BackgroundAfter viral infection and the stimulation of some pattern-recognition receptors, TANK-binding kinase I (TBK1) is activated by K63-linked polyubiquitination followed by trans-autophosphorylation. While the activated TBK1 induces type I interferon production by phosphorylating the transcription factor IRF3, the precise molecular mechanisms underlying TBK1 activation remain unclear.ResultsWe report here the localization of the ubiquitinated and phosphorylated active form of TBK1 to the Golgi apparatus after the stimulation of RIG-I-like receptors (RLRs) or Toll-like receptor-3 (TLR3), due to TBK1 K63-linked ubiquitination on lysine residues 30 and 401. The ubiquitin-binding protein optineurin (OPTN) recruits ubiquitinated TBK1 to the Golgi apparatus, leading to the formation of complexes in which TBK1 is activated by trans-autophosphorylation. Indeed, OPTN deficiency in various cell lines and primary cells impairs TBK1 targeting to the Golgi apparatus and its activation following RLR or TLR3 stimulation. Interestingly, the Bluetongue virus NS3 protein binds OPTN at the Golgi apparatus, neutralizing its activity and thereby decreasing TBK1 activation and downstream signaling.ConclusionsOur results highlight an unexpected role of the Golgi apparatus in innate immunity as a key subcellular gateway for TBK1 activation after RNA virus infection.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-016-0292-z) contains supplementary material, which is available to authorized users.

The innate immune system has a key role in the mammalian immune response. In the cytosol, RNA viruses are sensed by the retinoic acid‐inducible gene‐I‐like receptors, which trigger a complex signaling cascade in which mitochondrial antiviral signaling protein plays a central role in mediating the innate host response through the induction of antiviral and inflammatory responses. Hence, the mitochondrion is now emerging as a fundamental hub for innate antiviral immunity beyond its known roles in metabolic processes and the control of programmed cell death. This review summarizes the findings related to mitochondrial antiviral signaling protein, and mitochondria and their dynamics, in the innate immune response to RNA viruses.

Mitochondria are dynamic organelles with a morphology resulting from the balance between two opposing processes: fusion and fission. Little is known about the function of mitochondrial fusion, beside its role in the maintenance of mitochondrial DNA. We report here that enforced mitochondrial hyperfusion, due to the expression of a dominant-negative mutant of Drp1 or of MARCH5, promotes NF-jB activation in a TAK1-and IKK-dependent manner, through the mitochondrial E3 ubiquitin ligase MULAN. The capability of MULAN to activate NF-jB depends on its RING domain and on the E3 ubiquitin ligase TRAF2. Under physiological conditions, stress-induced mitochondrial hyperfusion (SIMH) is also accompanied by NF-jB activation, and the prevention of SIMH or the knockdown of MULAN impairs NF-jB activation. During SIMH, MU-LAN forms a complex with TRAF2 and modulates its ubiquitylation, signifying that TRAF2 may serve as an ubiquitylated transmitter of NF-jB signaling in this pathway. Our results suggest that mitochondria, through their dynamics, convert stress signals into a cell response leading to NF-jB activation. Structured digital abstractTRAF2 physically interacts with MULAN by anti bait coip (View interaction)

Pathogen-associated molecular pattern (PAMP) recognition leads to TANK-binding kinase (TBK1) polyubiquitination and activation by trans-autophosphorylation, resulting in IFN-β production. Here we describe a mouse model of optineurin insufficiency (OptnΔ157) in which the TBK1-interacting N-terminus of optineurin was deleted. PAMP-stimulated cells from OptnΔ157 mice had reduced TBK1 activity, no phosphorylation of optineurin Ser187, and mounted low IFN-β responses. In contrast to pull-down assays where the presence of N-terminus was sufficient for TBK1 binding, both the N-terminal and the ubiquitin-binding regions of optineurin were needed for PAMP-induced binding. This report establishes optineurin as a positive regulator TBK1 via a bipartite interaction between these molecules.

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26Bluetongue virus (BTV) is an arbovirus transmitted by blood-feeding midges to a wide 27 range of wild and domestic ruminants. In this report, we showed that BTV, through its virulence 28 non-structural protein NS3 (BTV-NS3), is able to activate the MAPK/ERK pathway. In 29 response to growth factors, the MAPK/ERK pathway activates cell survival, differentiation, 30 proliferation and protein translation but can also lead to the production of several inflammatory 31 cytokines. By combining immunoprecipitation of BTV-NS3 and mass spectrometry analysis 32 from both BTV-infected and NS3-transfected cells, we identified the serine/threonine-protein 33 kinase B-Raf (BRAF), a crucial player of the MAPK/ERK pathway, as a new cellular interactor 34 of BTV-NS3. BRAF silencing led to a significant decrease of the MAPK/ERK activation by 35 BTV supporting a model where BTV-NS3 interacts with BRAF to activate this signaling 36 cascade. Furthermore, the intrinsic ability of BTV-NS3 to bind BRAF and activate the 37 MAPK/ERK pathway is conserved throughout multiple serotypes/strains but appears to be 38 specific to BTV compared to other members of Orbivirus genus. Inhibition of MAPK/ERK 39 pathway with U0126 reduced viral titers, suggesting that BTV manipulates this pathway for its 40 own replication. Therefore, the activation of the MAPK/ERK pathway by BTV-NS3 could 41 benefit to BTV replication by promoting its own viral protein synthesis but could also explain 42 the deleterious inflammation associated with tissue damages as already observed in severe cases 43 of BT disease. Altogether, our data provide molecular mechanisms to explain the role of BTV-44 NS3 as a virulence factor and determinant of pathogenesis. 45 author/funder. All rights reserved. No reuse allowed without permission. Importance 46 47Bluetongue Virus (BTV) is responsible of the non-contagious arthropod-borne disease 48 Bluetongue (BT) transmitted to ruminants by blood-feeding midges. Despite the fact that BTV 49 has been extensively studied, we still have little understanding of the molecular determinants 50 of BTV virulence. In this report, we found that the virulence protein NS3 interacts with BRAF, 51a key component of the MAPK/ERK pathway. In response to growth factors, this pathway 52 promotes cell survival, increases protein translation but also contributes to the production of 53 inflammatory cytokines. We showed that BTV-NS3 enhances the MAPK/ERK pathway and 54 this activation is BRAF-dependent. Our results demonstrate, at the molecular level, how a 55 single virulence factor has evolved to target a cellular function to ensure its viral replication. 56On the other hand, our findings could also explain the deleterious inflammation associated with 57 tissue damages as already observed in severe cases of BT disease. 58 author/funder. All rights reserved. No reuse allowed without permission.

Bluetongue virus (BTV), an arbovirus transmitted by Culicoides biting midges, is a major concern of wild and domestic ruminants. While BTV induces type I interferon (alpha/beta interferon [IFN-α/β]) production in infected cells, several reports have described evasion strategies elaborated by this virus to dampen this intrinsic, innate response. In the present study, we suggest that BTV VP3 is a new viral antagonist of the IFN-β synthesis. Indeed, using split luciferase and coprecipitation assays, we report an interaction between VP3 and both the mitochondrial adapter protein MAVS and the IRF3-kinase IKKε. Overall, this study describes a putative role for the BTV structural protein VP3 in the control of the antiviral response.

BackgroundThe nuclear factor κB (NF-κB) family members regulate several biological processes as cell proliferation and differentiation, inflammation, immunity and tumor progression. Ubiquitination plays a key role in NF-κB activation and the ubiquitylated transmitters of the NF-κB signaling cascade accumulate in close proximity to endomembranes.FindingsWe performed an unbiased siRNA library screen targeting the 46 E3 ubiquitin ligases bearing transmembrane domains to uncover new modulators of NF-κB activation, using tumor necrosis factor–α (TNF-α) receptor (TNFR) stimulation as a model. We report here the identification of a new Golgi Apparatus-resident protein, RNF121, as an enhancer of NF-κB promoter activity through the catalytic function of its RING domain. From a molecular standpoint, while knocking down RNF121 did not alter RIP1 ubiquitination and IKK activation, the proteasomal degradation of IκBα was impaired suggesting that this E3 ubiquitin ligase regulates this process. However, RNF121 did not directly ubiquitinate IκBα While they were found in the same complex. Finally, we discovered that RNF121 acts as a broad regulator of NF-κB signaling since its silencing also dampens NF-κB activation following stimulation of Toll-Like Receptors (TLRs), Nod-Like Receptors (NLRs), RIG-I-Like Receptors (RLRs) or after DNA damages.ConclusionsThese results unveil an unexpected role of Golgi Apparatus and reveal RNF121 as a new player involved in the signaling leading to NF-κB activation.Electronic supplementary materialThe online version of this article (doi:10.1186/s12964-014-0072-8) contains supplementary material, which is available to authorized users.