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J. Biol. Chem., Vol. 280, Issue 38, 32930-32943, September 23, 2005

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Regulation of Actin Ring Formation by Rho GTPases in Osteoclasts^*

From the Department of Biomedical Sciences, Dental School, University of Maryland, Baltimore, Maryland 21201

Actin ring formation is a prerequisite for osteoclast bone resorption. Although gelsolin null osteoclasts failed to exhibit podosomes, actin ring was observed in these osteoclasts. Wiscott-Aldrich syndrome protein (WASP) was observed in the actin ring of gelsolin null osteoclast. Osteoclasts stimulated with osteopontin simulated the effects of Rho and Cdc42 in phosphatidylinositol 4,5-bisphosphate (PIP₂) association with WASP as well as formation of podosomes, peripheral microfilopodia-like structures, and actin ring. To explore the potential functions of Rho and Cdc42, TAT-mediated delivery of Rho proteins into osteoclasts was performed. Although Rho and Cdc42 are required for actin ring formation, transduction of either one of the proteins alone is insufficient for this process. Addition of osteopontin to osteoclasts transduced with Cdc42^Val12 or transduction of osteoclasts with both Rho^Val14 and Cdc42^Val12 augments the formation of WASP-Arp2/3 complex and actin ring. Neomycin, an antibiotic, blocked the effects of osteopontin or TAT-Rho^Val14 on PIP₂ interaction with WASP. WASP distribution was found to be cytosolic in these osteoclasts. Depletion of WASP by short interfering RNA-mediated gene silencing blocked actin polymerization as well as actin ring formation in osteoclasts. These results suggest that Rho-mediated PIP₂ interaction with WASP may contribute to the activation and membrane targeting of WASP. Subsequent interaction of Cdc42 and Arp2/3 with WASP may enhance cortical actin polymerization in the process of actin ring formation in osteoclasts.

Received for publication, January 5, 2005 , and in revised form, June 17, 2005.

^* This work was supported by National Institutes of Health Grant R01-AR46292. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Biomedical Sciences, University of Maryland Dental School, Baltimore, MD 21201. Tel.: 410-706-2083; Fax: 410-706-0193; E-mail: mac001{at}dental.umaryland.edu.

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