Folic Acid Reduces Tau Phosphorylation by Regulating PP2A Methylation in Streptozotocin-Induced Diabetic Mice

Zebrafish models for studying cognitive enhancers

Cognitive decline is commonly seen both in normal aging and in neurodegenerative and neuropsychiatric diseases. Various experimental animal models represent a valuable tool to study brain cognitive processes and their deficits. Equally important is the search for novel drugs to treat cognitive deficits and improve cognitions. Complementing rodent and clinical findings, studies utilizing zebrafish (Danio rerio) are rapidly gaining popularity in translational cognitive research and neuroactive drug screening. Here, we discuss the value of zebrafish models and assays for screening nootropic (cognitive enhancer) drugs and the discovery of novel nootropics. We also discuss the existing challenges, and outline future directions of research in this field.

Methyl donor supplementation reduces phospho-Tau, Fyn and demethylated protein phosphatase 2A levels and mitigates learning and motor deficits in a mouse model of tauopathy

BACKGROUND

Reduced folate status and elevated levels of circulating homocysteine are modifiable risk factors for cognitive decline and dementia. Disturbances in one-carbon metabolism are associated with the pathological accumulation of phosphorylated tau, a hallmark feature of prevalent dementia, including Alzheimer's disease and subgroups of frontotemporal dementia.

METHODS

Here, using transgenic TAU58/2 mouse models of human tauopathy, we tested whether dietary supplementation with L-methylfolate (the active folate form), choline and betaine can reduce tau phosphorylation and associated behavioural phenotypes.

RESULTS

TAU58/2 mice fed with the methyl donor-enriched diet showed reduced phosphorylation of tau at the pathological S202 (CP13) and S396/S404 (PHF-1) epitopes and alleviation of associated motor and learning deficits. Compared with mice on the control diet, the decrease in cortical phosphorylated tau levels in mice fed with the methyl donor-enriched diet was associated with enhanced methylation of protein phosphatase 2A, the major brain tau Ser/Thr phosphatase. It also correlated with a reduction in protein levels of Fyn, a tau tyrosine kinase that plays a central role in mediating pathological tau-induced neurodegeneration. Conversely, Fyn expression levels were increased in mice with deficiencies in folate metabolism.

CONCLUSIONS

Our findings provide the first experimental evidence that boosting one-carbon metabolism with L-methylfolate, choline and betaine can mitigate key pathological, learning and motor deficits in a tauopathy mouse model. They give support to using a combination of methyl donors as a preventive or disease-modifying strategy for tauopathies.

Reversal of Lipid Metabolism Dysregulation by Selenium and Folic Acid Co-Supplementation to Mitigate Pathology in Alzheimer’s Disease

Aberrant lipid metabolism is reported to be closely related to the pathogenesis of neurodegenerative diseases, such as Alzheimer’s disease (AD). Selenium (Se) and folate are two ideal and safe nutritional supplements, whose biological effects include regulating redox and homocysteine (Hcy) homeostasis in vivo. Here, to achieve effective multitarget therapy for AD, we combined Se and folic acid in a co-supplementation regimen (Se-FA) to study the therapeutic potential and exact mechanism in two transgenic mouse models of AD (APP/Tau/PSEN and APP/PS1). In addition to a reduction in Aβ generation and tau hyperphosphorylation, a restoration of synaptic plasticity and cognitive ability was observed in AD mice upon Se-FA administration. Importantly, by using untargeted metabolomics, we found that these improvements were dependent on the modulation of brain lipid metabolism, which may be associated with an antioxidant effect and the promotion of Hcy metabolism. Thus, from mechanism to effects, this study systematically investigated Se-FA as an intervention for AD, providing important mechanistic insights to inform its potential use in clinical trials.

Homocysteine, Vitamins B6 and Folic Acid in Experimental Models of Myocardial Infarction and Heart Failure—How Strong Is That Link?

Cardiovascular diseases are the leading cause of death and the main cause of disability. In the last decade, homocysteine has been found to be a risk factor or a marker for cardiovascular diseases, including myocardial infarction (MI) and heart failure (HF). There are indications that vitamin B6 plays a significant role in the process of transsulfuration in homocysteine metabolism, specifically, in a part of the reaction in which homocysteine transfers a sulfhydryl group to serine to form α-ketobutyrate and cysteine. Therefore, an elevated homocysteine concentration (hyperhomocysteinemia) could be a consequence of vitamin B6 and/or folate deficiency. Hyperhomocysteinemia in turn could damage the endothelium and the blood vessel wall and induce worsening of atherosclerotic process, having a negative impact on the mechanisms underlying MI and HF, such as oxidative stress, inflammation, and altered function of gasotransmitters. Given the importance of the vitamin B6 in homocysteine metabolism, in this paper, we review its role in reducing oxidative stress and inflammation, influencing the functions of gasotransmitters, and improving vasodilatation and coronary flow in animal models of MI and HF.

Deep Sea Water Alleviates Tau Phosphorylation and Cognitive Impairment via PI3K/Akt/GSK-3β Pathway

Deep sea water (DSW), as a noticeable natural resource, has been demonstrated to contain high levels of beneficial minerals and exert marked anti-diabetes effects. Epidemiological studies show that type 2 diabetes mellitus (T2DM) is closely related to high danger of Alzheimer's disease (AD); moreover, Akt/GSK-3β signaling is the main underlying pathway that connects these two diseases. Besides, it has been demonstrated that minerals in DSW, such as Mg, Se, and Zn, could effectively treat cognitive deficits associated with AD. Herein, we first observed the protection of DSW against cognitive dysfunction in T2DM rats, then furtherly explored the neuroprotective mechanism in SH-SY5Y cell model. In T2DM rats, DSW obviously elevated the concentrations of elements Mg, V, Cr, Zn, and Se in brain and improved learning and memory dysfunction in behavior assays, including Morris water maze (MWM) and new object recognition (NOR). Western blot (WB) results demonstrated that DSW could stimulate PI3K/Akt/GSK-3β signaling, arrest Tau hyperphosphorylation at serine (Ser) 396 and threonine (Thr)231, which was confirmed by immunohistochemistry (IHC). In order to further confirm the mechanism, we employed wortmannin to inhibit PI3K in SH-SY5Y cells; results showed that pretreatment with wortmannin almost abolished DSW-induced decreases in phosphorylated Tau. Taken together, these data elucidated that DSW could improve Tau hyperphosphorylation and cognitive impairment, which were closely related with the stimulation of Akt/GSK-3β signaling, and the neuroprotective effects of DSW should be contributed to the synergistic effects of major and trace elements in it, such as Mg, V, Cr, Zn, and Se. These experimental evidence indicated that DSW may be explored as natural neuroprotective food for the prevention and treatment of AD.

One-Carbon Metabolism: Pulling the Strings behind Aging and Neurodegeneration

One-carbon metabolism (OCM) is a network of biochemical reactions delivering one-carbon units to various biosynthetic pathways. The folate cycle and methionine cycle are the two key modules of this network that regulate purine and thymidine synthesis, amino acid homeostasis, and epigenetic mechanisms. Intersection with the transsulfuration pathway supports glutathione production and regulation of the cellular redox state. Dietary intake of micronutrients, such as folates and amino acids, directly contributes to OCM, thereby adapting the cellular metabolic state to environmental inputs. The contribution of OCM to cellular proliferation during development and in adult proliferative tissues is well established. Nevertheless, accumulating evidence reveals the pivotal role of OCM in cellular homeostasis of non-proliferative tissues and in coordination of signaling cascades that regulate energy homeostasis and longevity. In this review, we summarize the current knowledge on OCM and related pathways and discuss how this metabolic network may impact longevity and neurodegeneration across species.

Targeting DNA Methylation in the Adult Brain through Diet

Metabolism and nutrition have a significant role in epigenetic modifications such as DNA methylation, which can influence gene expression. Recently, it has been suggested that bioactive nutrients and gut microbiota can alter DNA methylation in the central nervous system (CNS) through the gut-brain axis, playing a crucial role in modulating CNS functions and, finally, behavior. Here, we will focus on the effect of metabolic signals in shaping brain DNA methylation during adulthood. We will provide an overview of potential interactions among diet, gastrointestinal microbiome and epigenetic alterations on brain methylation and behavior. In addition, the impact of different diet challenges on cytosine methylation dynamics in the adult brain will be discussed. Finally, we will explore new ways to modulate DNA hydroxymethylation, which is particularly abundant in neural tissue, through diet.

Rutaecarpine Ameliorated High Sucrose-Induced Alzheimer's Disease Like Pathological and Cognitive Impairments in Mice

High sucrose can induce tau hyperphosphorylation and cognitive dysfunction/memory impairment as observed in Alzheimer's disease (AD). Rutaecarpine, a specific (transient receptor potential vanilloid 1 [TRPV1]) agonist, is neuroprotective against high sucrose diet-induced impairment, but detailed mechanisms are still elusive. In this study, we investigated whether rutaecarpine mitigates high sucrose diet-induced pathological alterations and cognitive in AD-like mice. Mice were administered fodder containing 0.01% rutaecarpine and 20% sucrose solution. Our results showed that rutaecarpine significantly attenuated high sucrose diet-induced spatial memory impairment and enhanced synaptic plasticity; rutaecarpine prevented high sucrose diet-induced tau hyperphosphorylation by decreasing glycogen synthase kinase-3β (GSK-3β) activity; activation of GSK-3β reversed the protective effect of rutaecarpine on learning and memory deficits, synaptic plasticity, and tau hyperphosphorylation induced by high-glucose diet significantly, suggesting that GSK-3β activation is required for high glucose-induced tau hyperphosphorylation. These results demonstrated that rutaecarpine can mitigate high sucrose diet-induced hyperphosphorylation of AD-associated tau protein and cognitive impairment by inhibiting GSK-3β, which supported that dietary rutaecarpine might have a promising use for therapeutic intervention of AD.

Chronic hyperglycemia induces tau hyperphosphorylation by downregulating OGT-involved O-GlcNAcylation in vivo and in vitro

OBJECTIVE

Diabetes mellitus (DM) can increase the risk of cognitive dysfunction, but its exact mechanisms remain unclear. The involvement of aberrant O-GlcNAcylation has been identified in hyperglycemia and DM, as well as the pathogenesis of Alzheimer's disease via competition with tau phosphorylation. This study was designed to investigate the role of O-GlcNAcylation in diabetes-associated cognitive dysfunction (DACD).

METHODS

Fifteen-week old male KK-Ay mice were used as DACD models, and advanced glycation end product (AGE)-treated HT22 cells were used as a model of high glucose toxicity. Morris water maze tests, histological staining, real-time quantitative PCR, and Western blot were also applied.

RESULTS

Mice with DACD exhibited evident obesity, hyperinsulinemia, hyperglycemia, and impaired learning and memory function. O-GlcNAcylation levels decreased and tau phosphorylation levels at Ser396, Ser404, Thr212, and Thr231 increased in the hippocampus of mice with DACD, as well as in AGE-treated HT22 cells. Hypoglycemic therapy improved these anomalies and elevated O-GlcNAc transferase (OGT) levels in mice with DACD. OGT plasmid transfection in HT22 cells partially reversed AGE-induced decreases in O-GlcNAcylation levels and increased tau phosphorylation levels.

CONCLUSIONS

Chronic hyperglycemia can induce tau hyperphosphorylation by downregulating OGT-involved O-GlcNAcylation in vivo and in vitro, which mediates DACD.

Drugs for Targeted Therapies of Alzheimer's Disease

Alzheimer's disease (AD) is one type of neurodegenerative diseases, which is prevalent in the elderly. Beta-amyloid (Aβ) plaques and phosphorylated tau-induced neurofibrillary tangles are two pathological hallmarks of this disease and the corresponding pathological pathways of these hallmarks are considered as the therapeutic targets. There are many drugs scheduled for pre-clinical and clinical trial that target to inhibit the initiators of pathological Aβ and tau aggregates as well as critical Aβ secretases and kinases in tau hyperphosphorylation. In addition, studies in disease gene variations, and detection of key prognostic effectors in early development are also important for AD control. The discovery of potential drug targets contributed to targeted therapy in a stage-dependent manner, However, there are still some issues that cause concern such as the low bioavailability and low efficacy of candidate drugs from clinical trial reports. Therefore, modification of drug candidates and development of delivery agents are essential and critical. With other medical advancements like cell replacement therapy, there is hope for the cure of Alzheimer's disease in the foreseeable future.

Protein phosphatase 2A and tau: an orchestrated 'Pas de Deux'

The neuronal microtubule-associated protein tau serves a critical role in regulating axonal microtubule dynamics to support neuronal and synaptic functions. Furthermore, it contributes to glutamatergic regulation and synaptic plasticity. Emerging evidence also suggests that tau serves as a signaling scaffold. Tau function and subcellular localization are tightly regulated, in part, by the orchestrated interplay between phosphorylation and dephosphorylation events. Significantly, protein phosphatase type 2A (PP2A), encompassing the regulatory PPP2R2A (or Bα) subunit, is a major brain heterotrimeric enzyme and the primary tau Ser/Thr phosphatase in vivo. Herein, we closely examine how the intimate and compartmentalized interactions between PP2A and tau regulate tau phosphorylation and function, and play an essential role in neuronal homeostasis. We also review evidence supporting a strong link between deregulation of tau-PP2A functional interactions and the molecular underpinnings of various neurodegenerative diseases collectively called tauopathies. Lastly, we discuss the opportunities and associated challenges in more specifically targeting PP2A-tau interactions for drug development for tauopathies.