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Advances in Use of Capsule-Based Fluorescent Sensors for Measuring Acidification of Endocytic Compartments in Cells with Altered Expression of V-ATPase Subunit V1G1

De Luca, Maria, Ferraro, Marzia M., Hartmann, Raimo, Rivera-Gil, Pilar, Klingl, Andreas, Nazarenus, Moritz, Ramirez, Agnese, Parak, Wolfgang J., Bucci, Cecilia, Rinaldi, Rosaria, del Mercato, Loretta L.
ACS Applied Materials & Interfaces 2015 v.7 no.27 pp. 15052-15060
H-transporting ATP synthase, acidification, cytoskeleton, eukaryotic cells, fluorescence, fluorescence microscopy, human diseases, pH, proton pump, transmission electron microscopy, vacuoles
Acidification of eukaryotic cell compartments is accomplished by vacuolar H⁺-ATPases (V-ATPases), large multisubunit complexes able to pump protons into the lumen of organelles or in the extracellular medium. V-ATPases are involved in a number of physiological cellular processes, and thus regulation of V-ATPase activity is of crucial importance for the cell. Indeed, dysfunction of V-ATPase or alterations of acidification have been recently recognized as key factors in a variety of human diseases. In this study, we applied capsule-based pH sensors and a real-time tracking method for investigating the role of the V₁G₁ subunit of V-ATPases in regulating the activity of the proton pump. We first constructed stable cell lines overexpressing or silencing the subunit V₁G₁. Second, we used fluorescent capsule-based pH sensors to monitor acidification before and during internalization by modified and control living cells. By using a simple real-time method for tracking capsule internalization, we were able to identify different capsule acidification levels with respect to each analyzed cell and to establish the kinetics for each. The intracellular pH measurements indicate a delay in acidification in either V₁G₁-overexpressing or V₁G₁-silenced cells compared to controls. Finally, in an independent set of experiments, we applied transmission electron microscopy and confocal fluorescence microscopy to further investigate the internalization of the capsules. Both analyses confirm that capsules are engulfed in acidic vesicular structures in modified and control cell lines. The use of capsule-based pH sensors allowed demonstration of the importance of the V₁G₁ subunit in V-ATPase activity concerning intravesicular acidification. We believe that the combined use of these pH-sensor system and such a real-time method for tracking their internalization path would contribute to systematically measure the proton concentration changes inside the endocytic compartments in various cell systems. This approach would provide fundamental information regarding molecular mechanisms and factors that regulate intracellular acidification, vesicular trafficking, and cytoskeletal reorganizations.