Combining valosin-containing protein (VCP) inhibition and suberanilohydroxamic acid (SAHA) treatment additively enhances the folding, trafficking, and function of epilepsy-associated γ-aminobutyric acid, type A (GABAA) receptors

ATPase Valosin-Containing Protein (VCP) Is Involved During the Replication and Egress of Sialodacryoadenitis Virus (SDAV) in Neurons

Sialodacryoadenitis virus (SDAV) has been identified as the etiological agent responsible for the respiratory system and salivary gland infections in rats. The existing literature on SDAV infections is insufficient to address the topic adequately, particularly in relation to the central nervous system. In order to ascertain how SDAV gains access to neuronal cells and subsequently exits, our attention was focused on the small molecule valosin-containing protein (VCP), which is an ATPase. VCP is acknowledged for its function in the ubiquitin-mediated proteasomal degradation of proteins, including those of viral origin. To ascertain the potential influence of VCP on SDAV replication and egress, high-content screening was employed to determine the viral titer and protein content. Western blot analysis was employed to ascertain the relative expression of VCP. Real-time imaging of SDAV-infected cells and confocal imaging for qualitative morphological analysis were conducted. The Eeyarestatin I (EerI) inhibitor was employed to disrupt VCP involvement in the endoplasmic reticulum-associated protein degradation pathway (ERAD) in both pre- and post-incubation systems, with concentrations of 5 μM/mL and 25 μM/mL, respectively. We demonstrated for the first time that SDAV productively replicates in cultured primary neurons. VCP expression is markedly elevated during SDAV infection. The application of 5 μM/mL EerI in the post-treatment system yielded a statistically significant inhibition of the SDAV yield. It is likely that this modulates the efficacy of virion assembly by arresting viral proteins in the submembrane area.

Pharmacological chaperones restore proteostasis of epilepsy-associated GABAA receptor variants

Recent advances in genetic diagnosis identified variants in genes encoding GABAA receptors as causative for genetic epilepsy. Here, we selected eight disease-associated variants in the α1 subunit of GABAA receptors causing mild to severe clinical phenotypes and showed that they are loss of function, mainly by reducing the folding and surface trafficking of the α1 protein. Furthermore, we sought client protein-specific pharmacological chaperones to restore the function of pathogenic receptors. Applications of positive allosteric modulators, including Hispidulin and TP003, increase the functional surface expression of the α1 variants. Mechanism of action study demonstrated that they enhance the folding, assembly, and trafficking and reduce the degradation of GABAA variants without activating the unfolded protein response in HEK293T cells and human iPSC-derived neurons. Since these compounds cross the blood-brain barrier, such a pharmacological chaperoning strategy holds great promise to treat genetic epilepsy in a GABAA receptor-specific manner.

Preliminary study on the mechanism of SAHA in the treatment of refractory epilepsy induced by GABRG2(F343L) mutation

Mutations in the γ-amino butyric acid type A (GABAA) receptor γ2 subunit gene, GABRG2, have been associated with refractory epilepsy. Increasing evidence indicates that suberoylanilide hydroxamic acid (SAHA), a broad-spectrum histone acetyltransferases (HDACs) inhibitor, can inhibit seizure onset. However, the mechanisms involved remains unknown. The present study aimed to explore the anti-epileptic effect and underlying mechanisms of SAHA in the treatment of refractory epilepsy induced by GABRG2 mutation. In the zebrafish line expressing human mutant GABRG2(F343L), Tg(hGABRG2F343L), SAHA was found to reduce seizure onset, swimming activity, and neuronal activity. In both Tg(hGABRG2F343L) zebrafish and HEK293T cells transfected with GABAA receptor subunits, SAHA could improve the pan-acetylation level and reduce the expression of HDAC1/10. The decreased expressions of GABAA receptor subunits could be rescued by SAHA treatment both in vivo and in vitro, which might be the result of increased gene transcription and protein trafficking. The up-regulated acetylation of histone H3 and H4 as well as Bip expression might be involved in the process. Taken together, our data proved that both histone and non-histone acetylation might contribute to the anti-epileptic effect of SAHA in refractory epilepsy caused by GABRG2(F343L) mutation, demonstrating SAHA as a promising therapeutic agent for refractory epilepsy.

Hsp47 promotes biogenesis of multi-subunit neuroreceptors in the endoplasmic reticulum

Protein homeostasis (proteostasis) deficiency is an important contributing factor to neurological and metabolic diseases. However, how the proteostasis network orchestrates the folding and assembly of multi-subunit membrane proteins is poorly understood. Previous proteomics studies identified Hsp47 (Gene: SERPINH1), a heat shock protein in the endoplasmic reticulum lumen, as the most enriched interacting chaperone for gamma-aminobutyric acid type A (GABAA) receptors. Here, we show that Hsp47 enhances the functional surface expression of GABAA receptors in rat neurons and human HEK293T cells. Furthermore, molecular mechanism study demonstrates that Hsp47 acts after BiP (Gene: HSPA5) and preferentially binds the folded conformation of GABAA receptors without inducing the unfolded protein response in HEK293T cells. Therefore, Hsp47 promotes the subunit-subunit interaction, the receptor assembly process, and the anterograde trafficking of GABAA receptors. Overexpressing Hsp47 is sufficient to correct the surface expression and function of epilepsy-associated GABAA receptor variants in HEK293T cells. Hsp47 also promotes the surface trafficking of other Cys-loop receptors, including nicotinic acetylcholine receptors and serotonin type 3 receptors in HEK293T cells. Therefore, in addition to its known function as a collagen chaperone, this work establishes that Hsp47 plays a critical and general role in the maturation of multi-subunit Cys-loop neuroreceptors.

Targeting endoplasmic reticulum associated degradation pathway combined with radiotherapy enhances the immunogenicity of esophageal cancer cells

Immunogenic cell death (ICD) is essential for the activation of immune system against cancer. We aimed to investigate the efficacy of endoplasmic reticulum (ER)-associated protein degradation (ERAD) inhibitors (EerI and NMS-873) in enhancing radiation-induced ICD in esophageal cancer (EC). EC cells were administered with ERAD inhibitors, radiation therapy (RT), and the combination treatment. ICD hallmarks including calreticulin (CALR), adenosine triphosphate (ATP), and high mobility group protein B1 (HMGB1) were detected. The efficacy of ERAD inhibitors combined with RT in stimulating ICD was analyzed. Additionally, the role of ICD hallmarks in immune cell infiltration and patient survival was investigated. Inhibiting ERAD pathways was able to stimulate ICD component emission from dying EC cells in a dose-dependent pattern. Radiation-induced ICD was significantly increased after high doses RT (≥10 Gy). ERAD inhibitor combined with moderate dose RT (≥6 Gy) was capable of stimulating increased ICD in EC cells. Dual therapy could elicit the antitumor immune response by enhancing dendritic cells maturation and phagocytosis. Further investigation revealed a significant correlation between CALR and tumor-infiltrating immune cells. Low expression of ATP and HMGB1 and high expression of CALR were associated with favorable survival in patients with EC. The immunogenicityof EC can be enhanced by ERAD inhibitors combined with moderate doses of RT. ICD hallmark genes, especially CALR, are correlated to immune cell infiltration and clinical outcomes in EC. The present results demonstrated an important method to improve the immunogenicity of EC cells for enhanced antitumor immune response.

Pharmacological chaperones restore proteostasis of epilepsy-associated GABAA receptor variants

Recent advances in genetic diagnosis identified variants in genes encoding GABAA receptors as causative for genetic epilepsy. Here, we selected eight disease-associated variants in the α 1 subunit of GABAA receptors causing mild to severe clinical phenotypes and showed that they are loss of function, mainly by reducing the folding and surface trafficking of the α 1 protein. Furthermore, we sought client protein-specific pharmacological chaperones to restore the function of pathogenic receptors. Applications of positive allosteric modulators, including Hispidulin and TP003, increase the functional surface expression of the α 1 variants. Mechanism of action study demonstrated that they enhance the folding and assembly and reduce the degradation of GABAA variants without activating the unfolded protein response in HEK293T cells and human iPSC-derived neurons. Since these compounds cross the blood-brain barrier, such a pharmacological chaperoning strategy holds great promise to treat genetic epilepsy in a GABAA receptor-specific manner.

Control of cell surface expression of GABAA receptors by a conserved region at the end of the N-terminal extracellular domain of receptor subunits

Type A γ-aminobutyric acid receptors (GABA_ARs) represent a family of pentameric GABA-gated Cl^-/HCO₃^- ion channels which mediate inhibitory transmission in the central nervous system. Cell surface expression of GABA_ARs, a prerequisite for their function, is dependent on the appropriate assembly of the receptor subunits and their transient interactions with molecular chaperones within the endoplasmic reticulum (ER) and Golgi apparatus. Here, we describe a highly conserved amino acid sequence within the extracellular N-terminal domain of the receptor subunits adjoining the first transmembrane domain as a region important for GABA_AR processing within the ER. Modifications of this region in the α1, β3, and γ2 subunits using insertion or site-directed mutagenesis impaired GABA_AR trafficking to the cell surface in heterologous cell systems although they had no effect on the subunit assembly. We found that mutated receptors accumulated in the ER where they were shown to associate with chaperones calnexin, BiP, and Grp94. However, their surface expression was increased when ER-associated degradation or proteosome function was inhibited, while modulation of ER calcium stores had little effect. When compared to the wt, mutated receptors showed decreased interaction with calnexin, similar binding to BiP, and increased association with Grp94. Structural modeling of calnexin interaction with the wt or mutated GABA_AR revealed that disruption in structure caused by mutations in the conserved region adjoining the first transmembrane domain may impair calnexin binding. Thus, this previously uncharacterized region plays an important role in intracellular processing of GABA_ARs at least in part by stabilizing their interaction with calnexin.

Quantitative interactome proteomics identifies a proteostasis network for GABAA receptors

Gamma-aminobutyric acid type A (GABAA) receptors are the primary inhibitory neurotransmitter-gated ion channels in the mammalian central nervous system. Maintenance of GABAA receptor protein homeostasis (proteostasis) in cells utilizing its interacting proteins is essential for the function of GABAA receptors. However, how the proteostasis network orchestrates GABAA receptor biogenesis in the endoplasmic reticulum is not well understood. Here, we employed a proteomics-based approach to systematically identify the interactomes of GABAA receptors. We carried out a quantitative immunoprecipitation-tandem mass spectrometry analysis utilizing stable isotope labeling by amino acids in cell culture. Furthermore, we performed comparative proteomics by using both WT α1 subunit and a misfolding-prone α1 subunit carrying the A322D variant as the bait proteins. We identified 125 interactors for WT α1-containing receptors, 105 proteins for α1(A322D)-containing receptors, and 54 overlapping proteins within these two interactomes. Our bioinformatics analysis identified potential GABAA receptor proteostasis network components, including chaperones, folding enzymes, trafficking factors, and degradation factors, and we assembled a model of their potential involvement in the cellular folding, degradation, and trafficking pathways for GABAA receptors. In addition, we verified endogenous interactions between α1 subunits and selected interactors by using coimmunoprecipitation in mouse brain homogenates. Moreover, we showed that TRIM21 (tripartite motif containing-21), an E3 ubiquitin ligase, positively regulated the degradation of misfolding-prone α1(A322D) subunits selectively. This study paves the way for understanding the molecular mechanisms as well as fine-tuning of GABAA receptor proteostasis to ameliorate related neurological diseases such as epilepsy.

The endoplasmic reticulum membrane complex promotes proteostasis of GABAA receptors

The endoplasmic reticulum membrane complex (EMC) plays a critical role in the biogenesis of tail-anchored proteins and a subset of multi-pass membrane proteins in the endoplasmic reticulum (ER). However, because of nearly exclusive expression of neurotransmitter-gated ion channels in the central nervous system (CNS), the role of the EMC in their biogenesis is not well understood. In this study, we demonstrated that the EMC positively regulates the surface trafficking and thus function of endogenous γ-aminobutyric acid type A (GABAA) receptors, the primary inhibitory ion channels in the mammalian brain. Moreover, among ten EMC subunits, EMC3 and EMC6 have the most prominent effect, and overexpression of EMC3 or EMC6 is sufficient to restore the function of epilepsy-associated GABAA receptor variants. In addition, EMC3 and EMC6 demonstrate endogenous interactions with major neuroreceptors, which depends on their transmembrane domains, suggesting a general role of the EMC in the biogenesis of neuroreceptors.

Disruption of the Ubiquitin-Proteasome System and Elevated Endoplasmic Reticulum Stress in Epilepsy

The epilepsies are a broad group of conditions characterized by repeated seizures, and together are one of the most common neurological disorders. Additionally, epilepsy is comorbid with many neurological disorders, including lysosomal storage diseases, syndromic intellectual disability, and autism spectrum disorder. Despite the prevalence, treatments are still unsatisfactory: approximately 30% of epileptic patients do not adequately respond to existing therapeutics, which primarily target ion channels. Therefore, new therapeutic approaches are needed. Disturbed proteostasis is an emerging mechanism in epilepsy, with profound effects on neuronal health and function. Proteostasis, the dynamic balance of protein synthesis and degradation, can be directly disrupted by epilepsy-associated mutations in various components of the ubiquitin-proteasome system (UPS), or impairments can be secondary to seizure activity or misfolded proteins. Endoplasmic reticulum (ER) stress can arise from failed proteostasis and result in neuronal death. In light of this, several treatment modalities that modify components of proteostasis have shown promise in the management of neurological disorders. These include chemical chaperones to assist proper folding of proteins, inhibitors of overly active protein degradation, and enhancers of endogenous proteolytic pathways, such as the UPS. This review summarizes recent work on the pathomechanisms of abnormal protein folding and degradation in epilepsy, as well as treatment developments targeting this area.

Endoplasmic Reticulum Associated Protein Degradation (ERAD) in the Pathology of Diseases Related to TGFβ Signaling Pathway: Future Therapeutic Perspectives

The transforming growth factor signaling pathway (TGFβ) controls a wide range of cellular activities in adulthood as well as during embryogenesis including cell growth, differentiation, apoptosis, immunological responses and other cellular functions. Therefore, germline mutations in components of the pathway have given rise to a heterogeneous spectrum of hereditary diseases with variable phenotypes associated with malformations in the cardiovascular, muscular and skeletal systems. Our extensive literature and database searches revealed 47 monogenic diseases associated with germline mutations in 24 out of 41 gene variant encoding for TGFβ components. Most of the TGFβ components are membrane or secretory proteins and they are therefore expected to pass through the endoplasmic reticulum (ER), where fidelity of proteins folding is stringently monitored via the ER quality control machineries. Elucidation of the molecular mechanisms of mutant proteins’ folding and trafficking showed the implication of ER associated protein degradation (ERAD) in the pathogenesis of some of the diseases. For example, hereditary hemorrhagic telangiectasia types 1 and 2 (HHT1 and HHT2) and familial pulmonary arterial hypertension (FPAH) associated with mutations in Endoglin, ALK1 and BMPR2 components of the signaling pathway, respectively, have all exhibited loss of function phenotype as a result of ER retention of some of their disease-causing variants. In some cases, this has led to premature protein degradation through the proteasomal pathway. We anticipate that ERAD will be involved in the mechanisms of other TGFβ signaling components and therefore warrants further research. In this review, we highlight advances in ER quality control mechanisms and their modulation as a potential therapeutic target in general with particular focus on prospect of their implementation in the treatment of monogenic diseases associated with TGFβ components including HHT1, HHT2, and PAH. In particular, we emphasis the need to establish disease mechanisms and to implement such novel approaches in modulating the molecular pathway of mutant TGFβ components in the quest for restoring protein folding and trafficking as a therapeutic approach.

Decanoic acid inhibits mTORC1 activity independent of glucose and insulin signaling

The mTORC1 complex provides a critical role in cell function, regulating a variety of processes including growth and autophagy. mTORC1 signaling is hyperactivated in a range of common diseases including cancer, epilepsy, and neurodegenerative disorders. Hence, mTORC1 signaling provides an important target for regulation in many contexts. Here, we show that decanoic acid, a key component of a widely used medicinal diet, reduces mTORC1 activity. We identify this in a tractable model system, and validate it in ex vivo rat brain tissue and in human iPSC-derived astrocytes from patients with a clinically relevant disease. Thus, we provide insight into an easily accessible therapeutic approach for a range of diseases.

Low-glucose and -insulin conditions, associated with ketogenic diets, can reduce the activity of the mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway, potentially leading to a range of positive medical and health-related effects. Here, we determined whether mTORC1 signaling is also a target for decanoic acid, a key component of the medium-chain triglyceride (MCT) ketogenic diet. Using a tractable model system, Dictyostelium, we show that decanoic acid can decrease mTORC1 activity, under conditions of constant glucose and in the absence of insulin, measured by phosphorylation of eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1). We determine that this effect of decanoic acid is dependent on a ubiquitin regulatory X domain-containing protein, mediating inhibition of a conserved Dictyostelium AAA ATPase, p97, a homolog of the human transitional endoplasmic reticulum ATPase (VCP/p97) protein. We then demonstrate that decanoic acid decreases mTORC1 activity in the absence of insulin and under high-glucose conditions in ex vivo rat hippocampus and in tuberous sclerosis complex (TSC) patient-derived astrocytes. Our data therefore indicate that dietary decanoic acid may provide a new therapeutic approach to down-regulate mTORC1 signaling.

Chaperoning Endoplasmic Reticulum-Associated Degradation (ERAD) and Protein Conformational Diseases

Misfolded proteins compromise cellular homeostasis. This is especially problematic in the endoplasmic reticulum (ER), which is a high-capacity protein-folding compartment and whose function requires stringent protein quality-control systems. Multiprotein complexes in the ER are able to identify, remove, ubiquitinate, and deliver misfolded proteins to the 26S proteasome for degradation in the cytosol, and these events are collectively termed ER-associated degradation, or ERAD. Several steps in the ERAD pathway are facilitated by molecular chaperone networks, and the importance of ERAD is highlighted by the fact that this pathway is linked to numerous protein conformational diseases. In this review, we discuss the factors that constitute the ERAD machinery and detail how each step in the pathway occurs. We then highlight the underlying pathophysiology of protein conformational diseases associated with ERAD.

Folding and Misfolding of Human Membrane Proteins in Health and Disease: From Single Molecules to Cellular Proteostasis

Advances over the past 25 years have revealed much about how the structural properties of membranes and associated proteins are linked to the thermodynamics and kinetics of membrane protein (MP) folding. At the same time biochemical progress has outlined how cellular proteostasis networks mediate MP folding and manage misfolding in the cell. When combined with results from genomic sequencing, these studies have established paradigms for how MP folding and misfolding are linked to the molecular etiologies of a variety of diseases. This emerging framework has paved the way for the development of a new class of small molecule "pharmacological chaperones" that bind to and stabilize misfolded MP variants, some of which are now in clinical use. In this review, we comprehensively outline current perspectives on the folding and misfolding of integral MPs as well as the mechanisms of cellular MP quality control. Based on these perspectives, we highlight new opportunities for innovations that bridge our molecular understanding of the energetics of MP folding with the nuanced complexity of biological systems. Given the many linkages between MP misfolding and human disease, we also examine some of the exciting opportunities to leverage these advances to address emerging challenges in the development of therapeutics and precision medicine.

Adapting Proteostasis and Autophagy for Controlling the Pathogenesis of Cystic Fibrosis Lung Disease

Cystic fibrosis (CF), a fatal genetic disorder predominant in the Caucasian population, is caused by mutations in the cystic fibrosis transmembrane conductance regulator (Cftr) gene. The most common mutation is the deletion of phenylalanine from the position-508 (F508del-CFTR), resulting in a misfolded-CFTR protein, which is unable to fold, traffic and retain its plasma membrane (PM) localization. The resulting CFTR dysfunction, dysregulates variety of key cellular mechanisms such as chloride ion transport, airway surface liquid (ASL) homeostasis, mucociliary-clearance, inflammatory-oxidative signaling, and proteostasis that includes ubiquitin-proteasome system (UPS) and autophagy. A collective dysregulation of these key homoeostatic mechanisms contributes to the development of chronic obstructive cystic fibrosis lung disease, instead of the classical belief focused exclusively on ion-transport defect. Hence, therapeutic intervention(s) aimed at rescuing chronic CF lung disease needs to correct underlying defect that mediates homeostatic dysfunctions and not just chloride ion transport. Since targeting all the myriad defects individually could be quite challenging, it will be prudent to identify a process which controls almost all disease-promoting processes in the CF airways including underlying CFTR dysfunction. There is emerging experimental and clinical evidence that supports the notion that impaired cellular proteostasis and autophagy plays a central role in regulating pathogenesis of chronic CF lung disease. Thus, correcting the underlying proteostasis and autophagy defect in controlling CF pulmonary disease, primarily via correcting the protein processing defect of F508del-CFTR protein has emerged as a novel intervention strategy. Hence, we discuss here both the rationale and significant therapeutic utility of emerging proteostasis and autophagy modulating drugs/compounds in controlling chronic CF lung disease, where targeted delivery is a critical factor-influencing efficacy.

Remodeling the endoplasmic reticulum proteostasis network restores proteostasis of pathogenic GABAA receptors

Biogenesis of membrane proteins is controlled by the protein homeostasis (proteostasis) network. We have been focusing on protein quality control of γ-aminobutyric acid type A (GABA_A) receptors, the major inhibitory neurotransmitter-gated ion channels in mammalian central nervous system. Proteostasis deficiency in GABA_A receptors causes loss of their surface expression and thus function on the plasma membrane, leading to epilepsy and other neurological diseases. One well-characterized example is the A322D mutation in the α1 subunit that causes its extensive misfolding and expedited degradation in the endoplasmic reticulum (ER), resulting in autosomal dominant juvenile myoclonic epilepsy. We aimed to correct misfolding of the α1(A322D) subunits in the ER as an approach to restore their functional surface expression. Here, we showed that application of BIX, a specific, potent ER resident HSP70 family protein BiP activator, significantly increases the surface expression of the mutant receptors in human HEK293T cells and neuronal SH-SY5Y cells. BIX attenuates the degradation of α1(A322D) and enhances their forward trafficking and function. Furthermore, because BiP is one major target of the two unfolded protein response (UPR) pathways: ATF6 and IRE1, we continued to demonstrate that modest activations of the ATF6 pathway and IRE1 pathway genetically enhance the plasma membrane trafficking of the α1(A322D) protein in HEK293T cells. Our results underlie the potential of regulating the ER proteostasis network to correct loss-of-function protein conformational diseases.

An engineered, quantifiable in vitro model for analysing the effect of proteostasis-targeting drugs on tissue physical properties

Cellular function depends on the maintenance of protein homeostasis (proteostasis) by regulated protein degradation. Chronic dysregulation of proteostasis is associated with neurodegenerative and age-related diseases, and drugs targeting components of the protein degradation apparatus are increasingly used in cancer therapies. However, as chronic imbalances rather than loss of function mediate their pathogenesis, research models that allow for the study of the complex effects of drugs on tissue properties in proteostasis-associated diseases are almost completely lacking. Here, to determine the functional effects of impaired proteostatic fine-tuning, we applied a combination of materials science characterisation techniques to a cell-derived, in vitro model of bone-like tissue formation in which we pharmacologically perturbed protein degradation. We show that low-level inhibition of VCP/p97 and the proteasome, two major components of the degradation machinery, have remarkably different effects on the bone-like material that human bone-marrow derived mesenchymal stromal cells (hMSC) form in vitro. Specifically, whilst proteasome inhibition mildly enhances tissue formation, Raman spectroscopic, atomic force microscopy-based indentation, and electron microscopy imaging reveal that VCP/p97 inhibition induces the formation of bone-like tissue that is softer, contains less protein, appears to have more crystalline mineral, and may involve aberrant micro- and ultra-structural tissue organisation. These observations contrast with findings from conventional osteogenic assays that failed to identify any effect on mineralisation. Taken together, these data suggest that mild proteostatic impairment in hMSC alters the bone-like material they form in ways that could explain some pathologies associated with VCP/p97-related diseases. They also demonstrate the utility of quantitative materials science approaches for tackling long-standing questions in biology and medicine, and could form the basis for preclinical drug testing platforms to develop therapies for diseases stemming from perturbed proteostasis or for cancer therapies targeting protein degradation. Our findings may also have important implications for the field of tissue engineering, as the manufacture of cell-derived biomaterial scaffolds may need to consider proteostasis to effectively replicate native tissues.

Adapting Secretory Proteostasis and Function Through the Unfolded Protein Response

Cells address challenges to protein folding in the secretory pathway by engaging endoplasmic reticulum (ER)-localized protective mechanisms that are collectively termed the unfolded protein response (UPR). By the action of the transmembrane signal transducers IRE1, PERK, and ATF6, the UPR induces networks of genes whose products alleviate the burden of protein misfolding. The UPR also plays instructive roles in cell differentiation and development, aids in the response to pathogens, and coordinates the output of professional secretory cells. These functions add to and move beyond the UPR's classical role in addressing proteotoxic stress. Thus, the UPR is not just a reaction to protein misfolding, but also a fundamental driving force in physiology and pathology. Recent efforts have yielded a suite of chemical genetic methods and small molecule modulators that now provide researchers with both stress-dependent and -independent control of UPR activity. Such tools provide new opportunities to perturb the UPR and thereby study mechanisms for maintaining proteostasis in the secretory pathway. Numerous observations now hint at the therapeutic potential of UPR modulation for diseases related to the misfolding and aggregation of ER client proteins. Growing evidence also indicates the promise of targeting ER proteostasis nodes downstream of the UPR. Here, we review selected advances in these areas, providing a resource to inform ongoing studies of secretory proteostasis and function as they relate to the UPR.

Interactions between GHRH and GABAARs in the brains of patients with epilepsy and in animal models of epilepsy

Growth hormone releasing hormone (GHRH) has recently been shown to increase the level of γ-aminobutyric acid (GABA) and activate GABA receptors (GABARs) in the cerebral cortex. GABA is an inhibitory neurotransmitter that can inhibit seizures. Does GHRH enhance the inhibitory effect of GABA to prevent epilepsy by increasing the GABA level and activating GABARs? In this study, patients with epilepsy and C57/BL6 mice with epilepsy induced by kainic acid (KA) or pentylenetetrazol (PTZ) served as the research subjects. Western blots were used to observe the differences in GHRH expression between the normal group and the epilepsy group, immunofluorescence was performed to explore the localization of GHRH in the brain, and coimmunoprecipitation was used to observe the interaction between GHRH and GABARs. GHRH expression was significantly increased in both patients with temporal lobe epilepsy (TLE) and in two mouse models induced by KA or PTZ compared with that in the normal groups (P < 0.05 or P < 0.01). GHRH was expressed in neurons in both humans and mice. Additionally, GHRH co-localized with presynaptic and postsynaptic sites of inhibitory neurons. Coimmunoprecipitation confirmed that GHRH interacted with GABAAα1 and GABAAβ2 + 3. GHRH may play an important role in inhibiting seizures by activating GABAARs.

Cilostazol Suppresses Aβ-induced Neurotoxicity in SH-SY5Y Cells through Inhibition of Oxidative Stress and MAPK Signaling Pathway

Alzheimer's disease (AD) is a slowly progressive form of dementia, characterized by memory impairment and cognitive dysfunction. AD is mainly characterized by the deposition of amyloid β (Aβ) plaques and intracellular neurofibrillary tangles in the brain, along with neuronal degeneration and high levels of oxidative stress. Cilostazol (CSZ) was recently found to suppress the progression of cognitive decline in patients with stable AD receiving acetylcholinesterase inhibitors. This present study aimed to clarify the mechanism by which CSZ protects neurons from degeneration associated with Aβ(1-42). We used Aβ(1-42) to induce neurotoxicity in human neuroblastoma SH-SY5Y cells. Cells were pretreated with CSZ before co-treatment with Aβ. To evaluate the effect of CSZ on oxidative stress, we examined levels of reactive oxygen species (ROS), nicotinamide adenine dinucleotide phosphate oxidase (Nox) activity, mRNA expression of NOX4, and Cu/Zn-Superoxide Dismutase (SOD), as well as apoptosis biomarkers [MTT, (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide), caspase-3 and -9 activities and staining of annexin V]. We also assayed the activity of mitogen-activated protein kinases (MAPK): p38 MAPK and extracellular signal-regulated kinase1/2 (ERK1/2), and biomarkers of mitochondrial function (Bcl-2 and Bax), and cyclic adenosine monophosphate response element-binding protein (CREB). Aβ-induced oxidative stress (ROS, NOX4 activity, and expression of NOX mRNA), caspase activation (caspase-3 and -9), and p38 MAPK phosphorylation were suppressed by co-treatment with CSZ, but not by ERK1/2 activation. In addition, pretreatment with CSZ suppressed Aβ-induced apoptosis and increased cell viability via suppression of Bax (a proapoptotic protein), upregulation of Bcl-2 (an antiapoptotic protein) and Cu/Zn-SOD (a superoxide scavenging enzyme), and phosphorylation of CREB. These findings suggested that CSZ could counteract neurotoxicity through multiple mechanisms, one mechanism involving the attenuation of oxidative stress by suppressing NOX activity and Nox mRNA expression in Aβ-induced neurotoxicity and another involving the anti-neurotoxic effect via the ERK1/2/phosphorylated CREB pathway.

Regulating Secretory Proteostasis through the Unfolded Protein Response: From Function to Therapy

Imbalances in secretory proteostasis induced by genetic, environmental, or aging-related insults are pathologically associated with etiologically diverse protein misfolding diseases. To protect the secretory proteome from these insults, organisms evolved stress-responsive signaling pathways that regulate the composition and activity of biologic pathways involved in secretory proteostasis maintenance. The most prominent of these is the endoplasmic reticulum (ER) unfolded protein response (UPR), which functions to regulate ER proteostasis in response to ER stress. While the signaling mechanisms involved in UPR activation are well defined, the impact of UPR activation on secretory proteostasis is only now becoming clear. Here, we highlight recent reports defining how activation of select UPR signaling pathways influences proteostasis within the ER and downstream secretory environments. Furthermore, we describe recent evidence that highlights the therapeutic potential for targeting UPR signaling pathways to correct pathologic disruption in secretory proteostasis associated with diverse types of protein misfolding diseases.

Lycopene protects human SH‑SY5Y neuroblastoma cells against hydrogen peroxide‑induced death via inhibition of oxidative stress and mitochondria‑associated apoptotic pathways

Oxidative stress, which is characterized by excessive production of reactive oxygen species (ROS), is a common pathway that results in neuronal injury or death due to various types of pathological stress. Although lycopene has been identified as a potent antioxidant, its effect on hydrogen peroxide (H₂O₂)-induced neuronal damage remains unclear. In the present study, pretreatment with lycopene was observed to protect SH-SY5Y neuroblastoma cells against H₂O₂-induced death via inhibition of apoptosis resulting from activation of caspase-3 and translocation of apoptosis inducing factor (AIF) to the nucleus. Furthermore, the over-produced ROS, as well as the reduced activities of anti-oxidative enzymes, superoxide dismutase and catalase, were demonstrated to be alleviated by lycopene. Additionally, lycopene counteracted H₂O₂-induced mitochondrial dysfunction, which was evidenced by suppression of mitochondrial permeability transition pore opening, attenuation of the decline of the mitochondrial membrane potential, and inhibition of the increase of Bax and decrease of Bcl-2 levels within the mitochondria. The release of cytochrome c and AIF from the mitochondria was also reduced. These results indicate that lycopene is a potent neuroprotectant against apoptosis, oxidative stress and mitochondrial dysfunction, and could be administered to prevent neuronal injury or death.

Grp94 Protein Delivers γ-Aminobutyric Acid Type A (GABAA) Receptors to Hrd1 Protein-mediated Endoplasmic Reticulum-associated Degradation

Proteostasis maintenance of γ-aminobutyric acid type A (GABAA) receptors dictates their function in controlling neuronal inhibition in mammalian central nervous systems. However, as a multisubunit, multispan, integral membrane protein, even wild type subunits of GABAA receptors fold and assemble inefficiently in the endoplasmic reticulum (ER). Unassembled and misfolded subunits undergo ER-associated degradation (ERAD), but this degradation process remains poorly understood for GABAA receptors. Here, using the α1 subunits of GABAA receptors as a model substrate, we demonstrated that Grp94, a metazoan-specific Hsp90 in the ER lumen, uses its middle domain to interact with the α1 subunits and positively regulates their ERAD. OS-9, an ER-resident lectin, acts downstream of Grp94 to further recognize misfolded α1 subunits in a glycan-dependent manner. This delivers misfolded α1 subunits to the Hrd1-mediated ubiquitination and the valosin-containing protein-mediated extraction pathway. Repressing the initial ERAD recognition step by inhibiting Grp94 enhances the functional surface expression of misfolding-prone α1(A322D) subunits, which causes autosomal dominant juvenile myoclonic epilepsy. This study clarifies a Grp94-mediated ERAD pathway for GABAA receptors, which provides a novel way to finely tune their function in physiological and pathophysiological conditions.

L-type Calcium Channel Blockers Enhance Trafficking and Function of Epilepsy-associated α1(D219N) Subunits of GABA(A) Receptors

Gamma-aminobutyric acid type A (GABAA) receptors are the primary inhibitory ion channels in the mammalian central nervous system and play an essential role in regulating inhibition-excitation balance in neural circuits. The α1 subunit harboring the D219N mutation of GABAA receptors was reported to be retained in the endoplasmic reticulum (ER) and traffic inefficiently to the plasma membrane, leading to a loss of function of α1(D219N) subunits and thus idiopathic generalized epilepsy (IGE). We present the use of small molecule proteostasis regulators to enhance the forward trafficking of α1(D219N) subunits to restore their function. We showed that treatment with verapamil (4 μM, 24 h), an L-type calcium channel blocker, substantially increases the α1(D219N) subunit cell surface level in both HEK293 cells and neuronal SH-SY5Y cells and remarkably restores the GABA-induced maximal chloride current in HEK293 cells expressing α1(D219N)β2γ2 receptors to a level that is comparable to wild type receptors. Our drug mechanism study revealed that verapamil treatment promotes the ER to Golgi trafficking of the α1(D219N) subunits post-translationally. To achieve that, verapamil treatment enhances the interaction between the α1(D219N) subunit and β2 subunit and prevents the aggregation of the mutant protein by shifting the protein from the detergent-insoluble fractions to detergent-soluble fractions. By combining (35)S pulse-chase labeling and MG-132 inhibition experiments, we demonstrated that verapamil treatment does not inhibit the ER-associated degradation of the α1(D219N) subunit. In addition, its effect does not involve a dynamin-1 dependent endocytosis. To gain further mechanistic insight, we showed that verapamil increases the interaction between the mutant protein and calnexin and calreticulin, two major lectin chaperones in the ER. Moreover, calnexin binding promotes the forward trafficking of the mutant subunit. Taken together, our data indicate that verapamil treatment enhances the calnexin-assisted forward trafficking and subunit assembly, which leads to substantially enhanced functional surface expression of the mutant receptors. Since verapamil is an FDA-approved drug that crosses blood-brain barrier and has been used as an additional medication for some epilepsies, our findings suggest that verapamil holds great promise to be developed to ameliorate IGE resulting from α1(D219N) subunit trafficking deficiency.

A novel UGGT1 and p97-dependent checkpoint for native ectodomains with ionizable intramembrane residue

There is unexpected collaboration of the cytosolic AAA-ATPase p97 and the luminal quality control factor UGGT1 in a novel, BiP- and CNX-independent protein quality checkpoint. This prevents Golgi transport of a chimera with a native ectodomain that passes the luminal quality control scrutiny but displays an intramembrane defect.

Only native polypeptides are released from the endoplasmic reticulum (ER) to be transported at the site of activity. Persistently misfolded proteins are retained and eventually selected for ER-associated degradation (ERAD). The paradox of a structure-based protein quality control is that functional polypeptides may be destroyed if they are architecturally unfit. This has health-threatening implications, as shown by the numerous “loss-of-function” proteopathies, but also offers chances to intervene pharmacologically to promote bypassing of the quality control inspection and export of the mutant, yet functional protein. Here we challenged the ER of human cells with four modular glycopolypeptides designed to alert luminal and membrane protein quality checkpoints. Our analysis reveals the unexpected collaboration of the cytosolic AAA-ATPase p97 and the luminal quality control factor UDP-glucose:glycoprotein glucosyltransferase (UGGT1) in a novel, BiP- and CNX-independent checkpoint. This prevents Golgi transport of a chimera with a native ectodomain that passes the luminal quality control scrutiny but displays an intramembrane defect. Given that human proteopathies may result from impaired transport of functional polypeptides with minor structural defects, identification of quality checkpoints and treatments to bypass them as shown here upon silencing or pharmacologic inhibition of UGGT1 or p97 may have important clinical implications.