PIAS1 S510G variant acts as a genetic modifier of spinocerebellar ataxia type 3 by selectively impairing mutant ataxin-3 proteostasis.

Dysregulated protein homeostasis, characterized by abnormal protein accumulation and aggregation, is a key contributor to the progression of neurodegenerative disorders such as Huntington's disease and spinocerebellar ataxia type 3 (SCA3). Previous studies have identified PIAS1 gene variants in patients with late-onset SCA3 and Huntington's disease. This study aims to elucidate the role of PIAS1 and its S510G variant in modulating the pathogenic mechanisms of SCA3. Through in vitro biochemical analyses and in vivo assays, we demonstrate that PIAS1 stabilizes both wild-type and mutant ataxin-3 (ATXN3). The PIAS1 S510G variant, however, selectively reduces the stability and SUMOylation of mutant ATXN3, thereby decreasing its aggregation and toxicity while maintaining the stability of wild-type ATXN3. This effect is mediated by a weakened interaction with the SUMO-conjugating enzyme UBC9 in the presence of mutant ATXN3. In Drosophila models, downregulation of dPIAS1 resulted in reduced levels of mutant ATXN3 and alleviated associated phenotypes, including retinal degeneration and motor dysfunction. Our findings suggest that the PIAS1 S510G variant acts as a genetic modifier of SCA3, highlighting the potential of targeting SUMOylation as a therapeutic strategy for this disease.

Chemical interference with DSIF complex formation lowers synthesis of mutant huntingtin gene products and curtails mutant phenotypes.

Earlier work has shown that siRNA-mediated reduction of the SUPT4H or SUPT5H proteins, which interact to form the DSIF complex and facilitate transcript elongation by RNA polymerase II (RNAPII), can decrease expression of mutant gene alleles containing nucleotide repeat expansions differentially. Using luminescence and fluorescence assays, we identified chemical compounds that interfere with the SUPT4H-SUPT5H interaction and then investigated their effects on synthesis of mRNA and protein encoded by mutant alleles containing repeat expansions in the huntingtin gene (HTT), which causes the inherited neurodegenerative disorder, Huntington's Disease (HD). Here we report that such chemical interference can differentially affect expression of HTT mutant alleles, and that a prototypical chemical, 6-azauridine (6-AZA), that targets the SUPT4H-SUPT5H interaction can modify the biological response to mutant HTT gene expression. Selective and dose-dependent effects of 6-AZA on expression of HTT alleles containing nucleotide repeat expansions were seen in multiple types of cells cultured in vitro, and in a Drosophila melanogaster animal model for HD. Lowering of mutant HD protein and mitigation of the Drosophila "rough eye" phenotype associated with degeneration of photoreceptor neurons in vivo were observed. Our findings indicate that chemical interference with DSIF complex formation can decrease biochemical and phenotypic effects of nucleotide repeat expansions.

A conserved hydrogen-bond network in the catalytic centre of animal glutaminyl cyclases is critical for catalysis.

QCs (glutaminyl cyclases; glutaminyl-peptide cyclotransferases, EC 2.3.2.5) catalyse N-terminal pyroglutamate formation in numerous bioactive peptides and proteins. The enzymes were reported to be involved in several pathological conditions such as amyloidotic disease, osteoporosis, rheumatoid arthritis and melanoma. The crystal structure of human QC revealed an unusual H-bond (hydrogen-bond) network in the active site, formed by several highly conserved residues (Ser(160), Glu(201), Asp(248), Asp(305) and His(319)), within which Glu(201) and Asp(248) were found to bind to substrate. In the present study we combined steady-state enzyme kinetic and X-ray structural analyses of 11 single-mutation human QCs to investigate the roles of the H-bond network in catalysis. Our results showed that disrupting one or both of the central H-bonds, i.e., Glu(201)...Asp(305) and Asp(248)...Asp(305), reduced the steady-state catalysis dramatically. The roles of these two COOH...COOH bonds on catalysis could be partly replaced by COOH...water bonds, but not by COOH...CONH(2) bonds, reminiscent of the low-barrier Asp...Asp H-bond in the active site of pepsin-like aspartic peptidases. Mutations on Asp(305), a residue located at the centre of the H-bond network, raised the K(m) value of the enzyme by 4.4-19-fold, but decreased the k(cat) value by 79-2842-fold, indicating that Asp(305) primarily plays a catalytic role. In addition, results from mutational studies on Ser(160) and His(319) suggest that these two residues might help to stabilize the conformations of Asp(248) and Asp(305) respectively. These data allow us to propose an essential proton transfer between Glu(201), Asp(305) and Asp(248) during the catalysis by animal QCs.