Inhibitory regulation of osteoclast bone resorption by signal regulatory protein α

Osteoclasts mediate bone resorption, which is critical for bone development, maintenance, and repair. Proper control of osteoclast development and function is important and deregulation of these processes may lead to bone disease, such as osteoporosis. Previous studies have shown that the cytosolic protein tyrosine phosphatase SHP-1 acts as a suppressor of osteoclast differentiation and function, but putative inhibitory receptors that mediate recruitment and activation of SHP-1 in osteoclasts have remained unknown. In the present study, we identify the SHP-1-recruiting inhibitory immunoreceptor signal regulatory protein (SIRP) α as a negative regulator of osteoclast activity. SIRPα is expressed by osteoclasts, and osteoclasts from mice lacking the SIRPα cytoplasmic tail and signaling capacity display enhanced bone resorption in vitro. Consequently, SIRPα-mutant mice have a significantly reduced cortical bone mass. Furthermore, osteoclasts from SIRPα-mutant mice show an enhanced formation of actin rings, k...