Ubiquitin, Autophagy and Neurodegenerative Diseases

Ubiquitin signals play various roles in proteolytic and non-proteolytic functions. Ubiquitin signals are recognized as targets of the ubiquitin–proteasome system and the autophagy–lysosome pathway. In autophagy, ubiquitin signals are required for selective incorporation of cargoes, such as proteins, organelles, and microbial invaders, into autophagosomes. Autophagy receptors possessing an LC3-binding domain and a ubiquitin binding domain are involved in this process. Autophagy activity can decline as a result of genetic variation, aging, or lifestyle, resulting in the onset of various neurodegenerative diseases. This review summarizes the selective autophagy of neurodegenerative disease-associated protein aggregates via autophagy receptors and discusses its therapeutic application for neurodegenerative diseases.

The blood-brain barrier is disrupted in Machado-Joseph disease/spinocerebellar ataxia type 3: evidence from transgenic mice and human post-mortem samples

Blood-brain barrier (BBB) disruption is a common feature in neurodegenerative diseases. However, BBB integrity has not been assessed in spinocerebellar ataxias (SCAs) such as Machado-Joseph disease/SCA type 3 (MJD/SCA3), a genetic disorder, triggered by polyglutamine-expanded ataxin-3. To investigate that, BBB integrity was evaluated in a transgenic mouse model of MJD and in human post-mortem brain tissues.

Firstly, we investigated the BBB permeability in MJD mice by: i) assessing the extravasation of the Evans blue (EB) dye and blood-borne proteins (e.g fibrinogen) in the cerebellum by immunofluorescence, and ii) in vivo Dynamic Contrast Enhanced-Magnetic Resonance Imaging (DCE-MRI). The presence of ataxin-3 aggregates in brain blood vessels and the levels of tight junction (TJ)-associated proteins were also explored by immunofluorescence and western blotting. Human brain samples were used to confirm BBB permeability by evaluating fibrinogen extravasation, co-localization of ataxin-3 aggregates with brain blood vessels and neuroinflammation.

In the cerebellum of the mouse model of MJD, there was a 5-fold increase in EB accumulation when compared to age-matched controls. Moreover, vascular permeability displayed a 13-fold increase demonstrated by DCE-MRI. These results were validated by the 2-fold increase in fibrinogen extravasation in transgenic animals comparing to controls. Interestingly, mutant ataxin-3 aggregates were detected in cerebellar blood vessels of transgenic mice, accompanied by alterations of TJ-associated proteins in cerebellar endothelial cells, namely a 29% decrease in claudin-5 oligomers and a 10-fold increase in an occludin cleavage fragment. These results were validated in post-mortem brain samples from MJD patients as we detected fibrinogen extravasation across BBB, the presence of ataxin-3 aggregates in blood vessels and associated microgliosis.

Altogether, our results prove BBB impairment in MJD/SCA3. These findings contribute for a better understanding of the disease mechanisms and opens the opportunity to treat MJD with medicinal products that in normal conditions would not cross the BBB.

Huntington disease: new insights into molecular pathogenesis and therapeutic opportunities

Huntington disease (HD) is a neurodegenerative disease caused by CAG repeat expansion in the huntingtin gene (HTT) and involves a complex web of pathogenic mechanisms. Mutant HTT (mHTT) disrupts transcription, interferes with immune and mitochondrial function, and is aberrantly modified post-translationally. Evidence suggests that the mHTT RNA is toxic, and at the DNA level, somatic CAG repeat expansion in vulnerable cells influences the disease course. Genome-wide association studies have identified DNA repair pathways as modifiers of somatic instability and disease course in HD and other repeat expansion diseases. In animal models of HD, nucleocytoplasmic transport is disrupted and its restoration is neuroprotective. Novel cerebrospinal fluid (CSF) and plasma biomarkers are among the earliest detectable changes in individuals with premanifest HD and have the sensitivity to detect therapeutic benefit. Therapeutically, the first human trial of an HTT-lowering antisense oligonucleotide successfully, and safely, reduced the CSF concentration of mHTT in individuals with HD. A larger trial, powered to detect clinical efficacy, is underway, along with trials of other HTT-lowering approaches. In this Review, we discuss new insights into the molecular pathogenesis of HD and future therapeutic strategies, including the modulation of DNA repair and targeting the DNA mutation itself.

Is the Immunological Response a Bottleneck for Cell Therapy in Neurodegenerative Diseases?

Neurodegenerative disorders such as Parkinson's (PD) and Huntington's disease (HD) are characterized by a selective detrimental impact on neurons in a specific brain area. Currently, these diseases have no cures, although some promising trials of therapies that may be able to slow the loss of brain cells are underway. Cell therapy is distinguished by its potential to replace cells to compensate for those lost to the degenerative process and has shown a great potential to replace degenerated neurons in animal models and in clinical trials in PD and HD patients. Fetal-derived neural progenitor cells, embryonic stem cells or induced pluripotent stem cells are the main cell sources that have been tested in cell therapy approaches. Furthermore, new strategies are emerging, such as the use of adult stem cells, encapsulated cell lines releasing trophic factors or cell-free products, containing an enriched secretome, which have shown beneficial preclinical outcomes. One of the major challenges for these potential new treatments is to overcome the host immune response to the transplanted cells. Immune rejection can cause significant alterations in transplanted and endogenous tissue and requires immunosuppressive drugs that may produce adverse effects. T-, B-lymphocytes and microglia have been recognized as the main effectors in striatal graft rejection. This review aims to summarize the preclinical and clinical studies of cell therapies in PD and HD. In addition, the precautions and strategies to ensure the highest quality of cell grafts, the lowest risk during transplantation and the reduction of a possible immune rejection will be outlined. Altogether, the wide-ranging possibilities of advanced therapy medicinal products (ATMPs) could make therapeutic treatment of these incurable diseases possible in the near future.

Gene therapy and immunotherapy as promising strategies to combat Huntington's disease-associated neurodegeneration: emphasis on recent updates and future perspectives

INTRODUCTION

Modulation of gene expression using gene therapy as well as modulation of immune activation using immunotherapy has attracted considerable attention as rapidly emerging potential therapeutic intervention for the treatment of HD. Several preclinical and clinical trials for gene-based therapy and immunotherapy/antibody-based have been conducted.

AREAS COVERED

This review focused on the potential use of gene therapy and immuno-based therapies to treat HD, including the current status, the rationale for these approaches as well as preclinical and clinical data supporting it. Growing knowledge of HD pathogenesis has resulted in the discovery of new therapeutic targets, some of which are now in clinical trials. Focus has been allocated to RNA and DNA-based gene therapies for the reduction of mutant huntingtin (mHTT), using Immuno/antibody-based therapies.

EXPERT OPINION

While safety and efficacy of gene therapy and immunotherapy has been well demonstrated for HD, therefore much focus has now been shifted to disease-modifying therapies. This review defines the current status and future directions of gene therapy and immunotherapies. The review summarizes by what means HD genetic root cause modification and functional restoration of mHtt protein could be achieved by using targeted multimodality gene therapy and immunotherapy to target intracellular and extracellular mHtt.

Restoring Function to Dopaminergic Neurons: Progress in the Development of Cell-Based Therapies for Parkinson's Disease

There is escalating interest in cell-based therapies to restore lost dopamine inputs in Parkinson's disease. This is based upon the rationale that implanting dopamine progenitors into the striatum can potentially improve dopamine-responsive motor symptoms. A rich body of data describing clinical trials of previous cell transplantation exists. These have included multiple cell sources for transplantation including allogeneic (human embryonic mesencephalic tissue, retinal pigment epithelial cells) and autologous (carotid body, adrenal medullary tissue) cells, as well as xenotransplantation. However, there are multiple limitations related to these cell sources, including availability of adequate numbers of cells for transplant, heterogeneity within cells transplanted, imprecisely defined mechanisms of action, and poor cell survival after transplantation in some cases. Nonetheless, evidence has accrued from a subset of trials to support the rationale for such a regenerative approach. Recent rapid advances in stem cell technology may now overcome these prior limitations. For example, dopamine neuron precursor cells for transplant can be generated from induced pluripotent cells and human embryonic stem cells. The benefits of these innovative approaches include: the possibility of scalability; a high degree of quality control; and improved understanding of mechanisms of action with rigorous preclinical testing. In this review, we focus on the potential for cell-based therapies in Parkinson's disease to restore the function of dopaminergic neurons, we critically review previous attempts to harness such strategies, we discuss potential benefits and predicted limitations, and we address how previous roadblocks may be overcome to bring a cell-based approach to the clinic.

Rhes Tunnels: A Radical New Way of Communication in the Brain's Striatum?

Ras homolog enriched in the striatum (Rhes) is a striatal enriched protein that promotes the formation of thin membranous tubes resembling tunneling nanotubes (TNT)-"Rhes tunnels"-that connect neighboring cell and transport cargoes: vesicles and proteins between the neuronal cells. Here the literature on TNT-like structures is reviewed, and the implications of Rhes-mediated TNT, the mechanisms of its formation, and its potential in novel cell-to-cell communication in regulating striatal biology and disease are emphasized. Thought-provoking ideas regarding how Rhes-mediated TNT, if it exists, in vivo, would radically change the way neurons communicate in the brain are discussed.

Use of adeno-associated virus-mediated delivery of mutant huntingtin to study the spreading capacity of the protein in mice and non-human primates

In order to model various aspects of Huntington's disease (HD) pathology, in particular protein spread, we administered adeno-associated virus (AAV) expressing green fluorescent protein (GFP) or GFP coupled to HTT-Exon1 (19Q or 103Q) to the central nervous system of adult wild-type (WT) mice and non-human primates. All animals underwent behavioral testing and post-mortem analyses to determine the long-term consequences of AAV injection. Both mice and non-human primates demonstrated behavioral changes at 2-3 weeks post-surgery. In mice, these changes were absent after 3 months while in non-human primates, they persisted in the majority of tested animals. Post-mortem analysis revealed that spreading of the aggregates was limited, although the virus did spread between synaptically-connected brain regions. Despite circumscribed spreading, the presence of mHTT generated changes in endogenous huntingtin (HTT) levels in both models. Together, these results suggest that viral expression of mHTTExon1 can induce spreading and seeding of HTT in both mice and non-human primates.

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

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

Modeling of cumulative effects of time-varying drug exposures on within-subject changes in a continuous outcome

An accurate assessment of the safety or effectiveness of drugs in pharmaco-epidemiological studies requires defining an etiologically correct time-varying exposure model, which specifies how previous drug use affects the outcome of interest. To address this issue, we develop, and validate in simulations, a new approach for flexible modeling of the cumulative effects of time-varying exposures on repeated measures of a continuous response variable, such as a quantitative surrogate outcome, or a biomarker. Specifically, we extend the linear mixed effects modeling to estimate how past and recent drug exposure affects the way individual values of the outcome change throughout the follow-up. To account for the dosage, duration and timing of past exposures, we rely on a flexible weighted cumulative exposure methodology to model the cumulative effects of past drug use, as the weighted sum of past doses. Weights, modeled with unpenalized cubic regression B-splines, reflect the relative importance of doses taken at different times in the past. In simulations, we evaluate the performance of the model under different assumptions concerning (i) the shape of the weight function, (ii) the sample size, (iii) the number of the longitudinal observations and (iv) the intra-individual variance. Results demonstrate the accuracy of our estimates of the weight function and of the between- and within-patients variances, and good correlation between the observed and predicted longitudinal changes in the outcome. We then apply the proposed method to re-assess the association between time-varying glucocorticoid exposure and weight gain in people living with rheumatoid arthritis.

Prion-like mechanisms in neurodegenerative disease: Implications for Huntington's disease therapy

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a CAG repeat expansions in the huntingtin gene resulting in the synthesis of a misfolded form of the huntingtin protein (mHTT) which is toxic. The current treatments for HD are only palliative. Some of the potential therapies for HD include gene therapy (using antisense oligonucleotides and clustered regularly interspaced short palindromic repeats-Cas9 system) and stem-cell-based therapies. Various types of stem cell transplants, such as mesenchymal stem cells, neural stem cells, and reprogrammed stem cells, have the potential to either replace the lost neurons or support the existing neurons by releasing trophic factors. Most of the transplants are xenografts and allografts; however, recent reports on HD patients who received grafts suggest that the mHTT aggregates are transferred from the host neurons to the grafted cells as well as to the surrounding areas of the graft by a "prion-like" mechanism. This observation seems to be true for autotransplantation paradigms, as well. This article reviews the different types of stem cells that have been transplanted into HD patients and their therapeutic efficacy, focusing on the transfer of mHTT from the host cells to the graft. Autotransplants of reprogramed stem cells in HD patients are a promising therapeutic option. However, this needs further attention to ensure a better understanding of the transfer of mHTT aggregates following transplantation of the gene-corrected cells back into the patient.

Molecular mechanisms of proteinopathies across neurodegenerative disease: a review

Background

Although there is a range of different symptoms across neurodegenerative diseases, they have been noted to have common pathogenic features. An archetypal feature shared between these diseases is protein misfolding; however, the mechanism behind the proteins abnormalities is still under investigation. There is an emerging hypothesis in the literature that the mechanisms that lead to protein misfolding may be shared across neurodegenerative processes, suggesting a common underlying pathology.

Main body

This review discusses the literature to date of the shared features of protein misfolding, failures in proteostasis, and potential propagation pathways across the main neurodegenerative disorders.

Conclusion

The current data suggests, despite overarching processes being shared, that the molecular events implicated in protein pathology are distinct across common neurodegenerative disorders.

The role of mutations associated with familial neurodegenerative disorders on blood-brain barrier function in an iPSC model

Background

Blood–brain barrier dysfunction is associated with many late-stage neurodegenerative diseases. An emerging question is whether the mutations associated with neurodegenerative diseases can independently lead to blood–brain barrier (BBB) dysfunction. Studies from patient-derived induced pluripotent stem cells suggest that mutations associated with neurodegenerative disease are non-cell autonomous, resulting in gain of toxic function in derived neurons and astrocytes. Here we assess whether selected mutations associated with neurodegenerative diseases can contribute to impairment of the blood–brain barrier.

Methods

We assessed barrier function of confluent monolayers of human brain microvascular endothelial cells (hBMECs) derived from induced pluripotent stem cells (iPSC) from three healthy individuals and eight individuals with neurodegenerative disease. We systematically assessed protein and gene expression of BBB biomarkers, transendothelial resistance (TEER), permeability of Lucifer yellow, permeability of d-glucose, permeability of rhodamine 123, the efflux ratio of rhodamine 123, and P-gp inhibition using Tariquidar for confluent monolayers of human brain microvascular endothelial cell (hBMECs).

Results

We provide evidence supporting the hypothesis that mutations associated with neurodegenerative disease can independently cause BBB dysfunction. These functional changes are not catastrophic since barrier breakdown would result in BBB impairment during development. Synergistic interactions between non-cell autonomous cerebrovascular dysfunction and the effects of gain-of-toxic function in neurons (e.g. toxic oligomers) are likely to increase disease burden through a positive feedback mechanism.

Conclusions

These results suggest that the accumulation of defects in brain microvascular endothelial cells may ultimately lead to impairment of the BBB. Small changes in barrier function over time could lead to accumulated defects that result in positive feedback to unrelated central nervous system diseases.

Electronic supplementary material

The online version of this article (10.1186/s12987-019-0139-4) contains supplementary material, which is available to authorized users.

A brief history of antibody-based therapy

Active and passive immunization have been used to treat human disease for hundreds of years and improvements in technology and knowledge is only increasing the number of therapeutic applications. The current and future use of immunization to treat neurodegenerative diseases are briefly described herein to serve as an introduction to this special issue.

Demonstration of prion-like properties of mutant huntingtin fibrils in both in vitro and in vivo paradigms

In recent years, evidence has accumulated to suggest that mutant huntingtin protein (mHTT) can spread into healthy tissue in a prion-like fashion. This theory, however, remains controversial. To fully address this concept and to understand the possible consequences of mHTT spreading to Huntington’s disease pathology, we investigated the effects of exogenous human fibrillar mHTT (Q48) and huntingtin (HTT) (Q25) N-terminal fragments in three cellular models and three distinct animal paradigms. For in vitro experiments, human neuronal cells [induced pluripotent stem cell-derived GABA neurons (iGABA) and (SH-SY5Y)] as well as human THP1-derived macrophages, were incubated with recombinant mHTT fibrils. Recombinant mHTT and HTT fibrils were taken up by all cell types, inducing cell morphology changes and death. Variations in HTT aggregation were further observed following incubation with fibrils in both THP1 and SH-SY5Y cells. For in vivo experiments, adult wild-type (WT) mice received a unilateral intracerebral cortical injection and R6/2 and WT pups were administered fibrils via bilateral intraventricular injections. In both protocols, the injection of Q48 fibrils resulted in cognitive deficits and increased anxiety-like behavior. Post-mortem analysis of adult WT mice indicated that most fibrils had been degraded/cleared from the brain by 14 months post-surgery. Despite the absence of fibrils at these later time points, a change in the staining pattern of endogenous HTT was detected. A similar change was revealed in post-mortem analysis of the R6/2 mice. These effects were specific to central administration of fibrils, as mice receiving intravenous injections were not characterized by behavioral changes. In fact, peripheral administration resulted in an immune response mounting against the fibrils. Together, the in vitro and in vivo data indicate that exogenously administered mHTT is capable of both causing and exacerbating disease pathology.

Molecular mechanisms of heterogeneous oligomerization of huntingtin proteins

There is still no successful strategy to treat Huntington's disease, an inherited autosomal disorder associated with the aggregation of mutated forms of the huntingtin protein containing polyglutamine tracts with more than 36 repeats. Recent experimental evidence is challenging the conventional view of the disease by revealing transcellular transfer of mutated huntingtin proteins which are able to seed oligomers involving wild type forms of the protein. Here we decipher the molecular mechanism of this unconventional heterogeneous oligomerization by performing discrete molecular dynamics simulations. We identify the most probable oligomer conformations and the molecular regions that can be targeted to destabilize them. Our computational findings are complemented experimentally by fluorescence-lifetime imaging microscopy/fluorescence resonance energy transfer (FLIM-FRET) of cells co-transfected with huntingtin proteins containing short and large polyglutamine tracts. Our work clarifies the structural features responsible for heterogeneous huntingtin aggregation with possible implications to contrast the prion-like spreading of Huntington's disease.

Rhes travels from cell to cell and transports Huntington disease protein via TNT-like protrusion

Tunneling nanotubes (TNT) are membranous tubes that connect two cells, but their functional roles and mechanism of biogenesis remain obscure. Sharma and Subramaniam demonstrate that Rhes, a brain-enriched protein, increases biogenesis of TNT-like cellular protrusions or “Rhes tunnels” through which Rhes travels from cell to cell and transports Huntington disease (HD) protein.

Tunneling nanotubes (TNT) are thin, membranous, tunnel-like cell-to-cell connections, but the mechanisms underlying their biogenesis or functional role remains obscure. Here, we report, Rhes, a brain-enriched GTPase/SUMO E3-like protein, induces the biogenesis of TNT-like cellular protrusions, “Rhes tunnels,” through which Rhes moves from cell to cell and transports Huntington disease (HD) protein, the poly-Q expanded mutant Huntingtin (mHTT). The formation of TNT-like Rhes tunnels requires the Rhes’s serine 33, C-terminal CAAX, and a SUMO E3-like domain. Electron microscopy analysis revealed that TNT-like Rhes tunnels appear continuous, cell–cell connections, and <200 nm in diameter. Live-cell imaging shows that Rhes tunnels establish contact with the neighboring cell and deliver Rhes-positive cargoes, which travel across the plasma membrane of the neighboring cell before entering it. The Rhes tunnels carry Rab5a/Lyso 20-positive vesicles and transport mHTT, but not normal HTT, mTOR, or wtTau proteins. SUMOylation-defective mHTT, Rhes C263S (cannot SUMOylate mHTT), or CRISPR/Cas9-mediated depletion of three isoforms of SUMO diminishes Rhes-mediated mHTT transport. Thus, Rhes promotes the biogenesis of TNT-like cellular protrusions and facilitates the cell–cell transport of mHTT involving SUMO-mediated mechanisms.

Propagation of Protein Aggregation in Neurodegenerative Diseases

Most common neurodegenerative diseases feature deposition of protein amyloids and degeneration of brain networks. Amyloids are ordered protein assemblies that can act as templates for their own replication through monomer addition. Evidence suggests that this characteristic may underlie the progression of pathology in neurodegenerative diseases. Many different amyloid proteins, including Aβ, tau, and α-synuclein, exhibit properties similar to those of infectious prion protein in experimental systems: discrete and self-replicating amyloid structures, transcellular propagation of aggregation, and transmissible neuropathology. This review discusses the contribution of prion phenomena and transcellular propagation to the progression of pathology in common neurodegenerative diseases such as Alzheimer's and Parkinson's. It reviews fundamental events such as cell entry, amplification, and transcellular movement. It also discusses amyloid strains, which produce distinct patterns of neuropathology and spread through the nervous system. These concepts may impact the development of new diagnostic and therapeutic strategies.

Disease-Modification in Huntington's Disease: Moving Away from a Single-Target Approach

To date, no candidate intervention has demonstrated a disease-modifying effect in Huntington's disease, despite promising results in preclinical studies. In this commentary we discuss disease-modifying therapies that have been trialled in Huntington's disease and speculate that these failures may be attributed, in part, to the assumption that a single drug selectively targeting one aspect of disease pathology will be universally effective, regardless of disease stage or "subtype". We therefore propose an alternative approach for effective disease-modification that uses 1) a combination approach rather than monotherapy, and 2) targets the disease process early on - before it is clinically manifest. Finally, we will consider whether this change in approach that we propose will be relevant in the future given the recent shift to targeting more proximal disease processes-e.g., huntingtin gene expression; a timely question given Roche's recent decision to take on the clinical development of a promising new drug candidate in Huntington's disease, IONIS-HTTRx.

Combating Neurodegenerative Diseases with the Plant Alkaloid Berberine: Molecular Mechanisms and Therapeutic Potential

Neurodegenerative diseases are among the most serious health problems affecting millions of people worldwide. Such diseases are characterized by a progressive degeneration and / or death of neurons in the central nervous system. Currently, there are no therapeutic approaches to cure or even halt the progression of neurodegenerative diseases. During the last two decades, much attention has been paid to the neuroprotective and anti-neurodegenerative activities of compounds isolated from natural products with high efficacy and low toxicity. Accumulating evidence indicates that berberine, an isoquinoline alkaloid isolated from traditional Chinese medicinal herbs, may act as a promising anti-neurodegenerative agent by inhibiting the activity of the most important pathogenic enzymes, ameliorating intracellular oxidative stress, attenuating neuroinflammation, triggering autophagy and protecting neurons against apoptotic cell death. This review attempts to summarize the current state of knowledge regarding the therapeutic potential of berberine against neurodegenerative diseases, with a focus on the molecular mechanisms that underlie its effects on Alzheimer's, Parkinson's and Huntington's diseases.

Blood-Brain Barrier: From Physiology to Disease and Back

The blood-brain barrier (BBB) prevents neurotoxic plasma components, blood cells, and pathogens from entering the brain. At the same time, the BBB regulates transport of molecules into and out of the central nervous system (CNS), which maintains tightly controlled chemical composition of the neuronal milieu that is required for proper neuronal functioning. In this review, we first examine molecular and cellular mechanisms underlying the establishment of the BBB. Then, we focus on BBB transport physiology, endothelial and pericyte transporters, and perivascular and paravascular transport. Next, we discuss rare human monogenic neurological disorders with the primary genetic defect in BBB-associated cells demonstrating the link between BBB breakdown and neurodegeneration. Then, we review the effects of genes underlying inheritance and/or increased susceptibility for Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease, and amyotrophic lateral sclerosis (ALS) on BBB in relation to other pathologies and neurological deficits. We next examine how BBB dysfunction relates to neurological deficits and other pathologies in the majority of sporadic AD, PD, and ALS cases, multiple sclerosis, other neurodegenerative disorders, and acute CNS disorders such as stroke, traumatic brain injury, spinal cord injury, and epilepsy. Lastly, we discuss BBB-based therapeutic opportunities. We conclude with lessons learned and future directions, with emphasis on technological advances to investigate the BBB functions in the living human brain, and at the molecular and cellular level, and address key unanswered questions.

Connectomic imaging reveals Huntington-related pathological and pharmaceutical effects in a mouse model

Recent studies suggest that neurodegenerative diseases could affect brain structure and function in disease-specific network patterns; however, how spontaneous activity affects structural covariance network (SC) is not clear. We hypothesized that hyper-excitability in Huntington disease (HD) disrupts the coordinated structural and functional connectivity, and treatment with memantine helps to reduce excitotoxicity and normalize the connectivity. MRI was conducted to measure somatosensory activation, resting-state functional-connectivity (rsFC), SC, amplitude of low frequency fluctuation (ALFF) and ALFF covariance (ALFFC) in the YAC128 mouse model of HD. We found somatosensory activation was unchanged but the subcortical ALFF was increased in HD mice, indicating subcortical but not cortical hyperactivity. The reduced sensorimotor rsFC but spared hippocampal and default mode networks in the HD mice was consistent with the more pronounced impairment in motor function compared with cognitive performance. The disease suppressed SC globally and reduced ALFFC in the basal ganglia network as well as its anti-correlation with the default mode network. By comparing these connectivity measures, we found that the originally coupled rsFC-SC relationship was impaired whereas SC-ALFFC correlation was increased by HD, suggesting disease facilitated covariation of brain volume and activity amplitude but not neural synchrony. The comparison with mono-synaptic axonal projection supports the hypothesis that rsFC, but not SC or ALFFC, is highly dependent on structural connectivity under healthy conditions. Treatment with memantine had a strong effect on normalizing the SC and reducing ALFF while slightly increasing other connectivity measures and restoring the rsFC-SC coupling, which is consistent with its effect on alleviating hyper-excitability and improving the coordinated neural growth. These results indicate that HD affects the cerebral structure-function relationship which could be partially reverted by NMDA antagonism. These connectivity measures provide unique insights into pathological and pharmaceutical effects in brain circuitry, and could be translatable biomarkers for evaluating drug effect and refining its efficacy.

Nonhuman Primate Models of Neurodegenerative Disorders

Alzheimer's (AD), Huntington's (HD), and Parkinson's (PD) disease are age-related neurodegenerative disorders characterized by progressive neuronal cell death. Although each disease has particular pathologies and symptoms, accumulated evidence points to similar mechanisms of neurodegeneration, including inflammation, oxidative stress, and protein aggregation. A significant body of research is ongoing to understand how these pathways affect each other and what ultimately triggers the onset of the disease. Experiments in nonhuman primates (NHPs) account for only 5% of all research in animals. Yet the impact of NHP studies for clinical translation is much greater, especially for neurodegenerative disorders, as NHPs have a complex cognitive and motor functions and highly developed neuroanatomy. New NHP models are emerging to better understand pathology and improve the platform in which to test novel therapies. The goal of this report is to review NHP models of AD, HD, and PD in the context of the current understanding of these diseases and their contribution to the development of novel therapies.

Outcome of cell suspension allografts in a patient with Huntington's disease

For patients with incurable neurodegenerative disorders such as Huntington's (HD) and Parkinson's disease, cell transplantation has been explored as a potential treatment option. Here, we present the first clinicopathological study of a patient with HD in receipt of cell-suspension striatal allografts who took part in the NEST-UK multicenter clinical transplantation trial. Using various immunohistochemical techniques, we found a discrepancy in the survival of grafted projection neurons with respect to grafted interneurons as well as major ongoing inflammatory and immune responses to the grafted tissue with evidence of mutant huntingtin aggregates within the transplant area. Our results indicate that grafts can survive more than a decade post-transplantation, but show compromised survival with inflammation and mutant protein being observed within the transplant site. Ann Neurol 2018;84:950-956.

Propagation and spread of pathogenic protein assemblies in neurodegenerative diseases

Many neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis, are characterized by the progressive appearance of abnormal proteinaceous assemblies in the nervous system. Studies in experimental systems indicate that the assemblies originate from the prion-like seeded aggregation of specific misfolded proteins that proliferate and amass to form the intracellular and/or extracellular lesions typical of each disorder. The host in which the proteopathic seeds arise provides the biochemical and physiological environment that either supports or restricts their emergence, proliferation, self-assembly, and spread. Multiple mechanisms influence the spatiotemporal spread of seeds and the nature of the resulting lesions, one of which is the cellular uptake, release, and transport of seeds along neural pathways and networks. The characteristics of cells and regions in the affected network govern their vulnerability and thereby influence the neuropathological and clinical attributes of the disease. The propagation of pathogenic protein assemblies within the nervous system is thus determined by the interaction of the proteopathic agent and the host milieu.

N-terminal Huntingtin (Htt) phosphorylation is a molecular switch regulating Htt aggregation, helical conformation, internalization, and nuclear targeting

Huntington's disease is a fatal neurodegenerative disorder resulting from a CAG repeat expansion in the first exon of the gene encoding the Huntingtin protein (Htt). Phosphorylation of this protein region (Httex1) has been shown to play important roles in regulating the structure, toxicity, and cellular properties of N-terminal fragments and full-length Htt. However, increasing evidence suggests that phosphomimetic substitutions in Htt result in inconsistent findings and do not reproduce all aspects of true phosphorylation. Here, we investigated the effects of bona fide phosphorylation at Ser-13 or Ser-16 on the structure, aggregation, membrane binding, and subcellular properties of the Httex1-Q18A variant and compared these effects with those of phosphomimetic substitutions. We show that phosphorylation at either Ser-13 and/or Ser-16 or phosphomimetic substitutions at both these residues inhibit the aggregation of mutant Httex1, but that only phosphorylation strongly disrupts the amphipathic α-helix of the N terminus and prompts the internalization and nuclear targeting of preformed Httex1 aggregates. In synthetic peptides, phosphorylation at Ser-13, Ser-16, or both residues strongly disrupted the amphipathic α-helix of the N-terminal 17 residues (Nt17) of Httex1 and Nt17 membrane binding. Experiments with peptides bearing different combinations of phosphorylation sites within Nt17 revealed a phosphorylation-dependent switch that regulates the Httex1 structure, involving cross-talk between phosphorylation at Thr-3 and Ser-13 or Ser-16. Our results provide crucial insights into the role of phosphorylation in regulating Httex1 structure and function, and underscore the critical importance of identifying the enzymes responsible for regulating Htt phosphorylation, and their potential as therapeutic targets for managing Huntington's disease.

Intercellular Spread of Protein Aggregates in Neurodegenerative Disease

Most neurodegenerative diseases are characterized by the accumulation of protein aggregates, some of which are toxic to cells. Mounting evidence demonstrates that in several diseases, protein aggregates can pass from neuron to neuron along connected networks, although the role of this spreading phenomenon in disease pathogenesis is not completely understood. Here we briefly review the molecular and histopathological features of protein aggregation in neurodegenerative disease, we summarize the evidence for release of proteins from donor cells into the extracellular space, and we highlight some other mechanisms by which protein aggregates might be transmitted to recipient cells. We also discuss the evidence that supports a role for spreading of protein aggregates in neurodegenerative disease pathogenesis and some limitations of this model. Finally, we consider potential therapeutic strategies to target spreading of protein aggregates in the treatment of neurodegenerative diseases.

Unconventional Secretion and Intercellular Transfer of Mutant Huntingtin

The mechanism of intercellular transmission of pathological agents in neurodegenerative diseases has received much recent attention. Huntington’s disease (HD) is caused by a monogenic mutation in the gene encoding Huntingtin (HTT). Mutant HTT (mHTT) harbors a CAG repeat extension which encodes an abnormally long polyglutamine (polyQ) repeat at HTT’s N-terminus. Neuronal pathology in HD is largely due to the toxic gain-of-function by mHTT and its proteolytic products, which forms both nuclear and cytoplasmic aggregates that perturb nuclear gene transcription, RNA splicing and transport as well cellular membrane dynamics. The neuropathological effects of mHTT have been conventionally thought to be cell-autonomous in nature. Recent findings have, however, indicated that mHTT could be secreted by neurons, or transmitted from one neuronal cell to another via different modes of unconventional secretion, as well as via tunneling nanotubes (TNTs). These modes of transmission allow the intercellular spread of mHTT and its aggregates, thus plausibly promoting neuropathology within proximal neuronal populations and between neurons that are connected within neural circuits. Here, the various possible modes for mHTT’s neuronal cell exit and intercellular transmission are discussed.

New directions in therapeutics for Huntington disease

Huntington disease (HD) is an autosomal dominantly inherited neurodegenerative disease that affects motor, cognitive and psychiatric functions, and ultimately leads to death. The pathology of the disease is based on an expansion of CAG repeats in exon 1 of the huntingtin gene on chromosome 4, which produces a mutant huntingtin protein (mHtt). This protein is involved in neurotoxicity and brain atrophy, and can form β-sheets and abnormal mHtt aggregates. Currently, there are no approved effective treatments for HD, although tetrabenazine (Xenazine™) and deutetrabenazine (AUSTEDO™) have been approved for treatment of the motor symptom chorea in HD. This literature review aims to address the latest research on promising therapeutics based on influencing the hypothesized pathological mechanisms.

Tau: A Common Denominator and Therapeutic Target for Neurodegenerative Disorders

There is compelling evidence that a number of neurodegenerative diseases share common pathogenic mechanisms. Better understanding these mechanisms will allow us to develop new therapeutic strategies. This commentary follows up on our recent findings that tau pathology can be found in healthy fetal tissue transplanted into the brain of patients with either Huntington or Parkinson disease. We will examine how tau appears to be shared in a number of different conditions and how its expression relates to cognitive decline and disease progression. We will further review pathogenic mechanisms and especially the relevance of the possible prion-like behavior of tau. We will conclude by discussing how all this work opens up novel therapeutic approaches to treating the cognitive impairments related to neurodegenerative diseases using a common strategy.

Dose-Dependent Lowering of Mutant Huntingtin Using Antisense Oligonucleotides in Huntington Disease Patients

On December 11 of 2017, Ionis Pharmaceuticals published a press release announcing dose-dependent reductions of mutant huntingtin protein in their HTTRx Phase 1/2a study in Huntington disease (HD) patients. The results from this Ionis trial have gained much attention from the patient community and the oligonucleotide therapeutics field, since it is the first trial targeting the cause of HD, namely the mutant huntingtin protein, using antisense oligonucleotides (ASOs). The press release also states that the primary endpoints of the study (safety and tolerability) were met, but does not contain data. This news follows the approval of another therapeutic ASO nusinersen (trade name Spinraza) for a neurological disease, spinal muscular atrophy, by the U.S. Food and Drug Administration and European Medicines Agency, in 2016 and 2017, respectively. Combined, this offers hope for the development of the HTTRx therapy for HD patients.

Blood-brain barrier breakdown in Alzheimer disease and other neurodegenerative disorders

The blood-brain barrier (BBB) is a continuous endothelial membrane within brain microvessels that has sealed cell-to-cell contacts and is sheathed by mural vascular cells and perivascular astrocyte end-feet. The BBB protects neurons from factors present in the systemic circulation and maintains the highly regulated CNS internal milieu, which is required for proper synaptic and neuronal functioning. BBB disruption allows influx into the brain of neurotoxic blood-derived debris, cells and microbial pathogens and is associated with inflammatory and immune responses, which can initiate multiple pathways of neurodegeneration. This Review discusses neuroimaging studies in the living human brain and post-mortem tissue as well as biomarker studies demonstrating BBB breakdown in Alzheimer disease, Parkinson disease, Huntington disease, amyotrophic lateral sclerosis, multiple sclerosis, HIV-1-associated dementia and chronic traumatic encephalopathy. The pathogenic mechanisms by which BBB breakdown leads to neuronal injury, synaptic dysfunction, loss of neuronal connectivity and neurodegeneration are described. The importance of a healthy BBB for therapeutic drug delivery and the adverse effects of disease-initiated, pathological BBB breakdown in relation to brain delivery of neuropharmaceuticals are briefly discussed. Finally, future directions, gaps in the field and opportunities to control the course of neurological diseases by targeting the BBB are presented.

Translation of MicroRNA-Based Huntingtin-Lowering Therapies from Preclinical Studies to the Clinic

The single mutation underlying the fatal neuropathology of Huntington's disease (HD) is a CAG triplet expansion in exon 1 of the huntingtin (HTT) gene, which gives rise to a toxic mutant HTT protein. There have been a number of not yet successful therapeutic advances in the treatment of HD. The current excitement in the HD field is due to the recent development of therapies targeting the culprit of HD either at the DNA or RNA level to reduce the overall mutant HTT protein. In this review, we briefly describe short-term and long-term HTT-lowering strategies targeting HTT transcripts. One of the most advanced HTT-lowering strategies is a microRNA (miRNA)-based gene therapy delivered by a single administration of an adeno-associated viral (AAV) vector to the HD patient. We outline the outcome measures for the miRNA-based HTT-lowering therapy in the context of preclinical evaluation in HD animal and cell models. We highlight the strengths and ongoing queries of the HTT-lowering gene therapy as an HD intervention with a potential disease-modifying effect. This review provides a perspective on the fast-developing HTT-lowering therapies for HD and their translation to the clinic based on existing knowledge in preclinical models.

Prion-Like Characteristics of Polyglutamine-Containing Proteins

Transmissible spongiform encephalopathies are infectious neurodegenerative diseases caused by the conversion of prion protein (PrP) into a self-replicating conformation that spreads via templated conversion of natively folded PrP molecules within or between cells. Recent studies provide compelling evidence that prion-like behavior is a general property of most protein aggregates associated with neurodegenerative diseases. Many of these disorders are associated with spontaneous protein aggregation, but genetic mutations can increase the aggregation propensity of specific proteins, including expansion of polyglutamine (polyQ) tracts, which is causative of nine inherited neurodegenerative diseases. Aggregates formed by polyQ-expanded huntingtin (Htt) in Huntington's disease can transfer between cells and seed the aggregation of cytoplasmic wild-type Htt in a prion-like manner. Additionally, prion-like properties of glutamine-rich proteins underlie nonpathological processes in yeast and higher eukaryotes. Here, we review current evidence supporting prion-like characteristics of polyQ and glutamine-rich proteins.

Huntington disease

Huntington disease is a monogenic neurodegenerative disorder that displays an autosomal-dominant pattern of inheritance. It is characterized by motor, psychiatric, and cognitive symptoms that progress over 15-20 years. Since the identification of the causative genetic mutation in 1993 much has been discovered about the underlying pathogenic mechanisms, but as yet there are no disease-modifying therapies available. This chapter reviews the epidemiology, genetic basis, pathogenesis, presentation, and clinical management of Huntington disease. The principles of genetic testing are explained. We also describe recent developments in the ongoing search for therapeutics and for biomarkers to track disease progression.

Possible Role of the Polyglutamine Elongation in Evolution of Amyloid-Related Evolvability

The polyglutamine (polyQ) diseases, such as Huntington's disease and the spinocerebellar ataxias, are characterized by the accumulation of elongated polyQ sequences (epolyQ) and mostly occur during midlife. Considering that polyQ disorders have not been selected out in evolution, there might be important physiological functions of epolyQ during development and/or reproduction. In a similar context, the physiological functions of neurodegeneration-associated amyloidogenic proteins (APs), such as β-amyloid in Alzheimer's disease and α-synuclein in Parkinson's disease, remain elusive. In this regard, we recently proposed that evolvability for coping with diverse stressors in the brain, which is beneficial for offspring, might be relevant to the physiological functions of APs. Given analogous properties of APs and epolyQ in terms of neurotoxic amyloid-fibril formation, the objective of this paper is to determine whether evolvability could also be applied to the physiological functions of epolyQ. Indeed, APs and epolyQ are similar in many ways, including functional redundancy of non-amyloidogenic homologues, hormesis conferred by the heterogeneity of the stress-induced protein aggregates, the transgenerational prion-like transmission of the protein aggregates via germ cells, and the antagonistic pleiotropy relationship between evolvability and neurodegenerative disease. Given that epolyQ is widely expressed from microorganisms to human brain, whereas APs are only identified in vertebrates, evolvability of epolyQ is considered to be much more primitive compared to those of APs during evolution. Collectively, epolyQ may be not only be important in the pathophysiology of polyQ diseases, but also in the evolution of amyloid-related evolvability.

Clinical Features of Huntington's Disease

Huntington's disease (HD) is the most common monogenic neurodegenerative disease and the commonest genetic dementia in the developed world. With autosomal dominant inheritance, typically mid-life onset, and unrelenting progressive motor, cognitive and psychiatric symptoms over 15-20 years, its impact on patients and their families is devastating. The causative genetic mutation is an expanded CAG trinucleotide repeat in the gene encoding the Huntingtin protein, which leads to a prolonged polyglutamine stretch at the N-terminus of the protein. Since the discovery of the gene over 20 years ago much progress has been made in HD research, and although there are currently no disease-modifying treatments available, there are a number of exciting potential therapeutic developments in the pipeline. In this chapter we discuss the epidemiology, genetics and pathogenesis of HD as well as the clinical presentation and management of HD, which is currently focused on symptomatic treatment. The principles of genetic testing for HD are also explained. Recent developments in therapeutics research, including gene silencing and targeted small molecule approaches are also discussed, as well as the search for HD biomarkers that will assist the validation of these potentially new treatments.

Mutant huntingtin protein expression and blood-spinal cord barrier dysfunction in huntington disease

OBJECTIVE

The aim of the study was to assess the distribution, frequency, and specific location of mutant huntingtin protein (mHTT) aggregates-the pathological hallmark of Huntington disease (HD)-within the various compartments of the spinal cord and their potential impact on the local vasculature and blood-spinal cord barrier (BSCB).

METHODS

We performed a series of postmortem immunohistochemical and immunofluorescent stainings, as well as Western blot analyses, on cervical and lumbar sections of the spinal cord in patients diagnosed with HD (n = 11 of all grades of disease severity) along with sex- and age-matched healthy controls (n = 9).

RESULTS

We observed that mHTT was preferably expressed within the anterior horn of the gray matter, in both cervical and lumbar sections. At the cellular level, mHTT aggregates were more often encountered in the extracellular matrix but could also be observed within cell bodies and neurites as well as within the endothelium of blood vessels with an increase in the density of small blood vessels in cervical sections of HD cases. These vasculature changes were accompanied with features of BSCB leakage, as assessed by the presence of increased levels of fibrinogen in the surrounding parenchyma and enhanced leukocyte infiltration.

INTERPRETATION

This alteration in BSCB integrity may be explained, in part, by the dysregulation we found in some of the main proteins associated with it such as junctional adhesion molecule-1 and vascular endothelial cadherin. These observations have important implications for our understanding of HD pathology and may also have significant therapeutic implications. Ann Neurol 2017;82:981-994.

Cell-to-cell Transmission of Polyglutamine Aggregates in C. elegans

Huntington disease (HD) is an inherited neurodegenerative disorder characterized by motor and cognitive dysfunction caused by expansion of polyglutamine (polyQ) repeat in exon 1 of huntingtin (HTT). In patients, the number of glutamine residues in polyQ tracts are over 35, and it is correlated with age of onset, severity, and disease progression. Expansion of polyQ increases the propensity for HTT protein aggregation, process known to be implicated in neurodegeneration. These pathological aggregates can be transmitted from neuron to another neuron, and this process may explain the pathological spreading of polyQ aggregates. Here, we developed an in vivo model for studying transmission of polyQ aggregates in a highly quantitative manner in real time. HTT exon 1 with expanded polyQ was fused with either N-terminal or C-terminal fragments of Venus fluorescence protein and expressed in pharyngeal muscles and associated neurons, respectively, of C. elegans. Transmission of polyQ proteins was detected using bimolecular fluorescence complementation (BiFC). Mutant polyQ (Q97) was transmitted much more efficiently than wild type polyQ (Q25) and forms numerous inclusion bodies as well. The transmission of Q97 was gradually increased with aging of animal. The animals with polyQ transmission exhibited degenerative phenotypes, such as nerve degeneration, impaired pharyngeal pumping behavior, and reduced life span. The C. elegans model presented here would be a useful in vivo model system for the study of polyQ aggregate propagation and might be applied to the screening of genetic and chemical modifiers of the propagation.

Human Neural Stem Cell Transplantation Rescues Functional Deficits in R6/2 and Q140 Huntington's Disease Mice

Huntington's disease (HD) is an inherited neurodegenerative disorder with no disease-modifying treatment. Expansion of the glutamine-encoding repeat in the Huntingtin (HTT) gene causes broad effects that are a challenge for single treatment strategies. Strategies based on human stem cells offer a promising option. We evaluated efficacy of transplanting a good manufacturing practice (GMP)-grade human embryonic stem cell-derived neural stem cell (hNSC) line into striatum of HD modeled mice. In HD fragment model R6/2 mice, transplants improve motor deficits, rescue synaptic alterations, and are contacted by nerve terminals from mouse cells. Furthermore, implanted hNSCs are electrophysiologically active. hNSCs also improved motor and late-stage cognitive impairment in a second HD model, Q140 knockin mice. Disease-modifying activity is suggested by the reduction of aberrant accumulation of mutant HTT protein and expression of brain-derived neurotrophic factor (BDNF) in both models. These findings hold promise for future development of stem cell-based therapies.

The Evidence for the Spread and Seeding Capacities of the Mutant Huntingtin Protein in in Vitro Systems and Their Therapeutic Implications

Neurodegenerative disorders are not only characterized by specific patterns of cell loss but the presence and accumulation of various pathological proteins—both of which correlate with disease evolution. There is now mounting evidence to suggest that these pathological proteins present with toxic, at times prion-like, properties and can therefore seed pathology in neighboring as well remotely connected healthy neurons as they spread across the brain. What is less clear, at this stage, is how much this actually contributes to, and drives, the core pathogenic events. In this review, we present a comprehensive, up-to-date summary of the reported in vitro studies that support the spreading and seeding capacities of pathological proteins, with an emphasis on mutant huntingtin protein in the context of Huntington's disease, although in vivo work remains to be performed to validate this theory in this particular disease. We have further reviewed these findings in light of their potential implications for the development of novel therapeutic approaches.

Indications and prospects of neural transplantation for chronic neurological diseases

PURPOSE OF REVIEW

The replacement of damaged cells in the central nervous system (CNS) affected by degenerative disorders represents an attractive therapeutic strategy. The advent of stem cell technology may offer the possibility of generating a large number of renewable, specifically differentiated cells to potentially cure large cohorts of patients. In this review, we discuss current knowledge and issues involved in neural cell transplantation. The most important preclinical and clinical results of cellular transplantation applied to Parkinson's, Huntington's disease and amyotrophic lateral sclerosis will be summarized.

RECENT FINDINGS

Cellular transplantation is emerging as a possible therapy for a variety of incurable neurological disorders. The disorders that will primarily take advantage from neural stem cell grafting are those involving a well defined cell population in a restricted area of the CNS. Several clinical trials have been initiated to assess safety and efficacy of different stem cell-derived products, and promising results have been obtained for disorders such as Parkinson's disease. However, several scientific questions remain unanswered. Among these, the impact of the immunological interaction between host and graft in the particular environment of the CNS still requires additional investigations.

SUMMARY

Several chronic neurological disorders appear to be amenable to cell regenerative therapies. However, safety, efficacy and immunological issues will need to be carefully evaluated beforehand.

Tissue specific microenvironments: a key tool for tissue engineering and regenerative medicine

The accumulated evidence points to the microenvironment as the primary mediator of cellular fate determination. Comprised of parenchymal cells, stromal cells, structural extracellular matrix proteins, and signaling molecules, the microenvironment is a complex and synergistic edifice that varies tissue to tissue. Furthermore, it has become increasingly clear that the microenvironment plays crucial roles in the establishment and progression of diseases such as cardiovascular disease, neurodegeneration, cancer, and ageing. Here we review the historical perspectives on the microenvironment, and how it has directed current explorations in tissue engineering. By thoroughly understanding the role of the microenvironment, we can begin to correctly manipulate it to prevent and cure diseases through regenerative medicine techniques.

Induced Pluripotent Stem Cell-Derived Neural Stem Cell Transplantations Reduced Behavioral Deficits and Ameliorated Neuropathological Changes in YAC128 Mouse Model of Huntington's Disease

Huntington's disease (HD) is a genetic neurodegenerative disorder characterized by neuronal loss and motor dysfunction. Although there is no effective treatment, stem cell transplantation offers a promising therapeutic strategy, but the safety and efficacy of this approach needs to be optimized. The purpose of this study was to test the potential of intra-striatal transplantation of induced pluripotent stem cell-derived neural stem cells (iPS-NSCs) for treating HD. For this purpose, we developed mouse adenovirus-generated iPSCs, differentiated them into neural stem cells in vitro, labeled them with Hoechst, and transplanted them bilaterally into striata of 10-month old wild type (WT) and HD YAC128 mice. We assessed the efficiency of these transplanted iPS-NSCs to reduce motor deficits in YAC128 mice by testing them on an accelerating rotarod task at 1 day prior to transplantation, and then weekly for 10 weeks. Our results showed an amelioration of locomotor deficits in YAC128 mice that received iPS-NSC transplantations. Following testing, the mice were sacrificed, and their brains were analyzed using immunohistochemistry and Western blot (WB). The results from our histological examinations revealed no signs of tumors and evidence that many iPS-NSCs survived and differentiated into region-specific neurons (medium spiny neurons) in both WT and HD mice, as confirmed by co-labeling of Hoechst-labeled transplanted cells with NeuN and DARPP-32. Also, counts of Hoechst-labeled cells revealed that a higher proportion were co-labeled with DARPP-32 and NeuN in HD-, compared to WT- mice, suggesting a dissimilar differentiation pattern in HD mice. Whereas significant decreases were found in counts of NeuN- and DARPP-32-labeled cells, and for neuronal density measures in striata of HD vehicle controls, such decrements were not observed in the iPS-NSCs-transplanted-HD mice. WB analysis showed increase of BDNF and TrkB levels in striata of transplanted HD mice compared to HD vehicle controls. Collectively, our data suggest that iPS-NSCs may provide an effective option for neuronal replacement therapy in HD.

Presence of tau pathology within foetal neural allografts in patients with Huntington's and Parkinson's disease

Cell replacement has been explored as a therapeutic strategy to repair the brain in patients with Huntington's and Parkinson's disease. Post-mortem evaluations of healthy grafted tissue in such cases have revealed the development of Huntington- or Parkinson-like pathology including mutant huntingtin aggregates and Lewy bodies. An outstanding question remains if tau pathology can also be seen in patients with Huntington's and Parkinson's disease who had received foetal neural allografts. This was addressed by immunohistochemical/immunofluorescent stainings performed on grafted tissue of two Huntington's disease patients, who came to autopsy 9 and 12 years post-transplantation, and two patients with Parkinson's disease who came to autopsy 18 months and 16 years post-transplantation. We show that grafts also contain tau pathology in both types of transplanted patients. In two patients with Huntington's disease, the grafted tissue showed the presence of hyperphosphorylated tau [both AT8 (phospho-tau Ser202 and Thr205) and CP13 (pSer202) immunohistochemical stainings] pathological inclusions, neurofibrillary tangles and neuropil threads. In patients with Parkinson's disease, the grafted tissue was characterized by hyperphosphorylated tau (AT8; immunofluorescent staining) pathological inclusions, neurofibrillary tangles and neuropil threads but only in the patient who came to autopsy 16 years post-transplantation. Abundant tau-related pathology was observed in the cortex and striatum of all cases studied. While the striatum of the grafted Huntington's disease patient revealed an equal amount of 3-repeat and 4-repeat isoforms of tau, the grafted tissue showed elevated 4-repeat isoforms by western blot. This suggests that transplants may have acquired tau pathology from the host brain, although another possibility is that this was due to acceleration of ageing. This finding not only adds to the recent reports that tau pathology is a feature of these neurodegenerative diseases, but also that tau pathology can manifest in healthy neural tissue transplanted into the brains of patients with two distinct neurodegenerative disorders.