Microglia-mediated Inflammation Research Articles

Sinomenine, a natural dextrorotatory morphinan analog, is anti-inflammatory and neuroprotective through inhibition of microglial NADPH oxidase

BackgroundThe mechanisms involved in the induction and regulation of inflammation resulting in dopaminergic (DA) neurotoxicity in Parkinson's disease (PD) are complex and incompletely understood. Microglia-mediated inflammation has recently been implicated as a critical mechanism responsible for progressive neurodegeneration.MethodsMesencephalic neuron-glia cultures and reconstituted cultures were used to investigate the molecular mechanisms of sinomenine (SN)-mediated anti-inflammatory and neuroprotective effects in both the lipopolysaccharide (LPS)- and the 1-methyl-4-phenylpyridinium (MPP+)-mediated models of PD.ResultsSN showed equivalent efficacy in protecting against DA neuron death in rat midbrain neuron-glial cultures at both micro- and sub-picomolar concentrations, but no protection was seen at nanomolar concentrations. The neuroprotective effect of SN was attributed to inhibition of microglial activation, since SN significantly decreased tumor necrosis factor-α (TNF-α, prostaglandin E2 (PGE2) and reactive oxygen species (ROS) production by microglia. In addition, from the therapeutic point of view, we focused on sub-picomolar concentration of SN for further mechanistic studies. We found that 10-14 M of SN failed to protect DA neurons against MPP+-induced toxicity in the absence of microglia. More importantly, SN failed to show a protective effect in neuron-glia cultures from mice lacking functional NADPH oxidase (PHOX), a key enzyme for extracellular superoxide production in immune cells. Furthermore, we demonstrated that SN reduced LPS-induced extracellular ROS production through the inhibition of the PHOX cytosolic subunit p47phoxtranslocation to the cell membrane.ConclusionOur findings strongly suggest that the protective effects of SN are most likely mediated through the inhibition of microglial PHOX activity. These findings suggest a novel therapy to treat inflammation-mediated neurodegenerative diseases.

A limited role for microglia in antibody mediated plaque clearance in APP mice

Amyloid-beta (Abeta) accumulation in senile plaques is a hallmark of Alzheimer's disease (AD). Immunotherapy is a leading approach for amyloid clearance, despite the early termination of the Elan clinical trial with active immunization due to a few cases of meningoencephalitis. The mechanisms of immunotherapy-mediated amyloid clearance and this deleterious side effect are largely unknown. While clearance of Abeta probably results in part from microglia-mediated inflammation, it can be microglia independent. Therefore, establishing the role of microglia in Abeta clearance is important for the treatment of AD. We analyzed the effects of direct microglia activation and inhibition on antibody-mediated Abeta clearance. Robust microglia activation with interferon-gamma led to modest Abeta clearance alone but did not potentiate antibody-mediated clearance. Microglia elimination/inactivation with immunotoxin or minocycline only partially limited antibody-induced Abeta clearance suggesting that although there is a role for microglia in Abeta clearance, it does not account for the majority of the effect observed after anti-Abeta antibody treatment.

Functional ryanodine receptors are expressed by human microglia and THP‐1 cells: Their possible involvement in modulation of neurotoxicity

Ryanodine receptors (RyRs) are intracellular Ca(2+) channels that mediate the release of calcium from internal stores and therefore play an important role in Ca(2+) signaling and homeostasis. Three RyR isoforms have been described thus far, and various areas of brain are known to express each of them. It is well established that neurons can express different RyR isoforms, but it is not known whether microglial cells do so. In the present study we showed that cultured human microglia from both fetal and adult brain specimens express mRNA for RyR1 and RyR2, whereas RyR3 mRNA can be detected only in fetal microglial cells. Calcium spectrofluorometry showed that high levels of the RyR agonist 4-chloro-m-cresol (4-CmC, 1-5 mM) induced elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)) in both types of cultured human microglial cells. This effect was attenuated by the RyR antagonist 1,1'-diheptyl-4,4'-bipyridinium dibromide (DHBP, 10 microM). Neurotoxicity of conditioned medium from human microglia and THP-1 monocytic cells stimulated with a combination of interferon-gamma (IFN-gamma) with either lipopolysaccharide (LPS) or alpha-synuclein was diminished by DHBP. It was also diminished by 4-CmC at concentrations approximately 100-fold lower than those used to stimulate intracellular Ca(2+) release. These data indicate that human microglial cells express functional RyRs and that selective RyR ligands exert antineurotoxic action on this cell type. Therefore, RyR ligands may represent a novel class of compounds that have utility in reducing microglial-mediated inflammation, which is believed to contribute to the pathogenesis of a number of neurodegenerative disorders including Alzheimer's disease and Parkinson's disease.

Microglial Activation and its Implications in the Brain Diseases

An inflammatory process in the central nervous system (CNS) is believed to play an important role in the pathway leading to neuronal cell death in a number of neurodegenerative diseases including Parkinson's disease, Alzheimer's disease, prion diseases, multiple sclerosis and HIV-dementia. The inflammatory response is mediated by the activated microglia, the resident immune cells of the CNS, which normally respond to neuronal damage and remove the damaged cells by phagocytosis. Activation of microglia is a hallmark of brain pathology. However, it remains controversial whether microglial cells have beneficial or detrimental functions in various neuropathological conditions. The chronic activation of microglia may in turn cause neuronal damage through the release of potentially cytotoxic molecules such as proinflammatory cytokines, reactive oxygen intermediates, proteinases and complement proteins. Therefore, suppression of microglia-mediated inflammation has been considered as an important strategy in neurodegenerative disease therapy. Several anti-inflammatory drugs of various chemical ingredients have been shown to repress the microglial activation and to exert neuroprotective effects in the CNS following different types of injuries. However, the molecular mechanisms by which these effects occur remain unclear. In recent years, several research groups including ours have attempted to explain the potential mechanisms and signaling pathways for the repressive effect of various drugs, on activation of microglial cells in CNS injury. We provide here a comprehensive review of recent findings of mechanisms and signaling pathways by which microglial cells are activated in CNS inflammatory diseases. This review article further summarizes the role of microglial cells in neurodegenerative diseases and various forms of potential therapeutic options to inhibit the microglial activation which amplifies the inflammation-related neuronal injury in neurodegenerative diseases.

Microglia Activation in the Brain as Inflammatory Biomarker of Alzheimer’s Disease Neuropathology and Clinical Dementia

The role of microglia-mediated inflammation in the progression of Alzheimer’s disease (AD) neuropathology remains unclear. In this study, postmortem brain sections from AD and control cases were subjected to Human Leukocyte Antigen (HLA)-DR immunohistochemistry to examine microglia activation in the progression of AD assessed by pre-mortem clinical dementia rating (CDR) and postmortem pathological manifestations of neuritic plaque (NP) and neurofibrillary tangle (NT) according to the Consortium to Establish a Registry for Alzheimer’s Disease (CERAD). In both gray and white matter of the entorhinal cortex (EC) and HLA-DR immunostaining increased with the progression of CDR or CERAD NP, and to a lesser degree with CERAD NT. Between CDR stages HLA-DR significance was found in moderate (CDR 2) to severe dementia (CDR 5) where as between CERAD NP stages staining increased significantly from NP 0 (no plaque) to NP 1 (sparse plaques), suggesting increased microglia activation begins with amyloid NP deposition. In the hippocampus, a significant increase in microglia immunostaining was found in the pyramidal cell layer of CA1 as early as CDR 1, and in the upper molecular layer of the dentate gyrus in CDR 0.5. This increase continues with the progression of CDR and reaches maximum in CDR 5. When assessed by CERAD NP stages however, a significant increase in microglia immunostaining was found only in mid-to-late stages (NP 3) and reduced staining was seen in NP 5. These results suggest that microglia activation increases with the progression of AD, with the increase varying depending on the involved brain region.

Wortmannin enhances lipopolysaccharide-induced inducible nitric oxide synthase expression in microglia in the presence of astrocytes in rats

Microglia, the primary inflammatory cells in the brain, are activated upon brain injury. Activated microglia produce nitric oxide (NO), a major toxin to neuronal cells. It has been reported that astrocytes inhibit microglial activation. In this study, we found that wortmannin, a natural inhibitor of phosphatidylinositol 3-kinase, significantly increased lipopolysaccharide (LPS)-induced NO release and inducible nitric oxide synthase (iNOS) expression in microglia in the presence but not in the absence of astrocytes. In response to LPS even in the presence of wortmannin, iNOS immunoreactivity was detected in microglia but not in astrocytes. These results suggest that astrocytes could regulate microglia-mediated brain inflammation by inhibiting microglial NO release/iNOS expression via a wortmannin-sensitive mechanism.

Inhibition of Alzheimer's β-Amyloid Induced Vasoactivity and Proinflammatory Response in Microglia by a cGMP-Dependent Mechanism

β-amyloid (Aβ) peptides are the major protein components of senile plaques in Alzheimer's disease (AD) brains. Vascular damage and reactive gliosis are found colocalized with amyloid deposits in AD brains, suggesting that the vasculature may be a clinically significant site of AD pathology. Our results show that freshly solubilized Aβ1–40enhances the vasoconstriction induced by endothelin-1 (ET-1) and increases resistance to relaxation triggered by nitric oxide (NO), suggesting that Aβ may oppose the NO/cGMP pathway. Using specific inhibitors and activators of the NO/cGMP pathway, we show that Aβ vasoactivity is not due to a modulation of nitric oxide synthase (NOS) or soluble guanylyl cyclase (sGC). However, we find that a selective cGMP phosphodiesterase (cGMP-PDE) inhibitor (dipyridamole) is able to interactively block the enhanced vasoconstriction as well as the opposition to relaxation induced by Aβ, suggesting that Aβ could effect the activity of this enzyme. Cyclic GMP levels, but not cAMP concentrations, are reduced after Aβ treatment of rat aortic rings, further substantiating this hypothesis. Moreover, in examination of this pathway in another cell type pertinent to AD, we find that Aβ induces a proinflammatory response in microglia as evidenced by increased leukotriene B4 release. We show that both dipyridamole and compounds which increase cGMP levels prevent Aβ-induced microglial inflammation. Our results suggest that therapeutic intervention aimed at reduction of microglial-mediated inflammation via inhibition of cGMP-PDE or elevation of cGMP may be beneficial in the treatment of AD.