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Department of General and Environmental Physiology (G.T., G.P., A.S., M.S., G.V.), University of Bari, 70126 Bari, Italy; Department of Biomolecular Science and Biotechnology (E.B., G.M., M.P.), University of Milan, 20122 Milan, Italy; Department of Physiology and Medical Physics (G.T., M.P.), Medical University of Innsbruck, A-6020 Innsbruck, Austria; and Department of Physics and Laboratorio Regionale LICRYL (Liquid Crystal Laboratory) (V.F.), Istituto Nazionale Fisica della Materia-Consiglio Nazionale della Ricerca (INFM-CNR), University of Calabria, 87036 Calabria, Italy
Address all correspondence and requests for reprints to: Giovanna Valenti, Ph.D., Department of General and Environmental Physiology, University of Bari, Via Amendola 165/A, 70126 Bari, Italy; or Markus Paulmichl, Department of Biomolecular Sciences and Biotechnology, University of Milan, Via Celoria 26, 20133 Milano, Italy. E-mail: g.valenti{at}biologia.uniba.it; markus.paulmichl{at}i-med.ac.at; or markus.paulmichl{at}unimi.it.
Kidney collecting-duct cells swell in response to changes in medulla osmolality caused by the transition from antidiuresis to diuresis. Regulatory volume decrease (RVD) mechanisms must be activated to face this hypotonic stress. In Aquaporin-2 (AQP2)-expressing renal CD8 cells, hypotonicity decreased cell surface expression of AQP2 and increased the amount of AQP2 localized intracellularly, whereas the total amount of AQP2 phosphorylated at ser-256 decreased. Analysis of cAMP dynamics using fluorescence resonance energy transfer (FRET) showed that hypotonicity causes a reduction of cAMP, consistent with a decrease in phospho-AQP2. Moreover, hypotonicity caused a profound actin reorganization, associated with the loss of stress fibers and formation of F-actin patches (microspikes) at the cell border. Those changes were regulated by the monomeric GTPase Cdc42. Interestingly, expression of the dominant-negative Cdc42 (N17-Cdc42) prevented the hypotonicity-induced microspike formation and the generation of Cl– currents. Hypotonicity also caused the relocation from the cytosol to the plasma membrane and increase in interaction with actin of ICln (nucleotide-sensitive chloride current protein), which is essential for the generation of ion currents activated during RVD. Together, the profound actin remodeling, internalization of AQP2 and translocation of ICln to the plasma membrane during hypotonicity may contribute to RVD after cell swelling in renal medulla.
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