Zinc Status Alters Alzheimer's Disease Progression through NLRP3-Dependent Inflammation.

Alzheimer's disease is a devastating neurodegenerative disease with a dramatically increasing prevalence and no disease-modifying treatment. Inflammatory lifestyle factors increase the risk of developing Alzheimer's disease. Zinc deficiency is the most prevalent malnutrition in the world and may be a risk factor for Alzheimer's disease potentially through enhanced inflammation, although evidence for this is limited. Here we provide epidemiological evidence suggesting that zinc supplementation was associated with reduced risk and slower cognitive decline, in people with Alzheimer's disease and mild cognitive impairment. Using the APP/PS1 mouse model of Alzheimer's disease fed a control (35 mg/kg zinc) or diet deficient in zinc (3 mg/kg zinc), we determined that zinc deficiency accelerated Alzheimer's-like memory deficits without modifying amyloid ß plaque burden in the brains of male mice. The NLRP3-inflammasome complex is one of the most important regulators of inflammation, and we show here that zinc deficiency in immune cells, including microglia, potentiated NLRP3 responses to inflammatory stimuli in vitro, including amyloid oligomers, while zinc supplementation inhibited NLRP3 activation. APP/PS1 mice deficient in NLRP3 were protected against the accelerated cognitive decline with zinc deficiency. Collectively, this research suggests that zinc status is linked to inflammatory reactivity and may be modified in people to reduce the risk and slow the progression of Alzheimer's disease.SIGNIFICANCE STATEMENT Alzheimer's disease is a common condition mostly affecting the elderly. Zinc deficiency is also a global problem, especially in the elderly and also in people with Alzheimer's disease. Zinc deficiency contributes to many clinical disorders, including immune dysfunction. Inflammation is known to contribute to the risk and progression of Alzheimer's disease; thus, we hypothesized that zinc status would affect Alzheimer's disease progression. Here we show that zinc supplementation reduced the prevalence and symptomatic decline in people with Alzheimer's disease. In an animal model of Alzheimer's disease, zinc deficiency worsened cognitive decline because of an enhancement in NLRP3-driven inflammation. Overall, our data suggest that zinc status affects Alzheimer's disease progression, and that zinc supplementation could slow the rate of cognitive decline.

Hallmarks of NLRP3 inflammasome activation are observed in organotypic hippocampal slice culture.

Microglial inflammation driven by the NACHT, LRR and PYD domain-containing protein 3 (NLRP3) inflammasome contributes to brain disease and is a therapeutic target. Most mechanistic studies on NLRP3 activation use two-dimensional pure microglial cell culture systems. Here we studied the activation of the NLRP3 inflammasome in organotypic hippocampal slices, which allowed us to investigate microglial NLRP3 activation in a three-dimensional, complex tissue architecture. Toll-like receptor 2 and 4 activation primed microglial inflammasome responses in hippocampal slices by increasing NLRP3 and interleukin-1ß expression. Nigericin-induced NLRP3 inflammasome activation was dynamically visualized in microglia through ASC speck formation. Downstream caspase-1 activation, gasdermin D cleavage, pyroptotic cell death and interleukin-1ß release were also detected, and these findings were consistent when using different NLRP3 stimuli such as ATP and imiquimod. NLRP3 inflammasome pathway inhibitors were effective in organotypic hippocampal slices. Hence, we have highlighted organotypic hippocampal slice culture as a valuable ex vivo tool to allow the future study of NLRP3 inflammasomes in a representative tissue section, aiding the discovery of further mechanistic insights and drug development.

Inflammasome-derived cytokine IL18 suppresses amyloid-induced seizures in Alzheimer-prone mice.

Alzheimer's disease (AD) is characterized by the progressive destruction and dysfunction of central neurons. AD patients commonly have unprovoked seizures compared with age-matched controls. Amyloid peptide-related inflammation is thought to be an important aspect of AD pathogenesis. We previously reported that NLRP3 inflammasome KO mice, when bred into APPswe/PS1ΔE9 (APP/PS1) mice, are completely protected from amyloid-induced AD-like disease, presumably because they cannot produce mature IL1ß or IL18. To test the role of IL18, we bred IL18KO mice with APP/PS1 mice. Surprisingly, IL18KO/APP/PS1 mice developed a lethal seizure disorder that was completely reversed by the anticonvulsant levetiracetam. IL18-deficient AD mice showed a lower threshold in chemically induced seizures and a selective increase in gene expression related to increased neuronal activity. IL18-deficient AD mice exhibited increased excitatory synaptic proteins, spine density, and basal excitatory synaptic transmission that contributed to seizure activity. This study identifies a role for IL18 in suppressing aberrant neuronal transmission in AD.

Targeting the IL33-NLRP3 axis improves therapy for experimental cerebral malaria.

Cerebral malaria (CM) is a serious neurological complication caused by Plasmodium falciparum infection. Currently, the only treatment for CM is the provision of antimalarial drugs; however, such treatment by itself often fails to prevent death or development of neurological sequelae. To identify potential improved treatments for CM, we performed a nonbiased whole-brain transcriptomic time-course analysis of antimalarial drug chemotherapy of murine experimental CM (ECM). Bioinformatics analyses revealed IL33 as a critical regulator of neuroinflammation and cerebral pathology that is down-regulated in the brain during fatal ECM and in the acute period following treatment of ECM. Consistent with this, administration of IL33 alongside antimalarial drugs significantly improved the treatment success of established ECM. Mechanistically, IL33 treatment reduced inflammasome activation and IL1ß production in microglia and intracerebral monocytes in the acute recovery period following treatment of ECM. Moreover, treatment with the NLRP3-inflammasome inhibitor MCC950 alongside antimalarial drugs phenocopied the protective effect of IL33 therapy in improving the recovery from established ECM. We further showed that IL1ß release from macrophages was stimulated by hemozoin and antimalarial drugs and that this was inhibited by MCC950. Our results therefore demonstrate that manipulation of the IL33-NLRP3 axis may be an effective therapy to suppress neuroinflammation and improve the efficacy of antimalarial drug treatment of CM.

A Fluorescent Reporter Mouse for Inflammasome Assembly Demonstrates an Important Role for Cell-Bound and Free ASC Specks during In Vivo Infection.

Inflammasome activation is associated with numerous diseases. However, in vivo detection of the activated inflammasome complex has been limited by a dearth of tools. We have developed transgenic mice that ectopically express the fluorescent adaptor protein, apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) and characterized the formation of assembled inflammasome complexes ("specks") in primary cells and tissues. In addition to hematopoietic cells, we have found that a stromal population in the lung tissues formed specks during the early phase of influenza infection, whereas myeloid cells showed speck formation after 2 days. In a peritonitis and group B streptococcus infection model, a higher percentage of neutrophils formed specks at early phases of infection, while dendritic cells formed specks at later time points. Furthermore, speck-forming cells underwent pyroptosis and extensive release of specks to the extracellular milieu in vivo. These data underscore the importance of free specks during inflammatory processes in vivo.

A dendritic-cell-stromal axis maintains immune responses in lymph nodes.

Within secondary lymphoid tissues, stromal reticular cells support lymphocyte function, and targeting reticular cells is a potential strategy for controlling pathogenic lymphocytes in disease. However, the mechanisms that regulate reticular cell function are not well understood. Here we found that during an immune response in lymph nodes, dendritic cells (DCs) maintain reticular cell survival in multiple compartments. DC-derived lymphotoxin beta receptor (LTßR) ligands were critical mediators, and LTßR signaling on reticular cells mediated cell survival by modulating podoplanin (PDPN). PDPN modulated integrin-mediated cell adhesion, which maintained cell survival. This DC-stromal axis maintained lymphocyte survival and the ongoing immune response. Our findings provide insight into the functions of DCs, LTßR, and PDPN and delineate a DC-stromal axis that can potentially be targeted in autoimmune or lymphoproliferative diseases.

RNA and ß-hemolysin of group B Streptococcus induce interleukin-1ß (IL-1ß) by activating NLRP3 inflammasomes in mouse macrophages.

The inflammatory cytokine IL-1ß is critical for host responses against many human pathogens. Here, we define Group B Streptococcus (GBS)-mediated activation of the Nod-like receptor-P3 (NLRP3) inflammasome in macrophages. NLRP3 activation requires GBS expression of the cytolytic toxin, ß-hemolysin, lysosomal acidification, and leakage. These processes allow the interaction of GBS RNA with cytosolic NLRP3. The present study supports a model in which GBS RNA, along with lysosomal components including cathepsins, leaks out of lysosomes and interacts with NLRP3 to induce IL-1ß production.

Synthetic oligodeoxynucleotides containing suppressive TTAGGG motifs inhibit AIM2 inflammasome activation.

Synthetic oligodeoxynucleotides (ODNs) comprised of the immunosuppressive motif TTAGGG block TLR9 signaling, prevent STAT1 and STAT4 phosphorylation and attenuate a variety of inflammatory responses in vivo. In this study, we demonstrate that such suppressive ODN abrogate activation of cytosolic nucleic acid-sensing pathways. Pretreatment of dendritic cells and macrophages with the suppressive ODN-A151 abrogated type I IFN, TNF-α, and ISG induction in response to cytosolic dsDNA. In addition, A151 abrogated caspase-1-dependent IL-1ß and IL-18 maturation in dendritic cells stimulated with dsDNA and murine CMV. Inhibition was dependent on A151's phosphorothioate backbone, whereas substitution of the guanosine residues for adenosine negatively affected potency. A151 mediates these effects by binding to AIM2 in a manner that is competitive with immune-stimulatory DNA and as a consequence prevents AIM2 inflammasome complex formation. Collectively, these findings reveal a new route by which suppressive ODNs modulate the immune system and unveil novel applications for suppressive ODNs in the treatment of infectious and autoimmune diseases.

NLRP3 is activated in Alzheimer's disease and contributes to pathology in APP/PS1 mice.

Alzheimer's disease is the world's most common dementing illness. Deposition of amyloid-ß peptide drives cerebral neuroinflammation by activating microglia. Indeed, amyloid-ß activation of the NLRP3 inflammasome in microglia is fundamental for interleukin-1ß maturation and subsequent inflammatory events. However, it remains unknown whether NLRP3 activation contributes to Alzheimer's disease in vivo. Here we demonstrate strongly enhanced active caspase-1 expression in human mild cognitive impairment and brains with Alzheimer's disease, suggesting a role for the inflammasome in this neurodegenerative disease. Nlrp3(-/-) or Casp1(-/-) mice carrying mutations associated with familial Alzheimer's disease were largely protected from loss of spatial memory and other sequelae associated with Alzheimer's disease, and demonstrated reduced brain caspase-1 and interleukin-1ß activation as well as enhanced amyloid-ß clearance. Furthermore, NLRP3 inflammasome deficiency skewed microglial cells to an M2 phenotype and resulted in the decreased deposition of amyloid-ß in the APP/PS1 model of Alzheimer's disease. These results show an important role for the NLRP3/caspase-1 axis in the pathogenesis of Alzheimer's disease, and suggest that NLRP3 inflammasome inhibition represents a new therapeutic intervention for the disease.

CD11c(hi) dendritic cells regulate the re-establishment of vascular quiescence and stabilization after immune stimulation of lymph nodes.

Lymph node expansion during immune responses is accompanied by rapid vascular expansion. The re-establishment of quiescence and stabilization of the newly expanded vasculature and the regulatory mechanisms involved have not been well studied. We show that although initiation of vascular expansion in immune-stimulated nodes is associated with upregulated endothelial cell proliferation, increased high endothelial venule trafficking efficiency and VCAM-1 expression, and disrupted perivascular fibroblastic reticular cell organization, the re-establishment of vascular quiescence and stabilization postexpansion is characterized by reversal of these phenomena. Although CD11c(med) cells are associated with the initiation of vascular expansion, CD11c(hi)MHC class II (MHC II)(med) dendritic cells (DCs) accumulate later, and their short-term depletion in mice abrogates the re-establishment of vascular quiescence and stabilization. CD11c(hi)MHC II(med) cells promote endothelial cell quiescence in vitro and, in vivo, mediate quiescence at least in part by mediating reduced lymph node vascular endothelial growth factor. Disrupted vascular quiescence and stabilization in expanded nodes is associated with attenuated T cell-dependent B cell responses. These results describe a novel mechanism whereby CD11c(hi)MHC II(med) DCs regulate the re-establishment of vascular quiescence and stabilization after lymph node vascular expansion and suggest that these DCs function in part to orchestrate the microenvironmental alterations required for successful immunity.

Fibroblast-type reticular stromal cells regulate the lymph node vasculature.

The lymph node vasculature is essential to immune function, but mechanisms regulating lymph node vascular maintenance and growth are not well understood. Vascular endothelial growth factor (VEGF) is an important mediator of lymph node endothelial cell proliferation in stimulated lymph nodes. It is expressed basally in lymph nodes and up-regulated upon lymph node stimulation, but the identity of VEGF-expressing cells in lymph nodes is not known. We show that, at homeostasis, fibroblast-type reticular stromal cells (FRC) in the T zone and medullary cords are the principal VEGF-expressing cells in lymph nodes and that VEGF plays a role in maintaining endothelial cell proliferation, although peripheral node addressin (PNAd)(+) endothelial cells are less sensitive than PNAd(-) endothelial cells to VEGF blockade. Lymphotoxin beta receptor (LTbetaR) blockade reduces homeostatic VEGF levels and endothelial cell proliferation, and LTbetaR stimulation of murine fibroblast-type cells up-regulates VEGF expression, suggesting that LTbetaR signals on FRC regulate lymph node VEGF levels and, thereby, lymph node endothelial cell proliferation. At the initiation of immune responses, FRC remain the principal VEGF mRNA-expressing cells in lymph nodes, suggesting that FRC may play an important role in regulating vascular growth in stimulated nodes. In stimulated nodes, VEGF regulates the proliferation and expansion of both PNAd(+) and PNAd(-) endothelial cells. Taken together, these data suggest a role for FRC as paracrine regulators of lymph node endothelial cells and suggest that modulation of FRC VEGF expression may be a means to regulate lymph node vascularity and, potentially, immune function.