Abstract
BackgroundHuntington's disease (HD) is a polyglutamine-expanded related neurodegenerative disease. Despite the ubiquitous expression of expanded, polyQ-Huntingtin (ExpHtt) in the brain, striatal neurons present a higher susceptibility to the mutation. A commonly admitted hypothesis is that Dopaminergic inputs participate to this vulnerability. We previously showed that D2 receptor stimulation increased aggregate formation and neuronal death induced by ExpHtt in primary striatal neurons in culture, and chronic D2 antagonist treatment protects striatal dysfunctions induced by ExpHtt in a lentiviral-induced model system in vivo. The present work was designed to elucidate the signalling pathways involved, downstream D2 receptor (D2R) stimulation, in striatal vulnerability to ExpHtt.Methodology/Principal FindingsUsing primary striatal neurons in culture, transfected with a tagged-GFP version of human exon 1 ExpHtt, and siRNAs against D2R or D1R, we confirm that DA potentiates neuronal dysfunctions via D2R but not D1R stimulation. We demonstrate that D2 agonist treatment induces neuritic retraction and growth cone collapse in Htt- and ExpHtt expressing neurons. We then tested a possible involvement of the Rho/ROCK signalling pathway, which plays a key role in the dynamic of the cytoskeleton, in these processes. The pharmacological inhibitors of ROCK (Y27632 and Hydroxyfasudil), as well as siRNAs against ROCK-II, reversed D2-related effects on neuritic retraction and growth cone collapse. We show a coupling between D2 receptor stimulation and Rho activation, as well as hyperphosphorylation of Cofilin, a downstream effector of ROCK-II pathway. Importantly, D2 agonist-mediated potentiation of aggregate formation and neuronal death induced by ExpHtt, was totally reversed by Y27632 and Hydroxyfasudil and ROCK-II siRNAs.Conclusions/SignificanceOur data provide the first demonstration that D2R-induced vulnerability in HD is critically linked to the activation of the Rho/ROCK signalling pathway. The inclusion of Rho/ROCK inhibitors could be an interesting therapeutic option aimed at forestalling the onset of the disease.
Highlights
Huntington’s disease (HD), a neurodegenerative disorder characterized by motor, cognitive, and psychiatric disorders (1) is caused by abnormal expansion of a CAG tract in exon 1 of the IT15 gene
Using primary cultures of striatal neurons expressing exon 1 of Huntingtin with a polyglutamine stretch (103Q: ExpHtt) fused to EGFP, we recently showed that DA, via D2 receptor stimulation, potentiates aggregate formation and neurodegenerescence induced by ExpHtt
We first confirm our previous results [10,11] showing that DA potentiation of ExpHtt toxicity is mediated via D2 receptor (D2R) but not D1 receptors (D1R) stimulation, in our model system, i.e. in striatal neurons expressing a truncated version of ExpHtt
Summary
Huntington’s disease (HD), a neurodegenerative disorder characterized by motor, cognitive, and psychiatric disorders (1) is caused by abnormal expansion of a CAG tract in exon 1 of the IT15 gene This mutation leads to an abnormal polyglutamine expansion in the N-terminal part of the huntingtin (Htt) protein. When mated to knock-in HD mice these mice showed exacerbation of HD pathophysiology and acceleration of aggregate formation [9] We recently extended these observations and demonstrated, in vitro, that DA accelerates two neuropathological hallmarks induced by expHtt: aggregate formation and striatal neuron death [10]. On this in vitro model system, DA exerted a dual role on striatal death, via two independent pathways. The present work was designed to elucidate the signalling pathways involved, downstream D2 receptor (D2R) stimulation, in striatal vulnerability to ExpHtt
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