miR-324 mediates bone homeostasis through the regulation of osteoblast and osteoclast differentiation and activity

Abstract

Abstract microRNAs (miRNAs) modulate the expression of other RNA molecules. One miRNA can target many transcripts, allowing each miRNA to play key roles in many biological pathways. miR-324 is implicated in bone and cartilage maintenance, defects of which result in the common age-related diseases osteoporosis and osteoarthritis. Here, in global miR-324-null mice cartilage damage was increased in both surgically and ageing-induced osteoarthritis, despite minimal changes to the cartilage transcriptome. However, in vivo micro-computed tomography and histology demonstrated that the mice showed increased bone mineral density and both trabecular and cortical thickness, with effect magnitudes increasing with age. The bone marrow of miR-324-null mice also had reduced lipid content while in vivo TRAP staining revealed a decrease in osteoclasts, with histomorphometry demonstrating an increased rate of bone formation. Ex vivo assays showed that the high bone mass phenotype of miR-324-null mice resulted from increased osteoblast activity and decreased osteoclastogenesis. RNA-seq analysis of osteoblasts, osteoclasts and bone marrow macrophages and target validation assays identified that the osteoclast fusion regulator Pin1 and the master osteogenic regulator were targets of miR-324-5p in osteoclast lineage cells and osteoblasts, respectively. Indeed, in vitro Runx2 overexpression recapitulated the increased osteogenesis and decreased adipogenesis phenotype observed in vivo by the loss of miR-324. Overall, these data demonstrate the importance of miR-324 in skeletal biology and that altered bone homeostasis is likely causal for the increased cartilage damage observed during osteoarthritis and ageing. Elucidation of pathways regulated by miR-324 offer promise for the treatment of bone diseases such as osteoporosis.