Reactome | Visual phototransduction

Visual phototransduction

Stable Identifier

R-HSA-2187338

DOI

10.3180/REACT_160125.1

Type

Pathway

Species

Homo sapiens

ReviewStatus

5/5

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Visual phototransduction is the process by which photon absorption by visual pigment molecules in photoreceptor cells is converted to an electrical cellular response. The events in this process are photochemical, biochemical and electrophysiological and are highly conserved across many species. This process occurs in two types of photoreceptors in the retina, rods and cones. Each type consists of two parts, the outer segment which detects a photon signal and the inner segment which contains the necessary machinery for cell metabolism. Each type of cell functions differently. Rods are very light sensitive but their flash response is slow so they work best in twilight conditions but are not good at detecting objects moving quickly. Cones are less light-sensitive and have a fast flash response so they work best in daylight conditions and are better at detecting fast moving objects than rods.

The visual pigment consists of a chromophore (11-cis-retinal, 11cRAL, A1) covalently attached to a GPCR opsin family member. The linkage is via a Schiff base forming retinylidene protein. Upon photon absorption, 11cRAL isomerises to all-trans retinal (atRAL), changing the conformation of opsin to an activated form which can activate the regulatory G protein transducin (Gt). The alpha subunit of Gt activates phosphodiesterase which hydrolyses cGMP to 5'-GMP. As high level of cGMP keep cGMP-gated sodium channels open, the lowering of cGMP levels closes these channels which causes hyperpolarization of the cell and subsequently, closure of voltage-gated calcium channels. As calcium levels drop, the level of the neurotransmitter glutamate also drops causing depolarization of the cell. This effectively relays the light signal to postsynaptic neurons as electrical signal (Burns & Pugh 2010, Korenbrot 2012, Pugh & Lamb 1993).

11cRAL cannot be synthesised in vertebrates. Vitamin A from many dietary sources is the precursor for 11cRAL. It is taken from food in the form of esters such as retinyl acetate or palmitate or one of four caretenoids (alpha-carotene, beta-carotene, gamma-carotene and beta-cryptoxanthin). Retinoids are transported from the gut to be stored in liver, until required by target organs such as the eye (Harrison & Hussain 2001, Harrison 2005). In the eye, in the form 11cRAL, it is used in the retinoid (visual) cycle to initiate phototransduction and for visual pigment regeneration to ready the photoreceptor for the next phototransduction event (von Lintig 2012, Blomhoff & Blomhoff 2006, von Lintig et al. 2010, D'Ambrosio et al. 2011, Wang & Kefalov 2011, Kefalov 2012, Wolf 2004).

Literature References

PubMed ID	Title	Journal	Year
16011460	Mechanisms of digestion and absorption of dietary vitamin A Harrison, EH	Annu Rev Nutr	2005
22074927	Metabolism of carotenoids and retinoids related to vision von Lintig, J	J. Biol. Chem.	2012
16688755	Overview of retinoid metabolism and function Blomhoff, R, Blomhoff, HK	J. Neurobiol.	2006
20188572	The biochemical and structural basis for trans-to-cis isomerization of retinoids in the chemistry of vision von Lintig, J, Kiser, PD, Golczak, M, Palczewski, K	Trends Biochem. Sci.	2010
21350678	Vitamin A metabolism: an update D'Ambrosio, DN, Clugston, RD, Blaner, WS	Nutrients	2011
15384919	The visual cycle of the cone photoreceptors of the retina Wolf, G	Nutr. Rev.	2004
11340090	Mechanisms involved in the intestinal digestion and absorption of dietary vitamin A Harrison, EH, Hussain, MM	J Nutr	2001
20430952	Lessons from photoreceptors: turning off G-protein signaling in living cells Burns, ME, Pugh, EN	Physiology (Bethesda)	2010
21111842	The cone-specific visual cycle Wang, JS, Kefalov, VJ	Prog Retin Eye Res	2011
22074928	Rod and cone visual pigments and phototransduction through pharmacological, genetic, and physiological approaches Kefalov, VJ	J. Biol. Chem.	2012
22658984	Speed, sensitivity, and stability of the light response in rod and cone photoreceptors: facts and models Korenbrot, JI	Prog Retin Eye Res	2012
8382952	Amplification and kinetics of the activation steps in phototransduction Pugh, EN, Lamb, TD	Biochim. Biophys. Acta	1993