Positively charged retinoids are potent and selective inhibitors of the trans-cis isomerization in the retinoid (visual) cycle

Supplementation with vitamin a derivatives to rescue vision in animal models of degenerative retinal diseases

The perception of light begins when photons reach retinal tissue located at the back of the eye and photoisomerize the visual chromophore 11-cis-retinal to all-trans-retinal within photoreceptor cells. Isomerization of 11-cis-retinal activates the protein rhodopsin located in photoreceptor outer segments, thereby inducing a phototransduction cascade leading to visual perception. To maintain vision, 11-cis-retinal is regenerated in the retinal pigmented epithelium (RPE) via the visual cycle and delivered back to the photoreceptor cells where it may again bind to rhodopsin. Distinct pathological mechanisms have been observed to contribute to inherited retinal degenerative diseases including severe delay in 11-cis-retinal regeneration and delayed clearance of all-trans-retinal, which leads to the accumulation of harmful retinoid by-products. In the last decade, our group has conducted several proof-of-concept (POC) studies with retinoid derivatives aimed at developing treatments for retinal degenerative diseases caused by an impaired visual cycle. Here, we will introduce experimental procedures, which have been developed for POC studies involving retinoid biology.

Expansion of first-in-class drug candidates that sequester toxic all-trans-retinal and prevent light-induced retinal degeneration

All-trans-retinal, a retinoid metabolite naturally produced upon photoreceptor light activation, is cytotoxic when present at elevated levels in the retina. To lower its toxicity, two experimentally validated methods have been developed involving inhibition of the retinoid cycle and sequestration of excess of all-trans-retinal by drugs containing a primary amine group. We identified the first-in-class drug candidates that transiently sequester this metabolite or slow down its production by inhibiting regeneration of the visual chromophore, 11-cis-retinal. Two enzymes are critical for retinoid recycling in the eye. Lecithin:retinol acyltransferase (LRAT) is the enzyme that traps vitamin A (all-trans-retinol) from the circulation and photoreceptor cells to produce the esterified substrate for retinoid isomerase (RPE65), which converts all-trans-retinyl ester into 11-cis-retinol. Here we investigated retinylamine and its derivatives to assess their inhibitor/substrate specificities for RPE65 and LRAT, mechanisms of action, potency, retention in the eye, and protection against acute light-induced retinal degeneration in mice. We correlated levels of visual cycle inhibition with retinal protective effects and outlined chemical boundaries for LRAT substrates and RPE65 inhibitors to obtain critical insights into therapeutic properties needed for retinal preservation.

Phase 1, dose-ranging study of emixustat hydrochloride (ACU-4429), a novel visual cycle modulator, in healthy volunteers

BACKGROUND

Emixustat hydrochloride (formerly ACU-4429) is a nonretinoid compound with a unique mode of action in the retinal pigment epithelium, where it modulates the biosynthesis of visual chromophore through its effect on retinal pigment epithelium-specific 65 kDa protein isomerase. This study provides clinicians with a background for understanding the pharmacokinetics and safety profile of orally administered emixustat.

METHODS

This randomized, double-masked, placebo-controlled Phase 1b study evaluated the pharmacokinetics, tolerability, and safety of a 14-day course of oral emixustat (5, 10, 20, 30, or 40 mg) or placebo (3:1 ratio) once daily in healthy volunteers.

RESULTS

A total of 40 subjects were enrolled (mean age, 38 years; 75% male). Emixustat (n = 30) was rapidly absorbed (median T(max), 3.0-5 hours) and readily eliminated (mean t(1/2), 4.6-7.9 hours), and mean C(max) and AUC(0-24) generally increased in proportion to dose. No significant accumulation of emixustat was observed with multiple-dose administration. Ocular adverse events occurred in 67% of the subjects who received emixustat; all were considered mild and resolved after study completion. Systemic adverse events were minimal.

CONCLUSION

Oral emixustat was safe and well tolerated when administered once daily for 14 days with minimal systemic adverse events reported. These data support evaluation of emixustat in subjects with geographic atrophy associated with dry age-related macular degeneration.

The role of 11-cis-retinyl esters in vertebrate cone vision

A cycle of cis-to-trans isomerization of the chromophore is intrinsic to vertebrate vision where rod and cone photoreceptors mediate dim- and bright-light vision, respectively. Daylight illumination can greatly exceed the rate at which the photoproduct can be recycled back to the chromophore by the canonical visual cycle. Thus, an additional supply pathway(s) must exist to sustain cone-dependent vision. Two-photon microscopy revealed that the eyes of the zebrafish (Danio rerio) contain high levels of 11-cis-retinyl esters (11-REs) within the retinal pigment epithelium. HPLC analyses demonstrate that 11-REs are bleached by bright light and regenerated in the dark. Pharmacologic treatment with all-trans-retinylamine (Ret-NH2), a potent and specific inhibitor of the trans-to-cis reisomerization reaction of the canonical visual cycle, impeded the regeneration of 11-REs. Intervention with 11-cis-retinol restored the regeneration of 11-REs in the presence of all-trans-Ret-NH2. We used the XOPS:mCFP transgenic zebrafish line with a functional cone-only retina to directly demonstrate that this 11-RE cycle is critical to maintain vision under bright-light conditions. Thus, our analyses reveal that a dark-generated pool of 11-REs helps to supply photoreceptors with the chromophore under the varying light conditions present in natural environments.

Noninvasive two-photon microscopy imaging of mouse retina and retinal pigment epithelium through the pupil of the eye

Two-photon excitation microscopy can image retinal molecular processes in vivo. Intrinsically fluorescent retinyl esters in subcellular structures called retinosomes are an integral part of the visual chromophore regeneration pathway. Fluorescent condensation products of all-trans-retinal accumulate in the eye with age and are also associated with age-related macular degeneration (AMD). Here, we report repetitive, dynamic imaging of these compounds in live mice through the pupil of the eye. By leveraging advanced adaptive optics, we developed a data acquisition algorithm that permitted the identification of retinosomes and condensation products in the retinal pigment epithelium by their characteristic localization, spectral properties and absence in genetically modified or drug-treated mice. This imaging approach has the potential to detect early molecular changes in retinoid metabolism that trigger light- and AMD-induced retinal defects and to assess the effectiveness of treatments for these conditions.

Two-photon microscopy reveals early rod photoreceptor cell damage in light-exposed mutant mice

Atrophic age-related and juvenile macular degeneration are especially devastating due to lack of an effective cure. Two retinal cell types, photoreceptor cells and the adjacent retinal pigmented epithelium (RPE), reportedly display the earliest pathological changes. Abca4(-/-)Rdh8(-/-) mice, which mimic many features of human retinal degeneration, allowed us to determine the sequence of light-induced events leading to retinal degeneration. Using two-photon microscopy with 3D reconstruction methodology, we observed an initial strong retinoid-derived fluorescence and expansion of Abca4(-/-)Rdh8(-/-) mouse rod cell outer segments accompanied by macrophage infiltration after brief exposure of the retina to bright light. Additionally, light-dependent fluorescent compounds produced in rod outer segments were not transferred to the RPE of mice genetically defective in RPE phagocytosis. Collectively, these findings suggest that for light-induced retinopathies in mice, rod photoreceptors are the primary site of toxic retinoid accumulation and degeneration, followed by secondary changes in the RPE.

Identification of nonvisual photomotor response cells in the vertebrate hindbrain

Nonvisual photosensation enables animals to sense light without sight. However, the cellular and molecular mechanisms of nonvisual photobehaviors are poorly understood, especially in vertebrate animals. Here, we describe the photomotor response (PMR), a robust and reproducible series of motor behaviors in zebrafish that is elicited by visual wavelengths of light but does not require the eyes, pineal gland, or other canonical deep-brain photoreceptive organs. Unlike the relatively slow effects of canonical nonvisual pathways, motor circuits are strongly and quickly (seconds) recruited during the PMR behavior. We find that the hindbrain is both necessary and sufficient to drive these behaviors. Using in vivo calcium imaging, we identify a discrete set of neurons within the hindbrain whose responses to light mirror the PMR behavior. Pharmacological inhibition of the visual cycle blocks PMR behaviors, suggesting that opsin-based photoreceptors control this behavior. These data represent the first known light-sensing circuit in the vertebrate hindbrain.

A1120, a nonretinoid RBP4 antagonist, inhibits formation of cytotoxic bisretinoids in the animal model of enhanced retinal lipofuscinogenesis

PURPOSE

Excessive accumulation of lipofuscin is associated with pathogenesis of atrophic age-related macular degeneration (AMD) and Stargardt disease. Pharmacologic inhibition of the retinol-induced interaction of retinol-binding protein 4 (RBP4) with transthyretin (TTR) in the serum may decrease the uptake of serum retinol to the retina and reduce formation of lipofuscin bisretinoids. We evaluated in vitro and in vivo properties of the new nonretinoid RBP4 antagonist, A1120.

METHODS

RBP4 binding potency, ability to antagonize RBP4-TTR interaction, and compound specificity were analyzed for A1120 and for the prototypic RBP4 antagonist fenretinide. A1120 ability to inhibit RPE65-mediated isomerohydrolase activity was assessed in the RPE microsomes. The in vivo effect of A1120 administration on serum RBP4, visual cycle retinoids, lipofuscin bisretinoids, and retinal visual function was evaluated using a combination of biochemical and electrophysiologic techniques.

RESULTS

In comparison to fenretinide, A1120 did not act as a RARα agonist, while exhibiting superior in vitro potency in RBP4 binding and RBP4-TTR interaction assays. A1120 did not inhibit isomerohydrolase activity in the RPE microsomes. A1120 dosing in mice induced 75% reduction in serum RBP4, which correlated with reduction in visual cycle retinoids and ocular levels of lipofuscin fluorophores. A1120 dosing did not induce changes in kinetics of dark adaptation.

CONCLUSIONS

A1120 significantly reduces accumulation of lipofuscin bisretinoids in the Abca4(-/-) animal model. This activity correlates with reduction in serum RBP4 and visual cycle retinoids confirming the mechanism of action for A1120. In contrast to fenretinide, A1120 does not act as a RARα agonist indicating a more favorable safety profile for this nonretinoid compound.

Retinal degeneration in animal models with a defective visual cycle

Continuous generation of visual chromophore through the visual (retinoid) cycle is essential to maintain eyesight and retinal heath. Impairments in this cycle and related pathways adversely affect vision. In this review, we summarize the chemical reactions of vitamin A metabolites involved in the retinoid cycle and describe animal models of associated human diseases. Development of potential therapies for retinal disorders in these animal models is also introduced.

Pharmacological inhibition of lipofuscin accumulation in the retina as a therapeutic strategy for dry AMD treatment

Age-related macular degeneration (AMD) is the leading cause of blindness in the western world. There is no FDA-approved treatment for the most prevalent dry (atrophic) form of AMD. Photoreceptor degeneration in dry AMD is triggered by abnormalities in the retinal pigment epithelium (RPE). It has been suggested that excessive accumulation of fluorescent lipofuscin pigment in the RPE represents an important pathogenic factor in etiology and progression of dry AMD. Cytotoxic lipofuscin bisretinoids, such as A2E, are formed in the retina in a non-enzymatic way from visual cycle retinoids. Inhibition of toxic bisretinoid production in the retina seems to be a sound treatment strategy for dry AMD. In this review we discuss the following classes of pharmacological treatments inhibiting lipofuscin bisretinoid formation in the retina: direct inhibitors of key visual cycle enzymes, RBP4 antagonists, primary amine-containing aldehyde traps, and deuterated analogs of vitamin A.

Structure of RPE65 isomerase in a lipidic matrix reveals roles for phospholipids and iron in catalysis

RPE65 is a key metalloenzyme responsible for maintaining visual function in vertebrates. Despite extensive research on this membrane-bound retinoid isomerase, fundamental questions regarding its enzymology remain unanswered. Here, we report the crystal structure of RPE65 in a membrane-like environment. These crystals, obtained from enzymatically active, nondelipidated protein, displayed an unusual packing arrangement wherein RPE65 is embedded in a lipid-detergent sheet. Structural differences between delipidated and nondelipidated RPE65 uncovered key residues involved in substrate uptake and processing. Complementary iron K-edge X-ray absorption spectroscopy data established that RPE65 as isolated contained a divalent iron center and demonstrated the presence of a tightly bound ligand consistent with a coordinated carboxylate group. These results support the hypothesis that the Lewis acidity of iron could be used to promote ester dissociation and generation of a carbocation intermediate required for retinoid isomerization.

Dry age-related macular degeneration: A currently unmet clinical need

Age-related macular degeneration (AMD) is a leading cause of severe visual impairment and disability in older people worldwide. Although considerable advances in the management of the neovascular form of AMD have been made in the last decade, no therapy is yet available for the advanced dry form of AMD (geographic atrophy). This review focuses on current trends in the development of new therapies targeting specific pathophysiological pathways of dry AMD. Increased understanding of the complex mechanisms that underlie dry AMD will help to address this largely unmet clinical need.

Retinal photodamage mediated by all-trans-retinal

Accumulation of all-trans-retinal (all-trans-RAL), reactive vitamin A aldehyde, is one of the key factors in initiating retinal photodamage. This photodamage is characterized by progressive retinal cell death evoked by light exposure in both an acute and chronic fashion. Photoactivated rhodopsin releases all-trans-RAL, which is subsequently transported by ATP-binding cassette transporter 4 and reduced to all-trans-retinol by all-trans-retinol dehydrogenases located in photoreceptor cells. Any interruptions in the clearing of all-trans-RAL in the photoreceptors can cause an accumulation of this reactive aldehyde and its toxic condensation products. This accumulation may result in the manifestation of retinal dystrophy including human retinal degenerative diseases such as Stargardt's disease and age-related macular degeneration. Herein, we discuss the mechanisms of all-trans-RAL clearance in photoreceptor cells by sequential enzymatic reactions, the visual (retinoid) cycle, and potential molecular pathways of retinal photodamage. We also review recent imaging technologies to monitor retinal health status as well as novel therapeutic strategies preventing all-trans-RAL-associated retinal photodamage.

Mechanism of all-trans-retinal toxicity with implications for stargardt disease and age-related macular degeneration

Compromised clearance of all-trans-retinal (atRAL), a component of the retinoid cycle, increases the susceptibility of mouse retina to acute light-induced photoreceptor degeneration. Abca4(-/-)Rdh8(-/-) mice featuring defective atRAL clearance were used to examine the one or more underlying molecular mechanisms, because exposure to intense light causes severe photoreceptor degeneration in these animals. Here we report that bright light exposure of Abca4(-/-)Rdh8(-/-) mice increased atRAL levels in the retina that induced rapid NADPH oxidase-mediated overproduction of intracellular reactive oxygen species (ROS). Moreover, such ROS generation was inhibited by blocking phospholipase C and inositol 1,4,5-trisphosphate-induced Ca(2+) release, indicating that activation occurs upstream of NADPH oxidase-mediated ROS generation. Because multiple upstream G protein-coupled receptors can activate phospholipase C, we then tested the effects of antagonists of serotonin 2A (5-HT(2A)R) and M(3)-muscarinic (M(3)R) receptors and found they both protected Abca4(-/-)Rdh8(-/-) mouse retinas from light-induced degeneration. Thus, a cascade of signaling events appears to mediate the toxicity of atRAL in light-induced photoreceptor degeneration of Abca4(-/-)Rdh8(-/-) mice. A similar mechanism may be operative in human Stargardt disease and age-related macular degeneration.

Primary amines protect against retinal degeneration in mouse models of retinopathies

Vertebrate vision is initiated by photoisomerization of the visual pigment chromophore 11-cis-retinal and is maintained by continuous regeneration of this retinoid through a series of reactions termed the retinoid cycle. However, toxic side reaction products, especially those involving reactive aldehyde groups of the photoisomerized product, all-trans-retinal, can cause severe retinal pathology. Here we lowered peak concentrations of free all-trans-retinal with primary amine-containing Food and Drug Administration (FDA)-approved drugs that did not inhibit chromophore regeneration in mouse models of retinal degeneration. Schiff base adducts between all-trans-retinal and these amines were identified by MS. Adducts were observed in mouse eyes only when an experimental drug protected the retina from degeneration in both short-term and long-term treatment experiments. This study demonstrates a molecular basis of all-trans-retinal-induced retinal pathology and identifies an assemblage of FDA-approved compounds with protective effects against this pathology in a mouse model that shows features of Stargardt's disease and age-related retinal degeneration.

The bisretinoids of retinal pigment epithelium

The retina exhibits an inherent autofluorescence that is imaged ophthalmoscopically as fundus autofluorescence. In clinical settings, fundus autofluorescence examination aids in the diagnosis and follow-up of many retinal disorders. Fundus autofluorescence originates from the complex mixture of bisretinoid fluorophores that are amassed by retinal pigment epithelial (RPE) cells as lipofuscin. Unlike the lipofuscin found in other cell-types, this material does not form as a result of oxidative stress. Rather, the formation is attributable to non-enzymatic reactions of vitamin A aldehyde in photoreceptor cells; transfer to RPE occurs upon phagocytosis of photoreceptor outer segments. These fluorescent pigments accumulate even in healthy photoreceptor cells and are generated as a consequence of the light capturing function of the cells. Nevertheless, the formation of this material is accelerated in some retinal disorders including recessive Stargardt disease and ELOVL4-related retinal degeneration. As such, these bisretinoid side-products are implicated in the disease processes that threaten vision. In this article, we review our current understanding of the composition of RPE lipofuscin, the structural characteristics of the various bisretinoids, their related spectroscopic features and the biosynthetic pathways by which they form. We will revisit factors known to influence the extent of the accumulation and therapeutic strategies being used to limit bisretinoid formation. Given their origin from vitamin A aldehyde, an isomer of the visual pigment chromophore, it is not surprising that the bisretinoids of retina are light sensitive molecules. Accordingly, we will discuss recent findings that implicate the photodegradation of bisretinoid in the etiology of age-related macular degeneration.

Influence of blue light on photoreceptors in a live retinal explant system

PURPOSE

The present study was performed to investigate the early effects of blue light irradiation of photoreceptors in retinal explant cultures.

METHODS

Murine retinal explant cultures were irradiated with visible blue light (405 nm) with an output power of 1 mW/cm2. Dihydroethidium was used to determine the production of reactive oxygen species. Morphological alterations of photoreceptor outer segments were determined by live imaging microscopy with mitochondrial dye JC-1. Transmission and scanning electron microscopy were used for ultrastructural evaluations. Cell death in the retina was assessed by the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate (dUTP) nick end labeling (TUNEL) assay method.

RESULTS

Live retinal explants displayed an increase in reactive oxygen species production, as revealed by fluorescent dihydroethidium products in photoreceptor cells after 30 min of blue light exposure. After 3 h of exposure, blue light caused disorganization of the normally neatly stacked outer segments of living photoreceptors. Ultrastructural analysis revealed breaks in the cell membrane surrounding the outer segments, especially in the middle section. The outer segments appeared tortuous, and the lamellar structures had been disrupted. TUNEL-staining revealed that long-term blue light exposure induced photoreceptor cell death.

CONCLUSIONS

In vitro blue light irradiation of retinal explants is a suitable model system for investigating early ultrastructural changes, as well as damage that leads to cell death in photoreceptor cells.

Key enzymes of the retinoid (visual) cycle in vertebrate retina

A major goal in vision research over the past few decades has been to understand the molecular details of retinoid processing within the retinoid (visual) cycle. This includes the consequences of side reactions that result from delayed all-trans-retinal clearance and condensation with phospholipids that characterize a variety of serious retinal diseases. Knowledge of the basic retinoid biochemistry involved in these diseases is essential for development of effective therapeutics. Photoisomerization of the 11-cis-retinal chromophore of rhodopsin triggers a complex set of metabolic transformations collectively termed phototransduction that ultimately lead to light perception. Continuity of vision depends on continuous conversion of all-trans-retinal back to the 11-cis-retinal isomer. This process takes place in a series of reactions known as the retinoid cycle, which occur in photoreceptor and RPE cells. All-trans-retinal, the initial substrate of this cycle, is a chemically reactive aldehyde that can form toxic conjugates with proteins and lipids. Therefore, much experimental effort has been devoted to elucidate molecular mechanisms of the retinoid cycle and all-trans-retinal-mediated retinal degeneration, resulting in delineation of many key steps involved in regenerating 11-cis-retinal. Three particularly important reactions are catalyzed by enzymes broadly classified as acyltransferases, short-chain dehydrogenases/reductases and carotenoid/retinoid isomerases/oxygenases. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Retinyl ester storage particles (retinosomes) from the retinal pigmented epithelium resemble lipid droplets in other tissues

Levels of many hydrophobic cellular substances are tightly regulated because of their potential cytotoxicity. These compounds tend to self-aggregate in cytoplasmic storage depots termed lipid droplets/bodies that have well defined structures that contain additional components, including cholesterol and various proteins. Hydrophobic substances in these structures become mobilized in a specific and regulated manner as dictated by cellular requirements. Retinal pigmented epithelial cells in the eye produce retinyl ester-containing lipid droplets named retinosomes. These esters are mobilized to replenish the visual chromophore, 11-cis-retinal, and their storage ensures proper visual function despite fluctuations in dietary vitamin A intake. But it remains unclear whether retinosomes are structures specific to the eye or similar to lipid droplets in other organs/tissues that contain substances other than retinyl esters. Thus, we initially investigated the production of these lipid droplets in experimental cell lines expressing lecithin:retinol acyltransferase, a key enzyme involved in formation of retinyl ester-containing retinosomes from all-trans-retinol. We found that retinosomes and oleate-derived lipid droplets form and co-localize concomitantly, indicating their intrinsic structural similarities. Next, we isolated native retinosomes from bovine retinal pigmented epithelium and found that their protein and hydrophobic small molecular constituents were similar to those of lipid droplets reported for other experimental cell lines and tissues. These unexpected findings suggest a common mechanism for lipid droplet formation that exhibits broad chemical specificity for the hydrophobic substances being stored.

Inhibition of the visual cycle by A2E through direct interaction with RPE65 and implications in Stargardt disease

Stargardt disease (STGD) is the major form of inherited juvenile macular degeneration. Pyridinium bis-retinoid A2E is a major component of lipofuscin which accumulates in retinal pigment epithelium (RPE) cells in STGD and contributes to the disease pathogenesis. However, the precise role of A2E in vision loss is unclear. Here we report that A2E efficiently inhibits RPE65 isomerohydrolase, a key enzyme in the visual cycle. Subretinal injection of A2E significantly inhibited retinoid isomerohydrolase activity in mice. Likewise, A2E also inhibited isomerohydrolase activity in cells coexpressing RPE65, lecithin retinol acyltransferase (LRAT), and cellular retinaldehyde-binding protein. In vitro isomerohydrolase activity assays confirmed that A2E inhibited enzymatic activity of recombinant RPE65 in a concentration-dependent manner, but did not inhibit LRAT activity. The inhibition type for isomerohydrolase was found to be reversible and competitive with K(i) = 13.6 μM. To determine the direct interaction of A2E with RPE65 protein, fluorescence binding assays were performed. As shown by fluorimetric titration, binding of purified RPE65 with A2E enhanced the bis-retinoid fluorescence. Consistently, the fluorescence of RPE65 decreased upon incubation with A2E. Both of these experiments suggest a direct, specific binding of A2E to RPE65. The binding constant for A2E and purified RPE65 was calculated to be 250 nM. These results demonstrate that A2E inhibits the regeneration of 11-cis retinal, the chromophore of visual pigments, which represents a unique mechanism by which A2E may impair vision in STGD.

Retinoids for treatment of retinal diseases

Knowledge about retinal photoreceptor signal transduction and the visual cycle required for normal eyesight has increased exponentially over the past decade. Substantial progress in human genetics has facilitated the identification of candidate genes and complex networks underlying inherited retinal diseases. Natural mutations in animal models that mimic human diseases have been characterized and advanced genetic manipulation can now be used to generate small mammalian models of human retinal diseases. Pharmacological repair of defective visual processes in animal models not only validates their involvement in vision, but also provides great promise for the development of improved therapies for millions who are progressing towards blindness or are almost completely robbed of their eyesight.

Beta,beta-carotene decreases peroxisome proliferator receptor gamma activity and reduces lipid storage capacity of adipocytes in a beta,beta-carotene oxygenase 1-dependent manner

Increasing evidence has been provided for a connection between retinoid metabolism and the activity of peroxisome proliferator receptors (Ppars) in the control of body fat reserves. Two different precursors for retinoids exist in the diet as preformed vitamin A (all-trans-retinol) and provitamin A (beta,beta-carotene). For retinoid production, beta,beta-carotene is converted to retinaldehyde by beta,beta-carotene monooxygenase 1 (Bcmo1). Previous analysis showed that Bcmo1 knock-out mice develop dyslipidemia and are more susceptible to diet-induced obesity. However, the role of Bcmo1 for adipocyte retinoid metabolism has yet not been well defined. Here, we showed that Bcmo1 mRNA and protein expression are induced during adipogenesis in NIH 3T3-L1 cells. In mature adipocytes, beta,beta-carotene but not all-trans-retinol was metabolized to retinoic acid (RA). RA decreased the expression of Ppar gamma and CCAAT/enhancer-binding protein alpha, key lipogenic transcription factors, and reduced the lipid content of mature adipocytes. This process was inhibited by the retinoic acid receptor antagonist LE450, showing that it involves canonical retinoid signaling. Accordingly, gavage of beta,beta-carotene but not all-trans-retinol induced retinoid signaling and decreased Ppar gamma expression in white adipose tissue of vitamin A-deficient mice. Our study identifies beta,beta-carotene as a critical physiological precursor for RA production in adipocytes and implicates provitamin A as a dietary regulator of body fat reserves.

Stable ion complexes of vitamin A species (retinol) in solution as detected by mass spectrometry

Mechanisms of retinol (ROL) transport in plasma alternative to that involving the Retinol-Binding Protein (RBP) have been hypothesized after RBP-knockout mice were shown to be viable and fertile. Accordingly, the possibility of a ROL fraction circulating free in plasma has been suggested. In this study, stable complex formation between ROL and the physiological relevant ions Na(+), Li(+), and K(+) was investigated by using mass spectrometry and assessed in regard to the aforementioned alternative mechanism. The ROL-Na complex was found to be kinetically favoured with respect to the ROL-Li and ROL-K complexes. Fragmentation of the [ROL-Na](+) adduct rendered the carbocation [ROL+H-H(2)O](+) (m/z 269) and NaOH as neutral loss. In consonance with these results, it is hypothesized on the possibility of the complex [ROL-Na](+) being an alternative way of ROL transport to cells, as well as an intermediate in cis/trans isomerism.

Proteomic changes in the photoreceptor outer segment upon intense light exposure

Acute light-induced photoreceptor degeneration has been studied in experimental animals as a model for photoreceptor cell loss in human retinal degenerative diseases. Light absorption by rhodopsin in rod photoreceptor outer segments (OS) induces oxidative stress and initiates apoptotic cell death. However, the molecular events that induce oxidative stress and initiate the apoptotic cascade remain poorly understood. To better understand the molecular mechanisms of light-induced photoreceptor cell death, we studied the proteomic changes in OS upon intense light exposure by using a proteolytic (18)O labeling method. Of 171 proteins identified, the relative abundance of 98 proteins in light-exposed and unexposed OS was determined. The quantities of 11 proteins were found to differ by more than 2-fold between light-exposed OS and those remaining in darkness. Among the 11 proteins, 8 were phototransduction proteins and 7 of these were altered such that the efficiency of phototransduction would be reduced or quenched during light exposure. In contrast, the amount of OS rhodopsin kinase was reduced by 2-fold after light exposure, suggesting attenuation in the mechanism of quenching phototransduction. Liquid chromatography multiple reaction monitoring (LC-MRM) was performed to confirm this reduction in the quantity of rhodopsin kinase. As revealed by immunofluorescence microscopy, this reduction of rhodopsin kinase is not a result of protein translocation from the outer to the inner segment. Collectively, our findings suggest that the absolute quantity of rhodopsin kinase in rod photoreceptors is reduced upon light stimulation and that this reduction may be a contributing factor to light-induced photoreceptor cell death. This report provides new insights into the proteomic changes in the OS upon intense light exposure and creates a foundation for understanding the mechanisms of light-induced photoreceptor cell death.

Visual cycle modulation in neurovascular retinopathy

Rats with oxygen-induced retinopathy (OIR) model the pediatric retinal disease retinopathy of prematurity (ROP). Recent findings in OIR rats imply a causal role for the rods in the ROP disease process, although only experimental manipulation of rod function can establish this role conclusively. Accordingly, a visual cycle modulator (VCM) - with no known direct effect on retinal vasculature - was administered to "50/10 model" OIR Sprague-Dawley rats to test the hypotheses that it would 1) alter rod function and 2) consequently alter vascular outcome. Four litters of pups (N=46) were studied. For two weeks, beginning on postnatal day (P) 7, the first and fourth litters were administered 6 mg kg(-1) N-retinylacetamide (the VCM) intraperitoneally; the second and third litters received vehicle (DMSO) alone. Following a longitudinal design, retinal function was assessed by electroretinography (ERG) and the status of the retinal vessels was monitored using computerized fundus photograph analysis. Rod photoreceptor and post-receptor response amplitudes were significantly higher in VCM-treated than in vehicle-treated rats; deactivation of phototransduction was also significantly more rapid. Notably, the arterioles of VCM-treated rats showed significantly greater recovery from OIR. Presuming that the VCM did not directly affect the retinal vessels, a causal role for the neural retina - particularly the rod photoreceptors - in OIR was confirmed. There was no evidence of negative alteration of photoreceptor function consequent to VCM treatment. This finding implicates the rods as a possible therapeutic target in neurovascular diseases such as ROP.

Membrane-binding and enzymatic properties of RPE65

Regeneration of visual pigments is essential for sustained visual function. Although the requirement for non-photochemical regeneration of the visual chromophore, 11-cis-retinal, was recognized early on, it was only recently that the trans to cis retinoid isomerase activity required for this process was assigned to a specific protein, a microsomal membrane enzyme called RPE65. In this review, we outline progress that has been made in the functional characterization of RPE65. We then discuss general concepts related to protein-membrane interactions and the mechanism of the retinoid isomerization reaction and describe some of the important biochemical and structural features of RPE65 with respect to its membrane-binding and enzymatic properties.

The biochemical and structural basis for trans-to-cis isomerization of retinoids in the chemistry of vision

Recently, much progress has been made in elucidating the chemistry and metabolism of retinoids and carotenoids, as well as the structures of processing proteins related to vision. Carotenoids and their retinoid metabolites are isoprenoids, so only a limited number of chemical transformations are possible, and just a few of these occur naturally. Although there is an intriguing evolutionary conservation of the key components involved in the production and recycling of chromophores, these genes have also adapted to the specific requirements of insect and vertebrate vision. These 'ancestral footprints' in animal genomes bear witness to the common origin of the chemistry of vision, and will further stimulate research across evolutionary boundaries.

RPE65, visual cycle retinol isomerase, is not inherently 11-cis-specific: support for a carbocation mechanism of retinol isomerization

The mechanism of retinol isomerization in the vertebrate retina visual cycle remains controversial. Does the isomerase enzyme RPE65 operate via nucleophilic addition at C(11) of the all-trans substrate, or via a carbocation mechanism? To determine this, we modeled the RPE65 substrate cleft to identify residues interacting with substrate and/or intermediate. We find that wild-type RPE65 in vitro produces 13-cis and 11-cis isomers equally robustly. All Tyr-239 mutations abolish activity. Trp-331 mutations reduce activity (W331Y to approximately 75% of wild type, W331F to approximately 50%, and W331L and W331Q to 0%) establishing a requirement for aromaticity, consistent with cation-pi carbocation stabilization. Two cleft residues modulate isomerization specificity: Thr-147 is important, because replacement by Ser increases 11-cis relative to 13-cis by 40% compared with wild type. Phe-103 mutations are opposite in action: F103L and F103I dramatically reduce 11-cis synthesis relative to 13-cis synthesis compared with wild type. Thr-147 and Phe-103 thus may be pivotal in controlling RPE65 specificity. Also, mutations affecting RPE65 activity coordinately depress 11-cis and 13-cis isomer production but diverge as 11-cis decreases to zero, whereas 13-cis reaches a plateau consistent with thermal isomerization. Lastly, experiments using labeled retinol showed exchange at 13-cis-retinol C(15) oxygen, thus confirming enzymatic isomerization for both isomers. Thus, RPE65 is not inherently 11-cis-specific and can produce both 11- and 13-cis isomers, supporting a carbocation (or radical cation) mechanism for isomerization. Specific visual cycle selectivity for 11-cis isomers instead resides downstream, attributable to mass action by CRALBP, retinol dehydrogenase 5, and high affinity of opsin apoproteins for 11-cis-retinal.

Phospholipid meets all-trans-retinal: the making of RPE bisretinoids

The lipid phase of the photoreceptor outer segment membrane is essential to the photon capturing and signaling functions of rhodopsin. Rearrangement of phospholipids in the bilayer accompanies the formation of the active intermediates of rhodopsin following photon absorption. Furthermore, evidence for the formation of a condensation product between the photolyzed chromophore all-trans-retinal and phosphatidylethanolamine indicates that phospholipid may also participate in the movement of the retinoid in the membrane. The downside of these interactions is the formation of bisretinoid-phosphatidylethanolamine compounds that accumulate in retinal pigment epithelial cells with age and that are particularly abundant in some retinal disorders. The propensity of these compounds to negatively impact on the cells has been linked to the pathogenesis of some retinal disorders including juvenile onset recessive Stargardt disease and age-related macular degeneration.

Evaluation of potential therapies for a mouse model of human age-related macular degeneration caused by delayed all-trans-retinal clearance

PURPOSE

Evaluate the efficacy of potential therapeutics in Rdh8(-/-)Abca4(-/-) mice, a rodent model of human age-related macular degeneration (AMD).

METHODS

Therapeutic efficacy of several antioxidant agents (ascorbic acid, alpha-lipoic acid, alpha-tocopherol, Mn(III)-tetrakis(4-benzoic acid)-porphyrin, and butylated hydroxytoluene), an immunosuppressive agent with antivascular endothelial growth factor (VEGF) activity (sirolimus, also known as rapamycin), a retinoid cycle inhibitor (retinylamine), and an artificial chromophore (9-cis-retinyl acetate) were evaluated side by side in a recently described murine model of AMD, the Rdh8(-/-)Abca4(-/-) mouse. This animal exhibits a retinopathy caused by delayed all-trans-retinal clearance resulting from the absence of both ATP-binding cassette transporter 4 (Abca4) and retinol dehydrogenase 8 (Rdh8) activities. Drug efficacy was evaluated by retinal histologic analyses and electroretinograms (ERGs).

RESULTS

All tested agents partially prevented atrophic changes in the Rdh8(-/-)Abca4(-/-) retina with retinylamine demonstrating the greatest efficacy. A significant reduction of complement deposition on Bruch's membrane was observed in sirolimus-treated mice, although the severity of retinal degeneration was similar to that observed in antioxidant- and 9-cis-retinyl acetate-treated mice. Sirolimus treatment of 6-month-old Rdh8(-/-)Abca4(-/-) mice for 4 months prevented choroidal neovascularization without changing retinal VEGF levels.

CONCLUSIONS

Mechanism-based therapy with retinylamine markedly attenuated degenerative retinopathy in Rdh8(-/-)Abca4(-/-) mice. Further understanding of pathogenic mechanisms involved in AMD is needed to develop more effective therapeutics.

Evaluation of 9-cis-retinyl acetate therapy in Rpe65-/- mice

PURPOSE

Mice lacking retinal pigment epithelium-specific 65-kDa protein (RPE65) develop retinopathy and blindness resembling Leber congenital amaurosis. Effects of 9-cis-retinyl acetate (9-cis-R-Ac) on visual function and retinopathy progression were tested in Rpe65(-/-) mice.

METHODS

Young C57Bl/6 mice were given 9-cis-R-Ac in each of four different oil-based vehicle solutions by gastric gavage to identify the vehicle most suitable for drug delivery by measuring retinoid levels in plasma. Then doses of 9-cis-R-Ac ranging from 1 to 100 mg/kg were administered to 5- to 12-week-old Rpe65(-/-) mice by different treatment regimens, including single doses and either intermittent or daily doses for various periods up to 8 weeks. Retinoid effects on visual function were evaluated by electroretinography, retinoid analyses, histologic methods, and vision-dependent behavioral testing.

RESULTS

Soybean oil vehicle provided the highest 9-cis-R-Ac metabolite levels in plasma. Single doses of 9-cis-R-Ac (6.25-50 mg/kg) provided significant dose-dependent improvement in electroretinographic responses. Well-tolerated daily doses (1-12.5 mg/kg) for 2 weeks induced remarkable improvement of retinal function. Significant dose-dependent improvement of electroretinographic responses was observed 6 days after administration of 9-cis-R-Ac daily for 3 days at 1 to 12.5 mg/kg. Mice given either daily or intermittent 9-cis-R-Ac treatment at 1 and 4 mg/kg and evaluated 8 weeks later displayed dose-dependent improvement of retinal function and morphology, whereas retinal function deteriorated in control animals. Treated mice also performed better than control animals in vision-dependent behavioral tests.

CONCLUSIONS

Treatment with 9-cis-R-Ac improves visual function and preserves retinal morphology in Rpe65(-/-) mice.

Loss of cone photoreceptors caused by chromophore depletion is partially prevented by the artificial chromophore pro-drug, 9-cis-retinyl acetate

Inactivating mutations in the retinoid isomerase (RPE65) or lecithin:retinol acyltransferase (LRAT) genes cause Leber congenital amaurosis (LCA), a severe visual impairment in humans. Both enzymes participate in the retinoid (visual) cycle, the enzymatic pathway that continuously generates 11-cis-retinal, the chromophore of visual pigments in rod and cone photoreceptor cells needed for vision. We investigated human RPE65-LCA patients and mice with visual cycle abnormalities to determine the impact of chronic chromophore deprivation on cones. Young patients with RPE65 mutations showed foveal cone loss along with shortened inner and outer segments of remaining cones; cone cell loss also was dramatic in young mice lacking Rpe65 or Lrat gene function. To selectively evaluate cone pathophysiology, we eliminated the rod contribution to electroretinographic (ERG) responses by generating double knockout mice lacking Lrat or Rpe65 together with an inactivated rod-specific G protein transducin gene (Gnat1-/-). Cone ERG responses were absent in Gnat1-/-Lrat-/- mice which also showed progressive degeneration of cones. Cone ERG responses in Gnat1-/-Rpe65-/- mice were markedly reduced and declined over weeks. Treatment of these mice with the artificial chromophore pro-drug, 9-cis-retinyl acetate, partially protected inferior retinal cones as evidenced by improved ERGs and retinal histochemistry. Gnat1-/- mice chronically treated with retinylamine, a selective inhibitor of RPE65, also showed a decline in the number of cones that was ameliorated by 9-cis-retinyl acetate. These results suggest that chronic lack of chromophore leads to progressive loss of cones in mice and humans. Therapy for LCA patients should be geared toward early adequate delivery of chromophore to cone photoreceptors.

Involvement of all-trans-retinal in acute light-induced retinopathy of mice

Exposure to bright light can cause visual dysfunction and retinal photoreceptor damage in humans and experimental animals, but the mechanism(s) remain unclear. We investigated whether the retinoid cycle (i.e. the series of biochemical reactions required for vision through continuous generation of 11-cis-retinal and clearance of all-trans-retinal, respectively) might be involved. Previously, we reported that mice lacking two enzymes responsible for clearing all-trans-retinal, namely photoreceptor-specific ABCA4 (ATP-binding cassette transporter 4) and RDH8 (retinol dehydrogenase 8), manifested retinal abnormalities exacerbated by light and associated with accumulation of diretinoid-pyridinium-ethanolamine (A2E), a condensation product of all-trans-retinal and a surrogate marker for toxic retinoids. Now we show that these mice develop an acute, light-induced retinopathy. However, cross-breeding these animals with lecithin:retinol acyltransferase knock-out mice lacking retinoids within the eye produced progeny that did not exhibit such light-induced retinopathy until gavaged with the artificial chromophore, 9-cis-retinal. No significant ocular accumulation of A2E occurred under these conditions. These results indicate that this acute light-induced retinopathy requires the presence of free all-trans-retinal and not, as generally believed, A2E or other retinoid condensation products. Evidence is presented that the mechanism of toxicity may include plasma membrane permeability and mitochondrial poisoning that lead to caspase activation and mitochondria-associated cell death. These findings further understanding of the mechanisms involved in light-induced retinal degeneration.

The role of the photoreceptor ABC transporter ABCA4 in lipid transport and Stargardt macular degeneration

ABCA4 is a member of the ABCA subfamily of ATP binding cassette (ABC) transporters that is expressed in rod and cone photoreceptors of the vertebrate retina. ABCA4, also known as the Rim protein and ABCR, is a large 2,273 amino acid glycoprotein organized as two tandem halves, each containing a single membrane spanning segment followed sequentially by a large exocytoplasmic domain, a multispanning membrane domain and a nucleotide binding domain. Over 500 mutations in the gene encoding ABCA4 are associated with a spectrum of related autosomal recessive retinal degenerative diseases including Stargardt macular degeneration, cone-rod dystrophy and a subset of retinitis pigmentosa. Biochemical studies on the purified ABCA4 together with analysis of abca4 knockout mice and patients with Stargardt disease have implicated ABCA4 as a retinylidene-phosphatidylethanolamine transporter that facilitates the removal of potentially reactive retinal derivatives from photoreceptors following photoexcitation. Knowledge of the genetic and molecular basis for ABCA4 related retinal degenerative diseases is being used to develop rationale therapeutic treatments for this set of disorders.

Quantitative mapping of ion channel regulation by visual cycle activity in rodent photoreceptors in vivo

PURPOSE

To test the hypothesis that the extent of outer retina uptake of manganese, measured noninvasively with manganese-enhanced MRI (MEMRI), is a quantitative biomarker of photoreceptor ion channel regulation by visual cycle activity.

METHODS

Four groups of animals were studied: control rats adapted to three different background light intensities, dark-adapted control mice systemically pretreated with retinylamine, and dark-adapted mice with a nonsense mutation in exon 3 of the RPE65 gene (RPE65(rd12)) with and without systemic 11-cis-retinal pretreatment. In all cases, rodents were anesthetized and studied with MEMRI 4 hours after manganese administration IP. Central retinal thickness and intraretinal ion channel regulation were measured from the MEMRI data.

RESULT

No differences (P>0.05) in retinal thickness were noted within any arm of this study. In rats, manganese uptake was inversely proportional to the background light intensity in the outer retina but not in the inner retina. Specific inhibition at the level of RPE65 activity, either acutely with retinylamine or chronically in RPE65(rd12) mice, similarly reduced (P<0.05) outer retinal manganese uptake compared with that in control mice. In RPE65(rd12) mice, outer retinal manganese uptake returned to normal (P>0.05) after 11-cis retinal treatment. Inner retinal uptake was supernormal (P<0.05) in retinylamine-treated mice but normal in untreated or 11-cis treated RPE65(rd12) mice.

CONCLUSIONS

The present data support measuring the extent of manganese uptake in the outer retina as an analytic noninvasive metric of visual cycle regulation of photoreceptor ion channel activity in vivo.

Retinopathy in mice induced by disrupted all-trans-retinal clearance

The visual (retinoid) cycle is a fundamental metabolic process in vertebrate retina responsible for production of 11-cis-retinal, the chromophore of rhodopsin and cone pigments. 11-cis-Retinal is bound to opsins, forming visual pigments, and when the resulting visual chromophore 11-cis-retinylidene is photoisomerized to all-trans-retinylidene, all-trans-retinal is released from these receptors. Toxic byproducts of the visual cycle formed from all-trans-retinal often are associated with lipofuscin deposits in the retinal pigmented epithelium (RPE), but it is not clear whether aberrant reactions of the visual cycle participate in RPE atrophy, leading to a rapid onset of retinopathy. Here we report that mice lacking both the ATP-binding cassette transporter 4 (Abca4) and enzyme retinol dehydrogenase 8 (Rdh8), proteins critical for all-trans-retinal clearance from photoreceptors, developed severe RPE/photoreceptor dystrophy at an early age. This phenotype includes lipofuscin, drusen, and basal laminar deposits, Bruch's membrane thickening, and choroidal neovascularization. Importantly, the severity of visual dysfunction and retinopathy was exacerbated by light but attenuated by treatment with retinylamine, a visual cycle inhibitor that slows the flow of all-trans-retinal through the visual cycle. These findings provide direct evidence that aberrant production of toxic condensation byproducts of the visual cycle in mice can lead to rapid, progressive retinal degeneration.

Metabolic basis of visual cycle inhibition by retinoid and nonretinoid compounds in the vertebrate retina

In vertebrate retinal photoreceptors, the absorption of light by rhodopsin leads to photoisomerization of 11-cis-retinal to its all-trans isomer. To sustain vision, a metabolic system evolved that recycles all-trans-retinal back to 11-cis-retinal. The importance of this visual (retinoid) cycle is underscored by the fact that mutations in genes encoding visual cycle components induce a wide spectrum of diseases characterized by abnormal levels of specific retinoid cycle intermediates. In addition, intense illumination can produce retinoid cycle by-products that are toxic to the retina. Thus, inhibition of the retinoid cycle has therapeutic potential in physiological and pathological states. Four classes of inhibitors that include retinoid and nonretinoid compounds have been identified. We investigated the modes of action of these inhibitors by using purified visual cycle components and in vivo systems. We report that retinylamine was the most potent and specific inhibitor of the retinoid cycle among the tested compounds and that it targets the retinoid isomerase, RPE65. Hydrophobic primary amines like farnesylamine also showed inhibitory potency but a short duration of action, probably due to rapid metabolism. These compounds also are reactive nucleophiles with potentially high cellular toxicity. We also evaluated the role of a specific protein-mediated mechanism on retinoid cycle inhibitor uptake by the eye. Our results show that retinylamine is transported to and taken up by the eye by retinol-binding protein-independent and retinoic acid-responsive gene product 6-independent mechanisms. Finally, we provide evidence for a crucial role of lecithin: retinol acyltransferase activity in mediating tissue specific absorption and long lasting therapeutic effects of retinoid-based visual cycle inhibitors.

Redundant and unique roles of retinol dehydrogenases in the mouse retina

Highly abundant short-chain alcohol dehydrogenases (RDHs) in the retina were assumed to be involved in the recycling of 11-cis-retinal chromophore in the visual cycle. Mutations in human RDH genes are associated with Fundus albipunctatus, a mild form of night blindness (RDH5) and an autosomal recessive, childhood-onset severe retinal dystrophy (RDH12). Rdh12 knockout mice were found to be susceptible to light-induced photoreceptor apoptosis, whereas Rdh5 and Rdh8 knockout mice displayed only delayed dark adaptation. However, each knockout mouse eventually regenerated normal levels of visual pigments, suggesting that RDHs compensate for each other in the visual cycle. Here, we established RDH double knockout (Rdh8(-/-)Rdh12(-/-)) and triple knockout (Rdh5(-/-)Rdh8(-/-)Rdh12(-/-)) mice generated on various genetic backgrounds including a rod alpha-transducin knockout to test cone function. RDH activity was severely reduced in Rdh8(-/-)Rdh12(-/-) retina extracts, whereas Rdh8(-/-) RDH activity was intermediate and Rdh12(-/-) RDH activity was reduced only slightly. Surprisingly, all multiple knockout mice produced sufficient amounts of the chromophore to regenerate rhodopsin and cone pigments in vivo. Three-month-old Rdh8(-/-)Rdh12(-/-) mice characteristically displayed a slowly progressing rod-cone dystrophy accompanied by accumulation of N-retinylidene-N-retinylethanolamine (A2E), a toxic substance known to contribute to retinal degeneration. A2E accumulation and retinal degeneration were prevented by application of retinylamine, a potent retinoid cycle inhibitor. The results suggest that RDH8 and RDH12 are dispensable in support of the visual cycle but appear to be key components in clearance of free all-trans-retinal, thereby preventing A2E accumulation and photoreceptor cell death.

Evidence for RPE65-independent vision in the cone-dominated zebrafish retina

An enzyme-based cyclic pathway for trans to cis isomerization of the chromophore of visual pigments (11-cis-retinal) is intrinsic to vertebrate cone and rod vision. This process, called the visual cycle, is mostly characterized in rod-dominated retinas and essentially depends on RPE65, an all-trans to 11-cis-retinoid isomerase. Here we analysed the role of RPE65 in zebrafish, a species with a cone-dominated retina. We cloned zebrafish RPE65 and showed that its expression coincided with photoreceptor development. Targeted gene knockdown of RPE65 resulted in morphologically altered rod outer segments and overall reduced 11-cis-retinal levels. Cone vision of RPE65-deficient larvae remained functional as demonstrated by behavioural tests and by metabolite profiling for retinoids. Furthermore, all-trans retinylamine, a potent inhibitor of the rod visual cycle, reduced 11-cis-retinal levels of control larvae to a similar extent but showed no additive effects in RPE65-deficient larvae. Thus, our study of zebrafish provides in vivo evidence for the existence of an RPE65-independent pathway for the regeneration of 11-cis-retinal for cone vision.

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.

Reduced-illuminance autofluorescence imaging in ABCA4-associated retinal degenerations

The health of the retinal pigment epithelium (RPE) can be estimated with autofluorescence (AF) imaging of lipofuscin, which accumulates as a byproduct of retinal exposure to light. Lipofuscin may be toxic to the RPE, and its toxicity may be enhanced by short-wavelength (SW) illumination. The high-intensity and SW excitation light used in conventional AF imaging could, at least in principle, increase the rate of lipofuscin accumulation and/or increase its toxicity. We considered two reduced-illuminance AF imaging (RAFI) methods as alternatives to conventional AF imaging. RAFI methods use either near-infrared (NIR) light or reduced-radiance SW illumination for excitation of fluorophores. We quantified the distribution of RAFI signals in relation to retinal structure and function in patients with the prototypical lipofuscin accumulation disease caused by mutations in ABCA4. There was evidence for two subclinical stages of macular ABCA4 disease involving hyperautofluorescence of both SW- and NIR-RAFI with and without associated loss of visual function. Use of RAFI methods and microperimetry in future clinical trials involving lipofuscinopathies should allow quantification of subclinical disease expression and progression without subjecting the diseased retina/RPE to undue light exposure.

Structure elucidation of retinoids in biological samples using postsource decay laser desorption/ionization mass spectrometry after high-performance liquid chromatography separation

Retinoids [retinol (vitamin A) and its metabolites] function in the visual cycle, embryonic development, cellular differentiation, and tissue homeostasis. Notwithstanding pivotal roles of retinoids in mammals, the limited number of commercially available retinoid standards is a major roadblock to identifying and studying retinoids in biological samples. Therefore, a need exists for improved methods to identify retinoid metabolites. We analyzed polar and nonpolar retinoids, including retinoic acid, retinol, retinyl acetate, and other retinyl esters, using postsource decay laser desorption/ionization mass spectrometry (PSD-LDI MS). PSD analysis was employed to examine the PSD fragmentation patterns of retinoids, as these patterns can be used for the characterization of retinoids from biological samples without the need for matching retention time with a commercially available or synthetic retinoid. Mechanisms for the formation of these PSD fragment ions are proposed. The feasibility of employing PSD after HPLC separation was demonstrated by characterizing the endogenous retinoids in canine kidney epithelial cell extracts and in mouse lung. We show that the PSD-LDI MS approach described here can facilitate the identification and characterization of retinoids from mammalian cells and tissues.

Diseases caused by defects in the visual cycle: retinoids as potential therapeutic agents

Absorption of a photon by an opsin pigment causes isomerization of the chromophore from 11-cis-retinaldehyde to all-trans-retinaldehyde. Regeneration of visual chromophore following light exposure is dependent on an enzyme pathway called the retinoid or visual cycle. Our understanding of this pathway has been greatly facilitated by the identification of disease-causing mutations in the genes coding for visual cycle enzymes. Defects in nearly every step of this pathway are responsible for human-inherited retinal dystrophies. These retinal dystrophies can be divided into two etiologic groups. One involves the impaired synthesis of visual chromophore. The second involves accumulation of cytotoxic products derived from all-trans-retinaldehyde. Gene therapy has been successfully used in animal models of these diseases to rescue the function of enzymes involved in chromophore regeneration, restoring vision. Dystrophies resulting from impaired chromophore synthesis can also be treated by supplementation with a chromophore analog. Dystrophies resulting from the accumulation of toxic pigments can be treated pharmacologically by inhibiting the visual cycle, or limiting the supply of vitamin A to the eyes. Recent progress in both areas provides hope that multiple inherited retinal diseases will soon be treated by pharmaceutical intervention.

Delivery of retinoid-based therapies to target tissues

Through its various metabolites, vitamin A controls essential physiological functions. Both naturally occurring metabolites and novel retinoid analogues have shown effectiveness in many clinical settings that include skin diseases and cancer, and in animal models of human conditions affecting vision. In this review, we analyze several potential retinoid-based therapies from the point of view of drug metabolism and transport to target tissues. We focus on the endogenous factors that affect the absorption, transport, and metabolism of retinoids by taking into account data obtained from the analysis of animal models that lack the enzymes or proteins involved in the storage and absorption of retinoids. We also discuss findings of toxicity associated with retinoids in an effort to improve the outcome of retinoid-based therapies. In this context, we review evidence that esterification of retinol and retinol-based drugs within target tissues provides one of the most efficient means to improve the absorption and to reduce the toxicity associated with pharmacological doses of retinoids. Future retinoid-based therapeutic strategies could involve targeted delivery mechanisms leading to lower toxicity and improved effectiveness of retinoids.