Loss of the <scp>RNA</scp> Binding Protein <scp>HuR</scp> in Early Murine Limb Mesenchyme Does Not Affect Development but Leads to Impaired Bone Homeostasis in Adulthood

In this study, we examined how a critical posttranscriptional regulator, the RNA-binding protein HuR (gene name Elavl1), contributes to the development and maintenance of limb skeletal tissue. Using the Prx1-Cre knockout model, we examined the effect of germline knockout (Elavl1KO) and limb mesenchyme-specific knockout (MSC-Elavl1KO) of HuR on limb development. We found that Elavl1KO disrupted the development of the limb skeleton and was associated with a loss of signaling from the apical ectodermal ridge (AER). In contrast, MSC-Elavl1KO did not appear to affect skeletal development. Mature MSC-Elavl1KO mice appeared healthy, but their limb skeleton exhibited abnormal bone structure in both males and females at 2.5 months of age. Osteoblasts isolated from MSC-Elavl1KO mice exhibited lower expression of osteoblastic marker genes, and their ability to generate a mineralized matrix was markedly impaired. RNA-Seq analysis of these osteoblasts demonstrated that loss of HuR substantially influenced their transcriptome, affecting genes associated with a wide range of cellular processes. Finally, using siRNA knockdown in the human MG63 cell line, we identified that loss of HuR leads to increased mRNA turnover of the osteoblastic transcription factor Runx2. Overall, the study has demonstrated a critical role for HuR-mediated posttranscriptional control in skeletal development and homeostasis, but finds that its expression in mesenchyme-derived cells only becomes critical in mature skeletal tissue.