Nirinjini Naidoo

The endoplasmic reticulum stress response in aging and age-related diseases

The endoplasmic reticulum(ER) is a multifunctional organelle within which protein folding, lipid biosynthesis, and calcium storage occurs. Perturbations such as energy or nutrient depletion, disturbances in calcium or redox status that disrupt ER homeostasis lead to the misfolding of proteins, ER stress and up-regulation of several signaling pathways coordinately called the unfolded protein response (UPR). The UPR is characterized by the induction of chaperones, degradation of misfolded proteins and attenuation of protein translation. The UPR plays a fundamental role in the maintenance of cellular homeostasis and thus is central to normal physiology. However, sustained unresolved ER stress leads to apoptosis. Aging linked declines in expression and activity of key ER molecular chaperones and folding enzymes compromise proper protein folding and the adaptive response of the UPR. One mechanism to explain age associated declines in cellular functions and age-related diseases is a progressive failure of chaperoning systems. In many of these diseases, proteins or fragments of proteins convert from their normally soluble forms to insoluble fibrils or plaques that accumulate in a variety of organs including the liver, brain or spleen. This group of diseases, which typically occur late in life includes Alzheimers, Parkinsons, type II diabetes and a host of less well known but often equally serious conditions such as fatal familial insomnia. The UPR is implicated in many of these neurodegenerative and familial protein folding diseases as well as several cancers and a host of inflammatory diseases including diabetes, atherosclerosis, inflammatory bowel disease and arthritis. This review will discuss age-related changes in the ER stress response and the role of the UPR in age-related diseases.

ER and aging—Protein folding and the ER stress response

The endoplasmic reticulum (ER) is a multifunctional organelle which co-ordinates protein folding, lipid biosynthesis, calcium storage and release. Perturbations that disrupt ER homeostasis lead to the misfolding of proteins, ER stress and up-regulation of a signaling pathway called the ER stress response or the unfolded protein response (UPR). The UPR is characterized by the induction of chaperones, degradation of misfolded proteins and attenuation of protein translation. Age-related declines and activity in key molecular chaperones and folding enzymes compromise proper protein folding and the adaptive response of the UPR. This review will highlight age-related changes in the protein folding machinery and in the UPR.

Aging Impairs the Unfolded Protein Response to Sleep Deprivation and Leads to Proapoptotic Signaling

Protein misfolding, accumulation, and aggregation characterize many aging-related diseases. Protein aggregates do not accumulate in unstressed cells primarily because of the existence of competent cellular “quality control” machinery. The endoplasmic reticulum (ER) is a major part of this quality control system. Accumulation of misfolded proteins in the ER causes ER stress and activates a signaling pathway called the unfolded protein response (UPR). The UPR limits protein load by upregulating ER chaperones such as Ig binding protein (BiP)/glucose-regulated protein 78 (GRP78) and by attenuating protein translation through eukaryotic initiation factor 2 α (eIF2α) phosphorylation. Acute sleep deprivation (6 h) in young mice leads to induction of the UPR with upregulation of BiP/GRP78 and attenuation of protein translation. We demonstrate here that aging impairs this adaptive response to sleep deprivation. Aged mice do not display an increase in BiP expression with acute sleep deprivation. In addition, there is decreased basal expression of BiP/GRP78 in aged mice. There is a decline in eIF2α phosphorylation in aged mouse cerebral cortex that is associated with higher levels of GADD34 (growth arrest and DNA damage 34) and proapoptotic proteins such as CCAAT/enhancer-binding protein-homologous protein and activated caspase-12, suggesting that young animals possess an efficient ER adaptive response that declines with aging.

Mechanisms of sleep-dependent consolidation of cortical plasticity.

Sleep is thought to consolidate changes in synaptic strength, but the underlying mechanisms are unknown. We investigated the cellular events involved in this process during ocular dominance plasticity (ODP)-a canonical form of in vivo cortical plasticity triggered by monocular deprivation (MD) and consolidated by sleep via undetermined, activity-dependent mechanisms. We find that sleep consolidates ODP primarily by strengthening cortical responses to nondeprived eye stimulation. Consolidation is inhibited by reversible, intracortical antagonism of NMDA receptors (NMDARs) or cAMP-dependent protein kinase (PKA) during post-MD sleep. Consolidation is also associated with sleep-dependent increases in the activity of remodeling neurons and in the phosphorylation of proteins required for potentiation of glutamatergic synapses. These findings demonstrate that synaptic strengthening via NMDAR and PKA activity is a key step in sleep-dependent consolidation of ODP.

Sleep deprivation induces the unfolded protein response in mouse cerebral cortex

Little is known about the molecular mechanisms underlying sleep. We show the induction of key regulatory proteins in a cellular protective pathway, the unfolded protein response (UPR), following 6 h of induced wakefulness. Using C57/B6 male mice maintained on a 12:12 light/dark cycle, we examined, in cerebral cortex, the effect of different durations of prolonged wakefulness (0, 3, 6, 9 and 12 h) from the beginning of the lights‐on inactivity period, on the protein expression of BiP/GRP78, a chaperone and classical UPR marker. BiP/GRP78 expression is increased with increasing durations of sleep deprivation (6, 9 and 12 h). There is no change in BiP/GRP78 levels in handling control experiments carried out during the lights‐off period. PERK, the transmembrane kinase responsible for attenuating protein synthesis, which is negatively regulated by binding to BiP/GRP78, is activated by dissociation from BiP/GRP78 and by autophosphorylation. There is phosphorylation of the elongation initiation factor 2α and alteration in ribosomal function. These changes are first observed after 6 h of induced wakefulness. Thus, prolonging wakefulness beyond a certain duration induces the UPR indicating a physiological limit to wakefulness.

The energy hypothesis of sleep revisited

One of the proposed functions of sleep is to replenish energy stores in the brain that have been depleted during wakefulness. Benington and Heller formulated a version of the energy hypothesis of sleep in terms of the metabolites adenosine and glycogen. They postulated that during wakefulness, adenosine increases and astrocytic glycogen decreases reflecting the increased energetic demand of wakefulness. We review recent studies on adenosine and glycogen stimulated by this hypothesis. We also discuss other evidence that wakefulness is an energetic challenge to the brain including the unfolded protein response, the electron transport chain, NPAS2, AMP-activated protein kinase, the astrocyte-neuron lactate shuttle, production of reactive oxygen species and uncoupling proteins. We believe the available evidence supports the notion that wakefulness is an energetic challenge to the brain, and that sleep restores energy balance in the brain, although the mechanisms by which this is accomplished are considerably more complex than envisaged by Benington and Heller.

Conservation of sleep: insights from non-mammalian model systems

The past 10 years have seen new approaches to elucidating genetic pathways regulating sleep. The emerging theme is that sleep-like states are conserved in evolution, with similar signaling pathways playing a role in animals as distantly related as flies and humans. We review the evidence for the presence of sleep states in non-mammalian species including zebrafish (Danio rerio), fruitflies (Drosophila melanogaster) and roundworms (Caenorhabditis elegans). We describe conserved sleep-regulatory molecular pathways with a focus on cAMP and epidermal growth factor signaling; neurotransmitters with conserved effects on sleep and wake regulation, including dopamine and GABA; and a conserved molecular response to sleep deprivation involving the chaperone protein BiP/GRP78.

Relationships among dietary nutrients and subjective sleep, objective sleep, and napping in women.

OBJECTIVE To describe which dietary nutrient variables are related to subjective and objective habitual sleep and subjective and objective napping. METHODS Participants were 459 post-menopausal women enrolled in the Womens Health Initiative. Objective sleep was estimated using one week of actigraphy. Subjective sleep was prospectively estimated with a daily sleep diary. Dietary nutrients were calculated from food frequency questionnaires. RESULTS The most significant correlations were with subjective napping, including (from strongest to weakest): total fat, calories, saturated fat, monounsaturated fat, trans fat, water, proline, serine, tyrosine, phenylalanine, valine, cholesterol, leucine, glutamic acid, ash, isoleucine, histidine, sodium, tryptophan, protein, threonine, cystine, methionine, phosphorous, polyunsaturated fat, animal protein, aspartic acid, arginine, lysine, alanine, caffeine, riboflavin, gamma-tocopherol, glycine, retinol, delta-tocopherol, Vitamin D, and selenium. Actigraphic nocturnal sleep duration was negatively associated with total fat, monounsaturated fat, trans fat, saturated fat, polyunsaturated fat, calories, gamma-tocopherol, cholesterol, and alpha-tocopherol-eq. CONCLUSIONS Actigraphic total sleep time was negatively associated with intake of fats. Subjective napping, which may be a proxy for subjective sleepiness, was significantly related to fat intake as well as intake of meat.

Protein Synthesis during Sleep Consolidates Cortical Plasticity In Vivo

Sleep consolidates experience-dependent brain plasticity, but the precise cellular mechanisms mediating this process are unknown [1]. De novo cortical protein synthesis is one possible mechanism. In support of this hypothesis, sleep is associated with increased brain protein synthesis [2, 3] and transcription of messenger RNAs (mRNAs) involved in protein synthesis regulation [4, 5]. Protein synthesis in turn is critical for memory consolidation and persistent forms of plasticity in vitro and in vivo [6, 7]. However, it is unknown whether cortical protein synthesis in sleep serves similar functions. We investigated the role of protein synthesis in the sleep-dependent consolidation of a classic form of cortical plasticity in vivo (ocular dominance plasticity, ODP; [8, 9]) in the cat visual cortex. We show that intracortical inhibition of mammalian target of rapamycin (mTOR)-dependent protein synthesis during sleep abolishes consolidation but has no effect on plasticity induced during wakefulness. Sleep also promotes phosphorylation of protein synthesis regulators (i.e., 4E-BP1 and eEF2) and the translation (but not transcription) of key plasticity related mRNAs (ARC and BDNF). These findings show that sleep promotes cortical mRNA translation. Interruption of this process has functional consequences, because it abolishes the consolidation of experience in the cortex.

Cellular stress/the unfolded protein response: Relevance to sleep and sleep disorders

Recent transcript profiling and microarray studies are beginning to unveil some of the mysteries of sleep. One of the most important clues has been the identification of the endoplasmic reticulum (ER) resident chaperone, immunoglobulin binding protein (BiP), that increases with sleep deprivation in all species studied. BiP, an ER resident chaperone, is the key cellular marker and master regulator of a signaling pathway called the ER stress response or unfolded protein response. The ER stress response occurs in 3 phases. It is healthy, protective and adaptive when the ER stress is moderate. Failure of the adaptive response leads to the activation of an inflammatory response. When the ER stress burden is great and prolonged, executioner pathways are activated. Collectively this work provides new evidence that modest sleep deprivation induces cellular stress that activates an adaptive response. Aging tilts the response to sleep deprivation from one that is adaptive and protective to one that is maladaptive. Understanding the pathways activated by sleep loss and the mechanisms by which they occur will allow the development of therapies to protect the brain during prolonged wakefulness and specifically in sleep disorders including those associated with aging.