Lithium inhibits stress-induced changes in tau phosphorylation in the mouse hippocampus

Heavy metals toxicity on epigenetic modifications in the pathogenesis of Alzheimer's disease (AD)

Alzheimer's disease (AD) is a progressive decline in cognitive ability and behavior which eventually disrupts daily activities. AD has no cure and the progression rate varies unlikely. Among various causative factors, heavy metals are reported to be a significant hazard in AD pathogenesis. Metal-induced neurodegeneration has been focused globally with thorough research to unravel the mechanistic insights in AD. Recently, heavy metals suggested to play an important role in epigenetic alterations which might provide evidential results on AD pathology. Epigenetic modifications are known to play towards novel therapeutic approaches in treating AD. Though many studies focus on epigenetics and heavy metal implications in AD, there is a lack of research on heavy metal influence on epigenetic toxicity in neurological disorders. The current review aims to elucidate the plausible role of cadmium (Cd), iron (Fe), arsenic (As), copper (Cu), and lithium (Li) metals on epigenetic factors and the increase in amyloid beta and tau phosphorylation in AD. Also, the review discusses the common methods of heavy metal detection to implicate in AD pathogenesis. Hence, from this review, we can extend the need for future research on identifying the mechanistic behavior of heavy metals on epigenetic toxicity and to develop diagnostic and therapeutic markers in AD.

Effects of early tooth loss on chronic stress and progression of neuropathogenesis of Alzheimer's disease in adult Alzheimer's model AppNL-G-F mice

Introduction

Alzheimer's disease (AD), the most common neurodegenerative disease, is characterized by accumulated amyloid-β (Aβ) plaques, aggregated phosphorylated tau protein, gliosis-associated neuroinflammation, synaptic dysfunction, and cognitive impairment. Many cohort studies indicate that tooth loss is a risk factor for AD. The detailed mechanisms underlying the association between AD and tooth loss, however, are not yet fully understood.

Methods

We explored the involvement of early tooth loss in the neuropathogenesis of the adult AppNL-G-F mouse AD model. The maxillary molars were extracted bilaterally in 1-month-old male mice soon after tooth eruption.

Results

Plasma corticosterone levels were increased and spatial learning memory was impaired in these mice at 6 months of age. The cerebral cortex and hippocampus of AD mice with extracted teeth showed an increased accumulation of Aβ plaques and phosphorylated tau proteins, and increased secretion of the proinflammatory cytokines, including interleukin 1β (IL-1β) and tumor necrosis factor α (TNF-α), accompanied by an increased number of microglia and astrocytes, and decreased synaptophysin expression. AD mice with extracted teeth also had a shorter lifespan than the control mice.

Discussion

These findings revealed that long-term tooth loss is a chronic stressor, activating the recruitment of microglia and astrocytes; exacerbating neuroinflammation, Aβ deposition, phosphorylated tau accumulation, and synaptic dysfunction; and leading to spatial learning and memory impairments in AD model mice.

Tauopathy and neurodegeneration: A role for stress

Neurodegenerative diseases are characterized by an irreversible and progressive loss of neuronal structure and function. While many alterations to normal cellular processes occur during neurodegeneration, a pathological accumulation of aggregated proteins constitutes a hallmark of several neurodegenerative disorders. Alzheimer's disease, specifically, is pathologically defined by the formation of amyloid plaques and tangles of hyperphosphorylated tau protein. Stress has emerged as an important factor in the development and progression of neurodegenerative diseases, including Alzheimer's. Very little is known, however, regarding the effects of stress on the mechanisms controlling abnormal protein aggregation and clearance. Chronic stress activates the hypothalamic-pituitary-adrenal (HPA) axis, causing an excessive secretion of glucocorticoids that are capable of impacting diverse physiological and cellular processes. The present review focuses on the influence of stress on a key feature of Alzheimer's disease pathology, emphasizing the relationship between tau phosphorylation and accumulation and its connection to HPA axis dysfunction.

Aging as a Precipitating Factor in Chronic Restraint Stress-Induced Tau Aggregation Pathology, and the Protective Effects of Rosmarinic Acid

Stress is an important risk factor of Alzheimer's disease (AD). It has been evidenced that stress could induce tau phosphorylation and increase tau insolubility in brain; however, little is known about the interactional effect of stress with aging on tauopathy. Therefore, we explored the effects of aging on stress-induced tauopathy and the potential mechanism in mouse model of chronic restraint stress (CRS). Here we found that in general, the level of phosphorylated tau (P-tau) was higher in brain of middle-aged mice than that in adult mice under physiological conditions. CRS-induced tau phosphorylation and its insolubility were more prominent in middle-aged mice. The increase of AT8-labeled insoluble P-tau was dramatic in middle-aged mice, which was highly ubiquitinated but did not form PHF structures. The levels of chaperones were relatively lower in middle-aged mice brain; CRS further reduced the expression, especially for HDJ2/HSP40. CRS also suppressed the expression of Pin1, the peptidylprolyl cis/trans isomerase, in middle-aged mice but not in adult mice. Downregulation of HSP40 or Pin1 caused an increase of transfected extraneous tau in 293 cells. Rosmarinic acid (RA) could effectively suppress the elevation of P-tau and insoluble P-tau formation induced by CRS, and reversed the abnormal changes of chaperones and Pin1 particularly in middle-aged mice. Taken together, our findings provided evidence that aging could be a promoting factor in stress-induced tauopathy, which was relevant with malregulation of chaperones and Pin1, and RA might be a promising beneficial agent for stress-induced tauopathy.

Increased tau phosphorylation and aggregation in the hippocampus of mice overexpressing corticotropin-releasing factor

Clinical and basic science research suggests that stress and/or changes in central stress signaling intermediates may be involved in Alzheimer's disease (AD) pathogenesis. Although the links between stress and AD remain unsettled, data from our group and others have established that stress exposure in rodents may confer susceptibility to AD pathology by inducing hippocampal tau phosphorylation (tau-P). Work in our laboratory has shown that stress-induced tau-P requires activation of the type-1 corticotropin-releasing factor receptor (CRFR1). CRF overexpressing (CRF-OE) mice are a model of chronic stress that display cognitive impairment at 9-10 month of age. In this study we used 6-7 month old CRF-OE mice to examine whether sustained exposure to CRF and stress steroids would impact hippocampal tau-P and kinase activity in the presence or absence of the CRFR1-specific antagonist, R121919, given daily for 30 days. CRF-OE mice had significantly elevated tau-P compared to wild type (WT) mice at the AT8 (S202/T204), PHF-1 (S396/404), S262, and S422 sites. Treating CRF-OE mice with R121919 blocked phosphorylation at the AT8 (S202/T204) and PHF-1 (S396/404) sites, but not at the S262 and S422 sites and reduced phosphorylation of c-Jun N Terminal Kinase (JNK). Examination of hippocampal extracts from CRF-OE mice at the ultrastructural level revealed negatively stained round/globular aggregates that were positively labeled by PHF-1. These data suggest critical roles for CRF and CRFR1 in tau-P and aggregation and may have implications for the development of AD cognitive decline.

KCNQ/Kv7 channel activator flupirtine protects against acute stress-induced impairments of spatial memory retrieval and hippocampal LTP in rats

Spatial memory retrieval and hippocampal long-term potentiation (LTP) are impaired by stress. KCNQ/Kv7 channels are closely associated with memory and the KCNQ/Kv7 channel activator flupirtine represents neuroprotective effects. This study aims to test whether KCNQ/Kv7 channel activation prevents acute stress-induced impairments of spatial memory retrieval and hippocampal LTP. Rats were placed on an elevated platform in the middle of a bright room for 30 min to evoke acute stress. The expression of KCNQ/Kv7 subunits was analyzed at 1, 3 and 12 h after stress by Western blotting. Spatial memory was examined by the Morris water maze (MWM) and the field excitatory postsynaptic potential (fEPSP) in the hippocampal CA1 area was recorded in vivo. Acute stress transiently decreased the expression of KCNQ2 and KCNQ3 in the hippocampus. Acute stress impaired the spatial memory retrieval and hippocampal LTP, the KCNQ/Kv7 channel activator flupirtine prevented the impairments, and the protective effects of flupirtine were blocked by XE-991 (10,10-bis(4-Pyridinylmethyl)-9(10H)-anthracenone), a selective KCNQ channel blocker. Furthermore, acute stress decreased the phosphorylation of glycogen synthase kinase-3β (GSK-3β) at Ser9 in the hippocampus, and flupirtine inhibited the reduction. These results suggest that the KCNQ/Kv7 channels may be a potential target for protecting both hippocampal synaptic plasticity and spatial memory retrieval from acute stress influences.

Chronic noise exposure causes persistence of tau hyperphosphorylation and formation of NFT tau in the rat hippocampus and prefrontal cortex

The non-auditory effects of noise exposure on the central nervous system have been established both epidemiologically and experimentally. Chronic noise exposure (CNE) has been associated with tau hyperphosphorylation and Alzheimer's disease (AD)-like pathological changes. However, experimental evidence for these associations remains limited. The aim of the current study was to explore the effects of CNE [100 dB sound pressure level (SPL) white noise, 4 h/d×14 d] on tau phosphorylation in the rat hippocampus and the prefrontal cortex. Forty-eight male Wistar rats were randomly assigned to two groups: a noise-exposed group and a control group. The levels of radioimmunoprecipitation assay (RIPA)-soluble and RIPA-insoluble phosphorylated tau at Ser202, Ser396, Ser404, and Ser422 in the hippocampus and the prefrontal cortex were measured at different time points (days 0, 3, 7, and 14) after the end of the last noise exposure. Exposure to white noise for 14 consecutive days significantly increased the levels of tau phosphorylation at Ser202, Ser396, Ser404, and Ser422, the sites typically phosphorylated in AD brains, in the hippocampus and the prefrontal cortex. Tau hyperphosphorylation persisted for 7 to 14 d after the cessation of noise exposure. These alterations were also concomitant with the generation of pathological neurofibrillary tangle (NFT) tau 3, 7 and 14 d after the end of the stimulus. Furthermore, lasting increases in proteins involved in hyperphosphorylation, namely glycogen synthase kinase 3β (GSK3β) and protein phosphatase 2A (PP2A), were found to occur in close correspondence with increase in tau hyperphosphorylation. The results of this study show that CNE leads to long-lasting increases in non-NFT hyperphosphorylated tau and delayed formation of misfolded NFT tau in the hippocampus and the prefrontal cortex. Our results also provide evidence for the involvement of GSK3β and PP2A in these processes.

Small-Molecule Inhibitors of GSK-3: Structural Insights and Their Application to Alzheimer's Disease Models

The world health organization (WHO) estimated that 18 million people are struck by Alzheimer's disease (AD). The USA, France, Germany, and other countries launched major programmes targeting the identification of risk factors, the improvement of caretaking, and fundamental research aiming to postpone the onset of AD. The glycogen synthase kinase 3 (GSK-3) is implicated in multiple cellular processes and has been linked to the pathogenesis of several diseases including diabetes mellitus, cancer, and AD. Inhibition of GSK-3 leads to neuroprotective effects, decreased β-amyloid production, and a reduction in tau hyperphosphorylation, which are all associated with AD. Various classes of small molecule GSK-3 inhibitors have been published in patents and original publications. Herein, we present a comprehensive summary of small molecules reported to interact with GSK-3. We illustrate the interactions of the inhibitors with the active site. Furthermore, we refer to the biological characterisation in terms of activity and selectivity for GSK-3, elucidate in vivo studies and pre-/clinical trials.

GSK3β negatively regulates oligodendrocyte differentiation and myelination in vivo

Glycogen synthase kinase 3β (GSK3β) is an essential integrating molecule for multiple proliferation and differentiation signals that regulate cell fate. Here, we have examined the effects of inhibiting GSK3β on the development of oligodendrocytes (OLs) from their oligodendrocyte precursors (OP) in vivo by injection into the lateral ventricle of postnatal mice and ex vivo in organotypic cultures of isolated intact rodent optic nerve. Our results show that a range of GSK3β inhibitors (ARA-014418, lithium, indirubin, and L803-mt) increase OPs and OLs and promote myelination. Inhibition of GSK3β stimulates OP proliferation and is prosurvival and antiapoptotic. The effects of GSK3β inhibition in OPs is via the canonical Wnt signaling pathway by stimulating nuclear translocation of β-catenin. However, direct comparison of the effects of Wnt3a and GSK3β inhibition in optic nerves shows that they have opposing actions on OLs, whereby GSK3β inhibition strikingly increases OL differentiation, whereas Wnt3a inhibits OL differentiation. Notably, GSK3β inhibition overrides the negative effects of Wnt3a on OLs, indicating novel GSK3β signaling mechanisms that negatively regulate OL differentiation. We identify that two mechanisms of GSK3β inhibition are to stimulate cAMP response element binding (CREB) and decrease Notch1 signaling, which positively and negatively regulate OL differentiation and myelination, respectively. A key finding is that GSK3β inhibition has equivalent effects in the adult and stimulates the regeneration of OLs and remyelination following chemically induced demyelination. This study identifies GSK3β as a profound negative regulator of OL differentiation that contributes to inefficient regeneration of OLs and myelin repair in demyelination.

Chronic restraint stress alters the expression and distribution of phosphorylated tau and MAP2 in cortex and hippocampus of rat brain

Microtubule-associated proteins (MAPs) play a critical role in maintaining normal cytoskeletal architecture and functions. In the present study, we aim to explore the effects of the emotional stressor, chronic restraint stress, on the expression levels and localization of tau and MAP2. We found that after chronic restraint stress, soluble hyperphosphorylated tau was greatly increased, whereas MAP2 was decreased. Moreover, immunohistochemistry analysis demonstrated that phosphorylated tau and MAP2 displayed the similar subcellular distribution pattern after chronic restraint stress. Robust hyperphosphorylated tau immunolabeling was observed both in cortex and hippocampus of stressed animals and mainly located to perikaryal/dendritic elements. After stress, the MAP2 was mainly distributed in the perikaryal compartments, discontinuous dendrites and neuropil. Moreover, the distribution pattern of MAP2 in hippocampus significantly changed. Immunofluorescence double labeling indicated that hyperphosphorylated tau increased in the regions where there displayed an decrease of MAP2. These results suggest that the involvement of repeated restraint stress may not only induce phosphorylation state of tau and distribution of phosphorylated tau, but also alter the content and neuronal distribution of MAP2. Tau and MAP2 are most important MAPs for neuronal cells, the subcellular distribution change of them might be link to functional change of neurons after emotional stress.

2-{3-[4-(Alkylsulfinyl)phenyl]-1-benzofuran-5-yl}-5-methyl-1,3,4-oxadiazole derivatives as novel inhibitors of glycogen synthase kinase-3beta with good brain permeability

Glycogen synthase kinase 3beta (GSK-3beta) inhibition is expected to be a promising therapeutic approach for treating Alzheimer's disease. Previously we reported a series of 1,3,4-oxadiazole derivatives as potent and highly selective GSK-3beta inhibitors, however, the representative compounds 1a,b showed poor pharmacokinetic profiles. Efforts were made to address this issue by reducing molecular weight and lipophilicity, leading to the identification of oxadiazole derivatives containing a sulfinyl group, (S)-9b and (S)-9c. These compounds exhibited not only highly selective and potent inhibitory activity against GSK-3beta but also showed good pharmacokinetic profiles including favorable BBB penetration. In addition, (S)-9b and (S)-9c given orally to mice significantly inhibited cold water stress-induced tau hyperphosphorylation in mouse brain.

Efficacy of a novel, orally active GSK-3 inhibitor 6-Methyl-N-[3-[[3-(1-methylethoxy)propyl]carbamoyl]-1H-pyrazol-4-yl]pyridine-3-carboxamide in tau transgenic mice

Neurofibrillary tangles (NFTs) composed of hyperphosphorylated and aggregated tau are common pathological characteristics in Alzheimer's disease (AD) and other tauopathies. Aberrant tau phosphorylation is an early and pivotal event in the pathogenesis of tauopathies, and since GSK-3 is a key factor implicated in aberrant tau phosphorylation, GSK-3 inhibition is expected to suppress tauopathy disease progression. In the present study, we report the efficacy of a newly discovered small molecule GSK-3 inhibitor, 6-methyl-N-[3-[[3-(1-methylethoxy)propyl]carbamoyl]-1H-pyrazol-4-yl]pyridine-3-carboxamide (compound A), to inhibit tau phosphorylation and to reduce the amount of pathological aggregated tau in JNPL3 mice that overexpress a mutant form of human tau. Compound A is a highly potent and selective inhibitor of GSK-3 with an IC(50) of 2 nM, with at least 230-fold lower potency against 27 other kinases. Oral administration of compound A resulted in a significant reduction of tau phosphorylation at several GSK-3 directed sites. Furthermore, chronic oral administration of compound A markedly reduced aggregated tau in old JNPL3 mice. These results suggest that a novel, orally active GSK-3 inhibitor, compound A, has potency in the prevention of tau pathology.

Functional interference between glycogen synthase kinase-3 beta and the transcription factor Nrf2 in protection against kainate-induced hippocampal cell death

Excitotoxicity mediated by glutamate receptors may underlay the pathology of several neurologic diseases. Considering that oxidative stress is central to excitotoxic damage, in this study we sought to analyze if the transcription factor Nrf2, guardian of redox homeostasis, might be targeted to prevent kainate-induced neuron death. Hippocampal slices from Nrf2 knockout mice exhibited increased oxidative stress and cell death compared to those of control mice in response to kainate, as determined with the redox sensitive probes 2,7-dichlorodihydrofluorescein diacetate (H(2)DCFAC) and propidium iodide and lactate dehydrogenase release, respectively, therefore demonstrating a role of Nrf2 in antioxidant protection against excitotoxicity. In the hippocampus of mice intraperitoneally injected with kainate we observed a rapid activation of Akt, inhibition of GSK-3beta and translocation of Nrf2 to the nucleus, but after 4 h Akt was inactive, GSK-3beta was active and Nrf2 was mostly cytosolic, therefore extending our previous studies which indicate that GSK-3beta excludes Nrf2 from the nucleus. Lithium, a GSK-3beta inhibitor, promoted Nrf2 transcriptional activity towards an Antioxidant-Response-Element (ARE) luciferase reporter and cooperated with sulforaphane (SFN) to induce this reporter and to increase the protein levels of heme oxygenase-1 (HO-1), coded by a representative ARE-containing gene. Conversely, ARE activation by SFN was attenuated by over-expression of active GSK-3beta. Finally, combined treatment with SFN and lithium attenuated oxidative stress and cell death in kainate-treated hippocampal slices of wild type mice but not Nrf2 null littermates. Our findings identify the axis GSK-3beta/Nrf2 as a pharmacological target in prevention of excitotoxic neuronal death.

Enhanced activity of hippocampal BACE1 in a mouse model of postmenopausal memory deficits

Ovarian hormone decline after menopause may influence cognitive performance and increase the risk for Alzheimer's disease (AD) in women. We have recently demonstrated that a combination of ovariectomy and chronic stress (OVX/stress) causes hippocampus-associated cognitive dysfunction in mice. In this study, we examined whether OVX/stress could affect the levels of AD-related molecules in the mouse hippocampus. Female ICR mice were ovariectomized or sham-operated, and then randomly divided into a daily restraint stress (21 days, 6 h/day) or non-stress group. Although OVX or stress alone did not affect beta-site amyloid precursor protein (APP)-cleaving enzyme-1 (BACE1) activity, OVX/stress increased activity in hippocampal CA1 and CA3 regions, compared with other groups. In contrast, OVX/stress did not affect gamma-secretase activity, Abeta(1-40), and phosphorylated-tau levels in the hippocampus. These findings suggest that a stressful life after menopause can influence the levels of AD-related molecules and that BACE1 is the most sensitive molecule for such a situation.

GSK-3beta activity is increased in skeletal muscle after burn injury in rats

Previous reports suggest that burn-induced muscle proteolysis can be inhibited by treatment with GSK-3beta inhibitors, suggesting that burn injury may be associated with increased GSK-3beta activity. The influence of burn injury on muscle GSK-3beta activity, however, is not known. We determined the effect of a 30% total body surface full-thickness burn injury in rats on muscle GSK-3beta activity by measuring GSK-3beta activity and tissue levels of serine 9 phosphorylated GSK-3beta, p(Ser9)-GSK-3beta, by Western blot analysis and immunohistochemistry. Because burn-induced muscle wasting is, at least in part, mediated by glucocorticoids, we used dexamethasone-treated cultured muscle cells in which GSK-3beta expression was reduced with small interfering RNA (siRNA) to further assess the role of GSK-3beta in muscle atrophy. Burn injury resulted in a seven-fold increase in GSK-3beta activity in skeletal muscle. This effect of burn was accompanied by reduced tissue levels of p(Ser9)-GSK-3beta, suggesting that burn injury stimulates GSK-3beta in skeletal muscle secondary to inhibited phosphorylation of the enzyme. In addition, burn injury resulted in inhibited phosphorylation and activation of Akt, an upstream regulatory mechanism of GSK-3beta activity. Reducing the expression of GSK-3beta in cultured muscle cells with siRNA inhibited dexamethasone-induced protein degradation by approximately 50%. The results suggest that burn injury stimulates GSK-3beta activity in skeletal muscle and that GSK-3beta may, at least in part, regulate glucocorticoid-mediated muscle wasting.

Corticotropin-releasing factor receptors differentially regulate stress-induced tau phosphorylation

Hyperphosphorylation of the microtubule-associated protein tau is a key event in the development of Alzheimer's disease (AD) neuropathology. Acute stress can induce hippocampal tau phosphorylation (tau-P) in rodents, but the mechanisms and pathogenic relevance of this response are unclear. Here, we find that hippocampal tau-P elicited by an acute emotional stressor, restraint, was not affected by preventing the stress-induced rise in glucocorticoids but was blocked by genetic or pharmacologic disruption of signaling through the type 1 corticotropin-releasing factor receptor (CRFR1). Conversely, these responses were exaggerated in CRFR2-deficient mice. Parallel CRFR dependence was seen in the stress-induced activation of specific tau kinases. Repeated stress exposure elicited cumulative effects on tau-P and its sequestration in an insoluble, and potentially pathogenic, form. These findings support differential regulatory roles for CRFRs in an AD-relevant form of neuronal plasticity and may link datasets documenting alterations in the CRF signaling system in AD and implicating chronic stress as a risk factor in age-related neurological disorders.