Ascorbic acid transporter MmSLC23A2 functions to inhibit apoptosis via ROS scavenging in hard clam (Mercenaria mercenaria) under acute hypo-salinity stress.

Solute carrier family 23 (SLC23) mediates cellular uptake of ascorbic acid, a crucial antioxidant protecting organisms against oxidative stress. Despite advances in understanding SLC23 in mammals, its physiological roles in bivalves remain poorly understood. Notably, euryhaline bivalves exhibit a significant expansion and positive selection of SLC23, highlighting the need for deeper investigation. Here, we identified 25 MmSLC23 in the hard clam genome. These genes predominantly cluster on chromosomes 3 and 14, with tandem duplications driving their expansion. All MmSLC23 localize to the plasma membrane, containing 9-14 transmembrane domains. Syntenic conservation of SLC23 was limited across order Venerida, with most expanded members being lineage-specific paralogs. Transcriptome analysis and fluorescence in situ hybridization revealed that MmSLC23 exhibited divergent expression patterns under acute and chronic salinity stress. Notably, RNA interference of MmSLC23A2 led to a significant reduction in intracellular ascorbic acid levels. Under acute hypo-salinity stress, increased ROS levels and elevated apoptosis rate were observed in MmSLC23A2 knockdown clams, as assessed by flow cytometry and transmission electron microscopy. These findings underscore the crucial role of SLC23 in mitigating oxidative damage and preventing premature apoptosis under acute salinity stress, offering new insights into the molecular mechanisms underlying the remarkable salinity adaptability of euryhaline bivalves.