Extension of lipid-linked oligosaccharides is a high-priority aspect of the unfolded protein response: endoplasmic reticulum stress in Type I congenital disorder of glycosylation fibroblasts

Manipulating mannose metabolism as a potential anticancer strategy

Cancer cells acquire metabolic advantages over their normal counterparts regarding the use of nutrients for sustained cell proliferation and cell survival in the tumor microenvironment. Notable among the metabolic traits in cancer cells is the Warburg effect, which is a reprogrammed form of glycolysis that favors the rapid generation of ATP from glucose and the production of biological macromolecules by diverting glucose into various metabolic intermediates. Meanwhile, mannose, which is the C-2 epimer of glucose, has the ability to dampen the Warburg effect, resulting in slow-cycling cancer cells that are highly susceptible to chemotherapy. This anticancer effect of mannose appears when its catabolism is compromised in cancer cells. Moreover, de novo synthesis of mannose within cancer cells has also been identified as a potential target for enhancing chemosensitivity through targeting glycosylation pathways. The underlying mechanisms by which alterations in mannose metabolism induce cancer cell vulnerability are just beginning to emerge. This review summarizes the current state of our knowledge of mannose metabolism and provides insights into its manipulation as a potential anticancer strategy.

The Therapeutic Future for Congenital Disorders of Glycosylation

The past decade, novel treatment options for congenital disorders of glycosylation (CDG) have advanced rapidly. Innovative therapies, targeting both the root cause, the affected metabolic pathways, and resulting manifestations, have transitioned from the research stage to practical applications. However, with novel therapeutic abilities, novel challenges await, specifically when it concerns the large number of clinical trials that need to be performed in order to treat all 190 genetic defects that cause CDG known to date. The present paper aims to provide an overview of how the CDG field can keep advancing its therapeutic strategies over the coming years with these challenges in mind. We focus on three important pillars that may shape the future of CDG: the use of disease models, clinical trial readiness, and the possibility to make individualized treatments scalable to the entire CDG cohort.

Deficient glycan extension and endoplasmic reticulum stresses in ALG3-CDG

ALG3-CDG is a rare congenital disorder of glycosylation (CDG) with a clinical phenotype that includes neurological manifestations, transaminitis, and frequent infections. The ALG3 enzyme catalyzes the first step of endoplasmic reticulum (ER) luminal glycan extension by adding mannose from Dol-P-Man to Dol-PP-Man5GlcNAc2 (Man5) forming Dol-PP-Man6. Such glycan extension is the first and fastest cellular response to ER stress, which is deficient in ALG3-CDG. In this study, we provide evidence that the unfolded protein response (UPR) and ER-associated degradation activities are increased in ALG3-CDG patient-derived cultured skin fibroblasts and there is constitutive activation of UPR mediated by the IRE1-α pathway. In addition, we show that N-linked Man3-4 glycans are increased in cellular glycoproteins and secreted plasma glycoproteins with hepatic or non-hepatic origin. We found that like other CDGs such as ALG1- or PMM2-CDG, in transferrin, the assembling intermediate Man5 in ALG3-CDG, are likely further processed into a distinct glycan, NeuAc1Gal1GlcNAc1Man3GlcNAc2, probably by Golgi mannosidases and glycosyltransferases. We predict it to be a mono-antennary glycan with the same molecular weight as the truncated glycan described in MGAT2-CDG. In summary, this study elucidates multiple previously unrecognized biochemical consequences of the glycan extension deficiency in ALG3-CDG which will have important implications in the pathogenesis of CDG.

Interplay of Impaired Cellular Bioenergetics and Autophagy in PMM2-CDG

Congenital disorders of glycosylation (CDG) and mitochondrial disorders are multisystem disorders with overlapping symptomatology. Pathogenic variants in the PMM2 gene lead to abnormal N-linked glycosylation. This disruption in glycosylation can induce endoplasmic reticulum stress, contributing to the disease pathology. Although impaired mitochondrial dysfunction has been reported in some CDG, cellular bioenergetics has never been evaluated in detail in PMM2-CDG. This prompted us to evaluate mitochondrial function and autophagy/mitophagy in vitro in PMM2 patient-derived fibroblast lines of differing genotypes from our natural history study. We found secondary mitochondrial dysfunction in PMM2-CDG. This dysfunction was evidenced by decreased mitochondrial maximal and ATP-linked respiration, as well as decreased complex I function of the mitochondrial electron transport chain. Our study also revealed altered autophagy in PMM2-CDG patient-derived fibroblast lines. This was marked by an increased abundance of the autophagosome marker LC3-II. Additionally, changes in the abundance and glycosylation of proteins in the autophagy and mitophagy pathways further indicated dysregulation of these cellular processes. Interestingly, serum sorbitol levels (a biomarker of disease severity) and the CDG severity score showed an inverse correlation with the abundance of the autophagosome marker LC3-II. This suggests that autophagy may act as a modulator of biochemical and clinical markers of disease severity in PMM2-CDG. Overall, our research sheds light on the complex interplay between glycosylation, mitochondrial function, and autophagy/mitophagy in PMM2-CDG. Manipulating mitochondrial dysfunction and alterations in autophagy/mitophagy pathways could offer therapeutic benefits when combined with existing treatments for PMM2-CDG.

Metabolic adaptation of human skin fibroblasts to ER stress caused by glycosylation defect in PMM2-CDG

PMM2-CDG is the most prevalent type of congenital disorders of glycosylation (CDG). It is caused by pathogenic variants in the gene encoding phosphomannomutase 2 (PMM2), which converts mannose-6-phosphate to mannose-1-phosphate and thus activates this saccharide for further glycosylation processes. Defective glycosylation can lead to an abnormal accumulation of unfolded proteins in endoplasmic reticulum (ER) and cause its stress. The ER is a key compartment for glycosylation, and its connection and communication with mitochondria has been described extensively in literature. Their crosstalk is important for cell proliferation, calcium homeostasis, apoptosis, mitochondrial fission regulation, bioenergetics, autophagy, lipid metabolism, inflammasome formation and unfolded protein response. Therefore, in the present study we posed a question, whether defective glycosylation leads to bioenergetic disruption. Our data reveal possible chronic stress in ER and activated unfolded protein response via PERK pathway in PMM2-CDG fibroblasts. Presumably, it leads to bioenergetic reorganization and increased assembly of respiratory chain complexes into supercomplexes together with suppressed glycolysis in PMM2-CDG patient cells. These changes cause alterations in Krebs cycle, which is tightly connected to electron transport system in mitochondria. In summary, we present data showing metabolic adaptation of cells to glycosylation defect caused by various pathogenic variants in PMM2.

DDOST-CDG: Clinical and molecular characterization of a third patient with a milder and a predominantly movement disorder phenotype

Congenital disorders of glycosylation (CDG) are a group of heterogeneous inherited metabolic disorders affecting posttranslational protein modification. DDOST-CDG, caused by biallelic pathogenic variants in DDOST which encodes dolichyl-diphospho-oligosaccharide-protein glycosyltransferase, a subunit of N-glycosylation oligosaccharyltransferase (OST) complex, is an ultra-rare condition that has been described in two patients only. The main clinical features in the two reported patients include profound developmental delay, failure to thrive, and hypotonia. In addition, both patients had abnormal transferrin glycosylation. Here, we report an 18-year-old male who presented with moderate developmental delay, progressive opsoclonus, myoclonus, ataxia, tremor, and dystonia. Biochemical studies by carbohydrate deficient transferrin analysis showed a type I CDG pattern. Exome sequencing identified compound heterozygous variants in DDOST: a maternally inherited variant, c.1142dupT (p.Leu381Phefs*11), and a paternally inherited variant, c.661 T > C (p.Ser221Pro). Plasma N-glycan profiling showed mildly increased small high mannose glycans including Man0-5 GlcNAc2, a pattern consistent with what was previously reported in DDOST-CDG or defects in other subunits of OST complex. Western blot analysis on patient's fibroblasts revealed decreased expression of DDOST and reduced intracellular N-glycosylation, as evident by the biomarkers ICAM-1 and LAMP2. Our study highlights the clinical variability, expands the clinical and biochemical phenotypes, and describes new genotype, which all are essential for diagnosing and managing patients with DDOST-CDG.

A genome-wide CRISPR screen identifies DPM1 as a modifier of DPAGT1 deficiency and ER stress

Partial loss-of-function mutations in glycosylation pathways underlie a set of rare diseases called Congenital Disorders of Glycosylation (CDGs). In particular, DPAGT1-CDG is caused by mutations in the gene encoding the first step in N-glycosylation, DPAGT1, and this disorder currently lacks effective therapies. To identify potential therapeutic targets for DPAGT1-CDG, we performed CRISPR knockout screens in Drosophila cells for genes associated with better survival and glycoprotein levels under DPAGT1 inhibition. We identified hundreds of candidate genes that may be of therapeutic benefit. Intriguingly, inhibition of the mannosyltransferase Dpm1, or its downstream glycosylation pathways, could rescue two in vivo models of DPAGT1 inhibition and ER stress, even though impairment of these pathways alone usually causes CDGs. While both in vivo models ostensibly cause cellular stress (through DPAGT1 inhibition or a misfolded protein), we found a novel difference in fructose metabolism that may indicate glycolysis as a modulator of DPAGT1-CDG. Our results provide new therapeutic targets for DPAGT1-CDG, include the unique finding of Dpm1-related pathways rescuing DPAGT1 inhibition, and reveal a novel interaction between fructose metabolism and ER stress.

Dissecting the transcriptional program of phosphomannomutase 2 deficient cells: B-LCL as a valuable model for congenital disorders of glycosylation studies

Congenital disorders of glycosylation (CDG) include 150 disorders constituting in genetically and clinically heterogeneous diseases, showing significant glycoprotein hypoglycosylation that leads to pathological consequences on multiple organs and systems which underlying mechanisms are not yet understood. A few cellular and animal models have been used to study specific CDG characteristics although they have given limited information due to the few CDG mutations tested and the still missing comprehensive molecular and cellular basic research. Here we provide specific gene expression profiles, based on RNA microarray analysis, together with some biochemical and cellular characteristics of a total of 9 control EBV-transformed lymphoblastoid B cell lines (B-LCL) and 13 CDG B-LCL from patients carrying severe mutations in the PMM2 gene, strong serum protein hypoglycosylation and neurological symptoms. Significantly dysregulated genes in PMM2-CDG cells included those regulating stress responses, transcription factors, glycosylation, motility, cell junction and, importantly, those related to development and neuronal differentiation and synapse such as CA2 and ADAM23. PMM2-CDG associated biological consequences involved the unfolded protein response, RNA metabolism and the endoplasmic reticulum, Golgi apparatus and mitochondria components. Changes in transcriptional and CA2 protein levels are consistent with CDG physiopathology. These results demonstrate the global transcriptional impact in phosphomannomutase 2 deficient cells, reveal CA2 as a potential cellular biomarker and confirm B-LCL as an advantageous model for CDG studies.

Bi-allelic variants in the ER quality-control mannosidase gene EDEM3 cause a congenital disorder of glycosylation

EDEM3 encodes a protein that converts Man8GlcNAc2 isomer B to Man7-5GlcNAc2. It is involved in the endoplasmic reticulum-associated degradation pathway, responsible for the recognition of misfolded proteins that will be targeted and translocated to the cytosol and degraded by the proteasome. In this study, through a combination of exome sequencing and gene matching, we have identified seven independent families with 11 individuals with bi-allelic protein-truncating variants and one individual with a compound heterozygous missense variant in EDEM3. The affected individuals present with an inherited congenital disorder of glycosylation (CDG) consisting of neurodevelopmental delay and variable facial dysmorphisms. Experiments in human fibroblast cell lines, human plasma, and mouse plasma and brain tissue demonstrated decreased trimming of Man8GlcNAc2 isomer B to Man7GlcNAc2, consistent with loss of EDEM3 enzymatic activity. In human cells, Man5GlcNAc2 to Man4GlcNAc2 conversion is also diminished with an increase of Glc1Man5GlcNAc2. Furthermore, analysis of the unfolded protein response showed a reduced increase in EIF2AK3 (PERK) expression upon stimulation with tunicamycin as compared to controls, suggesting an impaired unfolded protein response. The aberrant plasma N-glycan profile provides a quick, clinically available test for validating variants of uncertain significance that may be identified by molecular genetic testing. We propose to call this deficiency EDEM3-CDG.

Inhibition of protein glycosylation is a novel pro-angiogenic strategy that acts via activation of stress pathways

Endothelial cell (EC) metabolism is thought to be one of the driving forces for angiogenesis. Here we report the identification of the hexosamine D-mannosamine (ManN) as an EC mitogen and survival factor for bovine and human microvascular EC, with an additivity with VEGF. ManN inhibits glycosylation in ECs and induces significant changes in N-glycan and O-glycan profiles. We further demonstrate that ManN and two N-glycosylation inhibitors stimulate EC proliferation via both JNK activation and the unfolded protein response caused by ER stress. ManN results in enhanced angiogenesis in a mouse skin injury model. ManN also promotes angiogenesis in a mouse hindlimb ischemia model, with accelerated limb blood flow recovery compared to controls. In addition, intraocular injection of ManN induces retinal neovascularization. Therefore, activation of stress pathways following inhibition of protein glycosylation can promote EC proliferation and angiogenesis and may represent a therapeutic strategy for treatment of ischemic disorders.

Therapeutic angiogenesis has the potential of inducing and maintaining new blood vessels and thus improving outcomes in patients with ischemic disorders. Mannosamine functions as an endothelial cell mitogen/survival factor through activation of stress pathways and might be useful to protect and regenerate the vascular endothelium in a variety of disorders.

Mechanisms of Congenital Malformations in Pregnancies with Pre-existing Diabetes

PURPOSE OF REVIEW

Fetuses of diabetic mothers are at increased risk for congenital malformations. Research in recent decades using animal and embryonic stem cell models has revealed many embryonic developmental processes that are disturbed by maternal diabetes. The aim of this review is to give clinicians a better understanding of the reasons for rigorous glycemic control in early pregnancy, and to provide background to guide future research.

RECENT FINDINGS

Mouse models of diabetic pregnancy have revealed mechanisms for altered expression of tissue-specific genes that lead to malformations that are more common in diabetic pregnancies, such as neural tube defects (NTDs) and congenital heart defects (CHDs), and how altered gene expression causes apoptosis that leads to malformations. Embryos express the glucose transporter, GLUT2, which confers susceptibility to malformation, due to high rates of glucose uptake during maternal hyperglycemia and subsequent oxidative stress; however, the teleological function of GLUT2 for mammalian embryos may be to transport the amino sugar glucosamine (GlcN) from maternal circulation to be used as substrate for glycosylation reactions and to promote embryo cell growth. Malformations in diabetic pregnancy may be not only due to excess glucose uptake but also due to insufficient GlcN uptake. Avoiding maternal hyperglycemia during early pregnancy should prevent excess glucose uptake via GLUT2 into embryo cells, and also permit sufficient GLUT2-mediated GlcN uptake.

Nrf2 activation attenuates genetic endoplasmic reticulum stress induced by a mutation in the phosphomannomutase 2 gene in zebrafish

Nrf2 plays critical roles in animals' defense against electrophiles and oxidative stress by orchestrating the induction of cytoprotective genes. We previously isolated the zebrafish mutant it768, which displays up-regulated expression of Nrf2 target genes in an uninduced state. In this paper, we determine that the gene responsible for it768 was the zebrafish homolog of phosphomannomutase 2 (Pmm2), which is a key enzyme in the initial steps of N-glycosylation, and its mutation in humans leads to PMM2-CDG (congenital disorders of glycosylation), the most frequent type of CDG. The pmm2^it768 larvae exhibited mild defects in N-glycosylation, indicating that the pmm2^it768 mutation is a hypomorph, as in human PMM2-CDG patients. A gene expression analysis showed that pmm2^it768 larvae display up-regulation of endoplasmic reticulum (ER) stress, suggesting that the activation of Nrf2 was induced by the ER stress. Indeed, the treatment with the ER stress-inducing compounds up-regulated the gstp1 expression in an Nrf2-dependent manner. Furthermore, the up-regulation of gstp1 by the pmm2 inactivation was diminished by knocking down or out double-stranded RNA-activated protein kinase (PKR)-like ER kinase (PERK), one of the main ER stress sensors, suggesting that Nrf2 was activated in response to the ER stress via the PERK pathway. ER stress-induced activation of Nrf2 was reported previously, but the results have been controversial. Our present study clearly demonstrated that ER stress can indeed activate Nrf2 and this regulation is evolutionarily conserved among vertebrates. Moreover, ER stress induced in pmm2^it768 mutants was ameliorated by the treatment of the Nrf2-activator sulforaphane, indicating that Nrf2 plays significant roles in the reduction of ER stress.

DPAGT1-CDG: Functional analysis of disease-causing pathogenic mutations and role of endoplasmic reticulum stress

Pathogenic mutations in DPAGT1 are manifested as two possible phenotypes: congenital disorder of glycosylation DPAGT1-CDG (also known as CDG-Ij), and limb-girdle congenital myasthenic syndrome (CMS) with tubular aggregates. UDP-N-acetylglucosamine-dolichyl-phosphate N-acetylglucosamine phosphotransferase (GPT), the protein encoded by DPAGT1, is an endoplasmic reticulum (ER)-resident protein involved in an initial step in the N-glycosylation pathway. The aim of the present study was to examine the effect of six variants in DPAGT1 detected in patients with DPAGT1-CDG, and the role of endoplasmic reticulum stress, as part of the search for therapeutic strategies to use against DPAGT1-CDG. The effect of the six mutations, i.e., c.358C>A (p.Leu120Met), c.791T>G (p.Val264Gly), c.901C>T (p.Arg301Cys), c.902G>A (p.Arg301His), c.1154T>G (p.Leu385Arg), and of the novel mutation c.329T>C (p.Phe110Ser), were examined via the analysis of DPAGT1 transcriptional profiles and GTP levels in patient-derived fibroblasts. In addition, the transient expression of different mutations was analysed in COS-7 cells. The results obtained, together with those of bioinformatic studies, revealed these mutations to affect the splicing process, the stability of GTP, or the ability of this protein to correctly localise in the ER membrane. The unfolded protein response (UPR; the response to ER stress) was found not to be active in patient-derived fibroblasts, unlike that seen in cells from patients with PMM2-CDG or DPM1-CDG. Even so, the fibroblasts of patients with DPAGT1-CDG seemed to be more sensitive to the stressor tunicamycin. The present work improves our knowledge of DPAGT1-CDG and provides bases for developing tailored splicing and folding therapies.

Glucosamine induces ER stress by disrupting lipid-linked oligosaccharide biosynthesis and N-linked protein glycosylation

Glucosamine is an essential substrate for N-linked protein glycosylation. However, elevated levels of glucosamine can induce endoplasmic reticulum (ER) stress. Glucosamine-induced ER stress has been implicated in the development of diabetic complications, including atherosclerosis and hepatic steatosis. In this study, we investigate the potential relationship between the effects of glucosamine on lipid-linked oligosaccharide (LLO) biosynthesis, N-linked glycosylation, and ER homeostasis. Mouse embryonic fibroblasts (MEFs) were cultured in the presence of 0-5 mM glucosamine for up to 18 h, and LLO biosynthesis was monitored by fluorescence-assisted carbohydrate electrophoresis. ER stress was determined by quantification of unfolded protein response (UPR) gene expression. We found that exposure of MEFs to ≥1 mM glucosamine significantly impaired the biosynthesis of mature (Glc₃Man₉GlcNAc₂) LLOs before the activation of the UPR, which resulted in the accumulation of an LLO intermediate (Man₃GlcNAc₂). The addition of 4-phenylbutyric acid (4-PBA), a chemical chaperone, was able to alleviate ER stress but did not rescue LLO biosynthesis. Other ER stress-inducing agents, including dithiothreitol and thapsigargin, had no effect on LLO levels. Together, these data suggest that elevated concentrations of glucosamine induce ER stress by interfering with lipid-linked oligosaccharide biosynthesis and N-linked glycosylation. We hypothesize that this pathway represents a causative link between hyperglycemia and the development of diabetic complications.

Cell Recognition Molecule L1 Regulates Cell Surface Glycosylation to Modulate Cell Survival and Migration

Background: Cell recognition molecule L1 (L1) plays an important role in cancer cell differentiation, proliferation, migration and survival, but its mechanism remains unclear. Methodology/Principal: Our previous study has demonstrated that L1 enhanced cell survival and migration in neural cells by regulating cell surface glycosylation. In the present study, we show that L1 affected cell migration and survival in CHO (Chinese hamster ovary) cell line by modulation of sialylation and fucosylation at the cell surface via the PI3K (phosphoinositide 3-kinase) and Erk (extracellularsignal-regulated kinase) signaling pathways. Flow cytometry analysis indicated that L1 modulated cell surface sialylation and fucosylation in CHO cells. Activated L1 upregulated the protein expressions of ST6Gal1 (β-galactoside α-2,6-sialyltransferase 1) and FUT9 (Fucosyltransferase 9) in CHO cells. Furthermore, activated L1 promoted CHO cells migration and survival as shown by transwell assay and MTT assay. Inhibitors of sialylation and fucosylation blocked L1-induced cell migration and survival, while decreasing FUT9 and ST6Gal1 expressions via the PI3K-dependent and Erk-dependent signaling pathways. Conclusion : L1 modulated cell migration and survival by regulation of cell surface sialylation and fucosylation via the PI3K-dependent and Erk-dependent signaling pathways.

Endoplasmic Reticulum Stress and Autophagy in Homocystinuria Patients with Remethylation Defects

Proper function of endoplasmic reticulum (ER) and mitochondria is crucial for cellular homeostasis, and dysfunction at either site as well as perturbation of mitochondria-associated ER membranes (MAMs) have been linked to neurodegenerative and metabolic diseases. Previously, we have observed an increase in ROS and apoptosis levels in patient-derived fibroblasts with remethylation disorders causing homocystinuria. Here we show increased mRNA and protein levels of Herp, Grp78, IP₃R1, pPERK, ATF4, CHOP, asparagine synthase and GADD45 in patient-derived fibroblasts suggesting ER stress and calcium perturbations in homocystinuria. In addition, overexpressed MAM-associated proteins (Grp75, σ-1R and Mfn2) were found in these cells that could result in mitochondrial calcium overload and oxidative stress increase. Our results also show an activation of autophagy process and a substantial degradation of altered mitochondria by mitophagy in patient-derived fibroblasts. Moreover, we have observed that autophagy was partially abolished by antioxidants suggesting that ROS participate in this process that may have a protective role. Our findings argue that alterations in Ca²⁺ homeostasis and autophagy may contribute to the development of this metabolic disorder and suggest a therapeutic potential in homocystinuria for agents that stabilize calcium homeostasis and/or restore the proper function of ER-mitochondria communications.

Disruption of glycosylation enhances ubiquitin-mediated proteasomal degradation of Shadoo in Scrapie-infected rodents and cultured cells

Shadoo (Sho) is an N-glycosylated glycophosphatidylinositol-anchored protein that is expressed in the brain and exhibits neuroprotective properties. Recently, research has shown that a reduction of Sho levels may reflect the presence of PrPSc in the brain. However, the possible mechanism by which prion infection triggers down-regulation of Sho remains unclear. In the present study, Western blot and immunohistochemical assays revealed that Sho, especially glycosylated Sho, declined markedly in the brains of five scrapie agent-infected hamsters and mice at the terminal stages. Analyses of the down-regulation of Sho levels with the emergence of PrPSc C2 proteolytic fragments did not identify close association in all tested scrapie-infected models. To further investigate the mechanism of depletion of Sho in prion disease, a Sho-expressing plasmid with HA tag was introduced into a scrapie-infected cell line, SMB-S15, and its normal cell line, SMB-PS. Western blot assay revealed dramatically decreased Sho in SMB-S15 cells, especially its glycosylated form. Proteasome inhibitor MG132 reversed the decrease of nonglycosylated Sho, but had little effect on glycosylated Sho. N-acetylglucosamine transferase inhibitor tunicamycin efficiently reduced the glycosylations of Sho and PrPC in SMB-PS cells, while two other endoplasmic reticulum stress inducers showed clear inhibition of diglycosylated PrPC, but did not change the expression level and profile of Sho. Furthermore, immunoprecipitation of HA-Sho illustrated ubiquitination of Sho in SMB-S15 cells, but not in SMB-PS cells. We propose that the depletions of Sho in scrapie-infected cell lines due to inhibition of glycosylation mediate protein destabilization and subsequently proteasome degradation after modification by ubiquitination.

Phenotype-genotype correlations in patients with Marinesco-Sjögren syndrome

Marinesco-Sjögren syndrome (MSS; MIM 248800) is an autosomal recessive disorder characterized by congenital cerebellar ataxia, early cataracts, developmental delay, myopathy and short stature. Alterations in the gene SIL1 cause MSS in some patients with typical findings. In this study, molecular investigations including sequencing of the SIL1 gene, western blotting and microscopic investigations in fibroblast cultures were carried out in a cohort of 15 patients from 14 unrelated families, including the large, inbred family reported by Superneau et al., having the clinical features of MSS to provide insights into the pathophysiology of the disorder. A total of seven different mutations were found in eight of the patients from seven families. The mutations caused loss of the BIP-associated protein (BAP) protein in four patients by western blot. Novel clinical features such as dental abnormalities, iris coloboma, eczema and hormonal abnormalities were noticed in some patients, but there was no clear way to distinguish those with and without SIL1 mutations. Cultured fibroblasts contained numerous cytoplasmic inclusion bodies, similar to those identified in the brain of the whoozy mouse in five unrelated patients, three with and two without SIL1 mutations, suggesting some SIL1 negative patients share a common cellular pathogenesis with those who are SIL1 positive.

Understanding human glycosylation disorders: biochemistry leads the charge

Nearly 70 inherited human glycosylation disorders span a breathtaking clinical spectrum, impacting nearly every organ system and launching a family-driven diagnostic odyssey. Advances in genetics, especially next generation sequencing, propelled discovery of many glycosylation disorders in single and multiple pathways. Interpretation of whole exome sequencing results, insights into pathological mechanisms, and possible therapies will hinge on biochemical analysis of patient-derived materials and animal models. Biochemical diagnostic markers and readouts offer a physiological context to confirm candidate genes. Recent discoveries suggest novel perspectives for textbook biochemistry and novel research opportunities. Basic science and patients are the immediate beneficiaries of this bidirectional collaboration.

A zebrafish model of PMM2-CDG reveals altered neurogenesis and a substrate-accumulation mechanism for N-linked glycosylation deficiency

PMM2-CDG patients have phosphomannomutase (Pmm2) deficiency, with developmental and N-linked glycosylation defects attributed to depletion of mannose-1-phosphate and downstream lipid-linked oligosaccharides (LLOs). This, the first PMM2-CDG zebrafish model, shows, unexpectedly, that accumulation of the Pmm2 substrate mannose-6-phosphate explains LLO deficiency.

Congenital disorder of glycosylation (PMM2-CDG) results from mutations in pmm2, which encodes the phosphomannomutase (Pmm) that converts mannose-6-phosphate (M6P) to mannose-1-phosphate (M1P). Patients have wide-spectrum clinical abnormalities associated with impaired protein N-glycosylation. Although it has been widely proposed that Pmm2 deficiency depletes M1P, a precursor of GDP-mannose, and consequently suppresses lipid-linked oligosaccharide (LLO) levels needed for N-glycosylation, these deficiencies have not been demonstrated in patients or any animal model. Here we report a morpholino-based PMM2-CDG model in zebrafish. Morphant embryos had developmental abnormalities consistent with PMM2-CDG patients, including craniofacial defects and impaired motility associated with altered motor neurogenesis within the spinal cord. Significantly, global N-linked glycosylation and LLO levels were reduced in pmm2 morphants. Although M1P and GDP-mannose were below reliable detection/quantification limits, Pmm2 depletion unexpectedly caused accumulation of M6P, shown earlier to promote LLO cleavage in vitro. In pmm2 morphants, the free glycan by-products of LLO cleavage increased nearly twofold. Suppression of the M6P-synthesizing enzyme mannose phosphate isomerase within the pmm2 background normalized M6P levels and certain aspects of the craniofacial phenotype and abrogated pmm2-dependent LLO cleavage. In summary, we report the first zebrafish model of PMM2-CDG and uncover novel cellular insights not possible with other systems, including an M6P accumulation mechanism for underglycosylation.

A zebrafish model of congenital disorders of glycosylation with phosphomannose isomerase deficiency reveals an early opportunity for corrective mannose supplementation

Individuals with congenital disorders of glycosylation (CDG) have recessive mutations in genes required for protein N-glycosylation, resulting in multi-systemic disease. Despite the well-characterized biochemical consequences in these individuals, the underlying cellular defects that contribute to CDG are not well understood. Synthesis of the lipid-linked oligosaccharide (LLO), which serves as the sugar donor for the N-glycosylation of secretory proteins, requires conversion of fructose-6-phosphate to mannose-6-phosphate via the phosphomannose isomerase (MPI) enzyme. Individuals who are deficient in MPI present with bleeding, diarrhea, edema, gastrointestinal bleeding and liver fibrosis. MPI-CDG patients can be treated with oral mannose supplements, which is converted to mannose-6-phosphate through a minor complementary metabolic pathway, restoring protein glycosylation and ameliorating most symptoms, although liver disease continues to progress. Because Mpi deletion in mice causes early embryonic lethality and thus is difficult to study, we used zebrafish to establish a model of MPI-CDG. We used a morpholino to block mpi mRNA translation and established a concentration that consistently yielded 13% residual Mpi enzyme activity at 4 days post-fertilization (dpf), which is within the range of MPI activity detected in fibroblasts from MPI-CDG patients. Fluorophore-assisted carbohydrate electrophoresis detected decreased LLO and N-glycans in mpi morphants. These deficiencies resulted in 50% embryonic lethality by 4 dpf. Multi-systemic abnormalities, including small eyes, dysmorphic jaws, pericardial edema, a small liver and curled tails, occurred in 82% of the surviving larvae. Importantly, these phenotypes could be rescued with mannose supplementation. Thus, parallel processes in fish and humans contribute to the phenotypes caused by Mpi depletion. Interestingly, mannose was only effective if provided prior to 24 hpf. These data provide insight into treatment efficacy and the broader molecular and developmental abnormalities that contribute to disorders associated with defective protein glycosylation.

Quantitative measurement of events in the mammalian unfolded protein response

When the homeostasis of endoplasmic reticulum (ER) is disturbed by the accumulation of unfolded or misfolded proteins, a series of signaling responses collectively called the unfolded protein response (UPR) is triggered. UPR transducers IRE1, PERK, ATF6, and UPR-responsive genes such as GRP78/BiP, ERAD genes such as EDEM, and synthesis of the protein N-linked glycosylation donor lipid-linked oligosaccharides (LLOs) are mobilized. This chapter provides methods used in our laboratory to quantitatively measure the accumulation of mRNAs encoding BiP and EDEM, splicing of XBP1, cleavage of ATF6, inhibition of protein synthesis by PERK, and extension of LLOs under control and stress conditions.

Long isoforms of NRF1 contribute to arsenic-induced antioxidant response in human keratinocytes

BACKGROUND

Human exposure to inorganic arsenic (iAs), a potent oxidative stressor, causes various dermal disorders, including hyperkeratosis and skin cancer. Nuclear factor-erythroid 2-related factor 1 (NRF1, also called NFE2L1) plays a critical role in regulating the expression of many antioxidant response element (ARE)-dependent genes.

OBJECTIVES

We investigated the role of NRF1 in arsenic-induced antioxidant response and cytotoxicity in human keratinocytes.

RESULTS

In cultured human keratinocyte HaCaT cells, inorganic arsenite (iAs3+) enhanced the protein accumulation of long isoforms (120-140 kDa) of NRF1 in a dose- and time-dependent fashion. These isoforms accumulated mainly in the nuclei of HaCaT cells. Selective deficiency of NRF1 by lentiviral short-hairpin RNAs in HaCaT cells [NRF1-knockdown (KD)] led to decreased expression of γ-glutamate cysteine ligase catalytic subunit (GCLC) and regulatory subunit (GCLM) and a reduced level of intracellular glutathione. In response to acute iAs3+ exposure, induction of some ARE-dependent genes, including NAD(P)H:quinone oxidoreductase 1 (NQO1), GCLC, and GCLM, was significantly attenuated in NRF1-KD cells. However, the iAs3-induced expression of heme oxygenase 1 (HMOX-1) was unaltered by silencing NRF1, suggesting that HMOX-1 is not regulated by NRF1. In addition, the lack of NRF1 in HaCaT cells did not disturb iAs3+-induced NRF2 accumulation but noticeably decreased Kelch-like ECH-associated protein 1 (KEAP1) levels under basal and iAs3+-exposed conditions, suggesting a potential interaction between NRF1 and KEAP1. Consistent with the critical role of NRF1 in the transcriptional regulation of some ARE-bearing genes, knockdown of NRF1 significantly increased iAs3+-induced cytotoxicity and apoptosis.

CONCLUSIONS

Here, we demonstrate for the first time that long isoforms of NRF1 contribute to arsenic-induced antioxidant response in human keratinocytes and protect the cells from acute arsenic cytotoxicity.

Stimulation of N-Linked Glycosylation and Lipid-Linked Oligosaccharide Synthesis by Stress Responses in Metazoan Cells

Endoplasmic reticulum (ER) stress responses comprising the unfolded protein response (UPR) are activated by conditions that disrupt folding and assembly of proteins inside the ER lumenal compartment. Conditions known to be proximal triggers of the UPR include saturation of chaperones with misfolded protein, redox imbalance, disruption of Ca2+ levels, interference with N-linked glycosylation, and failure to dispose of terminally misfolded proteins. Potentially, ER stress responses can reprogram cells to correct all of these problems and thereby restore ER function to normal. This article will review literature on stimulation of N-linked glycosylation by ER stress responses, focusing on metazoan systems. The mechanisms involved will be contrasted with those mediating stimulation of N-linked glycosylation by cytoplasmic stress responses. This information will interest readers who study the biological roles of stress responses, the functions of N-linked glycans, and potential strategies for treatment of genetic disorders of N-linked glycosylation.

Chemical induction of the unfolded protein response in the liver increases glucose production and is activated during insulin-induced hypoglycaemia in rats

AIMS/HYPOTHESIS

Endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) can regulate insulin secretion, insulin action and in vitro hepatocyte glucose release. The aims of this study were to determine whether chemical agents that induce ER stress regulate glucose production in vivo and to identify a physiological setting in which this may be important.

METHODS

A pancreatic clamp test was performed in anaesthetised rats, and insulin and glucagon were replaced at basal levels. [6,6-(2)H(2)]Glucose was infused in the absence (CON, n = 10) or presence of ER stress-inducing agents, namely, tunicamycin (Tun, n = 10) or thapsigargin (Thap, n = 10).

RESULTS

Arterial insulin, glucagon, corticosterone and NEFA concentrations were constant throughout experiments and not different among groups. After 1 h, the glucose concentration was significantly increased in Tun and Thap rats (1.5 +/- 0.2 and 2.1 +/- 0.3 mmol/l, respectively; mean +/- SD), but did not change in CON rats. Glucose production increased (p < 0.05) by 11.0 +/- 1.6 and 13.2 +/- 2.2 micromol kg(-1) min(-1) in Tun and Thap rats, respectively, but did not change in CON rats. When glucose was infused in a fourth group (HYPER) to match the increase in glucose observed in the Tun and Thap rats, glucose production decreased by approximately 22 micromol kg(-1) min(-1). Liver phosphorylase activity was increased and glycogen decreased in the Tun and Thap groups compared with the CON and HYPER groups. Given that glucose deprivation induces ER stress in cells, we hypothesised that hypoglycaemia, a condition that elicits increased glucose production, would activate the UPR in the liver. Three hour hyperinsulinaemic (5 mU kg(-1) min(-1)) -euglycaemic (EUG, approximately 7.2 mmol/l, n = 6) or -hypoglycaemic (HYPO, approximately 2.8 mmol/l, n = 6) clamps were performed in conscious rats. Several biochemical markers of the UPR were significantly increased in the liver, but not in kidney or pancreas, in HYPO vs EUG rats.

CONCLUSIONS/INTERPRETATION

Based on our findings that the chemical induction of the UPR increased glucose production and that prolonged hypoglycaemia activated the UPR in the liver, we propose that the UPR in the liver may contribute to the regulation of glucose production during prolonged hypoglycaemia.

Congenital Disorders of Glycosylation: A Rapidly Expanding Disease Family

Congenital disorders of glycosylation (CDG) are a large family of genetic diseases resulting from defects in the synthesis of glycans and in the attachment of glycans to other compounds. These disorders cause a wide range of human diseases, with examples emanating from all medical subspecialties. Since our 2001 review on CDG ( 36 ), this field has seen substantial growth: The number of N-glycosylation defects has doubled (from 6 to 12), five new O-glycosylation defects have been added to the two previously known ones, three combined N- and O-glycosylation defects have been identified, the first lipid glycosylation defects have been discovered, and a new domain, that of the hyperglycosylation defects, has been introduced. A number of CDG are due to defects in enzymes with a putative glycosyltransferase function. There is also a growing group of patients with unidentified defects (CDG-x), some with typical clinical presentations and others with presentations not seen before in CDG. This review focuses on the clinical, biochemical, and genetic characteristics of CDG and on advances expected in their future study and clinical management.

Translation attenuation by PERK balances ER glycoprotein synthesis with lipid-linked oligosaccharide flux

Endoplasmic reticulum (ER) homeostasis requires transfer and subsequent processing of the glycan Glc₃Man₉GlcNAc₂ (G₃M₉Gn₂) from the lipid-linked oligosaccharide (LLO) glucose₃mannose₉N-acetylglucosamine₂-P-P-dolichol (G₃M₉Gn₂-P-P-Dol) to asparaginyl residues of nascent glycoprotein precursor polypeptides. However, it is unclear how the ER is protected against dysfunction from abnormal accumulation of LLO intermediates and aberrant N-glycosylation, as occurs in certain metabolic diseases. In metazoans phosphorylation of eukaryotic initiation factor 2α (eIF2α) on Ser⁵¹ by PERK (PKR-like ER kinase), which is activated by ER stress, attenuates translation initiation. We use brief glucose deprivation to simulate LLO biosynthesis disorders, and show that attenuation of polypeptide synthesis by PERK promotes extension of LLO intermediates to G₃M₉Gn₂-P-P-Dol under these substrate-limiting conditions, as well as counteract abnormal N-glycosylation. This simple mechanism requires eIF2α Ser⁵¹ phosphorylation by PERK, and is mimicked by agents that stimulate cytoplasmic stress-responsive Ser⁵¹ kinase activity. Thus, by sensing ER stress from defective glycosylation, PERK can restore ER homeostasis by balancing polypeptide synthesis with flux through the LLO pathway.

Multiple genes and factors associated with bipolar disorder converge on growth factor and stress activated kinase pathways controlling translation initiation: implications for oligodendrocyte viability

Famine and viral infection, as well as interferon therapy have been reported to increase the risk of developing bipolar disorder. In addition, almost 100 polymorphic genes have been associated with this disease. Several form most of the components of a phosphatidyl-inositol signalling/AKT1 survival pathway (PIK3C3, PIP5K2A, PLCG1, SYNJ1, IMPA2, AKT1, GSK3B, TCF4) which is activated by growth factors (BDNF, NRG1) and also by NMDA receptors (GRIN1, GRIN2A, GRIN2B). Various other protein products of genes associated with bipolar disorder either bind to or are affected by phosphatidyl-inositol phosphate products of this pathway (ADBRK2, HIP1R, KCNQ2, RGS4, WFS1), are associated with its constituent elements (BCR, DUSP6, FAT, GNAZ) or are downstream targets of this signalling cascade (DPYSL2, DRD3, GAD1, G6PD, GCH1, KCNQ2, NOS3, SLC6A3, SLC6A4, SST, TH, TIMELESS). A further pathway relates to endoplasmic reticulum-stress (HSPA5, XBP1), caused by problems in protein glycosylation (ALG9), growth factor receptor sorting (PIK3C3, HIP1R, SYBL1), or aberrant calcium homoeostasis (WFS1). Key processes relating to these pathways appear to be under circadian control (ARNTL, CLOCK, PER3, TIMELESS). DISC1 can also be linked to many of these pathways. The growth factor pathway promotes protein synthesis, while the endoplasmic reticulum stress pathway, and other stress pathways activated by viruses and cytokines (IL1B, TNF, Interferons), oxidative stress or starvation, all factors associated with bipolar disorder risk, shuts down protein synthesis via control of the EIF2 alpha and beta translation initiation complex. For unknown reasons, oligodendrocytes appear to be particularly prone to defects in the translation initiation complex (EIF2B) and the convergence of these environmental and genomic signalling pathways on this area might well explain their vulnerability in bipolar disorder.

Endoplasmic reticulum stress increases glucose-6-phosphatase and glucose cycling in liver cells

Impaired regulation of hepatic glucose production is a characteristic feature of the metabolic syndrome, a cluster of diseases that includes obesity, insulin resistance, type 2 diabetes, and cardiovascular disease. It has been proposed that sustained endoplasmic reticulum stress, which appears to occur in obesity and diabetes, modulates insulin action in the liver. In this study, we show that experimental induction of endoplasmic reticulum stress increases expression and activity of glucose-6-phosphatase and the capacity for glucose release and glucose cycling in primary rat hepatocytes and H4IIE liver cells. Increased expression of the catalytic subunit of glucose-6-phosphatase was largely a result of increased transcription. Deletion analysis of the glucose-6-phosphatase promoter identified an endoplasmic reticulum stress-responsive region located between -233 and -187 with respect to the transcriptional start site. Experimental induction of endoplasmic reticulum stress increased the activity of c-jun N-terminal kinase. Prevention of endoplasmic reticulum stress-mediated activation of c-jun N-terminal kinase reduced the expression of the catalytic subunit of glucose-6-phosphatase, glucose-6-phosphatase activity, glucose release, and glucose cycling. These data demonstrate that sustained endoplasmic reticulum stress in the hepatocyte provokes adaptations, mediated in part via activation of c-jun N-terminal kinase, that act to increase hepatocellular capacity for glucose release and glucose cycling.

HLA-B27 misfolding in transgenic rats is associated with activation of the unfolded protein response

The mechanism by which the MHC class I allele, HLA-B27, contributes to spondyloarthritis pathogenesis is unknown. In contrast to other alleles that have been examined, HLA-B27 has a tendency to form high m.w. disulfide-linked H chain complexes in the endoplasmic reticulum (ER), bind the ER chaperone BiP/Grp78, and undergo ER-associated degradation. These aberrant characteristics have provided biochemical evidence that HLA-B27 is prone to misfold. Recently, similar biochemical characteristics of HLA-B27 were reported in cells from HLA-B27/human beta2-microglobulin transgenic (HLA-B27 transgenic) rats, an animal model of spondyloarthritis, and correlated with disease susceptibility. In this study, we demonstrate that the unfolded protein response (UPR) is activated in macrophages derived from the bone marrow of HLA-B27 transgenic rats with inflammatory disease. Microarray analysis of these cells also reveals an IFN response signature. In contrast, macrophages derived from premorbid rats do not exhibit a strong UPR or evidence of IFN exposure. Activation of macrophages from premorbid HLA-B27 transgenic rats with IFN-gamma increases HLA-B27 expression and leads to UPR induction, while no UPR is seen in cells from nondisease-prone HLA-B7 transgenic or wild-type (nontransgenic) animals. This is the first demonstration, to our knowledge, that HLA-B27 misfolding is associated with ER stress that results in activation of the UPR. These observations link HLA-B27 expression with biological effects that are independent of immunological recognition, but nevertheless may play an important role in the pathogenesis of inflammatory diseases associated with this MHC class I allele.

Defect of N-glycosylation is not directly related to congenital disorder of glycosylation Ia fibroblast sensitivity to staurosporine-induced cell death

Congenital disorder of glycosylation Ia (CDGIa) is an autosomal recessive disease that is caused by mutations in the gene PMM2 encoding phosphomannomutase, an enzyme that synthesizes mannose-1-phosphate, an important intermediate for the N-glycan biosynthesis. Here, we investigated the susceptibility of CDGIa fibroblasts to cell death induction. CDGIa fibroblasts were more sensitive than control fibroblasts to staurosporine-induced apoptosis. Supplementation with mannose, which corrects N-glycosylation in CDGIa fibroblasts, did not abrogate their higher sensitivity to staurosporine. These results show that the sensitivity of CDGIa fibroblasts to apoptosis is not directly related to their defective N-glycosylation.

Quantitative measurement of events in the mammalian unfolded protein response

The accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) triggers the unfolded protein response. IRE1, PERK, ATF6, BiP, EDEM, lipid-linked oligosaccharides (LLOs), and XBP1 directly or indirectly participate in this process. This article provides methods used in our laboratory to quantitatively measure the accumulation of mRNAs encoding BiP and EDEM; splicing of XBP-1; cleavage of ATF6; inhibition of protein synthesis by PERK; and extension of LLOs under control and stress conditions.

Analysis of glycosylation in CDG-Ia fibroblasts by fluorophore-assisted carbohydrate electrophoresis: implications for extracellular glucose and intracellular mannose 6-phosphate

Phosphomannomutase (PMM) deficiency causes congenital disorder of glycosylation (CDG)-Ia, a broad spectrum disorder with developmental and neurological abnormalities. PMM converts mannose 6-phosphate (M6P) to mannose-1-phosphate, a precursor of GDP-mannose used to make Glc(3)Man(9)GlcNAc(2)-P-P-dolichol (lipid-linked oligosaccharide; LLO). LLO, in turn, is the donor substrate of oligosaccharyltransferase for protein N-linked glycosylation. Hepatically produced N-linked glycoproteins in CDG-Ia blood are hypoglycosylated. Upon labeling with [(3)H]mannose, CDG-Ia fibroblasts have been widely reported to accumulate [(3)H]LLO intermediates. Since these are thought to be poor oligosaccharyltransferase substrates, LLO intermediate accumulation has been the prevailing explanation for hypoglycosylation in patients. However, this is discordant with sporadic reports of specific glycoproteins (detected with antibodies) from CDG-Ia fibroblasts being fully glycosylated. Here, fluorophore-assisted carbohydrate electrophoresis (FACE, a nonradioactive technique) was used to analyze steady-state LLO compositions in CDG-Ia fibroblasts. FACE revealed that low glucose conditions accounted for previous observations of accumulated [(3)H]LLO intermediates. Additional FACE experiments demonstrated abundant Glc(3)Man(9)GlcNAc(2)-P-P-dolichol, without hypoglycosylation, CDG-Ia fibroblasts grown with physiological glucose. This suggested a "missing link" to explain hypoglycosylation in CDG-Ia patients. Because of the possibility of its accumulation, the effects of M6P on glycosylation were explored in vitro. Surprisingly, M6P was a specific activator for cleavage of Glc(3)Man(9)GlcNAc(2)-P-P-dolichol. This led to futile cycling the LLO pathway, exacerbated by GDP-mannose/PMM deficiency. The possibilities that M6P may accumulate in hepatocytes and that M6P-stimulated LLO cleavage may account for both hypoglycosylation and the clinical failure of dietary mannose therapy with CDG-Ia patients are discussed.

Genome‐wide analysis of the unfolded protein response in fibroblasts from congenital disorders of glycosylation type‐I patients

Congenital disorders of glycosylation (CDG) are a family of diseases characterized by defects of N-linked glycosylation. In CDG-I, several genetic defects cause a shortage of dolichol-linked oligosaccharides, which leads to underglycosylation of nascent glycoproteins. N-linked glycosylation is important for proper folding and trafficking of glycoproteins. Inhibition of glycosylation results in the buildup of misfolded proteins in the endoplasmic reticulum, which induces a protective reaction known as the unfolded protein response (UPR). To investigate whether UPR components are induced in CDG, we have performed a transcriptome analysis of primary fibroblasts from unaffected control subjects and from CDG-I patients using oligonucleotide gene expression arrays. The stress imposed by CDG was also compared with the stress induced by tunicamycin and glucose deprivation. Whereas tunicamycin elicited a strong transcriptional response typical for the UPR, CDG fibroblasts displayed a qualitatively similar yet moderate induction of genes encoding components of the UPR. Among these genes, the PERK kinase inhibitor DNAJC3/P58(IPK) gene showed the highest induction throughout all CDG-I types tested. This was paralleled by elevated expression of genes involved in amino acid biosynthesis and transport, which defined a new component of the cellular response to glycosylation stress.

Discordance of UPR signaling by ATF6 and Ire1p-XBP1 with levels of target transcripts

Accumulation of misfolded proteins within the lumen of the mammalian endoplasmic reticulum (ER) activates the unfolded protein response (UPR). ATF6 and Ire1p are ER-associated proteins that control UPR-specific transcription systems in mammals; UPR signaling involves cleavage of ATF6 and splicing of XBP1 mRNA initiated by Ire1p. We tested the hypothesis that activation of ATF6 and/or Ire1p determines the levels of mRNAs derived from target genes encoding GRP78/BiP and EDEM. By subjecting dermal fibroblasts to multiple stresses, strong correlations were found between ATF6 activation and XBP1 splicing, and between GRP78/BiP mRNA and EDEM mRNA accumulation. Surprisingly, there was no reasonable correlation between activation of either signal transducer with accumulation of either target transcript. Thus, ATF6 and Ire1p signaling do not define the magnitude of UPR-dependent mRNA increases, even though they may be necessary for gene activation, suggesting the existence of additional stress-sensitive factors acting as "coincidence detectors" for transcript accumulation.

Metformin-stimulated Mannose Transport in Dermal Fibroblasts

The biguanide drug metformin stimulates AMP-activated protein kinase, a master regulator of cellular energy metabolism, and has antihyperglycemic activity due to attenuation of gluconeogenesis in hepatocytes and 2-fold stimulation of glucose transport by skeletal muscle. Here we identify a metformin-stimulated d-mannose transport (MSMT) activity in dermal fibroblasts. MSMT increased mannose uptake 1.8-fold and had greater affinity for mannose than basal mannose transport activity. It was attributed to robust stimulation of a transporter expressed weakly in untreated cells. MSMT was not explained by greater glucose transporter activity because metformin unexpectedly decreased transport of 2-deoxy-d-glucose and 3-O-methyl-d-glucose by fibroblasts. Effective inhibitors of MSMT retained specificity for the 3-, 4-, and 6-OH groups of the mannose ring but not the 2-OH group. Thus, MSMT could be strongly inhibited by glucose and 2-deoxy-d-glucose even though the latter was not a good transport substrate. MSMT was significant because in the presence of 2.5 microm mannose, metformin corrected experimentally induced deficiencies in the synthesis of glucose(3)mannose(9)GlcNAc(2)-P-P-dolichol and N-linked glycosylation. MSMT was also identified in congenital disorder of glycosylation types Ia and Ib fibroblasts, and metformin acted synergistically with 100 microm mannose to correct lipid-linked oligosaccharide synthesis and N-glycosylation in the Ia cells. In conclusion, metformin activates a novel fibroblast mannose-selective transport system. This suggests that AMP-activated protein kinase may be a regulator of mannose metabolism and implies a therapy for congenital disorders of glycosylation-Ia.

The Mechanism of Oxidative Stress Stabilization of the Thromboxane Receptor in COS-7 Cells

The 8-iso-prostaglandin F(2alpha), a prostanoid produced in vivo by cyclooxygenase-independent free-radical-catalyzed lipid peroxidation, acts as a partial agonist on the thromboxane receptor (TXA(2)R) and is a potent vasoconstrictor in the oxidatively stressed isolated perfused rat heart. We hypothesized that the response in the isolated heart may be due to augmentation of TXA(2)R density, which may be initiated by the presence of oxidative radicals. Previous studies have shown that TXA(2)R density is increased during atherosclerosis on both the medial and intimal smooth muscle layers in human coronary arteries. Here we describe the effect of oxidative stress on TXA(2)R. The thromboxane A(2) receptor beta isoform (TXA(2)Rbeta) was transiently expressed in COS-7 cells. Immunofluorescence suggested that the presence of H(2)O(2) increased translocation of TXA(2)Rbeta from the endoplasmic reticulum (ER) to the Golgi complex. H(2)O(2) treatment also increased binding of a TXA(2)R antagonist ([(3)H]SQ29548) to membranes. Degradation kinetics of TXA(2)Rbeta following cycloheximide treatment, a protein synthesis inhibitor, suggested not only that TXA(2)Rbeta is a short-lived protein predominantly localized to the ER but also that TXA(2)Rbeta degradation is modulated in the presence of H(2)O(2). Our results indicate that oxidative stress induces maturation and stabilization of the TXA(2)Rbeta protein probably by intracellular translocation. Importantly, these observations also suggest that TXA(2)Rbeta levels are modulated by ER-associated degradation and controlled by the efficiency of transport to post-ER compartments. Stabilization of the TXA(2)Rbeta by translocation from a degradative compartment, i.e. the ER, can account for the augmentation of receptor density observed in vivo.

Activation of glycogen phosphorylase with 5-aminoimidazole-4-carboxamide riboside (AICAR). Assessment of glycogen as a precursor of mannosyl residues in glycoconjugates

The experimental evaluation of the contribution of glycogen phosphorylase (GP) to biochemical pathways is limited to methods that raise cAMP, activating the cAMP-dependent protein kinase/phosphorylase kinase/GP cascade. Such methods convert the unphosphorylated form, "GPb," which catalyzes glycogenolysis only in the presence of appropriate allosteric activators such as AMP, to the phosphorylated, constitutively activated form, "GPa." However, activation of GP in this way is indirect, requires a functional cAMP kinase cascade, and is complicated by other actions of cAMP. Here, we demonstrate a strategy for the experimental manipulation of GP in intact dermal fibroblasts, involving activation by the membrane-permeable adenosine analog 5-aminoimidazole-4-carboxamide riboside (AICAR) and inhibition by caffeine and Pfizer compound CP-91149, which bind to GP at distinct sites. Potential complications because of activation of AMP-activated protein kinase by AICAR were assessed with metformin, which activates this kinase but does not activate GP. Using this strategy, we show that glycogen can be a significant and regulatable precursor of mannosyl units in lipid-linked oligosaccharides and glycoproteins.

Endoplasmic reticulum-associated degradation of glycoproteins bearing Man5GlcNAc2 and Man9GlcNAc2 species in the MI8-5 CHO cell line

Endoplasmic reticulum-associated degradation of newly synthesized glycoproteins has been demonstrated previously using various mammalian cell lines. Depending on the cell type, glycoproteins bearing Man9 glycans and glycoproteins bearing Man5 glycans can be efficiently degraded. A wide variety of variables can lead to defective synthesis of lipid-linked oligosaccharides and, therefore, in mammalian cells, species derived from Man9GlcNAc2 or Man5GlcNAc2 are often recovered on newly synthesized glycoproteins. The degradation of glycoproteins bearing these two species has not been studied. We used a Chinese hamster ovary cell line lacking Glc-P-Dol-dependent glucosyltransferase I to generate various proportions of Man5GlcNAc2 and Man9GlcNAc2 on newly synthesized glycoproteins. By studying the structure of the soluble oligomannosides produced by degradation of these glycoproteins, we demonstrated the presence of a higher proportion of soluble oligomannosides originating from truncated glycans, showing that glycoproteins bearing Man5GlcNAc2 glycans are degraded preferentially.

Rapid activation of glycogen phosphorylase by the endoplasmic reticulum unfolded protein response

Endoplasmic reticulum (ER) stress is associated with misfolding of ER proteins and triggers the unfolded protein response (UPR). The UPR, in turn, helps restore normal ER function. Since fastidious N-linked glycosylation is critical for folding of most ER proteins, this study examined whether metabolic interconversions of precursors used for glycan assembly were controlled by the UPR. Thus, eight enzymes and factors with key roles in hexose phosphate metabolism were assayed in cytoplasmic extracts from primary dermal fibroblasts treated with UPR inducers. Stimulation of only one activity by the UPR was detected, AMP-independent glycogen phosphorylase (GP). GP activation required only 20 min of ER stress, with concurrent decreases in cellular glycogen and elevations of its metabolites Glc-1-P and Glc-6-P. Addition of phosphatase inhibitors to enzyme extracts from unstressed cells mimicked the effect of ER stress on GP activity, suggesting that phosphorylation of GP or a regulatory factor was involved. These data show that the UPR can modulate hexose metabolism in a manner beneficial for protein glycosylation. Since activation of GP appears to occur by a rapid post-translational process, it may be part of a general strategy of ER damage control, preceding the well-known transcription-dependent processes of the UPR that are manifested hours after the occurrence of ER stress.

Coupling of the dolichol-P-P-oligosaccharide pathway to translation by perturbation-sensitive regulation of the initiating enzyme, GlcNAc-1-P transferase

In mammalian cells, inhibition of translation interferes with synthesis of the lipid-linked oligosaccharide (LLO) Glc3Man9GlcNAc2-P-P-dolichol as measured with radioactive sugar precursors. Conflicting hypotheses have been proposed, and the fundamental basis for this regulation has remained elusive. Here, fluorophore-assisted carbohydrate electrophoresis (FACE) was used to measure LLO concentrations directly in cells treated with translation blockers. Further, LLO biosynthetic enzymes were assayed in vitro with endogenous acceptor substrates using either cells gently permeabilized with streptolysin-O (SLO) or microsomes from homogenized cells. In Chinese hamster ovary (CHO)-K1 cells treated with translation blockers, FACE did not detect changes in concentrations of Glc3Man9GlcNAc2-P-P-dolichol or early LLO intermediates. These results do not support earlier proposals for feedback repression of LLO initiation by accumulated Glc3Man9GlcNAc2-P-P-dolichol, or inhibition of a GDP-mannose dependent transferase. With microsomes from cells treated with translation blockers, there was no interference with LLO initiation by GlcNAc-1-P transferase (GPT), mannose-P-dolichol synthase, glucose-P-dolichol synthase, or LLO synthesis in vitro, as reported previously. Surprisingly, inhibition of all of these was detected with the SLO in vitro system. Additional experiments with the SLO system showed that the three transferases shared a limited pool of dolichol-P that was trapped as Glc3Man9GlcNAc2-P-P-dolichol by translation arrest. Overexpression of GPT was unable to reverse the effects of translation arrest on LLO initiation, and experiments with FACE and the SLO system showed that overexpressed GPT was not functional in vivo, although it was highly active in microsomal assays. Thus, the combined use of the SLO in vitro system and FACE showed that LLO biosynthesis depends upon a limited primary pool of dolichol-P. Physical perturbation associated with microsome preparation appears to make available a secondary pool of dolichol-P, masking inhibition by translation arrest, as well as activating a nonfunctional fraction of GPT. The implications of these results for the organization of the LLO pathway are discussed.