Preserving immune homeostasis with A20.

A20/TNFAIP3 is a TNF induced gene that plays a profound role in preserving cellular and organismal homeostasis (Lee, et al., 2000; Opipari etal., 1990).

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

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 zinc finger 7 (ZF7) ubiquitin-binding motif uniformly developed digit arthritis with features common to psoriatic arthritis, while mice expressing point mutations in A20's OTU or ZF4 motifs did not exhibit this phenotype. Arthritis in A20 mice required T cells and MyD88, was exquisitely sensitive to tumor necrosis factor and interleukin-17A, and persisted in germ-free conditions.

A20 in dendritic cells restrains intestinal anti-bacterial peptide expression and preserves commensal homeostasis.

Microbial dysbiosis commonly occurs in patients with inflammatory bowel diseases (IBD). Exogenous causes of dysbiosis such as antibiotics and diet are well described, but host derived causes are understudied. A20 is a potent regulator of signals triggered by microbial pattern molecules, and A20 regulates susceptibility to intestinal inflammation in mice and in humans.

A20: A multifunctional tool for regulating immunity and preventing disease.

A20, also known as TNFAIP3, is a potent regulator of ubiquitin (Ub) dependent signals. A20 prevents multiple human diseases, indicating that the critical functions of this protein are clinically as well as biologically impactful. As revealed by mouse models, cell specific functions of A20 are linked to its ability to regulate diverse signaling pathways.

OTUB1 non-catalytically stabilizes the E2 ubiquitin-conjugating enzyme UBE2E1 by preventing its autoubiquitination.

OTUB1 is a deubiquitinating enzyme that cleaves Lys-48-linked polyubiquitin chains and also regulates ubiquitin signaling through a unique, noncatalytic mechanism. OTUB1 binds to a subset of E2 ubiquitin-conjugating enzymes and inhibits their activity by trapping the E2∼ubiquitin thioester and preventing ubiquitin transfer. The same set of E2s stimulate the deubiquitinating activity of OTUB1 when the E2 is not charged with ubiquitin.

A20 and ABIN-1 synergistically preserve intestinal epithelial cell survival.

A20 () and ABIN-1 () are candidate susceptibility genes for inflammatory bowel disease and other autoimmune or inflammatory diseases, but it is unclear how these proteins interact in vivo to prevent disease. Here we show that intestinal epithelial cell (IEC)-specific deletion of either A20 or ABIN-1 alone leads to negligible IEC loss, whereas simultaneous deletion of both A20 and ABIN-1 leads to rapid IEC death and mouse lethality. Deletion of both A20 and ABIN-1 from enteroids causes spontaneous cell death in the absence of microbes or hematopoietic cells.

Ubiquitin-Modifying Enzymes and Regulation of the Inflammasome.

Ubiquitin and ubiquitin-modifying enzymes play critical roles in a wide variety of intracellular signaling pathways. Inflammatory signaling cascades downstream of TNF, TLR agonists, antigen receptor cross-linking, and cytokine receptors, all rely on ubiquitination events to direct subsequent immune responses. In the past several years, inflammasome activation and subsequent signal transduction have emerged as an excellent example of how ubiquitin signals control inflammatory responses.

Hypomorphic A20 expression confers susceptibility to psoriasis.

Psoriasis is a common inflammatory skin disease that affects approximately 1% of the population worldwide. Tumor necrosis factor-alpha-induced protein 3 (TNFAIP3) gene polymorphisms have been strongly associated with psoriasis susceptibility. In this study, we investigate how TNFAIP3, also known as A20, may regulate psoriasis susceptibility.

The Ubiquitin Binding Protein TAX1BP1 Mediates Autophagasome Induction and the Metabolic Transition of Activated T Cells.

During immune responses, naive T cells transition from small quiescent cells to rapidly cycling cells. We have found that T cells lacking TAX1BP1 exhibit delays in growth of cell size and cell cycling. TAX1BP1-deficient T cells exited G but stalled in S phase, due to both bioenergetic and biosynthetic defects.

The ubiquitin-modifying enzyme A20 restricts ubiquitination of the kinase RIPK3 and protects cells from necroptosis.

A20 is an anti-inflammatory protein linked to multiple human diseases; however, the mechanisms by which A20 prevents inflammatory disease are incompletely defined. We found that A20-deficient T cells and fibroblasts were susceptible to caspase-independent and kinase RIPK3-dependent necroptosis. Global deficiency in RIPK3 significantly restored the survival of A20-deficient mice.

A20 restricts ubiquitination of pro-interleukin-1β protein complexes and suppresses NLRP3 inflammasome activity.

Inappropriate inflammasome activation contributes to multiple human diseases, but the mechanisms by which inflammasomes are suppressed are poorly understood. The NF-κB inhibitor A20 is a ubiquitin-modifying enzyme that might be critical in preventing human inflammatory diseases. Here, we report that A20-deficient macrophages, unlike normal cells, exhibit spontaneous NLRP3 inflammasome activity to LPS alone.

Cutting edge: ABIN-1 protects against psoriasis by restricting MyD88 signals in dendritic cells.

Psoriasis is a chronic, inflammatory skin disease caused by a combination of environmental and genetic factors. The Tnip1 gene encodes A20 binding and inhibitor of NF-κB-1 (ABIN-1) protein and is strongly associated with susceptibility to psoriasis in humans. ABIN-1, a widely expressed ubiquitin-binding protein, restricts TNF- and TLR-induced signals.

A20 restricts wnt signaling in intestinal epithelial cells and suppresses colon carcinogenesis.

Colon carcinogenesis consists of a multistep process during which a series of genetic and epigenetic adaptations occur that lead to malignant transformation. Here, we have studied the role of A20 (also known as TNFAIP3), a ubiquitin-editing enzyme that restricts NFκB and cell death signaling, in intestinal homeostasis and tumorigenesis. We have found that A20 expression is consistently reduced in human colonic adenomas than in normal colonic tissues.

Dimerization and ubiquitin mediated recruitment of A20, a complex deubiquitinating enzyme.

A20 is an anti-inflammatory protein linked to multiple human autoimmune diseases and lymphomas. A20 possesses a deubiquitinating motif and a zinc finger, ZF4, that binds ubiquitin and supports its E3 ubiquitin ligase activity. To understand how these activities mediate A20's physiological functions, we generated two lines of gene-targeted mice, abrogating either A20's deubiquitinating activity (Tnfaip3(OTU) mice) or A20's ZF4 (Tnfaip3(ZF4) mice).

A20: linking a complex regulator of ubiquitylation to immunity and human disease.

A20 (also known as TNFAIP3) is a potent anti-inflammatory signalling molecule that restricts multiple intracellular signalling cascades. Recent studies in three general areas have converged to highlight the clinical and biological importance of A20. First, human genetic studies have strongly linked polymorphisms and mutations in the gene encoding A20 to inflammatory, autoimmune and malignant diseases.

Expression of A20 by dendritic cells preserves immune homeostasis and prevents colitis and spondyloarthritis.

Dendritic cells (DCs), which are known to support immune activation during infection, may also regulate immune homeostasis in resting animals. Here we show that mice lacking the ubiquitin-editing molecule A20 specifically in DCs spontaneously showed DC activation and population expansion of activated T cells. Analysis of DC-specific epistasis in compound mice lacking both A20 and the signaling adaptor MyD88 specifically in DCs showed that A20 restricted both MyD88-independent signals, which drive activation of DCs and T cells, and MyD88-dependent signals, which drive population expansion of T cells.

Ubiquitin makes its mark on immune regulation.

Ubiquitination, the covalent attachment of ubiquitin molecules to proteins, is emerging as a widely utilized mechanism for rapidly regulating cell signaling. Recent studies indicate that ubiquitination plays potent roles in regulating a variety of signals in both innate and adaptive immune cells. Here, we will review recent studies of ubiquitin ligases, ubiquitin chain linkages, and ubiquitin binding proteins that highlight the diversity and specificity of ubiquitin dependent functions in immune cells.

The ubiquitin modifying enzyme A20 restricts B cell survival and prevents autoimmunity.

A20 is a ubiquitin modifying enzyme that restricts NF-kappaB signals and protects cells against tumor necrosis factor (TNF)-induced programmed cell death. Given recent data linking A20 (TNFAIP3) with human B cell lymphomas and systemic lupus erythematosus (SLE), we have generated mice bearing a floxed allele of Tnfaip3 to interrogate A20's roles in regulating B cell functions. A20-deficient B cells are hyperresponsive to multiple stimuli and display exaggerated NF-kappaB responses to CD40-induced signals.

Macrophage- and dendritic-cell-derived interleukin-15 receptor alpha supports homeostasis of distinct CD8+ T cell subsets.

Interleukin-15 receptor alpha (IL-15R alpha) is a pleiotropically expressed molecule that chaperones and trans-presents IL-15 to NK and T cells. To investigate whether IL-15R alpha presented by different cells perform distinct physiological functions, we have generated four lines of mice lacking IL-15R alpha in various cell types. We find that IL-15R alpha expression on macrophages but not dendritic cells (DCs) supports the early transition of antigen specific effector CD8(+) T cells to memory cells.

A20 takes on tumors: tumor suppression by an ubiquitin-editing enzyme.

Many B cell cancers are characterized in part by the dysregulation of the NF-kappaB signaling pathway. A new study identifies somatic mutations in TNFAIP3, the gene encoding the NF-kappaB inhibitor A20, in Hodgkin lymphomas and primary mediastinal lymphomas. These data reveal the role of A20 as a tumor suppressor protein.

ABIN-1 is a ubiquitin sensor that restricts cell death and sustains embryonic development.

Proteins that directly regulate tumour necrosis factor receptor (TNFR) signalling have critical roles in regulating cellular activation and survival. ABIN-1 (A20 binding and inhibitor of NF-kappaB) is a novel protein that is thought to inhibit NF-kappaB signalling. Here we show that mice deficient for ABIN-1 die during embryogenesis with fetal liver apoptosis, anaemia and hypoplasia.

The ubiquitin-editing enzyme A20 restricts nucleotide-binding oligomerization domain containing 2-triggered signals.

Muramyl dipeptide (MDP), a product of bacterial cell-wall peptidoglycan, activates innate immune cells by stimulating nucleotide-binding oligomerization domain containing 2 (NOD2) -dependent activation of the transcription factor NFkappaB and transcription of proinflammatory genes. A20 is a ubiquitin-modifying enzyme that restricts tumor necrosis factor (TNF) receptor and Toll-like receptor (TLR) -induced signals. We now show that MDP induces ubiquitylation of receptor- interacting protein 2 (RIP2) in primary macrophages.