The use of the R6 transgenic mouse models of Huntington's disease in attempts to develop novel therapeutic strategies

NMDA receptor-mediated Ca2+ signaling: Impact on cell cycle regulation and the development of neurodegenerative diseases and cancer

NMDA receptors are Ca2+-permeable ligand-gated ion channels that mediate fast excitatory transmission in the central nervous system. NMDA receptors regulate the proliferation and differentiation of neural progenitor cells and also play critical roles in neural plasticity, memory, and learning. In addition to their physiological role, NMDA receptors are also involved in glutamate-mediated excitotoxicity, which results from excessive glutamate stimulation, leading to Ca2+ overload, and ultimately to neuronal death. Thus, NMDA receptor-mediated excitotoxicity has been linked to several neurodegenerative diseases such as Alzheimer's, Parkinson's, Huntington's, dementia, and stroke. Interestingly, in addition to its effects on cell death, aberrant expression or activation of NMDA receptors is also involved in pathological cellular proliferation, and is implicated in the invasion and proliferation of various types of cancer. These disorders are thought to be related to the contribution of NMDA receptors to cell proliferation and cell death through cell cycle modulation. This review aims to discuss the evidence implicating NMDA receptor activity in cell cycle regulation and the link between aberrant NMDA receptor activity and the development of neurodegenerative diseases and cancer due to cell cycle dysregulation. The information presented here will provide insights into the signaling pathways and the contribution of NMDA receptors to these diseases, and suggests that NMDA receptors are promising targets for the prevention and treatment of these diseases, which are leading causes of death and disability worldwide.

Brain-Periphery Interactions in Huntington's Disease: Mediators and Lifestyle Interventions

Prominent pathological features of Huntington's disease (HD) are aggregations of mutated Huntingtin protein (mHtt) in the brain and neurodegeneration, which causes characteristic motor (such as chorea and dystonia) and non-motor symptoms. However, the numerous systemic and peripheral deficits in HD have gained increasing attention recently, since those factors likely modulate disease progression, including brain pathology. While whole-body metabolic abnormalities and organ-specific pathologies in HD have been relatively well described, the potential mediators of compromised inter-organ communication in HD have been insufficiently characterized. Therefore, we applied an exploratory literature search to identify such mediators. Unsurprisingly, dysregulation of inflammatory factors, circulating mHtt, and many other messenger molecules (hormones, lipids, RNAs) were found that suggest impaired inter-organ communication, including of the gut-brain and muscle-brain axis. Based on these findings, we aimed to assess the risks and potentials of lifestyle interventions that are thought to improve communication across these axes: dietary strategies and exercise. We conclude that appropriate lifestyle interventions have great potential to reduce symptoms and potentially modify disease progression (possibly via improving inter-organ signaling) in HD. However, impaired systemic metabolism and peripheral symptoms warrant particular care in the design of dietary and exercise programs for people with HD.

Intravenous MSC-Treatment Improves Impaired Brain Functions in the R6/2 Mouse Model of Huntington's Disease via Recovered Hepatic Pathological Changes

Huntington's disease (HD), a congenital neurodegenerative disorder, extends its pathological damages beyond the nervous system. The systematic manifestation of HD has been extensively described in numerous studies, including dysfunction in peripheral organs and peripheral inflammation. Gut dysbiosis and the gut-liver-brain axis have garnered greater emphasis in neurodegenerative research, and increased plasma levels of pro-inflammatory cytokines have been identified in HD patients and various in vivo models, correlating with disease progression. In the present study, we investigated hepatic pathological markers in the liver of R6/2 mice which convey exon 1 of the human mutant huntingtin gene. Furthermore, we evaluated the impact of intravenously administered Mesenchymal Stromal Cells (MSCs) on the liver enzymes, changes in hepatic inflammatory markers, as well as brain pathology and behavioral deficits in R6/2 mice. Our results revealed altered enzyme expression and increased levels of inflammatory mediators in the liver of R6/2 mice, which were significantly attenuated in the MSC-treated R6/2 mice. Remarkably, neuronal pathology and altered motor activities in the MSC-treated R6/2 mice were significantly ameliorated, despite the absence of MSCs in the postmortem brain. Our data highlight the importance of hepatic pathological changes in HD, providing a potential therapeutic approach. Moreover, the data open new perspectives for the search in blood biomarkers correlating with liver pathology in HD.

Mitochondrial Dysfunction: A Key Player in Brain Aging and Diseases

Mitochondria are thought to have become incorporated within the eukaryotic cell approximately 2 billion years ago and play a role in a variety of cellular processes, such as energy production, calcium buffering and homeostasis, steroid synthesis, cell growth, and apoptosis, as well as inflammation and ROS production. Considering that mitochondria are involved in a multitude of cellular processes, mitochondrial dysfunction has been shown to play a role within several age-related diseases, including cancers, diabetes (type 2), and neurodegenerative diseases, although the underlying mechanisms are not entirely understood. The significant increase in lifespan and increased incidence of age-related diseases over recent decades has confirmed the necessity to understand the mechanisms by which mitochondrial dysfunction impacts the process of aging and age-related diseases. In this review, we will offer a brief overview of mitochondria, along with structure and function of this important organelle. We will then discuss the cause and consequence of mitochondrial dysfunction in the aging process, with a particular focus on its role in inflammation, cognitive decline, and neurodegenerative diseases, such as Huntington's disease, Parkinson's disease, and Alzheimer's disease. We will offer insight into therapies and interventions currently used to preserve or restore mitochondrial functioning during aging and neurodegeneration.

Brain transcriptomic, metabolic and mitohormesis properties associated with N-propargylglycine treatment: A prevention strategy against neurodegeneration

INTRODUCTION

There is an urgent need for new or repurposed therapeutics that protect against or significantly delay the clinical progression of neurodegenerative diseases, such as Huntington's disease (HD), Parkinson's disease and Alzheimer's disease. In particular, preclinical studies are needed for well tolerated and brain-penetrating small molecules capable of mitigating the proteotoxic mitochondrial processes that are hallmarks of these diseases. We identified a unique suicide inhibitor of mitochondrial proline dehydrogenase (Prodh), N-propargylglycine (N-PPG), which has anticancer and brain-enhancing mitohormesis properties, and we hypothesize that induction of mitohormesis by N-PPG protects against neurodegenerative diseases. We carried out a series of mouse studies designed to: i) compare brain and metabolic responses while on oral N-PPG treatment (50 mg/kg, 9-14 days) of B6CBA wildtype (WT) and short-lived transgenic R6/2 (HD) mice; and ii) evaluate potential brain and systemwide stress rebound responses in WT mice 2 months after cessation of extended mitohormesis induction by well-tolerated higher doses of N-PPG (100-200 mg/kg x 60 days). WT and HD mice showed comparable global evidence of N-PPG induced brain mitohormesis characterized by Prodh protein decay and increased mitochondrial expression of chaperone and Yme1l1 protease proteins. Interestingly, transcriptional analysis (RNAseq) showed partial normalization of HD whole brain transcriptomes toward those of WT mice. Comprehensive metabolomic profiles performed on control and N-PPG treated blood, brain, and kidney samples revealed expected N-PPG-induced tissue increases in proline levels in both WT and HD mice, accompanied by surprising parallel increases in hydroxyproline and sarcosine. Two months after cessation of the higher dose N-PPG stress treatments, WT mouse brains showed robust rebound increases in Prodh protein levels and mitochondrial transcriptome responses, as well as altered profiles of blood amino acid-related metabolites. Our HD and WT mouse preclinical findings point to the brain penetrating and mitohormesis-inducing potential of the drug candidate, N-PPG, and provide new rationale and application insights supporting its further preclinical testing in various models of neurodegenerative diseases characterized by loss of mitochondrial proteostasis.

Neurogenesis in primates versus rodents and the value of non-human primate models

Neurogenesis, the process of generating neurons from neural stem cells, occurs during both embryonic and adult stages, with each stage possessing distinct characteristics. Dysfunction in either stage can disrupt normal neural development, impair cognitive functions, and lead to various neurological disorders. Recent technological advancements in single-cell multiomics and gene-editing have facilitated investigations into primate neurogenesis. Here, we provide a comprehensive overview of neurogenesis across rodents, non-human primates, and humans, covering embryonic development to adulthood and focusing on the conservation and diversity among species. While non-human primates, especially monkeys, serve as valuable models with closer neural resemblance to humans, we highlight the potential impacts and limitations of non-human primate models on both physiological and pathological neurogenesis research.

Stress response mechanisms in protein misfolding diseases: Profiling a cellular model of Huntington's disease

Stress response pathways like the integrated stress response (ISR), the mitochondrial unfolded protein response (UPRmt) and the heat shock response (HSR) have emerged as part of the pathophysiology of neurodegenerative diseases, including Huntington's disease (HD) - a currently incurable disease caused by the production of mutant huntingtin (mut-Htt). Previous data from HD patients suggest that ISR is activated while UPRmt and HSR are impaired in HD. The study of these stress response pathways as potential therapeutic targets in HD requires cellular models that mimic the activation status found in HD patients of such pathways. PC12 cells with inducible expression of the N-terminal fragment of mut-Htt are among the most used cell lines to model HD, however the activation of stress responses remains unclear in this model. The goal of this study is to characterize the activation of ISR, UPRmt and HSR in this HD cell model and evaluate if it mimics the activation status found in HD patients. We show that PC12 HD cell model presents reduced levels of Hsp90 and mitochondrial chaperones, suggesting an impaired activation or function of HSR and UPRmt. This HD model also presents increased levels of phosphorylated eIF2α, the master regulator of the ISR, but overall similar levels of ATF4 and decreased levels of CHOP - transcription factors downstream to eIF2α - in comparison to control, suggesting an initial activation of ISR. These results show that this model mimics the ISR activation and the impaired UPRmt and HSR found in HD patients. This work suggests that the PC12 N-terminal HD model is suitable for studying the role of stress response pathways in the pathophysiology of HD and for exploratory studies investigating the therapeutic potential of drugs targeting stress responses.

An enquiry to the role of CB1 receptors in neurodegeneration

Neurodegenerative disorders are debilitating conditions that impair patient quality of life and that represent heavy social-economic burdens to society. Whereas the root of some of these brain illnesses lies in autosomal inheritance, the origin of most of these neuropathologies is scantly understood. Similarly, the cellular and molecular substrates explaining the progressive loss of brain functions remains to be fully described too. Indeed, the study of brain neurodegeneration has resulted in a complex picture, composed of a myriad of altered processes that include broken brain bioenergetics, widespread neuroinflammation and aberrant activity of signaling pathways. In this context, several lines of research have shown that the endocannabinoid system (ECS) and its main signaling hub, the type-1 cannabinoid (CB1) receptor are altered in diverse neurodegenerative disorders. However, some of these data are conflictive or poorly described. In this review, we summarize the findings about the alterations in ECS and CB1 receptors signaling in three representative brain illnesses, the Alzheimer's, Parkinson's and Huntington's diseases, and we discuss the relevance of these studies in understanding neurodegeneration development and progression, with a special focus on astrocyte function. Noteworthy, the analysis of ECS defects in neurodegeneration warrant much more studies, as our conceptual understanding of ECS function has evolved quickly in the last years, which now include glia cells and the subcellular-specific CB1 receptors signaling as critical players of brain functions.

Key Features in the Design and Function of Nanocarriers for Intranasal Administration of Gene Therapy in Huntington Disease

A major obstacle to fulfilling the therapeutic promise of gene therapies for hereditary brain diseases, such as Huntington' Disease (HD), is the requirement for viral vectors and/or an invasive delivery system (stereotaxic injection into brain or infusion into the intrathecal space). HD is an autosomal dominant neurodegenerative disease for which several clinical trials have demonstrated gene-lowering effects following intrathecal administration. These technical limitations have given impetus to the development of alternative non-invasive delivery systems for gene therapy of brain diseases. The overall objective of this review is to discuss the key features in the design of nanocarriers for intranasal administration of gene-therapy for HD, focusing primarily on our series of published work on the use of nanocarriers for gene therapy. Design and development of nanocarriers packaged with gene-lowering agents represents a significant advance towards non-invasive nose-to-brain delivery of gene therapy for HD and other hereditary brain disorders.

The Emerging Landscape of Small-Molecule Therapeutics for the Treatment of Huntington's Disease

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a CAG repeat expansion in the huntingtin gene (HTT). The new insights into HD's cellular and molecular pathways have led to the identification of numerous potent small-molecule therapeutics for HD therapy. The field of HD-targeting small-molecule therapeutics is accelerating, and the approval of these therapeutics to combat HD may be expected in the near future. For instance, preclinical candidates such as naphthyridine-azaquinolone, AN1, AN2, CHDI-00484077, PRE084, EVP4593, and LOC14 have shown promise for further optimization to enter into HD clinical trials. This perspective aims to summarize the advent of small-molecule therapeutics at various stages of clinical development for HD therapy, emphasizing their structure and design, therapeutic effects, and specific mechanisms of action. Further, we have highlighted the key drivers involved in HD pathogenesis to provide insights into the basic principle for designing promising anti-HD therapeutic leads.

Nrf2 Pathway in Huntington's Disease (HD): What Is Its Role?

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease that occurs worldwide. Despite some progress in understanding the onset of HD, drugs that block or delay symptoms are still not available. In recent years, many treatments have been proposed; among them, nuclear transcriptional factor-2 (Nrf2) enhancer compounds have been proposed as potential therapeutic agents to treat HD. Nrf2 triggers an endogenous antioxidant pathway activated in different neurodegenerative disorders. Probably, the stimulation of Nrf2 during either the early phase or before HD symptoms' onset, could slow or prevent striatum degeneration. In this review, we present the scientific literature supporting the role of Nrf2 in HD and the potential prophylactic and therapeutic role of this compound.

A Novel Huntington's Disease Assessment Platform to Support Future Drug Discovery and Development

Huntington's disease (HD) is a lethal neurodegenerative disorder without efficient therapeutic options. The inefficient translation from preclinical and clinical research into clinical use is mainly attributed to the lack of (i) understanding of disease initiation, progression, and involved molecular mechanisms; (ii) knowledge of the possible HD target space and general data awareness; (iii) detailed characterizations of available disease models; (iv) better suitable models; and (v) reliable and sensitive biomarkers. To generate robust HD-like symptoms in a mouse model, the neomycin resistance cassette was excised from zQ175 mice, generating a new line: zQ175^Δneo. We entirely describe the dynamics of behavioral, neuropathological, and immunohistological changes from 15-57 weeks of age. Specifically, zQ175^Δneo mice showed early astrogliosis from 15 weeks; growth retardation, body weight loss, and anxiety-like behaviors from 29 weeks; motor deficits and reduced muscular strength from 36 weeks; and finally slight microgliosis at 57 weeks of age. Additionally, we collected the entire bioactivity network of small-molecule HD modulators in a multitarget dataset (HD_MDS). Hereby, we uncovered 358 unique compounds addressing over 80 different pharmacological targets and pathways. Our data will support future drug discovery approaches and may serve as useful assessment platform for drug discovery and development against HD.

Evidences for Mutant Huntingtin Inducing Musculoskeletal and Brain Growth Impairments via Disturbing Testosterone Biosynthesis in Male Huntington Disease Animals

Body weight (BW) loss and reduced body mass index (BMI) are the most common peripheral alterations in Huntington disease (HD) and have been found in HD mutation carriers and HD animal models before the manifestation of neurological symptoms. This suggests that, at least in the early disease stage, these changes could be due to abnormal tissue growth rather than tissue atrophy. Moreover, BW and BMI are reported to be more affected in males than females in HD animal models and patients. Here, we confirmed sex-dependent growth alterations in the BACHD rat model for HD and investigated the associated contributing factors. Our results showed growth abnormalities along with decreased plasma testosterone and insulin-like growth factor 1 (IGF-1) levels only in males. Moreover, we demonstrated correlations between growth parameters, IGF-1, and testosterone. Our analyses further revealed an aberrant transcription of testosterone biosynthesis-related genes in the testes of BACHD rats with undisturbed luteinizing hormone (LH)/cAMP/PKA signaling, which plays a key role in regulating the transcription process of some of these genes. In line with the findings in BACHD rats, analyses in the R6/2 mouse model of HD showed similar results. Our findings support the view that mutant huntingtin may induce abnormal growth in males via the dysregulation of gene transcription in the testis, which in turn can affect testosterone biosynthesis.

Calcium imaging: A versatile tool to examine Huntington's disease mechanisms and progression

Huntington's disease (HD) is a fatal, hereditary neurodegenerative disorder that causes chorea, cognitive deficits, and psychiatric symptoms. It is characterized by accumulation of mutant Htt protein, which primarily impacts striatal medium-sized spiny neurons (MSNs), as well as cortical pyramidal neurons (CPNs), causing synapse loss and eventually cell death. Perturbed Ca²⁺ homeostasis is believed to play a major role in HD, as altered Ca²⁺ homeostasis often precedes striatal dysfunction and manifestation of HD symptoms. In addition, dysregulation of Ca²⁺ can cause morphological and functional changes in MSNs and CPNs. Therefore, Ca²⁺ imaging techniques have the potential of visualizing changes in Ca²⁺ dynamics and neuronal activity in HD animal models. This minireview focuses on studies using diverse Ca²⁺ imaging techniques, including two-photon microscopy, fiber photometry, and miniscopes, in combination of Ca²⁺ indicators to monitor activity of neurons in HD models as the disease progresses. We then discuss the future applications of Ca²⁺ imaging to visualize disease mechanisms and alterations associated with HD, as well as studies showing how, as a proof-of-concept, Ca²⁺imaging using miniscopes in freely-behaving animals can help elucidate the differential role of direct and indirect pathway MSNs in HD symptoms.

Effects of endocannabinoid system modulation on social behaviour: A systematic review of animal studies

There is a clear link between psychiatric disorders and social behaviour, and evidence suggests the involvement of the endocannabinoid system (ECS). A systematic review of preclinical literature was conducted using MEDLINE (PubMed) and PsychINFO databases to examine whether pharmacological and/or genetic manipulations of the ECS alter social behaviours in wildtype (WT) animals or models of social impairment (SIM). Eighty studies were included. Risk of bias (RoB) was assessed using SYRCLE's RoB tool. While some variability was evident, studies most consistently found that direct cannabinoid receptor (CBR) agonism decreased social behaviours in WT animals, while indirect CBR activation via enzyme inhibition or gene-knockout increased social behaviours. Direct and, more consistently, indirect CBR activation reversed social deficits in SIM. These CBR-mediated effects were often sex- and developmental-phase-dependent and blocked by CBR antagonism. Overall, ECS enzyme inhibition may improve social behaviour in SIM, suggesting the potential usefulness of ECS enzyme inhibition as a therapeutic approach for social deficits. Future research should endeavour to elucidate ECS status in neuropsychiatric disorders characterized by social deficits.

Current and Possible Future Therapeutic Options for Huntington’s Disease

Huntington’s disease (HD) is an autosomal neurodegenerative disease that is characterized by an excessive number of CAG trinucleotide repeats within the huntingtin gene ( HTT). HD patients can present with a variety of symptoms including chorea, behavioural and psychiatric abnormalities and cognitive decline. Each patient has a unique combination of symptoms, and although these can be managed using a range of medications and non-drug treatments there is currently no cure for the disease. Current therapies prescribed for HD can be categorized by the symptom they treat. These categories include chorea medication, antipsychotic medication, antidepressants, mood stabilizing medication as well as non-drug therapies. Fortunately, there are also many new HD therapeutics currently undergoing clinical trials that target the disease at its origin; lowering the levels of mutant huntingtin protein (mHTT). Currently, much attention is being directed to antisense oligonucleotide (ASO) therapies, which bind to pre-RNA or mRNA and can alter protein expression via RNA degradation, blocking translation or splice modulation. Other potential therapies in clinical development include RNA interference (RNAi) therapies, RNA targeting small molecule therapies, stem cell therapies, antibody therapies, non-RNA targeting small molecule therapies and neuroinflammation targeted therapies. Potential therapies in pre-clinical development include Zinc-Finger Protein (ZFP) therapies, transcription activator-like effector nuclease (TALEN) therapies and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated system (Cas) therapies. This comprehensive review aims to discuss the efficacy of current HD treatments and explore the clinical trial progress of emerging potential HD therapeutics.

Exploring multifunctional antioxidants as potential agents for management of neurological disorders

Free radical or oxidative stress may be a fundamental mechanism underlying several human neurologic diseases. Therapy using free radical scavengers (antioxidants) has the potential to prevent, delay, or ameliorate many neurologic disorders. However, the biochemistry of oxidative pathobiology is complex, and optimum antioxidant therapeutic options may vary and need to be tailored to individual diseases. In vitro and animal model studies support the potential beneficial role of various antioxidant compounds in neurological disease. Antioxidants generally play an important role in reducing or preventing the cell damage and other changes which occur in the cells like mitochondrial dysfunction, DNA mutations, and lipid peroxidation in the cell membrane. Based on their mechanism of action, antioxidants can be used to treat various neurological disorders like Huntington's disease, Alzheimer's disease, and Parkinson's disease. Vitamin E has a scavenging action for reactive oxygen species (ROS) and also prevents the lipid peroxidation. Creatine generally reduces the mitochondrial dysfunction in Parkinson's disease (PD) patients. Various metal chelators are used in PD for the prevention of accumulation of the metals. Superoxidase dismutase (SOD), lipases, and proteases act as repair enzymes in patients with AD. Accordingly, the antioxidant defense system is found to be most useful for treating various neurological disorders.

Gene-environment-gut interactions in Huntington's disease mice are associated with environmental modulation of the gut microbiome

Gut dysbiosis in Huntington's disease (HD) has recently been reported using microbiome profiling in R6/1 HD mice and replicated in clinical HD. In HD mice, environmental enrichment (EE) and exercise (EX) were shown to have therapeutic impacts on the brain and associated symptoms. We hypothesize that these housing interventions modulate the gut microbiome, configuring one of the mechanisms that mediate their therapeutic effects observed in HD. We exposed R6/1 mice to a protocol of either EE or EX, relative to standard-housed control conditions, before the onset of gut dysbiosis and motor deficits. We characterized gut structure and function, as well as gut microbiome profiling using 16S rRNA sequencing. Multivariate analysis identified specific orders, namely Bacteroidales, Lachnospirales and Oscillospirales, as the main bacterial signatures that discriminate between housing conditions. Our findings suggest a promising role for the gut microbiome in mediating the effects of EE and EX exposures, and possibly other environmental interventions, in HD mice.

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Gastrointestinal structure and motility are intact at an early stage in a HD mouse model

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There is sexual dimorphism in the presentation of the HD gut dysbiosis phenotype

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Bacteroidales, Lachnospirales and Oscillospirales bacteria are affected by experience

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Environmental enrichment and exercise may modulate HD via the microbiota-gut-brain axis

Targeted gene silencing in the nervous system with CRISPR-Cas13

Cas13 nucleases are a class of programmable RNA-targeting CRISPR effector proteins that are capable of silencing target gene expression in mammalian cells. Here, we demonstrate that RfxCas13d, a Cas13 ortholog with favorable characteristics to other family members, can be delivered to the mouse spinal cord and brain to silence neurodegeneration-associated genes. Intrathecally delivering an adeno-associated virus vector encoding an RfxCas13d variant programmed to target superoxide dismutase 1 (SOD1), a protein whose mutation can cause amyotrophic lateral sclerosis, reduced SOD1 mRNA and protein in the spinal cord by >50% and improved outcomes in a mouse model of the disorder. We further show that intrastriatally delivering an RfxCas13d variant programmed to target huntingtin (HTT), a protein whose mutation is causative for Huntington’s disease, led to a ~50% reduction in HTT protein in the mouse brain. Our results establish RfxCas13d as a versatile platform for knocking down gene expression in the nervous system.

Oxidative distress in aging and age-related diseases: Spatiotemporal dysregulation of protein oxidation and degradation

The concept of oxidative distress had arisen from the assessment of cellular response to high concentrations of reactive species that result from an imbalance between oxidants and antioxidants and cause biomolecular damage. The intracellular distribution and flux of reactive species dramatically change in time and space contributing to the remodeling of the redox landscape and sensitivity of protein residues to oxidants. Here, we hypothesize that compromised spatiotemporal control of generation, conversions, and removal of reactive species underlies protein damage and dysfunction of protein degradation machineries. This leads to the accumulation of oxidatively damaged proteins resulted in an age-dependent decline in the organismal adaptability to oxidative stress. We highlight recent data obtained with the use of various cell cultures, animal models, and patients on irreversible and non-repairable oxidation of key redox-sensitive residues. Multiple reaction products include peptidyl hydroperoxides, alcohols, carbonyls, and carbamoyl moieties as well as Tyr-Tyr, Trp-Tyr, Trp-Trp, Tyr-Cys, His-Lys, His-Arg, and Tyr-Lys cross-links. These lead to protein fragmentation, misfolding, covalent cross-linking, oligomerization, aggregation, and ultimately, causing impaired protein function and turnover. 20S proteasome and autophagy-lysosome pathways are two major types of machinery for the degradation and elimination of oxidatively damaged proteins. Spatiotemporal dysregulation of these pathways under oxidative distress conditions is implicated in aging and age-related disorders such as neurodegenerative and cardiovascular diseases and diabetes. Future investigations in this field allow the discovery of new drugs to target components of dysregulated cell signaling and protein degradation machinery to combat aging and age-related chronic diseases.

Myelinosome Organelles in the Retina of R6/1 Huntington Disease (HD) Mice: Ubiquitous Distribution and Possible Role in Disease Spreading

Visual deficit is one of the complications of Huntington disease (HD), a fatal neurological disorder caused by CAG trinucleotide expansions in the Huntingtin gene, leading to the production of mutant Huntingtin (mHTT) protein. Transgenic HD R6/1 mice expressing human HTT exon1 with 115 CAG repeats recapitulate major features of the human pathology and exhibit a degeneration of the retina. Our aim was to gain insight into the ultrastructure of the pathological HD R6/1 retina by electron microscopy (EM). We show that the HD R6/1 retina is enriched with unusual organelles myelinosomes, produced by retinal neurons and glia. Myelinosomes are present in all nuclear and plexiform layers, in the synaptic terminals of photoreceptors, in the processes of retinal neurons and glial cells, and in the subretinal space. In vitro study shows that myelinosomes secreted by human retinal glial Müller MIO-M1 cells transfected with EGFP-mHTT-exon1 carry EGFP-mHTT-exon1 protein, as revealed by immuno-EM and Western-blotting. Myelinosomes loaded with mHTT-exon1 are incorporated by naive neuronal/neuroblastoma SH-SY5Y cells. This results in the emergence of mHTT-exon1 in recipient cells. This process is blocked by membrane fusion inhibitor MDL 28170. Conclusion: Incorporation of myelinosomes carrying mHTT-exon1 in recipient cells may contribute to HD spreading in the retina. Exploring ocular fluids for myelinosome presence could bring an additional biomarker for HD diagnostics.

Huntington's disease mouse models: unraveling the pathology caused by CAG repeat expansion

Huntington's disease (HD) is a neurodegenerative disease that results in motor and cognitive dysfunction, leading to early death. HD is caused by an expansion of CAG repeats in the huntingtin gene (HTT). Here, we review the mouse models of HD. They have been used extensively to better understand the molecular and cellular basis of disease pathogenesis as well as to provide non-human subjects to test the efficacy of potential therapeutics. The first and best-studied in vivo rodent model of HD is the R6/2 mouse, in which a transgene containing the promoter and exon 1 fragment of human HTT with 150 CAG repeats was inserted into the mouse genome. R6/2 mice express rapid, robust behavioral pathologies and display a number of degenerative abnormalities in neuronal populations most vulnerable in HD. The first conditional full-length mutant huntingtin (mHTT) mouse model of HD was the bacterial artificial chromosome (BAC) transgenic mouse model of HD (BACHD), which expresses human full-length mHTT with a mixture of 97 CAG-CAA repeats under the control of endogenous HTT regulatory machinery. It has been useful in identifying the role of mHTT in specific neuronal populations in degenerative processes. In the knock-in (KI) model of HD, the expanded human CAG repeats and human exon 1 are inserted into the mouse Htt locus, so a chimera of the full-length mouse protein with the N-terminal human portion is expressed. Many of aspects of the pathology and behavioral deficits in the KI model better mimic disease characteristics found in HD patients than other models. Accordingly, some have proposed that these mice may be preferable models of the disease over others. Indeed, as our understanding of HD advances, so will the design of animal models to test and develop HD therapies.

Altered microRNA expression in animal models of Huntington's disease and potential therapeutic strategies

A review of recent animal models of Huntington's disease showed many microRNAs had altered expression levels in the striatum and cerebral cortex, and which were mostly downregulated. Among the altered microRNAs were miR-9/9*, miR-29b, miR-124a, miR-132, miR-128, miR-139, miR-122, miR-138, miR-23b, miR-135b, miR-181 (all downregulated) and miR-448 (upregulated), and similar changes had been previously found in Huntington's disease patients. In the animal cell studies, the altered microRNAs included miR-9, miR-9*, miR-135b, miR-222 (all downregulated) and miR-214 (upregulated). In the animal models, overexpression of miR-155 and miR-196a caused a decrease in mutant huntingtin mRNA and protein level, lowered the mutant huntingtin aggregates in striatum and cortex, and improved performance in behavioral tests. Improved performance in behavioral tests also occurred with overexpression of miR-132 and miR-124. In the animal cell models, overexpression of miR-22 increased the viability of rat primary cortical and striatal neurons infected with mutant huntingtin and decreased huntingtin -enriched foci of ≥ 2 µm. Also, overexpression of miR-22 enhanced the survival of rat primary striatal neurons treated with 3-nitropropionic acid. Exogenous expression of miR-214, miR-146a, miR-150, and miR-125b decreased endogenous expression of huntingtin mRNA and protein in Hdh^Q111/Hdh^Q111 cells. Further studies with animal models of Huntington's disease are warranted to validate these findings and identify specific microRNAs whose overexpression inhibits the production of mutant huntingtin protein and other harmful processes and may provide a more effective means of treating Huntington's disease in patients and slowing its progression.

From Neurodevelopmental to Neurodegenerative Disorders: The Vascular Continuum

Structural and functional integrity of the cerebral vasculature ensures proper brain development and function, as well as healthy aging. The inability of the brain to store energy makes it exceptionally dependent on an adequate supply of oxygen and nutrients from the blood stream for matching colossal demands of neural and glial cells. Key vascular features including a dense vasculature, a tightly controlled environment, and the regulation of cerebral blood flow (CBF) all take part in brain health throughout life. As such, healthy brain development and aging are both ensured by the anatomical and functional interaction between the vascular and nervous systems that are established during brain development and maintained throughout the lifespan. During critical periods of brain development, vascular networks remodel until they can actively respond to increases in neural activity through neurovascular coupling, which makes the brain particularly vulnerable to neurovascular alterations. The brain vasculature has been strongly associated with the onset and/or progression of conditions associated with aging, and more recently with neurodevelopmental disorders. Our understanding of cerebrovascular contributions to neurological disorders is rapidly evolving, and increasing evidence shows that deficits in angiogenesis, CBF and the blood-brain barrier (BBB) are causally linked to cognitive impairment. Moreover, it is of utmost curiosity that although neurodevelopmental and neurodegenerative disorders express different clinical features at different stages of life, they share similar vascular abnormalities. In this review, we present an overview of vascular dysfunctions associated with neurodevelopmental (autism spectrum disorders, schizophrenia, Down Syndrome) and neurodegenerative (multiple sclerosis, Huntington's, Parkinson's, and Alzheimer's diseases) disorders, with a focus on impairments in angiogenesis, CBF and the BBB. Finally, we discuss the impact of early vascular impairments on the expression of neurodegenerative diseases.

Preliminary examination of early neuroconnectivity features in the R6/1 mouse model of Huntington's disease by ultra-high field diffusion MRI

During the last decades, advances in the understanding of genetic, cellular, and microstructural alterations associated to Huntington's disease (HD) have improved the understanding of this progressive and fatal illness. However, events related to early neuropathological events, neuroinflammation, deterioration of neuronal connectivity and compensatory mechanisms still remain vastly unknown. Ultra-high field diffusion MRI (UHFD-MRI) techniques can contribute to a more comprehensive analysis of the early microstructural changes observed in HD. In addition, it is possible to evaluate if early imaging microstructural parameters might be linked to histological biomarkers. Moreover, qualitative studies analyzing histological complexity in brain areas susceptible to neurodegeneration could provide information on inflammatory events, compensatory increase of neuroconnectivity and mechanisms of brain repair and regeneration. The application of ultra-high field diffusion-MRI technology in animal models, particularly the R6/1 mice (a common preclinical mammalian model of HD), provide the opportunity to analyze alterations in a physiologically intact model of the disease. Although some disparities in volumetric changes across different brain structures between preclinical and clinical models has been documented, further application of different diffusion MRI techniques used in combination like diffusion tensor imaging, and neurite orientation dispersion and density imaging have proved effective in characterizing early parameters associated to alteration in water diffusion exchange within intracellular and extracellular compartments in brain white and grey matter. Thus, the combination of diffusion MRI imaging techniques and more complex neuropathological analysis could accelerate the discovery of new imaging biomarkers and the early diagnosis and neuromonitoring of patients affected with HD.

The Role of Vitamins in Neurodegenerative Disease: An Update

Acquiring the recommended daily allowance of vitamins is crucial for maintaining homeostatic balance in humans and other animals. A deficiency in or dysregulation of vitamins adversely affects the neuronal metabolism, which may lead to neurodegenerative diseases. In this article, we discuss how novel vitamin-based approaches aid in attenuating abnormal neuronal functioning in neurodegeneration-based brain diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease, Amyotrophic lateral sclerosis, and Prion disease. Vitamins show their therapeutic activity in Parkinson's disease by antioxidative and anti-inflammatory activity. In addition, different water- and lipid-soluble vitamins have also prevented amyloid beta and tau pathology. On the other hand, some results also show no correlation between vitamin action and the prevention of neurodegenerative diseases. Some vitamins also exhibit toxic activity too. This review discusses both the beneficial and null effects of vitamin supplementation for neurological disorders. The detailed mechanism of action of both water- and lipid-soluble vitamins is addressed in the manuscript. Hormesis is also an essential factor that is very helpful to determine the effective dose of vitamins. PubMed, Google Scholar, Web of Science, and Scopus were employed to conduct the literature search of original articles, review articles, and meta-analyses.

Oxidative Stress and Neurodegeneration: Interconnected Processes in PolyQ Diseases

Neurodegenerative polyglutamine (polyQ) disorders are caused by trinucleotide repeat expansions within the coding region of disease-causing genes. PolyQ-expanded proteins undergo conformational changes leading to the formation of protein inclusions which are associated with selective neuronal degeneration. Several lines of evidence indicate that these mutant proteins are associated with oxidative stress, proteasome impairment and microglia activation. These events may correlate with the induction of inflammation in the nervous system and disease progression. Here, we review the effect of polyQ-induced oxidative stress in cellular and animal models of polyQ diseases. Furthermore, we discuss the interplay between oxidative stress, neurodegeneration and neuroinflammation using as an example the well-known neuroinflammatory disease, Multiple Sclerosis. Finally, we review some of the pharmaceutical interventions which may delay the onset and progression of polyQ disorders by targeting disease-associated mechanisms.

Pharmacological characterization of mutant huntingtin aggregate-directed PET imaging tracer candidates

Huntington’s disease (HD) is caused by a CAG trinucleotide repeat expansion in the first exon of the huntingtin (HTT) gene coding for the huntingtin (HTT) protein. The misfolding and consequential aggregation of CAG-expanded mutant HTT (mHTT) underpin HD pathology. Our interest in the life cycle of HTT led us to consider the development of high-affinity small-molecule binders of HTT oligomerized/amyloid-containing species that could serve as either cellular and in vivo imaging tools or potential therapeutic agents. We recently reported the development of PET tracers CHDI-180 and CHDI-626 as suitable for imaging mHTT aggregates, and here we present an in-depth pharmacological investigation of their binding characteristics. We have implemented an array of in vitro and ex vivo radiometric binding assays using recombinant HTT, brain homogenate-derived HTT aggregates, and brain sections from mouse HD models and humans post-mortem to investigate binding affinities and selectivity against other pathological proteins from indications such as Alzheimer’s disease and spinocerebellar ataxia 1. Radioligand binding assays and autoradiography studies using brain homogenates and tissue sections from HD mouse models showed that CHDI-180 and CHDI-626 specifically bind mHTT aggregates that accumulate with age and disease progression. Finally, we characterized CHDI-180 and CHDI-626 regarding their off-target selectivity and binding affinity to beta amyloid plaques in brain sections and homogenates from Alzheimer’s disease patients.

Age-Dependent Degradation of Locomotion Encoding in Huntington's Disease R6/2 Model Mice

BACKGROUND

Huntington's disease (HD) is an inherited fatal neurodegenerative disease, leading to neocortical and striatal atrophy. The commonly studied R6/2 HD transgenic mouse model displays progressive motor and cognitive deficits in parallel to major pathological changes in corticostriatal circuitry.

OBJECTIVE

To study how disease progression influences striatal encoding of movement.

METHODS

We chronically recorded neuronal activity in the dorsal striatum of R6/2 transgenic (Tg) mice and their age-matched nontransgenic littermate controls (WTs) during novel environment exposure, a paradigm which engages locomotion to explore the novel environment.

RESULTS

Exploratory locomotion degraded with age in Tg mice as compared to WTs. We encountered fewer putative medium spiny neurons (MSNs)-striatal projection neurons, and more inhibitory interneurons-putative fast spiking interneurons (FSIs) in Tg mice as compared to WTs. MSNs from Tg mice fired less spikes in bursts without changing their firing rate, while FSIs from these mice had a lower firing rate and more of them were task-responsive as compared to WTs. Additionally, MSNs from Tg mice displayed a reduced ability to encode locomotion across age groups, likely associated with their low prevalence in Tg mice, whereas the encoding of locomotion by FSIs from Tg mice was substantially reduced solely in old Tg mice as compared to WTs.

CONCLUSION

Our findings reveal an age-dependent decay in striatal information processing in transgenic mice. We propose that the ability of FSIs to compensate for the loss of MSNs by processes of recruitment and enhanced task-responsiveness diminishes with disease progression, possibly manifested in the displayed age-dependent degradation of exploratory locomotion.

New Avenues for the Treatment of Huntington's Disease

Huntington’s disease (HD) is a neurodegenerative disorder caused by a CAG expansion in the HD gene. The disease is characterized by neurodegeneration, particularly in the striatum and cortex. The first symptoms usually appear in mid-life and include cognitive deficits and motor disturbances that progress over time. Despite being a genetic disorder with a known cause, several mechanisms are thought to contribute to neurodegeneration in HD, and numerous pre-clinical and clinical studies have been conducted and are currently underway to test the efficacy of therapeutic approaches targeting some of these mechanisms with varying degrees of success. Although current clinical trials may lead to the identification or refinement of treatments that are likely to improve the quality of life of those living with HD, major efforts continue to be invested at the pre-clinical level, with numerous studies testing novel approaches that show promise as disease-modifying strategies. This review offers a detailed overview of the currently approved treatment options for HD and the clinical trials for this neurodegenerative disorder that are underway and concludes by discussing potential disease-modifying treatments that have shown promise in pre-clinical studies, including increasing neurotropic support, modulating autophagy, epigenetic and genetic manipulations, and the use of nanocarriers and stem cells.

Transcriptional Dysregulation in Huntington's Disease: The Role in Pathogenesis and Potency for Pharmacological Targeting

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by a mutation in the gene that encodes a critical cell regulatory protein, huntingtin (Htt). The expansion of cytosine-adenine-guanine (CAG) trinucleotide repeats causes improper folding of functional proteins and is an initial trigger of pathological changes in the brain. Recent research has indicated that the functional dysregulation of many transcription factors underlies the neurodegenerative processes that accompany HD. These disturbances are caused not only by the loss of wild-type Htt (WT Htt) function but also by the occurrence of abnormalities that result from the action of mutant Htt (mHtt). In this review, we aim to describe the role of transcription factors that are currently thought to be strongly associated with HD pathogenesis, namely, RE1-silencing transcription factor, also known as neuron-restrictive silencer factor (REST/NRSF), forkhead box proteins (FOXPs), peroxisome proliferator-activated receptor gamma coactivator-1a (PGC1α), heat shock transcription factor 1 (HSF1), and nuclear factor κ light-chain-enhancer of activated B cells (NF- κB). We also take into account the role of these factors in the phenotype of HD as well as potential pharmacological interventions targeting the analyzed proteins. Furthermore, we considered whether molecular manipulation resulting in changes in transcription factor function may have clinical potency for treating HD.

Evaluation of early microstructural changes in the R6/1 mouse model of Huntington's disease by ultra-high field diffusion MR imaging

Diffusion MRI (dMRI) has been able to detect early structural changes related to neurological symptoms present in Huntington's disease (HD). However, there is still a knowledge gap to interpret the biological significance at early neuropathological stages. The purpose of this study is two-fold: (i) establish if the combination of Ultra-High Field Diffusion MRI (UHFD-MRI) techniques can add a more comprehensive analysis of the early microstructural changes observed in HD, and (ii) evaluate if early changes in dMRI microstructural parameters can be linked to cellular biomarkers of neuroinflammation. Ultra-high field magnet (16.7T), diffusion tensor imaging (DTI), and neurite orientation dispersion and density imaging (NODDI) techniques were applied to fixed ex-vivo brains of a preclinical model of HD (R6/1 mice). Fractional anisotropy (FA) was decreased in deep and superficial grey matter (GM) as well as white matter (WM) brain regions with well-known early HD microstructure and connectivity pathology. NODDI parameters associated with the intracellular and extracellular compartment, such as intracellular ventricular fraction (ICVF), orientation dispersion index (ODI), and isotropic volume fractions (IsoVF) were altered in R6/1 mice GM. Further, histological studies in these areas showed that glia cell markers associated with neuroinflammation (GFAP & Iba1) were consistent with the dMRI findings. dMRI can be used to extract non-invasive information of neuropathological events present in the early stages of HD. The combination of multiple imaging techniques represents a better approach to understand the neuropathological process allowing the early diagnosis and neuromonitoring of patients affected by HD.

In Vivo Expression of Reprogramming Factor OCT4 Ameliorates Myelination Deficits and Induces Striatal Neuroprotection in Huntington's Disease

White matter atrophy has been shown to precede the massive loss of striatal GABAergic neurons in Huntington’s disease (HD). This study investigated the effects of in vivo expression of reprogramming factor octamer-binding transcription factor 4 (OCT4) on neural stem cell (NSC) niche activation in the subventricular zone (SVZ) and induction of cell fate specific to the microenvironment of HD. R6/2 mice randomly received adeno-associated virus 9 (AAV9)-OCT4, AAV9-Null, or phosphate-buffered saline into both lateral ventricles at 4 weeks of age. The AAV9-OCT4 group displayed significantly improved behavioral performance compared to the control groups. Following AAV9-OCT4 treatment, the number of newly generated NSCs and oligodendrocyte progenitor cells (OPCs) significantly increased in the SVZ, and the expression of OPC-related genes and glial cell-derived neurotrophic factor (GDNF) significantly increased. Further, amelioration of myelination deficits in the corpus callosum was observed through electron microscopy and magnetic resonance imaging, and striatal DARPP32⁺ GABAergic neurons significantly increased in the AAV9-OCT4 group. These results suggest that in situ expression of the reprogramming factor OCT4 in the SVZ induces OPC proliferation, thereby attenuating myelination deficits. Particularly, GDNF released by OPCs seems to induce striatal neuroprotection in HD, which explains the behavioral improvement in R6/2 mice overexpressing OCT4.

Exosomes as a potential messenger unit during heterochronic parabiosis for amelioration of Huntington's disease

BACKGROUND

Huntington's disease (HD) starts its pathology long before clinical manifestation, however, there is no therapy to cure it completely and only a few studies have been reported for delaying the progression of HD. Recently, it has been shown that heterochronic parabiosis can modulate the neurodegenerative diseases. Despite the importance of the transportation process of positive factors during heterochronic parabiosis, there were limited understandings because the transportation process is nanoscale, which makes it difficult to identify the messenger unit. We demonstrated that heterochronic parabiosis could modulate HD in R6/2 mice model, and identified the messenger unit for transferring positive factors in the young blood serum.

METHODS

R6/2 mice were surgically connected with young wild-type mice (n = 13), old wild-type mice (n = 8), or R6/2 mice (n = 6) to examine the effect of heterochronic parabiosis. Parabionts composed of 5- to 6-week-old transgenic and wild-type mice were observed for 6 weeks in a single cage. The in vitro cellular model of HD cells were treated by the blood serum of the young or old mice, and by the exosomes isolated from thereof. The in vitro cellular model of HD were developed by differentiating neural stem cells cultured from SVZ of the brain.

RESULTS

After the heterochronic parabiosis, the weight loss and survival of HD mice was improved. Also, mutant Huntingtin aggregation (EM48 p < 0.005), improvement of mitochondria dysfunction (PGC-1a p < 0.05, p-CREB/CREB p < 0.005), cell death (p53 p < 0.05, Bax p < 0.05, Cleaved-caspase3 p < 0.05), and cognition (DCX p < 0.5) showed a near complete restoration. In addition, treating in vitro cellular model of HD by the exosomes from young blood serum improved mutant Huntingtin aggregation (EM48 p < 0.05), mitochondria biogenesis (p-CREB/CREB p < 0.005), cell death (p53 p < 0.05, Bax p < 0.005, Cleaved-caspase3 p < 0.05, Bcl-2 p < 0.05), and cell proliferation (WST-1 p < 0.005).

CONCLUSIONS

We found that the overall pathology of HD could be improved by the shared blood circulation through heterochronic parabiosis, furthermore, we demonstrated that the exosomes could be messengers for transferring positive factors, showing the potential of exosomes from young blood for the amelioration of HD.

Exposure of R6/2 mice in an enriched environment augments P42 therapy efficacy on Huntington's disease progression

Huntington's disease (HD) is due to a mutation in the gene encoding for Huntingtin protein generating polyQ domain extension. Mutant Htt (mHtt) leads to important dysfunction of the BDNF/TrkB signaling. We previously described the 23aa Htt fragment P42, that attenuated the pathological phenotypes induced by mHtt. We reported that, in the R6/2 mouse model of HD, P42 rescued striatal TrkB level but marginally increased cortical BDNF. In the present study, our aim was to address P42 neuroprotection in presence of an external input of BDNF. We combined P42 administration with environmental enrichment (EE), induced by training in the Hamlet test. We examined the consequences of P42 + EE combination on different phenotypes in R6/2 HD mice: motor and cognitive performances, recorded at early and late pathological stages, and analyzed aggregated mHtt and BDNF levels in forebrain structures. Hamlet exploration (i.e., entries in Run, Hide, Eat, Drink and Interact houses) was gradually impaired in R6/2 mice, but maintained by P42 treatment until week 8. Topographic memory alteration measured at week 7 was attenuated by P42. Motor performances (rotarod) were significantly ameliorated by the P42 + EE combination until late stage (week 12). The P42 + EE combination also significantly decreased aggregated Htt levels in the hippocampus, striatum and cortex, and increased BDNF levels in the cortex and striatum. We concluded that combination between P42 treatment, known to increase TrkB striatal expression, and a BDNF-enhancing therapy such as EE efficiently delayed HD pathology in R6/2 mice. Use of dual therapies might be a pertinent strategy to fight neurodegeneration in HD.

Spatiotemporal analysis of soluble aggregates and autophagy markers in the R6/2 mouse model

Maintenance of cellular proteostasis is vital for post-mitotic cells like neurons to sustain normal physiological function and homeostasis, defects in which are established hallmarks of several age-related conditions like AD, PD, HD, and ALS. The Spatio-temporal accumulation of aggregated proteins in the form of inclusion bodies/plaques is one of the major characteristics of many neurodegenerative diseases, including Huntington's disease (HD). Toxic accumulation of HUNTINGTIN (HTT) aggregates in neurons bring about the aberrant phenotypes of HD, including severe motor dysfunction, dementia, and cognitive impairment at the organismal level, in an age-dependent manner. In several cellular and animal models, aggrephagy induction has been shown to clear aggregate-prone proteins like HTT and ameliorate disease pathology by conferring neuroprotection. In this study, we used the mouse model of HD, R6/2, to understand the pathogenicity of mHTT aggregates, primarily focusing on autophagy dysfunction. We report that basal autophagy is not altered in R6/2 mice, whilst being functional at a steady-state level in neurons. Moreover, we tested the efficacy of a known autophagy modulator, Nilotinib (Tasigna™), presently in clinical trials for PD, and HD, in curbing mHTT aggregate growth and their potential clearance, which was ineffective in both inducing autophagy and rescuing the pathological phenotypes in R6/2 mice.

Mitochondria exert age-divergent effects on recovery from spinal cord injury

The extent that age-dependent mitochondrial dysfunction drives neurodegeneration is not well understood. This study tested the hypothesis that mitochondria contribute to spinal cord injury (SCI)-induced neurodegeneration in an age-dependent manner by using 2,4-dinitrophenol (DNP) to uncouple electron transport, thereby increasing cellular respiration and reducing reactive oxygen species (ROS) production. We directly compared the effects of graded DNP doses in 4- and 14-month-old (MO) SCI-mice and found DNP to have increased efficacy in mitochondria isolated from 14-MO animals. In vivo, all DNP doses significantly exacerbated 4-MO SCI neurodegeneration coincident with worsened recovery. In contrast, low DNP doses (1.0-mg/kg/day) improved tissue sparing, reduced ROS-associated 3-nitrotyrosine (3-NT) accumulation, and improved anatomical and functional recovery in 14-MO SCI-mice. By directly comparing the effects of DNP between ages we demonstrate that mitochondrial contributions to neurodegeneration diverge with age after SCI. Collectively, our data indicate an essential role of mitochondria in age-associated neurodegeneration.

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.

Hair dysmorphology in the R6/1 and R6/2 mouse models of Huntington's disease

Huntington's disease (HD), caused by expansion of CAG repeats in the 1st exon of the HTT gene, is a disorder inherited in an autosomal dominant manner. HD symptoms include chorea, behavioral disturbances and cognitive decline. Although it is described as a neurodegenerative disease, due to expression of HTT in all types of cells, peripheral symptoms also occur. R6/1 and R6/2 mouse lines, which demonstrate many different phenotypical disturbances, are among the most commonly used HD animal models. Nevertheless, in this report, we underlined, for the first time, a previously undescribed R6/1 and R6/2 feature, hair dysmorphology. We observed changes in the general view of pelage, as well as specific changes in the shape of hair, assessed under electron microscope (deep cavity and hilly hair surface or concave and convex areas on the long hair axis with an appearance of the hair as flat). Hair diameter was significantly increased in both HD mouse models relative to control animals. Moreover, loosened contact between the scales and loosened scale texture were observed in R6/1 and R6/2. Thus, this study highlighted that the hair morphology might be a useful, noninvasive and simple marker of a widely used HD mouse models, R6/1 and R6/2 lines, particularly in testing effects of potential therapeutics or disease progression.

Microglial depletion prevents extracellular matrix changes and striatal volume reduction in a model of Huntington's disease

Huntington's disease is associated with a reactive microglial response and consequent inflammation. To address the role of these cells in disease pathogenesis, we depleted microglia from R6/2 mice, a rapidly progressing model of Huntington's disease marked by behavioural impairment, mutant huntingtin (mHTT) accumulation, and early death, through colony-stimulating factor 1 receptor inhibition (CSF1Ri) with pexidartinib (PLX3397) for the duration of disease. Although we observed an interferon gene signature in addition to downregulated neuritogenic and synaptic gene pathways with disease, overt inflammation was not evident by microglial morphology or cytokine transcript levels in R6/2 mice. Nonetheless, CSF1Ri-induced microglial elimination reduced or prevented disease-related grip strength and object recognition deficits, mHTT accumulation, astrogliosis, and striatal volume loss, the latter of which was not associated with reductions in cell number but with the extracellular accumulation of chondroitin sulphate proteoglycans (CSPGs)-a primary component of glial scars. A concurrent loss of proteoglycan-containing perineuronal nets was also evident in R6/2 mice, and microglial elimination not only prevented this but also strikingly increased perineuronal nets in the brains of naïve littermates, suggesting a new role for microglia as homeostatic regulators of perineuronal net formation and integrity.

Lentiviral delivery of human erythropoietin attenuates hippocampal atrophy and improves cognition in the R6/2 mouse model of Huntington's disease

Huntington's disease (HD) is an incurable neurodegenerative disorder caused by a trinucleotide (CAG) repeat expansion in the huntingtin gene (HTT). The R6/2 transgenic mouse model of HD expresses exon 1 of the human HTT gene with approximately 150 CAG repeats. R6/2 mice develop progressive behavioural abnormalities, impaired neurogenesis, and atrophy of several brain regions. In recent years, erythropoietin (EPO) has been shown to confer neuroprotection and enhance neurogenesis, rendering it a promising molecule to attenuate HD symptoms. In this study, the therapeutic potential of EPO was evaluated in female R6/2 transgenic mice. A single bilateral injection of a lentivirus encoding human EPO (LV-hEPO) was performed into the lateral ventricles of R6/2 mice at disease onset (8 weeks of age). Control groups were either untreated or injected with a lentivirus encoding green fluorescent protein (LV-GFP). Thirty days after virus administration, hEPO mRNA and protein were present in injected R6/2 brains. Compared to control R6/2 mice, LV-hEPO-treated R6/2 mice exhibited reduced hippocampal atrophy, increased neuroblast branching towards the dentate granular cell layer, and improved spatial cognition. Our results suggest that LV-hEPO administration may be a promising strategy to reduce cognitive impairment in HD.

Calcium-Handling Defects and Neurodegenerative Disease

Calcium signaling is critical to neuronal function and regulates highly diverse processes such as gene transcription, energy production, protein handling, and synaptic structure and function. Because there are many common underlying calcium-mediated pathological features observed across several neurological conditions, it has been proposed that neurodegenerative diseases have an upstream underlying calcium basis in their pathogenesis. With certain diseases such as Alzheimer's, Parkinson's, and Huntington's, specific sources of calcium dysregulation originating from distinct neuronal compartments or channels have been shown to have defined roles in initiating or sustaining disease mechanisms. Herein, we will review the major hallmarks of these diseases, and how they relate to calcium dysregulation. We will then discuss neuronal calcium handling throughout the neuron, with special emphasis on channels involved in neurodegeneration.

Complementarity of gluCEST and 1 H-MRS for the study of mouse models of Huntington's disease

Identification of relevant biomarkers is fundamental to understand biological processes of neurodegenerative diseases and to evaluate therapeutic efficacy. Atrophy of brain structures has been proposed as a biomarker, but it provides little information about biochemical events related to the disease. Here, we propose to identify early and relevant biomarkers by combining biological specificity provided by ¹ H-MRS and high spatial resolution offered by gluCEST imaging. For this, two different genetic mouse models of Huntington's disease (HD)-the Ki140CAG model, characterized by a slow progression of the disease, and the R6/1 model, which mimics the juvenile form of HD-were used. Animals were scanned at 11.7 T using a protocol combining ¹ H-MRS and gluCEST imaging. We measured a significant decrease in levels of N-acetyl-aspartate, a metabolite mainly located in the neuronal compartment, in HD animals, and the decrease seemed to be correlated with disease severity. In addition, variations of tNAA levels were correlated with striatal volumes in both models. Significant variations of glutamate levels were also observed in Ki140CAG but not in R6/1 mice. Thanks to its high resolution, gluCEST provided complementary insights, and we highlighted alterations in small brain regions such as the corpus callosum in Ki140CAG mice, whereas the glutamate level was unchanged in the whole brain of R6/1 mice. In this study, we showed that ¹ H-MRS can provide key information about biological processes occurring in vivo but was limited by the spatial resolution. On the other hand, gluCEST may finely point to alterations in unexpected brain regions, but it can also be blind to disease processes when glutamate levels are preserved. This highlights in a practical context the complementarity of the two methods to study animal models of neurodegenerative diseases and to identify relevant biomarkers.

No symphony without bassoon and piccolo: changes in synaptic active zone proteins in Huntington's disease

Prominent features of HD neuropathology are the intranuclear and cytoplasmic inclusions of huntingtin and striatal and cortical neuronal cell death. Recently, synaptic defects have been reported on HD-related studies, including impairment of neurotransmitter release and alterations of synaptic components. However, the definite characteristics of synapse dysfunction and the underlying mechanisms remain largely unknown. We studied the gene expression levels and patterns of a number of proteins forming the cytoskeletal matrix of the presynaptic active zones in HD transgenic mice (R6/1), in hippocampal neuronal cultures overexpressing mutant huntingtin and in postmortem brain tissues of HD patients. To investigate the interactions between huntingtin and active proteins, we performed confocal microscopic imaging and immunoprecipitation in mouse and HEK 293 cell line models. The mRNA and protein levels of Bassoon were reduced in mouse and cell culture models of HD and in brain tissues of patients with HD. Moreover, a striking re-distribution of a complex of proteins including Bassoon, Piccolo and Munc 13-1 from the cytoplasm and synapses into intranuclear huntingtin aggregates with loss of active zone proteins and dendritic spines. This re-localization was age-dependent and coincided with the formation of huntingtin aggregates. Using co-immunoprecipitation, we demonstrated that huntingtin interacts with Bassoon, and that this interaction is likely mediated by a third linking protein. Three structural proteins involved in neurotransmitter release in the presynaptic active zones of neurons are altered in expression and that the proteins are redistributed from their normal functional site into mutant huntingtin aggregates.

Serum Raman spectroscopy as a diagnostic tool in patients with Huntington's disease

Huntington's disease is an inherited fatal progressive neurodegenerative disorder. A possible new Raman ‘spectral’ biomarker was identified that uses a tiny drop of patients' blood serum; thus can have immense diagnostic and therapeutic implications.

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by an abnormal CAG expansion in exon 1 of the huntingtin (HTT) gene. Given its genetic basis it is possible to study patients both in the pre-manifest and manifest stages of the condition. While disease onset can be modelled using CAG repeat size, there are no easily accessible biomarkers that can objectively track disease progression. Here, we employed a holistic approach using spectral profiles generated using both surface-enhanced Raman spectroscopy (SERS) and Raman Spectroscopy (RS), on the serum of healthy participants and HD patients covering a wide spectrum of disease stages. We found that there was both genotype- and gender-specific segregation on using the full range in the fingerprint region with both SERS and RS. On a more detailed interrogation using specific spectral intervals, SERS revealed significant correlations with disease progression, in particular progression from pre-manifest through to advanced HD was associated with serum molecules related to protein misfolding and nucleotide catabolism. Thus, this study shows the potential of Raman spectroscopy-based techniques for stratification of patients and, of SERS, in particular, to track disease status through provision of ‘spectral’ biomarkers in HD, with clinical applications for other diseases and trials looking at disease modifying therapies.

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.

Longitudinal Biochemical Assay Analysis of Mutant Huntingtin Exon 1 Protein in R6/2 Mice

BACKGROUND

Biochemical analysis of mutant huntingtin (mHTT) aggregation species in HD mice is a common measure to track disease. A longitudinal and systematic study of how tissue processing affects detection of conformers has not yet been reported. Understanding the homeostatic flux of mHTT over time and under different processing conditions would aid in interpretation of pre-clinical assessments of disease interventions.

OBJECTIVE

Provide a systematic evaluation of tissue lysis methods and molecular and biochemical assays in parallel with behavioral readouts in R6/2 mice to establish a baseline for HTT exon1 protein accumulation.

METHODS

Established biochemical methods were used to process tissue from R6/2 mice of specific ages following behavior tasks. Aggregation states and accumulation of mHTT exon 1 protein were evaluated using multiple break and assay methods to determine potential conformational flux assay specificity in detection of mHTT species, and tissue specificity of conformers.

RESULTS

Detection of mHTT exon 1 protein species varied based on biochemical processing and analysis providing a baseline for subsequent studies in R6/2 mice. Insoluble, high molecular weight species of mHTT exon 1 protein increased and tracked with onset of behavioral impairments in R6/2 mice using multiple assay methods.

CONCLUSIONS

Conformational flux from soluble monomer to high molecular weight, insoluble species of mHTT exon 1 protein was generally consistent for multiple assay methods throughout R6/2 disease progression; however, the results support the use of multiple biochemical techniques to detect mHTT exon 1 protein species for preclinical assessments in HD mouse models expressing mHTT exon 1 protein.