Human SFI1 and Centrin form a complex critical for centriole architecture and ciliogenesis

Over the course of evolution, the centrosome function has been conserved in most eukaryotes, but its core architecture has evolved differently in some clades, with the presence of centrioles in humans and a spindle pole body (SPB) in yeast. Similarly, the composition of these two core elements has diverged, with the exception of Centrin and SFI1, which form a complex in yeast to initiate SPB duplication. However, it remains unclear whether this complex exists at centrioles and whether its function has been conserved. Here, using expansion microscopy, we demonstrate that human SFI1 is a centriolar protein that associates with a pool of Centrin at the distal end of the centriole. We also find that both proteins are recruited early during procentriole assembly and that depletion of SFI1 results in the loss of the distal pool of Centrin, without altering centriole duplication. Instead, we show that SFI1/Centrin complex is essential for centriolar architecture, CEP164 distribution, and CP110 removal during ciliogenesis. Together, our work reveals a conserved SFI1/Centrin module displaying divergent functions between mammals and yeast.

Centrosome organization and functions

The centrosome, discovered near 1875, was named by Boveri when proposing the chromosomal theory of heredity. After a long eclipse, a considerable amount of molecular data has been accumulated on the centrosome and its biogenesis in the last 30 years, summarized regularly in excellent reviews. Major questions are still at stake in 2021 however, as we lack a comprehensive view of the centrosome functions. I will first try to see how progress towards a unified view of the role of centrosomes during evolution is possible, and then review recent data on only some of the many important questions raised by this organelle.

A helical inner scaffold provides a structural basis for centriole cohesion

The ninefold radial arrangement of microtubule triplets (MTTs) is the hallmark of the centriole, a conserved organelle crucial for the formation of centrosomes and cilia. Although strong cohesion between MTTs is critical to resist forces applied by ciliary beating and the mitotic spindle, how the centriole maintains its structural integrity is not known. Using cryo-electron tomography and subtomogram averaging of centrioles from four evolutionarily distant species, we found that MTTs are bound together by a helical inner scaffold covering ~70% of the centriole length that maintains MTTs cohesion under compressive forces. Ultrastructure Expansion Microscopy (U-ExM) indicated that POC5, POC1B, FAM161A, and Centrin-2 localize to the scaffold structure along the inner wall of the centriole MTTs. Moreover, we established that these four proteins interact with each other to form a complex that binds microtubules. Together, our results provide a structural and molecular basis for centriole cohesion and geometry.

A moment at the cell centre

I have been invited by the board of the French Society of Cell Biology (SBCF) to write a text around my presentation in the Symposium 'A day at the Cell Centre', held at the Curie Institute on May 17, 2019, and organized by four of my former students, namely Juliette Azimzadeh, Nathalie Delgehyr, Matthieu Piel and Manuel Théry. I have to thank them warmly for the quality of the science during this day. It was also a moving day for me indeed to listen to so many figures in the field.

[Cell polarity and the innovation of the primary cilium/centrosome organ in Metazoa]

Cell-autonomous polarity in Metazoans is inherited from ancestral unicellular organisms. We assume that permanent polarity in unicellular eukaryotes is required for cell motion and sensory reception and that the integration of these two activities corresponds to an evolutionary constrained cell function. While conserving the ancestral flagellum, Metazoans have co-opted a primary cilium/centrosome organ from it, ensuring similar functions, but in different cells, or in the same cell at different moments. We propose that the remodeling necessary to reach a new higher-level unit of selection in multi-cellular organisms, has been triggered by conflicts among individual cell polarities to reach an organismic polarity. We shall provisionally conclude that beyond critical consequences for embryo development, the conservation of cell-autonomous polarity in Metazoans has far reaching implications for the evolution of individuality.

The dual role of the centrosome in organizing the microtubule network in interphase

Here, we address the regulation of microtubule nucleation during interphase by genetically ablating one, or two, of three major mammalian γ-TuRC-binding factors namely pericentrin, CDK5Rap2, and AKAP450. Unexpectedly, we find that while all of them participate in microtubule nucleation at the Golgi apparatus, they only modestly contribute at the centrosome where CEP192 has a more predominant function. We also show that inhibiting microtubule nucleation at the Golgi does not affect centrosomal activity, whereas manipulating the number of centrosomes with centrinone modifies microtubule nucleation activity of the Golgi apparatus. In centrosome-free cells, inhibition of Golgi-based microtubule nucleation triggers pericentrin-dependent formation of cytoplasmic-nucleating structures. Further depletion of pericentrin under these conditions leads to the generation of individual microtubules in a γ-tubulin-dependent manner. In all cases, a conspicuous MT network forms. Strikingly, centrosome loss increases microtubule number independently of where they were growing from. Our results lead to an unexpected view of the interphase centrosome that would control microtubule network organization not only by nucleating microtubules, but also by modulating the activity of alternative microtubule-organizing centers.

Cell polarity: having and making sense of direction-on the evolutionary significance of the primary cilium/centrosome organ in Metazoa

Cell-autonomous polarity in Metazoans is evolutionarily conserved. I assume that permanent polarity in unicellular eukaryotes is required for cell motion and sensory reception, integration of these two activities being an evolutionarily constrained function. Metazoans are unique in making cohesive multicellular organisms through complete cell divisions. They evolved a primary cilium/centrosome (PC/C) organ, ensuring similar functions to the basal body/flagellum of unicellular eukaryotes, but in different cells, or in the same cell at different moments. The possibility that this innovation contributed to the evolution of individuality, in being instrumental in the early specification of the germ line during development, is further discussed. Then, using the example of highly regenerative organisms like planarians, which have lost PC/C organ in dividing cells, I discuss the possibility that part of the remodelling necessary to reach a new higher-level unit of selection in multi-cellular organisms has been triggered by conflicts among individual cell polarities to reach an organismic polarity. Finally, I briefly consider organisms with a sensorimotor organ like the brain that requires exceedingly elongated polarized cells for its activity. I conclude that beyond critical consequences for embryo development, the conservation of cell-autonomous polarity in Metazoans had far-reaching implications for the evolution of individuality.

Lens-free Video Microscopy for the Dynamic and Quantitative Analysis of Adherent Cell Culture

Here, we demonstrate that lens-free video microscopy enables us to simultaneously capture the kinetics of thousands of cells directly inside the incubator and that it is possible to monitor and quantify single cells along several cell cycles. We describe the full protocol used to monitor and quantify a HeLa cell culture for 2.7 days. First, cell culture acquisition is performed with a lens-free video microscope, and then the data is analyzed following a four-step process: multi-wavelength holographic reconstruction, cell-tracking, cell segmentation and cell division detection algorithms. As a result, we show that it is possible to gather a dataset featuring more than 10,000 cell cycle tracks and more than 2 x 10⁶ cell morphological measurements.

Alpha-catenin-dependent recruitment of the centrosomal protein CAP350 to adherens junctions allows epithelial cells to acquire a columnar shape

Epithelial morphogenesis involves a dramatic reorganisation of the microtubule cytoskeleton. How this complex process is controlled at the molecular level is still largely unknown. Here, we report that the centrosomal microtubule (MT)-binding protein CAP350 localises at adherens junctions in epithelial cells. By two-hybrid screening, we identified a direct interaction of CAP350 with the adhesion protein α-catenin that was further confirmed by co-immunoprecipitation experiments. Block of epithelial cadherin (E-cadherin)-mediated cell-cell adhesion or α-catenin depletion prevented CAP350 localisation at cell-cell junctions. Knocking down junction-located CAP350 inhibited the establishment of an apico-basal array of microtubules and impaired the acquisition of columnar shape in Madin-Darby canine kidney II (MDCKII) cells grown as polarised epithelia. Furthermore, MDCKII cystogenesis was also defective in junctional CAP350-depleted cells. CAP350-depleted MDCKII cysts were smaller and contained either multiple lumens or no lumen. Membrane polarity was not affected, but cortical microtubule bundles did not properly form. Our results indicate that CAP350 may act as an adaptor between adherens junctions and microtubules, thus regulating epithelial differentiation and contributing to the definition of cell architecture. We also uncover a central role of α-catenin in global cytoskeleton remodelling, in which it acts not only on actin but also on MT reorganisation during epithelial morphogenesis.

In epithelial cells, the normally centrosomal protein CAP350 binds to α-catenin at adherens junctions and helps to establish the cells' parallel apico-basal microtubule array and columnar shape.

Epithelia cover all the surfaces of and the cavities throughout the body and serve as barriers between the organism and its external environment. Epithelial differentiation requires the coordination in space and time of several mechanisms that ultimately lead to the acquisition of distinctive epithelial features, including apical-basal polarity, specialised cell-cell junctions, and columnar shape. Epithelial differentiation also induces the reorganisation of three cytoskeletal networks: actin filaments, intermediate filaments, and microtubules. In simple epithelia, cadherins and their cytoplasmic binding partners catenins play a crucial role in connecting cell-cell junctions to the actin cytoskeleton. The cadherin extracellular domain forms adhesive contacts between adjacent cells, and their cytoplasmic tail indirectly binds the actin-binding protein α-catenin, thus linking cell-cell junctions to the underlying actin cytoskeleton. We report here an additional role of α-catenin in remodelling microtubules during epithelial differentiation. In most epithelial cells, microtubules are organised as parallel bundles aligned along the apico-basal axis and as apical and basal plasma membrane-associated networks. We demonstrate that the microtubule-binding protein CAP350, which is only localised at the centrosome in most cells, is also recruited at cell–cell junctions in epithelial cells through its binding to α-catenin. In the absence of junctional CAP350, microtubules are unable to reorganise in bundles, and cells do not acquire columnar shape. Our results suggest that recruitment of centrosomal proteins to cell-cell junctions could be a general mechanism to control microtubule reorganisation in neighbour cells during epithelial differentiation.

Primary ciliogenesis requires the distal appendage component Cep123

Primary cilium formation is initiated at the distal end of the mother centriole in a highly co-ordinated manner. This requires the capping of the distal end of the mother centriole with a ciliary vesicle and the anchoring of the basal body (mother centriole) to the cell cortex, both of which are mediated by the distal appendages. Here, we show that the distal appendage protein Cep123 (Cep89/CCDC123) is required for the assembly, but not the maintenance, of a primary cilium. In the absence of Cep123 ciliary vesicle formation fails, suggesting that it functions in the early stages of primary ciliogenesis. Consistent with such a role, Cep123 interacts with the centriolar satellite proteins PCM-1, Cep290 and OFD1, all of which play a role in primary ciliogenesis. These interactions are mediated by a domain in the C-terminus of Cep123 (400-783) that overlaps the distal appendage-targeting domain (500-600). Together, the data implicate Cep123 as a new player in the primary ciliogenesis pathway and expand upon the role of the distal appendages in this process.

Cell confinement controls centrosome positioning and lumen initiation during epithelial morphogenesis

Epithelial organ morphogenesis involves sequential acquisition of apicobasal polarity by epithelial cells and development of a functional lumen. In vivo, cells perceive signals from components of the extracellular matrix (ECM), such as laminin and collagens, as well as sense physical conditions, such as matrix stiffness and cell confinement. Alteration of the mechanical properties of the ECM has been shown to promote cell migration and invasion in cancer cells, but the effects on epithelial morphogenesis have not been characterized. We analyzed the effects of cell confinement on lumen morphogenesis using a novel, micropatterned, three-dimensional (3D) Madin-Darby canine kidney cell culture method. We show that cell confinement, by controlling cell spreading, limits peripheral actin contractility and promotes centrosome positioning and lumen initiation after the first cell division. In addition, peripheral actin contractility is mediated by master kinase Par-4/LKB1 via the RhoA-Rho kinase-myosin II pathway, and inhibition of this pathway restores lumen initiation in minimally confined cells. We conclude that cell confinement controls nuclear-centrosomal orientation and lumen initiation during 3D epithelial morphogenesis.

The centrosome in cells and organisms

The centrosome acts as the main microtubule-nucleating organelle in animal cells and plays a critical role in mitotic spindle orientation and in genome stability. Yet, despite its central role in cell biology, the centrosome is not present in all multicellular organisms or in all cells of a given organism. The main outcome of centrosome reproduction is the transmission of polarity to daughter cells and, in most animal species, the sperm-donated centrosome defines embryo polarity. Here I will discuss the role of the centrosome in cell polarity, resulting from its ability to position the nucleus at the cell center, and discuss how centrosome innovation might have been critical during metazoan evolution.

Centrioles resist forces applied on centrosomes during G2/M transition

BACKGROUND INFORMATION

Centrosome movements at the onset of mitosis result from a balance between the pulling and pushing forces mediated by microtubules. The structural stability of the centrosome core structure, the centriole pair, is correlated with a heavy polyglutamylation of centriole tubulin.

RESULTS

Using HeLa cells stably expressing centrin-green fluorescent protein as a centriole marker, we monitored the effect of microinjecting an anti-(polyglutamylated tubulin) monoclonal antibody, GT335, in G1/S or G2 cells. In contrast with the slow effect of the monoclonal antibody GT335 during interphase, a dramatic and rapid centrosome fragmentation occurred in cells microinjected in G2 that was both Eg5- and dynein-dependent. Inhibition of either one of these two motors significantly decreased the scattering of centrosome fragments, and inhibition of centrosome segregation by impairing microtubule dynamics abolished centrosome fragmentation.

CONCLUSIONS

Our results demonstrate that the compact structure of the mitotic centrosome is capable of absorbing most of the pulling and pushing forces during G2/M transition and suggest that centrosomes could act as mechanosensors integrating tensions during cell division.

Assessing the localization of centrosomal proteins by PALM/STORM nanoscopy

The structure of the centrosome was resolved by EM many years ago to reveal a pair of centrioles embedded in a dense network of proteins. More recently, the molecular composition of the centrosome was catalogued by mass spectroscopy and many novel components were identified. Determining precisely where a novel component localizes to within the centrosome remains a challenge, and until now it has required the use of immuno-EM. This technique is both time-consuming and unreliable, as it often fails due to problems with antigen accessibility. We have investigated the use of two nanoscopic techniques, photoactivated localization microscopy (PALM) and stochastic optical reconstruction microscopy (STORM), as alternative techniques for localizing centrosomal proteins. The localization of a known centrosomal component, the distal appendage protein Cep164 was investigated by direct STORM (dSTORM) and resolved with a high spatial resolution. We further validated the use of nanoscopic PALM imaging by showing that the previously uncharacterized centrosomal protein CCDC123 (Cep123) localizes to the distal appendages, forming ring-like structures with a diameter of 500 nm. Our results demonstrate that both PALM and STORM imaging have great potential as alternatives to immuno-EM.

External forces control mitotic spindle positioning

The response of cells to forces is essential for tissue morphogenesis and homeostasis. This response has been extensively investigated in interphase cells, but it remains unclear how forces affect dividing cells. We used a combination of micro-manipulation tools on human dividing cells to address the role of physical parameters of the micro-environment in controlling the cell division axis, a key element of tissue morphogenesis. We found that forces applied on the cell body direct spindle orientation during mitosis. We further show that external constraints induce a polarization of dynamic subcortical actin structures that correlate with spindle movements. We propose that cells divide according to cues provided by their mechanical micro-environment, aligning daughter cells with the external force field.

Disconnecting the Golgi ribbon from the centrosome prevents directional cell migration and ciliogenesis

AKAP450 is a critical determinant of Golgi ribbon integrity, positioning, and function.

Mammalian cells exhibit a frequent pericentrosomal Golgi ribbon organization. In this paper, we show that two AKAP450 N-terminal fragments, both containing the Golgi-binding GM130-interacting domain of AKAP450, dissociated endogenous AKAP450 from the Golgi and inhibited microtubule (MT) nucleation at the Golgi without interfering with centrosomal activity. These two fragments had, however, strikingly different effects on both Golgi apparatus (GA) integrity and positioning, whereas the short fragment induced GA circularization and ribbon fragmentation, the large construct that encompasses an additional p150glued/MT-binding domain induced separation of the Golgi ribbon from the centrosome. These distinct phenotypes arose by specific interference of each fragment with either Golgi-dependent or centrosome-dependent stages of Golgi assembly. We could thus demonstrate that breaking the polarity axis by perturbing GA positioning has a more dramatic effect on directional cell migration than disrupting the Golgi ribbon. Both features, however, were required for ciliogenesis. We thus identified AKAP450 as a key determinant of pericentrosomal Golgi ribbon integrity, positioning, and function in mammalian cells.

[The evolutionary fates of the flagellum from the unicellular ancestral eukaryote]

The microtubule-based flagellum, or basal body/axoneme, is ancestral: it is a so-called "derived character" defining the extant eukaryote branch, present at the apparition of the early eukaryotic cell. It is a genuine cell compartment. Its structural conservation throughout evolution is remarkable, as well as the molecular conservation of the intraflagellar transport genes. From the start, it links cell sensation and cell locomotion, two properties which require integrated wiring at the cell level. The cell division machinery has been also tightly coupled with this organelle throughout evolution, possibly to ensure the continuity of this cellular polarized organization through division. The association of three basic cell modules, locomotion, sensation and division, all expressing a facet of cell polarity, within the same apparatus, could have given optimal survival value to primitive eukaryotic cells and could explain the evolutionary conservation and success of the basal body/axoneme. In vertebrates, primary cilia are critical sensory organelles, including during early mammalian development. Today the eukaryotic flagellum is not only an important issue for basic cell biology and developmental biology but also for human pathology. Many apparently unrelated severe developmental pathologies turned out to be all connected to the assembly or the function of the primary or beating cilia. The current efforts on human ciliopathies benefit directly from long lasting basic research on the flagellar assembly in experimental models like Chlamydomonas reinhardtii.

Cell shape and contractility regulate ciliogenesis in cell cycle-arrested cells

Adhesive micropatterns show the effect of spatial confinement and actin network architecture on basal body positioning and primary cilium formation.

In most lineages, cell cycle exit is correlated with the growth of a primary cilium. We analyzed cell cycle exit and ciliogenesis in human retinal cells and found that, contrary to the classical view, not all cells exiting the cell division cycle generate a primary cilium. Using adhesive micropatterns to control individual cell spreading, we demonstrate that cell spatial confinement is a major regulator of ciliogenesis. When spatially confined, cells assemble a contractile actin network along their ventral surface and a protrusive network along their dorsal surface. The nucleus–centrosome axis in confined cells is oriented toward the dorsal surface where the primary cilium is formed. In contrast, highly spread cells assemble mostly contractile actin bundles. The nucleus–centrosome axis of spread cells is oriented toward the ventral surface, where contractility prevented primary cilium growth. These results indicate that cell geometrical confinement affects cell polarity via the modulation of actin network architecture and thereby regulates basal body positioning and primary cilium growth.

GMAP210 and IFT88 are present in the spermatid golgi apparatus and participate in the development of the acrosome-acroplaxome complex, head-tail coupling apparatus and tail

We describe the localization of the golgin GMAP210 and the intraflagellar protein IFT88 in the Golgi of spermatids and the participation of these two proteins in the development of the acrosome-acroplaxome complex, the head-tail coupling apparatus (HTCA) and the spermatid tail. Immunocytochemical experiments show that GMAP210 predominates in the cis-Golgi, whereas IFT88 prevails in the trans-Golgi network. Both proteins colocalize in proacrosomal vesicles, along acrosome membranes, the HTCA and the developing tail. IFT88 persists in the acrosome-acroplaxome region of the sperm head, whereas GMAP210 is no longer seen there. Spermatids of the Ift88 mouse mutant display abnormal head shaping and are tail-less. GMAP210 is visualized in the Ift88 mutant during acrosome-acroplaxome biogenesis. However, GMAP210-stained vesicles, mitochondria and outer dense fiber material build up in the manchette region and fail to reach the abortive tail stump in the mutant. In vitro disruption of the spermatid Golgi and microtubules with Brefeldin-A and nocodazole blocks the progression of GMAP210- and IFT88-stained proacrosomal vesicles to the acrosome-acroplaxome complex but F-actin distribution in the acroplaxome is not affected. We provide the first evidence that IFT88 is present in the Golgi of spermatids, that the microtubule-associated golgin GMAP210 and IFT88 participate in acrosome, HTCA, and tail biogenesis, and that defective intramanchette transport of cargos disrupts spermatid tail development.

Autophosphorylation of polo-like kinase 4 and its role in centriole duplication

PLK4 is a key regulator of centriole duplication. Here, we show that PLK4 is active beyond the initiation of centriole duplication with the abundance of active kinase increasing to a peak in mitosis. Importantly, we show that differences in PLK4 abundance exist between mother and daughter centrioles and that active PLK4 is restricted to the centrosome.

Centrosome duplication occurs once every cell cycle in a strictly controlled manner. Polo-like kinase 4 (PLK4) is a key regulator of this process whose kinase activity is essential for centriole duplication. Here, we show that PLK4 autophosphorylation of serine S305 is a consequence of kinase activation and enables the active fraction to be identified in the cell. Active PLK4 is detectable on the replicating mother centriole in G1/S, with the proportion of active kinase increasing through interphase to reach a maximum in mitosis. Activation of PLK4 at the replicating daughter centriole is delayed until G2, but a level equivalent to the replicating mother centriole is achieved in M phase. Active PLK4 is regulated by the proteasome, because either proteasome inhibition or mutation of the degron motif of PLK4 results in the accumulation of S305-phosphorylated PLK4. Autophosphorylation probably plays a role in the process of centriole duplication, because mimicking S305 phosphorylation enhances the ability of overexpressed PLK4 to induce centriole amplification. Importantly, we show that S305-phosphorylated PLK4 is specifically sequestered at the centrosome contrary to the nonphosphorylated form. These data suggest that PLK4 activity is restricted to the centrosome to prevent aberrant centriole assembly and sustained kinase activity is required for centriole duplication.

Golgi localisation of GMAP210 requires two distinct cis-membrane binding mechanisms

BACKGROUND

The Golgi apparatus in mammals appears as a ribbon made up of interconnected stacks of flattened cisternae that is positioned close to the centrosome in a microtubule-dependent manner. How this organisation is achieved and retained is not well understood. GMAP210 is a long coiled-coil cis-Golgi associated protein that plays a role in maintaining Golgi ribbon integrity and position and contributes to the formation of the primary cilium. An amphipathic alpha-helix able to bind liposomes in vitro has been recently identified at the first 38 amino acids of the protein (amphipathic lipid-packing sensor motif), and an ARF1-binding domain (Grip-related Arf-binding domain) was found at the C-terminus. To which type of membranes these two GMAP210 regions bind in vivo and how this contributes to GMAP210 localisation and function remains to be investigated.

RESULTS

By using truncated as well as chimeric mutants and videomicroscopy we found that both the N-terminus and the C-terminus of GMAP210 are targeted to the cis-Golgi in vivo. The ALPS motif was identified as the N-terminal binding motif and appeared concentrated in the periphery of Golgi elements and between Golgi stacks. On the contrary, the C-terminal domain appeared uniformly distributed in the cis-cisternae of the Golgi apparatus. Strikingly, the two ends of the protein also behave differently in response to the drug Brefeldin A. The N-terminal domain redistributed to the endoplasmic reticulum (ER) exit sites, as does the full-length protein, whereas the C-terminal domain rapidly dissociated from the Golgi apparatus to the cytosol. Mutants comprising the full-length protein but lacking one of the terminal motifs also associated with the cis-Golgi with distribution patterns similar to those of the corresponding terminal end whereas a mutant consisting in fused N- and C-terminal ends exhibits identical localisation as the endogenous protein.

CONCLUSION

We conclude that the Golgi localisation of GMAP210 is the result of the combined action of the two N- and C-terminal domains that recognise different sub-regions of the cis-GA. Based on present and previous data, we propose a model in which GMAP210 would participate in homotypic fusion of cis-cisternae by anchoring the surface of cisternae via its C-terminus and projecting its distal N-terminus to bind the rims or to stabilise tubular structures connecting neighbouring cis-cisternae.

Simple and rapid process for single cell micro-patterning

We present a simple and environmentally friendly process for cell patterning on glass covered with an ultrathin layer of poly-l-lysine-grafted-polyethylene glycol (PLL-g-PEG) by exposure to deep UV light. The patterned substrates are stable for months in the lab atmosphere before incubation with proteins. Incubation with proteins resulted in well defined patterns, with high feature resolution. RPE-1 cells seeded on fibronectin/fibrinogen-Alexa 488 patterns were constrained for days on the deep UV exposed regions. Finally, large glass plates were patterned with high homogeneity enabling the assembly of micro-patterned microplates in 96-well format.

hPOC5 is a centrin-binding protein required for assembly of full-length centrioles

Centrin has been shown to be involved in centrosome biogenesis in a variety of eukaryotes. In this study, we characterize hPOC5, a conserved centrin-binding protein that contains Sfi1p-like repeats. hPOC5 is localized, like centrin, in the distal portion of human centrioles. hPOC5 recruitment to procentrioles occurs during G2/M, a process that continues up to the full maturation of the centriole during the next cell cycle and is correlated with hyperphosphorylation of the protein. In the absence of hPOC5, RPE1 cells arrest in G1 phase, whereas HeLa cells show an extended S phase followed by cell death. We show that hPOC5 is not required for the initiation of procentriole assembly but is essential for building the distal half of centrioles. Interestingly, the hPOC5 family reveals an evolutionary divergence between vertebrates and organisms like Drosophila melanogaster or Caenorhabditis elegans, in which the loss of hPOC5 may correlate with the conspicuous differences in centriolar structure.

Microtubule nucleation at the cis-side of the Golgi apparatus requires AKAP450 and GM130

We report that microtubule (MT) nucleation at the Golgi apparatus requires AKAP450, a centrosomal gamma-TuRC-interacting protein that also forms a distinct network associated with the Golgi. Depletion of AKAP450 abolished MT nucleation at the Golgi, whereas depletion of the cis-Golgi protein GM130 led to the disorganisation of AKAP450 network and impairment of MT nucleation. Brefeldin-A treatment induced relocalisation of AKAP450 to ER exit sites and concomitant redistribution of MT nucleation capacity to the ER. AKAP450 specifically binds the cis-side of the Golgi in an MT-independent, GM130-dependent manner. Short AKAP450-dependent growing MTs are covered by CLASP2. Like for centrosome, dynein/dynactin complexes are necessary to anchor MTs growing from the Golgi. We further show that Golgi-associated AKAP450 has a role in cell migration rather than in cell polarisation of the centrosome-Golgi apparatus. We propose that the recruitment of AKAP450 on the Golgi membranes through GM130 allows centrosome-associated nucleating activity to extend to the Golgi, to control the assembly of subsets of MTs ensuring specific functions within the Golgi or for transporting specific cargos to the cell periphery.

Physical mechanisms redirecting cell polarity and cell shape in fission yeast

The cylindrical rod shape of the fission yeast Schizosaccharomyces pombe is organized and maintained by interactions between the microtubule, cell membrane, and actin cytoskeleton [1]. Mutations affecting any components in this pathway lead to bent, branched, or round cells [2]. In this context, the cytoskeleton controls cell polarity and thus dictates cell shape. Here, we use soft-lithography techniques to construct microfluidic channels to control cell shape. We show that when wild-type rod-shaped cells are physically forced to grow in a bent fashion, they will reorganize their cytoskeleton and redirect cell polarity to make new ectopic cell tips. Moreover, when bent or round mutant cells are physically forced to conform to the wild-type rod-shape, they will reverse their mutational phenotypes by reorganizing their cytoskeleton to maintain proper wild-type-like localization of microtubules, cell-membrane proteins, and actin. Our study provides direct evidence that the cytoskeleton controls cell polarity and cell shape and demonstrates that cell shape also controls the organization of the cytoskeleton in a feedback loop. We present a model of the feedback loop to explain how fission yeast maintain a rod shape and how perturbation of specific parameters of the loop can lead to different cell shapes.

Organelle positioning and cell polarity

In spite of conspicuous differences in their polarized architecture, swimming unicellular eukaryotes and migrating cells from metazoa display a conserved hierarchical interlocking of the main cellular compartments, in which the microtubule network has a dominant role. A microtubule array can organize the distribution of endomembranes owing to a cell-wide and polarized extension around a unique nucleus-associated structure. The nucleus-associated structure in animal cells contains a highly conserved organelle, the centriole or basal body. This organelle has a defined polarity that can be transmitted to the cell. Its conservative mode of duplication seems to be a core mechanism for the transmission of polarities through cell division.

Origin and evolution of the centrosome

In this brief account we specifically address the question of how the plasma membrane-associated basal body/axoneme of the unicellular ancestor of eukaryotes has evolved into the centrosome organelle through the several attempts to multicellularity. We propose that the connection between the flagellar apparatus and the nucleus has been a critical feature for leading to the centriole-based centrosome of metazoa, the Spindle Pole Body of fungi, or to the absence of any centrosome in seed plants. We further suggest that the evolution of this connection could be reflected in the evolution of the centrin proteins. We then review evidence showing that the evolution of the centrosome-based tubulin network has been correlated with the evolution of the cortical actin-based cleavage apparatus. Finally we argue that this coevolution had a major impact on the cell individuation process and on the evolution of multicellular organisms. We conclude that only the metazoan lineage evolved multicellularity without loosing the ancestral association of three basic cellular functions of the basal body/axoneme or the derived centrosome organelle, namely sensation, motion and division.

Get round and stiff for mitosis

Cell rounding is a common feature of cell division. The spherical shape that cells adopt during mitosis is apparently neither a simple detachment nor a global softening or stiffening that allows cells to adopt what seems to be a mechanical equilibrium. It is a highly complex mechanical transformation by which membrane folding and peripheral signals focusing can match spindle size in order to ensure a proper cell division. Recent new insight into the mechanism involved will prompt the scientific community to focus on the regulation of the physical links that exist between the lipid bilayer membrane and the underlying actin cytoskeleton since it now appears that these will strongly influence some crucial cellular events such as the spatial organization of cell division.

Centrosomal CAP350 protein stabilises microtubules associated with the Golgi complex

A comprehensive model of how the centrosome organises the microtubule network in animal cells has not yet been elucidated. Here we show that the centrosomal large CAP-Gly protein CAP350 is not only present at the centrosome, but is also present as numerous dots in the pericentrosomal area. Using in vitro and in vivo expression of partial constructs, we demonstrated that CAP350 binds microtubules through an N-terminal basic region rather than through its CAP-Gly domain. CAP-Gly-containing domains of CAP350 are targeted not only to the centrosome but also to a Golgi-like network. Interestingly, full-length GFP-tagged CAP350 bound preferentially to microtubules in the pericentrosomal area. These results indicate that the large CAP350 protein has a dual binding ability. Overexpression of CAP350 promoted an increase in the stability of the whole microtubule network, as judged by a significant decrease in the number of EB1 comets and by an enhanced microtubule resistance to Nocodazole treatment. In support of this, CAP350 depletion decreased microtubule stability. Moreover, both depletion and overexpression of CAP350 induced specific fragmentation of the Golgi complex while maintaining a juxtanuclear localisation. We propose that CAP350 specifically stabilises Golgi-associated microtubules and in this way participates in the maintenance of a continuous pericentrosomal Golgi ribbon.

Comparative study and improvement of current cell micro-patterning techniques

The original micropatterning technique on gold, although very efficient, is not accessible to most biology labs and is not compatible with their techniques for image acquisition. Other solutions have been developed on silanized glass coverslips. These methods are still hardly accessible to biology labs and do not provide sufficient reproducibility to become incorporated in routine biological protocols. Here, we analyzed cell behavior on micro-patterns produced by various alternative techniques. Distinct cell types displayed different behavior on micropatterns, while some were easily constrained by the patterns others escaped or ripped off the patterned adhesion molecules. We report methods to overcome some of these limitations on glass coverslips and on plastic dishes which are compatible with our experimental biological applications. Finally, we present a new method based on UV crosslinking of adhesion proteins with benzophenone to easily and rapidly produce highly reproducible micropatterns without the use of a microfabricated elastomeric stamp.

Experimental and theoretical study of mitotic spindle orientation

The architecture and adhesiveness of a cell microenvironment is a critical factor for the regulation of spindle orientation in vivo. Using a combination of theory and experiments, we have investigated spindle orientation in HeLa (human) cells. Here we show that spindle orientation can be understood as the result of the action of cortical force generators, which interact with spindle microtubules and are activated by cortical cues. We develop a simple physical description of this spindle mechanics, which allows us to calculate angular profiles of the torque acting on the spindle, as well as the angular distribution of spindle orientations. Our model accounts for the preferred spindle orientation and the shape of the full angular distribution of spindle orientations observed in a large variety of different cellular microenvironment geometries. It also correctly describes asymmetric spindle orientations, which are observed for certain distributions of cortical cues. We conclude that, on the basis of a few simple assumptions, we can provide a quantitative description of the spindle orientation of adherent cells.

The universal dynamics of cell spreading

Cell adhesion and motility depend strongly on the interactions between cells and extracellular matrix (ECM) substrates. When plated onto artificial adhesive surfaces, cells first flatten and deform extensively as they spread. At the molecular level, the interaction of membrane-based integrins with the ECM has been shown to initiate a complex cascade of signaling events [1], which subsequently triggers cellular morphological changes and results in the generation of contractile forces [2]. Here, we focus on the early stages of cell spreading and probe their dynamics by quantitative visualization and biochemical manipulation with a variety of cell types and adhesive surfaces, adhesion receptors, and cytoskeleton-altering drugs. We find that the dynamics of adhesion follows a universal power-law behavior. This is in sharp contrast with the common belief that spreading is regulated by either the diffusion of adhesion receptors toward the growing adhesive patch [3-5] or by actin polymerization [6-8]. To explain this, we propose a simple quantitative and predictive theory that models cells as viscous adhesive cortical shells enclosing a less viscous interior. Thus, although cell spreading is driven by well-identified biomolecular interactions, it is dynamically limited by its mesoscopic structure and material properties.

Anisotropy of cell adhesive microenvironment governs cell internal organization and orientation of polarity

Control of the establishment of cell polarity is an essential function in tissue morphogenesis and renewal that depends on spatial cues provided by the extracellular environment. The molecular role of cell-cell or cell-extracellular matrix (ECM) contacts on the establishment of cell polarity has been well characterized. It has been hypothesized that the geometry of the cell adhesive microenvironment was directing cell surface polarization and internal organization. To define how the extracellular environment affects cell polarity, we analyzed the organization of individual cells plated on defined micropatterned substrates imposing cells to spread on various combinations of adhesive and nonadhesive areas. The reproducible normalization effect on overall cell compartmentalization enabled quantification of the spatial organization of the actin network and associated proteins, the spatial distribution of microtubules, and the positioning of nucleus, centrosome, and Golgi apparatus. By using specific micropatterns and statistical analysis of cell compartment positions, we demonstrated that ECM geometry determines the orientation of cell polarity axes. The nucleus-centrosome orientations were reproducibly directed toward cell adhesive edges. The anisotropy of the cell cortex in response to the adhesive conditions did not affect the centrosome positioning at the cell centroid. Based on the quantification of microtubule plus end distribution we propose a working model that accounts for that observation. We conclude that, in addition to molecular composition and mechanical properties, ECM geometry plays a key role in developmental processes.

Cell shape and cell division

The correlation between cell shape elongation and the orientation of the division axis described by early cell biologists is still used as a paradigm in developmental studies. However, analysis of early embryo development and tissue morphogenesis has highlighted the role of the spatial distribution of cortical cues able to guide spindle orientation. In vitro studies of cell division have revealed similar mechanisms. Recent data support the possibility that the orientation of cell division in mammalian cells is dominated by cell adhesion and the associated traction forces developed in interphase. Cell shape is a manifestation of these adhesive and tensional patterns. These patterns control the spatial distribution of cortical signals and thereby guide spindle orientation and daughter cell positioning. From these data, cell division appears to be a continuous transformation ensuring the maintenance of tissue mechanical integrity.

Tubulin tyrosination is a major factor affecting the recruitment of CAP-Gly proteins at microtubule plus ends

Tubulin-tyrosine ligase (TTL), the enzyme that catalyzes the addition of a C-terminal tyrosine residue to α-tubulin in the tubulin tyrosination cycle, is involved in tumor progression and has a vital role in neuronal organization. We show that in mammalian fibroblasts, cytoplasmic linker protein (CLIP) 170 and other microtubule plus-end tracking proteins comprising a cytoskeleton-associated protein glycine-rich (CAP-Gly) microtubule binding domain such as CLIP-115 and p150 Glued, localize to the ends of tyrosinated microtubules but not to the ends of detyrosinated microtubules. In vitro, the head domains of CLIP-170 and of p150 Glued bind more efficiently to tyrosinated microtubules than to detyrosinated polymers. In TTL-null fibroblasts, tubulin detyrosination and CAP-Gly protein mislocalization correlate with defects in both spindle positioning during mitosis and cell morphology during interphase. These results indicate that tubulin tyrosination regulates microtubule interactions with CAP-Gly microtubule plus-end tracking proteins and provide explanations for the involvement of TTL in tumor progression and in neuronal organization.

Cell distribution of stress fibres in response to the geometry of the adhesive environment

Cells display a large variety of shapes when plated in classical culture conditions despite their belonging to a common cell type. These shapes are transitory, since cells permanently disassemble and reassemble their cytoskeleton while moving. Adhesive micropatterns are commonly used to confine cell shape within a given geometry. In addition the micropattern can be designed so as to impose cells to spread upon adhesive and nonadhesive areas. Modulation of the pattern geometry allows the analysis of the mechanisms governing the determination of cell shape in response to external adhesive conditions. In this study, we show that the acquisition of cell shape follows two stages where initially the cell forms contact with the micropattern. Here, the most distal contacts made by the cell with the micropattern define the apices of the cell shape. Then secondly, the cell borders that link two apices move so as to minimise the distance between the two apices. In these cell borders, the absence of an underlying adhesive substrate is overcome by stress fibres forming between the apices, which in turn are marked by an accumulation of focal adhesions. By inhibiting myosin function, cell borders on nonadhesive zones become more concave, suggesting that the stress fibres work against the membrane tension in the cell border. Moreover, this suggested that traction forces are unevenly distributed in stationary, nonmigrating, cells. By comparing the stress fibres in cells with one, two, or three nonadherent cell borders it was reasoned that stress fibre strength is inversely proportional to number. We conclude that cells of a given area can generate the same total sum of tractional forces but that these tractional forces are differently spaced depending on the spatial distribution of its adherence contacts.

Nucleokinesis in tangentially migrating neurons comprises two alternating phases: forward migration of the Golgi/centrosome associated with centrosome splitting and myosin contraction at the rear

During rodent cortex development, cells born in the medial ganglionic eminence (MGE) of the basal telencephalon reach the embryonic cortex by tangential migration and differentiate as interneurons. Migrating MGE cells exhibit a saltatory progression of the nucleus and continuously extend and retract branches in their neuritic arbor. We have analyzed the migration cycle of these neurons using in vitro models. We show that the nucleokinesis in MGE cells comprises two phases. First, cytoplasmic organelles migrate forward, and second, the nucleus translocates toward these organelles. During the first phase, a large swelling that contains the centrosome and the Golgi apparatus separates from the perinuclear compartment and moves rostrally into the leading neurite, up to 30 mum from the waiting nucleus. This long-distance migration is associated with a splitting of the centrioles that line up along a linear Golgi apparatus. It is followed by the second, dynamic phase of nuclear translocation toward the displaced centrosome and Golgi apparatus. The forward movement of the nucleus is blocked by blebbistatin, a specific inhibitor of nonmuscle myosin II. Because myosin II accumulates at the rear of migrating MGE cells, actomyosin contraction likely plays a prominent role to drive forward translocations of the nucleus toward the centrosome. During this phase of nuclear translocation, the leading growth cone either stops migrating or divides, showing a tight correlation between leading edge movements and nuclear movements.

A role for the Rab6A' GTPase in the inactivation of the Mad2-spindle checkpoint

The two isoforms of the Rab6 GTPase, Rab6A and Rab6A', regulate a retrograde transport route connecting early endosomes and the endoplasmic reticulum via the Golgi complex in interphasic cells. Here we report that when Rab6A' function is altered cells are unable to progress normally through mitosis. Such cells are blocked in metaphase, despite displaying a normal Golgi fragmentation and with the Mad2-spindle checkpoint activated. Furthermore, the Rab6 effector p150(Glued), a subunit of the dynein/dynactin complex, remains associated with some kinetochores. A similar phenotype was observed when GAPCenA, a GTPase-activating protein of Rab6, was depleted from cells. Our results suggest that Rab6A' likely regulates the dynamics of the dynein/dynactin complex at the kinetochores and consequently the inactivation of the Mad2-spindle checkpoint. Rab6A', through its interaction with p150(Glued) and GAPCenA, may thus participate in a pathway involved in the metaphase/anaphase transition.

Dynamic modes of the cortical actomyosin gel during cell locomotion and division

Tight regulation of the contractility of the actomyosin cortex is essential for proper cell locomotion and division. Enhanced contractility leads, for example, to aberrations in the positioning of the mitotic spindle or to anomalous migration modes that allow tumor cells to escape anti-dissemination treatments. Spherical membrane protrusions called blebs occasionally appear during cell migration, cell division or apoptosis. We have shown that the cortex ruptures at sites where actomyosin cortical contractility is increased, leading to the formation of blebs. Here, we propose that bleb formation, which releases cortical tension, can be used as a reporter of cortical contractility. We go on to analyze the implications of spontaneous cortical contractile behaviors on cell locomotion and division and we particularly emphasize that variations in actomyosin contractility can account for a variety of migration modes.

The centrosome-nucleus complex and microtubule organization in the Drosophila oocyte

Molecular motors transport the axis-determining mRNAs oskar, bicoid and gurken along microtubules (MTs) in the Drosophila oocyte. However, it remains unclear how the underlying MT network is organized and how this transport takes place. We have identified a centriole-containing centrosome close to the oocyte nucleus. Remarkably, the centrosomal components, gamma-tubulin and Drosophila pericentrin-like protein also strongly accumulate at the periphery of this nucleus. MT polymerization after cold-induced disassembly in wild type and in gurken mutants suggests that in the oocyte the centrosome-nucleus complex is an active center of MT polymerization. We further report that the MT network comprises two perpendicular MT subsets that undergo dynamic rearrangements during oogenesis. This MT reorganization parallels the successive steps in localization of gurken and oskar mRNAs. We propose that in addition to a highly polarized microtubule scaffold specified by the cortex oocyte, the repositioning of the nucleus and its tightly associated centrosome could control MT reorganization and, hence, oocyte polarization.

The extracellular matrix guides the orientation of the cell division axis

The cell division axis determines the future positions of daughter cells and is therefore critical for cell fate. The positioning of the division axis has been mostly studied in systems such as embryos or yeasts, in which cell shape is well defined. In these cases, cell shape anisotropy and cell polarity affect spindle orientation. It remains unclear whether cell geometry or cortical cues are determinants for spindle orientation in mammalian cultured cells. The cell environment is composed of an extracellular matrix (ECM), which is connected to the intracellular actin cytoskeleton via transmembrane proteins. We used micro-contact printing to control the spatial distribution of the ECM on the substrate and demonstrated that it has a role in determining the orientation of the division axis of HeLa cells. On the basis of our analysis of the average distributions of actin-binding proteins in interphase and mitosis, we propose that the ECM controls the location of actin dynamics at the membrane, and thus the segregation of cortical components in interphase. This segregation is further maintained on the cortex of mitotic cells and used for spindle orientation.

Cortical actomyosin breakage triggers shape oscillations in cells and cell fragments

Cell shape and movements rely on complex biochemical pathways that regulate actin, microtubules, and substrate adhesions. Some of these pathways act through altering the cortex contractility. Here we examined cellular systems where contractility is enhanced by disassembly of the microtubules. We found that adherent cells, when detached from their substrate, developed a membrane bulge devoid of detectable actin and myosin. A constriction ring at the base of the bulge oscillated from one side of the cell to the other. The movement was accompanied by sequential redistribution of actin and myosin to the membrane. We observed this oscillatory behavior also in cell fragments of various sizes, providing a simplified, nucleus-free system for biophysical studies. Our observations suggest a mechanism based on active gel dynamics and inspired by symmetry breaking of actin gels growing around beads. The proposed mechanism for breakage of the actomyosin cortex may be used for cell polarization.