Noncompetitive affinity assays of glucagon and amylin using mirror-image aptamers as affinity probes

A microfluidic system for monitoring glucagon secretion from human pancreatic islets of Langerhans

Glucagon is a 29-amino acid peptide released from α-cells within pancreatic islets of Langerhans to help raise blood glucose levels. While a plethora of methodologies have been developed for quantitative measurement of insulin released from islets, such methods are not well developed for glucagon despite its importance in blood sugar regulation. In this work, a simple yet robust microfluidic device was developed for holding human pancreatic islets and perfuse them with glucose. The perfusate was collected into 2 min fractions and glucagon quantified using a homogeneous time-resolved Förster resonance energy transfer (TR-FRET) sandwich immunoassay. Simulation of fluid flow within the microfluidic device indicated the device produced low amounts of shear stress on islets, and characterization of the flow with standard glucagon solutions revealed response times within 2 fractions (<4 min). Results with human islets from multiple donors demonstrated either a "burst" of glucagon or a "sustained" glucagon release across the entire period of stimulation. The simplicity, yet robustness, of the device and method is expected to appeal to a number of researchers examining pancreatic islet physiology.

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.

A microfluidic platform integrating pressure-driven and electroosmotic-driven flow with inline filters for affinity separations

Pancreatic islets of Langerhans release glucagon to maintain blood glucose levels, and release of this peptide is dysregulated in diabetes mellitus. Although the importance of proper secretion of this peptide has been shown, no measurement of its release at the single islet level has been reported. In previous work, a non-competitive assay for glucagon was developed with a 6 pM limit of detection, low enough to measure from a single islet. To incorporate this method in an online assay, a microfluidic system with several distinct features was developed. To maintain appropriate flow rates in the presence of the high concentration of salt that was required for the assay, a piezo-actuated pressure transducer with in-line flow sensors was used to drive sample flow through 80 × 50 μm (width × depth) channels, while electroosmotic flow was used to gate the sample away from 15 × 5 μm separation channel. Flow rates tested with this system were 50 - 200 nL min^-1 with relative standard deviations (RSDs) ranging from 1 - 4 %. Use of the pressure-driven flow was found to increase the amount of clogs in the system, so a method to incorporate in-line filters into the channels was developed. A total of 4 low resistance, in-line microfabricated filters were evaluated, with all designs prolonging the operation time of the microfluidic device to more than 4 hours without clogs observed. Use of this system enabled highly reproducible injections (3-6% RSD). During initial incorporation of the noncompetitive assay for glucagon, it was determined that Joule heating was problematic and temperature measurements revealed the separation channel increased to more than 50°C during operation. A 3D-printed manifold was used to hold a Peltier cooler in place on the microfluidic device which produced a 2.6-fold improvement in the amount of the noncovalent glucagon complex that was detected compared to without cooling. These features are expected to be useful for not only long-term monitoring of the glucagon release from islets of Langerhans, but has the potential to be applied to a number of other microfluidic separation-based assays as well.

Mirror-Image Oligonucleotides: History and Emerging Applications

As chiral molecules, naturally occurring d-oligonucleotides have enantiomers, l-DNA and l-RNA, which are comprised of l-(deoxy)ribose sugars. These mirror-image oligonucleotides have the same physical and chemical properties as that of their native d-counterparts, yet are highly orthogonal to the stereospecific environment of biology. Consequently, l-oligonucleotides are resistant to nuclease degradation and many of the off-target interactions that plague traditional d-oligonucleotide-based technologies; thus making them ideal for biomedical applications. Despite a flurry of interest during the early 1990s, the inability of d- and l-oligonucleotides to form contiguous Watson-Crick base pairs with each other has ultimately led to the perception that l-oligonucleotides have only limited utility. Recently, however, scientists have begun to uncover novel strategies to harness the bio-orthogonality of l-oligonucleotides, while overcoming (and even exploiting) their inability to Watson-Crick base pair with the natural polymer. Herein, a brief history of l-oligonucleotide research is presented and emerging l-oligonucleotide-based technologies, as well as their applications in research and therapy, are presented.

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.

Aptamer chemistry

Aptamers are single-stranded DNA or RNA molecules capable of tightly binding to specific targets. These functional nucleic acids are obtained by an in vitro Darwinian evolution method coined SELEX (Systematic Evolution of Ligands by EXponential enrichment). Compared to their proteinaceous counterparts, aptamers offer a number of advantages including a low immunogenicity, a relative ease of large-scale synthesis at affordable costs with little or no batch-to-batch variation, physical stability, and facile chemical modification. These alluring properties have propelled aptamers into the forefront of numerous practical applications such as the development of therapeutic and diagnostic agents as well as the construction of biosensing platforms. However, commercial success of aptamers still proceeds at a weak pace. The main factors responsible for this delay are the susceptibility of aptamers to degradation by nucleases, their rapid renal filtration, suboptimal thermal stability, and the lack of functional group diversity. Here, we describe the different chemical methods available to mitigate these shortcomings. Particularly, we describe the chemical post-SELEX processing of aptamers to include functional groups as well as the inclusion of modified nucleoside triphosphates into the SELEX protocol. These methods will be illustrated with successful examples of chemically modified aptamers used as drug delivery systems, in therapeutic applications, and as biosensing devices.

Affinity capillary electrophoresis for studying interactions in life sciences

Affinity capillary electrophoresis (ACE) analyzes noncovalent interactions between ligands and analytes based on changes in their electrophoretic mobility. This technique has been widely used to investigate various biomolecules, mainly proteins, polysaccharides and hormones. ACE is becoming a technique of choice to validate high throughput screening results, since it is very predictively working in realistic and relevant media, e.g. in body fluids. It is highly recommended to incorporate ACE as a powerful analytical tool to properly prepare animal testing and preclinical studies. The interacting molecules can be found free in solution or can be immobilized to a solid support. Thus, ACE is classified in two modes, free solution ACE and immobilized ACE. Every ACE mode has advantages and disadvantages. Each can be used for a variety of applications. This review covers literature of scopus and SciFinder data base in the period from 2016 until beginning 2018, including the keywords "affinity capillary electrophoresis", "immunoaffinity capillary electrophoresis", "immunoassay capillary electrophoresis" and "immunosorbent capillary electrophoresis". More than 200 articles have been found and 112 have been selected and thoroughly discussed. During this period, the data processing and the underlying calculations in mobility shift ACE (ms ACE), frontal analysis ACE (FA ACE) and plug-plug kinetic capillary electrophoresis (ppKCE) as mostly applied free solution techniques have substantially improved. The range of applications in diverse free solution and immobilized ACE techniques has been considerably broadened.

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.