Visual Cascade

Wolfgang Baehr, University of Utah, Salt Lake City, Utah, USA
Paul Liebman, University of Pennsylvania, Philadelphia, Pennsylvania, USA

Published online: January 2006

DOI: 10.1038/npg.els.0004065

Vision begins in the outer segments of rod and cone photoreceptor cells when visual pigments absorb light photons and become activated. Approximately 104–105 cyclic nucleotide molecules are affected by activation of one rhodopsin molecule, so that signal generation requires enormous amplification through numerous molecules and mechanisms in the visual cascade.

Keywords: photoreceptors; phototransduction; hyperpolarization; rhodopsin

Figure 1. Model of rod phototransduction. Rhodopsin serially activates many copies of the G protein transducin (Gt) (gain 1), and the subunit of Gt, in turn, activates a cGMP-specific phosphodiesterase (PDE), which degrades cytoplasmic cGMP with rapid turnover (gain 2). The drop in (cGMP) causes cGMP-gated cation (CGC) channels in the plasma membrane to close, which produces hyperpolarization of the outer segment plasma membrane. Closure of cation channels prevents entry of cations, and cytoplasmic ( Ca2+) drops owing to its continued extrusion by the light-independent exchanger, also located in the plasma membrane. In a negative feedback loop, the drop in ( Ca2+) causes stimulation of a guanylate cyclase (GC) by one or several specific  Ca2+-binding proteins, termed guanylate cyclase-activating proteins (GCAPs). Sites where regulatory proteins (phosducin, Pdc; arrestin, Arr; recoverin, Rec) interact with components of the cascade are indicated by arrows. A simplified movie version of the visual cascade can be seen at http://insight.med.utah.edu (click first on webvision, then click on photoreceptors/transduction). A computer-generated simulation of the lateral diffusional activation of components of the visual cascade can be seen at http://athens.dental.upenn.edu/austinjp/eyereactioncascade/.
close
 References
    Baehr W, Devlin MJ and Applebury ML (1979) Isolation and characterization of cGMP phosphodiesterase from bovine rod outer segments. Journal of Biological Chemistry 254: 11669–11677.
    Dryja TP, McGee TL, Reichel E et al. (1990) A point mutation of the rhodopsin gene in one form of retinitis pigmentosa. Nature 343: 364–369.
    Fesenko EE, Kolesnikov SS and Lyubarski AL (1985) Induction by cGMP of cationic conductance in plasma membrane of retinal rod outer segment. Nature 313: 310–313.
    Godchaux W III and Zimmerman WF (1979) Membrane-dependent guanine nucleotide binding and GTPase activities of soluble protein from bovine rod cell outer segments. Journal of Biological Chemistry 254: 7874–7884.
    He W, Cowan CW and Wensel TG (1998) RGS9, a GTPase accelerator for phototransduction. Neuron 20: 95–102.
    Kaupp UB and Seifert R (2002) Cyclic nucleotide-gated ion channels. Physiological Review 82: 769–824.
    book Kuehne W (1878) "Ueber den Sehpurpur. In Untersuchungen aus dem Physiologischen". Heidelberg: Institut der Universitaet Heidelberg. pp. 15–113.
    Lambright DG, Sondek J, Bohm A et al. (1996) The 2.0 A crystal structure of a heterotrimeric G protein. Nature 379: 311–319.
    Liebman PA and Pugh EN Jr (1982) Gain, speed and sensitivity of GTP binding vs. PDE activation in visual excitation. Vision REs. 22: 1475–1480.
    Miki N, Baraban JM, Keirns JJ, Boyce JJ and Bitensky MW (1975) Purification and properties of the light-activated cyclic nucleotide phosphodiesterase of rod outer segments. Journal of Biological Chemistry 250: 6320–6327.
    proceedings Nathans J and Hogness DS (1984) Isolation and nucleotide sequence of the gene encoding human rhodopsin. Proceedings of the National Academy of Sciences of the USA 81: 4851–4855.
    Noel JP, Hamm HE and Sigler PB (1993) The 2.2 Å crystal structure of transducin- complexed with GTPgammaS. Nature 366: 654–663.
    Palczewski K, Kumasaka T, Hori T et al. (2000) Crystal structure of rhodopsin: A G protein-coupled receptor [see comments]. Science 289: 739–745.
    Polans A, Baehr W and Palczewski K (1996) Turned on by  Ca2+ ! The physiology and pathology of  Ca2+ binding proteins in the retina. Trends in Neurosciences 19: 547–554.
    Szerencsei RT, Winkfein RJ, Cooper CB et al. (2002) The Na/Ca-K exchanger gene family. Annals of the New York Academy of Sciences 976: 41–52.
    Wald G (1968) The molecular basis of visual excitation. Nature 219: 800–807.
    Yeagle PL and Albert AD (2003) A conformational trigger for activation of a G protein by a G protein-coupled receptor. Biochemistry 42: 1365–1368.
 Further Reading
    book Baehr W and Palczewski K (2002) Photoreceptors and Calcium. New York, Boston, Dordrecht, London, Moscow, Georgetown TX: Kluwer Academic/Plenum Publishers/Landes Bioscience/Eurekah.com.
    Palczewski K, Polans AS, Baehr W and Ames JB (2000b) Calcium binding proteins in the retina: Structure, function, and the etiology of human visual diseases. BioEssays 22: 337–350.
    Ridge KD, Abdulaev NG, Sousa M and Palczewski K (2003) Phototransduction: crystal clear. Trends in Biochemical Science 28: 479–487.

Related Sub-Topics:

Sensory Systems | Sensory Transduction Mechanisms
Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

Article Title: Visual Cascade
 
How to Cite close
Baehr, Wolfgang, and Liebman, Paul(Jan 2006) Visual Cascade. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0004065]