Molecular and structural architecture of polyQ aggregates in yeast

Abstract

Huntington's disease is caused by the expansion of a polyglutamine (polyQ) tract in the N-terminal exon of huntingtin (HttEx1), but the cellular mechanisms leading to neurodegeneration remain poorly understood. Here we present in situ structural studies by cryo-electron tomography of an established yeast model system of polyQ toxicity. We find that expression of polyQ-expanded HttEx1 results in the formation of unstructured inclusion bodies and in some cases fibrillar aggregates. This contrasts with recent findings in mammalian cells, where polyQ inclusions were exclusively fibrillar. In yeast, polyQ toxicity correlates with alterations in mitochondrial and lipid droplet morphology, which do not arise from physical interactions with inclusions or fibrils. Quantitative proteomic analysis shows that polyQ aggregates sequester numerous cellular proteins and cause a major change in proteome composition, most significantly in proteins related to energy metabolism. Thus, our data point to a multifaceted toxic gain-of-function of polyQ aggregates, driven by sequestration of endogenous proteins and mitochondrial and lipid droplet dysfunction.

Amyloid-like aggregates cause lysosomal defects in neurons via gain-of-function toxicity

“…In some cases, ER tubes surrounded the aggregate periphery and tunnelled through its interior ( Supplementary Fig. S4f), similar to previous observations for polyQ and heat-shock induced aggregates (Bauerlein et al, 2017;Gruber et al, 2018;Wagner et al, 2017). However, in contrast to polyQ fibrils in neurons (Bauerlein et al, 2017), β protein fibrils did not appear to deform cellular membranes ( Supplementary Fig.…”

“…The ER around the aggregates often engaged in membrane contact sites with mitochondria ( Supplementary Fig. S3b, d), as observed for stress-induced and polyQ aggregates (Gruber et al, 2018;Zhou et al, 2014). Altogether, the morphology and cellular interactions of β protein aggregates were reminiscent of those formed by natural aggregating proteins.…”

Studying Huntington’s Disease in Yeast: From Mechanisms to Pharmacological Approaches

“…In HD, aggregation of mHTT derives from an elongated poly(Q)-stretch in exon-1 ( Taylor et al, 2002 ), which can also be observed in yeast models of HD ( Krobitsch and Lindquist, 2000 ; Meriin et al, 2002 ; Ocampo and Barrientos, 2011 ; Kaiser et al, 2013 ). Recently, the ultrastructural architecture of a GFP-tagged human exon1-97Q construct was explored in yeast ( Gruber et al, 2018 ). Both fibrillar and – to a major part – unstructured inclusions were found, while no detrimental interactions with cellular membranes were seen ( Gruber et al, 2018 ).…”

“…Recently, the ultrastructural architecture of a GFP-tagged human exon1-97Q construct was explored in yeast ( Gruber et al, 2018 ). Both fibrillar and – to a major part – unstructured inclusions were found, while no detrimental interactions with cellular membranes were seen ( Gruber et al, 2018 ). This stands at odds with previous findings of a similar construct forming amyloid-like fibrils in mammalian cells that interact with endomembranes, preferentially those of the ER ( Bäuerlein et al, 2017 ).…”

“…However, this might also argue for the cellular context being highly relevant for aggregation and, thus, may provide another variable adding to the complexity of cell-specific effects by mHTT in human pathology. As Gruber et al (2018) have argued, it is indeed conceivable, that the dissimilar chaperone systems of yeast and mammalian cells might be one of the reasons for these differences.…”