Timothy D. Osslund

Efficiency of signalling through cytokine receptors depends critically on receptor orientation

Human erythropoietin is a haematopoietic cytokine required for the differentiation and proliferation of precursor cells into red blood cells. It activates cells by binding and orientating two cell-surface erythropoietin receptors (EPORs) which trigger an intracellular phosphorylation cascade. The half-maximal response in a cellular proliferation assay is evoked at an erythropoietin concentration of 10 pM (ref. 3), 10−2 of its K d value for erythropoietin–EPOR binding site 1 (Kd ≈ 1 nM), and 10−5 of the K d for erythropoietin–EPOR binding site 2 (Kd ≈ 1 μM). Overall half-maximal binding (IC50) of cell-surface receptors is produced with ∼0.18 nM erythropoietin, indicating that only ∼6% of the receptors would be bound in the presence of 10 pM erythropoietin. Other effective erythropoietin-mimetic ligands that dimerize receptors can evoke the same cellular responses, but much less efficiently, requiring concentrations close to their K d values (∼0.1 μM). The crystal structure of erythropoietin complexed to the extracellular ligand-binding domains of the erythropoietin receptor, determined at 1.9 Å from two crystal forms, shows that erythropoietin imposes a unique 120° angular relationship and orientation that is responsible for optimal signalling through intracellular kinase pathways.

Enhancement of therapeutic protein in vivo activities through glycoengineering

Delivery of protein therapeutics often requires frequent injections because of low activity or rapid clearance, thereby placing a burden on patients and caregivers. Using glycoengineering, we have increased and prolonged the activity of proteins, thus allowing reduced frequency of administration. Glycosylation analogs with new N-linked glycosylation consensus sequences introduced into the protein were screened for the presence of additional N-linked carbohydrates and retention of in vitro activity. Suitable consensus sequences were combined in one molecule, resulting in glycosylation analogs of rHuEPO, leptin, and Mpl ligand. All three molecules had substantially increased in vivo activity and prolonged duration of action. Because these proteins were of three different classes (rHuEPO is an N-linked glycoprotein, Mpl ligand an O-linked glycoprotein, and leptin contains no carbohydrate), glycoengineering may be generally applicable as a strategy for increasing the in vivo activity and duration of action of proteins. This strategy has been validated clinically for glycoengineered rHuEPO (darbopoetin alfa).

Sequences of genes encoding naphthalene dioxygenase in Pseudomonas putida strains G7 and NCIB 9816-4.

The multicomponent enzyme, naphthalene dioxygenase, initiates the metabolism of naphthalene by Pseudomonas putida strains G7 (PpG7) and NCIB 9816-4 (Pp9816-4). The genes involved (nahAaAbAcAd) are encoded by the NAH7 and pDTG1 plasmids, respectively, and form part of the nah operon. The locations of the structural genes were determined on previously cloned fragments of DNA. The nucleotide (nt) sequences were determined for nahAaAb from Pp9816-4 and for nahAaAbAcAd from PpG7. The appropriate open reading frames were identified using N-terminal amino acid sequences determined from the purified proteins. The two nt sequences showed 93% homology, with the least homology seen upstream from the promoter region.

Structure of the active core of human stem cell factor and analysis of binding to its receptor kit.

Stem cell factor (SCF) is an early‐acting hematopoietic cytokine that elicits multiple biological effects. SCF is dimeric and occurs in soluble and membrane‐bound forms. It transduces signals by ligand‐ mediated dimerization of its receptor, Kit, which is a receptor tyrosine kinase related to the receptors for platelet‐derived growth factor (PDGF), macrophage colony‐stimulating factor, Flt‐3 ligand and vascular endothelial growth factor (VEGF). All of these have extracellular ligand‐binding portions composed of immunoglobulin‐like repeats. We have determined the crystal structure of selenomethionyl soluble human SCF at 2.2 Å resolution by multiwavelength anomalous diffraction phasing. SCF has the characteristic helical cytokine topology, but the structure is unique apart from core portions. The SCF dimer has a symmetric ‘head‐to‐head’ association. Using various prior observations, we have located potential Kit‐binding sites on the SCF dimer. A superimposition of this dimer onto VEGF in its complex with the receptor Flt‐1 places the binding sites on SCF in positions of topographical and electrostatic complementarity with the Kit counterparts of Flt‐1, and a similar model can be made for the complex of PDGF with its receptor.

In Vivo Crystallization of Human IgG in the Endoplasmic Reticulum of Engineered Chinese Hamster Ovary (CHO) Cells

Protein synthesis and secretion are essential to cellular life. Although secretory activities may vary in different cell types, what determines the maximum secretory capacity is inherently difficult to study. Increasing protein synthesis until reaching the limit of secretory capacity is one strategy to address this key issue. Under highly optimized growth conditions, recombinant CHO cells engineered to produce a model human IgG clone started housing rod-shaped crystals in the endoplasmic reticulum (ER) lumen. The intra-ER crystal growth was accompanied by cell enlargement and multinucleation and continued until crystals outgrew cell size to breach membrane integrity. The intra-ER crystals were composed of correctly folded, endoglycosidase H-sensitive IgG. Crystallizing propensity was due to the intrinsic physicochemical properties of the model IgG, and the crystallization was reproduced in vitro by exposing a high concentration of IgG to a near neutral pH. The striking cellular phenotype implicated the efficiency of IgG protein synthesis and oxidative folding exceeded the capacity of ER export machinery. As a result, export-ready IgG accumulated progressively in the ER lumen until a threshold concentration was reached to nucleate crystals. Using an in vivo system that reports accumulation of correctly folded IgG, we showed that the ER-to-Golgi transport steps became rate-limiting in cells with high secretory activity.

Crystallization and liquid‐liquid phase separation of monoclonal antibodies and fc‐fusion proteins: Screening results

Crystallization holds the potential to be used for protein purification and low‐viscosity drug substance and drug product formulations. Twenty‐two different proteins (20 monoclonal antibodies and two Fc‐fusions) were examined to determine the breadth of applicability of crystallization to these therapeutic proteins. Vapor diffusion technique and an evaporative screening method were used to identify crystallization conditions using around a 100 initial conditions based on reagents that are generally regarded as safe (GRAS). Of 16 IgG2s examined, at least four formed diffraction‐quality crystals and four others formed crystal‐like particles. At least three of the IgG2s that crystallized well were also crystallized under the same set of operating conditions using inexpensive GRAS reagents. The crystals were formed to high‐yields in a few hours and were dissolved quickly without impacting product quality. Although only a fraction of the proteins examined crystallized, all exhibited liquid‐liquid phase separation (LLPS), which could be used for their concentration or possibly purification. One of the Fc‐fusions, for example, was concentrated by LLPS to a self‐buffering solution at 150 g/L. Crystallization and LLPS in the salting‐in region were shown to be feasible.

The Intermolecular Disulfide Bridge of Human Glial Cell Line-Derived Neurotrophic Factor: Its Selective Reduction and Biological Activity of the Modified Protein

Recombinant human glial cell line-derived neurotrophic factor has been implicated to have therapeutic potential in the treatment of neurodegenerative diseases. The mature protein is a single polypeptide of 134 amino acid residues and functions as a disulfide-linked dimer. Reduction of the protein with dithiothreitol at pH 7.0 and in the absence of denaturant showed that the single intermolecular cystine bridge was reduced preferentially. Direct alkylation of the generated free sulfhydryl group using iodoacetamide or iodoacetate without denaturant was incomplete. Unfolding the protein in 6 M guanidine hydrochloride prior to the modification showed rapid disulfide scrambling. However, the sulfhydryl-modifying reagent N-ethylmaleimide was able to label quantitatively the free cysteinyl residue in the absence of any added chaotropic agent. By a combination of peptide mapping, Edman degradation, and mass spectrometric analysis, the labeled residue was identified to be Cys101, hence verifying the location of the intermolecular disulfide bond. The modified protein behaved as a noncovalent dimer when chromatographed through a Superdex 75 column under nondenaturing conditions and was comparable in biological activity to an unmodified control sample. The results therefore indicate that the intermolecular disulfide bridge of the protein is not essential for its biological function.

Structural basis for the signal transduction of erythropoietin

The structures of recombinant erythropoietin (r-HuEPO), the extracellular ligand-binding portion of erythropoietin receptor (EPOR) and several novel peptides have been extensively studied using a variety of methods including X-ray crystallography and nuclear magnetic resonance (NMR) [1, 2] The receptor for EPO has been classified as a member of the class 1 cytokine receptor super family [3, 4]. EPOR is a transmembrane receptor that is activated when EPO binds to the extracellular portion of the receptor causing dimerization [5]. The dimerization of the extracellular portion of the receptor induces tyrosine kinase proteins associated with the cytoplasmic side of the receptor to initiate the signal transduction events that eventually cause the committed erythroid-colony-forming units (CFU-E) progenitor cells to eventually mature into red blood cells [6] (Fig. 1).

A systematic literature review of the efficacy, effectiveness, and safety of filgrastim

PurposeFilgrastim (NEUPOGEN®) is the originator recombinant human granulocyte colony-stimulating factor widely used for preventing neutropenia-related infections and mobilizing hematopoietic stem cells. This report presents findings of a systematic literature review and meta-analysis of efficacy and safety of originator filgrastim to update previous reports.MethodsA literature search of electronic databases, congress abstracts, and bibliographies of recent reviews was conducted to identify English-language reports of clinical trials and observational studies evaluating filgrastim in its US-approved indications up to February 2015. Two independent reviewers assessed titles/abstracts and full texts of publications, and extracted data from studies that compared originator filgrastim vs placebo or no treatment. For outcomes with sufficient homogeneous data reported across studies, meta-analysis was performed and relative risk (RR) determined. Data were summarized descriptively for all other evaluated outcomes.ResultsA total of 1194 unique articles evaluating originator filgrastim were identified, with 25 meeting eligibility criteria for data extraction: 18 randomized controlled trials, 2 nonrandomized clinical trials, and 5 observational studies. In chemotherapy-induced neutropenia (CIN), filgrastim vs placebo or no treatment significantly reduced febrile neutropenia incidence (RR 0.63, 95% CI 0.53–0.75) and grade 3 or 4 neutropenia incidence (RR 0.50, 95% CI 0.37–0.68). The most commonly reported adverse event (AE) with filgrastim was bone pain (RR 2.61, 95% CI 1.29–5.27 in CIN). Additional efficacy and safety outcomes are described within indications.ConclusionsThis systematic literature review and meta-analysis confirms and updates previous reports on the efficacy and safety of originator filgrastim. Bone pain was the commonly reported AE associated with filgrastim use.