NLRX1 Negatively Regulates TLR-Induced NF-κB Signaling by Targeting TRAF6 and IKK

Abstract

SUMMARY Tight regulation of NF-κB signaling is essential for innate and adaptive immune responses, yet the molecular mechanisms responsible for its negative regulation are not completely understood. Here we report that NLRX1, a NOD-like receptor family member, negatively regulates Toll-like receptor-mediated NF-κB activation. NLRX1 interacts with TRAF6 or IκB kinase (IKK) in an activation signal-dependent fashion. Upon LPS stimulation, NLRX1 is rapidly ubiquitinated, disassociates from TRAF6 and then binds to the IKK complex, resulting in inhibition of IKKα/β phosphorylation and NF-κB activation. Knockdown of NLRX1 in various cell types markedly enhances IKK phosphorylation and the production of NF-κB-responsive cytokines after LPS stimulation. We further provide in vivo evidence that NLRX1 knockdown in mice markedly enhances susceptibility to LPS-induced septic shock and plasma IL-6 level. Our study identifies a previously unrecognized role for NLRX1 in the negative regulation of TLR-induced NF-κB activation by dynamically interacting with TRAF6 and the IKK complex.

Cellular and molecular regulation of innate inflammatory responses

“…[101][102][103] On the contrary, NOD2 deficiency increases TLR2-mediated activation of NF-κB and dysregulated TLR2 in NOD2-deficient mice causes the development of antigenspecific colitis. 104,105 Not only NOD2, many other NLRs play inhibitory roles in regulating the signaling of TLRs: for NLRX1 and NLRC3 via interfering with the TRAF6-NF-κB signaling; 61,106,107 for NLRC5 via interacting with and blocking phosphorylation of IκB kinase α (IKKα) and IKKβ; 59 for NRLP6 and NLRP12 via targeting MAPK and NF-κB activation; 108,109 for NLRC4 via downregulating TLR5-mediated antibody immune responses against flagellin. 110 These synergistic or antagonistic interactions across these three PRR families contribute to a cross-linked and finely tuned network of PRR signaling in response to the large repertoire of PAMPs and ensures the most effective and proper outcomes of innate immune responses.…”

MicroRNAs in the regulation of TLR and RIG-I pathways

“…), ubiquitin-related proteins (A20, Nrdp1, CHIP, [15][16][17] etc. ), Nodlike family proteins (NLRX1, NLRC5, 18,19 etc. ), membrane molecules (CD300F, CD11b, PECAM-1, 20,21 etc.…”

Beyond the inflammasome: regulatory NOD-like receptor modulation of the host immune response following virus exposure

“…NLRP12 functions as a negative regulator of inflammation by modulating elements of the NF-k B signalling pathway ( Fig. 3) (Allen et al, 2012a, b;Arthur et al, 2007;Lei et al, 2012Lei et al, , 2013Moore et al, 2008;Tsuchiya et al, 2010;Wagner et al, 2009;Xia et al, 2011;Zaki et al, 2014). This hypothesis is supported by recent in vivo studies utilizing Nlrp12 2/2 mice in models of acute colitis and colitis-associated tumorigenesis.…”

“…The current hypothesis suggests that NLRX1 binds directly with MAVS, competing with RIG-I, which results in downstream NF-k B and IRF3 suppression (Moore et al, 2008). NLRX1 is also able to curtail NF-k B signalling in a MAVS-independent fashion through TRAF6 signalling (Allen et al, 2011;Xia et al, 2011). Given the diverse effects of NLRX1 on other proteins, there are likely to be many additional undiscovered indirect mechanisms through which this highly unique protein exerts its negative regulatory effects.…”

“…Using both overexpression and endogenous systems, these studies showed that NLRX1 can negatively regulate NF-k B signalling through its interactions with TRAF6 (Allen et al, 2011). Subsequent studies provided additional mechanistic insight that revealed, upon stimulation, that NLRX1 dissociates from TRAF6 to interact with the Ik B kinase (IKK) complex and downregulate NF-k B signalling (Xia et al, 2011). While NLRX1 appears to negatively regulate type I IFN and NF-k B signalling, it also seems to augment ROS production through interactions with the mitochondrial protein ubiquinol-cytochrome c reductase core protein II (UQCRC2; Arnoult et al, 2009).…”