Transmission-selective muscle pathology induced by active propagation of mutant huntingtin across the human neuromuscular synapse

Abstract

A potential explanation for the spatiotemporal accumulation of pathological lesions in the brain of patients with neurodegenerative protein misfolding diseases (PMDs) is cell-to-cell transmission of aggregation-prone, misfolded proteins. Little is known about central to peripheral transmission and its contribution to pathology. We show that transmission of Huntington’s disease- (HD-) associated mutant HTT exon 1 (mHTTEx1) occurs across the neuromuscular junctions in human iPSC cultures and in vivo in wild-type mice. We found that transmission is an active and dynamic process, that happens prior to aggregate formation and is regulated by synaptic activity. Furthermore, we find that transmitted mHTTEx1 causes HD-relevant pathology at a molecular and functional level in human muscle cells, even in the presence of ubiquitous expression mHTTEx1. With this work we uncover a casual-link between mHTTEx1 synaptic transmission and pathology, highlighting the therapeutic potential in blocking toxic protein transmission in PMDs.

Synaptic and functional alterations in the development of mutant huntingtin expressing hiPSC‐derived neurons

“…The fact that mHTT is present during neuronal development raises the interest to assess whether early developmental processes, important for appropriate assembly of neuronal networks, are altered in HD patients. We used an hiPSC line bearing a doxycycline-inducible pro-neuronal transcription factor, NGN2 ( iNGN2 ), and generated a double transgenic line by introducing a HTTEx1Q72 fused to mCherry ( iNgn2; HTTEx1Q72-mCherry , hereafter HTTEx1Q72-mCherry) ( Nehme et al, 2018 ; Dinamarca et al, 2021 ). We analyzed three clones for their expression of HTTEx1Q72-mCherry and decided to use, in this study, clone#72, which showed an intermediate level of protein expression and has been shown to have a moderate aggregate formation over time ( Supplementary Figure S1 ) ( Dinamarca et al, 2021 ).…”

“…We used an hiPSC line bearing a doxycycline-inducible pro-neuronal transcription factor, NGN2 ( iNGN2 ), and generated a double transgenic line by introducing a HTTEx1Q72 fused to mCherry ( iNgn2; HTTEx1Q72-mCherry , hereafter HTTEx1Q72-mCherry) ( Nehme et al, 2018 ; Dinamarca et al, 2021 ). We analyzed three clones for their expression of HTTEx1Q72-mCherry and decided to use, in this study, clone#72, which showed an intermediate level of protein expression and has been shown to have a moderate aggregate formation over time ( Supplementary Figure S1 ) ( Dinamarca et al, 2021 ). As a control line, we used iNGN2 hiPSCs expressing mCherry (Ctr-mCherry).…”

“…Multiple neurons from at least three coverslips and two independent differentiations were recorded. Firing patterns were categorized according to what has been described previously ( Zhao and Wu, 1997 ; Xie et al, 2018 ; Dinamarca et al, 2021 ) (for examples see Figure 3B ).…”