Migrational analysis of the constitutively proliferating subependyma population in adult mouse forebrain

Jedi-1/MEGF12-mediated phagocytosis controls the pro-neurogenic properties of microglia in the ventricular-subventricular zone

Microglia are the primary phagocytes in the central nervous system and clear dead cells generated during development or disease. The phagocytic process shapes the microglia phenotype, which affects the local environment. A unique population of microglia resides in the ventricular-subventricular zone (V-SVZ) of neonatal mice, but how they influence the neurogenic niche is not well understood. Here, we demonstrate that phagocytosis contributes to a pro-neurogenic microglial phenotype in the V-SVZ and that these microglia phagocytose apoptotic cells via the engulfment receptor Jedi-1. Deletion of Jedi-1 decreases apoptotic cell clearance, triggering a neuroinflammatory microglia phenotype that resembles dysfunctional microglia in neurodegeneration and aging and that reduces neural precursor proliferation via elevated interleukin-1β signaling; interleukin-1 receptor inhibition rescues precursor proliferation in vivo. Together, these results reveal a critical role for Jedi-1 in connecting microglial phagocytic activity to the maintenance of a pro-neurogenic phenotype in the developing V-SVZ.

Jedi-1/MEGF12-mediated phagocytosis controls the pro-neurogenic properties of microglia in the ventricular-subventricular zone

Microglia are the primary phagocytes in the central nervous system and are responsible for clearing dead cells generated during development or disease. The phagocytic process shapes the phenotype of the microglia, which affects the local environment. A unique population of microglia reside in the ventricular-subventricular zone (V-SVZ) of neonatal mice, but how they influence this neurogenic niche is not well-understood. Here, we demonstrate that phagocytosis creates a pro-neurogenic microglial phenotype in the V-SVZ and that these microglia phagocytose apoptotic cells via the engulfment receptor Jedi-1. Deletion of Jedi-1 decreases apoptotic cell clearance, triggering the development of a neuroinflammatory phenotype, reminiscent of neurodegenerative and-age-associated microglia, that reduces neural precursor proliferation via elevated interleukin (IL)-1β signaling; inhibition of IL-1 receptor rescues precursor proliferation in vivo. Together, these results reveal a critical role for Jedi-1 in connecting microglial phagocytic activity to a phenotype that promotes neurogenesis in the developing V-SVZ.

Constraint-induced movement therapy promotes motor recovery after neonatal stroke in the absence of neural precursor activation

Neonatal stroke is a leading cause of long-term disability and currently available rehabilitation treatments are insufficient to promote recovery. Activating neural precursor cells (NPCs) in adult rodents, in combination with rehabilitation, can accelerate functional recovery following stroke. Here, we describe a novel method of constraint-induced movement therapy (CIMT) in a rodent model of neonatal stroke that leads to improved functional outcomes, and we asked whether the recovery was correlated with expansion of NPCs. A hypoxia/ischemia (H/I) injury was induced on postnatal day 8 (PND8) via unilateral carotid artery ligation followed by systemic hypoxia. One week and two weeks post-H/I, CIMT was administered in the form of 3 botulinum toxin (Botox) injections, which induced temporary paralysis in the unaffected limb. Functional recovery was assessed using the foot fault task. NPC proliferation was assessed using the neurosphere assay and EdU immunohistochemistry. We found that neonatal H/I injury alone expands the NPC pool by >2.5-fold relative to controls. We determined that using Botox injections as a method to provide CIMT results in significant functional motor recovery after H/I. However, CIMT does not lead to enhanced NPC activation or migration into the injured parenchyma in vivo. At the time of functional recovery, increased numbers of proliferating inflammatory cells were found within the injured motor cortex. Together, these findings suggest that NPC activation following CIMT does not account for the observed functional improvement and suggests that CIMT-mediated modification of the CNS inflammatory response may play a role in the motor recovery.

Neural stem cell heterogeneity in the mammalian forebrain

The brain was long considered an organ that underwent very little change after development. It is now well established that the mammalian central nervous system contains neural stem cells that generate progeny that are capable of making new neurons, astrocytes, and oligodendrocytes throughout life. The field has advanced rapidly as it strives to understand the basic biology of these precursor cells, and explore their potential to promote brain repair. The purpose of this review is to present current knowledge about the diversity of neural stem cells in vitro and in vivo, and highlight distinctions between neural stem cell populations, throughout development, and within the niche. A comprehensive understanding of neural stem cell heterogeneity will provide insights into the cellular and molecular regulation of neural development and lifelong neurogenesis, and will guide the development of novel strategies to promote regeneration and neural repair.

Vascular pattern of the dentate gyrus is regulated by neural progenitors

Neurogenesis is a vital process that begins during early embryonic development and continues until adulthood, though in the latter case, it is restricted to the subventricular zone and the subgranular zone of the dentate gyrus (DG). In particular, the DG's neurogenic properties are structurally and functionally unique, which may be related to its singular vascular pattern. Neurogenesis and angiogenesis share molecular signals and act synergistically, supporting the concept of a neurogenic niche as a functional unit between neural precursors cells and their environment, in which the blood vessels play an important role. Whereas it is well known that vascular development controls neural proliferation in the embryonary and in the adult brain, by releasing neurotrophic factors; the potential influence of neural cells on vascular components during angiogenesis is largely unknown. We have demonstrated that the reduction of neural progenitors leads to a significant impairment of vascular development. Since VEGF is a potential regulator in the neurogenesis-angiogenesis crosstalk, we were interested in assessing the possible role of this molecule in the hippocampal neurovascular development. Our results showed that VEGF is the molecule involved in the regulation of vascular development by neural progenitor cells in the DG.

Home sweet home: the neural stem cell niche throughout development and after injury

Neural stem cells and their progeny reside in two distinct neurogenic niches within the mammalian brain: the subventricular zone and the dentate gyrus. The interplay between the neural stem cells and the niche in which they reside can have significant effects on cell kinetics and neurogenesis. A comprehensive understanding of the changes to the niche that occur through postnatal development and aging, as well as following injury, is relevant for developing therapeutics and interventions to promote neural repair. We discuss changes that occur within the neural stem and progenitor cell populations, the vasculature, extracellular matrix, microglia, and secreted proteins through aging which impact cell behavior within the neurogenic niches. We examine neural precursor cell and niche responses to injury in neonatal hypoxia-ischemia, juvenile cranial irradiation, and adult stroke. This review examines the interplay between the niche and stem cell behavior through aging and following injury as a means to understand intrinsic and extrinsic factors that regulate neurogenesis in vivo.

Activation of endogenous neural stem cells for multiple sclerosis therapy

Multiple sclerosis (MS) is a chronic inflammatory disorder of the central nervous system, leading to severe neurological deficits. Current MS treatment regimens, consist of immunomodulatory agents aiming to reduce the rate of relapses. However, these agents are usually insufficient to treat chronic neurological disability. A promising perspective for future therapy of MS is the regeneration of lesions with replacement of the damaged oligodendrocytes or neurons. Therapies targeting to the enhancement of endogenous remyelination, aim to promote the activation of either the parenchymal oligodendrocyte progenitor cells or the subventricular zone-derived neural stem cells (NSCs). Less studied but highly potent, is the strategy of neuronal regeneration with endogenous NSCs that although being linked to numerous limitations, is anticipated to ameliorate cognitive disability in MS. Focusing on the forebrain, this review highlights the role of NSCs in the regeneration of MS lesions.

Prosurvival factors derived from the embryonic brain promote adult neural stem cell survival

Temporally distinct populations of neural stem cells (NSCs; embryonic and adult) display the cardinal stem cell properties of self-renewal and multipotentiality; however, their relative frequency and cell kinetics vary through development and into old age. We asked whether changes in NSC behavior could be accounted for by changes in environmental signals over time. We identified a prosurvival signaling cascade that enhances adult-derived NSC survival using cues released from embryonic neurons. Specifically, we demonstrate that stromal-cell-derived factor-1α (SDF-1α) released by embryonic neurons leads to upregulation of neuronal nitric oxide synthase in adult neural precursor cells. The resulting increase in nitric oxide leads to the upregulation of the stem cell factor (SCF) receptor ckit on adult NSCs (ANSCs). SCF released from embryonic neurons results in enhanced NSC survival. Using both in vitro and in vivo assays, we have demonstrated expansion of the size of the NSC pool through this pathway, indicating that ANSCs retain their ability to respond to embryonic-derived cues into adulthood.

Type 1 metalloproteinase is selectively expressed in adult rat brain and can be rapidly up-regulated by kainate

The expression of metalloproteinase MMP-1 was traced in frontal sections of the rat brain in normal conditions and 4 h after an intraperitoneal injection of kainate. In the olfactory lobe, immunoreactivity was normally detected in the lateral olfactory tract. Kainate treatment led to the appearance of additional immunoreactivity in the neuropilar tracts. In the hippocampal part of brain, immunoreactive neurons were found exclusively after the kainate treatment in several hypothalamic and amygdalar nuclei, and in the restricted cortex areas (clusters of neurons in layers 3-4 of cortex, and a stripe of cells in layer 6). In the area between the hippocampus and cerebellum, MMP-1-like immunoreactivity was normally present in the entorhinal cortex, in the lateral periaqueductal gray, and in the pontine nucleus. After kainate treatment, the immunoreactive neurons were also found in the medial entorhinal cortex and in the dorsal raphe nucleus. In the brain stem, the immunoreactive cells were normally found in six nuclei. After kainate treatment, additional immunoreactivity appeared in the inferior olive neurons and in tracts supplying the cerebellar cortex. Thus, MMP-1 is present in several brain areas in normal conditions at a detectable level, and its expression increases after kainate-induced seizures.

Lineage tracing with Axin2 reveals distinct developmental and adult populations of Wnt/β-catenin-responsive neural stem cells

Since the discovery of neural stem cells in the mammalian brain, there has been significant interest in understanding their contribution to tissue homeostasis at both the cellular and molecular level. Wnt/β-catenin signaling is crucial for development of the central nervous system and has been implicated in stem cell maintenance in multiple tissues. Based on this, we hypothesized that the Wnt pathway likely controls neural stem cell maintenance and differentiation along the entire developmental continuum. To test this, we performed lineage tracing experiments using the recently developed tamoxifen-inducible Cre at Axin2 mouse strain to follow the developmental fate of Wnt/β-catenin-responsive cells in both the embryonic and postnatal mouse brain. From as early as embryonic day 8.5 onwards, Axin2(+) cells can give rise to spatially and functionally restricted populations of adult neural stem cells in the subventricular zone. Similarly, progeny from Axin2(+) cells labeled from E12.5 contribute to both the subventricular zone and the dentate gyrus of the hippocampus. Labeling in the postnatal brain, in turn, demonstrates the persistence of long-lived, Wnt/β-catenin-responsive stem cells in both of these sites. These results demonstrate the continued importance of Wnt/β-catenin signaling for neural stem and progenitor cell formation and function throughout developmental time.

Subventricular zone localized irradiation affects the generation of proliferating neural precursor cells and the migration of neuroblasts

Radiation therapy is a part of the standard treatment for brain tumor patients, often resulting in irreversible neuropsychological deficits. These deficits may be due to permanent damage to the neural stem cell (NSC) niche, damage to local neural progenitors, or neurotoxicity. Using a computed tomography-guided localized radiation technique, we studied the effects of radiation on NSC proliferation and neuroblast migration in the mouse brain. Localized irradiation of the subventricular zone (SVZ) eliminated the proliferating neural precursor cells and migrating neuroblasts. After irradiation, type B cells in the SVZ lacked the ability to generate migrating neuroblasts. Neuroblasts from the unirradiated posterior SVZ did not follow their normal migratory path through the irradiated anterior SVZ. Our results indicate that the migrating neuroblasts were not replenished, despite the presence of type B cells in the SVZ post-irradiation. This study provides novel insights into the effects of localized SVZ radiation on neurogenesis and cell migration that may potentially lead to the development of new radiotherapy strategies to minimize damage to NSCs and neuroblast migration.

Migration of neuronal precursors from the telencephalic ventricular zone into the olfactory bulb in adult zebrafish

In the brain of adult mammals, neuronal precursors are generated in the subventricular zone in the lateral wall of the lateral ventricles and migrate into the olfactory bulbs (OBs) through a well-studied route called the rostral migratory stream (RMS). Recent studies have revealed that a comparable neural stem cell niche is widely conserved at the ventricular wall of adult vertebrates. However, little is known about the migration route of neuronal precursors in nonmammalian adult brains. Here, we show that, in the adult zebrafish, a cluster of neuronal precursors generated in the telencephalic ventricular zone migrates into the OB via a route equivalent to the mammalian RMS. Unlike the mammalian RMS, these neuronal precursors are not surrounded by glial tubes, although radial glial cells with a single cilium lined the telencephalic ventricular wall, much as in embryonic and neonatal mammals. To observe the migrating neuronal precursors in living brain tissue, we established a brain hemisphere culture using a zebrafish line carrying a GFP transgene driven by the neurogenin1 (ngn1) promoter. In these fish, GFP was observed in the neuronal precursors migrating in the RMS, some of which were aligned with blood vessels. Numerous ngn1:gfp-positive cells were observed migrating tangentially in the RMS-like route medial to the OB. Taken together, our results suggest that the RMS in the adult zebrafish telencephalon is a functional migratory pathway. This is the first evidence for the tangential migration of neuronal precursors in a nonmammalian adult telencephalon.

Rapid transgene expression in multiple precursor cell types of adult rat subventricular zone mediated by adeno-associated type 1 vectors

The adult rat brain subventricular zone (SVZ) contains proliferative precursors that migrate to the olfactory bulb (OB) and differentiate into mature neurons. Recruitment of precursors constitutes a potential avenue for brain repair. We have investigated the kinetics and cellular specificity of transgene expression mediated by AAV2/1 vectors (i.e., adeno-associated virus type 2 pseudotyped with AAV1 capsid) in the SVZ. Self-complementary (sc) and single-stranded (ss) AAV2/1 vectors mediated efficient GFP expression, respectively, at 17 and 24 hr postinjection. Transgene expression was efficient in all the rapidly proliferating cells types, that is, Mash1(+) precursors (30% of the GFP(+) cells), Dlx2(+) neuronal progenitors (55%), Olig2(+) oligodendrocyte progenitors (35%), and doublecortin-positive (Dcx(+)) migrating cells (40%), but not in the slowly proliferating glial fibrillary acidic protein-positive (GFAP(+)) neural stem cell pool (5%). Because cell cycle arrest by wild-type and recombinant AAV has been described in primary cultures, we examined SVZ proliferative activity after vector injection. Indeed, cell proliferation was reduced immediately after vector injection but was normal after 1 month. In contrast, migration and differentiation of GFP(+) precursors were unaltered. Indeed, the proportion of Dcx(+) cells was similar in the injected and contralateral hemispheres. Furthermore, 1 month after vector injection into the SVZ, GFP(+) cells, found, as expected, in the OB granular cell layer, were mature GABAergic neurons. In conclusion, the rapid and efficient transgene expression in SVZ neural precursors mediated by scAAV2/1 vectors underlines their potential usefulness for brain repair via recruitment of immature cells. The observed transient precursor proliferation inhibition, not affecting their migration and differentiation, will likely not compromise this strategy.

Querkopf is a key marker of self-renewal and multipotency of adult neural stem cells

Adult neural stem cells (NSCs) reside in the subventricular zone (SVZ) and produce neurons throughout life. Although their regenerative potential has kindled much interest, few factors regulating NSCs in vivo are known. Among these is the histone acetyltransferase querkopf (QKF, also known as MYST4, MORF, KAT6B), which is strongly expressed in a small subset of cells in the neurogenic subventricular zone. However, the relationship between Qkf gene expression and the hierarchical levels within the neurogenic lineage is currently unknown. We show here that the 10% of SVZ cells with the highest Qkf expression possess the defining NSC characteristics of multipotency and self-renewal and express markers previously shown to enrich for NSCs. A fraction of cells expressing Qkf at medium to high levels is enriched for multipotent progenitor cells with limited self-renewal, followed by a population containing migrating neuroblasts. Cells low in Qkf promoter activity are predominantly ependymal cells. In addition, we show that mice deficient for Bmi1, a central regulator of NSC self-renewal, show an age-dependent decrease in the strongest Qkf-expressing cell population in the SVZ. Our results show a strong relationship between Qkf promoter activity and stem cell characteristics, and a progressive decrease in Qkf gene activity as lineage commitment and differentiation proceed in vivo.

Neurogliogenesis in the mature olfactory system: a possible protective role against infection and toxic dust

The outpost position of the olfactory bulb (OB) between the direct inputs from sensory neurons of the nasal epithelium and other parts of the brain suggests its highest vulnerability among all brain structures to penetration of exogenous agents. A number of neurotropic viruses have been found to invade the brain through the OB. There is growing evidence that microscopic particles of toxic dusts can propagate from the nasal epithelium to the OB and further into the brain. These harmful agents impair cellular elements of the brain. Apparently, cells in the OB are the most affected, as they are the first to encounter viral infections and toxic particles. It is well known that neuronal and glial progenitors are continuously generated from neuronal stem cells in the subventricular zone of the adult brain and then migrate predominantly into the OB. Therefore, it is feasible to suggest that substitution of injured or dead cells in the OB by new-born neurons, differentiating from progenitors, plays a role in protecting the OB neuronal microcircuits from destruction. Furthermore, some cytokines and chemokines released in response to infection and/or intoxication can modulate different stages of neurogenesis (proliferation, migration, and differentiation). We hypothesize that continuous neurogenesis in the olfactory system throughout adulthood evolved as a protective mechanism to prevent impairment of the most ancient but vitally important sensory system. In addition, differentiation of a substantial portion of progenitors to glial cells, including macrophages and microglia, may create an additional barrier to exogenous agents on their way deep to the brain.

Nestin in central nervous system cells

This literature review reflects current knowledge on the intermediate filament protein nestin, which most authors regard as a marker of "neural stem/progenitor cells." The structural-functional characteristics of nestin and its presence in various central nervous system cells at different stages of ontogenesis in normal and pathological conditions are discussed.

The rostral migratory stream is a neurogenic niche that predominantly engenders periglomerular cells: in vivo evidence in the adult rat brain

In vitro studies support the existence of adult neural stem cells in the rostral migratory stream (RMS). The evidence supporting this possibility in vivo is scarce. We then explore this issue by taking advantage of a rat model in which a physical barrier implanted in the brain interrupted the migration of neuroblasts derived from the SVZ along the RMS at the level of its vertical limb. The presence of local stem cells and neurogenesis were then established by estimating the number of nuclei labeled with bromo-deoxyuridine (BrdU), the number of doublecortin-positive neuroblasts and the existence of cells displaying co-localization of BrdU and Sox-2 immunoreactivity along the RMS, at different time points following barrier implantation. Estimations of the number of the granular and periglomerular neurons integrated into the corresponding layers of the olfactory bulb of implanted rats established that stem cells in the RMS give rise predominantly to periglomerular neurons. Our results then support the notion that the RMS is indeed a region in which neurogenesis is taking place in the adult brain. They also support that the relative location of the neurogenic niche might imprint, at least in some degree, the identity and lineage of the neuroblasts arising from them.

Protocols for studying adult neurogenesis: insights and recent developments

The first evidence that neurogenesis occurs in the adult brain was reported in rodents in the early 1960s, using [(3)H]-thymidine autoradiography. In the 1980s and 90s, the advent of new techniques and protocols for studying cell proliferation in situ, and particularly bromodeoxyuridine labeling, helped to confirm that neurogenesis occurs in the adult brain and neural stem cells reside in the adult CNS, including in humans. Bromodeoxyuridine labeling is currently the method most commonly used for studying neurogenesis in the adult brain. However, this procedure is not without limitations, and controversies. In this article, I will review recent protocols for studying adult neurogenesis, particularly new protocols for studying cell kinetics and cell proliferative history, using halopyrimidines. I will review these techniques, and discuss their implications for the field of adult neurogenesis.

A combined ex/in vivo assay to detect effects of exogenously added factors in neural stem cells

We describe a protocol developed/modified by our group for the ex vivo and in vivo assessment of the response to a soluble factor of murine neural stem cells from the adult sub-ventricular zone (SVZ). The procedure includes several experimental options that can be used either independently or in combination. Potential factor effects on self-renewal, survival and proliferation are assayed by means of neurosphere cultures, with the factor administered directly in vitro to the culture plates (Step 1) or infused in vivo immediately before tissue dissociation (Step 3). We also use bromodeoxiuridine (BrdU) retention to label slowly dividing cells in vivo and subsequently perform two different types of experiments. In one set of experiments, the factor is added to primary cultures of stem cells obtained from the BrdU-pulsed animals and effects are tested on label-retaining cells after immunocytochemistry (Step 2). In another set, prolonged intraventricular infusion of the factor in BrdU-pulsed animals is followed by immunohistochemical analysis of BrdU labeling in the intact SVZ (Step 4). The minimum estimated time for the full combined procedure is 45 d.

Growth factor-stimulated generation of new cortical tissue and functional recovery after stroke damage to the motor cortex of rats

Recent studies suggest that proliferation in the adult forebrain subventricular zone increases in response to a forebrain stroke and intraventricular infusions of growth factors enhance this response. The potential for growth factor infusions to regenerate the damaged motor cortex and promote recovery of motor function after stroke has not been examined. Here, we report that intraventricular infusions of epidermal growth factor and erythropoietin together, but not individually, promote substantial regeneration of the damaged cerebral cortex and reverse impairments in spontaneous and skilled motor tasks, in a rat model of stroke. Cortical regeneration and functional recovery occurred even when growth factor administration was delayed for up to 7 days after the stroke-induced lesion. Cell tracking demonstrated the contribution of neural precursors originating in the forebrain subventricular zone to the regenerated cortex. Strikingly, removal of the regenerated cortical tissue reversed the growth factor-induced functional recovery. These findings reveal that specific combinations of growth factors can mobilize endogenous adult neural stem cells to promote cortical tissue re-growth and functional recovery after stroke.

A progressive and cell non-autonomous increase in striatal neural stem cells in the Huntington's disease R6/2 mouse

Neural stem and progenitor cells are located in the subependyma of the adult forebrain. An increase in adult subependymal cell proliferation is reported after various kinds of brain injury. We demonstrate an expansion of neural precursor cells in the postnatal subependyma in a murine genetic disease model of Huntington's disease (HD), the R6/2 mouse. We used the in vitro neurosphere assay as an index of the number of neural stem cells in vivo and to assess proliferation kinetics in vitro and in vivo bromodeoxyuridine labeling to assess the progenitor cell population and their fates. Disease progression in this model leads to an increase in the numbers of neural stem cells in the adult striatal subependyma. This increase is produced cell non-autonomously by events in the R6/2 brains as the mice become increasingly symptomatic. Once the neural stem cell increase is induced in vivo, it is maintained during in vitro passaging of neural stem cells, but the neural stem cell increase is not reproduced during in vitro passaging of neural stem cells from presymptomatic R6/2 mice. In addition, we show that some of the R6/2 neural progenitor cells show a change from their normal migration destiny toward the olfactory bulb. Instead, some of these cells migrate into the striatum, one of the main affected areas in HD. Our findings demonstrate that HD damage recruits precursor cells in two ways: expansion of neural stem cells and altered migration of progenitor cells.

Neurogenesis in rats after focal cerebral ischemia is enhanced by indomethacin

BACKGROUND AND PURPOSE

Newborn cells may participate in repair following ischemic brain injury, but their survival and function may be influenced by inflammation.

METHODS

We investigated the effects of indomethacin, a nonsteroidal antiinflammatory drug, on the fate of newborn cells following transient focal ischemia.

RESULTS

Bromodeoxyuridine (BrdU)-labeled cells, including migrating neuroblasts, were observed in the neighboring striatum and overlying cortex 1 day poststroke. The density of BrdU+ cells labeled with doublecortin, nestin, glial fibrillary acidic protein, or NG2 was increased at 14 and 28 days. Indomethacin increased BrdU+ cells of all lineages and reduced microglial/monocyte activation.

CONCLUSIONS

Indomethacin enhanced the accumulation of newborn cells following stroke.

Localization of nestin in amygdaloid kindled rat: an immunoelectron microscopic study

BACKGROUND

Nestin is a class VI intermediate filament protein, expressed during early embryonic development in mammals. Postnatally, nestin and its mRNA are down-regulated and gradually disappear. Recently, nestin expression has been detected in the adult nervous system, and it has been suggested that this protein may be related to neurogenesis, although, its role in the mechanism of neurogenesis is not known.

METHODS

The present study examined the localization of nestin in CNS tissue of the amygdaloid kindled rat by light and electron microscopy.

RESULTS

Kindled animals showed nestin expression mainly in the piriform cortex and the perirhinal cortex. By light microscopy, nestin was shown to be expressed in astrocytes, neurons, and endothelial cells. Electron microscopy demonstrated nestin expression in endothelial cells, astrocytic perivascular end feet, the rare pericyte, neurons and oligodendrocytes.

CONCLUSION

We conclude that epilepsy causes widespread nestin expression in many cell types in the CNS, including non-neural cells.

Adult Neural Stem Cells: Attempting to Solve the Identity Crisis

A prevailing view in the literature is that the central nervous system (CNS) develops from a population of self-renewing, multipotential stem cells that persist throughout life. The in vivo identification and isolation of this rare subpopulation of cells has been the focus of much attention. This review will discuss the in vivo characterization of neural stem cells in the embryonic and adult CNS and some recent discrepancies and limitations between in vivo and in vitro assays used to identify neural stem cells.

Vascular adventitia generates neuronal progenitors in the monkey hippocampus after ischemia

In the adult hippocampus, neurogenesis proceeds in the subgranular zone (SGZ) of the dentate gyrus (DG), but not in the cornu Ammonis (CA). Recently, we demonstrated in monkeys that transient brain ischemia induces an increase of the neuronal progenitor cells in the SGZ, but not in CA1, in the second week after the insult. To identify the origin of primary neuronal progenitors in vivo, we compared the postischemic monkey DG and CA1, using light and electron microscopy, focusing on specific phenotype markers, as well as the expression of neurotrophic factors. Laser confocal microscopy showed that 1-3% of 5-bromo-2'-deoxyuridine (BrdU)-positive cells in the SGZ after 2-96 h labeling were also positive for neuronal markers such as TUC4, betaIII tubulin, and NeuN on days 9 and 15. In contrast, despite the presence of numerous BrdU-positive cells, CA1 showed no neurogenesis at any time points, and all the progenitors were positive for glial markers: Iba1 or S-100beta on days 4, 9, and 15. Highly polysialylated neural cell adhesion molecule (PSA-NCAM)-positive cells were abundant in the SGZ, but were absent in CA1. On day 9, most of the immature neurons positive for betaIII-tubulin in SGZ showed an increase in PSA-NCAM immunoreactivity. The immunoreactivity of brain-derived neurotrophic factor (BDNF) was abundant at the vascular adventitia of the SGZ, but was absent at the adventitia of CA1. BrdU-positive progenitor cells were frequently seen in the vicinity of proliferating blood vessels. Ultrastructural analysis indicated that most of the neuronal progenitor cells and microglia originated from the pericytes of capillaries and/or adventitial cells of arterioles (called vascular adventitia). The detaching adventitial cells showed mitotic figures in the perivascular space, and the resultant neuronal progenitor cells made contact with dendritic spines associated with synaptic vesicles or boutons. These data implicate the vascular adventitia as a novel potential source of neuronal progenitor cells in the postischemic primate SGZ.

Aging and neuronal replacement

Neural stem cells contribute to neurogenesis in both the embryonic and adult brain. However, while adult neural stem cells produce new neurons that populate the olfactory bulb and the granule cell layer of the hippocampus, they do not normally participate in reparative neurogenesis following injury or disease affecting regions distant from the subventricular zone or the dentate gyrus. Here we review differences between neural stem cells found in the embryo and the adult, and describe factors that enhance neuronal output from these cells in vivo. Additionally, we review evidence that neural stem cells can be transplanted into injured regions of the adult brain to enhance compensatory neurogenesis from endogenous precursors. Pre-differentiation of neural stem cells into immature neurons prior to transplantation can also aid in functional recovery following injury or disease.

Reactive gliosis in areas around third ventricle in association with epileptogenesis in amygdaloid-kindled rat

Repeated focal electrical stimulation of the brain is known to produce epileptogenesis, and this phenomenon is recognized as kindling. It is also considered to be a model of temporal lobe epilepsy. In the present study, morphological changes in areas around third ventricle in rat brain were examined according to the progression of kindling stage. Very few Glial Fibrillary Acidic Protein (GFAP)-positive astrocytes were present in ependymal cell layer at C0 stage. However, there was a specific increase in GFAP-positive cells in ependymal cell layer at stage C3 as compared to stage C0. Furthermore, GFAP-positive cells showed migration to subependymal zone (SEZ). By stage C5, almost all GFAP-positive cells had migrated to SEZ. While the precise mechanism of this cell migration is not clear, the results suggest a relationship between progression of kindling stage and astrogliosis.

Induction of neurogenesis in the adult rat subventricular zone and neostriatum following dopamine D3 receptor stimulation

Discrete regions of the adult CNS, including the subventricular zone (SVZ), do retain the capacity for neurogenesis. These progenitor cells may represent a potential new source of cells for replacement therapies in neuroregenerative diseases. An understanding of the microenvironmental signals regulating neurogenesis in the adult brain would facilitate the development of such therapeutic approaches. A particularly strong expression of dopamine D(3) receptor mRNA occurs in the proliferative SVZ during prenatal and early postnatal ontogeny. Although its expression diminishes following development, a restricted D(3) receptor expression persists in this region through adulthood, coincident with continued proliferation in this region. Here, we demonstrate a two-fold induction of cell proliferation (BrdU incorporation) in the SVZ and rostral migratory stream of the adult Sprague-Dawley rat brain following intrasubventricular administration of the dopamine D(3) receptor agonist, 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) for 2 weeks. The number of BrdU-positive cells was elevated ten-fold from very low baseline levels in the neighbouring neostriatum, another region known to express D(3) receptors. These striatal BrdU-positive cells appeared within 3 days following intracerebral infusion of 7-OH-DPAT and were distributed homogeneously throughout the striatum following systemic administration. This suggests that these cells originate from resident progenitor cells rather than the SVZ. Dopamine D(3) receptor activation may serve as a proneuronal differentiation signal as 60-70% of the new cells had neuronal markers following 7-OH-DPAT infusion. These results suggest that the dopamine D(3) receptor may be a good drug target for cell replacement strategies, particularly because of the fact that its expression is almost exclusively limited to the nervous system.

Alteration of gene expression by chromosome loss in the postnatal mouse brain

Frequent chromosomal aneuploidy has recently been discovered in normal neurons of the developing and mature murine CNS. Toward a more detailed understanding of aneuploidy and its effects on normal CNS cells, we examined the genomes of cells in the postnatal subventricular zone (SVZ), an area that harbors a large number of neural stem and progenitor cells (NPCs), which give rise to neurons and glia. Here we show that NPCs, neurons, and glia from the SVZ are frequently aneuploid. Karyotyping revealed that approximately 33% of mitotic SVZ cells lost or gained chromosomes in vivo, whereas interphase fluorescence in situ hybridization demonstrated aneuploidy in postnatal-born cells in the olfactory bulb (OB) in vivo, along with neurons, glia, and NPCs in vitro. One possible consequence of aneuploidy is altered gene expression through loss of heterozygosity (LOH). This was examined in a model of LOH: loss of transgene expression in mice hemizygous for a ubiquitously expressed enhanced green fluorescent protein (eGFP) transgene on chromosome 15. Concurrent examination of eGFP expression, transgene abundance, and chromosome 15 copy number demonstrated that a preponderance of living SVZ and OB cells not expressing eGFP lost one copy of chromosome 15; the eGFP transgene was lost in these cells as well. Although gene expression profiling revealed changes in expression levels of several genes relative to GFP-expressing controls, cells with LOH at chromosome 15 were morphologically normal and proliferated or underwent apoptosis at rates similar to those of euploid cells in vitro. These findings support the view that NPCs and postnatal-born neurons and glia can be aneuploid in vivo and functional gene expression can be permanently altered in living neural cells by chromosomal aneuploidy.

Marked differences in olfactory sensitivity and apparent speed of forebrain neuroblast migration in three inbred strains of mice

In the adult forebrain, new neuroblasts constantly migrate from the subventricular zone along the rostral migratory stream to the olfactory bulb, where many become neurons. It is unclear whether this process is different in commonly used mouse strains and whether it is related to olfactory function. Adult male BALB/c, C57BL/6, and 129/S1 (formerly 129SV) mice were tested for olfactory sensitivity plus discrimination, using male mouse urine from the two other strains. BALB/c mice had the greatest olfactory sensitivity, followed by 129/S1, and C57BL/6 mice, by an order of magnitude each. Newly formed cells were pulse-labeled for 3 h with i.p. 5-bromo-2'-deoxyuridine (BrdU) injections and the animals analyzed 24 h later. In 129/S1 mice, a greater proportion of neuroblasts were present closer to the olfactory bulb than in BALB/c mice, followed by C57BL/6 mice. The total number of BrdU-labeled cells did not differ, suggesting differences in migration and not proliferation. The impaired olfactory function in C57BL/6 mice might be caused by the reduced number of neuroblasts that reach the olfactory bulbs. However, olfactory function in BALB/c and 129/S1 mice did not correlate with their putative migration speed, suggesting a more complex nature of cellular processes that contribute to olfactory function. These results caution against comparing studies of olfactory function or neural precursors that use different strains of mice, and question the use of C57BL/6 mice as a "normal" strain or as transgenic background. Perhaps more importantly, the results point to an opportunity to identify genes that regulate olfactory function and neuroblast behavior.

Neural stem cells: an overview

This review focuses on the nature and functional properties of stem cells of the adult mammalian central nervous system (CNS). It has recently been shown that cell turnover, including neurons, does occur in the mature CNS, thanks to the persistence of precursor cells that possess the functional characteristics of bona-fide neural stem cells (NSCs) within restricted brain areas. We discuss how the subventricular zone of the forebrain (SVZ) is the most active neurogenetic area and the richest source of NSCs. These NSCs ensure a life-long contribution of new neurons to the olfactory bulb and, when placed in culture, can be grown and extensively expanded for months, allowing the generation of stem cell lines, which maintain stable and constant functional properties. A survey of the differentiation potential of these NSCs, both in vitro and in vivo, outlines their extreme plasticity that seems to outstretch the brain boundaries, so that these neuroectodermal stem cells may give rise to cells that derive from developmentally distinct tissues. A critical discussion of the latest, controversial findings regarding this surprising phenomenon is provided.

Effects of single low dose irradiation on subventricular zone cells in juvenile rat brain

Although the juvenile human brain is relatively radioresistant, irradiation can result in brain growth retardation, progressive mental disturbance, and neurologic abnormalities. As neural stem cells or progenitor cells may be a target of radiation injury and may play an important role in the brain's functional recovery, we examined the effects of whole brain irradiation on these cells in juvenile rat. Six-week-old Wistar rats, where the brain is still growing, were irradiated with single doses of 1, 2, or 3 Gy X-ray. We measured their body and brain weights at 30 or 60 days after irradiation. The chronological changes of the subventricular zone (SVZ) were examined at 6 h, 2, 7, 14, 30, or 60 days after irradiation by immunohistochemistry, specifically looking at the neural stem cells or progenitor cells using anti-nestin antibodies specific for these cells. The rate of brain weight gain of irradiated rats significantly decreased in comparison to controls, although that of body weight gain was similar among them. Multiple apoptotic cells appeared in the SVZ at 6 h after irradiation with simultaneous reduction in nestin-positive cells (69% of the control). The cell levels recovered within a week, with the nestin-positive cells reaching maximal numbers (182%) on Day 14. Nestin-positive cells returned to baseline levels within 30 days (96%) and remained unchanged for the subsequent 60 days. The X-ray dosage did not affect these findings. Our findings revealed that single low dose X-ray administration reversibly affected the levels of neural stem and progenitor cells in the SVZ region. These results suggest that continuous multiple administrations of X-rays in clinical treatment may affect irreversible changes on neural stem or progenitor cells, causing brain growth retardation, or dysfunction.

In vivo infusions of exogenous growth factors into the fourth ventricle of the adult mouse brain increase the proliferation of neural progenitors around the fourth ventricle and the central canal of the spinal cord

Stem cells isolated from the fourth ventricle and spinal cord form neurospheres in vitro in response to basic fibroblast growth factor (FGF2)+heparin (H) or epidermal growth factor (EGF)+FGF2 together. To determine whether these growth factor conditions are sufficient to induce stem cells within the fourth ventricle and spinal cord to proliferate and expand their progeny in vivo, we infused EGF and FGF2, alone or together, with or without H, into the fourth ventricle for 6 days via osmotic minipumps. Animals were injected with bromodeoxyuridine (BrdU) on days 4, 5 and 6 of infusion in order to label cells proliferating in response to the growth factors. Infusions of EGF+FGF2+H into the fourth ventricle resulted in the largest proliferative effect, a 10.8-fold increase in the number of BrdU+ cells around the fourth ventricle, and a 33.5-fold increase in the number of BrdU+ cells around the central canal of the spinal cord, as compared to vehicle infused controls. The majority of the cells were nestin+ after 6 days of infusion. Seven weeks post-infusion, 22 and 30% of the number of BrdU+ cells induced to proliferate after 6 days of EGF+FGF2+H infusions were still detected around the fourth ventricle and central canal of the spinal cord, respectively. Analysis of the fates of the remaining cells showed that a small percentage of BrdU+ cells around the fourth ventricle and in the white matter of the spinal cord differentiated into astrocytes and oligodendrocytes. BrdU+ neurons were not found in the brainstem or in the grey matter of the cervical spinal cord 7 weeks post-infusion. These results show that endogenous stem cells and progenitors around the fourth ventricle and central canal of the spinal cord proliferate in response to exogenously applied growth factors, but unlike in the lateral ventricle where they generate some new neurons, they only produce new astrocytes and oligodendrocytes at 7 weeks post-infusion.

PTEN in neural precursor cells: regulation of migration, apoptosis, and proliferation

PTEN is a lipid phosphatase, and PTEN mutations are associated with gliomas, macrocephaly, and mental deficiencies. We have used PTEN +/- mice to assess PTEN's role in subventricular zone (SVZ) precursor cells. For cultured SVZ neurosphere cells, haploinsufficiency for PTEN increases phosphorylation of Akt and forkhead transcription factor and slightly enhances proliferation. Based on a filter penetration assay, PTEN +/- cells are substantially more migratory and invasive than +/+ cells. The +/- cells also are more resistant to H(2)O(2)-induced apoptosis. Analysis of PTEN +/- and +/+ mice by BrdU labeling reveals no difference in the rate of cell proliferation in the SVZ. Exit of BrdU-labeled cells from the SVZ and radial migration to the outer layers of the olfactory bulb are more rapid for +/- cells. These observations indicate that PTEN regulates SVZ precursor cell function and is particularly important for migration and apoptosis in response to oxidative stress.

Multipotent neural stem cells reside into the rostral extension and olfactory bulb of adult rodents

The lateral walls of the forebrain lateral ventricles are the richest source of stem cells in the adult mammalian brain. These stem cells give rise to new olfactory neurons that are renewed throughout life. The neurons originate in the subventricular zone (SVZ), migrate within the rostral extension (RE) of the SVZ along the rostral migratory stream (RMS) within tube-like structures formed of glial cells, to eventually reach the olfactory bulb (OB). We demonstrate that, contrary to the current view, multipotential (neuronal-astroglial-oligodendroglial) precursors with stem cell features can be isolated not only from the SVZ but also from the entire RE, including the distal portion within the OB. Specifically, these stem cells do not derive from the migratory neuroblasts coming from the SVZ. Interestingly, stem cells isolated from the proximal RE generate significantly more oligodendrocytes, and those from the distal RE proliferate significantly more slowly than stem cells derived from the SVZ and other RE regions. These findings demonstrate that stem cells are not confined to the forebrain periventricular region and indicate that stem cells endowed with different functional characteristics occur at different levels of the SVZ-RE pathway.

NMDA receptor regulation of cell death in the rat olfactory bulb

Cell death is widespread in the developing nervous system and is under complex regulation by numerous intra- and intercellular mechanisms. Blockade of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor has been shown to promote cell death in the developing brain (Ikonomidou et al., 1999), suggesting that afferent functional activation is an important regulator of cell survival. The olfactory bulb, the first central relay for olfactory information from the nose, is well suited for examining the role of afferent activity in neuronal development. Functional deprivation is easily performed by surgical blockade of airflow to one side of the nasal passage, which results in dramatic alterations in postnatal development of the bulb (Brunjes, 1994), including enhanced neuronal loss (Frazier and Brunjes, 1988; Najbauer and Leon, 1995). The present report examined the specific role of NMDA receptor activation in regulating cell survival within the rat bulb. Pharmacological blockade of receptors with the noncompetitive channel blocker MK-801 (3 x 0.5 mg/kg i.p.) resulted in profound increases in cell death within 24 h. Furthermore, in contrast to other regions, where the effects of receptor blockade were confined to the first 2 postnatal weeks (Ikonomidou et al., 1999), enhancement of cell death was seen in the deeper granule cell-containing regions of the bulb with injections as late as postnatal day 28. In addition, the effects of MK-801 were much more dramatic than those seen after unilateral naris closure, suggesting that NMDA receptor activation may mediate additional survival pathways in the bulb beyond that provided by first nerve input.

Expression of active caspase-3 in mitotic and postmitotic cells of the rat forebrain

Active caspase-3 immunoreactivity was detected in the rat forebrain proliferative regions at birth and remained high in these areas for about 2 weeks, during which period labeled cells were present centroperipherally across the olfactory bulb. By the end of the third postnatal week, only a small number of immunolabeled cells remained in these forebrain structures. Active caspase-3 immunolabeling was localized mostly to cell nuclei and co-localized partially with TuJ1 and NeuN immunoreactivity, but not with glial fibrially acidic protein, OX-42, gamma-aminobutyric acid, or terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL)-positive labeling. Active caspase-3 and 5-bromo-2'-deoxyuridine (BrdU) double-labeled nuclei were seen in the proliferative regions after 2 hours and in the periglomerular region of the bulb after 7 days following BrdU injections. Examination of the cells with electron microscopy confirmed that the active caspase-3-containing nuclei in the proliferative regions often had infoldings and appeared to be undergoing division. Some of the cells with active caspase-3-labeled nuclei in the bulb had synapses on their somata or dendrites. Labeled dendritic spines and a few axon terminals were also observed in the olfactory bulb. Taken together, it appears that a wave of active caspase-3-positive cells are dividing in the proliferative zones and then migrating to the bulb as they differentiate into neurons. Therefore, active caspase-3 may play a role in cellular processes such as neuronal differentiation, migration, and plasticity, in addition to its role in cell death.

Vascular niche for adult hippocampal neurogenesis

The thin lamina between the hippocampal hilus and granule cell layer, or subgranule zone (SGZ), is an area of active proliferation within the adult hippocampus known to generate new neurons throughout adult life. Although the neuronal fate of many dividing cells is well documented, little information is available about the phenotypes of cells in S-phase or how the dividing cells might interact with neighboring cells in the process of neurogenesis. Here, we make the unexpected observation that dividing cells are found in dense clusters associated with the vasculature and roughly 37% of all dividing cells are immunoreactive for endothelial markers. Most of the newborn endothelial cells disappear over several weeks, suggesting that neurogenesis is intimately associated with a process of active vascular recruitment and subsequent remodeling. The present data provide the first evidence that adult neurogenesis occurs within an angiogenic niche. This environment may provide a novel interface where mesenchyme-derived cells and circulating factors influence plasticity in the adult central nervous system.

Cortical neurogenesis in adult rats after reversible photothrombotic stroke

Neurogenesis occurs throughout life in the dentate gyrus of hippocampus and subventricular zone, but this phenomenon has rarely been observed in other brain regions of adult mammals. The aim of the current study was to investigate the cell proliferation process in the ischemically challenged region-at-risk after focal cerebral ischemia in the adult rat brain. A reversible photothrombotic ring stroke model was used, which features sustained hypoperfusion followed by late spontaneous reperfusion and a remarkable morphologic tissue recovery in the anatomically well defined somatosensory cortical region-at-risk. Twelve-week-old male Wistar rats received repeated intraperitoneal injections of the cell proliferation specific marker 5-bromodeoxyuridine (BrdU) after stroke induction. Immunocytochemistry of coronal brain sections revealed that the majority of BrdU-positive cells were of glial, macrophage, and endothelial origin, whereas 3% to 6% of the BrdU-positive cells were double-labeled by BrdU and the neuronspecific marker Map-2 at 7 and 100 days after stroke onset in the region-at-risk. They were distributed randomly in cortical layers II-VI. Three-dimensional confocal analyses of BrdU and the neuronal-specific marker Neu N by double immunofluorescence confirmed their colocalization within the same cells at 72 hours and 30 days after stroke induction. This study suggests that, as a potential pathway for brain repair, new neurons can be generated in the cerebral cortex of adult rats after sublethal focal cerebral ischemia.

Physiological consequences of morphologically detectable synaptic plasticity: potential uses for examining recovery following damage

A growing literature indicates that brain structure is modified in various ways with experience. In this paper we briefly survey evidence that the brain retains the capacity to modify its organization in response to demands, including demands resulting from learning, throughout the lifetime. We attempt to address whether these experience-induced changes are accompanied by physiological changes that indicate a functional reorganization of the brain. The kinds of morphological changes that have been observed following brain injury appear to be very similar to those seen after learning. The similarity suggests that many of the basic mechanisms of synaptic change in the brain may be utilized for both functions. This suggests that we can take advantage of some of the methods used to test the changes in physiology with behavioral manipulations to examine the damaged brain. We advocate utilizing electrophysiological techniques to measure functional recovery from brain injury as these may be useful in evaluating both spontaneous recovery from damage and the therapeutic benefits of training, or other therapies.

A cell-survival factor (N-acetyl-L-cysteine) alters the in vivo fate of constitutively proliferating subependymal cells in the adult forebrain

The adult mouse brain contains a population of constitutively proliferating subependymal cells that surround the lateral ventricle and are the direct progeny of the neural stem cell. Constitutively proliferating cells divide rapidly; 6 days after labeling, 60% of their progeny undergo cell death, 25% migrate to the olfactory bulbs, and 15% continue to proliferate within the subependyma. We have intraventricularly infused a cell survival factor N-acetyl-L-cysteine (NAC), which is known to have survival effects without concomitant proliferative effects on cells in vitro, and examined the resulting fate of cells spared from the normally occurring cell death. NAC infusion for 5 days results in a five-fold increase in the number of retrovirally labeled subependymal cells compared to saline-infused controls. The increase in the number of subependymal cells is directly proportional to the amount of time during which NAC is present and is not due to increased proliferation. While NAC is able to keep all the normally dying progeny alive, the cells spared from death remain confined to the subependyma lining the lateral ventricles and do not migrate to the olfactory bulbs (one normal fate of constitutively proliferating progeny) or into the surrounding brain parenchyma. When animals survive for an additional 6 days following NAC infusion, the number of retrovirally labeled subependymal cells returns to control values, indicating that the continued presence of NAC is necessary for cell survival. These data suggest that preventing cell death is not sufficient to keep all of the progeny of these cells in a proliferative mode.