Effects of Ca2+ and ionophore A23187 on protein synthesis in intact rabbit reticulocytes

Regulatory mechanisms of skeletal muscle protein turnover during exercise

Skeletal muscle protein turnover is a relatively slow metabolic process that is altered by various physiological stimuli such as feeding, fasting, and exercise. During exercise, catabolism of amino acids contributes very little to ATP turnover in working muscle. With regard to protein turnover, there are now consistent data from tracer studies in rodents and humans showing that global protein synthesis is blunted in working skeletal muscle. Whether there is altered skeletal muscle protein breakdown during exercise remains unclear. The blunting of protein synthesis is believed to be mediated by suppressed mRNA translation initiation and elongation steps involving, but not limited to, changes in eukaryotic initiation factor 4E binding protein 1 and eukaryotic elongation factor 2 phosphorylation (eEF2), respectively. Evidence is provided that upstream signaling to translation factors is mediated by signaling downstream of changes in intracellular Ca2+ and energy turnover. In particular, a signaling cascade involving Ca2+/calmodulin-eEF2 kinase-eEF2 is implicated. The possible functional significance of altered protein turnover in working skeletal muscle during exercise is discussed. Further work with available and new techniques will undoubtedly reveal the functional significance and signaling mechanisms behind changes in skeletal muscle protein turnover during exercise.

Capsaicin causes protein synthesis inhibition and microtubule disassembly through TRPV1 activities both on the plasma membrane and intracellular membranes

TRPV1 is a non-selective cationic channel that is activated by capsaicin, acidic pH and thermal stimuli. Sustained TRPV1 channel activation causes severe cytotoxicity that leads to cell death. In this study, we investigated the mechanisms of capsaicin-induced cytotoxicity in HEK293 cells stably expressing TRPV1 with a focus on protein synthesis regulation and cytoskeleton reorganization. Capsaicin inhibited protein synthesis in TRPV1-expressing HEK cells with an IC(50) of 15.6nM and depolymerized microtubules within 10min after exposure. These effects were completely blocked by pretreatment of cells with the TRPV1 antagonist A-425619, both in the presence and absence of extracellular calcium. Protein synthesis inhibition induced by capsaicin was not a result of eIF2alpha hyperphosphorylation, but rather closely correlated with cytosolic calcium elevation caused by calcium flux through cell surface and intracellular TRPV1, and/or ER calcium depletion through intracellular TRPV1. Microtubule dependent cell process shrinkage may serve as a mechanism for rapid alteration of the neurotransmission network upon TRPV1 activation. Taken together, the present studies demonstrate that intracellular pool of TRPV1 plays an important role in regulating cell morphology and viability upon receptor activation.

Cellular abnormalities linked to endoplasmic reticulum dysfunction in cerebrovascular disease—therapeutic potential

Unfolded proteins accumulate in the lumen of the endoplasmic reticulum (ER) as part of the cellular response to cerebral hypoxia/ischemia and also to the overexpression of the mutant genes responsible for familial forms of degenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyothrophic lateral sclerosis, and Huntington's disease, as well as other disorders that are caused by an expanded CAG repeat. This accumulation arises from an imbalance between the load of proteins that need to be folded and processed in the ER lumen and the ER folding/processing capacity. To withstand such potentially lethal conditions, stress responses are activated that includes the shutdown of translation to reduce the ER work load and the activation of the expression of genes coding for proteins involved in the folding and processing reactions, to increase folding/processing capacity. In transient cerebral ischemia, ER stress-induced suppression of protein synthesis is believed to be too severe to permit sufficient activation of the genetic arm of the ER stress response. Mutations associated with Alzheimer's disease down-regulate the ER stress response and make cells more vulnerable to conditions associated with ER stress. When the functioning of the ER is severely impaired and affected cells can no longer withstand these stressful conditions, programmed cell death is induced, including a mitochondria-driven apoptotic pathway. Raising the resistance of cells to conditions that interfere with ER functions and activating the degradation and refolding of unfolded proteins accumulated in the ER lumen are possible strategies for blocking the pathological process leading to cell death at an early stage.

Exercise rapidly increases eukaryotic elongation factor 2 phosphorylation in skeletal muscle of men

Protein synthesis in skeletal muscle is known to decrease during contractions but the underlying regulatory mechanisms are unknown. Here, the effect of exercise on skeletal muscle eukaryotic elongation factor 2 (eEF2) phosphorylation, a key component in protein translation machinery, was examined. Eight healthy men exercised on a cycle ergometer at a workload eliciting approximately 67% peak pulmonary oxygen consumption (VO2 peak) with skeletal muscle biopsies taken from the vastus lateralis muscle at rest as well as after 1, 10, 30, 60 and 90 min of exercise. In response to exercise, there was a rapid (i.e. < 1 min) 5- to 7-fold increase in eEF2 phosphorylation at Thr56 that was sustained for 90 min of continuous exercise. The in vitro activity of skeletal muscle eEF2 kinase was not altered by exercise indicating that the increased activity of eEF2 kinase to eEF2 is not mediated by covalent mechanisms. In support of this, the increase in AMPK activity was temporally unrelated to eEF2 phosphorylation. However, skeletal muscle eEF2 kinase was potently activated by Ca(2)(+)-calmodulin in vitro, suggesting that the higher eEF2 phosphorylation in working skeletal muscle is mediated by allosteric activation of eEF2 kinase by Ca(2)(+) signalling via calmodulin. Given that eEF2 phosphorylation inhibits eEF2 activity and mRNA translation, these findings suggest that the inhibition of protein synthesis in contracting skeletal muscle is due to the Ca(2)(+)-induced stimulation of eEF2 kinase.

Endoplasmic reticulum: a primary target in various acute disorders and degenerative diseases of the brain

Changes in neuronal calcium activity in the various subcellular compartments have divergent effects on affected cells. In the cytoplasm and mitochondria, where calcium activity is normally low, a prolonged excessive rise in free calcium levels is believed to be toxic, in the endoplasmic reticulum (ER), in contrast, calcium activity is relatively high and severe stress is caused by a depletion of ER calcium stores. Besides its role in cellular calcium signaling, the ER is the site where membrane and secretory proteins are folded and processed. These calcium-dependent processes are fundamental to normal cell functioning. Under conditions of ER dysfunction unfolded proteins accumulate in the ER lumen, a signal responsible for activation of the unfolded protein response (UPR) and the ER-associated degradation (ERAD). UPR is characterized by activation of two ER-resident kinases, PKR-like ER kinase (PERK) and IRE1. PERK induces phosphorylation of the eukaryotic initiation factor (eIF2alpha), resulting in a shut-down of translation at the initiation step. This stress response is needed to block new synthesis of proteins that cannot be correctly folded, and thus to protect cells from the effect of unfolded proteins which tend to form toxic aggregates. IRE1, on the other hand, is turned after activation into an endonuclease that cuts out a sequence of 26 bases from the coding region of xbp1 mRNA. Processed xbp1 mRNA is translated into the respective protein, an active transcription factor specific for ER stress genes such as grp78. In acute disorders and degenerative diseases, the ER calcium pool is a primary target of toxic metabolites or intermediates, such as oxygen free radicals, produced during the pathological process. Affected neurons need to activate the entire UPR to cope with the severe form of stress induced by ER dysfunction. This stress response is however hindered under conditions where protein synthesis is suppressed to such an extent that processed xbp1 mRNA is not translated into the processed XBP1 protein (XBP1(proc)). Furthermore, activation of ERAD is important for the degradation of unfolded proteins through the ubiquitin/proteasomal pathway, which is impaired in acute disorders and degenerative diseases, resulting in further ER stress. ER functioning is thus impaired in two different ways: first by the direct action of toxic intermediates, produced in the course of the pathological process, hindering vital ER reactions, and second by the inability of cells to fully activate UPR and ERAD, leaving them unable to withstand the severe form of stress induced by ER dysfunction.

Mechanisms underlying suppression of protein synthesis induced by transient focal cerebral ischemia in mouse brain

Transient global cerebral ischemia triggers suppression of the initiation step of protein synthesis, a process which is controlled by endoplasmic reticulum (ER) function. ER function has been shown to be disturbed after transient cerebral ischemia, as indicated by an activation of the ER-resident eIF2alpha kinase PERK. In this study, we investigated ischemia-induced changes in protein levels and phosphorylation states of the initiation factors eIF2alpha, eIF2B epsilon, and eIF4G1 and of p70 S6 kinase, proteins playing a central role in the control of the initiation of translation. Transient focal cerebral ischemia was induced in mice by occlusion of the left middle cerebral artery. Transient ischemia caused a long-lasting suppression of global protein synthesis. eIF2alpha was transiently phosphorylated after ischemia, peaking at 1-3 h of recovery. eIF2B epsilon and p70 S6 kinase were completely dephosphorylated during ischemia and phosphorylation did not recover completely following reperfusion. In addition, eIF2B epsilon, eIF4G1, and p70 S6 kinase protein levels decreased progressively with increasing recirculation time. Thus, several different processes contributed to ischemia-induced suppression of the initiation of protein synthesis: a long-lasting dephosphorylation of eIF2B epsilon and p70 S6K starting during ischemia, a transient phosphorylation of eIF2alpha during early reperfusion, and a marked decrease of eIF2B epsilon, eIF4G1, and p70 S6K protein levels starting during vascular occlusion (eIF4G1). Study of the mechanisms underlying ischemia-induced suppression of the initiation step of translation will help to elucidate the role of protein synthesis inhibition in the development of neuronal cell injury triggered by transient cerebral ischemia.

Response of neurons to an irreversible inhibition of endoplasmic reticulum Ca(2+)-ATPase: relationship between global protein synthesis and expression and translation of individual genes

In the physiological state, there appears to be a regulatory link between endoplasmic reticulum (ER) Ca(2+) homoeostasis and the initiation of neuronal protein synthesis. Exposing neuronal cell cultures to thapsigargin (Tg), an irreversible inhibitor of sarcoplasmic/ER Ca(2+)-ATPase (SERCA), induced an almost complete suppression of protein synthesis, which recovered to approx. 60% of control 24 h after Tg exposure. This is an experimental model where the regulatory link between the initiation of protein synthesis and ER Ca(2+) homoeostasis recovers, despite an irreversible suppression of SERCA activity [Doutheil, Treiman, Oschlies and Paschen (1999) Cell Calcium 25, 419--428]. The model was used to investigate the relationship between transcription and translation of various stress genes that respond to conditions causing graded suppression of protein synthesis. Expression patterns revealed three groups of genes. The mRNA levels of genes responding to conditions of ER stress (grp78, grp94, gadd34 and gadd153) were increased up to 200-fold after Tg exposure, whereas those coding for ER-resident proteins (SERCA 2b and Bcl-2) were increased up to 6-fold in treated cultures, and those coding for cytoplasmic proteins (heat-shock protein 70 and p67) were increased only 2--4-fold. Analysis of translation of these mRNAs suggests an imbalance in the synthesis of apoptosis-inducing (GADD153) and tolerance-activating (GRP78 and Bcl-2) proteins after blocking of the ER Ca(2+) pump. The observation that the relationship between Tg-induced changes in mRNA and protein levels varied considerably for the various genes studied implies that translation of the respective transcripts is differently regulated under conditions causing graded suppression of global protein synthesis. Detailed analysis of the response of neuronal cells to transient disturbance of ER Ca(2+) homoeostasis may help to elucidate the mechanisms underlying neuronal cell injury in those neurological disorders in which an impairment of ER function is thought to contribute to the pathological process of deterioration.

Dependence of vital cell function on endoplasmic reticulum calcium levels: implications for the mechanisms underlying neuronal cell injury in different pathological states

The endoplasmic reticulum (ER) is a subcellular compartment playing a pivotal role in the control of vital calcium-related cell functions, including calcium storage and signalling. In addition, newly synthesized membrane and secretory proteins are folded and processed in the ER, reactions which are strictly calcium dependent. The ER calcium activity is therefore high, being several orders of magnitude above that of the cytoplasm. Depletion of ER calcium stores causes an accumulation of unfolded proteins in the ER lumen, a pathological situation which induces the activation of two highly conserved stress responses, the ER overload response (EOR) and the unfolded protein response (UPR). EOR triggers activation of the transcription factor NF kappa B, which, in turn, activates the expression of target genes. UPR triggers two downstream processes: it activates the expression of genes coding for ER-resident stress proteins, and it causes a suppression of the initiation of protein synthesis. A similar stress response is activated in pathological states of the brain including cerebral ischaemia, implying common underlying mechanisms. Depending on the extent and duration of the disturbance, an isolated impairment of ER function is sufficient to induce cell injury. In this review, evidence is presented that ER function is indeed disturbed in various diseases of the brain, including acute pathological states (e.g. cerebral ischaemia) and degenerative diseases (e.g. Alzheimer's disease). A body of evidence suggests that disturbances of ER function could be a global pathomechanism underlying neuronal cell injury in various acute and chronic disorders of the central nervous system. If that is true, restoration of ER function or attenuation of secondary disturbances induced by ER dysfunction could present a highly promising new avenue for pharmacological intervention to minimize neuronal cell injury in different pathological states of the brain.

Disturbances of the functioning of endoplasmic reticulum: a key mechanism underlying neuronal cell injury?

Cerebral ischemia leads to a massive increase in cytoplasmic calcium activity resulting from an influx of calcium ions into cells and a release of calcium from mitochondria and endoplasmic reticulum (ER). It is widely believed that this increase in cytoplasmic calcium activity plays a major role in ischemic cell injury in neurons. Recently, this concept was modified, taking into account that disturbances occurring during ischemia are potentially reversible: it then was proposed that after reversible ischemia, calcium ions are taken up by mitochondria, leading to disturbances of oxidative phosphorylation, formation of free radicals, and deterioration of mitochondrial functions. The current review focuses on the possible role of disturbances of ER calcium homeostasis in the pathologic process culminating in ischemic cell injury. The ER is a subcellular compartment that fulfills important functions such as the folding and processing of proteins, all of which are strictly calcium dependent. ER calcium activity is therefore relatively high, lying in the lower millimolar range (i.e., close to that of the extracellular space). Depletion of ER calcium stores is a severe form of stress to which cells react with a highly conserved stress response, the most important changes being a suppression of global protein synthesis and activation of stress gene expression. The response of cells to disturbances of ER calcium homeostasis is almost identical to their response to transient ischemia, implying common underlying mechanisms. Many observations from experimental studies indicate that disturbances of ER calcium homeostasis are involved in the pathologic process leading to ischemic cell injury. Evidence also has been presented that depletion of ER calcium stores alone is sufficient to activate the process of programmed cell death. Furthermore, it has been shown that activation of the ER-resident stress response system by a sublethal form of stress affords tolerance to other, potentially lethal insults. Also, disturbances of ER function have been implicated in the development of degenerative disorders such as prion disease and Alzheimer's disease. Thus, disturbances of the functioning of the ER may be a common denominator of neuronal cell injury in a wide variety of acute and chronic pathologic states of the brain. Finally, there is evidence that ER calcium homeostasis plays a key role in maintaining cells in their physiologic state, since depletion of ER calcium stores causes growth arrest and cell death, whereas cells in which the regulatory link between ER calcium homeostasis and protein synthesis has been blocked enter a state of uncontrolled proliferation.

Regulation of translational initiation during cellular responses to stress

Chemicals and conditions that damage proteins, promote protein misfolding, or inhibit protein processing trigger the onset of protective homeostatic mechanisms resulting in "stress responses" in mammalian cells. Included in these responses are an acute inhibition of mRNA translation at the initiation step, a subsequent induction of various protein chaperones, and the recovery of mRNA translation. Separate, but closely related, stress response systems exist for the endoplasmic reticulum (ER), relating to the induction of specific "glucose-regulated proteins" (GRPs), and for the cytoplasm, pertaining to the induction of the "heat shock proteins" (HSPs). Activators of the ER stress response system, including Ca(2+)-mobilizing and thiol-reducing agents, are discussed and compared to activators of the cytoplasmic stress system, such as arsenite, heavy metal cations, and oxidants. An emerging integrative literature is reviewed that relates protein chaperones associated with cellular stress response systems to the coordinate regulation of translational initiation and protein processing. Background information is presented describing the roles of protein chaperones in the ER and cytoplasmic stress response systems and the relationships of chaperones and protein processing to the regulation of mRNA translation. The role of chaperones in regulating eIF-2 alpha kinase activities, eIF-2 cycling, and ribosomal loading on mRNA is emphasized. The putative role of GRP78 in coupling rates of translation to processing is modeled, and functional relationships between the HSP and GRP chaperone systems are discussed.

Metaplasticity: a new vista across the field of synaptic plasticity

Over the past 20 years there has been an increasing understanding of the properties and mechanisms underlying long-term potentiation (LTP) and long-term depression (LTD) of synaptic efficacy, putative learning and memory mechanisms in the mammalian brain. More recently, however, it has become apparent that synaptic activity can also elicit persistent neuronal responses not manifest as changes in synaptic strength. Some of these changes may nonetheless modify the ability of synapses to undergo strength changes in response to subsequent episodes of synaptic activity. This kind of activity-dependent modulatory plasticity we have termed "metaplasticity". Metaplasticity has been observed physiologically as an inhibition of LTP and concomitant facilitation of LTD by prior N-methyl-D-aspartate receptor activation or, conversely, a facilitation of LTP induction by prior metabotropic glutamate receptor activation. The examples of metaplasticity described to date are input specific, and last as long as several hours. The mechanisms underlying such phenomena remain to be fully characterized, although some likely possibilities are an altered N-methyl-D-aspartate receptor function, altered calcium buffering, altered states of kinases or phosphatases, and a priming of protein synthesis machinery. While some details vary, experimentally observed metaplasticity bears some similarity to the "sliding threshold" feature of the Bienenstock, Cooper and Munro model of experience-dependent synaptic plasticity. Metaplasticity may serve several functions including (1) providing a way for synapses to integrate a response across temporally spaced episodes of synaptic activity and (2) keeping synapses within a dynamic functional range, and thus preventing them from entering states of saturated LTP or LTD.

Reversible phosphorylation of eukaryotic initiation factor 2 alpha in response to endoplasmic reticular signaling

Agents, such as EGTA, thapsigargin, and ionophore A23187, that mobilize sequestered Ca2+ from the endoplasmic reticulum (ER) or dithiothreitol (DTT) that compromises the oxidizing environment of the organelle, disrupt early protein processing and inhibit translational initiation. Increased phosphorylation of eIF-2 alpha (5-fold) and inhibition of eIF-2B activity (50%) occur in intact GH3 cells exposed to these agents for 15 min (Prostko et al. J. Biol. Chem. 267:16751-16754, 1992). Alterations in eIF-2 alpha phosphorylation and translational activity in response to EGTA were reversed by addition of Ca2+ in excess of chelator while responses to DTT were reversible by washing. Exposure for 3 h to either A23187 or DTT, previously shown to induce transcription-dependent translational recovery, resulted in dephosphorylation of eIF-2 alpha in a manner blocked by actinomycin D. Phosphorylation of eIF-2 alpha in response to A23187 or DTT was not prevented by conventional inhibitors of translation including cycloheximide, pactamycin, puromycin, or verrucarin. Prolonged inhibition of protein synthesis to deplete the ER of substrates for protein processing resulted in increased eIF-2 alpha phosphorylation, decreased eIF-2B activity, and reduced monosome content that were indicative of time-dependent blockade; these inhibitors did not abolish polysomal content. Superphosphorylation of eIF-2 alpha occurred upon exposure of these preparations to either A23187 or DTT. Tunicamycin, an inhibitor of co-translational transfer of core oligosaccharide, provoked rapid phosphorylation of eIF-2 alpha and inhibition of translational initiation whereas sugar analog inhibitors of glycoprotein processing did neither. A flow of processible protein to the ER does not appear to be required for the phosphorylation of eIF-2 alpha in response to ER perturbants. We hypothesize that perturbation of the translocon, rather than suppression of protein processing, initiates the signal emanating from the ER culminating in eIF-2 alpha phosphorylation and translational repression.