Pharmacological induction of heat shock proteins ameliorates toxicity of mutant PKC gamma in spinocerebellar ataxia type 14

Amyloid and amyloid-like protein aggregations are hallmarks of multiple, varied neurodegenerative disorders, including Alzheimer's and Parkinson's diseases. We previously reported that spinocerebellar ataxia type 14 (SCA14), a dominant-inherited neurodegenerative disease that affects cerebellar Purkinje cells, is characterized by the intracellular formation of neurotoxic amyloid-like aggregates of genetic variants of protein kinase C (PKC gamma). A number of protein chaperones, including heat shock protein 70 (Hsp70), promote the degradation and/or refolding of misfolded proteins and thereby prevent their aggregation. Here, we report that, in various SCA14-associated, aggregating PKC gamma variants, endogenous Hsp70 is incorporated into aggregates and that expression of these PKC gamma mutants up-regulates Hsp70 expression. We observed that PKC gamma binds Hsp70 and that this interaction is enhanced in the SCA14-associated variants, mediated by the kinase domain that is involved in amyloid-like fibril formation as well as the C2 domain of PKC gamma. Pharmacological up-regulation of Hsp70 by the Hsp90 inhibitors celastrol and herbimycin A attenuated the aggregation of mutant PKC gamma in primary cultured Purkinje cells. Up-regulation of Hsp70 diminished net PKC gamma aggregation by preventing aggregate formation, resulting in decreased levels of apoptotic cell death among primary cultured Purkinje cells expressing the PKC gamma variant. Of note, herbimycin A also ameliorated abnormal dendritic development. Extending our in vitro observations, administration of celastrol to mice up-regulated cerebellar Hsp70. Our findings identify heat shock proteins as important endogenous regulators of pathophysiological PKC gamma aggregation and point to Hsp90 inhibition as a potential therapeutic strategy in the treatment of SCA14.