Detection of prion protein using a capillary electrophoresis-based competitive immunoassay with laser-induced fluorescence detection and cyclodextrin-aided separation

Capillary electrophoresis-based immunoassay and aptamer assay: A review

Over the last two decades, the group of techniques called affinity probe CE has been widely used for the detection and the determination of several types of biomolecules with high sensitivity. These techniques combine the low sample consumption and high separation power of CE with the selectivity of the probe to the target molecule. The assays can be defined according to the type of probe used: CE immunoassays, with an antibody as the probe, or aptamer-based CE, with an aptamer as the probe. Immunoassays are generally divided into homogeneous and heterogeneous groups, and homogeneous variant can be further performed in competitive or noncompetitive formats. Interacting partners are free in solution at homogeneous assay, as opposed to heterogeneous analyses, where one of them is immobilized onto a solid support. Highly sensitive fluorescence, chemiluminescence or electrochemical detections were typically used in this type of study. The use of the aptamers as probes has several advantages over antibodies such as shorter generation time, higher thermal stability, lower price, and lower variability. The aptamer-based CE technique was in practice utilized for the determination of proteins in biological fluids and environmentally or clinically important small molecules. Both techniques were also transferred to microchip. This review is focused on theoretical principles of these techniques and a summary of their applications in research.

Application of capillary immunoelectrophoresis revealed an age- and gender-dependent regulated expression of PrPC in liver

The cellular prion protein (PrPC) is a glycoprotein, anchored to the plasma membrane and abundantly expressed in the central nervous system. The expression of PrPC in the peripheral tissues is low and only little information is available on its functions in the nonneuronal tissues. The antioxidant function of PrPC during the activation of hepatic stellate cells has already been reported. Therefore, the aim of the study was to expand our knowledge on the functions of PrPC by detailed characterization of its expressional profile in the liver. In a combined strategy by using capillary immunoelectrophoresis and standard techniques, we have shown a sexually dimorphic expression of PrPC in mice and human liver tissues. Further, we showed a significant age-dependent upregulation of PrPC expression in the liver of 14- and 9-month-old mice as compared to 3 months of age. Therefore, this study may provide new insights into the gender-specific role of PrPC in the liver, which may further be linked to its protective role against oxidative stress during aging. In addition, the current study also shows an application of immunoelectrophoresis with a low coefficient of variation to analyze the miniscule amount of PrPC in the mouse liver tissue.

Clinical applications of capillary electrophoresis based immunoassays

Immunoassays have long been an important set of tools in clinical laboratories for the detection, diagnosis, and treatment of disease. Over the last two decades, there has been growing interest in utilizing CE as a means for conducting immunoassays with clinical samples. The resulting method is known as a CE immunoassay. This approach makes use of the selective and strong binding of antibodies for their targets, as is employed in a traditional immunoassay, and combines this with the speed, efficiency, and small sample requirements of CE. This review discusses the variety of ways in which CE immunoassays have been employed with clinical samples. An overview of the formats and detection modes that have been employed in these applications is first presented. A more detailed discussion is then given on the type of clinical targets and samples that have been measured or studied by using CE immunoassays. Particular attention is given to the use of this method in the fields of endocrinology, pharmaceutical measurements, protein and peptide analysis, immunology, infectious disease detection, and oncology. Representative applications in each of these areas are described, with these examples involving work with both traditional and microanalytical CE systems.

Quantum dots and prion proteins: is this a new challenge for neurodegenerative diseases imaging?

A diagnostics of infectious diseases can be done by the immunologic methods or by the amplification of nucleic acid specific to contagious agent using polymerase chain reaction. However, in transmissible spongiform encephalopathies, the infectious agent, prion protein (PrP(Sc)), has the same sequence of nucleic acids as a naturally occurring protein. The other issue with the diagnosing based on the PrP(Sc) detection is that the pathological form of prion protein is abundant only at late stages of the disease in a brain. Therefore, the diagnostics of prion protein caused diseases represent a sort of challenges as that hosts can incubate infectious prion proteins for many months or even years. Therefore, new in vivo assays for detection of prion proteins and for diagnosis of their relation to neurodegenerative diseases are summarized. Their applicability and future prospects in this field are discussed with particular aim at using quantum dots as fluorescent labels.

Contribution of CE to the analysis of protein or peptide biomarkers

Biomarker analysis is pivotal for disease diagnosis and one important class of biomarkers is constituted by proteins and peptides. This review focuses on protein and peptide analyses from biological fluids performed by capillary electrophoresis. The various strategies that have been reported to prevent difficulties due to the handling of real samples are described. Innovative techniques to overcome the complexity of the sample, to prevent the adsorption of the analytes on the inner capillary wall, and to increase the sensibility of the analysis are summarized and illustrated by different applications. To fully illustrate the contribution of CE to the analysis of biomarkers from human sample, two detailed protocols are given: the analysis from CSF of five amyloid peptide, biomarkers of the Alzheimer disease, and the analysis of sialoforms of transferrin from human serum.

Capillary electrophoresis for the analysis of contaminants in emerging food safety issues and food traceability

This review presents an overview of the applicability of CE in the analysis of chemical and biological contaminants involved in emerging food safety issues. Additionally, CE-based genetic analyzers' usefulness as a unique tool in food traceability verification systems was presented. First, analytical approaches for the determination of melamine and specific food allergens in different foods were discussed. Second, natural toxin analysis by CE was updated from the last review reported in 2008. Finally, the analysis of prion proteins associated with the "mad cow" crises and the application of CE-based genetic analyzers for meat traceability were summarized.

Capillary electrophoresis-based immunoassays: principles and quantitative applications

The use of CE as a tool to conduct immunoassays has been an area of increasing interest over the last decade. This approach combines the efficiency, small sample requirements, and relatively high speed of CE with the selectivity of antibodies as binding agents. This review examines the various assay formats and detection modes that have been reported for these assays, along with some representative applications. Most CE immunoassays in the past have employed homogeneous methods in which the sample and reagents are allowed to react in solution. These homogeneous methods have been conducted as both competitive binding immunoassays and as noncompetitive binding immunoassays. Fluorescent labels are most commonly used for detection in these assays, but enzyme labels have also been utilized for such work. Some additional work has been performed in CE immunoassays with heterogeneous methods in which either antibodies or an analog of the analyte is immobilized to a solid support. These heterogeneous methods can be used for the selective isolation of analytes prior to their separation by CE or to remove a given species from a sample/reagent mixture prior to analysis by CE. These CE immunoassays can be used with a variety of detection modes, such as fluorescence, UV/Vis absorbance, chemiluminescence, electrochemical measurements, MS, and surface plasmon resonance.

Competitive affinity capillary electrophoresis assay based on a "hybrid" pre-incubation/on-capillary mixing format using an enantioselective aptamer as affinity ligand

In this paper, we describe an aptamer-based competitive affinity CE (ACE) assay involving (i) the pre-incubation of the target (D-arginine) and the specific ligand (anti-D-arginine-L-RNA aptamer) before (ii) the competition with the labeled target (dansylated D-arginine) through an on-capillary mixing strategy. The effects of some critical operating parameters such as the applied voltage and the sample-aptamer mixture plug length on the assay sensitivity were investigated. The ACE assay appeared particularly dependent on the plug length of the pre-incubated sample-aptamer solution. It was shown that this "hybrid" strategy significantly improved the assay sensitivity relative to that obtained with a "full" on-capillary mixing approach.

Capillary electrophoresis for the characterization of quantum dots after non-selective or selective bioconjugation with antibodies for immunoassay

Capillary electrophoresis coupled with laser-induced fluorescence was used for the characterization of quantum dots and their conjugates to biological molecules. The CE-LIF was laboratory-built and capable of injection (hydrodynamic and electrokinetic) from sample volumes as low as 4 μL via the use of a modified micro-fluidic chip platform. Commercially available quantum dots were bioconjugated to proteins and immunoglobulins through the use of established techniques (non-selective and selective). Non-selective techniques involved the use of EDCHCl/sulfo-NHS for the conjugation of BSA and myoglobin to carboxylic acid-functionalized quantum dots. Selective techniques involved 1) the use of heterobifunctional crosslinker, sulfo-SMCC, for the conjugation of partially reduced IgG to amine-functionalized quantum dots, and 2) the conjugation of periodate-oxidized IgGs to hydrazide-functionalized quantum dots. The migration times of these conjugates were determined in comparison to their non-conjugated QD relatives based upon their charge-to-size ratio values. The performance of capillary electrophoresis in characterizing immunoconjugates of quantum dot-labeled IgGs was also evaluated. Together, both QDs and CE-LIF can be applied as a sensitive technique for the detection of biological molecules. This work will contribute to the advancements in applying nanotechnology for molecular diagnosis in medical field.

Characterization of quantum dots using capillary zone electrophoresis

Commercially available quantum dots (QDs) were characterized using CE. The CE instruments were laboratory-built, each being capable of both electrokinetic and hydrodynamic injection. Modes of detection include UV absorption and LIF. The CE-LIF system was further modified to handle microliter sample volumes during injection. Sodium phosphate (5-25 mM, pH 7.5-11) was found to be a good buffer electrolyte. Sodium mercaptoproprionate CdTe/CdS (ADS620) QDs and carboxylic acid CdSe/ZnS (T2-Evitag) QDs yielded high separation efficiencies of N = 1.5x10(6) plates at t(M) = 10 min and N = 1.0x10(5) plates at t(M) = 3.8 min, respectively. Apparently the EDC/sulfo-NHS bioconjugation chemistry worked well with the neutral T2-Evitag QDs, but not so well with the negatively charged ADS620 QDs. This preliminary knowledge will serve as a basis for new CE immunoassay studies of QD-biomolecule conjugates and their immunocomplexes with target analytes.

Immunoassay by capillary electrophoresis with quantum dots

The application of quantum dots in capillary electrophoresis immunoassay was studied for the first time. Quantum dots were conjugated with antibody and subsequently tested by electrophoretic separation of free antibody and antibody-antigen complex. Antibody was fluorescently labeled by quantum dots via conjugation procedures and its electrophoretic characteristics were effectively modified due to the attachment of quantum dots. The determination of human IgM by direct CE based immunoassay could be easily achieved by simply changing the pH value of separation buffer. Polymer additive influenced the separation too but the effect was not as significant as buffer pH adjustment. Satisfactory separation of complex from free antibody could be achieved with 20mM sodium tetraborate as separation buffer, at pH 9.8. The immunoassay application of quantum dots in CE offers considerable advantages and can be readily applied to other large bio-molecules.

Aptamer-based enantioselective competitive binding assay for the trace enantiomer detection

The development of highly enantioselective assays and sensors has received much attention for the determination of enantiomeric impurities at a low level. For chiral compounds, the efficient monitoring of the in selection procedure has allowed the isolation of nucleic acid aptamers which are able to strongly discriminate the target enantiomers. In this paper, we demonstrated for the first time that an aptamer can be successfully used to design a highly enantioselective tool for the trace enantiomer detection. The aptamer-based stereoselective assay was developed using an affinity capillary electrophoresis-based competitive, homogeneous format and an on-capillary mixing approach. Detection of as low as 0.01% of the minor enantiomer in a nonracemic mixture can be achieved, in a short analysis time (<5 min).

Non-competitive immunoassay for luteinizing hormone in human serum using capillary electrophoresis with chemiluminescence detection

A non-competitive immunoassay based on capillary electrophoresis (CE) with chemiluminescence (CL) detection has been developed for the determination of luteinizing hormone (LH) in human serum. The work involved the development of separation and CL conditions, allowing for routine analysis of serum samples. In this study, horseradish peroxidase (HRP)-labelled monoclonal anti-LH can catalyse the luminol-hydrogen peroxide reaction. The determined LH can react with excessive amount of HRP-labelled anti-LH. Within 14 min, free enzyme conjugate and immune complex could be separated in alkaline borate buffer by means of a high voltage (15 kV). To improve sensitivity, a series of measures were adopted, including the choice of para-iodophenol as a CL enhancer, unique design in detect window. Under the optimal conditions, the calibration curve for LH was established in the concentration range 1-200 mIU/mL and the detection limit was 0.08 mIU/mL. Compared with ELISA, this method decreased the detection limit by about 12 times, and it has been successfully employed in the determination of LH in human serum.

A microfluidic flow-through device for high throughput electrical lysis of bacterial cells based on continuous dc voltage

Interest in electrical lysis of biological cells on a microfludic platform has increased because it allows for the rapid recovery of intracellular contents without introducing lytic agents. In this study we demonstrated a simple microfluidic flow-through device which lysed Escherichia coli cells under a continuous dc voltage. The E. coli cells had previously been modified to express green fluorescent protein (GFP). In our design, the cell lysis only happened in a defined section of a microfluidic channel due to the local field amplification by geometric modification. The geometric modification also effectively decreased the required voltage for lysis by several folds. We found that local field strength of 1000-1500 V/cm was required for nearly 100% cell death. This threshold field strength was considerably lower than the value reported in the literature, possibly due to the longer duration of the field [Lee, S.W., Tai, Y.C., 1999. Sens. Actuators A: Phys. 73, 74-79]. Cell lysis was detected by both plate count and fluorescence spectroscopy. The cell membrane was completely disintegrated in the lysis section of the microfluidic device, when the field strength was higher than 2000 V/cm. The devices were fabricated using low-cost soft lithography with channel widths considerably larger than the cell size to avoid clogging and ensure stable performance. Our tool will be ideal for high throughput processing of bacterial cells for chemical analysis of intracellular contents such as DNA and proteins. The application of continuous dc voltage greatly simplified the instrumentation compared to devices using electrical pulses for similar purposes. In principle, the same approach can also be applied for lysis of mammalian cells and electroporative transfection.

Detection of preclinical scrapie from serum by infrared spectroscopy and chemometrics

In this study we describe a methodology for diagnosing preclinical scrapie infection in hamsters from serum by a combination of Fourier-transform infrared (FT-IR) spectroscopy and chemometrics. Syrian hamsters (Mesocricetus auratus) were orally inoculated with the 263K scrapie agent, or mock-infected, and sera were obtained at 70, 100 and 130 days post infection (dpi) and at the terminal stage of scrapie (160 +/- 10 dpi). The analysis of hamster sera by FT-IR spectroscopy and artificial neural networks (ANN) confirmed results from earlier studies which had indicated the existence of disease-related structural and compositional alterations in the sera of infected donors in the terminal stage of scrapie [Schmitt et al. (2002) Anal Chem 74:3865-3868]. Experimental data from sera of animals in the preclinical stages of scrapie revealed subtle but reproducible spectral variations that permitted the identification of a preclinical scrapie infection at 100 dpi and later, but not at 70 dpi. The IR spectral features that were discriminatory for the preclinical stages differed from those of the terminally ill individuals. In order to reliably identify scrapie-negative as well as preclinical (100 and 130 dpi) and terminal scrapie-positive animals, a hierarchical classification system of independent artificial neural networks (ANN) was established. A "toplevel" ANN was designed which discriminates between animals in the terminal stage of scrapie and preclinical scrapie-positive or control animals. Spectra identified by the "toplevel" ANN as preclinical or controls were then further analyzed by a second classifier, the "sublevel" ANN. Using independent external validation procedures, the toplevel classifier produced an overall classification accuracy of 98%, while the sublevel classifier yielded an accuracy of 93%, indicating that scrapie-specific serum markers were also present for preclinical disease stages. Possible spectral markers responsible for the discrimination capacity of the two different ANNs are discussed.

Directed Evolution of an Anti-prion Protein scFv Fragment to an Affinity of 1 pM and its Structural Interpretation

Bovine spongiform encephalopathy (BSE) is a fatal neurodegenerative prion disease affecting cattle that is transmissible to humans, manifesting as a variant of Creutzfeldt-Jakob disease (vCJD) likely following the consumption of meat contaminated with BSE prions. High-affinity antibodies are a prerequisite for the development of simple, highly sensitive and non-invasive diagnostic tests that are able to detect even small amounts of the disease-associated PrP conformer (PrP(Sc)). We describe here the affinity maturation of a single-chain Fv antibody fragment with a binding affinity of 1 pM to a peptide derived from the unstructured region of bovine PrP (BoPrP (90-105)). This is the tightest peptide-binding antibody reported to date and may find useful application in diagnostics, especially when PrP(Sc) is pretreated by denaturation and/or proteolysis for peptide-like presentation. Several rounds of directed evolution and off-rate selection with ribosome display were performed using an antibody library generated from a single PrP binder with error-prone PCR and DNA-shuffling. As the correct determinations of affinities in this range are not straightforward, competition biosensor techniques and KinExA methods were both applied and compared. Structural interpretation of the affinity improvement was performed based on the crystal structure of the original prion binder in complex with the BoPrP (95-104) peptide by modeling the corresponding mutations.

Application of an immunocapillary electrophoresis assay to the detection of abnormal prion protein in brain, spleen and blood specimens from patients with variant Creutzfeldt-Jakob disease

Sensitive and specific detection of abnormal prion protein in blood could provide a diagnostic test or screening assay for animal and human prion diseases. Here, the application of an immunocapillary electrophoresis (ICE) method developed for sheep scrapie to brain, spleen and blood from patients with Creutzfeldt-Jakob disease (CJD) is described. The assay involves organic-solvent extraction, a competitive immunoassay using fluorescently labelled synthetic prion protein peptides and polyclonal antibodies specific for those sequences, and analysis by capillary electrophoresis using laser-induced fluorescence detection. The test was evaluated by using clinical blood specimens from patients with variant (n=5) or sporadic (n=4) CJD and patients initially suspected of having CJD who were given an alternative diagnosis (n=6). In this context, the ICE assay was specific, but incompletely sensitive (55%). The method was unable to detect abnormal prion protein in variant CJD brain or spleen reference materials due to its loss during the extraction process.

Capillary electrophoresis at the omics level: Towards systems biology

Emerging systems biology aims at integrating the enormous amount of existing omics data in order to better understand their functional relationships at a whole systems level. These huge datasets can be obtained through advances in high-throughput, sensitive, precise, and accurate analytical instrumentation and technological innovation. Separation sciences play an important role in revealing biological processes at various omic levels. From the perspective of systems biology, CE is a strong candidate for high-throughput, sensitive data generation which is capable of tackling the challenges in acquiring qualitative and quantitative knowledge through a system-level study. This review focuses on the applicability of CE to systems-based analytical data at the genomic, transcriptomic, proteomic, and metabolomic levels.

Confirmation of peak assignments in capillary electrophoresis using immunoprecipitation. Application to D-aspartate measurements in neurons

Capillary electrophoresis (CE) with laser-induced fluorescence (LIF) detection is a powerful tool for analysis of samples ranging from tissue extracts to single cells. However, accurate peak identification in electropherograms is challenging when complex biological samples are analyzed, as often matching a migration time between an analyte and corresponding standard may be insufficient to confirm the peak's identity. A method which combines single-step immunoprecipitation and CE-LIF analysis for investigation of the chiral amino acids in single cells and small tissue samples is demonstrated. D-Aspartate (D-Asp) has been reported in the central nervous system of the invertebrate neurobiological model Aplysia californica. In order to confirm the identity of D-Asp signal in the complex electropherograms of nerve tissue extracts and individual neurons, anti-D-Asp serum, preincubated with L-Asp conjugate, is added to the sample. This selectively binds the free D-Asp, creating an antibody-antigen complex with a migration time similar to that of antibody alone, but not that of D-Asp. The complete disappearance of the putative D-Asp peak confirms its identity and validates that there are no other detectable analytes co-migrating with D-Asp in the electropherogram.

Blood infectivity, processing and screening tests in transmissible spongiform encephalopathy

Surprising advances have been made in the areas of blood infectivity, infectivity removal and, especially, blood screening tests for transmissible spongiform encephalopathy (TSE) in the past few years. In fact, if anyone as recently as last year had suggested that a screening test for preclinical human infection might be available before the end of 2005, the statement would have been met with smiling disbelief. Nevertheless, it can be confidently predicted that the diagnostic misfolded 'prion' protein (PrP(TSE)) will soon be reliably detectable in blood during the symptomatic phase of disease, and it is highly probable that it will also be detectable in blood from at least a proportion of infected individuals during the preclinical phase of disease.