Reduced early hypoxic/ischemic brain damage is associated with increased GLT-1 levels in mice expressing mutant (P301L) human tau

Proteomic and Genomic Changes in Tau Protein, Which Are Associated with Alzheimer's Disease after Ischemia-Reperfusion Brain Injury

Recent evidence suggests that transient ischemia of the brain with reperfusion in humans and animals is associated with the neuronal accumulation of neurotoxic molecules associated with Alzheimer’s disease, such as all parts of the amyloid protein precursor and modified tau protein. Pathological changes in the amyloid protein precursor and tau protein at the protein and gene level due to ischemia may lead to dementia of the Alzheimer’s disease type after ischemic brain injury. Some studies have demonstrated increased tau protein immunoreactivity in neuronal cells after brain ischemia-reperfusion injury. Recent research has presented many new tau protein functions, such as neural activity control, iron export, protection of genomic DNA integrity, neurogenesis and long-term depression. This review discusses the potential mechanisms of tau protein in the brain after ischemia, including oxidative stress, apoptosis, autophagy, excitotoxicity, neurological inflammation, endothelium, angiogenesis and mitochondrial dysfunction. In addition, attention was paid to the role of tau protein in damage to the neurovascular unit. Tau protein may be at the intersection of many regulatory mechanisms in the event of major neuropathological changes in ischemic stroke. Data show that brain ischemia activates neuronal changes and death in the hippocampus in a manner dependent on tau protein, thus determining a new and important way to regulate the survival and/or death of post-ischemic neurons. Meanwhile, the association between tau protein and ischemic stroke has not been well discussed. In this review, we aim to update the knowledge about the proteomic and genomic changes in tau protein following ischemia-reperfusion injury and the connection between dysfunctional tau protein and ischemic stroke pathology. Finally we present the positive correlation between tau protein dysfunction and the development of sporadic Alzheimer’s disease type of neurodegeneration.

Tau as a potential therapeutic target for ischemic stroke

Tau is a protein mainly expressed in adult human brain. It plays important roles both in neurodegenerative diseases and stroke. Stroke is an important cause of adult death and disability, ischemic stroke almost account for 80% in all cases. Abundant studies have proven that the increase of dysfunctional tau may act as a vital factor in pathological changes after ischemic stroke. However, the relationship between tau and ischemic stroke remains ununified. Based on present studies, we firstly introduced the structure and biological function of tau protein. Secondly, we summarized the potential regulatory mechanisms of tau protein in the process of ischemic stroke. Thirdly, we discussed about the findings in therapeutic researches of ischemic stroke. This review may be helpful in implementing new therapies for ischemic stroke and may be beneficial for the clinical and experimental studies.

Schisantherin B Improves the Pathological Manifestations of Mice Caused by Behavior Desperation in Different Ages-Depression with Cognitive Impairment

Depression is a major mood disorder. Abnormal expression of glial glutamate transporter-1 (GLT-1) is associated with depression. Schisantherin B (STB) is one bioactive of lignans isolated from Schisandra chinensis (Turcz.) Baill which has been commonly used as a traditional herbal medicine for thousands of years. This paper was designed to investigate the effects of STB on depressive mice induced by forced swimming test (FST). Additionally, we also assessed the impairment of FST on cognitive function in mice with different ages. FST and open field test (OFT) were used for assessing depressive symptoms, and Y-maze was used for evaluating cognition processes. Our study showed that STB acting as an antidepressant, which increased GLT-1 levels by promoting PI3K/AKT/mTOR pathway. Although the damage is reversible, short-term learning and memory impairment caused by FST test is more serious in the aged mice, and STB also exerts cognition improvement ability in the meanwhile. Our findings suggested that STB might be a promising therapeutic agent of depression by regulating the GLT-1 restoration as well as activating PI3K/AKT/mTOR pathway.

Bexarotene targets autophagy and is protective against thromboembolic stroke in aged mice with tauopathy

Stroke is a highly debilitating, often fatal disorder for which current therapies are suitable for only a minor fraction of patients. Discovery of novel, effective therapies is hampered by the fact that advanced age, primary age-related tauopathy or comorbidities typical to several types of dementing diseases are usually not taken into account in preclinical studies, which predominantly use young, healthy rodents. Here we investigated for the first time the neuroprotective potential of bexarotene, an FDA-approved agent, in a co-morbidity model of stroke that combines high age and tauopathy with thromboembolic cerebral ischemia. Following thromboembolic stroke bexarotene enhanced autophagy in the ischemic brain concomitantly with a reduction in lesion volume and amelioration of behavioral deficits in aged transgenic mice expressing the human P301L-Tau mutation. In in vitro studies bexarotene increased the expression of autophagy markers and reduced autophagic flux in neuronal cells expressing P301L-Tau. Bexarotene also restored mitochondrial respiration deficits in P301L-Tau neurons. These newly described actions of bexarotene add to the growing amount of compelling data showing that bexarotene is a potent neuroprotective agent, and identify a novel autophagy-modulating effect of bexarotene.

tau protein, ischemic injury and vascular dementia

Clinical, neuroimaging and neuropathological studies have confirmed overlap between Alzheimer's disease (AD) and vascular dementia (VaD). Classical neuropathological changes of AD (plaques and tangles) can be present in VaD. We review neuroimaging, biochemical and animal studies to consider the role of tau protein in ischemic injury and VaD pathogenesis. The evidence comes largely from transgenic animal studies that confirm that tau transgenes influence cerebral vasculature. Clinicobiochemical studies in the cerebrospinal fluid (CSF) have, similarly, confirmed alterations in both total and phosphorylated tau protein in VaD. These data suggest that tau protein not only serves as a potential diagnostic tool for differential diagnosis of VaD from other types of dementia, but may also be a therapeutic target in ischemic stroke.

P301L tau expression affects glutamate release and clearance in the hippocampal trisynaptic pathway

Individuals at risk of developing Alzheimer's disease (AD) often exhibit hippocampal hyperexcitability. A growing body of evidence suggests that perturbations in the glutamatergic tripartite synapse may underlie this hyperexcitability. Here, we used a tau mouse model of AD (rTg(TauP301L)4510) to examine the effects of tau pathology on hippocampal glutamate regulation. We found a 40% increase in hippocampal vesicular glutamate transporter, which packages glutamate into vesicles, and has previously been shown to influence glutamate release, and a 40% decrease in hippocampal glutamate transporter 1, the major glutamate transporter responsible for removing glutamate from the extracellular space. To determine whether these alterations affected glutamate regulation in vivo, we measured tonic glutamate levels, potassium-evoked glutamate release, and glutamate uptake/clearance in the dentate gyrus, cornu ammonis 3(CA3), and cornu ammonis 1(CA1) regions of the hippocampus. P301L tau expression resulted in a 4- and 7-fold increase in potassium-evoked glutamate release in the dentate gyrus and CA3, respectively, and significantly decreased glutamate clearance in all three regions. Both release and clearance correlated with memory performance in the hippocampal-dependent Barnes maze task. Alterations in mice expressing P301L were observed at a time when tau pathology was subtle and before readily detectable neuron loss. These data suggest novel mechanisms by which tau may mediate hyperexcitability. Pre-synaptic vesicular glutamate transporters (vGLUTs) package glutamate into vesicles before exocytosis into the synaptic cleft. Once in the extracellular space, glutamate acts on glutamate receptors. Glutamate is removed from the extracellular space by excitatory amino acid transporters, including GLT-1, predominantly localized to glia. P301L tau expression increases vGLUT expression and glutamate release, while also decreasing GLT-1 expression and glutamate clearance.

Propofol inhibits inflammatory cytokine-mediated glutamate uptake dysfunction to alleviate learning/memory impairment in depressed rats undergoing electroconvulsive shock

Electroconvulsive therapy (ECT) is an effective treatment for major depression, but can result in memory impairment. Several studies have shown that anesthetic propofol can alleviate the impairment of memory induced by ECT. However, the underlying molecular mechanisms remain unclear. We aimed to investigate the effects of propofol and electroconvulsive shock (ECS, analog of ECT in animals) on hippocampal inflammatory cytokines and glutamate uptake in depressed rats. The chronic unpredictable mild stress (CUMS) procedure was adopted to establish a model of depression. Sixty adult Sprague-Dawley rats were randomly divided into 5 groups with the following assignments (n=12 for each group): group C: control group without treatment; group D: CUMS+sham ECS; group DE: CUMS+ECS; group DP: CUMS+propofol (80 mg/kg, i.p.); group DPE: CUMS+propofol (80 mg/kg, i.p.)+ECS. Sucrose preference test and Morris water maze were used to assess behavioral changes. Hippocampal glutamate levels were measured with high performance liquid chromatography and the expression levels of IL-1β, TNF-α, GLAST and GLT-1 was quantificational analyzed by real time PCR or Western Blotting. The results demonstrated that ECS increased the levels of IL-1β and TNF-α, down-regulated the expression of GLT-1, GLAST expression remains stable, heightened the concentration of glutamate in the hippocampus and aggravated learning and memory impairment of depressed rats. Propofol suppressed IL-1β and TNF-α production, up-regulated the expression of GLT-1, decreased the concentration of glutamate in the hippocampus and attenuated the impairment of learning and memory induced by ECS. Propofol alleviate the learning and memory impairment induced by ECS could be partly attributed to its anti-inflammatory effects. This article is part of a Special Issue entitled Brain and Memory.

Intermittent hypobaric hypoxia preconditioning induced brain ischemic tolerance by up-regulating glial glutamate transporter-1 in rats

Several studies showed that the up-regulation of glial glutamate transporter-1 (GLT-1) participates in the acquisition of brain ischemic tolerance induced by cerebral ischemic preconditioning or ceftriaxone pretreatment in rats. To explore whether GLT-1 plays a role in the acquisition of brain ischemic tolerance induced by intermittent hypobaric hypoxia (IH) preconditioning (mimicking 5,000 m high-altitude, 6 h per day, once daily for 28 days), immunohistochemistry and western blot were used to observe the changes in the expression of GLT-1 protein in hippocampal CA1 subfield during the induction of brain ischemic tolerance by IH preconditioning, and the effect of dihydrokainate (DHK), an inhibitor of GLT-1, on the acquisition of brain ischemic tolerance in rats. The basal expression of GLT-1 protein in hippocampal CA1 subfield was significantly up-regulated by IH preconditioning, and at the same time astrocytes were activated by IH preconditioning, which appeared normal soma and aplenty slender processes. The GLT-1 expression was decreased at 7 days after 8-min global brain ischemia. When the rats were pretreated with the IH preconditioning before the global brain ischemia, the down-regulation of GLT-1 protein was prevented clearly. Neuropathological evaluation by thionin staining showed that 200 nmol DHK blocked the protective role of IH preconditioning against delayed neuronal death induced normally by 8-min global brain ischemia. Taken together, the up-regulation of GLT-1 protein participates in the acquisition of brain ischemic tolerance induced by IH preconditioning in rats.

The role of glutamate transporter-1a in the induction of brain ischemic tolerance in rats

This study was undertaken to determine the role of glutamate transporter-1a (GLT-1a), one of the splice variants of glutamate transporter-1, in the induction of brain ischemic tolerance by cerebral ischemic preconditioning (CIP). We used a rat global cerebral ischemic model and assessed changes by neuropathological evaluation, Western blot analysis, immunohistochemistry, real-time PCR, in vivo brain microdialysis, and high performance liquid chromatography. We found that CIP induced a significant upregulation of GLT-1a expression in the CA1 hippocampus in a time course corresponding to that of neuroprotection of CIP against brain ischemia. Severe brain ischemia for 8 min induced delayed downregulation of GLT-1a, an obvious increase in glutamate concentration and delayed neuronal death of the pyramidal neurons in the CA1 hippocampus. When the animals were pretreated with CIP before the severe ischemia, the above changes normally induced by the severe ischemia were effectively prevented. Importantly, such a preventive effect of CIP on these changes was significantly inhibited by intracerebroventricular administration of GLT-1a antisense oligodeoxynucleotides, which have been proven to specifically inhibit the expression of GLT-1a protein and mRNA, and had no effect on the expression of GLT-1b. In addition, the concentration of aspartate was also elevated after severe brain ischemic insult. However, CIP had no effect on the elevated aspartate concentrations. These results indicate that GLT-1a participated in the brain ischemic tolerance induced by CIP in rats.

High expression of GLT-1 in hippocampal CA3 and dentate gyrus subfields contributes to their inherent resistance to ischemia in rats

It is well known that neurons in the CA3 and dentate gyrus (DG) subfields of the hippocampus are resistant to short period of ischemia which is usually lethal to pyramidal neurons in hippocampal CA1 subfield. The present study was undertaken to clarify whether the inherent higher resistance of neurons in CA3 and DG to ischemia is associated with glial glutamate transporter-1 (GLT-1) in rats. Western blot analysis and immunohistochemistry assay showed that the basal expressions of GLT-1 in both CA3 and DG were much higher than that in CA1 subfield. Mild global brain ischemia for 8 min induced delayed death of almost all CA1 pyramidal neurons and marked GLT-1 down-regulation in the CA1 subfield, but it was not lethal to the neurons in either CA3 or DG and induced GLT-1 up-regulation and astrocyte activation showed normal soma and aplenty slender processes in the both areas. When the global brain ischemia was prolonged to 25 min, neuronal death was clearly observed in CA3 and DG accompanied with down-regulation of GLT-1 expression and abnormal astrocytes represented with hypertrophic somas, but shortened processes. After down-regulating of GLT-1 expression and function by its antisense oligodeoxynucleotides or inhibiting GLT-1 function by dihydrokainate, an inhibitor of GLT-1, the mild global brain ischemia for 8 min, which usually was not lethal to CA3 and DG neurons, induced the neuronal death in CA3 and DG subfields. Taken together, the higher expression of GLT-1 in the CA3 and DG contributes to their inherent resistance to ischemia.

Tau as a potential novel therapeutic target in ischemic stroke

Stroke is associated with high mortality and major disability burdens worldwide, but there are few effective and widely available therapies. Tau plays an important role in promoting microtubule assembly and stabilizing microtubule networks with phosphorylation regulating these functions. Based on the "ischemia-reperfusion theory" of Alzheimer's disease, some previous studies have focused on the relationship of tau and Alzheimer lesions in experimental brain ischemia. Thus, we hypothesize that the alterations in phosphorylation of tau are critical to microtubule dynamics and metabolism, and contribute to the pathophysiologic mechanisms during brain ischemia and/or reperfusion processes. We infer that regulation of phosphorylation of tau may be considered as a potential new therapeutic target in ischemic stroke.