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

Ancient multiplicity in cyclic nucleotide-gated (CNG) cation channel repertoire was reduced in the ancestor of Olfactores before re-expansion by whole genome duplications in vertebrates

Cyclic nucleotide-gated (CNG) cation channels are important heterotetrameric proteins in the retina, with different subunit composition in cone and rod photoreceptor cells: three CNGA3 and one CNGB3 in cones and three CNGA1 and one CNGB1 in rods. CNGA and CNGB subunits form separate subfamilies. We have analyzed the evolution of the CNG gene family in metazoans, with special focus on vertebrates by using sequence-based phylogeny and conservation of chromosomal synteny to deduce paralogons resulting from the early vertebrate whole genome duplications (WGDs). Our analyses show, unexpectedly, that the CNGA subfamily had four sister subfamilies in the ancestor of bilaterians and cnidarians that we named CNGC, CNGD, CNGE and CNGF. Of these, CNGC, CNGE and CNGF were lost in the ancestor of Olfactores while CNGD was lost in the vertebrate ancestor. The remaining CNGA and CNGB genes were expanded by a local duplication of CNGA and the subsequent chromosome duplications in the basal vertebrate WGD events. Upon some losses, this resulted in the gnathostome ancestor having three members in the visual CNGA subfamily (CNGA1-3), a single CNGA4 gene, and two members in the CNGB subfamily (CNGB1 and CNGB3). The nature of chromosomal rearrangements in the vertebrate CNGA paralogon was resolved by including the genomes of a non-teleost actinopterygian and an elasmobranch. After the teleost-specific WGD, additional duplicates were generated and retained for CNGA1, CNGA2, CNGA3 and CNGB1. Furthermore, teleosts retain a local duplicate of CNGB3. The retention of duplicated CNG genes is explained by their subfunctionalisation and photoreceptor-specific expression. In conclusion, this study provides evidence for four previously unknown CNG subfamilies in metazoans and further evidence that the early vertebrate WGD events were instrumental in the evolution of the vertebrate visual and central nervous systems.

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.

CNGA3 is expressed in inner ear hair cells and binds to an intracellular C-terminus domain of EMILIN1

The molecular characteristics of CNG (cyclic nucleotide-gated) channels in auditory/vestibular hair cells are largely unknown, unlike those of CNG mediating sensory transduction in vision and olfaction. In the present study we report the full-length sequence for three CNGA3 variants in a hair cell preparation from the trout saccule with high identity to CNGA3 in olfactory receptor neurons/cone photoreceptors. A custom antibody targeting the N-terminal sequence immunolocalized CNGA3 to the stereocilia and subcuticular plate region of saccular hair cells. The cytoplasmic C-terminus of CNGA3 was found by yeast two-hybrid analysis to bind the C-terminus of EMILIN1 (elastin microfibril interface-located protein 1) in both the vestibular hair cell model and rat organ of Corti. Specific binding between CNGA3 and EMILIN1 was confirmed with surface plasmon resonance analysis, predicting dependence on Ca2+ with Kd=1.6×10-6 M for trout hair cell proteins and Kd=2.7×10-7 M for organ of Corti proteins at 68 μM Ca2+. Pull-down assays indicated that the binding to organ of Corti CNGA3 was attributable to the EMILIN1 intracellular sequence that follows a predicted transmembrane domain in the C-terminus. Saccular hair cells also express the transcript for PDE6C (phosphodiesterase 6C), which in cone photoreceptors regulates the degradation of cGMP used to gate CNGA3 in phototransduction. Taken together, the evidence supports the existence in saccular hair cells of a molecular pathway linking CNGA3, its binding partner EMILIN1 (and β1 integrin) and cGMP-specific PDE6C, which is potentially replicated in cochlear outer hair cells, given stereociliary immunolocalizations of CNGA3, EMILIN1 and PDE6C.

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.

Functional significance of the taper of vertebrate cone photoreceptors

Vertebrate photoreceptors are commonly distinguished based on the shape of their outer segments: those of cones taper, whereas the ones from rods do not. The functional advantages of cone taper, a common occurrence in vertebrate retinas, remain elusive. In this study, we investigate this topic using theoretical analyses aimed at revealing structure–function relationships in photoreceptors. Geometrical optics combined with spectrophotometric and morphological data are used to support the analyses and to test predictions. Three functions are considered for correlations between taper and functionality. The first function proposes that outer segment taper serves to compensate for self-screening of the visual pigment contained within. The second function links outer segment taper to compensation for a signal-to-noise ratio decline along the longitudinal dimension. Both functions are supported by the data: real cones taper more than required for these compensatory roles. The third function relates outer segment taper to the optical properties of the inner compartment whereby the primary determinant is the inner segment’s ability to concentrate light via its ellipsoid. In support of this idea, the rod/cone ratios of primarily diurnal animals are predicted based on a principle of equal light flux gathering between photoreceptors. In addition, ellipsoid concentration factor, a measure of ellipsoid ability to concentrate light onto the outer segment, correlates positively with outer segment taper expressed as a ratio of characteristic lengths, where critical taper is the yardstick. Depending on a light-funneling property and the presence of focusing organelles such as oil droplets, cone outer segments can be reduced in size to various degrees. We conclude that outer segment taper is but one component of a miniaturization process that reduces metabolic costs while improving signal detection. Compromise solutions in the various retinas and retinal regions occur between ellipsoid size and acuity, on the one hand, and faster response time and reduced light sensitivity, on the other.

Evolution of vertebrate rod and cone phototransduction genes

Vertebrate cones and rods in several cases use separate but related components for their signal transduction (opsins, G-proteins, ion channels, etc.). Some of these proteins are also used differentially in other cell types in the retina. Because cones, rods and other retinal cell types originated in early vertebrate evolution, it is of interest to see if their specific genes arose in the extensive gene duplications that took place in the ancestor of the jawed vertebrates (gnathostomes) by two tetraploidizations (genome doublings). The ancestor of teleost fishes subsequently underwent a third tetraploidization. Our previously reported analyses showed that several gene families in the vertebrate visual phototransduction cascade received new members in the basal tetraploidizations. We here expand these data with studies of additional gene families and vertebrate species. We conclude that no less than 10 of the 13 studied phototransduction gene families received additional members in the two basal vertebrate tetraploidizations. Also the remaining three families seem to have undergone duplications during the same time period but it is unclear if this happened as a result of the tetraploidizations. The implications of the many early vertebrate gene duplications for functional specialization of specific retinal cell types, particularly cones and rods, are discussed.

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.

Functional characterization and molecular cloning of the K+-dependent Na+/Ca2+ exchanger in intact retinal cone photoreceptors

Light-dependent changes in cytoplasmic free Ca(2+) are much faster in the outer segment of cone than rod photoreceptors in the vertebrate retina. In the limit, this rate is determined by the activity of an electrogenic Na(+)/Ca(2+) exchanger located in the outer segment plasma membrane. We investigate the functional properties of the exchanger activity in intact, single cone photoreceptors isolated from striped bass retina. Exchanger function is characterized through analysis both of the electrogenic exchanger current and cytoplasmic free Ca(2+) measured with optical probes. The exchanger in cones is K(+) dependent and operates both in forward and reverse modes. In the reverse mode, the K(+) dependence of the exchanger is described by binding to a single site with K(1/2) about 3.6 mM. From the retina of the fish we cloned exchanger molecules bassNCKX1 and bassNCKX2. BassNCKX1 is a single class of molecules, homologous to exchangers previously cloned from mammalian rods. BassNCKX2 exists in four splice variants that differ from each other by small sequence differences in the single, large cytoplasmic loop characteristic of these molecules. We used RT-PCR (reverse transcriptase polymerase chain reaction) of individual cells to identify the exchanger molecule specifically expressed in bass single and twin cone photoreceptors. Each and every one of the four bassNCKX2 splice variants is expressed in both single and twin cones indistinguishably. BassNCKX1 is not expressed in cones and, by exclusion, it is likely to be an exchanger expressed in rods.