Isolation, characterization and preclinical development of human glial-restricted progenitor cells for treatment of neurological disorders

Human iPSC-derived neural progenitor cells secreting GDNF provide protection in rodent models of ALS and retinal degeneration

Human induced pluripotent stem cells (iPSCs) are a renewable cell source that can be differentiated into neural progenitor cells (iNPCs) and transduced with glial cell line-derived neurotrophic factor (iNPC-GDNFs). The goal of the current study is to characterize iNPC-GDNFs and test their therapeutic potential and safety. Single-nuclei RNA-seq show iNPC-GDNFs express NPC markers. iNPC-GDNFs delivered into the subretinal space of the Royal College of Surgeons rodent model of retinal degeneration preserve photoreceptors and visual function. Additionally, iNPC-GDNF transplants in the spinal cord of SOD1G93A amyotrophic lateral sclerosis (ALS) rats preserve motor neurons. Finally, iNPC-GDNF transplants in the spinal cord of athymic nude rats survive and produce GDNF for 9 months, with no signs of tumor formation or continual cell proliferation. iNPC-GDNFs survive long-term, are safe, and provide neuroprotection in models of both retinal degeneration and ALS, indicating their potential as a combined cell and gene therapy for various neurodegenerative diseases.

Transplantation of Human Glial Progenitors to Immunodeficient Neonatal Mice with Amyotrophic Lateral Sclerosis (SOD1/rag2)

Amyotrophic lateral sclerosis (ALS) is a progressive, fatal disease with no effective therapy. The neurodegenerative character of ALS was an appealing target for stem cell-based regenerative approaches. Different types of stem cells have been transplanted in both preclinical and clinical settings, but no convincing outcomes have been noted. Human glial restricted precursors (hGRPs) transplanted intraventricularly to neonatal, immunodeficient mice rescued lifespan of dysmyelinated mice. Intraspinal injection of hGRPs also provided benefits in the mouse model of ALS. Therefore, we have recently developed an immunodeficient model of ALS (double mutant SOD1/rag2), and, in this study, we tested the strategy previously used in dysmyelinated mice of intraventricular transplantation of hGRPs to immunodeficient mice. To maximize potential therapeutic benefits, the cells were implanted into neonates. We used magnetic resonance imaging to investigate the progression of neurodegeneration and therapeutic responses. A cohort of animals was devoted to survival assessment. Postmortem analysis included immunohistochemistry, Nissl staining, and Western blots. Cell transplantation was not associated with improved animal survival, slowing neurodegeneration, or accumulation of misfolded superoxide dismutase 1. Postmortem analysis did not reveal any surviving hGRPs. Grafting into neonatal immunodeficient recipients did not prevent ALS-induced cell loss, which might explain the lack of positive therapeutic effects. The results of this study are in line with the modest effects of clinical neurotransplantations. Therefore, we urge stem cell and ALS communities to develop and implement cell tracking methods to better understand cell fates in the clinic.

Murine glial progenitor cells transplantation and synthetic PreImplantation Factor (sPIF) reduces inflammation and early motor impairment in ALS mice

Amyotrophic lateral sclerosis (ALS) is a progressive motor neuronal disorder characterized by neuronal degeneration and currently no effective cure is available to stop or delay the disease from progression. Transplantation of murine glial-restricted precursors (mGRPs) is an attractive strategy to modulate ALS development and advancements such as the use of immune modulators could potentially extend graft survival and function. Using a well-established ALS transgenic mouse model (SOD1^G93A), we tested mGRPs in combination with the immune modulators synthetic PreImplantation Factor (sPIF), Tacrolimus (Tac), and Costimulatory Blockade (CB). We report that transplantation of mGRPs into the cisterna magna did not result in increased mice survival. The addition of immunomodulatory regimes again did not increase mice lifespan but improved motor functions and sPIF was superior compared to other immune modulators. Immune modulators did not affect mGRPs engraftment significantly but reduced pro-inflammatory cytokine production. Finally, sPIF and CB reduced the number of microglial cells and prevented neuronal number loss. Given the safety profile and a neuroprotective potential of sPIF, we envision its clinical application in near future.

Glial restricted precursor cells in central nervous system disorders: Current applications and future perspectives

The crosstalk between glial cells and neurons represents an exceptional feature for maintaining the normal function of the central nervous system (CNS). Increasing evidence has revealed the importance of glial progenitor cells in adult neurogenesis, reestablishment of cellular pools, neuroregeneration, and axonal (re)myelination. Several types of glial progenitors have been described, as well as their potentialities for recovering the CNS from certain traumas or pathologies. Among these precursors, glial-restricted precursor cells (GRPs) are considered the earliest glial progenitors and exhibit tripotency for both Type I/II astrocytes and oligodendrocytes. GRPs have been derived from embryos and embryonic stem cells in animal models and have maintained their capacity for self-renewal. Despite the relatively limited knowledge regarding the isolation, characterization, and function of these progenitors, GRPs are promising candidates for transplantation therapy and reestablishment/repair of CNS functions in neurodegenerative and neuropsychiatric disorders, as well as in traumatic injuries. Herein, we review the definition, isolation, characterization and potentialities of GRPs as cell-based therapies in different neurological conditions. We briefly discuss the implications of using GRPs in CNS regenerative medicine and their possible application in a clinical setting. MAIN POINTS: GRPs are progenitors present in the CNS with differentiation potential restricted to the glial lineage. These cells have been employed in the treatment of a myriad of neurodegenerative and traumatic pathologies, accompanied by promising results, herein reviewed.

Induction of immunological tolerance to myelinogenic glial-restricted progenitor allografts

The immunological barrier currently precludes the clinical utilization of allogeneic stem cells. Although glial-restricted progenitors have become attractive candidates to treat a wide variety of neurological diseases, their survival in immunocompetent recipients is limited. In this study, we adopted a short-term, systemically applicable co-stimulation blockade-based strategy using CTLA4-Ig and anti-CD154 antibodies to modulate T-cell activation in the context of allogeneic glial-restricted progenitor transplantation. We found that co-stimulation blockade successfully prevented rejection of allogeneic glial-restricted progenitors from immunocompetent mouse brains. The long-term engrafted glial-restricted progenitors myelinated dysmyelinated adult mouse brains within one month. Furthermore, we identified a set of plasma miRNAs whose levels specifically correlated to the dynamic changes of immunoreactivity and as such could serve as biomarkers for graft rejection or tolerance. We put forward a successful strategy to induce alloantigen-specific hyporesponsiveness towards stem cells in the CNS, which will foster effective therapeutic application of allogeneic stem cells.

Evaluation of cell transplant-mediated attenuation of diffuse injury in experimental autoimmune encephalomyelitis using onVDMP CEST MRI

The development and translation of cell therapies have been hindered by an inability to predict and evaluate their efficacy after transplantation. Using an experimental autoimmune encephalomyelitis (EAE) mouse model of multiple sclerosis (MS), we studied attenuation of the diffuse injury characteristic of EAE and MS by transplanted glial-restricted precursor cells (GRPs). We assessed the potential of on-resonance variable delay multiple pulse (onVDMP) chemical exchange saturation transfer (CEST) MRI to visualize this attenuation. Allogeneic GRPs transplanted in the motor cortex or lateral ventricles attenuated paralysis in EAE mice and attenuated differences compared to naïve mice in onVDMP CEST signal 5 days after transplantation near the transplantation site. Histological analysis revealed that transplanted GRPs co-localized with attenuated astrogliosis. Hence, diffuse injury-sensitive onVDMP CEST MRI may complement conventional MRI to locate and monitor tissue regions responsive to GRP therapy.

Perfluorocarbon Labeling of Human Glial-Restricted Progenitors for 19 F Magnetic Resonance Imaging

One of the fundamental limitations in assessing potential efficacy in Central Nervous System (CNS) transplantation of stem cells is the capacity for monitoring cell survival and migration noninvasively and longitudinally. Human glial‐restricted progenitor (hGRP) cells (Q‐Cells) have been investigated for their utility in providing neuroprotection following transplantation into models of amyotrophic lateral sclerosis (ALS) and have been granted a Food and Drug Administration (FDA) Investigational New Drug (IND) for intraspinal transplantation in ALS patients. Furthermore, clinical development of these cells for therapeutic use will rely on the ability to track the cells using noninvasive imaging methodologies as well as the verification that the transplanted GRPs have disease‐relevant activity. As a first step in development, we investigated the use of a perfluorocarbon (PFC) dual‐modal (¹⁹F magnetic resonance imaging [MRI] and fluorescence) tracer agent to label Q‐Cells in culture and following spinal cord transplantation. PFCs have a number of potential benefits that make them appealing for clinical use. They are quantitative, noninvasive, biologically inert, and highly specific. In this study, we developed optimized PFC labeling protocols for Q‐Cells and demonstrate that PFCs do not significantly alter the glial identity of Q‐Cells. We also show that PFCs do not interfere with the capacity for differentiation into astrocytes either in vitro or following transplantation into the ventral horn of the mouse spinal cord, and can be visualized in vivo by hot spot ¹⁹F MRI. These studies provide a foundation for further preclinical development of PFCs within the context of evaluating Q‐Cell transplantation in the brain and spinal cord of future ALS patients using ¹⁹F MRI. stem cells translational medicine2019;8:355–365

Chemobrain as a Product of Growing Success in Chemotherapy - Focus on Glia as both a Victim and a Cure

Chemotherapy-induced cognitive impairment or chemobrain is a frequent consequence of cancer treatment with many psychiatric features. Ironically, the increasing efficacy of chemotherapy leaves growing number of patients alive with chemobrain. Therefore, there is an urgent need for strategies capable of returning cancer survivors back to their pre-morbid quality of life. Molecular mechanisms of chemobrain are largely unknown. Over the last decade there was a lot of emphasis in preclinical research on inflammatory consequences of chemotherapy and oxidative stress but so far none of these approaches were translated into clinical scenario. The co-administration of chemotherapy with protective agents was evaluated preclinically but it should be introduced with caution as potential interference was not yet studied and that could blunt therapeutic efficacy. Stem cell-based regenerative medicine approach has so far been exploited very sparsely in the context of chemobrain and the focus was on indirect mechanisms or neuronal replacement in the hippocampus. However, there is evidence for widespread white matter abnormalities in patients with chemobrain. This is quite logical considering life-long proliferation and turnover of glial cells, which makes them vulnerable to chemotherapeutic agents. Feasibility of glia replacement has been established in mice with global dysmyelination where profound therapeutic effect has been observed but only in case of global cell engraftment (across the entire brain). While global glia replacement has been achieved in mice translation to clinical setting might be challenging due to much larger brain size. Therefore, a lot of attention should be directed towards the route of administration to accomplish widespread cell delivery. Techniques facilitating that broad cell distribution including intra-arterial and intrathecal methods should be considered as very compelling options. Summarizing, chemobrain is a rapidly growing medical problem and global glia replacement should be considered as worthwhile therapeutic strategy.

Axonal regeneration of different tracts following transplants of human glial restricted progenitors into the injured spinal cord in rats

The goal of this study was to compare the efficacy of human glial restricted progenitors (hGRPs) in promoting axonal growth of different tracts. We examined the potential of hGRPs grafted into a cervical (C4) dorsal column lesion to test sensory axons, and into a C4 hemisection to test motor tracts. The hGRPs, thawed from frozen stocks, were suspended in a PureCol matrix and grafted acutely into a C4 dorsal column or hemisection lesion. Control rats received PureCol only. Five weeks after transplantation, all transplanted cells survived in rats with the dorsal column lesion but only about half of the grafts in the hemisection. In the dorsal column lesion group, few sensory axons grew short distances into the lesion site of control animals. The presence of hGRPs transplants enhanced axonal growth significantly farther into the transplants. In the hemisection group, coerulospinal axons extended similarly into both control and transplant groups with no enhancement by the presence of hGRPs. Rubrospinal axons did not grow into the lesion even in the presence of hGRPs. However, reticulospinal and raphespinal axons grew for a significantly longer distance into the transplants. These results demonstrate the differential capacity of axonal growth/regeneration of the motor and sensory tracts based on their intrinsic abilities as well as their response to the modified environment induced by the hGRPs transplants. We conclude that hGRP transplants can modify the injury site for axon growth of sensory and some motor tracts, and suggest they could be combined with other interventions to restore connectivity.

Transplanted human glial-restricted progenitors can rescue the survival of dysmyelinated mice independent of the production of mature, compact myelin

The therapeutic effect of glial progenitor transplantation in diseases of dysmyelination is currently attributed to the formation of new myelin. Using magnetic resonance imaging (MRI), we show that the therapeutic outcome in dysmyelinated shiverer mice is dependent on the extent of cell migration but not the presence of mature and compact myelin. Human or mouse glial restricted progenitors (GRPs) were transplanted into rag2^-/- shiverer mouse neonates and followed for over one year. Mouse GRPs produced mature myelin as detected with multi-parametric MRI, but showed limited migration without extended animal lifespan. In sharp contrast, human GRPs migrated extensively and significantly increased animal survival, but production of mature myelin did not occur until 46weeks post-grafting. We conclude that human GRPs can extend the survival of transplanted shiverer mice prior to production of mature myelin, while mouse GRPs fail to extend animal survival despite the early presence of mature myelin. This paradox suggests that transplanted GRPs provide therapeutic benefits through biological processes other than the formation of mature myelin capable to foster rapid nerve conduction, challenging the current dogma of the primary role of myelination in regaining function of the central nervous system.

Stem cell therapy for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by the loss of motor neurons. Currently, no effective therapy is available to treat ALS, except for Riluzole, which has only limited clinical benefits. Stem-cell-based therapy has been intensively and extensively studied as a potential novel treatment strategy for ALS and has been shown to be effective, at least to some extent. In this article, we will review the current state of research on the use of stem cell therapy in the treatment of ALS and discuss the most promising stem cells for the treatment of ALS.

Stem and Progenitor Cell-Derived Astroglia Therapies for Neurological Diseases

Astroglia are a major cellular constituent of the central nervous system (CNS) and play crucial roles in brain development, function, and integrity. Increasing evidence demonstrates that astroglia dysfunction occurs in a variety of neurological disorders ranging from CNS injuries to genetic diseases and chronic degenerative conditions. These new insights herald the concept that transplantation of astroglia could be of therapeutic value in treating the injured or diseased CNS. Recent technological advances in the generation of human astroglia from stem and progenitor cells have been prominent. We propose that a better understanding of the suitability of astroglial cells in transplantation as well as of their therapeutic effects in animal models may lead to the establishment of astroglia-based therapies to treat neurological diseases.

Neural stem cell therapy for cancer

Cancers of the brain remain one of the greatest medical challenges. Traditional surgery and chemo-radiation therapy are unable to eradicate diffuse cancer cells and tumor recurrence is nearly inevitable. In contrast to traditional regenerative medicine applications, engineered neural stem cells (NSCs) are emerging as a promising new therapeutic strategy for cancer therapy. The tumor-homing properties allow NSCs to access both primary and invasive tumor foci, creating a novel delivery platform. NSCs engineered with a wide array of cytotoxic agents have been found to significantly reduce tumor volumes and markedly extend survival in preclinical models. With the recent launch of new clinical trials, the potential to successfully manage cancer in human patients with cytotoxic NSC therapy is moving closer to becoming a reality.

Neural and glial progenitor transplantation as a neuroprotective strategy for Amyotrophic Lateral Sclerosis (ALS)

ALS is a neurodegenerative disease with a prevalence rate of up to 7.4/100,000 and the overall risk of developing ALS over a lifetime is 1:400. Most patients die from respiratory failure following a course of progressive weakness. To date, only one traditional pharmaceutical agent-riluzole, has been shown to afford a benefit on survival but numerous pharmaceutical interventions have been studied in preclinical models of ALS without subsequent translation to patient efficacy. Despite the relative selectivity of motor neuron cell death, animal and tissue culture models of familial ALS suggest that non-neuronal cells significantly contribute to neuronal dysfunction and death. Early efforts to transplant stem cells had focused on motor neuron replacement. More practically for this aggressive neurodegenerative disease, recent studies, preclinical efforts, and early clinical trials have focused on the transplantation of neural stem cells, mesenchymal stem cells, or glial progenitors. Using transgenic mouse or rat models of ALS, a number of studies have shown neuroprotection through a variety of different mechanisms that have included neurotrophic factor secretion, glutamate transporter regulation, and modulation of neuroinflammation, among others. However, given that cell replacement could involve a number of biologically relevant factors, identifying the key pathway(s) that may contribute to neuroprotection remains a challenge. Nevertheless, given the abundant data supporting the interplay between non-neuronal cell types and motor neuron disease propagation, the replacement of disease-carrying host cells by normal cells may be sufficient to confer neuroprotection. Key preclinical issues that currently are being addressed include the most appropriate methods and routes for delivery of cells to disease-relevant regions of the neuraxis, cell survival and migration, and tracking the cells following transplantation. Central to the initial development of stem cell transplantation into patients with ALS is the demonstration that transplanted cells lack tumorigenicity and have the appropriate biodistribution to ensure the safety of ALS patients receiving these therapies. Here, we review preclinical and clinical studies focusing on the transplantation of neural and glial progenitor cells as a promising neuroprotective therapy for ALS. The rationale for stem cell transplantation for neuroprotection, proof-of-concept animal studies, and current challenges facing translation of these therapies to the clinic is presented. Lastly, we discuss advancements on the horizon including induced pluripotent stem cell technology and developments for cellular tracking and detection post-transplantation. With the safe completion of the first-in-human Phase I clinical trial for intraspinal stem cell transplantation for ALS in the United States, the time is ripe for stem cell therapies to be translated to the clinic and excitingly, evaluated for neuroprotection for ALS. This article is part of a Special Issue entitled SI: Neuroprotection.

Human glial progenitor engraftment and gene expression is independent of the ALS environment

Although Amyotrophic Lateral Sclerosis (ALS) is a motor neuron disease, basic research studies have highlighted that astrocytes contribute to the disease process. Therefore, strategies which replace the diseased astrocyte population with healthy astrocytes may protect against motor neuron degeneration. Our studies have sought to evaluate astrocyte replacement using glial-restricted progenitors (GRPs), which are lineage-restricted precursors capable of differentiating into astrocytes after transplantation. The goal of our current study was to evaluate how transplantation to the diseased ALS spinal cord versus a healthy, wild-type spinal cord may affect human GRP engraftment and selected gene expression. Human GRPs were transplanted into the spinal cord of either an ALS mouse model or wild-type littermate mice. Mice were sacrificed for analysis at either the onset of disease course or at the endstage of disease. The transplanted GRPs were analyzed by immunohistochemistry and NanoString gene profiling which showed no gross differences in the engraftment or gene expression of the cells. Our data indicate that human glial progenitor engraftment and gene expression is independent of the neurodegenerative ALS spinal cord environment. These findings are of interest given that human GRPs are currently in clinical development for spinal cord transplantation into ALS patients.

Combined transplantation of GDAs(BMP) and hr-decorin in spinal cord contusion repair

Following spinal cord injury, astrocyte proliferation and scar formation are the main factors inhibiting the regeneration and growth of spinal cord axons. Recombinant decorin suppresses inflammatory reactions, inhibits glial scar formation, and promotes axonal growth. Rat models of T₈ spinal cord contusion were created with the NYU impactor and these models were subjected to combined transplantation of bone morphogenetic protein-4-induced glial-restricted precursor-derived astrocytes and human recombinant decorin transplantation. At 28 days after spinal cord contusion, double-immunofluorescent histochemistry revealed that combined transplantation inhibited the early inflammatory response in injured rats. Furthermore, brain-derived neurotrophic factor, which was secreted by transplanted cells, protected injured axons. The combined transplantation promoted axonal regeneration and growth of injured motor and sensory neurons by inhibiting astrocyte proliferation and glial scar formation, with astrocytes forming a linear arrangement in the contused spinal cord, thus providing axonal regeneration channels.

Astrocyte transplantation for spinal cord injury: current status and perspective

Spinal cord injury (SCI) often causes incurable neurological dysfunction because axonal regeneration in adult spinal cord is rare. Astrocytes are gradually recognized as being necessary for the regeneration after SCI as they promote axonal growth under both physiological and pathophysiological conditions. Heterogeneous populations of astrocytes have been explored for structural and functional restoration. The results range from the early variable and modest effects of immature astrocyte transplantation to the later significant, but controversial, outcomes of glial-restricted precursor (GRP)-derived astrocyte (GDA) transplantation. However, the traditional neuron-centric view and the concerns about the inhibitory roles of astrocytes after SCI, along with the sporadic studies and the lack of a comprehensive review, have led to some confusion over the usefulness of astrocytes in SCI. It is the purpose of the review to discuss the current status of astrocyte transplantation for SCI based on a dialectical view of the context-dependent manner of astrocyte behavior and the time-associated characteristics of glial scarring. Critical issues are then analyzed to reveal the potential direction of future research.

The past, present and future of stem cell clinical trials for ALS

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disorder that is characterized by progressive degeneration of motor neurons in the cortex, brainstem and spinal cord. This leads to paralysis, respiratory insufficiency and death within an average of 3 to 5 years from disease onset. While the genetics of ALS are becoming more understood in familial cases, the mechanisms underlying disease pathology remain unclear and there are no effective treatment options. Without understanding what causes ALS it is difficult to design treatments. However, in recent years stem cell transplantation has emerged as a potential new therapy for ALS patients. While motor neuron replacement remains a focus of some studies trying to treat ALS with stem cells, there is more rationale for using stem cells as support cells for dying motor neurons as they are already connected to the muscle. This could be through reducing inflammation, releasing growth factors, and other potential less understood mechanisms. Prior to moving into patients, stringent pre-clinical studies are required that have at least some rationale and efficacy in animal models and good safety profiles. However, given our poor understanding of what causes ALS and whether stem cells may ameliorate symptoms, there should be a push to determine cell safety in pre-clinical models and then a quick translation to the clinic where patient trials will show if there is any efficacy. Here, we provide a critical review of current clinical trials using either mesenchymal or neural stem cells to treat ALS patients. Pre-clinical data leading to these trials, as well as those in development are also evaluated in terms of mechanisms of action, validity of conclusions and rationale for advancing stem cell treatment strategies for this devastating disorder.

Effects of adult neural precursor-derived myelination on axonal function in the perinatal congenitally dysmyelinated brain: optimizing time of intervention, developing accurate prediction models, and enhancing performance

Stem cell repair shows substantial translational potential for neurological injury, but the mechanisms of action remain unclear. This study aimed to investigate whether transplanted stem cells could induce comprehensive functional remyelination. Subventricular zone (SVZ)-derived adult neural precursor cells (aNPCs) were injected bilaterally into major cerebral white matter tracts of myelin-deficient shiverer mice on postnatal day (P) 0, P7, and P21. Tripotential NPCs, when transplanted in vivo, integrated anatomically and functionally into local white matter and preferentially became Olig2+, Myelin Associated Glycoprotein-positive, Myelin Basic Protein-positive oligodendrocytes, rather than Glial Fibrillary Acidic Protein-positive astrocytes or Neurofiliment 200-positive neurons. Processes interacted with axons and transmission electron microscopy showed multilamellar axonal ensheathment. Nodal architecture was restored and by quantifying these anatomical parameters a computer model was generated that accurately predicted action potential velocity, determined by ex vivo slice recordings. Although there was no obvious phenotypic improvement in transplanted shi/shis, myelinated axons exhibited faster conduction, lower activation threshold, less refractoriness, and improved response to high-frequency stimulation than dysmyelinated counterparts. Furthermore, they showed improved resilience to ischemic insult, a promising finding in the context of perinatal brain injury. This study describes, for the first time mechanistically, the functional characteristics and anatomical integration of nonimmortalized donor SVZ-derived murine aNPCs in the dysmyelinated brain at key developmental time points.

Functional screening assays with neurons generated from pluripotent stem cell-derived neural stem cells

Rapid and effective drug discovery for neurodegenerative disease is currently impeded by an inability to source primary neural cells for high-throughput and phenotypic screens. This limitation can be addressed through the use of pluripotent stem cells (PSCs), which can be derived from patient-specific samples and differentiated to neural cells for use in identifying novel compounds for the treatment of neurodegenerative diseases. We have developed an efficient protocol to culture pure populations of neurons, as confirmed by gene expression analysis, in the 96-well format necessary for screens. These differentiated neurons were subjected to viability assays to illustrate their potential in future high-throughput screens. We have also shown that organelles such as nuclei and mitochondria could be live-labeled and visualized through fluorescence, suggesting that we should be able to monitor subcellular phenotypic changes. Neurons derived from a green fluorescent protein-expressing reporter line of PSCs were live-imaged to assess markers of neuronal maturation such as neurite length and co-cultured with astrocytes to demonstrate further maturation. These studies confirm that PSC-derived neurons can be used effectively in viability and functional assays and pave the way for high-throughput screens on neurons derived from patients with neurodegenerative disorders.

Human astrocytes derived from glial restricted progenitors support regeneration of the injured spinal cord

Cellular transplantation using neural stem cells and progenitors is a promising therapeutic strategy that has the potential to replace lost cells, modulate the injury environment, and create a permissive environment for the regeneration of injured host axons. Our research has focused on the use of human glial restricted progenitors (hGRP) and derived astrocytes. In the current study, we examined the morphological and phenotypic properties of hGRP prepared from the fetal central nervous system by clinically-approved protocols, compared with astrocytes derived from hGRP prepared by treatment with ciliary neurotrophic factor or bone morphogenetic protein 4. These differentiation protocols generated astrocytes that showed morphological differences and could be classified along an immature to mature spectrum, respectively. Despite these differences, the cells retained morphological and phenotypic plasticity upon a challenge with an alternate differentiation protocol. Importantly, when hGRP and derived astrocytes were transplanted acutely into a cervical dorsal column lesion, they survived and promoted regeneration of long ascending host sensory axons into the graft/lesion site, with no differences among the groups. Further, hGRP taken directly from frozen stocks behaved similarly and also supported regeneration of host axons into the lesion. Our results underscore the dynamic and permissive properties of human fetal astrocytes to promote axonal regeneration. They also suggest that a time-consuming process of pre-differentiation may not be necessary for therapeutic efficacy, and that the banking of large quantities of readily available hGRP can be an appropriate source of permissive cells for transplantation.

ICV-transplanted human glial precursor cells are short-lived yet exert immunomodulatory effects in mice with EAE

Human glial precursor cells (hGPs) have potential for remyelinating lesions and are an attractive cell source for cell therapy of multiple sclerosis (MS). To investigate whether transplanted hGPs can affect the pathogenesis of experimental autoimmune encephalomyelitis (EAE), an animal model of MS, we evaluated the therapeutic effects of transplanted hGPs together with the in vivo fate of these cells using magnetic resonance imaging (MRI) and bioluminescence imaging (BLI). At 14 days post-EAE induction, mice (n = 19) were intracerebroventricularly (ICV) injected with 5 × 10(5) hGPs that were magnetically labeled with superparamagnetic iron oxide (SPIO) particles as MR contrast agent and transduced with firefly luciferase for BLI of cell survival. Control mice (n = 18) received phosphate buffered saline (PBS) vehicle only. The severity of EAE clinical disability in the hGP-transplanted group was significantly suppressed (P < 0.05) with concomitant inhibition of ConA and MOG-specific T cell proliferation in the spleen. Astrogliosis was reduced and a lower activity of macrophages and/or microglia was observed in the spinal cord (P < 0.05). On MRI, SPIO signal was detected within the lateral ventricle from 1 day post-transplantation and remained there for up to 34 days. BLI indicated that most cells did not survive beyond 5-10 days, consistent with the lack of detectable migration into the brain parenchyma and the histological presence of an abundance of apoptotic cells. Transplanted hGPs could not be detected in the spleen. We conclude that ICV transplantation of short-lived hGPs can have a remote therapeutic effect through immunomodulation from within the ventricle, without cells directly participating in remyelination.

Phenotypic analysis of astrocytes derived from glial restricted precursors and their impact on axon regeneration

Although astrocytes are involved in the production of an inhibitory glial scar following injury, they are also capable of providing neuroprotection and supporting axonal growth. There is growing appreciation for a diverse and dynamic population of astrocytes, specified by a variety of glial precursors, whose function is regulated regionally and temporally. Consequently, the therapeutic application of glial precursors and astrocytes by effective transplantation protocols requires a better understanding of their phenotypic and functional properties and effective protocols for their preparation. We present a systematic analysis of astrocyte differentiation using multiple preparations of glial-restricted precursors (GRP), evaluating their morphological and phenotypic properties following treatment with fetal bovine serum (FBS), bone morphogenetic protein 4 (BMP-4), or ciliary neurotrophic factor (CNTF) in comparison to controls treated with basic fibroblast growth factor (bFGF), which maintains undifferentiated GRP. We found that treatments with FBS or BMP-4 generated similar profiles of highly differentiated astrocytes that were A2B5-/GFAP+. Treatment with FBS generated the most mature astrocytes, with a distinct and near-homogeneous morphology of fibroblast-like flat cells, whereas BMP-4 derived astrocytes had a stellate, but heterogeneous morphology. Treatment with CNTF induced differentiation of GRP to an intermediate state of GFAP+cells that maintained immature markers and had relatively long processes. Furthermore, astrocytes generated by BMP-4 or CNTF showed considerable experimental plasticity, and their morphology and phenotypes could be reversed with complementary treatments along a wide range of mature-immature states. Importantly, when GRP or GRP treated with BMP-4 or CNTF were transplanted acutely into a dorsal column lesion of the spinal cord, cells from all 3 groups survived and generated permissive astrocytes that supported axon growth and regeneration of host sensory axons into, but not out of the lesion. Our study underscores the dynamic nature of astrocytes prepared from GRP and their permissive properties, and suggest that future therapeutic applications in restoring connectivity following CNS injury are likely to require a combination of treatments.

Human glial-restricted progenitor transplantation into cervical spinal cord of the SOD1 mouse model of ALS

Cellular abnormalities are not limited to motor neurons in amyotrophic lateral sclerosis (ALS). There are numerous observations of astrocyte dysfunction in both humans with ALS and in SOD1(G93A) rodents, a widely studied ALS model. The present study therapeutically targeted astrocyte replacement in this model via transplantation of human Glial-Restricted Progenitors (hGRPs), lineage-restricted progenitors derived from human fetal neural tissue. Our previous findings demonstrated that transplantation of rodent-derived GRPs into cervical spinal cord ventral gray matter (in order to target therapy to diaphragmatic function) resulted in therapeutic efficacy in the SOD1(G93A) rat. Those findings demonstrated the feasibility and efficacy of transplantation-based astrocyte replacement for ALS, and also show that targeted multi-segmental cell delivery to cervical spinal cord is a promising therapeutic strategy, particularly because of its relevance to addressing respiratory compromise associated with ALS. The present study investigated the safety and in vivo survival, distribution, differentiation, and potential efficacy of hGRPs in the SOD1(G93A) mouse. hGRP transplants robustly survived and migrated in both gray and white matter and differentiated into astrocytes in SOD1(G93A) mice spinal cord, despite ongoing disease progression. However, cervical spinal cord transplants did not result in motor neuron protection or any therapeutic benefits on functional outcome measures. This study provides an in vivo characterization of this glial progenitor cell and provides a foundation for understanding their capacity for survival, integration within host tissues, differentiation into glial subtypes, migration, and lack of toxicity or tumor formation.

Precursor cell biology and the development of astrocyte transplantation therapies: lessons from spinal cord injury

This review summarizes current progress on development of astrocyte transplantation therapies for repair of the damaged central nervous system. Replacement of neurons in the injured or diseased central nervous system is currently one of the most popular therapeutic goals, but if neuronal replacement is attempted in the absence of appropriate supporting cells (astrocytes and oligodendrocytes), then the chances of restoring neurological functional are greatly reduced. Although the past 20 years have offered great progress on oligodendrocyte replacement therapies, astrocyte transplantation therapies have been both less explored and comparatively less successful. We have now developed successful astrocyte transplantation therapies by pre-differentiating glial restricted precursor (GRP) cells into a specific population of GRP cell-derived astrocytes (GDAs) by exposing the GRP cells to bone morphogenetic protein-4 (BMP) prior to transplantation. When transplanted into transected rat spinal cord, rat and human GDAs(BMP) promote extensive axonal regeneration, rescue neuronal cell survival, realign tissue structure, and restore behavior to pre-injury levels on a grid-walk analysis of volitional foot placement. Such benefits are not provided by GRP cells themselves, demonstrating that the lesion environment does not direct differentiation in a manner optimally beneficial for the restoration of function. Such benefits also are not provided by transplantation of a different population of astrocytes generated from GRP cells exposed to ciliary neurotrophic factor (GDAs(CNTF)), thus providing the first transplantation-based evidence of functional heterogeneity in astrocyte populations. Moreover, lessons learned from the study of rat cells are strongly predictive of outcomes using human cells. Thus, these studies provide successful strategies for the use of astrocyte transplantation therapies for restoration of function following spinal cord injury.

Translating stem cell studies to the clinic for CNS repair: current state of the art and the need for a Rosetta stone

Since their discovery twenty years ago and prospective isolation a decade later, neural stem cells (NSCs), their progenitors, and differentiated cell derivatives along with other stem-cell based strategies have advanced steadily toward clinical trials, spurred by the immense need to find reparative therapeutics for central nervous system (CNS) diseases and injury. Current phase I/II trials using stem cells in the CNS are the vanguard for the widely anticipated next generation of regenerative therapies and as such are pioneering the stem cell therapy process. While translation has typically been the purview of industry, academic researchers are increasingly driven to bring their findings toward treatments and face challenges in knowledge gap and resource access that are accentuated by the unique financial, manufacturing, scientific, and regulatory aspects of cell therapy. Solutions are envisioned that both address the significant unmet medical need and lead to increased funding for basic and translational research.

Transplantation of human glial restricted progenitors and derived astrocytes into a contusion model of spinal cord injury

Transplantation of neural progenitors remains a promising therapeutic approach to spinal cord injury (SCI), but the anatomical and functional evaluation of their effects is complex, particularly when using human cells. We investigated the outcome of transplanting human glial-restricted progenitors (hGRP) and astrocytes derived from hGRP (hGDA) in spinal cord contusion with respect to cell fate and host response using athymic rats to circumvent xenograft immune issues. Nine days after injury hGRP, hGDA, or medium were injected into the lesion center and rostral and caudal to the lesion, followed by behavioral testing for 8 weeks. Both hGRP and hGDA showed robust graft survival and extensive migration. The total number of cells increased 3.5-fold for hGRP, and twofold for hGDA, indicating graft expansion, but few proliferating cells remained by 8 weeks. Grafted cells differentiated into glia, predominantly astrocytes, and few remained at progenitor state. About 80% of grafted cells around the injury were glial fibrillary acidic protein (GFAP)-positive, gradually decreasing to 40-50% at a distance of 6 mm. Conversely, there were few graft-derived oligodendrocytes at the lesion, but their numbers increased away from the injury to 30-40%. Both cell grafts reduced cyst and scar formation at the injury site compared to controls. Microglia/macrophages were present at and around the lesion area, and axons grew along the spared tissue with no differences among groups. There were no significant improvements in motor function recovery as measured by the Basso, Beattie, and Bresnahan (BBB) scale and grid tests in all experimental groups. Cystometry revealed that hGRP grafts attenuated hyperactive bladder reflexes. Importantly, there was no increased sensory or tactile sensitivity associated with pain, and the hGDA group showed sensory function returning to normal. Although the improved lesion environment was not sufficient for robust functional recovery, the permissive properties and lack of sensory hypersensitivity indicate that human GRP and astrocytes remain promising candidates for therapy after SCI.

Human glial-restricted progenitors survive, proliferate, and preserve electrophysiological function in rats with focal inflammatory spinal cord demyelination

Transplantation of glial progenitor cells results in transplant-derived myelination and improved function in rodents with genetic dysmyelination or chemical demyelination. However, glial cell transplantation in adult CNS inflammatory demyelinating models has not been well studied. Here we transplanted human glial-restricted progenitor (hGRP) cells into the spinal cord of adult rats with inflammatory demyelination, and monitored cell fate in chemically immunosuppressed animals. We found that hGRPs migrate extensively, expand within inflammatory spinal cord lesions, do not form tumors, and adopt a mature glial phenotype, albeit at a low rate. Human GRP-transplanted rats, but not controls, exhibited preserved electrophysiological conduction across the spinal cord, though no differences in behavioral improvement were noted between the two groups. Although these hGRPs myelinated extensively after implantation into neonatal shiverer mouse brain, only marginal remyelination was observed in the inflammatory spinal cord demyelination model. The low rate of transplant-derived myelination in adult rat spinal cord may reflect host age, species, transplant environment/location, and/or immune suppression regime differences. We conclude that hGRPs have the capacity to myelinate dysmyelinated neonatal rodent brain and preserve conduction in the inflammatory demyelinated adult rodent spinal cord. The latter benefit is likely dependent on trophic support and suggests further exploration of potential of glial progenitors in animal models of chronic inflammatory demyelination.