A CD46-like Molecule Functional in Teleost Fish Represents an Ancestral Form of Membrane-Bound Regulators of Complement Activation

Abstract

In the complement system, the regulators of complement activation (RCA) play crucial roles in controlling excessive complement activation and in protecting host cell from misdirected attack of complement. Several members of RCA family have been cloned from cyclostome and bony fish species and classified into soluble and membrane-bound type as in mammalian RCA factors. Complement-regulatory functions have been described only for soluble RCA of lamprey and barred sand bass; however, little is known on the biological function of the membrane-bound RCA proteins in the lower vertebrates. In this study, a membrane-bound RCA protein, designated teleost complement-regulatory membrane protein (Tecrem), was cloned and characterized for its complement-regulatory roles. Carp Tecrem, an ortholog of a zebrafish type 2 RCA, ZCR1, consists of four short consensus repeat modules, a serine/threonine/proline-rich domain, a transmembrane region, and a cytoplasmic domain, from the N terminus, as does mammalian CD46. Tecrem showed a ubiquitous mRNA expression in carp tissues, agreeing well with the putative regulatory role in complement activation. A recombinant Chinese hamster ovary cell line bearing carp Tecrem showed a significantly higher tolerance against lytic activity of carp complement and less deposition of C3-S, the major C3 isotypes acting on the target cell, than control Chinese hamster ovary (mock transfectant). Anti-Tecrem mAb enhanced the depositions of carp C3 and two C4 isotypes on autologous erythrocytes. Thus, the present findings provide the evidence of complement regulation by a membrane-bound group 2 RCA in bony fish, implying the host–cell protection is an evolutionarily conserved mechanism in regulation of the complement system.

Evolution of the complement system: from defense of the single cell to guardian of the intravascular space

“…These are composed of approximately 60 amino acids of which 10–18 are highly conserved, as well as four invariant cysteines that define the CCP architecture (a loop within a loop structure) . The genomic organization and clustering of RCA genes are diverse among species that include mice, chicken, frog, and teleost (the most common group of ray‐finned bony fish [reviewed in ]). Both phylogenetic and genomic evidence indicates the emergence of the RCA by the time of cyclostomes (the most ancient class of vertebrates).…”

“…Both phylogenetic and genomic evidence indicates the emergence of the RCA by the time of cyclostomes (the most ancient class of vertebrates). A CD46‐like molecule has been identified and characterized in bony fish . Moreover, genes containing the complement control protein (CCP) domains common to human complement regulatory elements have been identified as early as in ascidians …”

Complement regulator CD46: genetic variants and disease associations

“…Complement systems similar to that in mammals also have been identified in birds, fish, amphibians, and reptiles. An AP is also found in more primitive species, even those lacking a circulatory system [13]. The complement system consists of three major activating pathways that are independently triggered, yet all have the common goal of modifying the target membrane by depositing C3 activation products and then engaging a common terminal membrane-attack complex (Fig.…”

“…These are separated by a 14–59 cM segment that includes a number of genes unrelated to complement [21]. Complement regulatory genes share a common ancestral motif from which they arose by multiple gene duplication events [13, 19, 21]. …”

Complement regulatory protein genes in channel catfish and their involvement in disease defense response

“…Furthermore, three membrane-bound complement regulatory protein isoforms gTecrem-1, gTecrem-2 and gTecrem-3 were identified in ginbuna crucian carp (Carassius auratus langsdorfii) (Nur et al, 2013). A CD46-like complement-regulatory membrane protein (cTecrem), an ortholog of a zebrafish RCA group 2 gene ZRC1, was cloned and characterized in common carp (Tsujikura et al, 2015). In channel catfish (Ictalurus punctatus), the primary aquaculture species in the United States, complement factors Bf/C2 and Df , and three complement regulatory protein genes, CD59, factor I, and C1INH have been reported (Abernathy et al, 2009;Yeh and Klesius, 2007), and C1INH was found to be significantly up-regulated at early stages after bacterial infection with Edwardsiella ictaluri .…”