Regulation of Osteoclast Multinucleation by the Actin Cytoskeleton Signaling Network

Abstract

Although it is known that osteoclasts are multinucleated cells that are responsible for bone resorption, the mechanism by which their size is regulated is unclear. We previously reported that an actin-rich superstructure, termed the zipper-like structure, specifically appears during the fusion of large osteoclast-like cells (OCLs). Actin cytoskeleton reorganization in osteoclasts is regulated by a signaling network that includes the macrophage colony-stimulating factor (M-CSF) receptor, a proto-oncogene, Src, and small GTPases. Here, we examined the role of actin reorganization in the multinucleation of OCLs differentiated from RAW 264.7 cells using various pharmacological agents. Jasplakinolide, which stabilizes actin stress fibers, induced the development of small OCLs, and the Src inhibitor SU6656 and the dynamin inhibitor dynasore impaired the maintenance of the podosome belt and the zipper-like structure. These inhibitors decreased the formation of large OCLs but increased the number of small OCLs. M-CSF is known to stimulate osteoclast fusion. M-CSF signaling via Src up-regulated Rac1 activity but down-regulated Rho activity. Rac1 and Rho localized to the center of the zipper-like structure. Rho activator II promoted the formation of small OCLs, whereas the Rho inhibitor Y27632 promoted the generation of large OCLs. These results suggest that the status of the actin cytoskeleton signaling network determines the size of OCLs during cell fusion.

Regulation of Embryonic and Postnatal Development by the CSF-1 Receptor

“…Downstream of CSF-1R, c-Src activates Rac and inhibits Rho, two small GTPases that regulate the cytoskeleton. While Rac activity is dispensable for osteoclast development and multinucleation (Croke et al, 2011), Src-mediated inhibition of Rho activity is necessary for efficient fusion (Takito et al, 2015). Activated RANK upregulates the expression of the Rho family GTP-ase Wrch (a.k.a.…”

DC-STAMP: A Key Regulator in Osteoclast Differentiation

“…An active rearrangement of the cytoskeleton is taking place during each cell‐to‐cell fusion, which involves the actin‐dependent cytoskeleton signaling network (Takito et al, ). In order to form multi‐nucleated osteoclasts, single nucleated osteoclast precursors need to go through many rounds of cell–cell fusion.…”

“…The molecular mechanisms underlying the control of osteoclast cell size remain largely unknown. By using various inhibitors that specifically block the actin cytoskeleton signaling network (Takito et al, ), it has been shown that the size of osteoclasts is regulated by the actin‐mediated cytoskeleton signaling network. Osteoclast polarization is accompanied by extensive reorganization of the actin cytoskeleton.…”

Quantitative proteomics and integrative network analysis identified novel genes and pathways related to osteoporosis

“…A previous study showed that osteoclast undergoes internal structure changes such as rearrangements of actin cytoskeleton that prepare it to resorb bone [38]. Another study suggested that the status of actin cytoskeleton signaling network determines the size of osteoclast-like cells during cell fusion [41]. As one of the top 10 pathways in our study, 8 genes out of the target gene list were enriched in the pathway “Regulation of actin cytoskeleton”.…”