Defining the layers of a sensory cilium with STORM and cryoelectron nanoscopy

Mapping Protein Distribution in the Canine Photoreceptor Sensory Cilium and Calyceal Processes by Ultrastructure Expansion Microscopy

Purpose

Photoreceptors are highly polarized sensory neurons, possessing a unique ciliary structure known as the photoreceptor sensory cilium (PSC). Vertebrates have two subtypes of photoreceptors: rods, which are responsible for night vision, and cones, which enable daylight vision and color perception. Despite the identification of functional and morphological differences between these subtypes, ultrastructural analysis of the PSC molecular architecture between rods and cones is still lacking. This study employed ultrastructure expansion microscopy (U-ExM) to characterize the PSC molecular architecture in canine retina.

Methods

Canine neuroretinas (5-mm punches) were fixed in paraformaldehyde solution for either short or long durations. Additionally, 20-µm-thick cryosections from frozen archival retinal tissues fixed using the longer protocol were analyzed. A U-ExM protocol previously developed for mouse retina was adapted to these canine tissues with a battery of specific antibodies that label the various compartments of the PSC.

Results

We demonstrated that U-ExM is applicable to both non-frozen and cryopreserved retinal tissues processed with standard paraformaldehyde fixation. Using this validated U-ExM protocol, we revealed the molecular localization of numerous ciliopathy-related proteins in canine photoreceptors. Furthermore, we identified significant architectural differences in the PSC, ciliary rootlet, and calyceal processes between canine rods and cones.

Conclusions

U-ExM is a powerful tool for studying the PSC molecular architecture using frozen archival retinas that are processed following standard paraformaldehyde fixation and embedding protocols. The findings gained from this study pave the way for a better understanding of alterations in the molecular architecture of the PSC in canine models of retinal ciliopathies.

Ciliopathy-associated protein, CEP290, is required for ciliary necklace and outer segment membrane formation in retinal photoreceptors

The most common genetic cause of the childhood blinding disease Leber Congenital Amaurosis is mutation of the ciliopathy gene CEP290. Though studied extensively, the photoreceptor-specific roles of CEP290 remain unclear. Using advanced microscopy techniques, we investigated the sub-ciliary localization of CEP290 and its role in mouse photoreceptors during development. CEP290 was found throughout the connecting cilium between the microtubules and membrane, with nine-fold symmetry. In the absence of CEP290 ciliogenesis occurs, but the connecting cilium membrane is aberrant, and sub-structures, such as the ciliary necklace and Y-links, are defective or absent throughout the mid to distal connecting cilium. Transition zone proteins AHI1 and NPHP1 were abnormally restricted to the proximal connecting cilium in the absence of CEP290, while others like NPHP8 and CEP89 were unaffected. Although outer segment disc formation is inhibited in CEP290 mutant retina, we observed large numbers of extracellular vesicles. These results suggest roles for CEP290 in ciliary membrane structure, outer segment disc formation and photoreceptor-specific spatial distribution of a subset of transition zone proteins, which collectively lead to failure of outer segment formation and photoreceptor degeneration.

Glutamylation imbalance impairs the molecular architecture of the photoreceptor cilium

Microtubules, composed of conserved α/β-tubulin dimers, undergo complex post-translational modifications (PTMs) that fine-tune their properties and interactions with other proteins. Cilia exhibit several tubulin PTMs, such as polyglutamylation, polyglycylation, detyrosination, and acetylation, with functions that are not fully understood. Mutations in AGBL5, which encodes the deglutamylating enzyme CCP5, have been linked to retinitis pigmentosa, suggesting that altered polyglutamylation may cause photoreceptor cell degeneration, though the underlying mechanisms are unclear. Using super-resolution ultrastructure expansion microscopy (U-ExM) in mouse and human photoreceptor cells, we observed that most tubulin PTMs accumulate at the connecting cilium that links outer and inner photoreceptor segments. Mouse models with increased glutamylation (Ccp5-/- and Ccp1-/-) or loss of tubulin acetylation (Atat1-/-) showed that aberrant glutamylation, but not acetylation loss, disrupts outer segment architecture. This disruption includes exacerbation of the connecting cilium, loss of the bulge region, and destabilization of the distal axoneme. Additionally, we found significant impairment in tubulin glycylation, as well as reduced levels of intraflagellar transport proteins and of retinitis pigmentosa-associated protein RPGR. Our findings indicate that proper glutamylation levels are crucial for maintaining the molecular architecture of the photoreceptor cilium.

Mapping protein distribution in the canine photoreceptor sensory cilium and calyceal processes by ultrastructure expansion microscopy

Photoreceptors are highly polarized sensory neurons, possessing a unique ciliary structure known as the photoreceptor sensory cilium (PSC). Vertebrates have two subtypes of photoreceptors: rods, which are responsible for night vision, and cones, which support daylight vision and color perception. Despite identifying functional and morphological differences between these subtypes, ultrastructural analyses of the PSC molecular architecture in rods and cones are still lacking. In this study, we employed ultrastructure expansion microscopy (U-ExM) to characterize the molecular architecture of the PSC in canine retina. We demonstrated that U-ExM is applicable to both non-frozen and cryopreserved retinal tissues with standard paraformaldehyde fixation. Using this validated U-ExM protocol, we revealed the molecular localization of numerous ciliopathy-related proteins in canine photoreceptors. Furthermore, we identified significant architectural differences in the PSC, ciliary rootlet, and calyceal processes between canine rods and cones. These findings pave the way for a better understanding of alterations in the molecular architecture of the PSC in canine models of retinal ciliopathies.

Ciliary tip actin dynamics regulate photoreceptor outer segment integrity

As signalling organelles, cilia regulate their G protein-coupled receptor content by ectocytosis, a process requiring localised actin dynamics to alter membrane shape. Photoreceptor outer segments comprise an expanse of folded membranes (discs) at the tip of highly-specialised connecting cilia, into which photosensitive GPCRs are concentrated. Discs are shed and remade daily. Defects in this process, due to mutations, cause retinitis pigmentosa (RP). Whilst fundamental for vision, the mechanism of photoreceptor disc generation is poorly understood. Here, we show membrane deformation required for disc genesis is driven by dynamic actin changes in a process akin to ectocytosis. We show RPGR, a leading RP gene, regulates actin-binding protein activity central to this process. Actin dynamics, required for disc formation, are perturbed in Rpgr mouse models, leading to aborted membrane shedding as ectosome-like vesicles, photoreceptor death and visual loss. Actin manipulation partially rescues this, suggesting the pathway could be targeted therapeutically. These findings help define how actin-mediated dynamics control outer segment turnover.

Centriole and transition zone structures in photoreceptor cilia revealed by cryo-electron tomography

Primary cilia mediate sensory signaling in multiple organisms and cell types but have structures adapted for specific roles. Structural defects in them lead to devastating diseases known as ciliopathies in humans. Key to their functions are structures at their base: the basal body, the transition zone, the "Y-shaped links," and the "ciliary necklace." We have used cryo-electron tomography with subtomogram averaging and conventional transmission electron microscopy to elucidate the structures associated with the basal region of the "connecting cilia" of rod outer segments in mouse retina. The longitudinal variations in microtubule (MT) structures and the lumenal scaffold complexes connecting them have been determined, as well as membrane-associated transition zone structures: Y-shaped links connecting MT to the membrane, and ciliary beads connected to them that protrude from the cell surface and form a necklace-like structure. These results represent a clearer structural scaffold onto which molecules identified by genetics, proteomics, and superresolution fluorescence can be placed in our emerging model of photoreceptor sensory cilia.

Super-resolution mapping in rod photoreceptors identifies rhodopsin trafficking through the inner segment plasma membrane as an essential subcellular pathway

Photoreceptor cells in the vertebrate retina have a highly compartmentalized morphology for efficient phototransduction and vision. Rhodopsin, the visual pigment in rod photoreceptors, is densely packaged into the rod outer segment sensory cilium and continuously renewed through essential synthesis and trafficking pathways housed in the rod inner segment. Despite the importance of this region for rod health and maintenance, the subcellular organization of rhodopsin and its trafficking regulators in the mammalian rod inner segment remain undefined. We used super-resolution fluorescence microscopy with optimized retinal immunolabeling techniques to perform a single molecule localization analysis of rhodopsin in the inner segments of mouse rods. We found that a significant fraction of rhodopsin molecules was localized at the plasma membrane, at the surface, in an even distribution along the entire length of the inner segment, where markers of transport vesicles also colocalized. Thus, our results collectively establish a model of rhodopsin trafficking through the inner segment plasma membrane as an essential subcellular pathway in mouse rod photoreceptors.

The tectonic complex regulates membrane protein composition in the photoreceptor cilium

The primary cilium is a signaling organelle with a unique membrane composition maintained by a diffusional barrier residing at the transition zone. Many transition zone proteins, such as the tectonic complex, are linked to preserving ciliary composition but the mechanism remains unknown. To understand tectonic's role, we generate a photoreceptor-specific Tctn1 knockout mouse. Loss of Tctn1 results in the absence of the entire tectonic complex and associated MKS proteins yet has minimal effects on the transition zone structure of rod photoreceptors. We find that the protein composition of the photoreceptor cilium is disrupted as non-resident membrane proteins accumulate in the cilium over time, ultimately resulting in photoreceptor degeneration. We further show that fluorescent rhodopsin moves faster through the transition zone in photoreceptors lacking tectonic, which suggests that the tectonic complex acts as a physical barrier to slow down membrane protein diffusion in the photoreceptor transition zone to ensure proper removal of non-resident membrane proteins.

Mapping rhodopsin trafficking in rod photoreceptors with quantitative super-resolution microscopy

Photoreceptor cells in the vertebrate retina have a highly compartmentalized morphology for efficient long-term phototransduction. Rhodopsin, the visual pigment in rod photoreceptors, is densely packaged into the rod outer segment sensory cilium and continuously renewed through essential synthesis and trafficking pathways housed in the rod inner segment. Despite the importance of this region for rod health and maintenance, the subcellular organization of rhodopsin and its trafficking regulators in the mammalian rod inner segment remain undefined. We used super-resolution fluorescence microscopy with optimized retinal immunolabeling techniques to perform a single molecule localization analysis of rhodopsin in the inner segments of mouse rods. We found that a significant fraction of rhodopsin molecules was localized at the plasma membrane in an even distribution along the entire length of the inner segment, where markers of transport vesicles also colocalized. Thus, our results collectively establish a model of rhodopsin trafficking through the inner segment plasma membrane as an essential subcellular pathway in mouse rod photoreceptors.

CEP162 deficiency causes human retinal degeneration and reveals a dual role in ciliogenesis and neurogenesis

Defects in primary or motile cilia result in a variety of human pathologies, and retinal degeneration is frequently associated with these so-called ciliopathies. We found that homozygosity for a truncating variant in CEP162, a centrosome and microtubule-associated protein required for transition zone assembly during ciliogenesis and neuronal differentiation in the retina, caused late-onset retinitis pigmentosa in 2 unrelated families. The mutant CEP162-E646R*5 protein was expressed and properly localized to the mitotic spindle, but it was missing from the basal body in primary and photoreceptor cilia. This impaired recruitment of transition zone components to the basal body and corresponded to complete loss of CEP162 function at the ciliary compartment, reflected by delayed formation of dysmorphic cilia. In contrast, shRNA knockdown of Cep162 in the developing mouse retina increased cell death, which was rescued by expression of CEP162-E646R*5, indicating that the mutant retains its role for retinal neurogenesis. Human retinal degeneration thus resulted from specific loss of the ciliary function of CEP162.

Single-molecule imaging in the primary cilium

The primary cilium is an important signaling organelle critical for normal development and tissue homeostasis. Its small dimensions and complexity necessitate advanced imaging approaches to uncover the molecular mechanisms behind its function. Here, we outline how single-molecule fluorescence microscopy can be used for tracking molecular dynamics and interactions and for super-resolution imaging of nanoscale structures in the primary cilium. Specifically, we describe in detail how to capture and quantify the 2D dynamics of individual transmembrane proteins PTCH1 and SMO and how to map the 3D nanoscale distributions of the inversin compartment proteins INVS, ANKS6, and NPHP3. This protocol can, with minor modifications, be adapted for studies of other proteins and cell lines to further elucidate the structure and function of the primary cilium at the molecular level.

Mechanistic Analysis of CCP1 in Generating ΔC2 α-Tubulin in Mammalian Cells and Photoreceptor Neurons

An important post-translational modification (PTM) of α-tubulin is the removal of amino acids from its C-terminus. Removal of the C-terminal tyrosine residue yields detyrosinated α-tubulin, and subsequent removal of the penultimate glutamate residue produces ΔC2-α-tubulin. These PTMs alter the ability of the α-tubulin C-terminal tail to interact with effector proteins and are thereby thought to change microtubule dynamics, stability, and organization. The peptidase(s) that produces ΔC2-α-tubulin in a physiological context remains unclear. Here, we take advantage of the observation that ΔC2-α-tubulin accumulates to high levels in cells lacking tubulin tyrosine ligase (TTL) to screen for cytosolic carboxypeptidases (CCPs) that generate ΔC2-α-tubulin. We identify CCP1 as the sole peptidase that produces ΔC2-α-tubulin in TTLΔ HeLa cells. Interestingly, we find that the levels of ΔC2-α-tubulin are only modestly reduced in photoreceptors of ccp1-/- mice, indicating that other peptidases act synergistically with CCP1 to produce ΔC2-α-tubulin in post-mitotic cells. Moreover, the production of ΔC2-α-tubulin appears to be under tight spatial control in the photoreceptor cilium: ΔC2-α-tubulin persists in the connecting cilium of ccp1-/- but is depleted in the distal portion of the photoreceptor. This work establishes the groundwork to pinpoint the function of ΔC2-α-tubulin in proliferating and post-mitotic mammalian cells.

Expansion Microscopy of Mouse Photoreceptor Cilia

The small size of ciliary structures that underlies photoreceptor function and inherited ciliopathies requires imaging techniques adapted to visualizing them at the highest possible resolution. In addition to powerful super-resolution imaging modalities, emerging approaches to sample preparation, including expansion microscopy (ExM), can provide a robust route to imaging specific molecules at the nanoscale level in the retina. We describe a protocol for applying ExM to whole retinas in order to achieve nanoscale fluorescence imaging of ciliary markers, including tubulin, CEP290, centrin, and CEP164. The results are consistent with those from other super-resolution fluorescence techniques and reveal new insights into their arrangements with respect to the subcompartments of photoreceptor cilia. This technique is complimentary to other imaging modalities used in retinal imaging, and can be carried out in virtually any laboratory, without the need for expensive specialized equipment.

Composition, organization and mechanisms of the transition zone, a gate for the cilium

The cilium evolved to provide the ancestral eukaryote with the ability to move and sense its environment. Acquiring these functions required the compartmentalization of a dynein-based motility apparatus and signaling proteins within a discrete subcellular organelle contiguous with the cytosol. Here, we explore the potential molecular mechanisms for how the proximal-most region of the cilium, termed transition zone (TZ), acts as a diffusion barrier for both membrane and soluble proteins and helps to ensure ciliary autonomy and homeostasis. These include a unique complement and spatial organization of proteins that span from the microtubule-based axoneme to the ciliary membrane; a protein picket fence; a specialized lipid microdomain; differential membrane curvature and thickness; and lastly, a size-selective molecular sieve. In addition, the TZ must be permissive for, and functionally integrates with, ciliary trafficking systems (including intraflagellar transport) that cross the barrier and make the ciliary compartment dynamic. The quest to understand the TZ continues and promises to not only illuminate essential aspects of human cell signaling, physiology, and development, but also to unravel how TZ dysfunction contributes to ciliopathies that affect multiple organ systems, including eyes, kidney, and brain.

In situ architecture of the ciliary base reveals the stepwise assembly of intraflagellar transport trains

The cilium is an antenna-like organelle that performs numerous cellular functions, including motility, sensing, and signaling. The base of the cilium contains a selective barrier that regulates the entry of large intraflagellar transport (IFT) trains, which carry cargo proteins required for ciliary assembly and maintenance. However, the native architecture of the ciliary base and the process of IFT train assembly remain unresolved. In this work, we used in situ cryo-electron tomography to reveal native structures of the transition zone region and assembling IFT trains at the ciliary base in Chlamydomonas. We combined this direct cellular visualization with ultrastructure expansion microscopy to describe the front-to-back stepwise assembly of IFT trains: IFT-B forms the backbone, onto which bind IFT-A, dynein-1b, and finally kinesin-2 before entry into the cilium.

Rapid 3D-STORM imaging of diverse molecular targets in tissue

Fine-scale molecular architecture is critical for nervous system and other biological functions. Methods to visualize these nanoscale structures would benefit from enhanced accessibility, throughput, and tissue compatibility. Here, we report RAIN-STORM, a rapid and scalable nanoscopic imaging optimization approach that improves three-dimensional visualization for subcellular targets in tissue at depth. RAIN-STORM uses conventional tissue samples and readily available reagents and is suitable for commercial instrumentation. To illustrate the efficacy of RAIN-STORM, we utilized the retina. We show that RAIN-STORM imaging is versatile and provide 3D nanoscopic data for over 20 synapse, neuron, glia, and vasculature targets. Sample preparation is also rapid, with a 1-day turnaround from tissue to image, and parameters are suitable for multiple tissue sources. Finally, we show that this method can be applied to clinical samples to reveal nanoscale features of human cells and synapses. RAIN-STORM thus paves the way for high-throughput studies of nanoscopic targets in tissue.

Mapping molecular complexes with super-resolution microscopy and single-particle analysis

Understanding the structure of supramolecular complexes provides insight into their functional capabilities and how they can be modulated in the context of disease. Super-resolution microscopy (SRM) excels in performing this task by resolving ultrastructural details at the nanoscale with molecular specificity. However, technical limitations, such as underlabelling, preclude its ability to provide complete structures. Single-particle analysis (SPA) overcomes this limitation by combining information from multiple images of identical structures and producing an averaged model, effectively enhancing the resolution and coverage of image reconstructions. This review highlights important studies using SRM-SPA, demonstrating how it broadens our knowledge by elucidating features of key biological structures with unprecedented detail.

Subcellular localization of mutant P23H rhodopsin in an RFP fusion knock-in mouse model of retinitis pigmentosa

The P23H mutation in rhodopsin (Rho), the rod visual pigment, is the most common allele associated with autosomal-dominant retinitis pigmentosa (adRP). The fate of misfolded mutant Rho in rod photoreceptors has yet to be elucidated. We generated a new mouse model, in which the P23H-Rho mutant allele is fused to the fluorescent protein Tag-RFP-T (P23HhRhoRFP). In heterozygotes, outer segments formed, and wild-type (WT) rhodopsin was properly localized, but mutant P23H-Rho protein was mislocalized in the inner segments. Heterozygotes exhibited slowly progressing retinal degeneration. Mislocalized P23HhRhoRFP was contained in greatly expanded endoplasmic reticulum (ER) membranes. Quantification of mRNA for markers of ER stress and the unfolded protein response revealed little or no increases. mRNA levels for both the mutant human rhodopsin allele and the WT mouse rhodopsin were reduced, but protein levels revealed selective degradation of the mutant protein. These results suggest that the mutant rods undergo an adaptative process that prolongs survival despite unfolded protein accumulation in the ER. The P23H-Rho-RFP mouse may represent a useful tool for the future study of the pathology and treatment of P23H-Rho and adRP.

This article has an associated First Person interview with the first author of the paper.

Summary: A mouse line with a knock-in of the human rhodopsin gene altered to contain the P23H mutation and a red fluorescent protein fusion provides a new model for autosomal-dominant retinitis pigmentosa.

Mechanical stimulation promotes enthesis injury repair by mobilizing Prrx1+ cells via ciliary TGF-β signaling

Proper mechanical stimulation can improve rotator cuff enthesis injury repair. However, the underlying mechanism of mechanical stimulation promoting injury repair is still unknown. In this study, we found that Prrx1⁺ cell was essential for murine rotator cuff enthesis development identified by single-cell RNA sequence and involved in the injury repair. Proper mechanical stimulation could promote the migration of Prrx1⁺ cells to enhance enthesis injury repair. Meantime, TGF-β signaling and primary cilia played an essential role in mediating mechanical stimulation signaling transmission. Proper mechanical stimulation enhanced the release of active TGF-β1 to promote migration of Prrx1⁺ cells. Inhibition of TGF-β signaling eliminated the stimulatory effect of mechanical stimulation on Prrx1⁺ cell migration and enthesis injury repair. In addition, knockdown of Pallidin to inhibit TGF-βR2 translocation to the primary cilia or deletion of Ift88 in Prrx1⁺ cells also restrained the mechanics-induced Prrx1⁺ cells migration. These findings suggested that mechanical stimulation could increase the release of active TGF-β1 and enhance the mobilization of Prrx1⁺ cells to promote enthesis injury repair via ciliary TGF-β signaling.

Recent Developments in Correlative Super-Resolution Fluorescence Microscopy and Electron Microscopy

The recently developed correlative super-resolution fluorescence microscopy (SRM) and electron microscopy (EM) is a hybrid technique that simultaneously obtains the spatial locations of specific molecules with SRM and the context of the cellular ultrastructure by EM. Although the combination of SRM and EM remains challenging owing to the incompatibility of samples prepared for these techniques, the increasing research attention on these methods has led to drastic improvements in their performances and resulted in wide applications. Here, we review the development of correlative SRM and EM (sCLEM) with a focus on the correlation of EM with different SRM techniques. We discuss the limitations of the integration of these two microscopy techniques and how these challenges can be addressed to improve the quality of correlative images. Finally, we address possible future improvements and advances in the continued development and wide application of sCLEM approaches.

Protein Kinase A in Human Retina: Differential Localization of Cβ, Cα, RIIα, and RIIβ in Photoreceptors Highlights Non-redundancy of Protein Kinase A Subunits

Protein kinase A (PKA) signaling is essential for numerous processes but the subcellular localization of specific PKA regulatory (R) and catalytic (C) subunits has yet to be explored comprehensively. Additionally, the localization of the Cβ subunit has never been spatially mapped in any tissue even though ∼50% of PKA signaling in neuronal tissues is thought to be mediated by Cβ. Here we used human retina with its highly specialized neurons as a window into PKA signaling in the brain and characterized localization of PKA Cα, Cβ, RIIα, and RIIβ subunits. We found that each subunit presented a distinct localization pattern. Cα and Cβ were localized in all cell layers (photoreceptors, interneurons, retinal ganglion cells), while RIIα and RIIβ were selectively enriched in photoreceptor cells where both showed distinct patterns of co-localization with Cα but not Cβ. Only Cα was observed in photoreceptor outer segments and at the base of the connecting cilium. Cβ in turn, was highly enriched in mitochondria and was especially prominent in the ellipsoid of cone cells. Further investigation of Cβ using RNA BaseScope technology showed that two Cβ splice variants (Cβ4 and Cβ4ab) likely code for the mitochondrial Cβ proteins. Overall, our data indicates that PKA Cα, Cβ, RIIα, and RIIβ subunits are differentially localized and are likely functionally non-redundant in the human retina. Furthermore, Cβ is potentially important for mitochondrial-associated neurodegenerative diseases previously linked to PKA dysfunction.

Super-resolution microscopy reveals photoreceptor-specific subciliary location and function of ciliopathy-associated protein CEP290

Mutations in the cilium-associated protein CEP290 cause retinal degeneration as part of multiorgan 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 super-resolution fluorescence microscopy and electron microscopy to localize CEP290 in the CC and in the primary cilia of cultured cells with subdiffraction resolution and to determine effects of CEP290 deficiency in 3 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 human retinal pigment epithelium-1 cells. We found Y-shaped links, ciliary substructures 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.

Structure and dynamics of photoreceptor sensory cilia

The rod and cone photoreceptor cells of the vertebrate retina have highly specialized structures that enable them to carry out their function of light detection over a broad range of illumination intensities with optimized spatial and temporal resolution. Most prominent are their unusually large sensory cilia, consisting of outer segments packed with photosensitive disc membranes, a connecting cilium with many features reminiscent of the primary cilium transition zone, and a pair of centrioles forming a basal body which serves as the platform upon which the ciliary axoneme is assembled. These structures form a highway through which an enormous flux of material moves on a daily basis to sustain the continual turnover of outer segment discs and the energetic demands of phototransduction. After decades of study, the details of the fine structure and distribution of molecular components of these structures are still incompletely understood, but recent advances in cellular imaging techniques and animal models of inherited ciliary defects are yielding important new insights. This knowledge informs our understanding both of the mechanisms of trafficking and assembly and of the pathophysiological mechanisms of human blinding ciliopathies.

Pathogenic STX3 variants affecting the retinal and intestinal transcripts cause an early-onset severe retinal dystrophy in microvillus inclusion disease subjects

Biallelic STX3 variants were previously reported in five individuals with the severe congenital enteropathy, microvillus inclusion disease (MVID). Here, we provide a significant extension of the phenotypic spectrum caused by STX3 variants. We report ten individuals of diverse geographic origin with biallelic STX3 loss-of-function variants, identified through exome sequencing, single-nucleotide polymorphism array-based homozygosity mapping, and international collaboration. The evaluated individuals all presented with MVID. Eight individuals also displayed early-onset severe retinal dystrophy, i.e., syndromic-intestinal and retinal-disease. These individuals harbored STX3 variants that affected both the retinal and intestinal STX3 transcripts, whereas STX3 variants affected only the intestinal transcript in individuals with solitary MVID. That STX3 is essential for retinal photoreceptor survival was confirmed by the creation of a rod photoreceptor-specific STX3 knockout mouse model which revealed a time-dependent reduction in the number of rod photoreceptors, thinning of the outer nuclear layer, and the eventual loss of both rod and cone photoreceptors. Together, our results provide a link between STX3 loss-of-function variants and a human retinal dystrophy. Depending on the genomic site of a human loss-of-function STX3 variant, it can cause MVID, the novel intestinal-retinal syndrome reported here or, hypothetically, an isolated retinal dystrophy.

Functional compartmentalization of photoreceptor neurons

Retinal photoreceptors are neurons that convert dynamically changing patterns of light into electrical signals that are processed by retinal interneurons and ultimately transmitted to vision centers in the brain. They represent the essential first step in seeing without which the remainder of the visual system is rendered moot. To support this role, the major functions of photoreceptors are segregated into three main specialized compartments-the outer segment, the inner segment, and the pre-synaptic terminal. This compartmentalization is crucial for photoreceptor function-disruption leads to devastating blinding diseases for which therapies remain elusive. In this review, we examine the current understanding of the molecular and physical mechanisms underlying photoreceptor functional compartmentalization and highlight areas where significant knowledge gaps remain.

Compartmentalization of Photoreceptor Sensory Cilia

Functional compartmentalization of cells is a universal strategy for segregating processes that require specific components, undergo regulation by modulating concentrations of those components, or that would be detrimental to other processes. Primary cilia are hair-like organelles that project from the apical plasma membranes of epithelial cells where they serve as exclusive compartments for sensing physical and chemical signals in the environment. As such, molecules involved in signal transduction are enriched within cilia and regulating their ciliary concentrations allows adaptation to the environmental stimuli. The highly efficient organization of primary cilia has been co-opted by major sensory neurons, olfactory cells and the photoreceptor neurons that underlie vision. The mechanisms underlying compartmentalization of cilia are an area of intense current research. Recent findings have revealed similarities and differences in molecular mechanisms of ciliary protein enrichment and its regulation among primary cilia and sensory cilia. Here we discuss the physiological demands on photoreceptors that have driven their evolution into neurons that rely on a highly specialized cilium for signaling changes in light intensity. We explore what is known and what is not known about how that specialization appears to have driven unique mechanisms for photoreceptor protein and membrane compartmentalization.

Photoreceptor cilia, in contrast to primary cilia, grant entry to a partially assembled BBSome

The BBSome is a protein complex consisting of BBS1, BBS2, BBS4, BBS5, BBS7, BBS8, BBS9 and BBS18 that associates with intraflagellar transport complexes and specializes in ciliary trafficking. In primary cilia, ciliary entry requires the fully assembled BBSome as well as the small GTPase, ARL6 (BBS3). Retinal photoreceptors possess specialized cilia. In light of key structural and functional differences between primary and specialized cilia, we examined the principles of BBSome recruitment to photoreceptor cilia. We performed sucrose gradient fractionation using retinal lysates of Bbs2-/-, Bbs7-/-, Bbs8-/- and Bbs3-/- mice to determine the status of BBSome assembly, then determined localization of BBSome components using immunohistochemistry. Surprisingly, we found that a subcomplex of the BBSome containing at least BBS1, BBS5, BBS8 and BBS9 is recruited to cilia in the absence of BBS2 or BBS7. In contrast, a BBSome subcomplex consisting of BBS1, BBS2, BBS5, BBS7 and BBS9 is found in Bbs8-/- retinas and is denied ciliary entry in photoreceptor cells. In addition, the BBSome remains fully assembled in Bbs3-/- retinas and can be recruited to photoreceptor cilia in the absence of BBS3. We compared phenotypic severity of their retinal degeneration phenotypes. These findings demonstrate that unlike primary cilia, photoreceptor cilia admit a partially assembled BBSome meeting specific requirements. In addition, the recruitment of the BBSome to photoreceptor cilia does not require BBS3. These findings indicate that the ciliary entry of the BBSome is subjected to cell-specific regulation, particularly in cells with highly adapted forms of cilia such as photoreceptors.

Primary cilia biogenesis and associated retinal ciliopathies

The primary cilium is a ubiquitous microtubule-based organelle that senses external environment and modulates diverse signaling pathways in different cell types and tissues. The cilium originates from the mother centriole through a complex set of cellular events requiring hundreds of distinct components. Aberrant ciliogenesis or ciliary transport leads to a broad spectrum of clinical entities with overlapping yet highly variable phenotypes, collectively called ciliopathies, which include sensory defects and syndromic disorders with multi-organ pathologies. For efficient light detection, photoreceptors in the retina elaborate a modified cilium known as the outer segment, which is packed with membranous discs enriched for components of the phototransduction machinery. Retinopathy phenotype involves dysfunction and/or degeneration of the light sensing photoreceptors and is highly penetrant in ciliopathies. This review will discuss primary cilia biogenesis and ciliopathies, with a focus on the retina, and the role of CP110-CEP290-CC2D2A network. We will also explore how recent technologies can advance our understanding of cilia biology and discuss new paradigms for developing potential therapies of retinal ciliopathies.