Neuroscience. Is more neurogenesis always better?

Increasing adult-born neurons protects mice from epilepsy

Neurogenesis occurs in the adult brain in the hippocampal dentate gyrus, an area that contains neurons which are vulnerable to insults and injury, such as severe seizures. Previous studies showed that increasing adult neurogenesis reduced neuronal damage after these seizures. Because the damage typically is followed by chronic life-long seizures (epilepsy), we asked if increasing adult-born neurons would prevent epilepsy. Adult-born neurons were selectively increased by deleting the pro-apoptotic gene Bax from Nestin-expressing progenitors. Tamoxifen was administered at 6 weeks of age to conditionally delete Bax in Nestin-CreERT2Baxfl/fl mice. Six weeks after tamoxifen administration, severe seizures (status epilepticus; SE) were induced by injection of the convulsant pilocarpine. After mice developed epilepsy, seizure frequency was quantified for 3 weeks. Mice with increased adult-born neurons exhibited fewer chronic seizures. Postictal depression was reduced also. These results were primarily in female mice, possibly because they were more affected by Bax deletion than males, consistent with sex differences in Bax. The female mice with enhanced adult-born neurons also showed less neuronal loss of hilar mossy cells and hilar somatostatin-expressing neurons than wild-type females or males, which is notable because loss of these two hilar cell types is implicated in epileptogenesis. The results suggest that selective Bax deletion to increase adult-born neurons can reduce experimental epilepsy, and the effect shows a striking sex difference. The results are surprising in light of past studies showing that suppressing adult-born neurons can also reduce chronic seizures.

Hippocampal dentate granule cells in temporal lobe epilepsy: A morphometry and transcriptomic study

The dentate gyrus (DG) plays a critical role in hippocampal circuitry, providing a "gate-like" function to the downstream cornu ammonis (CA) sectors. Despite this critical role, pathologies in DG are less commonly described than those in the CA sectors in the diagnosis of mesial temporal lobe epilepsy (mTLE). To elucidate the role of the DG in mTLE, we analysed hippocampal sclerosis (HS), no-HS, non-TLE epilepsy control, and non-epilepsy control cohorts using morphometry and gene expression profiling techniques. Morphometry techniques analysed DG cell spacing, nucleus size, and nucleus circularity. Our data show distinct DG morphometry and RNA expression profiles between HS and No-HS. Dentate granule cells are more dispersed in patients with HS, and the DG shows an elevated expression of the complement system, apoptosis, and extracellular matrix remodelling-related RNA. We also observe an overall decrease in neurogenesis-related RNA in HS DG. Interestingly, regardless of the pathological diagnosis, the DG morphometry correlates with post-operative outcomes. Increased cell spacing is observed in the DG of mTLE cases that achieve seizure freedom post-operatively. This study reveals the possible prognostic value of DG morphometry, as well as supporting the notion that HS and no-HS TLE may be distinct disease entities with differing contributing mechanisms.

Increasing adult-born neurons protects mice from epilepsy

Neurogenesis occurs in the adult brain in the hippocampal dentate gyrus, an area that contains neurons which are vulnerable to insults and injury, such as severe seizures. Previous studies showed that increasing adult neurogenesis reduced neuronal damage after these seizures. Because the damage typically is followed by chronic life-long seizures (epilepsy), we asked if increasing adult-born neurons would prevent epilepsy. Adult-born neurons were selectively increased by deleting the pro-apoptotic gene Bax from Nestin-expressing progenitors. Tamoxifen was administered at 6 weeks of age to conditionally delete Bax in Nestin-CreERT2 Bax fl/fl mice. Six weeks after tamoxifen administration, severe seizures (status epilepticus; SE) were induced by injection of the convulsant pilocarpine. After mice developed epilepsy, seizure frequency was quantified for 3 weeks. Mice with increased adult-born neurons exhibited fewer chronic seizures. Postictal depression was reduced also. These results were primarily in female mice, possibly because they were the more affected by Bax deletion than males, consistent with sex differences in Bax. The female mice with enhanced adult-born neurons also showed less neuronal loss of hilar mossy cells and hilar somatostatin-expressing neurons than wild type females or males, which is notable because these two hilar cell types are implicated in epileptogenesis. The results suggest that selective Bax deletion to increase adult-born neurons can reduce experimental epilepsy, and the effect shows a striking sex difference. The results are surprising in light of past studies showing that suppressing adult-born neurons can also reduce chronic seizures.

Application of P7C3 Compounds to Investigating and Treating Acute and Chronic Traumatic Brain Injury

Traumatic brain injury (TBI) is a leading worldwide cause of disability, and there are currently no medicines that prevent, reduce, or reverse acute or chronic neurodegeneration in TBI patients. Here, we review the target-agnostic discovery of nicotinamide adenine dinucleotide (NAD+)/NADH-stabilizing P7C3 compounds through a phenotypic screen in mice and describe how P7C3 compounds have been applied to advance understanding of the pathophysiology and potential treatment of TBI. We summarize how P7C3 compounds have been shown across multiple laboratories to mitigate disease progression safely and effectively in a broad range of preclinical models of disease related to impaired NAD+/NADH metabolism, including acute and chronic TBI, and note the reported safety and neuroprotective efficacy of P7C3 compounds in nonhuman primates. We also describe how P7C3 compounds facilitated the recent first demonstration that chronic neurodegeneration 1 year after TBI in mice, the equivalent of many decades in people, can be reversed to restore normal neuropsychiatric function. We additionally review how P7C3 compounds have facilitated discovery of new pathophysiologic mechanisms of neurodegeneration after TBI. This includes the role of rapid TBI-induced tau acetylation that drives axonal degeneration, and the discovery of brain-derived acetylated tau as the first blood-based biomarker of neurodegeneration after TBI that directly correlates with the abundance of a therapeutic target in the brain. We additionally review the identification of TBI-induced tau acetylation as a potential mechanistic link between TBI and increased risk of Alzheimer's disease. Lastly, we summarize historical accounts of other successful phenotypic-based drug discoveries that advanced medical care without prior recognition of the specific molecular target needed to achieve the desired therapeutic effect.

Beyond sex differences: short- and long-term effects of pregnancy on the brain

Growing attention has been directed to the inclusion of females in neuroscience studies, and to the importance of studying sex as a biological variable. However, how female-specific factors such as menopause and pregnancy, affect the brain remains understudied. In this review, we use pregnancy as a case in point of a female-unique experience that can alter neuroplasticity, neuroinflammation, and cognition. We examine studies in both humans and rodents indicating that pregnancy can modify neural function in the short term, as well as alter the trajectory of brain aging. Furthermore, we discuss the influence of maternal age, fetal sex, number of pregnancies, and presence of pregnancy complications on brain health outcomes. We conclude by encouraging the scientific community to prioritize researching female health by recognizing and including factors such as pregnancy history in research.

Neurogenesis in aging and age-related neurodegenerative diseases

Adult neurogenesis, the process by which neurons are generated in certain areas of the adult brain, declines in an age-dependent manner and is one potential target for extending cognitive healthspan. Aging is a major risk factor for neurodegenerative diseases and, as lifespans are increasing, these health challenges are becoming more prevalent. An age-associated loss in neural stem cell number and/or activity could cause this decline in brain function, so interventions that reverse aging in stem cells might increase the human cognitive healthspan. In this review, we describe the involvement of adult neurogenesis in neurodegenerative diseases and address the molecular mechanistic aspects of neurogenesis that involve some of the key aggregation-prone proteins in the brain (i.e., tau, Aβ, α-synuclein, …). We summarize the research pertaining to interventions that increase neurogenesis and regulate known targets in aging research, such as mTOR and sirtuins. Lastly, we share our outlook on restoring the levels of neurogenesis to physiological levels in elderly individuals and those with neurodegeneration. We suggest that modulating neurogenesis represents a potential target for interventions that could help in the fight against neurodegeneration and cognitive decline.

Areas of Convergence and Divergence in Adolescent Social Isolation and Binge Drinking: A Review

Adolescence is a critical developmental period characterized by enhanced social interactions, ongoing development of the frontal cortex and maturation of synaptic connections throughout the brain. Adolescents spend more time interacting with peers than any other age group and display heightened reward sensitivity, impulsivity and diminished inhibitory self-control, which contribute to increased risky behaviors, including the initiation and progression of alcohol use. Compared to adults, adolescents are less susceptible to the negative effects of ethanol, but are more susceptible to the negative effects of stress, particularly social stress. Juvenile exposure to social isolation or binge ethanol disrupts synaptic connections, dendritic spine morphology, and myelin remodeling in the frontal cortex. These structural effects may underlie the behavioral and cognitive deficits seen later in life, including social and memory deficits, increased anxiety-like behavior and risk for alcohol use disorders (AUD). Although the alcohol and social stress fields are actively investigating the mechanisms through which these effects occur, significant gaps in our understanding exist, particularly in the intersection of the two fields. This review will highlight the areas of convergence and divergence in the fields of adolescent social stress and ethanol exposure. We will focus on how ethanol exposure or social isolation stress can impact the development of the frontal cortex and lead to lasting behavioral changes in adulthood. We call attention to the need for more mechanistic studies and the inclusion of the evaluation of sex differences in these molecular, structural, and behavioral responses.

VEGF Modulates Neurogenesis and Microvascular Remodeling in Epileptogenesis After Status Epilepticus in Immature Rats

Neurogenesis and angiogenesis are widely recognized to occur during epileptogenesis and important in brain development. Because vascular endothelial growth factor (VEGF) is a critical neurovascular target in neurological diseases, its effect on neurogenesis, microvascular remodeling and epileptogenesis in the immature brain after lithium-pilocarpine-induced status epilepticus (SE) was investigated. The dynamic changes in and the correlation between hippocampal neurogenesis and microvascular remodeling after SE and the influence of VEGF or SU5416 injection into the lateral ventricles at different stages after SE on neurogenesis and microvascular remodeling through regulation of VEGF expression were assessed by immunofluorescence and immunohistochemistry. Western blot analysis revealed that the VEGFR2 signaling pathway promotes phosphorylated ERK and phosphorylated AKT expression. The effects of VEGF expression regulation at different stages after SE on pathological changes in hippocampal structure and spontaneous recurrent seizures (SRS) were evaluated by Nissl staining and electroencephalography (EEG). The results showed that hippocampal neurogenesis after SE is related to microvascular regeneration. VEGF promotion in the acute period and inhibition in the latent period after SE alleviates loss of hippocampal neuron, abnormal vascular regeneration and inhibits neural stem cells (NSCs) ectopic migration, which may effectively alleviate SRS severity. Interfering with VEGF via the AKT and ERK pathways in different phases after SE may be a promising strategy for treating and preventing epilepsy in children.

Androgen Effects on Neural Plasticity

Androgens are synthesized in the brain, gonads, and adrenal glands, in both sexes, exerting physiologically important effects on the structure and function of the central nervous system. These effects may contribute to the incidence and progression of neurological disorders such as autism spectrum disorder, schizophrenia, and Alzheimer's disease, which occur at different rates in males and females. This review briefly summarizes the current state of knowledge with respect to the neuroplastic effects of androgens, with particular emphasis on the hippocampus, which has been the focus of much of the research in this field.

Dissociable effects of complement C3 and C3aR on survival and morphology of adult born hippocampal neurons, pattern separation, and cognitive flexibility in male mice

Adult hippocampal neurogenesis (AHN) is a form of ongoing plasticity in the brain that supports specific aspects of cognition. Disruptions in AHN have been observed in neuropsychiatric conditions presenting with inflammatory components and are associated with impairments in cognition and mood. Recent evidence highlights important roles of the complement system in synaptic plasticity and neurogenesis during neurodevelopment and in acute learning and memory processes. In this work we investigated the impact of the complement C3/C3aR pathway on AHN and its functional implications for AHN-related behaviours. In C3-/- mice, we found increased numbers and accelerated migration of adult born granule cells, indicating that absence of C3 leads to abnormal survival and distribution of adult born neurons. Loss of either C3 or C3aR affected the morphology of immature neurons, reducing morphological complexity, though these effects were more pronounced in the absence of C3aR. We assessed functional impacts of the cellular phenotypes in an operant spatial discrimination task that assayed AHN sensitive behaviours. Again, we observed differences in the effects of manipulating C3 or C3aR, in that whilst C3aR-/- mice showed evidence of enhanced pattern separation abilities, C3-/- mice instead demonstrated impaired behavioural flexibility. Our findings show that C3 and C3aR manipulation have distinct effects on AHN that impact at different stages in the development and maturation of newly born neurons, and that the dissociable cellular phenotypes are associated with specific alterations in AHN-related behaviours.

Neuroprotective and Neurorestorative Effects of Epo and VEGF: Perspectives for New Therapeutic Approaches to Neurological Diseases

BACKGROUND

Erythropoietin (Epo) and vascular endothelial growth factor (VEGF) are two vasoactive molecules with essential trophic effects for brain development. The expression and secretion of both molecules increase in response to neuronal damage and they exert protective and restorative effects, which may also be accompanied by adverse side effects.

OBJECTIVE

We review the most relevant evidence on the neuroprotective and neurorestorative effects of Epo and VEGF in three of the most frequent neurological disorders, namely, stroke, epilepsy and Alzheimer's disease, to develop new therapeutic approaches.

METHODS

Several original scientific manuscripts and reviews that have discussed the evidence in critical way, considering both the beneficial and adverse effects of Epo and VEGF in the selected neurological disorders, were analysed. In addition, throughout this review, we propose several considerations to take into account in the design of therapeutic approaches based on Epo and VEGF signalling.

RESULTS

Although the three selected disorders are triggered by different mechanisms, they evolve through similar processes: excitotoxicity, oxidative stress, neuroinflammation, neuronal death, glial reactivity and vascular remodelling. Epo and VEGF exert neuroprotective and neurorestorative effects by acting on these processes due to their pleiotropism. In general, the evidence shows that both Epo and VEGF reduce neuronal death but that at the vascular level, their effects are contradictory.

CONCLUSION

Because the Epo and VEGF signalling pathways are connected in several ways, we conclude that more experimental studies, primarily studies designed to thoroughly assess the functional interactions between Epo and VEGF in the brain under both physiological and pathophysiological conditions, are needed.

The effects of microglia‐ and astrocyte‐derived factors on neurogenesis in health and disease

Hippocampal neurogenesis continues throughout life and has been suggested to play an essential role in maintaining spatial cognitive function under physiological conditions. An increasing amount of evidence has indicated that adult neurogenesis is tightly controlled by environmental conditions in the neurogenic niche, which consists of multiple types of cells including microglia and astrocytes. Microglia maintain the environment of neurogenic niche through their phagocytic capacity and interaction with neurons via fractalkine‐CX3CR1 signaling. In addition, microglia release growth factors such as brain‐derived neurotrophic factor (BDNF) and cytokines such as tumor necrosis factor (TNF)‐α to support the development of adult born neurons. Astrocytes also manipulate neurogenesis by releasing various soluble factors including adenosine triphosphate and lactate. Whereas, under pathological conditions such as Alzheimer's disease, depression, and epilepsy, microglia and astrocytes play a leading role in inflammation and are involved in attenuating the normal process of neurogenesis. The modulation of glial functions on neurogenesis in these brain diseases are attracting attention as a new therapeutic target. This review describes how these glial cells play a role in adult hippocampal neurogenesis in both health and disease, especially focusing glia‐derived factors.

Adult neurogenesis in the mouse dentate gyrus protects the hippocampus from neuronal injury following severe seizures

Previous studies suggest that reducing the numbers of adult-born neurons in the dentate gyrus (DG) of the mouse increases susceptibility to severe continuous seizures (status epilepticus; SE) evoked by systemic injection of the convulsant kainic acid (KA). However, it was not clear if the results would be the same for other ways to induce seizures, or if SE-induced damage would be affected. Therefore, we used pilocarpine, which induces seizures by a different mechanism than KA. Also, we quantified hippocampal damage after SE. In addition, we used both loss-of-function and gain-of-function methods in adult mice. We hypothesized that after loss-of-function, mice would be more susceptible to pilocarpine-induced SE and SE-associated hippocampal damage, and after gain-of-function, mice would be more protected from SE and hippocampal damage after SE. For loss-of-function, adult neurogenesis was suppressed by pharmacogenetic deletion of dividing radial glial precursors. For gain-of-function, adult neurogenesis was increased by conditional deletion of pro-apoptotic gene Bax in Nestin-expressing progenitors. Fluoro-Jade C (FJ-C) was used to quantify neuronal injury and video-electroencephalography (video-EEG) was used to quantify SE. Pilocarpine-induced SE was longer in mice with reduced adult neurogenesis, SE had more power and neuronal damage was greater. Conversely, mice with increased adult-born neurons had shorter SE, SE had less power, and there was less neuronal damage. The results suggest that adult-born neurons exert protective effects against SE and SE-induced neuronal injury.

Sustained somatostatin gene expression reverses kindling-induced increases in the number of dividing Type-1 neural stem cells in the hippocampi of behaviorally responsive rats

Neurogenesis persists throughout life in the hippocampi of all mammals, including humans. In the healthy hippocampus, relatively quiescent Type-1 neural stem cells (NSCs) can give rise to more proliferative Type-2a neural progenitor cells (NPCs), which generate neuronal-committed Type-2b NPCs that mature into Type-3 neuroblasts. Many Type-3 neuroblasts survive and mature into functionally integrated granule neurons over several weeks. In kindling models of epilepsy, neurogenesis is drastically upregulated and many new neurons form aberrant connections that could support epileptogenesis and/or seizures. We have shown that sustained vector-mediated hippocampal somatostatin (SST) expression can both block epileptogenesis and reverse seizure susceptibility in fully kindled rats. Here we test whether adeno-associated virus (AAV) vector-mediated sustained SST expression modulates hippocampal neurogenesis and microglial activation in fully kindled rats. We found significantly more dividing Type-1 NSCs and a corresponding increased number of surviving new neurons in the hippocampi of kindled versus sham-kindled rats. Increased numbers of activated microglia were found in the granule cell layer and hilus of kindled rats at both time points. After intrahippocampal injection with either eGFP or SST-eGFP vector, we found similar numbers of dividing Type-1 NSCs and -2 NPCs and surviving BrdU⁺ neurons and glia in the hippocampi of kindled rats. Upon observed variability in responses to SST-eGFP (2/4 rats exhibited Grade 0 seizures in the test session), we conducted an additional experiment. We found significantly fewer dividing Type-1 NSCs in the hippocampi of SST-eGFP vector-treated responder rats (5/13 rats) relative to SST-eGFP vector-treated non-responders and eGFP vector-treated controls that exhibited high-grade seizures on the test session. The number of activated microglia was upregulated in the GCL and hilus of kindled rats, regardless of vector treatment. These data support the hypothesis that sustained SST expression exerts antiepileptic effects potentially through normalization of neurogenesis and suggests that abnormally high proliferating Type-1 NSC numbers may be a cellular mechanism of epilepsy.

A Fatty Acid Oxidation-Dependent Metabolic Shift Regulates Adult Neural Stem Cell Activity

Hippocampal neurogenesis is important for certain forms of cognition, and failing neurogenesis has been implicated in neuropsychiatric diseases. The neurogenic capacity of hippocampal neural stem/progenitor cells (NSPCs) depends on a balance between quiescent and proliferative states. Here, we show that the rate of fatty acid oxidation (FAO) regulates the activity of NSPCs. Quiescent NSPCs show high levels of carnitine palmitoyltransferase 1a (Cpt1a)-dependent FAO, which is downregulated in proliferating NSPCs. Pharmacological inhibition and conditional deletion of Cpt1a in vitro and in vivo leads to altered NSPC behavior, showing that Cpt1a-dependent FAO is required for stem cell maintenance and proper neurogenesis. Strikingly, manipulation of malonyl-CoA, the metabolite that regulates levels of FAO, is sufficient to induce exit from quiescence and to enhance NSPC proliferation. Thus, the data presented here identify a shift in FAO metabolism that governs NSPC behavior and suggest an instructive role for fatty acid metabolism in regulating NSPC activity.

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A metabolic shift defines NSPC quiescence versus proliferation

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Quiescent NSPCs require high levels of FAO

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Changing levels of a single metabolite is sufficient to induce NSPC proliferation

Epilepsy as a Network Disorder (2): What can we learn from other network disorders such as dementia and schizophrenia, and what are the implications for translational research?

There is common agreement that many disorders of the central nervous system are 'complex', that is, there are many potential factors that influence the development of the disease, underlying mechanisms, and successful treatment. Most of these disorders, unfortunately, have no cure at the present time, and therapeutic strategies often have debilitating side effects. Interestingly, some of the 'complexities' of one disorder are found in another, and the similarities are often network defects. It seems likely that more discussions of these commonalities could advance our understanding and, therefore, have clinical implications or translational impact. With this in mind, the Fourth International Halifax Epilepsy Conference and Retreat was held as described in the prior paper, and this companion paper focuses on the second half of the meeting. Leaders in various subspecialties of epilepsy research were asked to address aging and dementia or psychosis in people with epilepsy (PWE). Commonalities between autism, depression, aging and dementia, psychosis, and epilepsy were the focus of the presentations and discussion. In the last session, additional experts commented on new conceptualization of translational epilepsy research efforts. Here, the presentations are reviewed, and salient points are highlighted.

Adult hippocampal neurogenesis and cognitive flexibility - linking memory and mood

Adult hippocampal neurogenesis has been implicated in cognitive processes, such as pattern separation, and in the behavioural effects of stress and antidepressants. Young adult-born neurons have been shown to inhibit the overall activity of the dentate gyrus by recruiting local interneurons, which may result in sparse contextual representations and improved pattern separation. We propose that neurogenesis-mediated inhibition also reduces memory interference and enables reversal learning both in neutral situations and in emotionally charged ones. Such improved cognitive flexibility may in turn help to decrease anxiety-like and depressive-like behaviour.

Enhanced Expression of Markers for Astrocytes in the Brain of a Line of GFAP-TK Transgenic Mice

GFAP-TK mice are widely used in studies on neurogenesis and reactive astrocytes. Previous studies reported that GCV treatment in GFAP-TK mice resulted in reduced neurogenesis and deletion of proliferating GFAP-expressing astrocytes without affecting mature GFAP-expressing astrocytes. In the present study, we found that GFAP- and vimentin-expressing astrocytes were dramatically increased in the cortex and hippocampus with or without GCV treatment in a line of GFAP-TK mice (Jackson Laboratory, Stock No. 005698), while the neurons and microglia were not affected. In a second line of GFAP-TK mice (MMRRC, Stock No. 037351-UNC) generated in Dr. Heather Cameron's laboratory in NIH, however, no difference in GFAP and vimentin expression was found in both hippocampus and cortex, regardless of GCV treatment or not. Furthermore, enhanced expression of aquaporin 4 (AQP4) was found in the cortex and hippocampus of the GFAP-TK mice from Jackson lab but not in the brain of GFAP-TK mice from NIH. Our data suggested that we should be careful to select different lines of GFAP-TK mice to study adult neurogenesis or reactive astrocytes.

Puerarin Ameliorates D-Galactose Induced Enhanced Hippocampal Neurogenesis and Tau Hyperphosphorylation in Rat Brain

Enhanced neurogenesis has been reported in the hippocampus of patients with Alzheimer's disease (AD), the most common neurodegenerative disorder characterized with amyloid-β (Aβ) aggregation, tau hyperphosphorylation, and progressive neuronal loss. Previously we reported that tau phosphorylation played an essential role in adult hippocampal neurogenesis, and activation of glycogen synthase kinase (GSK-3), a crucial tau kinase, could induce increased hippocampal neurogenesis. In the present study, we found that treatment of D-galactose rats with Puerarin could significantly improve behavioral performance and ameliorate the enhanced neurogenesis and microtubule-associated protein tau hyperphosphorylation in the hippocampus of D-galactose rat brains. FGF-2/GSK-3 signaling pathway might be involved in the effects of Puerarin on hippocampal neurogenesis and tau hyperphosphorylation. Our finding provides primary in vivo evidence that Puerarin can attenuate AD-like enhanced hippocampal neurogenesis and tau hyperphosphorylation. Our finding also suggests Puerarin can be served as a treatment for age-related neurodegenerative disorders, such as AD.

Stem Cell-Mediated Regeneration of the Adult Brain

Acute or chronic injury of the adult mammalian brain is often associated with persistent functional deficits as its potential for regeneration and capacity to rebuild lost neural structures is limited. However, the discovery that neural stem cells (NSCs) persist throughout life in discrete regions of the brain, novel approaches to induce the formation of neuronal and glial cells, and recently developed strategies to generate tissue for exogenous cell replacement strategies opened novel perspectives how to regenerate the adult brain. Here, we will review recently developed approaches for brain repair and discuss future perspectives that may eventually allow for developing novel treatment strategies in acute and chronic brain injury.

Neuroblast Distribution after Cortical Impact Is Influenced by White Matter Injury in the Immature Gyrencephalic Brain

Cortical contusions are a common type of traumatic brain injury (TBI) in children. Current knowledge of neuroblast response to cortical injury arises primarily from studies utilizing aspiration or cryoinjury in rodents. In infants and children, cortical impact affects both gray and white matter and any neurogenic response may be complicated by the large expanse of white matter between the subventricular zone (SVZ) and the cortex, and the large number of neuroblasts in transit along the major white matter tracts to populate brain regions. Previously, we described an age-dependent increase of neuroblasts in the SVZ in response to cortical impact in the immature gyrencephalic brain. Here, we investigate if neuroblasts target the injury, if white matter injury influences repair efforts, and if postnatal population of brain regions are disrupted. Piglets received a cortical impact to the rostral gyrus cortex or sham surgery at postnatal day (PND) 7, BrdU 2 days prior to (PND 5 and 6) or after injury (PND 7 and 8), and brains were collected at PND 14. Injury did not alter the number of neuroblasts in the white matter between the SVZ and the rostral gyrus. In the gray matter of the injury site, neuroblast density was increased in cavitated lesions, and the number of BrdU(+) neuroblasts was increased, but comprised less than 1% of all neuroblasts. In the white matter of the injury site, neuroblasts with differentiating morphology were densely arranged along the cavity edge. In a ventral migratory stream, neuroblast density was greater in subjects with a cavitated lesion, indicating that TBI may alter postnatal development of regions supplied by that stream. Cortical impact in the immature gyrencephalic brain produced complicated and variable lesions, increased neuroblast density in cavitated gray matter, resulted in potentially differentiating neuroblasts in the white matter, and may alter the postnatal population of brain regions utilizing a population of neuroblasts that were born prior to PND 5. This platform may be useful to continue to study potential complications of white matter injury and alterations of postnatal population of brain regions, which may contribute to the chronic effects of TBI in children.

The Epilepsy Spectrum: Targeting Future Research Challenges

There have been tremendous recent advances in our understanding of the biological underpinnings of epilepsy and associated comorbidities that justify its representation as a spectrum disorder. Advances in genetics, electrophysiology, and neuroimaging have greatly improved our ability to differentiate, diagnose, and treat individuals with epilepsy. However, we have made little overall progress in preventing epilepsy, and the number of patients who are cured remains small. Likewise, the comorbidities of epilepsy are often underdiagnosed or not adequately treated. In this article, we suggest a few areas in which additional research will likely pay big dividends for patients and their families.

Music application alleviates short-term memory impairments through increasing cell proliferation in the hippocampus of valproic acid-induced autistic rat pups

Autism is a neurodevelopmental disorder and this disorder shows impairment in reciprocal social interactions, deficits in communication, and restrictive and repetitive patterns of behaviors and interests. The effect of music on short-term memory in the view of cell proliferation in the hippocampus was evaluated using valproic acid-induced autistic rat pups. Animal model of autism was made by subcutaneous injection of 400-mg/kg valproic acid into the rat pups on the postnatal day 14. The rat pups in the music-applied groups were exposed to the 65-dB comfortable classic music for 1 hr once a day, starting postnatal day 15 and continued until postnatal day 28. In the present results, short-term memory was deteriorated by autism induction. The numbers of 5-bromo-2'-deoxyridine (BrdU)-positive, Ki-67-positive, and doublecortin (DCX)-positive cells in the hippocampal dentate gyrus were decreased by autism induction. Brain-derived neurotrophic factor (BDNF) and tyrosine kinase B (TrkB) expressions in the hippocampus were also suppressed in the autistic rat pups. Music application alleviated short-term memory deficits with enhancing the numbers of BrdU-positive, Ki-67-positive, and DCX-positive cells in the autistic rat pups. Music application also enhanced BDNF and TrkB expressions in the autistic rat pups. The present study show that application of music enhanced hippocampal cell proliferation and alleviated short-term memory impairment through stimulating BDNF-TrkB signaling in the autistic rat pups. Music can be suggested as the therapeutic strategy to overcome the autism-induced memory deficits.

Targeting Nicotinamide Phosphoribosyltransferase as a Potential Therapeutic Strategy to Restore Adult Neurogenesis

Adult neurogenesis is the process of generating new neurons throughout life in the olfactory bulb and hippocampus of most mammalian species, which is closely related to aging and disease. Nicotinamide phosphoribosyltransferase (NAMPT), also an adipokine known as visfatin, is the rate-limiting enzyme for mammalian nicotinamide adenine dinucleotide (NAD) salvage synthesis by generating nicotinamide mononucleotide (NMN) from nicotinamide. Recent findings from our laboratory and other laboratories have provided much evidence that NAMPT might serve as a therapeutic target to restore adult neurogenesis. NAMPT-mediated NAD biosynthesis in neural stem/progenitor cells is important for their proliferation, self-renewal, and formation of oligodendrocytes in vivo and in vitro. Therapeutic interventions by the administration of NMN, NAD, or recombinant NAMPT are effective for restoring adult neurogenesis in several neurological diseases. We summarize adult neurogenesis in aging, ischemic stroke, traumatic brain injury, and neurodegenerative disease and review the advances of targeting NAMPT in restoring neurogenesis. Specifically, we provide emphasis on the P7C3 family, a class of proneurogenic compounds that are potential NAMPT activators, which might shed light on future drug development in neurogenesis restoration.

Bacterial Peptidoglycan Traverses the Placenta to Induce Fetal Neuroproliferation and Aberrant Postnatal Behavior

Maternal infection during pregnancy is associated with adverse outcomes for the fetus, including postnatal cognitive disorders. However, the underlying mechanisms are obscure. We find that bacterial cell wall peptidoglycan (CW), a universal PAMP for TLR2, traverses the murine placenta into the developing fetal brain. In contrast to adults, CW-exposed fetal brains did not show any signs of inflammation or neuronal death. Instead, the neuronal transcription factor FoxG1 was induced, and neuroproliferation leading to a 50% greater density of neurons in the cortical plate was observed. Bacterial infection of pregnant dams, followed by antibiotic treatment, which releases CW, yielded the same result. Neuroproliferation required TLR2 and was recapitulated in vitro with fetal neuronal precursor cells and TLR2/6, but not TLR2/1, ligands. The fetal neuroproliferative response correlated with abnormal cognitive behavior in CW-exposed pups following birth. Thus, the bacterial CW-TLR2 signaling axis affects fetal neurodevelopment and may underlie postnatal cognitive disorders.

Neural repair in the adult brain

Acute or chronic injury to the adult brain often results in substantial loss of neural tissue and subsequent permanent functional impairment. Over the last two decades, a number of approaches have been developed to harness the regenerative potential of neural stem cells and the existing fate plasticity of neural cells in the nervous system to prevent tissue loss or to enhance structural and functional regeneration upon injury. Here, we review recent advances of stem cell-associated neural repair in the adult brain, discuss current challenges and limitations, and suggest potential directions to foster the translation of experimental stem cell therapies into the clinic.

Functional Integration of Adult-Born Hippocampal Neurons after Traumatic Brain Injury(1,2,3)

Traumatic brain injury (TBI) increases hippocampal neurogenesis, which may contribute to cognitive recovery after injury. However, it is unknown whether TBI-induced adult-born neurons mature normally and functionally integrate into the hippocampal network. We assessed the generation, morphology, and synaptic integration of new hippocampal neurons after a controlled cortical impact (CCI) injury model of TBI. To label TBI-induced newborn neurons, we used 2-month-old POMC-EGFP mice, which transiently and specifically express EGFP in immature hippocampal neurons, and doublecortin-CreER^T2 transgenic mice crossed with Rosa26-CAG-tdTomato reporter mice, to permanently pulse-label a cohort of adult-born hippocampal neurons. TBI increased the generation, outward migration, and dendritic complexity of neurons born during post-traumatic neurogenesis. Cells born after TBI had profound alterations in their dendritic structure, with increased dendritic branching proximal to the soma and widely splayed dendritic branches. These changes were apparent during early dendritic outgrowth and persisted as these cells matured. Whole-cell recordings from neurons generated during post-traumatic neurogenesis demonstrate that they are excitable and functionally integrate into the hippocampal circuit. However, despite their dramatic morphologic abnormalities, we found no differences in the rate of their electrophysiological maturation, or their overall degree of synaptic integration when compared to age-matched adult-born cells from sham mice. Our results suggest that cells born after TBI participate in information processing, and receive an apparently normal balance of excitatory and inhibitory inputs. However, TBI-induced changes in their anatomic localization and dendritic projection patterns could result in maladaptive network properties.

Aberrant hippocampal neurogenesis after limbic kindling: Relationship to BDNF and hippocampal-dependent memory

Seizures dramatically increase the number of adult generated neurons in the hippocampus. However, it is not known whether this effect depends on seizures that originate in specific brain regions or whether it is nonspecific to seizure activity regardless of origin. We used kindling of different brain sites to address this question. Rats received 99 kindling stimulations of the basolateral amygdala, dorsal hippocampus, or caudate nucleus over a 6-week period. After kindling, we counted the number of adult generated hippocampal neurons that were birth-dated with the proliferative marker bromodeoxyuridine (BrdU) to evaluate cell proliferation and survival under conditions of repeated seizures. Next, we counted the number of doublecortin immunoreactive (DCX-ir) cells and evaluated their dendritic complexity to determine if limbic and nonlimbic seizures have differential effects on neuronal maturation. We also quantified hippocampal brain-derived neurotrophin factor (BDNF) protein levels using an ELISA kit and assessed memory performance using a hippocampal-dependent fear conditioning paradigm. We found that limbic, but not nonlimbic, seizures dramatically increased hippocampal cell proliferation and the number of hilar-CA3 ectopic granule cells. Further, limbic kindling promoted dendritic outgrowth of DCX-ir cells and the number of DCX-ir cells containing basal dendrites. Limbic kindling also enhanced BDNF protein levels throughout the entire hippocampus and impaired the retrieval of fear memories. Collectively, our results suggest a relationship between limbic seizures, neurogenesis, BDNF protein, and cognition.

Adult neurogenesis and its role in neuropsychiatric disease, brain repair and normal brain function

Neural stem/progenitor cells (NSPCs) in the mammalian brain retain the ability to generate new neurones throughout life in discrete brain regions, through a process called adult neurogenesis. Adult neurogenesis, a dramatic form of adult brain circuitry plasticity, has been implicated in physiological brain function and appears to be of pivotal importance for certain forms of learning and memory. In addition, failing or altered neurogenesis has been associated with a variety of brain diseases such as major depression, epilepsy and age-related cognitive decline. Here we review recent advances in our understanding of the basic biology underlying the neurogenic process in the adult brain, focusing on mechanisms that regulate quiescence, proliferation and differentiation of NSPCs. In addition, we discuss how neurogenesis influences normal brain function, and in particular its role in memory formation, as well as its contribution to neuropsychiatric diseases. Finally, we evaluate the potential of targeting endogenous NSPCs for brain repair.

Adult neurogenesis: bridging the gap between mice and humans

Neural stem/progenitor cells (NSPCs) generate new neurons in the mammalian brain throughout life. Over the past two decades, substantial progress has been made in deciphering the cellular and molecular mechanisms underlying adult neurogenesis and in understanding the role played by new neurons in brain function in animal models of health and disease. By contrast, knowledge regarding the extent and relevance of neurogenesis in the adult human brain remains scant. Here we review new concepts about how new neurons shape adult brain circuits, discuss fundamental, unanswered questions about stem cell-associated neural plasticity, and illustrate how the gap between the animal-based basic research and current efforts to analyze life-long neuronal development of the human brain may be overcome by using novel experimental strategies.

Adult neurogenesis: mechanisms and functional significance

New neurons are generated throughout life in distinct regions of the mammalian brain. This process, called adult neurogenesis, has been implicated in physiological brain function, and failing or altered neurogenesis has been associated with a number of neuropsychiatric diseases. Here, we provide an overview of the mechanisms governing the neurogenic process in the adult brain and describe how new neurons may contribute to brain function in health and disease.

Mechanisms regulating neuronal excitability and seizure development following mTOR pathway hyperactivation

The phosphatidylinositol-3-kinase/phosphatase and tensin homolog (PTEN)-mammalian target of rapamycin (mTOR) pathway regulates a variety of neuronal functions, including cell proliferation, survival, growth, and plasticity. Dysregulation of the pathway is implicated in the development of both genetic and acquired epilepsies. Indeed, several causal mutations have been identified in patients with epilepsy, the most prominent of these being mutations in PTEN and tuberous sclerosis complexes 1 and 2 (TSC1, TSC2). These genes act as negative regulators of mTOR signaling, and mutations lead to hyperactivation of the pathway. Animal models deleting PTEN, TSC1, and TSC2 consistently produce epilepsy phenotypes, demonstrating that increased mTOR signaling can provoke neuronal hyperexcitability. Given the broad range of changes induced by altered mTOR signaling, however, the mechanisms underlying seizure development in these animals remain uncertain. In transgenic mice, cell populations with hyperactive mTOR have many structural abnormalities that support recurrent circuit formation, including somatic and dendritic hypertrophy, aberrant basal dendrites, and enlargement of axon tracts. At the functional level, mTOR hyperactivation is commonly, but not always, associated with enhanced synaptic transmission and plasticity. Moreover, these populations of abnormal neurons can affect the larger network, inducing secondary changes that may explain paradoxical findings reported between cell and network functioning in different models or at different developmental time points. Here, we review the animal literature examining the link between mTOR hyperactivation and epileptogenesis, emphasizing the impact of enhanced mTOR signaling on neuronal form and function.

Stress, glucocorticoid receptors, and adult neurogenesis: a balance between excitation and inhibition?

Adult neurogenesis, the birth of new neurons in the mature brain, has attracted considerable attention in the last decade. One of the earliest identified and most profound factors that affect adult neurogenesis both positively and negatively is stress. Here, we review the complex interplay between stress and adult neurogenesis. In particular, we review the role of the glucocorticoid receptor, the main mediator of the stress response in the proliferation, differentiation, migration, and functional integration of newborn neurons in the hippocampus. We review a multitude of mechanisms regulating glucocorticoid receptor activity in relationship to adult neurogenesis. We postulate a novel concept in which the level of glucocorticoid receptor expression directly regulates the excitation-inhibition balance, which is key for proper neurogenesis. We furthermore argue that an excitation-inhibition dis-balance may underlie aberrant functional integration of newborn neurons that is associated with psychiatric and paroxysmal brain disorders.

An examination of early neural and cognitive alterations in hippocampal-spatial function of ghrelin receptor-deficient rats

Ghrelin, a hormone implicated in the regulation of feeding and energy balance, has also been associated with neural function underlying learning and memory. These effects are thought to be mediated by ghrelin targeting receptors at extra hypothalamic sites such as the hippocampus. Exogenous ghrelin administration increases dendritic spine density in the hippocampal CA1 region and neurogenesis in the dentate gyrus (DG), while improving memory in rats. In the present study, we sought to determine whether rats lacking the ghrelin receptor would show early neural or cognitive decline measured via hippocampal integrity (spine density and neurogenesis) and spatial learning and memory. As such, we used young and middle-aged adult rats with mutations to the gene encoding for the ghrelin receptor (GHS-R KO) and wildtype (WT) littermates to determine differences in performance on hippocampal-dependent tasks (the water maze and radial arm maze). In addition, we examined the hippocampal dentate gyrus of these rats for differences in dendritic spine density and cell proliferation (doublecortin). Overall, results demonstrated that spine density and doublecortin staining in the dentate gyrus of the young GHS-R KO group was similar to that seen in middle-aged groups (both KO and WT) and lower than the young WT group. Middle-aged GHS-R KO and WT groups showed deficits on the radial arm maze food-motivated task but not the water maze task. These data suggest that impaired ghrelin signaling leads to an early onset decrement in hippocampal structural integrity that may manifest in non- spatial-related behavioral deficits.

Caspase 3 involves in neuroplasticity, microglial activation and neurogenesis in the mice hippocampus after intracerebral injection of kainic acid

BACKGROUND

The roles of caspase 3 on the kainic acid-mediated neurodegeneration, dendritic plasticity alteration, neurogenesis, microglial activation and gliosis are not fully understood. Here, we investigate hippocampal changes using a mouse model that receive a single kainic acid-intracerebral ventricle injection. The effects of caspase 3 inhibition on these changes were detected during a period of 1 to 7 days post kainic acid injection.

RESULT

Neurodegeneration was assessed by Fluoro-Jade B staining and neuronal nuclei protein (NeuN) immunostaining. Neurogenesis, gliosis, neuritic plasticity alteration and caspase 3 activation were examined using immunohistochemistry. Dendritic plasticity, cleavvage-dependent activation of calcineurin A and glial fibrillary acidic protein cleavage were analyzed by immunoblotting. We found that kainic acid not only induced neurodegeneration but also arouse several caspase 3-mediated molecular and cellular changes including dendritic plasticity, neurogenesis, and gliosis. The acute caspase 3 activation occurred in pyramidal neurons as well as in hilar interneurons. The delayed caspase 3 activation occurred in astrocytes. The co-injection of caspase 3 inhibitor did not rescue kainic acid-mediated neurodegeneration but seriously and reversibly disturb the structural integrity of axon and dendrite. The kainic acid-induced events include microglia activation, the proliferation of radial glial cells, neurogenesis, and calcineurin A cleavage were significantly inhibited by the co-injection of caspase 3 inhibitor, suggesting the direct involvement of caspase 3 in these events. Alternatively, the kainic acid-mediated astrogliosis is not caspase 3-dependent, although caspase 3 cleavage of glial fibrillary acidic protein occurred.

CONCLUSIONS

Our results provide the first direct evidence of a causal role of caspase 3 activation in the cellular changes during kainic acid-mediated excitotoxicity. These findings may highlight novel pharmacological strategies to arrest disease progression and control seizures that are refractory to classical anticonvulsant treatment.

p21Cip restrains hippocampal neurogenesis and protects neuronal progenitors from apoptosis during acute systemic inflammation

Altered neurogenesis in adult hippocampus is implicated in cognition impairment and depression. Inflammation is a potent inhibitor of neurogenesis. The cyclin-dependent kinase inhibitor p21(Cip1) (p21) restrains cell cycle progression and arrests the cell in the G1 phase. We recently showed that p21 is expressed in neuronal progenitors and regulates proliferation of these cells in the subgranular zone of the dentate gyrus of hippocampus where adult neurogenesis occurs. The current study suggests that p21 is induced in vivo in the hippocampus of WT mice in response to acute systemic inflammation caused by LPS injections, restrains neuronal progenitor proliferation and protects these cells from inflammation-induced apoptosis. In intact p21-/- hippocampus, neuronal progenitors proliferate more actively as assessed by BrdU incorporation, and give rise to increased number of DCX positive neuroblasts. However, when mice were treated with LPS, the number of neuroblasts decreased due to induced subgranular zone apoptosis. In vitro, differentiating Tuj-1 positive neuroblasts isolated from p21-/- hippocampus exhibited increased proliferation rate, measured by Ki-67 staining, as compared to WT cells (p<0.05). In WT neuronal progenitors treated with IL-6, the number of p21-positive cells was increased (p<0.05), and this led to Tuj-1(+) cell proliferation restraint, whereas the number of proliferating GFAP(+) astrocytes was increased ~ 2-fold. Thus, when p21 is intact, inflammation might divert neuronal progenitors towards astrogliogenesis by inducing p21. At the same time, when p21 is lacking, no effects of IL-6 on proliferation of Tuj-1(+) cells or GFAP(+) cells are detected in differentiating p21-/- neuronal progenitors. These results underscore the important role of p21 controlling hippocampal neuronal differentiation during inflammation.

Neurogenesis and increase in differentiated neural cell survival via phosphorylation of Akt1 after fluoxetine treatment of stem cells

Fluoxetine (FLX) is a selective serotonin reuptake inhibitor (SSRI). Its action is possibly through an increase in neural cell survival. The mechanism of improved survival rate of neurons by FLX may relate to the overexpression of some kinases such as Akt protein. Akt1 (a serine/threonine kinase) plays a key role in the modulation of cell proliferation and survival. Our study evaluated the effects of FLX on mesenchymal stem cell (MSC) fate and Akt1 phosphorylation levels in MSCs. Evaluation tests included reverse transcriptase polymerase chain reaction, western blot, and immunocytochemistry assays. Nestin, MAP-2, and β-tubulin were detected after neurogenesis as neural markers. Ten μM of FLX upregulated phosphorylation of Akt1 protein in induced hEnSC significantly. Also FLX did increase viability of these MSCs. Continuous FLX treatment after neurogenesis elevated the survival rate of differentiated neural cells probably by enhanced induction of Akt1 phosphorylation. This study addresses a novel role of FLX in neurogenesis and differentiated neural cell survival that may contribute to explaining the therapeutic action of fluoxetine in regenerative pharmacology.

The influence of ectopic migration of granule cells into the hilus on dentate gyrus-CA3 function

Postnatal neurogenesis of granule cells (GCs) in the dentate gyrus (DG) produces GCs that normally migrate from the subgranular zone to the GC layer. However, GCs can mismigrate into the hilus, the opposite direction. Previous descriptions of these hilar ectopic GCs (hEGCs) suggest that they are rare unless there are severe seizures. However, it is not clear if severe seizures are required, and it also is unclear if severe seizures are responsible for the abnormalities of hEGCs, which include atypical dendrites and electrophysiological properties. Here we show that large numbers of hEGCs develop in a transgenic mouse without severe seizures. The mice have a deletion of BAX, which normally regulates apoptosis. Surprisingly, we show that hEGCs in the BAX^-/- mouse have similar abnormalities as hEGCs that arise after severe seizures. We next asked if there are selective effects of hEGCs, i.e., whether a robust population of hEGCs would have any effect on the DG if they were induced without severe seizures. Indeed, this appears to be true, because it has been reported that BAX^-/- mice have defects in a behavior that tests pattern separation, which depends on the DG. However, inferring functional effects of hEGCs is difficult in mice with a constitutive BAX deletion because there is decreased apoptosis in and outside the DG. Therefore, a computational model of the normal DG and hippocampal subfield CA3 was used. Adding a small population of hEGCs (5% of all GCs), with characteristics defined empirically, was sufficient to disrupt a simulation of pattern separation and completion. Modeling results also showed that effects of hEGCs were due primarily to “backprojections” of CA3 pyramidal cell axons to the hilus. The results suggest that hEGCs can develop for diverse reasons, do not depend on severe seizures, and a small population of hEGCs may impair DG-dependent function.

New insights into the role of hilar ectopic granule cells in the dentate gyrus based on quantitative anatomic analysis and three-dimensional reconstruction

The dentate gyrus is one of two main areas of the mammalian brain where neurons are born throughout adulthood, a phenomenon called postnatal neurogenesis. Most of the neurons that are generated are granule cells (GCs), the major principal cell type in the dentate gyrus. Some adult-born granule cells develop in ectopic locations, such as the dentate hilus. The generation of hilar ectopic granule cells (HEGCs) is greatly increased in several animal models of epilepsy and has also been demonstrated in surgical specimens from patients with intractable temporal lobe epilepsy (TLE). Herein we review the results of our quantitative neuroanatomic analysis of HEGCs that were filled with Neurobiotin following electrophysiologic characterization in hippocampal slices. The data suggest that two types of HEGCs exist, based on a proximal or distal location of the cell body relative to the granule cell layer, and based on the location of most of the dendrites, in the molecular layer or hilus. Three-dimensional reconstruction revealed that the dendrites of distal HEGCs can extend along the transverse and longitudinal axis of the hippocampus. Analysis of axons demonstrated that HEGCs have projections that contribute to the normal mossy fiber innervation of CA3 as well as the abnormal sprouted fibers in the inner molecular layer of epileptic rodents (mossy fiber sprouting). These data support the idea that HEGCs could function as a "hub" cell in the dentate gyrus and play a critical role in network excitability.

Gene expression profiling of neural stem cells and their neuronal progeny reveals IGF2 as a regulator of adult hippocampal neurogenesis

Neural stem cells (NSCs) generate neurons throughout life in the hippocampal dentate gyrus (DG). How gene expression signatures differ among NSCs and immature neurons remains largely unknown. We isolated NSCs and their progeny in the adult DG using transgenic mice expressing a GFP reporter under the control of the Sox2 promoter (labeling NSCs) and transgenic mice expressing a DsRed reporter under the control of the doublecortin (DCX) promoter (labeling immature neurons). Transcriptome analyses revealed distinct gene expression profiles between NSCs and immature neurons. Among the genes that were expressed at significantly higher levels in DG NSCs than in immature neurons was the growth factor insulin-like growth factor 2 (IGF2). We show that IGF2 selectively controls proliferation of DG NSCs in vitro and in vivo through AKT-dependent signaling. Thus, by gene expression profiling of NSCs and their progeny, we have identified IGF2 as a novel regulator of adult neurogenesis.

From pattern to purpose: how comparative studies contribute to understanding the function of adult neurogenesis

The study of adult neurogenesis has had an explosion of fruitful growth. Yet numerous uncertainties and challenges persist. Our review begins with a survey of species that show evidence of adult neurogenesis. We then discuss how neurogenesis varies across brain regions and point out that regional specializations can indicate functional adaptations. Lifespan and aging are key life-history traits. Whereas 'adult neurogenesis' is the common term in the literature, it does not reflect the reality of neurogenesis being primarily a 'juvenile' phenomenon. We discuss the sharp decline with age as a universal trait of neurogenesis with inevitable functional consequences. Finally, the main body of the review focuses on the function of neurogenesis in birds and mammals. Selected examples illustrate how our understanding of avian and mammalian neurogenesis can complement each other. It is clear that although the two phyla have some common features, the function of adult neurogenesis may not be similar between them and filling the gaps will help us understand neurogenesis as an evolutionarily conserved trait to meet particular ecological pressures.

GRIK4/KA1 protein expression in human brain and correlation with bipolar disorder risk variant status

The kainate class of ionotropic glutamate receptors is involved in the regulation of neuronal transmission and synaptic plasticity. Previously we reported that a deletion variant within the gene GRIK4, which encodes the KA1 kainate receptor subunit, was associated with a reduced risk of bipolar disorder and increased GRIK4 mRNA abundance. Using a high resolution immunohistochemistry technique, we characterized KA1 protein localization in human brain and performed a genotype-protein expression correlation study. KA1 was expressed in specific populations of neuronal cells in the cerebellum and all layers of the frontal and parahippocampal cortices. In the hippocampus, strong KA1 expression was observed in the stratum pyramidale and stratum lucidum of CA3 and CA2, in cell processes in CA1, in the neuropil of the CA4 region, in polymorphic cells including mossy fiber neurons in the hilus, and dentate gyrus (DG) granule cells. Mean counts of KA1 positive DG granule cells, hippocampal CA3 pyramidal cells, and layer 1 of the frontal cortex were significantly increased in subjects with the deletion allele (P = 0.0005, 0.018, and 0.0058, respectively) compared to subjects homozygous for the insertion. Neuronal expression levels in all regions quantified were higher in the deletion group. These results support our hypothesis that the deletion allele affords protection against bipolar disorder through increased KA1 protein abundance in neuronal cells. Biological mechanisms which may contribute to this protective effect include KA1 involvement in adult hippocampal neurogenesis, HPA axis activation, or plasticity processes affecting neuronal circuitry.

Harnessing adult neurogenesis by cracking the epigenetic code

The adult brain contains a reservoir of neural stem cells (NSCs) that generates functional neurons in a process called adult neurogenesis. Integration of new neurons into mature neural circuits maintains brain tissue homeostasis essential for learning, olfaction and behavior. Even subtle disruptions in NSC self-renewal/differentiation can result in substantial changes in neuronal production rates, contributing to neuropsychiatric symptoms, cognitive dysfunction and epilepsy. Recent studies have revealed pivotal roles for epigenetic regulators of gene expression. Epigenetic and genetic regulation allows a rich array of possibilities to fine-tune neuronal gene expression and offers potential therapeutic opportunities to modulate brain function related to adult neurogenesis. Here we discuss the role of epigenetic mechanisms underlying NSC fate and translational strategies for the future.

Social instability stress in adolescent male rats alters hippocampal neurogenesis and produces deficits in spatial location memory in adulthood

The ongoing development of the hippocampus in adolescence may be vulnerable to stressors. The effects of social instability stress (SS) in adolescence (daily 1 h isolation and change of cage partner postnatal days 30-45) on cell proliferation in the dentate gyrus (DG) in adolescence (on days 33 and 46, experiment 1) and in adulthood (experiment 2) was examined in Long Evans male rats and compared to nonstressed controls (CTL). Additionally, in experiment 2, a separate group of SS and CTL rats was tested on either a spatial (hippocampal-dependent) or nonspatial (nonhippocampal dependent) version of an object memory test and also were used to investigate hippocampal expression of markers of synaptic plasticity. No memory impairment was evident until the SS rats were adults, and the impairment was only on the spatial test. SS rats initially (postnatal day 33) had increased cell proliferation based on counts of Ki67 immunoreactive (ir) cells and greater survival of immature neurons based on counts of doublecortin ir cells on day 46 and in adulthood, irrespective of behavioral testing. Counts of microglia in the DG did not differ by stress group, but behavioral testing was associated with reduced microglia counts compared to nontested rats. As adults, SS and CTL rats did not differ in hippocampal expression of synaptophysin, but compared to CTL rats, SS rats had higher expression of basal calcium/calmodulin-dependent kinase II (CamKII), and lower expression of the phosphorylated CamKII subunit threonine 286, signaling molecules related to synaptic plasticity. The results are contrasted with those from previous reports of chronic stress in adult rats, and we conclude that adolescent stress alters the ongoing development of the hippocampus leading to impaired spatial memory in adulthood, highlighting the heightened vulnerability to stressors in adolescence.

Modeling new neuron function: a history of using computational neuroscience to study adult neurogenesis

Adult neurogenesis is a sophisticated biological process whose function has remained a mystery to neuroscience researchers. To address this question, a number of unique modeling studies have explored the computational implications of adding new neurons to the adult dentate gyrus. Models of neurogenesis fall into two broad categories: abstract models that explore the function of new neurons in simple networks, and biologically based models that investigate the role of new neurons in networks based on the anatomy of the hippocampus. In this review, we summarize the strategies and results of these different modeling approaches, and we discuss their conclusions and limitations in the face of new biological findings.

Expression of neurogenesis genes in human temporal lobe epilepsy with hippocampal sclerosis

Both evoked and spontaneous seizures have been reported to increase neurogenesis in animal models. Less is known about whether neurogenesis and markers thereof are aberrantly expressed in human temporal lobe epilepsy (TLE) with hippocampal sclerosis. In the present study we measured protein levels of multiple neurogenesis marker genes using Western blotting. Tissue homogenates from sclerotic hippocampus surgically resected from patients with pharmacoresistantTLE (n = 7) were compared to hippocampal samples from a group of age- and gender-matched autopsy controls (n = 6). Expression of the mature neuron marker NeuN was significantly lower in TLE samples compared to controls. In contrast, levels of neurogenesis-associated genes including TUC-4, doublecortin, Neu-roD and Numb, were all similarly expressed in TLE and control hippocampus samples. The present study suggests hippocampal expression levels of proteins associated with neurogenesis are not notably different in human TLE, implying the sclerotic hippocampus may retain neurogenic potential.