Multiple aspects of homocysteine neurotoxicity: glutamate excitotoxicity, kinase hyperactivation and DNA damage

Potentiation of arsenic neurotoxicity by folate deprivation: protective role of S-adenosyl methionine

Folate deficiency contributes to a variety of age-related neurological and psychological disorders including amyotrophic lateral sclerosis (ALS). The environmental neurotoxin arsenic has recently been linked with decreased neurofilament (NF) content in peripheral nerve. We examined herein, whether or not folate deprivation potentiated the impact of arsenic on NF dynamics. Arsenic inhibited translocation of NFs into axonal neurites in culture and increased perikaryal NF phosphoepitopes. Folate deprivation potentiated the impact of arsenic on these phenomena. Supplementation with S-adenosyl methionine (SAM) attenuated the impact of folate deprivation on arsenic neurotoxicity, consistent with the decrease in SAM following folate deprivation and the requirement for SAM-mediated methylation for arsenic bioelimination. These findings demonstrate how key nutritional deficiencies can potentiate the impact of enrivonmental neurotoxins.

Effects of homocysteine on metabolic pathways in cultured astrocytes

Homocysteine is an amino acid that is an important risk factor for several neurodegenerative diseases such as Alzheimer's and Parkinson's disease. Increased homocysteine levels induce neuronal cell death in a variety of neuronal types. However, very few studies have probed the effects of homocysteine in astrocytes. The present study investigated the effects of homocysteine on primary cultures of astrocytes by exposing astrocytes to 400 microM homocysteine for 20 h. Metabolic extracts of cells were prepared following a 4-h incubation in minimum medium with 5.5 mM [U-(13)C]glucose in the presence or absence of homocysteine and analysed using (13)C NMR. The expression level of pyruvate dehydrogenase kinase isoform 2 (PDK-2), NAD(P)H levels and mitochondrial membrane potential responses were investigated following culture with homocysteine. Metabolomic analysis was performed using (1)H NMR spectroscopy and pattern recognition analysis. Following incubation with homocysteine there was a significant decrease (48%) in the ratio of flux through pyruvate carboxylase (PC) and pyruvate dehydrogenase (PDH) which was due to an increased flux through PDH. In addition, homocysteine culture resulted in a significant reduction in PDK-2 protein expression. Following stimulation with glucose there was a significant increase in NAD(P)H levels and an impaired hyperpolarisation of the mitochondrial membrane in homocysteine-treated cells. Metabolomic analysis showed that the most discriminating metabolites following homocysteine treatment were choline and hypotaurine. In summary, the results demonstrated that sub-lethal concentrations of homocysteine caused significant metabolic changes and altered mitochondrial function in primary cultures of astrocytes.

Homocysteine activates calcium-mediated cell signaling mechanisms targeting the cytoskeleton in rat hippocampus

Homocysteine is considered to be neurotoxic and a risk factor for neurodegenerative diseases. Despite the increasing evidences of excitotoxic mechanisms of homocysteine (Hcy), little is known about the action of Hcy on the cytoskeleton. In this context, the aim of the present work was to investigate the signaling pathways involved in the mechanism of action of Hcy on cytoskeletal phosphorylation in cerebral cortex and hippocampus of rats during development. Results showed that 100 microM Hcy increased the intermediate filament (IF) phosphorylation only in 17-day-old rat hippocampal slices without affecting the cerebral cortex from 9- to 29-day-old animals. Stimulation of (45)Ca(2+) uptake supported the involvement of NMDA receptors and voltage-dependent channels in extracellular Ca(2+) flux, as well as Ca(2+) release from intracellular stores through inositol-3-phosphate and ryanodine receptors. Moreover, the mechanisms underlying the Hcy effect on hippocampus cytoskeleton involved the participation of phospholipase C, protein kinase C, mitogen-activated protein kinase, phosphoinositol-3 kinase and calcium/calmodulin-dependent protein kinase II. The Hcy-induced IF hyperphosphorylation was also related to G(i) protein and inhibition of cAMP levels. These findings demonstrate that Hcy at a concentration described to induce neurotoxicity activates the IF-associated phosphorylating system during development in hippocampal slices of rats through different cell signaling mechanisms. These results probably suggest that hippocampal rather than cortical cytoskeleton is susceptible to neurotoxical concentrations of Hcy during development and this could be involved in the neural damage characteristic of mild homocystinuric patients.

Melatonin prevents gestational hyperhomocysteinemia-associated alterations in neurobehavioral developments in rats

Chronic hyperhomocysteinemia is a risk factor in cardiovascular diseases and neurodegeneration. Among the putative mechanisms of homocysteine-induced neurotoxicity, an increased production of reactive oxygen species has been suggested. However, elevated homocysteine levels might disturb neurogenesis during brain development and lead to persistent congenital malformations in the fetus. In this study, we examined whether administration of melatonin inhibits maternal hyperhomocysteinemia-induced cognitive deficits in offspring. Hyperhomocysteinemia was induced in female rats by administration of methionine during pregnancy at a dose of 1 g/kg body weight dissolved in drinking water. Some animals received methionine plus 10 mg/kg/day melatonin subcutaneously throughout pregnancy. The levels of glial fibrillary acidic protein, S100B protein, and neural cell adhesion molecules were determined in the brain tissue from the pups. Learning and memory performances of the young-adult offspring were tested using the Morris water maze test. There were significant reductions in the expression of glial fibrillary acidic protein and S100 B protein in the brains of pups from hyperhomocysteinemic rat dams. Furthermore, maternal hyperhomocysteinemia altered the expression pattern of neural cell adhesion molecules in the fetal brain. In addition, maternal hyperhomocysteinemia significantly reduced learning abilities in offspring. Treatment with melatonin during pregnancy improved learning deficits and prevented the reduction of glial and neuronal markers induced by hyperhomocysteinemia. In conclusion, administration of melatonin throughout pregnancy reduces the effects of hyperhomocysteinemia on the development of fetal brain; therefore, it might be beneficial in preventing persistent congenital malformations.

Folate deprivation increases tau phosphorylation by homocysteine-induced calcium influx and by inhibition of phosphatase activity: Alleviation by S-adenosyl methionine

Several recent studies have indicated that increased levels of homocysteine (HC), including that resulting from deficiency in folate, increases tau phosphorylation. Some studies indicate that this is accomplished via HC-dependent activation of NMDA channels and resultant activation of calcium-dependent kinase pathways, while others suggest that the increase in tau phosphorylation is derived via HC-dependent inhibition of methylation of phosphatases and resultant inhibition of phosphatase activity. We demonstrate herein in SH-SY-5Y human neuroblastoma that both of these phenomena contribute to the increase in phospho-tau immunoreactivity following folate deprivation, and that supplementation with S-adenosyl methionine (SAM) prevents both the increase in kinase activity and the decrease in phosphatase activity. These findings demonstrate that the divergent neuropathological consequences of folate deprivation includes multiple pathways that converge upon tau phosphorylation, and further support the notion that dietary supplementation with SAM may reduce or delay neurodegeneration.

Folate-mediated one-carbon metabolism and neural tube defects: balancing genome synthesis and gene expression

Neural tube defects (NTDs) refer to a cluster of neurodevelopmental conditions associated with failure of neural tube closure during embryonic development. Worldwide prevalence of NTDs ranges from approximately 0.5 to 60 per 10,000 births, with regional and population-specific variation in prevalence. Numerous environmental and genetic influences contribute to NTD etiology; accumulating evidence from population-based studies has demonstrated that folate status is a significant determinant of NTD risk. Folate-mediated one-carbon metabolism (OCM) is essential for de novo nucleotide biosynthesis, methionine biosynthesis, and cellular methylation reactions. Periconceptional maternal supplementation with folic acid can prevent occurrence of NTDs in the general population by up to 70%; currently several countries fortify their food supply with folic acid for the prevention of NTDs. Despite the unambiguous impact of folate status on NTD risk, the mechanism by which folic acid protects against NTDs remains unknown. Identification of the mechanism by which folate status affects neural tube closure will assist in developing more efficacious and better targeted preventative measures. In this review, we summarize current research on the relationship between folate status and NTDs, with an emphasis on linking genetic variation, folate nutriture, and specific metabolic and/or genomic pathways that intersect to determine NTD outcomes.

Homocysteine as a biomarker for cognitive dysfunction in the elderly

PURPOSE OF REVIEW

Homocysteine and B vitamins have been investigated in association with cognitive dysfunction in healthy and in multimorbid elderly patients. Whether reduction of hyperhomocystemia is reducing the risk of dementia or Alzheimer's disease is still under investigation.

RECENT FINDINGS

High homocysteine concentrations are associated with poorer cognitive function but can be influenced by a number of factors. The results of epidemiological studies are inconsistent in showing an association between elevated homocysteine levels and dementia or Alzheimer disease. Although prospective studies show a trend towards a benefit of homocysteine-related B vitamin substitution, consistent data are expected from upcoming clinical intervention trials. Data from recent clinical randomized trials including various cognitive tests, different aging groups and supplements in different doses are not sufficient to allow recommendation of homocysteine-reducing therapy with folate or vitamin B12 substitution. According to the published data it remains to be proven whether a reduction in homocysteine will improve cognitive performance.

SUMMARY

Homocysteine by itself is not a useful marker for screening cognitive decline, or Alzheimer disease but works as a surrogate parameter for malnutrition and organ insufficiency in the cognitive-declining patient.

Melatonin inhibits oxidative stress and apoptosis in fetal brains of hyperhomocysteinemic rat dams

Moderate hyperhomocysteinemia is a risk factor for neurodegenerative diseases and complications during pregnancy. Increased homocysteine levels during pregnancy may elevate developmental risk on fetal brain structure and function. However, little is known about the mechanism of action of homocysteine on the degeneration of the fetal brain. Hence in this study, we examined the effects of maternal hyperhomocysteinemia on oxidative stress and apoptosis in brain tissues and investigated whether administration of melatonin to the mother would prevent homocysteine-induced oxidative cerebral damage in pups. Hyperhomocysteinemia was induced in female rats by administration of methionine at a dose of 1 g/kg body weight dissolved in drinking water during pregnancy. Some animals received methionine plus 10 mg/kg/day melatonin subcutaneously throughout pregnancy. After delivery, the level of lipid peroxidation (malondialdehyde + 4-hydroxyalkenals) was determined in different subfractions of pup brains. Furthermore, DNA fragmentation, levels of Bcl-2 protein and p53 mRNA expression were determined to evaluate apoptosis. Significant elevation was found in the levels of lipid peroxidation in subcellular fractions of the brain of pups of hyperhomocysteinemic dams. Increased DNA fragmentation and p53 mRNA expression was observed in the brain of pups of homocysteine-treated rats, while a significant reduction was seen in the levels of anti-apoptotic Bcl-2 levels. Melatonin administration prevented markers of oxidative stress and biochemical signs of apoptosis. In conclusion, therapeutic administration of melatonin protects against the induction of oxidative stress and neural tissue injury and might prevent congenital malformations of fetal brain caused by maternal hyperhomocysteinemia.

Plasma homocysteine, folate and B12 in chronic schizophrenia

Elevated plasma levels of the amino acid homocysteine have been associated with schizophrenia, particularly in young male patients. Among other factors, low folate and vitamin B12 levels have been implicated in the increase in homocysteine. In order to investigate this association, we determined plasma homocysteine, folate and B12 levels in 97 (67 males and 30 females) inpatients with chronic schizophrenia and 103 (46 males and 57 females) controls. Patients and controls did not differ in folate or B12 levels, after adjusting for age. Patients with schizophrenia had higher plasma homocysteine than controls (mean=15.42 micromol/l in cases versus 11.54 micromol/l in controls: F(1,195)=17.978; p<0.001). This difference persisted after controlling for folate and B12 concentrations. Both male and female patients had increased plasma homocysteine compared to controls [(males: mean=16.61 micromol/l in cases versus mean=13.72 in controls: F(1,110)=5.54; p=0.020) (females: mean=12.78 micromol/l in cases versus mean=9.79 micromol/l in controls: F(1,84)=13.54; p<0.001)]. When dividing our sample into two age groups (age < and > or =50 years), both young and older females and younger males with schizophrenia had increased plasma homocysteine compared to controls. We therefore suggest that homocysteinemia is a general risk factor for schizophrenia. We further suggest that it is not limited to young male patients and is not necessarily associated with low folate or B12 levels.

Neuroprotective strategies to avert seizure-induced neurodegeneration in epilepsy

Neurodegeneration in limbic circuits is a hallmark feature of chronic temporal lobe epilepsy (TLE). Studies in experimental animal models and human patients indicate that seizure-induced neuronal injury involves some active, as well as passive cell death processes. Experimental approaches that inhibit active steps in cell death programs have been shown to reduce neuronal cell death and sclerosis, but not to prevent epileptogenesis in animal models of TLE. These findings suggest that we need additional research using both animal models and brain slices from human patients to understand the pathological mechanisms underlying seizure generation. Such comparative studies will also aid in evaluating the potential therapeutic value of inhibiting cell death in seizure disorders.

Folate deprivation increases presenilin expression, gamma-secretase activity, and Abeta levels in murine brain: potentiation by ApoE deficiency and alleviation by dietary S-adenosyl methionine

Folate deficiency has been associated with age-related neurodegeneration. One direct consequence of folate deficiency is a decline in the major methyl donor, S-adenosyl methionine (SAM). We demonstrate herein that dietary deficiency in folate and vitamin E, coupled pro-oxidant stress induced by dietary iron, increased presenilin-1 expression, gamma-secretase activity, and Abeta levels in normal adult mice. These increases were potentiated by apolipoprotein E deficiency as shown by treatment of transgenic mice homozygously lacking murine apolipoprotein E. Dietary supplementation with SAM in the absence of folate attenuated or alleviated these deleterious consequences. These findings link nutritional and genetic risk factors for age-related neurodegeneration and underscore that dietary supplementation with SAM may be useful to augment therapeutic approaches.

Prolonged exposure to homocysteine results in diminished but reversible pancreatic beta-cell responsiveness to insulinotropic agents

BACKGROUND

Plasma homocysteine levels may be elevated in poorly controlled diabetes with pre-existing vascular complications and/or nephropathy. Since homocysteine has detrimental effects on a wide diversity of cell types, the present study examined the effects of long-term homocysteine exposure on the secretory function of clonal BRIN-BD11 beta-cells.

METHODS

Acute insulin secretory function, cellular insulin content and viability of BRIN-BD11 cells were assessed following long-term (18 h) exposure to homocysteine in culture. RT-PCR and Western blot analysis were used to determine the expression of key beta-cell genes and proteins. Cells were cultured for a further 18 h without homocysteine to determine any long-lasting effects.

RESULTS

Homocysteine (250-1000 micromol/L) exposure reduced insulin secretion at both moderate (5.6 mmol/L) and stimulatory (16.7 mmol/L) glucose by 48-63%. Similarly, insulin secretory responsiveness to stimulatory concentrations of alanine, arginine, 2-ketoisocaproate, tolbutamide, KCl, elevated Ca2+, forskolin and PMA, GLP-1, GIP and CCK-8 were reduced by 11-62% following culture with 100-250 micromol/L homocysteine. These inhibitory effects could not simply be attributed to changes in cellular insulin content, cell viability, H2O2 generation or any obvious alterations of gene/protein expression for insulin, glucokinase, GLUT2, VDCC, or Kir6.2 and SUR1. Additional culture for 18 h in standard culture media after homocysteine exposure restored secretory responsiveness to all agents tested.

CONCLUSION

These findings suggest that long-term exposure to high homocysteine levels causes a reversible impairment of pancreatic beta-cell insulinotropic pathways. The in vivo actions of hyperhomocysteinaemia on islet cell function merit investigation.

Protein phosphatase 2A methyltransferase links homocysteine metabolism with tau and amyloid precursor protein regulation

Alzheimer's disease (AD) neuropathology is characterized by the accumulation of phosphorylated tau and amyloid-beta peptides derived from the amyloid precursor protein (APP). Elevated blood levels of homocysteine are a significant risk factor for many age-related diseases, including AD. Impaired homocysteine metabolism favors the formation of S-adenosylhomocysteine, leading to inhibition of methyltransferase-dependent reactions. Here, we show that incubation of neuroblastoma cells with S-adenosylhomocysteine results in reduced methylation of protein phosphatase 2A (PP2A), a major brain Ser/Thr phosphatase, most likely by inhibiting PP2A methyltransferase (PPMT). PP2A methylation levels are also decreased after ectopic expression of PP2A methylesterase in Neuro-2a (N2a) cells. Reduced PP2A methylation promotes the downregulation of B alpha-containing holoenzymes, thereby affecting PP2A substrate specificity. It is associated with the accumulation of both phosphorylated tau and APP isoforms and increased secretion of beta-secretase-cleaved APP fragments and amyloid-beta peptides. Conversely, incubation of N2a cells with S-adenosylmethionine and expression of PPMT enhance PP2A methylation. This leads to the accumulation of dephosphorylated tau and APP species and increased secretion of neuroprotective alpha-secretase-cleaved APP fragments. Remarkably, hyperhomocysteinemia induced in wild-type and cystathionine-beta-synthase +/- mice by feeding a high-methionine, low-folate diet is associated with increased brain S-adenosylhomocysteine levels, PPMT downregulation, reduced PP2A methylation levels, and tau and APP phosphorylation. We reported previously that downregulation of neuronal PPMT and PP2A methylation occur in affected brain regions from AD patients. The link between homocysteine, PPMT, PP2A methylation, and key CNS proteins involved in AD pathogenesis provides new mechanistic insights into this disorder.

Gestational vitamin B deficiency leads to homocysteine-associated brain apoptosis and alters neurobehavioral development in rats

Hyperhomocysteinemia has been identified as a risk factor for neurological disorders. To study the influence of early deficiency in nutritional determinants of hyperhomocysteinemia on the developing rat brain, dams were fed a standard diet or a diet lacking methyl groups during gestation and lactation. Homocysteinemia progressively increased in the offspring of the deficient group and at 21 days reached 13.3+/-3.7 micromol/L versus 6.8+/-0.3 micromol/L in controls. Homocysteine accumulated in both neurons and astrocytes of selective brain structures including the hippocampus, the cerebellum, the striatum, and the neurogenic subventricular zone. Most homocysteine-positive cells expressed p53 and displayed fragmented DNA indicative of apoptosis. Righting reflex and negative geotaxis revealed a delay in the onset of integration capacities in the deficient group. Between 19 and 21 days, a poorer success score was recorded in deficient animals in a locomotor coordination test. A switch to normal food after weaning allowed restoration of normal homocysteinemia. Nevertheless, at 80 days of age, the exploratory behavior in the elevated-plus maze and the learning and memory behavior in the eight-arm maze revealed that early vitamin B deprivation is associated with persistent functional disabilities, possibly resulting from the ensuing neurotoxic effects of homocysteine.

Effects of maternal hyperhomocysteinemia induced by high methionine diet on the learning and memory performance in offspring

In this study, we suggest that chronic maternal hyperhomocysteinemia results in learning deficits in the offspring due to delayed brain maturation and altered expression pattern of neural cell adhesion molecule. Although the deleterious effects of hyperhomocysteinemia were extensively investigated in the adults, there is no clear evidence suggesting its action on the developing fetal rat brain and cognitive functions of the offspring. Therefore, in the present work we aimed to investigate effects of maternal hyperhomocysteinemia on the fetal brain development and on the behavior of the offspring. A group of pregnant rats received daily methionine (1 g/kg body weight) dissolved in drinking water to induce maternal hyperhomocysteinemia, starting in the beginning of gestational day 0. The levels of glial fibrillary acidic protein, S100B protein, and neural cell adhesion molecule were determined in the tissue samples from the pups. Learning and memory performances of the young-adult offsprings were tested using Morris water maze test. There were significant reductions in the expressions of glial fibrillary acidic protein and S100B protein in the brains of maternally hyperhomocysteinemic pups on postnatal day 1, suggesting that hyperhomocysteinemia delays brain maturation. In conclusion, maternal hyperhomocysteinemia changes the expression pattern of neural cell adhesion molecule and therefore leads to an impairment in the learning performance of the offspring.

Homocysteine and Parkinson's disease: a dangerous liaison?

Homocysteine, a sulphur-containing amino acid formed by demethylation of methionine, is involved in numerous processes of methyl group transfer, all playing pivotal roles in the biochemistry of the human body. Increased levels of plasma homocysteine (hyperhomocysteinemia) - which may result from a deficiency of folate, vitamin B6 or B12 or mutations in enzymes regulating the catabolism of homocysteine - are associated with a wide range of clinical manifestations, mostly affecting the central nervous system (e.g., mental retardation, cerebral atrophy and epileptic seizures). Recent evidence suggests that changes in the metabolic fate of homocysteine, leading to hyperhomocysteinemia, may also play a role in the pathophysiology of neurodegenerative disorders, particularly Parkinson's disease (PD). The nervous system might be particularly sensitive to homocysteine, due to the excitotoxic-like properties of the amino acid. However, experimental findings have shown that homocysteine does not seem to posses direct, cytotoxic activity, while the amino acid has proven able to synergize with more specific neurotoxic insults. Hyperhomocysteinemia has been repeatedly reported in PD patients; the increase, however, seems mostly related to the methylated catabolism of l-Dopa, the main pharmacological treatment of PD. Therefore, hyperhomocysteinemia may not be specific to movement disorders or other neurological diseases, the condition being, in fact, rather the result of the combinations of different factors, mainly metabolic, but also genetic and pharmacological, intervening in the neurodegenerative process.

Homocysteine-induced impairment of insulin secretion from clonal pancreatic BRIN-BD11 beta-cells is not prevented by catalase

OBJECTIVES

Although detrimental effects of homocysteine attributed to homocysteine auto-oxidation and generation of hydrogen peroxide (H2O2) have been reported in various cell types, such actions have not been considered in pancreatic beta-cells. This study investigates the acute effects of homocysteine on beta-cell integrity and regulation, in particular, the role of H2O2 generated by auto-oxidation.

METHODS

Assessment of beta-cell function was examined during acute 20- or 40-minute incubations with homocysteine using clonal BRIN-BD11 beta-cells.

RESULTS

Homocysteine (50-1000 micromol/L) inhibited basal and glucose-induced insulin secretion in a concentration-dependent manner. Insulinotropic responses to alanine, arginine, 2-ketoisocaproate, elevated Ca, tolbutamide, potassium chloride (KCl), forskolin, and phorbol 12-myristate 13-acetate were also significantly reduced by homocysteine. Likewise, preincubation with homocysteine caused a reduction in the insulinotropic responses to glucose and each of the secretagogues tested. Notably, excess catalase (100 microg/mL) in the buffer, although sufficient to remove homocysteine-derived H2O2, did not alleviate the detrimental effects of homocysteine.

CONCLUSIONS

Collectively, these data suggest that homocysteine impairs insulin secretory function by mechanisms independent of H2O2 generation. Although homocysteine may give rise to reactive oxygen species, these observations indicate detrimental non-oxidative pancreatic beta-cell actions of homocysteine.

No association between MTHFR A1298C and MTRR A66G polymorphisms, and MS in an Australian cohort

Multiple sclerosis (MS) is a complex neurological disease that affects the central nervous system (CNS) resulting in debilitating neuropathology. Pathogenesis is primarily defined by CNS inflammation and demyelination of nerve axons. Methionine synthase reductase (MTRR) is an enzyme that catalyzes the remethylation of homocysteine (Hcy) to methionine via cobalamin and folate dependant reactions. Cobalamin acts as an intermediate methyl carrier between methylenetetrahydrofolate reductase (MTHFR) and Hcy. MTRR plays a critical role in maintaining cobalamin in an active form and is consequently an important determinant of total plasma Hcy (pHcy) concentrations. Elevated intracellular pHcy levels have been suggested to play a role in CNS dysfunction, neurodegenerative, and cerebrovascular diseases. Our investigation entailed the genotyping of a cohort of 140 cases and matched controls for MTRR and MTHFR, by restriction length polymorphism (RFLP) techniques. Two polymorphisms: MTRR A66G and MTHFR A1298C were investigated in an Australian age and gender matched case-control study. No significant allelic frequency difference was observed between cases and controls at the alpha = 0.05 level (MTRR chi2 = 0.005, P = 0.95, MTHFR chi2 = 1.15, P = 0.28). Our preliminary findings suggest no association between the MTRR A66G and MTHFR A1298C polymorphisms and MS.

Neuroprotective effects of ginsenoside Rg3 against homocysteine-induced excitotoxicity in rat hippocampus

We previously demonstrated that ginsenoside Rg(3) (Rg(3)), one of the active ingredients in Panax ginseng, attenuates NMDA receptor-mediated currents and NMDA-induced neurotoxicity (Kim, S., Kim, T., Ahn, K., Park, W.K., Nah, S.Y., Rhim, H., 2004. Ginsenoside Rg(3) antagonizes NMDA receptors through a glycine modulatory site in rat cultured hippocampal neurons. Biochem. Biophys. Res. Commun. 323, 416-424). Accumulating evidence suggests that homocysteine (HC), a metabolite of methionine, exerts its excitotoxicity through NMDA receptor activation. In the present study, we examined the neuroprotective effects of Rg(3) on HC-induced hippocampal excitotoxicity in vitro and in vivo. Our in vitro studies using rat cultured hippocampal neurons revealed that Rg(3) treatment significantly and dose-dependently inhibited HC-induced hippocampal cell death, with an EC(50) value of 28.7+/-7.5 muM. Rg(3) treatment not only significantly reduced HC-induced DNA damage, but also dose-dependently attenuated HC-induced caspase-3 activity in vitro. Our in vivo studies revealed that intracerebroventricular (i.c.v.) pre-administration of Rg(3) significantly and dose-dependently reduced i.c.v. HC-induced hippocampal damage in rats. To examine the mechanisms underlying the in vitro and in vivo neuroprotective effects of Rg(3) against HC-induced hippocampal excitotoxicity, we examined the effect of Rg(3) on HC-induced intracellular Ca(2+) elevations in cultured hippocampal cells and found that Rg(3) treatment dose-dependently inhibited HC-induced intracellular Ca(2+) elevation, with an IC(50) value of 41.5+/-17.5 muM. In addition, Rg(3) treatment dose-dependently inhibited HC-induced currents in Xenopus oocytes expressing the NMDA receptor, with an IC(50) of 47.3+/-14.2 muM. These results collectively indicate that Rg(3)-induced neuroprotection against HC in rat hippocampus might be achieved via inhibition of HC-mediated NMDA receptor activation.

Synergy of genes and nutrients: the case of homocysteine

PURPOSE OF REVIEW

Folic acid is now considered an important functional food component: it lowers potentially toxic homocysteine, prevents birth defects and modulates the risk of several cancers. The complexity of interactions involved, however, means we still have much to learn about the role of folate and homocysteine in both health and disease.

RECENT FINDINGS

This review examines the emergence of homocysteine as a public health issue, and places this in context by exploring recent developments in the field of homocysteine as a vasculo, neuro and embryotoxic thiol. The paper also examines the homocysteine nexus in relation to mood disorders and cancer. It ends with an assessment of the issues associated with government-mandated folate fortification.

SUMMARY

Folate fortification as a population measure may mask B12 deficiency, affect antiepileptic drug seizure control, and influence the genetic selection of a potentially deleterious genotype, albeit over a number of generations. It is likely that only large studies with a comprehensive battery of endpoints that fully address the complexity of nutrient-gene and gene-gene interactions will be able to answer all the necessary questions fully.

L-NAME has opposite effects on the productions of S-adenosylhomocysteine and S-adenosylmethionine in V12-H-Ras and M-CR3B-Ras pheochromocytoma cells

Homocysteine is a sulfur-containing, nonproteinogenic, neurotoxic amino acid biosynthesized during methyl cycles after demethylation of S-adenosylmethionine (SAM) to S-adenosylhomocysteine (SAH) and subsequent hydrolysis of SAH into homocysteine and adenosine. Formed homocysteine is either catabolized into cystathionine (transsulfuration pathway) by cystathionine beta-synthase, or remethylated into methionine (remethylation pathway) by methionine synthase. To demonstrate the specificity of Ras-elicited effects on the activity of methyl cycles, wild-type pheochromocytoma PC12, mutant oncogenic rasH gene (MVR) expressing PC12 pheochromocytoma and normal c-rasH stably transfected M-CR3B cells were incubated with the N(omega)-nitro-L-arginine methyl ester (L-NAME), and manumycin, (inhibitors of nitric oxide synthase and farnesyltransferase, respectively). We have found that L-NAME significantly changes the SAM/SAH ratio in both MCR and MVR cells. Moreover, these alterations have reciprocal character; in the MCR cells, the SAM/SAH ratio was raised, whereas in the MVR cells this ratio was decreased. We conclude that depletion of endogenous NO with L-NAME increased the production of SAH only in cells with mutated oncogenic RasH, possibly through enhancement of production of reactive oxygen species (ROS). Oxidative stress can increase cystathionine beta-synthase activity that switches methyl cycles from remethylation into transsulfuration pathway to maintain the intracellular glutathione pool (essential for the redox-regulating capacity of cells) via an adaptive process.

Homocysteine and other structurally-diverse amino thiols can alter pancreatic beta cell function without evoking cellular damage

Homocysteine and related amino thiols, homocysteic acid, cysteic acid, homocysteine sulphinic acid and cysteine sulphinic acid have been labelled as neurotoxins. Homocysteine thiolactone, a metabolic derivative of homocysteine, is cytotoxic to endothelial cells and other cell lineages. Since pancreatic beta cells share many phenotypic similarities with neuronal cells, the present study uses clonal pancreatic BRIN-BD11 cells to investigate possible detrimental effects of these amino thiols on insulin secretion and pancreatic beta cell function. Insulin secretion was concentration-dependently inhibited at both basal (1.1 mM) and stimulatory (16.7 mM) glucose by homocysteine, homocysteine thiolactone and homocysteine sulphinic acid. Cysteic acid concentration-dependently inhibited insulin secretion at 16.7 mM glucose. Cell viability was not compromised by any of the amino thiols. Insulin secretory responses to alanine were inhibited by homocysteine, homocysteine thiolactone, homocysteic acid and cysteic acid. Insulin secretion in the presence of elevated Ca(2+) and forskolin were lowered by all amino thiols, except homocysteic acid. The secretory responsiveness to PMA, GLP-1 and KCl were only impaired in the presence of homocysteine and homocysteine thiolactone. These findings indicate that homocysteine, homocysteine thiolactone and, to a lesser extent, other amino thiols cause dysfunctional insulin secretion from pancreatic beta cells.

Mechanisms of homocysteine neurotoxicity in neurodegenerative diseases with special reference to dementia

Mild to moderate hyperhomocysteinemia is a risk factor for neurodegenerative diseases. Human studies suggest that homocysteine (Hcy) plays a role in brain damage, cognitive and memory decline. Numerous studies in recent years investigated the role of Hcy as a cause of brain damage. Hcy itself or folate and vitamin B12 deficiency can cause disturbed methylation and/or redox potentials, thus promoting calcium influx, amyloid and tau protein accumulation, apoptosis, and neuronal death. The Hcy effect may also be mediated by activating the N-methyl-D-aspartate receptor subtype. Numerous neurotoxic effects of Hcy can be blocked by folate, glutamate receptor antagonists, or various antioxidants. This review describes the most important mechanisms of Hcy neurotoxicity and pharmacological agents known to reverse Hcy effects.

Hyperhomocysteinemia increases damage on brain slices exposed to in vitro model of oxygen and glucose deprivation: prevention by folic acid

In the present study we evaluate the effects of homocysteine on cellular damage using hippocampal slices from Wistar rats exposed to oxygen and glucose deprivation (OGD, followed by reoxygenation), an in vitro model of hypoxic-ischemic events. For chronic treatment, we induced elevated levels of homocysteine in blood (500 microM), comparable to those of human homocystinuria, and in brain (60 nmol/g wet tissue) of young rats by subcutaneous injections of homocysteine (0.3-0.6 micromol/g of body weight), twice a day with 8 h intervals, from the 6 th to the 28 th postpartum day and controls received saline. Rats were sacrificed 1, 3 or 12 h after the last injection. For acute treatment, 29-day-old rats received one single injection of homocysteine (0.6 micromol homocysteine/g body weight) or saline and were sacrificed 1h later. In another set of experiments rats were pretreated with Vitamins E (40 mg/kg) and C (100 mg/kg) or folic acid (5 mg/kg) during 1 week; 12 h after the last administration they received a single injection of homocysteine or saline and were sacrificed 1 h later. Results showed that both chronic (1 h after homocysteine administration) and acute hyperhomocysteinemia increased the cellular damage measured by LDH released to de incubation medium, suggesting an increase of tissue damage caused by OGD. Pretreatment with folic acid completely prevented the damage caused by acute hyperhomocysteinemia, whereas Vitamin E just partially prevented such effect. These findings may be relevant to explain, at least in part, the higher susceptibility of hyperhomocysteinemic patients to be susceptible to ischemic events and point to a possible preventive treatment.

Homocysteine, methylenetetrahydrofolate reductase and risk of schizophrenia: a meta-analysis

Elevated plasma homocysteine concentration has been suggested as a risk factor for schizophrenia, but the results of epidemiological studies have been inconsistent. The most extensively studied genetic variant in the homocysteine metabolism is the 677C>T polymorphism in the methylenetetrahydrofolate reductase (MTHFR) gene, resulting in reduced enzyme activity and, subsequently, in elevated homocysteine. A meta-analysis of eight retrospective studies (812 cases and 2113 control subjects) was carried out to examine the association between homocysteine and schizophrenia. In addition, a meta-analysis of 10 studies (2265 cases and 2721 control subjects) on the homozygous (TT) genotype of the MTHFR 677C>T polymorphism was carried out to assess if this association is causal. A 5 micromol/l higher homocysteine level was associated with a 70% (95% confidence interval, CI: 27-129) higher risk of schizophrenia. The TT genotype was associated with a 36% (95% CI: 7-72) higher risk of schizophrenia compared to the CC genotype. The performed meta-analyses showed no evidence of publication bias or excessive influence attributable to any given study. In conclusion, our study provides evidence for an association of homocysteine with schizophrenia. The elevated risk of schizophrenia associated with the homozygous genotype of the MTHFR 677C>T polymorphism provides support for causality between a disturbed homocysteine metabolism and risk of schizophrenia.

Plasma homocysteine levels in multiple sclerosis

BACKGROUND

There is evidence that homocysteine contributes to various neurodegenerative disorders, and elevated plasma homocysteine levels have been observed in patients with multiple sclerosis (MS).

OBJECTIVE

To investigate if and why plasma homocysteine levels are increased in MS, and whether they play a role in the disease course.

METHODS

We compared plasma levels of homocysteine in 88 patients with MS and 57 healthy controls. In the MS group, 28 had a benign course, 37 were secondary progressive, and 23 primary progressive. To explore the underlying mechanisms, we measured serum levels of vitamins B6 and B12, folate, interleukin (IL)-12, tumour necrosis factor (TNF)-alpha, leukocyte nitric oxide production, and plasma diene conjugate levels (measure of oxidative stress).

RESULTS

Mean (SD) plasma homocysteine concentration was higher in patients (13.8 (4.9) micromol/l) than in controls (10.1 (2.5) micromol/l; p<0.0001). However, there were no significant differences in homocysteine levels between the three clinical subgroups of MS. Serum concentrations of vitamin B6, vitamin B12, and folate were not different between patients with MS and controls. In the MS group, there were no correlations between plasma homocysteine levels and the serum concentrations of IL-12 or TNF-alpha, leukocyte nitric oxide production, or plasma diene conjugate levels.

CONCLUSIONS

Elevated plasma homocysteine occurs in both benign and progressive disease courses of MS, and seems unrelated to immune activation, oxidative stress, or a deficiency in vitamin B6, vitamin B12, or folate.

Oxidative imbalance in Alzheimer's disease

Oxidative stress is a striking feature of susceptible neurons in the Alzheimer's disease brain. Importantly, because oxidative stress is an early event in Alzheimer's disease, proximal to the development of hallmark pathologies, it likely plays an important role in the pathogenesis of the disease. Investigations into the cause of such oxidative stress show that interactions between abnormal mitochondria and disturbed metal metabolism are, at least in part, responsible for cytoplasmic oxidative damage observed in these susceptible neurons, which could ultimately lead to their demise. Oxidative stress not only temporally precedes the pathological lesions of the disease but could also contribute to their formation, which, in turn, could provide some protective mechanism to reduce oxidative stress and ensure that neurons do not rapidly succumb to oxidative insults. In this review, we present the evidence for oxidative stress in Alzheimer's disease and its likely sources and consequence in relation to other pathological changes.

Mining literature for a comprehensive pathway analysis: a case study for retrieval of homocysteine related genes for genetic and epigenetic studies

Homocysteine is an independent risk factor for cardiovascular diseases. It is also known to be associated with a variety of complex disorders. While there are a large number of independent studies implicating homocysteine in isolated pathways, the mechanism of homocysteine induced adverse effects are not clear. Homocysteine-induced modulation of gene expression through alteration of methylation status or by hitherto unknown mechanisms is predicted to lead to several pathological conditions either directly or indirectly. In the present manuscript, using literature mining approach, we have identified the genes that are modulated directly or indirectly by an elevated level of homocysteine. These genes were then placed in appropriate pathways in an attempt to understand the molecular basis of homocysteine induced complex disorders and to provide a resource for selection of genes for polymorphism screening and analysis of mutations as well as epigenetic modifications in relation to hyperhomocysteinemia. We have identified 135 genes in 1137 abstracts that either modulate the levels of homocysteine or are modulated by elevated levels of homocysteine. Mapping the genes to their respective pathways revealed that an elevated level of homocysteine leads to the atherosclerosis either by directly affecting lipid metabolism and transport or via oxidative stress and/or Endoplasmic Reticulum (ER) stress. Elevated levels of homocysteine also decreases the bioavailability of nitric oxide and modulates the levels of other metabolites including S-adenosyl methionine and S-adenosyl homocysteine which may result in cardiovascular or neurological disorders. The ER stress emerges as the common pathway that relates to apoptosis, atherosclerosis and neurological disorders and is modulated by levels of homocysteine. The comprehensive network collated has lead to the identification of genes that are modulated by homocysteine indicating that homocysteine exerts its effect not only through modulating the substrate levels for various catalytic processes but also through regulation of expression of genes involved in complex diseases.

Melatonin prevents oxidative stress and inhibits reactive gliosis induced by hyperhomocysteinemia in rats

Homocysteine (Hcy), an independent risk factor for atherosclerosis, undergoes auto-oxidation and generates reactive oxygen species, which are thought to be main cause of Hcy neurotoxicity. However, the mechanisms leading to neurodegenerative disorders are poorly understood because studies that have investigated the potential neurotoxicity of hyperhomocysteinemia in vivo are scarce. The purpose of this study was to test whether daily administration of methionine, which induces hyperhomocysteinemia, causes glial hyperactivity, and also to investigate the protective effects of melatonin on the brain tissue against oxidative stress of Hcy in rats. There was a significant development of oxidative stress as indicated by an increase in malondialdehyde + 4-hydroxyalkenals in hippocampus and cortex of hyperhomocysteinemic rats, whereas significant reduction was found in the activity of glutathione peroxidase (GSH-Px). Co-treatment with melatonin inhibited the elevation of lipid peroxidation and significantly increased GSH-Px activity in the brain regions studied. Western blot analysis revealed an increase in glial fibrillary acidic protein (GFAP) contents both in hippocampus and frontal cortex (p < 0.001) of hyperhomocysteinemic rats compared to the controls. Administration of melatonin significantly decreased GFAP contents in hippocampus and cortex (p < 0.05). S100B contents increased only in frontal cortex in hyperhomocysteinemic rats compared to the control (p < 0.01) and was inhibited by melatonin treatment (p < 0.01). The present findings show that Hcy can sensitize glial cells, a mechanism which might contribute to the pathogenesis of neurodegenerative disorders, and further suggest that melatonin can be involved in protecting against the toxicity of Hcy by inhibiting free radical generation and stabilizing glial cell activity.

Low folate status is associated with impaired cognitive function and dementia in the Sacramento Area Latino Study on Aging

BACKGROUND

Low folate status is associated with poor cognitive function and dementia in the elderly. Since 1998, grain products in the United States have been fortified with folic acid, which has reduced the prevalence of folate deficiency and hyperhomocysteinemia.

OBJECTIVE

We investigated whether folate status is associated with cognitive function and dementia in a cohort of elderly Latinos (aged >or= 60 y; n = 1789) exposed to folic acid fortification.

DESIGN

Global cognitive function was assessed by the Modified Mini-Mental State Examination (3MSE) and specific cognitive functions by cross-culturally validated neuropsychological tests. Dementia was diagnosed according to the American Psychiatric Association Diagnostic and Statistical Manual of Mental Disorders, 3rd edition revised, and California Alzheimer Disease Diagnostic and Treatment criteria. Red blood cell (RBC) folate was measured by automated chemiluminescence and total plasma homocysteine by HPLC.

RESULTS

The prevalence of folate deficiency (RBC folate <or= 160 ng/mL) was <1%. After control for confounding by homocysteine, vitamin B-12, creatinine, demographic variables, and depressive symptom score, RBC folate was directly associated with 3MSE (P = 0.005) and delayed recall (P = 0.007) scores. In addition, adjusted odds ratios for low 3MSE score (<or=78) and dementia diagnosis per unit increase in RBC folate were significantly below unity (P <or= 0.008), which indicated that the relative risks of cognitive impairment and dementia decreased with increasing RBC folate concentration. In contrast, adjusted odds ratios for low 3MSE score and dementia diagnosis per unit increase in homocysteine were not significant.

CONCLUSION

RBC folate is directly associated with cognitive function scores and is inversely associated with dementia in elderly Latinos despite folic acid fortification.

Plasma homocysteine levels are not increased in murine models of Alzheimer's disease

Alzheimer's disease (AD) patients tend to have increased plasma levels of homocysteine. However, it is unclear whether abnormality in homocysteine levels is a primary cause of Alzheimer's disease or a disease marker. In order to investigate the relative impact of Alzheimer's disease on plasma homocysteine levels, total plasma homocysteine levels were evaluated in transgenic mouse models that exhibit abnormalities in their brains that are similar to Alzheimer's patients. No significant difference was observed in blood of murine models compared to control mice, indicating that elevated plasma homocysteine level seems to be a risk marker at the most.

Folate, vitamin E, and acetyl-L-carnitine provide synergistic protection against oxidative stress resulting from exposure of human neuroblastoma cells to amyloid-beta

Oxidative stress is an early and pivotal factor in Alzheimer's disease (AD). The neurotoxic peptide amyloid-beta (Abeta) contributes to oxidative damage in AD by inducing lipid peroxidation, which in turn generates additional downstream cytosolic free radicals and reactive oxygen species (ROS), leading to mitochondrial and cytoskeletal compromise, depletion of ATP, and ultimate apoptosis. Timely application of antioxidants can prevent all downstream consequences of Abeta exposure in culture, but in situ efficacy is limited, due in part to prior damage as well as difficulty in delivery. Herein, we demonstrate that administration of a combination of vitamin E (which prevents de novo membrane oxidative damage), folate (which maintains levels of the endogenous antioxidant glutathione), and acetyl-L-carnitine (which prevents Abeta-induced mitochondrial damage and ATP depletion) provides superior protection to that derived from each agent alone. These findings support a combinatorial approach in Alzheimer's therapy.

Melatonin improves learning and memory performances impaired by hyperhomocysteinemia in rats

Oxidative stress has been proposed as a possible mechanism underlying many neurodegenerative diseases associated with hyperhomocysteinemia. In the present study, we investigated the possible link between oxidative stress, expression of neural cell adhesion molecules and spatial learning deficits induced by chronic hyperhomocysteinemia. Furthermore, the effectiveness of antioxidant melatonin against homocysteine neurotoxicity was also examined. Male Wistar rats were treated with either saline or methionine to induce hyperhomocysteinemia and half of methionine-treated rats administered daily melatonin in a dose of 10 mg/kg. We observed that chronic administration of melatonin significantly reduced the lipid peroxidation and restored the decreased glutathione levels induced by chronic hyperhomocysteinemia. Chronic hyperhomocysteinemia significantly impaired learning and memory performance in the passive avoidance test and Morris water maze task. We also found that these cognitive deficits were reversed by chronic treatment with antioxidant melatonin. Furthermore, melatonin administration was able to modulate the expression pattern of neural cell adhesion molecules in hippocampus. The results provide evidence that homocysteine induces long-lasting behavioral deficits, which are possibly caused by oxygen reactive species generation, and by changing in synaptic plasticity and also suggest that melatonin treatment has the ability to prevent nervous system against homocysteine toxicity.

Production of homocysteine in serum-starved apoptotic PC12 cells depends on the activation and modification of Ras

PC12 pheochromocytoma cells expressing a dominant inhibitory mutant of Ha-Ras (M-M17-26) and PC12 cells transfected with normal c-RasH (M-CR3B) have been used to investigate the role of nitrosylation and farnesylation of Ras on the production of homocysteine and the activities of the redox-sensitive transcription factors NF-kappaB and c-Fos. We found that under serum and nerve growth factor withdrawal conditions undifferentiated apoptotic M-CR3B cells accumulated more homocysteine than M-M17-26 cells, and the production of homocysteine decreased in the presence of manumycin and increased in the presence of l-NAME. Furthermore, we have shown that manumycin increased the activity of c-Fos in the M-CR3B cells and decreased the activity of NF-kappaB, while l-NAME decreased the activities of both transcription factors, and accelerated apoptosis of M-CR3B cells. In contrast, in M-M17-26 cells manumycin did not change the activity of c-Fos, nor the activity of NF-kappaB. We conclude that trophic factor withdrawal stimulates Ras, which apparently through the Rac/NADPH oxidase system induces permanent oxidative stress, modulates the activities of NF-kappaB and c-Fos, induces production of homocysteine and accelerates apoptosis. Nitrosylation of Ras is necessary for maintaining the survival of PC12 cells, while farnesylation of Ras stimulates apoptosis under withdrawal conditions.

Cystathionine beta-synthase, a key enzyme for homocysteine metabolism, is preferentially expressed in the radial glia/astrocyte lineage of developing mouse CNS

Cystathionine beta-synthase (CBS; EC 4.2.1.22) is a key enzyme in the generation of cysteine from methionine. A deficiency of CBS leads to homocystinuria, an inherited human disease characterized by mental retardation, seizures, psychiatric disturbances, skeletal abnormalities, and vascular disorders; however, the underlying mechanisms remain largely unknown. Here, we show the regional and cellular distribution of CBS in the adult and developing mouse brain. In the adult mouse brain, CBS was expressed ubiquitously, but it is expressed most intensely in the cerebellar molecular layer and hippocampal dentate gyrus. Immunohistochemical analysis revealed that CBS is preferentially expressed in cerebellar Bergmann glia and in astrocytes throughout the brain. At early developmental stages, CBS was expressed in neuroepithelial cells in the ventricular zone, but its expression changed to radial glial cells and then to astrocytes during the late embryonic and neonatal periods. CBS was most highly expressed in juvenile brain, and a striking induction was observed in cultured astrocytes in response to EGF, TGF-alpha, cAMP, and dexamethasone. Moreover, CBS was significantly accumulated in reactive astrocytes in the hippocampus after kainic acid-induced seizures, and cerebellar morphological abnormalities were observed in CBS-deficient mice. Taken together, these results suggest that CBS plays a crucial role in the development and maintenance of the CNS and that radial glia/astrocyte dysfunction might be involved in the complex neuropathological features associated with abnormal homocysteine metabolism.

High homocysteine and low B vitamins predict cognitive decline in aging men: the Veterans Affairs Normative Aging Study

BACKGROUND

Elevated homocysteine concentrations may contribute to cognitive impairment. Most elevations in homocysteine result from inadequate folate, vitamin B-12, or vitamin B-6 intake. It is not clear whether the observed associations between homocysteine and cognitive measures are causal or whether they are due to homocysteine, to independent actions of the B vitamins, or to both.

OBJECTIVE

We aimed to assess the individual and independent effects of baseline plasma homocysteine, folate, vitamin B-12, and vitamin B-6 and of dietary B vitamin intakes on 3-y changes in cognitive measures in 321 aging men.

DESIGN

Participants were from the Veterans Affairs Normative Aging Study. Cognitive function was assessed with the Mini-Mental State Examination and on the basis of measures of memory, verbal fluency, and constructional praxis, which were adapted from the revised Wechsler Adult Intelligence Scale and the Consortium to Establish a Registry for Alzheimer's Disease batteries at 2 time points. At baseline, dietary intakes were assessed with a food-frequency questionnaire, and blood was drawn for the measurement of B vitamins and homocysteine.

RESULTS

Over a mean 3-y follow-up, declines in constructional praxis, measured by spatial copying, were significantly associated with plasma homocysteine, folate, and vitamins B-6 and B-12 and with the dietary intake of each vitamin. Folate (plasma and dietary) remained independently protective against a decline in spatial copying score after adjustment for other vitamins and for plasma homocysteine. Dietary folate was also protective against a decline in verbal fluency. A high homocysteine concentration was associated with a decline in recall memory.

CONCLUSIONS

Low B vitamin and high homocysteine concentrations predict cognitive decline. Spatial copying measures appear to be most sensitive to these effects in a general population of aging men.

The central nervous system in animal models of hyperhomocysteinemia

Growing epidemiological evidence of associations between mildly elevated plasma homocysteine with age-related cognitive impairment, neurodegenerative and cerebrovascular disease has stimulated interest in the role of homocysteine in neurological and neuropsychiatric disorders. Homocysteine is an intermediate in the folate, vitamin B12 and B6 dependent pathways of one-carbon and sulfur amino acid metabolism. Impairments of these pathways may cause CNS dysfunction by promoting the intracellular generation of homocysteine, which is postulated to have vasotoxic and neurotoxic properties. It might also inhibit the methylation of myelin basic protein and membrane phospholipids, or disrupt biogenic amine metabolism and many other vital CNS reactions. However, it is unclear which, if any, of these putative mechanisms underlies the epidemiological associations. Genetic mouse models of hyperhomocysteinemia suggest that the primary metabolic disturbances rather than homocysteine per se may be important in determining neurological outcomes. However, severe and early developmental abnormalities in these mice limit their usefulness for understanding the relation of hyperhomocysteinemia to adult CNS disorders. Pharmacologic and dietary studies on homocysteine in rodents have reported heightened neuronal sensitivity to neurotoxic insults, neurochemical abnormalities and cerebrovascular dysfunction. Such studies are consistent with a causal relationship, but they fail to distinguish between effects that might result from a dietary imbalance and those that might be caused by homocysteine per se. Future work should be directed towards refining these models in order to distinguish between the effects of homocysteine and its determinants on neurological and behavioral outcomes that represent different CNS disorders.

Homocysteine and brain atrophy

Homocysteine (Hcy) has been implicated as a risk factor for vascular disease as well as brain atrophy. There is evidence to implicate Hcy in increased oxidative stress, DNA damage, the triggering of apoptosis and excitotoxicity, all important mechanisms in neurodegeneration. Hcy is also prothrombotic and proatherogenic, and causes damage to the vessel wall. It is related to brain atrophy in older individuals, and possibly to white matter hyperintensities (WMH) in the brain. Epidemiological evidence and longitudinal data support Hcy as a risk factor for cognitive impairment and Alzheimer's Disease (AD). This may be due to cerebrovascular as well as direct neurotoxic mechanisms. Its role in Parkinson Disease (PD) is less well supported. High Hcy has been suggested as a mediating factor in alcohol-related brain atrophy. The high prevalence of hyperhomocysteinemia in the population and its easy treatability make Hcy an interesting amino acid for future intervention studies in the prevention of degenerative brain disorders. Intervention studies are necessary to confirm its aetiological role.

Association of the C677T and A1298C polymorphisms of methylenetetrahydrofolate reductase gene with schizophrenia: association is significant in men but not in women

Schizophrenia is a complex and common psychiatric disorder with a polygenic inheritance. In our previous report, we showed an association between the methylenetetrahydrofolate reductase (MTHFR) gene C677T and A1298C polymorphisms and schizophrenia in patients from Bakirkoy in Istanbul, Turkey [Sazci, A., Ergul, E., Guzelhan, Y., Kaya, G., Kara, I., 2003. Methylenetetrahydrofolate reductase gene polymorphisms in patients with schizophrenia. Mol. Brain Res. 117, 104-107]. We wanted also independently to confirm this study in a gender-specific manner with schizophrenic patients from Erenkoy in Istanbul, Turkey. To investigate the role of the C677T and A1298C polymorphisms of methylenetetrahydrofolate reductase gene in schizophrenia in a gender-specific manner, we analyzed the genotypes of MTHFR677 and MTHFR1298 of 297 schizophrenic patients and 341 healthy controls, using a polymerase chain reaction restriction fragment length polymorphism method. The MTHFR 677T allele was significantly distributed (chi2=7.312; P=0.026), between schizophrenic patients and healthy controls. The T677T genotype was overrepresented in the total schizophrenic patients (OR=1.938; 95%CI=1.133-3.315; chi2=5.996; P=0.014). Similarly, the T677T/A1298A compound genotype was the most significant one in the total schizophrenic patients (OR=2.397; 95% CI=1.327-4.330; chi2=8.821; P=0.003). The C1298C genotype was overrepresented in the total schizophrenic patients (OR=1.706; 95%CI=1.014-2.870; chi2=4.126; P=0.042). Likewise, the C677C/C1298C compound genotype was significant in the total schizophrenic patients (OR=1.689; 95%CI=0.985-2.894; chi2=3.695; P=0.055). When schizophrenic patients and healthy controls were stratified according to gender difference, the T677T genotype and T677T/A1298A compound genotype were significantly overrepresented (OR=2.184; 95% CI=1.069-4.462; chi2=4.767; P=0.029; OR=2.748; 95% CI=1.215-6.214; chi2=6.301; P=0.012, respectively) in men schizophrenic patients. However, neither the MTHFR C677T nor the A1298C polymorphisms are associated with schizophrenia in women. In conclusion, the MTHFR 677T allele and T677T, C1298C genotypes, and T677T/A1298A, C677C/C1298C compound genotypes are genetic risk factors for schizophrenia in men but not in women in a gender-specific manner.

Homocysteine and serum markers of immune activation in primary dystonia

The cause of primary dystonia remains unknown. Several reports point to immune system disturbances in primary dystonia and a recent study demonstrated hyperhomocysteinemia in cervical dystonia. Homocysteine (HCY) is an amino acid and elevated HCY concentrations were shown to be associated with immune system activation and increased neopterin serum concentrations. We examined HCY serum concentrations together with serum markers of immune activation in patients with different types of primary dystonia. Eighty-three patients with different types of primary dystonia were included and investigated at least 3 months following botulinum toxin treatment. Thirty-six healthy volunteers with similar age and sex distribution served as controls. Total serum HCY, kynurenine, and tryptophan concentrations were determined by high-performance liquid chromatography; neopterin, folate, and vitamin B12 concentrations were measured by immunoassays. Routine blood analysis, including C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), and white blood count (WBC), was performed. Patients with primary dystonia had significantly higher HCY concentrations compared to controls. Among the dystonia subtypes, no significant difference of HCY serum concentrations was observed. CRP and ESR were within the normal range in >90% of the patients and all had normal WBC. Neopterin, kynurenine, and tryptophan serum concentrations were similar in patients and controls and not correlated with HCY serum concentrations. The results provide evidence against enhanced cellular immune activation in patients with primary dystonia. However, hyperhomocysteinemia was present in all dystonia subtypes and unrelated to immune activation in this study. HCY is a neuronal excitotoxic amino acid and hyperhomocysteinemia is considered an independent vascular risk factor. Further studies are required to define the background of hyperhomocysteinemia in primary dystonia.

Activation of catechol-O-methyltransferase in astrocytes stimulates homocysteine synthesis and export to neurons

Elevation of the total homocysteine (tHcy) concentration in plasma has been implicated in neurodegeneration in patients with stroke, dementia, Alzheimer disease, and Parkinson disease. Because the mechanisms controlling brain tHcy are unknown, the present study investigated its synthesis and transport in primary rat brain cell cultures. We found that the catechol-O-methyltransferase (COMT) substrate 3,4-dihydroxybenzoic acid (DHB) increased export of tHcy in astrocytes, but not in neurons. The export mechanism was selective for tHcy over cyst(e)ine, total glutathione (tGSH) or cysteinylglycine (Cys-Gly). tHcy export from astrocytes was also induced by the COMT substrates levodopa (L-DOPA), dopamine and quercetin, and it was blocked by the COMT inhibitors tropolone and entacapone. This export was associated with increased synthesis of tHcy because both intracellular and extracellular tHcy concentrations rose during COMT activation. Incubation in cyst(e)ine-deficient medium inhibited the tHcy export response to COMT activation. Exogenous tHcy (100 muM) was accumulated into neurons, but not into astrocytes. We conclude that activation of COMT causes sustained synthesis of Hcy in astrocytes and transport of this amino acid to neurons.

Identification of a homocysteine receptor in the peripheral endothelium and its role in proliferation

BACKGROUND

Homocysteine, a risk factor for atherosclerosis, increases intimal hyperplasia after carotid endarterectomy with associated smooth muscle cell proliferation and modulation of cytokines. The N-methyl-D-aspartate receptor (NMDAr), a glutamate-gated ion channel receptor, is associated with homocysteine-induced cerebrovascular injury; however, the receptor has not been identified in peripheral vascular cells, nor has any interaction with homocysteine been clarified. Our objectives were first, to identify NMDAr in rat carotid artery and rat aorta endothelial cells (RAEC); and second, to determine whether homocysteine activates NMDAr in the endothelium.

METHODS

NR1 and NR2A, two NMDAr subunits, were probed in rat carotid arteries by immunohistochemistry. RNA was isolated from RAECs, and expression of all NMDAr subunits (NR1, 2A, 2B, 2C, and 2D) were examined by RT-PCR and sequencing. For receptor protein expression, RAEC were incubated with different homocysteine concentrations and incubation times and also were treated with 50 microM homocysteine and/or preincubated with 50 microM dizocilpine MK-801, an NMDAr inhibitor.

RESULTS

Both NR1 and NR2A were expressed in rat carotid arteries. All NMDAr subunits were expressed in the RAECs, and there was 92% to 100% similarity compared with rat NMDAr from the National Center for Biotechnology Information (NCBI) GenBank. Homocysteine upregulated NR1 expression and increased cell proliferation. RAEC pretreatment with MK-801 reduced homocysteine-mediated cell proliferation.

CONCLUSION

This study is the first to show that NMDAr exists in the peripheral vasculature, and that homocysteine may act via NMDAr to increase intimal hyperplasia.

CLINICAL RELEVANCE

Our objectives included the identification of a homocysteine receptor in the peripheral vasculature. The possible inhibition of a homocysteine receptor to prevent intimal hyperplasia rather than treat established stenosis would make a significant clinical impact. This will open further avenues of study in determining the role of homocysteine in the pathogenesis of intimal hyperplasia.

Homocysteine and redox signaling

Homocysteine is a thiol-containing amino acid that has gained notoriety because its elevation in the plasma is correlated with complex and multifactorial diseases, including cardiovascular diseases, neurodegenerative diseases, and neural tube defects. Homocysteine is redox-active, and its toxic effects have been frequently attributed to direct or indirect perturbation of redox homeostasis. Although the literature on the pathophysiology of elevated homocysteine is rather extensive, a very wide range of concentrations of this amino acid has been used in these studies ranging from normal to pathophysiological to unphysiological. It is clear that homocysteine induces varied responses that are specific to cell type and that cells, depending on their origin, display a wide range of sensitivity to homocysteine. In this review, we focus on the redox signaling pathways that have been connected to homocysteine in vascular (endothelial and smooth muscle) cells and in neuronal cells. We also discuss redox regulation of the key enzymes involved in homocysteine clearance: methionine synthase, betaine-homocysteine methyltranferase, and cystathionine beta-synthase.

Melatonin inhibits neural apoptosis induced by homocysteine in hippocampus of rats via inhibition of cytochrome c translocation and caspase-3 activation and by regulating pro- and anti-apoptotic protein levels

In the present study, we examined the molecular mechanism by which homocysteine causes neuronal cell apoptosis. We further investigated the mechanisms of melatonin's ability to reduce homocysteine-induced apoptosis. Consistent with its antioxidant properties, melatonin reduced homocysteine-induced lipid peroxidation and stimulated glutathione peroxidase enzyme activity in hippocampus of rats with hyperhomocysteinemia. Furthermore, melatonin treatment diminished cytochrome c release from mitochondria and reduced caspase 3 and caspase 9 activation induced by hyperhomocysteinemia. Chronic hyperhomocysteinemia also led to poly(ADP-ribose) polymerase cleavage and subsequently DNA fragmentation. Treatment with melatonin markedly inhibited poly(ADP-ribose) polymerase cleavage and reduced DNA damage. Hyperhomocysteinemia caused an elevation of pro-apoptotic Bax levels while reducing anti-apoptotic protein, Bcl-2, levels. Daily administration of melatonin up-regulated Bcl-2 and down-regulated Bax levels. We propose that, in addition to its antioxidant properties, melatonin has the ability to protect neuronal cells against apoptosis mediated homocysteine neurotoxicity by modulating apoptosis-regulatory proteins in the hippocampus of rats.

Methylenetetrahydrofolate reductase C677T genotype and PD

In a prospective, population-based cohort study among 5,920 participants aged 55 years or older, we observed that the TT variant of the methylenetetrahydrofolate reductase C677T polymorphism is associated with an increased risk for Parkinson's disease in smokers. Both smoking and the TT genotype are known to induce hyperhomocystinemia, and synergistic effects on homocysteine levels have been reported. Increased plasma levels of homocysteine through direct neurotoxic effects might accelerate the selective dopaminergic cell death underlying Parkinson's disease. Our findings support the hypothesis that homocysteine plays a role in the pathogenesis of Parkinson's disease.