title: Effect of Rhodopsin Phosphorylation on Dark Adaptation in Mouse Rods.
abstract: Rhodopsin is a prototypical G-protein-coupled receptor (GPCR) that is activated when its 11-cis-retinal moiety is photoisomerized to all-trans retinal. This step initiates a cascade of reactions by which rods signal changes in light intensity. Like other GPCRs, rhodopsin is deactivated through receptor phosphorylation and arrestin binding. Full recovery of receptor sensitivity is then achieved when rhodopsin is regenerated through a series of steps that return the receptor to its ground state. Here, we show that dephosphorylation of the opsin moiety of rhodopsin is an extremely slow but requisite step in the restoration of the visual pigment to its ground state. We make use of a novel observation: isolated mouse retinae kept in standard media for routine physiologic recordings display blunted dephosphorylation of rhodopsin. Isoelectric focusing followed by Western blot analysis of bleached isolated retinae showed little dephosphorylation of rhodopsin for up to 4 h in darkness, even under conditions when rhodopsin was completely regenerated. Microspectrophotometeric determinations of rhodopsin spectra show that regenerated phospho-rhodopsin has the same molecular photosensitivity as unphosphorylated rhodopsin and that flash responses measured by trans-retinal electroretinogram or single-cell suction electrode recording displayed dark-adapted kinetics. Single quantal responses displayed normal dark-adapted kinetics, but rods were only half as sensitive as those containing exclusively unphosphorylated rhodopsin. We propose a model in which light-exposed retinae contain a mixed population of phosphorylated and unphosphorylated rhodopsin. Moreover, complete dark adaptation can only occur when all rhodopsin has been dephosphorylated, a process that requires >3 h in complete darkness.
  
  
  
  G-protein-coupled receptors (GPCRs) constitute the largest superfamily of proteins that compose ∼4% of the mammalian genome whose members share a common membrane topology. Signaling by GPCRs regulate a wide variety of physiological processes, including taste, smell, hearing, vision, and cardiovascular, endocrine, and reproductive homeostasis. An important feature of GPCR signaling is its timely termination. This normally occurs when, after their activation, GPCRs are rapidly phosphorylated by specific receptor kinases and subsequently bound by cognate arrestins. Recovery of receptor sensitivity to the ground state then requires dephosphorylation of the receptor and unbinding of arrestin, processes that are poorly understood. Here we investigate in mouse rod photoreceptors the relationship between rhodopsin dephosphorylation and recovery of visual sensitivity.
authors.affiliation: Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts 02118.
authors.forename: Justin
authors.identifier: 0000-0002-7137-2886
authors.initials: J
authors.lastname: Berry
authors.affiliation: Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts 02118.
authors.forename: Rikard
authors.identifier: 0000-0001-7999-8480
authors.initials: R
authors.lastname: Frederiksen
authors.affiliation: Zilkha Neurogenetic Institute, Department of Cell and Neurobiology, Keck School of Medicine, University of Southern California, Los Angeles, California 90033, and.
authors.forename: Yun
authors.initials: Y
authors.lastname: Yao
authors.affiliation: Department of Electronics and Communications Engineering, BioMediTech, Tampere University of Technology, 33520 Tampere, Finland.
authors.forename: Soile
authors.initials: S
authors.lastname: Nymark
authors.affiliation: Zilkha Neurogenetic Institute, Department of Cell and Neurobiology, Keck School of Medicine, University of Southern California, Los Angeles, California 90033, and cornwall@bu.edu jeannie@usc.edu.
authors.forename: Jeannie
authors.initials: J
authors.lastname: Chen
authors.affiliation: Department of Physiology and Biophysics, Boston University School of Medicine, Boston, Massachusetts 02118, cornwall@bu.edu jeannie@usc.edu.
authors.forename: Carter
authors.initials: C
authors.lastname: Cornwall
mesh_terms: D000818:Animals; D001703:Biophysics; D003623:Dark Adaptation / Q000187:drug effects / Q000235:genetics*; D004596:Electroretinography; D051606:G-Protein-Coupled Receptor Kinase 1 / Q000235:genetics / Q000378:metabolism; D020962:Heterotrimeric GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism; D066298:In Vitro Techniques; D007525:Isoelectric Focusing; D008027:Light; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D015591:Microspectrophotometry; D009154:Mutation / Q000235:genetics; D055355:Opsins / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects / Q000235:genetics; D012160:Retina / Q000166:cytology / Q000187:drug effects; D017948:Retinal Rod Photoreceptor Cells / Q000502:physiology*; D012172:Retinaldehyde / Q000494:pharmacology; D012243:Rhodopsin / Q000378:metabolism*
chemical_list: C515337:Gnat2 protein, mouse; D055355:Opsins; D012243:Rhodopsin; D051606:G-Protein-Coupled Receptor Kinase 1; D020962:Heterotrimeric GTP-Binding Proteins; D012172:Retinaldehyde
keywords: GPCR; dark adaptation; photoreceptor; phototransduction; retina; rhodopsin