1
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Berkowitz BA, Podolsky RH, Childers KL, Roberts R, Katz R, Waseem R, Robbings BM, Hass DT, Hurley JB, Sweet IR, Goodman C, Qian H, Alvisio B, Heaps S. Transducin-Deficient Rod Photoreceptors Evaluated With Optical Coherence Tomography and Oxygen Consumption Rate Energy Biomarkers. Invest Ophthalmol Vis Sci 2022; 63:22. [PMID: 36576748 PMCID: PMC9804021 DOI: 10.1167/iovs.63.13.22] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022] Open
Abstract
Purpose To test the hypothesis that rod energy biomarkers in light and dark are similar in mice without functional rod transducin (Gnat1rd17). Methods Gnat1rd17 and wildtype (WT) mice were studied in canonically low energy demand (light) and high energy demand (dark) conditions. We measured rod inner segment ellipsoid zone (ISez) profile shape, external limiting membrane-retinal pigment epithelium (ELM-RPE) thickness, and magnitude of a hyporeflective band (HB) intensity dip located between photoreceptor tips and apical RPE; antioxidants were given in a subset of mice. Oxygen consumption rate (OCR) and visual performance indexes were also measured. Results The lower energy demand expected in light-adapted wildtype retinas was associated with an elongated ISez, thicker ELM-RPE, and higher HB magnitude, and lower OCR compared to high energy demand conditions in the dark. Gnat1rd17 mice showed a wildtype-like ISez profile shape at 20 minutes of light that became rounder at 60 minutes; at both times, ELM-RPE was smaller than wildtype values, and the HB magnitude was unmeasurable. OCR was higher than in the dark. Light-adapted Gnat1rd17 mice biomarkers were unaffected by anti-oxidants. Gnat1rd17 mice showed modest outer nuclear layer thinning and no reduction in visual performance indexes. Conclusions Light-stimulated changes in all biomarkers in WT mice are consistent with the established light-induced decrease in net energy demand. In contrast, biomarker changes in Gnat1rd17 mice raise the possibility that light increases net energy demand in the absence of rod phototransduction.
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Affiliation(s)
- Bruce A Berkowitz
- Department of Ophthalmology, Visual and Anatomical Sciences, Wayne State University School of Medicine, Detroit, Michigan, United States
| | - Robert H Podolsky
- Biostatistics and Study Methodology, Children's National Hospital, Silver Spring, Maryland, United States
| | - Karen Lins Childers
- Beaumont Research Institute, Beaumont Health, Royal Oak, Michigan, United States
| | - Robin Roberts
- Department of Ophthalmology, Visual and Anatomical Sciences, Wayne State University School of Medicine, Detroit, Michigan, United States
| | - Ryan Katz
- Department of Ophthalmology, Visual and Anatomical Sciences, Wayne State University School of Medicine, Detroit, Michigan, United States
| | - Rida Waseem
- Department of Ophthalmology, Visual and Anatomical Sciences, Wayne State University School of Medicine, Detroit, Michigan, United States
| | - Brian M Robbings
- Department of Biochemistry, Department of Ophthalmology, University of Washington, Seattle, Washington, United States.,Department of Medicine, UW Medicine Diabetes Institute, University of Washington, Seattle, Washington, United States
| | - Daniel T Hass
- Department of Biochemistry, Department of Ophthalmology, University of Washington, Seattle, Washington, United States
| | - James B Hurley
- Department of Biochemistry, Department of Ophthalmology, University of Washington, Seattle, Washington, United States
| | - Ian R Sweet
- Department of Medicine, UW Medicine Diabetes Institute, University of Washington, Seattle, Washington, United States
| | - Cole Goodman
- Department of Ophthalmology, Visual and Anatomical Sciences, Wayne State University School of Medicine, Detroit, Michigan, United States
| | - Haohua Qian
- Visual Function Core, National Eye Institute, National Institutes of Health, Bethesda, Maryland, United States
| | - Bruno Alvisio
- OSIO Bioinformatics Core, National Eye Institute, National Institutes of Health, Bethesda, Maryland, United States
| | - Sam Heaps
- OSIO Bioinformatics Core, National Eye Institute, National Institutes of Health, Bethesda, Maryland, United States
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2
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Mathematical analysis of phototransduction reaction parameters in rods and cones. Sci Rep 2022; 12:19529. [PMID: 36376413 PMCID: PMC9663442 DOI: 10.1038/s41598-022-23069-0] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2021] [Accepted: 10/25/2022] [Indexed: 11/16/2022] Open
Abstract
Retinal photoreceptor cells, rods and cones, convert photons of light into chemical and electrical signals as the first step of the visual transduction cascade. Although the chemical processes in the phototransduction system are very similar to each other in these photoreceptors, the light sensitivity and time resolution of the photoresponse in rods are functionally different than those in the photoresponses of cones. To systematically investigate how photoresponses are divergently regulated in rods and cones, we have developed a detailed mathematical model on the basis of the Hamer model. The current model successfully reconstructed light intensity-, ATP- and GTP-dependent changes in concentrations of phosphorylated visual pigments (VPs), activated transducins (Tr*s) and phosphodiesterases (PDEs) in rods and cones. In comparison to rods, the lower light sensitivity of cones was attributed not only to the lower affinity of activated VPs for Trs but also to the faster desensitization of the VPs. The assumption of an intermediate inactive state, MIIi, in the thermal decay of activated VPs was essential for inducing faster inactivation of VPs in rods, and possibly also in cones.
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3
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Kawamura S, Tachibanaki S. Molecular basis of rod and cone differences. Prog Retin Eye Res 2021; 90:101040. [PMID: 34974196 DOI: 10.1016/j.preteyeres.2021.101040] [Citation(s) in RCA: 14] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/28/2021] [Revised: 12/22/2021] [Accepted: 12/27/2021] [Indexed: 12/15/2022]
Abstract
In the vertebrate retina, rods and cones both detect light, but they are different in functional aspects such as light sensitivity and time resolution, for example, and in some of cell biological aspects. For functional aspects, both photoreceptors are known to share a common mechanism, phototransduction cascade, consisting of a series of enzyme reactions to convert a photon-capture signal to an electrical signal. To understand the mechanisms of the functional differences between rods and cones at the molecular level, we compared biochemically each of the reactions in the phototransduction cascade between rods and cones using the cells isolated and purified from carp retina. Although proteins in the cascade are functionally similar between rods and cones, their activities together with their expression levels are mostly different between these photoreceptors. In general, reactions to generate a response are slightly less effective, as a total, in cones than in rods, but each of the reactions for termination and recovery of a response are much more effective in cones. These findings explain lower light sensitivity and briefer light responses in cones than in rods. In addition, our considerations suggest that a Ca2+-binding protein, S-modulin or recoverin, has a currently unnoticed role in shaping light responses. With comparison of the expression levels of proteins and/or mRNAs using purified cells, several proteins were found to be specifically or predominantly expressed in cones. These proteins would be of interest for future studies on the difference between rods and cones.
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Affiliation(s)
- Satoru Kawamura
- Graduate School of Frontier Biosciences, Osaka University, Yamada-oka 1-3, Suita, Osaka, 565-0871, Japan; Department of Biological Sciences, Graduate School of Science, Osaka University, Yamada-oka 1-3, Suita, Osaka, 565-0871, Japan.
| | - Shuji Tachibanaki
- Graduate School of Frontier Biosciences, Osaka University, Yamada-oka 1-3, Suita, Osaka, 565-0871, Japan; Department of Biological Sciences, Graduate School of Science, Osaka University, Yamada-oka 1-3, Suita, Osaka, 565-0871, Japan.
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4
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Genovese F, Reisert J, Kefalov VJ. Sensory Transduction in Photoreceptors and Olfactory Sensory Neurons: Common Features and Distinct Characteristics. Front Cell Neurosci 2021; 15:761416. [PMID: 34690705 PMCID: PMC8531253 DOI: 10.3389/fncel.2021.761416] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/19/2021] [Accepted: 09/20/2021] [Indexed: 12/24/2022] Open
Abstract
The past decades have seen tremendous progress in our understanding of the function of photoreceptors and olfactory sensory neurons, uncovering the mechanisms that determine their properties and, ultimately, our ability to see and smell. This progress has been driven to a large degree by the powerful combination of physiological experimental tools and genetic manipulations, which has enabled us to identify the main molecular players in the transduction cascades of these sensory neurons, how their properties affect the detection and discrimination of stimuli, and how diseases affect our senses of vision and smell. This review summarizes some of the common and unique features of photoreceptors and olfactory sensory neurons that make these cells so exciting to study.
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Affiliation(s)
| | | | - Vladimir J Kefalov
- Department of Ophthalmology, Gavin Herbert Eye Institute, University of California, Irvine, Irvine, CA, United States.,Department of Physiology and Biophysics, University of California, Irvine, Irvine, CA, United States
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5
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Structural Insights into Retinal Guanylate Cyclase Activator Proteins (GCAPs). Int J Mol Sci 2021; 22:ijms22168731. [PMID: 34445435 PMCID: PMC8395740 DOI: 10.3390/ijms22168731] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2021] [Revised: 08/09/2021] [Accepted: 08/10/2021] [Indexed: 11/16/2022] Open
Abstract
Retinal guanylate cyclases (RetGCs) promote the Ca2+-dependent synthesis of cGMP that coordinates the recovery phase of visual phototransduction in retinal rods and cones. The Ca2+-sensitive activation of RetGCs is controlled by a family of photoreceptor Ca2+ binding proteins known as guanylate cyclase activator proteins (GCAPs). The Mg2+-bound/Ca2+-free GCAPs bind to RetGCs and activate cGMP synthesis (cyclase activity) at low cytosolic Ca2+ levels in light-activated photoreceptors. By contrast, Ca2+-bound GCAPs bind to RetGCs and inactivate cyclase activity at high cytosolic Ca2+ levels found in dark-adapted photoreceptors. Mutations in both RetGCs and GCAPs that disrupt the Ca2+-dependent cyclase activity are genetically linked to various retinal diseases known as cone-rod dystrophies. In this review, I will provide an overview of the known atomic-level structures of various GCAP proteins to understand how protein dimerization and Ca2+-dependent conformational changes in GCAPs control the cyclase activity of RetGCs. This review will also summarize recent structural studies on a GCAP homolog from zebrafish (GCAP5) that binds to Fe2+ and may serve as a Fe2+ sensor in photoreceptors. The GCAP structures reveal an exposed hydrophobic surface that controls both GCAP1 dimerization and RetGC binding. This exposed site could be targeted by therapeutics designed to inhibit the GCAP1 disease mutants, which may serve to mitigate the onset of retinal cone-rod dystrophies.
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6
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Abbas F, Vinberg F. Transduction and Adaptation Mechanisms in the Cilium or Microvilli of Photoreceptors and Olfactory Receptors From Insects to Humans. Front Cell Neurosci 2021; 15:662453. [PMID: 33867944 PMCID: PMC8046925 DOI: 10.3389/fncel.2021.662453] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2021] [Accepted: 03/02/2021] [Indexed: 12/11/2022] Open
Abstract
Sensing changes in the environment is crucial for survival. Animals from invertebrates to vertebrates use both visual and olfactory stimuli to direct survival behaviors including identification of food sources, finding mates, and predator avoidance. In primary sensory neurons there are signal transduction mechanisms that convert chemical or light signals into an electrical response through ligand binding or photoactivation of a receptor, that can be propagated to the olfactory and visual centers of the brain to create a perception of the odor and visual landscapes surrounding us. The fundamental principles of olfactory and phototransduction pathways within vertebrates are somewhat analogous. Signal transduction in both systems takes place in the ciliary sub-compartments of the sensory cells and relies upon the activation of G protein-coupled receptors (GPCRs) to close cyclic nucleotide-gated (CNG) cation channels in photoreceptors to produce a hyperpolarization of the cell, or in olfactory sensory neurons open CNG channels to produce a depolarization. However, while invertebrate phototransduction also involves GPCRs, invertebrate photoreceptors can be either ciliary and/or microvillar with hyperpolarizing and depolarizing responses to light, respectively. Moreover, olfactory transduction in invertebrates may be a mixture of metabotropic G protein and ionotropic signaling pathways. This review will highlight differences of the visual and olfactory transduction mechanisms between vertebrates and invertebrates, focusing on the implications to the gain of the transduction processes, and how they are modulated to allow detection of small changes in odor concentration and light intensity over a wide range of background stimulus levels.
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Affiliation(s)
- Fatima Abbas
- Vinberg Lab, Department of Ophthalmology and Visual Science, John A. Moran Center, University of Utah, Salt Lake City, UT, United States
| | - Frans Vinberg
- Vinberg Lab, Department of Ophthalmology and Visual Science, John A. Moran Center, University of Utah, Salt Lake City, UT, United States
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7
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Zang J, Neuhauss SCF. Biochemistry and physiology of zebrafish photoreceptors. Pflugers Arch 2021; 473:1569-1585. [PMID: 33598728 PMCID: PMC8370914 DOI: 10.1007/s00424-021-02528-z] [Citation(s) in RCA: 18] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/03/2020] [Revised: 01/25/2021] [Accepted: 01/28/2021] [Indexed: 02/06/2023]
Abstract
All vertebrates share a canonical retina with light-sensitive photoreceptors in the outer retina. These photoreceptors are of two kinds: rods and cones, adapted to low and bright light conditions, respectively. They both show a peculiar morphology, with long outer segments, comprised of ordered stacks of disc-shaped membranes. These discs host numerous proteins, many of which contribute to the visual transduction cascade. This pathway converts the light stimulus into a biological signal, ultimately modulating synaptic transmission. Recently, the zebrafish (Danio rerio) has gained popularity for studying the function of vertebrate photoreceptors. In this review, we introduce this model system and its contribution to our understanding of photoreception with a focus on the cone visual transduction cascade.
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Affiliation(s)
- Jingjing Zang
- Department of Molecular Life Sciences, University of Zurich, Winterthurerstrase 190, CH - 8057, Zürich, Switzerland
| | - Stephan C F Neuhauss
- Department of Molecular Life Sciences, University of Zurich, Winterthurerstrase 190, CH - 8057, Zürich, Switzerland.
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8
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Abstract
Rod photoreceptors are composed of a soma and an inner segment (IS) connected to an outer segment (OS) by a thin cilium. OSs are composed of a stack of ∼800 lipid discs surrounded by the plasma membrane where phototransduction takes place. Intracellular calcium plays a major role in phototransduction and is more concentrated in the discs, where it can be incorporated and released. To study calcium dynamics in rods, we used the fluorescent calcium dye CaSiR-1 AM working in the near-infrared (NIR) (excitation at 650 and emission at 664 nm), an advantage over previously used dyes. In this way, we investigated calcium dynamics with an unprecedented accuracy and most importantly in semidark-adapted conditions. We observed light-induced drops in [Ca2+]i with kinetics similar to that of photoresponses recorded electrophysiologically. We show three properties of the rods. First, intracellular calcium and key proteins have concentrations that vary from the OS base to tip. At the OS base, [Ca2+]i is ∼80 nM and increases up to ∼200 nM at the OS tip. Second, there are spontaneous calcium flares in healthy and functional rod OSs; these flares are highly localized and are more pronounced at the OS tip. Third, a bright flash of light at 488 nm induces a drop in [Ca2+]i at the OS base but often a flare at the OS tip. Therefore, rod OSs are not homogenous structures but have a structural and functional gradient, which is a fundamental aspect of transduction in vertebrate photoreceptors.
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9
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Zang J, Neuhauss SCF. The Binding Properties and Physiological Functions of Recoverin. Front Mol Neurosci 2018; 11:473. [PMID: 30618620 PMCID: PMC6306944 DOI: 10.3389/fnmol.2018.00473] [Citation(s) in RCA: 27] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2018] [Accepted: 12/04/2018] [Indexed: 12/14/2022] Open
Abstract
Recoverin (Rcv) is a low molecular-weight, neuronal calcium sensor (NCS) primarily located in photoreceptor outer segments of the vertebrate retina. Calcium ions (Ca2+)-bound Rcv has been proposed to inhibit G-protein-coupled receptor kinase (GRKs) in darkness. During the light response, the Ca2+-free Rcv releases GRK, which in turn phosphorylates visual pigment, ultimately leading to the cessation of the visual transduction cascade. Technological advances over the last decade have contributed significantly to a deeper understanding of Rcv function. These include both biophysical and biochemical approaches that will be discussed in this review article. Furthermore, electrophysiological experiments uncovered additional functions of Rcv, such as regulation of the lifetime of Phosphodiesterase-Transducin complex. Recently, attention has been drawn to different roles in rod and cone photoreceptors.This review article focuses on Rcv binding properties to Ca2+, disc membrane and GRK, and its physiological functions in phototransduction and signal transmission.
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Affiliation(s)
- Jingjing Zang
- Institute of Molecular Life Sciences, University of Zurich, Zurich, Switzerland
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10
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Vinberg F, Kefalov VJ. Investigating the Ca 2+-dependent and Ca 2+-independent mechanisms for mammalian cone light adaptation. Sci Rep 2018; 8:15864. [PMID: 30367097 PMCID: PMC6203770 DOI: 10.1038/s41598-018-34073-8] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/23/2017] [Accepted: 10/10/2018] [Indexed: 12/15/2022] Open
Abstract
Vision is mediated by two types of photoreceptors: rods, enabling vision in dim light; and cones, which function in bright light. Despite many similarities in the components of their respective phototransduction cascades, rods and cones have distinct sensitivity, response kinetics, and adaptation capacity. Cones are less sensitive and have faster responses than rods. In addition, cones can function over a wide range of light conditions whereas rods saturate in moderately bright light. Calcium plays an important role in regulating phototransduction and light adaptation of rods and cones. Notably, the two dominant Ca2+-feedbacks in rods and cones are driven by the identical calcium-binding proteins: guanylyl cyclase activating proteins 1 and 2 (GCAPs), which upregulate the production of cGMP; and recoverin, which regulates the inactivation of visual pigment. Thus, the mechanisms producing the difference in adaptation capacity between rods and cones have remained poorly understood. Using GCAPs/recoverin-deficient mice, we show that mammalian cones possess another Ca2+-dependent mechanism promoting light adaptation. Surprisingly, we also find that, unlike in mouse rods, a unique Ca2+-independent mechanism contributes to cone light adaptation. Our findings point to two novel adaptation mechanisms in mouse cones that likely contribute to the great adaptation capacity of cones over rods.
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Affiliation(s)
- Frans Vinberg
- Ophthalmology and Visual Sciences, Washington University School of Medicine, St. Louis, Missouri, USA. .,John A. Moran Eye Center, University of Utah, Salt Lake City, Utah, USA.
| | - Vladimir J Kefalov
- Ophthalmology and Visual Sciences, Washington University School of Medicine, St. Louis, Missouri, USA
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11
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Vinberg F, Chen J, Kefalov VJ. Regulation of calcium homeostasis in the outer segments of rod and cone photoreceptors. Prog Retin Eye Res 2018; 67:87-101. [PMID: 29883715 DOI: 10.1016/j.preteyeres.2018.06.001] [Citation(s) in RCA: 39] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/05/2018] [Revised: 05/30/2018] [Accepted: 06/04/2018] [Indexed: 12/11/2022]
Abstract
Calcium plays important roles in the function and survival of rod and cone photoreceptor cells. Rapid regulation of calcium in the outer segments of photoreceptors is required for the modulation of phototransduction that drives the termination of the flash response as well as light adaptation in rods and cones. On a slower time scale, maintaining proper calcium homeostasis is critical for the health and survival of photoreceptors. Decades of work have established that the level of calcium in the outer segments of rods and cones is regulated by a dynamic equilibrium between influx via the transduction cGMP-gated channels and extrusion via rod- and cone-specific Na+/Ca2+, K+ exchangers (NCKXs). It had been widely accepted that the only mechanism for extrusion of calcium from rod outer segments is via the rod-specific NCKX1, while extrusion from cone outer segments is driven exclusively by the cone-specific NCKX2. However, recent evidence from mice lacking NCKX1 and NCKX2 have challenged that notion and have revealed a more complex picture, including a NCKX-independent mechanism in rods and two separate NCKX-dependent mechanisms in cones. This review will focus on recent findings on the molecular mechanisms of extrusion of calcium from the outer segments of rod and cone photoreceptors, and the functional and structural changes in photoreceptors when normal extrusion is disrupted.
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Affiliation(s)
- Frans Vinberg
- Ophthalmology and Visual Sciences, Washington University School of Medicine, St. Louis, Missouri, USA; John A. Moran Eye Center, University of Utah, Salt Lake City, UT, USA
| | - Jeannie Chen
- Zilkha Neurogenetic Institute, Department of Physiology and Neuroscience, University of Southern California Keck School of Medicine, Los Angeles, CA, USA
| | - Vladimir J Kefalov
- Ophthalmology and Visual Sciences, Washington University School of Medicine, St. Louis, Missouri, USA.
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12
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Vinberg F, Wang T, De Maria A, Zhao H, Bassnett S, Chen J, Kefalov VJ. The Na +/Ca 2+, K + exchanger NCKX4 is required for efficient cone-mediated vision. eLife 2017; 6:e24550. [PMID: 28650316 PMCID: PMC5515578 DOI: 10.7554/elife.24550] [Citation(s) in RCA: 26] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2016] [Accepted: 06/23/2017] [Indexed: 12/24/2022] Open
Abstract
Calcium (Ca2+) plays an important role in the function and health of neurons. In vertebrate cone photoreceptors, Ca2+ controls photoresponse sensitivity, kinetics, and light adaptation. Despite the critical role of Ca2+ in supporting the function and survival of cones, the mechanism for its extrusion from cone outer segments is not well understood. Here, we show that the Na+/Ca2+, K+ exchanger NCKX4 is expressed in zebrafish, mouse, and primate cones. Functional analysis of NCKX4-deficient mouse cones revealed that this exchanger is essential for the wide operating range and high temporal resolution of cone-mediated vision. We show that NCKX4 shapes the cone photoresponse together with the cone-specific NCKX2: NCKX4 acts early to limit response amplitude, while NCKX2 acts late to further accelerate response recovery. The regulation of Ca2+ by NCKX4 in cones is a novel mechanism that supports their ability to function as daytime photoreceptors and promotes their survival.
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Affiliation(s)
- Frans Vinberg
- Department of Ophthalmology and Visual Sciences, Washington University, St. Louis, United States
| | - Tian Wang
- Zilkha Neurogenetic Institute, Keck School of Medicine, University of Southern California, Los Angeles, United States
- Department of Cell and Neurobiology, Keck School of Medicine, University of Southern California, Los Angeles, United States
- Department of Ophthalmology, Keck School of Medicine, University of Southern California, Los Angeles, United States
| | - Alicia De Maria
- Department of Ophthalmology and Visual Sciences, Washington University, St. Louis, United States
| | - Haiqing Zhao
- Department of Biology, Johns Hopkins University, Baltimore, United States
| | - Steven Bassnett
- Department of Ophthalmology and Visual Sciences, Washington University, St. Louis, United States
| | - Jeannie Chen
- Zilkha Neurogenetic Institute, Keck School of Medicine, University of Southern California, Los Angeles, United States
- Department of Cell and Neurobiology, Keck School of Medicine, University of Southern California, Los Angeles, United States
- Department of Ophthalmology, Keck School of Medicine, University of Southern California, Los Angeles, United States
| | - Vladimir J Kefalov
- Department of Ophthalmology and Visual Sciences, Washington University, St. Louis, United States
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13
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Wensel TG, Zhang Z, Anastassov IA, Gilliam JC, He F, Schmid MF, Robichaux MA. Structural and molecular bases of rod photoreceptor morphogenesis and disease. Prog Retin Eye Res 2016; 55:32-51. [PMID: 27352937 PMCID: PMC5112133 DOI: 10.1016/j.preteyeres.2016.06.002] [Citation(s) in RCA: 42] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2015] [Revised: 06/14/2016] [Accepted: 06/20/2016] [Indexed: 12/15/2022]
Abstract
The rod cell has an extraordinarily specialized structure that allows it to carry out its unique function of detecting individual photons of light. Both the structural features of the rod and the metabolic processes required for highly amplified light detection seem to have rendered the rod especially sensitive to structural and metabolic defects, so that a large number of gene defects are primarily associated with rod cell death and give rise to blinding retinal dystrophies. The structures of the rod, especially those of the sensory cilium known as the outer segment, have been the subject of structural, biochemical, and genetic analysis for many years, but the molecular bases for rod morphogenesis and for cell death in rod dystrophies are still poorly understood. Recent developments in imaging technology, such as cryo-electron tomography and super-resolution fluorescence microscopy, in gene sequencing technology, and in gene editing technology are rapidly leading to new breakthroughs in our understanding of these questions. A summary is presented of our current understanding of selected aspects of these questions, highlighting areas of uncertainty and contention as well as recent discoveries that provide new insights. Examples of structural data from emerging imaging technologies are presented.
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Affiliation(s)
- Theodore G Wensel
- Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030, USA.
| | - Zhixian Zhang
- Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030, USA
| | - Ivan A Anastassov
- Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030, USA
| | - Jared C Gilliam
- Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030, USA
| | - Feng He
- Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030, USA
| | - Michael F Schmid
- Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030, USA
| | - Michael A Robichaux
- Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030, USA
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14
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Zang J, Keim J, Kastenhuber E, Gesemann M, Neuhauss SCF. Recoverin depletion accelerates cone photoresponse recovery. Open Biol 2016; 5:rsob.150086. [PMID: 26246494 PMCID: PMC4554923 DOI: 10.1098/rsob.150086] [Citation(s) in RCA: 29] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
The neuronal Ca2+-binding protein Recoverin has been shown to regulate phototransduction termination in mammalian rods. Here we identify four recoverin genes in the zebrafish genome, rcv1a, rcv1b, rcv2a and rcv2b, and investigate their role in modulating the cone phototransduction cascade. While Recoverin-1b is only found in the adult retina, the other Recoverins are expressed throughout development in all four cone types, except Recoverin-1a, which is expressed only in rods and UV cones. Applying a double flash electroretinogram (ERG) paradigm, downregulation of Recoverin-2a or 2b accelerates cone photoresponse recovery, albeit at different light intensities. Exclusive recording from UV cones via spectral ERG reveals that knockdown of Recoverin-1a alone has no effect, but Recoverin-1a/2a double-knockdowns showed an even shorter recovery time than Recoverin-2a-deficient larvae. We also showed that UV cone photoresponse kinetics depend on Recoverin-2a function via cone-specific kinase Grk7a. This is the first in vivo study demonstrating that cone opsin deactivation kinetics determine overall photoresponse shut off kinetics.
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Affiliation(s)
- Jingjing Zang
- Institute of Molecular Life Sciences, Neuroscience Center Zurich and Center for Integrative Human Physiology, University of Zurich, Winterthurerstrasse 190, Zurich 8057, Switzerland
| | - Jennifer Keim
- Institute of Molecular Life Sciences, Neuroscience Center Zurich and Center for Integrative Human Physiology, University of Zurich, Winterthurerstrasse 190, Zurich 8057, Switzerland
| | - Edda Kastenhuber
- Institute of Molecular Life Sciences, Neuroscience Center Zurich and Center for Integrative Human Physiology, University of Zurich, Winterthurerstrasse 190, Zurich 8057, Switzerland
| | - Matthias Gesemann
- Institute of Molecular Life Sciences, Neuroscience Center Zurich and Center for Integrative Human Physiology, University of Zurich, Winterthurerstrasse 190, Zurich 8057, Switzerland
| | - Stephan C F Neuhauss
- Institute of Molecular Life Sciences, Neuroscience Center Zurich and Center for Integrative Human Physiology, University of Zurich, Winterthurerstrasse 190, Zurich 8057, Switzerland
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15
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Vinberg F, Wang T, Molday RS, Chen J, Kefalov VJ. A new mouse model for stationary night blindness with mutant Slc24a1 explains the pathophysiology of the associated human disease. Hum Mol Genet 2015; 24:5915-29. [PMID: 26246500 DOI: 10.1093/hmg/ddv319] [Citation(s) in RCA: 28] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/05/2015] [Accepted: 07/31/2015] [Indexed: 11/12/2022] Open
Abstract
Mutations that affect calcium homeostasis (Ca(2+)) in rod photoreceptors are linked to retinal degeneration and visual disorders such as retinitis pigmentosa and congenital stationary night blindness (CSNB). It is thought that the concentration of Ca(2+) in rod outer segments is controlled by a dynamic balance between influx via cGMP-gated (CNG) channels and extrusion via Na(+)/Ca(2+), K(+) exchangers (NCKX1). The extrusion-driven lowering of rod [Ca(2+)]i following light exposure controls their light adaptation and response termination. Mutant NCKX1 has been linked to autosomal-recessive stationary night blindness. However, whether NCKX1 contributes to light adaptation has not been directly tested and the mechanisms by which human NCKX1 mutations cause night blindness are not understood. Here, we report that the deletion of NCKX1 in mice results in malformed outer segment disks, suppressed expression and function of rod CNG channels and a subsequent 100-fold reduction in rod responses, while preserving normal cone responses. The compensating loss of CNG channel function in the absence of NCKX1-mediated Ca(2+) extrusion may prevent toxic Ca(2+) buildup and provides an explanation for the stationary nature of the associated disorder in humans. Surprisingly, the lack of NCKX1 did not compromise rod background light adaptation, suggesting additional Ca(2+)-extruding mechanisms exist in these cells.
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Affiliation(s)
- Frans Vinberg
- Ophthalmology and Visual Sciences, Washington University School of Medicine, St. Louis, MO, USA
| | - Tian Wang
- Cell and Neurobiology, Zilkha Neurogenetic Institute, University of Southern California, Los Angeles, CA, USA and
| | - Robert S Molday
- Biochemistry/Molecular Biology, University of British Columbia, Vancouver, Canada
| | - Jeannie Chen
- Cell and Neurobiology, Zilkha Neurogenetic Institute, University of Southern California, Los Angeles, CA, USA and
| | - Vladimir J Kefalov
- Ophthalmology and Visual Sciences, Washington University School of Medicine, St. Louis, MO, USA,
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16
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Gross OP, Pugh EN, Burns ME. cGMP in mouse rods: the spatiotemporal dynamics underlying single photon responses. Front Mol Neurosci 2015; 8:6. [PMID: 25788876 PMCID: PMC4349151 DOI: 10.3389/fnmol.2015.00006] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/28/2014] [Accepted: 02/12/2015] [Indexed: 02/04/2023] Open
Abstract
Vertebrate vision begins when retinal photoreceptors transduce photons into electrical signals that are then relayed to other neurons in the eye, and ultimately to the brain. In rod photoreceptors, transduction of single photons is achieved by a well-understood G-protein cascade that modulates cGMP levels, and in turn, cGMP-sensitive inward current. The spatial extent and depth of the decline in cGMP during the single photon response (SPR) have been major issues in phototransduction research since the discovery that single photons elicit substantial and reproducible changes in membrane current. The spatial profile of cGMP decline during the SPR affects signal gain, and thus may contribute to reduction of trial-to-trial fluctuations in the SPR. Here we summarize the general principles of rod phototransduction, emphasizing recent advances in resolving the spatiotemporal dynamics of cGMP during the SPR.
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Affiliation(s)
- Owen P Gross
- Center for Neuroscience, University of California Davis Davis, CA, USA
| | - Edward N Pugh
- Departments of Ophthalmology and Vision Science, University of California Davis Davis, CA, USA ; Physiology and Membrane Biology, University of California Davis Davis, CA, USA ; Cell Biology and Human Anatomy, University of California Davis Davis, CA, USA
| | - Marie E Burns
- Center for Neuroscience, University of California Davis Davis, CA, USA ; Departments of Ophthalmology and Vision Science, University of California Davis Davis, CA, USA ; Cell Biology and Human Anatomy, University of California Davis Davis, CA, USA
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17
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Wen XH, Dizhoor AM, Makino CL. Membrane guanylyl cyclase complexes shape the photoresponses of retinal rods and cones. Front Mol Neurosci 2014; 7:45. [PMID: 24917784 PMCID: PMC4040495 DOI: 10.3389/fnmol.2014.00045] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2014] [Accepted: 05/01/2014] [Indexed: 12/02/2022] Open
Abstract
In vertebrate rods and cones, photon capture by rhodopsin leads to the destruction of cyclic GMP (cGMP) and the subsequent closure of cyclic nucleotide gated ion channels in the outer segment plasma membrane. Replenishment of cGMP and reopening of the channels limit the growth of the photon response and are requisite for its recovery. In different vertebrate retinas, there may be as many as four types of membrane guanylyl cyclases (GCs) for cGMP synthesis. Ten neuronal Ca2+ sensor proteins could potentially modulate their activities. The mouse is proving to be an effective model for characterizing the roles of individual components because its relative simplicity can be reduced further by genetic engineering. There are two types of GC activating proteins (GCAPs) and two types of GCs in mouse rods, whereas cones express one type of GCAP and one type of GC. Mutant mouse rods and cones bereft of both GCAPs have large, long lasting photon responses. Thus, GCAPs normally mediate negative feedback tied to the light-induced decline in intracellular Ca2+ that accelerates GC activity to curtail the growth and duration of the photon response. Rods from other mutant mice that express a single GCAP type reveal how the two GCAPs normally work together as a team. Because of its lower Ca2+ affinity, GCAP1 is the first responder that senses the initial decrease in Ca2+ following photon absorption and acts to limit response amplitude. GCAP2, with a higher Ca2+ affinity, is recruited later during the course of the photon response as Ca2+ levels continue to decline further. The main role of GCAP2 is to provide for a timely response recovery and it is particularly important after exposure to very bright light. The multiplicity of GC isozymes and GCAP homologs in the retinas of other vertebrates confers greater flexibility in shaping the photon responses in order to tune visual sensitivity, dynamic range and frequency response.
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Affiliation(s)
- Xiao-Hong Wen
- Department of Ophthalmology, Massachusetts Eye and Ear Infirmary and Harvard Medical School Boston, MA, USA
| | - Alexander M Dizhoor
- Department of Basic Sciences Research and Pennsylvania College of Optometry, Salus University Elkins Park, PA, USA
| | - Clint L Makino
- Department of Ophthalmology, Massachusetts Eye and Ear Infirmary and Harvard Medical School Boston, MA, USA
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18
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Lim S, Dizhoor AM, Ames JB. Structural diversity of neuronal calcium sensor proteins and insights for activation of retinal guanylyl cyclase by GCAP1. Front Mol Neurosci 2014; 7:19. [PMID: 24672427 PMCID: PMC3956117 DOI: 10.3389/fnmol.2014.00019] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/27/2014] [Accepted: 02/27/2014] [Indexed: 01/08/2023] Open
Abstract
Neuronal calcium sensor (NCS) proteins, a sub-branch of the calmodulin superfamily, are expressed in the brain and retina where they transduce calcium signals and are genetically linked to degenerative diseases. The amino acid sequences of NCS proteins are highly conserved but their physiological functions are quite different. Retinal recoverin controls Ca2+-dependent inactivation of light-excited rhodopsin during phototransduction, guanylyl cyclase activating proteins 1 and 2 (GCAP1 and GCAP2) promote Ca2+-dependent activation of retinal guanylyl cyclases, and neuronal frequenin (NCS-1) modulates synaptic activity and neuronal secretion. Here we review the molecular structures of myristoylated forms of NCS-1, recoverin, and GCAP1 that all look very different, suggesting that the attached myristoyl group helps to refold these highly homologous proteins into different three-dimensional folds. Ca2+-binding to both recoverin and NCS-1 cause large protein conformational changes that ejects the covalently attached myristoyl group into the solvent exterior and promotes membrane targeting (Ca2+-myristoyl switch). The GCAP proteins undergo much smaller Ca2+-induced conformational changes and do not possess a Ca2+-myristoyl switch. Recent structures of GCAP1 in both its activator and Ca2+-bound inhibitory states will be discussed to understand structural determinants that control their Ca2+-dependent activation of retinal guanylyl cyclases.
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Affiliation(s)
- Sunghyuk Lim
- Department of Chemistry, University of California at Davis Davis, CA, USA
| | - Alexander M Dizhoor
- Basic Sciences, Pennsylvania College of Optometry, Salus University Elkins Park, PA, USA
| | - James B Ames
- Department of Chemistry, University of California at Davis Davis, CA, USA
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19
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20
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Zang J, Matthews HR. Origin and control of the dominant time constant of salamander cone photoreceptors. ACTA ACUST UNITED AC 2012; 140:219-33. [PMID: 22802362 PMCID: PMC3409105 DOI: 10.1085/jgp.201110762] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
Recovery of the light response in vertebrate photoreceptors requires the shutoff of both active intermediates in the phototransduction cascade: the visual pigment and the transducin–phosphodiesterase complex. Whichever intermediate quenches more slowly will dominate photoresponse recovery. In suction pipette recordings from isolated salamander ultraviolet- and blue-sensitive cones, response recovery was delayed, and the dominant time constant slowed when internal [Ca2+] was prevented from changing after a bright flash by exposure to 0Ca2+/0Na+ solution. Taken together with a similar prior observation in salamander red-sensitive cones, these observations indicate that the dominance of response recovery by a Ca2+-sensitive process is a general feature of amphibian cone phototransduction. Moreover, changes in the external pH also influenced the dominant time constant of red-sensitive cones even when changes in internal [Ca2+] were prevented. Because the cone photopigment is, uniquely, exposed to the external solution, this may represent a direct effect of protons on the equilibrium between its inactive Meta I and active Meta II forms, consistent with the notion that the process dominating recovery of the bright flash response represents quenching of the active Meta II form of the cone photopigment.
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Affiliation(s)
- Jingjing Zang
- Physiological Laboratory, Department of Physiology, Development, and Neuroscience, University of Cambridge, Cambridge CB2 3EG, England, UK
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21
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Arshavsky VY, Burns ME. Photoreceptor signaling: supporting vision across a wide range of light intensities. J Biol Chem 2011; 287:1620-6. [PMID: 22074925 DOI: 10.1074/jbc.r111.305243] [Citation(s) in RCA: 148] [Impact Index Per Article: 11.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
For decades, photoreceptors have been an outstanding model system for elucidating basic principles in sensory transduction and biochemistry and for understanding many facets of neuronal cell biology. In recent years, new knowledge of the kinetics of signaling and the large-scale movements of proteins underlying signaling has led to a deeper appreciation of the photoreceptor's unique challenge in mediating the first steps in vision over a wide range of light intensities.
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Affiliation(s)
- Vadim Y Arshavsky
- Departments of Ophthalmology and Pharmacology, Duke University, Durham, North Carolina 27710, USA.
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22
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Abstract
A negative phototransduction feedback in rods and cones is critical for the timely termination of their light responses and for extending their function to a wide range of light intensities. The calcium feedback mechanisms that modulate phototransduction in rods have been studied extensively. However, the corresponding modulation mechanisms that enable cones to terminate rapidly their light responses and to adapt in bright light, properties critical for our daytime vision, are still not understood. In cones, calcium feedback to guanylyl cyclase is potentially a key step in phototransduction modulation. The guanylyl cyclase activity is modulated by the calcium-binding guanylyl cyclase activating proteins (GCAP1 and GCAP2). Here, we used single-cell and transretinal recordings from mouse to determine how GCAPs modulate dark-adapted responses as well as light adaptation in mammalian cones. Deletion of GCAPs increased threefold the amplitude and dramatically prolonged the light responses in dark-adapted mouse cones. It also reduced the operating range of mouse cones in background illumination and severely impaired their light adaptation. Thus, GCAPs exert powerful modulation on the mammalian cone phototransduction cascade and play an important role in setting the functional properties of cones in darkness and during light adaptation. Surprisingly, despite their better adaptation capacity and wider calcium dynamic range, mammalian cones were modulated by GCAPs to a lesser extent than mammalian rods. We conclude that a disparity in the strength of GCAP modulation cannot explain the differences in the dark-adapted properties or in the operating ranges of mammalian rods and cones.
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23
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Arinobu D, Tachibanaki S, Kawamura S. Larger inhibition of visual pigment kinase in cones than in rods. J Neurochem 2010; 115:259-68. [PMID: 20649847 DOI: 10.1111/j.1471-4159.2010.06925.x] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
Abstract
In the carp retina, visual pigment kinase, GRK1 (G-protein coupled receptor kinase 1) in rods and GRK7 in cones, is inhibited by a photoreceptor neuronal Ca(2+)-sensor protein, S-modulin (or recoverin) in rods and visinin (formerly named s26) in cones. Here, we compared Ca(2+)-dependent inhibition of GRK1 by S-modulin and that of GRK7 by visinin. First, the concentrations of S-modulin and visinin in the outer segment were estimated: the concentration of visinin (1.2 mM) was 20 times higher than that of S-modulin (53 μM). Based on the determined concentrations of the Ca(2+)-sensor proteins and the known dark Ca(2+) concentrations, we estimated that in situ Ca(2+)-dependent inhibition on GRK in cones would be 2.5 times higher than that in rods at the Ca(2+) concentration in the dark. Because GRK activity is approximately 100 times higher in cones than in rods [Proc. Natl Acad. Sci. USA 102 (2005) 21359], the range of Ca(2+)-dependent inhibition on GRK activity is more than 100 times wider in cones than in rods. The inhibitory effects of S-modulin and visinin on photoreceptor GRKs were indistinguishable, although these Ca(2+)-sensor proteins are expressed in a cell-type specific manner. The inhibition by these Ca(2+)-sensor proteins was slightly higher on GRK7 than GRK1 probably because of a characteristic specific to GRK7.
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Affiliation(s)
- Daisuke Arinobu
- Graduate School of Frontier Biosciences, Osaka University, Suita, Osaka, Japan
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24
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Antolin S, Reisert J, Matthews HR. Olfactory response termination involves Ca2+-ATPase in vertebrate olfactory receptor neuron cilia. ACTA ACUST UNITED AC 2010; 135:367-78. [PMID: 20351061 PMCID: PMC2847921 DOI: 10.1085/jgp.200910337] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
In vertebrate olfactory receptor neurons (ORNs), odorant-induced activation of the transduction cascade culminates in production of cyclic AMP, which opens cyclic nucleotide–gated channels in the ciliary membrane enabling Ca2+ influx. The ensuing elevation of the intraciliary Ca2+ concentration opens Ca2+-activated Cl− channels, which mediate an excitatory Cl− efflux from the cilia. In order for the response to terminate, the Cl− channel must close, which requires that the intraciliary Ca2+ concentration return to basal levels. Hitherto, the extrusion of Ca2+ from the cilia has been thought to depend principally on a Na+–Ca2+ exchanger. In this study, we show using simultaneous suction pipette recording and Ca2+-sensitive dye fluorescence measurements that in fire salamander ORNs, withdrawal of external Na+ from the solution bathing the cilia, which incapacitates Na+–Ca2+exchange, has only a modest effect on the recovery of the electrical response and the accompanying decay of intraciliary Ca2+ concentration. In contrast, exposure of the cilia to vanadate or carboxyeosin, a manipulation designed to block Ca2+-ATPase, has a substantial effect on response recovery kinetics. Therefore, we conclude that Ca2+-ATPase contributes to Ca2+ extrusion in ORNs, and that Na+–Ca2+exchange makes only a modest contribution to Ca2+ homeostasis in this species.
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Affiliation(s)
- Salome Antolin
- Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge, England, UK
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25
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Matthews HR, Sampath AP. Photopigment quenching is Ca2+ dependent and controls response duration in salamander L-cone photoreceptors. ACTA ACUST UNITED AC 2010; 135:355-66. [PMID: 20231373 PMCID: PMC2847922 DOI: 10.1085/jgp.200910394] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/02/2022]
Abstract
The time scale of the photoresponse in photoreceptor cells is set by the slowest of the steps that quench the light-induced activity of the phototransduction cascade. In vertebrate photoreceptor cells, this rate-limiting reaction is thought to be either shutoff of catalytic activity in the photopigment or shutoff of the pigment's effector, the transducin-GTP–phosphodiesterase complex. In suction pipette recordings from isolated salamander L-cones, we found that preventing changes in internal [Ca2+] delayed the recovery of the light response and prolonged the dominant time constant for recovery. Evidence that the Ca2+-sensitive step involved the pigment itself was provided by the observation that removal of Cl− from the pigment's anion-binding site accelerated the dominant time constant for response recovery. Collectively, these observations indicate that in L-cones, unlike amphibian rods where the dominant time constant is insensitive to [Ca2+], pigment quenching rate limits recovery and provides an additional mechanism for modulating the cone response during light adaptation.
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Affiliation(s)
- Hugh R Matthews
- Department of Physiology, Development and Neuroscience, Physiological Laboratory, University of Cambridge, Cambridge, England, UK.
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26
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Shen L, Caruso G, Bisegna P, Andreucci D, Gurevich V, Hamm H, DiBenedetto E. Dynamics of mouse rod phototransduction and its sensitivity to variation of key parameters. IET Syst Biol 2010; 4:12-32. [PMID: 20001089 PMCID: PMC3833298 DOI: 10.1049/iet-syb.2008.0154] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
The deep understanding of the biochemical and biophysical basis of visual transduction, makes it ideal for systems-level analysis. A sensitivity analysis is presented for a self-consistent set of parameters involved in mouse phototransduction. The organising framework is a spatio-temporal mathematical model, which includes the geometry of the rod outer segment (ROS), the layered array of the discs, the incisures, the biochemistry of the activation/deactivation cascade and the biophysics of the diffusion of the second messengers in the cytoplasm and the closing of the cyclic guanosine monophosphate (cGMP) gated cationic channels. These modules include essentially all the relevant geometrical, biochemical and biophysical parameters. The parameters are selected from within experimental ranges, to obey basic first principles such as conservation of mass and energy fluxes. By means of the model they are compared to a large set of experimental data, providing a strikingly close match. Following isomerisation of a single rhodopsin R * (single photon response), the sensitivity analysis was carried out on the photo-response, measured both in terms of number of effector molecules produced, and photocurrent suppression, at peak time and the activation and recovery phases of the cascade. The current suppression is found to be very sensitive to variations of the catalytic activities, Hill's coefficients and hydrolysis rates and the geometry of the ROS, including size and shape of the incisures. The activated effector phosphodiesterase (PDE *) is very sensitive to variations of catalytic activity of G-protein activation and the average lifetimes of activated rhodopsin R * and PDE *; however, they are insensitive to geometry and variations of the transduction parameters. Thus the system is separated into two functional modules, activation/deactivation and transduction, each confined in different geometrical domains, communicating through the hydrolysis of cGMP by PDE *, and each sensitive to variations of parameters only in its own module.
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Affiliation(s)
- L. Shen
- Department of Pharmacology, Vanderbilt University Medical Center, Nashville, TN, USA
| | - G. Caruso
- Construction Technologies Institute, National Research Council, Rome, Italy
| | - P. Bisegna
- Department of Civil Engineering, University of Rome Tor Vergata, Italy
| | - D. Andreucci
- Department of Mathematical Methods and Models, University of Rome La Sapienza, Italy
| | - V.V. Gurevich
- Department of Pharmacology, Vanderbilt University Medical Center, Nashville, TN, USA
| | - H.E. Hamm
- Department of Pharmacology, Vanderbilt University Medical Center, Nashville, TN, USA
| | - E. DiBenedetto
- Department of Mathematics, Vanderbilt University, Nashville, TN, USA
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27
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Ca(2+)-modulated vision-linked ROS-GC guanylate cyclase transduction machinery. Mol Cell Biochem 2009; 334:105-15. [PMID: 19943184 DOI: 10.1007/s11010-009-0330-z] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/10/2009] [Accepted: 11/04/2009] [Indexed: 02/02/2023]
Abstract
Vertebrate phototransduction depends on the reciprocal relationship between two-second messengers, cyclic GMP and Ca(2+). The concentration of both is reciprocally regulated including the dynamic synthesis of cyclic GMP by a membrane bound guanylate cyclase. Different from hormone receptor guanylate cyclases, the cyclases operating in phototransduction are regulated by the intracellular Ca(2+)-concentration via small Ca(2+)-binding proteins. Based on the site of their expression and their Ca(2+) modulation, this sub-branch of the cyclase family was named sensory guanylate cyclases, of which the retina specific forms are named ROS-GCs (rod outer segment guanylate cyclases). This review focuses on the structure and function of the ROS-GC subfamily present in the mammalian retinal neurons: photoreceptors and inner layers of the retinal neurons. Portions and excerpts of the review are from a previous chapter (Curr Top Biochem Res 6:111-144, 2004).
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28
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Solessio E, Umino Y, Cameron DA, Loew E, Engbretson GA, Knox BE, Barlow RB. Light responses in rods of vitamin A-deprived Xenopus. Invest Ophthalmol Vis Sci 2009; 50:4477-86. [PMID: 19407019 DOI: 10.1167/iovs.08-3186] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
Abstract
PURPOSE Accumulation of free opsin by mutations in rhodopsin or insufficiencies in the visual cycle can lead to retinal degeneration. Free opsin activates phototransduction; however, the link between constitutive activation and retinal degeneration is unclear. In this study, the photoresponses of Xenopus rods rendered constitutively active by vitamin A deprivation were examined. Unlike their mammalian counterparts, Xenopus rods do not degenerate. Contrasting phototransduction in vitamin A-deprived Xenopus rods with phototransduction in constitutively active mammalian rods may provide new understanding of the mechanisms that lead to retinal degeneration. METHODS The photocurrents of Xenopus tadpole rods were measured with suction electrode recordings, and guanylate cyclase activity was measured with the IBMX (3-isobutyl-1-methylxanthine) jump technique. The amount of rhodopsin in rods was determined by microspectrophotometry. RESULTS The vitamin A-deprived rod outer segments were 60% to 70% the length and diameter of the rods in age-matched animals. Approximately 90% of its opsin content was in the free or unbound form. Analogous to bleaching adaptation, the photoresponses were desensitized (10- to 20-fold) and faster. Unlike bleaching adaptation, the vitamin A-deprived rods maintained near normal saturating (dark) current densities by developing abnormally high rates of cGMP synthesis. Their rate of cGMP synthesis in the dark (15 seconds(-1)) was twofold greater than the maximum levels attainable by control rods ( approximately 7 seconds(-1)). CONCLUSIONS Preserving circulating current density and response range appears to be an important goal for rod homeostasis. However, the compensatory changes associated with vitamin A deprivation in Xenopus rods come at the high metabolic cost of a 15-fold increase in basal ATP consumption.
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Affiliation(s)
- Eduardo Solessio
- Department of Ophthalmology, SUNY Upstate Medical University, Center for Vision Research, Syracuse, New York 13210, USA.
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29
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Diffusion of the second messengers in the cytoplasm acts as a variability suppressor of the single photon response in vertebrate phototransduction. Biophys J 2008; 94:3363-83. [PMID: 18400950 DOI: 10.1529/biophysj.107.114058] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The single photon response in vertebrate phototransduction is highly reproducible despite a number of random components of the activation cascade, including the random activation site, the random walk of an activated receptor, and its quenching in a random number of steps. Here we use a previously generated and tested spatiotemporal mathematical and computational model to identify possible mechanisms of variability reduction. The model permits one to separate the process into modules, and to analyze their impact separately. We show that the activation cascade is responsible for generation of variability, whereas diffusion of the second messengers is responsible for its suppression. Randomness of the activation site contributes at early times to the coefficient of variation of the photoresponse, whereas the Brownian path of a photoisomerized rhodopsin (Rh*) has a negligible effect. The major driver of variability is the turnoff mechanism of Rh*, which occurs essentially within the first 2-4 phosphorylated states of Rh*. Theoretically increasing the number of steps to quenching does not significantly decrease the corresponding coefficient of variation of the effector, in agreement with the biochemical limitations on the phosphorylated states of the receptor. Diffusion of the second messengers in the cytosol acts as a suppressor of the variability generated by the activation cascade. Calcium feedback has a negligible regulatory effect on the photocurrent variability. A comparative variability analysis has been conducted for the phototransduction in mouse and salamander, including a study of the effects of their anatomical differences such as incisures and photoreceptors geometry on variability generation and suppression.
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30
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Liang L, Yan RT, Li X, Chimento M, Wang SZ. Reprogramming progeny cells of embryonic RPE to produce photoreceptors: development of advanced photoreceptor traits under the induction of neuroD. Invest Ophthalmol Vis Sci 2008; 49:4145-53. [PMID: 18469196 DOI: 10.1167/iovs.07-1380] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
Abstract
PURPOSE In examining the prospect of producing functional photoreceptors by reprogramming the differentiation of RPE progeny cells, this study was conducted to investigate whether reprogrammed cells can develop highly specialized ultrastructural and physiological traits that characterize retinal photoreceptors. METHODS Cultured chick RPE cells were reprogrammed to differentiate along the photoreceptor pathway by ectopic expression of neuroD. Cellular ultrastructure was examined with electron microscopy. Cellular physiology was studied by monitoring cellular free calcium (Ca(2+)) levels in dark-adapted cells in response to light and in light-bleached cells in response to 9-cis-retinal. RESULTS Reprogrammed cells were found to localize red opsin protein appropriately to the apex. These cells developed inner segments rich in mitochondria, and while in culture, some formed rudimentary outer segments, analogous to those of developing photoreceptors in the retina. In response to light, reprogrammed cells reduced their Ca(2+) levels, as observed with developing retinal photoreceptors in culture. Further, on exposure to 9-cis-retinal, the light-bleached, reprogrammed cells increased their Ca(2+) levels, reminiscent of visual cycle recovery. CONCLUSIONS These results indicate the potential of reprogrammed cells to develop advanced ultrastructural and physiological traits of photoreceptors and point to reprogramming progeny cells of embryonic RPE as a possible alternative in producing developing photoreceptors.
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Affiliation(s)
- Lina Liang
- Department of Ophthalmology, University of Alabama at Birmingham, Birmingham, Alabama
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31
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Gomez-Villafuertes R, Mellström B, Naranjo JR. Searching for a role of NCX/NCKX exchangers in neurodegeneration. Mol Neurobiol 2008; 35:195-202. [PMID: 17917108 DOI: 10.1007/s12035-007-0007-0] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/05/2006] [Revised: 11/30/1999] [Accepted: 02/02/2007] [Indexed: 01/03/2023]
Abstract
Control of intracellular calcium signaling is essential for neuronal development and function. Maintenance of Ca2+ homeostasis depends on the functioning of specific transport systems that remove calcium from the cytosol. Na+/Ca2+ exchange is the main calcium export mechanism across the plasma membrane that restores resting levels of calcium in neurons after stimulation. Two families of Na+/Ca2+ exchangers exist, one of which requires the co-transport of K+ and Ca2+ in exchange for Na+ ions. The malfunctioning of Na+/Ca2+ exchangers has been related to the development of pathological conditions in the regulation of neuronal death after hypoxia-anoxia, brain trauma, and nerve injury. In addition, the Na+/Ca2+ exchanger function has been associated with impaired Ca2+ homeostasis during aging of the brain, as well as with a role in Alzheimer's disease by regulating beta-amyloid toxicity. In this review, we summarize the current knowledge about the Na+/Ca2+ exchanger families and their implications in neurodegenerative disorders.
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Leung YT, Fain GL, Matthews HR. Simultaneous measurement of current and calcium in the ultraviolet-sensitive cones of zebrafish. J Physiol 2006; 579:15-27. [PMID: 17124271 PMCID: PMC2075373 DOI: 10.1113/jphysiol.2006.120162] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023] Open
Abstract
In rods and visible cone photoreceptors, multiple measurements cannot be made of intracellular Ca2+ concentration from the same cell using fluorescent dyes, because a single exposure of the measuring light bleaches too large a fraction of the rod or cone photopigment. We have therefore identified and characterized UV-sensitive cones of the zebrafish, whose wavelength of maximum sensitivity is at 360 nm which is far enough from the wavelength of our measuring light (514.5 nm) so that it has been possible to make multiple determinations of photocurrent and Ca2+ concentration from the same cells. We show that for a limited number of measurements, for which the bleaching of the cone photopigment is too small to affect flash kinetics, the outer segment Ca2+ concentration closely follows the wave form of the flash response convolved with the dominant time constant for Ca2+ removal by Na+-Ca2+-K+ exchange. For a larger number of measurements, significant acceleration of the response kinetics by pigment bleaching inevitably occurs, but the Ca2+ concentration nevertheless rises and falls in approximate agreement with the flash wave form. During exposure to steady background light, the Ca2+ concentration falls in proportion to the steady-state current for dim backgrounds at all times and for bright backgrounds at steady state. At early times following the onset of bright backgrounds, however, the Ca2+ concentration is markedly higher than expected from the current of the cone. We show this to be the result of light-dependent Ca2+ release by bright background light, which can be abolished by pre-exposure of the cone to the membrane-permeant acetoxymethyl ester of the Ca2+ chelator BAPTA. Our results therefore demonstrate that the cone outer segment Ca2+ concentration is predominantly a function of the rate of influx and efflux of Ca2+ across the plasma membrane, but that a release of Ca2+ in bright light most probably from buffer sites within the cell can transiently elevate the Ca2+ concentration above the level expected from the open probability of the light-dependent channels.
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Affiliation(s)
- Yiu Tak Leung
- Physiological Laboratory, Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge, UK
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Hamer RD, Nicholas SC, Tranchina D, Lamb TD, Jarvinen JLP. Toward a unified model of vertebrate rod phototransduction. Vis Neurosci 2006; 22:417-36. [PMID: 16212700 PMCID: PMC1482458 DOI: 10.1017/s0952523805224045] [Citation(s) in RCA: 72] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/17/2004] [Accepted: 01/27/2005] [Indexed: 11/07/2022]
Abstract
Recently, we introduced a phototransduction model that was able to account for the reproducibility of vertebrate rod single-photon responses (SPRs) (Hamer et al., 2003). The model was able to reproduce SPR statistics by means of stochastic activation and inactivation of rhodopsin (R*), transducin (G alpha ), and phosphodiesterase (PDE). The features needed to capture the SPR statistics were (1) multiple steps of R* inactivation by means of multiple phosphorylations (followed by arrestin capping) and (2) phosphorylation dependence of the affinity between R* and the three molecules competing to bind with R* (G alpha, arrestin, and rhodopsin kinase). The model was also able to account for several other rod response features in the dim-flash regime, including SPRs obtained from rods in which various elements of the cascade have been genetically disabled or disrupted. However, the model was not tested under high light-level conditions. We sought to evaluate the extent to which the multiple phosphorylation model could simultaneously account for single-photon response behavior, as well as responses to high light levels causing complete response saturation and/or significant light adaptation (LA). To date no single model, with one set of parameters, has been able to do this. Dim-flash responses and statistics were simulated using a hybrid stochastic/deterministic model and Monte-Carlo methods as in Hamer et al. (2003). A dark-adapted flash series, and stimulus paradigms from the literature eliciting various degrees of light adaptation (LA), were simulated using a full differential equation version of the model that included the addition of Ca2+-feedback onto rhodopsin kinase via recoverin. With this model, using a single set of parameters, we attempted to account for (1) SPR waveforms and statistics (as in Hamer et al., 2003); (2) a full dark-adapted flash-response series, from dim flash to saturating, bright flash levels, from a toad rod; (3) steady-state LA responses, including LA circulating current (as in Koutalos et al., 1995) and LA flash sensitivity measured in rods from four species; (4) step responses from newt rods ( Forti et al., 1989) over a large dynamic range; (5) dynamic LA responses, such as the step-flash paradigm of Fain et al. (1989), and the two-flash paradigm of Murnick and Lamb (1996); and (6) the salient response features from four knockout rod preparations. The model was able to meet this stringent test, accounting for almost all the salient qualitative, and many quantitative features, of the responses across this broad array of stimulus conditions, including SPR reproducibility. The model promises to be useful in testing hypotheses regarding both normal and abnormal photoreceptor function, and is a good starting point for development of a full-range model of cone phototransduction. Informative limitations of the model are also discussed.
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Affiliation(s)
- R D Hamer
- Smith-Kettlewell Eye Research Institute, San Francisco, CA 94115, USA.
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34
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Bradley J, Reisert J, Frings S. Regulation of cyclic nucleotide-gated channels. Curr Opin Neurobiol 2005; 15:343-9. [PMID: 15922582 DOI: 10.1016/j.conb.2005.05.014] [Citation(s) in RCA: 111] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2005] [Accepted: 05/06/2005] [Indexed: 11/20/2022]
Abstract
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.
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Affiliation(s)
- Jonathan Bradley
- Department of Neuroscience, Johns Hopkins School of Medicine, 725 North Wolfe Street, Baltimore, MD 21205, USA
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35
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Cilluffo MC, Matthews HR, Brockerhoff SE, Fain GL. Light-induced Ca2+ release in the visible cones of the zebrafish. Vis Neurosci 2005; 21:599-609. [PMID: 15579223 DOI: 10.1017/s0952523804214092] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/19/2004] [Indexed: 11/06/2022]
Abstract
We used suction-pipette recording and fluo-4 fluorescence to study light-induced Ca2+ release from the visible double cones of zebrafish. In Ringer, light produces a slow decrease in fluorescence which can be fitted by the sum of two decaying exponentials with time constants of 0.5 and 3.8 s. In 0Ca2+-0Na+ solution, for which fluxes of Ca2+ across the outer segment plasma membrane are greatly reduced, light produces a slow increase in fluorescence. Both the decrease and increase are delayed after incorporation of the Ca2+ chelator BAPTA, indicating that both are produced by a change in Ca2+. If the Ca2+ pool is first released by bright light in 0Ca2+-0Na+ solution and the cone returned to Ringer, the time course of Ca2+ decline is much faster than in Ringer without previous light exposure. This indicates that the time constants of 0.5 and 3.8 s actually reflect a sum of Na+/Ca2+-K+ exchange and light-induced release of Ca2+. The Ca2+ released by light appears to come from at least two sites, the first comprising 66% of the total pool and half-released by bleaching 4.8% of the pigment. Release of the remaining Ca2+ from the second site requires the bleaching of nearly all of the pigment. If, after release, the cone is maintained in darkness, a substantial fraction of the Ca2+ returns to the release pool even in the absence of pigment regeneration. The light-induced release of Ca2+ can produce a modulation of the dark current as large as 0.75 pA independently of the normal transduction cascade, though the rise time of the current is considerably slower than the normal light response. These experiments show that Ca2+ can be released within the cone outer segment by light intensities within the physiological range of photopic vision. The role this Ca2+ release plays remains unresolved.
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Affiliation(s)
- Marianne C Cilluffo
- Department of Physiological Science, University of California-Los Angeles, Los Angeles, CA 90095-1606, USA
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36
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Olshevskaya EV, Calvert PD, Woodruff ML, Peshenko IV, Savchenko AB, Makino CL, Ho YS, Fain GL, Dizhoor AM. The Y99C mutation in guanylyl cyclase-activating protein 1 increases intracellular Ca2+ and causes photoreceptor degeneration in transgenic mice. J Neurosci 2005; 24:6078-85. [PMID: 15240799 PMCID: PMC6729660 DOI: 10.1523/jneurosci.0963-04.2004] [Citation(s) in RCA: 85] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Guanylyl cyclase-activating proteins (GCAPs) are Ca2+-binding proteins that activate guanylyl cyclase when free Ca2+ concentrations in retinal rods and cones fall after illumination and inhibit the cyclase when free Ca2+ reaches its resting level in the dark. Several forms of retinal dystrophy are caused by mutations in GUCA1A, the gene coding for GCAP1. To investigate the cellular mechanisms affected by the diseased state, we created transgenic mice that express GCAP1 with a Tyr99Cys substitution (Y99C GCAP1) found in human patients with a late-onset retinal dystrophy (Payne et al., 1998). Y99C GCAP1 shifted the Ca2+ sensitivity of the guanylyl cyclase in photoreceptors, keeping it partially active at 250 nM free Ca2+, the normal resting Ca2+ concentration in darkness. The enhanced activity of the cyclase in the dark increased cyclic nucleotide-gated channel activity and elevated the rod outer segment Ca2+ concentration in darkness, measured by using fluo-5F and laser spot microscopy. In different lines of transgenic mice the magnitude of this effect rose with the Y99C GCAP1 expression. Surprisingly, there was little change in the rod photoresponse, indicating that dynamic Ca2+-dependent regulation of cGMP synthesis was preserved. However, the photoreceptors in these mice degenerated, and the rate of the cell loss increased with the level of the transgene expression, unlike in transgenic mice that overexpressed normal GCAP1. These results provide the first direct evidence that a mutation linked to congenital blindness increases Ca2+ in the outer segment, which may trigger the apoptotic process.
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Affiliation(s)
- Elena V Olshevskaya
- Hafter Research Laboratories, Pennsylvania College of Optometry, Elkins Park, Pennsylvania 19027, USA
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37
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Li H, Ferrari MB, Kuenzel WJ. Light-induced reduction of cytoplasmic free calcium in neurons proposed to be encephalic photoreceptors in chick brain. BRAIN RESEARCH. DEVELOPMENTAL BRAIN RESEARCH 2004; 153:153-61. [PMID: 15527883 DOI: 10.1016/j.devbrainres.2004.08.006] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Accepted: 08/03/2004] [Indexed: 10/26/2022]
Abstract
A population of cerebrospinal fluid-contacting neurons (CSFcn) in the lateral septal organ (LSO) may serve as encephalic photoreceptors (EPRs) functioning to signal the onset of seasonal reproductive development in birds. Previous studies on CSFcn in the LSO have focused on identification of retinal protein components in fixed brain tissue. In order to understand better the mechanisms underlying the light-induced photosexual response in birds, a physiological characterization is required. In this study, changes of intracellular free calcium concentration ([Ca2+]i) were monitored during light stimulation of CSFcn in the LSO in live brain slices from embryonic chicks. Using the fluorescent calcium indicator fluo-4, a reduced [Ca2+]i within CSFcn was recorded in response to photostimulation, which is consistent with what has been demonstrated in rods and cones following illumination. Results support the hypothesis that CSFcn in the LSO function as EPRs in the avian brain.
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Affiliation(s)
- Hongyan Li
- Center of Excellence for Poultry Science, University of Arkansas, Fayetteville, AR 72701, USA
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38
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Abstract
Calcium acts as a second messenger in vertebrate rods, regulating the recovery phase of the light response and modulating sensitivity during light-adaptation. Since light not only decreases the outer segment calcium concentration ([Ca2+]i) by closing cyclic nucleotide-gated channels but can also increase [Ca2+]i by releasing Ca2+ from buffer sites or intracellular stores, we examined in detail the effect of light and circulating current on [Ca2+]i by making simultaneous measurements of suction pipette current and [Ca2+]i from isolated rods of the salamander Ambystoma tigrinum after incorporation of the fluorescent dye fluo-5F. When the release of Ca2+ is measured in 0 Ca2+-0 Na+ solution, minimising fluxes of Ca2+ across the plasma membrane, it is substantial only for light bright enough to bleach a significant fraction of the photopigment and is restricted to the part of the outer segment in which the bleach occurred. It is unlikely, therefore, to make a large contribution to [Ca2+]i for most of the physiological operating range of the rod. Nevertheless, since release is half-maximal for a bleach of less than 10 %, it cannot be produced by a simple mechanism such as a change in the affinity of a binding site on rhodopsin itself but must instead require some more complex interaction. In Ringer solution, the Ca2+ in the light-releasable pool can be discharged merely by the decrease in [Ca2+]i that occurs as the outer segment channels close. In steady background light or after exposure to saturating illumination, the fraction of Ca2+ in the pool decreases essentially in proportion to [Ca2+]i as if Ca2+ were being removed from a buffer site within the cytoplasm. Furthermore, [Ca2+]i itself changes in proportion to the circulating current, with little evidence for a contribution from Ca2+ release or other mechanisms of Ca2+ homeostasis. This indicates that flux of Ca2+ across the plasma membrane is the major determinant of outer segment Ca2+ concentration within the rod's normal operating light intensity range. Once Ca2+ has been discharged from the releasable pool, it is restored following dim illumination apparently as the simple result of the subsequent restoration of dark [Ca2+]i and the rebinding of Ca2+ to its release site, but after brighter light perhaps also as a consequence of regeneration of the photopigment.
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Affiliation(s)
- Hugh R Matthews
- Physiological Laboratory, University of Cambridge, Downing Street, Cambridge CB2 3EG, UK.
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Hamer RD, Nicholas SC, Tranchina D, Liebman PA, Lamb TD. Multiple steps of phosphorylation of activated rhodopsin can account for the reproducibility of vertebrate rod single-photon responses. J Gen Physiol 2003; 122:419-44. [PMID: 12975449 PMCID: PMC1480412 DOI: 10.1085/jgp.200308832] [Citation(s) in RCA: 60] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Single-photon responses (SPRs) in vertebrate rods are considerably less variable than expected if isomerized rhodopsin (R*) inactivated in a single, memoryless step, and no other variability-reducing mechanisms were available. We present a new stochastic model, the core of which is the successive ratcheting down of R* activity, and a concomitant increase in the probability of quenching of R* by arrestin (Arr), with each phosphorylation of R* (Gibson, S.K., J.H. Parkes, and P.A. Liebman. 2000. Biochemistry. 39:5738-5749.). We evaluated the model by means of Monte-Carlo simulations of dim-flash responses, and compared the response statistics derived from them with those obtained from empirical dim-flash data (Whitlock, G.G., and T.D. Lamb. 1999. Neuron. 23:337-351.). The model accounts for four quantitative measures of SPR reproducibility. It also reproduces qualitative features of rod responses obtained with altered nucleotide levels, and thus contradicts the conclusion that such responses imply that phosphorylation cannot dominate R* inactivation (Rieke, F., and D.A. Baylor. 1998a. Biophys. J. 75:1836-1857; Field, G.D., and F. Rieke. 2002. Neuron. 35:733-747.). Moreover, the model is able to reproduce the salient qualitative features of SPRs obtained from mouse rods that had been genetically modified with specific pathways of R* inactivation or Ca2+ feedback disabled. We present a theoretical analysis showing that the variability of the area under the SPR estimates the variability of integrated R* activity, and can provide a valid gauge of the number of R* inactivation steps. We show that there is a heretofore unappreciated tradeoff between variability of SPR amplitude and SPR duration that depends critically on the kinetics of inactivation of R* relative to the net kinetics of the downstream reactions in the cascade. Because of this dependence, neither the variability of SPR amplitude nor duration provides a reliable estimate of the underlying variability of integrated R* activity, and cannot be used to estimate the minimum number of R* inactivation steps. We conclude that multiple phosphorylation-dependent decrements in R* activity (with Arr-quench) can confer the observed reproducibility of rod SPRs; there is no compelling need to invoke a long series of non-phosphorylation dependent state changes in R* (as in Rieke, F., and D.A. Baylor. 1998a. Biophys. J. 75:1836-1857; Field, G.D., and F. Rieke. 2002. Neuron. 35:733-747.). Our analyses, plus data and modeling of others (Rieke, F., and D.A. Baylor. 1998a. Biophys. J. 75:1836-1857; Field, G.D., and F. Rieke. 2002. Neuron. 35:733-747.), also argue strongly against either feedback (including Ca2+-feedback) or depletion of any molecular species downstream to R* as the dominant cause of SPR reproducibility.
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Affiliation(s)
- R D Hamer
- Smith-Kettlewell Eye Research Institute, 2318 Fillmore Street, San Francisco, CA 94115, USA.
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40
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Woodruff ML, Wang Z, Chung HY, Redmond TM, Fain GL, Lem J. Spontaneous activity of opsin apoprotein is a cause of Leber congenital amaurosis. Nat Genet 2003; 35:158-64. [PMID: 14517541 DOI: 10.1038/ng1246] [Citation(s) in RCA: 143] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/25/2003] [Accepted: 08/26/2003] [Indexed: 12/19/2022]
Abstract
Mutations in Rpe65 disrupt synthesis of the opsin chromophore ligand 11-cis-retinal and cause Leber congenital amaurosis (LCA), a severe, early-onset retinal dystrophy. To test whether light-independent signaling by unliganded opsin causes the degeneration, we used Rpe65-null mice, a model of LCA. Dark-adapted Rpe65-/- mice behaved as if light adapted, exhibiting reduced circulating current, accelerated response turn-off, and diminished intracellular calcium. A genetic block of transducin signaling completely rescued degeneration irrespective of an elevated level of retinyl ester. These studies clearly show that activation of sensory transduction by unliganded opsin, and not the accumulation of retinyl esters, causes light-independent retinal degeneration in LCA. A similar mechanism may also be responsible for degeneration induced by vitamin A deprivation.
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Affiliation(s)
- Michael L Woodruff
- Department of Physiological Science, University of California Los Angeles, Los Angeles, California 90095, USA
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41
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Chen C, Nakatani K, Koutalos Y. Free magnesium concentration in salamander photoreceptor outer segments. J Physiol 2003; 553:125-35. [PMID: 14500766 PMCID: PMC2343491 DOI: 10.1113/jphysiol.2003.053280] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Magnesium ions (Mg2+) play an important role in biochemical functions. In vertebrate photoreceptor outer segments, numerous reactions utilize MgGTP and MgATP, and Mg2+ also regulates several of the phototransduction enzymes. Although Mg2+ can pass through light-sensitive channels under certain conditions, no clear extrusion mechanism has been identified and removing extracellular Mg2+ has no significant effect on the light sensitivity or the kinetics of the photoresponse. We have used the fluorescent Mg2+ dye Furaptra to directly measure and monitor the free Mg2+ concentration in photoreceptor outer segments and examine whether the free Mg2+ concentration changes under physiological conditions. Resting free Mg2+ concentrations in bleached salamander rod and cone photoreceptor cell outer segments were 0.86 +/- 0.06 and 0.81 +/- 0.09 mM, respectively. The outer segment free Mg2+ concentration was not significantly affected by changes in extracellular pH, Ca2+ and Na+, excluding a significant role for the respective exchangers in the regulation of Mg2+ homeostasis. The resting free Mg2+ concentration was also not significantly affected by exposure to 0 Mg2+, suggesting the lack of significant basal Mg2+ flux. Opening the cGMP-gated channels led to a significant increase in the Mg2+ concentration in the absence of Na+ and Ca2+, but not in their presence, indicating that depolarization can cause a significant Mg2+ influx only in the absence of other permeant ions, but not under physiological conditions. Finally, light stimulation did not change the Mg2+ concentration in the outer segments of dark-adapted photoreceptors. The results suggest that there are no influx and efflux pathways that can significantly affect the Mg2+ concentration in the outer segment under physiological conditions. Therefore, it is unlikely that Mg2+ plays a significant role in the dynamic modulation of phototransduction.
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Affiliation(s)
- Chunhe Chen
- Department of Physiology and Biophysics, University of Colorado Health Sciences Center, Denver, CO 80262, USA
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42
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Abstract
Usually, photoreceptors interact with other retinal cells through the neurotransmitter glutamate. Here we describe a nonsynaptic interaction via a secreted protein, retinoschisin. Using in situ hybridization, we found that from early postnatal life retinoschisin mRNA is present only in the outer retina of the mouse, and with single-cell RT-PCR we demonstrated its localization in rod and cone photoreceptor cells but not in Müller cells. Western blot analyses of proteins from cultured ocular tissues and microdissected outer and inner retinas, as well as from the culture media of these samples, showed that retinoschisin is secreted from the photoreceptor cells. Immunostaining of permeabilized and nonpermeabilized dissociated retinal cells revealed that retinoschisin is mainly inside and outside the photoreceptors, outside bipolar cells, and associated with plasma membranes of Müller cells and inside their distal processes. Because we showed previously that retinoschisin is distributed all over the retina, our current data suggest that after synthesis and secretion by the photoreceptors, retinoschisin reaches the surface of retinal cells and mediates interactions/adhesion between photoreceptor, bipolar, and Müller cells, contributing to the maintenance of the cytoarchitectural integrity of the retina. These interactions may not occur when the gene encoding retinoschisin is mutated, as it occurs in X-linked juvenile retinoschisis, a disease that results in morphological and electrophysiological defects of the retina.
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43
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Nakatani K, Chen C, Yau KW, Koutalos Y. Calcium and phototransduction. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2003; 514:1-20. [PMID: 12596912 DOI: 10.1007/978-1-4615-0121-3_1] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Visual phototransduction, the conversion of incoming light to an electrical signal, takes place in the outer segments of the rod and cone photoreceptor cells. Light reduces the concentration of cGMP, which, in darkness, keeps open cationic channels present in the plasma membrane of the outer segment. Ca2+ plays an important role in phototransduction by modulating the cGMP-gated channels as well as cGMP synthesis and breakdown. Ca2+ is involved in a negative feedback that is essential for photoreceptor adaptation to background illumination. The effects of Ca2+ on the different components of rod phototransduction have been characterized and can quantitatively account for the steady state responses of the rod cell to background illumination. The propagation of the Ca2+ feedback signal from the periphery toward the center of the outer segment depends on the Ca2+ diffusion coefficient, which has a value of 15 +/- 1 microm2 s(-1). This value shows that diffusion of Ca2+ in the radial direction is quite slow providing a significant barrier in the propagation of the feedback signal. Also, because the diffusion coefficient of Ca2+ is much smaller than that of cGMP, the decline of Ca2+ in the longitudinal direction lags behind the propagation of excitation by the decline of cGMP.
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Affiliation(s)
- Kei Nakatani
- Institute of Biological Sciences, University of Tsukuba, Japan
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44
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Abstract
S-Modulin is a Ca2+-binding protein found in frog rod photoreceptors (1,2) and its bovine homologue is known as recoverin (3,4). In the Ca2+-bound form, S-modulin inhibits rhodopsin phosphorylation5 through inhibition of rhodopsin kinase. (6-9) Because rhodopsin phosphorylation is the quench mechanism of light-activated rhodopsin (R*), (10,11) the inhibition of the phosphorylation by S-modulin probably contributes to increase the lifetime of R* to result in sustained hydrolysis of cGMP5. The Ca2+ concentration decreases in the light in vertebrate photoreceptors, (12-14) and this decrease is essential for light-adaptation. (15,16) Thus, S-modulin is expected to regulate the lifetime of R* and thereby regulate the extent and the time course of hydrolysis of cGMP depending on the intensity of background light. With this mechanism, S-modulin is believed to regulate the waveform of a photoresponse and the efficiency of the light in the generation of a photoresponse.
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Affiliation(s)
- Satoru Kawamura
- Department of Biology, Graduate School of Science, Osaka University, Machikane-yama 1-1, Toyonaka, Osaka 560-0043, Japan
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45
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Reid SNM, Yamashita C, Farber DB. Retinoschisin, a photoreceptor-secreted protein, and its interaction with bipolar and muller cells. J Neurosci 2003; 23:6030-40. [PMID: 12853421 PMCID: PMC6740352] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/03/2023] Open
Abstract
Usually, photoreceptors interact with other retinal cells through the neurotransmitter glutamate. Here we describe a nonsynaptic interaction via a secreted protein, retinoschisin. Using in situ hybridization, we found that from early postnatal life retinoschisin mRNA is present only in the outer retina of the mouse, and with single-cell RT-PCR we demonstrated its localization in rod and cone photoreceptor cells but not in Müller cells. Western blot analyses of proteins from cultured ocular tissues and microdissected outer and inner retinas, as well as from the culture media of these samples, showed that retinoschisin is secreted from the photoreceptor cells. Immunostaining of permeabilized and nonpermeabilized dissociated retinal cells revealed that retinoschisin is mainly inside and outside the photoreceptors, outside bipolar cells, and associated with plasma membranes of Müller cells and inside their distal processes. Because we showed previously that retinoschisin is distributed all over the retina, our current data suggest that after synthesis and secretion by the photoreceptors, retinoschisin reaches the surface of retinal cells and mediates interactions/adhesion between photoreceptor, bipolar, and Müller cells, contributing to the maintenance of the cytoarchitectural integrity of the retina. These interactions may not occur when the gene encoding retinoschisin is mutated, as it occurs in X-linked juvenile retinoschisis, a disease that results in morphological and electrophysiological defects of the retina.
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Affiliation(s)
- Silvia N M Reid
- Jules Stein Eye Institute, University of California Los Angeles School of Medicine, Center for the Health Sciences, Los Angeles, California 90095-7008, USA.
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46
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Schnetkamp PPM. The SLC24 Na+/Ca2+-K+ exchanger family: vision and beyond. Pflugers Arch 2003; 447:683-8. [PMID: 14770312 DOI: 10.1007/s00424-003-1069-0] [Citation(s) in RCA: 61] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2003] [Accepted: 03/21/2003] [Indexed: 01/30/2023]
Abstract
Na(+)/Ca(2+)-K(+) exchange (NCKX) was first discovered in the outer segments of vertebrate rod photoreceptors (ROS), where it is the only mechanism for extruding the Ca(2+) that enters ROS via the light-sensitive and cGMP-gated channels. ROS NCKX1 is the only NCKX gene family member studied extensively in situ. ROS NCKX1 cDNAs have been cloned subsequently from a number of species including man and shown to be the first member of a new gene family ( SLCA24). Three further members of the human NCKX gene family have been cloned subsequently ( NCKX2- 4) by homology with NCKX1, while a partial sequence of a fifth human NCKX gene has appeared in the data base. NCKX-related genes have also been identified in lower animals including fruit flies, worms and sea urchins. NCKX2 is expressed in the brain, in retinal cone photoreceptors and in retinal ganglion cells, while NCKX3 and NCKX4 show a broader expression pattern. In situ NCKX1 and heterologously expressed NCKX2 operate at a 4Na(+):1Ca(2+)+1 K(+) stoichiometry; both NCKX1 and NCKX2 are bidirectional transporters normally extruding Ca(2+) from the cell (forward exchange), but also able to carry Ca(2+) into the cell (reverse exchange) when the transmembrane Na(+) gradient is reversed. Sequence changes have been observed for both NCKX1 and NCKX2 in patients with retinal diseases, but a definitive association with retinal disease has not been shown.
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Affiliation(s)
- Paul P M Schnetkamp
- Department of Physiology and Biophysics, Faculty of Medicine, University of Calgary, 3330 Hospital Drive, Alberta, T2N 4N1, N.W. Calgary, Canada.
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47
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Kang K, Bauer PJ, Kinjo TG, Szerencsei RT, Bönigk W, Winkfein RJ, Schnetkamp PPM. Assembly of retinal rod or cone Na(+)/Ca(2+)-K(+) exchanger oligomers with cGMP-gated channel subunits as probed with heterologously expressed cDNAs. Biochemistry 2003; 42:4593-600. [PMID: 12693957 DOI: 10.1021/bi027276z] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Proper control of intracellular free Ca(2+) is thought to involve subsets of proteins that co-localize to mediate coordinated Ca(2+) entry and Ca(2+) extrusion. The outer segments of vertebrate rod and cone photoreceptors present one example: Ca(2+) influx is exclusively mediated via cGMP-gated channels (CNG), whereas the Na(+)/Ca(2+)-K(+) exchanger (NCKX) is the only Ca(2+) extrusion protein present. In situ, a rod NCKX homodimer and a CNG heterotetramer are thought to be part of a single protein complex. However, NCKX-NCKX and NCKX-CNG interactions have been described so far only in bovine rod outer segment membranes. We have used thiol-specific cross-linking and co-immunoprecipitation to examine NCKX self-assembly and CNG-NCKX co-assembly after heterologous expression of either the rod or cone NCKX/CNG isoforms. Co-immunoprecipitation clearly demonstrated both NCKX homooligomerization and interactions between NCKX and CNG. The NCKX-NCKX and NCKX-CNG interactions were observed for both the rod and the cone isoforms. Thiol-specific cross-linking led to rod NCKX1 dimers and to cone NCKX2 adducts of an apparent molecular weight higher than that predicted for a NCKX2 dimer. The mass of the cross-link product critically depended on the location of the particular cysteine residue used by the cross-linker, and we cannot exclude that NCKX forms a higher oligomer. The NCKX-NCKX and NCKX-CNG interactions were not isoform-specific (i.e., rod NCKX could interact with cone NCKX, rod NCKX could interact with cone CNGA, and vice versa). Deletion of the two large hydrophilic loops from the NCKX protein did not abolish the NCKX oligomerization, suggesting that it is mediated by the highly conserved transmembrane spanning segments.
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Affiliation(s)
- KyeongJin Kang
- Department of Physiology and Biophysics, University of Calgary, Faculty of Medicine, Calgary, Alberta, Canada
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48
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Light stimulates a transducin-independent increase of cytoplasmic Ca2+ and suppression of current in cones from the zebrafish mutant nof. J Neurosci 2003. [PMID: 12533607 DOI: 10.1523/jneurosci.23-02-00470.2003] [Citation(s) in RCA: 74] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Transducins couple visual pigments to cGMP hydrolysis, the only recognized phototransduction pathway in vertebrate photoreceptors. Here we describe a zebrafish mutant, no optokinetic response f(w21) (nof), with a nonsense mutation in the gene encoding the alpha subunit of cone transducin. Retinal morphology and levels of phototransduction enzymes are normal in nof retinas, but cone transducin is undetectable. Dark current in nof cones is also normal, but it is insensitive to moderate intensity light. The nof cones do respond, however, to bright light. These responses are produced by a light-stimulated, but transducin-independent, release of Ca2+ into the cone cytoplasm. Thus, in addition to stimulating transducin, light also independently induces release of Ca2+ into the photoreceptor cytoplasm.
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Brockerhoff SE, Rieke F, Matthews HR, Taylor MR, Kennedy B, Ankoudinova I, Niemi GA, Tucker CL, Xiao M, Cilluffo MC, Fain GL, Hurley JB. Light stimulates a transducin-independent increase of cytoplasmic Ca2+ and suppression of current in cones from the zebrafish mutant nof. J Neurosci 2003; 23:470-80. [PMID: 12533607 PMCID: PMC6741873] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/28/2023] Open
Abstract
Transducins couple visual pigments to cGMP hydrolysis, the only recognized phototransduction pathway in vertebrate photoreceptors. Here we describe a zebrafish mutant, no optokinetic response f(w21) (nof), with a nonsense mutation in the gene encoding the alpha subunit of cone transducin. Retinal morphology and levels of phototransduction enzymes are normal in nof retinas, but cone transducin is undetectable. Dark current in nof cones is also normal, but it is insensitive to moderate intensity light. The nof cones do respond, however, to bright light. These responses are produced by a light-stimulated, but transducin-independent, release of Ca2+ into the cone cytoplasm. Thus, in addition to stimulating transducin, light also independently induces release of Ca2+ into the photoreceptor cytoplasm.
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Affiliation(s)
- Susan E Brockerhoff
- Department of Biochemistry, University of Washington, Seattle, Washington 98195, USA.
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50
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Webel R, Haug-Collet K, Pearson B, Szerencsei RT, Winkfein RJ, Schnetkamp PPM, Colley NJ. Potassium-dependent sodium-calcium exchange through the eye of the fly. Ann N Y Acad Sci 2002; 976:300-14. [PMID: 12502573 DOI: 10.1111/j.1749-6632.2002.tb04753.x] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
In this review, we describe the characterization of a Drosophila sodium/calcium-potassium exchanger, Nckx30C. Sodium/calcium (-potassium) exchangers (NCX and NCKX) are required for the rapid removal of calcium in excitable cells. The deduced protein topology for NCKX30C is similar to that of mammalian NCKX, with 5 hydrophobic domains in the amino terminus separated from 6 at the carboxy-terminal end by a large intracellular loop. NCKX30C functions as a potassium-dependent sodium-calcium exchanger and is expressed in adult neurons and during ventral nerve cord development in the embryo. Nckx30C is expressed in a dorsal/ventral pattern in the eye-antennal disc, suggesting that large fluxes of calcium may be occurring during imaginal disc development in the larvae. NCKX30C may play a critical role in modulating calcium during development as well as in the removal of calcium and maintenance of calcium homeostasis in adults.
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Affiliation(s)
- R Webel
- Department of Ophthalmology & Visual Science, University of Wisconsin, Madison, Wisconsin 53792, USA
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