Specific Evolution and Gene Family Expansion of Complement 3 and Regulatory Factor H in Fish

Abstract

The complement system comprises a large family of plasma proteins that play a central role in innate and adaptive immunity. To better understand the evolution of the complement system in vertebrates and the contribution of complement to fish immunity comprehensive in silico and expression analysis of the gene repertoire was made. Particular attention was given to C3 and the evolutionary related proteins C4 and C5 and to one of the main regulatory factors of C3b, factor H (Cfh). Phylogenetic and gene linkage analysis confirmed the standing hypothesis that the ancestral c3/c4/c5 gene duplicated early. The duplication of C3 (C3.1 and C3.2) and C4 (C4.1 and C4.2) was likely a consequence of the (1R and 2R) genome tetraploidization events at the origin of the vertebrates. In fish, gene number was not conserved and multiple c3 and cfh sequence related genes were encountered, and phylogenetic analysis of each gene generated two main clusters. Duplication of c3 and cfh genes occurred across the teleosts in a species-specific manner. In common, with other immune gene families the c3 gene expansion in fish emerged through a process of tandem gene duplication. Gilthead sea bream (Sparus aurata), had nine c3 gene transcripts highly expressed in liver although as reported in other fish, extra-hepatic expression also occurs. Differences in the sequence and protein domains of the nine deduced C3 proteins in the gilthead sea bream and the presence of specific cysteine and N-glycosylation residues within each isoform was indicative of functional diversity associated with structure. The diversity of C3 and other complement proteins as well as Cfh in teleosts suggests they may have an enhanced capacity to activate complement through direct interaction of C3 isoforms with pathogenic agents.

Domain-Dependent Evolution Explains Functional Homology of Protostome and Deuterostome Complement C3-Like Proteins

“…The structural similarities between deduced C3-like proteins and the TEP superfamily are consistent with its proposed origin from the TEP gene family duplication (1,9,40). The C3-like gene duplication in Hexacorallia and loss of the a-g cleavage site in one of the duplicates is reminiscent of the situation that arose in the vertebrate duplication that generated the C3, C4 and C5 genes (11)(12)(13)(14)(15). Although in early vertebrates like the hagfish (Eptatretus burgeri), the a-g cleavage site is still evident in the deduced C3 protein (41,42) and generates a mature protein with 3 chains.…”

“…The C3 gene has been identified in all deuterostomes studied so far (1). Studies directed at deciphering the evolution of the complement system suggest a common ancestral molecule gave rise to C3 and to two others complement molecules C4 and C5 in vertebrates (11)(12)(13)(14)(15). The C3 prototype gene underwent several rounds of duplication before the cyclostome (lampreys and hagfish) divergence and in the vertebrates C3, C4 and C5 gene members emerged (11,13).…”

“…Studies directed at deciphering the evolution of the complement system suggest a common ancestral molecule gave rise to C3 and to two others complement molecules C4 and C5 in vertebrates (11)(12)(13)(14)(15). The C3 prototype gene underwent several rounds of duplication before the cyclostome (lampreys and hagfish) divergence and in the vertebrates C3, C4 and C5 gene members emerged (11,13). Evolution and function of the complement pathway in the species rich protostome clade is less well resolved (16)(17)(18)(19)(20).…”

Complement as driver of systemic inflammation and organ failure in trauma, burn, and sepsis

“…The systems have been developed through gene duplications either through the two genome duplications that have taken place during evolution or through gene duplications taking place locally, which creates clusters of genes coding for proteins with similar functions. One poignant example is C3 [ 63 , 64 ]: a C3-like molecule is found early in evolution, and after two gene duplications, four homologous proteins are now found in vertebrates, i.e., C3, C4, C5, and also the proteinase inhibitor α2M (with a similar mechanism of action). Local duplications have led to the cluster of genes for complement inhibitors found on chromosome 1 that encode FH, DAF, MCP, C4BP, etc.…”

Liver regeneration observed across the different classes of vertebrates from an evolutionary perspective

“…The appearance of immunoglobulins in cartilaginous fish marked the emergence of the classical and lytic pathways of complement activation ( Sunyer and Lambris, 1998 ). Although complement system is conserved in vertebrates, fish present a more complex complement cascade most likely in order to compensate for the lack of developed acquired immune mechanisms ( Najafpour et al., 2020 ). In higher vertebrates, the complement system participates as effector of innate and adaptive immune responses.…”