Editorial

Abstract

The present issue of the journal is based on the fifth international symposium on cell volume regulation and its physiological and pathophysiological implications. This symposium, being sponsored by the Scandinavian Physiological Society as the 12th Acta Physiologica International Symposium, was held in Copenhagen, September 2005. In the symposium, scientists studying the mechanisms and implications of cell volume regulation in a wide variety of systems (yeast, plants and numerous vertebrate cell types and tissues) came together to join forces in increasing the understanding of this fundamental physiological phenomenon. The present issue contains a series of review papers and original articles based on the presentations at this symposium. In the following, we outline major themes from the symposium collected in this special issue of Acta Physiologica. The initial rate of change in cell volume upon a change in the intra- or extracellular content of osmolytes is dependent on the cellular water permeability, which in many tissues is largely determined by the presence of aquaporin water channels. Substantial progress has been made in recent years in the understanding of these channels, and several presentations at the symposium addressed this issue in both plant and animal species. The mechanisms by which an increase or a decrease in cell volume are sensed and subsequently transduced into activation of volume-regulatory ion transport proteins are the subject of intense studies in the field. In yeast, the mechanisms of osmosensing are well elucidated. In higher eukaryotes, the available evidence indicates that there are probably multiple mechanisms by which volume changes are initially sensed. These include two major categories of mechanisms, which may co-exist in a given cell type, and indeed also appear to be related in the sense that they modulate each other. First, there is strong evidence for a role for changes in intracellular concentrations of macromolecules (macromolecular crowding), in intracellular ionic strength, or in the intracellular concentrations of specific ions, of which Cl− may be of particular importance. Second, several lines of evidence point to the involvement of mechanical changes involving reorganization of cytoskeletal elements and changes in the organization of membrane subdomains in volume sensing. In the present issue, a number of contributions deal with various factors in this latter category, including the cholesterol-containing plasma membrane microdomains (lipid rafts and/or caveolae) and cytoskeletal elements including those linking the cytoskeleton to the plasma membrane and extracellular matrix (F-actin, myosin, microtubules, ERM proteins and integrins). A class of proteins emerging as major points of convergence between cell volume changes, cytoskeletal reorganization, regulation of ion transport and long-term adaptive responses are the Rho family of small GTP binding proteins, also reviewed in this issue. While there is no question of the major role of these proteins in, e.g. volume-sensitive cytoskeletal reorganization, it is still an issue of debate whether these proteins as such mediate the volume signals activating ion transport, or rather serve a modulatory function in this regard. Similarly, a number of other volume- and/or mechanosensitive signals have been shown to modulate the activity of volume-sensitive ion transporters. These include reactive oxygen and nitrogen species (hydrogen peroxide and NO), arachidonic acid metabolites (e.g. leukotrienes and 5′,6′-epoxyeicosatrienoic acid), and ATP released to the extracellular space and acting as paracrine/autocrine signals. Important new evidence has also been obtained for the regulation of several classes of phospholipases (such as both the Ca2+-dependent and –independent phospholipase A2) by cell volume changes and for a major role of the above-mentioned lipid mediators in regulation of volume-sensitive transporters for ions and organic osmolytes. Additionally, while the involvement of protein kinases and phosphatases in the regulation of volume-sensitive ion transporters has been known for many years, much progress has been made recently with respect to the nature of the kinases involved and the mechanisms by which these kinases regulate ion transport. An emerging picture is that in many cases, the volume-sensitive kinases must not only phosphorylate but also in fact directly associate with the transporters, in what at least sometimes appears to be multicomponent assemblies of transporter, kinase and corresponding phosphatase and cytoskeletal components. Examples include the proposed complex formation between the family of cation-chloride cotransporters, the ste20-related protein kinases, the mitogen-activated protein kinases and the actin-based cytoskeleton, as well as the regulation of the sodium-proton exchangers NHE1 and NHE3 by protein–protein interactions. Exciting advances in the understanding of volume sensing have also come with the discovery that several members of the family of transient receptor potential (TRP) channels are sensitive to changes in cell volume (both swelling and shrinkage-activated TRPs have been reported) and that the Ca2+ influx resulting from their activation is, at least in some cell types, important for cell volume regulation. The roles of TRPM7 and TRPV4 in this regard are discussed in an original paper and a review paper, respectively, in this issue. Another area in which considerable new evidence has been presented in recent years is that of the transcription factors involved in the long-term adaptive responses to cell volume changes, including but not limited to changes in the expression of volume-sensitive ion transporters. A major such transcription factor is TonEBP, the roles and regulation of which are also discussed in the current issue. The regulation of cell volume after cell swelling or shrinkage is mediated by net release or net uptake, respectively, of ions and organic osmolytes via specific, membrane associated osmolyte transporters. Following acute cell swelling, volume is restored by the parallel activation of specific efflux pathways for anions, cations, and organic osmolytes, and/or the activation of an electroneutral cation-chloride cotransporter (KCC). With respect to the swelling-activated chloride current (VRAC), considerable new evidence has been presented regarding its regulation/modulation. The molecular identity of VRAC is, however, still unknown, although several putative candidates were discussed (e.g. CLC3, CFTR and ICln). In contrast, several molecular candidates for the swelling-activated potassium current have been identified in various tissues (e.g. TASK2, TREK-1, BK, SK and IK), the roles and regulation of which were discussed extensively at the symposium and is presented in several contributions in the current issue. Following acute cell shrinkage, cell volume is restored predominantly by the parallel activity of sodium-proton exchange (NHE1) and chloride-bicarbonate exchange (AE) and/or by activation of sodium–potassium–chloride cotransport (NKCC1), and in some tissues also by the activation of non-selective cation channels (HICs). In addition to the above-mentioned findings regarding the signaling pathways regulating the activity of these shrinkage-activated transporters, novel findings regarding their osmosensitivity, inhibitor profile, and molecular identity was presented at the symposium and are reviewed in the present issue. Following long-term changes in extracellular osmolarity/cell volume, the expression of several volume-sensitive osmolyte transporters has been shown to be regulated by osmosensitive changes in the activity of transcription factors such as TonEBP. Examples reviewed in this issue are the organic osmolyte uptake pathways (TauT, SNAT2). An interesting aspect of the symposium was the presentation of a number of new techniques which will make it possible to address so far unanswered questions regarding, e.g. the specific molecular mechanisms behind the ion transduction by volume-sensitive transporters (e.g. single-molecule FRET), and the sequence of events activated by cell volume perturbations and leading to transporter activation and other volume-sensitive physiological events (e.g. gene-specific knockdown/rescue of specific elements in these volume-sensitive cascades of events). A number of important physiological events have been shown to involve changes in cell volume. Examples include hormone signaling and secretion, several aspects of which are presented in this issue. Moreover, cell volume regulation is now established to be a central factor in cell migration, which plays a basic role in many physiological and pathophysiological processes such as embryogenesis, immune defense, wound healing or metastasis. The mechanisms by which NHE1 is involved in cell migration was addressed at the symposium and is discussed in the present issue. Epithelia are physiologically exposed to osmotic stress, resulting in alteration of cell volume associated with several aspects of their function, and rapid cell volume regulation plays a fundamental role in their physiology. In this issue of Acta Physiologica, papers are presented which deal with specific aspects of this, namely: (i) the role and regulation (specifically by cytoskeletal components and ste20 related kinases) of shrinkage-activated Na+–K+–2Cl− cotransport in transepithelial salt transport, (ii) water transport across epithelia, including the principles behind isotonic water transport, and the formation of aqueous humour. A session at the symposium was dedicated to the physiology and patophysiology of the relation between cellular and extracellular volume and signal transduction in the brain. Due to the restricted extracellular space of the brain, cell volume regulation is of particular importance for the physiological function of the brain. Two of the present contributions address this issue, one with respect to the role of glial cells in pain transduction and the other concerning inter-neural communication. Apoptosis is a fundamental physiological mechanism, the derangement of which is involved in many cardiovascular, neurological and renal diseases. Because cell shrinkage is a hallmark in programmed cell death (apoptosis and apoptosis-like), it is very important to understand the deregulation of cell volume and ion homeostasis that occurs during apoptosis. It is as yet unclear to what extent cell shrinkage as such and/or the ion concentration changes seen during apoptotic volume decrease (AVD) are the primary activation signals at certain steps in the apoptotic process. Several interesting contributions in the present issue deal with this subject. It is found that cell shrinkage as such can induce apoptosis and that hyperosmotic shrinkage sensitizes the cells towards CD95 ligand-induced apoptosis by activating the CD95 system. The central importance of specifically perturbing of Na+ and K+ homeostasis during apoptosis, and the roles of Na+/K+ ATPase dysfunction and of activation of non-selective cation channels in particular, are also discussed in the present issue. We are very grateful to all contributors and participants in the conference. We dare hope that the novel insights arising from this symposium and presented in this volume, will provoke a lot of new experiments, the results of which will lead to exciting new advances in the understanding of the mechanisms and implications of cell volume regulation. The Symposium was supported by The Danish Natural Sciences Research Foundation, The Augustinus Foundation, The ‘‘Bikube’’ Foundation, The August Krogh Institute, Elizabeth and Knud Petersens Foundation, International Society for Neurochemistry, Roche Diagnostics and the Scandinavian Physiological Society.