CD14 and Toll-Like Receptors 2 and 4 Are Required for Fibrillar Aβ-Stimulated Microglial Activation

Abstract

Microglia are the brain's tissue macrophages and are found in an activated state surrounding ␤-amyloid plaques in the Alzheimer's disease brain. Microglia interact with fibrillar ␤-amyloid (fA␤) through an ensemble of surface receptors composed of the ␣ 6 ␤ 1 integrin, CD36, CD47, and the class A scavenger receptor. These receptors act in concert to initiate intracellular signaling cascades and phenotypic activation of these cells. However, it is unclear how engagement of this receptor complex is linked to the induction of an activated microglial phenotype. We report that the response of microglial cells to fibrillar forms of A␤ requires the participation of Toll-like receptors (TLRs) and the coreceptor CD14. The response of microglia to fA␤ is reliant upon CD14, which act together with TLR4 and TLR2 to bind fA␤ and to activate intracellular signaling. We find that cells lacking these receptors could not initiate a Src-Vav-Rac signaling cascade leading to reactive oxygen species production and phagocytosis. The fA␤-mediated activation of p38 MAPK also required CD14, TLR4, and TLR2. Inhibition of p38 abrogated fA␤-induced reactive oxygen species production and attenuated the induction of phagocytosis. Microglia lacking CD14, TLR4, and TLR2 showed no induction of phosphorylated IB␣ following fA␤. These data indicate these innate immune receptors function as members of the microglial fA␤ receptor complex and identify the signaling mechanisms whereby they contribute to microglial activation.

Differential Activation of Innate Immune Pathways by Distinct Islet Amyloid Polypeptide (IAPP) Aggregates

“…We found that activation of TLR2 serves as a critical trigger for hIAPP-induced Il1b expression in both BMDMs and cultured islets. Interestingly, diverse amyloidogenic peptides appear to act as endogenous ligands for TLR2: activation of TLR1/2 has been described for A␤ (32,33), serum amyloid A (34), lysozyme fibrils (12), curli fibrils (11), and ␣-synuclein (35). Lysozyme may also activate TLR2/6 (12), and our data suggest that heterodimerization with TLR6 is required for sensing of IAPP by TLR2.…”

The Evolving Biology of Microglia in Alzheimer's Disease

“…Phagocytosis is one of the key mechanisms for microgliamediated fibrillar and oligomeric Aβ removal [22,41,50,[64][65][66][67]. Large particles like fAβ are taken up by phagocytosis through engagement of innate immune surface receptors and activation of the cell's phagocytic machinery.…”

Evaluating the role of Toll-like receptors in diseases of the central nervous system

“…Further evidence suggesting a role for TLRs in microglial responses to Aβ was provided in a study where microglia derived from TLR2, 4 and CD14 KO mice were defective in Src-Vav-Rac and p38 MAPK signalling and showed reduced ROS production and phagocytosis in response to FAβ [39]. This suggests that TLR2, TLR4 and CD-14 are part of the receptor complex that responds to FAβ.…”

“…a physical interaction between CD14 and fibrillar Aβ (FAβ) was demonstated by Reed Geaghan et al [39]. The signal transduction cascades triggered by FAβ is identical to those triggered by TLR agonists [39] and Aβ induction of NFκB dependent genes requires TLR2, TLR4 and CD-14 [39], thus indicating that TLRs are important in sensing and responding to the presence of Aβ.…”

“…This suggests that TLR2, TLR4 and CD-14 are part of the receptor complex that responds to FAβ. This cellular complex also consists of cell surface receptors such as CD36, α 6 integrin, CD47 and scavenger receptor A, which, along with TLRs 2, 4 and CD14 can bind to FAβ to trigger multiple intracellular signaling cascades [39][40][41].…”