Different interaction modes for protein-disulfide isomerase (PDI) as an efficient regulator and a specific substrate of endoplasmic reticulum oxidoreductin-1α (Ero1α)

Initiation of ERAD by the bifunctional complex of Mnl1/Htm1 mannosidase and protein disulfide isomerase

Misfolded glycoproteins in the endoplasmic reticulum (ER) lumen are translocated into the cytosol and degraded by the proteasome, a conserved process called ER-associated protein degradation (ERAD). In Saccharomyces cerevisiae, the glycan of these proteins is trimmed by the luminal mannosidase Mnl1 (Htm1) to generate a degradation signal. Interestingly, Mnl1 is associated with protein disulfide isomerase (Pdi1). Here we used cryo-electron microscopy, biochemical and in vivo experiments to elucidate how this complex initiates ERAD. The Mnl1-Pdi1 complex first demannosylates misfolded, globular proteins that are recognized through the C-terminal domain (CTD) of Mnl1; Pdi1 causes the CTD to ignore completely unfolded polypeptides. The disulfides of these globular proteins are then reduced by the Pdi1 component of the complex. Mnl1 blocks the canonical oxidative function of Pdi1, allowing it to function as a disulfide reductase in ERAD. The generated unfolded polypeptides can then be translocated across the membrane into the cytosol.

Enhancing S-nitrosoglutathione reductase decreases S-nitrosylation of ERO1α and reduces neuronal death in secondary traumatic brain injury

Traumatic brain injury (TBI) has the highest incidence of all common neurological disorders, along with high mortality and disability rates. Pathological conversion of excess nitric oxide (NO) to S-nitrosoglutathion (GSNO) after TBI leads to high S-nitrosylation of intracellular proteins, causing nitrative stress. GSNO reductase (GSNOR) plays an important role by regulating GSNO and SNO-proteins (PSNOs) and as a redox regulator of the nervous system. However, the effect of GSNOR on protein S-nitrosylation in secondary brain injury after TBI is not clear. In vivo TBI model was established in male C57BL/6 mice via controlled cortical impact (CCI). Neuron-targeted GSNOR-overexpression adeno-associated virus (AAV) was constructed and administered to mice by stereotactic cortical injection. The results showed that NO, GSNO, neuronal protein S-nitrosylation and neuronal death increased after TBI, while the level and activity of GSNOR decreased. Overexpression of GSNOR by AAV decreased GSNO and NO and improved short-term neurobehavioral outcomes in mice. GSNOR overexpression can reduce endoplasmic reticulum stress and neuronal death by reducing the S-nitrosylation of ERO1α via H2O2 generation and plays a neuroprotective role. In conclusion, our results suggest that GSNOR regulating S-nitrosylation of ERO1α may participate in neuronal death, and overexpression of GSNOR in neurons after experimental brain injury alleviates secondary brain injury. Our research provides a potential therapeutic approach for the treatment of TBI.

Initiation of ERAD by the bifunctional complex of Mnl1 mannosidase and protein disulfide isomerase

Misfolded glycoproteins in the endoplasmic reticulum (ER) lumen are translocated into the cytosol and degraded by the proteasome, a conserved process called ER-associated protein degradation (ERAD). In S. cerevisiae, the glycan of these proteins is trimmed by the luminal mannosidase Mnl1 (Htm1) to generate a signal that triggers degradation. Curiously, Mnl1 is permanently associated with protein disulfide isomerase (Pdi1). Here, we have used cryo-electron microscopy, biochemical, and in vivo experiments to clarify how this complex initiates ERAD. The Mnl1-Pdi1 complex first de-mannosylates misfolded, globular proteins that are recognized through a C-terminal domain (CTD) of Mnl1; Pdi1 causes the CTD to ignore completely unfolded polypeptides. The disulfides of these globular proteins are then reduced by the Pdi1 component of the complex, generating unfolded polypeptides that can be translocated across the membrane. Mnl1 blocks the canonical oxidative function of Pdi1, but allows it to function as the elusive disulfide reductase in ERAD.

A Conformational-Dependent Interdomain Redox Relay at the Core of Protein Disulfide Isomerase Activity

Aims: Protein disulfide isomerases (PDIs) are a family of chaperones resident in the endoplasmic reticulum (ER). In addition to holdase function, some members catalyze disulfide bond formation and isomerization, a crucial step for native folding and prevention of aggregation of misfolded proteins. PDIs are characterized by an arrangement of thioredoxin-like domains, with the canonical protein disulfide isomerase A1 (PDIA1) organized as four thioredoxin-like domains forming a horseshoe with two active sites, a and a', at the extremities. We aimed to clarify important aspects underlying the catalytic cycle of PDIA1 in the context of the full pathways of oxidative protein folding operating in the ER. Results: Using two fluorescent redox sensors, redox green fluorescent protein 2 (roGFP2) and HyPer (circularly permutated yellow fluorescent protein containing the regulatory domain of the H2O2-sensing protein OxyR), either unfolded or native, as client substrates, we identified the N-terminal a active site of PDIA1 as the main oxidant of thiols. From there, electrons can flow to the C-terminal a' active site, with the redox-dependent conformational flexibility of PDIA1 allowing the formation of an interdomain disulfide bond. The a' active site then acts as a crossing point to redirect electrons to ER downstream oxidases or back to client proteins to reduce scrambled disulfide bonds. Innovation and Conclusions: The two active sites of PDIA1 work cooperatively as an interdomain redox relay mechanism that explains PDIA1 oxidative activity to form native disulfides and PDIA1 reductase activity to resolve scrambled disulfides. This mechanism suggests a new rationale for shutting down oxidative protein folding under ER redox imbalance. Whether it applies to physiological substrates in cells remains to be shown.

Integrated Transcriptome and Proteome Analysis Reveals the Regulatory Mechanism of Root Growth by Protein Disulfide Isomerase in Arabidopsis

Protein disulfide isomerase (PDI, EC 5.3.4.1) is a thiol-disulfide oxidoreductase that plays a crucial role in catalyzing the oxidation and rearrangement of disulfides in substrate proteins. In plants, PDI is primarily involved in regulating seed germination and development, facilitating the oxidative folding of storage proteins in the endosperm, and also contributing to the formation of pollen. However, the role of PDI in root growth has not been previously studied. This research investigated the impact of PDI gene deficiency in plants by using 16F16 [2-(2-Chloroacetyl)-2,3,4,9-tetrahydro-1-methyl-1H-pyrido[3,4-b]indole-1-carboxylic acid methyl ester], a small-molecule inhibitor of PDI, to remove functional redundancy. The results showed that the growth of Arabidopsis roots was significantly inhibited when treated with 16F16. To further investigate the effects of 16F16 treatment, we conducted expression profiling of treated roots using RNA sequencing and a Tandem Mass Tag (TMT)-based quantitative proteomics approach at both the transcriptomic and proteomic levels. Our analysis revealed 994 differentially expressed genes (DEGs) at the transcript level, which were predominantly enriched in pathways associated with "phenylpropane biosynthesis", "plant hormone signal transduction", "plant-pathogen interaction" and "starch and sucrose metabolism" pathways. Additionally, we identified 120 differentially expressed proteins (DEPs) at the protein level. These proteins were mainly enriched in pathways such as "phenylpropanoid biosynthesis", "photosynthesis", "biosynthesis of various plant secondary metabolites", and "biosynthesis of secondary metabolites" pathways. The comprehensive transcriptome and proteome analyses revealed a regulatory network for root shortening in Arabidopsis seedlings under 16F16 treatment, mainly involving phenylpropane biosynthesis and plant hormone signal transduction pathways. This study enhances our understanding of the significant role of PDIs in Arabidopsis root growth and provides insights into the regulatory mechanisms of root shortening following 16F16 treatment.

Biochemical analysis of Komagataella phaffii oxidative folding proposes novel regulatory mechanisms of disulfide bond formation in yeast

Oxidative protein folding in the endoplasmic reticulum (ER) is driven mainly by protein disulfide isomerase PDI and oxidoreductin Ero1. Their activity is tightly regulated and interconnected with the unfolded protein response (UPR). The mechanisms of disulfide bond formation have mainly been studied in human or in the yeast Saccharomyces cerevisiae. Here we analyze the kinetics of disulfide bond formation in the non-conventional yeast Komagataella phaffii, a common host for the production of recombinant secretory proteins. Surprisingly, we found significant differences with both the human and S. cerevisiae systems. Specifically, we report an inactive disulfide linked complex formed by K. phaffii Ero1 and Pdi1, similarly to the human orthologs, but not described in yeast before. Furthermore, we show how the interaction between K. phaffii Pdi1 and Ero1 is unaffected by the introduction of unfolded substrate into the system. This is drastically opposed to the previously observed behavior of the human pathway, suggesting a different regulation of the UPR and/or possibly different interaction mechanics between K. phaffii Pdi1 and Ero1.

Reducing oxidative protein folding alleviates senescence by minimizing ER-to-nucleus H2 O2 release

Oxidative protein folding occurs in the endoplasmic reticulum (ER) to generate disulfide bonds, and the by-product is hydrogen peroxide (H2 O2 ). However, the relationship between oxidative protein folding and senescence remains uncharacterized. Here, we find that the protein disulfide isomerase (PDI), a key oxidoreductase that catalyzes oxidative protein folding, accumulated in aged human mesenchymal stem cells (hMSCs) and deletion of PDI alleviated hMSCs senescence. Mechanistically, knocking out PDI slows the rate of oxidative protein folding and decreases the leakage of ER-derived H2 O2 into the nucleus, thereby decreasing the expression of SERPINE1, which was identified as a key driver of cell senescence. Furthermore, we show that depletion of PDI alleviated senescence in various cell models of aging. Our findings reveal a previously unrecognized role of oxidative protein folding in promoting cell aging, providing a potential target for aging and aging-related disease intervention.

Sulfenylation links oxidative stress to protein disulfide isomerase oxidase activity and thrombus formation

BACKGROUND

Oxidative stress contributes to thrombosis in atherosclerosis, inflammation, infection, aging, and malignancy. Oxidant-induced cysteine modifications, including sulfenylation, can act as a redox-sensitive switch that controls protein function. Protein disulfide isomerase (PDI) is a prothrombotic enzyme with exquisitely redox-sensitive active-site cysteines.

OBJECTIVES

We hypothesized that PDI is sulfenylated during oxidative stress, contributing to the prothrombotic potential of PDI.

METHODS

Biochemical and enzymatic assays using purified proteins, platelet and endothelial cell assays, and in vivo murine thrombosis studies were used to evaluate the role of oxidative stress in PDI sulfenylation and prothrombotic activity.

RESULTS

PDI exposure to oxidants resulted in the loss of PDI reductase activity and simultaneously promoted sulfenylated PDI generation. Following exposure to oxidants, sulfenylated PDI spontaneously converted to disulfided PDI. PDI oxidized in this manner was able to transfer disulfides to protein substrates. Inhibition of sulfenylation impaired disulfide formation by oxidants, indicating that sulfenylation is an intermediate during PDI oxidation. Agonist-induced activation of platelets and endothelium resulted in the release of sulfenylated PDI. PDI was also sulfenylated by oxidized low-density lipoprotein (oxLDL). In an in vivo model of thrombus formation, oxLDL markedly promoted platelet accumulation following an arteriolar injury. PDI oxidoreductase inhibition blocked oxLDL-mediated augmentation of thrombosis.

CONCLUSION

PDI sulfenylation is a critical posttranslational modification that is an intermediate during disulfide PDI formation in the setting of oxidative stress. Oxidants generated by vascular cells during activation promote PDI sulfenylation, and interference with PDI during oxidative stress impairs thrombus formation.

Silica nanoparticles induce ovarian granulosa cell apoptosis via activation of the PERK-ATF4-CHOP-ERO1α pathway-mediated IP3R1-dependent calcium mobilization

Ambient particulate matters (PMs) have adverse effects in human and animal female reproductive health. Silica nanoparticles (SNPs), as a major component of PMs, can induce follicular atresia via the promotion of ovarian granulosa cell apoptosis. However, the molecular mechanisms of apoptosis induced by SNPs are not very clear. This work focuses on revealing the mechanisms of ER stress on SNP-induced apoptosis. Our results showed that spherical Stöber SNPs (110 nm, 25.0 mg/kg b.w.) induced follicular atresia via the promotion of granulosa cell apoptosis by intratracheal instillation in vivo; meanwhile, SNPs decreased the viability and increase apoptosis in granulosa cells in vitro. SNPs were taken up and accumulated in the vesicles of granulosa cells. Additionally, our results found that SNPs increased calcium ion (Ca2+) concentration in granulosa cell cytoplasm. Furthermore, SNPs activated ER stress via an increase in the PERK and ATF6 pathway-related protein levels and IP3R1-dependent calcium mobilization via an increase in IP3R1 level. In addition, 4-PBA restored IP3R1-dependent calcium mobilization and decreased apoptosis via the inhibition of ER stress. The ATF4-C/EBP homologous protein (CHOP)-ER oxidoreductase 1 alpha (ERO1α) pathway regulated SNP-induced IP3R1-dependent calcium mobilization and cell apoptosis via ATF4, CHOP, and ERO1α depletion in ovarian granulosa cells. Herein, we demonstrate that ER stress cooperated in SNP-induced ovarian toxicity via activation of IP3R1-mediated calcium mobilization, leading to apoptosis, in which the PERK-ATF4-CHOP-ERO1α pathway plays an essential role in ovarian granulosa cells.

FAT-switch-based quantitative S-nitrosoproteomics reveals a key role of GSNOR1 in regulating ER functions

Reversible protein S-nitrosylation regulates a wide range of biological functions and physiological activities in plants. However, it is challenging to quantitively determine the S-nitrosylation targets and dynamics in vivo. In this study, we develop a highly sensitive and efficient fluorous affinity tag-switch (FAT-switch) chemical proteomics approach for S-nitrosylation peptide enrichment and detection. We quantitatively compare the global S-nitrosylation profiles in wild-type Arabidopsis and gsnor1/hot5/par2 mutant using this approach, and identify 2,121 S-nitrosylation peptides in 1,595 protein groups, including many previously unrevealed S-nitrosylated proteins. These are 408 S-nitrosylated sites in 360 protein groups showing an accumulation in hot5-4 mutant when compared to wild type. Biochemical and genetic validation reveal that S-nitrosylation at Cys337 in ER OXIDOREDUCTASE 1 (ERO1) causes the rearrangement of disulfide, resulting in enhanced ERO1 activity. This study offers a powerful and applicable tool for S-nitrosylation research, which provides valuable resources for studies on S-nitrosylation-regulated ER functions in plants.

Redox states in the endoplasmic reticulum directly regulate the activity of calcium channel, inositol 1,4,5-trisphosphate receptors

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are one of the two types of tetrameric ion channels that release calcium ion (Ca2+) from the endoplasmic reticulum (ER) into the cytosol. Ca2+ released via IP3Rs is a fundamental second messenger for numerous cell functions. Disturbances in the intracellular redox environment resulting from various diseases and aging interfere with proper calcium signaling, however, the details are unclear. Here, we elucidated the regulatory mechanisms of IP3Rs by protein disulfide isomerase family proteins localized in the ER by focusing on four cysteine residues residing in the ER lumen of IP3Rs. First, we revealed that two of the cysteine residues are essential for functional tetramer formation of IP3Rs. Two other cysteine residues, on the contrary, were revealed to be involved in the regulation of IP3Rs activity; its oxidation by ERp46 and the reduction by ERdj5 caused the activation and the inactivation of IP3Rs activity, respectively. We previously reported that ERdj5 can activate the sarco/endoplasmic reticulum Ca2+-ATPase isoform 2b (SERCA2b) using its reducing activity [Ushioda et al., Proc. Natl. Acad. Sci. U.S.A. 113, E6055-E6063 (2016)]. Thus, we here established that ERdj5 exerts the reciprocal regulatory function for IP3Rs and SERCA2b by sensing the ER luminal Ca2+ concentration, which contributes to the calcium homeostasis in the ER.

Transcriptome, proteome, and protein synthesis within the intracellular cytomatrix

Despite the knowledge that protein translation and various metabolic reactions that create and sustain cellular life occur in the cytoplasm, the structural organization within the cytoplasm remains unclear. Recent models indicate that cytoplasm contains viscous fluid and elastic solid phases. We separated these viscous fluid and solid elastic compartments, which we call the cytosol and cytomatrix, respectively. The distinctive composition of the cytomatrix included structural proteins, ribosomes, and metabolome enzymes. High-throughput analysis revealed unique biosynthetic pathways within the cytomatrix. Enrichment of biosynthetic pathways in the cytomatrix indicated the presence of immobilized biocatalysis. Enzymatic immobilization and segregation can surmount spatial impediments, and the local pathway segregation may form cytoplasmic organelles. Protein translation was reprogrammed within the cytomatrix under the restriction of protein synthesis by drug treatment. The cytosol and cytomatrix are an elaborately interconnected network that promotes operational flexibility in healthy cells and the survival of malignant cells.

Splice variants of protein disulfide isomerase - identification, distribution and functional characterization in the rat

BACKGROUND

Protein Disulfide Isomerase (PDI) enzyme is an emerging therapeutic target in oncology and hematology. Although PDI reductase activity has been studied with isolated fragments of the protein, natural structural variations affecting reductase activity have not been addressed.

METHODS

In this study, we discovered four coding splice variants of the Pdi pre-mRNA in rats. In vitro Michaelis constants and apparent maximum steady-state rate constants after purification and distribution in different rat tissues were determined.

RESULTS

The consensus sequence was found to be the most expressed splice variant while the second most expressed variant represents 15 to 35% of total Pdi mRNA. The third variant shows a quasi-null expression profile and the fourth was not quantifiable. The consensus sequence splice variant and the second splice variant are widely expressed (transcription level) in the liver and even more present in males. Measurements of the reductase activity of recombinant PDI indicate that the consensus sequence and third splice variant are fully active variants. The second most expressed variant, differing by a lack of signal peptide, was found active but less than the consensus sequence.

GENERAL SIGNIFICANCE

Our work emphasizes the importance of taking splice variants into account when studying PDI-like proteins to understand the full biological functionalities of PDI.

Oxidative protein folding fidelity and redoxtasis in the endoplasmic reticulum

In eukaryotic cells, oxidative protein folding occurs in the lumen of the endoplasmic reticulum (ER), catalyzed by ER sulfhydryl oxidase 1 (Ero1) and protein disulfide isomerase (PDI). The efficiency and fidelity of oxidative protein folding are vital for the function of secretory cells. Here, we summarize oxidative protein folding in yeast, plants, and mammals, and discuss how the conformation and activity of human Ero1-PDI machinery is regulated through various post-translational modifications (PTMs). We propose that oxidative protein folding fidelity and ER redox homeostasis are maintained by both the precise control of Ero1 oxidase activity and the division of labor between PDI family members. We also discuss how deregulated Ero1-PDI functions contribute to human diseases and can be leveraged for therapeutic interventions.

Acute toxic effects of diclofenac exposure on freshwater crayfish (Procambarus clarkii): Insights from hepatopancreatic pathology, molecular regulation and intestinal microbiota

In this study, we exposed adult male crayfish (Procambarus clarkii) to different concentrations of diclofenac (DCF) for 96 h. In the meantime, we investigated the alternations of hepatopancreatic pathology, molecular regulation and intestinal microbiota of P. clarkii exposed to DCF. The results demonstrated DCF led to histological changes including epithelium vacuolization and tubule lumen dilatation in the hepatopancreas. Transcriptome sequencing analysis showed that 642 and 586 genes were differentially expressed in the hepatopancreas of P. clarkii exposed to 1 and 10 mg/L DCF, respectively. DCF could affect the functions of antioxidation, immunity and metabolism of hepatopancreas by inducing the abnormal expressions of immune- and redox-related genes. GO enrichment results demonstrated that 10 mg/L DCF exposure could modulate the processes of molting, amino sugar metabolism, protein hydrolysis and intracellular protein translocation of P. clarkii. Additionally, the abundances of bacterial families including Shewanellaceae, Bacteroidaceae, Vibrionaceae, Erysipelotrichaceae, Aeromonadaceae, Moraxellaceae, etc. in the intestine were significantly changed after DCF exposure, and the disruption of intestinal flora might further cause abnormal intestinal metabolism in P. clarkii. This study provides novel mechanistic insights into the toxic effects of anti-inflammatory drugs on aquatic crustaceans.

Homocysteine facilitates endoplasmic reticulum stress and apoptosis of hepatocytes by suppressing ERO1α expression via cooperation between DNMT1 and G9a

Endoplasmic reticulum (ER) stress and apoptosis play a critical role in liver injury. Endoplasmic reticulum oxidoreductase 1α (ERO1α) is an oxidase that exists in the luminal side of the ER membrane, participating in protein folding and secretion and inhibiting apoptosis, but the underlying mechanism on liver injury induced by homocysteine (Hcy) remains obscure. In this study, hyperhomocysteinemia (HHcy) mice model was established in cbs^+/− mice by feeding a high‐methionine diet for 12 weeks; and cbs^+/− mice fed with high‐methionine diet exhibited more severe liver injury compared to cbs^+/+ mice. Mechanistically, we found that Hcy promoted ER stress and apoptosis of hepatocytes and thereby aggravated liver injury through inhibiting ERO1α expression; accordingly, overexpression of ERO1α remarkably alleviated ER stress and apoptosis of hepatocytes induced by Hcy. Epigenetic modification analysis revealed that Hcy significantly increased levels of DNA methylation and H3 lysine 9 dimethylation (H3K9me2) on ERO1α promoter, which attributed to upregulated DNA methyltransferase 1 (DNMT1) and G9a, respectively. Further study showed that DNMT1 and G9a cooperatively regulated ERO1α expression in hepatocytes exposed to Hcy. Taken together, our work demonstrates that Hcy activates ER stress and apoptosis of hepatocytes by downregulating ERO1α expression via cooperation between DNMT1 and G9a, which provides new insight into the mechanism of Hcy‐induced ER stress and apoptosis of hepatocytes in liver injury.

ER reductive stress caused by Ero1α S-nitrosation accelerates senescence

Oxidative stress in aging has attracted much attention; however, the role of reductive stress in aging remains largely unknown. Here, we report that the endoplasmic reticulum (ER) undergoes reductive stress during replicative senescence, as shown by specific glutathione and H₂O₂ fluorescent probes. We constructed an ER-specific reductive stress cell model by ER-specific catalase overexpression and observed accelerated senescent phenotypes accompanied by disrupted proteostasis and a compromised ER unfolded protein response (UPR). Mechanistically, S-nitrosation of the pivotal ER sulfhydryl oxidase Ero1α led to decreased activity, therefore resulting in reductive stress in the ER. Inhibition of inducible nitric oxide synthase decreased the level of Ero1α S-nitrosation and decreased cellular senescence. Moreover, the expression of constitutively active Ero1α restored an oxidizing state in the ER and successfully rescued the senescent phenotypes. Our results uncover a new mechanism of senescence promoted by ER reductive stress and provide proof-of-concept that maintaining the oxidizing power of the ER and organelle-specific precision redox regulation could be valuable future geroprotective strategies.

Roles of the protein disulphide isomerases AccPDIA1 and AccPDIA3 in response to oxidant stress in Apis cerana cerana

Protein disulphide isomerase (PDI) plays an important role in a variety of physiological processes through its oxidoreductase activity and molecular chaperone activity. In this study, we cloned two PDI family members, AccPDIA1 and AccPDIA3, from Apis cerana cerana. AccPDIA1 and AccPDIA3 had typical sequence features of PDI family members and were constitutively expressed in A. cerana cerana. The expression levels of AccPDIA1 and AccPDIA3 were generally upregulated after treatment with a variety of environmental stress factors. Inhibition assays showed that E. coli expressing recombinant AccPDIA1 and AccPDIA3 proteins was more resistant to oxidative stress than control E. coli. In addition, silencing AccPDIA1 or AccPDIA3 in A. cerana cerana resulted in significant changes in the expression levels of several antioxidant-related genes as well as the enzymatic activities of peroxidase (POD), superoxide dismutase (SOD) and catalase (CAT) and reduced the survival rate of A. cerana cerana under oxidative stress caused by high temperature. In conclusion, our results suggest that AccPDIA1 and AccPDIA3 may play important roles in the antioxidant activities of A. cerana cerana.

Two protein disulfide isomerase subgroups work synergistically in catalyzing oxidative protein folding

Disulfide bonds play essential roles in the folding of secretory and plasma membrane proteins in the endoplasmic reticulum (ER). In eukaryotes, protein disulfide isomerase (PDI) is an enzyme catalyzing the disulfide bond formation and isomerization in substrates. The Arabidopsis (Arabidopsis thaliana) genome encodes diverse PDIs including structurally distinct subgroups PDI-L and PDI-M/S. It remains unclear how these AtPDIs function to catalyze the correct disulfide formation. We found that one Arabidopsis ER oxidoreductin-1 (Ero1), AtERO1, can interact with multiple PDIs. PDI-L members AtPDI2/5/6 mainly serve as an isomerase, while PDI-M/S members AtPDI9/10/11 are more efficient in accepting oxidizing equivalents from AtERO1 and catalyzing disulfide bond formation. Accordingly, the pdi9/10/11 triple mutant exhibited much stronger inhibition than pdi1/2/5/6 quadruple mutant under dithiothreitol treatment, which caused disruption of disulfide bonds in plant proteins. Furthermore, AtPDI2/5 work synergistically with PDI-M/S members in relaying disulfide bonds from AtERO1 to substrates. Our findings reveal the distinct but overlapping roles played by two structurally different AtPDI subgroups in oxidative protein folding in the ER.

The extracellular Ero1α/PDI electron transport system regulates platelet function by increasing glutathione reduction potential

Protein disulfide isomerase (PDI), an oxidoreductase, possesses two vicinal cysteines in the -Cys-Gly-His-Cys-motif that either form a disulfide bridge (S–S) or exist in a sulfhydryl form (-SH), forming oxidized or reduced PDI, respectively. PDI has been proven to be critical for platelet aggregation, thrombosis, and hemostasis, and PDI inhibition is being evaluated as a novel antithrombotic strategy. The redox states of functional PDI during the regulation of platelet aggregation, however, remain to be elucidated. Endoplasmic reticulum (ER) oxidoreductin-1α (Ero1α) and PDI constitute the pivotal oxidative folding pathway in the ER and play an important role in ER redox homeostasis. Whether Ero1α and PDI constitute an extracellular electron transport pathway to mediate platelet aggregation is an open question. Here, we found that oxidized but not reduced PDI promotes platelet aggregation. On the platelet surface, Ero1α constitutively oxidizes PDI and further regulates platelet aggregation in a glutathione-dependent manner. The Ero1α/PDI system oxidizes reduced glutathione (GSH) and establishes a reduction potential optimal for platelet aggregation. Therefore, platelet aggregation is mediated by the Ero1α-PDI-GSH electron transport system on the platelet surface. We further showed that targeting the functional interplay between PDI and Ero1α by small molecule inhibitors may be a novel strategy for antithrombotic therapy.

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Oxidized but not reduced PDI promotes platelet aggregation.

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Ero1α and PDI constitute an electron transport pathway on platelet surface.

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Ero1α and PDI provide a redox environment optimal for platelet aggregation.

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The functional interplay between Ero1α and PDI can be a new target for antiplatelet therapy.

ERO1-PDI Redox Signaling in Health and Disease

Significance: Protein disulfide isomerase (PDI) and endoplasmic reticulum oxidoreductase 1 (ERO1) are crucial for oxidative protein folding in the endoplasmic reticulum (ER). These enzymes are frequently overexpressed and secreted, and they contribute to the pathology of neurodegenerative, cardiovascular, and metabolic diseases. Recent Advances: Tissue-specific knockout mouse models and pharmacologic inhibitors have been developed to advance our understanding of the cell-specific functions of PDI and ERO1. In addition to their roles in protecting cells from the unfolded protein response and oxidative stress, recent studies have revealed that PDI and ERO1 also function outside of the cells. Critical Issues: Despite the well-known contributions of PDI and ERO1 to specific disease pathology, the detailed molecular and cellular mechanisms underlying these activities remain to be elucidated. Further, although PDI and ERO1 inhibitors have been identified, the results from previous studies require careful evaluation, as many of these agents are not selective and may have significant cytotoxicity. Future Directions: The functions of PDI and ERO1 in the ER have been extensively studied. Additional studies will be required to define their functions outside the ER.

Protein disulfide isomerase is regulated in multiple ways: Consequences for conformation, activities, and pathophysiological functions

Protein disulfide isomerase (PDI) is one of the most abundant and critical protein folding catalysts in the endoplasmic reticulum of eukaryotic cells. PDI consists of four thioredoxin domains and interacts with a wide range of substrate and partner proteins due to its intrinsic conformational flexibility. PDI plays multifunctional roles in a variety of pathophysiological events, both as an oxidoreductase and a molecular chaperone. Recent studies have revealed that the conformation and activity of PDI can be regulated in multiple ways, including posttranslational modification and substrate/ligand binding. Here, we summarize recent advances in understanding the function and regulation of PDI in different pathological and physiological events. We propose that the multifunctional roles of PDI are regulated by multiple mechanisms. Furthermore, we discuss future directions for the study of PDI, emphasizing how different regulatory modes are linked to the conformational changes and biological functions of PDI in the context of diverse pathophysiologies.

ERO1α inhibits cell apoptosis and regulates steroidogenesis in mouse granulosa cells

ER oxidoreduclin 1α (ERO1α), an oxidase that exists in the ER, participates in protein folding and secretion and inhibiting apoptosis, and regulates tumor progression, which is a novel factor of poor cancer prognosis. However, the other physiological functions of ERO1α remain undiscovered. Although our preliminary results of this study indicated that ERO1α revealed the robust expression in ovary, especially in granulosa cells, the role of ERO1α in follicular development is not well known. Therefore, the aims of the present study were to explore the role of ERO1α and the possible mechanisms in regulating cell apoptosis and steroidogenesis in ovarian granulosa cells. ERO1α was mainly localized in granulosa cells and oocytes in the adult ovary by immunohistochemistry. Western blot analysis showed that the expression of ERO1α was highest at oestrous stage during the estrous cycle. The effect of ERO1α on cell apoptosis and steroidogenesis was detected by transduction of ERO1α overexpression and knockdown lentiviruses into primary cultured granulosa cells. Flow cytometry analysis showed that ERO1α decreased granulosa cells apoptosis. Western bolt and RT-qPCR analysis found that ERO1α increased the ratio of BCL-2/BAX, and decreased BAD and Caspase-3 expression. ELISA analysis showed that ERO1α enhanced estrogen (E2) secretion. Western bolt and RT-qPCR analysis found that ERO1α increased StAR, CYP11A1, 3β-HSD, CYP17A1, and CYP19A1 expression, and decreased CYP1B1 expression. Furthermore, Western bolt analysis found that ERO1αincreased PDI and PRDX 4 expression, and activated the PI3K/AKT/mTOR signaling pathway through increasing the phosphorylation of AKT and P70 S6 kinase. In summary, these results suggested that ERO1α might play an anti-apoptotic role and regulate steroidogenesis in granulosa cells, at least partly, via activation of the PI3K/AKT/mTOR signaling pathway.

Phosphorylation switches protein disulfide isomerase activity to maintain proteostasis and attenuate ER stress

Accumulated unfolded proteins in the endoplasmic reticulum (ER) trigger the unfolded protein response (UPR) to increase ER protein folding capacity. ER proteostasis and UPR signaling need to be regulated in a precise and timely manner. Here, we identify phosphorylation of protein disulfide isomerase (PDI), one of the most abundant and critical folding catalysts in the ER, as an early event during ER stress. The secretory pathway kinase Fam20C phosphorylates Ser357 of PDI and responds rapidly to various ER stressors. Phosphorylation of Ser357 induces an open conformation of PDI and turns it from a "foldase" into a "holdase", which is critical for preventing protein misfolding in the ER. Phosphorylated PDI also binds to the lumenal domain of IRE1α, a major UPR signal transducer, and attenuates excessive IRE1α activity. Importantly, PDI-S359A knock-in mice display enhanced IRE1α activation and liver damage under acute ER stress. We conclude that the Fam20C-PDI axis constitutes a post-translational response to maintain ER proteostasis and plays a vital role in protecting against ER stress-induced cell death.

Phase Separation and Cytotoxicity of Tau are Modulated by Protein Disulfide Isomerase and S-nitrosylation of this Molecular Chaperone

Cells have evolved molecular chaperones that modulate phase separation and misfolding of amyloidogenic proteins to prevent neurodegenerative diseases. Protein disulfide isomerase (PDI), mainly located at the endoplasmic reticulum and also present in the cytosol, acts as both an enzyme and a molecular chaperone. PDI is observed to be S-nitrosylated in the brain of Alzheimer's disease patients, but the mechanism has remained elusive. We herein report that both wild-type PDI and its quadruple cysteine mutant only having chaperone activity, significantly inhibit pathological phosphorylation and abnormal aggregation of Tau in cells, and significantly decrease the mitochondrial damage and Tau cytotoxicity resulting from Tau aberrant aggregation, highlighting the chaperone property of PDI. More importantly, we show that wild-type PDI is selectively recruited by liquid droplets of Tau, which significantly inhibits phase separation and stress granule formation of Tau, whereas S-nitrosylation of PDI abrogates the recruitment and inhibition. These findings demonstrate how phase separation of Tau is physiologically regulated by PDI and how S-nitrosylation of PDI, a perturbation in this regulation, leads to disease.

ERO1α promotes testosterone secretion in hCG-stimulated mouse Leydig cells via activation of the PI3K/AKT/mTOR signaling pathway

ER oxidoreduclin 1α (ERO1α) is an oxidase, participating in formation of secretory and membrane proteins. However, the other physiological functions ERO1α is not well known. We found that ERO1α is high in the Leydig cells of the testis. Therefore, the purposes of the current study are to explore the role of ERO1α and the possible mechanisms in regulating cell proliferation, apoptosis, and testosterone secretion of Leydig cells. ERO1α was mainly localized in Leydig cells in the adult mice testes by immunofluorescence staining. Western blot analysis showed that ERO1α was higher in Leydig cells than that in the seminiferous tubules. The effect of ERO1α on cell proliferation, apoptosis, and testosterone secretion was detected by transducing ERO1α overexpression and knockdown lentiviruses into cultured primary Leydig cells (PLCs) together with hCG exposure. Flow cytometry analysis showed that ERO1α promoted cell proliferation by increasing cell distribution at the S phase and decreasing that at the G0/G1 phase. Western bolt analysis showed that ERO1α increased CDK2 and CDK6 expression. Cell apoptosis determination found that ERO1α inhibited PLC apoptosis. Western bolt analysis showed that ERO1α increased the ratio of BCL-2/BAX, and decreased BAD and Caspase-3 expression. Enzyme-linked immunosorbent assay analysis demonstrated that ERO1α enhanced testosterone secretion. Western bolt analysis found that ERO1α increased StAR, 3β-HSD, and CYP17A1 expression. Furthermore, ERO1α could activate the PI3K/AKT/mTOR signaling pathway. In summary, these results suggest that ERO1α might play proliferation promotion and antiapoptotic roles and enhance testosterone secretion in PLC, at least partly, via activation of the PI3K/AKT/mTOR signaling pathway.

The Role of ERO1α in Modulating Cancer Progression and Immune Escape

Endoplasmic reticulum oxidoreductin-1 alpha (ERO1α) was originally shown to be an endoplasmic reticulum (ER) resident protein undergoing oxidative cycles in concert with protein disulfide isomerase (PDI) to promote proper protein folding and to maintain homeostasis within the ER. ERO1α belongs to the flavoprotein family containing a flavin adenine dinucleotide utilized in transferring of electrons during oxidation-reduction cycles. This family is used to maintain redox potentials and protein homeostasis within the ER. ERO1α's location and function has since been shown to exist beyond the ER. Originally thought to exist solely in the ER, it has since been found to exist in the golgi apparatus, as well as in exosomes purified from patient samples. Besides aiding in protein folding of transmembrane and secretory proteins in conjunction with PDI, ERO1α is also known for formation of de novo disulfide bridges. Public databases, such as the Cancer Genome Atlas (TCGA) and The Protein Atlas, reveal ERO1α as a poor prognostic marker in multiple disease settings. Recent evidence indicates that ERO1α expression in tumor cells is a critical determinant of metastasis. However, the impact of increased ERO1α expression in tumor cells extends into the tumor microenvironment. Secretory proteins requiring ERO1α expression for proper folding have been implicated as being involved in immune escape through promotion of upregulation of programmed death ligand-1 (PD-L1) and stimulation of polymorphonuclear myeloid derived suppressor cells (PMN-MDSC's) via secretion of granulocytic colony stimulating factor (G-CSF). Hereby, ERO1α plays a pivotal role in cancer progression and potentially immune escape; making ERO1α an emerging attractive putative target for the treatment of cancer.

Endoplasmic Reticulum Stress Cooperates in Silica Nanoparticles-Induced Macrophage Apoptosis via Activation of CHOP-Mediated Apoptotic Signaling Pathway

While silica nanoparticles (SiNPs) have wide applications, they inevitably increase atmospheric particulate matter and human exposure to this nanomaterial. Numerous studies have focused on how to disclose SiNP toxicity and on understanding its toxic mechanisms. However, there are few studies in the literature reporting the interaction between endoplasmic reticulum (ER) stress and SiNP exposure, and the corresponding detailed mechanisms have not been clearly determined. In this study, CCK-8 and flow cytometry assays demonstrated that SiNPs gradually decreased cell viability and increased cell apoptosis in RAW 264.7 macrophage cells in dose- and time-dependent manners. Western blot analysis showed that SiNPs significantly activated ER stress by upregulating GRP78, CHOP, and ERO1α expression. Meanwhile, western blot analysis also showed that SiNPs activated the mitochondrial-mediated apoptotic signaling pathway by upregulating BAD and Caspase-3, and downregulating the BCL-2/BAX ratio. Moreover, 4-phenylbutyrate (4-PBA), an ER stress inhibitor, significantly decreased GRP78, CHOP, and ERO1α expression, and inhibited cell apoptosis in RAW 264.7 macrophage cells. Furthermore, overexpression of CHOP significantly enhanced cell apoptosis, while knockdown of CHOP significantly protected RAW 264.7 macrophage cells from apoptosis induced by SiNPs. We found that the CHOP-ERO1α-caspase-dependent apoptotic signaling pathway was activated by upregulating the downstream target protein ERO1α and caspase-dependent mitochondrial-mediated apoptotic signaling pathway by upregulating Caspase-3 and downregulating the ratio of BCL-2/BAX. In summary, ER stress participated in cell apoptosis induced by SiNPs and CHOP regulated SiNP-induced cell apoptosis, at least partly, via activation of the CHOP-ERO1α-caspase apoptotic signaling pathway in RAW 264.7 macrophage cells.

Regulation of plant ER oxidoreductin 1 (ERO1) activity for efficient oxidative protein folding

In the endoplasmic reticulum (ER), ER oxidoreductin 1 (ERO1) catalyzes intramolecular disulfide-bond formation within its substrates in coordination with protein-disulfide isomerase (PDI) and related enzymes. However, the molecular mechanisms that regulate the ERO1-PDI system in plants are unknown. Reduction of the regulatory disulfide bonds of the ERO1 from soybean, GmERO1a, is catalyzed by enzymes in five classes of PDI family proteins. Here, using recombinant proteins, vacuum-ultraviolet circular dichroism spectroscopy, biochemical and protein refolding assays, and quantitative immunoblotting, we found that GmERO1a activity is regulated by reduction of intramolecular disulfide bonds involving Cys-121 and Cys-146, which are located in a disordered region, similarly to their locations in human ERO1. Moreover, a GmERO1a variant in which Cys-121 and Cys-146 were replaced with Ala residues exhibited hyperactive oxidation. Soybean PDI family proteins differed in their ability to regulate GmERO1a. Unlike yeast and human ERO1s, for which PDI is the preferred substrate, GmERO1a directly transferred disulfide bonds to the specific active center of members of five classes of PDI family proteins. Of these proteins, GmPDIS-1, GmPDIS-2, GmPDIM, and GmPDIL7 (which are group II PDI family proteins) failed to catalyze effective oxidative folding of substrate RNase A when there was an unregulated supply of disulfide bonds from the C121A/C146A hyperactive mutant GmERO1a, because of its low disulfide-bond isomerization activity. We conclude that regulation of plant ERO1 activity is particularly important for effective oxidative protein folding by group II PDI family proteins.

The role of thiols in antioxidant systems

The sulfur biochemistry of the thiol group endows cysteines with a number of highly specialized and unique features that enable them to serve a variety of different functions in the cell. Typically highly conserved in proteins, cysteines are predominantly found in functionally or structurally crucial regions, where they act as stabilizing, catalytic, metal-binding and/or redox-regulatory entities. As highly abundant low molecular weight thiols, cysteine thiols and their oxidized disulfide counterparts are carefully balanced to maintain redox homeostasis in various cellular compartments, protect organisms from oxidative and xenobiotic stressors and partake actively in redox-regulatory and signaling processes. In this review, we will discuss the role of protein thiols as scavengers of hydrogen peroxide in antioxidant enzymes, use thiol peroxidases to exemplify how protein thiols contribute to redox signaling, provide an overview over the diverse set of low molecular weight thiol-based redox systems found in biology, and illustrate how thiol-based redox systems have evolved not only to protect against but to take full advantage of a world full of molecular oxygen.

AtERO1 and AtERO2 Exhibit Differences in Catalyzing Oxidative Protein Folding in the Endoplasmic Reticulum

Disulfide bonds are essential for the folding of the eukaryotic secretory and membrane proteins in the endoplasmic reticulum (ER), and ER oxidoreductin-1 (Ero1) and its homologs are the major disulfide donors that supply oxidizing equivalents in the ER. Although Ero1 homologs in yeast (Saccharomyces cerevisiae) and mammals have been extensively studied, the mechanisms of plant Ero1 functions are far less understood. Here, we found that both Arabidopsis (Arabidopsis thaliana) ERO1 and its homolog AtERO2 are required for oxidative protein folding in the ER. The outer active site, the inner active site, and a long-range noncatalytic disulfide bond are required for AtERO1's function. Interestingly, AtERO1 and AtERO2 also exhibit significant differences. The ero1 plants are more sensitive to reductive stress than the ero2 plants. In vivo, both AtERO1 and AtERO2 have two distinct oxidized isoforms (Ox1 and Ox2), which are determined by the formation or breakage of the putative regulatory disulfide. AtERO1 is mainly present in the Ox1 redox state, while more AtERO2 exists in the Ox2 state. Furthermore, AtERO1 showed much stronger oxidative protein-folding activity than AtERO2 in vitro. Taken together, both AtERO1 and AtERO2 are required to regulate efficient and faithful oxidative protein folding in the ER, but AtERO1 may serves as the primary sulfhydryl oxidase relative to AtERO2.

Quantitative Analyses of the Yeast Oxidative Protein Folding Pathway In Vitro and In Vivo

Aims: Efficient oxidative protein folding (OPF) in the endoplasmic reticulum (ER) is a key requirement of the eukaryotic secretory pathway. In particular, protein folding linked to the formation of disulfide bonds, an activity dependent on the enzyme protein disulfide isomerase (PDI), is crucial. For the de novo formation of disulfide bonds, reduced PDI must be reoxidized by an ER-located oxidase (ERO1). Despite some knowledge of this pathway, the kinetic parameters with which these components act and the importance of specific parameters, such as PDI reoxidation by Ero1, for the overall performance of OPF in vivo remain poorly understood. Results: We established an in vitro system using purified yeast (Saccharomyces cerevisiae) PDI (Pdi1p) and ERO1 (Ero1p) to investigate OPF. This necessitated the development of a novel reduction/oxidation processing strategy to generate homogenously oxidized recombinant yeast Ero1p. This new methodology enabled the quantitative assessment of the interaction of Pdi1p and Ero1p in vitro by measuring oxygen consumption and reoxidation of reduced RNase A. The resulting quantitative data were then used to generate a simple model that can describe the oxidizing capacity of Pdi1p and Ero1p in vitro and predict the in vivo effect of modulation of the levels of these proteins. Innovation: We describe a model that can be used to explore the OPF pathway and its control in a quantitative way. Conclusion: Our study informs and provides new insights into how OPF works at a molecular level and provides a platform for the design of more efficient heterologous protein expression systems in yeast.

Targeting the functional interplay between endoplasmic reticulum oxidoreductin-1α and protein disulfide isomerase suppresses the progression of cervical cancer

BACKGROUND

Endoplasmic reticulum (ER) oxidoreductin-1α (Ero1α) and protein disulfide isomerase (PDI) constitute the pivotal pathway of oxidative protein folding, and are highly expressed in many cancers. However, whether targeting the functional interplay between Ero1α and PDI could be a new approach for cancer therapy remains unknown.

METHODS

We performed wound healing assays, transwell migration and invasion assays and xenograft assays to assess cell migration, invasion and tumorigenesis; gel filtration chromatography, oxygen consumption assay and in cells folding assays were used to detect Ero1α-PDI interaction and Ero1α oxidase activity.

FINDINGS

Here, we report that elevated expression of Ero1α is correlated with poor prognosis in human cervical cancer. Knockout of ERO1A decreases the growth, migration and tumorigenesis of cervical cancer cells, through downregulation of the H2O2-correlated epithelial-mesenchymal transition. We identify that the conserved valine (Val) 101 of Ero1α is critical for Ero1α-PDI complex formation and Ero1α oxidase activity. Val101 of Ero1α is specifically involved in the recognition of PDI catalytic domain. Mutation of Val101 results in a reduced ER, retarded oxidative protein folding and decreased H2O2 levels in the ER of cervical cancer cells and further impairs cell migration, invasion, and tumor growth.

INTERPRETATION

Our study identifies the critical residue of Ero1α for recognizing PDI, which underlines the molecular mechanism of oxidative protein folding for tumorigenesis and provides a proof-of-concept for cancer therapy by targeting Ero1α-PDI interaction. FUND: This work was supported by National Key R&D Program of China, National Natural Science Foundation of China, and Youth Innovation Promotion Association, CAS.

The b' domain of protein disulfide isomerase cooperates with the a and a' domains to functionally interact with platelets

Essentials Protein disulfide isomerase (PDI) interacts with the αIIbβ3 integrin on platelets We generated PDI domain fragments and full-length PDI containing point mutations PDI interacts with αIIbβ3 through the b' domain, with the a and a' domains contributing This is the first report demonstrating PDI binding to a native protein on intact cells SUMMARY: Background Protein disulfide isomerase (PDI) is an oxidoreductase consisting of four domains arranged in the order a-b-b'-a' with an x-linker between the b' and a' domains. PDI binds to αIIb β3 integrin on activated platelets, and potentiates activation of this integrin through the C-terminal CGHC active-site motif. How PDI binds to platelet αIIb β3 is unknown. Objective and methods We used PDI domain fragments and full-length PDI containing point mutations to study inhibition of Alexa 488-labeled PDI binding to thrombin-activated platelets. The effect of the PDI variants on platelet aggregation was tested. Results Only PDI fragments containing the b' domain bound to activated platelets. A double mutant of the b' domain had decreased binding, confirming the essential role of the b' domain. Addition of mutations in the a and a' domains further decreased binding, suggesting that these domains contribute to the interaction of PDI with platelets. The ability of the b' domain to interact directly with αIIb β3 was demonstrated with surface plasmon resonance, with contributions from the a and a' domains. The abb'x PDI fragment that binds to platelets but lacks the critical C-terminal active site inhibited platelet aggregation and in vivo thrombosis. Moreover, site mutations in the a, b' and a' domains that resulted in partial binding to platelets partially recovered aggregation of PDI-null platelets. PDI mutants that did not bind showed no recovery. Conclusion PDI functionally interacts with αIIb β3 on platelets through the substrate-binding b' domain, with the a and a' domains contributing to efficient binding.

Homocysteine causes vascular endothelial dysfunction by disrupting endoplasmic reticulum redox homeostasis

Endothelial dysfunction induced by hyperhomocysteinemia (HHcy) plays a critical role in vascular pathology. However, little is known about the role of endoplasmic reticulum (ER) redox homeostasis in HHcy-induced endothelial dysfunction. Here, we show that Hcy induces ER oxidoreductin-1α (Ero1α) expression with ER stress and inflammation in human umbilical vein endothelial cells and in the arteries of HHcy mice. Hcy upregulates Ero1α expression by promoting binding of hypoxia-inducible factor 1α to the ERO1A promoter. Notably, Hcy rather than other thiol agents markedly increases the GSH/GSSG ratio in the ER, therefore allosterically activating Ero1α to produce H₂O₂ and trigger ER oxidative stress. By contrast, the antioxidant pathway mediated by ER glutathione peroxidase 7 (GPx7) is downregulated in HHcy mice. Ero1α knockdown and GPx7 overexpression protect the endothelium from HHcy-induced ER oxidative stress and inflammation. Our work suggests that targeting ER redox homeostasis could be used as an intervention for HHcy-related vascular diseases.

Molecular analysis of human Ero1 reveals novel regulatory mechanisms for oxidative protein folding

Oxidative protein folding in the ER is driven mainly by oxidases of the endoplasmic reticulum oxidoreductin 1 (Ero1) family. Their action is regulated to avoid cell stress, including hyperoxidation. Previously published regulatory mechanisms are based on the rearrangement of active site and regulatory disulfides. In this study, we identify two novel regulatory mechanisms. First, both human Ero1 isoforms exist in a dynamic mixed disulfide complex with protein disulfide isomerase, which involves cysteines (Cys166 in Ero1α and Cys165 in Ero1β) that have previously been regarded as being nonfunctional. Second, our kinetic studies reveal that Ero1 not only has a high affinity for molecular oxygen as the terminal acceptor of electrons but also that there is a high cooperativity of binding (Hill coefficient >3). This allows Ero1 to maintain high activity under hypoxic conditions, without compromising cellular viability under hyper-hypoxic conditions. These data, together with novel mechanistic details of differences in activation between the two human Ero1 isoforms, provide important new insights into the catalytic cycle of human Ero1 and how they have been fine-tuned to operate at low oxygen concentrations.

Secretory kinase Fam20C tunes endoplasmic reticulum redox state via phosphorylation of Ero1α

Family with sequence similarity 20C (Fam20C), the physiological Golgi casein kinase, phosphorylates numerous secreted proteins that are involved in a wide variety of biological processes. However, the role of Fam20C in regulating proteins in the endoplasmic reticulum (ER) lumen is largely unknown. Here, we report that Fam20C interacts with various luminal proteins and that its depletion results in a more reduced ER lumen. We further show that ER oxidoreductin 1α (Ero1α), the pivotal sulfhydryl oxidase that catalyzes disulfide formation in the ER, is phosphorylated by Fam20C in the Golgi apparatus and retrograde-transported to the ER mediated by ERp44. The phosphorylation of Ser145 greatly enhances Ero1α oxidase activity and is critical for maintaining ER redox homeostasis and promoting oxidative protein folding. Notably, phosphorylation of Ero1α is induced under hypoxia, reductive stress, and secretion-demanding conditions such as mammalian lactation. Collectively, our findings open a door to uncover how oxidative protein folding is regulated by phosphorylation in the secretory pathway.

The Endoplasmic Reticulum Stress Response in Neuroprogressive Diseases: Emerging Pathophysiological Role and Translational Implications

The endoplasmic reticulum (ER) is the main cellular organelle involved in protein synthesis, assembly and secretion. Accumulating evidence shows that across several neurodegenerative and neuroprogressive diseases, ER stress ensues, which is accompanied by over-activation of the unfolded protein response (UPR). Although the UPR could initially serve adaptive purposes in conditions associated with higher cellular demands and after exposure to a range of pathophysiological insults, over time the UPR may become detrimental, thus contributing to neuroprogression. Herein, we propose that immune-inflammatory, neuro-oxidative, neuro-nitrosative, as well as mitochondrial pathways may reciprocally interact with aberrations in UPR pathways. Furthermore, ER stress may contribute to a deregulation in calcium homoeostasis. The common denominator of these pathways is a decrease in neuronal resilience, synaptic dysfunction and even cell death. This review also discusses how mechanisms related to ER stress could be explored as a source for novel therapeutic targets for neurodegenerative and neuroprogressive diseases. The design of randomised controlled trials testing compounds that target aberrant UPR-related pathways within the emerging framework of precision psychiatry is warranted.

Characterization of wheat endoplasmic reticulum oxidoreductin 1 and its application in Chinese steamed bread

This study investigated characteristics of recombinant wheat Endoplasmic Reticulum Oxidoreductin 1 (wEro1) and its influence on Chinese steamed bread (CSB) qualities. The purified wEro1 monomer, which contained two conserved redox active motif sites, bound to flavin adenine dinucleotide (FAD) cofactor with a molecular weight of ∼47 kDa. wEro1 catalyzed the reduction of both bound and free FAD, and its reduction activity of free FAD reached 7.8 U/mg. Moreover, wEro1 catalyzed the oxidation of dithiothreitol and wheat protein disulfide isomerase (wPDI). Both glutathione and the reduced ribonuclease could work as electron donors for wEro1 in catalyzing the oxidation of wPDI. Additionally, wEro1 supplementation improved the CSB qualities with an increased specific volume of CSB and decreased crumb hardness, which was attributed to water-insoluble wheat proteins increasing and gluten network strengthening. The results give an understanding of the properties and function of wEro1 to facilitate its application especially in the flour-processing industry.

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.

Influenza virus genome reaches the plasma membrane via a modified endoplasmic reticulum and Rab11-dependent vesicles

Transport of neo-synthesized influenza A virus (IAV) viral ribonucleoproteins (vRNPs) from the nucleus to the plasma membrane involves Rab 11 but the precise mechanism remains poorly understood. We used metal-tagging and immunolabeling to visualize viral proteins and cellular endomembrane markers by electron microscopy of IAV-infected cells. Unexpectedly, we provide evidence that the vRNP components and the Rab11 protein are present at the membrane of a modified, tubulated endoplasmic reticulum (ER) that extends all throughout the cell, and on irregularly coated vesicles (ICVs). Some ICVs are found very close to the ER and to the plasma membrane. ICV formation is observed only in infected cells and requires an active Rab11 GTPase. Against the currently accepted model in which vRNPs are carried onto Rab11-positive recycling endosomes across the cytoplasm, our findings reveal that the endomembrane organelle that is primarily involved in the transport of vRNPs is the ER.

Transport of neo-synthesized influenza A virus viral ribonucleoproteins (vRNPs) from the nucleus to the plasma membrane involves Rab 11 but the mechanism is unclear. Here the authors show that vRNPs are transported through a modified Rab11-positive endoplasmic reticulum and Rab11-dependent vesicles.

Chemistry and Enzymology of Disulfide Cross-Linking in Proteins

Cysteine thiols are among the most reactive functional groups in proteins, and their pairing in disulfide linkages is a common post-translational modification in proteins entering the secretory pathway. This modest amino acid alteration, the mere removal of a pair of hydrogen atoms from juxtaposed cysteine residues, contrasts with the substantial changes that characterize most other post-translational reactions. However, the wide variety of proteins that contain disulfides, the profound impact of cross-linking on the behavior of the protein polymer, the numerous and diverse players in intracellular pathways for disulfide formation, and the distinct biological settings in which disulfide bond formation can take place belie the simplicity of the process. Here we lay the groundwork for appreciating the mechanisms and consequences of disulfide bond formation in vivo by reviewing chemical principles underlying cysteine pairing and oxidation. We then show how enzymes tune redox-active cofactors and recruit oxidants to improve the specificity and efficiency of disulfide formation. Finally, we discuss disulfide bond formation in a cellular context and identify important principles that contribute to productive thiol oxidation in complex, crowded, dynamic environments.

Tanshinone IIA increases levels of NeuN, protein disulfide isomerase, and Na+/K+-ATPase and decreases evidence of microglial activation after cerebral ischemic injury

This study was designed to clarify the neuroprotective effects of tanshinone IIA (TSA) following cerebral ischemic insult. Adult Sprague-Dawley rats were operated upon to achieve a middle cerebral artery occlusion to cause transient focal cerebral ischemia, which were then randomly divided into the sham-operated control group and cerebral ischemia/reperfusion (I/R) groups receiving a 2 h occlusion. The treatment groups received daily intraperitoneal injections of high or low doses of TSA, for 7 or 15 days. NeuN immunostaining revealed neuronal loss following I/R, which was partially prevented with subsequent TSA dosing. Protein disulfide isomerase and adenosine triphosphatase (Na(+)/K(+)-ATPase) levels were all depressed by means of I/R. TSA treatment markedly reversed the depression of all indices examined. The intensity of microglial activation, as evidenced with CD11b staining, was increased by means of cerebral artery occlusion, but this was partially reversed with subsequent TSA treatment. TSA may affect neuroprotection by way of minimizing deficits in energy metabolism and reduction of the extent of cell death within affected regions.

Identification and functional characterization of an endoplasmic reticulum oxidoreductin 1-α gene in Litopenaeus vannamei

In the current study, full-length sequence of endoplasmic reticulum oxidoreductin 1-α (LvERO1-α) was cloned from Litopenaeus vannamei. Real-time RT-PCR results showed that LvERO1-α was highly expressed in hemocytes, gills, and intestines. White spot syndrome virus (WSSV) challenge was performed, and the expression of LvERO1-α and two other downstream genes of the double-stranded RNA-activated protein kinase-like ER kinase-eIF2α (PERK-α) pathway, namely, homocysteine-induced endoplasmic reticulum protein (LvHERP) and acylamino-acid-releasing enzyme (LvAARE), strongly increased in the hemocytes. Flow cytometry assay results indicated that the apoptosis rate of L. vannamei hemocytes in the LvERO1-α knockdown group was significantly lower than that of the controls. Moreover, shrimps with knockdown expression of LvERO1-α exhibited decreased cumulative mortality upon WSSV infection. Downregulation of L. vannamei immunoglobulin-binding protein (LvBip), which had been proven to induce unfolded protein response (UPR) in L. vannamei, did not only upregulate LvERO1-α, LvHERP, and LvAARE in hemocytes, but also increased their apoptosis rate, as well as the shrimp cumulative mortality. Furthermore, reporter gene assay results showed that the promoter of LvERO1-α was activated by L. vannamei activating transcription factor 4, thereby confirming that LvERO1-α was regulated by the PERK-eIF2α pathway. These results suggested that LvERO1-α plays a critical role in WSSV-induced apoptosis, which likely occurs through the WSSV-activated PERK-eIF2α pathway.

Interplay between redox and protein homeostasis

The subcellular compartments of eukaryotic cells are characterized by different redox environments. Whereas the cytosol, nucleus and mitochondria are more reducing, the endoplasmic reticulum represents a more oxidizing environment. As the redox level controls the formation of intra- and inter-molecular disulfide bonds, the folding of proteins is tightly linked to its environment. The proteostasis network of each compartment needs to be adapted to the compartmental redox properties. In addition to chaperones, also members of the thioredoxin superfamily can influence the folding of proteins by regulation of cysteine reduction/oxidation. This review will focus on thioredoxin superfamily members and chaperones of C. elegans, which play an important role at the interface between redox and protein homeostasis. Additionally, this review will highlight recent methodological developments on in vivo and in vitro assessment of the redox state and their application to provide insights into the high complexity of redox and proteostasis networks of C. elegans.

Novel Roles of the Non-catalytic Elements of Yeast Protein-disulfide Isomerase in Its Interplay with Endoplasmic Reticulum Oxidoreductin 1

The formation of disulfide bonds in the endoplasmic reticulum (ER) of eukaryotic cells is catalyzed by the sulfhydryl oxidase, ER oxidoreductin 1 (Ero1), and protein-disulfide isomerase (PDI). PDI is oxidized by Ero1 to continuously introduce disulfides into substrates, and feedback regulates Ero1 activity by manipulating the regulatory disulfides of Ero1. In this study we find that yeast Ero1p is enzymatically active even with its regulatory disulfides intact, and further activation of Ero1p by reduction of the regulatory disulfides requires the reduction of non-catalytic Cys(90)-Cys(97)disulfide in Pdi1p. The principal client-binding site in the Pdi1pb' domain is necessary not only for the functional Ero1p-Pdi1p disulfide relay but also for the activation of Ero1p. We also demonstrate by complementary activation assays that the regulatory disulfides in Ero1p are much more stable than those in human Ero1α. These new findings on yeast Ero1p-Pdi1p interplay reveal significant differences from our previously identified mode of human Ero1α-PDI interplay and provide insights into the evolution of the eukaryotic oxidative protein folding pathway.

Nicotine Directly Induces Endoplasmic Reticulum Stress Response in Rat Placental Trophoblast Giant Cells

Nicotine exposure during pregnancy leads to placental insufficiency impairing both fetal and neonatal development. Previous studies from our laboratory have demonstrated that in rats, nicotine augmented endoplasmic reticulum (ER) stress in association with placental insufficiency; however, the underlying mechanisms remain elusive. Therefore, we sought to investigate the possible direct effect of nicotine on ER stress in Rcho-1 rat placental trophoblast giant (TG) cells during differentiation. Protein and/or mRNA expression of markers involved in ER stress (eg, phosphorylated PERK, eIF2α, CHOP, and BiP/GRP78) and TG cell differentiation and function (eg, Pl-1, placental growth factor [Pgf], Hsd11b1, and Hsd11b2) were quantified via Western blot or real-time polymerase chain reaction. Nicotine treatment led to dose-dependent increases in the phosphorylation of PERK[Thr981] and eIF2α[Ser51], whereas pretreatment with a nicotinic acetylcholine receptor (nAChR) antagonist (mecamylamine hydrochloride) blocked the induction of PERK phosphorylation, verifying the direct involvement of nicotine and nAChR binding. We next investigated select target genes known to play essential roles in placental TG cell differentiation and function (Pl-1, Pgf, Hsd11b1, and Hsd11b2), and found that nicotine significantly augmented the mRNA levels of Hsd11b1 in a dose-dependent manner. Furthermore, using tauroursodeoxycholic acid, a safe bile acid known to improve protein chaperoning and folding, we were able to prevent nicotine-induced increases in both PERK phosphorylation and Hsd11b1 mRNA levels, revealing a potential novel therapeutic approach to reverse the deleterious effects of nicotine exposure in pregnancy. Collectively, these results implicate that nicotine, acting through its receptor, can directly augment ER stress and impair placental function.

Protein disulfide-isomerase, a folding catalyst and a redox-regulated chaperone

Protein disulfide-isomerase (PDI) was the first protein-folding catalyst to be characterized, half a century ago. It plays critical roles in a variety of physiological events by displaying oxidoreductase and redox-regulated chaperone activities. This review provides a brief history of the identification of PDI as both an enzyme and a molecular chaperone and of the recent advances in studies on the structure and dynamics of PDI, the substrate binding and release, and the cooperation with its partners to catalyze oxidative protein folding and maintain ER redox homeostasis. In this review, we highlight the structural features of PDI, including the high interdomain flexibility, the multiple binding sites, the two synergic active sites, and the redox-dependent conformational changes.