Paired octamer rings of retinoschisin suggest a junctional model for cell–cell adhesion in the retina

Abstract

Retinoschisin (RS1) is involved in cell-cell junctions in the retina, but is unique among known cell-adhesion proteins in that it is a soluble secreted protein. Loss-of-function mutations in RS1 lead to early vision impairment in young males, called X-linked retinoschisis. The disease is characterized by separation of inner retinal layers and disruption of synaptic signaling. Using cryo-electron microscopy, we report the structure at 4.1 Å, revealing double octamer rings not observed before. Each subunit is composed of a discoidin domain and a small N-terminal (RS1) domain. The RS1 domains occupy the centers of the rings, but are not required for ring formation and are less clearly defined, suggesting mobility. We determined the structure of the discoidin rings, consistent with known intramolecular and intermolecular disulfides. The interfaces internal to and between rings feature residues implicated in X-linked retinoschisis, indicating the importance of correct assembly. Based on this structure, we propose that RS1 couples neighboring membranes together through octamer-octamer contacts, perhaps modulated by interactions with other membrane components.retinoschisin | X-linked retinoschisis | discoidin domain | cryo-electron microscopy | single particle analysis T he structural integrity and functioning of tissues are highly dependent on cell-cell junctions. These are typically composed of two similar opposing layers of interacting proteins embedded as oligomers or arrays in the cellular membranes. For example, paired arrays of cadherins are found in adherens junctions and desmosomes (1); of claudins and occludins in tight junctions (2, 3); and of connexin channels in gap junctions (4). The back-to-back architecture of these junctions is fundamental to their adhesive function.The retina features another protein implicated in cell-cell adhesion, retinoschisin (RS1). RS1 is a 24-kDa protein expressed exclusively by photoreceptors and bipolar cells in the retina (5) and pineal gland (6). Loss-of-function mutants of RS1 lead to progressive visual impairment in a disease called X-linked retinoschisis (XLRS) (7-9). The clinical manifestation is a pathological separation of retinal layers, leading to structural splitting and cavity (or "schisis") formation (10). Additionally, the bipolar cells show disorganization (11) and failure to maintain synaptic organization (9). Both XLRS patients and RS1 gene knockout mouse models (Rs1-KO) show similar phenotypes: schisis/splitting through retinal layers and synaptic transmission defects manifested as reduced b-wave amplitudes on the clinical electroretinogram ( Fig. S1) (8, 12). RS1 gene therapy rescues the XLRS mouse phenotype (9, 13) and human RS1 gene therapy trials are underway (ClinicalTrials.gov: NCT02317887; NCT02416622).RS1 is secreted as a soluble disulfide bond-stabilized octamer (14, 15). Most of the monomer comprises a discoidin (DS) domain, a globular fold that is highly conserved over many proteins with diverse functions (16). Little progress has been made toward...

Retinoschisin is linked to retinal Na/K-ATPase signaling and localization

“…It encodes a protein of 224 amino acids (aa) termed retinoschisin, which is exported as oligomers ( Wu et al. , 2005 ; Tolun et al. , 2016 ).…”

Retinoschisin Facilitates the Function of L-Type Voltage-Gated Calcium Channels

“…Retinoschisin (RS1) is an extracellular adhesion protein secreted mainly from photoreceptors and bipolar cells (Reid et al, 1999 , 2003 ; Reid and Farber, 2005 ), and it tightly binds to the surface of these cells to maintain retinal cellular organization (Sauer et al, 1997 ; Wu et al, 2005 ). RS1 contains discoidin domains that allow itself to form homo-octameric complexes (Wu et al, 2005 ; Wang et al, 2006 ; Dyka et al, 2008 ; Bush et al, 2016 ; Tolun et al, 2016 ). In addition, RS1 is able to interact with various molecules on the plasma membrane including phosphatidylserine (Kotova et al, 2010 ), the sodium/potassium-ATPase and sterile alpha and TIR motif-containing protein (Na/K-ATPase-SARM1) complex (Molday et al, 2007 ), and avian Cav1.3 (Shi et al, 2009 ).…”

The X‐linked juvenile retinoschisis protein retinoschisin is a novel regulator of mitogen‐activated protein kinase signalling and apoptosis in the retina

“…It is specifically expressed in the retina by photoreceptor and bipolar cells, as well as in pinealocytes of the pineal gland . During protein synthesis, a 23‐aa signal sequence is cleaved to produce a 201‐aa mature polypeptide which is secreted from photoreceptors and bipolar cells as a homooctamer held together by intermolecular disulphide bonds between aa 223 and aa 59 . So far, over 190 unique XLRS‐associated sequence variants in RS1 have been reported (Leiden Open Variation Database, http://grenada.lumc.nl/LOVD2/eye/home.php?select_db=RS1, accessed May 2016).…”