Spectrofluorometric assay for the quantitation of cell-tissue electrofusion

Hydroethidine- and MitoSOX-derived red fluorescence is not a reliable indicator of intracellular superoxide formation: another inconvenient truth

Hydroethidine (HE; or dihydroethidium) is the most popular fluorogenic probe used for detecting intracellular superoxide radical anion. The reaction between superoxide and HE generates a highly specific red fluorescent product, 2-hydroxyethidium (2-OH-E(+)). In biological systems, another red fluorescent product, ethidium, is also formed, usually at a much higher concentration than 2-OH-E(+). In this article, we review the methods to selectively detect the superoxide-specific product (2-OH-E(+)) and the factors affecting its levels in cellular and biological systems. The most important conclusion of this review is that it is nearly impossible to assess the intracellular levels of the superoxide-specific product, 2-OH-E(+), using confocal microscopy or other fluorescence-based microscopic assays and that it is essential to measure by HPLC the intracellular HE and other oxidation products of HE, in addition to 2-OH-E(+), to fully understand the origin of red fluorescence. The chemical reactivity of mitochondria-targeted hydroethidine (Mito-HE, MitoSOX red) with superoxide is similar to the reactivity of HE with superoxide, and therefore, all of the limitations attributed to the HE assay are applicable to Mito-HE (or MitoSOX) as well.

In vivo cell electrofusion

In vitro electrofusion of cells brought into contact and exposed to electric pulses is an established procedure. Here we report for the first time the occurrence of fusion of cells within a tissue exposed in vivo to permeabilizing electric pulses. The dependence of electrofusion on the ratio of applied voltage to distance between the electrodes, and thus on the achievement of in vivo cell electropermeabilization (electroporation) is demonstrated in the metastasizing B16 melanoma tumor model. The kinetics of the morphological changes induced by cell electrofusion (appearance of syncytial areas or formation of giant cells) are also described, as well as the kinetics of mitosis and cell death occurrence. Finally, tissue dependence of in vivo cell electrofusion is reported and discussed, since electrofusion has been observed neither in liver nor in another tumor type. Particular microenvironmental conditions, such as the existence of reduced extracellular matrices, could be necessary for electrofusion achievement. Since biomedical applications of in vivo cell electropermeabilization are rapidly developing, we also discuss the influence of cell electrofusion on the efficacy of DNA electrotransfer for gene therapy and of antitumor electrochemotherapy, in which electrofusion could be an interesting advantage to treat metastasizing tumors.