Huntington's disease: can mice lead the way to treatment?

TRIM37 is a primate-specific E3 ligase for Huntingtin and accounts for the striatal degeneration in Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease characterized by preferential neuronal loss in the striatum. The mechanism underlying striatal selective neurodegeneration remains unclear, making it difficult to develop effective treatments for HD. In the brains of nonhuman primates, we examined the expression of Huntingtin (HTT), the gene responsible for HD. We found that HTT protein is highly expressed in striatal neurons due to its slow degradation in the striatum. We also identified tripartite motif-containing 37 (TRIM37) as a primate-specific protein that interacts with HTT and is selectively reduced in the primate striatum. TRIM37 promotes the ubiquitination and degradation of mutant HTT (mHTT) in vitro and modulates mHTT aggregation in mouse and monkey brains. Our findings suggest that nonhuman primates are crucial for understanding the mechanisms of human diseases such as HD and support TRIM37 as a potential therapeutic target for treating HD.

Application and perspective of CRISPR/Cas9 genome editing technology in human diseases modeling and gene therapy

The Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR) mediated Cas9 nuclease system has been extensively used for genome editing and gene modification in eukaryotic cells. CRISPR/Cas9 technology holds great potential for various applications, including the correction of genetic defects or mutations within the human genome. The application of CRISPR/Cas9 genome editing system in human disease research is anticipated to solve a multitude of intricate molecular biology challenges encountered in life science research. Here, we review the fundamental principles underlying CRISPR/Cas9 technology and its recent application in neurodegenerative diseases, cardiovascular diseases, autoimmune related diseases, and cancer, focusing on the disease modeling and gene therapy potential of CRISPR/Cas9 in these diseases. Finally, we provide an overview of the limitations and future prospects associated with employing CRISPR/Cas9 technology for diseases study and treatment.

Comparative analysis of primate and pig cells reveals primate-specific PINK1 expression and phosphorylation

PTEN-induced putative kinase 1 (PINK1), a mitochondrial kinase that phosphorylates Parkin and other proteins, plays a crucial role in mitophagy and protection against neurodegeneration. Mutations in PINK1 and Parkin can lead to loss of function and early onset Parkinson's disease. However, there is a lack of strong in vivo evidence in rodent models to support the theory that loss of PINK1 affects mitophagy and induces neurodegeneration. Additionally, PINK1 knockout pigs ( Sus scrofa) do not appear to exhibit neurodegeneration. In our recent work involving non-human primates, we found that PINK1 is selectively expressed in primate brains, while absent in rodent brains. To extend this to other species, we used multiple antibodies to examine the expression of PINK1 in pig tissues. In contrast to tissues from cynomolgus monkeys ( Macaca fascicularis), our data did not convincingly demonstrate detectable PINK1 expression in pig tissues. Knockdown of PINK1 in cultured pig cells did not result in altered Parkin and BAD phosphorylation, as observed in cultured monkey cells. A comparison of monkey and pig striatum revealed more PINK1-phosphorylated substrates in the monkey brain. Consistently, PINK1 knockout in pigs did not lead to obvious changes in the phosphorylation of Parkin and BAD. These findings provide new evidence that PINK1 expression is specific to primates, underscoring the importance of non-human primates in investigating PINK1 function and pathology related to PINK1 deficiency.

Large animal models for Huntington's disease research

Huntington's disease (HD) is a hereditary neurodegenerative disorder for which there is currently no effective treatment available. Consequently, the development of appropriate disease models is critical to thoroughly investigate disease progression. The genetic basis of HD involves the abnormal expansion of CAG repeats in the huntingtin ( HTT) gene, leading to the expansion of a polyglutamine repeat in the HTT protein. Mutant HTT carrying the expanded polyglutamine repeat undergoes misfolding and forms aggregates in the brain, which precipitate selective neuronal loss in specific brain regions. Animal models play an important role in elucidating the pathogenesis of neurodegenerative disorders such as HD and in identifying potential therapeutic targets. Due to the marked species differences between rodents and larger animals, substantial efforts have been directed toward establishing large animal models for HD research. These models are pivotal for advancing the discovery of novel therapeutic targets, enhancing effective drug delivery methods, and improving treatment outcomes. We have explored the advantages of utilizing large animal models, particularly pigs, in previous reviews. Since then, however, significant progress has been made in developing more sophisticated animal models that faithfully replicate the typical pathology of HD. In the current review, we provide a comprehensive overview of large animal models of HD, incorporating recent findings regarding the establishment of HD knock-in (KI) pigs and their genetic therapy. We also explore the utilization of large animal models in HD research, with a focus on sheep, non-human primates (NHPs), and pigs. Our objective is to provide valuable insights into the application of these large animal models for the investigation and treatment of neurodegenerative disorders.

CRISPR/Cas9 system and its applications in nervous system diseases

The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) system is an acquired immune system of many bacteria and archaea, comprising CRISPR loci, Cas genes, and its associated proteins. This system can recognize exogenous DNA and utilize the Cas9 protein's nuclease activity to break DNA double-strand and to achieve base insertion or deletion by subsequent DNA repair. In recent years, multiple laboratory and clinical studies have revealed the therapeutic role of the CRISPR/Cas9 system in neurological diseases. This article reviews the CRISPR/Cas9-mediated gene editing technology and its potential for clinical application against neurological diseases.

Neurotoxicity of Pyrethroids in neurodegenerative diseases: From animals' models to humans' studies

Neurodegenerative diseases are associated with diverse symptoms, both motor and mental. Genetic and environmental factors can trigger neurodegenerative diseases. Chemicals as pesticides are constantly used in agriculture and also domestically. In this regard, pyrethroids (PY), are a class of insecticides in which its main mechanism of action is through disruption of voltage-dependent sodium channels function in insects. However, in mammals, they can also induce oxidative stress and enzyme dysfunction. This review investigates the association between PY and neurodegenerative diseases as Alzheimer's, Huntington's, Parkinson's, Amyotrophic Lateral Sclerosis, and Autism in animal models and humans. Published works using specific and non-specific models for these diseases were selected. We showed a tendency toward the development and/or aggravating of these neurodegenerative diseases following exposure to PYs. In animal models, the biochemical mechanisms of the diseases and their interaction with the insecticides are more deeply investigated. Nonetheless, only a few studies considered the specific model for each type of disease to analyze the impacts of the exposure. The choice of a specific model during the research is an important step and our review highlights the knowledge gaps of PYs effects using these models reinforcing the importance of them during the design of the experiments.

Modeling Huntington's disease: An insight on in-vitro and in-vivo models

Huntington's disease is a neurodegenerative illness that causes neuronal death most extensively within the basal ganglia. There is a broad class of neurologic disorders associated with the expansion of polyglutamine (polyQ) repeats in numerous proteins. Several other molecular mechanisms have also been implicated in HD pathology, including brain-derived neurotrophic factor (BDNF), mitochondrial dysfunction, and altered synaptic plasticity in central spiny neurons. HD pathogenesis and the effectiveness of therapy approaches have been better understood through the use of animal models. The pathological manifestations of the disease were reproduced by early models of glutamate analog toxicity and mitochondrial respiration inhibition. Because the treatments available for HD are quite limited, it is important to have a definite preclinical model that mimics all the aspects of the disease. It can be used to study mechanisms and validate candidate therapies. Although there hasn't been much success in translating animal research into clinical practice, each model has something special to offer in the quest for a deeper comprehension of HD's neurobehavioral foundations. This review provides insight into various in-vitro-and in-vivo models of HD which may be useful in the screening of newer therapeutics for this incapacitating disorder.

Comparing HD knockin pigs and mice reveals the pathological role of IL-17

Our previous work has established a knockin (KI) pig model of Huntington's disease (HD) that can replicate the typical pathological features of HD, including selective striatal neuronal loss, reactive gliosis, and axonal degeneration. However, HD KI mice exhibit milder neuropathological phenotypes and lack overt neurodegeneration. By performing RNA sequencing to compare the gene expression profiles between HD KI pigs and mice, we find that genes related to interleukin-17 (IL-17) signaling are upregulated in the HD pig brains compared to the mouse brains. Delivery of IL-17 into the brain striatum of HD KI mice causes greater reactive gliosis and synaptic deficiency compared to HD KI mice that received PBS. These findings suggest that the upregulation of genes related to IL-17 signaling in HD pig brains contributes to severe glial pathology in HD and identify this as a potential therapeutic target for treating HD.

Microglia and complement mediate early corticostriatal synapse loss and cognitive dysfunction in Huntington's disease

Huntington's disease (HD) is a devastating monogenic neurodegenerative disease characterized by early, selective pathology in the basal ganglia despite the ubiquitous expression of mutant huntingtin. The molecular mechanisms underlying this region-specific neuronal degeneration and how these relate to the development of early cognitive phenotypes are poorly understood. Here we show that there is selective loss of synaptic connections between the cortex and striatum in postmortem tissue from patients with HD that is associated with the increased activation and localization of complement proteins, innate immune molecules, to these synaptic elements. We also found that levels of these secreted innate immune molecules are elevated in the cerebrospinal fluid of premanifest HD patients and correlate with established measures of disease burden.In preclinical genetic models of HD, we show that complement proteins mediate the selective elimination of corticostriatal synapses at an early stage in disease pathogenesis, marking them for removal by microglia, the brain's resident macrophage population. This process requires mutant huntingtin to be expressed in both cortical and striatal neurons. Inhibition of this complement-dependent elimination mechanism through administration of a therapeutically relevant C1q function-blocking antibody or genetic ablation of a complement receptor on microglia prevented synapse loss, increased excitatory input to the striatum and rescued the early development of visual discrimination learning and cognitive flexibility deficits in these models. Together, our findings implicate microglia and the complement cascade in the selective, early degeneration of corticostriatal synapses and the development of cognitive deficits in presymptomatic HD; they also provide new preclinical data to support complement as a therapeutic target for early intervention.

Pathological insights from amyotrophic lateral sclerosis animal models: comparisons, limitations, and challenges

In order to dissect amyotrophic lateral sclerosis (ALS), a multigenic, multifactorial, and progressive neurodegenerative disease with heterogeneous clinical presentations, researchers have generated numerous animal models to mimic the genetic defects. Concurrent and comparative analysis of these various models allows identification of the causes and mechanisms of ALS in order to finally obtain effective therapeutics. However, most genetically modified rodent models lack overt pathological features, imposing challenges and limitations in utilizing them to rigorously test the potential mechanisms. Recent studies using large animals, including pigs and non-human primates, have uncovered important events that resemble neurodegeneration in patients' brains but could not be produced in small animals. Here we describe common features as well as discrepancies among these models, highlighting new insights from these models. Furthermore, we will discuss how to make rodent models more capable of recapitulating important pathological features based on the important pathogenic insights from large animal models.

A novel HDAC6 inhibitor, CKD-504, is effective in treating preclinical models of huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder, of which pathogenesis is caused by a polyglutamine expansion in the amino-terminus of huntingtin gene that resulted in the aggregation of mutant HTT proteins. HD is characterized by progressive motor dysfunction, cognitive impairment and neuropsychiatric disturbances. Histone deacetylase 6 (HDAC6), a microtubule-associated deacetylase, has been shown to induce transport- and release-defect phenotypes in HD models, whilst treatment with HDAC6 inhibitors ameliorates the phenotypic effects of HD by increasing the levels of α-tubulin acetylation, as well as decreasing the accumulation of mutant huntingtin (mHTT) aggregates, suggesting HDAC6 inhibitor as a HD therapeutics. In this study, we employed in vitro neural stem cell (NSC) model and in vivo YAC128 transgenic (TG) mouse model of HD to test the effect of a novel HDAC6 selective inhibitor, CKD-504, developed by Chong Kun Dang (CKD Pharmaceutical Corp., Korea). We found that treatment of CKD-504 increased tubulin acetylation, microtubule stabilization, axonal transport, and the decrease of mutant huntingtin protein in vitro. From in vivo study, we observed CKD-504 improved the pathology of Huntington's disease: alleviated behavioral deficits, increased axonal transport and number of neurons, restored synaptic function in corticostriatal (CS) circuit, reduced mHTT accumulation, inflammation and tau hyperphosphorylation in YAC128 TG mouse model. These novel results highlight CKD-504 as a potential therapeutic strategy in HD. [BMB Reports 2023; 56(3): 178-183].

Vitamin B6, B12 and folate modulate deregulated pathways and protein aggregation in yeast model of Huntington disease

Huntington's disease (HD) is an incurable and progressive neurodegenerative disease affecting the basal ganglia of the brain. HD is caused due to expansion of the polyglutamine tract in the protein Huntingtin resulting in aggregates. The increased PolyQ length results in aggregation of protein Huntingtin leading to neuronal cell death. Vitamin B6, B12 and folate are deficient in many neurodegenerative diseases. We performed an integrated analysis of transcriptomic, metabolomic and cofactor-protein network of vitamin B6, B12 and folate was performed. Our results show considerable overlap of pathways modulated by Vitamin B6, B12 and folate with those obtained from transcriptomic and metabolomic data of HD patients and model systems. Further, in yeast model of HD we showed treatment of B6, B12 or folate either alone or in combination showed impaired aggregate formation. Transcriptomic analysis of yeast model treated with B6, B12 and folate showed upregulation of pathways like ubiquitin mediated proteolysis, autophagy, peroxisome, fatty acid, lipid and nitrogen metabolism. Metabolomic analysis of yeast model shows deregulation of pathways like aminoacyl-tRNA biosynthesis, metabolism of various amino acids, nitrogen metabolism and glutathione metabolism. Integrated transcriptomic and metabolomic analysis of yeast model showed concordance in the pathways obtained. Knockout of Peroxisomal (PXP1 and PEX7) and Autophagy (ATG5) genes in yeast increased aggregates which is mitigated by vitamin B6, B12 and folate treatment. Taken together our results show a role for Vitamin B6, B12 and folate mediated modulation of pathways important for preventing protein aggregation with potential implications for HD.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03525-y.

Beneficial behavioral effects of chronic cerebral dopamine neurotrophic factor (CDNF) infusion in the N171-82Q transgenic model of Huntington's disease

Huntington's disease (HD) is a progressive inherited neurological disease characterized by the degeneration of basal ganglia and the accumulation of mutant huntingtin (mHtt) aggregates in specific brain areas. Currently, there is no treatment for halting the progression of HD. Cerebral dopamine neurotrophic factor (CDNF) is a novel endoplasmic reticulum located protein with neurotrophic factor properties that protects and restores dopamine neurons in rodent and non-human primate models of Parkinson's disease. Our recent study showed that CDNF improves motor coordination and protects NeuN positive cells in a Quinolinic acid toxin rat model of HD. Here we have investigated the effect of chronic intrastriatal CDNF administration on behavior and mHtt aggregates in the N171-82Q mouse model of HD. Data showed that CDNF did not significantly decrease the number of mHtt aggregates in most brain regions studied. Notably, CDNF significantly delayed the onset of symptoms and improved motor coordination in N171-82Q mice. Furthermore, CDNF increased BDNF mRNA level in hippocampus in vivo in the N171-82Q model and BDNF protein level in cultured striatal neurons. Collectively our results indicate that CDNF might be a potential drug candidate for the treatment of HD.

Neuroinflammation in Huntington's disease: From animal models to clinical therapeutics

Huntington's disease (HD) is a progressive neurodegenerative disease characterized by preferential loss of neurons in the striatum in patients, which leads to motor and cognitive impairments and death that often occurs 10-15 years after the onset of symptoms. The expansion of a glutamine repeat (>36 glutamines) in the N-terminal region of huntingtin (HTT) has been defined as the cause of HD, but the mechanism underlying neuronal death remains unclear. Multiple mechanisms, including inflammation, may jointly contribute to HD pathogenesis. Altered inflammation response is evident even before the onset of classical symptoms of HD. In this review, we summarize the current evidence on immune and inflammatory changes, from HD animal models to clinical phenomenon of patients with HD. The understanding of the impact of inflammation on HD would help develop novel strategies to treat HD.

CRISPR-Based Genome-Editing Tools for Huntington's Disease Research and Therapy

Huntington's disease (HD) is an autosomal dominantly-inherited neurodegenerative disease, which is caused by CAG trinucleotide expansion in exon 1 of the Huntingtin (HTT) gene. Although HD is a rare disease, its monogenic nature makes it an ideal model in which to understand pathogenic mechanisms and to develop therapeutic strategies for neurodegenerative diseases. Clustered regularly-interspaced short palindromic repeats (CRISPR) is the latest technology for genome editing. Being simple to use and highly efficient, CRISPR-based genome-editing tools are rapidly gaining popularity in biomedical research and opening up new avenues for disease treatment. Here, we review the development of CRISPR-based genome-editing tools and their applications in HD research to offer a translational perspective on advancing the genome-editing technology to HD treatment.

A novel rhesus macaque model of Huntington's disease recapitulates key neuropathological changes along with motor and cognitive decline

We created a new nonhuman primate model of the genetic neurodegenerative disorder Huntington's disease (HD) by injecting a mixture of recombinant adeno-associated viral vectors, serotypes AAV2 and AAV2.retro, each expressing a fragment of human mutant HTT (mHTT) into the caudate and putamen of adult rhesus macaques. This modeling strategy results in expression of mutant huntingtin protein (mHTT) and aggregate formation in the injected brain regions, as well as dozens of other cortical and subcortical brain regions affected in human HD patients. We queried the disruption of cortico-basal ganglia circuitry for 30 months post-surgery using a variety of behavioral and imaging readouts. Compared to controls, mHTT-treated macaques developed working memory decline and progressive motor impairment. Multimodal imaging revealed circuit-wide white and gray matter degenerative processes in several key brain regions affected in HD. Taken together, we have developed a novel macaque model of HD that may be used to develop disease biomarkers and screen promising therapeutics.

New pathogenic insights from large animal models of neurodegenerative diseases

Animal models are essential for investigating the pathogenesis and developing the treatment of human diseases. Identification of genetic mutations responsible for neurodegenerative diseases has enabled the creation of a large number of small animal models that mimic genetic defects found in the affected individuals. Of the current animal models, rodents with genetic modifications are the most commonly used animal models and provided important insights into pathogenesis. However, most of genetically modified rodent models lack overt neurodegeneration, imposing challenges and obstacles in utilizing them to rigorously test the therapeutic effects on neurodegeneration. Recent studies that used CRISPR/Cas9-targeted large animal (pigs and monkeys) have uncovered important pathological events that resemble neurodegeneration in the patient’s brain but could not be produced in small animal models. Here we highlight the unique nature of large animals to model neurodegenerative diseases as well as the limitations and challenges in establishing large animal models of neurodegenerative diseases, with focus on Huntington disease, Amyotrophic lateral sclerosis, and Parkinson diseases. We also discuss how to use the important pathogenic insights from large animal models to make rodent models more capable of recapitulating important pathological features of neurodegenerative diseases.

A novel and accurate full-length HTT mouse model for Huntington's disease

Here, we report the generation and characterization of a novel Huntington's disease (HD) mouse model BAC226Q by using a bacterial artificial chromosome (BAC) system, expressing full-length human HTT with ~226 CAG-CAA repeats and containing endogenous human HTT promoter and regulatory elements. BAC226Q recapitulated a full-spectrum of age-dependent and progressive HD-like phenotypes without unwanted and erroneous phenotypes. BAC226Q mice developed normally, and gradually exhibited HD-like psychiatric and cognitive phenotypes at 2 months. From 3 to 4 months, BAC226Q mice showed robust progressive motor deficits. At 11 months, BAC226Q mice showed significant reduced life span, gradual weight loss and exhibited neuropathology including significant brain atrophy specific to striatum and cortex, striatal neuronal death, widespread huntingtin inclusions, and reactive pathology. Therefore, the novel BAC226Q mouse accurately recapitulating robust, age-dependent, progressive HD-like phenotypes will be a valuable tool for studying disease mechanisms, identifying biomarkers, and testing gene-targeting therapeutic approaches for HD.

Genetically modified large animal models for investigating neurodegenerative diseases

Neurodegenerative diseases represent a large group of neurological disorders including Alzheimer’s disease, amyotrophic lateral sclerosis, Parkinson’s disease, and Huntington’s disease. Although this group of diseases show heterogeneous clinical and pathological phenotypes, they share important pathological features characterized by the age-dependent and progressive degeneration of nerve cells that is caused by the accumulation of misfolded proteins. The association of genetic mutations with neurodegeneration diseases has enabled the establishment of various types of animal models that mimic genetic defects and have provided important insights into the pathogenesis. However, most of genetically modified rodent models lack the overt and selective neurodegeneration seen in the patient brains, making it difficult to use the small animal models to validate the effective treatment on neurodegeneration. Recent studies of pig and monkey models suggest that large animals can more faithfully recapitulate pathological features of neurodegenerative diseases. In this review, we discuss the important differences in animal models for modeling pathological features of neurodegenerative diseases, aiming to assist the use of animal models to better understand the pathogenesis and to develop effective therapeutic strategies.

C57BL/6 Background Attenuates mHTT Toxicity in the Striatum of YAC128 Mice

Mouse models are frequently used to study Huntington’s disease (HD). The onset and severity of neuronal and behavioral pathologies vary greatly between HD mouse models, which results from different huntingtin expression levels and different CAG repeat length. HD pathology appears to depend also on the strain background of mouse models. Thus, behavioral deficits of HD mice are more severe in the FVB than in the C57BL/6 background. Alterations in medium spiny neuron (MSN) morphology and function have been well documented in young YAC128 mice in the FVB background. Here, we tested the relevance of strain background for mutant huntingtin (mHTT) toxicity on the cellular level by investigating HD pathologies in YAC128 mice in the C57BL/6 background (YAC128/BL6). Morphology, spine density, synapse function and membrane properties were not or only subtly altered in MSNs of 12-month-old YAC128/BL6 mice. Despite the mild cellular phenotype, YAC128/BL6 mice showed deficits in motor performance. More pronounced alterations in MSN function were found in the HdhQ150 mouse model in the C57BL/6 background (HdhQ150/BL6). Consistent with the differences in HD pathology, the number of inclusion bodies was considerably lower in YAC128/BL6 mice than HdhQ150/BL6 mice. This study highlights the relevance of strain background for mHTT toxicity in HD mouse models.

Mitochondrial Abnormalities and Synaptic Damage in Huntington's Disease: a Focus on Defective Mitophagy and Mitochondria-Targeted Therapeutics

Huntington's disease (HD) is a fatal and pure genetic disease with a progressive loss of medium spiny neurons (MSN). HD is caused by expanded polyglutamine repeats in the exon 1 of HD gene. Clinically, HD is characterized by chorea, seizures, involuntary movements, dystonia, cognitive decline, intellectual impairment, and emotional disturbances. Several years of intense research revealed that multiple cellular changes, including defective axonal transport, protein-protein interactions, defective bioenergetics, calcium dyshomeostasis, NMDAR activation, synaptic damage, mitochondrial abnormalities, and selective loss of medium spiny neurons are implicated in HD. Recent research on mutant huntingtin (mHtt) and mitochondria has found that mHtt interacts with the mitochondrial division protein, dynamin-related protein 1 (DRP1), enhances GTPase DRP1 enzymatic activity, and causes excessive mitochondrial fragmentation and abnormal distribution, leading to defective axonal transport of mitochondria and selective synaptic degeneration. Recent research also revealed that failure to remove dead and/or dying mitochondria is an early event in the disease progression. Currently, efforts are being made to reduce abnormal protein interactions and enhance synaptic mitophagy as therapeutic strategies for HD. The purpose of this article is to discuss recent research in HD progression. This article also discusses recent developments of cell and mouse models, cellular changes, mitochondrial abnormalities, DNA damage, bioenergetics, oxidative stress, mitophagy, and therapeutics strategies in HD.

Large Animal Models of Huntington's Disease: What We Have Learned and Where We Need to Go Next

Genetically modified rodent models of Huntington’s disease (HD) have been especially valuable to our understanding of HD pathology and the mechanisms by which the mutant HTT gene alters physiology. However, due to inherent differences in genetics, neuroanatomy, neurocircuitry and neurophysiology, animal models do not always faithfully or fully recapitulate human disease features or adequately predict a clinical response to treatment. Therefore, conducting translational studies of candidate HD therapeutics only in a single species (i.e. mouse disease models) may not be sufficient. Large animal models of HD have been shown to be valuable to the HD research community and the expectation is that the need for translational studies that span rodent and large animal models will grow. Here, we review the large animal models of HD that have been created to date, with specific commentary on differences between the models, the strengths and disadvantages of each, and how we can advance useful models to study disease pathophysiology, biomarker development and evaluation of promising therapeutics.

Lack of association of somatic CAG repeat expansion with striatal neurodegeneration in HD knock-in animal models

Our previous work has established a huntingtin knock-in (KI) pig model that displays striatal neuronal loss, allowing us to examine if somatic CAG expansion in striatum accounts for the preferential neurodegeneration in Huntington disease (HD). We found that HD KI pigs do not display somatic CAG expansion in striatum as HD KI mice and that the majority of polyQ repeats in exon 1 HTT in the striatum of HD KI mice are fairly stable. We also found that striatal MSH2 and MLH3, which are involved in DNA repair, are more abundant in mouse brains than pig brains. Consistently inhibiting MSH2 and MLH3 reduced the somatic CAG expansion in HD KI mouse striatum with no influence on neuropathology. Our findings suggest that somatic CAG expansion is species-dependent, occurs in a small fraction of the HD gene in mice, and does not critically contribute to HD neuropathology.

Hypothalamic pathology in Huntington disease

Huntington's disease (HD), an autosomal dominant hereditary disorder associated with the accumulation of mutant huntingtin, is classically associated with cognitive decline and motor symptoms, notably chorea. However, growing evidence suggests that nonmotor symptoms are equally prevalent and debilitating. Some of these symptoms may be linked to hypothalamic pathology, demonstrated by findings in HD animal models and HD patients showing specific changes in hypothalamic neuropeptidergic populations and their associated functions. At least some of these alterations are likely due to local mutant huntingtin expression and toxicity, while others are likely caused by disturbed hypothalamic circuitry. Common problems include circadian rhythm disorders, including desynchronization of daily hormone excretion patterns, which could be targeted by novel therapeutic interventions, such as timed circadian interventions with light therapy or melatonin. However, translation of these findings from bench-to-bedside is hampered by differences in murine HD models and HD patients, including mutant huntingtin trinucleotide repeat length, which is highly heterogeneous across the various models. In this chapter, we summarize the current knowledge regarding hypothalamic alterations in HD patients and animal models, and the potential for these findings to be translated into clinical practice and management.

What is the Pathogenic CAG Expansion Length in Huntington's Disease?

Huntington's disease (HD) (OMIM 143100) is caused by an expanded CAG repeat tract in the HTT gene. The inherited CAG length is known to expand further in somatic and germline cells in HD subjects. Age at onset of the disease is inversely correlated with the inherited CAG length, but is further modulated by a series of genetic modifiers which are most likely to act on the CAG repeat in HTT that permit it to further expand. Longer repeats are more prone to expansions, and this expansion is age dependent and tissue-specific. Given that the inherited tract expands through life and most subjects develop disease in mid-life, this implies that in cells that degenerate, the CAG length is likely to be longer than the inherited length. These findings suggest two thresholds- the inherited CAG length which permits further expansion, and the intracellular pathogenic threshold, above which cells become dysfunctional and die. This two-step mechanism has been previously proposed and modelled mathematically to give an intracellular pathogenic threshold at a tract length of 115 CAG (95% confidence intervals 70- 165 CAG). Empirically, the intracellular pathogenic threshold is difficult to determine. Clues from studies of people and models of HD, and from other diseases caused by expanded repeat tracts, place this threshold between 60- 100 CAG, most likely towards the upper part of that range. We assess this evidence and discuss how the intracellular pathogenic threshold in manifest disease might be better determined. Knowing the cellular pathogenic threshold would be informative for both understanding the mechanism in HD and deploying treatments.

New developments in Huntington's disease and other triplet repeat diseases: DNA repair turns to the dark side

Huntington’s disease (HD) is a fatal, inherited neurodegenerative disease that causes neuronal death, particularly in medium spiny neurons. HD leads to serious and progressive motor, cognitive and psychiatric symptoms. Its genetic basis is an expansion of the CAG triplet repeat in the HTT gene, leading to extra glutamines in the huntingtin protein. HD is one of nine genetic diseases in this polyglutamine (polyQ) category, that also includes a number of inherited spinocerebellar ataxias (SCAs). Traditionally it has been assumed that HD age of onset and disease progression were solely the outcome of age-dependent exposure of neurons to toxic effects of the inherited mutant huntingtin protein. However, recent genome-wide association studies (GWAS) have revealed significant effects of genetic variants outside of HTT. Surprisingly, these variants turn out to be mostly in genes encoding DNA repair factors, suggesting that at least some disease modulation occurs at the level of the HTT DNA itself. These DNA repair proteins are known from model systems to promote ongoing somatic CAG repeat expansions in tissues affected by HD. Thus, for triplet repeats, some DNA repair proteins seem to abandon their normal genoprotective roles and, instead, drive expansions and accelerate disease. One attractive hypothesis—still to be proven rigorously—is that somatic HTT expansions augment the disease burden of the inherited allele. If so, therapeutic approaches that lower levels of huntingtin protein may need blending with additional therapies that reduce levels of somatic CAG repeat expansions to achieve maximal effect.

Ellagic acid rescues motor and cognitive deficits in the R6/2 mouse model of Huntington's disease by lowering mutant huntingtin protein

Huntington's disease (HD) is a genetic neurodegenerative disorder caused by a highly polymorphic CAG trinucleotide repeat expansion encoding an extended polyglutamine (polyQ) tract at the N-terminus of huntingtin protein (HTT). The polyQ tract promotes the formation of toxic oligomers and aggregates of HTT, which leads to neuronal dysfunction and death. Therapies to lower mutant HTT (mHTT) and its aggregates appear to be the most promising strategies. Ellagic acid (EA) has been marketed as a dietary supplement with various claimed benefits and neuroprotective effects on several neurodegenerative disorders, while its effect on mHTT pathology is still unknown. Here we reported that EA significantly attenuated motor and cognitive deficits in R6/2 mice. Moreover, EA significantly lowered mHTT levels, reduced neuroinflammation, rescued synapse loss, and decreased oxidative stress in R6/2 mouse brains. These findings indicated that EA has promising therapeutic potential for HD treatment.

Diffusion Tensor Imaging in Preclinical and Human Studies of Huntington's Disease: What Have we Learned so Far?

BACKGROUND

Huntington's Disease is an irreversible neurodegenerative disease characterized by the progressive deterioration of specific brain nerve cells. The current evaluation of cellular and physiological events in patients with HD relies on the development of transgenic animal models. To explore such events in vivo, diffusion tensor imaging has been developed to examine the early macro and microstructural changes in brain tissue. However, the gap in diffusion tensor imaging findings between animal models and clinical studies and the lack of microstructural confirmation by histological methods has questioned the validity of this method.

OBJECTIVE

This review explores white and grey matter ultrastructural changes associated to diffusion tensor imaging, as well as similarities and differences between preclinical and clinical Huntington's Disease studies.

METHODS

A comprehensive review of the literature using online-resources was performed (Pub- Med search).

RESULTS

Similar changes in fractional anisotropy as well as axial, radial and mean diffusivities were observed in white matter tracts across clinical and animal studies. However, comparative diffusion alterations in different grey matter structures were inconsistent between clinical and animal studies.

CONCLUSION

Diffusion tensor imaging can be related to specific structural anomalies in specific cellular populations. However, some differences between animal and clinical studies could derive from the contrasting neuroanatomy or connectivity across species. Such differences should be considered before generalizing preclinical results into the clinical practice. Moreover, current limitations of this technique to accurately represent complex multicellular events at the single micro scale are real. Future work applying complex diffusion models should be considered.

Functional genomics, genetic risk profiling and cell phenotypes in neurodegenerative disease

Human genetics provides unbiased insights into the causes of human disease, which can be used to create a foundation for effective ways to more accurately diagnose patients, stratify patients for more successful clinical trials, discover and develop new therapies, and ultimately help patients choose the safest and most promising therapeutic option based on their risk profile. But the process for translating basic observations from human genetics studies into pathogenic disease mechanisms and treatments is laborious and complex, and this challenge has particularly slowed the development of interventions for neurodegenerative disease. In this review, we discuss the many steps in the process, the important considerations at each stage, and some of the latest tools and technologies that are available to help investigators translate insights from human genetics into diagnostic and therapeutic strategies that will lead to the sort of advances in clinical care that make a difference for patients.

Reactive Species in Huntington Disease: Are They Really the Radicals You Want to Catch?

Huntington disease (HD) is a neurodegenerative condition and one of the so-called rare or minority diseases, due to its low prevalence (affecting 1–10 of every 100,000 people in western countries). The causative gene, HTT, encodes huntingtin, a protein with a yet unknown function. Mutant huntingtin causes a range of phenotypes, including oxidative stress and the activation of microglia and astrocytes, which leads to chronic inflammation of the brain. Although substantial efforts have been made to find a cure for HD, there is currently no medical intervention able to stop or even delay progression of the disease. Among the many targets of therapeutic intervention, oxidative stress and inflammation have been extensively studied and some clinical trials have been promoted to target them. In the present work, we review the basic research on oxidative stress in HD and the strategies used to fight it. Many of the strategies to reduce the phenotypes associated with oxidative stress have produced positive results, yet no substantial functional recovery has been observed in animal models or patients with the disease. We discuss possible explanations for this and suggest potential ways to overcome it.

Nemo-like kinase reduces mutant huntingtin levels and mitigates Huntington's disease

Nemo-like kinase (NLK), an evolutionarily conserved serine/threonine kinase, is highly expressed in the brain, but its function in the adult brain remains not well understood. In this study, we identify NLK as an interactor of huntingtin protein (HTT). We report that NLK levels are significantly decreased in HD human brain and HD models. Importantly, overexpression of NLK in the striatum attenuates brain atrophy, preserves striatal DARPP32 levels and reduces mutant HTT (mHTT) aggregation in HD mice. In contrast, genetic reduction of NLK exacerbates brain atrophy and loss of DARPP32 in HD mice. Moreover, we demonstrate that NLK lowers mHTT levels in a kinase activity-dependent manner, while having no significant effect on normal HTT protein levels in mouse striatal cells, human cells and HD mouse models. The NLK-mediated lowering of mHTT is associated with enhanced phosphorylation of mHTT. Phosphorylation defective mutation of serine at amino acid 120 (S120) abolishes the mHTT-lowering effect of NLK, suggesting that S120 phosphorylation is an important step in the NLK-mediated lowering of mHTT. A further mechanistic study suggests that NLK promotes mHTT ubiquitination and degradation via the proteasome pathway. Taken together, our results indicate a protective role of NLK in HD and reveal a new molecular target to reduce mHTT levels.

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.

CD200 is up-regulated in R6/1 transgenic mouse model of Huntington's disease

In Huntington's disease (HD), striatal medium spiny neurons (MSNs) are particularly sensitive to the presence of a CAG repeat in the huntingtin (HTT) gene. However, there are many evidences that cells from the peripheral immune system and central nervous system (CNS) immune cells, namely microglia, play an important role in the etiology and the progression of HD. However, it remains unclear whether MSNs neurodegeneration is mediated by a non-cell autonomous mechanism. The homeostasis in the healthy CNS is maintained by several mechanisms of interaction between all brain cells. Neurons can control microglia activation through several inhibitory mechanisms, such as the CD200-CD200R1 interaction. Due to the complete lack of knowledge about the CD200-CD200R1 system in HD, we determined the temporal patterns of CD200 and CD200R1 expression in the neocortex, hippocampus and striatum in the HD mouse models R6/1 and HdhQ111/7 from pre-symptomatic to manifest stages. In order to explore any alteration in the peripheral immune system, we also studied the levels of expression of CD200 and CD200R1 in whole blood. Although CD200R1 expression was not altered, we observed and increase in CD200 gene expression and protein levels in the brain parenchyma of all the regions we examined, along with HD pathogenesis in R6/1 mice. Interestingly, the expression of CD200 mRNA was also up-regulated in blood following a similar temporal pattern. These results suggest that canonical neuronal-microglial communication through CD200-CD200R1 interaction is not compromised, and CD200 up-regulation in R6/1 brain parenchyma could represent a neurotrophic signal to sustain or extend neuronal function in the latest stages of HD as pro-survival mechanism.

Genetic deletion of S6k1 does not rescue the phenotypic deficits observed in the R6/2 mouse model of Huntington's disease

Huntington’s disease (HD) is a fatal inherited autosomal dominant neurodegenerative disorder caused by an expansion in the number of CAG trinucleotide repeats in the huntingtin gene. The disease is characterized by motor, behavioural and cognitive symptoms for which at present there are no disease altering treatments. It has been shown that manipulating the mTOR (mammalian target of rapamycin) pathway using rapamycin or its analogue CCI-779 can improve the cellular and behavioural phenotypes of HD models. Ribosomal protein S6 kinase 1 (S6K1) is a major downstream signalling molecule of mTOR, and its activity is reduced by rapamycin suggesting that deregulation of S6K1 activity may be beneficial in HD. Furthermore, S6k1 knockout mice have increased lifespan and improvement in age-related phenotypes. To evalute the potential benefit of S6k1 loss on HD-related phenotypes, we crossed the R6/2 HD model with the long-lived S6k1 knockout mouse line. We found that S6k1 knockout does not ameliorate behavioural or physiological phenotypes in the R6/2 mouse model. Additionally, no improvements were seen in brain mass reduction or mutant huntingtin protein aggregate levels. Therefore, these results suggest that while a reduction in S6K1 signalling has beneficial effects on ageing it is unlikely to be a therapeutic strategy for HD patients.

Differential regulation of Kidins220 isoforms in Huntington's disease

Huntington's disease (HD) is an inherited progressive neurodegenerative disease characterized by brain atrophy particularly in the striatum that produces motor impairment, and cognitive and psychiatric disturbances. Multiple pathogenic mechanisms have been proposed including dysfunctions in neurotrophic support and calpain-overactivation, among others. Kinase D-interacting substrate of 220 kDa (Kidins220), also known as ankyrin repeat-rich membrane spanning (ARMS), is an essential mediator of neurotrophin signaling. In adult brain, Kidins220 presents two main isoforms that differ in their carboxy-terminal length and critical protein-protein interaction domains. These variants are generated through alternative terminal exon splicing of the conventional exon 32 (Kidins220-C32) and the recently identified exon 33 (Kidins220-C33). The lack of domains encoded by exon 32 involved in key neuronal functions, including those controlling neurotrophin pathways, pointed to Kidins220-C33 as a form detrimental for neurons. However, the functional role of Kidins220-C33 in neurodegeneration or other pathologies, including HD, has not been explored. In the present work, we discover an unexpected selective downregulation of Kidins220-C33, in the striatum of HD patients, as well as in the R6/1 HD mouse model starting at early symptomatic stages. These changes are C33-specific as Kidins220-C32 variant remains unchanged. We also find the early decrease in Kidins220-C33 levels takes place in neurons, suggesting an unanticipated neuroprotective role for this isoform. Finally, using ex vivo assays and primary neurons, we demonstrate that Kidins220-C33 is downregulated by mechanisms that depend on the activation of the protease calpain. Altogether, these results strongly suggest that calpain-mediated Kidins220-C33 proteolysis modulates onset and/or progression of HD.

Emerging links between cell competition and Alzheimer's disease

Alzheimer's disease (AD) causes a progressive loss of memory and other cognitive functions, which inexorably debilitates patients. There is still no cure for AD and effective treatments to delay or revert AD are urgently needed. On a molecular level, the excessive accumulation of amyloid-β (Aβ) peptides triggers a complex cascade of pathological events underlying neuronal death, whose details are not yet completely understood. Our laboratory recently discovered that cell competition may play a protective role against AD by eliminating less fit neurons from the brain of Aβ-transgenic flies. Loss of Aβ-damaged neurons through fitness comparison with healthy counterparts is beneficial for the organism, delaying cognitive decline and motor disability. In this Review, we introduce the molecular mechanisms of cell competition, including seminal works on the field and latest advances regarding genetic triggers and effectors of cell elimination. We then describe the biological relevance of competition in the nervous system and discuss how competitive interactions between neurons may arise and be exacerbated in the context of AD. Selection of neurons through fitness comparison is a promising, but still emerging, research field that may open new avenues for the treatment of neurological disorders.

IKKβ slows Huntington's disease progression in R6/1 mice

Neuroinflammation is an important contributor to neuronal pathology and death in neurodegenerative diseases and neuronal injury. Therapeutic interventions blocking the activity of the inflammatory kinase IKKβ, a key regulator of neuroinflammatory pathways, is protective in several animal models of neurodegenerative disease and neuronal injury. In Huntington's disease (HD), however, significant questions exist as to the impact of blocking or diminishing the activity of IKKβ on HD pathology given its potential role in Huntingtin (HTT) degradation. In cell culture, IKKβ phosphorylates HTT serine (S) 13 and activates HTT degradation, a process that becomes impaired with polyQ expansion. To investigate the in vivo relationship of IKKβ to HTT S13 phosphorylation and HD progression, we crossed conditional tamoxifen-inducible IKKβ knockout mice with R6/1 HD mice. Behavioral assays in these mice showed a significant worsening of HD pathological phenotypes. The increased behavioral pathology correlated with reduced levels of endogenous mouse full-length phospho-S13 HTT, supporting the importance of IKKβ in the phosphorylation of HTT S13 in vivo. Notably, many striatal autophagy genes were up-regulated in HD vs. control mice; however, IKKβ knockout partially reduced this up-regulation in HD, increased striatal neurodegeneration, and enhanced an activated microglial response. We propose that IKKβ is protective in striatal neurons early in HD progression via phosphorylation of HTT S13. As IKKβ is also required for up-regulation of some autophagy genes and HTT is a scaffold for selective autophagy, IKKβ may influence autophagy through multiple mechanisms to maintain healthy striatal function, thereby reducing neuronal degeneration to slow HD onset.

Longitudinal characterization of cognitive and motor deficits in an excitotoxic lesion model of striatal dysfunction in non-human primates

As research progresses in the understanding of the molecular and cellular mechanisms underlying neurodegenerative diseases like Huntington's disease (HD) and expands towards preclinical work for the development of new therapies, highly relevant animal models are increasingly needed to test new hypotheses and to validate new therapeutic approaches. In this light, we characterized an excitotoxic lesion model of striatal dysfunction in non-human primates (NHPs) using cognitive and motor behaviour assessment as well as functional imaging and post-mortem anatomical analyses. NHPs received intra-striatal stereotaxic injections of quinolinic acid bilaterally in the caudate nucleus and unilaterally in the left sensorimotor putamen. Post-operative MRI scans showed atrophy of the caudate nucleus and a large ventricular enlargement in all 6 NHPs that correlated with post-mortem measurements. Behavioral analysis showed deficits in 2 analogues of the Wisconsin card sorting test (perseverative behavior) and in an executive task, while no deficits were observed in a visual recognition or an episodic memory task at 6 months following surgery. Spontaneous locomotor activity was decreased after lesion and the incidence of apomorphine-induced dyskinesias was significantly increased at 3 and 6 months following lesion. Positron emission tomography scans obtained at end-point showed a major deficit in glucose metabolism and D2 receptor density limited to the lesioned striatum of all NHPs compared to controls. Post-mortem analyses revealed a significant loss of medium-sized spiny neurons in the striatum, a loss of neurons and fibers in the globus pallidus, a unilateral decrease in dopaminergic neurons of the substantia nigra and a loss of neurons in the motor and dorsolateral prefrontal cortex. Overall, we show that this robust NHP model presents specific behavioral (learning, execution and retention of cognitive tests) and metabolic functional deficits that, to the best of our knowledge, are currently not mimicked in any available large animal model of striatal dysfunction. Moreover, we used non-invasive, translational techniques like behavior and imaging to quantify such deficits and found that they correlate to a significant cell loss in the striatum and its main input and output structures. This model can thus significantly contribute to the pre-clinical longitudinal evaluation of the ability of new therapeutic cell, gene or pharmacotherapy approaches in restoring the functionality of the striatal circuitry.

Brain structure in juvenile-onset Huntington disease

OBJECTIVE

To assess brain morphometry in a sample of patients with juvenile-onset Huntington disease (JOHD) and several mouse models of Huntington disease (HD) that likely represent the human JOHD phenotype.

METHODS

Despite sharing the mutation in the Huntingtin gene, adult-onset HD characteristically presents as a hyperkinetic motor disorder, while JOHD typically presents as a hypokinetic motor disease. The University of Iowa Kids-JHD program enrolls individuals 5 to 25 years of age who have already received the clinical diagnosis. A total of 19 children with juvenile HD (JHD) (mean CAG = 72) were studied. Patients with JHD were compared to healthy controls (n = 234) using a cross-sectional study design. Volumetric data from structural MRI was compared between groups. In addition, we used the same procedure to evaluate brain morphology of R6/2, zQ175, HdhQ250 HD mice models.

RESULTS

Participants with JHD had substantially reduced intracranial volumes. After controlling for the small intracranial volume size, the volumes of subcortical regions (caudate, putamen, globus pallidus, and thalamus) and of cortical white matter were significantly decreased in patients with JHD. However, the cerebellum was proportionately enlarged in the JHD sample. The cerebral cortex was largely unaffected. Likewise, HD mice had a lower volume of striatum and a higher volume of cerebellum, mirroring the human MRI results.

CONCLUSIONS

The primary pathology of JOHD extends beyond changes in the striatal volume. Brain morphology in both mice and human patients with JHD shows proportional cerebellar enlargement. This pattern of brain changes may explain the unique picture of hypokinetic motor symptoms in JHD, which is not seen in the hyperkinetic chorea-like phenotype of adult-onset HD.

Overview of Huntington's Disease Models: Neuropathological, Molecular, and Behavioral Differences

Transgenic mouse models of Huntington's disease (HD), a neurodegenerative condition caused by a single gene mutation, have been transformative in their ability to reveal the molecular processes and pathophysiological mechanisms underlying the HD behavioral phenotype. Three model categories have been generated depending on the genetic context in which the mutation is expressed: truncated, full-length, and knock-in. No single model, however, broadly replicates the behavioral symptoms and massive neuronal loss that occur in human patients. The disparity between model and patient requires careful consideration of what each model has to offer when testing potential treatments. Although the translation of animal data to the clinic has been limited, each model can make unique contributions toward an improved understanding of the neurobehavioral underpinnings of HD. Thus, conclusions based on data obtained from more than one model are likely to have the most success in the search for new treatment targets. © 2018 by John Wiley & Sons, Inc.

Use of large animal models to investigate Huntington's diseases

Animal models that can mimic human diseases are the important tools for investigating the pathogenesis of the diseases and finding a way for treatment. There is no doubt that small animal models have provided a wealth of information regarding disease pathogenesis and also offered widely used tools to develop therapeutic strategies. Rodent models have been very valuable for investigators to understand the mechanisms underlying misfolded protein-mediated neuronal dysfunction and behavioral phenotypes in a variety of neurodegenerative diseases including Alzheimer's, Parkinson's, and Huntington's diseases (HD). However, most of genetically modified rodent models of these diseases lack the overt and selective neurodegeneration seen in the patient brains. Since large animals are more similar to humans than small animals and rodents, the large animal models are likely to mimic important neuropathological features in humans. Here we discuss the application of large animal models in neurodegenerative disease research with focus on the HD large animal models, aiming to provide insight into the application of animal models to study neurodegenerative diseases.

Metformin reverses early cortical network dysfunction and behavior changes in Huntington's disease

Catching primal functional changes in early, ‘very far from disease onset’ (VFDO) stages of Huntington’s disease is likely to be the key to a successful therapy. Focusing on VFDO stages, we assessed neuronal microcircuits in premanifest Hdh150 knock-in mice. Employing in vivo two-photon Ca²⁺ imaging, we revealed an early pattern of circuit dysregulation in the visual cortex - one of the first regions affected in premanifest Huntington’s disease - characterized by an increase in activity, an enhanced synchronicity and hyperactive neurons. These findings are accompanied by aberrations in animal behavior. We furthermore show that the antidiabetic drug metformin diminishes aberrant Huntingtin protein load and fully restores both early network activity patterns and behavioral aberrations. This network-centered approach reveals a critical window of vulnerability far before clinical manifestation and establishes metformin as a promising candidate for a chronic therapy starting early in premanifest Huntington’s disease pathogenesis long before the onset of clinical symptoms.

Huntington’s disease is a devastating brain disorder that causes severe mood disorders, problems with moving, and dementia. Most people develop the condition between their thirties and fifties, and die a decade or two after the symptoms first appear.

The disease emerges because of a mutation in the gene for the Huntingtin protein, which leads to neurons slowly dying in the brain. While genetic testing can reveal who carries the faulty gene, no treatment addresses the root of the disorder or prevents it from appearing. Instead, most therapies for Huntington’s disease aim to reduce brain damage once the telltale symptoms are already present. However, the disease-causing protein is expressed early during the life of a patient, which could give it time to damage the brain long before neurons die and the disorder reveals itself. Treatments that start after the first signs of the disease may be too late to reverse the damage. Detecting and preventing early brain changes in people that carry the mutation may thus help to stop the disease from progressing.

Here, Arnoux, Willam, Griesche et al. set out to detect the minute changes that the faulty Huntingtin protein may cause in the brain network of young mice with the mutation. State-of-the-art imaging tools helped to examine individual neurons in the brain area that processes visual information. These experiments revealed that a group of brain cells had become hyperactive; once this change had occurred, the mutant animals were less anxious than is typical for mice.

Metformin is a drug used to treat diabetes, but it also interferes with a structure that is required to produce the disease-causing Huntingtin protein. Arnoux et al. therefore explored whether the compound could rescue the early brain alterations observed in mutant mice. Adding metformin in the water of the animals for three weeks halted the production of the mutant protein, reversed the brain changes and stopped the abnormal behavior.

Further work is now required in humans to confirm that Huntington’s disease starts with a change in the activity of networks in the brain, and to verify that metformin can stop the disorder in its track.

Adeno-Associated Viral Vector Serotype DJ-Mediated Overexpression of N171-82Q-Mutant Huntingtin in the Striatum of Juvenile Mice Is a New Model for Huntington's Disease

Huntington's disease (HD) is an autosomal-dominant inherited neurodegenerative disorder characterized by motor, psychiatric and cognitive symptoms. HD is caused by an expansion of CAG repeats in the huntingtin (HTT) gene in various areas of the brain including striatum. There are few suitable animal models to study the pathogenesis of HD and validate therapeutic strategies. Recombinant adeno-associated viral (AAV) vectors successfully transfer foreign genes to the brain of adult mammalians. In this article, we report a novel mouse model of HD generated by bilateral intrastriatal injection of AAV vector serotype DJ (AAV-DJ) containing N171-82Q mutant HTT (82Q) and N171-18Q wild type HTT (18Q; sham). The AAV-DJ-82Q model displayed motor dysfunctions in pole and rotarod tests beginning 4 weeks after viral infection in juvenile mice (8 weeks after birth). They showed behaviors reflecting neurodegeneration. They also showed increased apoptosis, robust glial activation and upregulated representative inflammatory cytokines (tumor necrosis factor-alpha (TNF-α) and interleukin (IL)-6), mediators (cyclooxygenase-2 and inducible nitric oxide synthase) and signaling pathways (nuclear factor kappa B and signal transducer and activator of transcription 3 (STAT3)) in the striatum at 10 weeks after viral infection (14 weeks after birth) via successful transfection of mutant HTT into neurons, microglia, and astrocytes in the striatum. However, little evidence of any of these events was found in mice infected with the AAV-DJ-18Q expressing construct. Intrastriatal injection of AAV-DJ-82Q might be useful as a novel in vivo model to investigate the biology of truncated N-terminal fragment (N171) in the striatum and to explore the efficacy of therapeutic strategies for HD.

Intravenous immunoglobulin ameliorates motor and cognitive deficits and neuropathology in R6/2 mouse model of Huntington's disease by decreasing mutant huntingtin protein level and normalizing NF-κB signaling pathway

Huntington's disease (HD) is a fatal neurodegenerative disorder characterized by progressive movement disorders and cognitive deficits, which is caused by a CAG-repeat expansion encoding an extended polyglutamine (polyQ) tract in the huntingtin protein (HTT). Reduction of mutant HTT levels and inhibition of neuroinflammation has been proposed as a major therapeutic strategy in treating HD. Intravenous immunoglobulin (IVIg) therapy has been firmly established for the treatment of several autoimmune or inflammatory neurological diseases, either as adjunctive treatment or as first-line therapy. However, whether IVIg has therapeutic potential on HD remains unclear. Here we for the first time demonstrated that IVIg treatment remarkably rescued motor and cognitive deficits, prevented synaptic degeneration, attenuated neuroinflammation and oxidative stress in R6/2 mouse model. Further investigation showed that the beneficial effects of IVIg resulted from the reduced levels of mutant HTT and inhibition of NF-κB signalling pathway. These findings suggest that IVIg is a promising therapeutic potential for HD.

Cannabidiol as a Promising Strategy to Treat and Prevent Movement Disorders?

Movement disorders such as Parkinson's disease and dyskinesia are highly debilitating conditions linked to oxidative stress and neurodegeneration. When available, the pharmacological therapies for these disorders are still mainly symptomatic, do not benefit all patients and induce severe side effects. Cannabidiol is a non-psychotomimetic compound from Cannabis sativa that presents antipsychotic, anxiolytic, anti-inflammatory, and neuroprotective effects. Although the studies that investigate the effects of this compound on movement disorders are surprisingly few, cannabidiol emerges as a promising compound to treat and/or prevent them. Here, we review these clinical and pre-clinical studies and draw attention to the potential of cannabidiol in this field.

Autophagy induction by trehalose: Molecular mechanisms and therapeutic impacts

The balance between synthesis and degradation is crucial to maintain cellular homeostasis and different mechanisms are known to keep this balance. In this review, we will provide a short overview on autophagy as an intracellular homeostatic degradative machinery. We will also describe the involvement of downregulation of autophagy in numerous diseases including neurodegenerative diseases, cancer, aging, metabolic disorders, and other infectious diseases. Therefore, modulation of autophagic processes can represent a promising way of intervention in different diseases including neurodegeneration and cancer. Trehalose, also known as mycose, is a natural disaccharide found extensively but not abundantly among several organisms. It is described that trehalose can work as an important autophagy modulator and can be proficiently used in the control several diseases in which autophagy plays an important role. On these bases, we describe here the role of trehalose as an innovative drug in the treatment of neurodegenerative diseases and other illnesses opening a new scenario of intervention in conditions difficult to be treated.

Brain pericyte activation occurs early in Huntington's disease

Microvascular changes have recently been described for several neurodegenerative disorders, including Huntington's disease (HD). HD is characterized by a progressive neuronal cell loss due to a mutation in the Huntingtin gene. However, the temporal and spatial microvascular alterations in HD remain unclear. Also, knowledge on the implication of pericytes in HD pathology is still sparse and existing findings are contradictory. Here we examine alterations in brain pericytes in the R6/2 mouse model of HD and in human post mortem HD brain sections. To specifically track activated pericytes, we crossbred R6/2 mice with transgenic mice expressing the Green fluorescent protein gene under the Regulator of G-protein signaling 5 (Rgs5) promoter. We demonstrate an increase in activated pericytes in the R6/2 brain and in post mortem HD brain tissue. Importantly, pericyte changes are already detected before striatal neuronal cell loss, weight loss or behavioural deficits occur in R6/2 mice. This is associated with vascular alterations, whereby striatal changes precede cortical changes. Our findings suggest that pericyte activation may be one of the initial steps contributing to the observed vascular modifications in HD. Thus, pericytes may constitute an important target to address early microvascular changes contributing to disease progression in HD.

Longitudinal Anthropometric Assessment of Rhesus Macaque (Macaca mulatta) Model of Huntington Disease

The neurodegeneration associated with Huntington disease (HD) leads to the onset of motor and cognitive impairment and their advancement with increased age in humans. In children at risk for HD, body measurement growth abnormalities include a reduction in BMI, weight, height, and head circumference. The transgenic HD NHP model was first reported in 2008, and progressive decline in cognitive behaviors and motor impairment have been reported. This study focuses on longitudinal body measurements in HD macaques from infancy through adulthood. The growth of HD macaques was assessed through head circumference, sagittal and transverse head, and crown-to-rump ('height') measurements and BMI. The animals were measured monthly from 0 to 72 mo of age and every 3 mo from 72 mo of age onward. A mixed-effect model was used to assess subject-specific effects in our nonlinear serial data. Compared with WT controls, HD macaques displayed different developmental trajectories characterized by increased BMI, head circumference, and sagittal head measurements beginning around 40 mo of age. The physiologic comparability between NHP and humans underscores the translational utility of our HD macaques to evaluate growth and developmental patterns associated with HD.