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

αδ-4 is an auxiliary subunit of voltage-gated Ca1.4 L-type channels that regulate the development and mature exocytotic function of the photoreceptor ribbon synapse. In humans, mutations in the gene encoding αδ-4 cause heterogeneous forms of vision impairment in humans, the underlying pathogenic mechanisms of which remain unclear. To investigate the retinal function of αδ-4, we used genome editing to generate an αδ-4 knock-out (αδ-4 KO) mouse. In male and female αδ-4 KO mice, rod spherules lack ribbons and other synaptic hallmarks early in development. Although the molecular organization of cone synapses is less affected than rod synapses, horizontal and cone bipolar processes extend abnormally in the outer nuclear layer in αδ-4 KO retina. In reconstructions of αδ-4 KO cone pedicles by serial block face scanning electron microscopy, ribbons appear normal, except that less than one-third show the expected triadic organization of processes at ribbon sites. The severity of the synaptic defects in αδ-4 KO mice correlates with a progressive loss of Ca1.4 channels, first in terminals of rods and later cones. Despite the absence of b-waves in electroretinograms, visually guided behavior is evident in αδ-4 KO mice and better under photopic than scotopic conditions. We conclude that αδ-4 plays an essential role in maintaining the structural and functional integrity of rod and cone synapses, the disruption of which may contribute to visual impairment in humans with mutations. In the retina, visual information is first communicated by the synapse formed between photoreceptors and second-order neurons. The mechanisms that regulate the structural integrity of this synapse are poorly understood. Here we demonstrate a role for αδ-4, a subunit of voltage-gated Ca channels, in organizing the structure and function of photoreceptor synapses. We find that presynaptic Ca channels are progressively lost and that rod and cone synapses are disrupted in mice that lack αδ-4. Our results suggest that alterations in presynaptic Ca signaling and photoreceptor synapse structure may contribute to vision impairment in humans with mutations in the gene encoding αδ-4.

TUDCA treatment preserved ERG b-waves and the outer nuclear layer in Bbs1(M390R/M390R) mice, and prevented obesity assessed at P120. TUDCA treatment preserved ERG b-waves and the outer nuclear layer in the rd10 mice to P30. TUDCA is a prime candidate for treatment of humans with retinal degeneration, especially those with Bardet-Biedl syndrome, whom it may help not only with the vision loss, but with the debilitating obesity as well.

In a knock-in model of BBS1, subretinal delivery of AAV-Bbs1 rescues BBSome formation and rhodopsin localization, and shows a trend toward improved ERG. BBS is challenging to treat with gene therapy due to the stoichiometry of the BBSome protein complex and overexpression toxicity.