The RNA binding protein Nab2 regulates splicing of the RhoGEFtriotranscript to govern axon and dendrite morphology

Abstract

ABSTRACTTheDrosophilaRNA 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 theSex-lethal(Sxl) mRNA revealed that Nab2 promotes an exon-skipping event and regulates m6A methylation onSxlpre-mRNA by the Mettl3 methyltransferase. Mettl3 heterozygosity broadly rescuesNab2nullphenotypes 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 intron in thetriopre-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 inNab2nullnervous tissue. Expression of Trio-GEF2 rescues projection defects inNab2nullaxons and dendrites, while the GEF1 Rac1-regulatory domain exacerbates these defects, suggesting Nab2-mediated regulation Trio-GEF activities. Collectively, these data identify Nab2-regulated splicing as a key step in balancing Trio GEF1 and GEF2 activity and show that Nab2, Mettl3, and Trio function in a common pathway that shapes axon and dendrite morphology.Significance StatementDrosophilaNab2, ortholog of the human RBP ZC3H14 mutated in inherited intellectual disability, acts through unknown RNA targets to control axon and dendrite morphology.This study shows that Nab2 and the Mettl3 methyltransferase guide splicing oftriomRNA, which encodes a conserved GEF-domain protein. Intron retention intriomRNA leads to an imbalance in levels of two Trio GEF domains in Nab2-deficient neurons and restoring this balance rescues neuronal defects.The authors conclude that Nab2 control oftriosplicing is required to pattern axon and dendrite growth and suggests that ZC3H14 may play a similar role in the vertebrate brain.