The centrosome is a dynamic structure that ejects PCM flares

Native molecular architectures of centrosomes in C. elegans embryos

Centrosomes organize microtubules that are essential for mitotic divisions in animal cells. They consist of centrioles surrounded by Pericentriolar Material (PCM). Questions related to mechanisms of centriole assembly, PCM organization, and microtubule formation remain unanswered, in part due to limited availability of molecular-resolution structural analyses in situ. Here, we use cryo-electron tomography to visualize centrosomes across the cell cycle in cells isolated from C. elegans embryos. We describe a pseudo-timeline of centriole assembly and identify distinct structural features including a cartwheel in daughter centrioles, and incomplete microtubule doublets surrounded by a star-shaped density in mother centrioles. We find that centriole and PCM microtubules differ in protofilament number (13 versus 11) indicating distinct nucleation mechanisms. This difference could be explained by atypical γ-tubulin ring complexes with 11-fold symmetry identified at the minus ends of short PCM microtubules. We further characterize a porous and disordered network that forms the interconnected PCM. Thus, our work builds a three-dimensional structural atlas that helps explain how centrosomes assemble, grow, and achieve function.

Multivalent coiled-coil interactions enable full-scale centrosome assembly and strength

The outermost layer of centrosomes, called pericentriolar material (PCM), organizes microtubules for mitotic spindle assembly. The molecular interactions that enable PCM to assemble and resist external forces are poorly understood. Here, we use crosslinking mass spectrometry (XL-MS) to analyze PLK-1-potentiated multimerization of SPD-5, the main PCM scaffold protein in C. elegans. In the unassembled state, SPD-5 exhibits numerous intramolecular crosslinks that are eliminated after phosphorylation by PLK-1. Thus, phosphorylation induces a structural opening of SPD-5 that primes it for assembly. Multimerization of SPD-5 is driven by interactions between multiple dispersed coiled-coil domains. Structural analyses of a phosphorylated region (PReM) in SPD-5 revealed a helical hairpin that dimerizes to form a tetrameric coiled-coil. Mutations within this structure and other interacting regions cause PCM assembly defects that are partly rescued by eliminating microtubule-mediated forces, revealing that PCM assembly and strength are interdependent. We propose that PCM size and strength emerge from specific, multivalent coiled-coil interactions between SPD-5 proteins.

The PCM scaffold enables RNA localization to centrosomes

As microtubule-organizing centers, centrosomes direct assembly of the bipolar mitotic spindle required for chromosome segregation and genome stability. Centrosome activity requires the dynamic assembly of pericentriolar material (PCM), the composition and organization of which changes throughout the cell cycle. Recent studies highlight the conserved localization of several mRNAs encoded from centrosome-associated genes enriched at centrosomes, including Pericentrin-like protein (Plp) mRNA. However, relatively little is known about how RNAs localize to centrosomes and influence centrosome function. Here, we examine mechanisms underlying the subcellular localization of Plp mRNA. We find that Plp mRNA localization is puromycin-sensitive, and the Plp coding sequence is both necessary and sufficient for RNA localization, consistent with a co-translational transport mechanism. We identify regions within the Plp coding sequence that regulate Plp mRNA localization. Finally, we show that protein-protein interactions critical for elaboration of the PCM scaffold permit RNA localization to centrosomes. Taken together, these findings inform the mechanistic basis of Plp mRNA localization and lend insight into the oscillatory enrichment of RNA at centrosomes.

The organization and function of the Golgi apparatus in dendrite development and neurological disorders

Dendrites are specialized neuronal compartments that sense, integrate and transfer information in the neural network. Their development is tightly controlled and abnormal dendrite morphogenesis is strongly linked to neurological disorders. While dendritic morphology ranges from relatively simple to extremely complex for a specified neuron, either requires a functional secretory pathway to continually replenish proteins and lipids to meet dendritic growth demands. The Golgi apparatus occupies the center of the secretory pathway and is regulating posttranslational modifications, sorting, transport, and signal transduction, as well as acting as a non-centrosomal microtubule organization center. The neuronal Golgi apparatus shares common features with Golgi in other eukaryotic cell types but also forms distinct structures known as Golgi outposts that specifically localize in dendrites. However, the organization and function of Golgi in dendrite development and its impact on neurological disorders is just emerging and so far lacks a systematic summary. We describe the organization of the Golgi apparatus in neurons, review the current understanding of Golgi function in dendritic morphogenesis, and discuss the current challenges and future directions.

A central helical hairpin in SPD-5 enables centrosome strength and assembly

Centrosomes organize microtubules for mitotic spindle assembly and positioning. Forces mediated by these microtubules create tensile stresses on pericentriolar material (PCM), the outermost layer of centrosomes. How PCM resists these stresses is unclear at the molecular level. Here, we use cross-linking mass spectrometry (XL-MS) to map interactions underlying multimerization of SPD-5, an essential PCM scaffold component in C. elegans . We identified an interaction hotspot in an alpha helical hairpin motif in SPD-5 (a.a. 541-677). XL-MS data, ab initio structural predictions, and mass photometry suggest that this region dimerizes to form a tetrameric coiled-coil. Mutating a helical section (a.a. 610-640) or a single residue (R592) inhibited PCM assembly in embryos. This phenotype was rescued by eliminating microtubule pulling forces, revealing that PCM assembly and material strength are interrelated. We propose that interactions mediated by the helical hairpin strongly bond SPD-5 molecules to each other, thus enabling PCM to assemble fully and withstand stresses generated by microtubules.

Multivalent coiled-coil interactions enable full-scale centrosome assembly and strength

During mitotic spindle assembly, microtubules generate tensile stresses on pericentriolar material (PCM), the outermost layer of centrosomes. The molecular interactions that enable PCM to assemble rapidly and resist external forces are unknown. Here we use cross-linking mass spectrometry to identify interactions underlying supramolecular assembly of SPD-5, the main PCM scaffold protein in C. elegans . Crosslinks map primarily to alpha helices within the phospho-regulated region (PReM), a long C-terminal coiled-coil, and a series of four N-terminal coiled-coils. PLK-1 phosphorylation of SPD-5 creates new homotypic contacts, including two between PReM and the CM2-like domain, and eliminates numerous contacts in disordered linker regions, thus favoring coiled-coil-specific interactions. Mutations within these interacting regions cause PCM assembly defects that are partly rescued by eliminating microtubule-mediated forces. Thus, PCM assembly and strength are interdependent. In vitro , self-assembly of SPD-5 scales with coiled-coil content, although there is a defined hierarchy of association. We propose that multivalent interactions among coiled-coil regions of SPD-5 build the PCM scaffold and contribute sufficient strength to resist microtubule-mediated forces.

Self-assembly of pericentriolar material in interphase cells lacking centrioles

The major microtubule-organizing center (MTOC) in animal cells, the centrosome, comprises a pair of centrioles surrounded by pericentriolar material (PCM), which nucleates and anchors microtubules. Centrosome assembly depends on PCM binding to centrioles, PCM self-association and dynein-mediated PCM transport, but the self-assembly properties of PCM components in interphase cells are poorly understood. Here, we used experiments and modeling to study centriole-independent features of interphase PCM assembly. We showed that when centrioles are lost due to PLK4 depletion or inhibition, dynein-based transport and self-clustering of PCM proteins are sufficient to form a single compact MTOC, which generates a dense radial microtubule array. Interphase self-assembly of PCM components depends on γ-tubulin, pericentrin, CDK5RAP2 and ninein, but not NEDD1, CEP152, or CEP192. Formation of a compact acentriolar MTOC is inhibited by AKAP450-dependent PCM recruitment to the Golgi or by randomly organized CAMSAP2-stabilized microtubules, which keep PCM mobile and prevent its coalescence. Linking of CAMSAP2 to a minus-end-directed motor leads to the formation of an MTOC, but MTOC compaction requires cooperation with pericentrin-containing self-clustering PCM. Our data reveal that interphase PCM contains a set of components that can self-assemble into a compact structure and organize microtubules, but PCM self-organization is sensitive to motor- and microtubule-based rearrangement.

Orb-dependent polyadenylation contributes to PLP expression and centrosome scaffold assembly

As the microtubule-organizing centers of most cells, centrosomes engineer the bipolar mitotic spindle required for error-free mitosis. Drosophila Pericentrin-like protein (PLP) directs formation of a pericentriolar material (PCM) scaffold required for PCM organization and microtubule-organizing center function. Here, we investigate the post-transcriptional regulation of Plp mRNA. We identify conserved binding sites for cytoplasmic polyadenylation element binding (CPEB) proteins within the Plp 3′-untranslated region and examine the role of the CPEB ortholog Oo18 RNA-binding protein (Orb) in Plp mRNA regulation. Our data show that Orb interacts biochemically with Plp mRNA to promote polyadenylation and PLP protein expression. Loss of orb, but not orb2, diminishes PLP levels in embryonic extracts. Consequently, PLP localization to centrosomes and its function in PCM scaffolding are compromised in orb mutant embryos, resulting in genomic instability and embryonic lethality. Moreover, we find that PLP overexpression restores centrosome scaffolding and rescues the cell division defects caused by orb depletion. Our data suggest that Orb modulates PLP expression at the level of Plp mRNA polyadenylation and demonstrates that the post-transcriptional regulation of core, conserved centrosomal mRNAs is crucial for centrosome function.

Centrioles generate a local pulse of Polo/PLK1 activity to initiate mitotic centrosome assembly

Mitotic centrosomes are formed when centrioles start to recruit large amounts of pericentriolar material (PCM) around themselves in preparation for mitosis. This centrosome "maturation" requires the centrioles and also Polo/PLK1 protein kinase. The PCM comprises several hundred proteins and, in Drosophila, Polo cooperates with the conserved centrosome proteins Spd-2/CEP192 and Cnn/CDK5RAP2 to assemble a PCM scaffold around the mother centriole that then recruits other PCM client proteins. We show here that in Drosophila syncytial blastoderm embryos, centrosomal Polo levels rise and fall during the assembly process-peaking, and then starting to decline, even as levels of the PCM scaffold continue to rise and plateau. Experiments and mathematical modelling indicate that a centriolar pulse of Polo activity, potentially generated by the interaction between Polo and its centriole receptor Ana1 (CEP295 in humans), could explain these unexpected scaffold assembly dynamics. We propose that centrioles generate a local pulse of Polo activity prior to mitotic entry to initiate centrosome maturation, explaining why centrioles and Polo/PLK1 are normally essential for this process.

Loss of telomere silencing is accompanied by dysfunction of Polo kinase and centrosomes during Drosophila oogenesis and early development

Telomeres are nucleoprotein complexes that protect the ends of eukaryotic linear chromosomes from degradation and fusions. Telomere dysfunction leads to cell growth arrest, oncogenesis, and premature aging. Telomeric RNAs have been found in all studied species; however, their functions and biogenesis are not clearly understood. We studied the mechanisms of development disorders observed upon overexpression of telomeric repeats in Drosophila. In somatic cells, overexpression of telomeric retrotransposon HeT-A is cytotoxic and leads to the accumulation of HeT-A Gag near centrosomes. We found that RNA and RNA-binding protein Gag encoded by the telomeric retrotransposon HeT-A interact with Polo and Cdk1 mitotic kinases, which are conserved regulators of centrosome biogenesis and cell cycle. The depletion of proteins Spindle E, Ccr4 or Ars2 resulting in HeT-A overexpression in the germline was accompanied by mislocalization of Polo as well as its abnormal stabilization during oogenesis and severe deregulation of centrosome biogenesis leading to maternal-effect embryonic lethality. These data suggest a mechanistic link between telomeric HeT-A ribonucleoproteins and cell cycle regulators that ensures the cell response to telomere dysfunction.

A novel mitosis-specific Cep215 domain interacts with Cep192 and phosphorylated Aurora A for organization of spindle poles

The centrosome, which consists of centrioles and pericentriolar material (PCM), becomes mature and assembles mitotic spindles by increasing the number of microtubules (MTs) emanating from the PCM. Among the molecules involved in centrosome maturation, Cep192 and Aurora A (AurA, also known as AURKA) are primarily responsible for recruitment of γ-tubulin and MT nucleators, whereas pericentrin (PCNT) is required for PCM organization. However, the role of Cep215 (also known as CDK5RAP2) in centrosome maturation remains elusive. Cep215 possesses binding domains for γ-tubulin, PCNT and MT motors that transport acentrosomal MTs towards the centrosome. We identify a mitosis-specific centrosome-targeting domain of Cep215 (215N) that interacts with Cep192 and phosphorylated AurA (pAurA). Cep192 is essential for targeting 215N to centrosomes, and centrosomal localization of 215N and pAurA is mutually dependent. Cep215 has a relatively minor role in γ-tubulin recruitment to the mitotic centrosome. However, it has been shown previously that this protein is important for connecting mitotic centrosomes to spindle poles. Based on the results of rescue experiments using versions of Cep215 with different domain deletions, we conclude that Cep215 plays a role in maintaining the structural integrity of the spindle pole by providing a platform for the molecules involved in centrosome maturation.

Regulated changes in material properties underlie centrosome disassembly during mitotic exit

Centrosomes must resist microtubule-mediated forces for mitotic chromosome segregation. During mitotic exit, however, centrosomes are deformed and fractured by those same forces, which is a key step in centrosome disassembly. How the functional material properties of centrosomes change throughout the cell cycle, and how they are molecularly tuned, remain unknown. Here, we used optically induced flow perturbations to determine the molecular basis of centrosome strength and ductility in C. elegans embryos. We found that both properties declined sharply at anaphase onset, long before natural disassembly. This mechanical transition required PP2A phosphatase and correlated with inactivation of PLK-1 (Polo kinase) and SPD-2 (Cep192). In vitro, PLK-1 and SPD-2 directly protected centrosome scaffolds from force-induced disassembly. Our results suggest that, before anaphase, PLK-1 and SPD-2 respectively confer strength and ductility to the centrosome scaffold so that it can resist microtubule-pulling forces. In anaphase, centrosomes lose PLK-1 and SPD-2 and transition to a weak, brittle state that enables force-mediated centrosome disassembly.

The material state of centrosomes: lattice, liquid, or gel?

Centrosomes are micron-scale structures that nucleate microtubule arrays for chromosome segregation and mitotic spindle positioning. For these jobs, centrosomes must be dynamic enough to grow, yet stable enough to resist microtubule-mediated forces. How do centrosomes achieve such seemingly contradictory features? While much is understood about the molecular parts of centrosomes, very little is known about their functional material properties. Two prevalent hypotheses pose that the centrosome is either a liquid droplet or a solid lattice. However, many material states exist between a pure Newtonian liquid and a crystalline solid, and it is not clear where centrosomes lie along this spectrum. Furthermore, broad terms like "liquid" or "solid" do not reveal functional properties like strength, ductility, elasticity, and toughness, which are more relevant to understand how centrosomes resist forces. This review covers recent findings and new rheology techniques that reveal the material characteristics of centrosomes and how they are regulated.

centrocortin RNA localization to centrosomes is regulated by FMRP and facilitates error-free mitosis

Centrosomes are microtubule-organizing centers required for error-free mitosis and embryonic development. The microtubule-nucleating activity of centrosomes is conferred by the pericentriolar material (PCM), a composite of numerous proteins subject to cell cycle-dependent oscillations in levels and organization. In diverse cell types, mRNAs localize to centrosomes and may contribute to changes in PCM abundance. Here, we investigate the regulation of mRNA localization to centrosomes in the rapidly cycling Drosophila melanogaster embryo. We find that RNA localization to centrosomes is regulated during the cell cycle and developmentally. We identify a novel role for the fragile-X mental retardation protein in the posttranscriptional regulation of a model centrosomal mRNA, centrocortin (cen). Further, mistargeting cen mRNA is sufficient to alter cognate protein localization to centrosomes and impair spindle morphogenesis and genome stability.

Drosophila pericentrin-like protein promotes the formation of primordial germ cells

Primordial germ cells (PGCs) are the precursors to the adult germline stem cells that are set aside early during embryogenesis and specified through the inheritance of the germ plasm, which contains the mRNAs and proteins that function as the germline fate determinants. In Drosophila melanogaster, formation of the PGCs requires the microtubule and actin cytoskeletal networks to actively segregate the germ plasm from the soma and physically construct the pole buds (PBs) that protrude from the posterior cortex. Of emerging importance is the central role of centrosomes in the coordination of microtubule dynamics and actin organization to promote PGC development. We previously identified a requirement for the centrosome protein Centrosomin (Cnn) in PGC formation. Cnn interacts directly with Pericentrin-like protein (PLP) to form a centrosome scaffold structure required for pericentriolar material recruitment and organization. In this study, we identify a role for PLP at several discrete steps during PGC development. We find PLP functions in segregating the germ plasm from the soma by regulating microtubule organization and centrosome separation. These activities further contribute to promoting PB protrusion and facilitating the distribution of germ plasm in proliferating PGCs.

A two-step mechanism for the inactivation of microtubule organizing center function at the centrosome

The centrosome acts as a microtubule organizing center (MTOC), orchestrating microtubules into the mitotic spindle through its pericentriolar material (PCM). This activity is biphasic, cycling through assembly and disassembly during the cell cycle. Although hyperactive centrosomal MTOC activity is a hallmark of some cancers, little is known about how the centrosome is inactivated as an MTOC. Analysis of endogenous PCM proteins in C. elegans revealed that the PCM is composed of partially overlapping territories organized into an inner and outer sphere that are removed from the centrosome at different rates and using different behaviors. We found that phosphatases oppose the addition of PCM by mitotic kinases, ultimately catalyzing the dissolution of inner sphere PCM proteins at the end of mitosis. The nature of the PCM appears to change such that the remaining aging PCM outer sphere is mechanically ruptured by cortical pulling forces, ultimately inactivating MTOC function at the centrosome.

New cells are created when existing cells divide, a process that is critical for life. A structure called the spindle is an important part of cell division, helping to orient the division and separate parts of the old cell into the newly generated ones. The spindle is built using filamentous protein structures called microtubules which are arranged by microtubule organizing centers (or MTOCs for short). In animals, an MTOC forms at each end of the spindle around two structures called centrosomes.

A network of proteins called the pericentriolar material (PCM) form around centrosomes, converting them into MTOCs. The PCM grows around centrosomes as a cell prepares to divide and is removed again afterward. Enzymes called kinases are important in controlling cell division and PCM assembly; they are opposed by other enzymes known as phosphatases. The processes involved in organization and removal of the PCM are not well understood.

The microscopic worm Caenorhabditis elegans provides an opportunity to study details of cell division in a living animal. Magescas et al. used fluorescent labels to view proteins from the PCM under a microscope. The images showed two partially overlapping spherical parts to the PCM – inner and outer. Further examination revealed that the inner PCM is maintained by a careful balance of kinase and phosphatase activity. When kinases shut down at the end of cell division, the phosphatases break down the inner PCM. By contrast, the outer PCM is physically torn apart by forces acting through the attached microtubules.

Future work will seek to examine which proteins are specifically affected by phosphatases to identify the key regulators of PCM persistence in the cell and to reveal the proteins needed for MTOC activity at the centrosome. Since poor MTOC regulation can play a part in the growth and spread of cancer, this could lead to targets for new treatments.

Junctional tumor suppressors interact with 14-3-3 proteins to control planar spindle alignment

Nakajima et al. reveal a novel mechanism of planar spindle alignment through junctional tumor suppressors Scrib/Dlg and 14-3-3 proteins in the Drosophila wing disc epithelium. Their results suggest that 14-3-3 proteins interact with Scrib/Dlg to control planar spindle orientation and maintain epithelial architecture.

Proper orientation of the mitotic spindle is essential for cell fate determination, tissue morphogenesis, and homeostasis. During epithelial proliferation, planar spindle alignment ensures the maintenance of polarized tissue architecture, and aberrant spindle orientation can disrupt epithelial integrity. Nevertheless, in vivo mechanisms that restrict the mitotic spindle to the plane of the epithelium remain poorly understood. Here we show that the junction-localized tumor suppressors Scribbled (Scrib) and Discs large (Dlg) control planar spindle orientation via Mud and 14-3-3 proteins in the Drosophila wing disc epithelium. During mitosis, Scrib is required for the junctional localization of Dlg, and both affect mitotic spindle movements. Using coimmunoprecipitation and mass spectrometry, we identify 14-3-3 proteins as Dlg-interacting partners and further report that loss of 14-3-3s causes both abnormal spindle orientation and disruption of epithelial architecture as a consequence of basal cell delamination and apoptosis. Combined, these biochemical and genetic analyses indicate that 14-3-3s function together with Scrib, Dlg, and Mud during planar cell division.

Centrosomal and Non-Centrosomal Microtubule-Organizing Centers (MTOCs) in Drosophila melanogaster

The centrosome is the best-understood microtubule-organizing center (MTOC) and is essential in particular cell types and at specific stages during Drosophila development. The centrosome is not required zygotically for mitosis or to achieve full animal development. Nevertheless, centrosomes are essential maternally during cleavage cycles in the early embryo, for male meiotic divisions, for efficient division of epithelial cells in the imaginal wing disc, and for cilium/flagellum assembly in sensory neurons and spermatozoa. Importantly, asymmetric and polarized division of stem cells is regulated by centrosomes and by the asymmetric regulation of their microtubule (MT) assembly activity. More recently, the components and functions of a variety of non-centrosomal microtubule-organizing centers (ncMTOCs) have begun to be elucidated. Throughout Drosophila development, a wide variety of unique ncMTOCs form in epithelial and non-epithelial cell types at an assortment of subcellular locations. Some of these cell types also utilize the centrosomal MTOC, while others rely exclusively on ncMTOCs. The impressive variety of ncMTOCs being discovered provides novel insight into the diverse functions of MTOCs in cells and tissues. This review highlights our current knowledge of the composition, assembly, and functional roles of centrosomal and non-centrosomal MTOCs in Drosophila.

Phosphatase PP2A and microtubule-mediated pulling forces disassemble centrosomes during mitotic exit

Centrosomes are microtubule-nucleating organelles that facilitate chromosome segregation and cell division in metazoans. Centrosomes comprise centrioles that organize a micron-scale mass of protein called pericentriolar material (PCM) from which microtubules nucleate. During each cell cycle, PCM accumulates around centrioles through phosphorylation-mediated assembly of PCM scaffold proteins. During mitotic exit, PCM swiftly disassembles by an unknown mechanism. Here, we used Caenorhabditis elegans embryos to determine the mechanism and importance of PCM disassembly in dividing cells. We found that the phosphatase PP2A and its regulatory subunit SUR-6 (PP2A^SUR-6), together with cortically directed microtubule pulling forces, actively disassemble PCM. In embryos depleted of these activities, ∼25% of PCM persisted from one cell cycle into the next. Purified PP2A^SUR-6 could dephosphorylate the major PCM scaffold protein SPD-5 in vitro. Our data suggest that PCM disassembly occurs through a combination of dephosphorylation of PCM components and force-driven fragmentation of the PCM scaffold.

Summary: Centrosomes acquire and lose pericentriolar material during each cell cycle. We show that dephosphorylation and cortically directed forces disassemble pericentriolar material at the end of mitosis.

Structural Basis for Mitotic Centrosome Assembly in Flies

In flies, Centrosomin (Cnn) forms a phosphorylation-dependent scaffold that recruits proteins to the mitotic centrosome, but how Cnn assembles into a scaffold is unclear. We show that scaffold assembly requires conserved leucine zipper (LZ) and Cnn-motif 2 (CM2) domains that co-assemble into a 2:2 complex in vitro. We solve the crystal structure of the LZ:CM2 complex, revealing that both proteins form helical dimers that assemble into an unusual tetramer. A slightly longer version of the LZ can form micron-scale structures with CM2, whose assembly is stimulated by Plk1 phosphorylation in vitro. Mutating individual residues that perturb LZ:CM2 tetramer assembly perturbs the formation of these micron-scale assemblies in vitro and Cnn-scaffold assembly in vivo. Thus, Cnn molecules have an intrinsic ability to form large, LZ:CM2-interaction-dependent assemblies that are critical for mitotic centrosome assembly. These studies provide the first atomic insight into a molecular interaction required for mitotic centrosome assembly.

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The conserved PReM and CM2 domains of Cnn co-assemble into micron-scale structures

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The crystal structure of the PReM-LZ:CM2 complex is solved to 1.82 Å

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Mutations that block PReM-LZ:CM2 assembly in vitro block centrosome assembly in vivo

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Phosphorylation of PReM by Polo/Plk1 promotes scaffold assembly in vitro and in vivo

The Centrioles, Centrosomes, Basal Bodies, and Cilia of Drosophila melanogaster

Centrioles play a key role in the development of the fly. They are needed for the correct formation of centrosomes, the organelles at the poles of the spindle that can persist as microtubule organizing centers (MTOCs) into interphase. The ability to nucleate cytoplasmic microtubules (MTs) is a property of the surrounding pericentriolar material (PCM). The centriole has a dual life, existing not only as the core of the centrosome but also as the basal body, the structure that templates the formation of cilia and flagellae. Thus the structure and functions of the centriole, the centrosome, and the basal body have an impact upon many aspects of development and physiology that can readily be modeled in Drosophila Centrosomes are essential to give organization to the rapidly increasing numbers of nuclei in the syncytial embryo and for the spatially precise execution of cell division in numerous tissues, particularly during male meiosis. Although mitotic cell cycles can take place in the absence of centrosomes, this is an error-prone process that opens up the fly to developmental defects and the potential of tumor formation. Here, we review the structure and functions of the centriole, the centrosome, and the basal body in different tissues and cultured cells of Drosophila melanogaster, highlighting their contributions to different aspects of development and cell division.

Developmental Expression of 4-Repeat-Tau Induces Neuronal Aneuploidy in Drosophila Tauopathy Models

Tau-mediated neurodegeneration in Alzheimer’s disease and tauopathies is generally assumed to start in a normally developed brain. However, several lines of evidence suggest that impaired Tau isoform expression during development could affect mitosis and ploidy in post-mitotic differentiated tissue. Interestingly, the relative expression levels of Tau isoforms containing either 3 (3R-Tau) or 4 repeats (4R-Tau) play an important role both during brain development and neurodegeneration. Here, we used genetic and cellular tools to study the link between 3R and 4R-Tau isoform expression, mitotic progression in neuronal progenitors and post-mitotic neuronal survival. Our results illustrated that the severity of Tau-induced adult phenotypes depends on 4R-Tau isoform expression during development. As recently described, we observed a mitotic delay in 4R-Tau expressing cells of larval eye discs and brains. Live imaging revealed that the spindle undergoes a cycle of collapse and recovery before proceeding to anaphase. Furthermore, we found a high level of aneuploidy in post-mitotic differentiated tissue. Finally, we showed that overexpression of wild type and mutant 4R-Tau isoform in neuroblastoma SH-SY5Y cell lines is sufficient to induce monopolar spindles. Taken together, our results suggested that neurodegeneration could be in part linked to neuronal aneuploidy caused by 4R-Tau expression during brain development.

Centrosome centering and decentering by microtubule network rearrangement

Numerical simulations are used to investigate the role of microtubule network architecture in centrosome positioning. Microtubule gliding along cell edges and pivoting around the centrosome are key regulators of the orientation of pushing forces, the magnitude of which depends on the number, dynamics, and stiffness of microtubules.

The centrosome is positioned at the cell center by pushing and pulling forces transmitted by microtubules (MTs). Centrosome decentering is often considered to result from asymmetric, cortical pulling forces exerted in particular by molecular motors on MTs and controlled by external cues affecting the cell cortex locally. Here we used numerical simulations to investigate the possibility that it could equally result from the redistribution of pushing forces due to a reorientation of MTs. We first showed that MT gliding along cell edges and pivoting around the centrosome regulate MT rearrangement and thereby direct the spatial distribution of pushing forces, whereas the number, dynamics, and stiffness of MTs determine the magnitude of these forces. By modulating these parameters, we identified different regimes, involving both pushing and pulling forces, characterized by robust centrosome centering, robust off-centering, or “reactive” positioning. In the last-named conditions, weak asymmetric cues can induce a misbalance of pushing and pulling forces, resulting in an abrupt transition from a centered to an off-centered position. Taken together, these results point to the central role played by the configuration of the MTs on the distribution of pushing forces that position the centrosome. We suggest that asymmetric external cues should not be seen as direct driver of centrosome decentering and cell polarization but instead as inducers of an effective reorganization of the MT network, fostering centrosome motion to the cell periphery.

An Amino-Terminal Polo Kinase Interaction Motif Acts in the Regulation of Centrosome Formation and Reveals a Novel Function for centrosomin (cnn) in Drosophila

The formation of the pericentriolar matrix (PCM) and a fully functional centrosome in syncytial Drosophila melanogaster embryos requires the rapid transport of Cnn during initiation of the centrosome replication cycle. We show a Cnn and Polo kinase interaction is apparently required during embryogenesis and involves the exon 1A-initiating coding exon, suggesting a subset of Cnn splice variants is regulated by Polo kinase. During PCM formation exon 1A Cnn-Long Form proteins likely bind Polo kinase before phosphorylation by Polo for Cnn transport to the centrosome. Loss of either of these interactions in a portion of the total Cnn protein pool is sufficient to remove native Cnn from the pool, thereby altering the normal localization dynamics of Cnn to the PCM. Additionally, Cnn-Short Form proteins are required for polar body formation, a process known to require Polo kinase after the completion of meiosis. Exon 1A Cnn-LF and Cnn-SF proteins, in conjunction with Polo kinase, are required at the completion of meiosis and for the formation of functional centrosomes during early embryogenesis.

Centrosome biology: the ins and outs of centrosome assembly

As a microtubule-organizing center, the centrosome undergoes a dramatic increase in size - via expansion of the pericentriolar material - during mitosis. Recent work reveals shared assembly properties of a protein scaffold that facilitates and supports this expansion, a process critical to spindle assembly.

The end of a monolith: Deconstructing the Cnn-Polo interaction

In Drosophila melanogaster a functional pericentriolar matrix (PCM) at mitotic centrosomes requires Centrosomin-Long Form (Cnn-LF) proteins. Moreover, tissue culture cells have shown that the centrosomal localization of both Cnn-LF and Polo kinase are co-dependent, suggesting a direct interaction. Our recent study found Cnn potentially binds to and is phosphorylated by Polo kinase at 2 residues encoded by Exon1A, the initiating exon of a subset of Cnn isoforms. These interactions are required for the centrosomal localization of Cnn-LF in syncytial embryos and a mutation of either phosphorylation site is sufficient to block localization of both mutant and wild-type Cnn when they are co-expressed. Immunoprecipitation experiments show that Cnn-LF interacts directly with mitotically activated Polo kinase and requires the 2 phosphorylation sites in Exon1A. These IP experiments also show that Cnn-LF proteins form multimers. Depending on the stoichiometry between functional and mutant peptides, heteromultimers exhibit dominant negative or positive trans-complementation (rescue) effects on mitosis. Additionally, following the completion of meiosis, Cnn-Short Form (Cnn-SF) proteins are required for polar body formation in embryos, a process previously shown to require Polo kinase. These findings, when combined with previous work, clearly demonstrate the complexity of cnn and show that a view of cnn as encoding a single peptide is too simplistic.

A CEP215-HSET complex links centrosomes with spindle poles and drives centrosome clustering in cancer

Numerical centrosome aberrations underlie certain developmental abnormalities and may promote cancer. A cell maintains normal centrosome numbers by coupling centrosome duplication with segregation, which is achieved through sustained association of each centrosome with a mitotic spindle pole. Although the microcephaly- and primordial dwarfism-linked centrosomal protein CEP215 has been implicated in this process, the molecular mechanism responsible remains unclear. Here, using proteomic profiling, we identify the minus end-directed microtubule motor protein HSET as a direct binding partner of CEP215. Targeted deletion of the HSET-binding domain of CEP215 in vertebrate cells causes centrosome detachment and results in HSET depletion at centrosomes, a phenotype also observed in CEP215-deficient patient-derived cells. Moreover, in cancer cells with centrosome amplification, the CEP215-HSET complex promotes the clustering of extra centrosomes into pseudo-bipolar spindles, thereby ensuring viable cell division. Therefore, stabilization of the centrosome-spindle pole interface by the CEP215-HSET complex could promote survival of cancer cells containing supernumerary centrosomes.

Interphase centrosome organization by the PLP-Cnn scaffold is required for centrosome function

Cnn and PLP directly interact at two defined sites to coordinate the cell cycle–dependent rearrangement and scaffolding activity of the centrosome to permit normal centrosome organization, cell division, and embryonic viability.

Pericentriolar material (PCM) mediates the microtubule (MT) nucleation and anchoring activity of centrosomes. A scaffold organized by Centrosomin (Cnn) serves to ensure proper PCM architecture and functional changes in centrosome activity with each cell cycle. Here, we investigate the mechanisms that spatially restrict and temporally coordinate centrosome scaffold formation. Focusing on the mitotic-to-interphase transition in Drosophila melanogaster embryos, we show that the elaboration of the interphase Cnn scaffold defines a major structural rearrangement of the centrosome. We identify an unprecedented role for Pericentrin-like protein (PLP), which localizes to the tips of extended Cnn flares, to maintain robust interphase centrosome activity and promote the formation of interphase MT asters required for normal nuclear spacing, centrosome segregation, and compartmentalization of the syncytial embryo. Our data reveal that Cnn and PLP directly interact at two defined sites to coordinate the cell cycle–dependent rearrangement and scaffolding activity of the centrosome to permit normal centrosome organization, cell division, and embryonic viability.

Centrosome function and assembly in animal cells

It has become clear that the role of centrosomes extends well beyond that of important microtubule organizers. There is increasing evidence that they also function as coordination centres in eukaryotic cells, at which specific cytoplasmic proteins interact at high concentrations and important cell decisions are made. Accordingly, hundreds of proteins are concentrated at centrosomes, including cell cycle regulators, checkpoint proteins and signalling molecules. Nevertheless, several observations have raised the question of whether centrosomes are essential for many cell processes. Recent findings have shed light on the functions of centrosomes in animal cells and on the molecular mechanisms of centrosome assembly, in particular during mitosis. These advances should ultimately allow the in vitro reconstitution of functional centrosomes from their component proteins to unlock the secrets of these enigmatic organelles.

Centrosomin represses dendrite branching by orienting microtubule nucleation

Neuronal dendrite branching is fundamental for building nervous systems. Branch formation is genetically encoded by transcriptional programs to create dendrite arbor morphological diversity for complex neuronal functions. In Drosophila sensory neurons, the transcription factor Abrupt represses branching via an unknown effector pathway. Targeted screening for branching-control effectors identified Centrosomin, the primary centrosome-associated protein for mitotic spindle maturation. Centrosomin repressed dendrite branch formation and was used by Abrupt to simplify arbor branching. Live imaging revealed that Centrosomin localized to the Golgi cis face and that it recruited microtubule nucleation to Golgi outposts for net retrograde microtubule polymerization away from nascent dendrite branches. Removal of Centrosomin enabled the engagement of wee Augmin activity to promote anterograde microtubule growth into the nascent branches, leading to increased branching. The findings reveal that polarized targeting of Centrosomin to Golgi outposts during elaboration of the dendrite arbor creates a local system for guiding microtubule polymerization.

The Drosophila Pericentrin-like-protein (PLP) cooperates with Cnn to maintain the integrity of the outer PCM

Centrosomes comprise a pair of centrioles surrounded by a matrix of pericentriolar material (PCM). In vertebrate cells, Pericentrin plays an important part in mitotic PCM assembly, but the Drosophila Pericentrin-like protein (PLP) appears to have a more minor role in mitotic fly cells. Here we investigate the function of PLP during the rapid mitotic cycles of the early Drosophila embryo. Unexpectedly, we find that PLP is specifically enriched in the outer-most regions of the PCM, where it largely co-localizes with the PCM scaffold protein Cnn. In the absence of PLP the outer PCM appears to be structurally weakened, and it rapidly disperses along the centrosomal microtubules (MTs). As a result, centrosomal MTs are subtly disorganized in embryos lacking PLP, although mitosis is largely unperturbed and these embryos develop and hatch at near-normal rates. Y2H analysis reveals that PLP can potentially form multiple interactions with itself and with the PCM recruiting proteins Asl, Spd-2 and Cnn. A deletion analysis suggests that PLP participates in a complex network of interactions that ultimately help to strengthen the PCM.

Time-lapse recording of centrosomes and other organelles in Drosophila neuroblasts

Drosophila larval neuroblasts (NBs) are an excellent model for asymmetric division and cell cycle studies in general. For decades, visualizing relevant structures like centrosomes, chromosomes, or the mitotic spindle relied exclusively on immunofluorescence on fix samples. More recently, improvements on sensitivity and acquisition speed of different confocal systems have made it possible to acquire time-resolved images of combined fluorescent reporters from single larval NBs. Here, we provide protocols to visualize centrosomes and other organelles from both primary cultures of isolated single NBs and ex vivo, whole-mounted larval brains.

Pericentriolar material structure and dynamics

A centrosome consists of two barrel-shaped centrioles embedded in a matrix of proteins known as the pericentriolar material (PCM). The PCM serves as a platform for protein complexes that regulate organelle trafficking, protein degradation and spindle assembly. Perhaps most important for cell division, the PCM concentrates tubulin and serves as the primary organizing centre for microtubules in metazoan somatic cells. Thus, similar to other well-described organelles, such as the nucleus and mitochondria, the cell has compartmentalized a multitude of vital biochemical reactions in the PCM. However, unlike these other organelles, the PCM is not membrane bound, but rather a dynamic collection of protein complexes and nucleic acids that constitute the organelle's interior and determine its boundary. How is the complex biochemical machinery necessary for the myriad centrosome functions concentrated and maintained in the PCM? Recent advances in proteomics and RNAi screening have unveiled most of the key PCM components and hinted at their molecular interactions ( table 1). Now we must understand how the interactions between these molecules contribute to the mesoscale organization and the assembly of the centrosome. Among outstanding questions are the intrinsic mechanisms that determine PCM shape and size, and how it functions as a biochemical reaction hub.

Identification of a new stem cell population that generates Drosophila flight muscles

How myoblast populations are regulated for the formation of muscles of different sizes is an essentially unanswered question. The large flight muscles of Drosophila develop from adult muscle progenitor (AMP) cells set-aside embryonically. The thoracic segments are all allotted the same small AMP number, while those associated with the wing-disc proliferate extensively to give rise to over 2500 myoblasts. An initial amplification occurs through symmetric divisions and is followed by a switch to asymmetric divisions in which the AMPs self-renew and generate post-mitotic myoblasts. Notch signaling controls the initial amplification of AMPs, while the switch to asymmetric division additionally requires Wingless, which regulates Numb expression in the AMP lineage. In both cases, the epidermal tissue of the wing imaginal disc acts as a niche expressing the ligands Serrate and Wingless. The disc-associated AMPs are a novel muscle stem cell population that orchestrates the early phases of adult flight muscle development.

Live imaging of Drosophila larval neuroblasts

Stem cells divide asymmetrically to generate two progeny cells with unequal fate potential: a self-renewing stem cell and a differentiating cell. Given their relevance to development and disease, understanding the mechanisms that govern asymmetric stem cell division has been a robust area of study. Because they are genetically tractable and undergo successive rounds of cell division about once every hour, the stem cells of the Drosophila central nervous system, or neuroblasts, are indispensable models for the study of stem cell division. About 100 neural stem cells are located near the surface of each of the two larval brain lobes, making this model system particularly useful for live imaging microscopy studies. In this work, we review several approaches widely used to visualize stem cell divisions, and we address the relative advantages and disadvantages of those techniques that employ dissociated versus intact brain tissues. We also detail our simplified protocol used to explant whole brains from third instar larvae for live cell imaging and fixed analysis applications.

The centriolar protein Bld10/Cep135 is required to establish centrosome asymmetry in Drosophila neuroblasts

Centrosome asymmetry has been implicated in stem cell fate maintenance in both flies and vertebrates [1, 2]. Drosophila neuroblasts, the neural precursors of the fly's central nervous system [3], contain molecularly and physically asymmetric centrosomes, established through differences in pericentriolar matrix (PCM) retention [4-7]. For instance, the daughter centriole maintains PCM and thus microtubule-organizing center (MTOC) activity through Polo-mediated phosphorylation of Centrobin (Cnb) [7, 8]. The mother centriole, however, quickly downregulates PCM and moves away from the apical cortex, randomly migrating through the cytoplasm until maturation sets in at prophase [4-6, 8]. How PCM downregulation is molecularly controlled is currently unknown, but it involves Pericentrin (PCNT)-like protein (PLP) to prevent premature Polo localization and thus MTOC activity [9]. Here, we report that the centriolar protein Bld10, the fly ortholog of Cep135, is required to establish centrosome asymmetry in Drosophila neuroblasts through shedding of Polo from the mother centrosome. bld10 mutants fail to downregulate Polo and PCM, generating two active, improperly positioned MTOCs. Failure to shed Polo and PCM causes spindle alignment and centrosome segregation defects, resulting in neuroblasts incorrectly retaining the older mother centrosome. Since Cep135 is implicated in primary microcephaly, we speculate that perturbed centrosome asymmetry could contribute to this rare neurodevelopmental disease.

The centrosome-specific phosphorylation of Cnn by Polo/Plk1 drives Cnn scaffold assembly and centrosome maturation

Centrosomes are important cell organizers. They consist of a pair of centrioles surrounded by pericentriolar material (PCM) that expands dramatically during mitosis-a process termed centrosome maturation. How centrosomes mature remains mysterious. Here, we identify a domain in Drosophila Cnn that appears to be phosphorylated by Polo/Plk1 specifically at centrosomes during mitosis. The phosphorylation promotes the assembly of a Cnn scaffold around the centrioles that is in constant flux, with Cnn molecules recruited continuously around the centrioles as the scaffold spreads slowly outward. Mutations that block Cnn phosphorylation strongly inhibit scaffold assembly and centrosome maturation, whereas phosphomimicking mutations allow Cnn to multimerize in vitro and to spontaneously form cytoplasmic scaffolds in vivo that organize microtubules independently of centrosomes. We conclude that Polo/Plk1 initiates the phosphorylation-dependent assembly of a Cnn scaffold around centrioles that is essential for efficient centrosome maturation in flies.

The novel zinc finger protein dASCIZ regulates mitosis in Drosophila via an essential role in dynein light-chain expression

The essential zinc finger protein ASCIZ (also known as ATMIN, ZNF822) plays critical roles during lung organogenesis and B cell development in mice, where it regulates the expression of dynein light chain (DYNLL1/LC8), but its functions in other species including invertebrates are largely unknown. Here we report the identification of the Drosophila ortholog of ASCIZ (dASCIZ) and show that loss of dASCIZ function leads to pronounced mitotic delays with centrosome and spindle positioning defects during development, reminiscent of impaired dynein motor functions. Interestingly, similar mitotic and developmental defects were observed upon knockdown of the DYNLL/LC8-type dynein light chain Cutup (Ctp), and dASCIZ loss-of-function phenotypes could be suppressed by ectopic Ctp expression. Consistent with a genetic function of dASCIZ upstream of Ctp, we show that loss of dASCIZ led to reduced endogenous Ctp mRNA and protein levels and dramatically reduced Ctp-LacZ reporter gene activity in vivo, indicating that dASCIZ regulates development and mitosis as a Ctp transcription factor. We speculate that the more severe mitotic defects in the absence of ASCIZ in flies compared to mice may be due to redundancy with a second, ASCIZ-independent, Dynll2 gene in mammals in contrast to a single Ctp gene in Drosophila. Altogether, our data demonstrate that ASCIZ is an evolutionary highly conserved transcriptional regulator of dynein light-chain levels and a novel regulator of mitosis in flies.

Live imaging of neuroblast lineages within intact larval brains in Drosophila

Neuroblasts are the precursors of the Drosophila central nervous system and undergo repeated physical and molecular asymmetric cell divisions. Live imaging of neuroblast lineages within intact Drosophila larval brains has dramatically improved our current understanding of basic cellular processes such as the establishment of cell polarity, spindle orientation, and cytokinesis. The analysis of mutant phenotypes using live imaging can enlarge our understanding of asymmetric neuroblast division and self-renewal. Although much live neuroblast imaging is performed using green fluorescent protein only, the generation of improved fluorescent proteins has led to an increase in the use of two-color imaging. Here we present a simple protocol for isolating and imaging larval brain neuroblasts. We describe procedures for the dissection and mounting of brains from third-instar Drosophila larvae in explant solution and their subsequent live imaging. The method provides a close approximation to the in vivo environment and produces data with high temporal and spatial resolutions. We also discuss potential problems and pitfalls and provide examples of how this technique is used.

Epithelial junctions maintain tissue architecture by directing planar spindle orientation

During epithelial cell proliferation, planar alignment of the mitotic spindle coordinates the local process of symmetric cell cleavage with the global maintenance of polarized tissue architecture. Although the disruption of planar spindle alignment is proposed to cause epithelial to mesenchymal transition and cancer, the in vivo mechanisms regulating mitotic spindle orientation remain elusive. Here we demonstrate that the actomyosin cortex and the junction-localized neoplastic tumour suppressors Scribbled and Discs large 1 have essential roles in planar spindle alignment and thus the control of epithelial integrity in the Drosophila imaginal disc. We show that defective alignment of the mitotic spindle correlates with cell delamination and apoptotic death, and that blocking the death of misaligned cells is sufficient to drive the formation of basally localized tumour-like masses. These findings indicate a key role for junction-mediated spindle alignment in the maintenance of epithelial integrity, and also reveal a previously unknown cell-death-mediated tumour-suppressor function inherent in the polarized architecture of epithelia.

Regulation of centrosome movements by Numb and the Collapsin Response Mediator Protein during Drosophila sensory progenitor asymmetric division

Asymmetric cell division generates cell fate diversity during development and adult life. Recent findings have demonstrated that during stem cell divisions, the movement of centrosomes is asymmetric in prophase and that such asymmetry participates in mitotic spindle orientation and cell polarization. Here, we have investigated the dynamics of centrosomes during Drosophila sensory organ precursor asymmetric divisions and find that centrosome movements are asymmetric during cytokinesis. We demonstrate that centrosome movements are controlled by the cell fate determinant Numb, which does not act via its classical effectors, Sanpodo and α-Adaptin, but via the Collapsin Response Mediator Protein (CRMP). Furthermore, we find that CRMP is necessary for efficient Notch signalling and that it regulates the duration of the pericentriolar accumulation of Rab11-positive endosomes, through which the Notch ligand, Delta is recycled. Our work characterizes an additional mode of asymmetric centrosome movement during asymmetric divisions and suggests a model whereby the asymmetry in centrosome movements participates in differential Notch activation to regulate cell fate specification.

Sas-4 provides a scaffold for cytoplasmic complexes and tethers them in a centrosome

Centrosomes are conserved organelles that are essential for accurate cell division and cilium formation. A centrosome consists of a pair of centrioles surrounded by a protein network of pericentriolar material (PCM) that is essential for the centrosome's function. In this study, we show that Sas-4 provides a scaffold for cytoplasmic complexes (named S-CAP), which include CNN, Asl and D-PLP, proteins that are all found in the centrosomes at the vicinity of the centriole. When Sas-4 is absent, nascent procentrioles are unstable and lack PCM, and functional centrosomes are not generated. When Sas-4 is mutated, so that it cannot form S-CAP complexes, centrosomes are present but with dramatically reduced levels of PCM. Finally, purified S-CAP complexes or recombinant Sas-4 can bind centrosomes stripped of PCM, whereas recombinant CNN or Asl cannot. In summary, PCM assembly begins in the cytosol where Sas-4 provides a scaffold for pre-assembled cytoplasmic complexes before tethering of the complexes in a centrosome.

Genetic interaction of centrosomin and bazooka in apical domain regulation in Drosophila photoreceptor

Background

Cell polarity genes including Crumbs (Crb) and Par complexes are essential for controlling photoreceptor morphogenesis. Among the Crb and Par complexes, Bazooka (Baz, Par-3 homolog) acts as a nodal component for other cell polarity proteins. Therefore, finding other genes interacting with Baz will help us to understand the cell polarity genes' role in photoreceptor morphogenesis.

Methodology/Principal Findings

Here, we have found a genetic interaction between baz and centrosomin (cnn). Cnn is a core protein for centrosome which is a major microtubule-organizing center. We analyzed the effect of the cnn mutation on developing eyes to determine its role in photoreceptor morphogenesis. We found that Cnn is dispensable for retinal differentiation in eye imaginal discs during the larval stage. However, photoreceptors deficient in Cnn display dramatic morphogenesis defects including the mislocalization of Crumbs (Crb) and Bazooka (Baz) during mid-stage pupal eye development, suggesting that Cnn is specifically required for photoreceptor morphogenesis during pupal eye development. This role of Cnn in apical domain modulation was further supported by Cnn's gain-of-function phenotype. Cnn overexpression in photoreceptors caused the expansion of the apical Crb membrane domain, Baz and adherens junctions (AJs).

Conclusions/Significance

These results strongly suggest that the interaction of Baz and Cnn is essential for apical domain and AJ modulation during photoreceptor morphogenesis, but not for the initial photoreceptor differentiation in the Drosophila photoreceptor.

Centrioles regulate centrosome size by controlling the rate of Cnn incorporation into the PCM

BACKGROUND

centrosomes are major microtubule organizing centers in animal cells, and they comprise a pair of centrioles surrounded by an amorphous pericentriolar material (PCM). Centrosome size is tightly regulated during the cell cycle, and it has recently been shown that the two centrosomes in certain stem cells are often asymmetric in size. There is compelling evidence that centrioles influence centrosome size, but how centrosome size is set remains mysterious.

RESULTS

we show that the conserved Drosophila PCM protein Cnn exhibits an unusual dynamic behavior, because Cnn molecules only incorporate into the PCM closest to the centrioles and then spread outward through the rest of the PCM. Cnn incorporation into the PCM is driven by an interaction with the conserved centriolar proteins Asl (Cep152 in humans) and DSpd-2 (Cep192 in humans). The rate of Cnn incorporation into the PCM is tightly regulated during the cell cycle, and this rate influences the amount of Cnn in the PCM, which in turn is an important determinant of overall centrosome size. Intriguingly, daughter centrioles in syncytial embryos only start to incorporate Cnn as they disengage from their mothers; this generates a centrosome size asymmetry, with mother centrioles always initially organizing more Cnn than their daughters.

CONCLUSIONS

centrioles can control the amount of PCM they organize by regulating the rate of Cnn incorporation into the PCM. This mechanism can explain how centrosome size is regulated during the cell cycle and also allows mother and daughter centrioles to set centrosome size independently of one another.

Polarity in stem cell division: asymmetric stem cell division in tissue homeostasis

Many adult stem cells divide asymmetrically to balance self-renewal and differentiation, thereby maintaining tissue homeostasis. Asymmetric stem cell divisions depend on asymmetric cell architecture (i.e., cell polarity) within the cell and/or the cellular environment. In particular, as residents of the tissues they sustain, stem cells are inevitably placed in the context of the tissue architecture. Indeed, many stem cells are polarized within their microenvironment, or the stem cell niche, and their asymmetric division relies on their relationship with the microenvironment. Here, we review asymmetric stem cell divisions in the context of the stem cell niche with a focus on Drosophila germ line stem cells, where the nature of niche-dependent asymmetric stem cell division is well characterized.

Control of ciliogenesis by FOR20, a novel centrosome and pericentriolar satellite protein

Cilia and flagella are evolutionary conserved organelles that generate fluid movement and locomotion, and play roles in chemosensation, mechanosensation and intracellular signalling. In complex organisms, cilia are highly diversified, which allows them to perform various functions; however, they retain a 9+0 or 9+2 microtubules structure connected to a basal body. Here, we describe FOR20 (FOP-related protein of 20 kDa), a previously uncharacterized and highly conserved protein that is required for normal formation of a primary cilium. FOR20 is found in PCM1-enriched pericentriolar satellites and centrosomes. FOR20 contains a Lis1-homology domain that promotes self-interaction and is required for its satellite localization. Inhibition of FOR20 expression in RPE1 cells decreases the percentage of ciliated cells and the length of the cilium on ciliated cells. It also modifies satellite distribution, as judged by PCM1 staining, and displaces PCM1 from a detergent-insoluble to a detergent-soluble fraction. The subcellular distribution of satellites is dependent on both microtubule integrity and molecular motor activities. Our results suggest that FOR20 could be involved in regulating the interaction of PCM1 satellites with microtubules and motors. The role of FOR20 in primary cilium formation could therefore be linked to its function in regulating pericentriolar satellites. A role for FOR20 at the basal body itself is also discussed.