Metabolomics and biochemical assays reveal the metabolic responses to hypo-salinity stress and osmoregulatory role of cAMP-PKA pathway in Mercenaria mercenaria

Hypo-salinity events frequently occur in marine ecosystem due to persistent rainfall and freshwater inflow, reducing the cytosol osmolarity and triggering cellular stress responses in aquatic organisms. Euryhaline bivalves have developed sophisticated regulatory mechanisms to adapt to salinity fluctuations over a long period of evolution. In this study, we performed multiple biochemical assays, widely targeted metabolomics, and gene expression analysis to investigate the comprehensive metabolic responses to hypo-salinity stress and osmoregulation mechanisms in hard clam Mercenaria mercenaria, which is a euryhaline bivalve species widely cultured in China. During hypo-salinity stress, increased vacuoles appeared in gill filaments. The Na+ and Cl- concentrations in gills significantly decreased because of the up-regulation of Na+/K+-ATPase (NKA) activity. The cAMP content dramatically decreased at 5 d post hypo-salinity stress. Meanwhile, the gene expression levels of adenylate cyclase, proteinkinase A, and sodium and calcium channel proteins were evidently down-regulated, suggesting that cAMP-PKA pathway was inhibited to prevent ambient inorganic ions from entering the gill cells. Antioxidant metabolites, such as serine and Tyr-containing dipeptides, were significantly up-regulated to resist oxidative stress. Glycerolipid metabolism was strengthened to stabilize membrane structure when hypo-salinity stress was prolonged to 5 days. At 1 d post hypo-salinity stress, an increase in alanine and lactate contents marked the initiation of anaerobic metabolism. Acylcarnitines accumulation indicated that fatty acids b-oxidation was promoted to provide energy for osmoregulation. The potential biomarkers of hyposalinity stress were identified in hard clams. This study provides novel insights into the metabolic regulatory mechanisms to hypo-salinity stress in euryhaline bivalves. (c) 2022 The Authors. Published by Elsevier B.V. on behalf of Research Network of Computational and Structural Biotechnology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

This study investigated the metabolic responses to hypo-salinity stress and osmoregulation mechanisms in the hard clam Mercenaria mercenaria, a widely cultured euryhaline bivalve species in China. The study identified several changes in response to hypo-salinity stress, including increased vacuoles in gill filaments, up-regulation of Na+/K+-ATPase activity, and up-regulation of antioxidant metabolites. Potential biomarkers of hypo-salinity stress were also identified in the hard clams.