Apoptotic mesenchymal stem cells drive myocardial repair by activating RUNX1-TK1-mediated pyrimidine metabolic reprogramming in macrophages

Abstract Background Mesenchymal stem cell (MSC) therapy is promising for acute myocardial infarction (AMI), yet low post-transplantation survival remains a major hurdle. Emerging evidence suggests that apoptotic MSCs (aMSCs) are not mere "treatment failures" but actively modulate immune responses. Based on our preliminary findings that aMSC transplantation improves cardiac function in AMI mice and induces a unique macrophage subset with both proliferative and reparative phenotypes, this study aims to elucidate how aMSCs repair infarcted myocardium through macrophage metabolic reprogramming. Methods aMSCs were transplanted into the myocardium of AMI mice. scRNA-seq, metabolic pathway enrichment analysis, metabolic flux analysis, and cell-sorting-based transplantation were employed to identify and validate the aMSC-induced specific macrophage subset. siRNA-mediated gene knockdown, flow cytometry, transcription factor activity prediction, and ChIP were used to dissect the molecular regulatory axis, establishing a functional link between MerTK-PI3K/Akt-mTOR signaling and RUNX1-mediated transcriptional regulation of thymidine kinase 1 (TK1). Results In vivo imaging and flow cytometry confirmed that aMSCs were rapidly cleared by cardiac macrophages, significantly improving cardiac function and reducing fibrosis. scRNA-seq identified a unique aMSC-induced macrophage subset characterized by high expression of cell proliferation-related genes. Metabolic pathway analysis revealed significant enrichment of pyrimidine metabolism in this subset, with the most pronounced upregulation of TK1. Direct transplantation of the sorted specific macrophage subset into AMI hearts significantly improved post-infarction cardiac function. Mechanistically, aMSCs activated the MerTK-PI3K/Akt-mTOR axis, driving nuclear enrichment of RUNX1. RUNX1 acted as a metabolic master regulator, transcriptionally upregulating TK1-the rate-limiting enzyme of the pyrimidine salvage pathway. This metabolic reprogramming provided the necessary for macrophage DNA synthesis and sustained expansion. TK1 inhibition specifically abolished aMSC-induced macrophage proliferation without affecting phagocytic capacity or cell viability, indicating that TK1 plays a key role in aMSC-promoted macrophage proliferation. Conclusion This study demonstrates that aMSCs couple cellular clearance with nucleotide metabolic reprogramming by activating the MerTK-PI3K/Akt-mTOR-RUNX1-TK1-pyrimidine metabolism axis in macrophages. This mechanism transforms macrophages into a proliferative and reparative phenotype, offering a novel immunometabolic strategy to foster "beneficial inflammation" and promote myocardial repair post-AMI. These findings provide new insights into post-infarction immunometabolic remodeling and reveal the potential therapeutic value of targeting macrophage metabolism.Mechanism Diagram