Menadione-induced tau dephosphorylation in cultured human neuroblastoma cells

Anti-amyloid potential of some phytochemicals against Aβ-peptide and α-synuclein, tau, prion, and Huntingtin protein

Some molecules self-assemble to create complex structures through molecular self-assembly. Hydrogel preparation, tissue repair, and therapeutic drug delivery are a few applications of molecular self-assembly. However, the self-assembly of amino acids, peptides, and proteins forms amyloid fibrils, resulting in various disorders, most notably neurodegenerative ailments. Examples include the self-assembly of phenylalanine, which causes phenylketonuria; Aβ, which causes Alzheimer's disease; the tau protein, which causes both Alzheimer's and Parkinson's diseases; and α-synuclein, which causes Parkinson's illness. This review provides information related to phytochemicals of great significance that can prevent the formation of, or destabilize, amino acid, peptide, and protein self-assemblies.

Revisiting the Role of Vitamins and Minerals in Alzheimer's Disease

Alzheimer's disease (AD) is the most common type of dementia that affects millions of individuals worldwide. It is an irreversible neurodegenerative disorder that is characterized by memory loss, impaired learning and thinking, and difficulty in performing regular daily activities. Despite nearly two decades of collective efforts to develop novel medications that can prevent or halt the disease progression, we remain faced with only a few options with limited effectiveness. There has been a recent growth of interest in the role of nutrition in brain health as we begin to gain a better understanding of what and how nutrients affect hormonal and neural actions that not only can lead to typical cardiovascular or metabolic diseases but also an array of neurological and psychiatric disorders. Vitamins and minerals, also known as micronutrients, are elements that are indispensable for functions including nutrient metabolism, immune surveillance, cell development, neurotransmission, and antioxidant and anti-inflammatory properties. In this review, we provide an overview on some of the most common vitamins and minerals and discuss what current studies have revealed on the link between these essential micronutrients and cognitive performance or AD.

The war against Alzheimer, the mitochondrion strikes back!

Alzheimer's disease (AD) is a leading neurodegenerative pathology associated with aging worldwide. It is estimated that AD prevalence will increase from 5.8 million people today to 13.8 million by 2050 in the United States alone. AD effects in the brain are well known; however, there is still a lack of knowledge about the cellular mechanisms behind the origin of AD. It is known that AD induces cellular stress affecting the energy metabolism in brain cells. During the pathophysiological advancement of AD, damaged mitochondria enter a vicious cycle, producing reactive oxygen species (ROS), harming mitochondrial DNA and proteins, leading to more ROS and cellular death. Additionally, mitochondria are interconnected with the plaques formed by amyloid-β in AD and have underlying roles in the progression of the disease and severity. For years, the biomedical field struggled to develop new therapeutic options for AD without a significant advancement. However, mitochondria are striking back existing outside cells in a new mechanism of intercellular communication. Extracellular mitochondria are exchanged from healthy to damaged cells to rescue those with a perturbed metabolism in a process that could be applied as a new therapeutic option to repair those brain cells affected by AD. In this review we highlight key aspects of mitochondria's role in CNS' physiology and neurodegenerative disorders, focusing on AD. We also suggest how mitochondria strikes back as a therapeutic target and as a potential agent to be transplanted to repair neurons affected by AD.

Transplantation of engineered exosomes derived from bone marrow mesenchymal stromal cells ameliorate diabetic peripheral neuropathy under electrical stimulation

Diabetic peripheral neuropathy (DPN) is a long-term complication associated with nerve dysfunction and uncontrolled hyperglycemia. In spite of new drug discoveries, development of effective therapy is much needed to cure DPN. Here, we have developed a combinatorial approach to provide biochemical and electrical cues, considered to be important for nerve regeneration. Exosomes derived from bone marrow mesenchymal stromal cells (BMSCs) were fused with polypyrrole nanoparticles (PpyNps) containing liposomes to deliver both the cues in a single delivery vehicle. We developed DPN rat model and injected intramuscularly the fused exosomal system to understand its long-term therapeutic effect. We found that the fused system along with electrical stimulation normalized the nerve conduction velocity (57.60 ± 0.45 m/s) and compound muscle action potential (16.96 ± 0.73 mV) similar to healthy control (58.53 ± 1.10 m/s; 18.19 ± 1.45 mV). Gastrocnemius muscle morphology, muscle mass, and integrity were recovered after treatment. Interestingly, we also observed paracrine effect of delivered exosomes in controlling hyperglycemia and loss in body weight and also showed attenuation of damage to the tissues such as the pancreas, kidney, and liver. This work provides a promising effective treatment and also contribute cutting edge therapeutic approach for the treatment of DPN.

Co-Expression of Three Wild-Type 3R-Tau Isoforms Induces Memory Deficit via Oxidation-Related DNA Damage and Cell Death: A Promising Model for Tauopathies

The three isoforms of 3R-tau are predominantly deposited in neurons bearing neurofibrillary tangles in Alzheimer's disease (AD), while only 3R-tau accumulation has been detected in Pick's disease (PiD), suggesting the involvement of 3R-tau in neurodegeneration. However, both the role and the molecular mechanism of 3R-tau in neurodegeneration are elusive. Here, we co-expressed three isoforms of human wild-type 3R-tau in adult mouse hippocampal to mimic the pathologic tau accumulating observed in PiD patients. We found that co-expressing three 3R-tau isoforms induced hyperphosphorylation and accumulation of tau proteins; simultaneously, the mice showed remarkable neuron death with synapse and memory deficits. Further in vitro and in vivo studies demonstrated that co-expressing 3R-tau isoforms caused oxidative stress evidenced by an increased malondialdehyde, and the decreased superoxide dismutase and glutathione peroxidase; the 3R-tau accumulation also induced significant glial activation and DNA double-strand breaks (DSBs). Notably, the toxic effects of 3R-tau accumulation were efficiently reversed by administration of antioxidants Vitamin E (VitE) and Vitamin C (VitC), respectively. These data reveal that intracellular accumulation of 3R-tau isoforms in adult brain induces significant neuron death and memory deficits with the mechanism involving oxidation-mediated DSBs; and the antioxidants VitE and VitC can efficiently attenuate the toxicities of 3R-tau. Given that no significant cell death has been detected in the currently available wild-type tau-accumulating models, co-expressing 3R-tau isoforms could be a promising model for drug development of tauopathies, such as PiD.

Exposure to 3-Nitropropionic Acid Mitochondrial Toxin Induces Tau Pathology in Tangle-Mouse Model and in Wild Type-Mice

Oxidative stress, particularly of mitochondrial origin, plays an important role in the pathogenesis of neurodegenerative disorders, including Alzheimer’s disease (AD) and other tauopathies. Controversies regarding the responses of tau phosphorylation state to various stimuli causing oxidative stress have been reported. Here we investigated the effect of 3-nitropropionic acid (3NP), a mitochondrial toxin which induces oxidative stress, on the tangle-pathology in our previously generated double mutant (E257T/P301S, DM) -Tau-tg mice and in WT-mice. We detected an increase in tangle pathology in the hippocampus and cortex of the DM-Tau-tg mice following exposure of the mice to the toxin, as well as generation of tangles in WT-mice. This increase was accompanied with alterations in the level of the glycogen synthase kinase 3β (GSK3β), the kinase which phosphorylates the tau protein, and in the phosphorylation state of this kinase. A response of microglial cells was noticed. These results point to the involvement of mitochondrial dysfunction in the development of the tangle-pathology and may suggest that interfering with mitochondrial dysfunction may have an anti-tangle therapeutic potential.

Free d-aspartate triggers NMDA receptor-dependent cell death in primary cortical neurons and perturbs JNK activation, Tau phosphorylation, and protein SUMOylation in the cerebral cortex of mice lacking d-aspartate oxidase activity

In mammals, free d-aspartate (D-Asp) is abundant in the embryonic brain, while levels remain very low during adulthood as a result of the postnatal expression and activity of the catabolizing enzyme d-aspartate oxidase (DDO). Previous studies have shown that long-lasting exposure to nonphysiological, higher D-Asp concentrations in Ddo knockout (Ddo^-/-) mice elicits a precocious decay of synaptic plasticity and cognitive functions, along with a dramatic age-dependent expression of active caspase 3, associated with increased cell death in different brain regions, including hippocampus, prefrontal cortex, and substantia nigra pars compacta. Here, we investigate the yet unclear molecular and cellular events associated with the exposure of abnormally high D-Asp concentrations in cortical primary neurons and in the brain of Ddo^-/- mice. For the first time, our in vitro findings document that D-Asp induces in a time-, dose-, and NMDA receptor-dependent manner alterations in JNK and Tau phosphorylation levels, associated with pronounced cell death in primary cortical neurons. Moreover, observations obtained in Ddo^-/- animals confirmed that high in vivo levels of D-Asp altered cortical JNK signaling, Tau phosphorylation and enhanced protein SUMOylation, indicating a robust indirect role of DDO activity in regulating these biochemical NMDA receptor-related processes. Finally, no gross modifications in D-Asp concentrations and DDO mRNA expression were detected in the cortex of patients with Alzheimer's disease when compared to age-matched healthy controls.

Glutathione depletion triggers actin cytoskeleton changes via actin-binding proteins

The importance of glutathione (GSH) in alternative cellular roles to the canonically proposed, were analyzed in a model unable to synthesize GSH. Gene expression analysis shows that the regulation of the actin cytoskeleton pathway is strongly impacted by the absence of GSH. To test this hypothesis, we evaluate the effect of GSH depletion via buthionine sulfoximine (5 and 12.5 mM) in human neuroblastoma MSN cells. In the present study, 70% of GSH reduction did not induce reactive oxygen species, lipoperoxidation, or cytotoxicity, which enabled us to evaluate the effect of glutathione in the absence of oxidative stress. The cells with decreasing GSH levels acquired morphology changes that depended on the actin cytoskeleton and not on tubulin. We evaluated the expression of three actin-binding proteins: thymosin β4, profilin and gelsolin, showing a reduced expression, both at gene and protein levels at 24 hours of treatment; however, this suppression disappears after 48 hours of treatment. These changes were sufficient to trigger the co-localization of the three proteins towards cytoplasmic projections. Our data confirm that a decrease in GSH in the absence of oxidative stress can transiently inhibit the actin binding proteins and that this stimulus is sufficient to induce changes in cellular morphology via the actin cytoskeleton.

Combined expression of tau and the Harlequin mouse mutation leads to increased mitochondrial dysfunction, tau pathology and neurodegeneration

Mitochondrial dysfunction and oxidative stress play an important role in ageing and have been implicated in several age-related neurodegenerative conditions including Alzheimer's disease (AD) and other tauopathies characterized by the presence of intracellular accumulations of the hyperphosphorylated microtubule-associated protein tau. To study the interaction between mitochondrial dysfunction and tau pathology in vivo, we generated a novel mouse model by crossbreeding two existing lines: the Harlequin (Hq) mutant mice which suffer from mitochondrial dysfunction and oxidative stress due to a lack of the mitochondrial apoptosis-inducing factor (AIF), and the P301L tau transgenic mice, a mouse model of human tau pathology. Combined expression of the Hq mouse mutation and the tau transgene in the Tau/Hq double mutant mice led to an increase in tau pathology and apoptotic neurodegeneration when compared to single expression of the two mutations. Neurodegeneration was most prominent in the dentate gyrus and was significantly increased in the cerebellum leading to aggravated motor deficits. Functional activity measurements of the mitochondrial respiratory chain (MRC) in the Tau/Hq mice revealed early decreased activities of multiple MRC complexes and depleted ATP levels which preceded neurodegeneration and elevated oxidative stress markers. These results suggest an age-dependent mutual reinforcement of the tau pathology and mitochondrial dysfunction in vivo, which may contribute to neurodegeneration in patients suffering from AD and other age-related tauopathies.

Coenzyme q induces tau aggregation, tau filaments, and Hirano bodies

Tau aggregation is a common feature of tauopathies such as Alzheimer disease (AD). In AD, tau assembles into fibrillar polymers; it may also be present in other aberrant aggregates, including Hirano bodies. The mechanisms leading to tau polymerization in vivo are not understood. In this study, we found that coenzyme Q (ubiquinone) facilitates tau aggregation after binding to tau molecules at the region of the tau molecule involved in self-assembly. Consequently, after tau-tau interactions, this region is masked in fibrillar tau polymers. Further in vitro studies showed that ubiquinone facilitates the interaction of tau protein with actin to form structures that are morphologically similar to Hirano bodies. Finally, studies in AD brains show that Hirano bodies react with an antibody raised against ubiquinone, indicating that ubiquinone is a component of Hirano bodies. Taken together, the in vitro models and findings in AD suggest that in the presence of ubiquinone, Hirano bodies may result from the interaction of actin and other proteins, including tau.

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.

Vitamin K and sphingolipid metabolism: evidence to date

The brain is enriched with sphingolipids, which are important membrane constituents and major lipid signaling molecules that have a role in motor and cognitive behavior. Vitamin K has been implicated in brain sphingolipid metabolism for more than 30 years. The in vitro and in vivo studies to date suggest a role of vitamin K in the regulation of multiple enzymes involved in sphingolipid metabolism within the myelin-rich regions in the brain. However, the precise mechanisms of action are not well understood. Further, the physiological consequences of the observed effects of vitamin K on sphingolipid metabolism have not been systematically studied.

A distinct death mechanism is induced by 1-methyl-4-phenylpyridinium or by 6-hydroxydopamine in cultured rat cortical neurons: degradation and dephosphorylation of tau

We examined whether the well-known neurotoxins 6-hydroxydopamine (6-OHDA) and 1-methyl-4-phenylpyridinium ion (MPP(+)) recruit distinct cell death mechanisms using primary cultured neurons derived from day 16 embryonic rat cortices. Electron microscopy revealed that cell death induced by both 6-OHDA and MPP(+) was typified by a condensation of chromatin while prominent mitochondrial swelling was observed only in those cells treated with MPP(+). Co-treatment of cells with a pan-caspase inhibitor, Z-VAD-fmk, attenuated 6-OHDA-induced chromatin condensation and neuronal death. Co-treatment with such antioxidants as N-acetylcysteine or Mn-TBAP also suppressed 6-OHDA-induced cell death. None of these treatments attenuated MPP(+)-induced cell death although caspase inhibition abolished MPP(+)-induced chromatin condensation. Interestingly, in these paradigms of cell death, the N-terminus of tau was specifically cleaved and the levels of phosphorylated tau were markedly decreased following 6-OHDA treatment. By contrast, the C-terminus of tau was cleaved in MPP(+)-induced cell death while the levels of phosphorylated tau remained largely unaltered. Taken together, our results indicate that distinct cellular mechanisms appear to underlie neurotoxin-induced cortical neuronal cell death.

Activation of PP2A-like phosphatase and modulation of tau phosphorylation accompany stress-induced apoptosis in cultured oligodendrocytes

In a number of neurodegenerative diseases, tau-positive glial cytoplasmic inclusions (GCIs), immunochemically labeled with antibodies to the small heat shock protein (HSP) alphaB-crystallin, occur in oligodendrocytes. The microtubule-associated protein tau is functionally modulated by phosphorylation. We have shown previously that oxidative stress (OS) and heat shock (HS) induce apoptotic cell death in oligodendrocytes. The present study was undertaken to test whether stress responses in oligodendrocytes cause abnormalities in the expression and posttranslational modification of tau proteins, and whether the dynamic phosphorylation and dephosphorylation of tau are involved in the pathogenesis of glial cells. Cultured rat brain oligodendrocytes were subjected to OS, exerted by hydrogen peroxide, or HS (44 degrees C, 30 min). Immunoblot analysis with a panel of phosphorylation-dependent antibodies shows that OS and HS caused the rapid dephosphorylation of tau proteins at multiple sites, before characteristic features of apoptosis were observed. Concomitantly, ERK1,2 (extracellular signal-regulated kinase) was activated. Tau phosphorylation and rephosphorylation after stress was mediated by glycogen synthase kinase 3beta (GSK-3beta), and not by ERK1,2 and could be suppressed by lithium chloride, a specific inhibitor of GSK-3beta. Stress-induced dephosphorylation could be mimicked by alkaline phosphatase and suppressed by the protein phosphatase inhibitor okadaic acid (OA), indicating that PP2A in oligodendrocytes is activated by stress. OA at low concentrations could prevent stress-induced DNA fragmentation, but eventually exerted cytotoxic effects. Hence, stress-induced activation of PP2A in oligodendrocytes and tau dephosphorylation constitute a major feature of the response to injury in these cells, which eventually undergo apoptotic cell death.

Functional differences of tau isoforms containing 3 or 4 C-terminal repeat regions and the influence of oxidative stress

We report functional differences between tau isoforms with 3 or 4 C-terminal repeats and a difference in susceptibility to oxidative conditions, with respect to the regulation of microtubule dynamics in vitro and tau-microtubule binding in cultured cells. In the presence of dithiothreitol in vitro, a 3-repeat tau isoform promotes microtubule nucleation, reduces the tubulin critical concentration for microtubule assembly, and suppresses dynamic instability. Under non-reducing conditions, threshold concentrations of 3-repeat tau and tubulin exist below which this isoform still promotes microtubule nucleation and assembly but fails to reduce the tubulin critical concentration or suppress dynamic instability; above these threshold concentrations, amorphous aggregates of 3-repeat tau and tubulin can be produced at the expense of microtubule formation. A 4-repeat tau isoform is less sensitive to the oxidative potential of the environment, behaving under oxidative conditions similarly to the 3-repeat isoform under reducing conditions. Under conditions of oxidative stress, in Chinese hamster ovary cells stably expressing either 3- or 4-repeat tau, 3-repeat tau disassociates from microtubules more readily than the 4-repeat isoform, and tau-containing high molecular weight aggregates are preferentially observed in lysates from the Chinese hamster ovary cells expressing 3-repeat tau, indicating greater susceptibility of 3-repeat tau to oxidative conditions, compared with 4-repeat tau in vivo.

Multiple-label immunocytochemistry for the evaluation of nature of cell death in experimental models of neurodegeneration

A prominent feature of neurodegenerative diseases is a loss of specific neuronal populations. The pathophysiological mechanisms responsible are, however, poorly understood. Primary cultures of rodent embryonic neurons represent a useful experimental system for investigation of molecular pathways of neurodegeneration and mechanisms of cell death. Here, we report a technique utilizing triple-label immunocytochemistry with confocal immunofluorescence detection designed to simultaneously assess multiple parameters of cell injury in individual hippocampal neurons in primary culture. This method combines detection of DNA damage (TUNEL or Klenow assay) with double-label immunocytochemistry for the activated form of caspase-3 or, alternatively, caspase-cleaved actin (fractin), and microtubule-associated protein-2 (MAP-2) or beta-tubulin. The combined evaluation of the form of nuclear damage (karyorrhexis, pyknosis), the presence or absence of activated caspase-3, and the extent of the damage to cell cytoskeleton, allows for precise assessment of the extent of injury and the mode of cell death (apoptosis, oncosis) for individual neurons.

Up-regulation of the lysosomal system in experimental models of neuronal injury: implications for Alzheimer's disease

Previous studies established that the populations of neurons that frequently degenerate in Alzheimer's disease exhibit robust up-regulation of the lysosomal system. In this study, we investigated alterations of the lysosomal system during different forms of experimental injury in rat hippocampal neurons in culture, utilizing a combination of immunocytochemical and biochemical methods. Using triple-label immnocytochemistry for activated caspase-3, fragmentation of DNA and the microtubule-associated protein-2, we characterized treatment paradigms as models of the apoptotic (staurosporine, camptothecin), the oncotic (high-dose menadione, glutamate), and the mixed apoptotic and oncotic (low-dose menadione) pathways of neuronal death. Slowly developing apoptotic or slowly developing mixed apoptotic and oncotic forms of neuronal injury were associated with substantial increases in the number and size of cathepsin D-positive vesicles (late endosomes and mature lysosomes) as determined by immunocytochemistry, and elevated levels of cathepsin D by western blotting. In agreement with our previous findings in Alzheimer's disease, where lysosomal system activation was not restricted to overtly degenerating neurons, up-regulation of this system was also detected quite early during the course of experimental neuronal injury, preceding the development of dystrophic neurites, nuclear segmentation or fragmentation of DNA. These findings implicate lysosomal system activation, both in Alzheimer's disease and in experimental models of neuronal injury, as an important event associated with early stages of neurodegeneration.

Sensitization of neuronal cells to oxidative stress with mutated human alpha-synuclein

Linkage of alpha-synuclein (alpha-SN) mutations to familial Parkinson's disease (PD) and presence of alpha-SN as a major constituent of Lewy body in both sporadic and familial PD implicate alpha-SN abnormality in PD pathogenesis. Here we demonstrate that overexpression of wild-type or mutant alpha-SN does not cause any deleterious effect on the growth or continued propagation of transfected human cells, but overproduction of mutant alpha-SN heightens their sensitivity to menadione-induced oxidative injury. Such enhanced vulnerability is more pronounced in neuronal transfectants than in their nonneuronal counterparts and is associated with increased production of reactive oxygen species. The data suggest that mutated alpha-SN, especially with an alanine-to-proline substitution at residue 30, sensitizes neuronal cells to oxidative damage.

Amyloid-beta injection in rat amygdala alters tau protein but not mRNA expression

Previously we demonstrated local and distant changes in tau protein immunoreactivity reminiscent of those seen in Alzheimer's disease (AD) following a unilateral injection of amyloid-beta (Abeta)(25-35) into the rat amygdala. To explore the relevance of these findings to AD, we compared the effects of Abeta(1-42) to those of Abeta(25-35). Injections of both Abeta(1-42) and Abeta(25-35) into rat amygdala resulted in increased tau-2 immunolabeling in neurons. To determine whether these alterations were due to changes in the expression of tau, we measured tau protein expression by Western blotting and tau mRNA isoform expression by the reverse transcription-polymerase chain reaction in the amygdala, hippocampus, and cerebellum following a unilateral injection of Abeta(25-35) or vehicle into the amygdala. The levels of tau proteins were increased bilaterally in the amygdala of Abeta(25-35)- compared to vehicle-treated animals 8 and 16 days following treatment. The molecular weights of tau proteins were decreased in the Abeta(25-35)-treated (59-69 kDa) compared to the vehicle-treated (67-72 kDa) animals 8 days following treatment. There were no changes in tau mRNA expression in any brain region examined. In this model, just as in AD, there is an increase in tau protein levels without a change in tau mRNA expression, suggesting that Abeta peptides may influence tau protein stability in both the rat and the human brain.

An immunochemical study on tau glycation in paired helical filaments

Glycation is a non-enzymatic posttranslational modification that involves a covalent linkage between a sugar and an amino group of protein molecule forming ketoamine. Subsequent oxidation, fragmentation and/or crosslinking of ketoamine leads to the production of advanced glycation endproducts (AGEs). Formation of AGEs causes detrimental effects on the structure and function of affected proteins. Accumulation of AGEs has been implicated in normal aging and in the pathogenesis of diabetes-associated complications and Alzheimer's disease (AD). Of all AGEs, Nepsilon-(carboxymethyl)lysine (CML) is a major glycoxidation product known to be stable and accumulate progressively in vivo. In order to determine if tau is glycated in AD, we raised a rabbit antibody to CML that demonstrated its usefulness in detecting glycation of different proteins in vitro, including BSA, ribonuclease, lysozyme and recombinant tau. Immunochemical analyses indicated that ribose and glucose-6-phosphate are more effective than glucose in generating CML formation in these proteins. We used this antibody to probe for glycation in the following human tau preparations: tau of normal brains and preparations of soluble PHF-tau as well as insoluble PHF from AD brains. All three principal tau components resolved from PHF-tau on Western blots showed CML immunoreactivity indicating that tau is glycated in PHF-tau; and insoluble PHF exhibited prominent CML immunoreactivity on top of the stacking gel. Moreover, immunoelectron microscopic analyses indicate that the anti-CML antibody labels predominantly PHF in aggregates. Taken together, these results suggest that tau becomes glycated in PHF-tau and glycation may play a role in stabilizing PHF aggregation leading to tangle formation in AD.