Functional analysis of optineurin and some of its disease-associated mutants

Abstract

Optineurin is a multifunctional protein involved in a variety of cellular functions such as protein trafficking by vesicles, autophagy, and signal transduction. Certain mutations in optineurin (gene OPTN) are associated with neurodegenerative diseases like glaucoma and amyotrophic lateral sclerosis (ALS). Optineurin is also seen in pathological structures present in several other neurodegenerative diseases. In glaucoma, loss of vision occurs due to progressive degeneration of retinal ganglion cells, and perhaps loss of photoreceptor cone cells as well. Most of the glaucoma-associated mutations of optineurin are heterozygous missense mutations, whereas the ALS-associated mutations include deletion, truncation, and missense mutations. Optineurin mediates its functions by interacting with various proteins, often acting as an adaptor to provide a link between two or more proteins. Disease-causing mutations alter these interactions leading to functional defects in membrane vesicle trafficking, autophagy, signaling, aggregate formation, and other processes. Some of these functional defects, caused by glaucomaassociated mutants of optineurin, led to retinal cell death mediated by apoptosis and therefore may contribute to pathogenesis directly. Other mutations are likely to cause glaucoma by indirect mechanisms involving other cell types. Mechanisms of ALS pathogenesis by optineurin mutations are yet to be investigated in detail; however, some ALSassociated mutants cause defects in signaling, autophagy, and ubiquitin binding, which might contribute to pathogenesis. V C 2015 IUBMB Life, 67(2): [120][121][122][123][124][125][126][127][128] 2015

Linear ubiquitination is involved in the pathogenesis of optineurin-associated amyotrophic lateral sclerosis

“…However, the domains and pathways involved in the pathogenesis of OPTN-associated diseases still remain unclear. At present, missense mutations of OPTN , such as H26D, E50K, M98K, E322K, H486R and R545Q, were linked to POAG, and R96L, Q165X, Q398X, Q454E and E478G (X denotes a stop codon) mutations were identified from familial ALS patients1229 (Fig. 1a).…”

“…This suggested that impaired NF-κB regulation due to OPTN mutations is correlated with ALS pathogenesis. In OPTN-associated POAG, OPTN mutants such as E50K and M98K induce defects in vesicle trafficking and autophagy, and increase retinal ganglion cell death by apoptosis29. In contrast, in OPTN-associated ALS, such as the E478G mutation of OPTN , perturbations in NF-κB signalling, IRF3 activation, autophagy, mitophagy and intracellular trafficking have been reported1229.…”

“…In OPTN-associated POAG, OPTN mutants such as E50K and M98K induce defects in vesicle trafficking and autophagy, and increase retinal ganglion cell death by apoptosis29. In contrast, in OPTN-associated ALS, such as the E478G mutation of OPTN , perturbations in NF-κB signalling, IRF3 activation, autophagy, mitophagy and intracellular trafficking have been reported1229. A recent study using OPTN -KO mice indicated that the deficiency of OPTN affects IFN signalling and Salmonella -induced autophagy, but not NF-κB signalling39.…”

Identification of the Stress Which Causes Optineurin Aggregation

“…A previous study found that OPTN E50K , a mutated form of OPTN, aggregates, and that this aggregation might cause the death of RGCs [45]. In this study, we [48] and [49].…”

“…To investigate the mechanism underlying OPTN aggregation, we first evaluated autophagic flux and TBK1 expression, because OPTN E50K impairs autophagolysosome and interacts strongly with TBK1, leading to aggregation of OPTN [45]. We found that bafilomycin increased the ratio of LC3B-II/LC3B-I (autophagosome markers, Figure 2 expression, suggesting that they had no influence on autophagic flux in this assay (Figures 2(a)-(c)).…”