Cyclic nucleotide-gated channels: an expanding new family of ion channels.

Cyclic nucleotide-gated ion channels are relatively new entries in the world of ion channels. Their discovery began with the surprising finding in the mid-1980s that the cation conductance (the "light-sensitive" conductance) mediating visual transduction in retinal rod photoreceptors is directly activated by guanosine 3',5'-cyclic monophosphate (cGMP) (1, 2). Before then, cyclic nucleotides had been known to affect the functional characteristics of some ion channels, but these were indirect effects mediated by cyclic nucleotide-dependent protein kinases and channel phosphorylationsi.e. , along the conventional course of action of cyclic nucleotides (for a recent review, see ref. 3). The discovery of an ion channel directly activated by a cyclic nucleotide thus represented a departure from this rule. When the nature of the phototransduction channel first came to light, it was viewed to some extent as a curiosity, considered perhaps unique to visual transduction. However, a similar channel was subsequently identified in olfactory transduction (4), followed by the same in the pineal gland (5). Furthermore, molecular cloning has provided a powerful way to search for homologous channels in other tissues. For example, a protein apparently identical to the olfactory channel was recently cloned from the aorta (6). In this issue of the Proceedings, Biel et al. (7) report the cloning of a homologous but distinct channel from the kidney. Concurrently, and reported elsewhere, Weyand et al. (8) have cloned the same gene from testis. Thus, this new family of ion channels continues to expand. To appreciate the functional roles of these channels, one might start with its archetype, the rod phototransduction channel. Retinal rod photoreceptors are known to respond to light with a membrane hyperpolarization, caused by the closure of a cation conductance that is open in darkness and was thought to be selective for Na+ (for review, see ref. 9). At the same time, light is known to trigger a G protein-mediated signaling cascade that leads to the activation of a cGMP phosphodiesterase and the lowering of cytoplasmic cGMP level (for review, see ref. 10). The exact connection between the light-sensitive conductance and cGMP, however, was unclear for a long time until Fesenko et al. (1) found, with an excised patch of rod plasma membrane, that cGMP directly activates a conductance on the membrane. The same conductance was also identified by using ion-flux measurements (11). Soon afterwards, it was established that this conductance and the light-sensitive conductance are indeed identical (2). One surprising feature of this channel that turns out to be of great importance is its high permeability to Ca2+. The inward membrane current through the open channel in darkness is 80%o Na+ and 15% Ca2+ (with the rest apparently carried by Mg2+); however, because Na+ is -100 times more concentrated than Ca2+ extracellularly, the channel really prefers Ca2+ over Na+ by -10 to one (12). This permeability to Ca2+ is what underlies the ability of the rod cell to adapt to steady illumination (13, 14), a fundamental property of visual function. Briefly, in darkness, there is a balance between the Ca2+ influx through these channels and a Ca2+ efflux through a Na2+/Ca+,K+ exchange carrier (15, 16); in the light, closure of the channels stops the Ca2+ influx, but the efflux continues, thus causing a decline in the cytoplasmic Ca2+ concentration (15). This Ca2+ decrease then activates a rather elaborate negative-feedback mechanism to produce light adaptation (for the most recent review, see ref. 17). Shortly after the discovery of the cGMP-activated channel in rods, a similar channel was found in retinal cones, which are the photoreceptor cells responsible for vision in bright light (18, 19). There is, however, a subtle difference in electrical properties between the rod and the cone channels (18), suggesting that the two may be distinct proteins. On the other hand, the Ca2+ permeability and its involvement in light adaptation still hold true for the cone channel (14, 20, 21). For both the rod and the cone channels, cGMP is the most effective activating ligand among the various cyclic nucleotides (22), consistent with its being the second messenger in phototransduction. cAMP can also open the channels, but an -50-fold higher concentration is necessary (22). Remarkably, a similar channel turns out to be involved in olfactory transduction (4). In this process, at least some odorants are known to activate a G protein-mediated signaling cascade leading to activation of an adenylate cyclase and production ofcAMP, which in turn opens a cyclic nucleotide-activated channel to generate a membrane depolarization (for review, see ref. 23). In this manner, visual and olfactory transductions have a similar motif, although they differ with respect to the cyclic nucleotide involved and the polarity of its concentration change resulting from sensory stimulation. Like the photoreceptor channels, the olfactory channel is highly permeable to Ca2+, a feature again important for sensory adaptation, which involves, in this case, a rise in cytoplasmic Ca2+ upon odorant-induced channel opening (24). Unlike the photoreceptor channels, however, the olfactory channel requires much lower ligand concentrations (4). Even though cAMP is the second messenger in olfactory transduction, it is actually slightly less effective than cGMP as the activating ligand (4); the significance of this peculiarity is unclear at present. Molecular cloning of these channels first became possible when the rod channel protein was successfully purified from bovine retina (25). The deduced amino acid sequence of this channel based on the cloned cDNA (26) shows a domain on the cytoplasmic C-terminal segment with homology to the cyclic nucleotide-binding domains in cGMPand cAMP-dependent protein kinases. Closer inspection of the sequence has also identified a region resembling the voltagesensing S4 domain in voltage-gated channels (27), even though the rod cGMPgated channel cannot be activated by voltage in the absence of cyclic nucleotide. Moreover, it has a region homologous to the pore of voltage-gated K+ channels (28). Unlike K+ channels, however, the rod channel does not discriminate between Na+ and K+, and it is partially blocked by (as well as, of course, being permeant to) divalent cations (9). From mutagenesis studies, both of these differences appear to arise from two amino acid residues, tyrosine and