Z-DNA Binding Protein-1 (ZBP1) is a cytoplasmic innate immune sensor, which contains nucleic acid binding domains (Za) and RIP Homotypic Interaction Motif (RHIM). ZBP1 mediates host defense against some viruses by sensing viral nucleic acids. Here we have identified a pro-inflammatory role of ZBP1 in inducing necroptosis-mediated skin inflammation in mice.
RIPK1 controls inflammation and cell death and is important to maintain tissue homeostasis. Keratinocyte-specific deletion of RIPK1 (RIPK1E-KO) leads to progressive inflammatory skin disease in mice, which is characterized by epidermal hyper-proliferation, infiltration of immune cells and up-regulation of pro-inflammatory cytokines. Genetic deletion of RIPK3/MLKL, which are essential for necroptosis, completely protected the inflammatory skin lesions in RIPK1E-KO mice (Nature, 2014), showing necroptosis as a major trigger of skin inflammation. However, the mechanism by which RIPK1 counteracts RIPK3-MLKL-dependent inflammation remains unknown. We observed up-regulation of ZBP1 protein in the epidermis and reasoned that ZBP1 may regulate inflammation in RIPK1E-KO mice. To address the specific role of ZBP1 and its Za and RHIM domains, we generated RIPK1E-KO mice lacking ZBP1 (RIPK1E-KOZbp1-/-) or having mutation in the Za domain (RIPK1E-KOZbp1mZa2/mZa2) or RHIM domain (RIPK1E-KOZbp1mR1/mR1). Remarkably, inhibition of ZBP1 or its Za- or RHIM- mediated function prevented the development of inflammatory skin lesions, and inhibited the up-regulation of pro-inflammatory factors in the skin of RIPK1E-KO mice, demonstrating that endogenous nucleic acid sensing and RHIM-domain of ZBP1 trigger skin inflammation in RIPK1E-KO mice. Additionally, we detected endogenous retroelements-derived complementary reads in the epidermal RNA, and treatment with a combination of reverse transcriptase inhibitors ameliorated the skin inflammation in RIPK1E-KO mice, suggesting that double-stranded RNA derived from retroelements may trigger Za-domain-dependent activation of ZBP1 (Nature, 2016; Nature, 2020).
Collectively, we identified that epidermal RIPK1 counteracts RIPK3/MLKL-dependent necroptosis by inhibiting Za-dependent sensing of endogenous nucleic acid and RHIM-dependent function to prevent skin inflammation.