The retromer complex is required for photoreceptor maintenance and Rhodopsin recycling. Shiuan Wang¹, Bo Xiong¹, Shinya Yamamoto², Kai Li Tan¹, Hector Sandoval², Manish Jaiswal², Vafa Bayat³, Ke Zhang⁴, Wu Lin Charng¹, Gabriela David¹, Hugo Bellen^1,2,4,5,6. 1) Program in Developmental Biology; 2) Department of Molecular and Human Genetics; 3) MSTP; 4) SCBMB Program; 5) HHMI; 6) Department of Neuroscience; Neurological Research Institute at Baylor of College of Medicine, Houston, TX.

   Rhodopsin 1 (Rh1) is internalized and degraded upon light exposure, but it remains unknown if Rh1 can be recycled and if Rh1 recycling is required to maintain photoreceptor (PR) function. In a forward genetic screen for mutations that cause neurodegeneration we isolated mutations in Vps26. Vps26 mutant PRs exhibit progressively worsening electroretinograms (ERGs) and morphological defects when kept in the light/dark cycle (LD). However, darkness strongly suppresses both defects, suggesting that PR degeneration of Vps26 mutants are light dependent. Vps26 encodes a protein which together with Vps35 is part of the retromer complex. To address if retromer function is required to maintain PRs, we performed ERGs and morphological assays in Vps35 mutants. Similar to Vps26, Vps35 mutant PRs show degenerative phenotypes in LD, implicating the retromer in PR maintenance. The retromer recycles membrane proteins from endosomes and prevent their degradation in lysosomes. Indeed the number and size of lysosomes in mutant PRs is vastly expanded. Moreover, Western Blots revealed a decreased level of Rh1 in Vps26 mutants, suggesting that more Rh1 is delivered and degraded in lysosomes and that Rh1 recycling is impaired in Vps26 mutants. Increased Rh transport into lysosomes may increase the burden on the endolysosomal pathway. To reduce Rh1 endocytosis, we performed ERG in Vps26 shibire double mutants and observed a strong suppression of the degenerative phenotype. Therefore, we propose that the retromer complex recycles Rh1 and that impaired Rh1 recycling increases Rh1 transport to lysosomes. This in turn causes the demise of Vps26 mutant PRs. Reducing internalized Rh1 alleviates the stress and suppresses PR degeneration.