Retinoschisin (RS1), the Protein Encoded by the X-linked Retinoschisis Gene, Is Anchored to the Surface of Retinal Photoreceptor and Bipolar Cells through Its Interactions with a Na/K ATPase-SARM1 Complex

Abstract

Retinoschisin or RS1 is a discoidin domain-containing protein encoded by the gene responsible for X-linked retinoschisis (XLRS), an early onset macular degeneration characterized by a splitting of the retina. Retinoschisin, expressed and secreted from photoreceptors and bipolar cells as a homo-octameric complex, associates with the surface of these cells where it serves to maintain the cellular organization of the retina and the photoreceptor-bipolar synaptic structure. To gain insight into the role of retinoschisin in retinal cell adhesion and the pathogenesis of XLRS, we have investigated membrane components in retinal extracts that interact with retinoschisin. Unlike the discoidin domain-containing blood coagulation proteins Factor V and Factor VIII, retinoschisin did not bind to phospholipids or retinal lipids reconstituted into unilamellar vesicles or immobilized on microtiter plates. Instead, co-immunoprecipitation studies together with mass spectrometric-based proteomics and Western blotting showed that retinoschisin is associated with a complex consisting of Na/K ATPase (␣3, ␤2 isoforms) and the sterile alpha and TIR motif-containing protein SARM1. Double labeling studies for immunofluorescence microscopy confirmed the co-localization of retinoschisin with Na/K ATPase and SARM1 in photoreceptors and bipolar cells of retina tissue. We conclude that retinoschisin binds to Na/K ATPase on photoreceptor and bipolar cells. This interaction may be part of a novel SARM1-mediated cell signaling pathway required for the maintenance of retinal cell organization and photoreceptor-bipolar synaptic structure.Retinoschisin, also known as RS1, 3 is a retinal-specific protein encoded by the gene associated with X-linked retinoschisis (XLRS), a leading cause of early onset macular degeneration in males (1). It is primarily expressed in photoreceptors and to a lesser extent bipolar cells of the adult retina (2-4). Retinoschisin consists of a 23-amino acid N-terminal signal peptide and a 157-amino acid discoidin domain flanked by a unique 39-amino acid Rs1 domain and a 5-amino acid C-terminal segment (5, 6). The signal peptide directs the nascent polypeptide across the ER membrane before its removal by a signal peptidase in the lumen of the ER. The processed polypeptide folds into its native conformation and further assembles into a disulfide-linked homo-octameric complex prior to secretion from cells (7). The secreted, disulfide-linked octamer associates with the external surface of rod and cone photoreceptor cells of the outer retina and bipolar cells of the inner retina (3).The function of retinoschisin is not known at the present time. However, the characteristic features of XLRS patients and retinoschisin knock-out mice suggest that this extracellular protein plays a crucial role in maintaining the cellular organization and synaptic structure of the retina. XLRS is characterized by a splitting of the retinal cell layers, a loss in central vision, and a decrease in the b-wave amplitude of the ERG (8 -10). To date over 1...

Impaired Association of Retinal Degeneration-3 with Guanylate Cyclase-1 and Guanylate Cyclase-activating Protein-1 Leads to Leber Congenital Amaurosis-1

“…, and rd3 Ϫ/Ϫ mice, as well as COS-7 cells transfected with these genes, were labeled as described previously (42,43). Details of the antibody concentrations are shown in Table 1.…”

RD3, the protein associated with Leber congenital amaurosis type 12, is required for guanylate cyclase trafficking in photoreceptor cells

“…Retinal cryosections from 21-d-old homozygous rd3 and WT mice and COS-7-transfected cells were labeled as described previously (23,30). Antibodies used for these studies included rabbit anti-calnexin antibody (Sigma-Aldrich) diluted 1:200, mouse anti-rab11 monoclonal antibody (BD) diluted 1:25, GC-2H6 hybridoma fluid diluted 1:5 (29), and rabbit anti-Rd3 antibody diluted 1:250 (this study).…”

“…The unbound fraction was retained, and the bound protein was eluted from the matrix with 4% (wt/ vol) SDS in PBS. The fractions were analyzed by SDS/PAGE and Western blot analysis as described previously (30). The immunoprecipitated protein fraction was prepared for LC-MS/MS as described previously (30).…”

“…A membrane fraction from 40-80 retinas of 21-d-old WT and rd3 mice were prepared as described previously (30). In brief, for immunoprecipitation studies, a mouse retinal membrane extract (∼3.5 mg of protein) was incubated in hypotonic buffer [20 mM Hepes (pH 7.4) containing complete protease inhibitor) for 20 min on ice and then solubilized with 700 μL of 1% Triton X-100 in 20 mM Hepes (pH 7.4) and 0.3 M NaCl.…”

SARM inhibits both TRIF‐ and MyD88‐mediated AP‐1 activation

“…Yet another study showed a viral infection-mediated immune activation of SARM in the mouse brain [26]. Besides immune function, SARM has also been implicated in the neuronal system [27,28]. Overall, the conundrum of the function of SARM remains unsolved.…”