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Oxidative stress, altered glutathione levels, and hyperhomocysteinemia play critical roles in Alzheimer's disease. We studied the relationships between hyperhomocysteinemia, glutathione, and oxidative stress in TgCRND8 mice maintained in conditions of folate, B12, and B6 deficiency and the effect of S-adenosylmethionine supplementation. We found that hyperhomocysteinemia was correlated with increased reduced/oxidized brain glutathione ratio, with decreased glutathione S-transferase activity and increased lipid peroxidation. S-adenosylmethionine potentiated superoxide dismutase and glutathione S-transferase activity and restored altered brain glutathione and erythrocytes lipid peroxidation. These results underline the importance of S-adenosylmethionine as neuroprotective compound, acting both on methylation and oxidation metabolism.

Neurofibrillary tangles (NFTs), composed of intracellular filamentous aggregates of hyperphosphorylated protein tau, are one of the pathological hallmarks of Alzheimer's disease (AD). Tau phosphorylation is regulated by the equilibrium between activities of its protein kinases and phosphatases; unbalance of these activities is proposed to be a reasonable causative factor to the disease process. Glycogen synthase kinase 3beta (GSK3beta) is one of the most important protein kinase in regulating tau phosphorylation; overexpression of active GSK3beta causes ADlike hyperphosphorylation of tau. Protein phosphatase 2A (PP2A) is the major phosphatase that dephosphorylates tau; it was demonstrated that highly conserved carboxyl-terminal sequence of PP2A C-subunit is a focal point for phosphatase regulation. This is the site of a reversible methyl esterification reaction that controls AB_{alpha}C heterotrimers formation. Here we demonstrate that GSK3beta and PP2A genes were upregulated by inhibiting methylation reactions through B vitamin deficiency. In this condition, methylated catalytic subunit PP2Ac was decreased, leading to reduced PP2A activity. By contrast, we observed GSK3beta protein increase and a modulation in phosphorylation sites that regulate GSK3beta activity. Therefore, one-carbon metabolism alteration seems to be a cause of deregulation of the equilibrium between GSK3beta and PP2A, leading to abnormal hyperphosphorylated tau.

By means of functional genomics analysis, we recently described the mRNA expression profiles of various genes involved in the neuroinflammatory response in the brains of subjects with late-onset Alzheimer Disease (LOAD). Some of these genes, namely interleukin (IL)-1β and IL-6, showed distinct expression profiles with peak expression during the first stages of the disease and control-like levels at later stages. IL-1β and IL-6 genes are modulated by DNA methylation in different chronic and degenerative diseases; it is also well known that LOAD may have an epigenetic basis. Indeed, we and others have previously reported gene-specific DNA methylation alterations in LOAD and in related animal models. Based on these data, we studied the DNA methylation profiles, at single cytosine resolution, of IL-1β and IL-6 5'-flanking region by bisulphite modification in the cortex of healthy controls and LOAD patients at 2 different disease stages: Braak I-II/A and Braak V-VI/C. Our analysis provides evidence that neuroinflammation in LOAD is associated with (and possibly mediated by) epigenetic modifications.

The Presenilin1 (PSEN1) gene encodes the catalytic peptide of the γ-secretase complex, a key enzyme that cleaves the amyloid-β protein precursor (AβPP), to generate the amyloid-β (Aβ) peptides, involved in Alzheimer's Disease (AD). Other substrates of the γ-secretase, such as Ecadherin and Notch1, are involved in neurodevelopment and haematopoiesis. Gene-specific DNA methylation influences PSEN1 expression in AD animal models. Here we evaluated canonical and non-canonical cytosine methylation patterns of the PSEN1 5ʹ-flanking during brain development and AD progression, in DNA extracted from the frontal cortex of AD transgenic mice (TgCRND8) and post-mortem human brain. Mapping CpG and non-CpG methylation revealed different methylation profiles in mice and humans. PSEN1 expression only correlated with DNA methylation in adult female mice. However, in post-mortem human brain, lower methylation, both at CpG and non-CpG sites, correlated closely with higher PSEN1 expression during brain development and in disease progression. PSEN1 methylation in blood DNA was significantly lower in AD patients than in controls. The present study is the first to demonstrate a temporal correlation between dynamic changes in PSEN1 CpG and non-CpG methylation patterns and mRNA expression during neurodevelopment and AD neurodegeneration. These observations were made possible by the use of an improved bisulphite methylation assay employing primers that are not biased towards non-CpG methylation. Our findings deepen the understanding of γ-secretase regulation and support the hypothesis that epigenetic changes can promote the pathophysiology of AD. Moreover, they suggest that PSEN1 DNA methylation in peripheral blood may provide a biomarker for AD.