Abstract

PIWI-interacting RNAs (piRNAs) and their associated PIWI proteins play an important role in repressing transposable elements in animal germlines. However, little is known about the function of PIWI proteins and piRNAs in the developing brain. Here, we investigated the role of an important PIWI family member, Piwi-like protein 1 (Piwil1; also known as Miwi in mouse) in the developing mouse neocortex. Using a Piwil1 knock-out (Piwil1 KO) mouse strain, we found that Piwil1 is essential for several steps of neocorticogenesis, including neocortical cell cycle, neuron migration and dendritogenesis. Piwil1 deletion resulted in increased cell cycle re-entry at embryonic day 17 (E17) when predominantly intracortically projecting neurons are being produced. Prenatal Piwil1 deletion increased the number of Pax6+ radial glia at postnatal day 0 (P0). Furthermore, Piwil1 deletion disrupted migration of Satb2+ neurons within deep layers at E17, P0 and P7. Satb2+ neurons showed increased co-localization with Bcl11b (also known as Ctip2), marker of subcortically projecting neurons. Piwil1 knockouts had disrupted neocortical circuitry represented by thinning of the corpus callosum and altered dendritogenesis. We further investigated if Piwil1 deletion disrupted expression levels of neocortical piRNAs by small RNA-sequencing in neocortex. We did not find differential expression of piRNAs in the neocortices of Piwil1 KO, while differences were observed in other Piwil1 KO tissues. This result suggests that Piwil1 may act independently of piRNAs and have novel roles in higher cognitive centers, such as neocortex. In addition, we report a screen of piRNAs derived from tRNA fragments in developing neocortices. Our result is the first report of selective subsets of piRNAs and tRNA fragments in developing prenatal neocortices and helps clarify some outstanding questions about the role of the piRNA pathway in the brain.