Mutations in triggering receptor expressed on myeloid cells 2 (TREM2) have been linked to increased Alzheimer's disease (AD) risk. Neurobiological functions of TREM2 and its pathophysiological ligands remain elusive. Here we found that TREM2 directly binds to β-amyloid (Aβ) oligomers with nanomolar affinity, whereas AD-associated TREM2 mutations reduce Aβ binding. TREM2 deficiency impairs Aβ degradation in primary microglial culture and mouse brain. Aβ-induced microglial depolarization, K inward current induction, cytokine expression and secretion, migration, proliferation, apoptosis, and morphological changes are dependent on TREM2. In addition, TREM2 interaction with its signaling adaptor DAP12 is enhanced by Aβ, regulating downstream phosphorylation of SYK and GSK3β. Our data demonstrate TREM2 as a microglial Aβ receptor transducing physiological and AD-related pathological effects associated with Aβ.

Using physiological, pharmacological, and gene disruption approaches, we demonstrate that proteinase-activated receptor-2 (PAR-2) plays a pivotal role in mediating chronic inflammation. Using an adjuvant monoarthritis model of chronic inflammation, joint swelling was substantially inhibited in PAR-2-deficient mice, being reduced by more than fourfold compared with wild-type mice, with virtually no histological evidence of joint damage. Mice heterozygous for PAR-2 gene disruption showed an intermediate phenotype. PAR-2 expression, normally limited to endothelial cells in small arterioles, was substantially upregulated 2 weeks after induction of inflammation, both in synovium and in other periarticular tissues. PAR-2 agonists showed potent proinflammatory effects as intra-articular injection of ASKH95, a novel synthetic PAR-2 agonist, induced prolonged joint swelling and synovial hyperemia. Given the absence of the chronic inflammatory response in the PAR-2-deficient mice, our findings demonstrate a key role for PAR-2 in mediating chronic inflammation, thereby identifying a novel and important therapeutic target for the management of chronic inflammatory diseases such as rheumatoid arthritis.

Using physiological, pharmacological, and gene disruption approaches, we demonstrate that proteinase-activated receptor-2 (PAR-2) plays a pivotal role in mediating chronic inflammation. Using an adjuvant monoarthritis model of chronic inflammation, joint swelling was substantially inhibited in PAR-2-deficient mice, being reduced by more than fourfold compared with wild-type mice, with virtually no histological evidence of joint damage. Mice heterozygous for PAR-2 gene disruption showed an intermediate phenotype. PAR-2 expression, normally limited to endothelial cells in small arterioles, was substantially upregulated 2 weeks after induction of inflammation, both in synovium and in other periarticular tissues. PAR-2 agonists showed potent proinflammatory effects as intra-articular injection of ASKH95, a novel synthetic PAR-2 agonist, induced prolonged joint swelling and synovial hyperemia. Given the absence of the chronic inflammatory response in the PAR-2-deficient mice, our findings demonstrate a key role for PAR-2 in mediating chronic inflammation, thereby identifying a novel and important therapeutic target for the management of chronic inflammatory diseases such as rheumatoid arthritis

Lipopolysaccharides (LPS) of the outer membrane of Gram-negative bacteria represent a primary target for innate immune responses. We demonstrate here that the antimicrobial/anti-neutrophil elastase full-length elafin (FL-EL) is able to bind both smooth and rough forms of LPS. The N-terminus was shown to bind both forms of LPS more avidly. We demonstrate that the lipid A core-binding proteins polymyxin B (PB) and LPS-binding protein (LBP) compete with elafin for binding, and that LBP is able to displace prebound elafin from LPS. When PB, FL-EL, N-EL, and C-EL were pre-incubated with LPS before addition to immobilized LBP, PB was the most potent inhibitor of LPS transfer to LBP. These data prompted us to examine the biological consequences of elafin binding to LPS, using tumor necrosis factor (TNF)-alpha release by murine macrophages. In serum-containing conditions, N-EL had no effect, whereas both C-EL and FL-EL inhibited TNF-alpha production. In serum-free conditions, however, all moieties had a stimulatory activity on TNF-alpha release, with C-EL being the most potent at the highest concentration. The differential biological activity of elafin in different conditions suggests a role for this molecule in either LPS detoxification or activation of innate immune responses, depending on the external cellular environment.