Why is CTX all the RAGE?

0

INSTITUT

PASTEURIELSEVIER

Res. Immunol.

Paris 1996

1996, 147, 261-266

Why is CTX all the RAGE? L. Du Pasquier Base1 Institute for Immunology,

From several contributions to the Forum published in this issue (Medzhitov and Janeway, Ohno, Makody), it seems that the origin of T-cell receptors (TCR) and immunoglobulins (Ig) of vertebrates would be best explained by the existence of a primitive ancestral receptor probably resembling or being a non-dimeric adhesion molecule. Made of Ig domains without somatic rearrangement and therefore not clonally expressed, this molecule would transduce, through its own cytoplasmic tail, some signals to the inside of a primitive lymphocyte. Other contributions (Litman and Rast, Du Pasquier and Chretien) point out candidates that would partially fulfill these criteria. We proposed that CTX (for cortical thymocyte marker in Xenopus) might resemble the early lymphocyte receptor, but indeed it seems to be already a differentiated type of receptor. Its restricted tissue distribution is certainly a sign of specialization, and being part of a dimer, which CTX is likely to be (Robert er uZ., submitted), is already an advanced character. For more primitive genes with ancestral characteristics, data base search did not yield many other candidates. We therefore took a different approach, neglecting primary sequence homology at least in a first step. We recently found that CTX segregates with the Xenopus major histocompatibility complex (MHC) in crosses (unpublished). We then looked to see whether other Ig superfamily

Submitted May 17, 1996, accepted May 30, 1996. (*) For correspondence.

(*) and I. Chretien PO Box, CH 4005, Base1 (Switzerland)

(IgSF) members had been reported to segregate with MHC. B-G (Kaufman et al., 1991) (already mentioned in our paper), MOG (the myelin/oligodendrocyte glycoprotein of Gardinier and Matthieu, 1995), and butyrophilin (Vernet et al., 1993) are all obviously homologous in the V region of CTX, but they are otherwise very different. RAGE (receptor for advanced glycosylation end products) (Neeper et al., 1992) is a member of the IgSF; its apparently monomeric molecule consists of one V, domain (according to our nomenclature) and two C, domains. Although it resembles the molecule named “Amalgam” from Drosophila (already picked up as homologous to CTX, see our contribution), RAGE’s external domains, unlike Amalgam, are associated with a transmembrane region and a long cytoplasmic tail. RAGE is also known in rat and cattle and is located within the MHC (3’ to DR), at least in human beings (Sugaya et al., 1994). Aside from the highly conserved V and C2 specific residues, the homology of RAGE with CTX is poor but its exon-intron organization bears striking similarities to CTX. Its V domain is encoded by 2 half-domain exons (fig. 1). RAGE Vb and CTX Vb have the same length, glycosylation sites are in the same position, and splicing between Va and Vb is, in both cases, of type 0. The splicing between transmembrane and cytoplasmic domains is of type 2 (rarely encountered in

L. DU PASQUIER AND I. CHRkTIEN

262

Va, Vb CTX and RAGE exons

CTXVa

I i .v. i + I VQVTIQNPII~SGQ~YCTYILNNQNKNNLVIQWNIFQAKSQNQET

RAGEVa GAVVGAQ-INIRIGEPLVLKCKGAPKKPPQ---RLEWKL

CTXVb

** I * * ** Ii i I I II VFFYQNGQSLSGPSYKNRVTAAMSP~TISNMQSQDTGIYTCEVLNLPESSGQGKILLTVL

RAGEVb NTGRTEAWKVLSPQGGGPWDSVARVL~LFLPAVGIQDEGIFRCRAMNGKETKSNYRVRVY **

Splicing of CTX and RAGE exons CTX

Vi3

E

CTX

vb

RAGE Va T V F K L GAA ACA gt...ag GTG TTC AAA CTG gt...ag

TM M K ATG AA

RAGE

CY gt...ag

A

T ACT

vb N T AAC ACA

TM R GAG AG gt...ag

CY

E

G

Fig. 1. Alignment of Va and Vb sequences of RAGE and CTX according to the exon-intron ture (top), and comparison of splicing sites between CTX and RAGE exons (bottom).

K

AAG struc-

Top : Note the conservation of size and of the glycosylation sites in Vb. Stars point to conserved Ig superfamily V domain characteristic residues. Slashes indicate further similarities (for a more thorough comparison, see fig. 3). Boxes represent possible glycosylation sites.

Ig genes) in both cases. Splicing between V and C, or between the two C domains, is, in both cases, of type 1. RAGE C, domains are encoded by either 3 or 2 exons. The membrane proximal C, domain of RAGE with its 2 exons resembles the CTX C domain. Thus far, we could not find any particular receptor transmembrane domain (TM) related to that of CTX. It now seems that RAGE TM is rather closely related to it (fig. 2).

In terms of a relationship to the early lymphocyte receptor, interesting news comes from RAGE’s function and tissue distribution. Expressed on some endothelia and on cells of the haematopoietic lineage (T cells and monocytes), RAGE acts as a receptor for non-enzymatically glycosylated molecules, enabling the phagocytosis of aged red cells, and the elimination of “old’ albumin (Imani et al., 1993 ; Vlassara et al., 1987) and probably many other proteins. We

WHY IS CTX ALL THE RAGE?

263

been

Makody. It could resemble a primitive non-dimeric lymphocyte receptor, even more primitive than CTX (monomer V, versus a potential dimer of V,) but related to it more than to any other Ig superfamily members (e.g. MUC18 half domain exons do not match as well with RAGE or CTX). Is there a relationship among all the V domains of the Ig superfamily members encoded in the MHC? Alignments of the V domain of CTX, RAGE, B-G, MOG, and butyrophilin are shown in figure 3.

might not be very far from the receptor requested by Ohno. Its lack of rearrangement and its apparent ability to signal, by itself, through a long cytoplasmic tail, also make it fit the categories predicted by Medzhitov and Janeway or by

Perhaps the role of RAGE could now stimulate some functional studies to discover the role of B-G and CTX. Moreover, RAGE’s involvement in the inner world of the individual may reactivate the debate about how Ig and TCR developed. Stewart (1992), who argues that these receptors did not arise in the first place to fight pathogens, would seem to be vindicated; RAGE does not deal with pathogens. RAGE’s function

CTX and RAGE transmembrane

t

Fig. 2. Alignment

domains

t

of the transmembrane and RAGE.

domains

t

of CTX

Strong conservations and similarities have boxed. Arrows point to less conserved residues.

CTX/B-G/RAGE/Butyrophilin/lu100

A [-------------------------I CTXV B-G RAGE Buty. MOG

-----_----------

Q--VTIQNFIINVTSGQNATLYCTYILNNQKKNNL~IQWNIFQA' QIIVVAPSLRVTAIVGQDVVLRCHLSPCKDVR-NSDIRW-IQQR GAVVGAQN--ITARIGEPLVLKCKGAP---KKPPQRLEWKLNTAPFDVIGPEPILAVVGEDAELPCRLSPNASAE-HLELRWFRKKV QFRVIGPGHPIRALVGDEAELPCRISPGKNAT-GMEVGWYRSPF Cl

cll

[---------mm]

D [------------]

KSQNQETVFFYQNGQSLSGPS--YKNRVTAA---MSPG SsRL---VHHYRNGVDL-GQMEEYKQRTELLRDOLSDG -GR---TEAWKVLSPQGGEPWD~S~ARV--LPNG-S--SPAVL-VHRDGREQ-EAEQMPEYRGRAFLVQDGIAKG -SR---VVHLYRNGKDQAEQAPEYRGRTELLKESIGEG

E [ -m----------CTXV B-G RAGE Buty. MOG

c

-B [--------------]

[--------------1 CTXV B-G RAGE Buty. MOG

COMPARISON

-f

1

[---------------,

C-------------------l

G

NATITISNMQSQDTGIYTCEVLNLPESSGQGKILLTVL NLDLRITAVTSSDSGSYSCAVQDGDAYAEAVVNLEVSD - - - LFLPAVGIQDEGIFRCRAMNRNGKETKSNYRVRVRVALRIRGVRVSDDGEYTCFFREDGSYEEALVHLKVAA KVALRIQNVRFSDEGGYTCFFRDHSYQEEAVVELKVED

Fig. 3. Alignments

of the V domains

of MHC-encoded molecules CTX, B-G, RAGE, butyrophilin and MOG. The diglycine bulge in the G strand that distinguishes CTX from the other members is indicated. Bold letters correspond to amino acids shared in more than two members or shared between CTX and any other member, in order to put the emphasis on homologies with CTX.

264

L. DU PASQUIER AND I. CHRETIEN

is also reminiscent of the old idea of Grabar (1947 ; reviewed in 1983) that the precursors of antibody developed as transporters of catabolism products. If RAGE, as we propose, resembles an early lymphocyte receptor, then a homologue should be found in cyclostomes and invertebrates. The fact that no Ig superfamily member has ever been found in this class may be due to the poor choice of probes or PCR primers designed too much in function of their Ig and TCR origin. Indeed, the amino acid primary sequence RAGE is not homologous to Ig and TCR, but only to some cell adhesion molecules (Neeper et al., 1992). Perhaps the border between cell adhesion molecules and bona fide receptors will turn out to be more tenuous than originally thought. More clarification will come when the signalling or the absence of signalling properties of these molecules will be elucidated. The recurrent presence in the MHC of members of the Ig of members is, given the number of chromosomes of vertebrates and the number of IgSF members, not a total surprise ! However, the close linkage of B-G, RAGE, butyrophilin, MOG, and now CTX might not be coincidental. This point had already excited the curiosity of Linsley et al. (1994), who pointed to the relationship between B, and all the V domains encoded in the MHC (RAGE was not considered). Following the discovery of a duplication of the MHC class III region in mouse and man, a brilliant new insight into the evolution of the MHC genetic region has recently been proposed (Kasahara et al., 1996a). The primitive MHC seems to have existed without class I and class II, which were latecomers in a duplicated version of the MHC (this is at least one of the scenarios proposed by Kasahara et al. to explain the evolution of the MHC). Were these Ig superfamily members in the original MHC? Have receptors of the RAGE-CTX type preceded the appearance of class I and class II? If our idea about the primordial necessity to select against autoreactivity after introducing somatic rearrangement (see our contribution) is correct, then it is not inconceivable that the following scenario (which takes advantage of some of Medzhitov

and Janeway’s conclusions al’s model) took place.

and of Kasahara

et

The somatic rearrangement machinery was introduced in genes related to CTX and RAGE that were then present in the primordial MHC and the exons of which, in the early stage, corresponded to entire domains. “Because, to make use of this specific recognition [acquired by rearrangement], the specificities of clonotypic receptors must somehow be correlated with the semantic information provided by [a] non-clonal recognition system” (Medzhitov and Janeway, this Forum), the new receptor had to adapt to a system of signalling determining what effector mechanism will be activated. We propose that at the level of the receptor, this went together with the “differentiation” of its constant C2 into a Cl domain, assuming that the adjacent signalling molecules have to be associated with the constant region of the receptor. To explain the presence of Cl in the class I and II molecules, the gene for such a C 1 would be recruited and assembled later by exon shuffling to the primordial class II or class I peptide binding domain gene as already proposed by Flajnik et al. (1992) and rediscussed by Kasahara et al. (1996b). Perhaps the structural characteristics of Cl that made it suitable for interaction with coreceptors of the type of CD3 and MB l-B29 were also useful in some way for the interaction of MHC with CD4 and CD8, even though, in this case, the interacting molecule would come from the other cell partner. This is, of course, purely hypothetical, but we cannot help realizing that the CD8 binding region of the MHC class I for instance, is in a region where precisely Cl differs from C2 in term of helices and strand arrangement. Unlike Kasahara et al., we do not think that the RAGE or equivalent molecule C domains were donated directly to the future MHC class I or II genes because of the differences between Cl and C2. As a complement to the history of C domains, it is worth mentioning that recently the gene for butyrophilin has been completely sequenced. This putative receptor involved in lactation is made of an Ig V, domain and of what has been proposed to be a Cl domain (Taylor et al., 1996). This would be the first occurrence of a Cl domain outside bona fide Ig, TCR, and MHC class I-11 molecules. It should be

265

WHY IS CTX ALL THE RAGE?

scrutinized carefully to find out whether it bears some primitive characters that could explain the origin of Cl. The homology is not very pronounced, and several Cl key residues are missing, In any case, the specialization and the mammalian specificity of this molecule argue for a rather late evolutionary appearance. If it turns out to be a bona fide C 1, then perhaps the conservation of Cl in butyrophilin, TCR, Ig and MHC or beta-, microglobulin is less due to selection of Cl specific characteristics than to invention of Cl in this region of the genome and its distribution to other members. It would then be a marker of the “geographical” origin of this type of domain. The idea remains that the original somatically rearranging receptor developed from an MHC-linked primordial ancestor. With the increasing number of Ig superfamily members being discovered either with primitive characters in vertebrates (such as V2 and C2 or CTX-like V domains), or with evolved characters in invertebrates (V domains, molecules described by Hoek et al., 1996, involved in the immune system of invertebrates such as the mollusk defence molecule (MDM)). New strategies should be planned based on homology with these molecules in order to discover IgSF members in cyclostomes and find out whether some of them will be linked to their primordial MHC (i.e., without class I and class II according to Kasahara et al.). Whether or not adaptive immunity evolved as a consequence of jaw development, as audaciously proposed by Anderson and Matsunaga, it seems increasingly likely that the rearranging machinery was introduced in jawed ancestors of the Chondrichthyans, the totally extinct Placoderms. In Kasahara et d’s model, it is also proposed that the above mentioned MHC duplication took place after the emergence of the jawless fish but before that of Chondrichthyans. This reinforces the idea of a “simultaneous” or “linked” appearance of the somatically rearranging receptor at a certain step of vertebrate evolution and that of the molecules and the processing pathways necessary for their selection. Key-words: Ig superfamily.

T lymphocyte,

TCR, Evolution

;

Acknowledgements We thank M. Kasahara for pointing out to us the existence of RAGE and C.M. Steinberg for critical reading of the manuscript. The Base1 Institute for Immunology is supported by F. Hoffmann-LaRoche

was founded AG, Basel.

and

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ducts, PBX2 homeobox gene and a notch homolog, human counterpart of mouse mammary tumor gene int-3. Genomics, 23, 408-419. Taylor, M.R., Peterson, J.A., Ceriani, R.L. & Couto, J.R. (1996), Cloning and sequence analysis of human butyrophilin reveals a potential receptor function. Biochim. Biophys. Acta, 1306, l-4. Vemet, C., Boretto, J., Mattei, M.G., Takahashi, M., Jack, L.J., Mather, I.H., Rouquier, S. & Pontarotti, P.

Boehringer

(1993), Evolutionary study of multigenic families mapping close to the human MHC class I region. J. Mol. Evol., 37, 600-612. Vlassara, H., Valinsky, J., Brownlee, M., Cerami, C., Nishimoto, S. & Cerami, A. (1987), Advanced glycosylation endproducts on the erythrocyte cell surface induce receptor-mediated phagocytosis by macrophages. A model for turnover of aging cells. J. Exp. Med., 166, 539-549.

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