TLR4 signaling and macrophage inflammatory responses are dampened by GIV/Girdin

Abstract

AbstractSensing of pathogens by Toll-like receptor 4 (TLR4) induces an inflammatory response; controlled responses confer immunity but uncontrolled responses cause harm. Here we define how a multi-modular scaffold, GIV (a.k.a Girdin) titrates such inflammatory response in macrophages. Upon challenge with either live microbes or microbe-derived lipopolysaccharides (LPS, a ligand for TLR4), macrophages with GIV mount a more tolerant (hypo-reactive) transcriptional response and suppress pro-inflammatory cytokines and signaling pathways (i.e., NFkB and CREB) downstream of TLR4 compared to their GIV-depleted counterparts. Myeloid-specific gene depletion studies confirmed that the presence of GIV ameliorates DSS-induced colitis and sepsis-induced death. The anti-inflammatory actions of GIV are mediated via its C-terminally located TIR-like BB-loop (TILL)-motif which binds the cytoplasmic TIR-modules of TLR4 in a manner that precludes receptor dimerization; the latter is a pre-requisite for pro-inflammatory signaling. Binding of GIV’s TILL motif to other TIR modules inhibits pro-inflammatory signaling via other TLRs, suggesting a convergent paradigm for fine-tuning macrophage inflammatory responses.SignificanceTo ensure immunity, and yet limit pathology, inflammatory responses must be confined within the proverbial ‘Goldilocks zone’. TLR4 is the prototypical sensor that orchestrates inflammatory responses through a series of well characterized downstream cascades. How TLR4 signals are confined remain incompletely understood. Using trans-scale approaches ranging from disease modeling in live animals, through cell-based interventional studies, to structure-guided biochemical studies that offer an atomic-level resolution, this study unravels the existence of a ‘brake’ within the TLR4 signaling cascade, i.e., GIV; the latter is a prototypical member of an emerging class of scaffold proteins. By showing that GIV uses conserved mechanisms to impact multi-TLR signaling, this work unravels a multi-scale point of convergence of immune signaling of broader impact beyond TLR4.