Targeting the Gpr125-Gper1 axis: a novel osteogenic signaling pathway with therapeutic potential against osteoporosis

Osteoporosis, characterized by reduced bone mass and increased fracture risk, underscores the urgent need for novel anabolic therapies. G protein-coupled receptors (GPCRs) are major drug targets, but the functions of adhesion GPCRs (aGPCRs) in bone remain largely unexplored. G-protein coupled receptor 125 (Gpr125) is an orphan aGPCR, and its role in osteoblast-mediated bone formation is entirely unknown. We employed lentiviral-mediated knockdown and overexpression of Gpr125 in primary mouse calvarial osteoblasts and bone marrow stromal cells (BMSCs). Osteogenic and adipogenic differentiation were assessed by staining and marker analysis. Transcriptomic profiling (RNA-seq) and pathway analysis were used to identify downstream mechanisms, validated by rescue experiments with Gper1 overexpression, PI3K/AKT inhibitors (LY294002), and Wnt/β-catenin activation (Wnt3a CM). The osteogenic role of Gpr125 was tested in ovariectomized and aged mouse osteoporosis models via osteoblast-targeted adeno-associated virus (AAV) delivery. Gpr125 was highly expressed in osteoblasts, peaking during differentiation. Its knockdown severely impaired osteogenesis while promoting adipogenesis in vitro. Conversely, its overexpression enhanced bone formation. RNA-seq identified G protein-coupled estrogen receptor 1 (Gper1) as the key downstream target. We defined a novel signaling axis where Gpr125 upregulates Gper1, which activates PI3K/AKT signaling, leading to β-catenin stabilization and osteogenic transcription. Rescue experiments established a strict hierarchy: Gper1 overexpression fully rescued the osteogenic defect caused by Gpr125 loss, but not Gpr125 expression itself. PI3K inhibition blocked Gpr125-induced β-catenin activation and osteogenesis. β-catenin activation partially rescued osteogenesis but failed to restore upstream signaling. Critically, osteoblast-specific Gpr125 overexpression in vivo effectively ameliorated bone loss and reduced marrow adiposity in both postmenopausal and senile osteoporosis mouse models. Our study unveils a complete Gpr125-Gper1-PI3K/AKT-β-catenin signaling axis essential for osteoblast differentiation and bone formation. This work identifies the aGPCR Gpr125 as a novel positive regulator of bone anabolism and proposes the Gpr125-Gper1 axis as a promising therapeutic target for developing new treatments against osteoporosis.