MicroRNA-9 modulates Cajal-Retzius cell differentiation by suppressing Foxg1 expression in mouse medial pallium

Vertebrate brain hosts a diverse collection of microRNAs, but little is known about their functions in vivo. Here we propose that mouse microRNA-9 (miR-9) targets Foxg1 mRNAs for proper generation of Cajal-Retzius cells in the medial pallium. miR-9 expression is mediolaterally graded, being most intense in the cortical hem; it contrasts with the Foxg1 expression in a reciprocal gradient. The 3' untranslated regions of tetrapod, but not of teleost, Foxg1 mRNAs conserve miR-9 target sequences and are regulated by miR-9. Gain- and loss-of-function analyses of miR-9 showed that miR-9 negatively regulates endogenous Foxg1 protein level. Moreover, miR-9 overexpression in developing telencephalon at E11.5 by electroporation resulted in ectopic Reelin-positive cells over the cortex beyond the marginal zone. In addition, inhibition of endogenous miR-9 function by antisense oligonucleotides caused the regression of Wnt3a-positive cortical hem and reduction of reelin-, p73-, and NeuroD1-positive cells.