Shrinkage-induced activation of Na+/H+ exchange in primary rat astrocytes: role of myosin light-chain kinase

Regulation of the Na+/H+ exchanger in fibroblasts overexpressing the Na+/Ca2+ exchanger

To assess the role of Ca2+ in regulation of the Na+/H+ exchanger (NHE1),we used CCL-39 fibroblasts overexpressing the Na+/Ca2+ exchanger (NCX1). Expression of NCX1 markedly inhibited the transient cytoplasmic Ca2+ rise and long-lasting cytoplasmic alkalinization (60-80% inhibition) induced by alpha-thrombin. In contrast, coexpression of NCX1 did not inhibit this alkalinization in cells expressing the NHE1 mutant with the calmodulin (CaM)-binding domain deleted (amino acids 637-656), suggesting that the effect of NCX1 transfection involves Ca2+-CaM binding. Expression of NCX1 only slightly inhibited platelet-derived growth factor BB-induced alkalinization and did not affect hyperosmolarity- or phorbol 12-myristate 13-acetate-induced alkalinization. Downregulation of protein kinase C (PKC) inhibited thrombin-induced alkalinization partially in control cells and abolished it completely in NCX1-transfected cells, suggesting that the thrombin effect is mediated exclusively via Ca2+ and PKC. On the other hand, deletion mutant study revealed that PKC-dependent regulation occurs through a small cytoplasmic segment (amino aids 566-595). These data suggest that a mechanism involving direct Ca2+-CaM binding lasts for a relatively long period after agonist stimulation, despite apparent short-lived Ca2+ mobilization, and further support our previous conclusion that Ca2+- and PKC-dependent mechanisms are mediated through distinct segments of the NHE1 cytoplasmic domain.

Human cytomegalovirus infection enhances osmotic stimulation of Na+/H+ exchange in human fibroblasts

Infection with human cytomegalovirus (HCMV) causes an enlargement (cytomegaly) of human fibroblasts (MRC-5). As a first step toward determining whether solute uptake, mediated in part by Na+/H+ exchange, is responsible for the development of cytomegaly, we studied the effects of HCMV infection on intracellular pH (pHi) regulation (nominal CO2/ HCO3- concn = 0) by comparing cytomegalic cells with mock-infected cells. Seventy-two hours after HCMV infection of MRC-5 cells we observed the following changes relative to mock-infected cells: resting pHi is 0.1-0.2 pH unit more alkaline; the intrinsic buffering power of the cytoplasm was reduced by approximately 40-50%; acid-loading H(+)-equivalent fluxes were reduced; and there were alterations of Na+/H+ exchanger (NHE) properties, including an alkaline shift of the pHi dependence of activity, a reduction of the apparent affinity for extracellular Na+, and an increase of the apparent maximum velocity and a large increase in stimulation by a hyperosmotic challenge. These results indicate that HCMV infection exerts a profound effect on functional properties of the NHE, on acid-loading mechanisms, and on intrinsic cellular buffering power. These effects are consistent with a role for the NHE in the development of cytomegaly.

NO3--induced pH changes in mammalian cells. Evidence for an NO3--H+ cotransporter

The effect of NO3- on intracellular pH (pHi) was assessed microfluorimetrically in mammalian cells in culture. In cells of human, hamster, and murine origin addition of extracellular NO3- induced an intracellular acidification. This acidification was eliminated when the cytosolic pH was clamped using ionophores or by perfusing the cytosol with highly buffered solutions using patch-pipettes, ruling out spectroscopic artifacts. The NO3-- induced pH change was not due to modulation of Na+/H+ exchange, since it was also observed in Na+/H+ antiport-deficient mutants. Though NO3- is known to inhibit vacuolar-type (V) H+-ATPases, this effect was not responsible for the acidification since it persisted in the presence of the potent V-ATPase inhibitor bafilomycin A1. NO3-/HCO3- exchange as the underlying mechanism was ruled out because acidification occurred despite nominal removal of HCO3-, despite inhibition of the anion exchanger with disulfonic stilbenes and in HEK 293 cells, which seemingly lack anion exchangers (Lee, B. S., R.B. Gunn, and R.R. Kopito. 1991. J. Biol. Chem. 266:11448- 11454). Accumulation of intracellular NO3-, measured by the Greiss method after reduction to NO2-, indicated that the anion is translocated into the cells along with the movement of acid equivalents. The simplest model to explain these observations is the cotransport of NO3- with H+ (or the equivalent counter-transport of NO3- for OH-). The transporter appears to be bi-directional, operating in the forward as well as reverse directions. A rough estimate of the fluxes of NO3- and acid equivalents suggests a one-to-one stoichiometry. Accordingly, the rate of transport was unaffected by sizable changes in transmembrane potential. The cytosolic acidification was a saturable function of the extracellular concentration of NO3- and was accentuated by acidification of the extracellular space. The putative NO3--H+ cotransport was inhibited markedly by ethacrynic acid and by alpha-cyano-4-hydroxycinnamate, but only marginally by 4, 4'-diisothiocyanostilbene-2,2' disulfonate or by p-chloromercuribenzene sulfonate. The transporter responsible for NO3--induced pH changes in mammalian cells may be related, though not identical, to the NO3--H+ cotransporter described in Arabidopsis and Aspergillus. The mammalian cotransporter may be important in eliminating the products of NO metabolism, particularly in cells that generate vast amounts of this messenger. By cotransporting NO3- with H+ the cells would additionally eliminate acid equivalents from activated cells that are metabolizing actively, without added energetic investment and with minimal disruption of the transmembrane potential, inasmuch as the cotransporter is likely electroneutral.

Localization of an actin binding domain in smooth muscle myosin light chain kinase

Phosphorylation of the regulatory light chain of myosin II by myosin light chain kinase is important for regulating many contractile processes. Smooth muscle myosin light chain kinase has been shown to be associated with both actin and myosin filaments in vitro and in vivo. In this report we define an actin binding region by using molecular deletions to generate recombinant mutant proteins that were analyzed by co-sedimentation with F-actin. An actin binding region restricted to residues 2-42 in the amino terminus of the rabbit smooth muscle myosin light chain kinase was identified.

Induction of tyrosine phosphorylation and Na+/H+ exchanger activation during shrinkage of human neutrophils

The ubiquitous isoform of the Na+/H+ exchanger (NHE1) is essential for the regulation of cellular volume. The underlying molecular mechanism, which is poorly understood, was studied in human polymorphonuclear leukocytes (PMN). Suspension of PMN in hypertonic media induced rapid cellular shrinkage and activation of NHE1, which is measurable as a cytosolic alkalinization. Concomitantly, hypertonic stress also induced extensive tyrosine phosphorylation of several proteins. Pretreatment of PMN with genistein, a tyrosine kinase inhibitor, prevented not only the tyrosine phosphorylation in response to a hypertonic shock but also the activation of NHE1. The signal elicited by hyperosmolarity that induces activation of tyrosine kinases and NHE1 was investigated. Methods were devised to change medium osmolarity without altering cell volume and vice versa. Increasing medium and intracellular osmolarity in normovolemic cells failed to activate tyrosine kinases or NHE1. However, shrinkage of cells under iso-osmotic conditions stimulated both tyrosine phosphorylation and NHE1 activity. These findings imply that cells detect alterations in cell size but not changes in osmolarity or ionic strength. The identity of the proteins that were tyrosine-phosphorylated in response to cell shrinkage was also investigated. Unexpectedly, the mitogen-activated protein kinases SAPK, p38, erk1, and erk2 were not detectably phosphorylated or activated. In contrast, the tyrosine kinases p59(fgr) and p56/59(hck) were phosphorylated and activated upon hypertonic challenge. We propose that cells respond to alterations in cell size, but not to changes in osmolarity, with increased tyrosine phosphorylation, which in turn leads to the activation of NHE1. The resulting changes in ion content and cytosolic pH contribute to the restoration of cell volume in shrunken cells.