title: Defective trafficking of cone photoreceptor CNG channels induces the unfolded protein response and ER-stress-associated cell death.
abstract: Mutations that perturb the function of photoreceptor CNG (cyclic nucleotide-gated) channels are associated with several human retinal disorders, but the molecular and cellular mechanisms leading to photoreceptor dysfunction and degeneration remain unclear. Many loss-of-function mutations result in intracellular accumulation of CNG channel subunits. Accumulation of proteins in the ER (endoplasmic reticulum) is known to cause ER stress and trigger the UPR (unfolded protein response), an evolutionarily conserved cellular programme that results in either adaptation via increased protein processing capacity or apoptotic cell death. We hypothesize that defective trafficking of cone photoreceptor CNG channels can induce UPR-mediated cell death. To test this idea, CNGA3 subunits bearing the R563H and Q655X mutations were expressed in photoreceptor-derived 661W cells with CNGB3 subunits. Compared with wild-type, R563H and Q655X subunits displayed altered degradation rates and/or were retained in the ER. ER retention was associated with increased expression of UPR-related markers of ER stress and with decreased cell viability. Chemical and pharmacological chaperones {TUDCA (tauroursodeoxycholate sodium salt), 4-PBA (sodium 4-phenylbutyrate) and the cGMP analogue CPT-cGMP [8-(4-chlorophenylthio)-cGMP]} differentially reduced degradation and/or promoted plasma-membrane localization of defective subunits. Improved subunit maturation was concordant with reduced expression of ER-stress markers and improved viability of cells expressing localization-defective channels. These results indicate that ER stress can arise from expression of localization-defective CNG channels, and may represent a contributing factor for photoreceptor degeneration.
authors: Department of Veterinary and Comparative Anatomy, Pharmacology and Physiology, Washington State University, Pullman, WA 99164, USA.
authors: Deborah L
authors: DL
authors: Duricka
authors: R Lane
authors: RL
authors: Brown
authors: Michael D
authors: MD
authors: Varnum
mesh_terms: D000818:Animals; D016923:Cell Death / Q000187:drug effects; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D006152:Cyclic GMP / Q000031:analogs & derivatives / Q000494:pharmacology; D054815:Cyclic Nucleotide-Gated Cation Channels / Q000235:genetics*; D004721:Endoplasmic Reticulum / Q000502:physiology*; D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins / Q000378:metabolism; D006801:Humans; D051379:Mice; D018832:Molecular Chaperones / Q000494:pharmacology; D021381:Protein Transport / Q000187:drug effects; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology*; D051742:Transcription Factor CHOP / Q000378:metabolism; D056811:Unfolded Protein Response / Q000187:drug effects*
chemical_list: C518238:CNGA3 protein, human; C474799:CPT-cGMP; D054815:Cyclic Nucleotide-Gated Cation Channels; C497250:Ddit3 protein, mouse; D000091342:Endoplasmic Reticulum Chaperone BiP; D006360:Heat-Shock Proteins; D018832:Molecular Chaperones; D051742:Transcription Factor CHOP; D006152:Cyclic GMP