Hindered diffusion in excised membrane patches from retinal rod outer segments

Currents through Hv1 channels deplete protons in their vicinity

Proton channels have evolved to provide a pH regulatory mechanism, affording the extrusion of protons from the cytoplasm at all membrane potentials. Previous evidence has suggested that channel-mediated acid extrusion could significantly change the local concentration of protons in the vicinity of the channel. In this work, we directly measure the proton depletion caused by activation of Hv1 proton channels using patch-clamp fluorometry recordings from channels labeled with the Venus fluorescent protein at intracellular domains. The fluorescence of the Venus protein is very sensitive to pH, thus behaving as a genetically encoded sensor of local pH. Eliciting outward proton currents increases the fluorescence intensity of Venus. This dequenching is related to the magnitude of the current and not to channel gating and is dependent on the pH gradient. Our results provide direct evidence of local proton depletion caused by flux through the proton-selective channel.

Physical basis of apparent pore dilation of ATP-activated P2X receptor channels

The selectivity of ion channels is fundamental for their roles in electrical and chemical signaling and in ion homeostasis. Although most ion channels exhibit stable ion selectivity, the prevailing view of purinergic P2X receptor channels, transient receptor potential V1 (TRPV1) channels and acid-sensing ion channels (ASICs) is that their ion conduction pores dilate upon prolonged activation. We investigated this mechanism in P2X receptors and found that the hallmark shift in equilibrium potential observed with prolonged channel activation does not result from pore dilation, but from time-dependent alterations in the concentration of intracellular ions. We derived a physical model to calculate ion concentration changes during patch-clamp recordings, which validated our experimental findings and provides a quantitative guideline for effectively controlling ion concentration. Our results have fundamental implications for understanding ion permeation and gating in P2X receptor channels, as well as more broadly for using patch-clamp techniques to study ion channels and neuronal excitability.

Light-dependent changes in outer retinal water diffusion in rats in vivo

PURPOSE

To test the hypothesis that in rats, intraretinal light-dependent changes on diffusion-weighted magnetic resonance imaging (MRI) in vivo are consistent with known retinal layer-specific physiology.

METHODS

In male Sprague-Dawley rats, retinal morphology (thickness, extent, surface area, volume) and intraretinal profiles of the apparent diffusion coefficient (ADC, i.e., water mobility) parallel and perpendicular to the optic nerve were measured in vivo using quantitative MRI methods during light and dark stimulation.

RESULTS

The parallel ADC in the posterior half of the avascular, photoreceptor-dominated outer retina was significantly higher in light than dark, and this pattern was reversed (dark>light) in the anterior outer retina. The perpendicular ADC in the posterior outer retina was similar in light and dark, but was significantly higher in dark than light in the anterior outer retina. No light-dark changes in the inner retina were noted.

CONCLUSIONS

We identified light-dependent intraretinal diffusion changes that reflected established stimulation-based changes in outer retinal hydration. These findings are expected to motivate future applications of functional diffusion-based MRI in blinding disorders of the outer retina.

Cyclic nucleotide-gated channel block by hydrolysis-resistant tetracaine derivatives

To meet a pressing need for better cyclic nucleotide-gated (CNG) channel antagonists, we have increased the biological stability of tetracaine-based blockers by synthesizing amide and thioamide linkage substitutions of tetracaine (1) and a higher affinity octyl tail derivative (5). We report the apparent K(D) values, the mechanism of block, and the in vitro hydrolysis rates for these compounds. The ester linkage substitutions did not adversely affect CNG channel block; unexpectedly, thioamide substitution in 1 (compound 8) improved block significantly. Furthermore, the ester linkage substitutions did not appear to affect the mechanism of block in terms of the strong state preference for closed channels. All ester substituted compounds, especially the thioamide substitutions, were more resistant to hydrolysis by serum cholinesterase than their ester counterparts. These findings have implications for dissecting the physiological roles of CNG channels, treating certain forms of retinal degeneration, and possibly the current clinical uses of compound 1.

A novel cyclic nucleotide-gated ion channel enriched in synaptic terminals of isotocin neurons in zebrafish brain and pituitary

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels opened by binding of intracellular cyclic GMP or cyclic AMP. CNG channels mediate sensory transduction in the rods and cones of the retina and in olfactory sensory neurons, but in addition, CNG channels are also expressed elsewhere in the CNS, where their physiological roles have not yet been well defined. Besides the CNG channel subtypes that mediate vision and olfaction, zebrafish has an additional subtype, CNGA5, which is expressed almost exclusively in the brain. We have generated CNGA5-specific monoclonal antibodies, which we use here to show that immunoreactivity for CNGA5 channels is highly enriched in synaptic terminals of a discrete set of neurons that project to a subregion of the pituitary, as well as diffusely in the brain and spinal cord. Double labeling with a variety of antibodies against pituitary hormones revealed that CNGA5 is located in the terminals of neuroendocrine cells that secrete the nonapeptide hormone/transmitter isotocin in the neurohypophysis, brain, and spinal cord. Furthermore, we show that CNGA5 channels expressed in Xenopus oocytes are highly permeable to Ca(2+), which suggests that the channels are capable of modulating isotocin release in the zebrafish brain and pituitary. Isotocin is the teleost homolog of the mammalian hormone oxytocin, and like oxytocin, it regulates reproductive and social behavior. Therefore, the high calcium permeability of CNGA5 channels and their strategic location in isotocin-secreting synaptic terminals suggest that activation of CNGA5 channels in response to cyclic nucleotide signaling may have wide-ranging neuroendocrine and behavioral effects.

Cyclic nucleotide-gated ion channels in rod photoreceptors are protected from retinoid inhibition

In vertebrate rods, photoisomerization of the 11-cis retinal chromophore of rhodopsin to the all-trans conformation initiates a biochemical cascade that closes cGMP-gated channels and hyperpolarizes the cell. All-trans retinal is reduced to retinol and then removed to the pigment epithelium. The pigment epithelium supplies fresh 11-cis retinal to regenerate rhodopsin. The recent discovery that tens of nanomolar retinal inhibits cloned cGMP-gated channels at low [cGMP] raised the question of whether retinoid traffic across the plasma membrane of the rod might participate in the signaling of light. Native channels in excised patches from rods were very sensitive to retinoid inhibition. Perfusion of intact rods with exogenous 9- or 11-cis retinal closed cGMP-gated channels but required higher than expected concentrations. Channels reopened after perfusing the rod with cellular retinoid binding protein II. PDE activity, flash response kinetics, and relative sensitivity were unchanged, ruling out pharmacological activation of the phototransduction cascade. Bleaching of rhodopsin to create all-trans retinal and retinol inside the rod did not produce any measurable channel inhibition. Exposure of a bleached rod to 9- or 11-cis retinal did not elicit channel inhibition during the period of rhodopsin regeneration. Microspectrophotometric measurements showed that exogenous 9- or 11-cis retinal rapidly cross the plasma membrane of bleached rods and regenerate their rhodopsin. Although dark-adapted rods could also take up large quantities of 9-cis retinal, which they converted to retinol, the time course was slow. Apparently cGMP-gated channels in intact rods are protected from the inhibitory effects of retinoids that cross the plasma membrane by a large-capacity buffer. Opsin, with its chromophore binding pocket occupied (rhodopsin) or vacant, may be an important component. Exceptionally high retinoid levels, e.g., associated with some retinal degenerations, could overcome the buffer, however, and impair sensitivity or delay the recovery after exposure to bright light.

Clustering of cyclic-nucleotide-gated channels in olfactory cilia

Olfactory cilia contain the known components of olfactory signal transduction, including a high density of cyclic-nucleotide-gated (CNG) channels. CNG channels play an important role in mediating odor detection. The channels are activated by cAMP, which is formed by a G-protein-coupled transduction cascade. Frog olfactory cilia are 25-200 microm in length, so the spatial distribution of CNG channels along the length should be important in determining the sensitivity of odor detection. We have recorded from excised cilia and modeled diffusion of cAMP into a cilium to determine the spatial distribution of the CNG channels along the ciliary length. The proximal segment, which in frog is the first 20% of the cilium, appears to express a small fraction of the CNG channels, whereas the distal segment contains the majority, mostly clustered in one region.

Modifications to the tetracaine scaffold produce cyclic nucleotide-gated channel blockers with widely varying efficacies

Five new tetracaine analogues were synthesized and evaluated for potency of blockade of cyclic nucleotide-gated channels relative to a multiply charged tetracaine analogue described previously. Increased positive charge at the tertiary amine end of tetracaine results in higher potency and voltage dependence of block. Modifications that reduce the hydrophobic character at the butyl tail are deleterious to block. The tetracaine analogues described here have apparent affinities for CNGA1 channels that vary over nearly 8 orders of magnitude.

Defining the retinoid binding site in the rod cyclic nucleotide-gated channel

Rod vision is initiated when 11-cis-retinal, bound within rhodopsin, absorbs a photon and isomerizes to all-trans-retinal (ATR). This triggers an enzyme cascade that lowers cGMP, thereby closing cyclic nucleotide-gated (CNG) channels. ATR then dissociates from rhodopsin, with bright light releasing millimolar levels of ATR. We have recently shown that ATR is a potent closed-state inhibitor of the rod CNG channel, and that it requires access to the cytosolic face of the channel (McCabe, S.L., D.M. Pelosi, M. Tetreault, A. Miri, W. Nguitragool, P. Kovithvathanaphong, R. Mahajan, and A.L. Zimmerman. 2004. J. Gen. Physiol. 123:521–531). However, the details of the interaction between the channel and ATR have not been resolved. Here, we explore the nature of this interaction by taking advantage of specific retinoids and retinoid analogues, namely, β-ionone, all-trans-C15 aldehyde, all-trans-C17 aldehyde, all-trans-C22 aldehyde, all-trans-retinol, all-trans-retinoic acid, and all-trans-retinylidene-n-butylamine. These retinoids differ in polyene chain length, chemical functionality, and charge. Results obtained from patch clamp and NMR studies have allowed us to better define the characteristics of the site of retinoid–channel interaction. We propose that the cytoplasmic face of the channel contains a retinoid binding site. This binding site likely contains a hydrophobic region that allows the ionone ring and polyene tail to sit in an optimal position to promote interaction of the terminal functional group with residues ∼15 Å away from the ionone ring. Based on our functional data with retinoids possessing either a positive or a negative charge, we speculate that these amino acid residues may be polar and/or aromatic.

The carboxyl-terminal region of cyclic nucleotide-modulated channels is a gating ring, not a permeation path

The recent elucidation of the structure of the carboxyl-terminal region of the hyperpolarization-activated cyclic nucleotide-modulated (HCN2) channel has prompted us to investigate a curious feature of this structure in HCN2 channels and in the related CNGA1 cyclic nucleotide-gated (CNG) channels. The crystallized fragment of the HCN2 channel contains both the cyclic nucleotide-binding domain (CNBD) and the C-linker region, which connects the CNBD to the pore. At the center of the fourfold-symmetric structure is a tunnel that runs perpendicular to the membrane. The narrowest part of the tunnel is approximately 10 A in diameter and is lined by a ring of negatively charged amino acids: D487, E488, and D489. Many ion channels have "charge rings" that focus permeant ions at the mouth of the pore and increase channel conductance. We used nonstationary fluctuation analysis and single-channel recording, coupled with site-directed mutagenesis and cysteine modification, to determine whether this part of HCN and CNG channels might be an extension of the permeation pathway. Our results indicate that modifying charge-ring amino acids affects gating but not ion permeation in HCN2 and CNG channels. Thus, this portion of the channel is not an obligatory part of the ion path but instead acts as a "gating ring." The carboxyl-terminal region of these channels must hang below the pore much like the "hanging gondola" of voltage-gated potassium channels, but the permeation pathway must exit the protein before the level of the ring of charged amino acids.

All-trans-retinal is a closed-state inhibitor of rod cyclic nucleotide-gated ion channels

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.

The Ca-activated Cl channel and its control in rat olfactory receptor neurons

Odorants activate sensory transduction in olfactory receptor neurons (ORNs) via a cAMP-signaling cascade, which results in the opening of nonselective, cyclic nucleotide-gated (CNG) channels. The consequent Ca2+ influx through CNG channels activates Cl channels, which serve to amplify the transduction signal. We investigate here some general properties of this Ca-activated Cl channel in rat, as well as its functional interplay with the CNG channel, by using inside-out membrane patches excised from ORN dendritic knobs/cilia. At physiological concentrations of external divalent cations, the maximally activated Cl current was approximately 30 times as large as the CNG current. The Cl channels on an excised patch could be activated by Ca2+ flux through the CNG channels opened by cAMP. The magnitude of the Cl current depended on the strength of Ca buffering in the bath solution, suggesting that the CNG and Cl channels were probably not organized as constituents of a local transducisome complex. Likewise, Cl channels and the Na/Ca exchanger, which extrudes Ca2+, appear to be spatially segregated. Based on the theory of buffered Ca2+ diffusion, we determined the Ca2+ diffusion coefficient and calculated that the CNG and Cl channel densities on the membrane were approximately 8 and 62 micro m-2, respectively. These densities, together with the Ca2+ diffusion coefficient, demonstrate that a given Cl channel is activated by Ca2+ originating from multiple CNG channels, thus allowing low-noise amplification of the olfactory receptor current.

A cysteine scan of the inner vestibule of cyclic nucleotide-gated channels reveals architecture and rearrangement of the pore

Cyclic nucleotide-gated (CNG) channels belong to the P-loop-containing family of ion channels that also includes KcsA, MthK, and Shaker channels. In this study, we investigated the structure and rearrangement of the CNGA1 channel pore using cysteine mutations and cysteine-specific modification. We constructed 16 mutant channels, each one containing a cysteine mutation at one of the positions between 384 and 399 in the S6 region of the pore. By measuring currents activated by saturating concentrations of the full agonist cGMP and the partial agonists cIMP and cAMP, we show that mutating S6 residues to cysteine caused both favorable and unfavorable changes in the free energy of channel opening. The time course of cysteine modification with 2-aminoethylmethane thiosulfonate hydrochloride (MTSEA) was complex. For many positions we observed decreases in current activated by cGMP and concomitant increases in current activated by cIMP and cAMP. A model where modification affected both gating and permeation successfully reproduced the complex time course of modification for most of the mutant channels. From the model fits to the time course of modification for each mutant channel, we quantified the following: (a) the bimolecular rate constant of modification in the open state, (b) the change in conductance, and (c) the change in the free energy of channel opening for modification of each cysteine. At many S6 cysteines, modification by MTSEA caused a decrease in conductance and a favorable change in the free energy of channel opening. Our results are interpreted within the structural framework of the known structures of KcsA and MthK. We conclude that: (a) MTSEA modification affects both gating and permeation, (b) the open configuration of the pore of CNGA1 channels is consistent with the structure of MthK, and (c) the modification of S6 residues disrupts the helical packing of the closed channel, making it easier for channels to open.

Multimerization of the ligand binding domains of cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels comprise four subunits and are activated by the direct binding of cyclic nucleotide to an intracellular domain on each subunit. This ligand binding domain is thought to contain a beta roll followed by two alpha helices, designated the B and C helices. To examine the quaternary structure of CNG channels and how it changes during ion channel gating, we introduced single cysteines along the C helix of each subunit in an otherwise cysteineless channel. We found that cysteines on the C helices could form intersubunit disulfide bonds, even between diagonal subunits. Disulfide bond formation occurred primarily in closed channels and inhibited channel opening. These data suggest that the C helices from all four channel subunits are in close proximity in the closed state and move apart during channel opening.

All-trans-retinal shuts down rod cyclic nucleotide-gated ion channels: a novel role for photoreceptor retinoids in the response to bright light?

In retinal rods, light-induced isomerization of 11-cis-retinal to all-trans-retinal within rhodopsin triggers an enzyme cascade that lowers the concentration of cGMP. Consequently, cyclic nucleotide-gated (CNG) ion channels close, generating the first electrical response to light. After isomerization, all-trans-retinal dissociates from rhodopsin. We now show that all-trans-retinal directly and markedly inhibits cloned rod CNG channels in excised patches. 11-cis-retinal and all-trans-retinol also inhibited the channels, but at somewhat higher concentrations. Single-channel analysis suggests that all-trans-retinal reduces average open probability of rod CNG channels by inactivating channels for seconds at a time. At physiological cGMP levels, all-trans-retinal inhibited in the nanomolar range. Our results suggest that all-trans-retinal may be a potent regulator of the channel in rods during the response to bright light, when there is a large surge in the concentration of all-trans-retinal.

The cGMP-gated channel and related glutamic acid-rich proteins interact with peripherin-2 at the rim region of rod photoreceptor disc membranes

The rod cGMP-gated channel is localized in the plasma membrane of rod photoreceptor outer segments, where it plays a central role in phototransduction. It consists of alpha- and beta-subunits that assemble into a heterotetrameric protein. Each subunit contains structural features characteristic of nucleotide-gated channels, including a cGMP-binding domain, multiple membrane-spanning segments, and a pore region. In addition, the beta-subunit has a large glutamic acid- and proline-rich region called GARP that is also expressed as two soluble protein variants. Using monoclonal antibodies in conjunction with immunoprecipitation, cross-linking, and electrophoretic techniques, we show that the cGMP-gated channel associates with the Na/Ca-K exchanger in the rod outer segment plasma membrane. This complex and soluble GARP proteins also interact with peripherin-2 oligomers in the rim region of outer segment disc membranes. These results suggest that channel/peripherin protein interactions mediated by the GARP part of the channel beta-subunit play a role in connecting the rim region of discs to the plasma membrane and in anchoring the channel.exchanger complex in the rod outer segment plasma membrane.

Probing the interactions between cAMP and cGMP in cyclic nucleotide-gated channels using covalently tethered ligands

Cyclic nucleotide-gated channels contain four ligand-binding subunits, and they are directly activated by the binding of cGMP or cAMP. Channels with different combinations of subunits are known to have different sensitivities to the two nucleotides. However, the consequences of mixed occupancy by cGMP and cAMP are not well understood, and may have important implications for understanding the functions of these channels in different cell types. We studied the activation of homomeric and heteromeric retinal rod cyclic nucleotide-gated channels with the four ligand-binding sites occupied by different combinations of cGMP (a strong agonist) and cAMP (a weak agonist). Control of occupancy was obtained by covalently tethering different numbers of cGMP moieties using the photoaffinity analogue 8-p-azidophenacylthio-cGMP; the remaining sites were then saturated with cAMP, or cGMP, for comparison. The fractional current activated by cAMP increased dramatically as the number of tethered cGMP moieties increased. In homomeric channels comprised of the alpha subunit, cAMP became an effective agonist only after three of the four sites were occupied by tethered cGMP moieties. In contrast, in heteromeric channels comprised of two alpha and two beta subunits, cAMP caused significant activation after two sites were occupied by tethered cGMP moieties. In agreement with earlier work, a single residue on the beta subunit, N1201, accounted for much of the increased efficacy of cAMP on heteromeric channels. The results are consistent with significant interactions between subunits, including the two types of subunits in heteromeric channels.

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.

A point mutation in the pore region alters gating, Ca(2+) blockage, and permeation of olfactory cyclic nucleotide-gated channels

Upon stimulation by odorants, Ca(2+) and Na(+) enter the cilia of olfactory sensory neurons through channels directly gated by cAMP. Cyclic nucleotide-gated channels have been found in a variety of cells and extensively investigated in the past few years. Glutamate residues at position 363 of the alpha subunit of the bovine retinal rod channel have previously been shown to constitute a cation-binding site important for blockage by external divalent cations and to control single-channel properties. It has therefore been assumed, but not proven, that glutamate residues at the corresponding position of the other cyclic nucleotide-gated channels play a similar role. We studied the corresponding glutamate (E340) of the alpha subunit of the bovine olfactory channel to determine its role in channel gating and in permeation and blockage by Ca(2+) and Mg(2+). E340 was mutated into either an aspartate, glycine, glutamine, or asparagine residue and properties of mutant channels expressed in Xenopus laevis oocytes were measured in excised patches. By single-channel recordings, we demonstrated that the open probabilities in the presence of cGMP or cAMP were decreased by the mutations, with a larger decrease observed on gating by cAMP. Moreover, we observed that the mutant E340N presented two conductance levels. We found that both external Ca(2+) and Mg(2+) powerfully blocked the current in wild-type and E340D mutants, whereas their blockage efficacy was drastically reduced when the glutamate charge was neutralized. The inward current carried by external Ca(2+) relative to Na(+) was larger in the E340G mutant compared with wild-type channels. In conclusion, we have confirmed that the residue at position E340 of the bovine olfactory CNG channel is in the pore region, controls permeation and blockage by external Ca(2+) and Mg(2+), and affects channel gating by cAMP more than by cGMP.

Modification of cyclic nucleotide-gated ion channels by ultraviolet light

We irradiated cyclic nucleotide-gated ion channels in situ with ultraviolet light to probe the role of aromatic residues in ion channel function. UV light reduced the current through excised membrane patches from Xenopus oocytes expressing the alpha subunit of bovine retinal cyclic nucleotide-gated channels irreversibly, a result consistent with permanent covalent modification of channel amino acids by UV light. The magnitude of the current reduction depended only on the total photon dose delivered to the patches, and not on the intensity of the exciting light, indicating that the functionally important photochemical modification(s) occurred from an excited state reached by a one-photon absorption process. The wavelength dependence of the channels' UV light sensitivity (the action spectrum) was quantitatively consistent with the absorption spectrum of tryptophan, with a small component at long wavelengths, possibly due to cystine absorption. This spectral analysis suggests that UV light reduced the currents at most wavelengths studied by modifying one or more "target" tryptophans in the channels. Comparison of the channels' action spectrum to the absorption spectrum of tryptophan in various solvents suggests that the UV light targets are in a water-like chemical environment. Experiments on mutant channels indicated that the UV light sensitivity of wild-type channels was not conferred exclusively by any one of the 10 tryptophan residues in a subunit. The similarity in the dose dependences of channel current reduction and tryptophan photolysis in solution suggests that photochemical modification of a small number of tryptophan targets in the channels is sufficient to decrease the currents.

Coexpression of alpha and beta subunits of the rod cyclic GMP-gated channel restores native sensitivity to cyclic AMP: role of D604/N1201

Coexpression of the betawt and alphawt subunits of the bovine rod channel restores two characteristics of the native channels: higher sensitivity to cAMP and potentiation of cGMP-induced currents by low cAMP concentrations. To test whether the increased sensitivity to cAMP is due to the uncharged nature of the asparagine residue (N1201) situated in place of aspartate D604 in the beta subunit as previously suggested (, Neuron. 15:619-625), we compared currents from wild-type (alphawt and alphawt/betawt) and from mutated channels (alphaD604N, alphaD604N/betawt, and alphawt/betaN1201D). The results show that the sensitivity to cAMP and cAMP potentiation is partly but not entirely determined by the charge of residue 1201 in the beta subunit. The D604N mutation in the alpha subunit and, to a lesser extent, coexpression of the betawt subunit with the alphawt subunit reduce the open probability for cGMP compared to that of the alphawt channel. Interpretation of the data with the MWC allosteric model (model of Monod, Wyman, Changeux;, J. Mol. Biol. 12:88-118) suggests that the D604N mutation in the alpha subunits and coassembly of alpha and beta subunits alter the free energy of gating by cAMP more than that of cAMP binding.

Constraining the subunit order of rod cyclic nucleotide-gated channels reveals a diagonal arrangement of like subunits

Retinal rod cyclic nucleotide-gated channels are composed of alpha and beta subunits. We have explored possible subunit arrangements by expressing tandemly linked dimers of both subunits and examining their responses to three different modulating agents. Channels formed from either alpha-beta or beta-alpha heterodimers had functional properties similar to those formed from coexpressed alpha and beta monomers and to native channels. These results point to an alpha-beta-alpha-beta arrangement. To ensure that heterodimers had not flipped around, we coexpressed alpha-alpha dimers with an excess of either beta monomers or beta-beta dimers. Our data indicate that heteromultimers do not form efficiently in an alpha-alpha-beta-beta arrangement. Thus, we propose that native rod cyclic nucleotide-gated channels are arranged with like subunits diagonally opposed: alpha-beta-alpha-beta.

Gating by cyclic GMP and voltage in the alpha subunit of the cyclic GMP-gated channel from rod photoreceptors

Gating by cGMP and voltage of the alpha subunit of the cGMP-gated channel from rod photoreceptor was examined with a patch-clamp technique. The channels were expressed in Xenopus oocytes. At low [cGMP] (<20 microM), the current displayed strong outward rectification. At low and high (700 microM) [cGMP], the channel activity was dominated by only one conductance level. Therefore, the outward rectification at low [cGMP] results solely from an increase in the open probability, P(o). Kinetic analysis of single-channel openings revealed two exponential distributions. At low [cGMP], the larger P(o) at positive voltages with respect to negative voltages is caused by an increased frequency of openings in both components of the open-time distribution. In macroscopic currents, depolarizing voltage steps, starting from -100 mV, generated a time-dependent current that increased with the step size (activation). At low [cGMP] (20 microM), the degree of activation was large and the time course was slow, whereas at saturating [cGMP] (7 mM) the respective changes were small and fast. The dose-response relation at -100 mV was shifted to the right and saturated at significantly lower P(o) values with respect to that at +100 mV (0.77 vs. 0.96). P(o) was determined as function of the [cGMP] (at +100 and -100 mV) and voltage (at 20, 70, and 700 microM, and 7 mM cGMP). Both relations could be fitted with an allosteric state model consisting of four independent cGMP-binding reactions and one voltage-dependent allosteric opening reaction. At saturating [cGMP] (7 mM), the activation time course was monoexponential, which allowed us to determine the individual rate constants for the allosteric reaction. For the rapid rate constants of cGMP binding and unbinding, lower limits are determined. It is concluded that an allosteric model consisting of four independent cGMP-binding reactions and one voltage-dependent allosteric reaction, describes the cGMP- and voltage-dependent gating of cGMP-gated channels adequately.

Molecular rearrangements in the ligand-binding domain of cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels are activated in response to the direct binding of cyclic nucleotides to an intracellular domain. This domain is thought to contain a beta roll and two alpha helices, designated the B and C helices. To probe the conformational changes occurring in the ligand-binding domain during channel activation, we used the substituted cysteine accessibility method (SCAM). We found that a residue in the beta roll, C505, is more accessible in unliganded channels than in liganded channels, whereas a residue in the C helix, G597C, is more accessible in closed channels than in open channels. These results support a molecular mechanism for channel activation in which the ligand initially binds to the beta roll, followed by an opening allosteric transition involving the relative movement of the C helix toward the beta roll.

Divalent cation selectivity is a function of gating in native and recombinant cyclic nucleotide-gated ion channels from retinal photoreceptors

The selectivity of Ca2+ over Na+ is approximately 3.3-fold larger in cGMP-gated channels of cone photoreceptors than in those of rods when measured under saturating cGMP concentrations, where the probability of channel opening is 85-90%. Under physiological conditions, however, the probability of opening of the cGMP-gated channels ranges from its largest value in darkness of 1-5% to essentially zero under continuous, bright illumination. We investigated the ion selectivity of cGMP-gated channels as a function of cyclic nucleotide concentration in membrane patches detached from the outer segments of rod and cone photoreceptors and have found that ion selectivity is linked to gating. We determined ion selectivity relative to Na+ (PX/PNa) from the value of reversal potentials measured under ion concentration gradients. The selectivity for Ca2+ over Na+ increases continuously as the probability of channel opening rises. The dependence of PCa/PNa on cGMP concentration, in both rods and cones, is well described by the same Hill function that describes the cGMP dependence of current amplitude. At the cytoplasmic cGMP concentrations expected in dark-adapted intact photoreceptors, PCa/PNa in cone channels is approximately 7.4-fold greater than that in rods. The linkage between selectivity and gating is specific for divalent cations. The selectivity of Ca2+ and Sr2+ changes with cGMP concentration, but the selectivity of inorganic monovalent cations, Cs+ and NH4+, and organic cations, methylammonium+ and dimethylammonium+, is invariant with cGMP. Cyclic nucleotide-gated channels in rod photoreceptors are heteromeric assemblies of alpha and beta subunits. The maximal PCa/PNa of channels formed from alpha subunits of bovine rod channels is less than that of heteromeric channels formed from alpha and beta subunits. In addition, Ca2+ is a more effective blocker of channels formed by alpha subunits than of channels formed by alpha and beta subunits. The cGMP-dependent shift in divalent cation selectivity is a property of alphabeta channels and not of channels formed from alpha subunits alone.

Ca2+ permeation in cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels conduct Na+, K+ and Ca2+ currents under the control of cGMP and cAMP. Activation of CNG channels leads to depolarization of the membrane voltage and to a concomitant increase of the cytosolic Ca2+ concentration. Several polypeptides were identified that constitute principal and modulatory subunits of CNG channels in both neurons and non-excitable cells, co-assembling to form a variety of heteromeric proteins with distinct biophysical properties. Since the contribution of each channel type to Ca2+ signaling depends on its specific Ca2+ conductance, it is necessary to analyze Ca2+ permeation for each individual channel type. We have analyzed Ca2+ permeation in all principal subunits of vertebrates and for a principal subunit from Drosophila melanogaster. We measured the fractional Ca2+ current over the physiological range of Ca2+ concentrations and found that Ca2+ permeation is determined by subunit composition and modulated by membrane voltage and extracellular pH. Ca2+ permeation is controlled by the Ca2+-binding affinity of the intrapore cation-binding site, which varies profoundly between members of the CNG channel family, and gives rise to a surprising diversity in the ability to generate Ca2+ signals.

Movement of gating machinery during the activation of rod cyclic nucleotide-gated channels

In the visual and olfactory systems, cyclic nucleotide-gated (CNG) ion channels convert stimulus-induced changes in the internal concentrations of cGMP and cAMP into changes in membrane potential. Although it is known that significant activation of these channels requires the binding of three or more molecules of ligand, the detailed molecular mechanism remains obscure. We have probed the structural changes that occur during channel activation by using sulfhydryl-reactive methanethiosulfonate (MTS) reagents and N-ethylmaleimide (NEM). When expressed in Xenopus oocytes, the alpha-subunit of the bovine retinal channel forms homomultimeric channels that are activated by cGMP with a K1/2 of approximately 100 microM. Cyclic AMP, on the other hand, is a very poor activator; a saturating concentration elicits only 1% of the maximum current produced by cGMP. Treatment of excised patches with MTS-ethyltrimethylamine (MTSET) or NEM dramatically potentiated the channel's response to both cyclic nucleotides. After MTSET treatment, the dose-response relation for cGMP was shifted by over two orders of magnitude to lower concentrations. The effect on channel activation by cAMP was even more striking. After modification, the channels were fully activated by cAMP with a K1/2 of approximately 60 microM. This potentiation was abolished by conversion of Cys481 to a nonreactive alanine residue. Potentiation occurred more rapidly in the presence of saturating cGMP, indicating that this region of the channel is more accessible when the channel is open. Cys481 is located in a linker region between the transmembrane and cGMP-binding domains of the channel. These results suggest that this region of the channel undergoes significant movement during the activation process and is critical for coupling ligand binding to pore opening. Potentiation, however, is not mediated by the recently reported interaction between the amino- and carboxy-terminal regions of the alpha-subunit. Deletion of the entire amino-terminal domain had little effect on potentiation by MTSET.

Three amino acids in the C-linker are major determinants of gating in cyclic nucleotide-gated channels

The activation of cyclic nucleotide-gated (CNG) channels is a complex process comprising the initial ligand binding and a consecutive allosteric transition from a closed to an open configuration. The cone and olfactory CNG channels differ considerably in cyclic nucleotide affinity and efficacy. In each channel, the cyclic nucleotide-binding site is connected to the last transmembrane segment of the channel by a linker peptide (C-linker) of approximately 90 amino acids. Here we report that replacement of three amino acids in the cone C-linker by the corresponding amino acids of the olfactory channel (I439V, D481A and D494S) profoundly enhanced the cAMP efficacy and increased the affinities for cAMP and cGMP. Unlike the wild-type cone channel, the mutated channel exhibited similar single-channel kinetics for both cGMP and cAMP, explaining the increase in cAMP efficacy. We thus conclude that the identified amino acids are major determinants of channel gating.

Tetracaine reports a conformational change in the pore of cyclic nucleotide-gated channels

Local anesthetics are a diverse group of clinically useful compounds that act as pore blockers of both voltage- and cyclic nucleotide-gated (CNG) ion channels. We used the local anesthetic tetracaine to probe the nature of the conformational change that occurs in the pore of CNG channels during the opening allosteric transition. When applied to the intracellular side of wild-type rod CNG channels expressed in Xenopus oocytes from the alpha subunit, the local anesthetic tetracaine exhibits state-dependent block, binding with much higher affinity to closed states than to open states. Here we show that neutralization of a glutamic acid in the conserved P region (E363G) eliminated this state dependence of tetracaine block. Tetracaine blocked E363G channels with the same effectiveness at high concentrations of cGMP, when the channel spent more time open, and at low concentrations of cGMP, when the channel spent more time closed. In addition, Ni2+, which promotes the opening allosteric transition, decreased the effectiveness of tetracaine block of wild-type but not E363G channels. Similar results were obtained in a chimeric CNG channel that exhibits a more favorable opening allosteric transition. These results suggest that E363 is accessible to internal tetracaine in the closed but not the open configuration of the pore and that the conformational change that accompanies channel opening includes a change in the conformation or accessibility of E363.

Mechanism of tetracaine block of cyclic nucleotide-gated channels

Local anesthetics are a diverse group of ion channel blockers that can be used to probe conformational changes in the pore. We examined the effects of the local anesthetic tetracaine on rod and olfactory cyclic nucleotide-gated channels expressed from subunit 1 in Xenopus oocytes. We found that 40 microM tetracaine effectively blocked the bovine rod channel but not the rat olfactory channel at saturating concentrations of cGMP. By testing chimeric channels containing regions of sequence from both rod and olfactory channels, we found that determinants of apparent affinity for tetracaine at saturating cGMP did not map to any one region of the channel sequence. Rather, the differences in apparent affinity could be explained by differences between the chimeras in the free energy of the opening allosteric transition. If a channel construct (such as the rod channel) spent appreciable time in the closed state at saturating cGMP, then it had a high apparent affinity for tetracaine. If, on the other hand, a channel construct (such as the olfactory channel) spent little time in the closed state at saturating cGMP, then it had a low apparent affinity for tetracaine. Furthermore, tetracaine became more effective at low concentrations of cGMP and at saturating concentrations of cAMP, conditions which permit the channels to spend more time in the closed configuration. These results were well fit by a model in which tetracaine binds more tightly to the closed channel than to the open channel. Dose-response curves for tetracaine in the presence of saturating cGMP are well fit with a Michaelis-Menten binding scheme indicating that a single tetracaine molecule is sufficient to produce block. In addition, tetracaine block is voltage dependent with an effective z delta of +0.56. These data are consistent with a pore-block hypothesis. The finding that tetracaine is a state-dependent pore blocker suggests that the inner mouth of the pore of cyclic nucleotide-gated channels undergoes a conformational change during channel opening.

Voltage-dependent block by L- cis -diltiazem of the cyclic GMP-activated conductance of salamander rods

Block by L-cis-diltiazem of the cyclic GMP-activated conductance was studied in excised inside-out patches from the salamander rod outer segment. When L-cis-diltiazem was applied from the cytoplasmic face of the patch, current suppression increased monotonically with membrane depolarization, the ratio of blocked to unblocked current varying e-fold in 50 mV. This suggests that L-cis-diltiazem interacts with a binding site located about half-way across the membrane field, and is unable to fully traverse the cyclic GMP-activated channel. The kinetics of block were accelerated by increasing L-cis-diltiazem concentration and by depolarization. These results can be fitted by a single barrier model in which the barrier peak is located about a third of the way across the membrane field from the cytoplasmic face. Application of L-cis-diltiazem from the extracellular face of the patch also resulted in an enhancement of block with membrane depolarization. Indirect evidence supports the notion that this block resulted from partition of the unchanged form of the blocker across the membrane, and its subsequent interaction with the cytoplasmic face of the conductance.

Subunit interactions in the activation of cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) ion channels of retinal photoreceptors and olfactory neurons are multimeric proteins of unknown stoichiometry. To investigate the subunit interactions that occur during CNG channel activation, we have used tandem cDNA constructs of the rod CNG channel to generate heteromultimeric channels composed of wild-type and mutant subunits. We introduced point mutations that affect channel activation: 1) D604M, which alters the relative ability of agonists to promote the allosteric conformational change(s) associated with channel opening, and 2) T560A, which primarily affects the initial binding affinity for cGMP, and to a lesser extent, the allosteric transition. At saturating concentrations of agonist, heteromultimeric channels were intermediate between wild-type and mutant homomultimers in agonist efficacy and apparent affinity for cGMP, cIMP, and cAMP, consistent with a model for the allosteric transition involving a concerted conformational change in all of the channel subunits. Results were also consistent with a model involving independent transitions in two or three, but not one or four, of the channel subunits. The behavior of the heterodimers implies that the channel stoichiometry is some multiple of 2 and is consistent with a tetrameric quaternary structure for the functional channel complex. Steady-state dose-response relations for homomultimeric and heteromultimeric channels were well fit by a Monod, Wyman, and Changeux model with a concerted allosteric opening transition stabilized by binding of agonist.

Altered ligand specificity by protonation in the ligand binding domain of cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) ion channels are the critical mediators between the second messengers of sensory transduction and the cell's membrane potential. The photoreceptor CNG channels are activated by the direct binding of cGMP but can also be activated to a much lesser extent by cAMP. In rod CNG channels expressed in Xenopus oocytes, we demonstrate two types of potentiation by protons. One type potentiated cGMP-bound and cAMP-bound channels to the same extent, while another potentiated only cAMP-bound channels. Both types of potentiation could be described by a mechanism in which protons bound primarily to the channel open configuration. The potentiation specific to cAMP-bound channels could be accounted for by protonation of aspartic acid 604 (D604). It is the unfavorable electrostatic interaction between the carboxylate of D604 and the purine ring of cAMP that accounts for the normally poor activation of the channels by cAMP. Protonation at this site removed the unfavorable interaction and allowed cAMP to act as nearly a full agonist. Protonation of a second amino acid, H468, contributed to the nucleotide-nonspecific potentiation and is likely to be an element of the channel gating assembly. Protons potentiate native rod channels less than channels formed from subunit 1. In heteromultimeric channels formed by coexpressing subunit 1 with subunit 2, we found a similar attenuation of potentiation. The absence of protonatable amino acids in subunit 2 at positions corresponding to H468 and D604 can explain the reduced effects of pH on native channels.

Covalent activation of retinal rod cGMP-gated channels reveals a functional heterogeneity in the ligand binding sites

Ion channels gated by the binding of multiple ligands play a critical role in synaptic transmission and sensory transduction. It has been difficult to resolve the contribution of individual binding events to channel gating because ligands are continuously binding and unbinding at each site. In examining the allosteric mechanism of retinal rod cGMP-gated channels, we have circumvented this problem by making use of a cGMP derivative, 8-p-azidophenacylthio-cGMP (APT-cGMP), that can be covalently tethered to the binding sites in the presence of long-wavelength UV light. In excised membrane patches, a population of channels was isolated that contained covalently-attached ligands at all but one site. Activation of these channels by cGMP revealed a previously unknown heterogeneity in the ligand-binding sites. The dose-response relations were much shallower than predicted by single-site activation models, but were well described by models in which there are two populations of sites, in roughly equal proportion, that bind cGMP with apparent affinities that differ by a factor of 25. The two apparent affinities, incorporated into a four-site model of the channel, provided an accurate description of the patch's original dose-response relation. A comparison of results on native and expressed channels suggests that the heterogeneity in the native channel arises at least in part from the presence of two different cGMP-binding subunits.

Subunit interactions in coordination of Ni2+ in cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels present a unique model for studying the molecular mechanisms of channel gating. We have studied the mechanism of potentiation of expressed rod CNG channels by Ni2+ as a first step toward understanding the channel gating process. Here we report that coordination of Ni2+ between histidine residues (H420) on adjacent channel subunits occurs when the channels are open. Mutation of H420 to lysine completely eliminated the potentiation by Ni2+ but did not markedly alter the apparent cGMP affinity of the channel, indicating that the introduction of positive charge at the Ni(2+)-binding site was not sufficient to produce potentiation. Deletion or mutation of most of the other histidines present in the channel did not diminish potentiation by Ni2+. We studied the role of subunit interactions in Ni2+ potentiation by generating heteromultimeric channels using tandem dimers of the rod CNG channel sequence. Injection of single heterodimers in which one subunit contained H420 and the other did not (wt/H420Q or H420Q/wt) resulted in channels that were not potentiated by Ni2+. However, coinjection of both heterodimers into Xenopus oocytes resulted in channels that exhibited potentiation. The H420 residues probably occurred predominantly in nonadjacent subunits when each heterodimer was injected individually, but, when the two heterodimers were coinjected, the H420 residues could occur in adjacent subunits as well. These results suggest that the mechanism of Ni2+ potentiation involves intersubunit coordination of Ni2+ by H420. Based on the preferential binding of Ni2+ to open channels, we suggest that alignment of H420 residues of neighboring subunits into the Ni(2+)-coordinating position may be associated with channel opening.

Molecular mechanism for ligand discrimination of cyclic nucleotide-gated channels

Cyclic nucleotide-gated ion channels of retinal photoreceptors and olfactory neurons are differentially activated by ligands that vary only in their purine ring structure. The nucleotide selectivity of the bovine rod cyclic nucleotide-gated channel (cGMP > cIMP >> cAMP) was significantly altered by neutralization of a single aspartic acid residue in the binding domain (cGMP > or = cAMP > cIMP). Substitution by a nonpolar residue at this position inverted agonist selectivity (cAMP >> cIMP > or = cGMP). These effects resulted from an alteration in the relative ability of the agonists to promote the allosteric conformational change associated with channel activation, not from a modification in their initial binding affinity. We propose a general mechanism for guanine nucleotide discrimination, in common with that observed in high affinity GTP-binding proteins, involving the formation of a pair of hydrogen bonds between the aspartic acid side chain and N1 and N2 of the guanine ring.

Modulation of the cGMP-gated ion channel in frog rods by calmodulin and an endogenous inhibitory factor

1. Outer segment patches excised in the light were used to investigate the effects of exogenous calmodulin and an endogenous inhibitory factor on the cGMP-gated channel of frog rods. 2. Calmodulin shifted to the right the dose-response relation for activation of the channels by 8-Br-cGMP, but did not change the maximum current or the form of the relation. Reversal of this effect by removal of calmodulin was accelerated by brief exposure to saturating [8-Br-cGMP]. Inhibition by calmodulin required calcium and gave as much as a 5-fold decrease in current for an [8-Br-cGMP] functionally comparable to the presumed physiological [cGMP]. 3. Exposure to low [Ca2+]i (tens of nanomolar) appeared to irreversibly remove or inactivate an endogenous channel inhibitory factor from the patches, increasing the current at low [8-Br-cGMP]. Like calmodulin, this factor slowed the voltage-dependent channel-gating kinetics and did not change the maximum current. However, unlike calmodulin, the endogenous factor remained stably associated with the patches at high [Ca2+]i (1 microM), even with exposure to saturating [8-Br-cGMP]. 4. After the low-Ca2+ treatment increased the current, calmodulin reduced the current to about the same level as it had before the low-Ca2+ treatment, giving a larger fractional suppression. Furthermore, patches with high initial sensitivity to 8-Br-cGMP had small low-Ca2+ effects and large calmodulin effects, while the reverse was true for patches with low initial agonist sensitivity. 5. Application of trypsin to the intracellular surface of the patch prevented the responses to calmodulin and to low [Ca2+]i, suggesting involvement of a cytoplasmic portion of the channel. However, trypsin also reduced the total agonist-induced patch current. 6. Our results are consistent with a model in which calmodulin and an endogenous calcium-binding protein compete for the same site, inhibiting channel opening or cGMP binding. The tight association of the endogenous factor with the channel even at relatively low [Ca2+]i suggests that in the transducing rod it may inhibit the channels most of the time in darkness and in dim light, preventing any potential inhibitory effects of calmodulin. The endogenous factor would be expected to leave the channel only in bright or prolonged light, when the [Ca2+]i is thought to be very low.

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

The electrical response to light in retinal rods is mediated by cyclic nucleotide-gated, nonselective cation channels in the outer segment plasma membrane. Although cGMP appears to be the primary light-regulated second messenger, cellular levels of other substances, including Ca2+ and phosphatidylinositol-4,5-bisphosphate, are also sensitive to the level of illumination. We now show that diacylglycerol (DAG) analogs reversibly suppress the cGMP-activated conductance in excised patches from frog rod outer segments. This suppression did not require nucleoside triphosphates, indicating that a phosphorylation reaction was not involved. DAG was more effective at low than at high [cGMP]: with 50 microM 8-Br-cGMP, the DAG analog 1,2-dioctanoyl-sn-glycerol (1,2-DiC8) reduced the current with an IC50 of approximately 22 microM (Hill coefficient, 0.8), whereas with 1.2 microM 8-Br-cGMP, only approximately 1 microM 1,2-DiC8 was required to halve the current. DAG reduced the apparent affinity of the channels for cGMP: 4 microM 1,2-DiC8 produced a threefold increase in the K1/2 for channel activation by 8-Br-cGMP, as well as a threefold reduction in the maximum current, without changing the apparent stoichiometry or cooperativity of cGMP binding. Inhibition by 1,2-DiC8 was not relieved by supersaturating concentrations of 8-Br-cGMP, suggesting that DAG did not act by competitive inhibition of cGMP binding. Furthermore, DAG did not seem to significantly reduce single-channel conductance. A DAG analog similar to 1,2-DiC8--1,3-dioctanoyl-sn-glycerol (1,3-DiC8)--suppressed the current with the same potency as 1,2-DiC8, whereas an ethylene glycol of identical chain length (DiC8-EG) was much less effective. Our results suggest that DAG allosterically interferes with channel opening, and raise the question of whether DAG is involved in visual transduction.

The permeation of organic cations through cAMP-gated channels in mammalian olfactory receptor neurons

The permeation of monovalent organic cations through adenosine 3',5'-cyclic monophosphate-(cAMP) activated channels was studied by recording macroscopic currents in excised inside-out membrane patches from the dendritic knobs of isolated mammalian olfactory receptor neurons (ORNs). Current-voltage relations were measured when bathing solution Na+ was replaced by monovalent organic cations. Permeability ratios relative to Na+ ions were calculated from changes in reversal potentials. Some of the small organic cations tested included ammonium (NH4+), hydroxylammonium and formamidinium, with relative permeability ratios of 1.41, 2.3 and 1.01 respectively. The larger methylated and ethylated ammonium ions studied included: DMA (dimethylammonium), TMA (tetramethylammonium) and TEA (tetraethylammonium) and they all had permeability ratios larger than 0.09. Even large cations such as choline, arginine and tris(hydroxymethyl)aminomethane (Tris) were appreciably permeant through the cAMP-activated channel with permeability ratios ranging from 0.19 to 0.7. The size of the permeating cations, as assessed by molecular weight, was a good predictor of the permeability. The permeability sequence of the cAMP-activated channel in our study was PNH4 > PNa > PDMA > PTMA > PCholine > PTEA. Higher permeability ratios of hydroxylammonium, arginine and tris(hydroxymethyl)aminomethane cannot be explained by ionic size alone. Our results indicate that: (i) cAMP-activated channels poorly select between monovalent cations; (ii) the pore dimension must be at least 6.5 x 6.5 A, in order to allow TEA and Tris to permeate and (iii) molecular sieving must be an important mechanism for the permeation of large organic ions through the channels with specific ion binding playing a smaller role than in other structurally similar channels. In addition, the results clearly indicate that cyclic nucleotide-gated (CNG) channels in different cells are not the same, the olfactory CNG channel being different from that of the photoreceptors, particularly with respect to the permeation of large organic cations, which the ORN channels allow to permeate readily.

Localization of regions affecting an allosteric transition in cyclic nucleotide-activated channels

Sensory transduction in olfactory receptors and photoreceptors is mediated by cyclic nucleotide-activated ion channels. We have studied the gating mechanism in olfactory and rod channels expressed in Xenopus oocytes. We report that the differences in cyclic nucleotide affinity and efficacy between these channels resulted from sequence differences outside the cyclic nucleotide-binding domain, especially in the amino-terminal domain, influencing the free energy of the closed to open allosteric conformational change. In addition, Ni2+ inhibited activation of the olfactory channel, decreasing both the maximum current and the apparent affinity for cyclic nucleotides. Ni2+ exerted its effect by binding preferentially to the closed configuration of the channel, thereby destabilizing the opening conformational change. We have localized this inhibition to a single histidine (H396) following the last transmembrane segment, suggesting a role for this region in channel gating.

A histidine residue associated with the gate of the cyclic nucleotide-activated channels in rod photoreceptors

Ion channels directly activated by the binding of cGMP mediate the electrical response to light in rod photoreceptors. Here, we identify a region of the channel associated with the activation gate using potentiation by intracellular Ni2+. Low concentrations of Ni2+ caused a dramatic increase in the response of rod channels expressed in Xenopus oocytes to both cGMP and cAMP. Whereas saturating cAMP normally activated less than 1% of the channels, Ni2+ increased the cAMP potency nearly 50-fold. Ni2+ did not produce potentiation in the related channel from the olfactory epithelium. We localized the Ni(2+)-binding site to a histidine residue in the putative intracellular mouth of the rod channel (H420). We propose a mechanism for potentiation in which Ni2+ binds to H420 primarily when the channel is open, stabilizing the open conformation. These experiments suggest that H420 is associated with the activation gate.

Modulation of guanylate cyclase and phosphodiesterase by monovalent cations and nucleoside triphosphates in light-sensitive excised patches of rod outer segments

Excised inside-out patches of vertebrate rod outer segment can support phototransduction. I have examined how ionic and metabolic conditions influence the functional properties of light-sensitive patches from Gekko gekko. I find that such patches retain a variable level of basal phosphodiesterase activity, which lowers the cyclic guanosine monophosphate (cGMP) concentration reaching the channels and reduces the dark current. The dose/response relationship for channel opening by cGMP varies among patches and this variability is only reduced by working in darkness with the phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX), suggesting that it is only partially due to phosphodiesterase activity. MgATP or MgGTP, but not Mg or ATP separately, increase this activity but a kinase does not appear to be involved. Intracellular monovalent cations also influence dark current intensity and light response kinetics. With 5 mM MgGTP, 1 mM IBMX, and 144 mM Li+, Na+, K+, or Rb+, dark current intensity and recovery time follow the respective sequences K+ > Rb+ > Na+ > Li+ and K+ < Rb+ < Li+ < Na+. Without IBMX, a dark current develops with K+ but not with Na+. These effects are not due to altered channel permeability (P) [PLi+:Na+:K+:Rb+:guanidinium)/PNa+ = 0.84:1.00:1.01:1.09:0.42], or differential Mg2+ block, but to modulation of guanylate cyclase, which overcomes phosphodiesterase when the major cation is K+ but not when it is Na+.

Analysis of fluctuations in the cGMP-dependent currents of cone photoreceptor outer segments

We measured cGMP-dependent currents, under voltage clamp, in membrane patches detached from the outer segment of single-cone photoreceptors isolated from the retina of striped bass. We analyzed the variance of the current about its mean and the spectral density distribution of the current fluctuations. From the analysis of variance, we determined that the cGMP-gated channels increase their probability of opening with increasing cGMP up to a maximum value of 0.87 +/- 0.03. The dependence on cGMP of the probability of opening is well described by a Hill equation with Km = 60.2 +/- 3.7 microM and n = 2.33 +/- 0.32 at -50 mV. At the same voltage, the spectral density distribution is well fit by the sum of two Lorentzians with corner frequencies at 26 +/- 18 and 318 +/- 58 Hz. The single-channel conductance calculated from the current noise by two different methods suggests that the most frequently occupied conductance state has an amplitude of about 18 pS.

The permeability of the cGMP-activated channel to organic cations in retinal rods of the tiger salamander

1. The permeability of the channel activated by guanosine 3',5'-cyclic monophosphate (cGMP) to many organic monovalent cations was determined by recording macroscopic currents in excised inside-out patches of plasma membrane from isolated retinal rod outer segments of the tiger salamander. 2. Current-voltage relations were measured when the NaCl of the bathing medium was replaced by salts of organic cations. Permeability ratios relative to Na+ ions were calculated with the Goldman-Hodgkin-Katz potential equation from the measured changes of reversal potentials. 3. Hydroxylammonium+, hydrazinium+ and methylammonium+, which are molecules of very similar shape and size, permeate the channel with very different permeability ratios: 5.92, 1.99 and 0.60 respectively. 4. Methylated and ethylated ammonium+ compounds were investigated. It was found that, not only methylammonium+, but also dimethylammonium+ and ethylammonium+ were permeant with permeability ratios of 0.6, 0.14 and 0.16 respectively. Trimethylammonium+, tetramethylammonium+, diethylammonium+, triethylammonium+, and tetraethylammonium+ were not permeant. 5. Guanidinium+ and its derivatives formamidinium+, aminoguanidinium+, acetamidinium+ and methylguanidinium+ were all permeant with permeability ratios 1.12, 1.00, 0.63, 0.36 and 0.33 respectively. 6. The cGMP-activated channel was found to be permeable to at least thirteen organic cations. Molecular models of the permeant cations indicate that the cross-section of the narrowest part of the pore must be at least as large as a rectangle of 0.38 x 0.5 nm dimensions.

Interactions between divalent cations and the gating machinery of cyclic GMP-activated channels in salamander retinal rods

The effects of divalent cations on the gating of the cGMP-activated channel, and the effects of gating on the movement of divalent cations in and out of the channel's pore were studied by recording macroscopic currents in excised membrane patches from salamander retinal rods. The fractional block of cGMP-activated Na+ currents by internal and external Mg2+ as well as internal Ca2+ was nearly independent of cGMP concentration. This indicates that Mg2+ and Ca2+ bind with similar affinity to open and closed states of the channel. In contrast, the efficiency of block by internal Cd2+ or Zn2+ increased in proportion to the fraction of open channels, indicating that these ions preferentially occupy open channels. The kinetics of block by internal Ni2+, which competes with Mg2+ but blocks more slowly, were found to be unaffected by the fraction of channels open. External Ni2+, however, blocked and unblocked much more rapidly when channels were mostly open. This suggests that within the pore a gate is located between the binding site(s) for ions and the extracellular mouth of the channel. Micromolar concentrations of the transition metal divalent cations Ni2+, Cd2+, Zn2+, and Mn2+ applied to the cytoplasmic surface of a patch potentiated the response to subsaturating concentrations of cGMP without affecting the maximum current induced by saturating cGMP. The concentration of cGMP that opened half the channels was often lowered by a factor of three or more. Potentiation persisted after the experimental chamber was washed with divalent-free solution and fresh cGMP was applied, indicating that it does not result from an interaction between divalent cations and cGMP in solution; 1 mM EDTA or isotonic MgCl2 reversed potentiation. Voltage-jump experiments suggest that potentiation results from an increase in the rate of cGMP binding. Lowering the ionic strength of the bathing solution enhanced potentiation, suggesting that it involves electrostatic interactions. The strong electrostatic effect on cGMP binding and absence of effect on ion permeation through open channels implies that the cGMP binding sites on the channel are well separated from the permeation pathway.

Protein phosphatases modulate the apparent agonist affinity of the light-regulated ion channel in retinal rods

Ion channels directly activated by cGMP mediate the light response in retinal rods. Several components of the enzyme cascade controlling cGMP concentration are regulated, but there are no accepted mechanisms for modulation of the response of the channel to cGMP. Here we report evidence that in excised patches an endogenous protein phosphatase converts the channel from a state with low cGMP sensitivity to a state with almost 3 orders of magnitude higher sensitivity in the predicted physiological range of cGMP concentration. The action of this endogenous phosphatase was blocked by specific serine/threonine phosphatase inhibitors (microcystin-LR, okadaic acid, and calyculin A). An increase in apparent agonist affinity also was produced by addition of purified protein phosphatase 1. In contrast, protein phosphatase 2A decreased apparent agonist affinity, suggesting that two phosphorylation sites may regulate the agonist sensitivity of the channel in a reciprocal manner. This regulation may be involved in fine-tuning the light response or in light or dark adaptation.

Cation interactions within the cyclic GMP-activated channel of retinal rods from the tiger salamander

1. The ionic dependence of current through the 3',5'-cyclic guanosine monophosphate (cyclic GMP)-activated channels of salamander rods was studied in excised inside-out membrane patches from isolated outer segments. Voltage-clamp experiments on transducing rods were performed so that the channels in intact cells could be compared with those in excised patches. 2. The reversal potential of the cyclic GMP-induced patch current was close to the Na+ equilibrium potential when the concentration of NaCl on the cytoplasmic surface of a patch was varied at constant external NaCl concentration. Fitting the Goldman-Hodgkin-Katz equation indicated that the apparent ratio of permeabilities for Na+ and Cl- was at least 50. This confirms a previous report that the channel's Na+ permeability is much larger than its Cl- permeability. 3. Na+ currents through the channel did not obey the independence principle. The outward patch current at large positive potential began to saturate with increasing concentrations of internal Na+, as if permeation required Na+ to bind to a site with an apparent dissociation constant around 180 mM. 4. In symmetrical NaCl solutions containing very low concentrations of divalent cations the current-voltage relation measured from excised patches 50 microseconds after switching the voltage showed mild outward rectification. By 1 ms the rectification was more pronounced. The rectification at 50 microseconds is attributed to voltage dependence of Na+ permeation. The additional rectification at later times is attributed to voltage dependence of the channel's probability of being open, depolarization favouring the open state. 5. In symmetrical Mg2+ solutions the cyclic GMP-induced patch currents were smaller and the outward rectification was more pronounced. 6. Addition of Mg2+ or Ca2+ to an internal Na+ solution blocked the cyclic GMP-induced Na+ current through the channels, as if by occupying a single binding site with an affinity in the 0.1-2 mM range. Block by Mg2+ was voltage dependent, suggesting that the binding site was within the channel's transmembrane electric field. Raising the Mg2+ concentration on the external surface of the patch increased the apparent dissociation constant of block by internal Mg2+, as expected if external and internal Mg2+ compete for the same binding site. 7. Block by internal Ca2+ had an opposite and weaker voltage dependence than block by internal Mg2+. 8. In symmetrical solutions containing both Na+ and Mg2+ the outward rectification was more pronounced than in solutions containing Na+ alone. In solutions thought to be close to physiological the outward patch current increased e-fold for a depolarization of 24-30 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Cyclic GMP-activated channels of salamander retinal rods: spatial distribution and variation of responsiveness

1. Patch-clamp methods were used to investigate the areal density and spatial location of cyclic GMP-activated channels in the surface membrane of salamander rod outer segments. 2. The density of active channels (i.e. channels able to respond to cyclic GMP) in patches excised from outer segments was determined from the number of active channels, N, and the membrane area, A. N was estimated from the current induced by a saturating concentration of cyclic GMP, while A was estimated from the electrical capacitance of the patch. 3. In patches excised from forty-one isolated outer segments prepared in the light the active channel density varied over a remarkable range: 0.34-629 microns-2, with a mean of 166 microns-2. Density was not correlated with patch area in this or any of the conditions studied. 4. The spatial distribution of open channels on the outer segment of a transducing rod was measured by recording the local dark current at various positions with a loose-patch electrode. The apparent density of open channels varied by only about +/- 50% around the circumference of the outer segment and up and down its length. This indicates that the wide range of densities in excised patches did not result from sampling a non-uniform spatial distribution of channels. 5. Patches excised from sixteen dark-adapted whole cells with healthy appearances and saturating light responses of normal size had active channel densities of 1.1-200 microns-2, with a mean of 60 microns-2. Patches from twenty light-adapted whole cells had similar densities. Many densities from the whole cells were much lower than expected. This, and the wide variation in densities, suggests that obtaining a patch often lowered the density of active channels. The number of channels in a patch was quite stable from 1 s to 30 min after excision, ruling out progressive denaturation or adsorption of channels to the glass as a cause for this effect. 6. The mean active channel density in patches excised from whole cells was lower with calcium present in the external solution than with calcium absent (80 vs. 152 microns-2, n = 36 and 30 respectively). 7. We conclude that copies of the channel protein were present at a density of at least 650 microns-2 in the surface membrane of the outer segment and that the distribution of channels was fairly uniform on a 1 micron scale.(ABSTRACT TRUNCATED AT 400 WORDS)

Association of cyclic GMP-gated channels and Na(+)-Ca(2+)-K+ exchangers in bovine retinal rod outer segment plasma membranes

1. Cyclic GMP-gated channels and Na(+)-Ca(2+)-K+ exchangers from bovine photoreceptors were examined by investigation of the Ca2+ fluxes from vesicles of rod outer segment (ROS) membranes and from proteoliposomes obtained by solubilization of the ROS membrane proteins and reconstitution in soy bean L-alpha-phosphatidylcholine (PC). 2. Whereas vesicles obtained by mild sonication of ROS membranes in a Ca(2+)-containing buffer yielded a maximal cyclic GMP-induced Ca2+ release of about 2.5% and a maximal Na(+)-induced Ca2+ release of about 7%, freezing and thawing of ROS membranes prior to sonication elevated these maximal Ca2+ releases to about 17% for cyclic GMP, and to about 34% for Na+. These observations are in agreement with the view that cyclic GMP-gated channels and Na(+)-Ca(2+)-K+ exchangers are localized only in the plasma membrane of the photoreceptors (which in bovine ROS makes up about 6% of the total membrane), whereas freezing and thawing results in fusion of disc and plasma membranes, thus leading to a distribution of these proteins over a much larger membrane area. 3. For fused ROS membranes, the cyclic GMP-releasable fraction of Ca2+ of 17% is an upper bound; assuming that the cyclic GMP-gated channels are randomly distributed we estimate that about 37% of the vesicles contain at least one cyclic GMP-gated channel. The mean diameter of the vesicles prepared by sonication was determined to be 0.12 +/- 0.04 micron, and therefore the fused ROS membranes contain about sixteen cyclic GMP-gated channels/microns 2. If all cyclic GMP-gated channels originated from the plasma membrane, we estimate that the plasma membrane contains about 270 cyclic GMP-gated channels/microns 2. 4. In vesicles prepared from fused ROS membranes, Na(+)-Ca2+ exchange after activation of the cyclic GMP-gated channels. On the other hand, after an exhaustive Na(+)-Ca2+ exchange, only little, if any, Ca2+ was released upon addition of cyclic GMP, demonstrating that cyclic GMP-gated channels and Na(+)-Ca(2+)-K+ exchangers occur on the same vesicle fraction. This observation suggests that Na(+)-Ca(2+)-K+ exchangers do not distribute independently of the cyclic GMP-gated channels upon membrane fusion but are apparently associated with the cyclic GMP-gated channels.(ABSTRACT TRUNCATED AT 400 WORDS)