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Proc Natl Acad Sci U S A. 2015 Dec 1;112(48):14870-5. doi: 10.1073/pnas.1516309112. Epub 2015 Nov 17.

Three-dimensional organization of nascent rod outer segment disk membranes.

Author information

1
Department of Ophthalmology and Stein Eye Institute, University of California, Los Angeles, CA 90095;
2
Department of Ophthalmology and Stein Eye Institute, University of California, Los Angeles, CA 90095; Department of Microbiology, Immunology, and Molecular Genetics, University of California, Los Angeles, CA 90095; Electron Imaging Center for Nanoscience, California NanoSystems Institute, University of California, Los Angeles, CA 90095;
3
Neuroscience Research Institute, University of California, Santa Barbara, CA 93106;
4
Department of Microbiology, Immunology, and Molecular Genetics, University of California, Los Angeles, CA 90095; Electron Imaging Center for Nanoscience, California NanoSystems Institute, University of California, Los Angeles, CA 90095; Molecular Biology Institute, University of California, Los Angeles, CA 90095;
5
Neuroscience Research Institute, University of California, Santa Barbara, CA 93106; Department of Molecular, Cellular, and Developmental Biology, University of California, Santa Barbara, CA 93106;
6
Department of Ophthalmology and Stein Eye Institute, University of California, Los Angeles, CA 90095; Molecular Biology Institute, University of California, Los Angeles, CA 90095; Department of Neurobiology, David Geffen School of Medicine at UCLA, University of California, Los Angeles, CA 90095; Brain Research Institute, University of California, Los Angeles, CA 90095 dswilliams@ucla.edu.

Abstract

The vertebrate photoreceptor cell contains an elaborate cilium that includes a stack of phototransductive membrane disks. The disk membranes are continually renewed, but how new disks are formed remains poorly understood. Here we used electron microscope tomography to obtain 3D visualization of the nascent disks of rod photoreceptors in three mammalian species, to gain insight into the process of disk morphogenesis. We observed that nascent disks are invariably continuous with the ciliary plasma membrane, although, owing to partial enclosure, they can appear to be internal in 2D profiles. Tomographic analyses of the basal-most region of the outer segment show changes in shape of the ciliary plasma membrane indicating an invagination, which is likely a first step in disk formation. The invagination flattens to create the proximal surface of an evaginating lamella, as well as membrane protrusions that extend between adjacent lamellae, thereby initiating a disk rim. Immediately distal to this initiation site, lamellae of increasing diameter are evident, indicating growth outward from the cilium. In agreement with a previous model, our data indicate that mature disks are formed once lamellae reach full diameter, and the growth of a rim encloses the space between adjacent surfaces of two lamellae. This study provides 3D data of nascent and mature rod photoreceptor disk membranes at unprecedented z-axis depth and resolution, and provides a basis for addressing fundamental questions, ranging from protein sorting in the photoreceptor cilium to photoreceptor electrophysiology.

KEYWORDS:

EM tomography; cilium; disk morphogenesis; photoreceptor

PMID:
26578801
PMCID:
PMC4672767
DOI:
10.1073/pnas.1516309112
[Indexed for MEDLINE]
Free PMC Article

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