Kinetic analysis of the regulation of the Na+/H+ exchanger NHE-1 by osmotic shocks.
NHE-1 is a ubiquitous, mitogen-activatable, mammalian Na+/H+ exchanger that maintains cytosolic pH and regulates cell volume. We have previously shown that the kinetics of NHE-1 positive cooperative activation by intracellular acidifications fit best with a Monod-Wyman-Changeux mechanism, in which a dimeric NHE-1 oscillates between a low- and a high-affinity conformation for intracellular protons. The ratio between these two forms, the allosteric equilibrium constant L0, is in favor of the low-affinity form, making the system inactive at physiological pH. Conversely the high-affinity form is stabilized by intracellular protons, resulting in the observed positive cooperativity. The aim of the present study was to investigate the kinetics and mechanism of NHE-1 regulation by osmotic shocks. We show that they modify the L0 parameter (865 +/- 95 and 3757 +/- 328 for 500 and 100 mOsM, respectively, vs 1549 +/- 57 in isotonic conditions).This results in an activation of NHE-1 by hypertonic shocks and, conversely, in an inhibition by hypotonic media. Quantitatively, this modulation of L0 follows an exponential distribution relative to osmolarity, that is, additive to the activation of NHE-1 by intracellular signaling pathways. These effects can be mimicked by the asymmetric insertion of amphiphilic molecules into the lipid bilayer. Finally, site-directed mutagenesis of NHE-1 shows that neither its association with membrane PIP2 nor its interaction with cortical actin are required for mechanosensation. In conclusion, NHE-1 allosteric equilibrium and, thus, its cooperative response to intracellular acidifications is extremely sensitive to modification of its membrane environment.
Lacroix
Jérôme
J
Université de Nice-Sophia Antipolis, CNRS FRE3093 Transport Ionique aspects normaux et pathologiques, Faculté des Sciences Parc Valrose, 06108 Nice cedex 2, France.
Poët
Mallorie
M
Huc
Laurence
L
Morello
Vincent
V
Djerbi
Nadir
N
Ragno
Michel
M
Rissel
Mary
M
Tekpli
Xavier
X
Gounon
Pierre
P
Lagadic-Gossmann
Dominique
D
Counillon
Laurent
L
D000818:Animals; D027682:Cation Transport Proteins / Q000378:metabolism; D002460:Cell Line; D006224:Cricetinae; D003600:Cytosol / Q000378:metabolism; D005347:Fibroblasts / Q000378:metabolism; D005786:Gene Expression Regulation; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008856:Microscopy, Fluorescence; D008954:Models, Biological; D009997:Osmotic Pressure; D020033:Protein Isoforms; D015398:Signal Transduction; D000074082:Sodium-Hydrogen Exchanger 1; D017923:Sodium-Hydrogen Exchangers / Q000737:chemistry* / Q000378:metabolism
D027682:Cation Transport Proteins; D020033:Protein Isoforms; C490998:SLC9A1 protein, human; D000074082:Sodium-Hydrogen Exchanger 1; D017923:Sodium-Hydrogen Exchangers