Acute Hypoxia Stress-Induced Apoptosis in Gill of Japanese Flounder (Paralichthys olivaceus) by Modulating the Epas1/Bad Pathway

SIMPLE SUMMARY: Hypoxia is perhaps of the most pressing global environmental challenge in aquatic systems. Hypoxic stress can cause physiological changes in fish, resulting in injury and even death to them. Japanese flounder is a crucial benthic economic fish, but the mechanisms of its response to hypoxia are poorly understood. Here, we show that Japanese flounder can alter gill morphology, and induce apoptosis to weaken the risk caused by hypoxic stress. The mechanism of the EPAS1/Bad signal pathway in response to hypoxia was investigated from the perspective of transcription and epigenetics. Therefore, our results would enrich the theory of hypoxic stress on Japanese flounder and provide a reference for its healthy cultivation. ABSTRACT: The physiological responses and molecular mechanisms of apoptosis in Japanese flounder under hypoxic stress remain unclear. In the present study, we performed acute hypoxia stress on Japanese flounder (2.39 ± 0.84 mg/L) and detected gills responses in histomorphology and molecular mechanisms. The results showed that the volume of the interlamellar cell mass decreased and the gill lamellae prolonged, indicating the expansion of the respiratory surface area. Additionally, the fluorescence signal of apoptosis increased under hypoxic stress. In addition, the expression of two genes (EPAS1 and Bad) related to apoptosis increased about four-fold and two-fold, respectively, at 6 h of hypoxia. Meanwhile, the result of the dual-luciferase reporter assay showed that EPAS1 is a transcription factor, which could regulate (p < 0.05) the expression of the Bad gene, and we identified the binding site of EPAS1 was the AATGGAAAC sequence located near −766. DNA methylation assay showed that hypoxia affected the methylation status of CpG islands of EPAS1 and Bad genes. All results indicated that hypoxia could activate the EPAS1/Bad signal pathway to induce gill apoptosis of Japanese flounder. Our study provides new light on understanding the molecular mechanism of hypoxia-induced apoptosis in Japanese flounder.