Most of our knowledge of phototransduction comes from information known about rods, which are sensitive nocturnal light detectors. Considerably more biochemical material can be obtained from rods than from cones because rods are much more numerous in most retinas. In addition, rods contain far more membrane (ie, surface area) than do cones, which contributes to the rods’ greater sensitivity.
The outer segment of photoreceptors contains all the components required for phototransduction. It is composed primarily of plasma-membrane material organized into discs flattened perpendicular to the long axis of the outer segment (see Chapter 2, Fig 2-33). There are approximately 1000 discs within a rod outer segment and 1 million membrane-bound rhodopsin molecules in each disc. The discs float within the cytoplasm of the outer segment like a stack of coins disconnected from the plasma membrane. The discs contain the protein machinery to capture and amplify light energy. This abundance of outer-segment membrane increases the number of rhodopsin molecules, which can absorb light. Some deep-sea fish, which need considerable sensitivity to detect small amounts of light, rely on longer rod segments than those found in humans.
Rhodopsin is a freely diffusible membrane protein with 7 helical loops that is embedded in the lipid membrane (Fig 12-2). Rhodopsin absorbs green light best at wavelengths of approximately 510 nm. It absorbs blue and yellow light less well and is insensitive to longer wavelengths (red light). Rhodopsin is tuned to this part of the electromagnetic spectrum by its amino-acid sequence and by the binding of its chromophore 11-cis-retinal (also called 11-cis-retinaldehyde), which creates a molecular antenna.
The plasma membrane of the outer segment contains the cationic cyclic nucleotide–gated (CNG) channels, which are gated by cyclic guanosine monophosphate (cGMP). This channel controls the flow of sodium (Na+) and calcium (Ca2+) ions into the outer segment. In the dark, Na+ and Ca2+ flow in through the channel, which is kept open by cGMP. Ionic balance is maintained by Na+,K+-ATPase (also called sodium-potassium pump) in the inner segment and a Na+,K+-Ca2+ exchanger in the outer-segment membrane, both of which require metabolic energy. This flow of ions sets up the circulating dark current that keeps the photoreceptor’s membrane potential in a relatively depolarized state. The depolarized state of the photoreceptors causes a steady release of the transmitter glutamate from its synaptic terminal in the dark (Fig 12-3).
Figure 12-2 The rhodopsin molecule is embedded in the lipid membrane of the outer segment with 7 helical loops. Each circle represents an amino acid, and the highly conserved ones are shown in black. The red arrow represents the lysine to which the vitamin A chromophore is linked. Phosphorylation sites occur on the cytoplasmic and sugar attachment sites on the intradiscal (extracellular) ends of the rhodopsin molecule. Insets show the structures of 11-cis-retinal and all-trans-retinal.
(Courtesy of Peter Gouras, MD.)
Light activation of rhodopsin starts a series of reactions that lead to hyperpolarization of the photoreceptor’s membrane potential (Fig 12-4). Once rhodopsin absorbs a quantum of light, the 11-cis double bond of retinal is reconfigured (creating all-trans-retinal, also called all-trans-retinaldehyde), and the opsin molecule undergoes a series of rapid configurational changes to an activated state known as metarhodopsin II. Light-activated rhodopsin triggers a second molecule, transducin, by causing an exchange of guanosine diphosphate (GDP) for guanosine triphosphate (GTP) (see Fig 12-4A). One rhodopsin molecule can activate 100 transducin molecules, amplifying the reaction. Activated transducin excites a third protein, cGMP phosphodiesterase (PDE), which hydrolyzes cGMP to 5′-noncyclic GMP. The decrease in cGMP closes the CNG channels, which stops entry of Na+ and Ca2+ and hyperpolarizes the rod. Hyperpolarization stops the release of glutamate from the synaptic terminal.
When the light is extinguished, the rod returns to its dark state as the reaction cascade turns off. Recovery of the dark current requires that the catalytically active components of the phototransduction cascade be fully quenched and cGMP resynthesized to allow opening of the CNG channels. Rhodopsin is inactivated by phosphorylation at its C-terminal end by rhodopsin kinase and subsequent binding to arrestin (see Fig 12-4B). Inactivation of rhodopsin is aided by recoverin, a highly conserved Ca2+-binding protein found in both rods and cones. Transducin is inactivated by the hydrolysis of GTP to GDP via transducin’s intrinsic GTPase activity, which reduces PDE activity. Closure of the CNG channels with light activation also causes a drop in intracellular Ca2+ levels, which in turn stimulates retinal guanylate cyclase (also called guanylyl cyclase), the enzyme that synthesizes cGMP from GTP; the enzyme’s action is assisted by guanylate cyclase–assisting proteins (see Fig 12-4C). As cGMP levels increase, the CNG channels open and the rod is depolarized again. The corresponding rise in intracellular Ca2+ levels inhibits retinal guanylate cyclase activity to its dark level.
The discs of rod outer segments differ from those of cones in that they are disconnected from the outer plasma membrane. The rim of each rod disc has a collection of proteins. Two such proteins are peripherin and rod outer segment protein 1 (ROM1), which play a role in the development and maintenance of the disc’s curvature. Peripherin and ROM1 are also found in cone outer segments. Another protein in rod discs is ABCA4, an ATP-binding cassette (ABC) transporter. It is a transmembrane protein involved in the energy-dependent transport of substrates from the disc lumen to the rod cytosol. ABCA4 is unique to rod discs and is not found in cones. It functions as a transporter of all-trans-retinal.
Figure 12-3 Dark current and light response. (Left) In the dark, rhodopsin is inactive; the cyclic nucleotide–gated (CNG) channels in the outer segment are open; and the rod is depolarized with a steady release of glutamate from its axonal terminal. (Right) Rhodopsin is activated by light, which leads to closing of the CNG channels, rod membrane hyperpolarization, and inhibition of glutamate release from the axon terminal.
(Illustration by Mark Miller.)
Figure 12-4 Schematic representation of the phototransduction cascade in photoreceptor outer segments. A, Light-activated rhodopsin (R+) causes levels of cGMP to be reduced via transducin-disinhibited phosphodiesterase (PDE), leading to closure of cGMP voltage–gated channels (CNG) and subsequent hyperpolarization of the photoreceptor cell. B, R+ is deactivated through phosphorylation (indicated by Ps) and the binding of the protein arrestin (Arr). Phosphorylation is mediated by rhodopsin kinase (RK), which is regulated by recoverin (RV). RV dissociates from RK as calcium levels decrease following closure of cGMP voltage–gated channels. Arrestin binds to phosphorylated R+, completing the process. C, cGMP levels are restored through deactivation of transducin (T) via its intrinsic GTPase activity. PDE activity then decreases and guanylate cyclase activity increases, allowing cGMP levels to rise and opening the voltage-gated channels. cGMP = cyclic guanine monophosphate; GCAP = guanylate cyclase–activating protein; GDP = guanosine diphosphate; GTP = guanosine triphosphate; Tα, Tβ, Tγ = subunits of transducin.
(Redrawn from Ryan SJ, Schachat AP, Wilkinson CP, Hinton DR, Sadda SR, Wiedemann P.
5th ed. London: Saunders/Elsevier; 2013:Fig 14-4.)
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.