GIT1 is critical for formation of the CD31hiEmcnhi vessel subtype in coupling osteogenesis with angiogenesis via modulating preosteoclasts secretion of PDGF-BB

Abstract G protein-coupled receptor kinase 2 interacting protein-1 (GIT1) is a scaffold protein that plays a vital role in bone modeling and remodeling during osteogenesis coupled with angiogenesis. Recent studies have shown that a specialized subset of vascular endothelium strongly positive for CD31 and Endomucin (CD31 hi Emcn hi ) is coupled with anabolic bone formation. Based on our previous finding that GIT1 knockout (GIT1 KO) mice have impaired angiogenesis and bone formation, we hypothesized that GIT1 affects formation of the CD31 hi Emcn hi vessel subtype. In the current study, GIT1 knockout (GIT1 KO) mice displayed a significant decrease in trabecular bone mass and CD31 hi Emcn hi vessel number, compared to their wild-type counterparts. In the fracture healing mouse model, GIT1 KO mice contained a lower number of CD31 hi Emcn hi vessels in fracture callus at days 7 and 14. However, no significant differences in the number of preosteoclasts in bone marrow, trabecular bone and callus in GIT1 KO mice were observed, compared with wild-type mice. Notably, concentrations of serum platelet-derived growth factor-BB(PDGF-BB) secreted by preosteoclasts associated with CD31 hi Emcn hi vessel formation were lower in GIT1 KO mice. In addition, PDGF-BB-associated expression of phosphorylated extracellular signal-regulated kinase- 1/2 (ERK1/2) and specificity protein 1 (SP1) was significantly decreased in preosteoclasts of GIT1 KO mice. These results collectively suggest that GIT1 is a critical participant in formation of the CD31 hi Emcn hi vessel subtype, highlighting a novel biologic function of this scaffold protein in preosteoclasts.