The role of OTULIN in skin inflammation and cell death

The skin epithelium provides a physical and immunologically active multilayered barrier that protects the body from environmental factors and mediates cellular immune responses. Especially mechanisms regulating the cell survival and cell death of epidermal keratinocytes play a crucial role in maintaining tissue homeostasis. Inflammatory and cell death signalling is tightly controlled by linear ubiquitination, mediated via the E3 ligase Linear Ubiquitin Assembly Complex (LUBAC) to initiate innate immune responses. LUBAC itself is controlled by OTU deubiquitinase with linear specificity (OTULIN), a deubiquitinating enzyme that specifically degrades linear ubiquitin chains. Mouse model studies suggested that OTULIN deficiency results in loss of LUBAC components and cell death-mediated inflammation. Interestingly, mutations in the gene encoding OTULIN cause the OTULIN-related autoinflammatory syndrome (ORAS), a human genetic disease presented with complex pathological features and systemic inflammation affecting multiple organs, including the skin. The role of OTULIN in regulating skin homeostasis, inflammation and cell death is poorly understood and remains to be elucidated. The aim of this study was to investigate the biological function of OTULIN in the skin and to unravel the underlying cellular processes. The results acquired in this thesis reveal that OTULIN prevents skin inflammation by inhibiting keratinocyte necroptosis. In detail, deleting OTULIN in the mouse epidermis causes skin lesions that develop in different parts of the skin, including the tail. Those lesions resemble papilloma-like structures and are displayed with an inflammatory gene expression profile. Our genetic studies demonstrate that this skin pathology is driven by TNFR1 mediated RIPK1 kinase-dependent cell death. Furthermore, RIPK3 and MLKL deficiency strongly protect mice lacking OTULIN in keratinocytes from skin inflammation, thereby identifying necroptosis as a key driver, whereas FADD-dependent apoptosis plays a minor role in the skin disease development. Additionally, MyD88 deficiency strongly delayed and ameliorated skin pathology, implying that microbiota-dependent stimuli contribute to the pathogenesis in OTULINE-KO mice. Taken together, the data presented in this study uncover a key role for OTULIN in the epidermis in preventing cell death and inflammation to maintain skin homeostasis

Autophosphorylation at serine 166 regulates RIP kinase 1-mediated cell death and inflammation

Receptor interacting protein kinase 1 (RIPK1) regulates cell death and inflammatory responses downstream of TNFR1 and other receptors, and has been implicated in the pathogenesis of inflammatory and degenerative diseases. RIPK1 kinase activity induces apoptosis and necroptosis, however the mechanisms and phosphorylation events regulating RIPK1-dependent cell death signaling remain poorly understood. Here we show that RIPK1 autophosphorylation at serine 166 plays a critical role for the activation of RIPK1 kinase-dependent apoptosis and necroptosis. Moreover, we show that S166 phosphorylation is required for RIPK1 kinase-dependent pathogenesis of inflammatory pathologies in vivo in four relevant mouse models. Mechanistically, we provide evidence that trans autophosphorylation at S166 modulates RIPK1 kinase activation but is not by itself sufficient to induce cell death. These results show that S166 autophosphorylation licenses RIPK1 kinase activity to induce downstream cell death signaling and inflammation, suggesting that S166 phosphorylation can serve as a reliable biomarker for RIPK1 kinase-dependent pathologies