Optimizing memory performance and emotional states: multi-level enhancement of adult hippocampal neurogenesis

The possible role of neurogenesis activators in temporal lobe epilepsy: State of art and future perspective

Neurogenesis is a complex process by which the neurons and supporting cells of the central nervous system (CNS) are generated by neural stem cells. Adult hippocampal neurogenesis (AHN) in the human brain is an active process during life and plays a critical role in the regulation of memory, cognition, and mood. It has been shown that epilepsy is linked with dysregulation of AHN. Of note, AHN is very sensitive to the pathological electrical stimuli during epileptic seizures, which result in the induction of neurogenesis in acute epilepsy and inhibition of neurogenesis in chronic epilepsy. Epileptic seizure-induced neurodegeneration activates the mobilization of neural stem cells during neurogenesis to substitute for neural loss in temporal lobe epilepsy (TLE), which is the most refractory type of epilepsy. Moreover, recurrent epileptic seizures in TLE trigger neurogenesis in certain brain regions. However, AHN is a transient acute epileptic seizure that terminated with 1-4 weeks following status epilepticus (SE). Nevertheless, adult AHN is dramatically reduced in chronic epilepsy and associated with the development of cognitive impairment in TLE. These findings indicate that impairment of AHN is linked with the severity of epileptic seizures. Hence, neurogenesis activators may attenuate the pathogenesis of TLE. Therefore, this review aims to discuss and explain the beneficial role of AHN in TLE and how neurogenesis activators could be effective in the management of epilepsy.

Neural stem and progenitor cells support and protect adult hippocampal function via vascular endothelial growth factor secretion

Adult neural stem and progenitor cells (NSPCs) reside in the dentate gyrus (DG) of the hippocampus throughout the lifespan of most mammalian species. In addition to generating new neurons, NSPCs may alter their niche via secretion of growth factors and cytokines. We recently showed that adult DG NSPCs secrete vascular endothelial growth factor (VEGF), which is critical for maintaining adult neurogenesis. Here, we asked whether NSPC-derived VEGF alters hippocampal function independent of adult neurogenesis. We found that loss of NSPC-derived VEGF acutely impaired hippocampal memory, caused neuronal hyperexcitability and exacerbated excitotoxic injury. Conversely, we observed that overexpression of VEGF reduced microglial response to excitotoxic injury. We also found that NSPCs generate substantial proportions of total DG VEGF and VEGF disperses widely throughout the DG, both of which help explain how this anatomically-restricted cell population could modulate function broadly. These findings suggest that NSPCs actively support and protect DG function via secreted VEGF, thereby providing a non-neurogenic functional dimension to endogenous NSPCs.

Gastrodin enhances stress resilience through promoting Wnt/β-Catenin-dependent neurogenesis

BACKGROUND

Enhancing stress resilience constitutes a pivotal strategy in mitigating the risk of depression, making it a critical component of both prevention and treatment. In the current work, we identified a compound, gastrodin (GAS), as capable of enhancing stress resilience, as demonstrated by its ability to protect against depression following chronic stress exposure.

OBJECTIVES

To elucidate the potential of GAS to promote neurogenesis under chronic stress, along with the associated cellular and molecular processes involved.

METHOD

We evaluated the effect of GAS on NSPC proliferation and differentiation using both in vitro and in vivo investigations. Neurogenesis was inhibited using temozolomide to verify GAS's impact on stress resilience. Comprehensive methodologies, including hippocampal transcriptome analysis and western blotting, were utilized to identify the involvement of the Wnt/β-catenin pathway. Immunolocalization was conducted to confirm β-catenin's nuclear translocation in SOX2+ cells within the hippocampal dentate gyrus subgranular zone.

RESULTS

GAS demonstrated robust stimulation of NSPC proliferation and neuronal differentiation, enhancing adult hippocampal neurogenesis under conditions of chronic stress. Inhibition of neurogenesis negated GAS's protective effects on stress resilience. Integrated analysis pointed to the Wnt/β-catenin signaling pathway within NSPCs as a crucial mechanism facilitating GAS-promoted neurogenesis. Inhibiting Wnt expression or blocking β-catenin's nuclear translocation abolished GAS's neurogenic and stress-resilience enhancing effects.

CONCLUSION

These results suggested that GAS directly activates the Wnt/β-catenin signaling pathway, which promotes the proliferation and neuronal differentiation of NSPCs, thereby enhancing adult hippocampal neurogenesis and promoting stress resilience to mitigate the risk of depression.

Intracrine VEGF Signaling Is Required for Adult Hippocampal Neural Stem Cell Maintenance and Vascular Proximity

Adult neural stem cells (NSCs) in the mammalian dentate gyrus (DG) of the hippocampus rely on multiple signals for their preservation throughout the lifespan. While several studies have suggested that vascular endothelial growth factor (VEGF), in particular VEGF synthesized by NSCs themselves, is critical for NSC maintenance and adult neurogenesis, conflicting studies have left it uncertain how VEGF signals to NSCs. Here, we identified a VEGF-VEGFR2 intracrine signaling mechanism within adult DG NSCs that prevents NSC exhaustion and supports their proximity to local blood vessels. Using cell culture assays, we show that while intracellular VEGF stimulated receptor signaling cascades, extracellular VEGF did not. We found that this primary reliance on intracellular VEGF receptor signaling was most likely due to sheddase-mediated cleavage of extracellular VEGFR2 ligand binding domains, as phospho-signaling in response to extracellular VEGF could be restored using sheddase inhibitors. Using cultured adult DG NSCs and intact mice, we further show that NSC-VEGF loss caused cell-autonomous exhaustion of adult DG NSCs, along with impaired migration in cultured NSCs and reduced proximity of NSCs to local blood vessels in mouse DG. Our findings support an exclusively intracellular mechanism for VEGF signaling in adult DG NSCs, thereby providing resolution to previously conflicting studies and suggesting that cellular source can dictate the functional impact of soluble ligands in DG NSCs.

Comparative molecular landscapes of immature neurons in the mammalian dentate gyrus across species reveal special features in humans

Immature dentate granule cells (imGCs) arising from adult hippocampal neurogenesis contribute to plasticity, learning and memory, but their evolutionary changes across species and specialized features in humans remain poorly understood. Here we performed machine learning-augmented analysis of published single-cell RNA-sequencing datasets and identified macaque imGCs with transcriptome-wide immature neuronal characteristics. Our cross-species comparisons among humans, monkeys, pigs, and mice showed few shared (such as DPYSL5), but mostly species-specific gene expression in imGCs that converged onto common biological processes regulating neuronal development. We further identified human-specific transcriptomic features of imGCs and demonstrated functional roles of human imGC-enriched expression of a family of proton-transporting vacuolar-type ATPase subtypes in development of imGCs derived from human pluripotent stem cells. Our study reveals divergent gene expression patterns but convergent biological processes in the molecular characteristics of imGCs across species, highlighting the importance of conducting independent molecular and functional analyses for adult neurogenesis in different species.

Imipramine, an Acid Sphingomyelinase Inhibitor, Promotes Newborn Neuron Survival in the Hippocampus After Seizure

Epilepsy, a chronic neurological disorder, is triggered by various insults, including traumatic brain injury and stroke. Acid sphingomyelinase (ASMase), an enzyme that hydrolyzes sphingomyelin into ceramides, is implicated in oxidative stress, neuroinflammation, and neuronal apoptosis. Ceramides, which have pro-apoptotic properties, contribute to oxidative damage and lysosomal dysfunction, exacerbating neuronal injury. This study investigates the role of ASMase in epilepsy, hypothesizing that seizure activity upregulates ASMase, increasing ceramide levels, DNA damage, and neuronal apoptosis. We employed a pilocarpine-induced rat seizure model and examined the effects of imipramine, an ASMase inhibitor, administered intraperitoneally (10 mg/kg) for four weeks post-seizure induction. Histological and cognitive analyses showed that while imipramine did not prevent early neuronal death within the first week, it significantly reduced markers of neuronal apoptosis by four weeks. Imipramine also promoted hippocampal neurogenesis and preserved cognitive function, which is often impaired following seizures. These findings suggest that ASMase inhibition could mitigate neuronal apoptosis and improve cognitive recovery after seizures. Imipramine may serve as a promising therapeutic strategy for epilepsy-associated neuronal damage and cognitive deficits. Further studies should delineate the molecular mechanisms of ASMase inhibition and evaluate its long-term efficacy in addressing epilepsy-related neurodegeneration and functional impairments.

Sequential physical and cognitive training disrupts cocaine-context associations via multi-level stimulation of adult hippocampal neurogenesis

Cocaine-related contextual cues are a recurrent source of craving and relapse. Extinction of cue-driven cocaine seeking remains a clinical challenge, and the search for adjuvants is ongoing. In this regard, combining physical and cognitive training is emerging as a promising strategy that has shown synergistic benefits on brain structure and function, including enhancement of adult hippocampal neurogenesis (AHN), which has been recently linked to reduced maintenance of maladaptive drug seeking. Here, we examined whether this behavioral approach disrupts cocaine-context associations via improved AHN. To this aim, C57BL/6J mice (N = 37) developed a cocaine-induced conditioned place preference (CPP) and underwent interventions consisting of exercise and/or spatial working memory training. Bromodeoxyuridine (BrdU) was administered during early running sessions to tag a subset of new dentate granule cells (DGCs) reaching a critical window of survival during spatial learning. Once these DGCs became functionally mature (∼ 6 weeks-old), mice received extinction training before testing CPP extinction and reinstatement. We found that single and combined treatments accelerated CPP extinction and prevented reinstatement induced by a low cocaine priming (2 mg/kg). Remarkably, the dual-intervention mice showed a significant decrease of CPP after extinction relative to untreated animals. Moreover, combining the two strategies led to increased number and functional integration of BrdU+ DGCs, which in turn maximized the effect of spatial training (but not exercise) to reduce CPP persistence. Together, our findings suggests that sequencing physical and cognitive training may redound to decreased maintenance of cocaine-context associations, with multi-level stimulation of AHN as a potential underlying mechanism.

Adult Hippocampal Neurogenesis as a Therapeutic Target in Fetal Alcohol Spectrum Disorder

This review is focused on adult hippocampal neurogenesis as a potential therapeutic target in fetal alcohol spectrum disorder (FASD). Adult hippocampal neurogenesis refers to the production of new hippocampal dentate granule cells (DGCs) from a replenishable pool of neural stem and progenitor cells throughout life. Adult-generated DGCs have been shown to exert a profound influence on hippocampal network activity in experimental animals and have been implicated in the regulation of many hippocampal-dependent behaviors and emotional states, including certain forms of learning and memory, anxiety, mood, and stress resilience. While adult hippocampal neurogenesis in humans remains controversial, many studies support its existence and impact on hippocampal function in human health and disease. Here, we review mechanisms of adult hippocampal neurogenesis under physiological conditions, as described primarily in rodent brain, its impact on network activity and behavior, and the negative effects of developmental alcohol exposure on this process. We then explore hippocampal neurogenesis as a potential target for FASD therapy using pharmacological and neurophysiological approaches known to stimulate adult hippocampal neurogenesis, currently available for clinical use in FASD patients.

IL-17A Mediates Depressive-Like Symptoms by Inducing Microglia Activation in Psoriasiform Dermatitis Mice

BACKGROUND

Psoriasis is recognized as a systemic disease for its accompanying comorbidities, among which psychological disorders present a high incidence rate and affect patients' life quality. Interleukin (IL)-17A is the central pathological factor in the pathogenesis and development of psoriasis.

OBJECTIVE

To clarify if psoriasis-induced systemic IL-17A increase can mediate the neuronal inflammation and result in depressive-like symptoms.

METHODS

Psoriasiform dermatitis model was established by imiquimod (IMQ) application on male BALB/c mice and IL-17A intervention was performed by lateral ventricular catheterization. Skin structural, histopathological characteristics, and behavioral tests were assessed. Serum IL-17A levels were detected by Enzyme-linked immunosorbent assay. mRNA expression of pro-inflammatory factors IL-1β, IL-6, and tumor necrosis factor-α (TNF-α) as well as anti-inflammatory factors IL-4 and IL-10 in the hippocampus and cortex were measured by RT-qPCR. The number of microglia and hippocampal neurons was quantified by immunofluorescent assay.

RESULTS

IMQ treatment resulted in significant skin structural and histopathological characters of psoriasiform dermatitis with elevated serum IL-17A levels, obvious depressive-like behaviors, microglia activation with increased IL-1β, IL-6, and TNF-α expression levels in the hippocampus and cortex, and notable inhibition of hippocampal neurogenesis. While, IL-17A neutralization by intracerebroventricular injection of anti-IL-17A antibody can remarkably inhibit microglia activation and decrease the abnormally increased expression levels of IL-1β, IL-6, and TNF-α in the hippocampus and cortex of psoriasiform dermatitis mice, promote hippocampal neurogenesis, thus alleviate the depressive-like behaviors.

CONCLUSION

In the pathological condition of psoriasis, systemic IL-17A elevation can trigger microglia activation, provoke pro-inflammation mediators to release, evoke neuroinflammation, subsequently inhibit hippocampal neurogenesis, and result in depression. IL-17A, as an important pathogenic factor in psoriasis, contributes to its critical role in mediating systemic inflammation and depression comorbidity.

Estrogens dynamically regulate neurogenesis in the dentate gyrus of adult female rats

Estrone and estradiol differentially modulate neuroplasticity and cognition. How they influence the maturation of new neurons in the adult hippocampus, however, is not known. The present study assessed the effects of estrone and estradiol on the maturation timeline of neurogenesis in the dentate gyrus (DG) of ovariectomized (a model of surgical menopause) young adult Sprague-Dawley rats using daily subcutaneous injections of 17β-estradiol, estrone or vehicle. Rats were injected with a DNA synthesis marker, 5-bromo-2-deoxyuridine (BrdU), and were perfused 1, 2, or 3 weeks after BrdU injection and daily hormone treatment. Brains were sectioned and processed for various markers including: sex-determining region Y-box 2 (Sox2), glial fibrillary acidic protein (GFAP), antigen kiel 67 (Ki67), doublecortin (DCX), and neuronal nuclei (NeuN). Immunofluorescent labeling or co-labelling of BrdU with Sox2 (progenitor cells), Sox2/GFAP (neural progenitor cells), Ki67 (cell proliferation), DCX (immature neurons), NeuN (mature neurons) was used to examine the trajectory and maturation of adult-born neurons over time. Estrogens had early (1 week of exposure) effects on different stages of neurogenesis (neural progenitor cells, cell proliferation and early maturation of new cells into neurons) but these effects were less pronounced after prolonged treatment. Estradiol enhanced, whereas estrone reduced cell proliferation after 1 week but not after longer exposure to either estrogen. Both estrogens increased the density of immature neurons (BrdU/DCX-ir) after 1 week of exposure compared to vehicle treatment but this increased density was not sustained over longer durations of treatments to estrogens, suggesting that the enhancing effects of estrogens on neurogenesis were short-lived. Longer duration post-ovariectomy, without treatments with either of the estrogens, was associated with reduced neural progenitor cells in the DG. These results demonstrate that estrogens modulate several aspects of adult hippocampal neurogenesis differently in the short term, but may lose their ability to influence neurogenesis after long-term exposure. These findings have potential implications for treatments involving estrogens after surgical menopause.

Septo-dentate gyrus cholinergic circuits modulate function and morphogenesis of adult neural stem cells through granule cell intermediaries

Cholinergic neurons in the basal forebrain play a crucial role in regulating adult hippocampal neurogenesis (AHN). However, the circuit and molecular mechanisms underlying cholinergic modulation of AHN, especially the initial stages of this process related to the generation of newborn progeny from quiescent radial neural stem cells (rNSCs), remain unclear. Here, we report that stimulation of the cholinergic circuits projected from the diagonal band of Broca (DB) to the dentate gyrus (DG) neurogenic niche promotes proliferation and morphological development of rNSCs, resulting in increased neural stem/progenitor pool and rNSCs with longer radial processes and larger busy heads. Interestingly, DG granule cells (GCs) are required for DB-DG cholinergic circuit-dependent modulation of proliferation and morphogenesis of rNSCs. Furthermore, single-nucleus RNA sequencing of DG reveals cell type-specific transcriptional changes in response to cholinergic circuit stimulation, with GCs (among all the DG niche cells) exhibiting the most extensive transcriptional changes. Our findings shed light on how the DB-DG cholinergic circuits orchestrate the key niche components to support neurogenic function and morphogenesis of rNSCs at the circuit and molecular levels.

Neurobiology and systems biology of stress resilience

Stress resilience is the phenomenon that some people maintain their mental health despite exposure to adversity or show only temporary impairments followed by quick recovery. Resilience research attempts to unravel the factors and mechanisms that make resilience possible and to harness its insights for the development of preventative interventions in individuals at risk for acquiring stress-related dysfunctions. Biological resilience research has been lagging behind the psychological and social sciences but has seen a massive surge in recent years. At the same time, progress in this field has been hampered by methodological challenges related to finding suitable operationalizations and study designs, replicating findings, and modeling resilience in animals. We embed a review of behavioral, neuroimaging, neurobiological, and systems biological findings in adults in a critical methods discussion. We find preliminary evidence that hippocampus-based pattern separation and prefrontal-based cognitive control functions protect against the development of pathological fears in the aftermath of singular, event-type stressors [as found in fear-related disorders, including simpler forms of posttraumatic stress disorder (PTSD)] by facilitating the perception of safety. Reward system-based pursuit and savoring of positive reinforcers appear to protect against the development of more generalized dysfunctions of the anxious-depressive spectrum resulting from more severe or longer-lasting stressors (as in depression, generalized or comorbid anxiety, or severe PTSD). Links between preserved functioning of these neural systems under stress and neuroplasticity, immunoregulation, gut microbiome composition, and integrity of the gut barrier and the blood-brain barrier are beginning to emerge. On this basis, avenues for biological interventions are pointed out.

Autocrine VEGF drives neural stem cell proximity to the adult hippocampus vascular niche

The vasculature is a key component of adult brain neural stem cell (NSC) niches. In the adult mammalian hippocampus, NSCs reside in close contact with a dense capillary network. How this niche is maintained is unclear. We recently found that adult hippocampal NSCs express VEGF, a soluble factor with chemoattractive properties for vascular endothelia. Here, we show that global and NSC-specific VEGF loss led to dissociation of NSCs and their intermediate progenitor daughter cells from local vasculature. Surprisingly, though, we found no changes in local vascular density. Instead, we found that NSC-derived VEGF supports maintenance of gene expression programs in NSCs and their progeny related to cell migration and adhesion. In vitro assays revealed that blockade of VEGF receptor 2 impaired NSC motility and adhesion. Our findings suggest that NSCs maintain their own proximity to vasculature via self-stimulated VEGF signaling that supports their motility towards and/or adhesion to local blood vessels.

Neural stem and progenitor cells support and protect adult hippocampal function via vascular endothelial growth factor secretion

Adult neural stem and progenitor cells (NSPCs) reside in the dentate gyrus (DG) of the hippocampus throughout the lifespan of most mammalian species. In addition to generating new neurons, NSPCs may alter their niche via secretion of growth factors and cytokines. We recently showed that adult DG NSPCs secrete vascular endothelial growth factor (VEGF), which is critical for maintaining adult neurogenesis. Here, we asked whether NSPC-derived VEGF alters hippocampal function independent of adult neurogenesis. We found that loss of NSPC-derived VEGF acutely impaired hippocampal memory, caused neuronal hyperexcitability and exacerbated excitotoxic injury. We also found that NSPCs generate substantial proportions of total DG VEGF and VEGF disperses broadly throughout the DG, both of which help explain how this anatomically-restricted cell population could modulate function broadly. These findings suggest that NSPCs actively support and protect DG function via secreted VEGF, thereby providing a non-neurogenic functional dimension to endogenous NSPCs.

Compensatory remodeling of a septo-hippocampal GABAergic network in the triple transgenic Alzheimer's mouse model

BACKGROUND

Alzheimer's disease (AD) is characterized by a progressive loss of memory that cannot be efficiently managed by currently available AD therapeutics. So far, most treatments for AD that have the potential to improve memory target neural circuits to protect their integrity. However, the vulnerable neural circuits and their dynamic remodeling during AD progression remain largely undefined.

METHODS

Circuit-based approaches, including anterograde and retrograde tracing, slice electrophysiology, and fiber photometry, were used to investigate the dynamic structural and functional remodeling of a GABAergic circuit projected from the medial septum (MS) to the dentate gyrus (DG) in 3xTg-AD mice during AD progression.

RESULTS

We identified a long-distance GABAergic circuit that couples highly connected MS and DG GABAergic neurons during spatial memory encoding. Furthermore, we found hyperactivity of DG interneurons during early AD, which persisted into late AD stages. Interestingly, MS GABAergic projections developed a series of adaptive strategies to combat DG interneuron hyperactivity. During early-stage AD, MS-DG GABAergic projections exhibit increased inhibitory synaptic strength onto DG interneurons to inhibit their activities. During late-stage AD, MS-DG GABAergic projections form higher anatomical connectivity with DG interneurons and exhibit aberrant outgrowth to increase the inhibition onto DG interneurons.

CONCLUSION

We report the structural and functional remodeling of the MS-DG GABAergic circuit during disease progression in 3xTg-AD mice. Dynamic MS-DG GABAergic circuit remodeling represents a compensatory mechanism to combat DG interneuron hyperactivity induced by reduced GABA transmission.

Activation of hypothalamic-enhanced adult-born neurons restores cognitive and affective function in Alzheimer's disease

Patients with Alzheimer's disease (AD) exhibit progressive memory loss, depression, and anxiety, accompanied by impaired adult hippocampal neurogenesis (AHN). Whether AHN can be enhanced in impaired AD brain to restore cognitive and affective function remains elusive. Here, we report that patterned optogenetic stimulation of the hypothalamic supramammillary nucleus (SuM) enhances AHN in two distinct AD mouse models, 5×FAD and 3×Tg-AD. Strikingly, the chemogenetic activation of SuM-enhanced adult-born neurons (ABNs) rescues memory and emotion deficits in these AD mice. By contrast, SuM stimulation alone or activation of ABNs without SuM modification fails to restore behavioral deficits. Furthermore, quantitative phosphoproteomics analyses reveal activation of the canonical pathways related to synaptic plasticity and microglia phagocytosis of plaques following acute chemogenetic activation of SuM-enhanced (vs. control) ABNs. Our study establishes the activity-dependent contribution of SuM-enhanced ABNs in modulating AD-related deficits and informs signaling mechanisms mediated by the activation of SuM-enhanced ABNs.