A photocross-linking fluorescent indicator of mitochondrial membrane potential

Absolute spectroscopic determination of cross-membrane potential

Spectroscopic determination of the cross-membrane electric potential has been used for more than 20 years. This method, which usually employs absorption or fluorescence measurements, allows for a rapid and noninvasive study of the electrical properties of the membranes of cells and liposomes. However, the usual fluorescence techniques preferably allow monitoring changes in the potential on triggerable or excitable membranes, and not the absolute value of the potential. They also do not provide means for measuring the potential on single cells. This paper reviews three methods that solve these issues. Nernstian dyes which partition between intra-and extracompartmental volumes enable a fluorescence microscopic determination of a single cell and even a single organelle. Dual-wavelength ratiometric recording from membrane-staining dyes also provides means for measuring the field on a single cell. Resonance Raman probes provide a spectroscopic method with a natural internal standard for the absolute measurement of membrane potential.

Membrane potential estimation by flow cytometry

Membrane potential (delta psi) is generated and maintained by concentration gradients of ions such as sodium, potassium, chloride, and hydrogen. Changes in cytoplasmic delta psi in the course of surface-receptor-mediated processes related to the development, function, and pathology of many cell types often play a role in transmembrane signaling. Cytoplasmic delta psi is also reduced to zero when the membrane is ruptured by chemical or physical agents. Mitochondrial delta psi is reduced when energy metabolism is disrupted, notably in apoptosis. In bacteria, which lack mitochondria, delta psi reflects both the state of energy metabolism and the physical integrity of the cytoplasmic membrane. Flow cytometry can be used to estimate membrane potential in eukaryotic cells, mitochondria in situ, isolated mitochondria, and bacteria. Older methods, using lipophilic cationic dyes such as the cyanines and rhodamine 123 or lipophilic anionic dyes such as the oxonols can detect relatively large changes in delta psi and identify heterogeneity of response in subpopulations comprising substantial fractions of a cell population. Newer ratiometric techniques allow precise measurement of delta psi to within 10 mV or less. Among other factors, action of efflux pumps, changes in membrane structure, and changes in protein or lipid concentration in the medium in which cells are suspended can produce changes in cellular fluorescence which may be misinterpreted as changes in delta psi. Techniques for estimation and measurement of Delta Psi therefore typically require careful control of cell and reagent concentrations and incubation times and selection of appropriate controls if they are to provide accurate information.

Slow fluorescent indicators of membrane potential: a survey of different approaches to probe response analysis

Basic tenets related to the use of three main classes of potentiometric redistribution fluorescent dyes (carbocyanines, oxonols, and rhodamines) are discussed in detail. They include the structure/function relationship, formation of nonfluorescent (H-type) and fluorescent (J-type) dimers and higher aggregates, probe partitioning between membranes and medium and binding to membranes and intracellular components (with attendant changes in absorption and emission spectra, fluorescence quantum yield and lifetime). The crucial importance of suitable probe-to-cell concentration ratio and selection of optimum monitored fluorescence wavelength is illustrated in schematic diagrams and possible artifacts or puzzling results stemming from faulty experimental protocol are pointed out. Special attention is paid to procedures used for probe-response calibration (potential clamping by potassium in the presence of valinomycin, use of gramicidin D in combination with N-methylglucamine, activation of Ca-dependent K-channels by A23187, the null-point technique). Among other problems treated are dye toxicity, interaction with mitochondria and other organelles, and possible effects of intracellular pH and the quantity of cytosolic proteins and/or RNA on probe response. Individual techniques using redistribution dyes (fluorescence measurements in cuvettes, flow cytometry and microfluorimetry of individual cells including fluorescence confocal microscopy) are discussed in terms of reliability, limitations and drawbacks, and selection of suitable probes. Up-to-date examples of application of slow dyes illustrate the broad range of problems in which these probes can be used.