Energy intake, meal frequency, and health: a neurobiological perspective

Lifespan extension in Caenorhabditis elegans by complete removal of food

A partial reduction in food intake has been found to increase lifespan in many different organisms. We report here a new dietary restriction regimen in the nematode Caenorhabditis elegans, based on the standard agar plate lifespan assay, in which adult worms are maintained in the absence of a bacterial food source. These findings represent the first report in any organism of lifespan extension in response to prolonged starvation. Removal of bacterial food increases lifespan to a greater extent than partial reduction of food through a mechanism that is distinct from insulin/IGF-like signaling and the Sir2-family deacetylase, SIR-2.1. Removal of bacterial food also increases lifespan when initiated in postreproductive adults, suggesting that dietary restriction started during middle age can result in a substantial longevity benefit that is independent of reproduction.

Gender differences in free radical homeostasis during aging: shorter-lived female C57BL6 mice have increased oxidative stress

Gender is a profound determinant of aging and lifespan, but little is known about gender differences in free radical homeostasis. Free radicals are proposed as key elements in the multifactorial process of aging and it is predicted that the longer-lived gender should have lower levels of oxidative stress. While the majority of studies on aging have included a single gender, recent studies in rats compared genders and found that females, the longer-lived sex, had lower oxidative stress and mitochondrial dysfunction than males. We explored the association between oxidative stress and gender-specific aging in C57BL6 mice, in which females are the shorter-lived gender. Reactive oxygen species (ROS) were measured in young and old mice by confocal imaging of dihydroethidium (DHE) oxidation in the brain, and by electron paramagnetic resonance (EPR) spectrometry of isolated brain mitochondria. Both genders exhibited significant age-dependent increases in ROS. However, females had a greater increase with age than males in DHE oxidation but not mitochondrial EPR. Superoxide dismutase 1 (Sod1) and glutathione peroxidase 1 (GPx1) protein levels were lower in old females. To determine whether enhancing antioxidant defenses would eliminate gender differences in lifespan, mice were treated chronically with a superoxide dismutase mimetic. Treatment blocked the age-dependent increase in ROS, with a greater effect in females on DHE oxidation, but not mitochondrial EPR. Treatment also increased lifespan to a greater degree in females. Our results indicate that differences in ROS homeostasis contribute to gender divergence in survival, but also suggest that mitochondrial superoxide production may not be primarily responsible for gender differences in lifespan.

Mitochondrial regulation of neuronal plasticity

The structure and function of neurons is dynamic during development and in adaptive responses of the adult nervous system to environmental demands. The mechanisms that regulate neuronal plasticity are poorly understood, but are believed to involve neurotransmitter and neurotrophic factor signaling pathways. In the present article, I review emerging evidence that mitochondria play important roles in regulating developmental and adult neuroplasticity. In neurons, mitochondria are located in axons, dendrites, growth cones and pre- and post-synaptic terminals where their movements and functions are regulated by local signals such as neurotrophic factors and calcium influx. Mitochondria play important roles in fundamental developmental processes including the establishment of axonal polarity and the regulation of neurite outgrowth, and are also involved in synaptic plasticity in the mature nervous system. Abnormalities in mitochondria are associated with neurodegenerative and psychiatric disorders, suggesting a therapeutic potential for approaches that target mitochondrial mechanisms.

The metabolic syndrome, omega-3 fatty acids and inflammatory processes in relation to schizophrenia

Although schizophrenia is normally regarded as a brain disease, there is clear evidence that schizophrenia is strongly associated with a variety of physical conditions. These include an increased rate of the metabolic syndrome and its physical complications including diabetes and coronary heart disease, and a reduced rate of rheumatoid arthritis. It is argued that these associations may point to a commonality of some aetiological factors. Evidence implicating omega-3 fatty acids in all of these disorders is presented. The associations may derive either from genetic or from environmental factors, including nutrition. Further investigation of these associations may give important clues regarding the aetiology of schizophrenia.

Dietary restriction changes behaviours in brain-derived neurotrophic factor heterozygous mice: role of serotonergic system

Accumulating evidence has suggested that brain-derived neurotrophic factor (BDNF) plays a role in eating behaviours, and that BDNF-heterozygous (+/-) mice exhibit abnormal behaviours (e.g. obesity, anxiety and aggression). The present study was undertaken to determine whether or not dietary restriction (DR) alters the behaviours in BDNF(+/-) mice, as DR has been shown to exert a number of beneficial effects on the brain. Eight-week-old male wild-type (+/+) and BDNF(+/-) mice were divided into two groups, ad libitum (AL) diet group and DR group, for 16 weeks. After carrying out a behavioural evaluation, we determined the BDNF mRNA levels, as well as mRNA levels for subtypes (5-HT(1A), 5-HT(1B), 5-HT(2A) and 5-HT(2C)) of the 5-HT receptor and 5-HT transporter (5-HTT), protein levels of BDNF and concentrations of 5-HT and 5-HIAA in the hypothalamus, hippocampus and frontal cortex. DR significantly ameliorated behaviours including obesity, anxiety and aggression in BDNF(+/-) mice. The concentrations of 5-HT and 5-HIAA in the frontal cortex, and 5-HT in the hippocampus, of BDNF(+/-) mice were significantly lower than those of wild-type mice. Interestingly, DR significantly increased the levels of 5-HT and 5-HIAA in the frontal cortex of BDNF(+/-) mice. These findings suggest that DR may alter the behaviours in BDNF(+/-) mice, and that the 5-HT system may be implicated in the beneficial effects of DR on these behaviours.

Neurohormetic phytochemicals: Low-dose toxins that induce adaptive neuronal stress responses

Diets rich in vegetables and fruits are associated with reduced risk of several major diseases, including neurodegenerative disorders. Although some beneficial phytochemicals might function solely as antioxidants, it is becoming clear that many of the beneficial chemicals in vegetables and fruits evolved as toxins (to dissuade insects and other predators) that, at subtoxic doses, activate adaptive cellular stress-response pathways in a variety of cells including neurons. Examples of such 'preconditioning' or 'neurohormesis' pathways include those involving cell-survival signaling kinases, the transcription factors NRF2 and CREB, and histone deacetylases of the sirtuin family. In these ways, neurohormetic phytochemicals such as resveratrol, sulforaphanes and curcumin might protect neurons against injury and disease by stimulating the production of antioxidant enzymes, neurotrophic factors, protein chaperones and other proteins that help cells to withstand stress. Thus, as we discuss in this review, highly conserved longevity and survival pathways in neurons are the targets of many phytochemicals.

[Glycotoxins and cellular dysfunction. A new mechanism for understanding the preventive effects of lifestyle modifications]

Recently the AGE-RAGE interaction was identified as a potential mechanism underlying chronic and inflammatory diseases like atherosclerosis, diabetes mellitus and kidney disease. Advanced glycation end products (AGEs) are the derivatives of glucose-protein or glucose-lipid reactions and are mainly generated from the diet (depending on intensity of heating, cooking time and oxygenation). Binding of AGEs or other ligands to the AGE receptor (RAGE) results in cellular activation, i.e. increased expression of inflammatory mediators and oxidative stress. Diet-derived AGEs thus induce deleterious effects on tissues and the cardiovascular system. Recent research also found that other lifestyle factors are associated with pronounced inflammatory activation, e.g. psychosocial stress and smoking. In addition, each intake of meals is associated with proinflammatory cellular changes. The AGE-RAGE model and investigations of the underlying cellular mechanisms thus may lead to a better understanding of the health benefits of diets (Mediterranean diet, uncooked vegetarian diets), caloric restriction and intermittent fasting. The clinical impact of low-AGE diets and fasting and the interaction between stress and food intake should be further investigated in controlled trials.

Interferon-gamma is up-regulated in the hippocampus in response to intermittent fasting and protects hippocampal neurons against excitotoxicity

Dietary restriction (DR) increases the life span of many different organisms, and recent findings have demonstrated neuroprotective effects of DR in rodent and nonhuman primate models of neurodegenerative disorders. The neuroprotective mechanism of action of DR is unknown, but it may result from a mild cellular stress response involving increased production of neurotrophic factors. Because several different cytokines are known to be up-regulated in brain cells in response to stress, we determined whether DR affected cytokine expression in the rat brain. Levels of expression of interferon-gamma (IFN-gamma) and its receptor were significantly increased in the hippocampus of rats that had been maintained on an intermittent fasting DR regimen compared with rats on the ad libitum control diet. Pretreatment of embryonic rat hippocampal cell cultures with IFN-gamma protected neurons against glutamate-induced death. IFN-gamma-mediated neuroprotection was associated with an enhanced recovery of intracellular Ca(2+) concentrations following exposure to glutamate. Our data show that intermittent fasting DR stimulates IFN-gamma-mediated neuroprotective signaling in the hippocampus, suggesting a role for this cytokine in the previously reported ability of DR to protect neurons in animal models of severe epileptic seizures, stroke, and neurodegenerative disorders.

The potential for dietary restriction to increase longevity in humans: extrapolation from monkey studies

Based on results emerging from long-term studies of dietary restriction in rhesus monkeys, we offer our views regarding whether dietary restriction can increase longevity in humans. Because lifespan data in monkeys remain inconclusive currently, we respond that "we do not for sure". Based on the vast literature regarding the effects of healthy, low calorie diets on health and longevity in a wide range of species, including humans, and based on data emerging from monkey studies suggesting that dietary restriction improves markers of disease risk and health, we respond that "we think so." Because it is unlikely that an experimental study will ever be designed to address this question in humans, we respond that "we think we will never know for sure." We suggest that debate of this question is clearly an academic exercise; thus, we would suggest that the more compelling discussion should focus on whether basic mechanisms of DR can be discovered and if such discoveries can lead to the development of effective DR mimetics. Even if proof that DR or DR mimetics can increase longevity in humans will likely never emerge, we would suggest that endpoints regarding disease risk and disease incidence as well as maintenance of function can be examined in human clinical trials, and that these will be highly relevant for evaluating the effectiveness of such treatments.

Ageing and neuronal vulnerability

Everyone ages, but only some will develop a neurodegenerative disorder in the process. Disease might occur when cells fail to respond adaptively to age-related increases in oxidative, metabolic and ionic stress, thereby resulting in the accumulation of damaged proteins, DNA and membranes. Determinants of neuronal vulnerability might include cell size and location, metabolism of disease-specific proteins and a repertoire of signal transduction pathways and stress resistance mechanisms. Emerging evidence on protein interaction networks that monitor and respond to the normal ageing process suggests that successful neural ageing is possible for most people, but also cautions that cures for neurodegenerative disorders are unlikely in the near future.

Calorie restriction mimetics: an emerging research field

When considering all possible aging interventions evaluated to date, it is clear that calorie restriction (CR) remains the most robust. Studies in numerous species have demonstrated that reduction of calories 30-50% below ad libitum levels of a nutritious diet can increase lifespan, reduce the incidence and delay the onset of age-related diseases, improve stress resistance, and decelerate functional decline. A current major focus of this research area is whether this nutritional intervention is relevant to human aging. Evidence emerging from studies in rhesus monkeys suggests that their response to CR parallels that observed in rodents. To assess CR effects in humans, clinical trials have been initiated. However, even if results from these studies could eventually substantiate CR as an effective pro-longevity strategy for humans, the utility of this intervention would be hampered because of the degree and length of restriction required. As an alternative strategy, new research has focused on the development of 'CR mimetics'. The objective of this strategy is to identify compounds that mimic CR effects by targeting metabolic and stress response pathways affected by CR, but without actually restricting caloric intake. For example, drugs that inhibit glycolysis (2-deoxyglucose), enhance insulin action (metformin), or affect stress signaling pathways (resveratrol), are being assessed as CR mimetics (CRM). Promising results have emerged from initial studies regarding physiological responses which resemble those observed in CR (e.g. reduced body temperature and plasma insulin) as well as protection against neurotoxicity (e.g. enhanced dopamine action and up-regulated neurotrophic factors). Ultimately, lifespan analyses in addition to expanded toxicity studies must be accomplished to fully assess the potential of any CRM. Nonetheless, this strategy clearly offers a very promising and expanding research endeavor.

Striatal cell signaling in chronically food-restricted rats under basal conditions and in response to brief handling

Chronic food restriction increases exploratory behavior, cognitive function, and the rewarding effects of abused drugs. Recently, striatal neuroadaptations that may be involved in these effects were observed. Specifically, D-1 dopamine (DA) receptor agonist challenge produced stronger activation of extracellular signal-regulated kinase (ERK), calcium-calmodulin-dependent kinase II (CaMKII), and the nuclear transcription factor cAMP response element binding protein (CREB) in nucleus accumbens (NAc) of food-restricted (FR) relative to ad libitum fed (AL) rats. Further, when FR rats were injected intracerebroventricularly (i.c.v.) with vehicle (saline) they displayed stronger activation of c-Jun N-terminal protein kinase (JNK), ERK and CaMKII than did AL rats. It is not known to what extent the latter effects represent the basal state of FR rats or an amplified response to the brief handling involved in the i.c.v. injection procedure. Using Western blotting it was found that basal phospho-JNK is higher in caudate-putamen (CPu) and NAc of FR relative to AL rats. Interestingly, brief handling decreased phospho-JNK levels in FR subjects. Basal phospho-ERK1/2 also tended to be elevated in CPu and NAc of FR rats but the elevation was not significant. However, phospho-MEK--the activated kinase upstream of ERK1/2--was significantly elevated in NAc of FR rats. Neither ERK1/2 nor MEK were activated by brief handling. CaMKII was selectively activated by handling in NAc of FR rats, suggesting a state-dependent response to a salient event. Given the established involvement of mitogen-activated protein kinase (MAPK) and CaMKII in synaptic plasticity, learning and memory, the increase in basal phospho-MEK and hyperresponsiveness of CaMKII in NAc may represent adaptive cellular responses to persistent negative energy balance that facilitate associative learning in connection with food-seeking.

Cardioprotection by intermittent fasting in rats

BACKGROUND

Intermittent fasting (IF), a dietary regimen in which food is available only every other day, increases the life span and reduces the incidence of age-associated diseases in rodents. We have reported neuroprotective effects of IF against ischemic injury of the brain. In this study, we examined the effects of IF on ischemic injury of the heart in rats.

METHODS AND RESULTS

After 3 months of IF or regular every-day feeding (control) diets started in 2-month-old rats, myocardial infarction (MI) was induced by coronary artery ligation. Twenty-four hours after MI, its size in the IF group was 2-fold smaller, the number of apoptotic myocytes in the area at risk was 4-fold less, and the inflammatory response was significantly reduced compared with the control diet group. Serial echocardiography revealed that during 10 weeks after MI (with continuation of the IF regimen), the left ventricular (LV) remodeling and MI expansion that were observed in the control diet group were absent in the IF group. In a subgroup of animals with similar MI size at 1 week after MI, further observation revealed less remodeling, better LV function, and no MI expansion in the IF group compared with the control group.

CONCLUSIONS

IF protects the heart from ischemic injury and attenuates post-MI cardiac remodeling, likely via antiapoptotic and antiinflammatory mechanisms.

Neurotrophic factors and regulation of mood: role of exercise, diet and metabolism

Results from basic and clinical studies demonstrate that stress and depression decrease neurotrophic factor expression and neurogenesis in brain, and that antidepressant treatment blocks or reverses these effects, leading to a neurotrophic hypothesis of depression. Neurotrophic factor expression and neurogenesis are also decreased during aging and could be risk factors for depression. In contrast, exercise and enriched environment increase neurotrophic support and neurogenesis, which could contribute to the blockade of the effects of stress and aging and produce antidepressant effects. A brief overview of this work and the specific neurotrophic factors involved are discussed in this review.