Sterol composition of bovine retinal rod outer segment membranes and whole retinas

Dynamic lipid turnover in photoreceptors and retinal pigment epithelium throughout life

The retinal pigment epithelium-photoreceptor interphase is renewed each day in a stunning display of cellular interdependence. While photoreceptors use photosensitive pigments to convert light into electrical signals, the RPE supports photoreceptors in their function by phagocytizing shed photoreceptor tips, regulating the blood retina barrier, and modulating inflammatory responses, as well as regenerating the 11-cis-retinal chromophore via the classical visual cycle. These processes involve multiple protein complexes, tightly regulated ligand-receptors interactions, and a plethora of lipids and protein-lipids interactions. The role of lipids in maintaining a healthy interplay between the RPE and photoreceptors has not been fully delineated. In recent years, novel technologies have resulted in major advancements in understanding several facets of this interplay, including the involvement of lipids in phagocytosis and phagolysosome function, nutrient recycling, and the metabolic dependence between the two cell types. In this review, we aim to integrate the complex role of lipids in photoreceptor and RPE function, emphasizing the dynamic exchange between the cells as well as discuss how these processes are affected in aging and retinal diseases.

Impaired ABCA1/ABCG1-mediated lipid efflux in the mouse retinal pigment epithelium (RPE) leads to retinal degeneration

Age-related macular degeneration (AMD) is a progressive disease of the retinal pigment epithelium (RPE) and the retina leading to loss of central vision. Polymorphisms in genes involved in lipid metabolism, including the ATP-binding cassette transporter A1 (ABCA1), have been associated with AMD risk. However, the significance of retinal lipid handling for AMD pathogenesis remains elusive. Here, we study the contribution of lipid efflux in the RPE by generating a mouse model lacking ABCA1 and its partner ABCG1 specifically in this layer. Mutant mice show lipid accumulation in the RPE, reduced RPE and retinal function, retinal inflammation and RPE/photoreceptor degeneration. Data from human cell lines indicate that the ABCA1 AMD risk-conferring allele decreases ABCA1 expression, identifying the potential molecular cause that underlies the genetic risk for AMD. Our results highlight the essential homeostatic role for lipid efflux in the RPE and suggest a pathogenic contribution of reduced ABCA1 function to AMD.

Regulated efflux of photoreceptor outer segment-derived cholesterol by human RPE cells

Genetic studies have linked age-related macular degeneration (AMD) to genes involved in high-density lipoprotein (HDL) metabolism, including ATP-binding cassette transporter A1 (ABCA1). The retinal pigment epithelium (RPE) handles large amounts of lipids, among others cholesterol, partially derived from internalized photoreceptor outer segments (OS) and lipids physiologically accumulate in the aging eye. To analyze the potential function of ABCA1 in the eye, we measured cholesterol efflux, the first step of HDL generation, in RPE cells. We show the expression of selected genes related to HDL metabolism in mouse and human eyecups as well as in ARPE-19 and human primary RPE cells. Immunofluorescence staining revealed localization of ABCA1 on both sides of polarized RPE cells. This was functionally confirmed by directional efflux to apolipoprotein AI (ApoA-I) of ³H-labeled cholesterol given to the cells via serum or via OS. ABCA1 expression and activity was modulated using a liver-X-receptor (LXR) agonist and an ABCA1 neutralizing antibody, demonstrating that the efflux was ABCA1-dependent. We concluded that the ABCA1-mediated lipid efflux pathway, and hence HDL biosynthesis, is functional in RPE cells towards both the basal (choroidal) and apical (subretinal) space. Impaired activity of the pathway might cause age-related perturbations of lipid homeostasis in the outer retina and thus may contribute to disease development and/or progression.

An inducible amphipathic helix within the intrinsically disordered C terminus can participate in membrane curvature generation by peripherin-2/rds

Peripherin-2/rds is required for biogenesis of vertebrate photoreceptor outer segment organelles. Its localization at the high-curvature rim domains of outer segment disk membranes suggests that it may act to shape these structures; however, the molecular function of this protein is not yet resolved. Here, we apply biochemical, biophysical, and imaging techniques to elucidate the role(s) played by the protein's intrinsically disordered C-terminal domain and an incipient amphipathic α-helix contained within it. We investigated a deletion mutant lacking only this α-helix in stable cell lines and Xenopus laevis photoreceptors. We also studied a soluble form of the full-length ∼7-kDa cytoplasmic C terminus in cultured cells and purified from Escherichia coli The α-helical motif was not required for protein biosynthesis, tetrameric subunit assembly, tetramer polymerization, localization at disk rims, interaction with GARP2, or the generation of membrane curvature. Interestingly, however, loss of the helical motif up-regulated membrane curvature generation in cellulo, introducing the possibility that it may regulate this activity in photoreceptors. Furthermore, the incipient α-helix (within the purified soluble C terminus) partitioned into membranes only when its acidic residues were neutralized by protonation. This suggests that within the context of full-length peripherin-2/rds, partitioning would most likely occur at a bilayer interfacial region, potentially adjacent to the protein's transmembrane domains. In sum, this study significantly strengthens the evidence that peripherin-2/rds functions directly to shape the high-curvature rim domains of the outer segment disk and suggests that the protein's C terminus may modulate membrane curvature-generating activity present in other protein domains.

Molecular basis for photoreceptor outer segment architecture

To serve vision, vertebrate rod and cone photoreceptors must detect photons, convert the light stimuli into cellular signals, and then convey the encoded information to downstream neurons. Rods and cones are sensory neurons that each rely on specialized ciliary organelles to detect light. These organelles, called outer segments, possess elaborate architectures that include many hundreds of light-sensitive membranous disks arrayed one atop another in precise register. These stacked disks capture light and initiate the chain of molecular and cellular events that underlie normal vision. Outer segment organization is challenged by an inherently dynamic nature; these organelles are subject to a renewal process that replaces a significant fraction of their disks (up to ∼10%) on a daily basis. In addition, a broad range of environmental and genetic insults can disrupt outer segment morphology to impair photoreceptor function and viability. In this chapter, we survey the major progress that has been made for understanding the molecular basis of outer segment architecture. We also discuss key aspects of organelle lipid and protein composition, and highlight distributions, interactions, and potential structural functions of key OS-resident molecules, including: kinesin-2, actin, RP1, prominin-1, protocadherin 21, peripherin-2/rds, rom-1, glutamic acid-rich proteins, and rhodopsin. Finally, we identify key knowledge gaps and challenges that remain for understanding how normal outer segment architecture is established and maintained.

Cholesterol in the rod outer segment: A complex role in a "simple" system

The rod outer segment (ROS) of retinal photoreceptor cells consists of disk membranes surrounded by the plasma membrane. It is a relatively uncomplicated system in which to investigate cholesterol distribution and its functional consequences in biologically relevant membranes. The light sensitive protein, rhodopsin is the major protein in both membranes, but the lipid compositions are significantly different in the disk and plasma membranes. Cholesterol is high in the ROS plasma membrane. Disk membranes are synthesized at the base of the ROS and are also high in cholesterol. However, cholesterol is rapidly depleted as the disks are apically displaced. During this apical displacement the disk phospholipid fatty acyl chains become progressively more unsaturated, which creates an environment unfavorable to cholesterol. Membrane cholesterol has functional consequences. The high cholesterol found in the plasma membrane and in newly synthesized disks inhibits the activation of rhodopsin. As disks are apically displaced and cholesterol is depleted rhodopsin becomes more responsive to light. This effect of cholesterol on rhodopsin activation has been shown in both native and reconstituted membranes. The modulation of activity can be at least partially explained by the effect of cholesterol on bulk lipid properties. Cholesterol decreases the partial free volume of the hydrocarbon region of the bilayer and thereby inhibits rhodopsin conformational changes required for activation. However, cholesterol binds to rhodopsin and may directly affect the protein also. Furthermore, cholesterol stabilizes rhodopsin to thermal denaturation. The membrane must provide an environment that allows rhodopsin conformational changes required for activation while also stabilizing the protein to thermal denaturation. Cholesterol thus plays a complex role in modulating the activity and stability of rhodopsin, which have implications for other G-protein coupled receptors.

Characterization of solid-supported dipalmitoylphosphatidylcholine membranes containing cholesterol

The incorporation of cholesterol into dipalmitoylphosphatidylcholine (DPPC) membranes, even in small amounts, has been shown to significantly alter the properties of the membrane. In this work, force-distance interaction profiles of DPPC membranes containing 8 mol % cholesterol obtained using the surface force apparatus are analyzed in the context of high-resolution structural characterization by atomic force microscopy and neutron reflectometry. The adhesion between the mixed membranes was greater than that for pure DPPC and was variable-depending on the number of defects in the outer membrane leaflets. These defects were only detectable by atomic force microscopy and had an average size of 230 ± 30 nm and 1-5% surface density in the outer leaflet. The adhesion between the membranes monotonically increased as the thickness of the membrane decreased-in direct correlation with the number of defects present (exposed hydrophobic groups) in the membrane contact region. Because of the low diffusion rate of gel-phase membranes, the interaction force profiles were stable and no membrane restructuring was observed.

Retinal and nonocular abnormalities in Cyp27a1(-/-)Cyp46a1(-/-) mice with dysfunctional metabolism of cholesterol

Cholesterol elimination from nonhepatic cells involves metabolism to side-chain oxysterols, which serve as transport forms of cholesterol and bioactive molecules modulating a variety of cellular processes. Cholesterol metabolism is tissue specific, and its significance has not yet been established for the retina, where cytochromes P450 (CYP27A1 and CYP46A1) are the major cholesterol-metabolizing enzymes. We generated Cyp27a1(-/-)Cyp46a1(-/-) mice, which were lean and had normal serum cholesterol and glucose levels. These animals, however, had changes in the retinal vasculature, retina, and several nonocular organs (lungs, liver, and spleen). Changes in the retinal vasculature included structural abnormalities (retinal-choroidal anastomoses, arteriovenous shunts, increased permeability, dilation, nonperfusion, and capillary degeneration) and cholesterol deposition and oxidation in the vascular wall, which also exhibited increased adhesion of leukocytes and activation of the complement pathway. Changes in the retina included increased content of cholesterol and its metabolite, cholestanol, which were focally deposited at the apical and basal sides of the retinal pigment epithelium. Retinal macrophages of Cyp27a1(-/-)Cyp46a1(-/-) mice were activated, and oxidative stress was noted in their photoreceptor inner segments. Our findings demonstrate the importance of retinal cholesterol metabolism for maintenance of the normal retina, and suggest new targets for diseases affecting the retinal vasculature.

Cholesterol in the retina: the best is yet to come

Historically understudied, cholesterol in the retina is receiving more attention now because of genetic studies showing that several cholesterol-related genes are risk factors for age-related macular degeneration (AMD) and because of eye pathology studies showing high cholesterol content of drusen, aging Bruch's membrane, and newly found subretinal lesions. The challenge before us is determining how the cholesterol-AMD link is realized. Meeting this challenge will require an excellent understanding these genes' roles in retinal physiology and how chorioretinal cholesterol is maintained. In the first half of this review, we will succinctly summarize physico-chemical properties of cholesterol, its distribution in the human body, general principles of maintenance and metabolism, and differences in cholesterol handling in human and mouse that impact on experimental approaches. This information will provide a backdrop to the second part of the review focusing on unique aspects of chorioretinal cholesterol homeostasis, aging in Bruch's membrane, cholesterol in AMD lesions, a model for lesion biogenesis, a model for macular vulnerability based on vascular biology, and alignment of AMD-related genes and pathobiology using cholesterol and an atherosclerosis-like progression as unifying features. We conclude with recommendations for the most important research steps we can take towards delineating the cholesterol-AMD link.

Membrane curvature generation by a C-terminal amphipathic helix in peripherin-2/rds, a tetraspanin required for photoreceptor sensory cilium morphogenesis

Vertebrate vision requires photon absorption by photoreceptor outer segments (OSs), structurally elaborate membranous organelles derived from non-motile sensory cilia. The structure and function of OSs depends on a precise stacking of hundreds of membranous disks. Each disk is fully (as in rods) or partially (as in cones) bounded by a rim, at which the membrane is distorted into an energetically unfavorable high-curvature bend; however, the mechanism(s) underlying disk rim structure is (are) not established. Here, we demonstrate that the intrinsically disordered cytoplasmic C-terminus of the photoreceptor tetraspanin peripherin-2/rds (P/rds) can directly generate membrane curvature. A P/rds C-terminal domain and a peptide mimetic of an amphipathic helix contained within it each generated curvature in liposomes with a composition similar to that of OS disks and in liposomes generated from native OS lipids. Association of the C-terminal domain with liposomes required conical phospholipids, and was promoted by membrane curvature and anionic surface charge, results suggesting that the P/rds C-terminal amphipathic helix can partition into the cytosolic membrane leaflet to generate curvature by a hydrophobic insertion (wedging) mechanism. This activity was evidenced in full-length P/rds by its induction of small-diameter tubulovesicular membrane foci in cultured cells. In sum, the findings suggest that curvature generation by the P/rds C-terminus contributes to the distinctive structure of OS disk rims, and provide insight into how inherited defects in P/rds can disrupt organelle structure to cause retinal disease. They also raise the possibility that tethered amphipathic helices can function for shaping cellular membranes more generally.

Spatial distribution of the pathways of cholesterol homeostasis in human retina

Background

The retina is a light-sensitive tissue lining the inner surface of the eye and one of the few human organs whose cholesterol maintenance is still poorly understood. Challenges in studies of the retina include its complex multicellular and multilayered structure; unique cell types and functions; and specific physico-chemical environment.

Methodology/Principal Findings

We isolated specimens of the neural retina (NR) and underlying retinal pigment epithelium (RPE)/choroid from six deceased human donors and evaluated them for expression of genes and proteins representing the major pathways of cholesterol input, output and regulation. Eighty-four genes were studied by PCR array, 16 genes were assessed by quantitative real time PCR, and 13 proteins were characterized by immunohistochemistry. Cholesterol distribution among different retinal layers was analyzed as well by histochemical staining with filipin. Our major findings pertain to two adjacent retinal layers: the photoreceptor outer segments of NR and the RPE. We demonstrate that in the photoreceptor outer segments, cholesterol biosynthesis, catabolism and regulation via LXR and SREBP are weak or absent and cholesterol content is the lowest of all retinal layers. Cholesterol maintenance in the RPE is different, yet the gene expression also does not appear to be regulated by the SREBPs and varies significantly among different individuals.

Conclusions/Significance

This comprehensive investigation provides important insights into the relationship and spatial distribution of different pathways of cholesterol input, output and regulation in the NR-RPE region. The data obtained are important for deciphering the putative link between cholesterol and age-related macular degeneration, a major cause of irreversible vision loss in the elderly.

Lipids and age-related macular degeneration

Given the considerable public health burden imposed by age-related macular degeneration (AMD), much effort has been directed towards elucidating principles of pathogenesis in order to identify risk factors and develop preventive measures and treatments. Together with epidemiological evidence linking cardiovascular risk factors with AMD risk and basic science work examining the role of lipid metabolism in AMD, numerous human studies have assayed a potential relationship between dietary lipids and the development of AMD. We examine the evidence for a role for lipid metabolism in AMD, highlighting key basic biochemical principles, work in animal models, and relevant human studies. The topics of lipoprotein modulation and omega-3 fatty acid intake receive special attention from both a basic science and clinical study standpoint. The evidence suggests that consumption of omega-3 fatty acids, perhaps in concert with antioxidants, may constitute a rational preventative strategy against AMD development, though, absent an appropriately developed double-blind, randomized control trial, insufficient data exist to recommend implementation in the clinical setting at this time.

The ins and outs of cholesterol in the vertebrate retina

The vertebrate retina has multiple demands for utilization of cholesterol and must meet those demands either by synthesizing its own supply of cholesterol or by importing cholesterol from extraretinal sources, or both. Unlike the blood-brain barrier, the blood-retina barrier allows uptake of cholesterol from the circulation via a lipoprotein-based/receptor-mediated mechanism. Under normal conditions, cholesterol homeostasis is tightly regulated; also, cholesterol exists in the neural retina overwhelmingly in unesterified form, and sterol intermediates are present in minimal to negligible quantities. However, under certain pathological conditions, either due to an inborn error in cholesterol biosynthesis or as a consequence of exposure to selective inhibitors of enzymes in the cholesterol pathway, the ratio of sterol intermediates to cholesterol in the retina can rise dramatically and persist, in some cases resulting in progressive degeneration that significantly compromises the structure and function of the retina. Although the relative contributions of de novo synthesis versus extraretinal uptake are not yet known, herein we review what is known about these processes and the dynamics of cholesterol in the vertebrate retina and indicate some future avenues of research in this area.

The role of cholesterol in rod outer segment membranes

The photoreceptor rod outer segment (ROS) provides a unique system in which to investigate the role of cholesterol, an essential membrane constituent of most animal cells. The ROS is responsible for the initial events of vision at low light levels. It consists of a stack of disk membranes surrounded by the plasma membrane. Light capture occurs in the outer segment disk membranes that contain the photopigment, rhodopsin. These membranes originate from evaginations of the plasma membrane at the base of the outer segment. The new disks separate from the plasma membrane and progressively move up the length of the ROS over the course of several days. Thus the role of cholesterol can be evaluated in two distinct membranes. Furthermore, because the disk membranes vary in age it can also be investigated in a membrane as a function of the membrane age. The plasma membrane is enriched in cholesterol and in saturated fatty acids species relative to the disk membrane. The newly formed disk membranes have 6-fold more cholesterol than disks at the apical tip of the ROS. The partitioning of cholesterol out of disk membranes as they age and are apically displaced is consistent with the high PE content of disk membranes relative to the plasma membrane. The cholesterol composition of membranes has profound consequences on the major protein, rhodopsin. Biophysical studies in both model membranes and in native membranes have demonstrated that cholesterol can modulate the activity of rhodopsin by altering the membrane hydrocarbon environment. These studies suggest that mature disk membranes initiate the visual signal cascade more effectively than the newly synthesized, high cholesterol basal disks. Although rhodopsin is also the major protein of the plasma membrane, the high membrane cholesterol content inhibits rhodopsin participation in the visual transduction cascade. In addition to its effect on the hydrocarbon region, cholesterol may interact directly with rhodopsin. While high cholesterol inhibits rhodopsin activation, it also stabilizes the protein to denaturation. Therefore the disk membrane must perform a balancing act providing sufficient cholesterol to confer stability but without making the membrane too restrictive to receptor activation. Within a given disk membrane, it is likely that cholesterol exhibits an asymmetric distribution between the inner and outer bilayer leaflets. Furthermore, there is some evidence of cholesterol microdomains in the disk membranes. The availability of the disk protein, rom-1 may be sensitive to membrane cholesterol. The effects exerted by cholesterol on rhodopsin function have far-reaching implications for the study of G-protein coupled receptors as a whole. These studies show that the function of a membrane receptor can be modulated by modification of the lipid bilayer, particularly cholesterol. This provides a powerful means of fine-tuning the activity of a membrane protein without resorting to turnover of the protein or protein modification.

Structural studies on rhodopsin

Bovine rhodopsin is the prototypical G protein coupled receptor (GPCR). It was the first GPCR to be obtained in quantity and studied in detail. It is also the first GPCR for which detailed three dimensional structural information has been obtained. Reviewed here are the experiments leading up to the high resolution structure determination of rhodopsin and the most recent structural information on the activation and stability of this integral membrane protein.

Rhodopsin trafficking and its role in retinal dystrophies

We review the sorting/targeting steps involved in the delivery of rhodopsin to the outer segment compartment of highly polarized photoreceptor cells. The transport of rhodopsin includes (1) the sorting/budding of rhodopsin-containing vesicles at the trans-Golgi network, (2) the directional translocation of rhodopsin-bearing vesicles through the inner segment, and (3) the delivery of rhodopsin across the connecting cilium to the outer segment. Several independent lines of evidence suggest that the carboxyl-terminal, cytoplasmic tail of rhodopsin is involved in the post-Golgi trafficking of rhodopsin. Inappropriate subcellular targeting of naturally occurring rhodopsin mutants in vivo leads to photoreceptor cell death. Thus, the genes encoding mutations in the cellular components involved in photoreceptor protein transport are likely candidate genes for retinal dystrophies.

Ontogenesis of lipids in chick embryo retina

PURPOSE

The effects of embryonic development on lipid composition in the retina were studied in 7, 11, 15, and 18-day-old chick embryos and newly hatched chicks.

METHODS

The proportions of phospholipids, free and esterified cholesterol, diacylglycerides, and free fatty acids were determined using the Iatroscan TLC/FID procedure. Gas chromatography and mass spectrometry were used to determine the fatty acid composition.

RESULTS

The major phospholipid species were phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, phosphatidylinositol, lysophosphatidylcholine, and sphingomyelin. Concentrations of the analyzed components have been related to the chronology of concrete stages of retinal development. The fatty acid composition of the total lipids, (n-6):(n-3) and saturated: unsaturated fatty acid ratios, and other parameters are reported. The proportions of total saturated and total monounsaturated fatty acids decreased very little from day 7 to hatching, whereas total polyunsaturated fatty acids nearly doubled over the same period. The increase in C18:2(n-6) from day 11 onwards was not followed by a similar increase in C20:4(n-6), hence the C20:4 to C18:2 ratio decreased with age.

CONCLUSIONS

The cholesterol:phospholipid ratio decreased from day 7 to day 15 and increased from day 15 to hatching. High proportions of esterified cholesterol, very probably originating in the retinal pigment epithelium, were also recorded. Total saturated and monounsaturated fatty acids decreased, while polyunsaturated fatty acids increased during the period of initial retinal growth.

Identification of beta-galactosidase activity in purified bovine retinal rod outer segments

We have identified beta-galactosidase activity in purified bovine rod outer segments (ROS), using rho-nitrophenyl-beta-D-galactopyranoside (PNPG) and chlorophenol red-beta-D-galactopyranoside (CPRG) as substrates. This glycosylhydrolase activity did not appear to represent contamination from other retinal subcellular fractions, based upon the relative specific activities of beta-galactosidase vs. other hydrolases (N-acetyl-beta-glucosaminidase, alpha- and beta-mannosidase, alpha-fucosidase, and acid phosphatase) in bovine retina and ROS homogenates. Using PNPG as a substrate, two pH optima were observed (at 3.5 and 5.5), while the hydrolysis of CPRG exhibited a single, broad pH optimum centered at 5.5. In contrast, hydrolysis of PNPG and CPRG by retinal homogenates exhibited single pH optima, at 3.5 and 5.5., respectively. ROS beta-galactosidase activity increased linearly with time, temperature, and protein concentration, and obeyed Michaelis-Menten kinetics with both substrates. For PNPG, Vmax approximately 88 nmol/h/mg protein and the apparent Km approximately 147 microM. For CPRG, Vmax approximately 33 nmol/h/mg protein and the apparent Km approximately 50 microM. ROS beta-galactosidase activity was affected by carbohydrates and their derivatives: glucose, fucose, sucrose, maltose and N-acetyl-galactosamine were found to stimulate the activity, while D-galactono-gamma-lactone and, to a lesser extent, D-galactose were inhibitory. The enzyme activity also was slightly stimulated by [Cl-] and markedly by dithiothreitol (DTT), while rho-chloro-mercuribenzoic acid (PCMB) and rho-hydroxymercuribenzoic acid (PHMB) inactivated the enzyme. In addition, the enzymatic activity was also found to be differentially sensitive to various anionic and nonionic detergents. However, n-octyl-beta-D-glucoside was slightly stimulatory.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo biosynthesis of cholesterol in the rat retina

Previous reports have suggested that the rate of de novo cholesterol synthesis in the adult vertebrate retina is extremely slow. We investigated cholesterol biosynthesis in the adult rat retina in vivo, following intravitreal injection of [3H]acetate. HPLC analysis of retinal non-saponifiable lipid extracts revealed co-elution of radioactivity with endogenous cholesterol mass within 4.5 h post-injection. Incorporation of [3H]acetate into cholesterol was markedly reduced by co-injection of known inhibitors of the cholesterol pathway. In contrast to previous results with retinas from other species, no radiolabel or mass corresponded to squalene, except in lipid extracts from retinas treated with NB-598, a squalene epoxidase inhibitor. These results demonstrate, for the first time, the capacity of the adult vertebrate retina to rapidly synthesize cholesterol de novo.

Cholesterol movement between bovine rod outer segment disk membranes and phospholipid vesicles

The ability of cholesterol to move between bovine rod outer segment disk membranes and phospholipid membranes was examined. Disk membranes were incubated with small unilamellar phospholipid vesicles containing varying amounts of cholesterol. Aliquots were removed at specific times, and then the disks and the vesicles were separated by centrifugation and assayed for phospholipid and cholesterol content. When incubated with vesicles containing no cholesterol, the cholesterol to phospholipid ratio in the disk membrane was reduced due to migration of cholesterol from the disks into the vesicles. The cholesterol content of these cholesterol depleted disks could be readily returned to the normal disk cholesterol content by incubation of the cholesterol-depleted disks with small unilamellar vesicles containing high cholesterol. An apparent partition coefficient K was calculated as the quotient of the cholesterol/phospholipid mole ratio in the donor membranes and the cholesterol/phospholipid mole ratio in the acceptor membranes. The value of K was approximately 1 at cholesterol levels below normal disk cholesterol content, for disk membranes and phosphatidylcholine small unilamellar vesicles. Inclusion of phosphatidylethanolamine in the small unilamellar vesicle acceptor raised K, indicating that phosphatidylethanolamine creates an unfavourable environment for cholesterol. The cholesterol to phospholipid ratio of native disks could be increased by incubation with phosphatidylcholine small unilamellar vesicles (donor) which contained higher amounts of cholesterol than the disk membrane acceptor. In these experiments the distribution of cholesterol between disks and small unilamellar vesicles always favored the vesicles. The apparent partition coefficient was 1.7 at several cholesterol levels above the native disk cholesterol content. Liposomes made from lipid extracted from the disk membrane behaved in the same manner as intact disks with respect to cholesterol distribution at equilibrium. The phospholipid content of the disk membrane may be an important factor in determining the cholesterol content of the disk membrane.

In vitro metabolism of mevalonic acid in the bovine retina

Bovine retinas were incubated with 3RS-[5-3H]-mevalonic acid under conditions similar to those previously shown to support opsin biosynthesis in vitro. TLC of the total lipids indicated the formation of numerous radiolabeled components, including sterols, hydrocarbons, and "fatty acid-like material." The nonsaponifiable lipids were analyzed by TLC, GLC, and chromatography on columns of silicic acid-Super Cel, silica gel G-Super Cel-silver nitrate, and alumina-Super Cel-silver nitrate. The major nonsaponifiable components had the chromatographic properties of squalene and "methylated sterols" (i.e., C30, C29, and C28 monohydroxy sterols). Cholesterol represented no more than 1% of the total radioactivity in the nonsaponifiable lipid fraction. The "fatty acid-like material" was derivatized with diazomethane, and the resulting methyl esters were analyzed by GLC before and after catalytic hydrogenation. The radioactivity did not correspond to the normal fatty acids endogenous to the retina, but rather had the chromatographic properties of C15 and C20 isoprenoid acids. These results obtained with intact retinas are consistent with our previous observations concerning mevalonic acid metabolism in cell-free homogenates of bovine retinas.

Freeze-fracture study of filipin binding in photoreceptor outer segments and pigment epithelium of dystrophic and normal retinas

We have studied sterol distribution in the retinal pigment epithelial (RPE) microvillous and outer segment disc membranes of rats with inherited retinal degeneration (RCS; RCS-p/+) and of normal genetic controls (RCS-rdy+, RCS-rdy+-p/+) by using the polyene antibiotic filipin, which binds specifically to 3-B-hydroxy-sterols, and freeze-fracture techniques. Retinas were perfusion-fixed, incubated with filipin in the same fixative, and prepared routinely for freeze-fracture electron microscopy. In the normal retina, the distribution of filipin binding sites on both RPE microvillous and outer segment disc membranes changes during development. Prior to outer segment elongation and the onset of phagocytosis (10 days postnatal), filipin sterol complexes are homogeneously distributed in both microvillous and outer segment membranes. With the onset of phagocytosis (2 weeks postnatal and later) filipin binding in both tissues forms a proximal-to-distal gradient, and binding sites decrease as distance from the cell body increases. In the normal RPE microvillous membranes, binding sites are numerous proximally and sparse on the distal tips. In the normal outer segment disc membranes, binding sites are often present on the basal discs, but are sparse on the intact apical discs prior to shedding. As the discs are cast off and engulfed by the RPE, however, filipin binding increases on both disc and phagosome membranes. In the dystrophic retina, the distribution of filipin binding sites differs from the normal. First, in the microvillous membranes, the proximal-to-distal gradient in filipin binding is rarely present at 2 weeks postnatal and becomes prominent only after the buildup of membranous debris has begun (3-5 weeks postnatal). Second, as the photoreceptors degenerate and the membrane debris disappears (4 months postnatal), filipin binding on the microvillous membranes becomes relatively sparse and homogeneous. Third, filipin binding on the intact disc membranes does not change with outer segment elongation, and numerous filipin binding sites are present on both apical and basal outer segment disc membranes. Fourth, large aggregates of filipin binding sites occupy the vast expanses of particle-free areas of debris membranes which accumulate between the photoreceptors and the RPE. These changes in the amount and distribution of filipin binding sites in the dystrophic retina add to the evidence that the disease process involves outer segment as well as RPE membranes and suggest that alterations in cholesterol distribution could contribute to the phagocytic defect.

Neutral lipids of frog and rat rod outer segments

Neutral lipids of rod outer segments (ROS) from frog and rat comprise roughly 10 mol% of the total lipids and consist of free sterol, free fatty acids, and 1,2-diglycerides, but contain no detectable sterol esters or triglycerides. Expressed as nmol per mg ROS protein, the levels of both free fatty acids and free sterol are higher in the rat than in the frog, whereas the diglycerides in frogs are about four times higher than in rats. The level of 22:6 omega 3 in the free fatty acids fraction of the frog is twice that of the rat, although the level of 22:6 omega 3 in both the frog and rat free fatty acids is lower than in any of the glycerolipids. The diglyceride fraction from both animals consists almost entirely of two molecular species: in the frog, the C-38 and C-40 type predominate in a molar distribution of 52 and 42% respectively, whereas in the rat, the C-36 and C-38 types are most abundant in a molar distribution of 28 and 60%, respectively. Comparison of the diglyceride fatty acid and molecular species compositions with those of phosphatidylinositol and phosphatidic acid led to the suggestion that ROS diglycerides are derived from phosphatidylinositol.

Incorporation of 14C acetate into the sterols of rabbit lens in organ culture

Rabbit lenses were incubated in organ culture with 14C acetate for 3, 6, 12 and 20 hours and the nonsaponifiable fraction, composed primarily of sterols, was isolated. The incorporation of acetate into the nonsaponifiable fraction was linear for periods of 3 to 20 hours incubation. The nonsaponifiable fraction was subjected to thin-layer chromatography and 6 bands were observed. On the basis of Rf values, one band was tentatively identified as squalene, another as lanosterol and a third as a cholesterol-desmosterol mixture. The percentage of the radioactivity incorporated into the cholesterol-desmosterol band increased with incubation time. Conversely the percentage of the radioactivity incorporated into the squalene and lanosterol bands decreased with incubation time, indicating a precursor-product relationship.