Beta-amyloid peptide induces tumor necrosis factor-alpha and nitric oxide production in murine macrophage cultures

Ergosterol promotes neurite outgrowth, inhibits amyloid-beta synthesis, and extends longevity: In vitro neuroblastoma and in vivo Caenorhabditis elegans evidence

AIMS

Alzheimer's disease (AD), the most common neurodegenerative disorder associated with aging, is characterized by amyloid-β (Aβ) plaques in the hippocampus. Ergosterol, a mushroom sterol, exhibits neuroprotective activities; however, the underlying mechanisms of ergosterol in promoting neurite outgrowth and preventing Aβ-associated aging have never been investigated. We aim to determine the beneficial activities of ergosterol in neuronal cells and Caenorhabditis elegans (C. elegans).

MATERIALS AND METHODS

The neuritogenesis and molecular mechanisms of ergosterol were investigated in wild-type and Aβ precursor protein (APP)-overexpressing Neuro2a cells. The anti-amyloidosis properties of ergosterol were determined by evaluating in vitro Aβ production and the potential inhibition of Aβ-producing enzymes. Additionally, AD-associated transgenic C. elegans was utilized to investigate the in vivo attenuating effects of ergosterol.

KEY FINDINGS

Ergosterol promoted neurite outgrowth in Neuro2a cells through the upregulation of the transmembrane protein Teneurin-4 (Ten-4) mRNA and protein expressions, phosphorylation of the extracellular signal-regulated kinases (ERKs), activity of cAMP response element (CRE), and growth-associated protein-43 (GAP-43). Furthermore, ergosterol enhanced neurite outgrowth in transgenic Neuro2A cells overexpressing either the wild-type APP (Neuro2a-APPwt) or the Swedish mutant APP (Neuro2a-APPswe) through the Ten-4/ERK/CREB/GAP-43 signaling pathway. Interestingly, ergosterol inhibited Aβ synthesis in Neuro2a-APPwt cells. In silico analysis indicated that ergosterol can interact with the catalytic sites of β- and γ-secretases. In Aβ-overexpressing C. elegans, ergosterol decreased Aβ accumulation, increased chemotaxis behavior, and prolonged lifespan.

SIGNIFICANCE

Ergosterol is a potential candidate compound that might benefit AD patients by promoting neurite outgrowth, inhibiting Aβ synthesis, and enhancing longevity.

Restraining tumor necrosis factor-alpha by thalidomide prevents the amyloid beta-induced impairment of recognition memory in mice

No effective remedy has currently been realized to prevent the cognitive impairments of Alzheimer's disease (AD). The interruption of the toxic pathways of amyloid beta peptide (Abeta) still remains promising for the treatment. The involvement of tumor necrosis factor-alpha (TNF-alpha) in the toxicity of Abeta(1-40) in recent reports provide a fresh target for the interruption. In the current study, we evaluated the feasibility of a strategy that target TNF-alpha to prevent the impairment of memory induced by Abeta. The i.c.v-injection of Abeta(25-35) increased the hippocampal mRNA expression of both TNF-alpha and inducible nitric oxide synthase (iNOS), of which the former was stronger. The knock-out of TNF-alpha (TNF-alpha (-/-)) in mouse prevented the increase of iNOS mRNA induced by Abeta(25-35). Not only the inhibition of iNOS activity but also TNF-alpha (-/-) prevented the nitration of proteins in the hippocampus and the impairment of recognition memory in mice induced by Abeta(25-35). Daily treatment with thalidomide (20 mg/kg), a preferential degrader of TNF-alpha mRNA, or i.c.v.-injection of an anti-TNF-alpha antibody (10 etag/mouse) prevented the nitration of proteins in the hippocampus and the impairment of recognition memory induced by Abeta(25-35) or Abeta(1-40) in mice. These results suggested the practicability of targeting TNF-alpha as a preventive strategy against Abeta-mediated cognitive impairments.

AP-1 mediates beta-amyloid-induced iNOS expression in PC12 cells via the ERK2 and p38 MAPK signaling pathways

Nitrosative stress with subsequent inflammatory cell death has been implicated in some neurodegenerative disorders such as Alzheimer's disease (AD). Expression of inducible nitric oxide synthase (iNOS) and production of nitric oxide (NO) have been frequently elevated in AD. In this study, we have investigated the molecular mechanisms underlying nitrosative stress induced by beta-amyloid (A beta), a neurotoxic peptide associated with senile plaques formed in the brains of patients with AD. Exposure of rat pheochromocytoma (PC12) cells to the A beta resulted in increased mRNA and protein expression of iNOS and generation of NO. NO can rapidly interact with superoxide anion, forming more reactive peroxynitrite. Treatment of PC12 cells with A beta led to increased peroxynitrite production and nitrotyrosine formation. A beta induced activation of redox sensitive transcription factor activator protein-1 (AP-1), and AP-1 antisense oligonucleotide abolished the A beta-induced iNOS expression. Moreover, A beta transiently activated extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (p38 MAPK) via phosphorylation. Pharmacologic inhibition of both enzymes or dominant-negative mutation of ERK2 or p38 MAPK effectively down-regulated DNA binding as well as transcriptional activity of AP-1 and subsequent iNOS expression and NO production. The above findings suggest that A beta induces iNOS expression in PC12 cells through activation of AP-1 which is regulated by upstream kinases, such as ERK and p38 MAPK.

Heparan sulfate proteoglycan induces the production of NO and TNF-alpha by murine microglia

Background

A common feature of Alzheimer's disease (AD) pathology is the abundance of activated microglia in neuritic plaques containing amyloid-beta protein (Aβ) and associated molecules including heparan sulfate proteoglycan (HSPG). Besides the role as pathological chaperone favouring amyloidogenesis, little is known about whether or not HSPG can induce microglial activation. Cultures of primary murine microglia were used to assess the effect of HSPG on production of proinflammatory molecules that are known to be present in neuritic plaques of AD.

Results

HSPG stimulated up-regulation of tumor necrosis factor-alpha (TNF-α), production of inducible nitric oxide synthase (iNOS) mRNA and accumulation of TNF-α protein and nitrite (NO₂^-) in a time- and concentration-dependent manner. The effects of HSPG were primarily due to the property of the protein core as indicated by the lack of microglial accumulation of TNF-α and NO₂^- in response to denaturated HSPG or heparan sulfate GAG chains (HS).

Conclusion

These data demonstrate that HSPG may contribute to chronic microglial activation and neurodegeneration seen in neuritic plaques of AD.

The cAMP-specific phosphodiesterase 4B mediates Abeta-induced microglial activation

Microglial activation is a key player in the degenerative process that accompanies the deposition of amyloid-beta (Abeta) peptide into senile plaques in Alzheimer's disease (AD) patients. The goal of this study is to identify novel genes involved in microglial activation in response to Abeta peptide. Prompted by the fact that soluble Abeta(1-42) (sAbeta(1-42))-stimulated primary rat microglia produce more tumor necrosis factor-alpha (TNF-alpha) than fibrillar Abeta(1-42) (fAbeta(1-42))-stimulated microglia, we examined gene expression in these cells following stimulation using cDNA arrays. This analysis confirms the upregulation caused by both sAbeta(1-42) and fAbeta(1-42) of pro-inflammatory molecules such as TNF-alpha, interleukin-1beta and macrophage inflammatory protein-1alpha. In addition, other transcripts not previously described in the context of Abeta-induced microglial activation were identified. The modulation of some of these genes within microglial cells seems to be specific to sAbeta(1-42) as compared to fAbeta(1-42) suggesting that different forms of Abeta may activate distinct pathways during the progression of AD. Importantly, we demonstrate that Pde4B, a cAMP-specific phosphodiesterase, is upregulated by Abeta and results in an increased production of TNF-alpha. Inhibition of Pde4B reduces by up to 70% the release of TNF-alpha from sAbeta-stimulated microglial cells, implicating cAMP as an important mediator of Abeta-induced microglial activation.

Differential effects of oligomeric and fibrillar amyloid-β1–42 on astrocyte-mediated inflammation

Activated glia, as a result of chronic inflammation, are associated with amyloid-beta peptide (Abeta) deposits in the brain of Alzheimer's disease (AD) patients. In vitro, glia are activated by Abeta inducing secretion of pro-inflammatory molecules. Recent studies have focused on soluble oligomers (or protofibrils) of Abeta as the toxic species in AD. In the present study, using rat astrocyte cultures, oligomeric Abeta induced initial high levels of IL-1beta decreasing over time and, in contrast, fibrillar Abeta increased IL-1beta levels over time. In addition, oligomeric Abeta, but not fibrillar Abeta, induced high levels of iNOS, NO, and TNF-alpha. Our results suggest that oligomers induced a profound, early inflammatory response, whereas fibrillar Abeta showed less increase of pro-inflammatory molecules, consistent with a more chronic form of inflammation.

P2X7 receptor modulation of beta-amyloid- and LPS-induced cytokine secretion from human macrophages and microglia

To test whether extracellular ATP can play a role in the neuroimmunopathology of Alzheimer's disease (AD), we evaluated the capacity of the ATP-binding purinoreceptor, P2X7, to modulate cytokine secretion on cultured human macrophages and microglia pre-activated 24 h with the 42 amino acid beta-amyloid peptide (Abeta(1-42)) or lipopolysaccharide. Thirty minutes of exposure to the selective P2X7 agonist 2'-3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (BzATP) resulted in the secretion of IL-1beta after either Abeta(1-42) or LPS stimulation of human macrophages that was dependent on the concentration of the stimulus used to pre-activate the cells. Further tests on human microglia treated with BzATP (300 microM) resulted in a 1.5- and 3.5-fold enhancement of IL-1alpha and IL-1beta secretion, respectively, from cells pre-activated by 10 microM Abeta(1-42) and a 1.6- and 3.9-fold enhancement of IL-1alpha and IL-1beta secretion, respectively, from cells pre-activated by 1 microg/ml LPS. BzATP induction of IL-1alpha and IL-1beta secretion from microglia was completely reversed by pre-incubation of the cells with the P2X7 antagonist, adenosine 5'-triphosphate 2',3'-acyclic dialcohol (oxidized ATP). In contrast to its effects on IL-1alpha and IL-1beta secretion, BzATP induced TNF-alpha after LPS stimulation, but not after stimulation with Abeta(1-42), induced IL-18 secretion regardless of whether microglia were pre-activated and attenuated IL-6 secretion after either LPS or Abeta(1-42) pre-activation. These results demonstrate that extracellular ATP can modulate Abeta-induced cytokine secretion from human macrophages and microglia and thus may play a role in the neuroimmunopathology of AD.

Platelet phagocytosis and processing of beta-amyloid precursor protein as a mechanism of macrophage activation in atherosclerosis

In human occluded saphenous vein grafts, we previously demonstrated cytotoxic foam cells, presumably derived from macrophages engulfing platelets. In the present study, we investigated whether platelet phagocytosis occurs in human atherosclerotic plaques, whether this activates macrophages, and whether the platelet constituent, amyloid precursor protein (APP), was involved. Immunohistochemistry documented the presence of APP, beta-amyloid peptide (Abeta, cleaved from APP), and platelets (CD9), along with inducible NO synthase (iNOS) and cyclooxygenase-2, two markers of macrophage activation, around microvessels in advanced human carotid artery plaques (n=18). Abeta colocalized with iNOS-expressing macrophages that were often surrounded by platelets. In vitro, murine J774 and human THP-1 macrophages were incubated with or without washed human platelets. Coincubation of macrophages and platelets led to platelet phagocytosis (electron and confocal microscopy) and formation of lipid-, APP-, and Abeta-containing foam cells. These expressed iNOS mRNA (reverse transcription-polymerase chain reaction) and protein and produced nitrite and tumor necrosis factor-alpha (ELISA). Macrophage pretreatment with 4-(2-aminoethyl)benzenesulfonyl fluoride, a protease inhibitor, reduced APP processing and inhibited NO biosynthesis induced by platelet phagocytosis but not by lipopolysaccharides. Human atherosclerotic plaques and J774 and THP-1 macrophages contained mRNA of the APP-cleaving enzyme beta-secretase. This is the first demonstration of Abeta, a peptide extensively studied in Alzheimer's disease, in human atherosclerotic plaques. It was present in activated iNOS-expressing perivascular macrophages that had phagocytized platelets. In vitro studies indicate that platelet phagocytosis leads to macrophage activation and suggest that platelet-derived APP is proteolytically processed to Abeta, resulting in iNOS induction. This represents a novel mechanism for macrophage activation in atherosclerosis.