The unfolded protein response in secretory cell function

The Aspergillus nidulans signalling mucin MsbA regulates starvation responses, adhesion and affects cellulase secretion in response to environmental cues

In the heterogeneous semi-solid environment naturally occupied by lignocellulolytic fungi the majority of nutrients are locked away as insoluble plant biomass. Hence, lignocellulolytic fungi must actively search for, and attach to, a desirable source of nutrients. During growth on lignocellulose a period of carbon deprivation provokes carbon catabolite derepression and scavenging hydrolase secretion. Subsequently, starvation and/or contact sensing was hypothesized to play a role in lignocellulose attachment and degradation. In Aspergillus nidulans the extracellular signalling mucin, MsbA, influences growth under nutrient-poor conditions including lignocellulose. Cellulase secretion and activity was affected by MsbA via a mechanism that was independent of cellulase transcription. MsbA modulated both the cell wall integrity and filamentous growth MAPK pathways influencing adhesion, biofilm formation and secretion. The constitutive activation of MsbA subsequently enhanced cellulase activity by increasing the secretion of the cellobiohydrolase, CbhA, while improved substrate attachment and may contribute to an enhanced starvation response. Starvation and/or contact sensing therefore represents a new dimension to the already multifaceted regulation of cellulase activity.

Endoplasmic reticulum stress and fungal pathogenesis

The gateway to the secretory pathway is the endoplasmic reticulum (ER), an organelle that is responsible for the accurate folding, post-translational modification and final assembly of up to a third of the cellular proteome. When secretion levels are high, errors in protein biogenesis can lead to the accumulation of abnormally folded proteins, which threaten ER homeostasis. The unfolded protein response (UPR) is an adaptive signaling pathway that counters a buildup in misfolded and unfolded proteins by increasing the expression of genes that support ER protein folding capacity. Fungi, like other eukaryotic cells that are specialized for secretion, rely upon the UPR to buffer ER stress caused by fluctuations in secretory demand. However, emerging evidence is also implicating the UPR as a central regulator of fungal pathogenesis. In this review, we discuss how diverse fungal pathogens have adapted ER stress response pathways to support the expression of virulence-related traits that are necessary in the host environment.

Ire1 mediated mRNA splicing in a C-terminus deletion mutant of Drosophila Xbp1

The Unfolded Protein Response is a homeostatic mechanism that permits eukaryotic cells to cope with Endoplasmic Reticulum (ER) stress caused by excessive accumulation of misfolded proteins in the ER lumen. The more conserved branch of the UPR relies on an ER transmembrane enzyme, Ire1, which, upon ER stress, promotes the unconventional splicing of a small intron from the mRNA encoding the transcription factor Xbp1. In mammals, two specific regions (the hydrophobic region 2 - HR2 - and the C-terminal translational pausing site) present in the Xbp1^unspliced protein mediate the recruitment of the Xbp1 mRNA-ribosome-nascent chain complex to the ER membrane, so that Xbp1 mRNA can be spliced by Ire1. Here, we generated a Drosophila Xbp1 deletion mutant (Excision101) lacking both HR2 and C-terminal region, but not the Ire1 splicing site. We show that Ire1-dependent splicing of Xbp1 mRNA is reduced, but not abolished in Excision101. Our results suggest the existence of additional mechanisms for ER membrane targeting of Xbp1 mRNA that are independent of the C-terminal domain of Drosophila Xbp1^unspliced.

The importance of connections between the cell wall integrity pathway and the unfolded protein response in filamentous fungi

In the external environment, or within a host organism, filamentous fungi experience sudden changes in nutrient availability, osmolality, pH, temperature and the exposure to toxic compounds. The fungal cell wall represents the first line of defense, while also performing essential roles in morphology, development and virulence. A polarized secretion system is paramount for cell wall biosynthesis, filamentous growth, nutrient acquisition and interactions with the environment. The unique ability of filamentous fungi to secrete has resulted in their industrial adoption as fungal cell factories. Protein maturation and secretion commences in the endoplasmic reticulum (ER). The unfolded protein response (UPR) maintains ER functionality during exposure to secretion and cell wall stress. UPR, therefore, influences secretion and cell wall homeostasis, which in turn impacts upon numerous fungal traits important to pathogenesis and biotechnology. Subsequently, this review describes the relevance of the cell wall and UPR systems to filamentous fungal pathogens or industrial microbes and then highlights interconnections between the two systems. Ultimately, the possible biotechnological applications of an enhanced understanding of such regulatory systems in combating fungal disease, or the removal of natural bottlenecks in protein secretion in an industrial setting, are discussed.

Redox signaling via the molecular chaperone BiP protects cells against endoplasmic reticulum-derived oxidative stress

Oxidative protein folding in the endoplasmic reticulum (ER) has emerged as a potentially significant source of cellular reactive oxygen species (ROS). Recent studies suggest that levels of ROS generated as a byproduct of oxidative folding rival those produced by mitochondrial respiration. Mechanisms that protect cells against oxidant accumulation within the ER have begun to be elucidated yet many questions still remain regarding how cells prevent oxidant-induced damage from ER folding events. Here we report a new role for a central well-characterized player in ER homeostasis as a direct sensor of ER redox imbalance. Specifically we show that a conserved cysteine in the lumenal chaperone BiP is susceptible to oxidation by peroxide, and we demonstrate that oxidation of this conserved cysteine disrupts BiP's ATPase cycle. We propose that alteration of BiP activity upon oxidation helps cells cope with disruption to oxidative folding within the ER during oxidative stress.

DOI:http://dx.doi.org/10.7554/eLife.03496.001

The endoplasmic reticulum is the cellular compartment where approximately one third of the cell's proteins are made. Inside, chaperone molecules bind to newly made protein chains and help them to fold into the three-dimensional structure required for the protein to work correctly. A chaperone called Ero1 helps to facilitate this folding process by catalyzing a reaction that forms strong chemical bonds, which help stabilize the final protein structures. However, this help from Ero1 comes at a cost: forming a stabilizing bond this way also produces a peroxide molecule as a byproduct.

Peroxide is a ‘reactive oxygen species’: a chemical that can oxidize and damage proteins and DNA, which can potentially kill the cell. Three other enzymes in the endoplasmic reticulum can convert peroxide into water, to protect the cells from reactive oxygen species build-up. However, not all cells that use Ero1 have these other enzymes, suggesting that other pathways must exist to manage reactive oxygen species.

Wang et al. took advantage of yeast cells containing a hyperactive mutant version of the Ero1 enzyme to look for alternative detoxifying mechanisms that occur when the cell is stressed by an excess of reactive oxygen species. In these cells, Wang et al. observed that the high levels of reactive oxygen species caused part of a chaperone molecule called BiP to oxidize. This modification of BiP acts like a switch that the reactive oxygen species flip on. When activated by the reactive oxygen species, BiP enhances its activity as a folding molecular chaperone, keeping proteins apart. This is thought to allow BiP to minimize the protein misfolding that may otherwise occur in the wake of the damage caused by the building levels of peroxide. Wang et al. created a mutant BiP chaperone that mimics the oxidized form, and found that it also protects cells from the damage inflicted by the excess of reactive oxygen species.

Wang et al. propose that the BiP chaperone may be an important sensor of reactive oxygen species that changes its activity when these harmful chemicals are present and helps to protect the cell from damage. The success in mimicking the protective effects of oxidized BiP with a mutant BiP suggest that in the future one may be able to design small molecule drugs that bind to BiP to produce the activity of the modified form.

DOI:http://dx.doi.org/10.7554/eLife.03496.002

The endoplasmic reticulum and unfolded protein response in the control of mammalian recombinant protein production

The endoplasmic reticulum (ER) of eukaryotic cells is involved in the synthesis and processing of proteins and lipids in the secretory pathway. These processing events that proteins undergo in the ER may present major limiting steps for recombinant protein production. Increased protein synthesis, accumulation of improperly processed or mis-folded protein can induce ER stress. To cope with ER stress, the ER has quality control mechanisms, such as the unfolded protein response (UPR) and ER-associated degradation to restore homeostasis. ER stress and UPR activation trigger multiple physiological cellular changes. Here we review cellular mechanisms that cope with ER stress and illustrate how this knowledge can be applied to increase the efficiency of recombinant protein expression.

Fibrosis--a lethal component of systemic sclerosis

Fibrosis is a pathological process characterized by excessive accumulation of connective tissue components in an organ or tissue. Fibrosis is produced by deregulated wound healing in response to chronic tissue injury or chronic inflammation, the hallmarks of rheumatic diseases. Progressive fibrosis, which distorts tissue architecture and results in progressive loss of organ function, is now recognized to be one of the major causes of morbidity and mortality in individuals with one of the most lethal rheumatic disease, systemic sclerosis (SSc). In this Review, we discuss the pathological role of fibrosis in SSc. We discuss the involvement of endothelium and pericyte activation, aberrant immune responses, endoplasmic reticulum stress and chronic tissue injury in the initiation of fibrosis in SSc. We then discuss fibroblast activation and myofibroblast differentiation that occurs in response to these initiating processes and is responsible for excessive accumulation of extracellular matrix. Finally, we discuss the chemical and mechanical signals that drive fibroblast activation and myofibroblast differentiation, which could serve as targets for new therapies for fibrosis in SSc.

Physiological roles of regulated Ire1 dependent decay

Inositol-requiring enzyme 1 (Ire1) is an important transducer of the unfolded protein response (UPR) that is activated by the accumulation of misfolded proteins in the endoplamic reticulum (ER stress). Activated Ire1 mediates the splicing of an intron from the mRNA of Xbp1, causing a frame-shift during translation and introducing a new carboxyl domain in the Xbp1 protein, which only then becomes a fully functional transcription factor. Studies using cell culture systems demonstrated that Ire1 also promotes the degradation of mRNAs encoding mostly ER-targeted proteins, to reduce the load of incoming ER “client” proteins during ER stress. This process was called RIDD (regulated Ire1-dependent decay), but its physiological significance remained poorly characterized beyond cell culture systems. Here we review several recent studies that have highlighted the physiological roles of RIDD in specific biological paradigms, such as photoreceptor differentiation in Drosophila or mammalian liver and endocrine pancreas function. These studies demonstrate the importance of RIDD in tissues undergoing intense secretory function and highlight the physiologic role of RIDD during UPR activation in cells and organisms.

Defective secretory-protein mRNAs take the RAPP

Secretory proteins destined for the lumen of the endoplasmic reticulum are key regulators of cellular functions and are thus subject to several levels of quality control. A recent study finds that the earliest step in secretory protein biogenesis--binding of the signal recognition particle to the signal sequence of the nascent peptide--is subject to a quality control process termed RAPP for 'regulation of aberrant protein production'. This process involves AGO2 and mRNA degradation.

Abnormal chondrocyte apoptosis in the cartilage growth plate is influenced by genetic background and deletion of CHOP in a targeted mouse model of pseudoachondroplasia

Pseudoachondroplasia (PSACH) is an autosomal dominant skeletal dysplasia caused by mutations in cartilage oligomeric matrix protein (COMP) and characterised by short limbed dwarfism and early onset osteoarthritis. Mouse models of PSACH show variable retention of mutant COMP in the ER of chondrocytes, however, in each case a different stress pathway is activated and the underlying disease mechanisms remain largely unknown. T585M COMP mutant mice are a model of moderate PSACH and demonstrate a mild ER stress response. Although mutant COMP is not retained in significant quantities within the ER of chondrocytes, both BiP and the pro-apoptotic ER stress-related transcription factor CHOP are mildly elevated, whilst bcl-2 levels are decreased, resulting in increased and spatially dysregulated chondrocyte apoptosis. To determine whether the abnormal chondrocyte apoptosis observed in the growth plate of mutant mice is CHOP-mediated, we bred T585M COMP mutant mice with CHOP-null mice to homozygosity, and analysed the resulting phenotype. Although abnormal apoptosis was alleviated in the resting zone following CHOP deletion, the mutant growth plates were generally more disorganised. Furthermore, the bone lengths of COMP mutant CHOP null mice were significantly shorter at 9 weeks of age when compared to the COMP mutant mice, including a significant difference in the skull length. Overall, these data demonstrate that CHOP-mediated apoptosis is an early event in the pathobiology of PSACH and suggest that the lack of CHOP, in conjunction with a COMP mutation, may lead to aggravation of the skeletal phenotype via a potentially synergistic effect on endochondral ossification.

Proteostasis and longevity: when does aging really begin?

Aging is a complex process regulated by multiple cellular pathways, including the proteostasis network. The proteostasis network consists of molecular chaperones, stress-response transcription factors, and protein degradation machines that sense and respond to proteotoxic stress and protein misfolding to ensure cell viability. A loss of proteostasis is associated with aging and age-related disorders in diverse model systems, moreover, genetic or pharmacological enhancement of the proteostasis network has been shown to extend lifespan and suppress age-related disease. However, our understanding of the relationship between aging, proteostasis, and the proteostasis network remains unclear. Here, we propose, from studies in Caenorhabditis elegans, that proteostasis collapse is not gradual but rather a sudden and early life event that triggers proteome mismanagement, thereby affecting a multitude of downstream processes. Furthermore, we propose that this phenomenon is not stochastic but is instead a programmed re-modeling of the proteostasis network that may be conserved in other species. As such, we postulate that changes in the proteostasis network may be one of the earliest events dictating healthy aging in metazoans.

A star is born: new insights into the mechanism of astrogenesis

Astrocytes emerge as crucial cells for proper neuronal functioning in the developing and adult brain. Neurons and astrocytes are sequentially generated from the same pool of neural stem cells (NSCs). Tight regulation of the neuron-to-astrocyte switch is critical for (1) the generation of a balanced number of astrocytes and neurons and (2) neuronal circuit formation, since newborn astrocytes regulate synapse formation. This review focuses on signaling pathways that instruct astrogenesis, incorporating recently discovered intrinsic and extrinsic regulators. The canonical pathway of astrocytic gene expression, JAK/STAT signaling, is inhibited during neurogenesis to prevent premature astrocyte differentiation. At the onset of astrogenesis, Notch signaling induces epigenetic remodeling of astrocytic genes like glial fibrillary acidic protein to change NSC competence. In turn, astrogenesis is initiated by signals received from newborn neurons. We highlight how key molecular pathways like JAK/STAT and Notch are integrated in a complex network of environmental signals and epigenetic and transcriptional regulators to determine NSC differentiation. It is essential to understand NSC differentiation in respect to future NSC-based therapies for brain diseases, as transplanted NSCs preferentially become astrocytes. As emphasized in this review, many clues in this respect can be learned from development.

Endoplasmic reticulum stress activates transglutaminase 2 leading to protein aggregation

Aberrant activation of transglutaminase 2 (TGase2) contributes to a variety of protein conformational disorders such as neurodegenerative diseases and age-related cataracts. The accumulation of improperly folded proteins in the endoplasmic reticulum (ER) triggers the unfolded protein response (UPR), which promotes either repair or degradation of the damaged proteins. Inadequate UPR results in protein aggregation that may contribute to the development of age-related degenerative diseases. TGase2 is a calcium-dependent enzyme that irreversibly modifies proteins by forming cross-linked protein aggregates. Intracellular TGase2 is activated by oxidative stress which generates large quantities of unfolded proteins. However, the relationship between TGase2 activity and UPR has not yet been established. In the present study, we demonstrated that ER stress activated TGase2 in various cell types. TGase2 activation was dependent on the ER stress-induced increase in the intracellular calcium ion concentration but not on the TGase2 protein expression level. Enzyme substrate analysis revealed that TGase2-mediated protein modification promoted protein aggregation concurrently with decreasing water solubility. Moreover, treatment with KCC009, a TGase2 inhibitor, abrogated ER stress-induced TGase2 activation and subsequent protein aggregation. However, TGase2 activation had no effect on ER stress-induced cell death. These results demonstrate that the accumulation of misfolded proteins activates TGase2, which further accelerates the formation of protein aggregates. Therefore, we suggest that inhibition of TGase2 may be a novel strategy by which to prevent the protein aggregation in age-related degenerative diseases.

Size and position matter

The 2013 Nobel Prize in Physiology or Medicine emphasizes the progress made in understanding the molecular mechanisms that underpin the vesicular movement of cargo through the exocytic and endocytic pathways. Attention now focuses on those mechanisms that govern the relative size and position of the many different membrane-bound compartments. These homeostatic mechanisms are discussed in this issue of Nature Reviews Molecular Cell Biology and must be integrated so as to satisfy the needs of the cell and the organism.

Chondrocyte-specific pathology during skeletal growth and therapeutics in a murine model of pseudoachondroplasia

Mutations in the gene encoding cartilage oligomeric matrix protein (COMP) cause pseudoachondroplasia (PSACH), a severe dwarfing condition. Pain, a significant complication, has generally been attributed to joint abnormalities and erosion and early onset osteoarthritis. Previously, we found that the inflammatory-related transcripts were elevated in growth plate and articular cartilages, indicating that inflammation plays an important role in the chondrocyte disease pathology and may contribute to the overall pain sequelae. Here, we describe the effects of D469-delCOMP expression on the skeleton and growth plate chondrocytes with the aim to define a treatment window and thereby reduce pain. Consistent with the human PSACH phenotype, skeletal development of D469del-COMP mice was normal and similar to controls at birth. By postnatal day 7 (P7), the D469del-COMP skeleton, limbs, skull and snout were reduced and this reduction was progressive during postnatal growth, resulting in a short-limbed dwarfed mouse. Modulation of prenatal and postnatal expression of D469del-COMP showed minimal retention/cell death at P7 with some retention/cell death by P14, suggesting that earlier treatment intervention at the time of PSACH diagnosis may produce optimal results. Important and novel findings were an increase in inflammatory proteins generally starting at P21 and that exercise exacerbates inflammation. These observations suggest that pain in PSACH may be related to an intrinsic inflammatory process that can be treated symptomatically and is not related to early joint erosion. We also show that genetic ablation of CHOP dampens the inflammatory response observed in mice expressing D469del-COMP. Toward identifying potential treatments, drugs known to decrease cellular stress (lithium, phenylbutyric acid, and valproate) were assessed. Interestingly, all diminished the chondrocyte pathology but had untoward outcomes on mouse growth, development, and longevity. Collectively, these results define an early treatment window in which chondrocytes can be salvaged, thereby potentially increasing skeletal growth and decreasing pain.

IRE1α is essential for Xenopus pancreas development

Inositol requiring enzyme-1 (IRE1) is highly conserved from yeasts to humans. Upon the endoplasmic reticulum (ER) stress, IRE1 activates X-box-binding protein 1 (XBP1) by unconventionally splicing XBP1 mRNA, which activates the unfolded protein response (UPR) to restore ER homeostasis. In mice, IRE1α inactivity leads to embryonic death and IRE1α plays an essential role in extraembryonic tissues and the placenta. However, its precise action in the embryo proper is still unknown. In this study, the loss of function analysis was performed to investigate the function of Xenopus IRE1α (xIRE1α) during pancreas development. Firstly, the complete open reading frame of xIRE1α was amplified and the expression pattern was detected. The effects of Xenopus IRE1α and XBP1 during embryo development were detected with whole-mount in situ hybridization. The results demonstrated that xIRE1α was much closer to human IRE1α when compared with their sequence alignment. xIRE1α was expressed strongly in developing pancreas and the knockdown of xIRE1α inhibited the differentiation and specification of the pancreas. xIRE1α, which was required for cytoplasmic splicing of XBP1 pre-mRNA and XBP1MO, also showed inhibitory effects on pancreas development. These results suggest that xIRE1α is essential for pancreas development during embryogenesis and functions via the XBP1 dependent pathway.

Endoplasmic reticulum stress in amelogenesis imperfecta and phenotypic rescue using 4-phenylbutyrate

Inherited diseases caused by genetic mutations can arise due to loss of protein function. Alternatively, mutated proteins may mis-fold, impairing endoplasmic reticulum (ER) trafficking, causing ER stress and triggering the unfolded protein response (UPR). The UPR attempts to restore proteostasis but if unsuccessful drives affected cells towards apoptosis. Previously, we reported that in mice, the p.Tyr64His mutation in the enamel extracellular matrix (EEM) protein amelogenin disrupts the secretory pathway in the enamel-forming ameloblasts, resulting in eruption of malformed tooth enamel that phenocopies human amelogenesis imperfecta (AI). Defective amelogenin post-secretory self-assembly and processing within the developing EEM has been suggested to underlie the pathogenesis of X chromosome-linked AI. Here, we challenge this concept by showing that AI pathogenesis associated with the p.Tyr64His amelogenin mutation involves ameloblast apoptosis induced by ER stress. Furthermore, we show that 4-phenylbutyrate can rescue the enamel phenotype in affected female mice by promoting cell survival over apoptosis such that they are able to complete enamel formation despite the presence of the mutation, offering a potential therapeutic option for patients with this form of AI and emphasizing the importance of ER stress in the pathogenesis of this inherited conformational disease.

A comparative systems analysis of polysaccharide-elicited responses in Neurospora crassa reveals carbon source-specific cellular adaptations

Filamentous fungi are powerful producers of hydrolytic enzymes for the deconstruction of plant cell wall polysaccharides. However, the central question of how these sugars are perceived in the context of the complex cell wall matrix remains largely elusive. To address this question in a systematic fashion we performed an extensive comparative systems analysis of how the model filamentous fungus Neurospora crassa responds to the three main cell wall polysaccharides: pectin, hemicellulose and cellulose. We found the pectic response to be largely independent of the cellulolytic one with some overlap to hemicellulose, and in its extent surprisingly high, suggesting advantages for the fungus beyond being a mere carbon source. Our approach furthermore allowed us to identify carbon source-specific adaptations, such as the induction of the unfolded protein response on cellulose, and a commonly induced set of 29 genes likely involved in carbon scouting. Moreover, by hierarchical clustering we generated a coexpression matrix useful for the discovery of new components involved in polysaccharide utilization. This is exemplified by the identification of lat-1, which we demonstrate to encode for the physiologically relevant arabinose transporter in Neurospora. The analyses presented here are an important step towards understanding fungal degradation processes of complex biomass.

Ox-LDL induces ER stress and promotes the adipokines secretion in 3T3-L1 adipocytes

Adipocytes behave as a rich source of adipokines, which may be the link between obesity and its complications. Endoplasmic reticulum (ER) stress in adipocytes can modulate adipokines secretion. The aim of this study is to evaluate the effect of oxidized low density lipoprotein（ox-LDL）treatment on ER stress and adipokines secretion in differentiated adipocytes. 3T3-L1 pre-adipocytes were cultured and differentiated into mature adipocytes in vitro. Differentiated adipocytes were incubated with various concentrations of ox-LDL (0-100 µg/ml) for 48 hours; 50µg/ml ox-LDL for various times (0-48 hours) with or without tauroursodeoxycholic acid (TUDCA) (0-400µM) pre-treatment. The protein expressions of ER stress markers, glucose regulated protein 78(GRP78) and CCAAT/enhancer binding protein [C/EBP] homologous protein (CHOP) in adipocytes were detected by Western blot. The mRNA expressions of visfatin and resistin were measured by real-time PCR and the protein release of visfatin and resistin in supernatant were determined by ELISA. Treatment with ox-LDL could increase the cholesterol concentration in adipocytes. Ox-LDL induced the expressions of GRP78 and CHOP protein in adipocytes and promoted visfatin and resistin secretion in culture medium in dose and time-dependent manner. TUDCA could attenuate the effect of ox-LDL on GRP78 and CHOP expressions and reduce visfatin and resistin at mRNA and protein level in dose-dependent manner. In conclusion, ox-LDL promoted the expression and secretion of visfatin and resistin through its activation of ER stress, which may be related to the increase of cholesterol load in adipocytes.

The fungal UPR: a regulatory hub for virulence traits in the mold pathogen Aspergillus fumigatus

Aspergillus fumigatus is an opportunistic pathogen that is responsible for a life-threatening fungal infection known as invasive aspergillosis. Current therapies for the treatment of this disease continue to be associated with a poor outcome, so there is a need for more information about aspects of the fungus-host interaction that could offer novel targets for drug intervention. One attractive possibility is the unfolded protein response (UPR), an intracellular signaling network that helps the fungus meet the demand for secretion in the host environment. The major function of the UPR is to mitigate ER stress by maintaining an equilibrium between the load of client proteins entering the endoplasmic reticulum (ER) and the protein folding capacity of the organelle. However, recent findings suggest that A. fumigatus, as well as several other pathogenic fungi, also rely upon this pathway for virulence. In this review, we provide an update on the A. fumigatus UPR, discuss emerging evidence that the UPR is situated at the nexus of a number of physiological functions that are vital for the virulence of this fungus, and suggest exciting possibilities for future therapeutic targeting of this pathway for the treatment of aspergillosis.

Progesterone regulates the expression and activity of two mouse isoforms of the glycoprotein folding sensor UDP-Glc: glycoprotein glucosyltransferase (UGGT)

UDP-Glucose:glycoprotein glucosyltransferase (UGGT) is a central component of the endoplasmic reticulum (ER) glycoprotein-folding quality control system, which prevents the exit of partially folded species. UGGT activity can be regulated by the accumulation of misfolded proteins in the ER, a stimulus that triggers a complex signaling pathway known as unfolded protein response (UPR) which is closely associated with inflammation and disease. In this work, we investigated the effect of progesterone (P4) on the expression and activity of UGGT in a mouse hybridoma. We detected the expression of two UGGT isoforms, UGGT1 and UGGT2, and demonstrated that both isoforms are active in these cells. Interestingly, the expression of each isoform is regulated by high physiological P4 concentrations. This work provides the first evidence of a hormonal regulation of UGGT isoform expression and activity, which might influence the glycoprotein quality control mechanism. These findings could contribute to the study of pathologies triggered by the accumulation of misfolded proteins.

Crosstalk between the unfolded protein response and pathways that regulate pathogenic development in Ustilago maydis

The unfolded protein response (UPR) is a conserved eukaryotic signaling pathway regulating endoplasmic reticulum (ER) homeostasis during ER stress, which results, for example, from an increased demand for protein secretion. Here, we characterize the homologs of the central UPR regulatory proteins Hac1 (for Homologous to ATF/CREB1) and Inositol Requiring Enzyme1 in the plant pathogenic fungus Ustilago maydis and demonstrate that the UPR is tightly interlinked with the b mating-type-dependent signaling pathway that regulates pathogenic development. Exact timing of UPR is required for virulence, since premature activation interferes with the b-dependent switch from budding to filamentous growth. In addition, we found crosstalk between UPR and the b target Clampless1 (Clp1), which is essential for cell cycle release and proliferation in planta. The unusual C-terminal extension of the U. maydis Hac1 homolog, Cib1 (for Clp1 interacting bZIP1), mediates direct interaction with Clp1. The interaction between Clp1 and Cib1 promotes stabilization of Clp1, resulting in enhanced ER stress tolerance that prevents deleterious UPR hyperactivation. Thus, the interaction between Cib1 and Clp1 constitutes a checkpoint to time developmental progression and increased secretion of effector proteins at the onset of biotrophic development. Crosstalk between UPR and the b mating-type regulated developmental program adapts ER homeostasis to the changing demands during biotrophy.

Regulation of sumo mRNA during endoplasmic reticulum stress

The unfolded protein response (UPR) is a collection of pathways that maintains the protein secretory pathway during the many physiological and pathological conditions that cause stress in the endoplasmic reticulum (ER). The UPR is mediated in part by Ire1, an ER transmembrane kinase and endoribonuclease that is activated when misfolded proteins accumulate in the ER. Ire1's nuclease initiates the cytosolic splicing of the mRNA encoding X-box binding protein (Xbp1), a potent transcription factor that then upregulates genes responsible for restoring ER function. This same nuclease is responsible for the degradation of many other mRNAs that are localized to the ER, through Regulated Ire1 Dependent Decay (RIDD). Here we show that Smt3, a homolog of small ubiquitin-like modifier (sumo), is a non-canonical RIDD target in Drosophila S2 cells. Unlike other RIDD targets, the sumo transcript does not stably associate with the ER membrane, but instead relies on an Xbp1-like stem loop and a second UPR mediator, Perk, for its degradation during stress.

Induction of the unfolded protein response drives enhanced metabolism and chemoresistance in glioma cells

The unfolded protein response (UPR) is an endoplasmic reticulum (ER)-based cytoprotective mechanism acting to prevent pathologies accompanying protein aggregation. It is frequently active in tumors, but relatively unstudied in gliomas. We hypothesized that UPR stress effects on glioma cells might protect tumors from additional exogenous stress (ie, chemotherapeutics), postulating that protection was concurrent with altered tumor cell metabolism. Using human brain tumor cell lines, xenograft tumors, human samples and gene expression databases, we determined molecular features of glioma cell UPR induction/activation, and here report a detailed analysis of UPR transcriptional/translational/metabolic responses. Immunohistochemistry, Western and Northern blots identified elevated levels of UPR transcription factors and downstream ER chaperone targets in gliomas. Microarray profiling revealed distinct regulation of stress responses between xenograft tumors and parent cell lines, with gene ontology and network analyses linking gene expression to cell survival and metabolic processes. Human glioma samples were examined for levels of the ER chaperone GRP94 by immunohistochemistry and for other UPR components by Western blotting. Gene and protein expression data from patient gliomas correlated poor patient prognoses with increased expression of ER chaperones, UPR target genes, and metabolic enzymes (glycolysis and lipogenesis). NMR-based metabolomic studies revealed increased metabolic outputs in glucose uptake with elevated glycolytic activity as well as increased phospholipid turnover. Elevated levels of amino acids, antioxidants, and cholesterol were also evident upon UPR stress; in particular, recurrent tumors had overall higher lipid outputs and elevated specific UPR arms. Clonogenicity studies following temozolomide treatment of stressed or unstressed cells demonstrated UPR-induced chemoresistance. Our data characterize the UPR in glioma cells and human tumors, and link the UPR to chemoresistance possibly via enhanced metabolism. Given the role of the UPR in the balance between cell survival and apoptosis, targeting the UPR and/or controlling metabolic activity may prove beneficial for malignant glioma therapeutics.

IRE1: ER stress sensor and cell fate executor

Cells operate a signaling network termed the unfolded protein response (UPR) to monitor protein-folding capacity in the endoplasmic reticulum (ER). Inositol-requiring enzyme 1 (IRE1) is an ER transmembrane sensor that activates the UPR to maintain the ER and cellular function. Although mammalian IRE1 promotes cell survival, it can initiate apoptosis via decay of antiapoptotic miRNAs. Convergent and divergent IRE1 characteristics between plants and animals underscore its significance in cellular homeostasis. This review provides an updated scenario of the IRE1 signaling model, discusses emerging IRE1 sensing mechanisms, compares IRE1 features among species, and outlines exciting future directions in UPR research.

Simultaneous transcriptome analysis of Sorghum and Bipolaris sorghicola by using RNA-seq in combination with de novo transcriptome assembly

The recent development of RNA sequencing (RNA-seq) technology has enabled us to analyze the transcriptomes of plants and their pathogens simultaneously. However, RNA-seq often relies on aligning reads to a reference genome and is thus unsuitable for analyzing most plant pathogens, as their genomes have not been fully sequenced. Here, we analyzed the transcriptomes of Sorghum bicolor (L.) Moench and its pathogen Bipolaris sorghicola simultaneously by using RNA-seq in combination with de novo transcriptome assembly. We sequenced the mixed transcriptome of the disease-resistant sorghum cultivar SIL-05 and B. sorghicola in infected leaves in the early stages of infection (12 and 24 h post-inoculation) by using Illumina mRNA-Seq technology. Sorghum gene expression was quantified by aligning reads to the sorghum reference genome. For B. sorghicola, reads that could not be aligned to the sorghum reference genome were subjected to de novo transcriptome assembly. We identified genes of B. sorghicola for growth of this fungus in sorghum, as well as genes in sorghum for the defense response. The genes of B. sorghicola included those encoding Woronin body major protein, LysM domain-containing intracellular hyphae protein, transcriptional factors CpcA and HacA, and plant cell-wall degrading enzymes. The sorghum genes included those encoding two receptors of the simple eLRR domain protein family, transcription factors that are putative orthologs of OsWRKY45 and OsWRKY28 in rice, and a class III peroxidase that is a homolog involved in disease resistance in the Poaceae. These defense-related genes were particularly strongly induced among paralogs annotated in the sorghum genome. Thus, in the absence of genome sequences for the pathogen, simultaneous transcriptome analysis of plant and pathogen by using de novo assembly was useful for identifying putative key genes in the plant-pathogen interaction.

Endoplasmic reticulum (ER) stress response and its physiological roles in plants

The endoplasmic reticulum (ER) stress response is a highly conserved mechanism that results from the accumulation of unfolded or misfolded proteins in the ER. The response plays an important role in allowing plants to sense and respond to adverse environmental conditions, such as heat stress, salt stress and pathogen infection. Since the ER is a well-controlled microenvironment for proper protein synthesis and folding, it is highly susceptible to stress conditions. Accumulation of unfolded or misfolded proteins activates a signaling pathway, called the unfolded protein response (UPR), which acts to relieve ER stress and, if unsuccessful, leads to cell death. Plants have two arms of the UPR signaling pathway, an arm involving the proteolytic processing of membrane-associated basic leucine zipper domain (bZIP) transcription factors and an arm involving RNA splicing factor, IRE1, and its mRNA target. These signaling pathways play an important role in determining the cell's fate in response to stress conditions.

Defects in IRE1 enhance cell death and fail to degrade mRNAs encoding secretory pathway proteins in the Arabidopsis unfolded protein response

The unfolded protein response (UPR) is a cellular response highly conserved in eukaryotes to obviate accumulation of misfolded proteins in the endoplasmic reticulum (ER). Inositol-requiring enzyme 1 (IRE1) catalyzes the cytoplasmic splicing of mRNA encoding bZIP transcription factors to activate the UPR signaling pathway. Arabidopsis IRE1 was recently shown to be involved in the cytoplasmic splicing of bZIP60 mRNA. In the present study, we demonstrated that an Arabidopsis mutant with defects in two IRE1 paralogs showed enhanced cell death upon ER stress compared with a mutant with defects in bZIP60 and wild type, suggesting an alternative function of IRE1 in the UPR. Analysis of our previous microarray data and subsequent quantitative PCR indicated degradation of mRNAs encoding secretory pathway proteins by tunicamycin, DTT, and heat in an IRE1-dependent manner. The degradation of mRNAs localized to the ER during the UPR was considered analogous to a molecular mechanism referred to as the regulated IRE1-dependent decay of mRNAs reported in metazoans. Another microarray analysis conducted in the condition repressing transcription with actinomycin D and a subsequent Gene Set Enrichment Analysis revealed the regulated IRE1-dependent decay of mRNAs-mediated degradation of a significant portion of mRNAs encoding the secretory pathway proteins. In the mutant with defects in IRE1, genes involved in the cytosolic protein response such as heat shock factor A2 were up-regulated by tunicamycin, indicating the connection between the UPR and the cytosolic protein response.

Evolution of the unfolded protein response

The unfolded protein response (UPR) is a network of signaling pathways that responds to stress in the endoplasmic reticulum (ER). The general output of the UPR is to upregulate genes involved in ER function, thus restoring and/or increasing the capacity of the ER to fold and process proteins. In parallel, many organisms have mechanisms for limiting the load on the ER by attenuating translation or degrading ER-targeted mRNAs. Despite broad conservation of these signaling pathways across eukaryotes, interesting variations demonstrate a variety of mechanisms for managing ER stress. How do early-diverging protozoa respond to stress when they lack traditional transcriptional regulation? What is the role of the ER stress sensor Ire1 in fungal species that are missing its main target? Here I describe how diverse species have optimized the UPR to fit their needs. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Cytoplasmic organelles on the road to mRNA decay

Localization of both mRNAs and mRNA decay factors to internal membranes of eukaryotic cells provides a means of coordinately regulating mRNAs with common functions as well as coupling organelle function to mRNA turnover. The classic mechanism of mRNA localization to membranes is the signal sequence-dependent targeting of mRNAs encoding membrane and secreted proteins to the cytoplasmic surface of the endoplasmic reticulum. More recently, however, mRNAs encoding proteins with cytosolic or nuclear functions have been found associated with various organelles, in many cases through unknown mechanisms. Furthermore, there are several types of RNA granules, many of which are sites of mRNA degradation; these are frequently found associated with membrane-bound organelles such as endosomes and mitochondria. In this review we summarize recent findings that link organelle function and mRNA localization to mRNA decay. This article is part of a Special Issue entitled: RNA Decay mechanisms.

Comparison of mRNA localization and regulation during endoplasmic reticulum stress in Drosophila cells

Ire1 is an endoplasmic reticulum (ER) transmembrane protein that senses disturbances in protein folding homeostasis and contributes to a multifaceted response to stress. The nuclease activity of Ire1, in addition to splicing the mRNA encoding the transcription factor Xbp1, mediates mRNA degradation in response to ER stress through a pathway termed regulated Ire1-dependent decay (RIDD). We previously showed that ER targeting of substrates is necessary for RIDD; in this paper, we show that ER localization is also sufficient to induce decay in a normally unaffected mRNA. Using microarrays, we also measured relative mRNA degradation in the presence and absence of ER stress in Drosophila S2 cells, and determined mRNA membrane association using detergent fractionation. The vast majority of mRNAs that were strongly associated with the ER were degraded faster during ER stress in an Ire1-dependent manner, suggesting that RIDD is the default pathway for ER-localized mRNAs during stress. We also show that the mRNA encoding plexin A remains highly polysome associated during stress and escapes degradation by RIDD, and that its 5' untranslated region can protect a strong RIDD target from degradation. These results suggest that while translation is generally attenuated during ER stress, continued translation of certain messages can protect them from degradation by RIDD.