Photoreceptor cell

Members of the first group of NCS are recoverin and visinin. Recoverin is a 23 kDa myristoylated protein found under normal conditions only in photoreceptor cells (rods and cones). The main function of recoverin is to bind to and inhibit rhodopsin kinase, thereby prolonging the light response. 

A second class of neuronal calcium sensors is formed by the guanylate cyclase-activating protein (GCAP). The GCAPs are expressed only in the photoreceptor cells of the retina of vertebrates. Recoverins and GCAPs have antagonistic roles in phototransduction. 

Properties of Shaker channels in Drosophila muscle and photoreceptor cells ... 

The compound eyes of insects are formed of a hexagonal array of small units, or ommatidia (in the case of the fruit fly, approximately 800 small eyes ). Each is composed of eight photoreceptor cells... 

Bryant There are no mutants in Cbl but it has been tested in an ectopic expression assay, and if it is expressed in the eye disc it interferes with photoreceptor cell development (Meisner et al 1997). 

Meisner H, Daga A, Buxton J et al 1997 Interactions of Drosophila Cbl with epidermal growth factor receptors and role of Cbl in R7 photoreceptor cell development. Mol Cell Biol 17 2217-... 

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). 

Kirschfeld, K. (1982). Carotenoid pigments Their possible role in protecting against photooxidation in eyes and photoreceptor cells. Proc. R. Soc. Lond. B Biol. Sci. 216(1202) 71-85. 

Thomson, L. R., Y. Toyoda et al. (2002). Elevated retinal zeaxanthin and prevention of light-induced photoreceptor cell death in quail. Invest. Ophthalmol. Vis. Sci. 43(11) 3538-3549. 

Allikmets, R, Singh, N, Sun, H, Shroyer, NE, Hutchinson, A, Chidambaram, A, Gerrard, B et al. 1997. A photoreceptor cell-specific ATP-binding transporter gene (ABCR) is mutated in recessive Stargardt macular dystrophy. Nat Genet 15, 236-246. 

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. 

Gerashchenko, D., et al (1998). Dominant expression of rat prostanoid DP receptor mRNA in leptomeninges, inner segments of photoreceptor cells, iris epithelium, and ciliary processes. J. Neurochem. 71, 937-45. 

Rhodopsin is a seven ot-helix trans-membrane protein and visual pigment of the vertebrate rod photoreceptor cells that mediate dim light vision. In this photoreceptor, retinal is the chromophore bound by opsin protein, covalently linked to Lys296 by a Schiff base linkage. Kpega et al.64 have studied NMR spectra of Schiff bases being derivatives of all-frans retinal and amino-p-cyclodextrins as a model of rhodopsin, where p-cyclodextrin plays a role of a binding pocket. On the basis of analysis of the chemical shift differences for the model compound in the presence and in the absence of adamantane carboxylate, it has been shown that the derivative of 3-amino-p-cyclodextrin forms dimer in water and retinoid is inserted into p-cyclodextrin cavity [31]. 

Multiple forms of heterotrimeric G proteins exist in the nervous system. Three types of heterotrimeric G protein were identified in early studies. G termed transducin, was identified as the G protein that couples rhodopsin to regulation of photoreceptor cell function (see Ch. 49), and Gs and G were identified as the G proteins that couple plasma membrane receptors to the stimulation and inhibition, respectively, of adenylyl cyclase, the enzyme that catalyzes the synthesis of cAMP (see Ch. 21). 

The ligands for the other forms of membrane-bound GC are less well characterized. In some cases the functions of these GC forms have not been identified. GC-D is expressed in olfactory sensory neurons but its function has not been determined. GC-E and GC-F are expressed in photoreceptor cells in the retina and play a role in phototransduction. GC-G is widely expressed in peripheral tissues and brain but its ligand binding domain is similar to that for GC-A/GC-B, suggesting that it may be regulated by an ANP/BNP-like ligand [34]. 

Absorption of light causes inhibition of vertebrate photoreceptor cells, which then initiate programs of responses among retinal neurons 807... 

Answer B, Only phosphodiesterase participates as a signaling molecule in the visual cycle of photoreceptor cells. 

In photoreceptor cells, the rods and cones of the human retina, the retinal is linked to a specific protein termed opsin. The resulting pigment is known as rhodopsin. When a photon of light of the proper wavelength hits a molecule of rhodopsin, two chemical events take place. First, the ll-c -retinal is converted to the all-trans form and, secondly, the all-trani-retinal is released from the rhodopsin ... 

The retina contains two distinct types of photoreceptor cell to detect light cones and rods. The rods are specialised... 

The Na" channel has a receptor site for cyclic GMP when cyclic GMP is bound, the channel is closed. This leads to a decrease in the intracellular Na ion concentration, resulting in hyperpolarisation of the cell membrane. This decreases the release of the neurotransmitter glutamate into the synapse that connects the photoreceptor cell to the bipolar neurones. In this specific case, a decrease in the neurotransmitter concentration in the synapse is a signal that results in depolarisation of the bipolar cell. The action potential in the bipolar cells communicate with ganglion cells, the axons of which form the optic nerve. Thus action potentials are generated in the axons which are... 

Figure 15.9 (a) The ds- and trans-retinal interconversions in the detection of light Within the photoreceptor cell, light is detected by the conversion of c/s-retinal to trans-retinal, components of the light-sensitive pigment rhodopsin. This apparently small chemical change is sufficient for trans-retinal to dissociate from rhodopsin. (b) The cis/trans q/cte. To continue the process, trans-retinal must be converted back to c/s-retinal. 

For vision to continue, c/s-retinal must be regenerated. The /rans-retinal is reduced to /rans-retinol (i.e. the aldehyde is converted to alcohol) and is isomerised to c/s-retinal this is oxidised to c/s-retinol (see Figure 15.9(b)). Two different cells are involved the oxidation of retinal to retinol occurs in the photoreceptor cell. The retinol is then released and is taken up by the adjacent epithelial cell where it is isomerised to c/s-retinol and then reduced to... 

Figure 15.11 The biochemical reactions that result in the conversion of trans-retinal to ds-retinal, to continue the detection of light To continue the process, trans-retinal must be converted back to c/s-retinal. This is achieved in three reactions a dehydrogenase converts trans-retinal to trans-retinol an isomerase converts the trans-retinol to c/s-retinol and another dehydrogenase converts c/s-retinol to c/s-retinal. To ensure the process proceeds in a clockwise direction (i.e. the process does not reverse) the two dehydrogenases are separated. The trans-retinal dehydrogenase is present in the photoreceptor cell where it catalyses the conversion of trans-retinal to trans-retinol which is released into the interstitial space, from where it is taken up by an epithelial cell. Here it is isomerised to c/s-retinol and the same dehydrogenase catalyses its conversion back to c/s-retinal. This is released by the epithelial cell into the interstitial space from where it is taken up by the photoreceptor cell. This c/s-retinal then associates with the protein opsin to produce the light-sensitive rhodopsin to initiate another cycle. The division of labour between the two cells may be necessary to provide different NADH/NAD concentration ratios in the two cells. A high ratio is necessary in the photoreceptor cell to favour reduction of retinal and a low ration in the epithelial cell for the oxidation reaction (Appendix 9.7).

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