Glycosylation of erythropoietin affects receptor binding kinetics: role of electrostatic interactions

Spatiotemporal expression and control of haemoglobin in space

It is now widely recognised that the environment in space activates a diverse set of genes involved in regulating fundamental cellular pathways. This includes the activation of genes associated with blood homoeostasis and erythropoiesis, with a particular emphasis on those involved in globin chain production. Haemoglobin biology provides an intriguing model for studying space omics, as it has been extensively explored at multiple -omic levels, spanning DNA, RNA, and protein analyses, in both experimental and clinical contexts. In this study, we examined the developmental expression of haemoglobin over time and space using a unique suite of multi-omic datasets available on NASA GeneLab, from the NASA Twins Study, the JAXA CFE study, and the Inspiration4 mission. Our findings reveal significant variations in globin gene expression corresponding to the distinct spatiotemporal characteristics of the collected samples. This study sheds light on the dynamic nature of globin gene regulation in response to the space environment and provides valuable insights into the broader implications of space omics research.

New insight into strategies used to develop long-acting G-CSF biologics for neutropenia therapy

Over the last 20 years, granulocyte colony-stimulating factors (G-CSFs) have become the major therapeutic option for the treatment of patients with neutropenia. Most of the current G-CSFs require daily injections, which are inconvenient and expensive for patients. Increased understanding of G-CSFs' structure, expression, and mechanism of clearance has been very instrumental in the development of new generations of long-acting G-CSFs with improved efficacy. Several approaches to reducing G-CSF clearance via conjugation techniques have been investigated. PEGylation, glycosylation, polysialylation, or conjugation with immunoglobulins or albumins have successfully increased G-CSFs' half-lives. Pegfilgrastim (Neulasta) has been successfully approved and marketed for the treatment of patients with neutropenia. The rapidly expanding market for G-CSFs has increased demand for G-CSF biosimilars. Therefore, the importance of this review is to highlight the principle, elimination's route, half-life, clearance, safety, benefits, and limitations of different strategies and techniques used to increase the half-life of biotherapeutic G-CSFs. Understanding these strategies will allow for a new treatment with more competitive manufacturing and lower unit costs compared with that of Neulasta.

Impact of N-Linked Glycosylation on Therapeutic Proteins

Therapeutic proteins have unique advantages over small-molecule drugs in the treatment of various diseases, such as higher target specificity, stronger pharmacological efficacy and relatively low side effects. These advantages make them increasingly valued in drug development and clinical practice. However, although highly valued, the intrinsic limitations in their physical, chemical and pharmacological properties often restrict their wider applications. As one of the most important post-translational modifications, glycosylation has been shown to exert positive effects on many properties of proteins, including molecular stability, and pharmacodynamic and pharmacokinetic characteristics. Glycoengineering, which involves changing the glycosylation patterns of proteins, is therefore expected to be an effective means of overcoming the problems of therapeutic proteins. In this review, we summarize recent efforts and advances in the glycoengineering of erythropoietin and IgG monoclonal antibodies, with the goals of illustrating the importance of this strategy in improving the performance of therapeutic proteins and providing a brief overview of how glycoengineering is applied to protein-based drugs.

A Novel Plant-Produced Asialo-rhuEPO Protects Brain from Ischemic Damage Without Erythropoietic Action

Mammalian cell-produced recombinant human erythropoietin (rhuEPO^M) has been shown to be a multimodal neuroprotectant targeting an array of key pathological mechanisms in experimental stroke models. However, the rhuEPO^M clinical trials were terminated due to increased risk of thrombosis, largely ascribed to its erythropoietic function. We recently took advantage of a plant-based expression system lacking sialylation capacity to produce asialo-rhuEPO^P, a rhuEPO derivative without sialic acid residues. In the present study, we proved that asialo-rhuEPO^P is non-erythropoietic by repeated intravenous injection (44 μg/kg bw) in mice showing no increase in hemoglobin levels and red blood cell counts, and confirmed that it is non-immunogenic by measuring humoral response after immunizing the mice. We demonstrate that it is neuroprotective in a cerebral ischemia and reperfusion (I/R) mouse model, exhibiting ~ 50% reduction in cerebral infarct volume and edema, and significant improvement in neurological deficits and histopathological outcome. Our studies further revealed that asialo-rhuEPO^P, like rhuEPO^M, displays pleiotropic neuroprotective effects, including restoring I/R-interrupted mitochondrial fission and fusion proteins, preventing I/R injury-induced increase in mitophagy and autophagy markers, and inhibiting apoptosis to benefit nerve cell survival. Most importantly, asialo-rhuEPO^P lacking erythropoietic activity and immunogenicity holds great translational potential as a multimodal neuroprotectant for stroke treatment.

Factors affecting the quality of therapeutic proteins in recombinant Chinese hamster ovary cell culture

Chinese hamster ovary (CHO) cells are the most widely used mammalian host cells for the commercial production of therapeutic proteins. Fed-batch culture is widely used to produce therapeutic proteins, including monoclonal antibodies, because of its operational simplicity and high product titer. Despite technical advances in the development of culture media and cell cultures, it is still challenging to maintain high productivity in fed-batch cultures while also ensuring good product quality. In this review, factors that affect the quality attributes of therapeutic proteins in recombinant CHO (rCHO) cell culture, such as glycosylation, charge variation, aggregation, and degradation, are summarized and categorized into three groups: culture environments, chemical additives, and host cell proteins accumulated in culture supernatants. Understanding the factors that influence the therapeutic protein quality in rCHO cell culture will facilitate the development of large-scale, high-yield fed-batch culture processes for the production of high-quality therapeutic proteins.

S1P Stimulates Erythropoietin Production in Mouse Renal Interstitial Fibroblasts by S1P1 and S1P3 Receptor Activation and HIF-2α Stabilization

Erythropoietin (Epo) is the critical hormone for erythropoiesis. In adults, Epo is mainly produced by a subset of interstitial fibroblasts in the kidney, with minor amounts being produced in the liver and the brain. In this study, we used the immortalized renal interstitial fibroblast cell line FAIK F3-5 to investigate the ability of the bioactive sphingolipid sphingosine 1-phosphate (S1P) to stimulate Epo production and to reveal the mechanism involved. Stimulation of cells with exogenous S1P under normoxic conditions (21% O₂) led to a dose-dependent increase in Epo mRNA and protein levels and subsequent release of Epo into the medium. S1P also enhanced the stabilization of HIF-2α, a key transcription factor for Epo expression. S1P-stimulated Epo mRNA and protein expression was abolished by HIF-2α mRNA knockdown or by the HIF-2 inhibitor compound 2. Furthermore, the approved S1P receptor modulator FTY720, and its active form FTY720-phosphate, both exerted a similar effect on Epo expression as S1P. The effect of S1P on Epo was antagonized by the selective S1P₁ and S1P₃ antagonists NIBR-0213 and TY-52156, but not by the S1P₂ antagonist JTE-013. Moreover, inhibitors of the classical MAPK/ERK, the p38-MAPK, and inhibitors of protein kinase (PK) C and D all blocked the effect of S1P on Epo expression. Finally, the S1P and FTY720 effects were recapitulated in the Epo-producing human neuroblastoma cell line Kelly, suggesting that S1P receptor-dependent Epo synthesis is of general relevance and not species-specific. In summary, these data suggest that, in renal interstitial fibroblasts, which are the primary source of plasma Epo, S1P_{1 and 3} receptor activation upregulates Epo under normoxic conditions. This may have a therapeutic impact on disease situations such as chronic kidney disease, where Epo production is impaired, causing anemia, but it may also have therapeutic value as Epo can mediate additional tissue-protective effects in various organs.

Glycosylation of Therapeutic Proteins: A Critical Quality Attribute

Glycosylation is a common posttranslational modification of therapeutic proteins. The glycosylation pattern is dependent on many parameters such as the host cell line or the culture conditions. N- and O-linked glycans usually play a great role on the stability, safety, and efficacy of the drug. For this reason, glycosylation is considered as a critical quality attribute of therapeutic glycoproteins, and a thorough characterization should be performed, as well as a systematic control for each batch produced. This chapter gives a short presentation of the structure of glycans commonly found on recombinant therapeutic proteins, and their role on the properties of the drug, in terms of stability, pharmacokinetics, safety, and efficacy. Lastly, the use of mass spectrometry for the analysis of glycoproteins is briefly described.

EPO and EPO-Receptor System as Potential Actionable Mechanism for the Protection of Brain and Heart in Refractory Epilepsy and SUDEP

The most important activity of erythropoietin (EPO) is the regulation of erythrocyte production by activation of the erythropoietin receptor (EPO-R), which triggers the activation of anti-apoptotic and proliferative responses of erythroid progenitor cells. Additionally, to erythropoietic EPO activity, an antiapoptotic effect has been described in a wide spectrum of tissues. EPO low levels are found in the central nervous system (CNS), while EPO-R is expressed in most CNS cell types. In spite of EPO-R high levels expressed during the hypoxicischemic brain, insufficient production of endogenous cerebral EPO could be the cause of determined circuit alterations that lead to the loss of specific neuronal populations. In the heart, high EPO-R expression in cardiac progenitor cells appears to contribute to myocardial regeneration under EPO stimulation. Several lines of evidence have linked EPO to an antiapoptotic role in CNS and in heart tissue. In this review, an antiapoptotic role of EPO/EPO-R system in both brain and heart under hypoxic conditions, such as epilepsy and sudden death (SUDEP) has been resumed. Additionally, their protective effects could be a new field of research and a novel therapeutic strategy for the early treatment of these conditions and avoid SUDEP.

Evaluation of erythropoietin biosimilars Epotin™, Hemax® and Jimaixin™ by electrophoretic methods used for doping control analysis and specific N-glycan analysis revealed structural differences from original epoetin alfa drug Eprex®

Recombinant human erythropoietin (rEPO) biosimilars are copies of epoetin drugs developed after the first patents ended. However differences in the process of production can result in small structural differences when compared to the reference product. Differences in N-glycosylation profiles are of particular importance for rEPOs, because they can drastically impact the half-life in circulation and activity. Changes of structure can also impact electrophoretic profiles that are used to reveal the presence of a rEPO in a doping control sample. In this study three not well characterized biosimilars were evaluated (Jimaixin™ authorized in China, and Hemax® and Epotin™ authorized in Algeria). As these products could be used for doping, first their EPO profiles were determined using the antidoping methods (electrophoretic separation by the charge (isolectric focusing, IEF-PAGE) or the molecular weight (SDS-PAGE) and specific EPO immunodetection). Compared to the original epoetin alfa Eprex®, it revealed more basic isoforms for Epotin™ and Jimaixin™ after IEF-PAGE and a slightly lower molecular weight after SDS-PAGE in particular for Hemax®. To better understand the reason for these differences, EPO specific N-glycans were evaluated using two complementary approaches: MALDI-TOF mass spectrometry (MS) and hydrophilic interaction liquid chromatography (HILIC) with fluorescence detection. All three biosimilars presented a significant decrease in the major glycan forms of Eprex® along with an increase in less complex forms. Jimaixin™ and Epotin™ presented also a lower amount of fully sialylated forms. HILIC method also showed that O-acetylation level of sialic acid residues might vary from one rEPO to the other.

The manifold roles of sialic acid for the biological functions of endothelial glycoproteins

Vascular endothelia are covered with a dense glycocalix that is heavily sialylated. Sialylation of vascular glycoconjugates is involved in the regulation of cell-cell interactions, be it among endothelial cells at cell junctions or between endothelial and blood-borne cells. It also plays important roles in modulating the binding of soluble ligands and the signaling by vascular receptors. Here, we provide an overview over the sialylation-function relationships of glycoproteins expressed in the blood and lymphatic vasculature. We first describe cellular interactions in which sialic acid contributes in a stereospecific manner to glycan epitopes recognized by glycan-binding proteins. Our major focus is however on the rarely discussed examples of vascular glycoproteins whose biological functions are modulated by sialylation through other mechanisms.

Multi-Attribute Monitoring of Complex Erythropoetin Beta Glycosylation by GluC Liquid Chromatography-Mass Spectrometry Peptide Mapping

Recombinant human erythropoetin (EPO) is an important biopharmaceutical mainly used for the treatment of anemia. It is highly heterogeneous because of common amino acid chemical degradations known to occur in protein therapeutics (e.g., oxidation and deamidation) and its complex glycosylation profile. Recently, multi-attribute monitoring (MAM), i.e., the quantification of multiple post-translational and chemical modifications in a single peptide mapping liquid chromatography-mass spectrometry (LC-MS)-based method, has received increased attention for the analysis of antibody-like biotherapeutic proteins. In this study, an MAM method for examination of residue-specific glycan profiles of EPO was established. The MAM method, by virtue of the increased sensitivity and selectivity provided with LC-MS, yielded additional site-specific information not afforded by the conventional quality control (QC) methods. Low abundant glycans as well as additional post-translational and chemical modifications could also be simultaneously detected by the MAM method. Our results demonstrate that desialylated N-oligosaccharides (DeNO) and N-acetylneuraminic acids (Neu5Ac) could be monitored by the developed MAM approach with data readout highly comparable to QC methods, while differences were observed for charge isoform distribution. In summary, the comparative data obtained demonstrate that MAM by LC-MS peptide mapping can, in principle, adequately replace selected QC methods and would add value to the in-process control and release testing strategy of EPO.

A Markov model of glycosylation elucidates isozyme specificity and glycosyltransferase interactions for glycoengineering

Glycosylated biopharmaceuticals are important in the global pharmaceutical market. Despite the importance of their glycan structures, our limited knowledge of the glycosylation machinery still hinders controllability of this critical quality attribute. To facilitate discovery of glycosyltransferase specificity and predict glycoengineering efforts, here we extend the approach to model N-linked protein glycosylation as a Markov process. Our model leverages putative glycosyltransferase (GT) specificity to define the biosynthetic pathways for all measured glycans, and the Markov chain modelling is used to learn glycosyltransferase isoform activities and predict glycosylation following glycosyltransferase knock-in/knockout. We apply our methodology to four different glycoengineered therapeutics (i.e., Rituximab, erythropoietin, Enbrel, and alpha-1 antitrypsin) produced in CHO cells. Our model accurately predicted N-linked glycosylation following glycoengineering and further quantified the impact of glycosyltransferase mutations on reactions catalyzed by other glycosyltransferases. By applying these learned GT-GT interaction rules identified from single glycosyltransferase mutants, our model further predicts the outcome of multi-gene glycosyltransferase mutations on the diverse biotherapeutics. Thus, this modeling approach enables rational glycoengineering and the elucidation of relationships between glycosyltransferases, thereby facilitating biopharmaceutical research and aiding the broader study of glycosylation to elucidate the genetic basis of complex changes in glycosylation.

Mechanisms Influencing the Pharmacokinetics and Disposition of Monoclonal Antibodies and Peptides

Monoclonal antibodies (mAbs) and peptides are an important class of therapeutic modalities that have brought improved health outcomes in areas with limited therapeutic optionality. Presently, there more than 90 mAb and peptide therapeutics on the United States market, with over 600 more in various clinical stages of development in a broad array of therapeutic areas, including diabetes, autoimmune disorders, oncology, neuroscience, and cardiovascular and infectious diseases. Notwithstanding this potential, there is high clinical rate of attrition, with approximately 10% reaching patients. A major contributor to the failure of the molecules is often times an incomplete or poor understanding of the pharmacokinetics (PK) and disposition profiles leading to limited or diminished efficacy. Increased and thorough characterization efforts directed at disseminating mechanisms influencing the PK and disposition of mAbs and peptides can aid in improving the design for their intended pharmacological activity, and thereby their clinical success. The PK and disposition factors for mAbs and peptides are broadly influenced by target-mediated drug disposition and nontarget-related clearance mechanisms related to the interplay between the relationship of the structure and physiochemical properties of mAbs and peptides with physiologic processes. This review focuses on nontarget-related factors influencing the disposition and PK of mAbs and peptides. Contemporary considerations around the increasing in silico approaches to identify nontarget-related molecule limitations and enhancing the druggability of mAbs and peptides, including parenteral and nonparenteral delivery strategies that are geared toward improving patient experience and compliance, are also discussed.

Glycan analysis for protein therapeutics

Glycosylation can be a critical quality attribute for protein therapeutics due to its extensive impact on product safety and efficacy. Glycan characterization is important in the process of protein drug development, from early stage candidate selection to late stage regulatory submission. It is also an indispensable part in the evaluation of biosimilarity. This review discusses the effects of glycosylation on the stability and activity of protein therapeutics, regulatory considerations corresponding to manufacturing and structural characterization of glycosylated protein therapeutics, and focuses on mass spectrometry compatible separation methods for glycan characterization of protein therapeutics. These approaches include hydrophilic interaction liquid chromatography, reversed-phase liquid chromatography, capillary electrophoresis, porous graphitic carbon liquid chromatography and two-dimensional liquid chromatography. Advances and novelties in each separation method, as well as associated challenges and limitations, are discussed at the released glycan, glycopeptide, glycoprotein subunit and intact glycoprotein levels.

A Glycoengineered Interferon-β Mutein (R27T) Generates Prolonged Signaling by an Altered Receptor-Binding Kinetics

The glycoengineering approach is used to improve biophysical properties of protein-based drugs, but its direct impact on binding affinity and kinetic properties for the glycoengineered protein and its binding partner interaction is unclear. Type I interferon (IFN) receptors, composed of IFNAR1 and IFNAR2, have different binding strengths, and sequentially bind to IFN in the dominant direction, leading to activation of signals and induces a variety of biological effects. Here, we evaluated receptor-binding kinetics for each state of binary and ternary complex formation between recombinant human IFN-β-1a and the glycoengineered IFN-β mutein (R27T) using the heterodimeric Fc-fusion technology, and compared biological responses between them. Our results have provided evidence that the additional glycan of R27T, located at the binding interface of IFNAR2, destabilizes the interaction with IFNAR2 via steric hindrance, and simultaneously enhances the interaction with IFNAR1 by restricting the conformational freedom of R27T. Consequentially, altered receptor-binding kinetics of R27T in the ternary complex formation led to a substantial increase in strength and duration of biological responses such as prolonged signal activation and gene expression, contributing to enhanced anti-proliferative activity. In conclusion, our findings reveal N-glycan at residue 25 of R27T is a crucial regulator of receptor-binding kinetics that changes biological activities such as long-lasting activation. Thus, we believe that R27T may be clinically beneficial for patients with multiple sclerosis.

Recombinant asialoerythropoetin protects HL-1 cardiomyocytes from injury via suppression of Mst1 activation

Background

Recombinant human erythropoietin (rhuEPO) and asialoerythropoietin (asialo-rhuEPO) are cardioprotective. However, the protective effects of rhuEPO could not be translated into clinical practice because of its hematopoiesis-associated side effects while non-erythropoietic asialo-rhuEPO is unavailable in large quantities for clinical studies. This study was designed to investigate the cardiomyocyte protective potential of plant-produced asialo-rhuEPO (asialo-rhuEPO^P) against staurosporine (STS)-induced injury in HL-1 murine cardiomyocytes and identify cellular pathway(s) responsible for its cardioprotection.

Methods

HL-1 cardiomyocytes were simultaneously treated with STS and asialo-rhuEPO^P. Cellular injury, apoptosis, and cell viabilities were measured by LDH assay, Hoechst staining and trypan blue exclusion method, respectively while western blotting was used to study its effects on apoptosis and autophagy hallmarks.

Results

Our results showed that 20 IU/ml asialo-rhuEPO^P provided 39% protection to cardiomyocytes compared to STS-treated cells, which is 2-fold better than that of mammalian cell-produce rhuEPO (rhuEPO^M). Asialo-rhuEPO^P was found to suppress activation of proapoptotic kinase Mst1 (mammalian Sterile-20-like kinase 1) and FOXO3, leading to inhibition of apoptotic pathway and restoration of autophagy as indicated by the reduction of fragmented/condensed nuclei, altered ratios of Bax/Bcl2, p-Bad/Bad, cytosol/mitochondrial cyt c and caspase-3 activation, and the restored levels of autophagy markers Beclin1, p62 and LC3B-II. Additionally, Akt was found to be activated and FOXO3 was phosphorylated on Ser253, suggesting inhibition of FOXO3 transcriptional function.

Conclusions

Asialo-rhuEPO^P-mediated cardioprotection occurs through activation of PI3K/Akt pathway leading to suppression of Mst1 activation and promoting cardiomyocyte survival.

General significance

Asialo-rhuEPO^P could be used to modulate Mst1 activity elevated under numerous pathological states.

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Recombinant asialo-rhuEPO protect HL-1 cardiomyocytes against STS-induced injury.

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Protective effect of recombinant asialo-rhuEPO is superior to sialylated EPO.

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Asialo-rhuEPO suppresses activation of proapoptotic kinase MSt1 by activating Akt.

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Asialo-rhuEPO restores autophagy and inhibits apoptosis to promote cell survival.

Hydrogen deuterium exchange reveals changes to protein dynamics of recombinant human erythropoietin upon N- and O- desialylation

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.

Science of Biosimilars

Biosimilar therapeutic proteins in oncology offer the potential to decrease costs while providing safety and efficacy profiles consistent with their respective reference or originator products. Biosimilars have a number of important differences from generic small-molecule drugs, including manufacturing processes that are unique from their reference products. These differences may affect biosimilars through posttranslational modifications that can occur in specific cellular production lines, and these modifications have potential effects on protein structure, function, clinical pharmacology, and immunogenicity. Regulatory agencies expect these differences to be identified, analyzed, and minimized through iterative processes and extensive preclinical efforts. Generic naming of biosimilars reflects the nonproprietary reference product name along with a meaningless four-letter suffix to ensure that each product can be uniquely identified for prescribing and pharmacovigilance purposes. Labeling information for biosimilars reflects a greater detail of comparisons to reference products than conventional generic drugs, which ensures that prescribers can understand the source of information and have a complete understanding of the therapeutic profile of each biosimilar agent. Postmarketing surveillance programs will be required to evolve and ensure optimal pharmacovigilance reporting, because the potential for unexpected adverse events with biosimilars is higher than with conventional generic agents as a result of different manufacturing processes and different clinical trial designs and durations. The existing filgrastim biosimilars are likely to be joined soon by therapeutic monoclonal antibodies, including rituximab, trastuzumab, and bevacizumab, on the basis of patent expiration dates and clinical trial results.

Electrostatic Interactions in Protein Structure, Folding, Binding, and Condensation

Charged and polar groups, through forming ion pairs, hydrogen bonds, and other less specific electrostatic interactions, impart important properties to proteins. Modulation of the charges on the amino acids, e.g., by pH and by phosphorylation and dephosphorylation, have significant effects such as protein denaturation and switch-like response of signal transduction networks. This review aims to present a unifying theme among the various effects of protein charges and polar groups. Simple models will be used to illustrate basic ideas about electrostatic interactions in proteins, and these ideas in turn will be used to elucidate the roles of electrostatic interactions in protein structure, folding, binding, condensation, and related biological functions. In particular, we will examine how charged side chains are spatially distributed in various types of proteins and how electrostatic interactions affect thermodynamic and kinetic properties of proteins. Our hope is to capture both important historical developments and recent experimental and theoretical advances in quantifying electrostatic contributions of proteins.

Evaluation of the combined effects of pegylation and glycosylation on the stability of erythropoietin using a stability-indicating SE-HPLC

Recombinant human erythropoietin (rhEPO) is a commonly used biopharmaceutical for the treatment of anemia-associated disorders. Epogen; glycosylated erythropoietin (G-EPO) has short half-life and poor stability. Pegylated Epogen (Peg-G-EPO) was introduced to the market to overcome these limitations. The combined effects of pegylation and glycosylation on the stability of Peg-G-EPO was studied. Determination of Peg-G-EPO in the presence of its degradation products was achieved using SE-HPLC. The assay was validated according to ICH guidelines over concentration range of 50.00-320.00 μg/mL (r 0.9999). A mobile phase of 50 mM phosphate buffer (pH 6.5) with 300 mM sodium chloride and 20% ethanol was employed. Isocratic elution was carried out at 0.5 mL/min over run time of 30 min. Peg-G-EPO was found stable towards mechanical agitation only at low concentrations while it was stable towards repeated freeze/thaw; regardless of the concentration. Effect of temperature and pH were also investigated and Peg-G-EPO was found stable within narrow ranges. Results indicated formation of small molecular weight and very high molecular weight aggregates that have been filtered-off the column. Although Peg-G-EPO was found relatively more stable than its non-pegylated but glycosylated version, results indicated the need for careful stability-assessment of Peg-G-EPO.

Plant-Produced Asialo-Erythropoietin Restores Pancreatic Beta-Cell Function by Suppressing Mammalian Sterile-20-like Kinase (MST1) and Caspase-3 Activation

Pancreatic beta-cell death adversely contributes to the progression of both type I and II diabetes by undermining beta-cell mass and subsequently diminishing endogenous insulin production. Therapeutics to impede or even reverse the apoptosis and dysfunction of beta-cells are urgently needed. Asialo-rhuEPO, an enzymatically desialylated form of recombinant human erythropoietin (rhuEPO), has been shown to have cardioprotective and neuroprotective functions but with no adverse effects like that of sialylated rhuEPO. Heretofore, the anti-apoptotic effect of asialo-rhuEPO on pancreatic beta-cells has not been reported. In the current study, we investigated the cytoprotective properties of plant-produced asialo-rhuEPO (asialo-rhuEPO^P) against staurosporine-induced cell death in the pancreatic beta-cell line RIN-m5F. Our results showed that 60 IU/ml asialo-rhuEPO^P provided 41% cytoprotection while 60 IU/ml rhuEPO yielded no effect. Western blotting results showed that asialo-rhuEPO^P treatment inhibited both MST1 and caspase-3 activation with the retention of PDX1 and insulin levels close to untreated control cells. Our study provides the first evidence indicating that asialo-rhuEPO^P-mediated protection involves the reduction of MST1 activation, which is considered a key mediator of apoptotic signaling in beta-cells. Considering the many advantages its plant-based expression, asialo-rhuEPO^P could be potentially developed as a novel and inexpensive agent to treat or prevent diabetes after further performing studies in cell-based and animal models of diabetes.

Rate Constants and Mechanisms of Protein-Ligand Binding

Whereas protein-ligand binding affinities have long-established prominence, binding rate constants and binding mechanisms have gained increasing attention in recent years. Both new computational methods and new experimental techniques have been developed to characterize the latter properties. It is now realized that binding mechanisms, like binding rate constants, can and should be quantitatively determined. In this review, we summarize studies and synthesize ideas on several topics in the hope of providing a coherent picture of and physical insight into binding kinetics. The topics include microscopic formulation of the kinetic problem and its reduction to simple rate equations; computation of binding rate constants; quantitative determination of binding mechanisms; and elucidation of physical factors that control binding rate constants and mechanisms.

Forces Driving Chaperone Action

It is still unclear what molecular forces drive chaperone-mediated protein folding. Here, we obtain a detailed mechanistic understanding of the forces that dictate the four key steps of chaperone-client interaction: initial binding, complex stabilization, folding, and release. Contrary to the common belief that chaperones recognize unfolding intermediates by their hydrophobic nature, we discover that the model chaperone Spy uses long-range electrostatic interactions to rapidly bind to its unfolded client protein Im7. Short-range hydrophobic interactions follow, which serve to stabilize the complex. Hydrophobic collapse of the client protein then drives its folding. By burying hydrophobic residues in its core, the client's affinity to Spy decreases, which causes client release. By allowing the client to fold itself, Spy circumvents the need for client-specific folding instructions. This mechanism might help explain how chaperones can facilitate the folding of various unrelated proteins.

Analytical detection and characterization of biopharmaceutical glycosylation by MS

Glycosylation plays an important role in ensuring the proper structure and function of most biotherapeutic proteins. Even small changes in glycan composition, structure, or location can have a drastic impact on drug safety and efficacy. Recently, glycosylation has become the subject of increased focus as biopharmaceutical companies rush to create not only biosimilars, but also biobetters based on existing biotherapeutic proteins. Against this backdrop of ongoing biopharmaceutical innovation, updated methods for accurate and detailed analysis of protein glycosylation are critical for biopharmaceutical companies and government regulatory agencies alike. This review summarizes current methods of characterizing biopharmaceutical glycosylation, including compositional mass profiling, isomer-specific profiling and structural elucidation by MS and hyphenated techniques.

Increased Synthesis of Liver Erythropoietin with CKD

Anemia of CKD seems to be related to impaired production of renal erythropoietin (Epo). The glycosylation pattern of Epo depends on the synthesizing cell and thus, can indicate its origin. We hypothesized that synthesis of Epo from nonkidney cells increases to compensate for insufficient renal Epo production during CKD. We determined plasma Epo levels and Epo glycosylation patterns in 33 patients with CKD before undergoing dialysis and nine patients with CKD undergoing dialysis. We compared these values with values obtained in healthy volunteers and other controls. Although patients with CKD before undergoing dialysis had median (interquartile range) Epo levels higher than those of healthy controls (13.8 IU/L; interquartile range, 10.0-20.7 IU/L versus 8.4 IU/L; interquartile range, 7.6-9.0 IU/L; P<0.01), these patients were moderately anemic (mean±SD; hemoglobin =118±17 g/L). Detected as the percentage of migrated isoforms (PMI), Epo glycosylation in patients with CKD before undergoing dialysis (PMI=36.1±11.7%) differed from that in healthy controls (PMI=9.2±3.8%; P<0.01) but not from that in umbilical cord plasma (PMI=53.9±10.6%; P>0.05), which contains mainly liver-derived Epo. Furthermore, glycosylation modification correlated with eGFR loss. These results suggest that patients with CKD maintain persistent Epo synthesis despite declining renal function, and this maintenance may result in part from increased liver Epo synthesis.

Key factors influencing ADME properties of therapeutic proteins: A need for ADME characterization in drug discovery and development

Protein therapeutics represent a diverse array of biologics including antibodies, fusion proteins, and therapeutic replacement enzymes. Since their inception, they have revolutionized the treatment of a wide range of diseases including respiratory, vascular, autoimmune, inflammatory, infectious, and neurodegenerative diseases, as well as cancer. While in vivo pharmacokinetic, pharmacodynamic, and efficacy studies are routinely carried out for protein therapeutics, studies that identify key factors governing their absorption, distribution, metabolism, and excretion (ADME) properties have not been fully investigated. Thorough characterization and in-depth study of their ADME properties are critical in order to support drug discovery and development processes for the production of safer and more effective biotherapeutics. In this review, we discuss the main factors affecting the ADME characteristics of these large macromolecular therapies. We also give an overview of the current tools, technologies, and approaches available to investigate key factors that influence the ADME of recombinant biotherapeutic drugs, and demonstrate how ADME studies will facilitate their future development.

The influence of artificially introduced N-glycosylation sites on the in vitro activity of Xenopus laevis erythropoietin

Erythropoietin (EPO), the primary regulator of erythropoiesis, is a heavily glycosylated protein found in humans and several other mammals. Intriguingly, we have previously found that EPO in Xenopus laevis (xlEPO) has no N-glycosylation sites, and cross-reacts with the human EPO (huEPO) receptor despite low homology with huEPO. In this study, we introduced N-glycosylation sites into wild-type xlEPO at the positions homologous to those in huEPO, and tested whether the glycosylated mutein retained its biological activity. Seven xlEPO muteins, containing 1–3 additional N-linked carbohydrates at positions 24, 38, and/or 83, were expressed in COS-1 cells. The muteins exhibited lower secretion efficiency, higher hydrophilicity, and stronger acidic properties than the wild type. All muteins stimulated the proliferation of both cell lines, xlEPO receptor-expressing xlEPOR-FDC/P2 cells and huEPO receptor-expressing UT-7/EPO cells, in a dose-dependent manner. Thus, the muteins retained their in vitro biological activities. The maximum effect on xlEPOR-FDC/P2 proliferation was decreased by the addition of N-linked carbohydrates, but that on UT-7/EPO proliferation was not changed, indicating that the muteins act as partial agonists to the xlEPO receptor, and near-full agonists to the huEPO receptor. Hence, the EPO-EPOR binding site in X. laevis locates the distal region of artificially introduced three N-glycosylation sites, demonstrating that the vital conformation to exert biological activity is conserved between humans and X. laevis, despite the low similarity in primary structures of EPO and EPOR.

Serum and intraocular concentrations of erythropoietin and vascular endothelial growth factor in patients with type 2 diabetes and proliferative retinopathy

AIM

This study compared systemic and intraocular concentrations of erythropoietin (EPO) and vascular endothelial growth factor (VEGF) in patients with type 2 diabetes (T2D) and proliferative diabetic retinopathy (PDR) with levels in patients without diabetes, and looked for possible correlations between the concentrations found and other variables analyzed.

METHODS

Concentrations of EPO and VEGF were measured in the aqueous and vitreous humours and serum of patients undergoing vitrectomy for PDR (33 patients) or for macular holes or puckers (20 control patients). EPO was assayed by radioimmunoassay, with a lower limit of detection (LOD) of 1.0 mIU/mL. VEGF was assayed using enzyme-linked immunosorbent assay (ELISA), with a lower LOD of 10.0 pg/mL.

RESULTS

EPO concentrations in serum did not differ significantly between the two groups, whereas EPO in vitreous and aqueous were higher in diabetic than in non-diabetic patients. VEGF in serum was lower in diabetic patients than in non-diabetics; conversely, VEGF concentrations in vitreous were significantly higher in diabetic patients. A direct correlation was found between vitreous and aqueous EPO concentrations, and between vitreous EPO and blood glucose concentrations. A significant, negative correlation between vitreous EPO concentration and age was also recorded.

CONCLUSION

High EPO concentrations in the vitreous of patients with PDR and its correlation with blood glucose suggest that EPO could play a role in the pathogenesis of PDR. All possible factors affecting serum and ocular concentrations of EPO and VEGF should be determined to identify compounds able to prevent and control this serious microvascular complication of diabetes.

Low concentrations of recombinant granulocyte macrophage-colony stimulating factor derived from Chinese hamster ovary cells augments long-term bioactivity with delayed clearance in vitro

To date, the biological activity of granulocyte macrophage-colony stimulating factor (GM-CSF) has been investigated by using mostly Escherichia coli- or yeast cell-derived recombinant human GM-CSF (erhGM-CSF and yrhGM-CSF, respectively). However, Chinese hamster ovary cell-derived recombinant human GM-CSF (crhGM-CSF), as well as natural human GM-CSF, is a distinct molecule that includes modifications by complicated oligosaccharide moieties. In the present study, we reevaluated the bioactivity of crhGM-CSF by comparing it with those of erhGM-CSF and yrhGM-CSF. The effect of short-term stimulation (0.5h) on the activation of neutrophils/monocytes or peripheral blood mononuclear cells (PBMCs) by crhGM-CSF was lower than those with erhGM-CSF or yrhGM-CSF at low concentrations (under 60pM). Intermediate-term stimulation (24h) among the different rhGM-CSFs with respect to its effect on the activation of TF-1 cells, a GM-CSF-dependent cell line, or PBMCs was not significantly different. In contrast, the proliferation/survival of TF-1 cells or PBMCs after long-term stimulation (72-168h) was higher at low concentrations of crhGM-CSF (15-30pM) than that of cells treated with other GM-CSFs. The proportion of apoptotic TF-1 cells after incubation with crhGM-CSF for 72h was lower than that of cells incubated with other rhGM-CSFs. These effects were attenuated by desialylation of crhGM-CSF. Clearance of crhGM-CSF but not desialylated-crhGM-CSF by both TF-1 cells and PBMCs was delayed compared with that of erhGM-CSF or yrhGM-CSF. These results suggest that sialylation of oligosaccharide moieties delayed the clearance of GM-CSF, thus eliciting increased long-term bioactivity in vitro.

Binding of Clostridium botulinum C3 exoenzyme to intact cells

C3 from Clostridium botulinum (C3) specifically modifies Rho GTPases RhoA, RhoB, and RhoC by mono-ADP-ribosylation. The confined substrate profile of C3 is the basis for its use as pharmacological tool in cell biology to study cellular functions of Rho GTPases. Although C3 exoenzyme does not possess a cell-binding/-translocation domain, C3 is taken up by intact cells via an unknown mechanism. In the present work, binding of C3 to the hippocampus-derived HT22 cells and J774A.1 macrophages was characterized. C3 bound concentration-dependent to HT22 and J774A.1 cells. Pronase treatment of intact cells significantly reduced both C3 binding and C3 cell entry. Removal of sugar residues by glycosidase F treatment resulted in an increased binding of C3, but a reduced cell entry. To explore the involvement of phosphorylation in the binding process of C3, intact HT22 and J774A.1 cells were pre-treated with vanadate prior to incubation with C3. Inhibition of de-phosphorylation by vanadate resulted in an increased binding of C3. To differentiate between intracellular and extracellular phosphorylation, intact cells were treated with CIP (calf intestine phosphatase) to remove extracellular phosphate residues. The removal of phosphate residues resulted in a strong reduction in binding of C3 to cells. In sum, the C3 membranous binding partner is proteinaceous, and the glycosylation as well as the phosphorylation state is critical for efficient binding of C3.

Emerging principles for the therapeutic exploitation of glycosylation

Glycosylation plays a key role in a wide range of biological processes. Specific modification to a glycan's structure can directly modulate its biological function. Glycans are not only essential to glycoprotein folding, cellular homeostasis, and immune regulation but are involved in multiple disease conditions. An increased molecular and structural understanding of the mechanistic role that glycans play in these pathological processes has driven the development of therapeutics and illuminated novel targets for drug design. This knowledge has enabled the treatment of metabolic disorders and the development of antivirals and shaped cancer and viral vaccine strategies. Furthermore, an understanding of glycosylation has led to the development of specific drug glycoforms, for example, monoclonal antibodies, with enhanced potency.

Fixed charges in the gel matrix of sensor chips and dissociation in diffusion gradients influence the detection of fast protein-protein interactions

In molecular interaction studies based on surface plasmon resonance (SPR) measurements, the ligand is often immobilized in a thin carboxydextran gel matrix. Here we investigated the influence of the charged gel on the results of such SPR measurements. At physiological ionic strength, analytes with a net charge of more than about 5 are considerably enriched or depleted due to the Donnan potential under commonly applied experimental conditions. Below physiological ionic strength, enrichment was found to be even stronger than predicted by Donnan theory. The influence of the gel matrix on the apparent binding is prevented in competition experiments, in which SPR measurements are only used to discriminate between free and complexed analyte while the interaction between analyte and ligand is studied in solution. However, if the analyte-ligand interaction is very fast, thermodynamic equilibrium is disturbed near the interface where free analyte binds to the immobilized ligand due to mass transport limitation. Consequently, the soluble analyte-ligand complex dissociates, which results in an overestimation of free analyte. In experiments of calmodulin binding to fragments of the KCNH1 ion channel protein this mass-transport-induced dissociation led to a systematic underestimation of the affinity. We conclude that the insufficient discrimination between the true analyte-ligand binding and the complex interactions of the analyte with the gel phase may result in systematic errors. The theoretical framework for recognizing and avoiding such errors is provided.

Glycosylation characterization of recombinant human erythropoietin produced in glycoengineered Pichia pastoris by mass spectrometry

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.

Expression of functionally active sialylated human erythropoietin in plants

Recombinant human erythropoietin (rhEPO), a glycohormone, is one of the leading biopharmaceutical products. The production of rhEPO is currently restricted to mammalian cell expression systems because of rhEPO's highly complex glycosylation pattern, which is a major determinant for drug-efficacy. Here we evaluate the ability of plants to produce different glycoforms of rhEPO. cDNA constructs were delivered to Nicotiana benthamiana (N. benthamiana) and transiently expressed by a viral based expression system. Expression levels up to 85 mg rhEPO/kg fresh leaf material were achieved. Moreover, co-expression of rhEPO with six mammalian genes required for in planta protein sialylation resulted in the synthesis of rhEPO decorated mainly with bisialylated N-glycans (NaNa), the most abundant glycoform of circulating hEPO in patients with anemia. A newly established peptide tag (ELDKWA) fused to hEPO was particularly well-suited for purification of the recombinant hormone based on immunoaffinity. Subsequent lectin chromatography allowed enrichment of exclusively sialylated rhEPO. All plant-derived glycoforms exhibited high biological activity as determined by a cell-based receptor-binding assay. The generation of rhEPO carrying largely homogeneous glycosylation profiles (GnGnXF, GnGn, and NaNa) will facilitate further investigation of functionalities with potential implications for medical applications.

Imbalanced levels of angiogenic and angiostatic factors in vitreous, plasma and postmortem retinal tissue of patients with proliferative diabetic retinopathy

A role for vascular endothelial growth factor (VEGF) has been clearly implicated in the pathogenesis of proliferative diabetic retinopathy (PDR). However, other molecules and mechanisms may be operating independently, or in conjunction with VEGF in the pathogenesis of this disease. Therefore, we made an attempt to comparatively investigate the levels of angiogenic and angiostatic factors in vitreous, plasma and postmortem retinal tissue of subjects with Proliferative Diabetic Retinopathy (PDR) compared to control subjects. The vitreous and plasma concentrations of VEGF, EPO (Erythropoietin) and PEDF (Pigment Epithelium Derived Factor) were measured using Enzyme Linked Immunosorbent Assay (ELISA) and the postmortem retinal tissue was subjected to Western blot analysis. The mean vitreous and plasma levels of VEGF and EPO in patients with PDR were significantly (p<0.001) higher than those in subjects without diabetes. Conversely, the vitreous and plasma levels of PEDF were significantly (p<0.001) lower in the PDR patients compared to control subjects. Multivariate logistic-regression analyses indicated that EPO was more strongly associated with PDR than VEGF. The protein expression of the VEGF and EPO in the retinal tissue was significantly higher in PDR and diabetes without complication groups compared to controls. Compared to controls, the protein expression of PEDF was significantly lower in retinal tissues from diabetes patients without complications and in patients with PDR. The fact that the vitreous and plasma levels and the retinal tissue protein expression of EPO were strongly associated with PDR implies a definite role of 'hypererythropoietinemia' in neovascularization processes.

Erythropoietin produced by the retina: its role in physiology and diabetic retinopathy

Erythropoietin (Epo) is the principal regulator of erythropoiesis by inhibiting apoptosis and by stimulating the proliferation and differentiation of erythroid precursor cells. However, Epo also performs extra-erythropoietic actions of which the neuroprotective effects are among the most relevant. Apart from kidney and liver, Epo is also produced by the brain and the retina. In addition, Epo receptor (Epo-R) expression has also been found in the brain and in the retina, thus suggesting an autocrine/paracrine action which seems essential for the physiological homeostasis of both brain and retina. In this review, we will give an overview of the current concepts of the physiology of Epo and will focus on its role in the retina in both normal conditions and in the setting of diabetic retinopathy. Finally, the reasons as to why Epo could be contemplated as a potential new treatment for the early stages of diabetic retinopathy will be given.

Rationalizing 5000-fold differences in receptor-binding rate constants of four cytokines

The four cytokines erythropoietin (EPO), interleukin-4 (IL4), human growth hormone (hGH), and prolactin (PRL) all form four-helix bundles and bind to type I cytokine receptors. However, their receptor-binding rate constants span a 5000-fold range. Here, we quantitatively rationalize these vast differences in rate constants by our transient-complex theory for protein-protein association. In the transient complex, the two proteins have near-native separation and relative orientation, but have yet to form the short-range specific interactions of the native complex. The theory predicts the association rate constant as k(a)=k(a0)exp(-ΔG(el)(∗)/k(B)T) where k(a0) is the basal rate constant for reaching the transient complex by random diffusion, and the Boltzmann factor captures the rate enhancement due to electrostatic attraction. We found that the vast differences in receptor-binding rate constants of the four cytokines arise mostly from the differences in charge complementarity among the four cytokine-receptor complexes. The basal rate constants (k(a0)) of EPO, IL4, hGH, and PRL were similar (5.2 × 10(5) M(-1)s(-1), 2.4 × 10(5) M(-1)s(-1), 1.7 × 10(5) M(-1)s(-1), and 1.7 × 10(5) M(-1)s(-1), respectively). However, the average electrostatic free energies (ΔG(e1)(∗)) were very different (-4.2 kcal/mol, -2.4 kcal/mol, -0.1 kcal/mol, and -0.5 kcal/mol, respectively, at ionic strength=160 mM). The receptor-binding rate constants predicted without adjusting any parameters, 6.2 × 10(8) M(-1)s(-1), 1.3 × 10(7) M(-1)s(-1), 2.0 × 10(5) M(-1)s(-1), and 7.6 × 10(4) M(-1)s(-1), respectively, for EPO, IL4, hGH, and PRL agree well with experimental results. We uncover that these diverse rate constants are anticorrelated with the circulation concentrations of the cytokines, with the resulting cytokine-receptor binding rates very close to the limits set by the half-lives of the receptors, suggesting that these binding rates are functionally relevant and perhaps evolutionarily tuned. Our calculations also reproduced well-observed effects of mutations and ionic strength on the rate constants and produced a set of mutations on the complex of hGH with its receptor that putatively enhances the rate constant by nearly 100-fold through increasing charge complementarity. To quantify charge complementarity, we propose a simple index based on the charge distribution within the binding interface, which shows good correlation with ΔG(e1)(∗). Together these results suggest that protein charges can be manipulated to tune k(a) and control biological function.