Organelles do not colocalize with mRNA granules in post-ischemic neurons.

Following global brain ischemia and reperfusion, it is well-established that neurons undergo a translation arrest that is reversible in surviving neurons, but irreversible in vulnerable neurons. We previously showed a correlation between translation arrest in reperfused neurons and the presence of granular mRNA-containing structures we termed "mRNA granules." Here we further characterized the mRNA granules in reperfused neurons by performing colocalization studies using fluorescent in situ hybridization for poly(A) mRNAs and immunofluorescence histochemistry for markers of organelles and mRNA-binding proteins. There was no colocalization between the mRNA granules and markers of endoplasmic reticulum, cis- or trans-Golgi apparatus, mitochondria, microtubules, intermediate filaments, 60S ribosomal subunits, or the HuR ligands APRIL and pp32. The mRNA granules colocalized with the neuronal marker NeuN regardless of the relative vulnerability of the neuron type. RNA immunoprecipitation of HuR from the cytoplasmic fraction of 8 h reperfused forebrains selectively isolated hsp70 mRNA suggesting the mRNA granules are soluble structures. Together, these results rule out several organelle systems and a known HuR pathway as being directly involved in mRNA granule function.

Persistent redistribution of poly-adenylated mRNAs correlates with translation arrest and cell death following global brain ischemia and reperfusion.

Although persistent translation arrest correlates with the selective vulnerability of post-ischemic hippocampal cornu ammonis 1 (Ammon's horn) (CA1) neurons, the mechanism of persistent translation arrest is not fully understood. Using fluorescent in situ hybridization and immunofluorescence histochemistry, we studied colocalization of polyadenylated mRNAs [poly(A)] with the following mRNA binding factors: eukaryotic initiation factor (eIF) 4G (translation initiation factor), HuR (ARE-containing mRNA stabilizing protein), poly-adenylated mRNA binding protein (PABP), S6 (small ribosomal subunit marker), T cell internal antigen (TIA-1) (stress granule marker), and tristetraprolin (TTP) (processing body marker). We compared staining in vulnerable CA1 and resistant CA3 from 1 to 48 h reperfusion, following 10 min global ischemia in the rat. In both CA1 and CA3 neurons, cytoplasmic poly(A) mRNAs redistributed from a homogenous staining pattern seen in controls to granular structures we term mRNA granules. The mRNA granules abated after 16 h reperfusion in CA3, but persisted in CA1 neurons to 48 h reperfusion. Protein synthesis inhibition correlated precisely with the presence of the mRNA granules. In both CA1 and CA3, the mRNA granules colocalized with eIF4G and PABP, but not S6, TIA-1 or TTP, indicating that they were neither stress granules nor processing bodies. Colocalization of HuR in the mRNA granules correlated with translation of 70 kDa inducible heat shock protein, which occurred early in CA3 (8 h) and was delayed in CA1 (36 h). Thus, differential compartmentalization of mRNA away from the 40S subunit correlated with translation arrest in post-ischemic neurons, providing a concise mechanism of persistent translation arrest in post-ischemic CA1.

Convergence of stress granules and protein aggregates in hippocampal cornu ammonis 1 at later reperfusion following global brain ischemia.

The delayed and selective vulnerability of post-ischemic hippocampal cornu ammonis (CA) 1 pyramidal neurons correlates with a lack of recovery of normal protein synthesis. Recent evidence implicates sequestration of translational machinery into protein aggregates and stress granules as factors underlying persistent translation arrest in CA1 neurons. However, the relationship between protein aggregates and stress granules during brain reperfusion is unknown. Here we investigated the colocalization of protein aggregates and stress granules using immunofluorescence microscopy and pair-wise double labeling for ubiquitin/T cell internal antigen (TIA-1), ubiquitin/small ribosomal subunit protein 6 (S6), and TIA-1/S6. We evaluated the rat dorsal hippocampus at 1, 2 or 3 days of reperfusion following a 10 min global brain ischemic insult. At 1 day of reperfusion, ubiquitin-containing aggregates (ubi-protein clusters) occurred in neurons but did not colocalize with stress granules. At 2 days' reperfusion, only in CA1, cytoplasmic protein aggregates colocalized with stress granules, and ubiquitin-containing inclusions accumulated in the nuclei of CA1 pyramidal neurons. Functionally, a convergence of stress granules and protein aggregates would be expected to sustain translation arrest and inhibit clearance of ubiquitinated proteins, both factors expected to contribute to CA1 pyramidal neuron vulnerability.

Immunohistochemical mapping of total and phosphorylated eukaryotic initiation factor 4G in rat hippocampus following global brain ischemia and reperfusion.

Partial proteolysis and phosphorylation of the translation initiation factor eukaryotic initiation factor 4G (eIF4G) occur in reperfused brain, but the contribution of eIF4G alterations to brain injury has not been established. A component of the complex delivering mRNA to the small ribosomal subunit, eIF4G is also found in stress granules. Stress granules sequester inactive 48S preinitiation complexes during stress-induced translation arrest. We performed double-labeling immunofluorescence histochemistry for total or ser 1108 phosphorylated eIF4G and the stress granule component T-cell internal antigen following normothermic, 10 min cardiac arrest-induced global brain ischemia and up to 4 h reperfusion in the rat. In cornu ammonis (Ammon's horn; CA) 1 at 90 min and 4 h reperfusion, eIF4G staining transformed from a homogeneous to an aggregated distribution. The number of eIF4G-containing stress granules differed between CA1 and CA3 during reperfusion. In hippocampal pyramidal neurons, phosphorylated eIF4G appeared exclusively in stress granules. Supragranular interneurons of the dentate gyrus showed a large increase in cytoplasmic eIF4G(P) following reperfusion. Immunoblot analysis with antisera against different portions of eIF4G showed a large increase in phosphorylated C-terminal eIF4G fragments, suggesting these accumulate in the cytoplasm of dentate gyrus interneurons. Thus, altered eIF4G subcellular compartmentalization may contribute to prolonged translation arrest in CA1 pyramidal neurons. Accumulation of phosphorylated eIF4G fragments may contribute to the vulnerability of dentate interneurons. Ischemia and reperfusion invoke different translational control responses in distinct hippocampal neuron populations, which may contribute to the differential ischemic vulnerabilities of these cells.

Prolonged translation arrest in reperfused hippocampal cornu Ammonis 1 is mediated by stress granules.

Global brain ischemia and reperfusion cause phosphorylation of the alpha subunit of eukaryotic initiation factor 2alpha, a reversible event associated with neuronal translation inhibition. However, the selective vulnerability of cornu Ammonis (CA) 1 pyramidal neurons correlates with irreversible translation inhibition. Phosphorylation of eukaryotic initiation factor 2alpha also leads to the formation of stress granules, cytoplasmic foci containing, in part, components of the 48S pre-initiation complex and the RNA binding protein T cell internal antigen-1 (TIA-1). Stress granules are sites of translationally inactive protein synthesis machinery. Here we evaluated stress granules in rat hippocampal formation neurons after 10 min global brain ischemia and 10 min, 90 min or 4 h of reperfusion by double-labeling immunofluorescence for two stress granule components: small ribosomal subunit protein 6 and TIA-1. Stress granules in CA3, hilus and dentate gyrus, but not CA1, increased at 10 min reperfusion and returned to control levels by 90 min reperfusion. Dynamic changes in the nuclear distribution of TIA-1 occurred in resistant neurons. At 4 h reperfusion, small ribosomal subunit protein 6 was solely localized within stress granules only in CA1 pyramidal neurons. Both TIA-1 and small ribosomal subunit protein 6 levels decreased approximately 50% in hippocampus homogenates. Electron microscopy showed stress granules to be composed of electron dense bodies 100-200 nm in diameter, that were not membrane bound, but were associated with endoplasmic reticulum. Alterations in stress granule behavior in CA1 pyramidal neurons provide a definitive mechanism for the continued inhibition of protein synthesis in reperfused CA1 pyramidal neurons following dephosphorylation of eukaryotic initiation factor 2alpha.

Brain ischemia and reperfusion activates the eukaryotic initiation factor 2alpha kinase, PERK.

Reperfusion after global brain ischemia results initially in a widespread suppression of protein synthesis in neurons, which persists in vulnerable neurons, that is caused by the inhibition of translation initiation as a result of the phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 (eIF2alpha). To identify kinases responsible for eIF2alpha phosphorylation [eIF2alpha(P)] during brain reperfusion, we induced ischemia by bilateral carotid artery occlusion followed by post-ischemic assessment of brain eIF2alpha(P) in mice with homozygous functional knockouts in the genes encoding the heme-regulated eIF2alpha kinase (HRI), or the amino acid-regulated eIF2alpha kinase (GCN2). A 10-fold increase in eIF2alpha(P) was observed in reperfused wild-type mice and in the HRI-/- or GCN2-/- mice. However, in all reperfused groups, the RNA-dependent protein kinase (PKR)-like endoplasmic reticulum eIF2alpha kinase (PERK) exhibited an isoform mobility shift on SDS-PAGE, consistent with the activation of the kinase. These data indicate that neither HRI nor GCN2 are required for the large increase in post-ischemic brain eIF2alpha(P), and in conjunction with our previous report that eIF2alpha(P) is produced in the brain of reperfused PKR-/- mice, provides evidence that PERK is the kinase responsible for eIF2alpha phosphorylation in the early post-ischemic brain.

Brain ischemia and reperfusion: molecular mechanisms of neuronal injury.

Brain ischemia and reperfusion engage multiple independently-fatal terminal pathways involving loss of membrane integrity in partitioning ions, progressive proteolysis, and inability to check these processes because of loss of general translation competence and reduced survival signal-transduction. Ischemia results in rapid loss of high-energy phosphate compounds and generalized depolarization, which induces release of glutamate and, in selectively vulnerable neurons (SVNs), opening of both voltage-dependent and glutamate-regulated calcium channels. This allows a large increase in cytosolic Ca(2+) associated with activation of mu-calpain, calcineurin, and phospholipases with consequent proteolysis of calpain substrates (including spectrin and eIF4G), activation of NOS and potentially of Bad, and accumulation of free arachidonic acid, which can induce depletion of Ca(2+) from the ER lumen. A kinase that shuts off translation initiation by phosphorylating the alpha-subunit of eukaryotic initiation factor-2 (eIF2alpha) is activated either by adenosine degradation products or depletion of ER lumenal Ca(2+). Early during reperfusion, oxidative metabolism of arachidonate causes a burst of excess oxygen radicals, iron is released from storage proteins by superoxide-mediated reduction, and NO is generated. These events result in peroxynitrite generation, inappropriate protein nitrosylation, and lipid peroxidation, which ultrastructurally appears to principally damage the plasmalemma of SVNs. The initial recovery of ATP supports very rapid eIF2alpha phosphorylation that in SVNs is prolonged and associated with a major reduction in protein synthesis. High catecholamine levels induced by the ischemic episode itself and/or drug administration down-regulate insulin secretion and induce inhibition of growth-factor receptor tyrosine kinase activity, effects associated with down-regulation of survival signal-transduction through the Ras pathway. Caspase activation occurs during the early hours of reperfusion following mitochondrial release of caspase 9 and cytochrome c. The SVNs find themselves with substantial membrane damage, calpain-mediated proteolytic degradation of eIF4G and cytoskeletal proteins, altered translation initiation mechanisms that substantially reduce total protein synthesis and impose major alterations in message selection, down-regulated survival signal-transduction, and caspase activation. This picture argues powerfully that, for therapy of brain ischemia and reperfusion, the concept of single drug intervention (which has characterized the approaches of basic research, the pharmaceutical industry, and clinical trials) cannot be effective. Although rigorous study of multi-drug protocols is very demanding, effective therapy is likely to require (1) peptide growth factors for early activation of survival-signaling pathways and recovery of translation competence, (2) inhibition of lipid peroxidation, (3) inhibition of calpain, and (4) caspase inhibition. Examination of such protocols will require not only characterization of functional and histopathologic outcome, but also study of biochemical markers of the injury processes to establish the role of each drug.

Cell death, calcium mobilization, and immunostaining for phosphorylated eukaryotic initiation factor 2-alpha (eIF2alpha) in neuronally differentiated NB-104 cells: arachidonate and radical-mediated injury mechanisms.

These experiments examine the effects of arachidonate with respect to cell death, radical-mediated injury, Ca2+ mobilization, and formation of ser-51-phosphorylated eukaryotic initiation factor 2alpha [eIF2alpha(P)]. It is known that during brain ischemia the concentration of free arachidonate can reach 180 microM, and during reperfusion oxidative metabolism of arachidonate leads to generation of superoxide that can reduce stored ferric iron and promote lipid peroxidation. During early brain reperfusion, we have shown an approximately 20-fold increase in eIF2alpha(P) which maps to vulnerable neurons that display inhibition of protein synthesis. Here in neuronally differentiated NB-104 cells, equivalent cell death (assessed by LDH release) was induced by 40 microM arachidonate and 20 microM cumene hydroperoxide (CumOOH, a known alkoxyl radical generator). In these injury models (1) radical inhibitors (BHA, BHT, and the lipophilic iron chelator EMHP) block CumOOH-induced cell death but do not block arachidonate-induced death; (2) 40 microM arachidonate (but not up to 40 microM CumOOH) rapidly induces Ca2+ release from intracellular stores; (3) both 40 microM arachidonate and 20 microM CumOOH induce intense immunostaining for eIF2alpha(P); and (4) the elF2alpha(P) immunostaining induced by CumOOH but not that induced by arachidonate is completely blocked by anti-radical intervention with EMHP. Arachidonate-induced formation of eIF2alpha(P) and cell death do not require iron-mediated radical mechanisms and are associated with Ca2+ release from intracellular stores; however, radical-mediated injury also induces both eIF2alpha(P) and cell death without release of intracellular Ca2+. Our data link eIF2alpha(P) formation during brain reperfusion to two established injury mechanisms that may operate concurrently.

Eukaryotic initiation factor 2alpha kinase and phosphatase activity during postischemic brain reperfusion.

When ischemic brain is reperfused, there is in vulnerable neurons immediate inhibition of protein synthesis associated with a large increase in phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 [eIF2alpha, phosphorylated form eIF2alpha(P)]. We examined eIF2alpha kinase and eIF2alpha(P) phosphatase activity in brain homogenate postmitochondrial supernatants obtained from rats after 3 to 30 min of global brain ischemia (cardiac arrest), after 5 min of ischemia and 5 min of reperfusion (5R), and after 10 min of ischemia and 90 min reperfusion (90R). Because it has been suggested that PKR might be specifically responsible for producing eIF2alpha(P) during reperfusion, we also examined in brain homogenates from wild-type and PKR0/0 C57BL/6J x 129/SV mice the effect of 5 min of ischemia and 5 min of reperfusion on eIF2alpha(P). Cytosolic brain eIF2alpha(P) in the 5R and 90R rats was 18- and 23-fold that of nonischemic controls without any change in the rate of eIF2alpha(P) dephosphorylation. There was no change in eIF2alpha kinase activity between 3 and 30 min of ischemia but an 85% decrease in the 5R group; the 90R group was similar to controls. In wild-type and PKR0/0 mice total eIF2alpha was identical, and there was an identical 16-fold increase in eIF2alpha(P) at 5 min of reperfusion. Our observations contradict hypotheses that PKR activation, loss of eIF2alpha(P) phosphatase activity, or any general increase in eIF2alpha kinase activity are responsible for reperfusion-induced phosphorylation of eIF2alpha, and we suggest that the mechanism may involve regulation of the availability of eIF2alpha to a kinase.

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

Protein synthesis (PS) has been considered essential to sustain mammalian life, yet was found to be virtually arrested for weeks in brain and other organs of the hibernating ground squirrel, Spermophilus tridecemlineatus. PS, in vivo, was below the limit of autoradiographic detection in brain sections and, in brain extracts, was determined to be 0.04% of the average rate from active squirrels. Further, it was reduced 3-fold in cell-free extracts from hibernating brain at 37 degreesC, eliminating hypothermia as the only cause for protein synthesis inhibition (active, 0.47 +/- 0.08 pmol/mg protein per min; hibernator, 0.16 +/- 0.05 pmol/mg protein per min, P < 0.001). PS suppression involved blocks of initiation and elongation, and its onset coincided with the early transition phase into hibernation. An increased monosome peak with moderate ribosomal disaggregation in polysome profiles and the greatly increased phosphorylation of eIF2alpha are both consistent with an initiation block in hibernators. The elongation block was demonstrated by a 3-fold increase in ribosomal mean transit times in cell-free extracts from hibernators (active, 2.4 +/- 0.7 min; hibernator, 7.1 +/- 1.4 min, P < 0.001). No abnormalities of ribosomal function or mRNA levels were detected. These findings implicate suppression of PS as a component of the regulated shutdown of cellular function that permits hibernating ground squirrels to tolerate "trickle" blood flow and reduced substrate and oxygen availability. Further study of the factors that control these phenomena may lead to identification of the molecular mechanisms that regulate this state.

Calpain mediates eukaryotic initiation factor 4G degradation during global brain ischemia.

Global brain ischemia and reperfusion result in the degradation of the eukaryotic initiation factor (eIF) 4G, which plays a critical role in the attachment of the mRNA to the ribosome. Because eIF-4G is a substrate of calpain, these studies were undertaken to examine whether calpain I activation during global brain ischemia contributes to the degradation of eIF-4G in vivo. Immunoblots with antibodies against calpain I and eIF-4G were prepared from rat brain postmitochondrial supernatant incubated at 37 degrees C with and without the addition of calcium and the calpain inhibitors calpastatin or MDL-28,170. Addition of calcium alone resulted in calpain I activation (as measured by autolysis of the 80-kDa subunit) and degradation of eIF-4G; this effect was blocked by either 1 micromol/L calpastatin or 10 micromol/L MDL-28,170. In rabbits subjected to 20 minutes of cardiac arrest, immunoblots of brain postmitochondrial supernatants showed that the percentage of autolyzed calpain I increased from 1.9% +/- 1.1% to 15.8% +/- 5.0% and that this was accompanied by a 68% loss of eIF-4G. MDL-28,170 pretreatment (30 mg/kg) decreased ischemia-induced calpain I autolysis 40% and almost completely blocked eIF-4G degradation. We conclude that calpain I degrades eIF-4G during global brain ischemia.

Effect of brain ischemia and reperfusion on the localization of phosphorylated eukaryotic initiation factor 2 alpha.

Postischemic brain reperfusion is associated with a substantial and long-lasting reduction of protein synthesis in selectively vulnerable neurons. Because the overall translation initiation rate is typically regulated by altering the phosphorylation of serine 51 on the alpha-subunit of eukaryotic initiation factor 2 (eIF-2 alpha), we used an antibody specific to phosphorylated eIF-2 alpha [eIF-2(alpha P)] to study the regional and cellular distribution of eIF-2(alpha P) in normal, ischemic, and reperfused rat brains. Western blots of brain postmitochondrial supernatants revealed that approximately 1% of all eIF-2 alpha is phosphorylated in controls, eIF-2(alpha P) is not reduced by up to 30 minutes of ischemia, and eIF-2(alpha P) is increased approximately 20-fold after 10 and 90 minutes of reperfusion. Immunohistochemistry shows localization of eIF-2(alpha P) to astrocytes in normal brains, a massive increase in eIF-2(alpha P) in the cytoplasm of neurons within the first 10 minutes of reperfusion, accumulation of eIF-2(alpha P) in the nuclei of selectively vulnerable neurons after 1 hour of reperfusion, and morphology suggesting pyknosis or apoptosis in neuronal nuclei that continue to display eIF-2(alpha P) after 4 hours of reperfusion. These observations, together with the fact that eIF-2(alpha P) inhibits translation initiation, make a compelling case that eIF-2(alpha P) is responsible for reperfusion-induced inhibition of protein synthesis in vulnerable neurons.

Global brain ischemia and reperfusion: modifications in eukaryotic initiation factors associated with inhibition of translation initiation.

We used in vitro translation and antibodies against phosphoserine and the eukaryotic initiation factors elF-4E, elF-4G, and elF-2 alpha to examine the effects of global brain ischemia and reperfusion on translation initiation and its regulation in a rat model of 10 min of cardiac arrest followed by resuscitation and 90 min of reperfusion. Translation reactions were performed on postmitochondrial supernatants from brain homogenates with and without aurintricarboxylic acid to separate incorporation due to run-off from incorporation due to peptide synthesis initiated in vitro. The rate of leucine incorporation due to in vitro-initiated protein synthesis in normal forebrain homogenates was approximately 0.4 fmol of leucine/min/microgram of protein and was unaffected by 10 min of cardiac arrest, but 90 min of reperfusion reduced this rate 83%. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and western blots of these homogenates showed that neither 10 min of global brain ischemia nor 90 min of reperfusion induced significant alterations in the quantity or serine phosphorylation of elF-4E. However, we observed in all 90-min-reperfused samples elF-4G fragments that also bound elF-4E. The amount of elF-2 alpha was not altered by ischemia or reperfusion, and immunoblotting after isoelectric focusing did not detect serine-phosphorylated elF-2 alpha in normal samples or in those obtained after ischemia without reperfusion. However, serine-phosphorylated elF-2 alpha was uniformly present after 90 min of reperfusion and represented 24 +/- 3% of the elF-2 alpha in these samples. The serine phosphorylation of elF-2 alpha and partial fragmentation of elF-4G observed after 90 min of reperfusion offer an explanation for the inhibition of protein synthesis.

Global brain ischemia and reperfusion.

Brain damage accompanying cardiac arrest and resuscitation is frequent and devastating. Neurons in the hippocampus CA1 and CA4 zones and cortical layers III and V are selectively vulnerable to death after injury by ischemia and reperfusion. Ultrastructural evidence indicates that most of the structural damage is associated with reperfusion, during which the vulnerable neurons develop disaggregation of polyribosomes, peroxidative damage to unsaturated fatty acids in the plasma membrane, and prominent alterations in the structure of the Golgi apparatus that is responsible for membrane assembly. Reperfusion is also associated with vulnerable neurons with prominent production of messenger RNAs for stress proteins and for the proteins of the activator protein-1 complex, but these vulnerable neurons fail to efficiently translate these messages into the proteins. The inhibition of protein synthesis during reperfusion involves alteration of translation initiation factors, specifically serine phosphorylation of the alpha-subunit of eukaryotic initiation factor-2 (elF-2 alpha). Growth factors--in particular, insulin--have the potential to reverse phosphorylation of elF-2 alpha, promote effective translation of the mRNA transcripts generated in response to ischemia and reperfusion, enhance neuronal defenses against radicals, and stimulate lipid synthesis and membrane repair. There is now substantial evidence that the insulin-class growth factors have neuron-sparing effects against damage by radicals and ischemia and reperfusion. This new knowledge may provide a fundamental basis for a rational approach to "cerebral resuscitation" that will allow substantial amelioration of the often dismal neurologic outcome now associated with resuscitation from cardiac arrest.

Brain mu-calpain autolysis during global cerebral ischemia.

Proteolytic degradation of numerous calpain substrates, including cytoskeletal and regulatory proteins, has been observed during brain ischemia and reperfusion. In addition, calpain inhibitors have been shown to decrease degradation of these proteins and decrease postischemic neuronal death. Although these observations support the inference of a role for mu-calpain in the pathophysiology of ischemic neuronal injury, the evidence is indirect. A direct indicator of mu-calpain proteolytic activity is autolysis of its 80-kDa catalytic subunit, and therefore we examined the mu-calpain catalytic subunit for evidence of autolysis during cerebral ischemia. Rabbit brain homogenates obtained after 0, 5, 10, and 20 min of cardiac arrest were electrophoresed and immunoblotted with a monoclonal antibody specific to the mu-calpain catalytic subunit. In nonischemic brain homogenates the antibody identified an 80-kDa band, which migrated identically with purified mu-calpain, and faint 78- and 76-kDa bands, which represent autolyzed forms of the 80-kDa subunit. The average density of the 80-kDa band decreased by 25 +/- 4 (p = 0.008) and 28 +/- 9% (p = 0.004) after 10 and 20 min of cardiac arrest, respectively, whereas the average density of the 78-kDa band increased by 111 +/- 50% (p = 0.02) after 20 min of cardiac arrest. No significant change in the density of the 76-kDa band was detected. These results provide direct evidence for autolysis of brain mu-calpain during cerebral ischemia. Further work is needed to characterize the extent, duration, and localization of mu-calpain activity during brain ischemia and reperfusion as well as its role in the causal pathway of postischemic neuronal injury.

Eukaryotic initiation factor 4E degradation during brain ischemia.

Suppression of protein synthesis in the brain following an ischemic insult has been thought to occur because of inhibition of translation initiation. All eukaryotic mRNAs, with the exception of heat-shock transcripts, require the activity of eukaryotic initiation factor (eIF) 4E for formation of the translation initiation complex, and eIF-4E availability is rate-limiting. The response of brain eIF-4E concentration and phosphorylation following decapitation ischemia was studied in rat brain homogenates after electrophoresis and western blotting with antibodies against eIF-4E and phosphoserine, respectively. There was no change in level of eIF-4E after 5 min of ischemia (p = 0.82 vs. time 0), but it had decreased 32 (p = 0.01) and 57% (p = 0.006) after 10 and 20 min of ischemia, respectively. There was no loss of serine phosphorylation on eIF-4E beyond signal loss observed due to degradation of the protein itself (p = 0.31). In vitro exposure of eIF-4E to activated mu-calpain resulted in a 50% loss in 10 min of eIF-4E on western blots. If active eIF-4E is required for translation of its own mRNA, degradation of this protein during ischemia, possibly by activated mu-calpain, could be a direct mechanism of irreversible neuronal injury, and the rate of proteolysis of eIF-4E could place an upper time limit on the maximal duration of global brain ischemia compatible with neurologic recovery.

Insulin induces tyrosine phosphorylation of a 90-kDa protein during postischemic brain reperfusion.

Rat brain nuclear proteins were examined for tyrosine phosphorylation after resuscitation from a 10-min cardiac arrest. Insulin (1 unit/kg intravenously), given immediately after resuscitation, caused a marked increase in tyrosine phosphorylation of a 90-kDa brain protein. This effect occurred without hypoglycemia and was not observed after insulin administration in previously insulinopenic, diabetic, nonischemic animals. Insulin-responsive tyrosine phosphorylation of a specific 90-kDa protein during reperfusion may represent insulin stimulation of a neuroprotective brain response to an ischemic insult, consistent with recent observations that insulin administration during reperfusion protects selectively vulnerable neurons from postischemic death.

Studies of the protein synthesis system in the brain cortex during global ischemia and reperfusion.

Previous studies have demonstrated that brain protein synthesis declines after global ischemia and reperfusion. To investigate the role of the translation system in this phenomenon, we examined the ability of partially purified ribosomes, ribosome-bound mRNA and translation cofactors derived from the transiently ischemic cerebral cortex to synthesize protein in vitro. Samples were prepared from canines subjected to 20-min cardiac arrest and after 2 or 8 h of post-resuscitation intensive care. There was no significant decrease in the rate of in vitro protein synthesis as a consequence of either ischemia or reperfusion. Northern hybridization of ribosome-bound RNA revealed a discrete band of mRNA for brain-specific creatine kinase (ck-bb) that was consistent in presence and intensity in all groups. However, mRNA for heat shock 70 protein (hsp-70) was observed only during reperfusion and markedly increased between 2 and 8 h reperfusion. Thus, we conclude that (1) the transcription system is intact during reperfusion and hsp-70 mRNA is made and translocated to the ribosomes during reperfusion, (2) mRNA for ck-bb is not displaced from ribosomes by the appearance of hsp-70 during reperfusion and (3) isolated ribosomes maintain their ability to translate in vitro during the first 8 h of reperfusion after global brain ischemia. Therefore, the early reduction in protein synthesis observed in vivo during post-ischemic brain reperfusion is not due to an intrinsic dysfunction of the ribosomes.

Fluorescent histochemical localization of lipid peroxidation during brain reperfusion following cardiac arrest.

Rats were subjected to cardiac arrest and resuscitation, 90 min of reperfusion, and in situ perfusion fixation. Thiobarbituric acid (TBA) was included in the aldehyde-free perfusion fixative, the TBA reaction was driven in situ by heating, and fluorescence microscopy was utilized to characterize the location of products of the TBA reaction. Absorbance-difference spectra were performed on butanol-extracted brain homogenates to confirm in situ formation of TBA adducts with aldehydic products of lipid peroxidation. Nissl-stained sections revealed good cellular fixation without shrinkage artifacts. Fluorescence was not seen microscopically when TBA was omitted from the perfusion fixative, and little fluorescence was present in normal brains or brains after ischemia only. However, after 90-min reperfusion, intense granular fluorescence was seen in the neuronal perikarya (especially at the base of the apical dendrite) of numerous pyramidal neurons in cortical layers 5 and 6 and in the pyramidal layer of Ammon's horn in the hippocampus. The nuclei of these cells exhibited no fluorescence. Fluorescence was also present in some striatal neurons, but was absent in the adjacent radial bundles. Neither glia nor white matter exhibited similar fluorescence. These observations indicate that neurons in the selectively vulnerable zones of the cortex and hippocampus are early and specific targets of lipid peroxidation during post-ischemic reperfusion.