An update on corneal hydration control

Corneal endothelial cell proliferation: a function of cell density

PURPOSE

To determine how stimuli that increase corneal endothelial cell proliferation of human corneas in culture relate to changes in endothelial cell density in the central and peripheral cornea.

METHODS

Human donor cadaver corneas not suitable for transplantation were divided into four pie-shaped wedges and incubated at 37 degrees C in medium supplemented with fetal bovine serum, epidermal growth factor, fibroblast growth factor, and gentamicin. To promote a proliferative response, samples were treated with EDTA at concentrations of 0, 0.5, 2.5, and 5.0 mM for 1 hour and then returned to culture medium. Endothelial cell proliferation was assayed with Ki-67 immunolocalization 48 and 96 hours after EDTA treatment. Samples were mounted with propidium iodide or DAPI. The total number of cells and the number of Ki-67-positive cells were counted in three regions, defined as central, mid, and peripheral cornea, to determine endothelial cell density and percentage of proliferation.

RESULTS

A proliferative response to EDTA was not found. However, increased proliferation was noted in the central compared with the peripheral corneal region. Unexpectedly, the increased proliferation in the central region corresponded to a trend of lower endothelial cell density in the central region compared with the peripheral region. Corneal endothelial cell proliferation under our culture conditions is noted primarily when cell density is less than 2000 cells/mm(2).

CONCLUSIONS

Corneal endothelial cell proliferation under our culture conditions does not lead to supranormal endothelial cell density. Rather, cell proliferation is noted in those regions that may be experiencing a greater burden of cell loss.

Volumetric and ionic regulation during the in vitro development of a corneal endothelial barrier

Corneal endothelium is responsible for generating an ion flux between the corneal stroma and the anterior chamber of the eye that is necessary for the cornea to remain transparent. However, the ion transport regulatory mechanisms that develop during the formation of the endothelial barrier are not known. In this study, we determined the influence of cell confluence on cell volume and intracellular ionic content on the corneal endothelial cells of rabbits. Our results demonstrate that non-confluent endothelial cells display a hypertrophic volume increase, with higher intracellular contents of potassium and chlorine than those of confluent cells. In contrast, when cells reach confluence and the endothelial barrier forms, cell volume decreases and the intracellular contents of potassium and chlorine decrease. Our genetic analysis showed a higher expression of CFTR and CA2 genes in non-confluent cells, and of the gene KCNC3 in confluent cells. These results suggest that the normal ionic current that keeps the corneal stroma dehydrated and transparent is regulated by cell-cell contacts and endothelial cell confluence, and could explain why the loss of corneal endothelial cells is often associated with corneal edema and even blindness.

Characterization of regulatory volume behavior by fluorescence quenching in human corneal epithelial cells

An in-depth understanding of the mechanisms underlying regulatory volume behavior in corneal epithelial cells has been in part hampered by the lack of adequate methodology for characterizing this phenomenon. Accordingly, we developed a novel approach to characterize time-dependent changes in relative cell volume induced by anisosmotic challenges in calcein-loaded SV40-immortalized human corneal epithelial (HCE) cells with a fluorescence microplate analyzer. During a hypertonic challenge, cells shrank rapidly, followed by a temperature-dependent regulatory volume increase (RVI), tau(c) = 19 min. In contrast, a hypotonic challenge induced a rapid (tau(c) = 2.5 min) regulatory volume decrease (RVD). Temperature decline from 37 to 24 degrees C reduced RVI by 59%, but did not affect RVD. Bumetanide (50 microM), ouabain (1 mM), DIDS (1 mM), EIPA (100 microM), or Na(+)-free solution reduced the RVI by 60, 61, 39, 32, and 69%, respectively. K+, Cl- channel and K(+)-Cl(-) cotransporter (KCC) inhibition obtained with either 4-AP (1 mM), DIDS (1 mM), DIOA (100 microM), high K+ (20 mM) or Cl(-)-free solution, suppressed RVD by 42, 47, 34, 52 and 58%, respectively. KCC activity also affects steady-state cell volume, since its inhibition or stimulation induced relative volume alterations under isotonic conditions. Taken together, K+ and Cl- channels in parallel with KCC activity are important mediators of RVD, whereas RVI is temperature-dependent and is essentially mediated by the Na(+)-K(+)-2Cl(-) cotransporter (Na(+)-K(+)-2Cl(-)) and the Na(+)-K(+) pump. Inhibition of K+ and Cl- channels and KCC but not Na(+)-K(+)-2Cl(-) affect steady-state cell volume under isotonic conditions. This is the first report that KCC activity is required for HCE cell volume regulation and maintenance of steady-state cell volume.

Evaluation of the viability of cultured corneal endothelial cells by quantitative electron probe X-ray microanalysis

Construction of artificial organs and tissues by tissue engineering is strongly dependent on the availability of viable cells. For that reason, the viability and the physiological status of cells kept in culture must be evaluated before the cells can be used for clinical purposes. In this work, we determined the viability of isolated rabbit corneal endothelial cells by trypan blue staining and quantitative electron probe X-ray microanalysis. Our results showed that the ionic content of potassium in cultured corneal endothelial cells tended to rise initially, but significantly decreased in cells in the fifth (and final) subculture, especially in comparison to cells in the fourth subculture (P < 0.001). However, the concentration of sulfur was higher in the fifth subculture than in the fourth subculture (P < 0.001), with a nonsignificant increase in sodium in the fifth subculture (P = 0.031). These data imply a remarkable decrease in the K/Na ratio from the fourth to the fifth subculture. Our microanalytical results, along with the morphological differences between cells in the last two subcultures, are compatible with an early phase of the preapoptotic process in the fifth subculture, and suggest that cells of the first four subcultures would be better candidates for tissue engineering.

Antioxidant function of corneal ALDH3A1 in cultured stromal fibroblasts

Aldehyde dehydrogenase 3A1 (ALDH3A1) is highly expressed in epithelial cells and stromal keratocytes of mammalian cornea and is believed to play an important role in cellular defense. To explore a potential protective role against oxidative damage, a rabbit corneal fibroblastic cell line (TRK43) was stably transfected with the human ALDH3A1 and subjected to oxidative stress induced by H(2)O(2), mitomycin C (MMC), or etoposide (VP-16). ALDH3A1-transfected cells were more resistant to H(2)O(2,) MMC, and VP-16 compared to the vector-transfected cells. All treatments induced apoptosis only in vector-transfected cells, which was associated with increased levels of 4-hydroxy-2-nonenal (4-HNE)-adducted proteins. Treatment with H(2)O(2) resulted in a rise in reduced glutathione (GSH) levels in all groups but was more pronounced in the ALDH3A1-expressing cells. Treatment with the DNA-damaging agents led to GSH depletion in control groups, although the depletion was significantly less in ALDH3A1-expressing cells. Increased carbonylation of ALDH3A1 but not significant decline in enzymatic activity was observed after all treatments. In conclusion, our results suggest that ALDH3A1 may act to protect corneal cells against cellular oxidative damage by metabolizing toxic lipid peroxidation products (e.g., 4-HNE), maintaining cellular GSH levels and redox balance, and operating as an antioxidant.

Reduced Corneal Swelling and Deswelling Response in the Iridocorneal Endothelial Syndrome

PURPOSE

Despite severe abnormalities of the corneal endothelium in the iridocorneal endothelial (ICE) syndrome, the cornea can remain clear and maintain its normal thickness for years before corneal decompensation occurs. The aim of this study is to analyze this discrepancy by studying corneal hydration control in the ICE syndrome.

METHODS

In four subjects with unilateral ICE syndrome, without signs of corneal decompensation, a "corneal stress test" was performed in both the affected and the unaffected eye. The stress test measures the recovery of corneal thickness after swelling induced by wearing a soft contact lens with the eyes closed. Corneal thickness was measured by noncontact pachymetry.

RESULTS

Mean baseline corneal thickness of affected eyes (535+/-56 microm) was not different from unaffected eyes (526+/-39 microm). On the other hand, the corneal stress test showed a smaller induced swelling in affected eyes (33+/-15 microm) compared with unaffected eyes (67+/-11 microm) and a slower recovery of corneal thickness in affected eyes. In one of four subjects, the cornea of the affected eye was calculated not yet to have recovered its original thickness before midnight. After this finding, although the cornea of this subject did not show any signs of clinical decompensation, the study was discontinued.

CONCLUSIONS

Corneal hydration control is altered in the ICE syndrome. The ICE cornea can maintain its normal thickness despite severe morphologic abnormalities of the endothelium. Because we found that recovery of corneal thickness can be extremely slow, it cannot be guaranteed that such corneal "stress tests" are completely safe for subjects with the ICE syndrome, and we therefore feel that they should no longer be performed in these subjects.

Extracellular ATP opposes thrombin-induced myosin light chain phosphorylation and loss of barrier integrity in corneal endothelial cells

Increased contractility of the actin cytoskeleton by phosphorylation of the regulatory myosin light chain (MLC) results in a loss of barrier integrity in corneal endothelial cells. This study has investigated the effect of extracellular ATP, which may influence both Ca2+ and cAMP signalling, on MLC phosphorylation and barrier integrity in cultured bovine corneal endothelial cells (BCEC) known to express A2B and P2Y purinergic receptors, and ecto-nucleotidases. Extracellular ATP (100 microM) promoted MLC dephosphorylation (pMLC=61.8% at 18 min; n=9). Pre-exposure to ARL-67156, an ecto-nucleotidase inhibitor, prevented ATP-induced dephosphorylation. Other P2Y agonists, UTP and ATPgammaS, also induced MLC dephosphorylation but to a lesser degree compared to ATP. Thrombin (2 U/ml), which activate Rho kinase through PAR-1 receptors in the endothelium, induced MLC phosphorylation (pMLC=129.2%; n=14). This phosphorylation was completely abolished by concomitant exposure to ATP. When cells were pretreated with adenosine (100 microM; A2B agonist) or forskolin (10 microM), thrombin-induced phosphorylation was suppressed. ATP also led to a significant increase in cAMP (> 3-fold compared to 10 microM adenosine). Thrombin-induced increase in trans-endothelial flux of horseradish peroxidase (44 kDa) and disruption of the cortical actin were suppressed by ATP. These findings indicate that in BCEC (1) ATP induces elevated cAMP through its metabolite adenosine leading to MLC dephosphorylation, (2) Stimulation of P2Y2 receptors also leads to activation of MLCP since UTP- and ATPgammaS caused MLC dephosphorylation, and (3) ATP is antagonistic to thrombin since the latter inhibits MLCP through increased activity of Rho kinase. These findings further emphasize the role of contractility of the actin cytoskeleton in regulating the barrier integrity of corneal endothelium.

Expression and functional evaluation of transient receptor potential channel 4 in bovine corneal endothelial cells

We previously found that activation of purinergic receptors mobilizes Ca2+ and enhances bicarbonate transport in bovine corneal endothelial cells (BCEC). Since transient receptor potential channel 4 (TRPC) has been reported to be a candidate for capacitative calcium entry (CCE) and receptor operated calcium entry (ROC), we examined the expression of TRPC4 and evaluated the potential involvement of TRPC4 in CCE or ROC in BCEC. The C-terminus of TRPC4 was fused into the glutathione S-transferase (GST) expression vector. The fusion protein GST-TRPC4c was induced in bacteria and purified by affinity chromatography. An antibody was raised in rabbit by using the purified GST-TRPC4c antigen. In Western blotting, the TRPC4 antibody recognized the fusion protein while the pre-immune IgG did not. The TRPC4 antibody recognized a band at around 80 kD for membrane proteins from both the fresh and cultured BCEC. The pre-immune IgG could not detect bands at the same size. Incubation with the TRPC4c antigen abolished the 80 kD band. Immunofluorescence using the TRPC4 antibody stained both fresh and cultured BCEC, while pre-immune IgG did not. RNAi knocked down the expression of TRPC4 in cultured BCEC. Ca2+ entry induced by the purinergic receptor agonist ATP, was increased in TRPC4-siRNA transfected cells compared with the scrambled siRNA control, while Ca2+ entry induced by store depletion through blocking the endoplasmic reticulum Ca2+ pump, did not differ between the siRNA and scrambled siRNA-treated cells. Taken together, these results show that TRPC4 protein is expressed in the bovine corneal endothelial cells and may be a negative regulator in ROC stimulated by purinergic activation, but not by store depletion itself.