TAPPISOLUJEN NÄKÖSYKLISTÄ

TAPPISOLUJEN NÄKÖPIGMENTTI
Sitaatti 16.4. 2015  jota on syytä suomentaa myöhemmin. 
http://www.jneurosci.org/content/31/12/4714.full.pdf

Cones function in constant light and are responsible for mediating daytime human vision. Like rods, cones use the photosensitive molecule 11-cis-retinal to detect light, and in constant illumination, a continuous supply of 11-cis-retinal is needed. A retina visual cycle

is thought to provide a privileged supply of 11-cis-retinal to cones by using 11-

cis-retinol generated in Müller cells. In the cycle, 11-cis-retinol is transported from Müller cells to cone inner segments,where it is oxidized to 11-cis-retinal.This oxidation step is only performed

in cones, thus rendering the cycle cone-specific.

Interphotoreceptor retinoid-binding protein (IRBP) is a retinoid-binding protein in the

subretinal space that binds 11-cis-retinol endogenously.

Cones in Irbp-/-mice are retinoid-deficient under photopic conditions, and it is possible that 11-

cis-retinol supplies are disrupted in the absence of IRBP. We tested the hypothesis that IRBP facilitates the delivery of 11-cis-retinol to cones by preserving the isomeric state of 11-

cis-retinol in light. With electrophysiology, we show that the cone-like photoreceptors of

Nrl-/-mice use the cone visual cycle similarly to wild-type cones.Then, using oxidation assays in isolatedNrl-/-Rpe65-/-retinas, we show that IRBP delivers 11-cis-retinol for oxidation in cones and improves the efficiency of the oxidation reaction.

Finally, we show that IRBP protects the isomeric state of 11-cis-retinol in the presence of light. Together, these findings suggest that IRBP plays an important role in the delivery of 11-

cis-retinol to cones and can facilitate cone function in the presence of light

Photoreceptors depend on 11-cis-retinal to detect light.

Within photoreceptor outer segments, 11-cis-retinal is bound to opsins to form visual pigments (Wald, 1935). When light strikes a visual pigment molecule, 11-cis-retinal is isomerized to all-trans-retinal, the opsin is activated, and phototransduction begins. Each photon is detected at the expense of a molecule of 11-cis-retinal, and new 11-cis-retinal must be regenerated from the all-trans-retinal photoproduct for continued photoreceptor function.

The classical visual cycle ( of the rods) regenerates 11-cis-retinal from all-trans-retinal through reactions occurring in the photoreceptors and retinal pigment epithelium (RPE),

but a second, cone-specific visual cycle exists in the retina (Wang and Kefalov, 2009;Wang et al., 2009).

Like the classical visual cycle, the cone visual cycle begins with the reduction of all-trans-retinal, but after leaving the outer segment, all-trans-retinol is transported to Müller cells and converted to 11-cis-retinol (Mata et al., 2002, 2005). 11-cis-Retinol from Müller cells is transported to cones and oxidized to 11-

cis-retinal (Jones et al., 1989; Wang et al., 2009).

This final step is critical to the cycle’s cone-specific nature, as only cones can oxidize 11-cis-retinol to 11-cis-retinal (Jones et al.,1989; Ala-Laurila et al., 2009)

Discussion

Importance of the oxidation reaction to the cone visual cycle

Cone dysfunction in Irbp/ mice is evident through reduced photopic ERGs (Ripps et al., 2000; Parker et al., 2009), and the improvement of cone responses to WT levels after treatment with exogenous 9-cis-retinal suggests that the cones are chromophore-deficient under photopic conditions (Parker et al., 2009). A cone visual cycle is thought to provide cones with a unique source of chromophore to facilitate the high demand for 11-cis-retinal under photopic conditions. The aspect of the cone visual cycle responsible for its cone-specific nature is the supply of 11-cis-retinol to cone inner segments, as only cones have the ability to oxidize 11-cis-retinol to 11-cis-retinal. The source of 11-cis-retinol is proposed to be Müller cells, and multiple studies have shown that Müller cells have the ability to generate 11-cis-retinol from all-trans-retinol (Mata et al., 2002, 2005). IRBP is known to bind 11-cis-retinol endogenously, and under photopic conditions IRBP is localized to the outer limiting membrane where the microvilli of Müller cells are in close proximity to cone inner segments (Uehara et al., 1990). As such, IRBP is located in the appropriate area to facilitate the transport of 11-cis-retinol from Müller cells to cones

Oxidation of 11-cis-retinol in Nrl-/-photoreceptors

Studying the cone visual cycle in Nrl/mice is potentially advantageous, as the large number of cone-like photoreceptors increases the sensitivity of assays. However, a fundamental question that must be addressed before using the model is whether the cone-like photoreceptors in Nrl-/-mice function as cones in the retina visual cycle. Here, we have shown that Nrl-/-retinas, like WT retinas, can recover both response amplitude and sensitivity in cones following a bleach (Fig. 1).

Furthermore, we have shown that isolated retinas from Nrl-/-Rpe65-/-mice can oxidize 11-cis-retinol into 11-cis-retinal (Fig. 2A ). Reduced 11-cis-retinal production as photoreceptors degenerate with age suggests that this reaction occurs within the photoreceptors and not within other cells containing RDH enzymes (Farjo et al., 2009; Parker and Crouch, 2010b) (Fig. 2B).

While cones in isolated Nrl-/ -retinas recover their sensitivity and response amplitude, the time course of this recovery is slower than that seen in WT retinas.This delay may be due to the fact that Nrl- /-photoreceptors are more numerous than cones in WT mice, and more time is required to regenerate the full complement of pigment needed to restore their dark-adapted state.

With exogenous 11-cis-retinol, however, the time course for recovery is rapid and comparable in kinetics to that in WT retinas. Thus, the generation of 11-cis-retinol is likely the rate-limiting step of the cone visual cycle in Nrl-/-retinas.

Characterization of the oxidation reaction in cones

The cone visual cycle depends on a relatively small amount of 11-cis-retinol reaching the cone innersegments in the presence of an overwhelming amount of all- trans-retinol (Saari et al., 1982).

One characteristic of intact retinas that facilitates the cone visual cycle is the preference for 11-cis-

retinol in oxidation reactions.

IRBP binds both all-trans-retinol and 11-cis-retinol under photopic conditions (Adler and Spencer, 1991). If all-trans-retinol were readily delivered to cones, its oxidation to all-trans-retinal could interfere with oxidation of 11-cis-retinol.While IRBP readily binds all-trans-retinol, promotes its release from both rods and cones, and effectively delivers all-trans-retinol to the RPE, we have shown here that it is ineffective at delivering all-trans-retinol to photoreceptors for oxidation (Fig. 3). Electron microscopy studies of IRBP bound to either all-trans-retinol or11-cis-retinol suggest that IRBP undergoes a structural change when bound by either ligand (Adler et al., 1987). Thus, the binding of different retinoids to IRBP results in conformation changes to the protein, and it is possible that these changes promote tissue-specific delivery of retinoids. Also, it is possible that an unidentified 11-cis-RDH is responsible for the oxidation of 11-cis-retinol in cones and all-trans-retinol is not ausable substrate for that RDH. Regardless of the mechanism, the selective delivery or oxidation of 11-cis-retinol would facilitate the cone visual cycle.

Importance of IRBP to the oxidation reaction of the cone visual cycle

11-cis-Retinol is highly unstable and undergoes rapid isomerization and degradation under physiological conditions. This is increased further in the presence of light. IRBP is known to preserve the isomeric state of 11-cis-retinal (Crouch et al., 1992), and here we have shown that it also helps preserve 11-cis-retinol under physiological conditions (Fig. 4). The result of this protection is the more efficient production of 11-cis-retinal and reduced levels all-trans-retinol. Furthermore, IRBP’s ability to protect 11-cis-retinol in the presence of light would allow 11-cis-retinal to be generated in light (Fig. 5). Because the cone visual cycle should be able to provide cones with 11-cis-retinal under photopic conditions, these findings, coupled with the reduced chromophore levels known to be present in Irbp-/-mice under photopic conditions (Parker et al., 2009; Parker and Crouch, 2010a), suggest that IRBP plays an important role in normal cone function.

Summary

The cone-like photoreceptors of Nrl-/-mice are similar to WT cones in their ability to oxidize 11-

cis-retinol to 11-cis-retinal and use the retina visual cycle. IRBP is an effective vehicle for deliv-

ering 11-cis-retinol to cones and increases the efficiency of 11-cis-retinal production under physiological conditions. The ability of IRBP to protect 11-cis-retinol from isomerization in light may

allow cones to produce 11-cis-retinal under photopic conditions and enable continuous cone function in constant light.