Z-nucleic acid sensing triggers ZBP1-dependent necroptosis and inflammation

Z-DNA and Z-RNA are left-handed double helix nucleic acid structures with poorly understood biological function(1–3). Z-DNA binding protein 1 (ZBP1, also known as DAI or DLM-1) is a nucleic acid sensor containing two Zα domains that bind Z-DNA(4,5) and Z-RNA(6–8). ZBP1 mediates host-defence against certain viruses(6,7,9–14) by sensing viral nucleic acids(6,7,10). RIPK1 deficiency or mutation of its RIP homotypic interaction motif (RHIM) triggers ZBP1-dependent necroptosis and inflammation in mice(15,16), however, the mechanisms inducing ZBP1 activation in the absence of viral infection remain elusive. Here we show that Zα-dependent sensing of endogenous ligands induces ZBP1-mediated perinatal lethality in mice expressing RIPK1 with mutated RHIM (Ripk1(mR/mR)) and skin inflammation in mice with epidermis-specific RIPK1 deficiency (RIPK1(E-KO)), as well as colitis in mice with intestinal epithelial-specific FADD deficiency (FADD(IEC-KO)). Consistently, functional Zα domains were required for ZBP1-induced necroptosis in fibroblasts that express RIPK1 with mutated RHIM or were treated with caspase inhibitors. Moreover, inhibition of nuclear export triggered Zα-dependent activation of RIPK3 in the nucleus resulting in cell death, suggesting that ZBP1 may recognise Z-form nucleic acids (Z-NA) in the nucleus. We found that ZBP1 constitutively bound cellular double stranded RNA (dsRNA) in a Zα-dependent manner. Furthermore, endogenous retroelement (ERE)-derived complementary reads were detected in epidermal RNA, suggesting that ERE-derived dsRNA may act as Zα domain ligand triggering ZBP1 activation. Collectively, our results provide evidence that sensing of endogenous Z-NA by ZBP1 triggers RIPK3-dependent necroptosis and inflammation, which could underlie the development of chronic inflammatory conditions particularly in patients with mutations in the RIPK1 and CASPASE-8 genes(17–20).