpins.
PMID: 11525014 [PubMed - indexed for MEDLINE]
< http://www.ncbi.nlm.nih.gov:80/entrez....bstract >
IUBMB Life 2002 Jul;54(1):1-7
Serpins: finely balanced conformational traps.
Pike RN, Bottomley SP, Irving JA, Bird PI, Whisstock JC.
Department of Biochemistry and Molecular Biology, Monash University, Melbourne, Victoria, Australia.
Serine protease inhibitors (serpins) play very important roles in the maintenance of various physiologically
important systems. As knowledge of the workings of proteins of this family grows, new understanding is
gained of the mechanisms by which they inhibit target proteases, using conformational changes for which the
structure of serpins is uniquely adapted. This finely balanced system is utilized to healthy benefit in the control
of serpin function by modulators, arguably the most striking examples of which occur in the control of
proteolytic cascades, such as the coagulation system. Serpins also play very important intracellular roles: one
example is the protection of immune cells from their own cytotoxic proteases. The finely balanced serpin
mechanism also means that it is prone to disastrous consequences if mutations should occur in vital positions
in the serpin structure. Many examples of disease-associated mutations have been shown, which has the dual
effect of highlighting how important these molecules are in the maintenance of health and the fine balance that
must be maintained in order to preserve their active, inhibitory conformation.
PMID: 12387568 [PubMed - in process]
< http://www.ncbi.nlm.nih.gov:80/entrez....bstract >
Curr Opin Struct Biol 2001 Dec;11(6):740-5
Serpins and other covalent protease inhibitors.
Ye S, Goldsmith EJ.
Department of Biochemistry, The University of Texas Southwestern Medical Center at Dallas, 5323 Harry
Hines Boulevard, Dallas, Texas 75390, USA.
Serpins are irreversible covalent 'suicide' protease inhibitors. In the past two years, important advances in the
structural biology of serpins have been forthcoming with the crystal structures of a covalent complex between
trypsin and alpha1-antitrypsin, and of a Michaelis encounter complex between trypsin S195A and serpin 1B
from Manduca sexta. These structures have helped elucidate many aspects of the mechanism of action of
serpins. Also, the crystal structure of the cysteine protease caspase-8 in complex with the inhibitor p35 has
revealed a new family of suicide protease inhibitors.
PMID: 11751056 [PubMed - indexed for MEDLINE]
< http://www.ncbi.nlm.nih.gov:80/entrez....bstract >
Bioessays 1993 Jul;15(7):461-7
The role of conformational change in serpin structure and function.
Gettins P, Patston PA, Schapira M.
Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, Tennessee.
Serpins are members of a family of structurally related protein inhibitors of serine proteinases, with molecular
masses between 40 and 100kDa. In contrast to other, simpler, proteinase inhibitors, they may interact with
proteinases as inhibitors, as substrates, or as both. They undergo conformational interconversions upon
complex formation with proteinase, upon binding of some members to heparin, upon proteolytic cleavage at
the reactive center, and under mild denaturing conditions. These conformational changes appear to be critical
in determining the properties of the serpin. The structures and stabilities of these various forms may differ
significantly. Although the detailed structural changes required for inhibition of proteinase have yet to be
worked out, it is clear that the serpin does undergo a major conformational change. This is in contrast to other,
simpler, families of protein inhibitors of serine proteinases, which bind in a substrate-like or product-like
manner. Proteolytic cleavage of the serpin can result in a much more stable protein with new biological
properties such as chemo-attractant behaviour. These structural transformations in serpins provide
opportunities for regulation of the activity and properties of the inhibitor and are likely be important in vivo,
where serpins are involved in blood coagulation, fibrinolysis, complement activation and inflammation.
PMID: 8379949 [PubMed - indexed for MEDLINE]
KREM Di CERA
< http://www.ncbi.nlm.nih.gov:80/entrez....bstract >
J Biol Chem. 2003 Jul 7 [Epub ahead of print].
Related Articles, Links
Conserved Ser residues, the shutter region, and speciation in serpin evolution.
Krem MM, Di Cera E.
Department of Biochemistry and Molecular Biophysics, Washington University, St. Louis, MO 63110.
The suicide inhibitory mechanism of serine protease inhibitors of the serpin superfamily depends heavily on
their structural flexibility, which is controlled in large part by the breach and shutter regions of the central
Ab-sheet. We examined codon usage by the highly conserved residues Ser53 and Ser56 of the shutter region
and found a TCN-AGY usage dichotomy for Ser56 that, remarkably, is linked to the
protostome-deuterostome split. Our results suggest that serpin evolution was driven by phylogenetic speciation
and not pressure to fulfill new physiologic functions, mitigating against coevolution with the family of serine
proteases they inhibit.
PMID: 12847097 [PubMed - as supplied by publisher]
< http://www.ncbi.nlm.nih.gov:80/entrez....bstract >
J Biol Chem 2002 Oct 25;277(43):40260-4
Related Articles, Links
Ser(214) is crucial for substrate binding to serine proteases.
Krem MM, Prasad S, Di Cera E.
Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis,
MO 63110, USA.
Highly conserved amino acids that form crucial structural elements of the catalytic apparatus can be used to
account for the evolutionary history of serine proteases and the cascades into which they are organized. One
such evolutionary marker in chymotrypsin-like proteases is Ser(214), located adjacent to the active site and
forming part of the primary specificity pocket. Here we report the mutation of Ser(214) in thrombin to Ala,
Thr, Cys, Asp, Glu, and Lys. None of the mutants seriously compromises active site catalytic function as
measured by the kinetic parameter k(cat). However, the least conservative mutations result in large increases
in K(m) because of lower rates of substrate diffusion into the active site. Therefore, the role of Ser(214) is to
promote the productive formation of the enzyme-substrate complex. The S214C mutant is catalytically
inactive, which suggests that during evolution the TCN-->AGY codon transitions for Ser(214) occurred
through Thr intermediates.
PMID: 12181318 [PubMed - indexed for MEDLINE]
< http://www.ncbi.nlm.nih.gov:80/entrez....bstract >
Trends Biochem Sci 2002 Feb;27(2):67-74
Related Articles, Links
Evolution of enzyme cascades from embryonic development to blood coagulation.
Krem MM, Cera ED.
Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, Box
8231, St Louis, MO 63110-1093, USA.
Recent delineation of the serine protease cascade controlling dorsal-ventral patterning during Drosophila
embryogenesis allows this cascade to be compared with those controlling clotting and complement in
vertebrates and invertebrates. The identification of discrete markers of serine protease evolution has made it
possible to reconstruct the probable chronology of enzyme evolution and to gain new insights into functional
linkages among the cascades. Here, it is proposed that a single ancestral developmental/immunity cascade gave
rise to the protostome and deuterostome developmental, clotting and complement cascades. Extensive
similarities suggest that these cascades were built by adding enzymes from the bottom of the cascade up and
from similar macromolecular building blocks.
PMID: 11852243 [PubMed - indexed for MEDLINE]
< http://www.ncbi.nlm.nih.gov:80/entrez....bstract >
J Biol Chem. 2002 May 31;277(22):19243-6. Epub 2002 Mar 29.
Related Articles, Links
Substrate recognition drives the evolution of serine proteases.
Rose T, Di Cera E.
Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis,
Missouri 63110, USA.
A method is introduced to identify amino acid residues that dictate the functional diversity acquired during
evolution in a protein family. Using over 80 enzymes of the chymotrypsin family, we demonstrate that the
general organization of the phylogenetic tree and its functional branch points are fully accounted for by a
limited number of residues that cluster around the active site of the protein and define the contact region with
the P1-P4 residues of substrate.
PMID: 11925426 [PubMed - indexed for MEDLINE]
< http://www.ncbi.nlm.nih.gov:80/entrez....bstract >
Trends Microbiol 2000 May;8(5):238-44
The eukaryotic-like Ser/Thr protein kinases of Mycobacterium tuberculosis.
Av-Gay Y, Everett M.
Divn of Infectious Diseases, University of British Columbia, 2733 Heather St, Vancouver, BC, Canada.
In bacteria, extracellular signals are generally transduced into cellular responses via a two-component system.
However, genome sequence data have now revealed the presence of 'eukaryotic-like' protein kinases and
phosphatases. Mycobacterium tuberculosis appears to be unique among bacteria in that its genome contains 11
members of a newly identified protein kinase family. These M. tuberculosis eukaryotic-like protein kinases
could be key regulators of metabolic processes, including transcription, cell development and interactions with
PMID: 10785641 [PubMed - indexed for MEDLINE]
< http://www.ncbi.nlm.nih.gov:80/entrez....bstract >
EMBO J 2002 Dec 1;21(23):6330-6337
Related Articles, Links
A serpin mutant links Toll activation to melanization in the host defence of Drosophila.
Ligoxygakis P, Pelte N, Ji C, Leclerc V, Duvic B, Belvin M, Jiang H, Hoffmann JA, Reichhart JM.
Corresponding author e-mail:
P.Ligoxygakis and N.Pelte contributed equally to this work
A prominent response during the Drosophila host defence is the induction of proteolytic cascades, some of
which lead to localized melanization of pathogen surfaces, while others activate one of the major players in the
systemic antimicrobial response, the Toll pathway. Despite the fact that gain-of-function mutations in the Toll
receptor gene result in melanization, a clear link between Toll activation and the melanization reaction has not
been firmly established. Here, we present evidence for the coordination of hemolymph-borne melanization
with activation of the Toll pathway in the Drosophila host defence. The melanization reaction requires Toll
pathway activation and depends on the removal of the Drosophila serine protease inhibitor Serpin27A. Flies
deficient for this serpin exhibit spontaneous melanization in larvae and adults. Microbial challenge induces its
removal from the hemolymph through Toll-dependent transcription of an acute phase immune reaction
PMID: 12456640 [PubMed - as supplied by publisher]
< http://www.ncbi.nlm.nih.gov:80/entrez....bstract >
Int J Biochem Cell Biol. 2003 Nov;35(11):1536-47.
Serpins: structure, function and molecular evolution.
van Gent D, Sharp P, Morgan K, Kalsheker N.
Division of Clinical Chemistry, Institute of Genetics, Queen's Medical Centre, University of Nottingham, NG7
2UH, Nottingham, UK
The superfamily of serine proteinase inhibitors (serpins) are involved in a number of fundamental biological
processes such as blood coagulation, complement activation, fibrinolysis, angiogenesis, inflammation and
tumor suppression and are expressed in a cell-specific manner. The average protein size of a serpin family
member is 350-400 amino acids, but gene structure varies in terms of number and size of exons and introns.
Previous studies of all known serpins identified 16 clades and 10 orphan sequences. Vertebrate serpins can be
conveniently classified into six sub-groups.We provide additional data that updates the phylogenetic analysis in
the context of structural and functional properties of the proteins. From these, we can conclude that the
functional classification of serpins relies on their protein structure and not on sequence similarity.
PMID: 12824063 [PubMed - in process]
< http://www.ncbi.nlm.nih.gov:80/entrez....bstract >
J Thromb Haemost. 2003 Jul;1(7):1535-49.
Mechanisms of glycosaminoglycan activation of the serpins in hemostasis.
Department of Haematology, University of Cambridge, Cambridge Institute for Medical Research, Cambridge,
Serpins are the predominant protease inhibitors in the higher organisms and are responsible, in humans, for the
control of many highly regulated processes including blood coagulation and fibrinolysis. The serpin inhibitory
mechanism has recently been revealed by the solution of a crystallographic structure of the final
serpin-protease complex. The serpin mechanism, in contrast to the classical lock-and-key mechanism, involves
dramatic conformational change in both the inhibitor and the inhibited protein. The final result is a stable
covalent complex in which the properties of each component are altered so as to allow clearance from the
circulation. Several serpins are involved in hemostasis: antithrombin (AT) inhibits many coagulation proteases,
most importantly factor Xa and thrombin; heparin cofactor II (HCII) inhibits thrombin; protein C inhibitor
(PCI) inhibits activated protein C and thrombin bound to thrombomodulin; plasminogen activator inhibitor 1
inhibits tissue plasminogen activator; and alpha2-antiplasmin inhibits plasmin. Nearly all of these reactions are
accelerated through interactions with glycosaminoglycans (GAGs) such as heparin or heparan sulfate. Recent
structures of AT, HCII and PCI have revealed how in each case the serpin mechanism has been fine-tuned by
evolution to bring about high levels of regulatory control, and how seemingly disparate mechanisms of GAG
binding and activation can share critical elements. By considering the serpins involved in hemostasis together it
is possible to develop a deeper understanding of their complex individual roles.
PMID: 12871289 [PubMed - in process]
< http://www.ncbi.nlm.nih.gov:80/entrez....bstract >
J Thromb Haemost. 2003 Jul;1(7):1343-8.
Inflammation and thrombosis.
Cardiovascular Biology Research Program, Oklahoma Medical Research Foundation; Departments of
Pathology, and Biochemistry & Molecular Biology, University of Oklahoma Health Sciences Center; and
Howard Hughes Medical Institute, Oklahoma City, Oklahoma, USA.
Systemic inflammation is a potent prothrombotic stimulus. Inflammatory mechanisms upregulate procoagulant
factors, downregulate natural anticoagulants and inhibit fibrinolytic activity. In addition to modulating plasma
coagulation mechanisms, inflammatory mediators appear to increase platelet reactivity. In vivo, however,
natural anticoagulants not only prevent thrombosis, but they also dampen inflammatory activity. Some insights
into the evolution and linkages between inflammatory mechanisms and the coagulation/anticoagulation
mechanisms have become evident from recent structural studies. This review will summarize the interactions
between inflammation and coagulation.
PMID: 12871267 [PubMed - in process]
Posted by: niiicholas on July 31 2003,13:51
Reposting from the II popcorn thread:
< http://www.iidb.org/vbb....1102688 >
Dude, I was just about to post on exactly this, because I couldn't stand it anymore, and here rafe has already gone and done it.
The original paragraph from < Roth 2000 > that Nelson quoted makes the context clear:
How did these pre-rearranged genes arise? One possibility is that they are descendants of the ancestral antigen-receptor gene, before integration of the putative transposable element. A second possibility is that these genes arose from RAG-mediated DNA rearrangement events that occurred in the germline, an operation that violates the precept that the RAG recombinase is functional only in developing lymphocytes. A recent paper by Lee et al.  addresses these questions by examining immunoglobulin genes in the nurse shark. The nurse shark NS4 immunoglobulin light chain gene family provided very useful information, as there are several highly homologous genes present both in pre-rearranged and unrearranged forms. These features allowed the authors to evaluate sequences in sufficient detail to ascertain whether the genes bear characteristic features of V(D)J recombination or footprints of transposition. Their analysis revealed that the pre-rearranged genes did indeed contain tell-tale signs of coding joints formed by V(D)J recombination, including both N nucleotides and P nucleotides, which strongly suggest a hairpin intermediate. The presence of these features in several junctions suggests several independent germline V(D)J recombination events, although analysis of multiple unrelated individuals suggests that these events are not frequent. Importantly, analysis of the unrearranged NS4 genes failed to detect the target site duplications that are hallmarks of transposon insertions. Thus, while the pre-rearranged NS4 genes appear to have been derived from unrearranged genes by germline V(D)J recombination, there is as yet no evidence to support the hypothesis that the unrearranged genes were derived from the pre-rearranged genes by insertion of a transposable element; this is discussed in a recent review by Lewis and Wu .
(1) In this paragraph, Roth is talking about the origin of the pre-rearranged receptor genes found sometimes in sharks, where the rearrangements are "hard-coded" into the germline.
(2) Roth says that the evidence favors the idea that these pre-rearranged genes are derived from V(D)J recombining genes via the normal action of RAG (normal except that it occurred in the germline cells)
(3) In the last bolded section, Roth is discussing an possible alternative hypothesis that says that the opposite occurred: that the unrearranged V(D)J genes were derived from pre-rearranged VDJ genes by *insertion* of RAG.
(4) We can be sure that this is what Roth meant by clicking < on the handy link to reference 16, Lewis and Wu 2000 >, which says:
In this issue, a study by Lee et al. (12) further investigates whether joined genes—later interrupted by a transposon insertion—or unjoined genes—subsequently connected by site-specific recombination—came first in the evolution of the Ig light chain locus in the nurse shark. There is increased interest in this issue, not only because it reveals the ways in which RAG-mediated events have reorganized Ig and TCR loci, but also because RAG-mediated transposition, though demonstrated in a purified in vitro system, has not yet been observed in any in vivo setting.
Lee et al. took advantage of the fact that certain Ig light chain genes in two shark species were apparent orthologs. Whereas all of the type III L chain genes in the horned shark were unjoined, the corresponding NS4 genes in the nurse shark occurred in both joined and unjoined form. A key feature of the analysis was that the NS4 genes in the nurse shark were highly homologous to one another, allowing evolutionary relationships to be established with some confidence. In combination, these circumstances enabled the authors to do two things: construct a phylogenetic tree of the NS4 family sequences, and provide through DNA sequence analysis a means to distinguish between the transposon integration and site-specific recombination scenarios.
As mentioned above, a sequence that has been interrupted by RAG-mediated transposition is expected to exhibit (relative to the uninterrupted form) a 5-bp target site duplication (or more rarely a 4- or 3-bp duplication; Fig 1 B, wavy lines) on either side of an RSS-bordered insertion (6) (7). A gene that has instead been joined through site-specific recombination is expected to exhibit (relative to the unjoined form) a loss of a small, unfixed number of bp from the ends of the joined coding sequences, along with the acquisition of junctional insertions (of two classes: one random in sequence, and termed an N insertion, another occurring only at ends that escaped trimming, and bearing a palindromic relationship to the cut end, termed a P insert; for a review, see reference 10). The two approaches taken by Lee et al. (12) returned the same answer: the joined NS4 genes arose through site-specific V(D)J recombination and not through germline RAG-mediated transposition. Their phylogenetic analyses indicated that germline joining occurred more than once, and in every case the unjoined form lacked any evidence of the DNA sequence duplications predicted for transposition. Instead joined genes exhibited junctions that appeared to reflect processing accompanying V(D)J recombination: trimming and P nucleotide addition.
In short, the germline "pre-rearranged" receptors of sharks (only some of the shark receptors are pre-rearranged) are derived from more typical V(D)J genes, and are not the ancestors of V(D)J genes. This was determined via phylogenetic analysis by Lee et al.
(5) Returning to the issue of the origin of V(D)J-RAG system, that both Roth (2000) and Lewis and Wu (2000) are supporters of the standard transposition-insertion-in-a-preexisting-nonrearranging-receptor hypothesis is shown by quotes from the respective papers:
Early on, it was suggested that the V(D)J recombination system might have arisen by the fortuitous integration of a transposable element into an ancestral antigen-receptor gene . This hypothesis was strengthened by the discovery that the RAG genes are tightly linked , and by the finding that the RAG proteins can act as a transposase. Thus, a plausible model for the acquisition of the V(D)J recombination system during vertebrate evolution is the integration of a transposable element carrying the linked RAG genes into a primordial antigen-receptor gene in an ancestral jawed vertebrate, approximately 450 million years ago (reviewed in [1,11]). Presumably, this initial integration event created the first rearranging antigen-receptor gene; subsequent gene duplication events then created the multiple immunoglobulin and T-cell receptor loci.
[Lewis and Wu 2000]
As recently as only a few years ago, any real information bearing on the actual genesis of the V(D)J recombination system seemed to be irretrievably lost. However, as more was learned about the molecular genetic and biochemical properties of the V(D)J recombination proteins, termed recombination activating gene (RAG)-1 and RAG-2, tantalizing suggestions of a transposon origin began to emerge. These clues included the following: first, the fact that the genes encoding RAG-1 and RAG-2, which are unrelated in sequence, are tightly linked, and as such share this property with genes that are known to undergo horizontal transfer (4). Second, the chemical mechanism of the recombination reaction, where DNA strand breakage and rejoining is accomplished through one step transesterification reactions, was like that of several well-described mobile elements (5). Finally, a surprising finding further indicated that RAG-1 and RAG-2 might have once been part of a transposon when two groups independently demonstrated that purified RAG-1 and RAG-2 proteins have a latent ability to carry out the transposition of DNA (6) (7).
Why this latter observation was so important is that there was no a priori expectation that a protein that can perform V(D)J recombination through site-specific recognition of recombination signal sequences (RSS) should also be able to transpose RSS-terminated DNA fragments. A quick description of both types of rearrangement is needed to appreciate this point, and will also come to bear later in this commentary. As diagrammed in Fig 1, the transpositional excision and reintegration of DNA (Fig 1 B) is a fairly different transaction from the creation of site-specific connections in V(D)J recombination (Fig 1 A). One difference is in the number and specificity of double strand DNA breaks; transposition not only entails the introduction of breaks at each of two RSS, as in V(D)J recombination, but also requires the generation of a third, non–sequence-specific cut at an undetermined integration site. The signature features of a transposition product versus those arising from site-specific V(D)J recombination are also quite distinct. After transposition, a transposed DNA fragment terminated by the RSS, is flanked by a five-nucleotide repeat created by a staggered cut at the target integration site ((6) (7); Fig 1 B, wavy lines). In contrast, after V(D)J recombination, a signal joint and a coding joint are created. Signal joints are formed from exact RSS fusions, and coding joints from fusions of the associated coding segments. The latter characteristically contain small, irregular nucleotide deletions and insertions, reflecting various processing operations performed on the coding end intermediates as they undergo joining (Fig 1 A). Thus, the unusual in vitro ability of RAG-1 and RAG-2 to do two quite different things, and in particular to transpose DNA, provided strong support for the original speculation that the V(D)J recombination system used to be a transposable element (3). The fact that the once portable genome now serves a different and highly utilitarian role in its new context suggests that the V(D)J recombination system stands as a prime example of the rehabilitation of "selfish DNA" (for a review, see reference (8)).
Figure 2. Postulated intermediates in the molecular evolution of the Ig and TCR loci. (A) One theory is that the event responsible for the existence of the modular recombination units characterizing today's Ig and TCR loci was a singularity that took place early in vertebrate evolution. With the integration of a mobile element, an ancient gene encoding an Ig superfamily domain was split into two parts. The mobile element imported RAG recombinase, its RSS, and possibly other element-related features, into the vertebrate genome. Reassembly of a functional exon required site-specific excision of the introduced mobile element, through a RAG-mediated recombination event targeting the RSS motifs. (B) After the RAG transposon integrated, the interrupted gene was duplicated. It has been suggested by Litman et al. (reference 2) that the presence of clustered arrays of duplicated genes, as found today in sharks, resembles an early locus configuration. On this view, V(D)J recombination activity in germline tissues could have led to the following derived features: (i) joined copies, as seen in cartilaginous fish, arose through a "standard" V(D)J joining event resulting in coding joint formation; (ii) the de novo creation of D segments could have arisen from intercluster recombinations resulting in the formation of signal joints with junctional insertions; (iii) the substitution of a 12-spacer RSS for a 23-spacer RSS or vice versa may have resulted from the "hybrid joint" outcome of germline joining. All of the postulated manipulations are RAG mediated and site specific. Events shown in i and iii are supported by evidence from sharks. MYA, million years ago.
(5) In his zeal to find something, anything, with which to object to Inlay's article and the devastating rebuttal to Behe's arguments it presents, Nelson is so blinded that he will blatantly misread articles (among his many other sins).
(6) jon_e hardly can tell his [bleep] from a tunicate when it comes to discussing the evolution of the immune system. He operates by free word association rather than actually understanding anything, and so he picked up on Nelson's quote containing the words 'no evidence' and ran with it because jon_e thought maybe Nelson had rafe stumped. This is just a subset of jon_e's main strategy of throwing out whatever ignorant objections occur to him over breakfast, and hoping that something sticks or at least is obscured enough in the pile of objections that rafe or mesk doesn't get around to educating him on the particular issue.
(7) Rafe is watching all of this with bemusement, and even egged them on a bit by asking them carefully if they really thought that Roth (an authority on the subject) supported what Nelson and jon_e were saying. Rafe is essentially giving them all the rope they need to hang themselves (again), not that it will have any impact at all with the blinkers they have on.
(8) When the above is revealed, Nelson and jon will continue on as if nothing had happened. Nelson will understand that he's been shown to be an ass but will deny that he meant what he said and begin obfuscating by changing the subject to other topics. jon_e won't even understand why Nelson was wrong, and will continue raising the "no evidence" objection as if it were established for the next three pages, until some other trivialities (e.g., "Hey guys, I just read that skin is required for effective immune system function, this just reinforces my point that the immune system is IC!")
(9) None of them will ever get around to admitting that they've given away the store by failing to defend Dembski's and Behe's false assertions that the literature is silent on the evolution of IC systems.
I am of course just an amateur on this topic and might have misunderstood something about the shark receptor issue, but I think I've got the gist of it.
(rafe, reference/quote this post as you like (or not) over in the ARN thread. I admire your forbearance...I'm just using this thread as a fix to get off the ARN habit...)
Posted by: theyeti on Jan. 05 2004,01:47
Here's a new article of obvious relevance:
< Mol Immunol. 2004 Feb;40(12):897-902. >
Evolution of the complement system.
Nonaka M, Yoshizaki F.
The human complement system is composed of more than 30 serum and cell surface components, and most of these components show a characteristic domain structure, enabling us to trace the evolution of the genes based on their structures. Ongoing genome projects in both vertebrates and invertebrates revealed that most domains used by mammalian complement components are found in both protostomes and deuterostomes. However, the unique combinations of them as found in mammalian complement components are present only in deuterostomes, indicating that the complement system was established in the deuterostome lineage. Unexpectedly, the complement system of an invertebrate deuterostome, ascidian, shows a similar level of complexity as that of mammals. However, phylogenetic analysis suggested that expansion of complement genes by gene duplications occurred independently both in the ascidian and vertebrate lineages. Although most characteristic domain structures of the mammalian complement components are found in ascidians, detailed evolutionary analysis casts doubt on their mutual reactivity. Thus, the vertebrate complement system seems to be established by integrating some independent parts into one reaction system.
Posted by: niiicholas on Feb. 17 2004,17:34
Interesting article comparing animal and plant immune systems:
< http://www.the-scientist.com/yr2004/feb/research4_040216.html >
Same Tools, Different BoxesConvergent evolution plays out in plant and animal innate immunity | By Philip Hunter
As life's diversity demonstrates, nature has a pretty large toolbox for designing adaptations. While in many ways an efficient builder, it often reuses blueprints, even if not starting with the same tools. Analogous wing structures in bird and bat suggest a why-mess-with-success ethos. New World cacti and desert-dwelling Euphorbiaceae in the Old World share protective spines and photosynthesizing stems even though the last common ancestor predates such modifications.
Beyond structural adaptations, researchers are investigating convergent evolution at the molecular level, and this may allow for broader comparisons even between plants and animals. Both, of course, share the building blocks and fundamental biochemistry that evolved before the two kingdoms presumably diverged from common single-celled ancestors. But with their radically different cell structures, plants and animals were thought to have pursued largely independent evolutionary routes. Such disparity was reflected in the lack of interaction between the respective research communities.
But much is changing, especially with respect to the study of innate immunity, which turns out to involve strikingly similar mechanisms in both plants and animals. One can find resemblances in the receptors that recognize pathogenic components such as lipopolysaccharide; in the signaling systems that initiate responses through kinase cascades; and in the defense mechanisms, including reactive molecules such as nitric oxide, says Jonathan Jones, senior scientist at the Sainsbury Laboratory of the John Innes Centre in Norwich, UK.
Moreover, says Jones, autoimmune disorders can develop in plants as well as animals. In many cases, researchers consider plant and animal innate-immunity analogs to have evolved independently, because the underlying genes involved are radically different. Here, convergence is occurring purely at the functional level, according to Daniel Klessig, president and CEO of Boyce Thompson Institute (BTI) for Plant Research in Ithaca, NY. But now, say some, both functional and genetic similarities between plant and animal immunity are leading to cross-pollination between the respective research fields.
NOS BY ANY OTHER NAME Functional similarities can exist without accompanying sequence homologies. Klessig offers parallels in nitric oxide production between plants and animals. Nitric oxide is believed to exert direct anti-microbial effects by interfering with protein function and forming cytotoxic oxidants.
Klessig's group recently discovered the enzyme responsible for nitric oxide production in plants, a variant of the glycine decarboxylase complex's P protein.1 This plant nitric oxide synthase (NOS) shares biochemical and kinetic features with animal NOSs, says Klessig.
"Like the mammalian NOS, it is induced or activated by infection and has very high specific activity. We have also recently, in collaboration with Greg Martin's group [also at BTI] shown that like in animals, silencing of the plant NOS suppresses disease resistance." Although the animal and plant NOSs are very similar in what they do, not much of the underlying genetic sequence is common. "This suggests they use different chemistry for nitric oxide production," says Klessig.
There are some proteins common to plants and animals with considerable sequence homology, but researchers dispute whether many of these evolved purely to serve innate immunity. Some hypothesize that the evolution of mitogen-activated protein kinases (MAPKs) was driven at least partly by the requirements of innate immunity. MAPKs largely regulate mitosis, but they also have been implicated in the innate immune response as a signaling intermediary connecting recognition and response mechanisms, with some apparently striking parallels in plants and animals. Interference with MAPK signaling depletes resistance in both plants and animals. But while there is evidence that MAPK signaling has a role in programmed cell death to stop the spread of bacterial or viral infection, the exact nature of the immune responses elicited by the signaling is unknown.2
According to plant immunologist Jeff Dangl at the department of biology, University of North Carolina, Chapel Hill, some sequence homology in the MAPKs would be expected, given that genes encoding for kinase cascades account for about 10% of most eukaryote genomes. MAPKs have been widely available throughout evolution for a variety of signaling functions. "If you're going to do something, you'll pull something out of the toolkit," says Dangl. Here parallel paths to innate immunity modified a common tool developed for other functions.
Yet, some aspects of innate immunity require convergence at the molecular level, notably in common pathogen receptors. This can involve direct recognition of factors on the pathogen's surface, or response to pathogen proteins injected into the cell. Pathogens produce proteins that interfere with the host defenses, says Jones. In turn, the host has evolved proteins in an effort to negate the effect of pathogen interference, leading to a long-term molecular arms race. "You've got these layers of yin and yang between the host and the parasite," Jones notes.
Some responses are conserved between plants and animals. To grow in any host, flora or fauna, bacteria must identify their environment and then switch on genes to encode the proteins required for replication under those conditions. A primary locus for Gram-negative bacteria is the HRP gene locus, which encodes the Type III machinery that secretes so-called effector proteins directly into the cell. Mary Beth Mudgett,2 assistant professor of biology at Stanford University, focuses on the molecular mechanisms used collectively by Type III effector proteins to suppress host defenses. Mudgett's work is in plants, but a class of Type III effector protein is conserved between the bacteria infecting both plants and animals. The animal-infecting bacteria produce YopJ, but plant-pathogen versions now have been discovered sharing a key, four-amino-acid catalytic site. YopJ and its counterpart operate by cutting a protein involved in the MAPK cascade, blocking the pathway as a result.3
HOMOLOGY IN THE CARDS Significant similarities exist between some plant and animal receptors involved in sensing pathogen surface proteins, Jones says. He cites the case of NB/LRR proteins in plants, which have similar functions and sequences to the CARD4 and CARD15 proteins in animals. These proteins recognize lipopolysaccharides found on the outer membranes of some bacteria, and transduce internal cellular responses.4 The molecular characterization of these proteins has revealed that in plants and animals, similar domains are used in pathogen recognition and response initiation. This has stimulated debate over whether these proteins evolved specifically for disease resistance or whether it is another case of innate immunity reaching into the common toolkit.
Whatever the case, the overlap between CARD15 and NB/LRR has had felicitous implications for research into Crohn disease. A mutation in CARD15 increases susceptibility to Crohn, possibly by creating an inflammatory response to benign gut bacteria antigens.4 And the similarity with NB/LRR makes it possible to study gene mutants in plants. Dangl says that the genetics of resistance is easier to study in plants than in animals such as mice. "The plant guys are well ahead of the animal guys in terms of knowing what genes in the host are needed to transduce a signal perceived by these receptors, because it is easier to do experiments to isolate mutants," says Dangl. In other words, the genes involved in signaling the innate response to gut bacteria in humans can be studied to some extent in rapidly growing plants such as Arabidopsis. For this reason, plant work has now been well cited by Crohn disease researchers, according to Dangl. >P> And though there's nothing analogous to Crohn disease, plants can experience autoimmune disorders, such as the so-called paranoid plant syndrome. Paranoid plants include a collection of disease lesion-mimic mutants (Les) in maize that spontaneously form necrotic lesions usually seen in diseased plants. Also, Arabidopsis mutants for MAPK 4 are stunted and display systemic defenses, such as salicylic-acid production, even in the absence of a pathogen.5 Here, the mutation of a negative immune regulator confers resistance to a number of infections.
"Defense pathways that are powerful need strong negative regulation, and autoimmune diseases can occur when this negative regulation does not work," says Jones. The existence of autoimmune disease in plants is interesting, but has no significance for human conditions such as arthritis, he says.
But the overlap between plant and animal immunity may have potential significance for piecing together the innate system's finer pathways. Dangl concedes that the plant research community, of which he is a part, has been quick to exploit these connections to compete for funding generally destined for animal studies. "The animal guys ignored us," he says. Now they must take notice.
Philip Hunter (firstname.lastname@example.org) is a freelance writer in London.
1. M.R. Chandok et al., "The pathogen-inducible nitric oxide synthase (iNOS) in plants is a variant of the P protein of the glycine decarboxylase complex," Cell, 113:469-82, 2003.
2. R.A. Alegado et al., "Characterization of mediators of microbial virulence and innate immunity using the Caenorhabditis elegans host-pathogen model," Cell Microbiol, 5:435-44, 2003.
3. A. Hotson et al., "Xanthomonas type III effector XopD targets SUMO-conjugated proteins in plants," Mol Microbiol, 50:377-89, 2003.
4. J.P. Roberts, "An immunological role in the CARDs," The Scientist, 17:29-30, Oct. 20, 2003.
5. M. Petersen et al., "Arabidopsis MAP kinase 4 negatively regulates systemic acquired resistance," Cell, 103:1111-20, 2000.
Posted by: theyeti on June 19 2004,12:16
< Immunol Rev. 2004 Apr;198:36-58. >
Evolution of the innate immune system: the worm perspective.
Schulenburg H, Kurz CL, Ewbank JJ.
Simple model organisms that are amenable to comprehensive experimental analysis can be used to elucidate the molecular genetic architecture of complex traits. They can thereby enhance our understanding of these traits in other organisms, including humans. Here, we describe the use of the nematode Caenorhabditis elegans as a tractable model system to study innate immunity. We detail our current understanding of the worm's immune system, which seems to be characterized by four main signaling cascades: a p38 mitogen-activated protein kinase, a transforming growth factor-beta-like, a programed cell death, and an insulin-like receptor pathway. Many details, especially regarding pathogen recognition and immune effectors, are only poorly characterized and clearly warrant further investigation. We additionally speculate on the evolution of the C. elegans immune system, taking into special consideration the relationship between immunity, stress responses and digestion, the diversification of the different parts of the immune system in response to multiple and/or coevolving pathogens, and the trade-off between immunity and host life history traits. Using C. elegans to address these different facets of host-pathogen interactions provides a fresh perspective on our understanding of the structure and complexity of innate immune systems in animals and plants.