Abstract
Purpose :
X-linked retinoschisis (XLRS) is caused by mutations in the gene encoding the secreted protein retinoschisin (RS1). Of these, nonsense or frame shift mutations are predicted to result in absence of a full-length protein. However, many disease-causing mutations are missense ones allowing for production of full length misfolded protein that is retained intracellularly. RS1 gene delivery via viral vectors to RS1 KO mice has resulted in significant preservation of retinal structure and function. However, there is no information available concerning subunit assembly by individual cells expressing a misfolded mutant and wild-type proteins. We used in vitro RS1 expression and secretion assays to investigate the co-assembly of mutant and wild type RS1 to better understand the potential of gene supplementation therapy.
Methods :
Study 1. HEK293 cells were transiently transfected with plasmids containing differentially tagged WT mouse RS1 (6xHis) and mutant RS1 (C59S and R141C RS1) (myc) singly and in combination. Protein expression, secretion and subunit assembly of his-RS1 and myc-RS1 were analyzed by western blot (WB) and ELISA. Study 2. Co-expression and interactions were also studied by transfecting WT RS1 plasmid into CHO cells stably expressing mutant RS1. The co-assembly of WT and mutant RS1 was further investigated by immuno-precipitating mutant RS1 using anti–myc antibody-Protein G beads, and the bound fraction was analyzed by WB.
Results :
WT RS1 from both transient and stable transfections showed a single octamer band on non-reducing WB, whereas C59S myc-RS1 only exhibited dimer, and no secreted product was observed from R141C myc-RS1 (intracellular protein was present). Co-expression of WT RS1 with both mutants RS1 after either transient or stable transfections demonstrated RS1 octamer detected by all antibodies, anti-RS1, anti-His, and anti-myc, indicating the presence of hetero-octamer. Co-assembly and secretion were confirmed by co-immunoprecipitation. ELISA showed co-expression of WT RS1 with either mutant types resulted in decreased amounts of RS1.
Conclusions :
Mutant RS1 interferes with the expression/secretion of WT RS1 when co-expressed, and co-assembly of mutant and WT RS1 subunits into hetero-octamers may have deleterious effects on RS1 function. The molecular physiology of WT RS1 and the consequences of hetero-octamer formation with disease causing mutants remain to be elucidated.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.