Members of the tumour necrosis factor (TNF) receptor superfamily have important functions in immunity and inflammation. Recently linear ubiquitin chains assembled by a complex containing HOIL-1 and HOIP (also known as RBCK1 and RNF31, respectively) were implicated in TNF signalling, yet their relevance in vivo remained uncertain. Here we identify SHARPIN as a third component of the linear ubiquitin chain assembly complex, recruited to the CD40 and TNF receptor signalling complexes together with its other constituents, HOIL-1 and HOIP. Mass spectrometry of TNF signalling complexes revealed RIP1 (also known as RIPK1) and NEMO (also known as IKKγ or IKBKG) to be linearly ubiquitinated. Mutation of the Sharpin gene (Sharpin(cpdm/cpdm)) causes chronic proliferative dermatitis (cpdm) characterized by inflammatory skin lesions and defective lymphoid organogenesis. Gene induction by TNF, CD40 ligand and interleukin-1β was attenuated in cpdm-derived cells which were rendered sensitive to TNF-induced death. Importantly, Tnf gene deficiency prevented skin lesions in cpdm mice. We conclude that by enabling linear ubiquitination in the TNF receptor signalling complex, SHARPIN interferes with TNF-induced cell death and, thereby, prevents inflammation. Our results provide evidence for the relevance of linear ubiquitination in vivo in preventing inflammation and regulating immune signalling.

Nucleotide-binding and oligomerization domain (NOD)-like receptors constitute a first line of defense against invading bacteria. X-linked Inhibitor of Apoptosis (XIAP) is implicated in the control of bacterial infections, and mutations in XIAP are causally linked to immunodeficiency in X-linked lymphoproliferative syndrome type-2 (XLP-2). Here, we demonstrate that the RING domain of XIAP is essential for NOD2 signaling and that XIAP contributes to exacerbation of inflammation-induced hepatitis in experimental mice. We find that XIAP ubiquitylates RIPK2 and recruits the linear ubiquitin chain assembly complex (LUBAC) to NOD2. We further show that LUBAC activity is required for efficient NF-κB activation and secretion of proinflammatory cytokines after NOD2 stimulation. Remarkably, XLP-2-derived XIAP variants have impaired ubiquitin ligase activity, fail to ubiquitylate RIPK2, and cannot facilitate NOD2 signaling. We conclude that XIAP and LUBAC constitute essential ubiquitin ligases in NOD2-mediated inflammatory signaling and propose that deregulation of NOD2 signaling contributes to XLP-2 pathogenesis.

FAS (APO-1/CD95) and its physiological ligand, FASL, regulate apoptotic death of unwanted or dangerous cells in many tissues, functioning as a guardian against autoimmunity and cancer development1-4. Distinct cell types differ in the mechanisms by which the ‘death receptor’ FAS triggers their apoptosis1-4. In type I cells, such as lymphocytes, activation of ‘effector caspases’ by FAS-induced activation of caspase-8 suffices for cell killing whereas in type II cells, including hepatocytes and pancreatic β-cells, amplification of the caspase cascade through caspase-8 mediated activation of the pro-apoptotic BCL-2 family member BID5 is essential6-8. Here we show, that loss of X-chromosome linked inhibitor of apoptosis (XIAP)9,10 function by gene-targeting or treatment with a second mitochondria-derived activator of caspases (SMAC11, also called DIABLO12: direct IAP binding protein with low pI) mimetic drug rendered hepatocytes independent of BID for FAS-induced apoptosis signalling. These results show that XIAP is the critical discriminator between type I versus type II apoptosis signalling and suggest that IAP inhibitors should be used with caution in cancer patients with underlying liver conditions.