Conformational studies of pathogenic expanded polyglutamine protein deposits from Huntington’s disease

Abstract

Huntington’s disease, like other neurodegenerative diseases, continues to lack an effective cure. Current treatments that address early symptoms ultimately fail Huntington’s disease patients and their families, with the disease typically being fatal within 10–15 years from onset. Huntington’s disease is an inherited disorder with motor and mental impairment, and is associated with the genetic expansion of a CAG codon repeat encoding a polyglutamine-segment-containing protein called huntingtin. These Huntington’s disease mutations cause misfolding and aggregation of fragments of the mutant huntingtin protein, thereby likely contributing to disease toxicity through a combination of gain-of-toxic-function for the misfolded aggregates and a loss of function from sequestration of huntingtin and other proteins. As with other amyloid diseases, the mutant protein forms non-native fibrillar structures, which in Huntington’s disease are found within patients’ neurons. The intracellular deposits are associated with dysregulation of vital processes, and inter-neuronal transport of aggregates may contribute to disease progression. However, a molecular understanding of these aggregates and their detrimental effects has been frustrated by insufficient structural data on the misfolded protein state. In this review, we examine recent developments in the structural biology of polyglutamine-expanded huntingtin fragments, and especially the contributions enabled by advances in solid-state nuclear magnetic resonance spectroscopy. We summarize and discuss our current structural understanding of the huntingtin deposits and how this information furthers our understanding of the misfolding mechanism and disease toxicity mechanisms. Impact statement Many incurable neurodegenerative disorders are associated with, and potentially caused by, the amyloidogenic misfolding and aggregation of proteins. Usually, complex genetic and behavioral factors dictate disease risk and age of onset. Due to its principally mono-genic origin, which strongly predicts the age-of-onset by the extent of CAG repeat expansion, Huntington’s disease (HD) presents a unique opportunity to dissect the underlying disease-causing processes in molecular detail. Yet, until recently, the mutant huntingtin protein with its expanded polyglutamine domain has resisted structural study at the atomic level. We present here a review of recent developments in HD structural biology, facilitated by breakthrough data from solid-state NMR spectroscopy, electron microscopy, and complementary methods. The misfolded structures of the fibrillar proteins inform our mechanistic understanding of the disease-causing molecular processes in HD, other CAG repeat expansion disorders, and, more generally, protein deposition disease.

HAP40 orchestrates huntingtin structure for differential interaction with polyglutamine expanded exon 1

“…This region of the protein is unresolved in the HTT-HAP40 cryo-EM model (PDBID: 6EZ8; Guo et al, 2018) and therefore the influence of the tract expansion on HTT structure-function remains unclear. Although many studies have focussed on understanding the effects of polyglutamine expansion on exon 1 in isolation [24][25][26] , there is still very little known about this region in the context of the full-length HTT protein molecule, either in the apo form or in the complex with HAP40. The intrinsically disordered region (IDR), which spans residues 407-665 is subject to a range of posttranslational modifications, is postulated to be critical in mediating various protein interactions 21,27,28 , and is also unresolved in the cryo-EM structure.…”

The polyglutamine amyloid nucleus in living cells is a monomer with competing dimensions of order

“…These appeared to increase in size and order with the expression level of the protein, implying an evolution of the Q zipper structure toward mature amyloid. This progression rationalizes why polyQ diseases are rate-limited by primary nucleation despite amyloids --the ultimate product of nucleation --having benign or even protective roles (Kim et al, 2016;Leitman et al, 2013;Lu and Palacino, 2013;Matlahov and van der Wel, 2019;Takahashi et al, 2008;Wetzel, 2020).…”

“…Interplay of Q zipper structure and surface tension may drive the accumulation of pre-amyloid oligomers Soluble oligomers accumulate during the aggregation of pathologic length polyQ and/or Htt in vitro (Auer et al, 2008;Hsieh et al, 2017;Levin et al, 2014;Liang et al, 2018;Li et al, 2010;Sil et al, 2018;Vitalis and Pappu, 2011;Yamaguchi et al, 2005;Zanjani et al, 2020), in cultured cells (Olshina et al, 2010;Takahashi et al, 2008), and in the brains of patients (Legleiter et al, 2010;Sathasivam et al, 2010), and are likely culprits of proteotoxicity (Kim et al, 2016;Leitman et al, 2013;Lu and Palacino, 2013;Matlahov and van der Wel, 2019;Takahashi et al, 2008;Wetzel, 2020).…”

Live-Cell Imaging and Quantification of PolyQ Aggregates by Stimulated Raman Scattering of Selective Deuterium Labeling

“…However, these methods rely on extensive post-processing such as aggregation purification and solubilization 9,10 . In vitro spectroscopic studies including IR 14 , UVresonance Raman 15,16 , NMR spectroscopy 17 and fluorescence 18,19 on model peptides provide crucial information, but they are limited to relatively short expansion lengths because of the difficulty in isolating peptides with long Q repeats [14][15][16][17][18][19] . More importantly, all these in vitro studies cannot recapitulate the native aggregation status in live cells, which have complex intracellular environment.…”