Two distinct immunoglobulin heavy chain isotypes in a primitive, cartilaginous fish,Raja erinacea

Abstract

Immunoglobulin heavy chain genes in Raja erinacea (little skate) are organized in clusters consisting of VH, DH, JH segments and CH exons (1). An immunoglobulin heavy chain mu-like isotype that exhibits 61-91% nucleotide sequence identity in coding segments to the Heterodontus francisci (horned shark) mu-type immunoglobulin is described. The overall length of the mu-type clusters is approximately 16 kb; transmembrane exons (TM1 and TM2) are located 3 to CH exon 4 (CH4). In three of four TM-containing genomic clones, a significant deletion is present in TM1. A second isotype of Raja immunoglobulin heavy chain genes has been detected by screening a spleen cDNA library with homologous Raja VH- and CH1-specific probes complementing the respective regions of the mu-like isotype. Weak hybridization with VH-specific probes and no discernable hybridization with C mu-specific probes were considered presumptive evidence for a second immunoglobulin isotype that nominally is designated as X-type. The Vx region of the X-type cDNA is approximately 60% identical at the nucleotide (nt) level to other Raja VH segments and thus represents a second VH family. Putative Dx and Jx sequences also have been identified. The constant region of the X-type immunoglobulin heavy chain gene consists of two characteristic immunoglobulin domains and a cysteine-rich carboxy terminal segment that are only partially homologous with the mu-like isotype. Genomic Southern blotting indicates that the V and C segments of both immunoglobulin heavy chain isotypes are encoded by complex multigene families. Vx- and different Cx-specific probes hybridize to different length transcripts in northern blot analyses of Raja spleen RNA suggesting that the regulation of expression of the X-type genes may involve differential RNA processing.

IgD, like IgM, is a primordial immunoglobulin class perpetuated in most jawed vertebrates

“…The IgH gene organization is not known in lungfish, but at least from teleosts through mammals the ␦ gene is downstream of , and the ␦ mRNA is generated by alternative splicing from a large pre-mRNA [this has not been shown definitively in ray-finned fish but must be true (7)]. Because cartilaginous fish have discrete clusters of and genes (10,13,31), each cluster with its own set of V exons, how did the genes for the two isotypes become closely linked? It is difficult to even speculate on this problem, and it may never be solved, but the ''functional clustering'' of these genes must have occurred before the advent of isotype switching in amphibians.…”

“…A similarly strange phylogenetic jump surrounds the isotype IgW, which was discovered first in skates (9,10) and later in sharks (11,12). The secreted version of the IgW H chain ( ) is present in long and short forms in all elasmobranchs tested to date, and the TM form is also differentially spliced (13).…”

Identification of IgF, a hinge-region-containing Ig class, and IgD inXenopus tropicalis

“…Antibodies containing two constant region domains have previously been found in cartilaginous fish, lungfish, and ducks (13,23,28,40); however, these antibodies are generated through the use of different transcription termination sites or through alternative RNA splicing of the full-length transcript (13,28). Identification of a two-domain Ig heavy-chain isotype has also recently been reported in fugu (12).…”

“…IgM is found in all vertebrates (13)(14)(15)(16)(17)(18); however, the phylogenetic origin of the remaining mammalian Igs is less well established, although Igs referred to as IgA͞IgX and IgY have been reported in birds, reptiles, and amphibians (1,2,(19)(20)(21)(22). Cartilaginous fish and lungfish express IgM, IgNAR, and͞or IgW͞ IgX, containing either two or more than four constant region domains (1,15,23). Bony fish express three heavy-chain isotypes, IgM, IgD, and IgZ͞IgT (24,25).…”

An evolutionarily mobile antigen receptor variable region gene: Doubly rearranging NAR-TcR genes in sharks

“…Cartilaginous fish are the oldest animals having an adaptive immune system centered on rearranging antigen receptors. They have all four types of TcR (␣, ␤, ␥, ␦) (1), three Ig isotypes [IgM (2,3), IgW (4), and IgNAR (new antigen receptor) (5)], the recombination-activating gene recombinase (6), and polymorphic MHC genes (7,8). Studies of modern sharks may shed light on the origins of adaptive immunity (9).…”