Light-evoked changes in the interphotoreceptor matrix

The Impact of Photopigment Bleaching on the Human Rod Photoreceptor Subretinal Space Measured Via Optical Coherence Tomography

Purpose

The purpose of this study was to investigate rod photopigment bleaching-driven intrinsic optical signals (IOS) in the human outer retina and its measurement repeatability based on a commercial optical coherence tomography (OCT) platform.

Methods

The optical path length of the rod photoreceptor subretinal space (SRS), that is, the distance between signal bands of rod outer segment tips and retinal pigment epithelium, was measured in 15 healthy subjects in ambient light and during a long-duration bleaching white-light exposure.

Results

On 2 identical study days (day 1 and day 2 [D1 and D2]), light stimulation resulted in a significant decrease in rod SRS by 21.3 ± 7.6% and 19.8 ± 8.5% (both P < 0.001), respectively. The test-retest reliability of the SRS maximum change of an individual subject was moderate for single measures (intraclass correlation coefficient [ICC] = 0.730, 95% confidence interval [CI] = 0.376, 0.900, P < 0.001) and good for average measures (ICC = 0.844, 95% CI = 0.546, 0.947, P < 0.001). The mean area under the stimulus response curve with values of 14.8 ± 9.4 and 15.5 ± 7.5 µm × minutes (P = 0.782) showed excellent agreement between the stimulus response on D1 and D2. Intermittent dark adaptation of the retina led to an initial increase of the SRS by 6.1% (P = 0.018) and thereafter showed a decrease toward baseline, despite continued dark adaptation.

Conclusions

The data indicate the potential of commercial OCT in measuring slow IOS in the outer retina suggesting that the rod SRS could serve as a biomarker for photoreceptor function. The presented approach could provide an easily implementable clinical tool for the early detection of diseases affecting photoreceptor health.

Volumetric Reconstruction of a Human Retinal Pigment Epithelial Cell Reveals Specialized Membranes and Polarized Distribution of Organelles

Purpose

Despite the centrality of the retinal pigment epithelium (RPE) in vision and retinopathy our picture of RPE morphology is incomplete. With a volumetric reconstruction of human RPE ultrastructure, we aim to characterize major membranous features including apical processes and their interactions with photoreceptor outer segments, basolateral infoldings, and the distribution of intracellular organelles.

Methods

A parafoveal retinal sample was acquired from a 21-year-old male organ donor. With serial block-face scanning electron microscopy, a tissue volume from the inner-outer segment junction to basal RPE was captured. Surface membranes and complete internal ultrastructure of an individual RPE cell were achieved with a combination of manual and automated segmentation methods.

Results

In one RPE cell, apical processes constitute 69% of the total cell surface area, through a dense network of over 3000 terminal branches. Single processes contact several photoreceptors. Basolateral infoldings facing the choriocapillaris resemble elongated filopodia and comprise 22% of the cell surface area. Membranous tubules and sacs of endoplasmic reticulum represent 20% of the cell body volume. A dense basal layer of mitochondria extends apically to partly overlap electron-dense pigment granules. Pores in the nuclear envelope form a distinct pattern of rows aligned with chromatin.

Conclusions

Specialized membranes at the apical and basal side of the RPE cell body involved in intercellular uptake and transport represent over 90% of the total surface area. Together with the polarized distribution of organelles within the cell body, these findings are relevant for retinal clinical imaging, therapeutic approaches, and disease pathomechanisms.

Identification of the zebrafish homologues of IMPG2, a retinal proteoglycan

Photoreceptor outer segments are surrounded by a carbohydrate-rich matrix, the interphotoreceptor matrix, necessary for physiological retinal function. Few roles for molecules characterizing the interphotoreceptor matrix have been clearly defined. Recent studies have found the presence of nonsense mutations in the interphotoreceptor matrix proteoglycan 2 (IMPG2) gene in patients affected by retinal dystrophies. IMPG2 encodes for a proteoglycan synthesized by photoreceptors and secreted in the interphotoreceptor matrix. Little is known about the structure and function of this protein, we thus decided to characterize zebrafish impg2. In zebrafish there are two Impg2 proteins, Impg2a and Impg2b. We generated a phylogenetic tree based on IMPG2 protein sequence similarity among vertebrates, showing a significant similarity between humans and teleosts. The human and zebrafish proteins share conserved domains, as also shown by homology models. Expression analyses of impg2a and impg2b show a continued expression in the photoreceptor layer starting from developmental stages and continuing through adulthood. Between 1 and 6 months post-fertilization, there is a significant shift of Impg2 expression toward the outer segment region, suggesting an increase in secretion. This raises intriguing hypotheses about its possible role(s) during retinal maturation, laying the groundwork for the generation of most needed models for the study of IMPG2-related inherited retinal dystrophies.

Towards a New Biomarker for Diabetic Retinopathy: Exploring RBP3 Structure and Retinoids Binding for Functional Imaging of Eyes In Vivo

Diabetic retinopathy (DR) is a severe disease with a growing number of afflicted patients, which places a heavy burden on society, both socially and financially. While there are treatments available, they are not always effective and are usually administered when the disease is already at a developed stage with visible clinical manifestation. However, homeostasis at a molecular level is disrupted before visible signs of the disease are evident. Thus, there has been a constant search for effective biomarkers that could signal the onset of DR. There is evidence that early detection and prompt disease control are effective in preventing or slowing DR progression. Here, we review some of the molecular changes that occur before clinical manifestations are observable. As a possible new biomarker, we focus on retinol binding protein 3 (RBP3). We argue that it displays unique features that make it a very good biomarker for non-invasive, early-stage DR detection. Linking chemistry to biological function and focusing on new developments in eye imaging and two-photon technology, we describe a new potential diagnostic tool that would allow rapid and effective quantification of RBP3 in the retina. Moreover, this tool would also be useful in the future to monitor therapeutic effectiveness if levels of RBP3 are elevated by DR treatments.

Photoreceptor Function and Structure in Autosomal Dominant Vitelliform Macular Dystrophy Caused by BEST1 Mutations

Purpose

The purpose of this study was to evaluate rod and cone function and outer retinal structure within macular lesions, and surrounding extralesional areas of patients with autosomal dominant Best vitelliform macular dystrophy caused by BEST1 mutations.

Methods

Seventeen patients from seven families were examined with dark- and light-adapted chromatic perimetry and optical coherence tomography. Subsets of patients had long-term follow-up (14-22 years, n = 6) and dark-adaptation kinetics measured (n = 5).

Results

Within central lesions with large serous retinal detachments, rod sensitivity was severely reduced but visual acuity and cone sensitivity were relatively retained. In surrounding extralesional areas, there was a mild but detectable widening of the subretinal space in some patients and some retinal areas. Available evidence was consistent with subretinal widening causing slower dark-adaptation kinetics. Over long-term follow-up, some eyes showed formation of de novo satellite lesions at retinal locations that years previously demonstrated subretinal widening. A subclinical abnormality consisting of a retina-wide mild thickening of the outer nuclear layer was evident in many patients and thickening increased in the subset of patients with long-term follow-up.

Conclusions

Outcome measures for future clinical trials should include evaluations of rod sensitivity within central lesions and quantitative measures of outer retinal structure in normal-appearing regions surrounding the lesions.

Correcting QUEST Magnetic Resonance Imaging-Sensitive Free Radical Production in the Outer Retina In Vivo Does Not Correct Reduced Visual Performance in 24-Month-Old C57BL/6J Mice

Purpose

To test the hypothesis that acutely correcting a sustained presence of outer retina free radicals measured in vivo in 24-month-old mice corrects their reduced visual performance.

Methods

Male C57BL/6J mice two and 24 months old were noninvasively evaluated for unremitted production of paramagnetic free radicals based on whether 1/T1 in retinal laminae are reduced after acute antioxidant administration (QUEnch-assiSTed [QUEST] magnetic resonance imaging [MRI]). Superoxide production was measured in freshly excised retina (lucigenin assay). Combining acute antioxidant administration with optical coherence tomography (i.e., QUEST OCT) tested for excessive free radical–induced shrinkage of the subretinal space volume. Combining antioxidant administration with optokinetic tracking tested for a contribution of uncontrolled free radical production to cone-based visual performance declines.

Results

At two months, antioxidants had no effect on 1/T1 in vivo in any retinal layer. At 24 months, antioxidants reduced 1/T1 only in superior outer retina. No age-related change in retinal superoxide production was measured ex vivo, suggesting that free radical species other than superoxide contributed to the positive QUEST MRI signal at 24 months. Also, subretinal space volume did not show evidence for age-related shrinkage and was unresponsive to antioxidants. Finally, visual performance declined with age and was not restored by antioxidants that were effective per QUEST MRI.

Conclusions

An ongoing uncontrolled production of outer retina free radicals as measured in vivo in 24 mo C57BL/6J mice appears to be insufficient to explain reductions in visual performance.

OCT imaging of rod mitochondrial respiration in vivo

There remains a need for high spatial resolution imaging indices of mitochondrial respiration in the outer retina that probe normal physiology and measure pathogenic and reversible conditions underlying loss of vision. Mitochondria are involved in a critical, but somewhat underappreciated, support system that maintains the health of the outer retina involving stimulus-evoked changes in subretinal space hydration. The subretinal space hydration light-dark response is important because it controls the distribution of vision-critical interphotoreceptor matrix components, including anti-oxidants, pro-survival factors, ions, and metabolites. The underlying signaling pathway controlling subretinal space water management has been worked out over the past 30 years and involves cGMP/mitochondria respiration/pH/RPE water efflux. This signaling pathway has also been shown to be modified by disease-generating conditions, such as hypoxia or oxidative stress. Here, we review recent advances in MRI and commercially available OCT technologies that can measure stimulus-evoked changes in subretinal space water content based on changes in the external limiting membrane-retinal pigment epithelium region. Each step within the above signaling pathway can also be interrogated with FDA-approved pharmaceuticals. A highlight of these studies is the demonstration of first-in-kind in vivo imaging of mitochondria respiration of any cell in the body. Future examinations of subretinal space hydration are expected to be useful for diagnosing threats to sight in aging and disease, and improving the success rate when translating treatments from bench-to-bedside.

Functional regulation of an outer retina hyporeflective band on optical coherence tomography images

Human and animal retinal optical coherence tomography (OCT) images show a hyporeflective band (HB) between the photoreceptor tip and retinal pigment epithelium layers whose mechanisms are unclear. In mice, HB magnitude and the external limiting membrane-retinal pigment epithelium (ELM-RPE) thickness appear to be dependent on light exposure, which is known to alter photoreceptor mitochondria respiration. Here, we test the hypothesis that these two OCT biomarkers are linked to metabolic activity of the retina. Acetazolamide, which acidifies the subretinal space, had no significant impact on HB magnitude but produced ELM-RPE thinning. Mitochondrial stimulation with 2,4-dinitrophenol reduced both HB magnitude and ELM-RPE thickness in parallel, and also reduced F-actin expression in the same retinal region, but without altering ERG responses. For mice strains with relatively lower (C57BL/6J) or higher (129S6/ev) rod mitochondrial efficacy, light-induced changes in HB magnitude and ELM-RPE thickness were correlated. Humans, analyzed from published data captured with a different protocol, showed a similar light-dark change pattern in HB magnitude as in the mice. Our results indicate that mitochondrial respiration underlies changes in HB magnitude upstream of the pH-sensitive ELM-RPE thickness response. These two distinct OCT biomarkers could be useful indices for non-invasively evaluating photoreceptor mitochondrial metabolic activity.

Coculture techniques for modeling retinal development and disease, and enabling regenerative medicine

Stem cell-derived retinal organoids offer the opportunity to cure retinal degeneration of wide-ranging etiology either through the study of in vitro models or the generation of tissue for transplantation. However, despite much work in animals and several human pilot studies, satisfactory therapies have not been developed. Two major challenges for retinal regenerative medicine are (a) physical cell-cell interactions, which are critical to graft function, are not formed and (b) the host environment does not provide suitable queues for development. Several strategies offer to improve the delivery, integration, maturation, and functionality of cell transplantation. These include minimally invasive delivery, biocompatible material vehicles, retinal cell sheets, and optogenetics. Optimizing several variables in animal models is practically difficult, limited by anatomical and disease pathology which is often different to humans, and faces regulatory and ethical challenges. High-throughput methods are needed to experimentally optimize these variables. Retinal organoids will be important to the success of these models. In their current state, they do not incorporate a representative retinal pigment epithelium (RPE)-photoreceptor interface nor vascular elements, which influence the neural retina phenotype directly and are known to be dysfunctional in common retinal diseases such as age-related macular degeneration. Advanced coculture techniques, which emulate the RPE-photoreceptor and RPE-Bruch's-choriocapillaris interactions, can incorporate disease-specific, human retinal organoids and overcome these drawbacks. Herein, we review retinal coculture models of the neural retina, RPE, and choriocapillaris. We delineate the scientific need for such systems in the study of retinal organogenesis, disease modeling, and the optimization of regenerative cell therapies for retinal degeneration.

Preventing diabetic retinopathy by mitigating subretinal space oxidative stress in vivo

Patients with diabetes continue to suffer from impaired visual performance before the appearance of overt damage to the retinal microvasculature and later sight-threatening complications. This diabetic retinopathy (DR) has long been thought to start with endothelial cell oxidative stress. Yet newer data surprisingly finds that the avascular outer retina is the primary site of oxidative stress before microvascular histopathology in experimental DR. Importantly, correcting this early oxidative stress is sufficient to restore vision and mitigate the histopathology in diabetic models. However, translating these promising results into the clinic has been stymied by an absence of methods that can measure and optimize anti-oxidant treatment efficacy in vivo. Here, we review imaging approaches that address this problem. In particular, diabetes-induced oxidative stress impairs dark-light regulation of subretinal space hydration, which regulates the distribution of interphotoreceptor binding protein (IRBP). IRBP is a vision-critical, anti-oxidant, lipid transporter, and pro-survival factor. We show how optical coherence tomography can measure subretinal space oxidative stress thus setting the stage for personalizing anti-oxidant treatment and prevention of impactful declines and loss of vision in patients with diabetes.

Light-induced changes of the subretinal space of the temporal retina observed via optical coherence tomography

Photoreceptor function is impaired in many retinal diseases like age-related macular degeneration. Currently, assessment of the photoreceptor function for the early diagnosis and monitoring of these diseases is either subjective, as in visual field testing, requires contact with the eye, like in electroretinography, or relies on research prototypes with acquisition speeds unattained by conventional imaging systems. We developed an objective, noncontact method to monitor photoreceptor function using a standard optical coherence tomography system. This method can be used with various white light sources for stimulation. The technique was applied in five volunteers and detected a decrease of volume of the subretinal space associated with light adaptation processes of the retina.

Mitochondrial Respiration in Outer Retina Contributes to Light-Evoked Increase in Hydration In Vivo

Purpose

To test the hypothesis that mitochondrial respiration contributes to local changes in hydration involved in phototransduction-driven expansion of outer retina, as measured by structural responses on optical coherence tomography (OCT) and diffusion magnetic resonance imaging (MRI).

Methods

Oxygen consumption rate and mitochondrial reserve capacity of freshly isolated C57BL/6 and 129S6/SvEvTac mouse retina were measured using a Seahorse Extracellular Flux Analyzer. Light-stimulated outer retina layer water content was determined by proton density MRI, structure and thickness by ultrahigh-resolution OCT, and water mobility by diffusion MRI.

Results

Compared with C57BL/6 mice, 129S6/SvEvTac retina demonstrated a less robust mitochondrial respiratory basal level, with a higher reserve capacity and lower oxygen consumption in the light, suggesting a relatively lower production of water. C57BL/6 mice showed a light-triggered surge in water content of outer retina in vivo as well as an increase in hyporeflective bands, thickness, and water mobility. In contrast, light did not evoke augmented hydration in this region or an increase in hyporeflective bands or water mobility in the 129S6/SvEvTac outer retina. Nonetheless, we observed a significant but small increase in outer retinal thickness.

Conclusions

These studies suggest that respiratory-controlled hydration in healthy retina is linked with a localized light-evoked expansion of the posterior retina in vivo and may serve as a useful biomarker of the function of photoreceptor/retinal pigment epithelium complex.

BEST1 gene therapy corrects a diffuse retina-wide microdetachment modulated by light exposure

One of the most common forms of monogenic macular degeneration worldwide is caused by dominant or recessive bestrophinopathies associated with mutations in the BEST1 gene. Disease expression is known to start with a retina-wide electrophysiological defect leading to localized vitelliform and atrophic lesions and vision loss. To develop lasting therapies for this incurable disease, there is a need for greater understanding of the early pathophysiology before lesion formation. Here we find that the loss of retinal pigment epithelium apical microvilli and resulting microdetachment of the retina represent the earliest features of canine bestrophinopathies. We show that retinal light exposure expands, and dark adaptation contracts, the microdetachments. Subretinal adeno-associated virus-based gene therapy corrects both the vitelliform lesions and the light-modulated microdetachments.

Mutations in the BEST1 gene cause detachment of the retina and degeneration of photoreceptor (PR) cells due to a primary channelopathy in the neighboring retinal pigment epithelium (RPE) cells. The pathophysiology of the interaction between RPE and PR cells preceding the formation of retinal detachment remains not well-understood. Our studies of molecular pathology in the canine BEST1 disease model revealed retina-wide abnormalities at the RPE-PR interface associated with defects in the RPE microvillar ensheathment and a cone PR-associated insoluble interphotoreceptor matrix. In vivo imaging demonstrated a retina-wide RPE–PR microdetachment, which contracted with dark adaptation and expanded upon exposure to a moderate intensity of light. Subretinal BEST1 gene augmentation therapy using adeno-associated virus 2 reversed not only clinically detectable subretinal lesions but also the diffuse microdetachments. Immunohistochemical analyses showed correction of the structural alterations at the RPE–PR interface in areas with BEST1 transgene expression. Successful treatment effects were demonstrated in three different canine BEST1 genotypes with vector titers in the 0.1-to-5E11 vector genomes per mL range. Patients with biallelic BEST1 mutations exhibited large regions of retinal lamination defects, severe PR sensitivity loss, and slowing of the retinoid cycle. Human translation of canine BEST1 gene therapy success in reversal of macro- and microdetachments through restoration of cytoarchitecture at the RPE–PR interface has promise to result in improved visual function and prevent disease progression in patients affected with bestrophinopathies.

Light-Induced Thickening of Photoreceptor Outer Segment Layer Detected by Ultra-High Resolution OCT Imaging

PURPOSE

We examined if light induces changes in the retinal structure that can be observed using optical coherence tomography (OCT).

METHODS

Normal C57BL/6J mice (age 3-6 months) adapted to either room light (15 minutes to ∼5 hours, 50-500 lux) or darkness (overnight) were imaged using a Bioptigen UHR-OCT system. Confocal histologic images were obtained from mice killed under light- or dark-adapted conditions.

RESULTS

The OCT image of eyes adapted to room light exhibited significant increases (6.1 ± 0.8 μm, n = 13) in total retina thickness compared to the same eyes after overnight dark adaptation. These light-adapted retinal thickness changes occurred mainly in the outer retina, with the development of a hyporeflective band between the RPE and photoreceptor-tip layers. Histologic analysis revealed a light-evoked elongation between the outer limiting membrane and Bruch's membrane from 45.8 ± 1.7 μm in the dark (n = 5) to 52.1 ± 3.7 μm (n = 5) in the light. Light-adapted retinas showed an increase of actin staining in RPE apical microvilli at the same location as the hyporeflective band observed in OCT images. Elongation of the outer retina could be detected even with brief light exposures, increasing 2.1 ± 0.3 μm after 15 minutes (n = 9), and 4.1 ± 1.0 μm after 2 hours (n = 6). Conversely, dark-adaptation caused outer retinal shortening of 1.4 ± 0.4 μm (n = 7) and 3.0 ± 0.5 μm (n = 8) after 15 minutes and 2 hours, respectively.

CONCLUSIONS

Light-adaption induces an increase in the thickness of the outer retina and the appearance of a hyporeflective band in the OCT image. This is consistent with previous reports of light-induced fluid accumulation in the subretinal space.

New insights into retinoid metabolism and cycling within the retina

The retinoid cycle is a series of biochemical reactions within the eye that is responsible for synthesizing the chromophore, 11-cis retinal, for visual function. The chromophore is bound to G-protein coupled receptors, opsins, within rod and cone photoreceptor cells forming the photosensitive visual pigments. Integral to the sustained function of photoreceptors is the continuous generation of chromophore by the retinoid cycle through two separate processes, one that supplies both rods and cones and another that exclusively supplies cones. Recent findings such as RPE65 localization within cones and the pattern of distribution of retinoid metabolites within mouse and human retinas have challenged previous proposed schemes. This review will focus on recent findings regarding the transport of retinoids, the mechanisms by which chromophore is supplied to both rods and cones, and the metabolism of retinoids within the posterior segment of the eye.

Cone outer segment extracellular matrix as binding domain for interphotoreceptor retinoid-binding protein

Cones are critically dependent on interphotoreceptor retinoid-binding protein (IRBP) for retinoid delivery in the visual cycle. Cone-dominant vertebrates offer an opportunity to uncover the molecular basis of IRBP's role in this process. Here, we explore the association of IRBP with the interphotoreceptor matrix (IPM) of cones vs. rods in cone dominant retinas from chicken (Gallus domesticus), turkey (Meleagris gallopavo), and pig (Sus scrofa). Retinas were detached and fixed directly or washed in saline prior to fixation. Disassociated photoreceptors with adherent matrix were also prepared. Under 2 mM CaCl(2) , insoluble matrix was delaminated from saline washed retinas. The distribution of IRBP, as well as glycans binding peanut agglutinin (cone matrix) and wheat germ agglutinin (rod/cone matrix), was defined by confocal microscopy. Retina flat mounts showed IRBP diffusely distributed in an interconnecting, lattice-like pattern throughout the entire matrix. Saline wash replaced this pattern with fluorescent annuli surrounding individual cone outer segments. In isolated cones and matrix sheets, IRBP colocalized with the peanut agglutinin binding matrix glycans. Our results reveal a wash-resistant association of IRBP with a matrix domain immediately surrounding cone outer segments. The cone matrix sheath may be responsible for IRBP-mediated cone targeting of 11-cis retinoids.

RPE65 is present in human green/red cones and promotes photopigment regeneration in an in vitro cone cell model

RPE65 is an abundantly expressed protein within the retinal pigment epithelium (RPE) of the eye that is required for retinoid metabolism to support vision. Its genetic mutations are linked to the congenital disease Leber congenital amaurosis Type 2 (LCA2) characterized by the early onset of central vision loss. Current gene therapy trials have targeted restoration of functional RPE65 within the RPE of these patients with some success. Recent data show that RPE65 is also present within mouse cones to promote function. In this study, we evaluated the presence of RPE65 in human cones and investigated its potential mechanism for supporting cone function in the 661W cone cell line. We found that RPE65 was selectively expressed in human green/red cones but absent from blue cones and mediated ester hydrolysis for photopigment synthesis in vitro. These data suggest that cone RPE65 supports human diurnal vision, potentially enhancing our strategies for treating LCA2.

Interphotoreceptor retinoid-binding protein as the physiologically relevant carrier of 11-cis-retinol in the cone visual cycle

Cones function in constant light and are responsible for mediating daytime human vision. Like rods, cones use the photosensitive molecule 11-cis-retinal to detect light, and in constant illumination, a continuous supply of 11-cis-retinal is needed. A retina visual cycle is thought to provide a privileged supply of 11-cis-retinal to cones by using 11-cis-retinol generated in Müller cells. In the cycle, 11-cis-retinol is transported from Müller cells to cone inner segments, where it is oxidized to 11-cis-retinal. This oxidation step is only performed in cones, thus rendering the cycle cone-specific. Interphotoreceptor retinoid-binding protein (IRBP) is a retinoid-binding protein in the subretinal space that binds 11-cis-retinol endogenously. Cones in Irbp(-/-) mice are retinoid-deficient under photopic conditions, and it is possible that 11-cis-retinol supplies are disrupted in the absence of IRBP. We tested the hypothesis that IRBP facilitates the delivery of 11-cis-retinol to cones by preserving the isomeric state of 11-cis-retinol in light. With electrophysiology, we show that the cone-like photoreceptors of Nrl(-/-) mice use the cone visual cycle similarly to wild-type cones. Then, using oxidation assays in isolated Nrl(-/-)Rpe65(-/-) retinas, we show that IRBP delivers 11-cis-retinol for oxidation in cones and improves the efficiency of the oxidation reaction. Finally, we show that IRBP protects the isomeric state of 11-cis-retinol in the presence of light. Together, these findings suggest that IRBP plays an important role in the delivery of 11-cis-retinol to cones and can facilitate cone function in the presence of light.

The interphotoreceptor retinoid binding (IRBP) is essential for normal retinoid processing in cone photoreceptors

11-cis Retinal is the light-sensitive component in rod and cone photoreceptors, and its isomerization to all-trans retinal in the presence of light initiates the visual response. For photoreceptors to function normally, all-trans retinal must be converted back into 11-cis retinal through the visual cycle. While rods are primarily responsible for dim light vision, the ability of cones to function in constant light is essential to human vision and may be facilitated by cone-specific visual cycle pathways. The interphotoreceptor retinoid-binding protein (IRBP) is a proposed retinoid transporter in the visual cycle, but rods in Irbp ( -/- ) mice have a normal visual cycle. However, there is evidence that IRBP has cone-specific functions. Cone electroretinogram (ERG) responses are reduced, despite having cone densities and opsin levels similar to C57Bl/6 (WT) mice. Treatment with 9-cis retinal rescues the cone response in Irbp ( -/- ) mice and shows that retinoid deficiency underlies cone dysfunction. These data indicate that IRBP is essential to normal cone function and demonstrate that differences exist in the visual cycle of rods and cones.

[The role of retinal pigment epithelium in visual functions]

The evolution of light sensitive cells probably began with a primitive functional unit composed of a photoreceptor cell and a pigmented cell. Even during embryonic development this functional unit is formed in a differentiation process in which the two interacting partners depend on each other. For some of the most important forms of retinal degeneration this knowledge on the functional cooperation between retinal pigment epithelium and photoreceptors is of great importance for analysis and development of therapeutic approaches. In this way mutations of genes which are expressed in photoreceptors can lead to diseases which start in the retinal pigment epithelium and vice versa. This article summarizes the variety of different functions of the retinal pigment epithelium and describes the failure of those functions which are of most clinical importance.

Normal cone function requires the interphotoreceptor retinoid binding protein

11-cis-retinal is the light-sensitive component in rod and cone photoreceptors, and its isomerization to all-trans retinal in the presence of light initiates the visual response. For photoreceptors to function normally, all-trans retinal must be converted back into 11-cis-retinal through a series of enzymatic steps known as the visual cycle. The interphotoreceptor retinoid-binding protein (IRBP) is a proposed retinoid transporter in the visual cycle, but rods in Irbp(-/-) mice have a normal visual cycle. While rods are primarily responsible for dim light vision, the ability of cones to function in constant light is essential to human vision and may be facilitated by cone-specific visual cycle pathways. We analyzed the cones in Irbp(-/-) mice to determine whether IRBP has a cone-specific visual cycle function. Cone electroretinogram (ERG) responses were reduced in Irbp(-/-) mice, but similar responses from Irbp(-/-) mice at all ages suggest that degeneration does not underlie cone dysfunction. Furthermore, cone densities and opsin levels in Irbp(-/-) mice were similar to C57BL/6 (wild-type) mice, and both cone opsins were properly localized to the cone outer segments. To test for retinoid deficiency in Irbp(-/-) mice, ERGs were analyzed before and after intraperitoneal injections of 9-cis-retinal. Treatment with 9-cis-retinal produced a significant recovery of the cone response in Irbp(-/-) mice and shows that retinoid deficiency underlies cone dysfunction. These data indicate that IRBP is essential to normal cone function and demonstrate that differences exist in the visual cycle of rods and cones.

In vivo measurement of retinal physiology with high-speed ultrahigh-resolution optical coherence tomography

Noninvasive in vivo functional optical imaging of the intact retina is demonstrated by using high-speed, ultrahigh-resolution optical coherence tomography (OCT). Imaging was performed with 2.8 microm resolution at a rate of 24,000 axial scans per second. A white-light stimulus was applied to the dark-adapted rat retina, and the average reflectivities from different intraretinal layers were monitored as a function of time. A 10%-15% increase in the average amplitude reflectance of the photoreceptor outer segments was observed in response to the stimulus. The spatial distribution of the change in the OCT signal is consistent with an increase in backscatter from the photoreceptor outer segments. To our knowledge, this is the first in vivo demonstration of OCT functional imaging in the intact retina.

The electro-oculogram

The retinal pigment epithelium (RPE) lying distal to the retina regulates the extracellular environment and provides metabolic support to the outer retina. RPE abnormalities are closely associated with retinal death and it has been claimed several of the most important diseases causing blindness are degenerations of the RPE. Therefore, the study of the RPE is important in Ophthalmology. Although visualisation of the RPE is part of clinical investigations, there are a limited number of methods which have been used to investigate RPE function. One of the most important is a study of the current generated by the RPE. In this it is similar to other secretory epithelia. The RPE current is large and varies as retinal activity alters. It is also affected by drugs and disease. The RPE currents can be studied in cell culture, in animal experimentation but also in clinical situations. The object of this review is to summarise this work, to relate it to the molecular membrane mechanisms of the RPE and to possible mechanisms of disease states.

Novel role for alphavbeta5-integrin in retinal adhesion and its diurnal peak

alpha(v)beta(5)-Integrin is the sole integrin receptor at the retinal pigment epithelium (RPE)-photoreceptor interface and promotes RPE phagocytic signaling to the tyrosine kinase Mer tyrosine kinase (MerTK) once a day in response to circadian photoreceptor shedding. Herein we identify a novel role for alpha(v)beta(5)-integrin in permanent RPE-photoreceptor adhesion that is independent of alpha(v)beta(5)'s function in retinal phagocytosis. To compare retinal adhesion of wild-type and beta(5)-integrin(-/-) mice, we mechanically separated RPE and neural retina and quantified RPE protein and pigment retention with the neural retina. Lack of alpha(v)beta(5)-integrin with normal expression of other RPE integrins greatly weakened retinal adhesion in young mice and accelerated its age-dependent decline. Unexpectedly, the strength of wild-type retinal adhesion varied with a diurnal rhythm that peaked 3.5 h after light onset, after the completion of phagocytosis, when integrin signaling to MerTK is minimal. Permanent alpha(v)beta(5) receptor deficiency attenuated the diurnal peak of retinal adhesion in beta(5)-integrin(-/-) mice. These results identify alpha(v)beta(5)-integrin as the first RPE receptor that contributes to retinal adhesion, a vital mechanism for long-term photoreceptor function and viability. Furthermore, they indicate that alpha(v)beta(5) receptors at the same apical plasma membrane domain of RPE cells fulfill two separate functions that are synchronized by different diurnal rhythms.

The retinal pigment epithelium in visual function

Located between vessels of the choriocapillaris and light-sensitive outer segments of the photoreceptors, the retinal pigment epithelium (RPE) closely interacts with photoreceptors in the maintenance of visual function. Increasing knowledge of the multiple functions performed by the RPE improved the understanding of many diseases leading to blindness. This review summarizes the current knowledge of RPE functions and describes how failure of these functions causes loss of visual function. Mutations in genes that are expressed in the RPE can lead to photoreceptor degeneration. On the other hand, mutations in genes expressed in photoreceptors can lead to degenerations of the RPE. Thus both tissues can be regarded as a functional unit where both interacting partners depend on each other.

Mouse retina explants after long-term culture in serum free medium

The neonatal mouse retina remains viable as an explant in serum-supplemented growth media for more than 4 weeks. Interpretation of drug effects on this tissue is compromised by the enigmatic composition of the serum. We sought to remove this ambiguity by culturing neonatal as well as late postnatal mouse retina in serum-free nutrient medium. In this study three important observations were made, (1) there is histotypic development of neonatal as well as preservation of late postnatal mouse retinal structure during long-term culture in serum-free medium, although the late postnatal tissue tends to show some loss of cells in the outer nuclear layer. (2) Protein expression in explant photoreceptor cells was similar to that in the litter-matched ones, except for green cone opsin and interphotoreceptor retinoid-binding protein, although mRNA of the latter is present at similar amounts as in age-matched in vivo controls. (3) Cells of the inner retina stained by antibodies to calcium-binding proteins display some novel sprouting of processes. The results show that the mouse retina can be cultured as an explant for more than 4 weeks in a serum-free medium. This represents an important step forward because, (1) the possibility of interference of drug effects by unknown serum factors has been eliminated; and (2) the spent culture medium can be analyzed to investigate biomolecules released by the retina in vitro.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

Soluble expression in E. coli of a functional interphotoreceptor retinoid-binding protein module fused to thioredoxin: correlation of vitamin A binding regions with conserved domains of C-terminal processing proteases

The exchange of all-trans retinol and 11-cis retinal between the photoreceptors and retinal pigmented epithelium is mediated by interphotoreceptor retinoid-binding protein (IRBP). IRBP contains binding sites for retinoids, docosahexaenoic acid and probably cell surface and matrix receptors. IRBP arose through the quadruplication of an ancient protein, represented by its carboxy-terminal module (module 4 in amphibians and mammals). Module 4 has retinol binding activity and is composed of regions coded for by each of IRBP's four exons. Determining the function of the exons has been hampered by insoluble expression of module 4 in Escherichia coli. Here, we found that module 4 of Xenopus IRBP (X4IRBP), as well as its exon segments, can be expressed in a soluble form as thioredoxin fusion proteins. The recombinant proteins were purified by ion exchange and arsenical-based affinity chromatography. Liquid chromatography/mass spectrometry confirmed that the sequence of X4IRBP is correct. All-trans retinol binding was characterized by monitoring enhancement of retinol fluorescence, quenching of intrinsic protein fluorescence, and transfer of energy to the bound retinol. Retinol bound to X4IRBP at 2.20+/-0.29 sites with a KD=1.25+/-0.39. One of the two sites was localized to Exons(2+3) and had a KD=0.26+/-0.13 micron. This site, which supported protein quenching and energy transfer, probably contains at least one of the two conserved tryptophans present in this segment. The second site was localized to Exon 4. This site supported the enhancement of retinol fluorescence but not protein quenching or energy transfer and had a KD=1.94+/-0.20 micron. Exon 1 had no retinol binding activity. The location of the retinol binding regions correlated with the distribution of domains conserved between IRBPs and the newly recognized family of C-terminal processing proteases (CtpAs), proteins which bind and cleave non-polar carboxy termini.

Matrix metalloproteinase-2 and tissue inhibitor of metalloproteinase-1 in the retinal pigment epithelium and interphotoreceptor matrix: vectorial secretion and regulation

Matrix metalloproteinases (MMPs) and the tissue inhibitors of metalloproteinases (TIMPs) play an essential role in both normal and pathological extracellular matrix degradation, and a TIMP has been associated with at least one type of retinal degeneration. We have studied expression of MMP-2 and TIMP-1 by zymography, immunocytochemistry, and immunoblotting in the retinal pigment epithelium (RPE) from normal, aged and diseased retinas. MMPs and TIMPs were found in the rat RPE, interphotoreceptor matrix (IPM), and in media conditioned by human and rat RPE in culture. In other polarized cells. MMPs and TIMP-2 are secreted vectorially towards the basal lamina. In the RPE, however, MMP-2 and TIMP-1 were secreted preferentially from the apical surface, the surface bordering the IPM. These findings provide new evidence that MMPs and TIMPs could play a role in the turnover of IPM components. Cell homogenates and conditioned media from RPE isolated from mutant Royal College of Surgeons (RCS) rats with inherited retinal dystrophy had similar amounts of MMP-2 and TIMP-1 as those from congenic control rats. The secretion of MMP-2 and TIMP-1 from RPE cell cultures isolated from young and aged human donors varied widely. However, with increasing cell passage number, secretion of MMPs and TIMPs from human RPE increased dramatically. Also, growing human RPE on bovine corneal endothelial cell-generated extracellular matrix instead of plastic reduced the secretion of both MMPs and TIMPs. These data suggest that the integrity of Bruch's membrane may serve to regulate RPE functions in MMP and TIMP secretion and that extracellular matrices contain signals that regulate MMP and TIMP synthesis and/or secretion by the RPE.

Preference of peanut agglutinin labeling for long-wavelength-sensitive cone photoreceptors in the dace retina

Peanut agglutinin (PNA) was known for its selective binding to cone cells. In the present study, we investigated whether there was any difference in PNA binding among various subtypes of cone photoreceptor cells in the dace retina. The outer segments of the long-double- and long-single-cone cells were preferentially labeled with PNA. Ultrastructural pre-embedding labeling revealed that the binding sites of PNA were confined to the calycal processes of these cells. By contrast, only slight labeling was discerned on the corresponding regions of other types of cone cells. The results indicate that PNA can distinguish the long-wavelength-sensitive cone from the short-to-middle-wavelength-sensitive cone cells.

Association of matrix metalloproteinases with interphotoreceptor retinoid binding protein

PURPOSE

Interphotoreceptor retinoid binding protein (IRBP) is one of the major components of the interphotoreceptor matrix (IPM) where it may function in retinoid transport between the photoreceptor cells and the retinal pigment epithelium. In the course of studies of the metalloproteinases (MPs) of the IPM, we found that some MPs remain associated with IRBP through the standard purification scheme. We wished to report this finding as a caution to those working with IRBP prepared from fresh tissue.

METHODS

IRBP was prepared from bovine IPM by procedures commonly used for its purification, including ion exchange, concanavalin A affinity and gel filtration chromatographies. The MPs were detected by zymography, both gelatin and casein, run with and without preactivation.

RESULTS

Through each step of the purification both gelatinase and, especially, caseinase (stromelysin) activities were associated with IRBP. Inclusion of gelatin affinity chromatography did not totally remove the gelatinases. Much of this activity was latent and was only revealed following fractionation or by preactivation before zymography. Whether the fractionation steps remove an inhibitor or simply provide conditions appropriate for the activation of the latent zymogen forms is not known.

CONCLUSIONS

The close association of the MPs and IRBP may suggest a functional role for this complex. As a practical consideration, it is likely that many preparations of IRBP may be contaminated with one or more MPs. We found no evidence for any other class of proteinase. Caution should thus be exercised in using an IRBP preparation purified from fresh tissue. It should be monitored for proteinase activity by zymography and/or prepared or stored in the presence of EDTA or dithiothreitol as much as possible.

The interphotoreceptor matrix, a space in sight

The interphotoreceptor matrix (IPM) has in recent years been receiving much attention due to its delicate localization between the photoreceptors and the retinal pigment epithelium (RPE). The IPM is a resilient, structure forming and hydrophilic matrix composed of large glycoproteins and proteoglycans, which occupies the subretinal space between the photoreceptors. The IPM is most likely assembled with components synthesized by all the surrounding cell types: the photoreceptor cells, the RPE cells, and the Müller cells. It has been implied to be involved in the development and maintenance of photoreceptors, and as a major factor in retinal adhesion. Therefore, it has been thoroughly studied also in several models of photoreceptor degeneration. Comparative studies have revealed some remarkably consistent features between different species, such as the presence of the rod and cone specific matrix domains. Studies made in the IPM of several species have measured large fluctuations in ion concentrations as a result of changes in illumination. In some species, these ionic fluctuations coincide with the intriguing dynamic redistributions of IPM constituents that can be visualized with histochemical techniques. It can be hypothesized that because of the intensive biochemical activity and the frequent changes in metabolic states of rods and cones the IPM may act as a kind of "buffer." These studies have brought a new extracellular aspect to photoreceptor studies and a new perspective to photoreceptor-RPE research.

Interphotoreceptor retinoid-binding protein (IRBP): expression in the adult and developing Xenopus retina

Apposition of the neural retina and pigment epithelium is critical to photoreceptor development and function. Interphotoreceptor retinoid-binding protein (IRBP) is a major component of the extracellular matrix separating these epithelia in the African clawed frog Xenopus laevis (Gonzalez-Fernandez et al., [1993], J. Cell Sci. 105:7-21). In the adult retina, IRBP appears to mediate the transport of hydrophobic molecules, particularly retinoids and fatty acids, within the hydrophilic extracellular domain. In this paper, we compare the distribution of IRBP and its mRNA in adult and embryonic Xenopus retina. Xenopus IRBP antisense RNA, labeled with tritium or digoxigenin, was used for in situ hybridizaton studies. For immunohistochemistry, we used an antiserum against Xenopus IRBP expressed in Escherichia coli. In the adult, we found that IRBP is synthesized at similar levels by both rods and cones. The protein is restricted to the interphotoreceptor matrix, with lesser amounts in the pigment epithelial cytoplasm. In the embryo, expression of the mRNA for IRBP is restricted to the central retina, where photoreceptor differentiation has taken place. By contrast, the protein is distributed throughout the embryonic subretinal space. Therefore, the presence of IRBP precedes photoreceptor differentiation. In summary, IRBP is synthesized by both rods and cones and may be internalized by the pigment epithelium. In the embryo, IRBP is synthesized by the central retina and diffuses through the matrix, reaching the undifferentiated peripheral retina. In view of its ligand-binding properties, diffusion of IRBP may provide the peripheral neural retina with a vehicle to transport retinoids and docosahexaenoic acid (molecules critical to normal retinal development) from the pigment epithelium.

Retinal pigment epithelial fine structure in the American crow (Corvus brachyrhynchos)

The fine structure of the retinal epithelial (RPE) region has been investigated by light and electron microscopy in the American crow (Corvus brachyrhynchos). In this species the RPE consists of a single layer of cuboidal cells which display numerous deep basal (scleral) infoldings and plentiful apical (vitreal) microvillar processes which surround photoreceptor inner and outer segments. The RPE cells are joined laterally by a series of tight junctions (Verhoeff's membrane) located in the mid to basal region. Within the epithelial cells, smooth endoplasmic reticulum is very abundant while rough endoplasmic reticulum (RER) is scarce. Mitochondria of various shapes are abundant basally while polysomes are plentiful and widespread. In the light-adapted state RPE nuclei are large and vesicular and basally located while the melanosomes of these cells are predominantly located with the apical processes indicating photomechanical movements. Myeloid bodies are large and numerous and often have ribosomes on their outer surface. Bruch's membrane (complexus basalis) is typical of avian species in that it is pentalaminate and the lamina densa is displaced near the choriocapillaris. The endothelium of the choriocapillaris is thin facing Bruch's membrane but is only moderately fenestrated. Some of these fenestrations display a double-layered diaphragm while the majority show the more typical single-layered diaphragm noted in most species.

Calcium gradients and light-evoked calcium changes outside rods in the intact cat retina

We have studied light-evoked changes in extracellular Ca2+ concentration ([Ca2+]o) in the intact cat eye using ion-sensitive double-barreled microelectrodes. Two prominent changes in Ca2+ concentration were observed that differed in retinal location. There was a light-evoked increase in [Ca2+]o, accompanied by brief ON and OFF transients, which was maximal in the inner plexiform layer and was not further studied. There was an unexpected sustained light-evoked decrease in [Ca2+]o, of relatively rapid onset and offset, which was maximal in the distalmost region of the subretinal space (SRS). [Ca2+]o in the SRS was 1.0 mM higher than in the vitreous humor during dark adaptation and this transretinal gradient disappeared during rod-saturating illumination. After correcting for the light-evoked increase in the volume of the SRS, an increase in the total Ca2+ content of the SRS during illumination was revealed, which presumably represents the Ca2+ released by rods. To explain the light-evoked [Ca2+]o changes, we used the diffusion model described in the accompanying paper (Li et al., 1994b), with the addition of light-dependent sources of Ca2+ at the retina/retinal pigment epithelium (RPE) border and rod outer segments. We conclude that a drop in [Ca2+]o around photoreceptors, which persists during illumination and reduces a transretinal Ca2+ gradient, is the combined effect of the light-evoked SRS volume increase, Ca2+ release from photoreceptors, and an unidentified mechanism(s), which is presumably Ca2+ transport by the RPE. The relatively rapid onset and offset of the [Ca2+]o decrease remains unexplained. These steady-state shifts in [Ca2+]o should have significant effects on photoreceptor function, especially adaptation.

Redistribution of insoluble interphotoreceptor matrix components during photoreceptor differentiation in the mouse retina

The development of the nervous system is largely influenced by the extracellular matrix (ECM). In the neural retina, the photoreceptors are surrounded by a unique ECM, the interphotoreceptor matrix (IPM). The IPM plays a central and possibly crucial role in the development, maintenance and specific function of the photoreceptors. Therefore, the characterization of IPM components is necessary to understand the mechanisms regulating photoreceptor differentiation. The IPM in the mouse retina was examined during photoreceptor morphogenesis with the monoclonal antibody (MAb) F22, which recognizes a 250 kDa component of the interphotoreceptor matrix. The binding pattern of MAb F22 revealed a striking redistribution in the expression of the 250 kDa F22 antigen in late stage of postnatal photoreceptor differentiation in the mouse retina. The F22 staining was detectable in the IPM around the inner segments on the third postnatal day (P3). The MAb F22 initially labeled the region around inner segments, but as the outer segments elongated, the F22 distribution became concentrated to the matrix around the rod and cone outer segments until P16-17. At P17, the F22 label around rods began to disappear, while the label around cones became more defined. The shift in label distribution was largely completed by P20. Residual rod-associated label disappeared within a few days. In the adult animal, the F22 antibody labeled the cone-associated matrix only, and this labeling pattern remained stationary. The change in the distribution of MAb F22 demonstrated by immunolabeling was not accompanied by changes in the size of the molecule; F22 antigen isolated from the IPM of P13-15, and from adult IPM migrated with the same molecular weight on SDS gels. The distribution of MAb F22 was compared to that of chondroitin sulfate proteoglycans which are abundant in the IPM. The labeling patterns of MAbs CS-56, C6-S and C4-S were distinct from that of MAb F22. A general decrease of the label intensity was seen with two chondroitin sulfate MAbs (CS-56 and C4-S) between 16 days and 4 months, but a total loss of rod-associated label was not observed. All three chondroitin sulfate MAbs labeled the retina at embryonic day (E) 11.5-13.5, a time of outgrowth of ganglion cell axons, but the F22 antigen was not detected in the retina at this stage of development. The results demonstrate that the F22 and the chondroitin sulfate antibodies are recognizing different molecules that have distinct roles in retinal morphogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Alterations in light-evoked dopamine metabolism in dystrophic retinas of mutant rds mice

In dystrophic retinas of rds mice, which are devoid of photoreceptor outer segments, high steady state levels of dopamine were found in dark and light periods. These levels were similar to those observed in normal, BALB/c mouse retinas. Major differences were determined, however, between dopamine turnover in normal and dystrophic retinas. While substantial light-evoked elevation of dopamine synthesis and utilization was observed in normal retinas, dopamine synthesis and metabolism in rds retinas was very low and response to light was depressed. Retinal dopamine metabolism was already depressed in 2 week old rds mice, prior to the onset of photoreceptor cell death, relative to that in age-matched BALB/c mice. At 1 month of age, robust light/dark differences in retinal dopamine metabolism were observed in BALB/c mice, while no significant effect of light was seen in rds mice. The limited ability of the dopaminergic system in rds retinas to respond to light may be due to the absence of normal outer segments. Interestingly, in old rds retinas, although most photoreceptor cells had degenerated, a small but significant light-evoked increase in dopamine metabolism was measured. The presence of relatively high steady state levels of dopamine in rds retinas, despite the reduced dopamine synthetic activity, is maintained by a compensatory reduction in dopamine utilization. Thus, although a considerable amount of dopamine is present in the rds retina, it might not be available to exert its biological functions.

pHi-dependent Cl-HCO3 exchange at the basolateral membrane of frog retinal pigment epithelium

Intracellular pH (pHi) measurements in frog retinal pigment epithelium using the pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein demonstrate that the basolateral membrane contains a pHi-sensitive Cl-HCO3 exchanger. In control Ringer solution, the removal of Cl from the basal bath alkalinized the cells by 0.07 +/- 0.03 (SD) pH units (n = 39) with an initial rate of 0.022 +/- 0.0013 pH units/min. This effect was blocked by 0.5 mM basal 4,4'-diisothiocyanostilbene-2,2'- disulfonic acid or the removal of HCO3 from both the apical and basal baths. The rate of the exchange is reduced by acidification and increased by alkalinization. Increasing apical bath K concentration ([K]o) from 2 to 5 mM approximates the [K]o change in the subretinal space of the intact eye following a transition from light to dark. This [K]o change alkalinized the cells by increasing the rate of the apical membrane Na-HCO3 cotransporter. In 5 mM apical [K]o, the initial rate of the 0 Cl-induced alkalinization was significantly increased to 304 +/- 13% (n = 4) of control (2 mM [K]o). These mechanisms regulate pHi and could also buffer changes in subretinal pH.

K+ and Cl- transport mechanisms in bovine pigment epithelium that could modulate subretinal space volume and composition

1. Conventional and ion-selective double-barrelled microelectrodes were used in an in vitro bovine retinal pigment epithelium (RPE)-choroid preparation to measure the changes in membrane voltage, resistance and intracellular K+ and Cl- activities produced by small, physiological changes in extracellular potassium ([K+]o). 2. In the intact eye, light-induced changes in [K+]o occur in the extracellular (or subretinal) space that separates the neural retina and the RPE apical membrane. These [K+]o changes can be approximated in vitro by decreasing apical bath [K+]o from 5 to 2 mM. 3. This in vitro change in [K+]o simultaneously decreased intracellular Cl- and K+ activities (aCli and aKi) by 25 +/- 6 mM (n = 8) and 19 +/- 7 mM (n = 4) (mean +/- S.D.), respectively. In control Ringer solution (5 mM [K+]o) aCli and aKi were 65 +/- 10 mM (n = 28) and 65 +/- 8 mM (n = 6), respectively. 4. The [K+]o-induced decreases in aCli and aKi were both significantly inhibited, either by blocking the apical membrane K+ conductance with Ba2+ or the basolateral membrane Cl- conductance with DIDS (4,4'-diisothiocyano-stilbene-2,2'-disulphonic acid). 5. Transepithelial current pulses were used to determine the relative basolateral membrane Cl- conductance, TClBAS, was approximately 0.6 (n = 3), and the relative apical membrane K+ conductance, TKAP, was approximately 0.7 (n = 2). Step changes in basal bath [K+]o were used to estimate the relative basolateral membrane K+ conductance, TKBAS, was approximately 0.34 (n = 3). 6. These data show that the apical membrane K+ conductance and the basolateral membrane Cl- conductance are electrically coupled. In vivo, this coupling could have significant functional importance by modulating the relative hydration of the subretinal space, regulating RPE cell volume, and buffering the chemical composition of the subretinal space.

Interphotoreceptor retinoid-binding protein (IRBP). Molecular biology and physiological role in the visual cycle of rhodopsin

The regeneration of visual pigment in rod photoreceptors of the vertebrate retina requires an exchange of retinoids between the neural retina and the retina pigment epithelium (RPE). It has been hypothesized that interphotoreceptor retinoid-binding protein (IRBP) functions as a two-way carrier of retinoid through the aqueous compartment (interphotoreceptor matrix) that separates the RPE and the photoreceptors. The first part of this review summarizes the cellular and molecular biology of IRBP. Work on the IRBP gene indicates that the protein contains a four-fold repeat structure that may be involved in binding multiple retinoid and fatty acid ligands. These repeats and other aspects of the gene structure indicate that the gene has had an active and complex evolutionary history. IRBP mRNA is detected only in retinal photoreceptors and in the pineal gland; expression is thus restricted to the two photosensitive tissues of vertebrate organisms. In the second part of this review, we consider the results obtained in experiments that have examined the activity of IRBP in the process of visual pigment regeneration. We also consider the results obtained on the bleaching and regeneration of rhodopsin in the acutely detached retina, as well as in experiments testing the ability of IRBP to protect its retinoid ligand from isomerization and oxidation. Taken together, the findings provide evidence that, in vivo, IRBP facilitates both the delivery of all-trans retinol to the RPE and the transfer of 11-cis retinal from the RPE to bleached rod photoreceptors, and thereby directly supports the regeneration of rhodopsin in the visual cycle.

Photoreceptor differentiation of isolated retinal precursor cells includes the capacity for photomechanical responses

Isolated retinal precursor cells, grown without pigment epithelial or glial cells and in the absence of intercellular contacts, develop a complex set of photoreceptor-specific properties, including polarized structural and molecular organization and opsin immunoreactivity. We report here that these isolated embryonic photoreceptors are also capable of responding to light. Sequential photography showed that 50% of the photoreceptors grown in a light cycle elongate when exposed to light and contract in response to darkness. A smaller population (20%) showed the opposite response. Responses of individual cells could be observed during several sequential light cycles and resemble photomechanical movements in vivo [Ali, M. A. (1971) Vision Res. 11, 1225-1288]. The differentiation program expressed by isolated precursor cells, therefore, includes the capacity for highly complex functional activities that require light sensitivity. These observations raise challenging questions regarding the nature of the chromophore and pigments that mediate light-regulated behaviors of cultured photoreceptors.

Effects of light and darkness on pH outside rod photoreceptors in the cat retina

We recorded pH in the extracellular space surrounding rod photoreceptors in the dark-adapted eye of the cat and during illumination with double-barreled H(+)-selective microelectrodes. A pH of 7.17 was recorded in the vitreous at the retinal surface of the dark-adapted eye and this became more alkaline during light adaptation. In dark adaptation, a pH close to 7.00 was recorded in a region of maximal acidity in the extracellular space surrounding rods in the outer nuclear layer (ONL). pH steeply alkalinized as the microelectrode was moved more distally towards the retinal pigment epithelium (RPE), and almost reached the pH of the arterial blood at the apical surface of the RPE. Illumination produced an intraretinal alkalinization that was largest (up to 0.2 pH units) in the ONL, maximal in amplitude at rod-saturating intensities, and that was sustained during steady background illumination. The light-evoked alkalinization was relatively slow in onset, having a time constant (1/e) of 64 sec, and took 8-12.5 min to return to the dark-adapted level after the offset of maintained illumination. These results show that acid production by cat rods is highest in the dark, reflecting a high rate of energy metabolism, and suggest that glycolysis is required to support the dark current. Illumination, by suppressing both glycolysis and respiration, alkalinizes the extracellular space surrounding rods. The substantial change in pH outside rods from dark to light could alter pH dependent properties of the interphotoreceptor matrix.

Effects of intravenous acetazolamide on retinal pH in the cat

Double-barreled H(+)-selective microelectrodes were used to study the effect on intravenous acetazolamide on intraretinal pH in the cat. Acetazolamide (11.4-27.8 mg kg-1 intravenously) caused a rapid acidification of the subretinal space. This change in pH originated in the most distal portion of the subretinal space and could not be attributed to a change in pH or PCO2 of the arterial blood. Slow light-evoked alkalinizations in distal retina, attributable to a decrease in rod photoreceptor energy metabolism, were relatively unaltered by acetazolamide. This result indicated that acetazolamide had not crossed the blood-retinal barrier in sufficient amounts to change this response. In time, following intravenous perfusion of acetazolamide, continuous depth profiles of intraretinal pH showed an acidification of the entire retina and the vitreous also became more acidic. These results indicate that the rapid or primary effect of acetazolamide is an acidification of the distal portion of the subretinal space, which is thought to originate in a change in the transport of H+ or HCO3- by the retinal pigment epithelium. This is followed by an acidification of the entire retina and vitreous, presumably due to diffusion of acid from the distal retina, although there could be additional causes.

A role for microglia in the maintenance of photoreceptors in retinal transplants lacking pigment epithelium

Studies on intact retina have pointed to a necessary role for retinal pigment epithelium in the maintenance of photoreceptor outer segments and for regeneration of visual pigment. However, it has been shown that when embryonic retinae are separated from the pigment epithelium and transplanted into the brain of neonatal rats, the transplanted photoreceptors develop outer segments and the retina responds to light in the apparent absence of pigment epithelial cells. We confirm that there are no retinal pigment epithelium cells associated with transplanted retinae in the present series of experiments and show that a row of cells, composed predominantly of microglia of host origin, border the graft. These cells can be seen to contain engulfed outer segments when they are apposed to the outer retina, suggesting that the microglia have assumed, at the least, the phagocytic function normally associated with retinal pigment epithelium. Microglial cells and their processes are also found within the transplant, but these cells are typically devoid of phagosomes, indicating an absence of phagocytic activity. The close physical association of these resting microglia with the transplant may facilitate their role in antigen presentation under specific conditions of immune provocation.

Expression of opsin and IRBP genes in mutant RCS rats

The retinal pigment epithelium of RCS rats bearing the autosomal recessive rdy mutation fails to ingest shed rod outer segment tips. Accumulation of disk debris in the subretinal space of the maturing mutant retina causes a secondary degeneration of photoreceptor cells. Two hypotheses have been offered as possible explanations of the death of photoreceptor cells in this disorder: (1) photoreceptors are starved for amino acids, retinal, oxygen, etc; and (2) that IRBP levels and synthesis may be decreased and interfere with retinal transport and this deficiency is lethal to these cells. To test these hypotheses, we have studied the effect of this mutation on the levels of expression of opsin and IRBP genes, and gene products and on rates of synthesis at various ages in dystrophic RCS p+ rats and compared the results to those obtained with normal Long Evans rats. The mutant rats and normal controls had comparable amounts of opsin and IRBP mRNA transcripts and rates of synthesis up to post-natal day 45 (P45) but opsin transcripts were barely detectable at P60 and thereafter. IRBP mRNA levels were also very low after P62 although somewhat higher than opsin mRNA. Opsin could be detected immunochemically, albeit at lower levels, at all the ages studied up to P310, but IRBP levels fell below detection after P45. We localized opsin and IRBP in the retina by post-embedding EM immunocytochemical procedures and found that opsin is present in the remnants of rod outer segment debris, even at P390, long after detectable opsin synthesis had ceased. These data suggest that expression of opsin and IRBP genes is not influenced by the shape and state of the outer segments, and that the rdy mutation does not influence the expression of the opsin and IRBP in these retinas until the photoreceptor cells are profoundly damaged. Thus, neither hypothesis about the causes of cell death in this disorder is supported.