Neuronal aggregates are associated with phenotypic onset in the R6/2 Huntington's disease transgenic mouse

Fragment-based virtual screening identifies a first-in-class preclinical drug candidate for Huntington's disease

Currently, there are no therapies available to modify the disease progression of Huntington's disease (HD). Recent clinical trial failures of antisense oligonucleotide candidates in HD have demonstrated the need for new therapeutic approaches. Here, we developed a novel in-silico fragment scanning approach across the surface of mutant huntingtin (mHTT) polyQ and predicted four hit compounds. Two rounds of compound analoging using a strategy of testing structurally similar compounds in an affinity assay rapidly identified GLYN122. In vitro, GLYN122 directly binds and reduces mHTT and induces autophagy in neurons. In vivo, our results confirm that GLYN122 can reduce mHTT in the cortex and striatum of the R/2 mouse model of Huntington's disease and subsequently improve motor symptoms. Thus, the in-vivo pharmacology profile of GLYN122 is a potential new preclinical candidate for the treatment of HD.

The Contribution of Somatic Expansion of the CAG Repeat to Symptomatic Development in Huntington's Disease: A Historical Perspective

The discovery in the early 1990s of the expansion of unstable simple sequence repeats as the causative mutation for a number of inherited human disorders, including Huntington’s disease (HD), opened up a new era of human genetics and provided explanations for some old problems. In particular, an inverse association between the number of repeats inherited and age at onset, and unprecedented levels of germline instability, biased toward further expansion, provided an explanation for the wide symptomatic variability and anticipation observed in HD and many of these disorders. The repeats were also revealed to be somatically unstable in a process that is expansion-biased, age-dependent and tissue-specific, features that are now increasingly recognised as contributory to the age-dependence, progressive nature and tissue specificity of the symptoms of HD, and at least some related disorders. With much of the data deriving from affected individuals, and model systems, somatic expansions have been revealed to arise in a cell division-independent manner in critical target tissues via a mechanism involving key components of the DNA mismatch repair pathway. These insights have opened new approaches to thinking about how the disease could be treated by suppressing somatic expansion and revealed novel protein targets for intervention. Exciting times lie ahead in turning these insights into novel therapies for HD and related disorders.

Potential therapeutic targets in polyglutamine-mediated diseases

Polyglutamine diseases are a group of inherited neurodegenerative disorders that are caused by an abnormal expansion of a trinucleotide CAG repeat, which encodes a polyglutamine tract in the protein-coding region of the respective disease genes. To date, nine polyglutamine diseases are known, including Huntington's disease, spinal and bulbar muscular atrophy, dentatorubral-pallidoluysian atrophy and six forms of spinocerebellar ataxia. These diseases share a salient molecular pathophysiology including the aggregation of the mutant protein followed by the disruption of cellular functions such as transcriptional regulation and axonal transport. The intraneuronal accumulation of mutant protein and resulting cellular dysfunction are the essential targets for the development of disease-modifying therapies, some of which have shown beneficial effects in animal models. In this review, the current status of and perspectives on therapy development for polyglutamine diseases will be discussed.

Genetic deletion of transglutaminase 2 does not rescue the phenotypic deficits observed in R6/2 and zQ175 mouse models of Huntington's disease

Huntington's disease (HD) is an autosomal dominant, progressive neurodegenerative disorder caused by expansion of CAG repeats in the huntingtin gene. Tissue transglutaminase 2 (TG2), a multi-functional enzyme, was found to be increased both in HD patients and in mouse models of the disease. Furthermore, beneficial effects have been reported from the genetic ablation of TG2 in R6/2 and R6/1 mouse lines. To further evaluate the validity of this target for the treatment of HD, we examined the effects of TG2 deletion in two genetic mouse models of HD: R6/2 CAG 240 and zQ175 knock in (KI). Contrary to previous reports, under rigorous experimental conditions we found that TG2 ablation had no effect on either motor or cognitive deficits, or on the weight loss. In addition, under optimal husbandry conditions, TG2 ablation did not extend R6/2 lifespan. Moreover, TG2 deletion did not change the huntingtin aggregate load in cortex or striatum and did not decrease the brain atrophy observed in either mouse line. Finally, no amelioration of the dysregulation of striatal and cortical gene markers was detected. We conclude that TG2 is not a valid therapeutic target for the treatment of HD.

Prostaglandin E2 EP1 receptor antagonist improves motor deficits and rescues memory decline in R6/1 mouse model of Huntington's disease

In this study, we evaluated the potential beneficial effects of antagonizing prostaglandin E2 (PGE2) EP1 receptor on motor and memory deficits in Huntington's disease (HD). To this aim, we implanted an osmotic mini-pump system to chronically administrate an EP1 receptor antagonist (SC-51089) in the R6/1 mouse model of HD, from 13 to 18 weeks of age, and used different paradigms to assess motor and memory function. SC-51089 administration ameliorated motor coordination and balance dysfunction in R6/1 mice as analyzed by rotarod, balance beam, and vertical pole tasks. Long-term memory deficit was also rescued after EP1 receptor antagonism as assessed by the T-maze spontaneous alternation and the novel object recognition tests. Additionally, treatment with SC-51089 improved the expression of specific synaptic markers and reduced the number of huntingtin nuclear inclusions in the striatum and hippocampus of 18-week-old R6/1 mice. Moreover, electrophysiological studies showed that hippocampal long-term potentiation was significantly recovered in R6/1 mice after EP1 receptor antagonism. Altogether, these results show that the antagonism of PGE2 EP1 receptor has a strong therapeutic effect on R6/1 mice and point out a new therapeutic candidate to treat motor and memory deficits in HD.

Aggregation formation in the polyglutamine diseases: protection at a cost?

Mutant protein aggregation is a hallmark of many neurodegenerative diseases, including the polyglutamine disorders. Although the correlation between aggregation formation and disease pathology originally suggested that the visible inclusions seen in patient tissue might directly contribute to pathology, additional studies failed to confirm this hypothesis. Current opinion in the field of polyglutamine disease research now favors a model in which large inclusions are cytoprotective and smaller oligomers or misfolded monomers underlie pathogenesis. Nonetheless, therapies aimed at reducing or preventing aggregation show promise. This review outlines the debate about the role of aggregation in the polyglutamine diseases as it has unfolded in the literature and concludes with a brief discussion on the manipulation of aggregation formation and clearance mechanisms as a means of therapeutic intervention.

HTRF analysis of soluble huntingtin in PHAROS PBMCs

OBJECTIVE

We measured the levels of mutant huntingtin (mtHtt) and total huntingtin (tHtt) in blood leukocytes from Prospective Huntington At-Risk Observational Study (PHAROS) subjects at 50% risk of carrying the Huntington disease mutation using a homogeneous time-resolved fluorescence (HTRF) assay to assess its potential as a biomarker.

METHODS

Peripheral blood mononuclear cells from consenting PHAROS subjects were analyzed by HTRF using antibodies that simultaneously measured mtHtt and tHtt. mtHtt levels were normalized to tHtt, double-stranded DNA, or protein and analyzed according to cytosine-adenine-guanine repeat length (CAGn), demographics, predicted time to clinical onset or known time since clinical onset, and available clinical measures.

RESULTS

From 363 assayed samples, 342 met quality control standards. Levels of mtHtt and mt/tHtt were higher in 114 subjects with expanded CAG repeats (CAG ≥ 37) compared with 228 subjects with nonexpanded CAG repeats (CAG <37) (p < 0.0001). Analysis of relationships to predicted time to onset or to phenoconversion suggested that the HTRF signal could mark changes during the Huntington disease prodrome or after clinical onset.

CONCLUSIONS

The HTRF assay can effectively measure mtHtt in multicenter sample sets and may be useful in trials of therapies targeting huntingtin.

A huntingtin peptide inhibits polyQ-huntingtin associated defects

BACKGROUND

Huntington's disease (HD) is caused by the abnormal expansion of the polyglutamine tract in the human Huntingtin protein (polyQ-hHtt). Although this mutation behaves dominantly, huntingtin loss of function also contributes to HD pathogenesis. Indeed, wild-type Huntingtin plays a protective role with respect to polyQ-hHtt induced defects.

METHODOLOGY/PRINCIPAL FINDINGS

The question that we addressed here is what part of the wild-type Huntingtin is responsible for these protective properties. We first screened peptides from the Huntingtin protein in HeLa cells and identified a 23 aa peptide (P42) that inhibits polyQ-hHtt aggregation. P42 is part of the endogenous Huntingtin protein and lies within a region rich in proteolytic sites that plays a critical role in the pathogenesis process. Using a Drosophila model of HD, we tested the protective properties of this peptide on aggregation, as well as on different polyQ-hHtt induced neuronal phenotypes: eye degeneration (an indicator of cell death), impairment of vesicular axonal trafficking, and physiological behaviors such as larval locomotion and adult survival. Together, our results demonstrate high protective properties for P42 in vivo, in whole animals. These data also demonstrate a specific role of P42 on Huntington's disease model, since it has no effect on other models of polyQ-induced diseases, such as spinocerebellar ataxias.

CONCLUSIONS/SIGNIFICANCE

Altogether our data show that P42, a 23 aa-long hHtt peptide, plays a protective role with respect to polyQ-hHtt aggregation as well as cellular and behavioral dysfunctions induced by polyQ-hHtt in vivo. Our study also confirms the correlation between polyQ-hHtt aggregation and neuronal defects. Finally, these results strongly suggest a therapeutic potential for P42, specific of Huntington's disease.

Pharmacological protein targets in polyglutamine diseases: mutant polypeptides and their interactors

Polyglutamine diseases are a group of pathologies affecting different parts of the brain and causing dysfunction and atrophy of certain neural cell populations. These diseases stem from mutations in various cellular genes that result in the synthesis of proteins with extended polyglutamine tracts. In particular, this concerns huntingtin, ataxins, and androgen receptor. These mutant proteins can form oligomers, aggregates, and, finally, aggresomes with distinct functions and different degrees of cytotoxicity. In this review, we analyze the effects of different forms of polyQ proteins on other proteins and their functions, which are considered as targets for therapeutic intervention.

Cerebellar soluble mutant ataxin-3 level decreases during disease progression in Spinocerebellar Ataxia Type 3 mice

Spinocerebellar Ataxia Type 3 (SCA3), also known as Machado-Joseph disease, is an autosomal dominantly inherited neurodegenerative disease caused by an expanded polyglutamine stretch in the ataxin-3 protein. A pathological hallmark of the disease is cerebellar and brainstem atrophy, which correlates with the formation of intranuclear aggregates in a specific subset of neurons. Several studies have demonstrated that the formation of aggregates depends on the generation of aggregation-prone and toxic intracellular ataxin-3 fragments after proteolytic cleavage of the full-length protein. Despite this observed increase in aggregated mutant ataxin-3, information on soluble mutant ataxin-3 levels in brain tissue is lacking. A quantitative method to analyze soluble levels will be a useful tool to characterize disease progression or to screen and identify therapeutic compounds modulating the level of toxic soluble ataxin-3. In the present study we describe the development and application of a quantitative and easily applicable immunoassay for quantification of soluble mutant ataxin-3 in human cell lines and brain samples of transgenic SCA3 mice. Consistent with observations in Huntington disease, transgenic SCA3 mice reveal a tendency for decrease of soluble mutant ataxin-3 during disease progression in fractions of the cerebellum, which is inversely correlated with aggregate formation and phenotypic aggravation. Our analyses demonstrate that the time-resolved Förster resonance energy transfer immunoassay is a highly sensitive and easy method to measure the level of soluble mutant ataxin-3 in biological samples. Of interest, we observed a tendency for decrease of soluble mutant ataxin-3 only in the cerebellum of transgenic SCA3 mice, one of the most affected brain regions in Spinocerebellar Ataxia Type 3 but not in whole brain tissue, indicative of a brain region selective change in mutant ataxin-3 protein homeostasis.

Role of androgen receptor polyQ chain elongation in Kennedy's disease and use of natural osmolytes as potential therapeutic targets

Instability of CAG triplet repeat encoding polyglutamine (polyQ) stretches in the gene for target protein has been implicated as a putative mechanism in several inherited neurodegenerative diseases. Expansion of polyQ chain length in the androgen receptor (AR) causes spinal and bulbar muscular atrophy (SBMA) or Kennedy's disease. Although the mechanisms underlying gain-of-neurotoxic function are not completely understood, suggested pathological mechanisms of SBMA involve the formation of AR nuclear and cytoplasmic aggregates, a characteristic feature of patients with SBMA. The fact that certain AR coactivators are sequestered into the nuclear inclusions in SBMA possibly through protein-protein interactions supports the notion that AR transcriptional dysregulation may be a potential pathological mechanism leading to SBMA. AR conformational states associated with aberrant polyQ tract also modulate the interaction of AR with several coactivators. In many cases, such diseases can be treated through protein replacement therapy; however, because recombinant proteins do not cross the blood-brain barrier, the effectiveness of such therapies is limited in case of neurodegenerative diseases that warrant alternative therapeutic approaches. Among different approaches, inhibiting protein aggregation with small molecules that can stimulate protein folding and reverse aggregation are the most promising ones. Thus, naturally occurring osmolytes or "chemical chaperones" that can easily cross the blood-brain barrier and stabilize the functional form of a mutated protein by shifting the folding equilibrium away from degradation and/or aggregation is a useful therapeutic approach. In this review, we discuss the role of polyQ chain length extension in the pathophysiology of SBMA and the use of osmolytes as potential therapeutic tool.