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

Abstract

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 .

ZBP1: A STARGᐰTE to decode the biology of Z-nucleic acids in disease

“…These findings provide evidence for the activation of ZBP1 by sensing Z-NAs. Importantly, deleting or mutating only the Zα2 domain of ZBP1 is sufficient to abolish its activation in both endogenous and infectious conditions (Jiao et al, 2020;Kesavardhana et al, 2020). Overall, these new studies provide genetic evidence that endogenous or viral RNAs which attain Z-NA conformation are sensed by ZBP1, leading to cell death and inflammation.…”

“…Discovery of these dynamics uncovered a previously unknown homeostatic regulatory function for RIPK1 in restricting endogenous, spontaneous ZBP1 activation. Several groups have recently explored how ZBP1 is activated in sterile conditions and proposed that Z-RNA acts as the ligand to activate ZBP1-mediated cell death and inflammation (Devos et al, 2020;Jiao et al, 2020;Kesavardhana et al, 2020;Wang et al, 2020). Understanding this pathway is essential to unlock key mechanisms, perhaps the sensing of Z-NAs, that prime skin inflammation and autoinflammatory diseases and identify specific signaling cascades for therapeutic targeting.…”

“…Understanding this pathway is essential to unlock key mechanisms, perhaps the sensing of Z-NAs, that prime skin inflammation and autoinflammatory diseases and identify specific signaling cascades for therapeutic targeting. Devos et al (2020) and other recent studies have identified that endogenous nucleic acid sensing is critical for ZBP1 activation to induce necroptosis, skin and colon inflammation, and perinatal lethality (Jiao et al, 2020;Kesavardhana et al, 2020;Wang et al, 2020). To understand ZBP1 activation mechanisms, Devos et al (2020) crossed Ripk1 EKO mice with ZBP1 knock-in mice, which contain mutations disrupting the nucleic acid binding of the ZBP1 Zα domains (N46A, Y50A, N122A, Y126A; Maelfait et al, 2017).…”

“…Thus, ZBP1 activation is cell intrinsic under sterile conditions, and endogenous nucleic acids may potentially be the ligands engaging its activation. Other recent studies also have shown that the Zα domains of ZBP1 are critical for triggering skin and bowel inflammation, the perinatal lethality of Ripk1 mRHIM mice, and IAV-induced PANoptosis via the assembly of a PANoptosome, suggesting a role for Z-NA sensing in regulating physiological functions (Table 1; Christgen et al, 2020;Jiao et al, 2020;Kesavardhana et al, 2020;Samir et al, 2020;Wang et al, 2020;Zhang et al, 2020). Under sterile conditions, double-stranded RNAs (dsRNAs) from endogenous retrovirus (ERV) elements bind ZBP1 and are implicated in its activation (Jiao et al, 2020;Wang et al, 2020).…”

The innate sensor ZBP1-IRF3 axis regulates cell proliferation in multiple myeloma

“…ZBP1 is one of a group of cellular DNA/RNA sensors that activate innate immunity in response to pathogen-derived or cellular nucleic acids (NA) [1][2][3] . Binding of double stranded Z-form RNA (dsRNA) to its Zα and Zβ domains is followed by a RHIM-RHIM domain interaction of ZBP1 with RIPK3 and induction of programmed cell death 4-6 . This process is antagonised by RIPK1, preventing for example, ZBP1-dependent cell death and inflammation in the developing skin 7,8 .…”