Periodic Reporting for period 4 - NECROPTOSIS (Necroptosis in immunity, inflammation and autoimmunity induced by nucleic acid sensors)
Reporting period: 2023-04-01 to 2023-09-30
Host defence against viral infection relies on receptors that detect viral nucleic acids and trigger anti-viral immunity by inducing interferons and inflammatory cytokines and chemokines. The receptors detecting viral nucleic acids also have the capacity to recognise host DNA and RNA. Multiple mechanisms are in place to prevent the sensing of endogenous nucleic acids by the anti-viral nucleic acid receptors. Failure of these mechanisms can result in aberrant sensing of endogenous DNA and RNA by cytosolic nucleic acid sensors and the induction of potent interferon responses that cause severe diseases collectively described as type I interferonopathies. Understanding the mechanisms regulating the activation of interferon responses to endogenous nucleic acids will be pivotal for the development of better therapeutic approaches for these conditions. Z-nucleic acids are DNA and RNA with an alternative, left-handed double helix structure, as opposed to the classical Watson-Crick right handed double helix. Although biochemical evidence that Z-DNA and Z-RNA can be generated in vitro has been provided more than forty years ago, their physiological role has remained unknown largely because of the lack of experimental methodologies allowing studying the mechanisms regulating their formation and biological function in the context of an organism. Studying the proteins that sense them is currently the best approach to interrogate the function of Z nucleic acids in vivo. The overall objective of this project was to study the mechanisms of regulation and physiological role of Z-nucleic acids by focusing on the two proteins that can detect them, namely Adenosine Deaminase Acting on RNA 1 (ADAR1) and Z-DNA binding protein 1 (ZBP1). ZBP1 detects Z-nucleic acids and triggers an inflammatory type of cell death called necroptosis. ADAR1 deaminates dsRNA to prevent its recognition of by MDA5 and the induction of interferon responses. Our studies revealed that ZBP1 detects endogenous Z-nucleic acids and triggers necroptosis and inflammation in mice, providing experimental evidence that endogenous Z-nucleic acids are produced in mammalian tissues and induce a physiologically relevant response in vivo. Moreover, our work revealed an important interplay between ADAR1 and ZBP1 in the regulation of interferon responses. Collectively, our work within this project provided functional proof that Z-nucleic acids are generated in mammalian tissues and cause severe pathology by activating ZBP1-mediated cell death and type I interferon responses.
In this project, we studied how Z-nucleic acids may be implicated in the pathogenesis of inflammatory conditions. In particular, we focused on the role of Z-nucleic acid sensing by ZBP1 in the regulation of cell death and inflammation in vivo in mouse models of inflammatory diseases. We could show that ZBP1 is activated by binding to endogenous Z-nucleic acids and triggers keratinocyte necroptosis and inflammation in mice lacking RIPK1 in the epidermis or expressing RIPK1 with mutated RIP homotypic interaction motif (RHIM). In addition, we found that Z-nucleic acid-induced ZBP1-mediated necroptosis is under negative regulation by caspase-8 and that inhibition of caspase-8 triggers ZBP1-dependent inflammation in the intestine. These results provided the first experimental evidence that sensing of endogenous Z-nucleic acids by ZBP1 causes inflammation in vivo in relevant models of inflammatory diseases. Furthermore, we found that ZBP1 binds cellular dsRNA via its Z domains, identifying endogenous Z-RNA as a key ligand triggering ZBP1-mediated necroptosis. Computational analysis revealed that endogenous retroelement-derived transcripts constitute the majority of putatively dsRNA species in mouse skin, suggesting that endogenous retroelement-derived Z-RNA is the most likely ligand activating ZBP1 in vivo. Furthermore, we could show that ZBP1 acts in a Zalpha domain-dependent manner to induce the activation of RIPK3 and its substrate MLKL within the nucleus, suggesting that sensing of nuclear Z-RNA provides a potent trigger activating ZBP1-mediated cell death. Collectively, these results obtained during the first period of the project revealed an important role of ZBP1 in sensing endogenous Z-RNA likely derived from endogenous retroelements and inducing necroptosis and inflammation, which is relevant for the better understanding of the mechanisms controlling the pathogenesis of chronic inflammatory diseases.
In a separate line of experiments, we studied how Zalpha-dependent sensing of endogenous Z-RNA regulates the function of ADAR1 and its capacity to edit endogenous ERE-derived dsRNA to prevent their recognition by MDA5 and the induction of pathogenic type I interferon responses. We could show that hemizygous mutation of the ADAR1 Zalpha domain resulted in severe, early postnatally lethal, pathology in mice, which was driven by MDA5-MAVS-mediated signalling. In this mouse model of type I interferonopathy, we found that Zalpha-dependent ZBP1 signalling also contributes to the severe pathology induced by mutation of the ADAR1 Zalpha domain, revealing a critical interplay between ADAR1 and ZBP1 in the regulation of Z-RNA-dependent pathology. An unexpected finding in our studies was the ZBP1 drives the pathology of mice with hemizygous ADAR1 mutant Zalpha expression not by inducing cell death but by promoting type I interferon responses. Together, these results revealed a novel role of ZBP1 in sensing Z-RNA and promoting pathogenic type I interferon responses in a mouse model of type I interferonopathy induced by ADAR1 Zalpha mutation, suggesting that ZBP1 may also contribute to the pathology in human patients suffering from type I interferonopathies caused by ADAR1 mutations.
In a separate line of experiments, we studied how Zalpha-dependent sensing of endogenous Z-RNA regulates the function of ADAR1 and its capacity to edit endogenous ERE-derived dsRNA to prevent their recognition by MDA5 and the induction of pathogenic type I interferon responses. We could show that hemizygous mutation of the ADAR1 Zalpha domain resulted in severe, early postnatally lethal, pathology in mice, which was driven by MDA5-MAVS-mediated signalling. In this mouse model of type I interferonopathy, we found that Zalpha-dependent ZBP1 signalling also contributes to the severe pathology induced by mutation of the ADAR1 Zalpha domain, revealing a critical interplay between ADAR1 and ZBP1 in the regulation of Z-RNA-dependent pathology. An unexpected finding in our studies was the ZBP1 drives the pathology of mice with hemizygous ADAR1 mutant Zalpha expression not by inducing cell death but by promoting type I interferon responses. Together, these results revealed a novel role of ZBP1 in sensing Z-RNA and promoting pathogenic type I interferon responses in a mouse model of type I interferonopathy induced by ADAR1 Zalpha mutation, suggesting that ZBP1 may also contribute to the pathology in human patients suffering from type I interferonopathies caused by ADAR1 mutations.
Our findings showed that sensing of endogenous Z-nucleic acids by ZBP1 causes necroptosis and inflammation in vivo, providing the first experimental evidence that endogenous Z-nucleic acids have important functions in regulating immune homeostasis and contributing to disease pathogenesis. Our ongoing studies aim to unravel the role of necroptosis induced by endogenous Z-nucleic acids and other pathways in the pathogenesis of inflammatory and autoimmune diseases, focusing on understanding the underlying mechanisms and identifying novel therapeutic targets.