GRP94: An HSP90-like protein specialized for protein folding and quality control in the endoplasmic reticulum

The Hsp70/Hsp90 Chaperone Machinery in Neurodegenerative Diseases

The accumulation of misfolded proteins in the human brain is one of the critical features of many neurodegenerative diseases, including Alzheimer's disease (AD). Assembles of beta-amyloid (Aβ) peptide—either soluble (oligomers) or insoluble (plaques) and of tau protein, which form neurofibrillary tangles, are the major hallmarks of AD. Chaperones and co-chaperones regulate protein folding and client maturation, but they also target misfolded or aggregated proteins for refolding or for degradation, mostly by the proteasome. They form an important line of defense against misfolded proteins and are part of the cellular quality control system. The heat shock protein (Hsp) family, particularly Hsp70 and Hsp90, plays a major part in this process and it is well-known to regulate protein misfolding in a variety of diseases, including tau levels and toxicity in AD. However, the role of Hsp90 in regulating protein misfolding is not yet fully understood. For example, knockdown of Hsp90 and its co-chaperones in a Caenorhabditis elegans model of Aβ misfolding leads to increased toxicity. On the other hand, the use of Hsp90 inhibitors in AD mouse models reduces Aβ toxicity, and normalizes synaptic function. Stress-inducible phosphoprotein 1 (STI1), an intracellular co-chaperone, mediates the transfer of clients from Hsp70 to Hsp90. Importantly, STI1 has been shown to regulate aggregation of amyloid-like proteins in yeast. In addition to its intracellular function, STI1 can be secreted by diverse cell types, including astrocytes and microglia and function as a neurotrophic ligand by triggering signaling via the cellular prion protein (PrP^C). Extracellular STI1 can prevent Aβ toxic signaling by (i) interfering with Aβ binding to PrP^C and (ii) triggering pro-survival signaling cascades. Interestingly, decreased levels of STI1 in C. elegans can also increase toxicity in an amyloid model. In this review, we will discuss the role of intracellular and extracellular STI1 and the Hsp70/Hsp90 chaperone network in mechanisms underlying protein misfolding in neurodegenerative diseases, with particular focus on AD.

Comparative Proteomic Analysis of Three Chinese Hamster Ovary (CHO) Host Cells

Chinese hamster ovary (CHO) cells have been widely used to express heterologous genes and produce therapeutic proteins in biopharmaceutical industry. Different CHO host cells have distinct cell growth rates and protein expression characteristics. In this study, the expression of about 1,307 host proteins in three sublines, i.e. CHO K1, CHO S and CHO/dihydrofolate reductase (dhfr)^- , were investigated and compared using proteomic analysis. The proteins involved in cell growth, glycolysis, tricarboxylic acid cycle, transcription, translation and glycosylation were quantitated using Liquid chromatography tandem-mass spectrometry (LC-MS/MS). The key host cell proteins that regulate the kinetics of cell growth and the magnitude of protein expression levels were identified. Furthermore, several rational cell engineering strategies on how to combine the desired features of fast cell growth and efficient production of therapeutic proteins into one new super CHO host cell have been proposed.

Maintaining the factory: the roles of the unfolded protein response in cellular homeostasis in plants

Much like a factory, the endoplasmic reticulum (ER) assembles simple cellular building blocks into complex molecular machines known as proteins. In order to protect the delicate protein folding process and ensure the proper cellular delivery of protein products under environmental stresses, eukaryotes have evolved a set of signaling mechanisms known as the unfolded protein response (UPR) to increase the folding capacity of the ER. This process is particularly important in plants, because their sessile nature commands adaptation for survival rather than escape from stress. As such, plants make special use of the UPR, and evidence indicates that the master regulators and downstream effectors of the UPR have distinct roles in mediating cellular processes that affect organism growth and development as well as stress responses. In this review we outline recent developments in this field that support a strong relevance of the UPR to many areas of plant life.

Clients and Oncogenic Roles of Molecular Chaperone gp96/grp94

As an endoplasmic reticulum heat shock protein (HSP) 90 paralogue, glycoprotein (gp) 96 possesses immunological properties by chaperoning antigenic peptides for activation of T cells. Genetic studies in the last decade have unveiled that gp96 is also an essential master chaperone for multiple receptors and secreting proteins including Toll-like receptors (TLRs), integrins, the Wnt coreceptor, Low Density Lipoprotein Receptor-Related Protein 6 (LRP6), the latent TGFβ docking receptor, Glycoprotein A Repetitions Predominant (GARP), Glycoprotein (GP) Ib and insulin-like growth factors (IGF). Clinically, elevated expression of gp96 in a variety of cancers correlates with the advanced stage and poor survival of cancer patients. Recent preclinical studies have also uncovered that gp96 expression is closely linked to cancer progression in multiple myeloma, hepatocellular carcinoma, breast cancer and inflammation-associated colon cancer. Thus, gp96 is an attractive therapeutic target for cancer treatment. The chaperone function of gp96 depends on its ATPase domain, which is structurally distinct from other HSP90 members, and thus favors the design of highly selective gp96-targeted inhibitors against cancer. We herein discuss the strategically important oncogenic clients of gp96 and their underlying biology. The roles of cell-intrinsic gp96 in T cell biology are also discussed, in part because it offers another opportunity of cancer therapy by manipulating levels of gp96 in T cells to enhance host immune defense.

Levels of Hepatic Activating Transcription Factor 6 and Caspase-3 Are Downregulated in Mice after Excessive Training

Recently, we demonstrated that different running overtraining (OT) protocols with the same external load, but performed downhill (OTR/down), uphill (OTR/up), and without inclination (OTR), led to hepatic fat accumulation. As the disruption of endoplasmic reticulum (ER) homeostasis is linked to animal models of fatty liver disease, we investigated the effects of these OT models on the proteins related to ER stress (i.e., BiP, inositol-requiring enzyme 1, protein kinase RNA-like endoplasmic reticulum kinase, eIF2alpha, ATF6beta, and glucose-regulated protein 94) and apoptosis (C/EBP-homologous protein, Caspase-3, 4, and 12, Bax, and tumor necrosis factor receptor-associated factor 2) in livers of C57BL/6 mice. Also, aerobic training can attenuate cardiac ER stress and improve exercise capacity. Therefore, we investigated whether the decrease in performance induced by our OT protocols is linked to ER stress and apoptosis in mouse hearts. The rodents were divided into six groups: naïve (N, sedentary mice), control (CT, sedentary mice submitted to the performance evaluations), trained (TR), OTR/down, OTR/up, and OTR groups. Rotarod, incremental load, exhaustive, and grip force tests were used to evaluate performance. After the grip force test, the livers and cardiac muscles (i.e., left ventricle) were removed and used for immunoblotting. All of the OT protocols led to similar responses in the performance parameters and displayed significantly lower hepatic ATF6beta values compared to the N group. The OTR/down group exhibited lower liver cleaved caspase-3 values compared to the CT group. However, the other proteins related to ER stress and apoptosis were not modulated. Also, the cardiac proteins related to ER stress and apoptosis were not modulated in the experimental groups. In conclusion, the OT protocols decreased the levels of hepatic ATF6beta, and the OTR/down group decreased the levels of hepatic cleaved caspase-3. Also, the decrease in performance induced by our OT models is not associated with ER stress and apoptosis in mice hearts.

HSP90 inhibitors in the context of heat shock and the unfolded protein response: effects on a primary canine pulmonary adenocarcinoma cell line

BACKGROUND

Agents targeting HSP90 and GRP94 are seldom tested in stressed contexts such as heat shock (HS) or the unfolded protein response (UPR). Tumor stress often activates HSPs and the UPR as pro-survival mechanisms. This begs the question of stress effects on chemotherapeutic efficacy, particularly with drugs targeting chaperones such as HSP90 or GRP94. We tested the utility of several HSP90 inhibitors, including PU-H71 (targeting GRP94), on a primary canine lung cancer line under HS/UPR stress compared to control conditions.

METHODS

We cultured canine bronchoalveolar adenocarcinoma cells that showed high endogenous HSP90 and GRP94 expression; these levels substantially increased upon HS or UPR induction. We treated cells with HSP90 inhibitors 17-DMAG, 17-AAG or PU-H71 under standard conditions, HS or UPR. Cell viability/survival was assayed. Antibody arrays measured intracellular signalling and apoptosis profiles.

RESULTS

HS and UPR had varying effects on cells treated with different HSP90 inhibitors; in particular, HS and UPR promoted resistance to inhibitors in short-term assays, but combinations of UPR stress and PU-H571 showed potent cytotoxic activity in longer-term assays. Array data indicated altered signalling pathways, with apoptotic and pro-survival implications. UPR induction + dual targeting of HSP90 and GRP94 swayed the balance toward apoptosis.

CONCLUSION

Cellular stresses, endemic to tumors, or interventionally inducible, can deflect or enhance chemo-efficacy, particularly with chaperone-targeting drugs. Stress is likely not held accountable when testing new pharmacologics or assessing currently-used drugs. A better understanding of stress impacts on drug activities should be critical in improving therapeutic targeting and in discerning mechanisms of drug resistance.

Depletion of the thiol oxidoreductase ERp57 in tumor cells inhibits proliferation and increases sensitivity to ionizing radiation and chemotherapeutics

Rapidly growing tumor cells must synthesize proteins at a high rate and therefore depend on an efficient folding and quality control system for nascent secretory proteins in the endoplasmic reticulum (ER). The ER resident thiol oxidoreductase ERp57 plays an important role in disulfide bond formation. Lentiviral, doxycycline-inducible ERp57 knockdown was combined with irradiation and treatment with chemotherapeutic agents. The knockdown of ERp57 significantly enhanced the apoptotic response to anticancer treatment in HCT116 colon cancer cells via a p53-dependent mechanism. Instead of a direct interaction with p53, depletion of ERp57 induced cell death via a selective activation of the PERK branch of the Unfolded Protein Response (UPR). In contrast, apoptosis was reduced in MDA-MB-231 breast cancer cells harboring mutant p53. Nevertheless, we observed a strong reduction of proliferation in response to ERp57 knockdown in both cell lines regardless of the p53 status. Depletion of ERp57 reduced the phosphorylation activity of the mTOR-complex1 (mTORC1) as demonstrated by reduction of p70S6K phosphorylation. Our data demonstrate that ERp57 is a promising target for anticancer therapy due to synergistic p53-dependent induction of apoptosis and p53-independent inhibition of proliferation.

Biology of Hsp47 (Serpin H1), a collagen-specific molecular chaperone

Hsp47, a collagen-specific molecular chaperone that localizes in the endoplasmic reticulum (ER), is indispensable for molecular maturation of collagen. Hsp47, which is encoded by the SERPINH1 gene, belongs to the serpin family and has the serpin fold; however, it has no serine protease inhibitory activity. Hsp47 transiently binds to procollagen in the ER, dissociates in the cis-Golgi or ER-Golgi intermediate compartment (ERGIC) in a pH-dependent manner, and is then transported back to the ER via its RDEL retention sequence. Hsp47 recognizes collagenous (Gly-Xaa-Arg) repeats on triple-helical procollagen and can prevent local unfolding and/or aggregate formation of procollagen. Gene disruption of Hsp47 in mice causes embryonic lethality due to impairments in basement membrane and collagen fibril formation. In Hsp47-knockout cells, the type I collagen triple helix forms abnormally, resulting in thin and frequently branched fibrils. Secretion of type I collagens is slow and plausible in making aggregates of procollagens in the ER of hsp47-knocked out fibroblasts, which are ultimately degraded by autophagy. Mutations in Hsp47 are causally associated with osteogenesis imperfecta. Expression of Hsp47 is strongly correlated with expression of collagens in multiple types of cells and tissues. Therefore, Hsp47 represents a promising target for treatment of collagen-related disorders, including fibrosis of the liver, lung, and other organs.

Size dependent effect of ZnO nanoparticles on endoplasmic reticulum stress signaling pathway in murine liver

ZnO nanoparticles (NPs) have been assessed to show adverse effects on the liver, but the molecular mechanisms and the role of nanoparticle properties in these adverse reactions have not been sufficiently studied. In this study, the toxicity of various sizes of ZnO particles (bulk, 90nm, and 30nm) that were ingested orally over a period of 3days were evaluated in mice. The blood biochemistry, hematological analyses, and histopathological evaluation showed that there was apparent toxicity caused by smaller ZnO NPs (30nm) in liver. The smallest ZnO NPs showed highest accumulation in the mice liver. The RT-qPCR data indicated that 30nm ZnO NPs can induce significant endoplasmic reticulum (ER) stress responses. The ER stress marker of PERK, eIF2α, ATF4, Chop, JNK, caspase-12, caspase-9, GRP94, and Bax at the mRNA levels were higher expression in 30nm ZnO NP than that in bulk or 90nm ZnO. These findings implied that the smaller ZnO NPs (30nm) activated ER stress responses that signified severe apoptosis in murine liver.

Chemical Tools to Investigate Mechanisms Associated with HSP90 and HSP70 in Disease

The chaperome is a large and diverse protein machinery composed of chaperone proteins and a variety of helpers, such as the co-chaperones, folding enzymes, and scaffolding and adapter proteins. Heat shock protein 90s and 70s (HSP90s and HSP70s), the most abundant chaperome members in human cells, are also the most complex. As we have learned to appreciate, their functions are context dependent and manifested through a variety of conformations that each recruit a subset of co-chaperone, scaffolding, and folding proteins and which are further diversified by the posttranslational modifications each carry, making their study through classic genetic and biochemical techniques quite a challenge. Chemical biology tools and techniques have been developed over the years to help decipher the complexities of the HSPs and this review provides an overview of such efforts with focus on HSP90 and HSP70.

Comparative venomics of Psyttalia lounsburyi and P. concolor, two olive fruit fly parasitoids: a hypothetical role for a GH1 β-glucosidase

Venom composition of parasitoid wasps attracts increasing interest - notably molecules ensuring parasitism success on arthropod pests - but its variation within and among taxa is not yet understood. We have identified here the main venom proteins of two braconid wasps, Psyttalia lounsburyi (two strains from South Africa and Kenya) and P. concolor, olive fruit fly parasitoids that differ in host range. Among the shared abundant proteins, we found a GH1 β-glucosidase and a family of leucine-rich repeat (LRR) proteins. Olive is extremely rich in glycoside compounds that are hydrolyzed by β-glucosidases into defensive toxic products in response to phytophagous insect attacks. Assuming that Psyttalia host larvae sequester ingested glycosides, the injected venom GH1 β-glucosidase could induce the release of toxic compounds, thus participating in parasitism success by weakening the host. Venom LRR proteins are similar to truncated Toll-like receptors and may possibly scavenge the host immunity. The abundance of one of these LRR proteins in the venom of only one of the two P. lounsburyi strains evidences intraspecific variation in venom composition. Altogether, venom intra- and inter-specific variation in Psyttalia spp. were much lower than previously reported in the Leptopilina genus (Figitidae), suggesting it might depend upon the parasitoid taxa.

Characterization of HSP90 isoforms in transformed bovine leukocytes infected with Theileria annulata

HSP90 chaperones are essential regulators of cellular function, as they ensure the appropriate conformation of multiple key client proteins. Four HSP90 isoforms were identified in the protozoan parasite Theileria annulata. Partial characterization was undertaken for three and localization confirmed for cytoplasmic (TA12105), endoplasmic reticulum (TA06470), and apicoplast (TA10720) forms. ATPase activity and binding to the HSP90 inhibitor geldanamycin were demonstrated for recombinant TA12105, and all three native forms could be isolated to varying extents by binding to geldanamycin beads. Because it is essential, HSP90 is considered a potential therapeutic drug target. Resistance to the only specific Theileriacidal drug is increasing, and one challenge for design of drugs that target the parasite is to limit the effect on the host. An in vitro cell culture system that allows comparison between uninfected bovine cells and the T. annulata‐infected counterpart was utilized to test the effects of geldanamycin and the derivative 17‐AAG. T. annulata‐infected cells had greater tolerance to geldanamycin than uninfected cells yet exhibited significantly more sensitivity to 17‐AAG. These findings suggest that parasite HSP90 isoform(s) can alter the drug sensitivity of infected host cells and that members of the Theileria HSP90 family are potential targets worthy of further investigation.

5'-N-ethylcarboxamidoadenosine is not a paralog-specific Hsp90 inhibitor

The molecular chaperone Hsp90 facilitates the folding and modulates activation of diverse substrate proteins. Unlike other heat shock proteins such as Hsp60 and Hsp70, Hsp90 plays critical regulatory roles by maintaining active states of kinases, many of which are overactive in cancer cells. Four Hsp90 paralogs are expressed in eukaryotic cells: Hsp90α/β (in the cytosol), Grp94 (in the endoplasmic reticulum), Trap1 (in mitochondria). Although numerous Hsp90 inhibitors are being tested in cancer clinical trials, little is known about why different Hsp90 inhibitors show specificity among Hsp90 paralogs. The paralog specificity of Hsp90 inhibitors is likely fundamental to inhibitor efficacy and side effects. In hopes of gaining insight into this issue we examined NECA (5'-N-ethylcarboxamidoadenosine), which has been claimed to be an example of a highly specific ligand that binds to one paralog, Grp94, but not cytosolic Hsp90. To our surprise we find that NECA inhibits many different Hsp90 proteins (Grp94, Hsp90α, Trap1, yeast Hsp82, bacterial HtpG). NMR experiments demonstrate that NECA can bind to the N-terminal domains of Grp94 and Hsp82. We use ATPase competition experiments to quantify the inhibitory power of NECA for different Hsp90 proteins. This scale: Hsp82 > Hsp90α > HtpG ≈ Grp94 > Trap1, ranks Grp94 as less sensitive to NECA inhibition. Because NECA is primarily used as an adenosine receptor agonist, our results also suggest that cell biological experiments utilizing NECA may have confounding effects from cytosolic Hsp90 inhibition.

Review: The HSP90 molecular chaperone-an enigmatic ATPase

The HSP90 molecular chaperone is involved in the activation and cellular stabilization of a range of 'client' proteins, of which oncogenic protein kinases and nuclear steroid hormone receptors are of particular biomedical significance. Work over the last two decades has revealed a conformational cycle critical to the biological function of HSP90, coupled to an inherent ATPase activity that is regulated and manipulated by many of the co-chaperones proteins with which it collaborates. Pharmacological inhibition of HSP90 ATPase activity results in degradation of client proteins in vivo, and is a promising target for development of new cancer therapeutics. Despite this, the actual function that HSP90s conformationally-coupled ATPase activity provides in its biological role as a molecular chaperone remains obscure. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 594-607, 2016.

A Human Variant of Glucose-Regulated Protein 94 That Inefficiently Supports IGF Production

IGFs are critical for normal intrauterine and childhood growth and sustaining health throughout life. We showed previously that the production of IGF-1 and IGF-2 requires interaction with the chaperone glucose-regulated protein 94 (GRP94) and that the amount of secreted IGFs is proportional to the GRP94 activity. Therefore, we tested the hypothesis that functional polymorphisms of human GRP94 affect IGF production and thereby human health. We describe a hypomorphic variant of human GRP94, P300L, whose heterozygous carriers have 9% lower circulating IGF-1 concentration. P300L was found first in a child with primary IGF deficiency and was later shown to be a noncommon single-nucleotide polymorphism with frequencies of 1%-4% in various populations. When tested in the grp94(-/-) cell-based complementation assay, P300L supported only approximately 58% of IGF secretion relative to wild-type GRP94. Furthermore, recombinant P300L showed impaired nucleotide binding activity. These in vitro data strongly support a causal relationship between the GRP94 variant and the decreased concentration of circulating IGF-1, as observed in human carriers of P300L. Thus, mutations in GRP94 that affect its IGF chaperone activity represent a novel causal genetic mechanism that limits IGF biosynthesis, quite a distinct mechanism from the known genes in the GH/IGF signaling network.

Vinclozolin alters the expression of hormonal and stress genes in the midge Chironomus riparius

Vinclozolin is a fungicide used in agriculture that can reach aquatic ecosystems and affect the organisms living there. Its effects have been intensively studied in vertebrates, where it acts as an antiandrogen, but there is a lack of information about its mechanistic effects on invertebrates. In this work, we analyzed the response of genes related to the endocrine system, the stress response, and the detoxification mechanisms of Chironomus riparius fourth instar larvae after 24h and 48h exposures to 20 (69.9nM), 200 (699nM), and 2000μg/L (6.99μM) of Vinclozolin. Survival analysis showed that this compound has low toxicity, as it was not lethal for this organism at the concentrations used. However, this fungicide was shown to modify the transcriptional activity of the ecdysone response pathway genes EcR, E74, and Kr-h1 by increasing their mRNA levels. While no changes were observed in disembodied, a gene related with the ecdysone synthesis metabolic pathway, Cyp18A1, which is involved in the inactivation of the active form of ecdysone, was upregulated. Additionally, the expression of two genes related to other hormones, FOXO and MAPR, did not show any changes when Vinclozolin was present. The analysis of stress response genes showed significant changes in the mRNA levels of Hsp70, Hsp24, and Gp93, indicating that Vinclozolin activates the cellular stress mechanisms. Finally, the expressions of the genes Cyp4G and GstD3, which encode enzymes involved in phase I and phase II detoxification, respectively, were analyzed. It was found that their mRNA levels were altered by Vinclozolin, suggesting their involvement in the degradation of this compound. For the first time, these results show evidence that Vinclozolin can modulate gene expression, leading to possible significant endocrine alterations of the insect endocrine system. These results also offer new clues about the mode of action of this compound in invertebrates.

Co- and Post-Translational Protein Folding in the ER

The biophysical rules that govern folding of small, single-domain proteins in dilute solutions are now quite well understood. The mechanisms underlying co-translational folding of multidomain and membrane-spanning proteins in complex cellular environments are often less clear. The endoplasmic reticulum (ER) produces a plethora of membrane and secretory proteins, which must fold and assemble correctly before ER exit - if these processes fail, misfolded species accumulate in the ER or are degraded. The ER differs from other cellular organelles in terms of the physicochemical environment and the variety of ER-specific protein modifications. Here, we review chaperone-assisted co- and post-translational folding and assembly in the ER and underline the influence of protein modifications on these processes. We emphasize how method development has helped advance the field by allowing researchers to monitor the progression of folding as it occurs inside living cells, while at the same time probing the intricate relationship between protein modifications during folding.

Development of Glucose Regulated Protein 94-Selective Inhibitors Based on the BnIm and Radamide Scaffold

Glucose regulated protein 94 (Grp94) is the endoplasmic reticulum resident of the heat shock protein 90 kDa (Hsp90) family of molecular chaperones. Grp94 associates with many proteins involved in cell adhesion and signaling, including integrins, Toll-like receptors, immunoglobulins, and mutant myocilin. Grp94 has been implicated as a target for several therapeutic areas including glaucoma, cancer metastasis, and multiple myeloma. While 85% identical to other Hsp90 isoforms, the N-terminal ATP-binding site of Grp94 possesses a unique hydrophobic pocket that was used to design isoform-selective inhibitors. Incorporation of a cis-amide bioisostere into the radamide scaffold led to development of the original Grp94-selective inhibitor, BnIm. Structure-activity relationship studies have now been performed on the aryl side chain of BnIm, which resulted in improved analogues that exhibit better potency and selectivity for Grp94. These analogues also manifest superior antimigratory activity in a metastasis model as well as enhanced mutant myocilin degradation in a glaucoma model compared to BnIm.

Characterization and comparison of proteomes of albino sea cucumber Apostichopus japonicus (Selenka) by iTRAQ analysis

Sea cucumber is a commercially important marine organism in China. Of the different colored varieties sold in China, albino sea cucumber has the greatest appeal among consumers. Identification of factors contributing to albinism in sea cucumber is therefore likely to provide a scientific basis for improving the cultivability of these strains. In this study, two-dimensional liquid chromatography-tandem mass spectrometry coupled with isobaric tags for relative and absolute quantification labeling was used for the first time to quantitatively define the proteome of sea cucumbers and reveal proteomic characteristics unique to albino sea cucumbers. A total of 549 proteins were identified and quantified in albino sea cucumber and the functional annotations of 485 proteins have been exhibited based on COG database. Compared with green sea cucumber, 12 proteins were identified as differentially expressed in the intestine and 16 proteins in the body wall of albino sea cucumber. Among them, 5 proteins were up-regulated in the intestine and 8 proteins were down-regulated in body wall. Gene ontology annotations of these differentially expressed proteins consisted mostly of 'biological process'. The large number of differentially expressed proteins identified here should be highly useful in further elucidating the mechanisms underlying albinism in sea cucumber.

Grp94 Protein Delivers γ-Aminobutyric Acid Type A (GABAA) Receptors to Hrd1 Protein-mediated Endoplasmic Reticulum-associated Degradation

Proteostasis maintenance of γ-aminobutyric acid type A (GABAA) receptors dictates their function in controlling neuronal inhibition in mammalian central nervous systems. However, as a multisubunit, multispan, integral membrane protein, even wild type subunits of GABAA receptors fold and assemble inefficiently in the endoplasmic reticulum (ER). Unassembled and misfolded subunits undergo ER-associated degradation (ERAD), but this degradation process remains poorly understood for GABAA receptors. Here, using the α1 subunits of GABAA receptors as a model substrate, we demonstrated that Grp94, a metazoan-specific Hsp90 in the ER lumen, uses its middle domain to interact with the α1 subunits and positively regulates their ERAD. OS-9, an ER-resident lectin, acts downstream of Grp94 to further recognize misfolded α1 subunits in a glycan-dependent manner. This delivers misfolded α1 subunits to the Hrd1-mediated ubiquitination and the valosin-containing protein-mediated extraction pathway. Repressing the initial ERAD recognition step by inhibiting Grp94 enhances the functional surface expression of misfolding-prone α1(A322D) subunits, which causes autosomal dominant juvenile myoclonic epilepsy. This study clarifies a Grp94-mediated ERAD pathway for GABAA receptors, which provides a novel way to finely tune their function in physiological and pathophysiological conditions.

Proinsulin and heat shock protein 90 as biomarkers of beta-cell stress in the early period after onset of type 1 diabetes

Rapid evaluation of therapies designed to preserve β cells in persons with type 1 diabetes (T1D) is hampered by limited availability of sensitive β-cell health biomarkers. In particular, biomarkers elucidating the presence and degree of β-cell stress are needed. We characterized β-cell secretory activity and stress in 29 new-onset T1D subjects (10.6 ± 3.0 years, 55% male) at diagnosis and then 8.2 ± 1.2 weeks later at first clinic follow-up. We did comparisons with 16 matched healthy controls. We evaluated hemoglobin A1c (HbA1c), β-cell function (random C-peptide [C] and proinsulin [PI]), β-cell stress (PI:C ratio), and the β-cell stress marker heat shock protein (HSP)90 and examined these parameters' relationships with clinical and laboratory characteristics at diagnosis. Mean diagnosis HbA1c was 11.3% (100 mmol/mol) and 7.6% (60 mmol/mol) at follow-up. C-peptide was low at diagnosis (P < 0.001 vs controls) and increased at follow-up (P < 0.001) to comparable with controls. PI did not differ from controls at diagnosis but increased at follow-up (P = 0.003) signifying increased release of PI alongside improved insulin secretion. PI:C ratios and HSP90 concentrations were elevated at both time points. Younger subjects had lower C-peptide and greater PI, PI:C, and HSP90. We also examined islets isolated from prediabetic nonobese diabetic mice and found that HSP90 levels were increased ∼4-fold compared with those in islets isolated from matched CD1 controls, further substantiating HSP90 as a marker of β-cell stress in T1D. Our data indicate that β-cell stress can be assessed using PI:C and HSP90. This stress persists after T1D diagnosis. Therapeutic approaches to reduce β-cell stress in new-onset T1D should be considered.

Limited Unfolded Protein Response and Inflammation in Neuroserpinopathy

Familial encephalopathy with neuroserpin inclusion bodies (FENIB) is a rare disease characterized by the deposition of multiple intracytoplasmic neuronal inclusions that contain mutated neuroserpin. Tg-Syracuse (Tg-Syr) mice express Ser49Pro mutated neuroserpin and develop clinical and neuropathological features of human FENIB. We used 8-, 34-, 45- and 80-week-old Tg-Syr mice to characterize neuroinflammation and the unfolded protein response (UPR) in a neurodegenerative disease in which abnormal protein aggregates accumulate within the endoplasmic reticulum (ER). There were scattered neuroserpin inclusions in Tg-Syr mice at 8 weeks of age; the numbers of neurons involved and the amount of neuroserpin per neuron increased with age throughout the CNS to 80 weeks of age; no similar inclusions were found in wild type (Tg-WT) mice at any age. Increases in numbers of astrocytes and microglia occurred at advanced disease stages. Among 22 markers in 80-week-old Tg-Syr mice, only II1b and II10rb mRNAs in the somatosensory cortex and CxCl10 and Il10rb mRNAs in the olfactory bulb were upregulated when compared with Tg-WT mice indicating a limited relationship between neuroserpin inclusions and inflammatory responses. The changes were accompanied by a transient increase in expression of Xbp1 spliced at 45 weeks and increased ERdJ4 mRNAs at 80 weeks. The sequestration of UPR activators GRP78 and GRP94 in neuroserpin inclusions might explain the limited UPR responses despite the accumulation of neuroserpin in the ER in this FENIB mouse model.

Chaperones in hepatitis C virus infection

The hepatitis C virus (HCV) infects approximately 3% of the world population or more than 185 million people worldwide. Each year, an estimated 350000-500000 deaths occur worldwide due to HCV-associated diseases including cirrhosis and hepatocellular carcinoma. HCV is the most common indication for liver transplantation in patients with cirrhosis worldwide. HCV is an enveloped RNA virus classified in the genus Hepacivirus in the Flaviviridae family. The HCV viral life cycle in a cell can be divided into six phases: (1) binding and internalization; (2) cytoplasmic release and uncoating; (3) viral polyprotein translation and processing; (4) RNA genome replication; (5) encapsidation (packaging) and assembly; and (6) virus morphogenesis (maturation) and secretion. Many host factors are involved in the HCV life cycle. Chaperones are an important group of host cytoprotective molecules that coordinate numerous cellular processes including protein folding, multimeric protein assembly, protein trafficking, and protein degradation. All phases of the viral life cycle require chaperone activity and the interaction of viral proteins with chaperones. This review will present our current knowledge and understanding of the role of chaperones in the HCV life cycle. Analysis of chaperones in HCV infection will provide further insights into viral/host interactions and potential therapeutic targets for both HCV and other viruses.

Altered machinery of protein synthesis is region- and stage-dependent and is associated with α-synuclein oligomers in Parkinson's disease

INTRODUCTION

Parkinson's disease (PD) is characterized by the accumulation of abnormal α-synuclein in selected regions of the brain following a gradient of severity with disease progression. Whether this is accompanied by globally altered protein synthesis is poorly documented. The present study was carried out in PD stages 1-6 of Braak and middle-aged (MA) individuals without alterations in brain in the substantia nigra, frontal cortex area 8, angular gyrus, precuneus and putamen.

RESULTS

Reduced mRNA expression of nucleolar proteins nucleolin (NCL), nucleophosmin (NPM1), nucleoplasmin 3 (NPM3) and upstream binding transcription factor (UBF), decreased NPM1 but not NPM3 nucleolar protein immunostaining in remaining neurons; diminished 18S rRNA, 28S rRNA; reduced expression of several mRNAs encoding ribosomal protein (RP) subunits; and altered protein levels of initiation factor eIF3 and elongation factor eEF2 of protein synthesis was found in the substantia nigra in PD along with disease progression. Although many of these changes can be related to neuron loss in the substantia nigra, selective alteration of certain factors indicates variable degree of vulnerability of mRNAs, rRNAs and proteins in degenerating sustantia nigra. NPM1 mRNA and 18S rRNA was increased in the frontal cortex area 8 at stage 5-6; modifications were less marked and region-dependent in the angular gyrus and precuneus. Several RPs were abnormally regulated in the frontal cortex area 8 and precuneus, but only one RP in the angular gyrus, in PD. Altered levels of eIF3 and eIF1, and decrease eEF1A and eEF2 protein levels were observed in the frontal cortex in PD. No modifications were found in the putamen at any time of the study except transient modifications in 28S rRNA and only one RP mRNA at stages 5-6. These observations further indicate marked region-dependent and stage-dependent alterations in the cerebral cortex in PD. Altered solubility and α-synuclein oligomer formation, assessed in total homogenate fractions blotted with anti-α-synuclein oligomer-specific antibody, was demonstrated in the substantia nigra and frontal cortex, but not in the putamen, in PD. Dramatic increase in α-synuclein oligomers was also seen in fluorescent-activated cell sorter (FACS)-isolated nuclei in the frontal cortex in PD.

CONCLUSIONS

Altered machinery of protein synthesis is altered in the substantia nigra and cerebral cortex in PD being the frontal cortex area 8 more affected than the angular gyrus and precuneus; in contrast, pathways of protein synthesis are apparently preserved in the putamen. This is associated with the presence of α-synuclein oligomeric species in total homogenates; substantia nigra and frontal cortex are enriched, albeit with different band patterns, in α-synuclein oligomeric species, whereas α-synuclein oligomers are not detected in the putamen.

GRP94 Regulates Circulating Cholesterol Levels through Blockade of PCSK9-Induced LDLR Degradation

Clearance of circulating low-density lipoprotein cholesterol (LDLc) by hepatic LDL receptors (LDLR) is central for vascular health. Secreted by hepatocytes, PCSK9 induces the degradation of LDLR, resulting in higher plasma LDLc levels. Still, it remains unknown why LDLR and PCSK9 co-exist within the secretory pathway of hepatocytes without leading to complete degradation of LDLR. Herein, we identified the ER-resident GRP94, and more precisely its client-binding C-terminal domain, as a PCSK9-LDLR inhibitory binding protein. Depletion of GRP94 did not affect calcium homeostasis, induce ER stress, nor did it alter PCSK9 processing or its secretion but greatly increased its capacity to induce LDLR degradation. Accordingly, we found that hepatocyte-specific Grp94-deficient mice have higher plasma LDLc levels correlated with ∼ 80% reduction in hepatic LDLR protein levels. Thus, we provide evidence that, in physiological conditions, binding of PCSK9 to GRP94 protects LDLR from degradation likely by preventing early binding of PCSK9 to LDLR within the ER.

Intracellular antigens as targets for antibody based immunotherapy of malignant diseases

This review discusses the potential use of intracellular tumor antigens as targets of antibody-based immunotherapy for the treatment of solid tumors. In addition, it describes the characteristics of the intracellular tumor antigens targeted with antibodies which have been described in the literature and have been identified in the authors' laboratory. Finally, the mechanism underlying the trafficking of the intracellular tumor antigens to the plasma membrane of tumor cells are reviewed.

TRNA mutations that affect decoding fidelity deregulate development and the proteostasis network in zebrafish

Mutations in genes that encode tRNAs, aminoacyl-tRNA syntheases, tRNA modifying enzymes and other tRNA interacting partners are associated with neuropathies, cancer, type-II diabetes and hearing loss, but how these mutations cause disease is unclear. We have hypothesized that levels of tRNA decoding error (mistranslation) that do not fully impair embryonic development can accelerate cell degeneration through proteome instability and saturation of the proteostasis network. To test this hypothesis we have induced mistranslation in zebrafish embryos using mutant tRNAs that misincorporate Serine (Ser) at various non-cognate codon sites. Embryo viability was affected and malformations were observed, but a significant proportion of embryos survived by activating the unfolded protein response (UPR), the ubiquitin proteasome pathway (UPP) and downregulating protein biosynthesis. Accumulation of reactive oxygen species (ROS), mitochondrial and nuclear DNA damage and disruption of the mitochondrial network, were also observed, suggesting that mistranslation had a strong negative impact on protein synthesis rate, ER and mitochondrial homeostasis. We postulate that mistranslation promotes gradual cellular degeneration and disease through protein aggregation, mitochondrial dysfunction and genome instability.

The effects of induced production of reactive oxygen species in organelles on endoplasmic reticulum stress and on the unfolded protein response in arabidopsis

BACKGROUND AND AIMS

Accumulation of unfolded proteins caused by inefficient chaperone activity in the endoplasmic reticulum (ER) is termed 'ER stress', and it is perceived by a complex gene network. Induction of these genes triggers a response termed the 'unfolded protein response' (UPR). If a cell cannot overcome the accumulation of unfolded proteins, the ER-associated degradation (ERAD) system is induced to degrade those proteins. In addition to other factors, reactive oxygen species (ROS) are also produced during oxidative protein-folding in the ER. It has been shown in animal systems that there is a tight association between mitochondrial ROS and ER stress. However, in plants there are no reports concerning how induced ROS production in mitochondria and chloroplasts affects ER stress and if there is a possible role of organelle-originated ROS as a messenger molecule in the unfolded protein response. To address this issue, electron transport in chloroplasts and mitochondria and carnitine acetyl transferase (CAT) activity in peroxisomes were inhibited in wild-type Arabidopsis thaliana to induce ROS production. Expression of UPR genes was then investigated.

METHODS

Plants of A. thaliana ecotype Col-0 were treated with various H2O2- and ROS-producing agents specific to different organelles, including the mitochondria, chloroplasts and peroxisomes. The expression of ER stress sensor/transducer genes (bZIP28, bZIP17, IRE1A, IRE1B, BiP1, BiP3), genes related to protein folding (CNX, ERO1) and ERAD genes (HRD1, SEL1, DER1, UBC32) were evaluated by qRT-PCR analysis.

KEY RESULTS

Relatively low concentrations of ROS were more effective for induction of the ER stress response. Mitochondrial and chloroplastic ROS production had different induction mechanisms for the UPR and ER stress responses.

CONCLUSIONS

Chloroplast- and mitochondria-originated ROS have distinct roles in triggering the ER stress response. In general, low concentrations of ROS induced the transcription of ER stress-related genes, which can be attributed to the roles of ROS as secondary messengers. This is the first time that ROS production in organelles has been shown to affect the ER stress response in a plant system.

Targeting the ER-autophagy system in the trabecular meshwork to treat glaucoma

A major drainage network involved in aqueous humor dynamics is the conventional outflow pathway, which is gated by the trabecular meshwork (TM). The TM acts as a molecular sieve, providing resistance to aqueous outflow, which is responsible for regulating intraocular pressure (IOP). If the TM is damaged, aqueous outflow is impaired, IOP increases and glaucoma can manifest. Mutations in the MYOC gene cause hereditary primary open-angle glaucoma (POAG) by promoting the abnormal amyloidosis of the myocilin protein in the endoplasmic reticulum (ER), leading to ER stress-induced TM cell death. Myocilin accumulation is observed in approximately 70-80% of all glaucoma cases suggesting that environmental or other genetic factors may also promote myocilin toxicity. For example, simply preventing myocilin glycosylation is sufficient to promote its abnormal accretion. These myocilin amyloids are unique as there are no other known pathogenic proteins that accumulate within the ER of TM cells and cause toxicity. Moreover, this pathogenic accumulation only kills TM cells, despite expression of this protein in other cell types, suggesting that another modifier exclusive to the TM participates in the proteotoxicity of myocilin. ER autophagy (reticulophagy) is one of the pathways essential for myocilin clearance that can be impacted dramatically by aging and other environmental factors such as nutrition. This review will discuss the link between myocilin and autophagy, evaluating the role of this degradation pathway in glaucoma as well as its potential as a therapeutic target.

A sweet code for glycoprotein folding

Glycoprotein synthesis is initiated in the endoplasmic reticulum (ER) lumen upon transfer of a glycan (Glc3Man9GlcNAc2) from a lipid derivative to Asn residues (N-glycosylation). N-Glycan-dependent quality control of glycoprotein folding in the ER prevents exit to Golgi of folding intermediates, irreparably misfolded glycoproteins and incompletely assembled multimeric complexes. It also enhances folding efficiency by preventing aggregation and facilitating formation of proper disulfide bonds. The control mechanism essentially involves four components, resident lectin-chaperones (calnexin and calreticulin) that recognize monoglucosylated polymannose protein-linked glycans, lectin-associated oxidoreductase acting on monoglucosylated glycoproteins (ERp57), a glucosyltransferase that creates monoglucosylated epitopes in protein-linked glycans (UGGT) and a glucosidase (GII) that removes the glucose units added by UGGT. This last enzyme is the only mechanism component sensing glycoprotein conformations as it creates monoglucosylated glycans exclusively in not properly folded glycoproteins or in not completely assembled multimeric glycoprotein complexes. Glycoproteins that fail to properly fold are eventually driven to proteasomal degradation in the cytosol following the ER-associated degradation pathway, in which the extent of N-glycan demannosylation by ER mannosidases play a relevant role in the identification of irreparably misfolded glycoproteins.

Large-Scale Measurement of Absolute Protein Glycosylation Stoichiometry

Protein glycosylation is one of the most important protein modifications. Glycosylation site occupancy alteration has been implicated in human diseases and cancers. However, current glycoproteomic methods focus on the identification and quantification of glycosylated peptides and glycosylation sites but not glycosylation occupancy or glycoform stoichiometry. Here we describe a method for large-scale determination of the absolute glycosylation stoichiometry using three independent relative ratios. Using this method, we determined 117 absolute N-glycosylation occupancies in OVCAR-3 cells. Finally, we investigated the possible functions and the determinants for partial glycosylation.

Altered protein expression pattern in skin fibroblasts from parkin-mutant early-onset Parkinson's disease patients

Parkinson's disease (PD) is the most common neurodegenerative movement disorder caused primarily by selective degeneration of the dopaminergic neurons in substantia nigra. In this work the proteomes extracted from primary fibroblasts of two unrelated, hereditary cases of PD patients, with different parkin mutations, were compared with the proteomes extracted from commercial adult normal human dermal fibroblasts (NHDF) and primary fibroblasts from the healthy mother of one of the two patients. The results show that the fibroblasts from the two different cases of parkin-mutant patients display analogous alterations in the expression level of proteins involved in different cellular functions, like cytoskeleton structure-dynamics, calcium homeostasis, oxidative stress response, protein and RNA processing.

Role of the unfolded protein response, GRP78 and GRP94 in organ homeostasis

The endoplasmic reticulum (ER) is a cellular organelle where secretory and membrane proteins, as well as lipids, are synthesized and modified. When cells are subjected to ER stress, an adaptive mechanism referred to as the Unfolded Protein Response (UPR) is triggered to allow the cells to restore homeostasis. Evidence has accumulated that the UPR pathways provide specialized and unique roles in diverse development and metabolic processes. The glucose regulated proteins (GRPs) are traditionally regarded as ER proteins with chaperone and calcium binding properties. The GRPs are constitutively expressed at basal levels in all organs, and as stress-inducible ER chaperones, they are major players in protein folding, assembly and degradation. This conventional concept is augmented by recent discoveries that GRPs can be actively translocated to other cellular locations such as the cell surface, where they assume novel functions that regulate signaling, proliferation, apoptosis and immunity. Recent construction and characterization of mouse models where the gene encoding for the UPR components and the GRPs is genetically altered provide new insights on the physiological contribution of these proteins in vivo. This review highlights recent progress towards the understanding of the role of the UPR and two major GRPs (GRP78 and GRP94) in regulating homeostasis of organs arising from the endoderm, mesoderm and ectoderm. GRP78 and GRP94 exhibit shared and unique functions, and in specific organs their depletion elicits adaptive responses with physiological consequences.

Hurt, tired and queasy: Specific variants in the ATPase domain of the TRAP1 mitochondrial chaperone are associated with common, chronic "functional" symptomatology including pain, fatigue and gastrointestinal dysmotility

Functional disorders are common conditions with a substantial impact on a patients' wellbeing, and can be diagnostically elusive. There are bidirectional associations between functional disorders and mitochondrial dysfunction. In this study, provided clinical information and the exon sequence of the TRAP1 mitochondrial chaperone were retrospectively reviewed with a focus on the functional categories of chronic pain, fatigue and gastrointestinal dysmotility. Very-highly conserved TRAP1 variants were identified in 73 of 930 unrelated patients. Functional symptomatology is strongly associated with specific variants in the ATPase binding pocket. In particular, the combined presence of all three functional categories is strongly associated with p.Ile253Val (OR 7.5, P = 0.0001) and with two other interacting variants (OR 18, P = 0.0005). Considering a 1-2% combined variant prevalence and high odds ratios, these variants may be an important factor in the etiology of functional symptomatology.

Structure-activity relationship in a purine-scaffold compound series with selectivity for the endoplasmic reticulum Hsp90 paralog Grp94

Grp94 is involved in the regulation of a restricted number of proteins and represents a potential target in a host of diseases, including cancer, septic shock, autoimmune diseases, chronic inflammatory conditions, diabetes, coronary thrombosis, and stroke. We have recently identified a novel allosteric pocket located in the Grp94 N-terminal binding site that can be used to design ligands with a 2-log selectivity over the other Hsp90 paralogs. Here we perform extensive SAR investigations in this ligand series and rationalize the affinity and paralog selectivity of choice derivatives by molecular modeling. We then use this to design 18c, a derivative with good potency for Grp94 (IC50 = 0.22 μM) and selectivity over other paralogs (>100- and 33-fold for Hsp90α/β and Trap-1, respectively). The paralog selectivity and target-mediated activity of 18c was confirmed in cells through several functional readouts. Compound 18c was also inert when tested against a large panel of kinases. We show that 18c has biological activity in several cellular models of inflammation and cancer and also present here for the first time the in vivo profile of a Grp94 inhibitor.

Hypothermic Preconditioning of Human Cortical Neurons Requires Proteostatic Priming

Hypothermia is potently neuroprotective but poor mechanistic understanding has restricted its clinical use. Rodent studies indicate that hypothermia can elicit preconditioning, wherein a subtoxic cellular stress confers resistance to an otherwise lethal injury. The molecular basis of this preconditioning remains obscure. Here we explore molecular effects of cooling using functional cortical neurons differentiated from human pluripotent stem cells (hCNs). Mild-to-moderate hypothermia (28–32 °C) induces cold-shock protein expression and mild endoplasmic reticulum (ER) stress in hCNs, with full activation of the unfolded protein response (UPR). Chemical block of a principal UPR pathway mitigates the protective effect of cooling against oxidative stress, whilst pre-cooling neurons abrogates the toxic injury produced by the ER stressor tunicamycin. Cold-stress thus preconditions neurons by upregulating adaptive chaperone-driven pathways of the UPR in a manner that precipitates ER-hormesis. Our findings establish a novel arm of neurocryobiology that could reveal multiple therapeutic targets for acute and chronic neuronal injury.

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Clinically-relevant cooling induces archetypal cold-shock and mild endoplasmic reticulum (ER) stress in human neurons.

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Hypothermic neuronal ER-stress elicits an adaptive unfolded protein response (UPR) with ER-hormesis.

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Hypothermic preconditioning of the ER provides cross-tolerance to oxidative neuronal injury and requires an intact UPR.

Hepatic Overexpression of GRP94 in a Rabbit Model of Parenteral Nutrition-Associated Liver Disease

Objective. To use a rabbit model of parenteral nutrition-associated liver disease (PNALD) to study changes of the endoplasmic reticulum stress (ERS) marker glucose regulatory protein 94 (GRP94) and determine its role in the pathogenesis of PNALD. Methods. A rabbit PNALD model total parenteral nutrition (TPN) group was established. A corresponding control group received breast-feeding for one week. Serum biochemical parameters were measured and liver histological examinations were performed. The level of GRP94 mRNA and protein were measured. Results. The results showed that the serum TBIL, DBIL, and γ-GT levels in the TPN group were significantly higher than those in the control group, while levels of serum ALB in TPN group were significantly lower than those in the control group. The immunohistochemistry results showed that the protein expression level of GRP94 in the liver of TPN group was significantly increased compared with the control group. The RT-PCR results showed that the level of GRP94 mRNA in the liver of the TPN group was significantly higher compared with the control group. Conclusions. The mRNA and protein levels of GRP94 in the TPN group were both significantly increased, indicating that ERS may be directly related to the occurrence and development of PNALD.

Plasma membrane gp96 enhances invasion and metastatic potential of liver cancer via regulation of uPAR

Targeted therapy is currently under intensive investigation due to the resistance of liver cancer to cytotoxic chemotherapies. Dissecting the molecular events that drive the progression of liver cancer and defining specific targets are urgently needed to develop efficient tailored therapies. Cell membrane gp96 (mgp96) has been implicated in tumor growth and malignancy. Here, we explored the functional and clinical relevance of mgp96 in liver cancer. We found that elevated mgp96 abundance was associated with tumor metastasis and recurrence in patients with primary liver tumors. Decreased KDELR1 levels in hepatoma cells contribute to cell membrane translocation of the normally ER-resident gp96. Urokinase-type plasminogen activator receptor (uPAR) was identified as a mgp96 client protein, and mgp96 stabilized uPAR protein. Our clinical results proved that elevated mgp96 abundance is positively correlated with uPAR expression levels in liver tumors. We further provided evidence that targeting mgp96 with siRNA or a specific mAb that blocked the mgp96-uPAR interaction led to inhibited cell growth, survival, and invasion in vitro, as well as the suppression of liver tumor growth and metastasis in vivo. mgp96 promotes liver cancer progression through increasing the protein stability and signaling of uPAR, and may be a new promising target for suppressing uPAR-mediated tumor growth and metastasis in liver cancer.

Reaction design, discovery, and development as a foundation to function-oriented synthesis

Convergent C-C bond-forming reactions define the fabric of organic synthesis and, when applied in complex molecule synthesis, can have a profound impact on efficiency by decreasing the longest linear sequence of transformations required to convert simple starting materials to complex targets. Despite their well-appreciated strategic significance, campaigns in natural product synthesis typically embrace only a small suite of reactivity to achieve such bond construction (i.e., nucleophilic addition to polarized π-bonds, nucleophilic substitution, cycloaddition, and metal-catalyzed "cross-coupling"), therefore limiting the sites at which convergent coupling chemistry can be strategically employed. In our opinion, it is far too often that triumphs in the field are defined by chemical sequences that do not address the challenges associated with discovery, development, and production of natural product-inspired agents. We speculated that advancing an area of chemical reactivity not represented in the few well-established strategies for convergent C-C bond formation may lead to powerful new retrosynthetic relationships that could simplify approaches to the syntheses of a variety of different classes of natural products. Our studies ultimately embraced the pursuit of strategies to control the course of metallacycle-mediated "cross-coupling" between substrates containing sites of simple π-unsaturation (ubiquitous functionality in organic chemistry including alkenes, alkynes, allenes, aldehydes, and imines, among others). In just eight years since our initial publication in this area, we have defined over 20 stereoselective intermolecular C-C bond-forming reactions that provide access to structural motifs of relevance for the synthesis of polyketides, fatty acids, alkaloids, and terpenes, while doing so in a direct and stereoselective fashion. These achievements continue to serve as the foundation of my group's activity in natural product and function-oriented synthesis, where our achievements in reaction development are challenged in the context of complex targets. Among our early efforts, we achieved the most concise synthesis of a benzoquinone ansamycin ever described (macbecin I), and moved beyond this achievement to explore the role of our chemistry in function-oriented synthesis targeting the discovery of natural product-inspired Hsp90 inhibitors. These later efforts have led to the discovery of a uniquely selective benzoquinone ansamycin-inspired Hsp90 inhibitor that lacks the problematic quinone present in the natural series. This achievement was made possible by a concise chemical synthesis pathway that had at its core the application of metallacycle-mediated cross-coupling chemistry.

Dysregulated Ca2+ homeostasis in Fanconi anemia cells

Fanconi Anemia (FA) is a rare and complex inherited blood disorder associated with bone marrow failure and malignancies. Many alterations in FA physiology appear linked to red-ox unbalance including alterations in the morphology and structure of nuclei, intermediate filaments and mitochondria, defective respiration, reduced ATP production and altered ATP/AMP ratio. These defects are consistently associated with impaired oxygen metabolism indeed treatment with antioxidants N-acetylcysteine (NAC) and resveratrol (RV) does rescue FA physiology. Due to the importance of the intracellular calcium signaling and its key function in the control of intracellular functions we were interested to study calcium homeostasis in FA. We found that FANCA cells display a dramatically low intracellular calcium concentration ([Ca²⁺]_i) in resting conditions. This condition affects cellular responses to stress. The flux of Ca²⁺ mobilized by H₂O₂ from internal stores is significantly lower in FANCA cells in comparison to controls. The low basal [Ca²⁺]_i in FANCA appears to be an actively maintained process controlled by a finely tuned interplay between different intracellular Ca²⁺ stores. The defects associated with the altered Ca²⁺ homeostasis appear consistently overlapping those related to the unbalanced oxidative metabolism in FA cells underlining a contiguity between oxidative stress and calcium homeostasis.

A highly charged region in the middle domain of plant endoplasmic reticulum (ER)-localized heat-shock protein 90 is required for resistance to tunicamycin or high calcium-induced ER stresses

Heat-shock protein 90 (HSP90) is a highly conserved molecular chaperone that is involved in modulating a multitude of cellular processes under both physiological and stress conditions. In Arabidopsis, there are seven HSP90 isoforms (HSP90.1-HSP90.7) that are localized in the cytoplasm/nucleus, mitochondrion, chloroplast, and endoplasmic reticulum (ER) where protein folding actively takes place. In this study, we analysed the sequence of ER-localized Arabidopsis HSP90.7 and the other ER GRP94 proteins from plants and animals, and identified a short, charged region that is specifically present in the middle domain of plant-derived GRP94 proteins. To understand the role of this charged region, we analysed transgenic plants that expressed a mutant protein, HSP90.7(Δ22), which had this charged region deleted. We showed that seedlings expressing HSP90.7(Δ22) had significantly enhanced sensitivity to ER stress induced by tunicamycin or a high concentration of calcium, although its general chaperone activity in preventing the model protein from heat-induced aggregation was not significantly affected. We also analysed the ATP-binding and hydrolysis activity of both wild-type and mutant HSP90.7 proteins, and found that they had slightly different ATP-binding affinities. Finally, using a yeast two-hybrid screen, we identified a small set of HSP90.7 interactors and showed that the charged region is not required for the candidate client interaction, although it may affect their binding affinity, thus providing potential targets for further investigation of HSP90.7 functions.

GRP94 is encoded by two differentially expressed genes during development of rainbow trout (Oncorhynchus mykiss)

The glucose-regulated protein, 94 kDa (GRP94), is an endoplasmic reticulum (ER)-localized heat shock protein that plays among other functions a crucial role in folding and exports of Toll-like receptors (TLRs) and some other immune-relevant factors. We identified two copies of the GRP94-encoding gene in rainbow trout sharing 91% DNA sequence identity. The conceptually translated ORFs encode a 795-aa GRP94a and a 510-aa GRP94b protein variant, respectively, with characteristic domains and amino acid residues. However, the shorter variant lacks motifs required for its localization in the ER and might thus represent an isoform of the putative mammalian ortholog GRP94a. Heat stress only slightly affects the expression of the two GRP94-encoding trout genes, as reported for mammals. We recorded the abundances of transcripts coding for both GRP94 variants as well as for a broad panel of TLRs representing their potential targets. In embryonic and larval trout, only the mRNAs encoding TLR1, -2, -9, and -20 were found in significant concentrations, while the expression of nine other TLRs was hardly detectable. The GRP94a-encoding gene showed constantly high expression levels indicating that this isoform is vitally required throughout the life cycle of rainbow trout. The concentration of the GRP94b-encoding mRNA was only ~0.1% compared to the GRP94a mRNA level. These structural and gene expression data together suggest that the two GRP94 gene products fulfill different physiological roles.

Parenteral nutrition-associated liver injury and increased GRP94 expression prevented by ω-3 fish oil-based lipid emulsion supplementation

OBJECTIVE

Parenteral nutrition in infants with gastrointestinal disorders can be lifesaving, but it is also associated with parenteral nutrition-associated liver disease. We investigated the effects of incorporating ω-3 fish oil in a parenteral nutrition mixture on signs of parenteral nutrition-associated liver disease and explored the mechanism involved in this process.

METHODS

Seven-day-old New Zealand rabbits were divided into 3 groups of 8, and for 1 week they were infused via the right jugular vein with standard total parenteral nutrition with soybean oil (TPN-soy) or TPN with ω-3 fish oil-based lipid emulsion (TPN-FO), or naturally nursed with rabbit milk (control). Serum and liver tissues were analyzed for serological indicators and pathology, respectively. Reverse-transcriptase polymerase chain reaction was used to evaluate the messenger RNA levels of the endoplasmic reticulum stress chaperone protein glucose-regulated protein 94 (GRP94) in liver tissues and GRP94 protein levels were compared through immunohistochemistry and Western blot assays.

RESULTS

TPN-soy animals had significantly higher serum total bilirubin, direct bilirubin, and γ-glutamyl transpeptidase and lower serum albumin than the controls (P < 0.01, each) or the TPN-FO group, which were similar to the controls (P < 0.01 cf. TPN). Damage to liver tissues of the TPN-FO group was much less than that of the TPN-soy group. GRP94 messenger RNA and protein levels in liver tissues of TPN-soy animals were significantly higher than that of the controls or TPN-FO rabbits, which were similar to the controls.

CONCLUSIONS

Incorporating ω-3 fish oil in parenteral nutrition emulsion greatly prevented liver dysfunction and liver tissue damage in week-old rabbit kits, possibly by preventing endoplasmic reticulum stress.

Endoplasmic reticulum proteins quality control and the unfolded protein response: the regulative mechanism of organisms against stress injuries

The endoplasmic reticulum is the cellular compartment in which secretory proteins are synthesized and folded. Perturbations of endoplasmic reticulum homeostasis lead to the accumulation of unfolded proteins. The activation of the unfolded protein response during endoplasmic reticulum stress transmits information about the status of protein folding to the cytosol and nucleus. The unfolded protein response leads to the upregulation of genes encoding endoplasmic reticulum chaperones, attenuation of translation, and initiation of the endoplasmic reticulum quality control system to restore endoplasmic reticulum homeostasis. When the unfolded protein response is insufficient to rebuild the steady state in endoplasmic reticulum, the programmed cell death or apoptosis would be initiated, by triggering cell injuries, even to cell death through apoptosis signals. In this review, we briefly outline research on the chaperones and foldases conserved in eukaryotes and plants, and describe the general principles and mechanisms of the endoplasmic reticulum quality control and the unfolded protein response. We describe the current models for the molecular mechanism of the unfolded protein response in plants, and emphasize the role of inositol requiring enzyme-1-dependent network in the unfolded protein response. Finally, we give a general overview of the directions for future research on the unfolded protein response in plants and its role in the response to environmental stresses.

Endoplasmic reticulum chaperones and oxidoreductases: critical regulators of tumor cell survival and immunorecognition

Endoplasmic reticulum (ER) chaperones and oxidoreductases are abundant enzymes that mediate the production of fully folded secretory and transmembrane proteins. Resisting the Golgi and plasma membrane-directed “bulk flow,” ER chaperones and oxidoreductases enter retrograde trafficking whenever they are pulled outside of the ER by their substrates. Solid tumors are characterized by the increased production of reactive oxygen species (ROS), combined with reduced blood flow that leads to low oxygen supply and ER stress. Under these conditions, hypoxia and the unfolded protein response upregulate their target genes. When this occurs, ER oxidoreductases and chaperones become important regulators of tumor growth. However, under these conditions, these proteins not only promote the folding of proteins, but also alter the properties of the plasma membrane and hence modulate tumor immune recognition. For instance, high levels of calreticulin serve as an “eat-me” signal on the surface of tumor cells. Conversely, both intracellular and surface BiP/GRP78 promotes tumor growth. Other ER folding assistants able to modulate the properties of tumor tissue include protein disulfide isomerase (PDI), Ero1α and GRP94. Understanding the roles and mechanisms of ER chaperones in regulating tumor cell functions and immunorecognition will lead to important insight for the development of novel cancer therapies.

Comparative effects of SNX-7081 and SNX-2112 on cell cycle, apoptosis and Hsp90 client proteins in human cancer cells

SNX-2112, a novel 2-aminobenzamide inhibitor of Hsp90, previously showed a broad spectrum of anticancer activity. However, subsequent development has been discontinued due to ocular toxicity as identified in a phase I study. SNX-7081, another closely related Hsp90 inhibitor with a side chain of indole instead of indazole, has recently attracted attention. The aim of the present study was to investigate the anticancer effects of SNX-7081 in eleven cell lines, as well as the mechanisms involved, with SNX-2112 serving as a reference. The cytotoxic effects were determined using an MTT assay and apoptosis was measured using flow cytometry. The results showed that SNX-7081 exerted better inhibitory effects than SNX-2112 in six eighths of the human cancer cell lines, with an average IC50 of 1 µM. The two inhibitors exerted low cytotoxicity in L-02, HDF and MRC5 normal human cells (IC50 >50 µM), and arrested cancer cells at the G2/M phase in a similar manner to normal cells. Compared with SNX-2112, SNX-7081 exhibited more potent effects on cell apoptosis in four sixths of the human cancer cell lines, and was more active in the downregulation of Hsp90 client proteins. In addition, SNX-7081 exhibited a stronger binding affinity to Hsp90 than SNX-2112 in molecular docking experiments. Considering the superior effects against Hsp90 affinity, cell growth, apoptosis, and Hsp90 client proteins in a majority of human cancer cells, the novel SNX-7081 may be a promising alternative to SNX-2112, which merits further evaluation.

Cellular mechanisms of endoplasmic reticulum stress signaling in health and disease. 2. Protein misfolding and ER stress

The endoplasmic reticulum (ER) is a major site of protein synthesis, most strikingly in the specialized secretory cells of metazoans, which can produce their own weight in proteins daily. Cells possess a diverse machinery to ensure correct folding, assembly, and secretion of proteins from the ER. When this machinery is overwhelmed, the cell is said to experience ER stress, a result of the accumulation of unfolded or misfolded proteins in the lumen of the organelle. Here we discuss the causes of ER stress and the mechanisms by which cells elicit a response, with an emphasis on recent discoveries.

Selective targeting of the stress chaperome as a therapeutic strategy

Normal cellular function is maintained by coordinated proteome machinery that performs a vast array of activities. Helping the proteome in such roles is the chaperome, a network of molecular chaperones and folding enzymes. The stressed cell contains, at any time, a complex mixture of chaperome complexes; a majority performs 'housekeeping functions' similarly to non-stressed, normal cells, but a finely-tuned fraction buffers the proteome altered by chronic stress. The stress chaperome is epigenetically distinct from its normal, housekeeping counterpart, providing a basis for its selective targeting by small molecules. We discuss here the development of chaperome inhibitors, and how agents targeting chaperome members in stressed cells are in fact being directed towards chaperome complexes, and their effect is therefore determined by their ability to sample and engage such complexes. A new approach is needed to target and implement chaperome modulators in the investigation of diseases, and we propose that the classical thinking in drug discovery needs adjustment when developing chaperome-targeting drugs.