Localization of Na+/K(+)-ATPase in the bovine corneal endothelium

The Neuropeptide α-Melanocyte-Stimulating Hormone Prevents Persistent Corneal Edema following Injury

Corneal endothelial cells (CEnCs) regulate corneal hydration and maintain tissue transparency through their barrier and pump function. However, these cells exhibit limited regenerative capacity following injury. Currently, corneal transplantation is the only established therapy for restoring endothelial function, and there are no pharmacologic interventions available for restoring endothelial function. This study investigated the efficacy of the neuropeptide α-melanocyte-stimulating hormone (α-MSH) in promoting endothelial regeneration during the critical window between ocular injury and the onset of endothelial decompensation using an established murine model of injury using transcorneal freezing. Local administration of α-MSH following injury prevented corneal edema and opacity, reduced leukocyte infiltration, and limited CEnC apoptosis while promoting their proliferation. These results suggest that α-MSH has a proregenerative and cytoprotective function on CEnCs and shows promise as a therapy for the prevention and management of corneal endothelial dysfunction.

Tissue engineering of the corneal endothelium: a review of carrier materials

Functional impairment of the human corneal endothelium can lead to corneal blindness. In order to meet the high demand for transplants with an appropriate human corneal endothelial cell density as a prerequisite for corneal function, several tissue engineering techniques have been developed to generate transplantable endothelial cell sheets. These approaches range from the use of natural membranes, biological polymers and biosynthetic material compositions, to completely synthetic materials as matrices for corneal endothelial cell sheet generation. This review gives an overview about currently used materials for the generation of transplantable corneal endothelial cell sheets with a special focus on thermo-responsive polymer coatings.

Lactate-H⁺ transport is a significant component of the in vivo corneal endothelial pump

PURPOSE

To confirm the expression of monocarboxylate transporters (MCT) 1, 2, and 4 in rabbit CE and to test the hypothesis that cellular buffering contributed by HCO₃⁻, NBCe1, and carbonic anhydrase (CA) activity facilitates lactate-H⁺ efflux thereby controlling corneal hydration in vivo.

METHODS

MCT1-4 expression of rabbit endothelium was examined by Western blotting and immunofluorescence staining. Lactate-induced acidification (LIA) was measured in perfused CE in the presence and absence of HCO₃⁻ and acetazolamide (ACTZ) using tissue treated with siRNA specific to MCT1, 2, and 4. Corneal thickness and lactate concentration were measured in New Zealand White rabbits treated with the topical CA inhibitor Azopt, and from eyes that were injected intracamerally with ouabain, disodium 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS), and shRNA specific to the 1Na⁺:2HCO₃⁻ cotransporter NBCe1.

RESULTS

MCT1 and MCT4 are localized to the lateral membrane, while MCT2 is apical. Cell pH measurements showed LIA in response to 40 mM lactate in bicarbonate free (BF) Ringer's that was inhibited by niflumic acid and by MCT siRNA knockdown, and significantly reduced in the presence of HCO₃⁻. Lactate-dependent proton flux in vitro was not significantly greater in the presence of HCO₃⁻ or reduced by ACTZ. However, when active transport, NBCe1, or CA activity was disrupted in vivo, corneal edema ensued and was associated with significant corneal lactate accumulation.

CONCLUSIONS

MCT1, 2, and 4 are expressed in rabbit CE on both the apical and basolateral surfaces and function to transport lactate-H⁺. Lactate-H⁺ flux is facilitated by active transport, HCO₃⁻ transport and CA activity, disruption of which causes corneal edema in vivo and indicates that facilitation of lactate efflux is a component of the endothelial pump.

Electrical signaling in control of ocular cell behaviors

Epithelia of the cornea, lens and retina contain a vast array of ion channels and pumps. Together they produce a polarized flow of ions in and out of cells, as well as across the epithelia. These naturally occurring ion fluxes are essential to the hydration and metabolism of the ocular tissues, especially for the avascular cornea and lens. The directional transport of ions generates electric fields and currents in those tissues. Applied electric fields affect migration, division and proliferation of ocular cells which are important in homeostasis and healing of the ocular tissues. Abnormalities in any of those aspects may underlie many ocular diseases, for example chronic corneal ulcers, posterior capsule opacity after cataract surgery, and retinopathies. Electric field-inducing cellular responses, termed electrical signaling here, therefore may be an unexpected yet powerful mechanism in regulating ocular cell behavior. Both endogenous electric fields and applied electric fields could be exploited to regulate ocular cells. We aim to briefly describe the physiology of the naturally occurring electrical activities in the corneal, lens, and retinal epithelia, to provide experimental evidence of the effects of electric fields on ocular cell behaviors, and to suggest possible clinical implications.

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.

Transporters involved in regulation of intracellular pH in primary cultured rat brain endothelial cells

Fluid secretion across the blood-brain barrier, critical for maintaining the correct fluid balance in the brain, entails net secretion of HCO(3)(-), which is brought about by the combined activities of ion transporters situated in brain microvessels. These same transporters will concomitantly influence intracellular pH (pH(i)). To analyse the transporters that may be involved in the maintenance of pH(i) and hence secretion of HCO(3)(-), we have loaded primary cultured endothelial cells derived from rat brain microvessels with the pH indicator BCECF and suspended them in standard NaCl solutions buffered with Hepes or Hepes plus 5% CO(2)/HCO(3)(-). pH(i) in the standard solutions showed a slow acidification over at least 30 min, the rate being less in the presence of HCO(3)(-) than in its absence. However, after accounting for the difference in buffering, the net rates of acid loading with and without HCO(3)(-) were similar. In the nominal absence of HCO(3)(-) the rate of acid loading was increased equally by removal of external Na(+) or by inhibition of Na(+)/H(+) exchange by ethylisopropylamiloride (EIPA). By contrast, in the presence of HCO(3)(-) the increase in the rate of acid loading when Na(+) was removed was much larger and the rate was then also significantly greater than the rate observed in the absence of both Na(+) and HCO(3)(-). Removal of Cl(-) in the presence of HCO(3)(-) produced an alkalinization followed by a resumption of the slow acid gain. Removal of Na(+) following removal of Cl(-) increased the rate of acid gain. In the presence of HCO(3)(-) and initial presence of Na(+) and Cl(-), DIDS inhibited the changes in pH(i) produced by removal of either Na(+) or Cl(-). These are the expected results if these cells possess an AE-like Cl(-)/HCO(3)(-) exchanger, a 'channel-like' permeability allowing slow influx of acid (or efflux of HCO(3)(-)), a NBC-like Cl(-)-independent Na(+)-HCO(3)(-) cotransporter, and a NHE-like Na(+)/H(+) exchanger. The in vitro rates of HCO(3)(-) loading via the Na(+)-HCO(3)(-) cotransporter could, if the transporter is located on the apical, blood-facing side of the cells, account for the net secretion of HCO(3)(-) into the brain.

Functional human corneal endothelial cell sheets harvested from temperature‐responsive culture surfaces

This study reports a new method for fabricating bioengineered human corneal endothelial cell sheets suitable for ocular surgery and repair. We have initially cultured human corneal endothelial cells on type IV collagen-coated dishes and, after several passages, expanded cells were then seeded onto novel temperature-responsive culture dishes. Four weeks after reaching confluence, these cultured endothelial cells were harvested as intact monolayer cell sheets by simple temperature reduction without enzymatic treatment. Scanning electron microscopy indicated that these cells were primarily hexagonal with numerous microvilli and cilia, similar to the native corneal endothelium. The Na+, K+-ATPase pump sites were located at the cell borders as in vivo. Moreover, cell densities and numbers of pump sites were identical to those of in vivo human corneal endothelium under optimized conditions. A 3H-ouabain binding analysis demonstrated a linear proportionality for cell pump density between confluent cell densities of 575 cells/mm2 and 3070 cells/mm2. We also confirmed Na+, K+-ATPase activity in the sheets in vitro. Xenograft transplantation results showed that the fabricated sheets retain their function of maintaining proper stromal hydration in vivo. We have established a regimen to culture and proliferate human corneal endothelial cells and fabricate endothelial sheets ex vivo morphologically and functionally similar to the native corneal endothelium. Our results support the value of harvested cell sheets for clinical applications in ocular reconstructive surgery in patients with ocular endothelial decompensation.

Altered expression of aquaporins in bullous keratopathy and Fuchs' dystrophy corneas

Corneas with edema-related diseases lose transparency, which causes significant vision loss. This study analyzed seven aquaporins (AQPs) in normal corneas, pseudophakic/aphakic bullous keratopathy (PBK/ABK) corneas, Fuchs' dystrophy corneas, keratoconus corneas, post-cataract surgery (PCS) corneas, and normal organ-cultured corneas. RNA levels for AQP1, AQP4, and beta2-microglobulin were measured by RT-PCR. AQP1 antibody localized to stromal cells of all corneas. PBK/ABK and Fuchs' dystrophy corneas had decreased endothelial cell staining compared with normal. AQP1 mRNA was found in whole corneas and cultured stromal fibroblasts but not in isolated epithelial cells. AQP3 staining was found in basal epithelial cells of the normal, Fuchs' dystrophy, and keratoconus corneas but throughout the entire epithelium of PBK/ABK corneas. AQP4 antibody localized to endothelial cells of all corneas and in stromal cells of PBK/ABK corneas. AQP4 mRNA was identified in whole human corneas. AQP5 was found in epithelial cells of all corneas. AQP0, AQP2, and AQP9 were not found in any corneas. Normal AQP distributions were found in PCS and organ-cultured corneas, although they showed signs of swelling. Our study demonstrates that AQP abnormalities are found in PBK/ABK corneas (decreased AQP1, increased AQP3 and AQP4) and Fuchs' dystrophy corneas (decreased AQP1). Although both have vision-disrupting corneal edema, the mechanisms of fluid accumulation may be different in each disease.

Extracellular matrix and Na+,K+-ATPase in human corneas following cataract surgery: comparison with bullous keratopathy and Fuchs' dystrophy corneas

PURPOSE

To examine the distribution of extracellular matrix (ECM) and basement membrane (BM) components and of Na+,K+-ATPase in postcataract surgery (PCS) corneas. These corneas were from patients who never developed pseudophakic or aphakic bullous keratopathy (PBK/ABK) after cataract surgery. PCS corneas were compared with PBK/ABK and Fuchs' dystrophy corneas.

METHODS

The distribution of PBKIABK ECM and BM markers and of all three Na+,K+-ATPase alpha subunits was studied by immunofluorescence in 10 healthy, 11 PCS, 16 PBK/ABK, and 12 Fuchs' dystrophy corneas.

RESULTS

Fibrotic ECM proteins, tenascin-C and fibrillin-1, were found in only 1 of 10 healthy and in 2 of 11 PCS corneas. In contrast, these proteins were expressed in all PBK/ABK and more than half of the Fuchs' dystrophy corneas. BM components in PCS corneas were altered to a greater extent (40-60%), especially fibronectin and laminin-10. A decreased epithelial immunostaining for Na+,K+-ATPase alpha subunits was seen in approximately 40% of PCS corneas and in approximately two thirds of PBK/ABK and Fuchs' dystrophy corneas. However, the endothelial staining was normal in all groups.

CONCLUSIONS

Because tenascin-C and fibrillin-1 were mostly found in diseased but not in PCS corneas, their expression may be related to later, clinical stages of corneal edema development. However, BM components abnormal in PBK/ABK and Fuchs' dystrophy corneas were also altered in PCS corneas without clinical evidence of ocular disease. This may result from subclinical corneal changes resulting from cataract surgery, lens removal, exposure to the intraocular lens, or endothelial cell damage. Alterations of epithelial Na+,K+-ATPase point to the importance of epithelial changes in the development of corneal edematous diseases.

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.

Determination of Na+/Cl-, Na+/HCO3- and Na+/K+/2Cl- co-transporter activity in corneal endothelial cell plasma membrane vesicles

Corneal endothelial cell derived plasma membrane vesicles were used to investigate the presence of Na+/Cl-, Na+/HCO3- and Na+/K+/2Cl- co-transporter activity in the plasma membranes of these cells. Na+/H+ exchange was blocked by the presence of 1 mM amiloride in all determinations. The rate of accumulation of Na+ in the presence of chloride or bicarbonate was not significantly different from its accumulation in the presence of acetate, thiocyanate or gluconate. The addition of K+ to Na+ plus Cl- did not stimulate Na+ accumulation into the vesicles. The present work provides no evidence for Na+/K+/2Cl-, Na+/Cl- or Na+/HCO3- co-transport in corneal endothelial cell plasma membrane vesicles.

Two pathways for electrogenic bicarbonate ion movement across the rabbit corneal endothelium

Amiloride (0.5 mM) inhibited the rate of entry of Na+ into corneal endothelial cells by more than half ((0.76 +/- 0.10) to (0.21 +/- 0.10) microEq cm(-2)h(-1)). The same concentration of amiloride caused only minimal disturbance to corneal hydration control by the endothelium (range 0-12%). Amiloride (0.5 mM) and acetazolamide (1 mM) reversibly inhibited trans-endothelial short circuit current by about a half. Their combined effect was not additive. Acetazolamide (1 mM) reduced net HCO3- flux across the short-circuited endothelium by about the same amount ((0.50 +/- 0.11) microEq cm(-2)h(-1)) that amiloride (0.5 mM) reduced Na+ entry into the cells ((0.55 +/- 0.14) microEq cm(-2)h(-1)). Low concentrations of amiloride (10 microM) had little effect on the transport characteristics of the endothelium, indicating that Na+ entry into the endothelial cells under physiological conditions is not primarily through Na+ channels. The data are consistent with an Na+/H+ exchanger acting in tandem with carbonic anhydrase through a pathway which could have a regulatory role on endothelial transport via its effect on Na+ re-entry. Residual trans-endothelial HCO3- transport, apparently unaffected by amiloride or acetazolamide inhibition, is calculated to be of sufficient magnitude to maintain corneal hydration.

Determination of pathways for sodium movement across corneal endothelial cell derived plasma membrane vesicles

A bovine corneal endothelial cell plasma membrane vesicle preparation was used to investigate passive Na+ transport across the plasma membrane of these cells. Sodium accumulation rate into the vesicle was not dependent on the presence of HCO3- or a HCO3- gradient, but was stimulated by a trans-vesicle pH gradient. Amiloride, furosemide and DIDS all reduced the rate of Na+ accumulation. The data indicate the presence of at least two independent pathways for passive sodium movement across the vesicle: the first probably via a Na+/H+ exchanger and the second a furosemide inhibitable Na+ entry mechanism. No evidence was found for direct Na(+)-HCO3- coupled transport.