MiR-124 synergism with ELAVL3 enhances target gene expression to promote neuronal maturity

Abstract

Significance MicroRNAs (miRNAs) are small noncoding RNAs that when loaded into Argonuate (AGO) form the RNA-induced silencing complex that represses target transcripts. Here, in addition to the canonical repressive miRNA function, we identified the activity of miR-124, a neuron-enriched miRNA, as a positive regulator of its target genes. Transcripts harboring both miR-124 and ELAVL3, an RNA-binding protein, target sites at the 3′ untranslated region (3′UTR) allowing for the synergism of AGO-miR124 and ELAVL3 to enhance its target gene expression. The switch of miR-124 as a positive regulator is essential for facilitating neuronal reprogramming of human fibroblasts and promoting the functional maturity of primary human neurons. Our study provides a conceptual advance in microRNA’s function in regulating neuronal identity and function. Neuron-enriched microRNAs (miRNAs), miR-9/9* and miR-124 (miR-9/9*-124), direct cell fate switching of human fibroblasts to neurons when ectopically expressed by repressing antineurogenic genes. How these miRNAs function after the repression of fibroblast genes for neuronal fate remains unclear. Here, we identified targets of miR-9/9*-124 as reprogramming cells activate the neuronal program and reveal the role of miR-124 that directly promotes the expression of its target genes associated with neuronal development and function. The mode of miR-124 as a positive regulator is determined by the binding of both AGO and a neuron-enriched RNA-binding protein, ELAVL3, to target transcripts. Although existing literature indicates that miRNA–ELAVL family protein interaction can result in either target gene up-regulation or down-regulation in a context-dependent manner, we specifically identified neuronal ELAVL3 as the driver for miR-124 target gene up-regulation in neurons. In primary human neurons, repressing miR-124 and ELAVL3 led to the down-regulation of genes involved in neuronal function and process outgrowth and cellular phenotypes of reduced inward currents and neurite outgrowth. Our results highlight the synergistic role between miR-124 and RNA-binding proteins to promote target gene regulation and neuronal function.