Redox-assisted regulation of Ca2+ homeostasis in the endoplasmic reticulum by disulfide reductase ERdj5

Protein Promiscuity in H2O2 Signaling

SIGNIFICANCE

Decrypting the cellular response to oxidative stress relies on a comprehensive understanding of the redox signaling pathways stimulated under oxidizing conditions. Redox signaling events can be divided into upstream sensing of oxidants, midstream redox signaling of protein function, and downstream transcriptional redox regulation. Recent Advances: A more and more accepted theory of hydrogen peroxide (H₂O₂) signaling is that of a thiol peroxidase redox relay, whereby protein thiols with low reactivity toward H₂O₂ are instead oxidized through an oxidative relay with thiol peroxidases.

CRITICAL ISSUES

These ultrareactive thiol peroxidases are the upstream redox sensors, which form the first cellular port of call for H₂O₂. Not all redox-regulated interactions between thiol peroxidases and cellular proteins involve a transfer of oxidative equivalents, and the nature of redox signaling is further complicated through promiscuous functions of redox-regulated "moonlighting" proteins, of which the precise cellular role under oxidative stress can frequently be obscured by "polygamous" interactions. An ultimate goal of redox signaling is to initiate a rapid response, and in contrast to prokaryotic oxidant-responsive transcription factors, mammalian systems have developed redox signaling pathways, which intersect both with kinase-dependent activation of transcription factors, as well as direct oxidative regulation of transcription factors through peroxiredoxin (Prx) redox relays.

FUTURE DIRECTIONS

We highlight that both transcriptional regulation and cell fate can be modulated either through oxidative regulation of kinase pathways, or through distinct redox-dependent associations involving either Prxs or redox-responsive moonlighting proteins with functional promiscuity. These protein associations form systems of crossregulatory networks with multiple nodes of potential oxidative regulation for H₂O₂-mediated signaling.

Ablation of the Chaperone Protein ERdj5 Results in a Sjögren's Syndrome-Like Phenotype in Mice, Consistent With an Upregulated Unfolded Protein Response in Human Patients

Objective: Sjögren's syndrome (SS) is a chronic autoimmune disorder that affects mainly the exocrine glands. Endoplasmic reticulum (ER) stress proteins have been suggested to participate in autoimmune and inflammatory responses, either acting as autoantigens, or by modulating factors of inflammation. The chaperone protein ERdj5 is an ER-resident disulfide reductase, required for the translocation of misfolded proteins during ER-associated protein degradation. In this study we investigated the role of ERdj5 in the salivary glands (SGs), in association with inflammation and autoimmunity.

Methods:In situ expression of ERdj5 and XBP1 activation were studied immunohistochemically in minor SG tissues from primary SS patients and non-SS sicca-complaining controls. We used the mouse model of ERdj5 ablation and characterized its features: Histopathological, serological (antinuclear antibodies and cytokine levels), and functional (saliva flow rate).

Results: ERdj5 was highly expressed in the minor SGs of SS patients, with stain intensity correlated to inflammatory lesion severity and anti-SSA/Ro positivity. Moreover, SS patients demonstrated higher XBP1 activation within the SGs. Remarkably, ablation of ERdj5 in mice conveyed many of the cardinal features of SS, like spontaneous inflammation in SGs with infiltrating T and B lymphocytes, distinct cytokine signature, excessive cell death, reduced saliva flow, and production of anti-SSA/Ro and anti-SSB/La autoantibodies. Notably, these features were more pronounced in female mice.

Conclusions: Our findings suggest a critical connection between the function of the ER chaperone protein ERdj5 and autoimmune inflammatory responses in the SGs and provide evidence for a new, potent animal model of SS.

Structures of the heart specific SERCA2a Ca2+-ATPase

The sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase 2a (SERCA2a) performs active reuptake of cytoplasmic Ca²⁺ and is a major regulator of cardiac muscle contractility. Dysfunction or dysregulation of SERCA2a is associated with heart failure, while restoring its function is considered as a therapeutic strategy to restore cardiac performance. However, its structure has not yet been determined. Based on native, active protein purified from pig ventricular muscle, we present the first crystal structures of SERCA2a, determined in the CPA-stabilized E2-AlF4- form (3.3 Å) and the Ca²⁺-occluded [Ca₂]E1-AMPPCP form (4.0 Å). The structures are similar to the skeletal muscle isoform SERCA1a pointing to a conserved mechanism. We seek to explain the kinetic differences between SERCA1a and SERCA2a. We find that several isoform-specific residues are acceptor sites for post-translational modifications. In addition, molecular dynamics simulations predict that isoform-specific residues support distinct intramolecular interactions in SERCA2a and SERCA1a. Our experimental observations further indicate that isoform-specific intramolecular interactions are functionally relevant, and may explain the kinetic differences between SERCA2a and SERCA1a.

Function, evolution, and structure of J-domain proteins

Hsp70 chaperone systems are very versatile machines present in nearly all living organisms and in nearly all intracellular compartments. They function in many fundamental processes through their facilitation of protein (re)folding, trafficking, remodeling, disaggregation, and degradation. Hsp70 machines are regulated by co-chaperones. J-domain containing proteins (JDPs) are the largest family of Hsp70 co-chaperones and play a determining role functionally specifying and directing Hsp70 functions. Many features of JDPs are not understood; however, a number of JDP experts gathered at a recent CSSI-sponsored workshop in Gdansk (Poland) to discuss various aspects of J-domain protein function, evolution, and structure. In this report, we present the main findings and the consensus reached to help direct future developments in the field of Hsp70 research.

The endoplasmic reticulum (ER) chaperone BiP is a master regulator of ER functions: Getting by with a little help from ERdj friends

The endoplasmic reticulum (ER) represents the entry point into the secretory pathway where nascent proteins encounter a specialized environment for their folding and maturation. Inherent to these processes is a dedicated quality-control system that detects proteins that fail to mature properly and targets them for cytosolic degradation. An imbalance in protein folding and degradation can result in the accumulation of unfolded proteins in the ER, resulting in the activation of a signaling cascade that restores proper homeostasis in this organelle. The ER heat shock protein 70 (Hsp70) family member BiP is an ATP-dependent chaperone that plays a critical role in these processes. BiP interacts with specific ER-localized DnaJ family members (ERdjs), which stimulate BiP's ATP-dependent substrate interactions, with several ERdjs also binding directly to unfolded protein clients. Recent structural and biochemical studies have provided detailed insights into the allosteric regulation of client binding by BiP and have enhanced our understanding of how specific ERdjs enable BiP to perform its many functions in the ER. In this review, we discuss how BiP's functional cycle and interactions with ERdjs enable it to regulate protein homeostasis in the ER and ensure protein quality control.

Methods to identify the substrates of thiol-disulfide oxidoreductases

The formation of a disulfide bond is a critical step in the folding of numerous secretory and membrane proteins and catalyzed in vivo. A variety of mechanisms and protein structures have evolved to catalyze oxidative protein folding. Those enzymes that directly interact with a folding protein to accelerate its oxidative folding are mostly thiol-disulfide oxidoreductases that belong to the thioredoxin superfamily. The enzymes of this class often use a CXXC active-site motif embedded in their thioredoxin-like fold to promote formation, isomerization, and reduction of a disulfide bond in their target proteins. Over the past decade or so, an increasing number of substrates of the thiol-disulfide oxidoreductases that are present in the ER of mammalian cells have been discovered, revealing that the enzymes play unexpectedly diverse physiological functions. However, functions of some of these enzymes still remain unclear due to the lack of information on their substrates. Here, we review the methods used by researchers to identify the substrates of these enzymes and provide data that show the importance of using trichloroacetic acid in sample preparation for the substrate identification, hoping to aid future studies. We particularly focus on successful studies that have uncovered physiological substrates and functions of the enzymes in the periplasm of Gram-negative bacteria and the endoplasmic reticulum of mammalian cells. Similar approaches should be applicable to enzymes in other cellular compartments or in other organisms.

Identification of the physiological substrates of PDIp, a pancreas-specific protein-disulfide isomerase family member

About 20 members of the protein-disulfide isomerase (PDI) family are present in the endoplasmic reticulum of mammalian cells. They are thought to catalyze thiol-disulfide exchange reactions within secretory or membrane proteins to assist in their folding or to regulate their functions. PDIp is a PDI family member highly expressed in the pancreas and known to bind estrogen in vivo and in vitro However, the physiological functions of PDIp remained unclear. In this study, we set out to identify its physiological substrates. By combining acid quenching and thiol alkylation, we stabilized and purified the complexes formed between endogenous PDIp and its target proteins from the mouse pancreas. MS analysis of these complexes helped identify the disulfide-linked PDIp targets in vivo, revealing that PDIp interacts directly with a number of pancreatic digestive enzymes. Interestingly, when pancreatic elastase, one of the identified proteins, was expressed alone in cultured cells, its proenzyme formed disulfide-linked aggregates within cells. However, when pancreatic elastase was co-expressed with PDIp, the latter prevented the formation of these aggregates and enhanced the production and secretion of proelastase in a form that could be converted to an active enzyme upon trypsin treatment. These findings indicate that the main targets of PDIp are digestive enzymes and that PDIp plays an important role in the biosynthesis of a digestive enzyme by assisting with the proper folding of the proenzyme within cells.

Redox crosstalk at endoplasmic reticulum (ER) membrane contact sites (MCS) uses toxic waste to deliver messages

Many cellular redox reactions housed within mitochondria, peroxisomes and the endoplasmic reticulum (ER) generate hydrogen peroxide (H₂O₂) and other reactive oxygen species (ROS). The contribution of each organelle to the total cellular ROS production is considerable, but varies between cell types and also over time. Redox-regulatory enzymes are thought to assemble at a “redox triangle” formed by mitochondria, peroxisomes and the ER, assembling “redoxosomes” that sense ROS accumulations and redox imbalances. The redoxosome enzymes use ROS, potentially toxic by-products made by some redoxosome members themselves, to transmit inter-compartmental signals via chemical modifications of downstream proteins and lipids. Interestingly, important components of the redoxosome are ER chaperones and oxidoreductases, identifying ER oxidative protein folding as a key ROS producer and controller of the tri-organellar membrane contact sites (MCS) formed at the redox triangle. At these MCS, ROS accumulations could directly facilitate inter-organellar signal transmission, using ROS transporters. In addition, ROS influence the flux of Ca²⁺ ions, since many Ca²⁺ handling proteins, including inositol 1,4,5 trisphosphate receptors (IP₃Rs), SERCA pumps or regulators of the mitochondrial Ca²⁺ uniporter (MCU) are redox-sensitive. Fine-tuning of these redox and ion signaling pathways might be difficult in older organisms, suggesting a dysfunctional redox triangle may accompany the aging process.

How Are Proteins Reduced in the Endoplasmic Reticulum?

The reversal of thiol oxidation in proteins within the endoplasmic reticulum (ER) is crucial for protein folding, degradation, chaperone function, and the ER stress response. Our understanding of this process is generally poor but progress has been made. Enzymes performing the initial reduction of client proteins, as well as the ultimate electron donor in the pathway, have been identified. Most recently, a role for the cytosol in ER protein reduction has been revealed. Nevertheless, how reducing equivalents are transferred from the cytosol to the ER lumen remains an open question. We review here why proteins are reduced in the ER, discuss recent data on catalysis of steps in the pathway, and consider the implications for redox homeostasis within the early secretory pathway.

Reexamining the Function of Glutathione in Oxidative Protein Folding and Secretion

SIGNIFICANCE

Disturbance of glutathione (GSH) metabolism is a hallmark of numerous diseases, yet GSH functions are poorly understood. One key to this question is to consider its functional compartmentation. GSH is present in the endoplasmic reticulum (ER), where it competes with substrates for oxidation by the oxidative folding machinery, composed in eukaryotes of the thiol oxidase Ero1 and proteins from the disulfide isomerase family (protein disulfide isomerase). Yet, whether GSH is required for proper ER oxidative protein folding is a highly debated question. Recent Advances: Oxidative protein folding has been thoroughly dissected over the past decades, and its actors and their mode of action elucidated. Genetically encoded GSH probes have recently provided an access to subcellular redox metabolism, including the ER.

CRITICAL ISSUES

Of the few often-contradictory models of the role of GSH in the ER, the most popular suggest it serves as reducing power. Yet, as a reductant, GSH also activates Ero1, which questions how GSH can nevertheless support protein reduction. Hence, whether GSH operates in the ER as a reductant, an oxidant, or just as a "blank" compound mirroring ER/periplasm redox activity is a highly debated question, which is further stimulated by the puzzling occurrence of GSH in the Escherichia coli periplasmic "secretory" compartment, aside from the Dsb thiol-reducing and oxidase pathways.

FUTURE DIRECTIONS

Addressing the mechanisms controlling GSH traffic in and out of the ER/periplasm and its recycling will help address GSH function in secretion. In addition, as thioredoxin reductase was recently implicated in ER oxidative protein folding, the relative contribution of each of these two reducing pathways should now be addressed. Antioxid. Redox Signal. 27, 1178-1199.

An Update on Sec61 Channel Functions, Mechanisms, and Related Diseases

The membrane of the endoplasmic reticulum (ER) of nucleated human cells harbors the protein translocon, which facilitates membrane integration or translocation of almost every newly synthesized polypeptide targeted to organelles of the endo- and exocytotic pathway. The translocon comprises the polypeptide-conducting Sec61 channel and several additional proteins and complexes that are permanently or transiently associated with the heterotrimeric Sec61 complex. This ensemble of proteins facilitates ER targeting of precursor polypeptides, modification of precursor polypeptides in transit through the Sec61 complex, and Sec61 channel gating, i.e., dynamic regulation of the pore forming subunit to mediate precursor transport and calcium efflux. Recently, cryoelectron tomography of translocons in native ER membrane vesicles, derived from human cell lines or patient fibroblasts, and even intact cells has given unprecedented insights into the architecture and dynamics of the native translocon and the Sec61 channel. These structural data are discussed in light of different Sec61 channel activities including ribosome receptor function, membrane insertion, and translocation of newly synthesized polypeptides as well as the putative physiological roles of the Sec61 channel as a passive ER calcium leak channel. Furthermore, the structural insights into the Sec61 channel are incorporated into an overview and update on Sec61 channel-related diseases—the Sec61 channelopathies—and novel therapeutic concepts for their treatment.

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.

Endoplasmic reticulum chaperones tweak the mitochondrial calcium rheostat to control metabolism and cell death

The folding of secretory proteins is a well-understood mechanism, based on decades of research on endoplasmic reticulum (ER) chaperones. These chaperones interact with newly imported polypeptides close to the ER translocon. Classic examples for these proteins include the immunoglobulin binding protein (BiP/GRP78), and the lectins calnexin and calreticulin. Although not considered chaperones per se, the ER oxidoreductases of the protein disulfide isomerase (PDI) family complete the folding job by catalyzing the formation of disulfide bonds through cysteine oxidation. Research from the past decade has demonstrated that ER chaperones are multifunctional proteins. The regulation of ER-mitochondria Ca²⁺ crosstalk is one of their additional functions, as shown for calnexin, BiP/GRP78 or the oxidoreductases Ero1α and TMX1. This function depends on interactions of this group of proteins with the ER Ca²⁺ handling machinery. This novel function makes perfect sense for two reasons: i. It allows ER chaperones to control mitochondrial apoptosis instantly without a lengthy bypass involving the upregulation of pro-apoptotic transcription factors via the unfolded protein response (UPR); and ii. It allows the ER protein folding machinery to fine-tune ATP import via controlling the speed of mitochondrial oxidative phosphorylation. Therefore, the role of ER chaperones in regulating ER-mitochondria Ca²⁺ flux identifies the progression of secretory protein folding as a central regulator of cell survival and death, at least in cell types that secrete large amount of proteins. In other cell types, ER protein folding might serve as a sentinel mechanism that monitors cellular well-being to control cell metabolism and apoptosis. The selenoprotein SEPN1 is a classic example for such a role. Through the control of ER-mitochondria Ca²⁺-flux, ER chaperones and folding assistants guide cellular apoptosis and mitochondrial metabolism.

Increased Intracellular Reactive Oxygen Species Mediates the Anti-Cancer Effects of WZ35 via Activating Mitochondrial Apoptosis Pathway in Prostate Cancer Cells

BACKGROUND

The limited treatment option for recurrent prostate cancer and eventual resistant to conventional chemotherapy drugs has fueled continued interest in finding new anti-neoplastic agents. WZ35, a chemical analog of curcumin, had been demonstrated to have high chemical stability and potential anticancer effects in gastric cancer cells. The present study aimed to investigate the anti-prostate cancer effects of WZ35 in vitro and in vivo as well as the underlying mechanism.

METHODS

Two prostate cancer cell lines RM-1 and DU145 were utilized to test the anti-cancer effects of WZ35 and the underlying mechanism. MTT assay was used to assess the cytotoxic effect of WZ35. Cell cycle distribution, apoptosis, alteration of ROS, and [Ca²⁺ ]_i level were evaluated using flow cytometry. Western blotting assay was applied to measure the levels of proteins associated with apoptosis and cell cycle. Immunofluorescence staining and Electron micrographs were used to evaluate activation of mitochondrial apoptosis pathway. Tumor models in nude mice were induced by injection of RM-1 prostate cancer cells to test the in vivo anticancer action of WZ35.

RESULTS

Our results showed that WZ35 treatment induced loss of cell viability, cell apoptosis, and G2/M cycle arrest in both RM-1 and DU145 cells, coupled with ROS overproduction, intracellular calcium surge, and activation of mitochondrial apoptosis pathway in RM-1 cells. Interestingly, all above changes induced by WZ35 were completely reversed by ROS blockage. In addition, prevention of [Ca²⁺ ]_i elevation by BAPTA/AM also inhibited activation of mitochondrial apoptosis pathway induced by WZ35. In vivo studies, WZ35 treatment significantly inhibited RM-1 homograft tumor growth along with increased ROS accumulation, mitochondrial disruption, and cell apoptosis in tumor tissues.

CONCLUSIONS

In conclusion, this work provides a novel anticancer candidate for the treatment of prostate cancer and demonstrated that increased ROS mediate the anti-cancer effects of WZ35 via activating mitochondrial apoptosis pathway. Importantly, this work also reveals that targeting ROS generation might be an effective strategy in human androgen-resistant prostate cancer treatment. Prostate 77:489-504, 2017. © 2016 Wiley Periodicals, Inc.

How Do J-Proteins Get Hsp70 to Do So Many Different Things?

Hsp70 chaperone machineries have pivotal roles in an array of fundamental biological processes through their facilitation of protein folding, disaggregation, and remodeling. The obligate J-protein co-chaperones of Hsp70s drive much of this remarkable multifunctionality, with most Hsp70s having multiple J-protein partners. Recent data suggest that J-protein-driven versatility is substantially due to precise localization within the cell and the specificity of substrate protein binding. However, this relatively simple view belies the intricacy of J-protein function. Examples are emerging of J-protein interactions with Hsp70s and other chaperones, as well as integration into broader cellular networks. These interactions fine-tune, in critical ways, the ability of Hsp70s to participate in diverse cellular processes.

Regulation of Calcium Homeostasis by ER Redox: A Close-Up of the ER/Mitochondria Connection

Calcium signaling plays an important role in cell survival by influencing mitochondria-related processes such as energy production and apoptosis. The endoplasmic reticulum (ER) is the main storage compartment for cell calcium (Ca²⁺; ~60-500μM), and the Ca²⁺ released by the ER has a prompt effect on the homeostasis of the juxtaposed mitochondria. Recent findings have highlighted a close connection between ER redox and Ca²⁺ signaling that is mediated by Ca²⁺-handling proteins. This paper describes the redox-regulated mediators and mechanisms that orchestrate Ca²⁺ signals from the ER to mitochondria.