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The evolutionarily conserved miR-302 family of microRNAs is expressed during early mammalian embryonic development. Here, we report that deletion of miR-302a-d in mice results in a fully penetrant late embryonic lethal phenotype. Knockout embryos have an anterior neural tube closure defect associated with a thickened neuroepithelium. The neuroepithelium shows increased progenitor proliferation, decreased cell death, and precocious neuronal differentiation. mRNA profiling at multiple time points during neurulation uncovers a complex pattern of changing targets over time. Overexpression of one of these targets, Fgf15, in the neuroepithelium of the chick embryo induces precocious neuronal differentiation. Compound mutants between mir-302 and the related mir-290 locus have a synthetic lethal phenotype prior to neurulation. Our results show that mir-302 helps regulate neurulation by suppressing neural progenitor expansion and precocious differentiation. Furthermore, these results uncover redundant roles for mir-290 and mir-302 early in development.

ObjectiveIn growth plate chondrocytes, loss of Dicer, a microRNA (miRNA)-processing enzyme, causes defects in proliferation and differentiation, leading to a lethal skeletal dysplasia. However roles of miRNAs in articular chondrocytes have not been defined in vivo. To investigate the role of miRNAs in articular chondrocytes and to explore the possibility of generating a novel mouse osteoarthritis (OA) model caused by intrinsic cellular dysfunction, we ablated Drosha, another essential enzyme for miRNA biogenesis, exclusively in articular chondrocytes of postnatal mice.DesignFirst, to confirm that the essential role of miRNAs in skeletal development, we ablated the miRNA biogenesis pathway by deleting Drosha or DGCR8 in growth plate chondrocytes. Next, to investigate the role of miRNAs in articular cartilage, we deleted Drosha using Prg4-CreER(T) transgenic mice expressing a tamoxifen-activated Cre recombinase (CreER(T)) exclusively in articular chondrocytes. Tamoxifen was injected at postnatal days, 7, 14, 21, and 28 to ablate Drosha.ResultsDeletion of Drosha or DGCR8 in growth plate chondrocytes caused a lethal skeletal defect similar to that of Dicer deletion, confirming the essential role of miRNAs in normal skeletogenesis. Early postnatal Drosha deletion in articular chondrocytes significantly increased cell death and decreased Safranin-O staining. Mild OA-like changes, including surface erosion and cleft formation, were found in male mice at 6 months of age; however such changes in females were not observed even at 9 months of age.ConclusionsEarly postnatal Drosha deficiency induces articular chondrocyte death and can cause a mild OA-like pathology.

The Extracellular RNA (exRNA) Communication Consortium was launched by the National Institutes of Health to focus on the extent to which RNA might function in a non-cell-autonomous manner. With the availability of increasingly sensitive tools, small amount