Bone Abstracts

C-C chemokine receptor 5 (CCR5) is a co-receptor of macrophage-tropic viruses including HIV. Epidemiological and pathological findings have reported that functional changes in CCR5 correlate with HIV transmission bone destruction disease. However, the roles of CCR5 in bone pathophysiology have not been well documented.

Ccr5-deficient osteoclasts showed decreased bone resorption activity accompanied with disorganized cellular architecture and impaired motility. Multimodal and multidimensional super-resolution microscopy facilitates to observe irregular microtubule network and podosome arrangement in Ccr5^−/− osteoclasts, suggesting malfunctions for cell polarity, cell adhesion and locomotion. Time-lapse imaging and subsequent numerical analysis of cell deformity index revealed that Ccr5-deficient osteoclasts reduced their stability of attachment to matrix as evidence by abnormal motility. Expression of integrins and signaling of FAK-Src complex were markedly reduced in Ccr5-deficient osteoclasts, which was concomitant to reduced activity of small GTPase. CCL5, a major ligand of CCR5, stimulated FAK phosphorylation not Src in wild type osteoclasts. RANKL-induced Src phosphorylation and downstream signals were enhanced by CCL5 stimuli, suggesting a cooperative role of CCR5-mediated signaling with RANKL-mediated signaling pathways. Forced expression of constitutive active forms of Rho and Rac in Ccr5-deficient osteoclasts rescued integrin expression and bone resorption. These findings suggested that CCR5-mediated signaling, with cooperating with RANKL-mediated signaling, regulate small GTPases, and thus cellular architecture and motility of differentiated osteoclasts.

Ccr5-deficient bone had significantly increased osteoclasts number, although they did not show difference in BMD compared to their wild-type littermates, indicating dysfunction of Ccr5-deficient osteoclasts in vivo. Interestingly, Ccr5-deficient osteoclasts were observed to be flattened in shape with covering wider bone surface compared to those in wild type in bone sections, supporting our in vitro findings. Furthermore, Ccr5-deficient mice were less susceptible to RANKL-induced bone loss model.

Our findings suggested critical role of CCR5-mediated signaling in pathological bone destruction, thus its implications of bone-specific therapeutic targets.