Emerging concepts in neural stem cell research: autologous repair and cell-based disease modelling

Cell Therapy and Functional Recovery of Stroke

Stroke is the most common cause of disability. Brain repair mechanisms are often insufficient to allow a full recovery. Stroke damage involve all brain cell type and extracellular matrix which represent the crucial "glio-neurovascular niche" useful for brain plasticity. Regenerative medicine including cell therapies hold great promise to decrease post-stroke disability of many patients, by promoting both neuroprotection and neural repair through direct effects on brain lesion and/or systemic effects such as immunomodulation. Mechanisms of action vary according to each grafted cell type: "peripheral" stem cells, such as mesenchymal stem cells (MSC), can provide paracrine trophic support, and neural stem/progenitor cells (NSC) or neurons can act as direct cells' replacements. Optimal time window, route, and doses are still debated, and may depend on the chosen medicinal product and its expected mechanism such as neuroprotection, delayed brain repair, systemic effects, or graft survival and integration in host network. MSC, mononuclear cells (MNC), umbilical cord stem cells and NSC are the most investigated. Innovative approaches are implemented concerning combinatorial approaches with growth factors and biomaterials such as injectable hydrogels which could protect a cell graft and/or deliver drugs into the post-stroke cavity at chronic stages. Through main publications of the last two decades, we provide in this review concepts and suggestions to improve future translational researches and larger clinical trials of cell therapy in stroke.

Cell membrane-targeting NIR fluorescent probes with large Stokes shifts for ultralong-term transplanted neural stem cell tracking

There is an emerging therapeutic strategy to transplant stem cells into diseased host tissue for various neurodegenerative diseases, owing to their self-renewal ability and pluripotency. However, the traceability of long-term transplanted cells limits the further understanding of the mechanism of the therapy. Herein, we designed and synthesized a quinoxalinone scaffold-based near-infrared (NIR) fluorescent probe named QSN, which exhibits ultra-strong photostability, large Stokes shift, and cell membrane-targeting capacity. It could be found that QSN-labeled human embryonic stem cells showed strong fluorescent emission and photostability both in vitro and in vivo. Additionally, QSN would not impair the pluripotency of embryonic stem cells, indicating that QSN did not perform cytotoxicity. Moreover, it is worth mentioning that QSN-labeled human neural stem cells held cellular retention for at least 6 weeks in the mouse brain striatum post transplantation. All these findings highlight the potential application of QSN for ultralong-term transplanted cell tracking.

Human mesenchymal amniotic fluid stem cells reveal an unexpected neuronal potential differentiating into functional spinal motor neurons

Human amniotic fluids stem cells (hAFSCs) can be easily isolated from the amniotic fluid during routinely scheduled amniocentesis. Unlike hiPSCs or hESC, they are neither tumorigenic nor immunogenic and their use does not rise ethical or safety issues: for these reasons they may represent a good candidate for the regenerative medicine. hAFSCs are generally considered multipotent and committed towards the mesodermal lineages; however, they express many pluripotent markers and share some epigenetic features with hiPSCs. Hence, we hypothesized that hAFSCs may overcome their mesodermal commitment differentiating into to ectodermal lineages. Here we demonstrated that by the sequential exposure to specific factors, hAFSCs can give rise to spinal motor neurons (MNs), as evidenced by the gradual gene and protein upregulation of early and late MN markers (PAX6, ISL1, HB9, NF-L, vAChT). When co-cultured with myotubes, hAFSCs-derived MNs were able to create functional neuromuscular junctions that induced robust skeletal muscle contractions. These data demonstrated the hAFSCs are not restricted to mesodermal commitment and can generate functional MNs thus outlining an ethically acceptable strategy for the study and treatment of the neurodegenerative diseases.

Biomaterials for Neural Tissue Engineering

The therapy of neural nerve injuries that involve the disruption of axonal pathways or axonal tracts has taken a new dimension with the development of tissue engineering techniques. When peripheral nerve injury (PNI), spinal cord injury (SCI), traumatic brain injury (TBI), or neurodegenerative disease occur, the intricate architecture undergoes alterations leading to growth inhibition and loss of guidance through large distance. To improve the limitations of purely cell-based therapies, the neural tissue engineering philosophy has emerged. Efforts are being made to produce an ideal scaffold based on synthetic and natural polymers that match the exact biological and mechanical properties of the tissue. Furthermore, through combining several components (biomaterials, cells, molecules), axonal regrowth is facilitated to obtain a functional recovery of the neural nerve diseases. The main objective of this review is to investigate the recent approaches and applications of neural tissue engineering approaches.

Recent advances of induced pluripotent stem cells application in neurodegenerative diseases

Stem cell is defined by its ability to self-renewal and generates differentiated functional cell types, which are derived from the embryo and various sources of postnatal animal. These cells can be divided according to their potential development into totipotent, unipotent, multipotent andpluripotent. Pluripotent is considered as the most important type due to its advantageous capability to create different cell types of the body in a similar behavior as embryonic stem cell. Induced pluripotent stem cells (iPSCs) are adult cells that maintain the characteristics of embryonic stem cells because it can be genetically reprogrammed to an embryonic stem cell-like state via express genes and transcription factors. Such cells provide an efficient pathway to explorehuman diseases and their corresponding therapy, particularly, neurodevelopmental disorders. Consequently, iPSCs can be investigated to check the specific mutations of neurodegenerative disease due to their unique ability to differentiate into neural cell types and/or neural organoids. The current review addresses the different neurodegenerative diseases model by using iPSCs approach such as Alzheimer's diseases (AD), Parkinson diseases (PD),multiplesclerosis(MS) and psychiatric disorders. We also highlight the importance of autophagy in neurodegenerative diseases.

Next-generation disease modeling with direct conversion: a new path to old neurons

Within just over a decade, human reprogramming-based disease modeling has developed from a rather outlandish idea into an essential part of disease research. While iPSCs are a valuable tool for modeling developmental and monogenetic disorders, their rejuvenated identity poses limitations for modeling age-associated diseases. Direct cell-type conversion of fibroblasts into induced neurons (iNs) circumvents rejuvenation and preserves hallmarks of cellular aging. iNs are thus advantageous for modeling diseases that possess strong age-related and epigenetic contributions and can complement iPSC-based strategies for disease modeling. In this review, we provide an overview of the state of the art of direct iN conversion and describe the key epigenetic, transcriptomic, and metabolic changes that occur in converting fibroblasts. Furthermore, we summarize new insights into this fascinating process, particularly focusing on the rapidly changing criteria used to define and characterize in vitro-born human neurons. Finally, we discuss the unique features that distinguish iNs from other reprogramming-based neuronal cell models and how iNs are relevant to disease modeling.

Potential therapeutic roles of stem cells in ischemia-reperfusion injury

Ischemia-reperfusion injury (I/RI), produced by an initial interruption of organ blood flow and its subsequent restoration, contributes significantly to the pathophysiologies of stroke, myocardial infarction, renal I/RI, intestinal I/RI and liver I/RI, which are major causes of disability (including transplant failure) and even mortality. While the restoration of blood flow is required to restore oxygen and nutrient requirements, reperfusion often triggers local and systemic inflammatory responses and subsequently elevate the ischemic insult where the duration of ischemia determines the magnitude of I/RI damage. I/RI increases vascular leakage, changes transcriptional and cell death programs, drives leukocyte entrapment and inflammation and oxidative stress in tissues. Therapeutic approaches which reduce complications associated with I/RI are desperately needed to address the clinical and economic burden created by I/RI. Stem cells (SC) represent ubiquitous and uncommitted cell populations with the ability to self-renew and differentiate into one or more developmental 'fates'. Like immune cells, stem cells can home to and penetrate I/R-injured tissues, where they can differentiate into target tissues and induce trophic paracrine signaling which suppress injury and maintain tissue functions perturbed by ischemia-reperfusion. This review article summarizes the present use and possible protective mechanisms underlying stem cell protection in diverse forms of ischemia-reperfusion.

Multidisciplinary Perspectives for Alzheimer's and Parkinson's Diseases: Hydrogels for Protein Delivery and Cell-Based Drug Delivery as Therapeutic Strategies

This review presents two intriguing multidisciplinary strategies that might make the difference in the treatment of neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. The first proposed strategy is based on the controlled delivery of recombinant proteins known to play a key role in these neurodegenerative disorders that are released in situ by optimized polymer-based systems. The second strategy is the use of engineered cells, encapsulated and delivered in situ by suitable polymer-based systems, that act as drug reservoirs and allow the delivery of selected molecules to be used in the treatment of Alzheimer's and Parkinson's diseases. In both these scenarios, the design and development of optimized polymer-based drug delivery and cell housing systems for central nervous system applications represent a key requirement. Materials science provides suitable hydrogel-based tools to be optimized together with suitably designed recombinant proteins or drug delivering-cells that, once in situ, can provide an effective treatment for these neurodegenerative disorders. In this scenario, only interdisciplinary research that fully integrates biology, biochemistry, medicine and materials science can provide a springboard for the development of suitable therapeutic tools, not only for the treatment of Alzheimer's and Parkinson's diseases but also, prospectively, for a wide range of severe neurodegenerative disorders.

In Vivo Reprogramming for CNS Repair: Regenerating Neurons from Endogenous Glial Cells

Neuroregeneration in the CNS has proven to be difficult despite decades of research. The old dogma that CNS neurons cannot be regenerated in the adult mammalian brain has been overturned; however, endogenous adult neurogenesis appears to be insufficient for brain repair. Stem cell therapy once held promise for generating large quantities of neurons in the CNS, but immunorejection and long-term functional integration remain major hurdles. In this Perspective, we discuss the use of in vivo reprogramming as an emerging technology to regenerate functional neurons from endogenous glial cells inside the brain and spinal cord. Besides the CNS, in vivo reprogramming has been demonstrated successfully in the pancreas, heart, and liver and may be adopted in other organs. Although challenges remain for translating this technology into clinical therapies, we anticipate that in vivo reprogramming may revolutionize regenerative medicine by using a patient's own internal cells for tissue repair.

A robust vitronectin-derived peptide for the scalable long-term expansion and neuronal differentiation of human pluripotent stem cell (hPSC)-derived neural progenitor cells (hNPCs)

Despite therapeutic advances, neurodegenerative diseases and disorders remain some of the leading causes of mortality and morbidity in the United States. Therefore, cell-based therapies to replace lost or damaged neurons and supporting cells of the central nervous system (CNS) are of great therapeutic interest. To that end, human pluripotent stem cell (hPSC) derived neural progenitor cells (hNPCs) and their neuronal derivatives could provide the cellular 'raw material' needed for regenerative medicine therapies for a variety of CNS disorders. In addition, hNPCs derived from patient-specific hPSCs could be used to elucidate the underlying mechanisms of neurodegenerative diseases and identify potential drug candidates. However, the scientific and clinical application of hNPCs requires the development of robust, defined, and scalable substrates for their long-term expansion and neuronal differentiation. In this study, we rationally designed a vitronectin-derived peptide (VDP) that served as an adhesive growth substrate for the long-term expansion of several hNPC lines. Moreover, VDP-coated surfaces allowed for the directed neuronal differentiation of hNPC at levels similar to cells differentiated on traditional extracellular matrix protein-based substrates. Overall, the ability of VDP to support the long-term expansion and directed neuronal differentiation of hNPCs will significantly advance the future translational application of these cells in treating injuries, disorders, and diseases of the CNS.

Clinical trial design for stem cell therapies in stroke: What have we learned?

Stem cells of various sources have been investigated in a series of small, safety and feasibility-focused studies over the past 15 years. Understanding of mechanisms of action has evolved and the trial paradigms have become focused on two different approaches - one being an early subacute delivery of cells to reduce acute tissue injury and modify the tissue environment in a direction favourable to reparative processes (for example by being anti-inflammatory, anti-apoptotic, and encouraging endogenous stem cell mobilisation); the other exploring later delivery of cells during the recovery phase after stroke to modulate the local environment in favour of angiogenesis and neurogenesis. The former approach has generally investigated intravenous or intra-arterial delivery of cells with an expected paracrine mode of action and no expected engraftment within the brain. The latter has explored direct intracerebral implantation adjacent to the infarct. Several relevant trials have been conducted, including two controlled trials of intravenously delivered bone marrow-derived cells in the early subacute stage, and two small single-arm phase 1 trials of intracerebrally implanted cells. The findings of these studies and their implications for future trial design are considered.

Exogenous Neural Stem Cells Transplantation as a Potential Therapy for Photothrombotic Ischemia Stroke in Kunming Mice Model

Stroke is considered as the second leading cause of death worldwide. The survivors of stroke experience different levels of impairment in brain function resulting in debilitating disabilities. Current therapies for stroke are primarily palliative and may be effective in only a small population of stroke patients. In this study, we explore the transplantation of exogenous neural stem cells (NSCs) as the potential therapy for the photothrombotic ischemia stroke in a Kunming mice model. After stroke, mice receiving NSC transplantation demonstrated a better recovery of brain function during the neurobehavioral tests. Histology analysis of the brain samples from NSC transplanted mice demonstrated a reduction of brain damage caused by stroke. Moreover, immunofluorescence assay for biomarkers in brain sections confirmed that transplanted NSCs indeed differentiated to neurons and astrocytes, consistent with the improved brain function after stroke. Taken together, our data suggested that exogenous NSC transplantation could be a promising therapy for stroke.

Tapping Stem Cells to Target AMD: Challenges and Prospects

Human pluripotent stem cells (hPSCs) are increasingly gaining attention in biomedicine as valuable resources to establish patient-derived cell culture models of the cell type known to express the primary pathology. The idea of "a patient in a dish" aims at basic, but also clinical, applications with the promise to mimic individual genetic and metabolic complexities barely reflected in current invertebrate or vertebrate animal model systems. This may particularly be true for the inherited and complex diseases of the retina, as this tissue has anatomical and physiological aspects unique to the human eye. For example, the complex age-related macular degeneration (AMD), the leading cause of blindness in Western societies, can be attributed to a large number of genetic and individual factors with so far unclear modes of mutual interaction. Here, we review the current status and future prospects of utilizing hPSCs, specifically induced pluripotent stem cells (iPSCs), in basic and clinical AMD research, but also in assessing potential treatment options. We provide an outline of concepts for disease modelling and summarize ongoing and projected clinical trials for stem cell-based therapy in late-stage AMD.

The role of microRNAs in human neural stem cells, neuronal differentiation and subtype specification

The impressive neuronal diversity found within the nervous system emerges from a limited pool of neural progenitor cells that proceed through different gene expression programs to acquire distinct cell fates. Here, we review recent evidence indicating that microRNAs (miRNAs) are critically involved in conferring neural cell identities during neural induction, neuronal differentiation and subtype specification. Several studies have shown that miRNAs act in concert with other gene regulatory factors and genetic switches to regulate the spatial and temporal expression profiles of important cell fate determinants. So far, most studies addressing the role of miRNAs during neurogenesis were conducted using animal models. With the advent of human pluripotent stem cells and the possibility to differentiate these into neural stem cells, we now have the opportunity to study miRNAs in a human context. More insight into the impact of miRNA-based regulation during neural fate choice could in the end be exploited to develop new strategies for the generation of distinct human neuronal cell types.

Angioblast Derived from ES Cells Construct Blood Vessels and Ameliorate Diabetic Polyneuropathy in Mice

BACKGROUND

Although numerous reports addressing pathological involvements of diabetic polyneuropathy have been conducted, a universally effective treatment of diabetic polyneuropathy has not yet been established. Recently, regenerative medicine studies in diabetic polyneuropathy using somatic stem/progenitor cell have been reported. However, the effectiveness of these cell transplantations was restricted because of their functional and numerical impairment in diabetic objects. Here, we investigated the efficacy of treatment for diabetic polyneuropathy using angioblast-like cells derived from mouse embryonic stem cells.

METHODS AND RESULTS

Angioblast-like cells were obtained from mouse embryonic stem cells and transplantation of these cells improved several physiological impairments in diabetic polyneuropathy: hypoalgesia, delayed nerve conduction velocities, and reduced blood flow in sciatic nerve and plantar skin. Furthermore, pathologically, the capillary number to muscle fiber ratios were increased in skeletal muscles of transplanted hindlimbs, and intraepidermal nerve fiber densities were ameliorated in transplanted plantar skin. Transplanted cells maintained their viabilities and differentiated to endothelial cells and smooth muscle cells around the injection sites. Moreover, several transplanted cells constructed chimeric blood vessels with recipient cells.

CONCLUSIONS

These results suggest that transplantation of angioblast like cells induced from embryonic stem cells appears to be a novel therapeutic strategy for diabetic polyneuropathy.

Stem cell reprogramming: basic implications and future perspective for movement disorders

The introduction of stem cell-associated molecular factors into human patient-derived cells allows for their reprogramming in the laboratory environment. As a result, human induced pluripotent stem cells (hiPSC) can now be reprogrammed epigenetically without disruption of their overall genomic integrity. For patients with neurodegenerative diseases characterized by progressive loss of functional neurons, the ability to reprogram any individual's cells and drive their differentiation toward susceptible neuronal subtypes holds great promise. Apart from applications in regenerative medicine and cell replacement-based therapy, hiPSCs are increasingly used in preclinical research for establishing disease models and screening for drug toxicities. The rapid developments in this field prompted us to review recent progress toward the applications of stem cell technologies for movement disorders. We introduce reprogramming strategies and explain the critical steps in the differentiation of hiPSCs to clinical relevant subtypes of cells in the context of movement disorders. We summarize and discuss recent discoveries in this field, which, based on the rapidly expanding basic science literature as well as upcoming trends in personalized medicine, will strongly influence the future therapeutic options available to practitioners working with patients suffering from such disorders.

Integrated platform for production and purification of human pluripotent stem cell-derived neural precursors

Human pluripotent stem cells (hPSCs) are a promising source of cells for clinical applications, such as transplantation of clinically engineered tissues and organs, and drug discovery programs due to their ability to self-renew and to be differentiated into cells from the three embryonic germ layers. In this study, the differentiation of two hPSC-lines into neural precursors (NPs) was accomplished with more than 80% efficiency, by means of the dual-SMAD inhibition protocol, based on the use of two small molecules (SB431542 and LDN193189) to generate Pax6 and Nestin-positive neural entities. One of the major hurdles related to the in vitro generation of PSC-derived populations is the tumorigenic potential of cells that remain undifferentiated. These remaining hPSCs have the potential to generate teratomas after being transplanted, and may interfere with the outcome of in vitro differentiation protocols. One strategy to tackle this problem is to deplete these "contaminating" cells during the differentiation process. Magnetic activated cell sorting (MACS) was used for the first time for purification of hPSC-derived NPs after the neural commitment stage using anti-Tra-1-60 micro beads for negative selection of the unwanted hPSCs. The depletion had an average efficiency of 80.4 ± 5% and less than 1.5% of Tra-1-60 positive cells were present in the purified populations. After re-plating, the purified neural precursors maintained their phenotype, and the success of the preparative purification with MACS was further confirmed with a decrease of 94.3% in the number of Oct4-positive proliferating hPSC colonies. Thus, the integration of the MACS depletion step with the neural commitment protocol paves the way towards the establishment of a novel bioprocess for production of purified populations of hPSC-derived neural cells for different applications.

Endogenous WNT signaling regulates hPSC-derived neural progenitor cell heterogeneity and specifies their regional identity

Neural progenitor cells (NPCs) derived from human pluripotent stem cells (hPSCs) are a multipotent cell population that is capable of nearly indefinite expansion and subsequent differentiation into the various neuronal and supporting cell types that comprise the CNS. However, current protocols for differentiating NPCs toward neuronal lineages result in a mixture of neurons from various regions of the CNS. In this study, we determined that endogenous WNT signaling is a primary contributor to the heterogeneity observed in NPC cultures and neuronal differentiation. Furthermore, exogenous manipulation of WNT signaling during neural differentiation, through either activation or inhibition, reduces this heterogeneity in NPC cultures, thereby promoting the formation of regionally homogeneous NPC and neuronal cultures. The ability to manipulate WNT signaling to generate regionally specific NPCs and neurons will be useful for studying human neural development and will greatly enhance the translational potential of hPSCs for neural-related therapies.

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Heterogeneous endogenous WNT signaling regulates hPSC-derived neuronal diversity

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Endogenous WNT signaling specifies the regional identity of hPSC-derived neurons

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Exogenous WNT signaling leads to uniform neuronal cultures from hPSCs

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Effects of WNT signaling on neurogenesis are recapitulated in an hPSC-based system

Remote control of induced dopaminergic neurons in parkinsonian rats

Direct lineage reprogramming through genetic-based strategies enables the conversion of differentiated somatic cells into functional neurons and distinct neuronal subtypes. Induced dopaminergic (iDA) neurons can be generated by direct conversion of skin fibroblasts; however, their in vivo phenotypic and functional properties remain incompletely understood, leaving their impact on Parkinson's disease (PD) cell therapy and modeling uncertain. Here, we determined that iDA neurons retain a transgene-independent stable phenotype in culture and in animal models. Furthermore, transplanted iDA neurons functionally integrated into host neuronal tissue, exhibiting electrically excitable membranes, synaptic currents, dopamine release, and substantial reduction of motor symptoms in a PD animal model. Neuronal cell replacement approaches will benefit from a system that allows the activity of transplanted neurons to be controlled remotely and enables modulation depending on the physiological needs of the recipient; therefore, we adapted a DREADD (designer receptor exclusively activated by designer drug) technology for remote and real-time control of grafted iDA neuronal activity in living animals. Remote DREADD-dependent iDA neuron activation markedly enhanced the beneficial effects in transplanted PD animals. These data suggest that iDA neurons have therapeutic potential as a cell replacement approach for PD and highlight the applicability of pharmacogenetics for enhancing cellular signaling in reprogrammed cell-based approaches.

Generation of retinal pigment epithelial cells from small molecules and OCT4 reprogrammed human induced pluripotent stem cells

Autologous retinal pigment epithelium (RPE) grafts derived from induced pluripotent stem cells (iPSCs) may be used to cure blinding diseases in which RPE dysfunction results in photoreceptor degeneration. Four-, two-, and one-factor-derived iPSCs (4F-, 2F-, and 1F-iPSCs, respectively) were differentiated into fully functional cuboidal pigmented cells in polarized monolayers that express RPE-specific markers. 1F-iPSCs-RPE (1F-iPS-RPE) strongly resembles primary human fetal RPE (hfRPE) based on proteomic and untargeted metabolomic analyses, and using novel in vivo imaging technology coupled with electroretinography, we demonstrated that 1F-iPS-RPE mediate anatomical and functional rescue of photoreceptors after transplantation in an animal model of RPE-mediated retinal degeneration. 1F-iPS0RPE cells were injected subretinally as a suspension and formed a monolayer dispersed between host RPE cells. Furthermore, 1F-iPS-RPE do not simply provide trophic support to rescue photoreceptors as previously speculated but actually phagocytose photoreceptor outer segments in vivo and maintain visual cycling. Thus, 1f-iPS-RPE grafts may be superior to conventional iPS-RPE for clinical use because 1F-IPS-RPE closely resemble hfRPE, mediate anatomical and functional photoreceptor rescue in vivo, and are generated using a reduced number of potentially oncogenic reprogramming factors.

Control of obesity and glucose intolerance via building neural stem cells in the hypothalamus

Neural stem cells (NSCs) were recently revealed to exist in the hypothalamus of adult mice. Here, following our observation showing that a partial loss of hypothalamic NSCs caused weight gain and glucose intolerance, we studied if NSCs-based cell therapy could be developed to control these disorders. While hypothalamus-implanted NSCs failed to survive in mice with obesity, NF-κB inhibition induced survival and neurogenesis of these cells, leading to effects in counteracting obesity and glucose intolerance. To generate an alternative cell source, we revealed that iPS-derived NSCs were converted into htNSCs by neuropeptide treatment. Of note, obesity condition potentiated the transfer of carotid artery-injected NSCs into the hypothalamus. These iPS-derived cells when engineered with NF-κB inhibition were also effective in reducing obesity and glucose intolerance, and neurogenesis towards POMCergic and GABAergic lineages was accountable. In conclusion, building NSCs in the hypothalamus represents a strategy for controlling obesity and glucose disorders.

Auto-attraction of neural precursors and their neuronal progeny impairs neuronal migration

Limited neuronal migration into host brain tissue is a key challenge in neural transplantation. We found that one important mechanism underlying this phenomenon is an intrinsic chemotactic interaction between the grafted neural precursor cells (NPCs) and their neuronal progeny. NPCs secrete the receptor tyrosine kinase ligands FGF2 and VEGF, which act as chemoattractants for neurons. Interference with these signaling pathways resulted in enhanced migration of human neurons from neural clusters.

Neural differentiation and support of neuroregeneration of non-neural adult stem cells

Although it is well established that neural stem cells (NSCs) or neural stem/progenitor cells differentiated from pluripotent stem cells can generate neurons, astrocytes, and oligodendrocytes, a number of other cell populations are also being considered for therapy of central nervous system disorders. Here, we describe the potential of (stem) cells from other postnatal tissues, including bone marrow, (umbilical cord) blood, fat tissue, or dental pulp, which themselves do not (robustly) generate neural progeny. However, these non-neuroectoderm derived cell populations appear to capable of inducing endogenous neurogenesis and angiogenesis. As these "trophic" effects are also, at least partly, responsible for some of the beneficial effects seen when NSC are grafted in the brain, these non-neuroectodermal cells may exert beneficial effects when used to treat neurodegenerative disorders.

Perspectives for computational modeling of cell replacement for neurological disorders

Mathematical modeling of anatomically-constrained neural networks has provided significant insights regarding the response of networks to neurological disorders or injury. A logical extension of these models is to incorporate treatment regimens to investigate network responses to intervention. The addition of nascent neurons from stem cell precursors into damaged or diseased tissue has been used as a successful therapeutic tool in recent decades. Interestingly, models have been developed to examine the incorporation of new neurons into intact adult structures, particularly the dentate granule neurons of the hippocampus. These studies suggest that the unique properties of maturing neurons, can impact circuit behavior in unanticipated ways. In this perspective, we review the current status of models used to examine damaged CNS structures with particular focus on cortical damage due to stroke. Secondly, we suggest that computational modeling of cell replacement therapies can be made feasible by implementing approaches taken by current models of adult neurogenesis. The development of these models is critical for generating hypotheses regarding transplant therapies and improving outcomes by tailoring transplants to desired effects.

The pharmacology of regenerative medicine

Regenerative medicine is a rapidly evolving multidisciplinary, translational research enterprise whose explicit purpose is to advance technologies for the repair and replacement of damaged cells, tissues, and organs. Scientific progress in the field has been steady and expectations for its robust clinical application continue to rise. The major thesis of this review is that the pharmacological sciences will contribute critically to the accelerated translational progress and clinical utility of regenerative medicine technologies. In 2007, we coined the phrase "regenerative pharmacology" to describe the enormous possibilities that could occur at the interface between pharmacology, regenerative medicine, and tissue engineering. The operational definition of regenerative pharmacology is "the application of pharmacological sciences to accelerate, optimize, and characterize (either in vitro or in vivo) the development, maturation, and function of bioengineered and regenerating tissues." As such, regenerative pharmacology seeks to cure disease through restoration of tissue/organ function. This strategy is distinct from standard pharmacotherapy, which is often limited to the amelioration of symptoms. Our goal here is to get pharmacologists more involved in this field of research by exposing them to the tools, opportunities, challenges, and interdisciplinary expertise that will be required to ensure awareness and galvanize involvement. To this end, we illustrate ways in which the pharmacological sciences can drive future innovations in regenerative medicine and tissue engineering and thus help to revolutionize the discovery of curative therapeutics. Hopefully, the broad foundational knowledge provided herein will spark sustained conversations among experts in diverse fields of scientific research to the benefit of all.

Cell-based therapy for ischemic stroke

INTRODUCTION

Stroke is a major cause of mortality and disability in adults worldwide. Unfortunately, current therapy which targets vessel recanalization has a narrow treatment window, and at this time neuroprotective approaches are not effective for stroke treatment. However, after stroke the parenchymal and endothelial cells in the central nervous system (CNS) respond in concert to ischemic stressors and create a microenvironment in which successful recovery may ensue. Neurogenesis, synaptogenesis, axonal sprouting, glial cell activation, angiogenesis and vascular remodeling within the brain and the spinal cord are stimulated post stroke. Cell based-therapy amplifies these endogenous restorative effects within the CNS to promote functional outcome.

AREAS COVERED

This article reviews current knowledge of cell-based therapy in the adult brain after stroke, including transplanted cell type, benefits and risks, with an emphasis on mechanisms of action.

EXPERT OPINION

Experimental studies and clinical trials with cell-based therapy in stroke appear promising. Cell-based therapy is not intended for the replacement of damaged cells, but for the remodeling of the CNS by promoting neuroplasticity, angiogenesis and immunomodulation. However, there are risks associated with the use of cell-based therapy, and adequate evaluation of these potential risks is a prerequisite before clinical application for stroke patients.

Human-induced pluripotent stem cells: in quest of clinical applications

In the field of regenerative medicine, the development of induced pluripotent stem (iPS) cells may represent a potential strategy to overcome the limitations of human embryonic stem cells (ESCs). iPS cells have the potential to mimic human disease, since they carry the genome of the donor. Hypothetically, with iPS cell technology it is possible to screen patients for a genetic cause of disease (genetic mutation), develop cell lines, reprogram them back to iPS cells, finally differentiate them into one or more cell types that develop the disease. Although the creation of multiple lineages with iPS cells can seem limitless, a number of challenges need to be addressed in order to effectively use these cell lines for disease modeling. These include the low efficiency of iPS cell generation without genetic alterations, the possibility of tumor formation in vivo, the random integration of retroviral-based delivery vectors into the genome, and unregulated growth of the remaining cells that are partially reprogrammed and refractory to differentiation. The establishment of protein or RNA-based reprogramming strategies will help generate human iPS cells without permanent genetic alterations. Finally, direct reprogramming strategies can provide rapid production of models of human "diseases in a dish", without first passing the cells through a pluripotent state, so avoiding the challenges of time-consumming and labor-intensive iPS cell line generation. This review will overview methods to develop iPS cells, current strategies for direct reprogramming, and main applications of iPS cells as human disease model, focusing on human cardiovascular diseases, with the aim to be a potential information resource for biomedical scientists and clinicians who exploit or intend to exploit iPS cell technology in a range of applications.

Proliferation and differentiation of neural stem cells co-cultured with cerebral microvascular endothelial cells after oxygen-glucose deprivation

Various stem cells, including neural stem cells (NSCs), have been extensively studied in stroke models, but how to increase neuronal differentiation rate of NSCs remains unresolved, particularly in a damaged environment. The purpose of this study was to investigate the effects of cerebral microvascular endothelial cells (CMECs) on the neurogenesis of NSCs with or without oxygen-glucose deprivation (OGD). The NSCs acquired from primary culture were immunostained to prove cell purity. Survival and proliferation of NSCs were determined after the co-culture with CMECs for 7 days. After removing the CMECs, NSCs were randomly divided into two groups as follows: OGD and non-OGD groups. Both groups were maintained in differentiation culture for 4 days to evaluate the differentiation rate. Mouse embryo fibroblast (MEF) cells co-cultured with NSCs served as control group. NSCs co-cultured with CMECs had an increase in size (on the 7th day: 89.80±26.12 μm vs. 73.08±15.01 μm, P<0.001) (n=12) and number [on the 7th day: 6.33±5.61/high power objective (HP) vs. 2.23±1.61/HP, P<0.001] (n=12) as compared with those co-cultured with MEF cells. After further differentiation culture for 4 days, NSCs co-cultured with CMECs had an increase in neuronal differentiation rate in OGD and non-OGD groups, but not in the control group (15.16% and 16.07% vs. 8.81%; both P<0.001) (n=6). This study provided evidence that OGD could not alter the effects of CMECs in promoting the neuronal differentiation potential of NSCs. These findings may have important implications for the development of new cell therapies for cerebral vascular diseases.

Neural progenitor cells derived from adult bone marrow mesenchymal stem cells promote neuronal regeneration

AIM

It is well known that neural stem/progenitor cells (NS/PC) are an ideal cell type for the treatment of central nervous system (CNS) disease. However, ethical problems have severely hampered fetal NS/PC from being widely used as a source for stem cell therapy. Recently, it has been demonstrated that autologous bone marrow mesenchymal stem cells (BMSC) can transdifferentiate into neural progenitor cells (NPC). The biological function of BMSC derived NPC (MDNPC) in neuronal systems remains unknown. In the present study, we aimed to investigate whether MDNPC can promote in vitro neural regeneration, a process comprising mainly the generation of neurons and neurotransmitters.

MAIN METHODS

We co-cultured BMSC, MDNPC or fetal NS/PC with PC12 cells and studied their roles on proliferation, neurite formation and dopamine release from PC12 cells. Furthermore, we also explored the mechanisms by which MDNPC regulate dopamine secretion from PC12 derived neural cells using Western blot.

KEY FINDINGS

We found that both MDNPC and NS/PC had similar morphologies and there were no significant differences between MDNPC and NS/PC in promoting PC12 cell proliferation, neurite outgrowth, and dopamine release. We also demonstrated that NS/PC induced dopamine secretion was associated with an upregulation of dopamine transporter (DAT) levels.

SIGNIFICANCE

In summary, MDNPC were comparable to NS/PC in promoting neural regeneration, indicating that MDNPC are a promising candidate source of neural stem cells for the treatment of neurological diseases.

Generation of regionally specified neural progenitors and functional neurons from human embryonic stem cells under defined conditions

To model human neural-cell-fate specification and to provide cells for regenerative therapies, we have developed a method to generate human neural progenitors and neurons from human embryonic stem cells, which recapitulates human fetal brain development. Through the addition of a small molecule that activates canonical WNT signaling, we induced rapid and efficient dose-dependent specification of regionally defined neural progenitors ranging from telencephalic forebrain to posterior hindbrain fates. Ten days after initiation of differentiation, the progenitors could be transplanted to the adult rat striatum, where they formed neuron-rich and tumor-free grafts with maintained regional specification. Cells patterned toward a ventral midbrain (VM) identity generated a high proportion of authentic dopaminergic neurons after transplantation. The dopamine neurons showed morphology, projection pattern, and protein expression identical to that of human fetal VM cells grafted in parallel. VM-patterned but not forebrain-patterned neurons released dopamine and reversed motor deficits in an animal model of Parkinson's disease.

Recent therapeutic strategies for spinal cord injury treatment: possible role of stem cells

Spinal cord injury (SCI) often results in significant dysfunction and disability. A series of treatments have been proposed to prevent and overcome the formation of the glial scar and inhibitory factors to axon regrowth. In the last decade, cell therapy has emerged as a new tool for several diseases of the nervous system. Stem cells act as minipumps providing trophic and immunomodulatory factors to enhance axonal growth, to modulate the environment, and to reduce neuroinflammation. This capability can be boosted by genetical manipulation to deliver trophic molecules. Different types of stem cells have been tested, according to their properties and the therapeutic aims. They differ from each other for origin, developmental stage, stage of differentiation, and fate lineage. Related to this, stem cells differentiating into neurons could be used for cell replacement, even though the feasibility that stem cells after transplantation in the adult lesioned spinal cord can differentiate into neurons, integrate within neural circuits, and emit axons reaching the muscle is quite remote. The timing of cell therapy has been variable, and may be summarized in the acute and chronic phases of disease, when stem cells interact with a completely different environment. Even though further experimental studies are needed to elucidate the mechanisms of action, the therapeutic, and the side effects of cell therapy, several clinical protocols have been tested or are under trial. Here, we report the state-of-the-art of cell therapy in SCI, in terms of feasibility, outcome, and side effects.

Presenilin-1 L166P mutant human pluripotent stem cell-derived neurons exhibit partial loss of γ-secretase activity in endogenous amyloid-β generation

Alzheimer's disease (AD) is the most frequent cause of dementia. There is compelling evidence that the proteolytic processing of the amyloid precursor protein (APP) and accumulation of amyloid-β (Aβ) peptides play critical roles in AD pathogenesis. Due to limited access to human neural tissue, pathogenetic studies have, so far, mostly focused on the heterologous overexpression of mutant human APP in non-human cells. In this study, we show that key steps in proteolytic APP processing are recapitulated in neurons generated from human embryonic and induced pluripotent stem cell-derived neural stem cells (NSC). These human NSC-derived neurons express the neuron-specific APP(695) splice variant, BACE1, and all members of the γ-secretase complex. The human NSC-derived neurons also exhibit a differentiation-dependent increase in Aβ secretion and respond to the pharmacotherapeutic modulation by anti-amyloidogenic compounds, such as γ-secretase inhibitors and nonsteroidal anti-inflammatory drugs. Being highly amenable to genetic modification, human NSCs enable the study of mechanisms caused by disease-associated mutations in human neurons. Interestingly, the AD-associated PS1(L166P) variant revealed a partial loss of γ-secretase function, resulting in the decreased production of endogenous Aβ40 and an increased Aβ42/40 ratio. The PS1(L166P) mutant is also resistant to γ-secretase modulation by nonsteroidal anti-inflammatory drugs. Pluripotent stem cell-derived neurons thus provide experimental access to key steps in AD pathogenesis and can be used to screen pharmaceutical compounds directly in a human neuronal system.

Cell-replacement therapy and neural repair in the retina

Visual impairment severely affects the quality of life of patients and their families and is also associated with a deep economic impact. The most common pathologies responsible for visual impairment and legally defined blindness in developed countries include age-related macular degeneration, glaucoma and diabetic retinopathy. These conditions share common pathophysiological features: dysfunction and loss of retinal neurons. To date, two main approaches are being taken to develop putative therapeutic strategies: neuroprotection and cell replacement. Cell replacement is a novel therapeutic approach to restore visual capabilities to the degenerated adult neural retina and represents an emerging field of regenerative neurotherapy. The discovery of a population of proliferative cells in the mammalian retina has raised the possibility of harnessing endogenous retinal stem cells to elicit retinal repair. Furthermore, the development of suitable protocols for the reprogramming of differentiated somatic cells to a pluripotent state further increases the therapeutic potential of stem-cell-based technologies for the treatment of major retinal diseases. Stem-cell transplantation in animal models has been most effectively used for the replacement of photoreceptors, although this therapeutic approach is also being used for inner retinal pathologies. In this review, we discuss recent advances in the development of cell-replacement approaches for the treatment of currently incurable degenerative retinal diseases.

Capture of neuroepithelial-like stem cells from pluripotent stem cells provides a versatile system for in vitro production of human neurons

Human embryonic stem cells (hESC) and induced pluripotent stem cells (iPSC) provide new prospects for studying human neurodevelopment and modeling neurological disease. In particular, iPSC-derived neural cells permit a direct comparison of disease-relevant molecular pathways in neurons and glia derived from patients and healthy individuals. A prerequisite for such comparative studies are robust protocols that efficiently yield standardized populations of neural cell types. Here we show that long-term self-renewing neuroepithelial-like stem cells (lt-NES cells) derived from 3 hESC and 6 iPSC lines in two independent laboratories exhibit consistent characteristics including i) continuous expandability in the presence of FGF2 and EGF; ii) stable neuronal and glial differentiation competence; iii) characteristic transcription factor profile; iv) hindbrain specification amenable to regional patterning; v) capacity to generate functionally mature human neurons. We further show that lt-NES cells are developmentally distinct from fetal tissue-derived radial glia-like stem cells. We propose that lt-NES cells provide an interesting tool for studying human neurodevelopment and may serve as a standard system to facilitate comparative analyses of hESC and hiPSC-derived neural cells from control and diseased genetic backgrounds.

In vivo tracking of human neural stem cells with 19F magnetic resonance imaging

Background

Magnetic resonance imaging (MRI) is a promising tool for monitoring stem cell-based therapy. Conventionally, cells loaded with ironoxide nanoparticles appear hypointense on MR images. However, the contrast generated by ironoxide labeled cells is neither specific due to ambiguous background nor quantitative. A strategy to overcome these drawbacks is ¹⁹F MRI of cells labeled with perfluorocarbons. We show here for the first time that human neural stem cells (NSCs), a promising candidate for clinical translation of stem cell-based therapy of the brain, can be labeled with ¹⁹F as well as detected and quantified in vitro and after brain implantation.

Methodology/Principal Findings

Human NSCs were labeled with perfluoropolyether (PFPE). Labeling efficacy was assessed with ¹⁹F MR spectroscopy, influence of the label on cell phenotypes studied by immunocytochemistry. For in vitro MRI, NSCs were suspended in gelatin at varying densities. For in vivo experiments, labeled NSCs were implanted into the striatum of mice. A decrease of cell viability was observed directly after incubation with PFPE, which re-normalized after 7 days in culture of the replated cells. No label-related changes in the numbers of Ki67, nestin, GFAP, or βIII-tubulin+ cells were detected, both in vitro and on histological sections. We found that 1,000 NSCs were needed to accumulate in one image voxel to generate significant signal-to-noise ratio in vitro. A detection limit of ∼10,000 cells was found in vivo. The location and density of human cells (hunu+) on histological sections correlated well with observations in the ¹⁹F MR images.

Conclusion/Significance

Our results show that NSCs can be efficiently labeled with ¹⁹F with little effects on viability or proliferation and differentiation capacity. We show for the first time that ¹⁹F MRI can be utilized for tracking human NSCs in brain implantation studies, which ultimately aim for restoring loss of function after acute and neurodegenerative disorders.

Microcarrier expansion of mouse embryonic stem cell-derived neural stem cells in stirred bioreactors

Neural stem cells (NSCs) are self-renewing multipotent cells, able to differentiate into the phenotypes present in the central nervous system. Applications of NSCs may include toxicology, fundamental research, or cell therapies. The culture of floating cell clusters, called "neurospheres," is widely used for the propagation of NSC populations in vitro but shows several limitations, which may be circumvented by expansion under adherent conditions. In particular, the derivation of distinct populations of NSCs from embryonic stem cells capable of long-term culture under adherent conditions without losing differentiation potential was recently described. However, the expansion of these cells in agitated bioreactors has not been addressed until now and was the aim of this study. Selected microcarriers were tested under dynamic conditions in spinner flasks. Superior performance was observed with polystyrene beads coated with a recombinant peptide containing the Arg-Gly-Asp (RGD) motif (Pronectin F). After optimization of the culture, a 35-fold increase in cell number was achieved after 6 days. High cellular viability and multipotency were maintained throughout the culture. The study presented here may be the basis for the development of larger scale bioprocesses for expansion of these and other populations of adherent NSCs, either from mouse or human origin.

The myelin mutants as models to study myelin repair in the leukodystrophies

The leukodystrophies are rare and serious genetic disorders of the central nervous system that primarily affect children who frequently die early in life or have significantly delayed motor and mental milestones that result in long-term disability. Although with some of these disorders, early intervention with bone marrow or cord blood transplantation has been proven useful, it has not yet been determined that such therapies promote myelin repair of the central nervous system. Research on experimental therapies aimed at myelin repair is aided by the ability to test cell replacement strategies in genetic models in which the mutations and neuropathology match the human disorder. Thus, models exist of Pelizaeus-Merzbacher disease and the lysosomal storage disorder, Krabbe disease, which reflect the clinical and pathological course of the human disorders. Collectively, animals with mutations in myelin genes are called the myelin mutants, and they include rodent models such as the shiverer mouse that have been extensively used to study myelination by exogenous cell transplantation. These studies have encompassed many permutations of the age of the recipient, type of transplanted cell, site of engraftment, and so forth, and they offer hope that the scaling up of myelin produced by transplanted cells will have clinical significance in treating patients. Here we review these models and discuss their relative importance and use in such translational approaches. We discuss how grafts are identified and functional outcomes are measured. Finally, we briefly discuss the cells that have been successfully transplanted, which may be used in future clinical trials.

Human embryonic stem cell-derived neurons establish region-specific, long-range projections in the adult brain

While the availability of pluripotent stem cells has opened new prospects for generating neural donor cells for nervous system repair, their capability to integrate with adult brain tissue in a structurally relevant way is still largely unresolved. We addressed the potential of human embryonic stem cell-derived long-term self-renewing neuroepithelial stem cells (lt-NES cells) to establish axonal projections after transplantation into the adult rodent brain. Transgenic and species-specific markers were used to trace the innervation pattern established by transplants in the hippocampus and motor cortex. In vitro, lt-NES cells formed a complex axonal network within several weeks after the initiation of differentiation and expressed a composition of surface receptors known to be instrumental in axonal growth and pathfinding. In vivo, these donor cells adopted projection patterns closely mimicking endogenous projections in two different regions of the adult rodent brain. Hippocampal grafts placed in the dentate gyrus projected to both the ipsilateral and contralateral pyramidal cell layers, while axons of donor neurons placed in the motor cortex extended via the external and internal capsule into the cervical spinal cord and via the corpus callosum into the contralateral cortex. Interestingly, acquisition of these region-specific projection profiles was not correlated with the adoption of a regional phenotype. Upon reaching their destination, human axons established ultrastructural correlates of synaptic connections with host neurons. Together, these data indicate that neurons derived from human pluripotent stem cells are endowed with a remarkable potential to establish orthotopic long-range projections in the adult mammalian brain.