Comparative transcriptome analysis provides new insights into the molecular mechanism of blunt snout bream (Megalobrama amblycephala) heart in response to hypoxic stress

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Abstract
Megalobrama amblycephala is a key freshwater fish species in Chinese aquaculture, valued for its high artificial propagation success and relative resistance to common aquaculture diseases. However, it is sensitive to hypoxia, and its molecular responses to hypoxic stress in the heart remain relatively unknown. Herein, we investigated the effects of hypoxia on myocardial enzyme activities (creatine kinase, creatine kinase myocardial band isoenzyme, lactate dehydrogenase, and α-hydroxybutyrate dehydrogenase), histological structure, and transcriptome changes in the heart of M. amblycephala. The fish were divided into five groups, including normoxia control (N0), hypoxia for 6, 12, and 24 h (H6, H12, and H24, respectively), and reoxygenation for 24 h (R24) groups. The results showed that hypoxia significantly increased myocardial enzyme activities. Histological analysis revealed that hypoxia induced slight swelling and disorganization of myocardial fibers, while reoxygenation partially restored their structural integrity. Transcriptome sequencing identified 4,269 DEGs, with 964, 1,159, 812, and 1,334 identified in the H6, H12, H24, and R24 groups, respectively, compared to the N0 group. KEGG pathway analysis identified circadian rhythm, cardiac muscle contraction, fatty acid degradation, hypertrophic cardiomyopathy, and fluid shear stress and atherosclerosis pathways as candidate molecular pathways involved in hypoxia tolerance. Notably, several key genes, including cry5, LOC125248347, cacng5b, atp2a1l, LOC125252961, aldh3a1, gcdha, edn1, atp2a1, gpc1a, and hsp90aa1.2, were significantly up-regulated in the H24 group, highlighting their potential roles in the adaptive responses to hypoxic stress. These findings provide new insights into the physiological, histological, and molecular responses underlying hypoxia adaptation in M. amblycephala, laying the groundwork for future studies on hypoxia-related molecular mechanisms in other economically important fish species.

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