Co-occurrence of mixed proteinopathies in late-stage Huntington’s disease

Abstract

Accumulating evidence highlights the potential role of mixed proteinopathies (i.e., abnormal protein aggregation) in the development of clinical manifestations of neurodegenerative diseases (NDD). Huntington's disease (HD) is an inherited NDD caused by autosomal-dominant expanded CAG trinucleotide repeat mutation in the gene coding for Huntingtin (Htt). Previous studies have suggested the coexistence of phosphorylated-Tau, α-synuclein (α-Syn) and TAR DNA-binding protein 43 (TDP-43) inclusions in HD. However, definite evidence that HD pathology in humans can be accompanied by other proteinopathies is still lacking. Using human post-mortem putamen samples from 31 controls and 56 HD individuals, we performed biochemical analyses of the expression, oligomerization and aggregation of Tau, α-Syn, TDP-43, and Amyloid precursor protein (APP)/Aβ. In HD brain, we observed reduced soluble protein (but not mRNA) levels of Htt, α-Syn, and Tau. Our results also support abnormal phosphorylation of Tau in more advanced stages of disease. Aberrant splicing of Tau exons 2, 3 (exclusion) and 10 (inclusion) was also detected in HD patients, leading to higher 0N4R and lower 1N3R isoforms. Finally, following formic acid extraction, we observed increased aggregation of TDP-43, α-Syn, and phosphorylated-Tau during HD progression. Notably, we observed that 88% of HD patients with Vonsattel grade 4 neuropathology displayed at least one non-Htt proteinopathy compared to 29% in controls. Interestingly, α-Syn aggregation correlated with Htt, TDP-43 and phosphorylated-Tau in HD but not in controls. The impact of this work is twofold: (1) it provides compelling evidences that Tau, α-Syn and TDP-43 proteinopathies are increased in HD, and (2) it suggests the involvement of common mechanisms leading to abnormal accumulation of aggregation-prone proteins in NDD. Further studies will be needed to decipher the impact of these proteinopathies on clinical manifestation of HD.

Altered Levels and Isoforms of Tau and Nuclear Membrane Invaginations in Huntington’s Disease

“…More precisely, they detect 1N3R and 2N4R mRNA and protein isoforms and found an increase in 4R-Tau isoforms that leads to an altered ratio of isoforms. Recently, using putamen samples from a higher number of patients (St-Amour et al, 2018), it was shown a 2.5-fold increase in 4R-Tau/3R-Tau ratio at the protein level and a 1.8-fold increase at the mRNA level, due to an upregulation of 4R-Tau isoforms. The high number of samples analyzed in that study allowed them to conclude that the top increment takes place in grade 2 cases regarding the mRNA and in grade 3 cases regarding the protein, resulting in higher 0N4R and lower 1N3R isoforms (St-Amour et al, 2018).…”

“…Recently, using putamen samples from a higher number of patients (St-Amour et al, 2018), it was shown a 2.5-fold increase in 4R-Tau/3R-Tau ratio at the protein level and a 1.8-fold increase at the mRNA level, due to an upregulation of 4R-Tau isoforms. The high number of samples analyzed in that study allowed them to conclude that the top increment takes place in grade 2 cases regarding the mRNA and in grade 3 cases regarding the protein, resulting in higher 0N4R and lower 1N3R isoforms (St-Amour et al, 2018). All these findings regarding altered isoform ratio in HD are very relevant in view of the fact that alteration in the ratio of 4R-Tau and 3R-Tau isoforms is sufficient to cause neurodegeneration (Hutton et al, 1998;Qian and Liu, 2014), as this might contribute per se to HD neurodegeneration independently of other deleterious effects of mHtt, thus becoming a therapeutic target for HD.…”

“…Regarding Tau protein levels, a high increase in the levels of total Tau (Tau-5 antibody) was found in the cortex of HD patients and this is accompanied with the appearance of lower molecular weight (35 and 39 KD) bands (Fernández-Nogales et al, 2014), while no changes were found in the striatum (Fernández-Nogales et al, 2014). However, a more recent report has shown elevated Tau total mRNA levels in the putamen of HD patients (St-Amour et al, 2018). Importantly, the presence of an excess of Tau in HD brains most likely contributes to disease as Tau knock-down has been demonstrated to attenuate motor abnormalities in an HD mouse model (Fernández-Nogales et al, 2014).…”

“…Recently, it has been observed in a cohort of 56 HD patients a decrease in the levels of total Htt levels (using EM48 and CH00146 antibodies) and an increase in the levels of the N-terminal fragment without alteration in Htt mRNA levels. This is accompanied by an increase in Htt oligomers without changes in monomers according to Sarkosyl based protein fractionation, suggesting that abnormal translation and/or protein turnover is responsible for Htt misregulation in HD (St-Amour et al, 2018).…”

“…Inclusions can be detected using antibodies against Htt but also against other epitopes like polyQ or ubiquitin (DiFiglia et al, 1997;Sapp et al, 1997;Vonsattel et al, 2008). There is growing evidence that HD inclusions, in addition to mHtt, can nucleate other proteins like those that are characteristic of other neurodegenerative diseases, like α-synuclein -found in Parkinson's disease (Corrochano et al, 2012;Herrera and Outeiro, 2012;Tomas-Zapico et al, 2012), TDP-43 -found in amyotrophic lateral sclerosis- (Schwab et al, 2008;Coudert et al, 2019), or Tau -found in Tauopathies such as Alzheimer's disease (Fernández-Nogales et al, 2014;Vuono et al, 2015;St-Amour et al, 2018).…”