Rac1 Inhibition Via Srgap2 Restrains Inflammatory Osteoclastogenesis and Limits the Clastokine, SLIT3

Abstract

The Rac1‐specific guanosine triphosphatase (GTPase)‐activating protein Slit‐Robo GAP2 (Srgap2) is dramatically upregulated during RANKL‐induced osteoclastogenesis. Srgap2 interacts with the cell membrane to locally inhibit activity of Rac1. In this study, we determined the role of Srgap2 in the myeloid lineage on bone homeostasis and the osteoclastic response to TNFα treatment. The bone phenotype of mice specifically lacking Srgap2 in the myeloid lineage (Srgap2 f/f:LysM‐Cre; Srgap2 conditional knockout [cKO]) was investigated using histomorphometric analysis, in vitro cultures and Western blot analysis. Similar methods were used to determine the impact of TNFα challenge on osteoclast formation in Srgap2 cKO mice. Bone parameters in male Srgap2 cKO mice were unaffected. However, female cKO mice displayed higher trabecular bone volume due to increased osteoblast surface and bone formation rate, whereas osteoclastic parameters were unaltered. In vitro, cells from Srgap2 cKO had strongly enhanced Rac1 activation, but RANKL‐induced osteoclast formation was unaffected. In contrast, conditioned medium from Srgap2 cKO osteoclasts promoted osteoblast differentiation and had increased levels of the bone anabolic clastokine SLIT3, providing a possible mechanism for increased bone formation in vivo. Rac1 is rapidly activated by the inflammatory cytokine TNFα. Supracalvarial injection of TNFα caused an augmented osteoclastic response in Srgap2 cKO mice. In vitro, cells from Srgap2 cKO mice displayed increased osteoclast formation in response to TNFα. We conclude that Srgap2 plays a prominent role in limiting osteoclastogenesis during inflammation through Rac1, and restricts expression of the paracrine clastokine SLIT3, a positive regulator of bone formation. © 2019 American Society for Bone and Mineral Research.

Puerarin inhibits titanium particle‐induced osteolysis and RANKL‐induced osteoclastogenesis via suppression of the NF‐κB signaling pathway

“…Our study provides evidence that the increase in osteoclasts caused by Ti particles around the prosthesis causes inflammatory cell infiltration, resulting in an increase in bone absorption, which accelerates bone dissolution around the prosthesis and ultimately leads to a decline in prosthesis stability. This result was consistent with a previous report that inhibiting osteoclast formation and inflammation attenuated wear particle‐induced implant loosening 36 . All these results proved that puerarin can effectively treat bone loss induced by wear particles in rats.…”

“…This result was consistent with a previous report that inhibiting osteoclast formation and inflammation attenuated wear particleinduced implant loosening. 36 All these results proved that puerarin can effectively treat bone loss induced by wear particles in rats.…”

Sexual Dimorphism in Osteoclasts

“…Rac1-specific guanosine triphosphatase (GTPase)-activating protein Slit-Robo GAP2 (Srgap2) is upregulated by RANKL during osteoclastogenesis. Targeted deletion of Srgap2 in osteoclast precursors produced a female-specific high bone mass phenotype [ 46 ]. Protein kinase C delta (PKC-δ) deletion in osteoclasts resulted in a high bone mass phenotype only in male mice and an associated decrease in osteoclastogenesis in cultures of male bone marrow cells [ 47 ].…”

Identification of the canonical and noncanonical role of miR-143/145 in estrogen-deficient bone loss

“…We used bioinformatic analysis to search for miR-143 and miR-145 targeting candidates that were associated with induction of osteoclastogenesis. Results predicted that Cd226, also called DNAM-1 (DNAX accessory molecule-1), which inhibits osteoclast differentiation by regulating cellular fusion process 21 , is the direct target of miR-143 and that SLIT ROBO Rho GTPase-activating protein 2 (Srgap2), which represses osteoclast formation by modulating cytoskeletal organization 22 , is the direct target of miR-145. We subsequently analyzed Cd226 and Srgap2expression and found both were reduced in osteoclasts treated with supernatant from OVX BMSCs (Figure 6 M).…”