Impaired RPE survival on aged submacular human Bruch's membrane

Stem cell-based treatment in geographic atrophy: promises and pitfalls

Geographic atrophy is a common and untreatable form of advanced age-related macular degeneration. The degeneration primarily affects the retinal pigment epithelium and photoreceptors of the retina and their restoration by cell transplantation seems attractive. Recently, a patient with geographic atrophy was the first human to receive cells derived from human embryonic stem cells. In this short review, the rationale, potential and obstacles for stem cell-derived therapy in geographic atrophy are discussed.

Current treatment limitations in age-related macular degeneration and future approaches based on cell therapy and tissue engineering

Age-related macular degeneration (AMD) is the leading cause of blindness in the Western world. With an ageing population, it is anticipated that the number of AMD cases will increase dramatically, making a solution to this debilitating disease an urgent requirement for the socioeconomic future of the European Union and worldwide. The present paper reviews the limitations of the current therapies as well as the socioeconomic impact of the AMD. There is currently no cure available for AMD, and even palliative treatments are rare. Treatment options show several side effects, are of high cost, and only treat the consequence, not the cause of the pathology. For that reason, many options involving cell therapy mainly based on retinal and iris pigment epithelium cells as well as stem cells are being tested. Moreover, tissue engineering strategies to design and manufacture scaffolds to mimic Bruch's membrane are very diverse and under investigation. Both alternative therapies are aimed to prevent and/or cure AMD and are reviewed herein.

Human embryonic stem cell applications for retinal degenerations

Loss of vision in severe retinal degenerations often is a result of photoreceptor cell or retinal pigment epithelial cell death or dysfunction. Cell replacement therapy has the potential to restore useful vision for these individuals especially after they have lost most or all of their light-sensing cells in the eye. A reliable, well-characterized source of retinal cells will be needed for replacement purposes. Human embryonic stem cells (ES cells) can provide an unlimited source of replacement retinal cells to take over the function of lost cells in the eye. The author's intent for this review is to provide an historical overview of the field of embryonic stem cells with relation to the retina. The review will provide a quick primer on key pathways involved in the development of the neural retina and RPE followed by a discussion of the various protocols out in the literature for generating these cells from non-human and human embryonic stem cells and end with in vivo application of ES cell-derived photoreceptors and RPE cells.

Characterization of the effects of retinal pigment epithelium-conditioned media on porcine and aged human retina

BACKGROUND

Retinal pigment epithelium (RPE) cells produce neurotrophic factors that rescue photoreceptors from degeneration. Previously, we showed that conditioned medium (CM) from fetal vs adult RPE cells resulted in significantly better porcine retinal preservation, and possessed significantly higher levels of hepatocyte growth factor (HGF) and pigment epithelium-derived factor (PEDF). This study aimed to further describe the effects of human fetal RPE-CM on porcine and aged human retina, and to characterize its effects biochemically.

METHODS

RPE-CM was harvested from passage-2 fetal RPE, 7 days after passage, 24-hours after exposure to basal medium. After culture in RPE-CM, porcine retinal morphology was assessed with confocal microscopy. The effects of RPE-CM on porcine and aged human retina survival were assessed by cytotoxicity and apoptosis biochemical assays. To characterize RPE-CM biochemically, effects of heating, digesting with proteinase-K, dilution, concentration, and fractionation were tested. Recombinant proteins and neutralizing antibodies were used to identify proteins that might contribute to the salutary effects of RPE-CM on porcine retina.

RESULTS

Culturing porcine retina in RPE-CM significantly preserved outer nuclear layer width and the number of nuclei in cross-section, and significantly decreased photoreceptor axon retraction. RPE-CM decreased porcine retinal death by 17-34 % (p<0.05) compared to basal medium. Human retina from age-related macular degeneration (AMD) and non-AMD donors responded similarly after culture in RPE-CM. Heating, proteinase-K digestion, and dilution significantly diminished RPE-CM-mediated preservation of porcine retina, whereas concentrating RPE-CM significantly enhanced its preservation of porcine retina. Molecular cut filtration identified retina-preserving activity in the 3-100 kDa filtrate. PEDF or HGF at 90 % receptor occupancy significantly improved retinal preservation over 48 h of culture compared to basal medium. Neutralizing PEDF in RPE-CM decreased its ability to reduce retinal apoptosis by 23-27 % (p<0.05).

CONCLUSION

RPE-CM reduced biochemically and histologically measured degeneration in porcine retinae. This effect was concentration-dependent, and can be attributed to a protein component(s) in a 3-100 kDa molecular cut fraction. Human retina (including non-AMD and AMD Caucasian and non-AMD African-American) responds to culture in RPE-CM similarly to porcine retina. Receptor occupancy calculations and retinal viability data indicate that PEDF may be one of the components that contribute to retina preservation by RPE-CM.

Stem cells: a new paradigm for disease modeling and developing therapies for age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of blindness in people over age 55 in the U.S. and the developed world. This condition leads to the progressive impairment of central visual acuity. There are significant limitations in the understanding of disease progression in AMD as well as a lack of effective methods of treatment. Lately, there has been considerable enthusiasm for application of stem cell biology for both disease modeling and therapeutic application. Human embryonic stem cells and induced pluripotent stem cells (iPSCs) have been used in cell culture assays and in vivo animal models. Recently a clinical trial was approved by FDA to investigate the safety and efficacy of the human embryonic stem cell-derived retinal pigment epithelium (RPE) transplantation in sub-retinal space of patients with dry AMD These studies suggest that stem cell research may provide both insight regarding disease development and progression, as well as direction for therapeutic innovation for the millions of patients afflicted with AMD.

Adhesion failures determine the pattern of choroidal neovascularization in the eye: a computer simulation study

Choroidal neovascularization (CNV) of the macular area of the retina is the major cause of severe vision loss in adults. In CNV, after choriocapillaries initially penetrate Bruch's membrane (BrM), invading vessels may regress or expand (CNV initiation). Next, during Early and Late CNV, the expanding vasculature usually spreads in one of three distinct patterns: in a layer between BrM and the retinal pigment epithelium (sub-RPE or Type 1 CNV), in a layer between the RPE and the photoreceptors (sub-retinal or Type 2 CNV) or in both loci simultaneously (combined pattern or Type 3 CNV). While most studies hypothesize that CNV primarily results from growth-factor effects or holes in BrM, our three-dimensional simulations of multi-cell model of the normal and pathological maculae recapitulate the three growth patterns, under the hypothesis that CNV results from combinations of impairment of: 1) RPE-RPE epithelial junctional adhesion, 2) Adhesion of the RPE basement membrane complex to BrM (RPE-BrM adhesion), and 3) Adhesion of the RPE to the photoreceptor outer segments (RPE-POS adhesion). Our key findings are that when an endothelial tip cell penetrates BrM: 1) RPE with normal epithelial junctions, basal attachment to BrM and apical attachment to POS resists CNV. 2) Small holes in BrM do not, by themselves, initiate CNV. 3) RPE with normal epithelial junctions and normal apical RPE-POS adhesion, but weak adhesion to BrM (e.g. due to lipid accumulation in BrM) results in Early sub-RPE CNV. 4) Normal adhesion of RBaM to BrM, but reduced apical RPE-POS or epithelial RPE-RPE adhesion (e.g. due to inflammation) results in Early sub-retinal CNV. 5) Simultaneous reduction in RPE-RPE epithelial binding and RPE-BrM adhesion results in either sub-RPE or sub-retinal CNV which often progresses to combined pattern CNV. These findings suggest that defects in adhesion dominate CNV initiation and progression.

Advances in Retinal Tissue Engineering

Retinal degenerations cause permanent visual loss and affect millions world-wide. Current treatment strategies, such as gene therapy and anti-angiogenic drugs, merely delay disease progression. Research is underway which aims to regenerate the diseased retina by transplanting a variety of cell types, including embryonic stem cells, fetal cells, progenitor cells and induced pluripotent stem cells. Initial retinal transplantation studies injected stem and progenitor cells into the vitreous or subretinal space with the hope that these donor cells would migrate to the site of retinal degeneration, integrate within the host retina and restore functional vision. Despite promising outcomes, these studies showed that the bolus injection technique gave rise to poorly localized tissue grafts. Subsequently, retinal tissue engineers have drawn upon the success of bone, cartilage and vasculature tissue engineering by employing a polymeric tissue engineering approach. This review will describe the evolution of retinal tissue engineering to date, with particular emphasis on the types of polymers that have routinely been used in recent investigations. Further, this review will show that the field of retinal tissue engineering will require new types of materials and fabrication techniques that optimize the survival, differentiation and delivery of retinal transplant cells.

Mechanisms for countering oxidative stress and damage in retinal pigment epithelium

Clinical and experimental evidence supports that chronic oxidative stress is a primary contributing factor to numerous retinal degenerative diseases, such as age-related macular degeneration (AMD). Eyes obtained postmortem from AMD patients have extensive free radical damage to the proteins, lipids, DNA, and mitochondria of their retinal pigment epithelial (RPE) cells. In addition, several mouse models of chronic oxidative stress develop many of the pathological hallmarks of AMD. However, the extent to which oxidative stress is an etiologic component versus its involvement in disease progression remains a major unanswered question. Further, whether the primary target of oxidative stress and damage is photoreceptors or RPE cells, or both, is still unclear. In this review, we discuss the major functions of RPE cells with an emphasis on the oxidative challenges these cells encounter and the endogenous antioxidant mechanisms employed to neutralize the deleterious effects that such stresses can elicit if left unchecked.

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.

Integration of tight junctions and claudins with the barrier functions of the retinal pigment epithelium

The retinal pigment epithelium (RPE) forms the outer blood-retinal barrier by regulating the movement of solutes between the fenestrated capillaries of the choroid and the photoreceptor layer of the retina. Blood-tissue barriers use various mechanisms to accomplish their tasks including membrane pumps, transporters, and channels, transcytosis, metabolic alteration of solutes in transit, and passive but selective diffusion. The last category includes tight junctions, which regulate transepithelial diffusion through the spaces between neighboring cells of the monolayer. Tight junctions are extraordinarily complex structures that are dynamically regulated. Claudins are a family of tight junctional proteins that lend tissue specificity and selectivity to tight junctions. This review discusses how the claudins and tight junctions of the RPE differ from other epithelia and how its functions are modulated by the neural retina. Studies of RPE-retinal interactions during development lend insight into this modulation. Notably, the characteristics of RPE junctions, such as claudin composition, vary among species, which suggests the physiology of the outer retina may also vary. Comparative studies of barrier functions among species should deepen our understanding of how homeostasis is maintained in the outer retina. Stem cells provide a way to extend these studies of RPE-retinal interactions to human RPE.

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.

Nanomedicine in ophthalmology: the new frontier

PURPOSE

To review the fields of nanotechnology and nanomedicine as they relate to the development of treatments for vision-threatening disorders.

DESIGN

Perspective following literature review.

METHODS

Analysis of relevant publications in the peer-reviewed scientific literature.

RESULTS

Nanotechnology involves the creation and use of materials and devices at the size scale of intracellular structures and molecules and involves systems and constructs on the order of <100 nm. The aim of nanomedicine is the comprehensive monitoring, control, construction, repair, defense, and improvement of human biological systems at the molecular level, using engineered nanodevices and nanostructures, operating massively in parallel at the single cell level, ultimately to achieve medical benefit. The earliest impact of nanomedicine is likely to involve the areas of biopharmaceuticals (eg, drug delivery, drug discovery), implantable materials (eg, tissue regeneration scaffolds, bioresorbable materials), implantable devices (eg, intraocular pressure monitors, glaucoma drainage valves), and diagnostic tools (eg, genetic testing, imaging, intraocular pressure monitoring). Nanotechnology will bring about the development of regenerative medicine (ie, replacement and improvement of cells, tissues, and organs), ultrahigh-resolution in vivo imaging, microsensors and feedback devices, and artificial vision. "Regenerative nanomedicine," a new subfield of nanomedicine, uses nanoparticles containing gene transcription factors and other modulating molecules that allow for the reprogramming of cells in vivo.

CONCLUSIONS

Nanotechnology already has been applied to the measurement and treatment of different disease states in ophthalmology (including early- and late-stage disease), and many additional innovations will occur during the next century.

Modulation of MCT3 expression during wound healing of the retinal pigment epithelium

PURPOSE

MCT3 is a proton-coupled monocarboxylate transporter preferentially expressed in the basolateral membrane of the retinal pigment epithelium (RPE) and has been shown to play an important role in regulating pH and lactate concentrations in the outer retina. Decreased expression of MCT3 in response to trauma or disease could contribute to pathologic changes in the retina. The present study followed the expression of MCT3 after wounding and re-epithelialization of chick RPE explant and human fetal (hf) RPE cultures.

METHODS

Immunofluorescence microscopy and immunoblotting were performed to determine changes in MCT expression after scratch wounding and re-epithelialization of chick RPE/choroid explant cultures and hfRPE cell monolayers.

RESULTS

MCT3 expression and basolateral polarity were maintained in chick RPE/choroid explant cultures and hfRPE monolayers. Wounding resulted in loss of MCT3 and the upregulation of MCT4 expression in migrating cells at the edge of the wound. On re-epithelialization, MCT3 was detected in chick and hfRPE cells when cells became hexagonally packed and pigmented. However, in hfRPE cells, MCT4 was consistently expressed throughout the epithelial monolayer. RPE cells at the edges of chick explants and hfRPE cultures with a free edge expressed MCT4 but not MCT3.

CONCLUSIONS

Wounding of RPE monolayers resulted in dedifferentiation of the cells at the edge of the wound, as evidenced by a loss of MCT3 and increased MCT4 expression. Collectively, these findings suggest that both cell-cell and cell-substrate interactions are essential in directing and maintaining differentiation of the RPE and expression of MCT3.

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.

Induced pluripotent stem cell therapies for retinal disease

PURPOSE OF REVIEW

This review will discuss how recent advances with induced pluripotent stem (iPS) cells have brought the science of stem cell biology much closer to clinical application for patients with retinal degeneration.

RECENT FINDINGS

The ability to generate embryonic stem cells by reprogramming DNA taken from adult cells was demonstrated by the cloning of Dolly, the sheep, by somatic cell nuclear transfer, over 10 years ago. Recently, it has been shown that adult cells can be reprogrammed directly, without the need for a surrogate oocyte, through the generation of iPS cells. The method of reprogramming has since been optimized to avoid the use of retroviruses, making the process considerably safer. Last year, human iPS cells were isolated from an 80-year-old patient with neurodegenerative disease and differentiated into neurons in vitro.

SUMMARY

For stem cell therapies, the retina has the optimal combination of ease of surgical access, combined with an ability to observe transplanted cells directly through the clear ocular media. The question now is which retinal diseases are most appropriate targets for clinical trials using iPS cell approaches.

Long-term safety and function of RPE from human embryonic stem cells in preclinical models of macular degeneration

Assessments of safety and efficacy are crucial before human ESC (hESC) therapies can move into the clinic. Two important early potential hESC applications are the use of retinal pigment epithelium (RPE) for the treatment of age-related macular degeneration and Stargardt disease, an untreatable form of macular dystrophy that leads to early-onset blindness. Here we show long-term functional rescue using hESC-derived RPE in both the RCS rat and Elov14 mouse, which are animal models of retinal degeneration and Stargardt, respectively. Good Manufacturing Practice-compliant hESC-RPE survived subretinal transplantation in RCS rats for prolonged periods (>220 days). The cells sustained visual function and photoreceptor integrity in a dose-dependent fashion without teratoma formation or untoward pathological reactions. Near-normal functional measurements were recorded at >60 days survival in RCS rats. To further address safety concerns, a Good Laboratory Practice-compliant study was carried out in the NIH III immune-deficient mouse model. Long-term data (spanning the life of the animals) showed no gross or microscopic evidence of teratoma/tumor formation after subretinal hESC-RPE transplantation. These results suggest that hESCs could serve as a potentially safe and inexhaustible source of RPE for the efficacious treatment of a range of retinal degenerative diseases.

Genetic basis of inherited macular dystrophies and implications for stem cell therapy

Untreatable hereditary macular dystrophy (HMD) presents a major burden to society in terms of the resulting patient disability and the cost to the healthcare provision system. HMD results in central vision loss in humans sufficiently severe for blind registration, and key issues in the development of therapeutic strategies to target these conditions are greater understanding of the causes of photoreceptor loss and the development of restorative procedures. More effective and precise analytical techniques coupled to the development of transgenic models of disease have led to a prolific growth in the identification and our understanding of the genetic mutations that underly HMD. Recent successes in driving differentiation of pluripotent cells towards specific somatic lineages have led to the development of more efficient protocols that can yield enriched populations of a desired phenotype. Retinal pigmented epithelial cells and photoreceptors derived from these are some of the most promising cells that may soon be used in the treatment of specific HMD, especially since rapid developments in the field of induced pluripotency have now set the stage for the production of patient-derived stem cells that overcome the ethical and methodological issues surrounding the use of embryonic derivatives. In this review we highlight a selection of HMD which appear suitable candidates for combinatorial restorative therapy, focusing specifically on where those photoreceptor loss occurs. This technology, along with increased genetic screening, opens up an entirely new pathway to restore vision in patients affected by HMD.

Genetic ablation of retinal pigment epithelial cells reveals the adaptive response of the epithelium and impact on photoreceptors

The retinal pigment epithelium (RPE) plays a critical role in the maintenance of the outer retina. RPE cell death or dysfunction drives the pathophysiology of many retinal diseases, but the physiological response of the retina to RPE cell loss is poorly understood, mainly because of the absence of suitable experimental models. Here, we generated a transgenic mouse in which an inducible Cre recombinase is expressed exclusively in the RPE under the control of the monocarboxylate transporter 3 gene promoter (RPE(CreER)). This was crossed with a transgenic mouse harboring a diphtheria toxin A (DTA) chain gene rendered transcriptionally silent by a floxed stop sequence. We show that activation of DTA in the double transgenic mouse (RPE(CreER)/DTA) led to 60-80% RPE cell death, with surviving cells maintaining the integrity of the monolayer by increasing their size. Despite the apparent morphological normality of the enlarged RPE cells in the RPE(CreER)/DTA mice, functional analysis revealed significant deficits on electroretinography, and retinal histopathology showed regions of photoreceptor rosetting and degeneration although with retention of a normal vascular network. Our study reveals that whilst the RPE monolayer has a remarkable intrinsic capacity to cope with cellular attrition, specific aspects of RPE multifunctionality essential for photoreceptor survival are compromised. The RPE(CreER)/DTA mouse offers advantages over models that employ chemical or mechanical strategies to kill RPE cells, and should be useful for the development and evaluation of RPE-based therapies, such as stem cell transplantation.

Stem cells as tools in regenerative therapy for retinal degeneration

OBJECTIVE

To describe the use of stem cells (SCs) for regeneration of retinal degenerations. Regenerative medicine intends to provide therapies for severe injuries or chronic diseases where endogenous repair does not sufficiently restore the tissue. Pluripotent SCs, with their capacity to give rise to specialized cells, are the most promising candidates for clinical application. Despite encouraging results, a combination with up-to-date tissue engineering might be critical for ultimate success.

DESIGN

The focus is on the use of SCs for regeneration of retinal degenerations. Cell populations include embryonic, neural, and bone marrow-derived SCs, and engineered grafts will also be described.

RESULTS

Experimental approaches have successfully replaced damaged photoreceptors and retinal pigment epithelium using endogenous and exogenous SCs.

CONCLUSIONS

Stem cells have the potential to significantly impact retinal regeneration. A combination with bioengineering may bear even greater promise. However, ethical and scientific issues have yet to be solved.

Improving RPE adhesion to Bruch's membrane

Age-related macular degeneration is the leading cause of blindness in the developing world. Retinal pigmented epithelium (RPE) transplantation in subretinal space, has been assessed in various animal models of age-related macular degeneration and in humans as a potential technique to preserve the visual function. However, the RPE cell survival posttransplantation is limited because of lack of attachment of the transplanted cells to the pathological Bruch's membrane and also partly because of iatrogenic removal of adhesive elements in the membrane during the removal of choroidal new vessels before transplantation procedure. Although pathological Bruch's membrane is well studied, there is still much debate as to why and how changes in the structure and components of this membrane leads to loss of RPE cells and disruption of their function and subsequent death of photoreceptors leading to visual loss. Integrins on RPE cells have been characterized and shown to be important for attachment of cells to Bruch's membrane. Considering the essential role of integrins in functions such as cell migration and adhesion, it is plausible that lack of attachment of RPE cells posttransplantation can be overcome by improving integrin function. Here, we have focused on some of the recent findings on the use of integrins and modulation of their function to improve the adhesion of RPE cells to normal and pathological Bruch's membrane. This work also aims at elucidating a potential mechanism by which accumulating inhibitory molecules in the Bruch's membrane in the pathological state, interferes with integrin function.

Reduction of iatrogenic RPE lesions in AMD patients: evidence for wound healing?

PURPOSE

Our purpose was to study retinal pigment epithelium (RPE) wound healing in patients with age-related macular degeneration (AMD).

PATIENTS AND METHODS

Abrasive debridement of nasal RPE was performed with a metal cannula during pars plana vitrectomy for foveal choroidal neovascularization (CNV) membrane excision combined with simultaneous autologous RPE transplantation. Fundus autofluorescence, fluorescein angiography images, and red-free pictures were taken initially within 1-2 weeks postoperatively, subsequently in 2-week intervals until 3 months, monthly until 6 months, and every 3 months thereafter. The borders of these lesions were measured; areas were calculated and compared using ArchiCad Software. Fourteen eyes of 14 patients suffering from AMD were included (nine women and four men, mean age 75.6 years +/-6.6 years).

RESULTS

Six of 14 (42.9 %) patients showed a reduction of the RPE debrided area. The size of these lesions reduced 5.6-20% within 2 postoperative months compared with their size at first examination (from a mean of 13.7 mm2 +/- 7.2 at baseline to a mean of 12.8 mm2 +/- 6.7 at 2 months postoperatively). No further reduction of the lesions was seen after the 2 months. In eight cases, borders of the RPE debrided areas stayed stable during observation time.

CONCLUSIONS

Wound healing of abrasively debrided RPE monolayer defects in patients with AMD occurs to a certain extent in nearly half of the cases. This process seems to stop after 2 months.

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.

Development and role of tight junctions in the retinal pigment epithelium

The outer blood-retinal barrier is formed by the retinal pigment epithelium. In any epithelial monolayer, the tight junctions enable the epithelium to form a barrier by joining neighboring cells together and regulating transepithelial diffusion through the paracellular spaces. Tight junctions are complex, dynamic structures that regulate cell proliferation, polarity, and paracellular diffusion. The specific properties of tight junctions vary among epithelia, according to the physiological role of the epithelium. Unlike other epithelia, the apical surface of the retinal pigment epithelium interacts with a solid tissue, the neural retina. Secretions of the developing neural retina regulate the assembly, maturation, and tissue-specific properties of these tight junctions. The slow time course of development allows investigators to dissect the mechanisms of junction assembly and function. These studies are aided by culture systems that model different stages of development.

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.

Polydimethylsiloxane as a substrate for retinal pigment epithelial cell growth

Retinal pigment epithelial (RPE) cell transplantation represents potential treatment for age-related macular degeneration (AMD). Because delivery of isolated cells can cause serious complications, it is necessary to develop a suitable transplant membrane that could support an intact functioning RPE monolayer. Polydimethylsiloxane (PDMS) possesses the physical properties required for a transplanting device and is widely used clinically. We have investigated the use of PDMS as a potential surface for the growth of healthy RPE monolayers. PDMS discs were surface modified by air and ammonia gas plasma treatments. Dynamic contact angles were measured to determine the changes in wettability. Human ARPE-19 cells were seeded onto untreated and treated samples. Cell number, morphology and monolayer formation, cytotoxicity, and phagocytosis of photoreceptor outer segments (POS) were assessed at set time-points. Air plasma treatment increased the wettability of PDMS. This significantly enhanced cell growth, reaching confluence by day 7. Immunofluorescence revealed well-defined actin staining, monolayer formation, and high cell viability on air plasma treated and untreated surfaces, and to a lesser extent, on ammonia plasma treated. Furthermore, RPE monolayers were able to demonstrate phagocytosis of POS in a time-dependent manner similar to control. PDMS can support an intact functional monolayer of healthy differentiated RPE cells.

Bone marrow-derived cells home to and regenerate retinal pigment epithelium after injury

PURPOSE

To determine whether hematopoietic stem and progenitor cells (HSCs/HPCs) can home to and regenerate the retinal pigment epithelium (RPE) after induced injury.

METHODS

Enriched HSCs/HPCs from green fluorescent protein (gfp) transgenic mice were transplanted into irradiated recipient mice to track bone marrow-derived cells. Physical damage was induced by breaching Bruch's membrane and inducing vascular endothelial growth factor A (VEGFa) expression to promote neovascularization. RPE damage was also induced by sodium iodate injection (40 mg/kg) into wild-type or albino C57Bl/6 mice. Cell morphology, gfp expression, the presence of the Y chromosome, and the presence of melanosomes were used to determine whether the injured RPE was being repaired by the donor bone marrow.

RESULTS

Injury to the RPE recruits HSC/HPC-derived cells to incorporate into the RPE layer and differentiate into an RPE phenotype. A portion of the HSCs/HPCs adopt RPE morphology, express melanosomes, and integrate into the RPE without cell fusion.

CONCLUSIONS

HSCs/HPCs can migrate to the RPE layer after physical or chemical injury and regenerate a portion of the damaged cell layer.

Fundus autofluorescence in patients with pseudoxanthoma elasticum

PURPOSE

To evaluate the autofluorescence findings of patients with pseudoxanthoma elasticum, a disease resulting from a defect in a reputed transport protein encoded by the gene adenosine triphosphate-binding cassette subtype C number 6.

DESIGN

Observational case series.

METHODS

Color, red-free monochromatic, and autofluorescence photography and fluorescein angiography of patients with pseudoxanthoma elasticum seen in a referral practice were evaluated.

MAIN OUTCOME MEASURES

Cataloging of the abnormalities as detected by autofluorescence photography.

RESULTS

The 8 subjects ranged in age from 26 to 60 years (mean, 55+/-12), and their best-corrected visual acuity ranged from 20/20 to 5/400 (mean, 20/50). Of the 16 eyes of the 8 patients, all had abnormalities typical of pseudoxanthoma elasticum, including angioid streaks in 14, peau d'orange in 4, and choroidal neovascularization in 11. Angioid streaks appeared as hypoautofluorescent fissures, sometimes showing expansion of the hypoautofluorescence suggestive of retinal pigment epithelium (RPE) absence or atrophy. Peau d'orange had a stippled appearance of autofluorescence, and drusen of the optic nerve appeared as hyperautofluorescent bodies. In addition to the expansion of RPE atrophy around angioid streaks, 3 additional configurations of RPE atrophy were recognized as RPE rips in 6 eyes, multilobular areas of atrophy in 9 eyes, and broad areas of poorly demarcated atrophy in 5 eyes. Some eyes had more than one manifestation of RPE atrophy, but the latter 3 types of atrophy occurred in eyes with, but not necessary contiguous to, concurrent choroidal neovascularization.

CONCLUSIONS

Autofluorescence photography demonstrated that patients with pseudoxanthoma elasticum have more widespread areas of RPE disturbance, particularly atrophy, than what is detectable by other means of ocular imaging, which suggests that the RPE disturbance may play a role in the pathogenesis of visual loss in patients with pseudoxanthoma elasticum.