Primary structure and chromosomal localization of human and mouse rod photoreceptor cGMP-gated cation channel

Novel compound heterozygous mutation in the CNGA1 gene underlie autosomal recessive retinitis pigmentosa in a Chinese family

A novel compound mutation in CNGA1 gene, coding for the cGMP-gated ion channel protein, results in a protein product that is not targeted to the plasma membrane, which would be deleterious to rod photoreceptors leading to retinitis pigmentosa (RP).

Retinitis pigmentosa (RP) describes a group of inherited retinopathies that are characterized by the progressive degeneration of photoreceptor neurons, which causes night blindness, a reduction in the peripheral visual field and decreased visual acuity. More than 50 RP-related genes have been identified. In the present study, we analysed a Chinese family with autosomal recessive RP. We identified a compound heterozygous mutation, c.265delC and c.1537G>A, in CNGA1 using targeted next-generation sequencing (NGS) of RP-causing genes. The mutations were validated in the family members by Sanger sequencing. The mutations co-segregated with the RP phenotype and were absent from ethnically-matched control chromosomes. The mutant (mut) CNGA1 p.(G513R) protein caused by the mis-sense novel mutation c.1537G>A was expressed in vitro. The mut CNGA1 p.(G513R) protein was largely retained inside the cell rather than being targeted to the plasma membrane, suggesting the absence of cGMP-gated cation channels in the plasma membrane would be deleterious to rod photoreceptors, leading lead to RP.

A Cyclic GMP-Dependent K+ Channel in the Blastocladiomycete Fungus Blastocladiella emersonii

Phototaxis in flagellated zoospores of the aquatic fungus Blastocladiella emersonii depends on a novel photosensor, Blastocladiella emersonii GC1 (BeGC1), comprising a type I (microbial) rhodopsin fused to a guanylyl cyclase catalytic domain, that produces the conserved second messenger cyclic GMP (cGMP). The rapid and transient increase in cGMP levels during the exposure of zoospores to green light was shown to be necessary for phototaxis and dependent on both rhodopsin function and guanylyl cyclase activity. It is noteworthy that BeGC1 was localized to the zoospore eyespot apparatus, in agreement with its role in the phototactic response. A putative cyclic nucleotide-gated channel (BeCNG1) was also identified in the genome of the fungus and was implicated in flagellar beating via the action of a specific inhibitor (l-cis-diltiazem) that compromised zoospore motility. Here we show that B. emersonii expresses a K(+) channel that is activated by cGMP. The use of specific channel inhibitors confirmed the activation of the channel by cGMP and its K(+) selectivity. These characteristics are consistent with the function of an ion channel encoded by the BeCNG1 gene. Other blastocladiomycete fungi, such as Allomyces macrogynus and Catenaria anguillulae, possess genes encoding a similar K(+) channel and the rhodopsin-guanylyl cyclase fusion protein, while the genes encoding both these proteins are absent in nonflagellated fungi. The presence of these genes as a pair seems to be an exclusive feature of blastocladiomycete fungi. Taken together, these data demonstrate that the B. emersonii cGMP-activated K(+) channel is involved in the control of zoospore motility, most probably participating in the cGMP-signaling pathway for the phototactic response of the fungus.

A rhodopsin-guanylyl cyclase gene fusion functions in visual perception in a fungus

Sensing light is the fundamental property of visual systems, with vision in animals being based almost exclusively on opsin photopigments [1]. Rhodopsin also acts as a photoreceptor linked to phototaxis in green algae [2, 3] and has been implicated by chemical means as a light sensor in the flagellated swimming zoospores of the fungus Allomyces reticulatus [4]; however, the signaling mechanism in these fungi remains unknown. Here we use a combination of genome sequencing and molecular inhibition experiments with light-sensing phenotype studies to examine the signaling pathway involved in visual perception in the closely related fungus Blastocladiella emersonii. Our data show that in these fungi, light perception is accomplished by the function of a novel gene fusion (BeGC1) of a type I (microbial) rhodopsin domain and guanylyl cyclase catalytic domain. Photobleaching of rhodopsin function prevents accumulation of cGMP levels and phototaxis of fungal zoospores exposed to green light, whereas inhibition of guanylyl cyclase activity negatively affects fungal phototaxis. Immunofluorescence microscopy localizes the BeGC1 protein to the external surface of the zoospore eyespot positioned close to the base of the swimming flagellum [4, 5], demonstrating this is a photoreceptive organelle composed of lipid droplets. Taken together, these data indicate that Blastocladiomycota fungi have a cGMP signaling pathway involved in phototaxis similar to the vertebrate vision-signaling cascade but composed of protein domain components arranged as a novel gene fusion architecture and of distant evolutionary ancestry to type II rhodopsins of animals.

Electrophysiologic and phenotypic features of an autosomal cone-rod dystrophy caused by a novel CRX mutation

PURPOSE

To reexamine a large Albertan family previously reported with a progressive cone dystrophy with variable phenotype and to map the disorder using molecular genetic techniques.

DESIGN

Observational case series.

PARTICIPANTS

Twenty-nine subjects (10 affected) from four generations of a large kindred were clinically examined. Twenty-three of these individuals, as well as two unaffected spouses, were included in the molecular genetic study. Subject ages ranged from 17 to 91 years of age.

METHODS

Disease status and associated ocular abnormalities were assessed primarily by measurement of visual acuity, color vision, fundus photography, and both full-field and multifocal electroretinography (ERG and mfERG). Linkage of the disorder to the rhodopsin gene was studied using microsatellites. A mutational screen of the CRX gene was performed to identify coding sequence changes.

MAIN OUTCOME MEASURES

Visual acuity and color discrimination were reduced in clinically affected individuals; full-field flash ERG was used to measure function of both cones and rods. mfERG and fundus photography allowed documentation of the observed macular changes.

RESULTS

We noted a variable, adult-onset macular dystrophy, progressing in some cases to a retinitis pigmentosa-like phenotype. Both photopic and scotopic full-field ERG amplitudes were reduced by approximately 50%, demonstrating involvement of both photoreceptor systems. A reduced b-wave amplitude with a relatively preserved a-wave was observed at both cone and rod levels. Macular involvement was confirmed by mfERG. The rhodopsin locus was excluded by haplotype analysis. A novel frameshift mutation was detected in exon III of the CRX retinal homeobox gene. ERG and molecular genetic findings were consistent with the reclassification of this disease as an autosomal dominant cone-rod dystrophy (CRD) CONCLUSIONS: We report a novel CRX mutation causing autosomal dominant CRD. Observed ERG changes suggest that this mutation primarily impairs inner retinal function. Because retinal expression of CRX is limited to photoreceptors, this dysfunction may be the result of faulty photoreceptor communication with second-order retinal neurons. We propose misexpression of gated cation channels caused by altered CRX activity as one putative mechanism by which a sole photoreceptor defect may selectively impair neurotransmission without disrupting the upstream events of phototransduction.

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.

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

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

Cloning and immunocytochemical localization of a cyclic nucleotide-gated channel alpha-subunit to all cone photoreceptors in the mouse retina

Cyclic nucleotide-gated channels (CNGC) are ligand-gated ion channels that open and close in response to changes in the intracellular concentration of the second messengers, 3;,5;-cyclic adenosine monophosphate and 3;,5;-cyclic guanosine monophosphate. Most notably, they transduce the chemical signal produced by the absorption of light in photoreceptors into a membrane potential change, which is then transmitted to the ascending visual pathway. CNGCs have also been implicated in the signal transduction of other neurons downstream of the photoreceptors, in particular the ON-bipolar cells, as well as in other areas of the central nervous system. We therefore undertook a search for additional cyclic nucleotide-gated channels expressed in the retina. Following a degenerate reverse transcription polymerase chain reaction approach to amplify low-copy number messages, a cDNA encoding a new splice variant of CNGC alpha-subunit was isolated from mouse retina and classified as mCNG3. An antiserum raised against the carboxy-terminal sequence identified the retinal cell type expressing mCNG3 as cone photoreceptors. Preembedding immunoelectron microscopy demonstrated its membrane localization in the outer segments, consistent with its role in phototransduction. Double-labeling experiments with cone-specific markers indicated that all cone photoreceptors in the murid retina use the same or a highly conserved cyclic nucleotide-gated channel. Therefore, defects in this channel would be predicted to severely impair photopic vision.

Mutating three residues in the bovine rod cyclic nucleotide-activated channel can switch a nucleotide from inactive to active

Cyclic nucleotide-gated (CNG) channels, which were initially studied in retina and olfactory neurons, are activated by cytoplasmic cGMP or cAMP. Detailed comparisons of nucleotide-activated currents using nucleotide analogs and mutagenesis revealed channel-specific residues in the nucleotide-binding domain that regulate the binding and channel-activation properties. Of particular interest are N(1)-oxide cAMP, which does not activate bovine rod channels, and Rp-cGMPS, which activates bovine rod, but not catfish, olfactory channels. Previously, we showed that four residues coordinate the purine interactions in the binding domain and that three of these residues vary in the alpha subunits of the bovine rod, catfish, and rat olfactory channels. Here we show that both N(1)-oxide cAMP and Rp-cGMPS activate rat olfactory channels. A mutant of the bovine rod alpha subunit, substituted with residues from the rat olfactory channel at the three variable positions, was weakly activated by N(1)-oxide cAMP, and a catfish olfactory-like bovine rod mutant lost activation by Rp-cGMPS. These experiments underscore the functional importance of purine contacts with three residues in the cyclic nucleotide-binding domain. Molecular models of nucleotide analogs in the binding domains, constructed with AMMP, showed differences in the purine contacts among the channels that might account for activation differences.

Genomic organization of the human rod photoreceptor cGMP-gated cation channel beta-subunit gene

We previously reported that the CNGB1 locus encoding the rod photoreceptor cGMP-gated channel beta-subunit is complex, comprising non-overlapping transcription units that give rise to at least six transcripts (Ardell, M.D., Aragon, I., Oliveira, L., Porche, G.E., Burke, E., Pittler, S.J., 1996. The beta subunit of human rod photoreceptor cGMP-gated cation channel is generated from a complex transcription unit. FEBS Lett. 389, 213-218). To further understand the transcriptional regulation of this extraordinarily complex locus, and to develop a screen for defects in the gene in patients with hereditary disease, we determined its genomic organization and DNA sequence. The CNGB1 locus consists of 33 exons, which span approximately 100kb of genomic DNA on chromosome 16. The beta-subunit comprises two domains, an N-terminal glutamic acid-rich segment (GARP), and a C-terminal channel-like portion. Two additional exons encoding a short GARP transcript and a truncated channel-like transcript have been identified. A major transcription start point was identified 79bp upstream of the initiator ATG. To begin analysis of the basis for the generation of multiple transcripts, and to identify promoters driving expression in retina, approximately 2.5kb of the upstream region were sequenced. Putative cis-elements, which can bind the retina-specific transcription factors Crx and Erx, were found immediately upstream of the transcription start point, and may be important for gene expression in this tissue. From our analysis, a model is reported to account for at least four of the retinal transcripts.

Cyclic nucleotide-gated channels in non-sensory organs

Cyclic nucleotide-gated channels represent a class of ion channels activated directly by the binding of either cyclic-GMP or cyclic-AMP. They carry both mono and divalent cations, but select calcium over sodium. In the majority of the cases studied, binding of cyclic nucleotides to the channel results in the opening of the channel and the influx of calcium. As a consequence, cytosolic free calcium levels increase leading to the modifications of calcium-dependent processes. This represents and important link in the chain of events leading to the physiological response. Cyclic nucleotide-gated channels were discovered in sensory cell types, in the retina, and in olfactory cells, and were extensively studied in those cells. However, it is becoming increasingly evident that such channels are present not only in sensory systems, but in most, if not all, cell types where cyclic nucleotides play a role in signal transduction. A hypothesis is presented here which attributes physiological importance to these channels in non-sensory organs. Four examples of such channels in non-sensory cells are discussed in detail: those in the liver, in the heart, in the brain, and in the testis with the emphasis on the possible physiological roles that these channels might have in these organs.

Selective loss of cone function in mice lacking the cyclic nucleotide-gated channel CNG3

Two types of photoreceptors, rods and cones, coexist in the vertebrate retina. An in-depth analysis of the retinal circuitry that transmits rod and cone signals has been hampered by the presence of intimate physical and functional connections between rod and cone pathways. By deleting the cyclic nucleotide-gated channel CNG3 we have generated a mouse lacking any cone-mediated photoresponse. In contrast, the rod pathway is completely intact in CNG3-deficient mice. The functional loss of cone function correlates with a progressive degeneration of cone photoreceptors but not of other retinal cell types. CNG3-deficient mice provide an animal model to dissect unequivocally the contribution of rod and cone pathways for normal retinal function.

Identification of a cyclic nucleotide- and voltage-activated ion channel from insect antennae

From an antennal cDNA library of Heliothis virescens a clone has been isolated encoding a polypeptide of 678 amino acids. Data base comparisons and primary structure analysis of the deduced protein sequence (HvCNG) indicated significant homology to cyclic nucleotide and voltage-activated ion channels including six putative membrane spanning domains, a putative cyclic nucleotide binding site, a pore region and a voltage-sensor motif. Heterologous expression of the cloned cDNA in Sf9 cells resulted in a polypeptide of the predicted molecular mass. Patch clamp analysis allowed to record the activity of the identified HvCNG channels; they were activated by cAMP but also by hyperpolarization. The channel displayed in potassium solution a conductance of 30 pS; the ion selectivity was calculated as PK/PNa approximately 3. Northern blot analysis revealed that the channel is highly expressed in the antennae; weaker signal were detected in heads and legs. In situ hybridization of tissue sections through the antennae showed a spatial distribution of reactive cells; they are located beneath sensillar hairs. Thus, a novel channel type has been identified which may play an important role in antennal cells.

Molecular cloning of a putative cyclic nucleotide-gated ion channel cDNA from Limulus polyphemus

Cyclic nucleotide-gated channels have been proposed to mediate the electrical response to light in the ventral photoreceptor cells of the horseshoe crab, Limulus polyphemus. However, a cyclic nucleotide-gated channel has not been identified from Limulus. We have cloned a putative full-length cyclic nucleotide-gated channel cDNA by screening cDNA libraries constructed from Limulus brain using a probe developed from Limulus ventral eye nerves. The putative full-length cDNA was derived from two overlapping partial cDNA clones. The open reading frame encodes 905 amino acids; the sequence shows 44% identity to that of the alpha subunit of the bovine rod cyclic GMP-gated channel over the region containing the transmembrane domains and the cyclic nucleotide binding domain. This Limulus channel has a novel C-terminal region of approximately 200 amino acids, containing three putative Src homology domain 3 binding motifs and a putative coiled-coil domain. The possibility that this cloned channel is the same as that detected previously in excised patches from the photoreceptive membrane of Limulus ventral photoreceptors is discussed in terms of its sequence and its expression in the ventral eye nerves.

Retinitis pigmentosa: defined from a molecular point of view

Retinitis pigmentosa (RP) denotes a group of hereditary retinal dystrophies, characterized by the early onset of night blindness followed by a progressive loss of the visual field. The primary defect underlying RP affects the function of the rod photoreceptor cell, and, subsequently, mostly unknown molecular and cellular mechanisms trigger the apoptotic degeneration of these photoreceptor cells. Retinitis pigmentosa is very heterogeneous, both phenotypically and genetically. In this review we propose a tentative classification of RP based on the functional systems affected by the mutated proteins. This classification connects the variety of phenotypes to the mutations and segregation patterns observed in RP. Current progress in the identification of the molecular defects underlying RP reveals that at least three distinct functional mechanisms may be affected: 1) the daily renewal and shedding of the photoreceptor outer segments, 2) the visual transduction cascade, and 3) the retinol (vitamin A) metabolism. The first group includes the rhodopsin and peripherin/RDS genes, and mutations in these genes often result in a dominant phenotype. The second group is predominantly associated with a recessive phenotype that results, as we argue, from continuous inactivation of the transduction pathway. Disturbances in the retinal metabolism seem to be associated with equal rod and cone involvement and the presence of deposits in the retinal pigment epithelium.

Molecular cloning of cDNA encoding the alpha unit of CNGC gene from human fetal heart

Cyclic nucleotide-gated ion channels (CNGCs) play crucial roles in visual and olfactory signal transduction. As a first step to explore the presence of a CNGC gene in human heart, we cloned a human heart CNGC gene. The sequence consists of 111 bp 5' non-coding region and a 2064 bp open reading frame which is followed by a 459 bp 3' non-coding region. The predicted protein consists of 688 amino acids with a short highly charged segment rich in lysine and glutamate. Sequence comparison indicates that the human heart cDNA is almost identical to the retinal rod photo receptor CNGC cDNA. However, the human cardiac cDNA is lacking a 205 bp Alu fragment in the 5'-uncoding region, has a glutamic acid residue at amino acid position 129, and has a replacement of glutamic acid with a lysine residue at amino acid position 99. Data obtained with northern blot analysis confirm the presence of RNA for the CNGC alpha chain. This channel might play a role in cyclic nucleotide-mediated cellular processes, such as the inotropic effect in the heart.

Molecular and pharmacological analysis of cyclic nucleotide-gated channel function in the central nervous system

Most functional studies of cyclic nucleotide-gated (CNG) channels have been confined to photoreceptors and olfactory epithelium, in which CNG channels are abundant and easy to study. The widespread distribution of CNG channels in tissues throughout the body has only recently been recognized and the functions of this channel family in many of these tissues remain largely unknown. The molecular biological and pharmacological properties of the CNG channel family are summarized in order to put in context studies aimed at probing CNG channel functions in these tissues using pharmacological and genetic methods. Compounds have now been identified that are useful in distinguishing CNG channel activated pathways from cAMP/cGMP dependent-protein kinases or other pathways. The ways in which these interact with CNG channels are understood and this knowledge is leading to the identification of more potent and more specific CNG channel subtype-specific agonists or antagonists. Recent molecular and genetic analyses have identified novel roles of CNG channels in neuronal development and plasticity in both invertebrates and vertebrates. Targeting CNG channels via specific drugs and genetic manipulation (such as knockout mice) will permit better understanding of the role of CNG channels in both basic and higher orders of brain function.

Total colourblindness is caused by mutations in the gene encoding the alpha-subunit of the cone photoreceptor cGMP-gated cation channel

Total colourblindness (OMIM 216900), also referred to as rod monochromacy (RM) or complete achromatopsia, is a rare, autosomal recessive inherited and congenital disorder characterized by photophobia, reduced visual acuity, nystagmus and the complete inability to discriminate between colours. Electroretinographic recordings show that in RM, rod photoreceptor function is normal, whereas cone photoreceptor responses are absent. The locus for RM has been mapped to chromosome 2q11 (ref. 2), however the gene underlying RM has not yet been identified. Recently, a suitable candidate gene, CNGA3, encoding the alpha-subunit of the cone photoreceptor cGMP-gated cation channel, a key component of the phototransduction pathway, has been cloned and assigned to human chromosome 2q11 (refs 3,4). We report the identification of missense mutations in CNGA3 in five families with RM. Homozygous mutations are present in two families, whereas the remaining families show compound heterozygous mutations. In all cases, the segregation pattern of the mutations is consistent with the autosomal recessive inheritance of the disease and all mutations affect amino acids that are highly conserved among cyclic nucleotide gated channels (CNG) in various species. This is the first report of a colour vision disorder caused by defects other than mutations in the cone pigment genes, and implies at least in this instance a common genetic basis for phototransduction in the three different cone photoreceptors of the human retina.

PEST sequences in proteins involved in cyclic nucleotide signalling pathways

There is growing evidence that PEST sequences act as proteolytic recognition signals within polypeptides. PEST sequences are rich in proline (P), glutamic acid (E), serine (S), and threonine (T) and can be identified by the PEST-FIND program. Both the catalytic and regulatory subunits of the cAMP-dependent protein kinase have been shown to have conditional PEST sequences which are exposed upon cAMP binding to the enzyme. cAMP binding leads to rapid dissociation of C- and R-subunits, and both subunits have increased sensitivity to proteolysis. It is not known whether other proteins that participate in the cyclic nucleotide signalling pathway have PEST regions in their amino acid sequences. Therefore, we have screened amino acid sequences of proteins that are directly involved in cyclic nucleotide cascade, including cGMP-dependent protein kinases, anchoring proteins for cAMP-dependent protein kinase, cyclic nucleotide-gated ion channels, and cyclic nucleotide phosphodiesterases, for PEST sequences using the PEST-FIND program. Many PEST sequences with high scores have been identified in these proteins. The occurrence of the PEST sequences is very high in proteins involved in cyclic nucleotide signalling pathways (approximately 80%). This value is much higher than the percentage (10%) of PEST sequences that can be found in the primary structures of the proteins listed in the data bank. This frequent occurrence of PEST sequences in proteins involved in cyclic nucleotide action and metabolism suggests an important role of proteolysis of these key proteins of signal transduction.

Characterization of canine rod photoreceptor cGMP-gated cation channel alpha-subunit gene and exclusion of its involvement in the hereditary retinal dystrophy of Swedish Briards

The nucleotide sequence of the canine rod photoreceptor cGMP-gated cation channel alpha-subunit (cCNCG1) cDNA has been determined. The open reading frame consists of 2073 nucleotides, which encode a putative protein of 691 amino acids. In addition, we have established the exon/intron boundaries of the cCNCG1 gene and determined the complete sequence of six introns of a total of eight. The exon/intron organization (location and length of exons and introns) of the cCNCG1 gene is very similar to that of the human rod photoreceptor cGMP-gated cation channel alpha-subunit gene. We used single-strand conformation polymorphism analysis to search for potential pathogenic sequence changes in the cCNCG1 gene in a Swedish Briard and Briard-Beagle dog kindred, in which an autosomal recessive retinal dystrophy is segregating, a disease which shows phenotypic similarities to retinitis pigmentosa, a heterogeneous group of hereditary and progressive retinal degeneration in human. In intron 3, we found several DNA polymorphisms, which do not cosegregate with the affected status of the dogs, thus excluding cCNCG1 as a candidate gene for the retinal dystrophy in this strain of Swedish Briards.

Gene-based approach to human gene-phenotype correlations

Elucidating the genetic basis of human phenotypes is a major goal of contemporary geneticists. Logically, two fundamental and contrasting approaches are available, one that begins with a phenotype and concludes with the identification of a responsible gene or genes; the other that begins with a gene and works toward identifying one or more phenotypes resulting from allelic variation of it. This paper provides a conceptual overview of phenotype-based vs. gene-based procedures with emphasis on gene-based methods. A key feature of a gene-based approach is that laboratory effort first is devoted to developing an assay for mutations in the gene under regard; the assay then is applied to the evaluation of large numbers of unrelated individuals with a variety of phenotypes that are deemed potentially resulting from alleles at the gene. No effort is directed toward chromosomally mapping the loci responsible for the phenotypes scanned. Example is made of my laboratory's successful use of a gene-based approach to identify genes causing hereditary diseases of the retina such as retinitis pigmentosa. Reductions in the cost and improvements in the speed of scanning individuals for DNA sequence anomalies may make a gene-based approach an efficient alternative to phenotype-based approaches to correlating genes with phenotypes.

Mouse extracellular superoxide dismutase: primary structure, tissue-specific gene expression, chromosomal localization, and lung in situ hybridization

Extracellular superoxide dismutase (EC-SOD) is the major extracellular antioxidant enzyme. We have determined the primary structure of mouse EC-SOD by characterization of complementary DNA (cDNA) clones and by amino-acid sequence analysis of purified protein. cDNA sequence analysis indicates that mouse EC-SOD is synthesized as a 251-amino-acid precursor protein with a predicted molecular weight of 27,400 D. Amino-terminal micro sequence analysis of purified mature mouse lung EC-SOD demonstrated the sequence to begin with SSFDLADRLDPV-. These results indicate that EC-SOD as initially synthesized contains a 24-amino-acid precursor peptide, and that the mature protein is 227 amino acids in length. Computer algorithms that predict the most likely site of cotranslational signal peptidase cleavage suggest that processing will occur between amino acids 18 and 19 or 20 and 21, which implies that EC-SOD may be initially synthesized as a pre-pro-protein. Like human EC-SOD, mature mouse EC-SOD is glycosylated. The full-length mouse EC-SOD cDNA is 1,834 base pairs long and is 82% (79% for protein) identical to rat EC-SOD, but only 60% (60% for protein) identical to human EC-SOD. The mouse EC-SOD gene locus (Sod3) was mapped by interspecific backcross haplotype analysis as being 0.9 +/- 0.9 centimorgans distal to the Qdpr locus on mouse Chromosome 5, a position suggesting that the human homologue of EC-SOD will map close to the human QDPR locus (4p15.3). Of nine tissues examined by Northern blot analysis, those of the kidney and lung are by far the major tissues that express EC-SOD messenger RNA. Using in situ hybridization in the mouse lung, we demonstrate EC-SOD gene expression to be highly localized to alveolar Type II epithelial cells. These data suggest that alveolar Type II cells play a central role in mediating EC-SOD antioxidant function in the lung.

Alternatively spliced forms of the cGMP-gated channel in human keratinocytes

Alternatively spliced forms of the alpha subunit of the cGMP-gated channel have been cloned from human keratinocytes. One form encodes a complete channel which is almost identical to the rod photoreceptor. A second spliced variant would encode a protein missing a portion of the intracellular hydrophilic domain and the putative first transmembrane domain. Both complete and spliced variants of the channel also were found in epidermis. The expression of the complete form of the channel was induced by levels of extracellular calcium which promote keratinocyte differentiation. The cGMP-gated channel may play an important role in calcium induced keratinocyte differentiation by mediating Ca2+ entry.

Canine rod photoreceptor cGMP-gated channel protein alpha-subunit: studies on the expression of the gene and characterization of the cDNA

Rod photoreceptor cyclic GMP gated-channel protein is a key component of the visual transduction cascade in the vertebrate retina. The protein is composed of at least two subunits (alpha and beta). Mutations in the alpha-subunit (CNGC1) have been shown to cause retinitis pigmentosa (RP) in humans. Several heterogeneous canine retinal diseases, which are clinically similar to RP, are known collectively as progressive retinal atrophy (PRA) and occur in dogs in a breed-specific manner. For the purpose of examining CNGC1 gene as a candidate for PRA, we report here the characterization of canine CNGC1 cDNA, and examine the expression of the gene in different tissues by northern analysis, reverse transcription and polymerase chain reaction (RT-PCR), and retinal immunocytochemistry. The characterized canine CNGC1 cDNA sequence contains 2717 nucleotides which include 211 bp 5"-untranslated region and 430 bp 3"-untranslated region including the poly A tail. It is predicted to encode a protein containing 691 amino acids which include six putative transmembrane domains, a pore loop and a cGMP binding domain as well as one potential extracellular site for N-linked glycosylation. Over the coding region, the canine CNGC1 shares 85-90% identity in the nucleotide sequence and 91-94% identity in the deduced amino acid sequence with its homologues in other mammalian species. However, the homology drops to only 71% and 78% of shared nucleotide and predicted amino acid sequences, respectively, when compared to the chicken CNGC1. Among all the tissues examined the gene is expressed at a much higher level in retina as a major transcript of 3.5 kb length. In addition, another minor transcript (9.8 kb) is consistently observed in the canine retinal RNA which may represent the canine homologue of the rod specific beta-subunit of the cyclic nucleotide-gated channel protein. Transcripts were detected only in retina by northern analysis but low level of expression of CNGC1 was detected in liver, kidney, heart and brain by RT-PCR. The expression of the CNGC1 protein was found to be localized specifically to the photoreceptor outer segment by immunocytochemistry.

Expression of photoreceptor cyclic nucleotide-gated cation channel alpha subunit (CNGCalpha) in the liver and skeletal muscle

Glucagon and beta-adrenergic agents increase cAMP levels and stimulate Ca2+ influx in liver cells. There is no consensus as to the mechanism by which these hormones stimulate the influx of Ca2+. Using mouse retinal rod CNGCalpha cDNA probes, we cloned rat liver and skeletal muscle, and human hepatic CNGCalpha subunit sequences showing 97-100% identity with the human rod channel. In order to assess channel activity, the effect of cyclic nucleotides on free intracellular Ca2+ levels of isolated hepatocytes was measured. Dibutyryl-cAMP was more effective in increasing free Ca2+ levels than dibutyryl-cGMP. These data indicate that the CNGCalpha subunit is expressed in both the liver and skeletal muscle possibly mediating hormonal effects on ion fluxes.

Molecular cloning, functional expression and chromosomal localization of a human homolog of the cyclic nucleotide-gated ion channel of retinal cone photoreceptors

We have cloned from human retina a cyclic nucleotide-gated (CNG) ion channel that is distinct from the one found in rod photoreceptors. This channel protein is highly homologous to the CNG channel that recently has been cloned from bovine testis and kidney and has been shown to be present in retinal cone photoreceptors. When expressed in human embryonic kidney cells, the protein forms functional ion channels with properties broadly similar to those described for the cloned bovine channel. The gene for this channel resides on chromosome 2.

The beta subunit of human rod photoreceptor cGMP-gated cation channel is generated from a complex transcription unit

Human and bovine rod photoreceptor cGMP-gated cation channel consists of two subunits: alpha (63 kDa) and beta (240 kDa). The human beta subunit was shown to consist partly of sequence encoded by the cDNA clone hRCNC2b. Here we present the complete sequence of the human beta subunit and demonstrate that the previously reported human GAR1 gene encoding a glutamate-rich protein (hGARP) encodes its N-terminal portion. Using PCR, RNA blot and genomic DNA analysis, we provide evidence that the beta subunit is produced from a complex locus on chromosome 16 which is also capable of generating independent transcripts corresponding to GAR1 and the C-terminal two-thirds of the beta subunit. The results indicate that the beta subunit of the cGMP-gated cation channel is produced from an unusual locus consisting of more than one transcription unit.

Molecular cloning and expression of the Modulatory subunit of the cyclic nucleotide-gated cation channel

The cDNA of three variants of a cyclic nucleotide-gated (CNG) channel modulatory subunit (CNG4c-CNG4e) has been cloned. CNG4c, CNG4d, and CNG4e differ slightly from each other within an amino-terminal sequence that was originally reported as part of the bovine retinal glutamic acid-rich protein (GARP). The core region of CNG4 is homologous to the second subunit of the human rod photoreceptor channel (hRCNC2b), suggesting that both proteins are alternatively spliced products of the bovine and human homologue of the same gene. CNG4 transcripts are present in retina, testis, kidney, heart, and brain. Expression of CNG4 in HEK293 cells did not lead to detectable currents. Coexpression of CNG4 with the principal subunit of the bovine testis CNG channel (CNG3) resulted in currents which differed in several aspects from that induced by CNG3 alone. The heterooligomeric CNG3/CNG4 and the homooligomeric CNG3 channels were modified by Ca2+-calmodulin and some calmodulin antagonists. The results suggest that CNG4 forms functional heterooligomeric channels with CNG3 in vitro and probably also in intact tissues.

Cell-attached patch clamping of the intact rabbit ciliary epithelium

Following thorough removal of adhering aqueous humor, we have succeeded in patch clamping the intact rabbit ciliary epithelium in the cell-attached and inside-out excised-patch modes. Rapidly fluctuating currents ("chatter activity') were observed during recordings conducted for as long as 1 h. Chatter activity did not reflect seal instability since interconversion was noted between chatter activity and transitions between stable open and closed states, excision of patches into the bath was associated with substantial shifts in the reversal potential, and chatter activity could be triggered by sustained hyperpolarization, but was insensitive to stretch. The chatter channel was identified as cation-nonselective from the reversal potentials both in the cell-attached and excised-patch modes. The channel's kinetics were similar to those of the cGMP-activated phototransduction channel. The results of PCR amplifications of fragments in cDNA libraries from both human ciliary body and human nonpigmented ciliary epithelial (NPE) cells indicated that human ciliary epithelial cells transcribe message for the retinal phototransduction channel. The possible role of the phototransduction channel in expressing chatter activity was further explored by perfusing preparations with a known activator of that channel (cGMP) and with a known inhibitor (L-cis-diltiazem). Neither agent significantly affected chatter behavior. We conclude that: (1) this is the first demonstration of the feasibility of patch-clamping the intact ciliary epithelium; (2) the NPE cells display chatter activity arising from rapidly fluctuating transitions of a cation-nonselective channel; (3) NPE cells can transcribe message for the cation-nonselective phototransduction channel; and (4) if the observed chatter activity is from a homologue of the photo-transduction channel, the homologue is pharmacologically distinct.

Cyclic Nucleotide-Gated Channels, Nitric Oxide, and Neural Function

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels that are activated by direct interaction of the second messengers cAMP and cGMP. Once activated, they provide for membrane depolarization and Ca2+ influx into cells. The functions of CNG channels are tightly coupled to the mechanisms by which cAMP and cGMP are regulated in neurons, namely through activation of G-protein–coupled membrane receptors and through the nitric oxide/guanylyl cyclase signaling system. These functions are best understood in sensory neurons of the vertebrate visual and olfactory system, where CNG channels are critical components of the signal transduction apparatus. The family of known CNG channels is expanding, and there is now increasing evidence that these channels are also present in many other areas of the nervous system. Based on their role in sensory neurons, a functional framework for the role of CNG channels in the CNS is provided.

Isolation and characterization of the alpha-subunit of the rat rod photoreceptor cGMP-gated cation channel

A combination of genomic and PCR clones has been used to derive the full-length coding sequence of the alpha-subunit of the rat rod photoreceptor cGMP-gated cation channel. The sequence encodes a protein of 683 amino acids with a predicted molecular weight of 79,221. The sequence shows extensive homology with other rod cGMP-gated channels and also with the rat olfactory cyclic nucleotide-gated cation channel. When the full-length sequence of the rat rod channel was expressed in Xenopus oocytes it gave a conductance that responded to cGMP with an EC(50) of 42 mu M. No response to 2 mM cAMP was detected. The conductance was decreased in the presence of increasing concentrations of calcium. Both monoclonal and polyclonal antibodies were generated against a C-terminal peptide of the rat rod channel. On immunoblots of adult rat retinal membranes the antibodies recognized a band of 71 kDa, suggesting that the rat channel may undergo proteolytic cleavage in the retina, as has previously been found for the bovine rod channel. Immunocytochemical labeling of adult rat retinal sections detected prominent labeling over the rod photoreceptor outer segments with both monoclonal and polyclonal antibodies.

The renal cGMP-gated cation channel: its molecular structure and physiological role

Cyclic nucleotide-gated cation channels, which are permeable to monovalent and divalent cations, are expressed in a number of tissues. cDNAs encoding cGMP-gated cation channel subunits have been cloned in retinal rods, cones, olfactory neuroepithelium, pineal gland, aorta, testis, heart, and most recently kidney. Patch clamp studies have identified and characterized cGMP-gated cation channels in the cortical collecting duct (CCD) and inner medullary collecting duct (IMCD). cGMP-gated cation channels in kidney share many biophysical and molecular properties with the retinal rod cGMP-gated channel. However, unlike the retinal rod channel, the cGMP-gated cation channel in kidney is inhibited by cGMP and stimulated by increased calcium levels. In the IMCD the cGMP-gated cation channel mediates electrogenic sodium absorption which is inhibited by ANP via cGMP. Recently, cGMP-gated cation channel poly(A+) RNA has been identified in other nephron segments by RT-PCR and in situ PCR hybridization. Furthermore, cGMP-gated cation channel protein has also been identified in all nephron segments by Western blot analysis. These observations suggest that cGMP-gated cation channels, or closely related gene products, may play an important physiological role in all nephron segments. Hormones that increase intracellular cGMP may regulate sodium, and perhaps calcium, uptake in nephron segments proximal to the IMCD. Increases in cell sodium and calcium may regulate other transport and signaling pathways.

Recoverin, a calcium-binding protein in photoreceptors

Recoverin is a Ca2+-binding protein found primarily in vertebrate photoreceptors. The proposed physiological function of recoverin is based on the finding that recoverin inhibits light-stimulated phosphorylation of rhodopsin. Recoverin interacts with rod outer segment membranes in a Ca2+-dependent manner. This interaction requires N-terminal acylation of recoverin. Four types of fatty acids have been detected on the N-terminus of recoverin, but the functional significance of this heterogeneous acylation is not yet clear.

Future directions for rhodopsin structure and function studies

NMR (nuclear magnetic resonance) may be useful for determining the structure of retinal and its environment in rhodopsin, but not for determining the complete protein structure. Aggregation and low yield of fragments of rhodopsin may make them difficult to study by NMR. A long-term multidisciplinary attack on rhodopsin structure is required.

More answers about cGMP-gated channels pose more questions

Our understanding of the molecular properties and cellular role of cGMP-gated channels in outer segments of vertebrate photo-receptors has come from over a decade of studies which have continuously altered and refined ideas about these channels. Further examination of this current view may lead to future surprises and further refine the understanding of cGMP-gated channels.

Cyclic nucleotides as regulators of light-adaptation in photoreceptors

Cyclic nucleotides can regulate the sensitivity of retinal rods to light through phosducin. The phosphorylation state of phosducin determines the amount of G available for activation by Rho*. Phosducin phosphorylation is regulated by cyclic nucleotides through their activation of cAMP-dependent protein kinase. The regulation of phosphodiesterase activity by the noncatalytic cGMP binding sites as well as Ca2+/calmodulin dependent regulation of cGMP binding to the cation channel are also discussed.

Long term potentiation and CaM-sensitive adenylyl cyclase: Long-term prospects

The type I CaM-sensitive adenylyl cyclase is in a position to integrate signals from multiple inputs, consistent with the requirements for mediating long term potentiation (LTP). Biochemical and genetic evidence supports the idea that this enzyme plays an important role inc LTP. However, more work is needed before we will be certain of the role that CaM-sensitive adenylyl cyclases play in LTP.

Modulation of the cGMP-gated channel by calcium

Calcium acting through calmodulin has been shown to regulate the affinity of cyclic nucleotide-gated channels expressed in cell lines. But is calmodulin the Ca-sensor that normally regulates these channels?

How many light adaptation mechanisms are there?

The generally positive response to our target article indicates that most of the commentators accept our contention that light adaptation consists of multiple and possibly redundant mechanisms. The commentaries fall into three general categories. The first deals with putative mechanisms that we chose not to emphasize. The second is a more extended discussion of the role of calcium in adaptation. Finally, additional aspects of cGMP involvement in adaptation are considered. We discuss each of these points in turn.