Bone Abstracts

Matrix vesicles (MVs) play a key role in bone mineralization. These nano-sized vesicles bud from apical microvilli of osteoblasts and accumulate in osteoid, and initial hydroxyapatite crystals form within these vesicles. MVs appear to share a common feature with exosomes. Recent studies have demonstrated that exosomes include microRNAs (miRNAs) as mediators of intercellular communication. We purified MVs from MC3T3-E1 cell cultures by ultracentrifugation and found approximately 200 miRNAs. MVs suppressed RANKL-dependent osteoclast formation and bone resorption in mouse bone marrow macrophage (BMM) and/or RAW-D cell cultures with decreased levels of osteoclast marker genes such as Acp5, Ctsk and Dcstamp. There were no obvious effects of MVs on rat and human osteoblast and MC3T3-E1 cell cultures. We then focused on miRNAs with predicted target genes involved in osteoclast formation. In both human and rodent models, miR-125b was highly abundant in MVs, compared to osteoblasts and BMMs. Moreover, we found that miR-125b was complexed with Ago2 and stored in the extracellular matrix of MC3T3-E1 cell cultures and mouse bone. Besides being a suppression of osteoclast formation, miR-125b decreased the expression of osteoclast marker genes. The transcriptional repressor Prdm1 was downregulated by miR-125b, resulting in increases in the expression of the anti-osteoclastogenic genes, Irf8 and Mafb. To determine whether miR-125b affects bone in vivo, we generated transgenic mice overexpressing miR-125b under the control of the human osteocalcin promoter (Tg). Overexpressing miR-125b in osteoblasts in Tg mice led to high bone mass by decreasing the number of osteoclasts without any changes in osteoblasts. miR-125b suppressed osteoclastic osteolysis in the lipopolysaccharide -induced mouse osteolysis model. These findings suggest an additional role of MVs and that miR-125b embedded in bone regulates osteoclastogenesis, which may serve as a novel mechanism for osteoblast-osteoclast communication.