Sleep rebound leads to marked recovery of prolonged sleep deprivation-induced adversities in the stress response and hippocampal neuroplasticity of male rats

BACKGROUND

Sleep disturbances are not only frequent symptoms, but also risk factors for major depressive disorder. We previously reported that depressed patients who experienced "Hypersomnia" showed a higher and more rapid response rate under paroxetine treatment, but the underlying mechanism remains unclear. The present study was conducted to clarify the beneficial effects of sleep rebound through an experimental "Hypersomnia" rat model on glucocorticoid and hippocampal neuroplasticity associated with antidepressive potency.

METHODS

Thirty-four male Sprague-Dawley rats were subjected to sham treatment, 72-h sleep deprivation, or sleep deprivation and subsequent follow-up for one week. Approximately half of the animals were sacrificed to evaluate adrenal weight, plasma corticosterone level, hippocampal content of mRNA isoforms, and protein of the brain-derived neurotrophic factor (Bdnf) gene. In the other half of the rats, Ki-67- and doublecortin (DCX)-positive cells in the hippocampus were counted via immunostaining to quantify adult neurogenesis.

RESULTS

Prolonged sleep deprivation led to adrenal hypertrophy and an increase in the plasma corticosterone level, which had returned to normal after one week follow-up. Of note, sleep deprivation-induced decreases in hippocampal Bdnf transcripts containing exons II, IV, VI, and IX and BDNF protein levels, Ki-67-(+)-proliferating cells, and DCX-(+)-newly-born neurons were not merely reversed, but overshot their normal levels with sleep rebound.

LIMITATIONS

The present study did not record electroencephalogram or assess behavioral changes of the sleep-deprived rats.

CONCLUSIONS

The present study demonstrated that prolonged sleep deprivation-induced adversities are reversed or recovered by sleep rebound, which supports "Hypersomnia" in depressed patients as having a beneficial pharmacological effect.

TAAR1 in dentate gyrus is involved in chronic stress-induced impairments in hippocampal plasticity and cognitive function

Multiple lines of evidence suggest that the trace amine-associated receptor 1 (TAAR1) holds promise as a potential target for stress-related disorders, such as treating major depressive disorder (MDD). The role of TAAR1 in the regulation of adult neurogenesis is recently supported by transcriptomic data. However, it remains unknown whether TAAR1 in dentate gyrus (DG) mediate chronic stress-induced negative effects on hippocampal plasticity and related behavior in mice. The present study consisted of a series of experiments using RNAscope, genetic approaches, behavioral tests, immunohistochemical staining, Golgi-Cox technique to unravel the effects of TAAR1 on alterations of dentate neuronal plasticity and cognitive function in the chronic social defeat stress model. The mice subjected to chronic defeat stress exhibited a noteworthy decrease in the mRNA level of TAAR1 in DG. Additionally, they exhibited compromised social memory and spatial object recognition memory, as well as impaired proliferation and maturation of adult-born dentate granule cells. Moreover, the selective knockout TAAR1 in DG mostly mimicked the cognitive function deficits and neurogenesis impairment induced by chronic stress. Importantly, the administration of the selective TAAR1 partial agonist RO5263397 during stress exposure attenuated the adverse effects of chronic stress on cognitive function, adult neurogenesis, dendritic arborization, and the synapse number of dentate neurons in DG. In summary, our findings suggest that TAAR1 plays a crucial role in mediating the detrimental effects of chronic stress on hippocampal plasticity and cognition. TAAR1 agonists exhibit therapeutic potential for individuals suffering from cognitive impairments associated with MDD.

Sex specific effects of buprenorphine on adult hippocampal neurogenesis and behavioral outcomes during the acute phase after pediatric traumatic brain injury in mice

Traumatic brain injury (TBI) in children often causes cognitive and mental dysfunctions, as well as acute and chronic pain. Adult hippocampal neurogenesis plays a key role in cognition, depression, and pain. Adult hippocampal neurogenesis can be modulated by genetic and environmental factors, such as TBI and opioids. Buprenorphine (BPN), a semisynthetic opioid, is commonly used for pain management in children, however, the effects of BPN on adult hippocampal neurogenesis after pediatric TBI are still unclear. This study investigated the sex-specific effects of BPN on adult hippocampal neurogenesis during acute phase after pediatric TBI. Male and female littermates were randomized on postnatal day 20-21(P20-21) into Sham, TBI+saline and TBI+BPN groups. BPN was administered intraperitoneally to the TBI+BPN mice at 30 min after injury, and then every 6-12 h (h) for 2 days (d). Bromodeoxyuridine (BrdU) was administered intraperitoneally to all groups at 2, 4, 6, and 8-h post-injury. All outcomes were evaluated at 3-d post-BrdU administration. We found that TBI induced significant cognitive impairment, depression, and reduced adult hippocampal neurogenesis in both male and female mice, with more prominent effects in females. BPN significantly improved adult hippocampal neurogenesis and depression in males, but not in females. We further demonstrated that differential expressions of opioid receptors, transcription factors and neuroinflammatory markers at the neurogenic niche might be responsible for the differential effects of BPN in males and females. In conclusion, this study elucidates the effects of BPN on adult hippocampal neurogenesis and behavioral outcomes at the acute phase after pediatric TBI.

Ketamine's rapid and sustained antidepressant effects are driven by distinct mechanisms

Administration of multiple subanesthetic doses of ketamine increases the duration of antidepressant effects relative to a single ketamine dose, but the mechanisms mediating this sustained effect are unclear. Here, we demonstrate that ketamine's rapid and sustained effects on affective behavior are mediated by separate and temporally distinct mechanisms. The rapid effects of a single dose of ketamine result from increased activity of immature neurons in the hippocampal dentate gyrus without an increase in neurogenesis. Treatment with six doses of ketamine over two weeks doubled the duration of behavioral effects after the final ketamine injection. However, unlike ketamine's rapid effects, this more sustained behavioral effect did not correlate with increased immature neuron activity but instead correlated with increased numbers of calretinin-positive and doublecortin-positive immature neurons. This increase in neurogenesis was associated with a decrease in bone morphogenetic protein (BMP) signaling, a known inhibitor of neurogenesis. Injection of a BMP4-expressing lentivirus into the dentate gyrus maintained BMP signaling in the niche and blocked the sustained - but not the rapid - behavioral effects of ketamine, indicating that decreased BMP signaling is necessary for ketamine's sustained effects. Thus, although the rapid effects of ketamine result from increased activity of immature neurons in the dentate gyrus without requiring an increase in neurogenesis, ketamine's sustained effects require a decrease in BMP signaling and increased neurogenesis along with increased neuron activity. Understanding ketamine's dual mechanisms of action should help with the development of new rapid-acting therapies that also have safe, reliable, and sustained effects.

Effect of a multi-strain probiotic mixture consumption on anxiety and depression symptoms induced in adult mice by postnatal maternal separation

BACKGROUND

Intestinal microbial composition not only affects the health of the gut but also influences centrally mediated systems involved in mood, through the "gut-brain" axis, a bidirectional communication between gut microbiota and the brain. In this context, the modulation of intestinal microbiota and its metabolites through the administration of probiotics seems to represent a very promising approach in the treatment of the central nervous system alterations. Early postnatal life is a critical period during which the brain undergoes profound and essential modulations in terms of maturation and plasticity. Maternal separation (MS), i.e., the disruption of the mother-pup interaction, represents a pivotal paradigm in the study of stress-related mood disorders, by inducing persistent changes in the immune system, inflammatory processes, and emotional behavior in adult mammals.

RESULTS

We conducted experiments to investigate whether sustained consumption of a multi-strain probiotic formulation by adult male mice could mitigate the effects of maternal separation. Our data demonstrated that the treatment with probiotics was able to totally reverse the anxiety- and depressive-like behavior; normalize the neuro-inflammatory state, by restoring the resting state of microglia; and finally induce a proneurogenic effect. Mice subjected to maternal separation showed changes in microbiota composition compared to the control group that resulted in permissive colonization by the administered multi-strain probiotic product. As a consequence, the probiotic treatment also significantly affected the production of SCFA and in particular the level of butyrate.

CONCLUSION

Gut microbiota and its metabolites mediate the therapeutic action of the probiotic mix on MS-induced brain dysfunctions. Our findings extend the knowledge on the use of probiotics as a therapeutic tool in the presence of alterations of the emotional sphere that significantly impact on gut microbiota composition. Video Abstract.

The neuronal transcription factor MEIS2 is a calpain-2 protease target

Tight control over transcription factor activity is necessary for a sensible balance between cellular proliferation and differentiation in the embryo and during tissue homeostasis by adult stem cells, but mechanistic details have remained incomplete. The homeodomain transcription factor MEIS2 is an important regulator of neurogenesis in the ventricular-subventricular zone (V-SVZ) adult stem cell niche in mice. We here identify MEIS2 as direct target of the intracellular protease calpain-2 (composed of the catalytic subunit CAPN2 and the regulatory subunit CAPNS1). Phosphorylation at conserved serine and/or threonine residues, or dimerization with PBX1, reduced the sensitivity of MEIS2 towards cleavage by calpain-2. In the adult V-SVZ, calpain-2 activity is high in stem and progenitor cells, but rapidly declines during neuronal differentiation, which is accompanied by increased stability of MEIS2 full-length protein. In accordance with this, blocking calpain-2 activity in stem and progenitor cells, or overexpression of a cleavage-insensitive form of MEIS2, increased the production of neurons, whereas overexpression of a catalytically active CAPN2 reduced it. Collectively, our results support a key role for calpain-2 in controlling the output of adult V-SVZ neural stem and progenitor cells through cleavage of the neuronal fate determinant MEIS2.

Neurogenesis impairment with glial activation in the hippocampus-connected regions of intracerebroventricular streptozotocin-injected mice

Adult neurogenesis in the hippocampus and subventricular zone (SVZ) is impaired by intracerebroventricular administration of streptozotocin (icv-STZ) to rodents. Although neural cells in the several brain regions which connect with the hippocampus or SVZ is thought to be involved in the adult neurogenesis, few studies have investigated morphological alterations of glial cells in these areas. The present study revealed that icv-STZ induces reduction of neural progenitor cells and a dramatic increase in reactive astrocytes and microglia especially in the hippocampus and various hippocampus-connected brain areas. In contrast, there was no significant neuronal damage excluding demyelination of the stria medullaris. The results indicate the hippocampal neurogenesis impairment of this model might be occurred by activated glial cells in the hippocampus, or hippocampus-connected regions.

Stimulation of the muscarinic receptor M4 regulates neural precursor cell proliferation and promotes adult hippocampal neurogenesis

Cholinergic signaling plays a crucial role in the regulation of adult hippocampal neurogenesis; however, the mechanisms by which acetylcholine mediates neurogenic effects are not completely understood. Here, we report the expression of muscarinic acetylcholine receptor subtype M4 (M4 mAChR) on a subpopulation of neural precursor cells (NPCs) in the adult mouse hippocampus, and demonstrate that its pharmacological stimulation promotes their proliferation, thereby enhancing the production of new neurons in vivo. Using a targeted ablation approach, we also show that medial septum (MS) and the diagonal band of Broca (DBB) cholinergic neurons support both the survival and morphological maturation of adult-born neurons in the mouse hippocampus. Although the systemic administration of an M4-selective allosteric potentiator fails to fully rescue the MS/DBB cholinergic lesion-induced decrease in hippocampal neurogenesis, it further exacerbates the impairment in the morphological maturation of adult-born neurons. Collectively, these findings reveal stage-specific roles of M4 mAChRs in regulating adult hippocampal neurogenesis, uncoupling their positive role in enhancing the production of new neurons from the M4-induced inhibition of their morphological maturation, at least in the context of cholinergic signaling dysfunction.

Perinatal ethanol exposure affects cell populations in adult dorsal hippocampal neurogenic niche

Neurodevelopment is highly affected by perinatal ethanol exposure (PEE). In the adult brain, neurogenesis takes place in the dentate gyrus (DG) of the hippocampus and in the subventricular zone. This work aimed to analyze the effect of PEE on the cellular types involved in adult dorsal hippocampal neurogenesis phases using a murine model. For this purpose, primiparous female CD1 mice consumed only ethanol 6% v/v from 20 days prior to mating and along pregnancy and lactation to ensure that the pups were exposed to ethanol throughout pre- and early postnatal development. After weaning, pups had no further contact with ethanol. Cell types of the adult male dorsal DG were studied by immunofluorescence. A lower percentage of type 1 cells and immature neurons and a higher percentage of type 2 cells were observed in PEE animals. This decrease in type 1 cells suggests that PEE reduces the population of remnant progenitors of the dorsal DG present in adulthood. The increase in type 2 cells and the decrease in immature neurons indicate that, during neurodevelopment, ethanol alters the capacity of neuroblasts to become neurons in the adult neurogenic niche. These results suggest that pathways implicated in cell determination are affected by PEE and remain affected in adulthood.

An analogue of the Prolactin Releasing Peptide reduces obesity and promotes adult neurogenesis

Hypothalamic Adult Neurogenesis (hAN) has been implicated in regulating energy homeostasis. Adult-generated neurons and adult Neural Stem Cells (aNSCs) in the hypothalamus control food intake and body weight. Conversely, diet-induced obesity (DIO) by high fat diets (HFD) exerts adverse influence on hAN. However, the effects of anti-obesity compounds on hAN are not known. To address this, we administered a lipidized analogue of an anti-obesity neuropeptide, Prolactin Releasing Peptide (PrRP), so-called LiPR, to mice. In the HFD context, LiPR rescued the survival of adult-born hypothalamic neurons and increased the number of aNSCs by reducing their activation. LiPR also rescued the reduction of immature hippocampal neurons and modulated calcium dynamics in iPSC-derived human neurons. In addition, some of these neurogenic effects were exerted by another anti-obesity compound, Liraglutide. These results show for the first time that anti-obesity neuropeptides influence adult neurogenesis and suggest that the neurogenic process can serve as a target of anti-obesity pharmacotherapy.

Neural stem cells persist to generate new neurons in the hippocampus of adult and aged human brain - Fiction or accurate?

Adult neurogenesis, comprising the generation, differentiation and integration of new neurons in the mature brain, has emerged as a dynamic area of research over decades. The discovery of adult neurogenesis was a paradigm shift to comprehend mechanisms underlying brain plasticity, cognitive function, and neurological disorders. This review deliberates significant findings from articles published over four decades on adult neurogenesis, highlighting key milestones, methodological advances, and controversies that have shaped our comprehension of the phenomenon of adult neurogenesis. Early skepticism gave way to a rich body of evidence via various reliable approaches. Studies on neurogenic niches, microenvironmental factors, molecular regulators, and functional implications have uncovered the involvement of adult neurogenesis in learning, memory, mood, and even neurological and neurodegenerative conditions. Despite significant progress, several questions still need to be answered, including the exact contributions of new neurons to brain function, their integration into existing circuits, and the impact of enhancing adult neurogenesis in the human hippocampus. While the existence of robust neurogenesis in the adult and aged human hippocampus is yet to be confirmed, this review highlights evidence from a significant number of studies supporting the persistence of hippocampal neurogenesis during adulthood and aging in humans, including in some neurological conditions, such as epilepsy and Alzheimer's disease. Nonetheless, additional large-scale studies using single cell-RNA-seq, single nucleus-RNA-seq, and spatial transcriptomics are critical to validate the presence and contribution of hippocampal neurogenesis in the pathophysiology of various neurological and neurodegenerative conditions at different stages of the disease. There is also a need to develop standardized protocols for analyzing postmortem hippocampal tissues for cellular and molecular analyses.

Exercise Reshapes the Brain: Molecular, Cellular, and Structural Changes Associated with Cognitive Improvements

Physical exercise is well known as a non-pharmacological and holistic therapy believed to prevent and mitigate numerous neurological conditions and alleviate ageing-related cognitive decline. To do so, exercise affects the central nervous system (CNS) at different levels. It changes brain physiology and structure, promoting cognitive improvements, which ultimately improves quality of life. Most of these effects are mediated by neurotrophins release, enhanced adult hippocampal neurogenesis, attenuation of neuroinflammation, modulation of cerebral blood flow, and structural reorganisation, besides to promote social interaction with beneficial cognitive outcomes. In this review, we discuss, based on experimental and human research, how exercise impacts the brain structure and function and how these changes contribute to cognitive improvements. Understanding the mechanisms by which exercise affects the brain is essential to understand the brain plasticity following exercise, guiding therapeutic approaches to improve the quality of life, especially in obesity, ageing, neurodegenerative disorders, and following traumatic brain injury.

Targeting epidermal growth factor receptor to recruit newly generated neuroblasts in cortical brain injuries

BACKGROUND

Neurogenesis is stimulated in the subventricular zone (SVZ) of mice with cortical brain injuries. In most of these injuries, newly generated neuroblasts attempt to migrate toward the injury, accumulating within the corpus callosum not reaching the perilesional area.

METHODS

We use a murine model of mechanical cortical brain injury, in which we perform unilateral cortical injuries in the primary motor cortex of adult male mice. We study neurogenesis in the SVZ and perilesional area at 7 and 14 dpi as well as the expression and concentration of the signaling molecule transforming growth factor alpha (TGF-α) and its receptor the epidermal growth factor (EGFR). We use the EGFR inhibitor Afatinib to promote neurogenesis in brain injuries.

RESULTS

We show that microglial cells that emerge within the injured area and the SVZ in response to the injury express high levels of TGF-α leading to elevated concentrations of TGF-α in the cerebrospinal fluid. Thus, the number of neuroblasts in the SVZ increases in response to the injury, a large number of these neuroblasts remain immature and proliferate expressing the epidermal growth factor receptor (EGFR) and the proliferation marker Ki67. Restraining TGF-α release with a classical protein kinase C inhibitor reduces the number of these proliferative EGFR+ immature neuroblasts in the SVZ. In accordance, the inhibition of the TGF-α receptor, EGFR promotes migration of neuroblasts toward the injury leading to an elevated number of neuroblasts within the perilesional area.

CONCLUSIONS

Our results indicate that in response to an injury, microglial cells activated within the injury and the SVZ release TGF-α, activating the EGFR present in the neuroblasts membrane inducing their proliferation, delaying maturation and negatively regulating migration. The inactivation of this signaling pathway stimulates neuroblast migration toward the injury and enhances the quantity of neuroblasts within the injured area. These results suggest that these proteins may be used as target molecules to regenerate brain injuries.

Unravelling a novel role for cannabidivarin in the modulation of subventricular zone postnatal neurogenesis

Postnatal neurogenesis has been shown to rely on the endocannabinoid system. Here we aimed at unravelling the role of Cannabidivarin (CBDV), a non-psychoactive cannabinoid, with high affinity for the non-classical cannabinoid receptor TRPV1, on subventricular zone (SVZ) postnatal neurogenesis. Using the neurosphere assay, SVZ-derived neural stem/progenitor cells (NSPCs) were incubated with CBDV and/or 5'-Iodoresinferotoxin (TRPV1 antagonist), and their role on cell viability, proliferation, and differentiation were dissected. CBDV was able to promote, through a TRPV1-dependent mechanism, cell survival, cell proliferation and neuronal differentiation. Furthermore, pulse-chase experiments revealed that CBDV-induced neuronal differentiation was a result of cell cycle exit of NSPCs. Regarding oligodendrocyte differentiation, CBDV inhibited oligodendrocyte differentiation and maturation. Since our data suggested that the CBDV-induced modulation of NSPCs acted via TRPV1, a sodium-calcium channel, and that intracellular calcium levels are known regulators of NSPCs fate and neuronal maturation, single cell calcium imaging was performed to evaluate the functional response of SVZ-derived cells. We observed that CBDV-responsive cells displayed a two-phase calcium influx profile, being the initial phase dependent on TRPV1 activation. Taken together, this work unveiled a novel and untapped neurogenic potential of CBDV via TRPV1 modulation. These findings pave the way to future neural stem cell biological studies and repair strategies by repurposing this non-psychoactive cannabinoid as a valuable therapeutic target.

Presymptomatic Reduction of Individuality in the AppNL-F Knockin Model of Alzheimer's Disease

BACKGROUND

One-third of the risk for Alzheimer's disease is explained by environment and lifestyle, but Alzheimer's disease pathology might also affect lifestyle and thereby impair the individual potential for health behavior and prevention.

METHODS

We examined in mice how the AppNL-F/NL-F (NL-F) knockin mutation affects the presymptomatic response to environmental enrichment (ENR) as an experimental paradigm addressing nongenetic factors. We assessed the emergence of interindividual phenotypic variation under the condition that both the genetic background and the shared environment were held constant, thereby isolating the contribution of individual behavior (nonshared environment).

RESULTS

After 4 months of ENR, the mean and variability of plasma ApoE were increased in NL-F mice, suggesting a presymptomatic variation in pathogenic processes. Roaming entropy as a measure of behavioral activity was continuously assessed with radiofrequency identification (RFID) technology and revealed reduced habituation and variance in NL-F mice compared with control animals, which do not carry a Beyreuther/Iberian mutation. Intraindividual variation decreased, while behavioral stability was reduced in NL-F mice. Seven months after discontinuation of ENR, we found no difference in plaque size and number, but ENR increased variance in hippocampal plaque counts in NL-F mice. A reactive increase in adult hippocampal neurogenesis in NL-F mice, known from other models, was normalized by ENR.

CONCLUSIONS

Our data suggest that while NL-F has early effects on individual behavioral patterns in response to ENR, there are lasting effects on cellular plasticity even after the discontinuation of ENR. Hence, early behavior matters for maintaining individual behavioral trajectories and brain plasticity even under maximally constrained conditions.

Chronic intranasal oxytocin increases acoustic eavesdropping and adult neurogenesis

Social information gathering is a complex process influenced by neuroendocrine-modulated neural plasticity. Oxytocin (OXT) is a key regulator of social decision-making processes such as information gathering, as it contextually modulates social salience and can induce long-term structural plasticity, including neurogenesis. Understanding the link between OXT-induced plasticity and communicative awareness is crucial, particularly because OXT is being considered for treatment of social pathologies. We investigated the role of chronic OXT-dependent plasticity in attention to novel social information by manipulating the duration of time following cessation of intranasal treatment to allow for the functional integration of adult-born neurons resulting from OXT treatment. Following a 3-week delay, chronic intranasal OXT (IN-OXT) increased approach behavior of both female and male mice towards aggressive vocal playbacks of two unseen novel conspecifics, while no effect was observed after a 3-day delay. Immature neurons increased in the ventral hippocampus of females and males treated with chronic IN-OXT after the 3-week delay, indicating a potential association between ventral hippocampal neurogenesis and approach/acoustic eavesdropping. The less the mouse approached, the higher the level of neurogenesis. Contrary to expectations, the correlation between ventral hippocampal neurogenesis and approach behavior was not affected by IN-OXT, suggesting that other plasticity mechanisms underlie the long-term effects of chronic OXT on social approach. Furthermore, we found a negative correlation between ventral hippocampal neurogenesis and freezing behavior. Overall, our results demonstrate that chronic IN-OXT-induced long-term plasticity can influence approach to vocal information and we further reinforced the link between neurogenesis and anxiety.

Dbx2, an Aging-Related Homeobox Gene, Inhibits the Proliferation of Adult Neural Progenitors

In the adult mouse brain, the subventricular zone (SVZ) underlying the lateral ventricles harbours a population of quiescent neural stem cells, which can be activated (aNSCs) to initiate proliferation and generate a neurogenic lineage consisting of transit amplifying progenitors (TAPs), neuroblasts (NBs) and newborn neurons. This process is markedly reduced during aging. Recent studies suggest that the aged SVZ niche decreases the pool of proliferating neural/stem progenitor cells (NSPCs), and hence adult neurogenesis, by causing transcriptomic changes that promote NSC quiescence. The transcription factors that mediate these changes, however, remain unclear. We previously found that the homeobox gene Dbx2 is upregulated in NSPCs of the aged mouse SVZ and can inhibit the growth of NSPC cultures. Here, we further investigate its role as a candidate transcriptional regulator of neurogenic decline. We show that Dbx2 expression is downregulated by Epidermal Growth Factor receptor signaling, which promotes NSPC proliferation and decreases in the aged SVZ. By means of transgenic NSPC lines overexpressing Dbx2, we also show that this gene inhibits NSPC proliferation by hindering the G2/M transition. Furthermore, we exploit RNA sequencing of transgenic NSPCs to elucidate the transcriptomic networks modulated by Dbx2. Among the top hits, we report the downregulation of the molecular pathways implicated in cell cycle progression. Accordingly, we find that Dbx2 function is negatively correlated with the transcriptional signatures of proliferative NSPCs (aNSCs, TAPs and early NBs). These results point to Dbx2 as a transcription factor relaying the anti-neurogenic input of the aged niche to the NSPC transcriptome.

Neural stem cells characterization in the vagal complex of adult ovine brain: A combined neurosphere assay/RTqPCR approach

Neural stem cells are the effectors of adult neurogenesis, which occurs in discrete restricted areas of adult mammalian brain. In ovine species, like in rodents, in vivo incorporation of labeled DNA precursor led to characterize neurogenic proliferation in the subventricular zone and progeny migration and differentiation into the olfactory bulb. The present study addresses directly the existence of neural stem cells in the neurogenic niche of the vagal centre (area postrema) by in vitro neurosphere assay and RT-qPCR of specific markers on ex-vivo adult tissue explants, comparatively with the canonical neurogenic niche: the subventricular zone (SVZ) of the forebrain. Explants defined from the neuroanatomical patterns of in vivo BrdU incorporation yielded expandable and self-renewing spheres from both SVZ and AP. Within SVZ though, the density of sphere-forming cells was higher in ventral SVZ (SVZ-V) than in its latero-dorsal (SVZ-D) and lateral (SVZ-L) regions, which differs from the distributions of neural stem cells in mouse and swine brains. Consistently, RT-qPCR of the biomarker of neural stem cells, Sox2, yields highest expression in SVZ-V ahead of SVZ-D, SVZ-L and AP. These results are discussed with regard to previously published dynamics of adult ovine neurogenesis in vivo, and in light of corresponding features in other mammalian species. This confirms existence of neurogenetic plasticity in the vagal complex of adult mammals.

Oxidative stress associated with spatial memory impairment and social olfactory deterioration in female mice reveals premature aging aroused by perinatal protein malnutrition

Early-life adversity, like perinatal protein malnutrition, increases the vulnerability to develop long-term alterations in brain structures and function. This study aimed to determine whether perinatal protein malnutrition predisposes to premature aging in a murine model and to assess the cellular and molecular mechanisms involved. To this end, mouse dams were fed either with a normal (NP, casein 20%) or a low-protein diet (LP, casein 8%) during gestation and lactation. Female offspring were evaluated at 2, 7 and 12 months of age. Positron emission tomography analysis showed alterations in the hippocampal CA3 region and the accessory olfactory bulb of LP mice during aging. Protein malnutrition impaired spatial memory, coinciding with higher levels of reactive oxygen species in the hippocampus and sirt7 upregulation. Protein malnutrition also led to higher senescence-associated β-galactosidase activity and p21 expression. LP-12-month-old mice showed a higher number of newborn neurons that did not complete the maturation process. The social-odor discrimination in LP mice was impaired along life. In the olfactory bulb of LP mice, the senescence marker p21 was upregulated, coinciding with a downregulation of Sirt2 and Sirt7. Also, LP-12-month-old mice showed a downregulation of catalase and glutathione peroxidase, and LP-2-month-old mice showed a higher number of newborn neurons in the subventricular zone, which then returned to normal values. Our results show that perinatal protein malnutrition causes long-term impairment in cognitive and olfactory skills through an accelerated senescence phenotype accompanied by an increase in oxidative stress and altered sirtuin expression in the hippocampus and olfactory bulb.

Rewarded Maze Training Increases Approach Behavior in Rats Through Neurogenesis-Dependent Growth of Ventral Hippocampus-Prelimbic Circuits

Background

Learning complex navigation routes increases hippocampal volume in humans, but it is not clear whether this growth impacts behaviors outside the learning situation or what cellular mechanisms are involved.

Methods

We trained rats with pharmacogenetic suppression of adult neurogenesis and littermate controls in 3 mazes over 3 weeks and tested novelty approach behavior several days after maze exposure. We then measured hippocampus and prelimbic cortex volumes using magnetic resonance imaging and assessed neuronal and astrocyte morphology. Finally, we investigated the activation and behavioral role of the ventral CA1 (vCA1)-to-prelimbic pathway using immediate-early genes and DREADDs (designer receptors exclusively activated by designer drugs).

Results

Maze training led to volume increase of both the vCA1 region of the hippocampus and the prelimbic region of the neocortex compared with rats that followed fixed paths. Growth was also apparent in individual neurons and astrocytes in these 2 regions, and behavioral testing showed increased novelty approach in maze-trained rats in 2 different tests. Suppressing adult neurogenesis prevented the effects on structure and approach behavior after maze training without affecting maze learning itself. The vCA1 neurons projecting to the prelimbic area were more activated by novelty in maze-trained animals, and suppression of this pathway decreased approach behavior.

Conclusions

Rewarded navigational learning experiences induce volumetric and morphologic growth in the vCA1 and prelimbic cortex and enhance activation of the circuit connecting these 2 regions. Both the structural and behavioral effects of maze training require ongoing adult neurogenesis, suggesting a role for new neurons in experience-driven increases in novelty exploration.

Fine-tuned rest: unveiling the regulatory landscape of adult quiescent neural stem cells

Cell quiescence is an essential mechanism that allows cells to temporarily halt proliferation while preserving the potential to resume it at a later time. The molecular mechanisms underlying cell quiescence are complex and involve the regulation of various signaling pathways, transcription factors and epigenetic modifications. The importance of unveiling the mechanisms regulating the quiescent state is undeniable, as its long-term maintenance is key to sustain tissue homeostasis throughout life. Neural stem cells (NSCs) are maintained in the subependymal zone (SEZ) niche of adult mammalian brains mostly as long-lasting quiescent cells, owing to multiple intrinsic and extrinsic cues that actively regulate this state. Differently from other non-proliferative states, quiescence is a reversible and tightly regulated condition that can re-activate to support the formation of new neurons throughout adult lifespan. Decoding its regulatory mechanisms in homeostasis and unveiling how it is modulated in the context of the aged brain or during tumorigenesis, could bring us closer to the development of new potential strategies to intervene in adult neurogenesis with therapeutic purposes. Starting with a general conceptualization of the quiescent state in different stem cell niches, we here review what we have learned about NSC quiescence in the SEZ, encompassing the experimental strategies used for its study, to end up discussing the modulation of quiescence in the context of a physiology or pathological NSC dysregulation.

Repetitive transcranial magnetic stimulation and fluoxetine reverse depressive-like behavior but with differential effects on Olig2-positive cells in chronically stressed mice

Depression is a mood disorder coursing with several behavioral, cellular, and neurochemical alterations. The negative impact of chronic stress may precipitate this neuropsychiatric disorder. Interestingly, downregulation of oligodendrocyte-related genes, abnormal myelin structure, and reduced numbers and density of oligodendrocytes in the limbic system have been identified in patients diagnosed with depression, but also in rodents exposed to chronic mild stress (CMS). Several reports have emphasized the importance of pharmacological or stimulation-related strategies in influencing oligodendrocytes in the hippocampal neurogenic niche. Repetitive transcranial magnetic stimulation (rTMS) has gained attention as an intervention to revert depression. Here, we hypothesized that 5 Hz (Hz) of rTMS or Fluoxetine (Flx) would revert depressive-like behaviors by influencing oligodendrocytes and revert neurogenic alterations caused by CMS in female Swiss Webster mice. Our results showed that 5 Hz rTMS or Flx revert depressive-like behavior. Only rTMS influenced oligodendrocytes by increasing the number of Olig2-positive cells in the hilus of the dentate gyrus and the prefrontal cortex. However, both strategies exerted effects on some events of the hippocampal neurogenic processes, such as cell proliferation (Ki67-positive cells), survival (CldU-positive cells), and intermediate stages (doublecortin-positive cells) along the dorsal-ventral axis of this region. Interestingly, the combination of rTMS-Flx exerted antidepressant-like effects, but the increased number of Olig2-positive cells observed in mice treated only with rTMS was canceled. However, rTMS-Flx exerted a synergistic effect by increasing the number of Ki67-positive cells. It also increased the number of CldU- and doublecortin-positive cells in the dentate gyrus. Our results demonstrate that 5 Hz rTMS has beneficial effects, as it reverted depressive-like behavior by increasing the number of Olig2-positive cells and reverting the decrement in hippocampal neurogenesis in CMS-exposed mice. Nevertheless, the effects of rTMS on other glial cells require further investigation.

Eucommiae cortex polysaccharides attenuate gut microbiota dysbiosis and neuroinflammation in mice exposed to chronic unpredictable mild stress: Beneficial in ameliorating depressive-like behaviors

BACKGROUND

Chronic stress alters gut microbiota composition, as well as induces inflammatory responses and behavioral deficits. Eucommiae cortex polysaccharides (EPs) have been reported to remodel gut microbiota and ameliorate obesogenic diet-induced systemic low-grade inflammation, but their role in stress-induced behavioral and physiological changes is poorly understood.

METHODS

Male Institute of Cancer Research (ICR) mice were exposed to chronic unpredictable stress (CUMS) for 4 weeks and then supplemented with EPs at a dose of 400 mg/kg once per day for 2 weeks. Behavioral test-specific antidepressant and anxiolytic effects of EPs were assessed in FST, TST, EPM, and OFT. Microbiota composition and inflammation were detected using 16S ribosomal RNA (rRNA) gene sequencing, quantitative RT-PCR, western blot, and immunofluorescence.

RESULTS

We found that EPs ameliorated gut dysbiosis caused by CUMS, as evidenced by increasing the abundance of Lactobacillaceae and suppressing the expansion of the Proteobacteria, thereby mitigating intestinal inflammation and barrier derangement. Importantly, EPs reduced the release of bacterial-derived lipopolysaccharides (LPS, endotoxin) and inhibited the microglia-mediated TLR4/NFκB/MAPK signaling pathway, thereby attenuating the pro-inflammatory response in the hippocampus. These contributed to restoring the rhythm of hippocampal neurogenesis and alleviating behavioral abnormalities in CUMS mice. Correlation analysis showed that the perturbed-gut microbiota was strongly correlated with behavioral abnormalities and neuroinflammation.

LIMITATIONS

This study did not clarify the causal relationship between EPs remodeling the gut microbiota and improved behavior in CUMS mice.

CONCLUSIONS

EPs exert ameliorative effects on CUMS-induced neuroinflammation and depression-like symptoms, which may be strongly related to their beneficial effects on gut microbial composition.

The ventricular-subventricular, subgranular and subcallosal zones: three niches of neural stem cells in the postnatal brain

In the postnatal brain, three regions show high mitotic activity. These brain areas are neurogenic niches, and each niche harbors a microenvironment favorable for the proliferation and differentiation of neural stem cells. These multipotential cells maintain the capacity to self-renew and generate intermediate precursors that will differentiate into neuronal and glial lineages (astrocytes and oligodendrocytes). The most well-studied niches are the ventricular-subventricular zone (V-SVZ) of the lateral ventricles, the subgranular zone (SGZ) of the dentate gyrus in the hippocampus, and the subcallosal zone (SCZ), located in the limit between the corpus callosum and the hippocampal formation. The discovery of these three neurogenic niches has gained much interest in the field because they may be a therapeutic alternative in neural regeneration and neurodegenerative disorders. In this review, we describe in brief all these regions and explain their potential impact on solving some neurological conditions.

Endogenous but not sensory-driven activity controls migration, morphogenesis and survival of adult-born juxtaglomerular neurons in the mouse olfactory bulb

The development and survival of adult-born neurons are believed to be driven by sensory signaling. Here, in vivo analyses of motility, morphology and Ca²⁺ signaling, as well as transcriptome analyses of adult-born juxtaglomerular cells with reduced endogenous excitability (via cell-specific overexpression of either Kv1.2 or Kir2.1 K⁺ channels), revealed a pronounced impairment of migration, morphogenesis, survival, and functional integration of these cells into the mouse olfactory bulb, accompanied by a reduction in cytosolic Ca²⁺ fluctuations, phosphorylation of CREB and pCREB-mediated gene expression. Moreover, K⁺ channel overexpression strongly downregulated genes involved in neuronal migration, differentiation, and morphogenesis and upregulated apoptosis-related genes, thus locking adult-born cells in an immature and vulnerable state. Surprisingly, cells deprived of sensory-driven activity developed normally. Together, the data reveal signaling pathways connecting the endogenous intermittent neuronal activity/Ca²⁺ fluctuations as well as enhanced Kv1.2/Kir2.1 K⁺ channel function to migration, maturation, and survival of adult-born neurons.

Cortical projection to the subventricular zone and its effect on adult neurogenesis in mice

Various brain regions/nuclei project axons to the subventricular zone (SVZ), a postnatal neurogenic niche. In adults, neurogenesis is controlled by neuronal activity, via neurotransmitters. Glutamate is a major excitatory neurotransmitter, and glutamate receptors are expressed in SVZ cells. Although the cerebral cortex is a major source of glutamate and the medial cortex projects axons to the medial striatum next to the SVZ, it remains unclear whether cortical neurons regulate adult neurogenesis in vivo. First, to analyze axonal projection, plasmid vector expressing DsRed was introduced to the medial cortex by in utero electroporation. At the adult stage, DsRed-labeled axons were detected in the dorsolateral, striatal, and septal areas of the SVZ, and where they were in contact with neuroblasts. Furthermore, maturation of the cortical projection and the SVZ appeared to synchronize during postnatal stages. Next, stab injuries were made in the bilateral medial cortex to interrupt cortical input to the SVZ. At 17 days post-injury, cell proliferation in the SVZ and tangential migration of neuroblasts to the olfactory bulb were not significantly affected. There were clusters of neuroblasts in the striatum close to the SVZ in all experimental groups, but the number and size of neuroblast clusters were significantly larger in the medial cortex-injured group compared with the other experimental groups. These neuroblast clusters had a morphology of tangentially migrating cells to the olfactory bulb. These results suggest that cortical input to the SVZ inhibits the radial migration of neuroblasts to converge with the migration pathway in vivo.

L-arginine homeostasis governs adult neural stem cell activation by modulating energy metabolism in vivo

Neurogenesis in the developing and adult brain is intimately linked to remodeling of cellular metabolism. However, it is still unclear how distinct metabolic programs and energy sources govern neural stem cell (NSC) behavior and subsequent neuronal differentiation. Here, we found that adult mice lacking the mitochondrial urea metabolism enzyme, Arginase-II (Arg-II), exhibited NSC overactivation, thereby leading to accelerated NSC pool depletion and decreased hippocampal neurogenesis over time. Mechanistically, Arg-II deficiency resulted in elevated L-arginine levels and induction of a metabolic shift from glycolysis to oxidative phosphorylation (OXPHOS) caused by impaired attachment of hexokinase-I to mitochondria. Notably, selective inhibition of OXPHOS ameliorated NSC overactivation and restored abnormal neurogenesis in Arg-II deficient mice. Therefore, Arg-II-mediated intracellular L-arginine homeostasis directly influences the metabolic fitness of neural stem cells that is essential to maintain neurogenesis with age.

Doublecortin-Expressing Neurons in Human Cerebral Cortex Layer II and Amygdala from Infancy to 100 Years Old

A cohort of morphologically heterogenous doublecortin immunoreactive (DCX +) "immature neurons" has been identified in the cerebral cortex largely around layer II and the amygdala largely in the paralaminar nucleus (PLN) among various mammals. To gain a wide spatiotemporal view on these neurons in humans, we examined layer II and amygdalar DCX + neurons in the brains of infants to 100-year-old individuals. Layer II DCX + neurons occurred throughout the cerebrum in the infants/toddlers, mainly in the temporal lobe in the adolescents and adults, and only in the temporal cortex surrounding the amygdala in the elderly. Amygdalar DCX + neurons occurred in all age groups, localized primarily to the PLN, and reduced in number with age. The small-sized DCX + neurons were unipolar or bipolar, and formed migratory chains extending tangentially, obliquely, and inwardly in layers I-III in the cortex, and from the PLN to other nuclei in the amygdala. Morphologically mature-looking neurons had a relatively larger soma and weaker DCX reactivity. In contrast to the above, DCX + neurons in the hippocampal dentate gyrus were only detected in the infant cases in parallelly processed cerebral sections. The present study reveals a broader regional distribution of the cortical layer II DCX + neurons than previously documented in human cerebrum, especially during childhood and adolescence, while both layer II and amygdalar DCX + neurons persist in the temporal lobe lifelong. Layer II and amygdalar DCX + neurons may serve as an essential immature neuronal system to support functional network plasticity in human cerebrum in an age/region-dependent manner.

Repeated administration of acrylamide for 28 days suppresses adult neurogenesis of the olfactory bulb in young-adult rats

Acrylamide (AA) is a neurotoxicant that inhibits synaptic function in distal axons. We previously found that AA decreased neural cell lineages during late-stage differentiation of adult hippocampal neurogenesis and downregulated genes related to neurotrophic factor, neuronal migration, neurite outgrowth, and synapse formation in the hippocampal dentate gyrus in rats. To investigate whether olfactory bulb (OB)-subventricular zone (SVZ) neurogenesis is similarly affected by AA exposure, AA was administered to 7-week-old male rats via oral gavage at doses of 0, 5, 10, and 20 mg/kg for 28 days. Immunohistochemical analysis revealed that AA decreased the numbers of doublecortin-positive ⁽⁺⁾ cells and polysialic acid-neural cell adhesion molecule⁺ cells in the OB. On the other hand, the numbers of doublecortin⁺ cells and polysialic acid-neural cell adhesion molecule⁺ cells in the SVZ did not change with AA exposure, suggesting that AA impaired neuroblasts migrating in the rostral migratory stream and OB. Gene expression analysis in the OB revealed that AA downregulated Bdnf and Ncam2, which are related to neuronal differentiation and migration. These results suggest that AA decreased neuroblasts in the OB by suppressing neuronal migration. Thus, AA decreased neuronal cell lineages during late-stage differentiation of adult neurogenesis in the OB-SVZ, similar to the effect on adult hippocampal neurogenesis.

Adult-born neurons inhibit developmentally-born neurons during spatial learning

Ongoing neurogenesis in the dentate gyrus (DG) subregion of the hippocampus results in a heterogenous population of neurons. Immature adult-born neurons (ABNs) have physiological and anatomical properties that may give them a unique role in learning. For example, compared to older granule neurons, they have greater somatic excitability, which could facilitate their recruitment into memory traces. However, recruitment is also likely to depend on interactions with other DG neurons through processes such as lateral inhibition. Immature ABNs target inhibitory interneurons and, compared to older neurons, they receive less GABAergic inhibition. Thus, they may induce lateral inhibition of mature DG neurons while being less susceptible to inhibition themselves. To test this we used a chemogenetic approach to silence immature ABNs as rats learned a spatial water maze task, and measured activity (Fos expression) in ABNs and developmentally-born neurons (DBNs). A retrovirus expressing the inhibitory DREADD receptor, hM4Di, was injected into the dorsal DG of male rats at 6w to infect neurons born in adulthood. Animals were also injected with BrdU to label DBNs or ABNs. DBNs were significantly more active than immature 4-week-old ABNs. Silencing 4-week-old ABNs did not alter learning but it increased activity in DBNs. However, silencing ABNs did not affect activation in other ABNs within the DG. Silencing ABNs also did not alter Fos expression in parvalbumin- and somatostatin-expressing interneurons. Collectively, these results suggest that ABNs may directly inhibit DBN activity during hippocampal-dependent learning, which may be relevant for maintaining sparse hippocampal representations of experienced events.

Fluoxetine increased adult neurogenesis is mediated by 5-HT3 receptor

Adult neurogenesis is an aspect of structural plasticity that remains active during adulthood in some brain regions. One of them is the subgranular zone (SGZ) of the dentate gyrus of the hippocampus. Adult neurogenesis is reduced by different factors and in disorders of the CNS, including major depression. Antidepressant treatments, such as chronic fluoxetine administration, recover the normal level of adult neurogenesis. Fluoxetine treatment increases the free concentration of the neurotransmitter serotonin and this monoamine is implicated in the regulation of the neurogenic process; however, the target of the action of this neurotransmitter has not been fully elucidated. In this study, we have tried to determine the relevance of the serotonin receptor 3 (5-HT3) in the hippocampal neurogenesis of adult rats. We have used fluorescent immunohistochemistry to study the expression of the 5-HT3 receptor in different neurogenesis stages in the SGZ, identifying its expression in stem cells, amplifying neural progenitors and immature neurons. Moreover, we have studied the impact of a 5-HT3 antagonist (ondansetron) in the fluoxetine-induced adult neurogenesis. We observed that fluoxetine alone increases the number of both proliferating cells (ki67 positive) and immature neurons (DCX positive) in the SGZ. By contrast, co-treatment with ondansetron blocked the increase in proliferation and neurogenesis. This study demonstrates that the activation of 5-HT3 receptors is necessary for the increase of adult neurogenesis induced by fluoxetine.

Lrig1 expression identifies quiescent stem cells in the ventricular-subventricular zone from postnatal development to adulthood and limits their persistent hyperproliferation

BACKGROUND

We previously identified Leucine-rich repeats and immunoglobulin-like domains 1 (Lrig1) as a marker of long-term neurogenic stem cells in the lateral wall of the adult mouse brain. The morphology of the stem cells thus identified differed from the canonical B1 type stem cells, raising a question about their cellular origin. Thus, we investigated the development of these stem cells in the postnatal and juvenile brain. Furthermore, because Lrig1 is a known regulator of quiescence, we also investigated the effect(s) of its deletion on the cellular proliferation in the lateral wall.

METHODS

To observe the development of the Lrig1-lineage stem cells, genetic inducible fate mapping studies in combination with thymidine analog administration were conducted using a previously published Lrig1^T2A-iCreERT2 mouse line. To identify the long-term consequence(s) of Lrig1 germline deletion, old Lrig1 knock-out mice were generated using two different Lrig1 null alleles in the C57BL/6J background. The lateral walls from these mice were analyzed using an optimized whole mount immunofluorescence protocol and confocal microscopy.

RESULTS

We observed the Lrig1-lineage labeled cells with morphologies consistent with neurogenic stem cell identity in postnatal, juvenile, and adult mouse brains. Interestingly, when induced at postnatal or juvenile ages, morphologically distinct cells were revealed, including cells with the canonical B1 type stem cell morphology. Almost all of the presumptive stem cells labeled were non-proliferative at these ages. In the old Lrig1 germline knock-out mice, increased proliferation was observed compared to wildtype littermates without concomitant increase in apoptosis.

CONCLUSIONS

Once set aside during embryogenesis, the Lrig1-lineage stem cells remain largely quiescent during postnatal and juvenile development until activation in adult age. The absence of premature proliferative exhaustion in the Lrig1 knock-out stem cell niche during aging is likely due to a complex cascade of effects on the adult stem cell pool. Thus, we suggest that the adult stem cell pool size may be genetically constrained via Lrig1.

Resveratrol Mediated Regulation of Hippocampal Neuroregenerative Plasticity via SIRT1 Pathway in Synergy with Wnt Signaling: Neurotherapeutic Implications to Mitigate Memory Loss in Alzheimer's Disease

Alzheimer's disease (AD) is a major form of dementia. Abnormal amyloidogenic event-mediated degeneration of cholinergic neurons in the cognitive centers of the brain has been attributed to neuropathological sequelae and behavioral deficits in AD. Besides, impaired adult neurogenesis in the hippocampus has experimentally been realized as an underlying cause of dementia regardless of neurodegeneration. Therefore, nourishing the neurogenic process in the hippocampus has been considered an effective therapeutic strategy to mitigate memory loss. In the physiological state, the Wnt pathway has been identified as a potent mitogenic generator in the hippocampal stem cell niche. However, downstream components of Wnt signaling have been noticed to be downregulated in AD brains. Resveratrol (RSV) is a potent Sirtuin1 (SIRT1) enhancer that facilitates neuroprotection and promotes neurogenesis in the hippocampus of the adult brain. While SIRT1 is an important positive regulator of Wnt signaling, ample reports indicate that RSV treatment strongly mediates the fate determination of stem cells through Wnt signaling. However, the possible therapeutic roles of RSV-mediated SIRT1 enhancement on the regulation of hippocampal neurogenesis and reversal of memory loss through the Wnt signaling pathway have not been addressed yet. Taken together, this review describes RSV-mediated effects on the regulation of hippocampal neurogenesis via the activation of SIRT1 in synergy with the Wnt signaling. Further, the article emphasizes a hypothesis that RSV treatment can provoke the activation of quiescent neural stem cells and prime their neurogenic capacity in the hippocampus via Wnt signaling in AD.

EGFL7 loss correlates with increased VEGF-D expression, upregulating hippocampal adult neurogenesis and improving spatial learning and memory

Neural stem cells reside in the subgranular zone, a specialized neurogenic niche of the hippocampus. Throughout adulthood, these cells give rise to neurons in the dentate gyrus, playing an important role in learning and memory. Given that these core cognitive processes are disrupted in numerous disease states, understanding the underlying mechanisms of neural stem cell proliferation in the subgranular zone is of direct practical interest. Here, we report that mature neurons, neural stem cells and neural precursor cells each secrete the neurovascular protein epidermal growth factor-like protein 7 (EGFL7) to shape this hippocampal niche. We further demonstrate that EGFL7 knock-out in a Nestin-CreERT2-based mouse model produces a pronounced upregulation of neurogenesis within the subgranular zone. RNA sequencing identified that the increased expression of the cytokine VEGF-D correlates significantly with the ablation of EGFL7. We substantiate this finding with intraventricular infusion of VEGF-D upregulating neurogenesis in vivo and further show that VEGF-D knock-out produces a downregulation of neurogenesis. Finally, behavioral studies in EGFL7 knock-out mice demonstrate greater maintenance of spatial memory and improved memory consolidation in the hippocampus by modulation of pattern separation. Taken together, our findings demonstrate that both EGFL7 and VEGF-D affect neurogenesis in the adult hippocampus, with the ablation of EGFL7 upregulating neurogenesis, increasing spatial learning and memory, and correlating with increased VEGF-D expression.

Impaired hippocampal neurogenesis and cognitive performance in adult DBC1-knock out mice

The protein DBC1 is the main SIRT1 regulator known so far, and by doing so, it is involved in the regulation of energy metabolism, especially in liver and fat adipose tissue. DBC1 also has an important function in cell cycle progression and regulation in cancer cells, affecting tumorigenesis. We recently showed that during quiescence, non-transformed cells need DBC1 in order to re-enter and progress through the cell cycle. Moreover, we showed that deletion of DBC1 affects cell cycle progression during liver regeneration. This novel concept prompted us to evaluate the role of DBC1 during adult neurogenesis, where transition from quiescence to proliferation in neuronal progenitors is key and tightly regulated. Herein, we analyzed several markers of cell cycle expressed in the dentate gyrus of the hippocampus of controls and DBC1 KO adult mice. Our results suggest a reduced number of neuroblasts therein present, probably due to a decline of neuroblast generation or an impairment in neural differentiation. In agreement with this, we also found that adult DBC1 KO mice had a reduction in the volume of the granule cell layer of the dentate gyrus. Interestingly, behavioral analysis of KO and control mice revealed that deletion of DBC1 parallels to specific cognitive impairments, concerning learning and possibly memory formation. Our results show, for the first time, that DBC1 plays an active role in the nervous system. In particular, specific anatomical and behavioral changes are observed when is absent.

Age- and sex-dependent effects of DNA glycosylase Neil3 on amyloid pathology, adult neurogenesis, and memory in a mouse model of Alzheimer's disease

Oxidative stress generating DNA damage has been shown to be a key characteristic in Alzheimer's disease (AD). However, how it affects the pathogenesis of AD is not yet fully understood. Neil3 is a DNA glycosylase initiating repair of oxidative DNA base lesions and with a distinct expression pattern in proliferating cells. In brain, its function has been linked to hippocampal-dependent memory and to induction of neurogenesis after stroke and in prion disease. Here, we generated a novel AD mouse model deficient for Neil3 to study the impact of impaired oxidative base lesion repair on the pathogenesis of AD. Our results demonstrate an age-dependent decrease in amyloid-β (Aβ) plaque deposition in female Neil3-deficient AD mice, whereas no significant difference was observed in male mice. Furthermore, male but not female Neil3-deficient AD mice show reduced neural stem cell proliferation in the adult hippocampus and impaired working memory compared to controls. These effects seem to be independent of DNA repair as both sexes show increased level of oxidative base lesions in the hippocampus upon loss of Neil3. Thus, our findings suggest an age- and sex-dependent role of Neil3 in the progression of AD by altering cerebral Aβ accumulation and promoting adult hippocampal neurogenesis to maintain cognitive function.

Brain-derived neurotrophic factor expression in serotonergic neurons improves stress resilience and promotes adult hippocampal neurogenesis

The neurotrophin brain-derived neurotrophic factor (BDNF) stimulates adult neurogenesis, but also influences structural plasticity and function of serotonergic neurons. Both, BDNF/TrkB signaling and the serotonergic system modulate behavioral responses to stress and can lead to pathological states when dysregulated. The two systems have been shown to mediate the therapeutic effect of antidepressant drugs and to regulate hippocampal neurogenesis. To elucidate the interplay of both systems at cellular and behavioral levels, we generated a transgenic mouse line that overexpresses BDNF in serotonergic neurons in an inducible manner. Besides displaying enhanced hippocampus-dependent contextual learning, transgenic mice were less affected by chronic social defeat stress (CSDS) compared to wild-type animals. In parallel, we observed enhanced serotonergic axonal sprouting in the dentate gyrus and increased neural stem/progenitor cell proliferation, which was uniformly distributed along the dorsoventral axis of the hippocampus. In the forced swim test, BDNF-overexpressing mice behaved similarly as wild-type mice treated with the antidepressant fluoxetine. Our data suggest that BDNF released from serotonergic projections exerts this effect partly by enhancing adult neurogenesis. Furthermore, independently of the genotype, enhanced neurogenesis positively correlated with the social interaction time after the CSDS, a measure for stress resilience.

5 Hz of repetitive transcranial magnetic stimulation improves cognition and induces modifications in hippocampal neurogenesis in adult female Swiss Webster mice

Adult hippocampal neurogenesis is regulated by several stimuli to promote the creation of a reserve that may facilitate coping with environmental challenges. In this regard, repetitive transcranial magnetic stimulation (rTMS), a neuromodulation therapy, came to our attention because in clinical studies it reverts behavioral and cognitive alterations related to changes in brain plasticity. Some preclinical studies emphasize the need to understand the underlying mechanism of rTMS to induce behavioral modifications. In this study, we investigated the effects of rTMS on cognition, neurogenic-associated modifications, and neuronal activation in the hippocampus of female Swiss Webster mice. We applied 5 Hz of rTMS twice a day for 14 days. Three days later, mice were exposed to the behavioral battery. Then, brains were collected and immunostained for Ki67-positive cells, doublecortin-positive (DCX+)-cells, calbindin, c-Fos and FosB/Delta-FosB in the dentate gyrus. Also, we analyzed mossy fibers and CA3 with calbindin immunostaining. Mice exposed to rTMS exhibited cognitive improvement, an increased number of proliferative cells, DCX cells, DCX cells with complex dendrite morphology, c-Fos and immunoreactivity of FosB/Delta-FosB in the granular cell layer. The volume of the granular cell layer, mossy fibers and CA3 in rTMS mice also increased. Interestingly, cognitive improvement correlated with DCX cells with complex dendrite morphology. Also, those DCX cells and calbindin immunoreactivity correlated with c-Fos in the granular cell layer. Our results suggest that 5 Hz of rTMS applied twice a day modify cell proliferation, doublecortin cells, mossy fibers and enhance cognitive behavior in healthy female Swiss Webster mice.

ERRγ Ligand Regulates Adult Neurogenesis and Depression-like Behavior in a LRRK2-G2019S-associated Young Female Mouse Model of Parkinson's Disease

Adult neurogenesis, a process controlling the proliferation to maturation of newly generated neurons in the post-developmental brain, is associated with various brain functions and pathogenesis of neuropsychological diseases, such as Parkinson's disease (PD) and depression. Because orphan nuclear receptor estrogen-related receptor γ (ERRγ) plays a role in the differentiation of neuronal cells, we investigated whether an ERRγ ligand enhances adult neurogenesis and regulates depressive behavior in a LRRK2-G2019S-associated mouse model of PD. Young female LRRK2-G2019S mice (7-9 weeks old) showed depression-like behavior without dopaminergic neuronal loss in the nigrostriatal pathway nor motor dysfunction. A significant decrease in adult hippocampal neurogenesis was detected in young female LRRK2-G2019S mice, but not in comparable male mice. A synthetic ERRγ ligand, (E)-4-hydroxy-N'-(4-(phenylethynyl)benzylidene)benzohydrazide (HPB2), ameliorated depression-like behavior in young female LRRK2-G2019S mice and enhanced neurogenesis in the hippocampus, as evidenced by increases in the number of bromodeoxyuridine/neuronal nuclei-positive cells and in the intensity and number of doublecortin-positive cells in the hippocampal dentate gyrus (DG). Moreover, HPB2 significantly increased the number of spines and the number and length of dendrites in the DG of young female LRRK2-G2019S mice. Furthermore, HPB2 upregulated brain-derived neurotrophic factor (BDNF)/tropomyosin receptor kinase B (TrkB) signaling, one of the important factors regulating neurogenesis, as well as phosphorylated cAMP-response element binding protein-positive cells in the DG of young female LRRK2-G2019S mice. Together, these results suggest ERRγ as a novel therapeutic target for PD-associated depression by modulating adult neurogenesis and BDNF/TrkB signaling.

Environmental enrichment: dissociated effects between physical activity and changing environmental complexity on anxiety and neurogenesis in adult male Balb/C mice

Several factors, including environmental modifications, stimulate neuroplasticity. One type of neuroplasticity consists in the generation of new neurons in the dentate gyrus of the hippocampus. Neurogenesis is modulated by environmental enrichment (ENR, tunnels plus running wheel) and affected by the time of exposure to ENR. Despite the wide use of ENR to stimulate neuroplasticity, the degree to which ENR variations modeled by temporally changing the level of environmental complexity affect hippocampal neurogenesis and anxiety is still unclear. Thus, we investigated the effects of five housing conditions on young adult male Balb/C mice exposed for 42 days. The groups were as follows: standard conditions without ENR, constant ENR complexity, gradual increase of ENR complexity followed by a gradual decrease of ENR complexity, gradual increase of ENR complexity followed by constant ENR complexity, and constant ENR complexity followed by a gradual decrease of ENR complexity. On day 44, mice were exposed to the elevated plus-maze to evaluate anxiety. Further, we analyzed neurogenesis and quantified corticosterone levels. In an additional experiment, we explored the effect of voluntary physical activity on anxiety, neurogenesis, and corticosterone during the variations in ENR complexity. Our results showed that any change in ENR complexity over time reduced anxiety. Also, voluntary physical activity alone or in the context of a complex environment increased doublecortin cell maturation in the granular cell layer of the hippocampus. Finally, our study supports that physical activity acts proneurogenic, whereas any change in environmental complexity decreases anxiety-like behavior. However, the decrease in corticosterone levels elicited by physical activity was lower than the decrease produced by the decrement in environmental complexity.

Sustained correction of hippocampal neurogenic and cognitive deficits after a brief treatment by Nutlin-3 in a mouse model of fragile X syndrome

BACKGROUND

Fragile X syndrome (FXS), the most prevalent inherited intellectual disability and one of the most common monogenic forms of autism, is caused by a loss of fragile X messenger ribonucleoprotein 1 (FMR1). We have previously shown that FMR1 represses the levels and activities of ubiquitin ligase MDM2 in young adult FMR1-deficient mice, and treatment by a MDM2 inhibitor Nutlin-3 rescues both hippocampal neurogenic and cognitive deficits in FMR1-deficient mice when analyzed shortly after the administration. However, it is unknown whether Nutlin-3 treatment can have long-lasting therapeutic effects.

METHODS

We treated 2-month-old young adult FMR1-deficient mice with Nutlin-3 for 10 days and then assessed the persistent effect of Nutlin-3 on both cognitive functions and adult neurogenesis when mice were 6-month-old mature adults. To investigate the mechanisms underlying the persistent effects of Nutlin-3, we analyzed the proliferation and differentiation of neural stem/progenitor cells isolated from these mice and assessed the transcriptome of the hippocampal tissues of treated mice.

RESULTS

We found that transient treatment with Nutlin-3 of 2-month-old young adult FMR1-deficient mice prevents the emergence of neurogenic and cognitive deficits in mature adult FXS mice at 6 months of age. We further found that the long-lasting restoration of neurogenesis and cognitive function might not be mediated by changing intrinsic properties of adult neural stem cells. Transcriptomic analysis of the hippocampal tissue demonstrated that transient Nultin-3 treatment leads to significant expression changes in genes related to the extracellular matrix, secreted factors, and cell membrane proteins in the FMR1-deficient hippocampus.

CONCLUSIONS

Our data indicates that transient Nutlin-3 treatment in young adults leads to long-lasting neurogenic and behavioral changes likely through modulating adult neurogenic niche that impact adult neural stem cells. Our results demonstrate that cognitive impairments in FXS may be prevented by an early intervention through Nutlin-3 treatment.

Breast carcinoma-amplified sequence 2 regulates adult neurogenesis via β-catenin

Background

Breast carcinoma-amplified sequence 2 (BCAS2) regulates β-catenin gene splicing. The conditional knockout of BCAS2 expression in the forebrain (BCAS2 cKO) of mice confers impaired learning and memory along with decreased β-catenin expression. Because β-catenin reportedly regulates adult neurogenesis, we wondered whether BCAS2 could regulate adult neurogenesis via β-catenin.

Methods

BCAS2-regulating neurogenesis was investigated by characterizing BCAS2 cKO mice. Also, lentivirus-shBCAS2 was intracranially injected into the hippocampus of wild-type mice to knock down BCAS2 expression. We evaluated the rescue effects of BCAS2 cKO by intracranial injection of adeno-associated virus encoding BCAS2 (AAV-DJ8-BCAS2) and AAV-β-catenin gene therapy.

Results

To show that BCAS2-regulating adult neurogenesis via β-catenin, first, BCAS2 cKO mice showed low SRY-box 2-positive (Sox2⁺) neural stem cell proliferation and doublecortin-positive (DCX⁺) immature neurons. Second, stereotaxic intracranial injection of lentivirus-shBCAS2 knocked down BCAS2 in the hippocampus of wild-type mice, and we confirmed the BCAS2 regulation of adult neurogenesis via β-catenin. Third, AAV-DJ8-BCAS2 gene therapy in BCAS2 cKO mice reversed the low proliferation of Sox2⁺ neural stem cells and the decreased number of DCX⁺ immature neurons with increased β-catenin expression. Moreover, AAV-β-catenin gene therapy restored neuron stem cell proliferation and immature neuron differentiation, which further supports BCAS2-regulating adult neurogenesis via β-catenin. In addition, cells targeted by AAV-DJ8 injection into the hippocampus included Sox2 and DCX immature neurons, interneurons, and astrocytes. BCAS2 may regulate adult neurogenesis by targeting Sox2⁺ and DCX⁺ immature neurons for autocrine effects and interneurons or astrocytes for paracrine effects.

Conclusions

BCAS2 can regulate adult neurogenesis in mice via β-catenin.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-02837-9.

Effects of Involuntary and Voluntary Exercise in Combination with Acousto-Optic Stimulation on Adult Neurogenesis in an Alzheimer's Mouse Model

Single-factor intervention, such as physical exercise and auditory and visual stimulation, plays a positive role on the prevention and treatment of Alzheimer's disease (AD); however, the therapeutic effects of single-factor intervention are limited. The beneficial effects of these multifactor combinations on AD and its molecular mechanism have yet to be elucidated. Here, we investigated the effect of multifactor intervention, voluntary wheel exercise, and involuntary treadmill running in combination with acousto-optic stimulation, on adult neurogenesis and behavioral phenotypes in a mouse model of AD. We found that 4 weeks of multifactor intervention can significantly increase the production of newborn cells (BrdU⁺ cells) and immature neurons (DCX⁺ cells) in the hippocampus and lateral ventricle of Aβ oligomer-induced mice. Importantly, the multifactor intervention could promote BrdU⁺ cells to differentiate into neurons (BrdU⁺ DCX⁺ cells or BrdU⁺ NeuN⁺ cells) and astrocytes (BrdU⁺GFAP⁺ cells) in the hippocampus and ameliorate Aβ oligomer-induced cognitive impairment and anxiety- and depression-like behaviors in mice evaluated by novel object recognition, Morris water maze tests, elevated zero maze, forced swimming test, and tail suspension test, respectively. Moreover, multifactor intervention could lead to an increase in the protein levels of PSD-95, SYP, DCX, NeuN, GFAP, Bcl-2, BDNF, TrkB, and pSer473-Akt and a decrease in the protein levels of BAX and caspase-9 in the hippocampal lysates of Aβ oligomer-induced mice. Furthermore, sequencing analysis of serum metabolites revealed that aberrantly expressed metabolites modulated by multifactor intervention were highly enriched in the biological process associated with keeping neurons functioning and neurobehavioral function. Additionally, the intervention-mediated serum metabolites mainly participated in glutamate metabolism, glucose metabolism, and the tricarboxylic acid cycle in mice. Our findings suggest the potential of multifactor intervention as a non-invasive therapeutic strategy for AD to anti-Aβ oligomer neurotoxicity.

Effects of exosomes on adult hippocampal neurogenesis and neuropsychiatric disorders

Exosomes are extracellular vesicles originating from the endosomal system, which are involved in intercellular substance transfer and cell waste elimination. Recent studies implicate the roles of exosomes in adult hippocampal neurogenesis, a process through which new granule cells are generated in the dentate gyrus, and which is closely related to mood and cognition, as well as psychiatric disorders. As such, exosomes are recognized as potential biomarkers of neurologic and psychiatric disorders. This review briefly introduces the synthesis and secretion mechanism of exosomes, and discuss the relationship between exosomes and hippocampal neurogenesis, and their roles in regulating depression, epilepsy and schizophrenia. Finally, we discuss the prospects of their application in diagnosing disorders of the central nervous system (CNS).

Olig2 defines a subset of neural stem cells that produce specific olfactory bulb interneuron subtypes in the subventricular zone of adult mice

Distinct neural stem cells (NSCs) reside in different regions of the subventricular zone (SVZ) and generate multiple olfactory bulb (OB) interneuron subtypes in the adult brain. However, the molecular mechanisms underlying such NSC heterogeneity remain largely unknown. Here, we show that the basic helix-loop-helix transcription factor Olig2 defines a subset of NSCs in the early postnatal and adult SVZ. Olig2-expressing NSCs exist broadly but are most enriched in the ventral SVZ along the dorsoventral axis complementary to dorsally enriched Gsx2-expressing NSCs. Comparisons of Olig2-expressing NSCs from early embryonic to adult stages using single cell transcriptomics reveal stepwise developmental changes in their cell cycle and metabolic properties. Genetic studies further show that cross-repression contributes to the mutually exclusive expression of Olig2 and Gsx2 in NSCs/progenitors during embryogenesis, but that their expression is regulated independently from each other in adult NSCs. Finally, lineage-tracing and conditional inactivation studies demonstrate that Olig2 plays an important role in the specification of OB interneuron subtypes. Altogether, our study demonstrates that Olig2 defines a unique subset of adult NSCs enriched in the ventral aspect of the adult SVZ.

Change of hypothalamic adult neurogenesis in mice by chronic treatment of fluoxetine

OBJECTIVE

More than half of patients with depression display eating disorders, such as bulimia nervosa and anorexia nervosa. Feeding centers are located in the hypothalamus, and hypothalamic adult neurogenesis has an important role in feeding and energy balance. Antidepressants, which can regulate adult neurogenesis in the hippocampus, olfactory bulb, and neocortex, are used for eating disorders, but it is unclear whether antidepressants change hypothalamic adult neurogenesis. In this study, we used immunohistological analysis to assess effects of the antidepressant fluoxetine (FLX) on hypothalamic adult neurogenesis of adult mice.

RESULTS

Expressions of the proliferating cell marker, Ki67, and the neural stem cell marker, nestin, were significantly decreased in the hypothalamus by FLX. As regard to postmitotic cells, the number of the neural marker, NeuN, positive cells was significantly upregulated by FLX, but that of the astrocytic marker, S100B, positive cells was significantly reduced by FLX. The number of the oligodendrocyte marker, Olig2, positive cells was not changed by FLX. Interestingly, FLX treatment did not affect the total number of newly generated cells in the hypothalamus, comparing that in controls. These results suggest that FLX treatment influence hypothalamic adult neurogenesis and shift the balance between the numbers of neurons and astrocytes under studied conditions.

Development of the mammalian main olfactory bulb

The mammalian main olfactory bulb is a crucial processing centre for the sense of smell. The olfactory bulb forms early during development and is functional from birth. However, the olfactory system continues to mature and change throughout life as a target of constitutive adult neurogenesis. Our Review synthesises current knowledge of prenatal, postnatal and adult olfactory bulb development, focusing on the maturation, morphology, functions and interactions of its diverse constituent glutamatergic and GABAergic cell types. We highlight not only the great advances in the understanding of olfactory bulb development made in recent years, but also the gaps in our present knowledge that most urgently require addressing.

Summary: This Review describes the morphological and functional maturation of cells in the mammalian main olfactory bulb, from embryonic development to adult neurogenesis.

Hypobaric Hypoxia Induces Deficits in Adult Neurogenesis and Social Interaction via Cyclooxygenase-1/ EP1 Receptor Pathway Activating NLRP3 Inflammasome

Low oxygen environments, like hypobaric hypoxia (HH), are common nodes in a number of diseases characterized by neuroinflammation, which is detrimental to the structural and functional aspects of hippocampal circuitry. Hypoxic conditions lead to elevation of inflammasome-mediated inflammation that may contribute to cognitive deficits. However, a systematic investigation of the impact of inflammasome-mediated neuroinflammation on the components of neurogenic niche during HH remains to be elusive. Cerebral hypoxia was induced in adult male Sprague Dawley rats via decreasing partial pressure of oxygen. The effect of HH (1, 3, and 7 days at 25,000 ft) on social memory, anxiety, adult neurogenesis, and NLRP3- (NLR family pyrin domain containing 3) mediated neuroinflammation in the dentate gyrus (DG) was explored in detail. Furthermore, we explored the therapeutic efficacy of cyclooxygenase-1 inhibitor (valeryl salicylate, 5 mg/kg/day, i.p.) and EP1 receptor (EP1R) antagonist (SC19220, 1 mg/kg/day, i.p.) on HH-induced deficits. Seven days of HH exposure induced alteration in social and anxiety-like behavior along with perturbation in adult neurogenesis. Elevation in NLRP3, caspase-1, and IL-1β levels was observed during HH from day 1. A notable increase in the COX-1/EP1R pathway in activated glial cells in DG was evident during HH. COX-1 inhibitor and EP1R antagonist mitigated the detrimental effects of HH on social memory, adult neurogenesis via blunting NLRP3-mediated inflammation. Our data showed induction of the COX-1/EP1R pathway in the glial cells, which is detrimental to neurogenesis and social memory, opening up the possibility that the COX-1/EP1R pathway is a plausible target for inflammasome-related neurogenesis impairments.

Oral administration of cyclic glycyl-proline facilitates task learning in a rat stroke model

Cyclic glycyl-proline (cGP) exerts neuroprotective effects against ischemic stroke and may promote neural plasticity or network remodeling. We sought to determine to what extent oral administration of cGP could facilitate task learning in rats with ischemic lesions. We trained rats to perform a choice reaction time task using their forepaws. One week after changing the food to pellets containing cGP (no cGP: 0 mg/kg; low cGP: 25 mg/kg; and high cGP: 75 mg/kg), we made a focal ischemic lesion on the left or right forepaw area of the sensorimotor cortex. After recovery of task performance, we altered the correct-response side of the task, and then analyzed the number of training days required for the rat to reach a learning criterion (error rate < 15%) and the regulation of adult neurogenesis in the subventricular zones (SVZs), taking lesion size into account. The low-cGP group required fewer training days for task learning than the no-cGP group. Unexpectedly, rats with larger lesions required fewer training days in the no-cGP and low-cGP groups, but more training days in the high-cGP group. The number of Ki67-immunopositive cells (indicating proliferative cells) in ipsilesional SVZ increased more rapidly in the low-cGP and high-cGP groups than in the no-cGP group. However, lesion size had only a small effect on required training days and the number of Ki67-immunopositive cells. We conclude that oral administration of cGP can facilitate task learning in rats with focal ischemic infarction through neural plasticity and network remodeling, even with minimal neuroprotective effects.

Effect of soluble amyloid precursor protein-alpha on adult hippocampal neurogenesis in a mouse model of Alzheimer's disease

Soluble amyloid precursor protein-alpha (sAPPα) is a regulator of neuronal and memory mechanisms, while also having neurogenic and neuroprotective effects in the brain. As adult hippocampal neurogenesis is impaired in Alzheimer’s disease, we tested the hypothesis that sAPPα delivery would rescue adult hippocampal neurogenesis in an APP/PS1 mouse model of Alzheimer’s disease. An adeno-associated virus-9 (AAV9) encoding murine sAPPα was injected into the hippocampus of 8-month-old wild-type and APP/PS1 mice, and later two different thymidine analogues (XdU) were systemically injected to label adult-born cells at different time points after viral transduction. The proliferation of adult-born cells, cell survival after eight weeks, and cell differentiation into either neurons or astrocytes was studied. Proliferation was impaired in APP/PS1 mice but was restored to wild-type levels by viral expression of sAPPα. In contrast, sAPPα overexpression failed to rescue the survival of XdU⁺-labelled cells that was impaired in APP/PS1 mice, although it did cause a significant increase in the area density of astrocytes in the granule cell layer across both genotypes. Finally, viral expression of sAPPα reduced amyloid-beta plaque load in APP/PS1 mice in the dentate gyrus and somatosensory cortex. These data add further evidence that increased levels of sAPPα could be therapeutic for the cognitive decline in AD, in part through restoration of the proliferation of neural progenitor cells in adults.