Protective effects of estradiol against amyloid beta protein-induced inhibition of neuronal Cl(-)-ATPase activity

Associations of vitamin D receptor polymorphisms with risk of Alzheimer's disease, Parkinson's disease, and mild cognitive impairment: a systematic review and meta-analysis

Vitamin D is a lipid soluble steroid hormone, which plays a critical role in the calcium homeostasis, neuronal development, cellular differentiation, and growth by binding to vitamin D receptor (VDR). Associations between VDR gene polymorphism and Alzheimer's disease (AD), Parkinson's disease (PD), and mild cognitive impairment (MCI) risk has been investigated extensively, but the results remain ambiguous. The aim of this study was to comprehensively assess the correlations between four VDR polymorphisms (FokI, BsmI, TaqI, and ApaI) and susceptibility to AD, PD, and MCI. Crude odds ratios (ORs) and 95% confidence intervals (CIs) were calculated to determine the relationship of interest. Pooled analyses suggested that the ApaI polymorphism decreased the overall AD risk, and the TaqI increased the overall PD susceptibility. In addition, the BsmI and ApaI polymorphisms were significantly correlated with the overall MCI risk. Stratified analysis by ethnicity further showed that the TaqI and ApaI genotypes reduced the AD predisposition among Caucasians, while the TaqI polymorphism enhanced the PD risk among Asians. Intriguingly, carriers with the BB genotype significantly decreased the MCI risk in Asian descents, and the ApaI variant elevated the predisposition to MCI in Caucasians and Asians. Further studies are need to identify the role of VDR polymorphisms in AD, PD, and MCI susceptibility.

Association of vitamin D receptor gene haplotypes with late‑onset Alzheimer's disease in a Southeastern European Caucasian population

Vitamin D receptor (VDR) gene single nucleotide polymorphisms (SNPs) have been investigated over the past years with the aim of identifying any association with the development of Alzheimer's disease (AD). However, information regarding the potential association of VDR SNP haplotypes with AD is limited. The aim of the present study was to provide additional knowledge on the effects of VDR haplotypes on the development of late-onset AD in a cohort of Southeastern European Caucasians (SECs). The study sample included 78 patients with late-onset AD and 103 healthy subjects as the control group. VDR SNPs that were analyzed were TaqI (rs731236), BsmI (rs1544410) and FokI (rs2228570). The CAC (TaqI, BsmI and FokI) haplotype was found to be associated with a 53% lower risk of developing the disease (OR, 0.47; 95% CI, 0.23-0.96; P=0.04) and the TAC (TaqI, BsmI and FokI) haplotype was associated with an ~6-fold greater risk of developing AD (OR, 6.19; 95% CI, 1.91-20.13; P=0.0028). Female subjects carrying the TAC haplotype had a ~9-fold greater risk of developing AD in comparison to female control subjects (OR, 9.27; 95% CI, 1.86-46.28; P<0.05). The TaqI and BsmI polymorphisms were in high linkage disequilibrium (D'=0.9717, r=0.8467) and produced a haplotype with a statistically significant different frequency between the control and AD group. The TA (TaqI and BsmI) haplotype was associated with an ~8-fold greater risk of developing AD (OR, 8.27; 95% CI, 2.70-25.28; P<0.05). Female TA carriers had an ~14-fold greater risk of developing the disease in comparison to female control subjects (OR, 13.93; 95% CI, 2.95-65.87; P<0.05). On the whole, the present study demonstrates that in the SEC population, TAC and TA are risk haplotypes for AD, while the CAC haplotype may act protectively. SEC women carrying the TAC or TA haplotype are at a greater risk of developing AD, thus suggesting that women are markedly affected by the poor utilization of vitamin D induced by the VDR haplotype.

[Amyloid β hypothesis in Alzheimer's disease and Cl--ATPase-Neuronal cell death via PI4KIIα inhibition and recovery agents]

In the brains of patients with Alzheimer's disease, a decrease in phosphatidylinositol phosphate (PIP) requiring Cl^--ATPase activity was found. In cultured rat hippocampal neurons, pathophysiological concentrations of amyloid β proteins (Aβs≤10 nM) lowered PIP levels and Cl^--ATPase activity with an increase in intracellular Cl^- concentrations, resulting in Cl^--dependent enhancements in glutamate neurotoxicity and, ultimately, neuronal cell death. Pathophysiological concentrations of Aβs(0.1-10 nM) directly lowered phosphatidylinositol-4-kinase. Non-toxic peptide fragments of Aβ, such as Ile-Gly-Leu, recovered Aβ-induced inhibition of recombinant human phosphatidylinositol-4-kinase IIα (PI4KIIα) and the intrahippocampally administered Aβ-induced degeneration of hippocampal neurons and impairment of spatial memory in mice. Agents with the potential to block these neurotoxic mechanisms of Aβ were summarized herein as (1) Aβ antagonists, (2) substrates of PI4K, (3) PI4K product, (4) PI4K activators, and (5) GABAc receptor stimulants.

Retinoid X receptor-mediated neuroprotection via CYP19 upregulation and subsequent increases in estradiol synthesis

Increasing evidence has shown that one of the major neurosteroids, estradiol, has potent neuroprotective actions. We have reported that estradiol synthesis was enhanced when retinoic acid was added into rat hippocampal slice culture. In this study, we investigated the effects of a potent retinoid X receptor (RXR) agonist, bexarotene, on estrogen synthesis and neuroprotective action in hippocampal slices. Treatment with bexarotene increased estradiol levels as well as estrogen-synthesizing enzymes and CYP19 expression in hippocampal slice cultures. Bexarotene significantly suppressed neuronal cell death induced by oxygen-glucose deprivation (OGD)/reoxygenation. RXR agonists other than bexarotene, such as CD3254, also suppressed neuronal cell death accompanied by OGD/reoxygenation. The RXR antagonists HX531 and UVI3003 and the CYP19 inhibitor letrozole abolished the neuroprotection elicited by bexarotene, indicating that estradiol produced by RXR stimulation protects neurons from ischemic insult. The human brain-specific CYP19 promoter had 6 RXR half sites, and 2 of 6 half sites were responsible for CYP19 expression induced by bexarotene. Bexarotene increased the expression of catalase and glutathione peroxidase 1 and inhibited lipid peroxidation elicited by OGD/reoxygenation, suggesting that the antioxidative property of estrogen contributes to RXR-mediated neuroprotection. Bexarotene also suppressed neuronal injury induced by lipopolysaccharide in the hippocampal slices. Taken together, RXR stimulation can protect neurons via enhanced synthesis of estradiol with antioxidative mechanisms. The RXR-estrogen axis might be a novel mechanism-based strategy to prevent or ameliorate ischemic and/or inflammatory neuronal disorders.

Protective effect of quercetin in primary neurons against Abeta(1-42): relevance to Alzheimer's disease

Quercetin, a flavonoid found in various foodstuffs, has antioxidant properties and increases glutathione (GSH) levels and antioxidant enzyme function. Considerable attention has been focused on increasing the intracellular GSH levels in many diseases, including Alzheimer's disease (AD). Amyloid beta-peptide [Abeta(1-42)], elevated in AD brain, is associated with oxidative stress and neurotoxicity. We aimed to investigate the protective effects of quercetin on Abeta(1-42)-induced oxidative cell toxicity in cultured neurons in the present study. Decreased cell survival in neuronal cultures treated with Abeta(1-42) correlated with increased free radical production measured by dichlorofluorescein fluorescence and an increase in protein oxidation (protein carbonyl, 3-nitrotyrosine) and lipid peroxidation (protein-bound 4-hydroxy-2-nonenal). Pretreatment of primary hippocampal cultures with quercetin significantly attenuated Abeta(1-42)-induced cytotoxicity, protein oxidation, lipid peroxidation and apoptosis. A dose-response study suggested that quercetin showed protective effects against Abeta(1-42) toxicity by modulating oxidative stress at lower doses, but higher doses were not only non-neuroprotective but also toxic. These findings provide motivation to test the hypothesis that quercetin may provide a promising approach for the treatment of AD and other oxidative-stress-related neurodegenerative diseases.

Protective effects of Aβ-derived tripeptide, Aβ32–34, on Aβ1–42-induced phosphatidylinositol 4-kinase inhibition and neurotoxicity

We previously reported that the neurotoxicity of pathophysiological concentrations of amyloid beta proteins (Abetas, 0.1-10nM) as assessed by the inhibition of type II phosphatidylinositol 4-kinase (PI4KII) activity and the enhancement of glutamate toxicity was blocked by a short fragment of Abeta, Abeta(31-35). Such protective effects of shorter fragments derived from Abeta(31-35) were examined in this study to reach the shortest effective peptide, using recombinant human PI4KII and primary cultured rat hippocampal neurons. Among the peptides tested (Abeta(31-34), Abeta(31-33), Abeta(31-32), Abeta(32-35), Abeta(33-35), Abeta(34-35), Abeta(32-34), Abeta(33-34) and Abeta(32-33)), Abeta(31-34), Abeta(32-35) and Abeta(32-34) blocked both the Abeta(1-42)-induced inhibition of PI4KII activity and enhancement of glutamate toxicity on cell viability. The shortest peptide among them, Abeta(32-34), showed a dose-dependent protective effect with 50% effective concentration near 1nM, while Abeta(34-32), with a reverse amino acid sequence for Abeta(32-34), showed no protective effects. Thus, a tripeptide, Abeta(32-34) i.e. Ile-Gly-Leu, may be available as a lead compound for designing effective Abeta antagonists.

Treatment with phytoestrogen alpha-zearalanol might protect neurons of hippocampus in ovariectomized rats

Although neuroprotective effects of estrogen on postmenopausal women have been recognized, an associated increased incidence of uterine and breast tumors has jeopardized the clinical use of estrogen. This study was designed to evaluate the neuroprotective effects of a novel phytoestrogen alpha-zearalanol (alpha-ZAL), on ovariectomized (OVX) rats. Adult Wistar rats were ovariectomized or sham-operated and treatment with equivalent doses of 17beta-estradiol or alpha-ZAL for 5 wk. Uteruses have been weighted and stained by hematoxylin and eosin for morphology analysis. The expression of synaptophysin and parvalbumin in hippocampus were evaluated by immunohistochemistry assays. Our experiments indicated that the synaptophysin and parvalbumin-positive areas were significantly decreased in the OVX group compared to the sham group, alpha-ZAL or 17beta-estradiol administration can reverse the effects. Although alpha-ZAL and 17beta-estradiol treatments reconciled uterus weight loss which was induced by ovariectomy, the effect of alpha-ZAL was less than 17beta-estradiol. This result suggests that alpha-ZAL may effectively abate neurons loss in the hippocampus while slightly promoting weight gain of the uterus.

Chloride-dependency of amyloid β protein-induced enhancement of glutamate neurotoxicity in cultured rat hippocampal neurons

In our previous studies, pathophysiological concentrations of amyloid-beta (Abeta) proteins increased intracellular Cl(-) concentration ([Cl(-)]i) and enhanced glutamate neurotoxicity in primary cultured neurons, suggesting Cl(-)-dependent changes in glutamate signaling. To test this possibility, we examined the effects of isethionate-replaced low Cl(-) medium on the Abeta-induced enhancement of glutamate neurotoxicity in the primary cultured rat hippocampal neurons. In a normal Cl(-) (135 mM) medium, treatment with 10 nM Abeta25-35 for 2 days increased neuronal [Cl(-)]i to a level three times higher than that of control as assayed using a Cl(-)-sensitive fluorescent dye, while in a low Cl(-) (16 mM) medium such an Abeta25-35-induced increase in [Cl(-)]i was not observed. The Abeta treatment aggravated glutamate neurotoxicity in a normal Cl(-) medium as measured by mitochondrial reducing activity and lactate dehydrogenase (LDH) release, while in a low Cl(-) medium the Abeta treatment did not enhance glutamate toxicity. Upon such Abeta plus glutamate treatment under a normal Cl(-) condition, activated anti-apoptotic molecule Akt (Akt-pS473) level monitored by Western blot significantly decreased to 74% of control. Under a low Cl(-) condition, a resting Akt-pS473 level was higher than that under a normal Cl(-) condition and did not significantly change upon Abeta plus glutamate treatment. Tyrosine phosphorylation levels of 110 and 60 kDa proteins (pp110 and pp60) increased upon Abeta plus glutamate treatment under a normal Cl(-), but not low Cl(-), condition. These findings indicated that Abeta-induced enhancement of glutamate neurotoxicity is Cl(-)-dependent. Chloride-sensitive Akt pathway and tyrosine phosphorylation of proteins (pp110 and pp60) may be involved in this process.

Attenuation of amyloid β (Aβ)-induced inhibition of phosphatidylinositol 4-kinase activity by Aβ fragments, Aβ20–29 and Aβ31–35

We previously reported that pathophysiological concentrations of amyloid beta protein (Abeta25-35, 0.1-10 nM) directly inhibited type II phosphatidylinositol 4-kinase (PI4KII) activity in neuronal plasma membranes, which resulted in the enhanced glutamate neurotoxicity. In the present study, we examined the effects of Abeta fragments, Abeta20-29 and Abeta31-35, on the 10 nM Abeta25-35- or Abeta1-42-induced inhibition of PI4KII activity. Both of the peptide fragments recovered the inhibition of rat brain plasma membrane PI4KII activity over the concentration range of 0.1-5 nM. Such protection by the Abeta fragments was observed in the 10 nM Abeta25-35-induced inhibition of recombinant human PI4KII, suggesting that these Abeta fragments blocked the inhibition on PI4KII molecule. The Abeta25-35-induced enhancement of glutamate neurotoxicity was also completely inhibited in the presence of these fragments. Thus, Abeta20-29 and Abeta31-35 ameliorated the Abeta-enhanced glutamate neurotoxicity probably through attenuation of Abeta-induced inhibition of PI4KII activity.

Pathophysiological concentrations of amyloid beta proteins directly inhibit rat brain and recombinant human type II phosphatidylinositol 4-kinase activity

We previously found that pathophysiological concentrations (< or = 10 nm) of an amyloid beta protein (Abeta25-35) reduced the plasma membrane phosphatidylinositol monophosphate level in cultured rat hippocampal neurons with a decrease in phosphatidylinositol 4-monophosphate-dependent Cl- -ATPase activity. As this suggested an inhibitory effect of Abeta25-35 on plasma membrane phosphatidylinositol 4-kinase (PI4K) activity, in vitro effects of Abetas on PI4K activity was examined using rat brain subcellular fractions and recombinant human type II PI4K (PI4KII). Abeta25-35 (10 nm) inhibited PI4KII activity, but neither PI 3-kinase (PI3K) nor type III PI4K (PI4KIII) activity, in microsomal fractions, while 100 nm Abeta25-35 inhibited PI3K activity in mitochondrial fractions. In plasma membrane-rich fractions, Abetas (> 0.5 nm) dose-dependently inhibited PI4KII activity, the maximal inhibition to 77-87% of control being reached around 10 nm of Abetas without significant changes in apparent Km values for ATP and PI, suggesting non-competitive inhibition by Abetas. The inhibition by 10 nm Abeta25-35 was reversible. In recombinant human PI4KIIalpha, inhibition profiles of Abetas were similar to those in rat brain plasma membranes. Therefore, pathophysiological concentrations of Abetas directly and reversibly inhibited plasma membrane PI4KII activity, suggesting that plasma membrane PI4KII is a target of Abetas in the pathogenesis of Alzheimer's disease.

Effects of estrogen and progesterone treatment on rat hippocampal NMDA receptors: relationship to Morris water maze performance

Estrogen modulates NMDA receptors function in the brain. It increases both dendritic spine density and synapse number in the hippocampus, an effect that can be blocked by NMDA antagonist. In this study, we investigated the effect of 17beta-estradiol and progesterone treatment on NMDA receptors in ovariectomized rats. Two different doses were used for 10 weeks. Receptor autoradiography was done on brain sections using [(3)H] MK-801 as a ligand. Our results showed a significant increase in [(3)H] MK-801 binding in the dentate gyrus, CA3 and CA4 areas of the hippocampus of ovariectomized compared to sham operated rats. In addition, we observed similar changes in CA1. 17beta-estradiol treatment in both doses reduced the binding back to the normal level while progesterone treatment did not show any effect. Spatial reference memory was tested on Morris water maze task. Ovariectomy severely impaired spatial reference memory. Estradiol but not progesterone treatment significantly improved the memory performance of the ovariectomized rats. Low dose treatment showed better learning than high dose estrogen treatment. The decrease in the antagonist sites by estradiol treatment could result in an increase in the sensitivity of the hippocampus to the excitatory stimulation by glutamate system and hence the effect of estradiol on learning and memory. The changes of NMDA receptors in the hippocampus support the concept that estrogen-enhancing effect on spatial reference memory could be through the enhancing of NMDA function.

Non-steroidal anti-inflammatory drugs protect amyloid beta protein-induced increase in the intracellular Cl- concentration in cultured rat hippocampal neurons

Long-term use of non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen reportedly decrease a risk for the progression of Alzheimer's disease (AD), whose mechanisms are still controversial. We previously reported that pathophysiological concentrations (1-10 nM) of amyloid beta proteins (Abetas) increased intracellular Cl- concentration ([Cl-]i) and aggravated glutamate neurotoxicity in the rat brain neuronal culture. In this study, we examined the effects of therapeutic concentrations of ibuprofen and other drugs with cyclo-oxygenase (COX)-1 and/or COX-2 inhibiting activities on 10 nM Abeta25-35-induced changes in cultured rat hippocampal neurons. Ibuprofen (10-100 microM) dose-dependently inhibited the Abeta25-35-induced increase in [Cl-]i in pyramidal cell-like neurons. Not only ibuprofen, aspirin (100 microM), indomethacin (50 microM), and selective COX-1 or COX-2 inhibitor (10 nM ketrolac or 2 microM NS398) also blocked the Abeta-induced increase in neuronal [Cl-]i, though such effects of COX-2 preferring drugs were limited in aggregated Abeta-induced changes. Further, ibuprofen as well as selective COX-1 or COX-2 inhibitor reduced Abeta-induced aggravation of glutamate toxicity as assessed by cell viability. These findings suggest that NSAIDs protect neurons from Abeta-induced degeneration via inhibition of neuronal COX-1 as well as COX-2.

[Ionic mechanisms underlying the regulation of cell proliferation, differentiation and death]

Ion channels and transporters act as major components that regulate membrane excitability in neurons, muscles, and some secretory glands, but may also contribute to the regulation of proliferation, differentiation, and death in a greater variety of cells including non-excitable ones. The molecular basis of ionic mechanisms underlying the later regulation has been partly identified in the last several years and is a hot issue now. In this short review, some of the molecular mechanisms underlying these regulations and novel compounds acting on the mechanisms were introduced as exciting topics in this area. Several types of transient receptor potential (TRP), identified as Ca(2+)-permeable, non-selective cation channels, may play obligatory roles in functional complexes, which regulate multiple signal transduction pathways triggering proliferation, differentiation, or death of many cell types. In addition, the relation between Cl(-) pump activity and the induction of beta-amyloid protein toxicity for neuronal cell death in Alzheimer disease was described. Unique functions of H(+) channel and pump in osteoclasts in bone mineral homeostasis and remodeling were also discussed. Finally, topics about activation of specific types of Cl(-) channels and K(+) channels, which are responsible for the induction of apoptosis or proliferation in several types of cells, were introduced.