The matrix vesicle cargo miR-125b accumulates in the bone matrix, inhibiting bone resorption in mice

Minamizaki et al. show that osteoblast-specific overexpression of miR-125b, a cargo of matrix vesicles, increases the bone mass from a reduced number of osteoclasts in mice. This study suggests that bone matrix stores miR-125b which inhibits bone resorption by downregulating PRDM1, a transcriptional repressor of anti-osteoclastogenic factors. Communication between osteoblasts and osteoclasts plays a key role in bone metabolism. We describe here an unexpected role for matrix vesicles (MVs), which bud from bone-forming osteoblasts and have a well-established role in initiation of bone mineralization, in osteoclastogenesis. We show that the MV cargo miR-125b accumulates in the bone matrix, with increased accumulation in transgenic (Tg) mice overexpressing miR-125b in osteoblasts. Bone formation and osteoblasts in Tg mice are normal, but the number of bone-resorbing osteoclasts is reduced, leading to higher trabecular bone mass. miR-125b in the bone matrix targets and degrades Prdm1, a transcriptional repressor of anti-osteoclastogenic factors, in osteoclast precursors. Overexpressing miR-125b in osteoblasts abrogates bone loss in different mouse models. Our results show that the MV cargo miR-125b is a regulatory element of osteoblast-osteoclast communication, and that bone matrix provides extracellular storage of miR-125b that is functionally active in bone resorption.