Optineurin is colocalized with ubiquitin in Marinesco bodies

Autophagy-independent cytoprotection by optineurin from toxicity of aggregates formed by mutant huntingtin and mutant ataxin-3

An important feature of several neurodegenerative diseases is the formation of pathological structures containing aggregated proteins. The autophagy receptor optineurin/OPTN is frequently observed in these structures. The role played by optineurin in these aggregates is not clear. In this study, we explored whether optineurin has a cytoprotective role in the cells having mutant protein aggregates. We overexpressed mutant huntingtin having 97 glutamine repeats (mHtt), and mutant ataxin-3 having 130 glutamine repeats (mAtax-3) in wild-type and optineurin-deficient neuronal (N2A) and non-neuronal cells (Optn-/- mouse embryonic fibroblasts), and determined the percentage of dead cells with mutant protein aggregates. Optineurin-deficient cells having mHtt or mAtax-3 aggregates showed higher cell death as compared to wild-type cells having mutant protein aggregates. Confocal microscopy revealed that optineurin formed a shell around mHtt and mAtax-3 aggregates through its C-terminal domain. The C-terminal domain of optineurin, which lacks LC3-interacting region required for autophagy, was necessary and sufficient to reduce cytotoxicity of mHtt and mAtax-3 aggregates. Our results show that in the absence of optineurin, mutant protein aggregates are highly toxic, revealing an autophagy-independent cytoprotective function of optineurin, which is mediated by its C-terminal domain.

Dysfunction of Optineurin in Amyotrophic Lateral Sclerosis and Glaucoma

Neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS), frontotemporal dementia, and glaucoma, affect millions of people worldwide. ALS is caused by the loss of motor neurons in the spinal cord, brainstem, and brain, and genetic mutations are responsible for 10% of all ALS cases. Glaucoma is characterized by the loss of retinal ganglion cells and is the most common cause of irreversible blindness. Interestingly, mutations in OPTN, encoding optineurin, are associated with both ALS and glaucoma. Optineurin is a highly abundant protein involved in a wide range of cellular processes, including the inflammatory response, autophagy, Golgi maintenance, and vesicular transport. In this review, we summarize the role of optineurin in cellular mechanisms implicated in neurodegenerative disorders, including neuroinflammation, autophagy, and vesicular trafficking, focusing in particular on the consequences of expression of mutations associated with ALS and glaucoma. This review, therefore showcases the impact of optineurin dysfunction in ALS and glaucoma.

Mutations in the ubiquitin-binding domain of OPTN/optineurin interfere with autophagy-mediated degradation of misfolded proteins by a dominant-negative mechanism

OPTN (optineurin) is an autophagy receptor and mutations in the OPTN gene result in familial glaucoma (E50K) and amyotrophic lateral sclerosis (ALS) (E478G). However, the mechanisms through which mutant OPTN leads to human diseases remain to be characterized. Here, we demonstrated that OPTN colocalized with inclusion bodies (IBs) formed by mutant HTT/huntingtin protein (mHTT) in R6/2 transgenic mice and IBs formed by 81QNmHTT (nuclear form), 109QmHTT (cytoplasmic form) or the truncated form of TARDBP/TDP-43 (TARDBP(ND251)) in Neuro2A cells. This colocalization required the ubiquitin (Ub)-binding domain (UbBD, amino acids 424 to 511) of OPTN. Overexpression of wild-type (WT) OPTN decreased IBs through K63-linked polyubiquitin-mediated autophagy. E50K or 210 to 410Δ (with amino acids 210 to 410 deleted) whose mutation or deletion was outside the UbBD decreased the IBs formed by 109QmHTT or TARDBP(ND251), as was the case with WT OPTN. In contrast, UbBD mutants, including E478G, D474N, UbBDΔ, 411 to 520Δ and 210 to 520Δ, increased accumulation of IBs. UbBD mutants (E478G, UbBDΔ) retained a substantial ability to interact with WT OPTN, and were found to colocalize with polyubiquitinated IBs, which might occur indirectly through their WT partner in a WT-mutant complex. They decreased autophagic flux evidenced by alteration in LC3 level and turnover and in the number of LC3-positive puncta under stresses like starvation or formation of IBs. UbBD mutants exhibited a weakened interaction with MYO6 (myosin VI) and TOM1 (target of myb1 homolog [chicken]), important for autophagosome maturation, in cells or sorted 109QmHtt IBs. Taken together, our data indicated that UbBD mutants acted as dominant-negative traps through the formation of WT-mutant hybrid complexes to compromise the maturation of autophagosomes, which in turn interfered with OPTN-mediated autophagy and clearance of IBs.

Oligogenic inheritance of optineurin (OPTN) and C9ORF72 mutations in ALS highlights localisation of OPTN in the TDP-43-negative inclusions of C9ORF72-ALS

Amyotrophic lateral sclerosis (ALS) is characterized by motor neurone loss resulting in muscle weakness, spasticity and ultimately death. 5-10% are caused by inherited mutations, most commonly C9ORF72, SOD1, TARDBP and FUS. Rarer genetic causes of ALS include mutation of optineurin (mt OPTN). Furthermore, optineurin protein has been localized to the ubiquitylated aggregates in several neurodegenerative diseases, including ALS. This study: (i) investigated the frequency of mt OPTN in ALS patients in England; (ii) characterized the clinical and neuropathological features of ALS associated with a mt OPTN; and (iii) investigated optineurin neuropathology in C9ORF72-related ALS (C9ORF72-ALS). We identified a heterozygous p.E322K missense mutation in exon 10 of OPTN in one familial ALS patient who additionally had a C9ORF72 mutation. This patient had bulbar, limb and respiratory disease without cognitive problems. Neuropathology revealed motor neurone loss, trans-activation response DNA protein 43 (TDP-43)-positive neuronal and glial cytoplasmic inclusions together with TDP-43-negative neuronal cytoplasmic inclusions in extra motor regions that are characteristic of C9ORF72-ALS. We have demonstrated that both TDP-43-positive and negative inclusion types had positive staining for optineurin by immunohistochemistry. We went on to show that optineurin was present in TDP-43-negative cytoplasmic extra motor inclusions in C9ORF72-ALS cases that do not carry mt OPTN. We conclude that: (i) OPTN mutations are associated with ALS; (ii) optineurin protein is present in a subset of the extramotor inclusions of C9ORF72-ALS; (iii) It is not uncommon for multiple ALS-causing mutations to occur in the same patient; and (iv) studies of optineurin are likely to provide useful dataregarding the pathophysiology of ALS and neurodegeneration.

Loss of optineurin in vivo results in elevated cell death and alters axonal trafficking dynamics

Mutations in Optineurin have been associated with ALS, glaucoma, and Paget’s disease of bone in humans, but little is known about how these mutations contribute to disease. Most of the cellular consequences of Optineurin loss have come from in vitro studies, and it remains unclear whether these same defects would be seen in vivo. To answer this question, we assessed the cellular consequences of Optineurin loss in zebrafish embryos to determine if they showed the same defects as have been described in the in vitro studies. We found that loss of Optineurin resulted in increased cell death, as well as subtle cell morphology, cell migration and vesicle trafficking defects. However, unlike experiments on cells in culture, we found no indication that the Golgi apparatus was disrupted or that NF-κB target genes were upregulated. Therefore, we conclude that in vivo loss of Optineurin shows some, but not all, of the defects seen in in vitro work.

Age-related appearance of dendritic inclusions in catecholaminergic brainstem neurons

We identified p62-immunoreactive inclusions in dendrites of catecholaminergic brainstem projection neurons using antibodies against p62, ubiquitin, α-synuclein, hyperphosphorylated tau, and tyrosine hydroxylase in 100-μm sections through the brainstem dorsal vagal area, locus coeruleus, and substantia nigra of 149 autopsy cases staged for intraneuronal Alzheimer's and Parkinson's disease-associated lesions. The inclusions resembled Marinesco bodies within cell nuclei of catecholaminergic neurons as well as the dot-like structures previously described by Dickson in specific neuropil areas in humans. The p62-positive inclusions were confined to dendrites of catecholaminergic neurons, lacked neuromelanin granules, and were tau- and α-synuclein-negative. Their immunoreactivity for ubiquitin varied and their prevalence significantly increased with advancing age. The presence or absence of Alzheimer's and/or Parkinson's disease-associated pathology did not influence their existence. There was a strong association between the presence of p62-positive inclusions and Marinesco bodies (p < 0.0001). Our results reveal a hitherto unknown alteration within specific neuronal types of the human brainstem that may be independent of the sequestosome-ubiquitin-proteasomal pathway and unrelated to proteinaceous aggregate-formation of neurodegenerative diseases.