RESUMEN
Recent studies have elucidated the crucial role for microRNAs in peripheral nerve myelination by ablating components of the microRNA synthesis machinery. Few studies have focused on the role of individual microRNAs. To fill this gap, we focused this study on miR-138, which was shown to be drastically reduced in Dicer1 and Dgcr8 knockout mice with hypomyelinating phenotypes and to potentially target the negative regulators of Schwann cell differentiation. Here, we show that of two miR-138 encoding loci, mir-138-1 is the predominant locus transcribed in Schwann cells. mir-138-1 is transcriptionally upregulated during myelination and downregulated upon nerve injury. EGR2 is required for mir-138-1 transcription during development, and both SOX10 and EGR2 bind to an active enhancer near the mir-138-1 locus. Based on expression analyses, we hypothesized that miR-138 facilitates the transition between undifferentiated Schwann cells and myelinating Schwann cells. However, in conditional knockouts, we could not detect significant changes in Schwann cell proliferation, cell cycle exit, or myelination. Overall, our results demonstrate that miR-138 is an Egr2-dependent microRNA but is dispensable for Schwann cell myelination.
Asunto(s)
Proteína 2 de la Respuesta de Crecimiento Precoz/metabolismo , MicroARNs/genética , Vaina de Mielina/fisiología , Nervios Periféricos/fisiología , Animales , Ciclo Celular/genética , Proliferación Celular/genética , ARN Helicasas DEAD-box/deficiencia , ARN Helicasas DEAD-box/genética , Regulación hacia Abajo , Técnicas de Inactivación de Genes , Sitios Genéticos/genética , Ratones , Nervios Periféricos/citología , Proteínas de Unión al ARN/genética , Proteínas de Unión al ARN/metabolismo , Ribonucleasa III/deficiencia , Ribonucleasa III/genética , Factores de Transcripción SOXE/metabolismo , Células de Schwann/citologíaRESUMEN
We investigated the role of a key component of the Microprocessor complex, DGCR8, in the regulation of myelin formation and maintenance. We found that conditionally ablating Dgcr8 in Schwann cells (SCs) during development results in an arrest of SC differentiation. Dgcr8 conditional knock-out (cKO) SCs fail to form 1:1 relationships with axons or, having achieved this, fail to form myelin sheaths. The expression of genes normally found in immature SCs, such as sex-determining region Y-box 2 (Sox2), is increased in Dgcr8 cKO SCs, whereas the expression of myelin-related genes, including the master regulatory transcription factor early growth response 2 (Egr2), is decreased. Additionally, expression of a novel gene expression program involving sonic hedgehog (Shh), activated de novo in injured nerves, is elevated in Dgcr8 cKOs but not in Egr2 null mice, a model of SC differentiation arrest, suggesting that the injury-related gene expression program in Dgcr8 cKOs cannot be attributed to differentiation arrest. Inducible ablation of Dgcr8 in adult SCs results in gene expression changes similar to those found in cKOs, including an increase in the expression of Sox2 and Shh. Analyses of these nerves mainly reveal normal myelin thickness and axon size distribution but some dedifferentiated SCs and increased macrophage infiltration. Together our data suggest that Dgcr8 is responsible for modulation of gene expression programs underlying myelin formation and maintenance as well as suppression of an injury-related gene expression program.
