In Situ Analysis of Membrane-Protein Binding Kinetics and Cell-Surface Adhesion Using Plasmonic Scattering Microscopy

[Progress in the analysis of hemolysis and coagulation models for interventional micro-axial flow blood pumps]

Interventional micro-axial flow blood pump is widely used as an effective treatment for patients with cardiogenic shock. Hemolysis and coagulation are vital concerns in the clinical application of interventional micro-axial flow pumps. This paper reviewed hemolysis and coagulation models for micro-axial flow blood pumps. Firstly, the structural characteristics of commercial interventional micro-axial flow blood pumps and issues related to clinical applications were introduced. Then the basic mechanisms of hemolysis and coagulation were used to study the factors affecting erythrocyte damage and platelet activation in interventional micro-axial flow blood pumps, focusing on the current models of hemolysis and coagulation on different scales (macroscopic, mesoscopic, and microscopic). Since models at different scales have different perspectives on the study of hemolysis and coagulation, a comprehensive analysis combined with multi-scale models is required to fully consider the influence of complex factors of interventional pumps on hemolysis and coagulation.

A fluorescence-based binding assay for proteins using the cell surface as a sensing platform

Protein-protein interaction (PPI) analysis is very important for elucidating the functions of proteins because many proteins execute their functions in living cells by interacting with one another. In PPI analysis, methods using the sensor chips are widely employed to obtain quantitative data. However, these methods require that the target proteins be immobilized on the sensor chips, and the immobilization processes can affect the binding of the target proteins to their binding partners. In the present work, we propose a PPI analysis system in which the surface of the living cells is utilized as a sensing platform. In our approach, the target protein is displayed on the cell surface by expressing it as a fusion protein with a membrane protein, and the PPI analysis is then conducted by applying its binding partner labeled with a fluorescent dye to the cell surface. We have constructed a model of this binding analysis system using the interaction between biotin protein ligase (BPL) and biotin carboxyl carrier protein (BCCP), where BCCP was displayed on the cell surface and BPL labeled with fluorescein was applied to the cell surface. Here, a red fluorescent protein, mApple, was attached to the C-terminus of the fusion protein of BCCP with a membrane protein. We evaluated the binding level of the labeled BPL by using the intensity ratios of fluorescence from fluorescein to that from mApple. We found that the binding level of the labeled BPL was stably evaluated at least across 60 min observation period and estimated the binding dissociation constant between BPL and BCCP by equilibrium analysis to be 0.33 ± 0.05 μM.

Label-Free Optical Imaging of Nanoscale Single Entities

The advancement of optical microscopy technologies has achieved imaging of nanoscale objects, including nanomaterials, virions, organelles, and biological molecules, at the single entity level. Recently developed plasmonic and scattering based optical microscopy technologies have enabled label-free imaging of single entities with high spatial and temporal resolutions. These label-free methods eliminate the complexity of sample labeling and minimize the perturbation of the analyte native state. Additionally, these imaging-based methods can noninvasively probe the dynamics and functions of single entities with sufficient throughput for heterogeneity analysis. This perspective will review label-free single entity imaging technologies and discuss their principles, applications, and key challenges.

Surface Plasmon Resonance Biosensors: A Review of Molecular Imaging with High Spatial Resolution

Surface plasmon resonance (SPR) is a powerful tool for determining molecular interactions quantitatively. SPR imaging (SPRi) further improves the throughput of SPR technology and provides the spatially resolved capability for observing the molecular interaction dynamics in detail. SPRi is becoming more and more popular in biological and chemical sensing and imaging. However, SPRi suffers from low spatial resolution due to the imperfect optical components and delocalized features of propagating surface plasmonic waves along the surface. Diverse kinds of approaches have been developed to improve the spatial resolution of SPRi, which have enormously impelled the development of the methodology and further extended its possible applications. In this minireview, we introduce the mechanisms for building a high-spatial-resolution SPRi system and present its experimental schemes from prism-coupled SPRi and SPR microscopy (SPRM) to surface plasmonic scattering microscopy (SPSM); summarize its exciting applications, including molecular interaction analysis, molecular imaging and profiling, tracking of single entities, and analysis of single cells; and discuss its challenges in recent decade as well as the promising future.

Ligand Binding-Induced Cellular Membrane Deformation is Correlated with the Changes in Membrane Stiffness

Study interaction between ligands and protein receptors is a key step for biomarker research and drug discovery. In situ measurement of cell surface membrane protein binding on whole cells eliminates the cost and pitfalls associated with membrane protein purification. Ligand binding to membrane protein was recently found to induce nanometer-scale cell membrane deformations, which can be monitored with real-time optical imaging to quantify ligand/protein binding kinetics. However, the insight into this phenomenon has still not been fully understood. We hypothesize that ligand binding can change membrane stiffness, which induces membrane deformation. To investigate this, cell height and membrane stiffness changes upon ligand binding are measured using atomic force microscopy (AFM). Wheat germ agglutinin (WGA) is used as a model ligand that binds to the cell surface glycoprotein. The changes in cell membrane stiffness and cell height upon ligand bindings are determined for three different cell lines (human A431, HeLa, and rat RBL-2H3) on two different substrates. AFM results show that cells become stiffer with increased height after WGA modification for all cases studied. The increase in cell membrane stiffness is further confirmed by plasmonic scattering microscopy, which shows an increased cell spring constant upon WGA binding. Therefore, this study provides direct experimental evidence that the membrane stiffness changes are directly correlated with ligand binding-induced cell membrane deformation.

Label-free analysis of membrane protein binding kinetics and cell adhesions using evanescent scattering microscopy

Measuring ligand interactions with membrane proteins in single live cells is critical for understanding many cellular processes and screening drugs. However, developing such a capability has been a difficult challenge. Here, we employ evanescent scattering microscopy (ESM) to show that ligand binding to membrane proteins can change the cell adhesion properties, which are intrinsic cell properties and independent of random cell micromotions and ligand mass, thus allowing the kinetics analyses of both proteins and small molecules binding to membrane proteins in both single fixed and live cells. In addition, utilizing the high spatiotemporal resolution of ESM, the positions of cell adhesion sites can be tracked in real-time to analyze the cell deformations and migrations, thus providing a potential approach for understanding the cell activity during the ligand binding process in detail. The presented method may pave the road for developing a versatile and easy-to-use label-free detection strategy for in situ analysis of molecular interaction dynamics in living biosystems with single-cell resolution.

Plasmonic Scattering Microscopy for Label-Free Imaging of Molecular Binding Kinetics: From Single Molecules to Single Cells

Measuring molecular binding kinetics represents one of the most important tasks in molecular interaction analysis. Surface plasmon resonance (SPR) is a popular tool for analyzing molecular binding. Plasmonic scattering microscopy (PSM) is a newly developed SPR imaging technology, which detects the out-of-plane scattering of surface plasmons by analytes and has pushed the detection limit of label-free SPR imaging down to a single-protein level. In addition, PSM also allows SPR imaging with high spatiotemporal resolution, making it possible to analyze cellular response to the molecular bindings. In this Mini Review, we present PSM as a method of choice for chemical and biological imaging, introduce its theoretical mechanism, present its experimental schemes, summarize its exciting applications, and discuss its challenges as well as the promising future.

Label-Free Optical Imaging of the Electron Transfer in Single Live Microbial Cells

Measurement of electron transfer at the single-particle or -cell level is crucial to the in situ study of basic chemical and biological processes. However, it remains challenging to directly probe the microbial extracellular electron transfer process due to the weakness of signals and the lack of techniques. Here, we present a label-free and noninvasive imaging method that is able to measure the electron transfer in microbial cells. We measured the extracellular electron transfer processes by imaging the redox reaction of c-type outer membrane cytochromes in microbial cells using a plasmonic imaging technique, and obtained the electrochemical activity parameters (formal potential and number of electrons transferred) of multiple individual microbial cells, allowing for unveiling ample heterogeneities in electron transfer at the single-cell level. We anticipate that this method will contribute to the study of electron transfer in various biological and chemical processes.

Label-free imaging and biomarker analysis of exosomes with plasmonic scattering microscopy

Exosome analysis is a promising tool for clinical and biological research applications. However, detection and biomarker quantification of exosomes is technically challenging because they are small and highly heterogeneous. Here, we report an optical approach for imaging exosomes and quantifying their protein markers without labels using plasmonic scattering microscopy (PSM). PSM can provide improved spatial resolution and distortion-free image compared to conventional surface plasmon resonance (SPR) microscopy, with the signal-to-noise ratio similar to objective coupled surface plasmon resonance (SPR) microscopy, and millimeter-scale field of view as a prism-coupled SPR system, thus allowing exosome size distribution analysis with high throughput. In addition, PSM retains the high specificity and surface sensitivity of the SPR sensors and thus allows selection of exosomes from extracellular vesicles with antibody-modified sensor surfaces and in situ analyzing binding kinetics between antibody and the surface protein biomarkers on the captured exosomes. Finally, the PSM can be easily constructed on a popular prism-coupled SPR system with commercially available components. Thus, it may provide an economical and powerful tool for clinical exosome analysis and exploration of fundamental issues such as exosome biomarker binding properties.