Model of ion transport regulation in chloride-secreting airway epithelial cells. Integrated description of electrical, chemical, and fluorescence measurements

Mathematical models of cystic fibrosis as a systemic disease

Cystic fibrosis (CF) is widely known as a disease of the lung, even though it is in truth a systemic disease, whose symptoms typically manifest in gastrointestinal dysfunction first. CF ultimately impairs not only the pancreas and intestine but also the lungs, gonads, liver, kidneys, bones, and the cardiovascular system. It is caused by one of several mutations in the gene of the epithelial ion channel protein CFTR. Intense research and improved antimicrobial treatments during the past eight decades have steadily increased the predicted life expectancy of a person with CF (pwCF) from a few weeks to over 50 years. Moreover, several drugs ameliorating the sequelae of the disease have become available in recent years, and notable treatments of the root cause of the disease have recently generated substantial improvements in health for some but not all pwCF. Yet, numerous fundamental questions remain unanswered. Complicating CF, for instance in the lung, is the fact that the associated insufficient chloride secretion typically perturbs the electrochemical balance across epithelia and, in the airways, leads to the accumulation of thick, viscous mucus and mucus plaques that cannot be cleared effectively and provide a rich breeding ground for a spectrum of bacterial and fungal communities. The subsequent infections often become chronic and respond poorly to antibiotic treatments, with outcomes sometimes only weakly correlated with the drug susceptibility of the target pathogen. Furthermore, in contrast to rapidly resolved acute infections with a single target pathogen, chronic infections commonly involve multi-species bacterial communities, called "infection microbiomes," that develop their own ecological and evolutionary dynamics. It is presently impossible to devise mathematical models of CF in its entirety, but it is feasible to design models for many of the distinct drivers of the disease. Building upon these growing yet isolated modeling efforts, we discuss in the following the feasibility of a multi-scale modeling framework, known as template-and-anchor modeling, that allows the gradual integration of refined sub-models with different granularity. The article first reviews the most important biomedical aspects of CF and subsequently describes mathematical modeling approaches that already exist or have the potential to deepen our understanding of the multitude aspects of the disease and their interrelationships. The conceptual ideas behind the approaches proposed here do not only pertain to CF but are translatable to other systemic diseases. This article is categorized under: Congenital Diseases > Computational Models.

Physiology and pathophysiology of human airway mucus

The mucus clearance system is the dominant mechanical host defense system of the human lung. Mucus is cleared from the lung by cilia and airflow, including both two-phase gas-liquid pumping and cough-dependent mechanisms, and mucus transport rates are heavily dependent on mucus concentration. Importantly, mucus transport rates are accurately predicted by the gel-on-brush model of the mucociliary apparatus from the relative osmotic moduli of the mucus and periciliary-glycocalyceal (PCL-G) layers. The fluid available to hydrate mucus is generated by transepithelial fluid transport. Feedback interactions between mucus concentrations and cilia beating, via purinergic signaling, coordinate Na+ absorptive vs Cl- secretory rates to maintain mucus hydration in health. In disease, mucus becomes hyperconcentrated (dehydrated). Multiple mechanisms derange the ion transport pathways that normally hydrate mucus in muco-obstructive lung diseases, e.g., cystic fibrosis (CF), chronic obstructive pulmonary disease (COPD), non-CF bronchiectasis (NCFB), and primary ciliary dyskinesia (PCD). A key step in muco-obstructive disease pathogenesis is the osmotic compression of the mucus layer onto the airway surface with the formation of adherent mucus plaques and plugs, particularly in distal airways. Mucus plaques create locally hypoxic conditions and produce airflow obstruction, inflammation, infection, and, ultimately, airway wall damage. Therapies to clear adherent mucus with hydrating and mucolytic agents are rational, and strategies to develop these agents are reviewed.

Mathematical model reveals role of nucleotide signaling in airway surface liquid homeostasis and its dysregulation in cystic fibrosis

Mucociliary clearance is composed of three components (i.e., mucin secretion, airway surface hydration, and ciliary-activity) which function coordinately to clear inhaled microbes and other foreign particles from airway surfaces. Airway surface hydration is maintained by water fluxes driven predominantly by active chloride and sodium ion transport. The ion channels that mediate electrogenic ion transport are regulated by extracellular purinergic signals that signal through G protein-coupled receptors. These purinoreceptors and the signaling pathways they activate have been identified as possible therapeutic targets for treating lung disease. A systems-level description of airway surface liquid (ASL) homeostasis could accelerate development of such therapies. Accordingly, we developed a mathematical model to describe the dynamic coupling of ion and water transport to extracellular purinergic signaling. We trained our model from steady-state and time-dependent experimental measurements made using normal and cystic fibrosis (CF) cultured human airway epithelium. To reproduce CF conditions, reduced chloride secretion, increased potassium secretion, and increased sodium absorption were required. The model accurately predicted ASL height under basal normal and CF conditions and the collapse of surface hydration due to the accelerated nucleotide metabolism associated with CF exacerbations. Finally, the model predicted a therapeutic strategy to deliver nucleotide receptor agonists to effectively rehydrate the ASL of CF airways.

Bicarbonate-rich fluid secretion predicted by a computational model of guinea-pig pancreatic duct epithelium

KEY POINTS

The ductal system of the pancreas secretes large volumes of alkaline fluid containing HCO₃^- concentrations as high as 140 mm during hormonal stimulation. A computational model has been constructed to explore the underlying ion transport mechanisms. Parameters were estimated by fitting the model to experimental data from guinea-pig pancreatic ducts. The model was readily able to secrete 140 mm HCO₃^- . Its capacity to do so was not dependent upon special properties of the cystic fibrosis transmembrane conductance regulator (CFTR) anion channels and solute carrier family 26 member A6 (SLC26A6) anion exchangers. We conclude that the main requirement for secreting high HCO₃^- concentrations is to minimize the secretion of Cl^- ions. These findings help to clarify the mechanism responsible for pancreatic HCO₃^- secretion, a vital process that prevents the formation of protein plugs and viscous mucus in the ducts, which could otherwise lead to pancreatic disease.

ABSTRACT

A computational model of guinea-pig pancreatic duct epithelium was developed to determine the transport mechanism by which HCO₃^- ions are secreted at concentrations in excess of 140 mm. Parameters defining the contributions of the individual ion channels and transporters were estimated by least-squares fitting of the model predictions to experimental data obtained from isolated ducts and intact pancreas under a range of experimental conditions. The effects of cAMP-stimulated secretion were well replicated by increasing the activities of the basolateral Na⁺ -HCO₃^- cotransporter (NBC1) and apical Cl^- /HCO₃^- exchanger (solute carrier family 26 member A6; SLC26A6), increasing the basolateral K⁺ permeability and apical Cl^- and HCO₃^- permeabilities (CFTR), and reducing the activity of the basolateral Cl^- /HCO₃^- exchanger (anion exchanger 2; AE2). Under these conditions, the model secreted ∼140 mm HCO₃^- at a rate of ∼3 nl min^-1  mm^-2 , which is consistent with experimental observations. Alternative 1:2 and 1:1 stoichiometries for Cl^- /HCO₃^- exchange via SLC26A6 at the apical membrane were able to support a HCO₃^- -rich secretion. Raising the HCO₃^- /Cl^- permeability ratio of CFTR from 0.4 to 1.0 had little impact upon either the secreted HCO₃^- concentration or the volume flow. However, modelling showed that a reduction in basolateral AE2 activity by ∼80% was essential in minimizing the intracellular Cl^- concentration following cAMP stimulation and thereby maximizing the secreted HCO₃^- concentration. The addition of a basolateral Na⁺ -K⁺ -2Cl^- cotransporter (NKCC1), assumed to be present in rat and mouse ducts, raised intracellular Cl^- and resulted in a lower secreted HCO₃^- concentration, as is characteristic of those species. We conclude therefore that minimizing the driving force for Cl^- secretion is the main requirement for secreting 140 mm HCO₃^- .

Simulation of Cl(-) Secretion in Epithelial Tissues: New Methodology Estimating Activity of Electro-Neutral Cl(-) Transporter

Transcellular Cl⁻ secretion is, in general, mediated by two steps; (1) the entry step of Cl⁻ into the cytosolic space from the basolateral space across the basolateral membrane by Cl⁻ transporters, such as Na⁺-K⁺-2Cl⁻ cotransporter (NKCC1, an isoform of NKCC), and (2) the releasing step of Cl⁻ from the cytosolic space into the luminal (air) space across the apical membrane via Cl⁻ channels, such as cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel. Transcellular Cl⁻ secretion has been characterized by using various experimental techniques. For example, measurements of short-circuit currents in the Ussing chamber and patch clamp techniques provide us information on transepithelial ion movements via transcellular pathway, transepithelial conductance, activity (open probability) of single channel, and whole cell currents. Although many investigators have tried to clarify roles of Cl⁻ channels and transporters located at the apical and basolateral membranes in transcellular Cl⁻ secretion, it is still unclear how Cl⁻ channels/transporters contribute to transcellular Cl⁻ secretion and are regulated by various stimuli such as Ca²⁺ and cAMP. In the present study, we simulate transcellular Cl⁻ secretion using mathematical models combined with electrophysiological measurements, providing information on contribution of Cl⁻ channels/transporters to transcellular Cl⁻ secretion, activity of electro-neutral ion transporters and how Cl⁻ channels/transporters are regulated.

A spatial model of fluid recycling in the airways of the lung

The genetic disease cystic fibrosis (CF) is a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, and results in viscous mucus and impaired mucociliary clearance leading to chronic recurring pulmonary infections. Although extensive experimental research has been conducted over the last few decades, CF lung pathophysiology remains controversial. There are two competing explanations for the observed depletion of periciliary liquid (PCL) in CF lungs. The low volume hypothesis assumes fluid hyperabsorption through surface epithelia due to an over-active epithelial Na(+) channel (ENaC), and the low secretion hypothesis assumes inspissated mucins secreted from glands due to lack of serous fluid secreted from gland acini. We present a spatial mathematical model that reflects in vivo fluid recycling via submucosal gland (SMG) secretion, and absorption through surface epithelia. We then test the model in CF conditions by increasing ENaC open probability and decreasing SMG flux while simultaneously reducing CFTR open probability. Increasing ENaC activity only results in increased fluid absorption across surface epithelia, as seen in in vitro experiments. However, combining potential CF mechanisms results in markedly less fluid absorbed while providing the largest reduction in PCL volume, suggesting that a compromise in gland fluid secretion dominates over increased ENaC activity to decrease the amount of fluid transported transcellularly in CF lungs in vivo. Model results also indicate that a spatial model is necessary for an accurate calculation of total fluid transport, as the effects of spatial gradients can be severe, particularly in close proximity to the SMGs.

Transepithelial glucose transport and Na+/K+ homeostasis in enterocytes: an integrative model

The uptake of glucose and the nutrient coupled transcellular sodium traffic across epithelial cells in the small intestine has been an ongoing topic in physiological research for over half a century. Driving the uptake of nutrients like glucose, enterocytes must have regulatory mechanisms that respond to the considerable changes in the inflow of sodium during absorption. The Na-K-ATPase membrane protein plays a major role in this regulation. We propose the hypothesis that the amount of active Na-K-ATPase in enterocytes is directly regulated by the concentration of intracellular Na(+) and that this regulation together with a regulation of basolateral K permeability by intracellular ATP gives the enterocyte the ability to maintain ionic Na(+)/K(+) homeostasis. To explore these regulatory mechanisms, we present a mathematical model of the sodium coupled uptake of glucose in epithelial enterocytes. Our model integrates knowledge about individual transporter proteins including apical SGLT1, basolateral Na-K-ATPase, and GLUT2, together with diffusion and membrane potentials. The intracellular concentrations of glucose, sodium, potassium, and chloride are modeled by nonlinear differential equations, and molecular flows are calculated based on experimental kinetic data from the literature, including substrate saturation, product inhibition, and modulation by membrane potential. Simulation results of the model without the addition of regulatory mechanisms fit well with published short-term observations, including cell depolarization and increased concentration of intracellular glucose and sodium during increased concentration of luminal glucose/sodium. Adding regulatory mechanisms for regulation of Na-K-ATPase and K permeability to the model show that our hypothesis predicts observed long-term ionic homeostasis.

A model of lysosomal pH regulation

Lysosomes must maintain an acidic luminal pH to activate hydrolytic enzymes and degrade internalized macromolecules. Acidification requires the vacuolar-type H(+)-ATPase to pump protons into the lumen and a counterion flux to neutralize the membrane potential created by proton accumulation. Early experiments suggested that the counterion was chloride, and more recently a pathway consistent with the ClC-7 Cl(-)/H(+) antiporter was identified. However, reports that the steady-state luminal pH is unaffected in ClC-7 knockout mice raise questions regarding the identity of the carrier and the counterion. Here, we measure the current-voltage characteristics of a mammalian ClC-7 antiporter, and we use its transport properties, together with other key ion regulating elements, to construct a mathematical model of lysosomal pH regulation. We show that results of in vitro lysosome experiments can only be explained by the presence of ClC-7, and that ClC-7 promotes greater acidification than Cl(-), K(+), or Na(+) channels. Our models predict strikingly different lysosomal K(+) dynamics depending on the major counterion pathways. However, given the lack of experimental data concerning acidification in vivo, the model cannot definitively rule out any given mechanism, but the model does provide concrete predictions for additional experiments that would clarify the identity of the counterion and its carrier.

Biophysical model of ion transport across human respiratory epithelia allows quantification of ion permeabilities

Lung health and normal mucus clearance depend on adequate hydration of airway surfaces. Because transepithelial osmotic gradients drive water flows, sufficient hydration of the airway surface liquid depends on a balance between ion secretion and absorption by respiratory epithelia. In vitro experiments using cultures of primary human nasal epithelia and human bronchial epithelia have established many of the biophysical processes involved in airway surface liquid homeostasis. Most experimental studies, however, have focused on the apical membrane, despite the fact that ion transport across respiratory epithelia involves both cellular and paracellular pathways. In fact, the ion permeabilities of the basolateral membrane and paracellular pathway remain largely unknown. Here we use a biophysical model for water and ion transport to quantify ion permeabilities of all pathways (apical, basolateral, paracellular) in human nasal epithelia cultures using experimental (Ussing Chamber and microelectrode) data reported in the literature. We derive analytical formulas for the steady-state short-circuit current and membrane potential, which are for polarized epithelia the equivalent of the Goldman-Hodgkin-Katz equation for single isolated cells. These relations allow parameter estimation to be performed efficiently. By providing a method to quantify all the ion permeabilities of respiratory epithelia, the model may aid us in understanding the physiology that regulates normal airway surface hydration.

Increased apical Na+ permeability in cystic fibrosis is supported by a quantitative model of epithelial ion transport

Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which encodes an anion channel. In the human lung CFTR loss causes abnormal ion transport across airway epithelial cells. As a result CF individuals produce thick mucus, suffer persistent bacterial infections and have a much reduced life expectancy. Trans-epithelial potential difference (V_t) measurements are routinely carried out on nasal epithelia of CF patients in the clinic. CF epithelia exhibit a hyperpolarised basal V_t and a larger V_t change in response to amiloride (a blocker of the epithelial Na⁺ channel, ENaC). Are these altered bioelectric properties solely a result of electrical coupling between the ENaC and CFTR currents, or are they due to an increased ENaC permeability associated with CFTR loss? To examine these issues we have developed a quantitative mathematical model of human nasal epithelial ion transport. We find that while the loss of CFTR permeability hyperpolarises V_t and also increases amiloride-sensitive V_t, these effects are too small to account for the magnitude of change observed in CF epithelia. Instead, a parallel increase in ENaC permeability is required to adequately fit observed experimental data. Our study provides quantitative predictions for the complex relationships between ionic permeabilities and nasal V_t, giving insights into the physiology of CF disease that have important implications for CF therapy.

A mechanochemical model for auto-regulation of lung airway surface layer volume

We develop a proof-of-principle model for auto-regulation of water volume in the lung airway surface layer (ASL) by coupling biochemical kinetics, transient ASL volume, and homeostatic mechanical stresses. The model is based on the hypothesis that ASL volume is sensed through soluble mediators and phasic stresses generated by beating cilia and air drag forces. Model parameters are fit based on the available data on human bronchial epithelial cell cultures. Simulations then demonstrate that homeostatic volume regulation is a natural consequence of the processes described. The model maintains ASL volume within a physiological range that modulates with phasic stress frequency and amplitude. Next, we show that the model successfully reproduces the responses of cell cultures to significant isotonic and hypotonic challenges, and to hypertonic saline, an effective therapy for mucus hydration in cystic fibrosis patients. These results compel an advanced airway hydration model with therapeutic value that will necessitate detailed kinetics of multiple molecular pathways, feedback to ASL viscoelasticity properties, and stress signaling from the ASL to the cilia and epithelial cells.

Biophysical model of ion transport across human respiratory epithelia allows quantification of ion permeabilities

Lung health and normal mucus clearance depend on adequate hydration of airway surfaces. Because transepithelial osmotic gradients drive water flows, sufficient hydration of the airway surface liquid depends on a balance between ion secretion and absorption by respiratory epithelia. In vitro experiments using cultures of primary human nasal epithelia and human bronchial epithelia have established many of the biophysical processes involved in airway surface liquid homeostasis. Most experimental studies, however, have focused on the apical membrane, despite the fact that ion transport across respiratory epithelia involves both cellular and paracellular pathways. In fact, the ion permeabilities of the basolateral membrane and paracellular pathway remain largely unknown. Here we use a biophysical model for water and ion transport to quantify ion permeabilities of all pathways (apical, basolateral, paracellular) in human nasal epithelia cultures using experimental (Ussing Chamber and microelectrode) data reported in the literature. We derive analytical formulas for the steady-state short-circuit current and membrane potential, which are for polarized epithelia the equivalent of the Goldman-Hodgkin-Katz equation for single isolated cells. These relations allow parameter estimation to be performed efficiently. By providing a method to quantify all the ion permeabilities of respiratory epithelia, the model may aid us in understanding the physiology that regulates normal airway surface hydration.

Fluid Transport Across Leaky Epithelia: Central Role of the Tight Junction and Supporting Role of Aquaporins

The mechanism of epithelial fluid transport remains unsolved, which is partly due to inherent experimental difficulties. However, a preparation with which our laboratory works, the corneal endothelium, is a simple leaky secretory epithelium in which we have made some experimental and theoretical headway. As we have reported, transendothelial fluid movements can be generated by electrical currents as long as there is tight junction integrity. The direction of the fluid movement can be reversed by current reversal or by changing junctional electrical charges by polylysine. Residual endothelial fluid transport persists even when no anions (hence no salt) are being transported by the tissue and is only eliminated when all local recirculating electrical currents are. Aquaporin (AQP) 1 is the only AQP present in these cells, and its deletion in AQP1 null mice significantly affects cell osmotic permeability (by ∼40%) but fluid transport much less (∼20%), which militates against the presence of sizable water movements across the cell. In contrast, AQP1 null mice cells have reduced regulatory volume decrease (only 60% of control), which suggests a possible involvement of AQP1 in either the function or the expression of volume-sensitive membrane channels/transporters. A mathematical model of corneal endothelium we have developed correctly predicts experimental results only when paracellular electro-osmosis is assumed rather than transcellular local osmosis. Our evidence therefore suggests that the fluid is transported across this layer via the paracellular route by a mechanism that we attribute to electro-osmotic coupling at the junctions. From our findings we have developed a novel paradigm for this preparation that includes 1) paracellular fluid flow; 2) a crucial role for the junctions; 3) hypotonicity of the primary secretion; and 4) an AQP role in regulation rather than as a significant water pathway. These elements are remarkably similar to those proposed by the laboratory of Adrian Hill for fluid transport across other leaky epithelia.

A biophysical model for integration of electrical, osmotic, and pH regulation in the human bronchial epithelium

A dynamical biophysical model for the functioning of an epithelium is presented. This model integrates the electrical and osmotic behaviors of the epithelium, taking into account intracellular conditions. The specific tissue modeled is the human bronchial epithelium, which is of particular interest, as it is the location of the most common lethal symptoms of cystic fibrosis. The model is implemented in a modular form to facilitate future application of the code to other epithelial tissue by inputting different transporters, channels, and geometric parameters. The model includes pH regulation as an integral component of overall regulation of epithelial function, through the interdependence of pH, bicarbonate concentration, and current. The procedures for specification, the validation of the model, and parametric studies are presented using available experimental data of cultured human bronchial epithelium. Parametric studies are performed to elucidate a), the contribution of basolateral chloride channels to the short-circuit current functional form, and b), the role that regulation of basolateral potassium conductance plays in epithelial function.

A mathematical model of calcium-induced fluid secretion in airway epithelium

Regulation of periciliary liquid (PCL) depth is of central importance to mucociliary clearance by the airway epithelium. Without adequate hydration mucociliary transport would cease, leading to build up of mucus in the airways, and impairing the clearance of any trapped inhaled particulates. Airway epithelial cells are known to release ATP under a number of stress conditions. Cell surface receptors bind ATP and trigger an intracellular calcium response which regulates the gating of specific ion channels on the apical and basolateral cell membranes. This shifts the electrochemical balance, resulting in the accumulation of Na(+) and Cl(-) in the periciliary liquid, and providing an osmotic driving force for water flux. In this study, we present a mathematical model of a single airway epithelial cell which describes the fluid secretion elicited after a rise in intracellular calcium. The model provides a basis to quantitatively analyse the influence of intracellular calcium signalling on fluid movement. The model demonstrates behaviour consistent with a number of experimental data on manipulating periciliary liquid volume and tonicity, and provides a quantitative basis for analysing the role of the different membrane ion channels in determining water flux following different physiological stimuli.

In situ measurement of airway surface liquid [K+] using a ratioable K+-sensitive fluorescent dye

The airway surface liquid (ASL) is the thin fluid layer lining airway surface epithelial cells, whose volume and composition are tightly regulated and may be abnormal in cystic fibrosis (CF). We synthesized a two-color fluorescent dextran to measure ASL [K(+)], TAC-Lime-dextran-TMR, consisting of a green-fluorescing triazacryptand K(+) ionophore-Bodipy conjugate, coupled to dextran, together with a red fluorescing tetramethylrhodamine reference chromophore. TAC-Lime-dextran-TMR fluorescence was K(+)-selective, increasing >4-fold with increasing [K(+)] from 0 to 40 mm. In well differentiated human airway epithelial cells, ASL [K(+)] was 20.8 +/- 0.3 mm and decreased by inhibition of the Na(+)/K(+) pump (ouabain), ENaC (amiloride), CF transmembrane conductance regulator (CFTR(inh)-172), or K(+) channels (TEA or XE991). ASL [K(+)] was increased by forskolin but not affected by Na(+)/K(+)/2Cl(-) cotransporter inhibition (bumetanide). Functional and expression studies indicated the involvement of [K(+)] channels KCNQ1, KCNQ3, and KCNQ5 as determinants of ASL [K(+)]. [K(+)] in CF cultures was similar to that in non-CF cultures, suggesting that abnormal ASL [K(+)] is not a factor in CF lung disease. In intact airways, ASL [K(+)] was also well above extracellular [K(+)]: 22 +/- 1 mm in pig trachea ex vivo and 16 +/- 1 mm in mouse trachea in vivo. Our results provide the first noninvasive measurements of [K(+)] in the ASL and indicate the involvement of apical and basolateral membrane ion transporters in maintaining a high ASL [K(+)].

Towards a virtual lung: multi-scale, multi-physics modelling of the pulmonary system

The essential function of the lung, gas exchange, is dependent on adequate matching of ventilation and perfusion, where air and blood are delivered through complex branching systems exposed to regionally varying transpulmonary and transmural pressures. Structure and function in the lung are intimately related, yet computational models in pulmonary physiology usually simplify or neglect structure. The geometries of the airway and vascular systems and their interaction with parenchymal tissue have an important bearing on regional distributions of air and blood, and therefore on whole lung gas exchange, but this has not yet been addressed by modelling studies. Models for gas exchange have typically incorporated considerable detail at the level of chemical reactions, with little thought for the influence of structure. To date, relatively little attention has been paid to modelling at the cellular or subcellular level in the lung, or to linking information from the protein structure/interaction and cellular levels to the operation of the whole lung. We review previous work in developing anatomically based models of the lung, airways, parenchyma and pulmonary vasculature, and some functional studies in which these models have been used. Models for gas exchange at several spatial scales are briefly reviewed, and the challenges and benefits from modelling cellular function in the lung are discussed.

CD31 delays phagocyte membrane repolarization to promote efficient binding of apoptotic cells

Homophilic ligation of CD31, a member of the Ig superfamily of adhesion receptors, promotes macrophage clearance of apoptotic leukocytes by a mechanism hitherto not described. In studying CD31-dependent regulation of beta1-integrin binding of fibronectin-coated Latex beads, we discovered a role for the voltage-gated potassium channel ether-à-go-go-related gene (ERG) as a downstream effector of CD31 signaling. ERG was identified by tandem mass spectrometry as a 140-kDa protein, which was selectively modified with biotin following the targeted delivery of a biotin-transfer reagent to CD31 using Fab fragments of an anti-CD31 mAb. Similar results were obtained with macrophages but not K562 cells, expressing a truncated cytoplasmic tail of CD31, which failed to regulate bead binding. Colocalization of CD31 with ERG was confirmed by immunofluorescence for K562 cells and macrophages. We now demonstrate that the resting membrane potential of macrophages is depolarized on contact with apoptotic cells and that CD31 inhibits the ERG current, which would otherwise function to repolarize. Sustained depolarization favored the firm binding of phagocytic targets, a prerequisite for efficient engulfment. Our results identify ERG as a downstream effector of CD31 in the regulation of integrin-dependent binding of apoptotic cells by macrophages.

The Role of the Tight Junction in Paracellular Fluid Transport across Corneal Endothelium. Electro-osmosis as a Driving Force

The mechanism of epithelial fluid transport is controversial and remains unsolved. Experimental difficulties pose obstacles for work on a complex phenomenon in delicate tissues. However, the corneal endothelium is a relatively simple system to which powerful experimental tools can be applied. In recent years our laboratory has developed experimental evidence and theoretical insights that illuminate the mechanism of fluid transport across this leaky epithelium. Our evidence points to fluid being transported via the paracellular route by a mechanism requiring junctional integrity, which we attribute to electro-osmotic coupling at the junctions. Fluid movements can be produced by electrical currents. The direction of the movement can be reversed by current reversal or by changing junctional electrical charges by polylysine. Aquaporin 1 (AQP1) is the only AQP present in these cells, and its deletion in AQP1 null mice significantly affects cell osmotic permeability but not fluid transport, which militates against the presence of sizable water movements across the cell. By contrast, AQP1 null mice cells have reduced regulatory volume decrease (only 60% of control), which suggests a possible involvement of AQP1 in either the function or the expression of volume-sensitive membrane channels/transporters. A mathematical model of corneal endothelium predicts experimental results only when based on paracellular electro-osmosis, and not when transcellular local osmosis is assumed instead. Our experimental findings in corneal endothelium have allowed us to develop a novel paradigm for this preparation that includes: (1) paracellular fluid flow; (2) a crucial role for the junctions; (3) hypotonicity of the primary secretion; (4) an AQP role in regulation and not as a significant water pathway. These elements are remarkably similar to those proposed by the Hill laboratory for leaky epithelia.

Proton leak and CFTR in regulation of Golgi pH in respiratory epithelial cells

Work addressing whether cystic fibrosis transmembrane conductance regulator (CFTR) plays a role in regulating organelle pH has remained inconclusive. We engineered a pH-sensitive excitation ratiometric green fluorescent protein (pHERP) and targeted it to the Golgi with sialyltransferase (ST). As determined by ratiometric imaging of cells expressing ST-pHERP, Golgi pH (pH(G)) of HeLa cells was 6.4, while pH(G) of mutant (DeltaF508) and wild-type CFTR-expressing (WT-CFTR) respiratory epithelia were 6.7-7.0. Comparison of genetically matched DeltaF508 and WT-CFTR cells showed that the absence of CFTR statistically increased Golgi acidity by 0.2 pH units, though this small difference was unlikely to be physiologically important. Golgi pH was maintained by a H(+) vacuolar (V)-ATPase countered by a H(+) leak, which was unaffected by CFTR. To estimate Golgi proton permeability (P(H(+))), we modeled transient changes in pH(G) induced by inhibiting the V-ATPase and by acidifying the cytosol. This analysis required knowing Golgi buffer capacity, which was pH dependent. Our in vivo estimate is that Golgi P(H(+)) = 7.5 x 10(-4) cm/s when pH(G) = 6.5, and surprisingly, P(H(+)) decreased as pH(G) decreased.

Mechanisms of pH regulation in the regulated secretory pathway

A precise pH gradient between organelles of the regulated secretory pathway is required for sorting and processing of prohormones. We studied pH regulation in live endocrine cells by targeting biotin-based pH indicators to cellular organelles expressing avidin-chimera proteins. In AtT-20 cells, we found that steady-state pH decreased from the endoplasmic reticulum (ER) (pH(ER) = 7.4 +/- 0.2, mean +/- S.D.) to Golgi (pH(G) = 6.2 +/- 0.4) to mature secretory granules (MSGs) (pH(MSG) = 5.5 +/- 0.4). Golgi and MSGs required active H(+) v-ATPases for acidification. ER, Golgi, and MSG steady-state pH values were also dependent upon the different H(+) leak rates across each membrane. However, neither steady-state pH(MSG) nor rates of passive H(+) leak were affected by Cl(-)-free solutions or valinomycin, indicating that MSG membrane potential was small and not a determinant of pH(MSG). Therefore, our data do not support earlier suggestions that organelle acidification is primarily regulated by Cl(-) conductances. Measurements of H(+) leak rates, buffer capacities, and estimates of surface areas and volumes of these organelles were applied to a mathematical model to determine the H(+) permeability (P(H+)) of each organelle membrane. We found that P(H+) decreased progressively from ER to Golgi to MSGs, and proper acidification of Golgi and MSGs required gradual decreases in P(H+) and successive increases in the active H(+) pump density.

Regulation of organelle acidity

Intracellular organelles have characteristic pH ranges that are set and maintained by a balance between ion pumps, leaks, and internal ionic equilibria. Previously, a thermodynamic study by Rybak et al. (Rybak, S., F. Lanni, and R. Murphy. 1997. Biophys. J. 73:674-687) identified the key elements involved in pH regulation; however, recent experiments show that cellular compartments are not in thermodynamic equilibrium. We present here a nonequilibrium model of lumenal acidification based on the interplay of ion pumps and channels, the physical properties of the lumenal matrix, and the organelle geometry. The model successfully predicts experimentally measured steady-state and transient pH values and membrane potentials. We conclude that morphological differences among organelles are insufficient to explain the wide range of pHs present in the cell. Using sensitivity analysis, we quantified the influence of pH regulatory elements on the dynamics of acidification. We found that V-ATPase proton pump and proton leak densities are the two parameters that most strongly influence resting pH. Additionally, we modeled the pH response of the Golgi complex to varying external solutions, and our findings suggest that the membrane is permeable to more than one dominant counter ion. From this data, we determined a Golgi complex proton permeability of 8.1 x 10(-6) cm/s. Furthermore, we analyzed the early-to-late transition in the endosomal pathway where Na,K-ATPases have been shown to limit acidification by an entire pH unit. Our model supports the role of the Na,K-ATPase in regulating endosomal pH by affecting the membrane potential. However, experimental data can only be reproduced by (1) positing the existence of a hypothetical voltage-gated chloride channel or (2) that newly formed vesicles have especially high potassium concentrations and small chloride conductance.

Fluorescent chloride indicators to assess the efficacy of CFTR cDNA delivery

Cl(-)-sensitive fluorescent indicators have been used extensively in cell culture systems to measure the Cl(-)-transporting function of the cystic fibrosis transmembrane conductance regulator protein CFTR. These indicators have been used in establishing a surrogate end point to assess the efficacy of CFTR cDNA delivery in human gene therapy trials. The ability to measure Cl- transport with high sensitivity in small and heterogeneous tissue samples makes the use of Cl- indicators potentially attractive in gene delivery studies. In this review article, the important technical aspects of Cl- transport measurements by fluorescent indicators such as SPQ are described, applications of Cl- indicators to assay CFTR function are critically evaluated, and new methodological developments are discussed. The available Cl- indicators have been effective in quantifying Cl- transport rates in cell culture models and in vitro systems such as isolated membrane vesicles and liposomes. However, the imperfect photophysical properties of existing Cl- indicators limit their utility in performing measurements in airway tissues, where gene transfer vectors are delivered in CF gene therapy trials. The low efficiency of gene transfer and the cellular heterogeneity in airway samples pose substantial obstacles to functional measurements of CFTR expression. Significant new developments in generating long-wavelength and dual-wavelength halide indicators are described, and recommendations are proposed for the use of the indicators in gene therapy trials.

Computational studies of ion-water flux coupling in the airway epithelium. I. Construction of model

A mathematical model of ion and water transport across the airway epithelium is presented. The model consists of 12 state variables representing ion concentrations, volumes, and membrane potentials. All osmotically significant membrane transport processes for which there is conclusive experimental evidence are included: passive apical sodium and chloride movement, basolateral sodium-potassium pumping, basolateral sodium-potassium-chloride cotransport, passive basolateral potassium movement, nonselective passive paracellular ion motion, and water transport across all membranes. Ion movements are described by Michaelis-Menten kinetics or by the constant field flux equation. Model parameters are established with Ussing chamber data. Model behavior is validated by comparing in vitro simulations with experimental results. The model accurately reproduces short-circuit chloride and sodium fluxes, short-circuit current, and open-circuit membrane potentials from Ussing chamber data in the secreting and nonsecreting states. The model is then used to describe the behavior of the airway epithelium in vivo, in which case the apical electrolyte compartment is small and of variable size and ionic composition.

Human uterine cervical epithelial cells grown on permeable support--a new model for the study of differentiation

The purpose of the present study was to establish culture conditions for human uterine cervical epithelial cells on permeable support and to determine how it affects cervical cell differentiation. Human ectocervical epithelial cells (hECE), HPV-16 immortalized hECE cells (ECE16-1) and Caski cells were grown on collagen-coated filters. Culture conditions, density of cells in culture and expression of epithelial and cervical-cell phenotypic markers were determined and compared in cells grown on filter and on solid support. Compared with the latter, cultures on filter had a higher cell density, hECE cells stratified to 5-12 cell layers compared to 1-3 on solid support, and cells of all three types expressed intercellular tight junctions. The cytokeratin profiles revealed differences between the three cell types as well as differences within the same cell species when grown on filter, compared to solid support. Of particular importance was the finding of a higher expression of K-13 in hECE grown on filter compared to solid support; K-13 is a marker of ectocervical cell differentiation. The cytokeratin profiles of the cultured hECE, ECE16-1 and Caski cells resembled those of ectocervical, squamous metaplastic and endocervical epithelia, respectively. hECE and ECE16-1 expressed involucrin protein, the level of which in both was higher in cells grown on filter compared to solid support. Polarization of the cultures was determined by morphology (stratification of hECE cells, expression of pseudomicrovilli in the apical cell membrane), selective apical vs. basolateral secretion of [35S]methionine- and [35S]cysteine-, [3H]fucose- and [14C]glucosamine-labeled molecules, and positive short-circuit current (Isc) under voltage-clamp conditions. Confluency of the cultures was determined by measuring transepithelial unidirectional fluxes of inert molecules with different molecular weights (MWs) through the paracellular pathway, and by measuring transepithelial conductance. The results indicated transepithelial permeability of 7-22.10(-6) cm.sec-1, which was 5-100 fold smaller compared to blank inserts, with a cut-off MW of 40-70 kDa for hECE and Caski cells. Transepithelial conductance ranged 18.5 to 51.5 mS.cm-2, indicating a leaky but confluent epithelia. Collectively the results indicate the epithelial nature of the cells and their improved differentiation when grown on filter support; hECE is a model for ectocervical epithelium while ECE16-1 and Caski express phenotypic characteristics of squamous metaplastic cervical epithelium and endocervical epithelium respectively.

Hormonal regulation of Cl transport in polar airway epithelia measured by a fluorescent indicator

Cl transport mechanisms in polarized cultures of canine tracheal epithelium were examined using an Ussing-type chamber with independent mucosal and serosal perfusion. Cl activity was monitored continuously from fluorescence of entrapped 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ). When added to the serosal (but not mucosal) solution, isoproterenol increased Cl fluxes across the apical membrane Cl more than fourfold. Apical Cl transport was sensitive to diphenylamine-2-carboxylate (DPC) but not to furosemide, whereas basolateral membrane Cl transport was sensitive to furosemide but not to DPC. Based on a mathematical model of Cl transport, we developed a sensitive protocol to measure hormone-sensitive Cl transport. In Cl-loaded cells in which basolateral Cl transport was partially inhibited by furosemide, mucosal Cl removal caused no Cl efflux before but rapid efflux (0.25 mM/s) after addition of isoproterenol or chlorophenylthio-cAMP. In the presence of indomethacin to block prostaglandin production, elevation of intracellular Ca by bradykinin or 4-bromo-A23187 did not cause Cl efflux, nor did Ca buffering with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid affect stimulation by the cAMP pathway. Phorbol 12-myristate 13-acetate increased Cl efflux submaximally (0.09 mM/s) but did not affect maximal stimulation by cAMP agonists. Methoxamine did not alter apical or basolateral membrane Cl transport.(ABSTRACT TRUNCATED AT 250 WORDS)

Development and biological applications of chloride-sensitive fluorescent indicators

Chloride movement across cell plasma and internal membranes, is of central importance for regulation of cell volume and pH, vectorial salt movement in epithelia, and, probably, intracellular traffic. Quinolinium-based chloride-sensitive fluorescent indicators provide a new approach to study chloride transport mechanisms and regulation that is complementary to 36Cl tracer methods, intracellular microelectrodes, and patch clamp. Indicator fluorescence is quenched by chloride by a collisional mechanism with Stern-Volmer constants of up to 220 M-1. Fluorescence is quenched selectively by chloride in physiological systems and responds to changes in chloride concentration in under 1 ms. The indicators are nontoxic and can be loaded into living cells for continuous measurement of intracellular chloride concentration by single-cell fluorescence microscopy. In this review, the structure-activity relationships for chloride-sensitive fluorescent indicators are described. Methodology for measurement of chloride transport in isolated vesicle and liposome systems and in intact cells is evaluated critically by use of examples from epithelial cell physiology. Future directions for synthesis of tailored chloride-sensitive indicators and new applications of indicators for studies of transport regulation and intracellular ion gradients are proposed.