CHN-1 ubiquitylates hTBP43Q and mediates its proteasomal degradation by collaborating with HSP-1 and UFD-2

The digenic form of spinocerebellar ataxia type 17 (SCA17-DI) is caused by intermediate polyglutamine (polyQ) repeat expansion (40-46Q) in the TATA-box binding protein (TBP) gene and a heterozygous mutation in STUB1 gene, encoding the E3-ubiquitin ligase CHIP. However, repeats ≥47 in TBP cause the monogenic form of SCA17. To define the interplay between TBP and CHIP, we used C. elegans by pan-neuronally expressing the cDNA of human TBP with 38 (wild-type), 43, and 54 polyQ repeats. While hTBP54Q alone alters response to gentle touch, intermediate repeats (hTBP43Q) require a second mutation in chn-1, the CHIP's ortholog, to lead to a similar phenotype. Therefore, these latter animals model SCA17-DI by mimicking human pathological digenic condition. The expression of only hTBP54Q in the mechanosensory neurons was sufficient to cause functional defects, indicating that hTBP54Q plays a role in these neurons leading to the observed phenotype. In addition, single-copy integrated models recapitulated the observed results confirming that the defect is mutation-driven rather than a copy-number artefact. The expression of wild-type human CHIP completely rescued defective phenotype of SCA17-DI animals, whereas disease-causative mutations in CHIP failed to restore neurons’ functionality. The treatment of hTBP43Q animals with proteasomal, but not lysosomal, inhibitor similarly impaired gentle touch response phenocopying SCA17-DI model. In addition, we have identified that CHN-1 collaborates with the molecular chaperone HSP-1 and the E4-ubiquitin ligase UFD-2 to degrade hTBP43Q. We also identified that CHN-1 specifically ubiquitylates hTBP43Q at lysine 195. Overall, our results indicate that CHN-1 ubiquitylates hTBP43Q at K195 and mediates its proteasomal degradation by collaborating with HSP-1 and UFD-2. If this pathway is interrupted, it results in neuronal dysfunction. This degradation axis can be a therapeutic target for phenotypic rescue. Fond. Cariplo 2021‑1544 to SM.