Infoscience

Thesis

Altering protein folding, intracellular signaling, or the post-transcriptional program as potential therapeutic strategies to counteract Huntington's disease

Huntington's disease (HD) is an autosomal, dominantly inherited neurodegenerative disorder caused by expansion of a glutamine (polyQ) repeat in the huntingtin (htt) protein. The selective striatal neurodegeneration induced by this disease causes choreic involuntary movements, and psychiatric and cognitive disturbances. There are currently no treatments available to change the course of this fatal disorder. However, studies performed in recent years on cellular and animal models of HD have provided new informations on htt function and on the molecular basis of mutant htt neurotoxicity. Several pharmacological, restorative and neuroprotective strategies targeting specific cellular dysfunction and death pathways have been evaluated in HD models. Despite promising results obtained in models, these approaches have not shown significant therapeutic benefit when carried to phase I clinical trials in HD patients. In the present project, we have evaluated three points of intervention to counteract HD, namely protein folding/degradation, intracellular signaling and post-transcriptional regulation of htt expression. The local and long-term expression of htt fragments and of the therapeutic candidates was achieved through lentiviral-mediated gene transfer. The first approach is based on growing evidence that misfolding of mutated htt leads to aggregate formation and induces dysfunction in the ubiquitin-proteasome pathway. Heat shock proteins (hsps), a family of chaperone proteins implicated in the folding of nascent polypeptides and refolding of denatured proteins might modulate polyQ pathogenesis through interaction with misfolded mutated htt. Two chaperones, hsp104 and hsp27, belonging to different hsps families were tested for their potential positive effect on aggregate formation/clearance and/or protective role against htt-mediated toxicity. The second approach targets a stress-activated signaling cascade, the Jun N-terminal kinase (JNK) pathway, which is activated in several pathological conditions of the central nervous system such as stroke or neurodegenerative disorders. Inhibition of this pathway by dominant-negative mutants proteins protects against stress-induced apoptosis or glutamate-mediated excitotoxicity in several in vitro and in vivo models of cellular stress. Three dominant-negative mutants of the JNK pathway (MEKK1, ASK1, c-Jun) and a natural inhibitor, the JNK interacting protein-1 (JIP-1), were tested for their ability to interfere with polyQ-induced toxicity. The last approach directly targets the origin of the disease, namely the expression of mutated htt. An RNA interference method was used for the post-transcriptional silencing of htt expression. Small interfering RNAs (siRNA) with various species specificities were designed to investigate the effect of an allele-specific (mutated) or general (wild type and mutated) down-regulation of htt. In addition, pharmacologically regulated expression vectors were created to study the reversibility of the siRNA treatment.

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