TLR2 Is a Primary Receptor for Alzheimer’s Amyloid β Peptide To Trigger Neuroinflammatory Activation

Abstract

Microglia activated by extracellularly deposited amyloid β peptide (Aβ) act as a two-edged sword in Alzheimer’s disease pathogenesis: on the one hand, they damage neurons by releasing neurotoxic proinflammatory mediators (M1 activation); on the other hand, they protect neurons by triggering anti-inflammatory/neurotrophic M2 activation and by clearing Aβ via phagocytosis. TLRs are associated with Aβ-induced microglial inflammatory activation and Aβ internalization, but the mechanisms remain unclear. In this study, we used real-time surface plasmon resonance spectroscopy and conventional biochemical pull-down assays to demonstrate a direct interaction between TLR2 and the aggregated 42-aa form of human Aβ (Aβ42). TLR2 deficiency reduced Aβ42-triggered inflammatory activation but enhanced Aβ phagocytosis in cultured microglia and macrophages. By expressing TLR2 in HEK293 cells that do not endogenously express TLR2, we observed that TLR2 expression enabled HEK293 cells to respond to Aβ42. Through site-directed mutagenesis of tlr2 gene, we identified the amino acids EKKA (741–744) as a critical cytoplasmic domain for transduction of inflammatory signals. By coexpressing TLR1 or TLR6 in TLR2-transgenic HEK293 cells or silencing tlrs genes in RAW264.7 macrophages, we observed that TLR2-mediated Aβ42-triggered inflammatory activation was enhanced by TLR1 and suppressed by TLR6. Using bone marrow chimeric Alzheimer’s amyloid precursor transgenic mice, we observed that TLR2 deficiency in microglia shifts M1- to M2-inflammatory activation in vivo, which was associated with improved neuronal function. Our study demonstrated that TLR2 is a primary receptor for Aβ to trigger neuroinflammatory activation and suggested that inhibition of TLR2 in microglia could be beneficial in Alzheimer’s disease pathogenesis.

CD14 Protein Acts as an Adaptor Molecule for the Immune Recognition of Salmonella Curli Fibers

“…Although various TLRs have been implicated in this inflammatory process, caused by the accumulation of ␤-amyloid (56,57,78), TLR2 has been demonstrated convincingly to recognize the quaternary fibrillar structure of ␤-amyloid as well as curli fibers (46,48). Furthermore, consistent with the data showing the activation of the TLR2-TLR1 complex by curli fibers, ␤-amyloid has also been shown to activate the TLR2-TLR1 complex (49), suggesting that the immune activation mechanisms by FIGURE 4. CD14, TLR2, and TLR1 bind purified curli fibers.…”

“…Furthermore, we demonstrated that TLR2 requires the formation of a heterocomplex with TLR1 to recognize curli fibers (46). Consistent with our findings, a recent study revealed the involvement of TLR1 in the recognition of ␤-amyloid 1-42 by TLR2 (49). In addition, TLR2-TLR1 complex activation by serum amyloid A, an acute phase protein, has been demonstrated (50).…”

“…It is interesting to note that several amyloids with a similar quaternary structure rich in ␤-sheets are also produced in humans, leading to inflammatory responses through the activation of TLRs. In the case of Alzheimer's disease, ␤-amyloid that forms plaques in the brain of the patients elicits nitric oxide and cytokine activation (49,57,76,77). Although various TLRs have been implicated in this inflammatory process, caused by the accumulation of ␤-amyloid (56,57,78), TLR2 has been demonstrated convincingly to recognize the quaternary fibrillar structure of ␤-amyloid as well as curli fibers (46,48).…”

Toll-Like Receptor 2 and NLRP3 Cooperate To Recognize a Functional Bacterial Amyloid, Curli

“…The fibrillar amyloid structure of both curli and human amyloids activates the TLR2/TLR1 receptor complex, leading to the activation of the immune system (22,25). Here, we have demonstrated that macrophages deficient in TLR2 failed to produce IL-1␤ (Fig.…”

Trafficking of LAG-3 to the Surface on Activated T Cells via Its Cytoplasmic Domain and Protein Kinase C Signaling

“…The primer pairs used for each expression vector are listed in Supplemental Table I. The expression vectors encoding two different types of fusion proteins between LAG-3 and CD4 (LAG-3-CD4 or CD4-LAG-3), LAG-3 mutants with a single amino acid substitution (LAG-3S484A, LAG-3S397A), and LAG-3 mutant with a double amino acid substitution (LAG-3S484, 497A) were constructed using PCR-mediated recombination of two DNA fragments amplified by PCR with the primers listed in Supplemental Table I ( 16,17). Also, the expression vectors encoding two different types of LAG-3/CD4-EGFP chimeric fusion proteins (LAG-3-CD4-EGFP or CD4-LAG-3-EGFP), a LAG-3-EGFP fusion protein bearing a single amino acid substitution within LAG-3 (LAG-3S484A-EGFP, LAG-3S397A-EGFP), and a LAG-3-EGFP fusion protein harboring a double amino acid substitution within LAG-3 (LAG-3S484, 497A-EGFP) were constructed by the PCR-mediated recombination using the primers listed in Supplemental Table I.…”