Shared and unique determinants of the erythropoietin (EPO) receptor are important for binding EPO and EPO mimetic peptide

Satisfying QTPP of Erythropoietin Biosimilar by QbD through DoE-Derived Downstream Process Engineering

Well-characterized and scalable downstream processes for the purification of biologics are extremely demanding for delivering quality therapeutics to patients at a reasonable price. Erythropoietin (EPO) is a blockbuster biologic with diverse clinical applications, but its application is limited to financially well-off societies due to its high price. The high price of EPO is associated with the technical difficulties related to the purification challenge to obtain qualified products with a cost-effective defined process. Though there are reports for the purification of EPO there is no report of a well-characterized downstream process with critical process parameters (CPPs) that can deliver EPO consistently satisfying the quality target product profile (QTPP), which is a critical regulatory requirement. To advance the field, we applied the quality by design (QbD) principle and design of experiment (DoE) protocol to establish an effective process, which is scalable up to 100× batch size satisfying QTPP. We have successfully transformed the process from static mode to dynamic mode and validated it. Insignificant variation (p > 0.05) within and between 1×, 10×, and 100× batches showed that the process is reproducible and seamlessly scalable. The biochemical analysis along with the biofunctionality data ensures that the products from different scale batches were indifferent and comparable to a reference product. Our study thereby established a robust and scalable downstream process of EPO biosimilar satisfying QTPP. The technological scheme presented here can speed up the production of not only EPO but also many other life-saving biologics and make them available to the mass population at a reduced cost.

Chemical and Enzymatic Synthesis of Sialylated Glycoforms of Human Erythropoietin

Recombinant human erythropoietin (EPO) is the main therapeutic glycoprotein for the treatment of anemia in cancer and kidney patients. The in‐vivo activity of EPO is carbohydrate‐dependent with the number of sialic acid residues regulating its circulatory half‐life. EPO carries three N‐glycans and thus obtaining pure glycoforms provides a major challenge. We have developed a robust and reproducible chemoenzymatic approach to glycoforms of EPO with and without sialic acids. EPO was assembled by sequential native chemical ligation of two peptide and three glycopeptide segments. The glycopeptides were obtained by pseudoproline‐assisted Lansbury aspartylation. Enzymatic introduction of the sialic acids was readily accomplished at the level of the glycopeptide segments but even more efficiently on the refolded glycoprotein. Biological recognition of the synthetic EPOs was shown by formation of 1:1 complexes with recombinant EPO receptor.

An Unusual Occurrence of Erythrocytosis in a Child with Nephrotic Syndrome and Advanced Chronic Kidney Disease

Background: Anemia is common in patients with nephrotic syndrome (NS) for various reasons. Furthermore, anemia can occur in patients with chronic kidney disease (CKD) predominantly owing to inappropriately low erythropoietin (EPO) production relative to the degree of anemia. However, erythrocytosis is uncommon in patients with NS and advanced CKD who are not treated with exogenous erythropoietin stimulating agents, and when present, will necessitate exploration of the other etiologies. Case summary: Here, we describe an 8-year-old girl with erythrocytosis in association with NS and advanced CKD. The patient was found to have erythrocytosis during the evaluation for hypertensive urgency. She also had nephrotic range proteinuria without edema. Serum hemoglobin and hematocrit were 17 gm/dL and 51%, respectively, despite hydration. Renal function test showed an estimated glomerular filtration rate of 30 mL/min/1.73 m². There was mild iron deficiency anemia with serum iron saturation of 18%. Serum EPO level was normal. Urine EPO was not measured. Renal biopsy showed evidence of focal segmental glomerulosclerosis. Genetic testing for NS showed mutations in podocyte genes: NUP93, INF2, KANK1, and ACTN4. Gene sequence analysis of genes associated with erythrocytosis showed no variants in any of these genes. She required chronic dialysis ten months later and, subsequently, a renal transplantation 14 months after the initial presentation. Conclusion: Since the serum EPO level was normal, an increased sensitivity to EPO is the most probable mechanism of erythrocytosis. The unusual association of erythrocytosis in patients with NS and advanced CKD needs to be studied further in larger studies.

Locust Hemolymph Conveys Erythropoietin-Like Cytoprotection via Activation of the Cytokine Receptor CRLF3

The cytokine receptor-like factor 3 (CRLF3) is an evolutionary conserved class 1 cytokine receptor present in all major eumetazoan groups. Endogenous CRLF3 ligands have not been identified and the physiological responses mediated by mammalian CRLF3 are poorly characterized. Insect CRLF3 is activated by erythropoietin (Epo) and several related molecules that protect mammalian neurons from stress-induced apoptosis. However, insects neither express Epo nor “classical” Epo receptor. Cell-protective effects of insect hemolymph have been described for several species. In this study, we explored the possibility that the endogenous CRLF3 ligand is contained in locust hemolymph. PCR analyses confirmed expression of crfl3-transcripts in neurons and hemocytes of Locusta migratoria and Tribolium castaneum. Survival of locust hemocytes in primary cultures was significantly increased by supplementation of culture medium with locust hemolymph serum. Locust primary neuron cultures were also protected by locust hemolymph, though preceding exposure to fetal bovine serum changed the hemolymph dose-dependency of neuroprotection. Direct comparison of 10% hemolymph serum with recombinant human Epo in its optimal neuroprotective concentration revealed equivalent anti-apoptotic effects on hypoxia-exposed locust neurons. The same concentration of locust hemolymph serum also protected hypoxia-exposed T. castaneum neurons. This indicates that the neuroprotective factor in locust hemolymph is sufficiently conserved in insects to allow activation of neuroprotective receptors in different species. Locust hemolymph-induced neuroprotection in both L. migratoria and T. castaneum was abolished after RNAi-mediated suppression of crlf3-expression. In summary, we report the presence of a conserved endogenous cytokine in locust hemolymph that activates CRLF3 and connected anti-apoptotic processes in hemocytes and neurons. Identification and characterization of the CRLF3 ligand will promote knowledge about cytokine evolution and may unravel cell-protective agents with potential clinical application.

ADDITIVE NEUROPROTECTIVE EFFECT OF 3-HYDROXYPYRIDINE DERIVATIVES AND HUMAN ERYTHROPOETIN ANALOGUE ON A HEMORRHAGIC STROKE MODEL IN RATS

The correction of free radical oxidation processes is one of the most promising strategies of neuroprotection in acute cerebrovascular disorders.

The aim of the study is an experimental study of the neuroprotective effects of 3-hydroxypyridine and erythropoietin derivatives, as well as their combined use.

Materials and methods. The study was performed on 109 male Wistar rats. The neuroprotective effect of the substances was studied on a hemorrhagic stroke model. The study drugs were administered to the animals intraperitoneally. Carbamylated darbepoetin was administered three times in advance at the dose of 100 µg/kg within intervals of 3 days, the last injection took place 1 hour before the operation (the total dose was 300 mg/kg). Etoxidol was administered once 1 hour before the surgery at the dose of 50 mg/kg. The survival rate, behavioral features and the state of the animals on the 1st, 3rd, 7th and 14th days were recorded, and the morphological assessment of the brain was carried out.

Results. The investigated substances had a positive effect on both the survival rate of the animals during the first day and on the 14th day. The best survival rates on the 14th day were recorded in the group of a combined use of ethoxydol and carbamylated darbepoetin (75%). Thus, in this group of rats, a faster recovery of neurological disorders was already distinguished from the first day on. By the 7th day, more than 50% of the rats receiving the combination of the studied drugs, had had a slight neurological deficit (up to 3 points on the McGrow scale); by the 14th day there had been only minor changes in the neurological status in the rats of this group. A pronounced neuroprotective effect of the combination of 3-hydroxypyridine and erythropoietin derivatives has been confirmed by a histological examination of brain slices – a more rapid decrease in the size of perifocal edema and microcirculation disorders, less damage to neurons and glial elements, and faster processes of resorption and organization of hemorrhage. A macroscopic examination of the brain sections stained with triphenyltetrazolium chloride of the dying rats, showed that perifocal necrosis had been the main cause of high mortality in the control group after the 3rd day.

Conclusion. As a result of the experiment, the nephroprotective effect of the studied derivatives of 3-hydroxypyridine and erythropoietin has been proved. Moreover, the combination of these drugs has shown a greater neuroprotective activity than their isolated use. The additive effect of these drugs was due to their action mechanism resulting from the synergism of various structures and components of the cells.

Structural Basis of Interleukin-5 Inhibition by the Small Cyclic Peptide AF17121

Interleukin-5 (IL-5) is a T-helper cell of subtype 2 cytokine involved in many aspects of eosinophil life. Eosinophilic granulocytes play a pathogenic role in the progression of atopic diseases, such as allergy, asthma and atopic dermatitis and hypereosinophilic syndromes. Here, eosinophils upon activation degranulate leading to the release of proinflammatory proteins and mediators stored in intracellular vesicles termed granula thereby causing local inflammation, which when persisting leads to tissue damage and organ failure. As a key regulator of eosinophil function, IL-5 therefore presents a major pharmaceutical target and approaches to interfere with IL-5 receptor activation are of great interest. Here we present the structure of the IL-5 inhibiting peptide AF17121 bound to the extracellular domain of the IL-5 receptor IL-5Rα. The small 18mer cyclic peptide snugly fits into the wrench-like cleft of the IL-5 receptor, thereby blocking access of key residues for IL-5 binding. While AF17121 and IL-5 seemingly bind to a similar epitope at IL-5Rα, functional studies show that recognition and binding of both ligands differ. Using the structure data, peptide variants with improved IL-5 inhibition have been generated, which might present valuable starting points for superior peptide-based IL-5 antagonists.

Loss of Forkhead box M1 promotes erythropoiesis through increased proliferation of erythroid progenitors

Forkhead box M1 (FOXM1) belongs to the forkhead/winged-helix family of transcription factors and regulates a network of proliferation-associated genes. Its abnormal upregulation has been shown to be a key driver of cancer progression and an initiating factor in oncogenesis. FOXM1 is also highly expressed in stem/progenitor cells and inhibits their differentiation, suggesting that FOXM1 plays a role in the maintenance of multipotency. However, the exact molecular mechanisms by which FOXM1 regulates human stem/progenitor cells are still uncharacterized. To understand the role of FOXM1 in normal hematopoiesis, human cord blood CD34⁺ cells were transduced with FOXM1 short hairpin ribonucleic acid (shRNA) lentivirus. Knockdown of FOXM1 resulted in a 2-fold increase in erythroid cells compared to myeloid cells. Additionally, knockdown of FOXM1 increased bromodeoxyuridine (BrdU) incorporation in erythroid cells, suggesting greater proliferation of erythroid progenitors. We also observed that the defective phosphorylation of FOXM1 by checkpoint kinase 2 (CHK2) or cyclin-dependent kinases 1/2 (CDK1/2) increased the erythroid population in a manner similar to knockdown of FOXM1. Finally, we found that an inhibitor of FOXM1, forkhead domain inhibitor-6 (FDI-6), increased red blood cell numbers through increased proliferation of erythroid precursors. Overall, our data suggest a novel function of FOXM1 in normal human hematopoiesis.

Designing a small molecule erythropoietin mimetic

Erythropoietin (EPO) is a protein made by the kidneys in response to low red blood cell count that is secreted into the bloodstream and binds to a receptor on hematopoietic stem cells in the bone marrow inducing them to become new red blood cells. EPO made with recombinant DNA technology was brought to market in the 1980s to treat anemia caused by kidney disease and cancer chemotherapy. Because EPO infusion was able to replace blood transfusions in many cases, it rapidly became a multibillion dollar per year drug and as the first biologic created with recombinant technology it launched the biotech industry. For many years intense research was focused on creating a small molecule orally available EPO mimetic. The Robert Wood Johnson (RWJ) group seemed to definitively establish that only large peptides with a minimum of 60 residues could replace EPO, as anything less was not a full agonist. An intense study of the published work led me to hypothesize that the size of the mimetic is not the real issue, but the symmetry making and breaking of the EPO receptor induced by the ligand is the key to activating the stem cells. This analysis meant that residues in the binding site of the receptor deemed absolutely essential for ligand binding and activation from mutagenesis experiments, were probably not really that important. My fundamental hypotheses were: (a) the symmetric state of the homodimeric receptor is the most stable state and thus must be the off-state, (b) a highly localized binding site exists at a pivot point where the two halves of the receptor meet, (c) small molecules can be created that have high potency for this site that will be competitive with EPO and thus can displace the protein-protein interaction, (d) small symmetric molecules will stabilize the symmetric off-state of the receptor, and (e) a key asymmetry in the small molecule will stabilize a mirror image asymmetry in the receptor resulting in the stabilization of the on-state and proliferation of the stem cells into red blood cells. Researchers at Amgen published a co-crystal structure of EPO bound to the EPO receptor, which has a beautiful twofold symmetry-it was argued that this is the active state of the receptor. Activating the EPO receptor with EPO induces an almost instantaneous shutdown mechanism to sharply curtail any proliferative signal transduction, and thus, my hypotheses lead to the conclusion that the Amgen co-crystal is actually the state after receptor downregulation and thus an off-state. To put these hypotheses to the test, my computational method of Simulated Annealing of Chemical Potential was run using the co-crystal created at RWJ, which is the receptor trapped in a partial agonist state. The simulations predicted a previously unknown high affinity binding site at the pivot point where the two halves of the dimeric receptor meet, and detailed analysis of the fragment patterns led to the prediction of a molecule less than 300 MW that is basically twofold symmetric with a chiral center on one side and not the other. Thus, to the degree that computer simulations can be taken seriously, these results support my hypotheses on small molecule receptor activation. When this small molecule was synthesized and tested it indeed induced human hematopoietic stems cells to become red blood cells. When the predicted chiral center of this molecule was removed eliminating its one asymmetric feature, the resulting molecule was an antagonist-it could potently displace hot EPO but could no longer induce stem cell proliferation and differentiation. These results provided strong support for my theories on how to create potent small molecule EPO agonists and were used to launch the new company Locus Pharmaceuticals. These molecules, however, required significant chemical changes in order to make them stable in other in vitro assays and to be in vivo active, but these alterations had to be done in a way that maintained the symmetry-asymmetry considerations that led to the creation of an in vitro active molecule. The combination of changing functional groups to enable good pharmacokinetics, while not changing the key intrinsic symmetry properties were never seriously pursued at Locus and the program died. Investigations into how red blood cells are created have occupied many prominent researchers for the entire twentieth century. In the second half of the century EPO was discovered and by the end of the century it became a blockbuster commercial product that launched the biotech revolution.

Less known pathophysiological mechanisms of anemia in patients with diabetic nephropathy

Diabetes mellitus (DM) is currently considered a modern global epidemic, and diabetic nephropathy (DN) is the most common cause of chronic kidney disease (CKD). Anemia is one of the most significant complications of CKD, and it is mainly attributed to insufficient erythropoietin (EPO) production. However, anemia develops earlier in the course of CKD among patients with DM, and the severity of anemia tends to be more marked in these patients compared to nondiabetic subjects, regardless of the stage of CKD. In this review, we focus on the "less known" complex interacting mechanisms which are involved in the pathophysiology of anemia associated with DN. Although the major cause of anemia in DN is considered to be an inappropriate response of the plasma EPO concentration to anemia, several other possible mechanisms have been suggested. Glomerular hyperfiltration, proteinuria, renal tubular dysfunction and interstitial fibrosis are among the main culprits. On the other hand, systemic effects such as chronic inflammation, autonomic neuropathy and the renin-angiotensin system are also involved. Finally, several medications are considered to aggravate anemia associated with DN. Since anemia is an important predictor of quality of life and is implicated in the increased burden of cardiovascular morbidity and mortality, further research is required to elucidate its pathogenesis in diabetic patients.

Resistance of dialyzed patients to erythropoietin

Resistance to recombinant human erythropoietin is a common condition in dialyzed patients with chronic kidney disease and is associated with more hospitalizations, increased mortality and frequent blood transfusions. The main cause of hyporesponsiveness to recombinant human erythropoietin in these patients is iron deficiency. However, a high proportion of patients does not respond to treatment, even to the use of intravenous iron, which indicates the presence of other important causes of resistance. In addition to the iron deficiency, the most common causes of resistance include inflammation, infection, malnutrition, inadequate dialysis, and hyperparathyroidism, although other factors may be associated. In the presence of adequate iron stores, other causes should be investigated and treated appropriately.

Functional characterization of c-Mpl ectodomain mutations that underlie congenital amegakaryocytic thrombocytopenia

Activation of the cell surface receptor, c-Mpl, by the cytokine, thrombopoietin (TPO), underpins megakaryocyte and platelet production in mammals. In humans, mutations in c-Mpl have been identified as the molecular basis of Congenital Amegakaryocytic Thrombocytopenia (CAMT). Here, we show that CAMT-associated mutations in c-Mpl principally lead to defective receptor presentation on the cell surface. In contrast, one CAMT mutant c-Mpl, F104S, was expressed on the cell surface, but showed defective TPO binding and receptor activation. Using mutational analyses, we examined which residues adjacent to F104 within the membrane-distal cytokine receptor homology module (CRM) of c-Mpl comprise the TPO-binding epitope, revealing residues within the predicted Domain 1 E-F and A-B loops and Domain 2 F'-G' loop as key TPO-binding determinants. These studies underscore the importance of the c-Mpl membrane-distal CRM to TPO-binding and suggest that mutations within this CRM that perturb TPO binding could give rise to CAMT.

Cytokines in radiobiological responses: a review

Cytokines function in many roles that are highly relevant to radiation research. This review focuses on how cytokines are structurally organized, how they are induced by radiation, and how they orchestrate mesenchymal, epithelial and immune cell interactions in irradiated tissues. Pro-inflammatory cytokines are the major components of immediate early gene programs and as such can be rapidly activated after tissue irradiation. They converge with the effects of ionizing radiation in that both generate free radicals including reactive oxygen and nitrogen species (ROS/RNS). "Self" molecules secreted or released from cells after irradiation feed the same paradigm by signaling for ROS and cytokine production. As a result, multilayered feedback control circuits can be generated that perpetuate the radiation tissue damage response. The pro-inflammatory phase persists until such times as perceived challenges to host integrity are eliminated. Antioxidant, anti-inflammatory cytokines then act to restore homeostasis. The balance between pro-inflammatory and anti-inflammatory forces may shift to and fro for a long time after radiation exposure, creating waves as the host tries to deal with persisting pathogenesis. Individual cytokines function within socially interconnected groups to direct these integrated cellular responses. They hunt in packs and form complex cytokine networks that are nested within each other so as to form mutually reinforcing or antagonistic forces. This yin-yang balance appears to have redox as a fulcrum. Because of their social organization, cytokines appear to have a considerable degree of redundancy and it follows that an elevated level of a specific cytokine in a disease situation or after irradiation does not necessarily implicate it causally in pathogenesis. In spite of this, "driver" cytokines are emerging in pathogenic situations that can clearly be targeted for therapeutic benefit, including in radiation settings. Cytokines can greatly affect intrinsic cellular radiosensitivity, the incidence and type of radiation tissue complications, bystander effects, genomic instability and cancer. Minor and not so minor, polymorphisms in cytokine genes give considerable diversity within populations and are relevant to causation of disease. Therapeutic intervention is made difficult by such complexity; but the potential prize is great.

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.

A peptide derived from the CD loop-D helix region of ciliary neurotrophic factor (CNTF) induces neuronal differentiation and survival by binding to the leukemia inhibitory factor (LIF) receptor and common cytokine receptor chain gp130

Ciliary neurotrophic factor (CNTF) induces neuronal differentiation and promotes the survival of various neuronal cell types by binding to a receptor complex formed by CNTF receptor α (CNTFRα), gp130, and the leukemia inhibitory factor (LIF) receptor (LIFR). The CD loop-D helix region of CNTF has been suggested to be important for the cytokine interaction with LIFR. We designed a peptide, termed cintrofin, that encompasses this region. Surface plasmon resonance analysis demonstrated that cintrofin bound to LIFR and gp130, but not to CNTFRα, with apparent KD values of 35 nM and 1.1 nM, respectively. Cintrofin promoted the survival of cerebellar granule neurons (CGNs), in which cell death was induced either by potassium withdrawal or H2O2 treatment. Cintrofin induced neurite outgrowth from CGNs, and this effect was inhibited by specific antibodies against both gp130 and LIFR, indicating that these receptors are involved in the effects of cintrofin. The C-terminal part of the peptide, corresponding to the D helix region of CNTF, was shown to be essential for the neuritogenic action of the peptide. CNTF and LIF induced neurite outgrowth in CGNs plated on laminin-coated slides. On uncoated slides, CNTF and LIF had no neuritogenic effect but were able to inhibit cintrofin-induced neuronal differentiation, indicating that cintrofin and cytokines compete for the same receptors. In addition, cintrofin induced the phosphorylation of STAT3, Akt, and ERK, indicating that it exerts cell signaling properties similar to those induced by CNTF and may be a valuable survival agent with possible therapeutic potential.

Erythropoietin as a novel brain and kidney protective agent

Erythropoietin is a 30.4 kDa glycoprotein produced by the kidney, which is mostly known for its physiological function in regulating red blood cell production in the bone marrow Accumulating evidence, however suggests that erythropoietin has additional organ protective effects, which may specifically be useful in protecting the brain and kidneys from injury. Experimental evidence suggests that these protective mechanisms are multi-factorial in nature and may include inhibition of apoptotic cell death, stimulation of cellular regeneration, inhibition of deleterious pathways and promotion of recovery. In this article we review the physiology of erythropoietin, assess previous work that supports the role of erythropoietin as a general tissue protective agent and explain the mechanisms by which it may achieve this tissue protective effect. We then focus on specific laboratory and clinical data that suggest that erythropoietin has a strong brain protective and kidney protective effect. In addition, we comment on the implications of these studies for clinicians at the bedside and for researchers designing controlled trials to further elucidate the true clinical utility of erythropoietin as a neuroprotective and nephroprotective agent. Finally, we describe EPO-TBI, a double-blinded multi-centre randomised controlled trial involving the authors that is being conducted to investigate the organ protective effects of erythropoietin on the brain, and also assesses its effect on the kidneys.

Erythropoietin (EPO) in acute kidney injury

Erythropoietin (EPO) is a 30.4 kDa glycoprotein produced by the kidney, and is mostly well-known for its physiological function in regulating red blood cell production in the bone marrow. Accumulating evidence, however, suggests that EPO has additional organ protective effects, which may be useful in the prevention or treatment of acute kidney injury. These protective mechanisms are multifactorial in nature and include inhibition of apoptotic cell death, stimulation of cellular regeneration, inhibition of deleterious pathways, and promotion of recovery.

In this article, we review the physiology of EPO, assess previous work that supports the role of EPO as a general tissue protective agent, and explain the mechanisms by which it may achieve this tissue protective effect. We then focus on experimental and clinical data that suggest that EPO has a kidney protective effect.

Identification of the residues in the extracellular domain of thrombopoietin receptor involved in the binding of thrombopoietin and a nuclear distribution protein (human NUDC)

Thrombopoietin (TPO) and its receptor (Mpl) have long been associated with megakaryocyte proliferation, differentiation, and platelet formation. However, studies have also shown that the extracellular domain of Mpl (Mpl-EC) interacts with human (h) NUDC, a protein previously characterized as a human homolog of a fungal nuclear migration protein. This study was undertaken to further delineate the putative binding domain on the Mpl receptor. Using the yeast two-hybrid system assay and co-immunoprecipitation, we identified that within the Mpl-EC domain 1 (Mpl-EC-D1), amino acids 102-251 were strongly involved in ligand binding. We subsequently expressed five subdomains within this region with T7 phage display. Enzyme-linked immunosorbent binding assays identified a short stretch of peptide located between residues 206 and 251 as the minimum binding domain for both TPO and hNUDC. A series of sequential Ala replacement mutations in the region were subsequently used to identify the specific residues most involved in ligand binding. Our results point to two hydrophobic residues, Leu(228) and Leu(230), as having substantial effects on hNUDC binding. For TPO binding, mutations in residues Asp(235) and Leu(239) had the largest effect on binding efficacy. In addition, deletion of the conservative motif WGSWS reduced binding capacity for hNUDC but not for TPO. These separate binding sites on the Mpl receptor for TPO and hNUDC raise interesting implications for the cytokine-receptor interactions.

A diversity of antibody epitopes can induce signaling through the erythropoietin receptor

Stimulation of red cell production through agonism of the erythropoietin receptor (EpoR) has historically been accomplished through administration of erythropoietin (EPO), the native ligand. The short half-life of EPO has led to the development of a variety of other agonists, including antibodies. It is of considerable interest to understand how these agents might activate the EpoR and whether or not it is important to bind in a manner similar to the native ligand. The binding epitopes of a panel of eight agonistic, single-chain antibody (scFv-Fc) constructs were determined through scanning alanine mutagenesis as well as more limited arginine mutagenesis of the receptor. It was found that while some of these constructs bound to receptor epitopes shared by the ligand, others bound in completely unique ways. The use of a panel of agonists and scanning mutagenesis can define the critical binding regions for signaling; in the case of the EpoR, these regions were remarkably broad.

Nonnatural protein-protein interaction-pair design by key residues grafting

Protein-protein interface design is one of the most exciting fields in protein science; however, designing nonnatural protein-protein interaction pairs remains difficult. In this article we report a de novo design of a nonnatural protein-protein interaction pair by scanning the Protein Data Bank for suitable scaffold proteins that can be used for grafting key interaction residues and can form stable complexes with the target protein after additional mutations. Using our design algorithm, an unrelated protein, rat PLCdelta(1)-PH (pleckstrin homology domain of phospholipase C-delta1), was successfully designed to bind the human erythropoietin receptor (EPOR) after grafting the key interaction residues of human erythropoietin binding to EPOR. The designed mutants of rat PLCdelta(1)-PH were expressed and purified to test their binding affinities with EPOR. A designed triple mutation of PLCdelta(1)-PH (ERPH1) was found to bind EPOR with high affinity (K(D) of 24 nM and an IC(50) of 5.7 microM) both in vitro and in a cell-based assay, respectively, although the WT PLCdelta(1)-PH did not show any detectable binding under the assay conditions. The in vitro binding affinities of the PLCdelta(1)-PH mutants correlate qualitatively to the computational binding affinities, validating the design and the protein-protein interaction model. The successful practice of finding a proper protein scaffold and making it bind with EPOR demonstrates a prospective application in protein engineering targeting protein-protein interfaces.

Hot-spot mimicry of a cytokine receptor by a small molecule

Protein-protein complexes remain enticing, but extremely challenging, targets for small-molecule drug discovery. In a rare example described earlier, a high-affinity small molecule, SP4206 (Kd approximately 70 nM), was found to block binding of the IL-2alpha receptor (IL-2Ralpha) to IL-2 (Kd approximately 10 nM). Recently, the structure of the IL-2/IL-2Ralpha complex was solved [Rickert, M., Wang, X., Boulanger, M. J., Goriatcheva, N., Garcia, K. C. (2005) Science 308:1477-1480]. Using structural and functional analysis, we compare how SP4206 mimics the 83-fold larger IL-2Ralpha in binding IL-2. The binding free energy per contact atom (ligand efficiency) for SP4206 is about twice that of the receptor because of a smaller, but overlapping, contact epitope that insinuates into grooves and cavities not accessed by the receptor. Despite its independent design, the small molecule has a similar, but more localized, charge distribution compared with IL-2Ralpha. Mutational studies show that SP4206 targets virtually the same critical "hot-spot" residues on IL-2 that drive binding of IL-2Ralpha. Moreover, a mutation that enhances binding to the IL-2Ralpha near these hot spots also enhances binding to SP4206. Although the protein and small molecule do bind the same hot spot, they trap very different conformations of IL-2 because of its flexible nature. Our studies suggest that precise structural mimics of receptors are not required for high-affinity binding of small molecules, and they show that there are multiple solutions to tight binding at shared and adaptive hot spots.

Preassembly and ligand-induced restructuring of the chains of the IFN-gamma receptor complex: the roles of Jak kinases, Stat1 and the receptor chains

We previously demonstrated using noninvasive technologies that the interferon-gamma (IFN-gamma) receptor complex is preassembled (1). In this report we determined how the receptor complex is preassembled and how the ligand-mediated conformational changes occur. The interaction of Stat1 with IFN-gammaR1 results in a conformational change localized to IFN-gammaR1. Jak1 but not Jak2 is required for the two chains of the IFN-gamma receptor complex (IFN-gammaR1 and IFN-gammaR2) to interact; however, the presence of both Jak1 and Jak2 is required to see any ligand-dependant conformational change. Two IFN-gammaR2 chains interact through species-specific determinants in their extracellular domains. Finally, these determinants also participate in the interaction of IFN-gammaR2 with IFN-gammaR1. These results agree with a detailed model of the IFN-gamma receptor that requires the receptor chains to be pre-associated constitutively for the receptor to be active.

IL-3, IL-5, and GM-CSF signaling: crystal structure of the human beta-common receptor

The cytokines, interleukin-3 (IL-3), interleukin-5 (IL-5), and granulocyte-macrophage colony stimulating factor (GM-CSF), are polypeptide growth factors that exhibit overlapping activities in the regulation of hematopoietic cells. They appear to be primarily involved in inducible hematopoiesis in response to infections and are involved in the pathogenesis of allergic and inflammatory diseases and possibly in leukemia. The X-ray structure of the beta common (betac) receptor ectodomain has given new insights into the structural biology of signaling by IL-3, IL-5, and GM-CSF. This receptor is shared between the three ligands and functions together with three ligand-specific alpha-subunits. The structure shows betac is an intertwined homodimer in which each chain contains four domains with approximate fibronectin type-III topology. The two betac-subunits that compose the homodimer are interlocked by virtue of the swapping of beta-strands between domain 1 of one subunit and domain 3 of the other subunit. Site-directed mutagenesis has shown that the interface between domains 1 and 4 in this unique structure forms the functional epitope. This epitope is similar to those of other members of the cytokine class I receptor family but is novel in that it is formed by two different receptor chains. The chapter also reviews knowledge on the closely related mouse beta(IL-3) receptor and on the alpha-subunit-ligand interactions. The knowledge on the two beta receptors is placed in context with advances in understanding of the structural biology of other members of the cytokine class I receptor family.

Interleukin-3 Binding to the Murine βIL-3 and Human βc Receptors Involves Functional Epitopes Formed by Domains 1 and 4 of Different Protein Chains

Interleukin-3 (IL-3) is a cytokine produced by activated T-cells and mast cells that is active on a broad range of hematopoietic cells and in the nervous system and appears to be important in several chronic inflammatory diseases. In this study, alanine substitutions were used to investigate the role of residues of the human beta-common (hbetac) receptor and the murine IL-3-specific (beta(IL-3)) receptor in IL-3 binding. We show that the domain 1 residues, Tyr(15) and Phe(79), of the hbetac receptor are important for high affinity IL-3 binding and receptor activation as shown previously for the related cytokines, interleukin-5 and granulocyte-macrophage colony-stimulating factor, which also signal through this receptor subunit. From the x-ray structure of hbetac, it is clear that the domain 1 residues cooperate with domain 4 residues to form a novel ligand-binding interface involving the two protein chains of the intertwined homodimer receptor. We demonstrate by ultracentrifugation that the beta(IL-3) receptor is also a homodimer. Its high sequence homology with hbetac suggests that their structures are homologous, and we identified an analogous binding interface in beta(IL-3) for direct IL-3 binding to the high affinity binding site in hbetac. Tyr(21) (A-B loop), Phe(85), and Asn(87) (E-F loop) of domain 1; Ile(320) of the interdomain loop; and Tyr(348) (B'-C' loop) and Tyr(401) (F'-G' loop) of domain 4 were shown to have critical individual roles and Arg(84) and Tyr(317) major secondary roles in direct murine IL-3 binding to the beta(IL-3)receptor. Most surprising, none of the key residues for direct IL-3 binding were critical for high affinity binding in the presence of the murine IL-3 alpha receptor, indicating a fundamentally different mechanism of high affinity binding to that used by hbetac.

Drugs for increasing oxygen and their potential use in doping: a review

Blood oxygenation is a fundamental factor in optimising muscular activity. Enhancement of oxygen delivery to tissues is associated with a substantial improvement in athletic performance, particularly in endurance sports. Progress in medical research has led to the identification of new chemicals for the treatment of severe anaemia. Effective and promising molecules have been created and sometimes used for doping purposes. The aim of this review is to present methods, and drugs, known to be (or that might be) used by athletes to increase oxygen transport in an attempt to improve endurance capacity. These methods and drugs include: (i) blood transfusion; (ii) endogenous stimulation of red blood cell production at altitude, or using hypoxic rooms, erythropoietins (EPOs), EPO gene therapy or EPO mimetics; (iii) allosteric effectors of haemoglobin; and (iv) blood substitutes such as modified haemoglobin solutions and perfluorochemicals. Often, new chemicals are used before safety tests have been completed and athletes are taking great health risks. Such new chemicals have also created the need for new instrumental strategies in doping control laboratories, but not all of these chemicals are detectable. Further progress in analytical research is necessary.

A novel functional epitope formed by domains 1 and 4 of the human common beta-subunit is involved in receptor activation by granulocyte macrophage colony-stimulating factor and interleukin 5

The receptors for human interleukins 3 and 5 and granulocyte macrophage colony-stimulating factor are composed of ligand-specific alpha-subunits and a common beta-subunit (betac), the major signaling entity. The way in which betac interacts with ligands in the respective activation complexes has remained poorly understood. The recently determined crystal structure of the extracellular domain of betac revealed a possible ligand-binding interface composed of domain 1 of one chain of the betac dimer and the adjacent domain 4 of the symmetry-related chain. We have used site-directed mutagenesis, in conjunction with ligand binding and proliferation studies, to demonstrate the critical requirement of the domain 1 residues, Tyr(15) (A-B loop) and Phe(79) (E-F loop), in high affinity complex formation and receptor activation. The novel ligand-receptor interface formed between domains 1 and 4 represents the first example of a class I cytokine receptor interface to be composed of two noncontiguous fibronectin III domains.

Structure, binding, and antagonists in the IL-4/IL-13 receptor system

Interleukin-4 (IL-4) and IL-13 are the only cytokines known to bind to the receptor chain IL-4Ralpha. Receptor sharing by these two cytokines is the molecular basis for their overlapping biological functions. Both are key factors in the development of allergic hypersensitivity, and they also play a major role in exacerbating allergic and asthmatic symptoms. Knowledge of structure and function of this system has allowed the development of inhibitors that block the interaction between the cytokines and their shared receptor. Mutational analysis of IL-4 has revealed variants with high-affinity binding to IL-4Ralpha but no detectable affinity for the second receptor subunit, which is either (gamma)c or IL-13Ralpha1. These IL-4 antagonists fail to induce signal transduction and block IL-4 and IL-13 effects in vitro. IL-4 antagonists prevent the development of allergic disease in vivo and an antagonistic variant of human IL-4 is now in clinical trials for asthma. Detailed knowledge of the site of interaction of IL-4 and IL-4Ralpha has been gained by structure analysis of the complex of these two proteins and through functional studies employing mutants of IL-4 and its receptor subunits. Based on these new data, the hitherto elusive goal of designing small molecular mimetics may be feasible.

Structural basis for binding multiple ligands by the common cytokine receptor gamma-chain

The common gamma-chain (gamma(c)) that functions both in ligand binding and signal transduction is a shared subunit of the multichain receptors for interleukin (IL)-2, IL-4, IL-7, IL-9, IL-15, and IL-21. The structural basis by which the ectodomain of gamma(c) contributes to binding six distinct cytokines is only partially defined. In the present study, epitope mapping of antagonistic anti-gamma(c) monoclonal antibodies led to the identification of Asn-128 of mouse gamma(c) that represents another potential contact residue that is required for binding IL-2, IL-7, and IL-15 but not IL-4. In addition, Tyr-103, Cys-161, Cys-210, and Cys-211, previously identified to contribute to binding IL-2 and IL-7, were also found to be involved in binding IL-4 and IL-15. Collectively, these data favor a model in which gamma(c) utilizes a common mechanism for its interactions with multiple cytokines, and the binding sites are largely overlapping but not identical. Asn-128 and Tyr-103 likely act as contact residues whereas Cys-161, Cys-210, and Gly-211 may stabilize the structure of the proposed ligand-interacting surface formed by the two extracytoplasmic domains.

Functional epitope of common gamma chain for interleukin-4 binding

Interleukin 4 (IL-4) can act on target cells through an IL-4 receptor complex consisting of the IL-4 receptor alpha chain and the common gamma chain (gamma(c)). An IL-4 epitope for gamma(c) binding has previously been identified. In this study, the gamma(c) residues involved in IL-4 binding were defined by alanine-scanning mutational analysis. The epitope comprises gamma(c) residues I100, L102, and Y103 on loop EF1 together with L208 on loop FG2 as the major binding determinants. These predominantly hydrophobic determinants interact with the hydrophobic IL-4 epitope composed of residues I11, N15, and Y124. Double-mutant cycle analysis revealed co-operative interaction between gamma(c) and IL-4 side chains. Several gamma(c) residues involved in IL-4 binding have been previously shown to be mutated in X-linked severe combined immunodeficiency. The importance of these binding residues for gamma(c) function is discussed. These results provide a basis for elucidating the molecular recognition mechanism in the IL-4 receptor system and a paradigm for other gamma(c)-dependent cytokine receptor systems.

Kinetics of small molecule inhibitor binding to p38 kinase

p38 mitogen-activated protein kinase (MAPK) (p38/p38-alpha/CSBP2/RK) has been implicated in the regulation of many proinflammatory pathways. Because of this, it has received much attention as a potential drug target for controlling diseases such as rheumatoid arthritis, endotoxic shock, inflammatory bowel disease, osteoporosis, and many others. A number of small molecule inhibitors of this kinase have been described, and in this paper we have used surface plasmon resonance to directly measure and quantitate their binding to p38. Despite the relatively low molecular mass (approximately 400 Da) of these inhibitors, specific binding can be observed. For the two most potent inhibitors studied, SB 203580 and RWJ 67657, dissociation constants, K(d)'s, of 22 and 10 nm, respectively, were obtained. These values closely match the IC(5)0 values observed in a cell-based TNF alpha release assay implying that p38 plays a major role in TNF alpha release. The association and dissociation rates for the binding of these inhibitors to p38 have also been quantitated. SB 203580 and RWJ 67657 have very similar association rates of around 8 x 10(5) m(-1) x s(-1), and the differences in affinity are determined by different dissociation rates. The weaker binding compounds have dissociation rates similar to SB 203580, but the association rates vary by an order of magnitude or more. The direct measurement of compounds binding to p38 may help in understanding the difference between potency and efficacy for these inhibitors. This in turn may yield clues on how to develop better inhibitors.

Hematopoietic growth factor mimetics

Hematopoietic growth factors are glycoproteins of 15-70 kDa. Although much clinical success has been obtained using recombinant proteins produced in mammalian cell lines and in microbial fermentation processes, the full-length polypeptides necessarily are expensive to produce, require parenteral administration, and in some cases have provoked detrimental immune responses. With the availability of high throughput biological function and receptor binding assays it has become possible to screen millions, if not billions, of randomly produced organic compounds and relatively short peptides to identify lead compounds for the development of small molecular mimetics of hematopoietic growth factors. Herein the strategies used to screen libraries of small molecules and peptides and the successes in finding mimetics and antagonists for/to erythropoietin, granulocyte colony-stimulating factor, and thrombopoietin are reviewed. Finally, the structural study of mimetic-receptor complexes has provided us with many molecular details of growth factor-induced receptor activation and is likely to yield new insights into the molecular basis of hematopoietic signal transduction.

Survey of the 1999 surface plasmon resonance biosensor literature

The application of surface plasmon resonance biosensors in life sciences and pharmaceutical research continues to increase. This review provides a comprehensive list of the commercial 1999 SPR biosensor literature and highlights emerging applications that are of general interest to users of the technology. Given the variability in the quality of published biosensor data, we present some general guidelines to help increase confidence in the results reported from biosensor analyses.

Three loops of the common gamma chain ectodomain required for the binding of interleukin-2 and interleukin-7

The common gamma chain (gammac), a subunit of the interleukin (IL)-2, IL-4, IL-7, IL-9, and IL-15 receptors, contributes to both cytokine binding and subsequent signal transduction. Using a model-based site-directed mutagenesis strategy, we have identified residues of the mouse gammac extracellular domain that are required for normal gammac-dependent enhancement of IL-2 and IL-7 binding. One of these sites, Tyr-103, is homologous to key ligand-interacting residues in the growth hormone and erythropoietin receptors, whereas Cys-161, Cys-210, and Gly-211 may function indirectly by maintaining the functional conformation of gammac via formation of an intramolecular disulfide bond. These two cysteines are also required for the integrity of a putative epitope recognized by TUGm2, an antagonistic monoclonal antibody that blocks gammac-dependent cytokine binding and bioactivity. These results are consistent with the involvement of three predicted loops in gammac that contribute to the binding of both IL-2 and IL-7. Mutations in these loops have also been noted in the gammac gene of patients with X-linked severe combined immunodeficiency.

Cell surface receptors

During the past year, the database of ligand-receptor complexes has essentially doubled. These new results have immeasurably extended and expanded our view of cell surface receptor structure and function. The flood of data has revealed new models for receptor cross-linking, demonstrating the potential stringency of the extracellular requirements for the initiation of intracellular signalling and highlighting unexpected interactions suggestive of higher order clustering.

The structure, organization, activation and plasticity of the erythropoietin receptor

Dimerization of the erythropoietin receptor has long been accepted as the singular step in its mechanism of activation. Recent studies have revealed a regulator process for activation that is dependent on the actual configuration of the receptor-ligand dimer assembly. This aspect of the receptor subunit assembly appears to extend to the unliganded receptor, which can dimerize on the cell surface and diminish any spontaneous background signaling in the absence of ligand. This self-recognition, as well as the multiple ligand binding capabilities of the receptor binding site, is consistent with an emerging theme of plasticity in protein-protein and ligand-receptor interactions.