Rac deletion in osteoclasts causes severe osteopetrosis

Abstract

SummaryCdc42 mediates bone resorption principally by stimulating osteoclastogenesis. Whether its sister GTPase, Rac, meaningfully impacts upon the osteoclast and, if so, by what means, is unclear. We find that whereas deletion of Rac1 or Rac2 alone has no effect, variable reduction of Rac1 in osteoclastic cells of Rac2 2/2 mice causes severe osteopetrosis. Osteoclasts lacking Rac1 and Rac2 in combination (Rac double-knockout, RacDKO), fail to effectively resorb bone. By contrast, osteoclasts are abundant in RacDKO osteopetrotic mice and, unlike those deficient in Cdc42, express the maturation markers of the cells normally. Hence, the osteopetrotic lesion of RacDKO mice largely reflects impaired function, and not arrested differentiation, of the resorptive polykaryon. The dysfunction of RacDKO osteoclasts represents failed cytoskeleton organization as evidenced by reduced motility of the cells and their inability to spread or generate the key resorptive organelles (i.e. actin rings and ruffled borders), which is accompanied by abnormal Arp3 distribution. The cytoskeleton-organizing capacity of Rac1 is mediated through its 20-amino-acid effector domain. Thus, Rac1 and Rac2 are mutually compensatory. Unlike Cdc42 deficiency, their combined absence does not impact upon differentiation but promotes severe osteopetrosis by dysregulating the osteoclast cytoskeleton.

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.…”

“…82,83 On the other hand, Rac1/Rac2 double knockout osteoclasts do not form sealing zones. 63 This process might depend on Rac activation of the WAVE complex which requires simultaneous interactions with prenylated Rac-GTP, acidic phospholipids and a specific state of phosphorylation.…”

Tensin 3 is a new partner of Dock5 that controls osteoclast podosome organization and activity

“…We show here that tensin 3 can potentiate Dock5 exchange activity towards Rac. We also found that tensin 3 is necessary for the correct patterning of osteoclast podosomes, which is known to also require Dock5 (Vives et al, 2011), Rac (Razzouk et al, 1999;Wang et al, 2008;Croke et al, 2011) and the activation of Rac by Dock5 (Vives et al, 2015).…”

Osteoclasts: New Insights

“…Cdc42, another Rho GTPase, exerts only a mild cytoskeletal effect through atypical PKCs, and its major impact is on proliferation of osteoclast precursors and survival of mature osteoclasts (170). In contrast, Rac1 and Rac2 function as two key effector molecules of the αvβ3 integrin, as their deletion results in a severe compromise of the osteoclast cytoskeleton and osteopetrosis (171). Integrin αvβ3 mediated cytoskeleton organization leads to the formation of the actin ring, which isolates the resorptive space from its surroundings (Figure 4).…”