Small RNA Silencing Pathways in Germ and Stem Cells

摘要

During the past several years, it has become clear that small RNAs guard germ cell genomes from the activity of mobilegenetic elements. Indeed, in mammals, a class of small RNAs, known as Piwi-interacting RNAs (piRNAs), forms an innateimmune system that discriminates transposons from endogenous genes and selectively silences the former. piRNAs enforcesilencing by directing transposon DNA mcthylation during male germ cell development. As such, piRNAs represent perhapsthe only currently known sequence-specific factor for deposition of methylcytosinc in mammals. The three mammalian Piwiproteins Miwi2, Mili, and Miwi are required at different stages of germ cell development. Moreover, distinct classes ofpiRNAs are expressed in developmental waves, with particular generative loci and different sequence content distinguishingpiRNAs populations in embryonic germ cells from those that appear during meiosis. Although our understanding of Piwi pro-teins and piRNA biology have deepened substantially during the last several years, major gaps still exist in our understand-ing of these enigmatic RNA species.