Redox control of endoplasmic reticulum function

Oxidative Stress and the ER Stress Response in a Murine Model for Early-Stage Alcoholic Liver Disease

Alcoholic liver disease (ALD) is a primary cause of morbidity and mortality in the United States and constitutes a significant socioeconomic burden. Previous work has implicated oxidative stress and endoplasmic reticulum (ER) stress in the etiology of ALD; however, the complex and interrelated nature of these cellular responses presently confounds our understanding of ethanol-induced hepatopathy. In this paper, we assessed the pathological contribution of oxidative stress and ER stress in a time-course mouse model of early-stage ALD. Ethanol-treated mice exhibited significant hepatic panlobular steatosis and elevated plasma ALT values compared to isocaloric controls. Oxidative stress was observed in the ethanol-treated animals through a significant increase in hepatic TBARS and immunohistochemical staining of 4-HNE-modified proteins. Hepatic glutathione (GSH) levels were significantly decreased as a consequence of decreased CBS activity, increased GSH utilization, and increased protein glutathionylation. At the same time, immunoblot analysis of the PERK, IRE1α, ATF6, and SREBP pathways reveals no significant role for these UPR pathways in the etiology of hepatic steatosis associated with early-stage ALD. Collectively, our results indicate a primary pathogenic role for oxidative stress in the early initiating stages of ALD that precedes the involvement of the ER stress response.

Förster resonance energy transfer-based sensor targeting endoplasmic reticulum reveals highly oxidative environment

The glutathione thiol/disulfide couple is the major redox buffer in the endoplasmic reticulum (ER); however, mechanisms by which it contributes to the tightly regulated redox environment of this intracellular organelle are poorly understood. The recent development of genetically encoded, ratiometric, single green fluorescent protein-based redox-sensitive (roGFP) sensors adjusted for more oxidative environments enables non-invasive measurement of the ER redox environment in living cells. In turn, Förster resonance energy transfer (FRET) sensors based on two fluorophore probes represent an alternative strategy for ratiometric signal acquisition. In previous work, we described the FRET-based redox sensor CY-RL7 with a relatively high midpoint redox potential of -143 mV, which is required for monitoring glutathione potentials in the comparatively high oxidative environment of the ER. Here, the efficacy of the CY-RL7 probe was ascertained in the cytosol and ER of live cells with fluorescence microscopy and flow cytometry. The sensor was found to be fully reduced at steady state in the cytosol and became fully oxidized in response to treatment with 1-chloro-2,4-dinitrobenzene, a depletor of reduced glutathione (GSH). In contrast, the probe was strongly oxidized (88%) upon expression in the ER of cultured cells. We also examined the responsiveness of the ER sensor to perturbations in cellular glutathione homeostasis. We observed that the reductive level of the FRET sensor was increased two-fold to about 28% in cells pretreated with N-acetylcysteine, a substrate for GSH synthesis. Finally, we evaluated the responsiveness of CY-RL7 and roGFP1-iL to various perturbations of cellular glutathione homeostasis to address the divergence in the specificity of these two probes. Together, the present data generated with genetically encoded green fluorescent protein (GFP)-based glutathione probes highlight the complexity of the ER redox environment and indicate that the ER glutathione pool may be more oxidized than is currently considered.

The endoplasmic reticulum as the extracellular space inside the cell: role in protein folding and glycosylation

SIGNIFICANCE

Proteins destined to secretion and exposure on the cell surface are synthesized and processed in the extracellular-like environment of the endoplasmic reticulum (ER) of higher eukaryotic cells. Compartmentation plays a crucial role in the post-translational modifications, such as oxidative folding and N-glycosylation in the ER lumen. Transport of the required intermediates across the ER membrane and maintenance of the luminal redox conditions and Ca(2+) ion concentration are indispensable for appropriate protein maturation.

RECENT ADVANCES

Cooperation of enzymes and transporters to maintain a thiol-oxidizing milieu in the ER lumen has been recently elucidated. Ca(2+)-dependence of certain ER chaperones is a subject of intensive research.

CRITICAL ISSUES

Mounting evidence supports the existence of a real barrier between the ER lumen and the cytosol. The unique set of enzymes, selection of metabolites, and characteristic ion and redox milieu of the luminal compartment strongly argue against the general permeability of the ER membrane.

FUTURE DIRECTIONS

Alterations in the luminal environment can trigger the unfolded protein response, a common event in a variety of pathological conditions. Therefore, redox and calcium homeostasis and protein glycosylation in the ER provide novel drug-targets for medical treatment in a wide array of diseases.

Redox-dependent protein quality control in the endoplasmic reticulum: folding to degradation

SIGNIFICANCE

Nascent polypeptides entering the endoplasmic reticulum (ER) are co- and post-translationally modified by N-glycosylation and the oxidation/isomerization of cysteine residues followed by folding with the aid of molecular chaperones. Only properly folded proteins reach their final destination. The oxidative environment in the ER enables ER-resident oxidoreductases to facilitate disulfide bond formation, which stabilizes protein structures. ER oxidoreductases involve in both the productive folding of newly synthesized proteins and ER-associated degradation (ERAD) of misfolded proteins.

RECENT ADVANCES

The ER luminal event of ERAD is composed of three major steps: the recognition and segregation of terminally misfolded proteins from folding intermediates, unfolding of misfolded substrates by oxidoreductases that cleave the disulfide bonds to enable the translocation of the substrates through the retrotranslocation channel, and transport of substrates to be degraded to the dislocon channel. The factors required for these three critical steps have been found to form a supramolecular complex in the ER.

CRITICAL ISSUES

This complex comprises EDEM1, a lectin-like molecule that recognizes mannose-trimming and segregates the identified substrates from the productive folding pathway into the degradation pathway; ER DnaJ (ERdj)5, a reductase that resides in the ER and reduces disulfides in misfolded proteins; and immunoglobulin heavy chain binding protein (BiP), an heat shock protein (Hsp)70 family molecular chaperone that recruits substrates to the dislocon channel after dissociation from the EDEM1/ERdj5 complex coupled with ATP hydrolysis.

FUTURE DIRECTIONS

The importance of disulfide bond reduction in misfolded proteins for retrotranslocation through the dislocon channel will be discussed by comparing the function of ERdj5 with that of other oxidoreductases in the ER.

Oxidative protein folding and unfolded protein response elicit differing redox regulation in endoplasmic reticulum and cytosol of yeast

Oxidative protein folding can exceed the cellular secretion machinery, inducing the unfolded protein response (UPR). Sustained endoplasmic reticulum (ER) stress leads to cell stress and disease, as described for Alzheimer, Parkinson, and diabetes mellitus, among others. It is currently assumed that the redox state of the ER is optimally balanced for formation of disulfide bonds using glutathione as the main redox buffer and that UPR causes a reduction of this organelle. The direct effect of oxidative protein folding in the ER, however, has not yet been dissected from UPR regulation. To measure in vivo redox conditions in the ER and cytosol of the yeast model organism Pichia pastoris we targeted redox-sensitive roGFP variants to the respective organelles. Thereby, we clearly demonstrate that induction of the UPR causes reduction of the cytosol in addition to ER reduction. Similarly, a more reduced redox state of the cytosol, but not of the ER, is observed during oxidative protein folding in the ER without UPR induction, as demonstrated by overexpressing genes of disulfide bond-rich secretory proteins such as porcine trypsinogen or protein disulfide isomerase (PDI1) and ER oxidase (ERO1). Cytosolic reduction seems not to be caused by the action of glutathione reductase (GLR1) and could not be compensated for by overexpression of cytosolic glutathione peroxidase (GPX1). Overexpression of GPX1 and PDI1 oxidizes the ER and increases the secretion of correctly folded proteins, demonstrating that oxidative protein folding per se is enhanced by a more oxidized ER and is counterbalanced by a more reduced cytosol. As the total glutathione concentration of these strains does not change significantly, but the ratio of GSH to GSSG is altered, either transport or redox signaling between the glutathione pools of ER and cytosol is assumed. These data clearly demonstrate that protein folding and ER stress have a severe impact on the cytosolic redox balance, which may be a major factor during development of folding-related diseases.

The essential role of protein kinase Cδ in diabetes-induced neural tube defects

BACKGROUND

Maternal diabetes causes neural tube defects (NTDs) in the embryos via activating protein kinase Cs (PKCs), which regulate programmed cell death (apoptosis). The aims of this study are to investigate the role of proapoptotic PKCδ in NTD formation and the underlying mechanisms.

METHODS

PKCδ heterozygous (pkcδ(+/-)) female mice were diabetic (DM) induced by intravenous injection of streptozotocin. Occurrence of NTDs was evaluated at embryonic day 11.5 and compared between wild type (WT) and PKCδ homozygous (pkcδ(-/-)) embryos. Changes in oxidative and endoplasmic reticulum (ER) stress-associated factors and stress-response c-Jun N-terminal kinases (JNKs) were assessed using Western blot assay.

RESULTS

Compared to DM/WT, the DM/PKCδ(-/-) embryos had significantly lower NTD rate and lower levels of oxidative and ER stress factors and JNK activation. These values were similar to those in the non-diabetic control group.

CONCLUSION

PKCδ plays a critical role in diabetes-induced NTDs, potentially through increasing oxidative and ER stress and JNK-associated stress-response pathways.

Crosstalk and barriers between the electron carriers of the endoplasmic reticulum

SIGNIFICANCE

The lumen of the endoplasmic reticulum (ER) constitutes a separate compartment with a special proteome and metabolome. The characteristic redox environment required for the optimal functioning of local pathways is defined by the redox couples of the main electron carriers. These molecules, glutathione, pyridine nucleotides, and ascorbic acid, are present within the ER, but their composition, concentration, and redox state are characteristically different from those observed in other subcellular compartments. Spatial and kinetic barriers contribute to the generation and maintenance of this special redox environment.

RECENT ADVANCES

The ER redox has usually been considered from the perspective of oxidative protein folding, one of the major functions of the ER. Thus, the lumen has been described as a relatively oxidizing subcellular compartment.

CRITICAL ISSUES

The ER redoxome has been scantily mapped. However, recent observations suggest that the redox systems in reduced and oxidized states are present simultaneously. The concerted actions of transmembrane uptake processes and local oxidoreductases as well as the absence of specific transport and enzyme activities maintain the oxidized state of the thiol-disulfide systems and the reduced state of the pyridine nucleotide redox systems. These states are prerequisites for the normal redox reactions localized in the ER.

FUTURE DIRECTIONS

An outline of the interactions between the major electron carriers of the ER will contribute to a better understanding of human diseases related to ER redox homeostasis.

Peroxides and peroxidases in the endoplasmic reticulum: integrating redox homeostasis and oxidative folding

SIGNIFICANCE

The endoplasmic reticulum (ER), the port of entry into the secretory pathway, is a complex organelle that performs many fundamental functions, including protein synthesis and quality control, Ca(2+) storage and signaling. Redox homeostasis is of paramount importance for allowing the efficient folding of secretory proteins, most of which contain essential disulfide bonds.

RECENT ADVANCES

revealed that an intricate protein network sustains the processes of disulfide bond formation and reshuffling in the ER. Remarkably, H(2)O(2), which is a known by-product of Ero1 flavoproteins in cells, is utilized by peroxiredoxin-4 and glutathione peroxidases-7 and -8, which reside in the mammalian secretory compartment and further fuel oxidative protein folding while limiting oxidative damage.

CRITICAL ISSUES

that remain to be addressed are the sources, diffusibility and signaling role(s) of H(2)O(2) in and between organelles and cells, how the emerging redundancy in the systems is coupled to precise regulation, and how the distinct pathways operating in the early secretory compartment are integrated with one another.

FUTURE DIRECTIONS

A further dissection of the pathways that integrate folding, redox homeostasis, and signaling in the early secretory pathway may allow to manipulate protein homeostasis and survival-death decisions in degenerative diseases or cancer.

Proteomics of hyposaline stress in blue mussel congeners (genus Mytilus): implications for biogeographic range limits in response to climate change

Climate change is affecting species' physiology, pushing environmental tolerance limits and shifting distribution ranges. In addition to temperature and ocean acidification, increasing levels of hyposaline stress due to extreme precipitation events and freshwater runoff may be driving some of the reported recent range shifts in marine organisms. Using 2D gel electrophoresis and tandem mass spectrometry, we characterized the proteomic responses of the cold-adapted blue mussel species Mytilus trossulus, a native to the Pacific coast of North America, and the warm-adapted M. galloprovincialis, a Mediterranean invader that has replaced the native from the southern part of its range, but may be limited from expanding north due to hyposaline stress. After exposing laboratory-acclimated mussels for 4 h to two different experimental treatments of hyposaline conditions and one control treatment (24.5 and 29.8 and 35.0 psu, respectively) followed by a 0 and 24 h recovery at ambient salinity (35 psu), we detected changes in the abundance of molecular chaperones of the endoplasmic reticulum (ER), indicating protein unfolding, during stress exposure. Other common responses included changes in small GTPases of the Ras-superfamily during recovery, which suggest a role for vesicle transport, and cytoskeletal adjustments associated with cell volume, as indicated by cytoskeletal elements such as actin, tubulin, intermediate filaments and several actin-binding regulatory proteins. Changes of proteins involved in energy metabolism and scavenging of reactive oxygen species (ROS) suggest a reduction in overall energy metabolism during recovery. Principal component analyses of protein abundances suggest that M. trossulus is able to respond to a greater hyposaline challenge (24.5 psu) than M. galloprovincialis (29.8 psu), as shown by changing abundances of proteins involved in protein chaperoning, vesicle transport, cytoskeletal adjustments by actin-regulatory proteins, energy metabolism and oxidative stress. While proteins involved in energy metabolism were lower in M. trossulus during recovery from hyposaline stress, M. galloprovincialis showed higher abundances of those proteins at 29.8 psu, suggesting an energetic constraint in the invader but not the native congener. Both species showed lower levels of oxidative stress proteins during recovery. In addition, oxidative stress proteins associated with protein synthesis and folding in the ER, showed lower levels during recovery in M. galloprovincialis, in parallel with ER chaperones, indicating a reduction in protein synthesis. These differences may enable the native M. trossulus to cope with greater hyposaline stress in the northern part of its range. Furthermore, these differences may help M. trossulus to outcompete M. galloprovincialis in the southern part of M. trossulus' current range, thereby preventing M. galloprovincialis from expanding further north.

Protein folding and quality control in the ER

The endoplasmic reticulum (ER) uses an elaborate surveillance system called the ER quality control (ERQC) system. The ERQC facilitates folding and modification of secretory and membrane proteins and eliminates terminally misfolded polypeptides through ER-associated degradation (ERAD) or autophagic degradation. This mechanism of ER protein surveillance is closely linked to redox and calcium homeostasis in the ER, whose balance is presumed to be regulated by a specific cellular compartment. The potential to modulate proteostasis and metabolism with chemical compounds or targeted siRNAs may offer an ideal option for the treatment of disease.

Boronate oxidation as a bioorthogonal reaction approach for studying the chemistry of hydrogen peroxide in living systems

Reactive oxygen species (ROS), such as hydrogen peroxide, are important products of oxygen metabolism that, when misregulated, can accumulate and cause oxidative stress inside cells. Accordingly, organisms have evolved molecular systems, including antioxidant metalloenzymes (such as superoxide dismutase and catalase) and an array of thiol-based redox couples, to neutralize this threat to the cell when it occurs. On the other hand, emerging evidence shows that the controlled generation of ROS, particularly H(2)O(2), is necessary to maintain cellular fitness. The identification of NADPH oxidase enzymes, which generate specific ROS and reside in virtually all cell types throughout the body, is a prime example. Indeed, a growing body of work shows that H(2)O(2) and other ROS have essential functions in healthy physiological signaling pathways. The signal-stress dichotomy of H(2)O(2) serves as a source of motivation for disentangling its beneficial from its detrimental effects on living systems. Molecular imaging of this oxygen metabolite with reaction-based probes is a powerful approach for real-time, noninvasive monitoring of H(2)O(2) chemistry in biological specimens, but two key challenges to studying H(2)O(2) in this way are chemoselectivity and bioorthogonality of probe molecules. Chemoselectivity is problematic because traditional methods for ROS detection suffer from nonspecific reactivity with other ROS. Moreover, some methods require enzymatic additives not compatible with live-cell or live-animal specimens. Additionally, bioorthogonality requires that the reactions must not compete with or disturb intrinsic cellular chemistry; this requirement is particularly critical with thiol- or metal-based couples mediating the major redox events within the cell. Chemoselective bioorthogonal reactions, such as alkyne-azide cycloadditions and related click reactions, the Staudinger-Bertozzi ligation, and the transformations used in various reaction-based molecular probes, have found widespread application in the modification, labeling, and detection of biological molecules and processes. In this Account, we summarize H(2)O(2) studies from our laboratory using the H(2)O(2)-mediated oxidation of aryl boronates to phenols as a bioorthogonal approach to detect fluxes of this important ROS in living systems. We have installed this versatile switch onto organic and inorganic scaffolds to serve as "turn-on" probes for visible and near-infrared (NIR) fluorescence, ratiometric fluorescence, time-gated lanthanide luminescence, and in vivo bioluminescence detection of H(2)O(2) in living cells and animals. Further chemical and genetic manipulations target these probes to specific organelles and other subcellular locales and can also allow them to be trapped intracellularly, enhancing their sensitivity. These novel chemical tools have revealed fundamental new biological insights into the production, localization, trafficking, and in vivo roles of H(2)O(2) in a wide variety of living systems, including immune, cancer, stem, and neural cell models.

Endoplasmic reticulum stress, redox, and a proinflammatory environment in athero-susceptible endothelium in vivo at sites of complex hemodynamic shear stress

Endothelial phenotype heterogeneity plays an important role in the susceptibility of arteries to atherosclerosis. Regions of blood flow disturbance correlate with the development of disease. Here, we briefly outline the association of endoplasmic reticulum stress with endothelium in regions of athero-susceptibility in vivo. It is an important example of susceptible cell phenotype that is likely linked to proinflammatory and oxidative stress pathways. The endothelium in such regions is chronically exposed to complex hemodynamic shear stresses that may be considered as a risk factor for atherosclerosis via these mechanisms.

Multiple ways to make disulfides

Our concept of how disulfides form in proteins entering the secretory pathway has changed dramatically in recent years. The discovery of endoplasmic reticulum (ER) oxidoreductin 1 (ERO1) was followed by the demonstration that this enzyme couples oxygen reduction to de novo formation of disulfides. However, mammals deficient in ERO1 survive and form disulfides, which suggests the presence of alternative pathways. It has recently been shown that peroxiredoxin 4 is involved in peroxide removal and disulfide formation. Other less well-characterized pathways involving quiescin sulfhydryl oxidase, ER-localized protein disulfide isomerase peroxidases and vitamin K epoxide reductase might all contribute to disulfide formation. Here we discuss these various pathways for disulfide formation in the mammalian ER and highlight the central role played by glutathione in regulating this process.

Glutathione- and non-glutathione-based oxidant control in the endoplasmic reticulum

The redox-active tripeptide glutathione is an endogenous reducing agent that is found in abundance and throughout the cell. In the endoplasmic reticulum (ER), the ratio of glutathione to glutathione disulfide is lower compared with non-secretory organelles. This relatively oxidizing thiol-disulfide milieu is essential for the oxidative folding of nascent proteins in the ER and, at least in part, maintained by the activity of ER-resident endoplasmic oxidoreductin 1 (Ero1) enzymes that oxidize cysteine side chains at the expense of molecular oxygen. Glutathione disulfide and hydrogen peroxide formed as a consequence of Ero1 activity are widely considered as being inoperative and potentially dangerous by-products of oxidative protein folding in the ER. In contrast to this common view, this Commentary highlights the importance of glutathione- and non glutathione-based homeostatic redox control mechanisms in the ER. Stability in the thiol-disulfide system that prominently includes the protein disulfide isomerases is ensured by the contribution of tightly regulated Ero1 activity, ER-resident peroxidases and the glutathione-glutathione-disulfide redox pair that acts as a potent housekeeper of redox balance. Accordingly, the widely held concept that Ero1-mediated over-oxidation in the ER constitutes a common cause of cellular demise is critically re-evaluated.

Profiling impaired hepatic endoplasmic reticulum glycosylation as a consequence of ethanol ingestion

Alcoholic liver disease (ALD) is a prominent cause of morbidity and mortality in the United States. Alterations in protein folding occur in numerous disease states, including ALD. The endoplasmic reticulum (ER) is the primary site of post-translational modifications (PTM) within the cell. Glycosylation, the most abundant PTM, affects protein stability, structure, localization, and activity. Decreases in hepatic glycosylation machinery have been observed in rodent models of ALD, but specific protein targets have not been identified. Utilizing two-dimensional gel electrophoresis and liquid chromatography-tandem mass spectrometry, glycoproteins were identified in hepatic microsomal fractions from control and ethanol-fed mice. This study reports for the first time a global decrease in ER glycosylation. Additionally, the identification of 30 glycoproteins within this fraction elucidates pathway-specific alterations in ALD impaired glycosylation. Among the identified proteins, triacylglycerol hydrolase (TGH) is positively affected by glycosylation, showing increased activity following the addition of sugar moieties. Impaired TGH activity is associated with increased cellular storage of lipids and provides a potential mechanism for the observed pathologies associated with ALD.

Adipokines as novel biomarkers and regulators of the metabolic syndrome

Over the past two decades our view of adipose tissue has undergone a dramatic change from an inert energy storage tissue to an active endocrine organ. Adipose tissue communicates with other central and peripheral organs by synthesis and secretion of a host of molecules that we generally refer to as adipokines. The levels of some adipokines correlate with specific metabolic states and have the potential to impact directly upon the metabolic homeostasis of the system. A dysregulation of adipokines has been implicated in obesity, type 2 diabetes, hypertension, cardiovascular disease, and an ever-growing larger list of pathological changes in a number of organs. Here, we review the recent progress regarding the synthesis, secretion, and physiological function of adipokines with perspectives on future directions and potential therapeutic goals.

Adipokines as novel biomarkers and regulators of the metabolic syndrome

Over the past two decades our view of adipose tissue has undergone a dramatic change from an inert energy storage tissue to an active endocrine organ. Adipose tissue communicates with other central and peripheral organs by synthesis and secretion of a host of molecules that we generally refer to as adipokines. The levels of some adipokines correlate with specific metabolic states and have the potential to impact directly upon the metabolic homeostasis of the system. A dysregulation of adipokines has been implicated in obesity, type 2 diabetes, hypertension, cardiovascular disease, and an ever-growing larger list of pathological changes in a number of organs. Here, we review the recent progress regarding the synthesis, secretion, and physiological function of adipokines with perspectives on future directions and potential therapeutic goals.

Exploring the influence of torsinA expression on protein quality control

DYT1 dystonia is caused by a glutamic acid deletion (ΔE) in the endoplasmic reticulum (ER) protein torsinA. Previous studies suggest that torsinA modulates the aggregation of cytosolic misfolded proteins and ER stress responses, although the mechanisms underlying those effects remain unclear. In order to investigate the bases of these observations, we analyzed the interaction between torsinA expression, protein aggregation and ER stress in PC6.3 cells. Unexpectedly, we found that expression of torsinA(wt) or (ΔE) does not influence the inclusion formation by an expanded polyglutamine reporter protein in this cellular model. Furthermore, torsinA does not prevent the activation of ER stress induced by thapsigargin or the reducing agent DTT. Interestingly, DTT induces post-translational changes in torsinA, more prominently for torsinA(wt) than (ΔE). This work highlights the importance of model system selection for the study of torsinA function. Furthermore, it provides additional evidence suggesting that torsinA is sensitive to changes in the cellular redox potential.

Decreased prereceptorial glucocorticoid activating capacity in starvation due to an oxidative shift of pyridine nucleotides in the endoplasmic reticulum

Redox state of pyridine nucleotides of the endoplasmic reticulum (ER) lumen was determined in different nutritional conditions. NADPH-dependent cortisone reduction and NADP(+)-dependent cortisol oxidation were measured in rat liver microsomes, by utilizing the luminal 11β-hydroxysteroid dehydrogenase type 1 activity. Cortisone reduction decreased, while cortisol oxidation increased during onward starvation, showing that the luminal NADPH/NADP(+) ratio was substantially decreased. Cortisone or metyrapone addition caused a smaller decrease in NADPH fluorescence in microsomes from starved rats. The results demonstrate that nutrient supply is mirrored by the redox state of ER luminal pyridine nucleotides.