Defective Discoidin Domain Structure, Subunit Assembly, and Endoplasmic Reticulum Processing of Retinoschisin are Primary Mechanisms Responsible for X-linked Retinoschisis

Abstract

Retinoschisin is a 24-kDa discoidin domain-containing protein that is secreted from photoreceptor and bipolar cells as a large disulfide-linked multisubunit complex. It functions as a cell adhesion protein to maintain the cellular organization and synaptic structure of the retina. Over 125 different mutations in the RS1 gene are associated with X-linked juvenile retinoschisis, the most common form of early onset macular degeneration in males. To identify molecular determinants important for retinoschisin structure and function and elucidate molecular and cellular mechanisms responsible for X-linked juvenile retinoschisis, we have analyzed the expression, protein folding, disulfide-linked subunit assembly, intracellular localization, and secretion of wild-type retinoschisin, 15 Cys-to-Ser variants and 12 disease-linked mutants. Our studies, together with molecular modeling of the discoidin domain, identify Cys residues involved in intramolecular and intermolecular disulfide bonds essential for protein folding and subunit assembly. We show that misfolding of the discoidin domain, defective disulfide-linked subunit assembly, and inability of retinoschisin to insert into the endoplasmic reticulum membrane as part of the protein secretion process are three primary mechanisms responsible for the loss in the function of retinoschisin as a cell adhesion protein and the pathogenesis of X-linked juvenile retinoschisis.X-linked juvenile retinoschisis (XLRS) 1 is the most common form of early onset macular degeneration in males (1, 2). It is characterized by a mild to severe decrease in visual acuity, radial streaks extending from the central retina due to a splitting of the inner retina, progressive macular atrophy, and reduction in the electroretinogram b-wave (2-5). Lesions in the peripheral retina associated with impairment in peripheral vision are observed in half the cases. During the course of the disease, complications can arise, which include retinal detachment, vitreal hemorrhaging, and neovascular glaucoma leading to a poor outcome.The RS1 gene responsible for XLRS was identified by positional cloning and found to encode a 24-kDa protein called retinoschisin, or RS1 (6), that is secreted from photoreceptor and bipolar cells as a disulfide-linked oligomeric complex (7,8).The polypeptide consists of a leader sequence with a putative signal peptidase cleavage site and a discoidin domain spanning most of the protein (6). Discoidin domains, first identified in the discoidin I protein of Dictyostelium discoidium (9, 10), have now been found in many secreted and transmembrane proteins, including blood coagulation factors, tyrosine kinase receptors, and proteins involved in neural development (11-13). The function of the discoidin domain is not well understood, but in some proteins it has been implicated in cell adhesion and cell signaling through protein-protein, protein-carbohydrate, or protein-lipid interactions. Recently, the three-dimensional crystal structures of the C2 discoidin domain of blood coagulation Factors...

X-linked retinoschisis: an update

“…76 The cysteine residues within the retinoschisis discoidin domain are critical for folding and the formation of functional retinoschisin dimers and ultimately octamers. 11 Disulphide bonds between two pairs of cysteine residues (Cys63-Cys219 and Cys110-Cys142) stablilise the folded monomeric retinoschisin subunits and additional disulphide bonded pairs of cysteines link the subunits into dimers (Cys40-Cys40) and octamers (Cys59-Cys223). 11 Disruption of these bonds interferes with dimer and octamer formation.…”

“…11 Disulphide bonds between two pairs of cysteine residues (Cys63-Cys219 and Cys110-Cys142) stablilise the folded monomeric retinoschisin subunits and additional disulphide bonded pairs of cysteines link the subunits into dimers (Cys40-Cys40) and octamers (Cys59-Cys223). 11 Disruption of these bonds interferes with dimer and octamer formation. 11 Retinoschisin is believd to function in cell adhesion in the development and maintenance of retinal architecture.…”

“…11 Disruption of these bonds interferes with dimer and octamer formation. 11 Retinoschisin is believd to function in cell adhesion in the development and maintenance of retinal architecture. This is in keeping with other proteins containing discoidin domains and is supported by the observations in patients and in the mouse models.…”

“…Investigation of the protein retinoschisin, encoded by RS1, has revealed it to be a secretory protein, containing a discoidin domain and functioning as an octamer. [10][11][12][13] Three retinoschisis mouse models have been developed and they have similar retinal pathology to the human disease. [14][15][16] This article aims to review the clinical, pathological and electrophysiological features of XLRS, our current understanding of its molecular basis and to consider future therapy.…”

X-linked retinoschisis maculopathy treated with topical dorzolamide, and relationship to genotype

“…It has a discoidin domain thought to have a role in cell adhesion. 5,11,12 Mutation in RS1 results in schisis (splitting) of the neural retina within the superficial retinal layers, the inner limiting membrane, the nerve fibre layer, and the ganglion cell layer, leading to reduced visual acuity (VA) in affected males. [13][14][15][16] Three main underlying pathological mechanisms are presumed responsible for retinoschisin protein dysfunction.…”