The molecular mechanisms underlying BiP-mediated gating of the Sec61 translocon of the endoplasmic reticulum

Selective activation of antioxidant resources and energy deficiency in Marinesco-Sjögren syndrome fibroblasts as an adaptive biological response to Sil1 loss

Marinesco-Sjögren syndrome (MSS) is a neuromuscular disease which presents with ataxia, muscle weakness and cataracts. This syndrome is typically caused by mutations in SIL1 gene, an ER co-chaperone that disrupts protein folding. Although it is known that accumulation of misfolded proteins in the ER profoundly affect reduction-oxidation (redox) homeostasis and energy production, the possible role of these processes in MSS was not investigated to date. In patient-derived fibroblasts, both maximal mitochondrial respiration and mitochondrial ATP production rates were diminished, while the glycolytic fraction remained unaffected. Catalase and superoxide dismutase activities were increased, while glutathione peroxidase and glutathione reductase were decreased. Oxidative damage to lipids, proteins, and DNA was comparable or even lower to that observed in control cells. Similar alterations were observed in the muscle tissue of the woozy mouse model of MSS. In conclusion, we identified a mitochondrial energy deficit and an adaptive cellular mechanism that effectively manage oxidative stress in Sil1-deficient cells.

Requirements for nuclear GRP78 transcriptional regulatory activities and interaction with nuclear GRP94

GRP78, a molecular chaperone primarily located in the endoplasmic reticulum (ER), has recently been discovered to translocate into the nucleus of stressed and cancer cells where it assumes a new function reprogramming the transcriptome. This study explores the requirements of GRP78 nuclear translocation and its transcriptional activity and investigates the role of ER-associated degradation in the process. We show that the ER-processed, mature form of GRP78 is the major form of nuclear GRP78 and is the form with transcriptional regulatory activity. In contrast, exogenously expressed GRP78 designed to lack its ER signal peptide, thus preventing it from entering the ER or undergoing any ER-related processing/modification, while able to enter the nucleus, lacks transcriptional regulatory activity toward E-Box containing target genes. Additionally, the ATP-binding and substrate-binding activities of GRP78 are critical for this transcriptional regulatory function. We further discover that GRP94, an ER chaperone that acts in concert with GRP78 on protein folding, can translocate to the nucleus and colocalize with nuclear GRP78 upon ER stress. These findings suggest that some form of ER processing of GRP78, in addition to cleavage of the ER signal peptide, is critical for its nuclear activity and that in stressed cells, ER chaperones may assume new functions in the nucleus yet to be explored.

The Essential Functions of Molecular Chaperones and Folding Enzymes in Maintaining Endoplasmic Reticulum Homeostasis

It has been estimated that up to one-third of the proteins encoded by the human genome enter the endoplasmic reticulum (ER) as extended polypeptide chains where they undergo covalent modifications, fold into their native structures, and assemble into oligomeric protein complexes. The fidelity of these processes is critical to support organellar, cellular, and organismal health, and is perhaps best underscored by the growing number of disease-causing mutations that reduce the fidelity of protein biogenesis in the ER. To meet demands encountered by the diverse protein clientele that mature in the ER, this organelle is populated with a cadre of molecular chaperones that prevent protein aggregation, facilitate protein disulfide isomerization, and lower the activation energy barrier of cis-trans prolyl isomerization. Components of the lectin (glycan-binding) chaperone system also reside within the ER and play numerous roles during protein biogenesis. In addition, the ER houses multiple homologs of select chaperones that can recognize and act upon diverse peptide signatures. Moreover, redundancy helps ensure that folding-compromised substrates are unable to overwhelm essential ER-resident chaperones and enzymes. In contrast, the ER in higher eukaryotic cells possesses a single member of the Hsp70, Hsp90, and Hsp110 chaperone families, even though several homologs of these molecules reside in the cytoplasm. In this review, we discuss specific functions of the many factors that maintain ER quality control, highlight some of their interactions, and describe the vulnerabilities that arise from the absence of multiple members of some chaperone families.

Crosstalk between protein misfolding and endoplasmic reticulum stress during ageing and their role in age-related disorders

Maintaining the proteome is crucial to retaining cell functionality and response to multiple intrinsic and extrinsic stressors. Protein misfolding increased the endoplasmic reticulum (ER) stress and activated the adaptive unfolded protein response (UPR) to restore cell homeostasis. Apoptosis occurs when ER stress is prolonged or the adaptive response fails. In healthy young cells, the ratio of protein folding machinery to quantities of misfolded proteins is balanced under normal circumstances. However, the age-related deterioration of the complex systems for handling protein misfolding is accompanied by ageing-related disruption of protein homeostasis, which results in the build-up of misfolded and aggregated proteins. This ultimately results in decreased cell viability and forms the basis of common age-related diseases called protein misfolding diseases. Proteins or protein fragments convert from their ordinarily soluble forms to insoluble fibrils or plaques in many of these disorders, which build up in various organs such as the liver, brain, or spleen. Alzheimer's, Parkinson's, type II diabetes, and cancer are diseases in this group commonly manifest in later life. Thus, protein misfolding and its prevention by chaperones and different degradation paths are becoming understood from molecular perspectives. Proteodynamics information will likely affect future interventional techniques to combat cellular stress and support healthy ageing by avoiding and treating protein conformational disorders. This review provides an overview of the diverse proteostasis machinery, protein misfolding, and ER stress involvement, which activates the UPR sensors. Here, we will discuss the crosstalk between protein misfolding and ER stress and their role in developing age-related diseases.

Characterization of the interaction between the Sec61 translocon complex and ppαF using optical tweezers

The Sec61 translocon allows the translocation of secretory preproteins from the cytosol to the endoplasmic reticulum lumen during polypeptide biosynthesis. These proteins possess an N-terminal signal peptide (SP) which docks at the translocon. SP mutations can abolish translocation and cause diseases, suggesting an essential role for this SP/Sec61 interaction. However, a detailed biophysical characterization of this binding is still missing. Here, optical tweezers force spectroscopy was used to characterize the kinetic parameters of the dissociation process between Sec61 and the SP of prepro-alpha-factor. The unbinding parameters including off-rate constant and distance to the transition state were obtained by fitting rupture force data to Dudko-Hummer-Szabo models. Interestingly, the translocation inhibitor mycolactone increases the off-rate and accelerates the SP/Sec61 dissociation, while also weakening the interaction. Whereas the translocation deficient mutant containing a single point mutation in the SP abolished the specificity of the SP/Sec61 binding, resulting in an unstable interaction. In conclusion, we characterize quantitatively the dissociation process between the signal peptide and the translocon, and how the unbinding parameters are modified by a translocation inhibitor.

Signal sequences encode information for protein folding in the endoplasmic reticulum

One-third of newly synthesized proteins in mammals are translocated into the endoplasmic reticulum (ER) through the Sec61 translocon. How protein translocation coordinates with chaperone availability in the ER to promote protein folding remains unclear. We find that marginally hydrophobic signal sequences and transmembrane domains cause transient retention at the Sec61 translocon and require the luminal BiP chaperone for efficient protein translocation. Using a substrate-trapping proteomic approach, we identify that nascent proteins bearing marginally hydrophobic signal sequences accumulate on the cytosolic side of the Sec61 translocon. Sec63 is co-translationally recruited to the translocation site and mediates BiP binding to incoming polypeptides. BiP binding not only releases translocationally paused nascent chains but also ensures protein folding in the ER. Increasing hydrophobicity of signal sequences bypasses Sec63/BiP-dependent translocation, but translocated proteins are prone to misfold and aggregate in the ER under limited BiP availability. Thus, the signal sequence-guided protein folding may explain why signal sequences are diverse and use multiple protein translocation pathways.

Co-chaperones of the Human Endoplasmic Reticulum: An Update

In mammalian cells, the rough endoplasmic reticulum (ER) plays central roles in the biogenesis of extracellular plus organellar proteins and in various signal transduction pathways. For these reasons, the ER comprises molecular chaperones, which are involved in import, folding, assembly, export, plus degradation of polypeptides, and signal transduction components, such as calcium channels, calcium pumps, and UPR transducers plus adenine nucleotide carriers/exchangers in the ER membrane. The calcium- and ATP-dependent ER lumenal Hsp70, termed immunoglobulin heavy-chain-binding protein or BiP, is the central player in all these activities and involves up to nine different Hsp40-type co-chaperones, i.e., ER membrane integrated as well as ER lumenal J-domain proteins, termed ERj or ERdj proteins, two nucleotide exchange factors or NEFs (Grp170 and Sil1), and NEF-antagonists, such as MANF. Here we summarize the current knowledge on the ER-resident BiP/ERj chaperone network and focus on the interaction of BiP with the polypeptide-conducting and calcium-permeable Sec61 channel of the ER membrane as an example for BiP action and how its functional cycle is linked to ER protein import and various calcium-dependent signal transduction pathways.

The manipulation of cell signaling and host cell biology by cholera toxin

Vibrio cholerae colonizes the small intestine and releases cholera toxin into the extracellular space. The toxin binds to the apical surface of the epithelium, is internalized into the host endomembrane system, and escapes into the cytosol where it activates the stimulatory alpha subunit of the heterotrimeric G protein by ADP-ribosylation. This initiates a cAMP-dependent signaling pathway that stimulates chloride efflux into the gut, with diarrhea resulting from the accompanying osmotic movement of water into the intestinal lumen. G protein signaling is not the only host system manipulated by cholera toxin, however. Other cellular mechanisms and signaling pathways active in the intoxication process include endocytosis through lipid rafts, retrograde transport to the endoplasmic reticulum, the endoplasmic reticulum-associated degradation system for protein delivery to the cytosol, the unfolded protein response, and G protein de-activation through degradation or the function of ADP-ribosyl hydrolases. Although toxin-induced chloride efflux is thought to be an irreversible event, alterations to these processes could facilitate cellular recovery from intoxication. This review will highlight how cholera toxin exploits signaling pathways and other cell biology events to elicit a diarrheal response from the host.

Sec61 complex/translocon: The role of an atypical ER Ca2+-leak channel in health and disease

The heterotrimeric Sec61 protein complex forms the functional core of the so-called translocon that forms an aqueous channel in the endoplasmic reticulum (ER). The primary role of the Sec61 complex is to allow protein import in the ER during translation. Surprisingly, a completely different function in intracellular Ca2+ homeostasis has emerged for the Sec61 complex, and the latter is now accepted as one of the major Ca2+-leak pathways of the ER. In this review, we first discuss the structure of the Sec61 complex and focus on the pharmacology and regulation of the Sec61 complex as a Ca2+-leak channel. Subsequently, we will pay particular attention to pathologies that are linked to Sec61 mutations, such as plasma cell deficiency and congenital neutropenia. Finally, we will explore the relevance of the Sec61 complex as a Ca2+-leak channel in various pathophysiological (ER stress, apoptosis, ischemia-reperfusion) and pathological (type 2 diabetes, cancer) settings.

Chloroquine alleviates the heat-induced to injure via autophagy and apoptosis mechanisms in skin cell and mouse models

Burns can cause cell death and irreversible tissue damage. We examined the pathway of human dermis fibroblasts cell death caused by skin burns and the roles of chloroquine in human skin keratinocytes HaCaT wound healing. Western blot assays were performed to assess expression of proteins associated with autophagy, apoptosis, and endoplasmic reticulum stress in skin cells following burns. Changes in apoptosis-related proteins were assessed using flow cytometry, and wound cell migration was examined using wound healing assays. The burn animal model was used to test whether chloroquine would promote wound healing. In human burned fibroblasts, expression of LC3B-II and Cleave-caspase-7 was increased, whereas expression of Beclin-1, p62, and Grp78 was decreased. Severe burn induced ER stress and ERK phosphorylation, but PD98059 or necrostatin-1 treatment cells did not affect expression of autophagy LC3B-II protein and can induce apoptosis. Even though added with TGF-β and FGF did not repair autophagy caused by burns. Suggesting that autophagy and apoptosis were involved in heat-injured mechanism. Recombinant Wnt3a protein can help restore expression of β-catenin which reduced following burns in keratinocytes. Wnt3a protein can promote migration of keratinocytes after burns. Interesting, chloroquine increased expression of LC3B-II protein and restored cell migration activity after 24 h of burns. Consistently, surgical dressing containing chloroquine promoted wound healing in a burn animal mode. Autophagy and Wnt/β-catenin is two signalling pathways that participate in cell repair and wound healing in human fibroblasts, keratinocytes. Surgical dressing containing chloroquine can recover wound healing in burned rats.

Regulation of calcium homeostasis and flux between the endoplasmic reticulum and the cytosol

The concentration of Ca²⁺ in the endoplasmic reticulum (ER) is critically important for maintaining its oxidizing environment as well as for maintaining luminal ATP levels required for chaperone activity. Therefore, local luminal Ca²⁺ concentrations and the dynamic Ca²⁺ flux between the different subcellular compartments are tightly controlled. Influx of Ca²⁺ into the ER is enabled by a reductive shift, which opens the sarcoendoplasmic reticulum calcium transport ATPase pump, building the Ca²⁺ gradient across the ER membrane required for ATP import. Meanwhile, Ca²⁺ leakage from the ER has been reported to occur via the Sec61 translocon following protein translocation. In this review, we provide an overview of the complex regulation of Ca²⁺ homeostasis, Ca²⁺ flux between subcellular compartments, and the cellular stress response (the unfolded protein response) induced upon dysregulated luminal Ca²⁺ metabolism. We also provide insight into the structure and gating mechanism at the Sec61 translocon and examine the role of ER-resident cochaperones in assisting the central ER-resident chaperone BiP in the control of luminal Ca²⁺ concentrations.

Regulation of Translation, Translocation, and Degradation of Proteins at the Membrane of the Endoplasmic Reticulum

The endoplasmic reticulum (ER) of mammalian cells is the central organelle for the maturation and folding of transmembrane proteins and for proteins destined to be secreted into the extracellular space. The proper folding of target proteins is achieved and supervised by a complex endogenous chaperone machinery. BiP, a member of the Hsp70 protein family, is the central chaperone in the ER. The chaperoning activity of BiP is assisted by ER-resident DnaJ (ERdj) proteins due to their ability to stimulate the low, intrinsic ATPase activity of BiP. Besides their co-chaperoning activity, ERdj proteins also regulate and tightly control the translation, translocation, and degradation of proteins. Disturbances in the luminal homeostasis result in the accumulation of unfolded proteins, thereby eliciting a stress response, the so-called unfolded protein response (UPR). Accumulated proteins are either deleterious due to the functional loss of the respective protein and/or due to their deposition as intra- or extracellular protein aggregates. A variety of metabolic diseases are known to date, which are associated with the dysfunction of components of the chaperone machinery. In this review, we will delineate the impact of ERdj proteins in controlling protein synthesis and translocation under physiological and under stress conditions. A second aspect of this review is dedicated to the role of ERdj proteins in the ER-associated degradation pathway, by which unfolded or misfolded proteins are discharged from the ER. We will refer to some of the most prominent diseases known to be based on the dysfunction of ERdj proteins.

Biogenesis of hepatitis B virus e antigen is driven by translocon-associated protein complex and regulated by conserved cysteine residues within its signal peptide sequence

Hepatitis B virus uses e antigen (HBe), which is dispensable for virus infectivity, to modulate host immune responses and achieve viral persistence in human hepatocytes. The HBe precursor (p25) is directed to the endoplasmic reticulum (ER), where cleavage of the signal peptide (sp) gives rise to the first processing product, p22. P22 can be retro-translocated back to the cytosol or enter the secretory pathway and undergo a second cleavage event, resulting in secreted p17 (HBe). Here, we report that translocation of p25 to the ER is promoted by translocon-associated protein complex. We have found that p25 is not completely translocated into the ER; a fraction of p25 is phosphorylated and remains in the cytoplasm and nucleus. Within the p25 sp sequence, we have identified three cysteine residues that control the efficiency of sp cleavage and contribute to proper subcellular distribution of the precore pool.

Inhibitors of the Sec61 Complex and Novel High Throughput Screening Strategies to Target the Protein Translocation Pathway

Proteins targeted to the secretory pathway start their intracellular journey by being transported across biological membranes such as the endoplasmic reticulum (ER). A central component in this protein translocation process across the ER is the Sec61 translocon complex, which is only intracellularly expressed and does not have any enzymatic activity. In addition, Sec61 translocon complexes are difficult to purify and to reconstitute. Screening for small molecule inhibitors impairing its function has thus been notoriously difficult. However, such translocation inhibitors may not only be valuable tools for cell biology, but may also represent novel anticancer drugs, given that cancer cells heavily depend on efficient protein translocation into the ER to support their fast growth. In this review, different inhibitors of protein translocation will be discussed, and their specific mode of action will be compared. In addition, recently published screening strategies for small molecule inhibitors targeting the whole SRP-Sec61 targeting/translocation pathway will be summarized. Of note, slightly modified assays may be used in the future to screen for substances affecting SecYEG, the bacterial ortholog of the Sec61 complex, in order to identify novel antibiotic drugs.

The Role of Hsp70s in the Development and Pathogenicity of Plasmodium falciparum

The main agent of human malaria, the protozoa, Plasmodium falciparum is known to infect liver cells, subsequently invading the host erythrocyte, leading to the manifestation of clinical outcomes of the disease. As part of its survival in the human host, P. falciparum employs several heat shock protein (Hsp) families whose primary purpose is to ensure cytoprotection through their molecular chaperone role. The parasite expresses six Hsp70s that localise to various subcellular organelles of the parasite, with one, PfHsp70-x, being exported to the infected human erythrocyte. The role of these Hsp70s in the survival and pathogenicity of malaria has received immense research attention. Several studies have reported on their structure-function features, network partnerships, and elucidation of their potential substrates. Apart from their role in cytoprotection and pathogenicity, Hsp70s are implicated in antimalarial drug resistance. As such, they are deemed potential antimalarial drug candidates, especially suited for co-targeting in combination therapies. In addition, Hsp70 is implicated in host immune modulation. The current report highlights the various structure-function features of these proteins, their roles in the development of malaria, current and prospective efforts being employed towards targeting them in malaria intervention efforts.

Complexity and Specificity of Sec61-Channelopathies: Human Diseases Affecting Gating of the Sec61 Complex

The rough endoplasmic reticulum (ER) of nucleated human cells has crucial functions in protein biogenesis, calcium (Ca2+) homeostasis, and signal transduction. Among the roughly one hundred components, which are involved in protein import and protein folding or assembly, two components stand out: The Sec61 complex and BiP. The Sec61 complex in the ER membrane represents the major entry point for precursor polypeptides into the membrane or lumen of the ER and provides a conduit for Ca2+ ions from the ER lumen to the cytosol. The second component, the Hsp70-type molecular chaperone immunoglobulin heavy chain binding protein, short BiP, plays central roles in protein folding and assembly (hence its name), protein import, cellular Ca2+ homeostasis, and various intracellular signal transduction pathways. For the purpose of this review, we focus on these two components, their relevant allosteric effectors and on the question of how their respective functional cycles are linked in order to reconcile the apparently contradictory features of the ER membrane, selective permeability for precursor polypeptides, and impermeability for Ca2+. The key issues are that the Sec61 complex exists in two conformations: An open and a closed state that are in a dynamic equilibrium with each other, and that BiP contributes to its gating in both directions in cooperation with different co-chaperones. While the open Sec61 complex forms an aqueous polypeptide-conducting- and transiently Ca2+-permeable channel, the closed complex is impermeable even to Ca2+. Therefore, we discuss the human hereditary and tumor diseases that are linked to Sec61 channel gating, termed Sec61-channelopathies, as disturbances of selective polypeptide-impermeability and/or aberrant Ca2+-permeability.

Neuroprotective Role of Akt in Hypoxia Adaptation in Andeans

Chronic mountain sickness (CMS) is a disease that potentially threatens a large segment of high-altitude populations during extended living at altitudes above 2,500 m. Patients with CMS suffer from severe hypoxemia, excessive erythrocytosis and neurologic deficits. The cellular mechanisms underlying CMS neuropathology remain unknown. We previously showed that iPSC-derived CMS neurons have altered mitochondrial dynamics and increased susceptibility to hypoxia-induced cell death. Genome analysis from the same population identified many ER stress-related genes that play an important role in hypoxia adaptation or lack thereof. In the current study, we showed that iPSC-derived CMS neurons have increased expression of ER stress markers Grp78 and XBP1s under normoxia and hyperphosphorylation of PERK under hypoxia, alleviating ER stress does not rescue the hypoxia-induced CMS neuronal cell death. Akt is a cytosolic regulator of ER stress with PERK as a direct target of Akt. CMS neurons exhibited lack of Akt activation and lack of increased Parkin expression as compared to non-CMS neurons under hypoxia. By enhancing Akt activation and Parkin overexpression, hypoxia-induced CMS neuronal cell death was reduced. Taken together, we propose that increased Akt activation protects non-CMS from hypoxia-induced cell death. In contrast, impaired adaptive mechanisms including failure to activate Akt and increase Parkin expression render CMS neurons more susceptible to hypoxia-induced cell death.

Endoplasmic Reticulum Calcium Homeostasis in Kidney Disease: Pathogenesis and Therapeutic Targets

Calcium (Ca²⁺) homeostasis is a crucial determinant of cellular function and survival. Endoplasmic reticulum (ER) acts as the largest intracellular Ca²⁺ store that maintains Ca²⁺ homeostasis through the ER Ca²⁺ uptake pump, sarco/ER Ca²⁺ ATPase, ER Ca²⁺ release channels, inositol 1,4,5-trisphosphate receptor channel, ryanodine receptor, and Ca²⁺-binding proteins inside of the ER lumen. Alterations in ER homeostasis trigger ER Ca²⁺ depletion and ER stress, which have been associated with the development of a variety of diseases. In addition, recent studies have highlighted the role of ER Ca²⁺ imbalance caused by dysfunction of sarco/ER Ca²⁺ ATPase, ryanodine receptor, and inositol 1,4,5-trisphosphate receptor channel in various kidney diseases. Despite progress in the understanding of the importance of these ER Ca²⁺ channels, pumps, and binding proteins in the pathogenesis of kidney disease, treatment is still lacking. This mini-review is focused on: i) Ca²⁺ homeostasis in the ER, ii) ER Ca²⁺ dyshomeostasis and apoptosis, and iii) altered ER Ca²⁺ homeostasis in kidney disease, including podocytopathy, diabetic nephropathy, albuminuria, autosomal dominant polycystic kidney disease, and ischemia/reperfusion-induced acute kidney injury.

Acute Induction of Translocon-Mediated Ca2+ Leak Protects Cardiomyocytes Against Ischemia/Reperfusion Injury

During myocardial infarction, dysregulation of Ca²⁺ homeostasis between the reticulum, mitochondria, and cytosol occurs in cardiomyocytes and leads to cell death. Ca²⁺ leak channels are thought to be key regulators of the reticular Ca²⁺ homeostasis and cell survival. The present study aimed to determine whether a particular reticular Ca²⁺ leak channel, the translocon, also known as translocation channel, could be a relevant target against ischemia/reperfusion-mediated heart injury. To achieve this objective, we first used an intramyocardial adenoviral strategy to express biosensors in order to assess Ca²⁺ variations in freshly isolated adult mouse cardiomyocytes to show that translocon is a functional reticular Ca²⁺ leak channel. Interestingly, translocon activation by puromycin mobilized a ryanodine receptor (RyR)-independent reticular Ca²⁺ pool and did not affect the excitation–concentration coupling. Second, puromycin pretreatment decreased mitochondrial Ca²⁺ content and slowed down the mitochondrial permeability transition pore (mPTP) opening and the rate of cytosolic Ca²⁺ increase during hypoxia. Finally, this translocon pre-activation also protected cardiomyocytes after in vitro hypoxia reoxygenation and reduced infarct size in mice submitted to in vivo ischemia-reperfusion. Altogether, our report emphasizes the role of translocon in cardioprotection and highlights a new paradigm in cardioprotection by functionally uncoupling the RyR-dependent Ca²⁺ stores and translocon-dependent Ca²⁺ stores.

Understanding the mammalian TRAP complex function(s)

In eukaryotic cells, about one-third of the synthesized proteins are translocated into the endoplasmic reticulum; they are membrane or lumen resident proteins and proteins direct to the Golgi apparatus. The co-translational translocation takes place through the heterotrimeric protein-conducting channel Sec61 which is associated with the ribosome and many accessory components, such as the heterotetrameric translocon-associated protein (TRAP) complex. Recently, microscopic techniques, such as cryo-electron microscopy and cryo-electron tomography, have enabled the determination of the translocation machinery structure. However, at present, there is a lack of understanding regarding the roles of some of its components; indeed, the TRAP complex function during co-translational translocation needs to be established. In addition, TRAP may play a role during unfolded protein response, endoplasmic-reticulum-associated protein degradation and congenital disorder of glycosylation (ssr4 CDG). In this article, I describe the current understanding of the TRAP complex in the light of its possible function(s).

Chemical chaperones improve the functional recovery of stunned myocardium by attenuating the endoplasmic reticulum stress

AIM

Myocardial ischaemia/reperfusion (I/R) produces structural and functional alterations depending on the duration of ischaemia. Brief ischaemia followed by reperfusion causes reversible contractile dysfunction (stunned heart) but long-lasting ischaemia followed by reperfusion can result in irreversible injury with cell death. Events during I/R can alter endoplasmic reticulum (ER) function leading to the accumulation of unfolded/misfolded proteins. The resulting ER stress induces activation of several signal transduction pathways, known as unfolded protein response (UPR). Experimental evidence shows that UPR contributes to cell death in irreversible I/R injury; however, there is still uncertainty for its occurrence in the stunned myocardium. This study investigated the ER stress response and its functional impact on the post-ischaemic cardiac performance of the stunned heart.

METHODS

Perfused rat hearts were subjected to 20 minutes of ischaemia followed by 30 minutes of reperfusion. UPR markers were evaluated by qRT-PCR and western blot. Post-ischaemic mechanical recovery was measured in absence and presence of two chemical chaperones: tauroursodeoxycholic acid (TUDCA) and 4-phenylbutyric acid (4-PBA).

RESULTS

Analysis of mRNA and protein levels of various ER stress effectors demonstrated that different UPR signalling cascades, involving both pro-survival and pro-apoptotic pathways, are activated. Inhibition of the UPR with chemical chaperones improved the post-ischaemic recovery of cardiac mechanical function without affecting the I/R-induced increase in oxidative stress.

CONCLUSION

Our results suggest that prevention of ER stress by chemical chaperones could be a therapeutic tool to limit deterioration of the contractile function in clinical settings in which the phenomenon of myocardial stunning is present.

Folded or Degraded in Endoplasmic Reticulum

In consistent with other membrane-bound and secretory proteins, immune checkpoint proteins go through a set of modifications in the endoplasmic reticulum (ER) to acquire their native functional structures before they function at their destinations. There are various ER-resident chaperones and enzymes synergistically regulate and catalyze the glycosylation, folding and transporting of proteins. The whole processing is under the surveillance of ER quality control system which allows the correctly folded proteins to exit from the ER with the help of coat proteinII(COPII) coated vesicles, while retains the rest of terminally misfolded ones in the ER and then eliminates them via ER-associated degradation (ERAD) or ER-to-lysosomes-associated degradation (ERLAD). The dysfunction of the ER causes ER stress which triggers unfolded protein response (UPR) to restore ER proteostasis. Unsolvable prolonged ER stress ultimately results in cell death. This chapter reviews the process that proteins undergo in the ER, and the glycosylation, folding and degradation of immune checkpoint proteins as well as the associated potential immunotherapies to date.

Organellar Calcium Handling in the Cellular Reticular Network

Ca²⁺ is an important intracellular messenger affecting diverse cellular processes. In eukaryotic cells, Ca²⁺ is handled by a myriad of Ca²⁺-binding proteins found in organelles that are organized into the cellular reticular network (CRN). The network is comprised of the endoplasmic reticulum, Golgi apparatus, lysosomes, membranous components of the endocytic and exocytic pathways, peroxisomes, and the nuclear envelope. Membrane contact sites between the different components of the CRN enable the rapid movement of Ca²⁺, and communication of Ca²⁺ status, within the network. Ca²⁺-handling proteins that reside in the CRN facilitate Ca²⁺ sensing, buffering, and cellular signaling to coordinate the many processes that operate within the cell.

ATPase activity of human binding immunoglobulin protein (BiP) variants is enhanced by signal sequence and physiological concentrations of Mn2

B‐cell immunoglobulin binding protein (BiP) is an essential endoplasmic reticulum (ER) chaperone normally found in the ER lumen. However, BiP also has other extracellular and intracellular functions. As it is unclear whether peripheral BiP has a signal and/or ER retention sequence, here we produced and biochemically characterised four variants of BiP. The variants differed depending on the presence or the absence of signal and ER retention peptides. Proteins were purified using nickel affinity chromatography, and variant size and quality were confirmed using SDS/PAGE gels. The thermal denaturing temperature of these proteins was found to be 46–47 °C. In addition, we characterised nucleotide binding properties in the absence and the presence of divalent cations. Interestingly, in the absence of cations, ADP has a higher binding affinity to BiP than ATP. The presence of divalent cations results in a decrease of the K_d values of both ADP and ATP, indicating higher affinities of both nucleotides for BiP. ATPase assays were carried out to study the enzyme activity of these variants and to characterise the kinetic parameters of BiP variants. Variants with the signal sequence had higher specific activities than those without. Both Mg²⁺ and Mn²⁺ efficiently stimulated the ATPase activity of these variants at low micromolar concentrations, whereas calcium failed to stimulate BiP ATPase. Our novel findings indicate the potential functionality of BiP variants that retain a signal sequence, and also reveal the effect of physiological concentrations of cations on the nucleotide binding properties and enzyme activities of all variants.

TPR-containing proteins control protein organization and homeostasis for the endoplasmic reticulum

The endoplasmic reticulum (ER) is a complex, multifunctional organelle comprised of a continuous membrane and lumen that is organized into a number of functional regions. It plays various roles including protein translocation, folding, quality control, secretion, calcium signaling, and lipid biogenesis. Cellular protein homeostasis is maintained by a complicated chaperone network, and the largest functional family within this network consists of proteins containing tetratricopeptide repeats (TPRs). TPRs are well-studied structural motifs that mediate intermolecular protein-protein interactions, supporting interactions with a wide range of ligands or substrates. Seven TPR-containing proteins have thus far been shown to localize to the ER and control protein organization and homeostasis within this multifunctional organelle. Here, we discuss the roles of these proteins in controlling ER processes and organization. The crucial roles that TPR-containing proteins play in the ER are highlighted by diseases or defects associated with their mutation or disruption.

Toxins Utilize the Endoplasmic Reticulum-Associated Protein Degradation Pathway in Their Intoxication Process

Several bacterial and plant AB-toxins are delivered by retrograde vesicular transport to the endoplasmic reticulum (ER), where the enzymatically active A subunit is disassembled from the holotoxin and transported to the cytosol. In this process, toxins subvert the ER-associated degradation (ERAD) pathway. ERAD is an important part of cellular regulatory mechanism that targets misfolded proteins to the ER channels, prior to their retrotranslocation to the cytosol, ubiquitination and subsequent degradation by a protein-degrading complex, the proteasome. In this article, we present an overview of current understanding of the ERAD-dependent transport of AB-toxins to the cytosol. We describe important components of ERAD and discuss their significance for toxin transport. Toxin recognition and disassembly in the ER, transport through ER translocons and finally cytosolic events that instead of overall proteasomal degradation provide proper folding and cytotoxic activity of AB-toxins are discussed as well. We also comment on recent reports presenting medical applications for toxin transport through the ER channels.

Decreased Protein Quality Control Promotes the Cognitive Dysfunction Associated With Aging and Environmental Insults

Background: Most neurodegenerative diseases are sporadic and develop with age. Degenerative neural tissues often contain intra- and extracellular protein aggregates, suggesting that the proteostasis network that combats protein misfolding could be dysfunctional in the setting of neurodegenerative disease. Binding immunoglobulin protein (BiP) is an endoplasmic reticulum (ER) chaperone that is crucial for protein folding and modulating the adaptive response in early secretory pathways. The interaction between BiP and unfolded proteins is mediated by the substrate-binding domain and nucleotide-binding domain with ATPase activity. The interaction facilitates protein folding and maturation. BiP has a recovery motif at the carboxyl terminus. The aim of this study is to examine cognitive function in model mice with an impaired proteostasis network by expressing a mutant form of BiP lacking the recovery motif. We also investigated if impairments of cognitive function were exacerbated by exposure to environmental insults, such as inhaled anesthetics.

Methods: We examined cognitive function by performing radial maze testing with mutant BiP mice and assessed the additional impact of general anesthesia in the context of proteostasis dysfunction. Testing over 8 days was performed 10 weeks, 6 months, and 1 year after birth.

Results: Age-related cognitive decline occurred in both forms of mice. The mutant BiP and anesthetic exposure promoted cognitive dysfunction prior to the senile period. After senescence, when mice were tested at 6 months of age and at 1 year old, there were no significant differences between the two genotypes in terms of the radial maze testing; furthermore, there was no significant difference when tested with and without anesthetic exposure.

Conclusion: Our data suggest that aging was the predominant factor underlying the impairment of cognitive function in this study. Impairment of the proteostasis network may promote age-related neurodegeneration, and this is exacerbated by external insults.

The enigmatic ATP supply of the endoplasmic reticulum

The endoplasmic reticulum (ER) is a functionally and morphologically complex cellular organelle largely responsible for a variety of crucial functions, including protein folding, maturation and degradation. Furthermore, the ER plays an essential role in lipid biosynthesis, dynamic Ca²⁺ storage, and detoxification. Malfunctions in ER‐related processes are responsible for the genesis and progression of many diseases, such as heart failure, cancer, neurodegeneration and metabolic disorders. To fulfill many of its vital functions, the ER relies on a sufficient energy supply in the form of adenosine‐5′‐triphosphate (ATP), the main cellular energy source. Despite landmark discoveries and clarification of the functional principles of ER‐resident proteins and key ER‐related processes, the mechanism underlying ER ATP transport remains somewhat enigmatic. Here we summarize ER‐related ATP‐consuming processes and outline our knowledge about the nature and function of the ER energy supply.

New insights into the cellular temporal response to proteostatic stress

Maintaining a healthy proteome involves all layers of gene expression regulation. By quantifying temporal changes of the transcriptome, translatome, proteome, and RNA-protein interactome in cervical cancer cells, we systematically characterize the molecular landscape in response to proteostatic challenges. We identify shared and specific responses to misfolded proteins and to oxidative stress, two conditions that are tightly linked. We reveal new aspects of the unfolded protein response, including many genes that escape global translation shutdown. A subset of these genes supports rerouting of energy production in the mitochondria. We also find that many genes change at multiple levels, in either the same or opposing directions, and at different time points. We highlight a variety of putative regulatory pathways, including the stress-dependent alternative splicing of aminoacyl-tRNA synthetases, and protein-RNA binding within the 3’ untranslated region of molecular chaperones. These results illustrate the potential of this information-rich resource.

The basic route of the nuclear translocation porcine growth hormone (GH)-growth hormone receptor (GHR) complex (pGH/GHR) in porcine hepatocytes

Traditional views suggest that growth hormone and the growth hormone receptor (GH/GHR complex) exert their functions only on the plasma membrane. This paradigm, however, has been challenged by recent new findings that the GH/GHR complex could translocate into cell nuclei where they could still exhibit important physiological functions. We also reported the nuclear localization of porcine GH/GHR and their potential functions in porcine hepatocytes. However, the basic path of pGH/GHR's nuclear translocation remains unclear. Combining previous research results and our current findings, we proposed two basic routes of pGH/GHR's nuclear transportation as follows: 1) after pGH binding to GHR, pGH/GHR enters into the cytoplasm though clathrin- or caveolin-mediated endocytosis, then the pGH/GHR complex enters into early endosomes (Rab5-positive), and the endosome carries the GH/GHR complex to the endoplasmic reticulum (ER). After endosome docking on the ER, the endosome starts fission, and the pGH/GHR complex enters into the ER lumen. Then the pGH/GHR complex transports into the cytoplasm, possibly by the ERAD pathway. Subsequently, the pGH/GHR complex interacts with IMPα/β, which, in turn, mediates GH/GHR nuclear localization; 2) pGH binds with the GHR on the cell membrane and, subsequently, pGH/GHR internalizes into the cell and enters into the endosome (this endosome may belong to a class of endosomes called envelope-associated endosomes (NAE)). Then, the endosome carries the pGH/GHR to the nuclear membrane. After docking on the nuclear membrane, the pGH/GHR complex fuses with the nuclear membrane and then enters into the cell nucleus.

AXER is an ATP/ADP exchanger in the membrane of the endoplasmic reticulum

To fulfill its role in protein biogenesis, the endoplasmic reticulum (ER) depends on the Hsp70-type molecular chaperone BiP, which requires a constant ATP supply. However, the carrier that catalyzes ATP uptake into the ER was unknown. Here, we report that our screen of gene expression datasets for member(s) of the family of solute carriers that are co-expressed with BiP and are ER membrane proteins identifies SLC35B1 as a potential candidate. Heterologous expression of SLC35B1 in E. coli reveals that SLC35B1 is highly specific for ATP and ADP and acts in antiport mode. Moreover, depletion of SLC35B1 from HeLa cells reduces ER ATP levels and, as a consequence, BiP activity. Thus, human SLC35B1 may provide ATP to the ER and was named AXER (ATP/ADP exchanger in the ER membrane). Furthermore, we propose an ER to cytosol low energy response regulatory axis (termed lowER) that appears as central for maintaining ER ATP supply.

Protein Toxins That Utilize Gangliosides as Host Receptors

Subsets of protein toxins utilize gangliosides as host receptors. Gangliosides are preferred receptors due to their extracellular localization on the eukaryotic cell and due to their essential nature in host physiology. Glycosphingolipids, including gangliosides, are mediators of signal transduction within and between eukaryotic cells. Protein toxins possess AB structure-function organization, where the A domain encodes a catalytic function for the posttranslational modification of a host macromolecule, including proteins and nucleic acids, and a B domain, which encodes host receptor recognition, including proteins and glycosphingolipids, alone or in combination. Protein toxins use similar strategies to bind glycans by pockets and loops, generally employing hydrogen bonding and aromatic stacking to stabilize interactions with sugars. In some cases, glycan binding facilitates uptake, while in other cases, cross-linking or a second receptor is necessary to stimulate entry. The affinity that protein toxins have for host glycans is necessary for tissue targeting, but not always sufficient to cause disease. In addition to affinity for binding the glycan, the lipid moiety also plays an important role in productive uptake and tissue tropism. Upon endocytosis, the protein toxin must escape to another intracellular compartment or into cytosol to modify a host substrate, modulating host signaling, often resulting in cytotoxic or apoptotic events in the cell, and a unique morbidity for the organism. The study of protein toxins that utilize gangliosides as host receptors has illuminated numerous eukaryotic cellular processes, identified the basis for developing interventions to prevent disease through vaccines and control bacterial diseases through therapies. In addition, subsets of these protein toxins have been utilized as therapeutic agents to treat numerous human inflictions.

Apratoxin Kills Cells by Direct Blockade of the Sec61 Protein Translocation Channel

Apratoxin A is a cytotoxic natural product that prevents the biogenesis of secretory and membrane proteins. Biochemically, apratoxin A inhibits cotranslational translocation into the ER, but its cellular target and mechanism of action have remained controversial. Here, we demonstrate that apratoxin A prevents protein translocation by directly targeting Sec61α, the central subunit of the protein translocation channel. Mutagenesis and competitive photo-crosslinking studies indicate that apratoxin A binds to the Sec61 lateral gate in a manner that differs from cotransin, a substrate-selective Sec61 inhibitor. In contrast to cotransin, apratoxin A does not exhibit a substrate-selective inhibitory mechanism, but blocks ER translocation of all tested Sec61 clients with similar potency. Our results suggest that multiple structurally unrelated natural products have evolved to target overlapping but non-identical binding sites on Sec61, thereby producing distinct biological outcomes.

Novel galeterone analogs act independently of AR and AR-V7 for the activation of the unfolded protein response and induction of apoptosis in the CWR22Rv1 prostate cancer cell model

The androgen receptor (AR) has long been the primary target for the treatment of prostate cancer (PC). Despite continuous efforts to block AR activity through ligand depletion, AR antagonism, AR depletion and combinations thereof, advanced PC tumors remain resilient. Herein, we evaluate two galeterone analogs, VNPT-178 and VNLG-74A, in PC cell models of diverse androgen and AR dependence attempting to delineate their mechanisms of action and potential clinical utility. Employing basic biochemical techniques, we determined that both analogs have improved antiproliferative and anti-AR activities compared to FDA-approved abiraterone and enzalutamide. However, induction of apoptosis in these models is independent of the AR and its truncated variant, AR-V7, and instead likely results from sustained endoplasmic reticulum stress and deregulated calcium homeostasis. Using in silico molecular docking, we predict VNPT-178 and VNLG-74A bind the ATPase domain of BiP/Grp78 and Hsp70-1A with greater affinity than the AR. Disruption of 70 kDa heat shock protein function may be the underlying mechanism of action for these galeterone analogs. Therefore, despite simultaneously antagonizing AR activity, AR and/or AR-V7 expression alone may inadequately predict a patient's response to treatment with VNPT-178 or VNLG-74A. Future studies evaluating the context-specific limitations of these compounds may provide clarity for their clinical application.

The Role of BiP Retrieval by the KDEL Receptor in the Early Secretory Pathway and its Effect on Protein Quality Control and Neurodegeneration

Protein quality control in the early secretory pathway is a ubiquitous eukaryotic mechanism for adaptation to endoplasmic reticulum (ER) stress. An ER molecular chaperone, immunoglobulin heavy chain-binding protein (BiP), is one of the essential components in this process. BiP interacts with nascent proteins to facilitate their folding. BiP also plays an important role in preventing aggregation of misfolded proteins and regulating the ER stress response when cells suffer various injuries. BiP is a member of the 70-kDa heat shock protein (HSP70) family of molecular chaperones that resides in the ER. Interaction between BiP and unfolded proteins is mediated by a substrate-binding domain and a nucleotide-binding domain for ATPase activity, leading to protein folding and maturation. BiP also possesses a retrieval motif in its carboxyl terminal. When BiP is secreted from the ER, the Lys-Asp-Glu-Leu (KDEL) receptor in the post-ER compartments binds with the carboxyl terminal KDEL sequence of BiP and returns BiP to the ER via coat protein complex I (COPI) vesicular transport. Although yeast studies showed that BiP retrieval by the KDEL receptor is not essential in single cells, it is crucial for multicellular organisms, where some essential proteins require retrieval to facilitate folding and maturation. Experiments in knock-in mice expressing mutant BiP with the retrieval motif deleted revealed a unique role of BiP retrieval by the KDEL receptor in neuronal development and age-related neurodegeneration.

Animal models of biliary injury and altered bile acid metabolism

In the last 25years, a number of animal models, mainly rodents, have been generated with the goal to mimic cholestatic liver injuries and, thus, to provide in vivo tools to investigate the mechanisms of biliary repair and, eventually, to test the efficacy of innovative treatments. Despite fundamental limitations applying to these models, such as the distinct immune system and the different metabolism regulating liver homeostasis in rodents when compared to humans, multiple approaches, such as surgery (bile duct ligation), chemical-induced (3,5-diethoxycarbonyl-1,4-dihydrocollidine, DDC, α-naphthylisothiocyanate, ANIT), viral infections (Rhesus rotavirustype A, RRV-A), and genetic manipulation (Mdr2, Cftr, Pkd1, Pkd2, Prkcsh, Sec63, Pkhd1) have been developed. Overall, they have led to a range of liver phenotypes recapitulating the main features of biliary injury and altered bile acid metabolisms, such as ductular reaction, peribiliary inflammation and fibrosis, obstructive cholestasis and biliary dysgenesis. Although with a limited translability to the human setting, these mouse models have provided us with the ability to probe over time the fundamental mechanisms promoting cholestatic disease progression. Moreover, recent studies from genetically engineered mice have unveiled 'core' pathways that make the cholangiocyte a pivotal player in liver repair. In this review, we will highlight the main phenotypic features, the more interesting peculiarities and the different drawbacks of these mouse models. This article is part of a Special Issue entitled: Cholangiocytes in Health and Disease edited by Jesus Banales, Marco Marzioni, Nicholas LaRusso and Peter Jansen.

Let's talk about Secs: Sec61, Sec62 and Sec63 in signal transduction, oncology and personalized medicine

The heterotrimeric Sec61 complex and the dimeric Sec62/Sec63 complex are located in the membrane of the human endoplasmic reticulum (ER) and play a central role in translocation of nascent and newly synthesized precursor polypeptides into the ER. This process involves targeting of the precursors to the membrane and opening of the polypeptide conducting Sec61 channel for translocation. Apart from this central role in the intracellular transport of polypeptides, several studies of the last decade uncovered additional functions of Sec proteins in intracellular signaling: Sec62 can induce ER-phagy in the process of recovery of cells from ER stress and the Sec61 channel can also act as a passive ER calcium leak channel. Furthermore, mutations, amplifications and an overexpression of the SEC genes were linked to various diseases including kidney and liver diseases, diabetes and human cancer. Studies of the last decade could not only elucidate the functional role of Sec proteins in the pathogenesis of these diseases, but also demonstrate a relevance of Sec62 as a prognostic and predictive biomarker in head and neck cancer, prostate and lung cancer including a basis for new therapeutic strategies. In this article, we review the current understanding of protein transport across the ER membrane as central function of Sec proteins and further focus on recent studies that gave first insights into the functional role and therapeutic relevance of Sec61, Sec62 and Sec63 in human diseases.

GRP78 at the Centre of the Stage in Cancer and Neuroprotection

The 78-kDa glucose-regulated protein GRP78, also known as BiP and HSP5a, is a multifunctional protein with activities far beyond its well-known role in the unfolded protein response (UPR) which is activated after endoplasmic reticulum (ER) stress in the cells. Most of these newly discovered activities depend on its position within the cell. GRP78 is located mainly in the ER, but it has also been observed in the cytoplasm, the mitochondria, the nucleus, the plasma membrane, and secreted, although it is dedicated mostly to engage endogenous cytoprotective processes. Hence, GRP78 may control either UPR and macroautophagy or may activated phosphatidylinositol 3-kinase (PI3K)/AKT pro-survival pathways. GRP78 influences how tumor cells survive, proliferate, and develop chemoresistance. In neurodegeneration, endogenous mechanisms of neuroprotection are frequently insufficient or dysregulated. Lessons from tumor biology may give us clues about how boosting endogenous neuroprotective mechanisms in age-related neurodegeneration. Herein, the functions of GRP78 are revealed at the center of the stage of apparently opposite sites of the same coin regarding cytoprotection: neurodegeneration and cancer. The goal is to give a comprehensive and critical review that may serve to guide future experiments to identify interventions that will enhance neuroprotection.

HSPA5 Gene encoding Hsp70 chaperone BiP in the endoplasmic reticulum

The HSPA5 gene encodes the binding immunoglobulin protein (BiP), an Hsp70 family chaperone localized in the ER lumen. As a highly conserved molecular chaperone, BiP assists in a wide range of folding processes via its two structural domains, a nucleotide-binding domain (NBD) and substrate-binding domain (SBD). BiP is also an essential component of the translocation machinery for protein import into the ER, a regulator for Ca²⁺ homeostasis in the ER, as well as a facilitator of ER-associated protein degradation (ERAD) via retrograde transportation of aberrant proteins across the ER membrane. When unfolded/misfolded proteins in the ER overwhelm the capacity of protein folding machinery, BiP can initiate the unfolded protein response (UPR), decrease unfolded/misfolded protein load, induce autophagy, and crosstalk with apoptosis machinery to assist in the cell survival decision. Post-translational modifications (PTMs) of BiP have been shown to regulate BiP's activity, turnover, and availability upon different extrinsic or intrinsic stimuli. As a master regulator of ER function, BiP is associated with cancer, cardiovascular disease, neurodegenerative disease, and immunological diseases. BiP has been targeted in cancer therapies and shows promise for application in other relevant diseases.

A Novel Method for Assessing the Chaperone Activity of Proteins

Protein chaperones are molecular machines which function both during homeostasis and stress conditions in all living organisms. Depending on their specific function, molecular chaperones are involved in a plethora of cellular processes by playing key roles in nascent protein chain folding, transport and quality control. Among stress protein families-molecules expressed during adverse conditions, infection, and diseases-chaperones are highly abundant. Their molecular functions range from stabilizing stress-susceptible molecules and membranes to assisting the refolding of stress-damaged proteins, thereby acting as protective barriers against cellular damage. Here we propose a novel technique to test and measure the capability for protective activity of known and putative chaperones in a semi-high throughput manner on a plate reader. The current state of the art does not allow the in vitro measurements of chaperone activity in a highly parallel manner with high accuracy or high reproducibility, thus we believe that the method we report will be of significant benefit in this direction. The use of this method may lead to a considerable increase in the number of experimentally verified proteins with such functions, and may also allow the dissection of their molecular mechanism for a better understanding of their function.

BiPPred: Combined sequence- and structure-based prediction of peptide binding to the Hsp70 chaperone BiP

Substrate binding to Hsp70 chaperones is involved in many biological processes, and the identification of potential substrates is important for a comprehensive understanding of these events. We present a multi-scale pipeline for an accurate, yet efficient prediction of peptides binding to the Hsp70 chaperone BiP by combining sequence-based prediction with molecular docking and MMPBSA calculations. First, we measured the binding of 15mer peptides from known substrate proteins of BiP by peptide array (PA) experiments and performed an accuracy assessment of the PA data by fluorescence anisotropy studies. Several sequence-based prediction models were fitted using this and other peptide binding data. A structure-based position-specific scoring matrix (SB-PSSM) derived solely from structural modeling data forms the core of all models. The matrix elements are based on a combination of binding energy estimations, molecular dynamics simulations, and analysis of the BiP binding site, which led to new insights into the peptide binding specificities of the chaperone. Using this SB-PSSM, peptide binders could be predicted with high selectivity even without training of the model on experimental data. Additional training further increased the prediction accuracies. Subsequent molecular docking (DynaDock) and MMGBSA/MMPBSA-based binding affinity estimations for predicted binders allowed the identification of the correct binding mode of the peptides as well as the calculation of nearly quantitative binding affinities. The general concept behind the developed multi-scale pipeline can readily be applied to other protein-peptide complexes with linearly bound peptides, for which sufficient experimental binding data for the training of classical sequence-based prediction models is not available. Proteins 2016; 84:1390-1407. © 2016 Wiley Periodicals, Inc.

Protection of Human Pancreatic Islets from Lipotoxicity by Modulation of the Translocon

Type 2 diabetes is characterized by peripheral insulin resistance and pancreatic beta cell dysfunction. Elevated free fatty acids (FFAs) may impair beta cell function and mass (lipotoxicity). Altered calcium homeostasis may be involved in defective insulin release. The endoplasmic reticulum (ER) is the major intracellular calcium store. Lipotoxicity induces ER stress and in parallel an ER calcium depletion through unknown ER calcium leak channels. The main purposes of this study is first to identify one of these channels and secondly, to check the opportunity to restore beta cells function (i.e., insulin secretion) after pharmacological inhibition of ER calcium store depletion. We investigated the functionality of translocon, an ER calcium leak channel and its involvement on FFAs-induced alterations in MIN6B1 cells and in human pancreatic islets. We evidenced that translocon acts as a functional ER calcium leak channel in human beta cells using anisomycin and puromycin (antibiotics), respectively blocker and opener of this channel. Puromycin induced a significant ER calcium release, inhibited by anisomycin pretreatment. Palmitate treatment was used as FFA model to induce a mild lipotoxic effect: ER calcium content was reduced, ER stress but not apoptosis were induced and glucose induced insulin secretion was decreased in our beta cells. Interestingly, translocon inhibition by chronic anisomycin treatment prevented dysfunctions induced by palmitate, avoiding reticular calcium depletion, ER stress and restoring insulin secretion. Our results provide for the first time compelling evidence that translocon actively participates to the palmitate-induced ER calcium leak and insulin secretion decrease in beta cells. Its inhibition reduces these lipotoxic effects. Taken together, our data indicate that TLC may be a new potential target for the treatment of type 2 diabetes.

Diversity of mitochondria-dependent dilator mechanisms in vascular smooth muscle of cerebral arteries from normal and insulin-resistant rats

Mitochondrial depolarization following ATP-sensitive potassium (mitoKATP) channel activation has been shown to induce cerebral vasodilation by generation of mitochondrial reactive oxygen species (ROS), which sequentially promotes frequency of calcium sparks and activation of large conductance calcium-activated potassium channels (BKCa) in vascular smooth muscle (VSM). We previously demonstrated that cerebrovascular insulin resistance accompanies aging and obesity. It is unclear whether mitochondrial depolarization without the ROS generation enhances calcium sparks and vasodilation in phenotypically normal [Sprague Dawley (SD); Zucker lean (ZL)] and insulin-resistant [Zucker obese (ZO)] rats. We compared the mechanisms underlying the vasodilation to ROS-dependent (diazoxide) and ROS-independent [BMS-191095 (BMS)] mitoKATP channel activators in normal and ZO rats. Arterial diameter studies from SD, ZL, and ZO rats showed that BMS as well as diazoxide induced vasodilation in endothelium-denuded cerebral arteries. In normal rats, BMS-induced vasodilation was mediated by mitochondrial depolarization and calcium sparks generation in VSM and was reduced by inhibition of BKCa channels. However, unlike diazoxide-induced vasodilation, scavenging of ROS had no effect on BMS-induced vasodilation. Electron spin resonance spectroscopy confirmed that diazoxide but not BMS promoted vascular ROS generation. BMS- as well as diazoxide-induced vasodilation, mitochondrial depolarization, and calcium spark generation were diminished in cerebral arteries from ZO rats. Thus pharmacological depolarization of VSM mitochondria by BMS promotes ROS-independent vasodilation via generation of calcium sparks and activation of BKCa channels. Diminished generation of calcium sparks and reduced vasodilation in ZO arteries in response to BMS and diazoxide provide new insights into mechanisms of cerebrovascular dysfunction in insulin resistance.

Co-chaperone Specificity in Gating of the Polypeptide Conducting Channel in the Membrane of the Human Endoplasmic Reticulum

In mammalian cells, signal peptide-dependent protein transport into the endoplasmic reticulum (ER) is mediated by a dynamic polypeptide-conducting channel, the heterotrimeric Sec61 complex. Previous work has characterized the Sec61 complex as a potential ER Ca(2+) leak channel in HeLa cells and identified ER lumenal molecular chaperone immunoglobulin heavy-chain-binding protein (BiP) as limiting Ca(2+) leakage via the open Sec61 channel by facilitating channel closing. This BiP activity involves binding of BiP to the ER lumenal loop 7 of Sec61α in the vicinity of tyrosine 344. Of note, the Y344H mutation destroys the BiP binding site and causes pancreatic β-cell apoptosis and diabetes in mice. Here, we systematically depleted HeLa cells of the BiP co-chaperones by siRNA-mediated gene silencing and used live cell Ca(2+) imaging to monitor the effects on ER Ca(2+) leakage. Depletion of either one of the ER lumenal BiP co-chaperones, ERj3 and ERj6, but not the ER membrane-resident co-chaperones (such as Sec63 protein, which assists BiP in Sec61 channel opening) led to increased Ca(2+) leakage via Sec6 complex, thereby phenocopying the effect of BiP depletion. Thus, BiP facilitates Sec61 channel closure (i.e. limits ER Ca(2+) leakage) via the Sec61 channel with the help of ERj3 and ERj6. Interestingly, deletion of ERj6 causes pancreatic β-cell failure and diabetes in mice and humans. We suggest that co-chaperone-controlled gating of the Sec61 channel by BiP is particularly important for cells, which are highly active in protein secretion, and that breakdown of this regulatory mechanism can cause apoptosis and disease.

MicroRNAs meet calcium: joint venture in ER proteostasis

The endoplasmic reticulum (ER) is a cellular compartment that has a key function in protein translation and folding. Maintaining its integrity is of fundamental importance for organism's physiology and viability. The dynamic regulation of intraluminal ER Ca(2+) concentration directly influences the activity of ER-resident chaperones and stress response pathways that balance protein load and folding capacity. We review the emerging evidence that microRNAs play important roles in adjusting these processes to frequently changing intracellular and environmental conditions to modify ER Ca(2+) handling and storage and maintain ER homeostasis.

Calcium trafficking integrates endoplasmic reticulum function with mitochondrial bioenergetics

Calcium homeostasis is central to all cellular functions and has been studied for decades. Calcium acts as a critical second messenger for both extracellular and intracellular signaling and is fundamental in cell life and death decisions (Berridge et al., 2000) [1]. The calcium gradient in the cell is coupled with an inherent ability of the divalent cation to reversibly bind multiple target biological molecules to generate an extremely versatile signaling system [2]. Calcium signals are used by the cell to control diverse processes such as development, neurotransmitter release, muscle contraction, metabolism, autophagy and cell death. "Cellular calcium overload" is detrimental to cellular health, resulting in massive activation of proteases and phospholipases leading to cell death (Pinton et al., 2008) [3]. Historically, cell death associated with calcium ion perturbations has been primarily recognized as necrosis. Recent evidence clearly associates changes in calcium ion concentrations with more sophisticated forms of cellular demise, including apoptosis (Kruman et al., 1998; Tombal et al., 1999; Lynch et al., 2000; Orrenius et al., 2003) [4-7]. Although the endoplasmic reticulum (ER) serves as the primary calcium store in the metazoan cell, dynamic calcium release to the cytosol, mitochondria, nuclei and other organelles orchestrate diverse coordinated responses. Most evidence supports that calcium transport from the ER to mitochondria plays a significant role in regulating cellular bioenergetics, production of reactive oxygen species, induction of autophagy and apoptosis. Recently, molecular identities that mediate calcium traffic between the ER and mitochondria have been discovered (Mallilankaraman et al., 2012a; Mallilankaraman et al., 2012b; Sancak et al., 2013)[8-10]. The next questions are how they are regulated for exquisite tight control of ER-mitochondrial calcium dynamics. This review attempts to summarize recent advances in the role of calcium in regulation of ER and mitochondrial function. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Fucoidan induces cancer cell apoptosis by modulating the endoplasmic reticulum stress cascades

BACKGROUND

Cancer metastasis is the main cause leading to disease recurrence and high mortality in cancer patients. Therefore, inhibiting metastasis process or killing metastatic cancer cells by inducing apoptosis is of clinical importance in improving cancer patient survival. Previous studies revealed that fucoidan, a fucose-rich polysaccharide isolated from marine brown alga, is a promising natural product with significant anti-cancer activity. However, little is known about the role of endoplasmic reticulum (ER) stress in fucoidan-induced cell apoptosis.

PRINCIPAL FINDINGS

We reported that fucoidan treatment inhibits cell growth and induces apoptosis in cancer cells. Fucoidan treatments resulted in down-regulation of the glucose regulated protein 78 (GRP78) in the metastatic MDA-MB-231 breast cancer cells, and of the ER protein 29 (ERp29) in the metastatic HCT116 colon cancer cells. However, fucoidan treatment promoted ER Ca2+-dependent calmodulin-dependent kinase II (CaMKII) phosphorylation, Bcl-associated X protein (Bax) and caspase 12 expression in MDA-MB-231 cells, but not in HCT116 cells. In both types of cancer cells, fucoidan activated the phosphorylation of eukaryotic initiation factor 2 alpha (p-eIF2α)\CCAAT/enhancer binding protein homologous protein (CHOP) pro-apoptotic cascade and inhibited the phosphorylation of inositol-requiring kinase 1 (p-IRE-1)\X-box binding proteins 1 splicing (XBP-1s) pro-survival cascade. Furthermore, CHOP knockdown prevented DNA damage and cell death induced by fucoidan.

CONCLUSION/SIGNIFICANCE

Fucoidan exerts its anti-tumor function by modulating ER stress cascades. Contribution of ER stress to the fucoidan-induced cell apoptosis augments our understanding of the molecular mechanisms underlying its anti-tumour activity and provides evidence for the therapeutic application of fucoidan in cancer.

Protein transport into the human endoplasmic reticulum

Protein transport into the endoplasmic reticulum (ER) is essential for all eukaryotic cells and evolutionary related to protein transport into and across the cytoplasmic membrane of eubacteria and archaea. It is based on amino-terminal signal peptides in the precursor polypeptides plus various transport components in cytosol plus ER and can occur either cotranslationally or posttranslationally. The two mechanisms merge at the heterotrimeric Sec61 complex in the ER membrane, which forms an aqueous polypeptide-conducting channel. Since the mammalian ER is also the main intracellular calcium storage organelle, the Sec61 complex is tightly regulated in its dynamics between the open and closed conformations by various ligands, such as precursor polypeptides at the cytosolic face and the Hsp70-type molecular chaperone BiP at the ER lumenal face (Hsp, heat shock protein). Furthermore, BiP binding to the incoming precursor polypeptide contributes to unidirectionality and efficiency of transport. Recent insights into the structural dynamics of the Sec61 complex and related complexes in eubacteria and archaea have various mechanistic and functional implications.

Protein transport into the human ER and related diseases, Sec61-channelopathies

Protein transport into the human endoplasmic reticulum (ER) is relevant to the biogenesis of most soluble and membrane proteins of organelles, which are involved in endo- or exo-cytsosis. It involves amino-terminal signal peptides in the precursor polypeptides and various transport components in the cytosol plus the ER, and can occur co- or post-translationally. The two mechanisms merge at the level of the ER membrane, specifically at the level of the heterotrimeric Sec61 complex, which forms a dynamic polypeptide-conducting channel in the ER membrane. Since the mammalian ER is also the main intracellular calcium storage organelle, and the Sec61 complex is calcium permeable, the Sec61 complex is tightly regulated in its equilibrium between the closed and open conformations, or "gated", by ligands, such as signal peptides of the transport substrates and the ER lumenal Hsp70-type molecular chaperone BiP. Furthermore, BiP binding to the incoming polypeptide contributes to the efficiency and unidirectionality of transport. Recent insights into the structure and dynamic equilibrium of the Sec61 complex have various mechanistic as well as medical implications.