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

Abstract

Sensing 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 GIVs 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.