Cell-based therapy for stroke

Current treatments for stroke, such as the use of thrombolytic agents, are often limited by a narrow therapeutic time window. However, the regeneration of the brain after damage is still active days even weeks after stroke occurs, which might provide a second window for treatment. Cell-based therapy can be categorized into two strategies. One is transplantation of exogenous cells into the injured brain to replace the lost cells or support the remaining cells. The other strategy is to enhance the proliferation, differentiation, migration of endogenous stem or progenitor cells. Recent development in adult stem cell research and advancement in the induction of pluripotent stem cells from somatic adult cells provide a tremendous opportunity for transplantation therapy. Understanding the mechanisms and regulations involved in the endogenous neurogenesis will also help develop novel therapeutic interventions to promote neurogenesis and functional recovery in stroke. This review describes up-to-date progresses in cell-based therapy for the treatment of stroke.     

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.     

干细胞疗法治疗缺血性卒中的研究进展

目前缺血性卒中所致神经损伤修复的传统治疗方法尚无突破,而以干细胞为基础的新型治疗策略正成为研究热点。大量动物实验和部分临床实验已经证实,无论是机体自身干、祖细胞的动员、募集,还是自体或异体干细胞移植,都显著改善了受损的神经功能。目前已证实局部植入、静脉注入、或全身动员的干细胞在基质细胞衍生因子-1/基质细胞衍生因子-1受体4（Stromal cell-derived Factor-1/CXC Chemokine Receptor-4,SDF-1/CXCR4）趋化分子和β2-整联蛋白等因子的作用下归巢至脑缺血区域;提供富含营养因子的微环境,保护坏死灶周围缺血半暗带组织;增强血管发生和血管生成;促进卒中后内源性干、祖细胞的迁移、存活和分化;并逐渐分化为神经细胞替代丢失的神经元。这几重因素可能共同参与了结构重建和功能修复。     

Investigational Therapies for Ischemic Stroke: Neuroprotection and Neurorecovery

Stroke is one of the leading causes of death and disability worldwide. Current treatment strategies for ischemic stroke primarily focus on reducing the size of ischemic damage and rescuing dying cells early after occurrence. To date, intravenous recombinant tissue plasminogen activator is the only United States Food and Drug Administration approved therapy for acute ischemic stroke, but its use is limited by a narrow therapeutic window. The pathophysiology of stroke is complex and it involves excitotoxicity mechanisms, inflammatory pathways, oxidative damage, ionic imbalances, apoptosis, angiogenesis, neuroprotection, and neurorestoration. Regeneration of the brain after damage is still active days and even weeks after a stroke occurs, which might provide a second window for treatment. A huge number of neuroprotective agents have been designed to interrupt the ischemic cascade, but therapeutic trials of these agents have yet to show consistent benefit, despite successful preceding animal studies. Several agents of great promise are currently in the middle to late stages of the clinical trial setting and may emerge in routine practice in the near future. In this review, we highlight select pharmacologic and cell-based therapies that are currently in the clinical trial stage for stroke.     

Neurogenesis and stroke

Stroke stimulates neurogenesis in select regions of the adult brain, and the newborn neurons that result can migrate to areas of ischemic injury, where they may have the capacity to enhance brain recovery. These observations suggest that stroke-induced neurogenesis may contribute to endogenous brain repair after stroke, and that the mechanisms that underlie neurogenesis may represent potential therapeutic targets. Alternatively, transplantation of exogenously derived neural cells might also be an approach to the treatment of stroke.     

Role of vascular endothelial growth factor and other growth factors in post-stroke recovery

Stroke is a major health problem world-wide and its burden has been rising in last few decades. Until now tissue plasminogen activator is only approved treatment for stroke. Angiogenesis plays a vital role for striatal neurogenesis after stroke. Administration of various growth factors in an early post ischemic phase, stimulate both angiogenesis and neurogenesis and lead to improved functional recovery after stroke. However vascular endothelial growth factors (VEGF) is the most potent angiogenic factor for neurovascularization and neurogenesis in ischemic injury can be modulated in different ways and thus can be used as therapy in stroke. In response to the ischemic injury VEGF is released by endothelial cells through natural mechanism and leads to angiogenesis and vascularization. This release can also be up regulated by exogenous administration of Mesenchymal stem cells, by various physical therapy regimes and electroacupuncture, which further potentiate the efficacy of VEGF as therapy in post stroke recovery. Recent published literature was searched using PubMed and Google for the article reporting on methods of up regulation of VEGF and therapeutic potential of growth factors in stroke.     

Bone marrow-derived mesenchymal stem cells for the treatment of ischemic stroke

Bone marrow-derived mesenchymal stem cells (MSCs) have great potential as therapeutic agents in stroke management, since they are easily accessible and can be rapidly expanded ex vivo for autologous transplantation. Increasing evidence suggests that bone marrow cells migrate throughout the brain and differentiate into neurons and glial cells. Both non-human and human MSCs have been used to treat stroke in murine models with satisfactory results. Several factors, such as transdifferentiation, induction of neurogenesis and angiogenesis, neuroprotection, and activation of endogenous neurorestorative processes, contribute to the benefits of MSCs in the ischemic brain. Many variables, including types of MSCs, cell dose, timing of treatment, route of cell delivery, and characteristics of stroke patients, influence the efficacy of MSC treatment of stroke. Although the first trials of autologous MSC therapy in stroke patients showed promising results, the optimal approach for different clinical settings has yet to be determined. The fundamental properties of MSCs and their potential short-term and long-term toxicities also need to be determined before moving forward to use of these cells in clinical practice.     

Forebrain neurogenesis after focal Ischemic and traumatic brain injury

Neural stem cells persist in the adult mammalian forebrain and are a potential source of neurons for repair after brain injury. The two main areas of persistent neurogenesis, the subventricular zone (SVZ)-olfactory bulb pathway and hippocampal dentate gyrus, are stimulated by brain insults such as stroke or trauma. Here we focus on the effects of focal cerebral ischemia on SVZ neural progenitor cells in experimental stroke, and the influence of mechanical injury on adult hippocampal neurogenesis in models of traumatic brain injury (TBI). Stroke potently stimulates forebrain SVZ cell proliferation and neurogenesis. SVZ neuroblasts are induced to migrate to the injured striatum, and to a lesser extent to the peri-infarct cortex. Controversy exists as to the types of neurons that are generated in the injured striatum, and whether adult-born neurons contribute to functional restoration remains uncertain. Advances in understanding the regulation of SVZ neurogenesis in general, and stroke-induced neurogenesis in particular, may lead to improved integration and survival of adult-born neurons at sites of injury. Dentate gyrus cell proliferation and neurogenesis similarly increase after experimental TBI. However, pre-existing neuroblasts in the dentate gyrus are vulnerable to traumatic insults, which appear to stimulate neural stem cells in the SGZ to proliferate and replace them, leading to increased numbers of new granule cells. Interventions that stimulate hippocampal neurogenesis appear to improve cognitive recovery after experimental TBI. Transgenic methods to conditionally label or ablate neural stem cells are beginning to further address critical questions regarding underlying mechanisms and functional significance of neurogenesis after stroke or TBI. Future therapies should be aimed at directing appropriate neuronal replacement after ischemic or traumatic injury while suppressing aberrant integration that may contribute to co-morbidities such as epilepsy or cognitive impairment.     

Investigational Therapies for Ischemic Stroke: Neuroprotection and Neurorecovery

Stroke is one of the leading causes of death and disability worldwide. Current treatment strategies for ischemic stroke primarily focus on reducing the size of ischemic damage and rescuing dying cells early after occurrence. To date, intravenous recombinant tissue plasminogen activator is the only United States Food and Drug Administration approved therapy for acute ischemic stroke, but its use is limited by a narrow therapeutic window. The pathophysiology of stroke is complex and it involves excitotoxicity mechanisms, inflammatory pathways, oxidative damage, ionic imbalances, apoptosis, angiogenesis, neuroprotection, and neurorestoration. Regeneration of the brain after damage is still active days and even weeks after a stroke occurs, which might provide a second window for treatment. A huge number of neuroprotective agents have been designed to interrupt the ischemic cascade, but therapeutic trials of these agents have yet to show consistent benefit, despite successful preceding animal studies. Several agents of great promise are currently in the middle to late stages of the clinical trial setting and may emerge in routine practice in the near future. In this review, we highlight select pharmacologic and cell-based therapies that are currently in the clinical trial stage for stroke.

Electronic supplementary material

The online version of this article (doi:10.1007/s13311-011-0040-6) contains supplementary material, which is available to authorized users.     

Adult stem cell therapy in stroke

PURPOSE OF REVIEW: Acute cerebral infarction causes irreversible locally restricted loss of the neuronal circuitry and supporting glial cells with consecutive functional deficits and disabilities. The currently available and effective therapy targets fast vessel recanalization accompanied by symptomatic measures. Research activities focusing on stem cells, which represent a promising source for organotypic cell replacement and functional recovery after stroke, have gained momentum in recent years, making regenerative cell-based therapies a much more feasible realistic approach. This review provides an update about preclinical and clinical cell-based studies in stroke focusing on stem cells derived from the adult central nervous and hematopoetic systems. RECENT FINDINGS: Endogenous neural stem cells, which have been shown to reside throughout life in the central nervous system, have the capacity to replace lost neurons in models for numerous disorders, including cerebral ischemia. Considering adult neural stem cell transplantation as a regenerative strategy after stroke, progress has been made in isolating human adult neural stem cells and demonstrating the feasibility of autologous neural stem cell transplantation. An increasing number of studies provide evidence that hematopoietic stem cells, either after stimulation of endogenous stem cell pools or after exogenous hematopoietic stem cell application (transplantation), improve functional outcome after ischemic brain lesions. Various underlying mechanisms such as transdifferentiation into neural lineages, neuroprotection through trophic support, and cell fusion have been deciphered. SUMMARY: Many preclinical studies employing adult stem cell-based strategies hold great promise. For endogenous approaches the correlate of cell replacement underlying functional improvement needs to be demonstrated. Transplantation approaches on the experimental level need further development before clinical application can be considered.     

Biotherapies in stroke

Stroke is the second leading cause of death worldwide and the most common cause of severe disability. Neuroprotection and repair mechanisms supporting endogenous brain plasticity are often insufficient to allow complete recovery. While numerous neuroprotective drugs trials have failed to demonstrate benefits for patients, they have provided interesting translational research lessons related to neurorestorative therapy mechanisms in stroke. Stroke damage is not limited to neurons but involve all brain cell type including the extracellular matrix in a “glio-neurovascular niche”. Targeting a range of host brain cells, biotherapies such as growth factors and therapeutic cells, currently hold great promise as a regenerative medical strategy for stroke. These techniques can promote both neuroprotection and delayed neural repair through neuro-synaptogenesis, angiogenesis, oligodendrogliogenesis, axonal sprouting and immunomodulatory effects. Their complex mechanisms of action are interdependent and vary according to the particular growth factor or grafted cell type. For example, while “peripheral” stem or stromal cells can provide paracrine trophic support, neural stem/progenitor cells (NSC) or mature neurons can act as more direct neural replacements. With a wide therapeutic time window after stroke, biotherapies could be used to treat many patients. However, guidelines for selecting the optimal time window, and the best delivery routes and doses are still debated and the answers may depend on the chosen product and its expected mechanism including early neuroprotection, delayed neural repair, trophic systemic transient effects or graft survival and integration. Currently, the great variety of growth factors, cell sources and cell therapy products form a therapeutic arsenal that is available for stroke treatment. Their effective clinical use will require prior careful considerations regarding safety (e.g. tumorgenicity, immunogenicity), potential efficacy, cell characterization, delivery route and in vivo biodistribution. Bone marrow-derived cell populations such as mesenchymal stromal/stem cells (MSC) or mononuclear cells (MNC), umbilical cord stem cells and NSC are most investigated notably in clinical trials. Finally, we discuss perspectives concerning potential novel biotherapies such as combinatorial approaches (growth factor combined with cell therapy, in vitro optimization of cell products, or co-transplantation) and the development of biomaterials, which could be used as injectable hydrogel scaffold matrices that could protect a cell graft or selectively deliver drugs and growth factors into the post-stroke cavity at chronic stages. Considering the remaining questions about the best procedure and the safety cautions, we can hope that future translational research about biotherapies will bring more efficient treatments that will decrease post-stroke disability for many patients.     

Human neural stem cells promote proliferation of endogenous neural stem cells and enhance angiogenesis in ischemic rat brain

Transplantation of human neural stem cells into the dentate gyrus or ventricle of rodents has been reportedly to enhance neurogenesis. In this study, we examined endogenous stem cell proliferation and angiogenesis in the ischemic rat brain after the transplantation of human neural stem cells. Focal cerebral ischemia in the rat brain was induced by middle cerebral artery occlusion. Human neural stem cells were transplanted into the subventricular zone. The behavioral performance of human neural stem cells-treated ischemic rats was significantly improved and cerebral infarct volumes were reduced compared to those in untreated animals. Numerous transplanted human neural stem cells were alive and preferentially localized to the ipsilateral ischemic hemisphere. Furthermore, 5-bromo-2′-deoxyuridine-labeled endogenous neural stem cells were observed in the subventricular zone and hippocampus, where they differentiated into cells immunoreactive for the neural markers doublecortin, neuronal nuclear antigen Neu N, and astrocyte marker glial fibrillary acidic protein in human neural stem cells-treated rats, but not in the untreated ischemic animals. The number of 5-bromo-2′-deoxyuridine-positive ? anti-von Willebrand factor-positive proliferating endothelial cells was higher in the ischemic boundary zone of human neural stem cells-treated rats than in controls. Finally, transplantation of human neural stem cells in the brains of rats with focal cerebral ischemia promoted the proliferation of endogenous neural stem cells and their differentiation into mature neural-like cells, and enhanced angiogenesis. This study provides valuable insights into the effect of human neural stem cell transplantation on focal cerebral ischemia, which can be applied to the development of an effective therapy for stroke.     

Activated neural stem cells contribute to stroke-induced neurogenesis and neuroblast migration toward the infarct boundary in adult rats.

Stroke increases neurogenesis. The authors investigated whether neural stem cells or progenitor cells in the adult subventricular zone (SVZ) of rats contribute to stroke-induced increase in neurogenesis. After induction of stroke in rats, the numbers of cells immunoreactive to doublecortin, a marker for immature neurons, increased in the ipsilateral SVZ and striatum. Infusion of an antimitotic agent (cytosine-beta-D-arabiofuranoside, Ara-C) onto the ipsilateral cortex eliminated more than 98% of actively proliferating cells in the SVZ and doublecortin-positive cells in the ipsilateral striatum. However, doublecortin-positive cells rapidly replenished after antimitotic agent depletion of actively proliferating cells. Depleting the numbers of actively proliferating cells in vivo had no effect on the numbers of neurospheres formed in vitro, yet the numbers of neurospheres derived from stroke rats significantly (P<0.05) increased. Neurospheres derived from stroke rats self-renewed and differentiated into neurons and glia. In addition, doublecortin-positive cells generated in the SVZ migrated in a chainlike structure toward ischemic striatum. These findings indicate that in the adult stroke brain, increases in recruitment of neural stem cells contribute to stroke-induced neurogenesis, and that newly generated neurons migrate from the SVZ to the ischemic striatum.     

Cell-based therapy in ischemic stroke

Cell-based therapy for stroke represents a third wave of therapeutics for stroke and one focused on restorative processes with a longer time window of opportunity than neuroprotective therapies. An early time window, within the first week after stroke, is an opportunity for intravenously delivered bone marrow and perinatally derived cells that can home to areas of tissue injury and target brain remodeling. Allogeneic cells will likely be the most scalable and commercially viable product. Later time windows, months after stroke, may be opportunities for intracerebral transplantation of neuronally differentiated cell types. An integrated approach of cell-based therapy with early-phase clinical trials and continued preclinical work with focus on mechanisms of action is needed.     

Therapeutic application of cell transplantation and increased neurogenesis in cerebral infarction]

Cerebral ischemia often results in neuronal loss, leading to the neurological deficits in stroke patients. To obtain the functional recovery after stroke, cell transplantation and enhancement of endogenous neurogenesis may have potential application. Recent evidence has demonstrated that neural stem cells exist in the adult mammalian brain. After cerebral ischemia, newly born neurons were found not only in hippocampal dentate and olfactory bulb but also in hippocampal CA1 and striatum, where neurons were lost after ischemia. Administration of neurotrophic factors or genes encoding them into the lateral venticule could enhance endogenous neurogenesis in experimental ischemia model. Furthermore, we have recently developed non-invasive gene transfer into macrophages infiltrating an infarct to stimulate proliferation of neural stem cells in cerebral infarction. Several strategies including gene therapy and pharmacological approach will be tried in stroke patients in near future. However, it remains unclear whether the number of new-born neurons from endogenous neural stem cells is sufficient for replacement of damaged neurons. Cell transplantation will have the advantage of preparing the large amount of transplanted cells. Human neural stem cells, embryonic stem cells and bone marrow-derived cells will be donor cells in stroke patients. Surprisingly, neuron-like cells derived from human teratoma cell line were already applied in stroke patients. However, ethical aspect will have to be discussed carefully before cells from other individuals are used as donor cells in stroke patients.     

Evidence for high translational potential of mesenchymal stromal cell therapy to improve recovery from ischemic stroke

Although ischemic stroke is a major cause of morbidity and mortality, current therapies benefit only a small proportion of patients. Transplantation of mesenchymal stromal cells (MSC, also known as mesenchymal stem cells or multipotent stromal cells) has attracted attention as a regenerative therapy for numerous diseases, including stroke. Mesenchymal stromal cells may aid in reducing the long-term impact of stroke via multiple mechanisms that include induction of angiogenesis, promotion of neurogenesis, prevention of apoptosis, and immunomodulation. In this review, we discuss the clinical rationale of MSC for stroke therapy in the context of their emerging utility in other diseases, and their recent clinical approval for treatment of graft-versus-host disease. An analysis of preclinical studies examining the effects of MSC therapy after ischemic stroke indicates near-universal agreement that MSC have significant favorable effect on stroke recovery, across a range of doses and treatment time windows. These results are interpreted in the context of completed and ongoing human clinical trials, which provide support for MSC as a safe and potentially efficacious therapy for stroke recovery in humans. Finally, we consider principles of brain repair and manufacturing considerations that will be useful for effective translation of MSC from the bench to the bedside for stroke recovery.     

The potential of neural stem cells to repair stroke-induced brain damage

Acute injuries to CNS such as stroke induce neural progenitor proliferation in adult brain which might be an endogenous attempt to self-repair. This process is known to be altered by several exogenous and endogenous modulators including growth factors that could help to reinforce the post-stroke neurogenesis. Increasing the neurogenesis may be a future therapeutic option to decrease the cognitive and behavioral deficits following stroke. In addition, transplantation of various types of stem cells into the injured brain is currently thought to be an exciting option to replace the neurons lost in the post-ischemic brain. These include immortalized stem cell lines, neural progenitors prepared from embryonic and adult animals and mesenchymal stem cells. Using exogenous stem cells in addition to modulating endogenous neurogenesis, we may be able to repair the injured brain after a devastating stroke. This article reviewed the current literature of these two issues.     

Using endogenous neural stem cells to enhance recovery from ischemic brain injury

The use of cell-based therapy may be a valid therapeutic approach to ischemic brain injury. Stem cells have been proposed as a new form of cell based therapy in a variety of disorders, including acute and degenerative brain diseases. Up to date most efforts have concentrated on transplantation of embryonic stem cells (ESC) or neural stem cells (NSCs) obtained from immortalized cell lines into the diseased brain. These procedures require harvesting the appropriate stem cell, expansion in vitro and transplantation. Endogenous NSCs have been identified in the central nervous system where they reside largely in the subventricular zone and in the subgranular zone of the hippocampus. Endogenous NSCs may be capable of self-renewal and differentiation into functional glia and neurons. Manipulation of endogenous NSCs may bypass the need to use ESC as a form of therapy thus avoiding the complex ethical and biological issues involved with ES cells or immortalized cell lines. This review summarizes the evidence recently gathered in support of a therapeutic role for endogenous NSCs in acute experimental stroke.     

Regeneration of the ischemic brain by engineered stem cells: Fuelling endogenous repair processes

After ischemic brain injury various cell types including neurons, glia and endothelial cells are damaged and lose their function. Effective regeneration of brain tissue requires that all these cell types have to be replenished and combined to form a new functional network. Recent advances in regenerative medicine show the ability of stem cells to differentiate into various cell lineages. Several types of stem cells have been used to treat ischemic brain injury in rodent models including neuronal stem cells, mesenchymal stem cells and hematopoietic stem cells. Although these studies show promising results, it remains to be determined whether the beneficial effect of cell-based therapies in ischemic brain injury results from direct replacement of damaged cells by the transplanted cells. On the basis of the current literature we propose that neuroprotection by activation of anti-apoptotic mechanisms as well as improvement of the trophic milieu necessary for endogenous repair processes may be more important mechanisms underlying the improved functional outcome after stem cell treatment. Transplantation of native unmodified stem cells as such may not be sufficient to boost repair mechanisms provided by the endogenous stem cell population. An important aim of this review is to discuss the literature on the possible enhancement of regenerative function by combining stem cell transplantation with gene transduction into stem cells to enhance their regenerative and neuroprotective therapeutic potential. Finally, we briefly discuss the possibility of translation of this therapy to the clinic.