HTT-lowering reverses Huntington’s disease immune dysfunction caused by NFκB pathway dysregulation

Abstract

Huntington's disease is an inherited neurodegenerative disorder caused by a CAG repeat expansion in the huntingtin gene. The peripheral innate immune system contributes to Huntington's disease pathogenesis and has been targeted successfully to modulate disease progression, but mechanistic understanding relating this to mutant huntingtin expression in immune cells has been lacking. Here we demonstrate that human Huntington's disease myeloid cells produce excessive inflammatory cytokines as a result of the cell-intrinsic effects of mutant huntingtin expression. A direct effect of mutant huntingtin on the NFκB pathway, whereby it interacts with IKKγ, leads to increased degradation of IκB and subsequent nuclear translocation of RelA. Transcriptional alterations in intracellular immune signalling pathways are also observed. Using a novel method of small interfering RNA delivery to lower huntingtin expression, we show reversal of disease-associated alterations in cellular function-the first time this has been demonstrated in primary human cells. Glucan-encapsulated small interfering RNA particles were used to lower huntingtin levels in human Huntington's disease monocytes/macrophages, resulting in a reversal of huntingtin-induced elevated cytokine production and transcriptional changes. These findings improve our understanding of the role of innate immunity in neurodegeneration, introduce glucan-encapsulated small interfering RNA particles as tool for studying cellular pathogenesis ex vivo in human cells and raise the prospect of immune cell-directed HTT-lowering as a therapeutic in Huntington's disease.

The choreography of neuroinflammation in Huntington's disease

“…Monocytes and monocyte-derived macrophages obtained from blood collected from HD patients express mutant HTT but do not show increased pro-inflammatory cytokines expression and/or production compared with monocytes obtained from healthy donors [12,34,85]. However, these cells are hyperactive in response to an external pro-inflammatory stimulation [12,85].…”

“…However, these cells are hyperactive in response to an external pro-inflammatory stimulation [12,85]. Upon LPS stimulation in vitro , mutant HTT in myeloid cells binds IKKγ and results in increased NF-κB activity, likely explaining the altered transcription of NF-κB target genes, and RNAi of HTT reverses the effects associated with NF-κB dysregulation [85]. Monocytes from HD individuals display phenotypic heterogeneity in terms of M1–M2 polarization throughout the clinical course of the disease [86].…”

Huntington’s Disease: Mechanisms of Pathogenesis and Therapeutic Strategies

“…In the peripheral immune system, mutant huntingtin also impacts inflammatory responses through inhibition of NF-kB signaling (Träger et al 2014). Signaling through CB2 cannabinoid receptors might also explain inflammation in HD (Palazuelos et al 2009;Bouchard et al 2012).…”

Cerebrospinal Fluid Biomarkers for Huntington’s Disease

“…Most of this work has been carried out in peripheral blood or ex vivo cells [58] but in 2007 Dalrymple and colleagues found that plasma elevations of clusterin were mirrored in CSF from 20 patients and 10 controls in the first report from a CSF collection with dedicated matched contemporaneous healthy controls from the general population, rather than from patients under investigation for other conditions [12]. Their finding of elevated IL-6 and IL-8 in blood plasma was reproduced in patient CSF in 2008 by Björkqvist, et al [15].…”