Active and passive properties of the rabbit corneal endothelium

Descemet Membrane Endothelial Keratoplasty and Bowman Layer Transplantation: An Anatomic Review and Historical Survey

For nearly a century, the definitive treatment of many corneal dystrophies and ectactic disorders was limited to penetrating keratoplasty, but over the past 2 decades, a surge of surgical innovation has propelled the treatment of many corneal diseases to more targeted approaches with significantly better visual outcomes. Anterior stromal diseases were first changed through endothelial-sparing techniques, such as deep anterior lamellar keratoplasty, but have more recently transitioned to stromal-sparing approaches. Ultraviolet corneal crosslinking strengthens the cornea and halts progression of keratoconus in >90% of cases. Intracorneal ring segment and corneal allogenic ring segment implantation offer methods to flatten ectatic corneas. However, Bowman layer transplantation - inlay and more recently onlay techniques - has shown promise for treating advanced keratoconus and preventing keratoplasty. The advent of endothelial keratoplasty radically changed the treatment of corneal endothelial dysfunction, and Descemet membrane endothelial keratoplasty specifically offers an average postoperative visual acuity of 20/25 (0.8) with only 8.8% of grafts requiring retransplantation in the first 5 years. Here, we review the rapid innovations for surgical treatment of corneal diseases, spanning from endothelial keratoplasty and endothelial regeneration to anterior lamellar keratoplasty and stromal augmentation, highlighting key steps which may be moving us closer to a "postkeratoplasty" world.

The Need for Improved Therapeutic Approaches to Protect the Cornea Against Chemotoxic Injuries

Cornea, a highly specialized transparent tissue, is the major refractive element of the eye. The cornea is highly susceptible to chemotoxic injury through topical exposure to vapors, microparticles, and aqueous drops, as well as through systemically absorbed chemicals that access the cornea via tear film, aqueous humor, and limbal vasculature. Corneal injury activates a carefully orchestrated series of repair processes capable of resolving minor lesions over time, but it often fails to overcome the menace of moderate, severe, and chronic injuries and secondary pathophysiologies that permanently impair vision. The most serious complications of chemical injuries-persistent corneal edema, neovascularization, scarring/haze, limbal stem cell deficiency, and corneal melting-often manifest over months to years, suggesting that a better understanding of endogenous regenerative mechanisms of corneal repair can lead to the development of improved treatments that may attenuate or prevent corneal defects and protect vision.

Corneal Endothelial Pump Coupling to Lactic Acid Efflux in the Rabbit and Mouse

Purpose

Confirm that the corneal endothelial pump uses a lactate-coupled water efflux mechanism.

Methods

Corneal thickness, lactate efflux, and stromal [lactate] were measured in de-epithelialized swollen and nonswollen ex vivo-mounted rabbit corneas perfused with bicarbonate-rich and bicarbonate-free Ringers, ouabain, or acetazolamide to determine if the relationships among these parameters were similar to previous data using intact corneas. The role of barrier function was tested by perfusion with calcium-free EGTA. Predictions of [lactate] in endothelial dystrophy were examined in the Slc4a11 knock out mouse.

Results

De-epithelialized corneal swelling, lactate efflux, and stromal [lactate] in response to bicarbonate-free Ringers, ouabain, and acetazolamide perfusion had the same relationship as in intact corneas. The absolute amount of lactate efflux and stromal [lactate] in the de-epithelialized corneas was about half of intact corneas. De-epithelialized, swollen corneas deswelled fully with bicarbonate-rich, partially in the presence of acetazolamide, but continued to swell with bicarbonate-free or ouabain. The relationship among corneal thickness, lactate efflux, and [lactate] was the same as with nonswollen de-epithelialized corneas. In intact corneas swollen by perfusion with calcium-free EGTA, the relationship between swelling and lactate flux was the inverse of control corneas. The relationship between corneal swelling and [lactate] of intact corneas exposed to ouabain, but perfused with 7 mM lactate to simulate aqueous humor, was the same as without lactate. Corneal [lactate] in Slc4a11 knock out was twice that of wild type.

Conclusions

The corneal endothelial pump works via a lactate efflux mechanism that requires an intact osmotic barrier.

Perforating corneal injury in rat and pentadecapeptide BPC 157

Based on its healing effects in various tissues, we hypothesized that the stable gastric pentadecapeptide BPC 157 heals corneal ulcerations in rats and effects corneal transparency. We made a penetrant linear 2-mm incision in the paralimbal region of the left cornea at the 5 o'clock position with a 20-gauge MVR incision knife at 45° under an operating microscope. Medication was BPC 157 (2 pg/mL, 2 ng/mL, and 2 μg/mL distilled water, two eye drops/left rat eye) immediately after injury induction and then every 8 h up to 120 h; controls received an equal volume of distilled water. In contrast to the poor healing response in controls, BPC 157 significantly accelerated the healing process in 2 μg and 2 ng BPC 157-treated eyes, starting 24 h after the injury, and the fluorescein and Seidel tests became negative. The epithelial defects were completely healed at 72 h (2 μg BPC 157-treated group) and at 96 h (2 ng BPC 157-treated group) after injury. Aqueous cells were absent at 96 h and 120 h after injury in the 2 μg and 2 ng BPC 157-treated groups, respectively. In conclusion, BPC 157 effects the rapid regaining of corneal transparency. Whereas controls developed new vessels that grew from the limbus to the penetrated area, BPC 157-treated rats generally had no new vessels, and those that did form in the limbus did not make contact with the penetrated area. Thus, BPC 157 eye drops successfully close perforating corneal incisions in rats.

Wavelet analysis of corneal endothelial electrical potential difference reveals cyclic operation of the secretory mechanism

The corneal endothelium is a fluid-transporting epithelium. As other similar tissues, it displays an electrical potential of ~1 mV (aqueous side negative) across the entire layer [transendothelial potential difference (TEPD)]. It appears that this electrical potential is mainly the result of the transport of anions across the cell layer (from stroma to aqueous). There is substantial evidence that the TEPD is related linearly to fluid transport; hence, under proper conditions, its measure could serve as a measure of fluid transport. Furthermore, the TEPD is not steady; instead, it displays a spectrum of frequency components (0-15 Hz) recognized recently using Fourier transforms. Such frequency components appear due to charge-separating (electrogenic) processes mediated by epithelial plasma membrane proteins (both ionic channels and ionic cotransporters). In particular, the endothelial TEPD oscillations of the highest amplitude (1-2 Hz) were linked to the operation of so-called sodium bicarbonate cotransporters. However, no time localization of that activity could be obtained with the Fourier methodology utilized. For that reason we now characterize the TEPD using wavelet analysis with the aim to localize in time the variations in TEPD. We find that the mentioned high-amplitude oscillatory components of the TEPD appear cyclically during the several hours that an endothelial preparation survives in vitro. They have a period of 4.6 ± 0.4 s on average (n=4). The wavelet power value at the peak of such oscillations is 1.5 ± 0.1 mV(2) Hz on average (n = 4), and is remarkably narrow in its distribution.

Measurement of corneal endothelial impedance with non-invasive external electrodes--a theoretical study

The corneal endothelial cell layer function is critical for the maintenance of hydration and transparency of the cornea. Recent advances in corneal lamellar transplantation point to the need for reliable, non-invasive and rapid endothelial function assessment. Findings using an invasive electrode in an experimental animal model have suggested an association between bioimpedance parameters and endothelial cell function. Currently, however there is no clinical device that allows for non-invasive measurements of endothelial layer electrical impedance. This report is a finite element simulation study that models the human eye. It evaluates the feasibility of using external non-invasive electrodes to detect changes in endothelial layer electrical properties as a function of electrode location and measurement frequencies. The findings show that the ratio between the potential recorded at low and high frequencies is sensitive to changes in endothelial resistivity as well as endothelial capacitance. Moreover, the optimal electrode configuration yielding the highest sensitivity is one where the current injecting electrodes are oppose to each other and the voltage recording electrodes are adjacent to the current injecting electrodes. This first-order theoretical study suggests that a non-invasive device which measures electrical properties of the endothelial layer from the exterior of the eye could be developed. Clearly further animal and human studies are required to develop a reliable clinical tool.

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.

Non-invasive assessment of corneal endothelial permeability by means of electrical impedance measurements

The permeability of the corneal endothelial layer has an important role in the correct function of the cornea. Since ionic permeability has a fundamental impact on the passive electrical properties of living tissues, here it is hypothesized that impedance methods can be employed for assessing the permeability of the endothelial layer in a minimally invasive fashion. Precisely, the main objective of the present study is to develop and to analyze a minimally invasive method for assessing the electrical properties of the corneal endothelium, as a possible diagnostic tool for the evaluation of patients with endothelial dysfunction. A bidimensional model consisting of the main corneal layers and a four-electrode impedance measurement setup placed on the epithelium has been implemented and analyzed by means of the finite elements method (FEM). In order to obtain a robust indicator of the permeability of the endothelium layer, the effect of the endothelium electrical properties on the measured impedance has been studied together with reasonable variations of the other model layers. Simulation results show that the impedance measurements by means of external electrodes are indeed sufficiently sensitive to the changes in the electrical properties of the endothelial layer. It is concluded that the method presented here can be employed as non-invasive method for assessing endothelial layer function.

Regeneration of functional nerves within full thickness collagen-phosphorylcholine corneal substitute implants in guinea pigs

Our objective was to evaluate promotion of tissue and nerve regeneration by extracellular matrix (ECM) mimics, using corneal implantation as a model system. Porcine type I collagen and 2-methacryloyloxyethyl phosphorylcholine (MPC) were crosslinked using 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS) and moulded into appropriate corneal dimensions to serve as substitutes for natural corneal ECM. These were implanted as full thickness grafts by penetrating keratoplasty into the corneas of guinea pigs after removal of the host tissue, and tracked over eight months, by clinical examination, slit-lamp biomicroscopy, and esthesiometry. Histopathology and ex vivo nerve terminal impulse recordings were performed at three months and at eight months. The implants promoted regeneration of corneal cells, nerves and the tear film, while retaining optical clarity. After three months, electrophysiological recordings showed evidence of mechano-nociceptors, and polymodal units inside the implants, while cold-sensitive units were present only on the peripheral host cornea. Following eight months, the incidence of nerve activity and the frequency of spontaneous firing were higher than in control eyes as reported for regenerating fibers. Active cold nerve terminals also innervated the implant area. We show that ECM mimetic materials can promote regeneration of corneal cells and functional nerves. The simplicity in fabrication and demonstrated functionality shows potential for ECM substitutes in future clinical applications.

Modulation of tight junction properties relevant to fluid transport across rabbit corneal endothelium

Paracellular junctions could play an important role in corneal endothelial fluid transport. In this study we explored the effects of different reagents on the tight junctional barrier by assessing the translayer specific electrical resistance (TER) across rabbit corneal endothelial preparations and cultured rabbit corneal endothelial cells' (CRCEC) monolayers, the paracellular permeability (Papp) for fluorescein isothiocyanate (FITC) dextrans across CRCEC, and fluid transport across de-epithelialized rabbit corneal endothelial preparations. Palmitoyl carnitine (PC), poly-L-lysine (PLL), adenosine triphosphate (ATP), and dibutyryl adenosine 3',5'-cyclic monophosphate (dB-cAMP) were used to modulate corneal endothelial fluid transport and tight junctions (TJs). After seeding, the TER across CRCEC reached maximal values (29.2+/-1.0 Omega cm2) only after the 10th day. PC (0.1 mM) caused decreases both in TER (by 40%) and fluid transport (swelling rate: 18.5+/-0.3 microm/h), and an increase in Papp. PLL resulted in increased TER rose and Papp but decreased fluid transport (swelling rate: 10+/-0.3 microm/h). dB-cAMP (0.1 mM) and ATP (0.1 mM) decreased TER by 16% and 6%, increased Papp slightly, and stimulated fluid transport; the rates of de-swelling (in microm/h) were -5.4+/-0.3 and -12.1+/-0.4, respectively. PC might cause the junctions to open up unspecifically and thus increase passive leak. PLL is a known junctional charge modifier that may be adding steric hindrance to the tight junctions. The results with dB-cAMP and ATP are consistent with fluid transport via the paracellular route.

Numerical simulation of corneal transport processes

This paper presents a numerical study on the transport of ions and ionic solution in human corneas and the corresponding influences on corneal hydration. The transport equations for each ionic species and ionic solution within the corneal stroma are derived based on the transport processes developed for electrolytic solutions, whereas the transport across epithelial and endothelial membranes is modelled by using phenomenological equations derived from the thermodynamics of irreversible processes. Numerical examples are provided for both human and rabbit corneas, from which some important features are highlighted.

Mathematical modelling of corneal swelling

This paper presents a differential model of the corneal transport system capable of modelling thickness changes in response to osmotic perturbations applied to either limiting membrane. The work is directed towards understanding corneal behaviour in vivo. The model considers the coupled viscous flows within the corneal stroma and across the epithelial and endothelial membranes. The flows within the stroma are established based on transport theory in porous media, while the flows across the membranes are described using the phenomenological equations of irreversible thermodynamics. The ability of the numerical model to reproduce corneal thickness changes in response to endothelial perturbations was tested against available experimental data. The sensitivity of the model to changes in stromal and membrane transport coefficients was examined.

NaCl osmotic perturbation can modulate hydration control in rabbit cornea

The corneal endothelium transports solute from the stroma to the aqueous humor, maintaining corneal hydration. Currently, little is known about how this active transport system is controlled. The purpose of this study is to investigate in greater detail the corneal response to small NaCl osmotic perturbations using a more refined automatic thickness measurement system in a search for response signatures of transport control. Adult New Zealand White rabbit corneas were debrided of their epithelium, excised and mounted in perfusion chambers. The endothelium, thus isolated, was bathed in isotonic Glutathione Bicarbonate Ringer's (GBR) solution and the bare anterior stroma was covered with silicone oil. Following stabilization in isotonic GBR, the endothelial perfusate was altered by +/-15 mOsm or+/-45 mOsm for 1 hr and 45 min by addition or removal of NaCl and returned (reversal) to GBR for 1 hr and 45 min. An enhanced, automatic scanning specular microscope monitored stromal thickness. The effective membrane transport coefficients were determined from the stromal thickness vs. time curves using an established numerical model of corneal hydration dynamics. It was found that the small (+/-15 mOsm) NaCl perturbations of the rabbit corneal endothelium resulted in a rapid trans-endothelial stromal volume control response that was not reversible after return to GBR. Long after the expected dissipation of the induced transients, this thickness 'controlling' response ultimately resulted in a sustained net thinning of 14 microm following the hypotonic perturbation and reversal, and a net swelling of 16 microm following the hypertonic perturbation and reversal. Model calculations indicated that the change induced by the perturbation could be explained by an immediate and persistent reduction of the passive endothelial NaCl permeability by 26% for the -15 mOsm perturbation compared to the +15 mOsm perturbation. This change persisted even after return to GBR. In contrast, the larger (+/-45 mOsm) perturbations did not elicit a similar response consistently. Our data suggest that trans-endothelial fluid transport can be rapidly modulated to control stromal hydration in response to small NaCl osmotic stresses in a way that cushions the shock and reduces the change in corneal thickness. Moreover, this behavior is not reversible in the short term, and may assist the regulation of corneal hydration homeostatically.

Expression of MUC1 on corneal endothelium of human

PURPOSE

To determine the expression of MUC1 in the human corneal endothelium.

METHODS

Reverse transcription-polymerase chain reaction (RT-PCR) for MUC1 was performed with total RNA from endothelial cells extracted from the human cornea. In situ hybridization with sense and antisense probes of human MUC1 was performed on the human corneal endothelium, immunoblot analysis using monoclonal antibody specific for human MUC1 (HMFG-1, or VU4H5) was performed on collected human corneal endothelial cells, and immunohistochemistry on the human cornea, using the same antibodies.

RESULTS

MUC1 mRNA expression was observed by RT-PCR in the human corneal endothelium, and the nucleotide sequence from the amplified band was matched with known human MUC1. In situ hybridization studies showed the localization of MUC1 mRNA in the human corneal endothelium, and immunoblot assay demonstrated the presence of MUC1 protein (MW > 200 kd). In addition, MUC1 protein was observed on the apical surface of cells and at the superficial layer of the cytoplasm in immunohistochemical studies.

CONCLUSIONS

Human corneal endothelial cells produce MUC1, which is known to have protective and lubricative roles.

Effects of humidified and dry air on corneal endothelial cells during vitreal fluid-air exchange

PURPOSE

To report the immediate anatomic and functional alterations in corneal endothelial cells following use of humidified air and dry air during vitreal fluid-air exchange in rabbits.

DESIGN

Experimental study.

METHODS

Rabbits undergoing pars plana vitrectomy and lensectomy were perfused with either dry or humidified air during fluid-air exchange for designated durations. Three different experiments were performed. First, control and experimental corneas were examined by scanning electron microscopy (SEM). Second, corneas were stained with Phalloidin-FITC and examined by fluorescein microscopy. Finally, third, transendothelial permeability for carboxyfluorescein was determined using a diffusion chamber.

RESULTS

While different from the corneal endothelial cells, those cells exposed to humidified air were less stressed than cells exposed to dry air by SEM. Actin cytoskeleton was found highly disorganized with dry air exposure. Humidified air maintained the normal actin cytoskeleton throughout the 20 minutes of fluid-air exchange. Paracellular carboxyfluorescein leakage was significantly higher in dry air insufflated eyes compared with that of the humidified air after 5, 10, and 20 minutes of fluid-air exchange (P =.002, P =.004, and P =.002, respectively).

CONCLUSIONS

Dry air stress during fluid-air exchange causes significant immediate alterations in monolayer appearance, actin cytoskeleton, and barrier function of corneal endothelium in aphakic rabbit eyes. Use of humidified air largely prevents the alterations in monolayer appearance, actin cytoskeleton, and barrier function of corneal endothelial cells.

Predicted permeability of the cornea to topical drugs

PURPOSE

To develop a theoretical model to predict the passive, steady-state permeability of cornea and its component layers (epithelium, stroma, and endothelium) as a function of drug size and distribution coefficient (phi). The parameters of the model should represent physical properties that can be independently estimated and have physically interpretable meaning.

METHODS

A model was developed to predict corneal permeability using 1) a newly developed composite porous-medium approach to model transport through the transcellular and paracellular pathways across the epithelium and endothelium and 2) previous work on modeling corneal stroma using a fiber-matrix approach.

RESULTS

The model, which predicts corneal permeability for molecules having a broad range of size and lipophilicity, was validated by comparison with over 150 different experimental data points and showed agreement with a mean absolute fractional error of 2.43, which is within the confidence interval of the data. In addition to overall corneal permeability, the model permitted independent analysis of transcellular and paracellular pathways in epithelium, stroma and endothelium. This yielded strategies to enhance corneal permeability by targeting epithelial paracellular pathways for hydrophilic compounds (phi < 0.1 - 1), epithelial transcellular pathways for intermediate compounds, and stromal pathways for hydrophobic compounds (phi > 10 - 100). The effects of changing corneal physical properties (e.g., to mimic disease states or animals models) were also examined.

CONCLUSIONS

A model based on physicochemical properties of the cornea and drug molecules can be broadly applied to predict corneal permeability and suggest strategies to enhance that permeability.

Corneal endothelial NKCC: molecular identification, location, and contribution to fluid transport

Although Na(+)-K(+)-2Cl(-) cotransport has been demonstrated in cultured bovine corneal endothelial cells, its presence and role in the native tissue have been disputed. Using RT-PCR we have now identified a partial clone of the cotransporter protein in freshly dissected as well as in cultured corneal endothelial and epithelial cells. The deduced amino acid sequence of this protein segment is 99% identical to that of the bovine isoform (bNKCC1). [(3)H]bumetanide binding shows that the cotransporter sites are located in the basolateral membrane region at a density of 1.6 pmol/mg of protein, close to that in lung epithelium. Immunocytochemistry confirms the basolateral location of the cotransporter. We calculate the turnover rate of the cotransporter to be 83 s(-1). Transendothelial fluid transport, determined from deepithelialized rabbit corneal thickness measurements, is partially inhibited (30%) by bumetanide in a dose-dependent manner. Our results demonstrate that Na(+)-K(+)-2Cl(-) cotransporters are present in the basolateral domain of freshly dissected bovine corneal endothelial cells and contribute to fluid transport across corneal endothelial preparations.

ML-7, chelerythrine and phorbol ester increase outflow facility in the monkey Eye

Baseline or post-drug outflow facility was measured by two-level constant pressure perfusion of the anterior chamber (AC). The AC of one eye of cynomolgus monkeys was exchanged with the myosin light chain kinase (MLCK) inhibitor ML-7, the protein kinase (PK) C inhibitor chelerythrine (CHEL), or the PKC activator phorbol myristate acetate (PMA), followed by continuous AC infusion of the drug. The opposite eye similarly received the corresponding vehicle solution. The facility-effectiveness of subthreshold doses of ML-7 or CHEL + a subthreshold dose of the serine-threonine kinase inhibitor H-7, and of facility-effective doses of CHEL + a subthreshold or effective dose of PMA, were also determined. In 45 min post-exchange perfusions, 100 and 500 microM ML-7 increased outflow facility by 32 and 76%, while 100 and 500 microM CHEL increased facility by 68 and 101%, respectively, adjusted for baseline and contralateral control eye resistance washout. In 90 min post-exchange perfusions, 100 microM ML-7 or CHEL time-dependently increased outflow facility by 23, 49 and 69%, or by 44, 108 and 125% in the first, second and third 30 min periods, respectively. At 50 microM, ML-7 was ineffective, but CHEL increased outflow facility by 36% in the third 30 min period. Ten microM H-7 potentiated the outflow facility effect of 50 microM ML-7 or 20 microM CHEL by 36 and 28%, respectively, in the second 30 min period, and that of 50 microM CHEL by 44% in the overall 60 min post-exchange perfusion, compared to the H-7 only-treated contralateral eye. Ten, 50 or 100 n M PMA dose-dependently increased outflow facility by 23, 62 or 174%. Ten n M PMA + 50 microM CHEL did not induce any additional significant changes in outflow facility compared to 50 n M CHEL alone, while the effect of 50 n M PMA and 100 microM CHEL together was 63% more than that of 100 microM CHEL alone. In conclusion, ML-7/CHEL may increase outflow facility by a cytoskeletal mechanism. Separate or combined treatment with CHEL and PMA increases outflow facility, suggesting that PKC inhibition may not be involved in the facility-increase with either drug.

Demonstration of a Na(+)/H(+) exchanger NHE1 in fresh bovine corneal endothelial cell basolateral plasma membrane

Apical and basolateral plasma membranes of fresh bovine corneal endothelial cells were isolated using positively charged polyacrylamide beads. Marker enzyme assays demonstrated that the isolated apical and basolateral plasma membrane domains could be isolated and separated with relative purity. Western blotting with a polyclonal anti-NHE1 antibody detected a protein of 70 kDa in the basolateral plasma membrane isolate. NHE1 immunoreactivity was not detected in the apical membrane sample. This suggests that the Na(+)/H(+) exchanger, NHE1, is strictly localised to the basolateral membrane of fresh bovine corneal endothelial cells.

The spatial organization of apical junctional complex-associated proteins in feline and human corneal endothelium

PURPOSE

Previous studies suggest that proteins associated with the apical junctional complex (AJC) play essential roles in the development, maintenance and regulation of barrier function in transport epithelium and vascular endothelium. The goal of this study is to identify and determine the spatial organization of several major AJC-associated proteins in normal human and feline corneal endothelium.

METHODS

Fresh corneal tissue was obtained from 4 recipient buttons removed during penetrating keratoplasty (two from keratoconus patients, and two from patients with post-traumatic stromal scarring) as well as from 16 cat eyes. En bloc double- and triple-labeling of corneas was performed using phalloidin, and mouse, rat or rabbit antibodies to ZO-1, occludin, pan-cadherin, alpha-catenin, beta-catenin and plakoglobin (gamma-catenin). The 3-D localization of the proteins was then determined in situ using laser confocal microscopy.

RESULTS

Similar staining patterns were obtained for the corneal endothelium of normal cat corneas and fresh human buttons. Apically, f-actin was arranged into dense peripheral bands (DPB) in individual cells that were separated from those in adjacent cells. Diffuse phalloidin staining also extended from the DPB into the cytoplasm apically. Although weaker, phalloidin staining also appeared to be associated with the basolateral cell borders. The adherens junction protein, cadherin, formed a thin pericellular band at the apical cell junctions between the DPB. In addition, cadherin staining also appeared to extend along the basolateral cell borders in a convoluted pattern. Staining for alpha-catenin, beta-catenin and plakoglobin each showed a nearly identical organization as cadherin. ZO-1 formed a single apical band that was localized between the DPB; however, no basolateral ZO-1 staining was detected. Interestingly, the distribution of ZO-1 was discontinuous around the cell, with the largest gaps occurring at the Y-junctions between adjacent endothelial cells. Positive staining for occludin was not detected in either human or feline corneal endothelium.

CONCLUSIONS

The composition and organization of the AJC of corneal endothelium appears to be different from that of classical transport epithelia; these findings may be related to functional differences between these two cell types.

Fluid transport across cultured bovine corneal endothelial cell monolayers

The mammalian corneal endothelium is known to transport fluid from the stromal compartment to the aqueous humor, thereby maintaining corneal transparency. Corneal endothelial cells have been cultured for some years now, but whether they preserve their in vivo ability to actively transport fluid is not known. We have now grown bovine corneal endothelial cell monolayers (BCECM) on permeable substrates (Transwell) and report that, just like their counterparts in vivo, these cultured cells pump fluid from the basal to the apical compartment and display measurable electrical resistance and potential difference across the monolayer. BCECM were grown on collagen-treated permeable supports using Dulbecco's modified Eagle's medium (DMEM)/20% fetal bovine serum with antibiotics. Cells grew to confluence in 5-7 days and displayed polygonal shape. Only cells from passages 1-3 were utilized. Inserts were fitted directly into Lucite chambers specially built. The rate of fluid pumping by BCECM was 3.96 +/- 0.49 (SE) microliter.h-1.cm-2 (n = 13) and could be measured continuously for several hours; fluid pumping was inhibited by 0.2 mM amiloride. The specific electrical resistance of the monolayers was 180 +/- 22 omega.cm2 (n = 11). A mean electrical potential difference of 63.8 +/- 3.7 microV (n = 15, range 40-100 microV, apical side negative) was recorded across the monolayers in DMEM. The availability of the commercial inserts makes this procedure practical; as a consequence, the rate of fluid transport by cultured corneal endothelium has been quantitated for the first time. This method can now be extended to other cultured layers.(ABSTRACT TRUNCATED AT 250 WORDS)

Ocular pharmacokinetics of calcium, chloride and sulphate ions after instillation in the rabbit

The ionic environment of the eye is involved in major biochemical processes which are essential for preserving the integrity of cornea and lens. The purpose of the present study was to determine the intra-ocular penetration and the pharmacokinetic parameters of calcium, chloride and sulphate ions in the cornea, iris-ciliary body (ICB) and lens, after administration by instillation in the rabbit eye. In order to extrapolate our results to the processes occurring in man, we followed a precise instillation protocol using a low volume (5 microliters) of 45Calcium, 36Chloride and 35Sulphate, which is less than the lacrimal volume determined during the palpebral closing and performing manual blinking at a frequency of 2 min-1. The results indicate an immediate trans-corneal permeability and a rapid ocular distribution of these ions. We observe that, relative to dose, an important percentage of calcium (67.20 per cent) was entrapped in the cornea; this parameter was less important for chloride (10.19 per cent) and for sulphate ions (3.25 per cent). These values are in agreement with those predicted theoretically for trans-corneal penetration by such compounds. On the other hand, the total degrees of penetration by chloride and sulphate ions in ICB (1.40 per cent and 0.90 per cent, respectively) and lens (0.35 per cent and 0.41 per cent, respectively) are quite similar. Calcium retention is much higher in these tissues (25.39 per cent in ICB and 16.03 per cent in lens).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of human neutrophil chemotaxis across human endothelial cell monolayers on the permeability of these monolayers to ions and macromolecules

We have developed a method for studying the permeability properties of human endothelia in vitro. Human umbilical vein endothelial cells (HUVEC) were cultured on a substrate of human amnion. Confluent monolayers of these cells demonstrated 6-12 delta.cm2 of electrical resistance (a measure of their permeability to ions) and restricted the transendothelial passage of albumin from their apical to their basal surface. To determine whether leukocyte emigration alters endothelial permeability in this model, we examined the effects of migrating human polymorphonuclear leukocytes (PMN) on these two parameters. Few PMN migrated across the HUVEC monolayers in the absence of chemoattractants. In response to chemoattractants, PMN migration through HUVEC monolayers was virtually complete within 10 minutes and occurred at random locations throughout the monolayer. PMN migrated across the monolayer via the paracellular pathway. Although one PMN migrated across the monolayer for each HUVEC, PMN migration induced no change in electrical resistance or albumin permeability of these monolayers. At this PMN:HUVEC ratio, these permeability findings were correlated morphologically to measurements that HUVEC paracellular pathway size increases by less than 0.22% with PMN migration. This increase is insufficient to effect a measurable change in the electrical resistance of the endothelial cell monolayer. These findings demonstrate that increased permeability of cultured endothelial cell monolayers is not a necessary consequence of PMN emigration.

Role of the intraocular irrigating solutions in the pathogenesis of the postvitrectomy retinal edema

Alterations in the vitreo-retinal barrier can modify the exchange of water taking place between the retina and the vitreous body and cause retinal edema to develop. To verify this fact, albino rabbits were subjected to vitrectomies perfusing various intraocular irrigating solutions for various durations. The following conclusions were drawn from these experiments: Ringer's Lactate and physiological saline solutions are more edematogenous than BSS plus; the induced edema is more severe, the longer the perfusion. We have also confirmed that this edema is inhibited by topically administered indomethacin, which proves that in the rupture of blood-retinal barriers during the vitrectomy, prostaglandins play an important role.

Swelling pressure in bovine cornea determined by dialysis method

The swelling pressure of 62 +/- 2 mmHg and the fixed charge concentration of 15.7 +/- 1.9 mmol/kg H2O were determined in bovine corneal stroma using a dialysis method. The hydration of the dialyzed stromal tissues was independent of ambient Na concentration. The same relationship was observed by incubation of whole corneas mounted in modified Ussing chambers.

Corneal epithelial Cl-dependent pump quantified

Pairs of rabbit corneas were perfused and corneal thickness monitored. The epithelial side of the preparation, and after stabilization also the endothelial side, was covered with silicone oil. A constant thinning of the cornea was observed at a rate of 11.1 microns hr-1 (= 1.0 micron L h-1 cm-2) S.D. 2.1, n = 9. This thinning was also present after endothelial removal: 12.3 microns hr-1, S.D. 1.6 (n = 5). Epithelial abrasion virtually abolished the thinning (2.5 microns hr-1, S.D. 1.89, n = 4), as did cooling from 34 to 0 degrees C. When the preparation was perfused with a Cl-free solution (SO4(2-) instead of Cl-, corrected for osmolarity with sucrose) no significant thinning of the preparation was observed (2.4 microns hr-1, S.D. 2.49, n = 4) after covering both surfaces with silicone oil. This simple set of experiments quantified the epithelial pump mechanism. The epithelial pump rate of about 1.2 microns L hr-1 cm-2 has to be taken into account when endothelial pump rates are measured in the in vitro preparation with intact epithelium.

Amiloride inhibition of Na+-entry into corneal endothelium

Amiloride in 10(-3) M concentration inhibits incompletely the short circuit current and active potential difference across the bovine corneal endothelium in vitro. The drug effect is reversible and unilateral, e.g. the drug is effective only from the aqueous side. The amiloride effect is compared to the effect of ouabain, nystatin and vasopressin on the same electrical parameters. The effect of these drugs support a model for active Na+ transport across the corneal endothelium with two separate pathways for Na+ transport - one for extrusion and one for reentry.

Passive ion fluxes across the corneal endothelium

The deepithelialized cornea is mounted in a chamber and perfused on both surfaces by identical closed systems. The technique allows the continuous measurement of the net water flux and the electrical potential across the preparation as well as the steady state flux of radioactive ions. The permeabilities of pairs of ions in the same direction were compared, and they were found to be closely proportional to their free diffusion constants, suggesting that they passed by way of the intercellular spaces. Technical difficulties prevented a valid determination of the existence of active ion transport, but there was no indication of its presence.

Ionic transport across corneal endothelium

The active potential difference across bovine corneal endothelium was measured in vitro at different temperatures, pH, osmolality and salt compositions. The measurements were made using either identical or different solutions on each side of the membrane. The experimental results are consistent with a model in which sodium is actively transported into the intercellular cleft. We propose that Na+ re-enters the cell electroneutrally by coupled co-transport with carbonate, derived from bicarbonate in the solution.

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

These experiments were undertaken to study the effects of ouabain on endothelial function in isolated perfused human and rabbit corneas. Both human and rabbit corneas swell at rates which are dose-dependent when perfused with ouabain (10(-8) to 10(-5) M). The human tissue, however, requires tenfold higher concentrations to achieve swelling rates comparable to rabbit. Dose-dependent ultrastructural changes are seen in ouabain-perfused endothelia progressing from moderate endothelial edema (7.5 X 10(-7) M) to marked edema with altered subcellular organelles and vacuolization (10(-5) M). Ouabain-perfused human corneas show marked endothelial edema even at the lowest concentrations of ouabain tested. Apical junctions remain intact, however, in both human and rabbit endothelia at all concentrations. These results suggest that endothelial Na/K ATPase is essential to pump function in both human and rabbit corneas.

Ionic selectivity of the paracellular shunt path across rabbit corneal endothelium

We have measured the dilution and biionic potentials across the isolated rabbit corneal endothelium in order to learn about the ionic selectivity of its intercellular junctions. Single-salt dilution potentials have been measured as a function of [NaCl] or [NaHCO3] gradients across the tissue. Biionic potentials were similarly measured by replacing Na+ with K+ on either side of the tissue. The potentials thus measured were fit to the constant field equation and to an approximation of it to obtain the ionic permeabilities for K+, HCO-3 and Cl- relative to Na+. The permeability sequence obtained was PK greater than PNa greater than PHCO3 approximately equal to PCl. Potentials were also measured after imposing an osmotic gradient across the preparation using sucrose. The results obtained with all these methods are consistent and suggest that this tissue is slightly more permeant to cations than anions, but that the selectivity of the intercellular junction is relatively low. From these experiments, a 30 mM gradient of salt across the endothelial layer would be needed in order to explain the observed spontaneous potential difference (about 1 mV, aqueous negative) across that layer if the potential was due to the selectivity of the intercellular junctions. Such a value for the gradient is much larger than theoretical estimates of it; therefore, we favor electrogenic transport of HCO-3 as a better explanation for the origin of the spontaneous potential difference.

Clinical investigations on the corneal endothelium-XXXVIII Edward Jackson Memorial Lecture

The normal thickness and transparency of the cornea is maintained by the barrier function and the active fluid pump of the corneal endothelium. The major barrier is the intercellular gap junctions, and the fluid pump depends on the active transport of bicarbonate ions. Three methods are currently available for studying this important cell layer. (1) Endothelial dysfunction produces corneal swelling, and measuring the thickness of the swelling permits the degree of damage and the repair processes to be evaluated. (2) The endothelium's permeability to fluorescein reflects the state of its barrier. (3) The morphometric measurements obtained by specular microscopy of the endothelial cells permit the cell size distribution pattern and any alterations in it to be studied. Although the cell size distribution is normal in the young adult, cellular pleomorphism increases and cell density decreases during the aging process. A follow-up study of cell transformation after surgical trauma disclosed that the human endothelium shows practically no proliferative activity and that the damaged area is covered by means of cell migration. This migration, however, is incomplete, resulting in persistent regional differences in the cell distribution pattern. The traumatized endothelium continues to lose cells at an accelerated rate and endothelial dysfunction may develop many years after injury.

Effects of inhibitors of passive Na+ and HCO3- fluxes on electrical potential and fluid transport across rabbit corneal endothelium

We found that amiloride and SITS, which are inhibitors of passive movements of Na+ and HCO3- across cell membranes, inhibited but did not abolish the electrical potential difference and net fluid transport across the rabbit corneal endothelium.

Electrical properties of rabbit corneal endothelium as determined from impedance measurements

Alternating- and direct-current electrical characteristics of rabbit corneal endothelium were studied under varying experimental conditions. The measurements were performed by sending a 10-microA current (AC or DC) across the tissue layer. Maximal values of transendothelial potential difference and resistance were 1.3 +/- 0.1 mV and 73 +/- 6 omega . cm2, respectively. The short-circuit current was estimated from the potential and resistance values. Impedance loci were obtained for the frequency range 0.5-100 kHz. A capacitive reactance (C = 0.63 +/- 0.02 microF/cm2) was observed in the 100 Hz-100 kHz range. To relate the impedance data to the electrical parameters of the cell membranes, the voltage-divider ratio was determined by sending square pulse across the tissue and measuring voltage responses across the apical and basal membranes with an intracellular microelectrode. The intracellular potential difference was on the average -61 +/- 1 mV, and the voltage-divider ratio was found to be between 0.33 and 4. Impedance data were fit by a computer to an equivalent circuit representing a "lumped" model, and the agreement between the model and the data was satisfactory. The results are discussed in terms of both the morphological characteristics and properties of the fluid transport mechanism across the preparation.

Endothelial physiology and intraocular lens implantation

The endothelium is the cellular monolayer which lines the posterior surface of the cornea. This layer is important in clinical ophthalmology because it is vital to maintenance of the transparency of the cornea and vision through its pump and barrier functions which limit the ingress of fluid into the cornea from the aqueous. When the function of the corneal endothelium becomes compromised, the corneal stroma swells as it hydrates. Subsequently, epithelial bullae form with painful recurring epithelial erosions, and finally corneal scarring and blindness result. The relatively vulnerable position of the corneal endothelium renders it susceptible to iatrogenic injury during intraocular procedures, especially IOL implantation: the poor regenerative (mitotic) capacity of the human corneal endothelium limits its ability to recover normal function once it is injured.

A model of epithelial water transport. The corneal endothelium

To try to understand how an epithelial tissue can transport water between bathing solutions of equal tonicity and how intracellular solute and protein concentration are related to the structural specialization of the cell membrane at its apical, basal, and lateral margins, we have formulated and solved, using approximate analytical techniques, a new model which combines the detailed transport of local osmotic flow in extracellular channel with the multicompartment approach of thermodynamic models requiring the overall conservation of water and solute for the entire cell layer. Thus, unlike most previous models, which dealt exclusively with either the average properties of the cell layer or the local transport in the extracellular channel, we are able to solve simultaneously for the interaction of the cell with its environments across its apical, basal, and lateral cell membranes as well as the detailed transport in the extracellular channel. The model is then applied to corneal endothelium to obtain new insight into the water flow movement in this tissue under in vitro and in vivo conditions. Then in vitro solution shows that the cell at 297 mosmol/liter is slightly hypotonic to the 300-mosmol/liter external bathing solutions which drive water equally out both the aqueous (apical) and stromal (basal) cell faces. This water is replaced from the extracellular channel. There is a net flow of water because more water enters the channel through its open stromal end than through the higher resistance tight junction. In vivo, the solution predicts that the stromal swelling pressure forces water through the tight junctions towards the stroma so that there is no net flow. The interesting new features of our solution are the water recirculation pattern and the role of the osmotically active proteins in making the cell hypertonic relative to the channel.

Priming of the fluid pump by osmotic gradients across rabbit corneal endothelium

The present study shows that the inclusion of 5% Dextran (average mol. wt. 40 000) in solutions to preserve in vitro rabbit corneal endothelium induces a sizable osmotic flow across the preparation which is superimposed on the existing fluid transport. Furthermore, even after fluid transport ceases due to in vitro deterioration, the Dextran-induced flow remains for some addition time. The osmotic permeability was 162 +/- 17 micrometer/s in the presence of glucose and 451 +/- 84 micrometer/s in its absence. The latter, comparatively high value suggests that such osmotic flow traverses the intracellular junctions. In addition, temporary (10--15 min) imposition of an osmotic gradient has a separate stimulatory 'priming' effect on the rate of fluid transport. Thus, the rate of fluid pumping increased by about 40% after challenge with Dextran. It was further noted that, after addition of Dextran, preparations in the absence of glucose escape gross deterioration for a time longer than those in the presence of glucose. On the other hand, mere addition of Dextran to a glucose-containing solution does not appear to prolong the estimated 'survival time' of the pumping mechanism. The sizable osmotic flows and the priming effect described here may provide a physiological context with which previously described Dextran effects on cornea preservation can now be compared.

Corneal endothelial changes under induced intraocular pressure elevation: a scanning and transmission electron microscopic study in rabbits

Intraocular pressure was artificially raised to 60--70 mmHg in 7 albino rabbits for periods of 15 minutes to 4 hours. The corneal endothelium of these eyes was studied by transmission and scanning electron microscopy. A correlation between exposure time to elevated IOP, clinical signs observed by slit-lamp examination, and extent of morphological damage is clearly shown. In eyes exposed to high pressure for 15 and 30 minutes corneas remained transparent and only minimal changes could be detected by SEM, which consisted of small areas of cell with unevenness of their surface, occasional cellular ruptures, and diminution of cilia and microvilli. After 1--2 hours of exposure small, solitary corneal opacifications appeared. In these eyes more severe morphological changes affecting larger areas were observed, with additional cellular blebbing, excariocytosis, cellular rupture, disintegration, and disappearance seen in SEM. Thin sections revealed swelling of mitochondria, disorganisation of endoplasmic reticulum, and the existence of myelin bodies. In eyes exposed for 3 and 4 hours to high IOP corneal haziness, implying stromal oedema, appeared. In these eyes the areas affected were larger, the extent of damage being more severe. Many areas were bare of endothelium, surrounded by scattered cellular debris, and showed cells with ballooning surfaces and multiple ruptures. Even in severe cellular damage cellular junctions appeared intact. It is assumed that endothelial cells are more sensitive to IOP elevation than the cellular junctions and that injury to the active pump system due to morphological damage is responsible for the resultant corneal oedema.

Biphasic effects of insulin and ouabain on fluid transport across rabbit corneal endothelium

1. Low levels of insuling stimulate transendothelial fluid transport from preswollen stroma to aqueous in rabbit corneal preparations. The rate of stromal thinning at the end of the first hour averages 30% faster with insulin, 3.5 x 10(-22) M (4.8 micromicron/ml.), than that of the paired control. This concentration is about the physiological level in rabbit aqueous. 2. The stimulation with insulin is transient. Rates of thinning average higher but not significantly different from control rates by the second hour. 3. High levels of insulin between 3.5 x 10(-9) M (480 micromicron/ml.) and 2.0 x 10(-6) M (2.75 X 10(5) micromicron/ml.) inhibit fluid transport. The inhibition at the low end of this range of concentrations becomes more pronounced with longer perfusion times but appears not to exceed ca. 50% of the control rate. 4. Ouabain also induces a biphasic effect on fluid transport which is characteristically different from that with insulin. The maximal stimulation observed at all times occurred with a fixed concentration of 10(-10) M. The stimulation is not transient but increases throughout the duration of the perfusion; the average rate is elevated 50% above the control rate by the third hour. 5. The transition from a stimulatory to an inhibitory effect occurs consistently at ca. 10(-8) M with ouabain, while a similar transition with insulin occurs at ca. 10(-9) M and appears to shift towards slightly higher concentrations during a 3 hr perfusion period. 6. Inhibition of fluid transport with ouabain, 3 x 10(-7) M, is increased from ca. 50% after 1 hr to more than 70% at the end of the third hour of perfusion. 7. The combined presence of stimulatory concentrations of ouabain and insulin affects tromal thinning in a manner resembling the effect of ouabain alone more than that of insulin; additive effects could not be discriminated. Progressively raising the concentration of insulin to a level (10(-8) M) that alone inhibits stromal thinning, ultimately abolishes the stimulatory effect of ouabain. Based on other evidence and current models of drug/hormone-membrane interaction, these results can be interpreted to indicate a concentration-dependent interaction between receptor complexes of ouabain and insulin with (Na+ + K+)-ATPase.