Suppression of protein synthesis in brain during hibernation involves inhibition of protein initiation and elongation

Differential expression of Akt, PPARγ, and PGC-1 during hibernation in bats

The effects of hibernation on the expression of Akt (protein kinase B), the peroxisome proliferator-activated receptor gamma isoform (PPARγ), and the PPARγ coactivator PGC-1 were assessed in seven tissues of the little brown bat, Myotis lucifugus. Western blotting revealed that the levels of active phosphorylated Akt were strongly reduced in brain, kidney, liver, and white adipose during torpor as compared with aroused animals and that total Akt protein was also reduced in white adipose during torpor. By contrast, both total and phospho-Akt were elevated in brown adipose tissue, the thermogenic organ. PPARγ and PGC-1 levels showed parallel changes in all organs. Both were strongly suppressed in brain, but levels increased significantly in all other organs during hibernation (except for PGC-1 in heart). Reduced Akt activity is consistent with a probable reduced insulin response during torpor that facilitates the mobilization of lipid reserves for fuel supply and is further supported by increased gene expression of enzymes and proteins involved in lipid catabolism under the stimulation of enhanced PPARγ and PGC-1 levels.Key words: Myotis lucifugus, mammalian hibernation, lipid metabolism in torpor, protein kinase B, peroxisome proliferator-activated receptor gamma, PPARγ coactivator.

The role of eukaryotic initiation factor 2alpha during the metabolic depression associated with estivation

We have investigated the role of eukaryotic initiation factor 2alpha (eIF2alpha) in two estivating organisms previously shown to downregulate protein synthesis during metabolic depression, the land snail Helix aspersa Müller and the desert frog Neobatrachus sutor Main 1957. We have developed a method using a single antibody (which binds specifically to the phosphorylated, conserved phosphorylation region) by which the total levels of eIF2alpha and the ratio of phosphorylated eIF2alpha [eIF2alpha(P)] to total (phosphorylated and unphosphorylated) eIF2alpha can be determined. In H. aspersa, we have shown that the level of eIF2alpha mRNA expression is unchanged between the awake and estivating states. The amount of total eIF2alpha is the same in the estivating and awake states, and eIF2alpha(P) is undetectable and must represent < or =10% of total eIF2alpha in both states. Conversely, in N. sutor during estivation, the level of total eIF2alpha increases approximately 1.6-fold and the ratio of eIF2alpha(P)/eIF2alpha increases from 0.22+/-0.11 to 0.52+/-0.08, implicating eIF2alpha phosphorylation in the downregulation of protein synthesis during estivation in this animal. The differences in the amounts of eIF2alpha and the level of its phosphorylation between these two species also suggest possible differences either in the mechanism by which protein synthesis is downregulated during estivation or in the sensitivity of the initiation of translation to eIF2alpha(P) levels.

Hippocampal synaptophysin immunoreactivity is reduced during natural hypothermia in ground squirrels

Natural hypothermia during hibernation results in physiological and behavioral deficits. These changes may be traced at the level of hippocampal signal transduction. We investigated synaptophysin immunoreactivity (SYN-ir) in the hippocampus after short and long periods of hypothermia and short and long periods of euthermy in hibernating ground squirrels. SYN-ir in the stratum lucidum of the hippocampus was transiently reduced during natural hypothermia. Natural hypothermia thus reduces synaptic efficacy. This may play a role in the reduced neuronal connectivity of CA3 pyramidal cell dendrites observed in hibernating ground squirrels.

Tissue-specific depression of mitochondrial proton leak and substrate oxidation in hibernating arctic ground squirrels

A significant proportion of standard metabolic rate is devoted to driving mitochondrial proton leak, and this futile cycle may be a site of metabolic control during hibernation. To determine if the proton leak pathway is decreased during metabolic depression related to hibernation, mitochondria were isolated from liver and skeletal muscle of nonhibernating (active) and hibernating arctic ground squirrels (Spermophilus parryii). At an assay temperature of 37 degrees C, state 3 and state 4 respiration rates and state 4 membrane potential were significantly depressed in liver mitochondria isolated from hibernators. In contrast, state 3 and state 4 respiration rates and membrane potentials were unchanged during hibernation in skeletal muscle mitochondria. The decrease in oxygen consumption of liver mitochondria was achieved by reduced activity of the set of reactions generating the proton gradient but not by a lowered proton permeability. These results suggest that mitochondrial proton conductance is unchanged during hibernation and that the reduced metabolism in hibernators is a partial consequence of tissue-specific depression of substrate oxidation.

Ischemic tolerance and endogenous neuroprotection

Practically any stimulus capable of causing injury to a tissue or organ can, when applied close to (but below) the threshold of damage, activate endogenous protective mechanisms--thus potentially lessening the impact of subsequent, more severe stimuli. A sub-threshold ischemic insult applied to the brain, for example, activates certain cellular pathways that can help to reduce damage caused by subsequent ischemic episodes--a phenomenon known as 'ischemic preconditioning' (IP) or 'ischemic tolerance' (IT). Although investigated for some time in model organisms, IP/IT has recently been shown in human brain. This opens a window into endogenous neuroprotection and, potentially, a window of opportunity to utilize these mechanisms in the clinic to treat patients with stroke and other CNS disorders.

Stress-induced gene expression requires programmed recovery from translational repression

Active repression of protein synthesis protects cells against protein malfolding during endoplasmic reticulum stress, nutrient deprivation and oxidative stress. However, long-term adaptation to these conditions requires synthesis of new stress-induced proteins. Phosphorylation of the alpha-subunit of translation initiation factor 2 (eIF2alpha) represses translation in diverse stressful conditions. GADD34 is a stress-inducible regulatory subunit of a holophosphatase complex that dephosphorylates eIF2alpha, and has been hypothesized to play a role in translational recovery. Here, we report that GADD34 expression correlated temporally with eIF2alpha dephosphorylation late in the stress response. Inactivation of both alleles of GADD34 prevented eIF2alpha dephosphorylation and blocked the recovery of protein synthesis, normally observed late in the stress response. Furthermore, defective recovery of protein synthesis markedly impaired translation of stress-induced proteins and interfered with programmed activation of stress-induced genes in the GADD34 mutant cells. These observations indicate that GADD34 controls a programmed shift from translational repression to stress-induced gene expression, and reconciles the apparent contradiction between the translational and transcriptional arms of cellular stress responses.

An integrated stress response regulates amino acid metabolism and resistance to oxidative stress

Eukaryotic cells respond to unfolded proteins in their endoplasmic reticulum (ER stress), amino acid starvation, or oxidants by phosphorylating the alpha subunit of translation initiation factor 2 (eIF2alpha). This adaptation inhibits general protein synthesis while promoting translation and expression of the transcription factor ATF4. Atf4(-/-) cells are impaired in expressing genes involved in amino acid import, glutathione biosynthesis, and resistance to oxidative stress. Perk(-/-) cells, lacking an upstream ER stress-activated eIF2alpha kinase that activates Atf4, accumulate endogenous peroxides during ER stress, whereas interference with the ER oxidase ERO1 abrogates such accumulation. A signaling pathway initiated by eIF2alpha phosphorylation protects cells against metabolic consequences of ER oxidation by promoting the linked processes of amino acid sufficiency and resistance to oxidative stress.

The role of energy availability in Mammalian hibernation: a cost-benefit approach

Hibernation is widely regarded as an adaptation to seasonal energy shortage, but the actual influence of energy availability on hibernation patterns is rarely considered. Here we review literature on the costs and benefits of torpor expression to examine the influence that energy may have on hibernation patterns. We first establish that the dichotomy between food- and fat-storing hibernators coincides with differences in diet rather than body size and show that small or large species pursuing either strategy have considerable potential scope in the amount of torpor needed to survive winter. Torpor expression provides substantial energy savings, which increase the chance of surviving a period of food shortage and emerging with residual energy for early spring reproduction. However, all hibernating mammals periodically arouse to normal body temperatures during hibernation. The function of these arousals has long been speculated to involve recovery from physiological costs accumulated during metabolic depression, and recent physiological studies indicate these costs may include oxidative stress, reduced immunocompetence, and perhaps neuronal tissue damage. Using an optimality approach, we suggest that trade-offs between the benefits of energy conservation and the physiological costs of metabolic depression can explain both why hibernators periodically arouse from torpor and why they should use available energy to minimize the depth and duration of their torpor bouts. On the basis of these trade-offs, we derive a series of testable predictions concerning the relationship between energy availability and torpor expression. We conclude by reviewing the empirical support for these predictions and suggesting new avenues for research on the role of energy availability in mammalian hibernation.

Phosphorylation of eukaryotic initiation factor-2alpha (eIF2alpha) is associated with neuronal degeneration in Alzheimer's disease

Inhibition of protein translation is a mode of inducing neuronal apoptosis and neurodegeneration in Alzheimer's disease (AD). Phosphorylation of eukaryotic initiation factor-2alpha (eIF2alpha) terminates global protein translation and induces apoptosis. We examined whether this signaling pathway occurs in degenerating neurons of AD. Brain sections from young individuals, age-matched control individuals and AD patients were examined for immunoreactivity of phosphorylated eIF2alpha by immunohistochemical analysis. While young brain sections did not display and age-matched brain sections have mild immunoreactive positive cells, AD brain sections revealed intense immunoreactivity for phosphorylated eIF2alpha. Most of the phosphorylated eIF2alpha immunoreactive positive neurons have high immunoreactivity for phosphorylated tau using AT8 antibody. Also, intense staining of phosphorylated eIF2alpha is associated vacuoles in degenerating neurons. This phenomenon was also observed for the immunohistochemical staining of phosphorylated PKR (double-stranded RNA-dependent protein kinase), the upstream kinase for eIF2alpha. Activation of PKR-eIF2alpha pathway is considered to be pro-apoptotic. In addition, formation of autophagy is regulated by eIF2alpha kinase. Therefore, it is concluded that phosphorylation of eIF2alpha is associated with the degeneration of neurons in AD.

Involvement of double-stranded RNA-dependent protein kinase and phosphorylation of eukaryotic initiation factor-2alpha in neuronal degeneration

Inhibition of protein translation plays an important role in apoptosis. While double-stranded RNA-dependent protein kinase (PKR) is named as it is activated by double-stranded RNA produced by virus, its activation induces an inhibition of protein translation and apoptosis via the phosphorylation of the eukaryotic initiation factor 2alpha (eIF2alpha). PKR is also a stress kinase and its levels increase during ageing. Here we show that PKR activation and eIF2alpha phosphorylation play a significant role in apoptosis of neuroblastoma cells and primary neuronal cultures induced by the beta-amyloid (Abeta) peptides, the calcium ionophore A23187 and flavonoids. The phosphorylation of eIF2alpha and the number of apoptotic cells were enhanced in over-expressed wild-type PKR neuroblastoma cells exposed to Abeta peptide, while dominant-negative PKR reduced eIF2alpha phosphorylation and apoptosis induced by Abeta peptide. Primary cultured neurons from PKR knockout mice were also less sensitive to Abeta peptide toxicity. Activation of PKR and eIF2alpha pathway by Abeta peptide are triggered by an increase in intracellular calcium because the intracellular calcium chelator BAPTA-AM significantly reduced PKR phosphorylation. Taken together, these results reveal that PKR and eIF2alpha phosphorylation could be involved in the molecular signalling events leading to neuronal apoptosis and death and could be a new target in neuroprotection.

Life in the slow lane: molecular mechanisms of estivation

Estivation is a state of aerobic hypometabolism used by organisms to endure seasonally arid conditions, often in desert environments. Estivating species are often active for only a few weeks each year to feed and breed and then retreat to estivate in sheltered sites, often underground. In general, estivation includes a strong reduction in metabolic rate, a primary reliance on lipid oxidation to fuel metabolism, and methods of water retention, both physical (e.g. cocoons) and metabolic (e.g. urea accumulation). The present review focuses on several aspects of metabolic adaptation during estivation including changes in the activities of enzymes of intermediary metabolism and antioxidant defenses, the effects of urea on estivator enzymes, enzyme regulation by reversible protein phosphorylation, protein kinases and phosphatases involved in signal transduction mechanisms, and the role of gene expression in estivation. The focus is on two species: the spadefoot toad, Scaphiopus couchii, from the Arizona desert; and the land snail, Otala lactea, a native of the Mediterranean region. The mechanisms of metabolic depression in estivators are similar to those seen in hibernation and anaerobiosis, and contribute to the development of a unified set of biochemical principles for the control of metabolic arrest in nature.

Regulation of protein synthesis by hypoxia via activation of the endoplasmic reticulum kinase PERK and phosphorylation of the translation initiation factor eIF2alpha

Hypoxia profoundly influences tumor development and response to therapy. While progress has been made in identifying individual gene products whose synthesis is altered under hypoxia, little is known about the mechanism by which hypoxia induces a global downregulation of protein synthesis. A critical step in the regulation of protein synthesis in response to stress is the phosphorylation of translation initiation factor eIF2alpha on Ser51, which leads to inhibition of new protein synthesis. Here we report that exposure of human diploid fibroblasts and transformed cells to hypoxia led to phosphorylation of eIF2alpha, a modification that was readily reversed upon reoxygenation. Expression of a transdominant, nonphosphorylatable mutant allele of eIF2alpha attenuated the repression of protein synthesis under hypoxia. The endoplasmic reticulum (ER)-resident eIF2alpha kinase PERK was hyperphosphorylated upon hypoxic stress, and overexpression of wild-type PERK increased the levels of hypoxia-induced phosphorylation of eIF2alpha. Cells stably expressing a dominant-negative PERK allele and mouse embryonic fibroblasts with a homozygous deletion of PERK exhibited attenuated phosphorylation of eIF2alpha and reduced inhibition of protein synthesis in response to hypoxia. PERK(-/-) mouse embryo fibroblasts failed to phosphorylate eIF2alpha and exhibited lower survival after prolonged exposure to hypoxia than did wild-type fibroblasts. These results indicate that adaptation of cells to hypoxic stress requires activation of PERK and phosphorylation of eIF2alpha and suggest that the mechanism of hypoxia-induced translational attenuation may be linked to ER stress and the unfolded-protein response.

Microbial origin of glutamate, hibernation and tissue trauma: an in vivo microdialysis study

Using quantitative microdialysis in hibernating Arctic ground squirrels (AGS), striatal glutamate concentrations ([glu](dia)) progressively increased to approximately 200 microM after 3 days of microdialysis in euthermic but not hibernating ground squirrels. Initially, the progressive increase in [glu](dia) was thought to be related to greater tissue response in euthermic animals. Alternatively, given the vastly different body temperatures between the two groups (37 vs. 3 degrees C), glutamate might have originated from microbes, replicating at a faster rate in the warmer animals. To test these hypotheses, microdialysis was repeated using sterile technique and tissue response surrounding the probe tract was assessed in hematoxylin and eosin stained sections. Using sterile microdialysis technique, traumatic tissue response was greater in euthermic compared to hibernating tissue. However, sterile microdialysis abolished the progressive increase in glutamate. To confirm the microbial origin of glutamate we monitored [glu](dia) collected in vitro from probes immersed in glutamine-rich liquid medium incubated at 37 degrees C. In vitro, [glu](dia) increased as much as in vivo. Two bacteria isolated from in vitro dialysate and liquid medium were both identified as Ralstonia pickettii. Growth of these isolates as well as glutamate release was enhanced when glutamine rather than NH(4)NO(3) was added to the medium suggesting the bacteria utilize glutamine preferentially over ammonium as a nitrogen source.

Quantitative assessment of ground squirrel mRNA levels in multiple stages of hibernation

Hibernators in torpor dramatically reduce their metabolic, respiratory, and heart rates and core body temperature. These extreme physiological conditions are frequently and rapidly reversed during the winter hibernation season via endogenous mechanisms. This phenotype must derive from regulated expression of the hibernator's genome; to identify its molecular components, a cDNA subtraction was used to enrich for seasonally upregulated mRNAs in liver of golden-mantled ground squirrels. The relative steady-state levels for seven mRNAs identified by this screen, plus five others, were measured and analyzed for seasonal and stage-specific differences using kinetic RT-PCR. Four mRNAs show seasonal upregulation in which all five winter stages differ significantly from and are higher than summer (alpha2-macroglobulin, apolipoprotein A1, cathepsin H, and thyroxine-binding globulin). One of these mRNAs, alpha2-macroglobulin, varies during the winter stages with significantly lower levels at late torpor. None of the 12 mRNAs increased during torpor. The implications for these newly recognized upregulated mRNAs for hibernation as well as more global issues of maintaining steady-state levels of mRNA during torpor are discussed.

Classical pathway serum complement activity throughout various stages of the annual cycle of a mammalian hibernator, the golden-mantled ground squirrel, Spermophilus lateralis

Little is known about the changes in the immune system that coincide with the annual cycle of hibernating mammals. This study investigates classical pathway complement activity in the serum of the golden-mantled ground squirrel, a mammalian hibernator. Complement activity varied significantly among discreet stages of the annual cycle and is lowest during torpor and greatest during stages of arousal. C3 mRNA levels follow a pattern similar to that of complement-mediated hemolysis throughout the year but do not vary significantly among hibernation states. The classical pathway of the serum complement system is able to function in vitro at 5 degrees C, although at a slower rate than at 34 degrees C. The classical pathway of the serum complement system is active throughout all phases of the annual cycle of the golden-mantled ground squirrel.

Differential expression of mitochondria-encoded genes in a hibernating mammal

A cDNA library constructed from kidney of the thirteen-lined squirrel, Spermophilus tridecemlineatus, was differentially screened for genes that were upregulated during hibernation. A clone encoding cytochrome c oxidase subunit 1 was found and confirmed to have been upregulated by northern blotting. Differential expression of Cox1 mRNA occurred in multiple organs during hibernation; in hibernating animals transcript levels were twofold higher in kidney and fourfold higher in heart and brown adipose tissue than in euthermic animals, but were unchanged in skeletal muscle. Transcript levels of mitochondrial-encoded ATP synthase 6/8 were similarly upregulated in these tissues whereas transcript levels of the nuclear encoded subunits Cox4 and ATP synthase α did not change during hibernation. Immunoblot analysis revealed a 2.4-fold increase in Cox 1 protein and a slight decrease in Cox 4 protein in kidney of hibernating squirrels, compared with euthermic controls. Hibernating mammals may increase the expression of the mitochondrial genome in general, and Cox1specifically, to prevent or minimize the damage to the electron transport chain caused by the cold and ischemia experienced during a hibernation bout.

Invited review: molecular adaptations in mammalian hibernators: unique adaptations or generalized responses?

Hibernators are unique among mammals in their ability to attain, withstand, and reverse low body temperatures. Hibernators repeatedly cycle between body temperatures near zero during torpor and 37 degrees C during euthermy. How do these mammals maintain cardiac function, cell integrity, blood fluidity, and energetic balance during their prolonged periods at low body temperature and avoid damage when they rewarm? Hibernation is often considered an example of a unique adaptation for low-temperature function in mammals. Although such adaptation is apparent at the level of whole animal physiology, it is surprisingly difficult to demonstrate clear examples of adaptations at the cellular and biochemical levels that improve function in the cold and are unique to hibernators. Instead of adaptation for improved function in the cold, the key molecular adaptations of hibernation may be to exploit the cold to depress most aspects of biochemical function and then rewarm without damage to restore optimal function of all systems. These capabilities are likely due to novel regulation of biochemical pathways shared by all mammals, including humans.

Increased oxidative stress and decreased activities of Ca(2+)/Mg(2+)-ATPase and Na(+)/K(+)-ATPase in the red blood cells of the hibernating black bear

During hibernation, animals undergo metabolic changes that result in reduced utilization of glucose and oxygen. Fat is known to be the preferential source of energy for hibernating animals. Malonyldialdehyde (MDA) is an end product of fatty acid oxidation, and is generally used as an index of lipid peroxidation. We report here that peroxidation of lipids is increased in the plasma and in the membranes of red blood cells in black bears during hibernation. The plasma MDA content was about four fold higher during hibernation as compared to that during the active, non-hibernating state (P < 0.0001). Similarly, MDA content of erythrocyte membranes was significantly increased during hibernation (P < 0.025). The activity of Ca(2+)/Mg(2+)-ATPase in the erythrocyte membrane was significantly decreased in the hibernating state as compared to the active state. Na(+)/K(+)-ATPase activity was also decreased, though not significant, during hibernation. These results suggest that during hibernation, the bears are under increased oxidative stress, and have reduced activities of membrane-bound enzymes such as Ca(2+)/Mg(2+)-ATPase and Na(+)/K(+)-ATPase. These changes can be considered part of the adaptive for survival process of metabolic depression.

The translation state of differentially expressed mRNAs in the hibernating 13-lined ground squirrel (Spermophilus tridecemlineatus)

The translation state of differentially expressed mRNAs were compared in kidney and brown adipose tissue of the hibernating ground squirrel, Spermophilus tridecemlineatus. Polysome analysis revealed a striking disaggregation of polyribosomes during hibernation and the redistribution of Cox4 (cytochrome c oxidase subunit 4) and Oct2 (organic cation transporter type 2) transcripts into monosome and mRNP fractions of kidney cytoplasmic extracts. Additionally, OCT2 protein levels decreased in kidney of hibernating animals in line with a strong decrease (85%) in translation rate compared with euthermic kidney. There was no translational depression in brown adipose tissue during hibernation and the H isoform of fatty-acid-binding protein (H-FABP), that is up-regulated during hibernation, was increasingly abundant in the heavy polyribosome fraction isolated from the brown adipose of hibernators. This may indicate the existence of a tissue-specific mechanism for the translational control of a subset of genes that are physiologically relevant to the survival of hibernation.

Reversible suppression of protein synthesis in concert with polysome disaggregation during anoxia exposure in Littorina littorea

Many marine invertebrates can live without oxygen for long periods of time, a capacity that is facilitated by the ability to suppress metabolic rate in anoxia to a value that is typically less than 10% of the normal aerobic rate. The present study demonstrates that a reduction in the rate of protein synthesis is one factor in the overall anoxia-induced metabolic suppression in the marine snail, Littorina littorea. The rate of [3H]leucine incorporation into newly translated protein in hepatopancreas isolated from 48 h anoxic snails was determined to be 49% relative to normoxic controls. However, protein concentration in hepatopancreas did not change during anoxia, suggesting a coordinated suppression of net protein turnover. Analysis of hepatopancreas samples from snails exposed to 24-72 h anoxia showed a gradual disaggregation of polysomes into monosomes. A re-aggregation of monosomes into polysomes was observed after 3 h of aerobic recovery. Analysis of fractions from the ribosome profile using radiolabeled probe to detect alpha-tubulin transcripts confirmed a general decrease in protein translation during anoxia exposure (transcript association with polysomes decreased) with a reversal during aerobic recovery. Western blotting of hepatopancreas samples from normoxic, 24 h anoxic, and 1 h aerobic recovered snails demonstrated that eIF-2alpha is substantially phosphorylated during anoxia exposure and dephosphorylated during normoxia and aerobic recovery, suggesting a decrease in translation initiation during anoxia exposure. These results suggest that metabolic suppression during anoxia exposure in L. littorea involves a decrease in protein translation.

Differential expression of adipose- and heart-type fatty acid binding proteins in hibernating ground squirrels

The up-regulation of heart- and adipose-type fatty acid binding proteins (H-FABPs and A-FABPs) was detected during hibernation in brown adipose tissue (BAT) of 13-lined ground squirrels, Spermophilus tridecemlineatus, using a commercial rat cDNA array. Full length cDNAs encoding H-FABPs and A-FABPs were subsequently retrieved from a BAT cDNA library. These cDNAs were used to probe Northern blots of total RNA from tissues of euthermic versus hibernating ground squirrels. H-FABP mRNA transcripts increased in BAT, skeletal muscle and heart of hibernating animals whereas A-FABP transcripts, which are normally expressed exclusively in adipose tissue, increased in both BAT and heart during torpor. It is proposed that the increased expression of H-FABPs and A-FABPs during hibernation accelerates the rate at which fatty acids can be transported to the mitochondria for oxidization, particularly in support of the huge increase in thermogenesis by BAT and rapid increase in heart rate that are required during arousal from torpor. Comparison of the deduced polypeptide sequence of ground squirrel H-FABP with that from other mammals also revealed three unique amino acid differences which may be important for protein function at low body temperatures during hibernation.

Translational initiation is uncoupled from elongation at 18 degrees C during mammalian hibernation

Cellular and organismal homeostasis must be maintained across a body temperature (Tb) range of 0 to 37 degrees C during mammalian hibernation. Hibernators depress biosynthetic activities including protein synthesis, concordant with limited energy availability and temperature effects on reaction rates. We used polysome analysis to show that initiation of protein synthesis ceases during entrance into torpor in golden-mantled ground squirrels (Spermophilus lateralis) when Tb reaches 18 degrees C. Elongation of preinitiated polypeptides continues slowly throughout the torpor bout. As Tb begins to rise, initiation resumes even at temperatures below 18 degrees C, although the euthermic polysome pattern is not reestablished. At precisely 18 degrees C, there is a large increase in initiation events and a complete restoration of euthermic polysome distribution patterns. These data indicate a role for both passive and active depression of translation during torpor and are consistent with a requirement for new protein biosynthesis during each interbout arousal.

Growth arrest and DNA damage-inducible protein GADD34 assembles a novel signaling complex containing protein phosphatase 1 and inhibitor 1

The growth arrest and DNA damage-inducible protein, GADD34, was identified by its interaction with human inhibitor 1 (I-1), a protein kinase A (PKA)-activated inhibitor of type 1 protein serine/threonine phosphatase (PP1), in a yeast two-hybrid screen of a human brain cDNA library. Recombinant GADD34 (amino acids 233 to 674) bound both PKA-phosphorylated and unphosphorylated I-1(1-171). Serial truncations mapped the C terminus of I-1 (amino acids 142 to 171) as essential for GADD34 binding. In contrast, PKA phosphorylation was required for PP1 binding and inhibition by the N-terminal I-1(1-80) fragment. Pulldowns of GADD34 proteins expressed in HEK293T cells showed that I-1 bound the central domain of GADD34 (amino acids 180 to 483). By comparison, affinity isolation of cellular GADD34/PP1 complexes showed that PP1 bound near the C terminus of GADD34 (amino acids 483 to 619), a region that shows sequence homology with the virulence factors ICP34.5 of herpes simplex virus and NL-S of avian sarcoma virus. While GADD34 inhibited PP1-catalyzed dephosphorylation of phosphorylase a, the GADD34-bound PP1 was an active eIF-2alpha phosphatase. In brain extracts from active ground squirrels, GADD34 bound both I-1 and PP1 and eIF-2alpha was largely dephosphorylated. In contrast, the I-1/GADD34 and PP1/GADD34 interactions were disrupted in brain from hibernating animals, in which eIF-2alpha was highly phosphorylated at serine-51 and protein synthesis was inhibited. These studies suggested that modification of the I-1/GADD34/PP1 signaling complex regulates the initiation of protein translation in mammalian tissues.

Endoplasmic reticulum stress prolongs GH-induced Janus kinase (JAK2)/signal transducer and activator of transcription (STAT5) signaling pathway

The desensitization of the GH-induced Janus kinase 2 (JAK2) and signal transducer and activator of transcription 5 (STAT5) signaling pathway plays a crucial role in GH regulation of hepatic genes. Previous studies have demonstrated that the inactivation of the GH-induced JAK2/STAT5 pathway is regulated by protein translation and suppressors of cytokine signaling (SOCS). In this study we sought to explore the relationships between endoplasmic reticulum stress, GH-induced JAK2/STAT5 activity and SOCS expression. 1,2-bis(o-Aminophenoxy)ethane-N,N,N,N-tetraacetic acid (acetoxymethyl)ester (BAPTA-AM), used to provoke endoplasmic reticulum stress, caused a drastic inhibition of protein translation that correlated with the phosphorylation of the eukaryotic translation initiation factor 2alpha. Both GH and BAPTA-AM caused a rapid induction of the transcription factor C/EBP homology protein (CHOP) and an additive effect was observed with combined treatment, which suggests a regulatory role of GH on endoplasmic reticulum stress. Endoplasmic reticulum stress did not interfere with the rapid GH activation of STAT5 DNA binding activity. However, BAPTA-AM prolonged the DNA binding activity of STAT5 without affecting STAT5 or JAK2 protein levels. GH-induced phosphorylation of JAK2 and STAT5 DNA binding activity were prolonged in the presence of BAPTA-AM, suggesting that endoplasmic reticulum stress prevents the inactivation of STAT5 DNA binding activity by modulating the rate of JAK2/STAT5 dephosphorylation. Like BAPTA-AM, the endoplasmic reticulum stressors dithiothreitol and A23187 also prolonged the GH-induced STAT5 DNA binding activity. We were not able to correlate BAPTA-AM effects to the GH-dependent expression of SOCS proteins or SOCS mRNA, suggesting that endoplasmic reticulum stress modulates the rate of JAK2/STAT5 dephosphorylation through mechanisms other than inhibition of SOCS expression. This study indicates that cellular stress may modulate transcription through the JAK/STAT pathway.

Neuroprotective adaptations in hibernation: therapeutic implications for ischemia-reperfusion, traumatic brain injury and neurodegenerative diseases

Brains of hibernating mammals are protected against a variety of insults that are detrimental to humans and other nonhibernating species. Such protection is associated with a number of physiological adaptations including hypothermia, increased antioxidant defense, metabolic arrest, leukocytopenia, immunosuppression, and hypocoagulation. It is intriguing that similar manipulations provide considerable protection as experimental treatments for central nervous system injury. This review focuses on neuroprotective mechanisms employed during hibernation that may offer novel approaches in the treatment of stroke, traumatic brain injury, and neurodegenerative diseases in humans.

Hibernation, a model of neuroprotection

Hibernation, a natural model of tolerance to cerebral ischemia, represents a state of pronounced fluctuation in cerebral blood flow where no brain damage occurs. Numerous neuroprotective aspects may contribute in concert to such tolerance. The purpose of this study was to determine whether hibernating brain tissue is tolerant to penetrating brain injury modeled by insertion of microdialysis probes. Guide cannulae were surgically implanted in striatum of Arctic ground squirrels before any of the animals began to hibernate. Microdialysis probes were then inserted in some animals after they entered hibernation and in others while they remained euthermic. The brain tissue from hibernating and euthermic animals was examined 3 days after implantation of microdialysis probes. Tissue response, indicated by examination of hematoxylin and eosin-stained tissue sections and immunocytochemical identification of activated microglia, astrocytes, and hemeoxygenase-1 immunoreactivity, was dramatically attenuated around probe tracks in hibernating animals compared to euthermic controls. No difference in tissue response around guide cannulae was observed between groups. Further study of the mechanisms underlying neuroprotective aspects of hibernation may lead to novel therapeutic strategies for stroke and traumatic brain injury.

Depression of protein synthesis during diapause in embryos of the annual killifish Austrofundulus limnaeus

Rates of protein synthesis are substantially depressed in diapause II embryos of Austrofundulus limnaeus. Inhibition of oxygen consumption and heat dissipation with cycloheximide indicates that 36% of the adenosine triphosphate (ATP) turnover in prediapausing embryos (8 d postfertilization [dpf]) is caused by protein synthesis; the contribution of protein synthesis to ATP turnover in diapause II embryos is negligible. In agreement with the metabolic data, incorporation of amino acids (radiolabeled via (14)CO(2)) into perchloric acid-precipitable protein decreases by over 93% in diapause II embryos compared with embryos at 8 dpf. This result represents a 36% reduction in energy demand because of depression of protein synthesis during diapause. Adjusting for changes in the specific radioactivity of the free amino acid pool at the whole-embryo level yields rates of protein synthesis that are artifactually high and not supportable by the observed rates of oxygen consumption and heat dissipation during diapause. This result indicates a regionalized distribution of labeled amino acids likely dictated by a pattern of anterior to posterior cell cycle arrest. AMP/ATP ratios are strongly correlated with the decrease in rates of protein synthesis, which suggests a role for adenosine monophosphate (AMP) in the control of anabolic processes. The major depression of protein synthesis during diapause II affords a considerable reduction in energy demand and extends the duration of dormancy attainable in these embryos.

mRNA stability and polysome loss in hibernating Arctic ground squirrels (Spermophilus parryii)

All small mammalian hibernators periodically rewarm from torpor to high, euthermic body temperatures for brief intervals throughout the hibernating season. The functional significance of these arousal episodes is unknown, but one suggestion is that rewarming may be related to replacement of gene products lost during torpor due to degradation of mRNA. To assess the stability of mRNA as a function of the hibernation state, we examined the poly(A) tail lengths of liver mRNA from arctic ground squirrels sacrificed during four hibernation states (early and late during a torpor bout and early and late following arousal from torpor) and from active ground squirrels sacrificed in the summer. Poly(A) tail lengths were not altered during torpor, suggesting either that mRNA is stabilized or that transcription continues during torpor. In mRNA isolated from torpid ground squirrels, we observed a pattern of 12 poly(A) residues at greater densities approximately every 27 nucleotides along the poly(A) tail, which is a pattern consistent with binding of poly(A)-binding protein. The intensity of this pattern was significantly reduced following arousal from torpor and undetectable in mRNA obtained from summer ground squirrels. Analyses of polysome profiles revealed a significant reduction in polyribosomes in torpid animals, indicating that translation is depressed during torpor.

Gene up-regulation in heart during mammalian hibernation

A cDNA library prepared from heart of hibernating golden-mantled ground squirrels, Spermophilus lateralis, was differentially screened to clone genes that were up-regulated during hibernation. Two differentially expressed clones were found after three rounds of screening and were confirmed as up-regulated by Northern blotting. Clone Ang6 encoded a polypeptide with 116 amino acids that was identified as the ventricular isoform of myosin light chain 1 (MLC1(v)). Clone Ang19 coded for 274 amino acid residues of the mitochondrially encoded protein subunit 2 of NADH-ubiquinone oxidoreductase (ND2). Both proteins showed high amino acid sequence identity with their human counterparts, 97.5% for MLC1(v) and 66% for ND2. Northern blot hybridization revealed differential expression of these genes in multiple organs during hibernation. Transcript levels of both were approximately twofold higher in heart and three- to fourfold higher in skeletal muscle of hibernating, versus euthermic, animals. ND2 was also up-regulated in hibernator liver. Hibernation-induced up-regulation of MLC1(v) suggests that a restructuring of myosin subunit composition could contribute to changes in muscle contractility needed for hypothermic function, whereas changes in ND subunit composition may affect the function of the electron transport chain during hibernation.

Mitochondrial genome-encoded ATPase subunit 6+8 mRNA increases in human hepatoblastoma cells in response to nonfatal cold stress

Cellular responses to cold stress have not been well clarified, compared with heat shock responses, especially in mammalian cells. We investigated cold-stress responses in human hepatoblastoma cells (HepG2) exposed to a nonfatal temperature of 17 degrees C. Under the condition, RNA and protein syntheses in the cells were highly, but incompletely, depressed and cell growth was impaired. A cDNA subtraction method was used to isolate mRNAs for which the levels were increased in cold-stressed cells compared with cells cultured at 37 degrees C. A transcript isolated by the screening was identified as ATPase subunit 6+8 mRNA that encodes components of a mitochondrial ATPase complex and that is transcribed from a mitochondrial genome. The copy number of the mitochondrial genome in cells was not changed by cold stress. Thus, HepG2 cells were treated with various concentrations of actinomycin D and chloramphenicol to assess the effects of transcriptional and translational reduction on the increased level of the ATPase subunit 6+8 mRNA. The mRNA level was increased in cells treated with low concentrations of the RNA or protein synthesis inhibitors. These results indicate that the increase in ATPase subunit 6+8 mRNA stimulated by cold stress could be mediated by a partial decline of transcription and/or translation in the cells. In addition, the degradation of ATPase subunit 6+8 mRNA was suppressed in cold-stressed cells compared with that in 37 degrees C-cultured cells. This result implies that posttranscriptional regulation is also involved in the cold-stimulated increase in ATPase subunit 6+8 mRNA in HepG2 cells.

Dynamics of regional brain metabolism and gene expression after middle cerebral artery occlusion in mice

The evolution of brain infarcts during permanent occlusion of the middle cerebral artery (MCA) was studied in mice using multiparametric imaging techniques. Regional protein synthesis and the regional tissue content of ATP were measured on adjacent cryostat sections at increasing intervals after vascular occlusion ranging from 1 hour to 3 days. The observed changes were correlated with the expression of the mRNA of hsp70, c-fos, c-jun, and junB, as well as the distribution of DNA double-strand breaks visualized by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labelling (TUNEL). One hour after MCA occlusion, the tissue volume with suppressed protein synthesis was distinctly larger than that in which ATP was depleted. With ongoing ischemia time, the ATP-depleted area gradually expanded and, within 1 day, merged with the region of suppressed protein synthesis. Expression of hsp70 mRNA occurred mainly in the penumbra (defined as the region of suppressed protein synthesis but preserved ATP), peaking at 3 hours after vascular occlusion. Expression of the immediate-early genes c-jun, c-fos, and junB increased both in the penumbra and the periinfarct normal tissue already at 1 hour after vascular occlusion, with slightly different regional and temporal patterns for each of these genes. DNA fragmentations were clearly confined to neurons; they appeared after 1 day in the infarct core (defined as the region of suppressed ATP) and never were detected in the penumbra. The late appearance of TUNEL after infarcts had reached their final size and the absence in the penumbra points against a major pathogenetic role of apoptosis. Permanent MCA occlusion in mice thus produces a gradually expanding infarct, the final size of which is heralded by the early inhibition of protein synthesis.

Ascorbate and glutathione regulation in hibernating ground squirrels

Ground squirrels withstand up to 90% reductions in cerebral blood flow during hibernation as well as rapid reperfusion upon periodic arousals from torpor. Metabolic suppression likely plays a primary adaptive role which allows hibernating species to tolerate such phenomena. However, several other aspects of hibernation physiology are also consistent with tolerance to dramatic fluctuations in cerebral blood flow, suggesting that multiple neuroprotective adaptations may work in concert during hibernation. The purpose of the present work was to study the dynamics of the low molecular weight antioxidants, ascorbate and glutathione (GSH), during hibernation. Alterations in concentrations of ascorbate during hibernation and arousal in two species of hibernating ground squirrels suggest that it could play a protective role during hibernation or arousal. Samples were collected during the hibernation season from arctic ground squirrels (AGS; Spermophilus parryii) and 13-lined ground squirrels (TLS; S. tridecemlineatus) during prolonged torpor and in squirrels that did not hibernate or had not been hibernating for several weeks. We determined antioxidant levels in plasma, cerebrospinal fluid (CSF), and in frontal cortex, hippocampus and cerebellum using high-performance liquid chromatography (HPLC). Plasma ascorbate concentrations increased dramatically (3-4-fold) in both species during hibernation and rapidly returned to prehibernation levels upon arousal. By contrast, plasma GSH concentrations fell slightly or remained stable during hibernation. Ascorbate levels in the CSF doubled in hibernating AGS (not determined in TLS), while brain ascorbate content fell slightly (10-15%) in both species. Substantial increases in plasma and CSF ascorbate concentrations suggest that this antioxidant could play a protective role during hibernation and reperfusion upon arousal from hibernation.