The conserved miR-8/miR-200 microRNA family and their role in invertebrate and vertebrate neurogenesis

Since their discovery in the early 1990s, microRNAs have emerged as key components of the post-transcriptional regulation of gene expression. MicroRNAs occur in the plant and animal kingdoms, with the numbers of microRNAs encoded in the genome increasing together with the evolutionary expansion of the phyla. By base-pairing with complementary sequences usually located within the 3' untranslated region, microRNAs target mRNAs for degradation, destabilization and/or translational inhibition. Because one microRNA can have many, if not hundreds, of target mRNAs and because one mRNA can, in turn, be targeted by many microRNAs, these small single-stranded RNAs can exert extensive pleiotropic functions during the development, adulthood and ageing of an organism. Specific functions of an increasing number of microRNAs have been described for the invertebrate and vertebrate nervous systems. Among these, the miR-8/miR-200 microRNA family has recently emerged as an important regulator of neurogenesis and gliogenesis and of adult neural homeostasis in the central nervous system of fruit flies, zebrafish and rodents. This highly conserved microRNA family consists of a single ortholog in the fruit fly (miR-8) and five members in vertebrates (miR-200a, miR-200b, miR-200c, miR-141 and miR-429). Here, we review our current knowledge about the functions of the miR-8/miR-200 microRNA family during invertebrate and vertebrate neural development and adult homeostasis and, in particular, about their role in the regulation of neural stem/progenitor cell proliferation, cell cycle exit, transition to a neural precursor/neuroblast state, neuronal differentiation and cell survival and during glial cell growth and differentiation into mature oligodendrocytes.