Ischemia-induced neurogenesis is preserved but reduced in the aged rodent brain

The adult mammalian brain retains the capacity for neurogenesis, by which new neurons may be generated to replace those lost through physiological or pathological processes. However, neurogenesis diminishes with aging, and this casts doubt on its feasibility as a therapeutic target for cell replacement therapy in stroke and neurodegenerative disorders, which disproportionately affect the aged brain. In previous studies, neurogenesis was stimulated by cerebral ischemia in young rodents, and the neurogenesis response of the aged rodent brain to physiological stimuli, such as hormonal manipulation and growth factors, was preserved. To investigate the effect of aging on ischemia-induced neurogenesis, transient (60 min) middle cerebral artery occlusion was induced in young adult (3-month) and aged (24-month) rats, who were also given bromodeoxyuridine to label newborn cells. As found in prior studies, basal neurogenesis in control, nonischemic rats was reduced with aging. Ischemia failed to stimulate neurogenesis in the dentate gyrus (DG) subgranular zone (SGZ), in contrast to results obtained previously after more prolonged (90-120 min) middle cerebral artery occlusion, but increased the number of BrdU-labeled cells in the forebrain subventricular zone (SVZ). This effect was less prominent in aged than in young adult rats, with fold-stimulation of BrdU incorporation reduced by approximately 20% and the total number of cells generated diminished by approximately 50%. BrdU-labeled cells in SVZ coexpressed neuronal lineage markers, consistent with newborn neurons. We conclude that ischemia-induced neurogenesis occurs in the aged brain, and that measures designed to augment this phenomenon might have therapeutic applications.     

Vascular endothelial growth factor-B (VEGFB) stimulates neurogenesis: evidence from knockout mice and growth factor administration

Vascular endothelial growth factor-B (VEGFB) is an angiogenic and neuroprotective protein that reduces hypoxic and ischemic neuronal injury. To determine if VEGFB also regulates neurogenesis in the adult brain, we studied the effects of VEGFB administration in vitro and in vivo, as well as the effect of VEGFB gene knockout (KO) in mice, on bromodeoxyuridine (BrdU) incorporation and expression of immature neuronal markers in the subgranular zone (SGZ) of the hippocampal dentate gyrus and the forebrain subventricular zone (SVZ). Intracerebroventricular VEGFB administration increased BrdU incorporation into cells of neuronal lineage both in vitro and in vivo, and VEGFB-KO mice showed impaired neurogenesis, consistent with a neurogenesis-promoting effect of VEGFB. In addition, intraventricular administration of VEGFB restored neurogenesis to wild-type levels in VEGFB-KO mice. These results suggest a role for VEGFB in the regulation of adult neurogenesis, which could have therapeutic implications for diseases associated with central neuronal loss.     

Notch1 signaling modulates neuronal progenitor activity in the subventricular zone in response to aging and focal ischemia

Neurogenesis diminishes with aging and ischemia‐induced neurogenesis also occurs, but reduced in aged brain. Currently, the cellular and molecular pathways mediating these effects remain largely unknown. Our previous study has shown that Notch1 signaling regulates neurogenesis in subventricular zone (SVZ) of young adult brain after focal ischemia, but whether a similar effect occurs in aged normal and ischemic animals is unknown. Here, we used normal and ischemic aged rat brains to investigate whether Notch1 signaling was involved in the reduction of neurogenesis in response to aging and modulates neurogenesis in aged brains after focal ischemia. By Western blot, we found that Notch1 and Jagged1 expression in the SVZ of aged brain was significantly reduced compared with young adult brain. Consistently, the activated form of Notch1 (Notch intracellular domain; NICD) expression was also declined. Immunohistochemistry confirmed that expression and activation of Notch1 signaling in the SVZ of aged brain were reduced. Double or triple immunostaining showed that that Notch1 was mainly expressed in doublecortin (DCX)‐positive cells, whereas Jagged1 was predominantly expressed in astroglial cells in the SVZ of normal aged rat brain. In addition, disruption or activation of Notch1 signaling altered the number of proliferating cells labeled by bromodeoxyuridine (BrdU) and DCX in the SVZ of aged brain. Moreover, ischemia‐induced cell proliferation in the SVZ of aged brain was enhanced by activating the Notch1 pathway and was suppressed by inhibiting the Notch1 signaling. Reduced infarct volume and improved motor deficits were also observed in Notch1 activator–treated aged ischemic rats. Our data suggest that Notch1 signaling modulates the SVZ neurogenesis in aged brain in normal and ischemic conditions.     

The window and mechanisms of major age-related decline in the production of new neurons within the dentate gyrus of the hippocampus

While it is well known that production of new neurons from neural stem/progenitor cells (NSC) in the dentate gyrus (DG) diminishes greatly by middle age, the phases and mechanisms of major age-related decline in DG neurogenesis are largely unknown. To address these issues, we first assessed DG neurogenesis in multiple age groups of Fischer 344 rats via quantification of doublecortin-immunopositive (DCX+) neurons and then measured the production, neuronal differentiation and initial survival of new cells in the subgranular zone (SGZ) of 4-, 12- and 24-month-old rats using four injections (one every sixth hour) of 5'-bromodeoxyuridine (BrdU), and BrdU-DCX dual immunostaining. Furthermore, we quantified the numbers of proliferating cells in the SGZ of these rats using Ki67 immunostaining. Numbers of DCX+ neurons were stable at 4-7.5 months of age but decreased progressively at 7.5-9 months (41% decline), 9-10.5 months (39% decline), and 10.5-12 months (34% decline) of age. Analyses of BrdU(+) cells at 6 h after the last BrdU injection revealed a 71-78% decline in the production of new cells per day between 4-month-old rats and 12- or 24-month-old rats. Numbers of proliferating Ki67+ cells (putative NSCs) in the SGZ also exhibited similar (72-85%) decline during this period. However, the extent of both neuronal differentiation (75-81%) and initial 12-day survival (67-74%) of newly born cells was similar in all age groups. Additional analyses of dendritic growth of 12-day-old neurons revealed that newly born neurons in the aging DG exhibit diminished dendritic growth compared with their age-matched counterparts in the young DG. Thus, major decreases in DG neurogenesis occur at 7.5-12 months of age in Fischer 344 rats. Decreased production of new cells due to proliferation of far fewer NSCs in the SGZ mainly underlies this decline.     

Glycogen Synthase Kinase-3β Regulates Equilibrium Between Neurogenesis and Gliogenesis in Rat Model of Parkinson’s Disease: a Crosstalk with Wnt and Notch Signaling

Neurogenesis involves generation of functional newborn neurons from neural stem cells (NSCs). Insufficient formation or accelerated degeneration of newborn neurons may contribute to the severity of motor/nonmotor symptoms of Parkinson’s disease (PD). However, the functional role of adult neurogenesis in PD is yet not explored and whether glycogen synthase kinase-3β (GSK-3β) affects multiple steps of adult neurogenesis in PD is still unknown. We investigated the possible underlying molecular mechanism of impaired adult neurogenesis associated with PD. Herein, we show that single intra-medial forebrain bundle (MFB) injection of 6-hydroxydopamine (6-OHDA) efficiently induced long-term activation of GSK-3β and reduced NSC self-renewal, proliferation, neuronal migration, and neuronal differentiation accompanied with increased astrogenesis in subventricular zone (SVZ) and hippocampal dentate gyrus (DG). Indeed, 6-OHDA also delayed maturation of neuroblasts in the DG as witnessed by their reduced dendritic length and arborization. Using a pharmacological approach to inhibit GSK-3β activation by specific inhibitor SB216763, we show that GSK-3β inhibition enhances radial glial cells, NSC proliferation, self-renewal in the SVZ, and the subgranular zone (SGZ) in the rat PD model. Pharmacological inhibition of GSK-3β activity enhances neuroblast population in SVZ and SGZ and promotes migration of neuroblasts towards the rostral migratory stream and lesioned striatum from dorsal SVZ and lateral SVZ, respectively, in PD model. GSK-3β inhibition enhances dendritic arborization and survival of granular neurons and stimulates NSC differentiation towards the neuronal phenotype in DG of PD model. The aforementioned effects of GSK-3β involve a crosstalk between Wnt/β-catenin and Notch signaling pathways that are known to regulate NSC dynamics.     

Stimulatory effects of prostaglandin E2 on neurogenesis in the dentate gyrus of the adult rat

Neurogenesis in the dentate gyrus of adult rodents is elicited by transient global ischemia. Cyclooxygenase (COX) -2, a rate-limiting enzyme for prostanoid synthesis, is also induced by ischemia. We recently found that the administration of a non-selective COX inhibitor to ischemic animals suppressed cell proliferation in the subgranular zone (SGZ) at the dentate gyrus of the hippocampus. To clarify whether prostaglandin E2 (PGE2) synthesis by COX's is involved in neurogenesis, sulprostone, an analogue of PGE2, was injected into the rat hippocampus. Sulprostone injection increased the number of 5-bromo-2'-deoxyuridine (BrdU)-positive cells in the SGZ. BrdU-positive cells also expressed polysialylated isoforms of neural cell adhesion molecule and neuronal nuclear antigen. These results suggest that PGE2 plays an important role in the proliferation of cells in the SGZ.     

Systemically delivered Erythropoietin transiently enhances adult hippocampal neurogenesis

Erythropoietin is a primary regulator of erythropoiesis in the hematopoietic system. More recently erythropoietin has been shown to play a role in neurogenesis and provide neurotrophic support to injured CNS tissue. Here the effects of large systemic doses of erythropoietin on basal levels of adult hippocampal neurogenesis in mice were examined. A 7-day period of recombinant human erythropoietin (rhEPO) administration increased the number of bromodeoxyuridine [BrdU(+)] cells in the sub-granular zone (SGZ) by 30%. Analysis of cell phenotype revealed an increase in mitotically active doublecortin(+) neuronal progenitor cells and glial fibrillary acidic protein(+) SGZ radial astrocytes/stem cells but not mature S100beta(+) astrocytes. These effects appeared to be mediated, in part, by mitogen-activated protein kinase signaling and potentially regulated by suppressor of cytokine signaling-3. Hippocampal levels of phosphorylated extracellular signal-related kinase 42/44 and suppressor of cytokine signaling-3 were increased 2-6 h after a single systemic rhEPO injection. However, rhEPO had no observed effect on the long-term survival of new born cells in the SGZ, with similar numbers of BrdU(+) cells and BrdU(+)/NeuN(+) co-labeled cells after 4 weeks. Therefore, systemically delivered rhEPO transiently increased adult hippocampal neurogenesis without any apparent long-term effects.     

Mutant α-synuclein and aging reduce neurogenesis in the acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine model of Parkinson's disease

Neurogenesis, the production of new neurons from less differentiated precursor cells, normally occurs in adult brains in the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone of the hippocampal dentate gyrus. Neurogenesis declines with aging. In previous studies, neurogenesis was stimulated by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride (MPTP) in young animals. In this study, we examined the effect of acute MPTP administration and mutant α-synuclein A53T on neurogenesis and migration of newborn neurons in the aged (23-month) vs. young (2-month) rodent brain. Cell proliferation and neurogenesis were assessed via bromodeoxyuridine labeling and immunostaining for cell type-specific markers. In the aged brain, neural precursor cells in the rostral SVZ retained the capacity for proliferation and migration in response to MPTP-induced Parkinsonism, although the response is less robust than in younger animals. Furthermore, in transgenic mice that overexpress mutant α-synuclein (A53T), brains examined day 21 after MPTP administration showed markedly decreased olfactory bulb and substantia nigra neurogenesis. Our data suggest that in addition to aging effects associated with decline in the number of newly generated cells, mutant α-synuclein reduces MPTP-induced neurogenesis. This could provide a novel therapeutic target for chronic brain repair in this condition.     

The aging neurogenic subventricular zone

In the adult mouse brain, the subventricular zone (SVZ) is a neurogenic stem cell niche only 4-5 cell diameters thick. Within this narrow zone, a unique microenvironment supports stem cell self-renewal, gliogenesis or neurogenesis lineage decisions and tangential migration of newly generated neurons out of the SVZ and into the olfactory bulb. However, with aging, SVZ neurogenesis declines. Here, we examine the dynamic interplay between SVZ cytoarchitecture and neurogenesis through aging. Assembly of high-resolution electron microscopy images of corresponding coronal sections from 2-, 10- and 22-month-old mice into photomontages reveal a thinning of the SVZ with age. Following a 2-h BrdU pulse, we detect a significant decrease in cell proliferation from 2 to 22 months. Neuroblast numbers decrease with age, as do transitory amplifying progenitor cells, while both SVZ astrocytes and adjacent ependymal cells remain relatively constant. At 22 months, only residual pockets of neurogenesis remain and neuroblasts become restricted to the anterior dorsolateral horn of the SVZ. Within this dorsolateral zone many key components of the younger neurogenic niche are maintained; however, in the aged SVZ, increased numbers of SVZ astrocytes are found interposed within the ependyma. These astrocytes co-label with markers to ependymal cells and astrocytes, form intercellular adherens junctions with neighboring ependymal cells, and some possess multiple basal bodies of cilia within their cytoplasm. Together, these data reveal an age-related, progressive restriction of SVZ neurogenesis to the dorsolateral aspect of the lateral ventricle with increased numbers of SVZ astrocytes interpolated within the ependyma.     

IGF-1对缺血性脑损伤大鼠脑内神经发生的影响

目的:建立大鼠单侧局灶脑缺血模型,观察胰岛素样生长因子-1(IGF-1)对局灶脑缺血后的鼠脑神经发生及增殖后细胞生存的影响.方法:用健康雄性SD大鼠建立大脑中动脉阻塞(MCAO)模型,随机分成假手术组,缺血对照组和IGF-1治疗组.各组再按不同的治疗时间分为7d、14d、28d、42d组.免疫组化法观察BrdU、PSA-NCAM的变化,免疫双标法观察BrdU/PSA-NCAM、BrdU/MAP2和BrdU/GFAP的共同表达变化.结果:BrdU标记细胞和PSA-NCAM标记细胞计数均在缺血后第7d最多,分别是缺血对照组的4.0倍和1.8倍,是假手术组的9.9倍和5.4倍.BrdU和PSA-NCAM双标细胞在缺血发生后双侧SVZ和DG区可以检测到,于第7d计数最多,之后逐渐降低;而BrdU和MAP2以及BrdU和GFAP双标细胞却从第14d开始逐渐增多,直到第42d.随着BrdU/PSA-NCAM双标阳性表达的逐渐降低,BrdU/MAP2双标阳性表达逐渐增高,呈现此消彼涨的变化.结论:IGF-1侧脑室注射后,在早期(7d内)诱导了缺血性脑损伤后神经细胞的增殖;在中期(7d-14d)诱导了新生细胞的迁移;在后期(14d后)伴随着迁移的进行新生细胞逐渐发生了分化.     

Locating and labeling neural stem cells in the brain

The phenomenon of adult neurogenesis has been demonstrated in most mammals including humans. At least two regions of the adult brain maintain stem cells throughout life; the subgranular zone (SGZ) of the hippocampal dentate gyrus, and the subventricular zone (SVZ) of the lateral ventricle wall. Both regions continuously produce neurons that mature and become integrated into functional networks that are involved in learning and memory and odor discrimination, respectively. Apart from these well‐studied regions neurogenesis has been reported in a number of other brain regions, such as amygdala and cortex. However, these studies have been contested and there is currently no well‐postulated function for non‐SVZ/SGZ neurogenesis. The studies of the regional localization of neurogenesis in the brain have been made possible due to several methods for detecting adult neurogenesis including; bromodeoxyuridine labeling (BrdU) together with markers of mature neurons, genetic labeling, by mouse transgenesis, or with the use of viral vectors. These techniques are already put to creative use and will be essential for the discovery of the nature of the adult neural stem cells. In this mini‐review, we will discuss the localization of neural stem/progenitor cells in the brain and their implications as well as discussing the pro's and con's of stem cell labeling techniques. J. Cell. Physiol. 226: 1–7, 2010. © 2010 Wiley‐Liss, Inc.     

Zinc chelation reduces traumatic brain injury-induced neurogenesis in the subgranular zone of the hippocampal dentate gyrus

Numerous studies have demonstrated that traumatic brain injury (TBI) increases hippocampal neurogenesis in the rodent brain. However, the mechanisms underlying increased neurogenesis after TBI remain unknown. Continuous neurogenesis occurs in the subgranular zone (SGZ) of the hippocampal dentate gyrus (DG) in the adult brain. The mechanism that maintains active neurogenesis in the hippocampal area is not known. A high level of vesicular zinc is localized in the presynaptic terminals of the SGZ (mossy fiber). The mossy fiber of dentate granular cells contains high levels of chelatable zinc in their terminal vesicles, which can be released into the extracellular space during neuronal activity. Previously, our lab presented findings indicating that a possible correlation may exist between synaptic zinc localization and high rates of neurogenesis in this area after hypoglycemia or epilepsy. Using a weight drop animal model to mimic human TBI, we tested our hypothesis that zinc plays a key role in modulating hippocampal neurogenesis after TBI. Thus, we injected a zinc chelator, clioquinol (CQ, 30 mg/kg), into the intraperitoneal space to reduce brain zinc availability twice per day for 1 week. Neuronal death was evaluated with Fluoro Jade-B and NeuN staining to determine whether CQ has neuroprotective effects after TBI. The number of degenerating neurons (FJB (+)) and live neurons (NeuN (+)) was similar in vehicle and in CQ-treated rats at 1 week after TBI. Neurogenesis was evaluated using BrdU, Ki67 and doublecortin (DCX) immunostaining 1 week after TBI. The number of BrdU, Ki67 and DCX positive cell was increased after TBI. However, the number of BrdU, Ki67 and DCX positive cells was significantly decreased by CQ treatment. The present study shows that zinc chelation did not prevent neurodegeneration but did reduce TBI-induced progenitor cell proliferation and neurogenesis. Therefore, this study suggests that zinc has an essential role for modulating hippocampal neurogenesis after TBI.     

Brain ischemia, neurogenesis, and neurotrophic receptor expression in primates

Generation of new neurons persists in the normal adult mammalian brain, with neural stem/progenitor cells residing in at least two brain regions: the subventricular zone (SVZ) of the lateral ventricle and the subgranular zone (SGZ) of the dentate gyrus (DG). Adult neurogenesis is well documented in the rodent, and has also been demonstrated in vivo in nonhuman primates and humans. Brain injuries such as ischemia affect neurogenesis in adult rodents as both global and focal ischemic insults enhance the proliferation of progenitor cells residing in SGZ or SVZ. We addressed the issue whether an injury triggered activation of endogenous neuronal precursors also takes place in the adult primate brain. We found that the ischemic insult increased the number of progenitor cells in monkey SGZ and SVZ, and caused gliogenesis in the ischemia-prone hippocampal CA1 sector. To better understand the mechanisms regulating precursor cell division and differentiation in the primate, we analyzed the expression at protein level of a panel of potential regulatory molecules, including neurotrophic factors and their receptors. We found that a fraction of mitotic progenitors were positive for the neurotrophin receptor TrkB, while immature neurons expressed the neurotrophin receptor TrkA. Astroglia, ependymal cells and blood vessels in SVZ were positive for distinctive sets of ligands/receptors, which we characterized. Thus, a network of neurotrophic signals operating in an autocrine or paracrine manner may regulate neurogenesis in adult primate SVZ. We also analyzed microglial and astroglial proliferation in postischemic hippocampal CA1 sector. We found that proliferating postischemic microglia in adult monkey CA1 sector express the neurotrophin receptor TrkA, while activated astrocytes were labeled for nerve growth factor (NGF), ligand for TrkA, and the tyrosine kinase TrkB, a receptor for brain derived neurotrophic factor (BDNF). These results implicate NGF and BDNF as regulators of postischemic glial proliferation in adult primate hippocampus.     

Cellular organization of adult neurogenesis in the Common Marmoset

Adult neurogenesis within the subgranular zone (SGZ) of the hippocampal dentate gyrus and the subventricular zone (SVZ) of the lateral ventricle (LV) has been most intensely studied within the brains of rodents such as mice and rats. However, little is known about the cell types and processes involved in adult neurogenesis within primates such as the common marmoset (Callithrix jacchus). Moreover, substantial differences seem to exist between the neurogenic niche of the LV between rodents and humans. Here, we set out to use immunohistochemical and autogradiographic analysis to characterize the anatomy of the neurogenic niches and the expression of cell type-specific markers in those niches in the adult common marmoset brain. Moreover, we demonstrate significant differences in the activity of neurogenesis in the adult marmoset brain compared to the adult mouse brain. Finally, we provide evidence for ongoing proliferation of neuroblasts within both the SGZ and SVZ of the adult brain and further show that the age-dependent decline of neurogenesis in the hippocampus is associated with a decrease in neuroblast cells.     

Heparin-Binding EGF-Like Growth Factor Induces Heart Interstitial Fibrosis via an Akt/mTor/p70s6k Pathway

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) is essential for maintaining normal function of the adult heart and is known to play an important role in myocardial remodeling. In the present study, we observed that heart-specific HB-EGF transgenic (TG) mice had systolic dysfunction with decreased fractional shortening (FS%), increased end-systolic diameter (LVIDs) at 5 months of age, increased heart fibrosis, and increased mRNA expression of Col1α1 and Col3α1 at 1, 3, 5 and 7 months of age compared to nontransgenic (NTG) littermates. However, the left ventricular anterior wall thickness at end-systole (LVAWs) of the TG mice was not different than the NTG mice. Phosphorylation levels of Akt, mTor and p70s6k were increased due to HB-EGF expression in TG mice compared with the NTG mice at 3 and 7 months of age. Additionally, activated Akt, mTor and p70s6k were co-localized with vimentin to cardiac fibroblasts isolated from TG mice. Furthermore, HB-EGF significantly increased phosphorylation levels of Akt, mTor and p70s6k and increased expression of type I collagen in cultured primary cardiac fibroblasts. Rapamycin (Rapa) and CRM197, inhibitors of mTor and HB-EGF respectively, could inhibit the expression of type I collagen in the cultured primary cardiac fibroblasts and Rapa suppressed interstitial fibrosis of the heart tissues in vivo. In addition, a BrdU assay showed that HB-EGF increased proliferation of cardiac fibroblasts by 30% compared with cells without HB-EGF treatment. HB-EGF-induced proliferation was completely diminished in the presence of Rapa. These results suggest that HB-EGF induced heart fibrosis and proliferation of cardiac fibroblasts occurs through activation of the Akt/mTor/p70s6k pathway.     

Reduced proliferation in the adult mouse subventricular zone increases survival of olfactory bulb interneurons

Neurogenesis in the adult brain is largely restricted to the subependymal zone (SVZ) of the lateral ventricle, olfactory bulb (OB) and the dentate subgranular zone, and survival of adult-born cells in the OB is influenced by factors including sensory experience. We examined, in mice, whether survival of adult-born cells is also regulated by the rate of precursor proliferation in the SVZ. Precursor proliferation was decreased by depleting the SVZ of dopamine after lesioning dopamine neurons in the substantia nigra compacta with 6-hydroxydopamine. Subsequently, we examined the effect of reduced SVZ proliferation on the generation, migration and survival of neuroblasts and mature adult-born cells in the SVZ, rostral migratory stream (RMS) and OB. Proliferating cells in the SVZ, measured by 5-bromo-2-deoxyuridine (BrdU) injected 2 hours prior to death or by immunoreactivity against Ki67, were reduced by 47% or 36%, respectively, 7 days after dopamine depletion, and by 29% or 31% 42 days after dopamine depletion, compared to sham-treated animals. Neuroblast generation in the SVZ and their migration along the RMS were not affected, neither 7 nor 42 days after the 6-hydroxydopamine injection, since the number of doublecortin-immunoreactive neuroblasts in the SVZ and RMS, as well as the number of neuronal nuclei-immunoreactive cells in the OB, were stable compared to control. However, survival analysis 15 days after 6-hydroxydopamine and 6 days after BrdU injections showed that the number of BrdU+ cells in the SVZ was 70% higher. Also, 42 days after 6-hydroxydopamine and 30 days after BrdU injections, we found an 82% increase in co-labeled BrdU+/γ-aminobutyric acid-immunoreactive cell bodies in the granular cell layer, while double-labeled BrdU+/tyrosine hydroxylase-immunoreactive cell bodies in the glomerular layer increased by 148%. We conclude that the number of OB interneurons following reduced SVZ proliferation is maintained through an increased survival of adult-born precursor cells, neuroblasts and interneurons.     

Retinoic acid regulates postnatal neurogenesis in the murine subventricular zone-olfactory bulb pathway

Neurogenesis persists throughout life in the rodent subventricular zone (SVZ)-olfactory bulb pathway. The molecular regulation of this neurogenic circuit is poorly understood. Because the components for retinoid signaling are present in this pathway, we examined the influence of retinoic acid (RA) on postnatal SVZ-olfactory bulb neurogenesis. Using both SVZ neurosphere stem cell and parasagittal brain slice cultures derived from postnatal mouse, we found that RA exposure increased neurogenesis by enhancing the proliferation and neuronal differentiation of forebrain SVZ neuroblasts. The RA precursor retinol had a similar effect, which was reversed by treating cultures with the RA synthesis inhibitor disulfiram. Electroporation of dominant-negative retinoid receptors into the SVZ of slice cultures also blocked neuroblast migration to the olfactory bulb and altered the morphology of the progenitors. Moreover, the administration of disulfiram to neonatal mice decreased in vivo cell proliferation in the striatal SVZ. These results indicate that RA is a potent mitogen for SVZ neuroblasts and is required for their migration to the olfactory bulb. The regulation of multiple steps in the SVZ-olfactory bulb neurogenic pathway by RA suggests that manipulation of retinoid signaling is a potential therapeutic strategy to augment neurogenesis after brain injury.     

Indicators of hippocampal neurogenesis are altered by 56Fe-particle irradiation in a dose-dependent manner

The health risks to astronauts exposed to high-LET radiation include possible cognitive deficits. The pathogenesis of radiation-induced cognitive injury is unknown but may involve loss of neural precursor cells from the subgranular zone (SGZ) of the hippocampal dentate gyrus. To address this hypothesis, adult female C57BL/6 mice received whole-body irradiation with a 1 GeV/nucleon iron-particle beam in a single fraction of 0, 1, 2 and 3 Gy. Two months later mice were given BrdU injections to label proliferating cells. Subsequently, hippocampal tissue was assessed using immunohistochemistry for detection of proliferating cells and immature neurons. Routine histopathological methods were used to qualitatively assess tissue/cell morphology in the hippocampal formation and adjacent areas. When compared to controls, irradiated mice showed progressively fewer BrdU-positive cells as a function of dose. This observation was confirmed by Ki-67 immunostaining in the SGZ showing reductions in a dose-dependent fashion. The progeny of the proliferating SGZ cells, i.e. immature neurons, were visualized by doublecortin staining and were significantly reduced by irradiation, with the decreases ranging from 34% after 1 Gy to 71% after 3 Gy. Histopathology showed that in addition to cell changes in the SGZ, (56)Fe particles induced a chronic and diffuse astrocytosis and changes in pyramidal neurons in and around the hippocampal formation. The present data provide the first evidence that high-LET radiation has deleterious effects on cells associated with hippocampal neurogenesis.