pHi dependence of Na-H exchange and H delivery in IEC-6 cells

Disordered enterocyte signaling and intestinal barrier dysfunction in the pathogenesis of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is the leading cause of death from gastrointestinal disease in neonates, and is characterized by the development of diffuse intestinal necrosis in the stressed, pre-term infant. Systemic stress causes a breakdown in the intestinal mucosal barrier, which leads to translocation of bacteria and endotoxin and the initiation of a signaling response within the enterocyte. This review summarizes recent evidence defining a clear role that defective enterocyte signaling plays in the pathogenesis of NEC through the following mechanisms: 1) The localized production of nitric oxide by villus enterocytes results in an increase in enterocyte apoptosis and impaired proliferation; 2) The translocation of endotoxin results in a PI3K-dependent activation of RhoA-GTPase within the enterocyte leading to decreased enterocyte migration and impaired restitution; 3) Dysregulated sodium-proton exchange within the enterocyte by endotoxin renders the enterocyte monolayer more susceptible to damage in the face of the acidic microenvironment characteristic of systemic sepsis; and 4) Endotoxin causes a p38-dependent release of the pro-inflammatory molecule COX-2 by the enterocyte, which potentiates the systemic inflammatory response. An understanding of the mechanisms by which disordered enterocyte signaling contributes to the pathogenesis of barrier failure and NEC--through these and other mechanisms--may lead to the identification of novel therapeutic approaches for this devastating disease.

Endotoxin differentially modulates the basolateral and apical sodium/proton exchangers (NHE) in enterocytes

BACKGROUND

Maintenance of enterocyte activity during extracellular acidosis requires functional sodium/proton exchangers (NHE), which are present at both basolateral and apical surfaces. Necrotizing enterocolitis is characterized by systemic hypoperfusion, metabolic acidosis, and the apical to basolateral translocation of endotoxin (lipopolysaccharide [LPS]). We hypothesized that LPS differentially impairs NHE activity at the basolateral or apical domains of enterocytes, leading to cellular acidification, and explored the mechanisms involved.

METHODS

Experimental necrotizing enterocolitis (NEC) was induced in newborn rats using a combination of gavage feeds and hypoxia. NHE isoforms were assessed in primary and cultured enterocytes by Western blot analysis and by confocal microscopy in the presence or absence of LPS. NHE activity was detected by single-cell fluorescent ratiometric imaging with the use of the pH-sensitive dye 2', 7'-bis-(2carboxyethyl) -5-(and-6)-carboxyfluorescein.

RESULTS

In both NEC and control rats, NHE1 was basolateral and NHE3 was apical. A similar distribution was observed in polarized IEC-6 cells. LPS caused a dose-dependent reduction in basolateral NHE1 activity in IEC-6 cells, but had no effect on apical NHE3 activity. This effect could not be accounted for by reduced expression or impaired plasma membrane localization of NHE isoforms. Strikingly, LPS-mediated NHE1 impairment caused marked cytoplasmic acidification under conditions of extracellular acidosis, whereas functional NHE1 maintained cytoplasmic pH homeostasis in control cells.

CONCLUSIONS

LPS selectively impairs basolateral NHE1 but not apical NHE3, leading to cytoplasmic acidification during extracellular acidosis. This effect could mediate impaired enterocyte function after LPS translocation and suggests a mechanism leading to barrier disruption in NEC.

pH of TGN and recycling endosomes of H+/K+-ATPase-transfected HEK-293 cells: implications for pH regulation in the secretory pathway

The influences of the gastric H+/K+ pump on organelle pH during trafficking to and from the plasma membrane were investigated using HEK-293 cells stably expressing the alpha- and beta-subunits of human H+/K+-ATPase (H+/K+-alpha,beta cells). The pH values of trans-Golgi network (pHTGN) and recycling endosomes (pHRE) were measured by transfecting H+/K+-alpha,beta cells with the pH-sensitive GFP pHluorin fused to targeting sequences of either TGN38 or synaptobrevin, respectively. Immunofluorescence showed that H+/K+-ATPase was present in the plasma membrane, TGN, and RE. The pHTGN was similar in both H+/K+-alpha,beta cells (pHTGN 6.36) and vector-transfected ("mock") cells (pHTGN 6.34); pHRE was also similar in H+/K+-alpha,beta (pHRE 6.40) and mock cells (pHRE 6.37). SCH28080 (inhibits H+/K+-ATPase) caused TGN to alkalinize by 0.12 pH units; subsequent addition of bafilomycin (inhibits H+ v-ATPase) caused TGN to alkalinize from pH 6.4 up to a new steady-state pHTGN of 7.0-7.5, close to pHcytosol. Similar results were observed in RE. Thus H+/K+-ATPases that trafficked to the plasma membrane were active but had small effects to acidify the TGN and RE compared with H+ v-ATPase. Mathematical modeling predicted a large number of H+ v-ATPases (8000) active in the TGN to balance a large, passive H+ leak (with PH approximately 10-3 cm/s) via unidentified pathways out of the TGN. We propose that in the presence of this effective, though inefficient, buffer system in the Golgi and TGN, H+/K+-ATPases (estimated to be approximately 4000 active in the TGN) and other transporters have little effect on luminal pH as they traffic to the plasma membrane.

Human duodenal spheroids for noninvasive intracellular pH measurement and quantification of regulation mechanisms under physiological conditions

Three-dimensional cell cultures (spheroids) of biopsies of human duodenum were used to develop a new noninvasive method for studying intercellular and intracellular mechanisms. Through examinations of intracellular pH regulation, high functional similarity to native tissue could be shown, as already evidenced morphologically. A special microperfusion chamber was developed to fix individual spheroids physically to a nylon net, via laminar perfusion flow through the chamber. A significant improvement over current fixation methods was shown by the increase of cell viability almost up to 100%. Viability of the spheroids was confirmed by trypan blue exclusion, by a LIVE/DEAD viability/cytotoxicity kit, and by BCECF distribution. Intracellular pH was measured by use of the pH-sensitive fluorescence dye BCECF. To investigate the intracellular pH regulation, spheroid-like vesicles were acidified by NH4Cl prepulse technique. The subsequent active intracellular pH recovery was blocked with Na+-free Krebs Henseleit (KH) solution, with amiloride KH (inhibitor of the Na+-H+-exchanger), or with H2DIDS KH (inhibitor of the HCO3(-)-Cl(-)-exchanger and Na+-HCO3(-)-cotransporter). The intracellular pH of the spheroids was 7.31 +/- 0.05. pH-backregulation after acidification was prevented by sodium-free buffer, amiloride, and H2DIDS. These experiments indicated the presence of a Na+-H+-exchanger and a Na+-HCO3(-)-cotransporter. In conclusion, the human duodenal spheroid is an excellent physiological system for in vitro studies of the human duodenum.

Na(+)/H(+) exchange in mosquito Malpighian tubules

Fluid secretion and intracellular pH were measured in isolated mosquito Malpighian tubules to determine the presence of Na(+)/H(+) exchange. Rates of fluid secretion by individual Malpighian tubules in vitro were inhibited by 78% of control in the presence of 100 microM 5-(N-ethyl-n-isopropyl)-amiloride (EIPA), a specific inhibitor of Na(+)/H(+) exchange. Steady-state intracellular pH was measured microfluorometrically by using 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein in individual Malpighian tubules. Bathing the Malpighian tubules in 0 mM extracellular Na(+) or in the presence of 100 microM EIPA reduced the steady-state intracellular pH by 0.5 pH units. Stimulation of the Na(+)/H(+) exchanger by using the NH(4)Cl pulse technique resulted in a rate of recovery from the NH(4)Cl-induced acute acid load of 8.7 +/- 1.0 x 10(-3) pH/s. The rates of recovery of intracellular pH after the acute acid load in the absence of extracellular Na(+) or in the presence of 100 microM EIPA were 0.7 +/- 0.6 and -0.3 +/- 0.3 x 10(-3) pH/s, respectively. These results indicate that mosquito Malpighian tubules possess a Na(+)/H(+) exchanger.

A kinetically defined Na+/H+ antiporter within a mathematical model of the rat proximal tubule

The luminal membrane antiporter of the proximal tubule has been represented using the kinetic formulation of E. Heinz (1978. Mechanics and Engergetics of Biological Transport. Springer-Verlag, Berlin) with the assumption of equilibrium binding and 1:1 stoichiometry. Competitive binding and transport of NH+4 is included within this model. Ion affinities and permeation velocities were selected in a least-squares fit to the kinetic parameters determined experimentally in renal membrane vesicles (Aronson, P.S., M.A. Suhm, and J. Nee. 1983. Journal of Biological Chemistry. 258:6767-6771). The modifier role of internal H+ to enhance transport beyond the expected kinetics (Aronson, P.S., J. Nee, and M. A. Suhm. 1982. Nature. 299:161-163) is represented as a velocity effect of H+ binding to a single site. This kinetic formulation of the Na+/H+ antiporter was incorporated within a model of the rat proximal tubule (Weinstein, A. M. 1994. American Journal of Physiology. 267:F237-F248) as a replacement for the representation by linear nonequilibrium thermodynamics (NET). The membrane density of the antiporter was selected to yield agreement with the rate of tubular Na+ reabsorption. Simulation of 0.5 cm of tubule predicts that the activity of the Na+/H+ antiporter is the most important force for active secretion of ammonia. Model calculations of metabolic acid-base disturbances are performed and comparison is made among antiporter representations (kinetic model, kinetic model without internal modifier, and NET formulation). It is found that the ability to sharply turn off Na+/H+ exchange in cellular alkalosis substantially eliminates the cell volume increase associated with high HCO3- conditions. In the tubule model, diminished Na+/H+ exchange in alkalosis blunts the axial decrease in luminal HCO3- and thus diminishes paracellular reabsorption of Cl-. In this way, the kinetics of the Na+/H+ antiporter could act to enhance distal delivery of Na+, Cl-, and HCO3- in acute metabolic alkalosis.

Rapid computerized analysis of Na+/H+ exchange flux

The regulation of intracellular pH (pHi) is mediated by membrane transporters whose activity is directly controlled by pHi. Therefore, transport rates must be compared at identical pHi values in functional studies of these transporters. This is conventionally performed using scatter plots showing initial rates of proton flux versus intracellular pH. We present justification for determining proton flux over a wide range of pHi, by digitally smoothing a pHi trace and then directly taking the first derivative versus time of smoothed data. Compared to conventional least-squares analysis of initial rates, the derivative method generates much more information per experiment. Compared to other methods which fit pHi traces to a defined equation prior to rate calculation, the new method does not require that the pHi trace be well fit by any given mathematical function. The derivative technique is illustrated in an analysis of Na+/H+ exchange in Caco-2 cells. Intracellular pH is measured fluorometrically in cells loaded with BCECF (2',7'-bis[2-carboxyethyl]-5-(and-6)carboxyfluorescein). To validate the analysis of Na+/H+ exchange over an extended time range, we demonstrate that cellular acidification with NH4Cl does not change steady state Na+ content. We find that proton flux rates analyzed by the derivative method are equivalent to initial rates measured by least-squares analysis.

Basolateral mechanisms of intracellular pH regulation in the colonic epithelial cell line HT29 clone 19A

The intracellular pH (pHi) of the colonic tumour cell line HT29 cl.19A was studied by microspectrofluorometry using the pH-sensitive dye BCECF. Single cells within a confluent monolayer, grown in a polarized manner on permeable supports, were examined. An amiloride-sensitive Na+/H+ exchange and a stilbene-insensitive Cl-/HCO3- exchange mechanism have been identified in the basolateral membrane. Removal of Na+ from the basolateral solution caused a decrease of pHi by 0.50 +/- 0.09 unit (n = 4). Amiloride or Na(+)-free solution at the apical side had no effect on pHi. Cl- removal at the basolateral side led to an increase of pHi by 0.20 +/- 0.03 unit (n = 4) whereas apical removal had no influence on pHi. This effect was independent of Na+ and was insensitive to 0.2 mM 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulphonic acid. A basolateral Cl-/HCO3- exchanger is the most likely explanation for this observation. The Na+/H+ exchange mechanism in the basolateral membrane is an acid extruder, whereas the Cl-/HCO3- exchanger is an acid loader. Both of these mechanisms are important for the maintenance of intracellular pH in HT29 cl.19A cells.