title: Rapamycin inhibits polyglutamine aggregation independently of autophagy by reducing protein synthesis.
abstract: Accumulation of misfolded proteins and protein assemblies is associated with neuronal dysfunction and death in several neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's disease (HD). It is therefore critical to understand the molecular mechanisms of drugs that act on pathways that modulate misfolding and/or aggregation. It is noteworthy that the mammalian target of rapamycin inhibitor rapamycin or its analogs have been proposed as promising therapeutic compounds clearing toxic protein assemblies in these diseases via activation of autophagy. However, using a cellular model of HD, we found that rapamycin significantly decreased aggregation-prone polyglutamine (polyQ) and expanded huntingtin and its inclusion bodies (IB) in both autophagy-proficient and autophagy-deficient cells (by genetic knockout of the atg5 gene in mouse embryonic fibroblasts). This result suggests that rapamycin modulates the levels of misfolded polyQ proteins via pathways other than autophagy. We show that rapamycin reduces the amount of soluble polyQ protein via a modest inhibition of protein synthesis that in turn significantly reduces the formation of insoluble polyQ protein and IB formation. Hence, a modest reduction in huntingtin synthesis by rapamycin may lead to a substantial decrease in the probability of reaching the critical concentration required for a nucleation event and subsequent toxic polyQ aggregation. Thus, in addition to its beneficial effect proposed previously of reducing polyQ aggregation/toxicity via autophagic pathways, rapamycin may alleviate polyQ disease pathology via its effect on global protein synthesis. This finding may have important therapeutic implications.
authors.affiliation: Southampton Neuroscience Group, School of Biological Sciences, University of Southampton. Bassett Crescent East, Southampton SO16 7PX, UK.
authors.forename: Matthew A
authors.initials: MA
authors.lastname: King
authors.forename: Sarah
authors.initials: S
authors.lastname: Hands
authors.forename: Farida
authors.initials: F
authors.lastname: Hafiz
authors.forename: Noboru
authors.initials: N
authors.lastname: Mizushima
authors.forename: Aviva M
authors.initials: AM
authors.lastname: Tolkovsky
authors.forename: Andreas
authors.initials: A
authors.lastname: Wyttenbach
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001343:Autophagy / Q000187:drug effects*; D000071187:Autophagy-Related Protein 5; D002478:Cells, Cultured; D003513:Cycloheximide / Q000494:pharmacology; D005347:Fibroblasts / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D017353:Gene Deletion; D049452:Green Fluorescent Proteins / Q000378:metabolism; D006801:Humans; D000071058:Huntingtin Protein; D002479:Inclusion Bodies / Q000378:metabolism; D051379:Mice; D008869:Microtubule-Associated Proteins / Q000172:deficiency / Q000378:metabolism; D009419:Nerve Tissue Proteins / Q000378:metabolism; D009687:Nuclear Proteins / Q000378:metabolism; D010455:Peptides / Q000737:chemistry* / Q000378:metabolism*; D014176:Protein Biosynthesis / Q000187:drug effects*; D020836:Protein Structure, Quaternary; D021381:Protein Transport / Q000187:drug effects; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D020123:Sirolimus / Q000494:pharmacology*; D012967:Sodium Dodecyl Sulfate / Q000494:pharmacology; D012995:Solubility / Q000187:drug effects; D025801:Ubiquitin / Q000378:metabolism; D014746:Vimentin / Q000378:metabolism
chemical_list: D000900:Anti-Bacterial Agents; C495773:Atg5 protein, mouse; D000071187:Autophagy-Related Protein 5; C086055:HTT protein, human; D000071058:Huntingtin Protein; D008869:Microtubule-Associated Proteins; D009419:Nerve Tissue Proteins; D009687:Nuclear Proteins; D010455:Peptides; D011993:Recombinant Fusion Proteins; D025801:Ubiquitin; D014746:Vimentin; C485184:enhanced green fluorescent protein; D049452:Green Fluorescent Proteins; C097188:polyglutamine; D012967:Sodium Dodecyl Sulfate; D003513:Cycloheximide; D020123:Sirolimus