Distribution of inositol 1,4,5-trisphosphate receptors in rat osteoclasts.

Inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) are Ca2+ channels that localize to intracellular Ca2+ stores such as the endoplasmic reticulum (ER). Recently, IP3Rs were found to participate in the formation of the cytoskeleton and cellular adhesions. In this study, we examined the cellular localization of type I, II, and III IP3Rs to assess their role in cellular adhesion in rat osteoclasts. Rat bone marrow cells were cultured in alpha-MEM with 10% fetal bovine serum, M-CSF, RANKL, and 1,25(OH)2D3 for 1 week to promote osteoclast formation. Type I, II, and III IP3R expression in the osteoclasts was then examined by RT-PCR. Double-staining was performed using antibodies against type I, II, and III IP3Rs and DiOC6, an ER marker, or TRITC-phalloidin, an actin filament marker. Expression of all three IP3Rs was detected in the newly formed osteoclasts; however, the localization of the type I and II IP3Rs was predominantly close to nuclear, and possibly colocalized with the ER, while the type III IP3Rs were localized to the ER and podosomes, actin-rich adhesion structures in osteoclasts. These findings suggest that type III IP3Rs are associated with osteoclast adhesion.