Identification of novel microRNA inhibiting actin cytoskeletal rearrangement thereby suppressing osteoblast differentiation

We report the role of miR-1187 in regulation of osteoblast functions. Over-expression of miR-1187 inhibited osteoblast differentiation. Target prediction analysis tools and experimental validation by luciferase 3' UTR reporter assay identified BMPR-II and ArhGEF-9 as direct targets of miR-1187. ArhGEF-9 activates Cdc42 which has a major role in actin reorganization. BMP-2 also induces actin polymerization. Role of miR-1187 in actin reorganization was determined by western blotting, immunofluorescence, and in vivo gene silencing studies. Reduced protein levels of BMPR-II, activated Cdc42, and downstream signaling molecules were observed in miR-1187-transfected osteoblasts. miR-1187 over-expression resulted in decreased actin polymerization. Additionally, P-cofilin, which does not bind F-actin, was decreased in miR-1187-transfected cells. These results were corroborated by administration of BMPR-II exogenously in miR-1187-transfected osteoblasts. Silencing of miR-1187 in neonatal mice mitigated all the inhibitory effects of miR-1187 on actin cytoskeletal rearrangement. Importantly, in vivo treatment of miR-1187 inhibitor to ovariectomized BALB/c mice led to significant improvement in trabecular bone microarchitecture. Overall, miR-1187 functions as a negative regulator of osteogenesis by repressing BMPR-II and ArhGEF-9 expression thus suppressing non-Smad BMP2/Cdc42 signaling pathway and inhibiting actin reorganization. miR-1187 functions as a negative regulator of osteogenesis by repressing BMPR-II expression, which in turn, suppresses non-Smad BMP2/Cdc42 signaling pathway, thus inhibiting actin cytoskeletal rearrangement. Silencing of miR-1187 significantly improves trabecular bone microarchitecture. As miR-1187 exerts a negative regulatory role in osteoblasts function, hence, we propose that therapeutic approaches targeting miR-1187 could be useful in enhancing the bone formation and treatment of pathological conditions of bone loss.