Ion activities in the lateral intercellular spaces of gallbladder epithelium transporting at low external osmolarities

Membrane transporters control cerebrospinal fluid formation independently of conventional osmosis to modulate intracranial pressure

Background

Disturbances in the brain fluid balance can lead to life-threatening elevation in the intracranial pressure (ICP), which represents a vast clinical challenge. Nevertheless, the details underlying the molecular mechanisms governing cerebrospinal fluid (CSF) secretion are largely unresolved, thus preventing targeted and efficient pharmaceutical therapy of cerebral pathologies involving elevated ICP.

Methods

Experimental rats were employed for in vivo determinations of CSF secretion rates, ICP, blood pressure and ex vivo excised choroid plexus for morphological analysis and quantification of expression and activity of various transport proteins. CSF and blood extractions from rats, pigs, and humans were employed for osmolality determinations and a mathematical model employed to determine a contribution from potential local gradients at the surface of choroid plexus.

Results

We demonstrate that CSF secretion can occur independently of conventional osmosis and that local osmotic gradients do not suffice to support CSF secretion. Instead, the CSF secretion across the luminal membrane of choroid plexus relies approximately equally on the Na⁺/K⁺/2Cl⁻ cotransporter NKCC1, the Na⁺/HCO₃⁻ cotransporter NBCe2, and the Na⁺/K⁺-ATPase, but not on the Na⁺/H⁺ exchanger NHE1. We demonstrate that pharmacological modulation of CSF secretion directly affects the ICP.

Conclusions

CSF secretion appears to not rely on conventional osmosis, but rather occur by a concerted effort of different choroidal transporters, possibly via a molecular mode of water transport inherent in the proteins themselves. Therapeutic modulation of the rate of CSF secretion may be employed as a strategy to modulate ICP. These insights identify new promising therapeutic targets against brain pathologies associated with elevated ICP.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12987-022-00358-4.

Cerebrospinal fluid production by the choroid plexus: a century of barrier research revisited

Cerebrospinal fluid (CSF) envelops the brain and fills the central ventricles. This fluid is continuously replenished by net fluid extraction from the vasculature by the secretory action of the choroid plexus epithelium residing in each of the four ventricles. We have known about these processes for more than a century, and yet the molecular mechanisms supporting this fluid secretion remain unresolved. The choroid plexus epithelium secretes its fluid in the absence of a trans-epithelial osmotic gradient, and, in addition, has an inherent ability to secrete CSF against an osmotic gradient. This paradoxical feature is shared with other 'leaky' epithelia. The assumptions underlying the classical standing gradient hypothesis await experimental support and appear to not suffice as an explanation of CSF secretion. Here, we suggest that the elusive local hyperosmotic compartment resides within the membrane transport proteins themselves. In this manner, the battery of plasma membrane transporters expressed in choroid plexus are proposed to sustain the choroidal CSF secretion independently of the prevailing bulk osmotic gradient.

The lateral intercellular space as osmotic coupling compartment in isotonic transport

Solute-coupled water transport and isotonic transport are basic functions of low- and high-resistance epithelia. These functions are studied with the epithelium bathed on the two sides with physiological saline of similar composition. Hence, at transepithelial equilibrium water enters the epithelial cells from both sides, and with the reflection coefficient of tight junction being larger than that of the interspace basement membrane, all of the water leaves the epithelium through the interspace basement membrane. The common design of transporting epithelia leads to the theory that an osmotic coupling of water absorption to ion flow is energized by lateral Na(+)/K(+) pumps. We show that the theory accounts quantitatively for steady- and time dependent states of solute-coupled fluid uptake by toad skin epithelium. Our experimental results exclude definitively three alternative theories of epithelial solute-water coupling: stoichiometric coupling at the molecular level by transport proteins like SGLT1, electro-osmosis and a 'junctional fluid transfer mechanism'. Convection-diffusion out of the lateral space constitutes the fundamental problem of isotonic transport by making the emerging fluid hypertonic relative to the fluid in the lateral intercellular space. In the Na(+) recirculation theory the 'surplus of solutes' is returned to the lateral space via the cells energized by the lateral Na(+)/K(+) pumps. We show that this theory accounts quantitatively for isotonic and hypotonic transport at transepithelial osmotic equilibrium as observed in toad skin epithelium in vitro. Our conclusions are further developed for discussing their application to solute-solvent coupling in other vertebrate epithelia such as small intestine, proximal tubule of glomerular kidney and gallbladder. Evidence is discussed that the Na(+) recirculation theory is not irreconcilable with the wide range of metabolic cost of Na(+) transport observed in fluid-transporting epithelia.

General models for water transport across leaky epithelia

The group of leaky epithelia, such as proximal tubule and small intestine, have several common properties in regard to salt and water transport. The fluid transport is isotonic, the transport rate increases in dilute solutions, and water can be transported uphill. Yet, it is difficult to find common features that could form the basis for a general transport model. The direction of transepithelial water transport does not correlate with the direction of the primary active Na+ transport, or with the ultrastucture as defined by the location of apical and basolateral membranes, of the junctional complex and the lateral intercellular spaces. The presence of specific water channels, aquaporins, increases the water permeability of the epithelial cell membranes, i.e., the kidney proximal tubule. Yet other leaky epithelia, for example, the retinal pigment epithelium, have no known aquaporins. There is, however, a general correlation between the direction of transepithelial transport and the direction of transport via cotransporters of the symport type. A simple epithelial model based on water permeabilities, a hyperosmolar compartment and restricted salt diffusion, is unable to explain epithelial transport phenomena, in particular the ability for uphill water transport. The inclusion of cotransporters as molecular water pumps in these models alleviates this problem.

Cotransporters as molecular water pumps

Molecular water pumps are membrane proteins of the cotransport type in which a flux of water is coupled to substrate fluxes by a mechanism within the protein. Free energy can be exchanged between the fluxes. Accordingly, the flux of water may be relatively independent of the external water chemical potential and can even proceed uphill. In short, water is being cotransported. The evidence for water cotransport is reviewed with particular emphasis on electrogenic cotransporters expressed in Xenopus oocytes under voltage clamped conditions. Phenomena such as uphill water transport, tight coupling between water transport and clamp current, cotransport of small hydrophilic molecules, and shifts in reversal potentials with osmolarity are discussed with examples from the Na+/glutamate and Na+/glucose cotransporters. Unstirred layers and electrode artifacts as alternative explanations for such cotransport can be ruled out for both experimental and theoretical reasons. Indeed, substrate fluxes mediated by channels or ionophores generate much smaller water fluxes than those observed with cotransporters. Theoretical models, using reasonable values for the intracellular diffusion coefficient, indicate the presence of only small unstirred layers in the membranes studied.

Paracellular fluid transport by epithelia

The evidence that a major fraction of water crosses the paracellular route during isotonic fluid transfer is reviewed together with a description of the theory and experimental results derived from extracellular probe studies. Four transporting epithelia which have been studied using the method are gallbladder, intestine, Malpighian tubule, and salivary gland. It is concluded that paracellular probe flows are not due to simple convection generated by osmotic flow through the junctions but are generated by active fluid transport within the junction: a mechano-osmotic process. The geometry of the pathway involved would indicate that some salt accompanies the paracellular fluid, representing a hypo-osmotic flow. Transport of salt by the cell route, which may be accompanied by some water, represents a hypertonic flow. The problem then becomes one of balancing the two to produce an isotonic transportate. We suggest, using recent data from knockout mice, that some aquaporins are functioning in different epithelial tissues as osmo-comparators within a feedback loop that regulates the paracellular fluid flow rate. This results in an overall quasi-isotonic transport by the epithelium. The model is applied to forward-facing systems such as proximal tubule and backward-facing systems such as exocrine glands.

Molecular water pumps

There is good evidence that cotransporters of the symport type behave as molecular water pumps, in which a water flux is coupled to the substrate fluxes. The free energy stored in the substrate gradients is utilized, by a mechanism within the protein, for the transport of water. Accordingly, the water flux is secondary active and can proceed uphill against the water chemical potential difference. The effect has been recognized in all symports studied so far (Table 1). It has been studied in details for the K+/Cl- cotransporter in the choroid plexus epithelium, the H+/lactate cotransporter in the retinal pigment epithelium, the intestinal Na+/glucose cotransporter (SGLT1) and the renal Na+/dicarboxylate cotransporter both expressed in Xenopus oocytes. The generality of the phenomenon among symports with widely different primary structures suggests that the property of molecular water pumps derives from a pattern of conformational changes common for this type of membrane proteins. Most of the data on molecular water pumps are derived from fluxes initiated by rapid changes in the composition of the external solution. There was no experimental evidence for unstirred layers in such experiments, in accordance with theoretical evaluations. Even the experimental introduction of unstirred layers did not lead to any measurable water fluxes. The majority of the experimental data supports a molecular model where water is cotransported: A well defined number of water molecules act as a substrate on equal footing with the non-aqueous substrates. The ratio of any two of the fluxes is constant, given by the properties of the protein, and is independent of the driving forces or other external parameters. The detailed mechanism behind the molecular water pumps is as yet unknown. It is, however, possible to combine well established phenomena for enzymes into a working model. For example, uptake and release of water is associated with conformational changes during enzymatic action; a specific sequence of allosteric conformations in a membrane bound enzyme would give rise to vectorial transport of water across the membrane. In addition to their recognized functions, cotransporters have the additional property of water channels. Compared to aquaporins, the unitary water permeability is about two orders of magnitude lower. It is suggested that the water permeability is determined from chemical associations between the water molecule and sites within the pore, probably in the form of hydrogen-bonds. The existence of a passive water permeability suggests an alternative model for the molecular water pump: The water flux couples to the flux of non-aqueous substrates in a hyperosmolar compartment within the protein. Molecular water pumps allow cellular water homeostasis to be viewed as a balance between pumps and leaks. This enables cells to maintain their intracellular osmolarity despite external variations. Molecular water pumps could be relevant for a wide range of physiological functions, from volume regulation in contractile vacuoles in amoeba to phloem transport in plants (Zeuthen 1992, 1996). They could be important building blocks in a general model for vectorial water transport across epithelia. A simplified model of a leaky epithelium incorporating K+/Cl-/H2O and Na+/glucose/H2O cotransport in combination with channels and primary active transport gives good quantitative predictions of several properties. In particular of how epithelial cell layers can transport water uphill.

Molecular mechanisms for passive and active transport of water

Water crosses cell membranes by passive transport and by secondary active cotransport along with ions. While the first concept is well established, the second is new. The two modes of transport allow cellular H2O homeostasis to be viewed as a balance between H2O leaks and H2O pumps. Consequently, cells can be hyperosmolar relative to their surroundings during steady states. Under physiological conditions, cells from leaky epithelia may be hyperosmolar by roughly 5 mosm liter-1, under dilute conditions, hyperosmolarities up to 40 mosm liter-1 have been recorded. Most intracellular H2O is free to serve as solvent for small inorganic ions. The mechanism of transport across the membrane depends on how H2O interacts with the proteinaceous or lipoid pathways. Osmotic transport of H2O through specific H2O channels such as CHIP 28 is hydraulic if the pore is impermeable to the solute and diffusive if the pore is permeable. Cotransport of ions and H2O can be a result of conformational changes in proteins, which in addition to ion transport also translocate H2O bound to or occlude in the protein. A cellular model of a leaky epithelium based on H2O leaks and H2O pumps quantitatively predicts a number of so-far unexplained observations of H2O transport.

Cotransport of K+, Cl- and H2O by membrane proteins from choroid plexus epithelium of Necturus maculosus

1. The interaction between K+, Cl- and H2O fluxes was studied in the ventricular membrane of the choroid plexus epithelium from Necturus maculosus by means of ion-selective microelectrodes. 2. Three experimental strategies were adopted: the osmolarity of the ventricular solution was increased abruptly by addition of (i) mannitol or (ii) KCl; (iii) Na+ in the ventricular solution was replaced isosmotically by K+. 3. The mannitol experiments showed that H2O had two pathways across the ventricular membrane. One was purely passive, with a water permeability, L'p, of 0.64 x 10(-4) cm s-1 (osmol l-1)-1. This operated in parallel with an ion-dependent pathway of similar magnitude which was abolished in Cl(-)-free solutions. 4. When KCl was added there was a flow of H2O into the cell. Surprisingly, this took place despite the osmotic gradient which favoured an efflux of H2O. The effect was blocked by frusemide (furosemide), in which case KCl had the same effects as applications of NaCl or mannitol. 5. Replacement of Na+ with K+ caused an influx of H2O. This flux could proceed against osmotic gradients implemented by mannitol. 6. The present data and those of earlier publications show that the interdependence of the fluxes of K+, Cl- and H2O in the exit membrane can be described as cotransport. The fluxes have a fixed stoichiometry of 1:1:500, the flux of one species is able to energize the flux of the two others, and the transport exhibits saturation and is specific for K+ and Cl-. 7. A molecular model based upon a mobile barrier in a membrane spanning protein gives an accurate quantitative description of the data.

Water permeability of ventricular cell membrane in choroid plexus epithelium from Necturus maculosus

1. The osmotic water permeability Lp and the relations between the flows of H2O, K+ and Cl- were studied in the ventricular membrane of the epithelium from the choroid plexus of Necturus maculosus. 2. The flows were induced by abrupt changes in external osmolarity of the ventricular solution. Solution changes were convective and no effects of unstirred layers could be detected on measured parameters. 3. The initial rate of change in intracellular concentrations of K+ and Cl- was monitored by double-barrelled ion-selective microelectrodes. 4. The initial rate of flux of H2O could be monitored as the changes in the concentration of intracellular choline ions (Ch+i). When 0.5 mmol l-1 of choline chloride was added to the external solutions, Ch+i attained values of 1-5 mmol l-1. The dilution or concentration of Ch+i could be recorded by K+ electrodes since the sensitivity of these to Ch+ is more than 50 times greater than to K+. 5. The Lp of the ventricular membrane of the epithelium was 1.4-2.1 x 10(-4) cm s-1 (osmol l-1)-1 and independent of the direction of the induced water flux. Lp was unchanged in tissues adapted to osmolarities of half the physiological value. 6. The efflux of H2O induced by mannitol was associated with an instantaneous efflux of K+ which was inhibited by furosemide. The fluxes had a ratio of 40 mmol l-1. The influx of H2O induced by the removal of NaCl from the ventricular solution was associated with an instantaneous influx of K+. The H2O influx had a ratio to the flux of K+ of 70 mmol l-1. 7. The efflux of H2O induced by mannitol was associated with an efflux of Cl- which was inhibited by furosemide. The ratio of the two fluxes was in the range 15-44 mmol l-1. 8. The conclusion is that the Ch+ method gives a reliable measure of the movement of H2O across the ventricular membrane. The magnitude of the Lp and its relevance to transepithelial transport are discussed. The osmotically induced H2O movement is accompanied by furosemide-sensitive fluxes of K+ and Cl- of the same magnitude. This suggests that co-transport between H2O and KCl can take place in the membrane.

Secondary active transport of water across ventricular cell membrane of choroid plexus epithelium of Necturus maculosus

1. The interaction between Cl-, K+ and H2O fluxes were studied in the ventricular membrane of the choroid plexus epithelium from Necturus maculosus by means of ion-selective microelectrodes. The flux of H2O was measured by means of K+ electrodes as the dilution or concentration of intracellular choline ions, Ch+i. 2. In one series of experiments Cl- was readministered to the ventricular solution of tissues incubated in media with low Cl- concentrations. The resulting influx of Cl- was associated with an instantaneous influx of K+ and H2O. 3. Both the Cl- and the K+ influxes were reduced by the diuretic furosemide but were unaffected by inhibitors of Na+, K(+)-ATPase or changes in membrane potentials induced by Ba2+. Since the influx of K+ proceeds against its electrochemical gradient and is unaffected by changes in membrane potentials, the membrane exhibits secondary active, electroneutral transport of K+. 4. The influx of water, initiated simultaneously with the influx of K+ and Cl-, commenced before these ions had changed the osmolarity of the intracellular solution significantly. The influx of H2O could proceed against an osmotic gradient. The influx stopped when 100 mmol l-1 of mannitol was added to the ventricular solution at the same time as the Cl- ions. The influx of H2O was inhibited by K+ removal, furosemide or high external Ba2+ (10 mmol l-1), but not by strophanthidin, ouabain or low concentrations of Ba2+ (0.5 mmol l-1). The influx could not continue with other permeable anions, NO3-, acetate- or SCN-, replacing Cl-. 5. In another series of experiments Cl- was removed from the ventricular solution of tissues bathed in saline solutions with normal concentrations of Cl-. The resulting efflux of Cl- was associated with an instantaneous efflux of K+ and H2O. This efflux of H2O could proceed against an osmotic gradient of up to 70 mosmol l-1. This effect was inhibited by furosemide, in which case the water fluxes were entirely dependent on the osmotic gradients and the osmotic water permeability Lp of the ventricular membrane. 6. The data suggest that there is a coupling between the flux of KCl and of water in the ventricular membrane, which implies that the reflection coefficient sigma for KCl under the given circumstances is less than one. I suggest that the ability of leaky epithelia to transport against osmotic gradients depends on such a coupling, which derives from the properties of the proteins through which K+, Cl- and H2O leave the cell.

Video enhanced imaging of the fluorescent Na+ probe SBFI indicates that colonic crypts absorb fluid by generating a hypertonic interstitial fluid

Extracellular accumulation of Na+ detected by video-enhanced microscopic imaging of the impermeant fluorescent probe SBFI confirms the view that colonic crypts produce a hypertonic ascorbate ca 1000 mOsm.kg-1, thereby generating a large osmotic pressure across the crypt wall. This creates a high fluid tension within the crypt lumen, sufficient to dehydrate faeces. When bathed in Tyrode the SBFI.Na+ fluorescence indicates a [Na+] ca 750 mM within the interstitial space of metabolizing rat descending colon. There is no evidence of interstitial Na+ accumulation in octanol (2 mM) or in rabbit colon incubated with 1.0 mM ouabain and no evidence of Na+ secretion via the crypt lumen during absorption.

Influence of lateral intercellular spaces on current propagation in tubular epithelia as estimated by a multi-cable model

A multiple cable model has been developed for tubular epithelia which allows current flow along the tubular lumen, along the cell layer and inside the lateral intercellular space (LIS) to be quantitatively assessed. In this model tubular lumen and cell layer are represented by two concentric cylinders and the LIS by n concentric interconnected fluid layers which are interposed between the cells, contact the lateral cell membranes and extend all along the tubular length. The innermost LIS layer connects to the tight junctions and the outermost layer to the peritubular space. Modelling each element by a cable-like structure the mathematical solution leads to n + 2 linear combinations of n + 2 exponential functions. Based on morphometric data and resistance measurements on Necturus proximal tubule [4,10] model calculations have been performed of the voltage attenuation along tubular lumen, cell layer and LIS for n = 3 or n = 6 assuming different LIS widths (0.02, 0.2, and 2.0 micron). The results show that the influence of LIS is insignificant in Necturus proximal tubule under control conditions, but may become significant in other functional states or other tubules. Collapsing the LIS increases predominantly the shunt resistance and the effective resistance of the lateral cell membrane but longitudinal current propagation along the LIS remains negligible at all space widths. In addition, model calculations are presented which allow errors in determining tight junction resistance and cell membrane resistances from a simple cable model to be quantified as function of LIS width.

Acetylcholine-induced Na+ influx in the mouse lacrimal gland acinar cells: demonstration of multiple Na+ transport mechanisms by intracellular Na+ activity measurements

In the isolated, superfused mouse lacrimal gland, intracellular Na+ activities (aNai) of the acinar cells were directly measured with double-barreled Na+-selective microelectrodes. In the nonstimulated condition aNai was 6.5 +/- 0.5 mM and membrane potential (Vm) was -38.9 +/- 0.4 mV. Addition of 1 mM ouabain or superfusion with a K+-free solution slightly depolarized the membrane and caused a gradual increase in aNai. Stimulation with acetylcholine (ACh, 1 microM) caused a membrane hyperpolarization by about 20 mV and an increase in aNai by about 9 mM in 5 min. The presence of amiloride (0.1 mM) reduced the ACh-induced increase in aNai by approximately 50%, without affecting Vm and input resistance in both nonstimulated and ACh-stimulated conditions. Acid loading the acinar cells by an addition/withdrawal of 20 mM NH4Cl or by replacement of Tris+-buffer saline solution with HCO3-/CO2-buffered solution increased aNai by a few mM. Superfusion with a Cl(-)-free NO3- solution or 1 mM furosemide or 0.5 mM bumetanide-containing solution had little effect on the resting aNai levels, however, it reduced the ACh-induced increase in aNai by about 30%. Elimination of metabolite anions (glutamate, fumarate and pyruvate) from the superfusate reduced both the resting aNai and the ACh-induced increase in aNai. The present results suggest the presence of multiple Na+ entry mechanisms activated by ACh, namely, Na+/H+ exchange, Na-K-Cl cotransport and organic substrate-coupled Na+ transport mechanisms.

Electrophysiological studies on lateral intercellular spaces of Necturus gallbladder epithelium

The lateral intercellular spaces of Necturus gallbladder epithelium were punctured with double-barrelled ion selective microelectrodes in order to determine the ion concentrations of lateral space fluid and the contribution of the lateral spaces to transepithelial resistance. Neither under control conditions, nor after diluting the bathing fluids to increase the rate of volume absorption, nor during passage of direct current of 200 microA/cm2, were any reliable concentration differences observed between lateral space fluid and external bathing fluids. These observations suggest that water can follow salt transport without requiring osmotic concentration gradients of greater than 1 or 2 mosmol/l and indicate that recently observed high values of water permeability must still be considered as underestimates. After developing a test to recognize and exclude leaky punctures, the contribution of the lateral spaces to transepithelial resistance could be determined. It amounted to around 29%. This value agrees well with results from recent impedance measurements which were performed under control conditions in the same preparation.