EB1 and EB3 promote cilia biogenesis by several centrosome-related mechanisms

Molecular organization of the distal tip of vertebrate motile cilia

The beating of cilia on multi-ciliated cells (MCCs) is essential for normal development and homeostasis in animals. Unlike basal bodies or axonemes, the distal tips of MCC cilia remain poorly defined. Here, we characterize the molecular organization of the distal tip of vertebrate MCC cilia, revealing two distinct domains occupied by distinct protein constituents. Using frog, mouse, and human MCCs, we find that two largely uncharacterized proteins, Ccdc78 and Ccdc33 occupy a specialized region at the extreme distal tip, and these are required for the normal organization of other tip proteins, including Spef1, Cep104, and Eb3. Ccdc78 and Cccdc33 are also independently required for normal length regulation of MCC cilia. Mechanistically, Ccdc78 and Ccdc33 display robust microtubule-bundling activity both in vivo and in vitro . Thus, we reveal that two previously undefined proteins form a key module for organizing and stabilizing the distal tip of motile cilia in vertebrate MCC. We propose that these proteins represent potential disease loci for motile ciliopathies.

Axonemal microtubule dynamics in the assembly and disassembly of cilia

Cilia and eukaryotic flagella (exchangeable terms) function in cell motility and signaling, which are pivotal for development and physiology. Cilia dysfunction can lead to ciliopathies. Cilia are usually assembled in quiescent and/or differentiated cells and undergo disassembly when cells enter cell cycle or in response to environmental stresses. Cilia contain a microtubule-based structure termed axoneme that comprises nine outer doublet microtubules with or without a pair of central microtubules, which is ensheathed by the ciliary membrane. Regulation of the axonemal microtubule dynamics is tightly associated with ciliary assembly and disassembly. In this short review, we discuss recent findings on the regulation of axonemal microtubules by microtubule-binding proteins and microtubule modulating kinesins during ciliary assembly and disassembly.

A novel GFAP frameshift variant identified in a family with optico-retinal dysplasia and vision impairment

Gain-of-function variants in GFAP leads to protein aggregation and is the cause of the severe neurodegenerative disorder Alexander Disease (AxD), while loss of GFAP function has been considered benign. Here, we investigated a six-generation family, where multiple individuals presented with gliosis of the optic nerve head and visual impairment. Whole genome sequencing (WGS) revealed a frameshift variant in GFAP (c.928dup, p.(Met310Asnfs*113)) segregating with disease. Analysis of human embryonic tissues revealed strong expression of GFAP in retinal neural progenitors. A zebrafish model verified that c.928dup does not result in extensive GFAP protein aggregation and zebrafish gfap loss-of-function mutants showed vision impairment and retinal dysplasia, characterized by a significant loss of Müller glia cells and photoreceptor cells. Our findings show how different mutational mechanisms can cause diverging phenotypes and reveal a novel function of GFAP in vertebrate eye development.

Kif9 is an active kinesin motor required for ciliary beating and proximodistal patterning of motile axonemes

Most motile cilia have a stereotyped structure of nine microtubule outer doublets and a single central pair of microtubules. The central pair of microtubules are surrounded by a set of proteins, termed the central pair apparatus. A specific kinesin, Klp1 projects from the central pair and contributes to ciliary motility in Chlamydomonas. The vertebrate ortholog, Kif9, is required for beating in mouse sperm flagella, but the mechanism of Kif9/Klp1 function remains poorly defined. Here, using Xenopus epidermal multiciliated cells, we show that Kif9 is necessary for ciliary motility and the proper distal localization of not only central pair proteins, but also radial spokes and dynein arms. In addition, single-molecule assays in vitro reveal that Xenopus Kif9 is a long-range processive motor, although it does not mediate long-range movement in ciliary axonemes in vivo. Together, our data suggest that Kif9 is integral for ciliary beating and is necessary for proper axonemal distal end integrity.

Structure and function of distal and subdistal appendages of the mother centriole

Centrosomes are composed of centrioles surrounded by pericentriolar material. The two centrioles in G1 phase are distinguished by the localization of their appendages in the distal and subdistal regions; the centriole possessing both types of appendage is older and referred to as the mother centriole, whereas the other centriole lacking appendages is the daughter centriole. Both distal and subdistal appendages in vertebrate cells consist of multiple proteins assembled in a hierarchical manner. Distal appendages function mainly in the initial process of ciliogenesis, and subdistal appendages are involved in microtubule anchoring, mitotic spindle regulation and maintenance of ciliary signaling. Mutations in genes encoding components of both appendage types are implicated in ciliopathies and developmental defects. In this Review, we discuss recent advances in knowledge regarding the composition and assembly of centriolar appendages, as well as their roles in development and disease.

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.

The ciliopathy protein CCDC66 controls mitotic progression and cytokinesis by promoting microtubule nucleation and organization

Precise spatiotemporal control of microtubule nucleation and organization is critical for faithful segregation of cytoplasmic and genetic material during cell division and signaling via the primary cilium in quiescent cells. Microtubule-associated proteins (MAPs) govern assembly, maintenance, and remodeling of diverse microtubule arrays. While a set of conserved MAPs are only active during cell division, an emerging group of MAPs acts as dual regulators in dividing and nondividing cells. Here, we elucidated the nonciliary functions and molecular mechanism of action of the ciliopathy-linked protein CCDC66, which we previously characterized as a regulator of ciliogenesis in quiescent cells. We showed that CCDC66 dynamically localizes to the centrosomes, the bipolar spindle, the spindle midzone, the central spindle, and the midbody in dividing cells and interacts with the core machinery of centrosome maturation and MAPs involved in cell division. Loss-of-function experiments revealed its functions during mitotic progression and cytokinesis. Specifically, CCDC66 depletion resulted in defective spindle assembly and orientation, kinetochore fiber stability, chromosome alignment in metaphase as well as central spindle and midbody assembly and organization in anaphase and cytokinesis. Notably, CCDC66 regulates mitotic microtubule nucleation via noncentrosomal and centrosomal pathways via recruitment of gamma-tubulin to the centrosomes and the spindle. Additionally, CCDC66 bundles microtubules in vitro and in cells by its C-terminal microtubule-binding domain. Phenotypic rescue experiments showed that the microtubule and centrosome-associated pools of CCDC66 individually or cooperatively mediate its mitotic and cytokinetic functions. Collectively, our findings identify CCDC66 as a multifaceted regulator of the nucleation and organization of the diverse mitotic and cytokinetic microtubule arrays and provide new insight into nonciliary defects that underlie ciliopathies.

The ciliopathy-linked protein CCDC66 is only known for its ciliary functions. This study reveals that CCDC66 also has extensive non-ciliary functions, localizing to the spindle poles, spindle midzone, central spindle and midbody throughout cell division, where it regulates mitosis and cytokinesis by promoting microtubule nucleation and organization.

Manipulation of the Tubulin Code Alters Directional Cell Migration and Ciliogenesis

Conjunction of epithelial cells into monolayer sheets implies the ability to migrate and to undergo apicobasal polarization. Both processes comprise reorganization of cytoskeletal elements and rearrangements of structural protein interactions. We modulated expression of tubulin tyrosin ligase (TTL), the enzyme that adds tyrosine to the carboxy terminus of detyrosinated α-tubulin, to study the role of tubulin detyrosination/-tyrosination in the orientation of cell motility and in epithelial morphogenesis. Oriented cell migration and the organization of focal adhesions significantly lose directionality with diminishing amounts of microtubules enriched in detyrosinated tubulin. On the other hand, increasing quantities of detyrosinated tubulin results in faster plus end elongation of microtubules in migrating and in polarized epithelial cells. These plus ends are decorated by the plus end binding protein 1 (EB1), which mediates interaction between microtubules enriched in detyrosinated tubulin and the integrin-ILK complex at focal adhesions. EB1 accumulates at the apical cell pole at the base of the primary cilium following apicobasal polarization. Polarized cells almost devoid of detyrosinated tubulin form stunted primary cilia and multiluminal cysts in 3D-matrices. We conclude that the balance between detyrosinated and tyrosinated tubulin alters microtubule dynamics, affects the orientation of focal adhesions and determines the organization of primary cilia on epithelial cells.

Cytoskeleton Protein EB3 Contributes to Dendritic Spines Enlargement and Enhances Their Resilience to Toxic Effects of Beta-Amyloid

EB3 protein is expressed abundantly in the nervous system and transiently enters the dendritic spines at the tip of the growing microtubule, which leads to spine enlargement. Nevertheless, the role of dynamic microtubules, and particularly EB3 protein, in synapse function is still elusive. By manipulating the EB3 expression level, we have shown that this protein is required for a normal dendritogenesis. Nonetheless, EB3 overexpression also reduces hippocampal neurons dendritic branching and total dendritic length. This effect likely occurs due to the speeding neuronal development cycle from dendrite outgrowth to the step when dendritic spines are forming. Implementing direct morphometric characterization of dendritic spines, we showed that EB3 overexpression leads to a dramatic increase in the dendritic spine head area. EB3 knockout oppositely reduces spine head area and increases spine neck length and spine neck/spine length ratio. The same effect is observed in conditions of amyloid-beta toxicity, modeling Alzheimer`s disease. Neck elongation is supposed to be a common detrimental effect on the spine's shape, which makes them biochemically and electrically less connected to the dendrite. EB3 also potentiates the formation of presynaptic protein Synapsin clusters and CaMKII-alpha preferential localization in spines rather than in dendrites of hippocampal neurons, while its downregulation has an opposite effect and reduces the size of presynaptic protein clusters Synapsin and PSD95. EB3's role in spine development and maturation determines its neuroprotective effect. EB3 overexpression makes dendritic spines resilient to amyloid-beta toxicity, restores altered PSD95 clustering, and reduces CaMKII-alpha localization in spines observed in this pathological state.

Bardet-Biedl syndrome proteins modulate the release of bioactive extracellular vesicles

Primary cilia are microtubule based sensory organelles important for receiving and processing cellular signals. Recent studies have shown that cilia also release extracellular vesicles (EVs). Because EVs have been shown to exert various physiological functions, these findings have the potential to alter our understanding of how primary cilia regulate specific signalling pathways. So far the focus has been on lgEVs budding directly from the ciliary membrane. An association between cilia and MVB-derived smEVs has not yet been described. We show that ciliary mutant mammalian cells demonstrate increased secretion of small EVs (smEVs) and a change in EV composition. Characterisation of smEV cargo identified signalling molecules that are differentially loaded upon ciliary dysfunction. Furthermore, we show that these smEVs are biologically active and modulate the WNT response in recipient cells. These results provide us with insights into smEV-dependent ciliary signalling mechanisms which might underly ciliopathy disease pathogenesis.

Extracellular vesicles (EV) are known to be released from the primary cilium, but the role ciliary proteins play in EV biogenesis remains unexplored. Here, the authors demonstrate increased secretion of small EVs with altered cargo composition from cells with known ciliarelated mutations. Wnt related molecules made up a majority of altered cargo

Evidence for a Syncytial Origin of Eukaryotes from Ancestral State Reconstruction

Modern accounts of eukaryogenesis entail an endosymbiotic encounter between an archaeal host and a proteobacterial endosymbiont, with subsequent evolution giving rise to a unicell possessing a single nucleus and mitochondria. The mononucleate state of the last eukaryotic common ancestor (LECA) is seldom, if ever, questioned, even though cells harboring multiple (syncytia, coenocytes, and polykaryons) are surprisingly common across eukaryotic supergroups. Here, we present a survey of multinucleated forms. Ancestral character state reconstruction for representatives of 106 eukaryotic taxa using 16 different possible roots and supergroup sister relationships, indicate that LECA, in addition to being mitochondriate, sexual, and meiotic, was multinucleate. LECA exhibited closed mitosis, which is the rule for modern syncytial forms, shedding light on the mechanics of its chromosome segregation. A simple mathematical model shows that within LECA's multinucleate cytosol, relationships among mitochondria and nuclei were neither one-to-one, nor one-to-many, but many-to-many, placing mitonuclear interactions and cytonuclear compatibility at the evolutionary base of eukaryotic cell origin. Within a syncytium, individual nuclei and individual mitochondria function as the initial lower-level evolutionary units of selection, as opposed to individual cells, during eukaryogenesis. Nuclei within a syncytium rescue each other's lethal mutations, thereby postponing selection for viable nuclei and cytonuclear compatibility to the generation of spores, buffering transitional bottlenecks at eukaryogenesis. The prokaryote-to-eukaryote transition is traditionally thought to have left no intermediates, yet if eukaryogenesis proceeded via a syncytial common ancestor, intermediate forms have persisted to the present throughout the eukaryotic tree as syncytia but have so far gone unrecognized.

Nonredundant roles of DIAPHs in primary ciliogenesis

Primary cilia are hubs for several signaling pathways, and disruption in cilia function and formation leads to a range of diseases collectively known as ciliopathies. Both ciliogenesis and cilia maintenance depend on vesicle trafficking along a network of microtubules and actin filaments toward the basal body. The DIAPH (Diaphanous-related) family of formins promote both actin polymerization and microtubule (MT) stability. Recently, we showed that the formin DIAPH1 is involved in ciliogenesis. However, the role of other DIAPH family members in ciliogenesis had not been investigated. Here we show that depletion of either DIAPH2 or DIAPH3 also disrupted ciliogenesis and cilia length. DIAPH3 depletion also reduced trafficking within cilia. To specifically examine the role of DIAPH3 at the base, we used fused full-length DIAPH3 to centrin, which targeted DIAPH3 to the basal body, causing increased trafficking to the ciliary base, an increase in cilia length, and formation of bulbs at the tips of cilia. Additionally, we confirmed that the microtubule-stabilizing properties of DIAPH3 are important for its cilia length functions and trafficking. These results indicate the importance of DIAPH proteins in regulating cilia maintenance. Moreover, defects in ciliogenesis caused by DIAPH depletion could only be rescued by expression of the specific family member depleted, indicating nonredundant roles for these proteins.

Moonlighting of mitotic regulators in cilium disassembly

Correct timing of cellular processes is essential during embryological development and to maintain the balance between healthy proliferation and tumour formation. Assembly and disassembly of the primary cilium, the cell’s sensory signalling organelle, are linked to cell cycle timing in the same manner as spindle pole assembly and chromosome segregation. Mitotic processes, ciliary assembly, and ciliary disassembly depend on the centrioles as microtubule-organizing centres (MTOC) to regulate polymerizing and depolymerizing microtubules. Subsequently, other functional protein modules are gathered to potentiate specific protein–protein interactions. In this review, we show that a significant subset of key mitotic regulator proteins is moonlighting at the cilium, among which PLK1, AURKA, CDC20, and their regulators. Although ciliary assembly defects are linked to a variety of ciliopathies, ciliary disassembly defects are more often linked to brain development and tumour formation. Acquiring a better understanding of the overlap in regulators of ciliary disassembly and mitosis is essential in finding therapeutic targets for the different diseases and types of tumours associated with these regulators.

Recent advances in understanding assembly of the primary cilium membrane

Ciliogenesis describes the assembly of cilia in interphase cells. Several hundred proteins have been linked to ciliogenesis, which proceeds through a highly coordinated multistage process at the distal end of centrioles requiring membranes. In this short review, we focus on recently reported insights into the biogenesis of the primary cilium membrane and its association with other ciliogenic processes in the intracellular ciliogenesis pathway.

The molecular structure of mammalian primary cilia revealed by cryo-electron tomography

Primary cilia are microtubule-based organelles that are important for signaling and sensing in eukaryotic cells. Unlike the thoroughly studied motile cilia, the three-dimensional architecture and molecular composition of primary cilia are largely unexplored. Yet, studying these aspects is necessary to understand how primary cilia function in health and disease. We developed an enabling method for investigating the structure of primary cilia isolated from MDCK-II cells at molecular resolution by cryo-electron tomography. We show that the textbook '9 + 0' arrangement of microtubule doublets is only present at the primary cilium base. A few microns out, the architecture changes into an unstructured bundle of EB1-decorated microtubules and actin filaments, putting an end to a long debate on the presence or absence of actin filaments in primary cilia. Our work provides a plethora of insights into the molecular structure of primary cilia and offers a methodological framework to study these important organelles.

DIAPH1 regulates ciliogenesis and trafficking in primary cilia

Primary cilia are critical hubs for several signaling pathways, and defects in ciliogenesis or cilia maintenance produce a range of diseases collectively known as ciliopathies. Ciliogenesis requires vesicle trafficking along a network of microtubules and actin filaments to the basal body. The DIAPH1 (Diaphanous-related formin) family of formins promotes both actin polymerization and EB1-dependent microtubule (MT) stability. EB1 and EB3 have previously been implicated in cilia biogenesis to carry out centrosome-related functions. However, the role of DIAPH1 proteins had not been examined. Here we show that the depletion of DIAPH1 decreased ciliogenesis, cilia length, and reduced trafficking within cilia. Additionally, both actin nucleating and microtubule-stabilizing properties of DIAPH1 are important for their cilia functions. To assess their roles in ciliogenesis in isolation, we targeted DIAPH1 specifically to the basal body, which caused an increase in cilia length and increased trafficking within cilia. Intriguingly, expression of DIAPH1 mutants associated with human deafness and microcephaly impaired ciliation and caused cilia elongation and bulb formation. These results suggest that the actin and microtubule functions of DIAPH1 proteins regulate cilia maintenance in part by regulating vesicular trafficking to the base of the primary cilia.

Roles of TOG and jelly-roll domains of centrosomal protein CEP104 in its functions in cilium elongation and Hedgehog signaling

Primary cilia are generated through the extension of the microtubule-based axoneme. Centrosomal protein 104 (CEP104) localizes to the tip of the elongating axoneme, and CEP104 mutations are linked to a ciliopathy, Joubert syndrome. Thus, CEP104 has been implicated in ciliogenesis. However, the mechanism by which CEP104 regulates ciliogenesis remains elusive. We report here that CEP104 is critical for cilium elongation but not for initiating ciliogenesis. We also demonstrated that the tumor-overexpressed gene (TOG) domain of CEP104 exhibits microtubule-polymerizing activity and that this activity is essential for the cilium-elongating activity of CEP104. Knockdown/rescue experiments showed that the N-terminal jelly-roll (JR) fold partially contributes to cilium-elongating activity of CEP104, but neither the zinc-finger region nor the SXIP motif is required for this activity. CEP104 binds to a centriole-capping protein, CP110, through the zinc-finger region and to a microtubule plus-end-binding protein, EB1, through the SXIP motif, indicating that the binding of CP110 and EB1 is dispensable for the cilium-elongating activity of CEP104. Moreover, CEP104 depletion does not affect CP110 removal from the mother centriole, which suggests that CEP104 functions after the removal of CP110. Last, we also showed that CEP104 is required for the ciliary entry of Smoothened and export of GPR161 upon Hedgehog signal activation and that the TOG domain plays a critical role in this activity. Our results define the roles of the individual domains of CEP104 in its functions in cilium elongation and Hedgehog signaling and should enhance our understanding of the mechanism underlying CEP104 mutation-associated ciliopathies.

Microtubule-associated proteins and emerging links to primary cilium structure, assembly, maintenance, and disassembly

The primary cilium is a microtubule-based structure that protrudes from the cell surface in diverse eukaryotic organisms. It functions as a key signaling center that decodes a variety of mechanical and chemical stimuli and plays fundamental roles in development and homeostasis. Accordingly, structural and functional defects of the primary cilium have profound effects on the physiology of multiple organ systems including kidney, retina, and central nervous system. At the core of the primary cilium is the microtubule-based axoneme, which supports the cilium shape and acts as the scaffold for bidirectional transport of cargoes into and out of cilium. Advances in imaging, proteomics, and structural biology have revealed new insights into the ultrastructural organization and composition of the primary cilium, the mechanisms that underlie its biogenesis and functions, and the pathologies that result from their deregulation termed ciliopathies. In this viewpoint, we first discuss the recent studies that identified the three-dimensional native architecture of the ciliary axoneme and revealed that it is considerably different from the well-known '9 + 0' paradigm. Moving forward, we explore emerging themes in the assembly and maintenance of the axoneme, with a focus on how microtubule-associated proteins regulate its structure, length, and stability. This far more complex picture of the primary cilium structure and composition, as well as the recent technological advances, open up new avenues for future research.

Physiological and Pathophysiological Aspects of Primary Cilia-A Literature Review with View on Functional and Structural Relationships in Cartilage

Cilia are cellular organelles that project from the cell. They occur in nearly all non-hematopoietic tissues and have different functions in different tissues. In mesenchymal tissues primary cilia play a crucial role in the adequate morphogenesis during embryological development. In mature articular cartilage, primary cilia fulfil chemo- and mechanosensitive functions to adapt the cellular mechanisms on extracellular changes and thus, maintain tissue homeostasis and morphometry. Ciliary abnormalities in osteoarthritic cartilage could represent pathophysiological relationships between ciliary dysfunction and tissue deformation. Nevertheless, the molecular and pathophysiological relationships of ‘Primary Cilia’ (PC) in the context of osteoarthritis is not yet fully understood. The present review focuses on the current knowledge about PC and provide a short but not exhaustive overview of their role in cartilage.

CEP41-mediated ciliary tubulin glutamylation drives angiogenesis through AURKA-dependent deciliation

The endothelial cilium is a microtubule-based organelle responsible for blood flow-induced mechanosensation and signal transduction during angiogenesis. The precise function and mechanisms by which ciliary mechanosensation occurs, however, are poorly understood. Although posttranslational modifications (PTMs) of cytoplasmic tubulin are known to be important in angiogenesis, the specific roles of ciliary tubulin PTMs play remain unclear. Here, we report that loss of centrosomal protein 41 (CEP41) results in vascular impairment in human cell lines and zebrafish, implying a previously unknown pro-angiogenic role for CEP41. We show that proper control of tubulin glutamylation by CEP41 is necessary for cilia disassembly and that is involved in endothelial cell (EC) dynamics such as migration and tubulogenesis. We show that in ECs responding to shear stress or hypoxia, CEP41 activates Aurora kinase A (AURKA) and upregulates expression of VEGFA and VEGFR2 through ciliary tubulin glutamylation, as well as leads to the deciliation. We further show that in hypoxia-induced angiogenesis, CEP41 is responsible for the activation of HIF1α to trigger the AURKA-VEGF pathway. Overall, our results suggest the CEP41-HIF1α-AURKA-VEGF axis as a key molecular mechanism of angiogenesis and demonstrate how important ciliary tubulin glutamylation is in mechanosense-responded EC dynamics.

Analysis of Caveolin in Primary Cilia

Recent evidence has indicated that caveolins are localized at the base of primary cilia, which are microtubule-based sensory organelles present on the cell surface, and that Caveolin-1 (CAV1) plays important roles in regulating ciliary membrane composition and function. Here we describe methods to analyze the localization and function of CAV1 in primary cilia of cultured mammalian cells. These include methods for culturing and transfecting mammalian cells with a CAV1-encoding plasmid or small interfering RNA (siRNA), analysis of mammalian cells by immunofluorescence microscopy (IFM) with antibodies against ciliary markers and CAV1, as well as methods for analyzing ciliary CAV1 function in siRNA-treated cells by IFM and cell-based signaling assays.

Dynamic Changes in Ultrastructure of the Primary Cilium in Migrating Neuroblasts in the Postnatal Brain

New neurons, referred to as neuroblasts, are continuously generated in the ventricular-subventricular zone of the brain throughout an animal's life. These neuroblasts are characterized by their unique potential for proliferation, formation of chain-like cell aggregates, and long-distance and high-speed migration through the rostral migratory stream (RMS) toward the olfactory bulb (OB), where they decelerate and differentiate into mature interneurons. The dynamic changes of ultrastructural features in postnatal-born neuroblasts during migration are not yet fully understood. Here we report the presence of a primary cilium, and its ultrastructural morphology and spatiotemporal dynamics, in migrating neuroblasts in the postnatal RMS and OB. The primary cilium was observed in migrating neuroblasts in the postnatal RMS and OB in male and female mice and zebrafish, and a male rhesus monkey. Inhibition of intraflagellar transport molecules in migrating neuroblasts impaired their ciliogenesis and rostral migration toward the OB. Serial section transmission electron microscopy revealed that each migrating neuroblast possesses either a pair of centrioles or a basal body with an immature or mature primary cilium. Using immunohistochemistry, live imaging, and serial block-face scanning electron microscopy, we demonstrate that the localization and orientation of the primary cilium are altered depending on the mitotic state, saltatory migration, and deceleration of neuroblasts. Together, our results highlight a close mutual relationship between spatiotemporal regulation of the primary cilium and efficient chain migration of neuroblasts in the postnatal brain.SIGNIFICANCE STATEMENT Immature neurons (neuroblasts) generated in the postnatal brain have a mitotic potential and migrate in chain-like cell aggregates toward the olfactory bulb. Here we report that migrating neuroblasts possess a tiny cellular protrusion called a primary cilium. Immunohistochemical studies with zebrafish, mouse, and monkey brains suggest that the presence of the primary cilium in migrating neuroblasts is evolutionarily conserved. Ciliogenesis in migrating neuroblasts in the rostral migratory stream is suppressed during mitosis and promoted after cell cycle exit. Moreover, live imaging and 3D electron microscopy revealed that ciliary localization and orientation change during saltatory movement of neuroblasts. Our results reveal highly organized dynamics in maturation and positioning of the primary cilium during neuroblast migration that underlie saltatory movement of postnatal-born neuroblasts.

The Response of Osteoblasts and Bone to Sinusoidal Electromagnetic Fields: Insights from the Literature

Electromagnetic fields (EMFs) have been proposed as a tool to ameliorate bone formation and healing. Despite their promising results, however, they have failed to enter routine clinical protocols to treat bone conditions where higher bone mass has to be achieved. This is no doubt also due to a fundamental lack of knowledge and understanding on their effects and the optimal settings for attaining the desired therapeutic effects. This review analysed the available in vitro and in vivo studies that assessed the effects of sinusoidal EMFs (SEMFs) on bone and bone cells, comparing the results and investigating possible mechanisms of action by which SEMFs interact with tissues and cells. The effects of SEMFs on bone have not been as thoroughly investigated as pulsed EMFs; however, abundant evidence shows that SEMFs affect the proliferation and differentiation of osteoblastic cells, acting on multiple cellular mechanisms. SEMFs have also proven to increase bone mass in rodents under normal conditions and in osteoporotic animals.

A Ciliary View of the Immunological Synapse

The primary cilium has gone from being a vestigial organelle to a crucial signaling hub of growing interest given the association between a group of human disorders, collectively known as ciliopathies, and defects in its structure or function. In recent years many ciliogenesis proteins have been observed at extraciliary sites in cells and likely perform cilium-independent functions ranging from regulation of the cytoskeleton to vesicular trafficking. Perhaps the most striking example is the non-ciliated T lymphocyte, in which components of the ciliary machinery are repurposed for the assembly and function of the immunological synapse even in the absence of a primary cilium. Furthermore, the specialization traits described at the immunological synapse are similar to those seen in the primary cilium. Here, we review common regulators and features shared by the immunological synapse and the primary cilium that document the remarkable homology between these structures.

CEP128 Localizes to the Subdistal Appendages of the Mother Centriole and Regulates TGF-β/BMP Signaling at the Primary Cilium

The centrosome is the main microtubule-organizing center in animal cells and comprises a mother and daughter centriole surrounded by pericentriolar material. During formation of primary cilia, the mother centriole transforms into a basal body that templates the ciliary axoneme. Ciliogenesis depends on mother centriole-specific distal appendages, whereas the role of subdistal appendages in ciliary function is unclear. Here, we identify CEP128 as a centriole subdistal appendage protein required for regulating ciliary signaling. Loss of CEP128 did not grossly affect centrosomal or ciliary structure but caused impaired transforming growth factor-β/bone morphogenetic protein (TGF-β/BMP) signaling in zebrafish and at the primary cilium in cultured mammalian cells. This phenotype is likely the result of defective vesicle trafficking at the cilium as ciliary localization of RAB11 was impaired upon loss of CEP128, and quantitative phosphoproteomics revealed that CEP128 loss affects TGF-β1-induced phosphorylation of multiple proteins that regulate cilium-associated vesicle trafficking.

Mutation of FOP/FGFR1OP in mice recapitulates human short rib-polydactyly ciliopathy

Skeletal dysplasias are a clinically and genetically heterogeneous group of bone and cartilage disorders. A total of 436 skeletal dysplasias are listed in the 2015 revised version of the nosology and classification of genetic skeletal disorders, of which nearly 20% are still genetically and molecularly uncharacterized. We report the clinical and molecular characterization of a lethal skeletal dysplasia of the short-rib group caused by mutation of the mouse Fop gene. Fop encodes a centrosomal and centriolar satellite (CS) protein. We show that Fop mutation perturbs ciliogenesis in vivo and that this leads to the alteration of the Hedgehog signaling pathway. Fop mutation reduces CSs movements and affects pericentriolar material composition, which probably participates to the ciliogenesis defect. This study highlights the role of a centrosome and CSs protein producing phenotypes in mice that recapitulate a short rib-polydactyly syndrome when mutated.

The cellular effects of Pulsed Electromagnetic Fields on osteoblasts: A review

Electromagnetic fields (EMFs) have long been known to interact with living organisms and their cells and to bear the potential for therapeutic use. Among the most extensively investigated applications, the use of Pulsed EMFs (PEMFs) has proven effective to ameliorate bone healing in several studies, although the evidence is still inconclusive. This is due in part to our still-poor understanding of the mechanisms by which PEMFs act on cells and affect their functions and to an ongoing lack of consensus on the most effective parameters for specific clinical applications. The present review has compared in vitro studies on PEMFs on different osteoblast models, which elucidate potential mechanisms of action for PEMFs, up to the most recent insights into the role of primary cilia, and highlight the critical issues underlying at least some of the inconsistent results in the available literature. Bioelectromagnetics. 2019;9999:XX-XX. © 2019 Bioelectromagnetics Society.

Ciliary kinesins beyond IFT: Cilium length, disassembly, cargo transport and signalling

Cilia and flagella are microtubule-based antenna which are highly conserved among eukaryotes. In vertebrates, primary and motile cilia have evolved to exert several key functions during development and tissue homoeostasis. Ciliary dysfunction in humans causes a highly heterogeneous group of diseases called ciliopathies, a class of genetic multisystemic disorders primarily affecting kidney, skeleton, retina, lung and the central nervous system. Among key ciliary proteins, kinesin family members (KIF) are microtubule-interacting proteins involved in many diverse cellular functions, including transport of cargo (organelles, proteins and lipids) along microtubules and regulating the dynamics of cytoplasmic and spindle microtubules through their depolymerising activity. Many KIFs are also involved in diverse ciliary functions including assembly/disassembly, motility and signalling. We here review these ciliary kinesins in vertebrates and focus on their involvement in ciliopathy-related disorders.

Cilia Distal Domain: Diversity in Evolutionarily Conserved Structures

Eukaryotic cilia are microtubule-based organelles that protrude from the cell surface to fulfill sensory and motility functions. Their basic structure consists of an axoneme templated by a centriole/basal body. Striking differences in ciliary ultra-structures can be found at the ciliary base, the axoneme and the tip, not only throughout the eukaryotic tree of life, but within a single organism. Defects in cilia biogenesis and function are at the origin of human ciliopathies. This structural/functional diversity and its relationship with the etiology of these diseases is poorly understood. Some of the important events in cilia function occur at their distal domain, including cilia assembly/disassembly, IFT (intraflagellar transport) complexes' remodeling, and signal detection/transduction. How axonemal microtubules end at this domain varies with distinct cilia types, originating different tip architectures. Additionally, they show a high degree of dynamic behavior and are able to respond to different stimuli. The existence of microtubule-capping structures (caps) in certain types of cilia contributes to this diversity. It has been proposed that caps play a role in axoneme length control and stabilization, but their roles are still poorly understood. Here, we review the current knowledge on cilia structure diversity with a focus on the cilia distal domain and caps and discuss how they affect cilia structure and function.

A global analysis of IFT-A function reveals specialization for transport of membrane-associated proteins into cilia

Intraflagellar transport (IFT), which is essential for the formation and function of cilia in most organisms, is the trafficking of IFT trains (i.e. assemblies of IFT particles) that carry cargo within the cilium. Defects in IFT cause several human diseases. IFT trains contain the complexes IFT-A and IFT-B. To dissect the functions of these complexes, we studied a Chlamydomonas mutant that is null for the IFT-A protein IFT140. The mutation had no effect on IFT-B but destabilized IFT-A, preventing flagella assembly. Therefore, IFT-A assembly requires IFT140. Truncated IFT140, which lacks the N-terminal WD repeats of the protein, partially rescued IFT and supported formation of half-length flagella that contained normal levels of IFT-B but greatly reduced amounts of IFT-A. The axonemes of these flagella had normal ultrastructure and, as investigated by SDS-PAGE, normal composition. However, composition of the flagellar 'membrane+matrix' was abnormal. Analysis of the latter fraction by mass spectrometry revealed decreases in small GTPases, lipid-anchored proteins and cell signaling proteins. Thus, IFT-A is specialized for the import of membrane-associated proteins. Abnormal levels of the latter are likely to account for the multiple phenotypes of patients with defects in IFT140.This article has an associated First Person interview with the first author of the paper.

Microtubule asters anchored by FSD1 control axoneme assembly and ciliogenesis

Defective ciliogenesis causes human developmental diseases termed ciliopathies. Microtubule (MT) asters originating from centrosomes in mitosis ensure the fidelity of cell division by positioning the spindle apparatus. However, the function of microtubule asters in interphase remains largely unknown. Here, we reveal an essential role of MT asters in transition zone (TZ) assembly during ciliogenesis. We demonstrate that the centrosome protein FSD1, whose biological function is largely unknown, anchors MT asters to interphase centrosomes by binding to microtubules. FSD1 knockdown causes defective ciliogenesis and affects embryonic development in vertebrates. We further show that disruption of MT aster anchorage by depleting FSD1 or other known anchoring proteins delocalizes the TZ assembly factor Cep290 from centriolar satellites, and causes TZ assembly defects. Thus, our study establishes FSD1 as a MT aster anchorage protein and reveals an important function of MT asters anchored by FSD1 in TZ assembly during ciliogenesis.

Microtubule asters originate from centrosomes but their role during interphase remains largely unknown. Here, the authors find that microtubule asters anchored by previously-uncharacterized FSD1 play a role in ciliogenesis by maintaining the dynamic localization of centriolar satellites.

Composition, structure and function of the eukaryotic flagellum distal tip

Cilia and flagella are long extensions commonly found on the surface of eukaryotic cells. In fact, most human cells have a flagellum, and failure to correctly form cilia leads to a spectrum of diseases gathered under the name ‘ciliopathies’. The cilium distal tip is where it grows and signals. Yet, out of the flagellar regions, the distal tip is probably the least intensively studied. In this review, we will summarise the current knowledge on the diverse flagellar tip structures, the dynamicity and signalling that occurs here and the proteins localising to this important cellular region.

Spatial positioning of EB family proteins at microtubule tips involves distinct nucleotide-dependent binding properties

EB proteins track the ends of growing microtubules and regulate microtubule dynamics both directly and by acting as the hub of the tip-tracking network. Mammalian cells express cell type-specific combinations of three EB proteins with different cellular roles. Here, we reconstitute EB1, EB2 and EB3 tip tracking in vitro. We find that all three EBs show rapid exchange at the microtubule tip and that their signal correlates to the microtubule assembly rate. However, the three signals differ in their maxima and position from the microtubule tip. Using microtubules built with nucleotide analogues and site-directed mutagenesis, we show that EB2 prefers binding to microtubule lattices containing a 1:1 mixture of different nucleotides and its distinct binding specificity is conferred by amino acid substitutions at the right-hand-side interface of the EB microtubule-binding domain with tubulin. Our data are consistent with the model that all three EB paralogues sense the nucleotide state of both β-tubulins flanking their binding site. Their different profile of preferred binding sites contributes to occupying spatially distinct domains at the temporally evolving microtubule tip structure.

Summary:In vitro reconstitution of tip tracking with EB1, EB2 and EB3 shows that these three proteins sense the nucleotide state of both β-tubulins flanking their binding site.

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.

Regulation of ciliary membrane protein trafficking and signalling by kinesin motor proteins

Primary cilia are antenna-like sensory organelles that regulate a substantial number of cellular signalling pathways in vertebrates, both during embryonic development as well as in adulthood, and mutations in genes coding for ciliary proteins are causative of an expanding group of pleiotropic diseases known as ciliopathies. Cilia consist of a microtubule-based axoneme core, which is subtended by a basal body and covered by a bilayer lipid membrane of unique protein and lipid composition. Cilia are dynamic organelles, and the ability of cells to regulate ciliary protein and lipid content in response to specific cellular and environmental cues is crucial for balancing ciliary signalling output. Here we discuss mechanisms involved in regulation of ciliary membrane protein trafficking and signalling, with main focus on kinesin-2 and kinesin-3 family members.

The E3 ubiquitin ligase SMURF1 regulates cell-fate specification and outflow tract septation during mammalian heart development

Smad ubiquitin regulatory factor 1 (SMURF1) is a HECT-type E3 ubiquitin ligase that plays a critical role in vertebrate development by regulating planar cell polarity (PCP) signaling and convergent extension (CE). Here we show that SMURF1 is involved in mammalian heart development. We find that SMURF1 is highly expressed in outflow tract cushion mesenchyme and Smurf1^−/− mouse embryos show delayed outflow tract septation. SMURF1 is expressed in smooth muscle cells of the coronary arteries and great vessels. Thickness of the aortic smooth muscle cell layer is reduced in Smurf1^−/− mouse embryos. We show that SMURF1 is a negative regulator of cardiomyogenesis and a positive regulator of smooth muscle cell and cardiac fibroblast differentiation, indicating that SMURF1 is important for cell-type specification during heart development. Finally, we provide evidence that SMURF1 localizes at the primary cilium where it may regulate bone morphogenetic protein (BMP) signaling, which controls the initial phase of cardiomyocyte differentiation. In summary, our results demonstrate that SMURF1 is a critical regulator of outflow tract septation and cell-type specification during heart development, and that these effects may in part be mediated via control of cilium-associated BMP signaling.

Primary Cilia Mediate Diverse Kinase Inhibitor Resistance Mechanisms in Cancer

Primary cilia are microtubule-based organelles that detect mechanical and chemical stimuli. Although cilia house a number of oncogenic molecules (including Smoothened, KRAS, EGFR, and PDGFR), their precise role in cancer remains unclear. We have interrogated the role of cilia in acquired and de novo resistance to a variety of kinase inhibitors, and found that, in several examples, resistant cells are distinctly characterized by an increase in the number and/or length of cilia with altered structural features. Changes in ciliation seem to be linked to differences in the molecular composition of cilia and result in enhanced Hedgehog pathway activation. Notably, manipulating cilia length via Kif7 knockdown is sufficient to confer drug resistance in drug-sensitive cells. Conversely, targeting of cilia length or integrity through genetic and pharmacological approaches overcomes kinase inhibitor resistance. Our work establishes a role for ciliogenesis and cilia length in promoting cancer drug resistance and has significant translational implications.

Sailing to and Docking at the Immune Synapse: Role of Tubulin Dynamics and Molecular Motors

The different cytoskeleton systems and their connecting molecular motors move vesicles and intracellular organelles to shape cells. Polarized cells with specialized functions display an exquisite spatio-temporal regulation of both cytoskeletal and organelle arrangements that support their specific tasks. In particular, T cells rapidly change their shape and cellular function through the establishment of cell surface and intracellular polarity in response to a variety of cues. This review focuses on the contribution of the microtubule-based dynein/dynactin motor complex, the tubulin and actin cytoskeletons, and different organelles to the formation of the antigen-driven immune synapse.

The HCMV Assembly Compartment Is a Dynamic Golgi-Derived MTOC that Controls Nuclear Rotation and Virus Spread

Human cytomegalovirus (HCMV), a leading cause of congenital birth defects, forms an unusual cytoplasmic virion maturation site termed the "assembly compartment" (AC). Here, we show that the AC also acts as a microtubule-organizing center (MTOC) wherein centrosome activity is suppressed and Golgi-based microtubule (MT) nucleation is enhanced. This involved viral manipulation of discrete functions of MT plus-end-binding (EB) proteins. In particular, EB3, but not EB1 or EB2, was recruited to the AC and was required to nucleate MTs that were rapidly acetylated. EB3-regulated acetylated MTs were necessary for nuclear rotation prior to cell migration, maintenance of AC structure, and optimal virus replication. Independently, a myristoylated peptide that blocked EB3-mediated enrichment of MT regulatory proteins at Golgi regions of the AC also suppressed acetylated MT formation, nuclear rotation, and infection. Thus, HCMV offers new insights into the regulation and functions of Golgi-derived MTs and the therapeutic potential of targeting EB3.

IFT20 modulates ciliary PDGFRα signaling by regulating the stability of Cbl E3 ubiquitin ligases

PDGFRα signals from cilia to control development and tumorigenesis. Schmid et al. now show that intraflagellar transport protein 20 (IFT20) interacts with and stabilizes the E3 ubiquitin ligases c-Cbl and Cbl-b to promote feedback inhibition of PDGFRα signaling at the primary cilium.

Primary cilia have pivotal roles as organizers of many different signaling pathways, including platelet-derived growth factor receptor α (PDGFRα) signaling, which, when aberrantly regulated, is associated with developmental disorders, tumorigenesis, and cancer. PDGFRα is up-regulated during ciliogenesis, and ciliary localization of the receptor is required for its appropriate ligand-mediated activation by PDGF-AA. However, the mechanisms regulating sorting of PDGFRα and feedback inhibition of PDGFRα signaling at the cilium are unknown. Here, we provide evidence that intraflagellar transport protein 20 (IFT20) interacts with E3 ubiquitin ligases c-Cbl and Cbl-b and is required for Cbl-mediated ubiquitination and internalization of PDGFRα for feedback inhibition of receptor signaling. In wild-type cells treated with PDGF-AA, c-Cbl becomes enriched in the cilium, and the receptor is subsequently ubiquitinated and internalized. In contrast, in IFT20-depleted cells, PDGFRα localizes aberrantly to the plasma membrane and is overactivated after ligand stimulation because of destabilization and degradation of c-Cbl and Cbl-b.

Hierarchical assembly of centriole subdistal appendages via centrosome binding proteins CCDC120 and CCDC68

In animal cells, the centrosome is the main microtubule-organizing centre where microtubules are nucleated and anchored. The centriole subdistal appendages (SDAs) are the key structures that anchor microtubules in interphase cells, but the composition and assembly mechanisms of SDAs are not well understood. Here, we reveal that centrosome-binding proteins, coiled-coil domain containing (CCDC) 120 and CCDC68 are two novel SDA components required for hierarchical SDA assembly in human cells. CCDC120 is anchored to SDAs by ODF2 and recruits CEP170 and Ninein to the centrosome through different coiled-coil domains at its N terminus. CCDC68 is a CEP170-interacting protein that competes with CCDC120 in recruiting CEP170 to SDAs. Furthermore, CCDC120 and CCDC68 are required for centrosome microtubule anchoring. Our findings elucidate the molecular basis for centriole SDA hierarchical assembly and microtubule anchoring in human interphase cells.

Patient-specific three-dimensional explant spheroids derived from human nasal airway epithelium: a simple methodological approach for ex vivo studies of primary ciliary dyskinesia

BACKGROUND

Three-dimensional explant spheroid formation is an ex vivo technique previously used in studies of airway epithelial ion and water transport. Explanted cells and sheets of nasal epithelium form fully differentiated spheroids enclosing a partly fluid-filled lumen with the ciliated apical surface facing the outside and accessible for analysis of ciliary function.

METHODS

We performed a two-group comparison study of ciliary beat pattern and ciliary beat frequency in spheroids derived from nasal airway epithelium in patients with primary ciliary dyskinesia (PCD) and in healthy controls. Nasal ciliary cells and sheets were removed on day 1 by nasal brush biopsy and analyzed with regard to ciliary beat pattern-and frequency using high-speed video imaging for standard reference values. Three-dimensional explant spheroid formation was initiated in the same individual on the same day by incubation of cells and sheets from a separate brush biopsy. Harvested spheroids were analyzed earliest possible and values of spheroid ciliary beat pattern and frequency were compared to the corresponding reference values from day 1.

RESULTS

Spheroids formed fast in serum-free culture medium. Formation was successful in 15 out of 18 (82%) sampled individuals. Thus, formation was successful in seven healthy controls and eight PCD patients, while unsuccessful in 3 with PCD due to infection. Median (range) number of days in culture before harvesting of spheroids was 4 (1-5) in healthy versus 2 (1-5) in PCD. Spheroid ciliary beat pattern and frequency were unchanged compared to their corresponding day 1 standard reference values. Spheroid ciliary beat frequency discriminated highly significant between healthy controls (9.3 Hz) and PCD patients (2.4 Hz) (P < 0.0001). Survival of spheroids was 16 days in a single healthy person.

CONCLUSION

Patient-specific three-dimensional explant spheroid formation from a minimal invasive nasal brush biopsy is a feasible, fast and valid ex vivo method to assess ciliary function with potential of aiding the diagnosis of PCD. In addition, it may be a useful model in the investigation of pathophysiological aspects and drug effects in human nasal airway epithelium.

GAS2L1 Is a Centriole-Associated Protein Required for Centrosome Dynamics and Disjunction

Mitotic spindle formation and chromosome segregation require timely separation of the two duplicated centrosomes, and this process is initiated in late G2 by centrosome disjunction. Here we report that GAS2L1, a microtubule- and actin-binding protein, associates with the proximal end of mature centrioles and participates in centriole dynamics and centrosome disjunction. GAS2L1 attaches microtubules and actin to centrosomes, and the loss of GAS2L1 inhibits centrosome disjunction in G2 and centrosome splitting induced by depletion of the centrosome linker rootletin. Conversely, GAS2L1 overexpression induces premature centrosome separation, and this activity requires GAS2L1 association with actin, microtubules, and the microtubule end-binding proteins. The centrosome-splitting effect of GAS2L1 is counterbalanced by rootletin, reflecting the opposing actions of GAS2L1 and the centrosome linker. Our work reveals a GAS2L1-mediated centriole-tethering mechanism of microtubules and actin, which provide the forces required for centrosome dynamics and separation.

Loss of MACF1 Abolishes Ciliogenesis and Disrupts Apicobasal Polarity Establishment in the Retina

Microtubule actin crosslinking factor 1 (MACF1) plays a role in the coordination of microtubules and actin in multiple cellular processes. Here, we show that MACF1 is also critical for ciliogenesis in multiple cell types. Ablation of Macf1 in the developing retina abolishes ciliogenesis, and basal bodies fail to dock to ciliary vesicles or migrate apically. Photoreceptor polarity is randomized, while inner retinal cells laminate correctly, suggesting that photoreceptor maturation is guided by polarity cues provided by cilia. Deletion of MACF1 in adult photoreceptors causes reversal of basal body docking and loss of outer segments, reflecting a continuous requirement for MACF1 function. MACF1 also interacts with the ciliary proteins MKKS and TALPID3. We propose that a disruption of trafficking across microtubles to actin filaments underlies the ciliogenesis defect in cells lacking MACF1 and that MKKS and TALPID3 are involved in the coordination of microtubule and actin interactions.

Proto-Oncogenic Src Phosphorylates EB1 to Regulate the Microtubule-Focal Adhesion Crosstalk and Stimulate Cell Migration

Cell migration, a complex process critical for tumor progression and metastasis, requires a dynamic crosstalk between microtubules (MTs) and focal adhesions (FAs). However, the molecular mechanisms underlying this event remain elusive. Herein we identify the proto-oncogenic protein Src as an important player in the regulation of the MT-FA crosstalk. Src interacts with and phosphorylates end-binding protein 1 (EB1), a member of MT plus end-tracking proteins (+TIPs), both in cells and in vitro. Systematic mutagenesis reveals that tyrosine-247 (Y247) is the primary residue of EB1 phosphorylated by Src. Interestingly, both constitutively activated Src and Y247-phosphorylated EB1 localize to the centrosome and FAs. Src-mediated EB1 phosphorylation diminishes its interactions with other +TIPs, including adenomatous polyposis coli (APC) and mitotic centromere associated kinesin (MCAK). In addition, EB1 phosphorylation at Y247 enhances the rate of MT catastrophe and significantly stimulates cell migration. These findings thus demonstrate that the Src-EB1 axis plays a crucial role in regulating the crosstalk between MTs and FAs to promote cell migration.

Orchestrating Lymphocyte Polarity in Cognate Immune Cell-Cell Interactions

The immune synapse (IS) is a specialized structure established between different immune cells that fulfills several functions, including a role as a communication bridge. This intimate contact between a T cell and an antigen-presenting cell promotes the proliferation and differentiation of lymphocytes involved in the contact. T-cell activation requires the specific triggering of the T-cell receptor (TCR), which promotes the activation of different signaling pathways inducing the polarization of the T cell. During this process, different adhesion and signaling receptors reorganize at specialized membrane domains, concomitantly to the polarization of the tubulin and actin cytoskeletons, forming stable polarization platforms. The centrosome also moves toward the IS, driving the movement of different organelles, such as the biosynthetic, secretory, degrading machinery, and mitochondria, to sustain T-cell activation. A proper orchestration of all these events is essential for T-cell effector functions and the accomplishment of a complete immune response.

Absence of Radial Spokes in Mouse Node Cilia Is Required for Rotational Movement but Confers Ultrastructural Instability as a Trade-Off

Determination of left-right asymmetry in mouse embryos is established by a leftward fluid flow that is generated by clockwise rotation of node cilia. How node cilia achieve stable unidirectional rotation has remained unknown, however. Here we show that brief exposure to the microtubule-stabilizing drug paclitaxel (Taxol) induces randomly directed rotation and changes the ultrastructure of node cilia. In vivo observations and a computer simulation revealed that a regular 9+0 arrangement of doublet microtubules is essential for stable unidirectional rotation of node cilia. The 9+2 motile cilia of the airway, which manifest planar beating, are resistant to Taxol treatment. However, the airway cilia of mice lacking the radial spoke head protein Rsph4a undergo rotational movement instead of planar beating, are prone to microtubule rearrangement, and are sensitive to Taxol. Our results suggest that the absence of radial spokes allows node cilia to rotate unidirectionally but, as a trade-off, renders them ultrastructurally fragile.

Single-particle imaging reveals intraflagellar transport-independent transport and accumulation of EB1 in Chlamydomonas flagella

The microtubule (MT) plus-end tracking protein EB1 is present at the tips of cilia and flagella; end-binding protein 1 (EB1) remains at the tip during flagellar shortening and in the absence of intraflagellar transport (IFT), the predominant protein transport system in flagella. To investigate how EB1 accumulates at the flagellar tip, we used in vivo imaging of fluorescent protein-tagged EB1 (EB1-FP) in Chlamydomonas reinhardtii. After photobleaching, the EB1 signal at the flagellar tip recovered within minutes, indicating an exchange with unbleached EB1 entering the flagella from the cell body. EB1 moved independent of IFT trains, and EB1-FP recovery did not require the IFT pathway. Single-particle imaging showed that EB1-FP is highly mobile along the flagellar shaft and displays a markedly reduced mobility near the flagellar tip. Individual EB1-FP particles dwelled for several seconds near the flagellar tip, suggesting the presence of stable EB1 binding sites. In simulations, the two distinct phases of EB1 mobility are sufficient to explain its accumulation at the tip. We propose that proteins uniformly distributed throughout the cytoplasm like EB1 accumulate locally by diffusion and capture; IFT, in contrast, might be required to transport proteins against cellular concentration gradients into or out of cilia.

PDGFRβ and oncogenic mutant PDGFRα D842V promote disassembly of primary cilia through a PLCγ- and AURKA-dependent mechanism

Primary cilia are microtubule-based sensory organelles projecting from most quiescent mammalian cells, which disassemble in cells cultured in serum-deprived conditions upon re-addition of serum or growth factors. Platelet-derived growth factors (PDGF) are implicated in deciliation, but the specific receptor isoforms and mechanisms involved are unclear. We report that PDGFRβ promotes deciliation in cultured cells and provide evidence implicating PLCγ and intracellular Ca(2+) release in this process. Activation of wild-type PDGFRα alone did not elicit deciliation. However, expression of constitutively active PDGFRα D842V mutant receptor, which potently activates PLCγ (also known as PLCG1), caused significant deciliation, and this phenotype was rescued by inhibiting PDGFRα D842V kinase activity or AURKA. We propose that PDGFRβ and PDGFRα D842V promote deciliation through PLCγ-mediated Ca(2+) release from intracellular stores, causing activation of calmodulin and AURKA-triggered deciliation.