In vivo induction of massive proliferation, directed migration, and differentiation of neural cells in the adult mammalian brain

28. Embryonic and adult stem cell therapy

Stem cells are characterized by the ability to remain undifferentiated and to self-renew. Embryonic stem cells derived from blastocysts are pluripotent (able to differentiate into many cell types). Adult stem cells, which were traditionally thought to be monopotent multipotent, or tissue restricted, have recently also been shown to have pluripotent properties. Adult bone marrow stem cells have been shown to be capable of differentiating into skeletal muscle, brain microglia and astroglia, and hepatocytes. Stem cell lines derived from both embryonic stem and embryonic germ cells (from the embryonic gonadal ridge) are pluripotent and capable of self-renewal for long periods. Therefore embryonic stem and germ cells have been widely investigated for their potential to cure diseases by repairing or replacing damaged cells and tissues. Studies in animal models have shown that transplantation of fetal, embryonic stem, or embryonic germ cells may be able to treat some chronic diseases. In this review, we highlight recent developments in the use of stem cells as therapeutic agents for three such diseases: Diabetes, Parkinson disease, and congestive heart failure. We also discuss the potential use of stem cells as gene therapy delivery cells and the scientific and ethical issues that arise with the use of human stem cells.

Growth factors and stem cells as treatments for stroke recovery

Both polypeptide growth factors and stem cell populations from bone marrow and umbilical cord blood hold promise as treatments to enhance neurologic recovery after stroke. Growth factors may exert their effects through stimulation of neural sprouting and enhancement of endogenous progenitor cell proliferation, migration, and differentiation in brain. Exogenous stem cells may exert their effects by acting as miniature "factories" for trophic substances in the poststroke brain. The combination of growth factors and stem cells may be more effective than either treatment alone. Stroke recovery represents a new and relatively untested target for stroke therapeutics. Whereas acute stroke treatments focus on agents that dissolve blot clots (thrombolytics) and antagonize cell death (neuroprotective agents), stroke recovery treatments are likely to enhance structural and functional reorganization (plasticity) of the damaged brain. Successful clinical trials of stroke recovery-promoting agents are likely to be quite different from trials testing acute stroke therapies. In particular, the time window of effective treatment to enhance stroke recovery is likely to be far longer than that for acute stroke treatments, perhaps days or weeks rather than minutes or hours after stroke. This longer time window means that time is available for careful screening and testing of potential subjects for stroke recovery trials, both in terms of size and location of cerebral infarcts and in type and severity of neurologic deficits. Detailed baseline information can be obtained for each patient against which eventual clinical outcome can be compared. Finally, separate and detailed outcome measures can be obtained in both the sensorimotor and cognitive neurologic spheres, because it is possible that these two kinds of function may recover differently or be differentially responsive to recovery-promoting treatments. Stroke recovery represents an important and underexplored opportunity for the development of new stroke treatments.

Neural stem cells and neuro-oncology: quo vadis?

Conventionally, gliomas are assumed to arise via transformation of an intraparenchymal glial cell that forms a mass that then expands centrifugally, eventually invading surrounding tissues. We propose an alternative model in which gliomas arise via initiation and promotion of cells within the brain's subependymal layer or subventricular zone, the source of a recently characterized pool of neural cells with the properties of self-renewal and multipotentiality (i.e., stem cells) that persists into adulthood. In this model, the particular histological subtype of glioma would represent the effects of temporal and spatial environmental influences rather than the particular cell of origin and the disease's centrifugal point would be the subependymal layer. The implications of such a model are discussed.

EGF converts transit-amplifying neurogenic precursors in the adult brain into multipotent stem cells

Neural stem cells in the subventricular zone (SVZ) continue to generate new neurons in the adult brain. SVZ cells exposed to EGF in culture grow to form neurospheres that are multipotent and self-renewing. We show here that the majority of these EGF-responsive cells are not derived from relatively quiescent stem cells in vivo, but from the highly mitotic, Dlx2(+), transit-amplifying C cells. When exposed to EGF, C cells downregulate Dlx2, arrest neuronal production, and become highly proliferative and invasive. Killing Dlx2(+) cells dramatically reduces the in vivo response to EGF and neurosphere formation in vitro. Furthermore, purified C cells are 53-fold enriched for neurosphere generation. We conclude that transit-amplifying cells retain stem cell competence under the influence of growth factors.

Stem cells in the treatment of amyotrophic lateral sclerosis (ALS)

Until fairly recently, interest in stem cells was restricted to neurobiology studies on the principles of embryonic development. This situation has changed rapidly in the last few years when neuronal stems and precursors were isolated in vitro, thus allowing expansion and controlled differentiation of selective populations of neuronal cells. This theoretically unlimited reserve would then supply specific cells for transplantation in diseases characterized by widespread degeneration of selective cell populations as motor neurons in Amyotrophic Lateral Sclerosis (ALS). The recent evidence of cell transdifferentiation has further amplified the potential therapeutic use of stem cells. Stem cell technology is at an early stage but the desperate need for a therapy in ALS patients may legitimize clinical trials in absence of conclusive scientific evidence. This paper discusses the premises for stem cell therapy in ALS.

Rat forebrain neurogenesis and striatal neuron replacement after focal stroke

The persistence of neurogenesis in the forebrain subventricular zone (SVZ) of adult mammals suggests that the mature brain maintains the potential for neuronal replacement after injury. We examined whether focal ischemic injury in adult rat would increase SVZ neurogenesis and direct migration and neuronal differentiation of endogenous precursors in damaged regions. Focal stroke was induced in adult rats by 90-minute right middle cerebral artery occlusion (tMCAO). Cell proliferation and neurogenesis were assessed with bromodeoxyuridine (BrdU) labeling and immunostaining for cell type-specific markers. Brains examined 10-21 days after stroke showed markedly increased SVZ neurogenesis and chains of neuroblasts extending from the SVZ to the peri-infarct striatum. Many BrdU-labeled cells persisted in the striatum and cortex adjacent to infarcts, but at 35 days after tMCAO only BrdU-labeled cells in the neostriatum expressed neuronal markers. Newly generated cells in the injured neostriatum expressed markers of medium spiny neurons, which characterize most neostriatal neurons lost after tMCAO. These findings indicate that focal ischemic injury increases SVZ neurogenesis and directs neuroblast migration to sites of damage. Moreover, neuroblasts in the injured neostriatum appear to differentiate into a region-appropriate phenotype, which suggests that the mature brain is capable of replacing some neurons lost after ischemic injury.

Brain NGF and EGF administration improves passive avoidance response and stimulates brain precursor cells in aged male mice

Nerve growth factor (NGF) has been shown to improve damage in spatial cognition following aging, whereas epidermal growth factor (EGF) is important in brain cell proliferation. It is also known that the adult mammalian central nervous system contains persistent progenitor cells with characteristics of stem cells. These studies suggest that under appropriate external stimuli progenitor cells may generate neuronal and glial cells promoting recovery of the injured nervous system. However, little is known about the presence and responsiveness of progenitor cells in the aged brain. In the present investigation, we studied the effect of brain intracerebroventricular injections of EGF and/or NGF on progenitor cells of the subventricular area (SVZ) in aged male mice to test learning performances in the passive avoidance apparatus. We found that neither NGF nor EGF improved learning responses. However, combined NGF and EGF administration in the brain improved learning responses of aged mice in the passive avoidance when compared with aged matched nontreated controls. These findings resulted to be associated with increased immunopositivity to progenitor cells in the SVZ. The possible functional implications of these data are discussed.

Chronic, controlled GDNF infusion promotes structural and functional recovery in advanced parkinsonian monkeys

The powerful trophic effects that glial cell line-derived neurotrophic factor (GDNF) exerts on midbrain dopamine neurones suggest its use in treating Parkinson's disease. However, some important questions remain about the possible therapeutic applications of GDNF. Here we demonstrate that the chronic infusion of 5 or 15 micro g/day GDNF into the lateral ventricle or the striatum, using programmable pumps, promotes restoration of the nigrostriatal dopaminergic system and significantly improves motor functions in rhesus monkeys with neural deficits modelling the terminal stages of Parkinson's disease. The functional improvements were associated with pronounced upregulation and regeneration of nigral dopamine neurones and their processes innervating the striatum. When compared with vehicle recipients, these functional improvements were associated with (i) >30% bilateral increase in nigral dopamine neurone cell size; (ii) >20% bilateral increase in the number of nigral cells expressing the dopamine marker tyrosine hydroxylase; (iii) >70 and >50% bilateral increase in dopamine metabolite levels in the striatum and the pallidum, respectively; (iv) 233 and 155% increase in dopamine levels in the periventricular striatal region and the globus pallidus, respectively, on the lesioned side; and (v) a five-fold increase in tyrosine hydroxylase-positive fibre density in the periventricular striatal region on the lesioned side. In addition, chronic GDNF treatment did not induce the side-effects generally associated with chronic administration of levodopa, the most widely used treatment for Parkinson's disease. Thus, the results suggest that the prolonged and controlled delivery of GDNF into the brain could be used to intervene in long-term neurodegenerative disease processes like Parkinson's disease. Additional studies are required to determine the potential differences between chronic, intraventricular and intraputamenal (or intranigral) delivery of GDNF to maximize the efficacy of infusion treatments.

Experimental autoimmune encephalomyelitis mobilizes neural progenitors from the subventricular zone to undergo oligodendrogenesis in adult mice

The destiny of the mitotically active cells of the subventricular zone (SVZ) in adult rodents is to migrate to the olfactory bulb, where they contribute to the replacement of granular and periglomerular neurons. However, these adult neural progenitors also can be mobilized in periventricular white matter and triggered to differentiate into astrocytes and oligodendrocytes in response to lysolecithin-induced demyelination. To mimic the environmental conditions of multiple sclerosis, we assessed the proliferation, migration, and differentiation potential of adult SVZ progenitor cells in response to experimental autoimmune encephalomyelitis (EAE) in mice. Inflammation and demyelination were observed in all mouse brains after EAE induction. EAE induced cell proliferation throughout the brain and especially within the lesions. Proliferating cells were neural progenitors, astrocytes, and oligodendrocyte precursors. EAE enhanced the migration of SVZ-derived neural progenitors to the olfactory bulb and triggered their mobilization in the periventricular white matter. The mobilized cells gave rise to neurons, astrocytes, and oligodendrocytes in the olfactory bulb but essentially to astrocytes and oligodendrocytes in the lesioned white matter. Our data indicate that the adult mouse SVZ is a source of newly generated oligodendrocytes and thus may contribute, along with oligodendrocyte precursors, to the replacement of oligodendrocytes in inflammatory demyelinating diseases of the central nervous system such as multiple sclerosis.

Cellular replacement therapy for Parkinson's disease--where we are today?

The concept of replacing lost dopamine neurons in Parkinson's disease using mesencephalic brain cells from fetal cadavers has been supported by over 20 years of research in animals and over a decade of clinical studies. The ambitious goal of these studies was no less than a molecular and cellular "cure" for Parkinson's disease, other neurodegenerative diseases, and spinal cord injury. Much research has been done in rodents, and a few studies have been done in nonhuman primate models. Early uncontrolled clinical reports were enthusiastic, but the outcome of the first randomized, double blind, controlled study challenged the idea that dopamine replacement cells can cure Parkinson's disease, although there were some significant positive findings. Were the earlier animal studies and clinical reports wrong? Should we give up on the goal? Some aspects of the trial design and implantation methods may have led to lack of effects and to some side effects such as dyskinesias. But a detailed review of clinical neural transplants published to date still suggests that neural transplantation variably reverses some aspects of Parkinson's disease, although differing methods make exact comparisons difficult. While the randomized clinical studies have been in progress, new methods have shown promise for increasing transplant survival and distribution, reconstructing the circuits to provide dopamine to the appropriate targets and with normal regulation. Selected promising new strategies are reviewed that block apoptosis induced by tissue dissection, promote vascularization of grafts, reduce oxidant stress, provide key growth factors, and counteract adverse effects of increased age. New sources of replacement cells and stem cells may provide additional advantages for the future. Full recovery from parkinsonism appears not only to be possible, but a reliable cell replacement treatment may finally be near.

Growth factor treatment promotes mobilization of young but not aged adult subventricular zone precursors in response to demyelination

Precursor cells of the adult mouse subventricular zone (SVZ) are mobilized and recruited by a lysolecithin (LPC)-induced demyelination of the corpus callosum. Because age decreases the proliferation of the SVZ neural precursors as well as the potential for myelin repair of the adult central nervous system, we have compared the ability of young and aged adult neural precursors to respond to LPC-induced demyelination. With age, the SVZ cells lost their capacity to proliferate and to be recruited by the lesion. Whereas a single injection of fibroblast growth factor-2 or transforming growth factor-alpha stimulated the proliferation of SVZ and rostral migratory stream precursors in both groups of animals after demyelination, they favored recruitment at the lesion in young mice but not in aged ones. In vitro experiments using neurospheres derived from young and aged animals indicated that both populations have the same migratory performances. Our in vivo data thus suggest that aged neural precursors may loose their intrinsic capacities to respond to demyelination-induced signals. Alternatively, their function may be altered by modification of the aged extracellular environment.

Regeneration of hippocampal pyramidal neurons after ischemic brain injury by recruitment of endogenous neural progenitors

The adult brain is extremely vulnerable to various insults. The recent discovery of neural progenitors in adult mammals, however, raises the possibility of repairing damaged tissue by recruiting their latent regenerative potential. Here we show that activation of endogenous progenitors leads to massive regeneration of hippocampal pyramidal neurons after ischemic brain injury. Endogenous progenitors proliferate in response to ischemia and subsequently migrate into the hippocampus to regenerate new neurons. Intraventricular infusion of growth factors markedly augments these responses, thereby increasing the number of newborn neurons. Our studies suggest that regenerated neurons are integrated into the existing brain circuitry and contribute to ameliorating neurological deficits. These results expand the possibility of novel neuronal cell regeneration therapies for stroke and other neurological diseases.

Brain repair by endogenous progenitors

Stem cells within the adult brain can be stimulated by injury and growth factor treatment to replace damaged neurons, even neurons that are not normally generated in adults. Coupled with recent insights into the mechanism by which Nogo inhibits axonal regeneration, this discovery may inspire new treatments for central nervous system injuries and neurodegenerative diseases.

REN: a novel, developmentally regulated gene that promotes neural cell differentiation

Expansion and fate choice of pluripotent stem cells along the neuroectodermal lineage is regulated by a number of signals, including EGF, retinoic acid, and NGF, which also control the proliferation and differentiation of central nervous system (CNS) and peripheral nervous system (PNS) neural progenitor cells. We report here the identification of a novel gene, REN, upregulated by neurogenic signals (retinoic acid, EGF, and NGF) in pluripotent embryonal stem (ES) cells and neural progenitor cell lines in association with neurotypic differentiation. Consistent with a role in neural promotion, REN overexpression induced neuronal differentiation as well as growth arrest and p27Kip1 expression in CNS and PNS neural progenitor cell lines, and its inhibition impaired retinoic acid induction of neurogenin-1 and NeuroD expression. REN expression is developmentally regulated, initially detected in the neural fold epithelium of the mouse embryo during gastrulation, and subsequently throughout the ventral neural tube, the outer layer of the ventricular encephalic neuroepithelium and in neural crest derivatives including dorsal root ganglia. We propose that REN represents a novel component of the neurogenic signaling cascade induced by retinoic acid, EGF, and NGF, and is both a marker and a regulator of neuronal differentiation.

Proliferating cells can differentiate into neurons in the striatum of normal adult monkey

In this study we used bromodeoxyuridine (BrdU), a thymidine analogue that is incorporated into the DNA of mitotic cells, to study the cytogenesis status of the striatum in normal, adult, squirrel monkeys (Saimiri sciureus). Three weeks following BrdU injection, numerous BrdU-labeled (+) cells were encountered within both the dorsal and the ventral striatum, including the nucleus accumbens. Their number ranged from 5 to 50 per 40 microm-thick section. These BrdU+ cells were more abundant medially than laterally and displayed a rostrocaudal-decreasing gradient in the caudate nucleus and putamen. Double-immunofluorescence confocal studies have revealed that about 5-10% of the BrdU+ striatal cells expressed the neuronal nuclear antigen (NeuN), a marker for mature neurons. These findings suggest that new neurons are produced throughout adult life in the striatum of normal, adult primates. This result raises the possibility of experimentally enhancing the recruitment of these newborn neurons as a means to alleviate the symptoms of neurodegenerative diseases that affect the striatum.

Lesion induced new neuron incorporation in the adult hypothalamus of the avian brain

Cell loss in most adult vertebrate brain regions is thought to be irreversible. Here, we explore the effects of electrolytic lesions on the induction of cell proliferation and newborn neurons in the ventromedial nuclei (VMN) of the hypothalamus in young and adult ring doves. The hypothalamus does not normally recruit new neurons. Bromodeoxyuridine (BrdU) and tritiated thymidine ([3H]Thy) were used to identify cells born before and after bilateral electrolytic lesions. Hu and NeuN were used to identify neurons. TUNEL test for apoptosis and 3A7 antibodies were used to identify morphological changes of pre-existing cells. Lesions produced significantly more newborn cells in the subventricular zone (SVZ). The rate of cell proliferation peaked at 7-14 days postlesion. A fraction of these newborn cells were neuronal precursor and began to migrate away along the radial glial fibers 2 weeks after lesion. During this period, the outer area of the lesion site was marked with massive apoptosis and re-expression of radial glial-like fibers. In birds that survived 5 months, we found newly differentiated neurons in the outer area of the lesion site. We conclude that electrolytic lesion can invoke neuronal recruitment in the adult hypothalamus. We further suggest that lesion-induced apoptosis and re-expression of developmental mechanisms might be involved in the recruitment process.

The role of seizure-induced neurogenesis in epileptogenesis and brain repair

Data accumulated over the past four decades have led to the widespread recognition that neurogenesis, the birth of new neurons, persists in the hippocampal dentate gyrus and rostral forebrain subventricular zone (SVZ) of the adult mammalian brain. Neural precursor cells located more caudally in the forebrain SVZ are thought to also give rise to glia throughout life. The continued production of neurons and glia suggests that the mature brain maintains an even greater potential for plasticity after injury than was previously recognized. Underscoring this idea are recent findings that seizures induced by various experimental manipulations increase neurogenesis in the adult rodent dentate gyrus. Although neurogenesis and gliogenesis in persistent germinative zones are altered in adult rodent models of temporal lobe epilepsy (TLE), the effects of seizure-induced neurogenesis in the epileptic brain, in terms of either a pathological or reparative role, are only beginning to be explored. Emerging data suggest that altered neurogenesis in the epileptic dentate gyrus may be pathological and promote abnormal hyperexcitability. However, the presence of endogenous neural progenitors in other proliferative regions may offer potential strategies for the development of anti-epileptogenic or neuronal replacement therapies.

Opinion: neural stem cell therapy for neurological diseases: dreams and reality

There is a pressing need for treatments for neurodegenerative diseases. Hopes have been raised by the prospect of neural stem cell therapy; however, despite intense research activities and media attention, stem cell therapy for neurological disorders is still a distant goal. Effective strategies must be developed to isolate, enrich and propagate homogeneous populations of neural stem cells, and to identify the molecules and mechanisms that are required for their proper integration into the injured brain. This article examines these requirements, discusses the results obtained so far, and considers the steps that need to be taken to provide instruction to donor cells and to elucidate the neurogenic potential of the adult central nervous system environment.

Prolonged seizures increase proliferating neuroblasts in the adult rat subventricular zone-olfactory bulb pathway

Neuronal precursors in the adult rodent forebrain subventricular zone (SVZ) proliferate, migrate to the olfactory bulb in a restricted pathway known as the rostral migratory stream (RMS), and differentiate into neurons. The effects of injury on this neurogenic region of the mature brain are poorly understood. To determine whether seizure-induced injury modulates SVZ neurogenesis, we induced status epilepticus (SE) in adult rats by systemic chemoconvulsant administration and examined patterns of neuronal precursor proliferation and migration in the SVZ-olfactory bulb pathway. Within 1-2 weeks after pilocarpine-induced SE, bromodeoxyuridine (BrdU) labeling and Nissl staining increased in the rostral forebrain SVZ. These changes were associated with an increase in cells expressing antigenic markers of SVZ neuroblasts 2-3 weeks after prolonged seizures. At these same time points the RMS expanded and contained more proliferating cells and immature neurons. BrdU labeling and stereotactic injections of retroviral reporters into the SVZ showed that prolonged seizures also increased neuroblast migration to the olfactory bulb and induced a portion of the neuronal precursors to exit the RMS prematurely. These findings indicate that SE expands the SVZ neuroblast population and alters neuronal precursor migration in the adult rat forebrain. Identification of the mechanisms underlying the response of neural progenitors to seizure-induced injury may help to advance brain regenerative therapies by using either transplanted or endogenous neural precursor cells.

Investigating the use of primary adult subventricular zone neural precursor cells for neuronal replacement therapies

With the relatively recent discovery that neurogenesis persists throughout life in restricted regions of the adult mammalian brain, including those of human beings, there has been great interest in the use of adult-derived neural stem cells for neuronal replacement. There are many great hurdles that must be overcome in order for such replacement strategies to succeed. In this review, we outline some of these hurdles and discuss recent experiments that investigate the potential of using neural precursor cells found in the subventricular zone of the adult brain for brain repair.

Critical issues in gene therapy for neurologic disease

Gene therapy for the nervous system is a newly emerging field with special issues related to modes of delivery, potential toxicity, and realistic expectations for treatment of this vital and highly complex tissue. This review focuses on the potential for gene delivery to the brain, as well as possible risks and benefits of these procedures. This includes discussion of appropriate vectors, such as adeno-associated virus, lentivirus, gutless adenovirus, and herpes simplex virus hybrid amplicons, and cell vehicles, such as neuroprogenitor cells. Routes of delivery for focal and global diseases are enumerated, including use of migratory cells, facilitation of vascular delivery across the blood-brain barrier, cerebrospinal fluid delivery, and convection injection. Attention is given to examples of diseases falling into different etiologic types: metabolic deficiency states, including Canavan disease and lysosomal storage disorders; and degenerative conditions, including Parkinson's disease and other neurodegenerative conditions.

Myocyte growth and cardiac repair

Introduced several decades ago, the dogma persists that ventricular myocytes are terminally differentiated cells and cardiac repair by myocyte regeneration is completely inhibited shortly after birth. On the basis that cardiac myocytes are unable to divide in the adult heart, myocyte growth under physiologic and pathologic conditions is believed to be restricted to cellular hypertrophy. Evidence is presented to indicate that this old paradigm has to be changed to include myocyte replication as a significant component of the cellular processes of ventricular remodeling. Importantly, myocyte death, apoptotic and necrotic in nature, has to be regarded as an additional critical variable of the multifactorial events implicated in the alterations of cardiac anatomy and myocardial structure of the decompensated heart. Methodologies are currently available to recognize and measure quantitatively the contribution of myocyte size, number and death to the adaptation of the overloaded heart and its progression to cardiac failure.

Fifty ways to make a neuron: shifts in stem cell hierarchy and their implications for neuropathology and CNS repair

During embryogenesis, the developmental potential of individual cells is continuously restricted. While embryonic stem (ES) cells derived from the inner cell mass of the blastocyst can give rise to all tissues and cell types, their progeny segregates into a multitude of tissue-specific stem and progenitor cells. Following organogenesis, a pool of resident "adult" stem cells is maintained in many tissues. In this hierarchical concept, transition through defined intermediate stages of decreasing potentiality is regarded as prerequisite for the generation of a somatic cell type. Several recent findings have challenged this view. First, adult stem cells have been shown to adopt properties of pluripotent cells and contribute cells to a variety of tissues. Second, a direct transition from a pluripotent ES cell to a defined somatic phenotype has been postulated for the neural lineage. Finally, nuclear transplantation has revealed that the transcriptional machinery associated with a distinct somatic cell fate can be reprogrammed to totipotency. The possibility to bypass developmental hierarchies in stem cell differentiation opens new avenues for the study of nervous system development, disease, and repair.

Spinal cord regeneration: from gene to transplants

The past 20 years has seen the emergence of many exciting and promising experimental therapeutic strategies to promote regeneration of the injured spinal cord in laboratory animals. A greater understanding of the pathophysiologic mechanisms that contribute to the initial and secondary cord injury may facilitate the development of neuroprotective strategies that preserve axonal function and prevent apoptotic cell death, thus optimizing neurologic function. Neurotrophic factors have been used to augment the poor intrinsic regenerative capacity of central nervous system neurons, and the need for sophisticated delivery of such trophic agents has stimulated the application of gene therapy in this context. In addition to augmenting the neuronal capacity to regenerate axons, many researchers are developing strategies to overcome the inhibitory environment into which these axons must grow. Characterizing the inhibitory elements of the glial scar at the site of injury and of myelin in the distal tracts is therefore a focus of intense scientific interest. To this effect, a number of strategies have also been developed to bridge the injury site and facilitate axonal growth across the lesion with a variety of cellular substrates. These include fetal tissue transplants, stem cells, Schwann cells, and olfactory ensheathing cells. With the collaboration of basic scientists and clinicians, it is hoped that these experimental strategies coupled with a greater understanding of the neurobiology of spinal cord injury will be translatable to the clinical setting in the near future.

Management of Parkinson's disease. Strategies, pitfalls, and future directions

In coming years Parkinson's disease will become increasingly prevalent as the baby boom generation grows older. Diagnosis often is complicated and requires careful consideration of symptoms and neurologic findings. Optimal symptomatic treatment of Parkinson's disease involves an individualized approach with each patient and ongoing evaluation of benefits versus side effects. Neurosurgical intervention is an option for some patients who are not adequately helped by medical therapy. New treatments (e.g., stem cell therapy) are currently being studied and may be available in the foreseeable future.

Neurodegeneration: a failure of neuroregeneration?

Evidence suggests that the brain, like many other tissues, is in a state of dynamic equilibrium. It has an endogenous population of stem cells that proliferate in response to environmental and pharmacological manipulations and that can replace cells lost in some experimental lesions. However, the fact remains that neurodegenerative disorders such as Alzheimer's, Parkinson's, and Huntington's diseases are characterised by continuous loss of neurons that are not replaced. In this hypothesis, we postulate that a primary deficit in neural stem-cell proliferation; migration, or differentiation, or both, might contribute to net cell loss and neuronal circuit disruption in these disorders. Experimental validation of this hypothesis would not only substantially advance understanding of the pathogenesis of these diseases, but could also have profound implications for future treatment of these incurable disorders.

Insulin-like growth factor-I is necessary for neural stem cell proliferation and demonstrates distinct actions of epidermal growth factor and fibroblast growth factor-2

Neural stem cells (NSCs), when stimulated with epidermal growth factor (EGF) or fibroblast growth factor-2 (FGF-2), have the capacity to renew, expand, and produce precursors for neurons, astrocytes, and oligodendrocytes. We postulated that the early appearance of insulin-like growth factor (IGF-I) receptors during mouse striatum development implies a role in NSC regulation. Thus, we tested in vitro the action of IGF-I on the proliferation of striatal NSCs. In the absence of IGF-I, neither EGF nor FGF-2 was able to induce the proliferation of E14 mouse striatal cells. However, addition of IGF-I generated large proliferative clusters, termed spheres, in a dose-dependent manner. The newly generated spheres were multipotent, and clonal analysis revealed that EGF or FGF-2, in the presence of IGF-I, acted directly on NSCs. The actions of IGF-I suggest distinct modes of action of EGF or FGF-2 on NSCs. First, continuous versus delayed administration of these neurotrophic factors showed that neither IGF-I nor EGF had an effect on NSC survival, whereas FGF-2 promoted the survival or maintenance of the stem cell state of 50% of NSCs for 6 d. Second, short-term exposure to IGF-I induced the proliferation of NSCs in the presence of EGF, but not of FGF-2, through an autocrine secretion of IGF-I. These findings suggest that IGF-I is a key factor in the regulation of NSC activation and that EGF and FGF-2 control striatal NSC proliferation, in part, through distinct intracellular mechanisms.

Gliogenesis in the striatum of the adult rat: alteration in neural progenitor population after psychostimulant exposure

Cytogenesis from proliferating progenitor cells is present in the rat brain throughout adulthood, and is regulated by a variety of environmental stimuli. To determine whether adult cytogenesis occurs in the intact rat striatum and to explore the possible regulatory role of psychostimulant exposure on striatal cytogenesis, immunohistochemistry with the thymidine analog bromodeoxyuridine (BrdU), a marker of DNA synthesis that labels dividing cells and their terminal progeny, was performed on the brain sections of normal adult rats and rats treated with a psychostimulant, amphetamine (AMPH). Scattered cells that incorporated BrdU were consistently seen throughout the dorsal (caudate putamen) and ventral (nucleus accumbens) striatum 24 h after BrdU injection. Three to four weeks after BrdU injection, approximately 10-20% of surviving newborn cells differentiated into astroglia according to their radial morphology of glia and co-expression of an astroglial marker, S100beta. However, none of BrdU-positive cells were found to co-localize with a neuronal marker, neuronal nuclear antigen (NeuN). Acute injection of AMPH at a behaviorally active dose (10 mg/kg, i.p.) produced a rapid and transient decrease in the number of BrdU-labeled cells in both the dorsal (70.6% of control) and ventral (66.7% of control) striatum, but not in the subventricular zone and the hippocampal dentate gyrus. However, the fraction of differentiated astrocytes was not altered 3-4 weeks after AMPH treatment. These results indicate an existence of active gliogenesis (both proliferation and differentiation) in the adult rat striatum. Vulnerability of striatal cytogenesis to psychostimulant exposure indicates a new approach to elucidate brain mechanisms responsible for addictive properties of drugs of abuse.

Insulin-like growth factor-I is necessary for neural stem cell proliferation and demonstrates distinct actions of epidermal growth factor and fibroblast growth factor-2

Neural stem cells (NSCs), when stimulated with epidermal growth factor (EGF) or fibroblast growth factor-2 (FGF-2), have the capacity to renew, expand, and produce precursors for neurons, astrocytes, and oligodendrocytes. We postulated that the early appearance of insulin-like growth factor (IGF-I) receptors during mouse striatum development implies a role in NSC regulation. Thus, we tested in vitro the action of IGF-I on the proliferation of striatal NSCs. In the absence of IGF-I, neither EGF nor FGF-2 was able to induce the proliferation of E14 mouse striatal cells. However, addition of IGF-I generated large proliferative clusters, termed spheres, in a dose-dependent manner. The newly generated spheres were multipotent, and clonal analysis revealed that EGF or FGF-2, in the presence of IGF-I, acted directly on NSCs. The actions of IGF-I suggest distinct modes of action of EGF or FGF-2 on NSCs. First, continuous versus delayed administration of these neurotrophic factors showed that neither IGF-I nor EGF had an effect on NSC survival, whereas FGF-2 promoted the survival or maintenance of the stem cell state of 50% of NSCs for 6 d. Second, short-term exposure to IGF-I induced the proliferation of NSCs in the presence of EGF, but not of FGF-2, through an autocrine secretion of IGF-I. These findings suggest that IGF-I is a key factor in the regulation of NSC activation and that EGF and FGF-2 control striatal NSC proliferation, in part, through distinct intracellular mechanisms.

Doublecortin expression in the adult rat telencephalon

Doublecortin (DCX) is a protein required for normal neuronal migration in the developing cerebral cortex, where it is widely expressed in both radially and tangentially migrating neuroblasts. Moreover, it has been observed in the adult rostral migratory stream, which contains the neuronal precursors traveling to the olfactory bulb. We have performed DCX immunocytochemistry in the adult rat brain to identify precisely the neuronal populations expressing this protein. Our observations confirm the presence of DCX immunoreactive cells with the characteristic morphology of migrating neuroblasts in the subventricular zone, rostral migratory stream and the main and accessory olfactory bulbs. We have also found putative migratory cells expressing DCX in regions were no adult neuronal migration has been described, as the corpus callosum, the piriform cortex layer III/endopiriform nucleus and the striatum. Surprisingly, many cells with the phenotype of differentiated neurons were DCX immunoreactive; e.g. certain granule neurons in the hilar border of the granular layer of the dentate gyrus, some neuronal types in the piriform cortex layer II, granule and periglomerular neurons in the main and accessory olfactory bulbs, and isolated cells in the striatum. Almost all DCX immunoreactive cells also express the polysialylated form of neural cell adhesion molecule and have a similar distribution to rat collapsin receptor-mediated protein-4, two molecules involved in neuronal structural plasticity. Given these results, we hypothesize that DCX expression in differentiated neurons could be related to its capacity for microtubule reorganization and that this fact could be linked to axonal outgrowth or synaptogenesis.

Evidence that human cardiac myocytes divide after myocardial infarction

BACKGROUND

The scarring of the heart that results from myocardial infarction has been interpreted as evidence that the heart is composed of myocytes that are unable to divide. However, recent observations have provided evidence of proliferation of myocytes in the adult heart. Therefore, we studied the extent of mitosis among myocytes after myocardial infarction in humans.

METHODS

Samples from the border of the infarct and from areas of the myocardium distant from the infarct were obtained from 13 patients who had died 4 to 12 days after infarction. Ten normal hearts were used as controls. Myocytes that had entered the cell cycle in preparation for cell division were measured by labeling of the nuclear antigen Ki-67, which is associated with cell division. The fraction of myocyte nuclei that were undergoing mitosis was determined, and the mitotic index (the ratio of the number of nuclei undergoing mitosis to the number not undergoing mitosis) was calculated. The presence of mitotic spindles, contractile rings, karyokinesis, and cytokinesis was also recorded.

RESULTS

In the infarcted hearts, Ki-67 expression was detected in 4 percent of myocyte nuclei in the regions adjacent to the infarcts and in 1 percent of those in regions distant from the infarcts. The reentry of myocytes into the cell cycle resulted in mitotic indexes of 0.08 percent and 0.03 percent, respectively, in the zones adjacent to and distant from the infarcts. Events characteristic of cell division--the formation of the mitotic spindles, the formation of contractile rings, karyokinesis, and cytokinesis--were identified; these features demonstrated that there was myocyte proliferation after myocardial infarction.

CONCLUSIONS

Our results challenge the dogma that the adult heart is a postmitotic organ and indicate that the regeneration of myocytes may be a critical component of the increase in muscle mass of the myocardium.

The generation, migration, and differentiation of olfactory neurons in the adult primate brain

In adult rodents, neural progenitor cells in the subependymal (SZ) zone of the lateral cerebral ventricle generate neuroblasts that migrate in chains via the rostral migratory stream (RMS) into the olfactory bulb (OB), where they differentiate into interneurons. However, the existence of this neurogenic migratory system in other mammals has remained unknown. Here, we report the presence of a homologue of the rodent SZ/RMS in the adult macaque monkey, a nonhuman Old World primate with a relatively smaller OB. Our results-obtained by using combined immunohistochemical detection of a marker for DNA replication (5-bromodeoxyuridine) and several cell type-specific markers-indicate that dividing cells in the adult monkey SZ generate neuroblasts that undergo restricted chain migration over an extended distance of more than 2 cm to the OB and differentiate into granule interneurons. These findings in a nonhuman primate extend and support the use of the SZ/RMS as a model system for studying neural regenerative mechanisms in the human brain.