RACK1 modulates polyglutamine-induced neurodegeneration by promoting ERK degradation in Drosophila.

Polyglutamine diseases are neurodegenerative diseases caused by the expansion of polyglutamine (polyQ) tracts within different proteins. Although multiple pathways have been found to modulate aggregation of the expanded polyQ proteins, the mechanisms by which polyQ tracts induced neuronal cell death remain unknown. We conducted a genome-wide genetic screen to identify genes that suppress polyQ-induced neurodegeneration when mutated. Loss of the scaffold protein RACK1 alleviated cell death associated with the expression of polyQ tracts alone, as well as in models of Machado-Joseph disease (MJD) and Huntington's disease (HD), without affecting proteostasis of polyQ proteins. A genome-wide RNAi screen for modifiers of this rack1 suppression phenotype revealed that knockdown of the E3 ubiquitin ligase, POE (Purity of essence), further suppressed polyQ-induced cell death, resulting in nearly wild-type looking eyes. Biochemical analyses demonstrated that RACK1 interacts with POE and ERK to promote ERK degradation. These results suggest that RACK1 plays a key role in polyQ pathogenesis by promoting POE-dependent degradation of ERK, and implicate RACK1/POE/ERK as potent drug targets for treatment of polyQ diseases. Author summary: Polyglutamine (polyQ) diseases are neurodegenerative diseases caused by the expansion of glutamine repeats in specific proteins. After >100 years of effort trying to decipher the pathogenesis of polyQ diseases, the mechanisms by which these mutant proteins induced neuronal death are still unknown. Here, we performed a genome-wide genetic screen in Drosophila for mutations that alleviate polyQ cytotoxicity. We found that a mutation in the gene rack1, which encodes a scaffold protein, suppressed polyQ-induced neurodegeneration when polyQ tracts were expresses, and in models of HD and MJD. Importantly, this effect was seen without clearance of the polyQ proteins. Further, we found that RACK1 binds both the E3 ubiquitin ligase POE and ERK, thereby promoting ERK degradation. These data suggest that the RACK1/POE/ERK pathway is involved in pathogenesis of polyQ diseases, and identify RACK1/POE/ERK as potential therapeutic targets for treating polyQ diseases. [ABSTRACT FROM AUTHOR]