Increased neurogenesis in the dentate gyrus after transient global ischemia in gerbils

Injury-induced neurogenesis in the mammalian forebrain

It has been accepted that new neurons are added to the olfactory bulb and the hippocampal dentate gyrus throughout life in the healthy adult mammalian brain. Recent studies have clarified that brain insult raises the proliferation of neural stem cells/neural progenitor cells existing in the subventricular zone and the subgranular zone, which become sources of new neurons for the olfactory bulb and the dentate gyrus, respectively. Interestingly, convincing data has shown that brain insult invokes neurogenesis in various brain regions, such as the hippocampal cornu ammonis region, striatum, and cortex. These reports suggest that neural stem cells/neural progenitor cells, which can be activated by brain injury, might be broadly located in the adult brain or that new neurons may migrate widely from the neurogenic regions. This review focuses on brain insult-induced neurogenesis in the mammalian forebrain, especially in the neocortex.

Forebrain specific heparin-binding epidermal growth factor-like growth factor knockout mice show exacerbated ischemia and reperfusion injury

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) is a hypoxia-inducible neuroprotective protein that also stimulates proliferation of neuronal precursor cells. In this study, we investigated the possible role of HB-EGF in ischemia and reperfusion injury by measuring the changes in its mRNA expression following focal cerebral ischemia. We also examined neural damage after a middle cerebral artery occlusion (MCAO) and reperfusion in ventral forebrain specific HB-EGF knockout (KO) mice. The levels of HB-EGF mRNA in the cerebral cortex of wild-type (WT) mice were significantly increased 3-24 h after MCAO and reperfusion. Cerebral infraction in HB-EGF KO mice was aggravated at 1 day and 6 days after MCAO and reperfusion compared with WT mice. The number of terminal deoxynucleotidyl transferase (TdT)-mediated dNTP nick end labeling (TUNEL) and an oxidative stress marker, 8-hydroxy-2'-deoxyguanosine (8-OHdG) positive cells, were higher in HB-EGF KO mice than in WT mice. On the other hand, fewer bromodeoxyuridine (BrdU) positive cells were found in the subventricular zone in HB-EGF KO mice compared with WT mice. These results indicate that HB-EGF may play a pivotal role in ischemia and reperfusion injury and that endogenously synthesized HB-EGF is necessary for both the neuroprotective effect and for regulation of cell proliferation in the subventricular zone.

Neurogenesis and glial proliferation are stimulated following diffuse traumatic brain injury in adult rats

Although increased neurogenesis has been described in rodent models of focal traumatic brain injury (TBI), the neurogenic response occurring after diffuse TBI uncomplicated by focal injury has not been examined to date, despite the pervasiveness of this distinct type of brain injury in the TBI patient population. Here we characterize multiple stages of neurogenesis following a traumatic axonal injury (TAI) model of diffuse TBI as well as the proliferative response of glial cells. TAI was induced in adult rats using an impact-acceleration model, and 5-bromo-2'-deoxyuridine (BrdU) was administered on days 1-4 posttrauma or sham operation to label mitotic cells. Using immunohistochemistry for BrdU combined with phenotype-specific markers, we found that proliferation was increased following TAI in the subventricular zone of the lateral ventricles and in the hippocampal subgranular zone, although the ultimate production of new dentate granule neurons at 8 weeks was not significantly enhanced. Also, abundant proliferating and reactive astrocytes, microglia, and polydendrocytes were detected throughout the brain following TAI, indicating that a robust glial response occurs in this model, although very few new cells in the nonneurogenic brain regions became mature neurons. We conclude that diffuse brain injury stimulates early stages of a neurogenic response similar to that described for models of focal TBI.

Functional integration of new hippocampal neurons following insults to the adult brain is determined by characteristics of pathological environment

We have previously shown that following severe brain insults, chronic inflammation induced by lipopolysaccharide (LPS) injection, and status epilepticus, new dentate granule cells exhibit changes of excitatory and inhibitory synaptic drive indicating that they may mitigate the abnormal brain function. Major inflammatory changes in the environment encountering the new neurons were a common feature of these insults. Here, we have asked how the morphology and electrophysiology of new neurons are affected by a comparably mild pathology: repetitive seizures causing hyperexcitability but not inflammation. Rats were subjected to rapid kindling, i.e., 40 rapidly recurring, electrically-induced seizures, and subsequently exposed to stimulus-evoked seizures twice weekly. New granule cells were labeled 1 week after the initial insult with a retroviral vector encoding green fluorescent protein. After 6-8 weeks, new neurons were analyzed using confocal microscopy and whole-cell patch-clamp recordings. The new neurons exposed to the pathological environment exhibited only subtle changes in their location, orientation, dendritic arborizations, and spine morphology. In contrast to the more severe insults, the new neurons exposed to rapid kindling and stimulus-evoked seizures exhibited enhanced afferent excitatory synaptic drive which could suggest that the cells that had developed in this environment contributed to hyperexcitability. However, the new neurons showed concomitant reduction of intrinsic excitability which may counteract the propagation of this excitability to the target cells. This study provides further evidence that following insults to the adult brain, the pattern of synaptic alterations at afferent inputs to newly generated neurons is dependent on the characteristics of the pathological environment.

Estrogen enhances neurogenesis and behavioral recovery after stroke

Stroke is a leading cause of permanent disability and death. It is well accepted that the principal mammalian estrogen (E2), 17-β estradiol, provides robust neuroprotection in a variety of brain injury models in animals of both sexes. E2 enhances neurogenesis after stroke in the subventricular zone; however, it is unknown if these cells survive long-term or enhance functional recovery. In this study, we examined stroke-induced neurogenesis in male, gonadally intact female, and ovariectomized female mice 2 and 6 weeks after stroke. Treatment with 17-β estradiol increased 5-bromo-2'-deoxyuridine-labeled cells at both time points in both the dentate gyrus and subventricular zone; the majority were colabeled with doublecortin at 2 weeks and with NeuN at 6 weeks. Stroke-induced neurogenesis was reduced in estrogen receptor knockout mice, as well as in mice lacking the gene for aromatase, which converts testosterone into E2. Improved behavioral deficits were seen in E2-treated mice, suggesting that E2-induced increases in poststroke neurogenesis contribute to poststroke recovery.

Prospects and limitations of using endogenous neural stem cells for brain regeneration

Neural stem cells (NSCs) are capable of producing a variety of neural cell types, and are indispensable for the development of the mammalian brain. NSCs can be induced in vitro from pluripotent stem cells, including embryonic stem cells and induced-pluripotent stem cells. Although the transplantation of these exogenous NSCs is a potential strategy for improving presently untreatable neurological conditions, there are several obstacles to its implementation, including tumorigenic, immunological, and ethical problems. Recent studies have revealed that NSCs also reside in the adult brain. The endogenous NSCs are activated in response to disease or trauma, and produce new neurons and glia, suggesting they have the potential to regenerate damaged brain tissue while avoiding the above-mentioned problems. Here we present an overview of the possibility and limitations of using endogenous NSCs in regenerative medicine.

The dual role of SRC kinases in intracerebral hemorrhage

Src kinase signaling has been implicated in multiple mechanisms of intracerebral hemorrhage (ICH). These include (1) thrombin-mediated mitogenic stress, (2) excitatory amino acid (AA)-mediated excitatory toxicity, (3) vascular endothelial growth factor (VEGF) and matrix metalloproteinases (MMPs)-mediated changes of vascular permeability, (4) cytokines-mediated inflammatory responses, and (5) others. These work together after ICH, causing brain injuries in the acute stage and self-repair in the recovery stage. We found that acute administration of the Src inhibitor, PP2, blocks the blood-brain barrier (BBB) breakdown and brain edema that occurs after ICH. However, delayed and chronic administration of PP2 prevents the BBB repair and edema resolution after ICH. These results led us to suggest that the two contradictory findings share the same principles at least in part via activation of Src kinases in acute or recovery stages after ICH. Acute Src kinase activation after ICH leads to BBB damage, and chronic Src kinase activation after ICH leads to BBB repair.

Energy metabolism in adult neural stem cell fate

The adult mammalian brain contains a population of neural stem cells that can give rise to neurons, astrocytes, and oligodendrocytes and are thought to be involved in certain forms of memory, behavior, and brain injury repair. Neural stem cell properties, such as self-renewal and multipotency, are modulated by both cell-intrinsic and cell-extrinsic factors. Emerging evidence suggests that energy metabolism is an important regulator of neural stem cell function. Molecules and signaling pathways that sense and influence energy metabolism, including insulin/insulin-like growth factor I (IGF-1)-FoxO and insulin/IGF-1-mTOR signaling, AMP-activated protein kinase (AMPK), SIRT1, and hypoxia-inducible factors, are now implicated in neural stem cell biology. Furthermore, these signaling modules are likely to cooperate with other pathways involved in stem cell maintenance and differentiation. This review summarizes the current understanding of how cellular and systemic energy metabolism regulate neural stem cell fate. The known consequences of dietary restriction, exercise, aging, and pathologies with deregulated energy metabolism for neural stem cells and their differentiated progeny will also be discussed. A better understanding of how neural stem cells are influenced by changes in energy availability will help unravel the complex nature of neural stem cell biology in both the normal and diseased state.

Cell-based therapy for stroke

Current treatments for stroke, such as the use of thrombolytic agents, are often limited by a narrow therapeutic time window. However, the regeneration of the brain after damage is still active days even weeks after stroke occurs, which might provide a second window for treatment. Cell-based therapy can be categorized into two strategies. One is transplantation of exogenous cells into the injured brain to replace the lost cells or support the remaining cells. The other strategy is to enhance the proliferation, differentiation, migration of endogenous stem or progenitor cells. Recent development in adult stem cell research and advancement in the induction of pluripotent stem cells from somatic adult cells provide a tremendous opportunity for transplantation therapy. Understanding the mechanisms and regulations involved in the endogenous neurogenesis will also help develop novel therapeutic interventions to promote neurogenesis and functional recovery in stroke. This review describes up-to-date progresses in cell-based therapy for the treatment of stroke.

Role of CCR5 and its ligands in the control of vascular inflammation and leukocyte recruitment required for acute excitotoxic seizure induction and neural damage

Chemokines may play a role in leukocyte migration across the blood-brain barrier (BBB) during neuroinflammation and other neuropathological processes, such as epilepsy. We investigated the role of the chemokine receptor CCR5 in seizures. We used a rat model based on intraperitoneal kainic acid (KA) administration. Four months before KA injection, adult rats were given femoral intramarrow inoculations of SV (RNAiR5-RevM10.AU1), which carries an interfering RNA (RNAi) against CCR5, plus a marker epitope (AU1), or its monofunctional RNAi-carrying homologue, SV(RNAiR5). This treatment lowered expression of CCR5 in circulating cells. In control rats, seizures induced elevated expression of CCR5 ligands MIP-1α and RANTES in the microvasculature, increased BBB leakage and CCR5(+) cells, as well as neuronal loss, inflammation, and gliosis in the hippocampi. Animals given either the bifunctional or the monofunctional vector were largely protected from KA-induced seizures, neuroinflammation, BBB damage, and neuron loss. Brain CCR5 mRNA was reduced. Rats receiving RNAiR5-bearing vectors showed far greater repair responses: increased neuronal proliferation, and decreased production of MIP-1α and RANTES. Controls received unrelated SV(BUGT) vectors. Decrease in CCR5 in circulating cells strongly protected from excitotoxin-induced seizures, BBB leakage, CNS injury, and inflammation, and facilitated neurogenic repair.

Induction of striatal neurogenesis and generation of region-specific functional mature neurons after ischemia by growth factors

Object

The capacity to replace lost neurons after insults is retained by several regions of adult mammalian brains. However, it is unknown how many neurons actually replace and mature into region-specific functional neurons to restore lost brain function. In this paper, the authors asked whether neuronal regeneration could be achieved efficaciously by growth factor treatment using a global ischemia model in rats, and they analyzed neuronal long-term maturation processes.

Methods

Rat global ischemia using a modified 4-vessel occlusion model was used to induce consistent ischemic neuronal injury in the dorsolateral striatum. To potentiate the proliferative response of neural progenitors, epidermal growth factor and fibroblast growth factor–2 were infused intraventricularly for 7 days from Day 2 after ischemia. Six weeks after ischemia, the number of neurons was counted in the defined dorsolateral striatum. To label the proliferating neural progenitors for tracing studies, 5-bromo-2′-deoxyuridine (BrdU; 150 mg/kg, twice a day) was injected intraperitoneally from Days 5 to 7, and immunohistochemical studies were conducted to explore the maturation of these progenitors. Migration of the progenitors was further studied by enhanced green fluorescent protein retrovirus injection. The effect of an antimitotic drug (cytosine arabinoside) on the neuronal count was also evaluated for contribution to regeneration. To see electrophysiological changes, treated rats were subjected to slice studies by whole-cell recordings. Finally, the effect of neural regeneration was assessed by motor performance by using the staircase test.

Results

Following epidermal growth factor and fibroblast growth factor–2 infusion into the lateral ventricles for 7 days beginning on Day 2, when severe neuronal loss in the adult striatum was confirmed (2.3% of normal controls), a significant increase of striatal neurons was observed at 6 weeks (~ 15% of normal controls) compared with vehicle controls (~ 5% of normal controls). Immunohistochemical studies by BrdU and enhanced green fluorescent protein retrovirus injection disclosed proliferation of neural progenitors in the subventricular zone and their migration to the ischemic striatum. By BrdU tracing study, NeuN- and BrdU-positive new neurons significantly increased at 6 and 12 weeks following the treatment. These accounted for 4.6 and 11.0% of the total neurons present, respectively. Antimitotic treatment demonstrated an approximately 66% reduction in neurons at 6 weeks. Further long-term studies showed dynamic changes of site-specific maturation among various neuronal subtypes even after 6 weeks. Electrophysiological properties of these newly appeared neurons underwent changes that conform to neonatal development. These regenerative changes were accompanied by a functional improvement of overall behavioral performance.

Conclusions

Treatment by growth factors significantly contributed to regeneration of mature striatal neurons after ischemia by endogenous neural progenitors, which was accompanied by electrophysiological maturation and improved motor performance. Recognition and improved understanding of these underlying dynamic processes will contribute to the development of novel and efficient regenerative therapies for brain injuries.

Reproductive age modulates the impact of focal ischemia on the forebrain as well as the effects of estrogen treatment in female rats

While human observational studies and animal studies report a neuroprotective role for estrogen therapy in stroke, the multicenter placebo-controlled Women's Health Initiative (WHI) study concluded that hormone therapy increased the risk for stroke in postmenopausal women. The present study therefore tested the hypothesis that estrogen replacement would increase the severity of a stroke-like injury in females when this replacement occurs after a prolonged hypoestrogenic period, such as the menopause or reproductive senescence, but not when given to females that were normally cycling immediately prior to the hormone replacement. Two groups of female rats were used: multiparous females with normal but lengthened estrus cycles (mature adults), and older multiparous females currently in a persistent acyclic state (reproductive senescent). Animals were either used intact, or were bilaterally ovariectomized and immediately replaced with a 17beta-estradiol pellet or control pellet. Animals were subject to a forelimb placing test (a test for sensorimotor deficit) and thereafter to middle cerebral artery occlusion (MCAo) by stereotaxic injection of the vasoconstrictive peptide endothelin-1, adjacent to the MCA. One week after stroke, behavioral tests were performed again. Cortical and striatal infarct volume, measured from brain slices, was significantly greater in intact reproductive senescent females as compared to intact mature adults. Furthermore, estrogen treatment to ovariectomized mature adult females significantly reduced the cortical infarct volume. Paradoxically, estrogen treatment to ovariectomized reproductive senescent females significantly increased cortical and striatal infarct volumes as compared to control pellet replaced senescent females. Significant post-stroke behavioral deficit was observed in all groups on the side contralateral to the lesion, while senescent females also exhibited deficits on the ipsilateral side, in the cross-midline forelimb placement test. Using an animal model that approximates the natural ovarian aging process, these findings strongly support the hypothesis that the effectiveness of estrogen therapy in protecting brain health may depend critically on the time of initiation with respect to a female's reproductive status.

Isolation of adult hippocampal neural progenitors

Adult neurogenesis, or the creation of new neurons in adult organisms, is an exciting recent area of neurological research. The subgranular zone of the adult hippocampus is one area where hippocampal neural progenitors generate new neurons that functionally integrate into existing neuronal circuitry. Given the role that the hippocampus plays in learning and memory consolidation and its vulnerability to neurological diseases and conditions, such as Alzheimer's disease, understanding the mechanisms controlling the self-renewal and differentiation of neural progenitor cells is a critical first step in developing novel disease treatments. In this and subsequent chapters, we describe many of the in vivo and in vitro techniques necessary to study hippocampal progenitors in the adult rat. Specifically, this chapter details isolation of progenitors from the adult rat for the establishment of in vitro culture.

[Donepezil-induced neuroprotection of acetylcholinergic neurons in olfactory bulbectomized mice]

In the brain of Alzheimer's patients, the cholinergic neurons innervated the hippocampus and cerebral cortex degenerates before accumulation of beta-amyloid protein. Donepezil, a potent acetylcholinesterase (AChE) inhibitor is reported to rescue neurons from excitotoxic injury in culture. However, there is no evidence to confirm its neuroprotective effect on ACh neurons in vivo. Using olfactory bulbectomy (OBX) mice, we defined the neuroprotective mechanisms of donepezil on the medial septum cholinergic neurons with concomitant improvement of the impaired cognitive function. Bilateral olfactory bulbs of DDY mouse were removed by surgery. After olfactory bulbectomized (OBX), donepezil (1 or 3 mg/kg/day) was administered for 15 days and mouse brain was fixed with paraformaldehyde perfusion at day 18. Then, the neuroprotective effect of donepezil was evaluated by counting the number of Chdine acetyltrans-ferase (ChAT) immunoreactive neurons in the medial septum. The number of ChAT immunoreactive neurons in the medial septum reduced by 40% of that in sham-operated animals. The reduced ChAT positive neurons were restored by donepezil treatments. Consistent with these observations, the cognitive deficits observed in OBX mice were significantly improved by the donepezil treatment. Taken together, donepezil treatment rescues the cholinergic neurons in the medial septum from the neurodegeneration by OBX. We will also discuss the mechanism underlying the donepezil-induced neuroprotection in the medial septum cholinergic neurons.

Systemic injection of recombinant human erythropoietin after focal cerebral ischemia enhances oligodendroglial and endothelial progenitor cells in rat brain

Erythropoietin (EPO) has been demonstrated the ability of recombinant human erythropoietin (r-Hu-EPO), when administered intracerebro-ventricularly, to improve stroke outcome through the reduction of stroke damage. In a brain ischemic model, however, systemic administration of r-Hu-EPO has not been intensely investigated given that in general, large glycosylated molecules have been deemed incapable of crossing the blood-brain barrier. In this study, administration of r-Hu-EPO for 4 days, intraperitoneally after ischemia-reperfusion (I-R) increased the number of bromodeoxyuridine (BrdU)-positive cells in the penumbra (10.1±1.4, n=5, P<0.05) and in the subventricular zone (SVZ) of the lateral ventricle (LV) (25±2.7, n=5, P<0.05) as compared with those of I-R (penumbra: 2.5±0.7; SVZ of LV: 3.8±1.5). A significant increase of BrdU-positive cells in these areas was coincident with a strong immunoreactivity of oligodendrocyte progenitor cell marker (2', 3'-cyclic nucleotide 3'-phosphodiesterase). Furthermore, r-Hu-EPO administration increased the number of BrdU-positive cells in the choroid plexus (7.8±2.3, n=5, P<0.05) and in cerebral blood vessels (3.5±1.3, n=5, P<0.05) when compared with those of I-R (choroid plexus: 1.2±0.5; cerebral blood vessels: 0.6±0.1). These results suggest that, even when systemically administered, r-Hu-EPO may have therapeutic potential for stroke via the proliferation of oligodendroglial and endothelial progenitor cells.

Expression and Functions of Fibroblast Growth Factor 2 (FGF-2) in Hippocampal Formation

Among the 23 members of the fibroblast growth factor (FGF) family, FGF-2 is the most abundant one in the central nervous system. Its impact on neural cells has been profoundly investigated by in vitro and in vivo studies as well as by gene knockout analyses during the past 2 decades. Key functions of FGF-2 in the nervous system include roles in neurogenesis, promotion of axonal growth, differentiation in development, and maintenance and plasticity in adulthood. From a clinical perspective, its prominent role for the maintenance of lesioned neurons (e.g., ischemia and following transection of fiber tracts) is of particular relevance. In the unlesioned brain, FGF-2 is involved in synaptic plasticity and processes attributed to learning and memory. The focus of this review is on the expression of FGF-2 and its receptors in the hippocampal formation and the physiological and pathophysiological roles of FGF-2 in this region during development and adulthood.

Stem cells as therapeutics for neurodegenerative disorders?

Aging is associated with a progressive increase in the risk of several prominent neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, stroke and amyotrophic lateral sclerosis. In each of these disorders specific populations of neurons become dysfunctional, then die and are not replaced. The adult brain and spinal cord contain populations of so-called neural stem cells (self-renewing and multipotent) and neural precursor cells (specified to a certain fate, but still mitotic) that may provide a continuing source of new neurons and glial cells during successful aging and after injury to the nervous system. Recent studies have shown that stem cells from embryos and adults can be transplanted into the nervous system, differentiate into neurons and glia and restore lost function in experimental models of neurodegenerative diseases. Embryonic stem cells may be a particularly effective donor cell type for transplantation-based therapies. Efficacy of stem cell therapies remains to be established in clinical trials in humans. Another approach is to mobilize endogenous neural stem cells. Animals studies have shown that dietary and behavioral modifications can indeed stimulate neurogenesis. Molecular and cellular mechanisms that regulate the proliferation, differentiation and survival of neural stem cells and neural precursor cells are being elucidated and are revealing novel targets for the development of pharmaceuticals that promote neurogenesis.

Vascular endothelial growth factor is involved in mediating increased de novo hippocampal neurogenesis in response to traumatic brain injury

Stimulating the endogenous repair process after traumatic brain injury (TBI) can be an important approach in neuroregenerative medicine. Vascular endothelial growth factor (VEGF) is one of the molecules that can increase de novo hippocampal neurogenesis. Here, we tested whether VEGF signaling through Flk1 (VEGF receptor 2) is involved in the neurogenic process after experimental TBI. We found that Flk1 is expressed both by neuroblasts in the subgranular layer (SGL) and by maturing granule neurons in the adult dentate gyrus (DG) of the hippocampus. After lateral fluid percussion TBI (LFP-TBI) in the rat, we detected elevated VEGF levels and also increased numbers of de novo neurons in the ipsilateral DG. To test the involvement of VEGF and Flk1 in the neurogenic process directly, we delivered recombinant VEGF or SU5416, an inhibitor to Flk1, into the ipsilateral cerebral ventricle of injured animals. We found that VEGF infusion significantly increased the number of BrdU+/Prox1+ new neurons, decreased the number of TUNEL+ cells, but did not change the number of BrdU+ newborn cells per se. Infusion with SU5416 caused no significant changes. Our results suggest that (a) VEGF is a part of the molecular signaling network that mediates de novo hippocampal neurogenesis after TBI; (b) VEGF predominantly mediates survival of de novo granule neurons rather than proliferation of neuroblasts in the injured brain; and (c) additional VEGF receptor(s) and/or other molecular mechanism(s) are also involved in mediating increased neurogenesis following injury.

Characterization of neural stem/progenitor cells expressing VEGF and its receptors in the subventricular zone of newborn piglet brain

Neural stem/progenitor cell (NSP) biology and neurogenesis in adult central nervous system (CNS) are important both towards potential future therapeutic applications for CNS repair, and for the fundamental function of the CNS. In the present study, we report the characterization of NSP population from subventricular zone (SVZ) of neonatal piglet brain using in vivo and in vitro systems. We show that the nestin and vimentin-positive neural progenitor cells are present in the SVZ of the lateral ventricles of neonatal piglet brain. In vitro, piglet NSPs proliferated as neurospheres, expressed the typical protein of neural progenitors, nestin and a range of well-established neurodevelopmental markers. Upon dissociation and subculture, piglet NSPs differentiated into neurons and glial cells. Clonal analysis demonstrates that piglet NSPs are multipotent and retain the capacity to generate both glia and neurons. These cells expressed VEGF, VEGFR1, VEGFR2 and Neuropilin-1 and -2 mRNAs. Real time PCR revealed that SVZ NSPs from newborn piglet expressed total VEGF and all VEGF splice variants. These findings show that piglet NSPs may be helpful to more effectively design growth factor based strategies to enhance endogenous precursor cells for cell transplantation studies potentially leading to the application of this strategy in the nervous system disease and injury.

Transplantation of TAT-Bcl-xL-transduced neural precursor cells: long-term neuroprotection after stroke

Neural precursor cells (NPC) are an interesting tool in experimental stroke research, but their therapeutic potential is limited due to poor long-term survival. We therefore in vitro transduced subventricular zone-(SVZ)-derived NPC with the anti-apoptotic fusion protein TAT-Bcl-x(L) and analyzed NPC survival, differentiation, and post-stroke functional deficits after experimental ischemia in mice. Survival of TAT-Bcl-x(L)-transduced NPC, which were injected at day 7 post-stroke into the ischemic striatum, was significantly increased at 4 weeks after stroke. Increased survival of NPC was associated with reduced infarct injury and decreased post-stroke functional deficits. Animals grafted with TAT-Bcl-x(L)-transduced NPC showed an increased number of immature cells expressing the neuronal marker doublecortin. Since mature neuronal differentiation of NPC was not observed, reduced post-stroke injury cannot be attributed to enhanced neuronal regeneration, but rather to indirect by-stander effects of grafted NPC. In line with this, NPC-mediated neuroprotection of cortical neurons in vitro was associated with increased secretion of growth factors. Thus, in vitro transduction of cultivated NPC with TAT-Bcl-x(L) results in enhanced resistance of transplanted NPC followed by long-term neuroprotection and ameliorated functional deficits after transient focal cerebral ischemia in mice.

Cholera toxin regulates a signaling pathway critical for the expansion of neural stem cell cultures from the fetal and adult rodent brains

Background

New mechanisms that regulate neural stem cell (NSC) expansion will contribute to improved assay systems and the emerging regenerative approach that targets endogenous stem cells. Expanding knowledge on the control of stem cell self renewal will also lead to new approaches for targeting the stem cell population of cancers.

Methodology/Principal Findings

Here we show that Cholera toxin regulates two recently characterized NSC markers, the Tie2 receptor and the transcription factor Hes3, and promotes the expansion of NSCs in culture. Cholera toxin increases immunoreactivity for the Tie2 receptor and rapidly induces the nuclear localization of Hes3. This is followed by powerful cultured NSC expansion and induction of proliferation both in the presence and absence of mitogen.

Conclusions/Significance

Our data suggest a new cell biological mechanism that regulates the self renewal and differentiation properties of stem cells, providing a new logic to manipulate NSCs in the context of regenerative disease and cancer.

Neuronal precursor cell proliferation in the hippocampus after transient cerebral ischemia: a comparative study of two rat strains using stereological tools

Background

We are currently investigating microglial activation and neuronal precursor cell (NPC) proliferation after transient middle cerebral artery occlusion (tMCAo) in rats. This study aimed: (1) to investigate differences in hippocampal NPC proliferation in outbred male spontaneously hypertensive rats (SHRs) and Sprague-Dawley rats (SDs) one week after tMCAo; (2) to present the practical use of the optical fractionator and 2D nucleator in stereological brain tissue analyses; and (3) to report our experiences with an intraluminal tMCAo model where the occluding filament is advanced 22 mm beyond the carotid bifurcation and the common carotid artery is clamped during tMCAo.

Methods

Twenty-three SDs and twenty SHRs were randomized into four groups subjected to 90 minutes tMCAo or sham. BrdU (50 mg/kg) was administered intraperitoneally twice daily on Day 4 to 7 after surgery. On Day 8 all animals were euthanized. NeuN-stained tissue sections were used for brain and infarct volume estimation with the 2D nucleator and Cavalieri principle. Brains were studied for the presence of activated microglia (ED-1) and hippocampal BrdU incorporation using the optical fractionator.

Results

We found no significant difference or increase in post-ischemic NPC proliferation between the two strains. However, the response to remote ischemia may differ between SDs and SHRs. In three animals increased post-stroke NPC proliferation was associated with hippocampal ischemic injury. The mean infarct volume was 89.2 ± 76.1 mm³ in SHRs and 16.9 ± 22.7 mm³ in SDs (p < 0.005). Eight out of eleven SHRs had ischemic neocortical damage in contrast to only one out of 12 SDs. We observed involvement of the anterior choroidal and hypothalamic arteries in several animals from both strains and the anterior cerebral artery in two SHRs.

Conclusions

We found no evidence of an early hippocampal NPC proliferation one week after tMCAo in both strains. Infarction within the anterior choroidal artery could induce hippocampal ischemia and increase NPC proliferation profoundly. NPC proliferation was not aggravated by the presence of activated microglia. Intraluminal tMCAo in SHRs gave a more reliable infarct with neocortical involvement, but affected territories supplied by the anterior cerebral, anterior choroidal and hypothalamic arteries.

Deafferentation enhances neurogenesis in the young and middle aged hippocampus but not in the aged hippocampus

Increased neurogenesis in the dentate gyrus (DG) after brain insults such as excitotoxic lesions, seizures, or stroke is a well known phenomenon in the young hippocampus. This plasticity reflects an innate compensatory response of neural stem cells (NSCs) in the young hippocampus to preserve function or minimize damage after injury. However, injuries to the middle-aged and aged hippocampi elicit either no or dampened neurogenesis response, which could be due to an altered plasticity of NSCs and/or the hippocampus with age. We examined whether the plasticity of NSCs to increase neurogenesis in response to a milder injury such as partial deafferentation is preserved during aging. We quantified DG neurogenesis in the hippocampus of young, middle-aged, and aged F344 rats after partial deafferentation. A partial deafferentation of the left hippocampus without any apparent cell loss was induced via administration of Kainic acid (0.5 μg in 1.0 μl) into the right lateral ventricle of the brain. In this model, degeneration of CA3 pyramidal neurons and dentate hilar neurons in the right hippocampus results in loss of commissural axons which leads to partial deafferentation of the dendrites of dentate granule cells and CA1-CA3 pyramidal neurons in the left hippocampus. Quantification of newly born cells that are added to the dentate granule cell layer at postdeafferentation days 4-15 using 5'-bromodeoxyuridine (BrdU) labeling revealed greatly increased addition of newly born cells (∼three fold increase) in the deafferented young and middle-aged hippocampi but not in the deafferented aged hippocampus. Measurement of newly born neurons using doublecortin (DCX) immunostaining also revealed similar findings. Analyses using BrdU-DCX dual immunofluorescence demonstrated no changes in neuronal fate-choice decision of newly born cells after deafferentation, in comparison to the age-matched naive hippocampus in all age groups. Thus, the plasticity of hippocampal NSCs to increase DG neurogenesis in response to a milder injury such as partial hippocampal deafferentation is preserved until middle age but lost at old age.

Directed neural lineage differentiation of adult hippocampal progenitor cells via modulation of hippocampal cholinergic neurostimulating peptide precursor expression

Hippocampal cholinergic neurostimulating peptide (HCNP), originally purified from young rat hippocampus, has been known to promote the differentiation of septo-hippocampal cholinergic neurons. Recently, the precursor protein of HCNP (HCNP-pp) has also received attention as a multifunctional protein with roles, in addition to serving as the HCNP precursor, such as acting as an ATP-binding protein, a Raf kinase inhibitor protein (RKIP), and phosphatidylethanolamine-binding protein (PEBP). In particular, the function of RKIP has attracted attention over several years for its role in controlling cellular proliferation and metastasis in cancer cells. HCNP-pp is also thought to be important in regulating the proliferation and differentiation of neuronal cells in vitro and in vivo by modification of the MAPK cascade. In the present study, we used cultured adult rat hippocampal progenitor cells (AHPs), which are thought to be important for memory formation, and focused on the role of HCNP-pp in adult neurogenesis, namely, the production of new neurons from neural stem/progenitor cells. We found that HCNP-pp expression in AHPs was closely associated with differentiation into MAP2ab-positive neurons and RIP-positive oligodendrocytes, but not into GFAP-positive astrocytes. By contrast, a down-regulated HCNP-pp expression in AHPs accompanied differentiation into GFAP-positive astrocytes. Direct manipulations of HCNP-pp via viral over-expression or siRNA downregulation further confirmed the HCNP-pp contribution to specific neural lineage commitment of AHPs. Our results show that the expression level of HCNP-pp acts as a key regulator for differentiation of cultured AHPs into specific neural lineages, indicating that the control of neural stem cell fate can be achieved via the HCNP-pp pathway.

Growth factors improve neurogenesis and outcome after focal cerebral ischemia

Stem cells have been proposed as a new form of cell-based therapy in a variety of disorders, including acute and degenerative brain diseases. Endogenous neural stem cells (eNSC) reside in the subventricular zone and in the subgranular zone of the hippocampus. eNSC are capable of self-renewal and differentiation into functional glia and neurons. Unfortunately, spontaneous brain regeneration is inefficient for clinically significant improvement following brain injury. However, eNSC responses may be augmented considerably by perturbing the pathways governing cell proliferation, migration and differentiation by application of exogenous growth factors. Importantly, current evidence suggests that such perturbations may lead to better functional outcome after stroke. This article summarizes the progress made in this field.

Protection of hippocampal neurogenesis by TAT-Bcl-x(L) after cerebral ischemia in mice

Endogenous neurogenesis persists in the subgranular zone (SGZ) of the adult rodent brain. Cerebral ischemia stimulates endogenous neurogenesis involving proliferation, migration and differentiation of SGZ-derived neural precursor cells (NPC). However, the biological meaning of this phenomenon is limited by poor survival of NPC. In order to study the effects of an acute neuroprotective treatment on hippocampal endogenous neurogenesis after transient cerebral ischemia in mice, we applied a fusion protein consisting of the TAT domain of the HI virus with the anti-apoptotic Bcl-x(L). Intravenous injection of TAT-Bcl-x(L) resulted in reduced hippocampal cell injury for up to 4weeks after stroke as assessed by TUNEL and NeuN staining. This was in line with a TAT-Bcl-x(L)-mediated reduced postischemic microglia activation. Analysis of endogenous hippocampal cell proliferation revealed an increased number of BrdU(+) cells in the TAT-Bcl-x(L) group 4weeks after stroke compared to animals treated with saline and TAT-HA (negative control). Cell proliferation in non-ischemic sham operated animals was not affected by TAT-Bcl-x(L). Twenty-eight days after stroke co-expression of BrdU(+) cells with the immature neuronal marker doublecortin was significantly increased in TAT-Bcl-x(L) animals. Although TAT-Bcl-x(L) treatment also resulted in an increased number of BrdU(+) cells expressing the mature neuronal marker NeuN, the total amount of these cells was low. These data show that TAT-Bcl-x(L) treatment yields both postischemic sustained hippocampal neuroprotection and increased survival of NPC rather than an induction of endogenous neurogenesis itself.

Enhanced expression of SOCS-2 in the rat hippocampus after transient forebrain ischemia

Suppressor of cytokine signaling-2 (SOCS-2) has recently been identified as an important regulator involved in neuronal differentiation and maturation. However, the role of SOCS-2 in ischemia-induced hippocampal neurogenesis remains to be clarified. Here we investigated the spatiotemporal expression of SOCS-2 in the rat hippocampus following transient forebrain ischemia, and particular attention was paid to changes in the dentate gyrus. SOCS-2 mRNA was constitutively expressed in hippocampal neurons and astrocytes in control animals. However, its upregulation occurred specifically in reactive astrocytes in the hippocampus proper, in particular the CA1 and dentate hilar regions, at day 3 after reperfusion, and was sustained for more than 2 weeks. In addition to the CA1 and hilar regions, SOCS-2 was transiently increased in the subgranular zone (SGZ) of the dentate gyrus on days 3-7 after reperfusion. This correlated with the post-ischemic upregulation of SOCS-2 in the CA1 or dentate gyrus subfield, including the SGZ detected by semiquantitative reverse transcriptase-polymerase chain reaction analysis. The majority of the SOCS-2-expressing cells in the SGZ were co-labeled with glial fibrillary acidic protein (GFAP), and a subpopulation of GFAP/SOCS-2 double-labeled cells in the SGZ co-expressed the neural progenitor marker nestin, or the proliferation marker proliferating cellular nuclear antigen. In addition, a subset of SOCS-2-labeled cells in the SGZ expressed the immature neuronal marker polysialic acid-neural cell adhesion molecule. These data suggest that SOCS-2 may be involved in glial reactions, and possibly adult hippocampal neurogenesis during ischemic insults.

Roles of neural stem cells and adult neurogenesis in adolescent alcohol use disorders

This review discusses the contributions of a newly considered form of plasticity, the ongoing production of new neurons from neural stem cells, or adult neurogenesis, within the context of neuropathologies that occur with excessive alcohol intake in the adolescents. Neural stem cells and adult neurogenesis are now thought to contribute to the structural integrity of the hippocampus, a limbic system region involved in learning, memory, behavioral control, and mood. In adolescents with alcohol use disorders (AUDs), the hippocampus appears to be particularly vulnerable to the neurodegenerative effects of alcohol, but the role of neural stem cells and adult neurogenesis in alcoholic neuropathology has only recently been considered. This review encompasses a brief overview of neural stem cells and the processes involved in adult neurogenesis, how neural stem cells are affected by alcohol, and possible differences in the neurogenic niche between adults and adolescents. Specifically, what is known about developmental differences in adult neurogenesis between the adult and adolescent is gleaned from the literature, as well as how alcohol affects this process differently among the age groups. Finally, this review suggests differences that may exist in the neurogenic niche between adults and adolescents and how these differences may contribute to the susceptibility of the adolescent hippocampus to damage. However, many more studies are needed to discern whether these developmental differences contribute to the vulnerability of the adolescent to developing an AUD.

Erythropoietin promotes hippocampal neurogenesis in in vitro models of neonatal stroke

The hippocampus is often injured in neonatal stroke. We have investigated the effect of erythropoietin (EPO) on oxygen-glucose deprived hippocampal slices and hypoxic progenitor cells. EPO improved survival of the organotypic hippocampal slices with significantly less cell death in the dentate gyrus and an increased number of proliferating cells 4-5 days after insult. Significantly fewer markers of neurogenesis were seen after the insult but when EPO was added to the culture medium, neurogenesis was sustained. When hippocampal progenitor cultures were stimulated into differentiation, more cells chose a neuronal cell fate when treated with EPO. These findings support the hypothesis that EPO not only prevents ischemia induced cell death but promotes neuronal cell fate commitment in in vitro models of neonatal stroke.

Increased hippocampal neurogenesis in the progressive stage of Alzheimer's disease phenotype in an APP/PS1 double transgenic mouse model

Alzheimer's disease (AD) is a progressive neurodegenerative disease associated with senile beta-amyloid (Abeta) plaques and cognitive decline. Neurogenesis in the adult hippocampus is implicated in regulating learning and memory, and is increased in human postmortem brain of AD patients. However, little is currently known about the changes of hippocampal neurogenesis in the progression of AD. As brain tissues from patients during the progression of AD are generally not available, an amyloid precursor protein (APP)/presenilin1 (PS1) double transgenic mouse model of AD was studied. Bromodeoxyuridine (BrdU) labeling supported by doublecortin staining was used to detect proliferating hippocampal cells in the mice. Compared with age-matched wild-type controls, 9-month-old transgenic mice with memory impairment and numerous brain Abeta deposits showed increased numbers of proliferating hippocampal cells. However, 3-month-old transgenic mice with normal memory and subtle brain Abeta deposits showed normal hippocampal proliferation. Double immunofluorescent labeling with BrdU and either NeuN or glial fibrillary acidic protein was conducted in mice at 10 months (28 days after the last BrdU injection) to determine the differentiation of proliferating cells. The number of hippocampal BrdU-positive cells and BrdU-positive cells differentiating into neurons (neurogenesis) in 10-month-old mice was greater in transgenic mice compared with age-matched controls, but the ratio of hippocampal BrdU-positive cells differentiating into neurons and astroglia was comparable. These results suggest hippocampal neurogenesis may increase during the progression of AD. Targeting this change in neurogenesis and understanding the underlying mechanism could lead to the development of a new treatment to control the progression of AD.

New achievements in ginseng research and its future prospects

In recent decades, scientists in Asian and Western countries have been paying great attention to ginseng research. Today, more than 200 ginsenosides and non-saponin constituents have been isolated and identified. Ginsenosides show biological activities only after being deglycosylated by intestinal bacteria. Aglycone protopanaxadiol and protopanaxatriol show the highest bioactivities. According to literature, the noticeable action of ginseng is that of delaying aging and especially increasing the nootropic effect, and it was found for the first time that Rg1 could increase hippocampal neurogenesis in vitro and in vivo under physiological and pathological circumstances. This is one of primary mechanisms underlying many of its pharmacological actions on the central nervous system. Rg1 was further shown to improve learning and memory in normal rats and mice. The nootropic signaling pathway has also been carried out in normal rats, and the Rg1-induced signaling pathway is similar to the memory formation that occurs in mammals, suggesting that Rg1 may have a potential effect in increasing intellectual capacity in normal people. Comparisons of chemical structures and pharmacologic functions between Panax ginseng and Panax quiquefolium were carried out by many scientists. The conclusion is that each has its own characteristics. There is no superiority or inferiority to the other.

Imipramine enhances cell proliferation and decreases neurodegeneration in the hippocampus after transient global cerebral ischemia in rats

This study was aimed to determine whether imipramine chronic treatment promotes neurogenesis in the dentate gyrus (DG) and interferes with neuronal death in the CA1 subfield of the hippocampus after transient global cerebral ischemia (TGCI) in rats. After TGCI, animals were treated with imipramine (20mg/kg, i.p.) or saline during 14 days. 5-Bromo-2'-deoxyuridine-5'-monophosphate (BrdU) was injected 24h after the last imipramine or saline injection to label proliferating cells. In order to confirm the effect of TGCI on neuronal death and cell proliferation, a group of animals was sacrificed 7 days after TGCI. Neurogenesis and neurodegeneration were evaluated by doublecortin (DCX)-immunohistochemistry and Fluoro-Jade C (FJC)-staining, respectively. The rate of cell proliferation increases 7 days but returns to basal levels 14 days after TGCI. There was a significant increase in the number of FJC-positive neurons in the CA1 of animals 7 and 14 days after TGCI. Chronic imipramine treatment increased cell proliferation in the SGZ of DG and reduced the neurodegeneration in the CA1 of the hippocampus 14 days after TGCI. Immunohistochemistry for DCX detected an increased number of newly generated neurons in the hippocampal DG 14 days after TGCI, which was not affected by imipramine treatment. Further studies are needed to evaluate whether imipramine treatment for longer time would be able to promote survival of newly generated neurons as well as to improve functional recovery after TGCI.

Fate and manipulations of endogenous neural stem cells following brain ischemia

Stem cells have been proposed as a new form of cell-based therapy in a variety of disorders, including acute and degenerative brain diseases. Endogenous neural stem cells (eNSCs) have been identified in the central nervous system where they reside largely in the subventricular zone and in the subgranular zone of the hippocampus. eNSCs are capable of self-renewal and differentiation into functional glia and neurons throughout life. However, spontaneous brain regeneration does not suffice to induce significant behavioral improvement in acute or chronic brain injury. Nevertheless, eNSCs responses can be considerably increased by tweaking the pathways governing cell proliferation, migration and differentiation. Contemporary evidence now suggests that such perturbations may lead to better functional outcome after brain injury.

Cell cycle inhibition without disruption of neurogenesis is a strategy for treatment of central nervous system diseases

Classically, the cell cycle is regarded as the process leading to cellular proliferation. However, increasing evidence over the last decade supports the notion that neuronal cell cycle re-entry results in post-mitotic death. A mature neuron that re-enters the cell cycle can neither advance to a new G0 quiescent state nor revert to its earlier G0 state. This presents a critical dilemma to the neuron from which death may be an unavoidable but necessary outcome for adult neurons attempting to complete the cell cycle. In contrast, tumor cells that undergo aberrant cell cycle re-entry divide and can survive. Thus, cell cycle inhibition strategies are of interest in cancer treatment but may also represent an important means of protecting neurons. In this review, we put forth the concept of the "expanded cell cycle" and summarize the cell cycle proteins, signal transduction events and mitogenic molecules that can drive a neuron into the cell cycle in various CNS diseases. We also discuss the pharmacological approaches that interfere with the mitogenic pathways and prevent mature neurons from attempting cell cycle re-entry, protecting them from cell death. Lastly, future attempts at blocking the cell cycle to rescue mature neurons from injury should be designed so as to not block normal neurogenesis.

Potential role for ligand-gated ion channels after seizure-induced neurogenesis

Epileptic seizures result in an increased generation of new neurons in the dentate gyrus of the adult mammalian hippocampus. The role of these seizure-induced newborn neurons in the process of epileptogenesis remains largely unknown. Recent work, however, suggests an aberrant incorporation of newborn cells into the existing hippocampal network in such a way that they promote hippocampal hyperexcitability. In the present review, we discuss current knowledge about the possible role of seizure-induced newly generated neurons and the putative involvement of ligand-gated ion channels in the process of epileptogenesis.