What drives cell morphogenesis: a look inside the vertebrate photoreceptor

Abstract

Vision mediating photoreceptor cells are specialized light-sensitive neurons in the outer layer of the vertebrate retina. The human retina contains approximately 130 million of such photoreceptors, which enable images of the external environment to be captured at high resolution and high sensitivity. Rod and cone photoreceptor subtypes are further specialized for sensing light in low and high illumination, respectively. To enable visual function, these photoreceptors have developed elaborate morphological domains for the detection of light (outer segments), for changing cell shape (inner segments), and for communication with neighboring retinal neurons (synaptic terminals). Furthermore, rod and cone subtypes feature unique morphological variations of these specialized characteristics. Here, we review the major aspects of vertebrate photoreceptor morphology and key genetic mechanisms that drive their formation. These mechanisms are necessary for cell differentiation as well as function. Their defects lead to cell death.

Fig. 1

Overview of photoreceptor morphology. Left-hand panel shows a photograph of an isolated rod photoreceptor from the rabbit retina. A schematized view of the same cell is shown to the right. Position of the OLM relative to the nucleus varies for different photoreceptor types. Image to the left reprinted with permission from (Townes-Anderson et al., 1988). Outer segment (OS), connecting cilium (CC), outer limiting membrane (OLM).

Fig. 2. Photoreceptor Outer Segments

A: Generalized schematic of photoreceptor morphology; outer segment membranes are coloured in blue. B: Scanning EM of frog photoreceptors showing intact rods (R) and cones (C), and rod inner segments (RIS) left behind from broken photoreceptors. Calycal processes (CP), intact double cone (DC), green rod inner segment (GRIS). Reprinted with permission (Peters et al., 1983). C: Transmission EM of a developing Xenopus cone outer segment highlighting distal invaginations occurring between adjacent membranes (white arrows). Reprinted with permission (Eckmiller, 1987). D. Schematic of a rod outer segment, showing stacks of disk membranes indented by multiple incisures, and a microtubule scaffold highlighted in red. The connecting cilium axoneme contains an array of microtubule doublets that extend from the inner segment into the outer segment, wherein a separate set of longitudinal microtubules run in the indentations of incisures. These microtubules are located in the cytoplasm between the curved rim of the OS disks and the external plasma membrane. Reprinted with permission from (Eckmiller, 2004). E. Schematic of a cone outer segment showing layers of membranes that are continuous with the rest of the plasma membrane on the ciliary face. Microtubules are highlighted in red. Reprinted with permission from (Eckmiller, 2004). F: Electron micrograph of a section through the outer segment from a rabbit rod photoreceptor shows membrane folds and closely apposed disks. Reprinted with permission from (Townes-Anderson et al., 1988). G: A model of protein-protein interactions thought to maintain the architecture of outer segment membranes. This model also shows membrane discs being added via evagination at the base of the outer segment–an alternative to the scenario shown in panel (I). Peripherin/rds (P/rds) tetramers localize to disc rims. cGMP-gated cation channel (cGGCC), which includes a GARP domain, localizes to the plasma membrane and is thought to interact with peripherin. A cytoplasmic GARP protein has been proposed to bridge adjacent discs by interacting with peripherin/rds. Reprinted with permission from (Goldberg, 2006). H: A detail of disc rim formation according to the open disc “evagination” model. Rim expansion encloses space between adjacent membrane evaginations. Reprinted with permission from Steinberg et al. (Steinberg et al., 1980). I: A “vesicle fusion” model of rod disk formation. Rod opsin carrying vesicles pinch off at the base of the outer segment, and fuse with rod discs. Reprinted with permission from (Chuang et al., 2007).

Fig. 3

Transport into the outer segment. Proteins are synthesized in the cytoplasmic reticulum and from there transported via the Golgi apparatus to the periciliary area at the base of the connecting cilium, and subsequently along ciliary microtubules into the outer segment. A: A schematic representation of the photoreceptor cell. Ciliary microtubules are highlighted in blue. The Golgi complex and post-Golgi vesicles are in red. B: Intraflagellar transport (IFT) is thought to translocate proteins into the outer segment along ciliary microtubules. It is mediated via so-called IFT particles, protein complexes that consist of several polypeptides. It is not clear whether IFT is involved in the transport of opsins. In addition to IFT particle components, several other IFT-related proteins, such as BBS gene products, are necessary for photoreceptor survival. Nephrocystins (NPHP) are also required for photoreceptor viability. Their relationship to IFT is not clear. C: Rod opsin is transported from the Golgi apparatus to the base of the photoreceptor connecting cilium in vesicles. In one proposed scenario, this transport is driven by dynein, a microtubule-dependant motor. The budding of RTCs from the trans Golgi network and subsequently their fusion at the base of the connecting cilium appear to require a number of proteins, including a small GTPases Arf4, Rab8, and Rab11, a GTPase effector FIP3, and a GAP factor, ASAP1. Several other proteins are also proposed to participate in these processes (listed in the figure). The representation of RTC budding provided courtesy of D. Deretic. D: Immunolocalization of IFT88 (green) to the base of the connecting cilium in photoreceptors of larval zebrafish (Malicki lab). E: Longitudinal section of a frog rod cut along the axis of the connecting cilium reveals structural details in the periciliary ridge complex. Apical plasmalemma (APM) of the inner segment. Basal bodies (BB), Centriole (C), Connecting cilium (CC), Disks (D) in the outer segment, Lip (L), Mitochondria (M), Ridge (R), Carrier vesicles (V), Reprinted with permission from (Peters et al., 1983). F: Ultrastructure of a salamander rod photoreceptor. The outer segment contains stacks of membranous disks. The inner segment consists of a mitochondria-rich ellipsoid and a myoid region containing a Golgi apparatus. The inset in the bottom right corner (magnified at location of asterisk in main panel) shows photoreceptor fins, that interdigitate with microvilli of Muller cells above the external limiting membrane. Reprinted with permission (Townes-Anderson et al., 1985). Inset in top left corner shows scanning electron micrograph of the periciliary ridge complex. Reprinted with permission from (Peters et al., 1983). Some aspect of the models presented in this figure should be considered hypothetical.

Fig. 4. Photoreceptor cell body

A: A schematic representation of the photoreceptor cell. The nucleus is highlighted in blue. A belt of cell junctions, erroneously called the outer limiting membrane (OLM), subdivides the photoreceptor cell surface into the apical and baso-lateral domains (highlighted in red). B: Confocal image of the outer limiting membrane in the retina of larval zebrafish. Cell junctions are visualized by phalloidin staining (in green, indicated with an arrowhead). The Crumbs polypeptide is detected via antibody staining (in red). It localizes apical to cell junctions. (Malicki lab) C: Electron micrograph of a section through the photoreceptor cell layer in larval zebrafish. Arrowheads indicate cell junctions, the nucleus is indicated with an asterisk. (Malicki lab) D: A schematic representation of the protein complex that regulates the formation of cell junctions in the outer limiting membrane. The formation of the apical cell membrane domain of the photoreceptor cell, and the integrity of the junctional complexes in the OLM require the function of Crumbs, a transmembrne protein that features a large extracellular domain and a short cytoplasmic tail. Crumbs cytoplasmic moiety binds a MAGUK protein Stardust/Nagie oko and a FERM-domain protein Mosaic eyes (Moe), which also bind each other. Par6, Par3, and aPKC are also thought to contribute to this protein complex. Crb, Crumbs; Moe, Mosaic eyes; Nok, Nagie oko; Std, Stardust. E: The nucleus is by far the most voluminous organelle in the cytoplasm of the photoreceptor cell. Its position is affected by the activity of a microtubule dependant motor, dynein, and nuclear envelope components that feature a C-terminal KASH domain (Syne family proteins in vertebrates). The KASH domain contains a lipophilic segment thought to span the outer membrane of the nuclear envelope. The cytoplasmic portion of many KASH-domain proteins is exceptionally long (close to 10,000 amino acids in some cases). It is not clear how they interact with the dynein complex.

Fig. 5. The Synaptic Terminal

A: Schematic of photoreceptor morphology; synaptic terminal is labeled in blue. B: Schematic of a rod photoreceptor synapse. Views of the rod terminal perpendicular (left) and parallel (right) to the face of ribbon. Presynaptically, the ribbon tethers several hundred vesicles (white circles in red ribbon) and the active zone docks ca. 100 vesicles (yellow circles) and tethers ca. 770 vesicles. Postsynaptically, processes of 2 horizontal (h₁, h₂) and 2 bipolar (b₁, b₂) cells occupy the invagination. Reprinted with permission from (Rao-Mirotznik et al., 1995). C. EM images of the “tetrad” ribbon synapse of a mammalian rod photoreceptor. Ribbon is indicated by an arrowhead and the inset is a confocal image of RIBEYE antibody staining. Reprinted with permission from (tom Dieck et al., 2005). D. EM images of the “triad” ribbon synapse of a mammalian cone photoreceptor. Ribbons are indicated by arrowheads and the inset is a confocal image of RIBEYE antibody staining. Reprinted with permission from (tom Dieck et al., 2005). E. Confocal image of a tangential section through macaque photoreceptor terminals. Dotted lines outline rod and cone presynaptic terminals immunostained for Bassoon (green), and the 1F subunit of an L-type Ca2C channel (red). The rod terminal typically comprises of a single, crescent-shaped ribbon, but can appear as two separate but linked ribbons (arrowheads). The cone terminal contains an array of smaller ribbons that serve separate invaginations. Reprinted with permission (Wassle, 2003).