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Abstract
Dysregulated miRNA expression and mutation of genes involved in miRNA biogenesis have been reported in motor neuron diseases including spinal muscular atrophy (SMA) and amyotrophic lateral sclerosis (ALS). Therefore, identifying molecular mechanisms governing miRNA expression is important to understand these diseases. Here, we report that expression of DROSHA, which is a critical enzyme in the microprocessor complex and essential for miRNA biogenesis, is reduced in motor neurons from an SMA mouse model. We show that DROSHA is degraded by neuronal activity induced autophagy machinery, which is also dysregulated in SMA. Blocking neuronal activity or the autophagy-lysosome pathway restores DROSHA levels in SMA motor neurons. Moreover, reducing DROSHA levels enhances axonal growth. As impaired axonal growth is a well described phenotype of SMA motor neurons, these data suggest that DROSHA reduction by autophagy may mitigate the phenotype of SMA. In summary, these findings suggest that autophagy regulates RNA metabolism and neuronal growth via the DROSHA/miRNA pathway and this pathway is dysregulated in SMA.
Highlights
MicroRNAs are a sub-set of non-coding RNAs, which bind to the 3′-untranslated region of messenger RNAs (mRNAs), and act as translational repressors. miRNAs play a significant role in a broad range of cellular and developmental processes such as neuronal development[1], learning and memory[2] and synaptic plasticity[3]
To understand the mechanisms underlying the dysregulated miRNA expression in spinal muscular atrophy (SMA), we measured the levels of proteins relevant in miRNA homeostasis in spinal motor neurons isolated from E13.5 WT and SMA mice (Smn−/−;SMN2tg/0) at 10 days in vitro (10DIV) culture (Supplementary Fig. 1)
We found that expression of Dgcr[8] mRNA was increased, while Drosha mRNA levels were not altered in SMA motor neurons (Fig. 1C)
Summary
MicroRNAs (miRNAs) are a sub-set of non-coding RNAs, which bind to the 3′-untranslated region of mRNAs, and act as translational repressors. miRNAs play a significant role in a broad range of cellular and developmental processes such as neuronal development[1], learning and memory[2] and synaptic plasticity[3]. Dysregulated miRNA expression and mutations of genes involved in miRNA biogenesis are reported in motor neuron disorders such as spinal muscular atrophy (SMA)[4,5,6] and amyotrophic lateral sclerosis (ALS)[7,8,9,10,11]. In addition to miRNAs, DROSHA can process other types of RNAs including messenger RNAs (mRNAs) and ribosomal RNAs (rRNAs)[14] This indicates that proper function of DROSHA is crucial for cells. Dysregulated miRNA expression has been reported in SMA animal models and SMN deficient cells including neurons from C. elegans, neurons and muscles from mice, as well as fibroblasts and serum from patients[4,5,35,36].
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