Protein Misfolding and ER Stress in Huntington's Disease

The Zn2+ transporter ZIP7 enhances endoplasmic-reticulum-associated protein degradation and prevents neurodegeneration in Drosophila

Proteotoxic stress drives numerous degenerative diseases. Cells initially adapt to misfolded proteins by activating the unfolded protein response (UPR), including endoplasmic-reticulum-associated protein degradation (ERAD). However, persistent stress triggers apoptosis. Enhancing ERAD is a promising therapeutic approach for protein misfolding diseases. The ER-localized Zn2+ transporter ZIP7 is conserved from plants to humans and required for intestinal self-renewal, Notch signaling, cell motility, and survival. However, a unifying mechanism underlying these diverse phenotypes was unknown. In studying Drosophila border cell migration, we discovered that ZIP7-mediated Zn2+ transport enhances the obligatory deubiquitination of proteins by the Rpn11 Zn2+ metalloproteinase in the proteasome lid. In human cells, ZIP7 and Zn2+ are limiting for deubiquitination. In a Drosophila model of neurodegeneration caused by misfolded rhodopsin (Rh1), ZIP7 overexpression degrades misfolded Rh1 and rescues photoreceptor viability and fly vision. Thus, ZIP7-mediated Zn2+ transport is a previously unknown, rate-limiting step for ERAD in vivo with therapeutic potential in protein misfolding diseases.

Selenium Nanoparticles as Neuroprotective Agents: Insights into Molecular Mechanisms for Parkinson's Disease Treatment

Oxidative stress and the accumulation of misfolded proteins in the brain are the main causes of Parkinson's disease (PD). Several nanoparticles have been used as therapeutics for PD. Despite their therapeutic potential, these nanoparticles induce multiple stresses upon entry. Selenium (Se), an essential nutrient in the human body, helps in DNA formation, stress control, and cell protection from damage and infections. It can also regulate thyroid hormone metabolism, reduce brain damage, boost immunity, and promote reproductive health. Selenium nanoparticles (Se-NPs), a bioactive substance, have been employed as treatments in several disciplines, particularly as antioxidants. Se-NP, whether functionalized or not, can protect mitochondria by enhancing levels of reactive oxygen species (ROS) scavenging enzymes in the brain. They can also promote dopamine synthesis. By inhibiting the aggregation of tau, α-synuclein, and/or Aβ, they can reduce the cellular toxicities. The ability of the blood-brain barrier to absorb Se-NPs which maintain a healthy microenvironment is essential for brain homeostasis. This review focuses on stress-induced neurodegeneration and its critical control using Se-NP. Due to its ability to inhibit cellular stress and the pathophysiologies of PD, Se-NP is a promising neuroprotector with its anti-inflammatory, non-toxic, and antimicrobial properties.

Polyglutamine-mediated ribotoxicity disrupts proteostasis and stress responses in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by expansion of a CAG trinucleotide repeat in the Huntingtin (HTT) gene, encoding a homopolymeric polyglutamine (polyQ) tract. Although mutant HTT (mHTT) protein is known to aggregate, the links between aggregation and neurotoxicity remain unclear. Here we show that both translation and aggregation of wild-type HTT and mHTT are regulated by a stress-responsive upstream open reading frame and that polyQ expansions cause abortive translation termination and release of truncated, aggregation-prone mHTT fragments. Notably, we find that mHTT depletes translation elongation factor eIF5A in brains of symptomatic HD mice and cultured HD cells, leading to pervasive ribosome pausing and collisions. Loss of eIF5A disrupts homeostatic controls and impairs recovery from acute stress. Importantly, drugs that inhibit translation initiation reduce premature termination and mitigate this escalating cascade of ribotoxic stress and dysfunction in HD.

Crosstalk between protein misfolding and endoplasmic reticulum stress during ageing and their role in age-related disorders

Maintaining the proteome is crucial to retaining cell functionality and response to multiple intrinsic and extrinsic stressors. Protein misfolding increased the endoplasmic reticulum (ER) stress and activated the adaptive unfolded protein response (UPR) to restore cell homeostasis. Apoptosis occurs when ER stress is prolonged or the adaptive response fails. In healthy young cells, the ratio of protein folding machinery to quantities of misfolded proteins is balanced under normal circumstances. However, the age-related deterioration of the complex systems for handling protein misfolding is accompanied by ageing-related disruption of protein homeostasis, which results in the build-up of misfolded and aggregated proteins. This ultimately results in decreased cell viability and forms the basis of common age-related diseases called protein misfolding diseases. Proteins or protein fragments convert from their ordinarily soluble forms to insoluble fibrils or plaques in many of these disorders, which build up in various organs such as the liver, brain, or spleen. Alzheimer's, Parkinson's, type II diabetes, and cancer are diseases in this group commonly manifest in later life. Thus, protein misfolding and its prevention by chaperones and different degradation paths are becoming understood from molecular perspectives. Proteodynamics information will likely affect future interventional techniques to combat cellular stress and support healthy ageing by avoiding and treating protein conformational disorders. This review provides an overview of the diverse proteostasis machinery, protein misfolding, and ER stress involvement, which activates the UPR sensors. Here, we will discuss the crosstalk between protein misfolding and ER stress and their role in developing age-related diseases.

The Role of NRF2 in Trinucleotide Repeat Expansion Disorders

Trinucleotide repeat expansion disorders, a diverse group of neurodegenerative diseases, are caused by abnormal expansions within specific genes. These expansions trigger a cascade of cellular damage, including protein aggregation and abnormal RNA binding. A key contributor to this damage is oxidative stress, an imbalance of reactive oxygen species that harms cellular components. This review explores the interplay between oxidative stress and the NRF2 pathway in these disorders. NRF2 acts as the master regulator of the cellular antioxidant response, orchestrating the expression of enzymes that combat oxidative stress. Trinucleotide repeat expansion disorders often exhibit impaired NRF2 signaling, resulting in inadequate responses to excessive ROS production. NRF2 activation has been shown to upregulate antioxidative gene expression, effectively alleviating oxidative stress damage. NRF2 activators, such as omaveloxolone, vatiquinone, curcumin, sulforaphane, dimethyl fumarate, and resveratrol, demonstrate neuroprotective effects by reducing oxidative stress in experimental cell and animal models of these diseases. However, translating these findings into successful clinical applications requires further research. In this article, we review the literature supporting the role of NRF2 in the pathogenesis of these diseases and the potential therapeutics of NRF2 activators.

Impact of Obesity-Related Endoplasmic Reticulum Stress on Cancer and Associated Molecular Targets

Obesity, characterized by excessive body fat, is closely linked to endoplasmic reticulum (ER) stress, leading to insulin resistance and type 2 diabetes. Inflammatory pathways like c-Jun N-terminal kinase (JNK) worsen insulin resistance, impacting insulin signaling. Moreover, ER stress plays a substantial role in cancer, influencing tumor cell survival and growth by releasing factors like vascular endothelial growth factor (VEGF). The unfolded protein response (UPR) is pivotal in this process, offering both pro-survival and apoptotic pathways. This review offers an extensive exploration of the sophisticated connection between ER stress provoked by obesity and its role in both the onset and advancement of cancer. It delves into the intricate interplay between oncogenic signaling and the pathways associated with ER stress in individuals who are obese. Furthermore, this review sheds light on potential therapeutic strategies aimed at managing ER stress induced by obesity, with a focus on addressing cancer initiation and progression. The potential to alleviate ER stress through therapeutic interventions, which may encompass the use of small molecules, FDA-approved medications, and gene therapy, holds great promise. A more in-depth examination of pathways such as UPR, ER-associated protein degradation (ERAD), autophagy, and epigenetic regulation has the potential to uncover innovative therapeutic approaches and the identification of predictive biomarkers.

Endoplasmic reticulum stress and therapeutic strategies in metabolic, neurodegenerative diseases and cancer

The accumulation of unfolded or misfolded proteins within the endoplasmic reticulum (ER), due to genetic determinants and extrinsic environmental factors, leads to endoplasmic reticulum stress (ER stress). As ER stress ensues, the unfolded protein response (UPR), comprising three signaling pathways-inositol-requiring enzyme 1, protein kinase R-like endoplasmic reticulum kinase, and activating transcription factor 6 promptly activates to enhance the ER's protein-folding capacity and restore ER homeostasis. However, prolonged ER stress levels propels the UPR towards cellular demise and the subsequent inflammatory cascade, contributing to the development of human diseases, including cancer, neurodegenerative disorders, and diabetes. Notably, increased expression of all three UPR signaling pathways has been observed in these pathologies, and reduction in signaling molecule expression correlates with decreased proliferation of disease-associated target cells. Consequently, therapeutic strategies targeting ER stress-related interventions have attracted significant research interest. In this review, we elucidate the critical role of ER stress in cancer, metabolic, and neurodegenerative diseases, offering novel therapeutic approaches for these conditions.

Endoplasmic reticulum stress and its role in various neurodegenerative diseases

The Endoplasmic reticulum (ER), a critical cellular organelle, maintains cellular homeostasis by regulating calcium levels and orchestrating essential functions such as protein synthesis, folding, and lipid production. A pivotal aspect of ER function is its role in protein quality control. When misfolded proteins accumulate within the ER due to factors like protein folding chaperone dysfunction, toxicity, oxidative stress, or inflammation, it triggers the Unfolded protein response (UPR). The UPR involves the activation of chaperones like calnexin, calreticulin, glucose-regulating protein 78 (GRP78), and Glucose-regulating protein 94 (GRP94), along with oxidoreductases like protein disulphide isomerases (PDIs). Cells employ the Endoplasmic reticulum-associated degradation (ERAD) mechanism to counteract protein misfolding. ERAD disruption causes the detachment of GRP78 from transmembrane proteins, initiating a cascade involving Inositol-requiring kinase/endoribonuclease 1 (IRE1), Activating transcription factor 6 (ATF6), and Protein kinase RNA-like endoplasmic reticulum kinase (PERK) pathways. The accumulation and deposition of misfolded proteins within the cell are hallmarks of numerous neurodegenerative diseases. These aberrant proteins disrupt normal neuronal signalling and contribute to impaired cellular homeostasis, including oxidative stress and compromised protein degradation pathways. In essence, ER stress is defined as the cellular response to the accumulation of misfolded proteins in the endoplasmic reticulum, encompassing a series of signalling pathways and molecular events that aim to restore cellular homeostasis. This comprehensive review explores ER stress and its profound implications for the pathogenesis and progression of neurodegenerative diseases.

Efficacy of therapy by MK-28 PERK activation in the Huntington's disease R6/2 mouse model

There is currently no disease-modifying therapy for Huntington's disease (HD). We recently described a small molecule, MK-28, which restored homeostasis in HD models by specifically activating PKR-like ER kinase (PERK). This activation boosts the unfolded protein response (UPR), thereby reducing endoplasmic reticulum (ER) stress, a central cytotoxic mechanism in HD and other neurodegenerative diseases. Here, we have tested the long-term effects of MK-28 in HD model mice. R6/2 CAG (160) mice were treated by lifetime intraperitoneal injections 3 times a week. CatWalk measurements of motor function showed strong improvement compared to untreated mice after only two weeks of MK-28 treatment and continued with time, most significantly at 1 ​mg/kg MK-28, approaching WT values. Seven weeks treatment significantly improved paw grip strength. Body weight recovered and glucose levels, which are elevated in HD mice, were significantly reduced. Treatment with another PERK activator, CCT020312 at 1 ​mg/kg, also caused amelioration, consistent with PERK activation. Lifespan, measured in more resilient R6/2 CAG (120) mice with daily IP injection, was much extended by 16 days (20%) with 0.3 ​mg/kg MK-28, and by 38 days (46%) with 1 ​mg/kg MK-28. No toxicity, measured by weight, blood glucose levels and blood liver function markers, was detectable in WT mice treated for 6 weeks with 6 ​mg/kg MK-28. Boosting of PERK activity by long-term treatment with MK-28 could be a safe and promising therapeutic approach for HD.

Protein-rich foods, sea foods, and gut microbiota amplify immune responses in chronic diseases and cancers - Targeting PERK as a novel therapeutic strategy for chronic inflammatory diseases, neurodegenerative disorders, and cancer

The endoplasmic reticulum (ER) is a cellular organelle that is physiologically responsible for protein folding, calcium homeostasis, and lipid biosynthesis. Pathological stimuli such as oxidative stress, ischemia, disruptions in calcium homeostasis, and increased production of normal and/or folding-defective proteins all contribute to the accumulation of misfolded proteins in the ER, causing ER stress. The adaptive response to ER stress is the activation of unfolded protein response (UPR), which affect a wide variety of cellular functions to maintain ER homeostasis or lead to apoptosis. Three different ER transmembrane sensors, including PKR-like ER kinase (PERK), activating transcription factor 6 (ATF6), and inositol-requiring enzyme-1 (IRE1), are responsible for initiating UPR. The UPR involves a variety of signal transduction pathways that reduce unfolded protein accumulation by boosting ER-resident chaperones, limiting protein translation, and accelerating unfolded protein degradation. ER is now acknowledged as a critical organelle in sensing dangers and determining cell life and death. On the other hand, UPR plays a critical role in the development and progression of several diseases such as cardiovascular diseases (CVD), metabolic disorders, chronic kidney diseases, neurological disorders, and cancer. Here, we critically analyze the most current knowledge of the master regulatory roles of ER stress particularly the PERK pathway as a conditional danger receptor, an organelle crosstalk regulator, and a regulator of protein translation. We highlighted that PERK is not only ER stress regulator by sensing UPR and ER stress but also a frontier sensor and direct senses for gut microbiota-generated metabolites. Our work also further highlighted the function of PERK as a central hub that leads to metabolic reprogramming and epigenetic modification which further enhanced inflammatory response and promoted trained immunity. Moreover, we highlighted the contribution of ER stress and PERK in the pathogenesis of several diseases such as cancer, CVD, kidney diseases, and neurodegenerative disorders. Finally, we discuss the therapeutic target of ER stress and PERK for cancer treatment and the potential novel therapeutic targets for CVD, metabolic disorders, and neurodegenerative disorders. Inhibition of ER stress, by the development of small molecules that target the PERK and UPR, represents a promising therapeutic strategy.

Morin suppresses mTORc1/IRE-1α/JNK and IP3R-VDAC-1 pathways: Crucial mechanisms in apoptosis and mitophagy inhibition in experimental Huntington's disease, supported by in silico molecular docking simulations

AIMS

Endoplasmic reticulum stress (ERS) with aberrant mitochondrial-ER contact (MERC), mitophagy, and apoptosis are interconnected determinants in neurodegenerative diseases. Previously, we proved the potential of Morin hydrate (MH), a potent antioxidant flavonoid, to mitigate Huntington's disease (HD)-3-nitropropionic acid (3-NP) model by modulating glutamate/calpain/Kidins220/BDNF trajectory. Extending our work, we aimed to evaluate its impact on combating the ERS/MERC, mitophagy, and apoptosis.

METHODS

Rats were subjected to 3-NP for 14 days and post-treated with MH and/or the ERS inducer WAG-4S for 7 days. Disease progression was assessed by gross inspection and striatal biochemical, histopathological, immunohistochemical, and transmission electron microscopical (TEM) examinations. A molecular docking study was attained to explore MH binding to mTOR, JNK, the kinase domain of IRE1-α, and IP3R.

KEY FINDINGS

MH decreased weight loss and motor dysfunction using open field and rotarod tests. It halted HD degenerative striatal neurons and nucleus/mitochondria ultra-microscopic alterations reflecting neuroprotection. Mechanistically, MH deactivated striatal mTOR/IRE1-α/XBP1s&JNK/IP3R, PINK1/Ubiquitin/Mfn2, and cytochrome c/caspase-3 signaling pathways, besides enhancing p-PGC-1α and p-VDAC1. WAG-4S was able to ameliorate all effects initiated by MH to different extents. Molecular docking simulations revealed promising binding patterns of MH and hence its potential inhibition of the studied proteins, especially mTOR, IP3R, and JNK.

SIGNIFICANCE

MH alleviated HD-associated ERS, MERC, mitophagy, and apoptosis. This is mainly achieved by combating the mTOR/IRE1-α signaling, IP3R/VDAC hub, PINK1/Ubiquitin/Mfn2, and cytochrome c/caspase 3 axis to be worsened by WAG-4S. Molecular docking simulations showed the promising binding of MH to mTOR and JNK as novel identified targets.

The Preventive Effects of Salubrinal against Pyrethroid-Induced Disruption of Adult Hippocampal Neurogenesis in Mice

Environmental factors, including pesticide exposure, have been identified as substantial contributors to neurodegeneration and cognitive impairments. Previously, we demonstrated that repeated exposure to deltamethrin induces endoplasmic reticulum (ER) stress, reduces hippocampal neurogenesis, and impairs cognition in adult mice. Here, we investigated the potential relationship between ER stress and hippocampal neurogenesis following exposure to deltamethrin, utilizing both pharmacological and genetic approaches. To investigate whether ER stress is associated with inhibition of neurogenesis, mice were given two intraperitoneal injections of eIf2α inhibitor salubrinal (1 mg/kg) at 24 h and 30 min prior to the oral administration of deltamethrin (3 mg/kg). Salubrinal prevented hippocampal ER stress, as indicated by decreased levels of C/EBP-homologous protein (CHOP) and transcription factor 4 (ATF4) and attenuated deltamethrin-induced reductions in BrdU-, Ki-67-, and DCX-positive cells in the dentate gyrus (DG) of the hippocampus. To further explore the relationship between ER stress and adult neurogenesis, we used caspase-12 knockout (KO) mice. The caspase-12 KO mice exhibited significant protection against deltamethrin-induced reduction of BrdU-, Ki-67-, and DCX-positive cells in the hippocampus. In addition, deltamethrin exposure led to a notable upregulation of CHOP and caspase-12 expression in a significant portion of BrdU- and Ki-67-positive cells in WT mice. Conversely, both salubrinal-treated mice and caspase-12 KO mice exhibited a considerably lower number of CHOP-positive cells in the hippocampus. Together, these findings suggest that exposure to the insecticide deltamethrin triggers ER stress-mediated suppression of adult hippocampal neurogenesis, which may subsequently contribute to learning and memory deficits in mice.

Losing, preserving, and restoring vision from neurodegeneration in the eye

The retina is a part of the brain that sits at the back of the eye, looking out onto the world. The first neurons of the retina are the rod and cone photoreceptors, which convert changes in photon flux into electrical signals that are the basis of vision. Rods and cones are frequent targets of heritable neurodegenerative diseases that cause visual impairment, including blindness, in millions of people worldwide. This review summarizes the diverse genetic causes of inherited retinal degenerations (IRDs) and their convergence onto common pathogenic mechanisms of vision loss. Currently, there are few effective treatments for IRDs, but recent advances in disparate areas of biology and technology (e.g., genome editing, viral engineering, 3D organoids, optogenetics, semiconductor arrays) discussed here enable promising efforts to preserve and restore vision in IRD patients with implications for neurodegeneration in less approachable brain areas.

Targeting calcium homeostasis and impaired inter-organelle crosstalk as a potential therapeutic approach in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta, leading to motor symptoms such as tremors, rigidity, and bradykinesia. Current therapeutic strategies for PD are limited and mainly involve symptomatic relief, with no available treatment for the underlying causes of the disease. Therefore, there is a need for new therapeutic approaches that target the underlying pathophysiological mechanisms of PD. Calcium homeostasis is an essential process for maintaining proper cellular function and survival, including neuronal cells. Calcium dysregulation is also observed in various organelles, including the endoplasmic reticulum (ER), mitochondria, and lysosomes, resulting in organelle dysfunction and impaired inter-organelle communication. The ER, as the primary calcium reservoir, is responsible for folding proteins and maintaining calcium homeostasis, and its dysregulation can lead to protein misfolding and neurodegeneration. The crosstalk between ER and mitochondrial calcium signaling is disrupted in PD, leading to neuronal dysfunction and death. In addition, a lethal network of calcium cytotoxicity utilizes mitochondria, ER and lysosome to destroy neurons. This review article focused on the complex role of calcium dysregulation and its role in aggravating functioning of organelles in PD so as to provide new insight into therapeutic strategies for treating this disease. Targeting dysfunctional organelles, such as the ER and mitochondria and lysosomes and whole network of calcium dyshomeostasis can restore proper calcium homeostasis and improve neuronal function. Additionally targeting calcium dyshomeostasis that arises from miscommunication between several organelles can be targeted so that therapeutic effects of calcium are realised in whole cellular territory.

The GRP78-PERK axis contributes to memory and synaptic impairments in Huntington's disease R6/1 mice

Increasing evidence indicates that a key factor in neurodegenerative diseases is the activation of the unfolded protein response (UPR) caused by an accumulation of misfolded proteins in the endoplasmic reticulum (ER stress). Particularly, in Huntington's disease (HD) mutant huntingtin (mHtt) toxicity involves disruption of the ER-associated degradation pathway and loss of the ER protein homeostasis leading to neuronal dysfunction and degeneration. Besides the role of the UPR in regulating cell survival and death, studies that demonstrate the contribution of sustained UPR activation, particularly of PERK signaling, in memory disturbances and synaptic plasticity deficiencies are emerging. Given the contribution of hippocampal dysfunction to emotional and cognitive deficits seen in HD, we have analyzed the involvement of ER stress in HD memory alterations. We have demonstrated that at early disease stages, ER stress activation manifested as an increase in GRP78 and CHOP is observed in the hippocampus of R6/1 mice. Genetic reduction of GRP78 expression resulted in preventing hippocampal-dependent memory alterations but no motor deficits. Accordingly, hippocampal neuropathology namely, dendritic spine loss and accumulation of mHtt aggregates was ameliorated by GRP78 reduction. To elucidate the signaling pathways, we found that the inactivation of PERK by GSK2606414 restored spatial and recognition memories in R6/1 mice and rescued dendritic spine density in CA1 pyramidal neurons and protein levels of some specific immediate early genes. Our study unveils the critical role of the GRP78/PERK axis in memory impairment in HD mice and suggests the modulation of PERK activation as a novel therapeutic target for HD intervention.

Rescue of proteotoxic stress and neurodegeneration by the Zn2+ transporter ZIP7

Proteotoxic stress drives numerous degenerative diseases. In response to misfolded proteins, cells adapt by activating the unfolded protein response (UPR), including endoplasmic reticulum-associated protein degradation (ERAD). However persistent stress triggers apoptosis. Enhancing ERAD is a promising therapeutic approach for protein misfolding diseases. From plants to humans, loss of the Zn2+ transporter ZIP7 causes ER stress, however the mechanism is unknown. Here we show that ZIP7 enhances ERAD and that cytosolic Zn2+ is limiting for deubiquitination of client proteins by the Rpn11 Zn2+ metalloproteinase as they enter the proteasome in Drosophila and human cells. ZIP7 overexpression rescues defective vision caused by misfolded rhodopsin in Drosophila. Thus ZIP7 overexpression may prevent diseases caused by proteotoxic stress, and existing ZIP inhibitors may be effective against proteasome-dependent cancers.

Mitochondrial dysfunction, oxidative stress, ER stress and mitochondria-ER crosstalk alterations in a chemical rat model of Huntington's disease: potential benefits of bezafibrate

Redox homeostasis, mitochondrial functions, and mitochondria-endoplasmic reticulum (ER) communication were evaluated in the striatum of rats after 3-nitropropionic acid (3-NP) administration, a recognized chemical model of Huntington's disease (HD). 3-NP impaired redox homeostasis by increasing malondialdehyde levels at 28 days, decreasing glutathione (GSH) concentrations at 21 and 28 days, and the activities of glutathione peroxidase (GPx), superoxide dismutase (SOD) and glutathione S-transferase at 7, 21, and 28 days, catalase at 21 days, and glutathione reductase at 21 and 28 days. Impairment of mitochondrial respiration at 7 and 28 days after 3-NP administration was also observed, as well as reduced activities of succinate dehydrogenase (SDH) and respiratory chain complexes. 3-NP also impaired mitochondrial dynamics and the interactions between ER and mitochondria and induced ER-stress by increasing the levels of mitofusin-1, and of DRP1, VDAC1, Grp75 and Grp78. Synaptophysin levels were augmented at 7 days but reduced at 28 days after 3-NP injection. Finally, bezafibrate prevented 3-NP-induced alterations of the activities of SOD, GPx, SDH and respiratory chain complexes, DCFH oxidation and on the levels of GSH, VDAC1 and synaptophysin. Mitochondrial dysfunction and synaptic disruption may contribute to the pathophysiology of HD and bezafibrate may be considered as an adjuvant therapy for this disorder.

Proteostasis failure exacerbates neuronal circuit dysfunction and sleep impairments in Alzheimer's disease

Failed proteostasis is a well-documented feature of Alzheimer's disease, particularly, reduced protein degradation and clearance. However, the contribution of failed proteostasis to neuronal circuit dysfunction is an emerging concept in neurodegenerative research and will prove critical in understanding cognitive decline. Our objective is to convey Alzheimer's disease progression with the growing evidence for a bidirectional relationship of sleep disruption and proteostasis failure. Proteostasis dysfunction and tauopathy in Alzheimer's disease disrupts neurons that regulate the sleep-wake cycle, which presents behavior as impaired slow wave and rapid eye movement sleep patterns. Subsequent sleep loss further impairs protein clearance. Sleep loss is a defined feature seen early in many neurodegenerative disorders and contributes to memory impairments in Alzheimer's disease. Canonical pathological hallmarks, β-amyloid, and tau, directly disrupt sleep, and neurodegeneration of locus coeruleus, hippocampal and hypothalamic neurons from tau proteinopathy causes disruption of the neuronal circuitry of sleep. Acting in a positive-feedback-loop, sleep loss and circadian rhythm disruption then increase spread of β-amyloid and tau, through impairments of proteasome, autophagy, unfolded protein response and glymphatic clearance. This phenomenon extends beyond β-amyloid and tau, with interactions of sleep impairment with the homeostasis of TDP-43, α-synuclein, FUS, and huntingtin proteins, implicating sleep loss as an important consideration in an array of neurodegenerative diseases and in cases of mixed neuropathology. Critically, the dynamics of this interaction in the neurodegenerative environment are not fully elucidated and are deserving of further discussion and research. Finally, we propose sleep-enhancing therapeutics as potential interventions for promoting healthy proteostasis, including β-amyloid and tau clearance, mechanistically linking these processes. With further clinical and preclinical research, we propose this dynamic interaction as a diagnostic and therapeutic framework, informing precise single- and combinatorial-treatments for Alzheimer's disease and other brain disorders.

Redox Imbalance in Neurological Disorders in Adults and Children

Oxygen is a central molecule for numerous metabolic and cytophysiological processes, and, indeed, its imbalance can lead to numerous pathological consequences. In the human body, the brain is an aerobic organ and for this reason, it is very sensitive to oxygen equilibrium. The consequences of oxygen imbalance are especially devastating when occurring in this organ. Indeed, oxygen imbalance can lead to hypoxia, hyperoxia, protein misfolding, mitochondria dysfunction, alterations in heme metabolism and neuroinflammation. Consequently, these dysfunctions can cause numerous neurological alterations, both in the pediatric life and in the adult ages. These disorders share numerous common pathways, most of which are consequent to redox imbalance. In this review, we will focus on the dysfunctions present in neurodegenerative disorders (specifically Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis) and pediatric neurological disorders (X-adrenoleukodystrophies, spinal muscular atrophy, mucopolysaccharidoses and Pelizaeus-Merzbacher Disease), highlighting their underlining dysfunction in redox and identifying potential therapeutic strategies.

Dimethyl fumarate abrogates striatal endoplasmic reticulum stress in experimentally induced late-stage Huntington’s disease: Focus on the IRE1α/JNK and PERK/CHOP trajectories

Introduction: Dimethyl fumarate (DMF) is FDA-approved for use in patients with relapsing multiple sclerosis, and it processes neuroprotection in several experimental settings; however, its impact on combating Huntington’s disease (HD) remains elusive. This study aimed to explore the role of DMF post-treatment on HD mediated endoplasmic reticulum (ER) stress response in a selective striatal degeneration HD model.

Methods: Rats, exposed to 3-nitropropionic acid, were either left untreated or post-treated with DMF for 14 days.

Results and Discussion: DMF reduced locomotion deficits in both the open field and beam walk paradigms, boosted the striatal dopamine (DA) content, improved its architecture at the microscopic level, and hindered astrogliosis. Mechanistically, DMF limited the activation of two of the ER stress arms in the striatum by reducing p-IRE1α, p-JNK, and p-PERK protein expressions besides the CHOP/GADD153 content. Downstream from both ER stress arms’ suppression, DMF inhibited the intrinsic apoptotic pathway, as shown by the decrease in Bax and active caspase-3 while raising Bcl-2. DMF also decreased oxidative stress markers indicated by a decline in both reactive oxygen species and malondialdehyde while boosting glutathione. Meanwhile, it enhanced p-AKT to activate /phosphorylate mTOR and stimulate the CREB/BDNF/TrkB trajectory, which, in a positive feedforward loop, activates AKT again. DMF also downregulated the expression of miRNA-634, which negatively regulates AKT, to foster survival kinase activation.

Conclusion: This study features a focal novel point on the DMF therapeutic ability to reduce HD motor manifestations via its ability to enhance DA and suppress the IRE1α/JNK and PERK/CHOP/GADD153 hubs to inhibit the mitochondrial apoptotic pathway through activating the AKT/mTOR and BDNF/TrkB/AKT/CREB signaling pathways and abating miRNA-634 and oxidative stress.

Protein quality control and aggregation in the endoplasmic reticulum: From basic to bedside

Endoplasmic reticulum (ER) is the largest membrane-bound compartment in all cells and functions as a key regulator in protein biosynthesis, lipid metabolism, and calcium balance. Mammalian endoplasmic reticulum has evolved with an orchestrated protein quality control system to handle defective proteins and ensure endoplasmic reticulum homeostasis. Nevertheless, the accumulation and aggregation of misfolded proteins in the endoplasmic reticulum may occur during pathological conditions. The inability of endoplasmic reticulum quality control system to clear faulty proteins and aggregates from the endoplasmic reticulum results in the development of many human disorders. The efforts to comprehensively understand endoplasmic reticulum quality control network and protein aggregation will benefit the diagnostics and therapeutics of endoplasmic reticulum storage diseases. Herein, we overview recent advances in mammalian endoplasmic reticulum protein quality control system, describe protein phase transition model, and summarize the approaches to monitor protein aggregation. Moreover, we discuss the therapeutic applications of enhancing endoplasmic reticulum protein quality control pathways in endoplasmic reticulum storage diseases.

Longitudinal spatial mapping of lipid metabolites reveals pre-symptomatic changes in the hippocampi of Huntington's disease transgenic mice

In Huntington's disease (HD), a key pathological feature includes the development of inclusion-bodies of fragments of the mutant huntingtin protein in the neurons of the striatum and hippocampus. To examine the molecular changes associated with inclusion-body formation, we applied MALDI-mass spectrometry imaging and deuterium pulse labelling to determine lipid levels and synthesis rates in the hippocampus of a transgenic mouse model of HD (R6/1 line). The R6/1 HD mice lacked inclusions in the hippocampus at 6 weeks of age (pre-symptomatic), whereas inclusions were pervasive by 16 weeks of age (symptomatic). Hippocampal subfields (CA1, CA3 and DG), which formed the highest density of inclusion formation in the mouse brain showed a reduction in the relative abundance of neuron-enriched lipids that have roles in neurotransmission, synaptic plasticity, neurogenesis, and ER-stress protection. Lipids involved in the adaptive response to ER stress (phosphatidylinositol, phosphatidic acid, and ganglioside classes) displayed increased rates of synthesis in HD mice relative to WT mice at all the ages examined, including prior to the formation of the inclusion bodies. Our findings, therefore, support a role for ER stress occurring pre-symptomatically and potentially contributing to pathological mechanisms underlying HD.

A Novel Huntington's Disease Assessment Platform to Support Future Drug Discovery and Development

Huntington's disease (HD) is a lethal neurodegenerative disorder without efficient therapeutic options. The inefficient translation from preclinical and clinical research into clinical use is mainly attributed to the lack of (i) understanding of disease initiation, progression, and involved molecular mechanisms; (ii) knowledge of the possible HD target space and general data awareness; (iii) detailed characterizations of available disease models; (iv) better suitable models; and (v) reliable and sensitive biomarkers. To generate robust HD-like symptoms in a mouse model, the neomycin resistance cassette was excised from zQ175 mice, generating a new line: zQ175^Δneo. We entirely describe the dynamics of behavioral, neuropathological, and immunohistological changes from 15-57 weeks of age. Specifically, zQ175^Δneo mice showed early astrogliosis from 15 weeks; growth retardation, body weight loss, and anxiety-like behaviors from 29 weeks; motor deficits and reduced muscular strength from 36 weeks; and finally slight microgliosis at 57 weeks of age. Additionally, we collected the entire bioactivity network of small-molecule HD modulators in a multitarget dataset (HD_MDS). Hereby, we uncovered 358 unique compounds addressing over 80 different pharmacological targets and pathways. Our data will support future drug discovery approaches and may serve as useful assessment platform for drug discovery and development against HD.

Neurodegenerative Diseases: From Dysproteostasis, Altered Calcium Signalosome to Selective Neuronal Vulnerability to AAV-Mediated Gene Therapy

Despite intense research into the multifaceted etiology of neurodegenerative diseases (ND), they remain incurable. Here we provide a brief overview of several major ND and explore novel therapeutic approaches. Although the cause (s) of ND are not fully understood, the accumulation of misfolded/aggregated proteins in the brain is a common pathological feature. This aggregation may initiate disruption of Ca⁺⁺ signaling, which is an early pathological event leading to altered dendritic structure, neuronal dysfunction, and cell death. Presently, ND gene therapies remain unidimensional, elusive, and limited to modifying one pathological feature while ignoring others. Considering the complexity of signaling cascades in ND, we discuss emerging therapeutic concepts and suggest that deciphering the molecular mechanisms involved in dendritic pathology may broaden the phenotypic spectrum of ND treatment. An innovative multiplexed gene transfer strategy that employs silencing and/or over-expressing multiple effectors could preserve vulnerable neurons before they are lost. Such therapeutic approaches may extend brain health span and ameliorate burdensome chronic disease states.

Regulation of Blos1 by IRE1 prevents the accumulation of Huntingtin protein aggregates

Huntington's disease is characterized by accumulation of the aggregation-prone mutant Huntingtin (mHTT) protein. Here, we show that expression of exon 1 of mHTT in mouse cultured cells activates IRE1, the transmembrane sensor of stress in the endoplasmic reticulum, leading to degradation of the Blos1 mRNA and repositioning of lysosomes and late endosomes toward the microtubule organizing center. Overriding Blos1 degradation results in excessive accumulation of mHTT aggregates in both cultured cells and primary neurons. Although mHTT is degraded by macroautophagy when highly expressed, we show that before the formation of large aggregates, mHTT is degraded via an ESCRT-dependent, macroautophagy-independent pathway consistent with endosomal microautophagy. This pathway is enhanced by Blos1 degradation and appears to protect cells from a toxic, less aggregated form of mHTT.

Sec61 complex/translocon: The role of an atypical ER Ca2+-leak channel in health and disease

The heterotrimeric Sec61 protein complex forms the functional core of the so-called translocon that forms an aqueous channel in the endoplasmic reticulum (ER). The primary role of the Sec61 complex is to allow protein import in the ER during translation. Surprisingly, a completely different function in intracellular Ca2+ homeostasis has emerged for the Sec61 complex, and the latter is now accepted as one of the major Ca2+-leak pathways of the ER. In this review, we first discuss the structure of the Sec61 complex and focus on the pharmacology and regulation of the Sec61 complex as a Ca2+-leak channel. Subsequently, we will pay particular attention to pathologies that are linked to Sec61 mutations, such as plasma cell deficiency and congenital neutropenia. Finally, we will explore the relevance of the Sec61 complex as a Ca2+-leak channel in various pathophysiological (ER stress, apoptosis, ischemia-reperfusion) and pathological (type 2 diabetes, cancer) settings.

Role of MicroRNAs, Aptamers in Neuroinflammation and Neurodegenerative Disorders

Exploring the microRNAs and aptamers for their therapeutic role as biological drugs has expanded the horizon of its applicability against various human diseases, explicitly targeting the genetic materials. RNA-based therapeutics are widely being explored for the treatment and diagnosis of multiple diseases, including neurodegenerative disorders (NDD). Latter includes microRNA, aptamers, ribozymes, and small interfering RNAs (siRNAs), which control the gene expression mainly at the transcriptional strata. One RNA transcript translates into different protein types; hence, therapies targeted at the transcriptional sphere may have prominent and more extensive effects than alternative therapeutics. Unlike conventional gene therapy, RNAs, upon delivery, can either altogether abolish or alter the synthesis of the protein of interest, therefore, regulating their activities in a controlled and diverse manner. NDDs like Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis, Prion disease, and others are characterized by deposition of misfolded protein such as amyloid-ß, tau, α-synuclein, huntingtin and prion proteins. Neuroinflammation, one of the perquisites for neurodegeneration, is induced during neurodegenerative pathogenesis. In this review, we discuss microRNAs and aptamers' role as two different RNA-based approaches for their unique ability to regulate protein production at the transcription level, hence offering many advantages over other biologicals. The microRNA acts either by alleviating the malfunctioning RNA expression or by working as a replacement to lost microRNA. On the contrary, aptamer act as a chemical antibody and forms an aptamer-target complex.

Age Worsens the Cognitive Phenotype in Mice Carrying the Thr92Ala-DIO2 Polymorphism

The Thr92Ala-Dio2 polymorphism has been associated with reduced cognition in 2-month-old male mice and increased risk for cognitive impairment and Alzheimer's disease in African Americans. This has been attributed to reduced thyroid hormone (TH) signaling and endoplasmic reticulum (ER) stress in the brain. Here we studied the Thr92Ala-Dio2 mouse model and saw that older male mice (7-8-month-old) exhibited a more severe cognition impairment, which extended to different aspects of declarative and working memories. A similar phenotype was observed in 4-5-month-old female mice. There were no structural alterations in the prefrontal cortex (PFC) and hippocampus of the Thr92Ala-Dio2 mouse. Nonetheless, in both male and female PFC, there was an enrichment in genes associated with TH-dependent processes, ER stress, and Golgi apparatus, while in the hippocampus there was additional enrichment in genes associated with inflammation and apoptosis. Reduced TH signaling remains a key mechanism of disease given that short-term treatment with L-T3 rescued the cognitive phenotype observed in males and females. We conclude that in mice, age is an additional risk factor for cognitive impairment associated with the Thr92Ala-Dio2 polymorphism. In addition to reduced TH signaling, ER-stress, and involvement of the Golgi apparatus, hippocampal inflammation and apoptosis were identified as potentially important mechanisms of a disease.

Endoplasmic Reticulum Stress and Its Role in Homeostasis and Immunity of Central and Peripheral Neurons

Neuronal cells are specialists for rapid transfer and translation of information. Their electrical properties relay on a precise regulation of ion levels while their communication via neurotransmitters and neuropeptides depends on a high protein and lipid turnover. The endoplasmic Reticulum (ER) is fundamental to provide these necessary requirements for optimal neuronal function. Accumulation of misfolded proteins in the ER lumen, reactive oxygen species and exogenous stimulants like infections, chemical irritants and mechanical harm can induce ER stress, often followed by an ER stress response to reinstate cellular homeostasis. Imbedded between glial-, endothelial-, stromal-, and immune cells neurons are constantly in communication and influenced by their local environment. In this review, we discuss concepts of tissue homeostasis and innate immunity in the central and peripheral nervous system with a focus on its influence on ER stress, the unfolded protein response, and implications for health and disease.

Research Progress on Mono-ADP-Ribosyltransferases in Human Cell Biology

ADP-ribosylation is a well-established post-translational modification that is inherently connected to diverse processes, including DNA repair, transcription, and cell signaling. The crucial roles of mono-ADP-ribosyltransferases (mono-ARTs) in biological processes have been identified in recent years by the comprehensive use of genetic engineering, chemical genetics, and proteomics. This review provides an update on current methodological advances in the study of these modifiers. Furthermore, the review provides details on the function of mono ADP-ribosylation. Several mono-ARTs have been implicated in the development of cancer, and this review discusses the role and therapeutic potential of some mono-ARTs in cancer.

Transcriptome analysis endoplasmic reticulum-stress response in Litopenaeus vannamei hemocytes

Numerous studies have proved that endoplasmic reticulum (ER)-stress is an important cause of aquatic animal diseases. Therefore, for effectively preventing and controlling aquatic animal diseases, a systematic and in-depth understanding of the environmental stress response in aquatic animals is necessary. In present study, the influence of ER-stress in Litopenaeus vannamei was investigated using Illumina HiSeq based RNA-Seq. Comparing to the cDNA library of hemocytes treated with DMSO in L. vannamei, 286 unigenes were significantly upregulated and 473 unigenes were significantly down-regulated in the Thapsigargin treated group. KEGG analysis indicated that the differentially expressed genes (DEGs) are mainly related to ER-stress, immune as well as metabolism. Besides the classical ER-stress response pathways, the regulation of cell cycle and DNA replication are also important measures of ER-stress response. It has been suggested that the influence of ER-stress on immune genes might be an important factor in environmental stress inducing shrimp disease. Our investigation exhibited that immune-related DEG Prophenoloxidase activating enzyme 2 (LvPPAE2) roled in anti-pathogen immunity of shrimp. This study provides a solid foundation for uncovering the environmental adaptation response and especially its relationship with L. vannamei immune system.

Haplotype mapping uncovers unexplored variation in wild and domesticated soybean at the major protein locus cqProt-003

KEY MESSAGE

The major soy protein QTL, cqProt-003, was analysed for haplotype diversity and global distribution, and results indicate 304 bp deletion and variable tandem repeats in protein coding regions are likely causal candidates. Here, we present association and linkage analysis of 985 wild, landrace and cultivar soybean accessions in a pan genomic dataset to characterize the major high-protein/low-oil associated locus cqProt-003 located on chromosome 20. A significant trait-associated region within a 173 kb linkage block was identified, and variants in the region were characterized, identifying 34 high confidence SNPs, 4 insertions, 1 deletion and a larger 304 bp structural variant in the high-protein haplotype. Trinucleotide tandem repeats of variable length present in the second exon of gene Glyma.20G085100 are strongly correlated with the high-protein phenotype and likely represent causal variation. Structural variation has previously been found in the same gene, for which we report the global distribution of the 304 bp deletion and have identified additional nested variation present in high-protein individuals. Mapping variation at the cqProt-003 locus across demographic groups suggests that the high-protein haplotype is common in wild accessions (94.7%), rare in landraces (10.6%) and near absent in cultivated breeding pools (4.1%), suggesting its decrease in frequency primarily correlates with domestication and continued during subsequent improvement. However, the variation that has persisted in under-utilized wild and landrace populations holds high breeding potential for breeders willing to forego seed oil to maximize protein content. The results of this study include the identification of distinct haplotype structures within the high-protein population, and a broad characterization of the genomic context and linkage patterns of cqProt-003 across global populations, supporting future functional characterization and modification.

Protein S-sulfhydration: Unraveling the prospective of hydrogen sulfide in the brain, vasculature and neurological manifestations

Hydrogen sulfide (H₂S) and hydrogen polysulfides (H₂S_n) are essential regulatory signaling molecules generated by the entire body, including the central nervous system. Researchers have focused on the classical H₂S signaling from the past several decades, whereas the last decade has shown the emergence of H₂S-induced protein S-sulfhydration signaling as a potential therapeutic approach. Cysteine S-persulfidation is a critical paradigm of post-translational modification in the process of H₂S signaling. Additionally, studies have shown the cross-relationship between S-sulfhydration and other cysteine-induced post-translational modifications, namely nitrosylation and carbonylation. In the central nervous system, S-sulfhydration is involved in the cytoprotection through various signaling pathways, viz. inflammatory response, oxidative stress, endoplasmic reticulum stress, atherosclerosis, thrombosis, and angiogenesis. Further, studies have demonstrated H₂S-induced S-sulfhydration in regulating different biological processes, such as mitochondrial integrity, calcium homeostasis, blood-brain permeability, cerebral blood flow, and long-term potentiation. Thus, protein S-sulfhydration becomes a crucial regulatory molecule in cerebrovascular and neurodegenerative diseases. Herein, we first described the generation of intracellular H₂S followed by the application of H₂S in the regulation of cerebral blood flow and blood-brain permeability. Further, we described the involvement of S-sulfhydration in different biological and cellular functions, such as inflammatory response, mitochondrial integrity, calcium imbalance, and oxidative stress. Moreover, we highlighted the importance of S-sulfhydration in cerebrovascular and neurodegenerative diseases.

Passive immunization against phosphorylated tau improves features of Huntington's disease pathology

Huntington's disease is classically described as a neurodegenerative disorder of monogenic aetiology. The disease is characterized by an abnormal polyglutamine expansion in the huntingtin gene, which drives the toxicity of the mutated form of the protein. However, accumulation of the microtubule-associated protein tau, which is involved in a number of neurological disorders, has also been observed in patients with Huntington's disease. In order to unravel the contribution of tau hyperphosphorylation to hallmark features of Huntington's disease, we administered weekly intraperitoneal injections of the anti-tau pS202 CP13 monoclonal antibody to zQ175 mice and characterized the resulting behavioral and biochemical changes. After 12 weeks of treatment, motor impairments, cognitive performance and general health were improved in zQ175 mice along with a significant reduction in hippocampal pS202 tau levels. Despite the lack of effect of CP13 on neuronal markers associated with Huntington's disease pathology, tau-targeting enzymes and gliosis, CP13 was shown to directly impact mutant huntingtin aggregation such that brain levels of amyloid fibrils and huntingtin oligomers were decreased, while larger huntingtin protein aggregates were increased. Investigation of CP13 treatment of Huntington's disease patient-derived induced pluripotent stem cells (iPSCs) revealed a reduction in pS202 levels in differentiated cortical neurons and a rescue of neurite length. Collectively, these findings suggest that attenuating tau pathology could mitigate behavioral and molecular hallmarks associated with Huntington's disease.

A review on Huntington protein Insight into protein aggregation and therapeutic interventions

Huntington disease (HD) is a distressing, innate neurodegenerative disease that descends from CAG repeat expansion in the huntingtin gene causing behavioral changes, motor dysfunction, and dementia in children and adults. Mutation in huntingtin (HTT) protein has been suggested to cause neuron loss in the cortex and striatum through various mechanisms including abnormal regulation of transcription, proteasomal dysfunction, post-translational modification, and other events, regulating toxicity. Pathogenesis of HD involves cleavage of the huntingtin protein followed by the neuronal accumulation of its aggregated form. Several research groups made possible efforts to reduce huntingtin gene expression, protein accumulation, and protein aggregation using inhibitors and molecular chaperones as developing drugs against HD. Herein, we review the mechanism proposed towards the formation of HTT protein aggregation and the impact of therapeutic strategies for the treatment of HD.

The Complex Mechanisms by Which Neurons Die Following DNA Damage in Neurodegenerative Diseases

Human cells are exposed to numerous exogenous and endogenous insults every day. Unlike other molecules, DNA cannot be replaced by resynthesis, hence damage to DNA can have major consequences for the cell. The DNA damage response contains overlapping signalling networks that repair DNA and hence maintain genomic integrity, and aberrant DNA damage responses are increasingly described in neurodegenerative diseases. Furthermore, DNA repair declines during aging, which is the biggest risk factor for these conditions. If unrepaired, the accumulation of DNA damage results in death to eliminate cells with defective genomes. This is particularly important for postmitotic neurons because they have a limited capacity to proliferate, thus they must be maintained for life. Neuronal death is thus an important process in neurodegenerative disorders. In addition, the inability of neurons to divide renders them susceptible to senescence or re-entry to the cell cycle. The field of cell death has expanded significantly in recent years, and many new mechanisms have been described in various cell types, including neurons. Several of these mechanisms are linked to DNA damage. In this review, we provide an overview of the cell death pathways induced by DNA damage that are relevant to neurons and discuss the possible involvement of these mechanisms in neurodegenerative conditions.

Oligomannose-Type Glycan Processing in the Endoplasmic Reticulum and Its Importance in Misfolding Diseases

Simple Summary

Glycans play many roles in biological processes. For instance, they mediate cell–cell interaction, viral infection, and protein folding of glycoproteins. Glycoprotein folding in the endoplasmic reticulum (ER) is closely related to the onset of diseases such as misfolding diseases caused by accumulation of misfolded proteins in the ER. In this review, we focused on oligomannose-type glycan processing in the ER, which has central roles in glycoprotein folding in the ER, and we summarise relationship between oligomannose-type glycan processing and misfolding diseases arising from the disruption of ER homeostasis.

Abstract

Glycoprotein folding plays a critical role in sorting glycoprotein secretion and degradation in the endoplasmic reticulum (ER). Furthermore, relationships between glycoprotein folding and several diseases, such as type 2 diabetes and various neurodegenerative disorders, are indicated. Patients’ cells with type 2 diabetes, and various neurodegenerative disorders induce ER stress, against which the cells utilize the unfolded protein response for protection. However, in some cases, chronic and/or massive ER stress causes critical damage to cells, leading to the onset of ER stress-related diseases, which are categorized into misfolding diseases. Accumulation of misfolded proteins may be a cause of ER stress, in this respect, perturbation of oligomannose-type glycan processing in the ER may occur. A great number of studies indicate the relationships between ER stress and misfolding diseases, while little evidence has been reported on the connection between oligomannose-type glycan processing and misfolding diseases. In this review, we summarize alteration of oligomannose-type glycan processing in several ER stress-related diseases, especially misfolding diseases and show the possibility of these alteration of oligomannose-type glycan processing as indicators of diseases.

A C-terminal ataxin-2 disordered region promotes Huntingtin protein aggregation and neurodegeneration in Drosophila models of Huntington’s disease

The Ataxin-2 (Atx2) protein contributes to the progression of neurodegenerative phenotypes in animal models of amyotrophic lateral sclerosis (ALS), type 2 spinocerebellar ataxia (SCA-2), Parkinson’s disease, and Huntington’s disease (HD). However, because the Atx2 protein contains multiple separable activities, deeper understanding requires experiments to address the exact mechanisms by which Atx2 modulates neurodegeneration (ND) progression. Recent work on two ALS models, C9ORF72 and FUS, in Drosophila has shown that a C-terminal intrinsically disordered region (cIDR) of Atx2 protein, required for assembly of ribonucleoprotein (RNP) granules, is essential for the progression of neurodegenerative phenotypes as well as for accumulation of protein inclusions associated with these ALS models. Here, we show that the Atx2-cIDR also similarly contributes to the progression of degenerative phenotypes and accumulation of Huntingtin protein aggregates in Drosophila models of HD. Because Huntingtin is not an established component of RNP granules, these observations support a recently hypothesized, unexpected protein-handling function for RNP granules, which could contribute to the progression of Huntington’s disease and, potentially, other proteinopathies.

Protein Aggregation in the ER: Calm behind the Storm

As one of the largest organelles in eukaryotic cells, the endoplasmic reticulum (ER) plays a vital role in the synthesis, folding, and assembly of secretory and membrane proteins. To maintain its homeostasis, the ER is equipped with an elaborate network of protein folding chaperones and multiple quality control pathways whose cooperative actions safeguard the fidelity of protein biogenesis. However, due to genetic abnormalities, the error-prone nature of protein folding and assembly, and/or defects or limited capacities of the protein quality control systems, nascent proteins may become misfolded and fail to exit the ER. If not cleared efficiently, the progressive accumulation of misfolded proteins within the ER may result in the formation of toxic protein aggregates, leading to the so-called “ER storage diseases”. In this review, we first summarize our current understanding of the protein folding and quality control networks in the ER, including chaperones, unfolded protein response (UPR), ER-associated protein degradation (ERAD), and ER-selective autophagy (ER-phagy). We then survey recent research progress on a few ER storage diseases, with a focus on the role of ER quality control in the disease etiology, followed by a discussion on outstanding questions and emerging concepts in the field.

Connection Lost, MAM: Errors in ER-Mitochondria Connections in Neurodegenerative Diseases

Mitochondria associated membranes (MAMs), as the name suggests, are the membranes that physically and biochemically connect mitochondria with endoplasmic reticulum. MAMs not only structurally but also functionally connect these two important organelles within the cell which were previously thought to exist independently. There are multiple points of communication between ER-mitochondria and MAMs play an important role in both ER and mitochondria functions such as Ca²⁺ homeostasis, proteostasis, mitochondrial bioenergetics, movement, and mitophagy. The number of disease-related proteins and genes being associated with MAMs has been continually on the rise since its discovery. There is an overwhelming overlap between the biochemical functions of MAMs and processes affected in neurodegenerative disorders such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD). Thus, MAMs have received well-deserving and much delayed attention as modulators for ER-mitochondria communication and function. This review briefly discusses the recent progress made in this now fast developing field full of promise for very exciting future therapeutic discoveries.

Esculetin Provides Neuroprotection against Mutant Huntingtin-Induced Toxicity in Huntington's Disease Models

Huntington’s disease (HD) is a neurodegenerative disorder caused by an abnormal CAG trinucleotide repeat expansion within exon 1 of the huntingtin (HTT) gene. This mutation leads to the production of mutant HTT (mHTT) protein which triggers neuronal death through several mechanisms. Here, we investigated the neuroprotective effects of esculetin (ESC), a bioactive phenolic compound, in an inducible PC12 model and a transgenic Drosophila melanogaster model of HD, both of which express mHTT fragments. ESC partially inhibited the progression of mHTT aggregation and reduced neuronal death through its ability to counteract the oxidative stress and mitochondria impairment elicited by mHTT in the PC12 model. The ability of ESC to counteract neuronal death was also confirmed in the transgenic Drosophila model. Although ESC did not modify the lifespan of the transgenic Drosophila, it still seemed to have a positive impact on the HD phenotype of this model. Based on our findings, ESC may be further studied as a potential neuroprotective agent in a rodent transgenic model of HD.

PGRMC1 acts as a size-selective cargo receptor to drive ER-phagic clearance of mutant prohormones

The reticulon-3 (RTN3)-driven targeting complex promotes clearance of misfolded prohormones from the endoplasmic reticulum (ER) for lysosomal destruction by ER-phagy. Because RTN3 resides in the cytosolic leaflet of the ER bilayer, the mechanism of selecting misfolded prohormones as ER-phagy cargo on the luminal side of the ER membrane remains unknown. Here we identify the ER transmembrane protein PGRMC1 as an RTN3-binding partner. Via its luminal domain, PGRMC1 captures misfolded prohormones, targeting them for RTN3-dependent ER-phagy. PGRMC1 selects cargos that are smaller than the large size of other reported ER-phagy substrates. Cargos for PGRMC1 include mutant proinsulins that block secretion of wildtype proinsulin through dominant-negative interactions within the ER, causing insulin-deficiency. Chemical perturbation of PGRMC1 partially restores WT insulin storage by preventing ER-phagic degradation of WT and mutant proinsulin. Thus, PGRMC1 acts as a size-selective cargo receptor during RTN3-dependent ER-phagy, and is a potential therapeutic target for diabetes.

All Roads Lead to Rome: Different Molecular Players Converge to Common Toxic Pathways in Neurodegeneration

Multiple neurodegenerative diseases (NDDs) such as Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS) and Huntington’s disease (HD) are being suggested to have common cellular and molecular pathological mechanisms, characterized mainly by protein misfolding and aggregation. These large inclusions, most likely, represent an end stage of a molecular cascade; however, the soluble misfolded proteins, which take part in earlier steps of this cascade, are the more toxic players. These pathological proteins, which characterize each specific disease, lead to the selective vulnerability of different neurons, likely resulting from a combination of different intracellular mechanisms, including mitochondrial dysfunction, ER stress, proteasome inhibition, excitotoxicity, oxidative damage, defects in nucleocytoplasmic transport, defective axonal transport and neuroinflammation. Damage within these neurons is enhanced by damage from the nonneuronal cells, via inflammatory processes that accelerate the progression of these diseases. In this review, while acknowledging the hallmark proteins which characterize the most common NDDs; we place specific focus on the common overlapping mechanisms leading to disease pathology despite these different molecular players and discuss how this convergence may occur, with the ultimate hope that therapies effective in one disease may successfully translate to another.

Involvement of Oxidative and Endoplasmic Reticulum Stress in RDH12-Related Retinopathies

Retinol dehydrogenase 12 (RDH12) is expressed in photoreceptor inner segments and catalyses the reduction of all-trans retinal (atRAL) to all-trans retinol (atROL), as part of the visual cycle. Mutations in RDH12 are primarily associated with autosomal recessive Leber congenital amaurosis. To further our understanding of the disease mechanisms, HEK-293 cell lines expressing wildtype (WT) and mutant RDH12 were created. The WT cells afforded protection from atRAL-induced toxicity and oxidative stress. Mutant RDH12 cells displayed reduced protein expression and activity, with an inability to protect cells from atRAL toxicity, inducing oxidative and endoplasmic reticulum (ER) stress, with upregulation of sXBP1, CHOP, and ATF4. Pregabalin, a retinal scavenger, attenuated atRAL-induced ER stress in the mutant RDH12 cell lines. A zebrafish rdh12 mutant model (rdh12^u533 c.17_23del; p.(Val6AlafsTer5)) was generated through CRISPR-Cas9 gene editing. Mutant fish showed disrupted phagocytosis through transmission electron microscopy, with increased phagosome size at 12 months post-fertilisation. Rhodopsin mislocalisation and reduced expression of atg12 and sod2 indicated early signs of a rod-predominant degeneration. A lack of functional RDH12 results in ER and oxidative stress representing key pathways to be targeted for potential therapeutics.

Protein misfolding, ER Stress and Chaperones: An approach to develop chaperone-based therapeutics for Alzheimer's Disease

Alzheimer's disease (AD) is a heterogeneous neurodegenerative disorder with complex etiology that eventually leads to dementia. The main culprit of AD is the extracellular deposition of β-amyloid (Aβ) and intracellular neurofibrillary tangles. The protein conformational change and protein misfolding are the key events of AD pathophysiology, therefore endoplasmic reticulum (ER) stress is an apparent consequence. ER, stress-induced unfolded protein response (UPR) mediators (viz. PERK, IRE1, and ATF6) have been reported widely in the AD brain. Considering these factors, preventing proteins misfolding or aggregation of tau or amyloidogenic proteins appears to be the best approach to halt its pathogenesis. Therefore, therapies through chemical and pharmacological chaperones came to light as an alternative for the treatment of AD. Diverse studies have demonstrated 4-phenylbutyric acid (4-PBA) as a potential therapeutic agent in AD. The current review outlined the mechanism of protein misfolding, different etiological features behind the progression of AD, the significance of ER stress in AD, and the potential therapeutic role of different chaperones to counter AD. The study also highlights the gaps in current knowledge of the chaperones-based therapeutic approach and the possibility of developing chaperones as a potential therapeutic agent for AD treatment.

miR-302 Attenuates Mutant Huntingtin-Induced Cytotoxicity through Restoration of Autophagy and Insulin Sensitivity

Huntington’s disease (HD) is an autosomal-dominant brain disorder caused by mutant huntingtin (mHtt). Although the detailed mechanisms remain unclear, the mutational expansion of polyglutamine in mHtt is proposed to induce protein aggregates and neuronal toxicity. Previous studies have shown that the decreased insulin sensitivity is closely related to mHtt-associated impairments in HD patients. However, how mHtt interferes with insulin signaling in neurons is still unknown. In the present study, we used a HD cell model to demonstrate that the miR-302 cluster, an embryonic stem cell-specific polycistronic miRNA, is significantly downregulated in mHtt-Q74-overexpressing neuronal cells. On the contrary, restoration of miR-302 cluster was shown to attenuate mHtt-induced cytotoxicity by improving insulin sensitivity, leading to a reduction of mHtt aggregates through the enhancement of autophagy. In addition, miR-302 also promoted mitophagy and stimulated Sirt1/AMPK-PGC1α pathway thereby preserving mitochondrial function. Taken together, these results highlight the potential role of miR-302 cluster in neuronal cells, and provide a novel mechanism for mHtt-impaired insulin signaling in the pathogenesis of HD.

Impaired Restoration of Global Protein Synthesis Contributes to Increased Vulnerability to Acute ER Stress Recovery in Huntington's Disease

Neurons are susceptible to different cellular stresses and this vulnerability has been implicated in the pathogenesis of Huntington's disease (HD). Accumulating evidence suggest that acute or chronic stress, depending on its duration and severity, can cause irreversible cellular damages to HD neurons, which contributes to neurodegeneration. In contrast, how normal and HD neurons respond during the resolution of a cellular stress remain less explored. In this study, we challenged normal and HD cells with a low-level acute ER stress and examined the molecular and cellular responses after stress removal. Using both striatal cell lines and primary neurons, we first showed the temporal activation of p-eIF2α-ATF4-GADD34 pathway in response to the acute ER stress and during recovery between normal and HD cells. HD cells were more vulnerable to cell death during stress recovery and were associated with increased number of apoptotic/necrotic cells and decreased cell proliferation. This is also supported by the Gene Ontology analysis from the RNA-seq data which indicated that "apoptosis-related Biological Processes" were more enriched in HD cells during stress recovery. We further showed that HD cells were defective in restoring global protein synthesis during stress recovery and promoting protein synthesis by an integrated stress response inhibitor, ISRIB, could attenuate cell death in HD cells. Together, these data suggest that normal and HD cells undergo distinct mechanisms of transcriptional reprogramming, leading to different cell fate decisions during the stress recovery.

Mitochondrial Dysfunction and Endoplasmic Reticulum Stress in Age Related Macular Degeneration, Role in Pathophysiology, and Possible New Therapeutic Strategies

Age related macular degeneration (AMD) is the main cause of legal blindness in developed countries. It is a multifactorial disease in which a combination of genetic and environmental factors contributes to increased risk of developing this vision-incapacitating condition. Oxidative stress plays a central role in the pathophysiology of AMD and recent publications have highlighted the importance of mitochondrial dysfunction and endoplasmic reticulum stress in this disease. Although treatment with vascular endothelium growth factor inhibitors have decreased the risk of blindness in patients with the exudative form of AMD, the search for new therapeutic options continues to prevent the loss of photoreceptors and retinal pigment epithelium cells, characteristic of late stage AMD. In this review, we explain how mitochondrial dysfunction and endoplasmic reticulum stress participate in AMD pathogenesis. We also discuss a role of several antioxidants (bile acids, resveratrol, melatonin, humanin, and coenzyme Q10) in amelioration of AMD pathology.

Pridopidine reduces mutant huntingtin-induced endoplasmic reticulum stress by modulation of the Sigma-1 receptor

The endoplasmic reticulum (ER)-localized Sigma-1 receptor (S1R) is neuroprotective in models of neurodegenerative diseases, among them Huntington disease (HD). Recent clinical trials in HD patients and preclinical studies in cellular and mouse HD models suggest a therapeutic potential for the high-affinity S1R agonist pridopidine. However, the molecular mechanisms of the cytoprotective effect are unclear. We have previously reported strong induction of ER stress by toxic mutant huntingtin (mHtt) oligomers, which is reduced upon sequestration of these mHtt oligomers into large aggregates. Here, we show that pridopidine significantly ameliorates mHtt-induced ER stress in cellular HD models, starting at low nanomolar concentrations. Pridopidine reduced the levels of markers of the three branches of the unfolded protein response (UPR), showing the strongest effects on the PKR-like endoplasmic reticulum kinase (PERK) branch. The effect is S1R-dependent, as it is abolished in cells expressing mHtt in which the S1R was deleted using CRISPR/Cas9 technology. mHtt increased the level of the detergent-insoluble fraction of S1R, suggesting a compensatory cellular mechanism that responds to increased ER stress. Pridopidine further enhanced the levels of insoluble S1R, suggesting the stabilization of activated S1R oligomers. These S1R oligomeric species appeared in ER-localized patches, and not in the mitochondria-associated membranes nor the ER-derived quality control compartment. The colocalization of S1R with the chaperone BiP was significantly reduced by mHtt, and pridopidine restored this colocalization to normal, unstressed levels. Pridopidine increased toxic oligomeric mHtt recruitment into less toxic large sodium dodecyl sulfate-insoluble aggregates, suggesting that this in turn reduces ER stress and cytotoxicity.

Huntington's disease mice and human brain tissue exhibit increased G3BP1 granules and TDP43 mislocalization

Chronic cellular stress associated with neurodegenerative disease can result in the persistence of stress granule (SG) structures, membraneless organelles that form in response to cellular stress. In Huntington's disease (HD), chronic expression of mutant huntingtin generates various forms of cellular stress, including activation of the unfolded protein response and oxidative stress. However, it has yet to be determined whether SGs are a feature of HD neuropathology. We examined the miRNA composition of extracellular vesicles (EVs) present in the cerebrospinal fluid (CSF) of patients with HD and show that a subset of their target mRNAs were differentially expressed in the prefrontal cortex. Of these targets, SG components were enriched, including the SG-nucleating Ras GTPase-activating protein-binding protein 1 (G3BP1). We investigated localization and levels of G3BP1 and found a significant increase in the density of G3BP1-positive granules in the cortex and hippocampus of R6/2 transgenic mice and in the superior frontal cortex of the brains of patients with HD. Intriguingly, we also observed that the SG-associated TAR DNA-binding protein 43 (TDP43), a nuclear RNA/DNA binding protein, was mislocalized to the cytoplasm of G3BP1 granule-positive HD cortical neurons. These findings suggest that G3BP1 SG dynamics may play a role in the pathophysiology of HD.