Dietary restriction increases hippocampal neurogenesis by molecular mechanisms independent of NMDA receptors

The 5:2 diet does not increase adult hippocampal neurogenesis or enhance spatial memory in mice

New neurones are generated throughout life in the mammalian brain in a process known as adult hippocampal neurogenesis (AHN). Since this phenomenon grants a high degree of neuroplasticity influencing learning and memory, identifying factors that regulate AHN may be important for ameliorating age-related cognitive decline. Calorie restriction (CR) has been shown to enhance AHN and improve memory, mediated by the stomach hormone, ghrelin. Intermittent fasting (IF), a dietary strategy offering more flexibility than conventional CR, has also been shown to promote aspects of AHN. The 5:2 diet is a popular form of IF; however, its effects on AHN are not well characterised. To address this, we quantified AHN in adolescent and adult wild-type and ghrelin-receptor-deficient mice following 6 weeks on a 5:2 diet. We report an age-related decline in neurogenic processes. However, the 5:2 diet does not increase AHN nor enhance memory performance, suggesting that this specific form of IF is ineffective in promoting brain plasticity to support learning.

Adult neural stem cells and neurogenesis are resilient to intermittent fasting

Intermittent fasting (IF) is a promising strategy to counteract ageing shown to increase the number of adult-born neurons in the dentate gyrus of mice. However, it is unclear which steps of the adult neurogenesis process are regulated by IF. The number of adult neural stem cells (NSCs) decreases with age in an activation-dependent manner and, to counteract this loss, adult NSCs are found in a quiescent state which ensures their long-term maintenance. We aimed to determine if and how IF affects adult NSCs in the hippocampus. To identify the effects of every-other-day IF on NSCs and all following steps in the neurogenic lineage, we combined fasting with lineage tracing and label retention assays. We show here that IF does not affect NSC activation or maintenance and, that contrary to previous reports, IF does not increase neurogenesis. The same results are obtained regardless of strain, sex, diet length, tamoxifen administration or new-born neuron identification method. Our data suggest that NSCs maintain homeostasis upon IF and that this intervention is not a reliable strategy to increase adult neurogenesis.

Programmed ageing: decline of stem cell renewal, immunosenescence, and Alzheimer's disease

The characteristic maximum lifespan varies enormously across animal species from a few hours to hundreds of years. This argues that maximum lifespan, and the ageing process that itself dictates lifespan, are to a large extent genetically determined. Although controversial, this is supported by firm evidence that semelparous species display evolutionarily programmed ageing in response to reproductive and environmental cues. Parabiosis experiments reveal that ageing is orchestrated systemically through the circulation, accompanied by programmed changes in hormone levels across a lifetime. This implies that, like the circadian and circannual clocks, there is a master 'clock of age' (circavital clock) located in the limbic brain of mammals that modulates systemic changes in growth factor and hormone secretion over the lifespan, as well as systemic alterations in gene expression as revealed by genomic methylation analysis. Studies on accelerated ageing in mice, as well as human longevity genes, converge on evolutionarily conserved fibroblast growth factors (FGFs) and their receptors, including KLOTHO, as well as insulin-like growth factors (IGFs) and steroid hormones, as key players mediating the systemic effects of ageing. Age-related changes in these and multiple other factors are inferred to cause a progressive decline in tissue maintenance through failure of stem cell replenishment. This most severely affects the immune system, which requires constant renewal from bone marrow stem cells. Age-related immune decline increases risk of infection whereas lifespan can be extended in germfree animals. This and other evidence suggests that infection is the major cause of death in higher organisms. Immune decline is also associated with age-related diseases. Taking the example of Alzheimer's disease (AD), we assess the evidence that AD is caused by immunosenescence and infection. The signature protein of AD brain, Aβ, is now known to be an antimicrobial peptide, and Aβ deposits in AD brain may be a response to infection rather than a cause of disease. Because some cognitively normal elderly individuals show extensive neuropathology, we argue that the location of the pathology is crucial - specifically, lesions to limbic brain are likely to accentuate immunosenescence, and could thus underlie a vicious cycle of accelerated immune decline and microbial proliferation that culminates in AD. This general model may extend to other age-related diseases, and we propose a general paradigm of organismal senescence in which declining stem cell proliferation leads to programmed immunosenescence and mortality.

Neurogenesis in the Maternal Rodent Brain: Impacts of Gestation-Related Hormonal Regulation, Stress, and Obesity

In order to maintain maternal behavior, it is important that the maternal rodent brain promotes neurogenesis. Maternal neurogenesis is altered by the dynamic shifts in reproductive hormone levels during pregnancy. Thus, lifestyle events such as gestational stress and obesity that can affect hormone production will affect neuroendocrine control of maternal neurogenesis. However, there is a lack of information about the regulation of maternal neurogenesis by placental hormones, which are key components of the reproductive hormonal profile during pregnancy. There is also little known about how maternal neurogenesis can be affected by health concerns such as gestational stress and obesity, and its relationship to peripartum mental health disorders. This review summarizes the changing levels of neurogenesis in mice and rats during gestation and postpartum as well as regulation of neurogenesis by pregnancy-related hormones. The influence of neurogenesis on maternal behavior is also discussed while bringing attention to the effect of health-related concerns during gestation, such as stress and obesity on neuroendocrine control of maternal neurogenesis. In doing so, this review identifies the gaps in the literature and specifically emphasizes the importance of further research on maternal brain physiology to address them.

Mini review: The relationship between energy status and adult hippocampal neurogenesis

The ability to generate new hippocampal neurons throughout adulthood and successfully integrate them into existing neural networks is critical to cognitive function, while disordered regulation of this process results in neurodegenerative or psychiatric disease. Consequently, identifying the molecular mechanisms promoting homeostatic hippocampal neurogenesis in adults is essential to understanding the etiologies of these disorders and developing therapeutic interventions. For example, recent evidence identifies a strong association between metabolic function and adult hippocampal neurogenesis. Hippocampal neural stem cell (NSC) fate dynamically fluctuates with changes in substrate availability and energy status (AMP/ATP and NAD⁺/NADH ratios). Furthermore, many metabolic hormones, such as insulin, insulin-like growth factors, and leptin exhibit dual functions also modulating hippocampal neurogenesis and neuron survivability. These diverse metabolic inputs to NSC's from various tissues seemingly suggest the existence of a system in which energy status can finely modulate hippocampal neurogenesis. Supporting this hypothesis, interventions promoting energy balance, such as caloric restriction, intermittent fasting, and exercise, have shown encouraging potential enhancing hippocampal neurogenesis and cognitive function. Overall, there is a clear relationship between whole body energy status, adult hippocampal neurogenesis, and neuron survival; however, the molecular mechanisms underlying this phenomenon are multifaceted. Thus, the aim of this review is to analyze the literature investigating energy status-mediated regulation of adult neurogenesis in the hippocampus, highlight the neurocircuitry and intracellular signaling involved, and propose impactful future directions in the field.

Neurogenesis in the adult hippocampus: history, regulation, and prospective roles

BACKGROUND

The hippocampus is one of the sites in the mammalian brain that is capable of continuously generating controversy. Adult neurogenesis is a remarkable process, and yet an intensely debatable topic in contemporary neuroscience due to its distinctiveness and conceivable impact on neural activity. The belief that neurogenesis continues through adulthood has provoked remarkable efforts to describe how newborn neurons differentiate and incorporate into the adult brain. It has also encouraged studies that investigate the consequences of inadequate neurogenesis in neuropsychiatric and neurodegenerative diseases and explore the potential role of neural progenitor cells in brain repair. The adult nervous system is not static; it is subjected to morphological and physiological alterations at various levels. This plastic mechanism guarantees that the behavioral regulation of the adult nervous system is adaptable in response to varying environmental stimuli. Three regions of the adult brain, the olfactory bulb, the hypothalamus, and the hippocampal dentate gyrus, contain new-born neurons that exhibit an essential role in the natural functional circuitry of the adult brain. Purpose/Aim: This article explores current advancements in adult hippocampal neurogenesis by presenting its history and evolution and studying its association with neural plasticity. The article also discusses the prospective roles of adult hippocampal neurogenesis and describes the intracellular, extracellular, pathological, and environmental factors involved in its regulation. Abbreviations AHN Adult hippocampal neurogenesis AKT Protein kinase B BMP Bone Morphogenic Protein BrdU Bromodeoxyuridine CNS Central nervous system DG Dentate gyrus DISC1 Disrupted-in-schizophrenia 1 FGF-2 Fibroblast Growth Factor 2 GABA Gamma-aminobutyric acid Mbd1 Methyl-CpG-binding domain protein 1 Mecp2 Methyl-CpG-binding protein 2 mTOR Mammalian target of rapamycin NSCs Neural stem cells OB Olfactory bulb; P21: cyclin-dependent kinase inhibitor 1 RBPj Recombination Signal Binding protein for Immunoglobulin Kappa J Region RMS Rostral migratory Stream SGZ Subgranular zone Shh Sonic hedgehog SOX2 SRY (sex determining region Y)-box 2 SVZ Subventricular zone Wnt3 Wingless-type mouse mammary tumor virus.

Stress and adolescent hippocampal neurogenesis: diet and exercise as cognitive modulators

Adolescence is a critical period for brain maturation. Deciphering how disturbances to the central nervous system at this time affect structure, function and behavioural outputs is important to better understand any long-lasting effects. Hippocampal neurogenesis occurs during development and continues throughout life. In adulthood, integration of these new cells into the hippocampus is important for emotional behaviour, cognitive function and neural plasticity. During the adolescent period, maturation of the hippocampus and heightened levels of hippocampal neurogenesis are observed, making alterations to neurogenesis at this time particularly consequential. As stress negatively affects hippocampal neurogenesis, and adolescence is a particularly stressful time of life, it is important to investigate the impact of stressor exposure at this time on hippocampal neurogenesis and cognitive function. Adolescence may represent not only a time for which stress can have long-lasting effects, but is also a critical period during which interventions, such as exercise and diet, could ameliorate stress-induced changes to hippocampal function. In addition, intervention at this time may also promote life-long behavioural changes that would aid in fostering increased hippocampal neurogenesis and cognitive function. This review addresses both the acute and long-term stress-induced alterations to hippocampal neurogenesis and cognition during the adolescent period, as well as changes to the stress response and pubertal hormones at this time which may result in differential effects than are observed in adulthood. We hypothesise that adolescence may represent an optimal time for healthy lifestyle changes to have a positive and long-lasting impact on hippocampal neurogenesis, and to protect against stress-induced deficits. We conclude that future research into the mechanisms underlying the susceptibility of the adolescent hippocampus to stress, exercise and diet and the consequent effect on cognition may provide insight into why adolescence may be a vital period for correct conditioning of future hippocampal function.

The Benefits of Exercise on Structural and Functional Plasticity in the Rodent Hippocampus of Different Disease Models

In this review, the benefits of physical exercise on structural and functional plasticity in the hippocampus are discussed. The evidence is clear that voluntary exercise in rats and mice can lead to increases in hippocampal neurogenesis and enhanced synaptic plasticity which ultimately result in improved performance in hippocampal-dependent tasks. Furthermore, in models of neurological disorders, including fetal alcohol spectrum disorders, traumatic brain injury, stroke, and neurodegenerative disorders including Alzheimer's, Parkinson's and Huntington's disease exercise can also elicit beneficial effects on hippocampal function. Ultimately this review highlights the multiple benefits of exercise on hippocampal function in both the healthy and the diseased brain.

Moderate maternal food restriction in mice impairs physical growth, behavior, and neurodevelopment of offspring

Intrauterine growth retardation (IUGR) occurs in 3% to 7% of all pregnancies. Recent human studies have indicated that neurodevelopmental disabilities, learning disorders, memory impairment, and mood disturbance are common in IUGR offspring. However, the interactions between IUGR and neurodevelopmental disorders are unclear because of the wide range of causes of IUGR, such as maternal malnutrition, placental insufficiency, pregnancy toxemia, and fetal malformations. Meanwhile, many studies have shown that moderate food restriction enhances spatial learning and improves mood disturbance in adult humans and animals. To date, the effects of maternal moderate food restriction on fetal brain remain largely unknown. In this study, we hypothesized that IUGR would be caused by even moderate food restriction in pregnant females and that the offspring would have neurodevelopmental disabilities. Mid-pregnant mice received moderate food restriction through the early lactation period. The offspring were tested for aspects of physical development, behavior, and neurodevelopment. The results showed that moderate maternal food restriction induced IUGR. Offspring had low birth weight and delayed development of physical and coordinated movement. Moreover, IUGR offspring exhibited mental disabilities such as anxiety and poor cognitive function. In particular, male offspring exhibited significantly impaired cognitive function at 3 weeks of age. These results suggested that a restricted maternal diet could be a risk factor for developmental disability in IUGR offspring and that male offspring might be especially susceptible.

Role of adult neurogenesis in hippocampal-cortical memory consolidation

Acquired memory is initially dependent on the hippocampus (HPC) for permanent memory formation. This hippocampal dependency of memory recall progressively decays with time, a process that is associated with a gradual increase in dependency upon cortical structures. This process is commonly referred to as systems consolidation theory. In this paper, we first review how memory becomes hippocampal dependent to cortical dependent with an emphasis on the interactions that occur between the HPC and cortex during systems consolidation. We also review the mechanisms underlying the gradual decay of HPC dependency during systems consolidation from the perspective of memory erasures by adult hippocampal neurogenesis. Finally, we discuss the relationship between systems consolidation and memory precision.

Nutrition, adult hippocampal neurogenesis and mental health

INTRODUCTION

Over the last 8 years, emerging studies bridging the gap between nutrition and mental health have resolutely established that learning and memory abilities as well as mood can be influenced by diet. However, the mechanisms by which diet modulates mental health are still not well understood. Sources of data In this article, a review of the literature was conducted using PubMed to identify studies that provide functional implications of adult hippocampal neurogenesis (AHN) and its modulation by diet.

AREAS OF AGREEMENT

One of the brain structures associated with learning and memory as well as mood is the hippocampus. Importantly, the hippocampus is one of the two structures in the adult brain where the formation of newborn neurons, or neurogenesis, persists.

AREAS OF CONTROVERSY

The exact roles of these newborn neurons in learning, memory formation and mood regulation remain elusive.

GROWING POINTS

Nevertheless, there has been accumulating evidence linking cognition and mood to neurogenesis occurring in the adult hippocampus. Therefore, modulation of AHN by diet emerges as a possible mechanism by which nutrition impacts on mental health.

AREAS TIMELY FOR DEVELOPING RESEARCH

This area of investigation is new and needs attention because a better understanding of the neurological mechanisms by which nutrition affect mental health may lead to novel dietary approaches for disease prevention, healthier ageing and discovery of new therapeutic targets for mental illnesses.

Secretin deficiency causes impairment in survival of neural progenitor cells in mice

Hippocampal neurogenesis is the lifelong production of new neurons in the central nervous system (CNS), and affects many physiological and pathophysiological conditions, including neurobehavioral disorders. The early postnatal stage is the most prominent neurogenesis period; however, the functional role of neurogenesis in this developing stage has not been well characterized. To understand the role of hippocampal neurogenesis in the postnatal developing period, we analyzed secretin, a neuropeptide, which is expressed significantly higher in the development stage. Secretin is a pleiotropic neuropeptide hormone that belongs to the secretin/VIP/glucagon peptide family. Although secretin was originally isolated in the gastrointestinal system, it has been found that secretin itself acts as a neuropeptide in the CNS. Here, we report a new function of secretin as a survival factor for neural progenitor cells in the hippocampus. We found that secretin-deficient mice exhibit decreased numbers of BrdU-labeled new neurons and dramatically increased apoptosis of doublecortin-positive neural progenitor cells in the subgranular zone of the dentate gyrus (DG) during the early postnatal period. Furthermore, we found that reduced survival of neural progenitor cells leads to decreased volume of DG, reduced long-term potentiation and impaired spatial learning ability in adults. Our studies demonstrate that secretin has important implications for neurogenesis in postnatal development, and affects neurobehavioral function in the adult mouse.

Impact of diet on adult hippocampal neurogenesis

Research over the last 5 years has firmly established that learning and memory abilities, as well as mood, can be influenced by diet, although the mechanisms by which diet modulates mental health are not well understood. One of the brain structures associated with learning and memory, as well as mood, is the hippocampus. Interestingly, the hippocampus is one of the two structures in the adult brain where the formation of newborn neurons, or neurogenesis, persists. The level of neurogenesis in the adult hippocampus has been linked directly to cognition and mood. Therefore, modulation of adult hippocampal neurogenesis (AHN) by diet emerges as a possible mechanism by which nutrition impacts on mental health. In this study, we give an overview of the mechanisms and functional implications of AHN and summarize recent findings regarding the modulation of AHN by diet.

Molecular Mechanism of Dietary Restriction in Neuroprevention and Neurogenesis: Involvement of Neurotrophic Factors

Dietary restriction (DR) is the most efficacious intervention for retarding the deleterious effects of aging. DR increases longevity, decreases the occurrence and severity of age-related diseases, and retards the physiological decline associated with aging. The beneficial effects of DR have been mostly studied in non-neuronal tissues. However, several studies have showed that DR attenuate neuronal loss after several different insults including exposure to kainate, ischemia, and MPTR Moreover, administration of the non-metabolizable glucose analog 2-deoxy-D-glucose (2DG) could mimic the neuroprotective effect of DR in rodent, presumably by limiting glucose availability at the cellular level. Based on the studies of chemically induced D.R., it has been proposed that the mechanism whereby DR and 2DG protect neurons is largely mediated by stress response proteins such as HSP70 and GRP78 which are increased in neurons of rats and mice fed a DR regimen. In addition, D.R., as mild metabolic stress, could lead to the increased activity in neuronal circuits and thus induce expression of neurotrophic factors. Interestingly, such increased neuronal activities also enhance neurogenesis in the brains of adult rodents. In this review, we focus on what is known regarding molecular mechanisms of the protective role of DR in neurodegenerative diseases and aging process. Also, we propose that DR is a mild cellular stress that stimulates production of neurotrophic factors, which are major regulators of neuronal survival, as well as neurogenesis in adult brain.

Driving GDNF expression: the green and the red traffic lights

Glial cell line-derived neurotrophic factor (GDNF) is widely recognized as a potent survival factor for dopaminergic neurons of the nigrostriatal pathway that degenerate in Parkinson's disease (PD). In animal models of PD, GDNF delivery to the striatum or the substantia nigra protects dopaminergic neurons against subsequent toxin-induced injury and rescues previously damaged neurons, promoting recovery of the motor function. Thus, GDNF was proposed as a potential therapy to PD aimed at slowing down, halting or reversing neurodegeneration, an issue addressed in previous reviews. However, the use of GDNF as a therapeutic agent for PD is hampered by the difficulty in delivering it to the brain. Another potential strategy is to stimulate the endogenous expression of GDNF, but in order to do that we need to understand how GDNF expression is regulated. The aim of this review is to do a comprehensive analysis of the state of the art on the control of endogenous GDNF expression in the nervous system, focusing mainly on the nigrostriatal pathway. We address the control of GDNF expression during development, in the adult brain and after injury, and how damaged neurons signal glial cells to up-regulate GDNF. Pharmacological agents or natural molecules that increase GDNF expression and show neuroprotective activity in animal models of PD are reviewed. We also provide an integrated overview of the signalling pathways linking receptors for these molecules to the induction of GDNF gene, which might also become targets for neuroprotective therapies in PD.

Eat less, live longer? New insights into the role of caloric restriction in the brain

Caloric restriction has wide-ranging health benefits and may offer protection against age-related neuronal loss and neurodegenerative disorders such as Alzheimer's disease, possibly via enhanced adult neurogenesis. While a variety of interventions such as exercise increase neurogenesis, recent work suggests that exercise and caloric restriction may work to enhance neurogenesis by different neurobiological mechanisms, suggesting a role for both diet and exercise in disease prevention.

Neurogenesis and schizophrenia: dividing neurons in a divided mind?

Forty years after the initial discovery of neurogenesis in the postnatal brain of the rat, convincing evidence has been accrued that functional neurons are generated throughout the entire lifespan, particularly in the dentate gyrus (DG) and the subventricular zone (SVZ). This phenomenon has been termed adult neurogenesis (AN) and while it was detected in all examined mammalian species including humans, the physiological role of this process remains unknown. Although a plethora of animal studies indicate an involvement of AN in the pathophysiology of depression, this view has recently kindled considerable controversy. Pertinent studies in humans failed to confirm a role of reduced hippocampal neural stem cell proliferation (NSP) in depression but suggest a contribution to the pathophysiology of schizophrenia. The functional relevance of disturbed AN may encompass erroneous temporal encoding of new memory traces, thereby contributing to cognitive deficits observed in schizophrenia. This AN-hypothesis of schizophrenia is supported by neuroimaging, as well as by several genetically modified rodent models, e.g. reelin and NPAS3 knockout mice. Furthermore, several genes impacting on AN, including NPAS3, were also found to be associated with schizophrenia by case-control studies. In conclusion, several lines of evidence suggest that reduced AN may contribute to the etiopathogenesis of schizophrenic disorders, whereas it does not seem to be a critical risk factor for affective disorders.

Running wheel exercises accelerate neuronal turnover in mouse dentate gyrus

Neurogenesis continues throughout adulthood in mouse dentate gyrus, and is influenced by environmental, endocrine, and pharmacological conditions. Although running wheel exercises have been reported to enhance neurogenesis, the effects on neuronal death in dentate gyrus are not well understood. The precise control of the production and elimination of neurons is thought to be important for the maintenance of a relatively constant number of neurons in the adult nervous system and for the regulation of adult brain function. We report here that running wheel exercises enhance the death of pre-existing neurons as well as neurogenesis in dentate gyrus. In addition, we analyzed mice lacking an NMDA receptor varepsilon1 subunit, and found that the enhancement of the neuronal death by the exercises is suppressed in the varepsilon1 subunit knockout mice. These results suggest that running wheel exercises accelerate neuronal turnover in mouse dentate gyrus, through the activation of NMDA receptors.