A simple detection method for low-affinity membrane protein interactions by baculoviral display

Expression and presentation of immune-related membrane proteins of fish by a cell surface display platform using insect cells

Cell surface display is a useful platform to examine the interactions between two proteins of interest, such as immune receptors and ligands. This technique is also useful for studies on the immune receptors of lower vertebrates and invertebrates. However, in many cases, the commonly used cell culture temperature is relatively high for proteins from such organisms. Since insect cells can be cultured at lower temperatures than many other cells, and since they are equipped with "quality control" system, which is advantageous for the presentation of properly folded proteins, we anticipated that the insect cell surface display system could be more suitable for that type of research. In the present study, multiple cloning site of the commercially available expression vector pIB/V5-His was modified, and whether this vector could be useful to present fish immune-related membrane proteins was investigated. Using this plasmid, fugu's CD8α and CC chemokine receptor 7 could be presented on the cell surface. The clones of the lamprey variable lymphocyte receptors obtained previously by the yeast surface display (YSD) system as hen's egg lysozyme (HEL) binders also could be presented on the cell surface and bound to HEL. These results suggest that functional immune-related membrane proteins can be presented on the insect cell surface, indicating that this system is useful for immunological studies on exothermal animals.

Kinetic exclusion assay of monoclonal antibody affinity to the membrane protein Roundabout 1 displayed on baculovirus

The reliable assessment of monoclonal antibody (mAb) affinity against membrane proteins in vivo is a major issue in the development of cancer therapeutics. We describe here a simple and highly sensitive method for the evaluation of mAbs against membrane proteins by means of a kinetic exclusion assay (KinExA) in combination with our previously developed membrane protein display system using budded baculovirus (BV). In our BV display system, the membrane proteins are displayed on the viral surface in their native form. The BVs on which the liver cancer antigen Roundabout 1 (Robo1) was displayed were adsorbed onto magnetic beads without fixative (BV beads). The dissociation constant (Kd, ∼10(-11) M) that was measured on the Robo1 expressed BV beads correlated well with the value from a whole cell assay (the coefficient of determination, R(2) = 0.998) but not with the value for the soluble extracellular domains of Robo1 (R(2) = 0.834). These results suggest that the BV-KinExA method described here provides a suitably accurate Kd evaluation of mAbs against proteins on the cell surface.

Baculovirus display for discovery of low-affinity extracellular receptor-ligand interactions using protein microarrays

When used in conjunction with multivalent protein probes, protein microarrays offer a robust technology for discovery of low-affinity extracellular protein-protein interactions. Probes for receptor-matching screens generally consist of purified extracellular domains fused to affinity tags. Given that approximately two-thirds of extracellular proteins are transmembrane domain-containing proteins, it would be desirable to develop a system to express and display probe receptors in a native-like membrane environment. Toward this end, we evaluated baculovirus display as a platform for generating multivalent probes for protein microarray screens. Virion particles were generated displaying single-transmembrane domain receptors BTLA, CD200, and EFNB2, representing a range of affinities for their interacting partners. Virions directly labeled with Cy5 fluorophore were screened against a microarray containing more than 600 extracellular proteins, and the results were compared with data derived from soluble Fc protein or probe-coated protein A microbeads. An optimized protocol employing a blocking step with a nonrelated probe-expressing control baculovirus allowed identification of the expected interactions with a signal-to-noise ratio similar to or higher than those obtained with the other formats. Our results demonstrate that baculovirus display is suitable for detection of high- and low-affinity extracellular protein-protein interactions on protein microarrays. This platform eliminates the need for protein purification and provides a native-like lipid environment for membrane-associated receptors.

Generation of antibodies against membrane proteins

The monoclonal antibody has become an important therapeutic in the treatment of both hematological malignancies and solid tumors. The recent success of antibody-drug conjugates (ADCs) has broadened the extent of the potential target molecules in cancer immunotherapy. As a result, even molecules of low abundance have become targets for cytotoxic reagents. The multi-pass membrane proteins are an emerging target for the next generation antibody therapeutics. One outstanding challenge is the difficulty in preparing a sufficient amount of these membrane proteins so as to be able to generate the functional antibody. We have pursued the expression of various membrane proteins on the baculovirus particle and the utilization of displayed protein for immunization. The strong antigenicity of the virus acts either as a friend or foe in the making of an efficient antibody against an immunologically tolerant antigen. This article is part of a Special Issue entitled: Recent advances in molecular engineering of antibody.

Improving T cell responses to modified peptides in tumor vaccines

Immune recognition and elimination of cancerous cells is the primary goal of cancer immunotherapy. However, obstacles including immune tolerance and tumor-induced immunosuppression often limit beneficial immune responses. Vaccination is one proposed intervention that may help to overcome these issues and is an active area of study in cancer immunotherapy. Immunizing with tumor antigenic peptides is a promising, straight-forward vaccine strategy hypothesized to boost preexisting antitumor immunity. However, tumor antigens are often weak T cell agonists, attributable to several mechanisms, including immune self-tolerance and poor immunogenicity of self-derived tumor peptides. One strategy for overcoming these mechanisms is vaccination with mimotopes, or peptide mimics of tumor antigens, which alter the antigen presentation and/or T cell activation to increase the expansion of tumor-specific T cells. Evaluation of mimotope vaccine strategies has revealed that even subtle alterations in peptide sequence can dramatically alter antigen presentation and T cell receptor recognition. Most of this research has been performed using T cell clones, which may not be accurate representations of the naturally occurring antitumor response. The relationship between clones generated after mimotope vaccination and the polyclonal T cell repertoire is unclear. Our work with mimotopes in a mouse model of colon carcinoma has revealed important insights into these issues. We propose that the identification of mimotopes based on stimulation of the naturally responding T cell repertoire will dramatically improve the efficacy of mimotope vaccination.

Protein microarrays, biosensors, and cell-based methods for secretome-wide extracellular protein-protein interaction mapping

Approximately one quarter of all human genes encode proteins that function in the extracellular space or serve to bridge the extracellular and intracellular environments. Physical associations between these secretome proteins serve to regulate a wide range of biological activities and consequently represent important therapeutic targets. Moreover, some extracellular proteins are targeted by pathogens to allow host access or immune evasion. Despite the importance of extracellular protein-protein interactions, our knowledge in this area has remained sparse. Weak affinities and low abundance have often hindered efforts to identify these interactions using traditional methods such as biochemical purification and cDNA library expression cloning. Moreover, current large-scale protein-protein interaction mapping techniques largely under represent extracellular protein-protein interactions. This review highlights emerging biosensor and protein microarray technology, along with more traditional cell-based techniques, that are compatible with secretome-wide screens for extracellular protein-protein interaction discovery. A combination of these approaches will serve to rapidly expand our knowledge of the extracellular protein-protein interactome.

Spiral progression in the development of absorption enhancers based on the biology of tight junctions

Epithelium covers the body and, therefore, separates the inner body from the outside environment. Passage across the epithelium is the first step in drug absorption. Tight junctions (TJs) seal the space between adjacent epithelial cells and prevent the free movement of solutes through the paracellular space. Modulation of the epithelial barrier is the most important strategy for enhancing drug absorption. Development of the strategy has accelerated with progress in understanding of the biology of the TJ seal. The first-generation absorption enhancers were screened on the basis of their absorption-enhancing activity in vivo. However, TJs were not well understood initially. The identification of TJ components, including those based on occludin and claudins, has led to the development of new strategies for drug absorption. Accumulation of knowledge of claudins has provided new insights into the paracellular transport of drugs. This review examines the relationship between advances in understanding of TJ biology and paracellular transport of drugs and discusses progress in the development of mucosal absorption enhancers.

Purification of functional baculovirus particles from silkworm larval hemolymph and their use as nanoparticles for the detection of human prorenin receptor (PRR) binding

Background

Baculovirus, which has a width of 40 nm and a length of 250-300 nm, can display functional peptides, receptors and antigens on its surface by their fusion with a baculovirus envelop protein, GP64. In addition, some transmembrane proteins can be displayed without GP64 fusion, using the native transmembrane domains of the baculovirus. We used this functionality to display human prorenin receptor fused with GFP_uv (GFP_uv-hPRR) on the surface of silkworm Bombyx mori nucleopolyhedrovirus (BmNPV) and then tested whether these baculovirus particles could be used to detect protein-protein interactions.

Results

BmNPV displaying GFP_uv-hPRR (BmNPV-GFP_uv-hPRR) was purified from hemolymph by using Sephacryl S-1000 column chromatography in the presence of 0.01% Triton X-100. Its recovery was 86% and the final baculovirus particles number was 4.98 × 10⁸ pfu. Based on the results of enzyme-linked immunosorbent assay (ELISA), 3.1% of the total proteins in BmNPV-GFP_uv-hPRR were GFP_uv-hPRR. This value was similar to that calculated from the result of western blot by a densitometry (2.7%). To determine whether BmNPV-GFP_uv-hPRR particles were bound to human prorenin, ELISA results were compared with those from ELISAs using protease negative BmNPV displaying β1,3-N-acetylglucosaminyltransferase 2 fused with the gene encoding GFP_uv (GGT2) (BmNPV-CP^--GGT2) particles, which do not display hPRR on their surfaces.

Conclusion

The display of on the surface of the BmNPV particles will be useful for the detection of protein-protein interactions and the screening of inhibitors and drugs in their roles as nanobioparticles.

A novel screening system for claudin binder using baculoviral display

Recent progress in cell biology has provided new insight into the claudin (CL) family of integral membrane proteins, which contains more than 20 members, as a target for pharmaceutical therapy. Few ligands for CL have been identified because it is difficult to prepare CL in an intact form. In the present study, we developed a method to screen for CL binders by using the budded baculovirus (BV) display system. CL4-displaying BV interacted with a CL4 binder, the C-terminal fragment of Clostridium perfringens enterotoxin (C-CPE), but it did not interact with C-CPE that was mutated in its CL4-binding region. C-CPE did not interact with BV and CL1-displaying BV. We used CL4-displaying BV to select CL4-binding phage in a mixture of a scFv-phage and C-CPE-phage. The percentage of C-CPE-phage in the phage mixture increased from 16.7% before selection to 92% after selection, indicating that CL-displaying BV may be useful for the selection of CL binders. We prepared a C-CPE phage library by mutating the functional amino acids. We screened the library for CL4 binders by affinity to CL4-displaying BV, and we found that the novel CL4 binders modulated the tight-junction barrier. These findings indicate that the CL-displaying BV system may be a promising method to produce a novel CL binder and modulator.

Hemagglutinin displayed baculovirus protects against highly pathogenic influenza

Baculovirus (BV) replicating in insect cells can express a foreign gene product as part of its genome. The influenza hemagglutinin (HA) can be expressed from BV and displayed on the surface of baculovirus (HA-DBV). In this study we first generated six recombinant baculoviruses that expressed chimeric HAs with segments of the BV glycoprotein (gp64). The signal peptide (SP) and cytoplasmic tail (CT) domains of gp64 can enhance the display of HA from A/PR8/34 on BV surface, while the transmembrane (TM) domain of gp64 impairs HA display. Different doses of either live or β-propiolactone (BPL)-inactivated HA-DBV were administered to BALB/c mice. Live HA-DBV elicited higher hemagglutination-inhibition (HAI) titers than BPL-inactivated HA-DBV, and provided sterilizing protection. A second generation recombinant BV simultaneously displaying four HAs derived from four subclades of H5N1 influenza viruses was constructed. This tetravalent H5N1 HA-DBV vaccine elicited HAI titers against all four homologous H5N1 viruses, significantly decreasing viral lung titers of challenged mice and providing 100% protection against lethal doses of homologous H5N1 viruses. Moreover, mice vaccinated with HA-DBV had high levels of IFNγ-secreting and HA-specific CD8+ T cells. Taken together, this study demonstrates that HA-DBV can stimulate strong humoral, as well as cellular immune responses, and is an effective vaccine candidate for influenza.

A novel tumor-targeted therapy using a claudin-4-targeting molecule

Carcinogenesis is often accompanied by dysfunctional tight junction (TJs), resulting in the loss of cellular polarity. Claudin, a tetra-transmembrane protein, plays a pivotal role in the barrier and fence functions of TJs. Claudin-4 is deregulated in various cancers, including breast, prostate, ovarian, and gastric cancer. Claudin-4 may be a promising target molecule for tumor therapy, but the claudin-targeting strategy has never been fully developed. In the present study, we prepared a claudin-4-targeting molecule by fusion of the C-terminal fragment of Clostridium perfringens enterotoxin (C-CPE) with the protein synthesis inhibitory factor (PSIF) derived from Pseudomonas aeruginosa exotoxin. PSIF was not cytotoxic to claudin-4-expressing cells, whereas C-CPE-PSIF was cytotoxic. Cells that express claudin-1, -2, and -5 were less sensitive to C-CPE-PSIF. Pretreatment of the cells with C-CPE attenuated C-CPE-PSIF-induced cytotoxicity, and mutation of C-CPE in the claudin-4-binding residues attenuated the cytotoxicity of C-CPE-PSIF. TJ-undeveloped cells were more sensitive to C-CPE-PSIF than TJ-developed cells. It is noteworthy that polarized epithelial cells are sensitive to C-CPE-PSIF applied to the basal side, whereas the cells were less sensitive to C-CPE-PSIF applied to the apical side. Intratumoral injection of C-CPE-PSIF reduced tumor growth. This is the first report to indicate that a claudin-4-targeting strategy may be a promising method to overcome the malignant tumors.