Mingyi Sui

Recently, the increase in marine temperatures has become an important global marine environmental issue. The ability of energy supply in marine animals plays a crucial role in avoiding the stress of elevated temperatures. The investigation into anaerobic metabolism, an essential mechanism for regulating energy provision under heat stress, is limited in mollusks. In this study, key enzymes of four anaerobic metabolic pathways were identified in the genome of scallop Chlamys farreri, respectively including five opine dehydrogenases (CfOpDHs), two aspartate aminotransferases (CfASTs) divided into cytoplasmic (CfAST1) and mitochondrial subtype (CfAST2), and two phosphoenolpyruvate carboxykinases (CfPEPCKs) divided into a primitive type (CfPEPCK2) and a cytoplasmic subtype (CfPEPCK1). It was surprising that lactate dehydrogenase (LDH), a key enzyme in the anaerobic metabolism of the glucose–lactate pathway in vertebrates, was absent in the genome of scallops. Phylogenetic analysis verified that CfOpDHs clustered according to the phylogenetic relationships of the organisms rather than substrate specificity. Furthermore, CfOpDHs, CfASTs, and CfPEPCKs displayed distinct expression patterns throughout the developmental process and showed a prominent expression in muscle, foot, kidney, male gonad, and ganglia tissues. Notably, CfASTs displayed the highest level of expression among these genes during the developmental process and in adult tissues. Under heat stress, the expression of CfASTs exhibited a general downregulation trend in the six tissues examined. The expression of CfOpDHs also displayed a downregulation trend in most tissues, except CfOpDH1/3 in striated muscle showing significant up-regulation at some time points. Remarkably, CfPEPCK1 was significantly upregulated in all six tested tissues at almost all time points. Therefore, we speculated that the glucose–succinate pathway, catalyzed by CfPEPCK1, serves as the primary anaerobic metabolic pathway in mollusks experiencing heat stress, with CfOpDH3 catalyzing the glucose–opine pathway in striated muscle as supplementary. Additionally, the high and stable expression level of CfASTs is crucial for the maintenance of the essential functions of aspartate aminotransferase (AST). This study provides a comprehensive and systematic analysis of the key enzymes involved in anaerobic metabolism pathways, which holds significant importance in understanding the mechanism of energy supply in mollusks.

Heat shock proteins 90 (HSP90s) are a class of ubiquitous, highly conserved, and multi-functional molecular chaperones present in all living organisms. They assist protein folding processes to form functional proteins. In the present study, three HSP90 genes, CfHSP90, CfGRP94 and CfTRAP1, were successfully identified in the genome of Chlamys farreri. The length of CfHSP90, CfGRP94 and CfTRAP1 were 7211 bp, 26,457 bp, and 28,699 bp, each containing an open reading frame (ORF) of 2181 bp, 2397 bp, and 2181 bp, and encoding proteins of 726, 798, and 726 amino acids, respectively. A transcriptomic database demonstrated that CfHSP90 and CfGRP94 were the primary functional executors with high expression during larval development and in adult tissues, while CfTRAP1 expression was low. Furthermore, all of the three CfHSP90s showed higher expression in gonads and ganglia as compared with other tissues, which indicated their probable involvement in gametogenesis and nerve signal transmission in C. farreri. In addition, under heat stress, the expressions of CfHSP90 and CfGRP94 were significantly up-regulated in the mantle, gill, and blood, but not in the heart. Nevertheless, the expression of CfTRAP1 did not change significantly in the four tested tissues. Taken together, in coping with heat stress, CfHSP90 and CfGRP94 could help correct protein folding or salvage damaged proteins for cell homeostasis in C. farreri. Collectively, a comprehensive analysis of CfHSP90s in C. farreri was conducted. The study indicates the functional diversity of CfHSP90s in growth, development, and environmental response, and our findings may have implications for the subsequent in-depth exploration of HSP90s in invertebrates.