Group 3 : Ubiquitination in NF-kB signaling
Alain ISRAËL (Research Director, CNRS)
Emmanuel LAPLANTINE (Researcher, CNRS)
Nadine TARANTINO (Technician, CNRS)
NF-kB activation represents a general stress response pathway, which has been essentially studied in the context of the immune, inflammatory and anti-apoptotic responses. NF-kB activity, like most signaling pathways, is controlled by a series of reversible post-translational modifications, the most recently studied being protein ubiquitination. This mode of regulation involves a complex network of ubiquitinated molecules and ubiquitin receptors among which the protein NEMO (NF-kB essential modulator), the core element of the canonical NF-kB cascade, plays a key role (figure 1). We are currently studying the mechanisms that control this network, both under normal and pathological conditions.
Figure 1: NF-κB activation in response toTNF
Following binding of TNF to the TNF-R1 receptor, TRADD and RIP1 are recruited to its death domain. TRADD then facilitates recruitment of TRAF2/5, two E3 ubiquitin ligases, which in turn allow the recruitment and activation of 2 other E3 ligases, cIAP1 and 2. cIAPs ubiquitylate themselves together with certain components of the TNF receptor complex, including RIP1. These polyUb chains serve as recruitment platforms for the E3 ligase complex LUBAC, and for the kinase complexes TAK1/TAB2 and NEMO/IKK (although it is unclear to which chains these complexes are recruited). Once recruited, LUBAC is able to linearly ubiquitylate substrates (possibly NEMO and RIP1) and thereby facilitate the recruitment of additional NEMO/IKK complexes. Phosphorylation/activation of the IKK complex by TAK1 leads to phosphorylation of the IκB inhibitors of NF-κB, followed by their ubiquitination and degradation by the proteasome, therefore allowing nuclear translocation of NF-κB and activation of its target genes.
Figure 2: Domain organization of NEMO. CC : coiled coil; ZF : Zinc Finger; NOA : Nemo, Optineurin, Abin. The NOAZ module is a functional entity combining the 2 UBDs of NEMO (NOA and ZF).
The C-terminal domain of NEMO contains 2 ubiquitin binding domains (UBDs). These include a NOA domain (present in the proteins NEMO, Optineurin and ABIN1-3) and a C-terminal Zinc-finger (ZF) domain. We have shown that the association of these 2 UBDs (designated NOAZ in figure 2) is required for interaction with K63-linked polyubiquitin chains and subsequent NF-kB activation, while NOA alone is involved in the interaction with M1-linked (or linear) polyubiquitin chains (see below). The exact contribution of K63-linked and M1-linked polyubiquitin chains in the activation of the IKK complex is not fully understood. Moreover, while the ability of NEMO to bind these two types of polyubiquitin chains is important for signal transduction, the observation that NEMO is also poly-ubiquitinated in response to many NF-kB activating stimuli remains unexplained. We are thus investigating the role of NEMO ubiquitination in the activation of the IKK complex, a prerequisite for NF-kB activation; our preliminary data indicate that NEMO ubiquitination could act as a negative switch in this activation.
Linear ubiquitination has emerged as a new type of post-translational modification involved in NF-kB activation. This type of polyubiquitin chain is synthesized by a complex called LUBAC (Linear Ubiquitination Assembly Complex) composed of three subunits: HOIL1, HOIP and Sharpin. LUBAC had been so far considered a positive regulator of NF-kB, in both TNF and IL-1b signaling. In a collaborative work, we recently discovered that mutations in the HOIL1 gene found in human patients lead to a severe pathology combining immunodeficiency, auto-inflammation and amylopectinosis. We observed that NF-kB activation is diminished in the patients' fibroblasts and B-cells due to a lack of recruitment of NEMO to the receptor signaling complexes, probably explaining the immunodeficiency. Unexpectedly this decreased activity could be observed in response to IL-1b, but not to TNF. More surprisingly, we found the patients' monocytes were hyper-responsive to the pro-inflammatory cytokine IL-1β, probably explaining the auto-inflammatory condition of the patients. This reveals a cell-specific role of LUBAC which we are we are currently investigating in more details.
Boisson*, B., E. Laplantine*, C. Prando*, S. Giliani, E. Israelsson, Z. Xu, A. Abhyankar, L. Israel, G. Trevejo-Nunez, D. Bogunovic, A.M. Cepika, D. Macduff, M. Chrabieh, M. Hubeau, F. Bajolle, M. Debre, E. Mazzolari, D. Vairo, F. Agou, H.W. Virgin, X. Bossuyt, C. Rambaud, F. Facchetti, D. Bonnet, P. Quartier, J.C. Fournet, V. Pascual, D. Chaussabel, L.D. Notarangelo, A. Puel, A. Israel*, J.L. Casanova*, and C. Picard*. (2012). Immunodeficiency, autoinflammation and amylopectinosis in humans with inherited HOIL-1 and LUBAC deficiency. Nat Immunol., 13, 1178-1186.
Kachaner, D., Genin, P., Laplantine, E., and Weil, R. (2012). Optineurin: Towards an integrative view of Optineurin functions. Cell Cycle 11, 2808-2818.(Review article)
Kachaner, D., Laplantine, E., Genin, P., and Weil, R. (2012). Optineurin: A new vision of cell division control. Cell Cycle 11, 1481-1482. (Review article)
Kachaner, D., Filipe, J., Laplantine, E., Bauch, A., Bennett, K.L., Superti-Furga, G., Israel, A., and Weil, R. (2012). Plk1-dependent phosphorylation of optineurin provides a negative feedback mechanism for mitotic progression. Mol Cell 45, 553-566.
Laplantine, E. (2011). Rôle de l’ubiquitination dans la voie de signalisation NF-κB. pp 313-333. In: Coux, O. "Protéasome, ubiquitine et protéines apparentées à l'ubiquitine", Edition Lavoisier, Paris. Collective Book (in French). (Review article)
Laplantine, E., Fontan, E., Chiaravalli, J., Lopez, T., Lakisic, G., Veron, M., Agou, F., and Israel, A. (2009). NEMO specifically recognizes K63-linked poly-ubiquitin chains through a new bipartite ubiquitin-binding domain. EMBO J 28, 2885-2895.
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