Optineurin (OPTN), also known as FIP2, HYPL, FIP-2, TFIIIA-INTP, HIP7 or NRP, is a multifunctional adaptor protein involved in various cellular processes. It contains several domains, including coiled-coil1 (CC1) domain and coiled-coil2 domain (CC2), a leucine zipper domain, an LC3-interacting region, and an ubiquitin (Ub)-binding domain (UBD), consisting of a ubiquitin-binding region of ABIN proteins and NEMO (UBAN) motif and a zinc finger (ZF) domain. OPTN functions through its ability to bind polyubiquitinated proteins via its C-terminal UBD with a preference for M1- and K63-linked polyUb chains (Gleason CE et al., 2011; Nakazawa S et al., 2016; Li F et al., 2018). OPTN has been implicated in regulating various pathways including autophagy, vesicle trafficking, innate immune signaling pathways, cell survival, and cell death (reviewed in Markovinovic A et al., 2017; Slowicka K & van Loo G 2018; Toth RP & Atkin JD 2018; Outlioua A et al., 2018).

In innate immune signaling pathways, OPTN controls the activation of type I interferons (IFNs) by regulating the activity of the serine/threonine kinase TBK1 (Gleason CE et al., 2011; Munitic I et al., 2013; Meena NP et al., 2016; Pourcelot M et al., 2016; Markovinovic A et al., 2018; O’Loughlin T et al., 2020). TBK1 is activated through phosphorylation and K63-linked polyubiquitination downstream of pattern recognition receptors (PRRs) such as Toll-like receptor 3 (TLR3), TLR4, or RIG-I-like receptors including DDX58 and IFIH1 (Fitzgerald et al. 2003; McWhirter et al., 2004; Sharma et al., 2003; Solis et al., 2007). Activated TBK1, in turn, phosphorylates and activates transcription factors IRF3 and IRF7, which are essential for the induction of type I IFNs.

This Reactome event shows interaction between OPTN and TBK1 downstream of the TLR4 signaling complex.

Coimmunoprecipitation assays show association between on OPTN and TBK1 in human and mouse cells (Morton S et al., 2008; Mankouri J et al., 2010; Gleason CE et al., 2011; Li F et al., 2016). Structural and biochemical studies reveal that the N-terminal region of OPTN interacts with the C-terminal domain of TBK1 (Morton S et al., 2008; Meena NP et al., 2016; Li F et al., 2016, 2018) with micromolar affinity (Li F et al., 2016). Both constitutive (Mankouri J et al., 2010; Gleason CE et al., 2011; Richter B et al., 2016) and stimuli-induced (Pourcelot M et al., 2016; Meena NP et al., 2016) interactions between OPTN and TBK1 have been observed. The functional UBD of OPTN is required for OPTN-mediated regulation of the TBK1:IRF3 signaling axis in human and mouse cells (Mankouri J et al. 2010; Gleason CE et al. 2011; Munitic I et al. 2013; Genin P et al. 2015; Meena NP et al., 2016; Pourcelot M et al., 2016; Markovinovic A et al., 2018; O’Loughlin T et al., 2020). Further, phosphorylation of OPTN by TBK1 at S177, S473 or S513 enhances the polyUb-binding function of OPTN (Richter B et al., 2016; Li F et al., 2018). Notably, TBK1-mediated phosphorylation of OPTN at S473 has been shown to broaden the binding specificity of OPTN to other polyUb chains including K48-linked (Li F et al., 2018). Additionally, the expression of OPTN is regulated by NF-kappa-B signaling (Sudhakar C et al., 2009) and is enhanced upon activation of either TLR3 or DDX58 or upon stimulation with tumor necrosis factor alpha (TNFα) in various human epithelial cells (Sudhakar C et al., 2009; Mankouri J et al., 2010; O’Loughlin T et al., 2020). These data suggest that a microbial infection leads to NF-kappa-B-mediated upregulation of OPTN, which directly interacts with TBK1. The C-terminal UBD of OPTN binds polyubiquitinated TBK1 enhancing the regulatory function of OPTN.

The role of OPTN in type I IFN responses remains controversial. While OPTN facilitates type I IFN responses by promoting TBK1 activation, IRF3 phosphorylation, and IFNβ production (Gleason CE et al., 2011; Munitic I et al., 2013; Slowicka K et al., 2016; Meena NP et al., 2016; Pourcelot M et al., 2016; Markovinovic A et al., 2018; Bakshi S et al., 2017), it can also have a negative impact on the PRR-induced TBK1-IRF3 signaling pathway (Mankouri J et al., 2010; Munitic I et al., 2013; Genin P et al., 2015; O’Loughlin T et al., 2020; Fukushi M et al., 2023).

The subcellular localization of OPTN has an influence on type I IFN and NF-kappa-B signaling pathway activity (Pourcelot M et al., 2016; O’Loughlin T et al., 2020).

Taken together, these data suggest that OPTN binds TBK1 to control induction of type I IFNs in response to microbial infection downstream of PRRs. Further research is needed to fully understand the physiological consequences of the TBK1:OPTN interaction.