Osteoclasts Lacking Rac2 Have Defective Chemotaxis and Resorptive Activity

Abstract

The role of the small Rho GTPase Rac2 in mature osteoclasts has not been extensively studied. Rac2−/− mice are of normal size and have normal tooth eruption. However, femoral cortical thickness was significantly greater in Rac2−/− compared to wild-type mice, while percent cortical porosity was lower. As assessed by histomorphometry, trabecular bone mass was significantly higher in male Rac2−/− than wild-type animals, although trabecular bone mass was similar when data from male and female animals were combined. There were no significant differences in the number of osteoblasts per bone surface; however, the number of osteoclasts per total bone area tended to be higher in Rac2−/− mice and was significantly higher in male Rac2−/− mice. In the aggregate, these data suggested a defect in osteoclast function and, consistent with that, rates of bone resorption were significantly reduced in Rac2−/− osteoclasts. In addition, Rac2−/− osteoclasts had a significantly delayed spreading response to treatment with CSF1 for 15 min. Phalloidin staining showed areas of abnormal actin accumulation and impaired actin ring formation in Rac2−/− osteoclasts. Finally, Rac2−/− osteoclasts showed a marked defect in chemotaxis toward a point source of CSF1, with a dramatic reduction in migratory rate. Together, these findings indicate an important role for Rac2 in mature osteoclasts.

Modulation of osteoclast differentiation and bone resorption by Rho GTPases

“…13 Although the results are somehow conflicting and their precise function not elucidated yet, both Rac1 and Rac2 seem to be important for the formation of the podosome belt and for bone resorption. 31,[63][64][65][66] This confirms earlier studies showing podosome belt disruption in osteoclasts where Rac1 and Rac2 have been neutralized by specific antibodies. 67 Using DNA microarray, we analyzed the expression profile of 76 GEFs of the Dbl and Dock families in osteoclasts.…”

“…Although the authors agree on the increase in bone density, there are discrepancies regarding the mechanisms involved that will need further investigation. 31,[63][64][65] Interestingly, the absence of a single Rac GEF, such as Vav3, Dock5 or FARP2, is sufficient to result in osteoclasts impaired resorption in vivo, through distinct mechanisms. [68][69][70] The higher bone density observed in mice deficient for Rho GTPase signaling highlights them as targets in diseases with increased bone resorption such as post-menopausal osteoporosis or cancer-associated osteolysis.…”

Selective deletion of the membrane-bound colony stimulating factor 1 isoform leads to high bone mass but does not protect against estrogen-deficiency bone loss

“…To further explore the mechanism underlying the impaired osteoclastogenesis in the k/o mice and to determine whether the genetic absence of mCSF1 in other cells impacts osteoclastogenesis in these animals, osteoclast precursors were isolated from wt and k/o animals and cultured in vitro in the absence of osteoblasts but in the presence of osteoclastogenic cytokines, as we have previously published [34]. As shown in Fig.…”

“…Cells at the interface were collected, washed in PBS twice, and plated at a final concentration of 1.3 × 10 6 cells/35-mm dish. Cells were cultured in phenol red-free alpha-MEM containing 10% FBS, 75 ng/ml CSF1, 75 ng/ml RANKL, 10 ng/ml TNF-α, and 50 μg/ml AICAR, which has been previously reported to induce osteoclastogenesis [34]. The medium was changed every 3 days.…”

New roles of filopodia and podosomes in the differentiation and fusion process of osteoclasts

“…The fusion first occurs to a portion of the filopodia. Previous reports have shown that FLNA, Rac1, and Rac2 are related to the OC chemotaxis during differentiation (Wang et al, 2008;Leung et al, 2010;Itokowa et al, 2011;Sun et al, 2013). FLNA allows Rho GTPases to accumulate at the front of filopodia, and a cascade amplifies the effect to Rho GTPases (Nobes and Hall, 1995;Ohta et al, 2006;Leung et al, 2010).…”