Diacylglycerol analogs inhibit the rod cGMP-gated channel by a phosphorylation-independent mechanism

Mechanism of ligand activation of a eukaryotic cyclic nucleotide-gated channel

Cyclic nucleotide-gated (CNG) channels convert cyclic nucleotide (CN) binding and unbinding into electrical signals in sensory receptors and neurons. The molecular conformational changes underpinning ligand activation are largely undefined. We report both closed- and open-state atomic cryo-EM structures of a full-length Caenorhabditis elegans cyclic GMP-activated channel TAX-4, reconstituted in lipid nanodiscs. These structures, together with computational and functional analyses and a mutant channel structure, reveal a double-barrier hydrophobic gate formed by two S6 amino acids in the central cavity. cGMP binding produces global conformational changes that open the cavity gate located ~52 Å away but do not alter the structure of the selectivity filter-the commonly presumed activation gate. Our work provides mechanistic insights into the allosteric gating and regulation of CN-gated and nucleotide-modulated channels and CNG channel-related channelopathies.

Effect of knocking down the insulin receptor on mouse rod responses

Previous experiments have shown that the insulin receptor (IR) is expressed in mammalian rods and contributes to the protection of photoreceptors during bright-light exposure. The role of the insulin receptor in the production of the light response is however unknown. We have used suction-electrode recording to examine the responses of rods after conditionally knocking down the insulin receptor. Our results show that these IR knock-down rods have an accelerated decay of the light response and a small decrease in sensitivity by comparison to littermate WT rods. Our results indicate that the insulin receptor may have some role in controlling the rate of rod response decay, but they exclude a major role of the insulin receptor pathway in phototransduction.

Two structural components in CNGA3 support regulation of cone CNG channels by phosphoinositides

Cyclic nucleotide-gated (CNG) channels in retinal photoreceptors play a crucial role in vertebrate phototransduction. The ligand sensitivity of photoreceptor CNG channels is adjusted during adaptation and in response to paracrine signals, but the mechanisms involved in channel regulation are only partly understood. Heteromeric cone CNGA3 (A3) + CNGB3 (B3) channels are inhibited by membrane phosphoinositides (PIP(n)), including phosphatidylinositol 3,4,5-triphosphate (PIP(3)) and phosphatidylinositol 4,5-bisphosphate (PIP(2)), demonstrating a decrease in apparent affinity for cyclic guanosine monophosphate (cGMP). Unlike homomeric A1 or A2 channels, A3-only channels paradoxically did not show a decrease in apparent affinity for cGMP after PIP(n) application. However, PIP(n) induced an ∼2.5-fold increase in cAMP efficacy for A3 channels. The PIP(n)-dependent change in cAMP efficacy was abolished by mutations in the C-terminal region (R643Q/R646Q) or by truncation distal to the cyclic nucleotide-binding domain (613X). In addition, A3-613X unmasked a threefold decrease in apparent cGMP affinity with PIP(n) application to homomeric channels, and this effect was dependent on conserved arginines within the N-terminal region of A3. Together, these results indicate that regulation of A3 subunits by phosphoinositides exhibits two separable components, which depend on structural elements within the N- and C-terminal regions, respectively. Furthermore, both N and C regulatory modules in A3 supported PIP(n) regulation of heteromeric A3+B3 channels. B3 subunits were not sufficient to confer PIP(n) sensitivity to heteromeric channels formed with PIP(n)-insensitive A subunits. Finally, channels formed by mixtures of PIP(n)-insensitive A3 subunits, having complementary mutations in N- and/or C-terminal regions, restored PIP(n) regulation, implying that intersubunit N-C interactions help control the phosphoinositide sensitivity of cone CNG channels.

Acute manipulation of diacylglycerol reveals roles in nuclear envelope assembly & endoplasmic reticulum morphology

The functions and morphology of cellular membranes are intimately related and depend not only on their protein content but also on the repertoire of lipids that comprise them. In the absence of in vivo data on lipid asymmetry in endomembranes, it has been argued that motors, scaffolding proteins or integral membrane proteins rather than non-lamellar bilayer lipids such as diacylglycerol (DAG), are responsible for shaping of organelles, local membrane curvature and fusion. The effects of direct alteration of levels of such lipids remain predominantly uninvestigated. Diacylglycerol (DAG) is a well documented second messenger. Here we demonstrate two additional conserved functions of DAG: a structural role in organelle morphology, and a role in localised extreme membrane curvature required for fusion for which proteins alone are insufficient. Acute and inducible DAG depletion results in failure of the nuclear envelope (NE) to reform at mitosis and reorganisation of the ER into multi-lamellar sheets as revealed by correlative light and electron microscopy and 3D reconstructions. Remarkably, depleted cells divide without a complete NE, and unless rescued by 1,2 or 1,3 DAG soon die. Attenuation of DAG levels by enzyme microinjection into echinoderm eggs and embryos also results in alterations of ER morphology and nuclear membrane fusion. Our findings demonstrate that DAG is an in vivo modulator of organelle morphology in mammalian and echinoderm cells, indicating a fundamental role conserved across the deuterostome superphylum.

Insulin receptor regulates photoreceptor CNG channel activity

Photoreceptor cyclic nucleotide gated (CNG) channels are critical elements in phototransduction and light adaptation. Here we report that insulin receptor (IR), an integral membrane protein, directly phosphorylates the CNGA1 subunit of CNG channels that in turn affects the function of these channels negatively. The IR phosphorylates Tyr(498) and Tyr(503) residues on CNGA1 that are situated at the membrane-cytoplasmic interface. The IR tyrosine kinase activity is essential for the inhibition of CNG channel. To maintain the channels in an off state, it is necessary not only to have a precise balance of the cGMP levels but also to have a control on the cGMP sensitivity of the CNG channels itself. In this study, we observed that the channel opens at a lower concentration of cGMP in IR(-/-) mice. These studies suggest that IR regulates the modulation of CNG channel activity in vivo.

Regulation of phosphatidic Acid metabolism by sphingolipids in the central nervous system

This paper explores the way ceramide, sphingosine, ceramide 1-phosphate, and sphingosine 1-phosphate modulate the generation of second lipid messengers from phosphatidic acid in two experimental models of the central nervous system: in vertebrate rod outer segments prepared from dark-adapted retinas as well as in rod outer segments prepared from light-adapted retinas and in rat cerebral cortex synaptosomes under physiological aging conditions. Particular attention is paid to lipid phosphate phosphatase, diacylglycerol lipase, and monoacylglycerol lipase. Based on the findings reported in this paper, it can be concluded that proteins related to phototransduction phenomena are involved in the effects derived from sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide and that age-related changes occur in the metabolism of phosphatidic acid from cerebral cortex synaptosomes in the presence of either sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide. The present paper demonstrates, in two different models of central nervous system, how sphingolipids influence phosphatidic acid metabolism under different physiological conditions such as light and aging.

Diacylglyceride lipase activity in rod outer segments depends on the illumination state of the retina

We have demonstrated that the competition between phosphatidic acid (PA) and lysophosphatidic acid (LPA), sphingosine 1-phosphate (S1P) and ceramide 1-phosphate (C1P) for lipid phosphate phosphatases (LPP) generates different levels of diacylglycerol (DAG) depending on the illumination state of the retina. The aim of the present research was to determine the diacylglyceride lipase (DAGL) activity in purified rod outer segments (ROS) obtained from dark-adapted retinas (DROS) or light-adapted retinas (BLROS) as well as in ROS membrane preparations depleted of soluble and peripheral proteins. [2-(3)H]monoacylglycerol (MAG), the product of DAGL, was evaluated from [2-(3)H]DAG generated by LPP action on [2-(3)H]PA in the presence of either LPA, S1P or C1P. MAG production was inhibited by 55% in BLROS and by 25% when the enzymatic assay was carried out in ROS obtained from dark-adapted retinas and incubated under room light (LROS). The most important events occurred in DROS where co-incubation of [2-(3)H]PA with LPA, S1P or C1P diminished MAG production. A higher level of DAGL activity was observed in LROS than in BLROS, though this difference was not apparent in the presence of LPA, S1P or C1P. DAGL activity in depleted DROS was diminished with respect to that in entire DROS. LPA, S1P and C1P produced a similar decrease in MAG production in depleted DROS whereas only C1P significantly diminished MAG generation in depleted BLROS. Sphingosine and ceramide inhibited MAG production in entire DROS and stimulated its generation in BLROS. Sphingosine and ceramide stimulated MAG generation in both depleted DROS and BLROS. Under our experimental conditions the degree of MAG production depended on the illumination state of the retina. We therefore suggest that proteins related to phototransduction phenomena are involved in the effects observed in the presence of S1P/sphingosine or C1P/ceramide.

Diacylglycerols, multivalent membrane modulators

Diacylglycerols are second messengers confined to biomembranes and, although relatively simple molecules from the structural point of view, they are able of triggering a surprisingly wide range of biological responses. Diacylglycerols are recognized by a well conserved protein motif, such as the C1 domain. This domain was observed for the first time in protein kinases C but is now known to be present in many other proteins. The effect of diacylglycerols is not limited to binding to C1 domains and they are able to alter the biophysical properties of biomembranes and hence modulate the activity of membrane associated proteins and also facilitate some processes like membrane fusion.

Interplay between PIP3 and calmodulin regulation of olfactory cyclic nucleotide-gated channels

Phosphatidylinositol-3,4,5-trisphosphate (PIP3) has been proposed to modulate the odorant sensitivity of olfactory sensory neurons by inhibiting activation of cyclic nucleotide-gated (CNG) channels in the cilia. When applied to the intracellular face of excised patches, PIP3 has been shown to inhibit activation of heteromeric olfactory CNG channels, composed of CNGA2, CNGA4, and CNGB1b subunits, and homomeric CNGA2 channels. In contrast, we discovered that channels formed by CNGA3 subunits from cone photoreceptors were unaffected by PIP3. Using chimeric channels and a deletion mutant, we determined that residues 61-90 within the N terminus of CNGA2 are necessary for PIP3 regulation, and a biochemical "pulldown" assay suggests that PIP3 directly binds this region. The N terminus of CNGA2 contains a previously identified calcium-calmodulin (Ca2+/CaM)-binding domain (residues 68-81) that mediates Ca2+/CaM inhibition of homomeric CNGA2 channels but is functionally silent in heteromeric channels. We discovered, however, that this region is required for PIP3 regulation of both homomeric and heteromeric channels. Furthermore, PIP3 occluded the action of Ca2+/CaM on both homomeric and heteromeric channels, in part by blocking Ca2+/CaM binding. Our results establish the importance of the CNGA2 N terminus for PIP3 inhibition of olfactory CNG channels and suggest that PIP3 inhibits channel activation by disrupting an autoexcitatory interaction between the N and C termini of adjacent subunits. By dramatically suppressing channel currents, PIP3 may generate a shift in odorant sensitivity that does not require prior channel activity.

The pharmacology of cyclic nucleotide-gated channels: emerging from the darkness

Cyclic nucleotide-gated (CNG) ion channels play a central role in vision and olfaction, generating the electrical responses to light in photoreceptors and to odorants in olfactory receptors. These channels have been detected in many other tissues where their functions are largely unclear. The use of gene knockouts and other methods have yielded some information, but there is a pressing need for potent and specific pharmacological agents directed at CNG channels. To date there has been very little systematic effort in this direction - most of what can be termed CNG channel pharmacology arose from testing reagents known to target protein kinases or other ion channels, or by accident when researchers were investigating other intracellular pathways that may regulate the activity of CNG channels. Predictably, these studies have not produced selective agents. However, taking advantage of emerging structural information and the increasing knowledge of the biophysical properties of these channels, some promising compounds and strategies have begun to emerge. In this review we discuss progress on two fronts, cyclic nucleotide analogs as both activators and competitive inhibitors, and inhibitors that target the pore or gating machinery of the channel. We also discuss the potential of these compounds for treating certain forms of retinal degeneration.

KCNQ-like Potassium Channels in Caenorhabditis elegans

The human KCNQ gene family encodes potassium channels linked to several genetic syndromes including neonatal epilepsy, cardiac arrhythmia, and progressive deafness. KCNQ channels form M-type potassium channels, which are critical regulators of neuronal excitability that mediate autonomic responses, pain, and higher brain function. Fundamental mechanisms of the normal and abnormal cellular roles for these channels may be gained from their study in simple model organisms. Here we report that a multigene family of KCNQ-like channels is present in the nematode, Caenorhabditis elegans. We show that many aspects of the functional properties, tissue expression pattern, and modulation of these C. elegans channels are conserved, including suppression by the M1 muscarinic receptor. We also describe a conserved mechanism of modulation by diacylglycerol for a subset of C. elegans and vertebrate KCNQ/KQT channels, which is dependent upon the carboxyl-terminal domains of channel subunits and activated protein kinase C.

Amplification of Ca2+ Signaling by Diacylglycerol-mediated Inositol 1,4,5-Trisphosphate Production

Stimulation of various cell surface receptors leads to the production of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) through phospholipase C (PLC) activation, and the IP3 and DAG in turn trigger Ca2+ release through IP3 receptors and protein kinase C activation, respectively. The amount of IP(3) produced is particularly critical to determining the spatio-temporally coordinated Ca(2+)-signaling patterns. In this paper, we report a novel signal cross-talk between DAG and the IP3-mediated Ca(2+)-signaling pathway. We found that a DAG derivative, 1-oleoyl-2-acyl-sn-glycerol (OAG), induces Ca2+ oscillation in various types of cells independently of protein kinase C activity and extracellular Ca2+. The OAG-induced Ca2+ oscillation was completely abolished by depletion of Ca2+ stores or inhibition of PLC and IP3 receptors, indicating that OAG stimulates IP3 production through PLC activation and thereby induces IP3-induced Ca2+ release. Furthermore, intracellular accumulation of endogenous DAG by a DAG-lipase inhibitor greatly increased the number of cells responding to agonist stimulation at low doses. These results suggest a novel physiological function of DAG, i.e. amplification of Ca2+ signaling by enhancing IP3 production via its positive feedback effect on PLC activity.

PKC-independent inhibition of neuronal nicotinic acetylcholine receptors by diacylglycerol

Diacylglycerol modulates cell functions primarily through activation of protein kinase C (PKC). In a previous study, however, we found that a diacylglycerol analogue, 1-oleoyl-2-acetylglycerol (OAG), accelerated desensitization of neuronal nicotinic acetylcholine receptors (nAchRs) independently of PKC activation in PC12 cells. In the present study, we investigated whether other analogues and endogenous diacylglycerol exert similar effects on neuronal nAchRs and characterized the modulation by diacylglycerol. We measured the nicotine-induced whole-cell current in the absence and presence of diacylglycerol analogues in PC12 cells. We also investigated the effects of a blockade of metabolic pathways of diacylglycerol by inhibiting diacylglycerol lipase and kinase. We found that all four diacylglycerol analogues studied promoted desensitization and depressed the nondesensitized component of the nicotine-induced current. These effects seemed independent of PKC activation because they were not antagonized by the PKC inhibitors staurosporine or bisindolylmaleimide I; one analogue that lacks the PKC-stimulating action was also effective. The effects of diacylglycerol analogues were not antagonized by high doses of nicotine and were independent of the membrane potential. Similar modulatory effects were observed by treatment with RHC80267, a blocker of diacylglycerol lipase, and R59949, an inhibitor of diacylglycerol kinase, in the presence of staurosporine. These results suggest that diacylglycerol, both exogenously applied and endogenously produced, modulates neuronal nAchRs independently of PKC activation in PC12 cells; further, these effects seemed consistent with a noncompetitive and voltage-independent block. They raised the possibility that PKC-independent inhibition of neuronal nAchRs by diacylglycerol may be a novel modulatory process.

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) ion channels were first discovered in rod photoreceptors, where they are responsible for the primary electrical signal of the photoreceptor in response to light. CNG channels are highly specialized membrane proteins that open an ion-permeable pore across the membrane in response to the direct binding of intracellular cyclic nucleotides. CNG channels have been identified in a number of other tissues, including the brain, where their roles are only beginning to be appreciated. Recently, significant progress has been made in understanding the molecular mechanisms underlying their functional specializations. From these studies, a picture is beginning to emerge for how the binding of cyclic nucleotide is transduced into the opening of the pore and how this allosteric transition is modulated by various physiological effectors.

Activation of phospholipase C increases intramembrane electric fields in N1E-115 neuroblastoma cells

We imaged the intramembrane potential (a combination of transmembrane, surface, and dipole potential) on N1E-115 neuroblastoma cells with a voltage-sensitive dye. After activation of the B(2) bradykinin receptor, the electric field sensed by the dye increased by an amount equivalent to a depolarization of 83 mV. The increase in intramembrane potential was blocked by the phospholipase C (PLC) inhibitors U-73122 and neomycin, and was invariably accompanied by a transient rise of [Ca(2+)](i). A depolarized inner surface potential, as the membrane loses negative charges via phosphatidylinositol 4,5-bisphosphate (PIP(2)) hydrolysis, and an increase in the dipole potential, as PIP(2) is hydrolyzed to 1,2-diacylglycerol (DAG), can each account for a small portion of the change in intramembrane potential. The primary contribution to the measured change in intramembrane potential may arise from an increased dipole potential, as DAG molecules are generated from hydrolysis of other phospholipids. We found bradykinin produced an inhibition of a M-type voltage-dependent K(+) current (I(K(M))). This inhibition was also blocked by the PLC inhibitors and had similar kinetics as the bradykinin-induced modulation of intramembrane potential. Our results suggest that the change in the local intramembrane potential induced by bradykinin may play a role in mediating the I(K(M)) inhibition.

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels first identified in retinal photoreceptors and olfactory sensory neurons (OSNs). They are opened by the direct binding of cyclic nucleotides, cAMP and cGMP. Although their activity shows very little voltage dependence, CNG channels belong to the superfamily of voltage-gated ion channels. Like their cousins the voltage-gated K+ channels, CNG channels form heterotetrameric complexes consisting of two or three different types of subunits. Six different genes encoding CNG channels, four A subunits (A1 to A4) and two B subunits (B1 and B3), give rise to three different channels in rod and cone photoreceptors and in OSNs. Important functional features of these channels, i.e., ligand sensitivity and selectivity, ion permeation, and gating, are determined by the subunit composition of the respective channel complex. The function of CNG channels has been firmly established in retinal photoreceptors and in OSNs. Studies on their presence in other sensory and nonsensory cells have produced mixed results, and their purported roles in neuronal pathfinding or synaptic plasticity are not as well understood as their role in sensory neurons. Similarly, the function of invertebrate homologs found in Caenorhabditis elegans, Drosophila, and Limulus is largely unknown, except for two subunits of C. elegans that play a role in chemosensation. CNG channels are nonselective cation channels that do not discriminate well between alkali ions and even pass divalent cations, in particular Ca2+. Ca2+ entry through CNG channels is important for both excitation and adaptation of sensory cells. CNG channel activity is modulated by Ca2+/calmodulin and by phosphorylation. Other factors may also be involved in channel regulation. Mutations in CNG channel genes give rise to retinal degeneration and color blindness. In particular, mutations in the A and B subunits of the CNG channel expressed in human cones cause various forms of complete and incomplete achromatopsia.

Modulation of cyclic-nucleotide-gated channels and regulation of vertebrate phototransduction

Cyclic-nucleotide-gated (CNG) channels are crucial for sensory transduction in the photoreceptors (rods and cones) of the vertebrate retina. Light triggers a decrease in the cytoplasmic concentration of cyclic GMP in the outer segments of these cells, leading to closure of CNG channels and hyperpolarization of the membrane potential. Hence, CNG channels translate a chemical change in cyclic nucleotide concentration into an electrical signal that can spread through the photoreceptor cell and be transmitted to the rest of the visual system. The sensitivity of phototransduction can be altered by exposing the cells to light, through adaptation processes intrinsic to photoreceptors. Intracellular Ca2+ is a major signal in light adaptation and, in conjunction with Ca2+-binding proteins, one of its targets for modulation is the CNG channel itself. However, other intracellular signals may be involved in the fine-tuning of light sensitivity in response to cues internal to organisms. Several intracellular signals are candidates for mediating changes in cyclic GMP sensitivity including transition metals, such as Ni2+ and Zn2+, and lipid metabolites, such as diacylglycerol. Moreover, CNG channels are associated with protein kinases and phosphatases that catalyze changes in phosphorylation state and allosterically modulate channel activity. Recent studies suggest that the effects of circadian rhythms and retinal transmitters on CNG channels may be mediated by such changes in phosphorylation. The goal of this paper is to review the molecular mechanisms underlying modulation of CNG channels and to relate these forms of modulation to the regulation of light sensitivity.

Growth factors regulate phototransduction in retinal rods by modulating cyclic nucleotide-gated channels through dephosphorylation of a specific tyrosine residue

Illumination of vertebrate rod photoreceptors leads to a decrease in the cytoplasmic cGMP concentration and closure of cyclic nucleotide-gated (CNG) channels. Except for Ca(2+), which plays a negative feedback role in adaptation, and 11-cis-retinal, supplied by the retinal pigment epithelium, all of the biochemical machinery of phototransduction is thought to be contained within rod outer segments without involvement of extrinsic regulatory molecules. Here we show that insulin-like growth factor-I (IGF-I), a paracrine factor released from the retinal pigment epithelium, alters phototransduction by rapidly increasing the cGMP sensitivity of CNG channels. The IGF-I-signaling pathway ultimately involves a protein tyrosine phosphatase that catalyzes dephosphorylation of a specific residue in the alpha-subunit of the rod CNG channel protein. IGF-I conjointly accelerates the kinetics and increases the amplitude of the light response, distinct from events that accompany adaptation. These effects of IGF-I could result from the enhancement of the cGMP sensitivity of CNG channels. Hence, in addition to long-term control of development and survival of rods, growth factors regulate phototransduction in the short term by modulating CNG channels.

Mechanism of inhibition of cyclic nucleotide-gated channel by protein tyrosine kinase probed with genistein

Rod cyclic nucleotide-gated (CNG) channels are modulated by changes in tyrosine phosphorylation catalyzed by protein tyrosine kinases (PTKs) and phosphatases (PTPs). We used genistein, a PTK inhibitor, to probe the interaction between the channel and PTKs. Previously, we found that in addition to inhibiting tyrosine phosphorylation of the rod CNG channel alpha-subunit (RETalpha), genistein triggers a noncatalytic inhibitory interaction between the PTK and the channel. These studies suggest that PTKs affects RETalpha channels in two ways: (1) by catalyzing phosphorylation of the channel protein, and (2) by allosterically regulating channel activation. Here, we study the mechanism of noncatalytic inhibition. We find that noncatalytic inhibition follows the same activity dependence pattern as catalytic modulation (phosphorylation): the efficacy and apparent affinity of genistein inhibition are much higher for closed than for fully activated channels. Association rates with the genistein-PTK complex were similar for closed and fully activated channels and independent of genistein concentration. Dissociation rates were 100 times slower for closed channels, which is consistent with a much higher affinity for genistein-PTK. Genistein-PTK affects channel gating, but not single channel conductance or the number of active channels. By analyzing single channel gating during genistein-PTK dissociation, we determined the maximal open probability for normal and genistein-PTK-bound channels. genistein-PTK decreases open probability by increasing the free energy required for opening, making opening dramatically less favorable. Ni(2+), which potentiates RETalpha channel gating, partially relieves genistein inhibition, possibly by disrupting the association between the genistein-PTK and the channel. Studies on chimeric channels containing portions of RETalpha, which exhibits genistein inhibition, and the rat olfactory CNG channel alpha-subunit, which does not, reveals that a domain containing S6 and flanking regions is the crucial for genistein inhibition and may constitute the genistein-PTK binding site. Thus, genistein-PTK stabilizes the closed state of the channel by interacting with portions of the channel that participate in gating.

Vertebrate photoreceptors

The basis of the duplex theory of vision is examined in view of the dazzling array of data on visual pigment sequences and the pigments they form, on the microspectrophotometry measurements of single photoreceptor cells, on the kinds of photoreceptor cascade enzymes, and on the electrophysiological properties of photoreceptors. The implications of the existence of five distinct visual pigment families are explored, especially with regard to what pigments are in what types of photoreceptors, if there are different phototransduction enzymes associated with different types of photoreceptors, and if there are electrophysiological differences between different types of cones.

Adaptation in vertebrate photoreceptors

When light is absorbed within the outer segment of a vertebrate photoreceptor, the conformation of the photopigment rhodopsin is altered to produce an activated photoproduct called metarhodopsin II or Rh(*). Rh(*) initiates a transduction cascade similar to that for metabotropic synaptic receptors and many hormones; the Rh(*) activates a heterotrimeric G protein, which in turn stimulates an effector enzyme, a cyclic nucleotide phosphodiesterase. The phosphodiesterase then hydrolyzes cGMP, and the decrease in the concentration of free cGMP reduces the probability of opening of channels in the outer segment plasma membrane, producing the electrical response of the cell. Photoreceptor transduction can be modulated by changes in the mean light level. This process, called light adaptation (or background adaptation), maintains the working range of the transduction cascade within a physiologically useful region of light intensities. There is increasing evidence that the second messenger responsible for the modulation of the transduction cascade during background adaptation is primarily, if not exclusively, Ca(2+), whose intracellular free concentration is decreased by illumination. The change in free Ca(2+) is believed to have a variety of effects on the transduction mechanism, including modulation of the rate of the guanylyl cyclase and rhodopsin kinase, alteration of the gain of the transduction cascade, and regulation of the affinity of the outer segment channels for cGMP. The sensitivity of the photoreceptor is also reduced by previous exposure to light bright enough to bleach a substantial fraction of the photopigment in the outer segment. This form of desensitization, called bleaching adaptation (the recovery from which is known as dark adaptation), seems largely to be due to an activation of the transduction cascade by some form of bleached pigment. The bleached pigment appears to activate the G protein transducin directly, although with a gain less than Rh(*). The resulting decrease in intracellular Ca(2+) then modulates the transduction cascade, by a mechanism very similar to the one responsible for altering sensitivity during background adaptation.

Mutation of a single residue in the S2-S3 loop of CNG channels alters the gating properties and sensitivity to inhibitors

We previously found that native cyclic nucleotide-gated (CNG) cation channels from amphibian rod cells are directly and reversibly inhibited by analogues of diacylglycerol (DAG), but little is known about the mechanism of this inhibition. We recently determined that, at saturating cGMP concentrations, DAG completely inhibits cloned bovine rod (Brod) CNG channels while only partially inhibiting cloned rat olfactory (Rolf) channels (Crary, J.I., D.M. Dean, W. Nguitragool, P.T. Kurshan, and A.L. Zimmerman. 2000. J. Gen. Phys. 116:755-768; in this issue). Here, we report that a point mutation at position 204 in the S2-S3 loop of Rolf and a mouse CNG channel (Molf) found in olfactory epithelium and heart, increased DAG sensitivity to that of the Brod channel. Mutation of this residue from the wild-type glycine to a glutamate (Molf G204E) or aspartate (Molf G204D) gave dramatic increases in DAG sensitivity without changing the apparent cGMP or cAMP affinities or efficacies. However, unlike the wild-type olfactory channels, these mutants demonstrated voltage-dependent gating with obvious activation and deactivation kinetics. Interestingly, the mutants were also more sensitive to inhibition by the local anesthetic, tetracaine. Replacement of the position 204 glycine with a tryptophan residue (Rolf G204W) not only gave voltage-dependent gating and an increased sensitivity to DAG and tetracaine, but also showed reduced apparent agonist affinity and cAMP efficacy. Sequence comparisons show that the glycine at position 204 in the S2-S3 loop is highly conserved, and our findings indicate that its alteration can have critical consequences for channel gating and inhibition.

Mechanism of inhibition of cyclic nucleotide-gated ion channels by diacylglycerol

Cyclic nucleotide-gated (CNG) channels are critical components in the visual and olfactory signal transduction pathways, and they primarily gate in response to changes in the cytoplasmic concentration of cyclic nucleotides. We previously found that the ability of the native rod CNG channel to be opened by cGMP was markedly inhibited by analogues of diacylglycerol (DAG) without a phosphorylation reaction (Gordon, S.E., J. Downing-Park, B. Tam, and A.L. Zimmerman. 1995. Biophys. J. 69:409-417). Here, we have studied cloned bovine rod and rat olfactory CNG channels expressed in Xenopus oocytes, and have determined that they are differentially inhibited by DAG. At saturating [cGMP], DAG inhibition of homomultimeric (alpha subunit only) rod channels was similar to that of the native rod CNG channel, but DAG was much less effective at inhibiting the homomultimeric olfactory channel, producing only partial inhibition even at high [DAG]. However, at low open probability (P(o)), both channels were more sensitive to DAG, suggesting that DAG is a closed state inhibitor. The Hill coefficients for DAG inhibition were often greater than one, suggesting that more than one DAG molecule is required for effective inhibition of a channel. In single-channel recordings, DAG decreased the P(o) but not the single-channel conductance. Results with chimeras of rod and olfactory channels suggest that the differences in DAG inhibition correlate more with differences in the transmembrane segments and their attached loops than with differences in the amino and carboxyl termini. Our results are consistent with a model in which multiple DAG molecules stabilize the closed state(s) of a CNG channel by binding directly to the channel and/or by altering bilayer-channel interactions. We speculate that if DAG interacts directly with the channel, it may insert into a putative hydrophobic crevice among the transmembrane domains of each subunit or at the hydrophobic interface between the channel and the bilayer.

Do phosphatidylinositides modulate vertebrate phototransduction?

Mammalian rod cyclic nucleotide gated (CNG) channels (i.e., alpha plus beta subunits) are strongly inhibited by phosphatidylinositol 4, 5-bisphosphate (PIP(2)) when they are expressed in Xenopus oocytes and studied in giant membrane patches. Cytoplasmic Mg-ATP inhibits CNG currents similarly, and monoclonal antibodies to PIP(2) reverse the effect and hyperactivate currents. When alpha subunits are expressed alone, PIP(2) inhibition is less strong; olfactory CNG channels are not inhibited. In giant patches from rod outer segments, inhibition by PIP(2) is intermediate. Other anionic lipids (e.g., phosphatidyl serine and phosphatidic acid), a phosphatidylinositol-specific phospholipase C, and full-length diacylglycerol have stimulatory effects. Although ATP also potently inhibits cGMP-activated currents in rod patches, the following findings indicate that ATP is used to transphosphorylate GMP, generated from cGMP, to GTP. First, a phosphodiesterase (PDE) inhibitor, Zaprinast, blocks inhibition by ATP. Second, inhibition can be rapidly reversed by exogenous regulator of G-protein signaling 9, suggesting G-protein activation by ATP. Third, the reversal of ATP effects is greatly slowed when cyclic inosine 5'-monophosphate is used to activate currents, as expected for slow inosine 5' triphosphate hydrolysis by G-proteins. Still, other results remain suggestive of regulatory roles for PIP(2). First, the cGMP concentration producing half-maximal CNG channel activity (K(1/2)) is decreased by PIP(2) antibody in the presence of PDE inhibitors. Second, the activation of PDE activity by several nucleotides, monitored electrophysiologically and biochemically, is reversed by PIP(2) antibody. Third, exogenous PIP(2) can enhance PDE activation by nucleotides.

Do phosphatidylinositides modulate vertebrate phototransduction?

Mammalian rod cyclic nucleotide gated (CNG) channels (i.e., alpha plus beta subunits) are strongly inhibited by phosphatidylinositol 4, 5-bisphosphate (PIP(2)) when they are expressed in Xenopus oocytes and studied in giant membrane patches. Cytoplasmic Mg-ATP inhibits CNG currents similarly, and monoclonal antibodies to PIP(2) reverse the effect and hyperactivate currents. When alpha subunits are expressed alone, PIP(2) inhibition is less strong; olfactory CNG channels are not inhibited. In giant patches from rod outer segments, inhibition by PIP(2) is intermediate. Other anionic lipids (e.g., phosphatidyl serine and phosphatidic acid), a phosphatidylinositol-specific phospholipase C, and full-length diacylglycerol have stimulatory effects. Although ATP also potently inhibits cGMP-activated currents in rod patches, the following findings indicate that ATP is used to transphosphorylate GMP, generated from cGMP, to GTP. First, a phosphodiesterase (PDE) inhibitor, Zaprinast, blocks inhibition by ATP. Second, inhibition can be rapidly reversed by exogenous regulator of G-protein signaling 9, suggesting G-protein activation by ATP. Third, the reversal of ATP effects is greatly slowed when cyclic inosine 5'-monophosphate is used to activate currents, as expected for slow inosine 5' triphosphate hydrolysis by G-proteins. Still, other results remain suggestive of regulatory roles for PIP(2). First, the cGMP concentration producing half-maximal CNG channel activity (K(1/2)) is decreased by PIP(2) antibody in the presence of PDE inhibitors. Second, the activation of PDE activity by several nucleotides, monitored electrophysiologically and biochemically, is reversed by PIP(2) antibody. Third, exogenous PIP(2) can enhance PDE activation by nucleotides.

Stoichiometry and arrangement of subunits in rod cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) ion channels mediate the response to light in retinal rods. They are tetramers of two homologous subunits (alpha and beta), each of which is essential for the function of the channels in vivo. We have investigated the stoichiometry and arrangement of these two subunits to determine how they come together within an individual channel complex. We exploited the very specific geometric and spatial requirements for forming a high-affinity Ni2+-binding site to examine the number and relative positions of the subunits. We found that only an order of alpha/alpha/beta/beta could account qualitatively and quantitatively for the observed intersubunit coordination of Ni2+ in wild-type and mutant alpha/beta channels. Furthermore, our results suggest a structural dimerization among like subunits, at least at the level of the Ni2+-binding site.

Activity-dependent modulation of rod photoreceptor cyclic nucleotide-gated channels mediated by phosphorylation of a specific tyrosine residue

Cyclic nucleotide-gated (CNG) channels are crucial for phototransduction in vertebrate rod photoreceptors. The cGMP sensitivity of these channels is modulated by diffusible intracellular messengers, including Ca2+/calmodulin, contributing to negative feedback during sensory adaptation. Membrane-associated protein tyrosine kinases and phosphatases also modulate rod CNG channels, but whether this results from direct changes in the phosphorylation state of the channel protein has been unclear. Here, we show that bovine rod CNG channel alpha-subunits (bRET) contain a tyrosine phosphorylation site crucial for modulation. bRET channels expressed in Xenopus oocytes exhibit modulation, whereas rat olfactory CNG channels (rOLF) do not. Chimeric channels reveal that differences in the C terminus, containing the cyclic nucleotide-binding domain, account for this difference. One specific tyrosine in bRET (Y498) appears to be crucial; replacement of this tyrosine in bRET curtails modulation, whereas installation into rOLF confers modulability. As the channel becomes dephosphorylated, there is an increase in the rate of spontaneous openings in the absence of ligand, indicating that changes in the phosphorylation state affect the allosteric gating equilibrium. Moreover, we find that dephosphorylation, which favors channel opening, requires open channels, whereas phosphorylation, which promotes channel closing, requires closed channels. Hence, modulation by changes in tyrosine phosphorylation is activity-dependent and may constitute a positive feedback mechanism, contrasting with negative feedback systems underlying adaptation.

Activity-dependent modulation of rod photoreceptor cyclic nucleotide-gated channels mediated by phosphorylation of a specific tyrosine residue

Cyclic nucleotide-gated (CNG) channels are crucial for phototransduction in vertebrate rod photoreceptors. The cGMP sensitivity of these channels is modulated by diffusible intracellular messengers, including Ca2+/calmodulin, contributing to negative feedback during sensory adaptation. Membrane-associated protein tyrosine kinases and phosphatases also modulate rod CNG channels, but whether this results from direct changes in the phosphorylation state of the channel protein has been unclear. Here, we show that bovine rod CNG channel alpha-subunits (bRET) contain a tyrosine phosphorylation site crucial for modulation. bRET channels expressed in Xenopus oocytes exhibit modulation, whereas rat olfactory CNG channels (rOLF) do not. Chimeric channels reveal that differences in the C terminus, containing the cyclic nucleotide-binding domain, account for this difference. One specific tyrosine in bRET (Y498) appears to be crucial; replacement of this tyrosine in bRET curtails modulation, whereas installation into rOLF confers modulability. As the channel becomes dephosphorylated, there is an increase in the rate of spontaneous openings in the absence of ligand, indicating that changes in the phosphorylation state affect the allosteric gating equilibrium. Moreover, we find that dephosphorylation, which favors channel opening, requires open channels, whereas phosphorylation, which promotes channel closing, requires closed channels. Hence, modulation by changes in tyrosine phosphorylation is activity-dependent and may constitute a positive feedback mechanism, contrasting with negative feedback systems underlying adaptation.

Noncatalytic inhibition of cyclic nucleotide-gated channels by tyrosine kinase induced by genistein

Rod photoreceptor cyclic nucleotide-gated (CNG) channels are modulated by tyrosine phosphorylation. Rod CNG channels expressed in Xenopus oocytes are associated with constitutively active protein tyrosine kinases (PTKs) and protein tyrosine phosphatases that decrease and increase, respectively, the apparent affinity of the channels for cGMP. Here, we examine the effects of genistein, a competitive inhibitor of the ATP binding site, on PTKs. Like other PTK inhibitors (lavendustin A and erbstatin), cytoplasmic application of genistein prevents changes in the cGMP sensitivity that are attributable to tyrosine phosphorylation of the CNG channels. However, unlike these other inhibitors, genistein also slows the activation kinetics and reduces the maximal current through CNG channels at saturating cGMP. These effects occur in the absence of ATP, indicating that they do not involve inhibition of a phosphorylation event, but rather involve an allosteric effect of genistein on CNG channel gating. This could result from direct binding of genistein to the channel; however, the time course of inhibition is surprisingly slow (>30 s), raising the possibility that genistein exerts its effects indirectly. In support of this hypothesis, we find that ligands that selectively bind to PTKs without directly binding to the CNG channel can nonetheless decrease the effect of genistein. Thus, ATP and a nonhydrolyzable ATP derivative competitively inhibit the effect of genistein on the channel. Moreover, erbstatin, an inhibitor of PTKs, can noncompetitively inhibit the effect of genistein. Taken together, these results suggest that in addition to inhibiting tyrosine phosphorylation of the rod CNG channel catalyzed by PTKs, genistein triggers a noncatalytic interaction between the PTK and the channel that allosterically inhibits gating.

Molecular and pharmacological analysis of cyclic nucleotide-gated channel function in the central nervous system

Most functional studies of cyclic nucleotide-gated (CNG) channels have been confined to photoreceptors and olfactory epithelium, in which CNG channels are abundant and easy to study. The widespread distribution of CNG channels in tissues throughout the body has only recently been recognized and the functions of this channel family in many of these tissues remain largely unknown. The molecular biological and pharmacological properties of the CNG channel family are summarized in order to put in context studies aimed at probing CNG channel functions in these tissues using pharmacological and genetic methods. Compounds have now been identified that are useful in distinguishing CNG channel activated pathways from cAMP/cGMP dependent-protein kinases or other pathways. The ways in which these interact with CNG channels are understood and this knowledge is leading to the identification of more potent and more specific CNG channel subtype-specific agonists or antagonists. Recent molecular and genetic analyses have identified novel roles of CNG channels in neuronal development and plasticity in both invertebrates and vertebrates. Targeting CNG channels via specific drugs and genetic manipulation (such as knockout mice) will permit better understanding of the role of CNG channels in both basic and higher orders of brain function.

Modulation of rod photoreceptor cyclic nucleotide-gated channels by tyrosine phosphorylation

Cyclic nucleotide-gated (CNG) channels in vertebrate photoreceptors are crucial for transducing light-induced changes in cGMP concentration into electrical signals. In this study, we show that both native and exogenously expressed CNG channels from rods are modulated by tyrosine phosphorylation. The cGMP sensitivity of CNG channels, composed of rod alpha-subunits expressed in Xenopus oocytes, gradually increases after excision of inside-out patches from the oocyte membrane. This increase in sensitivity is inhibited by a protein tyrosine phosphatase (PTP) inhibitor and is unaffected by three different Ser/Thr phosphatase inhibitors. Moreover, it is suppressed or reversed by application of ATP but not by a nonhydrolyzable ATP analog. Application of protein tyrosine kinase (PTK) inhibitors causes an increase in cGMP sensitivity, but only in the presence of ATP. Taken together, these results suggest that CNG channels expressed in oocytes are associated with active PTK(s) and PTP(s) that regulate their cGMP sensitivity by changing phosphorylation state. The cGMP sensitivity of native CNG channels from salamander rod outer segments also increases and decreases after incubation with inhibitors of PTP(s) and PTK(s), respectively. These results suggest that rod CNG channels are modulated by tyrosine phosphorylation, which may function as a novel mechanism for regulating the sensitivity of rods to light.

Modulation of rod photoreceptor cyclic nucleotide-gated channels by tyrosine phosphorylation

Cyclic nucleotide-gated (CNG) channels in vertebrate photoreceptors are crucial for transducing light-induced changes in cGMP concentration into electrical signals. In this study, we show that both native and exogenously expressed CNG channels from rods are modulated by tyrosine phosphorylation. The cGMP sensitivity of CNG channels, composed of rod alpha-subunits expressed in Xenopus oocytes, gradually increases after excision of inside-out patches from the oocyte membrane. This increase in sensitivity is inhibited by a protein tyrosine phosphatase (PTP) inhibitor and is unaffected by three different Ser/Thr phosphatase inhibitors. Moreover, it is suppressed or reversed by application of ATP but not by a nonhydrolyzable ATP analog. Application of protein tyrosine kinase (PTK) inhibitors causes an increase in cGMP sensitivity, but only in the presence of ATP. Taken together, these results suggest that CNG channels expressed in oocytes are associated with active PTK(s) and PTP(s) that regulate their cGMP sensitivity by changing phosphorylation state. The cGMP sensitivity of native CNG channels from salamander rod outer segments also increases and decreases after incubation with inhibitors of PTP(s) and PTK(s), respectively. These results suggest that rod CNG channels are modulated by tyrosine phosphorylation, which may function as a novel mechanism for regulating the sensitivity of rods to light.

Two opposite effects of ATP on the apparent sensitivity of the cGMP-gated channel of the carp retinal cone

Effects of ATP on the activity of cGMP-gated channels from carp cone photoreceptors were studied. In 29% of the patches examined (N = 45), ATP (1 mM) enhanced a current evoked by cGMP (20 microM, up to about 100%), in 33%. ATP suppressed it by up to about 90%, and in the remaining 38%, ATP had no effect. ATP showed similar effects on a current evoked by 8-bromoguanosine 3',5'-cyclic monophosphate (2 microM, enhancing in 42% of the patches, suppressing in 25%, no effect in 33%, N = 12), suggesting that the effects were not through modulation of the phosphodiesterase. Both of the effects, enhancement and suppression, were produced by a change in apparent affinity for cGMP, since (1) the maximum current evoked by cGMP of the saturating concentration (> or = 1 mM) was not affected, and (2) the K1/2 value decreased by approximately 45% (N = 2) or increased by approximately 25% (N = 2). A lower pH (approximately 6) facilitated the enhancing effect. ATP-gamma-S (1 mM) showed a suppressing effect in 80% of the patches and no effect in 20% of the patches (N = 10). However, ATP-gamma-S did not show an enhancing effect. Thus, ATP had two opposite effects through different mechanisms on the apparent sensitivity of the channel to cGMP; increasing and decreasing.

General anosmia caused by a targeted disruption of the mouse olfactory cyclic nucleotide-gated cation channel

Olfactory neurons transduce the binding of odorants into membrane depolarization. Two intracellular messengers, cyclic AMP (cAMP) and inositol trisphosphate (IP3), are thought to mediate this process, with cAMP generating responses to some odorants and IP3 mediating responses to others. cAMP causes membrane depolarization by activating a cation-selective cyclic nucleotide-gated (CNG) channel. We created a mutant "knockout" mouse lacking functional olfactory CNG channels to assess the roles of different second messenger pathways in olfactory transduction. Using an electrophysiological assay, we find that excitatory responses to both cAMP- and IP3-producing odorants are undetectable in knockout mice. Our results provide direct evidence that the CNG channel subserves excitatory olfactory signal transduction, and further suggest that cAMP is the sole second messenger mediating this process.

Caged compounds of hydrolysis-resistant analogues of cAMP and cGMP: synthesis and application to cyclic nucleotide-gated channels

Photolabile compounds which rapidly release cAMP or cGMP after photolysis are widely used for in situ studies of signaling pathways inside cells. We synthesized two novel caged compounds, 4,5-dimethoxy-2-nitrobenzyl 8-Br-cAMP (caged 8-Br-cAMP) and 4,5-dimethoxy-2-nitrobenzyl 8-Br-cGMP caged 8-BR-cGMP), which respectively release the hydrolysis-resistant analogues 8-Br-cAMP and 8-Br-cGMP. Their usefulness for physiological studies was examined in a mammalian cell line expressing the cyclic nucleotide-gated (CNG) ion channel of bovine olfactory sensory neurons. The synthesis procedure resulted in diastereomeric mixtures which were chromatographically separated into the axial and equatorial isomers of caged 8-BR-cAMP and of caged 8-BR-cGMP. The axial isomers which have a higher solubility and better solvolytic stability than the equatorial forms were used for experiments with CNG channels. Flashes of UV light produced steps in the concentration of 8-Br-cGMP which activated currents through CNG channels. Concentration steps inside the cell could be calibrated precisely using the relation between the ligand concentration and the normalized current. Similar results were obtained with caged 8-Br-cAMP. Control experiments with caged cGMP showed that flash-induced currents decayed within a few minutes because photoreleased cGMP was degraded by endogenous phosphodiesterase activity. The rise time of the 8-Br-cGMP-activated whole-cell current was consistent with a bimolecular reaction between channel and ligand.

Molecular mechanisms of cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels are highly specialized to carry out their unique role in cell signalling. Significant progress has been made in the last several years determining the molecular mechanisms for these specializations. The activation of the channels begins with the binding of cyclic nucleotide to a domain in the carboxyl terminal region. This binding, in turn, produces an induced fit of the protein that involves a movement of the C-helix portion of the binding domain. The induced fit of the binding domain is coupled to an allosteric conformational change that opens the channel pore. The pore is formed primarily from the sequence between the S5 and S6 segments. A single glutamic acid in the pore represents the binding site for multiple monovalent cations, the blocking site for external divalent cations, and the site for the effect of protons on permeation.