Characterization and functional analysis of hypoxia-inducible factor HIF1α and its inhibitor HIF1αn in tilapia

Hypoxia is a major cause of fish morbidity and mortality in the aquatic environment. Hypoxia-inducible factors are very important modulators in the transcriptional response to hypoxic stress. In this study, we characterized and conducted functional analysis of hypoxia-inducible factor HIF1 alpha and its inhibitor HIF1 alpha n in Nile tilapia (Oreochromis niloticus). By cloning and Sanger sequencing, we obtained the full length cDNA sequences for HIF1 alpha (2686bp) and HIF1 alpha n (1308bp), respectively. The CDS of HIF1 alpha includes 15 exons encoding 768 amino acid residues and the CDS of HIF1 alpha n contains 8 exons encoding 354 amino acid residues. The complete CDS sequences of HIF1 alpha and HIF1 alpha n cloned from tilapia shared very high homology with known genes from other fishes. HIF1 alpha show differentiated expression in different tissues (brain, heart, gill, spleen, liver) and at different hypoxia exposure times (6h, 12h, 24h). HIF1 alpha n expression level under hypoxia is generally increased (6h, 12h, 24h) and shows extremely highly upregulation in brain tissue under hypoxia. A functional determination site analysis in the protein sequences between fish and land animals identified 21 amino acid sites in HIF1 alpha and 2 sites in HIF1 alpha n as significantly associated sites (alpha = 0.05). Phylogenetic tree-based positive selection analysis suggested 22 sites in HIF1 alpha as positively selected sites with a p-value of at least 95% for fish lineages compared to the land animals. Our study could be important for clarifying the mechanism of fish adaptation to aquatic hypoxia environment.