Transplantation of human neural precursor cells in Matrigel scaffolding improves outcome from focal cerebral ischemia after delayed postischemic treatment in rats

Neuro-immune interactions of neural stem cell transplants: from animal disease models to human trials

Stem cell technology is a promising branch of regenerative medicine that is aimed at developing new approaches for the treatment of severely debilitating human diseases, including those affecting the central nervous system (CNS). Despite the increasing understanding of the mechanisms governing their biology, the application of stem cell therapeutics remains challenging. The initial idea that stem cell transplants work in vivo via the replacement of endogenous cells lost or damaged owing to disease has been challenged by accumulating evidence of their therapeutic plasticity. This new concept covers the remarkable immune regulatory and tissue trophic effects that transplanted stem cells exert at the level of the neural microenvironment to promote tissue healing via combination of immune modulatory and tissue protective actions, while retaining predominantly undifferentiated features. Among a number of promising candidate stem cell sources, neural stem/precursor cells (NPCs) are under extensive investigation with regard to their therapeutic plasticity after transplantation. The significant impact in vivo of experimental NPC therapies in animal models of inflammatory CNS diseases has raised great expectations that these stem cells, or the manipulation of the mechanisms behind their therapeutic impact, could soon be translated to human studies. This review aims to provide an update on the most recent evidence of therapeutically-relevant neuro-immune interactions following NPC transplants in animal models of multiple sclerosis, cerebral stroke and traumas of the spinal cord, and consideration of the forthcoming challenges related to the early translation of some of these exciting experimental outcomes into clinical medicines.

Increasing doublecortin expression promotes migration of human embryonic stem cell-derived neurons

Human embryonic stem cell-derived neuronal progenitors (hNPs) provide a potential source for cellular replacement following neurodegenerative diseases. One of the greatest challenges for future neuron replacement therapies will be to control extensive cell proliferation and stimulate cell migration of transplanted cells. The doublecortin (DCX) gene encodes the protein DCX, a microtubule-associated protein essential for the migration of neurons in the human brain. In this study, we tested whether increasing the expression of DCX in hNPs would favorably alter their proliferation and migration. Migration and proliferation of hNPs was compared between hNPs expressing a bicistronic DCX/IRES-GFP transgene and those expressing a green fluorescent protein (GFP) transgene introduced by piggyBac-mediated transposition. The DCX-transfected hNPs showed a significant decrease in their proliferation and migrated significantly further on two different substrates, Matrigel and brain slices. Additionally, a dense network of nestin-positive (+) and vimentin+ fibers were found to extend from neurospheres transplanted onto brain slices, and this fiber growth was increased from neurospheres containing DCX-transfected hNPs. In summary, our results show that increased DCX expression inhibits proliferation and promotes migration of hNPs.

Hyaluronan, neural stem cells and tissue reconstruction after acute ischemic stroke

Focal stroke is a disabling disease with lifelong sensory, motor and cognitive impairments. Given the paucity of effective clinical treatments, basic scientists are developing novel options for protection of the affected brain and regeneration of lost tissue. Tissue bioengineering and stem/progenitor cell treatments have both been individually pursued for stroke neural repair therapies, with some benefit in tissue recovery. Emerging directions in stroke neural repair approaches combine these two therapies to use biopolymers with stem/progenitor transplants to promote greater cell survival in the transplant and directed delivery of bioactive molecules to the transplanted cells and the adjacent injured tissue. In this review the background literature on a combined use of neural stem/progenitor cells encapsulated in hyaluronan gels is discussed and the way this therapeutic approach can affect the important processes involved in brain tissue reconstruction, such as angiogenesis, axon regeneration, neural differentiation and inflammation is clarified. The glycosaminoglycan hyaluronan can optimize those processes and be employed in a successful neural tissue engineering approach.

Pifithrin-α enhances the survival of transplanted neural stem cells in stroke rats by inhibiting p53 nuclear translocation

AIMS

To examine a novel strategy to enhance the survival of grafted neural stem cells (NSCs) in stroke model.

METHODS

Using a cell counting kit-8 (CCK-8) and TUNEL assay to test the protective effects of p53 inhibitor, pifithrin-α (PFT-α), on oxygen glucose deprivation (OGD) in NSCs. We compared the effects of vehicle + NSCs and FFT-α + NSCs on the efficacy of transplantation in stroke rat model using behavioral analysis, immunohistochemistry, etc.

RESULTS

Pifithrin-α increased viability and decreased apoptosis in NSCs after OGD in vitro. By in vivo studies, we showed that the best recovery of neurological function in the stroke rats and the maximum survival of grafted NSCs were found in the PFT-α + NSCs group. Twelve hours after cell transplantation, p53 was localized to the nuclei of grafted NSCs in the vehicle + NSCs group but was primarily localized to the cytoplasm in the PFT-α + NSCs group. The p53-upregulated modulator of apoptosis (PUMA) was highly expressed among the grafted cells in the vehicle + NSCs group compared with that in the PFT-α + NSCs group.

CONCLUSION

Our results indicate that PFT-α enhances the survival of grafted NSCs through the inhibition of p53 translocation into the nucleus.

[Cell therapy for stroke: from myth to reality]

INTRODUCTION

Stroke is one of the leading causes of death and disability worldwide. Intravenous recombinant tissue plasminogen activator is the only available therapy for acute ischemic stroke, but its use is limited by a narrow therapeutic window and cannot stimulate endogenous repair and regeneration of damaged brain tissue. Stem cell-based approaches hold much promise as potential novel treatments to restore neurological function after stroke.

STATE OF THE ART

In this review, we summarize data from preclinical and clinical studies to investigate the potential application of stem cell therapies for treatment of stroke. Stem cells have been proposed as a potential source of new cells to replace those lost due to central nervous system injury, as well as a source of trophic molecules to minimize damage and promote recovery. Various stem cells from multiple sources can generate neural cells that survive and form synaptic connections after transplantation in the stroke-injured brain. Stem cells also exhibit neurorevitalizing properties that may ameliorate neurological deficits through stimulation of neurogenesis, angiogenesis and inhibition of inflammation.

PERSPECTIVES/CONCLUSION

Performed in stroke, cell therapy would decrease brain damage and reduce functional deficits. After the damage has been done, it would still improve neurological functions by activating endogenous repair. Nevertheless, many questions raised by experimental studies particularly related to long-term safety and technical details of cell preparation and administration must be resolved before wider clinical use.

Building biocompatible hydrogels for tissue engineering of the brain and spinal cord

Tissue engineering strategies employing biomaterials have made great progress in the last few decades. However, the tissues of the brain and spinal cord pose unique challenges due to a separate immune system and their nature as soft tissue. Because of this, neural tissue engineering for the brain and spinal cord may require re-establishing biocompatibility and functionality of biomaterials that have previously been successful for tissue engineering in the body. The goal of this review is to briefly describe the distinctive properties of the central nervous system, specifically the neuroimmune response, and to describe the factors which contribute to building polymer hydrogels compatible with this tissue. These factors include polymer chemistry, polymerization and degradation, and the physical and mechanical properties of the hydrogel. By understanding the necessities in making hydrogels biocompatible with tissue of the brain and spinal cord, tissue engineers can then functionalize these materials for repairing and replacing tissue in the central nervous system.

SSEA4-positive pig induced pluripotent stem cells are primed for differentiation into neural cells

Neural cells derived from induced pluripotent stem cells (iPSCs) have the potential for autologous cell therapies in treating patients with severe neurological disorders or injury. However, further study of efficacy and safety are needed in large animal preclinical models that have similar neural anatomy and physiology to humans such as the pig. The pig model for pluripotent stem cell therapy has been made possible for the first time with the development of pig iPSCs (piPSCs) capable of in vitro and in vivo differentiation into tissues of all three germ layers. Still, the question remains if piPSCs are capable of undergoing robust neural differentiation using a system similar to those being used with human iPSCs. In this study, we generated a new line of piPSCs from fibroblast cells that expressed pluripotency markers and were capable of embryoid body differentiation into all three germ layers. piPSCs demonstrated robust neural differentiation forming βIII-TUB/MAP2+ neurons, GFAP+ astrocytes, and O4+ oligodendrocytes and demonstrated strong upregulation of neural cell genes representative of all three major neural lineages of the central nervous system. In the presence of motor neuron signaling factors, piPSC-derived neurons showed expression of transcription factors associated with motor neuron differentiation (HB9 and ISLET1). Our findings demonstrate that SSEA4 expression is required for piPSCs to differentiate into neurons, astrocytes, and oligodendrocytes and furthermore develop specific neuronal subtypes. This indicates that the pigs can fill the need for a powerful model to study autologous neural iPSC therapies in a system similar to humans.

Manipulating neural-stem-cell mobilization and migration in vitro

Neural stem-cell transplantation is a promising strategy for the treatment of neural diseases and injuries, since the central nervous system (CNS) has a very limited capacity to repopulate the lost cells. Transplantation strategies face many difficulties including low viability, lack of control of stem-cell fate, and low levels of cell engraftment after transplantation. An alternative strategy for CNS repair without transplantation is using endogenous neural stem cells (NSCs) and precursor cells. Hepatocyte growth factor (HGF), a pleiotropic cytokine of mesenchymal origin, exerts a strong chemoattractive effect on stem cells. Leukemia inhibitory factor (LIF), a key regulator for stem-cell proliferation, mobilization, and fate choices, is currently being characterized for endogenous NSC manipulation for brain regeneration. In this study, HGF and LIF have been loaded into hydrogels and degradable nanoparticles, respectively, for sustained, long-term, localized delivery. We examine the use of HGF-loaded hydrogels and LIF-loaded nanoparticles for manipulating migration and mobilization of human NSCs in vitro. The combination of LIF-loaded nanoparticles and HGF-loaded hydrogels significantly mobilized hNSCs and promoted their migration in vitro. Studies are in progress to evaluate endogenous NSC mobilization and migration in vivo with simultaneous, controlled delivery of LIF at the natural reservoir of endogenous NSCs and HGF at the injury or disease site for in situ tissue regeneration.

Guided migration of neural stem cells derived from human embryonic stem cells by an electric field

Small direct current (DC) electric fields (EFs) guide neurite growth and migration of rodent neural stem cells (NSCs). However, this could be species dependent. Therefore, it is critical to investigate how human NSCs (hNSCs) respond to EF before any possible clinical attempt. Aiming to characterize the EF-stimulated and guided migration of hNSCs, we derived hNSCs from a well-established human embryonic stem cell line H9. Small applied DC EFs, as low as 16 mV/mm, induced significant directional migration toward the cathode. Reversal of the field polarity reversed migration of hNSCs. The galvanotactic/electrotactic response was both time and voltage dependent. The migration directedness and distance to the cathode increased with the increase of field strength. (Rho-kinase) inhibitor Y27632 is used to enhance viability of stem cells and has previously been reported to inhibit EF-guided directional migration in induced pluripotent stem cells and neurons. However, its presence did not significantly affect the directionality of hNSC migration in an EF. Cytokine receptor [C-X-C chemokine receptor type 4 (CXCR4)] is important for chemotaxis of NSCs in the brain. The blockage of CXCR4 did not affect the electrotaxis of hNSCs. We conclude that hNSCs respond to a small EF by directional migration. Applied EFs could potentially be further exploited to guide hNSCs to injured sites in the central nervous system to improve the outcome of various diseases.

Intracerebral chondroitinase ABC and heparan sulfate proteoglycan glypican improve outcome from chronic stroke in rats

Physical and chemical constraints imposed by the periinfarct glial scar may contribute to the limited clinical improvement often observed after ischemic brain injury. To investigate the role of some of these mediators in outcome from cerebral ischemia, we treated rats with the growth-inhibitory chondroitin sulfate proteoglycan neurocan, the growth-stimulating heparan sulfate proteoglycan glypican, or the chondroitin sulfate proteoglycan-degrading enzyme chondroitinase ABC. Neurocan, glypican, or chondroitinase ABC was infused directly into the infarct cavity for 7 d, beginning 7 d after middle cerebral artery occlusion. Glypican and chondroitinase ABC reduced glial fibrillary acidic protein immunoreactivity and increased microtubule-associated protein-2 immunoreactivity in the periinfarct region, and glypican- and chondroitinase ABC-treated rats showed behavioral improvement compared with neurocan- or saline-treated rats. Glypican and chondroitinase ABC also increased neurite extension in cortical neuron cultures. Glypican increased fibroblast growth factor-2 expression and chondroitinase ABC increased brain-derived neurotrophic factor expression in these cultures, whereas no such effects were seen following neurocan treatment. Thus, treatment with glypican or enzymatic disruption of neurocan with chondroitinase ABC improves gross anatomical, histological, and functional outcome in the chronic phase of experimental stroke in rats. Changes in growth factor expression and neuritogenesis may help to mediate these effects.

Mitochondrial gene replacement in human pluripotent stem cell-derived neural progenitors

Human pluripotent stem cell-derived neural progenitor (hNP) cells are an excellent resource for understanding early neural development and neurodegenerative disorders. Given that many neurodegenerative disorders can be correlated with defects in the mitochondrial genome, optimal utilization of hNP cells requires an ability to manipulate and monitor changes in the mitochondria. Here, we describe a novel approach that uses recombinant human mitochondrial transcription factor A (rhTFAM) protein to transfect and express a pathogenic mitochondrial genome (mtDNA) carrying the G11778A mutation associated with Leber's hereditary optic neuropathy (LHON) disease, into dideoxycytidine (ddC)-treated hNPs. Treatment with ddC reduced endogenous mtDNA and gene expression, without loss of hNP phenotypic markers. Entry of G11778A mtDNA complexed with the rhTFAM was observed in mitochondria of ddC-hNPs. Expression of the pathogenic RNA was confirmed by restriction enzyme analysis of the SfaN1-digested cDNA. On the basis of the expression of neuron-specific class III beta-tubulin, neuronal differentiation occurred. Our results show for the first time that pathogenic mtDNA can be introduced and expressed into hNPs without loss of phenotype or neuronal differentiation potential. This mitochondrial gene replacement technology allows for creation of in vitro stem cell-based models useful for understanding neuronal development and treatment of neurodegenerative disorders.

Injectable hydrogels for central nervous system therapy

Diseases and injuries of the central nervous system (CNS) including those in the brain, spinal cord and retina are devastating because the CNS has limited intrinsic regenerative capacity and currently available therapies are unable to provide significant functional recovery. Several promising therapies have been identified with the goal of restoring at least some of this lost function and include neuroprotective agents to stop or slow cellular degeneration, neurotrophic factors to stimulate cellular growth, neutralizing molecules to overcome the inhibitory environment at the site of injury, and stem cell transplant strategies to replace lost tissue. The delivery of these therapies to the CNS is a challenge because the blood-brain barrier limits the diffusion of molecules into the brain by traditional oral or intravenous routes. Injectable hydrogels have the capacity to overcome the challenges associated with drug delivery to the CNS, by providing a minimally invasive, localized, void-filling platform for therapeutic use. Small molecule or protein drugs can be distributed throughout the hydrogel which then acts as a depot for their sustained release at the injury site. For cell delivery, the hydrogel can reduce cell aggregation and provide an adhesive matrix for improved cell survival and integration. Additionally, by choosing a biodegradable or bioresorbable hydrogel material, the system will eventually be eliminated from the body. This review discusses both natural and synthetic injectable hydrogel materials that have been used for drug or cell delivery to the CNS including hyaluronan, methylcellulose, chitosan, poly(N-isopropylacrylamide) and Matrigel.

Functional recovery of the murine brain ischemia model using human induced pluripotent stem cell-derived telencephalic progenitors

Induced pluripotent stem (iPS) cells possess the properties of self-renewal and pluripotency, similar to embryonic stem cells. They are a good candidate as a source of suitable cells for cell replacement therapy. In this study, we transplanted human iPS cell-derived neural progenitors into an ischemic mouse brain. Human iPS cells were differentiated into neuronal progenitors by serum-free culture of embryoid body-like aggregates (SFEBs). Focal cerebral ischemia was induced by occluding the middle cerebral artery using the intraluminal filament technique. Donor cells were transplanted into the ischemic lateral striatum 1 week after ischemia induction. Cells survived at the transplantation site, with migration of a proportion of cells along the external capsule and corpus callosum. Cells that were positive for the basal telencephalon marker, Nkx2.1, migrated into the basal part of the telencephalon. The pallial telencephalon marker, Emx1, was detected in cells that had migrated into the pallial part of the telencephalon. SFEBs differentiated into various types of neurons, and a retrograde tracer labeling study showed that differentiated cells integrated into host neural circuitry. Behavioral recovery was significantly enhanced in the transplanted group. Our results suggest that human iPS cell-derived neuronal progenitors survive and migrate in the ischemic brain, and contribute toward functional recovery via neural circuit reconstitution.

The effect of injectable gelatin-hydroxyphenylpropionic acid hydrogel matrices on the proliferation, migration, differentiation and oxidative stress resistance of adult neural stem cells

Transplanted or endogenous neural stem cells often lack appropriate matrix in cavitary lesions in the central nervous system. In this study, gelatin-hydroxyphenylpropionic acid (Gtn-HPA), which could be enzymatically crosslinked with independent tuning of crosslinking degree and gelation rate, was explored as an injectable hydrogel for adult neural stem cells (aNSCs). The storage modulus of Gtn-HPA could be tuned (449-1717 Pa) to approximate adult brain tissue. Gtn-HPA was cytocompatible with aNSCs (yielding high viability >93%) and promoted aNSC adhesion. Gtn-HPA demonstrated a crosslinking-based approach for preconditioning aNSCs and increased the resistance of aNSCs to oxidative stress, improving their viability from 8-15% to 84% when challenged with 500 μM H(2)O(2). In addition, Gtn-HPA was able to modulate proliferation and migration of aNSCs in relation to the crosslinking degree. Finally, Gtn-HPA exhibited bias for neuronal cells. In mixed differentiation conditions, Gtn-HPA increased the proportion of aNSCs expressing neuronal marker β-tubulin III to a greater extent than that for astrocytic marker glial fibrillary acidic protein, indicating an enhancement in differentiation towards neuronal lineage. Between neuronal and astrocytic differentiation conditions, Gtn-HPA also selected for higher survival in the former. Overall, Gtn-HPA hydrogels are promising injectable matrices for supporting and influencing aNSCs in ways that may be beneficial for brain tissue regeneration after injuries.

Advantages and challenges of alternative sources of adult-derived stem cells for brain repair in stroke

Considerable promise has been demonstrated by cell therapy for the treatment of stroke. Adult-derived stem cells avoid the ethical dilemmas of using embryonic and fetal stem cells and thus are the ideal type of cell to study. There are a number of different types of stem cells that could prove to be useful, but there are potential concerns associated with each one. This review summarizes the current knowledge on the use of the different possible adult-derived stem cell types including their benefits and challenges. While the optimal conditions are still to be determined, these cells may prove to be at the forefront of stem cell research and ultimately therapy for stroke and other disorders.

Engineering of adult human neural stem cells differentiation through surface micropatterning

Interaction between differentiating neural stem cells and the extracellular environment guides the establishment of cell polarity during nervous system development. Developing neurons read the physical properties of the local substrate in a contact-dependent manner and retrieve essential guidance cues. To restore damage brain area by tissue engineering, the biomaterial scaffold has to mimic this microenvironment to allow organized tissue regeneration. To establish the validity of using microgrooved surfaces in order to simultaneously provide to primary adult human neural stem cells a permissive growth environment and a guide for neurite outgrowth in a pre-established direction, we have studied the long-term culture of adult human neural stem cells from patient biopsies on microgrooved polymers. By exploiting polymer moulding techniques, we engineered non-cytotoxic deep microstructured surfaces of polydimethylsiloxane (PDMS) exhibiting microchannels of various widths. Our results demonstrate that precoated micropatterned PDMS surfaces can serve as effective neurite guidance surfaces for human neural stem cells. Immunocytochemistry analysis show that channel width can impact strongly development and differentiation. In particular we found an optimal microchannel width, that conciliates a high differentiation rate with a pronounced alignment of neurites along the edges of the microchannels. The impact of the microstructures on neurite orientation turned out to be strongly influenced by cell density, attesting that cell/surface interactions at the origin of the alignment effect, are in competition with cell/cell interactions tending to promote interconnected networks of cells. Considering all these effects, we have been able to design appropriate structures allowing to obtain neuron development and differentiation rate comparable to a plane unpatterned surface, with an efficient neurite guidance and a long-term cell viability.

Roles for HIF-1α in neural stem cell function and the regenerative response to stroke

Stroke represents a leading cause of long-term disability worldwide, with few therapeutic options available for improving behavioral recovery. Identification of endogenous neural stem and progenitor cells (NSPCs) that are capable of promoting reparative responses following brain injury and stroke make these cells attractive therapeutic targets for stimulating cell replacement and neuronal plasticity. Interest in the mechanisms that support NSPC survival and replenishment of damaged cells within the ischemic brain has led to elucidation of new roles for hypoxia-inducible factor-1α (HIF-1α) in NSPC function. HIF-1α is a well-studied mediator of adaptive cellular responses to hypoxia through direct transcriptional regulation of cellular metabolism and angiogenesis. Recent evidence also indicates novel roles for HIF-1α in stem cell differentiation through modulation of Notch and Wnt/β-catenin signaling pathways. In this review, we will explore the hypothesis that HIF-1α represents an important mediator of NSPC function under both non-pathological conditions and stroke; and plays a central role in the regulation of NSPC response to hypoxia, metabolism and maintenance of the vascular environment of the neural stem cell niche.

Neural stem cell-based therapy for ischemic stroke

Stem cell-based approaches for the treatment of stroke have been the subject of intensive research over the past decade. Based on accumulated experimental evidence, stem cell-based therapy is a very promising prospect for the development of a novel treatment to restore stroke-damaged brain and impaired neurological function. Studies performed on experimental animal models of stroke employed a variety of stem cell types from diverse sources and have demonstrated their ability to replace lost neurons and functionally integrate into the brain, modulate inflammation, and stimulate angiogenesis and neurogenesis from an endogenous stem cell pool, most likely through trophic actions. A few clinical trials in stroke patients using stem cell transplantation have been completed or are on-going but the results have not yet proven the effectiveness of the stem cell-based approaches. A joint effort of stroke researchers and clinicians is needed to further optimize treatment protocols using safe and reproducible stem cell sources tested in relevant animal models of stroke and showing substantial neurological recovery of stroke-impaired function.

Stem cell research in stroke: how far from the clinic?

Stem cell-based approaches hold much promise as potential novel treatments to restore function after stroke. Studies in animal models have shown that stem cell transplantation can improve function by replacing neurons or by trophic actions, modulation of inflammation, promotion of angiogenesis, remyelination and axonal plasticity, and neuroprotection. Endogenous neural stem cells are also potential therapeutic targets because they produce new neurons after stroke. Clinical trials are ongoing but there is currently no proven stem cell-based therapy for stroke. Preclinical studies and clinical research will be needed to optimize the therapeutic benefit and minimize the risks of stem cells in stroke.

Restoring neuronal function after stroke by cell replacement: anatomic and functional considerations

BACKGROUND AND PURPOSE

A major challenge to effective treatment after stroke is the restoration of neuronal function. In recent years, cell-based therapies for stroke have been explored in experimental animal models, and the results have suggested behavioral improvements. However, the anatomic targets of a cell-based stroke therapy and the relationship of cell grafts to post stroke reorganization are poorly understood, which results in difficulties defining strategies for neuronal substitution. Given that stroke causes a variety of secondary changes at locations beyond the infarct lesion, overcoming these difficulties is even more important.

SUMMARY OF REVIEW

We describe which brain structures and cell types are candidates for substitution and how new neuronal functionality could be implemented in a damaged brain by capitalizing on current concepts of post stroke plasticity.

Cell therapy for ischaemic stroke

While acute neuroprotection in acute stroke has proven difficult and ended in many failures, there is increasing interest in restorative therapies that target brain remodelling. Cell therapy (transplantation of cells) shows promise, with a growing body of pre-clinical evidence demonstrating improved functional outcomes in animal models; however, questions still remain concerning mechanisms of action. Clinical trials are already underway and will increase in the next few years; their appropriate design and execution along with continued pre-clinical work are necessary for the field to advance and satisfy a large unmet clinical need.

Intranasal delivery of stem cells to the brain

INTRODUCTION

Stem cell-based therapy has proved to be a promising treatment option for neurological disorders. However, there are difficulties in successfully administrating these stem cells. For example, the brain-blood barrier impedes the entrance of stem cells into the CNS after systemic administration. Direct transplantation or injection may result in brain injury, and these strategies are clinically less feasible. Intranasal administration is a non-invasive and effective alternative for the delivery of drugs, vector-encoded viruses or even phages to the CNS. Recent studies have in fact demonstrated that stem cells may enter the CNS after intranasal administration. These results suggest that intranasal delivery may provide an alternative strategy for stem cell-based therapy.

AREAS COVERED

This review summarizes current studies that have applied the intranasal delivery of stem cells into the brain. In addition, the distribution and fate of stem cells in the brain and the potential opportunities as well as challenges of intranasal stem cell delivery are also discussed.

EXPERT OPINION

Intranasal delivery of stem cells is a new method with great potential for the transplantation of stem cells into the brain, and it may provide an extraordinary approach to overcoming the existing barriers of stem cell delivery for the treatment of many neurological disorders. This potential benefit emphasizes the importance of future research into intranasal delivery of stem cells.

Stem cells and stroke: opportunities, challenges and strategies

INTRODUCTION

Stroke remains the leading cause of disability in the Western world. Despite decades of work, no clinically effective therapies exist to facilitate recovery from stroke. Stem cells may have the potential to minimize injury and promote recovery after stroke.

AREAS COVERED

Transplanted stem cells have been shown in animal models to migrate to the injured region, secrete neurotrophic compounds, promote revascularization, enhance plasticity and regulate the inflammatory response, thereby minimizing injury. Endogenous neural stem cells also have a remarkable propensity to respond to injury. Under select conditions, subventricular zone progenitors may be mobilized to replace lost neurons. In response to focal infarcts, neuroblasts play important trophic roles to minimize neural injury. Importantly, these endogenous repair mechanisms may be experimentally augmented, leading to robust improvements in function. Ongoing clinical studies are now assessing the safety and feasibility of cell-based therapies for stroke.

EXPERT OPINION

We outline the unique challenges and potential pitfalls in the clinical translation of stem cell research for stroke. We then detail what we believe to be the specific basic science and clinical strategies needed to overcome these challenges, fill remaining gaps in knowledge and facilitate development of clinically viable stem cell-based therapies for stroke.

Neuroplasticity in the context of motor rehabilitation after stroke

Approximately one-third of patients with stroke exhibit persistent disability after the initial cerebrovascular episode, with motor impairments accounting for most poststroke disability. Exercise and training have long been used to restore motor function after stroke. Better training strategies and therapies to enhance the effects of these rehabilitative protocols are currently being developed for poststroke disability. The advancement of our understanding of the neuroplastic changes associated with poststroke motor impairment and the innate mechanisms of repair is crucial to this endeavor. Pharmaceutical, biological and electrophysiological treatments that augment neuroplasticity are being explored to further extend the boundaries of poststroke rehabilitation. Potential motor rehabilitation therapies, such as stem cell therapy, exogenous tissue engineering and brain-computer interface technologies, could be integral in helping patients with stroke regain motor control. As the methods for providing motor rehabilitation change, the primary goals of poststroke rehabilitation will be driven by the activity and quality of life needs of individual patients. This Review aims to provide a focused overview of neuroplasticity associated with poststroke motor impairment, and the latest experimental interventions being developed to manipulate neuroplasticity to enhance motor rehabilitation.

Endogenously EGFP-Labeled Mouse Embryonic Stem Cells

Transplantation of embryonic stem cell (ESC)-derived precursors holds great promise for treating various disease conditions. Tracing of precursors derived from ESC after transplantation is important to determine their migration and fate. Chemical labeling, as well as transfection or viral-mediated transduction of tracer genes in ESC or in ESC-derived precursors, which are the methods that have been used in the generation of the vast majority of labeled ESCs, have serious drawbacks such as varying efficacy. To circumvent this problem we generated endogenously traceable mouse (m)ESC clones by direct derivation from blastocysts of transgenic mice expressing enhanced green fluorescent protein (EGFP) under control of the housekeeping β-actin promoter The only previous report of endogenously EGFP-labeled mESC derived directly from transgenic EGFP embryos is that of Ahn and colleagues (Ahn et al, 2008. Cytotherapy 10:759-769), who used embryos from a different transgenic line and used a significantly different protocol for derivation. Cells from a high-expressing EGFP-mESC clone, G11, retain high levels of EGFP expression after differentiation into derivatives of all three primary germ layers both in vitro and in vivo, and contribution to all tissues in chimeric progeny. To determine whether progenitor cells derived from G11 could be used in transplantation experiments, we differentiated them to early neuronal precursors and injected them into syngeneic mouse brains. Transplanted EGFP-expressing cells at different stages of differentiation along the neuronal lineage could be identified in brains by expression of EGFP twelve weeks after transplantation. Our results suggest that the EGFP-mESC(G11) line may constitute a useful tool in ESC-based cell and tissue replacement studies.

Effect of human neural precursor cell transplantation on endogenous neurogenesis after focal cerebral ischemia in the rat

Little is known about the relationship between neuronal cell transplantation and endogenous neurogenesis after experimental stroke. We found previously that transplantation of neuronal precursors derived from BG01 human embryonic stem cells reduced infarct volume and improved behavioral outcome after distal middle cerebral artery occlusion (MCAO) in rats. In this study, transplantation was performed 14 days after distal MCAO and doublecortin (Dcx)-expressing cells in the subventricular zone (SVZ) and subgranular zone of dentate gyrus (SGZ) were counted 60 days post-transplant. Transplantation increased neurogenesis (Dcx expression) in ipsilateral SVZ, but not in contralateral SVZ or either SGZ, in both young adult (3-month-old) and aged (24-month-old) rats. These findings suggest that cell-based therapy for stroke may be associated with changes in endogenous adaptive processes, including neurogenesis.

Delayed transplantation of human neural precursor cells improves outcome from focal cerebral ischemia in aged rats

Neural precursor cell (NPC) transplantation may have a role in restoring brain function after stroke, but how aging might affect the brain's receptivity to such transplants is unknown. We reported previously that transplantation of human embryonic stem cell (hESC)-derived NPCs together with biomaterial (Matrigel) scaffolding into the brains of young adult Sprague-Dawley rats 3 weeks after distal middle cerebral artery occlusion (MCAO) reduced infarct volume and improved neurobehavioral performance. In this study, we compared the effect of NPC and Matrigel transplants in young adult (3-month-old) and aged (24-month-old) Fisher 344 rats from the National Institute on Aging's aged rodent colony. Distal MCAO was induced by electrocoagulation, and hESC-derived NPCs were transplanted into the infarct cavity 3 weeks later. Aged rats developed larger infarcts, but infarct volume and performance on the cylinder and elevated body swing tests, measured 6-8 weeks post-transplant, were improved by transplantation. We conclude that advanced age does not preclude a beneficial response to NPC transplantation following experimental stroke.

Stem cell-derived neurons grafted in the striatum are expelled out of the brain after chronic cortical stroke

BACKGROUND AND PURPOSE

In humans and rodents, cortical stroke can lead to cortex atrophy in long-term survivors. In the rodent, fetal brain neural precursors or stem cell-derived neurons grafted in the stroke-lesioned brain integrate successfully and reduce infarct in the short term. We have examined the fate, in the long term, of mouse embryonic stem cell-derived neural precursors grafted after permanent middle cerebral artery occlusion in mice.

METHODS

Green fluorescent protein-labeled neural precursors were grafted in the striatum of control and lesioned mice and their fate examined 9 months later.

RESULTS

In control mice, the neuronal progeny of mouse embryonic stem cells innervated distant brain structures, in a way remarkably similar between animals, displayed a laterality preference and remained polysialated neural cell adhesion molecule-immunoreactive. In lesioned mice, grafted cells were expelled out of the brain.

CONCLUSIONS

Stroke-related brain atrophy and reshaping were not prevented by cell grafting and, eventually, led to the expulsion of the graft.

Hydrogel matrix to support stem cell survival after brain transplantation in stroke

Stroke is a leading cause of adult disability. Stem/progenitor cell transplantation improves recovery after stroke in rodent models. These studies have 2 main limitations to clinical translation. First, most of the cells in stem/progenitor transplants die after brain transplantation. Second, intraparenchymal approaches target transplants to normal brain adjacent to the stroke, which is the site of the most extensive natural recovery in humans. Transplantation may damage this tissue. The stroke cavity provides an ideal target for transplantation because it is a compartmentalized region of necrosis, can accept a high volume transplant without tissue damage, and lies directly adjacent to the most plastic brain area in stroke. However, direct transplantation into the stroke cavity has caused massive death in the transplant. To overcome these limitations, the authors tested stem/progenitor transplants within a specific biopolymer hydrogel matrix to create a favorable environment for transplantation into the infarct cavity after stroke, and they tested this in comparison to stem cell injection without hydrogel support. A biopolymer hydrogel composed of cross-linked hyaluronan and heparin sulfate significantly promoted the survival of 2 different neural progenitor cell lines in vitro in conditions of stress and in vivo into the infarct cavity. Quantitative analysis of the transplant and surrounding tissue indicates diminished inflammatory infiltration of the graft with the hydrogel transplant. This result indicates that altering the local environment in stem cell transplantation enhances survival and diminishes cell stress. Stem cell transplantation into the infarct cavity within a pro-survival hydrogel matrix may provide a translational therapy for stroke recovery.

Murine neural stem/progenitor cells protect neurons against ischemia by HIF-1alpha-regulated VEGF signaling

Focal cerebral ischemia following middle cerebral artery occlusion (MCAO) stimulates a robust cytogenic response from the adult subventricular zone (SVZ) that includes massive proliferation of neural stem/progenitor cells (NSPCs) and cellular migration into the injury area. To begin to explore beneficial roles of NSPCs in this response, we investigated the ability of embryonic and postnatal NSPCs to promote neuronal survival under conditions of in vivo and in vitro ischemia. Intracerebral transplantation of NSPCs attenuated neuronal apoptosis in response to focal ischemia induced by transient MCAO, and prevented neuronal cell death of cortical neurons in response to oxygen-glucose deprivation (OGD) in culture. NSPC-mediated neuroprotection was blocked by the pharmacological inhibitors of vascular endothelial growth factor (VEGF), SU1498 and Flt-1Fc. Embryonic and postnatal NSPCs were both intrinsically resistant to brief OGD exposure, and constitutively expressed both hypoxia-inducible factor 1α (HIF-1α) transcription factor and its downstream target, VEGF. Genomic deletion of HIF-1α by Cre-mediated excision of exon 2 in NSPC cultures resulted in >50% reduction of VEGF production and ablation of NSPC-mediated neuroprotection. These findings indicate that NSPCs promote neuronal survival under ischemic conditions via HIF-1α-VEGF signaling pathways and support a role for NSPCs in promotion of neuronal survival following stroke.

Intracarotid administration of human bone marrow mononuclear cells in rat photothrombotic ischemia

Background

Increasing evidence suggests that cell therapy improves functional recovery in experimental models of stroke and myocardial infarction. So far only small pilot trials tested the effects of cell therapy in stroke patients, whereas large clinical trials were conducted in patients with ischemic heart disease. To investigate the therapeutic benefit of cell therapy to improve the recovery after stroke, we determined the efficacy of bone marrow derived mononuclear cells, which were shown to improve the recovery in experimental and clinical acute myocardial infarction studies, in a rat stroke model.

Methods

Adult male Wistar rats were randomly assigned to receive either five million human bone marrow mononuclear cells (hBMC) or placebo intraarterially 3 days after photothrombotic ischemia. For immunosuppression the animals received daily injections of cyclosporine throughout the experiment, commencing 24 hours before the cell transplantation. A battery of behavioral tests was performed before and up to 4 weeks after ischemia.

Results

Body temperature and body weight revealed no difference between groups. Neurological deficits measured by the Rotarod test, the adhesive-removal test and the cylinder test were not improved by hBMC transplantation compared to placebo.

Conclusions

This study demonstrates that hBMC do not improve functional recovery when transplanted intraaterially 3 days after the onset of focal cerebral ischemia. A possible reason for the failed neurological improvement after cell therapy might be the delayed treatment initiation compared to other experimental stroke studies that showed efficacy of bone marrow mononuclear cells.