Endoplasmic Reticulum Associated Degradation (ERAD) Of Neurodegenerative Disease Proteins

The hallmark of many neurodegenerative disorders,such as Alzheimer's disease (AD),Parkinson's disease(PD),amyotrophic lateral sclerosis(ALS),polyglutamine diseases and prion diseases and familial encephalopathy with neuroserpin inclusion bodies(FENIB),is the intracellular aggregates formed by the accumulation of aberrant misfolded disease proteins.Mutations of these proteins may alter the conformation and result in their misfolding.Furthermore,these misfolded proteins can form aggregates which are highly ubiquitinated,along with molecular chaperones and proteasome subunits.Copper-zinc superoxide dismutase(SOD1) is an antioxidant enzyme.Mutations of SOD1 are associated with ALS.Ataxin-3 is another neurodegenerative disease protein in association with Machado-Joseph disease(also known as spinocerebellar ataxia type 3,SCA3).These two disease proteins are misfolded,aggregated and neurotoxic in diseased brains.It was recently reported that these two proteins are associated with ER(endoplasmic reticulum) and both of them are involved in ERAD (ER-associated degradation).Abnormal accumulation of them in the ER induces ER stress.ERAD is a protein quality control system in eukaryotes to degrade misfolded proteins in the ER.In ERAD process,misfolded or unassembled target proteins are selected as substrates and finally degraded by the cytoplasmic 26S proteasome through ubiquitin-proteasome system(UPS).Many unassembled or misfolded proteins are accumulated in the ER and degraded through ERAD.Ubiquitin ligase (E3)-mediated ubiquitination of proteins is an important process for protein degradation by ERAD system.Up to date,al least five E3s,including gp78 and Hrd1, have been identified to be involved in mammalian ERAD.Similar to other ERAD E3s, mammalian gp78 is an ER membrane-spanning protein with the RING finger-type ubiquitin ligase activity site that is localized on the ER surface towards the cytoplasmic side.In this thesis,we mainly explored the role of ERAD in ALS and sCA3/MJD process.We show that gp78 interacts with both SOD1 and ataxin-3.Overexpression of gp78 promotes the ubiquitination and degradation of these two proteins,whereas knockdown of gp78 stabilizes them.Moreover,gp78 is increased in cells transfected with these two mutant proteins as well as in ALS mice.Furthermore,gp78 represses aggregate formation of mutant SOD1 and protect cells against mutant SOD1-induced cell death.Thus,our results suggest that gp78 functions in the regulation of SOD1 and ataxin-3 to target them for ERAD.Potentially,this regulation may underlie the pathogenesis of ALS and SCA3/MJD.