rab8 in retinal photoreceptors may participate in rhodopsin transport and in rod outer segment disk morphogenesis

piRNAs in the human retina and retinal pigment epithelium reveal a potential role in intracellular trafficking and oxidative stress

Long considered active only in the germline, the PIWI/piRNA pathway is now known to play a significant role in somatic cells, especially neurons. In this study, piRNAs were profiled in the human retina and retinal pigment epithelium (RPE). Furthermore, RNA immunoprecipitation with HIWI2 (PIWIL4) in ARPE19 cells yielded 261 piRNAs, and the expression of selective piRNAs in donor eyes was assessed by qRT-PCR. Intriguingly, computational analysis revealed complete and partial seed sequence similarity between piR-hsa-26131 and the sensory organ specific miR-183/96/182 cluster. Furthermore, the expression of retina-enriched piR-hsa-26131 was positively correlated with miR-182 in HIWI2-silenced Y79 cells. In addition, the lnc-ZNF169 sequence matched with two miRNAs of the let-7 family, and piRNAs, piR-hsa-11361 and piR-hsa-11360, which could modulate the regulatory network of retinal differentiation. Interestingly, we annotated four enriched motifs among the piRNAs and found that the piRNAs containing CACAATG and CTCATCAKYG motifs were snoRNA-derived piRNAs, which are significantly associated with developmental functions. However, piRNAs consisting of ACCACTANACCAC and AKCACGYTCSC motifs were mainly tRNA-derived fragments linked to stress response and sensory perception. Additionally, co-expression network analysis revealed cell cycle control, intracellular transport and stress response as the important biological functions regulated by piRNAs in the retina. Moreover, loss of piRNAs in HIWI2 knockdown ARPE19 confirmed altered expression of targets implicated in intracellular transport, circadian clock, and retinal degeneration. Moreover, piRNAs were dysregulated under oxidative stress conditions, indicating their potential role in retinal pathology. Therefore, we postulate that piRNAs, miRNAs, and lncRNAs might have a functional interplay during retinal development and functions to regulate retinal homeostasis.

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.

Retinal degeneration in rpgra mutant zebrafish

Introduction: Pathogenic mutations in RPGR ^ORF15, one of two major human RPGR isoforms, were responsible for most X-linked retinitis pigmentosa cases. Previous studies have shown that RPGR plays a critical role in ciliary protein transport. However, the precise mechanisms of disease triggered by RPGR ^ORF15 mutations have yet to be clearly defined. There are two homologous genes in zebrafish, rpgra and rpgrb. Zebrafish rpgra has a single transcript homologous to human RPGR ^ORF15; rpgrb has two major transcripts: rpgrb ^ex1-17 and rpgrb ^ORF15, similar to human RPGR ^ex1-19 and RPGR ^ORF15, respectively. rpgrb knockdown in zebrafish resulted in both abnormal development and increased cell death in the dysplastic retina. However, the impact of knocking down rpgra in zebrafish remains undetermined. Here, we constructed a rpgra mutant zebrafish model to investigate the retina defect and related molecular mechanism. Methods: we utilized transcription activator-like effector nuclease (TALEN) to generate a rpgra mutant zebrafish. Western blot was used to determine protein expression. RT-PCR was used to quantify gene transcription levels. The visual function of embryonic zebrafish was detected by electroretinography. Immunohistochemistry was used to observe the pathological changes in the retina of mutant zebrafish and transmission electron microscope was employed to view subcellular structure of photoreceptor cells. Results: A homozygous rpgra mutant zebrafish with c.1675_1678delins21 mutation was successfully constructed. Despite the normal morphological development of the retina at 5 days post-fertilization, visual dysfunction was observed in the mutant zebrafish. Further histological and immunofluorescence assays indicated that rpgra mutant zebrafish retina photoreceptors progressively began to degenerate at 3-6 months. Additionally, the mislocalization of cone outer segment proteins (Opn1lw and Gnb3) and the accumulation of vacuole-like structures around the connecting cilium below the OSs were observed in mutant zebrafish. Furthermore, Rab8a, a key regulator of opsin-carrier vesicle trafficking, exhibited decreased expression and evident mislocalization in mutant zebrafish. Discussion: This study generated a novel rpgra mutant zebrafish model, which showed retinal degeneration. our data suggested Rpgra is necessary for the ciliary transport of cone-associated proteins, and further investigation is required to determine its function in rods. The rpgra mutant zebrafish constructed in this study may help us gain a better understanding of the molecular mechanism of retinal degeneration caused by RPGR ^ORF15 mutation and find some useful treatment in the future.

Ciliogenesis membrane dynamics and organization

Ciliogenesis is a complex multistep process used to describe assembly of cilia and flagella. These organelles play essential roles in motility and signaling on the surface of cells. Cilia are built at the distal ends of centrioles through the formation of an axoneme that is surrounded by the ciliary membrane. As is the case in the biogenesis of other cellular organelles, regulators of membrane trafficking play essential roles in ciliogenesis, albeit with a unique feature that membranes are organized around microtubule-based structures. Membrane association with the distal end of the centriole is a critical initiating step for ciliogenesis. Studies of this process in different cell types suggests that a singular mechanism may not be utilized to initiate cilium assembly. In this review, we focus on recent insights into cilium biogenesis and the roles membrane trafficking regulators play in described ciliogenesis mechanisms with relevance to human disease.

Specific motifs mediate post-synaptic and surface transport of G protein-coupled receptors

G protein-coupled receptors (GPCRs) are key regulators of synaptic functions. However, their targeted trafficking to synapses after synthesis is poorly understood. Here, we demonstrate that multiple motifs mediate α2B-adrenergic receptor transport to the dendritic and post-synaptic compartments in primary hippocampal neurons, with a single leucine residue on the first intracellular loop being specifically involved in synaptic targeting. The N-terminally located tyrosine-serine motif operates differently in neuronal and non-neuronal cells. We further show that the highly conserved dileucine (LL) motif in the C-terminus is required for the dendritic and post-synaptic traffic of all GPCRs studied. The LL motif also directs the export from the endoplasmic reticulum of a chimeric GPCR and confers its transport ability to vesicular stomatitis virus glycoprotein in cell lines. Collectively, these data reveal the intrinsic structural determinants for the synaptic targeting of nascent GPCRs and their cell-type-specific trafficking along the biosynthetic pathways.

The molecular chaperone Hsp90α deficiency causes retinal degeneration by disrupting Golgi organization and vesicle transportation in photoreceptors

Heat shock protein 90 (Hsp90) is an abundant molecular chaperone with two isoforms, Hsp90α and Hsp90β. Hsp90β deficiency causes embryonic lethality, whereas Hsp90α deficiency causes few abnormities except male sterility. In this paper, we reported that Hsp90α was exclusively expressed in the retina, testis, and brain. Its deficiency caused retinitis pigmentosa (RP), a disease leading to blindness. In Hsp90α-deficient mice, the retina was deteriorated and the outer segment of photoreceptor was deformed. Immunofluorescence staining and electron microscopic analysis revealed disintegrated Golgi and aberrant intersegmental vesicle transportation in Hsp90α-deficient photoreceptors. Proteomic analysis identified microtubule-associated protein 1B (MAP1B) as an Hsp90α-associated protein in photoreceptors. Hspα deficiency increased degradation of MAP1B by inducing its ubiquitination, causing α-tubulin deacetylation and microtubule destabilization. Furthermore, the treatment of wild-type mice with 17-DMAG, an Hsp90 inhibitor of geldanamycin derivative, induced the same retinal degeneration as Hsp90α deficiency. Taken together, the microtubule destabilization could be the underlying reason for Hsp90α deficiency-induced RP.

Biochemistry and physiology of zebrafish photoreceptors

All vertebrates share a canonical retina with light-sensitive photoreceptors in the outer retina. These photoreceptors are of two kinds: rods and cones, adapted to low and bright light conditions, respectively. They both show a peculiar morphology, with long outer segments, comprised of ordered stacks of disc-shaped membranes. These discs host numerous proteins, many of which contribute to the visual transduction cascade. This pathway converts the light stimulus into a biological signal, ultimately modulating synaptic transmission. Recently, the zebrafish (Danio rerio) has gained popularity for studying the function of vertebrate photoreceptors. In this review, we introduce this model system and its contribution to our understanding of photoreception with a focus on the cone visual transduction cascade.

The ins and outs of the Arf4-based ciliary membrane-targeting complex

The small GTPase Arf4-based ciliary membrane-targeting complex recognizes specific targeting signals within sensory receptors and regulates their directed movement to primary cilia. Activated Arf4 directly binds the VxPx ciliary-targeting signal (CTS) of the light-sensing receptor rhodopsin. Recent findings revealed that at the trans-Golgi, marked by the small GTPase Rab6, activated Arf4 forms a functional complex with rhodopsin and the Arf guanine nucleotide exchange factor (GEF) GBF1, providing positive feedback that drives further Arf4 activation in ciliary trafficking. Arf4 function is conserved across diverse cell types; however, it appears that not all its aspects are conserved across species, as mouse Arf4 is a natural mutant in the conserved α3 helix, which is essential for its interaction with rhodopsin. Generally, activated Arf4 regulates the assembly of the targeting nexus containing the Arf GAP ASAP1 and the Rab11a-FIP3-Rabin8 dual effector complex, which controls the assembly of the highly conserved Rab11a-Rabin8-Rab8 ciliary-targeting module. It was recently found that this module interacts with the R-SNARE VAMP7, likely in its activated, c-Src-phosphorylated form. Rab11 and Rab8 bind VAMP7 regulatory longin domain (LD), whereas Rabin8 interacts with the SNARE domain, capturing VAMP7 for delivery to the ciliary base and subsequent pairing with the cognate SNAREs syntaxin 3 and SNAP-25. This review will focus on the implications of these novel findings that further illuminate the role of well-ordered Arf and Rab interaction networks in targeting of sensory receptors to primary cilia. Abbreviations: CTS: Ciliary-Targeting Signal; GAP: GTPase Activating Protein; GEF: Guanine Nucleotide Exchange Factor; RTC(s), Rhodopsin Transport Carrier(s); SNARE: Soluble N-ethylmaleimide-sensitive Factor Attachment Protein Receptor; TGN: Trans-Golgi Network.

More Than Meets the Eye: Current Understanding of RPGR Function

This article summarizes the recent advances in our understanding of a major retinal disease gene RPGR (retinitis pigmentosa GTPase regulator), mutations in which are associated with majority of X-linked forms of retinal degenerations. A great deal of work has been done to uncover the ciliary localization of RPGR and its interacting proteins in the retina. However, the molecular mechanisms of action of RPGR in the photoreceptors are still unclear. Recent studies have begun to shed light on the intracellular pathways in which RPGR is likely involved. The deregulation of such pathways may underlie the pathogenesis of severe retinal degeneration associated with RPGR. With the recent advances in the gene augmentation therapy for RPGR-associated disease, there is a lot of excitement in the field. Patients with RPGR mutations, however, present with clinically heterogeneous manifestations. It is therefore imperative to examine the function of RPGR in detail, so that we can design patient-oriented therapeutic strategies for this disease.

NudC regulates photoreceptor disk morphogenesis and rhodopsin localization

The outer segment (OS) of rod photoreceptors consist of a highly modified primary cilium containing phototransduction machinery necessary for light detection. The delivery and organization of the phototransduction components within and along the cilium into the series of stacked, highly organized disks is critical for cell function and viability. How disks are formed within the cilium remains an area of active investigation. We have found nuclear distribution protein C (nudC), a key component of mitosis and cytokinesis during development, to be present in the inner segment region of these postmitotic cells in several species, including mouse, tree shrew, monkey, and frog. Further, we found nudC interacts with rhodopsin and the small GTPase rab11a. Here, we show through transgenic tadpole studies that nudC is integral to rod cell disk formation and photoreceptor protein localization. Finally, we demonstrate that short hairpin RNA knockdown of nudC in tadpole rod photoreceptors, which leads to the inability of rod cells to maintain their OS, is rescued through coexpression of murine nudC.-Boitet, E. R., Reish, N. J., Hubbard, M. G., Gross, A. K. NudC regulates photoreceptor disk morphogenesis and rhodopsin localization.

Mutagenesis of putative ciliary genes with the CRISPR/Cas9 system in zebrafish identifies genes required for retinal development

Cilia are conserved microtubule-based organelles that function as mechanical and chemical sensors in various cell types. By bioinformatic, genomic, and proteomic studies, more than 2000 proteins have been identified as cilium-associated proteins or putative ciliary proteins; these proteins are referred to as the ciliary proteome or the ciliome. However, little is known about the function of these numerous putative ciliary proteins in cilia. To identify the possible new functional proteins or pathways in cilia, we carried out a small-scale genetic screen targeting 54 putative ciliary genes by using the clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) system. We successfully constructed 54 zebrafish mutants, and 8 of them displayed microphthalmias. Three of these 8 genes encode proteins for protein transport, suggesting the important roles of protein transport in retinal development. In situ hybridization revealed that all these genes are expressed in zebrafish eyes. Furthermore, polo-like kinase 1 was required for ciliogenesis in neural tube. We uncovered the potential function of the ciliary genes for the retinal development of zebrafish.-Hu, R., Huang, W., Liu, J., Jin, M., Wu, Y., Li, J., Wang, J., Yu, Z., Wang, H., Cao, Y. Mutagenesis of putative ciliary genes with the CRISPR/Cas9 system in zebrafish identifies genes required for retinal development.

An interaction network between the SNARE VAMP7 and Rab GTPases within a ciliary membrane-targeting complex

The Arf4-rhodopsin complex (mediated by the VxPx motif in rhodopsin) initiates expansion of vertebrate rod photoreceptor cilia-derived light-sensing organelles through stepwise assembly of a conserved trafficking network. Here, we examine its role in the sorting of VAMP7 (also known as TI-VAMP) - an R-SNARE possessing a regulatory longin domain (LD) - into rhodopsin transport carriers (RTCs). During RTC formation and trafficking, VAMP7 colocalizes with the ciliary cargo rhodopsin and interacts with the Rab11-Rabin8-Rab8 trafficking module. Rab11 and Rab8 bind the VAMP7 LD, whereas Rabin8 (also known as RAB3IP) interacts with the SNARE domain. The Arf/Rab11 effector FIP3 (also known as RAB11FIP3) regulates VAMP7 access to Rab11. At the ciliary base, VAMP7 forms a complex with the cognate SNAREs syntaxin 3 and SNAP-25. When expressed in transgenic animals, a GFP-VAMP7ΔLD fusion protein and a Y45E phosphomimetic mutant colocalize with endogenous VAMP7. The GFP-VAMP7-R150E mutant displays considerable localization defects that imply an important role of the R-SNARE motif in intracellular trafficking, rather than cognate SNARE pairing. Our study defines the link between VAMP7 and the ciliary targeting nexus that is conserved across diverse cell types, and contributes to general understanding of how functional Arf and Rab networks assemble SNAREs in membrane trafficking.

Mechanisms of the anterograde trafficking of GPCRs: Regulation of AT1R transport by interacting proteins and motifs

Anterograde cell surface transport of nascent G protein-coupled receptors (GPCRs) en route from the endoplasmic reticulum (ER) through the Golgi apparatus represents a crucial checkpoint to control the amount of the receptors at the functional destination and the strength of receptor activation-elicited cellular responses. However, as compared with extensively studied internalization and recycling processes, the molecular mechanisms of cell surface trafficking of GPCRs are relatively less defined. Here, we will review the current advances in understanding the ER-Golgi-cell surface transport of GPCRs and use angiotensin II type 1 receptor as a representative GPCR to discuss emerging roles of receptor-interacting proteins and specific motifs embedded within the receptors in controlling the forward traffic of GPCRs along the biosynthetic pathway.

Photoreceptor actin dysregulation in syndromic and non-syndromic retinitis pigmentosa

Retinitis pigmentosa (RP) is the leading cause of inherited blindness. RP is a genetically heterogeneous disorder, with more than 100 different causal genes identified in patients. Central to disease pathogenesis is the progressive loss of retinal photoreceptors. Photoreceptors are specialised sensory neurons that exhibit a complex and highly dynamic morphology. The highly polarised and elaborated architecture of photoreceptors requires precise regulation of numerous cytoskeletal elements. In recent years, significant work has been placed on investigating the role of microtubules (specifically, the acetylated microtubular axoneme of the photoreceptor connecting cilium) and their role in normal photoreceptor function. This has been driven by the emerging field of ciliopathies, human diseases arising from mutations in genes required for cilia formation or function, of which RP is a frequently reported phenotype. Recent studies have highlighted an intimate relationship between cilia and the actin cystoskeleton. This review will focus on the role of actin in photoreceptors, examining the connection between actin dysregulation in RP.

Endosome maturation factors Rabenosyn-5/VPS45 and caveolin-1 regulate ciliary membrane and polycystin-2 homeostasis

Primary cilium structure and function relies on control of ciliary membrane homeostasis, regulated by membrane trafficking processes that deliver and retrieve ciliary components at the periciliary membrane. However, the molecular mechanisms controlling ciliary membrane establishment and maintenance, especially in relation to endocytosis, remain poorly understood. Here, using Caenorhabditis elegans, we describe closely linked functions for early endosome (EE) maturation factors RABS-5 (Rabenosyn-5) and VPS-45 (VPS45) in regulating cilium length and morphology, ciliary and periciliary membrane volume, and ciliary signalling-related sensory behaviour. We demonstrate that RABS-5 and VPS-45 control periciliary vesicle number and levels of select EE/endocytic markers (WDFY-2, CAV-1) and the ciliopathy membrane receptor PKD-2 (polycystin-2). Moreover, we show that CAV-1 (caveolin-1) also controls PKD-2 ciliary levels and associated sensory behaviour. These data link RABS-5 and VPS-45 ciliary functions to the processing of periciliary-derived endocytic vesicles and regulation of ciliary membrane homeostasis. Our findings also provide insight into the regulation of PKD-2 ciliary levels via integrated endosomal sorting and CAV-1-mediated endocytosis.

Role of Rab GTPases in the export trafficking of G protein-coupled receptors

G protein-coupled receptors (GPCRs) constitute a superfamily of cell surface receptors that regulate a variety of cell functions. As the cell surface is the functional destination for most GPCRs, the cell surface targeting process represents a crucial checkpoint in controlling the functionality of the receptors. However, the molecular mechanisms underlying the cell surface delivery of newly synthesized GPCRs remain poorly understood. In this review, we will highlight the role of Rab GTPases in GPCR cell surface transport, particularly post-Golgi traffic, and discuss the underlying molecular mechanisms.

Molecular Insights into the Roles of Rab Proteins in Intracellular Dynamics and Neurodegenerative Diseases

In eukaryotes, the cellular functions are segregated to membrane-bound organelles. This inherently requires sorting of metabolites to membrane-limited locations. Sorting the metabolites from ribosomes to various organelles along the intracellular trafficking pathways involves several integral cellular processes, including an energy-dependent step, in which the sorting of metabolites between organelles is catalyzed by membrane-anchoring protein Rab-GTPases (Rab). They contribute to relaying the switching of the secretory proteins between hydrophobic and hydrophilic environments. The intracellular trafficking routes include exocytic and endocytic pathways. In these pathways, numerous Rab-GTPases are participating in discrete shuttling of cargoes. Long-distance trafficking of cargoes is essential for neuronal functions, and Rabs are critical for these functions, including the transport of membranes and essential proteins for the development of axons and neurites. Rabs are also the key players in exocytosis of neurotransmitters and recycling of neurotransmitter receptors. Thus, Rabs are critical for maintaining neuronal communication, as well as for normal cellular physiology. Therefore, cellular defects of Rab components involved in neural functions, which severely affect normal brain functions, can produce neurological complications, including several neurodegenerative diseases. In this review, we provide a comprehensive overview of the current understanding of the molecular signaling pathways of Rab proteins and the impact of their defects on different neurodegenerative diseases. The insights gathered into the dynamics of Rabs that are described in this review provide new avenues for developing effective treatments for neurodegenerative diseases-associated with Rab defects.

Loss-of-function of the ciliopathy protein Cc2d2a disorganizes the vesicle fusion machinery at the periciliary membrane and indirectly affects Rab8-trafficking in zebrafish photoreceptors

Ciliopathies are human disorders caused by dysfunction of primary cilia, ubiquitous organelles involved in transduction of environmental signals such as light sensation in photoreceptors. Concentration of signal detection proteins such as opsins in the ciliary membrane is achieved by RabGTPase-regulated polarized vesicle trafficking and by a selective barrier at the ciliary base, the transition zone (TZ). Dysfunction of the TZ protein CC2D2A causes Joubert/Meckel syndromes in humans and loss of ciliary protein localization in animal models, including opsins in retinal photoreceptors. The link between the TZ and upstream vesicle trafficking has been little explored to date. Moreover, the role of the small GTPase Rab8 in opsin-carrier vesicle (OCV) trafficking has been recently questioned in a mouse model. Using correlative light and electron microscopy and live imaging in zebrafish photoreceptors, we provide the first live characterization of Rab8-mediated trafficking in photoreceptors in vivo. Our results support a possibly redundant role for both Rab8a/b paralogs in OCV trafficking, based on co-localization of Rab8 and opsins in vesicular structures, and joint movement of Rab8-tagged particles with opsin. We further investigate the role of the TZ protein Cc2d2a in Rab8-mediated trafficking using cc2d2a zebrafish mutants and identify a requirement for Cc2d2a in the latest step of OCV trafficking, namely vesicle fusion. Progressive accumulation of opsin-containing vesicles in the apical portion of photoreceptors lacking Cc2d2a is caused by disorganization of the vesicle fusion machinery at the periciliary membrane with mislocalization and loss of the t-SNAREs SNAP25 and Syntaxin3 and of the exocyst component Exoc4. We further observe secondary defects on upstream Rab8-trafficking with cytoplasmic accumulation of Rab8. Taken together, our results support participation of Rab8 in OCV trafficking and identify a novel role for the TZ protein Cc2d2a in fusion of incoming ciliary-directed vesicles, through organization of the vesicle fusion machinery at the periciliary membrane.

Ciliopathies are human disorders caused by dysfunction of primary cilia, ubiquitous organelles involved in transduction of environmental signals to the cells. Concentration and regulation of signal detection proteins in the ciliary membrane is therefore tightly regulated through polarized vesicle trafficking and through a selective barrier at the ciliary base called the transition zone (TZ). Dysfunction of TZ proteins leads to human ciliopathies and to aberrant localization of ciliary proteins in animal models. In this work, we use zebrafish retinal photoreceptors as a model to explore the relationship between the TZ and upstream vesicle trafficking. Relying on modern technologies such as correlative light and electron microscopy and live imaging of fluorescently-tagged proteins, we identify a role for the TZ protein CC2D2A in organizing the components required for vesicle fusion at the periciliary membrane. We also characterize the movement dynamics of vesicles carrying light-detection proteins (opsins) towards the ciliary compartment of photoreceptors in vivo and provide novel data in support of the recently questioned involvement of the small GTPase Rab8 in opsin-carrier vesicle trafficking in photoreceptors.

Rpgrip1 is required for rod outer segment development and ciliary protein trafficking in zebrafish

Mutations in the RPGR-interacting protein 1 (RPGRIP1) gene cause recessive Leber congenital amaurosis (LCA), juvenile retinitis pigmentosa (RP) and cone-rod dystrophy. RPGRIP1 interacts with other retinal disease-causing proteins and has been proposed to have a role in ciliary protein transport; however, its function remains elusive. Here, we describe a new zebrafish model carrying a nonsense mutation in the rpgrip1 gene. Rpgrip1homozygous mutants do not form rod outer segments and display mislocalization of rhodopsin, suggesting a role for RPGRIP1 in rhodopsin-bearing vesicle trafficking. Furthermore, Rab8, the key regulator of rhodopsin ciliary trafficking, was mislocalized in photoreceptor cells of rpgrip1 mutants. The degeneration of rod cells is early onset, followed by the death of cone cells. These phenotypes are similar to that observed in LCA and juvenile RP patients. Our data indicate RPGRIP1 is necessary for rod outer segment development through regulating ciliary protein trafficking. The rpgrip1 mutant zebrafish may provide a platform for developing therapeutic treatments for RP patients.

Rab GTPases in cilium formation and function

Cilia are microtubule-based organelles extending from a basal body at the surface of eukaryotic cells. Cilia regulate cell and fluid motility, sensation and developmental signaling, and ciliary defects cause human diseases (ciliopathies) affecting the formation and function of many tissues and organs. Over the past decade, various Rab and Rab-like membrane trafficking proteins have been shown to regulate cilia-related processes such as basal body maturation, ciliary axoneme extension, intraflagellar transport and ciliary signaling. In this review, we provide a comprehensive overview of Rab protein ciliary associations, drawing on findings from multiple model systems, including mammalian cell culture, mice, zebrafish, C. elegans, trypanosomes, and green algae. We also discuss several emerging mechanistic themes related to ciliary Rab cascades and functional redundancy.

The Arf GEF GBF1 and Arf4 synergize with the sensory receptor cargo, rhodopsin, to regulate ciliary membrane trafficking

The small GTPase Arf4 and the Arf GTPase-activating protein (GAP) ASAP1 cooperatively sequester sensory receptor cargo into transport carriers targeted to primary cilia, but the input that drives Arf4 activation in this process remains unknown. Here, we show, by using frog retinas and recombinant human proteins, that during the carrier biogenesis from the photoreceptor Golgi/trans-Golgi network (TGN) a functional complex is formed between Arf4, the Arf guanine nucleotide exchange factor (GEF) GBF1 and the light-sensing receptor, rhodopsin. Rhodopsin and Arf4 bind the regulatory N-terminal dimerization and cyclophillin-binding (DCB)-homology upstream of Sec7 (HUS) domain of GBF1. The complex is sensitive to Golgicide A (GCA), a selective inhibitor of GBF1 that accordingly blocks rhodopsin delivery to the cilia, without disrupting the photoreceptor Golgi. The emergence of newly synthesized rhodopsin in the endomembrane system is essential for GBF1-Arf4 complex formation in vivo Notably, GBF1 interacts with the Arf GAP ASAP1 in a GCA-resistant manner. Our findings indicate that converging signals on GBF1 from the influx of cargo into the Golgi/TGN and the feedback from Arf4, combined with input from ASAP1, control Arf4 activation during sensory membrane trafficking to primary cilia.

Integrin α8 and Pcdh15 act as a complex to regulate cilia biogenesis in sensory cells

The way an organism perceives its surroundings depends on sensory systems and the highly specialized cilia present in the neurosensory cells. Here, we describe the existence of an integrin α8 (Itga8) and protocadherin-15a (Pcdh15a) ciliary complex in neuromast hair cells in a zebrafish model. Depletion of the complex via downregulation or loss-of-function mutation leads to a dysregulation of cilia biogenesis and endocytosis. At the molecular level, removal of the complex blocks the access of Rab8a into the cilia as well as normal recruitment of ciliary cargo by centriolar satellites. These defects can be reversed by the introduction of a constitutively active form of Rhoa, suggesting that Itga8-Pcdh15a complex mediates its effect through the activation of this small GTPase and probably by the regulation of actin cytoskeleton dynamics. Our data points to a novel mechanism involved in the regulation of sensory cilia development, with the corresponding implications for normal sensory function.

The exocyst is required for photoreceptor ciliogenesis and retinal development

We previously have shown that the highly conserved eight-protein exocyst trafficking complex is required for ciliogenesis in kidney tubule cells. We hypothesized here that ciliogenic programs are conserved across organs and species. To determine whether renal primary ciliogenic programs are conserved in the eye, and to characterize the function and mechanisms by which the exocyst regulates eye development in zebrafish, we focused on exoc5, a central component of the exocyst complex, by analyzing both exoc5 zebrafish mutants, and photoreceptor-specific Exoc5 knock-out mice. Two separate exoc5 mutant zebrafish lines phenocopied exoc5 morphants and, strikingly, exhibited a virtual absence of photoreceptors, along with abnormal retinal development and cell death. Because the zebrafish mutant was a global knockout, we also observed defects in several ciliated organs, including the brain (hydrocephalus), heart (cardiac edema), and kidney (disordered and shorter cilia). exoc5 knockout increased phosphorylation of the regulatory protein Mob1, consistent with Hippo pathway activation. exoc5 mutant zebrafish rescue with human EXOC5 mRNA completely reversed the mutant phenotype. We accomplished photoreceptor-specific knockout of Exoc5 with our Exoc5 fl/fl mouse line crossed with a rhodopsin-Cre driver line. In Exoc5 photoreceptor-specific knock-out mice, the photoreceptor outer segment structure was severely impaired at 4 weeks of age, although a full-field electroretinogram indicated a visual response was still present. However, by 6 weeks, visual responses were eliminated. In summary, we show that ciliogenesis programs are conserved in the kidneys and eyes of zebrafish and mice and that the exocyst is necessary for photoreceptor ciliogenesis and retinal development, most likely by trafficking cilia and outer-segment proteins.

BLOC-1 is required for selective membrane protein trafficking from endosomes to primary cilia

Monis et al. demonstrate that the BLOC-1 protein complex is required for trafficking polycystin-2, but not fibrocystin or smoothened, from endosomes to primary cilia. This is the first demonstration of a role for BLOC-1 in ciliary assembly and emphasizes the complexity of distinct ciliary membrane protein trafficking routes.

Primary cilia perceive the extracellular environment through receptors localized in the ciliary membrane, but mechanisms directing specific proteins to this domain are poorly understood. To address this question, we knocked down proteins potentially important for ciliary membrane targeting and determined how this affects the ciliary trafficking of fibrocystin, polycystin-2, and smoothened. Our analysis showed that fibrocystin and polycystin-2 are dependent on IFT20, GMAP210, and the exocyst complex, while smoothened delivery is largely independent of these components. In addition, we found that polycystin-2, but not smoothened or fibrocystin, requires the biogenesis of lysosome-related organelles complex-1 (BLOC-1) for ciliary delivery. Consistent with the role of BLOC-1 in sorting from the endosome, we find that disrupting the recycling endosome reduces ciliary polycystin-2 and causes its accumulation in the recycling endosome. This is the first demonstration of a role for BLOC-1 in ciliary assembly and highlights the complexity of pathways taken to the cilium.

Structural and molecular bases of rod photoreceptor morphogenesis and disease

The rod cell has an extraordinarily specialized structure that allows it to carry out its unique function of detecting individual photons of light. Both the structural features of the rod and the metabolic processes required for highly amplified light detection seem to have rendered the rod especially sensitive to structural and metabolic defects, so that a large number of gene defects are primarily associated with rod cell death and give rise to blinding retinal dystrophies. The structures of the rod, especially those of the sensory cilium known as the outer segment, have been the subject of structural, biochemical, and genetic analysis for many years, but the molecular bases for rod morphogenesis and for cell death in rod dystrophies are still poorly understood. Recent developments in imaging technology, such as cryo-electron tomography and super-resolution fluorescence microscopy, in gene sequencing technology, and in gene editing technology are rapidly leading to new breakthroughs in our understanding of these questions. A summary is presented of our current understanding of selected aspects of these questions, highlighting areas of uncertainty and contention as well as recent discoveries that provide new insights. Examples of structural data from emerging imaging technologies are presented.

Photoreceptors at a glance

Retinal photoreceptor cells contain a specialized outer segment (OS) compartment that functions in the capture of light and its conversion into electrical signals in a process known as phototransduction. In rods, photoisomerization of 11-cis to all-trans retinal within rhodopsin triggers a biochemical cascade culminating in the closure of cGMP-gated channels and hyperpolarization of the cell. Biochemical reactions return the cell to its 'dark state' and the visual cycle converts all-trans retinal back to 11-cis retinal for rhodopsin regeneration. OS are continuously renewed, with aged membrane removed at the distal end by phagocytosis and new membrane added at the proximal end through OS disk morphogenesis linked to protein trafficking. The molecular basis for disk morphogenesis remains to be defined in detail although several models have been proposed, and molecular mechanisms underlying protein trafficking are under active investigation. The aim of this Cell Science at a Glance article and the accompanying poster is to highlight our current understanding of photoreceptor structure, phototransduction, the visual cycle, OS renewal, protein trafficking and retinal degenerative diseases.

Small GTPases Rab8a and Rab11a Are Dispensable for Rhodopsin Transport in Mouse Photoreceptors

Rab11a and Rab8a are ubiquitous small GTPases shown as required for rhodopsin transport in Xenopus laevis and zebrafish photoreceptors by dominant negative (dn) disruption of function. Here, we generated retina-specific Rab11a (^retRab11a) and Rab8a (^retRab8a) single and double knockout mice to explore the consequences in mouse photoreceptors. Rhodopsin and other outer segment (OS) membrane proteins targeted correctly to OS and electroretinogram (ERG) responses in all three mutant mouse lines were indistinguishable from wild-type (WT). Further, AAV (adeno-associated virus)-mediated expression of dnRab11b in ^retRab11a^-/- retina, or expression of dnRab8b in ^retRab8a^-/- retina did not cause OS protein mislocalization. Finally, a ^retRab8a^-/- retina injected at one month of age with AAVs expressing dnRab11a, dnRab11b, dnRab8b, and dnRab10 (four dn viruses on Rab8a^-/- background) and harvested three months later exhibited normal OS protein localization. In contrast to results obtained with dnRab GTPases in Xenopus and zebrafish, mouse Rab11a and Rab8a are dispensable for proper rhodopsin and outer segment membrane protein targeting. Absence of phenotype after expression of four dn Rab GTPases in a Rab8a^-/- retina suggests that Rab8b and Rab11b paralogs maybe dispensable as well. Our data thus demonstrate significant interspecies variation in photoreceptor membrane protein and rhodopsin trafficking.

Trafficking of ciliary G protein-coupled receptors

In the last decade highly conserved cellular appendages called cilia have enjoyed a renewed interest from basic, biomedical scientists, and clinicians alike. This interest has grown upon the elucidation that cilia throughout the body serve as important sensory and signaling centers in both development and adult homeostasis. Furthermore, the identification of several rare genetic disorders associated with cilia dysfunction has broadened the field. However, even though their potential role in human health and disease is now recognized many basic questions about their functions remain. This chapter seeks to explore the trafficking of cilia-specific G protein-coupled receptors (GPCRs) and discusses several model systems in which this has been explored. We open the chapter by briefly discussing cilia and GPCRs then begin discussing some aspects of rhodopsin trafficking, arguably the most well studied of cilia GPCRs. We continue with sections on neuronal cilia and olfactory cilia receptor trafficking. Finally, we conclude with the emerging area of dynamic ciliary GPCR trafficking and speculate about future directions and some of the questions that remain for ciliary GPCRs.

Loss of retinitis pigmentosa 2 (RP2) protein affects cone photoreceptor sensory cilium elongation in mice

Degeneration of photoreceptors (rods and cones) results in blindness. As we rely almost entirely on our daytime vision mediated by the cones, it is the loss of these photoreceptors that results in legal blindness and poor quality of life. Cone dysfunction is usually observed due to two mechanisms: noncell-autonomous due to the secondary effect of rod death if the causative gene is specifically expressed in rods and cell autonomous, if the mutation is in a cone-specific gene. However, it is difficult to dissect cone autonomous effect of mutations in the genes that are expressed in both rods and cones. Here we report a property of murine cone photoreceptors, which is a cone-autonomous effect of the genetic perturbation of the retinitis pigmentosa 2 (Rp2) gene mutated in human X-linked RP. Constitutive loss of Rp2 results in abnormal extension of the cone outer segment (COS). This effect is phenocopied when the Rp2 gene is ablated specifically in cones but not when ablated in rods. Furthermore, the elongated COS exhibits abnormal ultrastructure with disorganized lamellae. Additionally, elongation of both the outer segment membrane and the microtubule cytoskeleton was observed in the absence of RP2. Taken together, our studies identify a cone morphological defect in retinal degeneration due to ablation of RP2 and will assist in understanding cone-autonomous responses during disease and develop targeted therapies.

Novel topography of the Rab11-effector interaction network within a ciliary membrane targeting complex

Small GTPases function as universal molecular switches due to the nucleotide dependent conformational changes of their switch regions that allow interacting proteins to discriminate between the active GTP-bound and the inactive GDP-bound states. Guanine nucleotide exchange factors (GEFs) recognize the inactive GDP-bound conformation whereas GTPase activating proteins (GAPs), and the GTPase effectors recognize the active GTP-bound state. Small GTPases are linked to each other through regulatory and effector proteins into functional networks that regulate intracellular membrane traffic through diverse mechanisms that include GEF and GAP cascades, GEF-effector interactions, common effectors and positive feedback loops linking interacting proteins. As more structural and functional information is becoming available, new types of interactions between regulatory proteins, and new mechanisms by which GTPases are networked to control membrane traffic are being revealed. This review will focus on the structure and function of the novel Rab11-FIP3-Rabin8 dual effector complex and its implications for the targeting of sensory receptors to primary cilia, dysfunction of which causes cilia defects underlying human diseases and disorders know as ciliopathies.

RPGR: Its role in photoreceptor physiology, human disease, and future therapies

Mammalian photoreceptors contain specialised connecting cilia that connect the inner (IS) to the outer segments (OS). Dysfunction of the connecting cilia due to mutations in ciliary proteins are a common cause of the inherited retinal dystrophy retinitis pigmentosa (RP). Mutations affecting the Retinitis Pigmentosa GTPase Regulator (RPGR) protein is one such cause, affecting 10-20% of all people with RP and the majority of those with X-linked RP. RPGR is located in photoreceptor connecting cilia. It interacts with a wide variety of ciliary proteins, but its exact function is unknown. Recently, there have been important advances both in our understanding of RPGR function and towards the development of a therapy. This review summarises the existing literature on human RPGR function and dysfunction, and suggests that RPGR plays a role in the function of the ciliary gate, which controls access of both membrane and soluble proteins to the photoreceptor outer segment. We discuss key models used to investigate and treat RPGR disease and suggest that gene augmentation therapy offers a realistic therapeutic approach, although important questions still remain to be answered, while cell replacement therapy based on retinal progenitor cells represents a more distant prospect.

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.

The Arf and Rab11 effector FIP3 acts synergistically with ASAP1 to direct Rabin8 in ciliary receptor targeting

Primary cilia have gained considerable importance in biology and disease now that their involvement in a wide range of human ciliopathies has been abundantly documented. However, detailed molecular mechanisms for specific targeting of sensory receptors to primary cilia are still unknown. Here, we show that the Arf and Rab11 effector FIP3 (also known as RAB11FIP3) promotes the activity of Rab11a and the Arf GTPase-activating protein (GAP) ASAP1 in the Arf4-dependent ciliary transport of the sensory receptor rhodopsin. During its passage out of the photoreceptor Golgi and trans-Golgi network (TGN), rhodopsin indirectly interacts with FIP3 through Rab11a and ASAP1. FIP3 competes with rhodopsin for binding to ASAP1 and displaces it from the ternary complex with Arf4-GTP and ASAP1. Resembling the phenotype resulting from </emph>lack of ASAP1, ablation of FIP3 abolishes ciliary targeting and causes rhodopsin mislocalization. FIP3 coordinates the interactions of ASAP1 and Rab11a with the Rab8 guanine nucleotide exchange factor Rabin8 (also known as RAB3IP). Our study implies that FIP3 functions as a crucial targeting regulator, which impinges on rhodopsin-ASAP1 interactions and shapes the binding pocket for Rabin8 within the ASAP1-Rab11a-FIP3 targeting complex, thus facilitating the orderly assembly and activation of the Rab11-Rabin8-Rab8 cascade during ciliary receptor trafficking.

Compartments within a compartment: what C. elegans can tell us about ciliary subdomain composition, biogenesis, function, and disease

The primary cilium has emerged as a hotbed of sensory and developmental signaling, serving as a privileged domain to concentrate the functions of a wide number of channels, receptors and downstream signal transducers. This realization has provided important insight into the pathophysiological mechanisms underlying the ciliopathies, an ever expanding spectrum of multi-symptomatic disorders affecting the development and maintenance of multiple tissues and organs. One emerging research focus is the subcompartmentalised nature of the organelle, consisting of discrete structural and functional subdomains such as the periciliary membrane/basal body compartment, the transition zone, the Inv compartment and the distal segment/ciliary tip region. Numerous ciliopathy, transport-related and signaling molecules localize at these compartments, indicating specific roles at these subciliary sites. Here, by focusing predominantly on research from the genetically tractable nematode C. elegans, we review ciliary subcompartments in terms of their structure, function, composition, biogenesis and relationship to human disease.

Diverse cell type-specific mechanisms localize G protein-coupled receptors to Caenorhabditis elegans sensory cilia

The localization of signaling molecules such as G protein-coupled receptors (GPCRs) to primary cilia is essential for correct signal transduction. Detailed studies over the past decade have begun to elucidate the diverse sequences and trafficking mechanisms that sort and transport GPCRs to the ciliary compartment. However, a systematic analysis of the pathways required for ciliary targeting of multiple GPCRs in different cell types in vivo has not been reported. Here we describe the sequences and proteins required to localize GPCRs to the cilia of the AWB and ASK sensory neuron types in Caenorhabditis elegans. We find that GPCRs expressed in AWB or ASK utilize conserved and novel sequences for ciliary localization, and that the requirement for a ciliary targeting sequence in a given GPCR is different in different neuron types. Consistent with the presence of multiple ciliary targeting sequences, we identify diverse proteins required for ciliary localization of individual GPCRs in AWB and ASK. In particular, we show that the TUB-1 Tubby protein is required for ciliary localization of a subset of GPCRs, implying that defects in GPCR localization may be causal to the metabolic phenotypes of tub-1 mutants. Together, our results describe a remarkable complexity of mechanisms that act in a protein- and cell-specific manner to localize GPCRs to cilia, and suggest that this diversity allows for precise regulation of GPCR-mediated signaling as a function of external and internal context.

RAB GTPases and RAB-interacting proteins and their role in the control of cognitive functions

A RAS-related class of small monomeric G proteins, the RAB GTPases, is emerging as of key biological importance in compartment specific directional control of vesicles formation, transport and fusion. Thanks to human genetic observation and to the consequent dedicated biochemical work, substantial progress has been made on the understanding of the role played by RAB GTPases and their effector proteins on neuronal development and the shaping of cognitive functions. This review is highlighting these initial elements to broaden the current scope of research on developmental cognitive deficits and take the point of view of RAB GTPases control on membrane transport in neurons and astrocytes.

Rab8a and Rab8b are essential for several apical transport pathways but insufficient for ciliogenesis

The small GTP-binding protein Rab8 is known to play an essential role in intracellular transport and cilia formation. We have previously demonstrated that Rab8a is required for localising apical markers in various organisms. Rab8a has a closely related isoform, Rab8b. To determine whether Rab8b can compensate for Rab8a, we generated Rab8b-knockout mice. Although the Rab8b-knockout mice did not display an overt phenotype, Rab8a and Rab8b double-knockout mice exhibited mislocalisation of apical markers and died earlier than Rab8a-knockout mice. The apical markers accumulated in three intracellular patterns in the double-knockout mice. However, the localisation of basolateral and/or dendritic markers of the double-knockout mice seemed normal. The morphology and the length of various primary and/or motile cilia, and the frequency of ciliated cells appeared to be identical in control and double-knockout mice. However, an additional knockdown of Rab10 in double-knockout cells greatly reduced the percentage of ciliated cells. Our results highlight the compensatory effect of Rab8a and Rab8b in apical transport, and the complexity of the apical transport process. In addition, neither Rab8a nor Rab8b are required for basolateral and/or dendritic transport. However, simultaneous loss of Rab8a and Rab8b has little effect on ciliogenesis, whereas additional loss of Rab10 greatly affects ciliogenesis.

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.

Primary cilia and dendritic spines: different but similar signaling compartments

Primary non-motile cilia and dendritic spines are cellular compartments that are specialized to sense and transduce environmental cues and presynaptic signals, respectively. Despite their unique cellular roles, both compartments exhibit remarkable parallels in the general principles, as well as molecular mechanisms, by which their protein composition, membrane domain architecture, cellular interactions, and structural and functional plasticity are regulated. We compare and contrast the pathways required for the generation and function of cilia and dendritic spines, and suggest that insights from the study of one may inform investigations into the other of these critically important signaling structures.

Mutations in RAB28, encoding a farnesylated small GTPase, are associated with autosomal-recessive cone-rod dystrophy

The majority of the genetic causes of autosomal-recessive (ar) cone-rod dystrophy (CRD) are currently unknown. A combined approach of homozygosity mapping and exome sequencing revealed a homozygous nonsense mutation (c.565C>T [p.Glu189*]) in RAB28 in a German family with three siblings with arCRD. Another homozygous nonsense mutation (c.409C>T [p.Arg137*]) was identified in a family of Moroccan Jewish descent with two siblings affected by arCRD. All five affected individuals presented with hyperpigmentation in the macula, progressive loss of the visual acuity, atrophy of the retinal pigment epithelium, and severely reduced cone and rod responses on the electroretinogram. RAB28 encodes a member of the Rab subfamily of the RAS-related small GTPases. Alternative RNA splicing yields three predicted protein isoforms with alternative C-termini, which are all truncated by the nonsense mutations identified in the arCRD families in this report. Opposed to other Rab GTPases that are generally geranylgeranylated, RAB28 is predicted to be farnesylated. Staining of rat retina showed localization of RAB28 to the basal body and the ciliary rootlet of the photoreceptors. Analogous to the function of other RAB family members, RAB28 might be involved in ciliary transport in photoreceptor cells. This study reveals a crucial role for RAB28 in photoreceptor function and suggests that mutations in other Rab proteins may also be associated with retinal dystrophies.

Transmembrane protein OSTA-1 shapes sensory cilia morphology via regulation of intracellular membrane trafficking in C. elegans

The structure and function of primary cilia are critically dependent on intracellular trafficking pathways that transport ciliary membrane and protein components. The mechanisms by which these trafficking pathways are regulated are not fully characterized. Here we identify the transmembrane protein OSTA-1 as a new regulator of the trafficking pathways that shape the morphology and protein composition of sensory cilia in C. elegans. osta-1 encodes an organic solute transporter alpha-like protein, mammalian homologs of which have been implicated in membrane trafficking and solute transport, although a role in regulating cilia structure has not previously been demonstrated. We show that mutations in osta-1 result in altered ciliary membrane volume, branch length and complexity, as well as defects in localization of a subset of ciliary transmembrane proteins in different sensory cilia types. OSTA-1 is associated with transport vesicles, localizes to a ciliary compartment shown to house trafficking proteins, and regulates both retrograde and anterograde flux of the endosome-associated RAB-5 small GTPase. Genetic epistasis experiments with sensory signaling, exocytic and endocytic proteins further implicate OSTA-1 as a crucial regulator of ciliary architecture via regulation of cilia-destined trafficking. Our findings suggest that regulation of transport pathways in a cell type-specific manner contributes to diversity in sensory cilia structure and might allow dynamic remodeling of ciliary architecture via multiple inputs.

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.

Crosstalk of Arf and Rab GTPases en route to cilia

Small GTPases are versatile temporal and spatial regulators of virtually all cellular processes including signal transduction, cytoskeleton dynamics and membrane trafficking. They function as molecular switches, aided by a multitude of regulatory and effector proteins that link them into functional networks. A picture is beginning to emerge whereupon scaffold proteins with many functional domains perform the regulatory and effector functions, thus allowing the ordered recruitment and activation of small GTPases. This leads to the formation of scaffolding patches that coordinate cargo concentration and capture, with the recruitment and activation of the membrane tethering complexes and fusion regulators. This review will focus on the crosstalk of Arf and Rab GTPases at the Golgi complex and the scaffolds that facilitate their activation during trafficking of sensory receptors to primary cilia. The evolutionary conservation of the GTPase cascades in ciliogenesis and yeast budding will be discussed.

The Arf GAP ASAP1 provides a platform to regulate Arf4- and Rab11-Rab8-mediated ciliary receptor targeting

Dysfunctional trafficking to primary cilia is a frequent cause of human diseases known as ciliopathies, yet molecular mechanisms for specific targeting of sensory receptors to cilia are largely unknown. Here, we show that the targeting of ciliary cargo, represented by rhodopsin, is mediated by a specialized system, the principal component of which is the Arf GAP ASAP1. Ablation of ASAP1 abolishes ciliary targeting and causes formation of actin-rich periciliary membrane projections that accumulate mislocalized rhodopsin. We find that ASAP1 serves as a scaffold that brings together the proteins necessary for transport to the cilia including the GTP-binding protein Arf4 and the two G proteins of the Rab family--Rab11 and Rab8--linked by the Rab8 guanine nucleotide exchange factor Rabin8. ASAP1 recognizes the FR ciliary targeting signal of rhodopsin. Rhodopsin FR-AA mutant, defective in ASAP1 binding, fails to interact with Rab8 and translocate across the periciliary diffusion barrier. Our study implies that other rhodopsin-like sensory receptors may interact with this conserved system and reach the cilia using the same platform.

Molecular assemblies that control rhodopsin transport to the cilia

This review will focus on the conserved molecular mechanisms for the specific targeting of rhodopsin and rhodopsin-like sensory receptors to the primary cilia. We will discuss the molecular assemblies that control the movement of rhodopsin from the central sorting station of the cell, the trans-Golgi network (TGN), into membrane-enclosed rhodopsin transport carriers (RTCs), and their delivery to the primary cilia and the cilia-derived sensory organelle, the rod outer segment (ROS). Recent studies reveal that these processes are initiated by the synergistic interaction of rhodopsin with the active form of the G-protein Arf4 and the Arf GTPase activating protein (GAP) ASAP1. During rhodopsin progression, ASAP1 serves as an activation platform that brings together the proteins necessary for transport to the cilia, including the Rab11a-Rabin8-Rab8 complex involved in ciliogenesis. These specialized molecular assemblies, through successive action of discrete modules, cooperatively determine how rhodopsin and other rhodopsin-like signaling receptors gain access to primary cilia.

Trafficking in and to the primary cilium

Polarized vesicle trafficking is mediated by small GTPase proteins, such as Rabs and Arls/Arfs. These proteins have essential roles in maintaining normal cellular function, in part, through regulating intracellular trafficking. Moreover, these families of proteins have recently been implicated in the formation and function of the primary cilium. The primary cilium, which is found on almost every cell type in vertebrates, is an organelle that protrudes from the surface of the cell and functions as a signaling center. Interestingly, it has recently been linked to a variety of human diseases, collectively referred to as ciliopathies. The primary cilium has an exceptionally high density of receptors on its membrane that are important for sensing and transducing extracellular stimuli. Moreover, the primary cilium serves as a separate cellular compartment from the cytosol, providing for unique spatial and temporal regulation of signaling molecules to initiate downstream events. Thus, functional primary cilia are essential for normal signal transduction. Rabs and Arls/Arfs play critical roles in early cilia formation but are also needed for maintenance of ciliary function through their coordination with intraflagellar transport (IFT), a specialized trafficking system in primary cilia. IFT in cilia is pivotal for the proper movement of proteins into and out of this highly regulated organelle. In this review article, we explore the involvement of polarized vesicular trafficking in cilia formation and function, and discuss how defects in these processes could subsequently lead to the abnormalities observed in ciliopathies.

Endocytosis genes facilitate protein and membrane transport in C. elegans sensory cilia

BACKGROUND

Multiple intracellular transport pathways drive the formation, maintenance, and function of cilia, a compartmentalized organelle associated with motility, chemo-/mechano-/photosensation, and developmental signaling. These pathways include cilium-based intraflagellar transport (IFT) and poorly understood membrane trafficking events. Defects in ciliary transport contribute to the etiology of human ciliary disease such as Bardet-Biedl syndrome (BBS). In this study, we employ the genetically tractable nematode Caenorhabditis elegans to investigate whether endocytosis genes function in cilium formation and/or the transport of ciliary membrane or ciliary proteins.

RESULTS

Here we show that localization of the clathrin light chain, AP-2 clathrin adaptor, dynamin, and RAB-5 endocytic proteins overlaps with a morphologically discrete periciliary membrane compartment associated with sensory cilia. In addition, ciliary transmembrane proteins such as G protein-coupled receptors concentrate at periciliary membranes. Disruption of endocytic gene function causes expansion of ciliary and/or periciliary membranes as well as defects in the ciliary targeting and/or transport dynamics of ciliary transmembrane and IFT proteins. Finally, genetic analyses reveal that the ciliary membrane expansions in dynamin and AP-2 mutants require bbs-8 and rab-8 function and that sensory signaling and endocytic genes may function in a common pathway to regulate ciliary membrane volume.

CONCLUSIONS

These data implicate C. elegans endocytosis proteins localized at the ciliary base in regulating ciliary and periciliary membrane volume and suggest that membrane retrieval from these compartments is counterbalanced by BBS-8 and RAB-8-mediated membrane delivery.

Regulation of intraflagellar transport and ciliogenesis by small G proteins

Cilia rely on their distinctive protein compositions to function. Proteins gain access to the privileged ciliary compartment through two major routes, membrane trafficking and intraflagellar transport (IFT). Recent advances have provided two possible models for ciliary membrane transport: lateral diffusion and retention, and targeted vesicle transport. The Rab11-Rab8 cascade, which was originally discovered in the yeast's secretion pathway for bud formation, is shown to be required for cilia membrane assembly. Small GTPases, including two IFT particle subunits, and Ran, the master regulator for nuclear-cytoplasmic transport, are implicated in various aspects of IFT, a fundamental process required for the assembly of the microtubule-based backbone of cilia. This chapter reviews the key steps of ciliogenesis and possible mechanisms of IFT regulation, with emphasis on the regulatory roles of small GTPases and their regulators.