Caspase-6 does not contribute to the proteolysis of mutant huntingtin in the HdhQ150 knock-in mouse model of Huntington's disease

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.

A patient-derived cellular model for Huntington's disease reveals phenotypes at clinically relevant CAG lengths

The huntingtin protein participates in several cellular processes that are disrupted when the polyglutamine tract is expanded beyond a threshold of 37 CAG DNA repeats in Huntington’s disease (HD). Cellular biology approaches to understand these functional disruptions in HD have primarily focused on cell lines with synthetically long CAG length alleles that clinically represent outliers in this disease and a more severe form of HD that lacks age onset. Patient-derived fibroblasts are limited to a finite number of passages before succumbing to cellular senescence. We used human telomerase reverse transcriptase (hTERT) to immortalize fibroblasts taken from individuals of varying age, sex, disease onset, and CAG repeat length, which we have termed TruHD cells. TruHD cells display classic HD phenotypes of altered morphology, size and growth rate, increased sensitivity to oxidative stress, aberrant adenosine diphosphate/adenosine triphosphate (ADP/ATP) ratios, and hypophosphorylated huntingtin protein. We additionally observed dysregulated reactive oxygen species (ROS)-dependent huntingtin localization to nuclear speckles in HD cells. We report the generation and characterization of a human, clinically relevant cellular model for investigating disease mechanisms in HD at the single-cell level, which, unlike transformed cell lines, maintains functions critical for huntingtin transcriptional regulation and genomic integrity.

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.

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.

N-Terminal Fragments of Huntingtin Longer than Residue 170 form Visible Aggregates Independently to Polyglutamine Expansion

BACKGROUND

A hallmark of Huntington's disease is the progressive aggregation of full length and N-terminal fragments of polyglutamine (polyQ)-expanded Huntingtin (Htt) into intracellular inclusions. The production of N-terminal fragments appears important for enabling pathology and aggregation; and hence the direct expression of a variety of N-terminal fragments are commonly used to model HD in animal and cellular models.

OBJECTIVE

It remains unclear how the length of the N-terminal fragments relates to polyQ - mediated aggregation. We investigated the fundamental intracellular aggregation process of eight different-length N-terminal fragments of Htt in both short (25Q) and long polyQ (97Q).

METHODS

N-terminal fragments were fused to fluorescent proteins and transiently expressed in mammalian cell culture models. These included the classic exon 1 fragment (90 amino acids) and longer forms of 105, 117, 171, 513, 536, 552, and 586 amino acids based on wild-type Htt (of 23Q) sequence length nomenclature.

RESULTS

N-terminal fragments of less than 171 amino acids only formed inclusions in polyQ-expanded form. By contrast the longer fragments formed inclusions irrespective of Q-length, with Q-length playing a negligible role in extent of aggregation. The inclusions could be classified into 3 distinct morphological categories. One type (Type A) was universally associated with polyQ expansions whereas the other two types (Types B and C) formed independently of polyQ length expansion.

CONCLUSIONS

PolyQ-expansion was only required for fragments of less than 171 amino acids to aggregate. Longer fragments aggregated predominately through a non-polyQ mechanism, involving at least one, and probably more distinct clustering mechanisms.

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.

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.

Comparative study of naturally occurring huntingtin fragments in Drosophila points to exon 1 as the most pathogenic species in Huntington's disease

Although Huntington's disease is caused by the expansion of a CAG triplet repeat within the context of the 3144-amino acid huntingtin protein (HTT), studies reveal that N-terminal fragments of HTT containing the expanded PolyQ region can be produced by proteolytic processing and/or aberrant splicing. N-terminal HTT fragments are also prevalent in postmortem tissue, and expression of some of these fragments in model organisms can cause pathology. This has led to the hypothesis that N-terminal peptides may be critical modulators of disease pathology, raising the possibility that targeting aberrant splicing or proteolytic processing may present attractive therapeutic targets. However, many factors can contribute to pathology, including genetic background and differential expression of transgenes, in addition to intrinsic differences between fragments and their cellular effects. We have used Drosophila as a model system to determine the relative toxicities of different naturally occurring huntingtin fragments in a system in which genetic background, transgene expression levels and post-translational proteolytic processing can be controlled. These studies reveal that among the naturally occurring N-terminal HTT peptides, the exon 1 peptide is exceptionally pathogenic and exhibits unique structural and biophysical behaviors that do not appear to be incremental changes compared with other fragments. If this proves correct, efforts to specifically reduce the levels of exon 1 peptides or to target toxicity-influencing post-translational modifications that occur with the exon 1 context are likely to have the greatest impact on pathology.

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.

The regulatory mechanism of the caspase 6 pro-domain revealed by crystal structure and biochemical assays

Caspase 6 (CASP6) is a neuron degeneration-related protease and is widely considered to be a potential drug-design target against neurodegenerative diseases such as Huntington's disease and Alzheimer's disease. The N-terminal pro-peptide of CASP6, also referred to as the pro-domain, contains 23 residues and its functional role remains elusive. In this study, the crystal structure of a full-length CASP6 zymogen mutant, proCASP6H121A, was solved. Although the pro-domain was flexible in the crystal, without visible electron density, structural analyses combined with biochemical assays revealed that the pro-domain inhibited CASP6 auto-activation by inhibiting intramolecular cleavage at the intersubunit cleavage site TEVD(193) and also by preventing this site from intermolecular cleavage at low protein concentration through a so-called `suicide-protection' mechanism. Further experiments showed that the length of the pro-domain and the side chain of Asn18 played critical roles in suicide protection. These results disclosed a new inhibitory mechanism of CASP6 and shed light on the pathogenesis and therapeutically relevant study of CASP6-related neurodegenerative diseases.

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.