Identification of a Regulatory Acidic Motif as the Determinant of Membrane Localization of TICAM-2

P34L Mutation of swine TIM-1 enhances its ability to mediate Japanese encephalitis virus infection

Japanese encephalitis virus (JEV) is a major causative agent of neurological infection affecting humans and pigs. Human T Cell Immunoglobulin and Mucin Domain 1 (hTIM-1) enhances the infection of JEV through virion-associated phosphatidylserine (PS) binding. Here, five swine TIM-1 (sTIM-1) gene variants were cloned from pig lung tissues by reverse-transcriptase polymerase chain reaction (RT-PCR). Sequence alignment analysis revealed that the gene homology between the sTIM-1 and hTIM-1 was 42.3-43.8%. Furthermore, ectopic expression of all five sTIM-1 variants in 293 T cells can promote JEV entry and infection. However, sTIM-1 V3 exhibited significantly less potent at promoting virus entry compared to the other four variants. Further studies revealed that the 34th amino acid of sTIM-1is critical for the entry of JEV, which is Pro34 in sTIM-1V3 while Leu34 in other four sTIM-1 variants. Mechanically, leucine at locus 34 was associated with the membrane distribution of sTIM-1, thereby affecting viral entry and infection. In total, our findings provide evidence that the PS receptor sTIM-1 promotes the infection of JEV and that the 34th amino acid position is critical for sTIM-1 to mediate viral infection.

The TLR4 adaptor TRAM controls the phagocytosis of Gram-negative bacteria by interacting with the Rab11-family interacting protein 2

Phagocytosis is a complex process that eliminates microbes and is performed by specialised cells such as macrophages. Toll-like receptor 4 (TLR4) is expressed on the surface of macrophages and recognizes Gram-negative bacteria. Moreover, TLR4 has been suggested to play a role in the phagocytosis of Gram-negative bacteria, but the mechanisms remain unclear. Here we have used primary human macrophages and engineered THP-1 monocytes to show that the TLR4 sorting adapter, TRAM, is instrumental for phagocytosis of Escherichia coli as well as Staphylococcus aureus. We find that TRAM forms a complex with Rab11 family interacting protein 2 (FIP2) that is recruited to the phagocytic cups of E. coli. This promotes activation of the actin-regulatory GTPases Rac1 and Cdc42. Our results show that FIP2 guided TRAM recruitment orchestrates actin remodelling and IRF3 activation, two events that are both required for phagocytosis of Gram-negative bacteria.

The Gram-negative bacteria E. coli is the most common cause of severe human pathological conditions like sepsis. Sepsis is a clinical syndrome defined by pathological changes due to systemic inflammation, resulting in paralysis of adaptive T-cell immunity with IFN-β as a critical factor. TLR4 is a key sensing receptor of lipopolysaccharide on Gram-negative bacteria. Inflammatory signalling by TLR4 is initiated by the use of alternative pair of TIR-adapters, MAL-MyD88 or TRAM-TRIF. MAL-MyD88 signaling occurs mainly from the plasma membrane giving pro-inflammatory cytokines like TNF, while TRAM-TRIF signaling occurs from vacuoles like endosomes and phagosomes to give type I interferons like IFN-β. It has previously been shown that TLR4 can control phagocytosis and phagosomal maturation through MAL-MyD88 in mice, however, these data have been disputed and published before the role of TRAM was defined in the induction of IFN-β. A role for TRAM or TRIF in phagocytosis has not previously been reported. Here we describe a novel mechanism where TRAM and its binding partner Rab11-FIP2 control phagocytosis of E. coli and regulate IRF3 dependent production of IFN-β. The significance of these results is that we define Rab11-FIP2 as a potential target for modulation of TLR4-dependent signalling in different pathological states.

Functional interfaces between TICAM-2/TRAM and TICAM-1/TRIF in TLR4 signaling

Toll-like receptor 4 (TLR4) recognizes lipopolysaccharide (LPS), produces pro-inflammatory cytokines and type I interferons, and associates with a trigger of endotoxin shock. TLR4 is interacted with a TIR domain-containing adaptor molecule-2 (TICAM-2)/TRAM [TRIF (TIR domain-containing adaptor-inducing interferon-β)-related adaptor molecule] via its Toll–interleukin-1 receptor homology (TIR) domain. TICAM-2 acts as a scaffold protein and activates TIR domain-containing adaptor molecule-1 (TICAM-1)/TRIF. According to the structural analysis by NMR, TICAM-2 interacts with TICAM-1 by the acidic amino acids motif, E87/D88/D89. The TIR domain of TICAM-2 couples with the dimer of TIR domain of TLR4 beneath the membrane, and TICAM-2 itself also forms dimer and constitutes a binding site with TICAM-1. Endosomal localization of TICAM-2 is essential for TLR4-mediated type I interferon-inducing signal from the endosome. N-terminal myristoylation allows TICAM-2 to anchor to the endosomal membrane. Additionally, we have identified two acidic amino acids, D91/E92, as a functional motif that cooperatively determines endosomal localization of TICAM-2. This structural information of TICAM-2 suggests that the specific structure is indispensable for the endosomal localization and type I interferon production of TICAM-2. Taken together with the knowledge on cytoplasmic sensors for LPS, TICAM-2/TICAM-1 may conform to a signal network on TLR4 to facilitate induction of cytokine disorders.

Raftlin Controls Lipopolysaccharide-Induced TLR4 Internalization and TICAM-1 Signaling in a Cell Type-Specific Manner

The clathrin-dependent endocytic pathway is crucial for endosomal TLR3- and TLR4-mediated Toll-IL-1R domain-containing adaptor molecule-1 (TICAM-1) signaling. TLR4 uses a different signaling platform, plasma membrane and endosomes, for activation of TIRAP-MyD88 and TICAM-2-TICAM-1, respectively. LPS-induced endocytosis of TLR4 is mandatory for TICAM-1-mediated signaling including IFN-β production. Several molecules/mechanisms such as CD14, clathrin, and phosphatidylinositol metabolism have been reported to act as inducers of TLR4 translocation. However, the molecular mechanism of spatiotemporal regulation of TLR4 signaling remains unresolved. We have previously shown that Raftlin is essential for clathrin-dependent endocytosis of TLR3 ligand in human epithelial cells and myeloid dendritic cells (DCs). In this article, we demonstrate that Raftlin also mediated LPS-induced TLR4 internalization and TICAM-1 signaling in human monocyte-derived DCs and macrophages (Mo-Mϕs). When Raftlin was knocked down, LPS-induced TLR4-mediated IFN-β promoter activation, but not NF-κB activation, was decreased in HEK293 cells overexpressing TLR4/MD-2 or TLR4/MD-2/CD14. LPS-induced IFN-β production by monocyte-derived DCs and Mo-Mϕs was significantly decreased by knockdown of Raftlin. Upon LPS stimulation, Raftlin moved from the cytoplasm to the plasma membrane in Mo-Mϕs, where it colocalized with TLR4. Raftlin associated with clathrin-associated adaptor protein-2 in resting cells and transiently bound to TLR4 and clathrin at the cell surface in response to LPS. Thus, Raftlin appears to modulate cargo selection as an accessary protein of clathrin-associated adaptor protein-2 in clathrin-mediated endocytosis of TLR3/4 ligands.

LRRC59 Regulates Trafficking of Nucleic Acid-Sensing TLRs from the Endoplasmic Reticulum via Association with UNC93B1

Compartmentalization of nucleic acid (NA)-sensing TLR3, 7, 8, and 9 is strictly regulated to direct optimal response against microbial infection and evade recognition of host-derived NAs. Uncoordinated 93 homolog B1 (UNC93B1) is indispensable for trafficking of NA-sensing TLRs from the endoplasmic reticulum (ER) to endosomes/lysosomes. UNC93B1 controls loading of the TLRs into COPII vesicles to exit from the ER and traffics with the TLRs in the steady state. Ligand-induced translocation also happens on NA-sensing TLRs. However, the molecular mechanism for ligand-dependent trafficking of TLRs from the ER to endosomes/lysosomes remains unclear. In this study, we demonstrated that leucine-rich repeat containing protein (LRRC) 59, an ER membrane protein, participated in trafficking of NA-sensing TLRs from the ER. Knockdown of LRRC59 reduced TLR3-, 8-, and 9-mediated, but not TLR4-mediated, signaling. Upon ligand stimulation, LRRC59 associated with UNC93B1 in a TLR-independent manner, which required signals induced by ligand internalization. Endosomal localization of endogenous TLR3 was decreased by silencing of LRRC59, suggesting that LRRC59 promotes UNC93B1-mediated translocation of NA-sensing TLRs from the ER upon infection. These findings help us understand how NA-sensing TLRs control their proper distribution in the infection/inflammatory state.