title: Secretory defect and cytotoxicity: the potential disease mechanisms for the retinitis pigmentosa (RP)-associated interphotoreceptor retinoid-binding protein (IRBP).
abstract: Interphotoreceptor retinoid-binding protein (IRBP) secreted by photoreceptors plays a pivotal role in photoreceptor survival and function. Recently, a D1080N mutation in IRBP was found in patients with retinitis pigmentosa, a frequent cause of retinal degeneration. The molecular and cellular bases for pathogenicity of the mutation are unknown. Here, we show that the mutation abolishes secretion of IRBP and results in formation of insoluble high molecular weight complexes via disulfide bonds. Co-expression of protein disulfide isomerase A2 that regulates disulfide bond formation or introduction of double Cys-to-Ala substitutions at positions 304 and 1175 in D1080N IRBP promoted secretion of the mutated IRBP. D1080N IRBP was not transported to the Golgi apparatus, but accumulated in the endoplasmic reticulum (ER), bound with the ER-resident chaperone proteins such as BiP, protein disulfide isomerase, and heat shock proteins. Splicing of X-box-binding protein-1 mRNA, expression of activating transcription factor 4 (ATF4), and cleavage of ATF6 were significantly increased in cells expressing D1080N IRBP. Moreover, D1080N IRBP induced up-regulation and nuclear translocation of the C/EBP homologous protein, a proapoptotic transcription factor associated with the unfolded protein response. These results indicate that loss of normal function (nonsecretion) and gain of cytotoxic function (ER stress) are involved in the disease mechanisms of D1080N IRBP. Chemical chaperones and low temperature, which help proper folding of many mutated proteins, significantly rescued secretion of D1080N IRBP, suggesting that misfolding is the molecular basis for pathogenicity of D1080N substitution and that chemical chaperones are therapeutic candidates for the mutation-caused blinding disease.
authors.affiliation: Department of Ophthalmology, Louisiana State University Health Sciences Center, New Orleans, Louisiana 70112, USA.
authors.forename: Songhua
authors.initials: S
authors.lastname: Li
authors.forename: Zhihui
authors.initials: Z
authors.lastname: Yang
authors.forename: Jane
authors.initials: J
authors.lastname: Hu
authors.forename: William C
authors.initials: WC
authors.lastname: Gordon
authors.forename: Nicolas G
authors.initials: NG
authors.lastname: Bazan
authors.forename: Arthur L
authors.initials: AL
authors.lastname: Haas
authors.forename: Dean
authors.initials: D
authors.lastname: Bok
authors.forename: Minghao
authors.initials: M
authors.lastname: Jin
mesh_terms: D051701:Activating Transcription Factor 4 / Q000235:genetics / Q000378:metabolism; D021581:Active Transport, Cell Nucleus / Q000235:genetics; D019943:Amino Acid Substitution; D000818:Animals; D022762:CCAAT-Enhancer-Binding Proteins / Q000235:genetics / Q000378:metabolism; D002460:Cell Line; D002467:Cell Nucleus / Q000235:genetics / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000235:genetics / Q000378:metabolism; D000091342:Endoplasmic Reticulum Chaperone BiP; D005136:Eye Proteins / Q000235:genetics / Q000378:metabolism*; D006056:Golgi Apparatus / Q000235:genetics / Q000378:metabolism; D006360:Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D051379:Mice; D020125:Mutation, Missense; D019704:Protein Disulfide-Isomerases / Q000096:biosynthesis / Q000235:genetics; D017510:Protein Folding; D012174:Retinitis Pigmentosa / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D012177:Retinol-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D056811:Unfolded Protein Response; D000099214:Retinol-Binding Proteins, Interstitial
chemical_list: D051701:Activating Transcription Factor 4; D022762:CCAAT-Enhancer-Binding Proteins; D000091342:Endoplasmic Reticulum Chaperone BiP; D005136:Eye Proteins; D006360:Heat-Shock Proteins; D019704:Protein Disulfide-Isomerases; D012177:Retinol-Binding Proteins; D000099214:Retinol-Binding Proteins, Interstitial; C497109:ATF4 protein, human; C081452:Atf4 protein, mouse