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

Retinal Cyclic Nucleotide-Gated Channel Regulation by Calmodulin

Retinal cyclic nucleotide-gated (CNG) ion channels bind to intracellular cGMP and mediate visual phototransduction in photoreceptor rod and cone cells. Retinal rod CNG channels form hetero-tetramers comprised of three CNGA1 and one CNGB1 protein subunits. Cone CNG channels are similar tetramers consisting of three CNGA3 and one CNGB3 subunits. Calmodulin (CaM) binds to two distinct sites (CaM1: residues 565-587 and CaM2: residues 1120-1147) within the cytosolic domains of rod CNGB1. The binding of Ca²⁺-bound CaM to CNGB1 promotes the Ca²⁺-induced desensitization of CNG channels in retinal rods that may be important for photoreceptor light adaptation. Mutations that affect Ca²⁺-dependent CNG channel function are responsible for inherited forms of blindness. In this review, we propose structural models of the rod CNG channel bound to CaM that suggest how CaM might cause channel desensitization and how dysregulation of the channel may lead to retinal disease.

Redefining the role of Ca2+-permeable channels in photoreceptor degeneration using diltiazem

Hereditary degeneration of photoreceptors has been linked to over-activation of Ca²⁺-permeable channels, excessive Ca²⁺-influx, and downstream activation of Ca²⁺-dependent calpain-type proteases. Unfortunately, after more than 20 years of pertinent research, unequivocal evidence proving significant and reproducible photoreceptor protection with Ca²⁺-channel blockers is still lacking. Here, we show that both D- and L-cis enantiomers of the anti-hypertensive drug diltiazem were very effective at blocking photoreceptor Ca²⁺-influx, most probably by blocking the pore of Ca²⁺-permeable channels. Yet, unexpectedly, this block neither reduced the activity of calpain-type proteases, nor did it result in photoreceptor protection. Remarkably, application of the L-cis enantiomer of diltiazem even led to a strong increase in photoreceptor cell death. These findings shed doubt on the previously proposed links between Ca²⁺ and retinal degeneration and are highly relevant for future therapy development as they may serve to refocus research efforts towards alternative, Ca²⁺-independent degenerative mechanisms.

Structure of the human cone photoreceptor cyclic nucleotide-gated channel

Cyclic nucleotide-gated (CNG) channels transduce light-induced chemical signals into electrical signals in retinal cone and rod photoreceptors. Structures of native CNG channels, which are heterotetramers formed by CNGA and CNGB subunits, have not been obtained. In the present study, we report a high-resolution cryo-electron microscopy structure of the human cone CNG channel in the apo closed state. The channel contains three CNGA3 and one CNGB3 subunits. Arg403 in the pore helix of CNGB3 projects into an asymmetric selectivity filter and forms hydrogen bonds with two pore-lining backbone carbonyl oxygens. Arg442 in S6 of CNGB3 protrudes into and occludes the pore below the hydrophobic cavity gate previously observed in homotetrameric CNGA channels. It is interesting that Arg403Gln is a disease mutation, and Arg442 is replaced by glutamine in some animal species with dichromatic or monochromatic vision. These and other unique structural features and the disease link conferred by CNGB3 indicate a critical role of CNGB3 in shaping cone photoresponses.

CNGA3 acts as a cold sensor in hypothalamic neurons

Most mammals maintain their body temperature around 37°C, whereas in hibernators it can approach 0°C without triggering a thermogenic response. The remarkable plasticity of the thermoregulatory system allowed mammals to thrive in variable environmental conditions and occupy a wide range of geographical habitats, but the molecular basis of thermoregulation remains poorly understood. Here we leverage the thermoregulatory differences between mice and hibernating thirteen-lined ground squirrels (Ictidomys tridecemlineatus) to investigate the mechanism of cold sensitivity in the preoptic area (POA) of the hypothalamus, a critical thermoregulatory region. We report that, in comparison to squirrels, mice have a larger proportion of cold-sensitive neurons in the POA. We further show that mouse cold-sensitive neurons express the cyclic nucleotide-gated ion channel CNGA3, and that mouse, but not squirrel, CNGA3 is potentiated by cold. Our data reveal CNGA3 as a hypothalamic cold sensor and a molecular marker to interrogate the neuronal circuitry underlying thermoregulation.

The B3 Subunit of the Cone Cyclic Nucleotide-gated Channel Regulates the Light Responses of Cones and Contributes to the Channel Structural Flexibility

Cone photoreceptor cyclic nucleotide-gated (CNG) channels play a pivotal role in cone phototransduction, which is a process essential for daylight vision, color vision, and visual acuity. Mutations in the cone channel subunits CNGA3 and CNGB3 are associated with human cone diseases, including achromatopsia, cone dystrophies, and early onset macular degeneration. Mutations in CNGB3 alone account for 50% of reported cases of achromatopsia. This work investigated the role of CNGB3 in cone light response and cone channel structural stability. As cones comprise only 2-3% of the total photoreceptor population in the wild-type mouse retina, we used Cngb3(-/-)/Nrl(-/-) mice with CNGB3 deficiency on a cone-dominant background in our study. We found that, in the absence of CNGB3, CNGA3 was able to travel to the outer segments, co-localize with cone opsin, and form tetrameric complexes. Electroretinogram analyses revealed reduced cone light response amplitude/sensitivity and slower response recovery in Cngb3(-/-)/Nrl(-/-) mice compared with Nrl(-/-) mice. Absence of CNGB3 expression altered the adaptation capacity of cones and severely compromised function in bright light. Biochemical analysis demonstrated that CNGA3 channels lacking CNGB3 were more resilient to proteolysis than CNGA3/CNGB3 channels, suggesting a hindered structural flexibility. Thus, CNGB3 regulates cone light response kinetics and the channel structural flexibility. This work advances our understanding of the biochemical and functional role of CNGB3 in cone photoreceptors.

Ion Channels in the Eye: Involvement in Ocular Pathologies

The eye is the sensory organ of vision. There, the retina transforms photons into electrical signals that are sent to higher brain areas to produce visual sensations. In the light path to the retina, different types of cells and tissues are involved in maintaining the transparency of avascular structures like the cornea or lens, while others, like the retinal pigment epithelium, have a critical role in the maintenance of photoreceptor function by regenerating the visual pigment. Here, we have reviewed the roles of different ion channels expressed in ocular tissues (cornea, conjunctiva and neurons innervating the ocular surface, lens, retina, retinal pigment epithelium, and the inflow and outflow systems of the aqueous humor) that are involved in ocular disease pathophysiologies and those whose deletion or pharmacological modulation leads to specific diseases of the eye. These include pathologies such as retinitis pigmentosa, macular degeneration, achromatopsia, glaucoma, cataracts, dry eye, or keratoconjunctivitis among others. Several disease-associated ion channels are potential targets for pharmacological intervention or other therapeutic approaches, thus highlighting the importance of these channels in ocular physiology and pathophysiology.

Inherited macular degeneration-associated mutations in CNGB3 increase the ligand sensitivity and spontaneous open probability of cone cyclic nucleotide-gated channels

Cyclic nucleotide gated (CNG) channels are a critical component of the visual transduction cascade in the vertebrate retina. Mutations in the genes encoding these channels have been associated with a spectrum of inherited retinal disorders. To gain insight into their pathophysiological mechanisms, we have investigated the functional consequences of several CNGB3 mutations, previously associated with macular degeneration (Y469D and L595F) or complete achromatopsia (S156F, P309L, and G558C), by expressing these subunits in combination with wild-type CNGA3 in Xenopus oocytes and characterizing them using patch-clamp recordings in the inside-out configuration. These mutations did not prevent the formation of functional heteromeric channels, as indicated by sensitivity to block by L-cis-diltiazem. With the exception of S156F, each of the mutant channels displayed electrophysiological properties reflecting enhanced channel activity at physiological concentrations of cGMP (i.e., a gain-of-function phenotype). The increased channel activity produced by these mutations resulted from either increased functional expression levels, or increased sensitivity to cyclic nucleotides. Furthermore, L595F increased the spontaneous open probability in the absence of activating ligand, signifying a ligand independent gain-of-function change. In addition to the CNGB3 disease-associate mutations, we characterized the effects of several common CNGB3 and CNGA3 single-nucleotide polymorphisms (SNPs) on heteromeric CNGA3+CNGB3 channel function. Two of the SNPs examined (A3-T153M, and B3-W234C) produced decreased ligand sensitivity for heteromeric CNG channels. These changes may contribute to background disease susceptibility when combined with other genetic or non-genetic factors. Together, these studies help to define the underlying molecular phenotype for mutations relating to CNG channel disease pathogenesis.

CNGB3-achromatopsia clinical trial with CNTF: diminished rod pathway responses with no evidence of improvement in cone function

PURPOSE

Ciliary neurotrophic factor (CNTF) protects rod photoreceptors from retinal degenerative disease in multiple nonhuman models. Thus far, CNTF has failed to demonstrate rod protection in trials for human retinitis pigmentosa. Recently, CNTF was found to improve cone photoreceptor function in a canine CNGB3 achromatopsia model. This study explores whether this finding translates to humans with CNGB3 achromatopsia.

METHODS

A five-subject, open-label Phase I/II study was initiated by implanting intraocular microcapsules releasing CNTF (nominally 20 ng/d) into one eye each of CNGB3 achromat participants. Fellow eyes served as untreated controls. Subjects were followed for 1 year.

RESULTS

Pupil constriction in treated eyes gave evidence of intraocular CNTF release. Additionally, scotopic ERG responses were reduced, and dark-adapted psychophysical absolute thresholds were increased, attributable to diminished rod or rod pathway activity. Optical coherence tomography revealed that the cone-rich fovea underwent structural changes as the foveal hyporeflective zone (HRZ) became diminished in CNTF-treated eyes. No objectively measurable enhancement of cone function was found by assessments of visual acuity, mesopic increment sensitivity threshold, or the photopic ERG. Careful measurements of color hue discrimination showed no change. Nonetheless, subjects reported beneficial changes of visual function in the treated eyes, including reduced light sensitivity and aversion to bright light, which may trace to decreased effective ambient light from the pupillary constriction; further they noted slowed adaptation to darkness, consistent with CNTF action on rod photoreceptors.

CONCLUSIONS

Ciliary neurotrophic factor did not measurably enhance cone function, which reveals a species difference between human and canine CNGB3 cones in response to CNTF. (ClinicalTrials.gov number, NCT01648452.).

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.

A cyclic nucleotide-gated channel mutation associated with canine daylight blindness provides insight into a role for the S2 segment tri-Asp motif in channel biogenesis

Cone cyclic nucleotide-gated channels are tetramers formed by CNGA3 and CNGB3 subunits; CNGA3 subunits function as homotetrameric channels but CNGB3 exhibits channel function only when co-expressed with CNGA3. An aspartatic acid (Asp) to asparagine (Asn) missense mutation at position 262 in the canine CNGB3 (D262N) subunit results in loss of cone function (daylight blindness), suggesting an important role for this aspartic acid residue in channel biogenesis and/or function. Asp 262 is located in a conserved region of the second transmembrane segment containing three Asp residues designated the Tri-Asp motif. This motif is conserved in all CNG channels. Here we examine mutations in canine CNGA3 homomeric channels using a combination of experimental and computational approaches. Mutations of these conserved Asp residues result in the absence of nucleotide-activated currents in heterologous expression. A fluorescent tag on CNGA3 shows mislocalization of mutant channels. Co-expressing CNGB3 Tri-Asp mutants with wild type CNGA3 results in some functional channels, however, their electrophysiological characterization matches the properties of homomeric CNGA3 channels. This failure to record heteromeric currents suggests that Asp/Asn mutations affect heteromeric subunit assembly. A homology model of S1-S6 of the CNGA3 channel was generated and relaxed in a membrane using molecular dynamics simulations. The model predicts that the Tri-Asp motif is involved in non-specific salt bridge pairings with positive residues of S3/S4. We propose that the D262N mutation in dogs with CNGB3-day blindness results in the loss of these inter-helical interactions altering the electrostatic equilibrium within in the S1-S4 bundle. Because residues analogous to Tri-Asp in the voltage-gated Shaker potassium channel family were implicated in monomer folding, we hypothesize that destabilizing these electrostatic interactions impairs the monomer folding state in D262N mutant CNG channels during biogenesis.

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.

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

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

Disease-associated mutations in CNGB3 promote cytotoxicity in photoreceptor-derived cells

Purpose

To determine if achromatopsia associated F525N and T383fsX mutations in the CNGB3 subunit of cone photoreceptor cyclic nucleotide-gated (CNG) channels increases susceptibility to cell death in photoreceptor-derived cells.

Methods

Photoreceptor-derived 661W cells were transfected with cDNA encoding wild-type (WT) CNGA3 subunits plus WT or mutant CNGB3 subunits, and incubated with the membrane-permeable CNG channel activators 8-(4-chlorophenylthio) guanosine 3′,5′-cyclic monophosphate (CPT-cGMP) or CPT-adenosine 3′,5′-cyclic monophosphate (CPT-cAMP). Cell viability under these conditions was determined by measuring lactate dehydrogenase release. Channel ligand sensitivity was calibrated by patch-clamp recording after expression of WT or mutant channels in Xenopus oocytes.

Results

Coexpression of CNGA3 with CNGB3 subunits containing F525N or T383fsX mutations produced channels exhibiting increased apparent affinity for CPT-cGMP compared to WT channels. Consistent with these effects, cytotoxicity in the presence of 0.1 μM CPT-cGMP was enhanced relative to WT channels, and the increase in cell death was more pronounced for the mutation with the largest gain-of-function effect on channel gating, F525N. Increased susceptibility to cell death was prevented by application of the CNG channel blocker L-cis-diltiazem. Increased cytotoxicity was also found to be dependent on the presence of extracellular calcium.

Conclusions

These results indicate a connection between disease-associated mutations in cone CNG channel subunits, altered CNG channel-activation properties, and photoreceptor cytotoxicity. The rescue of cell viability via CNG channel block or removal of extracellular calcium suggests that cytotoxicity in this model depends on calcium entry through hyperactive CNG channels.

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.

Cyclic nucleotide-gated channel α-3 (CNGA3) interacts with stereocilia tip-link cadherin 23 + exon 68 or alternatively with myosin VIIa, two proteins required for hair cell mechanotransduction

Previously, we obtained evidence for a photoreceptor/olfactory type of CNGA3 transcript in a purified teleost vestibular hair cell preparation with immunolocalization of CNGA3 protein to stereocilia of teleost vestibular and mammalian cochlear hair cells. The carboxyl terminus of highly Ca(2+)-permeable CNGA3 expressed in the mammalian organ of Corti and saccular hair cells was found to interact with an intracellular domain of microfibril interface-located protein 1 (EMILIN 1), a member of the elastin superfamily, also immunolocalizd to hair cell stereocilia (Selvakumar, D., Drescher, M. J., Dowdall, J. R., Khan, K. M., Hatfield, J. S., Ramakrishnan, N. A., and Drescher, D. G. (2012) Biochem. J. 443, 463-476). Here, we provide evidence for organ of Corti proteins, of Ca(2+)-dependent binding of the amino terminus of CNGA3 specifically to the carboxyl terminus of stereocilia tip-link protein CDH23 +68 (cadherin 23 with expressed exon 68) by yeast two-hybrid mating and co-transformation protocols, pulldown assays, and surface plasmon resonance analysis. Myosin VIIa, required for adaptation of hair cell mechanotransduction (MET) channel(s), competed with CDH23 +68, with direct Ca(2+)-dependent binding to the amino terminus of CNGA3. Based upon the premise that hair cell stereocilia tip-link proteins are closely coupled with MET, these results are consistent with the possibility that CNGA3 participates in hair-cell MET. Together with the demonstration of protein-protein interaction between HCN1 and tip-link protein protocadherin 15 CD3 (Ramakrishnan, N. A., Drescher, M. J., Barretto, R. L., Beisel, K. W., Hatfield, J. S., and Drescher, D. G. (2009) J. Biol. Chem. 284, 3227-3238; Ramakrishnan, N. A., Drescher, M. J., Khan, K. M., Hatfield, J. S., and Drescher, D. G. (2012) J. Biol. Chem. 287, 37628-37646), a protein-protein interaction for CNGA3 and a second tip-link protein, CDH23 +68, further suggests possible association of two different channels with a single stereocilia tip link.

Matrix metalloproteinase-9 and -2 enhance the ligand sensitivity of photoreceptor cyclic nucleotide-gated channels

Photoreceptor cyclic nucleotide-gated (CNG) channels are the principal ion channels responsible for transduction of the light-induced change in cGMP concentration into an electrical signal. The ligand sensitivity of photoreceptor CNG channels is subject to regulation by intracellular signaling effectors, including calcium-calmodulin, tyrosine kinases and phosphoinositides. Little is known, however, about regulation of channel activity by modification to extracellular regions of CNG channel subunits. Extracellular proteases MMP9 and -2 are present in the interphotoreceptor matrix adjacent to photoreceptor outer segments. Given that MMPs have been implicated in retinal dysfunction and degeneration, we hypothesized that MMP activity may alter the functional properties of photoreceptor CNG channels. For heterologously expressed rod and cone CNG channels, extracellular exposure to MMPs dramatically increased the apparent affinity for cGMP and the efficacy of cAMP. These changes to ligand sensitivity were not prevented by destabilization of the actin cytoskeleton or by disruption of integrin mediated cell adhesion, but could be attenuated by inhibition of MMP catalytic activity. MMP-mediated gating changes exhibited saturable kinetic properties consistent with enzymatic processing of the CNG channels. In addition, exposure to MMPs decreased the abundance of full-length expressed CNGA3 subunits, with a concomitant increase in putative degradation products. Similar gating effects and apparent proteolysis were observed also for native rod photoreceptor CNG channels. Furthermore, constitutive apparent proteolysis of retinal CNGA1 and retinal MMP9 levels were both elevated in aged mice compared with young mice. Together, these results provide evidence that MMP-mediated proteolysis can regulate the ligand sensitivity of CNG channels.

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.

Distinct binding properties distinguish LQ-type calmodulin-binding domains in cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels operate as transduction channels in photoreceptors and olfactory receptor neurons. Direct binding of cGMP or cAMP opens these channels which conduct a mixture of monovalent cations and Ca(2+). Upon activation, CNG channels generate intracellular Ca(2+) signals that play pivotal roles in the transduction cascades of the visual and olfactory systems. Channel activity is controlled by negative feedback mechanisms that involve Ca(2+)-calmodulin, for which all CNG channels possess binding sites. Here we compare the binding properties of the two LQ-type calmodulin binding sites, both of which are thought to be involved in channel regulation. They reside on the isoforms CNGB1 and CNGA4. The CNGB1 subunit is present in rod photoreceptors and olfactory receptor neurons. The CNGA4 subunit is only expressed in olfactory receptor neurons, and there are conflicting results as to its role in calmodulin-mediated feedback inhibition. We examined the interaction of Ca(2+)-calmodulin with two recombinant proteins that encompass either of the two LQ sites. Comparing binding properties, we found that the LQ site of CNGB1 binds Ca(2+)-calmodulin at 10-fold lower Ca(2+) levels than the LQ site of CNGA4. Our data provide biochemical evidence against a contribution of CNGA4 to feedback inhibition. In accordance with previous work on photoreceptor CNG channels, our results indicate that feedback control is the exclusive role of the B-subunits in photoreceptors and olfactory receptor neurons.

Molecular mechanisms of cyclic nucleotide-gated ion channel gating

Cyclic nucleotide-gated ion channels (CNGs) are distributed most widely in the neuronal cell. Great progress has been made in molecular mechanisms of CNG channel gating in the recent years. Results of many experiments have indicated that the stoichiometry and assembly of CNG channels affect their property and gating. Experiments of CNG mutants and analyses of cysteine accessibilities show that cyclic nucleotide-binding domains (CNBD) bind cyclic nucleotides and subsequently conformational changes occurred followed by the concerted or cooperative conformational change of all four subunits during CNG gating. In order to provide theoretical assistances for further investigation on CNG channels, especially regarding the disease pathogenesis of ion channels, this paper reviews the latest progress on mechanisms of CNG channels, functions of subunits, processes of subunit assembly, and conformational changes of subunit regions during gating.

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.

Tyrosine phosphorylation of cGMP-gated ion channels is under circadian control in chick retina photoreceptors

PURPOSE

To investigate the role of tyrosine phosphorylation in circadian regulation of cGMP-gated cation channels (CNGCs) of chicken cone photoreceptors.

METHODS

Chick retinas were studied on the second day of constant darkness (DD) after several days of entrainment to 12:12 hr light-dark (LD) cycles in vitro. Inside-out patch recordings were made during the subjective day and subjective night to quantify circadian changes in the sensitivity of CNGCs to activation by cGMP after treatment with various tyrosine kinase and tyrosine phosphatase inhibitors. Immunoprecipitation and immunoblot analysis were also used to examine tyrosine phosphorylation of CNGCs and closely associated proteins after separation by conventional and two-dimensional SDS-PAGE.

RESULTS

Treatment with tyrosine kinase inhibitors caused a significant decrease in K(1/2) for cGMP activation of CNGCs in patches excised from cones during the subjective day, but had no effect on K(1/2) during the subjective night. Conversely, treatment with a tyrosine phosphatase inhibitor caused a significant increase in the K(1/2) of CNGCs in patches excised during the subjective night but had no effect on channel K(1/2) during the subjective day. Broad spectrum serine-threonine phosphatase inhibitors had no effect. An 85-kDa tyrosine polypeptide that coimmunoprecipitated with CNGC alpha-subunits was detectable at higher levels during the subjective day than during the subjective night. CNGC alpha-subunits were not tyrosine phosphorylated as a function of the time of day.

CONCLUSIONS

Circadian control of cone CNGCs appears to entail elevated daytime tyrosine phosphorylation of an approximately 85-kDa auxiliary protein or another subunit of the CNGCs.

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

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

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.

Characterization of a novel cyclic nucleotide-gated channel from zebrafish brain

Cyclic nucleotide-gated (CNG) channels have been well characterized in the sensory receptors of vision and olfaction, but their characteristics in other tissues remain largely unknown. Here, we report characterization of a novel brain-specific CNG channel from zebrafish. Unique among CNG channels, the transcript is expressed mainly in the brain. When expressed in Xenopus oocytes, the channel's electrophysiological properties are distinct compared to CNG channels from either rods (CNGA1), olfactory receptors (CNGA2), or cones (CNGA3). The channel is less sensitive to cAMP than cGMP (K(1/2) of 280 and 7 microM, respectively), with a maximum cAMP efficacy at least 80% of that with saturating levels of cGMP. The single-channel conductance of 58pS is larger than most other CNG channels. Like other CNG channels the channel is relatively nonselective among monovalent cations. However, unlike other CNG channels, there was rundown of the macroscopic current within 30-100 min after patch excision.

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.

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.

CNGB3 mutations account for 50% of all cases with autosomal recessive achromatopsia

Achromatopsia is a congenital, autosomal recessively inherited disorder characterized by a lack of color discrimination, low visual acuity (<0.2), photophobia, and nystagmus. Mutations in the genes for CNGA3, CNGB3, and GNAT2 have been associated with this disorder. Here, we analyzed the spectrum and prevalence of CNGB3 gene mutations in a cohort of 341 independent patients with achromatopsia. In 163 patients, CNGB3 mutations could be identified. A total of 105 achromats carried apparent homozygous mutations, 44 were compound (double) heterozygotes, and 14 patients had only a single mutant allele. The derived CNGB3 mutation spectrum comprises 28 different mutations including 12 nonsense mutations, eight insertions and/or deletions, five putative splice site mutations, and three missense mutations. Thus, the majority of mutations in the CNGB3 gene result in significantly altered and/or truncated polypeptides. Several mutations were found recurrently, in particular a 1 bp deletion, c.1148delC, which accounts for over 70% of all CNGB3 mutant alleles. In conclusion, mutations in the CNGB3 gene are responsible for approximately 50% of all patients with achromatopsia. This indicates that the CNGB3/ACHM3 locus on chromosome 8q21 is the major locus for achromatopsia in patients of European origin or descent.

Functional consequences of progressive cone dystrophy-associated mutations in the human cone photoreceptor cyclic nucleotide-gated channel CNGA3 subunit

Progressive cone dystrophies are a genetically heterogeneous group of disorders characterized by early deterioration of visual acuity and color vision, together with psychophysical and electrophysiological evidence of abnormal cone function and cone degeneration. Recently, three mutations in the gene encoding the CNGA3 subunit of cone photoreceptor cyclic nucleotide-gated (CNG) channels have been linked to progressive cone dystrophy in humans. To investigate the functional consequences of these mutations, we expressed mutant human CNGA3 subunits in Xenopus oocytes, alone or together with human CNGB3, and studied these channels using patch-clamp recording. Compared with wild-type channels, homomeric and heteromeric channels containing CNGA3-N471S or CNGA3-R563H subunits exhibited an increase in apparent affinity for cGMP and an increase in the relative agonist efficacy of cAMP compared with cGMP. In contrast, R277C subunits did not form functional homomeric or heteromeric channels. Cell surface expression levels, determined using confocal microscopy of green fluorescent protein-tagged subunits and patch-clamp recording, were significantly reduced for both R563H and R277C but unchanged for N471S. Overall, these results suggest that the plasma membrane localization and gating properties of cone CNG channels are altered by progressive cone dystrophy-associated mutations, providing evidence that supports the pathogenicity of these mutations.

Longitudinal diffusion in retinal rod and cone outer segment cytoplasm: the consequence of cell structure

Excitation signals spread along photoreceptor outer segments away from the site of photon capture because of longitudinal diffusion of cGMP, a cytoplasmic second messenger. The quantitative features of longitudinal diffusion reflect the anatomical structure of the outer segment, known to be profoundly different in rod and cone photoreceptors. To explore how structural differences affect cytoplasmic diffusion and to assess whether longitudinal diffusion may contribute to the difference in signal transduction between photoreceptor types, we investigated, both theoretically and experimentally, the longitudinal diffusion of small, hydrophilic molecules in outer segments. We developed a new theoretical analysis to explicitly compute the longitudinal diffusion constant, Dl, in terms of outer segment structure. Using time-resolved fluorescence imaging we measured Dl of Alexa488 and lucifer yellow in intact, single cones and validated the theoretical analysis. We used numerical simulations of the theoretical model to investigate cGMP diffusion in outer segments of various species. At a given time interval, cGMP spreads further in rod than in cone outer segments of the same dimensions. Across all species, the spatial spread of cGMP at the peak of the dim light photocurrent is 3-5 microm in rod outer segments, regardless of their absolute size. Similarly the cGMP spatial spread is 0.7-1 microm in cone outer segments, independently of their dimensions.

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.

Calmodulin permanently associates with rat olfactory CNG channels under native conditions

An important mechanism by which vertebrate olfactory sensory neurons rapidly adapt to odorants is feedback modulation of the Ca(2+)-permeable cyclic nucleotide-gated (CNG) transduction channels. Extensive heterologous studies of homomeric CNGA2 channels have led to a molecular model of channel modulation based on the binding of calcium-calmodulin to a site on the cytoplasmic amino terminus of CNGA2. Native rat olfactory CNG channels, however, are heteromeric complexes of three homologous but distinct subunits. Notably, in heteromeric channels, we found no role for CNGA2 in feedback modulation. Instead, an IQ-type calmodulin-binding site on CNGB1b and a similar but previously unidentified site on CNGA4 are necessary and sufficient. These sites seem to confer binding of Ca(2+)-free calmodulin (apocalmodulin), which is then poised to trigger inhibition of native channels in the presence of Ca(2+).

Regulation of the mouse epithelial Ca2(+) channel TRPV6 by the Ca(2+)-sensor calmodulin

TRPV5 and TRPV6 are members of the superfamily of transient receptor potential (TRP) channels and facilitate Ca(2+) influx in a variety of epithelial cells. The activity of these Ca(2+) channels is tightly controlled by the intracellular Ca(2+) concentration in close vicinity to the channel mouth. The molecular mechanism underlying the Ca(2+)-dependent activity of TRPV5/TRPV6 is, however, still unknown. Here, the putative role of calmodulin (CaM) as the Ca(2+) sensor mediating the regulation of channel activity was investigated. Overexpression of Ca(2+)-insensitive CaM mutants (CaM(1234) and CaM(34)) significantly reduced the Ca(2+) as well as the Na(+) current of TRPV6- but not that of TRPV5-expressing HEK293 cells. By combining pull-down assays and co-immunoprecipitations, we demonstrated that CaM binds to both TRPV5 and TRPV6 in a Ca(2+)-dependent fashion. The binding of CaM to TRPV6 was localized to the transmembrane domain (TRPV6(327-577)) and consensus CaM-binding motifs located in the N (1-5-10 motif, TRPV6(88-97)) and C termini (1-8-14 motif, TRPV6(643-656)), suggesting a mechanism of regulation involving multiple interaction sites. Subsequently, chimeric TRPV6/TRPV5 proteins, in which the N and/or C termini of TRPV6 were substituted by that of TRPV5, were co-expressed with CaM(34) in HEK293 cells. Exchanging, the N and/or the C termini of TRPV6 by that of TRPV5 did not affect the CaM(34)-induced reduction of the Ca(2+) and Na(+) currents. These results suggest that CaM positively affects TRPV6 activity upon Ca(2+) binding to EF-hands 3 and 4, located in the high Ca(2+) affinity CaM C terminus, which involves the N and C termini and the transmembrane domain of TRPV6.

Cellular processing of cone photoreceptor cyclic GMP-gated ion channels: a role for the S4 structural motif

We examined cellular protein processing and functional expression of photoreceptor cyclic nucleotide-gated (CNG) ion channels. In a mammalian cell line, wild type bovine cone photoreceptor channel alpha subunits (bCNGA3) convert from an unglycosylated state, at 90 kDa, to two glycosylated states at 93 and 102 kDa as they transit within the cell to their final location at the plasma membrane. Glycosylation per se is not required to yield functional channels, yet it is a milestone that distinguishes sequential steps in channel protein maturation. CNG ion channels are not gated by membrane voltage although their structure includes the transmembrane S4 motif known to function as the membrane voltage sensor in all voltage-gated ion channels. S4 must be functionally important because its natural mutation in cone photoreceptor CNG channels is associated with achromatopsia, a human autosomal inherited loss of cone function. Point mutation of specific, not all, charged and neutral residues within S4 cause failure of functional channel expression. Cellular channel protein processing fails in every one of the non-functional S4 mutations we studied. Mutant proteins do not reach the 102-kDa glycosylated state and do not arrive at the plasma membrane. They remain trapped within the endoplasmic reticulum and fail to transit out to the Golgi apparatus. Coexpression of cone CNG beta subunit (CNGB3) does not rescue the consequence of S4 mutations in CNGA3. It is likely that an intact S4 is required for proper protein folding and/or assembly in the endoplasmic reticulum membrane.

Achromatopsia-associated mutation in the human cone photoreceptor cyclic nucleotide-gated channel CNGB3 subunit alters the ligand sensitivity and pore properties of heteromeric channels

Cone photoreceptor cyclic nucleotide-gated (CNG) channels are thought to form by assembly of two different subunit types, CNGA3 and CNGB3. Recently, mutations in the gene encoding the CNGB3 subunit have been linked to achromatopsia in humans. Here we describe the functional consequences of two achromatopsia-associated mutations in human CNGB3 (hCNGB3). Co-expression in Xenopus oocytes of human CNGA3 (hCNGA3) subunits with hCNGB3 subunits containing an achromatopsia-associated mutation in the S6 transmembrane domain (S435F) generated functional heteromeric channels that exhibited an increase in apparent affinity for both cAMP and cGMP compared with wild type heteromeric channels. In contrast, co-expression of a presumptive null mutation of hCNGB3 (T383f.s.Delta C) with hCNGA3 produced channels with properties indistinguishable from homomeric hCNGA3 channels. The effect of hCNGB3 S435F subunits on cell-surface expression of green fluorescent protein-tagged hCNGA3 subunits and of non-tagged hCNGA3 subunits on surface expression of green fluorescent protein-hCNGB3 S435F subunits were similar to those observed for wild type hCNGB3 subunits, suggesting that the mutation does not grossly disturb subunit assembly or plasma membrane targeting. The S435F mutation was also found to produce changes in the pore properties of the channel, including decreased single channel conductance and decreased sensitivity to block by l-cis-diltiazem. Overall, these results suggest that the functional properties of cone CNG channels may be altered in patients with the S435F mutation, providing evidence supporting the pathogenicity of this mutation in humans. Thus, achromatopsia may arise from a disturbance of cone CNG channel gating and permeation or from the absence of functional CNGB3 subunits.