Coupling Amperometry and Total Internal Reflection Fluorescence Microscopy at ITO Surfaces for Monitoring Exocytosis of Single Vesicles

Mechanical Strain Induces and Increases Vesicular Release Monitored by Microfabricated Stretchable Electrodes

Exocytosis involving the fusion of intracellular vesicles with cell membrane, is thought to be modulated by the mechanical cues in the microenvironment. Single-cell electrochemistry can offer unique information about the quantification and kinetics of exocytotic events; however, the effects of mechanical force on vesicular release have been poorly explored. Herein, we developed a stretchable microelectrode with excellent electrochemical stability under mechanical deformation by microfabrication of functionalized poly(3,4-ethylenedioxythiophene) conductive ink, which achieved real-time quantitation of strain-induced vesicular exocytosis from a single cell for the first time. We found that mechanical strain could cause calcium influx via the activation of Piezo1 channels in chromaffin cell, initiating the vesicular exocytosis process. Interestingly, mechanical strain increases the amount of catecholamines released by accelerating the opening and prolonging the closing of fusion pore during exocytosis. This work is expected to provide revealing insights into the regulatory effects of mechanical stimuli on vesicular exocytosis.

Electrochemical Fluorescence Switch of Organic Fluorescent or Fluorogenic Molecules

This short review is aimed at emphasizing the most prominent recent works devoted to the fluorescence modulation of organic fluorescent or fluorogenic molecules by electrochemistry. This still expanding research field not only addresses the smart uses of known molecules or the design of new ones, but also investigates the development of instrumentation providing time- and space-resolved information at the molecular level. Important considerations including fluorescent/fluorogenic probes, reversible/irreversible fluorescence switch, direct/indirect fluorescence modulation, or environment properties are especially scrutinized in recent works dealing with bioanalysis perspectives.

A Fluorescent False Neurotransmitter as a Dual Electrofluorescent Probe for Secretory Cell Models

A dual electrofluorescent probe (FFN42) belonging to the fluorescent false neurotransmitter family was rationally designed for investigating cell secretion. This probe, which comprises a coumarin core with one amino and two hydroxy groups, is very promising due to its electroactive and fluorescent properties. The optimal excitation and emission wavelengths (380 nm and 470 nm respectively) make this probe adapted for use in fluorescence microscopy. FFN42 has a quantum yield of 0.18, a molar absorption coefficient of 12000 M^-1 cm^-1 and pKa values of 5.4 and 6.7 for the hydroxy groups. The electroactivity of FFN42 was evidenced on carbon fiber and ITO electrodes at relatively low oxidation potentials (0.24 V and 0.45 V vs Ag/AgCl respectively). Epifluorescence observations showed that FFN42 accumulated into secretory vesicles of PC12 and N13 cells. Toxicity tests further revealed that FFN42 had no lethal effect on these cells. Amperometric data obtained on carbon fiber electrodes proved that the probe is released by N13 cells.

Amorphous silicon on indium tin oxide: a transparent electrode for simultaneous light activated electrochemistry and optical microscopy

Herein is reported a new type of transparent electrode, prepared by depositing a thin layer of amorphous silicon film on indium tin oxide, which enables photoswitchable electrochemistry and optical imaging to be performed simultaneously. This offers the opportunity to visualise a spatially controlled electrochemical event on an unstructured electrode surface.

Simultaneous impedance spectroscopy and fluorescence microscopy for the real-time monitoring of the response of cells to drugs

A dual fluorescence microscopy and electrochemical strategy to investigate how cell-surface interactions influence the cellular responses to cues for the cell-based biosensing of drug efficacy is reported herein. The combined method can be used to not only monitor the importance of controlling the cellular adhesive environment on the cell response to drugs but it also provides biological information on the timescales of downstream outside-in signaling from soluble cues. As an example of the use of the combined method, we show how adhesive cues influence the signalling responses of cells to soluble cues. G-protein-coupled receptors were used as the target for the soluble cues. The changes in cell adhesion, cell morphology and Ca2+ flux induced by soluble histamine were simultaneously monitored as a function of the spacing of the adhesive ligand RGD on the interdigitated indium tin oxide electrodes. The simultaneous measurements revealed that the timescales of histamine-induced Ca2+ mobilization and the decrease in cell-cell adhesions are correlated. Furthermore, cells on the surfaces with an RGD spacing of 31 nm were shown to display a faster release of Ca2+ and change in cell adhesion upon histamine stimulation compared to cells on other surfaces.

A Dual Functional Electroactive and Fluorescent Probe for Coupled Measurements of Vesicular Exocytosis with High Spatial and Temporal Resolution

In this work, Fluorescent False Neurotransmitter 102 (FFN102), a synthesized analogue of biogenic neurotransmitters, was demonstrated to show both pH-dependent fluorescence and electroactivity. To study secretory behaviors at the single-vesicle level, FFN102 was employed as a new fluorescent/electroactive dual probe in a coupled technique (amperometry and total internal reflection fluorescence microscopy (TIRFM)). We used N13 cells, a stable clone of BON cells, to specifically accumulate FFN102 into their secretory vesicles, and then optical and electrochemical measurements of vesicular exocytosis were experimentally achieved by using indium tin oxide (ITO) transparent electrodes. Upon stimulation, FFN102 started to diffuse out from the acidic intravesicular microenvironment to the neutral extracellular space, leading to fluorescent emissions and to the electrochemical oxidation signals that were simultaneously collected from the ITO electrode surface. The correlation of fluorescence and amperometric signals resulting from the FFN102 probe allows real-time monitoring of single exocytotic events with both high spatial and temporal resolution. This work opens new possibilities in the investigation of exocytotic mechanisms.

A new diamond biosensor with integrated graphitic microchannels for detecting quantal exocytic events from chromaffin cells

An MeV ion-microbeam lithographic technique can be successfully employed for the fabrication of an all-carbon miniaturized cellular biosensor based on graphitic microchannels embedded in a single-crystal diamond matrix. The device is functionally characterized for the in vitro recording of quantal exocytic events from single chromaffin cells, with high sensitivity and signal-to-noise ratio, opening promising perspectives for the realization of monolithic all-carbon cellular biosensors.

Evaluating the diffusion coefficient of dopamine at the cell surface during amperometric detection: disk vs ring microelectrodes

During exocytosis, small quantities of neurotransmitters are released by the cell. These neurotransmitters can be detected quantitatively using electrochemical methods, principally with disk carbon fiber microelectrode amperometry. An exocytotic event then results in the recording of a current peak whose characteristic features are directly related to the mechanisms of exocytosis. We have compared two exocytotic peak populations obtained from PC12 cells with a disk carbon fiber microelectrode and with a pyrolyzed carbon ring microelectrode array, with a 500 nm ring thickness. The specific shape of the ring electrode allows for precise analysis of diffusion processes at the vicinity of the cell membrane. Peaks obtained with a ring microelectrode array show a distorted average shape, owing to increased diffusion pathways. This result has been used to evaluate the diffusion coefficient of dopamine at the surface of a cell, which is up to an order of magnitude smaller than that measured in free buffer. The lower rate of diffusion is discussed as resulting from interactions with the glycocalyx.