Fibrillar Amyloid-β Peptides Activate Microglia via TLR2: Implications for Alzheimer's Disease

Microglial activation is an important pathological component in brains of patients with Alzheimer's disease (AD), and fibrillar amyloid-beta (A beta) peptides play an important role in microglial activation in AD. However, mechanisms by which A beta peptides induce the activation of microglia are poorly understood. The present study underlines the importance of TLR2 in mediating A beta peptide-induced activation of microglia. Fibrillar A beta 1-42 peptides induced the expression of inducible NO synthase, proinflammatory cytokines (TNF-alpha, IL-1 beta, and IL-6), and integrin markers (CD11b, CD11c, and CD68) in mouse primary microglia and BV-2 microglial cells. However, either antisense knockdown of TLR2 or functional blocking Abs against TLR2 suppressed A beta 1-42-induced expression of proinflammatory molecules and integrin markers in microglia. A beta 1-42 peptides were also unable to induce the expression of proinflammatory molecules and increase the expression of CD11b in microglia isolated from TLR2(-/-) mice. Finally, the inability of A beta 1-42 peptides to induce the expression of inducible NO synthase and to stimulate the expression of CD11b in vivo in the cortex of TLR2(-/-) mice highlights the importance of TLR2 in A beta-induced microglial activation. In addition, ligation of TLR2 alone was also sufficient to induce microglial activation. Consistent to the importance of MyD88 in mediating the function of various TLRs, antisense knockdown of MyD88 also inhibited A beta 1-42 peptide-induced expression of proinflammatory molecules. Taken together, these studies delineate a novel role of TLR2 signaling pathway in mediating fibrillar A beta peptide-induced activation of microglia. The Journal of Immunology, 2008, 181: 7254-7262.