Single-cell transcriptomic analysis reveals AP-1 downregulation remodels bone marrow environment and contributes to osteopenia in ovariectomized mice.

Background: Estrogen deficiency-induced osteoporosis largely results from disrupted immune environment in bone marrow, yet the underlying cellular and molecular mechanisms remain incompletely understood. This study aimed to investigate how estrogen deficiency alters bone marrow cellular composition and signaling pathways, with a focus on the regulatory role of activator protein 1 (AP-1) and its impact on osteoclastogenesis. Methods: Single-cell RNA sequencing (scRNA-seq) was performed on bone marrow cells from sham-operated and ovariectomized (OVX) mice to map cell-type-specific changes. AP-1 inhibitor T5224 was administered to validate its functional role in vivo, while in vitro experiments assessed AP-1 activation via estrogen receptor signaling under estrogen stimulation. B lymphogenesis was pharmacologically inhibited using an IL-7 monoclonal antibody in OVX mice to evaluate its therapeutic potential. Results: OVX mice exhibited a marked expansion of proliferative B cells (enriched in protein translation/DNA replication pathways) and reduced neutrophil proportions. scRNA-seq revealed widespread downregulation of AP-1 subunits, namely Fos, Fosb, Jun, Junb, across multiple cell types. T5224-induced AP-1 inhibition recapitulated OVX-associated B cell/neutrophil imbalance and triggered significant bone loss. Mechanistically, estrogen receptor activation upregulated AP-1 under estrogen stimulation, whereas AP-1 inhibition promoted B cell proliferation and increased GM-CSF and RANKL levels, thereby facilitating osteoclastogenesis. Critically, IL-7 antibody-mediated suppression of B lymphogenesis in OVX mice substantially attenuated bone loss. Conclusion: AP-1 downregulation drives estrogen deficiency-related osteopenia by disrupting bone marrow homeostasis, primarily through excessive B cell expansion and elevated osteoclastogenic signaling. Targeting B cell proliferation via IL-7 blockade presents a potential therapeutic strategy for mitigating osteoporosis in estrogen-deficient conditions.