Potential Targets for the Treatment of MI: GRP75-mediated Ca2+transfer in MAM

Background After myocardial infarction (MI), there is a notable disruption in cellular calcium ion homeostasis and mitochondrial function. These alterations are believed to be linked to endoplasmic reticulum (ER) stress, though the specific mechanisms are not fully understood. This research endeavors to elucidate the involvement of glucose regulated protein 75 (GRP75) in post-MI calcium ion homeostasis and mitochondrial function. Results Excessive oxidative stress was activated in humans’ post-myocardial infarction, with most differentially expressed genes being enriched in metabolic pathways, especially the calcium signaling pathway. In MI rats, symptoms of myocardial injury were accompanied by an increase in the activation of PERK, ATF6, and IRE1, as well as elevated Binding immunoglobulin protein (Bip) expression. Moreover, in oxygen-glucose deprivation (OGD)-induced cardiomyocytes, it was confirmed that inhibiting PERK exacerbated intracellular Ca 2+ disruption and cell apoptosis. More importantly, in cardiomyocytes undergoing Tunicamycin-induced ER stress, Ca 2+ accumulated in both the ER and mitochondria. Concurrently, the co-localization of GRP75 with IP3R and VDAC1 increased under ER stress in cardiomyocytes. In OGD-induced cardiomyocytes, knockdown of GRP75 not only reduced the Ca 2+ levels in both the ER and mitochondria and improved the ultrastructure of cardiomyocytes, but it also increased the number of contact points between ER and mitochondria, reducing MAM formation, and decreased cell apoptosis. Significantly, knockdown of GRP75 did not affect the protein expression of PERK and hypoxia-inducible factor 1α (HIF-1α). Transcriptome analysis of cardiomyocytes revealed that knockdown of GRP75 mainly influenced the molecular functions of sialyltransferase and IP3R, as well as the biosynthesis of glycosphingolipids and lactate metabolism. In OGD-induced cardiomyocytes, the knockdown of GRP75 lowered the protein expression levels of glucose transporter-1 (Glut1), pyruvate kinase M2 (PKM2), and lactate dehydrogenase A (LDHA), and decreased the metabolic products of glycolysis. Conclusion The complex interaction between the ER and mitochondria, driven by the GRP75 and its associated IP3R1-GRP75-VDAC1 complex, is crucial for calcium homeostasis and cardiomyocyte’s adaptive response to ER stress. Modulating GRP75 could offer a strategy to regulate calcium dynamics, diminish glycolysis, and thereby mitigate cardiomyocyte apoptosis.