Highly sensitive determination of recombinant human erythropoietin-alpha in aptamer-based affinity probe capillary electrophoresis with laser-induced fluorescence detection

A comprehensive review of capillary electrophoresis-based techniques for erythropoietin isoforms analysis

Different CE techniques have been used to analyze erythropoietin. These techniques have been shown to be effective in differentiating and quantifying erythropoietin isoforms, including natural and recombinant origins. This review provides a comprehensive overview of various capillary electrophoresis-based techniques used for the analysis of erythropoietin isoforms. The importance of erythropoietin in clinical practice and the necessity for the accurate analysis of its isoforms are first discussed. Various techniques that have been used for erythropoietin isoform analysis are then described. The main body of the review focuses on the different capillary electrophoresis-based methods that have been developed for erythropoietin isoform analysis, including capillary zone electrophoresis and capillary isoelectric focusing. The advantages and drawbacks of each method as well as their applications are discussed. Suggestions into the future directions of the area are also described.

ELECTROCHEMICAL BIOSENSOR FOR ERYTHROPOIETIN DETECTION IN ATHLETES

ABSTRACT Introduction: The cytokine erythropoietin (EPO) is a crucial hormone for producing RBCs, which carry oxygenated blood to the rest of the body. Objective: This paper aimed to create an electrochemical detection based on Fe2O3-NiO nanoparticles and graphene oxide to measure EPO levels in athletes’ blood. Methods: On a glassy carbon electrode, Fe2O3-NiO@GO was synthesized using the electrochemical deposition method. Results: The Fe2O3-NiO@GO/GCE was validated by structural characterizations using scanning electron microscopy (SEM). The Fe2O3-NiO@GO/GCE was found to be a suitable and stable erythropoietin biosensor with a linear range of 0-500 ng/l and a detection limit of 0.03ng/l in electrochemical tests using the DPV technique. Fe2O3-NiO@GO/erythropoietin was investigated as a biosensor for erythropoietin in athlete's plasma. Conclusion: The results showed that the values obtained for recovery (94.56% to 98.40) and RSD (2.01% to 3.22%) were acceptable, indicating that the suggested technique can be used as a practical erythropoietin biosensor in blood samples. Level of evidence II; Therapeutic studies - investigation of treatment outcomes.

Enhanced light-emitting diode induced fluorescence detection system with capillary electrophoresis

An enhanced fluorescence detection system of capillary electrophoresis (CE) was equipped with a concave silver mirror, by which the detection sensitivity of light-emitting diode induced fluorescence (LEDIF) can be increased greatly. The silver concave mirror and the cathode window in photomultiplier tube (PMT) were accurately set face to face at the same axis. When the two labeled tumor markers exactly moved to the center of detection window, the emission from analytes are excitated by LED source. Currently, the analytes may be regarded as a luminescent source point. When the source point exactly moves to the focus of the concave mirror, the emission of the labeled sample was collected effectively, enhanced by convergence and reflected by the concave mirror. Then it was sensitively detected by the PMT. The optical mechanism of enhancing detection sensitivity was explored. A simple comparative test on sensitivity was carried out, which aimed to compare sensitivity of the new detection system with concave mirror to that without concave mirror but the other conditions were kept the same. Two tumor markers labeled with FITC were selected for the test, using the simple LEDIF detect system. The results (LOD, 150 nM for L-Leu and L-Val) showed that the detection sensitivity matched with concave mirror reached more 16 times than the detection method without concave mirror.

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 Electrophoresis with Laser-Induced Fluorescence to Immunoassays and Enzyme Assays

Capillary electrophoresis using laser-induced fluorescence detection (CE-LIF) is one of the most sensitive separation tools among electrical separation methods. The use of CE-LIF in immunoassays and enzyme assays has gained a reputation in recent years for its high detection sensitivity, short analysis time, and accurate quantification. Immunoassays are bioassay platforms that rely on binding reactions between an antigen (analyte) and a specific antibody. Enzyme assays measure enzymatic activity through quantitative analysis of substrates and products by the reaction of enzymes in purified enzyme or cell systems. These two category analyses play an important role in the context of biopharmaceutical analysis, clinical therapy, drug discovery, and diagnosis analysis. This review discusses the expanding portfolio of immune and enzyme assays using CE-LIF and focuses on the advantages and disadvantages of these methods over the ten years of existing technology since 2008.

Magnetic nanoparticles and polydopamine amplified FP aptasensor for the highly sensitive detection of rHuEPO-α

In this paper, an amplified fluorescence polarization (FP) aptasensor based on magnetic nanoparticles @polydopamine (MNP@PDA) was innovatively developed for sensitive detection of recombinant human erythropoietin-alpha(rHuEPO-α). The amplified FP signal was due to the large mass of protein and MNP@PDA. And this assay can be utilized for target separation or recycling based on the magnetic property of MNP@PDA through magnetic separation. Briefly, rHuEPO-α and MNP@PDA were added successively to react with the labeled aptamer (FAM-P1), which both contributed to the increase of FP signal via the formation of FAM-P1-rHuEPO-α and particularly FAM-P1-MNP@PDA complex. The strong interaction between MNP@PDA and FAM-P1 ensured the high efficiency of mass amplification and magnetic separation. As a result, the detection limit for rHuEPO-α was 0.12 pM, 4 orders of magnitude lower than original assay. Besides, three kinds of rHuEPO-α injections, NuPIAO, Epogen and ESPO were used to evaluate the selectivity of this assay in complex matrix with reasonable standard deviation. In a word, this work provides a simple, rapid, non-modified, highly sensitive and selective sensing platform for the detection of rHuEPO-α.

A Low-Cost Palmtop High-Speed Capillary Electrophoresis Bioanalyzer with Laser Induced Fluorescence Detection

In this work, we developed a miniaturized palmtop high-speed capillary electrophoresis (CE) system integrating whole modules, including picoliter-scale sample injection, short capillary-based fast CE, high-voltage power supply, orthogonal laser induced fluorescence (LIF) detection, battery, system control, on-line data acquisition, processing, storage, and display modules. A strategy of minimalist miniaturization combining minimal system design and low-cost system construction was adopted to achieve the instrument miniaturization with extremely low cost, which is differing from the current microfabrication strategy used in most reported miniaturized CE systems. With such a strategy, the total size of the bioanalyzer was minimized to 90 × 75 × 77 mm (length × width × height) and the instrument cost was reduced to ca. $500, which demonstrated the smallest and lowest-cost CE instrument with LIF detection in so far reported systems. The present bioanalyzer also exhibited comparable analytical performances to previously-reported high-speed CE systems. A limit of detection of 1.02 nM sodium fluorescein was obtained. Fast separations were achieved for multiple types of samples as amino acids, amino acid enantiomers, DNA fragments, and proteins with high efficiency. We applied this instrument in colorectal cancer diagnosis for detecting KRAS mutation status by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method.

Detection of human neutrophil elastase by aptamer affinity capillary electrophoresis coupled with laser-induced fluorescence using specified site fluorescently labeled aptamer

As a multifunctional serine protease, human neutrophil elastase (HNE) plays critical roles in a variety of physiopathological processes, such as acute lung injury, emphysema, atherosclerosis, and arthritis. The quantification of HNE is important in many applications. In this paper, we report an aptamer affinity capillary electrophoresis coupled with laser-induced fluorescence (CE-LIF) assay for detection of HNE using a tetramethylrhodamine (TMR)-labeled DNA aptamer probe. The affinity complex of HNE and DNA aptamer probe was well separated from the unbound aptamer probe in CE separation based on the difference of electrophoretic mobility. Broad complex peaks appeared due to possible multiple binding. The 45-mer aptamer having TMR labeling on the 40th T base was used as affinity probe, as larger complex peaks were obtained. We investigated the effects of various metal cations (Na⁺, K⁺, and Mg²⁺) in sample buffer on the binding of HNE and the aptamer in CE-LIF analysis. The presence of Na⁺, K⁺, or Mg²⁺ in sample buffer caused a decrease of complex peaks, and Mg²⁺ showed a larger effect. Under optimized conditions, this aptamer CE-LIF assay enabled the detection of HNE at 0.5 nM. This assay showed good specificity and allowed for detection of HNE spiked in diluted human serum sample. Graphical abstract The complex of HNE and DNA aptamer probe was isolated from the unbound aptamer probe in CE separation due to difference of electrophoretic mobility, allowing a CE-LIF assay for HNE.

Enhancing the Affinity of Anti-Human α-Thrombin 15-mer DNA Aptamer and Anti-Immunoglobulin E Aptamer by PolyT Extension

Aptamer affinity capillary electrophoresis-laser-induced fluorescence (CE-LIF) for protein detection takes advantage of aptamers for their ease of synthesis and labeling, small size, and having many negative charges. Its success relies on the high binding affinity of aptamers. One 15-mer DNA aptamer (5'-GGT TGG TGT GGT TGG-3', Apt15) shows desirable specificity for human α-thrombin, an important enzyme with multiple functions in blood. However, Apt15 has weak binding affinity, and the use of Apt15 in affinity CE-LIF analysis remains challenging. Here we reported that extension of Apt15 at the 3'-end with a polyT tail having length of 18 T or longer significantly enhanced its affinity and enabled a well-isolated and stable peak for thrombin-aptamer complex in affinity CE. It was likely that the improvement of binding affinity resulted from double binding, an additional interaction of the polyT tail with thrombin in addition to the Apt15 section binding to thrombin. With dye-labeled Apt15 having a T₂₅ tail, we achieved detection of thrombin at concentrations as low as 0.1 nM by affinity CE-LIF. This aptamer probe specifically bound to human α-thrombin, showing negligible affinity for human β- and γ-thrombin, which are proteolyzed derivatives of human alpha α-thrombin and share similar structure. This strategy of adding a polyT extension also enhanced the binding affinity of anti-immunoglobulin E aptamer in CE-LIF analysis, showing that the affinity enhancement approach is not limited to the thrombin-binding aptamer and has potential for more applications in bioanalysis.

Stepping Library-Based Post-SELEX Strategy Approaching to the Minimized Aptamer in SPR

When evolved from SELEX (systematic evolution of ligands by exponential enrichment), aptamers are generally about 70-130 nucleotides in length and needed to be effectively truncated for further diagnosis or therapeutic uses. Post-SELEX optimization is then aroused to simplify the aptamer sequence and improve the affinity property. In this work, we report a new post-SELEX strategy based on a stepping library for the first time. With a hypothesis that one nucleobase can influence the whole binding affinity through its adjacent base stacking and potential steric hydrogen bonding interaction, we designed a stepping library composed of all probable nucleotide truncation directions. We employed an aptamer 807-39nt toward EPO-α as a model, and surface plasmon resonance (SPR) as an efficient screening and evaluation method to optimize all label-free sequences in the library. We have successfully picked out In27 as the minimized aptamer from a mini library of only 35 sequences. Aptamer In27 has a sole loop, without the original stem portion of the initial aptamer, but retains the whole binding affinity. We have also defined the key nucleotide contribution by site mutagenesis with natural bases, and finally produced a degenerated sequence with higher or the same good affinities. Furthermore, we explored different binding behaviors between aptamer In27 and other recognition molecule such as agglutinin, monoclonal antibody, or receptor by competition or binding assays. Our work provides a new and efficient post-SELEX optimization strategy, as well as a minimized aptamer In27 with an explicit degenerated sequence and a defined binding behavior. That would enhance their great potential in future diagnosis and therapy.

Affinity capillary electrophoresis with laser induced fluorescence detection for thrombin analysis using nuclease-resistant RNA aptamers

Aptamer affinity capillary electrophoresis coupled with laser-induced fluorescence (CE-LIF) combines the advantages of affinity aptamer, rapid CE separation, and high sensitivity detection. Here we reported an affinity CE-LIF assay for thrombin by using a fluorophore-labeled RNA aptamer containing 2'-fluoro modification in sugar rings of pyrimidine nucleotides (C and U) as affinity ligand. This RNA aptamer has high binding affinity, specificity and biostability. Thrombin at 0.2nM was successfully detected. This RNA aptamer allowed for the detection of thrombin spiked in diluted human serum sample due to the nuclease resistance. The RNA aptamer has comparable binding affinity to a 29-mer DNA aptamer for thrombin, and the binding site of the RNA aptamer on thrombin partially overlaps with the binding site of the 29-mer DNA aptamer on thrombin. It shows the nuclease-resistant RNA aptamers are promising in assays for thrombin.

Aptamer based peptide enrichment for quantitative analysis of gonadotropin-releasing hormone by LC-MS/MS

Over recent years threats to racing have expanded to include naturally occurring biological molecules, such as peptides and proteins, and their synthetic analogues. Traditionally, antibodies have been used to enable detection of these compounds as they allow purification and concentration of the analyte of interest. The rapid expansion of peptide-based therapeutics necessitates a similarly rapid development of suitable antibodies or other means of enrichment. Potential alternative enrichment strategies include the use of aptamers, which offer the significant advantage of chemical synthesis once the nucleic acid sequence is known. A method was developed for the enrichment, detection and quantitation of gonadotropin-releasing hormone (GnRH) in equine urine using aptamer-based enrichment and LC-MS/MS. The method achieved comparable limits of detection (1 pg/mL) and quantification (2.5 pg/mL) to previously published antibody-based enrichment methods. The intra- and inter-assay precision achieved was less than 10% at both 5 and 20 pg/mL, and displayed a working dynamic range of 2.5-100 pg/mL. Significant matrix enhancement (170 ± 8%) and low analytical recovery (29 ± 15%) was observed, although the use of an isotopically heavy labelled GnRH peptide, GnRH (Pro(13)C5,(15)N), as the internal standard provides compensation for these parameters. Within the current limits of detection GnRH was detectable up to 1h post administration in urine and identification of a urinary catabolite extended this detection window to 4h. Based on the results of this preliminary investigation we propose the use of aptamers as a viable alternative to antibodies in the enrichment of peptide targets from equine urine.

Recent advances in affinity capillary electrophoresis for binding studies

The present review covers recent advances and important applications of affinity capillary electrophoresis (ACE). It provides an overview about various ACE types, including ACE-MS, the multiple injection mode, the use of microchips and field-amplified sample injection-ACE. The most common scenarios of the studied affinity interactions are protein-drug, protein-metal ion, protein-protein, protein-DNA, protein-carbohydrate, carbohydrate-drug, peptide-peptide, DNA-drug and antigen-antibody. Approaches for the improvements of ACE in term of precision, rinsing protocols and sensitivity are discussed. The combined use of computer simulation programs to support data evaluation is presented. In conclusion, the performance of ACE is compared with other techniques such as equilibrium dialysis, parallel artificial membrane permeability assay, high-performance affinity chromatography as well as surface plasmon resonance, ultraviolet, circular dichroism, nuclear magnetic resonance, Fourier transform infrared, fluorescence, MS and isothermal titration calorimetry.

Nanoscaled aptasensors for multi-analyte sensing

Introduction: Nanoscaled aptamers (Aps), as short single-stranded DNA or RNA oligonucleotides, are able to bind to their specific targets with high affinity, upon which they are considered as powerful diagnostic and analytical sensing tools (the so-called "aptasensors"). Aptamers are selected from a random pool of oligonucleotides through a procedure known as "systematic evolution of ligands by exponential enrichment".

Methods: In this work, the most recent studies in the field of aptasensors are reviewed and discussed with a main focus on the potential of aptasensors for the multianalyte detection(s).

Results: Due to the specific folding capability of aptamers in the presence of analyte, aptasensors have substantially successfully been exploited for the detection of a wide range of small and large molecules (e.g., drugs and their metabolites, toxins, and associated biomarkers in various diseases) at very low concentrations in the biological fluids/samples even in presence of interfering species.

Conclusion: Biological samples are generally considered as complexes in the real biological media. Hence, the development of aptasensors with capability to determine various targets simultaneously within a biological matrix seems to be our main challenge. To this end, integration of various key scientific dominions such as bioengineering and systems biology with biomedical researches are inevitable.

Capillary electrophoresis-chemiluminescence detection for carcino-embryonic antigen based on aptamer/graphene oxide structure

A new strategy is proposed for determination of carcino-embryonic antigen (CEA) based on aptamer/graphene oxide (Apt/GO) by capillary electrophoresis-chemiluminescence (CE-CL) detection system. CEA aptamer conjugated with horseradish peroxidase (HRP) firstly mixes with GO, and the CL will be quenched because the stack of HRP-Apt on GO leads to chemiluminescence resonance energy transfer (CRET). When CEA exists, the specific combination of HRP-Apt and CEA can form HRP-Apt-CEA complex, which dissociates from GO. Then, the CL catalyzed by HRP-Apt-CEA complex can be detected without any CRET, and the content of CEA can be estimated by the CL intensity. It has been proved that the interference issue resulted from free HRP-Apt is solved well by mixing GO firstly with HRP-Apt, which blocks the free HRP-Apt's CL signal due to CL quenching effect of GO; and the interference resulted from GO to CL is also solved by CE, then the sensitivity and accuracy can be greatly improved. Results also showed that the CL intensity had a linear relationship with the concentration of CEA in the range from 0.0654 to 6.54 ng/mL, and the limit of detection was approximately 4.8 pg/mL (S/N = 3). This proposed method with high specificity offers a new way for separation and determination of biomolecule, and has good potential in application of biochemistry and bioanalysis.

A conjugated aptamer-gold nanoparticle fluorescent probe for highly sensitive detection of rHuEPO-α

We present here a novel conjugated aptamer-gold nanoparticle (Apt-AuNPs) fluorescent probe and its application for specific detection of recombinant human erythropoietin-α (rHuEPO-α). In this nanobiosensor, 12 nm AuNPs function as both a nano-scaffold and a nano-quencher (fluorescent energy acceptor), on the surface of which the complementary sequences are linked (as cODN-AuNPs) and pre-hybridized with carboxymethylfluorescein (FAM)-labeled anti-rHuEPO-α aptamers. Upon target protein binding, the aptamers can be released from the AuNP surface and the fluorescence signal is restored. Key variables such as the length of linker, the hybridization site and length have been designed and optimized. Full performance evaluation including sensitivity, linear range and interference substances are also described. This nanobiosensor provides a promising approach for a simple and direct quantification of rHuEPO-α concentrations as low as 0.92 nM within a few hours.