Identification of thiostrepton as a pharmacological approach to rescue misfolded alpha-sarcoglycan mutant proteins from degradation

Limb-girdle muscular dystrophies type 2D (LGMD2D) are characterized by a progressive proximal muscle weakness. LGMD2D is due to mutations in the gene encoding a sarcoglycan (?-SG), a dystrophin-associated glycoprotein playing a key role in the maintenance of sarcolemma integrity in striated muscles. Here, we report the development of a new in vitro high-throughput screening assay that allows monitoring the proper localization of the most prevalent mutant form of ?-SG (R77C substitution). Using this assay, we screened a library of 2560 FDA-approved drugs and bioactive compounds and identified thiostrepton, a cyclic antibiotic, as a potential drug to repurpose for LGMD2D treatment. Characterization of thiostrepton effect revealed a positive impact on R77C-?-SG and other missense mutant proteins (R34H, I124T, V247M) localization in fibroblasts overexpressing these proteins. Finally, further investigations of the molecular mechanisms of action of the compound revealed an inhibition of the chymotrypsin-like activity of the proteasome 24h following thiostrepton treatment and a synergic effect with bortezomib, a FDA-approved proteasome inhibitor. This study reports the first in vitro model of LGMD2D compatible with high throughput screening and propose a new therapeutical option for LGMD2D caused by missense mutations of ?-SG. Overall design: Identification of Thiostrepton as a pharmacological approach to rescue misfolded alpha-sarcoglycan mutant proteins from degradation