Carboxypeptidase M modulates BMSCs osteogenesis-adipogenesis via the MAPK/ERK pathway: An integrated single-cell and bulk transcriptomic study

The pathogenesis of osteoporosis (OP) is closely associated with the disrupted balance between osteogenesis and adipogenesis in bone marrow-derived mesenchymal stem cells (BMSCs). We analyzed published single-cell RNA sequencing (scRNA-seq) data to dissect the transcriptomic profiles of bone marrow-derived cells in OP, reviewing 56,377 cells across eight scRNA-seq datasets from femoral heads (osteoporosis or osteopenia n=5, osteoarthritis n=3). Seventeen genes, including carboxypeptidase M (CPM), were identified as key osteogenesis-adipogenesis regulators through comprehensive gene set enrichment, differential expression, regulon activity, and pseudotime analyses. In vitro, CPM knockdown reduced osteogenesis and promoted adipogenesis in BMSCs, while adenovirus-mediated CPM overexpression had the reverse effects. In vivo, intraosseous injection of CPM-overexpressing BMSCs mitigated bone loss in ovariectomized mice. Integrated scRNA-seq and bulk RNA sequencing analyses provided insight into the MAPK/ERK pathway's role in the CPM-mediated regulation of BMSC osteogenesis and adipogenesis; specifically, CPM overexpression enhanced MAPK/ERK signaling and osteogenesis. In contrast, the ERK1/2 inhibitor binimetinib negated the effects of CPM overexpression. Overall, our findings identify CPM as a pivotal regulator of BMSC differentiation, which provide new clues for the mechanistic study of OP. Overall design: Total RNA was isolated from human bone marrow derived mesenchymal stem cells (BMSCs) subjected to osteogenic induction and CPM knockdown (siCPM, n=3) or control siRNA (siNC, n=3) on day 7. Samples were then processed for high-throughput sequencing on the NovaSeq 6000 PE150 platform.