Bilateral human fetal striatal transplantation in Huntington's disease

Evidence of mutant huntingtin and tau-related pathology within neuronal grafts in Huntington's disease cases

A number of post-mortem studies conducted in transplanted Huntington's disease (HD) patients from various trials have reported the presence of pathological and misfolded proteins, in particular mutant huntingtin (mHtt) and phosphorylated tau neuropil threads, in the healthy grafted tissue. Here, we extended these observations with histological analysis of post-mortem tissue from three additional HD patients who had received similar striatal allografts from the fetal tissue transplantation trial conducted in Los Angeles in 1998. Immunohistochemical staining was performed using anti-mHtt antibodies, EM48 and MW7, as well as anti-hyperphosphorylated tau antibodies, AT8 and CP13. Immunofluorescence was used to assess the colocalization of EM48+ mHtt aggregates with the neuronal marker MAP2 and/or the extracellular matrix protein phosphacan in both the host and grafts. We confirmed the presence of mHtt aggregates within grafts of all three cases as well as tau neuropil threads in the grafts of two of the three transplanted HD patients. Phosphorylated tau was also variably expressed in the host cerebral cortex of all three subjects. While mHtt inclusions were present within neurons (immunofluorescence co-localization of MAP2 and EM48) as well as within the extracellular matrix of the host (immunofluorescence co-localization of phosphacan and EM48), their localization was limited to the extracellular matrix in the grafted tissue. This study corroborates previous findings that both mHtt and tau pathology can be found in the host and grafts of HD patients years post-grafting.

Stem Cell Therapies in Movement Disorders: Lessons from Clinical Trials

Stem cell-based therapies (SCT) to treat neurodegenerative disorders have promise but clinical trials have only recently begun, and results are not expected for several years. While most SCTs largely lead to a symptomatic therapeutic effect by replacing lost cell types, there may also be disease-modifying therapeutic effects. In fact, SCT may complement a multi-drug, subtype-specific therapeutic approach, consistent with the idea of precision medicine, which matches molecular therapies to biological subtypes of disease. In this narrative review, we examine published and ongoing trials in SCT in Parkinson's Disease, atypical parkinsonian disorders, Huntington's disease, amyotrophic lateral sclerosis, and spinocerebellar ataxia in humans. We discuss the benefits and pitfalls of using this treatment approach within the spectrum of disease-modification efforts in neurodegenerative diseases. SCT may hold greater promise in the treatment of neurodegenerative disorders, but much research is required to determine the feasibility, safety, and efficacy of these complementary aims of therapeutic efforts.

Stem-Cell-Based Therapy: The Celestial Weapon against Neurological Disorders

Stem cells are a versatile source for cell therapy. Their use is particularly significant for the treatment of neurological disorders for which no definitive conventional medical treatment is available. Neurological disorders are of diverse etiology and pathogenesis. Alzheimer's disease (AD) is caused by abnormal protein deposits, leading to progressive dementia. Parkinson's disease (PD) is due to the specific degeneration of the dopaminergic neurons causing motor and sensory impairment. Huntington's disease (HD) includes a transmittable gene mutation, and any treatment should involve gene modulation of the transplanted cells. Multiple sclerosis (MS) is an autoimmune disorder affecting multiple neurons sporadically but induces progressive neuronal dysfunction. Amyotrophic lateral sclerosis (ALS) impacts upper and lower motor neurons, leading to progressive muscle degeneration. This shows the need to try to tailor different types of cells to repair the specific defect characteristic of each disease. In recent years, several types of stem cells were used in different animal models, including transgenic animals of various neurologic disorders. Based on some of the successful animal studies, some clinical trials were designed and approved. Some studies were successful, others were terminated and, still, a few are ongoing. In this manuscript, we aim to review the current information on both the experimental and clinical trials of stem cell therapy in neurological disorders of various disease mechanisms. The different types of cells used, their mode of transplantation and the molecular and physiologic effects are discussed. Recommendations for future use and hopes are highlighted.

Concise Review: Recent advances in neuroimaging techniques to assist clinical trials on cell-based therapies in neurodegenerative diseases

Neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), are progressive disorders for which a curative therapy is still lacking. Cell-based therapy aims at replacing dysfunctional cellular populations by repairing damaged tissue and by enriching the microenvironment of selective brain areas, and thus constitutes a promising disease-modifying treatment of neurodegenerative diseases. Scientific research has engineered a wide range of human-derived cellular populations to help overcome some of the logistical, safety, and ethical issues associated with this approach. Open-label studies and clinical trials in human participants have employed neuroimaging techniques, such as positron emission tomography (PET) and magnetic resonance imaging (MRI), to assess the success of the transplantation, to evaluate the functional integration of the implanted tissue into the host environment and to understand the pathophysiological changes associated with the therapy. Neuroimaging has constituted an outcome measure of large, randomized clinical trials, and has given answers to clarify the pathophysiology underlying some of the complications linked with this therapy. Novel PET radiotracers and MRI sequences for the staging of neurodegenerative diseases and to study alterations at molecular level significantly expands the translational potential of neuroimaging to assist pre-clinical and clinical research on cell-based therapy in these disorders. This concise review summarizes the current use of neuroimaging in human studies of cell-based replacement therapy and focuses on future application of PET and MRI techniques to evaluate the pathophysiology and treatment efficacy, as well as to aid patient selection and as an outcome measure to improve treatment success.

Translating cell therapies for neurodegenerative diseases: Huntington's disease as a model disorder

There has been substantial progress in the development of regenerative medicine strategies for CNS disorders over the last decade, with progression to early clinical studies for some conditions. However, there are multiple challenges along the translational pipeline, many of which are common across diseases and pertinent to multiple donor cell types. These include defining the point at which the preclinical data are sufficiently compelling to permit progression to the first clinical studies; scaling-up, characterization, quality control and validation of the cell product; design, validation and approval of the surgical device; and operative procedures for safe and effective delivery of cell product to the brain. Furthermore, clinical trials that incorporate principles of efficient design and disease-specific outcomes are urgently needed (particularly for those undertaken in rare diseases, where relatively small cohorts are an additional limiting factor), and all processes must be adaptable in a dynamic regulatory environment. Here we set out the challenges associated with the clinical translation of cell therapy, using Huntington's disease as a specific example, and suggest potential strategies to address these challenges. Huntington's disease presents a clear unmet need, but, importantly, it is an autosomal dominant condition with a readily available gene test, full genetic penetrance and a wide range of associated animal models, which together mean that it is a powerful condition in which to develop principles and test experimental therapeutics. We propose that solving these challenges in Huntington's disease would provide a road map for many other neurological conditions. This white paper represents a consensus opinion emerging from a series of meetings of the international translational platforms Stem Cells for Huntington's Disease and the European Huntington's Disease Network Advanced Therapies Working Group, established to identify the challenges of cell therapy, share experience, develop guidance and highlight future directions, with the aim to expedite progress towards therapies for clinical benefit in Huntington's disease.

Considerations for clinical trial design and conduct in the evaluation of novel advanced therapeutics in neurodegenerative disease

The recent advances in the development of potentially disease modifying cell and gene therapies for neurodegenerative disease has resulted in the production of a number of promising novel therapies which are now moving forward to clinical evaluation. The robust evaluation of these therapies pose a significant number of challenges when compared to more traditional evaluations of pharmacotherapy, which is the current mainstay of neurodegenerative disease symptom management. Indeed, there is an inherent complexity in the design and conduct of these trials at multiple levels. Here we discuss specific aspects requiring consideration in the context of investigating novel cell and gene therapies for neurodegenerative disease. This extends to overarching trial designs that could be employed and the factors that underpin design choices such outcome assessments, participant selection and methods for delivery of cell and gene therapies. We explore methods of data collection that may improve efficiency in trials of cell and gene therapy to maximize data sharing and collaboration. Lastly, we explore some of the additional context beyond efficacy evaluations that should be considered to ensure implementation across relevant healthcare settings.

Revisiting cell and gene therapies in Huntington's disease

Neurodegenerative movement disorders, such as Huntington's disease (HD), share a progressive and relentless course with increasing motor disability, linked with neuropsychiatric impairment. These diseases exhibit diverse pathophysiological processes and are a topic of intense experimental and clinical research due to the lack of therapeutic options. Restorative therapies are promising approaches with the potential to restore brain circuits. However, there were less compelling results in the few clinical trials. In this review, we discuss cell replacement therapies applied to animal models and HD patients. We thoroughly describe the initial trials using fetal neural tissue transplantation and recent approaches based on alternative cell sources tested in several animal models. Stem cells were shown to generate the desired neuron phenotype and/or provide growth factors to the degenerating host cells. Besides, genetic approaches such as RNA interference and the CRISPR/Cas9 system have been studied in animal models and human-derived cells. New genetic manipulations have revealed the capability to control or counteract the effect of human gene mutations as described by the use of antisense oligonucleotides in a clinical trial. In HD, innovative strategies are at forefront of human testing and thus other brain genetic diseases may follow similar therapeutic strategies.

Molecular Components of Store-Operated Calcium Channels in the Regulation of Neural Stem Cell Physiology, Neurogenesis, and the Pathology of Huntington's Disease

One of the major Ca²⁺ signaling pathways is store-operated Ca²⁺ entry (SOCE), which is responsible for Ca²⁺ flow into cells in response to the depletion of endoplasmic reticulum Ca²⁺ stores. SOCE and its molecular components, including stromal interaction molecule proteins, Orai Ca²⁺ channels, and transient receptor potential canonical channels, are involved in the physiology of neural stem cells and play a role in their proliferation, differentiation, and neurogenesis. This suggests that Ca²⁺ signaling is an important player in brain development. Huntington’s disease (HD) is an incurable neurodegenerative disorder that is caused by polyglutamine expansion in the huntingtin (HTT) protein, characterized by the loss of γ-aminobutyric acid (GABA)-ergic medium spiny neurons (MSNs) in the striatum. However, recent research has shown that HD is also a neurodevelopmental disorder and Ca²⁺ signaling is dysregulated in HD. The relationship between HD pathology and elevations of SOCE was demonstrated in different cellular and mouse models of HD and in induced pluripotent stem cell-based GABAergic MSNs from juvenile- and adult-onset HD patient fibroblasts. The present review discusses the role of SOCE in the physiology of neural stem cells and its dysregulation in HD pathology. It has been shown that elevated expression of STIM2 underlying the excessive Ca²⁺ entry through store-operated calcium channels in induced pluripotent stem cell-based MSNs from juvenile-onset HD. In the light of the latest findings regarding the role of Ca²⁺ signaling in HD pathology we also summarize recent progress in the in vitro differentiation of MSNs that derive from different cell sources. We discuss advances in the application of established protocols to obtain MSNs from fetal neural stem cells/progenitor cells, embryonic stem cells, induced pluripotent stem cells, and induced neural stem cells and the application of transdifferentiation. We also present recent progress in establishing HD brain organoids and their potential use for examining HD pathology and its treatment. Moreover, the significance of stem cell therapy to restore normal neural cell function, including Ca²⁺ signaling in the central nervous system in HD patients will be considered. The transplantation of MSNs or their precursors remains a promising treatment strategy for HD.

Cell therapy in Huntington's disease: Taking stock of past studies to move the field forward

Huntington's disease (HD) is a rare inherited neurodegenerative disease that manifests mostly in adulthood with progressive cognitive, behavioral, and motor dysfunction. Neuronal loss occurs predominantly in the striatum but also extends to other brain regions, notably the cortex. Most patients die around 20 years after motor onset, although there is variability in the rate of progression and some phenotypic heterogeneity. The most advanced experimental therapies currently are huntingtin-lowering strategies, some of which are in stage 3 clinical trials. However, even if these approaches are successful, it is unlikely that they will be applicable to all patients or will completely halt continued loss of neural cells in all cases. On the other hand, cellular therapies have the potential to restore atrophied tissues and may therefore provide an important complementary therapeutic avenue. Pilot studies of fetal cell grafts in the 2000s reported the most dramatic clinical improvements yet achieved for this disease, but subsequent studies have so far failed to identify methodology to reliably reproduce these results. Moving forward, a major challenge will be to generate suitable donor cells from (nonfetal) cell sources, but in parallel there are a host of procedural and trial design issues that will be important for improving reliability of transplants and so urgently need attention. Here, we consider findings that have emerged from clinical transplant studies in HD to date, in particular new findings emerging from the recent multicenter intracerebral transplant HD study, and consider how these data may be used to inform future cell therapy trials.

Cell Replacement Therapy for Huntington's Disease

Huntington's disease (HD) is an inherited neurodegenerative disorder which is characterised by a triad of highly debilitating motor, cognitive, and psychiatric symptoms. While cell death occurs in many brain regions, GABAergic medium spiny neurons (MSNs) in the striatum experience preferential and extensive degeneration. Unlike most neurodegenerative disorders, HD is caused by a single genetic mutation resulting in a CAG repeat expansion and the production of a mutant Huntingtin protein (mHTT). Despite identifying the mutation causative of HD in 1993, there are currently no disease-modifying treatments for HD. One potential strategy for the treatment of HD is the development of cell-based therapies. Cell-based therapies aim to restore neuronal circuitry and function by replacing lost neurons, as well as providing neurotropic support to prevent further degeneration. In order to successfully restore basal ganglia functioning in HD, cell-based therapies would need to reconstitute the complex signalling network disrupted by extensive MSN degeneration. This chapter will discuss the potential use of foetal tissue grafts, pluripotent stem cells, neural stem cells, and somatic cell reprogramming to develop cell-based therapies for treating HD.

Human stem cell models of polyglutamine diseases: Sources for disease models and cell therapy

Polyglutamine (polyQ) diseases are a group of neurodegenerative disorders involving expanded CAG repeats in pathogenic genes that are translated into extended polyQ tracts and lead to progressive neuronal degeneration in the affected brain. To date, there is no effective therapy for these diseases. Due to the complex pathologic mechanisms of these diseases, intensive research on the pathogenesis of their progression and potential treatment strategies is being conducted. However, animal models cannot recapitulate all aspects of neuronal degeneration. Pluripotent stem cells (PSCs), such as induced pluripotent stem cells (iPSCs) and embryonic stem cells (ESCs), can be used to study the pathological mechanisms of polyQ diseases, and the ability of autologous stem cell transplantation to treat these diseases. Differentiated PSCs, neuronal precursor cells/neural progenitor cells (NPCs) and mesenchymal stem cells (MSCs) are valuable resources for preclinical and clinical cell transplantation therapies. Here, we discuss diverse stem cell models and their ability to generate neurons involved in polyQ diseases, such as medium spiny neurons (MSNs), cortical neurons, cerebellar Purkinje cells (PCs) and motor neurons. In addition, we discuss potential therapeutic approaches, including stem cell replacement therapy and gene therapy.

Beyond the Hippocampus and the SVZ: Adult Neurogenesis Throughout the Brain

Convincing evidence has repeatedly shown that new neurons are produced in the mammalian brain into adulthood. Adult neurogenesis has been best described in the hippocampus and the subventricular zone (SVZ), in which a series of distinct stages of neuronal development has been well characterized. However, more recently, new neurons have also been found in other brain regions of the adult mammalian brain, including the hypothalamus, striatum, substantia nigra, cortex, and amygdala. While some studies have suggested that these new neurons originate from endogenous stem cell pools located within these brain regions, others have shown the migration of neurons from the SVZ to these regions. Notably, it has been shown that the generation of new neurons in these brain regions is impacted by neurologic processes such as stroke/ischemia and neurodegenerative disorders. Furthermore, numerous factors such as neurotrophic support, pharmacologic interventions, environmental exposures, and stem cell therapy can modulate this endogenous process. While the presence and significance of adult neurogenesis in the human brain (and particularly outside of the classical neurogenic regions) is still an area of debate, this intrinsic neurogenic potential and its possible regulation through therapeutic measures present an exciting alternative for the treatment of several neurologic conditions. This review summarizes evidence in support of the classic and novel neurogenic zones present within the mammalian brain and discusses the functional significance of these new neurons as well as the factors that regulate their production. Finally, it also discusses the potential clinical applications of promoting neurogenesis outside of the classical neurogenic niches, particularly in the hypothalamus, cortex, striatum, substantia nigra, and amygdala.

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.

Human Fetal Cell Therapy in Huntington's Disease: A Randomized, Multicenter, Phase II Trial

BACKGROUND

Huntington's disease is a rare, severe, inherited neurodegenerative disease in which we assessed the safety and efficacy of grafting human fetal ganglionic eminence intrastriatally.

METHODS

Patients at the early stage of the disease were enrolled in the Multicentric Intracerebral Grafting in Huntington's Disease trial, a delayed-start phase II randomized study. After a run-in period of 12 months, patients were randomized at month 12 to either the treatment group (transplanted at month 13-month 14) or the control group and secondarily treated 20 months later (month 33-month 34). The primary outcome was total motor score compared between both groups 20 months postrandomization (month 32). Secondary outcomes included clinical, imaging, and electrophysiological findings and a comparison of pregraft and postgraft total motor score slopes during the entire study period (month 0-month 52) regardless of the time of transplant.

RESULTS

Of 54 randomized patients, 45 were transplanted; 26 immediately (treatment) and 19 delayed (control). Mean total motor score at month 32 did not differ between groups (treated controls difference in means adjusted for M12: +2.9 [95% confidence interval, -2.8 to 8.6]; P = 0.31). Its rate of decline after transplantation was similar to that before transplantation. A total of 27 severe adverse events were recorded in the randomized patients, 10 of which were related to the transplant procedure. Improvement of procedures during the trial significantly decreased the frequency of surgical events.We found antihuman leucocytes antigen antibodies in 40% of the patients.

CONCLUSION

No clinical benefit was found in this trial. This may have been related to graft rejection. Ectopia and high track number negatively influence the graft outcome. Procedural adjustments substantially improved surgical safety. (ClinicalTrials.gov NCT00190450.) © 2020 International Parkinson and Movement Disorder Society.

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.

Deconstructing Neurogenesis, Transplantation and Genome-Editing as Neural Repair Strategies in Brain Disease

Neural repair in injury and disease presents a pressing unmet need in regenerative medicine. Due to the intrinsically reduced ability of the brain to replace lost and damaged neurons, reversing long-term cognitive and functional impairments poses a unique problem. Over the years, advancements in cellular and molecular understanding of neurogenesis mechanisms coupled with sophistication of biotechnology tools have transformed neural repair into a cross-disciplinary field that integrates discoveries from developmental neurobiology, transplantation and tissue engineering to design disease- and patient-specific remedies aimed at boosting either native rehabilitation or delivering exogenous hypoimmunogenic interventions. Advances in deciphering the blueprint of neural ontogenesis and annotation of the human genome has led to the development of targeted therapeutic opportunities that have the potential of treating the most vulnerable patient populations and whose findings from benchside suggest looming clinical translation. This review discusses how findings from studies of adult neurogenesis have informed development of interventions that target endogenous neural regenerative machineries and how advances in biotechnology, including the use of new gene-editing tools, have made possible the development of promising, complex neural transplant-based strategies. Adopting a multi-pronged strategy that is tailored to underlying neural pathology and that encompasses facilitation of endogenous regeneration, correction of patient’s genomic mutations and delivery of transformed neural precursors and mature disease-relevant neuronal populations to replace injured or lost neural tissue remains no longer a fantasy.

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.

The Effect of Tissue Preparation and Donor Age on Striatal Graft Morphology in the Mouse

Huntington's disease (HD) is a progressive neurodegenerative disease in which striatal medium spiny neurons (MSNs) are lost. Neuronal replacement therapies aim to replace MSNs through striatal transplantation of donor MSN progenitors, which successfully improve HD-like deficits in rat HD models and have provided functional improvement in patients. Transplants in mouse models of HD are more variable and have lower cell survival than equivalent rat grafts, yet mice constitute the majority of transgenic HD models. Improving the quality and consistency of mouse transplants would open up access to this wider range of rodent models and facilitate research to increase understanding of graft mechanisms, which is essential to progress transplantation as a therapy for HD. Here we determined how donor age, cell preparation, and donor/host strain choice influenced the quality of primary embryonic grafts in quinolinic acid lesion mouse models of HD. Both a within-strain (W-S) and a between-strain (B-S) donor/host paradigm were used to compare transplants of donor tissues derived from mice at embryonic day E12 and E14 prepared either as dissociated suspensions or as minimally manipulated tissue pieces (TP). Good graft survival was observed, although graft volume and cellular composition were highly variable. The effect of cell preparation on grafts differed significantly depending on donor age, with E14 cell suspensions yielding larger grafts compared to TP. Conversely, TP were more effective when derived from E12 donor tissue. A W-S model produced larger grafts with greater MSN content, and while high levels of activated microglia were observed across all groups, a greater number was found in B-S transplants. In summary, we show that the effect of tissue preparation on graft morphology is contingent on the age of donor tissue used. The presence of microglial activation in all groups highlights the host immune response as an important consideration in mouse transplantation.

Imaging Transplantation in Movement Disorders

Cell replacement therapy with graft transplantation has been tested as a disease-modifying treatment in neurodegenerative diseases characterized by the damage of a predominant cell type, such as substantia nigra dopaminergic neurons in Parkinson's disease (PD) or striatal medium spiny projection neurons in Huntington's disease (HD). The results of these trials are mixed with success in preclinical and pilot open-label trials, which were not consistently reproduced in randomized controlled trials. Positron emission tomography (PET) and single photon emission computed tomography (SPECT) molecular imaging and functional magnetic resonance imaging allow the graft survival, and its relationship with the host tissues to be studied in vivo. In PD, PET with [¹⁸F]DOPA showed that graft survival does not necessarily correlate with the clinical improvement and PD patients with worse outcome had lower binding in the ventral striatum and a high serotonin ([¹¹C]DASB PET) to dopamine ([¹⁸F]DOPA PET) ratio in the grafted neurons. In HD, PET with [¹¹C]PK11195 showed the graft survival and the clinical responses may be related to the reactive activation of the host inflammatory/immune system. Findings from these studies have been used to refine study protocols and patient selection in current clinical trials, which includes identifying suitable candidates for transplantation using imaging markers and employing multiple and/or novel PET tracers to better assess graft functions and inflammatory responses to grafts.

Human cellular models of medium spiny neuron development and Huntington disease

The loss of gamma-aminobutyric acid (GABA)-ergic medium spiny neurons (MSNs) in the striatum is the hallmark of Huntington disease (HD), an incurable neurodegenerative disorder characterized by progressive motor, psychiatric, and cognitive symptoms. Transplantation of MSNs or their precursors represents a promising treatment strategy for HD. In initial clinical trials in which HD patients received fetal neurografts directly into the striatum without a pretransplant cell-differentiation step, some patients exhibited temporary benefits. Meanwhile, major challenges related to graft overgrowth, insufficient survival of grafted cells, and limited availability of donated fetal tissue remain. Thus, the development of approaches that allow modeling of MSN differentiation and HD development in cell culture platforms may improve our understanding of HD and translate, ultimately, into HD treatment options. Here, recent advances in the in vitro differentiation of MSNs derived from fetal neural stem cells/progenitor cells (NSCs/NPCs), embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), and induced NSCs (iNSCs) as well as advances in direct transdifferentiation are reviewed. Progress in non-allele specific and allele specific gene editing of HTT is presented as well. Cell characterization approaches involving phenotyping as well as in vitro and in vivo functional assays are also discussed.

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.

Dissection and Preparation of Human Primary Fetal Ganglionic Eminence Tissue for Research and Clinical Applications

Here, we describe detailed dissection and enzymatic dissociation protocols for the ganglionic eminences from the developing human brain to generate viable quasi-single cell suspensions for subsequent use in transplantation or cell culture. These reliable and reproducible protocols can provide tissue for use in the study of the developing human brain, as well as for the preparation of donor cells for transplantation in Huntington's disease (HD). For use in the clinic as a therapy for HD, the translation of these protocols from the research laboratory to the GMP suite is described, including modification to reagents used and appropriate monitoring and tissue release criteria.

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.

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.

Restorative Strategies in Movement Disorders: the Contribution of Imaging

Purpose of Review

The purpose of this review was to review the imaging, particularly positron emission tomography (PET), findings in neurorestoration studies in movement disorders, with specific focus on neural transplantation in Parkinson’s disease (PD) and Huntington’s disease (HD).

Recent Findings

PET findings in PD transplantation studies have shown that graft survival as reflected by increases in dopaminergic PET markers does not necessarily correlate with clinical improvement. PD patients with more denervated ventral striatum and more imbalanced serotonin-to-dopamine ratio in the grafted neurons tended to have worse outcome. In HD transplantation studies, variable graft survival and clinical responses may be related to host inflammatory/immune responses to the grafts.

Summary

Information gleaned from imaging findings in previous neural transplantation studies has been used to refine study protocol and patient selection in future trials. This includes identifying suitable candidates for transplantation using imaging markers, employing multiple and/or novel PET tracers to better assess graft functions and inflammatory responses to grafts.

Stem Cells and Other Emerging Agents as Innovative "Drugs" in Neurodegenerative Diseases: Benefits and Limitations

The brain has a limited process of repair/regeneration linked to the restricted and localized activity of neuronal stem cells. Consequently, it shows a reduced capacity to counteract the age-related loss of neural and glial cells and to repair the consequent injuries/lesions of nervous system. This progressively determines nervous dysfunction and onset/progression of neurodegenerative diseases, which represent a serious social (and economic) problem of our populations. Thus, the research of efficient treatments is encouraged. Stem cell therapy might represent a solution. Today, it, indeed, represents the object of intensive research with the hope of using it, in a near future, as effective therapy for these diseases and preventive treatment in susceptible individuals. Here, we report and discuss the data of the recent studies on this field, underling the obstacles and benefits. We also illustrate alternative measures of intervention, which represent another parallel aim for the care of neurodegenerative pathology-affected individuals. Thus, the road for delaying or retarding these diseases appears hard and long, but the advances might be different.

Recombinant AAV Viral Vectors Serotype 1, 2, and 5 Mediate Differential Gene Transfer Efficiency in Rat Striatal Fetal Grafts

Intrastriatal grafts of fetal ganglionic eminences (GE) can reverse symptoms of striatal lesions in animal models of Huntington's disease. On the other hand, neurotrophic factors have been shown to protect host striatal neurons from ongoing degeneration. Neurotrophic gene transfer into GE prior to grafting could combine the benefits of striatal neuron replacement and in situ delivery of neurotrophic factors. Here we evaluate the potency of recombinant adeno-associated viruses (rAAV) as vectors for gene delivery into rat embryonic (E15) GE using the eGFP reporter gene under the control of the strong cytomegalovirus (CMV) promoter. We observed a very efficient expression of the eGFP reporter gene in organotypic cultures of GE infected with rAAV serotype 1 from 4 days until at least 4 weeks postinfection. In contrast, transduction was low and absent when using serotype 2 and serotype 5 rAAV, respectively. Two months after transplantation of rAAV2/1-infected embryonic GE in adult rat striatum, more than 20% of grafted cells expressed eGFP. The majority of transduced cells in the graft were neurons as indicated by colabeling of GFP-immunoreactive cells with the NeuN marker. Our study suggests that GE transduced by rAAV-serotype 1 vectors could be an interesting tool to mediate efficient expression of a gene coding a neurotrophic factor in Huntington's disease.

In Vivo Evaluation of Pharmacologically Active Microcarriers Releasing Nerve Growth Factor and Conveying PC12 Cells

Cell therapy will probably become a major therapeutic strategy in the coming years. Nevertheless, few cells survive transplantation when employed as a treatment for neuronal disorders. To address this problem, we have developed a new tool, the pharmacologically active microcarriers (PAM). PAM are biocompatible and biodegradable microparticles coated with cell adhesion molecules, conveying cells on their surface and presenting a controlled delivery of growth factor. Thus, the combined effect of growth factor and coating influences the transported cells by promoting their survival and differentiation and favoring their integration in the host tissue after their complete degradation. Furthermore, the released factor may also influence the microenvironment. In this study, we evaluated their efficacy using nerve growth factor (NGF)-releasing PAM and PC12 cells, in a Parkinson's disease paradigm. After implantation of NGF-releasing or unloaded PAM conveying PC12 cells, or PC12 cells alone, we studied cell survival, differentiation, and apoptosis, as well as behavior of the treated rats. We observed that the NGF-releasing PAM coated with two synthetic peptides (poly-D-lysine and fibronectin-like) induced PC12 cell differentiation and reduced cell death and proliferation. Moreover, the animals receiving this implant presented an improved amphetamine-induced rotational behavior. These findings indicate that PAM could be a promising strategy for cell therapy of neurological diseases and could be employed in other situations with fetal cell transplants or with stem cells.

Neural Transplantation in Huntington's Disease: The NEST-UK Donor Tissue Microbiological Screening Program and Review of the Literature

Neural transplantation of human fetal tissue for Huntington's disease (HD) is now entering the clinical arena. The safety of the procedure has now been demonstrated in a number of studies, although the efficacy of such an approach is still being investigated. Stringent but practicable screening of the donor tissue for potential pathogens is an essential prerequisite for successful implementation of any novel transplant program that uses human fetal tissue. In this article we summarize the UK-NEST protocol for the screening of human fetal tissue being grafted to patients with mild to moderate HD. We describe the results of microbiological screening of 87 potential tissue donors in a pilot study, and of the first four donor–recipient patients included in the UK-NEST series. The rationale for the adoption and interpretation of the various tests is described and our methodology is compared with those previously used by other centers. This article therefore presents a comprehensive, logical yet pragmatic screening program that could be employed in any clinical studies that use human fetal tissue for neurotransplantation.

In Vitro Exposure of Cultured Porcine Choroid Plexus Epithelial Cells to Immunosuppressant, Anti-Inflammatory, and Psychoactive Drugs

Delivery of neurotrophic molecules to the CNS is a potential treatment for preventing the neuronal loss in neurological disorders such as Huntington's disease (HD). Choroid plexus (CP) epithelial cell transplants secrete several neurotrophic factors and are neuroprotective in rat and monkey animal models of HD. HD patients receiving CP transplants would likely receive a course of immunosuppressant/anti-inflammatory treatment postsurgery and would remain on psychoactive medications to treat their motor, psychiatric, and emotional symptoms. Therefore, we examined whether CP epithelial cells are impacted by incubation with cyclosporine A (CsA), dexmethasone, haloperidol, fluoxetine, and carbamezapine. In each case, DNA was quantified to determine cell number, a formazen dye-based assay was used to quantify cell metabolism, and vascular endothelial growth factor (VEGF) levels were measured as a marker of protein secretion. Except for the highest dose of fluoxetine, none of the drugs tested exerted any detrimental effect on cell number. Incubation with CsA or dexamethasone did not have any consistent significant effect on VEGF secretion or cell metabolism. Carbamazepine was without effect while only the highest dose of haloperidol tested modestly lowered cell metabolism. VEGF secretion and cell metabolism was not measurable from CP cells exposed to 100 μM fluoxetine. These data continue to support the potential use of CP transplants in HD.

Pluripotent stem cell-derived neurons for transplantation in Huntington's disease

Pluripotent stem cells present a potentially unlimited source of cells for regenerative medicine, providing that they can be efficiently and accurately differentiated to the target cell type. The principle target cell for Huntington's disease is the striatal medium spiny neuron. In this chapter, we review strategies for directing medium spiny neuron differentiation, based on known developmental principles, and we discuss the remaining hurdles on the road to engineering such cells for therapeutic application in Huntington's disease.

Mechanisms and use of neural transplants for brain repair

Under appropriate conditions, neural tissues transplanted into the adult mammalian brain can survive, integrate, and function so as to influence the behavior of the host, opening the prospect of repairing neuronal damage, and alleviating symptoms associated with neuronal injury or neurodegenerative disease. Alternative mechanisms of action have been postulated: nonspecific effects of surgery; neurotrophic and neuroprotective influences on disease progression and host plasticity; diffuse or locally regulated pharmacological delivery of deficient neurochemicals, neurotransmitters, or neurohormones; restitution of the neuronal and glial environment necessary for proper host neuronal support and processing; promoting local and long-distance host and graft axon growth; formation of reciprocal connections and reconstruction of local circuits within the host brain; and up to full integration and reconstruction of fully functional host neuronal networks. Analysis of neural transplants in a broad range of anatomical systems and disease models, on simple and complex classes of behavioral function and information processing, have indicated that all of these alternative mechanisms are likely to contribute in different circumstances. Thus, there is not a single or typical mode of graft function; rather grafts can and do function in multiple ways, specific to each particular context. Consequently, to develop an effective cell-based therapy, multiple dimensions must be considered: the target disease pathogenesis; the neurodegenerative basis of each type of physiological dysfunction or behavioral symptom; the nature of the repair required to alleviate or remediate the functional impairments of particular clinical relevance; and identification of a suitable cell source or delivery system, along with the site and method of implantation, that can achieve the sought for repair and recovery.

Rehabilitation training in neural restitution

Over the last decade, neural transplantation has emerged as one of the more promising, albeit highly experimental, potential therapeutics in neurodegenerative disease. Preclinical studies in rat lesion models of Huntington's disease (HD) and Parkinson's disease (PD) have shown that transplanted precursor neuronal tissue from a fetus into the lesioned striatum can survive, integrate, and reconnect circuitry. Importantly, specific training on behavioral tasks that target striatal function is required to encourage functional integration of the graft to the host tissue. Indeed, "learning to use the graft" is a concept recently adopted in preclinical studies to account for unpredicted profiles of recovery posttransplantation and is an emerging strategy for improving graft functionality. Clinical transplant studies in HD and PD have resulted in mixed outcomes. Small sample sizes and nonstandardized experimental procedures from trial to trial may explain some of this variability. However, it is becoming increasingly apparent that simply replacing the lost neurons may not be sufficient to ensure the optimal graft effects. The knowledge gained from preclinical grafting and training studies suggests that lifestyle factors, including physical activity and specific cognitive and/or motor training, may be required to drive the functional integration of grafted cells and to facilitate the development of compensatory neural networks. The clear implications of preclinical studies are that physical activity and cognitive training strategies are likely to be crucial components of clinical cell replacement therapies in the future. In this chapter, we evaluate the role of general activity in mediating the physical ability of cells to survive, sprout, and extend processes following transplantation in the adult mammalian brain, and we consider the impact of general and specific activity at the behavioral level on functional integration at the cellular and physiological level. We then highlight specific research questions related to timing, intensity, and specificity of training in preclinical models and synthesize the current state of knowledge in clinical populations to inform the development of a strategy for neural transplantation rehabilitation training.

Regenerative medicine in Huntington's disease: Strengths and weaknesses of preclinical studies

Huntington's disease (HD) is an inherited neurodegenerative disorder, characterized by impairment in motor, cognitive and psychiatric domains. Currently, there is no specific therapy to act on the onset or progression of HD. The marked neuronal death observed in HD is a main argument in favour of stem cells (SCs) transplantation as a promising therapeutic perspective to replace the population of lost neurons and restore the functionality of the damaged circuitry. The availability of rodent models of HD encourages the investigation of the restorative potential of SCs transplantation longitudinally. However, the results of preclinical studies on SCs therapy in HD are so far largely inconsistent; this hampers the individuation of the more appropriate model and precludes the comparative analysis of transplant efficacy on behavioural end points. Thus, this review will describe the state of the art of in vivo research on SCs therapy in HD, analysing in a translational perspective the strengths and weaknesses of animal studies investigating the therapeutic potential of cell transplantation on HD progression.

Cell-based technologies for Huntington's disease

Huntington's disease (HD) is a fatal genetic disorder, which causes the progressive breakdown of neurons in the human brain. HD deteriorates human physical and mental abilities over time and has no cure. Stem cell-based technologies are promising novel treatments, and in HD, they aim to replace lost neurons and/or to prevent neural cell death. Herein we discuss the use of human fetal tissue (hFT), neural stem cells (NSCs) of hFT origin or embryonic stem cells (ESCs) and induced pluripotent stem cells (IPSCs), in clinical and pre-clinical studies. The in vivo use of mesenchymal stem cells (MSCs), which are derived from non-neural tissues, will also be discussed. All these studies prove the potential of stem cells for transplantation therapy in HD, demonstrating cell grafting and the ability to differentiate into mature neurons, resulting in behavioral improvements. We claim that there are still many problems to overcome before these technologies become available for HD patient treatment, such as:

a) safety regarding the use of NSCs and pluripotent stem cells, which are potentially teratogenic;

b) safety regarding the transplantation procedure itself, which represents a risk and needs to be better studied; and finally

c) technical and ethical issues regarding cells of fetal and embryonic origin.

PET Imaging in Huntington's Disease

To date, little is known about how neurodegeneration and neuroinflammation propagate in Huntington's disease (HD). Unfortunately, no treatment is available to cure or reverse the progressive decline of function caused by the disease, thus considering HD a fatal disease. Mutation gene carriers typically remain asymptomatic for many years although alterations in the basal ganglia and cortex occur early on in mutant HD gene-carriers. Positron Emission Tomography (PET) is a functional imaging technique of nuclear medicine which enables in vivo visualization of numerous biological molecules expressed in several human tissues. Brain PET is most powerful to study in vivo neuronal and glial cells function as well as cerebral blood flow in a plethora of neurodegenerative disorders including Parkinson's disease, Alzheimer's and HD. In absence of HD-specific biomarkers for monitoring disease progression, previous PET studies in HD were merely focused on the study of dopaminergic terminals, cerebral blood flow and glucose metabolism in manifest and premanifest HD-gene carriers. More recently, research interest has been exploring novel PET targets in HD including the state of phosphodiesterse expression and the role of activated microglia. Hence, a better understanding of the HD pathogenesis mechanisms may lead to the development of targeted therapies. PET imaging follow-up studies with novel selective PET radiotracers such as 11C-IMA-107 and 11C-PBR28 may provide insight on disease progression and identify prognostic biomarkers, elucidate the underlying HD pathology and assess novel pharmaceutical agents and over time.

Creatine supplementation improves neural progenitor cell survival in Huntington's disease

Preclinical and clinical studies suggest that striatal transplantation of neural stem cells (NSCs) and neural progenitor cells (NPCs) may be an appealing and valuable system for treating Huntington's disease. Nevertheless, for a neural replacement to become an effective translational treatment for Huntington's disease, a certain number of difficulties must be addressed, including how to improve the integration of transplanted cell grafts with the host tissue, to elevate the survival rates of transplanted cells, and to ensure their directed differentiation into specific neuronal phenotypes. Research focusing on the translational applications of creatine (Cr) supplementation in NSC and NPC cell replacement therapies continues to offer promising results, pointing to Cr as a factor with the potential to improve cell graft survivability and encourage differentiation toward GABAergic phenotypes in models of striatal transplantation. Here, we evaluate research examining the outcomes of Cr supplementation and how the timing of supplementation regimes may affect their efficacy. The recent studies indicate that Cr's effects vary according to the developmental stage of the cells being treated, noting the dynamic differences in creatine kinase expression over the developmental stages of differentiating NPCs. This research continues to move Cr supplementation closer to the widespread clinical application and suggests such techniques warrant further examination.

Human t-DARPP is induced during striatal development

Human Dopamine- and cAMP-regulated phosphoprotein of molecular weight 32kDa (DARPP-32, also known as PPP1R1B) gene codes for different transcripts that are mainly translated into two DARPP-32 protein isoforms, full length (fl)-DARPP-32 and truncated (t)-DARPP. The t-DARPP lacks the first 36 residues at the N-terminal, which alters its function. In the central nervous system, fl-DARPP-32 is highly expressed in GABAergic striatal medium spiny neurons (MSNs), where it integrates dopaminergic and glutamatergic input signaling. However, no information about human DARPP-32 isoform expression during MSNs maturation is available. In this study, our aim is to determine the expression of the two DARPP-32 isoforms in human fetal and adult striatal samples. We show that DARPP-32 isoform expression is differentially regulated during human striatal development, with the t-DARPP isoform being virtually absent from whole ganglionic eminence (WGE) and highly induced in the adult striatum (in both caudate and putamen). We next compared the four most common anti-DARPP-32 antibodies used in human specimens, to study their recognition of the two isoforms in fetal and adult human striatal samples by western blot and immunohistochemistry. The four antibodies specifically identify the fl-DARPP-32 in both fetal and adult samples, while t-DARPP form was only detected in adult striatal samples. In addition, the lack of t-DARPP recognition in human adult striatum by the antibody generated against the full-length domain produces in turn different efficacy by immunohistochemical analysis. In conclusion, our results show that expression of human DARPP-32 protein isoforms depends on the striatal neurodevelopmental stage with t-DARPP being specific for the human adult striatum.

Stem Cell Therapy for the Central Nervous System in Lysosomal Storage Diseases

Neurological diseases with genetic etiologies result in the loss or dysfunction of neural cells throughout the CNS. At present, few treatment options exist for the majority of neurogenetic diseases. Stem cell transplantation (SCT) into the CNS has the potential to be an effective treatment modality because progenitor cells may replace lost cells in the diseased brain, provide multiple trophic factors, or deliver missing proteins. This review focuses on the use of SCT in lysosomal storage diseases (LSDs), a large group of monogenic disorders with prominent CNS disease. In most patients the CNS disease results in intellectual disability that is refractory to current standard-of-care treatment. A large amount of preclinical work on brain-directed SCT has been performed in rodent LSD models. Cell types that have been used for direct delivery into the CNS include neural stem cells, embryonic and induced pluripotent stem cells, and mesenchymal stem cells. Hematopoietic stem cells have been an effective therapy for the CNS in a few LSDs and may be augmented by overexpression of the missing gene. Current barriers and potential strategies to improve SCT for translation into effective patient therapies are discussed.

Developing stem cell therapies for juvenile and adult-onset Huntington's disease

Stem cell therapies have been explored as a new avenue for the treatment of neurologic disease and damage within the CNS in part due to their native ability to mimic repair mechanisms in the brain. Mesenchymal stem cells have been of particular clinical interest due to their ability to release beneficial neurotrophic factors and their ability to foster a neuroprotective microenviroment. While early stem cell transplantation therapies have been fraught with technical and political concerns as well as limited clinical benefits, mesenchymal stem cell therapies have been shown to be clinically beneficial and derivable from nonembryonic, adult sources. The focus of this review will be on emerging and extant stem cell therapies for juvenile and adult-onset Huntington’s disease.

Novel targets for Huntington's disease: future prospects

Huntington's disease (HD) is an incurable, inherited, progressive, neurodegenerative disorder that is characterized by a triad of motor, cognitive, and psychiatric problems. Despite the noticeable increase in therapeutic trials in HD in the last 20 years, there have, to date, been very few significant advances. The main hope for new and emerging therapeutics for HD is to develop a neuroprotective compound capable of slowing down or even stopping the progression of the disease and ultimately prevent the subtle early signs from developing into manifest disease. Recently, there has been a noticeable shift away from symptomatic therapies in favor of more mechanistic-based interventions, a change driven by a better understanding of the pathogenesis of this disorder. In this review, we discuss the status of, and supporting evidence for, potential novel treatments of HD that are currently under development or have reached the level of early Phase I/II clinical trials.

The effects of creatine supplementation on striatal neural progenitor cells depend on developmental stage

Transplantation of neural progenitor cells (NPCs) is a promising experimental therapy for Huntington's disease (HD). The variables responsible for the success of this approach, including selection of the optimal developmental stage of the grafted cells, are however largely unknown. Supporting cellular energy metabolism by creatine (Cr) supplementation is a clinically translatable method for improving cell transplantation strategies. The present study aims at investigating differences between early (E14) and late (E18) developmental stages of rat striatal NPCs in vitro. NPCs were isolated from E14 and E18 embryos and cultured for 7 days with or without Cr [5 mM]. Chronic treatment significantly increased the percentage of GABA-immunoreactive neurons as compared to untreated controls, both in the E14 (170.4 ± 4.7 %) and the E18 groups (129.3 ± 9.3 %). This effect was greater in E14 cultures (p < 0.05). Similarly, short-term treatment for 24 h resulted in increased induction (p < 0.05) of the GABA-ergic phenotype in E14 (163.0 ± 10.4 %), compared to E18 cultures (133.3 ± 9.5 %). Total neuronal cell numbers and general viability were not affected by Cr (p > 0.05). Protective effects of Cr against a metabolic insult were equal in E14 and E18 NPCs (p > 0.05). Cr exposure promoted morphological differentiation of GABA-ergic neurons, including neurite length in both groups (p < 0.05), but the number of branching points was increased only in the E18 group (p < 0.05). Our results demonstrate that the role of Cr as a GABA-ergic differentiation factor depends on the developmental stage of striatal NPCs, while Cr-mediated neuroprotection is not significantly influenced. These findings have potential implications for optimizing future cell replacement strategies in HD.

Direct Comparison of Rat- and Human-Derived Ganglionic Eminence Tissue Grafts on Motor Function

Huntington's disease (HD) is a debilitating, genetically inherited neurodegenerative disorder that results in early loss of medium spiny neurons from the striatum and subsequent degeneration of cortical and other subcortical brain regions. Behavioral changes manifest as a range of motor, cognitive, and neuropsychiatric impairments. It has been established that replacement of the degenerated medium spiny neurons with rat-derived fetal whole ganglionic eminence (rWGE) tissue can alleviate motor and cognitive deficits in preclinical rodent models of HD. However, clinical application of this cell replacement therapy requires the use of human-derived (hWGE), not rWGE, tissue. Despite this, little is currently known about the functional efficacy of hWGE. The aim of this study was to directly compare the ability of the gold standard rWGE grafts, against the clinically relevant hWGE grafts, on a range of behavioral tests of motor function. Lister hooded rats either remained as unoperated controls or received unilateral excitotoxic lesions of the lateral neostriatum. Subsets of lesioned rats then received transplants of either rWGE or hWGE primary fetal tissue into the lateral striatum. All rats were tested postlesion and postgraft on the following tests of motor function: staircase test, apomorphine-induced rotation, cylinder test, adjusting steps test, and vibrissae-evoked touch test. At 21 weeks postgraft, brain tissue was taken for histological analysis. The results revealed comparable improvements in apomorphine-induced rotational bias and the vibrissae test, despite larger graft volumes in the hWGE cohort. hWGE grafts, but not rWGE grafts, stabilized behavioral performance on the adjusting steps test. These results have implications for clinical application of cell replacement therapies, as well as providing a foundation for the development of stem cell-derived cell therapy products.

Use of human stem cells in Huntington disease modeling and translational research

Huntington disease (HD) is a devastating neurological disorder caused by an extended CAG repeat in exon 1 of the gene that encodes the huntingtin (HTT) protein. HD pathology involves a loss of striatal medium spiny neurons (MSNs) and progressive neurodegeneration affects the striatum and other brain regions. Because HTT is involved in multiple cellular processes, the molecular mechanisms of HD pathogenesis should be investigated on multiple levels. On the cellular level, in vitro stem cell models, such as induced pluripotent stem cells (iPSCs) derived from HD patients and HD embryonic stem cells (ESCs), have yielded progress. Approaches to differentiate functional MSNs from ESCs, iPSCs, and neural stem/progenitor cells (NSCs/NPCs) have been established, enabling MSN differentiation to be studied and disease phenotypes to be recapitulated. Isolation of target stem cells and precursor cells may also provide a resource for grafting. In animal models, transplantation of striatal precursors differentiated in vitro to the striatum has been reported to improve disease phenotype. Initial clinical trials examining intrastriatal transplantation of fetal neural tissue suggest a more favorable clinical course in a subset of HD patients, though shortcomings persist. Here, we review recent advances in the development of cellular HD models and approaches aimed at cell regeneration with human stem cells. We also describe how genome editing tools could be used to correct the HTT mutation in patient-specific stem cells. Finally, we discuss the potential and the remaining challenges of stem cell-based approaches in HD research and therapy development.

The Dark Side of the Force - Constraints and Complications of Cell Therapies for Stroke

Cell therapies are increasingly recognized as a promising option to augment the limited therapeutic arsenal available to fight ischemic stroke. During the last two decades, cumulating preclinical evidence has indicated a substantial efficacy for most cell treatment paradigms and first clinical trials are currently underway to assess safety and feasibility in patients. However, the strong and still unmet demand for novel stroke treatment options and exciting findings reported from experimental studies may have drawn our attention away from potential side effects related to cell therapies and the ways by which they are commonly applied. This review summarizes common and less frequent adverse events that have been discovered in preclinical and clinical investigations assessing cell therapies for stroke. Such adverse events range from immunological and neoplastic complications over seizures to cell clotting and cell-induced embolism. It also describes potential complications of clinically applicable administration procedures, detrimental interactions between therapeutic cells, and the pathophysiological environment that they are placed into, as well as problems related to cell manufacturing. Virtually each therapeutic intervention comes at a certain risk for complications. Side effects do therefore not generally compromise the value of cell treatments for stroke, but underestimating such complications might severely limit therapeutic safety and efficacy of cell treatment protocols currently under development. On the other hand, a better understanding will provide opportunities to further improve existing therapeutic strategies and might help to define those circumstances, under which an optimal effect can be realized. Hence, the review eventually discusses strategies and recommendations allowing us to prevent or at least balance potential complications in order to ensure the maximum therapeutic benefit at minimum risk for stroke patients.

Update on Huntington's disease: advances in care and emerging therapeutic options

INTRODUCTION

Huntington's disease (HD) is the most common hereditary neurodegenerative disorder. Despite the fact that both the gene and the mutation causing this monogenetic disorder were identified more than 20 years ago, disease-modifying therapies for HD have not yet been established.

REVIEW

While intense preclinical research and large cohort studies in HD have laid foundations for tangible improvements in understanding HD and caring for HD patients, identifying targets for therapeutic interventions and developing novel therapeutic modalities (new chemical entities and advanced therapies using DNA and RNA molecules as therapeutic agents) continues to be an ongoing process. The authors review recent achievements in HD research and focus on approaches towards disease-modifying therapies, ranging from huntingtin-lowering strategies to improving huntingtin clearance that may be promoted by posttranslational HTT modifications.

CONCLUSION

The nature and number of upcoming clinical studies/trials in HD is a reason for hope for HD patients and their families.