Epidermal growth factor and fibroblast growth factor-2 have different effects on neural progenitors in the adult rat brain

Neurogenesis in dentate subgranular zone and rostral subventricular zone after focal cerebral ischemia in the rat

Because neurogenesis persists in the adult mammalian brain and can be regulated by physiological and pathological events, we investigated its possible involvement in the brain's response to focal cerebral ischemia. Ischemia was induced by occlusion of the middle cerebral artery in the rat for 90 min, and proliferating cells were labeled with 5-bromo-2'-deoxyuridine-5'-monophosphate (BrdUrd) over 2-day periods before sacrificing animals 1, 2 or 3 weeks after ischemia. Ischemia increased the incorporation of BrdUrd into cells in two neuroproliferative regions-the subgranular zone of the dentate gyrus and the rostral subventricular zone. Both effects were bilateral, but that in the subgranular zone was more prominent on the ischemic side. Cells labeled with BrdUrd coexpressed the immature neuronal markers doublecortin and proliferating cell nuclear antigen but did not express the more mature cell markers NeuN and Hu, suggesting that they were nascent neurons. These results support a role for ischemia-induced neurogenesis in what may be adaptive processes that contribute to recovery after stroke.

The subventricular zone: source of neuronal precursors for brain repair

The subventricular zone (SVZ) is a major germinal zone which persists in the adult brain. The SVZ contains cells that self renew and continuously produce new neurons and glia. In this chapter we discuss the development, architecture and function of the adult SVZ, as well as the fate of SVZ cells after transplantation. We focus on identification of neural stem cells, factors which regulate neurogenesis and mechanisms for neuronal migration through the adult brain. Detailed understanding of these processes is necessary to utilize the SVZ as a source of neuronal and glial precursors for genetic manipulation, transplantation or brain self repair.

Activity-dependent regulation of neuronal plasticity and self repair

Plasticity is an essential characteristic of the brain: it is part of how the brain functions and is continuous while the brain interacts with the outer world. The state of activation and the level of activity of the entire organism affect the brain's plastic response. Brain plasticity has many substrates, ranging from synapses to neurites and entire cells. The production of new neurons is part of plasticity even in the adult and old brain, but under normal conditions neurogenesis only occurs in two privileged regions of the adult brain: hippocampus and olfactory system. At least in the hippocampus, physical activity stimulates neurogenesis by acting on the proliferation of neuronal stem cells. More specific functions such as learning may be able to recruit new neurons from the pool of cells with neurogenic potential. In a broader context neuronal stem cells can likely be found throughout the brain. Therefore, novel approaches to neuroregeneration will, when most effective, make use of the activity-related effects on neuronal stem cells in the adult brain to activate these stem cells in a targeted manner to enhance brain function.

Activation of phosphatidylinositol-3 kinase (PI-3K) and extracellular regulated kinases (Erk1/2) is involved in muscarinic receptor-mediated DNA synthesis in neural progenitor cells

Muscarinic acetylcholine receptor (mAChR), a member of the G-protein-coupled receptors (GPCRs) gene superfamily, has been shown to mediate the effects of acetylcholine on differentiation and proliferation in the CNS. However, the mechanism or mechanisms whereby mAChRs regulate cell proliferation remain poorly understood. Here we show that in vitro bFGF-expanded neural progenitor cells dissociated from rat cortical neuroepithelium express muscarinic acetylcholine receptor subtype mRNAs. We demonstrate that stimulation of these mAChRs with carbachol, a muscarinic agonist, activated extracellular-regulated kinases (Erk1/2) and phosphatidylinositol-3 kinase (PI-3K). This, in turn, stimulated DNA synthesis in neural progenitor cells. MEK inhibitor PD98059 and PI-3K inhibitors wortmannin and LY294002 inhibited a carbachol-induced increase in DNA synthesis. These findings indicate that the activation of both PI-3 kinase and MEK signaling pathways via muscarinic receptors is involved in stimulating DNA synthesis in the neural progenitor cells during early neurogenesis.

Ataxia telangiectasia mutated is essential during adult neurogenesis

Ataxia telangiectasia (A-T) is an autosomal recessive disease characterized by normal brain development followed by progressive neurodegeneration. The gene mutated in A-T (ATM) is a serine protein kinase implicated in cell cycle regulation and DNA repair. The role of ATM in the brain and the consequences of its loss on neuronal survival remain unclear. We studied the role of ATM in adult neural progenitor cells in vivo and in vitro to define the role of ATM in dividing and postmitotic neural cells from Atm-deficient (Atm(-/-)) mice in a physiologic context. We demonstrate that ATM is an abundant protein in dividing neural progenitor cells but is markedly down-regulated as cells differentiate. In the absence of ATM, neural progenitor cells of the dentate gyrus show abnormally high rates of proliferation and genomic instability. Atm(-/-) cells in vivo, and in cell culture, show a blunted response to environmental stimuli that promote neural progenitor cell proliferation, survival, and differentiation along a neuronal lineage. This study defines a role for ATM during the process of neurogenesis, demonstrates that ATM is required for normal cell fate determination and neuronal survival both in vitro and in vivo, and points to a mechanism for neuronal cell loss in progressive neurodegenerative diseases.

Comparison of neural precursor cell fate in second trimester human brain and spinal cord

Neural transplantation holds promise for the treatment of traumatic brain and spinal cord injury by replacing lost cellular elements as well as repairing neural damage. Fetal human stem cells derived from central nervous system (CNS) tissue are potential transplantable sources for all cell types found in the mature human nervous system including neurons, astrocytes and oligodendroglia. Although nearly all areas of the fetal human neuraxis contain undifferentiated neural precursor cells, the phenotypic fate of the daughter cells might vary from one region to another during a specific developmental period. The purpose of this study was to compare the various cell types derived from neural precursors cultured from second trimester fetal human brain and spinal cord. To this end, brains (n = 8) and spinal cords (n = 8) of 15-24 week fetuses were dissociated and grown in culture medium supplemented with epidermal growth factor (EGF), basic fibroblast growth factor (FGF) and leukemia inhibitory factor (LIF). The proliferating precursor cells from both brain and spinal cord grew as spherical masses that were plated on laminin-coated dishes after seven days in culture. During the next 5-7 days, the cells that emerged from these spheres were fixed and processed for immunocytochemistry. Brain derived spheres gave rise to cells expressing antigens specific for neurons (MAP-2ab and neuron specific-intermediate filaments), astrocytes (GFAP) and oligodendrocytes (A007). In contrast, cells that emerged from spinal cord derived spheres were only immunoreactive for GFAP. These data suggest that neuroepithelial precursor cells from different CNS regions, although similar in their responsiveness to proliferative growth factors, might differ in their ability to generate different cell types in the adult CNS.

Increased number of BrdU-labeled neurons in the rostral migratory stream of the estrous prairie vole

In the mammalian forebrain, most neurons originate from proliferating cells in the ventricular zone lining the lateral ventricles, including a discrete area of the subventricular zone in which neurogenesis continues into adulthood. The majority of the cells generated in the anterior portion of the subventricular zone (SVZa) are neuronal precursors with progeny that migrate to the olfactory bulb (OB) along a pathway known as the rostral migratory stream (RMS). The list of factors that influence the proliferation and survival of neurons in the adult brain remains incomplete, but previous studies have implicated neurotrophins in mammals and estrogen in birds. This study examined the effect of estrus induction on the proliferation of SVZa neurons in female prairie voles. Prairie voles, unlike many other rodents, are induced into estrus by chemosensory cues from a male. This olfactory-mediated process results in an increase in serum estrogen levels and the consequent induction of behavioral estrus (sexual receptivity). Female prairie voles induced into estrus by male exposure had a 92% increase in BrdU-labeled cells in the SVZa compared to females exposed to a female. Double-label immunocytochemical studies demonstrated that 80% of the BrdU-labeled cells in the RMS displayed a neuronal phenotype. Ovariectomized females exposed to a male did not show an increase in serum estrogen or BrdU labeling in the RMS. Conversely, ovariectomized females injected with estrogen were sexually receptive and had more BrdU-labeled cells in the RMS than oil-injected females. These data suggest that, in female prairie voles, estrus induction is associated with increased numbers of dividing cells in the RMS, possibly via an estrogen-mediated process.

Neuroprotection by estradiol

This review highlights recent evidence from clinical and basic science studies supporting a role for estrogen in neuroprotection. Accumulated clinical evidence suggests that estrogen exposure decreases the risk and delays the onset and progression of Alzheimer's disease and schizophrenia, and may also enhance recovery from traumatic neurological injury such as stroke. Recent basic science studies show that not only does exogenous estradiol decrease the response to various forms of insult, but the brain itself upregulates both estrogen synthesis and estrogen receptor expression at sites of injury. Thus, our view of the role of estrogen in neural function must be broadened to include not only its function in neuroendocrine regulation and reproductive behaviors, but also to include a direct protective role in response to degenerative disease or injury. Estrogen may play this protective role through several routes. Key among these are estrogen dependent alterations in cell survival, axonal sprouting, regenerative responses, enhanced synaptic transmission and enhanced neurogenesis. Some of the mechanisms underlying these effects are independent of the classically defined nuclear estrogen receptors and involve unidentified membrane receptors, direct modulation of neurotransmitter receptor function, or the known anti-oxidant activities of estrogen. Other neuroprotective effects of estrogen do depend on the classical nuclear estrogen receptor, through which estrogen alters expression of estrogen responsive genes that play a role in apoptosis, axonal regeneration, or general trophic support. Yet another possibility is that estrogen receptors in the membrane or cytoplasm alter phosphorylation cascades through direct interactions with protein kinases or that estrogen receptor signaling may converge with signaling by other trophic molecules to confer resistance to injury. Although there is clear evidence that estradiol exposure can be deleterious to some neuronal populations, the potential clinical benefits of estrogen treatment for enhancing cognitive function may outweigh the associated central and peripheral risks. Exciting and important avenues for future investigation into the protective effects of estrogen include the optimal ligand and doses that can be used clinically to confer benefit without undue risk, modulation of neurotrophin and neurotrophin receptor expression, interaction of estrogen with regulated cofactors and coactivators that couple estrogen receptors to basal transcriptional machinery, interactions of estrogen with other survival and regeneration promoting factors, potential estrogenic effects on neuronal replenishment, and modulation of phenotypic choices by neural stem cells.

Pluripotent stem cells engrafted into the normal or lesioned adult rat spinal cord are restricted to a glial lineage

Proliferating populations of undifferentiated neural stem cells were isolated from the embryonic day 14 rat cerebral cortex or the adult rat subventricular zone. These cells were pluripotent through multiple passages, retaining the ability to differentiate in vitro into neurons, astrocytes, and oligodendrocytes. Two weeks to 2 months after engraftment of undifferentiated, BrdU-labeled stem cells into the normal adult spinal cord, large numbers of surviving cells were seen. The majority of the cells differentiated with astrocytic phenotype, although some oligodendrocytes and undifferentiated, nestin-positive cells were detected; NeuN-positive neurons were not seen. Labeled cells were also engrafted into the contused adult rat spinal cord (moderate NYU Impactor injury), either into the lesion cavity or into the white or gray matter both rostral and caudal to the injury epicenter. Up to 2 months postgrafting, the majority of cells either differentiated into GFAP-positive astrocytes or remained nestin positive. No BrdU-positive neurons or oligodendrocytes were observed. These results show robust survival of engrafted stem cells, but a differentiated phenotype restricted to glial lineages. We suggest that in vitro induction prior to transplantation will be necessary for these cells to differentiate into neurons or large numbers of oligodendrocytes.

Adult spinal cord stem cells generate neurons after transplantation in the adult dentate gyrus

The adult rat spinal cord contains cells that can proliferate and differentiate into astrocytes and oligodendroglia in situ. Using clonal and subclonal analyses we demonstrate that, in contrast to progenitors isolated from the adult mouse spinal cord with a combination of growth factors, progenitors isolated from the adult rat spinal cord using basic fibroblast growth factor alone display stem cell properties as defined by their multipotentiality and self-renewal. Clonal cultures derived from single founder cells generate neurons, astrocytes, and oligodendrocytes, confirming the multipotent nature of the parent cell. Subcloning analysis showed that after serial passaging, recloning, and expansion, these cells retained multipotentiality, indicating that they are self-renewing. Transplantation of an in vitro-expanded clonal population of cells into the adult rat spinal cord resulted in their differentiation into glial cells only. However, after heterotopic transplantation into the hippocampus, transplanted cells that integrated in the granular cell layer differentiated into cells characteristic of this region, whereas engraftment into other hippocampal regions resulted in the differentiation of cells with astroglial and oligodendroglial phenotypes. The data indicate that clonally expanded, multipotent adult progenitor cells from a non-neurogenic region are not lineage-restricted to their developmental origin but can generate region-specific neurons in vivo when exposed to the appropriate environmental cues.

What role(s) for TGFalpha in the central nervous system?

Transforming growth factor alpha (TGFalpha) is a member of the epidermal growth factor (EGF) family with which it shares the same receptor, the EGF receptor (EGFR or erbB1). Identified since 1985 in the central nervous system (CNS), its functions in this organ have started to be determined during the past decade although numerous questions remain unanswered. TGFalpha is widely distributed in the nervous system, both glial and neuronal cells contributing to its synthesis. Although astrocytes appear as its main targets, mediating in part TGFalpha effects on different neuronal populations, results from different studies have raised the possibility for a direct action of this growth factor on neurons. A large array of experimental data have thus pointed to TGFalpha as a multifunctional factor in the CNS. This review is an attempt to present, in a comprehensive manner, the very diverse works performed in vitro and in vivo which have provided evidences for (i) an intervention of TGFalpha in the control of developmental events such as neural progenitors proliferation/cell fate choice, neuronal survival/differentiation, and neuronal control of female puberty onset, (ii) its role as a potent regulator of astroglial metabolism including astrocytic reactivity, (iii) its neuroprotective potential, and (iv) its participation to neuropathological processes as exemplified by astroglial neoplasia. In addition, informations regarding the complex modes of TGFalpha action at the molecular level are provided, and its place within the large EGF family is precised with regard to the potential interactions and substitutions which may take place between TGFalpha and its kindred.

Neural consequences of environmental enrichment

Neuronal plasticity is a central theme of modern neurobiology, from cellular and molecular mechanisms of synapse formation in Drosophila to behavioural recovery from strokes in elderly humans. Although the methods used to measure plastic responses differ, the stimuli required to elicit plasticity are thought to be activity-dependent. In this article, we focus on the neuronal changes that occur in response to complex stimulation by an enriched environment. We emphasize the behavioural and neurobiological consequences of specific elements of enrichment, especially exercise and learning.

In vivo regulation of precursor cells in the subventricular zone of adult rat brain by thyroid hormone and retinoids

The mature central nervous system contains precursor cells in the subventricular zone of the lateral ventricle. In this study we examined the possibility to affect fate of precursor cells through exogenous manipulations. The results indicate that administration of thyroid hormone and retinoic acid increases the expression of Ki67, a nuclear antigen associated with cell proliferation, and of nestin, a marker protein for precursor cells in the subventricular zone of adult male rats. Moreover, retinoic acid increases polysialated-neural cell adhesion molecules (PSA-NCAM)-immunoreactivity. These data suggest that nuclear receptor ligands are potential candidates for fate determination of precursor cells in the subventricular zone also in the adult brain.

Noggin antagonizes BMP signaling to create a niche for adult neurogenesis

Large numbers of new neurons are born continuously in the adult subventricular zone (SVZ). The molecular niche of SVZ stem cells is poorly understood. Here, we show that the bone morphogenetic protein (BMP) antagonist Noggin is expressed by ependymal cells adjacent to the SVZ. SVZ cells were found to express BMPs as well as their cognate receptors. BMPs potently inhibited neurogenesis both in vitro and in vivo. BMP signaling cell-autonomously blocked the production of neurons by SVZ precursors by directing glial differentiation. Purified mouse Noggin protein promoted neurogenesis in vitro and inhibited glial cell differentiation. Ectopic Noggin promoted neuronal differentiation of SVZ cells grafted to the striatum. We thus propose that ependymal Noggin production creates a neurogenic environment in the adjacent SVZ by blocking endogenous BMP signaling.

Disruption of Eph/ephrin signaling affects migration and proliferation in the adult subventricular zone

The subventricular zone (SVZ) of the lateral ventricles, the largest remaining germinal zone of the adult mammalian brain, contains an extensive network of neuroblasts migrating rostrally to the olfactory bulb. Little is known about the endogenous proliferation signals for SVZ neural stem cells or guidance cues along the migration pathway. Here we show that the receptor tyrosine kinases EphB1-3 and EphA4 and their transmembrane ligands, ephrins-B2/3, are expressed by cells of the SVZ. Electron microscopy revealed ephrin-B ligands associated with SVZ astrocytes, which function as stem cells in this germinal zone. A three-day infusion of the ectodomain of either EphB2 or ephrin-B2 into the lateral ventricle disrupted migration of neuroblasts and increased cell proliferation. These results suggest that Eph/ephrin signaling is involved in the migration of neuroblasts in the adult SVZ and in either direct or indirect regulation of cell proliferation.

FGF-2-responsive neural stem cell proliferation requires CCg, a novel autocrine/paracrine cofactor

We have purified and characterized a factor, from the conditioned medium of neural stem cell cultures, which is required for fibroblast growth factor 2's (FGF-2) mitogenic activity on neural stem cells. This autocrine/paracrine cofactor is a glycosylated form of cystatin C (CCg), whose N-glycosylation is required for its activity. We further demonstrated that, both in vitro and in vivo, neural stem cells undergoing cell division are immunopositive for cystatin C. Finally, we showed in vivo functional activity of CCg by demonstrating that the combined delivery of FGF-2 and CCg to the adult dentate gyrus stimulated neurogenesis. We propose that the process of neurogenesis is controlled by the cooperation between trophic factors and autocrine/paracrine cofactors, of which CCg is a prototype.

Tumor necrosis factor-alpha modulates the proliferation of neural progenitors in the subventricular/ventricular zone of adult rat brain

Little is known about the response of neural progenitors to inflammation following injuries of the central nervous system. In combination with bromodeoxyuridine (BrdU) intraperitoneally (i.p.) injected, tumor necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine that increased ED1+ activated microglia/macrophage population at injured sites, was administrated into adult rat brains. No difference in the immunostaining for proliferating cell nuclear antigen (PCNA) was observed in the subventricular/ventricular zone (SVZ/VZ) between TNF-alpha injected sites and controls. However, BrdU+ cells were apparently observed in the SVZ/VZ proximal to TNF-alpha injected site, and the number of BrdU+ cells increased at 6 and 24 h post injection. Since cell apoptosis was rarely found in the SVZ/VZ after TNF-alpha injection, these observations suggest that the diffusible TNF-alpha may directly and/or indirectly modulate the proliferation of neural progenitors.

Therapeutic potential of neurotrophic factors and neural stem cells against ischemic brain injury

Development of neuronal and glial cells and their maintenance are under control of neurotrophic factors (NTFs). An exogenous administration of NTFs protects extremely sensitive brain tissue from ischemic damage. On the other hand, it is now known that neural stem cells are present in normal adult brain, and have a potential to compensate and recover neural functions that were lost due to ischemic stroke. These stem cells are also under control of NTFs to differentiate into a certain species of neural cells. Thus, the purpose of this review is to summarize the present understanding of the role of NTFs in normal and ischemic brain and the therapeutic potential of NTF protein itself or gene therapy, and then to summarize the role of NTFs in stem cell differentiation and a possible therapeutic potential with the neural stem cells against ischemic brain injury.

EGF infusion stimulates the proliferation and migration of embryonic progenitor cells transplanted in the adult rat striatum

Immature progenitor cells (generated by in vitro propagation) may provide a useful alternative to primary cells (from dissected embryonic tissue) for transplantation if their migratory and proliferative and differentiation properties can be controlled and directed in vivo. In this study E15 murine EGF-responsive progenitor cells were transplanted to the striatum of adult rats. Simultaneously, these animals received continuous infusion of either epidermal growth factor (EGF) or vehicle, to the lateral ventricle, for 8 days. In animals that received EGF, the transplanted progenitors migrated toward the lateral ventricle and proliferated, as evidenced by bromodeoxyuridine incorporation. Progenitor cells transplanted to rats that received vehicle infusions showed neither of these responses. In all animals, transplanted progenitors expressed an immature astrocyte or oligodendrocyte phenotype, the majority of cells being astrocytes. We conclude that EGF stimulates the migration and proliferation of murine progenitor cells in vivo, either directly or indirectly, but does not influence their phenotypic differentiation.

Stem Cells and Neuronal Progenitors and Their Diversity in the CNS: Are Time and Place Important?

Stem cells are multilineage progenitor cells that are capable of self-regenerating and giving rise to different cell types. The proper assembly of the CNS into functionally relevant circuits requires that stem cells produce the right types of cells in the right number and position at the appropriate time. We suggest that the positional specification of stem cells is provided by the pattern of expression of early transcriptional regulators along the body axes. These mechanisms restrict the competence of stem cells to programming a local cellular repertoire. Conversely, we argue that the specification of different cell types in the appropriate number and sequence is independently carried out within CNS domains by subprograms that progressively change the intrinsic properties of the stem cells. Temporal changes in proliferation and differentiation of stem cells are controlled by cascades of extracellular signals and basic helix-loop-helix (bHlH) transcription factors. These regulators in turn may activate homeodomain transcription factors with more restricted effector functions. Fibroblast growth factors (FGF) are among the earliest acting signals providing local changes in growth within the developing CNS. Basic FGF (FGF2) increases the proliferation of either stem cells or their immediate progeny, increasing the number of founder cells in the developing cerebral cortex.

Neurons from stem cells: Implications for understanding nervous system development and repair

Neurodegenerative diseases cost the economies of the developed world billions of dollars per annum. Given ageing population profiles and the increasing extent of this problem, there has been a surge of interest in neural stem cells and in neural differentiation protocols that yield neural cells for therapeutic transplantation. Due to the oncogenic potential of stem cells a better characterisation of neural differentiation, including the identification of new neurotrophic factors, is required. Stem cell cultures undergoing synchronous in vitro neural differentiation provide a valuable resource for gene discovery. Novel tools such as microarrays promise to yield information regarding gene expression in stem cells. With the completion of the yeast, C. elegans, Drosophila, human, and mouse genome projects, the functional characterisation of genes using genetic and bioinformatic tools will aid in the identification of important regulators of neural differentiation.Key words: neural differentiation, neural precursor cell, brain repair, central nervous system repair, CNS.

Neurogenesis in the dentate gyrus of the rat following electroconvulsive shock seizures

Electroconvulsive shock (ECS) seizures provide an animal model of electroconvulsive therapy (ECT) in humans. Recent evidence indicates that repeated ECS seizures can induce long-term structural and functional changes in the brain, similar to those found in other seizure models. We have examined the effects of ECS on neurogenesis in the dentate gyrus of the adult rat using bromodeoxyuridine (BrdU) immunohistochemistry, which identifies newly generated cells. Cells have also been labeled for neuronal nuclear protein (NeuN) to identify neurons. One month following eight ECS seizures, ECS-treated rats had approximately twice as many BrdU-positive cells as sham-treated controls. Eighty-eight percent of newly generated cells colabeled with NeuN in ECS-treated subjects, compared to 83% in sham-treated controls. These data suggest that there is a net increase in neurogenesis within the hippocampal dentate gyrus following ECS treatment. Similar increases have been reported following kindling and kainic acid- or pilocarpine-induced status epilepticus. Increased neurogenesis appears to be a general response to seizure activity and may play a role in the therapeutic effects of ECT.

FGF-2 Up-regulation and proliferation of neural progenitors in the regenerating amphibian spinal cord in vivo

Regeneration of the spinal cord occurs spontaneously in adult urodele amphibians. The key cells in this regenerative process appear to be the ependymal cells that following injury migrate and proliferate to form the ependymal tube from which the spinal cord regenerates. Very little is known about the signal(s) that initiates and maintains the proliferative response of these cells. Fibroblast growth factor 2 (FGF-2) has been shown to play a role in maintaining neural progenitor cell cycling in vitro and may be important for neuronal survival and axonal growth after injury. We have investigated its role in regeneration of the spinal cord in vivo following tail amputation in the adult salamander, Pleurodeles waltl. We show that only the low-molecular-weight form of FGF-2 is found in Pleurodeles and that in the normal cord it is expressed in a subset of neurons, but is hardly detectable in ependymal cells. Tail amputation results in induction of FGF-2 in the ependymal cells of the regenerating structure, and later in regeneration FGF-2 is up-regulated in some newborn neurons. FGF-2 pattern of expression in the ependymal tube parallels that of proliferation. Furthermore, exogenous FGF-2 significantly increases ependymal cell proliferation in vivo. Overall our results strongly support the view that one important role of FGF-2 during spinal cord regeneration in Pleurodeles is to induce proliferation of neural progenitor cells.

Analysis of neurogenesis and programmed cell death reveals a self-renewing capacity in the adult rat brain

The adult central nervous system was thought to be very limited in its regenerative potential; however, the discovery that stem cell populations produce neurons in the adult brain highlights the dynamics of a previously assumed 'static' organ. The continuous generation of new neurons in the adult brain, nevertheless, leads to the question of whether neurogenesis is counterbalanced by an accompanying cell death in the same regions. The objective of this study was to stereologically analyze neurogenesis and programmed cell death in adult brain regions with known neurogenic activity. Using bromodeoxyuridine (BrdU) to identify newborn cells we find that within a few days of BrdU-labeling the adult dentate gyrus and olfactory bulb generate high numbers of newborn neurons. More importantly, dUTP-nick end labeling (TUNEL) reveals that areas of adult neurogenesis also contain high numbers of apoptotic cells. We conclude that programmed cell death may have an important regulatory function by eliminating supernumerous cells from neurogenic regions and may thus contribute to a self-renewal mechanism in the adult mammalian brain.

Epidermal growth factor and fibroblast growth factor 2 cause proliferation of ependymal precursor cells in the adult rat spinal cord in vivo

We investigated the mitogenic effect of continuous intrathecal infusion of epidermal growth factor (EGF) or fibroblast growth factor 2 (FGF2) on ependymal precursor cells of the adult rat spinal cord in vivo. Either EGF, FGF2, EGF plus FGF2, or artificial cerebrospinal fluid (aCSF) was infused at a flow rate of 0.5 microl/h (15 ng/h of EGF or FGF2) for 3 or 14 days into the intrathecal space at T1 through a catheter attached to an osmotic minipump. To assess proliferation, the bromodeoxyuridine (BrdU) labeling index (LI) in the ependyma at T1 was calculated at 3 or 14 days. At 3 days there was no statistical difference in LI between these groups, but at 14 days the LI was significantly higher in the EGF plus FGF2 group (27.2% = 16.0%) than in the aCSF group (5.4% +/- 4.7%; p < 0.05). With EGF alone or FGF2 alone, the LI increases were not significantly different from the aCSF group. With EGF plus FGF2 for 14 days, some BrdU-positive cells in the ependyma also expressed nestin. These results suggest that the intrathecal infusion of EGF plus FGF2 has a mitogenic effect on precursor cells in the ependyma of the adult rat spinal cord.

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.

Differential modulation of proliferation in the neocortical ventricular and subventricular zones

Recent studies have implicated the classical neurotransmitters GABA and glutamate in the regulation of neural progenitor proliferation. We now show that GABA and glutamate have opposite effects on the two neural progenitor populations in the ventricular zones (VZs) and subventricular zones (SVZs) of the embryonic cerebrum. Application of either molecule to organotypic slice cultures dramatically increases proliferation in the VZ by shortening the cell cycle, whereas proliferation in the SVZ is decreased. These disparate effects, measured both by bromodeoxyuridine uptake and the expansion of retrovirally labeled progenitor clones, are mimicked by the application of specific GABA and glutamate agonists and are blocked by antagonists. Thus, the relative contributions of the VZ and SVZ to neocortical growth may be regulated by differential responsiveness to GABA and glutamate.

Localization of epidermal growth factor receptors and putative neuroblasts in human subependymal zone

Studies in rodents and monkeys suggest that neuronal precursor cells continue to exist and differentiate well into adulthood in these species. These results challenge the long held assumption that neurogenesis does not occur in the postnatal human brain. We examined the rostral subependymal zone (SEZ) of postnatal human brain for expression of cell phenotypic markers that have been associated with neuronal precursors and neuroblasts in rodent brain. We found epidermal growth factor receptor (EGF-R) mRNA and protein to be expressed in infant, teen, young adult, and adult human SEZ. Some SEZ cells expressed the polysialic acid form of neural cell adhesion molecule (PSA-NCAM), characteristic of migrating neuroblasts, as well as class III beta-tubulin and Hu protein, characteristic of neuroblasts and early neurons. These neuroblast-like cells were negative for glial fibrillary acidic protein (GFAP), 2;,3;-cyclic nucleotide 3;-phosphohydrolase (CNPase), and vimentin, suggesting that they were not differentiating as glia. Our results show that neuroblast-like cells exist in the human SEZ and support the theory that SEZ of postnatal human brain has neurogenic potential.

Inhibition of mitogen-activated protein kinase kinase blocks proliferation of neural progenitor cells

Nestin-expressing neural progenitor (NP) cells have been isolated from the subventricular zone (SVZ) of the brain and propagated with epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF). In other neural cell types it has been shown that EGF and bFGF activate cell surface receptors involved in the mitogen-activated protein kinase (MAPK) signal pathway. To examine this issue in NP cells, we isolated primary SVZ cells and stimulated them with EGF and bFGF and then used a phosphorylation-specific antibody to detect activated MAPK by immunofluorescent staining or Western blotting. The addition of growth factors activated MAPK transiently in cells that co-expressed nestin. A distinct phospho-MAPK signal was also detected in nestin-positive cells with mitotic chromosomes. A novel MAPK kinase (MEK1) inhibitor U0126 blocked the activation of MAPK and the proliferation of primary cells more effectively than the same concentration of PD98059. After exposure of cells to U0126 for 10 days, we noted that there was a significant reduction in the number of cells that expressed nestin and an increase in the percentage of apoptotic cells. These data provide evidence that activation of MAPK by MEK1 is important for the proliferation of NP cells.

Basic fibroblast growth factor (Fgf2) is necessary for cell proliferation and neurogenesis in the developing cerebral cortex

Little is known about regionally specific signals that control the number of neuronal progenitor cells in vivo. We have previously shown that the germline mutation of the basic fibroblast growth factor (Fgf2) gene results in a reduction in the number of cortical neurons in the adult. We show here that Fgf2 is expressed in the pseudostratified ventricular epithelium (PVE) in a dorsoventral gradient and that Fgf2 and its receptor, Fgfr-1, are downregulated by mid to late stages of neurogenesis. In Fgf2 knockout mice, the volume and cell number of the dorsal PVE (the cerebral cortical anlage) are substantially smaller, whereas the volume of the basal PVE is unchanged. The dorsal PVE of Fgf2 knockout mice has a 50% decrease in founder cells and a reduced expansion of the progenitor pool over the first portion of neurogenesis. Despite this reduction, the degree of apoptosis within the PVE is not changed in the Fgf2 knockouts. Cortical neuron number was decreased by 45% in Fgf2 knockout mice by the end of neurogenesis, whereas the number of neurons in the basal ganglia was unaffected. Microscopically, the frontal cerebral cortex of neonatal Fgf2 null mutant mice lacked large neurons in deep cortical layers. We suggest that Fgf2 is required for the generation of a specific class of cortical neurons arising from the dorsal PVE.

Immunocytochemical and physiological characterization of a population of cultured human neural precursors

Human neural precursor cells (HNPC) have recently become commercially available. In an effort to determine the usefulness of these cells for in vitro studies, we have grown cultured HNPCs (cHNPCs) according to the supplier specifications. Here we report our characterization of cHNPCs under nondifferentiating and differentiating growth conditions and make a comparison to primary HNPCs (pHNPCs) obtained at the same developmental time point from a different commercial supplier. We found that under nondifferentiating conditions, cHNPCs expressed nestin, divided rapidly, expressed few markers of differentiated cells, and displayed both 4-aminopyridine (4-AP)-sensitive and delayed-rectifier type K(+) currents. No inward currents were observed. On changing to differentiating culture conditions, a majority of the cells expressed neuronal markers, did not divide, expressed inward and outward time- and voltage-dependent currents, and responded to the application of the neurotransmitters acetylcholine and glutamate. The outward current densities were indistinguishable from those in undifferentiated cells. The inward currents included TTX-sensitive and -resistant Na(+) currents, sustained Ca(2+) currents, and an inwardly rectifying K(+) current. Comparison of the properties of differentiated cells from cHNPCs with neurons obtained from primary fetal cultures (pHNPCs) revealed two major differences: the differentiated cHNPCs did not express embryonic neural cell adhesion molecule (E-NCAM) immunoreactivity but did co-express GFAP immunoreactivity. The co-expression of neuronal and glial markers was likely due to the growth of cells in serum containing medium as the pHNPCs that were never exposed to serum did express E-NCAM and did not co-express glial fibrillary acidic protein (GFAP). The relevance of these results is discussed and compared with results from other neuronal progenitor populations and cultured human neuronal cells.

Induction of neurogenesis in the neocortex of adult mice

Neurogenesis normally only occurs in limited areas of the adult mammalian brain--the hippocampus, olfactory bulb and epithelium, and at low levels in some regions of macaque cortex. Here we show that endogenous neural precursors can be induced in situ to differentiate into mature neurons, in regions of adult mammalian neocortex that do not normally undergo any neurogenesis. This differentiation occurs in a layer- and region-specific manner, and the neurons can re-form appropriate corticothalamic connections. We induced synchronous apoptotic degeneration of corticothalamic neurons in layer VI of anterior cortex of adult mice and examined the fates of dividing cells within cortex, using markers for DNA replication (5-bromodeoxyuridine; BrdU) and progressive neuronal differentiation. Newly made, BrdU-positive cells expressed NeuN, a mature neuronal marker, in regions of cortex undergoing targeted neuronal death and survived for at least 28 weeks. Subsets of BrdU+ precursors expressed Doublecortin, a protein found exclusively in migrating neurons, and Hu, an early neuronal marker. Retrograde labelling from thalamus demonstrated that BrdU+ neurons can form long-distance corticothalamic connections. Our results indicate that neuronal replacement therapies for neurodegenerative disease and CNS injury may be possible through manipulation of endogenous neural precursors in situ.

Adult brain neurogenesis and psychiatry: a novel theory of depression

Neurogenesis (the birth of new neurons) continues postnatally and into adulthood in the brains of many animal species, including humans. This is particularly prominent in the dentate gyrus of the hippocampal formation. One of the factors that potently suppresses adult neurogenesis is stress, probably due to increased glucocorticoid release. Complementing this, we have recently found that increasing brain levels of serotonin enhance the basal rate of dentate gyrus neurogenesis. These and other data have led us to propose the following theory regarding clinical depression. Stress-induced decreases in dentate gyrus neurogenesis are an important causal factor in precipitating episodes of depression. Reciprocally, therapeutic interventions for depression that increase serotonergic neurotransmission act at least in part by augmenting dentate gyrus neurogenesis and thereby promoting recovery from depression. Thus, we hypothesize that the waning and waxing of neurogenesis in the hippocampal formation are important causal factors, respectively, in the precipitation of, and recovery from, episodes of clinical depression.

Serotonergic reinnervation reverses lesion-induced decreases in PSA-NCAM labeling and proliferation of hippocampal cells in adult rats

Serotonin (5-HT) is believed to play a role in structural plasticity in the adult brain, and cell adhesion molecules may be involved in such adaptive processes. The present study sought to determine the effects of 5-HT denervation and reinnervation of the hippocampal formation on the expression of glial and neuronal markers and neurogenesis in adult rats. Injections of 5,7-dihydroxytryptamine (5,7-DHT) in the dorsal and medial raphe nuclei, producing a partial loss of 5-HT neurons, induced rapid and transient increases in glial fibrillary acidic protein immunoreactivity indicative of a reactive gliosis, but no changes in the S100beta or tenascin-C normally secreted by astroglial cells. In contrast, as long as the hippocampal formation was deprived of 5-HT innervation, significant decreases were observed in the number of granule cells expressing the highly polysialylated form of the neural cell adhesion molecule (PSA-NCAM) as well as the PSA-NCAM staining of the hilus in the dentate gyrus. Similar decreases in the number of newly generated granule cells labeled with bromodeoxyuridine were also detected during this time. All these effects were reversed later, when the hippocampal formation was reinnervated by 5-HT fibers. These results indicate that 5-HT is one factor which may regulate the number of granule cells proliferating in the adult dentate gyrus and thereafter expressing PSA-NCAM immunoreactive at the level of cell bodies, dendrites, and axonal paths (mossy fibers). They emphasize the critical role played by 5-HT in the neuronal organization of the hippocampus.

The radioresponse of the central nervous system: a dynamic process

Radiation continues to be a major treatment modality for tumors located within and close to the central nervous system (CNS). Consequently, alleviating or protecting against radiation-induced CNS injury would be of benefit in cancer treatment. However, the rational development of such interventional strategies will depend on a more complete understand-ing of the mechanisms responsible for the development of this form of normal tissue injury. Whereas the vasculature and the oligodendrocyte lineage have traditionally been considered the primary radiation targets in the CNS, in this review we suggest that other phenotypes as well as critical cellular interactions may also be involved in determining the radio-response of the CNS. Furthermore, based on the assumption that the CNS has a limited repertoire of responses to injury, the reaction of the CNS to other types of insults is used as a framework for modeling the pathogenesis of radiation-induced damage. Evidence is then provided suggesting that, in addition to acute cell death, radiation induces an intrinsic recovery/repair response in the form of specific cytokines and may

Neural stem cells: from cell biology to cell replacement

A large number of crippling neurological conditions result from the loss of certain cell populations from the nervous system through disease or injury, and these cells are not intrinsically replaced. Mounting evidence now suggests that replacement of depleted cell populations by transplantation may be of functional benefit in many such diseases. A diverse range of cell populations is vulnerable, and the loss of specific populations results in circumscribed deficits in different conditions. This diversity presents a considerable challenge if cell replacement therapy is to become widely applicable in the clinical domain, because each condition has specific requirements for the phenotype, developmental stage, and number of cells required. An ideal cell for universal application in cell replacement therapy would possess several key properties: it would be highly proliferative, allowing the ex vivo production of large numbers of cells from minimal donor material; it would also remain immature and phenotypically plastic such that it could differentiate into appropriate neural and glial cell types on, or prior to, transplantation. Critically, both proliferation and differentiation would be controllable. This review considers some of the evidence that stem cells exist in the central nervous system and that they may possess characteristics that make them ideal for broad application in cell replacement therapy.

Mammalian neural stem cells

Neural stem cells exist not only in the developing mammalian nervous system but also in the adult nervous system of all mammalian organisms, including humans. Neural stem cells can also be derived from more primitive embryonic stem cells. The location of the adult stem cells and the brain regions to which their progeny migrate in order to differentiate remain unresolved, although the number of viable locations is limited in the adult. The mechanisms that regulate endogenous stem cells are poorly understood. Potential uses of stem cells in repair include transplantation to repair missing cells and the activation of endogenous cells to provide "self-repair. " Before the full potential of neural stem cells can be realized, we need to learn what controls their proliferation, as well as the various pathways of differentiation available to their daughter cells.

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.

Tracing human oligodendroglial development in vitro

Human neural precursor cell cultures (neurospheres) were established from fetal brain tissues of 15-20 gestation weeks and propagated for over a year in the presence of epidermal growth factor, basic fibroblast growth factor and leukemia inhibitory factor. Neurospheres were differentiated without the presence of above growth factors to follow the development of oligodendroglia. Oligodendroglial progenitors, identified by their bipolar morphology and expression of platelet-derived growth factor receptor-alpha (PDGFRalpha), emerged from spheres as early as 1 DIV; O4+ cells with bipolar to multipolar processes were observed at 3 DIV whereas O1+ multiprocess-bearing oligodendroglia did not appear until 5-7 DIV. They further differentiated to myelin basic protein-expressing oligodendrocytes after 2-3 weeks in culture. Thus, human oligodendroglial maturation in vitro follows the same pathway as rat cells but takes twice as long as their rodent counterparts. Bromodeoxyuridine incorporation indicated that PDGFRalpha-expressing cells but not O4+ oligodendroglia proliferated. More oligodendroglial progenitors incorporated BrdU and more O4+ cells survived when they were in contact with neurons and astrocytes than when they developed beyond the astrocyte layer. In addition, oligodendroglia on astrocytes had a complex process branching whereas those growing beyond astrocyte layer often formed membrane sheaths. Thus the survival, proliferation and maturation of oligodendroglia are influenced by other cell types.

Serotonin may stimulate granule cell proliferation in the adult hippocampus, as observed in rats grafted with foetal raphe neurons

The long-term effects of hippocampal serotonergic denervation and reinnervation by foetal raphe tissue were examined in the dentate gyrus where neurons are continously born in the adult. Complete lesion of serotonin neurons following injections of 5, 7-dihydroxytryptamine in the dorsal and medial raphe nuclei produced long-term decreases in the number of newly generated granule cells identified with 5-Bromo-2'-deoxyuridine (BrdU) and the polysialylated form of neural cell adhesion molecule (PSA-NCAM) immunostaining, as observed in 2-month-survival rats. The raphe grafts, but not the control grafts of embryonic spinal tissue, reversed the postlesion-induced decreases in the density of BrdU- and PSA-NCAM-labelled cells detected in the granule layer. Inhibition of serotonin synthesis in animals with raphe grafts reversed back to lesion-induced changes in granule cell proliferation. Furthermore, extensive serotonergic reinnervation of the dentate gyrus in the area proximal to the raphe graft could be associated with supranormal density of BrdU-labelled cells. These results indicate that serotonin may be considered a positive regulatory factor of adult granule cell proliferation. Finally, the lack of effect of embryonic nonserotonergic tissue grafted to serotonin-deprived rats suggests that neurotrophic factors may not be involved in the effects of serotonin on adult neurogenesis.

Macrophage cell-conditioned medium promotes cholinergic differentiation of undifferentiated progenitors and synergizes with nerve growth factor action in the developing basal forebrain

Conditioned medium from stimulated microglia and from the monocyte/macrophage cell line (RAW 264.7; MC-CM) promotes the differentiation of cholinergic neurons from undifferentiated progenitors in the septal nuclei and adjacent basal forebrain (BF). We have studied the regulation of this process by measuring the activity of choline acetyltransferase (ChAT) in cultured BF taken from embryonic day 16 rat brain. Inhibition of either xanthine oxidase with allopurinol or nitric oxide synthase with N(G)-monomethyl-l-arginine produces a small but significant improvement in the efficacy of MC-CM while inclusion of pyrrolidine dithiocarbamate, a hydroxyl radical scavenger widely used as an antioxidant, lowers MC-CM-induced ChAT activity. Addition of nerve growth factor (NGF) but not brain-derived neurotrophic factor or glial-derived neurotrophic factor together with MC-CM has a synergistic effect on both ChAT activity and ChAT mRNA, raising ChAT activity as much as 29-fold and ChAT mRNA almost 15-fold. While MC-CM raised mRNA for trkA, the effect was not synergistic with NGF. mRNA for the common neurotrophin receptor (p75NTR) showed a modest synergistic increase. Blockade of the Ras/Raf/ERK [extracellular signal-regulated kinase, also known as mitogen-activated protein [(MAP) kinase] signal transduction pathway with either PD28059 (an inhibitor of MAP kinase/ERK kinase kinase or MEK) or N-acetyl-S-farnesyl-l-cysteine (an inhibitor of Ras farnesylation and, hence, activation) inhibited the action of MC-CM. Moreover, a subpopulation of cells responded rapidly to MC-CM with an increased appearance of phosphorylated ERK. Because NGF also utilizes this pathway, synergy may occur along this signal transduction pathway.

Regeneration of olfactory receptor neurons following chemical lesion: time course and enhancement with growth factor administration

Although it has been known for over 50 years that olfactory receptor neuron (ORN) neurogenesis and subsequent reinnervation of the olfactory bulb (OB) occurs following ORN injury, the precise intrinsic and extrinsic factors that regulate this dynamic process have not yet been fully identified. In the first of two experiments, we characterized the time course of anatomical recovery following zinc sulfate (ZnSO(4)) lesion of ORNs in adult male Sprague-Dawley rats. ZnSO(4) produced a near complete deafferentation of OB within 3 days following intranasal administration. A time-dependent increase in ORN reinnervation of OB was observed following 10, 20, and 30 day recovery intervals. Given the evidence that bFGF, EGF, and TGF-alpha have mitogenic effects on ORNs in vitro, a second experiment examined the extent to which these growth factors (GFs) might enhance ORN regeneration and subsequent reinnervation of OB in vivo. Rats received intranasal infusions of ZnSO(4) on day 0, followed by subcutaneous injections of either bFGF (5, 10, or 50 microgram/kg), EGF (5, 10, or 50 microgram/kg), or TGF-alpha (5 or 10 microgram/kg) on days 3-6. Horseradish peroxidase (HRP) histochemistry of OB following a 10-day recovery period revealed a dose-related enhancement in reinnervation of OB for each of the three growth factors examined, with the greatest enhancement produced by TGF-alpha. These data suggest that GFs may regulate ORN mitogenesis in vivo in a way similar to that which has been characterized in vitro.

Characterization of CArG-binding protein A initially identified by differential display

While investigating differences in the pattern of gene expression in functionally distinct areas of the rat caudate-putamen employing differential display, we identified a gene that is highly enriched in tissue adjacent to the lateral ventricle. To characterize the gene, a complementary DNA containing the complete coding sequence was obtained and sequenced. In addition, radiolabelled DNA and riboprobes were generated to examine the expression levels and anatomical distribution of the identified gene in the brain. The sequencing data suggests that the identified gene is a member of the heterogeneous nuclear ribonucleoprotein family and likely represents the rat homolog of CArG-binding protein A initially isolated from mouse C2 myogenic cells. CArG-binding protein A is widely distributed and moderately expressed in the rat brain and present within both neurons and astrocytes. Since the CArG box motif forms the core of the serum response element and the serum response element is involved in immediate early gene regulation, the expression level of CArG-binding protein A was examined following treatment of PC12 cells with nerve growth factor and correlated with changes in c-fos and zif268 expression. The results show that CArG-binding protein A is up-regulated following nerve growth factor treatment and that the up-regulation of CArG-binding protein A can be correlated with the down-regulation of c-fos and zif268. The results of the current study leads us to suggest that CArG-binding protein A may be involved in brain development and the regulation of the serum response element.

Neuronal progenitor-like cells expressing polysialylated neural cell adhesion molecule are present on the ventricular surface of the adult rat brain and spinal cord

In the adult rodent brain, it is now well established that neurons are continuously generated from proliferating neuronal progenitor cells located in the subventricular zone of the lateral ventricle (SVZ) and the dentate gyrus of the hippocampus. Recently, it has been shown that neurons can also be generated in vitro from various regions of the adult brain and spinal cord ventricular neuroaxis. As the highly polysialylated neural cell adhesion molecule (PSA-NCAM) has been shown to be specifically expressed by neuronal progenitor cells of the SVZ and the hippocampus, the present study was designed to determine whether cells expressing this molecule could be detected in the vicinity of the ventricular system of the adult rat brain and spinal cord. After double or triple immunostaining for different neuronal and glial markers, confocal microscopy was used to examine the surface of the ventricular neuroaxis in either 40- to 50-microm-thick transverse vibratome sections cut through different brain regions, or in 200- to 300-microm-thick tissue slices including the intact surface of the brain ventricles or of the spinal cord central canal. In untreated rats, PSA-NCAM, microtubule associated protein 2 (MAP2) and class III-beta-tubulin were found to be associated with a number of neuron-like cells located on the surface of the third and fourth ventricles and of the spinal cord central canal. The proliferation of the PSA-NCAM-immunoreactive (IR) neuron-like cells detected on the surface of the third and fourth ventricles was not affected by injection of epidermal growth factor (EGF) or basic fibroblast growth factor (bFGF) into these ventricles, but was stimulated by the combined injection of EGF + bFGF. These data indicate that cells exhibiting features of neuronal progenitors are present on the ependymal surface of the adult rat brain and spinal cord ventricular axis.