All-trans-retinal Is a Closed-state Inhibitor of Rod Cyclic Nucleotide–gated Ion Channels

Abstract

Rod vision begins when 11-cis-retinal absorbs a photon and isomerizes to all-trans-retinal (ATR) within the photopigment, rhodopsin. Photoactivated rhodopsin triggers an enzyme cascade that lowers the concentration of cGMP, thereby closing cyclic nucleotide–gated (CNG) ion channels. After isomerization, ATR dissociates from rhodopsin, and after a bright light, this release is expected to produce a large surge of ATR near the CNG channels. Using excised patches from Xenopus oocytes, we recently showed that ATR shuts down cloned rod CNG channels, and that this inhibition occurs in the nanomolar range (aqueous concentration) at near-physiological concentrations of cGMP. Here we further characterize the ATR effect and present mechanistic information. ATR was found to decrease the apparent cGMP affinity, as well as the maximum current at saturating cGMP. When ATR was applied to outside-out patches, inhibition was much slower and less effective than when it was applied to inside-out patches, suggesting that ATR requires access to the intracellular surface of the channel or membrane. The apparent ATR affinity and maximal inhibition of heteromeric (CNGA1/CNGB1) channels was similar to that of homomeric (CNGA1) channels. Single-channel and multichannel data suggest that channel inhibition by ATR is reversible. Inhibition by ATR was not voltage dependent, and the form of its dose–response relation suggested multiple ATR molecules interacting per channel. Modeling of the data obtained with cAMP and cGMP suggests that ATR acts by interfering with the allosteric opening transition of the channel and that it prefers closed, unliganded channels. It remains to be determined whether ATR acts directly on the channel protein or instead alters channel–bilayer interactions.

Retinol Dehydrogenase (RDH12) Protects Photoreceptors from Light-induced Degeneration in Mice

“…Free all-trans-retinal can contribute to changes in the physiological responses of rod cells by activating ligand-free opsin (38 -40) or cyclic GMP-dependent cation channels of the plasma membrane (41,42). Thus, physiological responses such as these measured by ERG could be disconnected from the results of chemical analyses, e.g.…”

Functional Characterization of Mouse RDH11 as a Retinol Dehydrogenase Involved in Dark Adaptation in Vivo

“…Third, all-trans-retinal is a biologically active molecule that can bind and activate opsin, leading to a noisy phototransduction background. All-trans-retinal was also shown to directly and markedly inhibit photoreceptor ion channels at physiological concentrations (32)(33)(34). Therefore, disruption of RDH activity in photoreceptors leading to delayed reduction of all-trans-retinal could have a number of important consequences.…”

β-ionone activates and bleaches visual pigment in salamander photoreceptors

“…cRNA was made by in vitro transcription using the mMessage mMachine Kit ™ (Ambion, Austin, TX). Injected oocytes were incubated at 16°C for 4–12 days prior to use (McCabe et al, 2004). Native channels were studied in membrane patches excised from the outer segments of GS salamander rods.…”