NLRP3 Inflammasome Is Expressed and Functional in Mouse Brain Microglia but Not in Astrocytes.

Neuroinflammation is the local reaction of the brain to infection, trauma, toxic molecules or protein aggregates. The brain resident macrophages, microglia, are able to trigger an appropriate response involving secretion of cytokines and chemokines, resulting in the activation of astrocytes and recruitment of peripheral immune cells. IL-1ß plays an important role in this response; yet its production and mode of action in the brain are not fully understood and its precise implication in neurodegenerative diseases needs further characterization. Our results indicate that the capacity to form a functional NLRP3 inflammasome and secretion of IL-1ß is limited to the microglial compartment in the mouse brain. We were not able to observe IL-1ß secretion from astrocytes, nor do they express all NLRP3 inflammasome components. Microglia were able to produce IL-1ß in response to different classical inflammasome activators, such as ATP, Nigericin or Alum. Similarly, microglia secreted IL-18 and IL-1α, two other inflammasome-linked pro-inflammatory factors. Cell stimulation with α-synuclein, a neurodegenerative disease-related peptide, did not result in the release of active IL-1ß by microglia, despite a weak pro-inflammatory effect. Amyloid-ß peptides were able to activate the NLRP3 inflammasome in microglia and IL-1ß secretion occurred in a P2X7 receptor-independent manner. Thus microglia-dependent inflammasome activation can play an important role in the brain and especially in neuroinflammatory conditions.

An efficient method to limit microglia-dependent effects in astroglial cultures.

Microglia, the CNS resident macrophages, and astrocytes, the most abundant glial cell population, are both implicated in brain pathologies and can exhibit a pro-inflammatory phenotype. Microglial cells are known to rapidly and strongly react to brain insults. They will promote astrocyte activation and may lead to a vicious, self-perpetuating cycle of chronic inflammation. To obtain a better understanding of the individual role of both cell types, primary cells are frequently used in in vitro studies, but the purity of specific cell cultures remains rarely investigated. The aim of this study is to determine the effect of specific removal of microglial cells on the inflammatory properties of different glial cultures. Here, the removal of microglial contamination from mixed glial cultures to obtain astrocyte-enriched cultures was achieved using a magnetic cell sorting approach. Compared to mixed cultures, we clearly showed that these enriched cultures are only weakly activated by pro-inflammatory agents (lipopolysaccharide, interferon-γ or beta-amyloid peptide). This finding was confirmed using twice-sorted astrocyte-enriched cultures and microglia-free cultures composed of neurosphere-derived astrocytes. Thus, we present evidence that the magnitude of the pro-inflammatory response is linked to the percentage of microglia in cultures. Due to their high reactivity to various insults or pro-inflammatory stimuli, microglia-derived effects could be credited to astrocytes in mixed glial cultures. Therefore, we highlight the importance of monitoring the presence of microglia in glial cultures since they can affect the interpretation of the results, especially when inflammatory processes are studied.

Microglial activation depends on beta-amyloid conformation: role of the formylpeptide receptor 2.

Alzheimer's disease (AD) is characterized by the presence of extracellular deposits referred to beta-amyloid (Abeta) complexes or senile plaques. Abeta peptide is firstly produced as monomers, readily aggregating to form multimeric complexes, of which the smallest aggregates are known to be the most neurotoxic. In AD patients, abundant reactive microglia migrate to and surround the Abeta plaques. Though it is well known that microglia are activated by Abeta, little is known about the peptide conformation and the signaling cascades responsible for this activation. In this study, we have stimulated murine microglia with different Abeta(1-42) forms, inducing an inflammatory state, which was peptide conformation-dependent. The lightest oligomeric forms induced a more violent inflammatory response, whereas the heaviest oligomers and the fibrillar conformation were less potent inducers. BocMLF, a formylpeptide chemotactic receptor 2 antagonist, decreased the oligomeric Abeta-induced inflammatory response. The Abeta-induced signal transduction was found to depend on phosphorylation mechanisms mediated by MAPKs and on activator protein 1/nuclear factor kappa-light-chain-enhancer of activated B cells pathways activation. These results suggest that the reactive microgliosis intensity during AD might depend on the disease progression and consequently on the Abeta conformation production. The recognition of Abeta by the formylpeptide chemotactic receptor 2 seems to be a starting point of the signaling cascade inducing an inflammatory state.