The function of miR-324 in neurological and musculoskeletal biology

Abstract

microRNAs are non-coding RNAs which modulate the expression of other RNA molecules. One microRNA can target many transcripts, allowing each microRNA to play key roles in many biological pathways. Our group has produced a global knockout model of miR-324, a microRNA previously implicated in bone and cartilage maintenance, defects of which result in common age-related diseases, such as osteoporosis or osteoarthritis. miR-324 has also been discussed in relation to epilepsy and therefore the neurological effects of miR-324 deletion were also investigated. Ex vivo hippocampal electrophysiology revealed that miR-324-null mice show a hyperexcitability phenotype, and RNA sequencing in the murine hippocampus and neocortex revealed that Suox, a gene causal of the human disorder isolated sulfite oxidase deficiency, was a direct target of miR-324. In vivo micro-computed tomography and histology revealed an increase in bone mineral density, trabecular thickness and cortical thickness in miR-324-null mice, while also displaying a decreased number of osteocytes and lipid droplets. In vivo TRAP staining revealed a decrease in osteoclasts and calcein/alizarin red-S double-labelling demonstrated an increased rate of bone formation in miR-324-null mice. Ex vivo assays revealed that the high bone mass phenotype of the miR-324-null mice resulted from increased osteoblast activity and decreased osteoclastogenesis. miR-324 Target prediction and validation in osteoblasts, bone marrow macrophages and osteocytes, revealed that the osteoclast fusion regulator Pin1 was a miR-324 target in the osteoclast lineage, Hoxa9 and Samd5 were osteocyte target genes and the master osteogenic regulator Runx2 was a target of miR-324-5p in osteoblasts, the overexpression of which in vitro recapitulated the increased osteogenesis and decreased adipogenesis phenotype observed in vivo. These data point to important roles of miR-324 in bone and neurological biology. Elucidation of pathways regulated by miR-324 offers promise for the treatment of bone diseases such as osteoporosis and neurological disorders such as epilepsy.