N-terminal polyglutamine-containing fragments inhibit androgen receptor transactivation function.

Abstract Several neurodegenerative diseases, including Kennedy's disease (KD), are associated with misfolding and aggregation of polyglutamine (polyQ)-expansion proteins. KD is caused by a polyQ-expansion in the androgen receptor (AR), a key player in male sexual differentiation. Interestingly, KD patients often show signs of mild-to-moderate androgen insensitivity syndrome (AIS) resulting from AR dysfunction. Here, we used the yeast Saccharomyces cerevisiae to investigate the molecular mechanism behind AIS in KD. Upon expression in yeast, polyQ-expanded N-terminal fragments of AR lacking the hormone binding domain caused a polyQ length-dependent growth defect. Interestingly, while AR fragments with 67 Q formed large, SDS-resistant inclusions, the most pronounced toxicity was observed upon expression of 102 Q fragments which accumulated exclusively as soluble oligomers in the 100-600 kDa range. Analysis using a hormone-dependent luciferase reporter revealed that full-length polyQ-expanded AR is fully functional in transactivation, but becomes inactivated in the presence of the corresponding polyQ-expanded N-terminal fragment. Furthermore, the greatest impairment of AR activity was observed upon interaction of full-length AR with soluble AR fragments. Taken together, our results suggest that soluble polyQ-containing fragments bind to full-length AR and inactivate it, thus providing insight into the mechanism behind AIS in KD and possibly other polyglutamine diseases, such as Huntington's disease.

Identification of anti-prion compounds as efficient inhibitors of polyglutamine protein aggregation in a zebrafish model.

Several neurodegenerative diseases, including Huntington disease (HD), are associated with aberrant folding and aggregation of polyglutamine (polyQ) expansion proteins. Here we established the zebrafish, Danio rerio, as a vertebrate HD model permitting the screening for chemical suppressors of polyQ aggregation and toxicity. Upon expression in zebrafish embryos, polyQ-expanded fragments of huntingtin (htt) accumulated in large SDS-insoluble inclusions, reproducing a key feature of HD pathology. Real time monitoring of inclusion formation in the living zebrafish indicated that inclusions grow by rapid incorporation of soluble htt species. Expression of mutant htt increased the frequency of embryos with abnormal morphology and the occurrence of apoptosis. Strikingly, apoptotic cells were largely devoid of visible aggregates, suggesting that soluble oligomeric precursors may instead be responsible for toxicity. As in nonvertebrate polyQ disease models, the molecular chaperones, Hsp40 and Hsp70, suppressed both polyQ aggregation and toxicity. Using the newly established zebrafish model, two compounds of the N'-benzylidene-benzohydrazide class directed against mammalian prion proved to be potent inhibitors of polyQ aggregation, consistent with a common structural mechanism of aggregation for prion and polyQ disease proteins.