Amperometric ATP microbiosensors for the analysis of chemosensitivity at rat carotid bodies

Cellular basis of learning and memory in the carotid body

The carotid body is the primary peripheral chemoreceptor in the body, and critical for respiration and cardiovascular adjustments during hypoxia. Yet considerable evidence now implicates the carotid body as a multimodal sensor, mediating the chemoreflexes of a wide range of physiological responses, including pH, temperature, and acidosis as well as hormonal, glucose and immune regulation. How does the carotid body detect and initiate appropriate physiological responses for these diverse stimuli? The answer to this may lie in the structure of the carotid body itself. We suggest that at an organ-level the carotid body is comparable to a miniature brain with compartmentalized discrete regions of clustered glomus cells defined by their neurotransmitter expression and receptor profiles, and with connectivity to defined reflex arcs that play a key role in initiating distinct physiological responses, similar in many ways to a switchboard that connects specific inputs to selective outputs. Similarly, within the central nervous system, specific physiological outcomes are co-ordinated, through signaling via distinct neuronal connectivity. As with the brain, we propose that highly organized cellular connectivity is critical for mediating co-ordinated outputs from the carotid body to a given stimulus. Moreover, it appears that the rudimentary components for synaptic plasticity, and learning and memory are conserved in the carotid body including the presence of glutamate and GABAergic systems, where evidence pinpoints that pathophysiology of common diseases of the carotid body may be linked to deviations in these processes. Several decades of research have contributed to our understanding of the central nervous system in health and disease, and we discuss that understanding the key processes involved in neuronal dysfunction and synaptic activity may be translated to the carotid body, offering new insights and avenues for therapeutic innovation.

Neurotransmitters responsible for purinergic motor neurotransmission and regulation of GI motility

Classical concepts of peripheral neurotransmission were insufficient to explain enteric inhibitory neurotransmission. Geoffrey Burnstock and colleagues developed the idea that ATP or a related purine satisfies the criteria for a neurotransmitter and serves as an enteric inhibitory neurotransmitter in GI muscles. Cloning of purinergic receptors and development of specific drugs and transgenic mice have shown that enteric inhibitory responses depend upon P2Y₁ receptors in post-junctional cells. The post-junctional cells that transduce purinergic neurotransmitters in the GI tract are PDGFRα⁺ cells and not smooth muscle cells (SMCs). PDGFRα⁺ cells express P2Y₁ receptors, are activated by enteric inhibitory nerve stimulation and generate Ca²⁺ oscillations, express small-conductance Ca²⁺-activated K⁺ channels (SK3), and generate outward currents when exposed to P2Y₁ agonists. These properties are consistent with post-junctional purinergic responses, and similar responses and effectors are not functional in SMCs. Refinements in methodologies to measure purines in tissue superfusates, such as high-performance liquid chromatography (HPLC) coupled with etheno-derivatization of purines and fluorescence detection, revealed that multiple purines are released during stimulation of intrinsic nerves. β-NAD⁺ and other purines, better satisfy criteria for the purinergic neurotransmitter than ATP. HPLC has also allowed better detection of purine metabolites, and coupled with isolation of specific types of post-junctional cells, has provided new concepts about deactivation of purine neurotransmitters. In spite of steady progress, many unknowns about purinergic neurotransmission remain and require additional investigation to understand this important regulatory mechanism in GI motility.

Biological insights from the direct measurement of purine release

Although purinergic signalling has been a well-established and accepted mechanism of chemical communication for many years, it remains important to measure the extracellular concentration of ATP and adenosine in real time. In this review I summarize the reasons why such measurements are still needed, how they provide additional mechanistic insight and give an overview of the techniques currently available to make spatially localised measurements of ATP and adenosine in real time. To illustrate the impact of direct real-time measurements, I explore CO2 and nutrient sensing in the medulla oblongata and hypothalamus. In both of these examples, the sensing involves hemichannel mediated ATP release from glial cells. For CO2 the hemichannels involved, connexin26, are directly CO2-sensitive. This mechanism contributes to the chemosensory control of breathing. In the hypothamalus, specialised glial cells, tanycytes, directly contact the cerebrospinal fluid in the 3rd ventricle and sense nutrients via sweet and umami taste receptors. Nutrient sensing by tanycytes is likely to contribute to the control of body weight as their selective stimulation alters food intake. To illustrate the importance of direct adenosine measurements, I consider the complex and multiple mechanisms of activity-dependent adenosine release in different brain regions. This activity dependent release of adenosine is likely to mediate important feedback regulation and may also be involved in controlling the sleep-wake state. I finish by briefly considering the potential of whole blood purine measurements in clinical practice.

Time-resolved ATP measurements during vesicle respiration

In vitro synthesis of ATP catalyzed by the ATP-synthase requires membrane vesicles, in which the ATP-synthase is present within the bilayer membrane. Inverted vesicle prepared from Gram negative cells (e.g., Escherichia coli or Pseudomonas putida) can be readily obtained and used for in vitro ATP-synthesis. Up to now, quantification of ATP synthesized by membrane vesicles has been mostly analyzed via bioluminescence-based assays. Alternatively, vesicle respiration and the associated ATP level can be determined using biosensors, which not only provide high selectivity, but allow ATP measurements without the sample being illuminated. Here, we present a microbiosensor for ATP in combination with scanning electrochemical microscopy (SECM) using an innovative two-compartment electrochemical cell for the determination of ATP levels at E.coli or P. putida inverted vesicles. For a protein concentration of 22 mg/ml, a total amount of 0.29 ± 0.03 μM/μl ATP per vesicle was determined in case of E.coli; in turn, P. putida derived vesicles yielded 0.48 ± 0.02 μM/μl ATP per vesicle at a total protein concentration of 25.2 mg/ml. Inhibition experiments with Venturicidin A clearly revealed that the respiratory chain enzyme complex responsible for ATP generation is effectively involved.

Highly Selective Cerebral ATP Assay Based on Micrometer Scale Ion Current Rectification at Polyimidazolium-Modified Micropipettes

Development of new principles and methods for cerebral ATP assay is highly imperative not only for determining ATP dynamics in brain but also for understanding physiological and pathological processes related to ATP. Herein, we for the first time demonstrate that micrometer scale ion current rectification (MICR) at a polyimidazolium brush-modified micropipette can be used as the signal transduction output for the cerebral ATP assay with a high selectivity. The rationale for ATP assay is essentially based on the competitive binding ability between positively charged polyimidazolium and ATP toward negatively charged ATP aptamer. The method is well responsive to ATP with a good linearity within a concentration range from 5 nM to 100 nM, and high selectivity toward ATP. These properties essentially enable the method to determine the cerebral ATP by combining in vivo microdialysis. The basal dialysate level of ATP in rat brain cortex is determined to be 11.32 ± 2.36 nM (n = 3). This study demonstrates that the MICR-based sensors could be potentially used for monitoring neurochemicals in cerebral systems.

Scanning Electrochemical Microscopy (SECM): Fundamentals and Applications in Life Sciences

In recent years, scanning electrochemical microscopy (SECM) has made significant contributions to the life sciences. Innovative developments focusing on high-resolution imaging, developing novel operation modes, and combining SECM with complementary optical or scanning probe techniques renders SECM an attractive analytical approach. This chapter gives an introduction to the essential instrumentation and operation principles of SECM for studying biologically-relevant systems. Particular emphasis is given to applications aimed at imaging the activity of biochemical constituents such as enzymes, antibodies, and DNA, which play a pivotal role in biomedical diagnostics. Furthermore, the unique advantages of SECM and combined techniques for studying live cells is highlighted by discussion of selected examples.

Dual recognition unit strategy improves the specificity of the adenosine triphosphate (ATP) aptamer biosensor for cerebral ATP assay

Adenosine triphosphate (ATP) aptamer has been widely used as a recognition unit for biosensor development; however, its relatively poor specificity toward ATP against adenosine-5'-diphosphate (ADP) and adenosine-5'-monophosphate (AMP) essentially limits the application of the biosensors in real systems, especially in the complex cerebral system. In this study, for the first time, we demonstrate a dual recognition unit strategy (DRUS) to construct a highly selective and sensitive ATP biosensor by combining the recognition ability of aptamer toward A nucleobase and of polyimidazolium toward phosphate. The biosensors are constructed by first confining the polyimidazolium onto a gold surface by surface-initiated atom transfer radical polymerization (SI-ATRP), and then the aptamer onto electrode surface by electrostatic self-assembly to form dual-recognition-unit-functionalized electrodes. The constructed biosensor based on DRUS not only shows an ultrahigh sensitivity toward ATP with a detection limit down to the subattomole level but also an ultrahigh selectivity toward ATP without interference from ADP and AMP. The constructed biosensor is used for selective and sensitive sensing of the extracellular ATP in the cerebral system by combining in vivo microdialysis and can be used as a promising neurotechnology to probing cerebral ATP concentration.

Disk-shaped amperometric enzymatic biosensor for in vivo detection of D-serine

At the synapse, D-serine is an endogenous co-agonist for the N-methyl-D-aspartate receptor (NMDAR). It plays an important role in synaptic transmission and plasticity and has also been linked to several pathological diseases such as schizophrenia and Huntington's. The quantification of local changes in D-serine concentration is essential to further understanding these processes. We report herein the development of a disk-shaped amperometric enzymatic biosensor for detection of D-serine based on a 25 μm diameter platinum disk microelectrode with an electrodeposited poly-m-phenylenediamine (PPD) layer and an R. gracilis D-amino acid oxidase (RgDAAO) layer. The disk-shaped D-serine biosensor is 1-5 orders of magnitude smaller than previously reported probes and exhibits a sensitivity of 276 μA cm(-2) mM(-1) with an in vitro detection limit of 0.6 μM. We demonstrate its usefulness for in vivo applications by measuring the release of endogenous D-serine in the brain of Xenopus laevis tadpoles.

Mucosal adenosine triphosphate mediates serotonin release from ileal but not colonic guinea pig enterochromaffin cells

BACKGROUND

Mechanical stimulation of the mucosal epithelium results in increased serotonin (5-HT) release from enterochromaffin (EC) cells. Little is known about how this process varies in different regions of the intestinal tract; however, purines are felt to play a role. We studied the relationship between mechanical stimulation, adenosine triphosphate (ATP), and 5-HT release from ileal and colonic mucosal tissue.

METHODS

Amperometric recordings of ATP and 5-HT were carried out using an ATP biosensor and boron-doped diamond microelectrode. Levels of extracellular ATP and 5-HT were monitored using high performance liquid chromatography.

KEY RESULTS

Under basal conditions, 5-HT levels were significantly decreased in the ileum (p < 0.001) but not the colon in the presence of the P2 antagonist suramin (100 μM). Ecto-ATPase inhibitor ARL67156 (10 μM) elevated ATP levels in the ileum and colon (both p < 0.001), but only 5-HT levels in the ileum (p < 0.001). Exogenous ATP increased 5-HT release in the presence of tetrodotoxin in the ileum (p < 0.001), but had not effect in the colon. Mechanical stimulation increased levels of 5-HT in the ileum (p < 0.001) and colon (p < 0.01), but levels returned to baseline in the presence of suramin and MRS2179 in the ileum. The onset of 5-HT release was delayed following mechanical stimulation. The rise time of the ATP response was quicker than that of 5-HT during mechanical stimulation.

CONCLUSIONS & INFERENCES

During mechanical stimulation of the mucosal epithelium, ATP mediates 5-HT release from EC cells in the ileum, but not the colon. Mucosal 5-HT signaling following mechanical stimulation is varied in different regions of the intestinal tract.

A highly selective sandwich-type FRET assay for ATP detection based on silica coated photon upconverting nanoparticles and split aptamer

In this paper, we report a highly selective sandwich-type fluorescence resonance energy transfer (FRET) assay for ATP detection by combining the unique optical properties of silica coated photon upconverting NaYF4:Yb(3+), Er(3+) nanoparticles (Si@UCNPs) with the high specific recognition ability of ATP aptamer. In the protocol, a single aptamer of ATP was split into two fragments. One of which was covalently attached to the Si@UCNPs at the 5' end, and the other was labeled with Black Hole Quencher-1 (BHQ1) at the 3' end. In the presence of ATP, the two fragments bound ATP with high affinity to form the sandwich complexes on the surface of Si@UCNPs. ATP induced association of the two fragments, thus bringing the Si@UCNPs and BHQ1 into close proximity. Under the illumination of 980 nm laser, energy transfer took place between the Si@UCNPs as the donor and BHQ1 as the acceptor, creating an optical "sandwich-type" assay for ATP detection. By monitoring the fluorescence change of the Si@UCNPs at 550 nm, the presence of the ATP could be quantitatively detected with a detection limit of 1.70 μM. The linear response range was 2 μM-16 μM. The background of this assay was ignorable because the fluorescence intensity of Si@UCNPs at 550 nm was not changed in the absence of ATP. This assay was also able to discriminate ATP from its analogs.

Local detection of mechanically induced ATP release from bone cells with ATP microbiosensors

The mechanically induced release of adenosine-5'-triphosphate (ATP) from osteoblastic cells (MC3T3-E1) was measured in real time. A stretching device integrated into scanning electrochemical microscopy was developed to apply controlled mechanical strain to MC3T3-E1 cells. For ATP secretion, a stepwise yet uniform mechanical stress was imposed onto MC3T3-E1 cells. The ATP biosensors were positioned at a distance of approximately 30-40 μm above the cell surface. Calibration functions were recorded prior to the cell measurements and revealed a linear response up to 40 μM with a sensitivity of 1-5pA/μM ATP. Stretching MC3T3-E1 cells up to 21% resulted in a concentration of 30.57±4.82 μM of extracellular ATP (N=12) detected above the cell surface. As a control experiment, nifedipine, a L-type voltage sensitive calcium channel (L-VSCC) inhibitor was applied, which blocks Ca(2+)entry from the outer medium into the cell. Inhibition resulted in a significantly smaller amount of released ATP, i.e., 7.08±1.93 μM ATP (N=10). Further control experiments with glucose microbiosensors did not yield significant changes of the baseline current (N=8).

Effect of development on [Ca2+]i transients to ATP in petrosal ganglion neurons: a pharmacological approach using optical recording

ATP, acting through P2X(2)/P2X(3) receptor-channel complexes, plays an important role in carotid body chemoexcitation in response to natural stimuli in the rat. Since the channels are permeable to calcium, P2X activation by ATP should induce changes in intracellular calcium ([Ca(2+)](i)). Here, we describe a novel ex vivo approach using fluorescence [Ca(2+)](i) imaging that allows screening of retrogradely labeled chemoafferent neurons in the petrosal ganglion of the rat. ATP-induced [Ca(2+)](i) responses were characterized at postnatal days (P) 5-8 and P19-25. While all labeled cells showed a brisk increase in [Ca(2+)](i) in response to depolarization by high KCl (60 mM), only a subpopulation exhibited [Ca(2+)](i) responses to ATP. ATP (250-1,000 μM) elicited one of three temporal response patterns: fast (R1), slow (R2), and intermediate (R3). At P5-8, R2 predominated and its magnitude was attenuated 44% by the P2X(1) antagonist, NF449 (10 μM), and 95% by the P2X(1)/P2X(3)/P2X(2/3) antagonist, TNP-ATP (10 μM). At P19-25, R1 and R3 predominated and their magnitudes were attenuated 15% by NF449, 66% by TNP-ATP, and 100% by suramin (100 μM), a nonspecific P2 purinergic receptor antagonist. P2X(1) and P2X(2) protein levels in the petrosal ganglion decreased with development, while P2X(3) protein levels did not change significantly. We conclude that the profile of ATP-induced P2X-mediated [Ca(2+)](i) responses changes in the postnatal period, corresponding with changes in receptor isoform expression. We speculate that these changes may participate in the postnatal maturation of chemosensitivity.

Measurement of purine release with microelectrode biosensors

Purinergic signalling departs from traditional paradigms of neurotransmission in the variety of release mechanisms and routes of production of extracellular ATP and adenosine. Direct real-time measurements of these purinergic agents have been of great value in understanding the functional roles of this signalling system in a number of diverse contexts. Here, we review the methods for measuring purine release, introduce the concept of microelectrode biosensors for ATP and adenosine and explain how these have been used to provide new mechanistic insight in respiratory chemoreception, synaptic physiology, eye development and purine salvage. We finish by considering the association of purine release with pathological conditions and examine the possibilities that biosensors for purines may one day be a standard part of the clinical diagnostic tool chest.

Effects of applied potential on the mass of non-conducting poly(ortho-phenylenediamine) electro-deposited on EQCM electrodes: comparison with biosensor selectivity parameters

An electrochemical quartz-crystal microbalance (EQCM) was used to determine the mass of poly-(o-phenylenediamine) (PoPD) layers electro-deposited at different applied potentials in neutral buffered monomer solution, conditions that produce the insulating form of the polymer used as a permselective membrane in biosensor applications. There was a systematic increase in the total, steady state PoPD mass deposited for fixed applied potentials from 0.05 to 0.6 V vs. SCE, followed by a plateau up to 0.8 V. Comparison of PoPD mass and permselectivity parameters indicates that the ability of the passivating form of PoPD to block interference species in biosensor applications is not related in a simple way to the mass of material deposited on the surface. Instead, effects of the applied electropolymerisation potential in driving the electro-oxidation of oPD dimers and oligomers formed during the electro-deposition process are likely to have a more direct impact on the selectivity characteristics of the PoPD layer. The results highlight the usefulness of apparent permeabilities, especially of ascorbic acid, in revealing differences between PoPD layers electro-deposited under different conditions.

ATP microelectrode biosensor for stable long-term in vitro monitoring from gastrointestinal tissue

We have developed a stable and selective ATP biosensor for long-term in vitro tissue monitoring. The electrode was fabricated by entrapping glucose oxidase (GOx) and hexokinase (HEX) in a poly-phenol film on a Pt microelectrode. The biosensor was stable to a fixed concentration of glucose for over 20 min and had a limit of detection of 9.9 ± 3.2 nM, with a sensitivity of 45.8 ± 1.22 pA μM(-1). Most significantly of all, the response on the ATP biosensor did not alter in the presence of 1mM ascorbic acid, 5 μM dopamine, 5 μM serotonin, 5 μM ADP and 5 μM AMP. The ATP biosensor was also shown to have excellent stability over 7 days, and showed only a 23.92 ± 3.55% loss in sensitivity. The ATP biosensor was utilised for the in vitro detection of ATP from gastrointestinal tissue. The ATP biosensor response was stable for 5h during in vitro recordings from ileum tissue. ATP release was shown to be greater from the mucosal surface in the ileum compared to the colon.

DNA aptasensor for the detection of ATP based on quantum dots electrochemiluminescence

A novel and facile strategy for the fabrication of aptamer-based adenosine 5'-triphosphate (ATP) biosensor was developed by a quantum dot (QD) electrochemiluminescence (ECL) technique. Different from the existing strategies for the development of aptasensors based on electrochemical, fluorescent or other methods, the strategy proposed here is essentially based on the aptamer-ATP specific affinity and the rules of Watson-Crick base pairing. After the thiol modified anti-ATP probes were immobilized onto the pretreated Au electrode, the electrode was incubated in ATP solution to form aptamer-ATP bioaffinity complexes. The complementary DNA (cDNA) oligonucleotides were hybridized with the free probes. As a result, the avidin-modified QDs were bound to the aptasensor through the biotin-avidin system in the existence of biotin-modified cDNA. The ECL signal of the aptasensor was responsive to the amount of QDs bound to the cDNA oligonucleotides, which was inversely proportional to the combined target analyte ATP. The QDs were characterized by high resolution transmission electron microscopy (HRTEM), ultraviolet (UV) and photoluminescence (PL) spectra. The preparation process for the aptasensor was monitored by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). Possible interference, such as from the pH value of the electrolyte, the incubation time and the concentration of coreactant K(2)S(2)O(8), on the aptasensor ECL response were investigated. The ATP concentration was measured through the decrease of ECL intensity. The ECL intensity of the aptasensor decreased with the increase of the logarithm of the ATP concentration over the 0.018-90.72 microM range. In addition, the aptasensor exhibited excellent selectivity responses toward the target analyte. This study may offer a new and relatively general approach to expand the application of QD ECL in the aptasensor field.

Developmental aspects of amperometric ATP biosensors based on entrapped enzymes

A novel concept for a dual-enzyme-based microbiosensor for the detection of adenosine-5'-triphosphate (ATP) was developed. The employed enzymes pyrroloquinoline quinone-dependent glucose dehydrogenase (PQQ-GDH) and hexokinase were entrapped, using pH-shift-induced precipitation of electrodeposition paint (EDP) at platinum microelectrodes (diameter of 25 microm). PQQ-GDH is known showing a superior activity for glucose conversion at the relevant conditions (low oxygen concentration) for ATP detection in targeted biomedical studies. For immobilizing the two enzymes PQQ-GDH and hexokinase, the deposition conditions of EDP Resydrol AY498w/35WA were adapted to ensure high immobilization rates. Prior to ATP sensing, the conversion of glucose, which is the co-substrate for both enzymatic reactions, was optimized. Optimization was targeted towards ATP measurements in biomedical environments by optimizing the PQQ-GDH sensor for glucose. Therefore, different mediators were tested regarding their electron transfer rate and their compatibility with the enzyme: free-diffusing N-methylphenazonium methyl sulfate (PMS) and ferrocenemethanol, and an immobilized chromium hexacyanoferrate layer at platinum electrode. Free-diffusing ferrocenemethanol reveals high sensitivity towards glucose of 1.5 +/- 0.4 nA/mM. In a next step, hexokinase was co-entrapped in the polymer film resulting in a sensitivity of up to 290 pA/microM.

Developmental maturation of chemosensitivity to hypoxia of peripheral arterial chemoreceptors--invited article

Peripheral arterial chemoreceptors, particularly the carotid body chemoreceptors, are the primary sites for the detection of hypoxia and reflexly increase ventilatory drive and behavioral arousal during hypoxic or asphyxial events. Newborn infants are at risk for hypoxic and asphyxial events during sleep, yet, the strength of the chemoreceptor responses is low or absent at birth and then progressively increases with early postnatal development. This review summarizes the available data showing that even though the "oxygen sensor" in the glomus cells has not been unequivocally identified, it is clear that development affects many of the other properties of the chemoreceptor unit (glomus cell, afferent nerve fibers and neurotransmitter profile at the synapse) that are necessary and essential for the propagation of the "sensing" response, and exposure to hypoxia, hyperoxia and nicotine can modify normal development of each of the components leading to altered peripheral chemoreceptor responses.

Contributions by a novel edge effect to the permselectivity of an electrosynthesized polymer for microbiosensor applications

Pt electrodes of different sizes (2 x 10(-5)-2 x 10(-2) cm(2)) and geometries (disks and cylinders) were coated with the ultrathin non-conducting form of poly(o-phenylenediamine), PPD, using amperometric electrosynthesis. Analysis of the ascorbic acid (AA) and H(2)O(2) apparent permeabilities for these Pt/PPD sensors revealed that the PPD deposited near the electrode insulation (Teflon or glass edge) was not as effective as the bulk surface PPD for blocking AA access to the Pt substrate. This discovery impacts on the design of implantable biosensors where electrodeposited polymers, such as PPD, are commonly used as the permselective barrier to block electroactive interference by reducing agents present in the target medium. The undesirable "edge effect" was particularly marked for small disk electrodes which have a high edge density (ratio of PPD-insulation edge length to electrode area), but was essentially absent for cylinder electrodes with a length of >0.2 mm. Sample biosensors, with a configuration based on these findings (25 microm diameter Pt fiber cylinders) and designed for brain neurotransmitter L-glutamate, behaved well in vitro in terms of Glu sensitivity and AA blocking.