Identification and characterization of calreticulin (CRT) from Nile tilapia (Oreochromis niloticus) in response to bacterial infection

Calreticulin (CRT) as a multifunctional Ca 2+ -binding protein is highly conserved in variety species. Recently, the structure and function analysis of CRT in teleost has attracted wide attention. The present study first amplified the open reading frame of CRT mRNA from Nile tilapia ( Oreochromis niloticus ), then characterized its tissue distribution and expression pattern against the challenge of pathogenic bacteria. Furthermore, analyzed the function of OnCRT in regulating phagocytosis and inflammatory response by monocytes/macrophages (MO/Mø). The results showed that the ORF of OnCRT is 1269 bp of nucleotide and encoding a polypeptide with 422 amino acids. OnCRT protein is highly homologous to the CRT in other species, containing three conserved structure domain, the N-domain, P-domain and C domain. The OnCRT mRNA was widely exhibited in the collected tissues of natural tilapia, and with the maximum in the liver. Following the challenges of Streptococcus agalactiae and Aeromonas hydrophila in vivo and in vitro , the expression of OnCRT transcripts was significantly upregulated. The recombinant OnCRT protein was purified with the His Band Resin columns, and confirmed to have the capacity of binding with bacteria and promoted the phagocytosis of MO/Mø. In addition, the expressions of inflammatory factors and chemokines in MO/Mø were increased significantly upon the stimulation of (r)OnCRT. In summary, the present study indicated that OnCRT was likely to get involved in host defense against bacterial infection in O. niloticus .