RESUMEN
Toll-like receptors (TLRs) recognize pathogens and their components, thereby initiating immune responses to infectious organisms. TLR ligation leads to the activation of NF-κB and MAPKs through well-defined pathways, but it has remained unclear how TLR signaling activates PI3K, which provides an inhibitory pathway limiting TLR responses. Here, we show that the signaling adapter B-cell adaptor for PI3K (BCAP) links TLR signaling to PI3K activation. BCAP-deficient macrophages and mice are hyperresponsive to TLR agonists and have reduced PI3K activation. The ability of BCAP to inhibit TLR responses requires its capacity to bind PI3K. BCAP is constitutively phosphorylated and associated with the p85 subunit of PI3K in macrophages. This tyrosine-phosphorylated BCAP is transiently enriched in the membrane fraction in response to LPS treatment, suggesting a model whereby TLR signaling causes the phosphorylation of the small amount of BCAP that is associated with membranes in the resting state or the translocation of phosphorylated BCAP from the cytoplasm to the membrane. This accumulation of tyrosine-phosphorylated BCAP at the membrane with its associated PI3K would then allow for the catalysis of Ptd Ins P2 to Ptd Ins P3 and downstream PI3K-dependent signals. Therefore, BCAP is an essential activator of the PI3K pathway downstream of TLR signaling, providing a brake to limit potentially pathogenic excessive TLR responses.
Asunto(s)
Proteínas Adaptadoras Transductoras de Señales/metabolismo , Linfocitos B/enzimología , Fosfatidilinositol 3-Quinasas/metabolismo , Receptores Toll-Like/metabolismo , Proteínas Adaptadoras Transductoras de Señales/deficiencia , Animales , Linfocitos B/efectos de los fármacos , Citocinas/biosíntesis , Activación Enzimática/efectos de los fármacos , Proteínas I-kappa B/metabolismo , Inflamación/patología , Péptidos y Proteínas de Señalización Intracelular/metabolismo , Lipopolisacáridos/farmacología , Macrófagos/efectos de los fármacos , Macrófagos/enzimología , Ratones , Ratones Endogámicos C57BL , Proteínas Quinasas Activadas por Mitógenos/metabolismo , Inhibidor NF-kappaB alfa , Fosforilación/efectos de los fármacos , Fosfotirosina/metabolismo , Unión Proteica/efectos de los fármacos , Subunidades de Proteína/metabolismo , Proteínas Tirosina Quinasas/metabolismo , Proteolisis/efectos de los fármacos , Proteínas Proto-Oncogénicas c-akt/metabolismo , Receptor de Factor Estimulante de Colonias de Macrófagos/metabolismo , Fracciones Subcelulares/efectos de los fármacos , Fracciones Subcelulares/metabolismo , Quinasa SykRESUMEN
Microglia are the principal immune cells of the brain. In Alzheimer disease, these brain mononuclear phagocytes are recruited from the blood and accumulate in senile plaques. However, the role of microglia in Alzheimer disease has not been resolved. Microglia may be neuroprotective by phagocytosing amyloid-beta (Abeta), but their activation and the secretion of neurotoxins may also cause neurodegeneration. Ccr2 is a chemokine receptor expressed on microglia, which mediates the accumulation of mononuclear phagocytes at sites of inflammation. Here we show that Ccr2 deficiency accelerates early disease progression and markedly impairs microglial accumulation in a transgenic mouse model of Alzheimer disease (Tg2576). Alzheimer disease mice deficient in Ccr2 accumulated Abeta earlier and died prematurely, in a manner that correlated with Ccr2 gene dosage, indicating that absence of early microglial accumulation leads to decreased Abeta clearance and increased mortality. Thus, Ccr2-dependent microglial accumulation plays a protective role in the early stages of Alzheimer disease by promoting Abeta clearance.
Asunto(s)
Enfermedad de Alzheimer/inmunología , Enfermedad de Alzheimer/prevención & control , Encéfalo/inmunología , Quimiocinas/metabolismo , Microglía/inmunología , Modelos Inmunológicos , Receptores CCR2/deficiencia , Péptidos beta-Amiloides/metabolismo , Análisis de Varianza , Animales , Cruzamientos Genéticos , Ensayo de Inmunoadsorción Enzimática , Citometría de Flujo , Inmunohistoquímica , Ratones , Ratones Noqueados , Monocitos/inmunología , Receptores CCR2/inmunologíaRESUMEN
Accumulation of inflammatory microglia in Alzheimer's senile plaques is a hallmark of the innate response to beta-amyloid fibrils and can initiate and propagate neurodegeneration characteristic of Alzheimer's disease (AD). The molecular mechanism whereby fibrillar beta-amyloid activates the inflammatory response has not been elucidated. CD36, a class B scavenger receptor, is expressed on microglia in normal and AD brains and binds to beta-amyloid fibrils in vitro. We report here that microglia and macrophages, isolated from CD36 null mice, had marked reductions in fibrillar beta-amyloid-induced secretion of cytokines, chemokines, and reactive oxygen species. Intraperitoneal and stereotaxic intracerebral injection of fibrillar beta-amyloid in CD36 null mice induced significantly less macrophage and microglial recruitment into the peritoneum and brain, respectively, than in wild-type mice. Our data reveal that CD36, a major pattern recognition receptor, mediates microglial and macrophage response to beta-amyloid, and imply that CD36 plays a key role in the proinflammatory events associated with AD.