Participation of lectin chaperones and thiol oxidoreductases in protein folding within the endoplasmic reticulum

Calreticulin Regulates SARS-CoV-2 Spike Protein Turnover and Modulates SARS-CoV-2 Infectivity

Cardiovascular complications are major clinical hallmarks of acute and post-acute coronavirus disease 2019 (COVID-19). However, the mechanistic details of SARS-CoV-2 infectivity of endothelial cells remain largely unknown. Here, we demonstrate that the receptor binding domain (RBD) of the SARS-CoV-2 spike (S) protein shares a similarity with the proline-rich binding ena/VASP homology (EVH1) domain and identified the endoplasmic reticulum (ER) resident calreticulin (CALR) as an S-RBD interacting protein. Our biochemical analysis showed that CALR, via its proline-rich (P) domain, interacts with S-RBD and modulates proteostasis of the S protein. Treatment of cells with the proteasomal inhibitor bortezomib increased the expression of the S protein independent of CALR, whereas the lysosomal/autophagy inhibitor bafilomycin 1A, which interferes with the acidification of lysosome, selectively augmented the S protein levels in a CALR-dependent manner. More importantly, the shRNA-mediated knockdown of CALR increased SARS-CoV-2 infection and impaired calcium homeostasis of human endothelial cells. This study provides new insight into the infectivity of SARS-CoV-2, calcium hemostasis, and the role of CALR in the ER-lysosome-dependent proteolysis of the spike protein, which could be associated with cardiovascular complications in COVID-19 patients.

Structural and Dynamic Differences between Calreticulin Mutants Associated with Essential Thrombocythemia

Essential thrombocythemia (ET) is a blood cancer. ET is characterized by an overproduction of platelets that can lead to thrombosis formation. Platelet overproduction occurs in megakaryocytes through a signaling pathway that could involve JAK2, MPL, or CALR proteins. CALR mutations are associated with 25–30% of ET patients; CALR variants must be dimerized to induce ET. We classified these variants into five classes named A to E; classes A and B are the most frequent classes in patients with ET. The dynamic properties of these five classes using structural models of CALR’s C-domain were analyzed using molecular dynamics simulations. Classes A, B, and C are associated with frameshifts in the C-domain. Their dimers can be stable only if a disulfide bond is formed; otherwise, the two monomers repulse each other. Classes D and E cannot be stable as dimers due to the absence of disulfide bonds. Class E and wild-type CALR have similar dynamic properties. These results suggest that the disulfide bond newly formed in classes A, B, and C may be essential for the pathogenicity of these variants. They also underline that class E cannot be directly related to ET but corresponds to human polymorphisms.

Analysis of Selenoprotein F Binding to UDP-Glucose:Glycoprotein Glucosyltransferase (UGGT) by a Photoreactive Crosslinker

In the endoplasmic reticulum glycoprotein quality control system, UDP-glucose : glycoprotein glucosyltransferase (UGGT) functions as a folding sensor. Although it is known to form a heterodimer with selenoprotein F (SelenoF), the details of the complex formation remain obscure. A pulldown assay using co-transfected SelenoF and truncated mutants of human UGGT1 (HUGT1) revealed that SelenoF binds to the TRXL2 domain of HUGT1. Additionally, a newly developed photoaffinity crosslinker was selectively introduced into cysteine residues of recombinant SelenoF to determine the spatial orientation of SelenoF to HUGT1. The crosslinking experiments showed that SelenoF formed a covalent bond with amino acids in the TRXL3 region and the interdomain between βS2 and GT24 of HUGT1 via the synthetic crosslinker. SelenoF might play a role in assessing and refining the disulfide bonds of misfolded glycoproteins in the hydrophobic cavity of HUGT1 as it binds to the highly flexible region of HUGT1 to reach its long hydrophobic cavity. Clarification of the SelenoF-binding domain of UGGT and its relative position will help predict and reveal the function of SelenoF from a structural perspective.

Endoplasmic reticulum stress and ubiquitin-proteasome system impairment in natural scrapie

Chronic accumulation of misfolded proteins such as PrP^Sc can alter the endoplasmic reticulum homeostasis triggering the unfolded protein response (UPR). In this pathogenic event, the molecular chaperones play an important role. Several reports in humans and animals have suggested that neurodegeneration is related to endoplasmic reticulum stress in diseases caused by the accumulation of misfolded proteins. In this study, we investigated the expression of three endoplasmic reticulum stress markers: PERK (protein kinase R-like endoplasmic reticulum kinase), BiP (binding immunoglobulin protein), and PDI (Protein Disulfide Isomerase). In addition, we evaluated the accumulation of ubiquitin as a marker for protein degradation mediated by the proteasome. These proteins were studied in brain tissues of sheep affected by scrapie in clinical and preclinical stages of the disease. Results were compared with those observed in healthy controls. Scrapie-infected sheep showed significant higher levels of PERK, BiP/Grp78 and PDI than healthy animals. As we observed before in models of spontaneous prion disease, PDI was the most altered ER stress marker between scrapie-infected and healthy sheep. Significantly increased intraneuronal and neuropil ubiquitinated deposits were observed in certain brain areas in scrapie-affected animals compared to controls. Our results suggest that the neuropathological and neuroinflammatory phenomena that develop in prion diseases cause endoplasmic reticulum stress in brain cells triggering the UPR. In addition, the significantly higher accumulation of ubiquitin aggregates in scrapie-affected animals suggests an impairment of the ubiquitin-proteasome system in natural scrapie. Therefore, these proteins may contribute as biomarkers and/or therapeutic targets for prion diseases.

Calreticulin as a prognostic biomarker and correlated with immune infiltrate in kidney renal clear cell carcinoma

Background: Calreticulin (CALR) has been investigated in several malignant diseases and is associated with immune-cell infiltration. However, the prognostic value of CALR in kidney renal clear cell carcinoma (KIRC) is still unknown. Methods: Based on the computational analysis, data from 530 KIRC cases and 72 normal kidney samples from The Cancer Genome Atlas (TGCA-KIRC) database were analyzed in this study. The expression of CALR mRNA in pan-cancer and immune infiltrates was analyzed using the Tumor Immune Estimation Resource (TIMER) database. The CALR protein expression was obtained from the UALCAN and Human Protein Atlas (HPA) databases. Survival, functional, and statistical analyses were conducted using R software. Results: The CALR expression was higher in KIRC cases than in normal kidneys. A high CALR expression was correlated with TNM stage, pathological stage, and histological grade. Kaplan-Meier survival analysis showed that a high CALR expression was associated with poor overall survival, disease-specific survival, and progression-free interval. Gene set enrichment analysis (GSEA) indicated that CALR was enriched in IL-6 and IL-2 signaling, interferon signaling, TNF signaling, inflammatory response, apoptosis, and the p53 pathway. CALR is correlated with immune-infiltrating cells. A significant correlation was observed between CALR expression and immunomodulators. Conclusion: We identified CALR as a prognostic biomarker of KIRC. Meanwhile, the CALR expression associated with immune infiltration indicated that CALR might be a potential immunotherapy target for patients with KIRC.

To the Surface and Back: Exo- and Endocytic Pathways in Trypanosoma brucei

Trypanosoma brucei is one of only a few unicellular pathogens that thrives extracellularly in the vertebrate host. Consequently, the cell surface plays a critical role in both immune recognition and immune evasion. The variant surface glycoprotein (VSG) coats the entire surface of the parasite and acts as a flexible shield to protect invariant proteins against immune recognition. Antigenic variation of the VSG coat is the major virulence mechanism of trypanosomes. In addition, incessant motility of the parasite contributes to its immune evasion, as the resulting fluid flow on the cell surface drags immunocomplexes toward the flagellar pocket, where they are internalized. The flagellar pocket is the sole site of endo- and exocytosis in this organism. After internalization, VSG is rapidly recycled back to the surface, whereas host antibodies are thought to be transported to the lysosome for degradation. For this essential step to work, effective machineries for both sorting and recycling of VSGs must have evolved in trypanosomes. Our understanding of the mechanisms behind VSG recycling and VSG secretion, is by far not complete. This review provides an overview of the trypanosome secretory and endosomal pathways. Longstanding questions are pinpointed that, with the advent of novel technologies, might be answered in the near future.

Calreticulin mutations in myeloproliferative neoplasms

Calreticulin (CALR) is a chaperone present in the endoplasmic reticulum, which is involved in the quality control of N-glycosylated proteins and storage of calcium ions. In 2013, the C-terminal mutation in CALR was identified in half of the patients with essential thrombocythemia and primary myelofibrosis who did not have a JAK2 or MPL mutation. The results of 8 years of intensive research are changing the clinical practice associated with treating myeloproliferative neoplasms (MPNs). The presence or absence of CALR mutations and their mutation types already provide important information for diagnosis and treatment decision making. In addition, the interaction with the thrombopoietin receptor MPL, which is the main mechanism of transformation by CALR mutation, and the expression of the mutant protein on the cell surface have a great potential as targets for molecular-targeted drugs and immunotherapy. This chapter presents recent findings on the clinical significance of the CALR mutation and the molecular basis by which this mutation drives MPNs.

Ca2+ Regulates ERp57-Calnexin Complex Formation

ERp57, a member of the protein disulfide isomerase family, is a ubiquitous disulfide catalyst that functions in the oxidative folding of various clients in the mammalian endoplasmic reticulum (ER). In concert with ER lectin-like chaperones calnexin and calreticulin (CNX/CRT), ERp57 functions in virtually all folding stages from co-translation to post-translation, and thus plays a critical role in maintaining protein homeostasis, with direct implication for pathology. Here, we present mechanisms by which Ca²⁺ regulates the formation of the ERp57-calnexin complex. Biochemical and isothermal titration calorimetry analyses revealed that ERp57 strongly interacts with CNX via a non-covalent bond in the absence of Ca²⁺. The ERp57-CNX complex not only promoted the oxidative folding of human leukocyte antigen heavy chains, but also inhibited client aggregation. These results suggest that this complex performs both enzymatic and chaperoning functions under abnormal physiological conditions, such as Ca²⁺ depletion, to effectively guide proper oxidative protein folding. The findings shed light on the molecular mechanisms underpinning crosstalk between the chaperone network and Ca²⁺.

Calreticulin-Multifunctional Chaperone in Immunogenic Cell Death: Potential Significance as a Prognostic Biomarker in Ovarian Cancer Patients

Immunogenic cell death (ICD) is a type of death, which has the hallmarks of necroptosis and apoptosis, and is best characterized in malignant diseases. Chemotherapeutics, radiotherapy and photodynamic therapy induce intracellular stress response pathways in tumor cells, leading to a secretion of various factors belonging to a family of damage-associated molecular patterns molecules, capable of inducing the adaptive immune response. One of them is calreticulin (CRT), an endoplasmic reticulum-associated chaperone. Its presence on the surface of dying tumor cells serves as an "eat me" signal for antigen presenting cells (APC). Engulfment of tumor cells by APCs results in the presentation of tumor's antigens to cytotoxic T-cells and production of cytokines/chemokines, which activate immune cells responsible for tumor cells killing. Thus, the development of ICD and the expression of CRT can help standard therapy to eradicate tumor cells. Here, we review the physiological functions of CRT and its involvement in the ICD appearance in malignant disease. Moreover, we also focus on the ability of various anti-cancer drugs to induce expression of surface CRT on ovarian cancer cells. The second aim of this work is to discuss and summarize the prognostic/predictive value of CRT in ovarian cancer patients.

Mechanisms of productive folding and endoplasmic reticulum-associated degradation of glycoproteins and non-glycoproteins

BACKGROUND

The quality of proteins destined for the secretory pathway is ensured by two distinct mechanisms in the endoplasmic reticulum (ER): productive folding of newly synthesized proteins, which is assisted by ER-localized molecular chaperones and in most cases also by disulfide bond formation and transfer of an oligosaccharide unit; and ER-associated degradation (ERAD), in which proteins unfolded or misfolded in the ER are recognized and processed for delivery to the ER membrane complex, retrotranslocated through the complex with simultaneous ubiquitination, extracted by AAA-ATPase to the cytosol, and finally degraded by the proteasome.

SCOPE OF REVIEW

We describe the mechanisms of productive folding and ERAD, with particular attention to glycoproteins versus non-glycoproteins, and to yeast versus mammalian systems.

MAJOR CONCLUSION

Molecular mechanisms of the productive folding of glycoproteins and non-glycoproteins mediated by molecular chaperones and protein disulfide isomerases are well conserved from yeast to mammals. Additionally, mammals have gained an oligosaccharide structure-dependent folding cycle for glycoproteins. The molecular mechanisms of ERAD are also well conserved from yeast to mammals, but redundant expression of yeast orthologues in mammals has been encountered, particularly for components involved in recognition and processing of glycoproteins and components of the ER membrane complex involved in retrotranslocation and simultaneous ubiquitination of glycoproteins and non-glycoproteins. This may reflect an evolutionary consequence of increasing quantity or quality needs toward mammals.

GENERAL SIGNIFICANCE

The introduction of innovative genome editing technology into analysis of the mechanisms of mammalian ERAD, as exemplified here, will provide new insights into the pathogenesis of various diseases.

Oxidoreductases in Glycoprotein Glycosylation, Folding, and ERAD

N-linked glycosylation and sugar chain processing, as well as disulfide bond formation, are among the most common post-translational protein modifications taking place in the endoplasmic reticulum (ER). They are essential modifications that are required for membrane and secretory proteins to achieve their correct folding and native structure. Several oxidoreductases responsible for disulfide bond formation, isomerization, and reduction have been shown to form stable, functional complexes with enzymes and chaperones that are involved in the initial addition of an N-glycan and in folding and quality control of the glycoproteins. Some of these oxidoreductases are selenoproteins. Recent studies also implicate glycan machinery–oxidoreductase complexes in the recognition and processing of misfolded glycoproteins and their reduction and targeting to ER-associated degradation. This review focuses on the intriguing cooperation between the glycoprotein-specific cell machineries and ER oxidoreductases, and highlights open questions regarding the functions of many members of this large family.

Calreticulin and cancer

Calreticulin (CALR) is an endoplasmic reticulum (ER)-resident protein involved in a spectrum of cellular processes. In healthy cells, CALR operates as a chaperone and Ca²⁺ buffer to assist correct protein folding within the ER. Besides favoring the maintenance of cellular proteostasis, these cell-intrinsic CALR functions support Ca²⁺-dependent processes, such as adhesion and integrin signaling, and ensure normal antigen presentation on MHC Class I molecules. Moreover, cancer cells succumbing to immunogenic cell death (ICD) expose CALR on their surface, which promotes the uptake of cell corpses by professional phagocytes and ultimately supports the initiation of anticancer immunity. Thus, loss-of-function CALR mutations promote oncogenesis not only as they impair cellular homeostasis in healthy cells, but also as they compromise natural and therapy-driven immunosurveillance. However, the prognostic impact of total or membrane-exposed CALR levels appears to vary considerably with cancer type. For instance, while genetic CALR defects promote pre-neoplastic myeloproliferation, patients with myeloproliferative neoplasms bearing CALR mutations often experience improved overall survival as compared to patients bearing wild-type CALR. Here, we discuss the context-dependent impact of CALR on malignant transformation, tumor progression and response to cancer therapy.

Object-Based Analyses in FIJI/ImageJ to Measure Local RNA Translation Sites in Neurites in Response to Aβ1-42 Oligomers

Subcellular protein delivery is especially important in signal transduction and cell behavior, and is typically achieved by localization signals within the protein. However, protein delivery can also rely on localization of mRNAs that are translated at target sites. Although once considered heretical, RNA localization has proven to be highly conserved in eukaryotes. RNA localization and localized translation are especially relevant in polarized cells like neurons where neurites extend dozens to hundreds of centimeters away from the soma. Local translation confers dendrites and axons the capacity to respond to their environment in an acute manner without fully relying on somatic signals. The relevance of local protein synthesis in neuron development, maintenance and disease has not been fully acknowledged until recent years, partly due to the limited amount of locally produced proteins. For instance, in hippocampal neurons levels of newly synthesized somatic proteins can be more than 20–30 times greater than translation levels of neuritic proteins. Thus local translation events can be easily overlooked under the microscope. Here we describe an object-based analysis used to visualize and quantify local RNA translation sites in neurites. Newly synthesized proteins are tagged with puromycin and endogenous RNAs labeled with SYTO. After imaging, signals corresponding to neuritic RNAs and proteins are filtered with a Laplacian operator to enhance the edges. Resulting pixels are converted into objects and selected by automatic masking followed by signal smoothing. Objects corresponding to RNA or protein and colocalized objects (RNA and protein) are quantified along individual neurites. Colocalization between RNA and protein in neurites correspond to newly synthesized proteins arising from localized RNAs and represent localized translation sites. To test the validity of our analyses we have compared control neurons to Aβ_1–42-treated neurons. Aβ is involved in the pathology of Alzheimer’s disease and was previously reported to induce local translation in axons and dendrites which in turn contributes to the disease. We have observed that Aβ increases the synthesis of neuritic proteins as well as the fraction of translating RNAs in distal sites of the neurite, suggesting an induction of local protein synthesis. Our results thus confirm previous reports and validate our quantification method.

Mechanistic Connections between Endoplasmic Reticulum (ER) Redox Control and Mitochondrial Metabolism

The past decade has seen the emergence of endoplasmic reticulum (ER) chaperones as key determinants of contact formation between mitochondria and the ER on the mitochondria-associated membrane (MAM). Despite the known roles of ER–mitochondria tethering factors like PACS-2 and mitofusin-2, it is not yet entirely clear how they mechanistically interact with the ER environment to determine mitochondrial metabolism. In this article, we review the mechanisms used to communicate ER redox and folding conditions to the mitochondria, presumably with the goal of controlling mitochondrial metabolism at the Krebs cycle and at the electron transport chain, leading to oxidative phosphorylation (OXPHOS). To achieve this goal, redox nanodomains in the ER and the interorganellar cleft influence the activities of ER chaperones and Ca²⁺-handling proteins to signal to mitochondria. This mechanism, based on ER chaperones like calnexin and ER oxidoreductases like Ero1α, controls reactive oxygen production within the ER, which can chemically modify the proteins controlling ER–mitochondria tethering, or mitochondrial membrane dynamics. It can also lead to the expression of apoptotic or metabolic transcription factors. The link between mitochondrial metabolism and ER homeostasis is evident from the specific functions of mitochondria–ER contact site (MERC)-localized Ire1 and PERK. These functions allow these two transmembrane proteins to act as mitochondria-preserving guardians, a function that is apparently unrelated to their functions in the unfolded protein response (UPR). In scenarios where ER stress cannot be resolved via the activation of mitochondrial OXPHOS, MAM-localized autophagosome formation acts to remove defective portions of the ER. ER chaperones such as calnexin are again critical regulators of this MERC readout.

The Protein Disulfide Isomerase Family: from proteostasis to pathogenesis

In mammalian cells, nearly one-third of proteins are inserted into the endoplasmic reticulum (ER), where they undergo oxidative folding and chaperoning assisted by approximately 20 members of the protein disulfide isomerase family (PDIs). PDIs consist of multiple thioredoxin-like domains and recognize a wide variety of proteins via highly conserved interdomain flexibility. Although PDIs have been studied intensely for almost 50 years, exactly how they maintain protein homeostasis in the ER remains unknown, and is important not only for fundamental biological understanding but also for protein misfolding- and aggregation-related pathophysiology. Herein, we review recent advances in structural biology and biophysical approaches that explore the underlying mechanism by which PDIs fulfil their distinct functions to promote productive protein folding and scavenge misfolded proteins in the ER, the primary factory for efficient production of the secretome.

Oxidative protein folding: state-of-the-art and current avenues of research in plants

Contents Summary 1230 I. Introduction 1230 II. Formation and isomerization of disulfides in the ER and the Golgi apparatus 1231 III. The disulfide relay in the mitochondrial intermembrane space: why are plants different? 1236 IV. Disulfide bond formation on luminal proteins in thylakoids 1240 V. Conclusion 1242 Acknowledgements 1242 References 1242 SUMMARY: Disulfide bonds are post-translational modifications crucial for the structure and function of thousands of proteins. Their formation and isomerization, referred to as oxidative folding, require specific protein machineries found in oxidizing subcellular compartments, namely the endoplasmic reticulum and the associated endomembrane system, the intermembrane space of mitochondria and the thylakoid lumen of chloroplasts. At least one protein component is required for transferring electrons from substrate proteins to an acceptor that is usually molecular oxygen. For oxidation reactions, incoming reduced substrates are oxidized by thiol-oxidoreductase proteins (or domains in case of chimeric proteins), which are usually themselves oxidized by a single thiol oxidase, the enzyme generating disulfide bonds de novo. By contrast, the description of the molecular actors and pathways involved in proofreading and isomerization of misfolded proteins, which require a tightly controlled redox balance, lags behind. Herein we provide a general overview of the knowledge acquired on the systems responsible for oxidative protein folding in photosynthetic organisms, highlighting their particularities compared to other eukaryotes. Current research challenges are discussed including the importance and specificity of these oxidation systems in the context of the existence of reducing systems in the same compartments.

Endoplasmic reticulum stress: major player in size-dependent inhibition of P-glycoprotein by silver nanoparticles in multidrug-resistant breast cancer cells

Background

Development of multidrug resistance (MDR) is a major burden of successful chemotherapy, therefore, novel approaches to defeat MDR are imperative. Although the remarkable anti-cancer propensity of silver nanoparticles (AgNP) has been demonstrated and their potential application in MDR cancer has been proposed, the nanoparticle size-dependent cellular events directing P-glycoprotein (Pgp) expression and activity in MDR cancer have never been addressed. Hence, in the present study we examined AgNP size-dependent cellular features in multidrug resistant breast cancer cells.

Results

In this study we report that 75 nm AgNPs inhibited significantly Pgp efflux activity in drug-resistant breast cancer cells and potentiated the apoptotic effect of doxorubicin, which features were not observed upon 5 nm AgNP treatment. Although both sized AgNPs induced significant ROS production and mitochondrial damage, 5 nm AgNPs were more potent than 75 nm AgNPs in this respect, therefore, these effects can not to be accounted for the reduced transport activity of ATP-driven pumps observed after 75 nm AgNP treatments. Instead we found that 75 nm AgNPs depleted endoplasmic reticulum (ER) calcium stores, caused notable ER stress and decreased plasma membrane positioning of Pgp.

Conclusion

Our study suggests that AgNPs are potent inhibitors of Pgp function and are promising agents for sensitizing multidrug resistant breast cancers to anticancer drugs. This potency is determined by their size, since 75 nm AgNPs are more efficient than smaller counterparts. This is a highly relevant finding as it renders AgNPs attractive candidates in rational design of therapeutically useful agents for tumor targeting. In the present study we provide evidence that exploitation of ER stress can be a propitious target in defeating multidrug resistance in cancers.

Electronic supplementary material

The online version of this article (10.1186/s12951-019-0448-4) contains supplementary material, which is available to authorized users.

Myo-inositol oxygenase accentuates renal tubular injury initiated by endoplasmic reticulum stress

Besides oxidant stress, endoplasmic reticulum (ER) stress has been implicated in the pathogenesis of various metabolic disorders affecting the kidney. These two forms of stresses are not mutually exclusive to each other and may operate by a feedback loop in worsening the cellular injury. To attest to this contention, studies were performed to assess whether in such a setting, there is worsening of tubulointerstitial injury. We employed tunicamycin as a model of ER stress and used tubular cells and mice overexpressing myo-inositol oxygenase (MIOX), an enzyme involved in glycolytic events with excessive generation of ROS. Concomitant treatment of tunicamycin and transfection of cells with MIOX-pcDNA led to a marked generation of ROS, which was reduced by MIOX-siRNA. Likewise, an accentuated expression of ER stress sensors, GRP78, XBP1, and CHOP, was observed, which was reduced with MIOX-siRNA. These sensors were markedly elevated in MIOX-TG mice compared with WT treated with tunicamycin. This was accompanied with marked deterioration of tubular morphology, along with impairment of renal functions. Interestingly, minimal damage and elevation of ER stressors was observed in MIOX-KO mice. Downstream events that were more adversely affected in MIOX-TG mice included accentuated expression of proapoptogenic proteins, proinflammatory cytokines, and extracellular matrix constituents, although expression of these molecules was unaffected in MIOX-KO mice. Also, their tunicamycin-induced accentuated expression in tubular cells was notably reduced with MIOX-siRNA. These studies suggest that the biology of MIOX-induced oxidant stress and tunicamycin-induced ER stress are interlinked, and both of the events may feed into each other to amplify the tubulointerstitial injury.

The ER-localized Ca2+-binding protein calreticulin couples ER stress to autophagy by associating with microtubule-associated protein 1A/1B light chain 3

Autophagy is of key importance for eliminating aggregated proteins during the maintenance of cellular proteostasis in response to endoplasmic reticulum (ER) stress. However, the upstream signaling that mediates autophagy activation in response to ER stress is incompletely understood. In this study, in vivo and in vitro approaches were utilized that include gain- and loss-of-function assays and mouse livers and human cell lines with tunicamycin-induced pharmacological ER stress. We report that calreticulin, a quality control chaperone that binds to misfolded glycoproteins for refolding in the ER, is induced under ER stress. Calreticulin overexpression stimulated the formation of autophagosomes and increased autophagic flux. Interestingly, calreticulin was sufficient for attenuating ER stress in tunicamycin- or thapsigargin-treated HeLa cells, whereas lentivirus-mediated shRNA calreticulin knockdown exacerbated ER stress. Mechanistically, we noted that calreticulin induces autophagy by interacting with microtubule-associated protein 1A/1B-light chain 3 (LC3). Confocal microscopy revealed that the colocalization of calreticulin and LC3 at the autophagosome was enhanced under ER stress conditions. Importantly, a conserved LC3-interacting region was necessary for calreticulin-mediated stimulation of autophagy and for reducing ER stress. These findings indicate a calreticulin-based mechanism that couples ER stress to autophagy activation, which, in turn, attenuates cellular stress, likely by alleviating the formation of aberrantly folded proteins. Pharmacological or genetic approaches that activate calreticulin-autophagy signaling may have potential for managing ER stress and related cellular disorders.

Overexpression of quality control proteins reduces prion conversion in prion-infected cells

Prion diseases are fatal infectious neurodegenerative disorders in humans and other animals and are caused by misfolding of the cellular prion protein (PrP^C) into the pathological isoform PrP^Sc. These diseases have the potential to transmit within or between species, including zoonotic transmission to humans. Elucidating the molecular and cellular mechanisms underlying prion propagation and transmission is therefore critical for developing molecular strategies for disease intervention. We have shown previously that impaired quality control mechanisms directly influence prion propagation. In this study, we manipulated cellular quality control pathways in vitro by stably and transiently overexpressing selected quality control folding (ERp57) and cargo (VIP36) proteins and investigated the effects of this overexpression on prion propagation. We found that ERp57 or VIP36 overexpression in persistently prion-infected neuroblastoma cells significantly reduces the amount of PrP^Sc in immunoblots and prion-seeding activity in the real-time quaking-induced conversion (RT-QuIC) assay. Using different cell lines infected with various prion strains confirmed that this effect is not cell type– or prion strain–specific. Moreover, de novo prion infection revealed that the overexpression significantly reduced newly formed PrP^Sc in acutely infected cells. ERp57-overexpressing cells significantly overcame endoplasmic reticulum stress, as revealed by expression of lower levels of the stress markers BiP and CHOP, accompanied by a decrease in PrP aggregates. Furthermore, application of ERp57-expressing lentiviruses prolonged the survival of prion-infected mice. Taken together, improved cellular quality control via ERp57 or VIP36 overexpression impairs prion propagation and could be utilized as a potential therapeutic strategy.

Abnormal expression of ER quality control and ER associated degradation proteins in the dorsolateral prefrontal cortex in schizophrenia

Abnormalities in posttranslational protein modifications (PTMs) that regulate protein targeting, trafficking, synthesis, and function have been implicated in the pathophysiology of schizophrenia. The endoplasmic reticulum (ER) contains specialized machinery that facilitate protein synthesis, ER entry and exit, quality control, and post-translational processing, steps required for protein maturation. Dysregulation of these systems could represent potential mechanisms for abnormalities of neurotransmitter associated proteins in schizophrenia. We hypothesized that expression of ER processing pathways is dysregulated in schizophrenia. We characterized protein and complex expression of essential components from protein folding, ER quality control (ERQC), and ER associated degradation (ERAD) processes in the dorsolateral prefrontal cortex of 12 matched pairs of elderly schizophrenia and comparison subjects. We found increased expression of proteins associated with recognizing and modifying misfolded proteins, including UDP-glucose/glycoprotein glucosyltransferase 2 (UGGT2), ER degradation enhancing alpha-mannosidase like protein 2 (EDEM2), and synoviolin (SYVN1)/HRD1. As SYVN1/HRD1 is a component of the ubiquitin ligase HRD1-SEL1L complex that facilitates ERAD, we immunoprecipitated SEL1L and measured expression of other proteins in this complex. In schizophrenia, SYVN1/HRD1 and OS-9, ERAD promoters, have increased association with SEL1L, while XTP3-B, which can prevent ERAD of substrates, has decreased association. Abnormal expression of proteins associated with ERQC and ERAD suggests dysregulation in ER localized protein processing pathways in schizophrenia. Interestingly, the deficits we found are not in the protein processing machinery itself, but in proteins that recognize and target incompletely or misfolded proteins. These changes may reflect potential mechanisms of abnormal neurotransmitter associated protein expression previously observed in schizophrenia.

Two novel calreticulin-related molecules with microbial binding and phagocytosis enhancing capacity in the half-smooth tongue sole, Cynoglossus semilaevis

Calreticulin (CRT) is highly conserved chaperone located in the endoplasmic reticulum. It plays important roles in innate immunity. Although various immune-related functions of CRT have been reported in vertebrates and invertebrates, information on the potential functions of teleost CRT is very limited. In the present study, we characterized two calreticulin-related molecules from tongue sole (Cynoglossus semilaevis), calreticulin-like1 and calreticulin-like2 (CsCRTL1 and CsCRTL2). CsCRTL1and CsCRTL2 contain signature CRT motifs that are highly conserved in different species. CsCRTL1and CsCRTL2 were expressed in liver, head kidney, brain, spleen, heart, muscle, skin, intestine and gills. The expression levels of CsCRTL1and CsCRTL2 were highest in liver and spleen, respectively. After stimulation by Vibrio anguillarum and Streptococcus agalactiae, CsCRTL1 and CsCRTL2 were significantly up-regulated. The expression patterns depended on the tissue type, pathogen type, and infection time. The recombinant proteins rCsCRTL1and rCsCRTL2 bound to different pathogen-associated molecular patterns (PAMPs) including LPS and PGN, and to different bacteria, such as Gram-negative bacteria V. anguillarum and Gram-positive bacteria Staphylococcus aureus. Moreover, rCsCRTL1and rCsCRTL2 significantly enhanced the killing of V. anguillarum by tongue sole macrophages. Our results indicate that CsCRTL1and CsCRTL2 play important roles in antibacterial immunity of tongue sole.

The Lectin Chaperone Calnexin Is Involved in the Endoplasmic Reticulum Stress Response by Regulating Ca2+ Homeostasis in Aspergillus nidulans

The Ca²⁺-mediated signaling pathway is crucial for environmental adaptation in fungi. Here we show that calnexin, a molecular chaperone located in the endoplasmic reticulum (ER), plays an important role in regulating the cytosolic free calcium concentration ([Ca²⁺]_c) in Aspergillus nidulans Inactivation of calnexin (ClxA) in A. nidulans caused severe defects in hyphal growth and conidiation under ER stress caused by the ER stress-inducing agent dithiothreitol (DTT) or high temperature. Importantly, defects in the ΔclxA mutant were restored by the addition of extracellular calcium. Furthermore, the CchA/MidA complex (the high-affinity Ca²⁺ channels), calcineurin (calcium/calmodulin-dependent protein phosphatase), and PmrA (secretory pathway Ca²⁺ ATPase) were required for extracellular calcium-based restoration of the DTT/thermal stress sensitivity in the ΔclxA mutant. Interestingly, the ΔclxA mutant exhibited markedly reduced conidium formation and hyphal growth defects under the low-calcium condition, which is similar to defects caused by mutations in MidA/CchA. Moreover, the phenotypic defects were further exacerbated in the ΔclxA ΔmidA ΔcchA mutant, which suggested that ClxA and MidA/CchA are both required under the calcium-limiting condition. Using the calcium-sensitive photoprotein aequorin to monitor [Ca²⁺]_c in living cells, we found that ClxA and MidA/CchA complex synergistically coordinate transient increase in [Ca²⁺]_c in response to extracellular calcium. Moreover, ClxA, in particular its luminal domain, plays a role in mediating the transient [Ca²⁺]_c in response to DTT-induced ER stress in the absence of extracellular calcium, indicating ClxA may mediate calcium release from internal calcium stores. Our findings provide new insights into the role of calnexin in the regulation of calcium-mediated response in fungal ER stress adaptation.IMPORTANCE Calnexin is a well-known molecular chaperone conserved from yeast to humans. Although it contains calcium binding domains, little is known about the role of calnexin in Ca²⁺ regulation. In this study, we demonstrate that calnexin (ClxA) in the filamentous fungus Aspergillus nidulans, similar to the high-affinity calcium uptake system (HACS), is required for normal growth and conidiation under the calcium-limiting condition. The ClxA dysfunction decreases the transient cytosolic free calcium concentration ([Ca²⁺]_c) induced by a high extracellular calcium or DTT-induced ER stress. Our findings provide the direct evidence that calnexin plays important roles in regulating Ca²⁺ homeostasis in addition to its role as a molecular chaperone in fungi. These results provide new insights into the roles of calnexin and expand knowledge of fungal stress adaptation.

The endoplasmic reticulum: A hub of protein quality control in health and disease

One third of the eukaryotic proteome is synthesized at the endoplasmic reticulum (ER), whose unique properties provide a folding environment substantially different from the cytosol. A healthy, balanced proteome in the ER is maintained by a network of factors referred to as the ER quality control (ERQC) machinery. This network consists of various protein folding chaperones and modifying enzymes, and is regulated by stress response pathways that prevent the build-up as well as the secretion of potentially toxic and aggregation-prone misfolded protein species. Here, we describe the components of the ERQC machinery, investigate their response to different forms of stress, and discuss the consequences of ERQC break-down.

Dysregulated expression of proteins associated with ER stress, autophagy and apoptosis in tissues from nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is categorized into nonalcoholic fatty liver (NAFL) and nonalcoholic steatohepatitis (NASH) and has emerged as a risk factor for more critical clinical conditions. However, the underlying mechanisms of NAFLD pathogenesis are not fully understood. In this study, expression of proteins associated with endoplasmic reticulum (ER) stress, apoptosis and autophagy were analyzed in normal, NAFL and NASH human livers by western blotting. Levels of some ER stress-transducing transcription factors, including cleaved activating transcription factor 6, were higher in NASH than in the normal tissues. However, the expression of a majority of the ER chaperones and foldases analyzed, including glucose-regulated protein 78 and ER protein 44, was lower in NASH than in the normal tissues. Levels of apoptosis markers, such as cleaved poly (ADP-ribose) polymerase, were also lower in NASH tissues, in which expression of some B-cell lymphoma-2 family proteins was up- or down-regulated compared to the normal tissues. The level of the autophagy substrate p62 was not different in NASH and normal tissues, although some autophagy regulators were up- or down-regulated in the NASH tissues compared to the normal tissues. Levels of most of the proteins analyzed in NAFL tissues were either similar to those in one of the other two types, NASH and normal, or were somewhere in between. Together, these findings suggest that regulation of certain important tissues processes involved in protein quality control and cell survival were broadly compromised in the NAFLD tissues.

Systematic deletion of the ER lectin chaperone genes reveals their roles in vegetative growth and male gametophyte development in Arabidopsis

Calnexin (CNX) and calreticulin (CRT) are homologous lectin chaperones in the endoplasmic reticulum (ER) that facilitate glycoprotein folding and retain folding intermediates to prevent their transit via the secretary pathway. The Arabidopsis genome has two CNX (CNX1 and CNX2) and three CRT (CRT1, CRT2 and CRT3) homologs. Despite growing evidence of the biological roles of CNXs and CRTs, little is understood about their function in Arabidopsis growth and development under normal conditions. Here, we report that the deletion of CNX1, but not of CNX2, in the crt1 crt2 crt3 triple mutation background had an adverse effect on pollen viability and pollen tube growth, leading to a significant reduction in fertility. The cnx1 crt1 crt2 crt3 quadruple mutation also conferred severe defects in growth and development, including a shortened primary root, increased root hair length and density, and reduced plant height. Disruption of all five members of the CNX/CRT family was revealed to be lethal. Finally, the abnormal phenotype of the cnx1 crt1 crt2 crt3 quadruple mutants was completely rescued by either the CNX1 or CNX2 cDNA under the control of the CNX1 promoter, suggesting functional redundancy between CNX1 and CNX2. Taken together, these results provide genetic evidence that CNX and CRT play essential and overlapping roles during vegetative growth and male gametophyte development in Arabidopsis.

Interactions between intersubunit transmembrane domains regulate the chaperone-dependent degradation of an oligomeric membrane protein

In the kidney, the epithelial sodium channel (ENaC) regulates blood pressure through control of sodium and volume homeostasis, and in the lung, ENaC regulates the volume of airway and alveolar fluids. ENaC is a heterotrimer of homologous α-, β- and γ-subunits, and assembles in the endoplasmic reticulum (ER) before it traffics to and functions at the plasma membrane. Improperly folded or orphaned ENaC subunits are subject to ER quality control and targeted for ER-associated degradation (ERAD). We previously established that a conserved, ER lumenal, molecular chaperone, Lhs1/GRP170, selects αENaC, but not β- or γ-ENaC, for degradation when the ENaC subunits were individually expressed. We now find that when all three subunits are co-expressed, Lhs1-facilitated ERAD was blocked. To determine which domain-domain interactions between the ENaC subunits are critical for chaperone-dependent quality control, we employed a yeast model and expressed chimeric α/βENaC constructs in the context of the ENaC heterotrimer. We discovered that the βENaC transmembrane domain was sufficient to prevent the Lhs1-dependent degradation of the α-subunit in the context of the ENaC heterotrimer. Our work also found that Lhs1 delivers αENaC for proteasome-mediated degradation after the protein has become polyubiquitinated. These data indicate that the Lhs1 chaperone selectively recognizes an immature form of αENaC, one which has failed to correctly assemble with the other channel subunits via its transmembrane domain.

Two endoplasmic reticulum proteins (calnexin and calreticulin) are involved in innate immunity in Chinese mitten crab (Eriocheir sinensis)

Calnexin (Cnx) and calreticulin (Crt), which are important chaperones in the endoplasmic reticulum (ER), participate in the folding and quality control of client proteins. Cnx and Crt identified from Chinese mitten crab (Eriocheir sinensis) are designated as EsCnx and EsCrt, respectively. EsCnx and EsCrt are expressed in the hemocyte, hepatopancrea, gill, and intestine at the mRNA and protein level. Immunofluorescence analysis indicated that EsCnx and EsCRT are located in the ER. Moreover, the mRNA and protein expression levels of EsCnx and EsCrt were altered by challenge with lipopolysaccharides (LPS), peptidoglycans (PGN), Staphyloccocus aureus, and Vibrio parahaemolyticus. Recombinant EsCnx and EsCrt (rEsCnx and rEsCrt, respectively) proteins can bind to various Gram-positive and Gram-negative bacteria, as well as to different polysaccharides (LPS and PGN). rEsCnx and rEsCrt assisted in the clearance of V. parahaemolyticus in vivo, and the clearance efficiency was impaired after silencing of EsCnx and EsCrt. Our results suggest that the two ER proteins are involved in anti-bacterial immunity in E. sinensis.

Protein disulfide isomerases in the endoplasmic reticulum promote anchorage-independent growth of breast cancer cells

Metastatic breast cancer cells are exposed to stress of detachment from the extracellular matrix (ECM). Cultured breast cancer cells that survive this stress and are capable of anchorage-independent proliferation form mammospheres. The purpose of this study was to explore a link between mammosphere growth, ECM gene expression, and the protein quality control system in the endoplasmic reticulum (ER). We compared the mRNA and protein levels of ER folding factors in SUM159PT and MCF10DCIS.com breast cancer cells grown as mammospheres versus adherent conditions. Publicly available gene expression data for mammospheres formed by primary breast cancer cells and for circulating tumor cells (CTCs) were analyzed to assess the status of ECM/ER folding factor genes in clinically relevant samples. Knock-down of selected protein disulfide isomerase (PDI) family members was performed to examine their roles in SUM159PT mammosphere growth. We found that cells grown as mammospheres had elevated expression of ECM genes and ER folding quality control genes. CTC gene expression data for an index patient indicated that upregulation of ECM and ER folding factor genes occurred at the time of acquired therapy resistance and disease progression. Knock-down of PDI, ERp44, or ERp57, three members of the PDI family with elevated protein levels in mammospheres, in SUM159PT cells partially inhibited the mammosphere growth. Thus, breast cancer cell survival and growth under detachment conditions require enhanced assistance of the ER protein folding machinery. Targeting ER folding factors, in particular members of the PDI family, may improve the therapeutic outcomes in metastatic breast cancer.

ALS-linked protein disulfide isomerase variants cause motor dysfunction

Disturbance of endoplasmic reticulum (ER) proteostasis is a common feature of amyotrophic lateral sclerosis (ALS). Protein disulfide isomerases (PDIs) areERfoldases identified as possibleALSbiomarkers, as well as neuroprotective factors. However, no functional studies have addressed their impact on the disease process. Here, we functionally characterized fourALS-linked mutations recently identified in two majorPDIgenes,PDIA1 andPDIA3/ERp57. Phenotypic screening in zebrafish revealed that the expression of thesePDIvariants induce motor defects associated with a disruption of motoneuron connectivity. Similarly, the expression of mutantPDIs impaired dendritic outgrowth in motoneuron cell culture models. Cellular and biochemical studies identified distinct molecular defects underlying the pathogenicity of thesePDImutants. Finally, targetingERp57 in the nervous system led to severe motor dysfunction in mice associated with a loss of neuromuscular synapses. This study identifiesERproteostasis imbalance as a risk factor forALS, driving initial stages of the disease.

The C-Terminal Acidic Region of Calreticulin Mediates Phosphatidylserine Binding and Apoptotic Cell Phagocytosis

Calreticulin is a calcium-binding chaperone that is normally localized in the endoplasmic reticulum. Calreticulin is detectable on the surface of apoptotic cells under some apoptosis-inducing conditions, where it promotes the phagocytosis and immunogenicity of dying cells. However, the precise mechanism by which calreticulin, a soluble protein, localizes to the outer surface of the plasma membrane of dying cells is unknown, as are the molecular mechanisms that are relevant to calreticulin-induced cellular phagocytosis. Calreticulin comprises three distinct structural domains: a globular domain, an extended arm-like P-domain, and a C-terminal acidic region containing multiple low-affinity calcium binding sites. We show that calreticulin, via its C-terminal acidic region, preferentially interacts with phosphatidylserine (PS) compared with other phospholipids and that this interaction is calcium dependent. Additionally, exogenous calreticulin binds apoptotic cells via a higher-affinity calcium-dependent mode that is acidic region dependent. Exogenous calreticulin also binds live cells, including macrophages, via a second, lower-affinity P-domain and globular domain-dependent, but calcium-independent binding mode that likely involves its generic polypeptide binding site. Truncation constructs lacking the acidic region or arm-like P-domain of calreticulin are impaired in their abilities to induce apoptotic cell phagocytosis by murine peritoneal macrophages. Taken together, the results of this investigation provide the first molecular insights into the phospholipid binding site of calreticulin as a key anchor point for the cell surface expression of calreticulin on apoptotic cells. These findings also support a role for calreticulin as a PS-bridging molecule that cooperates with other PS-binding factors to promote the phagocytosis of apoptotic cells.

Transforming Lepidopteran Insect Cells for Improved Protein Processing and Expression

The lepidopteran insect cells used with the baculovirus expression vector system (BEVS) are capable of synthesizing and accurately processing foreign proteins. However, proteins expressed in baculovirus-infected cells often fail to be completely processed, or are not processed in a manner that meets a researcher's needs. This chapter discusses a metabolic engineering approach that addresses this problem. Basically, this approach involves the addition of new or enhancement of existing protein processing functions in established lepidopteran insect cell lines. In addition to improvements in protein processing, this approach has also been used to improve protein expression levels obtained with the BEVS. Methods for engineering cell lines and assessing their properties as improved hosts for the BEVS are detailed. Examples of lepidopteran insect cell lines engineered for improved protein N-glycosylation, folding/trafficking, and expression are described in detail.

Glycosylation-directed quality control of protein folding

Membrane-bound and soluble proteins of the secretory pathway are commonly glycosylated in the endoplasmic reticulum. These adducts have many biological functions, including, notably, their contribution to the maturation of glycoproteins. N-linked glycans are of oligomeric structure, forming configurations that provide blueprints to precisely instruct the folding of protein substrates and the quality control systems that scrutinize it. O-linked mannoses are simpler in structure and were recently found to have distinct functions in protein quality control that do not require the complex structure of N-linked glycans. Together, recent studies reveal the breadth and sophistication of the roles of these glycan-directed modifications in protein biogenesis.

Functional Role of the Disulfide Isomerase ERp57 in Axonal Regeneration

ERp57 (also known as grp58 and PDIA3) is a protein disulfide isomerase that catalyzes disulfide bonds formation of glycoproteins as part of the calnexin and calreticulin cycle. ERp57 is markedly upregulated in most common neurodegenerative diseases downstream of the endoplasmic reticulum (ER) stress response. Despite accumulating correlative evidence supporting a neuroprotective role of ERp57, the contribution of this foldase to the physiology of the nervous system remains unknown. Here we developed a transgenic mouse model that overexpresses ERp57 in the nervous system under the control of the prion promoter. We analyzed the susceptibility of ERp57 transgenic mice to undergo neurodegeneration. Unexpectedly, ERp57 overexpression did not affect dopaminergic neuron loss and striatal denervation after injection of a Parkinson’s disease-inducing neurotoxin. In sharp contrast, ERp57 transgenic animals presented enhanced locomotor recovery after mechanical injury to the sciatic nerve. These protective effects were associated with enhanced myelin removal, macrophage infiltration and axonal regeneration. Our results suggest that ERp57 specifically contributes to peripheral nerve regeneration, whereas its activity is dispensable for the survival of a specific neuronal population of the central nervous system. These results demonstrate for the first time a functional role of a component of the ER proteostasis network in peripheral nerve regeneration.

Identification of rare protein disulfide isomerase gene variants in amyotrophic lateral sclerosis patients

Disruption of endoplasmic reticulum (ER) proteostasis is a salient feature of amyotrophic lateral sclerosis (ALS). Upregulation of ER foldases of the protein disulfide isomerase (PDI) family has been reported in ALS mouse models and spinal cord tissue and body fluids derived from sporadic ALS cases. Although in vitro studies suggest a neuroprotective role of PDIs in ALS, the possible contribution of genetic mutations of these ER foldases in the disease process remains unknown. Interestingly, intronic variants of the PDIA1 gene were recently reported as a risk factor for ALS. Here, we initially screened for mutations in two major PDI genes (PDIA1/P4HB and PDIA3/ERp57) in a US cohort of 96 familial and 96 sporadic ALS patients using direct DNA sequencing. Then, 463 familial and 445 sporadic ALS patients from two independent cohorts were also screened for mutations in these two genes using whole exome sequencing. A total of nine PDIA1 missense variants and seven PDIA3 missense variants were identified in 16 ALS patients. We have identified several novel and rare single nucleotide polymorphisms (SNPs) in both genes that are enriched in ALS cases compared with a large group of control subjects showing a frequency of around 1% in ALS cases. The possible biological and structural impact of these ALS-linked PDI variants is also discussed.

Cooperative role of calnexin and TigA in Aspergillus oryzae glycoprotein folding

Calnexin (CNX), known as a lectin chaperone located in the endoplasmic reticulum (ER), specifically recognizes G1M9GN2-proteins and facilitates their proper folding with the assistance of ERp57 in mammalian cells. However, it has been left unidentified how CNX works in Aspergillus oryzae, which is a filamentous fungus widely exploited in biotechnology. In this study, we found that a protein disulfide isomerase homolog TigA can bind with A. oryzae CNX (AoCNX), which was revealed to specifically recognize monoglucosylated glycans, similarly to CNX derived from other species, and accelerate the folding of G1M9GN2-ribonuclease (RNase) in vitro. For refolding experiments, a homogeneous monoglucosylated high-mannose-type glycoprotein G1M9GN2-RNase was chemoenzymatically synthesized from G1M9GN-oxazoline and GN-RNase. Denatured G1M9GN2-RNase was refolded with highest efficiency in the presence of both soluble form of AoCNX and TigA. TigA contains two thioredoxin domains with CGHC motif, mutation analysis of which revealed that the one in N-terminal regions is involved in binding to AoCNX, while the other in catalyzing protein refolding. The results suggested that in glycoprotein folding process of A. oryzae, TigA plays a similar role as ERp57 in mammalian cells, as a partner protein of AoCNX.

Syntenin is involved in the bacteria clearance response of kuruma shrimp (Marsupenaeus japonicus)

Syntenin is a multifunctional cytosolic adaptor protein that contributes to cell migration, proliferation, attachment, and apoptosis, as well as immune response to virus, in vertebrates. However, the functions of syntenin in the antibacterial response of invertebrates remain unclear. In this study, we identified a syntenin-like gene (MjSyn) from the kuruma shrimp (Marsupenaeus japonicus) and detected its function in the antibacterial immunity of shrimp. The full-length MjSyn was 1223 bp with a 963 bp open reading frame that encodes 320 amino acids. The deduced MjSyn proteins contained two atypical PDZ domains (sequence repeat that was first reported in the postsynaptic density protein or PSD-95, DlgA, and ZO-1 protein), an N-terminal domain, and a C-terminal domain. Reverse transcription (RT)-PCR results showed that MjSyn was expressed in all tested tissues. Quantitative real-time PCR analysis revealed that MjSyn transcripts in the hemocyte, gill, and intestine were significantly induced at various time points after infection with Staphylococcus aureus and Vibrio anguillarum. The knockdown of the expression of MjSyn by RNA interference resulted in a significant decrease in the phagocytic ability and increased bacteria number in vivo of shrimp. Moreover, the expression of MjCnx, a cytoplasma and membrane location lectin chaperone protein, was inhibited in the MjSyn-knocked down shrimp, which indicated a possible calnexin-related way. Thus, the MjSyn participates in the bacterial clearance response of kuruma shrimp, thereby providing new insight into the function of this kind of important adaptor protein.

Calnexin is essential for survival under nitrogen starvation and stationary phase in Schizosaccharomyces pombe

Cell fate is determined by the balance of conserved molecular mechanisms regulating death (apoptosis) and survival (autophagy). Autophagy is a process by which cells recycle their organelles and macromolecules through degradation within the vacuole in yeast and plants, and lysosome in metazoa. In the yeast Schizosaccharomyces pombe, autophagy is strongly induced under nitrogen starvation and in aging cells. Previously, we demonstrated that calnexin (Cnx1p), a highly conserved transmembrane chaperone of the endoplasmic reticulum (ER), regulates apoptosis under ER stress or inositol starvation. Moreover, we showed that in stationary phase, Cnx1p is cleaved into two moieties, L_Cnx1p and S_Cnx1p. Here, we show that the processing of Cnx1p is regulated by autophagy, induced by nitrogen starvation or cell aging. The cleavage of Cnx1p involves two vacuolar proteases: Isp6, which is essential for autophagy, and its paralogue Psp3. Blocking autophagy through the knockout of autophagy-related genes (atg) results in inhibition of both, the cleavage and the trafficking of Cnx1p from the ER to the vacuole. We demonstrate that Cnx1p is required for cell survival under nitrogen-starvation and in chronological aging cultures. The death of the mini_cnx1 mutant (overlapping S_cnx1p) cells is accompanied by accumulation of high levels of reactive-oxygen species (ROS), a slowdown in endocytosis and severe cell-wall defects. Moreover, mutant cells expressing only S_Cnx1p showed cell wall defects. Co-expressing mutant overlapping the L_Cnx1p and S_Cnx1p cleavage products reverses the death, ROS phenotype and cell wall defect to wild-type levels. As it is involved in both apoptosis and autophagy, Cnx1p could be a nexus for the crosstalk between these pro-death and pro-survival mechanisms. Ours, and observations in mammalian systems, suggest that the multiple roles of calnexin depend on its sub-cellular localization and on its cleavage. The use of S. pombe should assist in further shedding light on the multiple roles of calnexin.

Glycoprotein folding and quality-control mechanisms in protein-folding diseases

Biosynthesis of proteins – from translation to folding to export – encompasses a complex set of events that are exquisitely regulated and scrutinized to ensure the functional quality of the end products. Cells have evolved to capitalize on multiple post-translational modifications in addition to primary structure to indicate the folding status of nascent polypeptides to the chaperones and other proteins that assist in their folding and export. These modifications can also, in the case of irreversibly misfolded candidates, signal the need for dislocation and degradation. The current Review focuses on the glycoprotein quality-control (GQC) system that utilizes protein N-glycosylation and N-glycan trimming to direct nascent glycopolypeptides through the folding, export and dislocation pathways in the endoplasmic reticulum (ER). A diverse set of pathological conditions rooted in defective as well as over-vigilant ER quality-control systems have been identified, underlining its importance in human health and disease. We describe the GQC pathways and highlight disease and animal models that have been instrumental in clarifying our current understanding of these processes.

Molecular basis of the dominant negative effect of a glycine transporter 2 mutation associated with hyperekplexia

Hyperekplexia or startle disease is a rare clinical syndrome characterized by an exaggerated startle in response to trivial tactile or acoustic stimuli. This neurological disorder can have serious consequences in neonates, provoking brain damage and/or sudden death due to apnea episodes and cardiorespiratory failure. Hyperekplexia is caused by defective inhibitory glycinergic neurotransmission. Mutations in the human SLC6A5 gene encoding the neuronal GlyT2 glycine transporter are responsible for the presynaptic form of the disease. GlyT2 mediates synaptic glycine recycling, which constitutes the main source of releasable transmitter at glycinergic synapses. Although the majority of GlyT2 mutations detected so far are recessive, a dominant negative mutant that affects GlyT2 trafficking does exist. In this study, we explore the properties and structural alterations of the S512R mutation in GlyT2. We analyze its dominant negative effect that retains wild-type GlyT2 in the endoplasmic reticulum (ER), preventing surface expression. We show that the presence of an arginine rather than serine 512 provoked transporter misfolding, enhanced association to the ER-chaperone calnexin, altered association with the coat-protein complex II component Sec24D, and thereby impeded ER exit. The S512R mutant formed oligomers with wild-type GlyT2 causing its retention in the ER. Overexpression of calnexin rescued wild-type GlyT2 from the dominant negative effect of the mutant, increasing the amount of transporter that reached the plasma membrane and dampening the interaction between the wild-type and mutant GlyT2. The ability of chemical chaperones to overcome the dominant negative effect of the disease mutation on the wild-type transporter was demonstrated in heterologous cells and primary neurons.