Regulation of the epithelial Na(+) /H(+) exchanger isoform by the cytoskeleton.

Members of the Na(+)/H(+) exchanger (NHE) family mediate electroneutral countertransport of H(+) for Na(+) across cellular membranes. The six known isoforms mediate transepithelial Na(+) transport processes and housekeeping functions such as the regulation of cellular and organellar pH and volume. NHE3 is found primarily in the apical membrane of epithelial cells of the kidney and gastrointestinal tract, where it mediates Na(+) (re)absorption. Its fine regulation, whether by hormones that utilize cAMP as a signalling mechanism, or by physical parameters such as the cell volume, provides the adjustments necessary for the maintenance of systemic salt and fluid balance. Although the exact molecular mechanism of this control is unknown, two major modes of regulation have been invoked: 1) alteration of the number of cell surface transporters by changes in the rate of endocytosis and/or exocytosis and 2) regulation of the intrinsic activity of the individual exchangers. NHE3 requires an intact cytoskeleton for its optimal function. Pharmacological interference with actin polymerization or myosin phosphorylation markedly inhibits the exchanger, without altering the number of transporters exposed at the surface. This effect is isoform specific and is mediated by the cytoplasmic tail of the transporter. The small GTP-binding protein, RhoA and its downstream effector, Rho kinase regulate NHE3, possibly by controlling the level of myosin phosphorylation, that in turn determines the organization of actin. The cytoskeleton may not only be involved in the maintenance of the basal rate of transport, but is also likely to sense physical alterations and transmit signals to modulate NHE3 activity, thus providing fast and effective control of the exchanger.