ABIN1 is a signal-induced autophagy receptor that attenuates NF-κB activation by recognizing linear ubiquitin chains

ABIN1 is a negative regulator of effector functions in cytotoxic T cells

T cells are pivotal in the adaptive immune defense, necessitating a delicate balance between robust response against infections and self-tolerance. Their activation involves intricate cross-talk among signaling pathways triggered by the T-cell antigen receptors (TCR) and co-stimulatory or inhibitory receptors. The molecular regulation of these complex signaling networks is still incompletely understood. Here, we identify the adaptor protein ABIN1 as a component of the signaling complexes of GITR and OX40 co-stimulation receptors. T cells lacking ABIN1 are hyper-responsive ex vivo, exhibit enhanced responses to cognate infections, and superior ability to induce experimental autoimmune diabetes in mice. ABIN1 negatively regulates p38 kinase activation and late NF-κB target genes. P38 is at least partially responsible for the upregulation of the key effector proteins IFNG and GZMB in ABIN1-deficient T cells after TCR stimulation. Our findings reveal the intricate role of ABIN1 in T-cell regulation.

STING induces LUBAC-mediated synthesis of linear ubiquitin chains to stimulate innate immune signaling

STING activation by cyclic dinucleotides in mammals induces interferon- and NFκB -related gene expression, and the lipidation of LC3B at Golgi membranes. While mechanisms of the interferon response are well understood, the mechanisms of NFκB activation mediated by STING remain unclear. We report that STING activation induces K63- and M1-linked/linear ubiquitin chain formation at LC3B-associated Golgi membranes. Loss of the LUBAC E3 ubiquitin ligase prevents formation of linear, but not K63-linked ubiquitin chains or STING activation and inhibits STING-induced NFκB and IRF3-mediated signaling in monocytic THP1 cells. The proton channel activity of STING is also important for both K63 and linear ubiquitin chain formation, and NFκB- and interferon-related gene expression. Thus, LUBAC synthesis of linear ubiquitin chains regulates STING-mediated innate immune signaling.

Repressive Control of Keratinocyte Cytoplasmic Inflammatory Signaling

The overactivity of keratinocyte cytoplasmic signaling contributes to several cutaneous inflammatory and immune pathologies. An important emerging complement to proteins responsible for this overactivity is signal repression brought about by several proteins and protein complexes with the native role of limiting inflammation. The signaling repression by these proteins distinguishes them from transmembrane receptors, kinases, and inflammasomes, which drive inflammation. For these proteins, defects or deficiencies, whether naturally arising or in experimentally engineered skin inflammation models, have clearly linked them to maintaining keratinocytes in a non-activated state or returning cells to a post-inflamed state after a signaling event. Thus, together, these proteins help to resolve acute inflammatory responses or limit the development of chronic cutaneous inflammatory disease. We present here an integrated set of demonstrated or potentially inflammation-repressive proteins or protein complexes (linear ubiquitin chain assembly complex [LUBAC], cylindromatosis lysine 63 deubiquitinase [CYLD], tumor necrosis factor alpha-induced protein 3-interacting protein 1 [TNIP1], A20, and OTULIN) for a comprehensive view of cytoplasmic signaling highlighting protein players repressing inflammation as the needed counterpoints to signal activators and amplifiers. Ebb and flow of players on both sides of this inflammation equation would be of physiological advantage to allow acute response to damage or pathogens and yet guard against chronic inflammatory disease. Further investigation of the players responsible for repressing cytoplasmic signaling would be foundational to developing new chemical-entity pharmacologics to stabilize or enhance their function when clinical intervention is needed to restore balance.

TNIP1 inhibits selective autophagy via bipartite interaction with LC3/GABARAP and TAX1BP1

Mitophagy is a form of selective autophagy that disposes of superfluous and potentially damage-inducing organelles in a tightly controlled manner. While the machinery involved in mitophagy induction is well known, the regulation of the components is less clear. Here, we demonstrate that TNIP1 knockout in HeLa cells accelerates mitophagy rates and that ectopic TNIP1 negatively regulates the rate of mitophagy. These functions of TNIP1 depend on an evolutionarily conserved LIR motif as well as an AHD3 domain, which are required for binding to the LC3/GABARAP family of proteins and the autophagy receptor TAX1BP1, respectively. We further show that phosphorylation appears to regulate its association with the ULK1 complex member FIP200, allowing TNIP1 to compete with autophagy receptors, which provides a molecular rationale for its inhibitory function during mitophagy. Taken together, our findings describe TNIP1 as a negative regulator of mitophagy that acts at the early steps of autophagosome biogenesis.

A20 binding and inhibitor of nuclear factor kappa B (NF-κB)-1 (ABIN-1): a novel modulator of mitochondrial autophagy

A20 binding inhibitor of nuclear factor kappa B (NF-κB)-1 (ABIN-1), a polyubiquitin-binding protein, is a signal-induced autophagy receptor that attenuates NF-κB-mediated inflammation and cell death. The present study aimed to elucidate the potential role of ABIN-1 in mitophagy, a biological process whose outcome is decisive in diverse physiological and pathological settings. Microtubule-associated proteins 1A/1B light chain 3B-II (LC3B-II) was found to be in complex with ectopically expressed hemagglutinin (HA)-tagged-full length (FL)-ABIN-1. Bacterial expression of ABIN-1 and LC3A and LC3B showed direct binding of ABIN-1 to LC3 proteins, whereas mutations in the LC3-interacting region (LIR) 1 and 2 motifs of ABIN-1 abrogated ABIN-1/LC3B-II complex formation. Importantly, induction of autophagy in HeLa cells resulted in colocalization of ABIN-1 with LC3B-II in autophagosomes and with lysosomal-associated membrane protein 1 (LAMP-1) in autophagolysosomes, leading to degradation of ABIN-1 with p62. Interestingly, ABIN-1 was found to translocate to damaged mitochondria in HeLa-mCherry-Parkin transfected cells. In line with this observation, clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9)-mediated deletion of ABIN-1 significantly inhibited the degradation of the mitochondrial outer membrane proteins voltage-dependent anion-selective channel 1 (VDAC-1), mitofusin-2 (MFN2), and translocase of outer mitochondrial membrane (TOM)20. In addition, short interfering RNA (siRNA)-mediated knockdown of ABIN-1 significantly decreased lysosomal uptake of mitochondria in HeLa cells expressing mCherry-Parkin and the fluorescence reporter mt-mKEIMA. Collectively, our results identify ABIN-1 as a novel and selective mitochondrial autophagy regulator that promotes mitophagy, thereby adding a new player to the complex cellular machinery regulating mitochondrial homeostasis.

TBK1 phosphorylation activates LIR-dependent degradation of the inflammation repressor TNIP1

Limitation of excessive inflammation due to selective degradation of pro-inflammatory proteins is one of the cytoprotective functions attributed to autophagy. In the current study, we highlight that selective autophagy also plays a vital role in promoting the establishment of a robust inflammatory response. Under inflammatory conditions, here TLR3-activation by poly(I:C) treatment, the inflammation repressor TNIP1 (TNFAIP3 interacting protein 1) is phosphorylated by Tank-binding kinase 1 (TBK1) activating an LIR motif that leads to the selective autophagy-dependent degradation of TNIP1, supporting the expression of pro-inflammatory genes and proteins. This selective autophagy efficiently reduces TNIP1 protein levels early (0-4 h) upon poly(I:C) treatment to allow efficient initiation of the inflammatory response. At 6 h, TNIP1 levels are restored due to increased transcription avoiding sustained inflammation. Thus, similarly as in cancer, autophagy may play a dual role in controlling inflammation depending on the exact state and timing of the inflammatory response.