Increased number of new neurons in the olfactory bulb and hippocampus of adult non-human primates after focal ischemia

Neurogenesis in primates versus rodents and the value of non-human primate models

Neurogenesis, the process of generating neurons from neural stem cells, occurs during both embryonic and adult stages, with each stage possessing distinct characteristics. Dysfunction in either stage can disrupt normal neural development, impair cognitive functions, and lead to various neurological disorders. Recent technological advancements in single-cell multiomics and gene-editing have facilitated investigations into primate neurogenesis. Here, we provide a comprehensive overview of neurogenesis across rodents, non-human primates, and humans, covering embryonic development to adulthood and focusing on the conservation and diversity among species. While non-human primates, especially monkeys, serve as valuable models with closer neural resemblance to humans, we highlight the potential impacts and limitations of non-human primate models on both physiological and pathological neurogenesis research.

Special Issue "Genomics of Stroke" 2022

Stroke is one of the greatest medical threats to human health and quality of life in modern society [...].

Expression of Transcription Factor ZBTB20 in the Adult Primate Neurogenic Niche under Physiological Conditions or after Ischemia

The Zbtb20 gene encodes for a transcription factor that plays an important role in mammalian cortical development. Recently, its expression was reported in the adult mouse subventricular zone (SVZ), a major neurogenic niche containing neural stem cells throughout life. Here, we analyzed its expression in the adult primate anterior SVZ (SVZa) and rostral migratory stream (RMS) using macaque monkeys (Macaca fuscata). We report that the majority of Ki67+ cells, 71.4% in the SVZa and 85.7% in the RMS, co-label for ZBTB20. Nearly all neuroblasts, identified by their Doublecortin expression, were positive for ZBTB20 in both regions. Nearly all GFAP+ neural stem cells/astrocytes were also positive for ZBTB20. Analysis of images derived from a public database of gene expression in control/ischemic monkey SVZa, showed evidence for ZBTB20 upregulation in postischemic monkey SVZa. Furthermore, the co-localization of ZBTB20 with Doublecortin and Ki67 was increased in the postischemic SVZa. Our results suggest that ZBTB20 expression is evolutionarily conserved in the mammalian neurogenic niche and is reactive to ischemia. This opens the possibility for further functional studies on the role of this transcription factor in neurogenesis in primates.

Life and Death of Immature Neurons in the Juvenile and Adult Primate Amygdala

In recent years, a large population of immature neurons has been documented in the paralaminar nucleus of the primate amygdala. A substantial fraction of these immature neurons differentiate into mature neurons during postnatal development or following selective lesion of the hippocampus. Notwithstanding a growing number of studies on the origin and fate of these immature neurons, fundamental questions about the life and death of these neurons remain. Here, we briefly summarize what is currently known about the immature neurons present in the primate ventral amygdala during development and in adulthood, as well as following selective hippocampal lesions. We provide evidence confirming that the distribution of immature neurons extends to the anterior portions of the entorhinal cortex and layer II of the perirhinal cortex. We also provide novel arguments derived from stereological estimates of the number of mature and immature neurons, which support the view that the migration of immature neurons from the lateral ventricle accompanies neuronal maturation in the primate amygdala at all ages. Finally, we propose and discuss the hypothesis that increased migration and maturation of neurons in the amygdala following hippocampal dysfunction may be linked to behavioral alterations associated with certain neurodevelopmental disorders.

Novel pharmacotherapy: NNI-362, an allosteric p70S6 kinase stimulator, reverses cognitive and neural regenerative deficits in models of aging and disease

Aging is known to slow the neurogenic capacity of the hippocampus, one of only two mammalian adult neurogenic niches. The reduction of adult-born neurons with age may initiate cognitive decline progression which is exacerbated in chronic neurodegenerative disorders, e.g., Alzheimer's disease (AD). With physiologic neurogenesis diminished, but still viable in aging, non-invasive therapeutic modulation of this neuron regeneration process remains possible. The discovery of truly novel neuron regenerative therapies could be identified through phenotypic screening of small molecules that promote adult-born neurons from human neural progenitor cells (hNPCs). By identifying neuron-generating therapeutics and potentially novel mechanism of actions, therapeutic benefit could be confirmed through in vivo proof-of-concept studies. The key aging and longevity mTOR/p70S6 kinase axis, a commonly targeted pathway, is substrate for potential selective kinase modulators to promote new hippocampal neurons from NPCs. The highly regulated downstream substrate of mTOR, p70S6 kinase, directly controls pleiotropic cellular activities, including translation and cell growth. Stimulating this kinase, selectively in an adult neurogenic niche, should promote NPC proliferation, and cell growth and survival in the hippocampus. Studies of kinase profiling and immunocytochemistry of human progenitor neurogenesis suggest that the novel small molecule NNI-362 stimulates p70S6 kinase phosphorylation, which, in turn, promotes proliferation and differentiation of NPCs to neurons. NNI-362 promoted the associative reversal of age- and disease-related cognitive deficits in aged mice and Down syndrome-modeled mice. This oral, allosteric modulator may ultimately be beneficial for age-related neurodegenerative disorders involving hippocampal-dependent cognitive impairment, specifically AD, by promoting endogenous hippocampal regeneration.

Understanding the Real State of Human Adult Hippocampal Neurogenesis From Studies of Rodents and Non-human Primates

The concept of adult hippocampal neurogenesis (AHN) has been widely accepted, and a large number of studies have been performed in rodents using modern experimental techniques, which have clarified the nature and developmental processes of adult neural stem/progenitor cells, the functions of AHN, such as memory and learning, and its association with neural diseases. However, a fundamental problem is that it remains unclear as to what extent AHN actually occurs in humans. The answer to this is indispensable when physiological and pathological functions of human AHN are deduced from studies of rodent AHN, but there are controversial data on the extent of human AHN. In this review, studies on AHN performed in rodents and humans will be briefly reviewed, followed by a discussion of the studies in non-human primates. Then, how data of rodent and non-human primate AHN should be applied for understanding human AHN will be discussed.

Developmental dynamics of neurogenesis and gliogenesis in the postnatal mammalian brain in health and disease: Historical and future perspectives

The mature mammalian brain has long been thought to be a structurally rigid, static organ since the era of Ramón y Cajal in the early 20th century. Evidence accumulated over the past three decades, however, has completely overturned this long-held view. We now know that new neurons and glia are continuously added to the brain at postnatal stages, even in mature adults of various mammalian species, including humans. Moreover, these newly added cells contribute to structural plasticity and play important roles in higher order brain function, as well as repair after damage. A major source of these new neurons and glia is neural stem cells (NSCs) that persist in specialized niches in the brain throughout life. With this new view, our understanding of normal brain physiology and interventional approaches to various brain disorders has changed markedly in recent years. This article provides a brief overview on the historical changes in our understanding of the developmental dynamics of neurogenesis and gliogenesis in the postnatal and adult mammalian brain and discusses the roles of NSCs and other progenitor populations in such cellular dynamics in health and disease of the postnatal mammalian brain. This article is categorized under: Adult Stem Cells, Tissue Renewal, and Regeneration > Stem Cell Differentiation and Reversion Adult Stem Cells, Tissue Renewal, and Regeneration > Tissue Stem Cells and Niches Adult Stem Cells, Tissue Renewal, and Regeneration > Regeneration Adult Stem Cells, Tissue Renewal, and Regeneration > Stem Cells and Disease.

Suppressor of Cytokine Signaling-2 (SOCS2) Regulates the Microglial Response and Improves Functional Outcome after Traumatic Brain Injury in Mice

Traumatic brain injury (TBI) is frequently characterized by neuronal, axonal and myelin loss, reactive gliosis and neuroinflammation, often associated with functional deficits. Endogenous repair mechanisms include production of new neurons from precursor cells, but usually the new neurons fail to integrate and survive more than a few weeks. This is in part mediated by the toxic and inflammatory environment present in the injured brain which activates precursor cells to proliferate and differentiate but limits survival of the newborn progeny. Therefore, an understanding of mechanisms that regulate production and survival of newborn neurons and the neuroinflammatory response after brain injury may lead to therapeutic options to improve outcomes. Suppressor of Cytokine Signaling 2 (SOCS2) promotes hippocampal neurogenesis and survival of newborn neurons in the adult brain and regulates anti-inflammatory responses in the periphery, suggesting it may be a useful candidate to improve outcomes of TBI. In this study the functional and cellular responses of SOCS2 over-expressing transgenic (SOCS2Tg) mice were compared to wildtype littermates following mild or moderately severe TBI. Unlike wildtype controls, SOCS2Tg mice showed functional improvement on a ladder test, with a smaller lesion volume at 7d post injury and increased numbers of proliferative CD11b⁺ microglia/macrophages at 35d post-injury in the mild injury paradigm. At 7d post-moderately severe injury there was an increase in the area covered by cells expressing an anti-inflammatory M2 phenotype marker (CD206⁺) but no difference in cells with a pro-inflammatory M1 phenotype marker (CD16/32⁺). No effect of SOCS2 overexpression was observed in production or survival of newborn neurons, even in the presence of the neuroprotective agent erythropoietin (EPO). Therefore, SOCS2 may improve outcome of TBI in mice by regulating aspects of the neuroinflammatory response, promoting a more anti-inflammatory environment, although this was not sufficient to enhance survival of newborn cortical neurons.

Impaired Arm Function and Finger Dexterity in a Nonhuman Primate Model of Stroke

Background and Purpose—

Ischemic stroke is the leading cause of upper extremity motor impairments. Although several well-characterized experimental stroke models exist, modeling of upper extremity motor impairments, which are unique to primates, is not well established. Cortical representation of dexterous movements in nonhuman primates is functionally and topographically similar to that in humans. In this study, we characterize the African green monkey model of focal ischemia reperfusion with a defined syndrome, impaired dexterous movements.

Methods—

Cerebral ischemia was induced by transient occlusion of the M3 segment of the left middle cerebral artery. Motor and cognitive functions after stroke were evaluated using the object retrieval task with barrier-detour. Postmortem magnetic resonance imaging and histopathology were performed to map and characterize the infarct.

Results—

The middle cerebral artery occlusion consistently produced a necrotic infarct localized in the sensorimotor cortex in the middle cerebral artery territory. The infarction was reproducible and resulted in significant loss of fine motor function characterized by impaired dexterity. No significant cognitive impairment was detected. Magnetic resonance imaging and histopathology demonstrated consistent and significant loss of tissue on the left parietal cortex by the central sulcus covering the sensorimotor area. The results suggest that this species has less collateralization, which closely resembles humans.

Conclusions—

The reported nonhuman primate model produces a defined and reproducible syndrome relevant to our understanding of ischemic stroke, cortical representation, and sensorimotor integration controlling dexterous movements. This model will be useful in basic and translational research addressing loss of arm function and dexterity.

Sp8 expression in putative neural progenitor cells in guinea pig and human cerebrum

Neural stem/progenitor cells have been characterized at neurogenic sites in adult mammalian brain with various molecular markers. Here it has been demonstrated that Sp8, a transcription factor typically expressed among mature GABAergic interneurons, also labels putative neural precursors in adult guinea pig and human cerebrum. In guinea pigs, Sp8 immunoreactive (Sp8+) cells were localized largely in the superficial layers of the cortex including layer I, as well as the subventricular zone (SVZ) and subgranular zone (SGZ). Sp8+ cells at the SGZ showed little colocalization with mature and immature neuronal markers, but co-expressed neural stem cell markers including Sox2. Some layer I Sp8+ cells also co-expressed Sox2. The amount of Sp8+ cells in the dentate gyrus was maintained 2 weeks after X-ray irradiation, while that of doublecortin (DCX+) cells was greatly reduced. Mild ischemic insult caused a transient increase of Sp8+ cells in the SGZ and layer I, with the subgranular Sp8+ cells exhibited an increased colabeling for the mitotic marker Ki67 and pulse-chased bromodeoxyuridine (BrdU). Sp8+ cells in the dentate gyrus showed an age-related decline in guinea pigs, in parallel with the loss of DCX+ cells in the same region. In adult humans, Sp8+ cells exhibited comparable morphological features as seen in guinea pigs, with those at the SGZ and some in cortical layer I co-expressed Sox2. Together, these results suggested that Sp8 may label putative neural progenitors in guinea pig and human cerebrum, with the labeled cells in the SGZ appeared largely not mitotically active under normal conditions. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 939-955, 2016.

Cytogenesis in the adult monkey motor cortex: perivascular NG2 cells are the major adult born cell type

We used confocal microscopy and immunohistochemistry (IHC) to look for new cells in the motor cortex of adult macaque monkeys that might form the cellular bases of improved brain function from exercise. Twenty-four female Macaca fascicularis monkeys divided into groups by age (10–12 years, 15–17 years), postexercise survival periods, and controls, received 10 weekly injections of the thymidine analog, bromodeoxyuridine (BrdU) to mark new cells. Sixteen monkeys survived 15 weeks (5 weeks postexercise) and 8 monkeys survived 27 weeks (12 weeks postexercise) after initial BrdU injections. Additionally, five Macaca mulatta female monkeys (∼5.5–7 years) received single injections of BrdU and survived 2 days, 2 weeks, and 6 weeks after BrdU injections. Neural and glial antibodies were used to identify new cell phenotypes and to look for changes in proportions of these cells with respect to time and experimental conditions. No BrdU⁺/DCx⁺ cells were found but about 7.5% of new cells were calretinin-positive (Cr⁺). BrdU⁺/GABA⁺ (gamma-aminobutyric acid) cells were also found but no new Cr⁺ or GABA⁺ cells colabeled with a mature neuron marker, NeuN or chondroitin sulfate antibody, NG2. The proportion of new cells that were NG2⁺ was about 85% for short and long survival monkeys of which two, newly described perivascular phenotypes (Pldv and Elu) and a small percentage of pericytes (2.5%) comprised 44% and 51% of the new NG2⁺ cells, respectively. Proportions of NG2⁺ phenotypes were affected by post-BrdU survival periods, monkey age, and possibly a postexercise sedentary period but no direct effect of exercise was found.

Early expressions of hypoxia-inducible factor 1alpha and vascular endothelial growth factor increase the neuronal plasticity of activated endogenous neural stem cells after focal cerebral ischemia

Endogenous neural stem cells become “activated” after neuronal injury, but the activation sequence and fate of endogenous neural stem cells in focal cerebral ischemia model are little known. We evaluated the relationships between neural stem cells and hypoxia-inducible factor-1α and vascular endothelial growth factor expression in a photothromobotic rat stroke model using immunohistochemistry and western blot analysis. We also evaluated the chronological changes of neural stem cells by 5-bromo-2′-deoxyuridine (BrdU) incorporation. Hypoxia-inducible factor-1α expression was initially increased from 1 hour after ischemic injury, followed by vascular endothelial growth factor expression. Hypoxia-inducible factor-1α immunoreactivity was detected in the ipsilateral cortical neurons of the infarct core and peri-infarct area. Vascular endothelial growth factor immunoreactivity was detected in bilateral cortex, but ipsilateral cortex staining intensity and numbers were greater than the contralateral cortex. Vascular endothelial growth factor immunoreactive cells were easily found along the peri-infarct area 12 hours after focal cerebral ischemia. The expression of nestin increased throughout the microvasculature in the ischemic core and the peri-infarct area in all experimental rats after 24 hours of ischemic injury. Nestin immunoreactivity increased in the subventricular zone during 12 hours to 3 days, and prominently increased in the ipsilateral cortex between 3–7 days. Nestin-labeled cells showed dual differentiation with microvessels near the infarct core and reactive astrocytes in the peri-infarct area. BrdU-labeled cells were increased gradually from day 1 in the ipsilateral subventricular zone and cortex, and numerous BrdU-labeled cells were observed in the peri-infarct area and non-lesioned cortex at 3 days. BrdU-labeled cells rather than neurons, were mainly co-labeled with nestin and GFAP. Early expressions of hypoxia-inducible factor-1α and vascular endothelial growth factor after ischemia made up the microenvironment to increase the neuronal plasticity of activated endogenous neural stem cells. Moreover, neural precursor cells after large-scale cortical injury could be recruited from the cortex nearby infarct core and subventricular zone.

Evidence of adult neurogenesis in non-human primates and human

Adult neurogenesis in rodents has been extensively studied. Here, we briefly summarize the studies of adult neurogenesis based on non-human primate brains and human postmortem brain samples in recent decades. The differences between rodent, primate and human neurogenesis are discussed. We conclude that these differences may contribute to distinct physiological roles and the self-repair mechanisms in the brain across species.

Gypenosides pre-treatment protects the brain against cerebral ischemia and increases neural stem cells/progenitors in the subventricular zone

Gypenosides (GPs) have been reported to have neuroprotective effects in addition to other bioactivities. The protective activity of GPs during stroke and their effects on neural stem cells (NSCs) in the ischemic brain have not been fully elucidated. Here, we test the effects of GPs during stroke and on the NSCs within the subventricular zone (SVZ) of middle cerebral artery occlusion (MCAO) rats. Our results show that pre-treatment with GPs can reduce infarct volume and improve motor function following MCAO. Pre-treatment with GPs significantly increased the number of BrdU-positive cells in the ipsilateral and contralateral SVZ of MCAO rats. The proliferating cells in both sides of the SVZ were glial fibrillary acidic protein (GFAP)/nestin-positive type B cells and doublecortin (DCX)/nestin-positive type A cells. Our data indicate that GPs have neuroprotective effects during stroke which might be mediated through the enhancement of neurogenesis within the SVZ. These findings provide new evidence for a potential therapy involving GPs for the treatment of stroke.

Maturation-dependent response of neurogenesis after traumatic brain injury in children

OBJECT

Traumatic brain injury (TBI) is the leading cause of acquired disability in children, yet innate repair mechanisms are incompletely understood. Given data from animal studies documenting neurogenesis in response to trauma and other insults, the authors investigated whether similar responses could be found in children of different ages after TBI.

METHODS

Immunohistochemistry was used to label doublecortin (DCX), a protein expressed by immature migrating neuroblasts (newborn neurons), in specimens from patients ranging in age from 3 weeks to 10 years who had died either after TBI or from other causes. Doublecortin-positive (DCX+) cells were examined in the subventricular zone (SVZ) and periventricular white matter (PWM) and were quantified within the granule cell layer (GCL) and subgranular zone (SGZ) of the dentate gyrus to determine if age and/or injury affect the number of DCX+ cells in these regions.

RESULTS

The DCX+ cells decreased in the SVZ as patient age increased and were found in abundance around a focal subacute infarct in a 1-month-old non-TBI patient, but were scarce in all other patients regardless of age or history of trauma. The DCX+ cells in the PWM and dentate gyrus demonstrated a migratory morphology and did not co-localize with markers for astrocytes, microglia, or macrophages. In addition, there were significantly more DCX+ cells in the GCL and SGZ of the dentate gyrus in children younger than 1 year old than in older children. The density of immature migrating neuroblasts in infants (under 1 year of age) was significantly greater than in young children (2-6 years of age, p = 0.006) and older children (7-10 years of age, p = 0.007).

CONCLUSIONS

The main variable influencing the number of migrating neuroblasts observed in the SVZ, PWM, and hippocampus was patient age. Trauma had no discernible effect on the number of migrating neuroblasts in this cohort of patients in whom death typically occurred within hours to days after TBI.

Postnatal Development of Subcallosal Zone Following Suppression of Programmed Cell Death in Bax-deficient Mice

Neural stem cells are found in adult mammalian brain regions including the subgranular zone (SGZ) of the dentate gyrus (DG) and the subventricular zone (SVZ). In addition to these two regions, other neurogenic regions are often reported in many species. Recently, the subcallosal zone (SCZ) has been identified as a novel neurogenic region where new neuroblasts are spontaneously generated and then, by Bax-dependent apoptosis, eliminated. However, the development of SCZ in the postnatal brain is not yet fully explored. The present study investigated the precise location and amount of neuroblasts in the developing brain. To estimate the importance of programmed cell death (PCD) for SCZ histogenesis, SCZ development in the Bax-knockout (KO) mouse was examined. Interestingly, an accumulation of extra neurons with synaptic fibers in the SCZ of Bax-KO mice was observed. Indeed, Bax-KO mice exhibited enhanced startle response to loud acoustic stimuli and reduced anxiety level. Considering the prevention of PCD in the SCZ leads to sensory-motor gating dysfunction in the Bax-KO mice, active elimination of SCZ neuroblasts may promote optimal brain function.

Noninvasive strategies to promote functional recovery after stroke

Stroke is a common and disabling global health-care problem, which is the third most common cause of death and one of the main causes of acquired adult disability in many countries. Rehabilitation interventions are a major component of patient care. In the last few years, brain stimulation, mirror therapy, action observation, or mental practice with motor imagery has emerged as interesting options as add-on interventions to standard physical therapies. The neural bases for poststroke recovery rely on the concept of plasticity, namely, the ability of central nervous system cells to modify their structure and function in response to external stimuli. In this review, we will discuss recent noninvasive strategies employed to enhance functional recovery in stroke patients and we will provide an overview of neural plastic events associated with rehabilitation in preclinical models of stroke.

Neuronal restoration following ischemic stroke: influences, barriers, and therapeutic potential

Neurogenesis, the birth of new neurons, occurs throughout life in the subventricular zone and produces immature neurons that migrate tangentially through the rostral migratory stream to the olfactory bulb. This migration is tightly regulated by both structural and chemical influences. Interestingly, brain insults such as ischemic stroke increase neurogenesis and redirect neuroblast migration to the injury site. This injury-redirected neurogenesis and migration is coupled with angiogenic vasculature and is influenced by many of the factors that positively and negatively affect migration under developmental or normal adult conditions. Additionally, cytokines and chemokines such as stromal cell-derived factor-1 strongly influence neuronal migration poststroke. However, neuronal repopulation or brain regeneration is extremely limited. This limitation may potentially be due to the hostile poststroke microenvironment including the formation of the physical and chemical barriers of glial scar. Furthermore, interspecies differences in poststroke neurogenesis between rodents and humans complicate the translation of experimental results to humans. Despite these challenges, many drugs and other potential therapies have recently been evaluated for potential neurogenic properties poststroke. Improved understanding of poststroke neurorepair may lead to new and more effective neurorestorative therapies.

How to repair an ischemic brain injury? Value of experimental models in search of answers

The major aim of experimental models of cerebral ischemia is to study the cerebral ischemic damage under controlled and reproducible conditions. Experimental studies have been fundamental in the establishment of new concepts regarding the mechanisms underlying the ischemic brain injury, such as the ischemic penumbra, the reperfusion injury, the cell death or the importance of the damage induced on mitochondria, glial cells and white matter. Disagreement between experimental and clinical studies regarding the benefit of drugs to reduce or restore the cerebral ischemic damage has created a growing controversy about the clinical value of the experimental models of cerebral ischemia. One of the major explanations for the failure of the clinical trials is the reductionist approach of most therapies, which are focused on the known effect of a single molecule within a specific pathway of ischemic damage. This philosophy contrasts to the complex morphological design of the cerebral tissue and the complex cellular and molecular physiopathology underlying the ischemic brain injury. We believe that the main objective of studies carried out in experimental models of cerebral ischemic injury must be a better understanding of the fundamental mechanisms underlying progression of the ischemic injury. Clinical trials should not be considered if the benefit obtained in experimental studies is limited or weak.

G-protein-coupled receptors in adult neurogenesis

The importance of adult neurogenesis has only recently been accepted, resulting in a completely new field of investigation within stem cell biology. The regulation and functional significance of adult neurogenesis is currently an area of highly active research. G-protein-coupled receptors (GPCRs) have emerged as potential modulators of adult neurogenesis. GPCRs represent a class of proteins with significant clinical importance, because approximately 30% of all modern therapeutic treatments target these receptors. GPCRs bind to a large class of neurotransmitters and neuromodulators such as norepinephrine, dopamine, and serotonin. Besides their typical role in cellular communication, GPCRs are expressed on adult neural stem cells and their progenitors that relay specific signals to regulate the neurogenic process. This review summarizes the field of adult neurogenesis and its methods and specifies the roles of various GPCRs and their signal transduction pathways that are involved in the regulation of adult neural stem cells and their progenitors. Current evidence supporting adult neurogenesis as a model for self-repair in neuropathologic conditions, adult neural stem cell therapeutic strategies, and potential avenues for GPCR-based therapeutics are also discussed.

Functions of microglia in the central nervous system--beyond the immune response

Microglia cells are the immune cells of the central nervous system and consequently play important roles in brain infections and inflammation. Recent in vivo imaging studies have revealed that in the resting healthy brain, microglia are highly dynamic, moving constantly to actively survey the brain parenchyma. These active microglia can rapidly respond to pathological insults, becoming activated to induce a range of effects that may contribute to both pathogenesis, or to confer neuronal protection. However, interactions between microglia and neurons are being recognized as important in shaping neural circuit activity under more normal, physiological conditions. During development and neurogenesis, microglia interactions with neurons help to shape the final patterns of neural circuits important for behavior and with implications for diseases. In the mature brain, microglia can respond to changes in sensory activity and can influence neuronal activity acutely and over the long term. Microglia seem to be particularly involved in monitoring the integrity of synaptic function. In this review, we discuss some of these new insights into the involvement of microglia in neural circuits.

Involvement of metabotropic glutamate receptor 5 signaling in activity-related proliferation of adult hippocampal neural stem cells

Adult hippocampal neural stem cells can be activated by hippocampal neural activities. When focal cerebral ischemia, known as middle cerebral artery occlusion (MCAO), occurs, neural stem cells are activated to promote their proliferation. However, the mechanism by which these cells are activated is still unclear. Here, we indicate the involvement of metabotropic glutamate receptor 5 (mGluR5) signaling in neural stem cells in their activity-related proliferation after MCAO. We found mGluR5 molecules on neural stem cells by using calcium imaging. We detected the activation of neural stem cells by adding the mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine. On a hippocampal slice, the activation of neural stem cells to promote their proliferation was initiated by theta-burst electrical stimulation at the perforant pathway, and this activation was significantly blocked by an mGluR5 antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP). In addition to this, the injection of the blood-brain barrier-permeable mGluR5 agonist 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide into live mice promoted the proliferation of neural stem cells. Moreover, in vivo theta-burst electrical stimulation induced proliferation of neural stem cells. A chronic field recording study showed that the activity of the hippocampal formation was elevated after MCAO. Finally, we observed that the mGluR5 antagonist MPEP significantly blocked the stimulated proliferation of neural stem cells induced by MCAO, by blocking mGluR5 signaling. Our results suggest that glutamates released by the elevated neural activities after MCAO may trigger mGluR5 signaling in neural stem cells to promote their proliferation.

Thrombotic stroke in the anesthetized monkey (Macaca mulatta): characterization by MRI--a pilot study

BACKGROUND

The lack of a relevant stroke model in large nonhuman primates hinders the development of innovative diagnostic/therapeutic approaches concerned with this cerebrovascular disease. Our objective was to develop a novel and clinically relevant model of embolic stroke in the anesthetized monkey that incorporates readily available clinical imaging techniques and that would allow the possibility of drug delivery including strategies of reperfusion.

METHODS

Thrombin was injected into the lumen of the middle cerebral artery (MCA) in 12 anesthetized (sevoflurane) male rhesus macaques (Macaca mulatta). Sequential MRI studies (including angiography, FLAIR, PWI, DWI, and gadolinium-enhanced T1W imaging) were performed in a 3T clinical MRI. Physiological and biochemical parameters were monitored throughout the investigations.

RESULTS

Once standardized, the surgical procedure induced transient occlusion of the middle cerebral artery in all operated animals. All animals studied showed spontaneous reperfusion, which occurred some time between 2 h and 7 days post-ictus. Eighty percent of the studied animals showed diffusion/perfusion mismatch. The ischemic lesions at 24 h spared both superficial and profound territories of the MCA. Some animals presented hemorrhagic transformation at 7 days post-ictus.

CONCLUSION

In this study, we developed a pre-clinically relevant model of embolic stroke in the anesthetized nonhuman primate.

Comparison of neurogenesis in the dentate gyrus between the adult and aged gerbil following transient global cerebral ischemia

In the present study, we compared differences in cell proliferation, neuroblast differentiation and neuronal maturation in the hippocampal dentate gyrus (DG) between the adult and aged gerbil induced by 5 min of transient global cerebral ischemia using Ki-67 and BrdU (markers for cell proliferation), doublecortin (DCX, a marker for neuroblast differentiation) and neuronal nuclei (NeuN, a marker for mature neuron). The number of Ki-67-immunoreactive (⁺) cells in the DG of both the groups peaked 7 days after ischemia/reperfusion (I/R). However, the number in the aged DG was 40.6 ± 1.8% of that in the adult DG. Thereafter, the number decreased with time. After ischemic damage, DCX immunoreactivity and its protein level in the adult and aged DG peaked at 10 and 15 days post-ischemia, respectively. However, DCX immunoreactivity and its protein levels in the aged DG were much lower than those in the adult. DCX immunoreactivity and its protein level in the aged DG were 11.1 ± 0.6% and 34.4 ± 2.1% of the adult DG, respectively. In addition, the number of Ki-67⁺ cells and DCX immunoreactivity in both groups were similar to those in the sham at 60 days postischemia. At 30 days post-ischemia, the number of BrdU⁺ cells and BrdU⁺/NeuN⁺ cells in the adult-group were much higher (281.2 ± 23.4% and 126.4 ± 7.4%, respectively) than the aged-group (35.6 ± 6.8% and 79.5 ± 6.1%, respectively). These results suggest that the ability of neurogenesis in the ischemic aged DG is much lower than that in the ischemic adult DG.

Advantages and challenges of alternative sources of adult-derived stem cells for brain repair in stroke

Considerable promise has been demonstrated by cell therapy for the treatment of stroke. Adult-derived stem cells avoid the ethical dilemmas of using embryonic and fetal stem cells and thus are the ideal type of cell to study. There are a number of different types of stem cells that could prove to be useful, but there are potential concerns associated with each one. This review summarizes the current knowledge on the use of the different possible adult-derived stem cell types including their benefits and challenges. While the optimal conditions are still to be determined, these cells may prove to be at the forefront of stem cell research and ultimately therapy for stroke and other disorders.

Maturation time of new granule cells in the dentate gyrus of adult macaque monkeys exceeds six months

We studied two groups of adult macaque monkeys to determine the time course of adult neurogenesis in the dentate gyrus of the hippocampus. In the first group, six adult monkeys (Macaca mulatta) received a single injection of the thymidine analog BrdU (75 mg/kg), which is incorporated into replicating DNA and serves as a marker for new cell birth. Brain tissue was collected 48 h, 2 wk, and 6 wk after BrdU injection to examine the initial stages of neurogenesis. Because mature neurons were not evident at 6 wk, we examined tissue collected over a longer time course in a second study. In this study, eight monkeys (Macaca fascicularis) who were subjects in a separate exercise study received 10 weekly injections of BrdU (75 mg/kg), and brain tissue was collected at 16 and 28 wk from the first injection. Based on the timing of expression of neuronal cell markers (βIII-tubulin, doublecortin, NeuN), the extent of dendritic arborization, and acquisition of mature cell body morphology, we show that granule cell maturation in the dentate gyrus of a nonhuman primate is protracted over a minimum of a 6-mo time period, more than 6 times longer than in rodents. The lengthened time course for new cell maturation in nonhuman primates may be appropriate for preservation of neural plasticity over their longer life span and is relevant to our understanding of antidepressant and other therapies that have been linked to neurogenesis in humans.

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.

Adult human neurogenesis: from microscopy to magnetic resonance imaging

Neural stem cells reside in well-defined areas of the adult human brain and are capable of generating new neurons throughout the life span. In rodents, it is well established that the new born neurons are involved in olfaction as well as in certain forms of memory and learning. In humans, the functional relevance of adult human neurogenesis is being investigated, in particular its implication in the etiopathology of a variety of brain disorders. Adult neurogenesis in the human brain was discovered by utilizing methodologies directly imported from the rodent research, such as immunohistological detection of proliferation and cell-type specific biomarkers in postmortem or biopsy tissue. However, in the vast majority of cases, these methods do not support longitudinal studies; thus, the capacity of the putative stem cells to form new neurons under different disease conditions cannot be tested. More recently, new technologies have been specifically developed for the detection and quantification of neural stem cells in the living human brain. These technologies rely on the use of magnetic resonance imaging, available in hospitals worldwide. Although they require further validation in rodents and primates, these new methods hold the potential to test the contribution of adult human neurogenesis to brain function in both health and disease. This review reports on the current knowledge on adult human neurogenesis. We first review the different methods available to assess human neurogenesis, both ex vivo and in vivo and then appraise the changes of adult neurogenesis in human diseases.

Transient decrease in cerebral motor pathway fractional anisotropy after focal ischemic stroke in monkey

In this study, diffusion tensor MRI was used to examine the restoration of the cerebral white matter of macaque monkeys after unilateral cerebral multiple microinfarctions. Post-stroke, the monkeys showed deficits in several neurological functions, including motor functions, but most of the deficits resolved within 6 weeks. Very interestingly, the fractional anisotropy (a value determined by diffusion tensor MRI), of the monkeys' affected motor pathways dropped transiently, indicating a damage in the neural tracts. However, it returned to normal levels within 6 weeks after the stroke, concomitant with the gradual recovery of motor functions at subacute phase.

Interruption of beta-catenin signaling reduces neurogenesis in Alzheimer's disease

The neuronal loss associated with Alzheimer's disease (AD) affects areas of the brain that are vital to cognition. Although recent studies have shown that new neurons can be generated from progenitor cells in the neocortices of healthy adults, the neurogenic potential of the stem/progenitor cells of AD patients is not known. To answer this question, we compared the properties of glial progenitor cells (GPCs) from the cortices of healthy control (HC) and AD subjects. The GPCs from AD brain samples displayed reduced renewal capability and reduced neurogenesis compared with GPCs from HC brains. To investigate the mechanisms underlying this difference, we compared beta-catenin signaling proteins in GPCs from AD versus HC subjects and studied the effect of amyloid beta peptide (Abeta, a hallmark of AD pathology) on GPCs. Interestingly, GPCs from AD patients exhibited elevated levels of glycogen synthase kinase 3beta (GSK-3beta, an enzyme known to phosphorylate beta-catenin), accompanied by an increase in phosphorylated beta-catenin and a decrease in nonphosphorylated beta-catenin compared with HC counterparts. Furthermore. we found that Abeta treatment impaired the ability of GPCs from HC subjects to generate new neurons and caused changes in beta-catenin signaling proteins similar to those observed in GPCs from AD patients. Similar results were observed in GPCs isolated from AD transgenic mice. These results suggest that Abeta-induced interruption of beta-catenin signaling may contribute to the impairment of neurogenesis in AD progenitor cells.

White matter activated glial cells produce BDNF in a stroke model of monkeys

Lacunar-type stroke accounts for approximately a quarter of all ischemic strokes, and is the most common cause of vascular dementia. Despite its importance, there are few specific treatments for lacunar stroke, probably due largely to a lack of animal models. In this study, we developed a stroke model in a higher primate, the Macaque monkey. This was achieved by occluding the deep subcortical penetrating arteries with agarose spheres of mean diameters around 50 microm, and the appropriateness of this model as a lacunar-type stroke was verified by MRI. We observed widespread gliosis in the ipsilateral white matter (WM) of the stroke monkey. We also analyzed the expression of neurotrophins in the activated glial cells, and found that their expression of BDNF was stimulated in the affected WM following ischemic injury. Our results support the idea that WM glial cells play an active role in protecting and promoting the regeneration of nerve fibers in the affected WM of the ischemic brain, by producing BDNF. These findings may be useful for the development of new therapeutic strategies aimed at preventing or treating stroke.

The partial 5-HT1A receptor agonist buspirone enhances neurogenesis in the opossum (Monodelphis domestica)

We demonstrate for the first time that neurogenesis in the adult Monodelphis opossum has a typical mammalian pattern and occurs only in the dentate gyrus (DG) and subventricular zone (SVZ) of the lateral ventricles. In these two brain regions neurogenesis is present throughout the lifespan, although its rate is reduced by half in the old age. Treatment with buspirone, a partial 5-HT1A receptor agonist which is used in human clinic as an anxiolytic agent, boosts proliferation in the SVZ and DG in both adult and aged opossums. The neuronal phenotype dominates among newly generated cells in both non-treated and buspirone-treated opossums. We suggest that if functional importance of adult neurogenesis is in improving olfactory discrimination and generation of hippocampus-dependent memory, both spatial and emotional, then administration of drugs increasing the rate of neurogenesis via activation of 5-HT1A receptors may be a valuable aid in combating problems of the advanced age.

The potential of neural stem cells to repair stroke-induced brain damage

Acute injuries to CNS such as stroke induce neural progenitor proliferation in adult brain which might be an endogenous attempt to self-repair. This process is known to be altered by several exogenous and endogenous modulators including growth factors that could help to reinforce the post-stroke neurogenesis. Increasing the neurogenesis may be a future therapeutic option to decrease the cognitive and behavioral deficits following stroke. In addition, transplantation of various types of stem cells into the injured brain is currently thought to be an exciting option to replace the neurons lost in the post-ischemic brain. These include immortalized stem cell lines, neural progenitors prepared from embryonic and adult animals and mesenchymal stem cells. Using exogenous stem cells in addition to modulating endogenous neurogenesis, we may be able to repair the injured brain after a devastating stroke. This article reviewed the current literature of these two issues.

Primate-specific alterations in neural stem/progenitor cells in the aged hippocampus

In the dentate gyrus of the hippocampus, new neurons are generated from neural stem/progenitor cells (NPCs) throughout life. As aging progresses, the rate of neurogenesis decreases exponentially, which might be responsible, in part, for age-dependent cognitive decline in animals and humans. However, few studies have analyzed the alterations in NPCs during aging, especially in primates. Here, we labeled NPCs by triple immunostaining for FABP7, Sox2, and GFAP and found that their numbers decreased in aged macaque monkeys (>20 years old), but not in aged mice. Importantly, we observed marked morphological alterations of the NPCs in only the aged monkeys. In the aged monkey hippocampus, the processes of the NPCs were short and ran horizontally rather than vertically. Despite these alterations, the proliferation rate of the NPCs in aged monkeys was similar to that in young monkeys. Thus, morphological alterations do not affect the proliferation rate of NPCs, but may be involved in the maintenance of NPCs in aged primates, including elderly humans.

Age-dependent impairment of hippocampal neurogenesis in chronic cerebral hypoperfusion

Acute ischaemic brain damages, including both strokes and local ischaemia, are powerful stimulators of neurogenesis in the dentate gyrus of the hippocampus in adult rats and mice. As no data are available in chronic cerebral hypoperfusion, we investigated neurogenesis in rats after bilateral chronic occlusion of the carotid arteries (2VO). 2VO was performed in 3- and 12-month-old rats. Proliferation was investigated by bromodeoxyuridine uptake and MCM2 nuclear immunoreactivity, neurogenesis by counting doublecortin-IR cells in the subgranular area of the dentate gyrus of the hippocampus. We found increased proliferation and neurogenesis in the subgranular area of the dentate gyrus of the hippocampus in adult (3-month-old) rats 8 days after 2VO. This capability is lost in middle-aged (12-month-old) rats. Our data suggest that 2VO ligation can be a useful model for studying neurogenesis in experimental conditions mimicking long-lasting human pathologies, and also in the exploration of the uncertain relation between chronic brain hypoperfusion and age-related changes of cognitive function.

How widespread is adult neurogenesis in mammals?

It is now widely accepted that neurogenesis occurs in two regions of the adult mammalian brain--the hippocampus and the olfactory bulb. There is evidence for adult neurogenesis in several additional areas, including the neocortex, striatum, amygdala and substantia nigra, but this has been difficult to replicate consistently other than in the damaged brain. The discrepancies may be due to variations in the sensitivity of the methods used to detect new neurons.

Antidepressant-induced neurogenesis in the hippocampus of adult nonhuman primates

New neurons are generated in the adult hippocampus of many species including rodents, monkeys, and humans. Conditions associated with major depression, such as social stress, suppress hippocampal neurogenesis in rodents and primates. In contrast, all classes of antidepressants stimulate neuronal generation, and the behavioral effects of these medications are abolished when neurogenesis is blocked. These findings generated the hypothesis that induction of neurogenesis is a necessary component in the mechanism of action of antidepressant treatments. To date, the effects of antidepressants on newborn neurons have been reported only in rodents and tree shrews. This study examines whether neurogenesis is increased in nonhuman primates after antidepressant treatment. Adult monkeys received repeated electroconvulsive shock (ECS), which is the animal analog of electroconvulsive therapy (ECT), the most effective short-term antidepressant. Compared with control conditions, ECS robustly increased precursor cell proliferation in the subgranular zone (SGZ) of the dentate gyrus in the monkey hippocampus. A majority of these precursors differentiated into neurons or endothelial cells, while a few matured into glial cells. The ECS-mediated induction of cell proliferation and neurogenesis was accompanied by increased immunoreactivity for the neuroprotective gene product BCL2 (B cell chronic lymphocytic lymphoma 2) in the SGZ. The ECS interventions were not accompanied by increased hippocampal cell death or injury. This study demonstrates that ECS is capable of inducing neurogenesis in the nonhuman primate hippocampus and supports the possibility that antidepressant interventions produce similar alterations in the human brain.

On neural plasticity, new neurons and the postischemic milieu: An integrated view on experimental rehabilitation

This review discusses actual and potential contributors to functional improvement after stroke injuries. Topics that will be covered are neuronal re-organization and sprouting, neural stem/progenitor cell activation and neuronal replacement, as well as the neuronal milieu defined by glia, inflammatory cells and blood vessel supply. It is well established that different types of neuronal plasticity ultimately lead to post-stroke recovery. However, an untapped potential which only recently has started to be extensively explored is neuronal replacement through endogenous or exogenous resources. Major experimental efforts are needed to achieve progress in this burgeoning area. The review stresses the importance of applying neurodevelopmental principles as well as performing a characterization of the role of the postischemic milieu when studying adult brain neural stem/progenitor cells. Integrated and multifaceted experimentation, incorporating actual and possible poststroke function modulators, will be necessary in order to determine future strategies that will ultimately enable considerable progress in the field of neurorehabilitation.

Glial activation in white matter following ischemia in the neonatal P7 rat brain

This study examines cell death and proliferation in the white matter after neonatal stroke. In postnatal day 7 injured rat, there was a marked reduction in myelin basic protein (MBP) immunostaining mainly corresponding to numerous pyknotic immature oligodendrocytes and TUNEL-positive astrocytes in the ipsilateral external capsule. In contrast, a substantial restoration of MBP, as indicated by the MBP ratio of left-to-right, occurred in the cingulum at 48 (1.27 +/- 0.12) and 72 (1.30 +/- 0.18, P < 0.05) h of recovery as compared to age-matched controls (1.03 +/- 0.14). Ki-67 immunostaining revealed a first peak of newly generated cells in the dorsolateral hippocampal subventricular zone and cingulum at 72 h after reperfusion. Double immunofluorescence revealed that most of the Ki-67-positive cells were astrocytes at 48 h and NG2 pre-oligodendrocytes at 72 h of recovery. Microglia infiltration occurs over several days in the cingulum, and a huge quantity of macrophages reached the subcortical white matter where they engulfed immature oligodendrocytes. The overall results suggest that the persistent activation of microglia involves a chronic component of immunoinflammation, which overwhelms repair processes and contributes to cystic growth in the developing brain.