Non-catalytic ubiquitin binding by A20 prevents psoriatic arthritis–like disease and inflammation

Abstract

A20 is an anti-inflammatory protein that is strongly linked to human disease. Here we find that mice expressing three distinct targeted mutations of A20’s ZF7 ubiquitin binding motif uniformly developed digit arthritis that shares features with psoriatic arthritis, while mice expressing point mutations in A20’s OTU or ZF4 motifs did not exhibit this phenotype. Arthritis in A20 ZF7 mice required T cells and MyD88, was exquisitely sensitive to tumor necrosis factor (TNF) and interleukin 17A, and persisted in germ-free conditions. A20 ZF7 cells exhibited prolonged IKK kinase activity that drove exaggerated transcription of late-phase NF-κB-response genes in vitro and in pre-diseased mouse paws in vivo . In addition, mice expressing double-mutant A20 proteins in A20’s ZF4 and ZF7 motifs died perinatally with multi-organ inflammation. Therefore, A20’s ZF4 and ZF7 motifs synergistically prevent inflammatory disease in a non-catalytic manner.

An incoherent feedforward loop interprets NFκB/RelA dynamics to determine TNF-induced necroptosis decisions

“…Previous studies established A20 as an inhibitor of TNF-induced cell death [58][59][60][61][62]68 . Via its ubiquitin binding domain ZnF7, A20 is believed to stabilize M1-linked ubiquitin chains in TNFR1induced complex I, which may restrict complex II formation and thereby apoptosis and/or necroptosis as shown in MEFs 58 , macrophages 59 , and intestinal epithelial cells 61 .…”

“…Several A20-dependent mechanisms to limit TNF-induced cell death have previously been reported [58][59][60][61][62] . In complex I, A20 binds and stabilizes M1-ubiquitin chains, which may limit formation of death-inducing complex II 58,59,61,62 . In addition, A20 integrates into the downstream necrosome and restricts RIPK3 activation to limit necroptosis 60 .…”

A20: a master regulator of arthritis

“…3 b-c). More recently, it has been shown that A20 restricts Th17 cell expansion and arthritis through its ZnF7 motif [ 28 ].

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“…In vitro studies suggest that A20 restricts NF-κB signals via C103-based DUB activity, while in vivo, DUB activity does not play a role in directly inhibiting NF-κB and is dispensable for the regulation of NF-κB signaling [ 15 ]. Interestingly, a recent study showed that A20’s ZnF7 domain probably also directly regulates NF-κB signaling and that it is independent of A20’s C103-based deubiquitinating activity [ 28 ]. A mixture of K63-linked and M1-linked Ub chains exists in IKK complexes, which could be recognized by ZnF4 and ZnF7, resulting in the restriction of IKKγ activity.…”

“…A recent study even revealed that increased expression of type I interferon (IFN) was correlated with autoinflammatory disease activity in HA20 patients, while A20 was capable of restraining the type I IFN response, though the underlying mechanism needs further exploration [ 27 ]. The highlight of these studies is that the ZnF4 and ZnF7 ubiquitin-binding domains in A20 have been proven to be critical to the function of preventing inflammation-dependent arthritis [ 28 – 30 ], and exploration of the specific role of these domains has gradually progressed. For example, several mouse models have proven that the ZnF7 domain of A20 has a particular status in regulating spontaneous immune activation and preventing cell necroptosis; further ZnF4 plays a role in the process, as it has the property of acting synergistically with ZnF7 [ 28 , 29 ].…”