Effect of human Hsp70 on polyglutamine aggregation and toxicity of mutant huntingtin protein in Drosophila melanogaster

Abstract

There are at least 9 human neurodegenerative diseases that are caused by mutations in proteins containing a tract of repeating glutamines. When mutations increase the number of glutamines above a certain threshold, the expanded polyglutamine tract causes the protein to aggregate, which can lead to the death of neuronal cells. One such polyglutamine disease is Huntington’s disease (HD), which is caused by a mutation that expands the polyglutamine stretch in exon 1 of the huntingtin protein beyond its wildtype length. Mutant huntingtin proteins with over 36 polyglutamine repeats show characteristics of pathogenicity that increase in severity as the polyglutamine stretch gets longer. The overarching goals of this project were to identify and characterize tools that have the potential to suppress polyglutamine aggregation and toxicity associated with HD. More specifically, I focused on a chaperone, Hsp70, which has been shown (both in vitro and in vivo) to suppress aggregation and toxicity of various proteins containing polyglutamine expansions. To investigate the usefulness of Hsp70 as a potential therapeutic tool for suppressing the development of HD, I created lines of a fruit fly species, Drosophila melanogaster, that coexpress pathogenic human huntingtin protein and human Hsp70. Confocal microscopy was used to image and compare aggregation in HD flies that express and do not express human Hsp70, and a larval crawling behavior assay was used to compare toxicity between fly lines. Results suggest that Hsp70 could reduce aggregation in fly lines with moderate aggregating tendencies; however, instead of reducing toxicity, Hsp70 expression caused an increase in toxicity. Such results suggest the necessity of assessing whether increased toxicity was caused by (1) human Hsp70 being incompatible with the fly system, in which case Hsp70 treatment might yield different results in other model systems; or (2) human Hsp70 having a general adverse effect when present in large quantities, in which case Hsp70 treatment may also have toxic effects on Huntington’s disease patients