Optineurin (OPTN) is a multifunctional, ubiquitin-binding adaptor protein which exhibits a high homology with IKBKG (NEMO, the regulatory subunit of the IkB kinase complex). OPTN has been implicated in regulating of various signaling pathways including NF-kappa-B activation, TBK1-mediated type I interferon production, programmed cell death, and autophagy (reviewed in Markovinovic A et al. 2017; Slowicka K & van Loo G 2018; Toth RP & Atkin JD 2018). In human and mouse cells, OPTN was identified as a negative regulator of the tumor necrosis factor α (TNF-α)-induced signaling pathway by targeting several downstream components including receptor-interacting serine/threonine protein kinase 1 (RIPK1), caspase 8 (CASP8), and ubiquitin (Ub) carboxyl-terminal hydrolase CYLD (Zhu G et al. 2007; Nagabhushana A et al. 2011; Nakazawa S et al. 2016). Direct association of OPTN with TNFR1 and RIPK1 was detected in TNF-α-stimulated human epithelial carcinoma KB cells and HeLa cells by coimmunoprecipitation suggesting that OPTN is recruited to the TNFR1 complex (Zhu G et al. 2007; Klingseisen L et al. 2012; Nakazawa S et al. 2016). Structural and biochemical studies showed that OPTN functions through its ability to bind ubiquitinated proteins such as RIPK1, and the UBAN domain of OPTN preferentially recognizes linear polyUb chains (Gleason CE et al. 2011; Nakazawa S et al. 2016; Li F et al. 2018). Although the highest affinity is seen for M1-linked polyUb chains, TBK1-mediated phosphorylation of OPTN on S473 has been shown to broaden its specificity to include K63- and K48-linked polyUb chains (Li F et al. 2018). The Ub-binding activity of OPTN is thought to downregulate the TNF-α mediated NF-kappa-B activation (Zhu G et al. 2007; Maruyama H et al. 2010; Nakazawa S et al. 2016) by suppressing a scaffold function of RIPK1 within the TNFR1 signaling complex (Zhu G et al. 2007; Nakazawa S et al. 2016). In addition, OPTN interacts with deubiquitinase CYLD, which acts as a negative regulator of the TNFR1-induced gene expression by hydrolyzing K63-, M1-linked polyUb chains of RIPK1. CoIP assay detected interaction of OPTN with CYLD upon co-expression of tagged proteins in HeLa cells (Nagabhushana A et al. 2011). Defective binding of the glaucoma-associated OPTN H486R variant to CYLD prevented CYLD-mediated inhibition of TNFα-induced NF-kappa-B activity upon co-expression of OPTN and CYLD in HeLa cells. A catalytically inactive CYLD variant co-immunoprecipitated polyUb-RIPK1 along with OPTN in HeLa cells. Further, OPTN knockdown prevented RIPK1 deubiquitination by overexpressed CYLD in TNF-α-induced HeLa cells (Nagabhushana A et al. 2011). The data suggest that OPTN directly interacts with CYLD and RIPK1 to mediate deubiquitination of RIPK1 by CYLD within the TNFR1 signaling complex thus suppressing the NF-kappa-B signaling pathway. However, the OPTN regulatory function on NF-kappa-B signaling observed in vitro has not been confirmed in vivo suggesting that the mechanism of OPTN-mediated regulation is indirect and/or depends on the NF-kappa-B-mediated upregulation of OPTN (Sudhakar C et al. 2009; Gleason CE et al. 2011; Munitic I et al. 2013; Slowicka K et al. 2016; Markovinovic A et al. 2018).

Similar to mutations in the OPTN gene, CYLD mutations have been linked to the development of neurodegenerative diseases (Dobson-Stone C et al. 2020; Gu X et al. 2021; Xiao X et al. 2022). For example, the missense CYLD mutation M719V was identified in a family with autosomal dominant inheritance of frontotemporal dementia and amyotrophic lateral sclerosis (FTD-ALS) (Dobson-Stone C et al. 2020). When overexpressed in HEK293 cells, CYLD M719V has been shown to increase deubiquitinating activity of CYLD, leading to enhanced inhibition of the NF-kappa-B pathway, suggesting that this gain-of-function mechanism of action may contribute to the development of FTD-ALS (Dobson-Stone C et al. 2020).

This Reactome event shows CYLD association with OPTN bound to the TNFR1 signaling complex.