Identification and expression analysis of HIF-1 and PHD2 genes in razor clam (Sinonovacula constricta) under hypoxia stress

The HIF (hypoxia-inducible factor) signaling pathway is an oxygen-sensing signaling pathway in animals, and plays important roles in metabolism and energy homeostasis in hypoxia environments. In this study, four key genes of HIF signaling pathway, HIF-1α, HIF-1β, PHD2A and PHD2B, were cloned using PCR from Sinonovacula constricta, and the physicochemical properties and domains of their encoded proteins were predicted, and analyzed their phylogenetic relationships. The results showed that HIF-1 contained typical HLH, PAS, PAC, and C-TAD domains, HIF-1α specific ODDD and N-TAD domains, and ScPHD2 contained zf-MYND and P4Hc domains; unlike other invertebrates, shellfish including razor clam PHD2 had two copies. Using real-time fluorescence quantitative PCR (RT-qPCR), the expression levels of HIF-1 and PHD2 were further analyzed at different developmental stages, in different tissues of adults and under hypoxia (0.5 mg/L and 2.0 mg/L) and air exposion (21 ℃ air exposion and 4 ℃ air exposion) stresses. The results showed that razor clam HIF-1 and PHD2 were expressed from the egg stage, ScHIF-1α was expressed at higher levels than other genes in the six tissues examined, and the highest expression was in gill, followed by hepatopancreas, while ScPHD2A and ScPHD2B were expressed at very low levels in all six tissues. Under hypoxia stress, ScHIF-1α and ScPHD2 expressions were significantly up-regulated, but the 0.5 mg/L group was more up-regulated than the 2 mg/L group, while ScHIF-1β expressions were not significantly changed. Under air exposion stress, ScHIF-1β expression did not change significantly, while ScHIF-1α and ScPHD2 expression were significantly up-regulated, and the up-regulation was greater in the 4 ℃ air exposion group than in the 21 ℃ air exposion group. It was shown that ScHIF-1 and ScPHD2 possessed typical HIF and PHD family characteristics, respectively, and their expression was induced to be up-regulated under hypoxia, suggesting that they may be involved in the response process of razor clam after low oxygen stress. The results of the study provide a reference value for further research on the hypoxia signaling pathway and hypoxia adaptation mechanism of razor clam.