Phospholipase C-related, but catalytically inactive protein (PRIP) up-regulates osteoclast differentiation via calcium-calcineurin-NFATc1 signaling.

Phospholipase C-related, but catalytically inactive protein (PRIP) was previously identified as a novel inositol 1,4,5-trisphosphate-binding protein with a domain organization similar to that of phospholipase C-δ but lacking phospholipase activity. We recently showed that PRIP gene knock-out (KO) in mice increases bone formation and concomitantly decreases bone resorption, resulting in increased bone mineral density and trabecular bone volume. However, the role of PRIP in osteoclastogenesis has not yet been fully elucidated. Here, we investigated the effects of PRIP on bone remodeling by investigating dynamic tooth movement in mice fitted with orthodontic devices. Morphological analysis indicated that the extent of tooth movement was smaller in the PRIP-KO mice than in wild-type mice. Histological analysis revealed fewer osteoclasts on the bone-resorption side in maxillary bones of PRIP-KO mice, and osteoclast formation assays and flow cytometry indicated lower osteoclast differentiation in bone marrow cells isolated from these mice. The expression of genes implicated in bone resorption was lower in differentiated PRIP-KO cells, and genes involved in osteoclast differentiation, such as the transcription factor NFATc1, exhibited lower expression in immature PRIP-KO cells initiated by M-CSF. Moreover, calcineurin expression and activity were also lower in the PRIP-KO cells. The PRIP-KO cells also displayed fewer M-CSF-induced changes in intracellular Ca²⁺ and exhibited reduced nuclear localization of NFATc1. Up-regulation of intracellular Ca²⁺ restored osteoclastogenesis of the PRIP-KO cells. These results indicate that PRIP deficiency impairs osteoclast differentiation, particularly at the early stages, and that PRIP stimulates osteoclast differentiation through calcium-calcineurin-NFATc1 signaling via regulating intracellular Ca².

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