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Reengineering of Aged Bruch’s Membrane to Enhance Retinal Pigment Epithelium Repopulation

¹From the Department of Ophthalmology and Visual Sciences, Kentucky Lions Eye Center, University of Louisville School of Medicine, Louisville, Kentucky; and ²The Edward S. Harkness Eye Institute, Department of Ophthalmology, Columbia University, New York, New York.

	Abstract

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PURPOSE. An earlier study showed that age-related changes in the inner collagen layer (ICL) inhibit RPE cell repopulation of human Bruch’s membrane. The present study was undertaken to determine the effect of cleaning and/or an extracellular matrix (ECM) protein coating on the reattachment, apoptosis, proliferation, and final surface coverage of the transplanted RPE cells.

METHODS. Explants of aged Bruch’s membrane with ICL exposed were prepared from five human cadaveric eyes (donor ages, 69–84 years) and treated with Triton X-100 and/or coated with a mixture of laminin (330 µg/mL), fibronectin (250 µg/mL), and vitronectin (33 µg/mL). Viable human fetal and ARPE-19 cells (n = 15,000) were plated onto the surface and the RPE reattachment, apoptosis, and proliferation ratios were determined on the modified surfaces. Cells were cultured up to 17 days to determine the surface coverage. Ultrastructure of the modified Bruch’s membrane and RPE morphology were studied with transmission and scanning electron microscopy.

RESULTS. Reattachment ratios of fetal human RPE and ARPE-19 cells were similar on aged ICL (41.5% ± 1.7% and 42.9% ± 2.7%, P > 0.05). The reattachment ratio increased with ECM protein coating and decreased with detergent treatment. Combined cleaning and coating restored the reattachment ratio of the fetal RPE cells, but failed to increase the reattachment ratio of ARPE-19 cells. The highest apoptosis was observed on untreated ICL. Cleaning and the combined procedure of cleaning and ECM protein coating decreased fetal RPE cell apoptosis. Only RPE cells plated on cleaned or cleaned and ECM-coated ICL demonstrated proliferation that led to substantial surface coverage at day 17.

CONCLUSIONS. Age-related changes that impair RPE repopulation of Bruch’s membrane can be significantly reversed by combined cleaning and ECM protein coating of the ICL. Development of biologically tolerant techniques for modifying the ICL in vivo may enhance reattachment of the RPE and its repopulation of aged ICL.

Currently, photodynamic therapy and thermal laser photocoagulation are the only clinically validated treatments for selected cases of exudative AMD.^{3 4} Thermal laser treatment coagulates new choroidal vessels at the cost of destroying the overlying sensory retina and creating an absolute central scotoma.⁵ Even so, only less than 20% of patients with exudative AMD are eligible for laser photocoagulation, and half of them experience persistent or recurrent neovascularization after laser photocoagulation.⁶ Photodynamic therapy reduces the rate of visual loss due to well-defined choroidal neovascularization but does not lead to significant visual improvement in most individuals.⁴

These limitations have led to the development of alternative treatment modalities, such as systemic interferon,⁷ radiotherapy,⁸ subfoveal membranectomy,⁹ macular translocation,¹⁰ and antiangiogenic pharmacological agents, such as anti-VEGF antibody,¹¹ anti-VEGF aptamer,¹² triamcinolone,¹³ and anecortave acetate.¹⁴ All these techniques are intended to obliterate new choroidal vessels and/or decrease plasma leakage from them. They often necessitate multiple treatment sessions because of recurrences and result at best in slowing visual deterioration with little significant improvement of central vision. Eventually, persistent exudation from the subretinal fibrovascular tissue leads to fibrovascular scar formation with continuing disruption of the relationship among the choriocapillaris, RPE, and photoreceptors, with subsequent photoreceptor cell death and loss of central vision.

Unfortunately, simple excision of the subfoveal neovascular membrane in AMD leaves a large RPE defect under the fovea, due to the removal of native RPE along with the surgically removed neovascular complex.¹⁵ Resultant persistent RPE defects in AMD lead to the development of progressive choriocapillaris and photoreceptor atrophy.¹⁶ Attempts to repopulate Bruch’s membrane defects with native or transplanted RPE cells have not been successful.¹⁷ Histopathology after adult human RPE transplantation in one eye with AMD showed failure of transplanted RPE cells to attach and form a complete monolayer under the fovea.¹⁸

RPE cells must reattach to a substrate to avoid apoptosis.¹⁹ The age of the Bruch’s membrane and the layer of Bruch’s membrane available for cell attachment have a profound effect on the fate of human RPE seeded onto Bruch’s membrane in vitro.²⁰ In the present study, we explored the possibility of enhancing the reattachment and repopulation of human RPE seeded on denuded, aged human Bruch’s membrane by cleaning and/or reconstituting the extracellular matrix (ECM). We used fetal human RPE and ARPE-19 cells in these studies to test whether they share the same fate on untreated aged human Bruch’s membrane.

	Materials and Methods

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Preparation of RPE Cultures
All study protocols adhered to the provisions of the Declaration of Helsinki for research involving human tissue. Human fetal RPE cells were harvested from 14- and 17-week-old human fetuses processed within 6 hours of elective abortion. The techniques for harvesting and culturing the RPE cells have been published.²¹ Briefly, on receipt, eyes were cleaned of extraocular tissue. A circumferential scleral incision was made 1.5 mm posterior to the limbus, and the sclera was peeled away. The eyecup was then incubated with 25 U/mL dispase (Invitrogen-Gibco, Grand Island, NY) for 30 minutes and rinsed with CO₂–free medium (Gibco). Loosened RPE sheets were collected with a Pasteur pipette and plated onto bovine corneal endothelium-ECM–coated, 60-mm treated plastic dishes (Falcon; BD Biosciences UK, Plymouth, UK). The cells were incubated in a humidified atmosphere of 5% CO₂ and 95% air at 37°C and maintained in Dulbecco’s modified Eagle’s medium (DMEM H16; Invitrogen-Gibco) supplemented with 15% FBS, 100 IU/mL penicillin G, 100 µg/mL streptomycin, 5 µg/mL gentamicin, 2.5 µg/mL amphotericin B, and 1 ng/mL recombinant human basic fibroblast growth factor (bFGF; Invitrogen-Gibco), to promote RPE cell growth. The medium was changed every other day and the cells observed daily. Cells became confluent in approximately 10 days, and confluent cultures were passaged by trypsinization. First-passage RPE cell lines were used in these experiments.

The ARPE-19 cell line was obtained from the American Type Culture Collection (Manassas, VA). This is a line of spontaneously immortalized RPE cells that have morphologic and functional characteristics similar to those of adult human RPE cells.²² Cells were maintained in a 1:1 mixture of DMEM and Ham’s F-12 with HEPES buffer containing 20% FBS (Invitrogen-Gibco), 56 mM final concentration sodium bicarbonate, and 2 mM L-glutamine (Sigma-Aldrich, St. Louis, MO) and incubated at 37°C in 10% CO₂.

Cytokeratin Labeling
Cells were stained with a pancytokeratin antibody to verify that all cells were of epithelial origin. For this purpose, harvested RPE sheets were rinsed in phosphate-buffered saline (PBS), fixed with 4% paraformaldehyde for 30 minutes, and washed again with PBS. The cells were treated for 1 hour at room temperature with 3% bovine serum albumin (Sigma-Aldrich) in PBS to block nonspecific binding sites. The cells were then incubated at 37°C for 1 hour with an FITC-conjugated monoclonal anti-pan cytokeratin antibody to cytokeratin-5, -6, and -8 (Sigma-Aldrich). The cells were washed three times with PBS and examined under a fluorescence microscope. An irrelevant isotypic IgG primary antibody (anti-human von Willebrand antibody; Sigma-Aldrich), coupled with an FITC-conjugated secondary antibody was also used and showed no background staining. All the harvested cells were positive for pancytokeratin, indicating that the cells were of epithelial origin.²³

Harvesting of Human Bruch’s Membrane Explants
Explants of the inner collagen layer (ICL) of human Bruch’s membrane were prepared from the peripheral retinas of eyes of four elderly donors (average age, 77 ± 6 years [SD]; range, 69–84 years old) obtained within 24 hours of death. The harvesting technique has been described.²⁴ Briefly, a full-thickness circumferential incision was made posterior to the ora serrata, and the anterior segment and vitreous were carefully removed. The posterior pole of each eyecup was inspected visually with direct and retroillumination under a dissecting microscope, and globes were discarded if there was any evidence of subretinal blood, previous surgery, or any extensive structural or vascular alteration of the posterior segment due to a disease process, such as proliferative diabetic retinopathy or proliferative vitreoretinopathy. The eyecups were put in CO₂-free medium (Invitrogen-Gibco), and a scleral incision was made 3 mm from the limbus and extended 360°. Four radial incisions were then made, and the sclera was peeled away. A circumferential incision was made into the subretinal space 1 mm posterior to the ora serrata. The choroid-Bruch’s membrane-RPE complex was then carefully peeled toward the optic disc and removed after its attachment to the optic nerve was trimmed. Native RPE cells were removed by bathing the explant with 0.02 N ammonium hydroxide in a 50-mm polystyrene Petri dish (Falcon; BD Biosciences) for 20 minutes at room temperature, followed by washing three times in phosphate-buffered saline (PBS). The Bruch’s membrane explant from the fellow eye was prepared by removing the RPE with 0.02 N ammonium hydroxide as just described. The Bruch’s membrane explant was then floated in carbon dioxide–free medium over a 12- to 18-µm-thick hydrophilic polycarbonate-polyvinylpyrrolidone membrane with 0.4-µm pores (Millipore, Bedford, MA) with the basal lamina facing the membrane. The curled edges were flattened from the choroidal side with fine forceps without touching Bruch’s membrane. Four percent agarose (Sigma-Aldrich) was poured on the Bruch’s membrane-choroid complex from the choroidal side, and the tissue was kept at 4°C for 2 to 3 minutes to solidify the agarose. The hydrophilic membrane was peeled off along with the basal lamina of the RPE, thus exposing the bare ICL. Circular buttons (6-mm diameter) were then trephined from the peripheral Bruch’s membrane on a Teflon sheet and placed on 4% agarose at 37°C in nontreated polystyrene wells of a 96-well plate (Corning Costar Corp., Cambridge, MA). The agarose solidified within 2 to 3 minutes at room temperature, thus stabilizing the Bruch’s membrane explant. The wells were gently rinsed with PBS three times for 5 minutes, gamma sterilized (20,000 rad), and then stored at 4°C.

Different Treatments of ICL of Bruch’s Membrane
Explants containing ICL of aged Bruch’s membrane on the apical surface were prepared as described earlier and processed further to create four experimental plating surfaces: (1) cleaned ICL: For this purpose, triplicate explants were treated with 0.1% Triton X-100/0.1% sodium citrate solution for 20 minutes at 4°C; (2) ECM-protein–coated ICL: To coat ICL with ECM protein, another set of triplicate buttons were incubated with an ECM protein mixture containing laminin (330 µg/mL), fibronectin (250 µg/mL), and vitronectin (33 µg/mL) at 37°C for 30 minutes; (3) cleaned and ECM-protein–coated ICL: Some buttons were first cleaned and then coated with ECM protein; and (4) untreated buttons: These were used to determine the fate of the fetal and ARPE-19 RPE cell lines on aged ICL. After the cleaning and/or coating process, the exposed surfaces were washed three times with PBS for 5 minutes, and explants were stored at 4°C.

RPE Reattachment Studies
Confluent cell cultures were synchronized by placing them in phenol-free MEM (Invitrogen-Gibco) without serum for 24 hours before harvesting with 0.25% trypsin/0.25% EDTA in Hanks’ balanced salt solution (HBSS) for 10 minutes. Two milliliters of 0.1 mg/mL aprotinin (Sigma-Aldrich) in HEPES buffer (pH 7.5) was added to quench the trypsin reaction, and the cell suspension was centrifuged for 5 minutes at 800 rpm. The cell pellet was washed three times and then resuspended in phenol red–free MEM without serum. The number of cells was determined by cell counter (model Z-1; Coulter Scientific, Hialeah, FL), and cell viability was assessed with a kit (Live/Dead Viability Kit; Molecular Probes, Eugene, OR). At least 250 cells were examined under 100x magnification, and the viability was expressed as the average ratio of live cells to the total number of cells in these three different areas.

Fifteen thousand viable RPE cells were plated on different layers of Bruch’s membrane explants and serum- and phenol-free MEM containing 100 IU/mL penicillin G, 100 µg/mL streptomycin, 5 µg/mL gentamicin, and 2.5 µg/mL amphotericin B was added to reach a final volume of 200 µL in each well. At this plating density, the RPE cells covered approximately 15% of the plating area, assuming a cell diameter of 20 µm. RPE cells were allowed to attach to the surface for 24 hours in a humidified atmosphere of 95% air/5% CO₂ at 37°C in phenol red–free MEM (Invitrogen-Gibco) without serum. Unattached cells were removed from the tissue culture plates by gently washing the wells three times with MEM.

Assay for RPE Adhesion
The number of attached live RPE cells in each well was determined with a colorimetric assay that indirectly estimates the number of live cells by measuring intracellular dehydrogenase activity (CellTiter 96 Aqueous nonradioactive cell proliferation assay; Promega, Madison, WI). Dehydrogenase enzymes found in live cells reduce MTS (3-(4,5dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) into the aqueous-soluble formazan in the presence of an electron-coupling agent (phenazine methosulfate; PMS). The quantity of the formazan product can be determined from the absorbance at 490 nm and is directly proportional to the number of living cells in culture.

The assay was performed in dark conditions, because of the light sensitivity of MTS and PMS. MEM (100 µL) without phenol red was added to each well. The added solution contained 1.0 g/mL glucose in a bicarbonate-based buffer that maintains the pH at 7.3 to 7.4 in 5% CO₂ and 95% air, thus minimizing the effects of changes in glucose and pH on the colorimetric assay.²⁵ Twenty microliters of freshly prepared MTS/PMS solution (20:1) was added to each well, resulting in a final concentration of 333 µg/mL MTS and 25 µM PMS. Plates were incubated for 4 hours at 37°C, and 100 µL of medium from each well was transferred to a corresponding well of another 96-well plate and read at 490 nm with an ELISA plate reader. The corrected absorbance was obtained by subtracting the average optical density reading from triplicate sets of controls containing the Bruch’s membrane explant on 4% agarose without plated cells. The number of viable cells was estimated from standardized curves obtained by plating 100 to 13,000 viable, synchronized fetal RPE and ARPE-19 cells separately in triplicates on Bruch’s membrane explants stabilized on 4% agarose. A linear relationship (r = 0.93, 0.96 for fetal RPE and ARPE-19 cells, respectively) was observed between the number of viable cells and the absorbance at 490 nm (data not shown). The RPE reattachment ratio for a substrate was the ratio of attached cells to the entire plated cell population for that substrate: ratio = [attached/(attached + unattached)].

Assay for RPE Apoptosis
Twenty-four hours after cells were plated in triplicate wells, the wells were washed gently three times with MEM and fixed with 4% paraformaldehyde for 4 hours. Apoptotic cells were identified using the TUNEL method.²⁶ For this purpose, cells were permeabilized with 0.2% Triton X-100 in 0.2 M sodium citrate solution at 4°C for 4 minutes. Explants were washed three times with PBS and incubated with a mixture of fluorescein-labeled nucleotides and terminal deoxynucleotidyl transferase (TdT) from calf thymus for 60 minutes. TdT catalyzes the polymerization of labeled nucleotides to free 3'-OH terminals of DNA fragments. DNA breaks were then observed under a fluorescence microscope. For this purpose, explants were carefully removed from the wells and flipped over on a coverslip. The total number of apoptotic cells was counted under a fluorescence microscope. The apoptosis ratio on each plating surface was the ratio of apoptotic cells to the total number of attached cells on that surface.

Assay for RPE Proliferation
Twenty-four hours after plating, RPE cell proliferation was stimulated by replacing the medium with MEM supplemented with 15% fetal bovine serum (FBS) and 1 ng/mL recombinant bFGF (Invitrogen-Gibco). The number of cells on each explant was determined with the MTS assay 24 hours after growth stimulation, as described earlier. The proliferation ratio was the ratio of the number of viable and attached cells 24 hours after growth stimulation to the initial number of viable and attached cells on a certain surface.

Assay for RPE Repopulation
In triplicate wells, RPE cells were maintained in 200 µL of MEM containing inert fluorescent beads (Lumafluor, Stony Point, NY)²⁷ andsupplemented with 15% FBS, 100 IU/mL penicillin G, 100 µg/mL streptomycin, 5 µg/mL gentamicin, 2.5 µg/mL amphotericin B, and 1 ng/mL recombinant human bFGF (Invitrogen-Gibco). The culture medium was changed every other day, and cell growth was monitored daily for up to 17 days with an inverted fluorescence microscope (Olympus, Tokyo, Japan) equipped with a 20x long-working-distance objective (numeric aperture [NA]: 0.4, ULWD CDPlano 20PL; Olympus). At the end of the observation period, explants were removed from the wells and mounted upside down on coverslips. Fluorescence microscopy was used to obtain images from 10 representative areas. Total surface coverage, expressed as the percentage of the total surface area, was calculated from the collected images, using image-analysis software (MetaMorph 4.5; Universal Imaging Corporation, Downingtown, PA). Results were confirmed with scanning electron microscopy (SEM).

Scanning Electron Microscopy
Explants with RPE cells were fixed in modified Karnovsky fixative (2.5% glutaraldehyde and 2% paraformaldehyde in 0.1 M cacodylate buffer [pH 7.4]) at 4°C overnight. They were then postfixed in 1% osmium tetroxide in 0.16 M cacodylate buffer (pH 7.4) for 1 hour, stained in 1% uranyl acetate in 0.1 M sodium acetate buffer, and dehydrated in a graded series of ethyl alcohol (30%–100%). The samples were then critical point dried (E3000; Polaron, Watford Hertfordshire, UK), mounted on aluminum specimen stubs with carbon-conductive tabs grounded with colloidal silver liquid paint, and sputter coated with 15.0 nm of gold (E5000; Polaron). Samples were examined by SEM (model S-4500 FEG; Hitachi, Tokyo, Japan) at 15 kV accelerating voltage and the images recorded (55 P/N film; Polaroid Corp., Cambridge, MA).

Statistical Analysis
Triplicate wells were used to calculate the average reattachment, apoptosis, and proliferation ratios and the final fate of RPE cells seeded onto each substrate. Because of the limited number of explants that could be harvested from an eye, typically, SEM studies were performed in duplicate only. Data from all experiments were pooled and expressed as the mean ± SD. The reattachment, apoptosis, and proliferation ratios and surface coverage on different substrates and RPE cell populations were analyzed in pairs by the Dunn’s multiple comparison test.²⁸ A confidence level of P < 0.05 was considered to be statistically significant.

	Results

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Reattachment Ratio
Reattachment ratios of fetal human RPE and ARPE-19 cells on treated and untreated aged (>60 years) human ICL are shown in Figure 1 . Twenty-four hours after plating, the overall reattachment ratios to untreated ICL were comparable for both cell lines (41.5% ± 1.7% for fetal human RPE and 42.9% ± 2.7% for ARPE-19, P > 0.05). Coating of the ICL with a mixture of ECM protein increased the reattachment ratios of both cell lines compared with untreated ICL (48.9% ± 2.3%, P < 0.01 for fetal human RPE and 53.0% ± 4.9%, P < 0.01 for ARPE-19). The cleaning procedure significantly decreased the reattachment ratio in both cell lines (33.4% ± 2.3%, P < 0.01 for fetal human RPE and 21.1% ± 2.3%, P < 0.01 for ARPE-19), although the reattachment of ARPE-19 cells was decreased more (P < 0.01). Combined cleaning and ECM protein coating restored the reattachment ratio of fetal human RPE to levels comparable to untreated ICL (42.3% ± 2.3%, P > 0.05), but still remained significantly lower than ECM-protein–coated ICL (P < 0.01). In contrast, ECM protein coating after cleaning failed to increase the reattachment ratio of ARPE-19 cells to the level of untreated ICL (22.6% ± 1.4%, P > 0.05).

View larger version (17K): [in this window] [in a new window]   FIGURE 1. Reattachment ratios of RPE on modified aged-ICL. ECM protein coating (ECM-P) increased the reattachment ratios of both RPE cell lines to aged ICL, whereas the cleaning process (0.2% Triton X-100 in 0.2 M sodium citrate solution at 4°C for 4 minutes) decreased the reattachment ratio. ECM protein coating after the cleaning procedure restored the reattachment ratio of fetal RPE cells but failed to show a similar effect on ARPE-19 cells.

View larger version (15K): [in this window] [in a new window]   FIGURE 2. Apoptosis ratios of RPE on modified aged ICL. Highest apoptosis ratios were observed on untreated ICL. The apoptosis rate of fetal RPE was higher than ARPE-19 cells on untreated ICL. ECM protein coating (ECM-P) lowered the apoptosis ratio significantly in both cell types. Cleaning or cleaning and coating further decreased the apoptosis ratios of fetal RPE cells, but did not alter the apoptosis rate of ARPE-19 cells.

View larger version (21K): [in this window] [in a new window]   FIGURE 3. Proliferation ratios of RPE on modified aged ICL. Twenty-four hours after growth induction, no significant proliferation was observed on untreated and ECM-protein–coated (ECM-P) ICL, whereas RPE cells plated on cleaned or cleaned and ECM-protein–coated ICL proliferated. Shaded area: failure of RPE proliferation.

View larger version (16K): [in this window] [in a new window]   FIGURE 4. Surface coverage of RPE on modified aged ICL 17 days after plating. Few cells survived on untreated and ECM-coated (ECM-P) ICL. Although the initial attachment ratios were low on cleaned ICL, RPE cells proliferated on this layer, resulting in higher surface coverage. The highest surface coverage was obtained with ARPE-19 cells on cleaned and ECM-protein–coated ICL.

View larger version (152K): [in this window] [in a new window]   FIGURE 5. SEM of ICL from an 84-year-old donor. (A) Untreated ICL revealed cross-linked bundles of collagen (arrows). Small globular structures on the collagen fibers probably represent aggregates of abnormal ECM proteins (arrowheads). Macrodeposits filled interfibrillar spaces (). (B) ECM protein coating without cleaning increased the amount of ECM protein aggregates on the collagen matrix (arrowhead). Cross-linking of collagen fibers was not affected by the coating (arrows). (C) Cleaning with Triton X-100 and sodium citrate removed the aggregates of ECM proteins and debris and resulted in gaps between collagen fibers (). Note that cross-links between collagen fibers were broken, yielding individual fibers (arrowheads). In some areas there was incomplete separation of the macrofibers (arrows). (D) Cleaning and subsequent ECM protein coating broke the cross-links between collagen fibers and resulted in deposition of ECM proteins onto the regenerated collagen framework (arrowheads). The ECM proteins coating the surface were smaller in size and did not form multimeric aggregates on the native matrix. The gaps (arrows) remained between the collagen fibers, probably corresponding to areas were macrodebris was removed (compare to A). Bars, 0.5 µm.

View larger version (109K): [in this window] [in a new window]   FIGURE 6. TEM of ICL from an 84-year old donor. (A) Untreated ICL composed of bundles of cross-linked collagen (arrows) accompanied by numerous electron-lucent deposits (arrowheads). (B) ECM protein coating increased the number of electron-dense particles (arrowheads) which formed aggregates among the cross-linked collagen bundles (arrows). (C) Cleaning broke the cross-links between collagen fibers (arrows) with resulting gaps within the collagen matrix (asterisk). An electron-dense halo surrounds the residual deep electron-lucent lipoproteinaceous debris, possibly representing exposure of the hydrophilic tails of the proteins after detergent treatment (arrowheads). (D) Combined cleaning and coating resulted in breakdown of the cross-links between collagen fibers and an increase in electron-dense particles that were evenly distributed within the collagen framework and did not form aggregates (arrowheads). Bars: (A, B) 1 µm; (C, D) 0.5 µm.

View larger version (74K): [in this window] [in a new window]   FIGURE 7. SEM of RPE on ICL. (A) RPE cells plated on untreated ICL failed to flatten within 24 hours. (B) ECM protein coating increased the number of attached cells; however, these cells remain rounded. (C) Cleaning of the ICL resulted in fewer attached RPE cells but they were flattened. (D) Combined cleaning and ECM protein coating yielded flattened RPE cells with differentiated cell features such as microvilli. RPE cells extended foot processes and formed focal adhesion plaques (arrows). Bars: (A) 7.5 µm; (B, C) 5.0 µm; (D) 3.0 µm.

View larger version (155K): [in this window] [in a new window]   FIGURE 8. SEM of RPE 17 days after plating. (A) Occasional cellular debris was noted on untreated aged ICL. (B) ECM protein coating did not increase RPE cell survival. Seventeen days after plating, surface showed numerous areas of cellular debris. (C) Surviving RPE on cleaned ICL showed long cellular extensions (arrows). (D) RPE cells plated on cleaned and ECM-protein–coated ICL proliferated and formed patches of an epithelial monolayer (arrows). Bars: (A) 5 µm; (B) 30 µm; (C) 25 µm; (D) 50 µm.

	Discussion

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View larger version (47K): [in this window] [in a new window]   FIGURE 9. Effects of coating and/or cleaning of the ICL on RPE reattachment, apoptosis, and surface repopulation. Column 1: young Bruch’s membrane contains normal collagen with globular ECM proteins (gray ellipsoids). Changes develop within Bruch’s membrane as a function of age, including collagen cross-linking (shown as squiggled lines between collagen fibers) and deposition of abnormal material (polygons with light shading; row 2). Coating alone did not remove the abnormal structures but simply increased the number of ligands available for cell attachment (row 3). Cleaning with Triton X-100 and sodium citrate removed the abnormal deposits and cross-links but also removed most of the ECM proteins (row 4). The combination of cleaning plus coating restored Bruch’s membrane ICL to its normal morphology (row 5). Column 2: RPE cell attachment was higher on young and intact Bruch’s membrane (row 1) than on older ICL (row 2). RPE attachment was higher after the surface was coated with ECM proteins without cleaning (row 3). The attachment rate was lowered by cleaning the surface, which removed the abnormal deposits and normal ECM molecules (row 4). The attachment rate was highest after the surface was cleaned and then coated with new ECM proteins (row 5). Column 3: postattachment behavior of RPE. RPE cells flatten and undergo proliferation on young Bruch’s membrane. However, on uncoated ICL from older individuals, RPE cells failed to flatten and proliferate, and had the highest apoptosis rate (row 2). Coating the surface with ECM proteins decreased the apoptosis ratio (row 3), but attached cells did not flatten on the surface and failed to proliferate. Cleaning alone decreased the attachment ratio significantly; however, attached cell proliferated (row 4). Cleaning of the surface followed by coating with ECM proteins resulted in RPE flattening and proliferationand the lowest RPE apoptosis rate (row 5). Column 4: fate of the seeded RPE. RPE cells easily resurfaced the young and intact Bruch’s membrane (row 1), whereas on old ICL, they were all eliminated ultimately by apoptosis (row 2). ECM protein coating increased RPE cell attachment but did not allow the cells to resurface the old ICL (row 3). Cleaning alone resulted in some surface coverage at 17 days (row 4), and this effect was maximized by cleaning followed by recoating (row 5).

Cleaning followed by ICL resurfacing is necessary to reverse the age-related changes partially and maximize the surface repopulation of the ICL (Fig. 9 , row 5). Cleaning and ECM protein coating restores RPE cell attachment, subsequent cell spreading and proliferation and decreases the apoptosis rate. Electron microscopy reveals that surface cleaning eliminates the collagen cross-links and removes ECM deposits. At the same time, removing the debris and regenerating the collagen framework probably vacates binding sites of ECM proteins on collagen fibers, onto which newly added ECM proteins can polymerize to create a structure that is closer to the native ultrastructural architecture.

Implications for Cellular Replacement in AMD
The neurosensory retina overlying choroidal neovascularization has the potential to recover, as evidenced by the preservation of foveal photoreceptors in eyes with exudative AMD.²⁹ Even at later stages of AMD, 25% to 30% of the photoreceptors remained structurally intact.³⁰ Foveal translocation surgery has also supplied clues that remaining photoreceptors are sufficient to restore central vision, as some patients achieve a final visual acuity of 20/40 or better once the photoreceptor-RPE interface is restored.¹⁰

Adult human RPE cells can attach and repopulate the innermost basal laminar layer of both young and aged (<60) Bruch’s membrane, whereas they fail to survive on the deeper layers of aged human Bruch’s membrane.²⁰ Thus, the fate of an RPE graft at the time of surgical excision of a neovascular complex is dependent on the layer of Bruch’s membrane exposed.²⁰ However, Bruch’s membrane may be abnormal in patients with AMD. Thickening of Bruch’s membrane and the formation of basal laminar deposits, basal linear deposits, and drusen occur early in the pathogenesis of AMD.³¹ Furthermore, surgical removal of subfoveal choroidal neovascularization in AMD may disrupt the inner layers of Bruch’s membrane,¹⁵ so that the lamellae of Bruch’s membrane available for RPE reattachment may not be uniform throughout the transplantation bed. RPE cells plated onto deeper layers of aged human Bruch’s membrane fail to attach and eventually die by apoptosis.²⁰ Failure of RPE to survive and repopulate diseased and damaged areas of Bruch’s membrane may be one of several factors accounting for the fact that uncontrolled series of human transplantation studies with allogeneic and autologous fetal and adult human RPE failed to show any biological benefit.^{17 18 32 33 34 35} This is in contrast to the anatomic and functional success of RPE transplantation in animal models that lack age-related ultrastructural alterations in human Bruch’s membrane.^{36 37} Therefore, restoration of foveal vision in exudative AMD may require modification of aged Bruch’s membrane to improve RPE repopulation of this structure.

We have previously shown that age-related alterations in the molecular composition and ultrastructure of human Bruch’s membrane make it an unfavorable substrate for the attachment and survival of grafted RPE cells.²⁰ Apoptotic mechanisms are activated within the harvested RPE graft as soon as cells are detached from their native substrate during the harvesting procedure.¹⁹ RPE cell death can be suppressed by RPE reattachment and subsequent spreading on a substrate through the interaction between integrin receptors on the basal surface of RPE and their specific ligands within the ECM.¹⁹ Failure to reestablish this interaction after RPE harvesting inevitably results in rapid RPE death by apoptosis.^{19 20} The current report shows that age-related structural alterations in human Bruch’s membrane can be at least partially reversed by cleaning and coating with ECM proteins. Such treatment can reestablish the native ECM framework to an extent adequate enough to alter the dismal fate of human RPE grafts seeded onto the ICL of aged Bruch’s membrane.

Previous studies have demonstrated that several structural and molecular alterations occur within human Bruch’s membrane as a function of age. These changes, which disrupt the delicate molecular architecture of Bruch’s membrane, include (1) structural changes in the main collagen framework, including cross-linking and deposition of long-spaced collagen³⁸ ; (2) qualitative and quantitative changes in the native ECM molecules³⁹ ; (3) deposition of abnormal extrinsic molecules,⁴⁰ ; and (4) macromolecular changes in the structure of Bruch’s membrane, such as drusen formation, calcifications, and cracks or loss of inner layers due to inadequate basal membrane regeneration, as in geographic atrophy.^{31 15} Additional structural alterations can be induced by submacular surgery, because excised neovascular membranes in AMD eyes contain fragments of the basal lamina and deeper layers of Bruch’s membrane, thus exposing the ICL and perhaps other layers. The chemical treatments we used to reengineer the aged human Bruch’s membrane act by (1) liquefying and extracting membranous lipoprotein debris from the ICL to expose ECM protein receptors on native collagen fibers^{40 41} ; (2) reestablishing the native collagen framework by dissolving long-spacing collagen⁴² and breaking collagen cross-links⁴³ ; and (3) polymerizing a layer of ECM proteins onto the rejuvenated core collagen matrix of Bruch’s membrane.⁴⁴ A nonionic detergent (Triton X-100) was used to extract membranous debris from the aged Bruch’s membrane while preserving the anionic glycosaminoglycan bridges between the collagen fibrils and the native structure of collagen.⁴⁵ At the concentrations we used, Triton X-100 dissolved the membranous debris of age-related photoreceptor outer segments⁴⁶ without disrupting the ultrastructure of the matrix.⁴⁷ It also did not interfere with the subsequent adhesion of ECM proteins to the collagen fibers⁴⁸ and allowed them to polymerize in their native form on the collagen matrix.⁴⁹ Detergent treatment before ECM protein coating avoided the binding of ECM molecules to lipoprotein debris with a consequent abnormal configuration.⁵⁰ The reducing agent sodium citrate was added to solubilize the lipid debris and to facilitate the breakdown of age-related pentosidine cross-links between collagen fibers.⁵¹ In theory, the removal of the lipoprotein debris and the secondary increase in anionic binding sites may induce a shift toward hydrophilicity and increased hydraulic conductivity of the ICL.⁵² Taking all results together, we believe that the chemical treatments we used to reengineer the aged human Bruch’s membrane acted by liquefying and extracting membranous lipoprotein debris from the ICL to expose ECM protein receptors on native collagen fibers,^{40 41} thus reestablishing the native collagen framework by dissolving long-spacing collagen⁴² and breaking collagen cross-links.⁴³ This allowed proteins subsequently placed on this surface to polymerize onto the rejuvenated core collagen matrix of Bruch’s membrane.⁴⁴

Our data suggest that simple cleaning of the aged ICL lowers the reattachment of both RPE cell types, possibly by removal of ECM proteins serving as adhesion molecules. This hypothesis is supported by the disappearance of globular proteins from and between collagen fibers detected by on SEM. This conclusion is further supported by our observation that replenishing the ECM proteins—namely, laminin, vitronectin, and fibronectin—significantly increased the attachment rate of fetal RPE on cleaned ICL. Failure to restore the reattachment of ARPE-19 cells by ECM protein coating of cleaned ICL suggests these cells may depend on different ECM receptors or an unique three-dimensional architecture of binding sites for attachment. This is not surprising, because different cell lines and even different passages of the same cell line may express different integrin heterodimers to attach to a substrate.⁵³

Although cleaning alone decreased the reattachment of both RPE cell lines, the cells flattened and had significantly lower apoptosis rates on cleaned ICL. Integrin-dependent reorganization of the cytoskeleton is thought to be responsible for cell flattening after attachment.⁵⁴ Flattening of RPE on cleaned ICL, where ECM proteins were removed by detergents, may be due to lower surface tension due to concurrent removal of lipid debris.⁵⁵ The fact that cleaning of the ICL lowered RPE apoptosis, despite also lowering cell attachment, implies that age-related changes in Bruch’s membrane may trigger RPE apoptosis by a mechanism more complex than simply interfering with cell attachment. This may involve interfering with the ability of a cell to spread along the surface and adopt a distinct morphology.⁵⁶

Supplying the ICL with high concentrations of ECM proteins alone increased RPE reattachment but did not result in RPE flattening. A random polymerization pattern of supplementary ECM proteins on the ICL may prevent them from acquiring their native three-dimensional organization and expose their binding epitopes at regular intervals. The resultant inadequately spaced multiple focal adhesion plaques may not generate cytoskeletal modifications to trigger RPE flattening.⁵⁷ Although the addition of ECM proteins on the ICL increased the RPE attachment rate, it did not decrease the apoptosis rate. This suggests that increased RPE attachment alone is not sufficient to increase survival on aged ICL. The decreased survival of RPE cells on ECM-protein–coated ICL was associated with a failure of RPE flattening, implying, as stated earlier, that cytoskeletal alterations are essential for suppression of detachment-induced apoptosis.

On the appropriate substrate, RPE cell adhesion to a surface is followed by cell spreading, formation of focal adhesions, and development of stress fibers with subsequent cell proliferation and migration.²⁴ Cell proliferation is controlled by many of the same signaling proteins that play a role in adhesion,⁵⁸ and also requires a proper interaction of integrin receptors with their ECM ligands.⁵⁹ Inadequate binding sites on aged ICL may prevent attached RPE from undergoing proliferation. We observed that only RPE cells attached to cleaned ICL flattened and proliferated. However, even on cleaned and ECM-protein–coated ICL, where RPE obtain the highest proliferation rate, we were able to populate only approximately one third of the bare ICL during the observation period. Further modification of the ICL or an increase in the number of RPE cells may allow complete resurfacing of the epithelial defect in a more timely fashion.

The fate of the human fetal and ARPE-19 cell lines seeded onto untreated aged human ICL is similar to that of adult human RPE cells,²⁰ although ARPE-19 cells are more resistant to detachment-induced apoptosis. The resistance of ARPE-19 cells to apoptosis on untreated ICL may be due to a deficiency in two major apoptosis execution pathways within these cells: induction of nuclear calcium-dependent endonucleases and activation of the interleukin-1ß–converting enzyme family of proteases.⁶⁰ Despite their increased resistance, alterations in the chemical composition of the aged ICL can still induce apoptosis in ARPE-19 cells⁶¹ and ultimately lead to the same fate as that of adult and fetal human RPE cells.

Age-related changes in the Bruch’s membrane, such as collagen cross-linking, elastin fragmentation, and deposits of abnormal material may precede cellular changes by decades.³¹ Similar age-related changes occur in the ECMs of various other tissues and organs. For example, skin wrinkling is characterized by collagen cross-linking, fragmentation of elastin, and alteration of matrix metalloproteinase activity.⁶² In Alzheimer’s disease, there is an age-related aggregation of ß-amyloid within the ECM of the brain that induces secondary changes in neural cells.⁶³ Aging of the ECM is also responsible for a pro-oncogenic milieu that manifests itself as an exponentially increasing incidence of epithelial cancers with aging.⁶⁴ In the eye, age-related changes in the glycosaminoglycan in the trabecular meshwork contribute to the development of open-angle glaucoma.⁶⁵ To our knowledge, the present study is the first to demonstrate that it is possible to reverse the age-related changes in Bruch’s membrane composition and structure in AMD. We demonstrated that correction of this change leads to changes in RPE behavior. This view offers a different perspective of the role of the age-related changes in the ECM in altering cell behavior and presents a unique opportunity to intervene by reversing this process.

In summary, we have shown that age-related changes within the ICL inhibit RPE cell repopulation after subfoveal membranectomy in AMD. We now demonstrate that we can partially reverse this process in vitro by reengineering Bruch’s membrane—namely, by cleaning the ICL with a nonionic detergent and refurbishing it with ECM proteins (laminin, vitronectin, fibronectin). This study has several limitations that should be addressed before in vivo application: (1) The use of detergents in vivo may be limited by cell membrane damage, although this may be avoided by using biodetergents, such as fluorinated surfactants⁶⁶ or lipid-degrading enzymes such as lipoprotein lipase (Curcio CA, personal communication, 2003); (2) the effect of the overlying sensory retina on RPE cell attachment and proliferation must be considered, although in vitro studies remain the only way to study RPE-human ICL interaction; and (3) the effects of other ECM proteins on RPE survival should be studied to determine the most effective combinations and concentrations. Despite these limitations, our study serves as an important proof of principle and demonstrates that reengineering Bruch’s membrane may result in enhanced resurfacing of iatrogenic or age-related defects in AMD.

	Footnotes

 
Supported by a grant from Research to Prevent Blindness, the Commonwealth of Kentucky Research Challenge Trust Fund (HJK), the Robert L. Burch III Fund, the Macula Foundation, and the Foundation Fighting Blindness.

Submitted for publication February 23, 2004; revised May 5, 2004; accepted May 10, 2004.

Disclosure: T.H. Tezel, None, L.V. Del Priore, None; H.J. Kaplan, None

The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C. 1734 solely to indicate this fact.

Corresponding author: Tongalp H. Tezel, Kentucky Lions Eye Center, Department of Ophthalmology and Visual Sciences, University of Louisville School of Medicine, 301 E. Muhammad Ali Boulevard, Louisville, KY 40202; tongalp.tezel{at}louisville.edu.

	References

Top Abstract Materials and Methods Results Discussion References

 

1. West SK. Looking forward to 20/20: a focus on the epidemiology of eye diseases. Epidemiol Rev. 2000;22:64–70.[Free Full Text]
2. Smith W, Assink J, Klein R, et al. Risk factors for age-related macular degeneration: Pooled findings from three continents. Ophthalmology. 2001;108:697–704.[CrossRef][ISI][Medline][Order article via Infotrieve]
3. Macular Photocoagulation Study Group. Laser photocoagulation of subfoveal neovascular lesions in age-related macular degeneration: results of a randomized clinical trial. Arch Ophthalmol. 1991;109:1220–1231.[Abstract]
4. Photodynamic therapy with verteporfin for age-related macular degeneration. American Academy of Ophthalmology. Ophthalmology. 2000;107:2314–2317.[CrossRef][ISI][Medline][Order article via Infotrieve]
5. Tezel TH, Del Priore LV, Flowers BE, et al. Correlation between scanning laser ophthalmoscope microperimetry and anatomic abnormalities in patients with subfoveal neovascularization. Ophthalmology. 1996;103:1829–1836.[ISI][Medline][Order article via Infotrieve]
6. Moisseiev J, Alhalel A, Masuri R, Treister G. The impact of the macular photocoagulation study results on the treatment of exudative age-related macular degeneration. Arch Ophthalmol. 1995;113:185–189.[Abstract]
7. Poliner LS, Tornambe PE, Michelson PE, Heitzmann JG. Interferon alpha-2a for subfoveal neovascularization in age-related macular degeneration. Ophthalmology. 1993;100:1417–1424.[ISI][Medline][Order article via Infotrieve]
8. Ciulla TA, Danis RP, Klein SB, et al. Proton therapy for exudative age-related macular degeneration: a randomized, sham-controlled clinical trial. Am J Ophthalmol. 2002;134:905–906.[CrossRef][ISI][Medline][Order article via Infotrieve]
9. Rao PK, Thomas MA. Update on surgical removal of choroidal neovascularization. Curr Opin Ophthalmol. 2000;11:180–185.[CrossRef][Medline][Order article via Infotrieve]
10. Lai JC, Lapolice DJ, Stinnett SS, et al. Visual outcomes following macular translocation with 360-degree peripheral retinectomy. Arch Ophthalmol. 2002;120:1317–1324.[Abstract/Free Full Text]
11. Krzystolik MG, Afshari MA, Adamis AP, et al. Prevention of experimental choroidal neovascularization with intravitreal anti-vascular endothelial growth factor antibody fragment. Arch Ophthalmol. 2002;120:338–346.[Abstract/Free Full Text]
12. TES TESG. Anti-vascular endothelial growth factor therapy for subfoveal choroidal neovascularization secondary to age-related macular degeneration: phase II study results. Ophthalmology. 2003;110:979–986.[CrossRef][ISI][Medline][Order article via Infotrieve]
13. Penfold PL, Wen L, Madigan MC, Gillies MC, King NJ, Provis JM. Triamcinolone acetonide modulates permeability and intercellular adhesion molecule-1 (ICAM-1) expression of the ECV304 cell line: implications for macular degeneration. Clin Exp Immunol. 2000;121:458–465.[CrossRef][ISI][Medline][Order article via Infotrieve]
14. D’Amico DJ, Goldberg MF, Hudson H, et al. Anecortave acetate as monotherapy for the treatment of subfoveal lesions in patients with exudative age-related macular degeneration (AMD): interim (month 6) analysis of clinical safety and efficacy. Retina. 2003;23:14–23.[CrossRef][ISI][Medline][Order article via Infotrieve]
15. Grossniklaus HE, Hutchinson AK, Capone A, Jr, Woolfson J, Lambert HM. Clinicopathologic features of surgically excised choroidal neovascular membranes. Ophthalmology. 1994;101:1099–1111.[ISI][Medline][Order article via Infotrieve]
16. Del Priore LV, Kaplan HJ, Silverman MS, Valentino TL, Mason G, Hornbeck R. Experimental and surgical aspects of retinal pigment epithelial cell transplantation. Eur J Implant Ref Surg. 1993;5:128–132.
17. Algvere PV, Gouras P, Dafgard Kopp E. Long-term outcome of RPE allografts in non-immunosuppressed patients with AMD. Eur J Ophthalmol. 1999;9:217–230.[ISI][Medline][Order article via Infotrieve]
18. Del Priore LV, Kaplan HJ, Tezel TH, Hayashi N, Berger AS, Green WR. Retinal pigment epithelial cell transplantation after subfoveal membranectomy in age-related macular degeneration: clinicopathologic correlation. Am J Ophthalmol. 2001;131:472–480.[CrossRef][ISI][Medline][Order article via Infotrieve]
19. Tezel TH, Del Priore LV. Reattachment to a substrate prevents apoptosis of human retinal pigment epithelium. Graefes Arch Clin Exp Ophthalmol. 1997;235:41–47.[CrossRef][ISI][Medline][Order article via Infotrieve]
20. Tezel TH, Kaplan HJ, Del Priore LV. Fate of human retinal pigment epithelial cells seeded onto layers of human Bruch’s membrane. Invest Ophthalmol Vis Sci. 1999;40:467–476.[Abstract]
21. Tezel TH, Del Priore LV, Kaplan HJ. Harvest and storage of adult human retinal pigment epithelial sheets. Curr Eye Res. 1997;16:802–809.[CrossRef][ISI][Medline][Order article via Infotrieve]
22. Dunn KC, Aotaki-Keen AE, Putkey FR, Hjelmeland LM. ARPE-19, a human retinal pigment epithelial cell line with differentiated properties. Exp Eye Res. 1996;62:155–169.[CrossRef][ISI][Medline][Order article via Infotrieve]
23. McKechnie NM, Boulton M, Robey HL, Savage FJ, Grierson I. The cytoskeletal elements of human retinal pigment epithelium: in vitro and in vivo. J Cell Sci. 1988;91:303–312.[Abstract/Free Full Text]
24. Tezel TH, Del Priore LV. Repopulation of different layers of host human Bruch’s membrane by retinal pigment epithelial cell grafts. Invest Ophthalmol Vis Sci. 1999;40:767–774.[Abstract/Free Full Text]
25. Marshall NJ, Goodwin CJ, Holt SJ. A critical assessment of the use of microculture tetrazolium assays to measure cell growth and function. Growth Regul. 1995;5:69–84.[ISI][Medline][Order article via Infotrieve]
26. Ben-Sasson SA, Sherman Y, Gavrieli Y. Identification of dying cells: in situ staining. Methods Cell Biol. 1995;46:29–39.[ISI][Medline][Order article via Infotrieve]
27. Katz LC, Burkhalter A, Dreyer WJ. Fluorescent latex microspheres as a retrograde neuronal marker for in vivo and in vitro studies of visual cortex. Nature. 1984;310:498–500.[Medline][Order article via Infotrieve]
28. Daniel W. Biostatistics: A Foundation for Analysis in the Health Sciences. 1995; 6th ed. John Wiley & Sons New York, NY.
29. Green WR, Enger C. Age-related macular degeneration histopathologic studies. The 1992 Lorenz E. Zimmerman Lecture. Ophthalmology. 1993;100:1519–1535.[ISI][Medline][Order article via Infotrieve]
30. Kim SY, Sadda S, Pearlman J, et al. Morphometric analysis of the macula in eyes with disciform age-related macular degeneration. Retina. 2002;22:471–477.[CrossRef][ISI][Medline][Order article via Infotrieve]
31. Feeney-Burns L, Ellersieck MR. Age-related changes in the ultrastructure of Bruch’s membrane. Am J Ophthalmol. 1985;100:686–697.[ISI][Medline][Order article via Infotrieve]
32. Peyman GA, Blinder KJ, Paris CL, Alturki W, Nelson NC, Jr, Desai U. A technique for retinal pigment epithelium transplantation for age-related macular degeneration secondary to extensive subfoveal scarring. Ophthalmic Surg. 1991;22:102–108.[ISI][Medline][Order article via Infotrieve]
33. Weisz JM, Humayun MS, De Juan E, Jr, et al. Allogenic fetal retinal pigment epithelial cell transplant in a patient with geographic atrophy. Retina. 1999;19:540–545.[CrossRef][Medline][Order article via Infotrieve]
34. Binder S, Stolba U, Krebs I, et al. Transplantation of autologous retinal pigment epithelium in eyes with foveal neovascularization resulting from age-related macular degeneration: a pilot study. Am J Ophthalmol. 2002;133:215–225.[CrossRef][ISI][Medline][Order article via Infotrieve]
35. van Meurs JC, ter Averst E, Hofland LJ, et al. Autologous peripheral retinal pigment epithelium translocation in patients with subfoveal neovascular membranes. Br J Ophthalmol. 2004;88:110–113.[Abstract/Free Full Text]
36. Gouras P, Kong J, Tsang SH. Retinal degeneration and RPE transplantation in Rpe65(–/–) mice. Invest Ophthalmol Vis Sci. 2002;43:3307–3011.[Abstract/Free Full Text]
37. Lund RD, Adamson P, Sauve Y, et al. Subretinal transplantation of genetically modified human cell lines attenuates loss of visual function in dystrophic rats. Proc Natl Acad Sci USA. 2001;98:9942–9947.[Abstract/Free Full Text]
38. Yamamoto T, Yamashita H. Scanning electron microscopic observation of Bruch’s membrane with the osmium tetroxide treatment. Br J Ophthalmol. 1989;73:162–167.[Abstract/Free Full Text]
39. Pauleikhoff D, Wojteki S, Muller D, Bornfeld N, Heiligenhaus A. Adhesive properties of basal membranes of Bruch’s membrane. Immunohistochemical studies of age-dependent changes in adhesive molecules and lipid deposits (in German). Ophthalmologe. 2000;97:243–250.[CrossRef][ISI][Medline][Order article via Infotrieve]
40. Ruberti JW, Curcio CA, Millican CL, Menco BP, Huang JD, Johnson M. Quick-freeze/deep-etch visualization of age-related lipid accumulation in Bruch’s membrane. Invest Ophthalmol Vis Sci. 2003;44:1753–1759.[Abstract/Free Full Text]
41. Wilson GJ, Courtman DW, Klement P, Lee JM, Yeger H. Acellular matrix: a biomaterials approach for coronary artery bypass and heart valve replacement. Ann Thorac Surg. 1995;60:S353–S358.
42. Franzblau C, Schmid K, Faris B, et al. The interaction of collagen with alpha1-acid glycoprotein. Biochim Biophys Acta. 1976;427:302–314.[Medline][Order article via Infotrieve]
43. Han S, Tanzer ML. Collagen cross-linking: purification of lysyl oxidase in solvents containing nonionic detergents. J Biol Chem. 1979;254:10438–10442.[Free Full Text]
44. Del Priore LV, Geng L, Tezel TH, Kaplan HJ. Extracellular matrix ligands promote RPE attachment to inner Bruch’s membrane. Curr Eye Res. 2002;25:79–89.[CrossRef][ISI][Medline][Order article via Infotrieve]
45. Scott JE, Thomlinson AM. The structure of interfibrillar proteoglycan bridges (shape modules’) in extracellular matrix of fibrous connective tissues and their stability in various chemical environments. J Anat. 1998;192:391–405.
46. Pauleikhoff D, Sheraidah G, Marshall J, Bird AC, Wessing A. Biochemical and histochemical analysis of age related lipid deposits in Bruch’s membrane (in German). Ophthalmologe. 1994;91:730–734.[Medline][Order article via Infotrieve]
47. Takami Y, Matsuda T, Yoshitake M, Hanumadass M, Walter RJ. Dispase/detergent treated dermal matrix as a dermal substitute. Burns. 1996;22:182–190.[CrossRef][ISI][Medline][Order article via Infotrieve]
48. Haas R, Culp LA. Binding of fibronectin to gelatin and heparin: effect of surface denaturation and detergents. FEBS Lett. 1984;174:279–283.[CrossRef][ISI][Medline][Order article via Infotrieve]
49. Caglio S, Chiari M, Righetti PG. Gel polymerization in detergents: conversion efficiency of methylene blue vs. persulfate catalysis, as investigated by capillary zone electrophoresis. Electrophoresis. 1994;15:209–214.[CrossRef][ISI]