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Stephen R, Filipek S, Palczewski K, Sousa MC. Ca2+ -dependent regulation of phototransduction. Photochem Photobiol 2008; 84:903-10. [PMID: 18346093 DOI: 10.1111/j.1751-1097.2008.00323.x] [Citation(s) in RCA: 45] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
Photon absorption by rhodopsin triggers the phototransduction signaling pathway that culminates in degradation of cGMP, closure of cGMP-gated ion channels and hyperpolarization of the photoreceptor membrane. This process is accompanied by a decrease in free Ca(2+) concentration in the photoreceptor cytosol sensed by Ca(2+)-binding proteins that modulate phototransduction and activate the recovery phase to reestablish the photoreceptor dark potential. Guanylate cyclase-activating proteins (GCAPs) belong to the neuronal calcium sensor (NCS) family and are responsible for activating retinal guanylate cyclases (retGCs) at low Ca(2+) concentrations triggering synthesis of cGMP and recovery of the dark potential. Here we review recent structural insight into the role of the N-terminal myristoylation in GCAPs and compare it to other NCS family members. We discuss previous studies identifying regions of GCAPs important for retGC1 regulation in the context of the new structural data available for myristoylated GCAP1. In addition, we present a hypothetical model for the Ca(2+)-triggered conformational change in GCAPs and retGC1 regulation. Finally, we briefly discuss the involvement of mutant GCAP1 proteins in the etiology of retinal degeneration as well as the importance of other Ca(2+) sensors in the modulation of phototransduction.
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Affiliation(s)
- Ricardo Stephen
- Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, CO, USA
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Abstract
Detailed biochemical, structural and physiological studies of the role of Ca2(+)-binding proteins in mammalian retinal neurons have yielded new insights into the function of these proteins in normal and pathological states. In phototransduction, a biochemical process that is responsible for the conversion of light into an electrical impulse, guanylate cyclases (GCs) are regulated by GC-activating proteins (GCAPs). These regulatory proteins respond to changes in cytoplasmic Ca2+ concentrations. Disruption of Ca2+ homeostasis in photoreceptor cells by genetic and environmental factors can result ultimately in degeneration of these cells. Pathogenic mutations in GC1 and GCAP1 cause autosomal recessive Leber congenital amaurosis and autosomal dominant cone dystrophy, respectively. This report provides a recent account of the advances, challenges, and possible future prospects of studying this important step in visual transduction that transcends to other neuronal Ca2+ homeostasis processes.
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Helten A, Säftel W, Koch KW. Expression level and activity profile of membrane bound guanylate cyclase type 2 in rod outer segments. J Neurochem 2007; 103:1439-46. [PMID: 17868328 DOI: 10.1111/j.1471-4159.2007.04923.x] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
Rod and cone cells of the mammalian retina harbor two types of a membrane bound guanylate cyclase (GC), rod outer segment guanylate cyclase type 1 (ROS-GC1) and ROS-GC2. Both enzymes are regulated by small Ca(2+)-binding proteins named GC-activating proteins that operate as Ca2+ sensors and enable cyclases to respond to changes of intracellular Ca2+after illumination. We determined the expression level of ROS-GC2 in bovine ROS preparations and compared it with the level of ROS-GC1 in ROSs. The molar ratio of a ROS-GC2 dimer to rhodopsin was 1 : 13 200. The amount of ROS-GC1 was 25-fold higher than the amount of ROS-GC2. Heterologously expressed ROS-GC2 was differentially activated by GC-activating protein 1 and 2 at low free Ca2+ concentrations. Mutants of GC-activating protein 2 modulated ROS-GC2 in a manner different from their action on ROS-GC1 indicating that the Ca2+ sensitivity of the Ca2+ sensor is controlled by the mode of target-sensor interaction.
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Affiliation(s)
- Andreas Helten
- Biochemistry group, Institute of Biology and Environmental Sciences, Faculty V, Carl von Ossietzky University Oldenburg, Oldenburg, Germany
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Sokal I, Dupps WJ, Grassi MA, Brown J, Affatigato LM, Roychowdhury N, Yang L, Filipek S, Palczewski K, Stone EM, Baehr W. A novel GCAP1 missense mutation (L151F) in a large family with autosomal dominant cone-rod dystrophy (adCORD). Invest Ophthalmol Vis Sci 2005; 46:1124-32. [PMID: 15790869 PMCID: PMC1352313 DOI: 10.1167/iovs.04-1431] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
Abstract
PURPOSE To elucidate the phenotypic and biochemical characteristics of a novel mutation associated with autosomal dominant cone-rod dystrophy (adCORD). METHODS Twenty-three family members of a CORD pedigree underwent clinical examinations, including visual acuity tests, standardized full-field ERG, and fundus photography. Genomic DNA was screened for mutations in GCAP1 exons using DNA sequencing and single-strand conformational polymorphism (SSCP) analysis. Function and stability of recombinant GCAP1-L151F were tested as a function of [Ca(2+)], and its structure was probed by molecular dynamics. RESULTS Affected family members experienced dyschromatopsia, hemeralopia, and reduced visual acuity by the second to third decade of life. Electrophysiology revealed a nonrecordable photopic response with later attenuation of the scotopic response. Affected family members harbored a C-->T transition in exon 4 of the GCAP1 gene, resulting in an L151F missense mutation affecting the EF hand motif 4 (EF4). This change was absent in 11 unaffected family members and in 100 unrelated normal subjects. GCAP1-L151F stimulation of photoreceptor guanylate cyclase was not completely inhibited at high physiological [Ca(2+)], consistent with a lowered affinity for Ca(2+)-binding to EF4. CONCLUSIONS A novel L151F mutation in the EF4 hand domain of GCAP1 is associated with adCORD. The clinical phenotype is characterized by early cone dysfunction and a progressive loss of rod function. The biochemical phenotype is best described as persistent stimulation of photoreceptor guanylate cyclase, representing a gain of function of mutant GCAP1. Although a conservative substitution, molecular dynamics suggests a significant change in Ca(2+)-binding to EF4 and EF2 and changes in the shape of L151F-GCAP1.
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Affiliation(s)
| | | | | | - Jeremiah Brown
- Department of Ophthalmology and Visual Sciences, and the
| | | | | | - Lili Yang
- Departments of Ophthalmology and Visual Sciences
| | - Slawomir Filipek
- International Institute of Molecular and Cell Biology, Warsaw, Poland
| | - Krzysztof Palczewski
- From the Departments of Ophthalmology
- Pharmacology, and
- Chemistry, University of Washington, Seattle, Washington; the
| | - Edwin M. Stone
- Department of Ophthalmology and Visual Sciences, and the
- Howard Hughes Medical Institute, University of Iowa Carver College of Medicine, Iowa City, Iowa; the
| | - Wolfgang Baehr
- Departments of Ophthalmology and Visual Sciences
- Biology, and
- Neurobiology and Anatomy, University of Utah, Salt Lake City, Utah; and the
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Nishiguchi KM, Sokal I, Yang L, Roychowdhury N, Palczewski K, Berson EL, Dryja TP, Baehr W. A novel mutation (I143NT) in guanylate cyclase-activating protein 1 (GCAP1) associated with autosomal dominant cone degeneration. Invest Ophthalmol Vis Sci 2004; 45:3863-70. [PMID: 15505030 PMCID: PMC1475955 DOI: 10.1167/iovs.04-0590] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
Abstract
PURPOSE To identify pathogenic mutations in the guanylate cyclase-activating protein 1 (GCAP1) and GCAP2 genes and to characterize the biochemical effect of mutation on guanylate cyclase (GC) stimulation. METHODS The GCAP1 and GCAP2 genes were screened by direct sequencing for mutations in 216 patients and 421 patients, respectively, with various hereditary retinal diseases. A mutation in GCAP1 segregating with autosomal dominant cone degeneration was further evaluated biochemically by employing recombinant proteins, immunoblotting, Ca2+-dependent stimulation of GC, fluorescence emission spectra, and limited proteolysis in the absence and presence of Ca2+. RESULTS A novel GCAP1 mutation, I143NT (substitution of Ile at codon 143 by Asn and Thr), affecting the EF4 Ca2+-binding loop, was identified in a heterozygote father and son with autosomal dominant cone degeneration. Both patients had much greater loss of cone function versus rod function; previous histopathologic evaluation of the father's eyes at autopsy (age 75 years) showed no foveal cones but a few, scattered cones remaining in the peripheral retina. Biochemical analysis showed that the GCAP1-I143NT mutant adopted a conformation susceptible to proteolysis, and the mutant inhibited GC only partially at high Ca2+ concentrations. Individual patients with atypical or recessive retinitis pigmentosa (RP) had additional heterozygous GCAP1-T114I and GCAP2 gene changes (V85M and F150C) of unknown pathogenicity. CONCLUSIONS A novel GCAP1 mutation, I143NT, caused a form of autosomal dominant cone degeneration that destroys foveal cones by mid-life but spares some cones in the peripheral retina up to 75 years. Properties of the GCAP1-I143NT mutant protein suggested that it is incompletely inactivated by high Ca2+ concentrations as should occur with dark adaptation. The continued activity of the mutant GCAP1 likely results in higher-than-normal scotopic cGMP levels which may, in turn, account for the progressive loss of cones.
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Affiliation(s)
- Koji M. Nishiguchi
- Ocular Molecular Genetics Institute, Harvard Medical School, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts
| | - Izabela Sokal
- Department of Ophthalmology, University of Washington, Seattle, Washington
| | - Lili Yang
- Department of Ophthalmology and Visual Sciences, University of Utah, Salt Lake City, Utah
| | - Nirmalya Roychowdhury
- Department of Ophthalmology and Visual Sciences, University of Utah, Salt Lake City, Utah
| | - Krzysztof Palczewski
- Department of Ophthalmology, University of Washington, Seattle, Washington
- Department of Pharmacology, University of Washington, Seattle, Washington
- Department of Chemistry, University of Washington, Seattle, Washington
| | - Eliot L. Berson
- Berman-Gund Laboratory for the Study of Retinal Degenerations, Harvard Medical School, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts
| | - Thaddeus P. Dryja
- Ocular Molecular Genetics Institute, Harvard Medical School, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts
| | - Wolfgang Baehr
- Department of Ophthalmology and Visual Sciences, University of Utah, Salt Lake City, Utah
- Department of Biology, University of Utah, Salt Lake City, Utah
- Department of Neurobiology and Anatomy, University of Utah, Salt Lake City, Utah
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Palczewski K, Sokal I, Baehr W. Guanylate cyclase-activating proteins: structure, function, and diversity. Biochem Biophys Res Commun 2004; 322:1123-30. [PMID: 15336959 DOI: 10.1016/j.bbrc.2004.07.122] [Citation(s) in RCA: 77] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/23/2004] [Indexed: 10/26/2022]
Abstract
The guanylate cyclase-activating proteins (GCAPs), Ca2+-binding proteins of the calmodulin gene superfamily, function as regulators of photoreceptor guanylate cyclases. In contrast to calmodulin, which is active in the Ca2+-bound form, GCAPs stimulate GCs in the [Ca2+]-free form and inhibit GCs upon Ca2+ binding. In vertebrate retinas, at least two GCAP1 and two GCs are present, a third GCAP3 is expressed in humans and fish, and at least five additional GCAP4-8 genes have been identified or are predicted in zebrafish and pufferfish. Missense mutations in GCAP1 (Y99C, I143NT, E155G, and P50L) have been associated with autosomal dominant cone dystrophy. Absence of GCAP1/2 in mice delays recovery of the photoresponse, a phenotype consistent with delay in cGMP synthesis. In the absence of GCAP2, GCAP1 supports the generation of wild-type flash responses in both rod and cone cells. Recent progress revealed an unexpected complexity of the GC-GCAP system, pointing, out a number of unsolved questions.
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Gorczyca WA, Sokal I. GCAPs: Ca2+-sensitive regulators of retGC. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2003; 514:319-32. [PMID: 12596930 DOI: 10.1007/978-1-4615-0121-3_19] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 03/01/2023]
Abstract
Lowered concentration of Ca2+ ions, resulting from illumination of the photoreceptor cell, is the signal for resynthesis of cGMP by retina-specific guanylyl cyclase (retGC). This Ca2+-dependent activation of retGC is mediated by Ca2+-binding proteins named GCAPs (guanylyl cyclase-activating proteins) and contributes to the recovery of photoreceptor cell to the dark state. Three different GCAPs (GCAP1, GCAP2 and GCAP3) are identified in vertebrate retina to date. In this chapter we describe their discovery, methods of purification, properties, and possible modes of action.
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Affiliation(s)
- Wojciech A Gorczyca
- Laboratory of Signaling Proteins, L. Hirszfeld Institute of Immunology and Experimental Therapy, Polish Academy of Sciences, Wroclaw, Poland.
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Sokal I, Alekseev A, Palczewski K. Photoreceptor guanylate cyclase variants: cGMP production under control. Acta Biochim Pol 2003; 50:1075-1095. [PMID: 14739996 PMCID: PMC1351243] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2003] [Revised: 10/11/2003] [Accepted: 10/14/2003] [Indexed: 05/24/2023]
Abstract
Changes in the Ca2+ concentration are thought to affect many processes, including signal transduction in a vast number of biological systems. However, only in few cases the molecular mechanisms by which Ca2+ mediates its action are as well understood as in phototransduction. In dark-adapted photoreceptor cells, the equilibrium level of cGMP is maintained by two opposing activities, such as phosphodiesterase (PDE) and guanylate cyclase (GC). Upon absorption of photons, rhodopsin-G-protein-mediated activation of PDE leads to a transient decrease in [cGMP] and subsequently to lowering of [Ca2+]. In turn, lower [Ca2+] increases net production of cGMP by stimulation of GC until dark conditions are re-established. This activation of GC is mediated by Ca2+ -free forms of Ca2+ -binding proteins termed GC-activating proteins (GCAPs). The last decade brought the molecular identification of GCs and GCAPs in the visual system. Recent efforts have been directed toward understanding the properties of GC at the physiological and structural levels. Here, we summarize the recent progress and present a list of topics of ongoing research.
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Key Words
- retina
- photoreceptor cells
- guanylate cyclase
- rhodopsin
- ca2+-binding proteins
- guanylate cyclase-activating protein
- ac, adenylate cyclase
- anp, atrial natriuretic peptide
- cam, calmodulin
- cd, catalytic domain
- dd, dimerization domain
- ecd, extracellular domain
- gc, guanylate cyclase
- gcap, guanylate cyclase-activating protein
- gt, rod photoreceptor g protein
- icd, intracellular domain
- khd, kinase-homology domain
- meta ii (or r*), metarhodopsin ii (photoactivated rhodopsin)
- npr, natriuretic peptide receptor
- pdb, protein data bank
- rmsd, root-mean-square deviation
- pde, phosphodiesterase
- ros, rod outer segments
- sta, heat-stable enterotoxin
- tm, transmembrane region
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Affiliation(s)
| | | | - Krzysztof Palczewski
- Departments of Ophthalmology
- Pharmacology, and
- Chemistry, University of Washington, Seattle, WA 98195, U.S.A
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