Intracellular signaling from the endoplasmic reticulum to the nucleus

Nucleated synthetic cells with genetically driven intercompartment communication

Eukaryotic cells are characterized by multiple chemically distinct compartments, one of the most notable being the nucleus. Within these compartments, there is a continuous exchange of information, chemicals, and signaling molecules, essential for coordinating and regulating cellular activities. One of the main goals of bottom-up synthetic biology is to enhance the complexity of synthetic cells by establishing functional compartmentalization. There is a need to mimic autonomous signaling between compartments, which in living cells, is often regulated at the genetic level within the nucleus. This advancement is key to unlocking the potential of synthetic cells as cell models and as microdevices in biotechnology. However, a technological bottleneck exists preventing the creation of synthetic cells with a defined nucleus-like compartment capable of genetically programmed intercompartment signaling events. Here, we present an approach for creating synthetic cells with distinct nucleus-like compartments that can encapsulate different biochemical mixtures in discrete compartments. Our system enables in situ protein expression of membrane proteins, enabling autonomous chemical communication between nuclear and cytoplasmic compartments, leading to downstream activation of enzymatic pathways within the cell.

Increased activity of IRE1 improves the clinical presentation of EAE

Activation of the endoplasmic reticulum (ER) stress sensor inositol-requiring enzyme-1α (IRE1α) contributes to neuronal development and is known to induce neuronal remodeling in vitro and in vivo. On the contrary, excessive IRE1 activity is often detrimental and may contribute to neurodegeneration. To determine the consequences of increased activation of IRE1α, we used a mouse model expressing a C148S variant of IRE1α with increased and sustained activation. Surprisingly, the mutation did not affect the differentiation of highly secretory antibody-producing cells but exhibited a beneficial effect in a mouse model of experimental autoimmune encephalomyelitis (EAE). Although mechanical allodynia was unaffected, significant improvement in motor function was found in IRE1C148S mice with EAE relative to wild type (WT) mice. Coincident with this improvement, there was reduced microgliosis in the spinal cord of IRE1C148S mice, with reduced expression of proinflammatory cytokine genes. This was accompanied by reduced axonal degeneration and enhanced 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) levels, suggesting improved myelin integrity. Interestingly, while the IRE1C148S mutation is expressed in all cells, the reduction in proinflammatory cytokines and in the microglial activation marker ionized calcium-binding adapter molecule (IBA1), along with preservation of phagocytic gene expression, all point to microglia as the cell type contributing to the clinical improvement in IRE1C148S animals. Our data suggest that sustained increase in IRE1α activity can be beneficial in vivo, and that this protection is cell type and context dependent. Considering the overwhelming but conflicting evidence for the role of ER stress in neurological diseases, a better understanding of the function of ER stress sensors in physiological contexts is clearly needed.

Characterization of endoplasmic reticulum-associated degradation in the human fungal pathogen Candida albicans

Background

Candida albicans is the most prevalent human fungal pathogen. In immunocompromised individuals, C. albicans can cause serious systemic disease, and patients infected with drug-resistant isolates have few treatment options. The ubiquitin-proteasome system has not been thoroughly characterized in C. albicans. Research from other organisms has shown ubiquitination is important for protein quality control and regulated protein degradation at the endoplasmic reticulum (ER) via ER-associated protein degradation (ERAD).

Methods

Here we perform the first characterization, to our knowledge, of ERAD in a human fungal pathogen. We generated functional knockouts of C. albicans genes encoding three proteins predicted to play roles in ERAD, the ubiquitin ligases Hrd1 and Doa10 and the ubiquitin-conjugating enzyme Ubc7. We assessed the fitness of each mutant in the presence of proteotoxic stress, and we used quantitative tandem mass tag mass spectrometry to characterize proteomic alterations in yeast lacking each gene.

Results

Consistent with a role in protein quality control, yeast lacking proteins thought to contribute to ERAD displayed hypersensitivity to proteotoxic stress. Furthermore, each mutant displayed distinct proteomic profiles, revealing potential physiological ERAD substrates, co-factors, and compensatory stress response factors. Among candidate ERAD substrates are enzymes contributing to ergosterol synthesis, a known therapeutic vulnerability of C. albicans. Together, our results provide the first description of ERAD function in C. albicans, and, to our knowledge, any pathogenic fungus.

The acyltransferase Gpc1 is both a target and an effector of the unfolded protein response in Saccharomyces cerevisiae

The unfolded protein response (UPR) is sensitive to proteotoxic and membrane bilayer stress, both of which are sensed by the ER protein Ire1. When activated, Ire1 splices HAC1 mRNA, producing a transcription factor that targets genes involved in proteostasis and lipid metabolism, among others. The major membrane lipid phosphatidylcholine (PC) is subject to phospholipase-mediated deacylation, producing glycerophosphocholine (GPC), followed by reacylation of GPC through the PC deacylation/reacylation pathway (PC-DRP). The reacylation events occur via a two-step process catalyzed first by the GPC acyltransferase Gpc1, followed by acylation of the lyso-PC molecule by Ale1. However, whether Gpc1 is critical for ER bilayer homeostasis is unclear. Using an improved method for C14-choline-GPC radiolabeling, we first show that loss of Gpc1 results in abrogation of PC synthesis through PC-DRP and that Gpc1 colocalizes with the ER. We then probe the role of Gpc1 as both a target and an effector of the UPR. Exposure to the UPR-inducing compounds tunicamycin, DTT, and canavanine results in a Hac1-dependent increase in GPC1 message. Further, cells lacking Gpc1 exhibit increased sensitivity to those proteotoxic stressors. Inositol limitation, known to induce the UPR via bilayer stress, also induces GPC1 expression. Finally, we show that loss of GPC1 induces the UPR. A gpc1Δ mutant displays upregulation of the UPR in strains expressing a mutant form of Ire1 that is unresponsive to unfolded proteins, indicating that bilayer stress is responsible for the observed upregulation. Collectively, our data indicate an important role for Gpc1 in yeast ER bilayer homeostasis.

Increased activity of IRE1 improves the clinical presentation of EAE

Activation of the ER stress sensor IRE1α contributes to neuronal development and is known to induce neuronal remodeling in vitro and in vivo. On the other hand, excessive IRE1 activity is often detrimental and may contribute to neurodegeneration. To determine the consequences of increased activation of IRE1α, we used a mouse model expressing a C148S variant of IRE1α with increased and sustained activation. Surprisingly, the mutation did not affect the differentiation of highly secretory antibody-producing cells, but exhibited a strong protective effect in a mouse model of experimental autoimmune encephalomyelitis (EAE). Significant improvement in motor function was found in IRE1C148S mice with EAE relative to WT mice. Coincident with this improvement, there was reduced microgliosis in the spinal cord of IRE1C148S mice, with reduced expression of pro-inflammatory cytokine genes. This was accompanied by reduced axonal degeneration and enhanced CNPase levels, suggestiing improved myelin integrity. Interestingly, while the IRE1C148S mutation is expressed in all cells, the reduction in proinflammatory cytokines and in the activation of microglial activation marker IBA1, along with preservation of phagocytic gene expression, all point to microglia as the cell type contributing to the clinical improvement in IRE1C148S animals. Our data suggest that sustained increase in IRE1α activity can be protective in vivo, and that this protection is cell type and context dependent. Considering the overwhelming but conflicting evidence for the role of the ER stress in neurological diseases, a better understanding of the function of ER stress sensors in physiological contexts is clearly needed.

Immunogenic cell death inducer peptides: A new approach for cancer therapy, current status and future perspectives

Immunogenic Cell Death (ICD) is a type of cell death that kills tumor cells by stimulating the adaptive immune response against other tumor cells. ICD depends on the endoplasmic reticulum (ER) stress and the secretion of Damage-Associated Molecular Patterns (DAMP) by the dying tumor cell. DAMPs recruit innate immune cells such as Dendritic Cells (DC), triggering a cancer-specific immune response such as cytotoxic T lymphocytes (CTLs) to eliminate remaining cancer cells. ICD is accompanied by several hallmarks in dying cells, such as surface translocation of ER chaperones, calreticulin (CALR), and extracellular secretion of DAMPs such as high mobility group protein B1 (HMGB1) and adenosine triphosphate (ATP). Therapeutic peptides can kill bacteria and tumor cells thus affecting the immune system. They have high specificity and affinity for their targets, small size, appropriate cell membrane penetration, short half-life, and simple production processes. Peptides are interesting agents for immunomodulation since they may overcome the limitations of other therapeutics. Thus, the development of peptides affecting the TME and active antitumoral immunity has been actively pursued. On the other hand, several peptides have been recently identified to trigger ICD and anti-cancer responses. In the present review, we review previous studies on peptide-induced ICD, their mechanism, their targets, and markers. They include anti-microbial peptides (AMPs), cationic or mitochondrial targeting, checkpoint inhibitors, antiapoptotic inhibitors, and "don't eat me" inhibitor peptides. Also, peptides will be investigated potentially inducing ICD that is divided into ER stressors, ATPase inhibitors, and anti-microbial peptides.

Activation of the UPR sensor ATF6α is regulated by its redox-dependent dimerization and ER retention by ERp18

Membrane and secretory proteins are synthesized in the endoplasmic reticulum (ER). Perturbations to ER function disrupts protein folding, causing misfolded proteins to accumulate, a condition known as ER stress. Cells adapt to stress by activating the unfolded protein response (UPR), which ultimately restores proteostasis. A key player in the UPR response is ATF6α, which requires release from ER retention and modulation of its redox status during activation. Here, we report that ER stress promotes formation of a specific ATF6α dimer, which is preferentially trafficked to the Golgi for processing. We show that ERp18 regulates ATF6α by mitigating its dimerization and trafficking to the Golgi and identify redox-dependent oligomerization of ATF6α as a key mechanism regulating its function during the UPR.

The unfolded protein response (UPR) maintains cellular proteostasis during stress by activating sensors located to the endoplasmic reticulum (ER) membrane. A major sensor for this response, ATF6α, is activated by release from ER retention and trafficking to the Golgi, where it is cleaved to generate a bZIP transactivator to initiate a transcriptional response. The reduction of a disulfide in monomeric ATF6α is thought to be necessary for release from retention, trafficking, and proteolysis. Here we show that, following ER stress, ATF6α undergoes a redox switch to form a disulfide bonded dimer, which traffics to the Golgi for cleavage by the S1P protease. Additionally, we find that overexpression of ERp18 attenuates dimer formation thereby limiting Golgi trafficking. Our results provide mechanistic insight into activation of the ATF6α pathway, revealing an unexpected role for redox-dependent oligomerization prior to Golgi trafficking.

The novel ER stress inducer Sec C triggers apoptosis by sulfating ER cysteine residues and degrading YAP via ER stress in pancreatic cancer cells

Pancreatic adenocarcinoma (PAAD) is one of the most lethal malignancies. Although gemcitabine (GEM) is a standard treatment for PAAD, resistance limits its application and therapy. Secoemestrin C (Sec C) is a natural compound from the endophytic fungus Emericella, and its anticancer activity has not been investigated since it was isolated. Our research is the first to indicate that Sec C is a broad-spectrum anticancer agent and could exhibit potently similar anticancer activity both in GEM-resistant and GEM-sensitive PAAD cells. Interestingly, Sec C exerted a rapid growth-inhibiting effect (80% death at 6 h), which might be beneficial for patients who need rapid tumor shrinkage before surgery. Liquid chromatography/mass spectrometry and N-acetyl-l-cysteine (NAC) reverse assays show that Sec C sulfates cysteines to disrupt disulfide-bonds formation in endoplasmic reticulum (ER) proteins to cause protein misfolding, leading to ER stress and disorder of lipid biosynthesis. Microarray data and subsequent assays show that ER stress-mediated ER-associated degradation (ERAD) ubiquitinates and downregulates YAP to enhance ER stress via destruction complex (YAP-Axin-GSK-βTrCP), which also elucidates a unique degrading style for YAP. Potent anticancer activity in GEM-resistant cells and low toxicity make Sec C a promising anti-PAAD candidate.

The H2A N-terminal tail is required to alleviate copper-induced stress in Saccharomyces cerevisiae

Histone tail residues drive many biological processes by regulating genome-wide transcription. Functions of histone H3 and H4 tail residues in stress-responsive gene transcriptional programs have been extensively studied. The H2A tail residues have been shown to regulate DNA damage repair and oxidative stress response, but the involvement of N-terminal tail of H2A (H2ANtT) in proteostasis regulation is unknown. The unfolded protein response pathway (UPR) is an essential mechanism adopted by cells to prevent protein toxicity in response to ER stress. The disturbance in ER can occur by various factors such as heat stress, redox imbalance, exposure to xenobiotics and metals. Copper is utilized as a cofactor by cellular enzymes, but excessive copper affects ER homeostasis. We found that cells lacking 1-20 residues of H2ANtT are intolerant to copper stress, owing to the accumulation of misfolded proteins in the mutant cells. H2A 1-20 truncation also reduces the physiological UPR, and copper exposure further aggravates this effect. Furthermore, the expression of a spliced version of HAC1 mRNA in H2A∆(1-20) cells, encoding the downstream transcription factor of UPR signalling, rescues their growth under copper stress. Altogether these results provide evidence that H2ANtT reduces copper-induced ER stress by regulating UPR signalling.

Mechanism of Endoplasmic Reticulum Stress in Cerebral Ischemia

Endoplasmic reticulum (ER) is the main organelle for protein synthesis, trafficking and maintaining intracellular Ca²⁺ homeostasis. The stress response of ER results from the disruption of ER homeostasis in neurological disorders. Among these disorders, cerebral ischemia is a prevalent reason of death and disability in the world. ER stress stemed from ischemic injury initiates unfolded protein response (UPR) regarded as a protection mechanism. Important, disruption of Ca²⁺ homeostasis resulted from cytosolic Ca²⁺ overload and depletion of Ca²⁺ in the lumen of the ER could be a trigger of ER stress and the misfolded protein synthesis. Brain cells including neurons, glial cells and endothelial cells are involved in the complex pathophysiology of ischemic stroke. This is generally important for protein underfolding, but even more for cytosolic Ca²⁺ overload. Mild ER stress promotes cells to break away from danger signals and enter the adaptive procedure with the activation of pro-survival mechanism to rescue ischemic injury, while chronic ER stress generally serves as a detrimental role on nerve cells via triggering diverse pro-apoptotic mechanism. What’s more, the determination of some proteins in UPR during cerebral ischemia to cell fate may have two diametrically opposed results which involves in a specialized set of inflammatory and apoptotic signaling pathways. A reasonable understanding and exploration of the underlying molecular mechanism related to ER stress and cerebral ischemia is a prerequisite for a major breakthrough in stroke treatment in the future. This review focuses on recent findings of the ER stress as well as the progress research of mechanism in ischemic stroke prognosis provide a new treatment idea for recovery of cerebral ischemia.

The consensus Nglyco -X-S/T motif and a previously unknown Nglyco -N-linked glycosylation are necessary for growth and pathogenicity of Phytophthora

Asparagine (Asn, N)-linked glycosylation within N_glyco -X-S/T; X ≠ P motif is a ubiquitously distributed post-translational modification that participates in diverse cellular processes. In this work, N-glycosylation inhibitor was shown to prevent Phytophthora sojae growth, suggesting that N-glycosylation is necessary for oomycete development. We conducted a glycoproteomic analysis of P. sojae to identify and map N-glycosylated proteins and to quantify differentially expressed glycoproteins associated with mycelia, asexual cyst, and sexual oospore developmental stages. A total of 355 N-glycosylated proteins was found, containing 496 glycosites, potentially involved in glycan degradation, carbon metabolism, glycolysis, or other metabolic pathways. Through PNGase F deglycosylation assays and site-directed mutagenesis of a GPI transamidase protein (GPI16) upregulated in cysts and a heat shock protein 70 (HSP70) upregulated in oospores, we demonstrated that both proteins were N-glycosylated and that the N_glyco -N motif is a target site for asparagine - oligosaccharide linkage. Glycosite mutations of Asn 94 N_glyco -X-S/T in the GPI16 led to impaired cyst germination and pathogenicity, while mutation of the previously unknown Asn 270 N_glyco -N motif in HSP70 led to decreased oospore production. In addition to providing a map of the oomycete N-glycoproteome, this work confirms that P. sojae has evolved multiple N-glycosylation motifs essential for growth.

Identification and characteristics of insulin-like growth factor system in the brain, liver, and gonad during development of a seasonal breeding teleost, Pampus argenteus

Reproductive activity is closely related to the development and function of the brain and liver in teleosts, particularly in seasonal breeding teleosts. This study measured the involvement of the insulin-like growth factor (IGF) system in controlling the reproduction of the silver pomfret Pampus argenteus, a seasonal breeding tropical to temperate commercial fish. We cloned and characterized the cDNAs of igfs (igf2 and igf3) and igfrs (igf1ra, igf1rb, and igf2r) and examined their transcript levels in relation to seasonal reproduction. Phylogenetic analyses revealed that two types of IGFs (IGF-1 and IGF-2) and three types of IGFRs (IGF1RA, IGF1RB, and IGF2R) of the silver pomfret were clustered with those of teleosts; however, IGF-3 was a transmembrane protein different with the IGF-3 of other teleosts. The expression of IGF-3 was gonad-specific in the silver pomfret. The transcript levels of igf1 in the female brain were the highest, and the levels of igfrs in both sexes' brains increased during gametogenesis. Meanwhile, igfs and igfrs maintained high transcript levels in both sexes' liver and gonad during vitellogenesis and spermatogonia proliferation. We concluded that the development and activities of brain, liver, and gonad were related to the IGF system (IGFs and IGFRs). And the IGFs were mainly expressed in the liver. Nevertheless, gonadal development, especially vitellogenesis and spermatogonia proliferation, were related with IGFs in this species.

Yeast MED2 is involved in the endoplasmic reticulum stress response and modulation of the replicative lifespan

Saccharomyces cerevisiae MED2/YDL005C is a subunit of the mediator complex (Mediator), which is responsible for tightly controlling the transcription of protein-coding genes by mediating the interaction of RNA polymerase II with gene-specific transcription factors. Although a high-throughput analysis in yeast showed that the MED2 protein exhibits altered cellular localization under hypoxic stress, no specific function of MED2 has been described to date. In this study, we first provided evidence that MED2 is involved in the endoplasmic reticulum (ER) stress response and modulation of the replicative life span. We showed that deletion of MED2 leads to sensitivity to the ER stress inducer tunicamycin (TM) as well as a shortened replicative lifespan (RLS), accompanied by increased intracellular ROS levels and hyperpolarization of mitochondria. On the other hand, overexpression of MED2 in wild-type (WT) yeast enhanced TM resistance and extended the RLS. In addition, the IRE1-HAC1 pathway was essential for the TM resistance of MED2-overexpressing cells. Moreover, we showed that MED2 deficiency enhances ER unfolded protein response (UPR) activity compared to that in WT cells. Collectively, these results suggest the novel role of MED2 as a regulator in maintaining ER homeostasis and longevity.

Beyond Trees: Regulons and Regulatory Motif Characterization

Trees and their seeds regulate their germination, growth, and reproduction in response to environmental stimuli. These stimuli, through signal transduction, trigger transcription factors that alter the expression of various genes leading to the unfolding of the genetic program. A regulon is conceptually defined as a set of target genes regulated by a transcription factor by physically binding to regulatory motifs to accomplish a specific biological function, such as the CO-FT regulon for flowering timing and fall growth cessation in trees. Only with a clear characterization of regulatory motifs, can candidate target genes be experimentally validated, but motif characterization represents the weakest feature of regulon research, especially in tree genetics. I review here relevant experimental and bioinformatics approaches in characterizing transcription factors and their binding sites, outline problems in tree regulon research, and demonstrate how transcription factor databases can be effectively used to aid the characterization of tree regulons.

Role of Endoplasmic Reticulum Stress in Otitis Media

Endoplasmic reticulum (ER) stress occurs in many inflammatory responses. Here, we investigated the role of ER stress and its associated apoptosis in otitis media (OM) to elucidate the mechanisms of OM and the signaling crosstalk between ER stress and other cell damage pathways, including inflammatory cytokines and apoptosis. We examined the expression of inflammatory cytokine- and ER stress-related genes by qRT-PCR, Western blotting, and immunohistochemistry (IHC) in the middle ear of C57BL/6J mice after challenge with peptidoglycan polysaccharide (PGPS), an agent inducing OM. We also evaluated the effect of the suppression of ER stress with tauroursodeoxycholic acid (TUDCA), an ER stress inhibitor. The study revealed the upregulation of ER stress- and apoptosis-related gene expression after the PGPS treatment, specifically ATF6, CHOP, BIP, caspase-12, and caspase-3. TUDCA treatment of PGPS-treated mice decreased OM; reduced the expression of CHOP, BIP, and caspase 3; and significantly decreased the proinflammatory gene expression of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6). These results suggest that PGPS triggers ER stress and downstream proinflammatory gene expression in OM and that inhibition of ER stress alleviates OM. We propose that ER stress plays a critical role in inflammation and cell death, leading to the development of OM and points to ER stress inhibition as a potential therapeutic approach for the prevention of OM.

Overexpression of the transcription factor HAC1 improves nerolidol production in engineered yeast

Increasing the metabolic flux of the mevalonate pathway, reducing the metabolic flux of competing pathway and utilizing the diauxie-inducible system constructed by GAL promoters are strategies commonly used in yeast metabolic engineering for the production of terpenoids. Using these strategies, we constructed a series of yeast strains with a strengthened mevalonate pathway and finally produced 336.5 mg/L nerolidol in a shake flask. The spliced HAC1 mRNA assay indicated that the unfolded protein response (UPR) occurred in the strains that we constructed. UPR strains exhibited the low transcriptional activities of GAL1 promoter. HAC1-overexpressing strain exhibited dramatically enhanced transcriptional activity of GAL1 promoter at 72 h of fermentation in flasks. HAC1 overexpression also increased the nerolidol titer by 47.7 %, reaching 497.0 mg/L and increased cell vitality. RNA-seq showed that the genes whose transcription responded to HAC1-overexpression were involved in the regulation of monocarboxylic acid metabolic processes and cellular amino acid biosynthetic process, indicating that the metabolic regulation may be part of the reason of the improved nerolidol synthesis. Our findings enrich the knowledge of the relationship between the construction of sesquiterpene-producing cell factories and UPR regulation. This study provides an effective strategy for sesquiterpene production in yeast.

Global Proteome Remodeling during ER Stress Involves Hac1-Driven Expression of Long Undecoded Transcript Isoforms

Cellular stress responses often require transcription-based activation of gene expression to promote cellular adaptation. Whether general mechanisms exist for stress-responsive gene downregulation is less clear. A recently defined mechanism enables both up- and downregulation of protein levels for distinct gene sets by the same transcription factor via coordinated induction of canonical mRNAs and long undecoded transcript isoforms (LUTIs). We analyzed parallel gene expression datasets to determine whether this mechanism contributes to the conserved Hac1-driven branch of the unfolded protein response (UPRER), indeed observing Hac1-dependent protein downregulation accompanying the upregulation of ER-related proteins that typifies UPRER activation. Proteins downregulated by Hac1-driven LUTIs include those with electron transport chain (ETC) function. Abrogated ETC function improves the fitness of UPRER-activated cells, suggesting functional importance to this regulation. We conclude that the UPRER drives large-scale proteome remodeling, including coordinated up- and downregulation of distinct protein classes, which is partly mediated by Hac1-induced LUTIs.

Quantitative Profiling of N-linked Glycosylation Machinery in Yeast Saccharomyces cerevisiae

Asparagine-linked glycosylation is a common posttranslational protein modification regulating the structure, stability and function of many proteins. The N-linked glycosylation machinery involves enzymes responsible for the assembly of the lipid-linked oligosaccharide (LLO), which is then transferred to the asparagine residues on the polypeptides by the enzyme oligosaccharyltransferase (OST). A major goal in the study of protein glycosylation is to establish quantitative methods for the analysis of site-specific extent of glycosylation. We developed a sensitive approach to examine glycosylation site occupancy in Saccharomyces cerevisiae by coupling stable isotope labeling (SILAC) approach to parallel reaction monitoring (PRM) mass spectrometry (MS). We combined the method with genetic tools and validated the approach with the identification of novel glycosylation sites dependent on the Ost3p and Ost6p regulatory subunits of OST. Based on the observations that alternations in LLO substrate structure and OST subunits activity differentially alter the systemic output of OST, we conclude that sequon recognition is a direct property of the catalytic subunit Stt3p, auxiliary subunits such as Ost3p and Ost6p extend the OST substrate range by modulating interfering pathways such as protein folding. In addition, our proteomics approach revealed a novel regulatory network that connects isoprenoid lipid biosynthesis and LLO substrate assembly.

JNK inhibitor alleviates apoptosis of fetal neural stem cells induced by emulsified isoflurane

Isoflurane can provide both neuroprotection and neurotoxicity in various culture models and in rodent developing brains. Emulsified Isoflurane (EI) is an emulsion formulation of isoflurane, while its underlying molecular mechanism of developemental nerve toxicity largely remains unclear. We hypothesized that EI induced fetal neural stem cells (FNSCs) apoptosis, endoplasmic reticulum (ER) stress and c-Jun N-terminal kinase (JNK) activation.

FNSCs were isolated from the cortex of SD rats during 14 days of gestation. The cell viability, cell apoptotic rates and the expression of apoptosis-related protein Caspase3, inositol requiring enzyme 1 (IRE1), poly (adenosine diphosphate-ribose) polymerase (PARP), Bax, Bcl-2, JNK, p-JNK and XBP1 were determined. Specific inhibition was performed by siRNA-targeting of JNK in FNSCs. EI could increase the p-JNK, JNK and caspase3 protein expression, the JNK pathway was activated by EI, and EI-induced apoptosis was blocked by inhibiting JNK pathway with SP600125 or JNK-small interfering RNA (siRNA), EI enhanced the level of IRE1, PARP, Bax/Bcl-2 and XBP1, which led FNSCs to apoptosis and ER stress. Meanwhile, dilatation of the ER lumens in FNSCs treated by EI for 24 h was significant. Green fluorescent protein (GFP) positive cell ratios were significantly decreased by FNSCs transfecting with JNK gene silencing. JNK was efficiently silenced in siRNA-JNK1 group.

The results provided in-vitro evidence which supports that the underlying mechanisms of EI-induced apoptosis are the induction of ER stress and sequent JNK activation. Together, these data suggest that JNK inhibiting might be applied for improving therapeutic outcomes in anesthestics-induced neurotoxicity.

Highlights:

1. Prolonged treatment with high-dose EI decreased the survival level of FNSCs by inducing apoptosis and inhibiting proliferation via the JNK signaling pathway.

2. EI induced ER stress and sequent JNK activation.

3. JNK inhibiting might be applied for improving therapeutic outcomes in anesthestics-induced neurotoxicity

The fail-safe mechanism of post-transcriptional silencing of unspliced HAC1 mRNA

HAC1 encodes a transcription factor that is the central effector of the unfolded protein response (UPR) in budding yeast. When the UPR is inactive, HAC1 mRNA is stored as an unspliced isoform in the cytoplasm and no Hac1 protein is detectable. Intron removal is both necessary and sufficient to relieve the post-transcriptional silencing of HAC1 mRNA, yet the precise mechanism by which the intron prevents Hac1 protein accumulation has remained elusive. Here, we show that a combination of inhibited translation initiation and accelerated protein degradation—both dependent on the intron—prevents the accumulation of Hac1 protein when the UPR is inactive. Functionally, both components of this fail-safe silencing mechanism are required to prevent ectopic production of Hac1 protein and concomitant activation of the UPR. Our results provide a mechanistic understanding of HAC1 regulation and reveal a novel strategy for complete post-transcriptional silencing of a cytoplasmic mRNA.

DOI:http://dx.doi.org/10.7554/eLife.20069.001

Molecular machines called ribosomes read the genetic instructions in an mRNA molecule and then translate them to make proteins. However, cells do not translate all of the template mRNAs that they have available into proteins; instead they have a number of ways to block the process to control when certain proteins are made.

In budding yeast, the mRNA that codes for a protein called Hac1 is always present in the cell but the protein is normally not detected. The Hac1 protein is responsible for helping the cell deal with certain types of stress, so it only accumulates when the cell is experiencing such stresses. The mRNA that encodes Hac1 (referred to as HAC1 mRNA) contains a sequence called an intron. These sequences are normally cut out of mRNAs before they are read by the ribosome. However, the intron in the HAC1 mRNA is unusual, because it is only removed when cells are subjected to stress. The rest of the time, this intron serves to block the production of Hac1 through a poorly understood mechanism.

Now, Di Santo et al. show the HAC1 mRNA uses two strategies to keep itself fully repressed—both of which involve its intron. One strategy relies on a structure formed in the HAC1 mRNA that prevents ribosomes from starting translation in the first place. However, this block is occasionally bypassed, causing some Hac1 protein to be produced when it should not be. To deal with this, the Hac1 protein that is produced contains a short protein sequence, encoded by the intron, that targets this unneeded protein for degradation. These two strategies together comprise a “fail-safe” mechanism to completely repress the HAC1 mRNA.

Following on from these findings, it will be important to determine whether other mRNAs – both in budding yeast and in other species including humans – use a similar fail-safe strategy to block proteins from being made when they should not be.

DOI:http://dx.doi.org/10.7554/eLife.20069.002

Valproate Induces the Unfolded Protein Response by Increasing Ceramide Levels

Bipolar disorder (BD), which is characterized by depression and mania, affects 1-2% of the world population. Current treatments are effective in only 40-60% of cases and cause severe side effects. Valproate (VPA) is one of the most widely used drugs for the treatment of BD, but the therapeutic mechanism of action of this drug is not understood. This knowledge gap has hampered the development of effective treatments. To identify candidate pathways affected by VPA, we performed a genome-wide expression analysis in yeast cells grown in the presence or absence of the drug. VPA caused up-regulation of FEN1 and SUR4, encoding fatty acid elongases that catalyze the synthesis of very long chain fatty acids (C24 to C26) required for ceramide synthesis. Interestingly, fen1Δ and sur4Δ mutants exhibited VPA sensitivity. In agreement with increased fatty acid elongase gene expression, VPA increased levels of phytoceramide, especially those containing C24-C26 fatty acids. Consistent with an increase in ceramide, VPA decreased the expression of amino acid transporters, increased the expression of ER chaperones, and activated the unfolded protein response element (UPRE), suggesting that VPA induces the UPR pathway. These effects were rescued by supplementation of inositol and similarly observed in inositol-starved ino1Δ cells. Starvation of ino1Δ cells increased expression of FEN1 and SUR4, increased ceramide levels, decreased expression of nutrient transporters, and induced the UPR. These findings suggest that VPA-mediated inositol depletion induces the UPR by increasing the de novo synthesis of ceramide.

Crosstalk between cellular compartments protects against proteotoxicity and extends lifespan

In cells living under optimal conditions, protein folding defects are usually prevented by the action of chaperones. Here, we investigate the cell-wide consequences of loss of chaperone function in cytosol, mitochondria or the endoplasmic reticulum (ER) in budding yeast. We find that the decline in chaperone activity in each compartment results in loss of respiration, demonstrating the dependence of mitochondrial activity on cell-wide proteostasis. Furthermore, each chaperone deficiency triggers a response, presumably via the communication among the folding environments of distinct cellular compartments, termed here the cross-organelle stress response (CORE). The proposed CORE pathway encompasses activation of protein conformational maintenance machineries, antioxidant enzymes, and metabolic changes simultaneously in the cytosol, mitochondria, and the ER. CORE induction extends replicative and chronological lifespan in budding yeast, highlighting its protective role against moderate proteotoxicity and its consequences such as the decline in respiration. Our findings accentuate that organelles do not function in isolation, but are integrated in a functional crosstalk, while also highlighting the importance of organelle communication in aging and age-related diseases.

Drosophila as a model for unfolded protein response research

Endoplasmic Reticulum (ER) is an organelle where most secretory and membrane proteins are synthesized, folded, and undergo further maturation. As numerous conditions can perturb such ER function, eukaryotic cells are equipped with responsive signaling pathways, widely referred to as the Unfolded Protein Response (UPR). Chronic conditions of ER stress that cannot be fully resolved by UPR, or conditions that impair UPR signaling itself, are associated with many metabolic and degenerative diseases. In recent years, Drosophila has been actively employed to study such connections between UPR and disease. Notably, the UPR pathways are largely conserved between Drosophila and humans, and the mediating genes are essential for development in both organisms, indicating their requirement to resolve inherent stress. By now, many Drosophila mutations are known to impose stress in the ER, and a number of these appear similar to those that underlie human diseases. In addition, studies have employed the strategy of overexpressing human mutations in Drosophila tissues to perform genetic modifier screens. The fact that the basic UPR pathways are conserved, together with the availability of many human disease models in this organism, makes Drosophila a powerful tool for studying human disease mechanisms. [BMB Reports 2015; 48(8): 445-453]

The Cek1‑mediated MAP kinase pathway regulates exposure of α‑1,2 and β‑1,2‑mannosides in the cell wall of Candida albicans modulating immune recognition

The Cek1 MAP kinase (MAPK) mediates vegetative growth and cell wall biogenesis in the fungal pathogen Candida albicans. Alterations in the fungal cell wall caused by a defective Cek1‑mediated signaling pathway leads to increased β‑1,3‑glucan exposure influencing dectin‑1 fungal recognition by immune cells. We show here that cek1 cells also display an increased exposure of α‑1,2 and β‑1,2‑mannosides (α‑M and β‑M), a phenotype shared by strains defective in the activating MAPKK Hst7, suggesting a general defect in cell wall assembly. cek1 cells display walls with loosely bound material as revealed by transmission electron microscopy and are sensitive to tunicamycin, an inhibitor of N‑glycosylation. Transcriptomal analysis of tunicamycin treated cells revealed a differential pattern between cek1 and wild type cells which involved mainly cell wall and stress related genes. Mapping α‑M and β‑M epitopes in the mannoproteins of different cell wall fractions (CWMP) revealed an important shift in the molecular weight of the mannan derived from mutants defective in this MAPK pathway. We have also assessed the role of galectin‑3, a member of a β‑galactoside‑binding protein family shown to bind to and kill C. albicans through β‑M recognition, in the infection caused by cek1 mutants. Increased binding of cek1 to murine macrophages was shown to be partially blocked by lactose. Galectin-3(-/-) mice showed increased resistance to fungal infection, although galectin-3 did not account for the reduced virulence of cek1 mutants in a mouse model of systemic infection. All these data support a role for the Cek1‑mediated pathway in fungal cell wall maintenance, virulence and antifungal discovery.

Advances in crop proteomics: PTMs of proteins under abiotic stress

Under natural conditions, crop plants are frequently subjected to various abiotic environmental stresses such as drought and heat wave, which may become more prevalent in the coming decades. Plant acclimation and tolerance to an abiotic stress are always associated with significant changes in PTMs of specific proteins. PTMs are important for regulating protein function, subcellular localization and protein activity and stability. Studies of plant responses to abiotic stress at the PTMs level are essential to the process of plant phenotyping for crop improvement. The ability to identify and quantify PTMs on a large-scale will contribute to a detailed protein functional characterization that will improve our understanding of the processes of crop plant stress acclimation and stress tolerance acquisition. Hundreds of PTMs have been reported, but it is impossible to review all of the possible protein modifications. In this review, we briefly summarize several main types of PTMs regarding their characteristics and detection methods, review the advances in PTMs research of crop proteomics, and highlight the importance of specific PTMs in crop response to abiotic stress.

Calcium Homeostasis and Ionic Mechanisms in Pulmonary Fibroblasts

Fibroblasts are key cellular mediators of many chronic interstitial lung diseases, including idiopathic pulmonary fibrosis, scleroderma, sarcoidosis, drug-induced interstitial lung disease, and interstitial lung disease in connective tissue disease. A great deal of effort has been expended to understand the signaling mechanisms underlying the various cellular functions of fibroblasts. Recently, it has been shown that Ca(2+) oscillations play a central role in the regulation of gene expression in human pulmonary fibroblasts. However, the mechanisms whereby cytosolic [Ca(2+)] are regulated and [Ca(2+)] oscillations transduced are both poorly understood. In this review, we present the general concepts of [Ca(2+)] homeostasis, of ionic mechanisms responsible for various Ca(2+) fluxes, and of regulation of gene expression by [Ca(2+)]. In each case, we then also summarize the original findings that pertain specifically to pulmonary fibroblasts. From these data, we propose an overall signaling cascade by which excitation of the fibroblasts triggers pulsatile release of internally sequestered Ca(2+), which, in turn, activates membrane conductances, including voltage-dependent Ca(2+) influx pathways. Collectively, these events produce recurring Ca(2+) oscillations, the frequency of which is transduced by Ca(2+)-dependent transcription factors, which, in turn, orchestrate a variety of cellular events, including proliferation, synthesis/secretion of extracellular matrix proteins, autoactivation (production of transforming growth factor-β), and transformation into myofibroblasts. That unifying hypothesis, in turn, allows us to highlight several specific cellular targets and therapeutic intervention strategies aimed at controlling unwanted pulmonary fibrosis. The relationships between Ca(2+) signaling events and the unfolded protein response and apoptosis are also explored.

Protein kinase Snf1 is involved in the proper regulation of the unfolded protein response in Saccharomyces cerevisiae

Glc7 is the only catalytic subunit of the protein phosphatase type 1 in the yeast S. cerevisiae and, together with its regulatory subunits, is involved in many essential processes. Analysis of the non-essential mutants in the regulatory subunits of Glc7 revealed that the lack of Reg1, and no other subunit, causes hypersensitivity to unfolded protein response (UPR)-inducers, which was concomitant with an augmented UPR element-dependent transcriptional response. The Glc7-Reg1 complex takes part in the regulation of the yeast AMP-activated serine/threonine protein kinase Snf1 in response to glucose. We demonstrate in the present study that the observed phenotypes of reg1 mutant cells are attributable to the inappropriate activation of Snf1. Indeed, growth in the presence of limited concentrations of glucose, where Snf1 is active, or expression of active forms of Snf1 in a wild-type strain increased the sensitivity to the UPR-inducer tunicamycin. Furthermore, reg1 mutant cells showed a sustained HAC1 mRNA splicing and KAR2 mRNA levels during the recovery phase of the UPR, and dysregulation of the Ire1-oligomeric equilibrium. Finally, overexpression of protein phosphatases Ptc2 and Ptc3 alleviated the growth defect of reg1 cells under endoplasmic reticulum (ER) stress conditions. Altogether, our results reveal that Snf1 plays an important role in the attenuation of the UPR, as well as identifying the protein kinase and its effectors as possible pharmacological targets for human diseases that are associated with insufficient UPR activation.

Unfolding the unfolded protein response: unique insights into brain ischemia

The endoplasmic reticulum (ER) is responsible for processing of proteins that are destined to be secreted, enclosed in a vesicle, or incorporated in the plasma membrane. Nascent peptides that enter the ER undergo a series of highly regulated processing steps to reach maturation as they transit the ER. Alterations in the intracellular environment that induce ER stress are thought to interrupt these processing steps, and result in unfolding of proteins in the ER. Accumulation of unfolded proteins concurrently activates three transmembrane stress sensors, IRE1, ATF6 and PERK, and is referred to as the Unfolded Protein Response (UPR). Our understanding of the mechanisms of UPR induction has been assembled primarily from experiments inducing ER stress with chemical and genetic manipulations. However, physiological stress often induces activation of ER stress sensors in a distinct manner from the canonical UPR. The unique activation profiles in vivo have prompted us to examine the mechanism of UPR activation in neurons following cerebral ischemia.

Making recombinant proteins in filamentous fungi- are we expecting too much?

Hosts used for the production of recombinant proteins are typically high-protein secreting mutant strains that have been selected for a specific purpose, such as efficient production of cellulose-degrading enzymes. Somewhat surprisingly, sequencing of the genomes of a series of mutant strains of the cellulolytic Trichoderma reesei, widely used as an expression host for recombinant gene products, has shed very little light on the nature of changes that boost high-level protein secretion. While it is generally agreed and shown that protein secretion in filamentous fungi occurs mainly through the hyphal tip, there is growing evidence that secretion of proteins also takes place in sub-apical regions. Attempts to increase correct folding and thereby the yields of heterologous proteins in fungal hosts by co-expression of cellular chaperones and foldases have resulted in variable success; underlying reasons have been explored mainly at the transcriptional level. The observed physiological changes in fungal strains experiencing increasing stress through protein overexpression under strong gene promoters also reflect the challenge the host organisms are experiencing. It is evident, that as with other eukaryotes, fungal endoplasmic reticulum is a highly dynamic structure. Considering the above, there is an emerging body of work exploring the use of weaker expression promoters to avoid undue stress. Filamentous fungi have been hailed as candidates for the production of pharmaceutically relevant proteins for therapeutic use. One of the biggest challenges in terms of fungally produced heterologous gene products is their mode of glycosylation; fungi lack the functionally important terminal sialylation of the glycans that occurs in mammalian cells. Finally, exploration of the metabolic pathways and fluxes together with the development of sophisticated fermentation protocols may result in new strategies to produce recombinant proteins in filamentous fungi.

Antiviral activity of chemical compound isolated from Artemisia morrisonensis against hepatitis B virus in vitro

The compound p-hydroxyacetophenone (PHAP) isolated from Artemisia morrisonensis was found to have potential anti-HBV effects in HepG2 2.2.15 cells. We clarified its antiviral mode further and HBV-transfected Huh7 cells were used as the platform. During viral gene expression, treatment with PHAP had no apparent effects on the viral precore/pregenomic RNA. However, the 2.4-kb preS RNA of viral surface gene increased significantly relative to the 2.1-kb S RNA with PHAP. Promoter activity analysis demonstrated that PHAP had a potent effect on augmenting the viral preS promoter activity. The subsequent increase in the large surface protein and induce endoplasmic reticular (ER) stress has been reported previously. Interestingly, PHAP specifically reduced ER stress related GRP78 RNA/protein levels, but not those of GRP94, in treated Huh7 cells while PHAP also led to the significant intracellular accumulation of virus. Moreover, treatment with the ER chaperone inducer thapsigargin relieved the inhibitory effect of PHAP based on the supernatant HBV DNA levels of HBV-expressed cells. In conclusion, this study suggests that the mechanism of HBV inhibition by PHAP might involve the regulation of viral surface gene expression and block virion secretion by interference with the ER stress signaling pathway.

The role of the unfolded protein response in diabetes mellitus

The endoplasmic reticulum (ER) plays a key role in the synthesis and modification of secretory and membrane proteins in all eukaryotic cells. Under normal conditions, these proteins are correctly folded and assembled in the ER. However, when cells are exposed to environmental factors such as overproduction of ER proteins, viral infections, or glucose deprivation, the secretory and membrane proteins can accumulate in unfolded or misfolded forms in the lumen of the ER, and consequently, cause stress in the ER. To maintain cellular homeostasis, cells induce several responses to ER stress. In mammalian cells, ER stress responses are induced by a diversity of signal pathways. There are three ER-located transmembrane proteins that play important roles in mammalian ER stress responses: activating transcription factor 6, inositol-requiring protein 1, and protein kinase RNA-like endoplasmic reticulum kinase. ER stress is linked to various diseases, including diabetes. This review highlights the particular importance of ER stress-responsive molecules in insulin biosynthesis, glyconeogenesis, insulin resistance, glucose intolerance, and pancreatic β-cell apoptosis. An understanding of the pathogenic mechanism of diabetes from the aspect of ER stress is crucial in formulating therapeutic strategies.

Optimization of the production of Aspergillus niger α-glucosidase expressed in Pichia pastoris

The α-glucosidase (AGL) from Aspergillus niger has been applied to produce isomaltooligosaccharides. In the present study, various factors which affect the yield of recombinant AGL, produced by engineered Pichia pastoris, were investigated. The expression level reached 5.5 U ml(-1) in bioreactor after optimization of parameters of initial induction cell density, induction temperature and methanol concentration. In addition, it was found that coexpression of protein disulfide isomerase (PDI) inhibited the growth of the engineered P. pastoris strains and had an adverse effect on the production of AGL, while codon optimization of native A. niger α-glucosidase encoding gene (aglu) resulted in a significant enhancement of enzyme production, which reached 10.1 U ml(-1). We believe that yield of AGL is increased by codon optimization as a result of enhanced translation efficiency as well as more stable mRNA secondary structure. In contrast, PDI coexpression under the control of alcohol oxidase promoter (PAOX1) seems to be less efficient in helping disulfide bond formation in AGL while probably induce unfolded protein response, which further leads to cell apoptosis and increased protein degradation.

Basic leucine zipper transcription factor Hac1 binds DNA in two distinct modes as revealed by microfluidic analyses

A quantitative understanding of how transcription factors interact with genomic target sites is crucial for reconstructing transcriptional networks in vivo. Here, we use Hac1, a well-characterized basic leucine zipper (bZIP) transcription factor involved in the unfolded protein response (UPR) as a model to investigate interactions between bZIP transcription factors and their target sites. During the UPR, the accumulation of unfolded proteins leads to unconventional splicing and subsequent translation of HAC1 mRNA, followed by transcription of UPR target genes. Initial candidate-based approaches identified a canonical cis-acting unfolded protein response element (UPRE-1) within target gene promoters; however, subsequent studies identified a large set of Hac1 target genes lacking this UPRE-1 and containing a different motif (UPRE-2). Using a combination of unbiased and directed microfluidic DNA binding assays, we established that Hac1 binds in two distinct modes: (i) to short (6-7 bp) UPRE-2-like motifs and (ii) to significantly longer (11-13 bp) extended UPRE-1-like motifs. Using a genetic screen, we demonstrate that a region of extended homology N-terminal to the basic DNA binding domain is required for this dual site recognition. These results establish Hac1 as the first bZIP transcription factor known to adopt more than one binding mode and unify previously conflicting and discrepant observations of Hac1 function into a cohesive model of UPR target gene activation. Our results also suggest that even structurally simple transcription factors can recognize multiple divergent target sites of very different lengths, potentially enriching their downstream target repertoire.

Secretory pathway genes assessed by high-throughput microscopy and synthetic genetic array analysis

We developed a procedure for automated confocal microscopy to image the effect of the non-essential yeast gene deletion set on the localisation of the plasma membrane GFP-labelled protein Mrh1p-GFP. To achieve this it was necessary to devise an expression system expressing Redstar2 RFP-fluorescence specifically in the nucleus, mCherry RFP at a lower intensity in the cytoplasm and Mrh1p-GFP in the plasma membrane. This fluorescence labelling scheme utilising specifically designed image analysis scripts allowed automated segmentation of the cells into sub-regions comprising nuclei, cytoplasm and cell-surface. From this high-throughput high content screening approach we were able to determine that gene deletions including emc1Δ, emc2Δ, emc3Δ, emc4Δ, emc5Δ and emc6Δ, caused intracellular mislocalisation at the ER of a plasma membrane protein Mrh1p-GFP. CPY processing patterns were unaffected in these mutants and collectively our data suggest a transport role for the EMC genes within the early secretory pathway. HAC1 is central to the unfolded protein response (UPR) and in its absence, i.e. the absence of UPR, emc1Δ-, emc3Δ-, emc4Δ-, emc5Δ-hac1Δ double mutants were specifically hypersensitive to ER-stress (tunicamycin) lending credence to the usefulness of the high content microscope screening for discovery of functional effects of single mutants.

Up-regulation of endoplasmic reticulum stress-related genes during the early phase of treatment of cultured cortical neurons by the proteasomal inhibitor lactacystin

Inhibition of proteasome degradation pathway has been implicated in neuronal cell death leading to neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease. We and others demonstrated that treatment of cortical neurons with the proteasomal inhibitor lactacystin leads to apoptosis. We discovered by microarray analysis that lactacystin treatment modulates the expression of both potentially neuroprotective as well as pro-apoptotic genes in neurons. However, the significance of the genes which upon transcriptional modulation contributed to proteasomal inhibition-induced apoptosis, remained unidentified. By employing microarray analysis to decipher the time-dependent changes in transcription of these genes in cultured cortical neurons, we discovered different groups of genes were transcriptionally regulated in the early and late phase of lactacystin-induced cell death. In the early phase, several neuroprotective genes such as those encoding the proteasome subunits and ubiquitin-associated enzymes, as well as the heat-shock proteins (HSP) were up-regulated. However, the pro-apoptotic endoplasmic reticulum (ER) stress-associated genes were also up-regulated at the early phase of lactacystin-induced neuronal cell death. In the late phase, genes encoding antioxidants and calcium-binding proteins were up-regulated while those associated with cholesterol biosynthesis were down-regulated. The data suggest that ER stress may participate in mediating the apoptotic responses induced by proteasomal inhibition. The up-regulation of the neuroprotective antioxidant genes and calcium-binding protein genes and down-regulation of the cholesterol biosynthesis genes in the later phase are likely consequences of stimulation of the pro-apoptotic signaling pathways in the early phase of lactacystin treatment.

The compartmentalisation of phosphorylated free oligosaccharides in cells from a CDG Ig patient reveals a novel ER-to-cytosol translocation process

BACKGROUND

Biosynthesis of the dolichol linked oligosaccharide (DLO) required for protein N-glycosylation starts on the cytoplasmic face of the ER to give Man(5)GlcNAc(2)-PP-dolichol, which then flips into the ER for further glycosylation yielding mature DLO (Glc(3)Man(9)GlcNAc(2)-PP-dolichol). After transfer of Glc(3)Man(9)GlcNAc(2) onto protein, dolichol-PP is recycled to dolichol-P and reused for DLO biosynthesis. Because de novo dolichol synthesis is slow, dolichol recycling is rate limiting for protein glycosylation. Immature DLO intermediates may also be recycled by pyrophosphatase-mediated cleavage to yield dolichol-P and phosphorylated oligosaccharides (fOSGN2-P). Here, we examine fOSGN2-P generation in cells from patients with type I Congenital Disorders of Glycosylation (CDG I) in which defects in the dolichol cycle cause accumulation of immature DLO intermediates and protein hypoglycosylation.

METHODS AND PRINCIPAL FINDINGS

In EBV-transformed lymphoblastoid cells from CDG I patients and normal subjects a correlation exists between the quantities of metabolically radiolabeled fOSGN2-P and truncated DLO intermediates only when these two classes of compounds possess 7 or less hexose residues. Larger fOSGN2-P were difficult to detect despite an abundance of more fully mannosylated and glucosylated DLO. When CDG Ig cells, which accumulate Man(7)GlcNAc(2)-PP-dolichol, are permeabilised so that vesicular transport and protein synthesis are abolished, the DLO pool required for Man(7)GlcNAc(2)-P generation could be depleted by adding exogenous glycosylation acceptor peptide. Under conditions where a glycotripeptide and neutral free oligosaccharides remain predominantly in the lumen of the ER, Man(7)GlcNAc(2)-P appears in the cytosol without detectable generation of ER luminal Man(7)GlcNAc(2)-P.

CONCLUSIONS AND SIGNIFICANCE

The DLO pools required for N-glycosylation and fOSGN2-P generation are functionally linked and this substantiates the hypothesis that pyrophosphatase-mediated cleavage of DLO intermediates yields recyclable dolichol-P. The kinetics of cytosolic fOSGN2-P generation from a luminally-generated DLO intermediate demonstrate the presence of a previously undetected ER-to-cytosol translocation process for either fOSGN2-P or DLO.

The activity of yeast Hog1 MAPK is required during endoplasmic reticulum stress induced by tunicamycin exposure

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) triggers the so-called unfolded protein response (UPR), a conserved signaling pathway that drives the transcription of genes such as chaperones and folding enzymes. Nevertheless, the activity of the UPR accounts only for a part of the gene expression program activated upon ER stress. Moreover, the mechanism(s) for how cells adapt and survive to this stress are largely unknown. Here, we show that the yeast high osmolarity glycerol (HOG) pathway plays a role in ER stress resistance. Strains lacking the MAPK Hog1p displayed sensitivity to tunicamycin or beta-mercaptoethanol, whereas hyperactivation of the pathway enhanced their resistance. However, these effects were not due to Hog1p-mediated regulation of the UPR. Northern blot analysis demonstrated that Hog1p controls the tunicamycin-induced transcriptional change of GPD1 and that wild-type cells exposed to the drug accumulated glycerol in a Hog1p-dependent manner. Consistent with this, deletion of genes involved in glycerol synthesis caused increased sensitivity to tunicamycin, whereas overexpression of GPD1 provided higher tolerance to both wild-type and hog1Delta mutant cells. Quite remarkably, these effects were mediated by the basal activity of the MAPK because tunicamycin exposure does not trigger the phosphorylation of Hog1p or its nuclear import. Hence, our results describe new aspects of the yeast response to ER stress and identify additional functions of glycerol and the Hog1p MAPK to provide stress resistance.

Usa1 functions as a scaffold of the HRD-ubiquitin ligase

Protein quality control in the endoplasmic reticulum is of central importance for cellular homeostasis in eukaryotes. Crucial for this process is the HRD-ubiquitin ligase (HMG-CoA reductase degradation), which singles out terminally misfolded proteins and routes them for degradation to cytoplasmic 26S-proteasomes. Certain functions of this enzyme complex are allocated to defined subunits. However, it remains unclear how these components act in a concerted manner. Here, we show that Usa1 functions as a major scaffold protein of the HRD-ligase. For the turnover of soluble substrates, Der1 binding to the C terminus of Usa1 is required. The N terminus of Usa1 associates with Hrd1 and thus bridges Der1 to Hrd1. Strikingly, the Usa1 N terminus also induces oligomerization of the HRD complex, which is an exclusive prerequisite for the degradation of membrane proteins. Our data demonstrate that scaffold proteins are required to adapt ubiquitin ligase activities toward different classes of substrates.

Synergistic apoptosis induction in leukemic cells by the phosphatase inhibitor salubrinal and proteasome inhibitors

BACKGROUND

Cells adapt to endoplasmic reticulum (ER)-stress by arresting global protein synthesis while simultaneously activating specific transcription factors and their downstream targets. These processes are mediated in part by the phosphorylation-dependent inactivation of the translation initiation factor eIF2alpha. Following restoration of homeostasis protein synthesis is resumed when the serine/threonine-protein phosphatase PP1 dephosphorylates and reactivates eIF2alpha. Proteasome inhibitors, used to treat multiple myeloma patients evoke ER-stress and apoptosis by blocking the ER-associated degradation of misfolded proteins (ERAD), however, the role of eIF2alpha phosphorylation in leukemic cells under conditions of proteasome inhibitor-mediated ER stress is currently unclear.

METHODOLOGY AND PRINCIPAL FINDINGS

Bcr-Abl-positive and negative leukemic cell lines were used to investigate the functional implications of PP1-related phosphatase activities on eIF2alpha phosphorylation in proteasome inhibitor-mediated ER stress and apoptosis. Rather unexpectedly, salubrinal, a recently identified PP1 inhibitor capable to protect against ER stress in various model systems, strongly synergized with proteasome inhibitors to augment apoptotic death of different leukemic cell lines. Salubrinal treatment did not affect the phosphorlyation status of eIF2alpha. Furthermore, the proapoptotic effect of salubrinal occurred independently from the chemical nature of the proteasome inhibitor, was recapitulated by a second unrelated phosphatase inhibitor and was unaffected by overexpression of a dominant negative eIF2alpha S51A variant that can not be phosphorylated. Salubrinal further aggravated ER-stress and proteotoxicity inflicted by the proteasome inhibitors on the leukemic cells since characteristic ER stress responses, such as ATF4 and CHOP synthesis, XBP1 splicing, activation of MAP kinases and eventually apoptosis were efficiently abrogated by the translational inhibitor cycloheximide.

CONCLUSIONS

Although PP1 activity does not play a major role in regulating the ER stress response in leukemic cells, phosphatase signaling nevertheless significantly limits proteasome inhibitor-mediated ER-stress and apoptosis. Inclusion of specific phosphatase inhibitors might therefore represent an option to improve current proteasome inhibitor-based treatment modalities for hematological cancers.

CD1d-restricted glycolipid antigens: presentation principles, recognition logic and functional consequences

Invariant natural killer T (iNKT) cells are innate lymphocytes whose functions are regulated by self and foreign glycolipid antigens presented by the antigen-presenting molecule CD1d. Activation of iNKT cells in vivo results in rapid release of copious amounts of effector cytokines and chemokines with which they regulate innate and adaptive immune responses to pathogens, certain types of cancers and self-antigens. The nature of CD1d-restricted antigens, the manner in which they are recognised and the unique effector functions of iNKT cells suggest an innate immunoregulatory role for this subset of T cells. Their ability to respond fast and our ability to steer iNKT cell cytokine response to altered lipid antigens make them an important target for vaccine design and immunotherapies against autoimmune diseases. This review summarises our current understanding of CD1d-restricted antigen presentation, the recognition of such antigens by an invariant T-cell receptor on iNKT cells, and the functional consequences of these interactions.

Endoplasmic reticulum (ER) chaperone regulation and survival of cells compensating for deficiency in the ER stress response kinase, PERK

The activity of PERK, an endoplasmic reticulum (ER) transmembrane protein kinase, assists in an ER stress response designed to inhibit general protein synthesis while allowing upregulated synthesis of selective proteins such as the ATF4 transcription factor. PERK null mice exhibit phenotypes that especially affect secretory cell types. Although embryonic fibroblasts from these mice are difficult to transfect with high efficiency, we have generated 293 cells stably expressing the PERK-K618A dominant negative mutant. 293/PERK-K618A cells, in response to ER stress: (a) do not properly inhibit general protein synthesis, (b) exhibit defective/delayed induction of ATF4 and BiP, and (c) exhibit exuberant splice activation of XBP1 and robust cleavage activation of ATF6, with abnormal regulation of calreticulin levels. The data suggest compensatory mechanisms allowing for cell survival in the absence of functional PERK. Interestingly, although induction of CHOP (a transcription factor implicated in apoptosis) is notably delayed after onset of ER stress, 293/PERK-K618A cells eventually produce CHOP at normal or even supranormal levels and exhibit increased apoptosis either in response to general ER stress or, more importantly, to specific misfolded secretory proteins.