4.2 Energy metabolism
(1) Mitochondrial dysfunction
“Oxidative phosphorylation” enables the generation of ATP in the mitochondrial inner membrane and is the main source of energy in eukaryotes. It is generally accepted that hypoxia or ischemia inhibits mitochondrial oxidative phosphorylation and reduces the overall efficiency of energy metabolism(Cerretelli& Gelfi 2011). We found that all 34 DEP of “Oxidative phosphorylation” were downregulated (Fig. 2 ), indicating that the coupling reaction of ADP and inorganic phosphate to ATP through the respiratory chain was inhibited to a certain extent. It also reflected that reduced locomotion powered by skeletal muscle might pose a minimal threat to survival under hypoxia when swimming activity will be minimized.
Meanwhile we found that downregulated expression of the regulator of mitochondrial biogenesis (PPRC1, GFM1, LONP1, SERAC1) and the downstream mitochondrial related functional gene/proteins (COX1/2/4, Mt-Cyb, miR-457a/15b-UQCRC2, UQCRC1, CYC1, Cyb561a3, miR-457a/338/15b/PC-7236-ATP5A1, ATP5B), whichsuggested that mitochondria may be dysfunctional. (Table 1 and Fig. 5 ). Such mitochondrial damage induced by hypoxia may lead to the production of ROS and the decline of membrane potential of mitochondria, causing a metabolic disorder and apoptosis (Wang et al. 2019).
(2) Catabolism and anabolism
P. vachelli has been shown to have an acute response to hypoxia, including activation of catabolic capacity for more energy, and reduction of biosynthetic capacity to reduce energy expenditure in liver. Similar results are found in P. vachelli muscles, for example, upregulation of PPARα/β is involved in lipid metabolism and lipid oxidation of skeletal muscle in low oxygen environments (A. et al. 2018)(Table 1 ). Also hypoxia induces the activation of riboflavin catabolism through upregulation of RFK as the key enzyme, which improves carbohydrate, lipid, amino acid metabolism in low oxygen environments(Wanget al. 2014). In this study, we also found ample evidence of significantly increased catabolism inP. vachelli muscles. The transport-related enriched pathways (e.g., SNARE interactions in vesicular transport, and ABC transporters) and catabolic pathways (e.g., pyrimidine metabolism, central carbon metabolism in cancer) contained a large proportion of upregulated genes (Fig. 2A and 2C ). Furthermore, we found a significant decrease in intermediate metabolites in catabolism in metabolome KEGG analysis (e.g., Metabolic pathways, Glutathione, Purine and Linoleic acid metabolism), which indicated that the consumption of these intermediate metabolites under hypoxia exceeds their synthesis (Fig. 2C ).
Furthermore, the upregulated intermediate metabolites were a big part among anabolic pathways (e.g., biosynthesis of aminoacyl-tRNA, plant hormone, alkaloids derived from histidine, purine, ornithine, lysine and nicotinic acid) in this study (Fig. 2C ). This indicated that consumption of these intermediate metabolites was reduced and accumulated. Additionally, we identified a large portion of downregulated genes/proteins enriched in amino acid biosynthesis pathways (e.g., aminoacyl-tRNA, ribosome, tryptophan, lysine, cysteine, and methionine). These results confirmed the downregulation of anabolism in fish muscle induced by hypoxia, especially in amino acid synthesis (Hardy et al. 2013)(Fig. 2A and 2B ).