Brain protein synthesis in the conscious rat using L-[35S]methionine: relationship of methionine specific activity between plasma and precursor compartment and evaluation of methionine metabolic pathways

The transsulfuration pathway: a source of cysteine for glutathione in astrocytes

Astrocyte cells require cysteine as a substrate for glutamate cysteine ligase (γ-glutamylcysteine synthase; EC 6.3.2.2) catalyst of the rate-limiting step of the γ-glutamylcycle leading to formation of glutathione (L: -γ-glutamyl-L: -cysteinyl-glycine; GSH). In both astrocytes and glioblastoma/astrocytoma cells, the majority of cysteine originates from reduction of cystine imported by the x (c) (-) cystine-glutamate exchanger. However, the transsulfuration pathway, which supplies cysteine from the indispensable amino acid, methionine, has recently been identified as a significant contributor to GSH synthesis in astrocytes. The purpose of this review is to evaluate the importance of the transsulfuration pathway in these cells, particularly in the context of a reserve pathway that channels methionine towards cysteine when the demand for glutathione is high, or under conditions in which the supply of cystine by the x (c) (-) exchanger may be compromised.

Crossed cerebellar diaschisis: a positron emission tomography study withl-[methyl-11C]methionine and 2-deoxy-2-[18F]fluoro-d-glucose

OBJECTIVE

Crossed cerebellar diaschisis (CCD) is defined as a depression of blood flow and oxidative metabolism of glucose in the cerebellum contralateral to a supratentorial brain lesion, as detected with positron emission tomography (PET) and single photon emission computed tomography. We examined whether L-[methyl-11C]methionine (MET) uptake is affected in CCD.

METHODS

In 12 patients with a unilateral supratentorial brain tumor, we evaluated the uptake of 2-deoxy-2-[18F]fluoro-D-glucose (FDG) and MET in the cerebellar hemispheres by means of PET. Asymmetry index (AI) was defined as a difference in the average count between the ipsilateral and contralateral cerebellar hemispheres divided by the average count in both cerebellar hemispheres. Patients with AI of FDG PET more than 0.1 and those with AI equal to 0.1 or less than 0.1 were classified as CCD-positive and CCD-negative, respectively.

RESULTS

Six patients were CCD-positive and others were CCD-negative in the FDG PET study. Between CCD-positive and CCD-negative patients, mean AI of MET was not significantly different (0.017 +/- 0.023 and 0.014 +/- 0.039, respectively).

CONCLUSIONS

Different from glucose metabolism, cerebellar MET uptake was not affected in CCD. The present study may indicate that cerebellar MET uptake is independent of suppression of cerebellar neuronal activity.

Ischemic cell death in brain neurons

This review is directed at understanding how neuronal death occurs in two distinct insults, global ischemia and focal ischemia. These are the two principal rodent models for human disease. Cell death occurs by a necrotic pathway characterized by either ischemic/homogenizing cell change or edematous cell change. Death also occurs via an apoptotic-like pathway that is characterized, minimally, by DNA laddering and a dependence on caspase activity and, optimally, by those properties, additional characteristic protein and phospholipid changes, and morphological attributes of apoptosis. Death may also occur by autophagocytosis. The cell death process has four major stages. The first, the induction stage, includes several changes initiated by ischemia and reperfusion that are very likely to play major roles in cell death. These include inhibition (and subsequent reactivation) of electron transport, decreased ATP, decreased pH, increased cell Ca(2+), release of glutamate, increased arachidonic acid, and also gene activation leading to cytokine synthesis, synthesis of enzymes involved in free radical production, and accumulation of leukocytes. These changes lead to the activation of five damaging events, termed perpetrators. These are the damaging actions of free radicals and their product peroxynitrite, the actions of the Ca(2+)-dependent protease calpain, the activity of phospholipases, the activity of poly-ADPribose polymerase (PARP), and the activation of the apoptotic pathway. The second stage of cell death involves the long-term changes in macromolecules or key metabolites that are caused by the perpetrators. The third stage of cell death involves long-term damaging effects of these macromolecular and metabolite changes, and of some of the induction processes, on critical cell functions and structures that lead to the defined end stages of cell damage. These targeted functions and structures include the plasmalemma, the mitochondria, the cytoskeleton, protein synthesis, and kinase activities. The fourth stage is the progression to the morphological and biochemical end stages of cell death. Of these four stages, the last two are the least well understood. Quite little is known of how the perpetrators affect the structures and functions and whether and how each of these changes contribute to cell death. According to this description, the key step in ischemic cell death is adequate activation of the perpetrators, and thus a major unifying thread of the review is a consideration of how the changes occurring during and after ischemia, including gene activation and synthesis of new proteins, conspire to produce damaging levels of free radicals and peroxynitrite, to activate calpain and other Ca(2+)-driven processes that are damaging, and to initiate the apoptotic process. Although it is not fully established for all cases, the major driving force for the necrotic cell death process, and very possibly the other processes, appears to be the generation of free radicals and peroxynitrite. Effects of a large number of damaging changes can be explained on the basis of their ability to generate free radicals in early or late stages of damage. Several important issues are defined for future study. These include determining the triggers for apoptosis and autophagocytosis and establishing greater confidence in most of the cellular changes that are hypothesized to be involved in cell death. A very important outstanding issue is identifying the critical functional and structural changes caused by the perpetrators of cell death. These changes are responsible for cell death, and their identity and mechanisms of action are almost completely unknown.

Early and prolonged widespread increase in brain protein synthesis following a single electroconvulsive shock in free-moving rats

The autoradiographic method with l-[35S] methionine ([35S]Met) was used to determine the effect of a single electroconvulsive shock (ECS) on local rates of protein synthesis in the adult rat brain in free-moving conditions. We have estimated the relative contribution of methionine derived from protein breakdown to the intracellular precursor amino acid pool (tRNA pool) for protein synthesis. In steady-state conditions, we showed a large contribution (around 60%) of Met recycling into the precursor pool (lambda=0.37+/-0.11), after a single ECS. In all the 36 brain regions examined, apparent rates of protein synthesis were greatly increased (21-50%) 3 h after a single ECS indicating a generalized effect in rat brain. This ECS-induced activation of the overall rate of brain protein synthesis persisted for at least 24 h after cessation of ECS. This is consistent with the hypothesis that electroconvulsive therapy is associated with long-term molecular changes in neuronal activity.

N-acetylcysteine, but not methionine or 2-oxothiazolidine-4-carboxylate, serves as cysteine donor for the synthesis of glutathione in cultured neurons derived from embryonal rat brain

The ability of neurons to metabolize sulfur-containing compounds to cysteine was investigated using as indicator the glutathione content in neuron-rich primary cultures derived from the brains of embryonal rats. The-glutathione content of these cultures was doubled during a 4-h incubation in a minimal medium containing cysteine, glutamine and glycine. In contrast, absence of cysteine or replacement of cysteine by methionine or 2-oxothiazolidine-4-carboxylate failed to increase the glutathione content of cultured neurons. Besides cysteine, N-acetylcysteine (NAC) also caused in the millimolar range, a concentration-dependent increase in the neuronal glutathione content during a 4-h incubation. These data suggest that neurons in culture, contain an acylase activity which allows them to generate from extracellular NAC as precursor intracellular cysteine in concentrations sufficient for glutathione synthesis. In contrast, generation of cysteine from 2-oxothiazolidine-4-carboxylate by the reaction of 5-oxoprolinase or from methionine by the transsulfuration pathway appears not to take place in these cultured neurons.

Paradoxical metabolic response of the human brain to a single electroconvulsive shock

Regional brain protein synthesis was evaluated with positron emission tomography (PET) and L-(S-[11C]methyl)methionine ([11C]MET) in depressive patients, before and 3 h after an electroconvulsive shock (ECS), when energy supply is restored, and in healthy volunteers. Depressive patients presented apparent lower protein synthesis than normals, in agreement with known reduction of cerebral activity. In contrast, ECS resulted in a significant increase (56%, P < 0.05) in global cortical protein synthesis. This paradoxical hyperactivation of cellular protein metabolism in response to seizures and the fact that synaptic activity is further reduced after electroconvulsive therapy (ECT), may provide new insights for understanding the mechanism of action of ECT.

Quantitative measurement of cerebral protein synthesis in vivo: theory and methodological considerations

The true rate of cerebral protein synthesis can be calculated from the ratio of labeled proteins to integrated arterial plasma amino acid specific activity (SA) only when the fraction of amino acid precursor pool dilution is known. In the following, current experimental designs on the measurement of cerebral protein synthesis are discussed and compared to our own approach in which the determination of regional precursor pool dilution by recycled unlabeled leucine is combined with the quantitation of regional cerebral protein synthesis rates. For this purpose, a constant arterial plasma leucine SA level is maintained for 45 min by programmed intravenous infusion which is sufficient for complete equilibrium between tissue leucine pool SAs and plasma free leucine SA. In addition to the regional assessment of the precursor dilution factor, protein radioactivity can be determined in the same tissue sample or in parallel brain sections of the same animal by quantitative autoradiography. It is then possible to calculate the actual rate of protein synthesis using the correct fraction of precursor pool dilution. This renders our approach particularly suitable for the quantitative measurement of regional CPS under pathological conditions.

Widespread increase in brain protein synthesis following acute immobilization stress in adult rat brain

The autoradiographic method with [L-35S]methionine was used to determine the effects of a 2 h acute immobilization stress followed by a 4 h recovery on local rates of protein synthesis in the adult rat brain. Methionine incorporation into proteins was significantly increased (from 17 to 86%) in 37 out of the 39 analyzed brain structures. These results show that the stress-induced activation of the overall rate of brain protein synthesis may persist for at least 4 h after cessation of the stimulus even though the stress-related physiological variables have returned to basal levels. They suggest that increased protein synthesis may play a key role in the molecular events which lead to the neuronal plastic changes following an acute stress.

Influence of vitamin B12 on brain methionine adenosyltransferase activity in senile dementia of the Alzheimer's type

The influence of vitamin B12 on the activity of methionine adenosyltransferase (MAT) in postmortem brains of patients with senile dementia of the Alzheimer's type (SDAT) was investigated. In samples of cortex gyrus frontalis from SDAT patients with normal and low levels of serum B12, MAT Vmax was significantly increased by 25% and 19%, respectively. MAT Vmax from a SDAT group chronically treated with B12 was similar to controls. In contrast to cortex gyrus frontalis, no significant alterations were seen in MAT activity in nucleus caudatus. This study provides evidence that SDAT is associated with significant alterations in transmethylation mechanisms in specific regions of the brain. The relationship between blood levels of B12 and the actual status of this vitamin in the brain influencing the rates of synthesis of both methionine and SAM may, however, be far more complex and cannot be directly clarified on the basis of the present human brain results.

Combined study of cerebral glucose metabolism and [11C]methionine accumulation in probable Alzheimer's disease using positron emission tomography

There is a characteristic decrease in glucose metabolism in associative frontal and temporo-parietal cortices of patients suffering from Alzheimer's disease (AD). The decrease in metabolism might result from local neuronal loss or from a decrease of synaptic activity. We measured in vivo [11C]methionine accumulation into proteins with positron emission tomography (PET) to assess cortical tissue loss in AD. Both global regional activity and compartmental analysis were used to express [11C]methionine accumulation into brain tissue. Glucose metabolism was measures with [18F]fluorodeoxyglucose and autoradiographic method. Combined studies were performed in 10 patients with probable AD, compared to age-matched healthy volunteers. There was a significant 45% decrease of temporo-parietal glucose metabolism in patients with AD, and frontal metabolism was lowered in most patients. Temporo-parietal metabolism correlated to dementia severity. [11C]methionine incorporation into temporo-parietal and frontal cortices was not significantly decreased in AD. There was no correlation with clinical symptoms. Data suggest that regional tissue loss, assessed by the decrease of [11C]methionine accumulation, is not sufficient to explain cortical glucose hypometabolism, which reflects, rather, reduced synaptic connectivity.

Restoration of brain protein synthesis in mature and aged rats by a DA agonist, piribedil

Brain ageing affects numerous cerebral metabolic pathways such as cerebral glucose consumption or protein synthesis rate. The pharmacological effect of a mixed D1-D2 dopaminergic agonist, piribedil, on this last metabolism is reported. Cerebral Protein Synthesis Rate (CPSR) was measured by the [35S]L-methionine autoradiographic procedure in 38 main brain regions of 11 and 26-month-old Wistar rats after a 2-month treatment per os at 9 and 30 mg/kg/day with piribedil. Mean decrease of CPSR was -21% during the 15-month ageing we followed, with important local variations. Mean CPSR increased with the two treatments, +25% in mature and +35% in aged rats. Treatments restored CPSR of aged rats to the exact mature subjects levels in quite all the brain regions. No dose-effect or asymetrical modification was statistically revealed for the two treatments. Metabolic increases involved particularly central brain gray structures, especially some DA-targeted brain nuclei concerned with behaviour and learning. This effect argued for a general metabotrophic effect of D1-D2 dopamine stimulation of the brain. The original pattern of local ageing of brain protein synthesis in rat was also incidentally reported. This was the first direct report of a wide and effective metabolic activation of CPSR in the brain during ageing by a curative dopaminergic agonist treatment.

Evaluation of radioselenium labeled selenomethionine, a potential tracer for brain protein synthesis by PET

The blood-brain transfer, protein incorporation and metabolism of L-[75Se]selenomethionine (SeMet) of relatively high specific activity (> 400 GBq mmol) were studied in male Wistar rats. The highest uptake was found in the pancreas, followed by the tumor, kidney, liver, brain and muscle. In addition, plasma and brain samples of rats were analyzed for labeled fractions of free SeMet, metabolites, and SeMet bound to t-RNA and proteins. For example, free SeMet represented more than 80% of brain radioactivity at 1.5 min while it was less than 15% at 360 min. A concomitant increase was observed for protein bound SeMet in brain. A three-compartment model was applied to calculate the blood-brain transfer constant (K1 (0.15 +/- 0.070 mL g-1 min-1) and the rate constant of SeMet incorporation into proteins (k3 = 0.026 +/- 0.008 min-1). The apparent incorporation of methionine into proteins was estimated to be about 0.73 nmol g-1 min-1. From the studies it is concluded that the use of L-[75Se]selenomethionine may be appropriate to measure brain protein incorporation in humans with PET.

Cerebral protein synthesis alterations in response to acute and chronic immobilization stress in the rat

The quantitative autoradiographic method with L-(35S)methionine was used to determine the effects of 1-acute (4h) and 2-chronic (14 days) immobilization stress followed by one week of recovery. Acute stress induced a significant decrease in methionine incorporation into proteins in 17 of the 35 brain structures examined (mean effect: -22%), and a significant increase in the prepositus hypoglossal nucleus (+23%). Chronic stress induced a significant decrease in methionine incorporation into proteins in 8 of the 35 structures analyzed. Only 4 structures were similarly affected in both these conditions. Our results indicate that stress-induced specific molecular changes in brain are also associated with changes in more general molecular components of cellular metabolism.

Molecular detection of methionine in rat brain using specific antibodies

In order to study the localization of methionine in rat brain, an immunological approach was developed by raising antibodies directed against this amino acid. Methionine was conjugated to bovine serum albumin (BSA) or human serum albumin (HSA) via glutaraldehyde. The conjugates were then reduced by sodium borohydride and injected alternately into rabbits. Antibody affinity and specificity were evaluated using an adapted ELISA method, by competition experiments between conjugated methionine and related conjugated compounds, pre-incubated with anti-methionine antibodies diluted at 1/20,000. The resulting cross-reactivity ratios, calculated at half-displacement, showed that glutaraldehyde-methionine conjugate (methionine-G-BSA) was the best recognized compound. Non-reduced methionine conjugate (methionine=G=BSA) and the related-conjugated molecules such as homocysteine, homocysteic acid, cysteine, cystathionine and glutamate were not recognized at all. Antibodies to methionine were directed against a glutaraldehyde-methionine epitope and their very high affinity and specificity made them reliable tools for molecular detection of methionine in rat brain. Using purified antibodies diluted at 1/20,000, motoneurons were found to be the most methionine-immunoreactive cell bodies in glutaraldehyde-fixed rat brain sections.

Triiodothyronine does not affect the average incorporation of L-[35S]methionine in rat brain structures

The autoradiographic method with L-[35S]methionine was used to examine the effect of acute administration of L-triiodothyronine on local rates of brain protein synthesis in free-moving adult rats. Triiodothyronine was given intraperitoneally at doses of 12.5 or 25 micrograms kg-1. It did not modify the rate of plasma methionine incorporation in the 40 brain regions examined, despite a 4- to 8-fold increase of plasma free triiodothyronine levels. Biochemical analysis confirmed that triiodothyronine (25 micrograms kg-1) had no apparent effect on the overall rate of protein synthesis in the brain as a whole. These results suggest that changes in the circulating levels of thyroid hormones do not exert a general and direct metabolic effect in brain of intact adult rats.

Adrenalectomy-induced increase of brain protein synthesis is antagonized by corticosterone replacements in free-moving rats

The autoradiographic method with L-[35S]-methionine was used to determine whether changes in glucocorticoid circulating levels were associated with changes in local rates of protein synthesis in rat brain. Chronic bilateral adrenalectomy induced an increase of methionine incorporation rates into proteins in 60 of the 62 brain regions examined (mean effect, +50%). This effect was confirmed biochemically and quantified by correcting for the relative contribution of methionine derived from protein degradation to the precursor pool for protein synthesis in the whole brain. Acute or chronic administration of corticosterone, at doses that normalize basal levels of adrenocorticotrophic hormone, reversed or prevented the adrenalectomy-induced increase of protein synthesis in most regions. However, in nearly all the regions studied (59 of 62), acute corticosterone administration to sham-operated rats did not change the apparent rate of protein synthesis. These results demonstrate that glucocorticoids exert a generalized inhibitory action on brain protein synthesis, because the stimulatory and persistent effect of adrenalectomy on protein synthesis was antagonized by corticosterone replacements at physiological doses. Thus, the regulation of overall brain protein synthesis by glucocorticoids emphasizes the role of neuroendocrine events on long-term neurochemical processes.