Diffusion of a soluble protein, photoactivatable GFP, through a sensory cilium

Hydrophobic interaction between the TM1 and H8 is essential for rhodopsin trafficking to vertebrate photoreceptor outer segments

A major unsolved question in vertebrate photoreceptor biology is the mechanism of rhodopsin transport to the outer segment. In rhodopsin-like class A G protein-coupled receptors, hydrophobic interactions between C-terminal α-helix 8 (H8), and transmembrane α-helix-1 (TM1) have been shown to be important for transport to the plasma membrane, however whether this interaction is important for rhodopsin transport to ciliary rod outer segments is not known. We examined the crystal structures of vertebrate rhodopsins and class A G protein-coupled receptors and found a conserved network of predicted hydrophobic interactions. In Xenopus rhodopsin (xRho), this interaction corresponds to F313, L317, and L321 in H8 and M57, V61, and L68 in TM1. To evaluate the role of H8-TM1 hydrophobic interactions in rhodopsin transport, we expressed xRho-EGFP where hydrophobic residues were mutated in Xenopus rods and evaluated the efficiency of outer segment enrichment. We found that substituting L317 and M57 with hydrophilic residues had the strongest impact on xRho mislocalization. Substituting hydrophilic amino acids at positions L68, F313, and L321 also had a significant impact. Replacing L317 with M resulted in significant mislocalization, indicating that the hydrophobic interaction between residues 317 and 57 is exquisitely sensitive. The corresponding experiment in bovine rhodopsin expressed in HEK293 cells had a similar effect, showing that the H8-TM1 hydrophobic network is essential for rhodopsin transport in mammalian species. Thus, for the first time, we show that a hydrophobic interaction between H8 and TM1 is critical for efficient rhodopsin transport to the vertebrate photoreceptor ciliary outer segment.

Control of protein and lipid composition of photoreceptor outer segments-Implications for retinal disease

Vision is arguably our most important sense, and its loss brings substantial limitations to daily life for affected individuals. Light is perceived in retinal photoreceptors (PRs), which are highly specialized neurons subdivided into several compartments with distinct functions. The outer segments (OSs) of photoreceptors represent highly specialized primary ciliary compartments hosting the phototransduction cascade, which transforms incoming light into a neuronal signal. Retinal disease can result from various pathomechanisms originating in distinct subcompartments of the PR cell, or in the retinal pigment epithelium which supports the PRs. Dysfunction of primary cilia causes human disorders known as "ciliopathies", in which retinal disease is a common feature. This chapter focuses on PR OSs, discussing the mechanisms controlling their complex structure and composition. A sequence of tightly regulated sorting and trafficking events, both upstream of and within this ciliary compartment, ensures the establishment and maintenance of the adequate proteome and lipidome required for signaling in response to light. We discuss in particular our current understanding of the role of ciliopathy proteins involved in multi-protein complexes at the ciliary transition zone (CC2D2A) or BBSome (BBS1) and how their dysfunction causes retinal disease. While the loss of CC2D2A prevents the fusion of vesicles and delivery of the photopigment rhodopsin to the ciliary base, leading to early OS ultrastructural defects, BBS1 deficiency results in precocious accumulation of cholesterol in mutant OSs and decreased visual function preceding morphological changes. These distinct pathomechanisms underscore the central role of ciliary proteins involved in multiple processes controlling OS protein and lipid composition.

Morphology and Transport in Eukaryotic Cells

Transport of intracellular components relies on a variety of active and passive mechanisms, ranging from the diffusive spreading of small molecules over short distances to motor-driven motion across long distances. The cell-scale behavior of these mechanisms is fundamentally dependent on the morphology of the underlying cellular structures. Diffusion-limited reaction times can be qualitatively altered by the presence of occluding barriers or by confinement in complex architectures, such as those of reticulated organelles. Motor-driven transport is modulated by the architecture of cytoskeletal filaments that serve as transport highways. In this review, we discuss the impact of geometry on intracellular transport processes that fulfill a broad range of functional objectives, including delivery, distribution, and sorting of cellular components. By unraveling the interplay between morphology and transport efficiency, we aim to elucidate key structure-function relationships that govern the architecture of transport systems at the cellular scale. Expected final online publication date for the Annual Review of Biophysics, Volume 51 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Monitoring basal autophagy in the retina utilizing CAG-mRFP-EGFP-MAP1LC3B reporter mouse: technical and biological considerations

We describe the utility of a tandem-tagged autophagy reporter mouse model (CAG-RFP-EGFP-MAP1LC3B) in investigating basal macroautophagic/autophagic flux in the neural retina. Western blot, in situ hybridization, immunohistochemistry, and confocal microscopy showed that CAG promoter-driven expression of RFP-EGFP-MAP1LC3B increased “cytosolic” RFP-EGFP-LC3B-I levels, whereas RFP-EGFP-LC3B-II decorates true phagosomes. We verified that the electroretinographic (ERG) responses of tandem-tagged LC3B mice were comparable to those of age-matched controls. Optimized microscope settings detected lipofuscin autofluorescence in retinas of abca4^−/- mice. The majority of retinal phagosomes in the reporter mice exhibited only RFP (not EGFP) fluorescence, suggesting rapid maturation of phagosomes. Only ~1.5% of the total phagosome population was EGFP-labeled; RFP-labeled (mature) phagosomes colocalized with lysosomal markers LAMP2 and CTSD. In the outer retina, phagosome sizes were as follows (in µm², ave ± SEM): RPE, 0.309 ± 0.015; photoreceptor inner segment-myoid, 0.544 ± 0.031; and outer nuclear layer, 0.429 ± 0.011. Detection of RPE phagosomes by fluorescence microscopy is challenging, due to the presence of melanin. Increased lipofuscin autofluorescence, such as observed in the abca4^−/- mouse model of Stargardt disease, is a strong confounding factor when attempting to study autophagy in the RPE. In addition to RPE and photoreceptor cells, phagosomes were detected in inner retinal cell types, microglia, astrocytes, and endothelial cells. We conclude that the tandem-tagged LC3B mouse model serves as a useful system for studying autophagy in the retina. This utility, however, is dependent upon several technical and biological factors, including microscope settings, transgene expression, choice of fluorophores, and lipofuscin autofluorescence.

Abbreviations: ACTB: actin, beta; AIF1: allograft inflammatory factor 1; ATG: autophagy related; CTSD: cathepsin D; DAPI: (4’,6-diamido-2-phenylindole); DIC: differential interference contrast; EGFP: enhanced green fluorescent protein; ELM: external limiting membrane; ERG: electroretinography; GCL: ganglion cell layer; GLUL: glutamine-ammonia ligase (glutamine synthetase); INL: inner nuclear layer; IS-E/M: inner segment – ellipsoid/myoid; ISH: in situ hybridization; LAMP2: lysosomal-associated membrane protein 2; L.I.: laser Intensity; MAP1LC3B: microtubule-associated protein 1 light chain 3 beta; MTOR: mechanistic target of rapamycin kinase; O.C.T.: optimal cutting temperature; OS: outer segment; ONL: outer nuclear layer; PE: phosphatidylethanolamine; RFP: red fluorescent protein; R.O.I.: region of interest; RPE: retinal pigment epithelium

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.

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.

On the Wrong Track: Alterations of Ciliary Transport in Inherited Retinal Dystrophies

Ciliopathies are a group of heterogeneous inherited disorders associated with dysfunction of the cilium, a ubiquitous microtubule-based organelle involved in a broad range of cellular functions. Most ciliopathies are syndromic, since several organs whose cells produce a cilium, such as the retina, cochlea or kidney, are affected by mutations in ciliary-related genes. In the retina, photoreceptor cells present a highly specialized neurosensory cilium, the outer segment, stacked with membranous disks where photoreception and phototransduction occurs. The daily renewal of the more distal disks is a unique characteristic of photoreceptor outer segments, resulting in an elevated protein demand. All components necessary for outer segment formation, maintenance and function have to be transported from the photoreceptor inner segment, where synthesis occurs, to the cilium. Therefore, efficient transport of selected proteins is critical for photoreceptor ciliogenesis and function, and any alteration in either cargo delivery to the cilium or intraciliary trafficking compromises photoreceptor survival and leads to retinal degeneration. To date, mutations in more than 100 ciliary genes have been associated with retinal dystrophies, accounting for almost 25% of these inherited rare diseases. Interestingly, not all mutations in ciliary genes that cause retinal degeneration are also involved in pleiotropic pathologies in other ciliated organs. Depending on the mutation, the same gene can cause syndromic or non-syndromic retinopathies, thus emphasizing the highly refined specialization of the photoreceptor neurosensory cilia, and raising the possibility of photoreceptor-specific molecular mechanisms underlying common ciliary functions such as ciliary transport. In this review, we will focus on ciliary transport in photoreceptor cells and discuss the molecular complexity underpinning retinal ciliopathies, with a special emphasis on ciliary genes that, when mutated, cause either syndromic or non-syndromic retinal ciliopathies.

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.

Getting around the cell: physical transport in the intracellular world

Eukaryotic cells face the challenging task of transporting a variety of particles through the complex intracellular milieu in order to deliver, distribute, and mix the many components that support cell function. In this review, we explore the biological objectives and physical mechanisms of intracellular transport. Our focus is on cytoplasmic and intra-organelle transport at the whole-cell scale. We outline several key biological functions that depend on physically transporting components across the cell, including the delivery of secreted proteins, support of cell growth and repair, propagation of intracellular signals, establishment of organelle contacts, and spatial organization of metabolic gradients. We then review the three primary physical modes of transport in eukaryotic cells: diffusive motion, motor-driven transport, and advection by cytoplasmic flow. For each mechanism, we identify the main factors that determine speed and directionality. We also highlight the efficiency of each transport mode in fulfilling various key objectives of transport, such as particle mixing, directed delivery, and rapid target search. Taken together, the interplay of diffusion, molecular motors, and flows supports the intracellular transport needs that underlie a broad variety of biological phenomena.

Establishing and regulating the composition of cilia for signal transduction

The primary cilium is a hair-like surface-exposed organelle of the eukaryotic cell that decodes a variety of signals - such as odorants, light and Hedgehog morphogens - by altering the local concentrations and activities of signalling proteins. Signalling within the cilium is conveyed through a diverse array of second messengers, including conventional signalling molecules (such as cAMP) and some unusual intermediates (such as sterols). Diffusion barriers at the ciliary base establish the unique composition of this signalling compartment, and cilia adapt their proteome to signalling demands through regulated protein trafficking. Much progress has been made on the molecular understanding of regulated ciliary trafficking, which encompasses not only exchanges between the cilium and the rest of the cell but also the shedding of signalling factors into extracellular vesicles.

An intrinsic compartmentalization code for peripheral membrane proteins in photoreceptor neurons

In neurons, peripheral membrane proteins are enriched in subcellular compartments, where they play key roles, including transducing and transmitting information. However, little is known about the mechanisms underlying their compartmentalization. To explore the roles of hydrophobic and electrostatic interactions, we engineered probes consisting of lipidation motifs attached to fluorescent proteins by variously charged linkers and expressed them in Xenopus rod photoreceptors. Quantitative live cell imaging showed dramatic differences in distributions and dynamics of the probes, including presynapse and ciliary OS enrichment, depending on lipid moiety and protein surface charge. Opposing extant models of ciliary enrichment, most probes were weakly membrane bound and diffused through the connecting cilium without lipid binding chaperone protein interactions. A diffusion-binding-transport model showed that ciliary enrichment of a rhodopsin kinase probe occurs via recycling as it perpetually leaks out of the ciliary OS. The model accounts for weak membrane binding of peripheral membrane proteins and a leaky connecting cilium diffusion barrier.

Second messenger molecules have a limited spread in olfactory cilia

Olfactory responses in the cilia of olfactory receptor cells last for longer than 10 s, which cannot be explained by free diffusion of second messengers. Takeuchi and Kurahashi show that these signaling molecules have a limited spread and remain at the site of generation for a long time.

Odorants are detected by olfactory receptors on the sensory cilia of olfactory receptor cells (ORCs). These cylindrical cilia have a diameters of 100–200 nm, within which the components required for signal transduction by the adenylyl cyclase–cAMP system are located. The kinetics of odorant responses are determined by the lifetimes of active proteins as well as the production, diffusion, and extrusion/degradation of second messenger molecules (cAMP and Ca²⁺). However, there is limited information about the molecular kinetics of ORC responses, mostly because of the technical limitations involved in studying such narrow spaces and fine structures. In this study, using a combination of electrophysiology, photolysis of caged substances, and spot UV laser stimulation, we show that second messenger molecules work only in the vicinity of their site of generation in the olfactory cilia. Such limited spreading clearly explains a unique feature of ORCs, namely, the integer multiple of unitary events that they display in low Ca²⁺ conditions. Although the small ORC uses cAMP and Ca²⁺ for various functions in different regions of the cell, these substances seem to operate only in the compartment that has been activated by the appropriate stimulus. We also show that these substances remain in the same vicinity for a long time. This enables the ORC to amplify the odorant signal and extend the lifetime of Ca²⁺-dependent adaptation. Cytoplasmic buffers and extrusion/degradation systems seem to play a crucial role in limiting molecular spreading. In addition, binding sites on the cytoplasmic surface of the plasma membrane may limit molecular diffusion in such a narrow space because of the high surface/volume ratio. Such efficient energy conversion may also be broadly used in other biological systems that have not yet been subjected to systematic experiments.

Actin filaments partition primary cilia membranes into distinct fluid corrals

Lee et al. examine the dynamics of membrane proteins within the ciliary membrane using quantum dots and 2P Super FRAP. They show that ciliary membrane proteins diffuse rapidly within highly fluid local membrane domains delimited by actin filaments.

Physical properties of primary cilia membranes in living cells were examined using two independent, high-spatiotemporal-resolution approaches: fast tracking of single quantum dot–labeled G protein–coupled receptors and a novel two-photon super-resolution fluorescence recovery after photobleaching of protein ensemble. Both approaches demonstrated the cilium membrane to be partitioned into corralled domains spanning 274 ± 20 nm, within which the receptors are transiently confined for 0.71 ± 0.09 s. The mean membrane diffusion coefficient within the corrals, D_m1 = 2.9 ± 0.41 µm²/s, showed that the ciliary membranes were among the most fluid encountered. At longer times, the apparent membrane diffusion coefficient, D_m2 = 0.23 ± 0.05 µm²/s, showed that corral boundaries impeded receptor diffusion 13-fold. Mathematical simulations predict the probability of G protein–coupled receptors crossing corral boundaries to be 1 in 472. Remarkably, latrunculin A, cytochalasin D, and jasplakinolide treatments altered the corral permeability. Ciliary membranes are thus partitioned into highly fluid membrane nanodomains that are delimited by filamentous actin.

Ift172 conditional knock-out mice exhibit rapid retinal degeneration and protein trafficking defects

Intraflagellar transport (IFT) is a bidirectional transport process that occurs along primary cilia and specialized sensory cilia, such as photoreceptor outersegments. Genes coding for various IFT components are associated with ciliopathies. Mutations in IFT172 lead to diseases ranging from isolated retinal degeneration to severe syndromic ciliopathies. In this study, we created a mouse model of IFT172-associated retinal degeneration to investigate the ocular disease mechanism. We found that depletion of IFT172 in rod photoreceptors leads to a rapid degeneration of the retina, with severely reduced electroretinography (ERG) responses by 1 month and complete outer-nuclear layer (ONL) degeneration by 2 months. We investigated molecular mechanisms of degeneration and show that IFT172 protein reduction leads to mislocalization of specific photoreceptor outersegment (OS) proteins (RHO, RP1, IFT139), aberrant light-driven translocation of alpha transducin and altered localization of glioma-associated oncogene family member 1 (GLI1). This mouse model exhibits key features of the retinal phenotype observed in patients with IFT172-associated blindness and can be used for in vivo testing of ciliopathy therapies.

How the Ciliary Membrane Is Organized Inside-Out to Communicate Outside-In

Cilia, organelles that move to execute functions like fertilization and signal to execute functions like photoreception and embryonic patterning, are composed of a core of nine-fold doublet microtubules overlain by a membrane. Distinct types of cilia display distinct membrane morphologies, ranging from simple domed cylinders to the highly ornate invaginations and membrane disks of photoreceptor outer segments. Critical for the ability of cilia to signal, both the protein and the lipid compositions of ciliary membranes are different from those of other cellular membranes. This specialization presents a unique challenge for the cell as, unlike membrane-bounded organelles, the ciliary membrane is contiguous with the surrounding plasma membrane. This distinct ciliary membrane is generated in concert with multiple membrane remodeling events that comprise the process of ciliogenesis. Once the cilium is formed, control of ciliary membrane composition relies on discrete molecular machines, including a barrier to membrane proteins entering the cilium at a specialized region of the base of the cilium called the transition zone and a trafficking adaptor that controls G protein-coupled receptor (GPCR) localization to the cilium called the BBSome. The ciliary membrane can be further remodeled by the removal of membrane proteins by the release of ciliary extracellular vesicles that may function in intercellular communication, removal of unneeded proteins or ciliary disassembly. Here, we review the structures and transport mechanisms that control ciliary membrane composition, and discuss how membrane specialization enables the cilium to function as the antenna of the cell.

Photoreceptor outer segment as a sink for membrane proteins: hypothesis and implications in retinal ciliopathies

The photoreceptor outer segment (OS) is a unique modification of the primary cilium, specialized for light perception. Being homologous organelles, the primary cilium and the OS share common building blocks and molecular machinery to construct and maintain them. The OS, however, has several unique structural features that are not seen in primary cilia. Although these unique features of the OS have been well documented, their implications in protein localization have been under-appreciated. In this review, we compare the structural properties of the primary cilium and the OS, and propose a hypothesis that the OS can act as a sink for membrane proteins. We further discuss the implications of this hypothesis in polarized protein localization in photoreceptors and mechanisms of photoreceptor degeneration in retinal ciliopathies.

Photoreceptor Cilia and Retinal Ciliopathies

Photoreceptors are sensory neurons designed to convert light stimuli into neurological responses. This process, called phototransduction, takes place in the outer segments (OS) of rod and cone photoreceptors. OS are specialized sensory cilia, with analogous structures to those present in other nonmotile cilia. Deficient morphogenesis and/or dysfunction of photoreceptor sensory cilia (PSC) caused by mutations in a variety of photoreceptor-specific and common cilia genes can lead to inherited retinal degenerations (IRDs). IRDs can manifest as isolated retinal diseases or syndromic diseases. In this review, we describe the structure and composition of PSC and different forms of ciliopathies with retinal involvement. We review the genetics of the IRDs, which are monogenic disorders but genetically diverse with regard to causality.

Simulation of intra-ciliary diffusion suggests a novel role of primary cilia as a cell-signaling enhancer

Besides the role to generate a fluid flow in the surrounding medium, eukaryotic cilia have a crucial function in sensing external signals such as chemical or mechanical stimuli. A large body of work has shown that cilia are frequently found in various types of sensory cells and are closely related to many regulatory mechanisms in differentiation and development. However, we do not yet have a definitive answer to the fundamental question, "why cilia?" It has been a long-standing mystery why cells use cilia for sensing external signals. To shed light on this, we sought to describe the kinetics of signaling with theoretical approaches. Based on the results, here we propose a new role of cilia as a cell-signaling enhancer. The enhancing effect comes from restricted volume for the free intra-ciliary diffusion of molecules due to the cylindrical shape of cilia, which can facilitate quick accumulation of intracellular signaling molecules. Our simulations demonstrate that both the rate and amplitude of response in signal transduction depend on where the membrane receptors or channels are located along the ciliary shaft. In addition, the calculated transfer function of cilia regarded as a transmitter of external signals also suggests the properties of cilia as a signal enhancer. Since such unique composition of receptors and channels in cilia is found in various types of eukaryotic cells, signal enhancing is presumably one of the most essential and conserved roles of cilia.

Cilia - The sensory antennae in the eye

Cilia are hair-like projections found on almost all cells in the human body. Originally believed to function merely in motility, the function of solitary non-motile (primary) cilia was long overlooked. Recent research has demonstrated that primary cilia function as signalling hubs that sense environmental cues and are pivotal for organ development and function, tissue hoemoestasis, and maintenance of human health. Cilia share a common anatomy and their diverse functional features are achieved by evolutionarily conserved functional modules, organized into sub-compartments. Defects in these functional modules are responsible for a rapidly growing list of human diseases collectively termed ciliopathies. Ocular pathogenesis is common in virtually all classes of syndromic ciliopathies, and disruptions in cilia genes have been found to be causative in a growing number of non-syndromic retinal dystrophies. This review will address what is currently known about cilia contribution to visual function. We will focus on the molecular and cellular functions of ciliary proteins and their role in the photoreceptor sensory cilia and their visual phenotypes. We also highlight other ciliated cell types in tissues of the eye (e.g. lens, RPE and Müller glia cells) discussing their possible contribution to disease progression. Progress in basic research on the cilia function in the eye is paving the way for therapeutic options for retinal ciliopathies. In the final section we describe the latest advancements in gene therapy, read-through of non-sense mutations and stem cell therapy, all being adopted to treat cilia dysfunction in the retina.

Cos2/Kif7 and Osm-3/Kif17 regulate onset of outer segment development in zebrafish photoreceptors through distinct mechanisms

Zebrafish morphants of osm-3/kif17, a kinesin-2 family member and intraflagellar transport motor, have photoreceptor outer segments that are dramatically reduced in number and size. However, two genetic mutant lines, osm-3/kif17^sa0119 and osm-3/kif17^sa18340, reportedly lack any observable morphological outer segment defects. In this work, we use TALENs to generate an independent allele, osm-3/kif17^mw405, and show that both osm-3/kif17^sa0119 and osm-3/kif17^mw405 have an outer segment developmental delay in both size and density that is fully recovered by 6 days post-fertilization. Additionally, we use CRISPRs to generate cos2/kif7^mw406, a mutation in the kinesin-4 family member cos2/kif7 that has been implicated in controlling ciliary architecture and Hedgehog signaling to test whether it may be functioning redundantly with osm-3/kif17. We show that cos2/kif7^mw406 has an outer segment developmental delay similar to the osm-3/kif17 mutants. Using a three-dimensional mathematical model of outer segments, we show that while cos2/kif7^mw406 and osm-3/kif17^mw405 outer segments are smaller throughout the first 6 days of development, the volumetric rates of outer segment morphogenesis are not different among wild-type, cos2/kif7^mw406, and osm-3/kif17^mw405 after 60hpf. Instead, our model suggests that cos2/kif7^mw406 and osm-3/kif17^mw405 impact outer segment morphogenesis through upstream events that that are different for each motor. In the case of cos2/kif7^mw406 mutants, we show that early defects in Hedgehog signaling lead to a general, non-photoreceptor-specific delay of retinal neurogenesis, which in turn causes the secondary phenotype of delayed outer segment morphogenesis. In contrast, the osm-3/kif17^mw405 outer segment morphogenesis delays are linked specifically to initial disc morphogenesis of photoreceptors rather than an upstream event. Further, we show that osm-3/kif17 mutant mice also exhibit a similarly delayed outer segment development, suggesting a role for osm-3/kif17 in early outer segment development that is conserved across species. In conclusion, we show that both osm-3/kif17 and cos2/kif7 have comparable outer segment developmental delays, although through independent mechanisms.

Untangling ciliary access and enrichment of two rhodopsin-like receptors using quantitative fluorescence microscopy reveals cell-specific sorting pathways

Quantitative microscopy shows that protein-sorting signals have opposite effects on ciliary enrichment of G protein–coupled receptors in different cell types, revealing distinct ciliary trafficking mechanisms among ciliated cells.

Resolution limitations of optical systems are major obstacles for determining whether proteins are enriched within cell compartments. Here we use an approach to determine the degree of membrane protein ciliary enrichment that quantitatively accounts for the differences in sampling of the ciliary and apical membranes inherent to confocal microscopes. Theory shows that cilia will appear more than threefold brighter than the surrounding apical membrane when the densities of fluorescently labeled proteins are the same, thus providing a benchmark for ciliary enrichment. Using this benchmark, we examined the ciliary enrichment signals of two G protein–coupled receptors (GPCRs)—the somatostatin receptor 3 and rhodopsin. Remarkably, we found that the C-terminal VxPx motif, required for efficient enrichment of rhodopsin within rod photoreceptor sensory cilia, inhibited enrichment of the somatostatin receptor in primary cilia. Similarly, VxPx inhibited primary cilium enrichment of a chimera of rhodopsin and somatostatin receptor 3, where the dual Ax(S/A)xQ ciliary targeting motifs within the third intracellular loop of the somatostatin receptor replaced the third intracellular loop of rhodopsin. Rhodopsin was depleted from primary cilia but gained access, without being enriched, with the dual Ax(S/A)xQ motifs. Ciliary enrichment of these GPCRs thus operates via distinct mechanisms in different cells.

Applications of phototransformable fluorescent proteins for tracking the dynamics of cellular components

In the past few decades, fluorescent proteins have revolutionized the field of cell biology. Phototransformable fluorescent proteins are capable of changing their excitation and emission spectra after being exposed to specific wavelength(s) of light. The majority of phototransformable fluorescent proteins have originated from marine organisms. Genetic engineering of these proteins has made available many choices for different colors, modes of conversion, and other biophysical properties. Their phototransformative property has allowed the highlighting and tracking of subpopulations of cells, organelles, and proteins in living systems. Furthermore, phototransformable fluorescent proteins have offered new methods for superresolution fluorescence microscopy and optogenetics manipulation of proteins. One of the major advantages of phototransformable fluorescent proteins is their applicability for visualizing newly synthesized proteins that are en route to their final destinations. In this paper, we will discuss the biological applications of phototransformable fluorescent proteins with special emphasis on the application of tracking membrane proteins in vertebrate photoreceptor cells.

Molecular basis for photoreceptor outer segment architecture

To serve vision, vertebrate rod and cone photoreceptors must detect photons, convert the light stimuli into cellular signals, and then convey the encoded information to downstream neurons. Rods and cones are sensory neurons that each rely on specialized ciliary organelles to detect light. These organelles, called outer segments, possess elaborate architectures that include many hundreds of light-sensitive membranous disks arrayed one atop another in precise register. These stacked disks capture light and initiate the chain of molecular and cellular events that underlie normal vision. Outer segment organization is challenged by an inherently dynamic nature; these organelles are subject to a renewal process that replaces a significant fraction of their disks (up to ∼10%) on a daily basis. In addition, a broad range of environmental and genetic insults can disrupt outer segment morphology to impair photoreceptor function and viability. In this chapter, we survey the major progress that has been made for understanding the molecular basis of outer segment architecture. We also discuss key aspects of organelle lipid and protein composition, and highlight distributions, interactions, and potential structural functions of key OS-resident molecules, including: kinesin-2, actin, RP1, prominin-1, protocadherin 21, peripherin-2/rds, rom-1, glutamic acid-rich proteins, and rhodopsin. Finally, we identify key knowledge gaps and challenges that remain for understanding how normal outer segment architecture is established and maintained.

Gated entry into the ciliary compartment

Cilia and flagella play important roles in cell motility and cell signaling. These functions require that the cilium establishes and maintains a unique lipid and protein composition. Recent work indicates that a specialized region at the base of the cilium, the transition zone, serves as both a barrier to entry and a gate for passage of select components. For at least some cytosolic proteins, the barrier and gate functions are provided by a ciliary pore complex (CPC) that shares molecular and mechanistic properties with nuclear gating. Specifically, nucleoporins of the CPC limit the diffusional entry of cytosolic proteins in a size-dependent manner and enable the active transport of large molecules and complexes via targeting signals, importins, and the small G protein Ran. For membrane proteins, the septin protein SEPT2 is part of the barrier to entry whereas the gating function is carried out and/or regulated by proteins associated with ciliary diseases (ciliopathies) such as nephronophthisis, Meckel–Gruber syndrome and Joubert syndrome. Here, we discuss the evidence behind these models of ciliary gating as well as the similarities to and differences from nuclear gating.

Measurements of rhodopsin diffusion within signaling membrane microcompartments in live photoreceptors

High-resolution multiphoton imaging of live cells has become an invaluable method to study protein dynamics in highly compartmentalized subcellular environments. Here we describe procedures that we recently developed to quantify rhodopsin mobility within and between retinal rod photoreceptor light signaling microcompartments, the disc membrane lobules, using multiphoton fluorescence relaxation after photoconversion.

Photoreceptor Sensory Cilium: Traversing the Ciliary Gate

Cilia are antenna-like extensions of the plasma membrane found in nearly all cell types. In the retina of the eye, photoreceptors develop unique sensory cilia. Not much was known about the mechanisms underlying the formation and function of photoreceptor cilia, largely because of technical limitations and the specific structural and functional modifications that cannot be modeled in vitro. With recent advances in microscopy techniques and molecular and biochemical approaches, we are now beginning to understand the molecular basis of photoreceptor ciliary architecture, ciliary function and its involvement in human diseases. Here, I will discuss the studies that have revealed new knowledge of how photoreceptor cilia regulate their identity and function while coping with high metabolic and trafficking demands associated with processing light signal.

Ablation of retinal ciliopathy protein RPGR results in altered photoreceptor ciliary composition

Cilia regulate several developmental and homeostatic pathways that are critical to survival. Sensory cilia of photoreceptors regulate phototransduction cascade for visual processing. Mutations in the ciliary protein RPGR (retinitis pigmentosa GTPase regulator) are a prominent cause of severe blindness disorders due to degeneration of mature photoreceptors. However, precise function of RPGR is still unclear. Here we studied the involvement of RPGR in ciliary trafficking by analyzing the composition of photoreceptor sensory cilia (PSC) in Rpgr^ko retina. Using tandem mass spectrometry analysis followed by immunoblotting, we detected few alterations in levels of proteins involved in proteasomal function and vesicular trafficking in Rpgr^ko PSC, prior to onset of degeneration. We also found alterations in the levels of high molecular weight soluble proteins in Rpgr^ko PSC. Our data indicate RPGR regulates entry or retention of soluble proteins in photoreceptor cilia but spares the trafficking of key structural and phototransduction-associated proteins. Given a frequent occurrence of RPGR mutations in severe photoreceptor degeneration due to ciliary disorders, our results provide insights into pathways resulting in altered mature cilia function in ciliopathies.

Analysis of soluble protein entry into primary cilia using semipermeabilized cells

The primary cilium is a protrusion from the cell surface that serves as a specialized compartment for signal transduction. Many signaling factors are known to be dynamically concentrated within cilia and to require cilia for their function. Yet protein entry into primary cilia remains poorly understood. To enable a mechanistic analysis of soluble protein entry into cilia, we developed a method for semipermeabilization of mammalian cells in which the plasma membrane is permeabilized while the ciliary membrane remains intact. Using semipermeabilized cells as the basis for an in vitro diffusion-to-capture assay, we uncovered a size-dependent diffusion barrier that restricts soluble protein exchange between the cytosol and the cilium. The manipulability of this in vitro system enabled an extensive characterization of the ciliary diffusion barrier and led us to show that the barrier is mechanistically distinct from those at the axon initial segment and the nuclear pore complex. Because semipermeabilized cells enable a range of experimental perturbations that would not be easily feasible in intact cells, we believe this methodology will provide a unique resource for investigating primary cilium function in development and disease.

The role of primary cilia in the development and disease of the retina

The normal development and function of photoreceptors is essential for eye health and visual acuity in vertebrates. Mutations in genes encoding proteins involved in photoreceptor development and function are associated with a suite of inherited retinal dystrophies, often as part of complex multi-organ syndromic conditions. In this review, we focus on the role of the photoreceptor outer segment, a highly modified and specialized primary cilium, in retinal health and disease. We discuss the many defects in the structure and function of the photoreceptor primary cilium that can cause a class of inherited conditions known as ciliopathies, often characterized by retinal dystrophy and degeneration, and highlight the recent insights into disease mechanisms.

Mitochondria contribute to NADPH generation in mouse rod photoreceptors

NADPH is the primary source of reducing equivalents in the cytosol. Its major source is considered to be the pentose phosphate pathway, but cytosolic NADP(+)-dependent dehydrogenases using intermediates of mitochondrial pathways for substrates have been known to contribute. Photoreceptors, a nonproliferating cell type, provide a unique model for measuring the functional utilization of NADPH at the single cell level. In these cells, NADPH availability can be monitored from the reduction of the all-trans-retinal generated by light to all-trans-retinol using single cell fluorescence imaging. We have used mouse rod photoreceptors to investigate the generation of NADPH by different metabolic pathways. In the absence of extracellular metabolic substrates, NADPH generation was severely compromised. Extracellular glutamine supported NADPH generation to levels comparable to those of glucose, but pyruvate and lactate were relatively ineffective. At low extracellular substrate concentrations, partial inhibition of ATP synthesis lowered, whereas suppression of ATP consumption augmented NADPH availability. Blocking pyruvate transport into mitochondria decreased NADPH availability, and addition of glutamine restored it. Our findings demonstrate that in a nonproliferating cell type, mitochondria-linked pathways can generate substantial amounts of NADPH and do so even when the pentose phosphate pathway is operational. Competing demands for ATP and NADPH at low metabolic substrate concentrations indicate a vulnerability to nutrient shortages. By supporting substantial NADPH generation, mitochondria provide alternative metabolic pathways that may support cell function and maintain viability under transient nutrient shortages. Such pathways may play an important role in protecting against retinal degeneration.

Measurement of three-dimensional anisotropic diffusion by multiphoton fluorescence recovery after photobleaching

The multiphoton fluorescence recovery after photobleaching (MP-FRAP) technique has been developed to measure the three-dimensional (3D) solute diffusion within biological systems. However, current 3D MP-FRAP models are based on isotropic diffusion and spatial domain analysis. The 3D anisotropic diffusion and frequency domain analysis for MP-FRAP measurements are rarely studied. In this study, a new technique is demonstrated for the quantitative and non-destructive determination of 3D anisotropic solute diffusion tensors within biological fibrosis tissues by multiphoton photobleaching and spatial Fourier analysis (SFA). Compared to the spatial domain analysis based MP-FRAP techniques, this SFA-based method has the capability for determining the 3D anisotropic diffusion tensors as well as the flexibility for satisfying initial and boundary conditions. First, a close-form solution of the 3D anisotropic diffusion equation is derived by solely using SFA. Next, this new method is validated by computer-simulated MP-FRAP experiments with pre-defined 3D anisotropic diffusion tensors as well as experimental diffusion measurements of FITC-Dextran (FD) molecules in aqueous glycerol solutions. Finally, this MP-FRAP technique is applied to the measurement of 3D anisotropic diffusion tensors of FD molecules within porcine tendon tissues. This study provides a new tool for complete determination of 3D anisotropic solute diffusion tensor in biological tissues.

Regulation of rhodopsin-eGFP distribution in transgenic xenopus rod outer segments by light

The rod outer segment (OS), comprised of tightly stacked disk membranes packed with rhodopsin, is in a dynamic equilibrium governed by a diurnal rhythm with newly synthesized membrane inserted at the OS base balancing membrane loss from the distal tip via disk shedding. Using transgenic Xenopus and live cell confocal imaging, we found OS axial variation of fluorescence intensity in cells expressing a fluorescently tagged rhodopsin transgene. There was a light synchronized fluctuation in intensity, with higher intensity in disks formed at night and lower intensity for those formed during the day. This fluctuation was absent in constant light or dark conditions. There was also a slow modulation of the overall expression level that was not synchronized with the lighting cycle or between cells in the same retina. The axial variations of other membrane-associated fluorescent proteins, eGFP-containing two geranylgeranyl acceptor sites and eGFP fused to the transmembrane domain of syntaxin, were greatly reduced or not detectable, respectively. In acutely light-adapted rods, an arrestin-eGFP fusion protein also exhibited axial variation. Both the light-sensitive Rho-eGFP and arrestin-eGFP banding were in phase with the previously characterized birefringence banding (Kaplan, Invest. Ophthalmol. Vis. Sci. 21, 395–402 1981). In contrast, endogenous rhodopsin did not exhibit such axial variation. Thus, there is an axial inhomogeneity in membrane composition or structure, detectable by the rhodopsin transgene density distribution and regulated by the light cycle, implying a light-regulated step for disk assembly in the OS. The impact of these results on the use of chimeric proteins with rhodopsin fused to fluorescent proteins at the carboxyl terminus is discussed.

Molecular complexes that direct rhodopsin transport to primary cilia

Rhodopsin is a key molecular constituent of photoreceptor cells, yet understanding of how it regulates photoreceptor membrane trafficking and biogenesis of light-sensing organelles, the rod outer segments (ROS) is only beginning to emerge. Recently identified sequence of well-orchestrated molecular interactions of rhodopsin with the functional networks of Arf and Rab GTPases at multiple stages of intracellular targeting fits well into the complex framework of the biogenesis and maintenance of primary cilia, of which the ROS is one example. This review will discuss the latest progress in dissecting the molecular complexes that coordinate rhodopsin incorporation into ciliary-targeted carriers with the recruitment and activation of membrane tethering complexes and regulators of fusion with the periciliary plasma membrane. In addition to revealing the fundamental principals of ciliary membrane renewal, recent advances also provide molecular insight into the ways by which disruptions of the exquisitely orchestrated interactions lead to cilia dysfunction and result in human retinal dystrophies and syndromic diseases that affect multiple organs, including the eyes.

An in vitro assay for entry into cilia reveals unique properties of the soluble diffusion barrier

The ciliary permeability barrier is mechanistically distinct from other cellular diffusion barriers and allows soluble proteins under ∼100 kD in size to enter cilia in the absence of active transport.

Specific proteins are concentrated within primary cilia, whereas others remain excluded. To understand the mechanistic basis of entry into cilia, we developed an in vitro assay using cells in which the plasma membrane was permeabilized, but the ciliary membrane was left intact. Using a diffusion-to-capture system and quantitative analysis, we find that proteins >9 nm in diameter (∼100 kD) are restricted from entering cilia, and we confirm these findings in vivo. Interference with the nuclear pore complex (NPC) or the actin cytoskeleton in permeabilized cells demonstrated that the ciliary diffusion barrier is mechanistically distinct from those of the NPC or the axon initial segment. Moreover, applying a mass transport model to this system revealed diffusion coefficients for soluble and membrane proteins within cilia that are compatible with rapid exploration of the ciliary space in the absence of active transport. Our results indicate that large proteins require active transport for entry into cilia but not necessarily for movement inside cilia.

Molecular connections between nuclear and ciliary import processes

As an organelle, the cilium contains a unique complement of protein and lipid. Recent work has begun to shed light on the mechanisms that regulate entry of ciliary proteins into the compartment. Here, we focus on the mechanisms that regulate ciliary entry of cytosolic molecules. Studies have revealed a size exclusion mechanism for ciliary entry that is similar to the barrier to nuclear entry. Active import into the ciliary compartment involves nuclear trafficking components including importins, a Ran-guanosine triphosphate gradient, and nucleoporins. Together, this work indicates that nuclei and cilia share molecular, structural and mechanistic components that regulate import into the compartments.

Light-dependent phosphorylation of Bardet-Biedl syndrome 5 in photoreceptor cells modulates its interaction with arrestin1

Arrestins are dynamic proteins that move between cell compartments triggered by stimulation of G-protein-coupled receptors. Even more dynamically in vertebrate photoreceptors, arrestin1 (Arr1) moves between the inner and outer segments according to the light conditions. Previous studies have shown that the light-driven translocation of Arr1 in rod photoreceptors is initiated by rhodopsin through a phospholipase C/protein kinase C (PKC) signaling cascade. The purpose of this study is to identify the PKC substrate that regulates the translocation of Arr1. Mass spectrometry was used to identify the primary phosphorylated proteins in extracts prepared from PKC-stimulated mouse eye cups, confirming the finding with in vitro phosphorylation assays. Our results show that Bardet-Biedl syndrome 5 (BBS5) is the principal protein phosphorylated either by phorbol ester stimulation or by light stimulation of PKC. Via immunoprecipitation of BBS5 in rod outer segments, Arr1 was pulled down; phosphorylation of BBS5 reduced this co-precipitation of Arr1. Immunofluorescence and immunoelectron microscopy showed that BBS5 principally localizes along the axonemes of rods and cones, but also in photoreceptor inner segments, and synaptic regions. Our principal findings in this study are threefold. First, we demonstrate that BBS5 is post-translationally regulated by phosphorylation via PKC, an event that is triggered by light in photoreceptor cells. Second, we find a direct interaction between BBS5 and Arr1, an interaction that is modulated by phosphorylation of BBS5. Finally, we show that BBS5 is distributed along the photoreceptor axoneme, co-localizing with Arr1 in the dark. These findings suggest a role for BBS5 in regulating light-dependent translocation of Arr1 and a model describing its role in Arr1 translocation is proposed.

Protein transport in the connecting cilium of a photoreceptor cell: modeling the effects of bidirectional protein transitions between the diffusion-driven and motor-driven kinetic states

Physics of protein transport through the connecting cilium (CC) of a photoreceptor cell is a long-standing question in cellular biology. There is evidence implicating both molecular motor-driven and diffusion-driven modes of intracellular transport. Based on available experimental clues, this paper develops a new model for intraflagellar transport (IFT) of proteins synthesized in the inner segment and transported through the CC to the outer segment of a photoreceptor cell. The model accounts for the competition between two modes of protein transport: molecular motor-driven transport and diffusion. The obtained solutions made it possible to calculate how the number of protein molecules transported through the CC at a given time depends on their diffusivity. Modeling results were compared with published experimental estimates, and conclusions about possible contributions of diffusion to IFT were made.

Protein sorting, targeting and trafficking in photoreceptor cells

Vision is the most fundamental of our senses initiated when photons are absorbed by the rod and cone photoreceptor neurons of the retina. At the distal end of each photoreceptor resides a light-sensing organelle, called the outer segment, which is a modified primary cilium highly enriched with proteins involved in visual signal transduction. At the proximal end, each photoreceptor has a synaptic terminal, which connects this cell to the downstream neurons for further processing of the visual information. Understanding the mechanisms involved in creating and maintaining functional compartmentalization of photoreceptor cells remains among the most fascinating topics in ocular cell biology. This review will discuss how photoreceptor compartmentalization is supported by protein sorting, targeting and trafficking, with an emphasis on the best-studied cases of outer segment-resident proteins.

Three-dimensional architecture of the rod sensory cilium and its disruption in retinal neurodegeneration

Defects in primary cilia lead to devastating disease because of their roles in sensation and developmental signaling but much is unknown about ciliary structure and mechanisms of their formation and maintenance. We used cryo-electron tomography to obtain 3D maps of the connecting cilium and adjacent cellular structures of a modified primary cilium, the rod outer segment, from wild-type and genetically defective mice. The results reveal the molecular architecture of the cilium and provide insights into protein functions. They suggest that the ciliary rootlet is involved in cellular transport and stabilizes the axoneme. A defect in the BBSome membrane coat caused defects in vesicle targeting near the base of the cilium. Loss of the proteins encoded by the Cngb1 gene disrupted links between the disk and plasma membranes. The structures of the outer segment membranes support a model for disk morphogenesis in which basal disks are enveloped by the plasma membrane.

Modeling of Transient Transport of Soluble Proteins in the Connecting Cilium of a Photoreceptor Cell

A minimal mathematical model describing mass transport in the connecting cilium (CC) of a photoreceptor cell in response to a suddenly increased protein concentration at the base of the CC is developed. Dimensionless governing equations and dimensionless parameters are identified. Analytical solutions are obtained for concentrations of free (diffusion-driven) and motor-driven proteins. The obtained solutions make it possible to predict mass transfer in the CC as a function of two dimensionless transport parameters involved in the model: the diffusivity of free soluble proteins and the transition rate from the diffusion-driven to the motor-driven state. Sensitivities of the obtained solutions to these two parameters are discussed.

The role of arrestins in visual and disease processes of the eye

Visual arrestins are well known for their function in quenching the phototransduction process in rods and cones. Perhaps not as well known is their participation in multiple other processes in the normal and disease states of the eye. This chapter covers the range of the known functions of the visual arrestins, beginning with their classical role in quenching light-activated visual pigments. The role of visual arrestins is also reviewed from the perspective of their dynamic mobility whereby they redistribute significantly between the compartments of highly polarized photoreceptor cells. Additional roles of the visual arrestins are also reviewed based on new interacting partners that have been discovered over the past decade. Finally, the contribution of the visual arrestins to diseases of the visual system is explored.

Analysis of ciliary import

Cilia and flagella are microtubule-based organelles that play important roles in human health by contributing to cellular motility as well as sensing and responding to environmental cues. Defects in cilia formation and function cause a broad class of human genetic diseases called ciliopathies. To carry out their specialized functions, cilia contain a unique complement of proteins that must be imported into the ciliary compartment. In this chapter, we describe methods to measure the permeability barrier of the ciliary gate by microinjection of fluorescent proteins and dextrans of different sizes into ciliated cells. We also describe a fluorescence recovery after photobleaching assay to measure the entry of ciliary proteins into the ciliary compartment. These assays can be used to determine the molecular mechanisms that regulate the formation and function of cilia in mammalian cells.

Generation of transgenic Xenopus using restriction enzyme-mediated integration

Transgenesis, the process of incorporating an exogenous gene (transgene) into an organism's genome, is a widely used tool to develop models of human diseases and to study the function and/or regulation of genes. Generating transgenic Xenopus is rapid and involves simple in vitro manipulations, taking advantage of the large size of the amphibian egg and external embryonic development. Restriction enzyme-mediated integration (REMI) has a number of advantages for transgenesis compared to other methods used to produce transgenic Xenopus, including relative efficiency, higher transgene expression levels, fewer genetic chimera in founder transgenic animals, and near-complete germ-line transgene transmission. This chapter explains the REMI method for generating transgenic Xenopus laevis tadpoles, including improvements developed to enable studies in the mature retina.

Light-dependent changes in outer retinal water diffusion in rats in vivo

PURPOSE

To test the hypothesis that in rats, intraretinal light-dependent changes on diffusion-weighted magnetic resonance imaging (MRI) in vivo are consistent with known retinal layer-specific physiology.

METHODS

In male Sprague-Dawley rats, retinal morphology (thickness, extent, surface area, volume) and intraretinal profiles of the apparent diffusion coefficient (ADC, i.e., water mobility) parallel and perpendicular to the optic nerve were measured in vivo using quantitative MRI methods during light and dark stimulation.

RESULTS

The parallel ADC in the posterior half of the avascular, photoreceptor-dominated outer retina was significantly higher in light than dark, and this pattern was reversed (dark>light) in the anterior outer retina. The perpendicular ADC in the posterior outer retina was similar in light and dark, but was significantly higher in dark than light in the anterior outer retina. No light-dark changes in the inner retina were noted.

CONCLUSIONS

We identified light-dependent intraretinal diffusion changes that reflected established stimulation-based changes in outer retinal hydration. These findings are expected to motivate future applications of functional diffusion-based MRI in blinding disorders of the outer retina.

Scoring a backstage pass: mechanisms of ciliogenesis and ciliary access

Cilia are conserved, microtubule-based cell surface projections that emanate from basal bodies, membrane-docked centrioles. The beating of motile cilia and flagella enables cells to swim and epithelia to displace fluids. In contrast, most primary cilia do not beat but instead detect environmental or intercellular stimuli. Inborn defects in both kinds of cilia cause human ciliopathies, diseases with diverse manifestations such as heterotaxia and kidney cysts. These diseases are caused by defects in ciliogenesis or ciliary function. The signaling functions of cilia require regulation of ciliary composition, which depends on the control of protein traffic into and out of cilia.

Ciliary signaling cascades in photoreceptors

For being a polarized neuron and having a sensory cilium, photoreceptors attract remarkable attention. This is due their highly polarized structure and active visual signal transduction cascades and for the enrichment of complex networks of proteins in the cilium. Structural and functional maintenance of the photoreceptor sensory cilium, also called outer segment, ensures that light signal is received and relayed appropriately to the brain. Any perturbations in the protein content of the outer segment result in photoreceptor dysfunction, degeneration and eventually, blindness. This review focuses on the importance of photoreceptor sensory cilium to carry out signal transduction cascade for vision.

Transport and localization of signaling proteins in ciliated cells

Most cells in the human body elaborate cilia which serve a wide variety of functions, including cell and tissue differentiation during development, sensing physical and chemical properties of the extracellular milieu and mechanical force generation. Common among cilia is the transduction of external stimuli into signals that regulate the activities of the cilia and the cells that possess them. These functions require the transport and localization of specialized proteins to the cilium, a process that many recent studies have shown to be vital for normal cell function and, ultimately, the health of the organism. Here we discuss several mechanisms proposed for the transport and localization of soluble and peripheral membrane proteins to, or their exclusion from the ciliary compartment with a focus on how the structure of the cytoplasm and the size and shape of proteins influence these processes. Additionally, we examine the impact of cell and protein structure on our ability to accurately measure the relative concentrations of fluorescently tagged proteins amongst various cellular domains, which is integral to our understanding of the molecular mechanisms underlying protein localization and transport.

Impact of signaling microcompartment geometry on GPCR dynamics in live retinal photoreceptors

G protein–coupled receptor (GPCR) cascades rely on membrane protein diffusion for signaling and are generally found in spatially constrained subcellular microcompartments. How the geometry of these microcompartments impacts cascade activities, however, is not understood, primarily because of the inability of current live cell–imaging technologies to resolve these small structures. Here, we examine the dynamics of the GPCR rhodopsin within discrete signaling microcompartments of live photoreceptors using a novel high resolution approach. Rhodopsin fused to green fluorescent protein variants, either enhanced green fluorescent protein (EGFP) or the photoactivatable PAGFP (Rho-E/PAGFP), was expressed transgenically in Xenopus laevis rod photoreceptors, and the geometries of light signaling microcompartments formed by lamellar disc membranes and their incisure clefts were resolved by confocal imaging. Multiphoton fluorescence relaxation after photoconversion experiments were then performed with a Ti–sapphire laser focused to the diffraction limit, which produced small sub–cubic micrometer volumes of photoconverted molecules within the discrete microcompartments. A model of molecular diffusion was developed that allows the geometry of the particular compartment being examined to be specified. This was used to interpret the experimental results. Using this unique approach, we showed that rhodopsin mobility across the disc surface was highly heterogeneous. The overall relaxation of Rho-PAGFP fluorescence photoactivated within a microcompartment was biphasic, with a fast phase lasting several seconds and a slow phase of variable duration that required up to several minutes to reach equilibrium. Local Rho-EGFP diffusion within defined compartments was monotonic, however, with an effective lateral diffusion coefficient D_lat = 0.130 ± 0.012 µm²s⁻¹. Comparison of rhodopsin-PAGFP relaxation time courses with model predictions revealed that microcompartment geometry alone may explain both fast local rhodopsin diffusion and its slow equilibration across the greater disc membrane. Our approach has for the first time allowed direct examination of GPCR dynamics within a live cell signaling microcompartment and a quantitative assessment of the impact of compartment geometry on GPCR activity.

Direct measurement of diffusion in olfactory cilia using a modified FRAP approach

The diffusion coefficient of fluorescein in detached cilia of Xenopus laevis olfactory receptor neurons was measured using spatially-resolved FRAP, where the dye along half of the ciliary length was photobleached and its spatiotemporal fluorescence redistribution recorded. Fitting a one-dimensional numerical simulation of diffusion and photobleaching for 35 cilia resulted in a mean value of the diffusion coefficient and thus a reduction by a factor of compared to free diffusion in aqueous solution.