Structural correlates of the transepithelial water transport

LRRC8 family proteins within lysosomes regulate cellular osmoregulation and enhance cell survival to multiple physiological stresses

LRRC8 family proteins on the plasma membrane play a critical role in cellular osmoregulation by forming volume-regulated anion channels (VRACs) necessary to prevent necrotic cell death. We demonstrate that intracellular LRRC8 proteins acting within lysosomes also play an essential role in cellular osmoregulation. LRRC8 proteins on lysosome membranes generate large lysosomal volume-regulated anion channel (Lyso-VRAC) currents in response to low cytoplasmic ionic strength conditions. When a double-leucine L⁷⁰⁶L⁷⁰⁷ motif at the C terminus of LRRC8A was mutated to alanines, normal plasma membrane VRAC currents were still observed, but Lyso-VRAC currents were absent. We used this targeting mutant, as well as pharmacological tools, to demonstrate that Lyso-VRAC currents are necessary for the formation of large lysosome-derived vacuoles, which store and then expel excess water to maintain cytosolic water homeostasis. Thus, Lyso-VRACs allow lysosomes of mammalian cells to act as the cell`s "bladder." When Lyso-VRAC current was selectively eliminated, the extent of necrotic cell death to sustained stress was greatly increased, not only in response to hypoosmotic stress, but also to hypoxic and hypothermic stresses. Thus Lyso-VRACs play an essential role in enabling cells to mount successful homeostatic responses to multiple stressors.

General models for water transport across leaky epithelia

The group of leaky epithelia, such as proximal tubule and small intestine, have several common properties in regard to salt and water transport. The fluid transport is isotonic, the transport rate increases in dilute solutions, and water can be transported uphill. Yet, it is difficult to find common features that could form the basis for a general transport model. The direction of transepithelial water transport does not correlate with the direction of the primary active Na+ transport, or with the ultrastucture as defined by the location of apical and basolateral membranes, of the junctional complex and the lateral intercellular spaces. The presence of specific water channels, aquaporins, increases the water permeability of the epithelial cell membranes, i.e., the kidney proximal tubule. Yet other leaky epithelia, for example, the retinal pigment epithelium, have no known aquaporins. There is, however, a general correlation between the direction of transepithelial transport and the direction of transport via cotransporters of the symport type. A simple epithelial model based on water permeabilities, a hyperosmolar compartment and restricted salt diffusion, is unable to explain epithelial transport phenomena, in particular the ability for uphill water transport. The inclusion of cotransporters as molecular water pumps in these models alleviates this problem.

Reversible vacuolation of T-tubules in skeletal muscle: mechanisms and implications for cell biology

The majority of investigations of the transverse tubules (T-system) of skeletal muscle have been devoted to their role in excitation-contraction coupling, with particular reference to contact with the sarcoplasmic reticulum and the mechanism of Ca2- release. By contrast, this review is concerned with structural and functional aspects of the vacuolation of T-tubules. It covers experimental procedures used in reversible vacuolation induced by the efflux-influx of glycerol and other small nonelectrolytes, sugars, and ions. The characteristics of the phenomenon, associated alterations in muscle function, and the swelling of analogous structures in nonmuscle cells are considered. Possible functions of reversible vacuolation in water balance, transport, membrane repair, muscle pathology, and fatigue are considered, and the potential application of reversible vacuolation in the transfection of skeletal muscle is discussed. In relation to the possible mechanisms involved in reversible vacuolation, particular attention is given to the dynamic and structural aspects of the opening and closing of T-tubules, the origin of vacuolar membranes, and the localized character of tubular swelling.