Targeting Irgm1 to combat osteoporosis: suppressing ROS and restoring bone remodeling

The accumulation of reactive oxygen species (ROS) leads to enhanced osteoclast activity, causing severe bone destruction in postmenopausal osteoporosis. Immunity-related GTPase family M member 1 (Irgm1) plays an essential role in affecting the production of intracellular ROS. To detect whether deletion of Irgm1 could suppress osteoclastogenesis through cellular redox regulation, we first evaluated whether the Irgm1 level was significantly elevated in mice bone marrow-derived monocytes/macrophages (BMDMs) from ovariectomy (OVX)-induced osteoporosis mice. Moreover, bioinformatics network analysis was performed to identify Irgm1 as a key upregulated gene during osteoclast differentiation. Next, we found that macrophage-specific Irgm1 knockout (Irgm1-cKO, Lyz2-Cre; Irgm1^flox/flox) in OVX mice resulted in slower bone loss compared with OVX mice from the control group (Irgm1^flox/flox). We then demonstrated that loss of Irgm1 inhibited osteoclast differentiation and bone resorption function via suppressing ROS accumulation. Further mechanism revealed that Irgm1 could endogenously bind to kelch-like ECH-associated protein 1 (Keap1) and keep Keap1 from ubiquitination and degradation. In the absence of Irgm1, Keap1 was downregulated and causing nuclear factor erythroid 2-related factor 2 (Nrf2) to translocate to the nucleus, thereby activating the level of the antioxidant system to combat oxidative stress. Moreover, Irgm1 deficiency in RAW264.7 promoted osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) through inhibiting the M1 phenotype polarization. Taken together, our results revealed that loss of Irgm1 significantly alleviates OVX-induced bone loss, thus laying the foundation for exploring Irgm1 as a novel targeting approach for the treatment of osteoporosis.