Heat shock response relieves ER stress

Unfolded protein responses with or without unfolded proteins?

The endoplasmic reticulum (ER) is the site of secretory protein biogenesis. The ER quality control (QC) machinery, including chaperones, ensures the correct folding of secretory proteins. Mutant proteins and environmental stresses can overwhelm the available QC machinery. To prevent and resolve accumulation of misfolded secretory proteins in the ER, cells have evolved integral membrane sensors that orchestrate the Unfolded Protein Response (UPR). The sensors, Ire1p in yeast and IRE1, ATF6, and PERK in metazoans, bind the luminal ER chaperone BiP during homeostasis. As unfolded secretory proteins accumulate in the ER lumen, BiP releases, and the sensors activate. The mechanisms of activation and attenuation of the UPR sensors have exhibited unexpected complexity. A growing body of data supports a model in which Ire1p, and potentially IRE1, directly bind unfolded proteins as part of the activation process. However, evidence for an unfolded protein-independent mechanism has recently emerged, suggesting that UPR can be activated by multiple modes. Importantly, dysregulation of the UPR has been linked to human diseases including Type II diabetes, heart disease, and cancer. The existence of alternative regulatory pathways for UPR sensors raises the exciting possibility for the development of new classes of therapeutics for these medically important proteins.

Transcriptomics of coping strategies in free-swimming Lepeophtheirus salmonis (Copepoda) larvae responding to abiotic stress

The salmon louse Lepeophtheirus salmonis is a marine ectoparasite of wild and farmed salmon in the Northern Hemisphere. Infections of farmed salmon are of economic and ecological concern. Nauplius and copepodid salmon lice larvae are free-swimming and disperse in the water column until they encounter a host. In this study, we characterized the sublethal stress responses of L. salmonis copepodid larvae by applying a 38K oligonucleotide microarray to profile transcriptomes following 24 h exposures to suboptimal salinity (30–10 parts per thousand (‰)) or temperature (16–4 °C) environments. Hyposalinity exposure resulted in large-scale gene expression changes relative to those elicited by a thermal gradient. Subsequently, transcriptome responses to a more finely resolved salinity gradient between 30 ‰ and 25 ‰ were profiled. Minimal changes occurred at 29 ‰ or 28 ‰, a threshold of response was identified at 27 ‰, and the largest response was at 25 ‰. Differentially expressed genes were clustered by pattern of expression, and clusters were characterized by functional enrichment analysis. Results indicate larval copepods adopt two distinct coping strategies in response to short-term hyposaline stress: a primary response using molecular chaperones and catabolic processes at 27 ‰; and a secondary response up-regulating ion pumps, transporters, a different suite of chaperones and apoptosis-related transcripts at 26 ‰ and 25 ‰. The results further our understanding of the tolerances of L. salmonis copepodids to salinity and temperature gradients and may assist in the development of salmon louse management strategies.

Perturbation of sphingolipid metabolism induces endoplasmic reticulum stress-mediated mitochondrial apoptosis in budding yeast

Sphingolipids are a class of membrane lipids conserved from yeast to mammals which determine whether a cell dies or survives. Perturbations in sphingolipid metabolism cause apoptotic cell death. Recent studies indicate that reduced sphingolipid levels trigger the cell death, but little is known about the mechanisms. In the budding yeast Saccharomyces cerevisiae, we show that reduction in complex sphingolipid levels causes loss of viability, most likely due to the induction of mitochondria-dependent apoptotic cell death pathway, accompanied by changes in mitochondrial and endoplasmic reticulum morphology and endoplasmic reticulum stress. Elevated cytosolic free calcium is required for the loss of viability. These results indicate that complex sphingolipids are essential for maintaining endoplasmic reticulum homeostasis and suggest that perturbation in complex sphingolipid levels activates an endoplasmic reticulum stress-mediated and calcium-dependent pathway to propagate apoptotic signals to the mitochondria.

Identification and characterization of Inositol-requiring enzyme-1 and X-box binding protein 1, two proteins involved in the unfolded protein response of Litopenaeus vannamei

The inositol-requiring enzyme-1 (IRE1)-X-box binding protein 1 (IRE1-XBP1) pathway is the key branch of the unfolded protein response (UPR). To investigate the role of the IRE1-XBP1 pathway in reducing environmental stress and increasing anti-viral immunity in Litopenaeus vannamei, homologues of IRE1 (designated as LvIRE1) and XBP1 (designated as LvXBP1) were identified and characterized. The full-length cDNA of LvIRE1 is 4908bp long, with an open reading frame (ORF) that encodies a putative 1174 amino acid protein. The full-length cDNA of LvXBP1 is 1746bp long. It contains two ORFs that encode putative 278 amino acid and 157 amino acid proteins, respectively. LvXBP1 mRNA has the predicted IRE1 splicing motifs CNG'CNGN located within the loop regions of two short hairpins. Sequencing of the splicing fragment induced by endoplasmic reticulum (ER)-stress showed a 3bp or 4bp frame shift from the predicted sites. The spliced form LvXBP1 (LvXBP1s) contained an ORF encodes a putative 463 amino acid protein. The reporter gene assays indicated that LvXBP1s activates the promoter of L. vannamei immunoglobulin heavy chain binding protein (LvBip), an important UPR effector. RT-PCR showed that LvXBP1 was spliced during the experiments. For heat shock treatment, the total LvXBP1 expression was increased and peaked at about 36h, whereas the percentages of the two isoforms were relatively stable. For the WSSV challenge, LvXBP1 was upregulated during the experiment and the percentage of the spliced form continuously declined after 18h of infection. Knock-down of LvXBP1 by RNA interference resulted in a lower cumulative mortality of L. vannamei under WSSV infection. Furthermore, the expression profiles of LvIRE1 and LvXBP1 in the gills, hemocytes, intestines, and hepatopancreas of the WSSV-challenged shrimp were detected using real-time RT-PCR. Taken together, these results confirm that the IRE1-XBP1 pathway is important for L. vannamei environmental stress resistance, suggest that L. vannamei IRE1-XBP1 may activated by WSSV and be annexed to serve the virus.

Small molecule activators of the heat shock response: chemical properties, molecular targets, and therapeutic promise

All cells have developed various mechanisms to respond and adapt to a variety of environmental challenges, including stresses that damage cellular proteins. One such response, the heat shock response (HSR), leads to the transcriptional activation of a family of molecular chaperone proteins that promote proper folding or clearance of damaged proteins within the cytosol. In addition to its role in protection against acute insults, the HSR also regulates lifespan and protects against protein misfolding that is associated with degenerative diseases of aging. As a result, identifying pharmacological regulators of the HSR has become an active area of research in recent years. Here, we review progress made in identifying small molecule activators of the HSR, what cellular targets these compounds interact with to drive response activation, and how such molecules may ultimately be employed to delay or reverse protein misfolding events that contribute to a number of diseases.

Translational infidelity-induced protein stress results from a deficiency in Trm9-catalyzed tRNA modifications

Correct codon-anticodon pairing promotes translational fidelity, with these interactions greatly facilitated by modified nucleosides found in tRNA. We hypothesized that wobble uridine modifications catalyzed by tRNA methyltransferase 9 (Trm9) are essential for translational fidelity. In support, we have used phenotypic, reporter and protein-based assays to demonstrate increased translational infidelity in trm9Δ Saccharomyces cerevisiae cells. Codon reengineering studies suggest that Trm9-catalyzed tRNA modifications promote fidelity during the translation of specific genes, those rich in arginine and glutamic acid codons from mixed boxes. Using quantitative tRNA modification analysis, we determined that trm9Δ cells are only deficient in 2 of 23 tRNA modifications, with those 2, 5-methoxycarbonylmethyluridine (mcm ( 5) U) and 5-methoxycarbonylmethyl-2-thiouridine (mcm ( 5) s ( 2) U), classified as key determinants of translational fidelity. We also show that in the absence of mcm ( 5) U and mcm ( 5) s ( 2) U, the resulting translational infidelity promotes protein errors and activation of unfolded protein and heat shock responses. These data support a model in which Trm9-catalyzed tRNA modifications promote fidelity during the translation of specific transcripts, with decreased wobble base modification leading to translational infidelity, protein errors and activation of protein stress response pathways.

Involvement of the Leishmania donovani virulence factor A2 in protection against heat and oxidative stress

Leishmania is an obligate intracellular protozoan parasite that infects cells of the reticulo-endothelial system. Host defences against Leishmania include fever and oxidant production, and the parasite has developed a number of defence mechanisms to neutralize the host response. The Leishmania donovani A2 family of proteins has been shown to be essential for survival in mammalian visceral organs. Here we provide evidence that A2 proteins protect the parasite against host defences, namely heat stress (fever) and oxidative stress. A2 is however unable to protect the cells from endoplasmic reticulum stress induced by dithiothreitol. To downregulate A2 protein expression, L. donovani was transfected with an A2 antisense RNA expressing-vector, resulting in significant reduction of A2 levels. The resulting A2-deficient cells were more sensitive to heat shock and this was associated with increased production of internal oxidants during heat shock. Moreover, axenic amastigotes with downregulated A2 expression had increased internal oxidants and decreased viability following treatment with hydrogen peroxide or a nitric oxide donor when compared to control cells. Overall, these results suggest that A2 protects L. donovani from a variety of stresses, thereby allowing it to survive in the internal organs of the mammalian host and to cause visceral disease.

Small-molecule proteostasis regulators for protein conformational diseases

Protein homeostasis (proteostasis) is essential for cellular and organismal health. Stress, aging, and the chronic expression of misfolded proteins, however, challenge the proteostasis machinery and the vitality of the cell. Enhanced expression of molecular chaperones, regulated by heat shock transcription factor-1 (HSF-1), has been shown to restore proteostasis in a variety of conformational disease models, suggesting a promising therapeutic approach. We describe the results of a ∼900,000 small molecule screen that identified novel classes of small molecule proteostasis regulators (PRs) that induce HSF-1-dependent chaperone expression and restore protein folding in multiple conformational disease models. The beneficial effects to proteome stability are mediated by HSF-1, DAF-16/FOXO, SKN-1/Nrf-2, and the chaperone machinery through mechanisms that are distinct from current known small molecule activators of the HSR. We suggest that modulation of the proteostasis network by PRs represents a promising therapeutic approach for the treatment of a variety of protein conformational diseases.

Embryo collection induces transient activation of XBP1 arm of the ER stress response while embryo vitrification does not

Embryo cryopreservation has become a standard procedure in the practice of assisted reproduction. While routinely performed in IVF labs, the effects of embryo vitrification on the molecular mechanisms governing preimplantation development remain largely unknown. The endoplasmic reticulum stress (ER stress) response is an evolutionary conserved mechanism that cells employ to manage ER stress. ER stress can be defined as an imbalance between protein synthesis and secretion within the ER. The primary focus of this study was to investigate whether standard embryo manipulations, including embryo collection, culture and vitrification, result in activation of the ER stress pathway in vitro and to determine whether the embryo utilizes the unfolded protein response as an adaptive response. Our results indicate that the major ER stress pathway constituents are present at all stages of preimplantation development and that the activation of ER stress pathways can be induced at the 8-cell, morula and blastocyst stages. Additionally, we have demonstrated that the IRE1α arm of the ER Stress pathway is activated in freshly collected embryos but contrastingly, this ER Stress arm is not activated following embryo vitrification. It is important to understand the possible stresses that Assisted Reproductive Technologies place on the embryo and the mechanisms the embryo employs to adapt to these stresses. This study indicates that among the adaptive pathways available, cultured mammalian embryos can employ the ER stress pathway. Assisted reproduction techniques should be aware that their activities may induce the ER stress pathway in their patients' early embryos.

Hyperthermia induces the ER stress pathway

BACKGROUND

The ER chaperone GRP78/BiP is a homolog of the Hsp70 family of heat shock proteins, yet GRP78/BiP is not induced by heat shock but instead by ER stress. However, previous studies had not considered more physiologically relevant temperature elevation associated with febrile hyperthermia. In this report we examine the response of GRP78/BiP and other components of the ER stress pathway in cells exposed to 40°C.

METHODOLOGY

AD293 cells were exposed to 43°C heat shock to confirm inhibition of the ER stress response genes. Five mammalian cell types, including AD293 cells, were then exposed to 40°C hyperthermia for various time periods and induction of the ER stress pathway was assessed.

PRINCIPAL FINDINGS

The inhibition of the ER stress pathway by heat shock (43°C) was confirmed. In contrast cells subjected to more mild temperature elevation (40°C) showed either a partial or full ER stress pathway induction as determined by downstream targets of the three arms of the ER stress pathway as well as a heat shock response. Cells deficient for Perk or Gcn2 exhibit great sensitivity to ER stress induction by hyperthermia.

CONCLUSIONS

The ER stress pathway is induced partially or fully as a consequence of hyperthermia in parallel with induction of Hsp70. These findings suggest that the ER and cytoplasm of cells contain parallel pathways to coordinately regulate adaptation to febrile hyperthermia associated with disease or infection.

An atypical unfolded protein response in heat shocked cells

Background

The heat shock response (HSR) and the unfolded protein response (UPR) are both activated by proteotoxic stress, although in different compartments, and share cellular resources. How these resources are allocated when both responses are active is not known. Insight in possible crosstalk will help understanding the consequences of failure of these systems in (age-related) disease.

Results

In heat stressed HEK293 cells synthesis of the canonical UPR transcription factors XBP1s and ATF4 was detected as well as HSF1 independent activation of the promoters of the ER resident chaperones HSPA5 (BiP) and DNAJB9 (ERdj4). However, the heat stress activation of the DNAJB9 promoter, a XBP1s target, was not blocked in cells expressing a dominant negative IRE1α mutant, and thus did not require XBP1s. Furthermore, the DNA element required for heat stress activation of the DNAJB9 promoter is distinct from the ATF4 and ATF6 target elements; even though inhibition of eIF2α phosphorylation resulted in a decreased activation of the DNAJB9 promoter upon heat stress, suggesting a role for an eIF2α phosphorylation dependent product.

Conclusions

The initial step in the UPR, synthesis of transcription factors, is activated by heat stress but the second step, transcriptional transactivation by these factors, is blocked and these pathways of the UPR are thus not productive. Expression of canonical ER chaperones is part of the response of heat stressed cells but another set of transcription factors has been recruited to regulate expression of these ER chaperones.

HSFs and regulation of Hsp70.1 (Hspa1b) in oocytes and preimplantation embryos: new insights brought by transgenic and knockout mouse models

Gene encoding heat shock protein (Hsps) are induced following a thermal stress thanks to the activation of heat shock transcription factor (HSF) which interacts with heat shock elements (HSE) located within the sequence of Hsp promoters. This cellular and protective response (heat shock response (HSR)) is well known and evolutionarily conserved. Nevertheless, HSR does not function in all the cells produced during the life of a multicellular organism, e.g., early mouse embryos. Taking advantage of mouse transgenic and knockout models, we investigated the roles of trans (HSF 1 and 2) and cis (HSE) regulatory elements in the control of Hsp70.1 (Hspa1b) through several developmental steps from oocytes to blastocysts. Our studies confirm that, even in absence of any stress, HSF1 regulates Hsp70.1 in oocytes and early embryos. Our data emphasize the role of maternal and paternal HSFs in the developmentally regulated expression of Hsp70.1 observed when the zygotic genome activation occurs. Furthermore, in this unstressed developmental condition, affinity and binding to HSEs might be more permissive than in the stress response. Finally, submitting blastocyst to different stress conditions, we show that HSF2 is differentially required for Hsp expression and cell survival. Taken together, our findings indicate that the role of heat shock trans and cis regulatory elements evolve along the successive steps of early embryonic development.

Glutathione revisited: a vital function in iron metabolism and ancillary role in thiol-redox control

Glutathione contributes to thiol-redox control and to extra-mitochondrial iron-sulphur cluster (ISC) maturation. To determine the physiological importance of these functions and sort out those that account for the GSH requirement for viability, we performed a comprehensive analysis of yeast cells depleted of or containing toxic levels of GSH. Both conditions triggered an intense iron starvation-like response and impaired the activity of extra-mitochondrial ISC enzymes but did not impact thiol-redox maintenance, except for high glutathione levels that altered oxidative protein folding in the endoplasmic reticulum. While iron partially rescued the ISC maturation and growth defects of GSH-depleted cells, genetic experiments indicated that unlike thioredoxin, glutathione could not support by itself the thiol-redox duties of the cell. We propose that glutathione is essential by its requirement in ISC assembly, but only serves as a thioredoxin backup in cytosolic thiol-redox maintenance. Glutathione-high physiological levels are thus meant to insulate its cytosolic function in iron metabolism from variations of its concentration during redox stresses, a model challenging the traditional view of it as prime actor in thiol-redox control.

Heat shock response in yeast involves changes in both transcription rates and mRNA stabilities

We have analyzed the heat stress response in the yeast Saccharomyces cerevisiae by determining mRNA levels and transcription rates for the whole transcriptome after a shift from 25 °C to 37 °C. Using an established mathematical algorithm, theoretical mRNA decay rates have also been calculated from the experimental data. We have verified the mathematical predictions for selected genes by determining their mRNA decay rates at different times during heat stress response using the regulatable tetO promoter. This study indicates that the yeast response to heat shock is not only due to changes in transcription rates, but also to changes in the mRNA stabilities. mRNA stability is affected in 62% of the yeast genes and it is particularly important in shaping the mRNA profile of the genes belonging to the environmental stress response. In most cases, changes in transcription rates and mRNA stabilities are homodirectional for both parameters, although some interesting cases of antagonist behavior are found. The statistical analysis of gene targets and sequence motifs within the clusters of genes with similar behaviors shows that both transcriptional and post-transcriptional regulons apparently contribute to the general heat stress response by means of transcriptional factors and RNA binding proteins.

Conserved endoplasmic reticulum-associated degradation system to eliminate mutated receptor-like kinases in Arabidopsis

Endoplasmic reticulum (ER)-associated degradation (ERAD) is an integral part of the ER quality-control system that removes toxic misfolded proteins via ubiquitin/proteasome-mediated degradation. Most of our knowledge on ERAD comes from biochemical and genetic studies in yeast and mammalian cells. Although ERAD is known to operate in plant cells, little is known about its molecular components and its biochemical mechanism. A genetic screen for suppressors of the Arabidopsis bri1-9, a weak dwarf mutant caused by ER retention of a structurally defective yet biochemically competent brassinosteroid (BR) receptor BRI1, resulted in identification of the EMS-mutagenized bri1 suppressor 5 (EBS5) gene that encodes an Arabidopsis homolog of the yeast Hrd3/mammalian Sel1L protein known to be involved in ERAD. Loss-of-function ebs5 mutations block the ERAD of bri1-9 and bri1-5, another ER-retained BR receptor. We showed that EBS5 complemented the ERAD defect of the yeast Δhrd3 mutant and interacted with the two mutated BR receptors in plant cells. Using a reverse genetic approach, we discovered that two Arabidopsis homologs of the yeast/mammalian Hrd1, an ER membrane-localized ubiquitin ligase, function redundantly in the ERAD of bri1-9. Together, our results revealed functional roles of two conserved ERAD components in degrading mutated/misfolded receptor-like kinases in Arabidopsis.

Verification of alternative splicing variants based on domain integrity, truncation length and intrinsic protein disorder

According to current estimations ∼95% of multi-exonic human protein-coding genes undergo alternative splicing (AS). However, for 4000 human proteins in PDB, only 14 human proteins have structures of at least two alternative isoforms. Surveying these structural isoforms revealed that the maximum insertion accommodated by an isoform of a fully ordered protein domain was 5 amino acids, other instances of domain changes involved intrinsic structural disorder. After collecting 505 minor isoforms of human proteins with evidence for their existence we analyzed their length, protein disorder and exposed hydrophobic surface. We found that strict rules govern the selection of alternative splice variants aimed to preserve the integrity of globular domains: alternative splice sites (i) tend to avoid globular domains or (ii) affect them only marginally or (iii) tend to coincide with a location where the exposed hydrophobic surface is minimal or (iv) the protein is disordered. We also observed an inverse correlation between the domain fraction lost and the full length of the minor isoform containing the domain, possibly indicating a buffering effect for the isoform protein counteracting the domain truncation effect. These observations provide the basis for a prediction method (currently under development) to predict the viability of splice variants.

Analysis of ER stress in developing rice endosperm accumulating beta-amyloid peptide

The common neurodegenerative disorder known as Alzheimer's disease is characterized by cerebral neuritic plaques of amyloid beta (Abeta) peptide. Plaque formation is related to the highly aggregative property of this peptide, because it polymerizes to form insoluble plaques or fibrils causing neurotoxicity. Here, we expressed Abeta peptide as a new causing agent to endoplasmic reticulum (ER) stress to study ER stress occurred in plant. When the dimer of Abeta(1-42) peptide was expressed in maturing seed under the control of the 2.3-kb glutelin GluB-1 promoter containing its signal peptide, a maximum of about 8 mug peptide per grain accumulated and was deposited at the periphery of distorted ER-derived PB-I protein bodies. Synthesis of Abeta peptide in the ER lumen severely inhibited the synthesis and deposition of seed storage proteins, resulting in the generation of many small and abnormally appearing PB bodies. This ultrastructural change was accounted for by ER stress leading to the accumulation of aggregated Abeta peptide in the ER lumen and a coordinated increase in ER-resident molecular chaperones such as BiPs and PDIs in Abeta-expressing plants. Microarray analysis also confirmed that expression of several BiPs, PDIs and OsbZIP60 containing putative transmembrane domains was affected by the ER stress response. Abeta-expressing transgenic rice kernels exhibited an opaque and shrunken phenotype. When grain phenotype and expression levels were compared among transgenic rice grains expressing several different recombinant peptides, such detrimental effects on grain phenotype were correlated with the expressed peptide causing ER stress rather than expression levels.

Heat shock proteins: cellular and molecular mechanisms in the central nervous system

Emerging evidence indicates that heat shock proteins (HSPs) are critical regulators in normal neural physiological function as well as in cell stress responses. The functions of HSPs represent an enormous and diverse range of cellular activities, far beyond the originally identified roles in protein folding and chaperoning. HSPs are now understood to be involved in processes such as synaptic transmission, autophagy, ER stress response, protein kinase and cell death signaling. In addition, manipulation of HSPs has robust effects on the fate of cells in neurological injury and disease states. The ongoing exploration of multiple HSP superfamilies has underscored the pluripotent nature of HSPs in the cellular context, and has demanded the recent revamping of the nomenclature referring to these families to reflect a re-organization based on structure and function. In keeping with this re-organization, we first discuss the HSP superfamilies in terms of protein structure, regulation, expression and distribution in the brain. We then explore major cellular functions of HSPs that are relevant to neural physiological states, and from there we discuss known and proposed HSP impacts on major neurological disease states. This review article presents a three-part discussion on the array of HSP families relevant to neuronal tissue, their cellular functions, and the exploration of therapeutic targets of these proteins in the context of neurological diseases.

A dominant negative mutant of PMA1 interferes with the folding of the wild type enzyme

Misfolded proteins are usually arrested in the endoplasmic reticulum (ER) and degraded by the ER-associated degradation (ERAD) machinery. Several mutant alleles of PMA1, the gene coding for the plasma membrane H (+)-ATPase, render misfolded proteins that are subjected to ERAD. A subset of misfolded PMA1 mutants exhibits a dominant negative effect on yeast growth since, when co-expressed with the wild type allele, both proteins are retained in the ER and degraded. We have used a PMA1-D378T dominant lethal allele to analyse the mechanism underlying the retention of the wild type enzyme by the dominant negative mutant. A genetic screen was performed for isolation of intragenic suppressors of PMA1-D378T allele. This analysis pointed to transmembrane helix 10 (TM10) as an important element in the establishment of the dominant lethality. Deletion of the TM10 was able to suppress not only the PMA1-D378T but all the dominant lethal alleles tested. Biochemical analyses suggest that dominant lethal proteins obstruct, through TM10, the correct folding of the wild type enzyme leading to its retention and degradation by ERAD.

The Rpd3L HDAC complex is essential for the heat stress response in yeast

To ensure cell survival and growth during temperature increase, eukaryotic organisms respond with transcriptional activation that results in accumulation of proteins that protect against damage and facilitate recovery. To define the global cellular adaptation response to heat stress, we performed a systematic genetic screen that yielded 277 yeast genes required for growth at high temperature. Of these, the Rpd3 histone deacetylase complex was enriched. Global gene expression analysis showed that Rpd3 partially regulated gene expression upon heat shock. The Hsf1 and Msn2/4 transcription factors are the main regulators of gene activation in response to heat stress. RPD3-deficient cells had impaired activation of Msn2/4-dependent genes, while activation of genes controlled by Hsf1 was deacetylase-independent. Rpd3 bound to heat stress-dependent promoters through the Msn2/4 transcription factors, allowing entry of RNA Pol II and activation of transcription upon stress. Finally, we found that the large, but not the small Rpd3 complex regulated cell adaptation in response to heat stress.

The endoplasmic reticulum-associated degradation of the epithelial sodium channel requires a unique complement of molecular chaperones

The epithelial sodium channel (ENaC) is composed of a single copy of an alpha-, beta-, and gamma-subunit and plays an essential role in water and salt balance. Because ENaC assembles inefficiently after its insertion into the ER, a substantial percentage of each subunit is targeted for ER-associated degradation (ERAD). To define how the ENaC subunits are selected for degradation, we developed novel yeast expression systems for each ENaC subunit. Data from this analysis suggested that ENaC subunits display folding defects in more than one compartment and that subunit turnover might require a unique group of factors. Consistent with this hypothesis, yeast lacking the lumenal Hsp40s, Jem1 and Scj1, exhibited defects in ENaC degradation, whereas BiP function was dispensable. We also discovered that Jem1 and Scj1 assist in ENaC ubiquitination, and overexpression of ERdj3 and ERdj4, two lumenal mammalian Hsp40s, increased the proteasome-mediated degradation of ENaC in vertebrate cells. Our data indicate that Hsp40s can act independently of Hsp70 to select substrates for ERAD.

Orm1 and Orm2 are conserved endoplasmic reticulum membrane proteins regulating lipid homeostasis and protein quality control

Yeast members of the ORMDL family of endoplasmic reticulum (ER) membrane proteins play a central role in lipid homeostasis and protein quality control. In the absence of yeast Orm1 and Orm2, accumulation of long chain base, a sphingolipid precursor, suggests dysregulation of sphingolipid synthesis. Physical interaction between Orm1 and Orm2 and serine palmitoyltransferase, responsible for the first committed step in sphingolipid synthesis, further supports a role for the Orm proteins in regulating sphingolipid synthesis. Phospholipid homeostasis is also affected in orm1Delta orm2Delta cells: the cells are inositol auxotrophs with impaired transcriptional regulation of genes encoding phospholipid biosynthesis enzymes. Strikingly, impaired growth of orm1Delta orm2Delta cells is associated with constitutive unfolded protein response, sensitivity to stress, and slow ER-to-Golgi transport. Inhibition of sphingolipid synthesis suppresses orm1Delta orm2Delta phenotypes, including ER stress, suggesting that disrupted sphingolipid homeostasis accounts for pleiotropic phenotypes. Thus, the yeast Orm proteins control membrane biogenesis by coordinating lipid homeostasis with protein quality control.

Inhibition of apoptosis signal-regulating kinase 1 reduces endoplasmic reticulum stress and nuclear huntingtin fragments in a mouse model of Huntington disease

Huntington's disease (HD) is characterized clinically by chorea, psychiatric disturbances, and dementia, while it is characterized pathologically by neuronal inclusions as well as striatal and cortical neurodegeneration. The neurodegeneration arises from the loss of long projection neurons in the cortex and striatum. In this study, we investigated the role of apoptosis signal-regulating kinase 1 (Ask1) in the pathogenesis of HD. We analyzed the expression of Ask1 and huntingtin (htt) within the striatum and cortex and also examined the interaction of Ask1 with htt fragments in HD (R6/2) mice. Additionally, we inhibited Ask1 and analyzed the resulting changes in brain-derived neurotrophic factor (BDNF) expression, motor function, and striatal atrophy. Ask1 activity was blocked using an Ask1 antibody raised against the C-terminus of the Ask1 protein. The anti-Ask1 antibody was infused into the striatum of the HD mice for four weeks using a micro-osmotic pump. The levels of Ask1 protein and endoplasmic reticulum (ER) stress were increased in HD mice. Binding of inactivated Ask1 to htt fragments was more prevalent in the cytosol than the nucleus of cortical neurons. Binding of inactivated Ask1 to htt fragments prevented translocation of the htt fragments into the nucleus, resulting in an improvement in motor dysfunction and atrophy. In the normal state, active Ask1 may help htt fragments enter the nucleus, while inactivated Ask1 hinders this translocation by binding to but not releasing fragmented htt into the nucleus. We propose that Ask1 may interact with htt fragments and subsequently induce ER stress. BDNF depletion may be prevented by targeting Ask1; this would decrease ER stress and possibly ameliorate behavioral or anatomical abnormalities that accompany HD. Therefore, regulating the amounts and activity of the Ask1 protein is a novel strategy for treatment of HD.

Cu, Zn superoxide dismutase and NADP(H) homeostasis are required for tolerance of endoplasmic reticulum stress in Saccharomyces cerevisiae

Genome-wide screening for sensitivity to chronic endoplasmic reticulum (ER) stress induced by dithiothreitol and tunicamycin (TM) identified mutants deleted for Cu, Zn superoxide dismutase (SOD) function (SOD1, CCS1) or affected in NADPH generation via the pentose phosphate pathway (TKL1, RPE1). TM-induced ER stress led to an increase in cellular superoxide accumulation and an increase in SOD1 expression and Sod1p activity. Prior adaptation of the hac1 mutant deficient in the unfolded protein response (UPR) to the superoxide-generating agent paraquat reduced cell death under ER stress. Overexpression of the ER oxidoreductase Ero1p known to generate hydrogen peroxide in vitro, did not lead to increased superoxide levels in cells subjected to ER stress. The mutants lacking SOD1, TKL1, or RPE1 exhibited decreased UPR induction under ER stress. Sensitivity of the sod1 mutant to ER stress and decreased UPR induction was partially rescued by overexpression of TKL1 encoding transketolase. These data indicate an important role for SOD and cellular NADP(H) in cell survival during ER stress, and it is proposed that accumulation of superoxide affects NADP(H) homeostasis, leading to reduced UPR induction during ER stress.

One step at a time: endoplasmic reticulum-associated degradation

Protein folding in the endoplasmic reticulum (ER) is monitored by ER quality control (ERQC) mechanisms. Proteins that pass ERQC criteria traffic to their final destinations through the secretory pathway, whereas non-native and unassembled subunits of multimeric proteins are degraded by the ER-associated degradation (ERAD) pathway. During ERAD, molecular chaperones and associated factors recognize and target substrates for retrotranslocation to the cytoplasm, where they are degraded by the ubiquitin-proteasome machinery. The discovery of diseases that are associated with ERAD substrates highlights the importance of this pathway. Here, we summarize our current understanding of each step during ERAD, with emphasis on the factors that catalyse distinct activities.

SOD1, a new Kluyveromyces lactis helper gene for heterologous protein secretion

Bottlenecks in protein expression and secretion often limit the development of industrial processes. By manipulating chaperone and foldase levels, improvements in yeast secretion were found for a number of proteins. Recently, sustained endoplasmic reticulum stress, occurring due to recombinant protein production, was reported to cause oxidative stress in yeast. Saccharomyces cerevisiae cells are able to trigger an adaptive response to oxidative-stress conditions, resulting in the upregulation of both primary and secondary antioxidant defenses. SOD1 encodes for a superoxide dismutase that catalyzes the dismutation of superoxide anions (O(2)(-)) into oxygen and hydrogen peroxide. It is a Cu(2+)/Zn(2+) metalloenzyme and represents an important antioxidant defense in nearly all aerobic and aerotolerant organisms. We found that overexpression of the Kluyveromyces lactis SOD1 (KlSOD1) gene was able to increase the production of two different heterologous proteins, human serum albumin (HSA) and glucoamylase from Arxula adeninivorans. In addition, KlSOD1 overexpression led to a significant decrease in the amount of reactive oxygen species (ROS) that originated during protein production. The yield of HSA also increased when K. lactis cells were grown in the presence of the antioxidant agent ascorbic acid and decreased when cells were challenged with menadione, a ROS generator compound. Moreover, we observed that, in high-osmolarity medium, cells overexpressing KlSOD1 showed higher growth rates than control cells. Our results thus further support the notion that the production of some heterologous proteins may be improved by manipulating genes involved in general stress responses.

Chemical and biological approaches synergize to ameliorate protein-folding diseases

Loss-of-function diseases are often caused by a mutation in a protein traversing the secretory pathway that compromises the normal balance between protein folding, trafficking, and degradation. We demonstrate that the innate cellular protein homeostasis, or proteostasis, capacity can be enhanced to fold mutated enzymes that would otherwise misfold and be degraded, using small molecule proteostasis regulators. Two proteostasis regulators are reported that alter the composition of the proteostasis network in the endoplasmic reticulum through the unfolded protein response, increasing the mutant folded protein concentration that can engage the trafficking machinery, restoring function to two nonhomologous mutant enzymes associated with distinct lysosomal storage diseases. Coapplication of a pharmacologic chaperone and a proteostasis regulator exhibits synergy because of the former's ability to further increase the concentration of trafficking-competent mutant folded enzymes. It may be possible to ameliorate loss-of-function diseases by using proteostasis regulators alone or in combination with a pharmacologic chaperone.

Trans-Golgi network and endosome dynamics connect ceramide homeostasis with regulation of the unfolded protein response and TOR signaling in yeast

Synthetic genetic array analyses identify powerful genetic interactions between a thermosensitive allele (sec14-1(ts)) of the structural gene for the major yeast phosphatidylinositol transfer protein (SEC14) and a structural gene deletion allele (tlg2Delta) for the Tlg2 target membrane-soluble N-ethylmaleimide-sensitive factor attachment protein receptor. The data further demonstrate Sec14 is required for proper trans-Golgi network (TGN)/endosomal dynamics in yeast. Paradoxically, combinatorial depletion of Sec14 and Tlg2 activities elicits trafficking defects from the endoplasmic reticulum, and these defects are accompanied by compromise of the unfolded protein response (UPR). UPR failure occurs downstream of Hac1 mRNA splicing, and it is further accompanied by defects in TOR signaling. The data link TGN/endosomal dynamics with ceramide homeostasis, UPR activity, and TOR signaling in yeast, and they identify the Sit4 protein phosphatase as a primary conduit through which ceramides link to the UPR. We suggest combinatorial Sec14/Tlg2 dysfunction evokes inappropriate turnover of complex sphingolipids in endosomes. One result of this turnover is potentiation of ceramide-activated phosphatase-mediated down-regulation of the UPR. These results provide new insight into Sec14 function, and they emphasize the TGN/endosomal system as a central hub for homeostatic regulation in eukaryotes.