The neuronal SAPK/JNK pathway is altered in a murine model of hyperhomocysteinemia

Hyperhomocysteinemia: Metabolic Role and Animal Studies with a Focus on Cognitive Performance and Decline-A Review

Disturbances in the one-carbon metabolism are often indicated by altered levels of the endogenous amino acid homocysteine (HCys), which is additionally discussed to causally contribute to diverse pathologies. In the first part of the present review, we profoundly and critically discuss the metabolic role and pathomechanisms of HCys, as well as its potential impact on different human disorders. The use of adequate animal models can aid in unravelling the complex pathological processes underlying the role of hyperhomocysteinemia (HHCys). Therefore, in the second part, we systematically searched PubMed/Medline for animal studies regarding HHCys and focused on the potential impact on cognitive performance and decline. The majority of reviewed studies reported a significant effect of HHCys on the investigated behavioral outcomes. Despite of persistent controversial discussions about equivocal findings, especially in clinical studies, the present evaluation of preclinical evidence indicates a causal link between HHCys and cognition-related- especially dementia-like disorders, and points out the further urge for large-scale, well-designed clinical studies in order to elucidate the normalization of HCys levels as a potential preventative or therapeutic approach in human pathologies.

Ability of dietary factors to affect homocysteine levels in mice: a review

Homocysteine is associated with several diseases, and a series of dietary factors are known to modulate homocysteine levels. As mice are often used as model organisms to study the effects of dietary hyperhomocysteinemia, we collected data about concentrations of vitamin B₁₂, vitamin B₆, folate, methionine, cystine, and choline in mouse diets and the associated plasma/serum homocysteine levels. In addition, we more closely examined the composition of the control diet, the impact of the mouse strain, sex and age, and the duration of the dietary intervention on homocysteine levels. In total, 113 out of 1103 reviewed articles met the inclusion criteria. In the experimental and control diets, homocysteine levels varied from 0.1 to 280 µmol/l. We found negative correlations between dietary vitamin B₁₂ (rho = − 0.125; p < 0.05), vitamin B₆ (rho = − 0.191; p < 0.01) and folate (rho = − 0.395; p < 0.001) and circulating levels of homocysteine. In contrast, a positive correlation was observed between dietary methionine and homocysteine (methionine: rho = 0.146; p < 0.05). No significant correlations were found for cystine or choline and homocysteine levels. In addition, there was no correlation between the duration of the experimental diets and homocysteine levels. More importantly, the data showed that homocysteine levels varied widely in mice fed control diets as well. When comparing control diets with similar nutrient concentrations (AIN-based), there were significant differences in homocysteine levels caused by the strain (ANOVA, p < 0.05) and age of the mice at baseline (r = 0.47; p < 0.05). When comparing homocysteine levels and sex, female mice tended to have higher homocysteine levels than male mice (9.3 ± 5.9 µmol/l vs. 5.8 ± 4.5 µmol/l; p = 0.069). To conclude, diets low in vitamin B₁₂, vitamin B₆, or folate and rich in methionine are similarly effective in increasing homocysteine levels. AIN recommendations for control diets are adequate with respect to the amounts of homocysteine-modulating dietary parameters. In addition, the mouse strain and the age of mice can affect the homocysteine level.

Homocysteine-lowering gene therapy rescues signaling pathways in brain of mice with intermediate hyperhomocysteinemia

Hyperhomocysteinemia due to cystathionine beta synthase (CBS) deficiency is associated with diverse cognitive dysfunction. Considering the role of the serine/threonine kinase DYRK1A, not only in developmental defects with life-long structural and functional consequences, but also in multiple neurodegenerative diseases, its protein expression and kinase activity has been analyzed in brain of heterozygous CBS deficient mice and found to be increased. We previously demonstrated that specific liver treatment with an adenovirus expressing Dyrk1A normalizes hepatic DYRK1A level and decreases hyperhomocysteinemia in mice with moderate to intermediate hyperhomocysteinemia. We here use a hepatocyte-specific recombinant adeno-associated viral (AAV) serotype 8-mediated DYRK1A gene therapy (AAV2/8-DYRK1A) to analyze the effect of hepatic Dyrk1A gene transfer on some altered molecular mechanisms in brain of mice with intermediate hyperhomocysteinemia. Our selective hepatic treatment alleviates altered DYRK1A protein level and signaling pathways in brain of mice, the MAPK/ERK and PI3K/Akt pathways initiated by receptor tyrosine kinase, the BDNF dependent TrkB pathway, and NFkB pathway. These results demonstrate the positive effect of AAV2/8-DYRK1A gene transfer on neuropathological and inflammatory processes in brain of mice with intermediate hyperhomocysteinemia.

H2S biosynthesis and catabolism: new insights from molecular studies

Hydrogen sulfide (H₂S) has profound biological effects within living organisms and is now increasingly being considered alongside other gaseous signalling molecules, such as nitric oxide (NO) and carbon monoxide (CO). Conventional use of pharmacological and molecular approaches has spawned a rapidly growing research field that has identified H₂S as playing a functional role in cell-signalling and post-translational modifications. Recently, a number of laboratories have reported the use of siRNA methodologies and genetic mouse models to mimic the loss of function of genes involved in the biosynthesis and degradation of H₂S within tissues. Studies utilising these systems are revealing new insights into the biology of H₂S within the cardiovascular system, inflammatory disease, and in cell signalling. In light of this work, the current review will describe recent advances in H₂S research made possible by the use of molecular approaches and genetic mouse models with perturbed capacities to generate or detoxify physiological levels of H₂S gas within tissues.

Molecular rescue of DYRK1A overexpression in cystathionine beta synthase-deficient mouse brain by enriched environment combined with voluntary exercise

Hyperhomocysteinemia resulting from cystathionine beta synthase (CBS) deficiency can produce cognitive dysfunction. We recently found that CBS-deficient mice exhibit increased expression of the serine/threonine kinase dual-specificity tyrosine-(Y)-phosphorylation-regulated kinase 1A (DYRK1A) in the brain. When dysregulated, DYRK1A contributes to the neurodegeneration, neuronal death, and loss of function observed in neurodegenerative diseases. However, brain plasticity can be improved by interventions like enriched environment combined with voluntary exercise (EE/VE). The present study sought to assess the effects of EE/VE on molecular mechanisms linked to DYRK1A overexpression in the brain of CBS-deficient mice. EE/VE was applied to 3-month-old female CBS-deficient mice for 1 month. Without intervention, CBS-deficient mice exhibited increased DYRK1A and decreased brain-derived neurotrophic factor (BDNF) levels in the cortex and hippocampus. However, EE/VE rescued these altered DYRK1A and BDNF levels in the hippocampus of CBS-deficient mice. We conclude that exercise combined with enriched environment can restore the altered molecular mechanisms in the brain of CBS-deficient mice.

Mice deficient in cystathionine beta synthase display increased Dyrk1A and SAHH activities in brain

Hyperhomocysteinemia is associated with brain disease. However, biological actions linking hyperhomocysteinemia to neuronal abnormalities are not well understood. We recently found a relationship between Dyrk1A protein expression, a serine/threonine kinase that might be responsible for cognitive functions in Down's syndrome, and hepatic S-adenosylhomocysteine hydrolase (SAHH) activity, which plays a key role in S-adenosylmethionine-dependent methylation reactions. Considering the role of methylation and Dyrk1A in cognitive functions, the aim of this study was to investigate the relationship between Dyrk1A and SAHH activity in brain of hyperhomocysteinemic mice. We found an increase in Dyrk1A protein expression and activity in brain of hyperhomocysteinemic mice, concomitant with an increased SAHH activity. The effect of overexpression of protein Dyrk1A on SAHH activity was confirmed in brain of Dyrk1A transgenic mice, and additionally we found a positive correlation between Dyrk1A and SAHH activity. These observations suggest a potential effect of Dyrk1A on brain phenotypes linked to hyperhomocysteinemia.

Dyrk1A, a serine/threonine kinase, is involved in ERK and Akt activation in the brain of hyperhomocysteinemic mice

Hyperhomocysteinemia due to cystathionine beta synthase (CBS) deficiency is associated with diverse brain disease. Whereas the biological actions linking hyperhomocysteinemia to the cognitive dysfunction are not well understood, we tried to establish relationships between hyperhomocysteinemia and alterations of signaling pathways. In the brain of CBS-deficient mice, a murine model of hyperhomocysteinemia, we previously found an activation of extracellular signal-regulated kinase (ERK) pathway and an increase of Dyrk1A, a serine/threonine kinase involved in diverse functions ranging from development and growth to apoptosis. We then investigated the relationship between Dyrk1A and the signaling pathways initiated by receptor tyrosine kinases (RTK), the ERK and PI3K/Akt pathways. We found a significant increase of phospho-ERK, phospho-MEK, and phospho-Akt in the brain of CBS-deficient and Dyrk1a-overexpressing mice. This increase was abolished when CBS-deficient and Dyrk1A-transgenic mice were treated with harmine, an inhibitor of Dyrk1A kinase activity, which emphasizes the role of Dyrk1A activity on ERK and Akt activation. Sprouty 2 protein level, a negative feedback loop modulator that limits the intensity and duration of RTK activation, is decreased in the brain of CBS-deficient mice, but not in the brain of Dyrk1A transgenic mice. Furthermore, a reduced Dyrk1A and Grb2 binding on sprouty 2 and an increased interaction of Dyrk1A with Grb2 were found in the brain of Dyrk1A transgenic mice. The consequence of Dyrk1A overexpression on RTK activation seems to be a decreased interaction of sprouty 2/Grb2. These observations demonstrate ERK and Akt activation induced by Dyrk1A in the brain of hyperhomocysteinemic mice and open new perspectives to understand the basis of the cognitive defects in hyperhomocysteinemia.

The Worldwide Challenge of the Dementias: A Role for B Vitamins and Homocysteine?

Dementia has reached epidemic proportions, with an estimated 4.6 million new cases worldwide each year. With an aging world population, the prevalence of dementia will increase dramatically in the next few decades. Of the predicted 114 million who will have dementia in 2050, about three-quarters will live in less developed regions. Although strongly age-related, dementia is not an inevitable part of aging but is a true disease, caused by exposure to several genetic and nongenetic risk factors. Prevention will be possible when the nongenetic risk factors have been identified. Apart from age, more than 20 nongenetic risk factors have been postulated, but very few have been established by randomized intervention studies. Elevated blood concentrations of total homocysteine and low-normal concentrations of B vitamins (folate, vitamin B12, and vitamin B6) are candidate risk factors for both Alzheimer's disease and vascular dementia. Seventy-seven cross-sectional studies on more than 34,000 subjects and 33 prospective studies on more than 12,000 subjects have shown associations between cognitive deficit or dementia and homocysteine and/or B vitamins. Biologically plausible mechanisms have been proposed to account for these associations, including atrophy of the cerebral cortex, but a definite causal pathway has yet to be shown. Raised plasma total homocysteine is a strong prognostic marker of future cognitive decline, and is common in world populations. Low-normal concentrations of the B vitamins, the main determinant of homocysteine concentrations, are also common and occur in particularly vulnerable sections of the population, such as infants and elderly. Large-scale randomized trials of homocysteine-lowering vitamins are needed to see if a proportion of dementia in the world can be prevented.

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.

Endoplasmic reticulum stress increases the expression of methylenetetrahydrofolate reductase through the IRE1 transducer

Methylenetetrahydrofolate reductase (MTHFR), an enzyme in folate and homocysteine metabolism, influences many cellular processes including methionine and nucleotide synthesis, methylation reactions, and maintenance of homocysteine at nontoxic levels. Mild deficiency of MTHFR is common in many populations and modifies risk for several complex traits including vascular disease, birth defects, and cancer. We recently demonstrated that MTHFR can be up-regulated by NF-kappaB, an important mediator of cell survival that is activated by endoplasmic reticulum (ER) stress. This observation, coupled with the reports that homocysteine can induce ER stress, prompted us to examine the possible regulation of MTHFR by ER stress. We found that several well characterized stress inducers (tunicamycin, thapsigargin, and A23187) as well as homocysteine could increase Mthfr mRNA and protein in Neuro-2a cells. The induction of MTHFR was also observed after overexpression of inositol-requiring enzyme-1 (IRE1) and was inhibited by a dominant-negative mutant of IRE1. Because IRE1 triggers c-Jun signaling, we examined the possible involvement of c-Jun in up-regulation of MTHFR. Transfection of c-Jun and two activators of c-Jun (LiCl and sodium valproate) increased MTHFR expression, whereas a reported inhibitor of c-Jun (SP600125) and a dominant-negative derivative of c-Jun N-terminal kinase-1 reduced MTHFR activation. We conclude that ER stress increases MTHFR expression and that IRE1 and c-Jun mediate this activation. These findings provide a novel mechanism by which the ER can regulate homeostasis and allude to an important role for MTHFR in cell survival.

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.

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.

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.

Homocysteine and cognitive function in a population-based study of older adults

OBJECTIVES

To assess the relations between homocysteine levels and neurobehavioral test scores representing a broad range of cognitive domains in a population-based study of older adults.

DESIGN

Cross-sectional analysis of first-visit data of subjects in the Baltimore Memory Study, a longitudinal study.

SETTING

Specific neighborhoods in Baltimore.

PARTICIPANTS

Participants were 1,140 randomly selected residents aged 50 to 70 with a mean age+/-standard deviation of 59.3+/-5.9; 65.9% were female, and 54.2% were white and 41.1% African-American.

MEASUREMENTS

Twenty neurobehavioral test scores in eight cognitive domains.

RESULTS

Linear regression models revealed that homocysteine was consistently and strongly associated with worse neurobehavioral test performance, in crude analysis and after control for a large set of important covariates. Associations were observed in all eight cognitive domains assessed but were strongest and most consistent in the domains of simple motor and psychomotor speed, eye-hand coordination/manual dexterity, and verbal memory and learning. On average, an increase in homocysteine levels from the 25th to the 75th percentile, all in the generally accepted normal range, was equivalent in its association with neurobehavioral test scores to an increase of 4.2 years of age. Logistic regression models revealed that, on average, for the neurobehavioral tests associated with homocysteine, subjects in the highest quartile of homocysteine levels were more than two times as likely to be in the lowest quartile of neurobehavioral test scores as those in the lowest quartile.

CONCLUSION

Higher homocysteine levels were associated with worse function across a broad range of cognitive domains, and the magnitude of the associations was large. The data suggest that homocysteine may be a potentially important modifiable cause of cognitive dysfunction.

Homocysteine levels in A/J and C57BL/6J mice: genetic, diet, gender, and parental effects

Increased levels of homocysteine in the blood have been associated with various birth defects and adult diseases. However, the extent to which genetic factors control homocysteine levels in healthy individuals is unclear. Laboratory mice are valuable models for dissecting the genetic and environmental controls of total homocysteine (tHcy) levels. We assessed the inheritance of tHcy levels in two inbred strains, A/J and C57BL/6J (B6), under controlled physiological conditions and assessed the relative importance of genetic, diet, gender, and parental effects. Diet affected mean tHcy levels, whereas gender affected both the mean and variance of tHcy levels. Moreover, gender of the parents influenced mean tHcy levels in reciprocal F1 hybrids, suggesting maternal effects. Finally, gene-diet interactions affected heritability of mean tHcy levels. These studies showed that each of these factors contributes to tHcy levels and provided important clues to understanding homocysteine homeostasis in humans.

Regulation of extracellular signal-regulated kinase by homocysteine in hippocampus

In several neurological disorders including hyperhomocysteinemia, homocysteine (Hcy) accumulates in the brain, and acts as a potent neurotoxin. However, the molecular mechanisms induced by increased levels of Hcy in brain are not well understood. Here we show an activation of the extracellular signal-regulated kinases (ERK1 and ERK2) and the downstream nuclear targets Elk-1 and calcium/cAMP response element binding protein, in the hippocampus of cystathionine beta synthase deficient mice, a murine model of hyperhomocysteinemia. An ex vivo model of hippocampal slices allowed us to reproduce Hcy -induced ERK activation and to unravel the mechanisms responsible of this activation. Of interest, N-methyl-d-aspartate (NMDA), non-NMDA and metabotropic glutamate receptor antagonists all blocked Hcy -induced ERK activation. Moreover, the ERK activation was blocked in the presence of Na+-channel blocker tetrodotoxin, indicating the existence of a trans-synaptic activity in ERK activation by Hcy in hippocampal slices. The effects of Hcy on ERK cascade activation were also dependent on calcium influx, CaMK-II, PKC as well as PKA activation. Thus, altogether these data support a role of Hcy on ERK activation, via complex mechanisms, starting with a control of glutamate release, which in turn activates ionotropic and metabotropic receptor subtypes and produces increases in intracellular calcium levels.