Possible role of increased brain methylation in methionine sulfoximine epileptogenesis: effects of administration of adenosine and homocysteine thiolactone

Ketogenic diet, neuroprotection, and antiepileptogenesis

High fat, low carbohydrate ketogenic diets (KD) have been in use for the treatment of epilepsy for almost a hundred years. Remarkably, seizures that are resistant to conventional anti-seizure drugs can in many cases be controlled by the KD therapy, and it has been shown that many patients with epilepsy become seizure free even after discontinuation of the diet. These findings suggest that KD combine anti-seizure effects with disease modifying effects. In addition to the treatment of epilepsy, KDs are now widely used for the treatment of a wide range of conditions including weight reduction, diabetes, and cancer. The reason for the success of metabolic therapies is based on the synergism of at least a dozen different mechanisms through which KDs provide beneficial activities. Among the newest findings are epigenetic mechanisms (DNA methylation and histone acetylation) through which KD exerts long-lasting disease modifying effects. Here we review mechanisms through which KD can affect neuroprotection in the brain, and how a combination of those mechanisms with epigenetic alterations can attenuate and possibly reverse the development of epilepsy.

Adenosinergic signaling in epilepsy

Despite the introduction of at least 20 new antiepileptic drugs (AEDs) into clinical practice over the past decades, about one third of all epilepsies remain refractory to conventional forms of treatment. In addition, currently used AEDs have been developed to suppress neuronal hyperexcitability, but not necessarily to address pathogenic mechanisms involved in epilepsy development or progression (epileptogenesis). For those reasons endogenous seizure control mechanisms of the brain may provide alternative therapeutic opportunities. Adenosine is a well characterized endogenous anticonvulsant and seizure terminator of the brain. Several lines of evidence suggest that endogenous adenosine-mediated seizure control mechanisms fail in chronic epilepsy, whereas therapeutic adenosine augmentation effectively prevents epileptic seizures, even those that are refractory to conventional AEDs. New findings demonstrate that dysregulation of adenosinergic mechanisms are intricately involved in the development of epilepsy and its comorbidities, whereas adenosine-associated epigenetic mechanisms may play a role in epileptogenesis. The first goal of this review is to discuss how maladaptive changes of adenosinergic mechanisms contribute to the expression of seizures (ictogenesis) and the development of epilepsy (epileptogenesis) by focusing on pharmacological (adenosine receptor dependent) and biochemical (adenosine receptor independent) mechanisms as well as on enzymatic and transport based mechanisms that control the availability (homeostasis) of adenosine. The second goal of this review is to highlight innovative adenosine-based opportunities for therapeutic intervention aimed at reconstructing normal adenosine function and signaling for improved seizure control in chronic epilepsy. New findings suggest that transient adenosine augmentation can have lasting epigenetic effects with disease modifying and antiepileptogenic outcome. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Diet-dependent survival of protein repair-deficient mice

Protein L-isoaspartyl (D-aspartyl) O-methyltransferase (PCMT1) is a protein-repair enzyme, and mice lacking this enzyme accumulate damaged proteins in multiple tissues, die at an early age from progressive epilepsy and have an increased S-adenosylmethionine (AdoMet) to S-adenosylhomocysteine (AdoHcy) ratio in brain tissue. It has been proposed that the alteration of AdoMet and AdoHcy levels might contribute to the seizure phenotype, particularly as AdoHcy has anticonvulsant properties. To investigate whether altered AdoMet and AdoHcy levels might contribute to the seizures and thus the survivability of the repair-deficient mice, a folate-deficient amino acid-based diet was administered to the mice in place of a standard chow diet. We found that the low-folate diet significantly decreases the AdoMet/AdoHcy ratio in brain tissue and results in an almost threefold extension of mean life span in the protein repair-deficient mice. These results indicate that the increased AdoMet/AdoHcy ratio may contribute to the lowered seizure threshold in young PCMT1-deficient mice. However, mean survival was also extended almost twofold for mice on a control folate-replete amino acid-based diet compared to mice on the standard chow diet. Survival after 40 days was similar in the mice on the low- and high-folate amino acid-based diets, suggesting that the survival of older PCMT1-deficient mice is not affected by the higher brain AdoMet/AdoHcy ratio. Additionally, the surviving older repair-deficient mice have a significant increase in body weight when compared to age-matched normal mice, independent of the type of diet. This weight increase was not accompanied by an increase in consumption levels, indicating that the repair-deficient mice may also have an altered metabolic state.

Altered levels of S-adenosylmethionine and S-adenosylhomocysteine in the brains of L-isoaspartyl (D-Aspartyl) O-methyltransferase-deficient mice

L-Isoaspartyl (D-aspartyl) O-methyltransferase (PCMT1) is a protein repair enzyme that initiates the conversion of abnormal D-aspartyl and L-isoaspartyl residues to the normal L-aspartyl form. In the course of this reaction, PCMT1 converts the methyl donor S-adenosylmethionine (AdoMet) to S-adenosylhomocysteine (AdoHcy). Due to the high level of activity of this enzyme, particularly in the brain, it seemed of interest to investigate whether the lack of PCMT1 activity might alter the concentrations of these small molecules. AdoMet and AdoHcy were measured in mice lacking PCMT1 (Pcmt1-/-), as well as in their heterozygous (Pcmt1+/-) and wild type (Pcmt1+/+) littermates. Higher levels of AdoMet and lower levels of AdoHcy were found in the brains of Pcmt1-/- mice, and to a lesser extent in Pcmt1+/- mice, when compared with Pcmt1+/+ mice. In addition, these levels appear to be most significantly altered in the hippocampus of the Pcmt1-/- mice. The changes in the AdoMet/AdoHcy ratio could not be attributed to increases in the activities of methionine adenosyltransferase II or S-adenosylhomocysteine hydrolase in the brain tissue of these mice. Because changes in the AdoMet/AdoHcy ratio could potentially alter the overall excitatory state of the brain, this effect may play a role in the progressive epilepsy seen in the Pcmt1-/- mice.

Down-regulation of S-adenosylhomocysteine hydrolase in the active methyl transfer system in the brain of genetically epileptic El mice

To study the role of cerebral methylation in epileptogenesis, we investigated the active methyl transfer pathway in the brain of genetically epileptic El mice. We examined S-adenosylhomocysteine (AdoHcy) hydrolase activity (by high performance liquid chromatography), the corresponding mRNA levels (by competitive reverse transcription-polymerase chain reaction), as well as S-adenosylmethionine (AdoMet) and AdoHcy levels in epileptic El and non-epileptic control ddY mice. The level of AdoHcy, a potent feedback inhibitor in the methyl transfer system, was relatively high throughout growth, particularly in 15 week old El mice where it was increased approximately 20% compared to control ddY mice. AdoHcy hydrolase activity in the El mice brain did not increase during growth, which was confirmed by the finding that mRNA synthesis from the hydrolase gene behaved likewise. In contrast, ddY mice exhibited a gradual increase in the mRNA synthesis up to three fold and 20% increase in the enzyme activity at 15 weeks of age compared to those of El mice in which the seizure frequency was 100%. The levels of AdoMet, a versatile methyl donor, did not change throughout growth. We concluded that the down-regulation of AdoHcy hydrolase results in the accumulation of AdoHcy, possibly inducing an unstable state including seizures in the El mouse brain. El mice predisposed to epilepsy may be characterized by disordered feedback regulation of the AdoMet-dependent methyl transfer pathway.

Possible link between brain serotonin metabolism and methionine sulfoximine-induced hypothermia and associated behavior in the rat

L-Methionine-D,L-sulfoximine (MSO) intraperitoneally or intracerebroventricularly (third ventricle) injected at convulsant doses induced a hypothermia, primarily associated with a syndrome of ataxia, in the restrained rat maintained at an ambient temperature of 23 degrees C. Depletion of brain serotonin (5-HT) by pretreatment with p-chlorophenylalanine (PCPA), p-chloroamphetamine (PCA), and d-fenfluramine (FFA) did not significantly modify the time course and magnitude of MSO-induced developing hypothermia but it enhanced abnormal motor behavior. Enhancement of 5-HT synthesis in MSO-submitted rats pretreated with 5-hydroxytryptophan (5-HTP) (200 mg/kg, IP) alone or 5-HTP (100 mg/kg, IP) preassociated with carbidopa (10 mg/kg, IP) suppressed significantly hypothermia, but it did not greatly modify motor disturbances. In conclusion, the neurocytochemical processes initiating hypothermia following administration of MSO to the rat appear to be linked to a slowdown of the rate of brain 5-HT turnover, maybe at the level of the midbrain raphe nuclei.

Involvement of serotonin receptors in methionine sulfoximine-induced hypothermia in the rat

L-Methionine-D,L-sulfoximine (MSO), intraperitoneally (i.p.) or intracerebroventricularly (i.c.v.) (third ventricle) injected at a convulsant dose, induced a centrally mediated body hypothermia in the restrained rat maintained at an ambient temperature of 23 degrees C. Pretreatment with (+/-)-pindolol (1.5-3 mg/kg s.c.) significantly attenuated MSO-induced hypothermia, but at a dose of 6 mg/kg s.c. hypothermia developed without any modification of its characteristics. Pretreatment with (-)-propranolol (16-25 mg/kg i.p.) potentiated MSO-induced hypothermia, but pretreatment of MSO-treated rats with ketanserin (0.7-4 mg/kg i.p.) did not significantly modify hypothermia. Selective antagonists for beta-adrenoceptors were used and their effects on MSO-induced hypothermia were compared with those of pindolol and propranolol. Pretreatment with betaxolol (1.5-4 mg/kg s.c.) did not modify the hypothermia following administration of MSO, but potentiation of hypothermia was recorded in rats pretreated with ICI 118,551 (2.26 mg/kg i.p.) then i.p. injected with MSO. These findings favour a control exerted by 5-HT1 receptors in the central development of MSO-induced hypothermia in the restrained rat.

What is the function of protein carboxyl methylation?

The following functions of protein carboxyl methylation seem to be reasonably well established: Multiple, stoichiometric methylation of chemotactic receptors in bacteria at glutamyl residues serves as one (but not the only) adaptation mechanism of the transduction chain to constant background levels of chemotactic stimuli. Stoichiometric methylation of hormones and hormone carrier proteins plays a role in hormone storage and secretion by the pituitary gland. Substoichiometric methylation at D-aspartyl residues is involved in a repair mechanism of aged proteins. Stoichiometric methylation of calmodulin modulates the sensitivity of calmodulin-dependent processes to calcium. Research of the past 3 years has indicated that in order to demonstrate an involvement of methylation in the coupling of surface receptors to intracellular events three new criteria have to be met: (a) the cell should possess a protein carboxyl methylase with relatively narrow substrate specificity; (b) methylation should take place at L-amino acid residues; (c) the methyl accepting proteins should be methylated in a stoichiometric fashion.