The culture of limbal stromal cells and corneal endothelial cells

The cornea is the transparent front part of the eye and comprises three distinct cell layers. One of these cell layers is a self-renewing epithelium long believed to harbor a resident stem cell population. The location and characteristics of corneal epithelial stem cells have now been confirmed by several research groups, and these cells are currently applied therapeutically. The corneal stroma and endothelium are largely quiescent after infancy, and until recently they were not considered to undergo self-renewal or to maintain stem cells. This view was overturned during the last two decades. At present, cell populations with characteristics of adult stem cells are routinely isolated and characterized from the limbal stroma and the corneal -endothelium. This chapter describes methods for isolation and culture of limbal stromal cells and corneal endothelial cells.

Ultraviolet light transmission through the human corneal stroma is reduced in the periphery

This article investigates in vitro light transmission through the human cornea in the ultraviolet (UV) portion of the electromagnetic spectrum as a function of position across the cornea from center to periphery. Spectrophotometry was used to measure UV transmission in the wavelength range 310-400 nm, from the central cornea to its periphery. UV transmission decreases away from the center, and this is attributed to scattering and absorbance. Corneal endothelial cells, which line the back of the cornea and are more numerous in the periphery, therefore receive a lower dose of UV than do those in the central cornea. This is consistent with the recent observation that endothelial cells in the corneal periphery exhibit less nuclear oxidative DNA damage than those in the central cornea.

Potential of human umbilical cord blood mesenchymal stem cells to heal damaged corneal endothelium

PURPOSE

To test the feasibility of altering the phenotype of umbilical cord blood mesenchymal stem cells (UCB MSCs) toward that of human corneal endothelial cells (HCEC) and to determine whether UCB MSCs can "home" to sites of corneal endothelial cell injury using an ex vivo corneal wound model.

METHODS

RNA was isolated and purified from UCB MSCs and HCECs. Baseline information regarding the relative gene expression of UCB MSCs and HCEC was obtained by microarray analysis. Quantitative real-time PCR (q-PCR) verified the microarray findings for a subset of genes. The ability of different culture media to direct UCB MSCs toward a more HCEC-like phenotype was tested in both tissue culture and ex vivo corneal endothelial wound models using three different media: MSC basal medium (MSCBM), a basal medium used to culture lens epithelial cells (LECBM), or lens epithelial cell-conditioned medium (LECCM). Morphology of the MSCs was observed by phase-contrast microscopy or by light microscopic observation of crystal violet-stained cells. Immunolocalization of the junction-associated proteins, zonula occludins-1 (ZO1) and N-cadherin, was visualized by fluorescence confocal microscopy. Formation of cell-cell junctions was tested by treatment with the calcium chelator, EGTA. A second microarray analysis compared gene expression between UCB MSCs grown in LECBM and LECCM to identify changes induced by the lens epithelial cell-conditioned culture medium. The ability of UCB MSCs to "home" to areas of endothelial injury was determined using ZO1 immunolocalization patterns in ex vivo corneal endothelial wounds.

RESULTS

Baseline microarray analysis provided information regarding relative gene expression in UCB MSCs and HCECs. MSCs attached to damaged, but not intact, corneal endothelium in ex vivo corneal wounds. The morphology of MSCs was consistently altered when cells were grown in the presence of LECCM. In tissue culture and in ex vivo corneal wounds, UCB MSC treated with LECCM were elongated and formed parallel sheets of closely apposed cells. In both tissue culture and ex vivo corneal endothelial wounds, ZO1 and N-cadherin localized mainly to the cytoplasm of UCB MSCs in the presence of MSCBM. However, both proteins localized to cell borders when UCB MSCs were grown in either LECBM or LECCM. This localization was lost when extracellular calcium levels were reduced by treatment with EGTA. A second microarray analysis showed that, when UCB MSCs were grown in LECCM instead of LECBM, the relative expression of a subset of genes markedly differed, suggestive of a more HCEC-like phenotype.

CONCLUSIONS

Results indicate that UCB MSCs are able to "home" to areas of injured corneal endothelium and that the phenotype of UCB MSCs can be altered toward that of HCEC-like cells. Further study is needed to identify the specific microenvironmental conditions that would permit tissue engineering of UCB MSCs to replace damaged or diseased corneal endothelium.

Age-related gene response of human corneal endothelium to oxidative stress and DNA damage

PURPOSE

Nuclear oxidative DNA damage increases with age in human corneal endothelial cells (HCECs) and contributes to their decreased proliferative capacity. These studies investigated whether HCECs respond to this damage by upregulating their expression of oxidative stress and DNA damage-signaling genes in an age-dependent manner.

METHODS

HCECs were dissected from the corneas of young (30 years and younger) and older (50 years and older) donors. Total RNA was isolated and reverse-transcribed. Oxidative stress and DNA damage-signaling gene expression were analyzed using commercial PCR-based microarrays. Western blot analyses were conducted on selected proteins to verify the microarray results. Nuclear DNA damage foci were detected in the endothelium of ex vivo corneas by immunostaining for H2AX-Ser139.

RESULTS

Four of 84 genes showed a statistically significant age-related difference in the expression of oxidative stress-related genes; however, Western blot analysis demonstrated an age-related increase in only 2 (cytoglobin and GPX-1) of 11 proteins tested. No age-related differences were detected in the expression of DNA damage-signaling genes. Western blot analysis of seven DNA damage-related proteins verified this finding. Intense nuclear staining of DNA damage foci was observed in nuclei within the central endothelium of older donors. Central endothelium from young donors consistently showed a low level of positive staining.

CONCLUSIONS

HCECs respond to age-related increases in oxidative nuclear DNA damage by forming DNA damage repair foci; however, they do not vigorously defend against or repair this damage by upregulating the expression of multiple oxidative stress or DNA damage-signaling genes.

Decreasing expression of the G1-phase inhibitors, p21Cip1 and p16INK4a, promotes division of corneal endothelial cells from older donors

PURPOSE

The current studies were conducted to determine whether the cyclin-dependent kinase inhibitors, p21Cip1 (p21 cyclin-dependent kinase-interacting protein 1) and p16INK4a (p16 cyclin-dependent kinase inhibitor 1A), help mediate G(1)-phase inhibition in human corneal endothelial cells (HCEC) by testing the effect of siRNA (small interfering RNA)-mediated down-regulation of the expression of these inhibitors on cell cycle entry and proliferation in HCEC cultured from older donors.

METHODS

HCEC were obtained from National Disease Research Interchange, Philadelphia, PA, and cultured according to published methods. Cells were electroporated in the presence of either a non-silencing siRNA control or p21+p16 siRNA. The efficiency of siRNA transfer was observed by fluorescence microscopy of Cy3-labeled control siRNA. Viability was determined by direct counting of cells before and after electroporation. The ability of p21+p16 siRNA to decrease the protein expression of p21Cip1 and p16INK4a was determined by semi-quantitative analysis of western blots. The effect of siRNA treatment on cell cycle progression and proliferation was determined 1, 5, and 11 days after electroporation by counting Ki67-positive cells and total DAPI-stained nuclei.

RESULTS

siRNA was efficiently transferred to HCEC by the electroporation method. The average cell loss was 41.25% at 24 h following electroporation. Protein levels of both p21Cip1 and p16INK4a were significantly decreased as the result of p21+p16 siRNA treatment. This treatment significantly increased the average number of Ki67-positive cells over controls and increased the total number of cells in a time-dependent manner.

CONCLUSIONS

Both p21Cip1 and p16INK4a are involved in negative regulation of the cell cycle in HCEC and, thereby, provide an effective barrier to cell division. The siRNA-induced reduction in expression of these proteins increased the number of cells entering the cell cycle, as well as total cell numbers. Thus, reduction of the levels of p21Cip1 and p16INK4a could be useful in the development of treatments to induce transient cell division to increase corneal endothelial cell density.

Differential protein expression in human corneal endothelial cells cultured from young and older donors

PURPOSE

To establish a baseline protein fingerprint of cultured human corneal endothelial cells (HCEC), to determine whether the protein profiles exhibit age-related differences, and to identify proteins differentially expressed in HCEC cultured from young and older donors.

METHODS

Corneas were obtained from five young (<30 years old) and five older donors (>50 years old). HCEC were cultured, and protein was extracted from confluent passage 3 cells. Extracts from each age group were pooled to form two samples. Proteins were separated on two-dimensional (2-D) gels and stained with SyproRuby. Resultant images were compared to identify protein spots that were either similarly expressed or differentially expressed by at least twofold. Protein spots were then identified by matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry.

RESULTS

Protein spots were well resolved, and patterns were reproducible on 2-D gels using either pH 3-10 or pH 4-7 IPG strips. Two-dimensional gels prepared with pH 4-7 IPG strips were used for differential display analysis, which was reproduced on three separate pairs of gels. MALDI-TOF identified 58 proteins with similar expression; 30 proteins were expressed twofold higher in HCEC from young donors; five proteins were expressed twofold higher in cells from older donors; and 10 proteins were identified in gels from young donors that did not match in gels from older donors. Several proteins expressed at higher levels in younger donors support metabolic activity, protect against oxidative damage, or mediate protein folding or degradation.

CONCLUSIONS

This is the first proteomic comparison of proteins expressed in HCEC cultured from young and older donors. Although restricted to proteins with isoelectric points between pH 4.0 and pH 7.0, the data obtained represent an initial step in the investigation of molecular mechanisms that underlie physiologically important age-related differences in cultured HCEC, including differences that may affect proliferative capacity. Results indicate that HCEC from older donors exhibit reduced expression of proteins that support important cellular functions such as metabolism, antioxidant protection, protein folding, and protein degradation. These differences may affect the ability to consistently obtain a sufficient number of healthy cultured HCEC for use in preparing bioengineered endothelium as an alternative method for the treatment of endothelial dysfunction.

Increased clusterin expression in Fuchs' endothelial dystrophy

PURPOSE

To investigate the differential expression of the glycoprotein clusterin/apoJ (CLU) in normal and Fuchs' endothelial dystrophy (FED) corneal endothelium and to compare the expression of various forms of CLU in normal and FED tissue.

METHODS

FED and pseudophakic bullous keratopathy (PBK) corneal buttons were removed during transplantation, and normal corneas were obtained from tissue banks. Human corneal endothelial cells and Descemet's membrane (HCEC-DM) complex was dissected from the stroma. Proteins were separated on 2-D gels and subjected to comparative proteomic analysis. Relative expression of presecretory CLU (pre-sCLU), secretory (s)CLU, and nuclear (n)CLU were compared between normal and FED HCEC-DM by Western blot analysis. Expression of CLU mRNA was compared by using RT-PCR. Subcellular localization of CLU was compared in corneal wholemounts from normal eyes and eyes with FED by immunocytochemistry followed by confocal microscopy.

RESULTS

Proteomic analysis revealed an apparent increase in CLU expression in FED HCEC-DM compared with the normal control. Western blot analysis demonstrated that pre-sCLU protein expression was 5.2 times higher in FED than in normal samples (P = 3.52E-05), whereas the mature form modified for secretion (sCLU) was not significantly elevated (P = 0.092). Expression of nCLU protein was significantly elevated in FED (P = 0.013). RT-PCR analysis revealed that CLU mRNA was significantly increased (P = 0.002) in FED samples, but not in PBK samples. CLU also had a distinctive localization in FED samples with enhanced intracellular staining around the guttae and in the nuclei of endothelial cells.

CONCLUSIONS

CLU expression is markedly elevated in FED-affected tissue, pointing to a yet undiscovered form of dysregulation of endothelial cell function involved in FED pathogenesis.

Decreased expression of peroxiredoxins in Fuchs' endothelial dystrophy

PURPOSE

To compare the relative expression of peroxiredoxin (Prx) proteins in normal human corneal endothelium with endothelium in corneas affected by Fuchs' endothelial dystrophy (FED) and between normal human endothelium and epithelial/stromal tissue.

METHODS

Human corneal endothelial cell-Descemet's membrane (HCEC-DM) complexes from normal and FED corneal buttons were dissected from the epithelium/stroma. For proteomic analysis, HCEC-DM protein extracts were separated by using two-dimensional gel electrophoresis. Relative differences in protein spot density was analyzed. Proteins of interest, including Prx isoforms, were identified by MALDI-TOF (matrix-assisted desorption ionization-time of flight) mass spectrometry. Western blot analysis compared the relative expression of Prx isoforms in normal and FED endothelium and between normal endothelium and normal epithelium/stroma. Expression of Prx-2 mRNA was compared by using real-time PCR.

RESULTS

Proteomic analysis identified differences in the relative expression of Prx isoforms between normal and FED endothelium. Western blot analysis confirmed that expression of Prx-2, -3, and -5 was significantly decreased (P < 0.05) in FED cells. Normal HCECs expressed significantly (P < 0.05) higher levels of Prx-2 and -3 than did the epithelium/stroma. Expression of Prx-5 was not significantly different (P > 0.05) in the endothelium versus the epithelium/stroma. Real-time PCR analysis revealed that Prx-2 mRNA was significantly decreased (P = 0.027) in FED samples.

CONCLUSIONS

Prx proteins were identified in human corneal endothelium. The fact that Prx-2 and -3 were expressed at significantly higher levels in HCEC-DM compared with the epithelium/stroma reflects the different physiologic activities of individual corneal cell types. Significantly decreased expression of Prx-2, -3, and -5 in FED may suggest an alteration in the ability of endothelial cells to withstand oxidant-induced damage and may be closely related to the pathogenesis of this disease.

Protein tyrosine phosphatase-1B (PTP1B) helps regulate EGF-induced stimulation of S-phase entry in human corneal endothelial cells

PURPOSE

Human corneal endothelial cells (HCEC), particularly from older donors, only proliferate weakly in response to EGF. The protein tyrosine phosphatase, PTP1B, is known to negatively regulate EGF-induced signaling in several cell types by dephosphorylating the epidermal growth factor receptor (EGFR). The current studies were conducted to determine whether PTP1B plays a role in regulating cell cycle entry in HCEC in response to EGF stimulation.

METHODS

Donor corneas were obtained from the National Disease Research Interchange and accepted for study based on established exclusion criteria. PTP1B was localized in the endothelium of ex vivo corneas and in cultured cells by immunocytochemistry. Western blot analysis verified PTP1B protein expression in HCEC and then compared the relative expression of EGFR and PTP1B in HCEC from young (<3 years old) and older donors (>60 years old). The effect of inhibiting the activity of PTP1B on S-phase entry was tested by comparing time-dependent BrdU incorporation in subconfluent HCEC incubated in the presence or absence of the PTP1B inhibitor, CinnGEL 2Me, before EGF stimulation.

RESULTS

PTP1B was localized in a punctate pattern mainly within the cytoplasm of HCEC in ex vivo corneas and cultured cells. Western blots revealed the presence of three PTP1B-positive bands in HCEC and the control. Further western blot analysis showed no significant age-related difference in expression of EGFR (p=0.444>0.05); however, PTP1B expression was significantly higher in HCEC from older donors (p=0.024<0.05). Pre-incubation of HCEC with the PTP1B inhibitor significantly increased (p=0.019<0.05) the number of BrdU positive cells by 48 h after EGF stimulation.

CONCLUSIONS

Both immunolocalization and western blot studies confirmed that PTP1B is expressed in HCEC. Staining patterns strongly suggest that at least a subset of PTP1B is localized to the cytoplasm and most likely to the endoplasmic reticulum, the known site of EGFR/PTP1B interaction following EGF stimulation. PTP1B expression, but not EGFR expression, was elevated in HCEC from older donors, suggesting that the reduced proliferative activity of these cells in response to EGF is due, at least in part, to increased PTP1B activity. The fact that inhibition of PTP1B increased the relative number of cells entering S-phase strongly suggests that PTP1B helps negatively regulate EGF-stimulated cell cycle entry in HCEC. These results also suggest that it may be possible to increase the proliferative activity of HCEC, particularly in cells from older donors, by inhibiting the activity of this important protein tyrosine phosphatase.

Age and topographical comparison of telomere lengths in human corneal endothelial cells

PURPOSE

Human corneal endothelium exhibits both age-related and topographical differences in relative proliferative capacity and in senescence characteristics. The purpose of these studies was to compare telomere lengths in human corneal endothelial cells (HCEC) from the central and peripheral areas of corneas from young and older donors to determine whether these changes may be due to replicative senescence or to stress-induced premature senescence.

METHODS

Pairs of corneas from five young (<30 years old) and six older donors (>65 years old) were separated into central and peripheral areas using a 9.5 mm diameter trephine to remove scleral tissue and a 6.0 mm diameter trephine to mark the central-peripheral boundary. One of the pair of corneas was cut into quarters and stained with a peptide nucleic acid (PNA)/fluorescein isothiocyanate (PNA/FITC) probe that specifically binds to telomere repeats. HCEC from the central (0-6.0 mm) and peripheral areas (6.0-9.5 mm) were isolated from the second cornea, mounted on slides by Cytospin, and stained with the PNA/FITC probe. Fluorescence confocal microscopy was used to obtain digital images. The average FITC intensity of nuclei was compared between the central and peripheral areas within and between the two age groups. Ccl185 and 1301 cells were analyzed as controls. Student's unpaired t-test was used to determine the statistical significance of the data.

RESULTS

Average FITC intensity from the central endothelium was 205.8+/-4.2 (younger) and 194.2+/-10.5 (older) and from the peripheral endothelium was 208.1+/-9.3 (younger) and 195.9+/-10.8 (older). Average intensity of single cells isolated from central endothelium was 113.9+/-31.1 (younger) and 107.9+/-26.1 (older) and from the periphery was 109.9+/-12.0 (younger) and 106.9+/-32.4 (older). Average FITC intensity of Ccl185 cells and 1301 cells was 50.5+/-5.0 and 206.9+/-19.4, respectively. Comparison of the results indicates no statistically significant difference between the central and peripheral areas within each group or between the young and older age group.

CONCLUSIONS

Results indicate that the age-related and topographical reduction in relative proliferative capacity and senescence characteristics observed in HCEC are not due to replicative senescence caused by critically short telomeres but implicate stress-induced premature senescence as a cause of these clinically important changes.

Protein tyrosine phosphatase, PTP1B, expression and activity in rat corneal endothelial cells

PURPOSE

The current studies were conducted to determine whether the protein tyrosine phosphatase, PTP1B, plays a role in regulating epidermal growth factor receptor (EGFR) Tyr992 phosphorylation and cell cycle entry in rat corneal endothelial cells.

METHODS

Corneas were obtained from male Sprague-Dawley rats. PTP1B mRNA and protein expression were compared in confluent and subconfluent cells by RT-PCR and western blots. Immunocytochemistry was used to determine the subcellular localization of both PTP1B and EGFR following epidermal growth factor (EGF) stimulation. Western blots were used to analyze the time-dependent effect of EGF on phosphorylation of EGFR Tyr992 plus or minus CinnGEL 2Me, an inhibitor of PTP1B activity. The effect of PTP1B inhibition on cell cycle entry was determined by calculating the percent of Ki67-positive cells following EGF treatment.

RESULTS

PTP1B mRNA expression was similar in confluent and subconfluent cells, but PTP1B protein was expressed at 3 fold higher levels in subconfluent cells. Positive staining for PTP1B was localized in vesicular structures below the plasma membrane. EGFR staining was located at cell-cell borders in untreated endothelium, but was mainly cytoplasmic by 15 min after EGF treatment. In control cultures, phosphorylation of EGFR Tyr992 peaked by 5 min following EGF stimulation and rapidly decreased to basal levels by 30 min. In cultures pretreated with CinnGEL 2Me, Tyr992 phosphorylation peaked 2 min following EGF addition and was consistently sustained at a higher level than controls until 60 min after treatment. By 18 h following EGF treatment, cultures pretreated with CinnGEL 2Me exhibited a 1.7 fold increase in the number of Ki67-positive cells compared with control cultures.

CONCLUSIONS

Comparison of PTP1B mRNA and protein levels indicates that PTP1B expression is regulated mainly at the protein level and is higher in subconfluent cells. PTP1B was located in vesicles below the plasma membrane. The fact that EGFR is internalized in response to EGF stimulation suggests that it could interact with and be regulated by PTP1B. The ability of PTP1B inhibitor to sustain EGFR Tyr992 phosphorylation and increase the number of Ki67-positive cells indicates that PTP1B plays a role in the negative regulation of EGF-induced signaling and helps suppress cell cycle entry.

p27kip1 siRNA induces proliferation in corneal endothelial cells from young but not older donors

PURPOSE

To determine whether small interfering (si)RNA downregulation of the cyclin-dependent kinase inhibitor p27kip1 overcomes G(1)-phase arrest and promotes cell-cycle progression in human corneal endothelial cells (HCECs) from young (<30 years old) and older (>60 years old) donors.

METHODS

Transfection of siRNA was confirmed by incubating confluent cultures of HCECs with FITC-labeled nonsilencing siRNA. Confluent cultures were transfected for 48 hours with p27kip1 siRNA (2.5, 5, 25, or 100 nM) or nonsilencing siRNA, with a lipid transfection reagent. As a comparison, cultures were also transfected for 48 hours with p27kip1 antisense (AS) or missense (MS) oligonucleotides (oligo). At various times after transfection, cells were fixed for immunocytochemical localization of p27kip1 or extracted for Western blot analysis to assess relative p27kip1 protein levels. Cultures were also prepared for ZO-1 immunolocalization, to assess the effect of transfection on the morphology of the monolayer. The number of cells was counted at 0, 48, 96, 144, and 192 hours after incubation, and a cell-viability assay was performed.

RESULTS

A dose-dependent decrease in p27kip1 protein level was observed in Western blot analysis, and nuclear staining for p27kip1 was greatly reduced in HCECs incubated with p27kip1 siRNA. No change in p27kip1 levels or in nuclear staining was observed in the nonsilencing control. p27kip1 siRNA (25 nM) appeared to be quantitatively more efficient than antisense oligonucleotide (500 nM) in reducing p27kip1 protein levels. Viability was less affected by siRNA treatment than by AS oligo transfection. ZO-1 staining showed no effect on morphology of the monolayer. The number of HCECs from young donors (<30 years old) transfected with p27kip1 siRNA increased up to 144 hours after incubation, whereas no change in the number of cells was observed in HCECs transfected with nonsilencing siRNA. In contrast to the results from young donors, no change in the number of cells was observed at any time point tested in HCECs from older donors (>60 years old) after p27kip1 siRNA transfection.

CONCLUSIONS

Transfection of p27kip1 siRNA was sufficient to promote proliferation in confluent cultures of HCECs from younger, but not older donors. These results suggest that inhibition of proliferation in older donors is regulated by other mechanisms in addition to p27kip1.

Age Differences in Cyclin-Dependent Kinase Inhibitor Expression and Rb Hyperphosphorylation in Human Corneal Endothelial Cells

PURPOSE

Human corneal endothelial cells (HCECs) are considered to be nonreplicative in vivo; however, isolated HCECs can be cultured and grown successfully, indicating that they retain proliferative capacity. This capacity to replicate tends to decrease with donor age. Cyclin-dependent kinase inhibitors (CKIs) are important negative regulators of the cell cycle. Of those CKIs, p16INK4a, p21WAF1/Cip1, and p27Kip1 are expressed in corneal endothelium. To help reveal the mechanism of this age-related difference, the relative expression of those CKIs and the kinetics of hyperphosphorylation of the retinoblastoma protein, Rb, were analyzed in HCECs from various aged donors.

METHODS

Fresh-frozen sections of corneas from an 18-year-old and a 74-year-old donor were immunostained to reveal the expression and localization of the three CKIs in corneal endothelium in situ. HCECs from eight donors of various ages were isolated and cultured until they reached passage 4. After the cells reached confluence, total protein was extracted, and the relative expression of p16(INK4a), p21WAF1/Cip1, and p27Kip1 was determined by Western blot analysis. A parallel analysis was performed with primary cultures of HCECs obtained from eight different donors. Subconfluent passage 2 HCECs from eight donors were serum starved and, at different times after growth factor stimulation, protein was extracted, and Western blot analysis was used to compare the overall expression of Rb protein and the kinetics of Rb hyperphosphorylation.

RESULTS

Immunocytochemistry confirmed the expression and nuclear localization of p16(INK4a), p21WAF1/Cip1, and p27Kip1 in HCECs in situ. Western blot studies revealed an age-related increase in p16INK4a and p21WAF1/Cip1 protein expression in cultured HCECs. Expression of p27Kip1 tended to decrease with the donor's age in passage-4 cells; however, there was no significant difference in p27Kip1 expression level between young and older donors in primary cultured HCECs. No age-related difference in total Rb protein was observed in the Western blots; however, the rate of Rb hyperphosphorylation was significantly slower in HCECs from older donors.

CONCLUSIONS

p16(INK4a), p21WAF1/Cip1, p27Kip1, and Rb were all expressed in HCECs, regardless of donor age. Age-related differences in the relative expression of p16INK4a and p21WAF1/Cip1 and in the kinetics of Rb hyperphosphorylation led to the conclusion that, in addition to the normal inhibitory activity of p27Kip1, there is an age-dependent increase in negative regulation of the cell cycle by p16INK4a and p21WAF1/Cip1. This additional molecular mechanism may be responsible, at least in part, for the reduced proliferative response observed in HCECs from older donors.

Replication competence and senescence in central and peripheral human corneal endothelium

PURPOSE

To compare replication competence and senescence in human corneal endothelial cells (HCECs) between the central and peripheral areas and between younger and older donors.

METHODS

Human corneas were obtained from the eye bank and separated into two groups: young (younger than 30 years) and old (older than 50 years). Corneas were cut in quarters and a 2-mm scrape wound was created in the endothelium from the periphery to the center. Unwounded endothelium acted as a negative control. Corneal pieces were incubated for 24, 36, 48, 60, 72, 84, and 96 hours in medium containing 8% fetal bovine serum (FBS) plus additional growth factors. Tissue was fixed, immunostained for minichromosome maintenance (MCM)-2, a marker of replication competence, and mounted in medium containing propidium iodide (PI) to visualize all nuclei. Fluorescence microscope images were used to count PI-stained and MCM2-positive HCECs in three 100-microm2 areas within the central and peripheral wound area. Results are expressed as mean number of cells/100 microm2. Senescent HCECs in ex vivo corneas were identified by staining for senescence-associated beta-galactosidase activity (SA-beta-Gal). Whole corneas were cut in quarters and incubated in staining solution containing SA-beta-Gal at pH 6.0. The number of cells stained for SA-beta-Gal and the grade of SA-beta-Gal intensity in three 100-microm2 areas were averaged for the central and peripheral areas from each donor. For all studies, results were compared between central and peripheral cornea and between younger and older donors.

RESULTS

In both age groups (n = 4/group), cells repopulated the wound area in a time-dependent manner. In corneas from older donors, significantly fewer HCECs migrated into the wound bed in the central cornea than in the periphery. At each time point, the density of cells in the central wound area was lower in corneas from older donors than from younger donors. In both age groups, the mean percentage of MCM2-positive cells increased with time until wound healing. In both age groups, more MCM2-positive cells were present in the wounded area of the peripheral than of the central cornea. At 36, 48, 60, and 72 hours after wounding, the percentage of MCM2-positive cells in the central or peripheral area of older corneas was significantly less than in the corresponding region in younger corneas. No MCM2-positive staining was observed in unwounded areas at any time point. HCECs in corneas from younger donors (n = 4) showed little to no SA-beta-Gal activity in either the central or peripheral area. SA-beta-Gal activity was easily detectable in corneas from older donors (n = 4) and a significantly higher percentage of central HCECs showed strong SA-beta-Gal activity compared with HCECs in the periphery.

CONCLUSIONS

In ex vivo corneas, HCECs from the peripheral area retain higher replication competence, regardless of donor age. HCECs in the central area of corneas from older donors retain replicative competence, but the relative percentage of cells that are competent to replicate is significantly lower than in the periphery or in the central area of corneas from younger donors. This reduction in replicative competence negatively correlates with the observed increase in the population of central HCECs exhibiting senescence-like characteristics.

Comparison of the proliferative capacity of human corneal endothelial cells from the central and peripheral areas

PURPOSE

To compare the relative proliferative capacity between human corneal endothelial cells (HCECs) cultured from the central and peripheral areas of the cornea.

METHODS

Human corneas were divided into two groups based on donor age (younger group, < or =30 years of age; older group, > or =50 years of age). Corneas were trephined, and Descemet's membrane with HCECs was stripped from the central (0-6.75 mm) and peripheral (6.75-9.5 mm) areas. HCECs were then isolated from Descemet's membrane and cultivated. An equal number of passage-1 endothelial cells from each area were seeded, and the number of cells was determined at various times after seeding. Doubling times of cells from each area were compared. The antibody against minichromosome maintenance-2 (MCM2) protein was tested for replication competence.

RESULTS

Morphologically, HCECs from the central area were similar to cells from the peripheral area. The doubling time (in hours) of HCECs from the central area was 35.20 in the younger group (n = 4) and 54.54 in the older group (n = 4) and from the peripheral area, 29.37 in the younger group and 46.23 in the older group. There was no significant difference (younger: P = 0.515; older: P = 0.222) between the central and peripheral area in each age group. MCM2-positive cells were consistently observed in cultures from the central, as well as peripheral, area. There was no significant difference (younger: P = 0.929; older: P = 0.613) in the percentage of MCM2-positive cells between these two areas in either age group. Even though there was no significant difference, there was a tendency toward increased doubling time and decreased percentage of MCM2 in the central area of the older group.

CONCLUSIONS

These results indicate that corneal endothelial cells from both the central and peripheral areas retain potential proliferative capacity.

Induction of replication in human corneal endothelial cells by E2F2 transcription factor cDNA transfer

PURPOSE

Corneal endothelial cells in humans do not replicate to any meaningful extent. Diminishing density of the cell monolayer with age and in the disease states is a major cause of loss of corneal transparency. This study was conducted to test the hypothesis that overexpression of the transcription factor E2F2 results in replication in nonproliferating human corneal endothelial cells.

METHODS

Whole human corneas were incubated for 2 hours in a solution of recombinant E1(-)/E3(-) adenovirus incorporating cDNA encoding E2F2 and green fluorescent protein (GFP) under control of a bidirectional promoter and subsequently maintained in ex vivo culture. Control specimens were incubated with an identical virus bearing the GFP sequence only, or virus-free medium. Efficiency of gene transfer and localization was examined by fluorescence microscopy. En face confocal microscopy of the corneal endothelial surface was used to image recombinant E2F2 expression. 5-bromodeoxyuridine (BrdU) incorporation was used to examine progression to the S phase. Changes in density of the corneal endothelium were quantified by specular microscopy and counting of trypan-blue-stained cells. Apoptosis was tested with a TUNEL assay.

RESULTS

Recombinant proteins were expressed predominantly in the endothelium and in a high proportion of endothelial cells in the first week after exposure to virus, diminishing thereafter. Compared with the control, transduction with E2F2 resulted in progression from the G(1) to the S phase in a significant number of cells and in increased cell density. Apoptosis was not found to any significant extent.

CONCLUSIONS

Overexpression of the transcription factor E2F2 in nonmitotic human corneal endothelial cells results in short-term expression, cell-cycle progression, and increased monolayer cell density.

Effects of SOV-induced phosphatase inhibition and expression of protein tyrosine phosphatases in rat corneal endothelial cells

Contact inhibition is an important mechanism for maintaining corneal endothelium in a non-replicative state. Protein tyrosine phosphatases (PTPs) play a role in regulating the integrity of cell-cell contacts, differentiation, and growth. In this study, we aimed to evaluate whether phosphatases are involved in the maintenance of contact-dependent inhibition of proliferation in corneal endothelial cells and to identify candidate PTPs that are expressed in these cells and might be involved in regulation of contact inhibition. Confluent cultures of rat corneal endothelial cells or endothelium in ex vivo corneas were treated with the general phosphatase inhibitor, sodium orthovanadate (SOV). Immunocytochemistry (ICC) evaluated the effect of SOV on cell-cell contacts by staining for ZO-1, and on cell cycle progression by staining for Ki67. Transverse sections of rat cornea and cultured rat corneal endothelial cells were used to test for expression of the candidate PTPs: PTP-mu, PTP-LAR, PTP1B, SHP-1, SHP-2, and PTEN using ICC and either Western blots or RT-PCR. ZO-1 staining demonstrated that SOV induced a time-dependent release of cell-cell contacts in confluent cultures of corneal endothelial cells and in the endothelium of ex vivo corneas. Staining for Ki67 indicated that SOV promoted limited cell cycle progression in the absence of serum. PTP-mu, PTP1B, SHP-1, SHP-2, and PTEN, but not PTP-LAR, were expressed in rat corneal endothelial cells in situ and in culture. The subcellular location of PTP-mu and PTP1B differed in subconfluent and confluent cells, while that of SHP-1, SHP-2, and PTEN was similar, regardless of confluent status. Western blots confirmed the expression of PTP1B, SHP-1, SHP-2, and PTEN. RT-PCR confirmed expression of PTP-mu mRNA. Phosphatases are involved in regulation of junctional integrity and of cell proliferation in corneal endothelial cells. PTP-mu, PTP1B, SHP-1, SHP-2, and PTEN are expressed in rat corneal endothelium and may be involved in regulation of contact inhibition in these normally non-proliferating cells.

Cell cycle status in human corneal endothelium

Corneal endothelium is the single-cell layer that forms a physical barrier between the corneal stroma and aqueous humour. The barrier and ionic 'pump' functions of corneal endothelium help regulate stromal hydration. Loss of endothelial cells due to increasing age, trauma, disease, dystrophy, or previous corneal transplants can reduce the density of endothelial cells to a critical point below which the stroma becomes edematous and visual acuity is lost. Throughout life, division of endothelial cells either does not occur or occurs at a rate too slow to adequately replace dead cells. Thus, the major means of repairing the monolayer is by cell migration and/or enlargement. The basis for the lack of endothelial cell proliferation is not yet fully understood, although it is clear that cells do retain proliferative capacity. Previous studies from this laboratory have identified certain environmental conditions that may be responsible for maintaining these cells in a non-replicative state in vivo. In addition, corneal endothelial cells exhibit intrinsic, age-related differences in relative proliferative capacity. The studies described below provide evidence to support the hypothesis that, with age, an increasing number of HCEC enter a replicative senescence-like state in which they become increasingly refractive to mitogenic stimulation. This decreasing sensitivity to mitogens appears to be mediated, at least in part, by age-dependent alterations in the relative expression and activity of the cyclin-dependent kinase inhibitors, p27KIP1, p16INK4A, and p21CIP1.

Human corneal endothelial cell proliferation: potential for use in regenerative medicine

PURPOSE

To review and update the experience of our laboratory in culturing human corneal endothelial cells (HCEC) from young and older donors.

METHODS

Corneas were obtained from National Disease Research Interchange, Philadelphia, PA. Data from the past 3 years were reviewed to develop criteria for selecting donor corneas to be used for endothelial cell culture. Immunocytochemical localization using mAb 9.3.E identified endothelial cells, and Ki67 staining demonstrated actively cycling cells. Cell counts demonstrated the effect of growth-promoting agents on proliferation of cells from young (<30 years old) and older (>50 years old) donors. Phase-contrast microscopy documented morphologic characteristics of cells in primary culture and the effect of growth factors on cell morphology.

RESULTS

Exclusion criteria were developed to increase the chance of successful culture of HCEC. Isolation methods to remove Descemet membrane with attached endothelial cells avoided contamination with other corneal cell types. EDTA treatment combined with mechanical disruption facilitated isolation of cells. Culture medium containing FBS, EGF, NGF, and bovine pituitary extract stimulated maximal growth and facilitated normal monolayer formation. Age-related differences were detected in the density of confluent cells in primary culture and in the proliferative response to growth-promoting agents.

CONCLUSIONS

Untransformed HCEC can be successfully cultured from the corneas of both young and older donors by using care in the selection of donor material. Care must also be taken in the early phases of endothelial cell isolation to obtain maximal numbers of healthy cells for culture. There appear to be true age-related differences in overall proliferative capacity; however, the relative response to specific growth factors was similar in cells from young and older donors. Results of these studies provide guidelines for successful growth of untransformed HCEC for use in regenerative medicine.

Expression of vascular endothelial growth factor receptor-3 (VEGFR-3) on monocytic bone marrow-derived cells in the conjunctiva

Vascular endothelial growth factor-3 (VEGFR-3), also known as Fms-like tyrosine kinase receptor 4 (FLT-4), was thought to be expressed exclusively on the lymphatic endothelium, high endothelial venules, and rarely on vascular endothelium. It plays a critical role in the development of lymphatics and cancer metastasis. Very recently, however, VEGFR-3 expression has been identified on dendritic cells (DCs) in the inflamed cornea, and related to the trafficking of these cells to lymphoid organs. The current study was performed to evaluate the expression of VEGFR-3 in the conjunctiva. The conjunctiva and limbus of normal and inflamed murine eyes were excised and stained for VEGFR-3. Immunofluorescence double staining for CD11b, CD11c, CD31, CD45, GR-1, CD3, CD80, LYVE-1 and class II major histocompatibility complex (MHC) antigen expression, using confocal microscopy, was performed to further phenotype the VEGFR-3+ cells. VEGFR-3 and LYVE-1 expression was observed on lymphatic, but not blood vessel, endothelium. In addition, we also detected expression of VEGFR-3 on non-endothelial CD45+ bone marrow-derived cells in the conjunctiva of normal and, in an increased number, in inflamed eyes. These cells were uniformly CD11b+, CD3-, and Gr-1-, suggesting a monocytic origin, similar to the VEGFR-3+ cells in the cornea. Nearly half of the VEGFR-3+ cells were also positive for MHC class II expression, and none were positive for CD80 (B7-1), indicating their relative immature status. In contrast to the recently described VEGFR3+ corneal cells, however, VEGFR-3+ conjunctival cells did not express the DC marker CD11c. We conclude that in addition to its known role in lymphangiogenesis, VEGFR-3 is also expressed by a conjunctival monocyte/macrophage lineage, implicating a potential relationship between lymphangiogenesis and leukocyte trafficking in the ocular surface.

p27kip1 Antisense-induced proliferative activity of rat corneal endothelial cells

PURPOSE

To determine whether antisense downregulation of p27(kip1) will overcome G(1)-phase arrest and promote cell cycle progression in rat corneal endothelial cells (CECs).

METHODS

Confluent cultures of rat CECs were incubated for 24 hours in the presence of p27(kip1) antisense (AS) oligonucleotides (oligoS) using nonliposomal lipid transfection. Control cultures were incubated under one of the following conditions: no oligos or lipid-containing buffer, lipid-containing buffer alone, or lipid-containing buffer plus missense (MS) p27(kip1) oligo. Viability was tested by a cell-viability assay after 0, 24, 48, and 72 hours. After postincubation for 0, 24, 48, or 72 hours, cultures were fixed and immunostained for p27(kip1), to test for downregulation, or for Ki67 or BrdU, to detect actively cycling cells. Western blot and immunocytochemistry (ICC) studies were conducted to determine the effect of p27(kip1) antisense treatment on the relative protein level and subcellular localization of several cell cycle proteins, including cyclin-D1, -E, -A, and -B1; CDK2 and -4; p21(cip1); and p15(INK4b). Proliferation was determined by direct counting of propidium iodide (PI) or 4',6'-diamino-2-phenylindole (DAPI)-stained cells.

RESULTS

Viability was not significantly affected by lipid-based oligo transfection for up to 48 hours, after which a decline was noted. The protein level of p27(kip1) was reduced after AS transfection in a time-dependent manner. Nuclear staining for p27(kip1) was greatly reduced in CECs incubated with AS oligo. No change in p27(kip1) levels was observed in controls at any time point tested. p27(kip1) AS oligo transfection increased cyclin-D1, -E, -A, and -B1 protein levels, and all cyclins were localized to the nucleus. No changes in protein level were observed for CDK2, CDK4, p21(cip1), or p15(INK4B). A time-dependent increase in the relative number of Ki67- and BrdU-positive cells was noted in CECs incubated with AS oligo. In contrast, no to few Ki67- or BrdU-positive cells were observed in CECs incubated with MS oligo or the buffer-treated control cells. The percentage increase in the number of cells transfected with AS oligo increased with time, compared with that of cells transfected with MS oligo.

CONCLUSIONS

Treatment with p27(kip1) antisense oligonucleotides followed by postincubation in 10% FBS lowers endogenous p27(kip1) protein levels and promotes proliferation in confluent cultures of rat CECs.

Proliferative response of corneal endothelial cells from young and older donors

PURPOSE

To compare the effect of epidermal growth factor (EGF), nerve growth factor (NGF), platelet-derived growth factor-BB (PDGF-BB), bovine pituitary extract, and fetal bovine serum (FBS), alone or in combination, on proliferation of human corneal endothelial cells (HCEC) cultured from young (<30 years old) and older donors (>50 years old).

METHODS

Corneas from donors 2 to 79 years old were obtained from the National Disease Research Interchange. Descemet's membrane with intact endothelium was dissected. Cells were isolated by EDTA treatment and cultured to confluence. The HCEC marker, antibody 9.3.E, tested for pure endothelial populations. Antibody Ki67 and ZO-1 tested either before or after cultured cells reached confluence to indicate cell proliferation and cell-cell contact formation. Cell morphology was documented by inverted phase-contrast microscopy. Passages I through VII were used to test the effect of various factors on cell proliferation. For each study, equal numbers of cells were seeded, maintained overnight in 4% FBS to permit cell attachment, washed, and incubated for up to 3 weeks in one of the following: modified Eagle's Minimum Essential Medium (Opti-MEM-I) alone; Opti-MEM-I plus EGF, NGF, PDGF-BB, bovine pituitary extract, or FBS; or a combination of factors. At various times after seeding, cell numbers were determined by electronic cell counter. For each condition, three separate wells were tested and each sample was counted three times. Studies were repeated at least twice using cells from different donors and age groups. Within each study, a one-way ANOVA test was performed to analyze statistical significance.

RESULTS

Cells stained positively with antibody 9.3.E, indicating isolation of HCEC and lack of contamination with epithelial cells or keratocytes. Positive staining of Ki67, indicating cycling cells, was found in subconfluent cultures. Plasma membrane-associated ZO-1 staining and lack of Ki67 staining indicated that cultured cells formed a contact-inhibited monolayer. Cultured cells decreased in density, increased in size, and became more heterogeneous depending on donor age and on the number of passages. Incubation in OptiMEM-I promoted attachment and induced a moderate proliferative response above that of MEM (P < 0.001). In general, proliferative responses to growth stimuli were relatively slow, with cell counts generally plateauing 10 to 14 days after exposure to growth-promoting agents. EGF yielded a broad, dose-dependent effect and, at 5-50 ng/mL, peak cell counts were significantly higher (P < 0.001) than basal levels. EGF consistently stimulated proliferation in cells from younger donors, but was less effective in stimulating growth of cells from older donors. NGF did not show a consistent significant stimulatory effect at any concentration tested. PDGF-BB (25 ng/mL) tended to stimulate growth to a greater extent than EGF (P < 0.05) in cultures from the same donor. Pituitary extract significantly increased counts at 1.0 (P < 0.05) to 100 ug/mL (P < 0.001). PDGF-BB plus pituitary extract demonstrated greater stimulation than pituitary extract (P < 0.01) or PDGF-BB alone (P < 0.01). FBS (1%-8%) increased cell numbers in a dose-dependent manner, and, at 4%-8%, yielded counts significantly higher (P < 0.001) than that of any single growth-promoting agent tested.

CONCLUSIONS

HCEC from both young and older donors can proliferate in vitro in response to growth-promoting agents. Proliferation in the presence of multiple mitogens ceased when confluence was reached, indicating the formation of a contact-inhibited monolayer. In general, cells cultured from young donors were more responsive to the agents tested, but the relative response of HCEC to these agents was similar, regardless of donor age. The relative difference in the extent of the response of the same cell population to different mitogens suggests that these mitogens induce different downstream signals. The relatively robust proliferative response of HCEC to FBS may involve stimulation of multiple downstream signaling pathways may involve stimulation of multiple downstream signaling pathways and/or induce more sustained downstream signaling than the other growth-promoting agents tested.

Effect of overexpressing the transcription factor E2F2 on cell cycle progression in rabbit corneal endothelial cells

PURPOSE

To test the hypothesis that overexpression of the transcription factor E2F2 promotes cell cycle progression in nonproliferating corneal endothelial cells.

METHODS

Ex vivo rabbit corneas were transfected with a lipid transfection reagent and either a control plasmid containing full-length cDNA for enhanced green fluorescent protein (pIRES2-EGFP) or a plasmid containing full-length cDNA for both E2F2 and EGFP (pIRES2-E2F2/EGFP). Transfection control experiments consisted of corneas incubated in buffer without transfection reagent or plasmid or incubated in reagent without plasmid. After transfection, corneas were incubated for various periods in 0.1% FBS (a concentration that maintains cell health, but does not promote proliferation). Immunocytochemical (ICC) localization tested for overexpression of E2F2 in transfected corneal endothelial cells and permitted calculation of transfection efficiency. Endothelial cell viability was tested in transfected ex vivo corneas and confluent cultures by using a cell-viability assay. Apoptosis was detected in confluent cultures by TUNEL assay. RT-PCR tested for mRNA expression of Ki67 (a marker of actively cycling cells) and cyclin B1 (a marker for the G2-phase of the cell cycle). Semiquantitative densitometric analysis compared the relative amounts of PCR reaction products.

RESULTS

ICC demonstrated the colocalization of E2F2 and EGFP in corneal endothelium with a transfection efficiency of 10% to 12%, using the pIRES2-based plasmid and transfection reagent. The cell-viability assay revealed very few dead cells in ex vivo corneal endothelium that overexpressed E2F2. Cell viability and TUNEL assays of confluent cultures revealed that approximately 27% of cells died in all cultures incubated with transfection reagent, but death appeared not to be due to apoptosis. No additional cell death was noted by either assay in cells that overexpressed E2F2. RT-PCR of endothelial samples obtained 48 hours after transfection showed the presence of higher levels of reaction product for Ki67 (a 5.1-fold increase) and cyclin B1 (a 2.3-fold increase) in cells that overexpressed E2F2 than in control samples.

CONCLUSIONS

Overexpression of the transcription factor E2F2 in nonproliferating rabbit corneal endothelial cells induces cell cycle progression without inducing significant apoptosis.

Proliferative capacity of the corneal endothelium

Corneal endothelium is the single layer of cells forming a boundary between the corneal stroma and anterior chamber. The barrier and "pump" functions of the endothelium are responsible for maintaining corneal transparency by regulating stromal hydration. Morphological studies have demonstrated an age-related decrease in endothelial cell density and indicate that the endothelium in vivo either does not proliferate at all or proliferates at a rate that does not keep pace with the rate of cell loss. Lack of a robust proliferative response to cell loss makes the endothelium, at best, a fragile tissue. As a result of excessive cell loss due to accidental or surgical trauma, dystrophy, or disease, the endothelium may no longer effectively act as a barrier to fluid flow from the aqueous humor to the stroma. This loss of function can cause corneal edema, decreased corneal clarity, and loss of visual acuity, thus requiring corneal transplantation to restore normal vision. Studies from this and other laboratories indicate that corneal endothelium in vivo DOES possess proliferative capacity, but is arrested in G1-phase of the cell cycle. It appears that several intrinsic and extrinsic factors together contribute to maintain the endothelium in a non-replicative state. Ex vivo studies comparing cell cycle kinetics in wounded endothelium of young (< 30 years old) and older donors ( > 50 years old) provide evidence that cells from older donors can enter and complete the cell cycle; however, the length of G1-phase appears to be longer and the cells require stronger mitogenic stimulation than cells from younger donors. In vivo conditions per se also contribute to maintenance of a non-replicative monolayer. Endothelial cells are apparently unable to respond to autocrine or paracrine stimulation even though they express mRNA and protein for a number of growth factors and their receptors. Exogenous transforming growth factor-beta (TGF-beta) and TGF-beta in aqueous humor suppress S-phase entry in cultured endothelial cells, suggesting that this cytokine could inhibit proliferation in vivo. In addition, cell-cell contact appears to inhibit endothelial cell proliferation during corneal development and to help maintain the mature endothelial monolayer in a non-proliferative state, in part, via the activity of p27kip1, a known G1-phase inhibitor. The fact that human corneal endothelium retains proliferative capacity has led to recent efforts to induce division and increase the density of these important cells. For example, recent studies have demonstrated that adult human corneal endothelial cells can be induced to grow in culture and then transplanted to recipient corneas ex vivo. The laboratory work that has been conducted up to now opens an exciting new door to the future. The time is right to apply the knowledge that has been gained regarding corneal endothelial cell proliferative capacity and regulation of its cell cycle to develop new therapies to treat patients at risk for vision loss due to low endothelial cells counts.

Mechanisms of mitotic inhibition in corneal endothelium: contact inhibition and TGF-beta2

PURPOSE

Contact inhibition has been implicated as an important antiproliferative mechanism in developing and mature corneal endothelium. Although exogenous TGF-beta2 and TGF-beta2 in aqueous humor suppress S-phase entry in cultured rat corneal endothelial cells, it is not known whether TGF-beta2 contributes to the mitotic inhibition that occurs during in vivo endothelial development. TGF-beta receptors I, II, and III must be coexpressed for a TGF-beta2-induced intracellular signal to be transmitted. The current study was conducted to determine whether TGF-beta2 contributes to mitotic inhibition during endothelial development, by investigating when these receptors become coexpressed in the endothelium of neonatal rats. Cyclin-dependent kinase inhibitors (CKIs), such as p27kip1 and p15INK4b, help mediate mitotic inhibition in other cell types. The role of CKIs in inhibiting proliferation in corneal endothelium was examined by first determining the kinetics of p27kip1 expression in neonatal rat corneal endothelium. Studies were then extended to cultured cells to more directly compare the effects of TGF-beta2 and cell-cell contact on the relative protein and mRNA expression of the CKIs, p27kip1, and p15INK4b.

METHODS

Immunocytochemistry (ICC) detected TGF-beta receptors I, II, and III (RI, RII, RIII, respectively) in the endothelium of rat corneas on postnatal days 1, 10, and 21, and in adult (3-month-old) rats. ICC for p27kip1 was conducted on postnatal days 1, 7, 14, and 21. Samples were taken for p27kip1 RT-PCR on postnatal days 7, 14, and 21 and from adult rats. The effect of TGF-beta2 on p27kip1 and p15INK4b expression was determined in G(0)-phase synchronized subconfluent rat corneal endothelial cells incubated for 24 hours in 10% serum +/- 5 ng/mL TGF-beta2. CKI expression was also examined in fully confluent cultures. RT-PCR and Western blot analysis detected p27kip1 and p15INK4b mRNA and protein expression, respectively. The effect of releasing cells from cell-cell contact on proliferation and p27kip1 protein expression was studied in confluent cultures treated for 1 hour with and without 2.0 mg/mL di-sodium EDTA and then maintained for 24 hours in 10% serum. Cultures were then either fixed for ICC of Ki67, a marker of actively cycling cells, or extracted for Western blot determination of p27kip1 protein.

RESULTS

Positive staining for TGF-beta RIII was detected on postnatal day 10, and staining for RI and RII was detected on postnatal day 21. The endothelium stained positively for p27kip1 on postnatal day 1 and thereafter, and p27kip1 PCR product was detectable at the earliest time point tested (postnatal day 7). In cultured cells, TGF-beta2 and cell-cell contact had relatively little effect on expression of p27kip1 or p15INK4b mRNA. TGF-beta2 lowered the levels of both proteins, but p27kip1 remained at a higher level than p15INK4b. In confluent cultures, p15INK4b protein was reduced; however, p27kip1 protein levels increased 20-fold. Positive staining for Ki67 was detected, and p27kip1 protein levels substantially decreased in EDTA-treated confluent cells compared with the untreated control.

CONCLUSIONS

Previous studies from this laboratory showed that corneal endothelial cell proliferation ceases in neonatal rat by postnatal day 13. This timing correlated with the formation of stable cell-cell contacts, implicating contact inhibition as an important mechanism of growth arrest during endothelial development. The current studies showed that coexpression of TGF-beta RI, RII, and RIII occurred too late for TGF-beta2 to have a significant role in inhibiting proliferation during endothelial development. Studies in cultured cells suggest that p27kip1 mediates inhibition of proliferation induced by TGF-beta2, although the response to this cytokine was relatively weak. ICC and RT-PCR of p27kip1 in neonatal endothelium and RT-PCR and Western blot studies in cultured cells indicate that contact inhibition is mediated, in large part, through the activity of p27kip1. These results, together with previous data from this laboratory, strongly suggest that contact inhibition is an important mechanism responsible for inducing cell cycle arrest during corneal endothelial development and for maintaining the mature monolayer in a nonproliferative state. In both cases, contact-induced inhibition is mediated, at least in part, by p27kip1. TGF-beta2 appears not to induce mitotic arrest in the developing endothelium, but may function to maintain the mature endothelium in a nonreplicative state should cell-cell contact be lost in the monolayer.

Transplantation of adult human corneal endothelium ex vivo: a morphologic study

PURPOSE

To investigate the feasibility of transplanting untransformed human corneal endothelial cells as a treatment strategy and possible alternative for penetrating keratoplasty by growing donor cells in culture and then transplanting them to denuded Descemet's membrane of recipient corneas.

METHODS

Corneas from adult donors (50-80 years old) were obtained from eye banks. To grow corneal endothelial cells, Descemet's membrane with associated cells was dissected from the stroma. Endothelial cells were released by ethylenediaminetetraacetic acid treatment, grown in medium containing multiple growth factors, and identified as being of endothelial origin by morphology and by reverse-transcription polymerase chain reaction for keratin 12 and collagen type VIII. In transplantation experiments, cultured cells were seeded onto denuded Descemet's membrane of a second donor cornea at 5 x 10(5) cells/mL. The recipient cornea was incubated in organ culture for as long as 2 weeks. The morphology and ultrastructure of the endothelium were evaluated 7 and 14 days after transplantation by transmission electron microscopy, and by immunolocalization of zonula occludins-1 (ZO-1). Endothelial cell density was calculated in transplants by counting ZO-1-stained cells.

RESULTS

Corneal endothelial cells cultured from adult donors consistently grew well in culture medium. Cells were identified as corneal endothelium by characteristic morphology and messenger RNA expression. Morphologic and ultrastructural studies of corneas containing transplanted endothelial cells demonstrated that with time there was an increase in endothelial cell-Descemet's membrane adhesion, in the extent of cell-cell contacts and lateral interdigitation, and in formation of a single cell layer. ZO-1 staining revealed tight junction formation similar to that of corneas in vivo. Mean endothelial cell density in transplanted corneas was 1,895 cells/mm(2) (range, 1,503-2,159 cells/mm(2) ).

CONCLUSION

Untransformed adult human corneal endothelial cells can be efficiently and consistently cultured and transplanted onto denuded Descemet's membrane. Transplanted cells in organ culture exhibit morphologic characteristics and cell densities similar to corneal endothelial cells in vivo. These results provide evidence for the feasibility of developing methods for in vivo transplantation of untransformed corneal endothelial cells cultured from adult donor tissue.

TGF-beta receptor types I and II are differentially expressed during corneal epithelial wound repair

PURPOSE

It has been demonstrated that cells migrating to cover an epithelial débridement wound exit the cell cycle and that the cell-cycle inhibitor p15(INK4b) is upregulated in these cells. TGF-beta signaling has been implicated in both of these processes, and this study was conducted to determine whether the expression and localization of TGF-beta receptor (TbetaR)-I and -II are altered during corneal epithelial wound repair.

METHODS

Three-millimeter superficial keratectomy wounds and 3-mm débridement wounds were made in central rat cornea and allowed to heal in vivo for 1 to 48 hours. Immunofluorescence microscopy and Western blot analysis were used to determine the localization and expression of TbetaR-I and -II. Unwounded rat corneas served as control samples. To determine the effect of epidermal growth factor (EGF) and TGF-beta1 on p15(INK4b) and TbetaR-I and -II expression, human corneal epithelial cells were grown in culture to 50% to 60% confluence, and EGF (5 ng/ml) and/or TGF-beta1 (2 ng/ml) were added for 6 hours. Cells were harvested and p15(INK4b) and TBR-I and -II levels were assayed by using Western blot analysis.

RESULTS

In unwounded corneas, TbetaR-I and TbetaR-II were present at low levels across the cornea, with higher levels in limbal epithelium. Both TbetaR-I and -II were upregulated after wounding. However, levels of TbetaR-II appeared to increase in the epithelial cells that had migrated to cover the wound area, whereas TbetaR-I was upregulated in the entire corneal epithelium. Western blot analysis indicated that both TbetaR-I and -II were upregulated threefold after wounding. In cultured cells, EGF and TGF-beta1 stimulated TbetaR-II; however, neither one stimulated TbetaR-I expression. TGF-beta1 stimulated p15(INK4b) protein levels threefold.

CONCLUSIONS

After wounding, TbetaR-I and TbetaR-II were both expressed at high levels in cells migrating to cover a corneal wound, suggesting that TGF-beta signaling is involved in blocking migrating cells from progressing through the cell cycle. This blockage, at least in part, involves the inhibitor p15(INK4b). In addition, although both TbetaR-I and TbetaR-II are upregulated during wound repair, they appear to be differentially regulated.

Protein kinase C regulation of corneal endothelial cell proliferation and cell cycle

PURPOSE

The purpose of this study was to determine the role of protein kinase C (PKC) in corneal endothelial cell proliferation.

METHODS

Immunocytochemistry and Western blotting were used to define the PKC isoforms expressed in primary cultures of rat corneal endothelial cells. For proliferation studies, primary cultures of rat corneal endothelial cells were serum-starved for 48 hours and incubated for 2 hours with the PKC inhibitors staurosporine (10(-9) to 10(-7) M), chelerythrine (10(-9) to 5 x 10(-8) M), or calphostin C (10(-9) to 10(-7) M). Individual PKC isoforms were inhibited using PKCalpha antisense oligonucleotide transfection or exposure for 1 hour to myristoylated, pseudosubstrate-derived peptide inhibitors against PKCalpha, -alphassgamma, -epsilon, and -delta (10(-8) to 10(-6) M). Cells were then stimulated with 2.5% serum for 24 hours. Cell proliferation was measured with bromodeoxyuridine (BrDU) and Ki67 immunocytochemistry. Protein level of cyclin E was determined by Western blotting.

RESULTS

PKCalpha, -ssII, -delta, -epsilon, -iota, -eta, -gamma, and -theta were detected in corneal endothelial cells. Maximum inhibition of PKC with staurosporine, calphostin C, and chelerythrine reduced cell proliferation to 7%, 31%, and 48% of control, respectively. Myristoylated peptide inhibition of PKCalpha and -epsilon reduced cell proliferation to 57% and 59% of control, respectively. PKCalpha antisense oligonucleotide reduced cell proliferation to 35% of control. Cyclin E protein level was decreased to 70%, 38%, 57%, and 43% of control in cells treated with calphostin C, staurosporine, chelerythrine, and PKCalpha antisense, respectively.

CONCLUSIONS

PKC activity, in particular PKCalpha and -epsilon activity, is important in promoting corneal endothelial cell proliferation. Inhibition of PKC activity prohibits G1/S-phase progression and reduces cyclin E protein levels.

Immune privilege and immunogenicity reside among different layers of the mouse cornea

PURPOSE

To determine the extent to which each layer of the mouse cornea displays alloimmunogenicity or immune privilege.

METHODS

Intact corneas or individual or combined layers of corneas from normal or cauterized eyes of BALB/c, C57BL/6, and CD95L-deficient B6-gld mice were grafted beneath the kidney capsule of normal BALB/c, B10.D2, BALB.B mice or of BALB/c mice presensitized to donor antigens. Graft fate was assessed clinically and histologically and acquisition of donor-specific delayed hypersensitivity (DH) was assessed at selected intervals after grafting.

RESULTS

Full-thickness allogeneic corneas induced vigorous DH and were rejected acutely. Similar results were obtained with allografts of corneal epithelium alone (if supported by syngeneic viable stroma), allografts of epithelium from cauterized corneas (containing Langerhans' cells), and stromal allografts deprived of endothelium. Grafts comprised of stroma plus endothelium (without epithelium) were not rejected, nor did they induce DH unless the graft had no CD95L expression. If stroma-endothelium grafts had no CD95L expression, DH directed against major histocompatibility complex (MHC), but not minor histocompatibility, alloantigens was induced. Moreover, CD95L expressed on stroma-endothelium grafts protected endothelial cells, but not stromal cells, from rejection in presensitized recipients.

CONCLUSIONS

When grafted to a heterotopic site, the alloimmunogenicity of the normal cornea resides within its epithelial and stromal layers, whereas immune privilege arises from the endothelium. In normal mice, CD95L-expressing endothelium can inhibit the stroma from inducing immunity directed at MHC alloantigens, but in presensitized mice the endothelium can protect itself only from immune rejection.

EDTA: a promoter of proliferation in human corneal endothelium

PURPOSE

To determine whether it is possible to induce proliferation in the endothelium of older donor corneas by treatment of the intact monolayer with EDTA.

METHODS

Corneas from donors 52 to 75 years of age were obtained from an eye bank and were usually cut in quarters to increase sample size. The effect of EDTA dose (0.02-2.0 mg/ml) and incubation time (6, 30, and 60 minutes) on endothelial cell-cell contacts was evaluated by staining for ZO-1, a cell junction marker. Cell death was tested by a commercial live-dead assay. Corneal pieces were incubated for 0, 24, 48, or 60 hours in culture medium (M-199, 10% fetal bovine serum, 10 ng/ml epidermal growth factor, 20 ng/ml fibroblast growth factor) before EDTA treatment. After treatment, pieces were incubated in the same medium for 24, 48, 72, or 96 hours to permit cell cycle entry. Tissue was fixed, stained for Ki67 (a marker for late G1-phase through the M-phase), and mounted in medium containing propidium iodide to visualize all nuclei. Confocal images were evaluated by computer (Image software; NIH, Bethesda, MD) to count Ki67-positive and propidium iodide-stained cells.

RESULTS

EDTA released corneal endothelial cell-cell contacts in a dose- and time-dependent manner. At doses and incubation times tested, EDTA did not induce significant cell death. Preincubation in culture medium for 24 hours was needed for endothelial cells to efficiently initiate proliferation in response to EDTA. The endothelium of corneas incubated in mitogen-containing medium for up to 108 hours without EDTA treatment did not stain for Ki67. EDTA at 2.0 mg/ml for 60 minutes appeared optimal and stimulated 16% to 18% of the cells to proliferate. Ki67-positive mitotic figures were visible 48 hours after exposure to EDTA. Formation of daughter cells was visible after double-staining for Ki67 and ZO-1.

CONCLUSIONS

EDTA released cells from contact inhibition and promoted proliferation in corneal endothelium from older donors. The authors hypothesize that corneal endothelium from older individuals divide in situ when exposed to positive growth factors under conditions in which cells have been transiently released from contact inhibition.

Expression patterns of retinoblastoma and E2F family proteins during corneal development

PURPOSE

To determine the expression patterns of the retinoblastoma protein and the E2F transcription factor families in limbal and corneal epithelia and in corneal keratocytes in situ during corneal development and differentiation.

METHODS

Retinoblastoma protein (pRb) and its family members p107 and p130; E2F-1, -2, and -4, members of the E2F family of transcription factors; and Ki67, a marker of actively cycling cells, were localized by indirect immunofluorescence microscopy, in corneas of neonatal, juvenile, and adult rats. Presence of mRNA for pRb, p107, p130, and E2F types 1 to 5 in adult corneal epithelium was determined by reverse transcription-polymerase chain reaction.

RESULTS

mRNA for all members of pRb and E2F families was present in adult corneal epithelium. The greatest number of Ki67-positive corneal and limbal epithelial cells were present at days 13 to 19, and Ki67-positive stromal keratocytes at day 2. pRb and E2F-2 were localized to all cells in neonatal, juvenile, and adult corneas. With age, p130 localization became more intense and nuclear in stromal keratocytes and suprabasal cells of corneal and limbal epithelia; p107, initially nuclear in limbal and corneal epithelia, became increasingly cytoplasmic in corneal epithelium. E2F-1 was initially nuclear in keratocytes and diminished after day 10. E2F-1 was localized in the basal cell layer of limbal and corneal epithelia after day 10. E2F4 was always nuclear in limbal epithelium and cytoplasmic in corneal epithelium.

CONCLUSIONS

Expression patterns of pRb and E2F family proteins vary with corneal cell differentiation, but are most apparent with p130 and p107. Nuclear localization of p130 appears to correlate with terminal differentiation in epithelium and entrance into a quiescent state by keratocytes. In contrast, p107 is nuclear in the undifferentiated limbal basal cells and is cytoplasmic in the remainder of the corneal epithelial cells.

Cell cycle kinetics in corneal endothelium from old and young donors

PURPOSE

To compare cell cycle kinetics in corneal endothelial cells from young and old donors.

METHODS

Human corneas were obtained from the eye bank and separated into two groups: young (19 corneas, <30 years of age) and old (40 corneas, >50 years of age). Corneas were cut in quarters, and the endothelium was released from contact inhibition by producing a 2-mm scrape wound. Unwounded endothelium acted as a negative control. Corneal pieces were exposed for 24, 36, 48, 60, 72, and 84 hours to medium containing 10% fetal bovine serum, 20 ng/ml fibroblast growth factor, and 50 mg/ml gentamicin or the same medium supplemented with 10 ng/ml epidermal growth factor (EGF). Tissue was fixed, immunostained for Ki67 (a marker for the late G1-through M-phase) or for 5-bromo-2'-deoxyuridine (BrdU; a marker for the S-phase), and mounted in medium containing propidium iodide (PI) to visualize all nuclei. Confocal images were evaluated using an image analysis program to count Ki67-positive and PI-stained cells and to evaluate cell cycle position. Cells were counted in 15x100 microm2 areas randomly selected from each wound, and the mean was used for subsequent calculations.

RESULTS

Human corneal endothelial cells could be reliably scored for their position within the cell cycle using Ki67 staining patterns. In both age groups, cells repopulating the wound area stained positively for Ki67, whereas no Ki67 staining was observed in unwounded areas under any condition tested. Cells from old donors treated with fetal bovine serum and FGF stained positively for Ki67, indicating that these cells were actively cycling. Compared with cells from young donors, old cells entered the cell cycle more slowly (48 versus 36 hours), the peak of Ki67 staining occurred later (72 versus 60 hours), and fewer cells proliferated (23% versus 47%) or exhibited mitotic figures (4% versus 7%). Addition of EGF to the culture medium increased Ki67 staining in both groups, but the effect on old cells was more dramatic. More cells from old donors entered the cell cycle by 36 hours after wounding, the number of proliferating cells increased 1.6-fold, and the relative number of mitotic figures increased 2.5-fold over cells treated in the absence of EGF.

CONCLUSIONS

Regardless of donor age, corneal endothelial cells can enter and complete the cell cycle. In the presence of fetal bovine serum and FGF, cells from old donors can proliferate but respond more slowly and to a lesser extent than cells from young donors. EGF added to the medium stimulates cells from old donors to enter the cell cycle faster, increases the relative number of actively cycling cells, and increases the number of cells exhibiting mitotic figures. The resultant hypothesis is that it is possible to stimulate a significant proliferative response in corneal endothelial cells from old individuals. Administration of an optimal combination of stimulatory growth factors is required under conditions in which cells have been transiently released from contact inhibition.

TGF-beta2 in aqueous humor suppresses S-phase entry in cultured corneal endothelial cells

PURPOSE

Corneal endothelium in vivo is arrested in G1, the phase of the cell cycle that prepares cells for DNA synthesis. In many cell types, transforming factor (TGF)-beta inhibits proliferation by inducing G1-phase arrest. Evidence indicates that corneal endothelial cells synthesize mRNA for TGF-beta1 and are also bathed in aqueous humor that contains TGF-beta2 (mainly in a latent form). As such, this cytokine may maintain the corneal endothelium in a G1-phase-arrested state in vivo. The purpose of these studies was to determine the effect of exogenous TGF-beta2 and TGF-beta2 in aqueous humor on DNA synthesis in cultured corneal endothelial cells.

METHODS

Rat corneal endothelial cells were grown in explant culture and identified by morphology and reverse transcription-polymerase chain reaction using primers for specific corneal cell markers. Subconfluent cells were synchronized in the G0 phase (quiescence) by serum starvation for 24 hours. Serum was then added to the cells in the presence or absence of exogenous TGF-beta2 or activated rat aqueous humor. [3H]Thymidine was added, and radioactivity was measured at various time points to detect DNA synthesis. Preincubation of exogenous TGF-beta2 or activated rat aqueous humor with neutralizing antibody was used to test for cytokine specificity.

RESULTS

A linear increase in [3H]thymidine incorporation began approximately 16 hours after serum addition, and peak incorporation occurred at approximately 24 hours. Exposure of cells to serum plus TGF-beta2 suppressed [3H]thymidine incorporation in a dose-dependent manner at concentrations ranging from 5 pg/ml to 5 ng/ml. [3H]Thymidine incorporation was also suppressed in cells exposed to serum plus rat aqueous humor diluted 1:10. Neutralizing antibody reversed the effects of both exogenous TGF-beta2 and aqueous humor.

CONCLUSIONS

Exogenous TGF-beta2 and TGF-beta2 in aqueous humor suppress S-phase entry of rat corneal endothelial cells. These results suggest that this cytokine in aqueous humor could help maintain the corneal endothelium in a G1-phase-arrested state in vivo.

Synchronization of the G1/S transition in response to corneal debridement

PURPOSE

This study's intention was to examine the progression of ocular surface epithelium through the G1/S transition of the cell cycle after corneal epithelial debridement.

METHODS

Three-millimeter debridements were made in central rat cornea and allowed to heal 4 to 48 hours in vivo. Unwounded contralateral eyes served as controls. Two hours before the animals were killed, 5-bromo-2-deoxyuridine (BrdU) was injected to detect S-phase cells. Incorporated BrdU was visualized by indirect immunofluorescence microscopy, and expression of G1 cell-cycle markers cyclins D and E was examined by indirect immunofluorescence and immunoblotting.

RESULTS

The number of BrdU-labeled cells in conjunctival, limbal, and peripheral epithelium peaked at 28 hours after wounding (3.9-, 4.5-, and 3.2-fold increases, respectively). In unwounded eyes, cyclin D showed diffuse cytoplasmic localization with occasional basal cells exhibiting a nuclear localization, while anti-cyclin E showed intense localization in limbal and conjunctival basal cells but only minimal labeling in corneal epithelium. Within 8 to 12 hours after wounding, the nuclei of most corneal basal cells outside the wound area were bound intensely by anti-cyclins D and E. Immunoblotting revealed that cyclin D and E protein levels increased 4.5- and 12.1-fold after wounding, respectively. Epithelium migrating into the wound area did not incorporate BrdU and did not exhibit nuclear localization of cyclins D and E.

CONCLUSIONS

Corneal epithelial debridement stimulates basal cells outside the wound area to synchronously enter the cell cycle. However, cells migrating to cover the wound area do not progress through the cell cycle. These data suggest a compartmentalization of the proliferative and migratory phases of wound repair.

Transforming growth factor-beta suppresses proliferation of rabbit corneal endothelial cells in vitro

Corneal endothelial cells in vivo appear to be inhibited in G1 phase of the cell cycle. Studies were carried out to determine whether cultured rabbit corneal endothelium expresses transforming growth factor-beta (TGF-beta) receptor types I, II, and III, suggesting they would be sensitive to a TGF-beta-induced signal. In addition, we explored if TGF-beta might mediate this G1 phase inhibition by implementing flow cytometry and 5-bromo-2'-deoxyuridine (BrdU) immunofluorescence. Reverse transcription-polymerase chain reaction (RT-PCR) products of the expected size were obtained for all three TGF-beta receptor types. Flow cytometry revealed a dose-dependent suppression in the percentage of S phase cells in cultures treated with TGF-beta1 or TGF-beta2. The lowest percentage of S phase cells was found for 10 ng/ml TGF-beta1 and 0.1 ng/ml TGF-beta2. BrdU, an S phase marker, was immunolocalized, and semiquantitative analysis of stained cells showed a maximum suppression of S phase entry at 18 h for 10 ng/ml of TGF-beta11 and 24 h for 10 ng/ml of TGF-beta2. In rabbit, the corneal endothelium expresses TGF-beta receptor types I, II, and III, permitting a TGF-beta signal to be transduced. Flow cytometry reveals a dose-dependent response to both TGF-beta1 and TGF-beta2, and the cells are more sensitive to TGF-beta2. At optimal TGF-beta concentrations, the percentage of S phase cells is comparable to that of a non-proliferating culture, suggesting TGF-beta prevents the cells from proceeding through the G1/S phase transition. This suppression was also seen with BrdU labeling. Together, these results indicate that TGF-beta could be one of the pathways that leads to G1 phase arrest in corneal endothelial cells.

Mitotic inhibition of corneal endothelium in neonatal rats

PURPOSE

Corneal endothelium in humans does not divide to any significant extent after birth; therefore, with age there is a gradual loss of cells. When cell density is reduced to a critical level, the endothelium cannot function to maintain corneal clarity, and the cornea becomes permanently cloudy. Currently, the blindness that results can be treated only by corneal transplantation. The long-term goal is to find methods to stimulate corneal endothelial proliferation in a clinically relevant manner. The first step toward achieving this goal is to identify mechanisms responsible for the induction and maintenance of mitotic inhibition of the corneal endothelium in vivo. During corneal development, the endothelium is formed by migration and proliferation of mesenchymal cells from the ocular periphery. Soon after the monolayer is formed, proliferation ceases. In tissue culture, many cell types cease proliferating upon formation of stable cell-cell and cell-substrate attachments. The goal of the present studies was to determine whether establishment of stable contacts correlates with cessation of endothelial proliferation during corneal development in vivo.

METHODS

Corneas from neonatal (days 1, 3, 7, 10, 13, 14, 17, 21, 28, and 42) and adult rats were used for immunolocalization of the following: bromodeoxyuridine (BrdU), an S-phase marker; p27kip1 and p21cip1, G1-phase inhibitors; connexin-43 and ZO-1, proteins associated with gap and tight junctions, respectively; Na+/K+-ATPase and beta3-integrin, markers of plasma membrane polarity; and fibronectin and collagen type IV, constituents of Descemet's membrane. Nuclei staining positively for BrdU were counted to determine the relative number of S-phase cells at various times after birth. Marker protein expression and localization were determined by conventional fluorescence microscopy and by confocal microscopy.

RESULTS

The number of endothelial cells staining positively for BrdU gradually decreased between postnatal days 1 and 13. After postnatal day 13, positive BrdU staining was no longer detectable. During the first postnatal week, cells stained positively for the G1-phase inhibitor p27kip1 but not for p21cip1. Connexin-43 achieved its mature location by postnatal day 1. ZO-1, Na+/K+-ATPase, beta3-integrin, fibronectin, and collagen type IV achieved their mature localization patterns between postnatal days 14 and 21.

CONCLUSIONS

In neonatal rat, corneal endothelial cells are still entering the cell cycle at birth, but cell cycle entry gradually decreases, so that by postnatal day 13 cells are no longer entering the S-phase. The G1-phase inhibitor p27kip1, but not p21cip1, may help mediate this inhibition. Stable cell-cell and cell-substrate contacts gradually form, and monolayer maturation is complete between postnatal days 14 and 21. The results lead to the hypothesis that, in developing rat cornea in vivo, the establishment of stable cell-cell and cell-substrate contacts initiates a cascade of events, mediated by p27kip1, which induces mitotic inhibition in the endothelial monolayer.

Transforming growth factor-beta receptor expression in human cornea

PURPOSE

Limbal basal cells and corneal endothelial cells appear to be inhibited in the G1 phase of the cell cycle. As a preliminary to determining whether transforming growth factor-beta (TGF-beta) might mediate this inhibition, investigation was made to determine whether human corneal and limbal cells express TGF-beta receptor types I (RI), II (RII), and III (RIII).

METHODS

Corneas from eight human donors, aged stillborn to 85 years, were fresh frozen, cryostat sectioned, and prepared for indirect immunofluorescence localization of RI, RII, and RIII, using an established protocol. Corneas from donors 50 years of age or older were used to prepare RNA from the epithelium and endothelium. Reverse transcription-polymerase chain reaction was conducted using primers specific for each TGF-beta receptor type.

RESULTS

Immunolocalization patterns for RI, RII, and RIII were similar, regardless of donor age. Binding of RI and RII antibodies was barely detectable in central corneal epithelium; however, most limbal basal cells stained positively for RI and RII. All layers of central corneal epithelium and the suprabasal layers of the limbus stained positively for RIII, whereas staining for this receptor was markedly decreased in limbal basal cells. Corneal endothelium bound the antibody for all three TGF-beta receptor types. In the same tissue sections, antibody staining for the RIII protein was more intense in corneal endothelial cells than in limbal basal cells. Polymerase chain reaction product for RI, RII, and RIII was detected in the epithelium and in the endothelium.

CONCLUSIONS

Limbal basal cells and corneal endothelial cells expressed mRNA and protein for TGF-beta receptor types I, II, and III, suggesting that both cell types can transmit a TGF-beta-induced signal. These two cell types may differ in their relative response to those TGF-beta isoforms that require binding to RIII for signal transduction, in that staining intensity for RIII was relatively low in limbal basal cells compared with that in the endothelium. That limbal basal and corneal endothelial cells express receptors for TGF-beta suggests that this cytokine could mediate G1 phase arrest in these two cell types.

Expression of cell cycle-associated proteins in human and rabbit corneal endothelium in situ

PURPOSE

It is unknown why human corneal endothelium exhibits limited capacity to divide while the endothelia of other species, such as rabbit, divide in vivo at wounding and in culture. A potentially valuable source of information concerning why human endothelium has such a limited proliferative capacity lies in elucidating any differences in the molecular events governing the cell cycle of these two species. A recent study of the relative expression of cell cycle-associated proteins in donor corneas suggests that human corneal endothelial cells in vivo have not exited the cell cycle but are arrested in G1-phase. The purpose of the current study was to identify differences in cell cycle protein expression in human and rabbit endothelium that would explain the difference in their relative proliferative capacities. Specifically, the authors first ascertained the relative proliferative status of rabbit corneal endothelial cells in vivo. The expression and intracellular distribution of G1-phase regulatory proteins was then determined in both species, and the results were compared.

METHODS

Corneas from New Zealand white rabbits (weight range, 2 to 3 kg) and from human donors (age range, 6 months to 67 years) were fresh frozen, cryostat sectioned, and prepared for indirect immunofluorescence microscopy using an established protocol. The following monoclonal antibodies were localized in rabbit corneal endothelium only: cyclins D, E, A, and B1; protein kinase p34cdc2; and Ki67, a marker of actively cycling cells. Localization patterns for the following G1-phase regulatory proteins were compared in both human and rabbit corneal endothelia: the tumor suppressors, pRb, p53, and p16INK4, and the transcription factor, E2F. Reverse transcription-polymerase chain reaction studies were conducted to detect mRNA for Ki67 in human and rabbit corneal cells.

RESULTS

Cyclins D, E, and A were localized in the cytoplasm of rabbit corneal endothelium, whereas cyclins B1 and p34cdc2 were detected in the nucleus. No Ki67 protein or mRNA expression was detected in the endothelium of either species. In human and rabbit endothelia, p53 and p16INK4 were localized to the cytoplasm, whereas pRb was detected in the nucleus. E2F exhibited a nuclear and a cytoplasmic localization in each species.

CONCLUSIONS

The corneal endothelium of rabbits stained positively for cyclins D, E, and A and did not stain for Ki67, suggesting that, as in humans, rabbit corneal endothelium in vivo is arrested in G1-phase upstream from Ki67 synthesis. Cyclin E was located in the cytoplasm of rabbit cells, whereas it was found in the nucleus in human endothelium. The apparent difference in cellular distribution of cyclin E in these two species may be significant because this cyclin is active during the G1-/S-phase transition. It is possible that in situ human and rabbit corneal endothelial cells are arrested at different points within G1-phase and/or that the difference in relative proliferative capacity exhibited by the corneal endothelium in these two species may be caused by differences in their relative ability to overcome G1-phase arrest.

Cell cycle protein expression and proliferative status in human corneal cells

PURPOSE

To determine the relative expression of cell cycle-association proteins in human corneal and limbal epithelium and corneal endothelium in situ, to correlate staining patterns of cell cycle-associated proteins with known proliferative status of corneal and limbal epithelial cells, and to determine the relative proliferative status of corneal endothelial cells in situ by comparing their staining patterns with those of corneal and limbal epithelial cells.

METHODS

Corneas from donors 6 weeks and 17, 27, 37, 53, 66, and 67 years of age were preserved in Optisol, received on ice within 24 to 36 hours of death, and immediately fresh frozen. Transverse 6-micron corneal sections were prepared for indirect immunofluorescence localization using commercial antibodies that specifically recognize the following cell cycle-associated proteins: cyclins D, E, A, and B1; protein kinases p33cdk2 and p34cdc2; and Ki67, a marker of actively cycling cells.

RESULTS

All cells of the corneal and limbal epithelium and corneal endothelium stained positively for protein kinases, p33cdk2 and p34cdc2, and for cyclin B1. Staining patterns for cyclins D, E, and A and for Ki67 differed depending on the relative proliferative status of the cells. Terminally differentiated, noncycling corneal epithelial subrabasal cells did not stain significantly for cyclins D, E, or A, or for Ki67. Some corneal epithelial basal cells showed nuclear staining, particularly for cyclin D and Ki67, indicating the presence of actively cycling cells in this regenerative cell layer. In peripheral corneal epithelium, the relative number of basal cells with positive cytoplasmic staining for cyclins D, E, and A increased with proximity to the limbus. Within this region, an occasional nucleus stained positively for Ki67. Limbal basal cells, which contain a slow-cycling stem cell population, stained positively for cyclins D, E, and A within the cytoplasm. Nuclear staining for cyclin D and Ki67 was observed in a few basal cells. Occasional cells within the suprabasal layers of the limbus stained positively for Ki67. The corneal endothelium, considered a nonrenewing population, exhibited staining patterns similar to those of limbal basal cells, except that in no specimen was Ki67 staining observed.

CONCLUSIONS

All corneal and limbal epithelial and corneal endothelial cells express protein kinases, p33cdk2 and p34cdc2, and cyclin B1. Relative staining patterns of the cell cycle-dependent proteins, cyclins D, E, and A, and of Ki67 acted as markers to distinguish terminally differentiated epithelial suprabasal cells that have exited the cell cycle, actively cycling epithelial basal cells, and slowly-cycling limbal basal (stem) cells. Staining patterns of the corneal endothelium most closely corresponded to those of limbal basal cells, suggesting that endothelial cells are arrested in G1-phase and have not exited the cell cycle.

EGF and PGE2: effects on corneal endothelial cell migration and monolayer spreading during wound repair in vitro

In vivo repair of the adult human corneal endothelium occurs mainly by movement of cells into the wound defect rather than by cell division. Two forms of cell movement contribute to endothelial wound repair: migration of individual cells into the defect and spreading of the confluent monolayer into the wound area. This laboratory has developed a tissue culture model using rabbit corneal endothelial cells pretreated with the mitotic inhibitor 5-fluorouracil to mimic the relatively amitotic state of human corneal endothelium in vivo. This model permits study of the effects of growth factors and other agents on individual cell migration and monolayer spreading in response to wounding. mRNA for epidermal growth factor (EGF) and its receptor has been detected in cultured corneal endothelial cells and EGF receptors have been detected on human corneal endothelial cells in situ, suggesting that this growth factor may act in an autocrine manner. Prostaglandin E2 (PGE2) is synthesized by cultured corneal endothelial cells and is present in relatively high quantity in aqueous humor in response to corneal wounding and to inflammation in the anterior chamber. Although corneal endothelial cells may be exposed to both EGF and PGE2, little is known about their effects on monolayer repair. The current study compared the effects of PGE2 alone, EGF alone, and both agents in combination on individual cell migration and monolayer spreading using the wound model system and also determined the effect of EGF on PGE2 secretion using a commercial immunoassay. A 15 min exposure of wounded cultures to exogenous PGE2 stimulated individual cell migration and suppressed monolayer spreading.(ABSTRACT TRUNCATED AT 250 WORDS)

PGE2: a mediator of corneal endothelial wound repair in vitro

Maintenance of the integrity of the corneal endothelial monolayer is essential for corneal clarity. Aging, trauma, inflammation, and diseases, such as diabetes, can compromise monolayer integrity, resulting in corneal edema and loss of visual acuity. In adult humans, repair of the monolayer occurs mainly by two forms of cell movement: "migration," in which individual cells move from the wound edge to repopulate the defect, and "spreading," in which cells enlarge and flatten, causing movement of the monolayer as a sheet to cover the defect. Previous studies from this laboratory have shown that these two forms of movement can be pharmacologically separated. In the current studies, an established tissue culture model was used to determine the effect of prostaglandin E2 (PGE2) and of mediators of the adenosine 3',5'-cyclic monophosphate (cAMP) pathway on individual cell migration. Indomethacin, an inhibitor of prostaglandin synthesis, significantly decreased individual cell migration below levels obtained when wounds were exposed to culture medium alone. PGE2, but not PGF2 alpha, restored this response in a dose-dependent manner. In the presence of indomethacin, forskolin, a direct adenylate cyclase activator, stimulated individual cell migration. 2',5'-Dideoxyadenosine, an adenylate cyclase inhibitor, reversed the stimulatory effects of both forskolin and PGE2. Dibutyryl cAMP (DBcAMP) also stimulated individual cell migration in the presence of indomethacin, whereas, H89, a protein kinase A inhibitor, reversed both the DBcAMP and PGE2-induced effects. These results provide evidence that stimulation of the cAMP pathway enhances individual cell migration and that PGE2, acting via this pathway, may be an endogenous stimulator of this response during corneal endothelial wound repair.

Protein kinase C activation during corneal endothelial wound repair

In previous studies, the authors have shown that the two forms of cell translocation that occur during corneal endothelial monolayer wound repair can be pharmacologically separated. Epidermal growth factor (EGF) enhanced the breaking of cell-cell contacts and movement of individual cells from the wound edge, while indomethacin, an inhibitor of PGE2 synthesis, promoted cell enlargement and spreading of the confluent monolayer sheet into the wound defect. From these findings, the authors hypothesized that the two forms of cell translocation were stimulated by different but coordinately regulated second messenger systems. The current studies used selected protein kinase C (PKC) stimulators and inhibitors, Rh-phalloidin staining of actin filaments, and immunofluorescent localization of PKC to show that: (1) PKC acts as a mediator of the EGF-induced enhancement of the migratory response; (2) the enhanced migratory response results, at least in part, from short-term EGF stimulation of PKC; (3) PKC is a mediator of the EGF-induced alterations in the actin cytoskeleton; and (4) PKC becomes activated in cells at the wound edge during normal, endogenously stimulated wound repair. The results of these studies provide suggestive evidence that wounding of the corneal endothelial monolayer must produce an endogenous, EGF-like stimulation of PKC activity in cells at the wound edge. One effect of PKC activation that must contribute to stimulation of individual cell migration is the induction of cytoplasmic changes that lead to alterations in actin filament organization.

In situ injury-induced release of basic-fibroblast growth factor from corneal epithelial cells

Basic-fibroblast growth factor (b-FGF) binds to heparan sulfate proteoglycan in Bowman's layer of the cornea. The mechanism by which the molecule is deposited in Bowman's layer is the subject of controversy since b-FGF lacks a signal peptide sequence for extracellular secretion. Using immunofluorescence, the authors studied the presence and distribution of b-FGF in the bovine cornea and the conditions under which it could be released and bound to Bowman's layer. The results indicate that corneal epithelium contains b-FGF but that uninjured corneas do not contain detectable levels of b-FGF in Bowman's layer. Injury to the corneal epithelium results in the binding of b-FGF to Bowman's layer. Removal of the intact corneal epithelium without cell injury does not result in the binding of b-FGF to Bowman's layer. These findings suggest that one mechanism for the release of b-FGF from corneal epithelial cells is passive leakage after cell injury with secondary binding to Bowman's layer.

In vitro pharmacologic separation of corneal endothelial migration and spreading responses

Repair of corneal endothelial wounds involves two forms of cell translocation: (1) "migration," in which individual cells at the wound edge break contacts with neighboring cells and move as individuals into the wound defect, and (2) "spreading," in which cells within the confluent monolayer adjacent to the wound move as a group into the wound area. The authors combined morphometric analysis of Giemsa-stained cultures, phase-contrast video microscopy, and Rh-phalloidin staining of actin filaments to study the effects of epidermal growth factor (EGF) and indomethacin on the migratory and spreading responses to wounding using an in vitro wound-closure model which mimics the amitotic state and general behavior of human corneal endothelium. They found that EGF stimulated the migration of individual cells from the wound edge, induced cellular elongation, and promoted a diffuse distribution of actin filaments. Indomethacin promoted spreading of the confluent monolayer into the wound defect, induced enlargement and flattening of cells, and promoted the formation of long, thick actin stress fibers. These results provide evidence that the migration and spreading responses of corneal endothelial cells to wounding can be pharmacologically separated. The findings suggest that migration of individual cells during wound repair may result from an endogenous form of EGF-like stimulation and that the elongated shape associated with this form of translocation results, at least in part, from an EGF-like alteration in actin-filament organization. Spreading of the confluent monolayer to cover the wound defect may result from a decrease in cyclic adenosine monophosphate induced by a transient reduction in prostaglandin E2 synthesis. This form of translocation may result, in part, from enlargement and flattening of corneal endothelial cells secondary to an enhancement of actin stress-fiber formation.

Effects of EGF and indomethacin on rabbit corneal endothelial wound closure in excised corneas

Corneal endothelial cells of rabbit corneas stored in M-K medium at 37 degrees C were wounded by touching them lightly with a micropipet under video specular microscope observation. Three groups were studied: control, with EGF, and with EGF + indomethacin. The wound closure process (initial wound area about 8500 microns 2) was observed and recorded with time-lapse videography for 6 hr. The recorded video images were digitized and computer assisted morphometric analysis was performed. (1) Addition of either EGF (10 ng/ml) + indomethacin (1 microM), or EGF (10 ng/ml) alone to the M-K medium statistically significantly shortened the wound closure time as compared with the control group. (2) Both EGF + indomethacin and EGF alone resulted in a greater average percent relative change of the shape factor, more than three times greater with EGF + indomethacin and more than two times greater with EGF alone, than in the control group 150 min after wounding. (3) The maximum cell shape change occurred at about 150 min after wounding in the groups EGF + indomethacin and EGF alone, and at about 200 min in the control group. After this time in all three groups the cells began to approach a normal shape. (4) The cells near the wound boundary moved faster in the EGF + indomethacin and the EGF groups as compared with the control group. These results suggest that EGF and indomethacin may be of therapeutic value in promoting closure of traumatized human corneal endothelium.

Corneal endothelial wound closure in vitro. Effects of EGF and/or indomethacin

In response to stress, the corneal endothelium must maintain or region its barrier function. To study cellular responses of the corneal endothelium, our laboratory has developed an in vitro model of rabbit corneal endothelial wound closure. When cells are free to divide, a 3 mm diameter wound closes within 4 days. 5-fluorouracil added to these cultures does not affect the cellular morphology or ultrastructure, but does inhibit cell division. In the presence of 5-fluorouracil, wounds close in approximately 7 days. These conditions mimic the amitotic state and general behavior of adult human corneal endothelium in vivo. Using this model, we studied the effects of epidermal growth factor (EGF) and/or indomethacin treatment on corneal endothelial wound closure in mitotically competent and inhibited cultures. EGF appeared to stimulate migration, whereas indomethacin appeared to enhance cell spreading in response to wounding, particularly in mitotically inhibited cultures. Treatment with the above agents at the time of wounding had little effect on wound closure rates, but did affect closure patterns. In contrast, pretreatment of cultures, particularly with indomethacin, significantly accelerated closure in mitotically inhibited cultures. In the presence of indomethacin, wounds closed in 3-4 days compared to 7-8 days for controls. These results indicate that the response of corneal endothelial cells to wounding can be pharmacologically manipulated, and perhaps accelerated, and suggest that the treatment of the endothelium with nonsteroidal anti-inflammatory drugs or EGF-like growth factors may be clinically useful.

Interaction of angiotensin II with functional smooth muscle cells in culture

In this study we report on the characterization of a highly enriched population of cultured vascular smooth muscle cells (SMC) prepared from collagenase-treated medial layer explant outgrowths of rabbit aortae. Studies done on cells from first passage explant outgrowths showed that the cells retain the fine structural features of vascular SMC in situ, can be immunostained with anti-smooth muscle myosin IgG, and bind [125I]angiotensin II (ANG II) in a specific and saturable manner with an apparent Kd of 1 nM. Addition of ANG II (0.1 microM) to the cultures causes obvious shape changes and retraction of cell processes. Electron microscopic autoradiography of cells labeled with [125I]ANG II show that the initial site of interaction of ANG II with the SMC is the plasma membrane. The distribution of ANG II receptors among cells in the population was studied using light microscopic autoradiography. The autoradiographical grain density varied among cells in the population ranging from cells that were heavily labeled to those that possessed virtually no label. These data imply that the expression of ANG II receptors may be limited to a certain progeny within the cell population or is a function of their stage within the cell cycle.