Characterization,Expression Pattern Analysis,and Cellular Localization of Two Homologous MUC2 Genes in Japanese Flounder(Paralichthys olivaceus)

As a member of mucins family,MUC2 is an important component of mucus,which is characterized by tandem and irregular repeat sequences rich in threonine and serine.It is strongly expressed by goblet cells that are involved in innate immunity.Japanese flounder is an economically important marine flatfish.In this study,two homologous genes of MUC2 were identified,named MUC2-1 and MUC2-2.Depending on phylogenetic and structural analysis,MUC2-1 and MUC2-2 were clustered in two clades,respectively.Paralichthys olivaceus MUC2-1 showed a closer relationship with MUC2 in the higher vertebrates.Various healthy tissues were analyzed to determine the expression patterns,and both MUC2 genes showed high expression levels in the gills and intestines.Following Edwardsiella tarda challenge,P.olivaceus MUC2-1 and MUC2-2 both showed significant up-regulated expression in intestine and kidney.Moreover,MUC2-1 was significantly up-regulated in gill cell lines following PGN and polyI:C stimulation,and MUC2-2 was significantly up-regulated in gill cell lines following LPS stimulation.In addition,ISH results revealed that MUC2-1 and MUC2-2 showed the same tissue localization in intestine tissues,but displayed different localization in gill.The siRNA-mediated knockdown of MUC2-1 and MUC2-2 genes in the gill cell lines of Japanese flounder affected the expressions of galnt family genes and smad pathway members that are related to mucosal immunity.The results provided a valuable insight into understanding the functions of MUC2 on mucosal immune system in response to the invasion of bacterial pathogen.