Nascent mutant Huntingtin exon 1 chains do not stall on ribosomes during translation but aggregates do recruit machinery involved in ribosome quality control

Abstract

Mutations that cause Huntingtons Disease involve a polyglutamine (polyQ) sequence expansion beyond 35 repeats in exon 1 of Huntingtin. Intracellular inclusion bodies of mutant Huntingtin protein are a key feature of Huntingtons disease brain pathology. We previously showed that in cell culture the formation of inclusions involved the assembly of disordered structures of mHtt exon 1 fragments (Httex1) and they were enriched with translational machinery when first formed. We hypothesized that nascent mutant Httex1 chains co-aggregate during translation by phase separation into liquid-like disordered aggregates and then convert to more rigid, amyloid structures. Here we further examined the mechanisms of inclusion assembly in a human epithelial kidney (AD293) cell culture model and examined whether ribosome quality control machinery previously implicated in stalled ribosomes were involved. We found mHttex1 did not appear to stall translation of its own nascent chain and there was no recruitment of RNA into inclusions. However, proteins involved in translation or ribosome quality control were co-recruited into the inclusions (Ltn1 and Rack1) compared to a protein not anticipated to be involved (NACAD). Furthermore, we observed co-aggregation with other proteins previously identified in inclusions, including Upf-1 and chaperone-like proteins Sgta and Hspb1, which also suppressed aggregation at high co-expression levels. The newly formed inclusions contained immobile mHttex1 molecules which points to the disordered aggregates being mechanically rigid prior to amyloid formation.