Intracellular [Na+], Na+ pathways, and fluid transport in cultured bovine corneal endothelial cells

Mitochondrial Antioxidant SkQ1 Improves Hypothermic Preservation of the Cornea

Diseases of the cornea are a frequent cause of blindness worldwide. Keratoplasty is an efficient method for treating severely damaged cornea. The functional competence of corneal endothelial cells is crucial for successful grafting, which requires improving the media for the hypothermic cornea preservation, as well as developing the methods for the evaluation of the corneal functional properties. The transport of water and ions by the corneal endothelium is important for the viability and optic properties of the cornea. We studied the impact of SkQ1 on the equilibrium sodium concentration in the endothelial cells after hypothermic preservation of pig cornea at 4°C for 1, 5, and 10 days in standard Eusol-C solution. The intracellular sodium concentration in the endothelial cells was assayed using the fluorescent dye Sodium Green; the images were analyzed with the custom-designed CytoDynamics computer program. The concentrations of sodium in the pig corneal endothelium significantly increased after 10 days of hypothermic preservation, while addition of 1.0 nM SkQ1 to the preservation medium decreased the equilibrium concentration of intracellular sodium (at 37°C). After 10 days of hypothermic preservation, the permeability of the plasma membrane for sodium decreased in the control cells, but not in the cells preserved in the presence of 1 nM SkQ1. Therefore, SkQ1 increased the ability of endothelial cells to restore the intracellular sodium concentration, which makes SkQ1 a promising agent for facilitating retention of the functional competence of endothelial cells during cold preservation.

STUDY OF THE EFFECT OF HYPOTHERMIC CONSERVATION ON THE INTRACELLULAR SODIUM CONCENTRATION IN THE ENDOTHELIUM OF CORNEAL TRANSPLANTS

Endothelial keratoplasty has become the treatment of choice for corneal endothelial dysfunction. Advancements in the surgical treatment of corneal endothelial diseases depend on progress in graft conservation and its related advantages in assessing the suitability of grafts for transplantation. Transport of water and ions by cornea endothelium is important for the optic properties of cornea. In this work, we study the intracellular sodium concentration in cornea endothelial cells in samples of pig cornea that underwent hypothermic conservation for 1 and 10 days and endothelial cells of human cornea grafts after 10-day conservation. The concentration of intracellular sodium in preparations of endothelial cells was assayed using fluorescent dye SodiumGreen. The fluorescent images were analyzed with the custom-made computer program CytoDynamics. An increased level of intracellular sodium was shown in the endothelium after 10-day conservation in comparison with one-day conservation (pig samples). Sodium permeability of pig endothelial cell plasma membranes significantly decreased in these samples. Assessment of intracellular sodium in human cornea endothelium showed a higher level – as was in analogues pig samples of the corneal endothelium. The assay of the intracellular sodium balance concentration established in endothelial cells after hypothermic conservation in mediums L-15 and Optisol-GS showed a significant advantage of specialized me dium Optisol-GS. The balanced intracellular concentration after 10 days of hypothermic conservation was significantly lower in cells incubated at 4 °C in Optisol-GS (L-15, 128 ± 14,  n = 15; Optisol-GS, 108 ± 14, n = 11; mM, p < 0.001). Intracellular sodium concentration could be a useful parameter for assessing cornea endothelium cell viability.

Co-expression of vasoactive intestinal peptide and protein gene product 9.5 surrounding the lumen of human Schlemm's canal

Previous studies of aqueous humor outflow have focused primarily on resistance at the trabecular meshwork (TM), and little is known about the function of Schlemm's canal (SC). Here, we aimed to investigate whether SC is innervated by the peripheral nervous system. Ten eye specimens from eight donors were processed for histological analysis. CD31 was used to identify SC, after which we used protein gene product (PGP) 9.5 as a marker to detect nerve fibers around SC. We then characterized the nerves by double staining for PGP9.5 and sympathetic nerve markers, such as tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DβH), or the parasympathetic marker vasoactive intestinal peptide (VIP), as well as sensory nerve marker calcitonin gene-related peptide (CGRP) and vesicular glutamate transporter 2 (VGLUT2). Immunohistochemistry and immunofluorescence were also performed to detect the expression of γ-epithelial Na⁺ channel (γ-ENaC) in SC. We found that different markers were expressed in the anterior chamber angle, with the luminal surface of SC were only positive stained for PGP9.5, VIP, and γ-ENaC. CGRP and VGLUT2 were expressed in TM and scleral spur (SS), whereas TH and DβH were absent in both TM and SC. Furthermore, PGP9.5 was co-expressed with VIP and γ-ENaC in the region surrounding the SC as well as in SS. Our findings indicate that the peripheral nerves anatomically spread in the tissues around the SC and the local nerve fibers may be parasympathetic or sensory rather than sympathetic.

Fluid transport by the cornea endothelium is dependent on buffering lactic acid efflux

Maintenance of corneal hydration is dependent on the active transport properties of the corneal endothelium. We tested the hypothesis that lactic acid efflux, facilitated by buffering, is a component of the endothelial fluid pump. Rabbit corneas were perfused with bicarbonate-rich (BR) or bicarbonate-free (BF) Ringer of varying buffering power, while corneal thickness was measured. Perfusate was collected and analyzed for lactate efflux. In BF with no added HEPES, the maximal corneal swelling rate was 30.0 ± 4.1 μm/h compared with 5.2 ± 0.9 μm/h in BR. Corneal swelling decreased directly with [HEPES], such that with 60 mM HEPES corneas swelled at 7.5 ± 1.6 μm/h. Perfusate [lactate] increased directly with [HEPES]. Similarly, reducing the [HCO3 (-)] increased corneal swelling and decreased lactate efflux. Corneal swelling was inversely related to Ringer buffering power (β), whereas lactate efflux was directly related to β. Ouabain (100 μM) produced maximal swelling and reduction in lactate efflux, whereas carbonic anhydrase inhibition and an monocarboxylic acid transporter 1 inhibitor produced intermediate swelling and decreases in lactate efflux. Conversely, 10 μM adenosine reduced the swelling rate to 4.2 ± 0.8 μm/h and increased lactate efflux by 25%. We found a strong inverse relation between corneal swelling and lactate efflux (r = 0.98, P < 0.0001). Introducing lactate in the Ringer transiently increased corneal thickness, reaching a steady state (0 ± 0.6 μm/h) within 90 min. We conclude that corneal endothelial function does not have an absolute requirement for bicarbonate; rather it requires a perfusing solution with high buffering power. This facilitates lactic acid efflux, which is directly linked to water efflux, indicating that lactate flux is a component of the corneal endothelial pump.

New trends in quantitative assessment of the corneal barrier function

The cornea is a very particular tissue due to its transparency and its barrier function as it has to resist against the daily insults of the external environment. In addition, maintenance of this barrier function is of crucial importance to ensure a correct corneal homeostasis. Here, the corneal epithelial permeability has been assessed in vivo by means of non-invasive tetrapolar impedance measurements, taking advantage of the huge impact of the ion fluxes in the passive electrical properties of living tissues. This has been possible by using a flexible sensor based in SU-8 photoresist. In this work, a further analysis focused on the validation of the presented sensor is performed by monitoring the healing process of corneas that were previously wounded. The obtained impedance measurements have been compared with the damaged area observed in corneal fluorescein staining images. The successful results confirm the feasibility of this novel method, as it represents a more sensitive in vivo and non-invasive test to assess low alterations of the epithelial permeability. Then, it could be used as an excellent complement to the fluorescein staining image evaluation.

Modulation of sodium-bicarbonate co-transporter (SLC4A4/NBCe1) protein and mRNA expression in rat's uteri by sex-steroids and at different phases of the oestrous cycle

Oestrogen-induced uterine fluid sodium (Na(+)) and bicarbonate (HCO3(-)) secretion may involve SLC4A4. We hypothesized that uterine SLC4A4 expression changes under different sex-steroid influence, therefore may account for the fluctuation in uterine fluid Na(+) and HCO3(-) content throughout the oestrous cycle. The aim of this study is to investigate the differential effects of sex-steroids and oestrous cycle phases on uterine SLC4A4 expression.

METHODS

Adult female WKY rats were ovariectomised and treated with different doses of 17β-oestradiol (E2) (0.2, 2, 20 and 50 μg/ml/day) or progesterone (P4) (4 mg/ml/day) for three consecutive days and 3 days treatment with 0.2 μg/ml/day E2 followed by another 3 days with P4 to mimic the hormonal changes in early pregnancy. Oestrous cycle phases in intact, non-ovariectomised rats were determined by vaginal smear. The animals were then sacrificed and uteri were removed for protein and mRNA expression analyses by Western blotting and Real Time PCR, respectively. SLC4A4 distribution was observed by immunohistochemistry.

RESULTS

Treatment with increasing E2 doses resulted in a dose-dependent increase in SLC4A4 protein expression. High SLC4A4 protein and mRNA expression can be seen at estrus. SLC4A4 is distributed mainly at the apical as well as basolateral membranes of the luminal and glandular epithelia following E2 treatment and at Es. Meanwhile, SLC4A4 expression was reduced following P4 treatment and was low at diestrus.

CONCLUSION

High SLC4A4 expression under estrogen dominance may contribute to the increase in uterine fluid Na(+) and HCO3(-) content, while its low expression under P4 dominance may result in vice versa.

Characteristics of the low density corneal endothelial monolayer

Corneal endothelial cells form a leaky barrier on the posterior surface of the cornea, allowing influx of nutrient-carrying aqueous humor through the paracellular space and efflux of excess fluid. Corneal edema arises when the density of these non-proliferative endothelial cells declines from endothelial disease or intraocular surgery. The cellular changes occurring at low densities are ill-defined. We therefore investigated the paracellular pathway of corneal endothelial cell monolayers of varying density to determine alterations occurring in paracellular permeability and monolayer morphology. Primary cultures of bovine corneal endothelial cells (BCECs) were passaged onto permeable supports under varying culture conditions to obtain confluent monolayers of <1000, 1000-1999 and >2000 cells/mm(2). Culture growth was monitored by transendothelial electrical resistance measurements. Diffusional permeability to sodium fluorescein, FITC-dextran MW 4000 or FITC-dextran MW 20,000 was measured. Confluent cultures were also analyzed by immunofluorescence localization of the tight junction protein ZO-1 and by transmission electron microscopy. For comparison, we evaluated ZO-1 for low and high density human corneal endothelium. Our results showed that all BCEC cultures grew to the same final transendothelial electrical resistance regardless of final density. In the diffusional permeability assay, permeability increased significantly only for the smallest tracer molecule (sodium fluorescein) in the lowest density monolayers (<1000 cells/mm(2)). ZO-1 immunofluorescence distinctly localized to intercellular junctions in high density BCEC cultures but had more diffuse localization at lower densities. Transmission electron microscopy imaging revealed cells with thinner cross-sectional profiles and longer overlapping intercellular processes at low density relative to high density cultures. Low density human corneal endothelium lacked the diffuse ZO-1 distribution seen in BCECs. Our data supports the hypothesis that barrier integrity is the primary function disrupted in low density corneal endothelial monolayers and contradicts the idea of a linear decline in barrier function with decreasing cell density.

Molecular mechanisms underlying the corneal endothelial pump

The corneal endothelium is responsible for maintaining the hydration of the cornea. This is through a "Pump-Leak" mechanism where the active transport properties of the endothelium represent the "Pump" and the stromal swelling pressure represents the "Leak". For the "Pump", Na(+), K(+) ATPase activity and the presence of HCO(3)(-), Cl(-), and carbonic anhydrase activity are required. Several basolateral (stromal side) anion transporters, apical (facing the aqueous humor) ion channels and water channels have been identified that could support a model for ion secretion as the basis for the endothelial pump, however evidence of sustained anion fluxes, osmotic gradients or the need for water channels is lacking. This has prompted consideration of other models, such as Electro-osmosis, and consideration of metabolite flux as components of the endothelial pump. Although the conditions under which the "Pump" is supported are known, a complete model of the endothelial "Pump" has yet to emerge.

Frequency spectrum of transepithelial potential difference reveals transport-related oscillations

How epithelia transport fluid is a fundamental issue that is unresolved. Explanations offered include molecular engines, local transcellular osmosis, local paracellular osmosis, and paracellular fluid transport. On the basis of experimental and theoretical work done on corneal endothelium, a fluid transporting epithelium, we suggest electroosmotic coupling at the level of the intercellular junctions driven by the transendothelial electrical potential difference as an explanation of paracellular fluid transport. We collect frequency spectra of that potential difference in real-time. For what we believe is the first time for any epithelium, we report that, unexpectedly, the potential difference displays oscillations at many characteristic frequencies. We also show that on both stimulating cell activity and inhibiting ion transport mechanisms, there are corresponding changes in the oscillations amplitudes that mirror changes known previously in rates of fluid transport. We believe these findings provide a novel tool to study the kinetics of electrogenic elements such as channels and transporters, which from this evidence would give rise to current oscillations with characteristic periods going from 150 ms to 8 s.

Ocular novel drug delivery: impacts of membranes and barriers

BACKGROUND

Ocular drug delivery is an extremely challenging area due to its restrictive barrier functionalities.

OBJECTIVE

Drug transport via corneal/non-corneal routes involves several intricate biological processes such as drug penetration across the ocular barriers and transfer to the anterior or posterior chambers, thus the influence of these processes on the pharmacotherapy of the eye should be fully addressed.

METHODS

To pursue the impacts of such impediments in novel drug therapy, recent publications were reviewed regarding advanced strategies such as nanomedicines.

CONCLUSION

The ocular barriers are highly specialized and selectively control the inward/outward traverse of compounds, hence a better understanding of these biological obstacles would provide a platform to advance ophthalmic drug therapy towards specified delivery/targeting with minimal adverse consequences.

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.

The human corneal endothelium: new insights into electrophysiology and ion channels

The corneal endothelium is a monolayer that mediates the flux of solutes and water across the posterior corneal surface. Thereby, it plays an essential role to maintain the transparency of the cornea. Unlike the epithelium, the human endothelium is an amitotic cell layer with a critical cell density and the risk of corneal decompensation. The number of endothelial cells subsequently decreases with age. Moreover, the endothelial cell loss is accelerated after various impairments such as surgical trauma (e.g. cataract extraction) and following corneal transplantation. This cell loss is associated with programmed cell death (apoptosis) and changed ion channel activity. However, little is known about the electrophysiology and ion channel expression (in particular Ca2+ channels) in corneal endothelial cells. This article reviews our current knowledge about the electrophysiology of the corneal endothelium. It highlights ion channel expression, which may have a major role in corneal cell physiology and pathological events. A better understanding of the (electro)physiological function of the cornea may lead to the development of clinical relevant new therapeutic and preventive measures.

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.

A possible role for membrane depolarization in epithelial wound healing

Linear narrow wounds produced on cultured bovine corneal endothelial monolayers heal by actin cable formation at the wound border and lamellar crawling of cells into the injured area. We report the novel finding that membrane potential depolarization occurs at the leading edge of wounds and gradually extends inward toward the neighboring cells. We have determined that the replacement of extracellular Na+by choline and the incorporation of phenamil, an inhibitor of the epithelial Na+channel (ENaC), provoke a decrease in the actin cable and depolarization areas and in the lamellar activity of the wound edges. To the contrary, extracellular Li+can successfully replace Na+in the determination of the depolarization and cytoskeletal responses. This finding supports the idea that membrane depolarization, not the increase in intracellular Na+concentration, is responsible for the formation of the actin cable, a result that is in agreement with previous evidence showing that nonspecific depolarization of the plasma membrane potential (PMP) of epithelial cells may promote characteristic cytoskeletal rearrangements per se (Chifflet S, Hernández JA, Grasso S, and Cirillo A. Exp Cell Res 282: 1–13, 2003). We suggest that spontaneous depolarization of the PMP of the cells at the wound borders determined by a rise in the ENaC activity of these cells constitutes an additional factor in the intermediate cellular processes leading to wound healing in some epithelia.