The role and regulation of the RNA-binding protein G3BP1 in synapse development

Abstract

Excitatory synapses are mostly located within dendritic spines and the proper morphology of dendritic spine is essential for synapse development and memory formation. Impaired spine morphology is related to neurological disorders including fragile X syndrome and autism. Both local protein synthesis and neuronal activity account for the regulation of dendritic spine maturation. The functions of synaptic proteins in synapse development are usually regulated by various post-translational modifications (PTMs). One less-studied PTM form at synapse is protein arginine methylation, which is mediated by protein arginine N-methyltransferase (PRMT). Our laboratory has previously identified PRMT8, a brain-restricted PRMT, as a key regulator in dendritic spine morphology. However, the underlying mechanism by which PRMT8 promotes spine maturation is not clear. Here, we find that loss of PRMT8 will contribute to the overactivation of Rac1-PAK signaling and reduces actin turnover in dendritic spines. Moreover, inhibiting PAK activity reverses the over-produced filopodia caused by PRMT8 knockdown. Our observations indicate that PRMT8 controls actin dynamics in dendritic spines through the suppression of Rac1-PAK signaling. Two homolog proteins, Ras-GAP SH3 domain binding protein (G3BP) 1 and 2 have previously been identified as PRMT8-interacting proteins by our laboratory. G3BP1 and G3BP2 are essential for stress granule assembly and modulate protein synthesis in response to cellular stresses. However, the physiological functions of G3BP1 and G3BP2 in neuron in the absence of cellular stresses remain elusive. Here, we find that G3BP1 and G3BP2 are present in dendritic spines and both are essential for the proper dendritic spine morphology. Interestingly, only loss of G3BP1 but not G3BP2 phenocopies the over-abundance of filopodia observed in PRMT8 knockdown neurons. Importantly, reduced asymmetric demethylation of G3BP1 was detected upon Prmt8 knockout, indicating G3BP1 is one of the PRMT8 substrates in the brain. Furthermore, the action of G3BP1 in maintaining dendritic spine maturation and proper filamentous actin turnover in dendritic spines is mediated through arginine methylation. Taken together, these findings address the important role of PRMT8 and its substrate G3BP1 in suppressing filopodia production during dendritic spine maturation, which is mediated by the control on actin dynamics through Rac1-PAK signaling in dendritic spines.