From Zero to Many: Control of Centriole Number in Development and Disease

Centrioles are essential for the formation of microtubule-derived structures, including cilia, flagella and centrosomes. These structures are involved in a variety of functions, from cell motility to division. In most dividing animal cells, centriole formation is coupled to the chromosome cycle. However, this is not the case in certain specialized divisions, such as meiosis, and in some differentiating cells. For example, oocytes loose their centrioles upon differentiation, whereas multiciliated epithelial cells make several of those structures after they exit the cell cycle. Aberrations of centriole number are seen in many cancer cells. Recent studies began to shed light on the molecular control of centriole number, its variations in development, and how centriole number changes in human disease. Here we review the recent developments in this field.     

CEP97 phosphorylation by Dyrk1a is critical for centriole separation during multiciliogenesis

Proper cilia formation in multiciliated cells (MCCs) is necessary for appropriate embryonic development and homeostasis. Multicilia share many structural characteristics with monocilia and primary cilia, but there are still significant gaps in our understanding of the regulation of multiciliogenesis. Using the Xenopus embryo, we show that CEP97, which is known as a negative regulator of primary cilia formation, interacts with dual specificity tyrosine phosphorylation regulated kinase 1A (Dyrk1a) to modulate multiciliogenesis. We show that Dyrk1a phosphorylates CEP97, which in turn promotes the recruitment of Polo-like kinase 1 (Plk1), which is a critical regulator of MCC maturation that functions to enhance centriole disengagement in cooperation with the enzyme Separase. Knockdown of either CEP97 or Dyrk1a disrupts cilia formation and centriole disengagement in MCCs, but this defect is rescued by overexpression of Separase. Thus, our study reveals that Dyrk1a and CEP97 coordinate with Plk1 to promote Separase function to properly form multicilia in vertebrate MCCs.     

MDM1 is a microtubule-binding protein that negatively regulates centriole duplication

Mouse double-minute 1 (Mdm1) was originally identified as a gene amplified in transformed mouse cells and more recently as being highly up-regulated during differentiation of multiciliated epithelial cells, a specialized cell type having hundreds of centrioles and motile cilia. Here we show that the MDM1 protein localizes to centrioles of dividing cells and differentiating multiciliated cells. 3D-SIM microscopy showed that MDM1 is closely associated with the centriole barrel, likely residing in the centriole lumen. Overexpression of MDM1 suppressed centriole duplication, whereas depletion of MDM1 resulted in an increase in granular material that likely represents early intermediates in centriole formation. We show that MDM1 binds microtubules in vivo and in vitro. We identified a repeat motif in MDM1 that is required for efficient microtubule binding and found that these repeats are also present in CCSAP, another microtubule-binding protein. We propose that MDM1 is a negative regulator of centriole duplication and that its function is mediated through microtubule binding.     

Drosophila asterless and vertebrate Cep152 Are orthologs essential for centriole duplication

The centriole is the core structure of centrosome and cilium. Failure to restrict centriole duplication to once per cell cycle has serious consequences and is commonly observed in cancer. Despite its medical importance, the mechanism of centriole formation is poorly understood. Asl was previously reported to be a centrosomal protein essential for centrosome function. Here we identify mecD, a severe loss-of-function allele of the asl gene, and demonstrate that it is required for centriole and cilia formation. Similarly, Cep152, the Asl ortholog in vertebrates, is essential for cilia formation and its function can be partially rescued by the Drosophila Asl. The study of Asl localization suggests that it is closely associated with the centriole wall, but is not part of the centriole structure. By analyzing the biogenesis of centrosomes in cells depleted of Asl, we found that, while pericentriolar material (PCM) function is mildly affected, Asl is essential for daughter centriole formation. The clear absence of several centriolar markers in mecD mutants suggests that Asl is critical early in centriole duplication.     

Cep120 is asymmetrically localized to the daughter centriole and is essential for centriole assembly

Centrioles form the core of the centrosome in animal cells and function as basal bodies that nucleate and anchor cilia at the plasma membrane. In this paper, we report that Cep120 (Ccdc100), a protein previously shown to be involved in maintaining the neural progenitor pool in mouse brain, is associated with centriole structure and function. Cep120 is up-regulated sevenfold during differentiation of mouse tracheal epithelial cells (MTECs) and localizes to basal bodies. Cep120 localizes preferentially to the daughter centriole in cycling cells, and this asymmetry between mother and daughter centrioles is relieved coincident with new centriole assembly. Photobleaching recovery analysis identifies two pools of Cep120, differing in their halftime at the centriole. We find that Cep120 is required for centriole duplication in cycling cells, centriole amplification in MTECs, and centriole overduplication in S phase-arrested cells. We propose that Cep120 is required for centriole assembly and that the observed defect in neuronal migration might derive from a defect in this process.     

C-NAP1 and rootletin restrain DNA damage-induced centriole splitting and facilitate ciliogenesis

Cilia are found on most human cells and exist as motile cilia or non-motile primary cilia. Primary cilia play sensory roles in transducing various extracellular signals, and defective ciliary functions are involved in a wide range of human diseases. Centrosomes are the principal microtubule-organizing centers of animal cells and contain two centrioles. We observed that DNA damage causes centriole splitting in non-transformed human cells, with isolated centrioles carrying the mother centriole markers CEP170 and ninein but not kizuna or cenexin. Loss of centriole cohesion through siRNA depletion of C-NAP1 or rootletin increased radiation-induced centriole splitting, with C-NAP1-depleted isolated centrioles losing mother markers. As the mother centriole forms the basal body in primary cilia, we tested whether centriole splitting affected ciliogenesis. While irradiated cells formed apparently normal primary cilia, most cilia arose from centriolar clusters, not from isolated centrioles. Furthermore, C-NAP1 or rootletin knockdown reduced primary cilium formation. Therefore, the centriole cohesion apparatus at the proximal end of centrioles may provide a target that can affect primary cilium formation as part of the DNA damage response.     

Centriole is the pivot co-ordinating dynamic signaling for cell proliferation and organization during early development in the vertebrates

Vertebrates have an elaborate and functionally segmented body. It evolves from a single cell by systematic cell proliferation but attains a complex body structure with exquisite precision. This development requires two cellular events: cell cycle and ciliogenesis. For these events, the dynamic molecular signaling is converged at the centriole. The cell cycle helps in cell proliferation and growth of the body and is a highly regulated and integrated process. Its errors cause malignancies and developmental disorders. The cells newly proliferated are organized during organogenesis. For a cellular organization, dedicated signaling hubs are developed in the cells, and most often cilia are utilized. The cilium is generated from one of the centrioles involved in cell proliferation. The developmental signaling pathways hosted in cilia are essential for the elaboration of the body plan. The cilium's compartmental seclusion is ideal for noise-free molecular signaling and is essential for the precision of the body layout. The dysfunctional centrioles and primary cilia distort the development of body layout that manifest as serious developmental disorders. Thus, centriole has a dual role in the growth and cellular organization. It organizes dynamically expressed molecules of cell cycle and ciliogenesis and plays a balancing act to generate new cells and organize them during development. A putative master molecule may regulate and co-ordinate the dynamic gene expression at the centrioles. The convergence of many critical signaling components at the centriole reiterates the idea that centriole is a major molecular workstation involved in elaborating the structural design and complexity in vertebrates.     

C-NAP1 and rootletin restrain DNA damage-induced centriole splitting and facilitate ciliogenesis

Cilia are found on most human cells and exist as motile cilia or non-motile primary cilia. Primary cilia play sensory roles in transducing various extracellular signals, and defective ciliary functions are involved in a wide range of human diseases. Centrosomes are the principal microtubule-organizing centers of animal cells and contain two centrioles. We observed that DNA damage causes centriole splitting in non-transformed human cells, with isolated centrioles carrying the mother centriole markers CEP170 and ninein but not kizuna or cenexin. Loss of centriole cohesion through siRNA depletion of C-NAP1 or rootletin increased radiation-induced centriole splitting, with C-NAP1-depleted isolated centrioles losing mother markers. As the mother centriole forms the basal body in primary cilia, we tested whether centriole splitting affected ciliogenesis. While irradiated cells formed apparently normal primary cilia, most cilia arose from centriolar clusters, not from isolated centrioles. Furthermore, C-NAP1 or rootletin knockdown reduced primary cilium formation. Therefore, the centriole cohesion apparatus at the proximal end of centrioles may provide a target that can affect primary cilium formation as part of the DNA damage response.     

Phosphatase inhibitor 2 promotes acetylation of tubulin in the primary cilium of human retinal epithelial cells

Background  

Primary cilia are flagella-like projections from the centriole of mammalian cells that have a key role in cell signaling. Human diseases are linked to defects in primary cilia. Microtubules make up the axoneme of cilia and are selectively acetylated and this is thought to contribute to the stability of the structure. However, mechanisms to regulate tubulin acetylation in cilia are poorly understood.     

The LIM protein Ajuba is required for ciliogenesis and left-right axis determination in medaka

Cilia are microtubule-based organelles that are present on the surfaces of almost all vertebrate cells. Most cilia function as sensory or molecular transport structures. Malfunctions of cilia have been implicated in several diseases of human development. The assembly of cilia is initiated by the centriole (or basal body), and several centrosomal proteins are involved in this process. The mammalian LIM protein Ajuba is a well-studied centrosomal protein that regulates cell division but its role in ciliogenesis is unknown. In this study, we isolated the medaka homolog of Ajuba and showed that Ajuba localizes to basal bodies of cilia in growth-arrested cells. Knockdown of Ajuba resulted in randomized left-right organ asymmetries and altered expression of early genes responsible for left-right body axis determination. At the cellular level, we found that Ajuba function was essential for ciliogenesis in the cells lining Kupffer’s vesicle; it is these cells that induce the asymmetric fluid flow required for left-right axis determination. Taken together, our findings identify a novel role for Ajuba in the regulation of vertebrate ciliogenesis and left-right axis determination.     

The ultrastructure of primary cilia in quiescent 3T3 cells

Electron microscopy was used to investigate primary cilia in quiescent 3T3 cells. As in the case of primary cilia of other cell types, their basal centriole was found to be a focal point of numerous cytoplasmic microtubules which terminate at the basal feet. There are also intermediate filaments which appear to converge at the basal centriole. Cross-striated fibers of microtubular diameter, reminiscent of striated rootlets of ordinary cilia, appear associated with the proximal end of the basal centriole. Usually less than nine cross-banded basal feet surround the basal centriole in a well-defined plane perpendicular to the centriolar axis. The ciliary shaft was found to be entirely enclosed in the cytoplasm of fully flattened cells. In rounded cells, it could be found extending to the outside of the cell. Periodic striations along the entire shaft were observed after preparing the cells in a special way. The tip of the shaft showed an electron-dense specialization. Several unusual forms of primary cilia were observed which were reminiscent of olfactory flagella or retinal rods.Using tubulin antibody for indirect immunofluorescence, a fluorescent rod is visible in the cells [18] which we demonstrate is identical with the primary cilium.     

Centriole maturation requires regulated Plk1 activity during two consecutive cell cycles

Newly formed centrioles in cycling cells undergo a maturation process that is almost two cell cycles long before they become competent to function as microtubule-organizing centers and basal bodies. As a result, each cell contains three generations of centrioles, only one of which is able to form cilia. It is not known how this long and complex process is regulated. We show that controlled Plk1 activity is required for gradual biochemical and structural maturation of the centrioles and timely appendage assembly. Inhibition of Plk1 impeded accumulation of appendage proteins and appendage formation. Unscheduled Plk1 activity, either in cycling or interphase-arrested cells, accelerated centriole maturation and appendage and cilia formation on the nascent centrioles, erasing the age difference between centrioles in one cell. These findings provide a new understanding of how the centriole cycle is regulated and how proper cilia and centrosome numbers are maintained in the cells.     

Basal body stability and ciliogenesis requires the conserved component Poc1

Centrioles are the foundation for centrosome and cilia formation. The biogenesis of centrioles is initiated by an assembly mechanism that first synthesizes the ninefold symmetrical cartwheel and subsequently leads to a stable cylindrical microtubule scaffold that is capable of withstanding microtubule-based forces generated by centrosomes and cilia. We report that the conserved WD40 repeat domain–containing cartwheel protein Poc1 is required for the structural maintenance of centrioles in Tetrahymena thermophila. Furthermore, human Poc1B is required for primary ciliogenesis, and in zebrafish, DrPoc1B knockdown causes ciliary defects and morphological phenotypes consistent with human ciliopathies. T. thermophila Poc1 exhibits a protein incorporation profile commonly associated with structural centriole components in which the majority of Poc1 is stably incorporated during new centriole assembly. A second dynamic population assembles throughout the cell cycle. Our experiments identify novel roles for Poc1 in centriole stability and ciliogenesis.     

The centrosome duplication cycle in health and disease

Centrioles function in the assembly of centrosomes and cilia. Structural and numerical centrosome aberrations have long been implicated in cancer, and more recent genetic evidence directly links centrosomal proteins to the etiology of ciliopathies, dwarfism and microcephaly. To better understand these disease connections, it will be important to elucidate the biogenesis of centrioles as well as the controls that govern centriole duplication during the cell cycle. Moreover, it remains to be fully understood how these organelles organize a variety of dynamic microtubule-based structures in response to different physiological conditions. In proliferating cells, centrosomes are crucial for the assembly of microtubule arrays, including mitotic spindles, whereas in quiescent cells centrioles function as basal bodies in the formation of ciliary axonemes. In this short review, we briefly introduce the key gene products required for centriole duplication. Then we discuss recent findings on the centriole duplication factor STIL that point to centrosome amplification as a potential root cause for primary microcephaly in humans. We also present recent data on the role of a disease-related centriole-associated protein complex, Cep164-TTBK2, in ciliogenesis.     

Stress-induced localization of HSPA6 (HSP70B′) and HSPA1A (HSP70-1) proteins to centrioles in human neuronal cells

The localization of yellow fluorescent protein (YFP)-tagged HSP70 proteins was employed to identify stress-sensitive sites in human neurons following temperature elevation. Stable lines of human SH-SY5Y neuronal cells were established that expressed YFP-tagged protein products of the human inducible HSP70 genes HSPA6 (HSP70B′) and HSPA1A (HSP70-1). Following a brief period of thermal stress, YFP-tagged HSPA6 and HSPA1A rapidly appeared at centrioles in the cytoplasm of human neuronal cells, with HSPA6 demonstrating a more prolonged signal compared to HSPA1A. Each centriole is composed of a distal end and a proximal end, the latter linking the centriole doublet. The YFP-tagged HSP70 proteins targeted the proximal end of centrioles (identified by γ-tubulin marker) rather than the distal end (centrin marker). Centrioles play key roles in cellular polarity and migration during neuronal differentiation. The proximal end of the centriole, which is involved in centriole stabilization, may be stress-sensitive in post-mitotic, differentiating human neurons.     

CP110 exhibits novel regulatory activities during centriole assembly in Drosophila

CP110 is a conserved centriole protein implicated in the regulation of cell division, centriole duplication, and centriole length and in the suppression of ciliogenesis. Surprisingly, we report that mutant flies lacking CP110 (CP110Δ) were viable and fertile and had no obvious defects in cell division, centriole duplication, or cilia formation. We show that CP110 has at least three functions in flies. First, it subtly influences centriole length by counteracting the centriole-elongating activity of several centriole duplication proteins. Specifically, we report that centrioles are ∼10% longer than normal in CP110Δ mutants and ∼20% shorter when CP110 is overexpressed. Second, CP110 ensures that the centriolar microtubules do not extend beyond the distal end of the centriole, as some centriolar microtubules can be more than 50 times longer than the centriole in the absence of CP110. Finally, and unexpectedly, CP110 suppresses centriole overduplication induced by the overexpression of centriole duplication proteins. These studies identify novel and surprising functions for CP110 in vivo in flies.     

PRESENCE OF MULTIPLE CENTRIOLES AND PRIMARY CILIA DURING GROWTH AND EARLY DIFFERENTIATION IN THE MYOBLAST CO25 CELL LINE

CO25 cells, a mouse myoblast line, contain multiple centrioles and primary cilia. A most unusual feature has been the finding of large numbers of separate structures in single cells—up to a maximum of nine centrioles, six primary cilia, and 12 of both organelles together. Aberrant multipolar spindles were occasionally seen containing variable numbers of centrioles. This strongly suggests that cells containing supernumerary centrioles and cilia are lost during mitosis, and that additional centriolar structures are generated during each interphase. No change in centriole or primary cilium frequency was detected after inducing the differentiation of myoblasts into myotubes. However, a significant migration of these structures occurred from a perinuclear to a supranuclear position prior to and during the phase of myoblast elongation. This shift was not maintained during cell fusion, when a net migration back to the periphery was observed, suggesting that it may have some function in relation to cell elongation and the change in the pattern of microtubule distribution which occurs as part of the process.