Calcium modulation of ligand affinity in the cyclic GMP-gated ion channels of cone photoreceptors

Structural basis of calmodulin modulation of the rod cyclic nucleotide-gated channel

Calmodulin (CaM) regulates many ion channels to control calcium entry into cells, and mutations that alter this interaction are linked to fatal diseases. The structural basis of CaM regulation remains largely unexplored. In retinal photoreceptors, CaM binds to the CNGB subunit of cyclic nucleotide-gated (CNG) channels and, thereby, adjusts the channel's Cyclic guanosine monophosphate (cGMP) sensitivity in response to changes in ambient light conditions. Here, we provide the structural characterization for CaM regulation of a CNG channel by using a combination of single-particle cryo-electron microscopy and structural proteomics. CaM connects the CNGA and CNGB subunits, resulting in structural changes both in the cytosolic and transmembrane regions of the channel. Cross-linking and limited proteolysis-coupled mass spectrometry mapped the conformational changes induced by CaM in vitro and in the native membrane. We propose that CaM is a constitutive subunit of the rod channel to ensure high sensitivity in dim light. Our mass spectrometry-based approach is generally relevant for studying the effect of CaM on ion channels in tissues of medical interest, where only minute quantities are available.

CNGA3 achromatopsia-associated mutation potentiates the phosphoinositide sensitivity of cone photoreceptor CNG channels by altering intersubunit interactions

Cyclic nucleotide-gated (CNG) channels are critical for sensory transduction in retinal photoreceptors and olfactory receptor cells; their activity is modulated by phosphoinositides (PIPn) such as phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylinositol 3,4,5-trisphosphate (PIP3). An achromatopsia-associated mutation in cone photoreceptor CNGA3, L633P, is located in a carboxyl (COOH)-terminal leucine zipper domain shown previously to be important for channel assembly and PIPn regulation. We determined the functional consequences of this mutation using electrophysiological recordings of patches excised from cells expressing wild-type and mutant CNG channel subunits. CNGA3-L633P subunits formed functional channels with or without CNGB3, producing an increase in apparent cGMP affinity. Surprisingly, L633P dramatically potentiated PIPn inhibition of apparent cGMP affinity for these channels. The impact of L633P on PIPn sensitivity depended on an intact amino (NH2) terminal PIPn regulation module. These observations led us to hypothesize that L633P enhances PIPn inhibition by altering the coupling between NH2- and COOH-terminal regions of CNGA3. A recombinant COOH-terminal fragment partially restored normal PIPn sensitivity to channels with COOH-terminal truncation, but L633P prevented this effect. Furthermore, coimmunoprecipitation of channel fragments, and thermodynamic linkage analysis, also provided evidence for NH2-COOH interactions. Finally, tandem dimers of CNGA3 subunits that specify the arrangement of subunits containing L633P and other mutations indicated that the putative interdomain interaction occurs between channel subunits (intersubunit) rather than exclusively within the same subunit (intrasubunit). Collectively, these studies support a model in which intersubunit interactions control the sensitivity of cone CNG channels to regulation by phosphoinositides. Aberrant channel regulation may contribute to disease progression in patients with the L633P mutation.

Temporal sampling, resetting, and adaptation orchestrate gradient sensing in sperm

Sperm use temporal sampling, resetting of intracellular calcium level, and adaptation of their sensitivity to respond to a wide range of chemoattractant concentrations during their voyage toward the egg.

Sperm, navigating in a chemical gradient, are exposed to a periodic stream of chemoattractant molecules. The periodic stimulation entrains Ca²⁺ oscillations that control looping steering responses. It is not known how sperm sample chemoattractant molecules during periodic stimulation and adjust their sensitivity. We report that sea urchin sperm sampled molecules for 0.2–0.6 s before a Ca²⁺ response was produced. Additional molecules delivered during a Ca²⁺ response reset the cell by causing a pronounced Ca²⁺ drop that terminated the response; this reset was followed by a new Ca²⁺ rise. After stimulation, sperm adapted their sensitivity following the Weber–Fechner law. Taking into account the single-molecule sensitivity, we estimate that sperm can register a minimal gradient of 0.8 fM/µm and be attracted from as far away as 4.7 mm. Many microorganisms sense stimulus gradients along periodic paths to translate a spatial distribution of the stimulus into a temporal pattern of the cell response. Orchestration of temporal sampling, resetting, and adaptation might control gradient sensing in such organisms as well.

Speed, sensitivity, and stability of the light response in rod and cone photoreceptors: facts and models

The light responses of rod and cone photoreceptors in the vertebrate retina are quantitatively different, yet extremely stable and reproducible because of the extraordinary regulation of the cascade of enzymatic reactions that link photon absorption and visual pigment excitation to the gating of cGMP-gated ion channels in the outer segment plasma membrane. While the molecular scheme of the phototransduction pathway is essentially the same in rods and cones, the enzymes and protein regulators that constitute the pathway are distinct. These enzymes and regulators can differ in the quantitative features of their functions or in concentration if their functions are similar or both can be true. The molecular identity and distinct function of the molecules of the transduction cascade in rods and cones are summarized. The functional significance of these molecular differences is examined with a mathematical model of the signal-transducing enzymatic cascade. Constrained by available electrophysiological, biochemical and biophysical data, the model simulates photocurrents that match well the electrical photoresponses measured in both rods and cones. Using simulation computed with the mathematical model, the time course of light-dependent changes in enzymatic activities and second messenger concentrations in non-mammalian rods and cones are compared side by side.

CNG-modulin: a novel Ca-dependent modulator of ligand sensitivity in cone photoreceptor cGMP-gated ion channels

The transduction current in several different types of sensory neurons arises from the activity of cyclic nucleotide-gated (CNG) ion channels. The channels in these sensory neurons vary in structure and function, yet each one demonstrates calcium-dependent modulation of ligand sensitivity mediated by the interaction of the channel with a soluble modulator protein. In cone photoreceptors, the molecular identity of the modulator protein was previously unknown. We report the discovery and characterization of CNG-modulin, a novel 301 aa protein that interacts with the N terminus of the β subunit of the cGMP-gated channel and modulates the cGMP sensitivity of the channels in cone photoreceptors of striped bass (Morone saxatilis). Immunohistochemistry and single-cell PCR demonstrate that CNG-modulin is expressed in cone but not rod photoreceptors. Adding purified recombinant CNG-modulin to cone membrane patches containing the native CNG channels shifts the midpoint of cGMP dependence from ∼91 μM in the absence of Ca(2+) to ∼332 μM in the presence of 20 μM Ca(2+). At a fixed cGMP concentration, the midpoint of the Ca(2+) dependence is ∼857 nM Ca(2+). These restored physiological features are statistically indistinguishable from the effects of the endogenous modulator. CNG-modulin binds Ca(2+) with a concentration dependence that matches the calcium dependence of channel modulation. We conclude that CNG-modulin is the authentic Ca(2+)-dependent modulator of cone CNG channel ligand sensitivity. CNG-modulin is expressed in other tissues, such as brain, olfactory epithelium, and the inner ear, and may modulate the function of ion channels in those tissues as well.

Native cone photoreceptor cyclic nucleotide-gated channel is a heterotetrameric complex comprising both CNGA3 and CNGB3: a study using the cone-dominant retina of Nrl-/- mice

Cone vision mediated by photoreceptor cyclic nucleotide-gated (CNG) channel activation is essential for central and color vision and visual acuity. Mutations in genes encoding the cone CNG channel subunits, CNGA3 and CNGB3, have been linked to various forms of achromatopsia and progressive cone dystrophy in humans. This study investigates the biochemical components of native cone CNG channels, using the cone-dominant retina in mice deficient in the transcription factor neural retina leucine zipper (Nrl). Abundant expression of CNGA3 and CNGB3 but no rod CNG channel expression was detected in Nrl-/- retina by western blotting and immunolabeling. Localization of cone CNG channel in both blue (S)- and red/green (M)-cones was shown by double immunolabeling using antibodies against the channel subunits and against the S- and M-opsins. Immunolabeling also showed co-localization of CNGA3 and CNGB3 in the mouse retina. Co-immunoprecipitation demonstrated the direct interaction between CNGA3 and CNGB3. Chemical cross-linking readily generated products at sizes consistent with oligomers of the channel complexes ranging from dimeric to tetrameric complexes, in a concentration- and time-dependent pattern. Thus this work provides the first biochemical evidence showing the inter-subunit interaction between CNGA3 and CNGB3 and the presence of heterotetrameric complexes of the native cone CNG channel in retina. No association between CNGA3 and the cone Na(+)/Ca(2+)-K(+) exchanger (NCKX2) was shown by co-immunoprecipitation and chemical cross-linking. This may implicate a distinct modulatory mechanism for Ca(2+) homeostasis in cones compared to rods.

Olfactory CNG channel desensitization by Ca2+/CaM via the B1b subunit affects response termination but not sensitivity to recurring stimulation

Ca2+/calmodulin-mediated negative feedback is a prototypical regulatory mechanism for Ca2+-permeable ion channels. In olfactory sensory neurons (OSNs), such regulation on the cyclic nucleotide-gated (CNG) channel is considered a major mechanism of OSN adaptation. To determine the role of Ca2+/calmodulin desensitization of the olfactory CNG channel, we introduced a mutation in the channel subunit CNGB1b in mice that rendered the channel resistant to fast desensitization by Ca2+/calmodulin. Contrary to expectations, mutant OSNs showed normal receptor current adaptation to repeated stimulation. Rather, they displayed slower response termination and, consequently, reduced ability to transmit olfactory information to the olfactory bulb. They also displayed reduced response decline during sustained odorant exposure. These results suggest that Ca2+/calmodulin-mediated CNG channel fast desensitization is less important in regulating the sensitivity to recurring stimulation than previously thought and instead functions primarily to terminate OSN responses.

Regulation of human cone cyclic nucleotide-gated channels by endogenous phospholipids and exogenously applied phosphatidylinositol 3,4,5-trisphosphate

Cyclic nucleotide-gated (CNG) channels are critical components of the vertebrate visual transduction cascade involved in converting light-induced changes in intracellular cGMP concentrations into electrical signals that can be interpreted by the brain as visual information. To characterize regulatory mechanisms capable of altering the apparent ligand affinity of cone channels, we have expressed heteromeric (CNGA3 + CNGB3) human cone CNG channels in Xenopus laevis oocytes and characterized the alterations in channel activity that occur after patch excision using patch-clamp recording in the inside-out configuration. We found that cone channels exhibit spontaneous changes in current at subsaturating cGMP concentrations; these changes are enhanced by application of ATP and seem to reflect alterations in channel gating. Similar to rod CNG channels, lavendustin A prevented this regulation, suggesting the involvement of a tyrosine phosphorylation event. However, the tyrosine residue in CNGB3 (Tyr545) that is equivalent to the critical tyrosine residues in rod and olfactory CNG channel subunits does not participate in cone channel regulation. Furthermore, the changes in ligand sensitivity of CNGA3 + CNGB3 channels were prevented by inhibition of phosphatidylinositol 3-kinase (PI3-kinase) using wortmannin or 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride (LY294002), which suggests that phospholipid metabolism can regulate the channels. Direct application of phosphatidylinositol 3,4,5-trisphosphate (PIP3) to the intracellular face of excised patches also resulted in down-regulation of channel activity. Thus, phospholipid metabolism and exogenously applied PIP3 can modulate heterologously expressed cone CNG channels.

Cloning and molecular characterization of cGMP-gated ion channels from rod and cone photoreceptors of striped bass ( M. saxatilis ) retina

Vertebrate photoreceptors respond to light with changes in membrane conductance that reflect the activity of cyclic-nucleotide gated channels (CNG channels). The functional features of these channels differ in rods and cones; to understand the basis of these differences we cloned CNG channels from the retina of striped bass, a fish from which photoreceptors can be isolated and studied electrophysiologically. Through a combination of experimental approaches, we recovered and sequenced three full-length cDNA clones. We made unambiguous assignments of the cellular origin of the clones through single photoreceptor RT-PCR. Synthetic peptides derived from the sequence were used to generate monospecific antibodies which labeled intact, unfixed photoreceptors and confirmed the cellular assignment of the various clones. In rods, we identified the channel alpha subunit gene product as 2040 bp in length, transcribed into two mRNA 1.8 kb and 2.9 kb in length and translated into a single 96-kDa protein. In cones we identified both alpha (CNGA3) and beta (CNGB3) channel subunits. For alpha, the gene product is 1956 bp long, the mRNA 3.4 kb, and the protein 74 kDa. For beta, the gene product is 2265 bp long and the mRNA 3.3 kb. Based on deduced amino acid sequence, we developed a phylogenetic map of the evolution of vertebrate rod and cone CNG channels. Sequence comparison revealed channels in striped bass, unlike those in mammals, are likely not N-linked-glycosylated as they are transported within the photoreceptor. Also bass cone channels lack certain residues that, in mammals, can be phosphorylated and, thus, affect the cGMP sensitivity of gating. On the other hand, functionally critical residues, such as positively charged amino acids within the fourth transmembrane helix (S4) and the Ca(2+)-binding glutamate in the pore loop are absolutely the same in mammalian and nonmammalian species.

Analysis of macroscopic ionic currents mediated by GABArho1 receptors during lanthanide modulation predicts novel states controlling channel gating

Lanthanide-induced modulation of GABA(C) receptors expressed in Xenopus oocytes was studied. We obtained two-electrode voltage-clamp recordings of ionic currents mediated by recombinant homomeric GABArho(1) receptors and performed numerical simulations of kinetic models of the macroscopic ionic currents.GABA-evoked chloride currents were potentiated by La(3+), Lu(3+) and Gd(3+) in the micromolar range. Lanthanide effects were rapid, reversible and voltage independent. The degree of potentiation was reduced by increasing GABA concentration.Lu(3+) also induced receptor desensitization and decreased the deactivation rate of GABArho(1) currents. In the presence of 300 microM Lu(3+), dose-response curves for GABA-evoked currents showed a significant enhancement of the maximum amplitude and an increase of the apparent affinity. The rate of onset of TPMPA and picrotoxin antagonism of GABArho(1) receptors was modulated by Lu(3+). These results suggest that the potentiation of the anionic current was the result of a direct lanthanide-receptor interaction at a site capable of allosterically modulating channel properties. Based on kinetic schemes, which included a second open state and a nonconducting desensitized state that closely reproduced the experimental results, two nonexclusive probable models of GABArho(1) channels gating are proposed.

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.

Regulation of cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels are found in several cell types, and are best studied in photoreceptors and olfactory sensory neurons. There, CNG channels are gated by the second messengers of the visual and olfactory signalling cascades, cGMP and cAMP respectively, and operate as transduction channels generating the stimulus-induced receptor potentials. In visual and olfactory sensory cells CNG channels conduct cationic currents. Calcium can contribute a large fraction of this current, and calcium influx serves a modulatory role in CNG-channel mediated signal transduction. There have been recent developments in our understanding of how the regulation of CNG channels contributes to the physiological properties of photoreceptors and olfactory sensory cells, and in particular on the role of calcium-mediated feedback.

In intact mammalian photoreceptors, Ca2+-dependent modulation of cGMP-gated ion channels is detectable in cones but not in rods

In the mammalian retina, cone photoreceptors efficiently adapt to changing background light intensity and, therefore, are able to signal small differences in luminance between objects and backgrounds, even when the absolute intensity of the background changes over five to six orders of magnitude. Mammalian rod photoreceptors, in contrast, adapt very little and only at intensities that nearly saturate the amplitude of their photoresponse. In search of a molecular explanation for this observation we assessed Ca²⁺-dependent modulation of ligand sensitivity in cyclic GMP–gated (CNG) ion channels of intact mammalian rods and cones. Solitary photoreceptors were isolated by gentle proteolysis of ground squirrel retina. Rods and cones were distinguished by whether or not their outer segments bind PNA lectin. We measured membrane currents under voltage-clamp in photoreceptors loaded with Diazo-2, a caged Ca²⁺ chelator, and fixed concentrations of 8Br-cGMP. At 600 nM free cytoplasmic Ca²⁺ the midpoint of the cone CNG channels sensitivity to 8BrcGMP, ^8BrcGMPK_1/2, is ∼2.3 μM. The ligand sensitivity is less in rod than in cone channels. Instantly decreasing cytoplasmic Ca²⁺ to <30 nM activates a large inward membrane current in cones, but not in rods. Current activation arises from a Ca²⁺ -dependent modulation of cone CNG channels, presumably because of an increase in their affinity to the cyclic nucleotide. The time course of current activation is temperature dependent; it is well described by a single exponential process of ∼480 ms time constant at 20–21°C and 138 ms at 32°C. The absence of detectable Ca²⁺-dependent CNG current modulation in intact rods, in view of the known channel modulation by calmodulin in-vitro, affirms the modulation in intact rods may only occur at low Ca²⁺ concentrations, those expected at intensities that nearly saturate the rod photoresponse. The correspondence between Ca²⁺ dependence of CNG modulation and the ability to light adapt suggest these events are correlated in photoreceptors.

Dynamics of Ca2+-calmodulin-dependent inhibition of rod cyclic nucleotide-gated channels measured by patch-clamp fluorometry

Cyclic nucleotide-gated (CNG) ion channels mediate cellular responses to sensory stimuli. In vertebrate photoreceptors, CNG channels respond to the light-induced decrease in cGMP by closing an ion-conducting pore that is permeable to cations, including Ca²⁺ ions. Rod CNG channels are directly inhibited by Ca²⁺-calmodulin (Ca²⁺/CaM), but the physiological role of this modulation is unknown. Native rod CNG channels comprise three CNGA1 subunits and one CNGB1 subunit. The single CNGB1 subunit confers several key properties on heteromeric channels, including Ca²⁺/CaM-dependent modulation. The molecular basis for Ca²⁺/CaM inhibition of rod CNG channels has been proposed to involve the binding of Ca²⁺/CaM to a site in the NH₂-terminal region of the CNGB1 subunit, which disrupts an interaction between the NH₂-terminal region of CNGB1 and the COOH-terminal region of CNGA1. Here, we test this mechanism for Ca²⁺/CaM-dependent inhibition of CNGA1/CNGB1 channels by simultaneously monitoring protein interactions with fluorescence spectroscopy and channel function with patch-clamp recording. Our results show that Ca²⁺/CaM binds directly to CNG channels, and that binding is the rate-limiting step for channel inhibition. Further, we show that the NH₂- and COOH-terminal regions of CNGB1 and CNGA1 subunits, respectively, are in close proximity, and that Ca²⁺/CaM binding causes a relative rearrangement or separation of these regions. This motion occurs with the same time course as channel inhibition, consistent with the notion that rearrangement of the NH₂- and COOH-terminal regions underlies Ca²⁺/CaM-dependent inhibition.

Subunit configuration of heteromeric cone cyclic nucleotide-gated channels

Cone photoreceptor cyclic nucleotide-gated (CNG) channels are thought to be tetrameric assemblies of CNGB3 (B3) and CNGA3 (A3) subunits. We have used functional and biochemical approaches to investigate the stoichiometry and arrangement of these subunits in recombinant channels. First, tandem dimers of linked subunits were used to constrain the order of CNGB3 and CNGA3 subunits; the properties of channels formed by B3/B3+A3/A3 dimers, or A3/B3+B3/A3 dimers, closely resembled those of channels arising from B3+A3 monomers. Functional markers in B3/B3 (or A3/A3) dimers confirmed that both B3 subunits (and both A3 subunits) gained membership into the pore-forming tetramer and that like subunits were positioned adjacent to each other. Second, chemical crosslinking and co-immunoprecipitation studies using epitope-tagged monomer subunits both demonstrated the presence of two CNGB3 subunits in cone channels. Together, these data support a preferred subunit arrangement for cone CNG channels (B3-B3-A3-A3) that is distinct from the 3A:1B configuration of rod channels.

Calcium sensitivity of a sodium-activated nonselective cation channel in lobster olfactory receptor neurons

We report that a Na+-activated nonselective cation channel described previously in lobster olfactory neurons, in which phosphoinositide signaling mediates olfactory transduction, can also be activated by Ca2+. Ca2+ activates the channel in the presence of Na+, increasing the open probability of the channel with a K1/2 of 490 nM and a Hill coefficient of 1.3. Ca2+ also increases the sensitivity of the channel to Na+. In some cells, the same channel is Ca2+ insensitive in a cell-specific manner. The nonspecific activator of protein phosphatases, protamine, applied to the intracellular face of patches containing the channel irreversibly eliminates the sensitivity to Ca2+. This effect can be blocked by okadaic acid, a nonspecific blocker of protein phosphatases, and restored by the catalytic subunit of protein kinase A in the presence of MgATP. The Ca2+-sensitive form of the channel is predominantly expressed in the transduction zone of the cells in situ. These findings imply that the Ca2+ sensitivity of the channel, and possibly its regulation by phosphorylation, play a role in olfactory transduction and help tie activation of the channel to the canonical phosphoinositide turnover pathway.

Disruption of an intersubunit interaction underlies Ca2+-calmodulin modulation of cyclic nucleotide-gated channels

Cyclic nucleotide-gated channels are key molecular elements for olfactory transduction. Olfactory adaptation caused by repeated exposure to an odorant has been proposed to be mediated by the binding of Ca2+-calmodulin to the NH2-terminal domain of the channel, breaking its interaction with the COOH-terminal domain and downregulating the channel. We used a fluorescence resonance energy transfer (FRET) approach to study the structural aspects of this domain-domain interaction under physiological conditions in real time. Fluorescent proteins enhanced cyan fluorescent protein and enhanced yellow fluorescent protein were genetically attached at sites adjacent to the NH2- and COOH-terminal interacting domains, respectively, allowing direct observation of molecular rearrangements in intact channels. FRET signals caused by the specific interdomain interaction were observed in both intact cells and excised patches. Comparison of the effective FRET efficiencies demonstrated that the interaction occurs specifically between subunits but not within the same subunit. Binding of Ca2+-calmodulin caused a reversible decrease in FRET with the same time course as channel downregulation. These results suggest that a separation or reorientation of the interacting domains between subunits by Ca2+-calmodulin leads to channel downregulation. The quaternary arrangement presents a structural framework for understanding the molecular mechanism of olfactory adaptation.

Disruption of an intersubunit interaction underlies Ca2+-calmodulin modulation of cyclic nucleotide-gated channels

Cyclic nucleotide-gated channels are key molecular elements for olfactory transduction. Olfactory adaptation caused by repeated exposure to an odorant has been proposed to be mediated by the binding of Ca2+-calmodulin to the NH2-terminal domain of the channel, breaking its interaction with the COOH-terminal domain and downregulating the channel. We used a fluorescence resonance energy transfer (FRET) approach to study the structural aspects of this domain-domain interaction under physiological conditions in real time. Fluorescent proteins enhanced cyan fluorescent protein and enhanced yellow fluorescent protein were genetically attached at sites adjacent to the NH2- and COOH-terminal interacting domains, respectively, allowing direct observation of molecular rearrangements in intact channels. FRET signals caused by the specific interdomain interaction were observed in both intact cells and excised patches. Comparison of the effective FRET efficiencies demonstrated that the interaction occurs specifically between subunits but not within the same subunit. Binding of Ca2+-calmodulin caused a reversible decrease in FRET with the same time course as channel downregulation. These results suggest that a separation or reorientation of the interacting domains between subunits by Ca2+-calmodulin leads to channel downregulation. The quaternary arrangement presents a structural framework for understanding the molecular mechanism of olfactory adaptation.

Tuning outer segment Ca2+ homeostasis to phototransduction in rods and cones

Cone photoreceptors respond to light with less sensitivity, faster kinetics and adapt over a much wider range of intensities than do rods. These differences can be explained, in part, by the quantitative differences in the molecular processes that regulate the cytoplasmic free Ca2+ concentration in the outer segment of both receptor types. Ca2+ concentration is regulated through the kinetic balance between the ions' influx and efflux and the action of intracellular buffers. Influx is passive and mediated by the cyclic-GMP gated ion channels. In cones, Ca2+ ions carry about 35% of the ionic current flowing through the channels in darkness. In rods, in contrast, this fraction is about 20%. We present a kinetic rate model of the ion channels that helps explain the differences in their Ca2+ fractional flux. In cones, but not in rods, the cGMP-sensitivity of the cyclic GMP-gated ion channels changes with Ca2+ at the concentrations expected in dark-adapted photoreceptors. Ca2+ efflux is active and mediated by a Na+ and K+-dependent exchanger. The rate of Ca2+ clearance mediated by the exchanger in cones, regardless of the absolute size of their outer segment is of the order of tens of milliseconds. In rod outer segments, and again independently of their size, Ca2+ clearance rate is of the order of hundreds of milliseconds to seconds. We investigate the functional consequences of these differences in Ca2+ homeostasis using computational models of the phototransduction signal in rods and cones. Consistent with experimental observation, differences in Ca2+ homeostasis can make the cone's flash response faster and less sensitive to light than that of rods. In the simulations, however, changing Ca2+ homeostasis is not sufficient to recreate authentic cone responses. Accelerating the rate of inactivation (but NOT activation) of the enzymes of the transduction cascade, in addition, to changes in Ca2+ homeostasis are needed to explain the differences between rod and cone photosignals. The large gain and precise kinetic control of the electrical photoresponse of rod and cone retinal receptors suggested a long time back that phototransduction is mediated by cytoplasmic second messengers that, in turn, control membrane ionic conductance. (1) The unquestionable identification of cyclic GMP as the phototransduction messenger, however, did not come until the mid 1980's with the discovery that the light-regulated membrane conductance in both rods and cones is gated by this nucleotide (2-4) and is, in fact, an ion channel. (7) The cyclic nucleotide gated (CNG) channels, now we know, are not just the compliant targets of light-dependent change in cytoplasmic cGMP, but actively participate in the regulation transduction through Ca2+ feedback signals. The precise magnitude and time course of the concentration changes of cGMP and Ca2+ in either rods or cones remains controversial. It is clear, however, that whereas cGMP directly controls the opening and closing of the plasma membrane channels, Ca2+ controls the light-sensitivity and kinetics of the transduction signal. (8,9) The modulatory role of Ca2+ is particularly apparent in the process of light adaptation: in light-adapted rods or cones, the transduction signal generated by a given flash is lower in sensitivity and faster in time course than in dark-adapted cells. Light adaptation is compromised if Ca2+ concentration changes are attenuated by cytopiasmic Ca2+ buffers (8,10,11) and does not occur if Ca2+ concentration changes are prevented by manipulation of the solution bathing the cells. (2,4) Several Ca2+-dependent biochemical reactions have been identified in photoreceptors, among them: 1. ATP-dependent deactivation. (15,16) 2 Phodopsin phospshorylation, through the action of recoverin (S-modulin). (17-19) 3. Catalytic activity of guanylyl cyclase, (20-22) through the action of GCAP proteins. (23,24,25) 4. cGMP-sensitivity of the CNG channels. (26-29,30) A challenge in contemporary phototransduction research is to understand the details of these reactions and their role in the control of the phototransduction signal. Transduction signals in cone photoreceptors are faster, lower in light sensitivity, and more robust in their adaptation features than those in rods (for review see refs. 31;32). A detailed molecular explanation for these differences is not at hand. However, biochemical and electrophysiological (33) studies indicate that the elements in the light-activated pathway that hydrolyzes cGMP are quantitatively similar in their function in rods and cones and unlikely to account for the functional differences. Also, within the limited exploration completed todate, the Ca2+-dependence of guanylyl cyclase (34) and visual pigment phosphorylation (19) do not differ in rods and cones. On the other hand, data accumulated over the past few years indicate that cytoplasmic Ca2+ homeostasis, while controlled through essentially identical mechanisms it is quantitatively very different in its features in the two photoreceptor types. Both Ca2+ influx through CNG channels and the rate of Ca2+ clearance from the outer segment differ between the two receptor cells. Also, the Ca2+-dependent modulation of cGMP sensitivity is larger in extent in cones than in rods. Most significantly, the concentration range of this Ca2+ dependence overlaps the physiological range of light-dependent changes in cytoplasmic Ca2+ level in cones, but not in rods. We briefly review some of the evidence that supports these assertions and we then provide a quantitative analysis of the possible significance of these known differences. We conclude that while differences in Ca2+ homeostasis contribute importantly to explaining the differences between the two receptor types, they are alone not sufficient to explain the differences in the photoreceptor's response. It is likely that Ca2+-independent inactivation of the transduction cascade enzymes is more rapid in cones than in rods.

Functionally important calmodulin-binding sites in both NH2- and COOH-terminal regions of the cone photoreceptor cyclic nucleotide-gated channel CNGB3 subunit

Whereas an important aspect of sensory adaptation in rod photoreceptors and olfactory receptor neurons is thought to be the regulation of cyclic nucleotide-gated (CNG) channel activity by calcium-calmodulin (Ca2+-CaM), it is not clear that cone photoreceptor CNG channels are similarly modulated. Cone CNG channels are composed of at least two different subunit types, CNGA3 and CNGB3. We have investigated whether calmodulin modulates the activity of these channels by direct binding to the CNGB3 subunit. Heteromeric channels were formed by co-expression of human CNGB3 with human CNGA3 subunits in Xenopus oocytes; CNGB3 subunits conferred sensitivity to regulation by Ca2+-CaM, whereas CaM regulation of homomeric CNGA3 channels was not detected. To explore the mechanism underlying this regulation, we localized potential CaM-binding sites in both NH2- and COOH-terminal cytoplasmic domains of CNGB3 using gel-overlay and glutathione S-transferase pull-down assays. For both sites, binding of CaM depended on the presence of Ca2+. Individual deletions of either CaM-binding site in CNGB3 generated channels that remained sensitive to regulation by Ca2+-CaM, but deletion of both together resulted in heteromeric channels that were not modulated. Thus, both NH2- and COOH-terminal CaM-binding sites in CNGB3 are functionally important for regulation of recombinant cone CNG channels. These studies suggest a potential role for direct binding and unbinding of Ca2+-CaM to human CNGB3 during cone photoreceptor adaptation and recovery.

Circadian phase-dependent modulation of cGMP-gated channels of cone photoreceptors by dopamine and D2 agonist

The affinity of cGMP-gated ion channels (CNGCs) for cGMP in chick retinal cone photoreceptors is under circadian control. Here we report that dopamine (DA) and D2 receptor agonists evoke phase-dependent shifts in the affinity of CNGCs for activating ligand. Inside-out patch recordings from cultured chick cones were performed at circadian time (CT) 4-7 and CT 16-19 on the second day of constant darkness. Exposing intact cells to DA or the D2 agonist quinpirole for 2 hr before patch excision caused a significant increase in the K(D) for cGMP during the night (CT 16-19) but had no effect during the day (CT 4-7). DA or quinpirole treatment had no effect on the Hill slope or the average number of channels per patch. The effect of DA was blocked by the D2 antagonist eticlopride and was not mimicked by D1 agonists or blocked by D1 antagonists. By contrast, a brief (15 min) exposure to DA or quinpirole caused a decrease in K(D) during the subjective day and had no effect during the subjective night. Thus, the effect of D2 agonists depends on both the duration of agonist exposure and the time of day. Application of DA or quinpirole evoked a transient activation of the MAP kinase Erk (extracellular signal-related kinase) during the day but caused a sustained inhibition during the night. Conversely, D2 agonists caused activation of Ca2+/calmodulin-dependent protein kinase II during the night and inhibited this enzyme during the day. A circadian oscillator in cones appears to regulate the nature of the transduction cascade used by D2 receptors.

Calcium/calmodulin modulation of olfactory and rod cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) ion channels mediate sensory transduction in olfactory sensory neurons and retinal photoreceptor cells. In these systems, internal calcium/calmodulin (Ca2+/CaM) inhibits CNG channels, thereby having a putative role in sensory adaptation. Functional differences in Ca2+/CaM-dependent inhibition depend on the different subunit composition of olfactory and rod CNG channels. Recent evidence shows that three subunit types (CNGA2, CNGA4, and CNGB1b) make up native olfactory CNG channels and account for the fast inhibition of native channels by Ca2+/CaM. In contrast, two subunit types (CNGA1 and CNGB1) appear sufficient to mirror the native properties of rod CNG channels, including the inhibition by Ca2+/CaM. Within CNG channel tetramers, specific subunit interactions also mediate Ca2+/CaM-dependent inhibition. In olfactory CNGA2 channels, Ca2+/CaM binds to an N-terminal region and disrupts an interaction between the N- and C-terminal regions, causing inhibition. Ca2+/CaM also binds the N-terminal region of CNGB1 subunits and disrupts an intersubunit, N- and C-terminal interaction between CNGB1 and CNGA1 subunits in rod channels. However, the precise N- and C-terminal regions that form these interactions in olfactory channels are different from those in rod channels. Here, we will review recent advances in understanding the subunit composition and the mechanisms and roles for Ca2+/CaM-dependent inhibition in olfactory and rod CNG channels.

Circadian phase-dependent modulation of cGMP-gated channels of cone photoreceptors by dopamine and D2 agonist

The affinity of cGMP-gated ion channels (CNGCs) for cGMP in chick retinal cone photoreceptors is under circadian control. Here we report that dopamine (DA) and D2 receptor agonists evoke phase-dependent shifts in the affinity of CNGCs for activating ligand. Inside-out patch recordings from cultured chick cones were performed at circadian time (CT) 4-7 and CT 16-19 on the second day of constant darkness. Exposing intact cells to DA or the D2 agonist quinpirole for 2 hr before patch excision caused a significant increase in the K(D) for cGMP during the night (CT 16-19) but had no effect during the day (CT 4-7). DA or quinpirole treatment had no effect on the Hill slope or the average number of channels per patch. The effect of DA was blocked by the D2 antagonist eticlopride and was not mimicked by D1 agonists or blocked by D1 antagonists. By contrast, a brief (15 min) exposure to DA or quinpirole caused a decrease in K(D) during the subjective day and had no effect during the subjective night. Thus, the effect of D2 agonists depends on both the duration of agonist exposure and the time of day. Application of DA or quinpirole evoked a transient activation of the MAP kinase Erk (extracellular signal-related kinase) during the day but caused a sustained inhibition during the night. Conversely, D2 agonists caused activation of Ca2+/calmodulin-dependent protein kinase II during the night and inhibited this enzyme during the day. A circadian oscillator in cones appears to regulate the nature of the transduction cascade used by D2 receptors.

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.

Calmodulin as an ion channel subunit

A surprising variety of ion channels found in a wide range of species from Homo to Paramecium use calmodulin (CaM) as their constitutive or dissociable Ca(2+)-sensing subunits. The list includes voltage-gated Ca(2+) channels, various Ca(2+)- or ligand-gated channels, Trp family channels, and even the Ca(2+)-induced Ca(2+) release channels from organelles. Our understanding of CaM chemistry and its relation to enzymes has been instructive in channel research, yet the intense study of CaM regulation of ion channels has also revealed unexpected CaM chemistry. The findings on CaM channel interactions have indicated the existence of secondary interaction sites in addition to the primary CaM-binding peptides and the functional differences between the N- and C-lobes of CaM. The study of CaM in channel biology will figure into our understanding on how this uniform, universal, vital, and ubiquitous Ca(2+) decoder coordinates the myriad local and global cell physiological transients.

Calcium-binding proteins: intracellular sensors from the calmodulin superfamily

In all eukaryotic cells, and particularly in neurons, Ca(2+) ions are important second messengers in a variety of cellular signaling pathways. In the retina, Ca(2+) modulation plays a crucial function in the development of the visual system's neuronal connectivity and a regulatory role in the conversion of the light signal received by photoreceptors into an electrical signal transmitted to the brain. Therefore, the study of retinal Ca(2+)-binding proteins, which frequently mediate Ca(2+) signaling, has given rise to the important discovery of two subfamilies of these proteins, neuronal Ca(2+)-binding proteins (NCBPs) and calcium-binding proteins (CaBPs), that display similarities to calmodulin (CaM). These and other Ca(2+)-binding proteins are integral components of cellular events controlled by Ca(2+). Some members of these subfamilies also play a vital role in signal transduction outside of the retina. The expansion of the CaM-like protein family reveals diversification among Ca(2+)-binding proteins that evolved on the basis of the classic molecule, CaM. A large number of NCBP and CaBP subfamily members would benefit from their potentially specialized role in Ca(2+)-dependent cellular processes. Pinpointing the role of these proteins will be a challenging task for further research.

Factors that affect regulation of cGMP synthesis in vertebrate photoreceptors and their genetic link to human retinal degeneration

Cyclic GMP is essential for the ability of rods and cones to respond to the light stimuli. Light triggers hydrolysis of cGMP and stops the influx of sodium and calcium through the cGMP-gated ion channels. The consequence of this event is 2-fold: first, the decrease in the inward sodium current plays the major role in an abrupt hyperpolarization of the cellular membrane; secondly, the decrease in the Ca2+ influx diminishes the free intracellular Ca2+ concentration. While the former constitutes the essence of the phototransduction pathway in rods and cones, the latter gives rise to a potent feedback mechanism that accelerates photoreceptor recovery and adaptation to background light. One of the most important events by which Ca2+ feedback controls recovery and light adaptation is synthesis of cGMP by guanylyl cyclase. Two isozymes of membrane photoreceptor guanylyl cyclase (retGC) have been identified in rods and cones that are regulated by Ca2+-binding proteins, GCAPs. At low intracellular concentrations of Ca2+ typical for light-adapted rods and cones GCAPs activate RetGC, but concentrations above 500 nM typical for dark-adapted photoreceptors turn them into inhibitors of retGC. A variety of mutations found in GCAP and retGC genes have been linked to several forms of human congenital retinal diseases, such as dominant cone degeneration, cone-rod dystrophy and Leber congenital amaurosis.

Activation, deactivation, and adaptation in vertebrate photoreceptor cells

Visual transduction captures widespread interest because its G-protein signaling motif recurs throughout nature yet is uniquely accessible for study in the photoreceptor cells. The light-activated currents generated at the photoreceptor outer segment provide an easily observed real-time measure of the output of the signaling cascade, and the ease of obtaining pure samples of outer segments in reasonable quantity facilitates biochemical experiments. A quiet revolution in the study of the mechanism has occurred during the past decade with the advent of gene-targeting techniques. These have made it possible to observe how transduction is perturbed by the deletion, overexpression, or mutation of specific components of the transduction apparatus.

Ligand sensitivity of the 2 subunit from the bovine cone cGMP-gated channel is modulated by protein kinase C but not by calmodulin

1. Homomeric cyclic nucleotide-gated (CNG) channels composed of alpha2 subunits from bovine cone photoreceptors were heterologously expressed in the human embryonic kidney (HEK) 293 cell line. Modulation of cGMP sensitivity by protein kinase C (PKC)-mediated phosphorylation and by binding of calmodulin (CaM) was investigated in inside-out patches. 2. A peptide encompassing the putative CaM-binding site within the N-terminus of the channel protein binds Ca(2+)-CaM with high affinity, yet the ligand sensitivity of alpha2 channels is not modulated by CaM. 3. PKC-mediated phosphorylation increased the activation constant (K(1/2)) for cGMP from 19 to 56 microM and decreased the Hill coefficient (from 2.5 to 1.5). The change in ligand sensitivity involves phosphorylation of the serine residues S577 and S579 in the cGMP-binding domain. The increase in K(1/2) was completely abolished in mutant channels in which the two serine residues were replaced by alanine. 4. An antibody specific for the delta isoform of PKC strongly labels the cone outer segments. 5. Modulation of cGMP affinity of bovine alpha2 CNG channels by phosphorylation could play a role in the regulation of photoreceptor sensitivity.

Vinpocetine-induced stimulation of calcium-activated potassium currents in rat pituitary GH3 cells

The effects of vinpocetine, an inhibitor of cyclic GMP phosphodiesterase, on ionic currents were examined in rat pituitary GH3 lactotrophs with the aid of the patch-clamp technique. In GH3 cells bathed in normal Tyrode's solution, vinpocetine (10 microM) reversibly increased the amplitude of Ca2+-activated K+ current (I(K)Ca) with an EC50 value of 4 microM. When the recording pipettes were filled with 10 mM EGTA, vinpocetine also stimulated I(K)Ca. In the cell-attached configuration, application of vinpocetine to the bath increased the activity of large-conductance Ca2+-activated K+ (BK(Ca)) channels. In excised membrane patches, application of vinpocetine (10 microM) to the bath did not change the single-channel conductance of BK(Ca) channels; however, it did increase channel activity. In the inside-out configuration, neither 8-bromo cyclic GMP nor YC-1 applied intracellularly affected BK(Ca) channel activity. The vinpocetine-induced change in the kinetic behavior of BK(Ca) channels was due to an increase in mean open time and a decrease in mean closed time. Vinpocetine (10 microM) caused a leftward shift in the midpoint for the voltage-dependent opening. Under the current-clamp mode, vinpocetine (10 microM) decreased the firing rate of spontaneous action potentials induced by thyrotropin-releasing hormone (10 microM) in GH3 cells. In pheochromocytoma PC12 cells, vinpocetine (10 microM) applied intracellularly also enhanced the activity of BK(Ca) channels without altering single-channel conductance. Thus, the present study suggests that vinpocetine-mediated stimulation of I(K)Ca may result from the direct activation of BK(Ca) channels and indirectly from elevated cytosolic Ca2+.

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.

Circadian regulation of cGMP-gated cationic channels of chick retinal cones. Erk MAP Kinase and Ca2+/calmodulin-dependent protein kinase II

cGMP-gated channels are essential for phototransduction in the vertebrate retina. Here we show that the affinity of these channels for cGMP in chick cones is substantially higher during the subjective night than during the subjective day. This effect persists in constant environmental conditions after entrainment to 12:12 hr light-dark cycles in vitro or in ovo. Circadian modulation of ligand affinity is a posttranslational effect and is driven by rhythms in the activities of two protein kinases: Erk and Ca2+/calmodulin-dependent protein kinase II (CaMKII). Erk is maximally active during the subjective night, whereas CaMKII is maximally active during the subjective day. Acute inhibition of these signaling pathways causes phase-dependent changes in the affinity of the channels for cGMP.

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.

Analysis of Ca++-dependent gain changes in PDE activation in vertebrate rod phototransduction

PURPOSE

Recent biochemical and physiological data point to the existence of one or more Ca++-mediated feedback mechanisms modulating gain at stages early in the vertebrate phototransduction cascade, i.e., prior to activation of cGMP-phosphodiesterase (PDE). The present study is a computational analysis that combines quantitative optimization to key data with a qualitative evaluation of each candidate model's ability to capture "signature" features of representative rod responses obtained under a broad range of dark- (DA) and light-adapted (LA) conditions. The primary data motivating the analyses were the two-flash data of Murnick & Lamb. These data exhibited strikingly nonlinear behavior: the period of complete photocurrent saturation (Tsat) in response to a Test flash was reduced substantially when preceded by a less-intense saturating Pre-flash. Depending on the delay between Pre- and Test flashes, the change in Tsat (DTsat) could exceed the magnitude of the delay, and could be reduced by as much as approximately 50%, corresponding to a large reduction in gain by a factor of 10-15. The overall goal of the study was to evaluate what model structure(s) were commensurate with both the Murnick & Lamb data and the salient qualitative features of rod responses obtained under a broad range of DA and LA conditions.

METHODS

Three candidate models were quantitatively optimized to the Murnick & Lamb saturated toad rod flash responses and, simultaneously, to a set of sub-saturated flash responses. Using the parameters from these optimizations, each candidate model was then used to simulate a suite of DA and LA responses.

RESULTS

The analyses showed that: (1) Within the context of a model with Ca++ feedback onto rhodopsin (R*) lifetime (tR), the salient features of the Murnick & Lamb data can only be accounted for if the rate-limiting step is not the Ca++-sensitive step in the early cascade reactions, i.e., if PDE* lifetime, and not tR, is rate-limiting. (2) With tR rate-limiting, the model cannot account for DTsat exceeding the delay. (3) The Ca++-dependent reduction in tR required to effect the large gain is incommensurate with the empirical dynamics of dim-flash responses. (4) Regardless of which reaction is rate-limiting, a model using solely modulation of R* lifetime puts strong constraints on the domain of biochemical parameters commensurate with the large gain changes Murnick & Lamb observed. (5) The analyses show that, in principle, the Murnick & Lamb data can be accounted for when tR is both rate-limiting and Ca++-sensitive if, in addition to the feedback onto tR, there is an earlier, stronger Ca++ feedback that does not affect R* inactivation kinetics (e.g., gain at R* activation or transducin (T*) activation). (6) Ca++-modulation of R* activation or T* activation as the sole early gain mechanism can also account for the Murnick & Lamb data, but fails to predict the data of Matthews, and can thus be rejected along with any model of comparable form.

CONCLUSIONS

The results imply that the Murnick & Lamb data per se are insufficient to rule out rate-limitation by (Ca++-sensitive) R* lifetime; evaluation of a broader set of responses is required. The analyses illustrate the importance of evaluating candidate models in relation to sets of data obtained under the broadest possible range of DA and LA conditions. The analyses are aided by the presence of reproducible signature, qualitative features in the data since these tend to constrain the domain of acceptable model structures and/or parameter sets. Some implications for vertebrate photoreceptor light-adaptation are discussed.

Fraction of the dark current carried by Ca(2+) through cGMP-gated ion channels of intact rod and cone photoreceptors

The selectivity for Ca(2+) over Na(+), PCa/PNa, is higher in cGMP-gated (CNG) ion channels of retinal cone photoreceptors than in those of rods. To ascertain the physiological significance of this fact, we determined the fraction of the cyclic nucleotide-gated current specifically carried by Ca(2+) in intact rods and cones. We activated CNG channels by suddenly (<5 ms) increasing free 8Br-cGMP in the cytoplasm of rods or cones loaded with a caged ester of the cyclic nucleotide. Simultaneous with the uncaging flash, we measured the cyclic nucleotide-dependent changes in membrane current and fluorescence of the Ca(2+)-binding dye, Fura-2, also loaded into the cells. The ratio of changes in fura-2 fluorescence and the integral of the membrane current, under a restricted set of experimental conditions, is a direct measure of the fractional Ca(2+) flux. Under normal physiological salt concentrations, the fractional Ca(2+) flux is higher in CNG channels of cones than in those of rods, but it differs little among cones (or rods) of different species. Under normal physiological conditions and for membrane currents </=200 pA, the Ca(2+) fractional flux in single cones of striped bass was 33 +/- 2%, and 34 +/- 6% in catfish cones. Under comparable conditions, the Ca(2+) fractional flux in rod outer segments of tiger salamander was 21 +/- 1%, and 14 +/- 1% in catfish rods. Fractional Ca(2+) flux increases as extracellular Ca(2+) rises, with a dependence well described by the Michaelis-Menten equation. KCa, the concentration at which Ca(2+) fractional flux is 50% was 1.98 mM in bass cones and 4.96 mM in tiger salamander rods. Because Ca(2+) fractional flux is higher in cones than in rods, light flashes that generate equal photocurrents will cause a larger change in cytoplasmic Ca(2+) in cones than in rods.

Time course and Ca(2+) dependence of sensitivity modulation in cyclic GMP-gated currents of intact cone photoreceptors

We determined the Ca(2+) dependence and time course of the modulation of ligand sensitivity in cGMP-gated currents of intact cone photoreceptors. In electro-permeabilized single cones isolated from striped bass, we measured outer segment current amplitude as a function of cGMP or 8Br-cGMP concentrations in the presence of various Ca(2+) levels. The dependence of current amplitude on nucleotide concentration is well described by the Hill function with values of K(1/2), the ligand concentration that half-saturates current, that, in turn, depend on Ca(2+). K(1/2) increases as Ca(2+) rises, and this dependence is well described by a modified Michaelis-Menten function, indicating that modulation arises from the interaction of Ca(2+) with a single site without apparent cooperativity. (Ca)K(m), the Michaelis-Menten constant for Ca(2+) concentration is 857 +/- 68 nM for cGMP and 863 +/- 51 for 8Br-cGMP. In single cones under whole-cell voltage clamp, we simultaneously measured changes in membrane current and outer segment free Ca(2+) caused by sudden Ca(2+) sequestration attained by uncaging diazo-2. In the presence of constant 8Br-cGMP, 15 micro, Ca(2+) concentration decrease was complete within 50 ms and membrane conductance was enhanced 2.33 +/- 0.95-fold with a mean time to peak of 1.25 +/- 0.23 s. We developed a model that assumes channel modulation is a pseudo-first-order process kinetically limited by free Ca(2+). Based on the experimentally measured changes in Ca(2+) concentration, model simulations match experimental data well by assigning the pseudo-first-order time constant a mean value of 0.40 +/- 0.14 s. Thus, Ca(2+)-dependent ligand modulation occurs over the concentration range of the normal, dark-adapted cone. Its time course suggests that its functional effects are important in the recovery of the cone photoresponse to a flash of light and during the response to steps of light, when cones adapt.

The mechanism of actions of 3-(5'-(hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1) on Ca(2+)-activated K(+) currents in GH(3) lactotrophs

The effects of 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1), an activator of soluble guanylyl cyclase, on ionic currents have been assessed in rat pituitary GH(3) lactotrophs. In GH(3) cells bathed in normal Tyrode's solution, YC-1 (1 microM) reversibly suppressed the amplitude of the Ca(2+)-activated K(+) current (I(K(Ca))). YC-1 at a concentration above 10 microM produced a biphasic response in the amplitude of I(K(Ca)), i.e., an initial decrease followed by a sustained increase. When the pipette solutions were filled with high EGTA (10 mM), the YC-1-induced stimulatory effect on I(K(Ca)) was abolished. Over a similar concentration range, YC-1 also effectively inhibited the voltage-dependent K(+) current (I(K(V))) in GH(3) cells. The IC(50) value required for the inhibition of I(K(V)) by YC-1 was 1 microM. Unlike YC-1, 8-bromo cGMP did not inhibit I(K(Ca)). However, YC-1 (10 microM) did not affect the amplitude of L-type Ca(2+) current. In the cell-attached configuration, application of YC-1 (10 microM) to the bath did not change the single-channel conductance of the large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels; however, it did increase the opening probability of BK(Ca) channels. In contrast, in the outside-out configuration, YC-1 (10 microM) significantly suppressed the opening probability of BK(Ca) channels. The present study shows dual effects of YC-1 on I(K(Ca)) in GH(3) cells. The YC-1-mediated stimulation of I(K(Ca)) may result from elevated cytosolic Ca(2+), whereas the inhibition of I(K(Ca)) and I(K(V)) by YC-1 appears to be direct and independent of the activation of soluble guanylyl cyclase. Caution thus needs to be used in attributing the YC-1-mediated response to the activation of soluble guanylyl cyclase.

Computational analysis of vertebrate phototransduction: combined quantitative and qualitative modeling of dark- and light-adapted responses in amphibian rods

We evaluated the generality of two models of vertebrate phototransduction. The approach was to quantitatively optimize each model to the full waveform of high-quality, dark-adapted (DA), salamander rod flash responses. With the optimal parameters, each model was then used to account for signature, qualitative features of rod responses from three experimental paradigms (stimulus/response, "S/R suite"): (1) step responses; (2) the intensity dependence of the period of photocurrent saturation (Tsat vs. ln(I)); and (3) light-adapted (LA) incremental flash sensitivity as a function of background intensity. The first model was the recent successful model of Nikonov et al. (1998). The second model replaced the instantaneous Ca2+ buffering used in the Nikonov et al. model with a dynamic buffer. The results showed that, in the absence of the dynamic Ca2+ buffer, the Nikonov et al. model does not have sufficient flexibility to provide a good fit to the flash responses, and, using the same parameters, reproduce the salient features of the S/R suite--critical features at step onset and offset are absent; the Tsat function has too shallow a slope; and the model cannot generate the empirically observed I-range of Weber-Fechner LA behavior. Some features could be recovered by changing parameters, but only at the expense of the fit to the reference (Ref) data. When the dynamic buffer is added, the model is able to achieve an acceptable fit to the Ref data while reproducing several features of the S/R suite, including an empirically observed Tsat function, and an extended range of LA flash sensitivity adhering to Weber's law. The overall improved behavior of the model with a dynamic Ca2+ buffer indicates that it is an important mechanism to include in a working model of phototransduction, and that, despite the slow kinetics of amphibian rods, Ca2+ buffering should not be simulated as an instantaneous process. However, neither model was able to capture all the features with the same parameters yielding the optimal fit to the Ref data. In addition, neither model could maintain a good fit to the Ref data when five key biochemical parameters were held at their current known values. Moreover, even after optimization, a number of important parameters remained outside their empirical estimates. We conclude that other mechanisms will need to be added, including additional Ca2+-feedback mechanisms. The present research illustrates the importance of a hybrid qualitative/quantitative approach to model development, and the limitations of modeling restricted sets of data.

Internal calcium modulates apparent affinity of metabotropic GABA receptors

The metabotropic GABA receptor (GABA(B)R) regulates calcium influx in neurons. Whole cell voltage-clamp techniques were employed to determine the effects of internal calcium on the activity of GABA(B)Rs. GABA(B)R receptor apparent affinity was maximal when bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) maintained internal calcium below 70 nM. Apparent affinity was reduced as internal calcium increased. EGTA did not produce similar effects, suggesting that localized increases in calcium influenced GABA(B)R apparent affinity. Confocal imaging disclosed relatively high internal calcium just below the plasma membrane of isolated neurons. BAPTA, but not EGTA, reduced this ring of high calcium. Heparin, dantrolene, and ryanodine increased GABA(B)R apparent affinity, effects similar to that of BAPTA. Calmodulin inhibitors also increased receptor apparent affinity. These results suggest that internally released calcium activates calmodulin, which reduces GABA(B)R apparent affinity. This identifies a reciprocal system in which the metabotropic GABA receptor can reduce calcium influx, but internal calcium can suppress this receptor pathway. Metabotropic glutamate receptors linked to inositol 1,4,5 trisphosphate (InsP(3)) raised internal calcium and suppressed the action of GABA(B)Rs. Thus negative feedback systems control the balance between excitatory and inhibitory metabotropic receptor pathways in retinal neurons.

Molecular determinants of the modulation of cyclic nucleotide-activated channels by calmodulin

The action of calmodulin (CaM) on target proteins is important for a variety of cellular functions. We demonstrate here, however, that the presence of a CaM-binding site on a protein does not necessarily imply a functional effect. The alpha-subunit of the cGMP-gated cation channel of human retinal cones has a CaM-binding site on its cytoplasmic N-terminal region, but the homomeric channel that it forms is not functionally modulated by CaM. Mutational analysis based on comparison to the highly homologous olfactory cyclic nucleotide-gated channel alpha-subunit, which does form a CaM-modulated channel, indicates that residues downstream of the CaM-binding domain on these channels are also important for CaM to have an effect. These findings suggest that a CaM-binding site and complementary structural features in a protein probably evolve independently, and an effect caused by CaM occurs only in the presence of both elements. More generally, the same may be true for other recognized binding sites on proteins for modulators or activators, so that a demonstrated physical interaction does not necessarily imply functional consequence.

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.

In intact cone photoreceptors, a Ca2+-dependent, diffusible factor modulates the cGMP-gated ion channels differently than in rods

We investigated the modulation of cGMP-gated ion channels in single cone photoreceptors isolated from a fish retina. A new method allowed us to record currents from an intact outer segment while controlling its cytoplasmic composition by superfusion of the electropermeabilized inner segment. The sensitivity of the channels to agonists in the intact outer segment differs from that measured in membrane patches detached from the same cell. This sensitivity, measured as the ligand concentration necessary to activate half-maximal currents, K1/2, also increases as Ca2+ concentration decreases. In electropermeabilized cones, K1/2 for cGMP is 335.5 +/- 64.4 microM in the presence of 20 microM Ca2+, and 84.3 +/- 12.6 microM in its absence. For 8Br-cGMP, K1/2 is 72.7 +/- 11.3 microM in the presence of 20 microM Ca2+ and 15.3 +/- 4.5 microM in its absence. The Ca2+-dependent change in agonist sensitivity is larger in extent than that measured in rods. In electropermeabilized tiger salamander rods, K1/2 for 8Br-cGMP is 17.9 +/- 3.8 microM in the presence of 20 microM Ca2+ and 7.2 +/- 1.2 microM in its absence. The Ca2+-dependent modulation is reversible in intact cone outer segments, but is progressively lost in the absence of divalent cations, suggesting that it is mediated by a diffusible factor. Comparison of data in intact cells and detached membrane fragments from cones indicates that this factor is not calmodulin. At 40 microM 8Br-cGMP, the Ca2+-dependent change in sensitivity in cones is half-maximal at KCa = 286 +/- 66 nM Ca2+. In rods, by contrast, KCa is approximately 50 nM Ca2+. The difference in magnitude and Ca2+ dependence of channel modulation between photoreceptor types suggests that this modulation may play a more significant role in the regulation of photocurrent gain in cones than in rods.