Sigma1R restores mitochondrial energy metabolism via the IRE1α/XBP1 pathway

To investigate the role of Sigma1 receptor (Sigma1R) in mitochondrial energy metabolism remodeling in atrial myocytes, elucidate the associated molecular mechanisms, and evaluate its therapeutic potential in atrial fibrillation (AF). HL-1 atrial myocytes were subjected to tachypacing at 5 Hz for 24 h to establish an AF model. Lentiviral vectors were used to modulate Sigma1R and IRE1α expression. Cell viability was assessed by CCK-8 assay, apoptosis by Annexin V-FITC/PI staining and flow cytometry, mitochondrial function by TMRE staining for membrane potential, MitoSOX Red for reactive oxygen species (ROS) detection, and ATP assays. Calcium dynamics were measured using Fura-2/AM and Fluo-3/AM imaging. Protein expression was analyzed by Western blot, and subcellular localization was confirmed by fluorescence in situ hybridization (FISH). Tachypacing induced significant damage in atrial myocytes, including a 32.16% apoptosis rate, decreased Sigma1R expression, mitochondrial swelling, a 38% reduction in ATP levels, a 37% increase in mitochondrial ROS, and a 122% increase in cytosolic calcium compared to control cells. Overexpression of Sigma1R significantly mitigated these effects: cell viability increased by 55% (P < 0.001), apoptosis was reduced by 55% (P < 0.01), ATP levels were restored to 84% of control values (P < 0.01), and mitochondrial ROS decreased by 55% (P < 0.05). Mechanistically, Sigma1R overexpression normalized calcium homeostasis, reducing cytosolic calcium to 134 ± 11 nM from 218 ± 16 nM in the AF group (P < 0.01) and suppressed pathological expansion of endoplasmic reticulum-mitochondria contact sites. The activation of the IRE1α/XBP1 pathway was inhibited by Sigma1R, as evidenced by reductions in IRE1α, phosphorylated IRE1α, and XBP1s protein levels by 39-47% (P < 0.05). Conversely, IRE1α overexpression abrogated the protective effects of Sigma1R, leading to a 22% increase in apoptosis (P < 0.01) and exacerbating mitochondrial and calcium dysfunction. Sigma1R protects atrial myocytes from tachypacing-induced injury by enhancing mitochondrial function, reducing oxidative stress, and regulating calcium homeostasis at mitochondria-associated membranes, primarily through inhibition of the IRE1α/XBP1 pathway. These findings highlight Sigma1R as a promising therapeutic target for mitigating mitochondrial remodeling in AF.