Weak Phd2-Hif-1α Affinity Coupled with High Basal Expression Is Predicted to Enhance HIF Pathway Flexibility in Nile...

To explore the molecular basis of hypoxia tolerance variation within euteleosts, we compared the hypoxia-inducible factor (HIF) pathways of the highly tolerant Nile tilapia (Oreochromis niloticus) and the hypoxia-sensitive rainbow trout (Oncorhynchus mykiss). Evolutionary analysis revealed that Nile tilapia possesses single copies of Hif-1α and prolyl hydroxylase domain protein 2 (Phd2), whereas rainbow trout retains two and three copies, respectively. The Leu-X-X-Leu-Ala-Pro (LXXLAP) motifs in the oxygen-dependent degradation (ODD) domain of Hif-1α and the interacting loop region of Phd2 are highly conserved, indicating a conserved core mechanism for regulating Hif-1α stability. However, differences in charged residue composition flanking the Phd2 loop (e.g., fewer positively charged residues in Nile tilapia) were identified. Molecular dynamics simulations revealed that the complex formed by Nile tilapia Phd2 and the Hif-1α LXXLAP motif was unstable across physiological temperatures, suggesting potential impairment of the catalytic geometry compatible with hydroxylation and elevated normoxic Hif-1α stability. In contrast, the corresponding complexes in rainbow trout were more stable, particularly at low temperatures. Expression profiling revealed that Nile tilapia tissues, including the heart, maintain higher basal expression of glycolytic genes, may help support energy production during hypoxia. Our findings indicate that a weakened protein interaction and high constitutive expression is predicted to enhance HIF pathway responsiveness, potentially priming vital tissues for glycolytic energy production and may contribute to this species' hypoxia tolerance.