Optineurin in Huntington's disease intranuclear inclusions

Emerging role and mechanism of HACE1 in the pathogenesis of neurodegenerative diseases: A promising target

HACE1 is a member of the HECT domain-containing E3 ligases with 909 amino acid residues, containing N-terminal ankyrin-repeats (ANK) and C-terminal HECT domain. Previously, it was shown that HACE1 is inactive in human tumors and plays a crucial role in the initiation, progression, and invasion of malignant tumors. Recent studies indicated that HACE1 might be closely involved in neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and Huntington's disease. HACE1 interacts with its substrates, including Ras-related C3 botulinum toxin substrate 1 (Rac1), nuclear factor erythroid 2-related factor 2 (Nrf2), tumor necrosis factor receptor (TNFR), and optineurin (OPTN), through which participates in several pathophysiological processes, such as oxidative stress, autophagy and inflammation. Therefore, in this review, we elaborately describe the essential substrates of HACE1 and illuminate the pathophysiological processes by which HACE1 is involved in neurodegenerative diseases. We provide a new molecular target for neurodegenerative diseases.

[Optineurin and mitochondrial dysfunction in neurodegeneration]

Optineurin (OPTN) is a multifunctional protein playing a crucial role as a receptor in selective autophagy. OPTN gene mutations are linked to diseases such as normal-tension glaucoma and amyotrophic lateral sclerosis. Recognized as a critical receptor for mitophagy, OPTN is pivotal in selectively degrading damaged mitochondria. This process is essential to prevent their accumulation, the generation of reactive oxygen species, and the release of pro-apoptotic factors. Mitophagy's quality control is governed by the PINK1 kinase and the cytosolic ubiquitin ligase Parkin, whose mutations are associated with Parkinson's disease. This review highlights recent insights emphasizing OPTN's role in mitophagy and its potential involvement in neurodegenerative diseases.

Autophagy disruption reduces mTORC1 activation leading to retinal ganglion cell neurodegeneration associated with glaucoma

Autophagy dysfunction has been associated with several neurodegenerative diseases including glaucoma, characterized by the degeneration of retinal ganglion cells (RGCs). However, the mechanisms by which autophagy dysfunction promotes RGC damage remain unclear. Here, we hypothesized that perturbation of the autophagy pathway results in increased autophagic demand, thereby downregulating signaling through mammalian target of rapamycin complex 1 (mTORC1), a negative regulator of autophagy, contributing to the degeneration of RGCs. We identified an impairment of autophagic-lysosomal degradation and decreased mTORC1 signaling via activation of the stress sensor adenosine monophosphate-activated protein kinase (AMPK), along with subsequent neurodegeneration in RGCs differentiated from human pluripotent stem cells (hPSCs) with a glaucoma-associated variant of Optineurin (OPTN-E50K). Similarly, the microbead occlusion model of glaucoma resulting in ocular hypertension also exhibited autophagy disruption and mTORC1 downregulation. Pharmacological inhibition of mTORC1 in hPSC-derived RGCs recapitulated disease-related neurodegenerative phenotypes in otherwise healthy RGCs, while the mTOR-independent induction of autophagy reduced protein accumulation and restored neurite outgrowth in diseased OPTN-E50K RGCs. Taken together, these results highlight an important balance between autophagy and mTORC1 signaling essential for RGC homeostasis, while disruption to these pathways contributes to neurodegenerative features in glaucoma, providing a potential therapeutic target to prevent neurodegeneration.

Mutant Huntingtin Protein Interaction Map Implicates Dysregulation of Multiple Cellular Pathways in Neurodegeneration of Huntington's Disease

BACKGROUND

Huntington's disease (HD) is a genetic neurodegenerative disease caused by trinucleotide repeat (CAG) expansions in the human HTT gene encoding the huntingtin protein (Htt) with an expanded polyglutamine tract.

OBJECTIVE

HD models from yeast to transgenic mice have investigated proteins interacting with mutant Htt that may initiate molecular pathways of cell death. There is a paucity of datasets of published Htt protein interactions that include the criteria of 1) defining fragments or full-length Htt forms, 2) indicating the number of poly-glutamines of the mutant and wild-type Htt forms, and 3) evaluating native Htt interaction complexes. This research evaluated such interactor data to gain understanding of Htt dysregulation of cellular pathways.

METHODS

Htt interacting proteins were compiled from the literature that meet our criteria and were subjected to network analysis via clustering, gene ontology, and KEGG pathways using rigorous statistical methods.

RESULTS

The compiled data of Htt interactors found that both mutant and wild-type Htt interact with more than 2,971 proteins. Application of a community detection algorithm to all known Htt interactors identified significant signal transduction, membrane trafficking, chromatin, and mitochondrial clusters, among others. Binomial analyses of a subset of reported protein interactor information determined that chromatin organization, signal transduction and endocytosis were diminished, while mitochondria, translation and membrane trafficking had enriched overall edge effects.

CONCLUSION

The data support the hypothesis that mutant Htt disrupts multiple cellular processes causing toxicity. This dataset is an open resource to aid researchers in formulating hypotheses of HD mechanisms of pathogenesis.

Macroautophagy in CNS health and disease

Macroautophagy is an evolutionarily conserved process that delivers diverse cellular contents to lysosomes for degradation. As our understanding of this pathway grows, so does our appreciation for its importance in disorders of the CNS. Once implicated primarily in neurodegenerative events owing to acute injury and ageing, macroautophagy is now also linked to disorders of neurodevelopment, indicating that it is essential for both the formation and maintenance of a healthy CNS. In parallel to understanding the significance of macroautophagy across contexts, we have gained a greater mechanistic insight into its physiological regulation and the breadth of cargoes it can degrade. Macroautophagy is a broadly used homeostatic process, giving rise to questions surrounding how defects in this single pathway could cause diseases with distinct clinical and pathological signatures. To address this complexity, we herein review macroautophagy in the mammalian CNS by examining three key features of the process and its relationship to disease: how it functions at a basal level in the discrete cell types of the brain and spinal cord; which cargoes are being degraded in physiological and pathological settings; and how the different stages of the macroautophagy pathway intersect with diseases of neurodevelopment and adult-onset neurodegeneration.

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.

Protein network analysis to prioritize key genes in amyotrophic lateral sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a fatal disease, progressive nature characterizes by loss of both upper and lower motor neuron functions. One of the major challenge is to understand the mechanism of ALS multifactorial nature. We aimed to explore some key genes related to ALS through bioinformatics methods for its therapeutic intervention. Here, we applied a systems biology approach involving experimentally validated 148 ALS-associated proteins and construct ALS protein-protein interaction network (ALS-PPIN). The network was further statistically analysed and identified bottleneck-hubs. The network is also subjected to identify modules which could have similar functions. The interaction between the modules and bottleneck-hubs provides the functional regulatory role of the ALS mechanism. The ALS-PPIN demonstrated a hierarchical scale-free nature. We identified 17 bottleneck-hubs, in which CDC5L, SNW1, TP53, SOD1, and VCP were the high degree nodes (hubs) in ALS-PPIN. CDC5L was found to control highly cluster modules and play a vital role in the stability of the overall network followed by SNW1, TP53, SOD1, and VCP. HSPA5 and HSPA8 acting as a common connector for CDC5L and TP53 bottleneck-hubs. The functional and disease association analysis showed ALS has a strong correlation with mRNA processing, protein deubiquitination, and neoplasms, nervous system, immune system disease classes. In the future, biochemical investigation of the observed bottleneck-hubs and their interacting partners could provide a further understanding of their role in the pathophysiology of ALS.

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Amyotrophic Lateral Sclerosis protein-protein interaction network (ALS-PPIN) followed a hierarchical scale-free nature.

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We identified 17 bottleneck-hubs in the ALS-PPIN.

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Among bottleneck-hubs we found CDC5L, SNW1, TP53, SOD1, and VCP were the high degree nodes (hubs) in the ALS-PPIN.

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CDC5L is the effective communicator with all five modules in the ALS-PPIN and followed by SNW1 and TP53.

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Modules are highly associated with various disease classes like neoplasms, nervous systems and others.

Nuclear and cytoplasmic huntingtin inclusions exhibit distinct biochemical composition, interactome and ultrastructural properties

Despite the strong evidence linking the aggregation of the Huntingtin protein (Htt) to the pathogenesis of Huntington's disease (HD), the mechanisms underlying Htt aggregation and neurodegeneration remain poorly understood. Herein, we investigated the ultrastructural properties and protein composition of Htt cytoplasmic and nuclear inclusions in mammalian cells and primary neurons overexpressing mutant exon1 of the Htt protein. Our findings provide unique insight into the ultrastructural properties of cytoplasmic and nuclear Htt inclusions and their mechanisms of formation. We show that Htt inclusion formation and maturation are complex processes that, although initially driven by polyQ-dependent Htt aggregation, also involve the polyQ and PRD domain-dependent sequestration of lipids and cytoplasmic and cytoskeletal proteins related to HD dysregulated pathways; the recruitment and accumulation of remodeled or dysfunctional membranous organelles, and the impairment of the protein quality control and degradation machinery. We also show that nuclear and cytoplasmic Htt inclusions exhibit distinct biochemical compositions and ultrastructural properties, suggesting different mechanisms of aggregation and toxicity.

Emerging views of OPTN (optineurin) function in the autophagic process associated with disease

Macroautophagy/autophagy is a highly conserved process in eukaryotic cells. It plays a critical role in cellular homeostasis by delivering cytoplasmic cargos to lysosomes for selective degradation. OPTN (optineurin), a well-recognized autophagy receptor, has received considerable attention due to its multiple roles in the autophagic process. OPTN is associated with many human disorders that are closely related to autophagy, such as rheumatoid arthritis, osteoporosis, and nephropathy. Here, we review the function of OPTN as an autophagy receptor at different stages of autophagy, focusing on cargo recognition, autophagosome formation, autophagosome maturation, and lysosomal quality control. OPTN tends to be protective in most autophagy associated diseases, though the molecular mechanism of OPTN regulation in these diseases is not well understood. A comprehensive review of the function of OPTN in autophagy provides valuable insight into the pathogenesis of human diseases related to OPTN and facilitates the discovery of potential key regulators and novel therapeutic targets for disease intervention in patients with autophagic diseases.Abbreviations: ATG: autophagy-related; APAP: acetaminophen; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; CC: coiled-coil; HACE1: HECT domain and ankyrin repeat containing E3 ubiquitin protein ligase 1; MYO6: myosin VI; IKBKG/NEMO: inhibitor of nuclear factor kappa B kinase regulatory subunit gamma; IKK: IκB kinase; LIR: LC3-interacting region; LZ: leucine zipper; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; NFKB/NF-κB: nuclear factor kappa B subunit; OPTN: optineurin; PIK3C3: phosphatidylinositol 3-kinase catalytic subunit type 3; PINK1: PTEN induced kinase 1; PRKN: parkin RBR E3 ubiquitin protein ligase; RTECs: renal tubular epithelial cells; SQSTM1/p62: sequestosome 1; TBK1: TANK binding kinase 1; TOM1: target of myb1 membrane trafficking protein; UBD: ubiquitin-binding domain; ULK1: unc-51 like autophagy activating kinase 1; WIPI2: WD repeat domain, phosphoinositide interacting 2; ZF: zinc finger.

Glaucoma and neuroinflammation: An overview

Glaucoma is an optic neuropathy characterized by well-defined optic disc morphological changes (i.e., cup enlargement, neuroretinal border thinning, and notching, papillary vessel modifications) consequent to retinal ganglion cell loss, axonal degeneration, and lamina cribrosa remodeling. These modifications tend to be progressive and are the main cause of functional damage in glaucoma. Despite the latest findings about the pathophysiology of the disease, the exact trigger mechanisms and the mechanism of degeneration of retinal ganglion cells and their axons have not been completely elucidated. Neuroinflammation may play a role in both the development and the progression of the disease as a result of its effects on retinal environment and retinal ganglion cells. We summarize the latest findings about neuroinflammation in glaucoma and examine the connection between risk factors, neuroinflammation, and retinal ganglion cell degeneration.

Optineurin deletion disrupts metabotropic glutamate receptor 5-mediated regulation of ERK1/2, GSK3β/ZBTB16, mTOR/ULK1 signaling in autophagy

Optineurin (OPTN) is a multifunctional protein that mediates a network of cellular processes regulating membrane trafficking, inflammatory responses and autophagy. The OPTN-rich interactome includes Group I metabotropic glutamate receptors (mGluR1 and 5), members of the Gα_q/11 protein receptor family. Recent evidence has shown that mGluR5, in addition to its canonical Gα_q/11 protein-coupled signaling, regulates autophagic machinery via mTOR/ULK1 and GSK3β/ZBTB16 pathways in both Alzheimer's and Huntington's disease mouse models. Despite its potential involvement, the role of OPTN in mediating mGluR5 downstream signaling cascades remains largely unknown. Here, we employed a CRISPR/Cas9 OPTN-deficient STHdh^Q7/Q7 striatal cell line and global OPTN knockout mice to investigate whether Optn gene deletion alters both mGluR5 canonical and noncanonical signaling. We find that OPTN is required for mGluR5-activated Ca²⁺ flux and ERK1/2 signaling following receptor activation in STHdh^Q7/Q7 cells and acute hippocampal slices. Deletion of OPTN impairs both GSK3β/ZBTB16 and mTOR/ULK1 autophagic signaling in STHdh^Q7/Q7 cells. Furthermore, mGluR5-dependent regulation of GSK3β/ZBTB16 and mTOR/ULK1 autophagic signaling is impaired in hippocampal slices of OPTN knockout mice. Overall, we show that the crosstalk between OPTN and mGluR5 can have major implication on receptor signaling and therefore potentially contribute to the pathophysiology of neurodegenerative diseases.

Stress granule subtypes: an emerging link to neurodegeneration

Stress Granules (SGs) are membraneless cytoplasmic RNA granules, which contain translationally stalled mRNAs, associated translation initiation factors and multiple RNA-binding proteins (RBPs). They are formed in response to various stresses and contribute to reprogramming of cellular metabolism to aid cell survival. Because of their cytoprotective nature, association with translation regulation and cell signaling, SGs are an essential component of the integrated stress response pathway, a complex adaptive program central to stress management. Recent advances in SG biology unambiguously demonstrate that SGs are heterogeneous in their RNA and protein content leading to the idea that various SG subtypes exist. These SG variants are formed in cell type- and stress-specific manners and differ in their composition, dynamics of assembly and disassembly, and contribution to cell viability. As aberrant SG dynamics contribute to the formation of pathological persistent SGs that are implicated in neurodegenerative diseases, the biology of different SG subtypes may be directly implicated in neurodegeneration. Here, we will discuss mechanisms of SG formation, their subtypes, and potential contribution to health and disease.

The diverse role of optineurin in pathogenesis of disease

Optineurin is a widely expressed protein that possesses multiple functions. Growing evidence suggests that mutation or dysregulation of optineurin can cause several neurodegenerative diseases, including amyotrophic lateral sclerosis, primary open-angle glaucoma, and Huntington's disease, as well as inflammatory digestive disorders such as Crohn's disease. Optineurin engages in vesicular trafficking, receptor regulation, immune reactions, autophagy, and distinct signaling pathways including nuclear factor kappa beta, by which optineurin contributes to cellular death and related diseases, indicating its potential as a therapeutic target. In this review, we discuss the major functions and signaling pathways of optineurin. Furthermore, we illustrate the influence of optineurin mutation or dysregulation to region-specific pathogenesis as well as potential applications of optineurin in therapeutic strategies.

Lowering Mutant Huntingtin Levels and Toxicity: Autophagy-Endolysosome Pathways in Huntington's Disease

Huntington's disease (HD) is a monogenetic neurodegenerative disease, which serves as a model of neurodegeneration with protein aggregation. Autophagy has been suggested to possess a great value to tackle protein aggregation toxicity and neurodegenerative diseases. Current studies suggest that autophagy-endolysosomal pathways are critical for HD pathology. Here we review recent advancement in the studies of autophagy and selective autophagy relating HD. Restoration of autophagy flux and enhancement of selective removal of mutant huntingtin/disease-causing protein would be effective approaches towards tackling HD as well as other similar neurodegenerative disorders.

Selective autophagy as a potential therapeutic target for neurodegenerative disorders

Cells rely on surveillance systems such as autophagy to handle protein alterations and organelle damage. Dysfunctional autophagy, an evolutionarily conserved cellular mechanism for degradation of intracellular components in lysosomes, frequently leads to neurodegeneration. The neuroprotective effect of autophagy stems from its ability to eliminate pathogenic forms of proteins such as α-synuclein or tau. However, the same pathogenic proteins often affect different types and steps of the autophagic process. Furthermore, genetic studies have shown that some proteins related to neurodegeneration, such as huntingtin, participate in autophagy as one of their physiological functions. This complex interplay between autophagy and neurodegeneration suggests that targeting autophagy as a whole might have limited applicability in neurodegenerative diseases, and that future efforts should focus instead on targeting specific types and steps of the autophagic process. This change of strategy in the modulation of autophagy might hold promise for future disease-modifying therapies for patients with neurodegenerative disorders.

Role of Optineurin in the Mitochondrial Dysfunction: Potential Implications in Neurodegenerative Diseases and Cancer

Optineurin (Optn) is a 577 aa protein encoded by the Optn gene. Mutations of Optn are associated with normal tension glaucoma and amyotrophic lateral sclerosis, and its gene has also been linked to the development of Paget’s disease of bone and Crohn’s disease. Optn is involved in diverse cellular functions, including NF-κB regulation, membrane trafficking, exocytosis, vesicle transport, reorganization of actin and microtubules, cell cycle control, and autophagy. Besides its role in xenophagy and autophagy of aggregates, Optn has been identified as a primary autophagy receptor, among the five adaptors that translocate to mitochondria during mitophagy. Mitophagy is a selective macroautophagy process during which irreparable mitochondria are degraded, preventing accumulation of defective mitochondria and limiting the release of reactive oxygen species and proapoptotic factors. Mitochondrial quality control via mitophagy is central to the health of cells. One of the important surveillance pathways of mitochondrial health is the recently defined signal transduction pathway involving the mitochondrial PTEN-induced putative kinase 1 (PINK1) protein and the cytosolic RING-between-RING ubiquitin ligase Parkin. Both of these proteins, when mutated, have been identified in certain forms of Parkinson’s disease. By targeting ubiquitinated mitochondria to autophagosomes through its association with autophagy related proteins, Optn is responsible for a critical step in mitophagy. This review reports recent discoveries on the role of Optn in mitophagy and provides insight into its link with neurodegenerative diseases. We will also discuss the involvement of Optn in other pathologies in which mitophagy dysfunctions are involved including cancer.

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.

Optineurin: A Coordinator of Membrane-Associated Cargo Trafficking and Autophagy

Optineurin is a multifunctional adaptor protein intimately involved in various vesicular trafficking pathways. Through interactions with an array of proteins, such as myosin VI, huntingtin, Rab8, and Tank-binding kinase 1, as well as via its oligomerisation, optineurin has the ability to act as an adaptor, scaffold, or signal regulator to coordinate many cellular processes associated with the trafficking of membrane-delivered cargo. Due to its diverse interactions and its distinct functions, optineurin is an essential component in a number of homeostatic pathways, such as protein trafficking and organelle maintenance. Through the binding of polyubiquitinated cargoes via its ubiquitin-binding domain, optineurin also serves as a selective autophagic receptor for the removal of a wide range of substrates. Alternatively, it can act in an ubiquitin-independent manner to mediate the clearance of protein aggregates. Regarding its disease associations, mutations in the optineurin gene are associated with glaucoma and have more recently been found to correlate with Paget’s disease of bone and amyotrophic lateral sclerosis (ALS). Indeed, ALS-associated mutations in optineurin result in defects in neuronal vesicular localisation, autophagosome–lysosome fusion, and secretory pathway function. More recent molecular and functional analysis has shown that it also plays a role in mitophagy, thus linking it to a number of other neurodegenerative conditions, such as Parkinson’s. Here, we review the role of optineurin in intracellular membrane trafficking, with a focus on autophagy, and describe how upstream signalling cascades are critical to its regulation. Current data and contradicting reports would suggest that optineurin is an important and selective autophagy receptor under specific conditions, whereby interplay, synergy, and functional redundancy with other receptors occurs. We will also discuss how dysfunction in optineurin-mediated pathways may lead to perturbation of critical cellular processes, which can drive the pathologies of number of diseases. Therefore, further understanding of optineurin function, its target specificity, and its mechanism of action will be critical in fully delineating its role in human disease.

Autophagy and lysosomal pathways in nervous system disorders

Autophagy is an evolutionarily conserved pathway for delivering cytoplasmic cargo to lysosomes for degradation. In its classically studied form, autophagy is a stress response induced by starvation to recycle building blocks for essential cellular processes. In addition, autophagy maintains basal cellular homeostasis by degrading endogenous substrates such as cytoplasmic proteins, protein aggregates, damaged organelles, as well as exogenous substrates such as bacteria and viruses. Given their important role in homeostasis, autophagy and lysosomal machinery are genetically linked to multiple human disorders such as chronic inflammatory diseases, cardiomyopathies, cancer, and neurodegenerative diseases. Multiple targets within the autophagy and lysosomal pathways offer therapeutic opportunities to benefit patients with these disorders. Here, I will summarize the mechanisms of autophagy pathways, the evidence supporting a pathogenic role for disturbed autophagy and lysosomal degradation in nervous system disorders, and the therapeutic potential of autophagy modulators in the clinic.

Structural insights into the ubiquitin recognition by OPTN (optineurin) and its regulation by TBK1-mediated phosphorylation

OPTN (optineurin), a ubiquitin-binding scaffold protein, functions as an important macroautophagy/autophagy receptor in selective autophagy processes. Mutations in OPTN have been linked with human neurodegenerative diseases including ALS and glaucoma. However, the mechanistic basis underlying the recognition of ubiquitin by OPTN and its regulation by TBK1-mediated phosphorylation are still elusive. Here, we demonstrate that the UBAN domain of OPTN preferentially recognizes linear ubiquitin chain and forms an asymmetric 2:1 stoichiometry complex with the linear diubiquitin. In addition, our results provide new mechanistic insights into how phosphorylation of UBAN would regulate the ubiquitin-binding ability of OPTN and how disease-associated mutations in the OPTN UBAN domain disrupt its interaction with ubiquitin. Finally, we show that defects in ubiquitin-binding may affect the recruitment of OPTN to linear ubiquitin-decorated mutant Huntington protein aggregates. Taken together, our findings clarify the interaction mode between UBAN and linear ubiquitin chain in general, and expand our knowledge of the molecular mechanism of ubiquitin-decorated substrates recognition by OPTN as well as the pathogenesis of neurodegenerative diseases caused by OPTN mutations.

Cellular Functions of Optineurin in Health and Disease

Optineurin (OPTN) was initially identified as a regulator of NF-κB and interferon signaling, but attracted most attention because of its association with various human disorders such as glaucoma, Paget disease of bone, and amyotrophic lateral sclerosis. Importantly, OPTN has recently been identified as an autophagy receptor important for the autophagic removal of pathogens, damaged mitochondria, and protein aggregates. This activity is most likely compromised in patients carrying OPTN mutations, and contributes to the observed phenotypes. In this review we summarize recent studies describing the molecular mechanisms by which OPTN controls immunity and autophagy, and discuss these findings in the context of several diseases that have been associated with OPTN (mal)function.

From the Cover: Alterations in Optineurin Expression and Localization in Pre-clinical Parkinson's Disease Models

Parkinson's disease (PD) is a progressive neurodegenerative disease that affects ∼5 million people around the world. PD etiopathogenesis is poorly understood and curative or disease modifying treatments are not available. Mechanistic studies have identified numerous pathogenic pathways that overlap with many other neurodegenerative diseases. Mutations in the protein optineurin (OPTN) have recently been identified as causative factors for glaucoma and amyotrophic lateral sclerosis. OPTN has multiple recognized roles in neurons, notably in mediating autophagic flux, which has been found to be disrupted in most neurodegenerative diseases. OPTN(+ )aggregates have preliminarily been identified in cytoplasmic inclusions in numerous neurodegenerative diseases, however, whether OPTN has a role in PD pathogenesis has yet to be tested. Thus, we chose to test the hypothesis that OPTN expression and localization would be modulated in pre-clinical PD models. To test our hypothesis, we characterized midbrain OPTN expression in normal rats and in a rat rotenone PD model. For the first time, we show that OPTN is enriched in dopamine neurons in the midbrain, and its expression is modulated by rotenone treatment in vivo Here, animals were sampled at time-points both prior to overt neurodegeneration and after severe behavioral deficits, where a lesion to the nigrostriatal dopamine system is present. The effect and magnitude of OPTN expression changes are dependent on duration of treatment. Furthermore, OPTN colocalizes with LC3 (autophagic vesicle marker) and alpha-synuclein positive puncta in rotenone-treated animals, potentially indicating an important role in autophagy and PD pathogenesis.

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.

Autophagy in motor neuron disease: Key pathogenetic mechanisms and therapeutic targets

Autophagy is a lysosome-dependant intracellular degradation process that eliminates long-lived proteins as well as damaged organelles from the cytoplasm. An increasing body of evidence suggests that dysregulation of this system plays a pivotal role in the etiology and/or progression of neurodegenerative diseases including motor neuron disorders. Herein, we review the latest findings that highlight the involvement of autophagy in the pathogenesis of amyotrophic lateral sclerosis (ALS) and the potential role of this pathway as a target of therapeutic purposes. Autophagy promotes the removal of toxic, cytoplasmic aggregate-prone pathogenetic proteins, enhances cell survival, and modulates inflammation. The existence of several drugs targeting this pathway can facilitate the translation of basic research to clinical trials for ALS and other motor neuron diseases.

Autophagy and neurodegeneration

Most neurodegenerative diseases that afflict humans are associated with the intracytoplasmic deposition of aggregate-prone proteins in neurons. Autophagy is a powerful process for removing such proteins. In this Review, we consider how certain neurodegenerative diseases may be associated with impaired autophagy and how this may affect pathology. We also discuss how autophagy induction may be a plausible therapeutic strategy for some conditions and review studies in various models that support this hypothesis. Finally, we briefly describe some of the signaling pathways that may be amenable to therapeutic targeting for these goals.

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.

Autophagy in Huntington disease and huntingtin in autophagy

Autophagy is an important biological process that is essential for the removal of damaged organelles and toxic or aggregated proteins by delivering them to the lysosome for degradation. Consequently, autophagy has become a primary target for the treatment of neurodegenerative diseases that involve aggregating proteins. In Huntington disease (HD), an expansion of the polyglutamine (polyQ) tract in the N-terminus of the huntingtin (HTT) protein leads to protein aggregation. However, HD is unique among the neurodegenerative proteinopathies in that autophagy is not only dysfunctional but wild type (wt) HTT also appears to play several roles in regulating the dynamics of autophagy. Herein, we attempt to integrate the recently described novel roles of wtHTT and altered autophagy in HD.

Genetics of amyotrophic lateral sclerosis: an update

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder involving both upper motor neurons (UMN) and lower motor neurons (LMN). Enormous research has been done in the past few decades in unveiling the genetics of ALS, successfully identifying at least fifteen candidate genes associated with familial and sporadic ALS. Numerous studies attempting to define the pathogenesis of ALS have identified several plausible determinants and molecular pathways leading to motor neuron degeneration, which include oxidative stress, glutamate excitotoxicity, apoptosis, abnormal neurofilament function, protein misfolding and subsequent aggregation, impairment of RNA processing, defects in axonal transport, changes in endosomal trafficking, increased inflammation, and mitochondrial dysfunction. This review is to update the recent discoveries in genetics of ALS, which may provide insight information to help us better understanding of the disease neuropathogenesis.

Ubiquitin-independent function of optineurin in autophagic clearance of protein aggregates

Aggregation of misfolded proteins and the associated loss of neurons are considered a hallmark of numerous neurodegenerative diseases. Optineurin is present in protein inclusions observed in various neurodegenerative diseases including amyotrophic lateral sclerosis (ALS), Huntington's disease, Alzheimer's disease, Parkinson's disease, Creutzfeld-Jacob disease and Pick's disease. Optineurin deletion mutations have also been described in ALS patients. However, the role of optineurin in mechanisms of protein aggregation remains unclear. In this report, we demonstrate that optineurin recognizes various protein aggregates via its C-terminal coiled-coil domain in a ubiquitin-independent manner. We also show that optineurin depletion significantly increases protein aggregation in HeLa cells and that morpholino-silencing of the optineurin ortholog in zebrafish causes the motor axonopathy phenotype similar to a zebrafish model of ALS. A more severe phenotype is observed when optineurin is depleted in zebrafish carrying ALS mutations. Furthermore, TANK1 binding kinase 1 (TBK1) is colocalized with optineurin on protein aggregates and is important in clearance of protein aggregates through the autophagy-lysosome pathway. TBK1 phosphorylates optineurin at serine 177 and regulates its ability to interact with autophagy modifiers. This study provides evidence for a ubiquitin-independent function of optineurin in autophagic clearance of protein aggregates as well as additional relevance for TBK1 as an upstream regulator of the autophagic pathway.

An in vitro perspective on the molecular mechanisms underlying mutant huntingtin protein toxicity

Huntington's disease (HD) is a devastating neurodegenerative disorder whose main hallmark is brain atrophy. However, several peripheral organs are considerably affected and their symptoms may, in fact, manifest before those resulting from brain pathology. HD is of genetic origin and caused by a mutation in the huntingtin gene. The mutated protein has detrimental effects on cell survival, but whether the mutation leads to a gain of toxic function or a loss of function of the altered protein is still highly controversial. Most currently used in vitro models have been designed, to a large extent, to investigate the effects of the aggregation process in neuronal-like cells. However, as the pathology involves several other organs, new in vitro models are critically needed to take into account the deleterious effects of mutant huntingtin in peripheral tissues, and thus to identify new targets that could lead to more effective clinical interventions in the early course of the disease. This review aims to present current in vitro models of HD pathology and to discuss the knowledge that has been gained from these studies as well as the new in vitro tools that have been developed, which should reflect the more global view that we now have of the disease.

Aggrephagy: selective disposal of protein aggregates by macroautophagy

Protein aggregation is a continuous process in our cells. Some proteins aggregate in a regulated manner required for different vital functional processes in the cells whereas other protein aggregates result from misfolding caused by various stressors. The decision to form an aggregate is largely made by chaperones and chaperone-assisted proteins. Proteins that are damaged beyond repair are degraded either by the proteasome or by the lysosome via autophagy. The aggregates can be degraded by the proteasome and by chaperone-mediated autophagy only after dissolution into soluble single peptide species. Hence, protein aggregates as such are degraded by macroautophagy. The selective degradation of protein aggregates by macroautophagy is called aggrephagy. Here we review the processes of aggregate formation, recognition, transport, and sequestration into autophagosomes by autophagy receptors and the role of aggrephagy in different protein aggregation diseases.