Zebrafish complement factor H and its related genes: identification, evolution, and expression.

The complement system consists of a number of serum and membrane-bound proteins that play a crucial role in protecting the host organism against microbial infection. Complement factor H (CFH) regulates the alternative pathway of complement in plasma and mediates discrimination of cellular surfaces to alternative pathway activators and non-activators. Although complement system of zebrafish has been extensively studied, the information regarding CFH and its related genes in this important model species remains lacking. In this study, we report the molecular cloning and identification of CFH and complement factor H-like 1-4 (CFHL1-4) in Danio rerio. Sequence comparison and phylogenetic, syntenic, as well as genomic structure analyses demonstrated that the scaffold encompassing CFH and CFHLs region was conserved during evolution from bony fish to humans, and CFH and CFHL1-4 originated by intra-chromosome duplication on chromosome 22. Besides, quantitative real-time polymerase chain reaction revealed that both zebrafish CFH and CFHLs were predominantly expressed in the liver in a tissue-specific manner, and their expression was inducible by lipopolysaccharide, an inducer of immune responses, suggesting that they are possibly involved in acute phase responses. These are the first such data in bony fish, laying a foundation for further study of their physiological functions.

Three SIBLINGs (small integrin-binding ligand, N-linked glycoproteins) enhance factor H's cofactor activity enabling MCP-like cellular evasion of complement-mediated attack.

Previously we have shown that two members of the newly named SIBLING (small integrin-binding ligand, N-linked glycoproteins) family of proteins, bone sialoprotein, and osteopontin, bound first to a cell surface receptor and then to complement Factor H thereby blocking the lytic activity of the alternative pathway of complement. Another member of this family, dentin matrix protein 1, is shown in this paper to be very similar to osteopontin in that it can bind strongly to Factor H (K(a) approximately 1 nm) and block the lytic activity through either the vitronectin receptor (alpha(V)beta(3) integrin) or CD44. Binding of Factor H to SIBLING localized to the cells surface was demonstrated by fluorescence-activated cell sorting. Extensive overlapping fragment analyses suggests that both dentin matrix protein 1 and osteopontin interact with cell surface CD44 through their amino termini. Similar fragments of bone sialoprotein, like the intact protein, did not functionally interact with CD44. All three proteins are shown to act in conjunction with Factor I, a serum protease that, when complexed to appropriate cofactors, stops the lytic pathway by digesting the bound C3b in a series of proteolytic steps. These results show that at least three members of this family confer membrane cofactor protein-like activity (MCP or CD46) upon cells expressing RGD-binding integrins or CD44. The required order of the assembly of the complex suggests that this cofactor activity is limited to short diffusional distances.