Microglia and macrophages in the CNS contain multimolecular complexes termed inflammasomes. Inflammasomes function as intracellular sensors for infectious agents as well as for host-derived danger signals that are associated with neurological diseases, including meningitis, stroke and Alzheimer's disease. Assembly of an inflammasome activates caspase 1 and, subsequently, the proteolysis and release of the cytokines interleukin-1β and interleukin-18, as well as pyroptotic cell death. Since the discovery of inflammasomes in 2002, there has been burgeoning recognition of their complexities and functions. Here, we review the current understanding of the functions of different inflammasomes in the CNS and their roles in neurological diseases.

Members of the caspase family of cysteine proteases play central roles in coordinating the stereotypical events that occur during apoptosis. Because the major executioner caspases, caspase-3 and caspase-7, exhibit almost indistinguishable activity toward certain synthetic peptide substrates, this has led to the widespread view that these proteases occupy functionally redundant roles within the cell death machinery. However, the distinct phenotypes of mice deficient in either of these caspases, as well as mice deficient in both, is at odds with this view. These distinct phenotypes could be related to differences in the relative expression levels of caspase-3 and caspase-7 in vivo, or due to more fundamental differences between these proteases in terms of their ability to cleave natural substrates. Here we show that caspase-3 and caspase-7 exhibit differential activity toward multiple substrate proteins, including Bid, XIAP, gelsolin, caspase-6, and cochaperone p23. Caspase-3 was found to be generally more promiscuous than caspase-7 and appears to be the major executioner caspase during the demolition phase of apoptosis. Our observations provide a molecular basis for the different phenotypes seen in mice lacking either caspase and indicate that these proteases occupy nonredundant roles within the cell death machinery.apoptosis ͉ caspase substrates ͉ proteolysis

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The myeloid cell-derived calcium-binding murine protein, S100A8, is secreted to act as a chemotactic factor at picomolar concentrations, stimulating recruitment of myeloid cells to inflammatory sites. S100A8 may be exposed to oxygen metabolites, particularly hypochlorite, the major oxidant generated by activated neutrophils at inflammatory sites. Here we show that hypochlorite oxidizes the single Cys residue (Cys 41 ) of S100A8. Electrospray mass spectrometry and SDS-polyacrylamide gel electrophoresis analysis indicated that low concentrations of hypochlorite (40 M) converted 70 -80% of S100A8 to the disulfide-linked homodimer. The mass was 20,707 Da, 92 Da more than expected, indicating additional oxidation of susceptible amino acids (possibly methionine). Phorbol 12-myristate 13-acetate activation of differentiated HL-60 granulocytic cells generated an oxidative burst that was sufficient to efficiently oxidize exogenous S100A8 within 10 min, and results implicate involvement of the myeloperoxidase system. Moreover, disulfide-linked dimer was identified in lung lavage fluid of mice with endotoxin-induced pulmonary injury. S100A8 dimer was inactive in chemotaxis and failed to recruit leukocytes in vivo. Positive chemotactic activity of recombinant Ala 41 S100A8 indicated that Cys 41 was not essential for function and suggested that covalent dimerization may structurally modify accessibility of the chemotactic hinge domain. Disulfidedependent dimerization may be a physiologically significant regulatory mechanism controlling S100A8-provoked leukocyte recruitment.Murine S100A8, also known as CP-10 (chemotactic protein, 10 kDa), myeloid-related protein 8, and calgranulin A (1), is a small acidic protein containing two Ca 2ϩ -binding EF hands belonging to the highly conserved S100 protein family (2). Most S100 proteins appear to function as intracellular calcium-modulated proteins that may regulate diverse functions including cell growth, differentiation, energy metabolism, cytoskeletalmembrane interactions, and kinase activities (1, 3). Extracellular activities have been ascribed to at least five family members, and since our description of the chemotactic activity of S100A8, similar functions for S100A2 (S100L, chemotactic for guinea pig eosinophils) and S100A7 (psoriasin, chemotactic for human CD4ϩ T lymphocytes and neutrophils) have been reported (4, 5). S100B is an extracellular neurotrophic factor, and mitogen (6) and human S100A8 and S100A9 are antimicrobial and cytostatic (7) and have been associated with inflammatory pathologies (reviewed in Ref. 8).S100A8 is constitutively expressed with S100A9 in neutrophils. It is up-regulated by bacterial lipopolysaccharide (LPS) 1 and interleukin 1 in macrophages (9) and microvascular endothelial cells (10) and has been associated with neutrophil recruitment in abscess formation (11) and bleomycin lung (12). Our recent experiments also indicate an important role for S100A8 in embryogenesis where it is expressed by migrating trophoblasts, and deletion of the gene was let...

BackgroundHuman immunodeficiency virus type 1(HIV-1) infects and activates innate immune cells in the brain resulting in inflammation and neuronal death with accompanying neurological deficits. Induction of inflammasomes causes cleavage and release of IL-1β and IL-18, representing pathogenic processes that underlie inflammatory diseases although their contribution HIV-associated brain disease is unknown.ResultsInvestigation of inflammasome-associated genes revealed that IL-1β, IL-18 and caspase-1 were induced in brains of HIV-infected persons and detected in brain microglial cells. HIV-1 infection induced pro-IL-1β in human microglia at 4 hr post-infection with peak IL-1β release at 24 hr, which was accompanied by intracellular ASC translocation and caspase-1 activation. HIV-dependent release of IL-1β from a human macrophage cell line, THP-1, was inhibited by NLRP3 deficiency and high extracellular [K+]. Exposure of microglia to HIV-1 gp120 caused IL-1β production and similarly, HIV-1 envelope pseudotyped viral particles induced IL-1β release, unlike VSV-G pseudotyped particles. Infection of cultured feline macrophages by the related lentivirus, feline immunodeficiency virus (FIV), also resulted in the prompt induction of IL-1β. In vivo FIV infection activated multiple inflammasome-associated genes in microglia, which was accompanied by neuronal loss in cerebral cortex and neurological deficits. Multivariate analyses of data from FIV-infected and uninfected animals disclosed that IL-1β, NLRP3 and caspase-1 expression in cerebral cortex represented key molecular determinants of neurological deficits.ConclusionsNLRP3 inflammasome activation was an early and integral aspect of lentivirus infection of microglia, which was associated with lentivirus-induced brain disease. Inflammasome activation in the brain might represent a potential target for therapeutic interventions in HIV/AIDS.

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