Vision

(lots of information on transparencies and some figures from Alberts et al.)

Retina, rods and cones
TRANSPARENCY Fig. 22.6 p. 1144 retina
Rod, periphery, dim black and white, sensitivity
Very sensitive - 1 photon
Cone, ..., fovea, color, acuity
Stacks of disks.TRANSPARENCY Fig.22-7, p.1145 & Fig.15-39, p.753
G-protein coupled receptor, with its 7 membrane spans, would have N-terminus on outside (and glycosylation site is near there)
shows N-terminal of opsin in lumen
Lumen is topologically like the outside of the cell TRANSPARENCY (home made)

Visual pigment
Halobacterium halobium TRANSPARENCY Fig. 10-30, p. 495
Bacteriorhodopsin TRANSPARENCY Fig. 10.31, p. 496

G. Wald, Molecular basis of Visual excitation (Nobel prize speech) Science 162, 230-239, 1968
TRANSPARENCY (Figs 3, 7 & 8)
Vitamin A cis->trans conversion (from Wald)
vitamin A is the chromophore
Metarhodopsins. "bleaching" into opsin and trans vitamin A
TRANSPARENCY conversion steps isolated by liquid nitrogen, etc.
old terminology (prelumirhodopsin now bathorhodopsin)
note, now it is known that MII is R*
TRANSPARENCY some carotenoids - two vitamin A's end to end
TRANSPARENCY Rhodopsin mutants- ADRP-
(autosomal dominant retinitis pigmentosa) (Dryja)
T. P. Dryja et al., Mutation spectrum of the rhodopsin gene among patients with autosomal retinitis pigmentosa, PNAS 88, 9370-9374, 1991

Color vision
turtles, fish, birds have good color vision, needed to re-evolve for mammals
evolutionary bottleneck hypothesis - selection pressure on color vision relaxed in early mammalian evolution when animals were nocturnal
Young -Helmholtz trichromatic theory
3 kinds - evolutionarily related 2 (red and green) on X
color blind people might be "dichromats" as opposed to trichromats
NW Monkeys - usually 2 but females may have variants on 2 X's
and retina would be mosaic because of Mary Lyon X-inactivation hypothesis and Barr body
J.D.Mollon "Tho' she kneel'd in that place where they grew..." The uses and origins of primate colour vision. J. Exp. Biol. 146, 21-38, 1989
Color blindness - on X in males - superfamily
TRANSPARENCY Fig. from Nathans et al.
J. Nathans et al., Molecular genetics of human color vision: the genes encoding blue, green and red pigments, Science, 232, 193-202, 1986
differences between yellow and green absorbing
does not show 2 palmitylated cysteines on C terminus
or phosphorylation sites
TRANSPARENCY Fig from Nathans et al. on color blindness genes
J. Nathans et al., Molecular genetics of inherited variation in human color vision, Science, 232, 1986, 203-232
odd in that numbers of gene copies is screwed up
more recent work confirms and extends - there can be many copies
M. Neitz and J. Neitz, Numbers and ratios of visual pigment genes for normal red-green color vision, Science 267, 1013-1016

Transduction and the dark current
mitochondria packed in inner segment pump sodium out, and it leaks into the inner segment, but less so when signal transduction cascade closes channels in the light in in the dark into the TRANSPARENCY (home made, like 15-40 shown early)
lots of information, wiring of retina, rod association with retinal pigment epithelium, involved in turnover of vitamin A, membrane shedding in the phagosomal-lysosomal system, and accumulation of lipofuscin, the aging pigment, as the indigestible residue of the phagolysosomal system in these post-mitotic r.p.e. cells which must last a lifetime)
Rod works backwards, physiologically, hyperpolarizes in response to light
TRANSPARENCY like Fig. 4-54, p. 182 - Baylor rod current recording - like a whole organelle patch clamp

Generations of Transduction models (very much updated from 1998)

TRANSPARENCY Here (Polans, Baehr and Palczewski 1996) is one diagram of the phototrandduction cascade (Rhodopsin gets excited by a quantum to R*; R* activates transducin. The alpha subunits of this heterotrimeric protein effect the removal of inhibitory gamma subunits from the PDE alpha-beta enzyme. The cGMP hydrolyzed would have opened channels, so, with light, channels close. Since Ca2+ comes in through the channel, it can affect adaptation (3 ways). (1) Ca2+ binds recoverin (Rec) which binds and inhibits rhodopsin kinase (RK) which phosphorylates rhodopsin making it ready for arrestin binding. [Arrestin is also known as S-antigen (S=soluable) or 48 k (kD) protein.] (2) Also Ca2+ binds GCAP (GC activating protein) inhibiting its activation of GC (gualyate cyclase) which makes cGMP. (3) Ca2+ binds to CaM (calmodulin) which binds to the beta subunit of the CNG channel. In summary, lowering calcium (after photon absorption) increases cGMP synthesis, increases channel sensitivity to cGMP and decreases the lifetime of R* function, helping to counteract excitation

It is useful to reiterate the cascade with different diagrams, emphasizing where things take place

TRANSPARENCY Fig 4 (Molday 1998) is another diagram

TRANSPARENCY Fig 1 (Lem 1998) is another diagram.

TRANSPARENCY Fig. 2 (Jindrova 1998) is yet another diagram

TRANSPARENCY The molecules peripherin/rds and rom-1 are thought to be involved in maintaining the structure of the rod disk (Fig. 11, same TRANSPARENCY as Fig. 4 above (Molday 1998))

TRANSPARENCY Now, peripherin/rds is mapped with respect to mutations which cause retinal degeneration (Fig. 13 (Molday 1998))

TRANSPARENCYThe ABCR/RIM protein of Stargardt's degeneration is shown (Fig. 15 (Molday 1998)). There are 12 transmembrane segments and 2 ATP binding casettes

TRANSPARENCY Fig. 17 (Molday 1998) shows the location and ATP dependence of ABCR/RIM. For orientation purposes, Peripherin/rds - rom-1, GC and rhodopsin are shown. "Peripherin" is the name based on EM immunocytochemistry, it is localized to the periphery of the disks. "rds" is the genetic name, "retinal degeneration slow."

Fig. 1 (Polans et al. 1996) is a diagram of the domains of GC. An intracellular kinase domain is where GCAP interacts. The intracellular catalytic domain is where GTP is converted to cGMP. What the extracellular domain does is not clear.

TRANSPARENCY This model of the phototransduction cascade emphasizes diseases (Fig. 18 (Molday 1998, same TRANSPARENCY as above). Abbreviations:
ADRP - autosomal dominant retinitis pigmentosa
ARRP - (recessive)
CSNB - congenital stationary night blindness
CRD - cone rod dystrophy
MD - macylar dystrophy
XLRP - X-linked retinitis pigmentosa
XLRS - X-linked retinoschisis

Fig 1 (Molday 1996) shows alpha and beta subunits of bovine rod CNG's. For rods (and in olfactory receptors) the channel probably is a hetero-oligimer. Many features are familiar, S1 - S6 with S4 having a voltage-sensor motif and a pore between S5 and S6. There are cGMP binding sites. It should be intuitively obvious to the most casual observer that "C" (alpha helix) stands for bacterial catabolite gene activator protein. By the same token, GARP = glutamic-acid-rich protein.

Fig. 7 (Molday 1998) repeats the previous ingormation but includes linear representatiuons for the protein domains and the now familiar representation of the tetrameric channel.

From (Lem 1998). Table 1 is a list of diseases resulting from mutations in G-protein-coupled receptors, Table 2 for G-protein subunits and Table 3 for other molecules of the cascade.

Several general and summary notes:
There are 10x lower numbers of molecules further into cascade R > G > PDE
S = S antigen = 48 kD protein = arrestin

TRANSPARENCY, PDE - rd mutant mouse is in beta subunit (Bowes)
C. Bowes et al., Isolation of a candidate cDNA for the gene causing retinal degeneration in the rd mouse, PNAS 86, 9722-9726, 1989
C. Bowes et al., Retinal degeneration in the rd mouse is caused by a defect in the beta subunit of rod cGMP-phosphodiesterase, Nature, 347, 677-680, 1990

Review of calcium involvement
first thought of as second messenger in 1970's until cGMP won out
then a calcium binding protein recoverin was discovered
after some confusion,
now known to inhibit rhodopsin kinase at high calcium
These calcium binding proteins have EF-hands with negatively
charged a.a.'s
GC's (there are 2 in retina) are big proteins with one membrane pass
GCAP (guanylate cyclase activating proteins [there are 2])
activate target in absence of calcium
contribute to adaptation
Calcium is more obviously important in invertebrate phototransduction

References:

C. Bowes et al., Isolation of a candidate cDNA for the gene causing retinal degeneration in the rd mouse, PNAS 86, 9722-9726, 1989

C. Bowes et al., Retinal degeneration in the rd mouse is caused by a defect in the beta subunit of rod cGMP-phosphodiesterase, Nature, 347, 677-680, 1990T. P. Dryja et al., Mutation spectrum of the rhodopsin gene among patients with autosomal retinitis pigmentosa, PNAS 88, 9370-9374, 1991

Jindrova, H., 1998 Vertebrate phototransduction: activation, recovery, and adaptation. Physiol Res. 47: 155-68.

Lem, J., 1998 Diseases of G-protein-coupled signal transduction pathways: The mammalian visual system as a model. Sem. Neurosci. 9: 232-239.

Molday, R. S., 1996 Calmodulin regulation of cyclic-nucleotide-gated channels. Curr Opin Neurobiol. 6: 445-52.

Molday, R. S., 1998 Photoreceptor membrane proteins, phototransduction, and retinal degenerative diseases. The Friedenwald Lecture. Invest Ophthalmol Vis Sci. 39: 2491-513.

J.D.Mollon "Tho' she kneel'd in that place where they grew..." The uses and origins of primate colour vision. J. Exp. Biol. 146, 21-38, 1989

J. Nathans et al., Molecular genetics of human color vision: the genes encoding blue, green and red pigments, Science, 232, 193-202, 1986

M. Neitz and J. Neitz, Numbers and ratios of visual pigment genes for normal red-green color vision, Science 267, 1013-1016

Polans, A., W. Baehr, and K. Palczewski, 1996 Turned on by Ca2+! The physiology and pathology of Ca(2+)-binding proteins in the retina. Trends Neurosci. 19: 547-54.

G. Wald, Molecular basis of Visual excitation (Nobel prize speech) Science 162, 230-239, 1968

This page was last updated on December 19, 2001

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