Retina, membranes

Nakanishi, H., Artificial retina membranes. Prog. Surf. Sci., 49, 107, 1995. 

Retina membrane phospholipids, particularly phosphatidylethanolamine, contain a high level of 22 6n-3 (Birch etal., 1992 Suh etal., 1994). In the rod outer segment of the retina, significant amounts of 22 6n-3 in phosphatidylserine and phosphatidylcholine also occur (Suh et al., 1994). Increased dietary intake of n-3 fatty acids increases the n-3... 

A general picture of the fractionation of retina membranes is shown in Fig. 1. The ROS (rod outer segment) membranes were isolated by the procedure of McConnell (1965) with slight modifications. In the sucrose density gradient that we used the ROS membranes floated as two well defined red colored layers, at the junction between 0.77 and 0.94 M sucrose and between 0.94 and 1.10 M sucrose. The proteins of these two membrane layers showed similar patterns by sodium dodecyl-sulfate polyacrylamide gel electrophoresis, being rhodopsin its major component. The retina membranes that did not sediment with the crude ROS fraction, namely ROS free membranes, were subfractionated by centrifugation in sucrose density gradients into subfractions Pla,Plb,P2,P3 and P4. The ROS and Plb layers were red colored while the sediment at the bottom of the tube was brownish. 

The transfer activity of retina membranes was preserved for at least six months upon storage at -15 in 0.24 M sucrose. 

Table 2. Incorporation of H-NeuNAc into endogenous acceptors from retina membrane fractions...

These results clearly indicate that the bulk of the in vitro sialic acid incorporation into endogenous gangliosides and sialo-glycoproteins was in retina membranes other than ROS. Centrifugation of ROS free membranes in sucrose density gradients resulted in the isolation of the more active membranes for the transfer of H-NeuNAc from CMP- H-NeuNAc, which were collected at the junction between the sucrose layers of densities 1.10 - 1.11. The labelled lipids and proteins found in the ROS membranes after they were incubated with CMP- H-NeuNAc could arise from Pla membranes contaminating the crude ROS fraction during fractionation of retinas. 

Photoreceptors in the eyes convert radiation in the visible band into neural signals that reach the brain. Photoreceptors are located throughout the retina, a sensoiy membrane that covers the entire back of the... 

AQP4 is the predominant water channel in the central nervous system (CNS), where it is involved in maintaining brain water balance and neural signal transduction. It is mainly expressed in astroglial cells, which support the neurons. Outside the CNS, AQP4 has been found in the basolateral membrane of renal principal cells as well as in various glandular epithelia, airways, skeletal muscle, stomach, retina and ear. 

Disorder characterized by atrophy ofthe choroid (the thin membrane covering most of the posterior of the eye between the retina and sclera) and degeneration of the retinal pigment epithelium resulting in night blindness. The disease is caused by mutations in Rab escort protein Repl (component A of Rab geranylgeranyl transferase). 

Methanol is a dangerous fire hazard when exposed to heat or flame, and a moderate expl hazard when exposed to flame. It is a dangerous disaster hazard upon exposure to heat or flame, and can react vigorously with oxidizing materials. Methanol possesses distinct narcotic props, and is also a slight irritant to the mucous membranes. Its main toxic effect is exerted upon the nervous system, particularly the optic nerves and possibly the retinae. In the body the products formed by its oxidn are formaldehyde and formic acid, both of which are toxic. Because of the slowness with which it is eliminated, methanol should be regarded as a cumulative poison (Ref 5)... 

The vitreous is a transparent extracellular matrix occupying the space between the posterior lens and the retina and, in the majority of vertebrate species, constitutes the major f)art of the volume of the eye. Embryo-logically it can be considered as the basement membrane of the retina. It provides a mechanical support for surrounding tissues and acts as a shock absorber by virtue of its viscoelastic properties (Balzas and Delinger, 1984). Vitreous consists mainly of water (98%) and colloids (0.1%) with ions and low molecular weight solutes making up the remainder. It is not fully developed at birth, and changes in both volume and chemical composition occur postnatally. 

Early macular changes are often called age-related macu-lopathy and are characterized by large drusen and pigmentary abnormalities in the macula.20 Age-related maculopathy accounts for 85% to 90% of all age-related macular changes. Ten to fifteen percent of these patients will develop advanced atrophy and/or develop abnormal blood vessels in and under the retina called choroidal neovascular membranes. Patients are then classified as having AMD.23... 

Choroidal neovascular membranes Development of abnormal blood vessels in and under the retina. 

Elimination from the vitreous occurs by one of two pathways. This can be visualized by injecting fluorescent compounds and examining the concentration distribution in frozen sections obtained after a steady state has been established [230]. If the major route of elimination is by means of the re-tina/choroid, at steady state the lowest concentration would be in the vicinity of the retina. The contours observed in frozen sections of the rabbit eye obtained after intravitreal injection of fluorescein exhibit this pattern, with the highest concentration immediately behind the lens (Fig. 16A). Compounds not chiefly eliminated through the retina exit the vitreous by passive diffusion and enter the posterior aqueous, where they are eliminated by the natural production and outflow of aqueous humor. In such a situation, the contours would be perpendicular to the retina, with the highest concentration towards the rear of the vitreous cavity. This appears to be the case for fluorescently labeled dextran polymer, whose contours decrease in concentration toward the hyaloid membrane (Fig. 16B). 

The vertebrate retina contains two classes of light-sensitive receptor cells called rods and cones. The rod is an elongated cylindrical cell containing several hundred thylakoids which support the visual pigment. The pigment system in the rod is confined to internal membranes situated close to the outer membrane of the cell. In the other type of visual receptor, the cone, the pigment is situated in the external membrane itself. In the cone the external... 

Carotenoids are also present in animals, including humans, where they are selectively absorbed from diet (Furr and Clark 1997). Because of their hydrophobic nature, carotenoids are located either in the lipid bilayer portion of membranes or form complexes with specific proteins, usually associated with membranes. In animals and humans, dietary carotenoids are transported in blood plasma as complexes with lipoproteins (Krinsky et al. 1958, Tso 1981) and accumulate in various organs and tissues (Parker 1989, Kaplan et al. 1990, Tanumihardjo et al. 1990, Schmitz et al. 1991, Khachik et al. 1998, Hata et al. 2000). The highest concentration of carotenoids can be found in the eye retina of primates. In the retina of the human eye, where two dipolar carotenoids, lutein and zeaxan-thin, selectively accumulate from blood plasma, this concentration can reach as high as 0.1-1.0mM (Snodderly et al. 1984, Landrum et al. 1999). It has been shown that in the retina, carotenoids are associated with lipid bilayer membranes (Sommerburg et al. 1999, Rapp et al. 2000) although, some macular carotenoids may be connected to specific membrane-bound proteins (Bernstein et al. 1997, Bhosale et al. 2004). 

Rapp, L. M., S. S. Maple, and J. H. Choi. 2000. Lutein and zeaxanthin concentrations in rod outer segment membranes from perifoveal and peripheral human retina. Invest. Ophthalmol. Vis. Sci. 41 1200-1209. 

Xanthophylls can further inhibit the peroxidation of membrane phospholipids (Lim et al. 1992) and reduce photooxidation of lipofuscin fluorophores (Kim et al. 2006), which are implicated in the pathogenesis of AMD (Sparrow and Boulton 2005). Furthermore, it was shown that light-induced damage to photoreceptors was reduced in quails fed zeaxanthin, with the number of apoptotic photoreceptor cells being inversely related to the concentration of zeaxanthin in the retina (Thomson et al. 2002). 

Moreover, efficient rhodopsin regeneration may precede enzymatic reduction of all-fran.v-retinal to all-trans-retinol in the aged retina (Figure 15.2c) (Schadel et al., 2003). Upon rhodopsin regeneration, all-trans-retinal is released from the exit site of the protein into the lipid membrane (Figure 15.2c) (Schadel et al., 2003). From here the removal of all-tnms-retinal to the outer leaflet of the disc membrane is dependent on activity of ATP-binding cassette trasporter A4 (ABCA4) present in the rim of photoreceptor disc, known also as ABCR protein. 

ApoJ is another protein component of HDL which is highly expressed by the RPE and neural retina, especially under oxidative stress conditions (Wong et al., 2000, 2001). It can act as a complement regulatory protein, which by binding to and inactivating the membrane-attack complex can prevent cytolysis (Bartl et al., 2001). ApoJ accumulation was identified in drusen in AMD patients (Sakaguchi et al., 2002 Wong et al., 2000). 

Yemelyanov, AY, Katz, NB, and Bernstein, PS, 2001. Ligand-binding characterization of xanthophyll carotenoids to solubilized membrane proteins derived from human retina. Exp Eye Res 72, 381-392. 

FIGURE 16.4 Precursors of RPE cell lipofuscin form in the outer segments of photoreceptor cells. The retina of normal rat (A, B) and the Royal College of Surgeons (RCS) rat (C, D) viewed under the phase contrast (A, C) and the epifluorescence microscopy (B, D). In the normal rat, autofluorescent material accumulates as lipofuscin in RPE cells (arrows). In the RCS, due to a defect in RPE cell phagocytosis, shed outer segment membrane builds up at the photoreceptor-RPE interface the autofluorescence in this debris is attributable to lipofuscin precursors that form in photoreceptor outer segments. 

Liu, J., Itagaki, Y., Ben-Shabat, S., Nakanishi, K., Sparrow, J.R., 2000. The biosynthesis of A2E, a fluorophore of aging retina, involves the formation of the precursor, A2-PE, in the photoreceptor outer segment membrane. J Biol Chem. 275, 29354-29360. 

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