Aggregate-prone proteins are cleared from the cytosol by autophagy: therapeutic implications

Intracellular protein misfolding/aggregation are features of many late-onset neurodegenerative diseases, called proteinopathies. These include Alzheimer's disease, Parkinson's disease, tauopathies, and polyglutamine expansion diseases [e.g., Huntington's disease; and various spinocerebellar ataxias (SCAs), like SCA3]. There are no effective strategies to slow or prevent the neurodegeneration resulting from these diseases in humans. The mutations causing many proteinopathies (e.g., polyglutamine diseases and tauopathies) confer novel toxic functions on the specific protein, and disease severity frequently correlates with the expression levels of the protein. Thus, the factors regulating the synthesis and clearance of these aggregate-prone proteins are putative therapeutic targets. The proteasome and autophagy-lysosomal pathways are the major routes for mutant huntingtin fragment clearance. While the narrow proteasome barrel precludes entry of oligomers/aggregates of mutant huntingtin (or other aggregate-prone intracellular proteins), such substrates can be degraded by macroautophagy (which we will call autophagy). We showed that the autophagy inducer rapamycin reduced the levels of soluble and aggregated huntingtin and attenuated its toxicity in cells, and in transgenic Drosophila and mouse models. We extended the range of intracellular proteinopathy substrates that are cleared by autophagy to a wide range of other targets, including proteins mutated in certain SCAs, forms of alpha-synuclein mutated in familial forms of Parkinson's disease, and tau mutants that cause frontotemporal dementia/tauopathy. In this chapter, we consider the therapeutic potential of autophagy upregulation for various proteinopathies, and describe how this strategy may act both by removing the primary toxin (the misfolded/aggregate-prone protein) and by reducing susceptibility to apoptotic insults.