Therapeutic strategies for Huntington's disease based on a molecular understanding of the disorder

Huntington's Disease: An Immune Perspective

Huntington's disease (HD) is a progressive neurodegenerative disorder that is caused by abnormal expansion of CAG trinucleotide repeats. Neuroinflammation is a typical feature of most neurodegenerative diseases that leads to an array of pathological changes within the affected areas in the brain. The neurodegeneration in HD is also caused by aberrant immune response in the presence of aggregated mutant huntingtin protein. The effects of immune activation in HD nervous system are a relatively unexplored area of research. This paper summarises immunological features associated with development and progression of HD.

Ribonucleic acid interference for neurological disorders: candidate diseases, potential targets, and current approaches

OBJECTIVE

Ribonucleic acid (RNA) interference (RNAi) is a conserved evolutionary defense mechanism that is gaining utility for therapeutic application by modulating gene expression or silencing disease-causing genes.

METHODS

This strategy has recently achieved success in mammalian cells via synthetic small interfering RNA or short hairpin RNA expressed in vectors for gene delivery. The vector-based RNAi strategy has particular potential because of the possibility of targeted gene delivery, long-term gene expression, and the potential means of penetrating the blood-brain barrier.

RESULTS

RNAi-based approaches have been proposed for a variety of neurological disorders, including dominant genetic diseases, neurodegenerative diseases, malignant brain tumors, pain, and viral-induced encephalopathies.

CONCLUSION

This review summarizes the current approaches of the RNAi strategy for neurological disorders, focusing on potential targets for therapeutic intervention.

Functional connectivity of the prefrontal cortex in Huntington's disease

BACKGROUND

Huntington's disease is a progressive neurodegenerative disorder that results in deterioration and atrophy of various brain regions.

AIM

To assess the functional connectivity between prefrontal brain regions in patients with Huntington's disease, compared with normal controls, using functional magnetic resonance imaging.

PATIENTS AND METHODS

20 patients with Huntington's disease and 17 matched controls performed a Simon task that is known to activate lateral prefrontal and anterior cingulate cortical regions. The functional connectivity was hypothesised to be impaired in patients with Huntington's disease between prefrontal regions of interest, selected from both hemispheres, in the anterior cingulate and dorsal lateral prefrontal cortex.

RESULTS

Controls showed a dynamic increase in interhemispheric functional connectivity during task performance, compared with the baseline state; patients with Huntington's disease, however, showed no such increase in prefrontal connectivity. Overall, patients with Huntington's disease showed significantly impaired functional connectivity between anterior cingulate and lateral prefrontal regions in both hemispheres compared with controls. Furthermore, poor task performance was predicted by reduced connectivity in patients with Huntington's disease between the left anterior cingulate and prefrontal regions.

CONCLUSIONS

This finding represents a loss of synchrony in activity between prefrontal regions in patients with Huntington's disease when engaged in the task, which predicted poor task performance. Results show that functional interactions between critical prefrontal regions, necessary for cognitive performance, are compromised in Huntington's disease. It is speculated whether significantly greater levels of activation in patients with Huntington's disease (compared with controls) observed in several brain regions partially compensate for the otherwise compromised interactions between cortical regions.

Therapeutic Strategies in Huntington's Disease

This article provides an overview of the therapeutic strategies, from ordinary classical drugs to the modern molecular strategy at experimental level, for Huntington's disease. The disease is characterized by choreic movements, psychiatric disorders, striatal atrophy with selective small neuronal loss, and autosomal dominant inheritance. The genetic abnormality is CAG expansion in huntingtin gene. Mutant huntingtin with abnormally long glutamine stretch aggregates and forms intranuclear inclusions. In this review, I summarize the results of previous trials from the following aspects; 1. symptomatic/palliative therapies including drugs, stereotaxic surgery and repetitive transcranial magnetic stimulation, 2. anti-degenerative therapies including anti-excitotoxicity, reversal of mitochondrial dysfunction and anti-apoptosis, 3. restorative/reparative therapies including neural trophic factors and tissue or stem cell transplantation, and 4. molecular targets in specific and radical therapies including inhibition of truncation of huntingtin, inhibition of aggregate formation, normalization of transcriptional dysregulation, enhancement of autophagic clearance of mutant huntingtin, and specific inhibition of huntingtin expression by sRNAi. Although the strategies mentioned in the latter two categories are mostly at laboratory level at present, we are pleased that one can discuss such "therapeutic strategies", a matter absolutely impossible before the causal gene of Huntington's disease was identified more than 10 years ago. It is also true, however, that some of the "therapeutic strategies" mentioned here would be found difficult to implement and abandoned in the future.

Pharmaceutical, cellular and genetic therapies for Huntington's disease

HD (Huntington's disease) is a devastating neurodegenerative disorder caused by a polyglutamine expansion in the gene encoding the huntingtin protein. Presently, there is no known cure for HD and existing symptomatic treatments are limited. However, recent advances have identified multiple pathological mechanisms involved in HD, some of which have now become the focus of therapeutic intervention. In this review, we consider progress made towards developing safe and effective pharmaceutical-, cell- and genetic-based therapies, and discuss the extent to which some of these therapies have been successfully translated into clinical trials. These new prospects offer hope for delaying and possibly halting this debilitating disease.

Clinico-pathological rescue of a model mouse of Huntington's disease by siRNA

Huntington's disease (HD) is an autosomal dominant inheritable neurodegenerative disorder currently without effective treatment. It is caused by an expanded polyglutamine (poly Q) tract in the corresponding protein, huntingtin (htt), and therefore suppressing the huntingtin expression in brain neurons is expected to delay the onset and mitigate the severity of the disease. Here, we have used small interfering RNAs (siRNAs) directed against the huntingtin gene to repress the transgenic mutant huntingtin expression in an HD mouse model, R6/2. Results showed that intraventricular injection of siRNAs at an early postnatal period inhibited transgenic huntingtin expression in brain neurons and induced a decrease in the numbers and sizes of intranuclear inclusions in striatal neurons. Treatments using this siRNA significantly prolonged model mice longevity, improved motor function and slowed down the loss of body weight. This work suggests that siRNA-based therapy is promising as a future treatment for HD.

Antisense downregulation of mutant huntingtin in a cell model

BACKGROUND

Huntington's disease (HD) is an inherited neurodegenerative disorder which is caused by an expansion of a CAG repeat sequence in the HD gene. The repeat encodes an expanded polyglutamine tract in the protein huntingtin. The still unknown pathological mechanisms leading to death of specific neurons in the brains of HD patients correlate with the expression of mutant huntingtin. Therefore, we have studied whether mutant huntingtin expression can be downregulated by antisense technique.

METHODS

NT2 precursor cells and differentiated postmitotic NT2-N neurons, respectively, were transfected with plasmid constructs containing exon 1 of the HD gene with expanded CAG repeats in frame with the reporter protein EGFP. The transfected cell cultures were treated with a phosphorothioated antisense oligonucleotide (PS-ASHD/20+) or a control oligonucleotide either by cotransfection or by addition to the culture medium.

RESULTS

Expression of the fusion protein containing the mutant huntingtin fragment resulted in diffuse green fluorescence in the cytoplasm and formation of aggregates in some of the NT2 cells and NT2-N neurons. We obtained antisense sequence-specific inhibition of expression of the fusion protein and/or suppression of the aggregate formation in both cell types. In the NT2 cells the antisense effect was dependent on the way of administration of the oligo.

CONCLUSIONS

The PS-antisense oligo is effective in downregulation of mutant huntingtin, and the reduction of aggregate formation is a sensitive biological marker. The findings suggest that antisense knockdown of huntingtin could be a useful strategy for treatment of HD, and could also be suitable for studies of the normal and pathological function of huntingtin in different cellular model systems.

Metabolic changes in quinolinic acid-lesioned rat striatum detected non-invasively by in vivo (1)H NMR spectroscopy

Intrastriatal injection of quinolinic acid (QA) provides an animal model of Huntington disease. In vivo (1)H NMR spectroscopy was used to measure the neurochemical profile non-invasively in seven animals 5 days after unilateral injection of 150 nmol of QA. Concentration changes of 16 metabolites were measured from 22 microl volume at 9.4 T. The increase of glutamine ((+25 +/- 14)%, mean +/- SD, n = 7) and decrease of glutamate (-12 +/- 5)%, N-acetylaspartate (-17 +/- 6)%, taurine (-14 +/- 6)% and total creatine (-9 +/- 3%) were discernible in each individual animal (P < 0.005, paired t-test). Metabolite concentrations in control striata were in excellent agreement with biochemical literature. The change in glutamate plus glutamine was not significant, implying a shift in the glutamate-glutamine interconversion, consistent with a metabolic defect at the level of neuronal-glial metabolic trafficking. The most significant indicator of the lesion, however, were the changes in glutathione ((-19 +/- 9)%, P < 0.002)), consistent with oxidative stress. From a comparison with biochemical literature we conclude that high-resolution in vivo (1)H NMR spectroscopy accurately reflects the neurochemical changes induced by a relatively modest dose of QA, which permits one to longitudinally follow mitochondrial function, oxidative stress and glial-neuronal metabolic trafficking as well as the effects of treatment in this model of Huntington disease.

Transplanted fetal striatum in Huntington's disease: phenotypic development and lack of pathology

Neural and stem cell transplantation is emerging as a potential treatment for neurodegenerative diseases. Transplantation of specific committed neuroblasts (fetal neurons) to the adult brain provides such scientific exploration of these new potential therapies. Huntington's disease (HD) is a fatal, incurable autosomal dominant (CAG repeat expansion of huntingtin protein) neurodegenerative disorder with primary neuronal pathology within the caudate-putamen (striatum). In a clinical trial of human fetal striatal tissue transplantation, one patient died 18 months after transplantation from cardiovascular disease, and postmortem histological analysis demonstrated surviving transplanted cells with typical morphology of the developing striatum. Selective markers of both striatal projection and interneurons such as dopamine and c-AMP-related phosphoprotein, calretinin, acetylcholinesterase, choline acetyltransferase, tyrosine hydroxylase, calbindin, enkephalin, and substance P showed positive transplant regions clearly innervated by host tyrosine hydroxylase fibers. There was no histological evidence of immune rejection including microglia and macrophages. Notably, neuronal protein aggregates of mutated huntingtin, which is typical HD neuropathology, were not found within the transplanted fetal tissue. Thus, although there is a genetically predetermined process causing neuronal death within the HD striatum, implanted fetal neural cells lacking the mutant HD gene may be able to replace damaged host neurons and reconstitute damaged neuronal connections. This study demonstrates that grafts derived from human fetal striatal tissue can survive, develop, and are unaffected by the disease process, at least for 18 months, after transplantation into a patient with HD.

Inhibition of Huntington synthesis by antisense oligodeoxynucleotides

The Huntington disease gone encodes the protein huntington, which is widely expressed during embryonic development and in mature tissues. In order to elucidate the physiological function of huntington, which so far is unknown, we intend to study the effect of antisense down-regulated huntington expression. We have found an inhibiting effect of a phosphorothioated oligodeoxynucleotide (PS-ODN) added to the culture medium of embryonic teratocarcinoma cells (NT2) and postmitotic neurons (NT2N neurons) differentiated from the NT2 cells. Specific inhibition of expression of endogenous huntington was achieved in NT2N neurons in the concentration range of 1-5 microM PS-ODN, whereas no inhibition was obtained in NT2 cells. We describe in detail the selection of the target sequence for the antisense oligo and the uptake, intracellular distribution, and stability of the antisense PS-ODN in the two cell types. Antisense down-regulation of huntington in this model of human neurons represents a suitable approach to study its normal function.

Increased complement biosynthesis by microglia and complement activation on neurons in Huntington's disease

In this study complement activation and biosynthesis have been analysed in the brains of Huntington's disease (HD) (n = 9) and normal (n = 3) individuals. In HD striatum, neurons, myelin and astrocytes were strongly stained with antibodies to C1q, C4, C3, iC3b-neoepitope and C9-neoepitope. In contrast, no staining for complement components was found in the normal striatum. Marked astrogliosis and microgliosis were observed in all HD caudate and the internal capsule samples but not in normal brain. RT-PCR analysis and in-situ hybridisation were carried out to determine whether complement was synthesised locally by activated glial cells. By RT-PCR, we found that complement activators of the classical pathway C1q C chain, C1r, C4, C3, as well as the complement regulators, C1 inhibitor, clusterin, MCP, DAF, CD59, were all expressed constitutively and at much higher level in HD brains compared to normal brain. Complement anaphylatoxin receptor mRNAs (C5a receptor and C3a receptor) were strongly expressed in HD caudate. In general, we found that the level of complement mRNA in normal control brains was from 2 to 5 fold lower compared to HD striatum. Using in-situ hybridisation, we confirmed that C3 mRNA and C9 mRNA were expressed by reactive microglia in HD internal capsule. We propose that complement produced locally by reactive microglia is activated on the membranes of neurons, contributing to neuronal necrosis but also to proinflammatory activities. Complement opsonins (iC3b) and anaphylatoxins (C3a, C5a) may be involved in the recruitment and stimulation of glial cells and phagocytes bearing specific complement receptors.

The IGF-I amino-terminal tripeptide glycine-proline-glutamate (GPE) is neuroprotective to striatum in the quinolinic acid lesion animal model of Huntington's disease

Huntington's disease is an incurable genetic neurological disorder characterized by the relatively selective degeneration of the striatum. Lesioning of the striatum in rodents using the excitatory amino acid agonist, quinolinic acid (QA), effectively mimics the human neuropathology seen in Huntington's disease. Using this animal model of Huntington's disease, we investigated the ability of the insulin-like growth factor-I (IGF-I) amino-terminal tripeptide glycine-proline-glutamate (GPE) to protect striatal neurons from degeneration. Adult rats received a single unilateral intrastriatal injection of QA (100 nmol) and then daily injection of either vehicle or GPE (0.3 microgram/microliter/day) into the striatum for 7 days. QA at this dose resulted in a partial lesioning of the striatum after 7 days to approximately 50% of cells of unlesioned levels in vehicle-treated animals. The major striatal neuronal phenotype, GABAergic projection neurons, were identified by immunocytochemical labeling of either glutamate decarboxylase 67 (GAD(67)) or the calcium binding protein calbindin in alternate sections. Treatment with GPE for 7 days reversed the loss in projection neurons when assessed by counts of calbindin-stained cells; however, these rescued cells did not regain immunologically detectable levels of GAD(67). GPE also significantly reversed the phenotypic degeneration of cholinergic interneurons identified by immunolabeling for choline acetyltransferase (ChAT) and NADPH diaphorase interneurons identified histochemically. GPE treatment failed to rescue the calcium binding protein interneuron populations of parvalbumin and calretinin neurons. These findings reveal that exogenous administration of GPE selectively prevents excitotoxin induced phenotypic degeneration of striatal projection neurons and cholinergic and NADPH diaphorase interneurons in an animal model of Huntington's disease.

Administration of recombinant human Activin-A has powerful neurotrophic effects on select striatal phenotypes in the quinolinic acid lesion model of Huntington's disease

Huntington disease is characterized by the selective loss of striatal neurons, particularly of medium-sized spiny glutamate decarboxylase67 staining/GABAergic projection neurons which co-contain the calcium binding protein calbindin. Lesioning of the adult rat striatum by intrastriatal injection of the N-methyl-D-aspartate receptor agonist quinolinic acid (100 nmol) results in a pattern of striatal neuropathology seven days later that resembles that seen in the Huntington brain. Using this animal model of human Huntington's disease we investigated the effect of daily intrastriatal infusion of the nerve cell survival molecule ActivinA (single bolus dose of 0.73 microg daily for seven days) on the quinolinic acid-induced degeneration of various striatal neuronal phenotypes. By seven days, unilateral intrastriatal infusion of quinolinic acid produced a partial but significant loss (P < 0.01) in the number of striatal neurons immunoreactive for glutamate decarboxylase (to 51.0+/-5.8% of unlesioned levels), calbindin (to 58.7+/-5.1%), choline acetyltransferase (to 68.6+/-6.1%), NADPH-diaphorase (to 47.4+/-5.4%), parvalbumin (to 58.8+/-4.1%) and calretinin (to 60.6+/-8.6%) in adult rats that were administered intrastriatal phosphate-buffered saline for seven days following quinolinic acid. In contrast, in rats that received intrastriatal recombinant human ActivinA once daily for seven days following quinolinic acid, phenotypic degeneration was significantly attenuated in several populations of striatal neurons. Treatment with ActivinA had the most potent protective effect on the striatal cholinergic interneuron population almost completely preventing the lesion induced decline in choline acetyltransferase expression (to 95.1+/-5.8% of unlesioned levels, P < 0.01). ActivinA also conferred a significant protective effect on parvalbumin (to 87.5+/-7.7%, P < 0.01) and NADPH-diaphorase (to 77.5+/-7.5%, P < 0.01) interneuron populations but failed to prevent the phenotypic degeneration of calretinin neurons (to 56.6+/-5.5%). Glutamate decarboxylase67 and calbindin-staining nerve cells represent largely overlapping populations and both identify striatal GABAergic projection neurons. We found that ActivinA significantly attenuated the loss in the numbers of neurons staining for calbindin (to 79.7+/-6.6%, P < 0.05) but not glutamate decarboxylase67 (to 61.1+/-5.9%) at seven days following quinolinic acid lesioning. Taken together these results suggest that exogenous administration of ActivinA can rescue both striatal interneurons (labelled with choline acetyltransferase, parvalbumin, NADPH-diaphorase) and striatal projection neurons (labelled by calbindin) from excitotoxic lesioning with quinolinic acid. Longer-term studies will be required to determine whether these surviving calbindin-expressing projection neurons recover their ability to express the glutamate decarboxylase67/GABAergic phenotype. These results therefore suggest that treatment with ActivinA may help to prevent the degeneration of vulnerable striatal neuronal populations in Huntington's disease.

Advances in the understanding of early Huntington's disease using the functional imaging techniques of PET and SPET

The functional imaging techniques of positron emission tomography (PET) and single photon emission tomography (SPET) have been used to study regional brain function in Huntington's disease (HD) in vivo. Reduced striatal glucose metabolism and dopamine receptor binding are evident in all symptomatic HD patients and in approximately 50% of asymptomatic adult mutation carriers. These characteristics correlate with clinical measures of disease severity. Reduced cortical glucose metabolism and dopamine receptor binding, together with reduced striatal and cortical opioid receptor binding, have also been demonstrated in symptomatic patients with HD. Repeat PET measures of striatal function have been used to monitor the progression of this disease objectively. In the future, functional imaging will provide a valuable way of assessing the efficacy of both fetal striatal cell implants and putative neuroprotective agents, such as nerve growth factors.

Antisense gene therapy for neurodegenerative disease?

Diseases resulting from defects in a single gene may be more amenable to treatment by conventional gene therapy strategies than idiopathic or polygenic disorders. We have attempted to reduce the expression in vivo of the Huntington's disease gene protein, Huntingtin, using an 18-mer fluorescein-labeled phosphorothiorated antisense oligodeoxynucleotide (ODN) targeted against the start site of the first exon of the IT15 gene. Animals were given repeated intrastriatal infusions (5 microliters of a 100 nmol/microliter solution daily over 4 days) of the antisense ODN. The treatments ended on Day 5 and the tissue was processed for immunohistochemical and Western blot analysis. The fluorescein-labeled ODN appeared to penetrate several cells and did not cause any obvious toxicity to the neurons. The average reduction in levels of Huntingtin (16.9 +/- 7.2%) did not differ significantly between striatal tissue of antisense ODN-treated animals compared to those treated with a sense ODN or vehicle. Improved methods for molecular modifications of the IT15 gene may be needed for therapeutic initiatives.