Early attachment of uncultured retinal pigment epithelium from aged donors onto Bruch's membrane explants

Regenerative treatment of ophthalmic diseases with stem cells: Principles, progress, and challenges

Background

Degenerate eye disorders, such as glaucoma, cataracts and age-related macular degeneration (AMD), are prevalent causes of blindness and visual impairment worldwide. Other eye disorders, including limbal stem cell deficiency (LSCD), dry eye diseases (DED), and retinitis pigmentosa (RP), result in symptoms such as ocular discomfort and impaired visual function, significantly impacting quality of life. Traditional therapies are limited, primarily focus on delaying disease progression, while emerging stem cell therapy directly targets ocular tissues, aiming to restore ocular function by reconstructing ocular tissue.

Main text

The utilization of stem cells for the treatment of diverse degenerative ocular diseases is becoming increasingly significant, owing to the regenerative and malleable properties of stem cells and their functional cells. Currently, stem cell therapy for ophthalmopathy involves various cell types, such as embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), mesenchymal stem cells (MSCs), and retinal progenitor cells (RPCs). In the current article, we will review the current progress regarding the utilization of stem cells for the regeneration of ocular tissue covering key eye tissues from the cornea to the retina. These therapies aim to address the loss of functional cells, restore damaged ocular tissue and or in a paracrine-mediated manner. We also provide an overview of the ocular disorders that stem cell therapy is targeting, as well as the difficulties and opportunities in this field.

Conclusions

Stem cells can not only promote tissue regeneration but also release exosomes to mitigate inflammation and provide neuroprotection, making stem cell therapy emerge as a promising approach for treating a wide range of eye disorders through multiple mechanisms.

Bioengineering approaches for modelling retinal pathologies of the outer blood-retinal barrier

Alterations of the junctional complex of the outer blood-retinal barrier (oBRB), which is integrated by the close interaction of the retinal pigment epithelium, the Bruch's membrane, and the choriocapillaris, contribute to the loss of neuronal signalling and subsequent vision impairment in several retinal inflammatory disorders such as age-related macular degeneration and diabetic retinopathy. Reductionist approaches into the mechanisms that underlie such diseases have been hindered by the absence of adequate in vitro models using human cells to provide the 3D dynamic architecture that enables expression of the in vivo phenotype of the oBRB. Conventional in vitro cell models are based on 2D monolayer cellular cultures, unable to properly recapitulate the complexity of living systems. The main drawbacks of conventional oBRB models also emerge from the cell sourcing, the lack of an appropriate Bruch's membrane analogue, and the lack of choroidal microvasculature with flow. In the last years, the advent of organ-on-a-chip, bioengineering, and stem cell technologies is providing more advanced 3D models with flow, multicellularity, and external control over microenvironmental properties. By incorporating additional biological complexity, organ-on-a-chip devices can mirror physiologically relevant properties of the native tissue while offering additional set ups to model and study disease. In this review we first examine the current understanding of oBRB biology as a functional unit, highlighting the coordinated contribution of the different components to barrier function in health and disease. Then we describe recent advances in the use of pluripotent stem cells-derived retinal cells, Bruch's membrane analogues, and co-culture techniques to recapitulate the oBRB. We finally discuss current advances and challenges of oBRB-on-a-chip technologies for disease modelling.

New Prospects for Retinal Pigment Epithelium Transplantation

ABSTRACT

Retinal pigment epithelium (RPE) transplants rescue photoreceptors in selected animal models of retinal degenerative disease. Early clinical studies of RPE transplants as treatment for age-related macular degeneration (AMD) included autologous and allogeneic transplants of RPE suspensions and RPE sheets for atrophic and neovascular complications of AMD. Subsequent studies explored autologous RPE-Bruch membrane-choroid transplants in patients with neovascular AMD with occasional marked visual benefit, which establishes a rationale for RPE transplants in late-stage AMD. More recent work has involved transplantation of autologous and allogeneic stem cell-derived RPE for patients with AMD and those with Stargardt disease. These early-stage clinical trials have employed RPE suspensions and RPE monolayers on biocompatible scaffolds. Safety has been well documented, but evidence of efficacy is variable. Current research involves development of better scaffolds, improved modulation of immune surveillance, and modification of the extracellular milieu to improve RPE survival and integration with host retina.

Retina stem cells, hopes and obstacles

Retinal degeneration is a major contributor to visual dysfunction worldwide. Although it comprises several eye diseases, loss of retinal pigment epithelial (RPE) and photoreceptor cells are the major contributors to their pathogenesis. Early therapies included diverse treatments, such as provision of anti-vascular endothelial growth factor and many survival and trophic factors that, in some cases, slow down the progression of the degeneration, but do not effectively prevent it. The finding of stem cells (SC) in the eye has led to the proposal of cell replacement strategies for retina degeneration. Therapies using different types of SC, such as retinal progenitor cells (RPCs), embryonic SC, pluripotent SCs (PSCs), induced PSCs (iPSCs), and mesenchymal stromal cells, capable of self-renewal and of differentiating into multiple cell types, have gained ample support. Numerous preclinical studies have assessed transplantation of SC in animal models, with encouraging results. The aim of this work is to revise the different preclinical and clinical approaches, analyzing the SC type used, their efficacy, safety, cell attachment and integration, absence of tumor formation and immunorejection, in order to establish which were the most relevant and successful. In addition, we examine the questions and concerns still open in the field. The data demonstrate the existence of two main approaches, aimed at replacing either RPE cells or photoreceptors. Emerging evidence suggests that RPCs and iPSC are the best candidates, presenting no ethical concerns and a low risk of immunorejection. Clinical trials have already supported the safety and efficacy of SC treatments. Serious concerns are pending, such as the risk of tumor formation, lack of attachment or integration of transplanted cells into host retinas, immunorejection, cell death, and also ethical. However, the amazing progress in the field in the last few years makes it possible to envisage safe and effective treatments to restore vision loss in a near future.

Bio-inspired human in vitro outer retinal models: Bruch's membrane and its cellular interactions

Retinal degenerative disorders, such as age-related macular degeneration (AMD), are one of the leading causes of blindness worldwide, however, treatments to completely stop the progression of these debilitating conditions are non-existent. Researchers require sophisticated models that can accurately represent the native structure of human retinal tissue to study these disorders. Current in vitro models used to study the retina are limited in their ability to fully recapitulate the structure and function of the retina, Bruch's membrane and the underlying choroid. Recent developments in the field of induced pluripotent stem cell technology has demonstrated the capability of retinal pigment epithelial cells to recapitulate AMD-like pathology. However, such studies utilise unsophisticated, bio-inert membranes to act as Bruch's membrane and support iPSC-derived retinal cells. This review presents a concise summary of the properties and function of the Bruch's membrane-retinal pigment epithelium complex, the initial pathogenic site of AMD as well as the current status for materials and fabrication approaches used to generate in vitro models of this complex tissue. Finally, this review explores required advances in the field of in vitro retinal modelling. STATEMENT OF SIGNIFICANCE: Retinal degenerative disorders such as age-related macular degeneration are worldwide leading causes of blindness. Previous attempts to model the Bruch's membrane-retinal pigment epithelial complex, the initial pathogenic site of age-related macular degeneration, have lacked the sophistication to elucidate valuable insights into disease mechanisms. Here we provide a detailed account of the morphological, physical and chemical properties of Bruch's membrane which may aid the fabrication of more sophisticated and physiologically accurate in vitro models of the retina, as well as various fabrication techniques to recreate this structure. This review also further highlights some recent advances in some additional challenging aspects of retinal tissue modelling including integrated fluid flow and photoreceptor alignment.

Interactions of the choroid, Bruch's membrane, retinal pigment epithelium, and neurosensory retina collaborate to form the outer blood-retinal-barrier

The three interacting components of the outer blood-retinal barrier are the retinal pigment epithelium (RPE), choriocapillaris, and Bruch's membrane, the extracellular matrix that lies between them. Although previously reviewed independently, this review integrates these components into a more wholistic view of the barrier and discusses reconstitution models to explore the interactions among them. After updating our understanding of each component's contribution to barrier function, we discuss recent efforts to examine how the components interact. Recent studies demonstrate that claudin-19 regulates multiple aspects of RPE's barrier function and identifies a barrier function whereby mutations of claudin-19 affect retinal development. Co-culture approaches to reconstitute components of the outer blood-retinal barrier are beginning to reveal two-way interactions between the RPE and choriocapillaris. These interactions affect barrier function and the composition of the intervening Bruch's membrane. Normal or disease models of Bruch's membrane, reconstituted with healthy or diseased RPE, demonstrate adverse effects of diseased matrix on RPE metabolism. A stumbling block for reconstitution studies is the substrates typically used to culture cells are inadequate substitutes for Bruch's membrane. Together with human stem cells, the alternative substrates that have been designed offer an opportunity to engineer second-generation culture models of the outer blood-retinal barrier.

Recent Advances in Retinal Stem Cell Therapy

Purpose of review

Progress in stem cell research for blinding diseases over the past decade is now being applied to patients with retinal degenerative diseases and soon perhaps, glaucoma. However, the field still has much to learn about the conversion of stem cells into various retinal cell types, and the potential delivery methods that will be required to optimize the clinical efficacy of stem cells delivered into the eye.

Recent findings

Recent groundbreaking human clinical trials have demonstrated both the opportunities and current limitations of stem cell transplantation for retinal diseases. New progress in developing in vitro retinal organoids, coupled with the maturation of bio-printing technology, and non-invasive high-resolution imaging have created new possibilities for repairing and regenerating the diseased retina and rigorously validating its clinical impact in vivo.

Summary

While promising progress is being made, meticulous clinical trials with cells derived using good manufacturing practice, novel surgical methods, and improved methods to derive all of the neuronal cell types present in the retina will be indispensable for developing stem cell transplantation as a paradigm shift for the treatment of blinding diseases.

Enhancing RPE Cell-Based Therapy Outcomes for AMD: The Role of Bruch's Membrane

Age-related macular degeneration (AMD) is the leading cause of legal blindness in older people in the developed world. The disease involves damage to the part of the retina responsible for central vision. Degeneration of retinal pigment epithelial (RPE) cells, photoreceptors, and choriocapillaris may contribute to visual loss. Over the past decades, scientists and clinicians have tried to replace lost RPE cells in patients with AMD using cells from different sources. In recent years, advances in generating RPE cells from stem cells have been made and clinical trials are currently evaluating the safety and efficiency of replacing the degenerated RPE cell layer with stem cell-derived RPE cells. However, the therapeutic success of transplantation of stem cell-derived RPE cells may be limited unless the transplanted cells can adhere and survive in the long term in the diseased eye. One hallmark of AMD is the altered extracellular environment of Bruch's membrane to which the grafted cells have to adhere. Here, we discuss recent approaches to overcome the inhibitory environment of the diseased eye and to enhance the survival rate of transplanted RPE cells. Our aim is to highlight novel approaches that may have the potential to improve the efficacy of RPE transplantation for AMD in the future.

Stem cells in retinal regeneration: past, present and future

Stem cell therapy for retinal disease is under way, and several clinical trials are currently recruiting. These trials use human embryonic, foetal and umbilical cord tissue-derived stem cells and bone marrow-derived stem cells to treat visual disorders such as age-related macular degeneration, Stargardt's disease and retinitis pigmentosa. Over a decade of analysing the developmental cues involved in retinal generation and stem cell biology, coupled with extensive surgical research, have yielded differing cellular approaches to tackle these retinopathies. Here, we review these various stem cell-based approaches for treating retinal diseases and discuss future directions and challenges for the field.

Ocular epithelial transplantation: current uses and future potential

Visual loss may be caused by a variety of ocular diseases and places a significant burden on society. Replacing or regenerating epithelial structures in the eye has been demonstrated to recover visual loss in a number of such diseases. Several types of cells (e.g., embryonic stem cells, adult stem/progenitor/differentiated epithelial cells and induced pluripotent cells) have generated much interest and research into their potential in restoring vision in a variety of conditions: from ocular surface disease to age-related macular degeneration. While there has been some success in clinical transplantation of conjunctival and particularly corneal epithelium utilizing ocular stem cells, in particular, from the limbus, the replacement of the retinal pigment epithelium by utilizing stem cell sources has yet to reach the clinic. Advances in our understanding of all of these cell types, their differentiation and subsequent optimization of culture conditions and development of suitable substrates for their transplantation will enable us to overcome current clinical obstacles. This article addresses the current status of knowledge concerning the biology of stem cells, their progeny and the use of differentiated epithelial cells to replace ocular epithelial cells. It will highlight the clinical outcomes to date and their potential for future clinical use.

Comparison of FRPE and human embryonic stem cell-derived RPE behavior on aged human Bruch's membrane

PURPOSE

To compare RPE derived from human embryonic stem cells (hES-RPE) and fetal RPE (fRPE) behavior on human Bruch's membrane (BM) from aged and AMD donors.

METHODS

hES-RPE of 3 degrees of pigmentation and fRPE were cultured on BM explants. Explants were assessed by light, confocal, and scanning electron microscopy. Integrin mRNA levels were determined by real-time polymerase chain reaction studies. Secreted proteins in media were analyzed by multiplex protein analysis after 48-hour exposure at culture day 21.

RESULTS

hES-RPE showed impaired initial attachment compared to fRPE; pigmented hES-RPE showed nuclear densities similar to fRPE at day 21. At days 3 and 7, hES-RPE resurfaced BM to a limited degree, showed little proliferation (Ki-67), and partial retention of RPE markers (MITF, cytokeratin, and CRALBP). TUNEL-positive nuclei were abundant at day 3. fRPE exhibited substantial BM resurfacing at day 3 with decreased resurfacing at later times. Most fRPE retained RPE markers. Ki-67-positive nuclei decreased with time in culture. TUNEL staining was variable. Increased integrin mRNA expression did not appear to affect cell survival at day 21. hES-RPE and fRPE protein secretion was similar on equatorial BM except for higher levels of nerve growth factor and thrombospondin-2 (TSP2) by hES-RPE. On submacular BM, fRPE secreted more vascular endothelial growth factor (VEGF), brain-derived neurotrophic factor, and platelet-derived growth factor; hES-RPE secreted more TSP2.

CONCLUSIONS

Although pigmented hES-RPE and fRPE resurfaced aged and AMD BM to a similar, limited degree at day 21, cell behavior at earlier times was markedly dissimilar. Differences in protein secretion may indicate that hES-RPE may not function identically to native RPE after seeding on aged or AMD BM.

Retinal pigment epithelium cell alignment on nanostructured collagen matrices

We investigated attachment and migration of human retinal pigment epithelial cells (primary, SV40-transfected and ARPE-19) on nanoscopically defined, two-dimensional matrices composed of parallel-aligned collagen type I fibrils. These matrices were used non-cross-linked (native) or after riboflavin/UV-A cross-linking to study cell attachment and migration by time-lapse video microscopy. Expression of collagen type I and IV, MMP-2 and of the collagen-binding integrin subunit α(2) were examined by immunofluorescence and Western blotting. SV40-RPE cells quickly attached to the nanostructured collagen matrices and aligned along the collagen fibrils. However, they disrupted both native and cross-linked collagen matrices within 5 h. Primary RPE cells aligned more slowly without destroying either native or cross-linked substrates. Compared to primary RPE cells, ARPE-19 cells showed reduced alignment but partially disrupted the matrices within 20 h after seeding. Expression of the collagen type I-binding integrin subunit α(2) was highest in SV40-RPE cells, lower in primary RPE cells and almost undetectable in ARPE-19 cells. Thus, integrin α(2) expression levels directly correlated with the degree of cell alignment in all examined RPE cell types. Specific integrin subunit α(2)-mediated matrix binding was verified by preincubation with an α(2)-function-blocking antibody, which impaired cell adhesion and alignment to varying degrees in primary and SV40-RPE cells. Since native matrices supported extended and directed primary RPE cell growth, optimizing the matrix production procedure may in the future yield nanostructured collagen matrices serving as transferable cell sheet carriers.

Cell-deposited matrix improves retinal pigment epithelium survival on aged submacular human Bruch's membrane

PURPOSE

To determine whether resurfacing submacular human Bruch's membrane with a cell-deposited extracellular matrix (ECM) improves retinal pigment epithelial (RPE) survival.

METHODS

Bovine corneal endothelial (BCE) cells were seeded onto the inner collagenous layer of submacular Bruch's membrane explants of human donor eyes to allow ECM deposition. Control explants from fellow eyes were cultured in medium only. The deposited ECM was exposed by removing BCE. Fetal RPE cells were then cultured on these explants for 1, 14, or 21 days. The explants were analyzed quantitatively by light microscopy and scanning electron microscopy. Surviving RPE cells from explants cultured for 21 days were harvested to compare bestrophin and RPE65 mRNA expression. Mass spectroscopy was performed on BCE-ECM to examine the protein composition.

RESULTS

The BCE-treated explants showed significantly higher RPE nuclear density than did the control explants at all time points. RPE expressed more differentiated features on BCE-treated explants than on untreated explants, but expressed very little mRNA for bestrophin or RPE65. The untreated young (<50 years) and African American submacular Bruch's membrane explants supported significantly higher RPE nuclear densities (NDs) than did the Caucasian explants. These differences were reduced or nonexistent in the BCE-ECM-treated explants. Proteins identified in the BCE-ECM included ECM proteins, ECM-associated proteins, cell membrane proteins, and intracellular proteins.

CONCLUSIONS

Increased RPE survival can be achieved on aged submacular human Bruch's membrane by resurfacing the latter with a cell-deposited ECM. Caucasian eyes seem to benefit the most, as cell survival is the worst on submacular Bruch's membrane in these eyes.

Integrin activation or alpha 9 expression allows retinal pigmented epithelial cell adhesion on Bruch's membrane in wet age-related macular degeneration

Retinal pigment epithelial cell malfunction is a causative feature of age-related macular degeneration, and transplantation of new retinal pigment epithelial cells is an attractive strategy to prevent further progression and visual loss. However, transplants have shown limited efficacy, mainly because transplanted cells fail to adhere and migrate onto pathological Bruch's membrane. Adhesion to Bruch's membrane is integrin-mediated. Ageing of Bruch's membrane leads to a decline in integrin ligands and, added to this, wet age-related macular degeneration leads to upregulation of anti-adhesive molecules such as tenascin-C. We have therefore investigated whether manipulation of integrin function in retinal pigment epithelial cells can restore their adhesion and migration on wet age-related macular degeneration-damaged Bruch's membrane. Using spontaneously immortalized human retinal pigment epithelial cells (adult retinal pigment epithelium-19), we show that adhesion and migration on the Bruch's membrane components is integrin-dependent and enhanced by integrin-activating agents manganese and TS2/16. These allowed cells to adhere and migrate on low concentrations of ligand, as would be found in aged Bruch's membrane. We next developed a method for stripping cells from Bruch's membrane so that adhesion and migration assays can be performed on its surface. Integrin activation had a moderate effect on enhancing retinal pigmented epithelial cell adhesion and migration on normal human and rat Bruch's membrane. However, on Bruch's membrane prepared from human wet age-related macular degeneration-affected eyes, adhesion was lower and integrin activation had a much greater effect. A candidate molecule for preventing retinal pigmented epithelial interaction with age-related macular degeneration-affected Bruch's membrane is tenascin-C which we confirm is present at high levels in wet age-related macular degeneration membrane. We show that tenascin-C is anti-adhesive for retinal pigmented epithelial cells, but after integrin activation, they can adhere and migrate on it using alphaVbeta3 integrin. Alternatively, we find that transduction of retinal pigmented epithelial cells with alpha9 integrin, a tenascin-C-binding integrin, led to a large increase in alpha9beta1-mediated adhesion and migration on tenascin-C. Both expression of alpha9 integrin and integrin activation greatly enhanced the ability of retinal pigment epithelial cells to adhere to tenascin-rich wet age-related macular degeneration-affected Bruch's membranes. Our results suggest that manipulation of retinal pigment epithelial cell integrins through integrin activating strategies, or expression of new integrins such as alpha9, could be effective in improving the efficacy of retinal pigment epithelial cell transplantation in wet age-related macular degeneration-affected eyes.

ACTA-EVER lecture 2007. The retinal pigment epithelium: friend or foe?

The retinal pigment epithelium (RPE) plays an important role in the physiology and pathophysiology of the vertebrate retina. The RPE absorbs fluid from the retinal extracellular space, via a proton-lactate-water co-transport mechanism located in the apical membrane of the epithelium. This mechanism can account for the apparent capability of the RPE to absorb water against an osmotic gradient. RPE cells participate in retinal wound healing. We have created a porcine model of experimental choroidal neovascularization (CNV). In this model, the CNV eventually becomes enveloped by seemingly proliferating RPE cells. By means of 5-bromo-2-deoxyuridine (BrdU) labelling, we studied the proliferation of RPE cells in the porcine eye after experimental posterior pole injury. Surprisingly, we found that only the peripheral RPE cells incorporated the BrdU label, indicating that central injury elicits peripheral RPE proliferation. This might suggest the existence of a peripheral pool of RPE stem cells. RPE cell proliferation plays a role in the pathological wound healing known as proliferative vitreoretinopathy. Antiproliferative agents have been tried to treat this condition but with little success so far. We report on a drug delivery system under development where a prodrug of the antimetabolite 5-fluoro-uracil (5-FU) is suspended in the silicone oil used as a surgical device in the treatment of proliferative vitreoretinopathy (PVR). The theoretical advantage of this approach is that it allows for long contact times between therapeutic, and non-toxic, concentrations of 5-FU and the RPE.

RPE transplantation and its role in retinal disease

Retinal pigment epithelial (RPE) transplantation aims to restore the subretinal anatomy and re-establish the critical interaction between the RPE and the photoreceptor, which is fundamental to sight. The field has developed over the past 20 years with advances coming from a large body of animal work and more recently a considerable number of human trials. Enormous progress has been made with the potential for at least partial restoration of visual function in both animal and human clinical work. Diseases that have been treated with RPE transplantation demonstrating partial reversal of vision loss include primary RPE dystrophies such as the merTK dystrophy in the Royal College of Surgeons (RCS) rat and in humans, photoreceptor dystrophies as well as complex retinal diseases such as atrophic and neovascular age-related macular degeneration (AMD). Unfortunately, in the human trials the visual recovery has been limited at best and full visual recovery has not been demonstrated. Autologous full-thickness transplants have been used most commonly and effectively in human disease but the search for a cell source to replace autologous RPE such as embryonic stem cells, marrow-derived stem cells, umbilical cord-derived cells as well as immortalised cell lines continues. The combination of cell transplantation with other modalities of treatment such as gene transfer remains an exciting future prospect. RPE transplantation has already been shown to be capable of restoring the subretinal anatomy and improving photoreceptor function in a variety of retinal diseases. In the near future, refinements of current techniques are likely to allow RPE transplantation to enter the mainstream of retinal therapy at a time when the treatment of previously blinding retinal diseases is finally becoming a reality.

Adult retinal pigment epithelial transplantation in exudative age-related macular degeneration

PURPOSE

To improve visual function by retinal pigment epithelial (RPE) cell transplantation and systemic immunosuppression at the time of surgical removal of subfoveal choroidal neovascularization in exudative age-related macular degeneration (AMD).

DESIGN

An interventional case series of RPE transplantation in exudative AMD.

METHODS

Twelve patients (one eye only) underwent subfoveal membranectomy with transplantation of a sheet of adult human allogeneic RPE cells at a single institution and were followed for one year. Eligibility criteria included age >60, best-corrected acuity < or =20/63 and subfoveal neovascularization < or =9 disk areas on preoperative fluorescein angiography. All patients were started on triple immunosuppression postoperatively. The primary outcome measure was best-corrected vision, with contrast sensitivity and reading speed as secondary outcome measures.

RESULTS

The best-corrected visual acuity (P = .085), contrast sensitivity (P = .204), and the reading speed (P = .077) did not change significantly at one year compared with preoperative values. Transplants showed no signs of rejection in patients who were able to continue the immunosuppressants for six months. Postoperative surgical complications included cataract progression requiring surgery (three of eight phakic eyes), retinal detachment (three eyes), intraoperative retinal breaks (two eyes), and macular pucker (two eyes). None of the patients developed cystoid macular edema on postoperative fluorescein angiography or postoperative inflammation.

CONCLUSIONS

A sheet of adult human allogeneic RPE can be transplanted into the subretinal space in AMD patients at the time of subfoveal membranectomy. Systemic immune suppression appeared to prevent rejection of the transplanted tissue, but did not lead to an improvement in visual function.

Transplantation of the RPE in AMD

The retinal pigment epithelium (RPE) maintains retinal function as the metabolic gatekeeper between photoreceptors (PRs) and the choriocapillaries. The RPE and Bruch's membrane (BM) suffer cumulative damage over lifetime, which is thought to induce age-related macular degeneration (AMD) in susceptible individuals. Unlike palliative pharmacologic treatments, replacement of the RPE has a curative potential for AMD. This article reviews mechanisms leading to RPE dysfunction in aging and AMD, laboratory studies on RPE transplantation, and surgical techniques used in AMD patients. Future strategies using ex vivo steps prior to transplantation, BM prosthetics, and stem cell applications are discussed. The functional peculiarity of the macular region, epigenetic phenomena leading to an age-related shift in protein expression, along with the accumulation of lipofuscin may affect the metabolism in the central RPE. Thickening of BM with age decreases its hydraulic conductivity. Drusen are deposits of extracellular material and formed in part by activation of the alternative complement pathway in individuals carrying a mutant allele of complement factor H. AMD likely represents an umbrella term for a disease entity with multifactorial etiology and manifestations. Presently, a slow progressing (dry) non-neovascular atrophic form and a rapidly blinding neovascular (wet) form are discerned. No therapy is currently available for the former, while RPE transplantation and promising (albeit non-causal) anti-angiogenic therapies are available for the latter. The potential of RPE transplantation was demonstrated in animal models. Rejection of allogeneic homologous transplants in patients focused further studies on autologous sources. In vitro studies elucidated cell adhesion and wound healing mechanisms on aged human BM. Currently, autologous RPE, harvested from the midperiphery, is being transplanted as a cell suspension or a patch of RPE and choroid in AMD patients. These techniques have been evaluated from several groups. Autologous RPE transplants may have the disadvantage of carrying the same genetic information that may have led to AMD manifestation. An intermittent culturing step would allow for in vitro therapy of the RPE, its rejuvenation and prosthesis of BM to improve the success RPE transplants. Recent advances in stem cell biology when combined with lessons learned from studies of RPE transplantation are intriguing future therapeutic modalities for AMD patients.

Impaired RPE survival on aged submacular human Bruch's membrane

Resurfacing of diseased or iatrogenically damaged Bruch's membrane with healthy retinal pigment epithelium (RPE) has been proposed as adjunctive treatment for age-related macular degeneration (AMD). The purpose of this study was to determine whether cultured fetal human RPE cells can attach and differentiate on aged submacular human Bruch's membrane. Bruch's membrane was debrided to expose native RPE basement membrane, the superficial inner collagenous layer directly below the RPE basement membrane, or the deep inner collagenous layer. These are three surfaces that transplanted RPE cells will encounter in situ. Approximately 3146 cultured fetal RPE cells mm(-2) were seeded onto these three surfaces and grown in organ culture for 1, 7, or 14 days. Explants were bisected and examined histologically or analyzed with a scanning electron microscope. RPE nuclear density was measured on stained sections. Morphology and cell density were compared to cells seeded onto bovine corneal endothelial cell-extracellular matrix (BCE-ECM). In situ submacular RPE nuclear density was also measured in tissue sections of donor eyes ranging from 18 weeks gestation to 88 years of age to determine the effect of age on RPE density. Compared to cells seeded onto BCE-ECM at similar density, RPE cell coverage and cellular morphology on aged submacular human Bruch's membrane was poor at all time points. In contrast to cells on BCE-ECM, RPE cell density on Bruch's membrane decreased with time. In general, cell morphology on all three Bruch's membrane surfaces worsened by day-7 compared to day-1. Although some cells were more pigmented on RPE basement membrane and the deep inner collagenous layer at day-7, poor cellular morphology indicated the remaining cells were not well differentiated. At day-14, the cells were uniform and cuboidal on BCE-ECM, with cell density similar to that at day-7 and similar to in situ density of young donors (<age 30 years). The morphology of cells on Bruch's membrane was variable, and the nuclear density declined over time. A Bruch's membrane explant from a donor with large soft drusen showed the poorest resurfacing at day-7 in organ culture. These data indicate that aged submacular human Bruch's membrane does not support transplanted RPE survival and differentiation readily. The formation of localized RPE defects, cell death, and worsening cellular morphology on aged Bruch's membrane indicates that modification of Bruch's membrane may be necessary to prevent graft failure in AMD patients receiving RPE transplants.

Amniotic membrane maintains the phenotype of rabbit retinal pigment epithelial cells in culture

The success of surgical removal of choroidal neovascularisation followed by transplantation of autologous retinal pigment epithelial cells (RPE) for age-related macular degeneration (ARMD) may be limited by damage in Bruch's membrane. We investigated whether amniotic membrane (AM) might be used as an alternative basement membrane-containing matrix to support RPE growth and differentiation. Primary RPE plastic cultures were established from freshly enucleated Dutch belted rabbit eyes in DMEM/F12 containing 0.1 mM Ca(++) and 10% dialysed FBS. Upon subconfluence, cells were subcultured at 5000-9000 cells cm(-2) in the above-mentioned culture medium on intact AM (iAM), epithelially denuded AM (dAM) or plastic. After confluence, the Ca(++) concentration in the medium was increased to 1.8 mm for 4 weeks. Growth and morphology were monitored by phase contrast microscopy, and the phenotype by immunostaining with antibodies against cytokeratin 18, tight junction protein ZO-1, and RPE65 protein, and by transepithelial resistance (TER) measurement. Immunostaining to cytokeratin 18 confirmed the epithelial origin of isolated cells in both primary culture and subcultures. Compared to plastic cultures, RPE increased pigmentation within 24 hr after seeding on AM, with iAM being more pronounced than dAM. RPE adopted a hexagonal epithelial phenotype with more organised pigmentation, strong expression of ZO-1 and RPE65, and a significantly higher TER 4 weeks after Ca(++) switch on dAM. Our results indicate that AM may be used as a basement membrane-containing matrix to maintain RPE phenotype in vitro, and may facilitate subsequent transplantation to treat ARMD.

Short-term study of retinal pigment epithelium sheet transplants onto Bruch's membrane

The purpose of this study is to investigate the survival and behaviour of retinal pigment epithelium sheets transplanted onto hydraulically debrided Bruch's membrane. Uncultured retinal pigment epithelium sheets obtained from male cats and sandwiched between two gelatin sheets were transplanted onto the tapetal area of female cats after native retinal pigment epithelium was debrided. For controls, the gelatin carrier was transplanted after debridement. Each transplant or control specimen was analyzed histologically and immunohistochemically. Transplanted male retinal pigment epithelial cells were identified by in situ labelling of the cat Y chromosome. Over half of the transplants appeared as retinal pigment epithelium multilayers in the subretinal space. Retinal pigment epithelium pigment dispersion into the subretinal space was seen in most of the transplants, and retinal pigment epithelium pigment infiltration into the neural retina was seen in all 7-day survival transplants. A few condensed darkly stained retinal pigment epithelium nuclei and Terminal Transferase dUTP Nick End Labelling-positive retinal pigment epithelium cells were observed in all transplants. Cellular retinaldehyde-binding protein was present up to day-7 in most transplanted RPE cells. In both transplant and control specimens, the antibody against the Ki-67 nuclear antigen labelled a few retinal pigment epithelium cells at day-3. Terminal Transferase dUTP Nick End Labelling-positive outer nuclear layer nuclei were most frequently observed at day-1 but were much less frequent at day-3 in both transplants and controls. The survival and effectiveness of retinal pigment epithelium sheet transplants appeared similar to the retinal pigment epithelium microaggregates transplants conducted previously in this model.

Amniotic membrane as support for human retinal pigment epithelium (RPE) cell growth

PURPOSE

The aim of this work was to culture human retinal pigment epithelium (hRPE) cells over human amniotic membrane (hAM). Human AM was studied for its viability as an adequate support for transplantation of an hRPE cell monolayer with preserved cell polarity to the subretinal space.

METHODS

Human AM was obtained from pregnant women during caesarean section. The hAM was sectioned and the pieces were fixed to culture dishes. Human RPE cells were cultured from adult corneal donors and were seeded over hAM. Phase-contrast photographs were obtained. Selected specimens were processed by transmission electronic microscopy (TEM).

RESULTS

The attachment and growth of hRPE cells over hAM was observed. Human RPE cells constituted tight colonies that maintained epithelial phenotype. Using TEM, we identified a monolayer of hRPE cells, with cuboidal to spheroidal morphology. These cells showed integration with the substrate and cell-cell contacts were detected.

CONCLUSION

Amniotic membrane may be a suitable substrate for hRPE growth. Further studies are required in order to determine the viability of hRPE on hAM in the subretinal space.

Autologous transplantation of retinal pigment epithelium after mechanical debridement of Bruch's membrane

PURPOSE

To determine whether transplantation of autologous retinal pigment epithelium (RPE) will prevent atrophy of the choriocapillaris and loss of photoreceptors in an area in which the RPE has been mechanically debrided from Bruch's membrane.

METHODS

Abrasive debridement of RPE was performed with a metal cannula after localized retinal bleb detachments in two separate areas of the rabbit retina. The RPE cell suspension aspirated from one of the debridement sites was transplanted to the other. The debridement-only site served as control. The transplant and control sites were evaluated after 30 days by color fundus photography, fluorescein angiography, light microscopy and transmission electron microscopy.

RESULTS

Compared with debridement only, debridement plus transplantation of RPE resulted in more complete repopulation of the bare Bruch's membrane surface with relative preservation of choriocapillaris and photoreceptors.

CONCLUSION

Autologous transplantation of RPE onto an abrasively debrided Bruch's membrane decreases choriocapillaris atrophy and photoreceptor loss.