Louis Du Pasquier

Immunoreceptor tyrosine‐based inhibition motifs: a quest in the past and future

Summary: Since an immunoreceptor tyrosine‐based inhibition motif (ITIM) was first identified in the intracytoplasmic domain of FcγRIIB, ITIMs have been found in a large number of inhibitory molecules that were shown to negatively regulate cell activation. Due to their wide tissue distribution and to the variety of their extracellular ligands, ITIM‐containing molecules are involved in the control of a large spectrum of biological functions, mostly but not exclusively related to immunity. On the basis of sequence comparison, ITIMs were structurally defined as 6‐amino acid sequences containing a tyrosine (Y) with loosely conserved N‐terminal (Y−2) and C‐terminal (Y+3) residues. Molecular analysis of signaling events demonstrated that when coaggregated with activating receptors, ITIMs are phosphorylated by Src‐family tyrosine kinases, which enables them to recruit Src homology 2 domain‐containing phosphatases that antagonize activation signals. Because ITIM‐dependent negative regulation seems to be a fundamental regulatory mechanism, both in rodents and in humans, and because it can be used either as a target or as a powerful tool in various diseases, we undertook (i) a genome‐wide search of potential novel ITIM‐containing molecules in humans, mice, frogs, birds, and flies and (ii) a comparative analysis of potential ITIMs in major animal phyla, from mammals to protozoa. We found a surprisingly high number of potential ITIM‐containing molecules, having a great diversity of extracellular domains, and being expressed by a variety of immune and non‐immune cells. ITIMs could be traced back to the most primitive metazoa. The genes that encode ITIM‐containing molecules that belong to the immunoglobulin superfamily or to the C‐lectin family seem to derive from a common set of ancestor genes and to have dramatically expanded and diverged in Gnathostomata (from fish to mammals).

Genetics of polyploid Xenopus

Abstract Polyploidy, in providing duplicate genetic information, might have been an important factor in vertebrate evolution. The African Clawed Frogs of the genus Xenopus , which comprises bisexual species of several different ploidy levels, represent a promising model for an experimental approach to the many problems associated with a polyploid condition.

Plasticity of Animal Genome Architecture Unmasked by Rapid Evolution of a Pelagic Tunicate

Ocean Dweller Sequenced The Tunicates, which include the solitary free-swimming larvaceans that are a major pelagic component of our oceans, are a basal lineage of the chordates. In order to investigate the major evolutionary transition represented by these organisms, Denoeud et al. (p. 1381, published online 18 November) sequenced the genome of Oikopleura dioica, a chordate placed by phylogeny between vertebrates and amphioxus. Surprisingly, the genome showed little conservation in genome architecture when compared to the genomes of other animals. Furthermore, this highly compacted genome contained intron gains and losses, as well as species-specific gene duplications and losses that may be associated with development. Thus, contrary to popular belief, global similarities of genome architecture from sponges to humans are not essential for the preservation of ancestral morphologies. A metazoan genome departs from the organization that appears rigidly established in other animal phyla. Genomes of animals as different as sponges and humans show conservation of global architecture. Here we show that multiple genomic features including transposon diversity, developmental gene repertoire, physical gene order, and intron-exon organization are shattered in the tunicate Oikopleura, belonging to the sister group of vertebrates and retaining chordate morphology. Ancestral architecture of animal genomes can be deeply modified and may therefore be largely nonadaptive. This rapidly evolving animal lineage thus offers unique perspectives on the level of genome plasticity. It also illuminates issues as fundamental as the mechanisms of intron gain.

An evolutionarily conserved target motif for immunoglobulin class-switch recombination

Immunoglobulin H class-switch recombination (CSR) occurs between switch regions and requires transcription and activation-induced cytidine deaminase (AID). Transcription through mammalian switch regions, because of their GC-rich composition, generates stable R-loops, which provide single-stranded DNA substrates for AID. However, we show here that the Xenopus laevis switch region Sμ, which is rich in AT and not prone to form R-loops, can functionally replace a mouse switch region to mediate CSR in vivo. X. laevis Sμ–mediated CSR occurred mostly in a region of AGCT repeats targeted by the AID–replication protein A complex when transcribed in vitro. We propose that AGCT is a primordial CSR motif that targets AID through a non-R-loop mechanism involving an AID–replication protein A complex.

Antibody diversity in lower vertebrates—why is it so restricted?

Recent studies have revealed that lower vertebrates such as fish, amphibians and reptiles have an antibody repertoire that is much less diverse than that of mammals. It is suggested that the various mechanisms generating antibody diversity are the same in all vertebrates but that non-immunological parameters such as mode of ontogenetic development and cell cycle properties allow fewer somatic events in generating the antibody repertoire of cold-blooded vertebrates.

CTX, a Xenopus thymocyte receptor, defines a molecular family conserved throughout vertebrates

CTX, a cortical thymocyte marker in Xenopus, is an immunoglobulin superfamily (Igsf) member comprising one variable and one constant C2‐type Igsf domain, a transmembrane segment and a cytoplasmic tail. Although resembling that of the TCR and immunoglobulins, the variable domain is not encoded by somatic rearrangement of the gene but by splicing of two half‐domain exons. The C2 domain, also encoded by two exons, has an extra pair of cysteines. The transmembrane segment is free of charged residues, and the cytoplasmic tail (70 amino acids) contains one tyrosine and many glutamic acid residues. ChT1, a chicken homologue of CTX, has the same structural and genetic features, and both molecules are expressed on the thymocyte surface. We cloned new mouse (CTM) and human (CTH) cDNA and genes which are highly homologous to CTX/ChT1 but not lymphocyte specific. Similarity with recently described human cell surface molecules, A33 antigen and CAR (coxsackie and adenovirus 5 receptor), and a number of expressed sequence tags leads us to propose that CTX defines a novel subset of the Igsf, conserved throughout vertebrates and extending beyond the immune system. Strong homologies within vertebrate sequences suggest that the V and C2 CTX domains are scions of a very ancient lineage.

The Dscam Homologue of the Crustacean Daphnia Is Diversified by Alternative Splicing Like in Insects

In insects, the homologue of the Down syndrome cell adhesion molecule (Dscam) is a unique case of a single-locus gene whose expression has extensive somatic diversification in both the nervous and immune systems. How this situation evolved is best understood through comparative studies. We describe structural, expression, and evolutionary aspects of a Dscam homolog in 2 species of the crustacean Daphnia. The Dscam of Daphnia generates up to 13,000 different transcripts by the alternative splicing of variable exons. This extends the taxonomic range of a highly diversified Dscam beyond the insects. Additionally, we have identified 4 alternative forms of the cytoplasmic tail that generate isoforms with or without inhibitory or activating immunoreceptor tyrosine-based motifs (ITIM and ITAM respectively), something not previously reported in insects Dscam. In Daphnia, we detected exon usage variability in both the brain and hemocytes (the effector cells of immunity), suggesting that Dscam plays a role in the nervous and immune systems of crustaceans, as it does in insects. Phylogenetic analysis shows a high degree of amino acid conservation between Daphnia and insects except in the alternative exons, which diverge greatly between these taxa. Our analysis shows that the variable exons diverged before the split of the 2 Daphnia species and is in agreement with the nearest-neighbor model for the evolution of the alternative exons. The genealogy of the Dscam gene family from vertebrates and invertebrates confirmed that the highly diversified form of the gene evolved from a nondiversified form before the split of insects and crustaceans.

The immune system of invertebrates and vertebrates.

All metazoans protect themselves from invasion of microorganisms, parasites, viruses and even cells from individuals of the same species. In all phyla, precise mechanisms of recognition allow for discrimination between self and non-self avoiding the danger of contamination. In that sense, all metazoans have an ‘immune system’. This does not mean that the recognition events and the resulting effector reactions are mediated by homologous systems across metazoans. Some features may be conserved while some will be specific to one phylum or even one class within a Ž phylum Fig. 1 and review in Du Pasquier and . Flajnik, 1999 . This overview will summarise both the conserved and the divergent aspects of the various immune systems of metazoans throughout their history ultimately revealing the unique characteristics of the immune system in vertebrates.

Changes in the immune system during metamorphosis of Xenopus

Profound immunological changes occur as tadpoles metamorphose into adult amphibians. These include the expression of a different antibody repertoire, a lessening of skin graft tolerance, the appearance on leukocytes of class I MHC antigens. Here Martin Flajnik and his colleagues review what is known of these changes in Xenopus and speculate on how they may occur.

Expression of MHC Class II Antigens During Xenopus Development

Larval and adult forms of the amphibian Xenopus differ in their MHC class II .expression. In tadpoles, class II epitopes can be detected by monoclonal antibodies only on B cells, macrophages (whatever their location), spleen reticulum, thymus epithelium, and the pharyngobuccal cavity. In contrast, all adult T cells express class II on their surface. The transitions in class II expression occur at metamorphosis and are accompanied by other changes. The skin is invaded by class II positive dendritic cells, and the skin glands differentiate and also express class II. The gut, which expressed class II in discrete areas of the embryonic tissue, becomes invaded with B cells, and its epithelium also becomes class II positive.