Inhibition of NADPH oxidase promotes alternative and anti-inflammatory microglial activation during neuroinflammation

Nutraceutical agents with anti-inflammatory properties prevent dietary saturated-fat induced disturbances in blood-brain barrier function in wild-type mice

Background

Emerging evidence suggests that disturbances in the blood–brain barrier (BBB) may be pivotal to the pathogenesis and pathology of vascular-based neurodegenerative disorders. Studies suggest that heightened systemic and central inflammations are associated with BBB dysfunction. This study investigated the effect of the anti-inflammatory nutraceuticals garlic extract-aged (GEA), alpha lipoic acid (ALA), niacin, and nicotinamide (NA) in a murine dietary-induced model of BBB dysfunction.

Methods

C57BL/6 mice were fed a diet enriched in saturated fatty acids (SFA, 40% fat of total energy) for nine months to induce systemic inflammation and BBB disturbances. Nutraceutical treatment groups included the provision of either GEA, ALA, niacin or NA in the positive control SFA-group and in low-fat fed controls. Brain parenchymal extravasation of plasma derived immunoglobulin G (IgG) and large macromolecules (apolipoprotein (apo) B lipoproteins) measured by quantitative immunofluorescent microscopy, were used as markers of disturbed BBB integrity. Parenchymal glial fibrillar acidic protein (GFAP) and cyclooxygenase-2 (COX-2) were considered in the context of surrogate markers of neurovascular inflammation and oxidative stress. Total anti-oxidant status and glutathione reductase activity were determined in plasma.

Results

Brain parenchymal abundance of IgG and apoB lipoproteins was markedly exaggerated in mice maintained on the SFA diet concomitant with significantly increased GFAP and COX-2, and reduced systemic anti-oxidative status. The nutraceutical GEA, ALA, niacin, and NA completely prevented the SFA-induced disturbances of BBB and normalized the measures of neurovascular inflammation and oxidative stress.

Conclusions

The anti-inflammatory nutraceutical agents GEA, ALA, niacin, or NA are potent inhibitors of dietary fat-induced disturbances of BBB induced by systemic inflammations.

The role of glutamate and the immune system in organophosphate-induced CNS damage

Organophosphate (OP) poisoning is associated with long-lasting neurological damage, which is attributed mainly to the excessive levels of glutamate caused by the intoxication. Glutamate toxicity, however, is not specific to OP poisoning, and is linked to propagation of damage in both acute and chronic neurodegenerative conditions in the central nervous system (CNS). In addition to acute excitotoxic effects of glutamate, there is now a growing amount of evidence of its intricate immunomodulatory effects in the brain, involving both the innate and the adaptive immune systems. Moreover, it was demonstrated that immunomodulatory treatments, aimed at regulating the interaction between the resident immune cells of the brain (microglia) and the peripheral immune system, can support buffering of excessive levels of glutamate and restoration of the homeostasis. In this review, we will discuss the role of glutamate as an excitotoxic agent in the acute phase of OP poisoning, and the possible functions it may have as both a neuroprotectant and an immunomodulator in the sub-acute and chronic phases of OP poisoning. In addition, we will describe the novel immune-based neuroprotective strategies aimed at counteracting the long-term neurodegenerative effects of glutamate in the CNS.

The role of vitamin D in Alzheimer's disease: possible genetic and cell signaling mechanisms

Alzheimer's disease (AD) is the most common form of dementia in the elderly individuals and is associated with progressive memory loss and cognitive dysfunction. A significant association between AD and low levels of vitamin D has been demonstrated. Furthermore, vitamin D supplements appear to have a beneficial clinical effect on AD by regulating micro-RNA, enhancing toll-like receptors, modulating vascular endothelial factor expression, modulating angiogenin, and advanced glycation end products. Vitamin D also exerts its effects on AD by regulating calcium-sensing receptor expression, enhancing amyloid-β peptides clearance, interleukin 10, downregulating matrix metalloproteinases, upregulating heme oxygenase 1, and suppressing the reduced form of nicotinamide adenine dinucleotide phosphate expression. In conclusion, vitamin D may play a beneficial role in AD. Calcitriol is the best vitamin D supplement for AD, because it is the active form of the vitamin D3 metabolite and modulates inflammatory cytokine expression. Therefore, further investigation of the role of calcitriol in AD is needed.

Strategy for dual-analyte luciferin imaging: in vivo bioluminescence detection of hydrogen peroxide and caspase activity in a murine model of acute inflammation

In vivo molecular imaging holds promise for understanding the underlying mechanisms of health, injury, aging, and disease, as it can detect distinct biochemical processes such as enzymatic activity, reactive small-molecule fluxes, or post-translational modifications. Current imaging techniques often detect only a single biochemical process, but, within whole organisms, multiple types of biochemical events contribute to physiological and pathological phenotypes. In this report, we present a general strategy for dual-analyte detection in living animals that employs in situ formation of firefly luciferin from two complementary caged precursors that can be unmasked by different biochemical processes. To establish this approach, we have developed Peroxy Caged Luciferin-2 (PCL-2), a H(2)O(2)-responsive boronic acid probe that releases 6-hydroxy-2-cyanobenzothiazole (HCBT) upon reacting with this reactive oxygen species, as well as a peptide-based probe, z-Ile-Glu-ThrAsp-D-Cys (IETDC), which releases D-cysteine in the presence of active caspase 8. Once released, HCBT and D-cysteine form firefly luciferin in situ, giving rise to a bioluminescent signal if and only if both chemical triggers proceed. This system thus constitutes an AND-type molecular logic gate that reports on the simultaneous presence of H(2)O(2) and caspase 8 activity. Using these probes, chemoselective imaging of either H(2)O(2) or caspase 8 activity was performed in vitro and in vivo. Moreover, concomitant use of PCL-2 and IETDC in vivo establishes a concurrent increase in both H(2)O(2) and caspase 8 activity during acute inflammation in living mice. Taken together, this method offers a potentially powerful new chemical tool for studying simultaneous oxidative stress and inflammation processes in living animals during injury, aging, and disease, as well as a versatile approach for concurrent monitoring of multiple analytes using luciferin-based bioluminescence imaging technologies.

Salidroside attenuates beta amyloid-induced cognitive deficits via modulating oxidative stress and inflammatory mediators in rat hippocampus

Beta amyloid (Aβ)-induced oxidative stress and chronic inflammation in the brain are considered to be responsible for the pathogenesis of Alzheimer's disease (AD). Salidroside, the major active ingredient of Rhodiola crenulata, has been previously shown to have antioxidant and neuroprotective properties in vitro. The present study aimed to investigate the protective effects of salidroside on Aβ-induced cognitive impairment in vivo. Rats received intrahippocampal Aβ1-40 injection were treated with salidroside (25, 50 and 75 mg/kg p.o.) once daily for 21 days. Learning and memory performance were assessed in the Morris water maze (days 17-21). After behavioral testing, the rats were sacrificed and hippocampi were removed for biochemical assays (reactive oxygen species (ROS), superoxide dismutase (SOD), glutathione peroxidase (GPx), malondialdehyde (MDA), acetylcholinesterase (AChE), acetylcholine (ACh)) and molecular biological analysis (Cu/Zn-SOD, Mn-SOD, GPx, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, nuclear factor κB (NF-κB), inhibitor of κB-alpha (IκBα), cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), receptor for advanced glycation end products (RAGE)). Our results confirmed that Aβ1-40 peptide caused learning and memory deficits in rats. Further analysis demonstrated that the NADPH oxidase-mediated oxidative stress was increased in Aβ1-40-injected rats. Furthermore, NF-κB was demonstrated to be activated in Aβ1-40-injected rats, and the COX-2, iNOS and RAGE expression were also induced by Aβ1-40. However, salidroside (50 and 75 mg/kg p.o.) reversed all the former alterations. Thus, the study indicates that salidroside may have a protective effect against AD via modulating oxidative stress and inflammatory mediators.

Anti-inflammatory and anti-amyloidogenic effects of a small molecule, 2,4-bis(p-hydroxyphenyl)-2-butenal in Tg2576 Alzheimer's disease mice model

BACKGROUND

Alzheimer's disease (AD) is pathologically characterized by excessive accumulation of amyloid-beta (Aβ) fibrils within the brain and activation of astrocytes and microglial cells. In this study, we examined anti-inflammatory and anti-amyloidogenic effects of 2,4-bis(p-hydroxyphenyl)-2-butenal (HPB242), an anti-inflammatory compound produced by the tyrosine-fructose Maillard reaction.

METHODS

12-month-old Tg2576 mice were treated with HPB242 (5 mg/kg) for 1 month and then cognitive function was assessed by the Morris water maze test and passive avoidance test. In addition, western blot analysis, Gel electromobility shift assay, immunostaining, immunofluorescence staining, ELISA and enzyme activity assays were used to examine the degree of Aβ deposition in the brains of Tg2576 mice. The Morris water maze task was analyzed using two-way ANOVA with repeated measures. Otherwise were analyzed by one-way ANOVA followed by Dunnett's post hoc test.

RESULTS

Treatment of HPB242 (5 mg/kg for 1 month) significantly attenuated cognitive impairments in Tg2576 transgenic mice. HPB242 also prevented amyloidogenesis in Tg2576 transgenic mice brains. This can be evidenced by Aβ accumulation, BACE1, APP and C99 expression and β-secretase activity. In addition, HPB242 suppresses the expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) as well as activation of astrocytes and microglial cells. Furthermore, activation of nuclear factor-kappaB (NF-κB) and signal transducer and activator of transcription 1/3 (STAT1/3) in the brain was potently inhibited by HPB242.

CONCLUSIONS

Thus, these results suggest that HPB242 might be useful to intervene in development or progression of neurodegeneration in AD through its anti-inflammatory and anti-amyloidogenic effects.

Traumatic brain injury in aged animals increases lesion size and chronically alters microglial/macrophage classical and alternative activation states

Traumatic brain injury (TBI) causes chronic microglial activation that contributes to subsequent neurodegeneration, with clinical outcomes declining as a function of aging. Microglia/macrophages (MG/Mɸ) have multiple phenotypes, including a classically activated, proinflammatory (M1) state that might contribute to neurotoxicity, and an alternatively activated (M2) state that might promote repair. In this study we used gene expression, immunohistochemical, and stereological analyses to show that TBI in aged versus young mice caused larger lesions associated with an M1/M2 balance switch and increased numbers of reactive (bushy and hypertrophic) MG/Mɸ in the cortex, hippocampus, and thalamus. Chitinase3-like 3 (Ym1), an M2 phenotype marker, displayed heterogeneous expression after TBI with amoeboid-like Ym1-positive MG/Mɸ at the contusion site and ramified Ym1-positive MG/Mɸ at distant sites; this distribution was age-related. Aged-injured mice also showed increased MG/Mɸ expression of major histocompatibility complex II and NADPH oxidase, and reduced antioxidant enzyme expression which was associated with lesion size and neurodegeneration. Thus, altered relative M1/M2 activation and an nicotinamide adenine dinucleotide phosphate oxidase (NADPH oxidase)-mediated shift in redox state might contribute to worse outcomes observed in older TBI animals by creating a more proinflammatory M1 MG/Mɸ activation state.

Oligomeric Aβ-induced microglial activation is possibly mediated by NADPH oxidase

Recent studies have shown that oligomeric amyloid-β (oAβ) peptide can potentially activate microglia in addition to inducing more potent neurotoxicity compared with fibrillar Aβ (fAβ); however, its mechanisms of action remain unclear. This study was designed to investigate the possible mechanisms involved in the microglial activation induced by oAβ in BV-2 microglial cells. The results showed that oAβ induced activated properties of microglia, including higher proliferative capacity as well as increased production of reactive oxygen species, nitric oxide (NO), tumor necrosis factor-α (TNF-α), and interleukin-1β (IL-1β). NADPH oxidase inhibitors [diphenylene iodonium (DPI) and apocynin (4-hydroxy-3-methoxy-acetophenone)] prevented the microglial activation induced by oAβ, suggesting that NADPH oxidase activation was involved in microglial activation. In addition, TNF-α and IL-1β, which are massively released by activated microglia, significantly induced the activation of microglia, thereby resulting in the production of NO and proliferation of microglia, respectively. These effects could be inhibited by diphenylene iodonium and apocynin, indicating a self-cycle regulated by NADPH oxidase in microglial activation in response to oAβ. In conclusion, microglial activation induced by oAβ is possibly mediated by NADPH oxidase, suggesting that oAβ, which is normally considered a neurotoxin, may also lead to indirect neuronal damage through the pro-inflammation activation of microglia in Alzheimer's disease and that NADPH oxidase could be a potential target to prevent oAβ-induced inflammatory neurodegeneration.

A self-propelling cycle mediated by reactive oxide species and nitric oxide exists in LPS-activated microglia

It has been widely accepted that microglia, the innate immune cells in the brain, can be chronically activated in response to neuron death, fuelling a self-renewing cycle of microglial activation followed by further neuron damage (reactive microgliosis), which has been considered as the main reason responsible for the progressive nature of neurodegenerative diseases. In the present study, it was found that LPS (lipopolysaccharide) significantly induced the activation of N9 microglia, and the increase of NO level induced by pretreatment of LPS could last after the removal of LPS. The culture medium of activated microglia significantly decreased the viability of rat primary cortical neuron. These results can be blocked by the antioxidant N-acetylcysteine (NAC) and nicotinamide adenine dinucleotide phosphate reduced (NADPH) oxidase inhibitor diphenyleneiodonium sulfate (DPI), suggesting that intracellular reactive oxide species (iROS) released from the activated microglial cells may continue to further activate microglia. Next, it was shown that the iROS level increased rapidly after the LPS treatment in microglia cells followed by the NO production through the regulation of iNOS (inducible nitric oxide synthase) expression. The increase of iROS could be reversed by gp91phox (the critical and catalytic subunit of NADPH oxidase) siRNA. Moreover, NO released from sodium nitroprusside (SNP) was able to increase the iROS production of N9 microglia by regulating of the activity and the expression of NADPH oxidase. In conclusion, our research suggests for the first time that there may exist a self-propelling cycle in microglial cells possibly mediated by iROS and NO when they become activated by LPS. It may be responsible partially for the ongoing microglial activation and the progressive nature of neurodegenerative diseases.

Neuroprotective effect of methyl lucidone against microglia-mediated neurotoxicity

Excessive microglial activation-mediated neurotoxicity has been implicated in playing a crucial role in the pathogenesis of stroke and neurodegenerative diseases. Therefore, much attention has been paid to therapeutic strategies aimed at suppressing neurotoxic microglial activation. The microglial regulatory mechanism of methyl lucidone, a cyclopentenedione isolated from the stem bark of Lindera erythrocarpa Makino, was investigated in the present study. Methyl lucidone treatment (0.1-10 μM) significantly inhibited lipopolysaccharide (LPS, 100 ng/ml, 24 h)-stimulated nitric oxide (NO) production in a dose-dependent manner in both primary cortical microglia and BV-2 cell line. Moreover, it strongly inhibited LPS-stimulated secretion of pro-inflammatory cytokines, such as interleukin 6 (IL-6) and tumor necrosis factor α (TNF-α). Methyl lucidone treatment markedly induced down-regulation of LPS-induced nuclear translocation of nuclear factor κB (NF-κB) through preventing the degradation of the inhibitory protein IκBα. In addition, phosphorylation of Akt and mitogen-activated protein kinases (MAPKs) such as extracellular signal-regulated kinase (ERK) and p38 kinases were also suppressed by methyl lucidone. The cell viabilities of HT-22 neurons were significantly attenuated by treatment of the conditioned media containing neurotoxic secretary molecules from LPS-stimulated microglia. However, methyl lucidone significantly blocked neuronal cell death induced by microglial conditioned media. These neuroprotective effects of methyl lucidone were also confirmed in a neuron-microglia co-culture system using EGFP-transfected B35 neuroblastoma cell line. Taken together, these results suggest that methyl lucidone may have a neuroprotective potential via inhibition of neurotoxic microglial activation implicated in neurodegeneration.

NADPH oxidases: novel therapeutic targets for neurodegenerative diseases

Oxidative stress is a key pathologic factor in neurodegenerative diseases such as Alzheimer and Parkinson diseases (AD, PD). The failure of free-radical-scavenging antioxidants in clinical trials pinpoints an urgent need to identify and to block major sources of oxidative stress in neurodegenerative diseases. As a major superoxide-producing enzyme complex in activated phagocytes, phagocyte NADPH oxidase (PHOX) is essential for host defense. However, recent preclinical evidence has underscored a pivotal role of overactivated PHOX in chronic neuroinflammation and progressive neurodegeneration. Deficiency in PHOX subunits mitigates neuronal damage induced by diverse insults/stresses relevant to neurodegenerative diseases. More importantly, suppression of PHOX activity correlates with reduced neuronal impairment in models of neurodegenerative diseases. The discovery of PHOX and non-phagocyte NADPH oxidases in astroglia and neurons further reinforces the crucial role of NADPH oxidases in oxidative stress-mediated chronic neurodegeneration. Thus, proper modulation of NADPH oxidase activity might hold therapeutic potential for currently incurable neurodegenerative diseases.