Deuterosome-mediated centriole biogenesis

The Rac1 regulator ELMO controls basal body migration and docking in multiciliated cells through interaction with Ezrin

Cilia are microtubule-based organelles that are present on most cells and are required for normal tissue development and function. Defective cilia cause complex syndromes with multiple organ manifestations termed ciliopathies. A crucial step during ciliogenesis in multiciliated cells (MCCs) is the association of future basal bodies with the apical plasma membrane, followed by their correct spacing and planar orientation. Here, we report a novel role for ELMO-DOCK1, which is a bipartite guanine nucleotide exchange factor complex for the small GTPase Rac1, and for the membrane-cytoskeletal linker Ezrin, in regulating centriole/basal body migration, docking and spacing. Downregulation of each component results in ciliopathy-related phenotypes in zebrafish and disrupted ciliogenesis in Xenopus epidermal MCCs. Subcellular analysis revealed a striking impairment of basal body docking and spacing, which is likely to account for the observed phenotypes. These results are substantiated by showing a genetic interaction between elmo1 and ezrin b. Finally, we provide biochemical evidence that the ELMO-DOCK1-Rac1 complex influences Ezrin phosphorylation and thereby probably serves as an important molecular switch. Collectively, we demonstrate that the ELMO-Ezrin complex orchestrates ciliary basal body migration, docking and positioning in vivo.

Cilia in the choroid plexus: their roles in hydrocephalus and beyond

Cilia are whip-like projections that are widely conserved in eukaryotes and function as a motile propeller and/or sensory platform to detect various extracellular stimuli. In vertebrates, cilia are ubiquitously found in most cells, showing structural and functional diversities depending on the cell type. In this review, we focus on the structure and function of cilia in choroid plexus epithelial cells (CPECs). CPECs form one or two dozen non-motile 9+0 cilia, which display transient acquisition of motility during development. Genetic malfunction of cilia can lead to failure of multiple organs including the brain. Especially, several groups have demonstrated that the defects in CPEC cilia cause the communicating form of hydrocephalus. In order to elucidate the molecular mechanisms underlying the hydrocephalus, we have previously demonstrated that the cilia possess an NPFF receptor for autocrine signaling to regulate transepithelial fluid transport. In this perspective, we also discuss the potential involvement of cilia in the other aspects of choroid plexus functions, such as the regulation of brain development and neuroinflammation.

Pathogenic mechanisms in centronuclear myopathies

Centronuclear myopathies (CNMs) are a genetically heterogeneous group of inherited neuromuscular disorders characterized by clinical features of a congenital myopathy and abundant central nuclei as the most prominent histopathological feature. The most common forms of congenital myopathies with central nuclei have been attributed to X-linked recessive mutations in the MTM1 gene encoding myotubularin (“X-linked myotubular myopathy”), autosomal-dominant mutations in the DNM2 gene encoding dynamin-2 and the BIN1 gene encoding amphiphysin-2 (also named bridging integrator-1, BIN1, or SH3P9), and autosomal-recessive mutations in BIN1, the RYR1 gene encoding the skeletal muscle ryanodine receptor, and the TTN gene encoding titin. Models to study and rescue the affected cellular pathways are now available in yeast, C. elegans, drosophila, zebrafish, mouse, and dog. Defects in membrane trafficking have emerged as a key pathogenic mechanisms, with aberrant T-tubule formation, abnormalities of triadic assembly, and disturbance of the excitation–contraction machinery the main downstream effects studied to date. Abnormal autophagy has recently been recognized as another important collateral of defective membrane trafficking in different genetic forms of CNM, suggesting an intriguing link to primary disorders of defective autophagy with overlapping histopathological features. The following review will provide an overview of clinical, histopathological, and genetic aspects of the CNMs in the context of the key pathogenic mechanism, outline unresolved questions, and indicate promising future lines of enquiry.

Identification of a cell cycle-dependent duplicating complex that assembles basal bodies de novo in Naegleria

During the differentiation of the amoeba Naegleria pringsheimi into a flagellate, a transient complex containing γ-tubulin, pericentrin-like protein, and myosin II (GPM complex) is formed, and subsequently a pair of basal bodies is assembled from the complex. It is not understood, however, how a single GPM is formed nor how the capability to form this complex is acquired by individual cells. We hypothesized that the GPM is formed from a precursor complex and developed an antibody that recognizes Naegleria (Ng)-transacylase, a component of the precursor complex. Immunostaining of differentiating cells showed that Ng-transacylase is concentrated at a site in the amoeba and that γ-tubulin is transiently co-concentrated at the site, suggesting that the GPM is formed from a precursor, GPMp, which contains Ng-transacylase and is already present in the amoeba. Immunostaining of growing N. pringsheimi with Ng-transacylase antibody revealed the presence of one GPMp in interphase cells, but two GPMps in mitotic cells, suggesting that N. pringsheimi maintains one GPMp per cell by duplicating and segregating the complex according to its cell cycle. Our results demonstrate the existence of a cell cycle-dependent duplicating complex that provides a site for the de novo assembly of the next generation of basal bodies.

Systematic discovery of novel ciliary genes through functional genomics in the zebrafish

Cilia are microtubule-based hair-like organelles that play many important roles in development and physiology, and are implicated in a rapidly expanding spectrum of human diseases, collectively termed ciliopathies. Primary ciliary dyskinesia (PCD), one of the most prevalent of ciliopathies, arises from abnormalities in the differentiation or motility of the motile cilia. Despite their biomedical importance, a methodical functional screen for ciliary genes has not been carried out in any vertebrate at the organismal level. We sought to systematically discover novel motile cilia genes by identifying the genes induced by Foxj1, a winged-helix transcription factor that has an evolutionarily conserved role as the master regulator of motile cilia biogenesis. Unexpectedly, we find that the majority of the Foxj1-induced genes have not been associated with cilia before. To characterize these novel putative ciliary genes, we subjected 50 randomly selected candidates to a systematic functional phenotypic screen in zebrafish embryos. Remarkably, we find that over 60% are required for ciliary differentiation or function, whereas 30% of the proteins encoded by these genes localize to motile cilia. We also show that these genes regulate the proper differentiation and beating of motile cilia. This collection of Foxj1-induced genes will be invaluable for furthering our understanding of ciliary biology, and in the identification of new mutations underlying ciliary disorders in humans.

Centriole amplification by mother and daughter centrioles differs in multiciliated cells

The semi-conservative centrosome duplication in cycling cells gives rise to a centrosome composed of a mother and a newly formed daughter centriole. Both centrioles are regarded as equivalent in their ability to form new centrioles and their symmetric duplication is crucial for cell division homeostasis. Multiciliated cells do not use the archetypal duplication program and instead form more than a hundred centrioles that are required for the growth of motile cilia and the efficient propelling of physiological fluids. The majority of these new centrioles are thought to appear de novo, that is, independently from the centrosome, around electron-dense structures called deuterosomes. Their origin remains unknown. Using live imaging combined with correlative super-resolution light and electron microscopy, we show that all new centrioles derive from the pre-existing progenitor cell centrosome through multiple rounds of procentriole seeding. Moreover, we establish that only the daughter centrosomal centriole contributes to deuterosome formation, and thus to over ninety per cent of the final centriole population. This unexpected centriolar asymmetry grants new perspectives when studying cilia-related diseases and pathological centriole amplification observed in cycling cells and associated with microcephaly and cancer.

Multiciliated cells

Cilia are microtubule-based projections that serve a wide variety of essential functions in animal cells. Defects in cilia structure or function have recently been found to underlie diverse human diseases. While many eukaryotic cells possess only one or two cilia, some cells, including those of many unicellular organisms, exhibit many cilia. In vertebrates, multiciliated cells are a specialized population of post-mitotic cells decorated with dozens of motile cilia that beat in a polarized and synchronized fashion to drive directed fluid flow across an epithelium. Dysfunction of human multiciliated cells is associated with diseases of the brain, airway and reproductive tracts. Despite their importance, multiciliated cells are relatively poorly studied and we are only beginning to understand the mechanisms underlying their development and function. Here, we review the general phylogeny and physiology of multiciliation and detail our current understanding of the developmental and cellular events underlying the specification, differentiation and function of multiciliated cells in vertebrates.

Triadopathies: an emerging class of skeletal muscle diseases

The triad is a skeletal muscle substructure responsible for the regulation of excitation-contraction coupling. It is formed by the close apposition of the T-tubule and the terminal sarcoplasmic reticulum. A rapidly growing list of skeletal myopathies, here referred to as triadopathies, are caused by gene mutations in components of the triad. These disorders, at their root, are caused by defects in excitation contraction coupling and intracellular calcium homeostasis. Secondary abnormalities in triad structure and/or function are also reported in several muscle diseases, most notably certain muscular dystrophies. This review highlights the current understanding of both primary and secondary triadopathies, and identifies important concepts yet to be fully addressed in the field. The emphasis of the review is both on the pathogenesis of triadopathies and their potential treatment.

Radial intercalation is regulated by the Par complex and the microtubule-stabilizing protein CLAMP/Spef1

The directed movement of cells is critical for numerous developmental and disease processes. A developmentally reiterated form of migration is radial intercalation; the process by which cells move in a direction orthogonal to the plane of the tissue from an inner layer to an outer layer. We use the radial intercalation of cells into the skin of Xenopus laevis embryos as a model to study directed cell migration within an epithelial tissue. We identify a novel function for both the microtubule-binding protein CLAMP and members of the microtubule-regulating Par complex during intercalation. Specifically, we show that Par3 and aPKC promote the apical positioning of centrioles, whereas CLAMP stabilizes microtubules along the axis of migration. We propose a model in which the Par complex defines the orientation of apical migration during intercalation and in which subcellular localization of CLAMP promotes the establishment of an axis of microtubule stability required for the active migration of cells into the outer epithelium.

miR-34/449 miRNAs are required for motile ciliogenesis by repressing cp110

The mir-34/449 family consists of six homologous miRNAs at three genomic loci. Redundancy of miR-34/449 miRNAs and their dominant expression in multiciliated epithelia suggest a functional significance in ciliogenesis. Here we report that mice deficient for all miR-34/449 miRNAs exhibited postnatal mortality, infertility and strong respiratory dysfunction caused by defective mucociliary clearance. In both mouse and Xenopus, miR-34/449-deficient multiciliated cells (MCCs) exhibited a significant decrease in cilia length and number, due to defective basal body maturation and apical docking. The effect of miR-34/449 on ciliogenesis was mediated, at least in part, by post-transcriptional repression of Cp110, a centriolar protein suppressing cilia assembly. Consistent with this, cp110 knockdown in miR-34/449-deficient MCCs restored ciliogenesis by rescuing basal body maturation and docking. Altogether, our findings elucidate conserved cellular and molecular mechanisms through which miR-34/449 regulate motile ciliogenesis.

Choosing sides--asymmetric centriole and basal body assembly

Centrioles and basal bodies (CBBs) are microtubule-rich cylindrical structures that nucleate and organize centrosomes and cilia, respectively. Despite their apparent ninefold rotational symmetry, the nine sets of triplet microtubules in CBBs possess asymmetries in their morphology and in the structures that associate with them. These asymmetries define the position of nascent CBB assembly, the orientation of ciliary beating, the orientation of spindle poles and the maintenance of cellular geometry. For some of these functions, the orientation of CBBs is first established during new CBB biogenesis when the daughter structure is positioned adjacent to the mother. The mother CBB organizes the surrounding environment that nascent CBBs are born into, thereby providing a nest for the new CBB to develop. Protists, including ciliates and algae, highlight the importance of this environment with the formation of asymmetrically placed scaffolds onto which new basal bodies assemble and are positioned. Recent studies illuminate the positioning of nascent centrioles relative to a modular pericentriolar material (PCM) environment and suggest that, like ciliates, centrosomes organize an immediate environment surrounding centrioles for their biogenesis and positioning. In this Commentary, I will explore the positioning of nascent CBB assembly as the first event in building cellular asymmetries and describe how the environment surrounding both basal bodies and centrioles may define asymmetric assembly.

Mutations in CCNO result in congenital mucociliary clearance disorder with reduced generation of multiple motile cilia

Using a whole-exome sequencing strategy, we identified recessive CCNO (encoding cyclin O) mutations in 16 individuals suffering from chronic destructive lung disease due to insufficient airway clearance. Respiratory epithelial cells showed a marked reduction in the number of multiple motile cilia (MMC) covering the cell surface. The few residual cilia that correctly expressed axonemal motor proteins were motile and did not exhibit obvious beating defects. Careful subcellular analyses as well as in vitro ciliogenesis experiments in CCNO-mutant cells showed defective mother centriole generation and placement. Morpholino-based knockdown of the Xenopus ortholog of CCNO also resulted in reduced MMC and centriole numbers in embryonic epidermal cells. CCNO is expressed in the apical cytoplasm of multiciliated cells and acts downstream of multicilin, which governs the generation of multiciliated cells. To our knowledge, CCNO is the first reported gene linking an inherited human disease to reduced MMC generation due to a defect in centriole amplification and migration.

Switching on cilia: transcriptional networks regulating ciliogenesis

Cilia play many essential roles in fluid transport and cellular locomotion, and as sensory hubs for a variety of signal transduction pathways. Despite having a conserved basic morphology, cilia vary extensively in their shapes and sizes, ultrastructural details, numbers per cell, motility patterns and sensory capabilities. Emerging evidence indicates that this diversity, which is intimately linked to the different functions that cilia perform, is in large part programmed at the transcriptional level. Here, we review our understanding of the transcriptional control of ciliary biogenesis, highlighting the activities of FOXJ1 and the RFX family of transcriptional regulators. In addition, we examine how a number of signaling pathways, and lineage and cell fate determinants can induce and modulate ciliogenic programs to bring about the differentiation of distinct cilia types.

Centriole biogenesis in multiciliated cells

Centrioles duplicate only once per cell cycle in proliferating cells, whereas in multiciliated cells, hundreds of centrioles form almost simultaneously. The molecular control mechanisms that govern centriole amplification in multiciliated cells are largely unknown. Two studies highlight Deup1 and CCDC78 as key players in this process.

C2orf62 and TTC17 are involved in actin organization and ciliogenesis in zebrafish and human

Vertebrate genomes contain around 20,000 protein-encoding genes, of which a large fraction is still not associated with specific functions. A major task in future genomics will thus be to assign physiological roles to all open reading frames revealed by genome sequencing. Here we show that C2orf62, a highly conserved protein with little homology to characterized proteins, is strongly expressed in testis in zebrafish and mammals, and in various types of ciliated cells during zebrafish development. By yeast two hybrid and GST pull-down, C2orf62 was shown to interact with TTC17, another uncharacterized protein. Depletion of either C2orf62 or TTC17 in human ciliated cells interferes with actin polymerization and reduces the number of primary cilia without changing their length. Zebrafish embryos injected with morpholinos against C2orf62 or TTC17, or with mRNA coding for the C2orf62 C-terminal part containing a RII dimerization/docking (R2D2) - like domain show morphological defects consistent with imperfect ciliogenesis. We provide here the first evidence for a C2orf62-TTC17 axis that would regulate actin polymerization and ciliogenesis.