Beta-amyloid-mediated inhibition of NMDA receptor-dependent long-term potentiation induction involves activation of microglia and stimulation of inducible nitric oxide synthase and superoxide

A key role for the microglial NADPH oxidase in APP-dependent killing of neurons

Reactive oxygen species (ROS) and deposition of cleaved products of amyloid precursor protein (APP) are thought to contribute to neuronal loss observed in Alzheimer's disease (AD). The relationship between these factors was studied in a neuroblastoma and microglia co-culture system. Overexpression of wild-type APP (APP-wt) or APP with three mutations typical of familial AD (APP-3m) in SH-SY5Y neuroblastoma cells did not directly alter their morphology, growth rate, cell cycle or H(2)O(2) sensitivity. In a co-culture of APP-wt neuroblastoma cells with microglia, microglial cells generated ROS and neuronal cells died. The cell death was more pronounced in APP-3m-expressing neurons. Neuroblastoma cell death was attenuated by ROS-scavengers and was dose-dependently inhibited by the NADPH oxidase inhibitor diphenyleneiodonium chloride (DPI). Macrophage cell lines behaved similarly to microglia in the co-culture model. However, a macrophage cell line deficient in the NADPH oxidase subunit, gp91phox, failed to kill neurons. These results suggest that APP-dependent microglia activation and subsequent ROS generation by the phagocyte NADPH oxidase play a crucial role in neuronal killing in a cellular model of AD.

SIRT1 protects against microglia-dependent amyloid-beta toxicity through inhibiting NF-kappaB signaling

Accumulating evidence suggests that neurodegeneration induced by pathogenic proteins depends on contributions from surrounding glia. Here we demonstrate that NF-kappaB signaling in microglia is critically involved in neuronal death induced by amyloid-beta (Abeta) peptides, which are widely presumed to cause Alzheimer disease. Constitutive inhibition of NF-kappaB signaling in microglia by expression of the nondegradable IkappaBalpha superrepressor blocked neurotoxicity, indicating a pivotal role for microglial NF-kappaB signaling in mediating Abeta toxicity. Stimulation of microglia with Abeta increased acetylation of RelA/p65 at lysine 310, which regulates the NF-kappaB pathway. Overexpression of SIRT1 deacetylase and the addition of the SIRT1 agonist resveratrol markedly reduced NF-kappaB signaling stimulated by Abeta and had strong neuroprotective effects. Our results support a glial loop hypothesis by demonstrating a critical role for microglial NF-kappaB signaling in Abeta-dependent neurodegeneration. They also implicate SIRT1 in this pathway and highlight the therapeutic potential of resveratrol and other sirtuin-activating compounds in Alzheimer disease.

Mechanism of impairment of long-term potentiation by amyloid beta is independent of NMDA receptors or voltage-dependent calcium channels in hippocampal CA1 pyramidal neurons

beta-Amyloid peptide (Abeta) is known to be involved in Alzheimer's disease (AD). Although the fibril form of Abeta is known to have neurotoxicity, it has been shown that not only the fibril form but also the oligomer form of Abeta may be related to the neuropathophysiology of AD, specifically to memory loss. Some studies have demonstrated that low concentrations of the Abeta oligomer impair long-term potentiation (LTP), a cellular model for learning and memory, after short exposure times in vivo and in vitro, although little is known about the mechanism involved in Abeta-mediated inhibition of LTP. In this study, we used the patch clamp whole-cell technique in rat hippocampal CA1 pyramidal neurons to study more precisely how the Abeta oligomer affects synaptic plasticity. The brief perfusion of slices with a low concentration (1microM) of Abeta(1-42) significantly impaired LTP induction of the excitatory input. The same concentration of Abeta did not affect basal transmission or paired-pulse facilitation. We also demonstrated that neither NMDAR-EPSCs nor the voltage-depended calcium channel (VDCC) currents were affected by the same concentration of Abeta(1-42) as used in the LTP experiments. These data suggest that Abeta mediated impairment of LTP induction is independent of NMDARs or VDCCs.

Investigations with cultured human microglia on pathogenic mechanisms of Alzheimer's disease and other neurodegenerative diseases

Inflammation-mediated mechanisms for human neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD) have evolved from being on the fringe of medical hypotheses to mainstream thinking. Pioneering immunopathology studies with human brain tissues identified microglia associated with neuropathologic hallmarks of these diseases. As activated macrophages were known to produce many potential toxic products, this gave rise to the hypothesis that activated microglia (brain resident macrophages) could be contributing to the degeneration of key target neurons in these diseases, as well as potential vascular dysfunction. Studies with microglia derived from different sources, including human brains, have confirmed that activated microglia can mediate neuronal cell death. Based on these theories, a number of human clinical trials with antiinflammatory agents have been carried out on AD patients. Results to date have indicated a lack of effectiveness at slowing disease progression and have begun to cast doubt on the significance of inflammation in AD. It has been shown recently that activating microglia through immunization of amyloid plaque-developing mice with amyloid beta peptide (Abeta) has promise as a therapeutic strategy and despite some setbacks, has potential as a treatment for AD patients. This article will consider experimental data with microglia to determine whether the additional targets need to be investigated. The use of human microglia cultures, in particular those derived from elderly diseased human brains, offers an experimental system that can closely model the cell type activated in human neurodegenerative diseases. Experimental data produced by our laboratory and others is reviewed to determine the contribution of this unique experimental model to understanding disease mechanisms and possibly discovering new therapeutic targets.

Mechanisms of the inhibitory effects of amyloid β-protein on synaptic plasticity

Alzheimer's disease can be considered a protein misfolding disease. In particular, inappropriate processing of a proteolytic fragment of amyloid precursor protein, amyloid beta-protein (Abeta), in early stages of Alzheimer's disease may lead to stabilization of small oligomers that are highly mobile and have a potential to be extremely toxic assemblies. Recently, the importance of such soluble species of Abeta in triggering synaptic dysfunction, long before neuronal loss occurs, has become apparent. Animal models have revealed that plasticity of hippocampal excitatory synaptic transmission is relatively selectively vulnerable to Abeta both in vitro and in vivo. This review focuses on the mechanisms of Abeta inhibition of long-term potentiation at synapses in the rodent hippocampus from two complimentary perspectives. Firstly, we examine evidence that the synaptic activity of this peptide resides primarily in oligomeric rather than monomeric or fibrillar Abeta species. Secondly, the importance of different oxidative/nitrosative stress-linked cascades including JNK, p38 MAPK and NADPH oxidase/iNOS-generated reactive oxygen/nitrogen free radicals in mediating the inhibition of LTP by Abeta is emphasised. These mechanistic studies provide a plausible explanation for the sensitivity of hippocampus-dependent memory to impairment in the early preclinical stages of Alzheimer's disease.