Multipotent progenitor cells in the adult dentate gyrus

Neuronal differentiation of hippocampus-derived neural stem cells cultured in conditioned medium of embryonic rat retina

PURPOSE

To investigate whether conditioned medium from embryonic rat retinas can induce differentiation of adult rat hippocampus-derived neural stem cells (AHSCs) into neurons and glia in vitro.

METHODS

AHSCs were cultured in 3 types of media: standard culture medium, conditioned medium from embryonic rat retina, and standard culture medium with retinoic acid. Neuronal and glial differentiation of the cultured cells was assessed by cell growth analysis, flow cytometric analysis, immunofluorescent staining, and RT-PCR analysis.

RESULTS

Cells cultured in the standard medium showed very little neuronal and glial differentiation. The cells cultured in the conditioned medium and the medium with retinoic acid showed neuronal morphology and growth inhibition. They also expressed mature neuronal markers and glial markers. In addition, the cells cultured in the conditioned medium expressed Thy-1, HPC-1, and calbindin, which were not found in the previous studies with postnatal retinas in vivo. Those cultured in the medium with retinoic acid expressed HPC-1 and calbindin, but not Thy-1.

CONCLUSIONS

Conditioned medium from embryonic rat retina contains factors that induce neuronal and glial cell differentiation of AHSCs, and promote up-regulation of some types of retinal cell markers.

Genetic programs and responses of neural stem/progenitor cells during demyelination: potential insights into repair mechanisms in multiple sclerosis

In recent years, it has become evident that the adult mammalian CNS contains a population of neural stem cells (NSCs) described as immature, undifferentiated, multipotent cells, that may be called upon for repair in neurodegenerative and demyelinating diseases. NSCs may give rise to oligodendrocyte progenitor cells (OPCs) and other myelinating cells. This article reviews recent progress in elucidating the genetic programs and dynamics of NSC and OPC proliferation, differentiation, and apoptosis, including the response to demyelination. Emerging knowledge of the molecules that may be involved in such responses may help in the design of future stem cell-based treatment of demyelinating diseases such as multiple sclerosis.

Neuropeptide Y is neuroproliferative for post-natal hippocampal precursor cells

New neurones are produced in the adult hippocampus throughout life and are necessary for certain types of hippocampal learning. Little, however, is known about the control of hippocampal neurogenesis. We used primary hippocampal cultures from early post-natal rats and neuropeptide Y Y1 receptor knockout mice as well as selective neuropeptide Y receptor antagonists and agonists to demonstrate that neuropeptide Y is proliferative for nestin-positive, sphere-forming hippocampal precursor cells and beta-tubulin-positive neuroblasts and that the neuroproliferative effect of neuropeptide Y is mediated via its Y1 receptor. Immunohistochemistry confirmed Y1 receptor staining on both nestin-positive cells and beta-tubulin-positive cells in culture and short pulse 5-bromo-2-deoxyuridine studies demonstrated that neuropeptide Y has a proliferative effect on both cell types. These studies suggest that the proliferation of hippocampal neuroblasts and precursor cells is increased by neuropeptide Y and, therefore, that hippocampal learning and memory may be modulated by neuropeptide Y-releasing interneurones.

Effects of stimulus frequency and age on bidirectional synaptic plasticity in the dentate gyrus of freely moving rats

We investigated the frequency-dependent transition from homosynaptic long-term depression (LTD) to long-term potentiation (LTP) at the lateral perforant pathway/dentate gyrus synapse in adult (90 days of age) and immature (15 days of age) awake, freely moving rats. Dentate-evoked field potentials were recorded and analyzed using the population spike amplitude and the field EPSP slope measures following sustained stimulation (900 pulses) of the lateral perforant pathway at various frequencies (1, 3, 7, 30, 50, or 200 Hz). Our results indicate that both the strength and the direction (LTP or LTD) of synaptic plasticity vary as a function of activation frequency: sustained low-frequency stimulation ranging from 1 to 7 Hz results in depression of activated synapses, whereas high-frequency stimulation (30-200 Hz) produces potentiation. In addition, a significant (P < 0.01) ontogenetic shift in the frequency of transition from LTD to LTP was observed; the transition frequency in immature animals was significantly lower than that obtained in adult animals. These observations agree strongly with the prediction of the Bienenstock-Cooper-Munro theory of synapse modification, indicating perhaps a neurophysiological basis for this theoretical model of learning in the dentate gyrus of awake behaving rats.

Injury-induced neurogenesis in the adult mammalian brain

The persistence of neurogenesis in the adult mammalian forebrain suggests that endogenous precursors may be a potential source for neuronal replacement after injury or neurodegeneration. Limited knowledge exists, however, regarding the normal function of neurogenesis in the adult and its alteration by brain injury. Neural precursors generate neurons throughout life in the mammalian forebrain subventricular zone (SVZ)-olfactory bulb pathway and hippocampal dentate gyrus. Accumulating evidence indicates that various brain insults increase neurogenesis in these persistent germinative zones. Two brain injury models in particular, experimental epilepsy and stroke in the adult rodent, have provided significant insight into the consequences of injury-induced neurogenesis. Studies of dentate gyrus neurogenesis in adult rodent epilepsy models suggest that seizure-induced neurogenesis involves aberrant neuroblast migration and integration that may contribute to persistent hippocampal hyperexcitability. In contrast, adult rat forebrain SVZ neurogenesis induced by stroke may have reparative effects. SVZ neural precursors migrate to regions of focal or global ischemic injury and appear to form appropriate neuronal subtypes to replace damaged neurons. These findings underscore the need for a better understanding of injury-induced neurogenesis in the adult and suggest that the manipulation of endogenous neural precursors is a potential strategy for brain reparative therapies.

Proliferation of neural and neuronal progenitors after global brain ischemia in young adult macaque monkeys

To investigate the effect of global cerebral ischemia on brain cell proliferation in young adult macaques, we infused 5-bromo-2'-deoxyuridine (BrdU), a DNA replication indicator, into monkeys subjected to ischemia or sham-operated. Subsequent quantification by BrdU immunohistochemistry revealed a significant postischemic increase in the number of BrdU-labeled cells in the hippocampal dentate gyrus, subventricular zone of the temporal horn of the lateral ventricle, and temporal neocortex. In all animals, 20-40% of the newly generated cells in the dentate gyrus and subventricular zone expressed the neural progenitor cell markers Musashi1 or Nestin. A few BrdU-positive cells in postischemic monkeys were double-stained for markers of neuronal progenitors (class III beta-tubulin, TUC4, doublecortin, or Hu), neurons (NeuN), or glia (S100beta or GFAP). Our results suggest that ischemia activates endogenous neuronal and glial precursors residing in diverse locations of the adult primate central nervous system.

Dentate gyrus: alterations that occur with hippocampal injury

Injury to the brain usually manifests not in a diffuse uniform manner but rather with selective sites of damage indicative of differential vulnerability. This question of neuronal susceptibility has been one of major interest both in disease processes as well as damage induced by environmental factors. For experimental examination, brain structures with obvious neuronal subpopulations and organization such as the cerebellum and the hippocampus have offered the most promise. In the hippocampus distinct neuronal populations exist that demonstrate differential vulnerability to various forms of insult including ischemia, excitotoxicity, and environmental factors. The more recent data regarding the presence of neuronal progenitor cells in the subgranular zone of the dentate offers the opportunity to expand such experimental examination to the process of injury-induced neurogenesis. Thus, more recent studies have expanded the examination of the hippocampus to include models of damage to the dentate neurons in addition to the highly vulnerable pyramidal neurons. A number of these models are presented for both human disease and experimental animal conditions. Examination of the responses between these distinct cell populations offers the potential for understanding factors that are critical in neuronal death and survival.

Neural stem cells constitutively secrete neurotrophic factors and promote extensive host axonal growth after spinal cord injury

Neural stem cells (NSCs) offer the potential to replace lost tissue after nervous system injury. This study investigated whether grafts of NSCs (mouse clone C17.2) could also specifically support host axonal regeneration after spinal cord injury and sought to identify mechanisms underlying such growth. In vitro, prior to grafting, C17.2 NSCs were found for the first time to naturally constitutively secrete significant quantities of several neurotrophic factors by specific ELISA, including nerve growth factor, brain-derived neurotrophic factor, and glial cell line-derived neurotrophic factor. When grafted to cystic dorsal column lesions in the cervical spinal cord of adult rats, C17.2 NSCs supported extensive growth of host axons of known sensitivity to these growth factors when examined 2 weeks later. Quantitative real-time RT-PCR confirmed that grafted stem cells expressed neurotrophic factor genes in vivo. In addition, NSCs were genetically modified to produce neurotrophin-3, which significantly expanded NSC effects on host axons. Notably, overexpression of one growth factor had a reciprocal effect on expression of another factor. Thus, stem cells can promote host neural repair in part by secreting growth factors, and their regeneration-promoting activities can be modified by gene delivery.

Molecular mechanisms of cerebral ischemia-induced neuronal death

The mode of neuronal death caused by cerebral ischemia and reperfusion appears on the continuum between the poles of catastrophic necrosis and apoptosis: ischemic neurons exhibit many biochemical hallmarks of apoptosis but remain cytologically necrotic. The position on this continuum may be modulated by the severity of the ischemic insult. The ischemia-induced neuronal death is an active process (energy dependent) and is the result of activation of cascades of detrimental biochemical events that include perturbion of calcium homeostasis leading to increased excitotoxicity, malfunction of endoplasmic reticulum and mitochondria, elevation of oxidative stress causing DNA damage, alteration in proapoptotic gene expression, and activation of the effector cysteine proteases (caspases) and endonucleases leading to the final degradation of the genome. In spite of strong evidence showing that brain infarction can be reduced by inhibiting any one of the above biochemical events, such as targeting excitotoxicity, up-regulation of an antiapoptotic gene, or inhibition of a down-stream effector caspase, it is becoming clear that targeting a single gene or factor is not sufficient for stroke therapeutics. An effective neuroprotective therapy is likely to be a cocktail aimed at all of the above detrimental events evoked by cerebral ischemia and the success of such therapeutic intervention relies upon the complete elucidation of pathways and mechanisms of the cerebral ischemia-induced active neuronal death.

Migration and differentiation of adult rat subventricular zone progenitor cells transplanted into the adult rat striatum

Adult brain subventricular zone progenitor cells undergo neurogenesis in the olfactory bulb. We tested the hypothesis that cultured adult subventricular zone progenitor cells migrate and differentiate into neurons when transplanted into the adult striatum. Cells in the adult rat subventricular zone were isolated and cultured for 8 days in medium containing basic fibroblast growth factor. These cells proliferated as assayed by bromodeoxyuridine immunostaining, and the majority of them were neuron-specific class III beta-tubulin (TuJ1) immunoreactive at 8 days of culture. These cultured cells were labeled in vitro with bromodeoxyuridine or with lipophilic dye-coated particles and were transplanted into the adult rat striatum. Twenty-eight days after transplantation, the cells migrated 0.5-1.5 mm from the midline of the graft to the surrounding host striatum. Migration of grafted cells in the host striatum was also detected on magnetic resonance imaging in living rats. Morphological analysis revealed that many of these migrated cells exhibited multibranched processes from the cell soma resembling host medium-size striatal projection neurons. Only a few astrocyte-like cells were detected. Double immunostaining showed that many bromodeoxyuridine immunoreactive cells were microtubule-associated protein 2 or immunoreactive with a mouse monoclonal antibody against neuronal nuclear protein, whereas only a few bromodeoxyuridine immunoreactive cells had glial fibrillary acidic protein immunoreactivity. Morphology of bromodeoxyuridine and microtubule-associated protein 2 immunoreactive cells was similar to those of host microtubule-associated protein 2 immunoreactive cells. These results suggest that transplanted cultured adult subventricular zone progenitor cells can migrate and differentiate in response to guidance cues within the adult striatum.

Expression of the ecto-ATPase NTPDase2 in the germinal zones of the developing and adult rat brain

In the adult nervous system, multipotential stem cells of the subventricular zone of the lateral ventricles generate neuron precursors (type-A cells) that migrate via the rostral migratory stream to the olfactory bulb where they differentiate into neurons. The migrating neuroblasts are surrounded by a sheath of astrocytes (type-B cells). Using immunostaining, in situ hybridization and enzyme histochemistry, we demonstrate that the ecto-ATPase nucleoside triphosphate diphosphohydrolase 2 (NTPDase2) is expressed in the subventricular zone and the rostral migratory stream of the adult rat brain. This enzyme hydrolyses extracellular nucleoside triphosphates to the respective nucleoside diphosphates and is thought to directly modulate ATP receptor-mediated cell communication. Double labelling for the astrocyte intermediate filament protein GFAP and the glial glutamate transporter GLAST identifies the NTPDase2-positive cells as type-B cells. During development the enzyme protein is first detected at E18, long before expression of the astrocyte marker GFAP. It gradually becomes expressed along the ventricular and subventricular zone of the brain, followed by complete retraction to the adult expression pattern at P21. NTPDase2 is transiently expressed in the outer molecular layer of the dentate gyrus and within the cerebellar white matter and is associated with select microvessels, tanycytes of the third ventricle, and subpial astrocytes of the adult brain. Our results suggest that NTPDase2 can serve as a novel marker for specifying subsets of cells during in vivo and in vitro studies of neural development and raise the possibility that ATP-mediated signalling pathways play a role in neural development and differentiation.

Agonistic encounters in aged male mouse potentiate the expression of endogenous brain NGF and BDNF: possible implication for brain progenitor cells' activation

The condition of dominance or submission following agonistic encounters in the adult male mouse is known to differentially affect brain nerve growth factor, a neurotrophin playing a role in brain remodeling, in the fine tuning of behaviour and in the regulation of the basal forebrain cholinergic neurons. During development and adult life nerve growth factor regulates brain expression of neurotransmitters and the stimulation of progenitor cells (stem cells) which, under different external stimuli, may differentiate into neuronal and/or glial cells promoting the recovery of the injured brain. However, little information is available for the aged brain. Thus in the present study we investigated the effect of the social status ('dominance' vs. 'submission') in the aged mouse on the presence of nerve growth factor, brain-derived neurotrophic factor, choline acetyltransferase, neuropeptide Y and progenitor cells of selected brain regions. We found that aged dominant mice showed increased brain-derived neurotrophic factor in the subventricular zone and hippocampus and increased choline acetyltransferase in the septum and basal nuclei, which were associated with increased presence of progenitor cells in the subventricular zone. Conversely, in aged subordinate mice the data showed a marked brain increase in nerve growth factor in the subventricular zone and hippocampus, choline acetyltransferase in the septum and basal nuclei and neuropeptide Y in the hippocampus and parietal cortex. The possible functional implications of these findings are discussed.

Proliferation and death of conditionally immortalized neural cells from murine neocortex: p53 alters the ability of neuron-like cells to re-enter the cell cycle

Neurons are distinctive in that they are generally considered to be permanently post-mitotic cells. The oncoprotein p53 is a key regulator in neuronal development, notably in cell proliferation and neuronal death. We hypothesize that p53 maintains the post-mitotic characteristic of differentiated neurons. New lines of conditionally immortalized cortical cells were generated to test this hypothesis. Populations of cells were obtained from the neocortices of dual transgenic mice that were null for p53 and expressed a temperature-sensitive SV40 large T antigen. At a permissive temperature (32 degrees C), the cells continued to proliferate and most expressed nestin and proteins associated with glia. At a non-permissive temperature (39 degrees C), the cells expressed cytoskeletal proteins associated with differentiated neurons such as microtubule associated protein 2 and neurofilament 200. Under permissive conditions, both p53(+/-) and p53(-/-) cells exhibited similar cycling behaviors; the length of the cell cycle was 13-15 h and >85% of the cells were actively cycling. In non-permissive conditions, most p53(+/-) cells stopped dividing, whereas the p53(-/-) cells continued to proliferate. The survival of the cells also differed. In the non-permissive conditions, many p53(+/-) cells died following treatment with a neurotoxin (ethanol, 400 mg/dl), whereas the p53(-/-) cells did not. After re-introduction to the permissive conditions, both cell lines expressed neuron-like characteristics, but only the p53(-/-) cells retained their ability to cycle. Therefore, p53-mediated activities appear to be involved in the proliferation, survival, and post-mitotic nature of neuron-like cells.

Distinguishing features of progenitor cells in the late embryonic and adult hippocampus

Adult neurogenesis occurs within the subgranular layer of the hippocampal dentate gyrus. In this study, we examined dividing cells in the late embryonic and adult rat hippocampus to identify distinguishing characteristics and potential neural stem cell population(s), as identified by the putative neural stem cell markers FGFR4 and Sox1. In embryonic hippocampal cells in primary culture, basic fibroblast factor caused cell proliferation, increased telomerase activity and upregulation of FGFR4 mRNA. In both the embryonic and adult brains, proliferating cells express Sox1, as well as markers for neuronal- and glial-restricted precursors. However, the cell markers associated with cells expressing proliferative cell nuclear antigen (PCNA) and Sox1 differed between late embryonic and adult hippocampus, suggesting that there are important differences between adult and embryonic neurogenesis.

Transplanted bone marrow generates new neurons in human brains

Adult bone marrow stem cells seem to differentiate into muscle, skin, liver, lung, and neuronal cells in rodents and have been shown to regenerate myocardium, hepatocytes, and skin and gastrointestinal epithelium in humans. Because we have demonstrated previously that transplanted bone marrow cells can enter the brain of mice and differentiate into neurons there, we decided to examine postmortem brain samples from females who had received bone marrow transplants from male donors. The underlying diseases of the patients were lymphocytic leukemia and genetic deficiency of the immune system, and they survived between 1 and 9 months after transplant. We used a combination of immunocytochemistry (utilizing neuron-specific antibodies) and fluorescent in situ hybridization histochemistry to search for Y chromosome-positive cells. In all four patients studied we found cells containing Y chromosomes in several brain regions. Most of them were nonneuronal (endothelial cells and cells in the white matter), but neurons were certainly labeled, especially in the hippocampus and cerebral cortex. The youngest patient (2 years old), who also lived the longest time after transplantation, had the greatest number of donor-derived neurons (7 in 10,000). The distribution of the labeled cells was not homogeneous. There were clusters of Y-positive cells, suggesting that single progenitor cells underwent clonal expansion and differentiation. We conclude that adult human bone marrow cells can enter the brain and generate neurons just as rodent cells do. Perhaps this phenomenon could be exploited to prevent the development or progression of neurodegenerative diseases or to repair tissue damaged by infarction or trauma.

Ependymal cell reactions in spinal cord segments after compression injury in adult rat

Recently, it has been suggested that neural stem cells and neural progenitor cells exist in the ependyma that forms the central canal of the spinal cord. In this study, we produced various degrees of thoracic cord injury in adult rats using an NYU-weight-drop device, assessed the degree of recovery of lower limb motor function based on a locomotor rating scale, and analyzed the kinetics of ependymal cell proliferation and differentiation by proliferating cell nuclear antigen (PCNA), nestin, glial fibrillary acidic protein (GFAP), or GAP-43 immunostaining. The results showed that the time course of the ependymal cell proliferation and differentiation reactions differed according to the severity of injury, and that the responses occurred not only in the neighborhood of the injury but in the entire spinal cord. An increase in the locomotor rating score was related to an increase in the number of PCNA-positive cells, and the differentiation of ependymal cells into reactive astrocytes was involved in injury repair. No apoptotic cells in the ependyma were detectable by the TUNEL method. These results indicate that the ependymal cells of the spinal central canal are themselves multipotent, can divide and proliferate according to the severity of injury, and differentiate into reactive astrocytes within the ependyma without undergoing apoptosis or cell death.

Possible regulation by N-methyl-d-aspartate receptors of proliferative progenitor cells expressed in adult mouse hippocampal dentate gyrus

An immunohistochemical technique was employed to analyze mechanisms underlying modulation by N-methyl-d-aspartate (NMDA) receptors of proliferation of neural progenitor cells in adult mouse brain. The systemic administration of NMDA at 100 mg/kg resulted in marked expression of c-Fos, Fra-2 and c-Jun proteins in the granule cell layers of the dentate gyrus in murine hippocampus 2 h later, followed by a significant reduction of the incorporation of 5-bromo-2'-deoxyuridine (BrdU) in a manner sensitive to the antagonist dizocilpine 2 days after administration. The administration of NMDA also suppressed constitutive expression of both nestin and proliferating cell nuclear antigen (PCNA) in the dentate granule cells 2 days later, without markedly affecting cell viability for up to 8 weeks after administration. In the subventricular zone and olfactory bulb, however, NMDA failed to affect either the incorporation of BrdU or the expression of nestin and PCNA. The NR1 subunit was highly expressed in the dentate gyrus in addition to the stratum oriens in the hippocampus, but not in the subventricular zone and olfactory bulb. These results suggest that NMDA receptors may play a role crucial for maintenance of the integrity and function of proliferative neural progenitor cells through expression of the nuclear transcription factor activator protein-1 in granule cells of the dentate gyrus in adult mouse brain.

Differentiation and migration of astrocytes in the spinal cord following dorsal root injury in the adult rat

Nerve fibre degeneration in the spinal cord is accompanied by astroglial proliferation. It is not known whether these cells proliferate in situ or are recruited from specific regions harbouring astroglial precursors. We found cells expressing nestin, characteristic of astroglial precursors, at the dorsal surface of the spinal cord on the operated side from 30 h after dorsal root injury. Nestin-expressing cells dispersed to deeper areas of the dorsal funiculus and dorsal horn on the operated side during the first few days after injury. Injection of bromodeoxyuridine (BrdU) 2 h before the end of the experiment, at 30 h after injury, revealed numerous BrdU-labelled, nestin-positive cells in the dorsal superficial region. In animals surviving 20 h after BrdU injection at 28 h postlesion, cells double-labelled with BrdU and nestin were also found in deeper areas. Labeling with BrdU 2 h before perfusion showed proliferation of microglia and radial astrocytes in the ventral and lateral funiculi on both sides of the spinal cord 30 h after injury. Nestin-positive cells coexpressed the calcium-binding protein Mts1, a marker for white matter astrocytes, in the dorsal funiculus, and were positive for glial fibrillary acidic protein (GFAP), but negative for Mts1 in the dorsal horn. One week after injury the level of nestin expression decreased and was undetectable after 3 months. Taken together, our data indicate that after dorsal root injury newly formed astrocytes in the degenerating white and grey matter first appear at the dorsal surface of the spinal cord from where some of them subsequently migrate ventrally, and differentiate into white- or grey-matter astrocytes.

Alterations in cyclin A, B, and D1 in mouse dentate gyrus following TMT-induced hippocampal damage

The interactions of glia and neurons during injury and subsequent neurodegeneration are a subject of interest both in disease and chemical-induced brain injury. One such model is the prototypical hippocampal toxicant trimethyltin (TMT). An acute injection of TMT (2.0 mg/kg, i.p.) to postnatal day 21 CD-1 male mice produced neuronal necrosis and loss of dentate granule cells, astrocyte hypertrophy, and microglia activation in the hippocampus within 24 hrs. Neuronal necrosis and microglia differentiation to a phagocytic phenotype is temporally correlated with peak elevations in TNF-alpha, cyclin A2, cyclin B1 and cyclin D1 at 72 h post-TMT. TNF-alpha mRNA levels were significantly elevated in the hippocampus by 12 h and remained elevated for 72 h. mRNA levels for cyclin A2 and cyclin B1 were elevated by approximately 2-fold at 72 h. Immunohistochemistry suggested a cellular localization of cyclin A to microglia in the region of neuronal necrosis in the dentate, cyclin B in glial cells in juxtaposition to neurons in the hilus of the hippocampus and cyclin D1 to non-glial cells in the dentate. mRNA levels for cyclin D1 were elevated approximately 1.5-fold by 72 h as determined by RNase protection assay. No changes were seen in mRNA levels for cyclins E, F, G1, G2, H or I nor cyclin dependent kinases. These elevations are not associated with proliferation of microglia as determined by BrdU incorporation and Ki-67 immunohistochemistry. Upregulation of cell cycle genes was associated with cellular processes other than proliferation and may contribute to the differentiation of microglia to a phagocytic phenotype. These data suggest an integrated role for cell cycle regulation of neural cells in the manifestation of hippocampal pathophysiology.

Sonic hedgehog regulates adult neural progenitor proliferation in vitro and in vivo

Neural stem cells exist in the developing and adult nervous systems of all mammals, but the basic mechanisms that control their behavior are not yet well understood. Here, we investigated the role of Sonic hedgehog (Shh), a factor vital for neural development, in regulating adult hippocampal neural stem cells. We found high expression of the Shh receptor Patched in both the adult rat hippocampus and neural progenitor cells isolated from this region. In addition, Shh elicited a strong, dose-dependent proliferative response in progenitors in vitro. Furthermore, adeno-associated viral vector delivery of shh cDNA to the hippocampus elicited a 3.3-fold increase in cell proliferation. Finally, the pharmacological inhibitor of Shh signaling cyclopamine reduced hippocampal neural progenitor proliferation in vivo. This work identifies Shh as a regulator of adult hippocampal neural stem cells.

Treadmill exercise increases cell proliferation in dentate gyrus of rats with streptozotocin-induced diabetes

Diabetes mellitus is a common and serious metabolic disorder in humans. Streptozotocin (STZ)-induced diabetic rats were shown to produce a significant reduction in the number of proliferating cells in the dentate gyrus. In the present study, the effect of treadmill exercise on cell proliferation in the dentate gyrus of rats with STZ-induced diabetes was investigated via immunohistochemistry. Male Sprague-Dawley rats were divided into four groups: the control-rest group, the control-exercise group, the diabetes-rest group, and the diabetes-exercise group. Each of the animals of the diabetes groups was given a single injection of STZ (50 mg/kg) intraperitoneally. All animals were injected with 5-bromo-2'-deoxyuridine (50 mg/kg) for six consecutive days, starting on the second day after STZ injection, and rats of the exercise groups were made to run on treadmill for 30 min each day over the same period. On the eighth day of the experiment, all animals were sacrificed. In the present results, it was shown that cell proliferation in the dentate gyrus is suppressed under diabetic conditions, and that treadmill is effective in enhancing hippocampal granular cell proliferation in rats with STZ-induced diabetes. These results raise the possibility that treadmill exercise is of help in the alleviation of the central neural sequelae of diabetes mellitus.

Neural precursor proliferation and newborn cell survival in the adult rat dentate gyrus are affected by vitamin E deficiency

The adult hippocampal neurogenesis is affected by vitamin E deficiency. In the present investigation we examined if neural precursor proliferation, newborn cell survival or both are altered by vitamin E deficiency. 5-Bromo-2'-deoxyuridine (BrdU) was employed as a marker of proliferating cells. BrdU-labelled cells were revealed 1 and 30 days after BrdU administration in order to evaluate proliferation and newborn cell survival, respectively. Cell proliferation decreased in controls from juvenile to adult age, and the decrease was lesser in vitamin E deficiency. Thus we found a higher number of proliferating cells in vitamin E-deficient rats than in age-matched controls at 5 months of age. Comparing the number of BrdU-positive cells between 1 and 30 days after the last BrdU injection revealed a remarkable decrease in all groups; this is the greatest in vitamin E-deficient rats and the lowest in control rats. Consistently cell death in the dentate gyrus, assessed by TUNEL technique, was found to decrease from 1 to 5 months of age, but at 5 months it was significantly higher in vitamin E-deficient rats than in age-matched controls. These data show that vitamin E deficiency enhances neural precursor proliferation and cell death during adult neurogenesis.

Ginseng radix increases cell proliferation in dentate gyrus of rats with streptozotocin-induced diabetes

In the present study, the effect of Ginseng radix on cell proliferation in the dentate gyrus of rats with streptozotocin-induced diabetes was investigated via immunohistochemistry. Aqueous extract of Ginseng radix was shown to exert no significant effect on weight in normal rats, while it prevented weight loss in rats with streptozotocin-induced diabetes. Cell proliferation in the dentate gyrus of diabetic rats was increased by Ginseng radix treatment, but it had no effect on cell proliferation in normal rats. These results suggest that Ginseng radix may help in improve the central nervous system complications of diabetes mellitus.

Caspase-mediated death of newly formed neurons in the adult rat dentate gyrus following status epilepticus

A large proportion of cells that proliferate in the adult dentate gyrus under normal conditions or in response to brain insults exhibit only short-term survival. Here, we sought to determine which cell death pathways are involved in the degeneration of newly formed neurons in the rat dentate gyrus following 2 h of electrically induced status epilepticus. We investigated the role of three families of cysteine proteases, caspases, calpains, and cathepsins, which can all participate in apoptotic cell death. Status epilepticus increased the number of bromodeoxyuridine (BrdU)-positive proliferated cells in the subgranular zone of the dentate gyrus. At the time of maximum cell proliferation, immunohistochemical analyses revealed protein expression of active caspase-cleaved poly (ADP-ribose) polymerase (PARP) in approximately 66% of the BrdU-positive cells, while none of them expressed cathepsin B or the 150-kDa calpain-produced fodrin breakdown product. To evaluate the importance of cysteine proteases in regulating survival of the newly formed neurons, we administered intracerebroventricular infusions of a caspase inhibitor cocktail (zVAD-fmk, zDEVD-fmk and zLEHD-fmk) over a 2-week period, sufficient to allow for neuronal differentiation, starting 1 week after the epileptic insult. Increased numbers of cells double-labelled with BrdU and neuron-specific nuclear protein (NeuN) marker were detected in the subgranular zone and granule cell layer of the caspase inhibitor-treated rats. Our data indicate that caspase-mediated cell death pathways are active in progenitor cell progeny generated by status epilepticus and compromise survival during neuronal differentiation.

Neural stem cells in aging and disease

Aging in the central nervous system is associated with progressive loss of function which is exacerbated by neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. The two primary cell replacement strategies involve transplantation of exogenous tissue, and activation of proliferation of endogenous cells. Transplanted tissue is used to either directly replace lost tissue, or to implant genetically engineered cells that secrete factors which promote survival and/or proliferation. However, successful application of any cell replacement therapy requires knowledge of the complex relationships between neural stem cells and the more restricted neural and glial progenitor cells. This review focuses on recent advances in the field of stem cell biology of the central nervous system, with an emphasis on cellular and molecular approaches to replacing cells lost in neurodegenerative disorders.

Chronic hypoxia up-regulates fibroblast growth factor ligands in the perinatal brain and induces fibroblast growth factor-responsive radial glial cells in the sub-ependymal zone

A number of signaling molecules have been implicated in the acute response to hypoxia/ischemia in the adult brain. In contrast, the reaction to chronic hypoxemia is largely unexplored. We used a protocol of chronic hypoxia in rat pups during the first three postnatal weeks, encompassing the period of cellular plasticity in the cerebral cortex. We find that the levels of fibroblast growth factor 1 (FGF1) and FGF2, two members of the FGF family, increase after 2 weeks of chronic hypoxia. In contrast, members of the neurotrophin family are unaffected. FGF2 is normally expressed in the nucleus of mature, glial fibrillary acidic protein (GFAP)-containing astrocytes. Under hypoxia, most FGF2-containing cells do not express detectable levels of GFAP, suggesting that chronic low O(2) induces their transformation into more immature glial phenotypes. Remarkably, hypoxia promotes the appearance of radial glia throughout the sub-ventricular and ependymal zones. Most of these cells express vimentin and brain lipid binding protein. A subset of these radial glial cells expresses FGF receptor 1, and are in close contact with FGF2-positive cells in the sub-ventricular zone. Thus, FGF receptor signaling in radial glia may foster cell genesis after chronic hypoxic damage. From the results of this study we suggest that after the chronic exposure to low levels of oxygen during development, the expression of radial glia increases in the forebrain periventricular region. We envision that astroglia, which are the direct descendants of radial glia, are reverting back to immature glial cells. Alternatively, hypoxia hinders the normal maturation of radial glia into GFAP-expressing astrocytes. Interestingly, hypoxia increases the levels of expression of FGF2, a factor that is essential for neuronal development. Furthermore, chronic hypoxia up-regulated FGF2's major receptor in the periventricular region. Because radial glia have been suggested to play a key role in neurogenesis and cell migration, our data suggests that hypoxia-induced FGF signaling in radial glia may represent part of a conserved program capable of regenerating neurons in the brain after injury.

Seizure-induced neurogenesis: are more new neurons good for an adult brain?

The idea that neural stem cells may play a role in the pathophysiology or potential treatment of specific epilepsy syndromes is relatively new. This notion relates directly to advances in the field of stem cell biology over the past decade, which have confirmed prior theories that both neural stem cells and neurogenesis, the birth of new neurons, persist in specific regions of the adult mammalian brain. The physiological role of persistent neurogenesis is not known, although recent work implicates this process in specific learning and memory tasks. Knowledge of the normal neurogenic pathways in the mature brain has led to recent studies of neurogenesis in rodent models of acute seizures or epileptogenesis. Most of these studies have examined neurogenesis in the adult rodent dentate gyrus, and current evidence indicates that single brief or prolonged seizures, as well as repeated kindled seizures, increase dentate granule cell (DGC) neurogenesis. The models studied to date include pilocarpine and kainic acid models of temporal lobe epilepsy, limbic kindling, and intermittent perforant path stimulation. Recent work also suggests that pilocarpine-induced status epilepticus increases rostral forebrain subventricular zone (SVZ) neurogenesis and caudal SVZ gliogenesis. Several lines of evidence implicate newly generated neurons in structural and functional network abnormalities in the epileptic hippocampal formation of adult rodents. These abnormalities include aberrant mossy fiber reorganization, persistence of immature DGC structure (e.g. basal dendrites), and the abnormal migration of newborn neurons to ectopic sites in the dentate gyrus. Taken together, these findings suggest a pro-epileptogenic role of seizure- or injury-induced neurogenesis in the epileptic hippocampal formation. However, the induction of forebrain SVZ neurogenesis and directed migration to injury after seizures and other brain insults underscores the potential therapeutic use of neural stem cells as a source for neuronal replacement after injury.

Spontaneous recurrent seizures after pilocarpine-induced status epilepticus activate calbindin-immunoreactive hilar cells of the rat dentate gyrus

Although it is now established that neurogenesis of dentate gyrus granule cells increases after experimental seizures, little is currently known about the function of the new granule cells. One question is whether they become integrated into the network around them. Recent experiments that focused on the newly born granule cells in the hilus showed that indeed the new cells appear to become synchronized with host hippocampal neurons [Scharfman et al. (2000) J. Neurosci. 20, 6144-6158]. To address this issue further, we asked whether the new hilar granule cells were active during spontaneous limbic seizures that follow status epilepticus induced by pilocarpine injection. Thus, we perfused rats after spontaneous seizures and stained sections using antibodies to c-fos, a marker of neural activity, and calbindin, a marker of the newly born hilar granule cells [Scharfman et al. (2000) J. Neurosci. 20, 6144-6158]. We asked whether calbindin-immunoreactive hilar neurons were also c-fos-immunoreactive.C-fos was highly expressed in calbindin-immunoreactive hilar neurons. Approximately 23% of hilar cells that expressed c-fos were double-labeled for calbindin. In addition, other types of hilar neurons, i.e. those expressing parvalbumin or neuropeptide Y, also expressed c-fos. Yet other hippocampal neurons, including granule cells and pyramidal cells, had weak expression of c-fos at the latency after the seizure that hilar neuron expression occurred. In controls, there was very little c-fos or calbindin expression in the hilus.These results indicate that calbindin-immunoreactive hilar cells are activated by spontaneous seizures. Based on the evidence that many of these cells are likely to be newly born, the data indicate that new cells can become functionally integrated into limbic circuits involved in recurrent seizure generation. Furthermore, they appear to do so in a manner similar to many neighboring hilar neurons, apparently assimilating into the local environment. Finally, the results show that a number of hilar cell types are activated during chronic recurrent seizures in the pilocarpine model, a surprising result given that many hilar neurons are thought to be damaged soon after pilocarpine-induced status epilepticus.

Poly(ADP-ribose) polymerase-2 (PARP-2) is required for efficient base excision DNA repair in association with PARP-1 and XRCC1

The DNA damage dependence of poly(ADP-ribose) polymerase-2 (PARP-2) activity is suggestive of its implication in genome surveillance and protection. Here we show that the PARP-2 gene, mainly expressed in actively dividing tissues follows, but to a smaller extent, that of PARP-1 during mouse development. We found that PARP-2 and PARP-1 homo- and heterodimerize; the interacting interfaces, sites of reciprocal modification, have been mapped. PARP-2 was also found to interact with three other proteins involved in the base excision repair pathway: x-ray cross complementing factor 1 (XRCC1), DNA polymerase beta, and DNA ligase III, already known as partners of PARP-1. XRCC1 negatively regulates PARP-2 activity, as it does for PARP-1, while being a polymer acceptor for both PARP-1 and PARP-2. To gain insight into the physiological role of PARP-2 in response to genotoxic stress, we developed by gene disruption mice deficient in PARP-2. Following treatment by the alkylating agent N-nitroso-N-methylurea (MNU), PARP-2-deficient cells displayed an important delay in DNA strand breaks resealing, similar to that observed in PARP-1 deficient cells, thus confirming that PARP-2 is also an active player in base excision repair despite its low capacity to synthesize ADP-ribose polymers.

Overexpression of wild type but not an FAD mutant presenilin-1 promotes neurogenesis in the hippocampus of adult mice

Mutations in the presenilin-1 (PS-1) gene are one cause of familial Alzheimer's disease (FAD). However, the functions of the PS-1 protein as well as how PS-1 mutations cause FAD are incompletely understood. Here we investigated if neuronal overexpression of wild-type or FAD mutant PS-1 in transgenic mice affects neurogenesis in the hippocampus of adult animals. We show that either a wild-type or an FAD mutant PS-1 transgene reduces the number of neural progenitors in the dentate gyrus. However, the wild-type, but not the FAD mutant PS-1 promoted the survival and differentiation of progenitors leading to more immature granule cell neurons being generated in PS-1 wild type expressing animals. These studies suggest that PS-1 plays a role in regulating neurogenesis in adult hippocampus and that FAD mutants may have deleterious properties independent of their effects on amyloid deposition.

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

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

Three steps of neural stem cells development in gerbil dentate gyrus after transient ischemia

The stage of neurogenesis can be divided into three steps: proliferation, migration, and differentiation. To elucidate detailed relations between these three steps after ischemia, the authors evaluated the three steps in the adult gerbil dentate gyrus (DG) after 5 minutes of transient global ischemia using bromodeoxyuridine (BrdU), highly polysialylated neural cell adhesion molecule (PSA-NCAM), and neuronal nuclear antigen (NeuN) and glial fibrillary acidic protein (GFAP) as markers for proliferation, migration, and differentiation, respectively. Bromodeoxyuridine-labeled cells increased approximately sevenfold, and PSA-NCAM-positive cells increased approximately threefold in the subgranular zone (SGZ) with a peak 10 days after ischemia. Bromodeoxyuridine-labeled cells with PSA-NCAM expression were first detected both in the SGZ and the granule cell layer (GCL) 20 days after ischemia and gradually decreased after that, whereas BrdU-labeled cells with NeuN gradually increased in the GCL until 60 days after ischemia. A few BrdU-labeled cells with GFAP expression were detected in DG after ischemia; no PSA-NCAM-positive cells with GFAP expression were detected, but the radial processes of glial cells were partly in contact with PSA-NCAM-positive cell bodies and dendrites. These results suggest that neural stem cell proliferation begins at the SGZ, and that the cells then migrate into the GCL and differentiate mainly into neuronal cells. The majority of these three steps finished in 2 months after transient global ischemia.

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

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

Identification of neural stem cells in the adult vertebrate brain

Neurogenesis continues into adult life in restricted germinal layers. The identification of the neural stem cells that give rise to these new neurons has important clinical implications and provides fundamental information to understand the origins of the new neurons. Work in adult birds and rodents yielded a surprising result: the neural stem cells appear to have characteristics of glia. In adult birds, the primary neuronal precursors are radial glia. In adult mammals, the primary neuronal precursors have properties of astrocytes. Radial glial cells have previously been shown to transform into astrocytes; both cell types are classically considered part of a committed astroglial lineage. Instead, we propose that neural stem cells are contained within this astroglial lineage. These findings in adult vertebrate brain, together with recent work in the developing mammalian cerebral cortex, force us to reexamine traditional concepts about the origin of neurons and glia in the central nervous system. In particular, neural stem cells possess a surprisingly elaborate structure, suggesting that in addition to their progenitor role, they have important structural and metabolic support functions. The very same cells that give birth to new neurons also seem to nurture their maturation and support their function.

Dynamics of cell proliferation in the adult dentate gyrus of two inbred strains of mice

The output potential of proliferating populations in either the developing or the adult nervous system is critically dependent on the length of the cell cycle (T(c)) and the size of the proliferating population. We developed a new approach for analyzing the cell cycle, the 'Saturate and Survive Method' (SSM), that also reveals the dynamic behaviors in the proliferative population and estimates of the size of the proliferating population. We used this method to analyze the proliferating population of the adult dentate gyrus in 60 day old mice of two inbred strains, C57BL/6J and BALB/cByJ. The results show that the number of cells labeled by exposure to BUdR changes dramatically with time as a function of the number of proliferating cells in the population, the length of the S-phase, cell division, the length of the cell cycle, dilution of the S-phase label, and cell death. The major difference between C57BL/6J and BALB/cByJ mice is the size of the proliferating population, which differs by a factor of two; the lengths of the cell cycle and the S-phase and the probability that a newly produced cell will die within the first 10 days do not differ in these two strains. This indicates that genetic regulation of the size of the proliferating population is independent of the genetic regulation of cell death among those newly produced cells. The dynamic changes in the number of labeled cells as revealed by the SSM protocol also indicate that neither single nor repeated daily injections of BUdR accurately measure 'proliferation.'

Neurogenesis following brain ischemia

Following 5 or 10 min of global ischemia in the adult gerbil there is a tenfold increase in the birth of new cells in the subgranular zone of dentate gyrus of the hippocampus as assessed using BrdU incorporation. This begins at 7 days, peaks at 11 days, and decreases thereafter. Over the next month approximately 25% of the newborn cells disappear. Of the remaining cells, 60% migrate into the granule cell layer where two-thirds become NeuN, calbindin and MAP-2 immunostained neurons. The remaining 40% of the cells migrate into the dentate hilus where 25% of these become GFAP labeled astrocytes. It is proposed that ischemia-induced neurogenesis contributes to the recovery of function, and specifically may serve to improve anterograde and retrograde recent memory function that is lost following global ischemia in animals and man.

Induction of neuronal type-specific neurogenesis in the cerebral cortex of adult mice: manipulation of neural precursors in situ

Over the past 3 decades, research exploring potential neuronal replacement therapies have focused on replacing lost neurons by transplanting cells or grafting tissue into diseased regions of the brain [Nat. Neurosci. 3 (2000) 67-78]. Over most of the past century of modern neuroscience, it was thought that the adult brain was completely incapable of generating new neurons. However, in the last decade, the development of new techniques has resulted in an explosion of new research showing that neurogenesis, the birth of new neurons, normally occurs in two limited and specific regions of the adult mammalian brain, and that there are significant numbers of multipotent neural precursors in many parts of the adult mammalian brain [Mol. Cell. Neurosci. 19 (1999) 474-486]. Recent findings from our laboratory demonstrate that it is possible to induce neurogenesis de novo in the adult mammalian brain, particularly in the neocortex where it does not normally occur, and that it may become possible to manipulate endogenous multipotent precursors in situ to replace lost or damaged neurons [Nature 405 (2000) 951-955; Neuron 25 (2000) 481-492]. Recruitment of new neurons can be induced in a region-specific, layer-specific, and neuronal type-specific manner, and newly recruited neurons can form long-distance connections to appropriate targets. Elucidation of the relevant molecular controls may both allow control over transplanted precursor cells and potentially allow the development of neuronal replacement therapies for neurodegenerative disease and other central nervous system injuries that do not require transplantation of exogenous cells.

Morphological and functional properties of rat dentate granule cells after adrenalectomy

After complete adrenalectomy, part of the granule cells in the dentate gyrus undergo apoptosis. Findings on morphological changes in non-apoptotic granule cells, though, have been equivocal. In the present study we examined the dendritic trees of dentate granule cells 7 days after adrenalectomy or sham operation, and tested the hypothesis that changes in dendritic trees have considerable consequences for ionic currents, as measured in the soma with whole cell recording. For the latter, we focussed on voltage-gated calcium currents, which are partly generated in distal dendrites. All cells were passively filled with a fluorescent dye via the patch pipette while recording calcium currents; subsequently the cells were three-dimensionally reconstructed with the use of a confocal microscope. In sham-operated rats, dendritic trees of cells with a soma located in the inner part of the granule cell layer (facing the hilus) were significantly smaller than trees of cells located in the outer part of the layer. Neurons from rats that had extremely low (undetectable-0.3 microg/dl) circulating levels of corticosterone displayed very small and simple dendritic trees compared to cells from adrenalectomized rats that still had residual levels of corticosterone (0.6-1.0 microg/dl), regardless of the location of their soma. Despite the observation that simple dendritic trees were seen in rats where corticosterone was extremely low, the whole cell calcium current amplitude recorded from the soma of these cells was not reduced compared to the remaining cells from adrenalectomized or sham-operated rats. Our data indicate that in the absence of corticosterone dendritic trees of dentate granule cells display atrophy but that this does not necessarily reduce ionic currents measured in the soma.

Presenilin-1 is expressed in neural progenitor cells in the hippocampus of adult mice

The functions of the presenilin-1 (PS-1) protein remain largely unknown. In adult brain PS-1 is expressed principally in neurons. However during development PS-1 is expressed more widely including in embryonic neural progenitors. To determine if PS-1 is expressed in neural progenitors in adult hippocampus we used bromodeoxyuridine (BrdU) labeling combined with immunostaining for BrdU, PS-1 and markers of neuronal or glial differentiation. Most BrdU labeled cells also expressed PS-1 at a time when few BrdU labeled cells expressed the early neuronal markers beta-III tubulin or TOAD-64 and none expressed mature neuronal (NeuN or calbindin) or astrocytic (GFAP) markers. Cells expressing PS-1 and the neural progenitor marker nestin were also found. Thus PS-1 is expressed in neural progenitor cells in adult hippocampus implying its possible role in neurogenesis in adult brain.

Stem cells: hype and reality

This update discusses what is known regarding embryonic and adult tissue-derived pluripotent stem cells, including the mechanisms underlying self-renewal without senescence, differentiation in multiple cell types both in vitro and in vivo, and future potential clinical uses of such stem cells. In Section I, Dr. Lansdorp reviews the structure and function of telomerase, the enzyme that restores telomeric ends of chromosomes upon cell division, highly present in embryonic stem cells but not adult stem cells. He discusses the structure and function of telomerase and signaling pathways activated by the enzyme, with special emphasis on normal and leukemic hematopoietic stem cells. In Section II, Dr. Pera reviews the present understanding of mammalian pluripotent embryonic stem cells. He discusses the concept of pluripotentiality in its embryonic context, derivation of stem cells from embryonic or fetal tissue, the basic properties of the stem cells, and methods to produce specific types of differentiated cell from stem cells. He examines the potential applications of stem cells in research and medicine and some of the barriers that must be crossed to achieve these goals. In Section III, Dr. Verfaillie reviews the present understanding of pluripotency of adult stem cells. She discusses the concept of stem cell plasticity, a term used to describe the greater potency described by several investigators of adult tissue-derived stem cells, critically reviews the published studies demonstrating stem cell plasticity, and possible mechanisms underlying such plasticity, and examines the possible role of pluripotent adult stem cells in research and medicine.

Hippocampal granule neuron production and population size are regulated by levels of bFGF

Numerous studies of the proliferative effects of basic fibroblast growth factor (bFGF) in culture, including neonatal and adult hippocampal precursors, suggest that the factor plays a ubiquitous and life-long role in neurogenesis. In contrast, in vivo, bFGF is devoid of effects on neurons in mature hippocampus, raising the possibility that bFGF exhibits developmental stage-specific activity in the complex animal environment. To define neurogenetic effects in the newborn, a single subcutaneous injection of bFGF (20 ng/gm) was administered to postnatal day 1 (P1) rats, and hippocampal DNA content was quantified: bFGF elicited an increase in total DNA throughout adulthood, by 48% at P4, 25% at P22, and 17% at P180, suggesting that bFGF increases hippocampal cell number. To define mechanisms, bromodeoxyuridine (BrdU) was injected at P1 and mitotically labelled cells were assessed at P22: there was a twofold increase in BrdU-positive cells in the dentate granule cell layer (GCL), indicating that bFGF enhanced the generation of neurons, or neuronogenesis, from a cohort of precursors. Moreover, enhanced mitosis and survival led to a 33% increase in absolute GCL neuron number, suggesting that neuron production depends on environmental levels of bFGF. To evaluate this possibility, bFGF-knockout mice were analyzed: hippocampal DNA content was decreased at all ages examined (P3, -42%; P21, -28%; P360, -18%), and total GCL neuron and glial fibrillary acidic protein (GFAP)-positive cell number were decreased by 30%, indicating that bFGF is necessary for normal hippocampal neurogenesis. We conclude that environmental levels of bFGF regulate neonatal hippocampal neurogenesis. As adult hippocampal neuronogenesis was unresponsive to bFGF manipulation in our previous study [Wagner, J.P., Black, I.B. & DiCicco-Bloom, E. (1999) J. Neurosci., 19, 6006], these observations suggest distinct, stage-specific roles of bFGF in the dentate gyrus granule cell lineage.

Regulation of olfactory neurogenesis by amidated neuropeptides

The existence of stem cells in the CNS raises issues concerning the ability of nervous tissues to regenerate in the adult mammal and provides new perspectives on the treatment of degenerative disease and traumatic injury of the nervous system. These cells have a relatively limited range of locations within the nervous system and include cells of the rostral migratory stream, hippocampus, retina, and olfactory epithelium. The olfactory epithelium has been studied as a model of adult neuronal regeneration, with neuronal precursor/basal cells serving as the olfactory "stem cells." The identification of factors that promote neuronal proliferation or regeneration within the olfactory epithelium can provide clues to the process of adult mammalian nervous system repair and treatment. Multiple factors have been examined that appear to influence the proliferation and subsequent maturation of basal cells. These factors include nerve growth factor, fibroblast growth factor-2, epidermal growth factor, and insulin/insulin-like growth factor-1. Recently, two amidated neuropeptides, neuropeptide Y (NPY) and pituitary adenylate cyclase-activating polypeptide (PACAP38), identified in the olfactory epithelium have been shown to promote dramatically neuronal proliferation. The effects of NPY and PACAP suggest that amidated neuropeptides may serve a broad developmental and regenerative role in the mammalian olfactory epithelium.

Proliferation and differentiation of progenitor cells in the cortex and the subventricular zone in the adult rat after focal cerebral ischemia

Progenitor cells in the subventricular zone of the lateral ventricle and in the dentate gyrus of the hippocampus can proliferate throughout the life of the animal. To examine the proliferation and fate of progenitor cells in the subventricular zone and dentate gyrus after focal cerebral ischemia, we measured the temporal and spatial profiles of proliferation of cells and the phenotypic fate of proliferating cells in ischemic brain in a model of embolic middle cerebral artery occlusion in the adult rat. Proliferating cells were labeled by injection of bromodeoxyuridine (BrdU) in a pulse or a cumulative protocol. To determine the temporal profile of proliferating cells, ischemic rats were injected with BrdU every 4 h for 12 h on the day preceding death. Rats were killed 2-14 days after ischemia. We observed significant increases in numbers of proliferating cells in the ipsilateral cortex and subventricular zone 2-14 days with a peak at 7 days after ischemia compared with the control group. To maximize labeling of proliferating cells, a single daily injection of BrdU was administered over a 14-day period starting the day after ischemia. Rats were killed either 2 h or 28 days after the last injection of BrdU. A significant increase in numbers of BrdU immunoreactive cells in the subventricular zone was coincident with a significant increase in numbers of BrdU immunoreactive cells in the olfactory bulb 14 days after ischemia and numbers of BrdU immunoreactive cells did not significantly increase in the dentate gyrus. However, 28 days after the last labeling, the number of BrdU labeled cells decreased by 90% compared with number at 14 days. Clusters of BrdU labeled cells were present in the cortex distal to the infarction. Numerous cells immunostained for the polysialylated form of the neuronal cell adhesion molecule were detected in the ipsilateral subventricular zone. Only 6% of BrdU labeled cells exhibited glial fibrillary acidic protein immunoreactivity in the cortex and subcortex and no BrdU labeled cells expressed neuronal protein markers (neural nuclear protein and microtubule associated protein-2). From these data we suggest that focal cerebral ischemia induces transient and regional specific increases in cell proliferation in the ipsilateral hemisphere and that proliferating progenitor cells may exist in the adult cortex.

p53 is dispensable for apoptosis but controls neurogenesis of mouse dentate gyrus cells following gamma-irradiation

Mammalian cells respond to DNA insults by activating cell-cycle checkpoints. This may result in a temporary cell growth arrest which allows DNA repair before proliferation or induces apoptosis. p53 is one of the main contributors in regulating these activities. To get a better insight on the molecular mechanism underlying these activities we studied the role of p53 in apoptosis and neurogenesis of brain cells from adult p53(+/+) or p53(-/-) mice exposed to gamma-irradiation. Apoptosis and neurogenesis were assessed up to 14 days following the injury. Five-ten hours following gamma-irradiation, cells with TUNEL positive nuclei were identified within the subgranular zone of dentate gyrus (DG) of both p53(+/+) and p53(-/-) mice. At the same time-points, pyknotic and shrinking nuclei were visualized by Hoechst 33258 staining. Furthermore, gamma-irradiation increased the number of proliferating cell nuclear antigen (PCNA) positive cells with a peak at 5-10 h in both animal groups. PCNA immunoreactivity was detected in cells exhibiting condensed nuclei as visualized by Hoechst 33258 staining. Neurogenesis, assessed by mitotic marker p34(cdc2) immunoreactivity, showed a biphasic response to gamma-irradiation both in p53(+/+) and p53(-/-) mice which was characterized by an early inhibition and a delayed stimulation. In p53(-/-) mice, the time required by DG granule cells to recover from the lesion and to stimulate proliferation was significantly shortened in comparison with wild-type mice thus resulting in an accelerated neurogenesis. Our data indicate that following gamma-radiation p53 plays a role in regulating cell-cycle progression rate but it is dispensable for promoting apoptosis of DG granule cells.