In atherosclerosis and Alzheimer’s disease, deposition of the altered-self components oxidized low-density lipoprotein (LDL) and β-amyloid triggers a protracted sterile inflammatory response. Although chronic stimulation of the innate immune system is believed to underlie the pathology of these diseases, the molecular mechanisms of activation remain unclear. Here we show that oxidized LDL and β-amyloid trigger inflammatory signaling through a heterodimer of Toll-like receptors 4 and 6. Assembly of this novel heterodimer is regulated by signals from the scavenger receptor CD36, a common receptor for these disparate ligands. Our results identify CD36-TLR4-TLR6 activation as a common molecular mechanism by which atherogenic lipids and β-amyloid stimulate sterile inflammation and suggest a new model of TLR heterodimerization triggered by co-receptor signaling events.

The increased resistance of various bacteria toward available antibiotic drugs has initiated intensive research efforts into identifying new sources of antimicrobial substances. Short antibiotic peptides (10-30 residues) are prevalent in nature as part of the intrinsic defense mechanisms of most organisms and have been proposed as a blueprint for the design of novel antimicrobial agents. Antimicrobial peptides are generally believed to kill bacteria through membrane permeabilization and extensive pore-formation. Assays providing rapid and easy evaluation of interactions between antimicrobial membrane peptides and lipid bilayers could significantly improve screening for substances with effective antibacterial properties, as well as contribute to the elucidation of structural and functional properties of antimicrobial peptides. Here we describe a colorimetric sensor in which particles composed of phospholipids and polymerized polydiacetylene (PDA) lipids were shown to exhibit striking color changes upon interactions with antimicrobial membrane peptides. The color changes in the system occur because of the structural perturbation of the lipids following their interactions with antimicrobial peptides. The assay was also sensitive to the antibacterial properties of structurally and functionally related peptide analogs.

Rac1 small GTPase controls essential aspects of cell biology and is a direct target of numerous bacterial virulence factors. The CNF1 toxin of pathogenic Escherichia coli addresses Rac1 to ubiquitin-proteasome system (UPS). We report the essential role of the tumor suppressor HACE1, a HECT-domain containing E3 ubiquitin-ligase, in the targeting of Rac1 to UPS. HACE1 binds preferentially GTP-bound Rac1 and catalyzes its polyubiquitylation. HACE1 expression increases the ubiquitylation of Rac1, when the GTPase is activated by point mutations or by the GEF-domain of Dbl. RNAi-mediated depletion of HACE1 blocks the ubiquitylation of active Rac1 and increases GTP-bound Rac1 cellular levels. HACE1 antagonizes cell isotropic spreading, a hallmark of Rac1 activation, and is required for endothelial cell monolayer invasion by bacteria. Together, these data establish the role of the HACE1 E3 ubiquitin-ligase in controlling Rac1 ubiquitylation and activity.