Unilateral transplantation of human primary fetal tissue in four patients with Huntington's disease: NEST-UK safety report ISRCTN no 36485475

Advancements in surgical treatments for Huntington disease: From pallidotomy to experimental therapies

Huntington disease (HD) is an autosomal dominant neurodegenerative disorder characterized by choreic movements, behavioral changes, and cognitive impairment. The pathogenesis of this process is a consequence of mutant protein toxicity in striatal and cortical neurons. Thus far, neurosurgical management of HD has largely been limited to symptomatic relief of motor symptoms using ablative and stimulation techniques. These interventions, however, do not modify the progressive course of the disease. More recently, disease-modifying experimental therapeutic strategies have emerged targeting intrastriatal infusion of neurotrophic factors, cell transplantation, HTT gene silencing, and delivery of intrabodies. Herein we review therapies requiring neurosurgical intervention, including those targeting symptom management and more recent disease-modifying agents, with a focus on safety, efficacy, and surgical considerations.

Therapeutic advances in neural regeneration for Huntington's disease

Huntington's disease is a neurodegenerative disease caused by the expansion mutation of a cytosine-adenine-guanine triplet in the exon 1 of the HTT gene which is responsible for the production of the huntingtin (Htt) protein. In physiological conditions, Htt is involved in many cellular processes such as cell signaling, transcriptional regulation, energy metabolism regulation, DNA maintenance, axonal trafficking, and antiapoptotic activity. When the genetic alteration is present, the production of a mutant version of Htt (mHtt) occurs, which is characterized by a plethora of pathogenic activities that, finally, lead to cell death. Among all the cells in which mHtt exerts its dangerous activity, the GABAergic Medium Spiny Neurons seem to be the most affected by the mHtt-induced excitotoxicity both in the cortex and in the striatum. However, as the neurodegeneration proceeds ahead the neuronal loss grows also in other brain areas such as the cerebellum, hypothalamus, thalamus, subthalamic nucleus, globus pallidus, and substantia nigra, determining the variety of symptoms that characterize Huntington's disease. From a clinical point of view, Huntington's disease is characterized by a wide spectrum of symptoms spanning from motor impairment to cognitive disorders and dementia. Huntington's disease shows a prevalence of around 3.92 cases every 100,000 worldwide and an incidence of 0.48 new cases every 100,000/year. To date, there is no available cure for Huntington's disease. Several treatments have been developed so far, aiming to reduce the severity of one or more symptoms to slow down the inexorable decline caused by the disease. In this context, the search for reliable strategies to target the different aspects of Huntington's disease become of the utmost interest. In recent years, a variety of studies demonstrated the detrimental role of neuronal loss in Huntington's disease condition highlighting how the replacement of lost cells would be a reasonable strategy to overcome the neurodegeneration. In this view, numerous have been the attempts in several preclinical models of Huntington's disease to evaluate the feasibility of invasive and non-invasive approaches. Thus, the aim of this review is to offer an overview of the most appealing approaches spanning from stem cell-based cell therapy to extracellular vesicles such as exosomes in light of promoting neurogenesis, discussing the results obtained so far, their limits and the future perspectives regarding the neural regeneration in the context of 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.

Huntington's Disease: Complex Pathogenesis and Therapeutic Strategies

Huntington's disease (HD) arises from the abnormal expansion of CAG repeats in the huntingtin gene (HTT), resulting in the production of the mutant huntingtin protein (mHTT) with a polyglutamine stretch in its N-terminus. The pathogenic mechanisms underlying HD are complex and not yet fully elucidated. However, mHTT forms aggregates and accumulates abnormally in neuronal nuclei and processes, leading to disruptions in multiple cellular functions. Although there is currently no effective curative treatment for HD, significant progress has been made in developing various therapeutic strategies to treat HD. In addition to drugs targeting the neuronal toxicity of mHTT, gene therapy approaches that aim to reduce the expression of the mutant HTT gene hold great promise for effective HD therapy. This review provides an overview of current HD treatments, discusses different therapeutic strategies, and aims to facilitate future therapeutic advancements in the field.

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.

Current and Possible Future Therapeutic Options for Huntington's Disease

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

What did we learn from neural grafts in Huntington disease?

Huntington's disease is a rare, severe, and inherited neurodegenerative disorder that affects young adults. To date, there is no treatment to stop its progression. The primary atrophy of the striatum in HD, is limited in space and centrally focalised in the brain and thus constitutes a good candidate for graft. Therefore, transplantation of foetal cells from the ganglionic eminence, the germinal zone of the striatum, has the potential to restore disrupted fronto-cortical circuits and corresponding clinical functions. The international Multicentric intracerebral Grafting in Huntington's disease trial was not as successful as two pilot trials (Créteil and London) which showed promising results in the 2000s, displaying stabilisation/recovery of symptoms in some patients. A point-by-point comparison of the differences between MIG-HD and the pilot trial from Créteil in which similar data are available provides lessons on the grafting procedure and allows for strategic thinking before embarking on future trials. MIG-HD demonstrated the existence of intracerebral alloimmunisation leading to acute or chronic graft rejection into the brain and showed the limitations of surgical standardisation and immunosuppression. It has also improved the safety of the procedure and provided guidance for the follow-up of future patients. Indeed, even if disease modifiers treatments are currently the focus of intense research, they may not stop or slow the progression of the disease sufficiently, or even be administered in all patients, to prevent brain atrophy in all cases. Although disease-modifying therapies are currently the subject of intense research, they may not stop or slow disease progression sufficiently, or may not be given to all patients to prevent brain atrophy. A combination with intracerebral transplantation to repair the damaged structures may thus prove beneficial. Altogether, pursuing research in intracerebral transplantation remains necessary.

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.

Protocol for an open label: phase I trial within a cohort of foetal cell transplants in people with Huntington's disease

Huntington's disease is a progressive neurodegenerative disorder characterized by motor, cognitive and psychiatric symptoms. Currently, no disease-modifying therapies are available to slow or halt disease progression. Huntington's disease is characterized by relatively focal and specific loss of striatal medium spiny neurons, which makes it suitable for cell-replacement therapy, a process involving the transplantation of donor cells to replace those lost due to disease. TRIal DEsigns for delivery of Novel Therapies in neurodegeneration is a phase I Trial Within a Cohort designed to assess safety and feasibility of transplanting human foetal striatal cells into the striatum of people with Huntington's disease. A minimum of 18 participants will be enrolled in the study cohort, and up to five eligible participants will be randomly selected to undergo transplantation of 12-22 million foetal cells in a dose escalation paradigm. Independent reviewers will assess safety outcomes (lack of significant infection, bleeding or new neurological deficit) 4 weeks after surgery, and ongoing safety will be established before conducting each subsequent surgery. All participants will undergo detailed clinical and functional assessment at baseline (6 and 12 months). Surgery will be performed 1 month after baseline, and transplant participants will undergo regular clinical follow-up for at least 12 months. Evaluation of trial processes will also be undertaken. Transplant participants and their carers will be interviewed ∼1 month before and after surgery. Interviews will also be conducted with non-transplanted participants and healthcare staff delivering the intervention and involved in the clinical care of participants. Evaluation of clinical and functional efficacy outcomes and intervention costs will be carried out to explore plausible trial designs for subsequent randomized controlled trials aimed at evaluating efficacy and cost-effectiveness of cell-replacement therapy. TRIal DEsigns for delivery of Novel Therapies in neurodegeneration will enable the assessment of the safety, feasibility, acceptability and cost of foetal cell transplants in people with Huntington's disease. The data collected will inform trial designs for complex intra-cranial interventions in a range of neurodegenerative conditions and facilitate the development of stable surgical pipelines for delivery of future stem cell trials. Trial Registration: ISRCTN52651778.

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.

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.

Intracerebral Transplantation of BDNF-overexpressing Human Neural Stem Cells (HB1.F3.BDNF) Promotes Migration, Differentiation and Functional Recovery in a Rodent Model of Huntington's Disease

Huntington's disease (HD) is a dominantly inherited neurodegenerative disorder caused by abnormally expanded CAG repeats in the huntingtin gene. The huntingtin gene mutation leads to the progressive degeneration of striatal GABAergic medium spiny neurons (MSN) and reduces the level of brain-derived neurotrophic factor (BDNF) in HD patient's brain. BDNF is an essential neurotrophic factor for the cortico-striatal synaptic activity and the survival of GABAergic neurons. In this study, we transplanted BDNF-overexpressing human neural stem cells (HB1.F3.BDNF) into the contra-lateral side of unilateral quinolinic acid (QA)-lesioned striatum of HD rat model. The results of in vivo transplantation were monitored using various behavioral tests, 4.7 T animal magnetic resonance imaging (MRI) and immunohistochemical staining. We observed that the QA-lesioned rats receiving HB1.F3.BDNF cells exhibited significant behavioral improvements in the stepping, rotarod and apomorphine-induced rotation tests. Interestingly, contralaterally transplanted cells were migrated to the QA-lesioned striatum and the size of lateral ventricle was reduced. Histological analyses further revealed that the transplanted cells, which had migrated to the QA lesion site, were differentiated into the cells of GABAergic, MSN-type neurons expressing DARPP-32, and neural networks were established between the transplanted cells and the host brain, as revealed by retrograde tracing. Finally, there was a significant reduction of inflammatory response in HB1.F3.BDNF-transplanted HD animal model, compared with vehicle-transplanted group. Taken together, these results suggest that HB1.F3.BDNF can be an effective therapeutic strategy to treat HD patients in the future.

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.

Donation of neural stem cells? Post mortal viability of spinal cord neuronal cells

Transplantation of cells into central nervous system (CNS) shows a potential for treatment of post-traumatic and neurodegenerative diseases. Cadaver-derived neural cells can help reducing deficit of allogeneic material ready for transplantation. In this study we analyze post-mortal survival of spinal cord neural cells. Maximal time when alive neuronal cells can be recovered form spinal cord of the animals was determined as 56hr for human-size animal and 18hr for rat. Cells with surface expression of ganglioside GD2 and antigen CD24 constituted up to one percent of all recovered alive cells in earlier samples with time dependent decline in percentage. GD2-positive cells from rat spinal cord demonstrated spontaneous and induced electrical activity, which reduces with time post mortem.

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.

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.

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.

Making Stem Cell Lines Suitable for Transplantation

Human stem cells, progenitor cells, and cell lines have been derived from embryonic, fetal, and adult sources in the search for graft tissue suitable for the treatment of CNS disorders. An increasing number of experimental studies have shown that grafts from several sources survive, differentiate into distinct cell types, and exert positive functional effects in experimental animal models, but little attention has been given to developing cells under conditions of good manufacturing practice (GMP) that can be scaled up for mass treatment. The capacity for continued division of stem cells in culture offers the opportunity to expand their production to meet the widespread clinical demands posed by neurodegenerative diseases. However, maintaining stem cell division in culture long term, while ensuring differentiation after transplantation, requires genetic and/or oncogenetic manipulations, which may affect the genetic stability and in vivo survival of cells. This review outlines the stages, selection criteria, problems, and ultimately the successes arising in the development of conditionally immortal clinical grade stem cell lines, which divide in vitro, differentiate in vivo, and exert positive functional effects. These processes are specifically exemplified by the murine MHP36 cell line, conditionally immortalized by a temperature-sensitive mutant of the SV40 large T antigen, and cell lines transfected with the c-myc protein fused with a mutated estrogen receptor (c-mycERTAM), regulated by a tamoxifen metabolite, but the issues raised are common to all routes for the development of effective clinical grade cells.

Huntington's Disease: Pathological Mechanisms and Therapeutic Strategies

Huntington's disease (HD) is a devastating neurodegenerative disorder that occurs in patients with a mutation in the huntingtin or IT15 gene. Patients are plagued by early cognitive signs, motor deficits, and psychiatric disturbances. Symptoms are attributed to cell death in the striatum and disruption of cortical–striatal circuitry. Mechanisms of cell death are unclear, but processes involving mitochondrial abnormalities, excitotoxicity, and abnormal protein degradation have been implicated. Many factors likely contribute to neuron death and dysfunction, and this has made it difficult to systematically address the pathology in HD. Pharmaceutical therapies are commonly used in patients to treat disease symptoms. These have limited benefit and do not address the inexorable disease progression. Several neuroprotective therapies are being evaluated in animal models of HD as well as in clinical trials. Similarly, cell replacement strategies such as fetal transplantation have been used in the clinic with minimal success, making future cell replacement strategies such as stem cell therapy uncertain. This review describes the disease pathology in HD and addresses many of the past and emerging therapeutic strategies.

Transplanted hNT Cells (“LBS Neurons”) in a Rat Model of Huntington's Disease: Good Survival, Incomplete Differentiation, and Limited Functional Recovery

A variety of immortalized cell lines have been proposed to exhibit sufficient phenotypic plasticity to allow them to replace primary embryonic neurons for restorative cell transplantation. In the present experiments we evaluate the functional viability of one particular cell line, the hNT cells developed by Layton Bioscience, to replace lost neurons and alleviate asymmetrical motor deficits in a unilateral excitotoxic lesion model of Huntington's disease. Because the grafts involved implantation of human-derived cells into a rat host environment, all animals were immunosuppressed. Cyclosporin A and FK-506 were similar in providing effective immunoprotection of the hNT xenografts, and whereas the lesions induced a marked inflammatory response in the host brain, this was not exacerbated by the presence of xenograft cells. The presence of grafted cells was determined with the human-specific antigen HuNu, and good graft survival was demonstrated in almost all animals up to the longest survival examined, 16 weeks posttransplantation. Although the cells exhibited progressively greater maturation and differentiation at 10-day, 4- and 16-week time points, staining for the mature neuronal marker NeuN was at best very weak, and we were unable to detect unequivocal staining with any markers of mature striatal phenotype, including DARPP-32, calbindin, parvalbumin, choline acetyl transferase, or NADPH diaphorase (with in all cases positive control provided by good staining on the intact contralateral side of the brain). Nor were we able to detect any differences between rats with lesions alone and rats with grafts in the contralateral motor deficits exhibited in a test of skilled paw reaching or cylinder placing. These results suggest that further and more extensive studies should be undertaken to assess whether hNT neurons can show more extensive and appropriate maturation and be associated with recovery in appropriate behavioral models, before they may be considered a suitable replacement for primary embryonic cells for clinical application in Huntington's disease.

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.

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.

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.

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.

Current status of PET imaging in Huntington's disease

Purpose

To review the developments of recent decades and the current status of PET molecular imaging in Huntington’s disease (HD).

Methods

A systematic review of PET studies in HD was performed. The MEDLINE, Web of Science, Cochrane and Scopus databases were searched for articles in all languages published up to 19 August 2015 using the major medical subject heading “Huntington Disease” combined with text and key words “Huntington Disease”, “Neuroimaging” and “PET”. Only peer-reviewed, primary research studies in HD patients and premanifest HD carriers, and studies in which clinical features were described in association with PET neuroimaging results, were included in this review. Reviews, case reports and nonhuman studies were excluded.

Results

A total of 54 PET studies were identified and analysed in this review. Brain metabolism ([¹⁸F]FDG and [¹⁵O]H₂O), presynaptic ([¹⁸F]fluorodopa, [¹¹C]β-CIT and [¹¹C]DTBZ) and postsynaptic ([¹¹C]SCH22390, [¹¹C]FLB457 and [¹¹C]raclopride) dopaminergic function, phosphodiesterases ([¹⁸F]JNJ42259152, [¹⁸F]MNI-659 and [¹¹C]IMA107), and adenosine ([¹⁸F]CPFPX), cannabinoid ([¹⁸F]MK-9470), opioid ([¹¹C]diprenorphine) and GABA ([¹¹C]flumazenil) receptors were evaluated as potential biomarkers for monitoring disease progression and for assessing the development and efficacy of novel disease-modifying drugs in premanifest HD carriers and HD patients. PET studies evaluating brain restoration and neuroprotection were also identified and described in detail.

Conclusion

Brain metabolism, postsynaptic dopaminergic function and phosphodiesterase 10A levels were proven to be powerful in assessing disease progression. However, no single technique may be currently considered an optimal biomarker and an integrative multimodal imaging approach combining different techniques should be developed for monitoring potential neuroprotective and preventive treatment in HD.

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.

Human neural progenitor cells in central nervous system lesions

Various immature cells can be isolated from human embryonic and fetal central nervous system (CNS) residual tissue and potentially be used in cell therapy for a number of neurological diseases and CNS insults. Transplantation of neural stem and progenitor cells is essential for replacing lost cells, particularly in the CNS with very limited endogenous regenerative capacity. However, while dopamine released from transplanted cells can substitute the lost dopamine neurons in the experimental models of Parkinson's disease, stem and progenitor cells primarily have a neuroprotective effect, probably through the release of trophic factors. Understanding the therapeutic effects of transplanted cells is crucial to determine the design of clinical trials. During the last few years, a number of clinical trials for CNS diseases and insults such as amyotrophic lateral sclerosis (ALS), stroke, and spinal cord trauma using neural progenitor cells have been initiated. Data from these early studies will provide vital information on the safety of transplanting these cells, which still is a major concern. That the beneficial results observed in experimental models also can be repeated in the clinical setting is highly hoped for.

Translation of Cell Therapies to the Clinic: Characteristics of Cell Suspensions in Large-Diameter Injection Cannulae

With the use of cell replacement therapies as a realistic prospect for conditions such as Parkinson's and Huntington's diseases, the logistics of the delivery of cell suspensions to deep brain targets is a topic for consideration. Because of the large cannulae required for such procedures, we need to consider the behavior of cell suspensions within the cannulae if we are to ensure that the injected cells are distributed as intended within the target tissue. We have investigated the behavior of primary embryonic cell suspensions of neural tissue, in cannulae of different diameters, using a protocol designed to mimic the handling and injection of cells during clinical application. Internal cannula diameter had a large effect on the distribution of cells during their dispensation from the syringe. In vertical or near vertical cannulae, cells settled toward the tip of the needle, and were dispensed unevenly, with the majority of cells emerging in the first 10-20% of the injectate. In horizontal or near-horizontal cannulae, we observed the opposite effect, such that few cells were dispensed in the first 80% of the injectate, and the majority emerged in the final 10-20%. Use of a glass cannula showed that the results obtained using the horizontal cannula were caused by settling and adherence of the cells on the side of the cannulae, such that during dispensation, the overlying, cell-free solution was dispensed first, prior to the emergence of the cells. We show that the behavior of cells in such cannulae is affected by the cannula diameter, and by the material of the cannula itself. In horizontal cannulae, uneven expulsion of cells from the needle can be ameliorated by regular rotation of the cannula during the procedure. We discuss the potential impact of these observations on the translation of cell therapies to the clinic.

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.

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.

Differentiation of pluripotent stem cells into striatal projection neurons: a pure MSN fate may not be sufficient

Huntington's disease (HD) is an autosomal dominant inherited disorder leading to the loss inter alia of DARPP-32 positive medium spiny projection neurons ("MSNs") in the striatum. There is no known cure for HD but the relative specificity of cell loss early in the disease has made cell replacement by neural transplantation an attractive therapeutic possibility. Transplantation of human fetal striatal precursor cells has shown "proof-of-principle" in clinical trials; however, the practical and ethical difficulties associated with sourcing fetal tissues have stimulated the need to identify alternative source(s) of donor cells that are more readily available and more suitable for standardization. We now have available the first generation of protocols to generate DARPP-32 positive MSN-like neurons from pluripotent stem cells and these have been successfully grafted into animal models of HD. However, whether these grafts can provide stable functional recovery to the level that can regularly be achieved with primary fetal striatal grafts remains to be demonstrated. Of particular concern, primary fetal striatal grafts are not homogenous; they contain not only the MSN subpopulation of striatal projection neurons but also include all the different cell types that make up the mature striatum, such as the multiple populations of striatal interneurons and striatal glia, and which certainly contribute to normal striatal function. By contrast, present protocols for pluripotent stem cell differentiation are almost entirely targeted at specifying just neurons of an MSN lineage. So far, evidence for the functionality and integration of stem-cell derived grafts is correspondingly limited. Indeed, consideration of the features of full striatal reconstruction that is achieved with primary fetal striatal grafts suggests that optimal success of the next generations of stem cell-derived replacement therapy in HD will require that graft protocols be developed to allow inclusion of multiple striatal cell types, such as interneurons and/or glia. Almost certainly, therefore, more sophisticated differentiation protocols will be necessary, over and above replacement of a specific population of MSNs. A rational solution to this technical challenge requires that we re-address the underlying question-what constitutes a functional striatal graft?

Fetal striatal grafting slows motor and cognitive decline of Huntington's disease

OBJECTIVE

To assess the clinical effect of caudate-putaminal transplantation of fetal striatal tissue in Huntington's disease (HD).

METHODS

We carried out a follow-up study on 10 HD transplanted patients and 16 HD not-transplanted patients. All patients were evaluated with the Unified HD Rating Scale (UHDRS) whose change in motor, cognitive, behavioural and functional capacity total scores were considered as outcome measures. Grafted patients also received morphological and molecular neuroimaging.

RESULTS

Patients were followed-up from disease onset for a total of 309.3 person-years (minimum 5.3, median 11.2 years, maximum 21.6 years). UHDRS scores have been available since 2004 (median time of 5.7 years since onset, minimum zero, maximum 17.2 years). Median post-transplantation follow-up was 4.3 years, minimum 2.8, maximum 5.1 years. Adjusted post-transplantation motor score deterioration rate was reduced compared to the pretransplantation period, and to that of not-transplanted patients by 0.9 unit/years (95% CI 0.2 to 1.6). Cognitive score deterioration was reduced of 2.7 unit/years (95% CI 0.1 to 5.3). For grafted patients the 2-year post-transplantation [(18)F]fluorodeoxyglucose positron emission tomography (PET) showed striatal/cortical metabolic increase compared to the presurgical evaluation; 4-year post-transplantation PET values were slightly decreased, but remained higher than preoperatively. [(123)I]iodobenzamide single photon emission CT demonstrated an increase in striatal D2-receptor density during postgrafting follow-up.

CONCLUSIONS

Grafted patients experienced a milder clinical course with less pronounced motor/cognitive decline and associated brain metabolism improvement. Life-time follow-up may ultimately clarify whether transplantation permanently modifies the natural course of the disease, allowing longer sojourn time at less severe clinical stage, and improvement of overall survival.

Transplantation of Human Fetal Tissue for Neurodegenerative Diseases: Validation of a New Protocol for Microbiological Analysis and Bacterial Decontamination

Restorative cell therapy concepts in neurodegenerative diseases are aimed at replacing lost neurons. Despite advances in research on pluripotent stem cells, fetal tissue from routine elective abortions is still regarded as the only safe cell source. Progenitor cells isolated from distinct first-trimester fetal CNS regions have already been used in clinical trials and will be used again in a new multicenter trial funded by the European Union (TRANSEURO). Bacterial contamination of human fetal tissue poses a potential risk of causing infections in the brain of the recipient. Thus, effective methods of microbial decontamination and validation of these methods are required prior to approval of a neurorestorative cell therapy trial. We have developed a protocol consisting of subsequent washing steps at different stages of tissue processing. Efficacy of microbial decontamination was assessed on rat embryonic tissue incubated with high concentrations of defined microbe solutions including representative bacterial and fungal species. Experimental microbial contamination was reduced by several log ranks. Subsequently, we have analyzed the spectrum of microbial contamination and the effect of subsequent washing steps on aborted human fetal tissue; 47.7% of the samples taken during human fetal tissue processing were positive for a microbial contamination, but after washing, no sample exhibited bacterial growth. Our data suggest that human fetal tissue for neural repair can carry microbes of various species, highlighting the need for decontamination procedures. The decontamination protocol described in this report has been shown to be effective as no microbes could be detected at the end of the procedure.

Neuroimaging in Huntington’s disease

Huntington’s disease (HD) is a progressive and fatal neurodegenerative disorder caused by an expanded trinucleotide CAG sequence in huntingtin gene (HTT) on chromosome 4. HD manifests with chorea, cognitive and psychiatric symptoms. Although advances in genetics allow identification of individuals carrying the HD gene, much is still unknown about the mechanisms underlying the development of overt clinical symptoms and the transitional period between premanifestation and manifestation of the disease. HD has no cure and patients rely only in symptomatic treatment. There is an urgent need to identify biomarkers that are able to monitor disease progression and assess the development and efficacy of novel disease modifying drugs. Over the past years, neuroimaging techniques such as magnetic resonance imaging (MRI) and positron emission tomography (PET) have provided important advances in our understanding of HD. MRI provides information about structural and functional organization of the brain, while PET can detect molecular changes in the brain. MRI and PET are able to detect changes in the brains of HD gene carriers years ahead of the manifestation of the disease and have also proved to be powerful in assessing disease progression. However, no single technique has been validated as an optimal biomarker. An integrative multimodal imaging approach, which combines different MRI and PET techniques, could be recommended for monitoring potential neuroprotective and preventive therapies in HD. In this article we review the current neuroimaging literature in HD.

Organization of the human fetal subpallium

The subpallium comprises large parts of the basal ganglia including striatum and globus pallidus. Genes and factors involved in the development of the subpallium have been extensively studied in most vertebrates, including amphibians, birds, and rodents. However, our knowledge on patterning of the human subpallium remains insufficient. Using double fluorescent immunohistochemistry, we investigated the protein distribution of transcription factors involved in patterning of the subventricular zone (SVZ) in the human forebrain at late embryonic development. Furthermore, we compared the development of cortical and striatal precursors between human fetal brain and E14 and E16 fetal rat brains. Our results reveal that DLX2 marks SVZ precursors in the entire subpallium. Individual subpallial subdomains can be identified based on co-expression of DLX2 with either PAX6 or NKX2-1. SVZ precursors in the dorsal LGE and preopto-hypothalamic boundary are characterized by DLX2/PAX6 co-expression, while precursors in the MGE and preoptic region co-express DLX2/NKX2-1. SVZ precursors in the ventral LGE are DLX2(+)/PAX6(-)/NKX2-1(-). In terms of staging comparisons, the development of the corpus striatum in the human fetal brain during late embryonic stages corresponds well with the development of the striatum observed in E14 fetal rat brains. Our study demonstrates that the pattern underlying the development of the subpallium is highly conserved between rodents and humans and suggests a similar function for these factors in human brain development. Moreover, our data directly influence the application of ganglionic eminence derived human tissue for cell therapeutic approaches in neurodegenerative disorders such as Huntington's disease.

Stem cell-based therapy for Huntington's disease

Huntington's disease (HD) is a late-onset neurodegenerative disease characterized by a progressive loss of medium spiny neurons in the basal ganglia. The development of stem cell-based therapies for HD aims to replace lost neurons and/or to prevent cell death. This review will discuss pre-clinical studies which have utilized stem or progenitor cells for transplantation therapy using HD animal models. In several studies, neural stem and progenitor cells used as allotransplants and xenografts have been shown to be capable of surviving transplantation and differentiating into mature GABAergic neurons, resulting in behavioral improvements. Beneficial effects have also been reported for transplantation of stem cells derived from non-neural tissue, for example, mesenchymal- and adipose-derived stem cells, which have mainly been attributed to their secretion of growth and neurotrophic factors. Finally, we review studies using stem cells genetically engineered to over-express defined neurotrophic factors. While these studies prove the potential of stem cells for transplantation therapy in HD, it also becomes clear that technical and ethical issues regarding the availability of stem cells must be solved before human trials can be conducted.

The long-term safety and efficacy of bilateral transplantation of human fetal striatal tissue in patients with mild to moderate Huntington's disease

Huntington's disease (HD) is a fatal autosomal dominant neurodegenerative disease involving progressive motor, cognitive and behavioural decline, leading to death approximately 20 years after motor onset. The disease is characterised pathologically by an early and progressive striatal neuronal cell loss and atrophy, which has provided the rationale for first clinical trials of neural repair using fetal striatal cell transplantation. Between 2000 and 2003, the 'NEST-UK' consortium carried out bilateral striatal transplants of human fetal striatal tissue in five HD patients. This paper describes the long-term follow up over a 3-10-year postoperative period of the patients, grafted and non-grafted, recruited to this cohort using the 'Core assessment program for intracerebral transplantations-HD' assessment protocol. No significant differences were found over time between the patients, grafted and non-grafted, on any subscore of the Unified Huntington's Disease Rating Scale, nor on the Mini Mental State Examination. There was a trend towards a slowing of progression on some timed motor tasks in four of the five patients with transplants, but overall, the trial showed no significant benefit of striatal allografts in comparison with a reference cohort of patients without grafts. Importantly, no significant adverse or placebo effects were seen. Notably, the raclopride positron emission tomography (PET) signal in individuals with transplants, indicated that there was no obvious surviving striatal graft tissue. This study concludes that fetal striatal allografting in HD is safe. While no sustained functional benefit was seen, we conclude that this may relate to the small amount of tissue that was grafted in this safety study compared with other reports of more successful transplants in patients with HD.

Devices for cell transplantation into the central nervous system: Design considerations and emerging technologies

Successful use of cell-based therapies for the treatment of neurological diseases is dependent upon effective delivery to the central nervous system (CNS). The CNS poses several challenges to the delivery of cell-based therapeutics, including the blood-brain barrier, anatomic complexity, and regional specificity. Targeted delivery methods are therefore required for the selective treatment of specific CNS regions. In addition, CNS tissues are mechanically and physiologically delicate and even minor injury to normal brain or spinal cord can cause devastating neurological deficits. Targeted delivery methods must therefore minimize tissue trauma. At present, direct injection into brain or spinal cord parenchyma promises to be the most versatile and accurate method of targeted CNS therapeutic delivery. While direct injection methods have already been employed in clinical trials of cell transplantation for a wide variety of neurological diseases, there are many shortcomings with the devices and surgical approaches currently used. Some of these technical limitations may hinder the clinical development of cell transplantation therapies despite validity of the underlying biological mechanisms. In this review, we discuss some of the important technical considerations of CNS injection devices such as targeting accuracy, distribution of infused therapeutic, and overall safety to the patient. We also introduce and discuss an emerging technology - radially branched deployment - that may improve our ability to safely distribute cell-based therapies and other therapeutic agents to the CNS. Finally, we speculate on future technological developments that may further enhance the efficacy of CNS therapeutic delivery.