Selective abundance of the stemness-promoting cluster miR-290-295 within the adult substantia nigra dopamine neurons is neuroprotective via preservation of protein synthesis

Locomotor, reward and other critical functions of the body are regulated by the ventral midbrain, with the central role played by dopamine (DA) neurons. The function of these cells from the early development to maturity is critically dependent on the orchestrated expression of coding and non-coding genes. For example, in the stem cells the miR-290-295 cluster constitutes the majority of expressed microRNAs and is critical for stemness in rodents. During development towards various terminally differentiated lineages, such as neurons, the cells typically switch off transcription of these stem cell-specific microRNAs. Here we report that within the adult substantia nigra pars compacta (SN), the miR-290-295 cluster is exclusively expressed in DA neurons (SN ^DA ), preventing the locomotor deficits and maintaining an adequate expression of enzymes involved in DA biogenesis, such as tyrosine hydroxylase (TH), dopa decarboxylase (DDC) and DA transporter (DAT). Importantly, a global knock-out of the miR-290-295 cluster leads to decreased numbers of SN ^DA neurons in adult mice. Using in vitro and in vivo DA cell-specific loss-of-function models, we demonstrated that miR-292a-3p, the most abundant microRNA in this cluster, directly targets Pten, a phosphatase antagonizing the neuroprotective phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)-Akt-mechanistic target of rapamycin kinase (mTOR) pathways regulating translation initiation. Mechanistically, when labelled with the click chemistry-compatible methionine analogue L-azidohomoalanine, miR-290-295 cluster-deficient SN ^DA neurons revealed a drastic impairment of protein synthesis, which is critical for DA biogenesis. Our surprising finding demonstrates for the first time a selective expression of stem cell-specific and stemness-promoting microRNAs in a distinct population of mature neurons to maintain their physiological functions in the adulthood, suggesting that similar epigenetic disinhibition mechanisms may be also critical for other terminally differentiated cells across species.