Activation of adenylate cyclase during swelling of S49 cells in hypotonic medium is not involved in subsequent volume regulation

Automated cell-based assay for screening of aquaporin inhibitors

Aquaporins form water channels that play major roles in a variety of physiological processes so that altered expression or function may underlie pathological conditions. In order to identify compounds that modulate aquaporin function, we have implemented a functional assay based on rapid measurement of osmotically induced cell volume changes to screen several libraries of diverse drugs. The time course of fluorescence changes in calcein-loaded cells was analyzed during an osmotic challenge using a 96-multiwell fluorescence plate reader. This system was validated using astrocyte primary cultures and fibroblasts that strongly express endogenous AQP4 and AQP1 proteins, respectively, as well as AQP4-transfected cells. We screened 3575 compounds, including 418 FDA-approved and commercially available drugs, for their effect on AQP-mediated water transport. Primary screening yielded 10 compounds that affected water transport activity in both astrocytes and AQP4-transfected cells and 42 compounds that altered cell volume regulation in astrocytes. Selected drugs were then analyzed on AQP1-expressing erythrocytes and AQP4-expressing membrane vesicles by stopped-flow light scattering. Four molecules of the National Cancer Institute's chemical library (NSC164914, NSC670229, NSC168597, NSC301460) were identified that differentially affected both AQP4 and AQP1 mediated water transport, with EC50 values between 20 and 50 microM. This fluorescence microplate reader-based assay may, thus, provide a platform for high-throughput screening which, when coupled to a secondary evaluation to confirm target specificity, should allow discovery of AQP-specific compounds for novel therapeutic strategies in the treatment of water balance disorders.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

The role of swelling-induced anion channels during neuronal volume regulation

Regulation of cell volume is an essential function of most mammalian cells. In the cells of the central nervous system, maintenance of cell osmolarity and, hence, volume, is particularly crucial because of the restrictive nature of the skull. Cell volume regulation involves a variety of pathways, with considerable differences between cell types. One common pathway activated during hypo-osmotic stress involves chloride (Cl-) channels. However, hypo-osmotically stimulated anion permeability can be regulated by a diverse array of second messengers. Although neuronal swelling can occur in a number of pathological and nonpathological conditions, our understanding of neuronal volume regulation is limited. This article summarizes our current understanding of the role of anion channels during neuronal volume regulation.