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.photoreceptor degeneration | trafficking | primary cilia | outer segment | retinitis pigmentosa

An Actin Network Dispatches Ciliary GPCRs into Extracellular Vesicles to Modulate Signaling

“…3D). This signal-independent ectocytosis is likely to reflect the existence of ciliary proteins that are constitutively cleared from cilia by the BBSome – the viral receptor CAR (Mick et al, 2015), Syntaxin 3 and Syntaxin BP1 (Datta et al, 2015) or phospholipase D (Lechtreck et al, 2013) – and become ectocytosed in Bbs mutants. Nevertheless, constitutive ectocytosis is neither as pervasive nor as efficient as signal-dependent ectocytosis as SSTR3 activation enhanced ectosome production in Arl6 −/− cells beyond the constitutive level (Fig.…”

Endocytic Control of Cellular Signaling at the Primary Cilium

“…Similarly, two separate studies in Chlamydomonas have shown that specific membrane proteins can accumulate in the cilia of that organism independently of IFT [26,54]. By contrast, it has been clearly demonstrated in Chlamydomonas [54] as well as in mouse fibroblasts [51] and photoreceptors [55] that IFT mediates export of ciliary membrane proteins via the BBSome, which functions as a cargo adaptor that links ciliary membrane proteins to the IFT system during retrograde transport. Consequently, mutations in genes encoding proteins that link the BBSome and retrograde IFT machinery cause aberrant accumulation of these membrane proteins in cilia [51,54,55].…”

Molecular basis for photoreceptor outer segment architecture

“…One route for non-vesicular intracellular lipid transport is mediated by phospholipid transfer proteins, and although not well-studied in photoreceptors, one example has been documented (Dudley and Anderson, 1978). A recent report suggests that photoreceptors may possess a means to transport proteins from the OS to the cell body (Datta et al, 2015); additional studies would be required to assess the potential impact of this retrograde pathway on lipid trafficking.…”