Changes in hippocampal neurogenesis throughout early development

LUZP1 Regulates Dendritic Spine Maturation and Synaptic Plasticity in the Hippocampal Dentate Gyrus of Mice

Leucine zipper protein 1 (LUZP1) functions in the maintenance and dynamics of the cytoskeleton by interacting with actin and microtubules. Deficiency or mutation of LUZP1 is associated with brain developmental disorders; however, its precise role in brain function remains unclear. We showed that LUZP1 localizes to actin and is highly expressed in CaMKIIα-expressing neurons within the mouse hippocampal dentate gyrus. Depletion of LUZP1 impedes dendritic spine maturation, which is characterized by excess immature filopodia and loss of mature mushroom spines both in vitro and in vivo. LUZP1 knockdown reduces spontaneous electrical activity and synaptic plasticity in hippocampal neurons. Conditional deletion of LUZP1 in CaMKIIα-expressing neurons causes impaired learning and memory behavior in mice of both sexes. Mechanistically, LUZP1 control dendritic maturation by directly interacting with filamin A and modulating the Rac1-PAK1 signaling pathway. These findings shed light on the role of LUZP1 in regulating synaptic plasticity and brain function.

Anti-RGMa neutralizing antibody ameliorates vascular cognitive impairment in mice

Repulsive Guidance Molecule A (RGMa) is well-recognized for its role in axon guidance. Recent studies have unveiled its diverse functions under pathological conditions within the central nervous system, such as spinal cord injury, multiple sclerosis, and Parkinson's disease. In this study, we explored the involvement of RGMa and the therapeutic effects of an anti-RGMa neutralizing antibody in a mouse model of vascular dementia (VaD). The VaD mouse model was established using the bilateral common carotid artery stenosis (BCAS) method. Immunohistochemical analysis revealed that these mice exhibited increased RGMa expression in the hippocampus, which coincided with reduced neurogenesis and impaired cholinergic innervation. These alterations manifested as cognitive impairments in the BCAS mice. Significantly, treatment with anti-RGMa neutralizing antibody reversed these pathological changes and cognitive deficits. Our findings suggest that RGMa plays a pivotal role in VaD pathology within the hippocampus and propose the anti-RGMa antibody as a promising therapeutic avenue for treating VaD.

Hippocampal rejuvenation by a single intracerebral injection of one-carbon metabolites in C57BL6 old wild-type mice

The Izpisua-Belmonte group identified a cocktail of metabolites that promote partial reprogramming in cultured muscle cells. We tested the effect of brain injection of these metabolites in the dentate gyrus of aged wild-type mice. The dentate gyrus is a brain region essential for memory function and is extremely vulnerable to aging. A single injection of the cocktail containing four compounds (putrescine, glycine, methionine and threonine) partially reversed brain aging phenotypes and epigenetic alterations in age-associated genes. Our analysis revealed three levels: chromatin methylation, RNA sequencing, and protein expression. Functional studies complemented the previous ones, showing cognitive improvement. In summary, we report the reversal of various age-associated epigenetic changes, such as the transcription factor Zic4, and several changes related to cellular rejuvenation in the dentate gyrus (DG). These changes include increased expression of the Sox2 protein. Finally, the increases in the survival of newly generated neurons and the levels of the NMDA receptor subunit GluN2B were accompanied by improvements in both short-term and long-term memory performance. Based on these results, we propose the use of these metabolites to explore new strategies for the development of potential treatments for age-related brain diseases.

Modelling adult neurogenesis in the aging rodent hippocampus: a midlife crisis

Contrary to humans, adult hippocampal neurogenesis in rodents is not controversial. And in the last three decades, multiple studies in rodents have deemed adult neurogenesis essential for most hippocampal functions. The functional relevance of new neurons relies on their distinct physiological properties during their maturation before they become indistinguishable from mature granule cells. Most functional studies have used very young animals with robust neurogenesis. However, this trait declines dramatically with age, questioning its functional relevance in aging animals, a caveat that has been mentioned repeatedly, but rarely analyzed quantitatively. In this meta-analysis, we use data from published studies to determine the critical functional window of new neurons and to model their numbers across age in both mice and rats. Our model shows that new neurons with distinct functional profile represent about 3% of the total granule cells in young adult 3-month-old rodents, and their number decline following a power function to reach less than 1% in middle aged animals and less than 0.5% in old mice and rats. These low ratios pose an important logical and computational caveat to the proposed essential role of new neurons in the dentate gyrus, particularly in middle aged and old animals, a factor that needs to be adequately addressed when defining the relevance of adult neurogenesis in hippocampal function.

Influence of chrysin on D-galactose induced-aging in mice: Up regulation of AMP kinase/liver kinase B1/peroxisome proliferator-activated receptor-γ coactivator 1-α signaling pathway

Adenosine monophosphate kinase/liver kinase B1/peroxisome proliferator-activated receptor-γ coactivator 1-α (AMPK/LKB1/PGC1α) pathway has a vital role in regulating age-related diseases. It controls neurogenesis, cell proliferation, axon outgrowth, and cellular energy homeostasis. AMPK pathway also regulates mitochondrial synthesis. The current study evaluated the effect of chrysin on D-galactose (D-gal) induced-aging, neuron degeneration, mitochondrial dysfunction, oxidative stress, and neuroinflammation in mice. The mice were allocated randomly into four groups (10 each group): Group 1: normal control group, Group 2: D-gal group, Groups 3 and 4: chrysin (125 and 250 mg/kg, respectively). Groups 2-4 were injected with D-gal (200 mg/kg/day; s.c) for 8 weeks to induce aging. Groups 3 and 4 were orally gavaged every day concurrent with D-gal. At the end of experiment, behavioral, brain biochemical and histopathological changes were monitored. Chrysin administration elevated discrimination ratio in object recognition, Y Maze percentage alternation, locomotor activity and brain contents of AMPK, LKB1, PGC1α, NAD (P)H quinone oxidoreductase 1 (NQO1), heme oxygenase 1 (HO-1), nerve growth factor (NGF) (neurotrophin-3; NT-3), and seretonin as well as reduced brain contents of tumor necrosis factor-alpha (TNF-α), nuclear factor kappa B (NF-κB), advanced glycation end products (AGEs) and glial fibrillary acidic protein (GFAP) compared to D-gal-treated mice. Chrysin also alleviated cerebral cortex and white matter neurons degeneration. Chrysin protects against neurodegeneration, improves mitochondrial autophagy and biogenesis as well as activates antioxidant genes expression. In addition, chrysin ameliorates neuroinflammation and stimulates the release of NGF and serotonin neurotransmitter. So, chrysin has a neuroprotective effect in D-gal induced-aging in mice.

Perturbations in neural stem cell function during a neurotropic viral infection in juvenile mice

Viral infections of the central nervous system (CNS) often cause worse neurological outcomes in younger hosts. Throughout childhood, the brain undergoes extensive development and refinement to produce functional neural networks. Network function is maintained partly with the help of neural stem cells (NSCs) that replace neuronal and glia subtypes in the two neurogenic niches of the brain (the hippocampus and subventricular zone). Accumulating evidence suggests that viruses disrupt NSC function in adulthood and infancy, but the in vivo impact of childhood infections on acute and long-term NSC function is unknown. Using a juvenile mouse model of measles virus (MeV) infection, where only mature neurons in the brain are infected, we defined the effects of the antiviral immune response on NSCs from juvenile to adult stages of life. We found that (a) virus persists in the brains of survivors despite an anti-viral immune response; (b) NSC numbers decrease dramatically during early infection, but ultimately stabilize in adult survivors; (c) infection is associated with mild apoptosis throughout the juvenile brain, but NSC proliferation is unchanged; (d) the loss of NSC numbers is dependent upon the stage of NSC differentiation; and (e) immature neurons increase early during infection, concurrent with depletion of NSC pools. Collectively, we show that NSCs are exquisitely sensitive to the inflammatory microenvironment created during neuron-restricted MeV infection in juveniles, responding with an early loss of NSCs but increased neurogenesis. These studies provide insight into potential cellular mechanisms associated with long-term neurological deficits in survivors of childhood CNS infections.

Epigenetic aging in adult neurogenesis

Neural stem cells (NSCs) in the hippocampus generate new neurons throughout life, which functionally contribute to cognitive flexibility and mood regulation. Yet adult hippocampal neurogenesis substantially declines with age and age-related impairments in NSC activity underlie this reduction. Particularly, increased NSC quiescence and consequently reduced NSC proliferation are considered to be major drivers of the low neurogenesis levels in the aged brain. Epigenetic regulators control the gene expression programs underlying NSC quiescence, proliferation and differentiation and are hence critical to the regulation of adult neurogenesis. Epigenetic alterations have also emerged as central hallmarks of aging, and recent studies suggest the deterioration of the NSC-specific epigenetic landscape as a driver of the age-dependent decline in adult neurogenesis. In this review, we summarize the recently accumulating evidence for a role of epigenetic dysregulation in NSC aging and propose perspectives for future research directions.

The Role of SOX Transcription Factors in Ageing and Age-Related Diseases

The quest for eternal youth and immortality is as old as humankind. Ageing is an inevitable physiological process accompanied by many functional declines that are driving factors for age-related diseases. Stem cell exhaustion is one of the major hallmarks of ageing. The SOX transcription factors play well-known roles in self-renewal and differentiation of both embryonic and adult stem cells. As a consequence of ageing, the repertoire of adult stem cells present in various organs steadily declines, and their dysfunction/death could lead to reduced regenerative potential and development of age-related diseases. Thus, restoring the function of aged stem cells, inducing their regenerative potential, and slowing down the ageing process are critical for improving the health span and, consequently, the lifespan of humans. Reprograming factors, including SOX family members, emerge as crucial players in rejuvenation. This review focuses on the roles of SOX transcription factors in stem cell exhaustion and age-related diseases, including neurodegenerative diseases, visual deterioration, chronic obstructive pulmonary disease, osteoporosis, and age-related cancers. A better understanding of the molecular mechanisms of ageing and the roles of SOX transcription factors in this process could open new avenues for developing novel strategies that will delay ageing and prevent age-related diseases.

Experimental and Clinical Biomarkers for Progressive Evaluation of Neuropathology and Therapeutic Interventions for Acute and Chronic Neurological Disorders

This article describes commonly used experimental and clinical biomarkers of neuronal injury and neurodegeneration for the evaluation of neuropathology and monitoring of therapeutic interventions. Biomarkers are vital for diagnostics of brain disease and therapeutic monitoring. A biomarker can be objectively measured and evaluated as a proxy indicator for the pathophysiological process or response to therapeutic interventions. There are complex hurdles in understanding the molecular pathophysiology of neurological disorders and the ability to diagnose them at initial stages. Novel biomarkers for neurological diseases may surpass these issues, especially for early identification of disease risk. Validated biomarkers can measure the severity and progression of both acute neuronal injury and chronic neurological diseases such as epilepsy, migraine, Alzheimer's disease, Parkinson's disease, Huntington's disease, traumatic brain injury, amyotrophic lateral sclerosis, multiple sclerosis, and other brain diseases. Biomarkers are deployed to study progression and response to treatment, including noninvasive imaging tools for both acute and chronic brain conditions. Neuronal biomarkers are classified into four core subtypes: blood-based, immunohistochemical-based, neuroimaging-based, and electrophysiological biomarkers. Neuronal conditions have progressive stages, such as acute injury, inflammation, neurodegeneration, and neurogenesis, which can serve as indices of pathological status. Biomarkers are critical for the targeted identification of specific molecules, cells, tissues, or proteins that dramatically alter throughout the progression of brain conditions. There has been tremendous progress with biomarkers in acute conditions and chronic diseases affecting the central nervous system.

Examination of Longitudinal Alterations in Alzheimer’s Disease-Related Neurogenesis in an APP/PS1 Transgenic Mouse Model, and the Effects of P33, a Putative Neuroprotective Agent Thereon

Neurogenesis plays a crucial role in cognitive processes. During aging and in Alzheimer’s disease (AD), altered neurogenesis and neuroinflammation are evident both in C57BL/6J, APP_Swe/PS1_dE9 (Tg) mice and humans. AD pathology may slow down upon drug treatment, for example, in a previous study of our group P33, a putative neuroprotective agent was found to exert advantageous effects on the elevated levels of APP, Aβ, and neuroinflammation. In the present study, we aimed to examine longitudinal alterations in neurogenesis, neuroinflammation and AD pathology in a transgenic (Tg) mouse model, and assessed the putative beneficial effects of long-term P33 treatment on AD-specific neurological alterations. Hippocampal cell proliferation and differentiation were significantly reduced between 8 and 12 months of age. Regarding neuroinflammation, significantly elevated astrogliosis and microglial activation were observed in 6- to 7-month-old Tg animals. The amounts of the molecules involved in the amyloidogenic pathway were altered from 4 months of age in Tg animals. P33-treatment led to significantly increased neurogenesis in 9-month-old animals. Our data support the hypothesis that altered neurogenesis may be a consequence of AD pathology. Based on our findings in the transgenic animal model, early pharmacological treatment before the manifestation of AD symptoms might ameliorate neurological decline.

Chronic IL-10 overproduction disrupts microglia-neuron dialogue similar to aging, resulting in impaired hippocampal neurogenesis and spatial memory

The subgranular zone of the dentate gyrus is an adult neurogenic niche where new neurons are continuously generated. A dramatic hippocampal neurogenesis decline occurs with increasing age, contributing to cognitive deficits. The process of neurogenesis is intimately regulated by the microenvironment, with inflammation being considered a strong negative factor for this process. Thus, we hypothesize that the reduction of new neurons in the aged brain could be attributed to the age-related microenvironmental changes towards a pro-inflammatory status. In this work, we evaluated whether an anti-inflammatory microenvironment could counteract the negative effect of age on promoting new hippocampal neurons. Surprisingly, our results show that transgenic animals chronically overexpressing IL-10 by astrocytes present a decreased hippocampal neurogenesis in adulthood. This results from an impairment in the survival of neural newborn cells without differences in cell proliferation. In parallel, hippocampal-dependent spatial learning and memory processes were affected by IL-10 overproduction as assessed by the Morris water maze test. Microglial cells, which are key players in the neurogenesis process, presented a different phenotype in transgenic animals characterized by high activation together with alterations in receptors involved in neuronal communication, such as CD200R and CX3CR1. Interestingly, the changes described in adult transgenic animals were similar to those observed by the effect of normal aging. Thus, our data suggest that chronic IL-10 overproduction mimics the physiological age-related disruption of the microglia-neuron dialogue, resulting in hippocampal neurogenesis decrease and spatial memory impairment.

Lamin B1 decline underlies age-related loss of adult hippocampal neurogenesis

Neurogenesis in the adult hippocampus declines with age, a process that has been implicated in cognitive and emotional impairments. However, the mechanisms underlying this decline have remained elusive. Here, we show that the age-dependent downregulation of lamin B1, one of the nuclear lamins in adult neural stem/progenitor cells (ANSPCs), underlies age-related alterations in adult hippocampal neurogenesis. Our results indicate that higher levels of lamin B1 in ANSPCs safeguard against premature differentiation and regulate the maintenance of ANSPCs. However, the level of lamin B1 in ANSPCs declines during aging. Precocious loss of lamin B1 in ANSPCs transiently promotes neurogenesis but eventually depletes it. Furthermore, the reduction of lamin B1 in ANSPCs recapitulates age-related anxiety-like behavior in mice. Our results indicate that the decline in lamin B1 underlies stem cell aging and impacts the homeostasis of adult neurogenesis and mood regulation.

GDNF and GFRα1 Are Required for Proper Integration of Adult-Born Hippocampal Neurons

The glial cell line-derived neurotrophic factor (GDNF) is required for the survival and differentiation of diverse neuronal populations during nervous system development. Despite the high expression of GDNF and its receptor GFRα1 in the adult hippocampus, the functional role of this system remains unknown. Here, we show that GDNF, acting through its GFRα1 receptor, controls dendritic structure and spine density of adult-born granule cells, which reveals that GFRα1 is required for their integration into preexisting circuits. Moreover, conditional mutant mice for GFRα1 show deficits in behavioral pattern separation, a task in which adult neurogenesis is known to play a critical role. We also find that running increases GDNF in the dentate gyrus and promotes GFRα1-dependent CREB (cAMP response element-binding protein) activation and dendrite maturation. Together, these findings indicate that GDNF/GFRα1 signaling plays an essential role in the plasticity of adult circuits, controlling the integration of newly generated neurons.

The neuroprotective effect of curcumin against Cd-induced neurotoxicity and hippocampal neurogenesis promotion through CREB-BDNF signaling pathway

Heavy metal neurotoxicity is one of the major challenges in today's era due to the large scale and widespread mechanisation of the production. However, the causative factors responsible for neurotoxicity are neither known nor do we have the availability of therapeutic approaches to deal with it. One of the major causative agents of neurotoxicity is a non-essential transition heavy metal, Cadmium (Cd), that reaches the central nervous system (CNS) through the nasal mucosa and olfactory pathway causing adverse structural and functional effects. In this study, we explored the neuroprotective efficacy of plant derived Curcumin which is reported to have pleiotropic biological activity including anti-oxidant, anti-inflammatory, anti-apoptotic, anti-carcinogenic and anti-angiogenic effects. Four different concentrations of curcumin (20, 40, 80 and 160 mg/kg of the body weight) were used to assess the behavioural, biochemical, hippocampal proteins (BDNF, CREB, DCX and Synapsin II) and histological changes in Swiss Albino mice that were pre-treated with Cd (2.5 mg/kg). The findings showed that Cd exposure led to the behavioural impairment through oxidative stress, reduction of hippocampal neurogenesis associated proteins, and degeneration of CA3 and cortical neurons. However, treatment of different curcumin concentrations had effectively restored the behavioural changes in Cd-exposed mice through regulation of oxidative stress and up-regulation of hippocampal proteins in a dose-dependent manner. Significantly, a dose of 160 mg/kg body weight was found to be glaringly effective. From this study, we infer that curcumin reverses the adverse effects of neurotoxicity induced by Cd and promotes neurogenesis.

Guanosine Promotes Proliferation in Neural Stem Cells from Hippocampus and Neurogenesis in Adult Mice

Neural stem cells can generate new neurons in the mouse adult brain in a complex multistep process called neurogenesis. Several factors regulate this process, including neurotransmitters, hormones, neurotrophic factors, pharmacological agents, and environmental factors. Purinergic signaling, mainly the adenosinergic system, takes part in neurogenesis, being involved in cell proliferation, migration, and differentiation. However, the role of the purine nucleoside guanosine in neurogenesis remains unclear. Here, we examined the effect of guanosine by using the neurosphere assay derived from neural stem cells of adult mice. We found that continuous treatment with guanosine increased the number of neurospheres, neural stem cell proliferation, and neuronal differentiation. The effect of guanosine to increase the number of neurospheres was reduced by removing adenosine from the culture medium. We next traced the neurogenic effect of guanosine in vivo. The intraperitoneal treatment of adult C57BL/6 mice with guanosine (8 mg/kg) for 26 days increased the number of dividing bromodeoxyuridine (BrdU)-positive cells and also increased neurogenesis, as identified by measuring doublecortin (DCX)-positive cells in the dentate gyrus (DG) of the hippocampus. Antidepressant-like behavior in adult mice accompanied the guanosine-induced neurogenesis in the DG. These results provide new evidence of a pro-neurogenic effect of guanosine on neural stem/progenitor cells, and it was associated in vivo with antidepressant-like effects.

Dysregulated expression of monoacylglycerol lipase is a marker for anti-diabetic drug metformin-targeted therapy to correct impaired neurogenesis and spatial memory in Alzheimer's disease

Rationale: Monoacylglycerol lipase (Mgll), a hydrolase that breaks down the endocannabinoid 2-arachidonoyl glycerol (2-AG) to produce arachidonic acid (ARA), is a potential target for neurodegenerative diseases, such as Alzheimer's disease (AD). Increasing evidence shows that impairment of adult neurogenesis by perturbed lipid metabolism predisposes patients to AD. However, it remains unknown what causes aberrant expression of Mgll in AD and how Mgll-regulated lipid metabolism impacts adult neurogenesis, thus predisposing to AD during aging. Here, we identify Mgll as an aging-induced factor that impairs adult neurogenesis and spatial memory in AD, and show that metformin, an FDA-approved anti-diabetic drug, can reduce the expression of Mgll to reverse impaired adult neurogenesis, prevent spatial memory decline and reduce β-amyloid accumulation.

Methods: Mgll expression was assessed in both human AD patient post-mortem hippocampal tissues and 3xTg-AD mouse model. In addition, we used both the 3xTg-AD animal model and the CbpS436A genetic knock-in mouse model to identify that elevated Mgll expression is caused by the attenuation of the aPKC-CBP pathway, involving atypical protein kinase C (aPKC)-stimulated Ser436 phosphorylation of histone acetyltransferase CBP through biochemical methods. Furthermore, we performed in vivo adult neurogenesis assay with BrdU/EdU labelling and Morris water maze task in both animal models following pharmacological treatments to show the key role of Mgll in metformin-corrected neurogenesis and spatial memory deficits of AD through reactivating the aPKC-CBP pathway. Finally, we performed in vitro adult neurosphere assays using both animal models to study the role of the aPKC-CBP mediated Mgll repression in determining adult neural stem/progenitor cell (NPC) fate.

Results: Here, we demonstrate that aging-dependent induction of Mgll is observed in the 3xTg-AD model and human AD patient post-mortem hippocampal tissues. Importantly, we discover that elevated Mgll expression is caused by the attenuation of the aPKC-CBP pathway. The accumulation of Mgll in the 3xTg-AD mice reduces the genesis of newborn neurons and perturbs spatial memory. However, we find that metformin-stimulated aPKC-CBP pathway decreases Mgll expression to recover these deficits in 3xTg-AD. In addition, we reveal that elevated Mgll levels in cultured adult NPCs from both 3xTg-AD and CbpS436A animal models are responsible for their NPC neuronal differentiation deficits.

Conclusion: Our findings set the stage for development of a clinical protocol where Mgll would serve as a biomarker in early stages of AD to identify potential metformin-responsive AD patients to restore their neurogenesis and spatial memory.

Bcl11 Transcription Factors Regulate Cortical Development and Function

Transcription factors regulate multiple processes during brain development and in the adult brain, from brain patterning to differentiation and maturation of highly specialized neurons as well as establishing and maintaining the functional neuronal connectivity. The members of the zinc-finger transcription factor family Bcl11 are mainly expressed in the hematopoietic and central nervous systems regulating the expression of numerous genes involved in a wide range of pathways. In the brain Bcl11 proteins are required to regulate progenitor cell proliferation as well as differentiation, migration, and functional integration of neural cells. Mutations of the human Bcl11 genes lead to anomalies in multiple systems including neurodevelopmental impairments like intellectual disabilities and autism spectrum disorders.

AMPK Signaling Regulates the Age-Related Decline of Hippocampal Neurogenesis

The global incidence of age-associated neurological diseases is expected to rise with increasingly greying societies. In the aged brain, there is a dramatic decrease in the number of stem cells, which is a main cause for the decrease in brain function. Intrinsic factors, such as cell metabolism, have been studied but its role in neurogenesis is still unknown. Therefore, this study sought to establish whether AMP-activated protein kinase (AMPK) signaling does indeed regulate hippocampal neurogenesis in the aged brain. We found that i) AMPKα2 was the predominant catalytic subunit in the subgranular and subventricular zones; ii) AMPK activation was at a significantly higher level in the aged vs. young hippocampus; iii) short term (7 days) treatment with selective AMPK signaling inhibitor Compound C (10 mg/kg/day, i.p.) significantly increased the numbers of newborn (BrdU+), Type 2 (MCM2+), and Type 3 (DCX+) neural stem cells, but not Type 1 (GFAP+/Sox2+) cells, in the aged hippocampus. Taken together, our results demonstrate that AMPK signaling plays a critical role in the age-related decline of hippocampal neurogenesis.

Experience-Dependent Structural Plasticity of Adult-Born Neurons in the Aging Hippocampus

Synaptic modification in cortical structures underlies the acquisition of novel information that results in learning and memory formation. In the adult dentate gyrus, circuit remodeling is boosted by the generation of new granule cells (GCs) that contribute to specific aspects of memory encoding. These forms of plasticity decrease in the aging brain, where both the rate of adult neurogenesis and the speed of morphological maturation of newly generated neurons decline. In the young-adult brain, a brief novel experience accelerates the integration of new neurons. The extent to which such degree of plasticity is preserved in the aging hippocampus remains unclear. In this work, we characterized the time course of functional integration of adult-born GCs in middle-aged mice. We performed whole-cell recordings in developing GCs from Ascl1CreERT2;CAGfloxStopTom mice and found a late onset of functional excitatory synaptogenesis, which occurred at 4 weeks (vs. 2 weeks in young-adult mice). Overall mature excitability and maximal glutamatergic connectivity were achieved at 10 weeks. In contrast, large mossy fiber boutons (MFBs) in CA3 displayed mature morphological features including filopodial extensions at 4 weeks, suggesting that efferent connectivity develops faster than afference. Notably, new GCs from middle-aged mice exposed to enriched environment for 7 days showed an advanced degree of maturity at 3 weeks, revealed by the high frequency of excitatory postsynaptic responses, complex dendritic trees, and large size of MFBs with filopodial extensions. These findings demonstrate that adult-born neurons act as sensors that transduce behavioral stimuli into major network remodeling in the aging brain.

Hyperthermia-induced seizures followed by repetitive stress are associated with age-dependent changes in specific aspects of the mouse stress system

Stress is among the most frequently self-reported factors provoking epileptic seizures in children and adults. It is still unclear, however, why some people display stress-sensitive seizures and others do not. Recently, we showed that young epilepsy patients with stress-sensitive seizures exhibit a dysregulated hypothalamic-pituitary-adrenal (HPA)-axis. Most likely, this dysregulation gradually develops, and is triggered by stressors occurring early in life (early-life stress [ELS]). ELS may be particularly impactful when overlapping with the period of epileptogenesis. To examine this in a controlled and prospective manner, the present study investigated the effect of repetitive variable stressors or control treatment between postnatal day (PND) 12 and 24 in male mice exposed on PND10 to hyperthermia (HT)-induced prolonged seizures (control: normothermia). A number of peripheral and central indices of HPA-axis activity were evaluated at pre-adolescent and young adult age (ie, at PND25 and 90, respectively). At PND25 but not at PND90, body weight gain and absolute as well as relative (to body weight) thymus weight were reduced by ELS (vs control), whereas relative adrenal weight was enhanced, confirming the effectiveness of the stress treatment. Basal and stress-induced corticosterone levels were unaffected, though, by ELS at both ages. HT by itself did not affect any of these peripheral markers of HPA-axis activity, nor did it interact with ELS. However, centrally we did observe age-specific interaction effects of HT and ELS with regard to hippocampal glucocorticoid receptor mRNA expression, neurogenesis with the immature neurone marker doublecortin and the number of hilar (ectopic) granule cells using Prox1 staining. This lends some support to the notion that exposure to repetitive stress after HT-induced seizures may dysregulate central components of the stress system in an age-dependent manner. Such dysregulation could be one of the mechanisms conferring higher vulnerability of individuals with epilepsy to develop seizures in the face of stress.

Molecular Mechanisms of Neurogenic Aging in the Adult Mouse Subventricular Zone

In the adult rodent brain, the continuous production of new neurons by neural stem/progenitor cells (NSPCs) residing in specialized neurogenic niches and their subsequent integration into pre-existing cerebral circuitries supports odour discrimination, spatial learning, and contextual memory capabilities. Aging is recognized as the most potent negative regulator of adult neurogenesis. The neurogenic process markedly declines in the aged brain, due to the reduction of the NSPC pool and the functional impairment of the remaining NSPCs. This decline has been linked to the progressive cognitive deficits of elderly individuals and it may also be involved in the onset/progression of neurological disorders. Since the human lifespan has been dramatically extended, the incidence of age-associated neuropsychiatric conditions in the human population has increased. This has prompted efforts to shed light on the mechanisms underpinning the age-related decline of adult neurogenesis, whose knowledge may foster therapeutic approaches to prevent or delay cognitive alterations in elderly patients. In this review, we summarize recent progress in elucidating the molecular causes of neurogenic aging in the most abundant NSPC niche of the adult mouse brain: the subventricular zone (SVZ). We discuss the age-associated changes occurring both in the intrinsic NSPC molecular networks and in the extrinsic signalling pathways acting in the complex environment of the SVZ niche, and how all these changes may steer young NSPCs towards an aged phenotype.

Adult Hippocampal Neurogenesis: A Coming-of-Age Story

What has become standard textbook knowledge over the last decade was a hotly debated matter a decade earlier: the proposition that new neurons are generated in the adult mammalian CNS. The early discovery by Altman and colleagues in the 1960s was vulnerable to criticism due to the lack of technical strategies for unequivocal demonstration, quantification, and physiological analysis of newly generated neurons in adult brain tissue. After several technological advancements had been made in the field, we published a paper in 1996 describing the generation of new neurons in the adult rat brain and the decline of hippocampal neurogenesis during aging. The paper coincided with the publication of several other studies that together established neurogenesis as a cellular mechanism in the adult mammalian brain. In this Progressions article, which is by no means a comprehensive review, we recount our personal view of the initial setting that led to our study and we discuss some of its implications and developments that followed. We also address questions that remain regarding the regulation and function of neurogenesis in the adult mammalian brain, in particular the existence of neurogenesis in the adult human brain.

Storage fidelity for sequence memory in the hippocampal circuit

Episodic memories have been suggested to be represented by neuronal sequences, which are stored and retrieved from the hippocampal circuit. A special difficulty is that realistic neuronal sequences are strongly correlated with each other since computational memory models generally perform poorly when correlated patterns are stored. Here, we study in a computational model under which conditions the hippocampal circuit can perform this function robustly. During memory encoding, CA3 sequences in our model are driven by intrinsic dynamics, entorhinal inputs, or a combination of both. These CA3 sequences are hetero-associated with the input sequences, so that the network can retrieve entire sequences based on a single cue pattern. We find that overall memory performance depends on two factors: the robustness of sequence retrieval from CA3 and the circuit's ability to perform pattern completion through the feedforward connectivity, including CA3, CA1 and EC. The two factors, in turn, depend on the relative contribution of the external inputs and recurrent drive on CA3 activity. In conclusion, memory performance in our network model critically depends on the network architecture and dynamics in CA3.

High Plasticity of New Granule Cells in the Aging Hippocampus

During aging, the brain undergoes changes that impair cognitive capacity and circuit plasticity, including a marked decrease in production of adult-born hippocampal neurons. It is unclear whether development and integration of those new neurons are also affected by age. Here, we show that adult-born granule cells (GCs) in aging mice are scarce and exhibit slow development, but they display a remarkable potential for structural plasticity. Retrovirally labeled 3-week-old GCs in middle-aged mice were small, underdeveloped, and disconnected. Neuronal development and integration were accelerated by voluntary exercise or environmental enrichment. Similar effects were observed via knockdown of Lrig1, an endogenous negative modulator of neurotrophin receptors. Consistently, blocking neurotrophin signaling by Lrig1 overexpression abolished the positive effects of exercise. These results demonstrate an unparalleled degree of plasticity in the aging brain mediated by neurotrophins, whereby new GCs remain immature until becoming rapidly recruited to the network by activity.

Neural stem cell heterogeneity in the mammalian forebrain

The brain was long considered an organ that underwent very little change after development. It is now well established that the mammalian central nervous system contains neural stem cells that generate progeny that are capable of making new neurons, astrocytes, and oligodendrocytes throughout life. The field has advanced rapidly as it strives to understand the basic biology of these precursor cells, and explore their potential to promote brain repair. The purpose of this review is to present current knowledge about the diversity of neural stem cells in vitro and in vivo, and highlight distinctions between neural stem cell populations, throughout development, and within the niche. A comprehensive understanding of neural stem cell heterogeneity will provide insights into the cellular and molecular regulation of neural development and lifelong neurogenesis, and will guide the development of novel strategies to promote regeneration and neural repair.

Therapeutic potential of systemic brain rejuvenation strategies for neurodegenerative disease

Neurodegenerative diseases are a devastating group of conditions that cause progressive loss of neuronal integrity, affecting cognitive and motor functioning in an ever-increasing number of older individuals. Attempts to slow neurodegenerative disease advancement have met with little success in the clinic; however, a new therapeutic approach may stem from classic interventions, such as caloric restriction, exercise, and parabiosis. For decades, researchers have reported that these systemic-level manipulations can promote major functional changes that extend organismal lifespan and healthspan. Only recently, however, have the functional effects of these interventions on the brain begun to be appreciated at a molecular and cellular level. The potential to counteract the effects of aging in the brain, in effect rejuvenating the aged brain, could offer broad therapeutic potential to combat dementia-related neurodegenerative disease in the elderly. In particular, results from heterochronic parabiosis and young plasma administration studies indicate that pro-aging and rejuvenating factors exist in the circulation that can independently promote or reverse age-related phenotypes. The recent demonstration that human umbilical cord blood similarly functions to rejuvenate the aged brain further advances this work to clinical translation. In this review, we focus on these blood-based rejuvenation strategies and their capacity to delay age-related molecular and functional decline in the aging brain. We discuss new findings that extend the beneficial effects of young blood to neurodegenerative disease models. Lastly, we explore the translational potential of blood-based interventions, highlighting current clinical trials aimed at addressing therapeutic applications for the treatment of dementia-related neurodegenerative disease in humans.

Mechanisms of Hippocampal Aging and the Potential for Rejuvenation

The past two decades have seen remarkable progress in our understanding of the multifactorial drivers of hippocampal aging and cognitive decline. Recent findings have also raised the possibility of functional rejuvenation in the aged hippocampus. In this review, we aim to synthesize the mechanisms that drive hippocampal aging and evaluate critically the potential for rejuvenation. We discuss the functional changes in synaptic plasticity and regenerative potential of the aged hippocampus, followed by mechanisms of microglia aging, and assess the cross talk between these proaging processes. We then examine proyouth interventions that demonstrate significant promise in reversing age-related impairments in the hippocampus and, finally, attempt to look ahead toward novel therapeutics for brain aging.

Transplantation of mesenchymal stem cells reverses behavioural deficits and impaired neurogenesis caused by prenatal exposure to valproic acid

Neurodevelopmental impairment can affect lifelong brain functions such as cognitive and social behaviour, and may contribute to aging-related changes of these functions. In the present study, we hypothesized that bone marrow-derived mesenchymal stem cells (MSC) administration may repair neurodevelopmental behavioural deficits by modulating adult hippocampal neurogenesis. Indeed, postnatal intracerebral transplantation of MSC has restored cognitive and social behaviour in mice prenatally exposed to valproic acid (VPA). MSC transplantation also restored post-developmental hippocampal neurogenesis, which was impaired in VPA-exposed mice displaying delayed differentiation and maturation of newly formed neurons in the granular cell layer of the dentate gyrus. Importantly, a statistically significant correlation was found between neuronal differentiation scores and behavioural scores, suggesting a mechanistic relation between the two. We thus conclude that post-developmental MSC administration can overcome prenatal neurodevelopmental deficits and restore cognitive and social behaviours via modulation of hippocampal adult neurogenesis.

Early pathogenic event of Alzheimer's disease documented in iPSCs from patients with PSEN1 mutations

Alzheimer's disease (AD) is the most common age-related dementia characterized by progressive neuronal loss. However, the molecular mechanisms for the neuronal loss is still debated. Here, we used induced pluripotent stem cells (iPSCs) derived from somatic cells of familial AD patients carrying PSEN1 mutations to study the early pathogenic event of AD. We found that premature neuronal differentiation with decreased proliferation and increased apoptosis occured in AD-iPSC-derived neural progenitor cells (AD-NPCs) once neuronal differentiation was initiated, together with higher levels of Aβ42 and phosphorylated tau. Premature neuronal differentiation in AD-NPCs was caused by PSEN1 mutations and might be correlated to multiple dysregulated processes including but not limited to Wnt-Notch pathway. Our study documented previously unappreciated early NPC dysfunction in AD-NPCs, providing valuable new insights into the early mechanisms underlying AD pathogenesis.

Lycopersicon esculentum Extract Enhances Cognitive Function and Hippocampal Neurogenesis in Aged Mice

A decrease in adult neurogenesis is associated with the aging process, and this decrease is closely related to memory impairment. Tomato (Lycopersicon esculentum) is a fruit with diverse bioactive nutrients that is consumed worldwide. In this study, we investigated the cognition-enhancing effect of tomato ethanolic extracts (TEE) in aged mice. Six weeks of oral TEE administration in 12-month-old aged mice significantly increased their exploration time of novel objects when compared to vehicle-treated mice. The TEE supplement increased doublecortin (DCX)-positive cells and postsynaptic density-95 (PSD95) expression in mice hippocampus. Moreover, we found an increased expression of brain-derived neurotrophic factor (BDNF) and subsequently-activated extracellular-signal-regulated kinase (ERK)/cAMP response element binding (CREB) signaling pathway in the TEE-supplemented mice hippocampus. In conclusion, the oral administration of TEE exhibits a cognition-enhancing effect, and the putative underlying mechanism is the induction of BDNF signaling-mediated proliferation and synapse formation in the hippocampus. These findings indicate that TEE could be a candidate for treatment of age-related memory impairment and neurodegenerative disorders.

The aPKC-CBP Pathway Regulates Adult Hippocampal Neurogenesis in an Age-Dependent Manner

While epigenetic modifications have emerged as attractive substrates to integrate environmental changes into the determination of cell identity and function, specific signals that directly activate these epigenetic modifications remain unknown. Here, we examine the role of atypical protein kinase C (aPKC)-mediated Ser436 phosphorylation of CBP, a histone acetyltransferase, in adult hippocampal neurogenesis and memory. Using a knockin mouse strain (CbpS436A) in which the aPKC-CBP pathway is deficient, we observe impaired hippocampal neuronal differentiation, maturation, and memory and diminished binding of CBP to CREB in 6-month-old CbpS436A mice, but not at 3 months of age. Importantly, elevation of CREB activity rescues these deficits, and CREB activity is reduced whereas aPKC activity is increased in the murine hippocampus as they age from 3 to 6 months regardless of genotype. Thus, the aPKC-CBP pathway is a homeostatic compensatory mechanism that modulates hippocampal neurogenesis and memory in an age-dependent manner in response to reduced CREB activity.

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The aPKC-CBP pathway maintains mature adult hippocampal neuronal differentiation

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The aPKC-CBP pathway is required for hippocampal-dependent memory in mature adult

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The aPKC-CBP pathway is required for CBP binding to CREB in mature adult hippocampi

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Increased CREB activity rescues the deficits due to the deficient aPKC-CBP pathway

Activity Dependency and Aging in the Regulation of Adult Neurogenesis

Age and activity might be considered the two antagonistic key regulators of adult neurogenesis. Adult neurogenesis decreases with age but remains present, albeit at a very low level, even in the oldest individuals. Activity, be it physical or cognitive, increases adult neurogenesis and thereby seems to counteract age effects. It is, thus, proposed that activity-dependent regulation of adult neurogenesis might contribute to some sort of "neural reserve," the brain's ability to compensate functional loss associated with aging or neurodegeneration. Activity can have nonspecific and specific effects on adult neurogenesis. Mechanistically, nonspecific stimuli that largely affect precursor cell stages might be related by the local microenvironment, whereas more specific, survival-promoting effects take place at later stages of neuronal development and require the synaptic integration of the new cell and its particular synaptic plasticity.

FGF Signaling Is Necessary for Neurogenesis in Young Mice and Sufficient to Reverse Its Decline in Old Mice

The mechanisms regulating hippocampal neurogenesis remain poorly understood. Particularly unclear is the extent to which age-related declines in hippocampal neurogenesis are due to an innate decrease in precursor cell performance or to changes in the environment of these cells. Several extracellular signaling factors that regulate hippocampal neurogenesis have been identified. However, the role of one important family, FGFs, remains uncertain. Although a body of literature suggests that FGFs can promote the proliferation of cultured adult hippocampal precursor cells, their requirement for adult hippocampal neurogenesis in vivo and the cell types within the neurogenic lineage that might depend on FGFs remain unclear. Here, specifically targeting adult neural precursor cells, we conditionally express an activated form of an FGF receptor or delete the FGF receptors that are expressed in these cells. We find that FGF receptors are required for neural stem-cell maintenance and that an activated receptor expressed in all precursors can increase the number of neurons produced. Moreover, in older mice, an activated FGF receptor can rescue the age-related decline in neurogenesis to a level found in young adults. These results suggest that the decrease in neurogenesis with age is not simply due to fewer stem cells, but also to declining signals in their niche. Thus, enhancing FGF signaling in precursors can be used to reverse age-related declines in hippocampal neurogenesis.

SIGNIFICANCE STATEMENT

Hippocampal deficits can result from trauma, neurodegeneration, or aging and can lead to loss of memory and mood control. The addition of new neurons to the hippocampus facilitates memory formation, but how this process is regulated and how we might manipulate it to reverse hippocampal dysfunction remains unclear. The FGF signaling pathway has been hypothesized to be important, but its role in generating new neurons had been poorly defined. Our study indicates that FGF signaling maintains and expands subsets of neural precursor cells. Moreover, in older mice, increasing FGF signaling is sufficient to reverse the decline in neuron production to levels found in young adults, providing a potential means of reversing age-related hippocampal deficits.