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.
Footnotes
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This study was supported by the Wellcome Trust before October 2006 (to M.A.K., A.M.T.), the Hereditary Disease Foundation (to F.H.), and the Medical Research Council (to A.W., S.H.).
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ABBREVIATIONS: polyQ, polyglutamine; CHX, cycloheximide; GFP, green fluorescent protein; EGFP, enhanced green fluorescent protein; Htt, huntingtin protein; Ex1Htt, huntingtin exon 1; HD, Huntington's disease; IB, inclusion body; MEF, mouse embryonic fibroblast; mRFP, monomeric red fluorescent protein; mTOR, mammalian target of rapamycin; BafA1, bafilomycin A1; ANOVA, analysis of variance; PAGE, polyacrylamide gel electrophoresis; Rap, rapamycin; FK506, tacrolimus; CCI-779, temsirolimus; ERK, extracellular signal-regulated kinase; tERK, total extracellular signal-regulated kinase.
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↵ The online version of this article (available at http://molpharm.aspetjournals.org) contains supplemental material.
- Received November 12, 2007.
- Accepted December 31, 2007.
- The American Society for Pharmacology and Experimental Therapeutics
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