Rac1 and Rac2 affects bone architecture and fragility in vivo

Introduction: Rac is a member of the family of Rho-small GTPases involved with intracellular signaling and actin remodeling. Recent studies have identified the family of RhoGTPases as the target of nitrogen-containing bisphosphonates (nBPs) used in the treatment of osteoporosis and osteolytic diseases; however, the function of these signaling molecules during osteoclastogenesis is largely undefined.

Objectives: Using transgenic mice with Rac1(Rac1null), Rac2(Rac2null), or both (Rac1/2null) genes deleted, our project focuses on elucidating the contributions of Rac1 and Rac2 to bone architecture and fragility in vivo. The deletion of either Rac isoform in osteoclasts is hypothesized to affect bone resorption and remodeling, thereby affecting bone mechanical and architectural properties.

Methods: 16 week old male mice (n=11 per group) were scanned using Dual Energy X-ray Absorptiometry (DEXA). Femurs and vertebrae were subjected to mechanical testing using torsion, three point bending, femoral neck fracture and compression assays. Bone architecture was assessed via scanning electron microscopy (SEM) and Micro-Computed Tomography (microCT). Statistical analysis using one-way ANOVA and post-hoc Fisher's LSD test was completed.

Results: Rac deletions differentially affect femoral and vertebral bone. Rac1 affects both cortical and trabecular bone architectural and mechanical properties (P<0.05), where as Rac2 diminishes only cortical bone (P<0.05). Rac1/2 double knock-outs demonstrate reduced femoral and vertebral length (P<0.05), but interestingly do not show severe deficits in bone properties beyond that seen in single knock-out groups.

Conclusion: Using an in vivo mouse model, we show that Rac1 and Rac2 contribute differently in determining the growth and mechanical properties of bone. Specifically, Rac1 regulates both femur and vertebral bone, while Rac2 affects only femurs. Rac1/2 double knock-outs demonstrate reduced femoral and vertebral size, but interestingly do not show severe deficits in mechanical properties beyond single knock-out groups, demonstrating that the loss of both isoforms is not additive.