Caspase-6 activity in a BACHD mouse modulates steady-state levels of mutant huntingtin protein but is not necessary for production of a 586 amino acid proteolytic fragment

Proteomic Analysis of Huntington's Disease Medium Spiny Neurons Identifies Alterations in Lipid Droplets

Huntington's disease (HD) is a neurodegenerative disease caused by a CAG repeat expansion in the Huntingtin (HTT) gene. The resulting polyglutamine (polyQ) tract alters the function of the HTT protein. Although HTT is expressed in different tissues, the medium spiny projection neurons (MSNs) in the striatum are particularly vulnerable in HD. Thus, we sought to define the proteome of human HD patient-derived MSNs. We differentiated HD72 induced pluripotent stem cells and isogenic controls into MSNs and carried out quantitative proteomic analysis. Using data-dependent acquisitions with FAIMS for label-free quantification on the Orbitrap Lumos mass spectrometer, we identified 6,323 proteins with at least two unique peptides. Of these, 901 proteins were altered significantly more in the HD72-MSNs than in isogenic controls. Functional enrichment analysis of upregulated proteins demonstrated extracellular matrix and DNA signaling (DNA replication pathway, double-strand break repair, G1/S transition) with the highest significance. Conversely, processes associated with the downregulated proteins included neurogenesis-axogenesis, the brain-derived neurotrophic factor-signaling pathway, Ephrin-A: EphA pathway, regulation of synaptic plasticity, triglyceride homeostasis cholesterol, plasmid lipoprotein particle immune response, interferon-γ signaling, immune system major histocompatibility complex, lipid metabolism and cellular response to stimulus. Moreover, proteins involved in the formation and maintenance of axons, dendrites, and synapses (e.g., Septin protein members) were dysregulated in HD72-MSNs. Importantly, lipid metabolism pathways were altered, and using quantitative image, we found analysis that lipid droplets accumulated in the HD72-MSN, suggesting a deficit in the turnover of lipids possibly through lipophagy. Our proteomics analysis of HD72-MSNs identified relevant pathways that are altered in MSNs and confirm current and new therapeutic targets for HD.

Truncation of mutant huntingtin in knock-in mice demonstrates exon1 huntingtin is a key pathogenic form

Polyglutamine expansion in proteins can cause selective neurodegeneration, although the mechanisms are not fully understood. In Huntington’s disease (HD), proteolytic processing generates toxic N-terminal huntingtin (HTT) fragments that preferentially kill striatal neurons. Here, using CRISPR/Cas9 to truncate full-length mutant HTT in HD140Q knock-in (KI) mice, we show that exon 1 HTT is stably present in the brain, regardless of truncation sites in full-length HTT. This N-terminal HTT leads to similar HD-like phenotypes and age-dependent HTT accumulation in the striatum in different KI mice. We find that exon 1 HTT is constantly generated but its selective accumulation in the striatum is associated with the age-dependent expression of striatum-enriched HspBP1, a chaperone inhibitory protein. Our findings suggest that tissue-specific chaperone function contributes to the selective neuropathology in HD, and highlight the therapeutic potential in blocking generation of exon 1 HTT.

The mechanisms by which mutant Huntington protein Htt leads to selective neurodegeneration are not fully understood. Here, using gene editing in HD140Q knock-in mice, the authors show that exon1 Htt is a critical pathological form of the protein.

Inhibiting cellular uptake of mutant huntingtin using a monoclonal antibody: Implications for the treatment of Huntington's disease

Huntington's disease (HD) is caused by a highly polymorphic CAG trinucleotide expansion in the gene encoding for the huntingtin protein (HTT). The resulting mutant huntingtin protein (mutHTT) is ubiquitously expressed but also exhibits the ability to propagate from cell-to-cell to disseminate pathology; a property which may serve as a new therapeutic focus. Accordingly, we set out to develop a monoclonal antibody (mAB) targeting a particularly exposed region close to the aa586 caspase-6 cleavage site of the HTT protein. This monoclonal antibody, designated C6-17, effectively binds mutHTT and is able to deplete the protein from cell culture supernatants. Using cell-based assays, we demonstrate that extracellular secretion of mutHTT into cell culture media and its subsequent uptake in recipient HeLa cells can be almost entirely blocked by mAB C6-17. Immunohistochemical stainings of post-mortem HD brain tissue confirmed the specificity of mAB C6-17 to human mutHTT aggregates. These findings demonstrate that mAB C6-17 not only successfully engages with its target, mutHTT, but also inhibits cell uptake suggesting that this antibody could interfere with the pathological processes of mutHTT spreading in vivo.

Targeting the proteostasis network in Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a polyglutamine expansion mutation in the huntingtin protein. Expansions above 40 polyglutamine repeats are invariably fatal, following a symptomatic period characterised by choreiform movements, behavioural abnormalities, and cognitive decline. While mutant huntingtin (mHtt) is widely expressed from early life, most patients with HD present in mid-adulthood, highlighting the role of ageing in disease pathogenesis. mHtt undergoes proteolytic cleavage, misfolding, accumulation, and aggregation into inclusion bodies. The emerging model of HD pathogenesis proposes that the chronic production of misfolded mHtt overwhelms the chaperone machinery, diverting other misfolded clients to the proteasome and the autophagy pathways, ultimately leading to a global collapse of the proteostasis network. Multiple converging hypotheses also implicate ageing and its impact in the dysfunction of organelles as additional contributing factors to the collapse of proteostasis in HD. In particular, mitochondrial function is required to sustain the activity of ATP-dependent chaperones and proteolytic machinery. Recent studies elucidating mitochondria-endoplasmic reticulum interactions and uncovering a dedicated proteostasis machinery in mitochondria, suggest that mitochondria play a more active role in the maintenance of cellular proteostasis than previously thought. The enhancement of cytosolic proteostasis pathways shows promise for HD treatment, protecting cells from the detrimental effects of mHtt accumulation. In this review, we consider how mHtt and its post translational modifications interfere with protein quality control pathways, and how the pharmacological and genetic modulation of components of the proteostasis network impact disease phenotypes in cellular and in vivo HD models.

Differential susceptibility of striatal, hippocampal and cortical neurons to Caspase-6

Active cysteinyl protease Caspase-6 is associated with early Alzheimer and Huntington diseases. Higher entorhinal cortex and hippocampal Caspase-6 levels correlate with lower cognitive performance in aged humans. Caspase-6 induces axonal degeneration in human primary neuron cultures and causes inflammation and neurodegeneration in mouse hippocampus, and age-dependent memory impairment. To assess whether Caspase-6 causes damage to another neuronal system, a transgenic knock-in mouse overexpressing a self-activated form of Caspase-6 five-fold in the striatum, the area affected in Huntington disease, and 2.5-fold in the hippocampus and cortex, was generated. Detection of Tubulin cleaved by Caspase-6 confirmed Caspase-6 activity. The Caspase-6 expressing mice and control littermates were subjected to behavioral tests to assess Huntington disease-relevant psychiatric, motor, and cognitive deficits. Depression was excluded with the forced swim and sucrose consumption tests. Motor deficits were absent in the nesting, clasping, rotarod, vertical pole, gait, and open field analyzes. However, Caspase-6 mice developed age-dependent episodic and spatial memory deficits identified by novel object recognition, Barnes maze and Morris water maze assays. Neuron numbers were maintained in the striatum, hippocampus, and cortex. Microglia and astrocytes were increased in the hippocampal stratum lacunosum molecular and in the cortex, but not in the striatum. Synaptic mRNA profiling identified two differentially expressed genes in transgenic hippocampus, but none in striatum. Caspase-6 impaired synaptic transmission and induced neurodegeneration in hippocampal CA1 neurons, but not in striatal medium spiny neurons. These data revealed that active Caspase-6 in the striatal medium spiny neurons failed to induce inflammation, neurodegeneration or behavioral abnormalities, whereas active Caspase-6 in the cortex and hippocampus impaired episodic and spatial memories, and induced inflammation, neuronal dysfunction, and neurodegeneration. The results indicate age and neuronal subtype-dependent Caspase-6 toxicity and highlight the importance of targeting the correct neuronal subtype to identify underlying molecular mechanisms of neurodegenerative diseases.

Salivary levels of total huntingtin are elevated in Huntington's disease patients

Patients with Huntington's disease (HD), an autosomal-dominant neurodegenerative disease, show substantial variability in age-of-onset, symptom severity and course of illness, warranting the need for biomarkers to anticipate and monitor these features. The HD gene encodes the disease protein huntingtin (Htt), a potentially useful biomarker for this disease. In the current study, we determined whether total Htt protein (normal plus mutant; "tHtt") could be reliably measured in human saliva, a body fluid that is much more accessible compared to cerebral spinal fluid or even blood, and whether salivary levels of tHtt were clinically meaningful. We collected 146 saliva samples from manifest HD patients, early-premanifest individuals, late-premanifest patients, gene-negative family members and normal controls. We found that tHtt protein could be reliably and stably detected in human saliva and that tHtt levels were significantly increased in saliva from HD individuals compared to normal controls. Salivary tHtt showed no gender effects, nor were levels correlated with total protein levels in saliva. Salivary tHtt was significantly positively correlated with age, but not age-of-onset or CAG-repeat length. Importantly, salivary tHtt was significantly correlated with several clinical measures, indicating relevance to disease symptom onset and/or severity. Measurements of salivary tHtt offer significant promise as a relevant, non-invasive disease biomarker for HD, and its use could be implemented into clinical applications.

Huntingtin protein: A new option for fixing the Huntington's disease countdown clock

Huntington's disease is a dreadful, incurable disorder. It springs from the autosomal dominant mutation in the first exon of the HTT gene, which encodes for the huntingtin protein (HTT) and results in progressive neurodegeneration. Thus far, all the attempted approaches to tackle the mutant HTT-induced toxicity causing this disease have failed. The mutant protein comes with the aberrantly expanded poly-glutamine tract. It is primarily to blame for the build-up of β-amyloid-like HTT aggregates, deleterious once broadened beyond the critical ∼35-37 repeats threshold. Recent experimental findings have provided valuable information on the molecular basis underlying this HTT-driven neurodegeneration. These findings indicate that the poly-glutamine siding regions and many post-translation modifications either abet or counter the poly-glutamine tract. This review provides an overall, up-to-date insight into HTT biophysics and structural biology, particularly discussing novel pharmacological options to specifically target the mutated protein and thus inhibit its functions and toxicity.

Preventing mutant huntingtin proteolysis and intermittent fasting promote autophagy in models of Huntington disease

Huntington disease (HD) is caused by the expression of mutant huntingtin (mHTT) bearing a polyglutamine expansion. In HD, mHTT accumulation is accompanied by a dysfunction in basal autophagy, which manifests as specific defects in cargo loading during selective autophagy. Here we show that the expression of mHTT resistant to proteolysis at the caspase cleavage site D586 (C6R mHTT) increases autophagy, which may be due to its increased binding to the autophagy adapter p62. This is accompanied by faster degradation of C6R mHTT in vitro and a lack of mHTT accumulation the C6R mouse model with age. These findings may explain the previously observed neuroprotective properties of C6R mHTT. As the C6R mutation cannot be easily translated into a therapeutic approach, we show that a scheduled feeding paradigm is sufficient to lower mHTT levels in YAC128 mice expressing cleavable mHTT. This is consistent with a previous model, where the presence of cleavable mHTT impairs basal autophagy, while fasting-induced autophagy remains functional. In HD, mHTT clearance and autophagy may become increasingly impaired as a function of age and disease stage, because of gradually increased activity of mHTT-processing enzymes. Our findings imply that mHTT clearance could be enhanced by a regulated dietary schedule that promotes autophagy.

Mouse Models of Huntington's Disease

The identification of the mutation causing Huntington's disease (HD) has led to the generation of a large number of mouse models. These models are used to further enhance our understanding of the mechanisms underlying the disease, as well as investigating and identifying therapeutic targets for this disorder. Here we review the transgenic, knock-in mice commonly used to model HD, as well those that have been generated to study specific disease mechanisms. We then provide a brief overview of the importance of standardizing the use of HD mice and describe brief protocols used for genotyping the mouse models used within the Bates Laboratory.

Caspase-6 mediates resistance against Burkholderia pseudomallei infection and influences the expression of detrimental cytokines

Caspase-6 is a member of the executioner caspases and known to play a role in innate and adaptive immune processes. However, its role in infectious diseases has rarely been addressed yet. We here examined the impact of caspase-6 in an in vivo infection model using the Gram-negative rod Burkholderia pseudomallei, causing the infectious disease melioidosis that is endemic in tropical and subtropical areas around the world. Caspase-6-/- and C57BL/6 wild type mice were challenged with B. pseudomallei for comparing mortality, bacterial burden and inflammatory cytokine expression. Bone-marrow derived macrophages were used to analyse the bactericidal activity in absence of caspase-6. Caspase-6 deficiency was associated with higher mortality and bacterial burden in vivo after B. pseudomallei infection. The bactericidal activity of caspase-6-/- macrophages was impaired compared to wild type cells. Caspase-6-/- mice showed higher expression of the IL-1β gene, known to be detrimental in murine melioidosis. Expression of the IL-10 gene was also increased in caspase-6-/- mice as early as 6 hours after infection. Treatment with exogenous IL-10 rendered mice more susceptible against B. pseudomallei challenge. Thus, caspase-6 seems to play a crucial role for determining resistance against the causative agent of melioidosis. To our knowledge this is the first report showing that caspase-6 is crucial for mediating resistance in an in vivo infection model. Caspase-6 influences the expression of detrimental cytokines and therefore seems to be important for achieving a well-balanced immune response that contributes for an efficient elimination of the pathogen.

Caspase 6 has a protective role in SOD1(G93A) transgenic mice

In amyotrophic lateral sclerosis (ALS), it has been suggested that the process of neurodegeneration starts at the neuromuscular junction and is propagated back along axons towards motor neurons. Caspase-dependent pathways are well established as a cause of motor neuron death, and recent work in other disease models indicated a role for caspase 6 in axonal degeneration. Therefore we hypothesised that caspase 6 may be involved in motor neuron death in ALS. To investigate the role of caspase 6 in ALS we profiled protein levels of caspase-6 throughout disease progression in the ALS mouse model SOD1(G93A); this did not reveal differences in caspase 6 levels during disease. To investigate the role of caspase 6 further we generated a colony with SOD1(G93A) transgenic mice lacking caspase 6. Analysis of the transgenic SOD1(G93A); Casp6(-/-) revealed an exacerbated phenotype with motor dysfunction occurring earlier and a significantly shortened lifespan when compared to transgenic SOD1(G93A); Casp6(+/+) mice. Immunofluorescence analysis of the neuromuscular junction revealed no obvious difference between caspase 6(+/+) and caspase 6(-/-) in non-transgenic mice, while the SOD1(G93A) transgenic mice showed severe degeneration compared to non-transgenic mice in both genotypes. Our data indicate that caspase-6 does not exacerbate ALS pathogenesis, but may have a protective role.

Motor deficits associated with Huntington's disease occur in the absence of striatal degeneration in BACHD transgenic mice

Huntington's disease (HD) is an incurable neurodegenerative condition characterized by progressive motor and cognitive dysfunction, and depletion of neurons in the striatum. Recently, BACHD transgenic mice expressing the full-length human huntingtin gene have been generated, which recapitulate some of the motor and cognitive deficits seen in HD. In this study, we carried out a series of extensive behavioural and neuropathological tests on BACHD mice, to validate this mouse for preclinical research. Transgenic C57BL/6J BACHD and litter-matched wild-type mice were examined in a battery of motor and cognitive function tests at regular intervals up to 12 months of age. Brains from these mice were also analysed for signs of neurodegeneration and striatal and cortical volume sizes compared using anatomic 16.4T magnetic resonance imaging (MRI) brain scans. BACHD mice showed progressive motor impairments on rotarod and balance beam tests starting from 3 months of age, were hypoactive in the open field tests starting from 6 months of age, however, showed no alterations in gait and grip strength at any age. Surprisingly, despite these distinct motor deficits, no signs of neuronal loss, gliosis or blood-brain barrier degeneration were observed in the striatum of 12-month-old mice. MRI brain scans confirmed no reduction in striatal or cortical volumes at 12 months of age, and BACHD mice had a normal lifespan. These results demonstrate that classical Huntington's-like motor impairments seen in this transgenic model, do not occur due to degeneration of the striatum, and thus caution against the use of this model for preclinical studies into HD.

New Views on the Misconstrued: Executioner Caspases and Their Diverse Non-apoptotic Roles

Initially characterized for their roles in apoptosis, executioner caspases have emerged as important regulators of an array of cellular activities. This is especially true in the nervous system, where sublethal caspase activity has been implicated in axonal pathfinding and branching, axonal degeneration, dendrite pruning, regeneration, long-term depression, and metaplasticity. Here we examine the roles of sublethal executioner caspase activity in nervous system development and maintenance, consider the mechanisms that locally activate and restrain these potential killers, and discuss how their activity be subverted in neurodegenerative disease.

Huntingtin proteolysis releases non-polyQ fragments that cause toxicity through dynamin 1 dysregulation

Cleavage of mutant huntingtin (HTT) is an essential process in Huntington's disease (HD), an inherited neurodegenerative disorder. Cleavage generates N-ter fragments that contain the polyQ stretch and whose nuclear toxicity is well established. However, the functional defects induced by cleavage of full-length HTT remain elusive. Moreover, the contribution of non-polyQ C-terminal fragments is unknown. Using time- and site-specific control of full-length HTT proteolysis, we show that specific cleavages are required to disrupt intramolecular interactions within HTT and to cause toxicity in cells and flies. Surprisingly, in addition to the canonical pathogenic N-ter fragments, the C-ter fragments generated, that do not contain the polyQ stretch, induced toxicity via dilation of the endoplasmic reticulum (ER) and increased ER stress. C-ter HTT bound to dynamin 1 and subsequently impaired its activity at ER membranes. Our findings support a role for HTT on dynamin 1 function and ER homoeostasis. Proteolysis-induced alteration of this function may be relevant to disease.

Integration-independent Transgenic Huntington Disease Fragment Mouse Models Reveal Distinct Phenotypes and Life Span in Vivo

The cascade of events that lead to cognitive decline, motor deficits, and psychiatric symptoms in patients with Huntington disease (HD) is triggered by a polyglutamine expansion in the N-terminal region of the huntingtin (HTT) protein. A significant mechanism in HD is the generation of mutant HTT fragments, which are generally more toxic than the full-length HTT. The protein fragments observed in human HD tissue and mouse models of HD are formed by proteolysis or aberrant splicing of HTT. To systematically investigate the relative contribution of the various HTT protein proteolysis events observed in vivo, we generated transgenic mouse models of HD representing five distinct proteolysis fragments ending at amino acids 171, 463, 536, 552, and 586 with a polyglutamine length of 148. All lines contain a single integration at the ROSA26 locus, with expression of the fragments driven by the chicken β-actin promoter at nearly identical levels. The transgenic mice N171-Q148 and N552-Q148 display significantly accelerated phenotypes and a shortened life span when compared with N463-Q148, N536-Q148, and N586-Q148 transgenic mice. We hypothesized that the accelerated phenotype was due to altered HTT protein interactions/complexes that accumulate with age. We found evidence for altered HTT complexes in caspase-2 fragment transgenic mice (N552-Q148) and a stronger interaction with the endogenous HTT protein. These findings correlate with an altered HTT molecular complex and distinct proteins in the HTT interactome set identified by mass spectrometry. In particular, we identified HSP90AA1 (HSP86) as a potential modulator of the distinct neurotoxicity of the caspase-2 fragment mice (N552-Q148) when compared with the caspase-6 transgenic mice (N586-Q148).

Transgenic animal models for study of the pathogenesis of Huntington's disease and therapy

Huntington’s disease (HD) is caused by a genetic mutation that results in polyglutamine expansion in the N-terminal regions of huntingtin. As a result, this polyQ expansion leads to the misfolding and aggregation of mutant huntingtin as well as age-dependent neurodegeneration. The genetic mutation in HD allows for generating a variety of animal models that express different forms of mutant huntingtin and show differential pathology. Studies of these animal models have provided an important insight into the pathogenesis of HD. Mouse models of HD include transgenic mice, which express N-terminal or full-length mutant huntingtin ubiquitously or selectively in different cell types, and knock-in mice that express full-length mutant Htt at the endogenous level. Large animals, such as pig, sheep, and monkeys, have also been used to generate animal HD models. This review focuses on the different features of commonly used transgenic HD mouse models as well as transgenic large animal models of HD, and also discusses how to use them to identify potential therapeutics. Since HD shares many pathological features with other neurodegenerative diseases, identification of therapies for HD would also help to develop effective treatment for different neurodegenerative diseases that are also caused by protein misfolding and occur in an age-dependent manner.

Proteolytic cleavage of ataxin-7 promotes SCA7 retinal degeneration and neurological dysfunction

The neurodegenerative disorder spinocerebellar ataxia type 7 (SCA7) is caused by a polyglutamine (polyQ) expansion in the ataxin-7 protein, categorizing SCA7 as one member of a large class of heritable neurodegenerative proteinopathies. Cleavage of ataxin-7 by the protease caspase-7 has been demonstrated in vitro, and the accumulation of proteolytic cleavage products in SCA7 patients and mouse models has been identified as an early pathological change. However, it remains unknown whether a causal relationship exists between ataxin-7 proteolysis and in vivo SCA7 disease progression. To determine whether caspase cleavage is a critical event in SCA7 disease pathogenesis, we generated transgenic mice expressing polyQ-expanded ataxin-7 with a second-site mutation (D266N) to prevent caspase-7 proteolysis. When we compared SCA7-D266N mice with SCA7 mice lacking the D266N mutation, we found that SCA7-D266N mice exhibited improved motor performance, reduced neurodegeneration and substantial lifespan extension. Our findings indicate that proteolysis at the D266 caspase-7 cleavage site is an important mediator of ataxin-7 neurotoxicity, suggesting that inhibition of caspase-7 cleavage of polyQ-ataxin-7 may be a promising therapeutic strategy for this untreatable disorder.

A Huntingtin-based peptide inhibitor of caspase-6 provides protection from mutant Huntingtin-induced motor and behavioral deficits

Over the past decade, increasing evidence has implied a significant connection between caspase-6 activity and the pathogenesis of Huntington's disease (HD). Consequently, inhibiting caspase-6 activity was suggested as a promising therapeutic strategy to reduce mutant Huntingtin toxicity, and to provide protection from mutant Huntingtin-induced motor and behavioral deficits. Here, we describe a novel caspase-6 inhibitor peptide based on the huntingtin caspase-6 cleavage site, fused with a cell-penetrating sequence. The peptide reduces mutant Huntingtin proteolysis by caspase-6, and protects cells from mutant Huntingtin toxicity. Continuous subcutaneous administration of the peptide protected pre-symptomatic BACHD mice from motor deficits and behavioral abnormalities. Moreover, administration of the peptide in an advanced disease state resulted in the partial recovery of motor performance, and an alleviation of depression-related behavior and cognitive deficits. Our findings reveal the potential of substrate-based caspase inhibition as a therapeutic strategy, and present a promising agent for the treatment of HD.

Partial rescue of some features of Huntington Disease in the genetic absence of caspase-6 in YAC128 mice

Huntington Disease (HD) is a progressive neurodegenerative disease caused by an elongated CAG repeat in the huntingtin (HTT) gene that encodes a polyglutamine tract in the HTT protein. Proteolysis of the mutant HTT protein (mHTT) has been detected in human and murine HD brains and is implicated in the pathogenesis of HD. Of particular importance is the site at amino acid (aa) 586 that contains a caspase-6 (Casp6) recognition motif. Activation of Casp6 occurs presymptomatically in human HD patients and the inhibition of mHTT proteolysis at aa586 in the YAC128 mouse model results in the full rescue of HD-like phenotypes. Surprisingly, Casp6 ablation in two different HD mouse models did not completely prevent the generation of this fragment, and therapeutic benefits were limited, questioning the role of Casp6 in the disease. We have evaluated the impact of the loss of Casp6 in the YAC128 mouse model of HD. Levels of the mHTT-586 fragment are reduced but not absent in the absence of Casp6 and we identify caspase 8 as an alternate enzyme that can generate this fragment. In vivo, the ablation of Casp6 results in a partial rescue of body weight gain, normalized IGF-1 levels, a reversal of the depression-like phenotype and decreased HTT levels. In the YAC128/Casp6-/- striatum there is a concomitant reduction in p62 levels, a marker of autophagic activity, suggesting increased autophagic clearance. These results implicate the HTT-586 fragment as a key contributor to certain features of HD, irrespective of the enzyme involved in its generation.

Novel BAC Mouse Model of Huntington's Disease with 225 CAG Repeats Exhibits an Early Widespread and Stable Degenerative Phenotype

BACKGROUND

Unusually large CAG repeat expansions (>60) in exon one of Huntingtin (HTT) are invariably associated with a juvenile-onset form of Huntington's disease (HD), characterized by a more extensive and rapidly progressing neuropathology than the more prevalent adult-onset form. However, existing mouse models of HD that express the full-length Htt gene with CAG repeat lengths associated with juvenile HD (ranging between ~75 to ~150 repeats in published models) exhibit selective neurodegenerative phenotypes more consistent with adult-onset HD. Objective: To determine if a very large CAG repeat (>200) in full-length Htt elicits neurodegenerative phenotypes consistent with juvenile HD.

METHODS

Using a …bacterial artificial chromosome (BAC) system, we generated mice expressing full-length mouse Htt with ~225 CAG repeats under control of the mouse Htt promoter. Mice were characterized using behavioral, neuropathological, biochemical and brain imaging methods.

RESULTS

BAC-225Q mice exhibit phenotypes consistent with a subset of features seen in juvenile-onset HD: very early motor behavior abnormalities, reduced body weight, widespread and progressive increase in Htt aggregates, gliosis, and neurodegeneration. Early striatal pathology was observed, including reactive gliosis and loss of dopamine receptors, prior to detectable volume loss. HD-related blood markers of impaired energy metabolism and systemic inflammation were also increased. Aside from an age-dependent progression of diffuse nuclear aggregates at 6 months of age to abundant neuropil aggregates at 12 months of age, other pathological and motor phenotypes showed little to no progression.

CONCLUSIONS

The HD phenotypes present in animals 3 to 12 months of age make the BAC-225Q mice a unique and stable model of full-length mutant Htt associated phenotypes, including body weight loss, behavioral impairment and HD-like neurodegenerative phenotypes characteristic of juvenile-onset HD and/or late-stage adult-onset HD.

Activation and regulation of caspase-6 and its role in neurodegenerative diseases

Caspases, a family of cysteine proteases, are major mediators of apoptosis and inflammation. Caspase-6 is classified as an apoptotic effector, and it mediates nuclear shrinkage during apoptosis, but it possesses unique activation and regulation mechanisms that differ from those of other effector caspases. Furthermore, increasing evidence has shown that caspase-6 is highly involved in axon degeneration and neurodegenerative diseases, such as Huntington's disease and Alzheimer's disease. Cleavage at the caspase-6 site in mutated huntingtin protein is a prerequisite for the development of the characteristic behavioral and neuropathological features of Huntington's disease. Active caspase-6 is present in early stages of Alzheimer's disease, and caspase-6 activity is associated with the disease's pathological lesions. In this review, we discuss the evidence relevant to the role of caspase-6 in neurodegenerative diseases and summarize its activation and regulation mechanisms. In doing so, we provide new insight about potential therapeutic approaches that incorporate the modulation of caspase-6 function for the treatment of neurodegenerative diseases.

Autophagy in Huntington disease and huntingtin in autophagy

Autophagy is an important biological process that is essential for the removal of damaged organelles and toxic or aggregated proteins by delivering them to the lysosome for degradation. Consequently, autophagy has become a primary target for the treatment of neurodegenerative diseases that involve aggregating proteins. In Huntington disease (HD), an expansion of the polyglutamine (polyQ) tract in the N-terminus of the huntingtin (HTT) protein leads to protein aggregation. However, HD is unique among the neurodegenerative proteinopathies in that autophagy is not only dysfunctional but wild type (wt) HTT also appears to play several roles in regulating the dynamics of autophagy. Herein, we attempt to integrate the recently described novel roles of wtHTT and altered autophagy in HD.

From pathways to targets: understanding the mechanisms behind polyglutamine disease

The history of polyglutamine diseases dates back approximately 20 years to the discovery of a polyglutamine repeat in the androgen receptor of SBMA followed by the identification of similar expansion mutations in Huntington's disease, SCA1, DRPLA, and the other spinocerebellar ataxias. This common molecular feature of polyglutamine diseases suggests shared mechanisms in disease pathology and neurodegeneration of disease specific brain regions. In this review, we discuss the main pathogenic pathways including proteolytic processing, nuclear shuttling and aggregation, mitochondrial dysfunction, and clearance of misfolded polyglutamine proteins and point out possible targets for treatment.

Identification of a post-translationally myristoylated autophagy-inducing domain released by caspase cleavage of huntingtin

Huntington disease (HD) is a debilitating neurodegenerative disease characterized by the loss of motor control and cognitive ability that ultimately leads to death. It is caused by the expansion of a polyglutamine tract in the huntingtin (HTT) protein, which leads to aggregation of the protein and eventually cellular death. Both the wild-type and mutant form of the protein are highly regulated by post-translational modifications including proteolysis, palmitoylation and phosphorylation. We now demonstrate the existence of a new post-translational modification of HTT: the addition of the 14 carbon fatty acid myristate to a glycine residue exposed on a caspase-3-cleaved fragment (post-translational myristoylation) and that myristoylation of this fragment is altered in a physiologically relevant model of mutant HTT. Myristoylated HTT553-585-EGFP, but not its non-myristoylated variant, initially localized to the ER, induced the formation of autophagosomes and accumulated in abnormally large autophagolysosomal/lysosomal structures in a variety of cell types, including neuronal cell lines under nutrient-rich conditions. Our results suggest that accumulation of myristoylated HTT553-586 in cells may alter the rate of production of autophagosomes and/or their clearance through the heterotypic autophagosomal/lysosomal fusion process. Overall, our novel observations establish a role for the post-translational myristoylation of a caspase-3-cleaved fragment of HTT, highly similar to the Barkor/ATG14L autophagosome-targeting sequence domain thought to sense, maintain and/or promote membrane curvature in the regulation of autophagy. Abnormal processing or production of this myristoylated HTT fragment might be involved in the pathophysiology of HD.

Preventing formation of toxic N-terminal huntingtin fragments through antisense oligonucleotide-mediated protein modification

Huntington's disease (HD) is a progressive autosomal dominant disorder, caused by a CAG repeat expansion in the HTT gene, which results in expansion of a polyglutamine stretch at the N-terminal end of the huntingtin protein. Several studies have implicated the importance of proteolytic cleavage of mutant huntingtin in HD pathogenesis and it is generally accepted that N-terminal huntingtin fragments are more toxic than full-length protein. Important cleavage sites are encoded by exon 12 of HTT. Here we report proof of concept using antisense oligonucleotides to induce skipping of exon 12 in huntingtin pre-mRNA, thereby preventing the formation of a 586 amino acid N-terminal huntingtin fragment implicated in HD toxicity. In vitro studies showed successful exon skipping and appearance of a shorter huntingtin protein. Cleavage assays showed reduced 586 amino acid N-terminal huntingtin fragments in the treated samples. In vivo studies revealed exon skipping after a single injection of antisense oligonucleotides in the mouse striatum. Recent advances to inhibit the formation of mutant huntingtin using oligonucleotides seem promising therapeutic strategies for HD. Nevertheless, huntingtin is an essential protein and total removal has been shown to result in progressive neurodegeneration in vivo. Our proof of concept shows a completely novel approach to reduce mutant huntingtin toxicity not by reducing its expressing levels, but by modifying the huntingtin protein.

Cerebrospinal fluid tau cleaved by caspase-6 reflects brain levels and cognition in aging and Alzheimer disease

Caspase-6 (Casp6) activation in the brain is implicated early in the pathogenesis of Alzheimer disease (AD). In view of the need for early AD diagnosis, brain Casp6 activity was investigated by measuring Tau cleaved by Casp6 (TauΔCasp6) protein in postmortem cerebrospinal fluid (CSF) of 7 non-cognitively impaired; 5 mild cognitively impaired; and 12 mild, moderate, and severe AD patients. Levels of TauΔCasp6 in CSF accurately reflected the levels of active Casp6 and TauΔCasp6 detected using immunohistochemistry in hippocampal sections from the same individuals. Levels of CSF TauΔCasp6 significantly correlated with AD severity and with lower Global Cognitive Scores; Mini-Mental State Examination scores; and episodic, semantic, and working memory scores. Regression analyses suggested that the CSF TauΔCasp6 levels combined with TauΔCasp6 brain pathology predict cognitive performance. These results indicate that CSF TauΔCasp6 levels hold promise as a novel early biomarker of AD.

Genetic manipulations of mutant huntingtin in mice: new insights into Huntington's disease pathogenesis

This year (2013) marks the 20th anniversary of identification of the causal genetic mutation for Huntington's disease (HD), a landmark discovery that heralded study of the biological underpinnings of this most common dominantly inherited neurodegenerative disorder. Among the variety of model organisms used to study HD pathogenesis, the mouse model is by far the most commonly used mammalian genetic model. Much of our current knowledge regarding mutant huntingtin (mHtt)-induced disease pathogenesis in mammalian models has been obtained by studying transgenic mouse models expressing mHtt N-terminal fragments, full-length murine or human mHtt. In this review, we focus on recent progress in using novel HD mouse models with targeted mHtt expression in specific brain cell types. These models help to address the role of distinct neuronal and non-neuronal cell types in eliciting cell-autonomous or non-cell-autonomous disease processes in HD. We also describe several mHtt transgenic mouse models with targeted mutations in Htt cis-domains to address specific pathogenic hypotheses, ranging from mHtt proteolysis to post-translational modifications. These novel mouse genetic studies, through direct manipulations of the causal HD gene, utilize a reductionist approach to systematically unravel the cellular and molecular pathways that are targeted by mHtt in disease pathogenesis, and to potentially identify novel targets for therapy.

PolyQ disease: misfiring of a developmental cell death program?

Polyglutamine (polyQ) repeat diseases are neurodegenerative ailments elicited by glutamine-encoding CAG nucleotide expansions within endogenous human genes. Despite efforts to understand the basis of these diseases, the precise mechanism of cell death remains stubbornly unclear. Much of the data seem to be consistent with a model in which toxicity is an inherent property of the polyQ repeat, whereas host protein sequences surrounding the polyQ expansion modulate severity, age of onset, and cell specificity. Recently, a gene, pqn-41, encoding a glutamine-rich protein, was found to promote normally occurring non-apoptotic cell death in Caenorhabditis elegans. Here we review evidence for toxic and modulatory roles for polyQ repeats and their host proteins, respectively, and suggest similarities with pqn-41 function. We explore the hypothesis that toxicity mediated by glutamine-rich motifs may be important not only in pathology, but also in normal development.