Stable isotope labeling of glycoprotein expressed in silkworms using immunoglobulin G as a test molecule

Specific isotopic labelling and reverse labelling for protein NMR spectroscopy: using metabolic precursors in sample preparation

The study of protein structure, dynamics and function by NMR spectroscopy commonly requires samples that have been enriched ('labelled') with the stable isotopes 13C and/or 15N. The standard approach is to uniformly label a protein with one or both of these nuclei such that all C and/or N sites are in principle 'NMR-visible'. NMR spectra of uniformly labelled proteins can be highly complicated and suffer from signal overlap. Moreover, as molecular size increases the linewidths of NMR signals broaden, which decreases sensitivity and causes further spectral congestion. Both effects can limit the type and quality of information available from NMR data. Problems associated with signal overlap and signal broadening can often be alleviated though the use of alternative, non-uniform isotopic labelling patterns. Specific isotopic labelling 'turns on' signals at selected sites while the rest of the protein is NMR-invisible. Conversely, specific isotopic unlabelling (also called 'reverse' labelling) 'turns off' selected signals while the rest of the protein remains NMR-visible. Both approaches can simplify NMR spectra, improve sensitivity, facilitate resonance assignment and permit a range of different NMR strategies when combined with other labelling tools and NMR experiments. Here, we review methods for producing proteins with enrichment of stable NMR-visible isotopes, with particular focus on residue-specific labelling and reverse labelling using Escherichia coli expression systems. We also explore how these approaches can aid NMR studies of proteins.

Structural insight and stability of TNFR-Fc fusion protein (Etanercept) produced by using transgenic silkworms

Therapeutic proteins expressed using transgenic animals have been of great interest for several years. Especially, transgenic silkworm has been studied intensively because of its ease in handling, low-cost, high-yield and unique glycosylation patterns. However, the physicochemical property of the therapeutic protein expressed in transgenic silkworm remains elusive. Here, we constructed an expression system for the TNFR-Fc fusion protein (Etanercept) using transgenic silkworm. The TNFR-Fc fusion protein was employed to N-glycan analysis, which revealed an increased amount of afucosylated protein. Evidence from surface plasmon resonance analysis showed that the TNFR-Fc fusion protein exhibit increased binding affinity for Fcγ receptor IIIa and FcRn compared to the commercial Etanercept, emphasizing the profit of expression system using transgenic silkworm. We have further discussed the comparison of higher order structure, thermal stability and aggregation of the TNFR-Fc fusion protein.

Silkworm Pupae Function as Efficient Producers of Recombinant Glycoproteins with Stable-Isotope Labeling

Baculovirus-infected silkworms are promising bioreactors for producing recombinant glycoproteins, including antibodies. Previously, we developed a method for isotope labeling of glycoproteins for nuclear magnetic resonance (NMR) studies using silkworm larvae reared on an artificial diet containing ¹⁵N-labeled yeast crude protein extract. Here, we further develop this method by introducing a technique for the expression of isotope-labeled glycoproteins by silkworm pupae, which has several potential advantages relative to larvae-based techniques in terms of production yield, ease of handling, and storage. Here, we fed fifth instar larvae an artificial diet with an optimized composition containing [methyl-¹³C]methionine, leading to pupation. Nine-day-old pupae were then injected with recombinant Bombyx mori nucleopolyhedrovirus (BmNPV) bacmid for expression of recombinant human immunoglobulin G (IgG). From the whole-body homogenates of pupae, 0.35 mg/pupa of IgG was harvested, which is a yield that is five times higher than can be obtained from larvae. Recombinant IgG, thus prepared, exhibited mainly three kinds of pauci-mannose-type oligosaccharides and had a ¹³C-enrichment ratio of approximately 80%. This enabled selective observation of NMR signals originating from the methionyl methyl group of IgG, confirming its conformational integrity. These data demonstrate the utility of silkworm pupae as factories for producing recombinant glycoproteins with amino-acid-selective isotope labeling.

[Structural Biological Approach to Biopharmaceuticals]

Detailed structural characterization of protein biopharmaceuticals is a critical step in research and development; however, this step is often hampered by the structural complexities associated with glycosylation. Most protein biopharmaceuticals are modified with structurally heterogeneous and dynamic oligosaccharides which govern the physicochemical properties, functionality, pharmacokinetics, and potential pathogenicity of these glycoproteins. Considering this, we have developed a structural biological approach to describe the dynamic three-dimensional structures and interactions of glycoproteins as biopharmaceuticals. We developed an NMR technique assisted by metabolic stable-isotope labeling that can provide useful atomic-level probes for detecting and characterizing structural perturbations of glycoproteins caused by alterations in solution conditions and production protocols, as well as by mutagenesis. We have applied this method in conjunction with X-ray crystallography to investigate the structural impacts of varying glycoforms of the Fc region of immunoglobulin G (IgG), thereby elucidating the functional roles of the Fc glycans. In particular, we have successfully elucidated the structural mechanisms by which defucosylation of the IgG-Fc region increases its affinity for Fcγ receptor IIIa, leading to an improvement in ameliorating antibody-dependent cell-mediated cytotoxicity. In addition, we applied our stable-isotope-assisted NMR method to analyzing biomolecular interactions in serum environments, which are characterized by molecular crowding and promiscuous intermolecular interactions. An integrative structural biological approach combining NMR spectroscopy, X-ray crystallography, neutron scattering, atomic force microscopy, and molecular dynamics simulation will provide new research tools that will enable the visualization of dynamic structures and interactions of glycoproteins of pharmaceutical interest, thereby providing valuable insights for the development of biopharmaceuticals.

Stable isotope labeling approaches for NMR characterization of glycoproteins using eukaryotic expression systems

Glycoproteins are characterized by the heterogeneous and dynamic nature of their glycan moieties, which hamper crystallographic analysis. NMR spectroscopy provides potential advantages in dealing with such complicated systems, given that the target molecules can be isotopically labeled. Methods of metabolic isotope labeling in recombinant glycoproteins have been developed recently using a variety of eukaryotic production vehicles, including mammalian, yeast, insect, and plant cells, each of which has a distinct N-glycan diversification pathway. Yeast genetic engineering has enabled the overexpression of homogeneous high-mannose-type oligosaccharides with ¹³C labeling for NMR characterization of their conformational dynamics. The utility of stable isotope-assisted NMR spectroscopy has also been demonstrated using the Fc fragment of immunoglobulin G (IgG) as a model glycoprotein, providing useful information regarding intramolecular carbohydrate-protein interactions. Transverse relaxation optimization of intact IgG with a molecular mass of 150 kDa has been achieved by tailored deuteration of selected amino acid residues using a mammalian expression system. This offers a useful probe for the characterization of molecular interaction networks in multimolecular crowded systems typified by serum. Perspectives regarding the development of techniques for tailoring glycoform designs and isotope labeling of recombinant glycoproteins are also discussed.

Structure and Dynamics of Immunoglobulin G Glycoproteins

Immunoglobulin G (IgG) is a major serum glycoprotein that exerts the role of antibody in the immune system. This multifunctional glycoprotein couples antigen recognition with a variety of effector functions promoted via interactions with various IgG-binding proteins. Given its versatile functionality, IgG has recently been used for therapeutic interventions. Evidence indicates that the carbohydrate moieties of IgG glycoproteins critically affect their antibody functions, particularly the effector functions mediated by the interactions with Fcγ receptors (FcγRs). N-glycans at specific positions of FcγR also contribute both positively and negatively to the interactions with IgG. The integration of multilateral biophysical approaches, including X-ray crystallography, nuclear magnetic resonance spectroscopy, and molecular dynamics simulations, has provided structural insights into the mechanisms underlying the glycofunctions of this interacting system. The N-glycans of IgG and FcγR mediate their interactions by either strengthening or weakening the affinity on the basis of their glycoforms. Moreover, the N-glycosylation of IgG-Fc is a prerequisite to maintain the integrity of the quaternary structure of the sites interacting with the effector molecules and can also control functionally relevant local conformations. The biopharmaceutical significance of these glycan functions is discussed from a structural point of view.

NMR Detection of Semi-Specific Antibody Interactions in Serum Environments

Although antibody functions are executed in heterogeneous blood streams characterized by molecular crowding and promiscuous intermolecular interaction, detailed structural characterizations of antibody interactions have thus far been performed under homogeneous in vitro conditions. NMR spectroscopy potentially has the ability to study protein structures in heterogeneous environments, assuming that the target protein can be labeled with NMR-active isotopes. Based on our successful development of isotope labeling of antibody glycoproteins, here we apply NMR spectroscopy to characterize antibody interactions in heterogeneous extracellular environments using mouse IgG-Fc as a test molecule. In human serum, many of the HSQC peaks originating from the Fc backbone exhibited attenuation in intensity of various magnitudes. Similar spectral changes were induced by the Fab fragment of polyclonal IgG isolated from the serum, but not by serum albumin, indicating that a subset of antibodies reactive with mouse IgG-Fc exists in human serum without preimmunization. The metaepitopes recognized by serum polyclonal IgG cover the entire molecular surface of Fc, including the binding sites to Fc receptors and C1q. In-serum NMR observation will offer useful tools for the detailed characterization of biopharamaceuticals, including therapeutic antibodies in physiologically relevant heterogeneous environments, also giving deeper insight into molecular recognition by polyclonal antibodies in the immune system.