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

Feedforward and feedback mechanisms cooperatively regulate rapid experience-dependent response adaptation in a single thermosensory neuron type

Sensory adaptation allows neurons to adjust their sensitivity and responses based on recent experience. The mechanisms that mediate continuous adaptation to stimulus history over seconds- to hours-long timescales, and whether these mechanisms can operate within a single sensory neuron type, are unclear. The single pair of AFD thermosensory neurons in Caenorhabditis elegans exhibits experience-dependent plasticity in their temperature response thresholds on both minutes- and hours-long timescales upon a temperature upshift. While long-term response adaptation requires changes in gene expression in AFD, the mechanisms driving rapid response plasticity are unknown. Here, we show that rapid thermosensory response adaptation in AFD is mediated via cGMP and calcium-dependent feedforward and feedback mechanisms operating at the level of primary thermotransduction. We find that either of two thermosensor receptor guanylyl cyclases (rGCs) alone is sufficient to drive rapid adaptation, but that each rGC drives adaptation at different rates. rGC-driven adaptation is mediated in part via phosphorylation of their intracellular domains, and calcium-dependent feedback regulation of basal cGMP levels via a neuronal calcium sensor protein. In turn, cGMP levels feedforward via cGMP-dependent protein kinases to phosphorylate a specific subunit of the cGMP-gated thermotransduction channel to further regulate rapid adaptation. Our results identify multiple molecular pathways that act in AFD to ensure rapid adaptation to a temperature change and indicate that the deployment of both transcriptional and nontranscriptional mechanisms within a single sensory neuron type can contribute to continuous sensory adaptation.

Feedforward and feedback mechanisms cooperatively regulate rapid experience-dependent response adaptation in a single thermosensory neuron type

Sensory adaptation allows neurons to adjust their sensitivity and responses based on recent experience. The mechanisms that mediate continuous adaptation to stimulus history over seconds to hours long timescales, and whether these mechanisms can operate within a single sensory neuron type, are unclear. The single pair of AFD thermosensory neurons in C. elegans exhibits experience-dependent plasticity in their temperature response thresholds on both minutes- and hours-long timescales upon a temperature upshift. While long-term response adaptation requires changes in gene expression in AFD, the mechanisms driving rapid response plasticity are unknown. Here, we show that rapid thermosensory response adaptation in AFD is mediated via cGMP and calcium-dependent feedforward and feedback mechanisms operating at the level of primary thermotransduction. We find that either of two thermosensor receptor guanylyl cyclases (rGCs) alone is sufficient to drive rapid adaptation, but that each rGC drives adaptation at different rates. rGC-driven adaptation is mediated in part via phosphorylation of their intracellular domains, and calcium-dependent feedback regulation of basal cGMP levels via a neuronal calcium sensor protein. In turn, cGMP levels feedforward via cGMP-dependent protein kinases to phosphorylate a specific subunit of the cGMP-gated thermotransduction channel to further regulate rapid adaptation. Our results identify multiple molecular pathways that act in AFD to ensure rapid adaptation to a temperature change, and indicate that the deployment of both transcriptional and non-transcriptional mechanisms within a single sensory neuron type can contribute to continuous sensory adaptation.

Exploration of cone cyclic nucleotide-gated channel-interacting proteins using affinity purification and mass spectrometry

Photopic (cone) vision essential for color sensation, central vision, and visual acuity is mediated by the activation of photoreceptor cyclic nucleotide-gated (CNG) channels. Naturally occurring mutations in the cone channel subunits CNGA3 and CNGB3 are associated with achromatopsia and cone dystrophies. This work investigated the functional modulation of cone CNG channel by exploring the channel-interacting proteins. Retinal protein extracts prepared from cone-dominant Nrl (- / -) mice were used in CNGA3 antibody affinity purification, followed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) separation and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry analysis. The peptide mass fingerprinting of the tryptic digests and database search identified a number of proteins including spectrin alpha-2, ATPase (Na(+)/K(+) transporting) alpha-3, alpha and beta subunits of ATP synthase (H(+) transporting, mitochondrial F1 complex), and alpha-2 subunit of the guanine nucleotide-binding protein. In addition, the affinity-binding assays demonstrated an interaction between cone CNG channel and calmodulin but not cone Na(+)/Ca(2+)-K(+) exchanger in the mouse retina. Results of this study provide insight into our understanding of cone CNG channel-interacting proteins and the functional modulations.

Cyclic nucleotide-gated channel subunit glycosylation regulates matrix metalloproteinase-dependent changes in channel gating

Cyclic-nucleotide gated (CNG) channels are essential for phototransduction within retinal photoreceptors. We have demonstrated previously that the enzymatic activity of matrix metalloproteinase-2 and -9, members of the matrix metalloproteinase (MMP) family of extracellular, Ca(2+)- and Zn(2+)-dependent proteases, enhances the ligand sensitivity of both rod (CNGA1 and CNGB1) and cone (CNGA3 and CNGB3) CNG channels. Additionally, we have observed a decrease in the maximal CNG channel current (Imax) that begins late during MMP-directed gating changes. Here we demonstrate that CNG channels become nonconductive after prolonged MMP exposure. Concurrent with the loss of conductive channels is the increased relative contribution of channels exhibiting nonmodified gating properties, suggesting the presence of a subpopulation of channels that are protected from MMP-induced gating effects. CNGA subunits are known to possess one extracellular core glycosylation site, located at one of two possible positions within the turret loop near the pore-forming region. Our results indicate that CNGA glycosylation can impede MMP-dependent modification of CNG channels. Furthermore, the relative position of the glycosylation site within the pore turret influences the extent of MMP-dependent proteolysis. Glycosylation at the site found in CNGA3 subunits was found to be protective, while glycosylation at the bovine CNGA1 site was not. Relocating the glycosylation site in CNGA1 to the position found in CNGA3 recapitulated CNGA3-like protection from MMP-dependent processing. Taken together, these data indicate that CNGA glycosylation may protect CNG channels from MMP-dependent proteolysis, consistent with MMP modification of channel function having a requirement for physical access to the extracellular face of the channel.

Phosphorylation via PKC Regulates the Function of the Drosophila Odorant Co-Receptor

Insect odorant receptors (ORs) have a unique design of heterodimers formed by an olfactory receptor protein and the ion channel Orco. Heterologously expressed insect ORs are activated via an ionotropic and a metabotropic pathway that leads to cAMP production and activates the Orco channel. The contribution of metabotropic signaling to the insect odor response remains to be elucidated. Disruption of the G(q) protein signaling cascade reduces the odor response (Kain et al., 2008). We investigated this phenomenon in HEK293 cells expressing Drosophila Orco and found that phospholipase C (PLC) inhibition reduced the sensitivity of Orco to cAMP. A similar effect was seen upon inhibition of protein kinase C (PKC), whereas PKC stimulation activated Orco even in the absence of cAMP. Mutation of the five PKC phosphorylation sites in Orco almost completely eliminated sensitivity to cAMP. To test the impact of PKC activity in vivo we combined single sensillum electrophysiological recordings with microinjection of agents affecting PLC and PKC function and observed an altered response of olfactory sensory neurons (OSNs) to odorant stimulation. Injection of the PLC inhibitor U73122 or the PKC inhibitor Gö6976 into sensilla reduced the OSN response to odor pulses. Conversely, injection of the PKC activators OAG, a diacylglycerol analog, or phorbol myristate acetate (PMA) enhanced the odor response. We conclude that metabotropic pathways affecting the phosphorylation state of Orco regulate OR function and thereby shape the OSN odor response.

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.

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.

Bestrophin-2 is a candidate calcium-activated chloride channel involved in olfactory transduction

Ca-activated Cl channels are an important component of olfactory transduction. Odor binding to olfactory receptors in the cilia of olfactory sensory neurons (OSNs) leads to an increase of intraciliary Ca concentration by Ca entry through cyclic nucleotide-gated (CNG) channels. Ca activates a Cl channel that leads to an efflux of Cl from the cilia, contributing to the amplification of the OSN depolarization. The molecular identity of this Cl channel remains elusive. Recent evidence has indicated that bestrophins are able to form Ca-activated Cl channels in heterologous systems. Here we have analyzed the expression of bestrophins in the mouse olfactory epithelium and demonstrated that only mouse bestrophin-2 (mBest2) was expressed. Single-cell RT-PCR showed that mBest2 was expressed in OSNs but not in supporting cells. Immunohistochemistry revealed that mBest2 was expressed on the cilia of OSNs, the site of olfactory transduction, and colocalized with the main CNGA2 channel subunit. Electrophysiological properties of Ca-activated Cl currents from native channels in dendritic knob/cilia of mouse OSNs were compared with those induced by the expression of mBest2 in HEK-293 cells. We found the same anion permeability sequence, small estimated single-channel conductances, a Ca sensitivity difference of one order of magnitude, and the same side-specific blockage of the two Cl channel blockers commonly used to inhibit the odorant-induced Ca-activated Cl current in OSNs, niflumic acid, and 4-acetamido-4'-isothiocyanato-stilben-2,2'-disulfonate (SITS). Therefore, our data suggest that mBest2 is a good candidate for being a molecular component of the olfactory Ca-activated Cl channel.

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.

Inhibition of human ether à go-go potassium channels by Ca2+/calmodulin binding to the cytosolic N- and C-termini

Human ether à go-go potassium channels (hEAG1) open in response to membrane depolarization and they are inhibited by Ca2+/calmodulin (CaM), presumably binding to the C-terminal domain of the channel subunits. Deletion of the cytosolic N-terminal domain resulted in complete abolition of Ca2+/CaM sensitivity suggesting the existence of further CaM binding sites. A peptide array-based screen of the entire cytosolic protein of hEAG1 identified three putative CaM-binding domains, two in the C-terminus (BD-C1: 674-683, BD-C2: 711-721) and one in the N-terminus (BD-N: 151-165). Binding of GST-fusion proteins to Ca2+/CaM was assayed with fluorescence correlation spectroscopy, surface plasmon resonance spectroscopy and precipitation assays. In the presence of Ca2+, BD-N and BD-C2 provided dissociation constants in the nanomolar range, BD-C1 bound with lower affinity. Mutations in the binding domains reduced inhibition of the functional channels by Ca2+/CaM. Employment of CaM-EF-hand mutants showed that CaM binding to the N- and C-terminus are primarily dependent on EF-hand motifs 3 and 4. Hence, closure of EAG channels presumably requires the binding of multiple CaM molecules in a manner more complex than previously assumed.

Diverse regulation of sensory signaling by C. elegans nPKC-epsilon/eta TTX-4

Molecular and pharmacological studies in vitro suggest that protein kinase C (PKC) family members play important roles in intracellular signal transduction. Nevertheless, the in vivo roles of PKC are poorly understood. We show here that nPKC-epsilon/eta TTX-4 in the nematode Caenorhabditis elegans is required for the regulation of signal transduction in various sensory neurons for temperature, odor, taste, and high osmolality. Interestingly, the requirement for TTX-4 differs in different sensory neurons. In AFD thermosensory neurons, gain or loss of TTX-4 function inactivates or hyperactivates the neural activity, respectively, suggesting negative regulation of temperature sensation by TTX-4. In contrast, TTX-4 positively regulates the signal sensation of ASH nociceptive neurons. Moreover, in AWA and AWC olfactory neurons, TTX-4 plays a partially redundant role with another nPKC, TPA-1, to regulate olfactory signaling. These results suggest that C. elegans nPKCs regulate different sensory signaling in various sensory neurons. Thus, C. elegans provides an ideal model to reveal genetically novel components of nPKC-mediated molecular pathways in sensory signaling.

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.

Cyclic nucleotide-gated ion channels

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

A cyclic GMP-dependent calcium-activated chloride current in smooth-muscle cells from rat mesenteric resistance arteries

We have previously demonstrated the presence of a cyclic GMP (cGMP)-dependent calcium-activated inward current in vascular smooth-muscle cells, and suggested this to be of importance in synchronizing smooth-muscle contraction. Here we demonstrate the characteristics of this current. Using conventional patch-clamp technique, whole-cell currents were evoked in freshly isolated smooth-muscle cells from rat mesenteric resistance arteries by elevation of intracellular calcium with either 10 mM caffeine, 1 microM BAY K8644, 0.4 microM ionomycin, or by high calcium concentration (900 nM) in the pipette solution. The current was found to be a calcium-activated chloride current with an absolute requirement for cyclic GMP (EC50 6.4 microM). The current could be activated by the constitutively active subunit of PKG. Current activation was blocked by the protein kinase G antagonist Rp-8-Br-PET-cGMP or with a peptide inhibitor of PKG, or with the nonhydrolysable ATP analogue AMP-PNP. Under biionic conditions, the anion permeability sequence of the channel was SCN- > Br- > I- > Cl- > acetate > F- >> aspartate, but the conductance sequence was I- > Br- > Cl- > acetate > F- > aspartate = SCN-. The current had no voltage or time dependence. It was inhibited by nickel and zinc ions in the micromolar range, but was unaffected by cobalt and had a low sensitivity to inhibition by the chloride channel blockers niflumic acid, DIDS, and IAA-94. The properties of this current in mesenteric artery smooth-muscle cells differed from those of the calcium-activated chloride current in pulmonary myocytes, which was cGMP-independent, exhibited a high sensitivity to inhibition by niflumic acid, was unaffected by zinc ions, and showed outward current rectification as has previously been reported for this current. Under conditions of high calcium in the patch-pipette solution, a current similar to the latter could be identified also in the mesenteric artery smooth-muscle cells. We conclude that smooth-muscle cells from rat mesenteric resistance arteries have a novel cGMP-dependent calcium-activated chloride current, which is activated by intracellular calcium release and which has characteristics distinct from other calcium-activated chloride currents.

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.

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.

Survey of the year 2001 commercial optical biosensor literature

We have assembled references of 700 articles published in 2001 that describe work performed using commercially available optical biosensors. To illustrate the technology's diversity, the citation list is divided into reviews, methods and specific applications, as well as instrument type. We noted marked improvements in the utilization of biosensors and the presentation of kinetic data over previous years. These advances reflect a maturing of the technology, which has become a standard method for characterizing biomolecular interactions.

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.

Hyperpolarization-activated channels HCN1 and HCN4 mediate responses to sour stimuli

Sour taste is initiated by protons acting at receptor proteins or channels. In vertebrates, transduction of this taste quality involves several parallel pathways. Here we examine the effects of sour stimuli on taste cells in slices of vallate papilla from rat. From a subset of cells, we identified a hyperpolarization-activated current that was enhanced by sour stimulation at the taste pore. This current resembled Ih found in neurons and cardio-myocytes, a current carried by members of the family of hyperpolarization-activated and cyclic-nucleotide-gated (HCN) channels. We show by in situ hybridization and immunohistochemistry that HCN1 and HCN4 are expressed in a subset of taste cells. By contrast, gustducin, the G-protein involved in bitter and sweet taste, is not expressed in these cells. Lowering extracellular pH causes a dose-dependent flattening of the activation curve of HCN channels and a shift in the voltage of half-maximal activation to more positive voltages. Our results indicate that HCN channels are gated by extracellular protons and may act as receptors for sour taste.

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

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