Autofluorescence fundus imaging using an adaptive optics scanning laser ophthalmoscope (AOSLO) allows for imaging of individual retinal pigment epithelial (RPE) cells in vivo. In this study, the potential of retinal damage was investigated by using radiant exposure levels that are 2 to 150 times those used for routine imaging.
Macaque retinas were imaged in vivo with a fluorescence AOSLO. The retina was exposed to 568- or 830-nm light for 15 minutes at various intensities over a square ½° per side. Pre-and immediate postexposure images of the photoreceptors and RPE cells were taken over a 2° field. Long-term AOSLO imaging was performed intermittently from 5 to 165 days after exposure. Exposures delivered over a uniform field were also investigated.
Exposures to 568-nm light caused an immediate decrease in autofluorescence of RPE cells. Follow-up imaging revealed either full recovery of autofluorescence or long-term damage in the RPE cells at the exposure. The outcomes of AOSLO exposures and uniform field exposures of equal average power were not significantly different. No effects from 830-nm exposures were observed.
The study revealed a novel change in RPE autofluorescence induced by 568-nm light exposure. Retinal damage occurred as a direct result of total average power, independent of the light-delivery method. Because the exposures were near or below permissible levels in laser safety standards, these results suggest that caution should be used with exposure of the retina to visible light and that the safety standards should be re-evaluated for these exposure conditions.
We have previously shown that long exposure to 568 nm light at levels below the maximum permissible exposure safety limit produces retinal damage preceded by a transient reduction in the autofluorescence of retinal pigment epithelial (RPE) cells in vivo. Here, we determine how the effects of exposure power and duration combine to produce this autofluorescence reduction and find the minimum exposure causing a detectable autofluorescence reduction.
Macaque retinas were imaged using a fluorescence adaptive optics scanning laser ophthalmoscope to resolve individual RPE cells in vivo. The retina was exposed to 568 nm light over a square subtending 0.5° with energies ranging from 1 J/cm2 to 788 J/cm2, where power and duration were independently varied.
In vivo exposures of 5 J/cm2 and higher caused an immediate decrease in autofluorescence followed by either full autofluorescence recovery (exposures ≤ 210 J/cm2) or permanent RPE cell damage (exposures ≥ 247 J/cm2). No significant autofluorescence reduction was observed for exposures of 2 J/cm2 and lower. Reciprocity of exposure power and duration held for the exposures tested, implying that the total energy delivered to the retina, rather than its distribution in time, determines the amount of autofluorescence reduction.
That reciprocity holds is consistent with a photochemical origin, which may or may not cause retinal degeneration. The implementation of safe methods for delivering light to the retina requires a better understanding of the mechanism causing autofluorescence reduction. Finally, RPE imaging was demonstrated using light levels that do not cause a detectable reduction in autofluorescence.
The retina exhibits an inherent autofluorescence that is imaged ophthalmoscopically as fundus autofluorescence. In clinical settings, fundus autofluorescence examination aids in the diagnosis and follow-up of many retinal disorders. Fundus autofluorescence originates from the complex mixture of bisretinoid fluorophores that are amassed by retinal pigment epithelial (RPE) cells as lipofuscin. Unlike the lipofuscin found in other cell-types, this material does not form as a result of oxidative stress. Rather, the formation is attributable to non-enzymatic reactions of vitamin A aldehyde in photoreceptor cells; transfer to RPE occurs upon phagocytosis of photoreceptor outer segments. These fluorescent pigments accumulate even in healthy photoreceptor cells and are generated as a consequence of the light capturing function of the cells. Nevertheless, the formation of this material is accelerated in some retinal disorders including recessive Stargardt disease and ELOVL-4-related retinal degeneration. As such, these bisretinoid side-products are implicated in the disease processes that threaten vision. In this article, we review our current understanding of the composition of RPE lipofuscin, the structural characteristics of the various bisretinoids, their related spectroscopic features and the biosynthetic pathways by which they form. We will revisit factors known to influence the extent of the accumulation and therapeutic strategies being used to limit bisretinoid formation. Given their origin from vitamin A aldehyde, an isomer of the visual pigment chromophore, it is not surprising that the bisretinoids of retina are light sensitive molecules. Accordingly, we will discuss recent findings that implicate the photodegradation of bisretinoid in the etiology of age-related macular degeneration.
A2E; all-trans-retinal; bisretinoid; retinal pigment epithelium; macular degeneration; retina
Non-invasive reflectance imaging of the human RPE cell mosaic is demonstrated using a modified
confocal adaptive optics scanning light ophthalmoscope (AOSLO). The confocal circular aperture in
front of the imaging detector was replaced with a combination of a circular aperture 4 to 16 Airy
disks in diameter and an opaque filament, 1 or 3 Airy disks thick. This arrangement reveals the RPE
cell mosaic by dramatically attenuating the light backscattered by the photoreceptors. The RPE cell
mosaic was visualized in all 7 recruited subjects at multiple retinal locations with varying degrees
of contrast and cross-talk from the photoreceptors. Various experimental settings were explored for
improving the visualization of the RPE cell boundaries including: pinhole diameter, filament
thickness, illumination and imaging pupil apodization, unmatched imaging and illumination focus,
wavelength and polarization. None of these offered an obvious path for enhancing image contrast. The
demonstrated implementation of dark-field AOSLO imaging using 790 nm light requires low light
exposures relative to light safety standards and it is more comfortable for the subject than the
traditional autofluorescence RPE imaging with visible light. Both these factors make RPE dark-field
imaging appealing for studying mechanisms of eye disease, as well as a clinical tool for screening
and monitoring disease progression.
(170.4460) Ophthalmic optics and devices; (170.4470) Ophthalmology; (290.4210) Multiple scattering; (110.1080) Active or adaptive optics
Daily phagocytosis by the retinal pigment epithelium (RPE) of spent photoreceptor outer segment fragments is critical for vision. In the retina, early morning circadian photoreceptor rod shedding precedes synchronized uptake of shed photoreceptor particles by RPE cells. In vitro, RPE cells use the integrin receptor αvβ5 for particle binding. Here, we tested RPE phagocytosis and retinal function in β5 integrin–deficient mice, which specifically lack αvβ5 receptors. Retinal photoresponses severely declined with age in β5−/− mice, whose RPE accumulated autofluorescent storage bodies that are hallmarks of human retinal aging and disease. β5−/− RPE in culture failed to take up isolated photoreceptor particles. β5−/− RPE in vivo retained basal uptake levels but lacked the burst of phagocytic activity that followed circadian photoreceptor shedding in wild-type RPE. Rhythmic activation of focal adhesion and Mer tyrosine kinases that mediate wild-type retinal phagocytosis was also completely absent in β5−/− retina. These results demonstrate an essential role for αvβ5 integrin receptors and their downstream signaling pathways in synchronizing retinal phagocytosis. Furthermore, they identify the β5−/− integrin mouse strain as a new animal model of age-related retinal dysfunction.
circadian rhythm; knockout; photoreceptors; retinal pigment; epithelium; vision
Usher syndrome (USH) is a genetically heterogeneous group of autosomal recessive deaf-blinding disorders. Pathophysiology leading to the blinding retinal degeneration in USH is uncertain. There is evidence for involvement of the photoreceptor cilium, photoreceptor synapse, the adjacent retinal pigment epithelium (RPE) cells, and the Crumbs protein complex, the latter implying developmental abnormalities in the retina. Testing hypotheses has been difficult in murine USH models because most do not show a retinal degeneration phenotype. We defined the retinal disease expression in vivo in human USH using optical imaging of the retina and visual function. In MYO7A (USH1B), results from young individuals or those at early stages indicated the photoreceptor was the first detectable site of disease. Later stages showed photoreceptor and RPE cell pathology. Mosaic retinas in Myo7a-deficient shaker1 mice supported the notion that the mutant photoreceptor phenotype was cell autonomous and not secondary to mutant RPE. Humans with PCDH15 (USH1F), USH2A or GPR98 (USH2C) had a similar retinal phenotype to MYO7A (USH1B). There was no evidence of photoreceptor synaptic dysfunction and no dysplastic phenotype as in CRB1 (Crumbs homologue1) retinopathy. The results point to the photoreceptor cell as the therapeutic target for USH treatment trials, such as MYO7A somatic gene replacement therapy.
Retinal pigment epithelial (RPE) cell dysfunction plays a central role in various retinal degenerative diseases, but knowledge is limited regarding the pathways responsible for adult RPE stress responses in vivo. RPE mitochondrial dysfunction has been implicated in the pathogenesis of several forms of retinal degeneration. Here we have shown that postnatal ablation of RPE mitochondrial oxidative phosphorylation in mice triggers gradual epithelium dedifferentiation, typified by reduction of RPE-characteristic proteins and cellular hypertrophy. The electrical response of the retina to light decreased and photoreceptors eventually degenerated. Abnormal RPE cell behavior was associated with increased glycolysis and activation of, and dependence upon, the hepatocyte growth factor/met proto-oncogene pathway. RPE dedifferentiation and hypertrophy arose through stimulation of the AKT/mammalian target of rapamycin (AKT/mTOR) pathway. Administration of an oxidant to wild-type mice also caused RPE dedifferentiation and mTOR activation. Importantly, treatment with the mTOR inhibitor rapamycin blunted key aspects of dedifferentiation and preserved photoreceptor function for both insults. These results reveal an in vivo response of the mature RPE to diverse stressors that prolongs RPE cell survival at the expense of epithelial attributes and photoreceptor function. Our findings provide a rationale for mTOR pathway inhibition as a therapeutic strategy for retinal degenerative diseases involving RPE stress.
Lipofuscin results from digestion of photoreceptor outer segments by the retinal pigment epithelium (RPE) and is the principal compound that causes RPE fluorescence during autofluorescence imaging. Absorption of the 488-nanometer blue light by macular pigments, especially by the carotenoids lutein and zeaxanthin, causes normal macular hypo-autofluorescence. Fundus autofluorescence imaging is being increasingly employed in ophthalmic practice to diagnose and monitor patients with a variety of retinal disorders. In macular edema for example, areas of hyper-autofluorescence are usually present which are postulated to be due to dispersion of macular pigments by pockets of intraretinal fluid. For this reason, the masking effect of macular pigments is reduced and the natural autofluorescence of lipofuscin can be observed without interference. In cystic types of macular edema, e.g. cystoid macular edema due to retinal vein occlusion, diabetic macular edema and post cataract surgery, hyper-autofluorescent regions corresponding to cystic spaces of fluid accumulation can be identified. In addition, the amount of hyper-autofluorescence seems to correspond to the severity of edema. Hence, autofluorescence imaging, as a noninvasive technique, can provide valuable information on cystoid macular edema in terms of diagnosis, follow-up and efficacy of treatment.
Autofluorescence; Cystoid Macular Edema; Lipofuscin
We provide our detailed, standardized in vitro protocol for culture and differentiation of human retinal pigment epithelial (RPE) cells into a highly polarized, functional monolayer. Disruption of polarized RPE function plays an important role in the pathogenesis of common blinding disorders of the retina. The availability of this polarized RPE monolayer allows for reproducible evaluation of RPE function, modeling of RPE dysfunction in retinal disease, and in vitro evaluation of novel therapies. The protocol, which takes approximately 6 weeks, describes the culture of RPE from human fetal donor eyes, and the differentiation of these cells into a polarized monolayer with high transepithelial resistance, and morphologic characteristics that mimic the RPE monolayer in vivo. By modifying the procedure for initial isolation of pure RPE cells, and culture conditions used in existing protocols, we have established a standardized protocol that provides highly reproducible RPE monolayers from the same donor eye.
The photoreceptor/RPE complex must maintain a delicate balance between maximizing the absorption of photons for vision and retinal image quality while simultaneously minimizing the risk of photodamage when exposed to bright light. We review the recent discovery of two new effects of light exposure on the photoreceptor/RPE complex in the context of current thinking about the causes of retinal phototoxicity. These effects are autofluorescence photobleaching in which exposure to bright light reduces lipofuscin autofluorescence and, at higher light levels, RPE disruption in which the pattern of autofluorescence is permanently altered following light exposure. Both effects occur following exposure to visible light at irradiances that were previously thought to be safe. Photopigment, retinoids involved in the visual cycle, and bisretinoids in lipofuscin have been implicated as possible photosensitizers for photochemical damage. The mechanism of RPE disruption may follow either of these paths. On the other hand, autofluorescence photobleaching is likely an indicator of photooxidation of lipofuscin. The permanent changes inherent in RPE disruption might require modification of the light safety standards. AF photobleaching recovers after several hours although the mechanisms by which this occurs are not yet clear. Understanding the mechanisms of phototoxicity is all the more important given the potential for increased susceptibility in the presence of ocular diseases that affect either the visual cycle and/or lipofuscin accumulation. In addition, knowledge of photochemical mechanisms can improve our understanding of some disease processes that may be influenced by light exposure, such as some forms of Leber’s congenital amaurosis, and aid in the development of new therapies. Such treatment prior to intentional light exposures, as in ophthalmic examinations or surgeries, could provide an effective preventative strategy.
Phototoxicity; Photochemical; Retina; Retinal pigment epithelium; Autofluorescence; Visual cycle; Lipofuscin; Bisretinoids
Optical coherence tomography (OCT) may revolutionize fundamental investigation and clinical management of age-related macular degeneration and other eye diseases. However, quantitative OCT interpretation is hampered due to uncertain sub-cellular correlates of reflectivity in the retinal pigment epithelium (RPE) and photoreceptor. The purpose of this study was twofold: 1) to test OCT correlates in the RPE, and 2) to demonstrate the feasibility of longitudinal OCT monitoring of sub-cellular RPE dynamics. A high resolution OCT was constructed to achieve dynamic imaging of frog eyes, in which light-driven translocation of RPE melanosomes occurred within the RPE cell body and apical processes. Comparative histological examination of dark- and light-adapted eyes indicated that the RPE melanin granule, i.e., melanosome, was a primary OCT correlate. In vivo OCT imaging of RPE melanosomes opens the opportunity for quantitative assessment of RPE abnormalities associated with disease, and enables longitudinal investigation of RPE kinetics correlated with visual function.
Chorioretinal folds typically involve the choroid, Bruch membrane, retinal pigment epithelium (RPE), and sometimes overlying neurosensory retina. von Winning hypothesized that the alternate banding pattern of choroidal folds shown by fluorescein angiography is explained by RPE density. To our knowledge, autofluorescence imaging of chorioretinal folds has not been previously described.
A 47-year-old healthy hyperopic man had best-corrected visual acuity of 20/30 in the right eye and 20/25 in the left eye. Posterior segment examination revealed bilateral chorioretinal folds with subtle streaks of RPE hyperpigmentation and hypopigmentation emanating from both optic nerve heads.
Early-phase fluorescein angiography revealed the characteristic pattern of alternating light and dark bands. Autofluorescence imaging disclosed a similar pattern as well as peripapillary mottling. The alternating patterns of light and dark bands observed using autofluorescence imaging and fluorescein angiography were found to be precisely in register but inverted.
Autofluorescence imaging noninvasively demonstrates the pathognomonic pattern of alternating light and dark bands shown by fluorescein angiography diagnostic of choroidal folds but in an inverse fashion. This observation provides independent support of von Winning’s hypothesis regarding the etiopathogenesis of the banding pattern.
chorioretinal folds; autofluorescence; fluorescein angiography; hyperopia; retinal pigment epithelium
Autologous retinal pigment epithelium (RPE) grafts derived from induced pluripotent stem cells (iPSCs) may be used to cure blinding diseases in which RPE dysfunction results in photoreceptor degeneration. Four-, two-, and one-factor-derived iPSCs (4F-, 2F-, and 1F-iPSCs, respectively) were differentiated into fully functional cuboidal pigmented cells in polarized monolayers that express RPE-specific markers. 1F-iPSC-RPE (1F-iPS-RPE) strongly resembles primary human fetal RPE (hfRPE) based on proteomic and untargeted metabolomic analyses, and using novel in vivo imaging technology coupled with electroretinography, we demonstrated that 1F-iPS-RPE mediate anatomical and functional rescue of photoreceptors after transplantation in an animal model of RPE-mediated retinal degeneration. 1F-iPS-RPE cells were injected subretinally as a suspension and formed a monolayer dispersed between host RPE cells. Furthermore, 1F-iPS-RPE do not simply provide trophic support to rescue photoreceptors as previously speculated but actually phagocytose photoreceptor outer segments in vivo and maintain visual cycling. Thus, 1F-iPS-RPE grafts may be superior to conventional iPS-RPE for clinical use because 1F-iPS-RPE closely resemble hfRPE, mediate anatomical and functional photoreceptor rescue in vivo, and are generated using a reduced number of potentially oncogenic reprogramming factors.
Retina; Induced pluripotent stem cells; Differentiation; Small molecules; Stem cell transplantation; Aging
Compared with neural crest-derived melanocytes, retinal pigment epithelium (RPE) cells in the back of the eye are pigment cells of a different kind. They are a part of the brain, form an epithelial monolayer, respond to distinct extracellular signals, and provide functions that far exceed those of a light-absorbing screen. For instance, they control nutrient and metabolite flow to and from the retina, replenish 11-cis-retinal by re-isomerizing all-trans-retinal generated during photoconversion, phagocytose daily a portion of the photoreceptors’ outer segments, and secrete cytokines that locally control the innate and adaptive immune systems. Not surprisingly, RPE cell damage is a major cause of human blindness worldwide, with age-related macular degeneration a prevalent example. RPE replacement therapies using RPE cells generated from embryonic or induced pluripotent stem cells provide a novel approach to a rational treatment of such forms of blindness. In fact, RPE-like cells can be obtained relatively easily when stem cells are subjected to a two-step induction protocol, a first step that leads to a neuroectodermal fate and a second to RPE differentiation. Here, we discuss the characteristics of such cells, propose criteria they should fulfill in order to be considered authentic RPE cells, and point out the challenges one faces when using such cells in attempts to restore vision.
ES cells; induced pluripotent stem cells; age-related macular degeneration; retinitis pigmentosa; cell-based therapy; retinal pigment epithelium
To investigate the capacity of mature retinal pigment epithelium (RPE) cells to enter the cell cycle in vivo using a range of RPE-specific and proliferative specific markers in both pigmented and albino rats.
Whole-mounted retinas of both Dark Agouti and albino rats were immunolabeled with cell cycle markers Ki67 or PCNA and double labeled with RPE cell marker RPE65 or CRALBP. The number and distribution of these cells was mapped. An additional group of Dark Agouti rats were given repeated intraperitoneal injections of Bromodeoxyuridine (BrdU )for 20 days and then sacrificed 30 days later. The retinas were then processed for BrdU detection and Otx, a RPE cell-specific marker. For comparison, human RPE tissue from a postmortem donor was also labeled for Ki67.
In both pigmentation phenotypes, a subpopulation of mature RPE cells in the periphery were positive for both cell cycle markers. These cells were negative for Caspase 3, hence were not apoptotic. Ki67-positive cells were also seen in human RPE. Further, many cells positive for BrdU were identified in similar retinal regions, confirming that RPE cells not only enter the cell cycle but also divide, albeit at a slow cell cycle rate. There was a ten fold increase in the number of RPE cells positive for cell cycle markers in albino (approximately 200 cells) compared to pigmented rats (approximately 20 cells).
Peripheral RPE cells in rats have the capacity to enter the cell cycle and complete cellular division.
The human retina is rich in autofluorescent species, such as lipofuscin and melanin. Consequently, it is difficult to localize antigens in the human retina using immunofluorescence microscopy. To address this issue, we have developed a methodology to tag retinal antigens using quantum dot nanoparticles that absorb in the ultraviolet and emit in the infrared, thereby avoiding the visible spectrum. This protocol dramatically improves signal-to-background autofluorescence ratios of immunofluorescence images of human retinal sections, thus enhancing the specific fluorescence in microscopic studies. Of particular note is the ability to detect antigens within the brightly autofluorescent RPE cell layer.
retina; fluorescence; nanoparticles; human
Cone photoreceptors have faster light responses than rods and a higher demand for 11-cis retinal (11cRAL), the chromophore of visual pigments. RPE65 is the isomerohydrolase in the retinal pigment epithelium (RPE) which converts all-trans retinyl ester (atRE) to 11-cis retinol, a key step in the visual cycle to regenerate 11cRAL. Accumulating evidence suggests that cone-dominant species express an alternative isomerase, likely in retinal Müller cells, in order to meet the high demand for the chromophore by cones. Herein we describe the identification and characterization of a novel isomerohydrolase, RPE65c, from the cone-dominant zebrafish retina. RPE65c shares 78% amino acid sequence identity with RPE-specific zebrafish RPE65a (orthologue of human RPE65) and retains all of the known key residues for the enzymatic activity of RPE65. Similar to the other RPE-specific RPE65, RPE65c was present in both the membrane and cytosolic fractions, used atRE as its substrate and required iron for its enzymatic activity. However, immunohistochemistry detected RPE65c in the inner retina including Müller cells, but not in the RPE. Furthermore, double-immunostaining of dissociated retinal cells using antibodies for RPE65c and glutamine synthetase (a Müller cell marker), showed that RPE65c co-localized with the Müller cell marker. These results suggest that RPE65c is the alternative isomerohydrolase in the intra-retinal visual cycle, providing 11cRAL to cone photoreceptors in cone-dominant species. Identification of an alternative visual cycle will contribute to the understanding of the functional differences of rod and cone photoreceptors.
retinoids; cone-dominant retina; isomerohydrolase; Müller cell; visual cycle
Within the vertebrate eye, the retinal pigment epithelium (RPE) juxtaposes with the retina, but how the RPE plays a role in retinal morphogenesis remains elusive. It has been shown that the loss of function of the polarity proteins, such as Nagie oko (Nok), disrupts RPE integrity and retinal lamination. However, it is unclear whether or not such defects are caused in a tissue-autonomous fashion. Here, by taking advantage of the nok mutation, we have generated a transgenic model to restore the Nok function in the RPE, but not in the retina. With this model, we show that Nok is required for RPE integrity in a tissue-autonomous manner. However, proper retinal epithelial polarity does not require retinal expression of Nok prior to embryonic photoreceptor genesis; rather, it requires a Nok-mediated intact RPE. Interestingly, sporadic wildtype RPE donor cells are not sufficient to maintain proper retinal polarity. We further show that RPE-mediated retinal epithelial polarity underlies proper patterning of retinal ganglion cells and the cells of the inner nuclear layer. Nevertheless, during embryonic photoreceptor genesis, an intact RPE is not sufficient to maintain retinal epithelial polarity and retinal cellular pattern formation. Our results show that the subcellular architecture and cellular pattern formation of a tissue may be regulated by neighboring tissues through tissue-tissue interactions.
RPE; retina; cellular pattern formation; Nok; polarity; transgenesis
Autologous retinal pigment epithelium (RPE) grafts derived from induced pluripotent stem cells (iPSCs) may be used to cure blinding diseases in which RPE dysfunction results in photoreceptor degeneration. Four, two, and one factor-derived iPS (4F-, 2F-, and 1F-iPSCs, respectively) were differentiated into fully functional cuboidal shaped pigmented cells in polarized monolayers that express RPE-specific markers. 1F-iPS-RPE strongly resemble primary human fetal RPE (hfRPE) based on proteomic and untargeted metabolomic analyses, and, utilizing novel in vivo imaging technology coupled with electroretinography, we demonstrate that 1F-iPS-RPE mediate anatomical and functional rescue of photoreceptors after transplantation in an animal model of RPE-mediated retinal degeneration. 1F-iPS-RPE cells were injected subretinally as a suspension and formed a monolayer dispersed between host RPE cells. Furthermore, 1F-iPS-RPE do not simply provide trophic support to rescue photoreceptors as previously speculated, but actually phagocytose photoreceptor outer segments in vivo and restore visual cycling (based on high-resolution mass spectrometry based detection of recycled photoreceptor protein and lipid end products and electron microscopic analysis). Thus, 1F-iPS-RPE grafts may be superior to conventional iPS-RPE for clinical use since 1F-iPS-RPE closely resemble hfRPE, mediate anatomical and functional photoreceptor rescue in vivo and are generated using a reduced number of potentially oncogenic reprogramming factors.
Retinal pigment epithelium; induced pluripotent stem cells; differentiation; small molecules
Mutations in MYO7A cause Usher syndrome type 1B, a disorder involving profound congenital deafness and progressive blindness. In the retina, most of MYO7A is localized in the apical region of the RPE (retinal pigmented epithelial) cells, and a small amount is associated with the ciliary and periciliary membrane of the photoreceptor cells. Its roles appear to be quite varied. Studies with MYO7A-null mice indicate that MYO7A participates in the apical localization of RPE melanosomes and in the removal of phagosomes from the apical RPE for their delivery to lysosomes in the basal RPE. In the first role, MYO7A competes with microtubule motors, but, in the second one, it may function cooperatively. An additional role of MYO7A in the RPE is indicated from its requirement for light-dependent translocation of the ER-associated, visual cycle enzyme, RPE65, and normal functioning of the visual retinoid cycle. In photoreceptor cells, lacking MYO7A, opsin accumulates abnormally in the transition zone of the cilium, suggesting that MYO7A functions in a selective barrier for membrane proteins at the distal end of the transition zone. It is likely that the progressive retinal degeneration that occurs in Usher 1B patients results from a combination of cellular defects in the RPE and photoreceptor cells.
Myosin VIIa; retina; melanosome; phagosome; endoplasmic reticulum; cilium
The retinal pigment epithelium (RPE) is unique among epithelia in that its apical surface does not face a lumen, but, instead, is specialized for interaction with the neural retina. The molecules involved in the interaction of the RPE with the neural retina are not known. We show here that the neural cell adhesion molecule (N-CAM) is found both on the apical surface of RPE in situ and on the outer segments of photoreceptors, fulfilling an important requisite for an adhesion role between both structures. Strikingly, culture of RPE results in rapid redistribution of N-CAM to the basolateral surface. This is not due to an isoform shift, since the N-CAM expressed by cultured cells (140 kD) is the same as that expressed by RPE in vivo. Rather, the reversed polarity of N-CAM appears to result from the disruption of the contact between the RPE and the photoreceptors of the neural retina. We suggest that N-CAM in RPE and photoreceptors participate in these interactions.
To correlate the degree of functional loss with structural changes in patients with Stargardt disease.
Eighteen eyes of 10 Stargardt patients were studied. Scanning laser ophthalmoscope (SLO) infrared images were compared to corresponding spectral domain optical coherence tomography (SD-OCT) scans. Additionally, SLO microperimetry was performed and results were superimposed on SLO infrared images and in selected cases on fundus autofluorescence (FAF) images.
Seventeen of 18 eyes showed a distinct hypo-reflective foveal and/or perifoveal area with distinct borders on SLO-infrared images which was less evident on funduscopy and incompletely depicted in FAF images. This hypo-reflective zone corresponded to areas of significantly elevated psychophysical thresholds on microperimetry testing, in addition to thinning of the retinal pigment epithelium (RPE), disorganization or loss of the photoreceptor cell inner-outer segment (IS-OS) junction and external limiting membrane (ELM) on SD-OCT.
SLO-infrared fundus images are useful for depicting retinal structural changes in Stargardt patients. An SD-OCT/SLO microperimetry device allows for a direct correlation of structural abnormalities with functional defects that will likely be applicable for the determination of retinal areas for potential improvement of retinal function in these patients during future clinical trials and for the monitoring of the diseases' natural history.
microperimetry; SLO infrared imaging; Stargardt disease; fundus autofluorescence imaging
Retinal pigment epithelial (RPE) cells play an important role in normal functioning of retina and photoreceptors, and some retinal degenerations arise due to malfunctioning RPE. Retinal pigment epithelium transplantation is being explored as a strategy to rescue degenerating photoreceptors in diseases such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP). Additionally, RPE-secreted factors could rescue degenerating photoreceptors by prolonging survival or by their ability to differentiate and give rise to photoreceptors by transdifferentiation. In this study, we have explored what role cell density could play in differentiation induced in a human retinal progenitor cell line, in response to RPE-secreted growth factors. Retinal progenitors plated at low (1 × 104 cells/cm2), medium (2–4 × 104 cells/cm2), and high (1 × 105 cells/cm2) cell density were exposed to various dilutions of RPE-conditioned medium (secreted factors) under conditions of defined medium culture. Progenitor cell differentiation was monitored phenotypically (morphological, biochemical analysis, and immunophenotyping, and western blot analysis were performed). Our data show that differentiation in response to RPE-secreted factors is modulated by cell density and dilutions of conditioned medium. We conclude that before embarking on RPE transplantation as a modality for treatment of RP and AMD, one will have to determine the role that cell density and inhibitory and stimulatory neurotrophins secreted by RPE could play in the efficacy of survival of transplants. We report that RPE-conditioned medium enhances neuronal phenotype (photoreceptors, bipolars) at the lowest cell density in the absence of cell–cell contact. Eighty percent to 90% of progenitor cells differentiate into photoreceptors and bipolars at 50% concentration of conditioned medium, while exposure to 100% conditioned medium might increase multipolar neurons (ganglionic and amacrine phenotypes) to a small degree. However, no clear-cut pattern of differentiation in response to RPE-secreted factors is noted at higher cell densities.
Retinal progenitors; Retinal pigment epithelium; Secreted factors; Differentiation; Density
The ability to resolve single retinal cells in rodents in vivo has applications in rodent models of the visual system and retinal disease. We have characterized the performance of a fluorescence adaptive optics scanning laser ophthalmoscope (fAOSLO) that provides cellular and subcellular imaging of rat retina in vivo.
Green fluorescent protein (eGFP) was expressed in retinal ganglion cells of normal Sprague Dawley rats via intravitreal injections of adeno-associated viral vectors. Simultaneous reflectance and fluorescence retinal images were acquired using the fAOSLO. fAOSLO resolution was characterized by comparing in vivo images with subsequent imaging of retinal sections from the same eyes using confocal microscopy.
Retinal capillaries and eGFP-labeled ganglion cell bodies, dendrites, and axons were clearly resolved in vivo with adaptive optics (AO). AO correction reduced the total root mean square wavefront error, on average, from 0.30 μm to 0.05 μm (1.7-mm pupil). The full width at half maximum (FWHM) of the average in vivo line-spread function (LSF) was ∼1.84 μm, approximately 82% greater than the FWHM of the diffraction-limited LSF.
With perfect aberration compensation, the in vivo resolution in the rat eye could be ∼2× greater than that in the human eye due to its large numerical aperture (∼0.43). While the fAOSLO corrects a substantial fraction of the rat eye's aberrations, direct measurements of retinal image quality reveal some blur beyond that expected from diffraction. Nonetheless, subcellular features can be resolved, offering promise for using AO to investigate the rodent eye in vivo with high resolution.
Retina organ cultures can be used as a valuable tool to study retina development ex vivo. Comparison between culture methods has revealed that timing the start of the culture and the presence of the retinal pigment epithelium (RPE) are critical for the development of the rods and cones, which are the two types of photoreceptors; rods can develop in the absence of the RPE, cones cannot. One of the necessary compounds produced by the RPE and essential for cone development and survival is the chromophore 11-cis retinal. Here, we further examined rod and cone development, chromophore production by the RPE, and photoreceptor signaling to the inner retina under organ culture conditions.
Retina-RPE cultures were prepared from 7-day-old C57BL/6 pups and maintained in culture for 11 days. Rod and cone structure was analyzed by immunohistochemistry, and cell-specific mRNA expression was analyzed by quantitative real-time PCR. We quantified 11-cis retinal spectrophotometrically by measuring rhodopsin. Signal transmission in the rod pathway was studied by analyzing c-fos expression in the inner retina in response to stroboscopic illumination.
In retina-RPE cultures analyzed after 11 days in culture, rod and cone numbers exhibited a similar ratio to those observed in the intact animal. Although photoreceptor outer segments were shorter when grown ex vivo, membrane proteins, such as cone opsin and transducin, were localized appropriately to the outer segment. Relative 11-cis retinal production ex vivo plateaued after 7 days in culture, resulting in approximately 30% of the in vivo level by day 11. The retinas responded to prolonged stroboscopic illumination with the normal nuclear expression of c-fos in cells in the inner retina.
Mouse retinal structure is maintained in retina–RPE organ cultures. The RPE in organ cultures produces sufficient amounts of 11-cis retinal to promote cone development and support signal transmission in the rod pathway. Organ cultures may be a powerful low-throughput screening tool to identify novel agents to promote photoreceptor cell survival and signaling.