title: Rod disc renewal occurs by evagination of the ciliary plasma membrane that makes cadherin-based contacts with the inner segment.
abstract: The outer segments of vertebrate rod photoreceptors are renewed every 10 d. Outer segment components are transported from the site of synthesis in the inner segment through the connecting cilium, followed by assembly of the highly ordered discs. Two models of assembly of discrete discs involving either successive fusion events between intracellular rhodopsin-bearing vesicles or the evagination of the plasma membrane followed by fusion of adjacent evaginations have been proposed. Here we use immuno-electron microscopy and electron tomography to show that rhodopsin is transported from the inner to the outer segment via the ciliary plasma membrane, subsequently forming successive evaginations that "zipper" up proximally, but at their leading edges are free to make junctions containing the protocadherin, PCDH21, with the inner segment plasma membrane. Given the physical dimensions of the evaginations, coupled with likely instability of the membrane cortex at the distal end of the connecting cilium, we propose that the evagination occurs via a process akin to blebbing and is not driven by actin polymerization. Disassembly of these junctions is accompanied by fusion of the leading edges of successive evaginations to form discrete discs. This fusion is topologically different to that mediated by the membrane fusion proteins, SNAREs, as initial fusion is between exoplasmic leaflets, and is accompanied by gain of the tetraspanin rim protein, peripherin.
authors.affiliation: Institute of Ophthalmology, University College London, London, EC1V9EL, United Kingdom;
authors.forename: Thomas
authors.initials: T
authors.lastname: Burgoyne
authors.affiliation: Institute of Ophthalmology, University College London, London, EC1V9EL, United Kingdom;
authors.forename: Ingrid P
authors.initials: IP
authors.lastname: Meschede
authors.affiliation: Laboratory for Molecular and Cell Biology, University College London, London, WC1E6BT, United Kingdom;
authors.forename: Jemima J
authors.identifier: http://orcid.org/0000-0001-5894-7043
authors.initials: JJ
authors.lastname: Burden
authors.affiliation: Institute of Ophthalmology, University College London, London, EC1V9EL, United Kingdom;
authors.forename: Maryse
authors.initials: M
authors.lastname: Bailly
authors.affiliation: National Heart and Lung Institute, Imperial College, London, SW72AZ, United Kingdom; Centro de Estudos de Doenças Crónicas, NOVA Medical School/Faculdade de Ciências Médicas, Universidade Nova de Lisboa, 1169-056, Lisboa, Portugal.
authors.forename: Miguel C
authors.initials: MC
authors.lastname: Seabra
authors.affiliation: Institute of Ophthalmology, University College London, London, EC1V9EL, United Kingdom; c.futter@ucl.ac.uk.
authors.forename: Clare E
authors.initials: CE
authors.lastname: Futter
mesh_terms: D000818:Animals; D000070585:Cadherin Related Proteins; D015820:Cadherins / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism* / Q000648:ultrastructure; D020285:Cryoelectron Microscopy; D055032:Electron Microscope Tomography; D005123:Eye / Q000378:metabolism / Q000648:ultrastructure; D005136:Eye Proteins / Q000378:metabolism; D008810:Mice, Inbred C57BL; D016253:Microscopy, Immunoelectron; D051938:Munc18 Proteins / Q000378:metabolism; D009419:Nerve Tissue Proteins / Q000378:metabolism; D010786:Photoreceptor Cells / Q000378:metabolism* / Q000648:ultrastructure; D050765:Qa-SNARE Proteins / Q000378:metabolism; D055211:Retinal Photoreceptor Cell Inner Segment / Q000378:metabolism* / Q000648:ultrastructure; D017948:Retinal Rod Photoreceptor Cells / Q000378:metabolism* / Q000648:ultrastructure; D012243:Rhodopsin / Q000378:metabolism; D012374:Rod Cell Outer Segment / Q000378:metabolism / Q000648:ultrastructure
chemical_list: C442481:CDHR1 protein, human; D000070585:Cadherin Related Proteins; D015820:Cadherins; D005136:Eye Proteins; D051938:Munc18 Proteins; D009419:Nerve Tissue Proteins; D050765:Qa-SNARE Proteins; D012243:Rhodopsin
keywords: disc renewal; protocadherin; rhodopsin; rod photoreceptors