© 2020. Published by The Company of Biologists Ltd | Journal of Cell Science (2020) 133, jcs239822. doi:10.1242/jcs.239822 RESEARCH ARTICLE OPTN recruitment to a Golgi-proximal compartment regulates immune signalling and cytokine secretion Thomas O’Loughlin1,2,‡, Antonina J. Kruppa1, Andre L. R. Ribeiro3,4, James R. Edgar1, Abdulaziz Ghannam3, Andrew M. Smith3,* and Folma Buss1,*,‡ ABSTRACT appears to be a key protein in a range of pathways downstream of Optineurin (OPTN) is a multifunctional protein involved in autophagy TLR3, participating in the innate immune response through the and secretion, as well as nuclear factor κB (NF-κB) and IRF3 secretion of cytokines, acting as a selective autophagy receptor, and κ signalling, and OPTN mutations are associated with several human regulating both NF- B and IRF3 signalling (Slowicka et al., 2016). κ κ diseases. Here, we show that, in response to viral RNA, OPTN NF- B signalling centres around the NF- B transcription factor translocates to foci in the perinuclear region, where it negatively complex which, under non-stimulated conditions, is inhibited κ regulates NF-κB and IRF3 signalling pathways and downstream pro- through binding to I B proteins. In response to stimuli, such as inflammatory cytokine secretion. These OPTN foci consist of a tight TLR3 or RIG-I ligation, the pathway is switched on leading to cluster of small membrane vesicles, which are positive for ATG9A. activation of the IKK complex, composed of two kinase subunits α β Disease mutations in OPTN linked to primary open-angle glaucoma (IKK and IKK ; also known as CHUK and IKBKB, respectively) γ κ (POAG) cause aberrant foci formation in the absence of stimuli, which and a regulatory subunit IKK [IKBKG; also known as NF- B κ correlates with the ability of OPTN to inhibit signalling. By using essential modulator (NEMO)], which phosphorylates I B proteins proximity labelling proteomics, we identify the linear ubiquitin and triggers their subsequent degradation. This degradation releases κ assembly complex (LUBAC), CYLD and TBK1 as part of the OPTN the NF- B complex, allowing it to translocate to the nucleus and interactome and show that these proteins are recruited to this OPTN- induce expression of numerous target genes (Perkins, 2007). An positive perinuclear compartment. Our work uncovers a crucial role additional critical step in this pathway is the linear M1-linked for OPTN in dampening NF-κB and IRF3 signalling through ubiquitylation of NEMO and receptor-interacting protein kinase 1 the sequestration of LUBAC and other positive regulators in this (RIPK1) by the linear ubiquitin assembly complex (LUBAC), viral RNA-induced compartment, leading to altered pro-inflammatory which consists of HOIP (RNF31), HOIL1 (RBCK1) and SHARPIN cytokine secretion. (Gerlach et al., 2011; Kirisako et al., 2006; Tokunaga et al., 2009, 2011). These linear ubiquitin chains can then function as scaffolds KEY WORDS: OPTN, BioID, Functional proteomics, NF-κB, TBK1, to recruit the IKK complex through the ubiquitin binding in ABIN IFN, Linear ubiquitin, LUBAC, ATG9A and NEMO (UBAN) domain of the IKK subunit NEMO (Fujita et al., 2014; Rahighi et al., 2009; Wagner et al., 2008). OPTN is INTRODUCTION highly similar to NEMO, with around 52% sequence similarity, and Pathogen associated molecular patterns (PAMPs) are recognised by shares its linear ubiquitin-binding UBAN domain (Schwamborn pattern recognition receptors (PRRs), such as Toll-like receptors et al., 2000; Wagner et al., 2008). However, unlike NEMO, OPTN (TLRs), and trigger a range of adaptive and innate immune cannot bind to IKKα or IKKβ and therefore cannot rescue NF-κB responses in the host (Takeuchi and Akira, 2010). For example, activity in NEMO-deficient cells (Schwamborn et al., 2000). activation of TLR3 or RIG-I (also known as DDX58) by double- Instead, OPTN appears to antagonise NEMO function by stranded viral RNA activates signalling cascades culminating in the competitively binding to ubiquitylated RIPK1 and can thereby activation of transcription factors including nuclear factor κB inhibit TNFα-induced NF-κB activation (Zhu et al., 2007). In (NF-κB) and IRF3 and gene expression programs composed of addition, OPTN interacts with CYLD, a deubiquitylase (DUB) for pro-inflammatory cytokines (e.g. IL6) and interferons (IFNs), linear and K63 ubiquitin chains, which is able to negatively regulate respectively (Alexopoulou et al., 2001). Optineurin (OPTN) NF-κB signalling via the deubiquitylation of a range of NF-κB signalling proteins including NEMO and RIPK1 (Lork et al., 2017; Nagabhushana et al., 2011). 1Cambridge Institute for Medical Research, The Keith Peters Building, University of Cambridge, Hills Road, Cambridge CB2 0XY, UK. 2Helen Diller Family Alternatively, OPTN can bind to the IKK-related kinase TBK1 or Comprehensive Cancer Center, San Francisco, CA 94158, USA. 3Microbial the E3 ligase TRAF3 to regulate IRF3 activity (Mankouri et al., Diseases, Eastman Dental Institute, University College London, London WC1X 8LD, UK. 4Department of Oral and Maxillofacial Surgery, University Centre of Pará, 2010; Morton et al., 2008). A complex composed of TBK1 and Belém, Brazil. IKKε is activated via TRAF3 downstream of PRRs, such as TLR3 *These authors contributed equally to this work or RIG-I. Once active, the TBK1–IKKε complex can phosphorylate ‡Authors for correspondence ([email protected]; [email protected]) its substrate, IRF3, which subsequently dimerises and translocates to the nucleus to induce expression of target genes T.O., 0000-0002-4783-2352; F.B., 0000-0003-4457-3479 such as type I IFNs (IFNs). Through its interactions with both β This is an Open Access article distributed under the terms of the Creative Commons Attribution TBK1 and TRAF3, OPTN appears to attenuate IFN- production License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, (Mankouri et al., 2010). distribution and reproduction in any medium provided that the original work is properly attributed. An increasing number of perturbations in OPTN gene function Handling Editor: Daniel Billadeau have been linked to diseases including primary open-angle Received 30 September 2019; Accepted 14 April 2020 glaucoma (POAG), amyotrophic lateral sclerosis (ALS), Paget’s Journal of Cell Science 1 RESEARCH ARTICLE Journal of Cell Science (2020) 133, jcs239822. doi:10.1242/jcs.239822 disease of bone (PDB) and Crohn’s disease (CD) (Albagha et al., and 2′,3′-cGAMP (cGAMP) were unable to elicit the release of 2010; Maruyama et al., 2010; Rezaie et al., 2002; Smith et al., CXCL8 from RPE cells, illustrating a lack of activation downstream of 2015). A common feature of the role of OPTN in these diseases TLR4, TLR2 and STING (also known as STING1). To determine the appears to be aberrant NF-κB signalling or cytokine secretion complete secretory response of RPE cells downstream of poly(I:C) profiles. Many ALS mutants show a loss of OPTN-mediated NF-κB stimulation, we analysed conditioned medium from unstimulated or suppression (Nakazawa et al., 2016), deficiencies in OPTN poly(I:C)-stimulated RPE cells using quantitative SILAC mass expression increase NF-κB activity and susceptibility to PDB spectrometry. These experiments identified 380 proteins in the (Obaid et al., 2015) and a subset of CD patients with reduced OPTN conditioned medium with 26 showing significant (P<0.05) expression display impaired secretion of TNF-α, IL6 and IFN-γ upregulation (Fig. 1B; Table S1). Among the upregulated proteins (Smith et al., 2015). were well-known pro-inflammatory cytokines, such as CXCL8 and, In this study, we address the role of OPTN in innate immune to a lesser extent, IL6 (Fig. 1B). We validated this data by ELISA and signalling and cytokine secretion, and the mechanism by which found poly(I:C) stimulation resulted in the induction of both CXCL8 perturbation of OPTN function in these processes may contribute to and IL6 protein secretion (Fig. 1Ci,ii). To assess the contribution of human inflammatory disease. We use a retinal pigment epithelial NF-κB signalling in regulation of cytokine secretion, we generated an (RPE) cell model, which is relevant to the role of OPTN in the RPE cell line expressing a NF-κB luciferase reporter. We found that pathogenesis of POAG, and show these cells respond to TLR3 and stimulating these cells with poly(I:C) induced NF-κBpromoter RIG-I ligands, leading to upregulation of OPTN and its activity and a similar elevation in phosphorylated p65 (also known as translocation to perinuclear foci. Our ultrastructural analysis of RELA; an NF-κB subunit) was observed using immunoblot analysis these foci by correlative light and electron microscopy reveals that (Fig. 1Ciii,D). Although no IFNs were detected in the proteomics this compartment consists of a tight cluster of small vesicles, which datasets, we predicted that IRF3 signalling would also be active appear positive for the autophagy protein ATG9A. This downstream of TLR3 (Doyle et al., 2002). Indeed, upon poly(I:C) multispanning membrane protein is present at the Golgi complex stimulation, we observed a rapid phosphorylation of IRF3, an and in clusters of small 30–40 nm vesicles, which are often found in elevation in IFN-β (IFNB1) mRNA levels, and could detect IFNs in close proximity to autophagosomes, but do not appear to be the supernatant at 2 h post-stimulation (Fig. 1Civ,v,D). incorporated into the growing phagophore (Orsi et al., 2012; Young et al., 2006). We demonstrate that wild-type or mutant variants of OPTN translocates