Single-cell RNA sequencing reveals that an imbalance in monocyte subsets rather than changes in gene expression patterns is a feature of postmenopausal osteoporosis

The role of monocytes in postmenopausal osteoporosis is widely recognized; however, the mechanisms underlying monocyte reprogramming remain unknown. In this study, single-cell RNA sequencing (scRNA-seq) was conducted on CD14+ bone marrow monocytes obtained from 3 postmenopausal women with normal BMD and 3 women with postmenopausal osteoporosis (PMOP). Monocle2 was used to classify the monocytes into 7 distinct clusters. The proportion of cluster 1 significantly decreased in PMOP patients, while the proportion of cluster 7 increased. Further analysis via GSEA, transcription factor activity analysis, and sc-metabolic analysis revealed significant differences between clusters 1 and 7. Cluster 7 exhibited upregulated pathways associated with inflammation, immunity, and osteoclast differentiation, whereas cluster 1 demonstrated the opposite results. Monocle2, TSCAN, VECTOR, and scVelo data indicated that cluster 1 represented the initial subset and that cluster 7 represents one of the terminal subsets. BayesPrism and ssGSEA were employed to analyze the bulk transcriptome data obtained from the GEO database. The observed alterations in the proportions of 1 and 7 were validated and found to have diagnostic significance. CD16 serves as the marker gene for cluster 7, thus leading to an increased proportion of CD16+ monocytes in women with PMOP. Flow cytometry was used to assess the consistency of outcomes with those of the bioinformatic analysis. Subsequently, an additional scRNA-seq analysis was conducted on bone marrow mononuclear cells obtained from 3 patients with PMOP and 3 postmenopausal women with normal BMD. The differential proportions of cluster 1 and cluster 7 were once again confirmed, with the pathological effect of cluster 7 may attribute to cell-cell communication. The scRNA-seq findings suggest that an imbalance in monocyte subsets is a characteristic feature of PMOP. These findings elucidate the limitations of utilizing bulk transcriptome data for detecting alterations in monocytes, which may influence novel research inquiries. Monocytes are a type of white blood cell that plays a role in postmenopausal osteoporosis (PMOP), a condition where bones become weak and brittle after menopause. However, how monocytes change in this condition is not fully understood. In this study, single-cell RNA sequencing was used to analyze bone marrow monocytes from postmenopausal women with normal bone density and those with osteoporosis. Two distinct types of monocytes were identified, which were called clusters 1 and 7. In women with PMOP, there was a decrease in cluster 1 monocytes and an increase in cluster 7 monocytes. This change was validated in external datasets and in peripheral blood. Further analysis showed that cluster 7 monocytes positively correlated with inflammation, immunity, and osteoclast differentiation (a process that leads to bone resorption). Cluster 1 monocytes were found to be the initial subset, while cluster 7 monocytes were one of the terminal subsets. Overall, this study suggests that an imbalance in monocyte subsets is a characteristic feature of postmenopausal osteoporosis. These findings have important implications for understanding the role of monocytes in bone health. Graphical Abstract