Observation of the middle intestinal tight junction structure, cloning and studying tissue distribution of the four Claudin genes of the grass carp (Ctenopharyngodon idellus).

To confirm the existence of the tight junction (TJ) in middle intestine and obtain the genetic information of Claudin-3, Claudin-15a, Claudinb and Claudinc of grass carp, we observed the physical structure of TJ by transmission electron microscopy and cloned the partial cDNAs of the four Claudins using reverse transcriptase PCR technique. The four partial cDNAs consist of 1,261, 490, 776 and 662 bp encoded 131, 150, 195 and 171 amino acids, respectively. Homology analysis showed that the grass carp Claudin shared high homology with other teleost species, especially with Danio rerio and Carassius auratus. Multi-alignments of the four Claudin amino acid sequences have seen the two conserved cysteines existing in the first extracellular loop of Claudin-15a, Claudinb and Claudinc, and the sequence diversity of the four Claudins mainly lies within the C-terminal tails, which usually end with the -Y-V motif, except the -F-V motif in Claudinb. Tissue distributions of the four Claudins were measured by applying quantitative real-time PCR technique. Results showed that Claudin-3 was mainly expressed in liver and middle intestine and Claudinb was ubiquitously expressed with a higher expression in middle intestine while Claudin-15a and Claudinc were mainly expressed in middle intestine. Our study revealed the existence of the TJ in the middle intestinal and obtained the genetic information of Claudin-3, Claudin-15a, Claudinb and Claudinc of grass carp, aiming to found the molecular biology basis for the further study of the intestinal barrier function of grass carp.