Intraflagellar Transport and Ciliary Dynamics

Rab35 controls cilium length, function and membrane composition

Rab and Arl guanine nucleotide-binding (G) proteins regulate trafficking pathways essential for the formation, function and composition of primary cilia, which are sensory devices associated with Sonic hedgehog (Shh) signalling and ciliopathies. Here, using mammalian cells and zebrafish, we uncover ciliary functions for Rab35, a multitasking G protein with endocytic recycling, actin remodelling and cytokinesis roles. Rab35 loss via siRNAs, morpholinos or knockout reduces cilium length in mammalian cells and the zebrafish left-right organiser (Kupffer's vesicle) and causes motile cilia-associated left-right asymmetry defects. Consistent with these observations, GFP-Rab35 localises to cilia, as do GEF (DENND1B) and GAP (TBC1D10A) Rab35 regulators, which also regulate ciliary length and Rab35 ciliary localisation. Mammalian Rab35 also controls the ciliary membrane levels of Shh signalling regulators, promoting ciliary targeting of Smoothened, limiting ciliary accumulation of Arl13b and the inositol polyphosphate 5-phosphatase (INPP5E). Rab35 additionally regulates ciliary PI(4,5)P2 levels and interacts with Arl13b. Together, our findings demonstrate roles for Rab35 in regulating cilium length, function and membrane composition and implicate Rab35 in pathways controlling the ciliary levels of Shh signal regulators.

Phosphorylation of CEP83 by TTBK2 is necessary for cilia initiation

Primary cilia are microtubule-based organelles that play important roles in development and tissue homeostasis. Tau-tubulin kinase-2 (TTBK2) is genetically linked to spinocerebellar ataxia type 11, and its kinase activity is crucial for ciliogenesis. Although it has been shown that TTBK2 is recruited to the centriole by distal appendage protein CEP164, little is known about TTBK2 substrates associated with its role in ciliogenesis. Here, we perform superresolution microscopy and discover that serum starvation results in TTBK2 redistribution from the periphery toward the root of distal appendages. Our biochemical analyses uncover CEP83 as a bona fide TTBK2 substrate with four phosphorylation sites characterized. We also demonstrate that CEP164-dependent TTBK2 recruitment to distal appendages is required for subsequent CEP83 phosphorylation. Specifically, TTBK2-dependent CEP83 phosphorylation is important for early ciliogenesis steps, including ciliary vesicle docking and CP110 removal. In summary, our results reveal a molecular mechanism of kinase regulation in ciliogenesis and identify CEP83 as a key substrate of TTBK2 during cilia initiation.

The Microtubule-Depolymerizing Kinesin-13 Klp10A Is Enriched in the Transition Zone of the Ciliary Structures of Drosophila melanogaster

The precursor of the flagellar axoneme is already present in the primary spermatocytes of Drosophila melanogaster. During spermatogenesis each primary spermatocyte shows a centriole pair that moves to the cell membrane and organizes an axoneme-based structure, the cilium-like region (CLR). The CLRs persist through the meiotic divisions and are inherited by young spermatids. During spermatid differentiation the ciliary caps elongate giving rise to the sperm axoneme. Mutations in Klp10A, a kinesin-13 of Drosophila, results in defects of centriole/CLR organization in spermatocytes and of ciliary cap assembly in elongating spermatids. Reduced Klp10A expression also results in strong structural defects of sensory type I neurons. We show, here, that this protein displays a peculiar localization during male gametogenesis. The Klp10A signal is first detected at the distal ends of the centrioles when they dock to the plasma membrane of young primary spermatocytes. At the onset of the first meiotic prometaphase, when the CLRs reach their full size, Klp10A is enriched in a distinct narrow area at the distal end of the centrioles and persists in elongating spermatids at the base of the ciliary cap. We conclude that Klp10A could be a core component of the ciliary transition zone in Drosophila.

The Autophagy-Cilia Axis: An Intricate Relationship

Primary cilia are microtubule-based organelles protruding from the surface of almost all vertebrate cells. This organelle represents the cell’s antenna which acts as a communication hub to transfer extracellular signals into intracellular responses during development and in tissue homeostasis. Recently, it has been shown that loss of cilia negatively regulates autophagy, the main catabolic route of the cell, probably utilizing the autophagic machinery localized at the peri-ciliary compartment. On the other side, autophagy influences ciliogenesis in a context-dependent manner, possibly to ensure that the sensing organelle is properly formed in a feedback loop model. In this review we discuss the recent literature and propose that the autophagic machinery and the ciliary proteins are functionally strictly related to control both autophagy and ciliogenesis. Moreover, we report examples of diseases associated with autophagic defects which cause cilia abnormalities, and propose and discuss the hypothesis that, at least some of the clinical manifestations observed in human diseases associated to ciliary disfunction may be the result of a perturbed autophagy.

How can memories last for days, years, or a lifetime? Proposed mechanisms for maintaining synaptic potentiation and memory

With memory encoding reliant on persistent changes in the properties of synapses, a key question is how can memories be maintained from days to months or a lifetime given molecular turnover? It is likely that positive feedback loops are necessary to persistently maintain the strength of synapses that participate in encoding. Such feedback may occur within signal-transduction cascades and/or the regulation of translation, and it may occur within specific subcellular compartments or within neuronal networks. Not surprisingly, numerous positive feedback loops have been proposed. Some posited loops operate at the level of biochemical signal-transduction cascades, such as persistent activation of Ca²⁺/calmodulin kinase II (CaMKII) or protein kinase Mζ. Another level consists of feedback loops involving transcriptional, epigenetic and translational pathways, and autocrine actions of growth factors such as BDNF. Finally, at the neuronal network level, recurrent reactivation of cell assemblies encoding memories is likely to be essential for late maintenance of memory. These levels are not isolated, but linked by shared components of feedback loops. Here, we review characteristics of some commonly discussed feedback loops proposed to underlie the maintenance of memory and long-term synaptic plasticity, assess evidence for and against their necessity, and suggest experiments that could further delineate the dynamics of these feedback loops. We also discuss crosstalk between proposed loops, and ways in which such interaction can facilitate the rapidity and robustness of memory formation and storage.

661W Photoreceptor Cell Line as a Cell Model for Studying Retinal Ciliopathies

The retina contains several ciliated cell types, including the retinal pigment epithelium (RPE) and photoreceptor cells. The photoreceptor cilium is one of the most highly modified sensory cilia in the human body. The outer segment of the photoreceptor is a highly elaborate primary cilium, containing stacks or folds of membrane where the photopigment molecules are located. Perhaps unsurprisingly, defects in cilia often lead to retinal phenotypes, either as part of syndromic conditions involving other organs, or in isolation in the so-called retinal ciliopathies. The study of retinal ciliopathies has been limited by a lack of retinal cell lines. RPE1 retinal pigment epithelial cell line is commonly used in such studies, but the existence of a photoreceptor cell line has largely been neglected in the retinal ciliopathy field. 661W cone photoreceptor cells, derived from mouse, have been widely used as a model for studying macular degeneration, but not described as a model for studying retinal ciliopathies such as retinitis pigmentosa. Here, we characterize the 661W cell line as a model for studying retinal ciliopathies. We fully characterize the expression profile of these cells, using whole transcriptome RNA sequencing, and provide this data on Gene Expression Omnibus for the advantage of the scientific community. We show that these cells express the majority of markers of cone cell origin. Using immunostaining and confocal microscopy, alongside scanning electron microscopy, we show that these cells grow long primary cilia, reminiscent of photoreceptor outer segments, and localize many cilium proteins to the axoneme, membrane and transition zone. We show that siRNA knockdown of cilia genes Ift88 results in loss of cilia, and that this can be assayed by high-throughput screening. We present evidence that the 661W cell line is a useful cell model for studying retinal ciliopathies.

Motional dynamics of single Patched1 molecules in cilia are controlled by Hedgehog and cholesterol

The Hedgehog-signaling pathway is an important target in cancer research and regenerative medicine; yet, on the cellular level, many steps are still poorly understood. Extensive studies of the bulk behavior of the key proteins in the pathway established that during signal transduction they dynamically localize in primary cilia, antenna-like solitary organelles present on most cells. The secreted Hedgehog ligand Sonic Hedgehog (SHH) binds to its receptor Patched1 (PTCH1) in primary cilia, causing its inactivation and delocalization from cilia. At the same time, the transmembrane protein Smoothened (SMO) is released of its inhibition by PTCH1 and accumulates in cilia. We used advanced, single molecule-based microscopy to investigate these processes in live cells. As previously observed for SMO, PTCH1 molecules in cilia predominantly move by diffusion and less frequently by directional transport, and spend a fraction of time confined. After treatment with SHH we observed two major changes in the motional dynamics of PTCH1 in cilia. First, PTCH1 molecules spend more time as confined, and less time freely diffusing. This result could be mimicked by a depletion of cholesterol from cells. Second, after treatment with SHH, but not after cholesterol depletion, the molecules that remain in the diffusive state showed a significant increase in the diffusion coefficient. Therefore, PTCH1 inactivation by SHH changes the diffusive motion of PTCH1, possibly by modifying the membrane microenvironment in which PTCH1 resides.

Polyubiquitylation of α-tubulin at K304 is required for flagellar disassembly in Chlamydomonas

Cilia/flagella are structurally conserved and dynamic organelles; their assembly and disassembly are coordinated with the cell cycle and cell differentiation. Several post-translational modifications, including acetylation, methylation, phosphorylation and ubiquitylation, participate in ciliary disassembly. However, the detailed mechanism and the role of ubiquitylation in ciliary disassembly are unclear. This study identified 20 proteins that were ubiquitylated in shortening flagella of Chlamydomonas α-Tubulin was the most abundant ubiquitylated protein and it was labeled with K63 polyubiquitin chains primarily at K304. Expression of an α-tubulin mutant (K304R), which could not be ubiquitylated, decreased the rate of flagellar disassembly and resulted in an enrichment of the mutant form in the axoneme, suggesting that ubiquitylation of α-tubulin is required for the normal kinetics of axonemal disassembly. Immunoprecipitation and glutathione-S-transferase pulldown assays demonstrated that the retrograde intraflagellar transport (IFT) protein, IFT139, interacted with a variety of ubiquitylated proteins, including α-tubulin, suggesting that IFT-A was responsible for transporting ubiquitylated proteins out of the flagella. Our data suggest an important role for ubiquitylation and retrograde IFT in ciliary disassembly.This article has an associated First Person interview with the first author of the paper.

Microscopy-Based Automated Live Cell Screening for Small Molecules That Affect Ciliation

The primary monocilium, or cilium, is a single antenna-like organelle that protrudes from the surface of most mammalian cell types, and serves as a signaling hub. Mutations of cilia-associated genes result in severe genetic disorders termed ciliopathies. Among these, the most common is autosomal dominant polycystic kidney disease (ADPKD); less common genetic diseases include Bardet–Biedl syndrome, Joubert syndrome, nephronophthisis, and others. Important signaling cascades with receptor systems localized exclusively or in part at cilia include Sonic Hedgehog (SHH), platelet derived growth factor alpha (PDGFRα), WNTs, polycystins, and others. Changes in ciliation during development or in pathological conditions such as cancer impacts signaling by these proteins. Notably, ciliation status of cells is coupled closely to the cell cycle, with cilia protruding in quiescent (G0) or early G1 cells, declining in S/G2, and absent in M phase, and has been proposed to contribute to cell cycle regulation. Because of this complex biology, the elaborate machinery regulating ciliary assembly and disassembly receives input from many cellular proteins relevant to cell cycle control, development, and oncogenic transformation, making study of genetic factors and drugs influencing ciliation of high interest. One of the most effective tools to investigate the dynamics of the cilia under different conditions is the imaging of live cells. However, developing assays to observe the primary cilium in real time can be challenging, and requires a consideration of multiple details related to the cilia biology. With the dual goals of identifying small molecules that may have beneficial activity through action on human diseases, and of identifying ciliary activities of existing agents that are in common use or development, we here describe creation and evaluation of three autofluorescent cell lines derived from the immortalized retinal pigmented epithelium parental cell line hTERT-RPE1. These cell lines stably express the ciliary-targeted fluorescent proteins L13-Arl13bGFP, pEGFP-mSmo, and tdTomato-MCHR1-N-10. We then describe methods for use of these cell lines in high throughput screening of libraries of small molecule compounds to identify positive and negative regulators of ciliary disassembly.

Mechanism and Regulation of Centriole and Cilium Biogenesis

The centriole is an ancient microtubule-based organelle with a conserved nine-fold symmetry. Centrioles form the core of centrosomes, which organize the interphase microtubule cytoskeleton of most animal cells and form the poles of the mitotic spindle. Centrioles can also be modified to form basal bodies, which template the formation of cilia and play central roles in cellular signaling, fluid movement, and locomotion. In this review, we discuss developments in our understanding of the biogenesis of centrioles and cilia and the regulatory controls that govern their structure and number. We also discuss how defects in these processes contribute to a spectrum of human diseases and how new technologies have expanded our understanding of centriole and cilium biology, revealing exciting avenues for future exploration.

Primary Cilia as Signaling Hubs in Health and Disease

Primary cilia detect extracellular cues and transduce these signals into cells to regulate proliferation, migration, and differentiation. Here, the function of primary cilia as signaling hubs of growth factors and morphogens is in focus. First, the molecular mechanisms regulating the assembly and disassembly of primary cilia are described. Then, the role of primary cilia in mediating growth factor and morphogen signaling to maintain human health and the potential mechanisms by which defects in these pathways contribute to human diseases, such as ciliopathy, obesity, and cancer are described. Furthermore, a novel signaling pathway by which certain growth factors stimulate cell proliferation through suppression of ciliogenesis is also described, suggesting novel therapeutic targets in cancer.

Dependency relationships between IFT-dependent flagellum elongation and cell morphogenesis in Leishmania

Flagella have multiple functions that are associated with different axonemal structures. Motile flagella typically have a 9 + 2 arrangement of microtubules, whereas sensory flagella normally have a 9 + 0 arrangement. Leishmania exhibits both of these flagellum forms and differentiation between these two flagellum forms is associated with cytoskeletal and cell shape changes. We disrupted flagellum elongation in Leishmania by deleting the intraflagellar transport (IFT) protein IFT140 and examined the effects on cell morphogenesis. Δift140 cells have no external flagellum, having only a very short flagellum within the flagellar pocket. This short flagellum had a collapsed 9 + 0 (9v) axoneme configuration reminiscent of that in the amastigote and was not attached to the pocket membrane. Although amastigote-like changes occurred in the flagellar cytoskeleton, the cytoskeletal structures of Δift140 cells retained their promastigote configurations, as examined by fluorescence microscopy of tagged proteins and serial electron tomography. Thus, Leishmania promastigote cell morphogenesis does not depend on the formation of a long flagellum attached at the neck. Furthermore, our data show that disruption of the IFT system is sufficient to produce a switch from the 9 + 2 to the collapsed 9 + 0 (9v) axonemal structure, echoing the process that occurs during the promastigote to amastigote differentiation.

Biallelic Mutations in LRRC56, Encoding a Protein Associated with Intraflagellar Transport, Cause Mucociliary Clearance and Laterality Defects

Primary defects in motile cilia result in dysfunction of the apparatus responsible for generating fluid flows. Defects in these mechanisms underlie disorders characterized by poor mucus clearance, resulting in susceptibility to chronic recurrent respiratory infections, often associated with infertility; laterality defects occur in about 50% of such individuals. Here we report biallelic variants in LRRC56 (known as oda8 in Chlamydomonas) identified in three unrelated families. The phenotype comprises laterality defects and chronic pulmonary infections. High-speed video microscopy of cultured epithelial cells from an affected individual showed severely dyskinetic cilia but no obvious ultra-structural abnormalities on routine transmission electron microscopy (TEM). Further investigation revealed that LRRC56 interacts with the intraflagellar transport (IFT) protein IFT88. The link with IFT was interrogated in Trypanosoma brucei. In this protist, LRRC56 is recruited to the cilium during axoneme construction, where it co-localizes with IFT trains and is required for the addition of dynein arms to the distal end of the flagellum. In T. brucei carrying LRRC56-null mutations, or a variant resulting in the p.Leu259Pro substitution corresponding to the p.Leu140Pro variant seen in one of the affected families, we observed abnormal ciliary beat patterns and an absence of outer dynein arms restricted to the distal portion of the axoneme. Together, our findings confirm that deleterious variants in LRRC56 result in a human disease and suggest that this protein has a likely role in dynein transport during cilia assembly that is evolutionarily important for cilia motility.

The regulation of cilium assembly and disassembly in development and disease

The primary cilium is an antenna-like organelle assembled on most types of quiescent and differentiated mammalian cells. This immotile structure is essential for interpreting extracellular signals that regulate growth, development and homeostasis. As such, ciliary defects produce a spectrum of human diseases, termed ciliopathies, and deregulation of this important organelle also plays key roles during tumor formation and progression. Recent studies have begun to clarify the key mechanisms that regulate ciliary assembly and disassembly in both normal and tumor cells, highlighting new possibilities for therapeutic intervention. Here, we review these exciting new findings, discussing the molecular factors involved in cilium formation and removal, the intrinsic and extrinsic control of cilium assembly and disassembly, and the relevance of these processes to mammalian cell growth and disease.

The "transition zone" of the cilium-like regions in the Drosophila spermatocytes and the role of the C-tubule in axoneme assembly

The fruit-fly Drosophila melanogaster harbours different types of ciliary structures: ciliary projections associated with neurons of type I and cilium-like regions (CLRs) found during male gametogenesis. The latter deserve particular attention since they are morphologically similar to vertebrate primary cilia and transform into the sperm axonemes during spermiogenesis. Although, all the centrioles are able to organize the CLRs, we found that the mother centriole docks first to the plasma membrane suggesting a new intrinsic functional asymmetry between the parent centrioles. We also show that the CLRs lack the Y-links that connect the axoneme doublets with the plasma membrane in conventional primary cilia. Moreover, the C-tubules, that are lacking in the axoneme of the primary cilia, persisted along the CLRs albeit modified into longitudinal blades. Remarkably, mutant flies in which the CLRs are devoid of the C-tubules or their number is reduced lack sperm axonemes or have incomplete axonemes. Therefore, the C-tubules are dispensable for the assembly of the CLRs but are essential for sperm axoneme elongation and maintenance in Drosophila.

Cilium structure, assembly, and disassembly regulated by the cytoskeleton

The cilium, once considered a vestigial structure, is a conserved, microtubule-based organelle critical for transducing extracellular chemical and mechanical signals that control cell polarity, differentiation, and proliferation. The cilium undergoes cycles of assembly and disassembly that are controlled by complex inter-relationships with the cytoskeleton. Microtubules form the core of the cilium, the axoneme, and are regulated by post-translational modifications, associated proteins, and microtubule dynamics. Although actin and septin cytoskeletons are not major components of the axoneme, they also regulate cilium organization and assembly state. Here, we discuss recent advances on how these different cytoskeletal systems­ affect cilium function, structure, and organization.

Ciliary Length Sensing Regulates IFT Entry via Changes in FLA8/KIF3B Phosphorylation to Control Ciliary Assembly

The length of cilia is robustly regulated [1]. Previous data suggest that cells possess a sensing system to control ciliary length [2-5]. However, the details of the mechanism are currently not known [6, 7]. Such a system requires a mechanism that responds to ciliary length, and consequently, disruption of that response system should alter ciliary length [1]. The assembly rate of cilium mediated by intraflagellar transport (IFT) gradually decreases as the cilium elongates and eventually is balanced by the constant rate of disassembly, at which point cilium elongation stops [8, 9]. Because the rate of IFT entry into the cilium also decreases as the cilium elongates [10], regulation of IFT entry could provide the mechanism for length control. Previously, we showed that phosphorylation of the FLA8/KIF3B subunit of the anterograde kinesin-II IFT motor blocks IFT entry and flagellar assembly in Chlamydomonas [11]. Here, we show in Chlamydomonas that cellular signaling in response to alteration of flagellar length regulates phosphorylation of FLA8/KIF3B, which restricts IFT entry and, thus, flagellar assembly to control flagellar length. Cellular levels of phosphorylated FLA8 (pFLA8) are tightly linked to flagellar length: FLA8 phosphorylation is reduced in cells with short flagella and elevated in cells with long flagella. Depletion of the phosphatases CrPP1 and CrPP6 increases the level of cellular pFLA8, leading to short flagella due to decreased IFT entry. The results demonstrate that ciliary length control is achieved by a cellular sensing system that controls IFT entry through phosphorylation of the anterograde IFT motor.

Microtubules and Microtubule-Associated Proteins

Microtubules act as "railways" for motor-driven intracellular transport, interact with accessory proteins to assemble into larger structures such as the mitotic spindle, and provide an organizational framework to the rest of the cell. Key to these functions is the fact that microtubules are "dynamic." As with actin, the polymer dynamics are driven by nucleotide hydrolysis and influenced by a host of specialized regulatory proteins, including microtubule-associated proteins. However, microtubule turnover involves a surprising behavior-termed dynamic instability-in which individual polymers switch stochastically between growth and depolymerization. Dynamic instability allows microtubules to explore intracellular space and remodel in response to intracellular and extracellular cues. Here, we review how such instability is central to the assembly of many microtubule-based structures and to the robust functioning of the microtubule cytoskeleton.

Holoprosencephaly in the genomics era

Holoprosencephaly (HPE) is the direct consequence of specific genetic and/or environmental insults interrupting the midline specification of the nascent forebrain. Such disturbances can lead to a broad range of phenotypic consequences for the brain and face in humans. This malformation sequence is remarkably common in utero (1 in 250 human fetuses), but 97% typically do not survive to birth. The precise molecular pathogenesis of HPE in these early human embryos remains largely unknown. Here, we outline our current understanding of the principal driving factors leading to HPE pathologies and elaborate our multifactorial integrated genomics approach. Overall, our understanding of the pathogenesis continues to become simpler, rather than more complicated. Genomic technologies now provide unprecedented insight into disease-associated variation, including the overall extent of genetic interactions (coding and noncoding) predicted to explain divergent phenotypes.

Rab23 and developmental disorders

Rab23 is a conserved member of the Rab family of small GTPases that regulates membrane trafficking in eukaryotes. It is unique amongst the Rabs in terms of its implicated role in mammalian development, as originally illustrated by the embryonic lethality and open neural tube phenotype of a spontaneous mouse mutant that carries homozygous mutation of open brain, a gene encoding Rab23. Rab23 was initially identified to act as an antagonist of Sonic hedgehog (Shh) signaling, and has since been implicated in a number of physiological and pathological roles, including oncogenesis. Interestingly, RAB23 null allele homozygosity in humans is not lethal, but instead causes the developmental disorder Carpenter’s syndrome (CS), which is characterized by craniofacial malformations, polysyndactyly, obesity and intellectual disability. CS bears some phenotypic resemblance to a spectrum of hereditary defects associated with the primary cilium, or the ciliopathies. Recent findings have in fact implicated Rab23 in protein traffic to the primary cilium, thus linking it with the primary cellular locale of Shh signaling. Rab23 also has Shh and cilia-independent functions. It is known to mediate the expression of Nodal at the mouse left lateral plate mesoderm and Kupffer’s vesicle, the zebrafish equivalent of the mouse node. It is thus important for the left-right patterning of vertebrate embryos. In this review, we discuss the developmental disorders associated with Rab23 and attempt to relate its cellular activities to its roles in development.

Motor Proteins

Myosin motors power movements on actin filaments, whereas dynein and kinesin motors power movements on microtubules. The mechanisms of these motor proteins differ, but, in all cases, ATP hydrolysis and subsequent release of the hydrolysis products drives a cycle of interactions with the track (either an actin filament or a microtubule), resulting in force generation and directed movement.

Multispecies exclusion process with fusion and fission of rods: A model inspired by intraflagellar transport

We introduce a multispecies exclusion model where length-conserving probabilistic fusion and fission of the hard rods are allowed. Although all rods enter the system with the same initial length ℓ=1, their length can keep changing, because of fusion and fission, as they move in a step-by-step manner towards the exit. Two neighboring hard rods of lengths ℓ_{1} and ℓ_{2} can fuse into a single rod of longer length ℓ=ℓ_{1}+ℓ_{2} provided ℓ≤N. Similarly, length-conserving fission of a rod of length ℓ^{'}≤N results in two shorter daughter rods. Based on the extremum current hypothesis, we plot the phase diagram of the model under open boundary conditions utilizing the results derived for the same model under periodic boundary condition using mean-field approximation. The density profile and the flux profile of rods are in excellent agreement with computer simulations. Although the fusion and fission of the rods are motivated by similar phenomena observed in intraflagellar transport (IFT) in eukaryotic flagella, this exclusion model is too simple to account for the quantitative experimental data for any specific organism. Nevertheless, the concepts of "flux profile" and "transition zone" that emerge from the interplay of fusion and fission in this model are likely to have important implications for IFT and for other similar transport phenomena in long cell protrusions.

Signaling through the Primary Cilium

The presence of single, non-motile “primary” cilia on the surface of epithelial cells has been well described since the 1960s. However, for decades these organelles were believed to be vestigial, with no remaining function, having lost their motility. It wasn't until 2003, with the discovery that proteins responsible for transport along the primary cilium are essential for hedgehog signaling in mice, that the fundamental importance of primary cilia in signal transduction was realized. Little more than a decade later, it is now clear that the vast majority of signaling pathways in vertebrates function through the primary cilium. This has led to the adoption of the term “the cells's antenna” as a description for the primary cilium. Primary cilia are particularly important during development, playing fundamental roles in embryonic patterning and organogenesis, with a suite of inherited developmental disorders known as the “ciliopathies” resulting from mutations in genes encoding cilia proteins. This review summarizes our current understanding of the role of these fascinating organelles in a wide range of signaling pathways.

Mechanisms of ciliary targeting: entering importins and Rabs

Primary cilium is a rod-like plasma membrane protrusion that plays important roles in sensing the cellular environment and initiating corresponding signaling pathways. The sensory functions of the cilium critically depend on the unique enrichment of ciliary residents, which is maintained by the ciliary diffusion barrier. It is still unclear how ciliary cargoes specifically enter the diffusion barrier and accumulate within the cilium. In this review, the organization and trafficking mechanism of the cilium are compared to those of the nucleus, which are much better understood at the moment. Though the cilium differs significantly from the nucleus in terms of molecular and cellular functions, analogous themes and principles in the membrane organization and cargo trafficking are notable between them. Therefore, knowledge in the nuclear trafficking can likely shed light on our understanding of the ciliary trafficking. Here, with a focus on membrane cargoes in mammalian cells, we briefly review various ciliary trafficking pathways from the Golgi to the periciliary membrane. Models for the subsequent import translocation across the diffusion barrier and the enrichment of cargoes within the ciliary membrane are discussed in detail. Based on recent discoveries, we propose a Rab-importin-based model in an attempt to accommodate various observations on ciliary targeting.

Trainspotting in a cilium

A new imaging technique sheds light on how cilia regulate their length and growth.

Neurodevelopmental disease mechanisms, primary cilia, and endosomes converge on the BLOC-1 and BORC complexes

The biogenesis of lysosome-related organelles complex-1 (BLOC-1) and the bloc-one-related complex (BORC) are the cytosolic protein complexes required for specialized membrane protein traffic along the endocytic route and the spatial distribution of endosome-derived compartments, respectively. BLOC-1 and BORC complex subunits and components of their interactomes have been associated with the risk and/or pathomechanisms of neurodevelopmental disorders. Thus, cellular processes requiring BLOC-1 and BORC interactomes have the potential to offer novel insight into mechanisms underlying behavioral defects. We focus on interactions between BLOC-1 or BORC subunits with the actin and microtubule cytoskeleton, membrane tethers, and SNAREs. These interactions highlight requirements for BLOC-1 and BORC in membrane movement by motors, control of actin polymerization, and targeting of membrane proteins to specialized cellular domains such as the nerve terminal and the primary cilium. We propose that the endosome-primary cilia pathway is an underappreciated hub in the genesis and mechanisms of neurodevelopmental disorders. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 311-330, 2018.

Intraflagellar transporter protein (IFT27), an IFT25 binding partner, is essential for male fertility and spermiogenesis in mice

Intraflagellar transport (IFT) is an evolutionarily conserved mechanism essential for the assembly and maintenance of most eukaryotic cilia and flagella. In mice, mutations in IFT proteins have been shown to cause several ciliopathies including retinal degeneration, polycystic kidney disease, and hearing loss. However, little is known about its role in the formation of the sperm tail, which has the longest flagella of mammalian cells. IFT27 is a component of IFT-B complex and binds to IFT25 directly. In mice, IFT27 is highly expressed in the testis. To investigate the role of IFT27 in male germ cells, the floxed Ift27 mice were bred with Stra8-iCre mice so that the Ift27 gene was disrupted in spermatocytes/spermatids. The Ift27: Stra8-iCre mutant mice did not show any gross abnormalities, and all of the mutant mice survived to adulthood. There was no difference between testis weight/body weight between controls and mutant mice. All adult homozygous mutant males examined were completely infertile. Histological examination of the testes revealed abnormally developed germ cells during the spermiogenesis phase. The epididymides contained round bodies of cytoplasm. Sperm number was significantly reduced compared to the controls and only about 2% of them remained significantly reduced motility. Examination of epididymal sperm by light microscopy and SEM revealed multiple morphological abnormalities including round heads, short and bent tails, abnormal thickness of sperm tails in some areas, and swollen tail tips in some sperm. TEM examination of epididymal sperm showed that most sperm lost the "9+2″ axoneme structure, and the mitochondria sheath, fibrous sheath, and outer dense fibers were also disorganized. Some sperm flagella also lost cell membrane. Levels of IFT25 and IFT81 were significantly reduced in the testis of the conditional Ift27 knockout mice, and levels of IFT20, IFT74, and IFT140 were not changed. Sperm lipid rafts, which were disrupted in the conditional Ift25 knockout mice, appeared to be normal in the conditional Ift27 knockout mice. Our findings suggest that like IFT25, IFT27, even though not required for ciliogenesis in somatic cells, is essential for sperm flagella formation, sperm function, and male fertility in mice. IFT25 and IFT27 control sperm formation/function through many common mechanisms, but IFT25 has additional roles beyond IFT27.

Early ciliary and prominin-1 dysfunctions precede neurogenesis impairment in a mouse model of type 2 diabetes

Diabetes mellitus (DM) is reaching epidemic conditions worldwide and increases the risk for cognition impairment and dementia. Here, we postulated that progenitors in adult neurogenic niches might be particularly vulnerable. Therefore, we evaluated the different components of the mouse subventricular zone (SVZ) during the first week after chemical induction of type 1 and type 2 diabetes-like (T1DM and T2DM) conditions. Surprisingly, only T2DM mice showed SVZ damage. The initial lesions were localized to ependymal cilia, which appeared disorientated and clumped together. In addition, they showed delocalization of the ciliary membrane protein prominin-1. Impairment of neuroprogenitor proliferation, neurogenic marker abnormalities and ectopic migration of neuroblasts were found at a later stage. To our knowledge, our data describe for the first time such an early impact of T2DM on the SVZ. This is consistent with clinical data indicating that brain damage in T2DM patients differs from that in T1DM patients.

The Ciliary Transition Zone: Finding the Pieces and Assembling the Gate

Eukaryotic cilia are organelles that project from the surface of cells to fulfill motility and sensory functions. In vertebrates, the functions of both motile and immotile cilia are critical for embryonic development and adult tissue homeostasis. Importantly, a multitude of human diseases is caused by abnormal cilia biogenesis and functions which rely on the compartmentalization of the cilium and the maintenance of its protein composition. The transition zone (TZ) is a specialized ciliary domain present at the base of the cilium and is part of a gate that controls protein entry and exit from this organelle. The relevance of the TZ is highlighted by the fact that several of its components are coded by ciliopathy genes. Here we review recent developments in the study of TZ proteomes, the mapping of individual components to the TZ structure and the establishment of the TZ as a lipid gate.