Primary cilia carry out numerous signaling and sensory functions, and defects in them, “ciliopathies,” cause a range of symptoms, including blindness. Understanding of their nanometer-scale ciliary substructures and their disruptions in ciliopathies has been hindered by limitations of conventional microscopic techniques. We have combined cryoelectron tomography, enhanced by subtomogram averaging, with superresolution stochastic optical reconstruction microscopy (STORM) to define subdomains within the light-sensing rod sensory cilium of mouse retinas and reveal previously unknown substructures formed by resident proteins. Domains are demarcated by structural features such as the axoneme and its connections to the ciliary membrane, and are correlated with molecular markers of subcompartments, including the lumen and walls of the axoneme, the membrane glycocalyx, and the intervening cytoplasm. Within this framework, we report spatial distributions of key proteins in wild-type (WT) mice and the effects on them of genetic deficiencies in 3 models of Bardet–Biedl syndrome.

Although the importance of human apolipoprotein E (apoE) in vascular diseases has clearly been established, most of the research on apoE has focused on its role in cholesterol metabolism. In view of the observation that apoE and its functional domains impact extracellular matrix (ECM) remodeling, we hypothesized that apoE could also confer protection against ECM degradation by mechanisms independent of its role in cholesterol and lipoprotein transport. The ECM degrading enzyme, heparanase, is secreted by cells as pro-heparanase that is internalized through low-density lipoprotein (LDL) receptor-related protein-1 (LRP-1) to become enzymatically active. Both apoE and pro-heparanase bind the LRP-1. We further hypothesized that an apoE mimetic peptide (apoEdp) would inhibit the production of active heparanase by blocking LRP-1-mediated uptake of pro-heparanase and thereby decrease degradation of the ECM. To test this hypothesis, we induced the expression of heparanase by incubating human retinal endothelial cells (hRECs) with high glucose (30 mM) for 72 hours. We found that elevated expression of heparanase by high glucose was associated with increased shedding of heparan sulfate (ΔHS) and the tight junction protein occludin. Treatment of hRECs with 100 µM apoEdp in the presence of high glucose significantly reduced the expression of heparanase, shedding of ΔHS, and loss of occludin as detected by Western blot analysis. Either eye drop treatment of 1% apoEdp topically 4 times a day for 14 consecutive days or intraperitoneal injection (40 mg/kg) of apoEdp daily for 14 consecutive days in an in vivo mouse model of streptozotocin-induced diabetes inhibited the loss of tight junction proteins occludin and zona occludin- 1 (ZO-1). These findings imply a functional relationship between apoE and endothelial cell matrix because the deregulation of these molecules can be inhibited by a short peptide derived from the receptor-binding region of apoE. Thus, strategies targeting ECM-degrading enzymes could be therapeutically beneficial for treating diabetic retinopathy.

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Mutations in the cilium-associated protein CEP290 cause retinal degeneration as part of multi-organ ciliopathies or as retina-specific diseases. The precise location and the functional roles of CEP290 within cilia and, specifically, the connecting cilia (CC) of photoreceptors, remain unclear. We used superresolution fluorescence microscopy and electron microscopy (TEM) to localize CEP290 in the CC and in primary cilia of cultured cells with sub-diffraction resolution, and to determine effects of CEP290 deficiency in three mutant models. Radially, CEP290 localizes in close proximity to the microtubule doublets in the region between the doublets and the ciliary membrane. Longitudinally, it is distributed throughout the length of the CC whereas it is confined to the very base of primary cilia in hRPE-1 cells. We found Y-shaped links, ciliary sub-structures between microtubules and membrane, throughout the length of the CC. Severe CEP290 deficiencies in mouse models did not prevent assembly of cilia or cause obvious mislocalization of ciliary components in early stages of degeneration. There were fewer cilia and no normal outer segments in the mutants, but the Y-shaped links were clearly present. These results point to photoreceptor-specific functions of CEP290 essential for CC maturation and stability following the earliest stages of ciliogenesis.

Primary cilia are cylindrical organelles extending from the surface of most animal cells that have been implicated in a host of signaling and sensory functions. Genetic defects in their component molecules, known as "ciliopathies" give rise to devastating symptoms, ranging from defective development, to kidney disease, to progressive blindness. The detailed structures of these organelles and the true functions of proteins encoded by ciliopathy genes are poorly understood because of the small size of cilia and the limitations of conventional microscopic techniques. We describe the combination of cryo-electron tomography, enhanced by sub-tomogram averaging, with super-resolution stochastic reconstruction microscopy (STORM) to define substructures and subdomains within the light-sensing rod sensory cilium of the mammalian retina.Longitudinal and radial domains are demarcated by structural features such as the axoneme and its connections to the ciliary membrane, and are correlated with molecular markers of these compartments, including Ca 2+ -binding protein centrin-2 in the lumen of the axoneme, acetylated tubulin forming the axoneme, the glycocalyx extending outward from the surface of the plasma membrane, and molecular residents of the space between axoneme and ciliary membrane, including Arl13B, intraflagellar transport proteins, BBS5, and syntaxin-3. Within this framework we document that deficiencies in the ciliopathy proteins BBS2, BBS7 and BBS9 lead to inappropriate accumulation of proteins in rod outer segments while largely preserving their sub-domain localization within the connecting cilium region, but alter the distribution of syntaxin-3 clusters.