The effects of ouabain on endothelial function in human and rabbit corneas

Molecular Mechanisms of Fuchs and Congenital Hereditary Endothelial Corneal Dystrophies

The cornea, the eye's outermost layer, protects the eye from the environment. The cornea's innermost layer is an endothelium separating the stromal layer from the aqueous humor. A central role of the endothelium is to maintain stromal hydration state. Defects in maintaining this hydration can impair corneal clarity and thus visual acuity. Two endothelial corneal dystrophies, Fuchs Endothelial Corneal Dystrophy (FECD) and Congenital Hereditary Endothelial Dystrophy (CHED), are blinding corneal diseases with varied clinical presentation in patients across different age demographics. Recessive CHED with an early onset (typically age: 0-3 years) and dominantly inherited FECD with a late onset (age: 40-50 years) have similar phenotypes, although caused by defects in several different genes. A range of molecular mechanisms have been proposed to explain FECD and CHED pathology given the involvement of multiple causative genes. This critical review provides insight into the proposed molecular mechanisms underlying FECD and CHED pathology along with common pathways that may explain the link between the defective gene products and provide a new perspective to view these genetic blinding diseases.

Feasibility Study of Human Corneal Endothelial Cell Transplantation Using an In Vitro Human Corneal Model

PURPOSE

To test the feasibility of a cell therapy approach to treat corneal endothelial (CE) disorders using an in vitro model of human corneal decompensation.

METHODS

A CE decompensation model was established by removal of the Descemet membrane/endothelium complex from cadaveric human corneas in an air interface organ culture system (group 2) and compared with normal corneas (group 1). The posterior stroma of decompensated corneas was seeded with immortalized human corneal endothelial cells (HCEC-12) in group 3 and passage 0 primary human CE cells in group 4 corneas. Functional effects on stromal thickness were determined with histological analysis 3 to 10 days after cell therapy treatment.

RESULTS

Removal of the Descemet membrane/endothelium complex in group 2 corneas resulted in a stromal thickness of 903 ± 86 μm at 12 hours compared with 557 ± 72 μm in group 1 corneas. Stromal thickness reduced from 1218 ± 153 μm to 458 ± 90 μm (63% ± 6%, P = 0.001) after cell transplantation in group 3 and from 1100 ± 86 μm to 489 ± 94 μm (55% ± 7%, P = 0.00004) in group 4. Posttransplantation histology demonstrated formation of a monolayer of corneal endothelium attached to the posterior stromal surface.

CONCLUSIONS

Direct transplantation of cultured human CE cells and immortalized HCEC-12 to bare posterior corneal stroma resulted in formation of an endothelial monolayer and restoration of stromal hydration to physiological thickness, demonstrating the feasibility of cell therapy in treatment of CE decompensation in a human in vitro model.

Acute ocular hypertension disrupts barrier integrity and pump function in rat corneal endothelial cells

Acute ocular hypertension (AOH) frequently compromises corneal endothelial cell (CEC) function in clinical practice. This type of stress induces corneal oedema and a decrease in the corneal endothelial cell density (ECD). The anterior chamber of the right eye of Sprague-Dawley rats was irrigated with Balanced Salt Solution (BSS) for two hours, and the left eye served as a control to determine the time-dependent effects of AOH on endothelial cell morphology and function. The average intraocular pressure (IOP) increased to 82.6 ± 2.3 mmHg (normal range: 10.2 ± 0.4 mmHg) during anterior irrigation. Very soon after initiating irrigation, corneal oedema became evident and the cornea exhibited a significant increase in permeability to FITC-dextran. The peripheral ECD was significantly reduced, and the morphology of CECs became irregular and multiform. The structures of the zonula occludens-1 (ZO-1) and F-actin were severely disrupted. In addtion, Na,K-ATPase exhibited a dispersed expression pattern. Two days after irrigation, obvious CEC proliferation was observed, the ECD recovered to a normal level, and F-actin was dispersed throughout the cytoplasm. Seven days later, the CEC structure and function were nearly normalized. Based on the results obtained using this model, an acute IOP crisis exerts transient deleterious effects on CEC structure and function in rats.

3D map of the human corneal endothelial cell

Corneal endothelial cells (CECs) are terminally differentiated cells, specialized in regulating corneal hydration and transparency. They are highly polarized flat cells that separate the cornea from the aqueous humor. Their apical surface, in contact with aqueous humor is hexagonal, whereas their basal surface is irregular. We characterized the structure of human CECs in 3D using confocal microscopy of immunostained whole corneas in which cells and their interrelationships remain intact. Hexagonality of the apical surface was maintained by the interaction between tight junctions and a submembraneous network of actomyosin, braced like a drum. Lateral membranes, which support enzymatic pumps, presented complex expansions resembling interdigitated foot processes at the basal surface. Using computer-aided design and drafting software, we obtained a first simplified 3D model of CECs. By comparing their expression with those in epithelial, stromal and trabecular corneal cells, we selected 9 structural or functional proteins for which 3D patterns were specific to CECs. This first 3D map aids our understanding of the morphologic and functional specificity of CECs and could be used as a reference for characterizing future cell therapy products destined to treat endothelial dysfunctions.

Isolation of a recombinant antibody specific for a surface marker of the corneal endothelium by phage display

Cell surface antigens are important targets for monoclonal antibodies, but they are often difficult to work with due to their association with the cell membrane. Phage display is a versatile technique that can be applied to generate binders against difficult targets. Here we used antibody phage display to isolate a binder for a rare and specialized cell, the human corneal endothelial cell. The human corneal endothelium is a medically important cell layer; defects in this layer account for about half of all corneal transplants. Despite its importance, no specific antigens have been found to mark this cell type. By panning a phage library directly on human corneal endothelial cells, we isolated an antibody that bound to these cells and not the other types of corneal cells. Subsequently, we identified the antibody's putative target to be CD166 by immunoprecipitation and mass spectrometry. This approach can be used to isolate antibodies against other poorly-characterized cell types, such as stem cells or cancer cells, without any prior knowledge of their discriminating markers.

Corneal oedema and its medical treatment

Corneal oedema is a common sign of acute or protracted corneal disease of various aetiologies. In this paper, we review the causes and pathophysiological bases of corneal oedema, as well as discussing the goals and modalities of its medical treatment. Corneal oedema, if adequately understood and appropriately treated, generally shows a good prognosis.

Aquaporins in complex tissues: distribution of aquaporins 1-5 in human and rat eye

Multiple physiological fluid movements are involved in vision. Here we define the cellular and subcellular sites of aquaporin (AQP) water transport proteins in human and rat eyes by immunoblotting, high-resolution immunocytochemistry, and immunoelectron microscopy. AQP3 is abundant in bulbar conjunctival epithelium and glands but is only weakly present in corneal epithelium. In contrast, AQP5 is prominent in corneal epithelium and apical membranes of lacrimal acini. AQP1 is heavily expressed in scleral fibroblasts, corneal endothelium and keratocytes, and endothelium covering the trabecular meshwork and Schlemm's canal. Although AQP1 is plentiful in ciliary nonpigmented epithelium, it is not present in ciliary pigmented epithelium. Posterior and anterior epithelium of the iris and anterior lens epithelium also contain significant amounts of AQP1, but AQP0 (major intrinsic protein of the lens) is expressed in lens fiber cells. Retinal Müller cells and astrocytes exhibit notable concentrations of AQP4, whereas neurons and retinal pigment epithelium do not display aquaporin immunolabeling. These studies demonstrate selective expression of AQP1, AQP3, AQP4, and AQP5 in distinct ocular epithelia, predicting specific roles for each in the complex network through which water movements occur in the eye.

Transient corneal edema induced by nitric oxide synthase inhibition

The aim of the study is to identify nitric oxide synthase (NOS) in the rabbit cornea and further investigate the physiological role of nitric oxide in the rabbit cornea. For histological identification, an immunohistochemical technique using anti-NOS monoclonal antibodies was employed. For the physiological study, we measured the corneal thickness in vivo as an indicator of corneal edema by ultrasonic pachymetry. The measurements were repeated before and after ipsilateral injections of N(G)-nitro-L-arginine methyl ester (L-NAME) or N(G)-nitro-D-arginine methyl ester (D-NAME) or 6-anilino-5,8-quinolinedione (LY-83583) with contralateral injection of vehicle (balanced salt solution) into the anterior chamber of the rabbit. We also monitored intraocular pressure (IOP) by pneumatonometry. Endothelial NOS (eNOS) immunoreactivity was demonstrated both in the corneal epithelium and the endothelium. The corneal thickness significantly increased after L-NAME or LY-83583 without significant rise of IOP, whereas no change was detected after vehicle or D-NAME. These results suggest that NO is spontaneously produced in the corneal endothelium and the NO/cyclic GMP pathway is involved in maintainance of corneal thickness.

Ionic channels in corneal endothelium

Single-channel patch-clamp techniques as well as standard and perforated-patch whole cell voltage-clamp techniques have been applied to the study of ionic channels in the corneal endothelium of several species. These studies have revealed two major K+ currents. One is due to an anion- and temperature-stimulated channel that is blocked by Cs+ but not by most other K+ channel blockers, and the other is similar to the family of A-currents found in excitable cells. The A-current is transient after a depolarizing voltage step and is blocked by both 4-aminopyridine and quinidine. These two currents are probably responsible for setting the -50 to -60 mV resting voltage reported for these cells. A Ca(2+)-activated ATP-inhibited nonselective cation channel and a tetrodotoxin-blocked Na+ channel are possible Na+ inflow pathways, but, given their gating properties, it is not certain that either channel works under physiological conditions. A large-conductance anion channel has also been identified by single-channel patch-clamp techniques. Single corneal endothelial cells have input resistances of 5-10 G omega and have steady-state K+ currents that are approximately 10 pA at the resting voltage. Pairs or monolayers of cells are electrically coupled and dye coupled through gap junctions.

Nonselective cation channel activation during wound healing in the corneal endothelium

Rabbit corneas were injured by mechanical or thermal trauma. At several time points after wounding, corneal endothelial cells were isolated and their ion channels examined using standard and amphotericin perforated-patch whole cell patch-clamp configurations. Within 15-24 h after mechanical or thermal trauma, a nonselective cation current was observed in 79% of the cells examined that was not present in unwounded or sham-wounded corneas. By 73 h postwounding, the current was present in only 10% of the cells examined. The wound healing-induced current is outwardly rectifying, activates at depolarized voltages, shows no sign of inactivation, and is inhibited by flufenamic acid, quinidine, and acetate. In addition to this new current, it was observed that endothelial cells from freeze-wounded corneas no longer expressed the transient K+ current seen in control, sham, and mechanically wounded corneas. Corneal endothelial superfusion experiments found no significant difference in swelling rates between control and flufenamic acid-superfused wounded corneas, indicating that the wound healing-induced channel is not involved in the stromal hydration maintenance function of the corneal endothelium.

Effect of hyperglycemia duration on rabbit corneal thickness and endothelial ATPase activity

This study compares corneal thickness and corneal endothelial Na,K ATPase activity in normal and age-matched diabetic rabbits with up to 10 weeks of hyperglycemia. Corneal endothelial Na,K ATPase activity in normal rabbits increased between 10 and 20 weeks of age. Corneal thickness increased after 5 weeks of hyperglycemia. Decreased corneal endothelial homogenate Na,K ATPase activity was first seen after 10 weeks of hyperglycemia, though an earlier onset was possible due to sampling restrictions. Corneal endothelial homogenate Mg++ ATPase activity showed a transient increase after 2 to 5 weeks of hyperglycemia. This study shows that hyperglycemia in rabbit is associated with complex dynamic interactions between corneal thickness and corneal endothelial Mg++ ATPase and Na,K ATPase activities.

Toxic effects of ouabain on the rabbit corneal endothelium

Rabbit corneas kept in a standard tissue culture medium, were continually exposed to ouabain 10(-3) M, and cultivated for 2, 3 and 4 days. The endothelium was still intact with a complete cell cover after 2 days of ouabain exposure. Total destruction of the endothelium was found in the corneas cultivated for 3 and 4 days. The corneal epithelium, however, appeared to be unaffected. Other rabbit corneas were exposed for three days to ouabain concentrations of 10(-4) M, 10(-5) M, 10(-6) M, 10(-7) M, 10(-8) M, and 0 M. The corneas exposed to 10(-4) M demonstrated total endothelial cell damage. The specimens exposed to concentrations from 10(-5) to 10(-8) M, presented dose dependent endothelial impairment as compared with the control corneas (0 M). Tracer experiments showed that the endothelial barrier was intact as long as there was a continuous cell cover, as estimated by light, transmission and scanning electron microscopy.

A method for registration of toxic drug effects on corneal endothelium. Effect of gentamicin on rabbit corneal endothelium

A weight recording system (Medin & Davanger 1988, 1989), was used to demonstrate possible toxic damage of a medicament to the endothelium of rabbit corneas stored in organ culture. Seven corneas were stored in organ culture medium containing gentamicin (3.0 mg/ml). Seven other corneas stored in identical organ culture medium without gentamicin served as controls. The corneas were followed with weight recordings for up to 76 h. A toxic effect of gentamicin was demonstrated by a rapid weight increase in the corneas stored in the presence of gentamicin. After 5.3 h there was a significant difference (P = 0.0002) between the average weights of the two groups, and this difference increased during the following 2-3 days. Corneas from the two groups were also examined by light microscopy and scanning electron microscopy. There was good accordance between the weight recordings and the morphology. The weight recording system detects clearly the toxic effect of gentamicin (3.0 mg/ml).

Metabolism of 12(R)-hydroxy-5,8,10,14-eicosatetraenoic acid (12(R)-HETE) in corneal tissues: formation of novel metabolites

12(R)-Hydroxy-5,8,10,14-eicosatetraenoic acid [12(R)-HETE], a cytochrome P450 arachidonate metabolite, is metabolized by corneal tissues via three distinct metabolic pathways: beta-oxidation, omega-hydroxylation, and keto-reduction. The major metabolite released from the intact rabbit corneal epithelium or cultured cells was identified by mass spectrometric analysis as 8-hydroxy-4,6,10-hexadecatrienoic acid, the tetranor metabolite derived following two steps of beta-oxidation from the carboxy terminus. The beta-oxidation pathway was expressed in both microsomes and mitochondria isolated from bovine corneal epithelium and was dependent on the addition of oxidizing equivalents. The major metabolite of 12(R)-HETE in subcellular fractions of bovine corneal epithelial cells was a dihydro compound, 12-hydroxy-5,8,14-eicosatrienoic acid (12-HETrE). This derivative is presumably formed by an oxidation of the hydroxyl group followed by two keto-reduction steps, since its formation was accompanied by the appearance of a keto metabolite identified as 12-oxo-5,8,14-eicosatrienoic acid. The omega-hydroxylation, in contrast to other cell types, was a minor route for 12(R)-HETE metabolism in these tissues. Since 12(R)-HETE has been implicated as a modulator of Na(+)-K(+)-ATPase activity and its related functions in ocular tissues, these findings raise the possibility that the newly described metabolites may be involved in regulating corneal functions. In addition, the presence of a keto reductase in the cornea may be of great importance following injury since 12(R)-HETrE resulting from 12(R)-HETE by this activity is a potent ocular proinflammatory compound.

The corneal endothelium

The endothelium is a monolayer of cells on the posterior corneal surface that transports water from the stroma into the anterior chamber. This movement of water counters a natural tendency for the stroma to swell and is necessary to maintain a transparent cornea. Embryologic studies, in particular the demonstration of the derivation of the endothelium from the neural crest, have provided insight into the factors that govern the response of this tissue to disease. In some species the endothelium can regenerate after injury, but in man cellular enlargement is the main mechanism of repair after cell loss. A clinical estimate of endothelial cell density and function is provided by specular microscopy, fluorophotometry and pachymetry. In this paper we review the development, structure and function of the corneal endothelium, and then consider the pathological processes that can affect this tissue.

Physiological state of the rabbit cornea following 4 degrees C moist chamber storage

Eyes from female albino rabbits (1.9-2.3 kg) were enucleated immediately following euthanasia along with the lids and conjunctiva. The globes were moist chamber-stored (in a 4 degrees C refrigerator with the lids closed and the cornea facing downwards) for 12-500 hr. Central corneal thickness (by ultrasound) increased from 350 to approximately 650 microns within 72 hr but changed little thereafter. In the latter period, the relative fluid pressure of the globe (pneumatonography) dropped to less than 10 mmHg, aqueous bicarbonate (tCO2) levels fell to less than 5 mM and anterior chamber fluid tonicity decreased progressively (especially after 24 hr storage) to reach values of around 200 mosmol l-1 by 7 days. Storma-endothelium preparations of the corneas, after 2 hr of in vitro equilibration with a bicarbonate-Ringer solution (supplemented with glucose, adenosine and glutathione) were evaluated for their ability to undergo deturgescence under silicone oil or to pump fluid against a hydrostatic pressure of 20 cmH2O. Corneal preparations from up to 7 days storage showed rapid (60 to 135 microns hr-1) and complete deturgescence (net change in thickness of 140-180 microns) that was maintained. Thereafter, the ability to show deturgescence declined to zero by 10 days. In marked contrast, endothelial fluid pump activity (of approximately 5 microliters hr-1) was manifest for only 36 hr after which time this function rapidly declined. Most corneas stored for periods longer than 48 hr exhibited a continuous leak (of -1 to -5 microliters hr-1). The results indicate that corneal deturgescence and endothelial fluid pump function are not necessarily coupled in vitro.

Endothelial pump function in rabbit corneas kept under eye bank conditions

Endothelial pump activity of rabbit corneas kept under eye bank conditions was examined. Different swelling rates for medium with and without ouabain were used for the calculation of endothelial pump rate. After an initial weight increase during 24 h, the control corneas kept approximately constant weight for 1 week. The ouabain-treated corneas increased continually, and during the time interval 24-48 h the increase was 41% of start weight. This corresponds to a pump activity of 2.3% of the weight of the normal corneal stroma per h, or to 9 microns of the corneal thickness per h. Each endothelial cell pumped fluid corresponding to 200-300% of the cell volume per h between day 1 (24 h) and day 8. The pump activity was gradually reduced after 8 days. The average weight was the same in both groups after 7 weeks.

Visualization of ouabain binding in bovine cornea

In these experiments, the binding of ouabain, a specific inhibitor of NA,K-ATPase is visualized in bovine cornea using an immunohistochemical method. Due to cross-reactivity between ouabain and digitoxin antiserum, the ouabain binding is demonstrated after an immunological labelling with fluorescein. As shown in the corneal tissue sections and in the epithelial and endothelial cell preparations, a bright fluorescence of the endothelial cell membranes is observed in corneae incubated in 10(-5)-10(-3) mol/l ouabain. In the epithelium, only a weak binding of the ouabain is observed at the basolateral membrane after incubation in 10(-3) mol/l ouabain.

Characterization of the Na, K-ATPase pump in cultured bovine corneal endothelial cells

Bovine corneal endothelial cells in culture possess Na, K-ATPase pump sites, as measured by [3H]ouabain binding, and demonstrated an active ouabain-sensitive 86Rb+ uptake. The binding of [3H]ouabain, a specific inhibitor of Na, K-ATPase, was used to quantitative the density of Na, K-ATPase pump sites in bovine corneal endothelial cell cultures. [3H]ouabain binding was time-dependent and reached saturation after 1-2 hr. The specific binding represented more than 99% of the total cell-associated [3H]ouabain, and about 85% of this binding was abolished in the presence of K+ ions. The binding was concentration-dependent and saturated at a ouabain concentration of 2 X 10(-8)M with a dissociation constant (Kd) of 1.0 X 10(-8)M. The number of [3H]ouabain binding sites was maximal in sparse, activity growing cultures and decreased accompanying the development of a confluent monolayer. A pump density of 2.2 X 10(6) pump sites cell-1 was estimated for sparse cultures, declining to 0.8 X 10(6) pump site cell-1 at confluence. The activity of the Na, K-ATPase pump in bovine corneal endothelial cell cultures was evaluated by measuring 86Rb+ influx. Sparse and confluent cultures demonstrated 86Rb+ ouabain-sensitive uptake at a rate of 4.2 nmol (10(6) cells)-1 min-1 and 1.5 nmol (10(6) cells)-1 min-1, respectively. The ouabain-sensitive 86Rb+ uptake was linear for at least 30 min, while the ouabain-insensitive 86Rb+ uptake was slower and declined during the 30-min time period.(ABSTRACT TRUNCATED AT 250 WORDS)

Pump function of the human corneal endothelium. Effects of age and cornea guttata

The specific binding of tritiated ouabain to endothelial Na/K ATPase was used to quantitate the density of pump sites in the human corneal endothelium. Donor eyes, unsuitable for use in keratoplasty, were obtained from the Wisconsin Lions Eye Bank. The endothelium of each donor eye was examined using wide-field specular microscopy, and the specular micrographs were traced and digitized for the determination of cell density. Ouabain binding was measured in matched pairs of isolated endothelial sheets. A total of 26 pairs of donor eyes, ranging in age from 11 through 91 years, were studied. Twenty pairs, determined to have normal endothelia, were found to have a constant pump site density which was independent of donor age. Six donor pairs had moderate guttata; in this group pump site density was significantly increased. These results indicate that, although pump site density is normally constant in the human corneal endothelium, conditions which increase endothelial permeability, such as guttata, can cause a compensatory increase in pump site density and presumably pump function.

Intracellular potential and pH of rabbit corneal endothelial cells

Rabbit corneal endothelial pH and electrical potential have both been determined using tracer distribution techniques. Intercellular pH was measured using the dimethyloxazolidine-dione method and intracellular potential was measured using tetraphenylphosphonium bromide. Intracellular pH was determined as 7.10 in an ambient solution of pH 7.5. The only solution variations which altered intracellular pH were variations in the external solution pH, bathing in sodium-free or bicarbonate-free solution, incubation for 3 hours with 10(-6) or 10(-4) M ouabain or for 1 hour with 10(-4) M ouabain or in a high (60 mM) bicarbonate solution. The data indicate a close correlation between sodium and bicarbonate needs for the endothelium which corresponds with known effects of these ions on transendothelial ion fluxes. Intracellular potentials were measured of -34 mV, which were stable in the face of all environmental perturbations except 1 mM acetazolamide and 10(-6) M ouabain exposure for 3 hours. These newer techniques may be employed to provide some clues into the mechanism of endothelial transport systems.