Preparation of a cell-free extract from rat brain which can initiate protein synthesis in vitro

Intracerebroventricular administration of ouabain, a Na/K-ATPase inhibitor, activates mTOR signal pathways and protein translation in the rat frontal cortex

Intracerebroventricular (ICV) injection of ouabain, a specific Na/K-ATPase inhibitor, induces behavioral changes in rats in a putative animal model of mania. The binding of ouabain to Na/K-ATPase affects signaling molecules in vitro, including ERK1/2 and Akt, which promote protein translation. We have also reported that ERK1/2 and Akt in the brain are involved in the ouabain-induced hyperactivity of rats. In this study, rats were given an ICV injection of ouabain, and then their frontal cortices were examined to determine the effects of ouabain on the mTOR/p70S6K/S6 signaling pathway and protein translation, which are important in modifications of neural circuits and behavior. Rats showed ouabain-induced hyperactivity up to 8h following injection, and increased phosphorylation levels of mTOR, p70S6K, S6, eIF4B, and 4E-BP at 1, 2, 4, and 8h following ouabain injection. Immunohistochemical analyses revealed that increased p-S6 immunoreactivity in the cytoplasm of neurons by ouabain was evident in the prefrontal, cingulate, and orbital cortex. These findings suggested increased translation initiation in response to ouabain. The rate of protein synthesis was measured as the amount of [(3)H]-leucine incorporation in the cell-free extracts of frontal cortical tissues, and showed a significant increase at 8h after ouabain injection. These results suggest that ICV injection of ouabain induced activation of the protein translation initiation pathway regulated by ERK1/2 and Akt, and prolonged hyperactivity in rats. In conclusion, protein translation pathway could play an important role in ouabain-induced hyperactivity in a rodent model of mania.

Effects of neonatal MK-801 treatment on p70S6K-S6/eIF4B signal pathways and protein translation in the frontal cortex of the developing rat brain

Systemic injections of MK-801, a selective NMDAR antagonist, into neonatal rats induces long-term neurochemical and behavioural changes. It has been suggested that these changes form the neurodevelopmental basis for schizophrenia-like behaviour in rats. In this study, 7-d-old rats were treated with MK-801, and their frontal cortices were examined to investigate the effects on p70S6K-S6 signal pathway and on protein translation, which play crucial roles in the neurodevelopmental process. MK-801, in doses of 0.5 and 1.0 mg/kg, induced a decrease in phosphorylation of p70S6K and its substrates, S6 and eIF4B, in the first 8 h, and no change at 24 and 48 h. These effects were more prominent after two injections of MK-801 than one. Decreased S6 phosphorylation by MK-801 was evident in the prefrontal, cingulate, and insular cortex. In two representative upstream p70S6K-S6 pathways related to ERK1/2 and Akt, changes in ERK1/2-p90RSK phosphorylation were accompanied by changes of p70S6K-S6. Although two MK-801 injections induced a dose-dependent decrease in phosphorylation of Akt and mTOR at 4 and 8 h, a single injection did not produce a significant effect. Protein synthesis rate, measured by [3H]leucine incorporation in frontal cortical tissue, was reduced until 24 h after two MK-801 (1.0 mg/kg) injections. In summary, this study found that neonatal MK-801 treatment induced dysregulation in the p70S6K-S6/eIF4B pathway and protein translation in the frontal cortex of the developing rat brain, which may suggest an important role of protein translation machinery in the MK-801 neurodevelopmental animal model of schizophrenia.

PKR-like endoplasmic reticulum kinase (PERK) activation following brain ischemia is independent of unfolded nascent proteins

Transient global brain ischemia results in an immediate inhibition of protein translation upon reperfusion. During early brain reperfusion protein synthesis is inhibited by alpha subunit of eukaryotic initiation factor 2 (eIF2alpha) phosphorylation by the PKR-like endoplasmic reticulum kinase (PERK). Normally, PERK is held in an inactive, monomeric state by the binding of the endoplasmic reticulum (ER) chaperone GRP78 to the lumenal end of PERK. The prevailing view is that ER stress leads to the accumulation of unfolded proteins in the ER lumen. GRP78 dissociates from PERK to bind these accumulated unfolded proteins, leading to PERK activation, phosphorylation of eIF2alpha, and inhibition of translation. To determine if an increase in unfolded nascent proteins following transient brain ischemia contributes to PERK activation, protein synthesis was blocked by intracerebral injection of anisomycin prior to induction of ischemia. Anisomycin inhibited protein synthesis by over 99% and reduced newly synthesized proteins in the ER to approximately 20% of controls. With an ER nearly devoid of newly synthesized proteins, PERK was still activated and was able to phosphorylate eIF2alpha in CA1 neurons during reperfusion. These data strongly argue that PERK activation is independent of the large increase in unfolded nascent proteins within the ER following transient global brain ischemia.

Proteasome inhibition induces reversible impairments in protein synthesis

Proteasome inhibition occurs during normal aging and in a variety of age-related diseases, with inhibition of proteasome function sufficient to induce physiological and pathological alterations observed in each of these conditions. It is presumed that proteasome inhibition induces cellular alterations by promoting rapid protein accumulation, as the direct result of impairments in protein removal, which assumes protein synthesis remains relatively unchanged during proteasome inhibition. We conducted experimentation using established proteasome inhibitors and primary rat neuron cultures in order to elucidate whether proteasome inhibition had any effect on neuronal protein synthesis. Proteasome inhibition impaired neuronal protein synthesis, with concentrations of inhibitor necessary to significantly inhibit protein synthesis similar to the concentrations necessary to induce subsequent neuron death. The inhibition of protein synthesis was reversible during the first 6 h of treatment, with the neurotoxicity of proteasome inhibition reversible during the first 12 h of treatment. These studies are the first to demonstrate a potentially important interplay between the proteasome and protein synthesis in neurons, and the first to identify that some effects of proteasome inhibition are reversible in neurons. Together these findings have important implications for understanding proteasome inhibition as a potential contributor to aging and age-related disease.

Decreased RNA, and Increased RNA Oxidation, in Ribosomes from Early Alzheimer’s Disease

All cells rely on efficient protein synthesis in order to maintain cellular homeostasis. Recent studies from our laboratory indicate that declines in protein synthesis and ribosome function occur in the earliest stage of Alzheimer's disease (AD). Additional studies indicate a potential role for ribosomal RNA oxidation as a potential mediator of decreased protein synthesis in AD. The ribosome is a complex of proteins and nucleic acids that mediates all protein synthesis. At present it is unclear if significant alterations in ribosomal RNA occurs within the ribosome complex during the progression of AD. In this study we examined the amount of ribosomal RNA in the different ribosomal fractions generated from control subjects, individuals with mild cognitive impairment (MCI), and individuals with AD. Studies were conducted in the inferior parietal lobule of each subject. Together, these data demonstrate that during the progression of AD there is a gross decline in the amount of ribosomal RNA within the ribosome complex. Additionally, these studies provide evidence for gross elevations in RNA oxidation within the ribosome complex of MCI and AD. Together, these data strongly suggest a role for RNA alterations within the ribosome as a mediator of decreased protein synthesis in both MCI and AD.

Ribosome dysfunction is an early event in Alzheimer's disease

Alzheimer's disease (AD) is a progressive and devastating disorder that is often preceded by mild cognitive impairment (MCI). In the present study, we report that in multiple cortical areas of MCI and AD subjects, there is a significant impairment in ribosome function that is not observed in the cerebellum of the same subjects. The impairment in ribosome function is associated with a decreased rate and capacity for protein synthesis, decreased ribosomal RNA and tRNA levels, and increased RNA oxidation. No alteration in the level of initiation factors was observed in the brain regions exhibiting impairments in protein synthesis. Together, these data indicate for the first time that impairments in protein synthesis may be one of the earliest neurochemical alterations in AD and directly demonstrate that the polyribosome complex is adversely affected early in the development of AD. These data have important implications for AD studies involving proteomics and studies analyzing proteolysis in AD, indicate that oxidative damage may contribute to decreased protein synthesis, and suggest a role for alterations in protein synthesis as a novel contributor to the onset and development of AD.

PERK is responsible for the increased phosphorylation of eIF2alpha and the severe inhibition of protein synthesis after transient global brain ischemia

Reperfusion after global brain ischemia results initially in a widespread suppression of protein synthesis in neurons that is due to inhibition of translation initiation as a result of the phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 (eIF2). To address the role of the eIF2alpha kinase RNA-dependent protein kinase-like endoplasmic reticulum kinase (PERK) in the reperfused brain, transgenic mice with a targeted disruption of the Perk gene were subjected to 20 min of forebrain ischemia followed by 10 min of reperfusion. In wild-type mice, phosphorylated eIF2alpha was detected in the non-ischemic brain and its levels were elevated threefold after 10 min of reperfusion. Conversely, there was no phosphorylated eIF2alpha detected in the non-ischemic transgenic mice and there was no sizeable rise in phosphorylated eIF2alpha levels in the forebrain after ischemia and reperfusion. Moreover, there was a substantial rescue of protein translation in the reperfused transgenic mice. Neither group showed any change in total eIF2alpha, phosphorylated eukaryotic elongation factor 2 or total eukaryotic elongation factor 2 levels. These data demonstrate that PERK is responsible for the large increase in phosphorylated eIF2alpha and the suppression of translation early in reperfusion after transient global brain ischemia.

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.

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.

The intraischemic and early reperfusion changes of protein synthesis in the rat brain. eIF-2 alpha kinase activity and role of initiation factors eIF-2 alpha and eIF-4E

Rats were subjected to the standard four-vessel occlusion model of transient cerebral ischemia (vertebral and carotid arteries). The effects of normothermic ischemia (37 degrees C) followed or not by 30-minute reperfusion, as well as 30-minute postdecapitative ischemia, on translational rates were examined. Protein synthesis rate, as measured in a cell-free system, was significantly inhibited in ischemic rats, and the extent of inhibition strongly depended on duration and temperature, and less on the model of ischemia used. The ability of reinitiation in vitro (by using aurintricarboxylic acid) decreased after ischemia, suggesting a failure in the synthetic machinery at the initiation level. Eukaryotic initiation factor 2 (eIF-2) presented almost basal activity and levels after 30-minute normothermic ischemia, and the amount of phosphorylated eIF-2 alpha in these samples, as well as in sham-control samples, was undetectable. The decrease in the levels of phosphorylated initiation factor 4E (eIF-4E) after 30-minute ischemia (from 32% to 16%) could explain, at least partially, the impairment of initiation during transient cerebral ischemia. After reperfusion, eIF-4E phosphorylation was almost completely restored to basal levels (29%), whereas the level of phosphorylated eIF-2 alpha was higher (13%) than in controls and ischemic samples (both less than 2%). eIF-2 alpha kinase activity in vitro as measured by phosphorylation of endogenous eIF-2 in the presence of ATP/Mg2+, was higher in ischemic samples (8%) than in controls (4%). It seems probable that the failure of the kinase in phosphorylating eIF-2 in vivo during ischemia is due to the depletion of ATP stores. The levels of the double-stranded activated eIF-2 alpha kinase were slightly higher in ischemic animals than in controls. Our results suggest that the modulation of eIF-4E phosphorylation could be implicated in the regulation of translation during ischemia. On the contrary, phosphorylation of eIF-2 alpha, by an eIF-2 alpha kinase already activated during ischemia, represents a plausible mechanism for explaining the inhibition of translation during reperfusion.

Phosphorylation of the alpha subunit of initiation factor 2 correlates with the inhibition of translation following transient cerebral ischaemia in the rat

Rats were subjected to the standard four-vessel occlusion model of cerebral transient ischaemia (vertebral and carotid arteries) for 15 and 30 min. After a 30 min recirculation period, protein synthesis rate, initiation factor 2 (eIF-2) and guanine nucleotide exchange factor (GEF) activities, and the level of phosphorylation of the alpha subunit of eIF-2 (eIF-2 alpha) were determined in the neocortex region of the brain from sham-operated controls and ischaemic animals. Following reversible cerebral ischaemia, the protein synthesis rate, as measured in a cell-free system, was significantly inhibited (70%) in the ischaemic animals. eIF-2 activity, as measured by its ability to form a ternary complex, also decrease parallel to the decrease in protein synthesis. As eIF-2 activity was assayed in the presence of Mg2+ and GTP-regenerating capacity, the decrease in ternary-complex formation indicated the possible impairment of GEF activity. Since phosphorylated eIF-2 [eIF-2(alpha P)] is a powerful inhibitor of GEF, the levels of phosphorylated eIF-2 alpha were determined, and an increase from 7% phosphorylation in sham control rats to 20% phosphorylation in 15 min and 29% phosphorylation in 30 min in ischaemic rats was observed, providing evidence for a tight correlation of phosphorylation of eIF-2 alpha and inhibition of protein synthesis. Moreover, GEF activity measured in the GDP-exchange assay was in fact inhibited in the ischaemic animals, proving that protein synthesis is impaired by the presence of eIF-2(alpha P), which blocks eIF-2 recycling.

In vitro translation in a hamster brain cell-free system

We have developed a cell-free translation system derived from hamster brain tissue. The optimal incorporation conditions were 160 mM K+, 2 mM Mg2+ and microM spermine. The absence of this latter compound could be compensated only by doubling (to 4 mM) Mg2+ concentration. This system was inhibited by cycloheximide and emetine (elongation inhibitors) as well as NaF and aurintricarboxilic acid (ATA) (initiation inhibitors). The nature of the ATA inhibiting effect was studied by kinetic comparison with cycloheximide and polyribosome sedimentation pattern.

In vitro localization of the protein synthesis defect associated with experimental phenylketonuria

We have used a cell-free system derived from hamster brain to investigate protein synthesis during experimental phenylketonuria. In such a system the elongation inhibitor emetine impeded translation in extracts derived from both treated and control animals. On the other hand the initiation inhibitor aurintricarboxylic acid showed no effects on protein synthesis activity of treated hamsters, although it was severely inhibiting in controls. This suggests that initiation is the altered step in brain protein synthesis failure consecutive to phenylketonuria.

Regional analysis of rat brain proteins during senescence

Brains of male Fischer-344 rats aged 3-4 months and 28-30 months were dissected into 11 regions, and the patterns of brain proteins in these regions were analyzed using two dimensional gel electrophoresis in conjunction with a sensitive silver stain detection method. Several hundred abundant and moderately abundant brain proteins were detected in each region using this method. At 3-4 months, most proteins were present in approximately equal amounts in each of the 11 regions. On the whole, this distribution was maintained as a function of age. One protein of 21 kdal pI 5.1 was present in the cerebellum in greater amount than the other regions at both 3-4 months and 28-30 months. Two proteins, 44 kdal pI 5.4 and 47 kdal pI 5.2 were present at increased levels in the inferior colliculus of 28-30 month animals compared to 3-4 month animals. Of the abundant and moderately abundant brain proteins detected by this method, there were none which showed major decreases in levels as a function of age. These results provide support for the concept that the molecular mechanisms which result in differential gene expression in different regions of the young adult rat brain are operative and are maintained in the brains of senescent rats.

Absence of age differences in protein synthesis by rat brain, measured with an initiating cell-free system

A cell-free protein synthesis system was derived from brains of young (3 month) and old (greater than 23 month) male Fischer-344 rats in order to examine brain protein synthesis in relation to age. The system was shown to be capable of reinitiating protein synthesis in vitro, and of synthesizing protein from exogenously added mRNA. Optimal ionic conditions for amino acid incorporation were 200 mM potassium ion and 5 mM magnesium ion, and amino acid incorporation depended on addition of ATP, GTP, and an energy-generating system (creatine phosphate and creatine phosphokinase). Amino acid incorporation was sensitive to the initiation inhibitors aurintricarboxylic acid and sodium fluoride. Optimal conditions were independent of the age of the rat from which the brain was taken. There was no statistically significant relation (p greater than 0.05) between capacity of amino acid incorporation and age. The aggregation state of brain polyribosomes also did not differ between young (3 month) and old (30 month) rats. The results suggest that overall brain protein synthesis capacity is age-invariant in the rat.