ROLE OF CLCA GENE FAMILY MEMBERS IN FISH BACTERIAL DISEASES

Bacterial diseases cause significant economic loss to the aquaculture industry. There is a need to find new solutions that can replace antibiotics. Epithelial tissues play  a vital role in host defense mechanisms. One of them is  a  barrier function that protects inhibits pathogenesis . Epithelial cells also secrete humoral factors into the fish mucus that fight bacteria and other pathogens. In this study we investigate the role of zebrafish CLCA genes in bacterial infections.

CLCA genes are well conserved across species, including fish. CLCAs are predominately expressed in epithelial cells and goblet cells. CLCA2 is expressed at the basolateral junctions of epithelial cells and is required for epithelial differentiation . Many studies have shown that dysregulation of CLCAs is significant in various pathologies including bacterial infections. CLCA1 has been shown to increase pro-inflammatory cytokines upon its activation in a Staphylococcus aureus disease model . We have evaluated the expression of three z CLCA family members in various zebrafish tissues. z CLCA1 is 903, zCLCA 5.1 is 888, and z CLCA 5.2 is 229 amino acids long . We found that the z CLCA1's highest expression is  in the intestine when compared to gills, or skin; unlike z CLCA5.1 or z CLCA 5.2. T here are ample studies showing the presence of proteases and metalloproteases in mucus that act as antibacterial peptides. CLCAs from other species are shown to be zn+2 metalloproteases. CLCAs have metal-binding motif HEXXH, which is conserved across species, including zebrafish.

Interestingly, we find that zCLCA 5.1 and zCLCA5.2 expression is significantly lower in tissues but upregulated in mucus. zCLCA5.2 protein sequence analysis shows that it possibly comprises of only the ectodomain. We speculate that zCLCAs like other CLCAs are Zn⁺² dependant metalloproteases expressed at cell-cell junctions zCLCA1, and zCLCA5.1. They also express transcript variants that act as antibacterial peptides (zCLCA 5.1 and 5.2) that are potential therapeutics for fish bacterial diseases.