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The accumulation of toxic protein aggregates is thought to play a key role in a range of degenerative pathologies, but it remains unclear why aggregation of polypeptides into non-native assemblies is toxic and why cellular clearance pathways offer ineffective protection. We here study the A4V mutant of SOD1, which forms toxic aggregates in motor neurons of patients with familial amyotrophic lateral sclerosis (ALS). A comparison of the location of aggregation prone regions (APRs) and Hsp70 binding sites in the denatured state of SOD1 reveals that ALS-associated mutations promote exposure of the APRs more than the strongest Hsc/Hsp70 binding site that we could detect. Mutations designed to increase the exposure of this Hsp70 interaction site in the denatured state promote aggregation but also display an increased interaction with Hsp70 chaperones. Depending on the cell type, in vitro this resulted in cellular inclusion body formation or increased clearance, accompanied with a suppression of cytotoxicity. The latter was also observed in a zebrafish model in vivo. Our results suggest that the uncontrolled accumulation of toxic SOD1A4V aggregates results from insufficient detection by the cellular surveillance network. [ABSTRACT FROM AUTHOR]

Fabry disease is a lysosomal storage disorder caused by loss of α-galactosidase function. More than 500 Fabry disease mutants have been identified, the majority of which are structurally destabilized. A therapeutic strategy under development for lysosomal storage diseases consists of using pharmacological chaperones to stabilize the structure of the mutant protein, thereby promoting lysosomal delivery over retrograde degradation. The substrate analog 1-deoxygalactonojirimycin (DGJ) has been shown to restore activity of mutant α-galactosidase and is currently in clinical trial for treatment of Fabry disease. However, only ∼65% of tested mutants respond to treatment in cultured patient fibroblasts, and the structural underpinnings of DGJ response remain poorly explained. Using computational modeling and cell culture experiments, we show that the DGJ response is negatively affected by protein aggregation of α-galactosidase mutants, revealing a qualitative difference between misfolding-associated and aggregation-associated loss of function. A scoring function combining predicted thermodynamic stability and intrinsic aggregation propensity of mutants captures well their aggregation behavior under overexpression in HeLa cells. Interestingly, the same classifier performs well on DGJ response data of patient-derived cultured lymphoblasts, showing that protein aggregation is an important determinant of chemical chaperone efficiency under endogenous expression levels as well. Our observations reinforce the idea that treatment of aggregation-associated loss of function observed for the more severe α-galactosidase mutants could be enhanced by combining pharmacological chaperone treatment with the suppression of mutant aggregation, e.g. via proteostatic regulator compounds that increase cellular chaperone expression. [ABSTRACT FROM AUTHOR]