Regulation Of Neurodegenerative Disease Protein Functions By Post-translational Modification

Neurodegenerative diseases are chronic disorders caused by the deterioration of certain neurons.Changes in these neurons cause a function disorder,eventually lead to neuronal cell death.Neurodegenerative diseases occur either in familial or sporadic forms.Familial neurodegenerative diseases are caused by mutations in association with diseasegenes.Mutated disease proteins form the intracellular,intranuclear or extracellular aggregates,which are the common pathologic hallmarks of these diseases.The mutations in the gene encoding copper-zinc superoxide dismutase (SOD1)causes approximately 20%cases of familial amyotrophic lateral sclerosis (FALS),characterized by selective loss of motor neurons.Mutant SOD1 forms inclusions in tissues from FALS patients.However,the precise mechanism of the accumulation of mutant SOD1 remains unclear.Spinocerebellar ataxia typeⅢ(SCA3)/Machado-Joseph disease(MJD)is a dominant neurodegenerative disorder caused by an expansion of the polyglutamine tract in MJD-1 gene product,ataxin-3. The expanded ataxin-3 forms nuclear inclusions in MJD brain.The precise mechanism of inclusion formation is also not well illuminated.It was reported that post-translational modifications of desease proteins play an important role in the aggregation process.Post-translational modifications of proteins, such as phophorylation,acetylation,methylation,ubiquitination and SUMOylation, are important for the regulation of protein functions,activities or localizations after their synthesis has been completed.Recent studies have revealed that some neurodegenerative disease proteins,such as huntingtin,ataxin-1,α-synuclein,DJ-1 and tau,are modified by SUMO,implicating that SUMO modification of these disease proteins may participate in regulation of these protens' functions and associate with their pathogenesis.And some studies show that phosphorylation of some polyglutamine disease proteins,such as huntingtin,ataxin-1 and DRPLA,plays an important role in regulating pathogenesis.Thus,post-translational modifications have critical roles in neurodegenerative diseases.In this thesis,we mainly focus on the SUMOylation of SOD1 and phosphorylation of ataxin-3.From these two aspects,we investigated the role that post-tranlational modifications act in neurodegenerative diseases.The main results are below: 1.We show that human SOD1 is a substrate modified by SUMO-1.A conversion of lysine 75 to an arginine within a SUMO consensus sequence in SOD1 completely abolishes SOD1 sumoylation.We further show that SUMO-1 modification,on both wild type and mutant SOD1,increases SOD1 steady state level and aggregation.Moreover,SUMO-1 co-localizes onto the aggregates formed by SOD1.These findings imply that SUMO-1 modification on lysine 75 may participate in regulating SOD1 stability and its aggregation process.Thus,our results suggest that sumoylation of SOD1 may be involved in the pathogenesis of FALS associated with mutant SOD1.2.We show that S256 site in ataxin-3 is phosphorylated by GSK 3β.Moreover, S256A mutant of expanded ataxin-3 forms high molecular weight protein aggregation,whereas S256D mutant and expanded ataxin-3 without mutation at this site are monomeric.The molecular chaperone Hsp70 represses the aggregation of S256A mutant.Our results imply that phosphorylation of ataxin-3 by GSK 3βat serine 256 regulates the aggregation of ataxin-3.Taking together,post-translational modifications regulate the specificities of neurodegenerative disease proteins,influence the aggregate formation and play a role in the pathogenesis of neurodegenerative diseses.