A11 Huntingtin-mediated axonal transport requires arginine methylation by PRMT6

Background Neuronal function depends heavily on the ability to transport various molecules along axons. The huntingtin (HTT) protein transports various organelles, including vesicles containing neurotrophic factors, from embryonic development throughout life. To better understand how HTT mediates axonal transport and why this function is disrupted in Huntington’s disease (HD), we studied the biology of vesicle-associated HTT. Given that HTT interacts with several arginine methyltransferases and that arginine methylation is particularly abundant in neurons, we wondered whether HTT itself is methylated and whether this influences HTT’s roles in axonal transport. Methods We immunopurified HTT from vesicular fractions isolated from brains of wild-type mice and analyzed HTT arginine methylation by mass spectrometry. We validated the presence of HTT arginine methylation by in vitro methylation assay and immunoblotting experiments in cellular models. Finally, we specifically modulated arginine methylation in vitro and in vivo to assess its effect on HTT-mediated vesicular trafficking, on mutant HTT (mHTT) toxicity and the HD phenotype. Results We found that HTT is dimethylated at a highly conserved arginine residue (R118) by the protein arginine methyltransferase 6 (PRMT6). Without R118 methylation, HTT associates less with vesicles, anterograde trafficking is diminished, and neuronal death ensues—very similar to what occurs in HD. Inhibiting PRMT6 in HD cells and neurons exacerbates mHTT toxicity and impairs axonal trafficking, whereas overexpressing PRMT6 restores axonal transport and neuronal viability, except in the presence of a methylation-defective variant of mHTT. In HD flies, overexpressing PRMT6 rescues axonal defects and eclosion. Conclusions Arginine methylation regulates HTT-mediated vesicular transport along the axon, and increasing HTT methylation could be of therapeutic interest for HD.