Analysis of the role of arginine methylation in the pathogenesis of Huntington’s disease

Huntington's disease (HD) is a fatal neurodegenerative disorder characterized by progressive loss of striatal and cortical neurons. HD is caused by an abnormal polyglutamine (polyQ) expansion in Huntingtin protein (HTT). HTT controls vesicular trafficking along axons in neurons through interaction with components of the molecular motor machinery. Arginine methylation is one of the most abundant post-translational modifications (PTMs) and is catalyzed by protein arginine methyltransferases (PRMTs). Recent evidence supports a key role for arginine methylation in neurodegeneration and particularly in polyglutamine diseases. However, whether HTT is methylated at arginine residues has not been investigated yet and the role of arginine methylation in HD pathogenesis remains to be fully elucidated. In this thesis, I show that vesicle-associated HTT is methylated in vivo at two evolutionarily conserved arginine residues, namely R101 and R118. Methylation of HTT at R118 is catalyzed by Protein Arginine Methyltransferase 6 (PRMT6), which localizes on vesicles together with HTT, whereas further analyses are required to identify the enzyme(s) responsible for R101 methylation. Interestingly, loss of PRMT6-mediated R118 methylation reduces the association of HTT with vesicles, impairs anterograde axonal transport and exacerbates polyQ-expanded HTT toxicity. Conversely, PRMT6 overexpression improves the global efficiency of anterograde axonal transport and rescues cell death in neurons expressing polyQ-expanded HTT. These findings establish a crucial role of arginine methylation as a modulator of both normal HTT function and polyQ-expanded HTT toxicity and identify PRMT6 as a novel modifier of HD pathogenesis. Importantly, defects in HTT methylation may contribute to neurodegeneration in HD and promoting arginine methylation of HTT might represent a new therapeutic strategy for HD.