Methylation and phosphorylation: a tangled relationship?

The role of PIMT in Alzheimer's disease pathogenesis: A novel hypothesis

There are multiple theories of Alzheimer's disease pathogenesis. One major theory is that oxidation of amyloid beta (Aβ) promotes plaque deposition that directly contributes to pathology. A competing theory is that hypomethylation of DNA (due to altered one carbon metabolism) results in pathology through altered gene regulation. Herein, we propose a novel hypothesis involving L-isoaspartyl methyltransferase (PIMT) that unifies the Aβ and DNA hypomethylation hypotheses into a single model. Importantly, the proposed model allows bidirectional regulation of Aβ oxidation and DNA hypomethylation. The proposed hypothesis does not exclude simultaneous contributions by other mechanisms (e.g., neurofibrillary tangles). The new hypothesis is formulated to encompass oxidative stress, fibrillation, DNA hypomethylation, and metabolic perturbations in one carbon metabolism (i.e., methionine and folate cycles). In addition, deductive predictions of the hypothesis are presented both to guide empirical testing of the hypothesis and to provide candidate strategies for therapeutic intervention and/or nutritional modification. HIGHLIGHTS: PIMT repairs L-isoaspartyl groups on amyloid beta and decreases fibrillation. SAM is a common methyl donor for PIMT and DNA methyltransferases. Increased PIMT activity competes with DNA methylation and vice versa. The PIMT hypothesis bridges a gap between plaque and DNA methylation hypotheses.

Non-Histone Protein Methylation: Biological Significance and Bioengineering Potential

Protein methylation is a key post-translational modification whose effects on gene expression have been intensively studied over the last two decades. Recently, renewed interest in non-histone protein methylation has gained momentum for its role in regulating important cellular processes and the activity of many proteins, including transcription factors, enzymes, and structural complexes. The extensive and dynamic role that protein methylation plays within the cell also highlights its potential for bioengineering applications. Indeed, while synthetic histone protein methylation has been extensively used to engineer gene expression, engineering of non-histone protein methylation has not been fully explored yet. Here, we report the latest findings, highlighting how non-histone protein methylation is fundamental for certain cellular functions and is implicated in disease, and review recent efforts in the engineering of protein methylation.

Histological study on the protective role of vitamin B complex on the cerebellum of diabetic rat

BACKGROUND

Disorder in cerebellar structure was reported in diabetes mellitus. B vitamins are involved in many significant metabolic processes within the brain.

AIM OF THE WORK

To detect the protective role of vitamin B complex on the histological structure of the cerebellum of experimentally induced diabetic rat.

MATERIAL & METHODS

Eighteen adult male Wistar rats were divided into two groups. Group I: normal vehicle control (n=6). Group II: streptozotocin-induced diabetic rats (n=12), which was equally divided into two subgroups; IIA (diabetic vehicle control), IIB (diabetic vitamin B complex-treated): streptozotocin-induced diabetic rats treated with vitamin B complex (1mg/kg/day) for 6 weeks. Specimens from the cerebellum were processed for light and electron microscopy.

RESULTS

In vitamin B complex treated group, the histological changes in Purkinje cells, astrocytes and oligodendrocytes were improved compared with the diabetic non-treated group. The white matter revealed intact myelinated axons. Inducible nitric oxide synthase (iNOS) and caspase-3 expression reduced. Glial fibrillary acidic protein (GFAP) expression revealed less activated astroglia. The number of Purkinje cells/mm(2) significantly increased. While, the number of GFAP positive astrocytes/mm(2) significantly decreased. In addition, the blood glucose level was reduced.

CONCLUSION

Vitamin B complex protected the cerebellum from the histological changes which occurred in STZ- induced diabetic rats.

Folate deficiency-induced oxidative stress contributes to neuropathy in young and aged zebrafish--implication in neural tube defects and Alzheimer's diseases

Folate is a nutrient essential for the development, function and regeneration of nervous systems. Folate deficiency has been linked to many neurological disorders including neural tube defects in fetus and Alzheimer's diseases in the elderly. However, the etiology underlying these folate deficiency-associated diseases is not completely understood. In this study, zebrafish transgenic lines with timing and duration-controllable folate deficiency were developed by ectopically overexpressing a recombinant EGFP-γ-glutamyl hydrolase (γGH). Impeded neural crest cell migration was observed in the transgenic embryos when folate deficiency was induced in early stages, leading to defective neural tube closure and hematopoiesis. Adding reduced folate or N-acetylcysteine reversed the phenotypic anomalies, supporting the causal link between the increased oxidative stress and the folate deficiency-induced abnormalities. When folate deficiency was induced in aged fish accumulation of beta-amyloid and phosphorylated Tau protein were found in the fish brain cryo-sections. Increased autophagy and accumulation of acidic autolysosome were apparent in folate deficient neuroblastoma cells, which were reversed by reduced folate or N-acetylcysteine supplementation. Decreased expression of cathepsin B, a lysosomal protease, was also observed in cells and tissue with folate deficiency. We concluded that folate deficiency-induced oxidative stress contributed to the folate deficiency-associated neuropathogenesis in both early and late stages of life.

Expression of genes encoding enzymes involved in the one carbon cycle in rat placenta is determined by maternal micronutrients (folic acid, vitamin B12) and omega-3 fatty acids

We have reported that folic acid, vitamin B₁₂, and omega-3 fatty acids are interlinked in the one carbon cycle and have implications for fetal programming. Our earlier studies demonstrate that an imbalance in maternal micronutrients influence long chain polyunsaturated fatty acid metabolism and global methylation in rat placenta. We hypothesize that these changes are mediated through micronutrient dependent regulation of enzymes in one carbon cycle. Pregnant dams were assigned to six dietary groups with varying folic acid and vitamin B₁₂ levels. Vitamin B₁₂ deficient groups were supplemented with omega-3 fatty acid. Placental mRNA levels of enzymes, levels of phospholipids, and glutathione were determined. Results suggest that maternal micronutrient imbalance (excess folic acid with vitamin B₁₂ deficiency) leads to lower mRNA levels of methylene tetrahydrofolate reductase (MTHFR) and methionine synthase , but higher cystathionine b-synthase (CBS) and Phosphatidylethanolamine-N-methyltransferase (PEMT) as compared to control. Omega-3 supplementation normalized CBS and MTHFR mRNA levels. Increased placental phosphatidylethanolamine (PE), phosphatidylcholine (PC), in the same group was also observed. Our data suggests that adverse effects of a maternal micronutrient imbalanced diet may be due to differential regulation of key genes encoding enzymes in one carbon cycle and omega-3 supplementation may ameliorate most of these changes.

Genetic amplification of PPME1 in gastric and lung cancer and its potential as a novel therapeutic target

Protein phosphatase methylesterase 1 (PPME1) is a protein phosphatase 2A (PP2A)-specific methyl esterase that negatively regulates PP2A through demethylation at its carboxy terminal leucine 309 residue. Emerging evidence shows that the upregulation of PPME1 is associated with poor prognosis in glioblastoma patients. By performing an array comparative genomic hybridization analysis to detect copy number changes, we have been the first to identify PPME1 gene amplification in 3.8% (5/131) of Chinese gastric cancer (GC) samples and 3.1% (4/124) of Chinese lung cancer (LC) samples. This PPME1 gene amplification was confirmed by fluorescence in situ hybridization analysis and is correlated with elevated protein expression, as determined by immunohistochemistry analysis. To further investigate the role of PPME1 amplification in tumor growth, short-hairpin RNA-mediated gene silencing was employed. A knockdown of PPME1 expression resulted in a significant inhibition of cell proliferation and induction of cell apoptosis in PPME1-amplified human cancer cell lines SNU668 (GC) and Oka-C1 (LC), but not in nonamplified MKN1 (GC) and HCC95 (LC) cells. The PPME1 gene knockdown also led to a consistent decrease in PP2A demethylation at leucine 309, which was correlated with the downregulation of cellular Erk and AKT phosphorylation. Our data indicate that PPME1 could be an attractive therapeutic target for a subset of GCs and LCs.

Vitamin B12 in neurology and ageing; clinical and genetic aspects

The classic neurological and psychiatric features associated with vitamin B12 deficiency have been well described and are the subject of many excellent review articles. The advent of sensitive diagnostic tests, including homocysteine and methylmalonic acid assays, has revealed a surprisingly high prevalence of a more subtle 'subclinical' form of B12 deficiency, particularly within the elderly. This is often associated with cognitive impairment and dementia, including Alzheimer's disease. Metabolic evidence of B12 deficiency is also reported in association with other neurodegenerative disorders including vascular dementia, Parkinson's disease and multiple sclerosis. These conditions are all associated with chronic neuro-inflammation and oxidative stress. It is possible that these clinical associations reflect compromised vitamin B12 metabolism due to such stress. Physicians are also increasingly aware of considerable inter-individual variation in the clinical response to B12 replacement therapy. Further research is needed to determine to what extent this is attributable to genetic determinants of vitamin B12 absorption, distribution and cellular uptake.

Evaluation of vitamin B12 effects on DNA damage induced by pioglitazone

Pioglitazone is a prototype of thiazolidinediones, used for the treatment of type 2 diabetes mellitus. Previous studies suggest that pioglitazone might cause DNA damage by generation of oxidative species. In this study, we investigated the mutagenic effects of pioglitazone using sister chromatid exchanges (SCEs), and chromosomal aberrations (CAs) assays in cultured human lymphocytes. In addition, oxidative DNA damage was evaluated in cells culture by measuring 8-hydroxy-2'-deoxyguanosine (8-OH-dG) marker. We also investigated the possible protective effects of vitamin B12, which is associated with DNA repair, on DNA damage induced by pioglitazone. Treatment of the human lymphocytes with pioglitazone (100μM) significantly increases the frequency of SCEs and CAs (p<0.01). In addition, significant elevation in 8-OH-dG release from lymphocytes was observed after treatment with pioglitazone (p<0.01). On the other hand, pretreatment of cultures with vitamin B12 (13.5μg/ml) protected lymphocytes from the genotoxic effect of pioglitazone. Therefore, we conclude that pioglitazone is genotoxic, and it induces chromosomal and oxidative DNA damage in cultured lymphocytes and this toxicity is prevented by pretreatment with vitamin B12.

S-adenosylmethionine reduces the progress of the Alzheimer-like features induced by B-vitamin deficiency in mice

Methylation reactions linked to homocysteine in the one-carbon metabolism are increasingly elicited in Alzheimer's disease, although the association of hyperhomocysteinemia and of low B vitamin levels with the disease is still debated. We previously demonstrated that hyperhomocysteinemia and DNA hypomethylation induced by B vitamin deficiency are associated with PSEN1 and BACE1 overexpression and amyloid production. The present study is aimed at assessing S-adenosylmethionine effects in mice kept under a condition of B vitamin deficiency. To this end, TgCRND8 mice and wild-type littermates were assigned to control or B vitamin deficient diet, with or without S-adenosylmethionine supplementation. We found that S-adenosylmethionine reduced amyloid production, increased spatial memory in TgCRND8 mice and inhibited the upregulation of B vitamin deficiency-induced PSEN1 and BACE1 expression and Tau phosphorylation in TgCRND8 and wild-type mice. Furthermore, S-adenosylmethionine treatment reduced plaque spreading independently on B vitamin deficiency. These results strengthen our previous observations on the possible role of one-carbon metabolism in Alzheimer's disease, highlighting hyperhomocysteinemia-related mechanisms in dementia onset/progression and encourage further studies aimed at evaluating the use of S-adenosylmethionine as a potential candidate drug for the treatment of the disease.

Alzheimer’s disease, oxidative stress and B-vitamin depletion

There is an association between cognitive function and vitamin B12 and folate status. Both vitamins participate in recycling the potentially toxic amino acid homocysteine to methionine and, ultimately, to the methyl donor S-adenosylmethionine (SAM). Consequently, B12 and folate indirectly influence glutathione synthesis – a major intracellular antioxidant. Neuroinflammation and oxidative stress are early features of Alzheimer’s disease (AD). Such stress impairs homocysteine recycling, degrades folate and decreases its cellular retention, resulting in limited SAM availability and increased homocysteine levels. Oxidized homocysteine derivatives, such as homocysteic acid, can initiate a vicious cycle by promoting free-radical formation. Decreased SAM also fosters development of characteristic AD neuropathologies – neurofibrillary tangles and amyloid plaques. The latter generate additional free radicals in a further feed-forward cascade. Future therapies should simultaneously halt neuroinflammation, restore redox homeostasis and replace depleted intracellular B vitamins. Developing early markers for these harmful processes will allow targeting of such therapy before irreversible cellular damage ensues.