All-trans retinol in rod photoreceptor outer segments moves unrestrictedly by passive diffusion

In situ autofluorescence lifetime assay of a photoreceptor stimulus response in mouse retina and human retinal organoids

Photoreceptors are the key functional cell types responsible for the initiation of vision in the retina. Phototransduction involves isomerization and conversion of vitamin A compounds, known as retinoids, and their recycling through the visual cycle. We demonstrate a functional readout of the visual cycle in photoreceptors within stem cell-derived retinal organoids and mouse retinal explants based on spectral and lifetime changes in autofluorescence of the visual cycle retinoids after exposure to light or chemical stimuli. We also apply a simultaneous two- and three-photon excitation method that provides specific signals and increases contrast between these retinoids, allowing for reliable detection of their presence and conversion within photoreceptors. This multiphoton imaging technique resolves the slow dynamics of visual cycle reactions and can enable high-throughput functional screening of retinal tissues and organoid cultures with single-cell resolution.

Interphotoreceptor retinoid-binding protein removes all-trans-retinol and retinal from rod outer segments, preventing lipofuscin precursor formation

Interphotoreceptor retinoid-binding protein (IRBP) is a specialized lipophilic carrier that binds the all-trans and 11-cis isomers of retinal and retinol, and this facilitates their transport between photoreceptors and cells in the retina. One of these retinoids, all-trans-retinal, is released in the rod outer segment by photoactivated rhodopsin after light excitation. Following its release, all-trans-retinal is reduced by the retinol dehydrogenase RDH8 to all-trans-retinol in an NADPH-dependent reaction. However, all-trans-retinal can also react with outer segment components, sometimes forming lipofuscin precursors, which after conversion to lipofuscin accumulate in the lysosomes of the retinal pigment epithelium and display cytotoxic effects. Here, we have imaged the fluorescence of all-trans-retinol, all-trans-retinal, and lipofuscin precursors in real time in single isolated mouse rod photoreceptors. We found that IRBP removes all-trans-retinol from individual rod photoreceptors in a concentration-dependent manner. The rate constant for retinol removal increased linearly with IRBP concentration with a slope of 0.012 min^-1 μm^-1 IRBP also removed all-trans-retinal, but with much less efficacy, indicating that the reduction of retinal to retinol promotes faster clearance of the photoisomerized rhodopsin chromophore. The presence of physiological IRBP concentrations in the extracellular medium resulted in lower levels of all-trans-retinal and retinol in rod outer segments following light exposure. It also prevented light-induced lipofuscin precursor formation, but it did not remove precursors that were already present. These findings reveal an important and previously unappreciated role of IRBP in protecting the photoreceptor cells against the cytotoxic effects of accumulated all-trans-retinal.

All-trans retinal levels and formation of lipofuscin precursors after bleaching in rod photoreceptors from wild type and Abca4-/- mice

The accumulation of lipofuscin in the cells of the retinal pigment epithelium (RPE) is thought to play an important role in the development and progression of degenerative diseases of the retina. The bulk of RPE lipofuscin originates in reactions of the rhodopsin chromophore, retinal, with components of the photoreceptor outer segment. The 11-cis retinal isomer is generated in the RPE and supplied to rod photoreceptor outer segments where it is incorporated as the chromophore of rhodopsin. It is photoisomerized during light detection to all-trans and subsequently released by photoactivated rhodopsin as all-trans retinal, which is removed through reduction to all-trans retinol in a reaction requiring metabolic input in the form of NADPH. Both 11-cis and all-trans retinal can form lipofuscin precursor fluorophores in rod photoreceptor outer segments. Increased accumulation of lipofuscin has been suggested to result from excess formation of lipofuscin precursors due to buildup of all-trans retinal released by light exposure. In connection with this suggestion, the Abca4 transporter protein, an outer segment protein defects in which result in recessive Stargardt disease, has been proposed to promote the removal of all-trans retinal by facilitating its availability for reduction. To examine this possibility, we have measured the outer segment levels of all-trans retinal, all-trans retinol, and of lipofuscin precursors after bleaching by imaging the fluorescence of single rod photoreceptors isolated from wild type and Abca4^-/- mice. We found that all-trans retinol and all-trans retinal levels increased after bleaching in both wild type and Abca4^-/- rods. At all times after bleaching, there was no significant difference in all-trans retinal levels between the two strains. All-trans retinol levels were not significantly different between the two strains at early times, but were lower in Abca4^-/- rods at times longer than 20 min after bleaching. Bleaching in the presence of lower metabolic substrate concentrations resulted in higher all-trans retinal levels and increased formation of lipofuscin precursors in both wild type and Abca4^-/- rods. The results show that conditions that result in buildup of all-trans retinal levels result in increased generation of lipofuscin precursors in both wild type and Abca4^-/- rods. The results are consistent with the proposal that Abca4 facilitates the reduction of all-trans retinal to retinol; absence of Abca4 however does not appear to be associated with higher all-trans retinal levels compared to wild type.

In Vivo Two-Photon Fluorescence Kinetics of Primate Rods and Cones

Purpose

The retinoid cycle maintains vision by regenerating bleached visual pigment through metabolic events, the kinetics of which have been difficult to characterize in vivo. Two-photon fluorescence excitation has been used previously to track autofluorescence directly from retinoids and pyridines in the visual cycle in mouse and frog retinas, but the mechanisms of the retinoid cycle are not well understood in primates.

Methods

We developed a two-photon fluorescence adaptive optics scanning light ophthalmoscope dedicated to in vivo imaging in anesthetized macaques. Using pulsed light at 730 nm, two-photon fluorescence was captured from rods and cones during light and dark adaptation through the eye's pupil.

Results

The fluorescence from rods and cones increased with light exposure but at different rates. During dark adaptation, autofluorescence declined, with cone autofluorescence decreasing approximately 4 times faster than from rods. Rates of autofluorescence decrease in rods and cones were approximately 4 times faster than their respective rates of photopigment regeneration. Also, subsets of sparsely distributed cones were less fluorescent than their neighbors immediately following bleach at 565 nm and they were comparable with the S cone mosaic in density and distribution.

Conclusions

Although other molecules could be contributing, we posit that these fluorescence changes are mediated by products of the retinoid cycle. In vivo two-photon ophthalmoscopy provides a way to monitor noninvasively stages of the retinoid cycle that were previously inaccessible in the living primate eye. This can be used to assess objectively photoreceptor function in normal and diseased retinas.

Coherent anti-stokes Raman scattering (CARS) microscopy: a novel technique for imaging the retina

PURPOSE

To image the cellular and noncellular structures of the retina in an intact mouse eye without the application of exogenous fluorescent labels using noninvasive, nondestructive techniques.

METHODS

Freshly enucleated mouse eyes were imaged using two nonlinear optical techniques: coherent anti-Stokes Raman scattering (CARS) and two-photon autofluorescence (TPAF). Cross sectional transverse sections and sequential flat (en face) sagittal sections were collected from a region of sclera approximately midway between the limbus and optic nerve. Imaging proceeded from the surface of the sclera to a depth of ∼60 μm.

RESULTS

The fluorescent signal from collagen fibers within the sclera was evident in the TPAF channel; the scleral collagen fibers showed no organization and appeared randomly packed. The sclera contained regions lacking TPAF and CARS fluorescence of ∼3 to 15 μm in diameter that could represent small vessels or scleral fibroblasts. Intense punctate CARS signals from the retinal pigment epithelial layer were of a size and shape of retinyl storage esters. Rod outer segments could be identified by the CARS signal from their lipid-rich plasma membranes.

CONCLUSIONS

CARS microscopy can be used to image the outer regions of the mammalian retina without the use of a fluorescent dye or exogenously expressed recombinant protein. With technical advancements, CARS/TPAF may represent a new avenue for noninvasively imaging the retina and might complement modalities currently used in clinical practice.

Low aqueous solubility of 11-cis-retinal limits the rate of pigment formation and dark adaptation in salamander rods

We report experiments designed to test the hypothesis that the aqueous solubility of 11-cis-retinoids plays a significant role in the rate of visual pigment regeneration. Therefore, we have compared the aqueous solubility and the partition coefficients in photoreceptor membranes of native 11-cis-retinal and an analogue retinoid, 11-cis 4-OH retinal, which has a significantly higher solubility in aqueous medium. We have then correlated these parameters with the rates of pigment regeneration and sensitivity recovery that are observed when bleached intact salamander rod photoreceptors are treated with physiological solutions containing these retinoids. We report the following results: (a) 11-cis 4-OH retinal is more soluble in aqueous buffer than 11-cis-retinal. (b) Both 11-cis-retinal and 11-cis 4-OH retinal have extremely high partition coefficients in photoreceptor membranes, though the partition coefficient of 11-cis-retinal is roughly 50-fold greater than that of 11-cis 4-OH retinal. (c) Intact bleached isolated rods treated with solutions containing equimolar amounts of 11-cis-retinal or 11-cis 4-OH retinal form functional visual pigments that promote full recovery of dark current, sensitivity, and response kinetics. However, rods treated with 11-cis 4-OH retinal regenerated on average fivefold faster than rods treated with 11-cis-retinal. (d) Pigment regeneration from recombinant and wild-type opsin in solution is slower when treated with 11-cis 4-OH retinal than with 11-cis-retinal. Based on these observations, we propose a model in which aqueous solubility of cis-retinoids within the photoreceptor cytosol can place a limit on the rate of visual pigment regeneration in vertebrate photoreceptors. We conclude that the cytosolic gap between the plasma membrane and the disk membranes presents a bottleneck for retinoid flux that results in slowed pigment regeneration and dark adaptation in rod photoreceptors.

The lipid translocase, ABCA4: seeing is believing

Mutations to members of the A subfamily of ATP binding cassette (ABC) proteins are responsible for a number of diseases; typically they are associated with aberrant cellular lipid transport processes. Mutations to the ABCA4 protein are linked to a number of visual disorders including Stargardt's disease and retinitis pigmentosa. Over 500 disease-associated mutations in ABCA4 have been demonstrated; however, the genotype-phenotype link has not been firmly established. This shortfall is primarily because the function of ABCA4 in the visual cycle is not yet fully understood. One hypothesis suggests that ABCA4 mediates the trans-bilayer translocation of retinal-phosphatidylethanolamine conjugates to facilitate the retinal regeneration process in the visual cycle. This review examines the evidence to support, or refute, this working hypothesis on the function of this clinically important protein.

Two-photon excited autofluorescence imaging of freshly isolated frog retinas

The purpose of this study was to investigate cellular sources of autofluorescence signals in freshly isolated frog (Rana pipiens) retinas. Equipped with an ultrafast laser, a laser scanning two-photon excitation fluorescence microscope was employed for sub-cellular resolution examination of both sliced and flat-mounted retinas. Two-photon imaging of retinal slices revealed autofluorescence signals over multiple functional layers, including the photoreceptor layer (PRL), outer nuclear layer (ONL), outer plexiform layer (OPL), inner nuclear layer (INL), inner plexiform layer (IPL), and ganglion cell layer (GCL). Using flat-mounted retinas, depth-resolved imaging of individual retinal layers further confirmed multiple sources of autofluorescence signals. Cellular structures were clearly observed at the PRL, ONL, INL, and GCL. At the PRL, the autofluorescence was dominantly recorded from the intracellular compartment of the photoreceptors; while mixed intracellular and extracellular autofluorescence signals were observed at the ONL, INL, and GCL. High resolution autofluorescence imaging clearly revealed mosaic organization of rod and cone photoreceptors; and sub-cellular bright autofluorescence spots, which might relate to connecting cilium, was observed in the cone photoreceptors only. Moreover, single-cone and double-cone outer segments could be directly differentiated.

Measurement of the mobility of all-trans-retinol with two-photon fluorescence recovery after photobleaching

The mobility of all-trans-retinol makes a crucial contribution to the rate of the reactions in which it participates. This is even more so because of its low aqueous solubility, which makes the presence of carrier proteins and the spatial arrangement of cellular membranes especially relevant. In rod photoreceptor outer segments, all-trans-retinol is generated after light exposure from the reduction of all-trans-retinal that is released from bleached rhodopsin. The mobility of all-trans-retinol in rod outer segments was measured with fluorescence recovery after photobleaching (FRAP), using two-photon excitation of its fluorescence. The values of the lateral and axial diffusion coefficients indicate that most of the all-trans-retinol in rod outer segments move unrestricted and without being aided by carriers.

2-Hydroxypropyl-beta-cyclodextrin removes all-trans retinol from frog rod photoreceptors in a concentration-dependent manner

PURPOSE

To determine whether a nonprotein lipophilic carrier, 2-hydroxypropyl-beta-cyclodextrin (HP-beta-CD), can remove all-trans retinol from rod photoreceptor outer segments. All-trans retinol is generated in rod outer segments after light exposure. It is highly insoluble, and its efficient transport across extra- and intracellular aqueous space requires specialized carriers.

METHODS

Experiments were carried out with isolated frog rod photoreceptor cells. The removal of all-trans retinol by different concentrations of this carrier was measured by imaging its fluorescence in single-rod photoreceptors.

RESULTS

HP-beta-CD concentrations >0.3 mM significantly increased the rate of all-trans retinol removal. The rate of removal increased linearly with carrier concentration, with a slope of 0.0058 min(-1)/mM.

CONCLUSIONS

The effectiveness of HP-beta-CD shows that a specialized interaction with the cell membrane is not necessary for the efficient transfer of all-trans retinol between the cell membrane and the carrier. The transfer occurs through a collision-based mechanism, as indicated by the linear increase of the rate of removal with the carrier concentration.

Visualization of retinoid storage and trafficking by two-photon microscopy

Vertebrate vision is maintained by the retinoid (visual) cycle, a complex enzymatic pathway that operates in the retina to regenerate the visual chromophore, 11-cis-retinal, a prosthetic group of rhodopsin that undergoes activation by light. Many different mutations in genes encoding retinoid cycle proteins can cause a variety of human blinding diseases. Two-photon microscopy is an evolving, non-invasive, and repetitive imaging technology that can be used to monitor biomolecules within the vertebrate retina at a subcellular resolution. This method has the great advantage of portraying endogenous retinoid fluorophores in their native state without the need for artificial staining. Such real-time retinal imaging permits rapid evaluation not only of various stages of retinal disease in live animal models of human retinopathies but also of the outcome from intended pharmacological therapies. Two-photon microscopy offers substantial potential for early detection of age- and disease-related changes in the eye, long before clinical or pathological manifestations become apparent.

Microfluorometric measurement of the formation of all-trans-retinol in the outer segments of single isolated vertebrate photoreceptors

The first step in the detection of light by vertebrate photoreceptors is the photoisomerization of the retinyl chromophore of their visual pigment from 11-cis to the all-trans configuration. This initial reaction leads not only to an activated form of the visual pigment, meta II, that initiates reactions of the visual transduction cascade but also to the photochemical destruction of the visual pigment. By a series of reactions termed the visual cycle, native visual pigment is regenerated. These coordinated reactions take place in the photoreceptors themselves as well as the adjacent pigment epithelium and Müller cells. The critical initial steps in the visual cycle are the release of all-trans-retinal from the photoactivated pigment and its reduction to all-trans-retinol. The goal of this monograph is to describe methods of fluorescence imaging that allow the measurement of changes in the concentration of all-trans-retinol as it is reduced from all-trans-retinal in isolated intact salamander and mouse photoreceptors. The kinetics of all-trans-retinol formation depend on cellular factors that include the visual pigment and photoreceptor cell type, as well as the cytoarchitecture of outer segments. In general, all-trans-retinol forms much faster in cone cells than in rods.

Two-photon microscopy: shedding light on the chemistry of vision

Two-photon microscopy (TPM) has come to occupy a prominent place in modern biological research with its ability to resolve the three-dimensional distribution of molecules deep inside living tissue. TPM can employ two different types of signals, fluorescence and second harmonic generation, to image biological structures with subcellular resolution. Two-photon excited fluorescence imaging is a powerful technique with which to monitor the dynamic behavior of the chemical components of tissues, whereas second harmonic imaging provides novel ways to study their spatial organization. Using TPM, great strides have been made toward understanding the metabolism, structure, signal transduction, and signal transmission in the eye. These include the characterization of the spatial distribution, transport, and metabolism of the endogenous retinoids, molecules essential for the detection of light, as well as the elucidation of the architecture of the living cornea. In this review, we present and discuss the current applications of TPM for the chemical and structural imaging of the eye. In addition, we address what we see as the future potential of TPM for eye research. This relatively new method of microscopy has been the subject of numerous technical improvements in terms of the optics and indicators used, improvements that should lead to more detailed biochemical characterizations of the eyes of live animals and even to imaging of the human eye in vivo.

Interphotoreceptor retinoid-binding protein is the physiologically relevant carrier that removes retinol from rod photoreceptor outer segments

Light detection by vertebrate rod photoreceptor outer segments results in the destruction of the visual pigment, rhodopsin, as its retinyl moiety is photoisomerized from 11-cis to all-trans. The regeneration of rhodopsin is necessary for vision and begins with the release of the all-trans retinal and its reduction to all-trans retinol. Retinol is then transported out of the rod outer segment for further processing. We used fluorescence imaging to monitor retinol fluorescence and quantify the kinetics of its formation and clearance after rhodopsin bleaching in the outer segments of living isolated frog (Rana pipiens) rod photoreceptors. We independently measured the release of all-trans retinal from bleached rhodopsin in frog rod outer segment membranes and the rate of all-trans retinol removal by the lipophilic carriers interphotoreceptor retinoid binding protein (IRBP) and serum albumin. We find that the kinetics of all-trans retinol formation in frog rod outer segments after rhodopsin bleaching are to a good first approximation determined by the kinetics of all-trans retinal release from the bleached pigment. For the physiological concentrations of carriers, the rate of retinol removal from the outer segment is determined by IRBP concentration, whereas the effect of serum albumin is negligible. The results indicate the presence of a specific interaction between IRBP and the rod outer segment, probably mediated by a receptor. The effect of different concentrations of IRBP on the rate of retinol removal shows no cooperativity and has an EC50 of 40 micromol/L.