title: Accumulation of non-outer segment proteins in the outer segment underlies photoreceptor degeneration in Bardet-Biedl syndrome.
abstract: Compartmentalization and polarized protein trafficking are essential for many cellular functions. The photoreceptor outer segment (OS) is a sensory compartment specialized for phototransduction, and it shares many features with primary cilia. As expected, mutations disrupting protein trafficking to cilia often disrupt protein trafficking to the OS and cause photoreceptor degeneration. Bardet-Biedl syndrome (BBS) is one of the ciliopathies associated with defective ciliary trafficking and photoreceptor degeneration. However, precise roles of BBS proteins in photoreceptor cells and the underlying mechanisms of photoreceptor degeneration in BBS are not well understood. Here, we show that accumulation of non-OS proteins in the OS underlies photoreceptor degeneration in BBS. Using a newly developed BBS mouse model [Leucine zipper transcription factor-like 1 (Lztfl1)/Bbs17 mutant], isolated OSs, and quantitative proteomics, we determined 138 proteins that are enriched more than threefold in BBS mutant OS. In contrast, only eight proteins showed a more than threefold reduction. We found striking accumulation of Stx3 and Stxbp1/Munc18-1 and loss of polarized localization of Prom1 within the Lztfl1 and Bbs1 mutant OS. Ultrastructural analysis revealed that large vesicles are formed in the BBS OS, disrupting the lamellar structure of the OS. Our findings suggest that accumulation (and consequent sequestration) of non-OS proteins in the OS is likely the primary cause of photoreceptor degeneration in BBS. Our data also suggest that a major function of BBS proteins in photoreceptors is to transport proteins from the OS to the cell body or to prevent entry of non-OS proteins into the OS.
authors.affiliation: Department of Ophthalmology and Visual Sciences, University of Iowa College of Medicine, Iowa City, IA 52242;
authors.forename: Poppy
authors.initials: P
authors.lastname: Datta
authors.affiliation: Central Microscopy Research Facility, University of Iowa, Iowa City, IA 52242;
authors.forename: Chantal
authors.initials: C
authors.lastname: Allamargot
authors.affiliation: Department of Ophthalmology and Visual Sciences, University of Iowa College of Medicine, Iowa City, IA 52242;
authors.forename: Joseph S
authors.initials: JS
authors.lastname: Hudson
authors.affiliation: Department of Ophthalmology and Visual Sciences, University of Iowa College of Medicine, Iowa City, IA 52242;
authors.forename: Emily K
authors.initials: EK
authors.lastname: Andersen
authors.affiliation: Department of Ophthalmology and Visual Sciences, University of Iowa College of Medicine, Iowa City, IA 52242;
authors.forename: Sajag
authors.initials: S
authors.lastname: Bhattarai
authors.affiliation: Department of Ophthalmology and Visual Sciences, University of Iowa College of Medicine, Iowa City, IA 52242;
authors.forename: Arlene V
authors.initials: AV
authors.lastname: Drack
authors.affiliation: Department of Pediatrics, University of Iowa College of Medicine, Iowa City, IA 52242; Howard Hughes Medical Institute, University of Iowa College of Medicine, Iowa City, IA 52242.
authors.forename: Val C
authors.initials: VC
authors.lastname: Sheffield
authors.affiliation: Department of Ophthalmology and Visual Sciences, University of Iowa College of Medicine, Iowa City, IA 52242; seongjin-seo@uiowa.edu.
authors.forename: Seongjin
authors.initials: S
authors.lastname: Seo
mesh_terms: D000818:Animals; D000918:Antibody Specificity; D020788:Bardet-Biedl Syndrome / Q000378:metabolism* / Q000473:pathology*; D015153:Blotting, Western; D002469:Cell Separation; D005136:Eye Proteins / Q000378:metabolism*; D008810:Mice, Inbred C57BL; D008817:Mice, Mutant Strains; D009765:Obesity / Q000150:complications / Q000473:pathology; D040901:Proteomics; D015203:Reproducibility of Results; D012162:Retinal Degeneration / Q000150:complications / Q000378:metabolism* / Q000473:pathology* / Q000503:physiopathology; D055214:Retinal Photoreceptor Cell Outer Segment / Q000378:metabolism* / Q000473:pathology* / Q000648:ultrastructure; D014157:Transcription Factors / Q000378:metabolism; D014461:Ultracentrifugation
chemical_list: D005136:Eye Proteins; C486612:Lztfl1 protein, mouse; D014157:Transcription Factors
keywords: outer segment; photoreceptor degeneration; primary cilia; retinitis pigmentosa; trafficking