Real-time detection of the chemokine CXCL12 in urine samples by surface plasmon resonance

Direct Binding Methods to Measure Receptor-Ligand Interactions

G-protein-coupled receptors (GPCRs) contribute to numerous physiological processes via complex network mechanisms. While indirect signaling assays (Ca2+ mobilization, cAMP production, and GTPγS binding) have been useful in identifying and characterizing downstream signaling mechanisms of GPCRs, these methods lack measurements of direct binding affinities, kinetics, binding specificity, and selectivity that are important parameters in GPCR drug discovery. In comparison to existing direct methods that use radio- or fluorescent labels, label-free techniques can closely emulate the native interactions around binding partners. Surface plasmon resonance (SPR) is a label-free technique that utilizes the refractive index (RI) property and is applied widely in quantitative GPCR-ligand binding kinetics measurement including small molecules screening. However, purified GPCRs are further embedded in a synthetic lipid environment which is immobilized through different tags to the SPR sensor surface, resulting in a non-native environment. Here, we introduced a methodology that also uses the RI property to measure binding interactions in a label-free, immobilization-free arrangement. The free-solution technique is successfully applied in quantifying the interaction of bioactive lipids to cognate lipid GPCRs, which is not purified but rather present in near-native conditions, i.e., in milieu of other cytoplasmic lipids and proteins. To further consider the wide applicability of these free-solution approaches in biomolecular interaction research, additional applications on a variety of receptor-ligand pairs are imperative.

Biodetection Techniques for Quantification of Chemokines

Chemokines are a class of cytokine whose special properties, together with their involvement and relevant role in various diseases, make them a restricted group of biomarkers suitable for diagnosis and monitoring. Despite their importance, biodetection techniques dedicated to the selective determination of one or more chemokines are very scarce. For some years now, the critical diagnosis of inflammatory diseases by detecting both cytokine and chemokine biomarkers, has had a strong impact on the development of multiple detection platforms. However, it would be desirable to implement methodologies with a higher degree of selectivity for chemokines, in order to provide more precise information. In addition, better development of biosensor technology applied to this specific field would make it possible to address the main challenges of detection methods for several diseases with a high incidence in the population, avoiding high costs and low sensitivity. Taking this into account, this review aims to present the state of the art of chemokine biodetection techniques and emphasize the role of these systems in the prevention, monitoring and treatment of various diseases associated with chemokines as a starting point for future developments that are also analyzed throughout the article.

Recent Advances in Antifouling Materials for Surface Plasmon Resonance Biosensing in Clinical Diagnostics and Food Safety

Strategies to develop antifouling surface coatings are crucial for surface plasmon resonance (SPR) sensing in many analytical application fields, such as detecting human disease biomarkers for clinical diagnostics and monitoring foodborne pathogens and toxins involved in food quality control. In this review, firstly, we provide a brief discussion with considerations about the importance of adopting appropriate antifouling materials for achieving excellent performances in biosensing for food safety and clinical diagnosis. Secondly, a non-exhaustive landscape of polymeric layers is given in the context of surface modification and the mechanism of fouling resistance. Finally, we present an overview of some selected developments in SPR sensing, emphasizing applications of antifouling materials and progress to overcome the challenges related to the detection of targets in complex matrices relevant for diagnosis and food biosensing.

Development of a Novel SPR Assay to Study CXCR4-Ligand Interactions

G protein-coupled receptors (GPCRs) are involved in a plethora of different diseases. Consequently, these proteins are considered as an important class of drug targets. Measuring detailed kinetic information on these types of proteins has been challenging. Surface plasmon resonance (SPR) can provide this information, however, the use of SPR on GPCRs remains a complex issue. Here, we report an SPR assay to investigate the interactions between the full-length chemokine receptor CXCR4 and nanobody-Fc (Nb-Fc) ligands. Nb-Fcs consist of two monovalent VHH domains fused with an Fc domain of a human IgG molecule. The CXCR4 protein used in this assay was produced with a C-terminal 10x-histidine tag and was immobilized on a nitrilotriacetic acid chip. In order to verify the sensitivity and effectiveness of this assay, the results were compared to data obtained from cellular assays as well as from another SPR assay using CXCR4 virus-like particles (VLPs). CXCR4 remained intact and stable for at least 12 h, and the kinetic results correlated well with both the cellular assays and the VLP SPR assay results. Apart from determining the binding kinetics of Nb-Fc with CXCR4, our results contributed to understanding CXCR4 interaction dynamics. In conclusion, this assay provides a viable experimental platform that has high potential to be expanded for studying other molecules as well as other histidine-tagged GPCRs.

A compact SPR biosensor device for the rapid and efficient monitoring of gluten-free diet directly in human urine

Celiac disease (CD) is a chronic autoimmune disorder induced in genetically susceptible individuals by the ingestion of gluten from wheat, rye, barley, or certain varieties of oats. A careful diet follow-up is necessary to avoid health complications associated with long-term gluten intake by the celiac patients. Small peptides (GIP, gluten immunogenic peptides) derived from gluten digestion, which are excreted in the urine and feces, have emerged as promising biomarkers to monitor gluten intake. We have implemented a simple and sensitive label-free point-of-care (POC) device based on surface plasmon resonance for the direct detection of these biomarkers in urine. The assay employs specific monoclonal antibodies and has been optimized for the detection of the 33-mer α2-gliadin, known as the main immunogenic peptide of wheat gluten, and for the detection of GIP. Direct detection in undiluted urine has been accomplished by using biosensing chips containing a robust and stable biorecognition layer, obtained after carefully optimizing the biofunctionalization protocol. Excellent limits of detection have been reached (1.6-4.0 ng mL^-1 using mAb G12 and A1, respectively), which ensures the detection of gluten peptides even when the gluten intake is around the maximum tolerable amount in the digestive tract (< 50 mg) for celiac individuals. No sample pretreatment, extraction, or dilution is required, and the analysis takes less than 15 min. The assays have excellent reproducibility' as demonstrated by measuring spiked urine samples containing the same target concentration using different biofunctionalized chips prepared and stored at different periods of time (i.e., CV% of 3.58% and 11.30%, for G12- and A1-based assays, respectively). The assay has been validated with real samples. These features pave the way towards an end-user easy-to-handle biosensor device for the rapid monitoring of gluten-free diet (GFD) and follow-up of the health status in celiac patients.

CysLT1 receptor antagonist alleviates pathogenesis of collagen-induced arthritis mouse model

Cysteinyl leukotrienes (CysLTs) play a key role in inflammatory diseases such as asthma and their receptors’ antagonists are currently used as anti-asthmatic drugs. CysLTs have also been found to participate in other inflammatory reactions. Here, we reported that in rheumatoid arthritis (RA) animals model, collagen-induced arthritis, (CIA), CysLT1, a receptor for CysLTs, was up-regulated in hind paw and lymph node, while CysLTs levels in the blood were also higher than normal mice. Montelukast, a drug targeting CysLT1, has been shown to effectively reduce the CIA incidence, peak severity, and cumulative disease scores. Further study indicated that CysLT1 signaling did not affect the differentiation of pathogenic T helper cells. We conclude that montelukast may play important roles in the pathogenesis of CIA, mainly by inducing infiltration of pathogenic T cells, increasing IL-17A secretion and expression of IL-17A, while these effects can be blocked by CysLT1 antagonists. Our findings indicate that antagonist of CysLT1 receptor may be used to treat rheumatoid arthritis.

Surface Plasmon Resonance Clinical Biosensors for Medical Diagnostics

The design and application of sensors for monitoring biomolecules in clinical samples is a common goal of the sensing research community. Surface plasmon resonance (SPR) and other plasmonic techniques such as localized surface plasmon resonance (LSPR) and imaging SPR are reaching a maturity level sufficient for their application in monitoring biomolecules in clinical samples. In recent years, the first examples for monitoring antibodies, proteins, enzymes, drugs, small molecules, peptides, and nucleic acids in biofluids collected from patients afflicted with a series of medical conditions (Alzheimer's, hepatitis, diabetes, leukemia, and cancers such as prostate and breast cancers, among others) demonstrate the progress of SPR sensing in clinical chemistry. This Perspective reviews the current status of the field, showcasing a series of early successes in the application of SPR for clinical analysis and detailing a series of considerations regarding sensing schemes, exposing issues with analysis in biofluids, and comparing SPR with ELISA, while providing an outlook of the challenges currently associated with plasmonic materials, instrumentation, microfluidics, bioreceptor selection, selection of a clinical market, and validation of a clinical assay for applying SPR sensors to clinical samples. Research opportunities are proposed to further advance the field and transition SPR biosensors from research proof-of-concept stage to actual clinical applications.

Nanoyeast and Other Cell Envelope Compositions for Protein Studies and Biosensor Applications

Rapid progress in disease biomarker discovery has increased the need for robust detection technologies. In the past several years, the designs of many immunoaffinity reagents have focused on lowering costs and improving specificity while also promoting stability. Antibody fragments (scFvs) have long been displayed on the surface of yeast and phage libraries for selection; however, the stable production of such fragments presents challenges that hamper their widespread use in diagnostics. Membrane and cell wall proteins similarly suffer from stability problems when solubilized from their native environment. Recently, cell envelope compositions that maintain membrane proteins in native or native-like lipid environment to improve their stability have been developed. This cell envelope composition approach has now been adapted toward stabilizing antibody fragments by retaining their native cell wall environment. A new class of immunoaffinity reagents has been developed that maintains antibody fragment attachment to yeast cell wall. Herein, we review recent strategies that incorporate cell wall fragments with functional scFvs, which are designed for easy production while maintaining specificity and stability when in use with simple detection platforms. These cell wall based antibody fragments are globular in structure, and heterogeneous in size, with fragments ranging from tens to hundreds of nanometers in size. These fragments appear to retain activity once immobilized onto biosensor surfaces for the specific and sensitive detection of pathogen antigens. They can be quickly and economically generated from a yeast display library and stored lyophilized, at room temperature, for up to a year with little effect on stability. This new format of scFvs provides stability, in a simple and low-cost manner toward the use of scFvs in biosensor applications. The production and "panning" of such antibody cell wall composites are also extremely facile, enabling the rapid adoption of stable and inexpensive affinity reagents for emerging infectious threats.

Immunosensors for Biomarker Detection in Autoimmune Diseases

Autoimmune diseases occur when the immune system generates proinflammatory molecules and autoantibodies that mistakenly attack their own body. Traditional diagnosis of autoimmune disease is primarily based on physician assessment combined with core laboratory tests. However, these tests are not sensitive enough to detect early molecular events, and quite often, it is too late to control these autoimmune diseases and reverse tissue damage when conventional tests show positivity for disease. It is fortunate that during the past decade, research in nanotechnology has provided enormous opportunities for the development of ultrasensitive biosensors in detecting early biomarkers with high sensitivity. Biosensors consist of a biorecognition element and a transducer which are able to facilitate an accurate detection of proinflammatory molecules, autoantibodies and other disease-causing molecules. Apparently, novel biosensors could be superior to traditional metrics in assessing the drug efficacy in clinical trials, especially when specific biomarkers are indicative of the pathogenesis of disease. Furthermore, the portability of a biosensor enables the development of point-of-care devices. In this review, various types of biomolecule sensing systems, including electrochemical, optical and mechanical sensors, and their applications and future potentials in autoimmune disease treatment were discussed.

Chemokine Detection Using Receptors Immobilized on an SPR Sensor Surface

Chemokines and their receptors take part in many physiological and pathological processes, and their dysregulated expression is linked to chronic inflammatory and autoimmune diseases, immunodeficiencies, and cancer. The chemokine receptors, members of the G protein-coupled receptor family, are integral membrane proteins, with seven-transmembrane domains that bind the chemokines and transmit signals through GTP-binding proteins. Many assays used to study the structure, conformation, or activation mechanism of these receptors are based on ligand-binding measurement, as are techniques to detect new agonists and antagonists that modulate chemokine function. Such methods require labeling of the chemokine and/or its receptor, which can alter their binding characteristics. Surface plasmon resonance (SPR) is a powerful technique for analysis of the interaction between immobilized receptors and ligands in solution, in real time, and without labeling. SPR measurements nonetheless require expression and purification steps that can alter the conformation, stability, and function of the chemokine and/or the chemokine receptor. In this review, we focus on distinct methods to immobilize chemokine receptors on the surface of an optical biosensor. We expose the advantages and disadvantages of different protocols used and describe in detail the method to retain viral particles as receptor carriers that can be used for SPR determinations.

Equilibrium-fluctuation analysis for interaction studies between natural ligands and single G protein-coupled receptors in native lipid vesicles

G protein-coupled receptors (GPCRs) constitute the most versatile family of cell-membrane receptors and have been increasingly identified as important mediators of many physiological functions. They also belong to one of the most central drug target classes, but current screening technologies are limited by the requirements of overexpression and stabilization of GPCRs. This calls for sensitivity-increased detection strategies preferably meeting single-molecule detection limits. This challenge is here addressed by employing total internal reflection fluorescence microscopy to characterize the interaction kinetics between CXCR3, a GPCR involved in inflammatory responses, and two of its chemokine ligands, CXCL10 and CXCL11. Fluorescence labeling of the lipid membrane, rather than the membrane protein itself, of GPCR-containing native vesicles, and immobilization of the corresponding ligand on the surface, enabled determination of the interaction kinetics using single-molecule equilibrium-fluctuation analysis. With a limit of detection of GPCR-containing vesicles in the low picomolar concentration regime, the results demonstrate the possibility to use inhibition in solution screening of high affinity ligands/drug candidates, which due to target-binding depletion of the inhibiting compounds is demanding using assays with more moderate detection limits.

Methods to immobilize GPCR on the surface of SPR sensors

The G protein-coupled receptors (GPCRs) form one of the largest membrane receptor families. The nature of the ligands that interact with these receptors is highly diverse; they include light, peptides and hormones, neurotransmitters, and small molecular weight compounds. The GPCRs are involved in a wide variety of physiological processes and thus hold considerable therapeutic potential.GPCR function is usually determined in cell-based assays, whose complexity nonetheless limits their use. The use of alternative, cell-free assays is hampered by the difficulties in purifying these seven-transmembrane domain receptors without altering their functional properties. Several methods have been proposed to immobilize GPCR on biosensor surfaces which use antibodies or avidin-/biotin-based capture procedures, alone or with reconstitution of the GPCR physiological microenvironment. Here we propose a method for GPCR immobilization in their native membrane microenvironment that requires no manipulation of the target receptor and maintains the many conformations GPCR can adopt in the cell membrane.

Surface plasmon resonance: advances of label-free approaches in the analysis of biological samples

Surface plasmon resonance sensors have made vast advancements in the sensing technology and the number of applications achievable. New developments in surface plasmon resonance sensors have gained considerable momentum promoted by the urgent needs of fast, reliable and label-free methods for detection and quantification of analytes in molecular biology, medicine and other life sciences. However, even if enormous improvements in the limits of detections have been achieved, this technology still faces important challenges to be translated to clinical practice or in-field measurements. This paper reviews the important recent advances of this technology for the label-free detection in real biological samples and we discussed the key challenges to be overcome to transit from prototypes to commercial biosensors.

The effects of lipids and surfactants on TLR5-proteoliposome functionality for flagellin detection using surface plasmon resonance biosensing

The use of proteoliposomes as affinity elements in conjunction with a surface plasmon resonance sensor is a high-sensitivity alternative for the detection of multiple analytes. However, one of the most important aspects of these conformations is maintaining the functionality of the immobilized protein, which is determined by the choice of lipids and surfactants employed in the reconstitutions. Previously, we demonstrated the functionality of TLR5-proteoliposomes as screening affinity elements of bacterial flagellin. In this new study we change the conditions of immobilization of TLR5 and evaluate how the fluidity of the membrane and the final size of the liposomes affect the functionality of the construct and thus increase their utility as an affinity element for design of new biosensors. In particular, we used reconstructions into preformed liposomes composed of the lipids POPC, POPC-DMPC and POPC-POPE mediated by the use of surfactants OG, Triton X100, and DDM, respectively. The affinity results were evaluated by SPR technology proteoliposomes and were correlated with the anisotropic change in the membrane status; the final sizes of the proteoliposomes were estimated. Our results clearly show the dependence of fluidity and final size of the proteoliposomes with surface plasmon resonance affinity measurements.