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Hydrogen Deuterium Exchange Reveals Changes to Protein Dynamics of Recombinant Human Erythropoietin upon N- and O- Desialylation
Abstract
Recombinant human erythropoietin (EPO) is a therapeutic glycoprotein widely used for treating anemia. EPO glycans carry extensive sialylation and the level of the modification is known to affect receptor binding, protein stability and pharmacokinetics. Nonetheless, a detailed understanding of the effects of sialylation on EPO conformation and dynamics is still lacking. Here we investigate the changes to EPO dynamics following enzymatic trimming of terminal sialic acid by amide hydrogen deuterium exchange mass spectrometry (HDX-MS). The results revealed that desialylation enhances structural flexibility near the glycosylation sites, with greater effects observed around the O-glycosylation site relative to the N-glycosylation sites. The affected regions are surface-exposed loops connecting the helix bundle, which do not appear to reduce the thermostability of the molecule as revealed from melting measurement. Our findings demonstrate the feasibility of HDX-MS technique in deciphering the function of specific type of glycosylation that can provide novel insights into the role of sialylation on protein therapeutics.
... 548−551 In other glycoproteins, the only observed changes were increased dynamics through the protein upon glycan removal. 541,552 Some studies observed only very minor changes in HDX kinetics despite known effects on overall thermal stability upon glycan removal. 553 The available studies have thus far revealed a complex and very context-dependent relationship between glycosylation and structural dynamics. ...
Solution-phase hydrogen/deuterium exchange (HDX) coupled to mass spectrometry (MS) is a widespread tool for structural analysis across academia and the biopharmaceutical industry. By monitoring the exchangeability of backbone amide protons, HDX-MS can reveal information about higher-order structure and dynamics throughout a protein, can track protein folding pathways, map interaction sites, and assess conformational states of protein samples. The combination of the versatility of the hydrogen/deuterium exchange reaction with the sensitivity of mass spectrometry has enabled the study of extremely challenging protein systems, some of which cannot be suitably studied using other techniques. Improvements over the past three decades have continually increased throughput, robustness, and expanded the limits of what is feasible for HDX-MS investigations. To provide an overview for researchers seeking to utilize and derive the most from HDX-MS for protein structural analysis, we summarize the fundamental principles, basic methodology, strengths and weaknesses, and the established applications of HDX-MS while highlighting new developments and applications.
... Previously, glycan related conformational change has also been reported for erythropoietin following desialylation. 25 Other analytical tools, such as differential scanning calorimetry (DSC), have been evaluated and are less informative ( Figure S-7 and Table S-2). The differences observed by DSC as related to thermal stability are not as easily differentiated as compared to HDX-MS and do not provide any site-specific structural information as does HDX-MS. ...
Characterization of the higher order structures in Idursulfase (iduronate-2-sulfatase, I2S) has been accomplished through the use of hydrogen-deuterium exchange mass spectrometry (HDX-MS). The method has over 97% sequence coverage, including 7 of the 8 glycosylation sites and has been used to study the impact of glycosylation on backbone proton exchange. In addition, the method adapted a well-used biophysical spectra comparison method (similarity scoring) to define quantitative acceptance criteria for analytical comparability of different batches of drug substance as well as samples with modulated glycans. Differences in HDX profile were induced by enzymatic removal of terminal sialic and phosphate groups on negatively charged glycans. These differences were mapped to the crystal structure and demonstrated synergistic HDX changes focused around the N221 and N255 glycosylation sites which contain mannose-6-phosphate motifs important for I2S uptake into cells.
Purpose:
Oligosaccharides play diverse and unpredictable functional roles when attached to proteins and are a largely unexplored scaffold for deconstructing and attributing novel functions to proteins during drug development. Here, the glycoprotein Artemin (ART) was carefully assessed by multiple analytical methods that allow us to provide a comprehensive understanding of how N-linked glycosylation impact the structural and functional properties of ART.
Methods:
Modification of the N-linked glycan of ART was performed by incubation with various enzymes. Biological assays and systems were used to examine the relative activity and pharmacokinetic properties of ART as a function of glycosylation. In order to reveal the conformational impact of glycosylation on ART, hydrogen/deuterium exchange mass spectrometry (HDX-MS) was employed in addition to differential scanning calorimetry. The colloidal stability of ART glycovariants was assessed by dynamic light scattering, viscometry, and solubility assays.
Results:
No difference in pharmacokinetics or relative potency was revealed between glycosylated and nonglycosylated ART. Surprisingly, the HDX-MS data indicated that the glycan does not greatly influence the conformation and dynamics of the protein. In contrast, differences in thermal and colloidal stability clearly revealed a role of glycosylation in increasing the solubility and stability of ART.
Conclusions:
Our findings demonstrate how careful analysis using multiple advanced techniques can be used to identify and dissect the multiple potential functions of protein glycosylation and form a prerequisite for glycoengineering and drug development of glycoproteins.
Protein glycosylation plays critical roles for the regulation of diverse biological processes, and determination of glycan structure-function relationships is important to better understand these events. However, characterization of glycan and glycopeptide structural isomers remains challenging and often relies on biosynthetic pathways being conserved. In glycoproteomic analysis with liquid chromatography - tandem mass spectrometry (LC-MS/MS), using collision-induced dissociation (CID), saccharide oxonium ions containing N-acetylhexosamine (HexNAc) residues are prominent. Through analysis of beam-type CID spectra and ion trap CID spectra of synthetic, and natively derived, N- and O-glycopeptides we found that the fragmentation patterns of oxonium ions characteristically differ between glycopeptides terminally substituted with GalNAcα1-O-, GlcNAcβ1-O-, Galβ3GalNAcα1-O-, Galβ4GlcNAcβ-O-, and Galβ3GlcNAcβ-O- structures. The difference in oxonium ion fragmentation profiles of such glycopeptides may thus be used to distinguish between these glycan structures, and be of importance in LC-MS/MS based glycoproteomic studies.
Recombinant human erythropoietin (rhEPO) has been extensively used as a pharmaceutical product for treating anemia. Glycosylation of rhEPO affects the biological activity, immunogenicity, pharmacokinetics, and in-vivo clearance rate of rhEPO. Characterization of the glycosylation status of rhEPO is of great importance for quality control. In this study, we established a fast and comprehensive approach for reliable characterization and relative quantitation of rhEPO glycosylation, which combines multiple-enzyme digestion, hydrophilic-interaction chromatography (HILIC) enrichment of glycopeptides, and tandem mass spectrometry (MS) analysis. The N-linked and O-linked intact glycopeptides were analyzed with high-resolution and high-accuracy (HR–AM) mass spectrometry using an Orbitrap. In total, 74 intact glycopeptides from four glycosylation sites at N24, N38, N83, and O126 were identified, with the simultaneous determination of peptide sequences and glycoform compositions. The extracted ion chromatograms based on the HR–AM data enabled relative quantification of glycoforms. Our results could be extended to quality control of rhEPO or could help establish detection approaches for glycosylation of other proteins.
Graphical Abstract
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Human α1-Acid Glycoprotein (AGP), an acute phase glycoprotein, exists predominantly in blood. With its ability to bind basic, lipophilic, and acidic drugs, AGP has served as a drug carrier. It has been shown that the carbohydrate composition of AGP changes in response to tissue injury, inflammation or infection, and can have great impact on AGP's drug-binding activities. The molecular-level details of the effects of desialylation on the AGP conformation and AGP-ligand interactions, however, are unknown. Here we report the use of hydrogen/deuterium exchange coupled with mass spectrometry (HDX-MS) to reveal the changes in AGP conformational dynamics induced by the removal of terminal sialic acid. HDX-MS also reveals the changes in conformational dynamics of sialylated and unsialylated AGP upon formation of holo-AGP-complexes with progesterone or propranolol. Our HDX-MS results demonstrate that desialylation stabilizes two loop regions that are exterior to the β-sheet barrel in AGP, and this stabilization minimizes the conformational changes of AGP upon binding with progesterone or propranolol.
Protein glycosylation commonly stabilizes proteins thereby increasing protein half-lives and protecting against denaturation or proteolytic degradation. While generally beneficial, such stabilization is potentially disadvantageous in the case of inhibitory serpins. These protease inhibitors are metastable and a conformational transition to a more stable form is key to their function. Instability is therefore essential for these inhibitory serpins and mutagenesis has demonstrated that substantial stabilization results in compromised function. We have used optical spectroscopy and hydrogen/deuterium exchange and mass spectrometry to investigate the effects of glycosylation on the human serpin alpha-1 antitrypsin (α(1)-AT). Previous studies found that unglycosylated recombinant α(1)-AT populates a molten globule at low denaturant and that the ability to populate this state is correlated with efficient protease inhibition. Further, a high degree of conformational flexibility was found in several important regions. Guanidine hydrochloride denaturation monitored by circular dichroism indicates that plasma α(1)-AT, which is glycosylated at 3 sites, is substantially stabilized relative to the unglycosylated form. However, hydrogen exchange reveals complete loss of protection in plasma α(1)-AT above 1 M GuHCl, similar to what is seen for the recombinant form. Sugars therefore appear to stabilize the compact denatured state of α(1)-AT without significant stabilization of the folded state. Native state hydrogen exchange reveals minor perturbations to native flexibility, but high flexibility in key regions such as the f helix is conserved. β-strand 1c is stabilized in plasma α(1)-AT, which may confer increased resistance to forming pathogenic polymers. Overall, our results indicate that glycosylation of inhibitory serpins does not interfere with either native state flexibility or the native instability that is required for efficient function, though it may confer resistance to degradation by proteases and thus extend the half-life of circulating serpins.
Erythropoietin is a glycoprotein hormone that stimulates the maturation of late erythroid progenitor cells. It has three N-linked and one O-linked carbohydrates which play an important role in the biosynthesis and biological activities of the protein. To determine the role the carbohydrate might have in maintaining the conformational stability of the protein, the protein expressed in mammalian cells (fully glycosylated), the asialo mammalian-expressed protein, and the protein expressed in Escherichia coli (no carbohydrate) were compared for their stability to guanidine HCl, pH, and temperature. Circular dichroism was used to follow protein unfolding. Both the intact and asialo mammalian-expressed proteins unfolded with a cooperative transition in guanidine HCl, with a midpoint at 1.75 M guanidine HCl. The E. coli-expressed material unfolded with a midpoint of 1.2 M guanidine HCl, and a delta G of unfolding which was 1.4 kcal/mol less than that of the two glycosylated molecules. The E. coli-derived protein was also significantly less stable to pH-induced conformational changes, showing a cooperative transition in 35% glycerol with a midpoint at pH 4.4, while both the intact and asialo mammalian-expressed molecules had a transition midpoint of pH 3.75 in the absence of glycerol, and approximately pH 3 in the presence of 35% glycerol. The E. coli-expressed molecule unfolded and precipitated upon heating to 44 degrees C, while the asialo and intact mammalian-expressed proteins remained soluble, with a Tm of 56 degrees C. From these experiments, the carbohydrate appears to play a critical role in stabilizing the erythropoietin molecule to denaturing conditions, and this increased stability does not depend on the presence of sialic acid.
Human erythropoietin is a haematopoietic cytokine required for the differentiation and proliferation of precursor cells into red blood cells. It activates cells by binding and orientating two cell-surface erythropoietin receptors (EPORs) which trigger an intracellular phosphorylation cascade. The half-maximal response in a cellular proliferation assay is evoked at an erythropoietin concentration of 10 pM, 10(-2) of its Kd value for erythropoietin-EPOR binding site 1 (Kd approximately equal to nM), and 10(-5) of the Kd for erythropoietin-EPOR binding site 2 (Kd approximately equal to 1 microM). Overall half-maximal binding (IC50) of cell-surface receptors is produced with approximately 0.18 nM erythropoietin, indicating that only approximately 6% of the receptors would be bound in the presence of 10 pM erythropoietin. Other effective erythropoietin-mimetic ligands that dimerize receptors can evoke the same cellular responses but much less efficiently, requiring concentrations close to their Kd values (approximately 0.1 microM). The crystal structure of erythropoietin complexed to the extracellular ligand-binding domains of the erythropoietin receptor, determined at 1.9 A from two crystal forms, shows that erythropoietin imposes a unique 120 degrees angular relationship and orientation that is responsible for optimal signalling through intracellular kinase pathways.
Erythropoietin (Epo) controls the proliferation, differentiation and survival of the erythroid progenitors. This cytokine was cloned in 1985 and rapidly became used for treatment of anemia of renal failure, opening the way to the first clinical trials of a hematopoietic growth factor. The clonage of one chain of the Epo receptor followed in 1989, thereby opening the research on intracellular signal transduction induced by Epo. Epo is synthesized mainly by the kidney and the liver and sequences required for tissue-specific expression have been localized in the Epo gene. A 3'enhancer is responsible for hypoxia-inducible Epo gene expression. HIF-1 alpha and beta proteins bind to this enhancer. Gene regulation by hypoxia is widespread in many cells and involves numerous genes in addition to the Epo gene. The Epo receptor belongs to the cytokine receptor family and includes a p66 chain which is dimerized upon Epo activation; two accessory proteins defined by cross-linking remain to be characterized. Epo binding induces the stimulation of Jak2 tyrosine kinase. Jak2 activation leads to the tyrosine phosphorylation of several proteins including the Epo receptor itself. As a result, different intracellular pathways are activated: Ras/MAP kinase, phosphatidylinositol 3-kinase and STAT transcription factors. However, the exact mechanisms by which the proliferation and/or the differentiation of erythroid cells are regulated after Epo stimulation are not known. Furthermore, target disruption of both Epo and Epo receptor showed that Epo was not involved in the commitment of the erythroid lineage and seemed to act mainly as a survival factor.
Erythropoietin (EPO) is a haemopoietic hormone specific to cells of erythroid lineage. EPO has recently become available for the treatment of anaemia as the first human recombinant biomedicine produced in heterologous mammalian cells. Human EPO is characterized by its large carbohydrate chains, which occupy close to 40% of its total mass. These sugar moieties were thought to be important for the biological activity of EPO, but detailed studies were not performed until the structures were elucidated. The variety of roles for the sugar chains were then immediately found once the structures were known. EPO is an excellent model for investigating the roles of sugar chains on glycoproteins, since its gene and its multiple glycoforms are available, as well as sensitive bioassays for testing. In this review, we will first summarize the known sugar chain structures of EPO from different host cells, and then discuss the host-cell dependent and peptide structure-dependent glycosylation of glycoproteins. We will then address how one investigates the roles of sugar chains of glycoproteins, show several examples of such investigations, and discuss the functional roles of HuEPO's sugar chains in its biosynthesis and secretion, its in vitro and in vivo biological activities, and its half-life in blood circulation.
N-Linked glycosylation plays important roles in modulating protein structure and function. The direct impact of the modification on protein conformation is not yet well understood.
Here we probed the dynamic changes following Endo H trimming of high mannose glycans in α-amylase by means of amide hydrogen/deuterium exchange mass spectrometry.
The results revealed that deglycosylation elicited extensive alterations in backbone dynamics, affecting regions both adjacent to and distal from the glycosylation site.
The overall exchange rate is reduced in the glycosylated state, which indicates rigidity enhancement due to the attached carbohydrates. Copyright © 2013 John Wiley & Sons, Ltd.
Glycosylation plays a critical role in the in vivo efficacy of both endogenous and recombinant erythropoietin (EPO). Using mass spectrometry, we characterized the N-/O-linked glycosylation of recombinant human EPO (rhEPO) produced in glycoengineered Pichia pastoris and compared with the glycosylation of Chinese hamster ovary (CHO) cell-derived rhEPO. While the three predicted N-linked glycosylation sites (Asn24, Asn38 and Asn83) showed complete site occupancy, Pichia- and CHO-derived rhEPO showed distinct differences in the glycan structures with the former containing sialylated bi-antennary glycoforms and the latter containing a mixture of sialylated bi-, tri- and tetra-antennary structures. Additionally, the N-linked glycans from Pichia-produced rhEPO were similar across all three sites. A low level of O-linked mannosylation was detected on Pichia-produced rhEPO at position Ser126, which is also the O-linked glycosylation site for endogenous human EPO and CHO-derived rhEPO. In summary, the mass spectrometric analyses revealed that rhEPO derived from glycoengineered Pichia has a highly uniform bi-antennary N-linked glycan composition and preserves the orthogonal O-linked glycosylation site present on endogenous human EPO and CHO-derived rhEPO. Copyright © 2013 John Wiley & Sons, Ltd.
The erythropoietin (EPO) molecule contains four carbohydrate chains. Three contain N-linkages to asparagines at positions 24, 38, and 83, and one contains an O-linkage to a serine at position 126. We constructed human EPO variants that eliminated the three N-glycosylation sites by replacing the asparagines with glutamines singly or in combination. The O-linked carbohydrate chain was removed by replacing the serine with glutamine, valine, histidine, or alanine. A variant with a double mutation (Gln38,83) and another with a triple mutation (Gln24,38,83) were secreted poorly from COS1 and CHO cells even though RNA encoding these variants was present. All other variants with mutations in N-linked glycosylation sites were secreted normally. Removal of any of the N-glycosylation sites reduced the in vivo but not the in vitro biological activity of the EPO molecule. All the mutations at Ser126, the O-glycosylation site, were secreted normally. In vitro activity was also unaffected except for Ala126 which had a 50-fold decrease. The Val126 variant was tested in vivo, and its specific activity was only slightly less than that of the native EPO, which indicates that the O-linked carbohydrate is not essential for activity.
The rates at which hydrogens located at peptide amide linkages in proteins undergo isotopic exchange when a protein is exposed to D2O depend on whether these amide hydrogens are hydrogen bonded and whether they are accessible to the aqueous solvent. Hence, amide hydrogen exchange rates are a sensitive probe for detecting changes in protein conformation and dynamics. Hydrogen exchange rates in proteins are most often measured by NMR or Fourier transform IR spectroscopy. After a brief introduction to model kinetics used to relate amide hydrogen exchange rates to protein structure and dynamics, information required to understand and implement a new method that uses acid proteases and mass spectrometry to determine amide hydrogen exchange rates in proteins is presented. Structural and dynamic features affecting isotopic exchange rates can be detected and localized from the deuterium levels detected by mass spectrometry in proteolytic fragments of the protein. Procedures used to adjust for isotopic exchange occurring during the analysis, to extract isotope exchange rate constants from mass spectra and to link bimodal isotope patterns to protein unfolding and structural heterogeneity are also discussed. In addition, the relative merits of using mass spectrometry or NMR combined with amide hydrogen exchange to study protein structure and dynamics are discussed. The spatial resolution of hydrogen exchange results obtained by this method is typically in the range of 1–10 residues, which is substantially less than that obtained by high-resolution NMR, but sufficient to detect many functionally significant structural changes. Advantages in the areas of sensitivity, protein solubility, detection of correlated exchange and high molecular mass proteins make this approach particularly attractive for a wide range of studies. © 1997 by John Wiley & Sons, Ltd.
1Recombinant human erythropoietin has N-linked sugar [Tsuda et al., (1988) Biochemistry 27, 5646–5654]. Here we have demonstrated the presence of O-linked sugar (0.85 mol/mol erythropoietin) composed of sialic acid and Galβ(1–3)GalNAc.2To investigate the role of these sugars, erythropoietins deglycosylated to different extents were prepared using specific glycosidases. Sugars are not essential for in vitro biological activity of erythropoietin, because the fully deglycosylated erythropoietin had the full activity when assayed with in vitro bioassay methods. Asialylation yielded erythropoietin with higher affinity to the receptor than the undigested hormone and therefore an increased in vitro activity. Although erythropoiein from which N-linked or total sugars were removed also had higher affinity for the receptor, their in vitro activity remained unchanged compared with that of the undigested erythropoietin for unknown reasons. On the other hand, removal of sialic acids or N-linked sugar abolished the in vivo biological activity completely, indicating that the presence of N-linked sugar with terminal sialic acids is required for the hormone to reach target sites; full deglycosylation resulted in total loss of the in vivo biological activity of erythropoietin.3Incubation of asialo-erythropoietin and fully deglycosylated recombinant human erythropoietin at 70 °C for 15 min decreased the biological activity to 35% and 11% of the initial activity, respectively, while the undigested erythropoietin lost no activity. Thus resistance of erythropoietin to thermal inactivation is largely due to the presence of sugars, and terminal sialic acids greatly contribute to the stability.
Collision-induced dissociation (CID) tandem mass spectrometry (MS/MS) does not allow the characterization of glycopeptides because of the fragmentation of glycan structures and limited fragmentation of peptide backbones. Electron transfer dissociation (ETD) MS/MS, on the other hand, offers a complementary approach, prompting only peptide backbone fragmentation while keeping post-translational modifications intact. Characterization of glycopeptides using both CID and ETD is summarized in this unit. While CID provides information related to the composition of glycan moieties attached to a peptide backbone, ETD permits de novo sequencing of peptides. Radical anion transfer of electrons to the peptide backbone in ETD induces cleavage of the N-Cα bond. The glycan moiety is retained on the peptide backbone, largely unaffected by the ETD process, thus allowing the identification of the amino acid sequence of a glycopeptide and its glycosylation site. This unit discusses the use of both CID and ETD for better characterization of glycopeptides.
Hydrogen/deuterium exchange coupled to mass spectrometry (HDX-MS) has emerged as a technique for studying glycoproteins, which are often refractory to classical methods. Glycan chains are generally assumed to exchange protons very rapidly, making them invisible to this technique. Here, we show that under conditions commonly used for HDX-MS, acetamido groups within glycan chains retain a significant amount of deuterium. Using mono- and polysaccharide standards along with glycopeptides from a panel of glycoproteins, we demonstrate that N-acetyl hexosamines, along with modified Asn side chains, are responsible for this effect. Model compounds for sialic acid also displayed similar exchange kinetics, but terminal sialic acids in the context of an entire glycan chain did not contribute to deuterium retention. Furthermore, the presence of sialic acid appears to enhance the exchange rate of the nearby N-acetyl glucosamines. The ability to detect deuterium exchange at the glycan level opens the possibility of applying HDX-MS to monitor glycan interactions and dynamics.
Erythropoietin (EPO) is a haemopoietic hormone specific to cells of erythroid lineage. EPO has recently become available for the treatment of anaemia as the first human recombinant biomedicine produced in heterologous mammalian cells. Human EPO is characterized by its large carbohydrate chains, which occupy close to 40% of its total mass. These sugar moieties were thought to be important for the biological activity of EPO, but detailed studies were not performed until the structures were elucidated. The variety of roles for the sugar chains were then immediately found once the structures were known. EPO is an excellent model for investigating the roles of sugar chains on glycoproteins, since its gene and its multiple glycoforms are available, as well as sensitive bioassays for testing. In this review, we will first summarize the known sugar chain structures of EPO from different host cells, and then discuss the host-cell dependent and peptide structure-dependent glycosylation of glycoproteins. We will then address how one investigates the roles of sugar chains of glycoproteins, show several examples of such investigations, and discuss the functional roles of HuEPO's sugar chains in its biosynthesis and secretion, its in vitro and in vivo biological activities, and its half-life in blood circulation.
Protein function is dictated by protein conformation. For the protein biopharmaceutical industry, therefore, it is important to have analytical tools that can detect changes in protein conformation rapidly, accurately, and with high sensitivity. In this paper we show that hydrogen/deuterium exchange mass spectrometry (H/DX-MS) can play an important role in fulfilling this need within the industry. H/DX-MS was used to assess both global and local conformational behavior of a recombinant monoclonal IgG1 antibody, a major class of biopharmaceuticals. Analysis of exchange into the intact, glycosylated IgG1 (and the Fab and Fc regions thereof) showed that the molecule was folded, highly stable, and highly amenable to analysis by this method using less than a nanomole of material. With improved chromatographic methods, peptide identification algorithms and data-processing steps, the analysis of deuterium levels in peptic peptides produced after labeling was accomplished in 1-2 days. On the basis of peptic peptide data, exchange was localized to specific regions of the antibody. Changes to IgG1 conformation as a result of deglycosylation were determined by comparing exchange into the glycosylated and deglycosylated forms of the antibody. Two regions of the IgG1 (residues 236-253 and 292-308) were found to have altered exchange properties upon deglycosylation. These results are consistent with previous findings concerning the role of glycosylation in the interaction of IgG1 with Fc receptors. Moreover, the data clearly illustrate how H/DX-MS can provide important characterization information on the higher order structure of antibodies and conformational changes that these molecules may experience upon modification.
Sialylation, the attachment of sialic acid residues to a protein, can affect the biological activity and in vivo circulatory half-life of glycoproteins. Human alpha2,3- sialyltransferase (alpha2,3-ST) and beta1,4-galactosyltransferase (beta1,4-GT) are responsible for terminal sialylation and galactosylation, respectively. Enhanced sialylation of human erythropoietin (EPO) by the expression of alpha2,3-ST and beta1,4-GT was achieved using recombinant Chinese hamster ovary (CHO) cells (EC1). The sialic acid content and sialylation of N-glycans were evaluated by HPLC. When alpha2,3-ST was expressed in CHO cells (EC1-ST2), the sialic acid content (moles of sialic acid/mole of EPO) increased from 6.7 to 7.5. In addition, the amount of trisialylated glycans increased from 17.3% to 26.1%. When alpha2,3-ST and beta1,4-GT were coexpressed in CHO cells (EC1-GTST15), the degree of sialylation was greater than that in EC1-ST2 cells. In the case of EC1-GTST15 cells, the sialic acid content increased to 8.2 and the proportion of trisialylated glycans was markedly increased from 17.3% to 35.5%. Interestingly, the amount of asialoglycans decreased only in the case of GTST15 cells (21.4% to 14.2%). These results show that coexpression of alpha2,3- ST and beta1,4-GT is more effective than the expression of alpha2,3-ST alone. Coexpression of alpha2,3-ST and beta1,4-GT did not affect CHO cell growth and metabolism or EPO production. Thus, coexpression of alpha2,3-ST and beta1,4-GT may be beneficial for producing therapeutic glycoproteins with enhanced sialylation in CHO cells.
Controversy exists regarding the functional role of N-linked oligosaccharides in the hormone erythropoietin. We have now examined the role of carbohydrates in the hormone's action using quantitative enzymatic deglycosylation. N-deglycosylated hormone exhibited full biological activity and potency in vitro. Denaturing with 6M urea and renaturing revealed that both the native and N-deglycosylated forms recovered full activity as long as the intrachain disulfide bonds remained intact. Therefore, receptor recognition, subsequent biological activity and maintenance of tertiary structure are intrinsic properties of the polypeptide chain of erythropoietin.
1. Recombinant human erythropoietin has N-linked sugar [Tsuda et al., (1988) Biochemistry 27, 5646-5654]. Here we have demonstrated the presence of O-linked sugar (0.85 mol/mol erythropoietin) composed of sialic acid and Gal beta(1-3)GalNAc. 2. To investigate the role of these sugars, erythropoietins deglycosylated to different extents were prepared using specific glycosidases. Sugars are not essential for in vitro biological activity of erythropoietin, because the fully deglycosylated erythropoietin had the full activity when assayed with in vitro bioassay methods. Asialylation yielded erythropoietin with higher affinity to the receptor than the undigested hormone and therefore an increased in vitro activity. Although erythropoietin from which N-linked or total sugars were removed also had higher affinity for the receptor, their in vitro activity remained unchanged compared with that of the undigested erythropoietin for unknown reasons. On the other hand, removal of sialic acids or N-linked sugar abolished the in vivo biological activity completely, indicating that the presence of N-linked sugar with terminal sialic acids is required for the hormone to reach target sites; full deglycosylation resulted in total loss of the in vivo biological activity of erythropoietin. 3. Incubation of asialo-erythropoietin and fully deglycosylated recombinant human erythropoietin at 70 degrees C for 15 min decreased the biological activity to 35% and 11% of the initial activity, respectively, while the undigested erythropoietin lost no activity. Thus resistance of erythropoietin to thermal inactivation is largely due to the presence of sugars, and terminal sialic acids greatly contribute to the stability.
We administered recombinant human erythropoietin to 25 anemic patients with end-stage renal disease who were undergoing hemodialysis. The recombinant human erythropoietin was given intravenously three times weekly after dialysis, and transfusion requirements, hematocrit, ferrokinetics, and reticulocyte responses were monitored. Over a range of doses between 15 and 500 units per kilogram of body weight, dose-dependent increases in effective erythropoiesis were noted. At 500 units per kilogram, changes in the hematocrit of as much as 10 percentage points were seen within three weeks, and increases in ferrokinetics of three to four times basal values, as measured by erythron transferrin uptake, were observed. Of 18 patients receiving effective doses of recombinant human erythropoietin, 12 who had required transfusions no longer needed them, and in 11 the hematocrit increased to 35 percent or more. Along with the rise in hematocrit, four patients had an increase in blood pressure, and a majority had increases in serum creatinine and potassium levels. No organ dysfunction or other toxic effects were observed, and no antibodies to the recombinant hormone were formed. These results demonstrate that recombinant human erythropoietin is effective, can eliminate the need for transfusions with their risks of immunologic sensitization, infection, and iron overload, and can restore the hematocrit to normal in many patients with the anemia of end-stage renal disease.
Recent advances in experimental hematology have provided new insight into the physiology of erythropoiesis. Techniques are now available for cloning in vitro the various hematopoietic progenitor cells of both animals and man. Erythropoietin has been purified and a radioimmunoassay for the hormone has been described. In this paper, we discuss the physiology of erythropoietin and erythropoiesis with special emphasis on data obtained through the employment of new technology.
Among bacterial, fungal and viral sialidases, the sialidase from Arthrobacter ureafaciens has the unique property of cleaving sialic acids linked to the internal galactose of gangliotetraose. In this study, we examined the ability to cleave the internal sialic acids of GM1 and fucosyl GM1 of sialidases from several bacterial and fungal origins, including Clostridium perfringens and Vibrio cholerae. We found that A. ureafaciens sialidase could liberate the sialic acid of GM1 at the highest rate, and was the only enzyme which could cleave fucosyl GM1 among the sialidases examined.
The affinity-purified sialidase derived from the culture medium of A. ureafaciens was comprised of four isoenzymes with different molecular weights and isoelectric points, the isoenzymes that cleaved fucosyl GM1 being L (88 kDa, pI 5.0), M1 (66 kDa, pI 6.2) and M2 (66 kDa, pI 5.5), but not S (52 kDa, pI 6.2) which showed the highest specific activity toward colominic acid among the four isoenzymes. Abbreviations: SA, sialic acid; PBS, phosphate-buffered saline; PVP, polyvinylpyrrolidone; FABMS, fast atom bombardment mass spectrometry; Galβint, internal galactose of Gg4Cer; Galβext, external galactose of Gg4Cer
Erythropoietin (EPO) is a cytokine produced by the kidney whose function is to stimulate red blood cell production in the bone marrow. Previously, it was shown that the affinity of EPO for its receptor, EPOR, is inversely related to the sialylation of EPO carbohydrate. To better understand the properties of EPO that modulate its receptor affinity, various glycoforms were analyzed using surface plasmon resonance. The system used has been well characterized and is based on previous reports employing an EPOR-Fc chimera captured on a Protein A surface. Using three variants of EPO containing different levels of sialylation, we determined that sialic acid decreased the association rate constant (k(on)) about 3-fold. Furthermore, glycosylated EPO had a 20-fold slower k(on) than nonglycosylated EPO, indicating that the core carbohydrate also negatively impacted k(on). The effect of electrostatic forces on EPO binding was studied by measuring binding kinetics in varying NaCl concentrations. Increasing NaCl concentration resulted in a slower k(on) while having little impact on k(off), suggesting that long-range electrostatic interactions are primarily important in determining the rate of association between EPO and EPOR. Furthermore, the glycosylation content (i.e., nonglycosylated vs glycosylated, sialylated vs desialylated) affected the overall sensitivities of k(on) to [NaCl], indicating that sialic acid and the glycan itself each impact the overall effect of these electrostatic forces.
Hydrogen exchange coupled to mass spectrometry (MS) has become a valuable analytical tool for the study of protein dynamics. By combining information about protein dynamics with more classical functional data, a more thorough understanding of protein function can be obtained. In many cases, protein dynamics are directly related to specific protein functions such as conformational changes during enzyme activation or protein movements during binding. The method is made possible because labile backbone hydrogens in a protein will exchange with deuterium atoms when the protein is placed in a D2O solution. The subsequent increase in protein mass over time is measured with high-resolution MS. The location of the deuterium incorporation is determined by monitoring deuterium incorporation in peptic fragments that are produced after the labeling reaction. In this review, we will summarize the general principles of the method, discuss the latest variations on the experimental protocol that probe different types of protein movements, and review other recent work and improvements in the field.
Anemia has an incidence both on the quality of life and the evolution of cancer. Anemia may result in cancer from either a bone marrow infiltration of cancer cells or a cytotoxic effect of chemotherapy and/or radiotherapy, or both. EPO is a glycoprotein which stimulates erythrocyte formation by bone marrow progenitory cells. Recombinant EPO has considerably improved treatment of anemic patients, by increasing hemoglobin serum levels and reducing the need for blood transfusion. The quality of life of cancer patients is thus improved and several studies highlight the beneficial role of EPO on the clinical outcome. A preclinical background and some clinical data suggest however a detrimental role of EPO in cancer by a possible stimulation of tumor growth. There is a need of more clinical trials in order to assess the effects of EPO on tumors and their treatment.
- C Lacombe
- P Mayeux
C. Lacombe, P. Mayeux, Biology of erythropoietin, Haematologica 83 (1998)
724-732.
- R J Darling
- U Kuchibhotla
- W Glaesner
- R Micanovic
- D R Witcher
- J M Beals
R.J. Darling, U. Kuchibhotla, W. Glaesner, R. Micanovic, D.R. Witcher, J.M.
Beals, Glycosylation of erythropoietin affects receptor binding kinetics: role of
electrostatic interactions, Biochemistry 41 (2002) 14524-14531.
Structure and role of carbohydrate in human erythropoietin
- M Fukuda
- H Sasaki
- M N Fukuda
M. Fukuda, H. Sasaki, M.N. Fukuda, Structure and role of carbohydrate in
human erythropoietin, Adv. Exp. Med. Biol. 271 (1989) 53-67.
Structure and role of carbohydrate in human erythropoietin
- Fukuda
Characterization of IgG1 conformation and conformational dynamics by hydrogen/deuterium exchange mass spectrometry
- Houde