The RNA-binding protein Nab2 regulates levels of the RhoGEF Trio to govern axon and dendrite morphology.

The RNA-binding protein (RBP) Nab2 acts in neurons to regulate neurodevelopment and is orthologous to the human intellectual disability-linked RBP, ZC3H14. Nab2 governs axon projection in mushroom body neurons and limits dendritic arborization of class IV sensory neurons in part by regulating splicing events in ∼150 mRNAs. Analysis of the () mRNA revealed that Nab2 promotes an exon-skipping event and regulates mA methylation on pre-mRNA by the Mettl3 methyltransferase. Mettl3 heterozygosity broadly rescues phenotypes implying that Nab2 acts through similar mechanisms on other RNAs, including unidentified targets involved in neurodevelopment. Here, we show that Nab2 and Mettl3 regulate the removal of a 5'UTR (untranslated region) intron in the pre-mRNA. Trio utilizes two GEF domains to balance Rac and RhoGTPase activity. Intriguingly, an isoform of Trio containing only the RhoGEF domain, GEF2, is depleted in nervous tissue. Expression of Trio-GEF2 rescues projection defects in axons and dendrites, while the GEF1 Rac1-regulatory domain exacerbates these defects, suggesting Nab2-mediated regulation Trio-GEF activities. Collectively, these data indicate that Nab2-regulated processing of is critical for balancing Trio-GEF1 and -GEF2 activity and show that Nab2, Mettl3, and Trio function in a common pathway that shapes axon and dendrite morphology.