Huntington’s disease: how does huntingtin, an anti-apoptotic protein, become toxic?

A combined approach for the assessment of cell viability and cell functionality of human fibrochondrocytes for use in tissue engineering

Temporo-mandibular joint disc disorders are highly prevalent in adult populations. Autologous chondrocyte implantation is a well-established method for the treatment of several chondral defects. However, very few studies have been carried out using human fibrous chondrocytes from the temporo-mandibular joint (TMJ). One of the main drawbacks associated to chondrocyte cell culture is the possibility that chondrocyte cells kept in culture tend to de-differentiate and to lose cell viability under in in-vitro conditions. In this work, we have isolated human temporo-mandibular joint fibrochondrocytes (TMJF) from human disc and we have used a highly-sensitive technique to determine cell viability, cell proliferation and gene expression of nine consecutive cell passages to determine the most appropriate cell passage for use in tissue engineering and future clinical use. Our results revealed that the most potentially viable and functional cell passages were P5–P6, in which an adequate equilibrium between cell viability and the capability to synthesize all major extracellular matrix components exists. The combined action of pro-apoptotic (TRAF5, PHLDA1) and anti-apoptotic genes (SON, HTT, FAIM2) may explain the differential cell viability levels that we found in this study. These results suggest that TMJF should be used at P5–P6 for cell therapy protocols.

Mutant CAG repeats of Huntingtin transcript fold into hairpins, form nuclear foci and are targets for RNA interference

The CAG repeat expansions that occur in translated regions of specific genes can cause human genetic disorders known as polyglutamine (poly-Q)-triggered diseases. Huntington's disease and spinobulbar muscular atrophy (SBMA) are examples of these diseases in which underlying mutations are localized near other trinucleotide repeats in the huntingtin (HTT) and androgen receptor (AR) genes, respectively. Mutant proteins that contain expanded polyglutamine tracts are well-known triggers of pathogenesis in poly-Q diseases, but a toxic role for mutant transcripts has also been proposed. To gain insight into the structural features of complex triplet repeats of HTT and AR transcripts, we determined their structures in vitro and showed the contribution of neighboring repeats to CAG repeat hairpin formation. We also demonstrated that the expanded transcript is retained in the nucleus of human HD fibroblasts and is colocalized with the MBNL1 protein. This suggests that the CAG repeats in the HTT mRNA adopt ds-like RNA conformations in vivo. The intracellular structure of the CAG repeat region of mutant HTT transcripts was not sufficiently stable to be protected from cleavage by an siRNA targeting the repeats and the silencing efficiency was higher for the mutant transcript than for its normal counterpart.

Allele-selective inhibition of mutant huntingtin expression with antisense oligonucleotides targeting the expanded CAG repeat

Huntington's disease (HD) is a currently incurable neurodegenerative disease caused by the expansion of a CAG trinucleotide repeat within the huntingtin (HTT) gene. Therapeutic approaches include selectively inhibiting the expression of the mutated HTT allele while conserving function of the normal allele. We have evaluated a series of antisense oligonucleotides (ASOs) targeted to the expanded CAG repeat within HTT mRNA for their ability to selectively inhibit expression of mutant HTT protein. Several ASOs incorporating a variety of modifications, including bridged nucleic acids and phosphorothioate internucleotide linkages, exhibited allele-selective silencing in patient-derived fibroblasts. Allele-selective ASOs did not affect the expression of other CAG repeat-containing genes and selectivity was observed in cell lines containing minimal CAG repeat lengths representative of most HD patients. Allele-selective ASOs left HTT mRNA intact and did not support ribonuclease H activity in vitro. We observed cooperative binding of multiple ASO molecules to CAG repeat-containing HTT mRNA transcripts in vitro. These results are consistent with a mechanism involving inhibition at the level of translation. ASOs targeted to the CAG repeat of HTT provide a starting point for the development of oligonucleotide-based therapeutics that can inhibit gene expression with allelic discrimination in patients with HD.

3-Nitropropionic acid as a tool to study the mechanisms involved in Huntington's disease: past, present and future

Huntington's disease (HD) is an inheritable autosomal-dominant disorder whose causal mechanisms remain unknown. Experimental models have begun to uncover these pathways, thus helping to understand the mechanisms implicated and allowing for the characterization of potential targets for new therapeutic strategies. 3-Nitropropionic acid is known to produce in animals behavioural, biochemical and morphologic changes similar to those occurring in HD. For this reason, this phenotypic model is gaining attention as a valuable tool to mimick this disorder and further developing new therapies. In this review, we will focus on the past and present research of this molecule, to finally bring a perspective on what will be next in this promising field of study.

A PET study of word generation in Huntington's disease: effects of lexical competition and verb/noun category

Huntington's disease (HD) patients show language production deficits that have been conceptualized as a consequence of executive disorders, e.g. selection deficit between candidate words or switching between word categories. More recently, a deficit of word generation specific to verbs has been reported, which might relate to impaired action representations in HD. We studied the brain correlates of language impairment in HD using H(2)O(15) positron emission tomography (PET). The activation task consisted of generation of semantically appropriate nouns and verbs in dominant (low lexical selection) and selective conditions (high lexical selection). Reaction times were longer and number of errors was higher in 12 non-demented HD than in 17 age-matched controls in all conditions. In both groups, the selective condition yielded longer reaction time and a greater number of errors than the dominant one. PET data revealed that, in control subjects, the left inferior temporal gyrus was involved in the selective condition whereas it was not in HD. Moreover, activity in the anterior cingulate and the inferior frontal gyri was correlated with behavioral performance in control subjects only. In HD, the lack of implication of these regions, already shown to be crucial in lexical selection, might have been partly compensated by the activation in the left supramarginal gyrus (phonological loop activity) and the right inferior frontal gyrus (effortful retrieval processes), which might support accessory language strategies allowing patients to achieve word generation.

Mitochondria and Huntington's disease pathogenesis: insight from genetic and chemical models

A mechanistic link between cellular energetic defects and the pathogenesis of Huntington's disease (HD) has long been hypothesized based on the cardinal observations of progressive weight loss in patients and metabolic defects in brain and muscle. Identification of respiratory chain deficits in HD postmortem brain led to the use of mitochondrial complex II inhibitors to generate acute toxicity models that replicate aspects of HD striatal pathology in vivo. Subsequently, the generation of progressive genetic animal models has enabled characterization of numerous cellular and systematic changes over disease etiology, including mitochondrial modifications that impact cerebral metabolism, calcium handling, oxidative damage, and apoptotic cascades. This review focuses on how HD animal models have influenced our understanding of mechanisms underlying HD pathogenesis, concentrating on insight gained into the roles of mitochondria in disease etiology. One outstanding question concerns the hierarchy of mitochondrial alterations in the cascade of events following mutant huntingtin (mhtt)-induced toxicity. One hypothesis is that a direct interaction of mhtt with mitochondria may trigger the neuronal damage and degeneration that occurs in HD. While there is evidence that mhtt associates with mitochondria, deleterious consequences of this interaction have not yet been established. Contrary evidence suggests that a primary nuclear action of mhtt may detrimentally influence mitochondrial function via effects on gene transcription. Irrespective of whether the principal toxic action of mhtt directly or secondarily impacts mitochondria, the repercussions of sufficient mitochondrial dysfunction are catastrophic to cells and may arguably underlie many of the other disruptions in cellular processes that evolve during HD pathogenesis.

Huntington's disease

Huntington's disease is an autosomal-dominant, progressive neurodegenerative disorder with a distinct phenotype, including chorea and dystonia, incoordination, cognitive decline, and behavioural difficulties. Typically, onset of symptoms is in middle-age after affected individuals have had children, but the disorder can manifest at any time between infancy and senescence. The mutant protein in Huntington's disease--huntingtin--results from an expanded CAG repeat leading to a polyglutamine strand of variable length at the N-terminus. Evidence suggests that this tail confers a toxic gain of function. The precise pathophysiological mechanisms of Huntington's disease are poorly understood, but research in transgenic animal models of the disorder is providing insight into causative factors and potential treatments.

Apoptotic cascades as possible targets for inhibiting cell death in Huntington's disease

Huntington's disease (HD) is a devastating autosomal dominant disorder characterized by progressive motor and neuropsychological symptoms. Evidence implicating the apoptotic cascades as a possible cause for the neurodegeneration seen in HD has directed researchers toward investigating therapeutic treatments targeting caspases and other proapoptotic factors. Cellular and murine models, which have demonstrated that caspase-mediated cleavage could be the cause for the neurodegeneration seen in HD, have evoked more research investigating the possible inhibition of apoptosis in HD. In particular, minocycline, a tetracycline-derived antibiotic that has been shown to increase survival in transgenic mouse models of HD, exhibits a neuroprotective feature in HD and demonstrates an anti-inflammatory as well as an anti-microbial effect by inhibiting microglial activation known to cause apoptosis.