Energy intake and exercise as determinants of brain health and vulnerability to injury and disease

Abstract

Evolution favored individuals with superior cognitive and physical abilities under conditions of limited food sources, and brain function can therefore be optimized by intermittent dietary energy restriction (ER) and exercise. Such energetic challenges engage adaptive cellular stress-response signaling pathways in neurons involving neurotrophic factors, protein chaperones, DNA-repair proteins, autophagy, and mitochondrial biogenesis. By suppressing adaptive cellular stress responses, overeating and a sedentary lifestyle may increase the risk of Alzheimer's and Parkinson's diseases, stroke, and depression. Intense concerted efforts of governments, families, schools, and physicians will be required to successfully implement brain-healthy lifestyles that incorporate ER and exercise.

Figure 1. Features of neuronal circuits involved in learning and memory, and how they can respond to changes in energy intake and expenditure, and cognitive challenges

A glutamatergic (excitatory) neuron in the hippocampus, with a long axon (red) and three major dendrites (blue) is shown. During exercise, energy restriction, or when engaged in cognitive challenges, the excitatory neurotransmitter glutamate is released from presynaptic terminals (red) and activates receptors located on postsynaptic spines (black projections on the dendrites). As a result, Ca²⁺ influx occurs which then activates signaling pathways that: 1) induce the expression of genes involved in synaptic plasticity and cell survival, including those encoding neurotrophic factors, protein chaperones, and antioxidant enzymes; 2) modify mitochondrial energy metabolism and free radical generation; and 3) trigger Ca²⁺ release from the endoplasmic reticulum (ER). The activity and function of glutamatergic neurons is subject to modification by intrinsic interneurons that employ the inhibitory neurotransmitter GABA, as well as by inputs from serotonergic and noradrenergic neurons located in the brainstem and cholinergic neurons in the basal forebrain. The arrows indicate the direction of information flow to and within the glutamatergic neuron.

Figure 2. Synaptic signaling mechanisms involved in neuronal network activity-dependent neuroplasticity

An excitatory synapse is comprised of a presynaptic axon terminal with vesicles containing the neurotransmitter glutamate (red) that is released into the synapse in response to Ca²⁺ influx resulting from Na⁺ influx-mediated depolarization of the neuron. Glutamate binds to ionotropic α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPAR) and N-methyl-D-aspartate receptors (NMDAR) located at the membrane surface of the postsynaptic dendrite. AMPAR flux Na⁺ resulting in membrane depolarization and opening of NMDAR which flux Ca²⁺, as well as opening of voltage-dependent Ca²⁺ channels (VDCC). Within the dendrite, Ca²⁺ functions as a second messenger to activate multiple signal transduction cascades including one involving Ca²⁺/calmodulin-dependent kinase (CaMK) and the transcription factor cyclic AMP response element-binding protein (CREB). Ligands that activate receptors coupled to cyclic AMP (cAMP) production (R) also activate CREB; examples include serotonin and glucagon-like peptide 1. CREB induces the expression of multiple nuclear genes that encode proteins involved in synaptic plasticity and cell survival including brain-derived neurotrophic factor (BDNF). BDNF mRNA is then transported to the postsynaptic region of the dendrite where it is translated into BDNF protein which is, in turn, released and activates BDNF receptors (trkB) located on the surface of both the post- and pre-synaptic terminals. One prominent signal transduction pathway activated when BDNF binds trkB involves phosphatidylinositol-3-kinase (PI3K) and Akt kinase. Glial cells (astrocytes and microglia) can modulate synaptic plasticity by producing neurotrophic factors such as fibroblast growth factor 2 (FGF2) and glial cell line-derived neurotrophic factor (GDNF), and cytokines such as tumor necrosis factor (TNFα) and interleukin-1β. In the dentate gyrus of the hippocampus, a brain region critical for learning and memory, resides a population of neural stem cells that are capable of self-renewal (promoted by FGF2) and differentiation into neurons (promoted by BDNF) and astrocytes (promoted by bone morphogenic protein 2; BMP2).

Figure 3. Endocrine and neurochemical pathways that mediate adaptive responses of neurons to exercise and energy restriction

In response to intermittent bouts of vigorous exercise and periods of energy restriction (ER), several prominent signaling pathways are activated in neurons that engender changes in metabolism, functional and structural plasticity, and cellular stress resistance. Collectively, these signaling pathways optimize brain function, reduce neuronal vulnerability to aging and disease, and enhance recovery from traumatic and ischemic insults. During exercise and ER, activity in many neuronal circuits increases and is mediated by glutamate release from presynaptic terminals, which then activates ionotropic glutamate receptors resulting in Ca²⁺ influx. Ca²⁺ may also be released from endoplasmic reticulum (EndRet) stores. The Ca²⁺ then activates kinases such as Ca²⁺/calmodulin-dependent kinases (CaMK) and CaMK, in turn, enhances the activity of the transcription factor cyclic AMP response element-binding protein (CREB) in the nucleus. CREB can also activated by ligands that bind to cell surface receptors that are coupled to the GTP-binding protein Gs which then activates adenylate cyclase resulting in the production of cyclic AMP (cAMP) and activation of protein kinase A (PKA). Glucagon-like peptide 1 (GLP-1) is an example of a hormone/neuropeptide that activates receptors (R) coupled to cAMP production in neurons. Ca²⁺ influx may also lead to the activation of the transcription factor NF-κB in the cytoplasm, resulting in the translocation of the activate NF-κB into the nucleus. Exercise and ER also increase signaling via receptors for brain-derived neurotrophic factor (BDNF) and insulin or insulin-like growth factor-1 (IGF-1); activation of these receptors engages two signaling cascades in neurons including those involving phosphatidylinositol-3-kinase – Akt, and extracellular signal-regulated kinases (ERKs). The mild stress associated with exercise and ER can also activate the transcription factors nuclear regulatory factor 2 (Nrf-2) and heat-shock factor 1 (HSF-1) by a mechanism involving mitochondrial reactive oxygen species (ROS) and/or depletion of cellular energy substrates. Moreover, exercise and ER can promote mitochondrial biogenesis (upper left) by a mechanism involving a transcription factor called peroxisome proliferator-activated receptor gamma co-activator-1α (PGC-1α). Examples of proteins synthesized from mRNAs up-regulated in response to exercise and ER are shown in the region labeled ‘protein synthesis’ and include: mitochondrial transcription factor A (TFAM); Nrf-2; peroxisome proliferator activator receptor α (PPARa); NAD(P)H:quinone oxidoreductase 1 (NQO1); heme oxygenase 1 (HO-1); heat-shock protein 70 (HSP70); BDNF; apurinic/apyrimidinic endonuclease 1 (APE1); Bcl-2; and manganese superoxide dismutase (SOD2).

Figure 4. The widespread incorporation of intermittent energetic challenges into daily life is a society-wide problem

Knowledge gained from biomedical research can provide a workable prescription for optimal brain health (and general health) that should be implemented beginning early in adult life. Effective approaches for intermittent energetic challenges (energy restriction and exercise) must be incorporated into the curriculum at medical schools, and should be practiced by physicians in general practice and specialists who treat diseases associated with a chronic positive energy balance. Interventions would be administered in a manner that results in a high level of compliance. Patients for whom the prescription fails could be referred to inpatient rehabilitation facilities for a sufficient time period for the patient to adapt to a regimen of intermittent energetic challenges. Primary and secondary education of youth should emphasize the importance of intermittent energetic challenges for optimal health, productivity and happiness. Parents should be counseled on their critical role in determining the diet and exercise habits of their children. The government plays multiple roles in promoting optimal brain health of societies through its support of education and biomedical research, and its control of the ‘dark, brain-wasting forces’ of the food, pharmaceutical, agriculture industries. High energy density addictive fat- and simple sugar-laden foods that are heavily advertised, together with an agriculture industry that dissuades the growing and marketing of vegetables and fruits, has results in widespread overeating. Because of the development of energy expenditure-sparing technologies (automobiles, mass transit, elevators, computers, etc.), exercise is unnecessary for the vast majority of the population to perform their jobs. The pharmaceutical industry is bolstered by the large market for drugs to treat diseases caused by excessive energy intake and effort-sparing technologies.