Arginine methylation of the RNA-binding protein G3BP1 promotes synapse development of neuron via control of actin cytoskeleton

Neurons in the brain communicate via neurotransmission at dendritic spines. These specialized protrusions on neuronal dendrites are initially formed as elongated filopodia, but refine to become mushroom-shaped as the brain matures. Failure of spine maturation is associated with neurodevelopmental disorders such as autism. Dendritic spine development depends on specific RNA-binding proteins (RBP) which transport selective mRNAs to dendrites and control their translation locally near synapses. How is the function of RBP regulated by posttranslational modification in dendrite is not well-understood. Arginine methylation, catalyzed by a group of enzymes called protein arginine methyltransferases (PRMTs), is best known to regulate chromatin structure and transcription in the nucleus. Its role outside the nucleus, particularly in the neuronal context, remains poorly defined. Since many RBPs contain RGG motifs that are hot spots of arginine methylation, we aim to investigate whether and how PRMTs regulate synapse development via arginine methylation of RBP in dendrites. Here we found that PRMT8 is present at neuronal synapses and its expression is up-regulated in the brain when dendritic spines mature. Depletion of PRMT8 leads to over-abundance of filopodia and mislocalization of synapses. G3BP1, a key RBP in the assembly of stress granules, is expressed in dendritic spines of developing neuron in the absence of stress. PRMT8 methylates G3BP1 to promote spine maturation by suppression of the Rac1-PAK1 signaling pathway and subsequent control of synaptic actin cytoskeleton. Photobleaching after recovery (FRAP) on mCherry-LifeAct revealed decrease in F-actin turnover in dendritic spines after knockdown of PRMT8 or G3BP1, and the phenotype was reversed by co-expressing wild-type but not methylation-deficient G3BP1. Our findings identify an important function of G3BP1 during neuronal development beyond the control of mRNA translation in stress response, and uncover a new nonnuclear function of PRMTs in the regulation of actin cytoskeleton.