Expression and functional characterization of PGRP6 splice variants in grass carp Ctenopharyngodon idella.

Peptidoglycan recognition proteins (PGRPs), which are evolutionarily conserved pattern recognition receptors from insects to mammals, recognize bacterial PGN and function in antibacterial innate immunity. The existence of alternative splicing is a common feature for PGRP family. Here the splicing pattern from the splicing at the 5' end of PGRP6 gene was identified in a teleost fish, the grass carp (Ctenopharyngodon idella). Four splice variants of grass carp PGRP6 were designated as gcPGRP6a, gcPGRP6b, gcPGRP6c and gcPGRP6d, respectively. Real-time PCR revealed the different expression of these variants in fish individuals and CIK cell line in response to stimulation with different microbial ligands. Immunofluorescence microscopy and Western blotting showed that the splice variants are intracellular protein. Cell lysates from Epithelioma papulosum cyprini (EPC) cells transfected with gcPGRP6 splice variants are able to bind microbial PAMPs including Lys-type PGN from Staphylococcus aureus, DAP-type PGN from Bacillus subtilis, glucan, mannan, and microorganisms including Streptococcus dysgalactiae, Flavobacterium columnare and Saccharomyces cerevisiae. Moreover, overexpression of gcPGRP6 variants inhibited earlier stage growth of intracellular bacteria. The data also identified a specific role for gcPGRP6c variant in the positive regulation of cytolytic molecule perforin, and for gcPGRP6a, gcPGRP6b and gcPGRP6c variants in positive regulation of antimicrobial peptides (AMPs). However, the gcPGRP6d variant, which encoded basically only the PGRP domain, failed to induce the expression of perforin and AMPs. It is suggested that fish PGRP6 splice variants have common and variant-specific function in innate immune response.