Molecular characterization and expression analysis of three hypoxia-inducible factor alpha subunits, HIF-1α,-2α and-3α in hypoxia-tolerant Indian catfish, Clarias batrachus [Linnaeus, 1758]

The present study aimed at characterization of three HIF-alpha subunits, HIF-1 alpha -2 alpha and -3 alpha from hypoxia-tolerant Clarias batrachus, as well as to elucidate their expression pattern under short and long-term hypoxic conditions and identification of biomarker candidate. The complete cDNAs of HIF-1 alpha, -2 alpha and -3 alpha were 2,833, 4,270 and 3,256 bp in length, encoding 774, 818 and 628 amino acid residues, respectively. In C. batrachus, HIF-alpha subunits were structurally similar in DNA binding, dimerization, degradation and transcriptional activation domains, but differed in their oxygen-dependent degradation domains. Presence of c-Jun N-terminal kinase binding domain in HIF-alpha subunits was reported here for the first time in fish. In adult C. batrachus, three HIF-alpha mRNAs were detected in different tissues under normoxic conditions, however HIF-1 alpha was highly expressed in all the tissues studied, in comparison to HIF-2 alpha and -3 alpha. Short-term hypoxia exposure caused significant increase in three HIF-alpha transcripts in brain, liver and head kidney, while after long-term hypoxia exposure, significant up-regulation of HIF-1 alpha in spleen and -2 alpha in muscle was observed and HIF-3 alpha significantly down-regulated in head kidney. These observations suggest that the differential expression of HIF-alpha subunits in C. batrachus was hypoxic time period dependent and may play specialized roles in adaptive response to hypoxia. HIF-2 alpha, with its highly elevated expression in muscle tissues, can be a robust biomarker candidate for exposure to hypoxic environment.