The Deubiquitinase OTULIN Is an Essential Negative Regulator of Inflammation and Autoimmunity

摘要

class="head no_bottom_margin" id="sec1title">IntroductionProtein ubiquitination regulates virtually every aspect of cellular homeostasis, in large part through structurally and functionally distinct polyubiquitin (polyUb) signals (). PolyUb chains can be linked via one of seven Ub Lys (K) residues (e.g., K63-linked chains) or via Ub Met1 (M1), forming M1-linked (also known as linear) chains. The latter have important roles in regulating the immune system, in which they contribute to regulating nuclear factor-κB (NF-κB) transcription factors that orchestrate immune responses ().Ub chains regulate canonical NF-κB activation by mediating timed degradation of the inhibitor of κB (IκB) proteins but also serve as a scaffolding, recruitment, and activation platform in receptor signaling complexes. Non-degradative K63- and M1-linked chains mediate the key upstream event of recruiting the TGFβ-activated kinase (TAK1) and the IκB kinase (IKK) complexes, respectively (). K63 and M1 linkages occur in the same Ub polymers (), facilitating TAK1 and IKK co-localization and cross-activation.M1-linked chains are generated by the linear ubiquitin chain assembly complex (LUBAC) consisting of HOIP, HOIL-1, and SHARPIN (, ). LUBAC is recruited to many immune receptors, including TNF-R1, IL-1R, CD40, TLRs, and NOD2, in a Ub-dependent manner. At the receptors, LUBAC ubiquitinates a host of targets, including RIPK1, RIPK2, MyD88, IRAKs, and NEMO, directly or on pre-existing Ub chains (, ).Genetic loss of LUBAC components leads to immunodeficiency () and inflammatory phenotypes in mice (, , , ), which can be rescued by co-deletion of TNF-R1 (, , href="#bib31" rid="bib31" class=" bibr popnode">Peltzer et al., 2014, href="#bib33" rid="bib33" class=" bibr popnode">Rickard et al., 2014). Mutations in LUBAC components also cause inflammatory conditions in humans (href="#bib2" rid="bib2" class=" bibr popnode">Boisson et al., 2015, href="#bib1" rid="bib1" class=" bibr popnode">Boisson et al., 2012). Hence, loss of M1-linked chains imbalances immune signaling.Several deubiquitinating enzymes (DUBs), including A20, CYLD, and Cezanne, act as negative regulators of NF-κB signaling and are essential for resolving inflammation and the return to homeostasis (href="#bib16" rid="bib16" class=" bibr popnode">Harhaj and Dixit, 2012). OTULIN (also known as FAM105B or Gumby) is the only DUB known to specifically cleave M1 linkages (href="#bib22" rid="bib22" class=" bibr popnode">Keusekotten et al., 2013, href="#bib34" rid="bib34" class=" bibr popnode">Rivkin et al., 2013). OTULIN directly binds the LUBAC component HOIP, and knockdown of OTULIN in human cell lines increases M1-linked chains on LUBAC and its substrates (href="#bib9" rid="bib9" class=" bibr popnode">Elliott et al., 2014, href="#bib12" rid="bib12" class=" bibr popnode">Fiil et al., 2013, href="#bib22" rid="bib22" class=" bibr popnode">Keusekotten et al., 2013, href="#bib34" rid="bib34" class=" bibr popnode">Rivkin et al., 2013, href="#bib38" rid="bib38" class=" bibr popnode">Schaeffer et al., 2014). Strikingly, while LUBAC translocates to receptor signaling complexes (RSCs), OTULIN is not stably associated with TNF or NOD2 RSCs (href="#bib8" rid="bib8" class=" bibr popnode">Draber et al., 2015), and how it regulates signaling complexes, e.g., TNF signaling, is unclear (href="#bib18" rid="bib18" class=" bibr popnode">Hrdinka et al., 2016). Indeed, the physiological role of OTULIN in the immune system has remained unstudied, since OTULIN loss-of-function mutations lead to early embryonic lethality (E12.5–E14) in mice due to defective Wnt signaling and angiogenesis (href="#bib34" rid="bib34" class=" bibr popnode">Rivkin et al., 2013).Here, we describe that a homozygous hypomorphic OTULIN mutation in a consanguineous family causes a potentially fatal autoinflammatory disorder termed OTULIN-related autoinflammatory syndrome (ORAS), which can be managed by Infliximab (anti-TNF neutralizing antibody). We recapitulate key features of ORAS in mouse models of OTULIN deficiency. In an acute model, induced loss of OTULIN in immune cells leads to multi-organ inflammation and deterioration of animals within a few days; this can be ameliorated by anti-TNF, but not by neutralization of other upregulated cytokines. In addition, loss of OTULIN in myeloid cells generates a chronic model in which mice display increased serum levels of inflammation-associated cytokines and chemokines and develop splenomegaly and autoimmunity. In bone-marrow-derived macrophages (BMDMs), loss of OTULIN leads to overproduction of M1-linked Ub chains and dysregulated NF-κB activation and cytokine secretion. Strikingly, while mice lacking OTULIN in B or T cells do not display overt inflammatory phenotypes, further analysis indicates that these OTULIN-deficient cells have downregulated LUBAC components HOIP and SHARPIN, but not HOIL-1.Together, the data from mouse models and human patients clearly establish OTULIN and M1-linked polyUb chains as key regulators of immune homeostasis, inflammation, and autoimmunity and reveal cell-type-specific effects of OTULIN in immune cells.