Centrosome centering and decentering by microtubule network rearrangement

Reduction of cortical pulling at mitotic entry facilitates aster centration

Equal cell division relies upon astral microtubule-based centering mechanisms, yet how the interplay between mitotic entry, cortical force generation and long astral microtubules leads to symmetric cell division is not resolved. We report that a cortically located sperm aster displaying long astral microtubules that penetrate the whole zygote does not undergo centration until mitotic entry. At mitotic entry, we find that microtubule-based cortical pulling is lost. Quantitative measurements of cortical pulling and cytoplasmic pulling together with physical simulations suggested that a wavelike loss of cortical pulling at mitotic entry leads to aster centration based on cytoplasmic pulling. Cortical actin is lost from the cortex at mitotic entry coincident with a fall in cortical tension from ∼300pN/µm to ∼100pN/µm. Following the loss of cortical force generators at mitotic entry, long microtubule-based cytoplasmic pulling is sufficient to displace the aster towards the cell center. These data reveal how mitotic aster centration is coordinated with mitotic entry in chordate zygotes.

Positioning centrioles and centrosomes

Centrosomes are the primary microtubule organizer in eukaryotic cells. In addition to shaping the intracellular microtubule network and the mitotic spindle, centrosomes are responsible for positioning cilia and flagella. To fulfill these diverse functions, centrosomes must be properly located within cells, which requires that they undergo intracellular transport. Importantly, centrosome mispositioning has been linked to ciliopathies, cancer, and infertility. The mechanisms by which centrosomes migrate are diverse and context dependent. In many cells, centrosomes move via indirect motor transport, whereby centrosomal microtubules engage anchored motor proteins that exert forces on those microtubules, resulting in centrosome movement. However, in some cases, centrosomes move via direct motor transport, whereby the centrosome or centriole functions as cargo that directly binds molecular motors which then walk on stationary microtubules. In this review, we summarize the mechanisms of centrosome motility and the consequences of centrosome mispositioning and identify key questions that remain to be addressed.

Building on-chip cytoskeletal circuits via branched microtubule networks

Controllable platforms to engineer robust cytoskeletal scaffolds have the potential to create novel on-chip nanotechnologies. Inspired by axons, we combined the branching microtubule (MT) nucleation pathway with microfabrication to develop "cytoskeletal circuits." This active matter platform allows control over the adaptive self-organization of uniformly polarized MT arrays via geometric features of microstructures designed within a microfluidic confinement. We build and characterize basic elements, including turns and divisions, as well as complex regulatory elements, such as biased division and MT diodes, to construct various MT architectures on a chip. Our platform could be used in diverse applications, ranging from efficient on-chip molecular transport to mechanical nano-actuators. Further, cytoskeletal circuits can serve as a tool to study how the physical environment contributes to MT architecture in living cells.

Autoinhibitory mechanism controls binding of centrosomin motif 1 to γ-tubulin ring complex

The γ-tubulin ring complex (γTuRC) is the principal nucleator of cellular microtubules, and the microtubule-nucleating activity of the complex is stimulated by binding to the γTuRC-mediated nucleation activator (γTuNA) motif. The γTuNA is part of the centrosomin motif 1 (CM1), which is widely found in γTuRC stimulators, including CDK5RAP2. Here, we show that a conserved segment within CM1 binds to the γTuNA and blocks its association with γTuRCs; therefore, we refer to this segment as the γTuNA inhibitor (γTuNA-In). Mutational disruption of the interaction between the γTuNA and the γTuNA-In results in a loss of autoinhibition, which consequently augments microtubule nucleation on centrosomes and the Golgi complex, the two major microtubule-organizing centers. This also causes centrosome repositioning, leads to defects in Golgi assembly and organization, and affects cell polarization. Remarkably, phosphorylation of the γTuNA-In, probably by Nek2, counteracts the autoinhibition by disrupting the γTuNA‒γTuNA-In interaction. Together, our data reveal an on-site mechanism for controlling γTuNA function.

Mechanistic model for nuclear migration in hyphae during mitosis

Saccharomyces cerevisiae and Candida albicans, the two well-known human pathogens, can be found in all three morphologies, i.e., yeast, pseudohyphae, and true hyphae. The cylindrical daughter-bud (germ tube) grows very long for true hyphae, and the cell cycle is delayed compared to the other two morphologies. The place of the nuclear division is specific for true hyphae determined by the position of the septin ring. However, the septin ring can localize anywhere inside the germ tube, unlike the mother-bud junction in budding yeast. Since the nucleus often migrates a long path in the hyphae, the underlying mechanism must be robust for executing mitosis in a timely manner. We explore the mechanism of nuclear migration through hyphae in light of mechanical interactions between astral microtubules and the cell cortex. We report that proper migration through constricted hyphae requires a large dynein pull applied on the astral microtubules from the hyphal cortex. This is achieved when the microtubules frequently slide along the hyphal cortex so that a large population of dyneins actively participate, pulling on them. Simulation shows timely migration when the dyneins from the mother cortex do not participate in pulling on the microtubules. These findings are robust for long migration and positioning of the nucleus in the germ tube at the septin ring.

Tubulin Post-Translational Modifications: The Elusive Roles of Acetylation

Microtubules (MTs), dynamic polymers of α/β-tubulin heterodimers found in all eukaryotes, are involved in cytoplasm spatial organization, intracellular transport, cell polarity, migration and division, and in cilia biology. MTs functional diversity depends on the differential expression of distinct tubulin isotypes and is amplified by a vast number of different post-translational modifications (PTMs). The addition/removal of PTMs to α- or β-tubulins is catalyzed by specific enzymes and allows combinatory patterns largely enriching the distinct biochemical and biophysical properties of MTs, creating a code read by distinct proteins, including microtubule-associated proteins (MAPs), which allow cellular responses. This review is focused on tubulin-acetylation, whose cellular roles continue to generate debate. We travel through the experimental data pointing to α-tubulin Lys40 acetylation role as being a MT stabilizer and a typical PTM of long lived MTs, to the most recent data, suggesting that Lys40 acetylation enhances MT flexibility and alters the mechanical properties of MTs, preventing MTs from mechanical aging characterized by structural damage. Additionally, we discuss the regulation of tubulin acetyltransferases/desacetylases and their impacts on cell physiology. Finally, we analyze how changes in MT acetylation levels have been found to be a general response to stress and how they are associated with several human pathologies.

Actin network architecture can ensure robust centering or sensitive decentering of the centrosome

The orientation of cell polarity depends on the position of the centrosome, the main microtubule-organizing center (MTOC). Microtubules (MTs) transmit pushing forces to the MTOC as they grow against the cell periphery. How the actin network regulates these forces remains unclear. Here, in a cell-free assay, we used purified proteins to reconstitute the interaction of a microtubule aster with actin networks of various architectures in cell-sized microwells. In the absence of actin filaments, MTOC positioning was highly sensitive to variations in microtubule length. The presence of a bulk actin network limited microtubule displacement, and MTOCs were held in place. In contrast, the assembly of a branched actin network along the well edges centered the MTOCs by maintaining an isotropic balance of pushing forces. An anisotropic peripheral actin network caused the MTOC to decenter by focusing the pushing forces. Overall, our results show that actin networks can limit the sensitivity of MTOC positioning to microtubule length and enforce robust MTOC centering or decentering depending on the isotropy of its architecture.

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.

Establishing neuronal polarity: microtubule regulation during neurite initiation

The initiation of nascent projections, or neurites, from the neuronal cell body is the first stage in the formation of axons and dendrites, and thus a critical step in the establishment of neuronal architecture and nervous system development. Neurite formation relies on the polarized remodelling of microtubules, which dynamically direct and reinforce cell shape, and provide tracks for cargo transport and force generation. Within neurons, microtubule behaviour and structure are tightly controlled by an array of regulatory factors. Although microtubule regulation in the later stages of axon development is relatively well understood, how microtubules are regulated during neurite initiation is rarely examined. Here, we discuss how factors that direct microtubule growth, remodelling, stability and positioning influence neurite formation. In addition, we consider microtubule organization by the centrosome and modulation by the actin and intermediate filament networks to provide an up-to-date picture of this vital stage in neuronal development.

Mechanics of microtubule organizing center clustering and spindle positioning in budding yeast Cryptococcus neoformans

The dynamic process of mitotic spindle assembly depends on multitudes of inter-dependent interactions involving kinetochores (KTs), microtubules (MTs), spindle pole bodies (SPBs), and molecular motors. Before forming the mitotic spindle, multiple visible microtubule organizing centers (MTOCs) coalesce into a single focus to serve as an SPB in the pathogenic budding yeast, Cryptococcus neoformans. To explain this unusual phenomenon in the fungal kingdom, we propose a "search and capture" model, in which cytoplasmic MTs (cMTs) nucleated by MTOCs grow and capture each other to promote MTOC clustering. Our quantitative modeling identifies multiple redundant mechanisms mediated by a combination of cMT-cell cortex interactions and inter-cMT coupling to facilitate MTOC clustering within the physiological time limit as determined by time-lapse live-cell microscopy. Besides, we screen various possible mechanisms by computational modeling and propose optimal conditions that favor proper spindle positioning-a critical determinant for timely chromosome segregation. These analyses also reveal that a combined effect of MT buckling, dynein pull, and cortical push maintains spatiotemporal spindle localization.

Cell division geometries as central organizers of early embryo development

Early cellular patterning is a critical step of embryonic development that determines the proper progression of morphogenesis in all metazoans. It relies on a series of rapid reductive divisions occurring simultaneously with the specification of the fate of different subsets of cells. Multiple species developmental strategies emerged in the form of a unique cleavage pattern with stereotyped division geometries. Cleavage geometries have long been associated to the emergence of canonical developmental features such as cell cycle asynchrony, zygotic genome activation and fate specification. Yet, the direct causal role of division positioning on blastomere cell behavior remain partially understood. Oriented and/or asymmetric divisions define blastomere cell sizes, contacts and positions, with potential immediate impact on cellular decisions, lineage specification and morphogenesis. Division positions also instruct daughter cells polarity, mechanics and geometries, thereby influencing subsequent division events, in an emergent interplay that may pattern early embryos independently of firm deterministic genetic programs. We here review the recent literature which helped to delineate mechanisms and functions of division positioning in early embryos.

Syndecan-4 in Tumor Cell Motility

Simple Summary

Cell migration is crucial fReaor metastasis formation and a hallmark of malignancy. The primary cause of high mortality among oncology patients is the ability of cancer cells to metastasize. To form metastasis, primary tumor cells must be intrinsically able to move. The transmembrane, heparan sulfate proteoglycan syndecan-4 (SDC4) exhibits multiple functions in signal transduction by regulating Rac1 GTPase activity and consequently actin remodeling, as well as regulating focal adhesion kinase, protein kinase C-alpha and the level of intracellular calcium. By affecting several signaling pathways and biological processes, SDC4 is involved in cell migration under physiological and pathological conditions as well. In this review, we discuss the SDC4-mediated cell migration focusing on the role of SDC4 in tumor cell movement.

Abstract

Syndecan-4 (SDC4) is a ubiquitously expressed, transmembrane proteoglycan bearing heparan sulfate chains. SDC4 is involved in numerous inside-out and outside-in signaling processes, such as binding and sequestration of growth factors and extracellular matrix components, regulation of the activity of the small GTPase Rac1, protein kinase C-alpha, the level of intracellular calcium, or the phosphorylation of focal adhesion kinase. The ability of this proteoglycan to link the extracellular matrix and actin cytoskeleton enables SDC4 to contribute to biological functions like cell adhesion and migration, cell proliferation, cytokinesis, cellular polarity, or mechanotransduction. The multiple roles of SDC4 in tumor pathogenesis and progression has already been demonstrated; therefore, the expression and signaling of SDC4 was investigated in several tumor types. SDC4 influences tumor progression by regulating cell proliferation as well as cell migration by affecting cell-matrix adhesion and several signaling pathways. Here, we summarize the general role of SDC4 in cell migration and tumor cell motility.

Self-organized optimal packing of kinesin-5-driven microtubule asters scales with cell size

Radial microtubule (MT) arrays or asters determine cell geometry in animal cells. Multiple asters interacting with motors, such as those in syncytia, form intracellular patterns, but the mechanical principles behind this are not clear. Here, we report that oocytes of the marine ascidian Phallusia mammillata treated with the drug BI-D1870 spontaneously form cytoplasmic MT asters, or cytasters. These asters form steady state segregation patterns in a shell just under the membrane. Cytaster centers tessellate the oocyte cytoplasm, that is divide it into polygonal structures, dominated by hexagons, in a kinesin-5-dependent manner, while inter-aster MTs form 'mini-spindles'. A computational model of multiple asters interacting with kinesin-5 can reproduce both tessellation patterns and mini-spindles in a manner specific to the number of MTs per aster, MT lengths and kinesin-5 density. Simulations predict that the hexagonal tessellation patterns scale with increasing cell size, when the packing fraction of asters in cells is ∼1.6. This self-organized in vivo tessellation by cytasters is comparable to the 'circle packing problem', suggesting that there is an intrinsic mechanical pattern-forming module that is potentially relevant to understanding the role of collective mechanics of cytoskeletal elements in embryogenesis. This article has an associated First Person interview with the first author of the paper.

Physiological relevance of post-translational regulation of the spindle assembly checkpoint protein BubR1

BubR1 is an essential component of the spindle assembly checkpoint (SAC) during mitosis where it functions to prevent anaphase onset to ensure proper chromosome alignment and kinetochore-microtubule attachment. Loss or mutation of BubR1 results in aneuploidy that precedes various potential pathologies, including cancer and mosaic variegated aneuploidy (MVA). BubR1 is also progressively downregulated with age and has been shown to be directly involved in the aging process through suppression of cellular senescence. Post-translational modifications, including but not limited to phosphorylation, acetylation, and ubiquitination, play a critical role in the temporal and spatial regulation of BubR1 function. In this review, we discuss the currently characterized post-translational modifications to BubR1, the enzymes involved, and the biological consequences to BubR1 functionality and implications in diseases associated with BubR1. Understanding the molecular mechanisms promoting these modifications and their roles in regulating BubR1 is important for our current understanding and future studies of BubR1 in maintaining genomic integrity as well as in aging and cancer.

Dynein self-organizes while translocating the centrosome in T-cells

T-cells massively restructure their internal architecture upon reaching an antigen-presenting cell (APC) to form the immunological synapse (IS), a cell-cell interface necessary for efficient elimination of the APC. This reorganization occurs through tight coordination of cytoskeletal processes: actin forms a peripheral ring, and dynein motors translocate the centrosome toward the IS. A recent study proposed that centrosome translocation involves a microtubule (MT) bundle that connects the centrosome perpendicularly to dynein at the synapse center: the "stalk." The synapse center, however, is actin-depleted, while actin was assumed to anchor dynein. We propose that dynein is attached to mobile membrane anchors, and investigate this model with computer simulations. We find that dynein organizes into a cluster in the synapse when translocating the centrosome, aligning MTs into a stalk. By implementing both a MT-capture-shrinkage and a MT-sliding mechanism, we explicitly demonstrate that this organization occurs in both systems. However, results obtained with MT-sliding dynein are more robust and display a stalk morphology consistent with our experimental data obtained with expansion microscopy. Thus, our simulations suggest that actin organization in T-cells during activation defines a specific geometry in which MT-sliding dynein can self-organize into a cluster and cause stalk formation.

Kinesin-4 KIF21B limits microtubule growth to allow rapid centrosome polarization in T cells

When a T cell and an antigen-presenting cell form an immunological synapse, rapid dynein-driven translocation of the centrosome toward the contact site leads to reorganization of microtubules and associated organelles. Currently, little is known about how the regulation of microtubule dynamics contributes to this process. Here, we show that the knockout of KIF21B, a kinesin-4 linked to autoimmune disorders, causes microtubule overgrowth and perturbs centrosome translocation. KIF21B restricts microtubule length by inducing microtubule pausing typically followed by catastrophe. Catastrophe induction with vinblastine prevented microtubule overgrowth and was sufficient to rescue centrosome polarization in KIF21B-knockout cells. Biophysical simulations showed that a relatively small number of KIF21B molecules can restrict mirotubule length and promote an imbalance of dynein-mediated pulling forces that allows the centrosome to translocate past the nucleus. We conclude that proper control of microtubule length is important for allowing rapid remodeling of the cytoskeleton and efficient T cell polarization.

The immune system is composed of many types of cells that can recognize foreign molecules and pathogens so they can eliminate them. When cells in the body become infected with a pathogen, they can process the pathogen’s proteins and present them on their own surface. Specialized immune cells can then recognize infected cells and interact with them, forming an ‘immunological synapse’. These synapses play an important role in immune response: they activate the immune system and allow it to kill harmful cells.

To form an immunological synapse, an immune cell must reorganize its internal contents, including an aster-shaped scaffold made of tiny protein tubes called microtubules. The center of this scaffold moves towards the immunological synapse as it forms. This re-orientation of the microtubules towards the immunological synapse is known as 'polarization' and it happens very rapidly, but it is not yet clear how it works.

One molecule involved in the polarization process is called KIF21B, a protein that can walk along microtubules, building up at the ends and affecting their growth. Whether KIF21B makes microtubules grow more quickly, or more slowly, is a matter of debate, and the impact microtubule length has on immunological synapse formation is unknown.

Here, Hooikaas, Damstra et al. deleted the gene for KIF21B from human immune cells called T cells to find out how it affected their ability to form an immunological synapse. Without KIF21B, the T cells grew microtubules that were longer than normal, and had trouble forming immunological synapses. When the T cells were treated with a drug that stops microtubule growth, their ability to form immunological synapses was restored, suggesting a role for KIF21B. To explore this further, Hooikaas, Damstra et al. replaced the missing KIF21B gene with a gene that coded for a version of the protein that could be seen using microscopy. This revealed that, when KIF21B reaches the ends of microtubules, it stops their growth and triggers their disassembly. Computational modelling showed that cells find it hard to reorient their microtubule scaffolding when the individual tubes are too long. It only takes a small number of KIF21B molecules to shorten the microtubules enough to allow the center of the scaffold to move.

Research has linked the KIF21B gene to autoimmune conditions like multiple sclerosis. Microtubules also play an important role in cell division, a critical process driving all types of cancer. Drugs that affect microtubule growth are already available, and a deeper understanding of KIF21B and microtubule regulation in immune cells could help to improve treatments in the future.

Microtubule-dependent pushing forces contribute to long-distance aster movement and centration in Xenopus laevis egg extracts

During interphase of the eukaryotic cell cycle, the microtubule (MT) cytoskeleton serves as both a supportive scaffold for organelles and an arborized system of tracks for intracellular transport. At the onset of mitosis, the position of the astral MT network, specifically its center, determines the eventual location of the spindle apparatus and ultimately the cytokinetic furrow. Positioning of the MT aster often results in its movement to the center of a cell, even in large blastomeres hundreds of microns in diameter. This translocation requires positioning forces, yet how these forces are generated and then integrated within cells of various sizes and geometries remains an open question. Here we describe a method that combines microfluidics, hydrogels, and Xenopus laevis egg extract to investigate the mechanics of aster movement and centration. We determined that asters were able to find the center of artificial channels and annular cylinders, even when cytoplasmic dynein-dependent pulling mechanisms were inhibited. Characterization of aster movement away from V-shaped hydrogel barriers provided additional evidence for a MT-based pushing mechanism. Importantly, the distance over which this mechanism seemed to operate was longer than that predicted by radial aster growth models, agreeing with recent models of a more complex MT network architecture within the aster.

Stoichiometric interactions explain spindle dynamics and scaling across 100 million years of nematode evolution

The spindle shows remarkable diversity, and changes in an integrated fashion, as cells vary over evolution. Here, we provide a mechanistic explanation for variations in the first mitotic spindle in nematodes. We used a combination of quantitative genetics and biophysics to rule out broad classes of models of the regulation of spindle length and dynamics, and to establish the importance of a balance of cortical pulling forces acting in different directions. These experiments led us to construct a model of cortical pulling forces in which the stoichiometric interactions of microtubules and force generators (each force generator can bind only one microtubule), is key to explaining the dynamics of spindle positioning and elongation, and spindle final length and scaling with cell size. This model accounts for variations in all the spindle traits we studied here, both within species and across nematode species spanning over 100 million years of evolution.

Mechanics of Multicentrosomal Clustering in Bipolar Mitotic Spindles

To segregate chromosomes in mitosis, cells assemble a mitotic spindle, a molecular machine with centrosomes at two opposing cell poles and chromosomes at the equator. Microtubules and molecular motors connect the poles to kinetochores, specialized protein assemblies on the centromere regions of the chromosomes. Bipolarity of the spindle is crucial for the proper cell division, and two centrosomes in animal cells naturally become two spindle poles. Cancer cells are often multicentrosomal, yet they are able to assemble bipolar spindles by clustering centrosomes into two spindle poles. Mechanisms of this clustering are debated. In this study, we computationally screen effective forces between 1) centrosomes, 2) centrosomes and kinetochores, 3) centrosomes and chromosome arms, and 4) centrosomes and cell cortex to understand mechanics that determines three-dimensional spindle architecture. To do this, we use the stochastic Monte Carlo search for stable mechanical equilibria in the effective energy landscape of the spindle. We find that the following conditions have to be met to robustly assemble the bipolar spindle in a multicentrosomal cell: 1) the strengths of centrosomes' attraction to each other and to the cell cortex have to be proportional to each other and 2) the strengths of centrosomes' attraction to kinetochores and repulsion from the chromosome arms have to be proportional to each other. We also find that three other spindle configurations emerge if these conditions are not met: 1) collapsed, 2) monopolar, and 3) multipolar spindles, and the computational screen reveals mechanical conditions for these abnormal spindles.

Molecular mechanisms of cell polarity in a range of model systems and in migrating neurons

Cell polarity is defined as the asymmetric distribution of cellular components along an axis. Most cells, from the simplest single-cell organisms to highly specialized mammalian cells, are polarized and use similar mechanisms to generate and maintain polarity. Cell polarity is important for cells to migrate, form tissues, and coordinate activities. During development of the mammalian cerebral cortex, cell polarity is essential for neurogenesis and for the migration of newborn but as-yet undifferentiated neurons. These oriented migrations include both the radial migration of excitatory projection neurons and the tangential migration of inhibitory interneurons. In this review, I will first describe the development of the cerebral cortex, as revealed at the cellular level. I will then define the core molecular mechanisms - the Par/Crb/Scrib polarity complexes, small GTPases, the actin and microtubule cytoskeletons, and phosphoinositides/PI3K signaling - that are required for asymmetric cell division, apico-basal and front-rear polarity in model systems, including C elegans zygote, Drosophila embryos and cultured mammalian cells. As I go through each core mechanism I will explain what is known about its importance in radial and tangential migration in the developing mammalian cerebral cortex.

Centering and Shifting of Centrosomes in Cells

Centrosomes have a nonrandom localization in the cells: either they occupy the centroid of the zone free of the actomyosin cortex or they are shifted to the edge of the cell, where their presence is justified from a functional point of view, for example, to organize additional microtubules or primary cilia. This review discusses centrosome placement options in cultured and in situ cells. It has been proven that the central arrangement of centrosomes is due mainly to the pulling microtubules forces developed by dynein located on the cell cortex and intracellular vesicles. The pushing forces from dynamic microtubules and actomyosin also contribute, although the molecular mechanisms of their action have not yet been elucidated. Centrosomal displacement is caused by external cues, depending on signaling, and is drawn through the redistribution of dynein, the asymmetrization of microtubules through the capture of their plus ends, and the redistribution of actomyosin, which, in turn, is associated with basal-apical cell polarization.

Prophase-Specific Perinuclear Actin Coordinates Centrosome Separation and Positioning to Ensure Accurate Chromosome Segregation

Centrosome separation in late G2/ early prophase requires precise spatial coordination that is determined by a balance of forces promoting and antagonizing separation. The major effector of centrosome separation is the kinesin Eg5. However, the identity and regulation of Eg5-antagonizing forces is less well characterized. By manipulating candidate components, we find that centrosome separation is reversible and that separated centrosomes congress toward a central position underneath the flat nucleus. This positioning mechanism requires microtubule polymerization, as well as actin polymerization. We identify perinuclear actin structures that form in late G2/early prophase and interact with microtubules emanating from the centrosomes. Disrupting these structures by breaking the interactions of the linker of nucleoskeleton and cytoskeleton (LINC) complex with perinuclear actin filaments abrogates this centrosome positioning mechanism and causes an increase in subsequent chromosome segregation errors. Our results demonstrate how geometrical cues from the cell nucleus coordinate the orientation of the emanating spindle poles before nuclear envelope breakdown.

Microtubules in cell migration

Directed cell migration is critical for embryogenesis and organ development, wound healing and the immune response. Microtubules are dynamic polymers that control directional migration through a number of coordinated processes: microtubules are the tracks for long-distance intracellular transport, crucial for delivery of new membrane components and signalling molecules to the leading edge of a migrating cell and the recycling of adhesion receptors. Microtubules act as force generators and compressive elements to support sustained cell protrusions. The assembly and disassembly of microtubules is coupled to Rho GTPase signalling, thereby controlling actin polymerisation, myosin-driven contractility and the turnover of cellular adhesions locally. Cross-talk of actin and microtubule dynamics is mediated through a number of common binding proteins and regulators. Furthermore, cortical microtubule capture sites are physically linked to focal adhesions, facilitating the delivery of secretory vesicles and efficient cross-talk. Here we summarise the diverse functions of microtubules during cell migration, aiming to show how they contribute to the spatially and temporally coordinated sequence of events that permit efficient, directional and persistent migration.

Centering and symmetry breaking in confined contracting actomyosin networks

Centering and decentering of cellular components is essential for internal organization of cells and their ability to perform basic cellular functions such as division and motility. How cells achieve proper localization of their organelles is still not well-understood, especially in large cells such as oocytes. Here, we study actin-based positioning mechanisms in artificial cells with persistently contracting actomyosin networks, generated by encapsulating cytoplasmic Xenopus egg extracts into cell-sized ‘water-in-oil’ droplets. We observe size-dependent localization of the contraction center, with a symmetric configuration in larger cells and a polar one in smaller cells. Centering is achieved via a hydrodynamic mechanism based on Darcy friction between the contracting network and the surrounding cytoplasm. During symmetry breaking, transient attachments to the cell boundary drive the contraction center to a polar location. The centering mechanism is cell-cycle dependent and weakens considerably during interphase. Our findings demonstrate a robust, yet tunable, mechanism for subcellular localization.

In order to survive, cells need to react to their environment and change their shape or the localization of their internal components. For example, the nucleus – the compartment that contains the genetic information – is often localized at the center of the cell, but it can also be positioned at the side, for instance when cells move or divide asymmetrically.

Cells use multiple positioning mechanisms to move their internal components, including a process that relies on networks of filaments made of a protein known as actin. These networks are constantly remodeled as actin proteins are added and removed from the network. Embedded molecular motors can cause the network of actin filaments to contract and push or pull on the compartments. Yet, the exact way these networks localize components in the cell remains unclear, especially in eggs and other large cells.

To investigate this question, Ierushalmi et al. studied the actin networks in artificial cells that they created by enclosing the contents of frog eggs in small droplets surrounded by oil. This showed that the networks contracted either to the center of the cell or to its side. Friction between the contracting actin network and the fluid in the cell generated a force that tends to push the contraction center towards the middle of the cell. In larger cells, this led to the centering of the actin network. In smaller cells however, the network transiently attached to the boundary of the cell, leading the contraction center to be pulled to one side.

By developing simpler artificial cells that mimic the positioning processes seen in real-life cells, Ierushalmi et al. discovered new mechanisms for how cells may center or de-center their components. This knowledge may be useful to understand diseases that can emerge when the nucleus or other compartments fail to move to the right location, and which are associated with certain organs developing incorrectly.

Chromosomes function as a barrier to mitotic spindle bipolarity in polyploid cells

Ploidy variations such as genome doubling are frequent in human tumors and have been associated with genetic instability favoring tumor progression. How polyploid cells deal with increased centrosome numbers and DNA content remains unknown. Using Drosophila neuroblasts and human cancer cells to study mitotic spindle assembly in polyploid cells, we found that most polyploid cells divide in a multipolar manner. We show that even if an initial centrosome clustering step can occur at mitotic entry, the establishment of kinetochore-microtubule attachments leads to spatial chromosome configurations, whereby the final coalescence of supernumerary poles into a bipolar array is inhibited. Using in silico approaches and various spindle and DNA perturbations, we show that chromosomes act as a physical barrier blocking spindle pole coalescence and bipolarity. Importantly, microtubule stabilization suppressed multipolarity by improving both centrosome clustering and pole coalescence. This work identifies inhibitors of bipolar division in polyploid cells and provides a rationale to understand chromosome instability typical of polyploid cancer cells.

The Centrosome Linker and Its Role in Cancer and Genetic Disorders

Centrosome cohesion, the joining of the two centrosomes of a cell, is increasingly appreciated as a major regulator of cell functions such as Golgi organization and cilia positioning. One major element of centrosome cohesion is the centrosome linker that consists of a growing number of proteins. The timely disassembly of the centrosome linker enables centrosomes to separate and assemble a functional bipolar mitotic spindle that is crucial for maintaining genomic integrity. Exciting new findings link centrosome linker defects to cell transformation and genetic disorders. We review recent data on the molecular mechanisms of the assembly and disassembly of the centrosome linker, and discuss how defects in the proper execution of these processes cause DNA damage and genomic instability leading to disease.

Cytoskeleton Force Exertion in Bulk Cytoplasm

The microtubule and actin cytoskeletons generate forces essential to position centrosomes, nuclei, and spindles for division plane specification. While the largest body of work has documented force exertion at, or close to the cell surface, mounting evidence suggests that cytoskeletal polymers can also produce significant forces directly from within the cytoplasm. Molecular motors such as kinesin or dynein may for instance displace cargos and endomembranes in the viscous cytoplasm yielding friction forces that pull or push microtubules. Similarly, the dynamics of bulk actin assembly/disassembly or myosin-dependent contractions produce cytoplasmic forces which influence the spatial organization of cells in a variety of processes. We here review the molecular and physical mechanisms supporting bulk cytoplasmic force generation by the cytoskeleton, their limits and relevance to organelle positioning, with a particular focus on cell division.

Tryptone-stabilized gold nanoparticles induce unipolar clustering of supernumerary centrosomes and G1 arrest in triple-negative breast cancer cells

Gold nanoparticles of different sizes, shapes, and decorations exert a variety of effects on biological systems. We report a novel mechanism of action of chemically modified, tryptone-stabilized gold nanoparticles (T-GNPs) in the triple-negative breast cancer (TNBC) cell line, MDA-MB-231. The T-GNPs, synthesized using HAuCl4.3H2O and tryptone and characterized by an assortment of spectroscopy techniques combined with high-resolution electron microscopy, demonstrated strong antiproliferative and anti-clonogenic potential against MDA-MB-231 cells, arresting them at the G1 phase of the cell cycle and promoting apoptosis. The molecular mechanism of action of these particles involved induction of unipolar clustering and hyper amplification of the supernumerary centrosomes (a distinctive feature of many tumour cells, including TNBC cells). The clustering was facilitated by microtubules with suppressed dynamicity. Mass spectrometry-assisted proteomic analysis revealed that the T-GNP-induced G1 arrest was facilitated, at least in part, by downregulation of ribosome biogenesis pathways. Due to the presence of supernumerary centrosomes in many types of tumour cells, we propose chemical induction of their unipolar clustering as a potential therapeutic strategy.

Generation and regulation of microtubule network asymmetry to drive cell polarity

Microtubules control cell architecture by serving as a scaffold for intracellular transport, signaling, and organelle positioning. Microtubules are intrinsically polarized, and their orientation, density, and post-translational modifications both respond and contribute to cell polarity. Animal cells that can rapidly reorient their polarity axis, such as fibroblasts, immune cells, and cancer cells, contain radially organized microtubule arrays anchored at the centrosome and the Golgi apparatus, whereas stably polarized cells often acquire non-centrosomal microtubule networks attached to the cell cortex, nucleus, or other structures. Microtubule density, longevity, and post-translational modifications strongly depend on the dynamics of their plus ends. Factors controlling microtubule plus-end dynamics are often part of cortical assemblies that integrate cytoskeletal organization, cell adhesion, and secretion and are subject to microtubule-dependent feedback regulation. Finally, microtubules can mechanically contribute to cell asymmetry by promoting cell elongation, a property that might be important for cells with dense microtubule arrays growing in soft environments.

Force balances between interphase centrosomes as revealed by laser ablation

Numerous studies have highlighted the self-centering activities of individual microtubule (MT) arrays in animal cells, but relatively few works address the behavior of multiple arrays that coexist in a common cytoplasm. In multinucleated Dictyostelium discoideum cells, each centrosome organizes a radial MT network, and these networks remain separate from one another. This feature offers an opportunity to reveal the mechanism(s) responsible for the positioning of multiple centrosomes. Using a laser microbeam to eliminate one of the two centrosomes in binucleate cells, we show that the unaltered array is rapidly repositioned at the cell center. This result demonstrates that each MT array is constantly subject to centering forces and infers a mechanism to balance the positions of multiple arrays. Our results address the limited actions of three kinesins and a cross-linking MAP that are known to have effects in maintaining MT organization and suggest a simple means used to keep the arrays separated.

Actin filaments regulate microtubule growth at the centrosome

The centrosome is the main microtubule-organizing centre. It also organizes a local network of actin filaments. However, the precise function of the actin network at the centrosome is not well understood. Here, we show that increasing densities of actin filaments at the centrosome of lymphocytes are correlated with reduced amounts of microtubules. Furthermore, lymphocyte activation resulted in disassembly of centrosomal actin and an increase in microtubule number. To further investigate the direct crosstalk between actin and microtubules at the centrosome, we performed in vitro reconstitution assays based on (i) purified centrosomes and (ii) on the co-micropatterning of microtubule seeds and actin filaments. These two assays demonstrated that actin filaments constitute a physical barrier blocking elongation of nascent microtubules. Finally, we showed that cell adhesion and cell spreading lead to lower densities of centrosomal actin, thus resulting in higher microtubule growth. We therefore propose a novel mechanism, by which the number of centrosomal microtubules is regulated by cell adhesion and actin-network architecture.

Asymmetric division through a reduction of microtubule centering forces

Asymmetric divisions are essential for the generation of cell fate and size diversity. They implicate cortical domains where minus end-directed motors, such as dynein, are activated to pull on microtubules to decenter asters attached to centrosomes, nuclei, or spindles. In asymmetrically dividing cells, aster decentration typically follows a centering phase, suggesting a time-dependent regulation in the competition between microtubule centering and decentering forces. Using symmetrically dividing sea urchin zygotes, we generated cortical domains of magnetic particles that spontaneously cluster endogenous dynein activity. These domains efficiently attract asters and nuclei, yielding marked asymmetric divisions. Remarkably, aster decentration only occurred after asters had first reached the cell center. Using intracellular force measurement and models, we demonstrate that this time-regulated imbalance results from a global reduction of centering forces rather than a local maturation of dynein activity at the domain. Those findings define a novel paradigm for the regulation of division asymmetry.

Mechanistic three-dimensional model to study centrosome positioning in the interphase cell

During the interphase in mammalian cells, the position of the centrosome is actively maintained at a small but finite distance away from the nucleus. The perinuclear positioning of the centrosome is crucial for cellular trafficking and progression into mitosis. Although the literature suggests that the contributions of the microtubule-associated forces bring the centrosome to the center of the cell, the position of the centrosome was merely investigated in the absence of the nucleus. Upon performing a coarse-grained simulation study with mathematical analysis, we show that the combined effect of the forces due to the cell cortex and the nucleus facilitate the centrosome positioning. Our study also demonstrates that in the absence of nucleus-based forces, the centrosome collapses on the nucleus due to cortical forces. Depending upon the magnitudes of the cortical forces and the nucleus-based forces, the centrosome appears to stay at various distances away from the nucleus. Such null force regions are found to be stable as well as unstable fixed points. This study uncovers a set of redundant schemes that the cell may adopt to produce the required cortical and nucleus-based forces stabilizing the centrosome at a finite distance away from the nucleus.

Spatio-temporal regulation of nuclear division by Aurora B kinase Ipl1 in Cryptococcus neoformans

The nuclear division takes place in the daughter cell in the basidiomycetous budding yeast Cryptococcus neoformans. Unclustered kinetochores gradually cluster and the nucleus moves to the daughter bud as cells enter mitosis. Here, we show that the evolutionarily conserved Aurora B kinase Ipl1 localizes to the nucleus upon the breakdown of the nuclear envelope during mitosis in C. neoformans. Ipl1 is shown to be required for timely breakdown of the nuclear envelope as well. Ipl1 is essential for viability and regulates structural integrity of microtubules. The compromised stability of cytoplasmic microtubules upon Ipl1 depletion results in a significant delay in kinetochore clustering and nuclear migration. By generating an in silico model of mitosis, we previously proposed that cytoplasmic microtubules and cortical dyneins promote atypical nuclear division in C. neoformans. Improving the previous in silico model by introducing additional parameters, here we predict that an effective cortical bias generated by cytosolic Bim1 and dynein regulates dynamics of kinetochore clustering and nuclear migration. Indeed, in vivo alterations of Bim1 or dynein cellular levels delay nuclear migration. Results from in silico model and localization dynamics by live cell imaging suggests that Ipl1 spatio-temporally influences Bim1 or/and dynein activity along with microtubule stability to ensure timely onset of nuclear division. Together, we propose that the timely breakdown of the nuclear envelope by Ipl1 allows its own nuclear entry that helps in spatio-temporal regulation of nuclear division during semi-open mitosis in C. neoformans.

Unlike the model ascomycetous budding yeast Saccharomyces cerevisiae, microtubule organizing centers (MTOCs) coalesce to form the spindle pole body (SPB) in C. neoformans. This process also ensures unclustered kinetochores to gradually cluster in this organism. As C. neoformans cells enter mitosis, the nuclear envelope ruptures and the nucleus eventually moves to the daughter bud before division. Here, we combine cell and systems biology techniques to understand the key determinants of nuclear division in C. neoformans. We show that the evolutionarily conserved Aurora B kinase Ipl1 enters the nucleus during the mitotic phase as cells undergo semi-open mitosis. Ipl1 regulates dynamics of cytoplasmic microtubules, cytosolic proteins such as Bim1 and dynein-mediated cortical forces and integrity of the nuclear envelope to ensure timely kinetochore clustering and nuclear division in this medically relevant human pathogenic budding yeast.

A computational model of the early stages of acentriolar meiotic spindle assembly

The mitotic spindle is an ensemble of microtubules responsible for the repartition of the chromosomal content between the two daughter cells during division. In metazoans, spindle assembly is a gradual process involving dynamic microtubules and recruitment of numerous associated proteins and motors. During mitosis, centrosomes organize and nucleate the majority of spindle microtubules. In contrast, oocytes lack canonical centrosomes but are still able to form bipolar spindles, starting from an initial ball that self-organizes in several hours. Interfering with early steps of meiotic spindle assembly can lead to erroneous chromosome segregation. Although not fully elucidated, this process is known to rely on antagonistic activities of plus end– and minus end–directed motors. We developed a model of early meiotic spindle assembly in mouse oocytes, including key factors such as microtubule dynamics and chromosome movement. We explored how the balance between plus end– and minus end–directed motors, as well as the influence of microtubule nucleation, impacts spindle morphology. In a refined model, we added spatial regulation of microtubule stability and minus-end clustering. We could reproduce the features of early stages of spindle assembly from 12 different experimental perturbations and predict eight additional perturbations. With its ability to characterize and predict chromosome individualization, this model can help deepen our understanding of spindle assembly.

Chronology of motor-mediated microtubule streaming

We introduce a filament-based simulation model for coarse-grained, effective motor-mediated interaction between microtubule pairs to study the time-scales that compose cytoplasmic streaming. We characterise microtubule dynamics in two-dimensional systems by chronologically arranging five distinct processes of varying duration that make up streaming, from microtubule pairs to collective dynamics. The structures found were polarity sorted due to the propulsion of antialigned microtubules. This also gave rise to the formation of large polar-aligned domains, and streaming at the domain boundaries. Correlation functions, mean squared displacements, and velocity distributions reveal a cascade of processes ultimately leading to microtubule streaming and advection, spanning multiple microtubule lengths. The characteristic times for the processes extend over three orders of magnitude from fast single-microtubule processes to slow collective processes. Our approach can be used to directly test the importance of molecular components, such as motors and crosslinking proteins between microtubules, on the collective dynamics at cellular scale.

Microtubule Dynamics Scale with Cell Size to Set Spindle Length and Assembly Timing

Successive cell divisions during embryonic cleavage create increasingly smaller cells, so intracellular structures must adapt accordingly. Mitotic spindle size correlates with cell size, but the mechanisms for this scaling remain unclear. Using live cell imaging, we analyzed spindle scaling during embryo cleavage in the nematode Caenorhabditis elegans and sea urchin Paracentrotus lividus. We reveal a common scaling mechanism, where the growth rate of spindle microtubules scales with cell volume, which explains spindle shortening. Spindle assembly timing is, however, constant throughout successive divisions. Analyses in silico suggest that controlling the microtubule growth rate is sufficient to scale spindle length and maintain a constant assembly timing. We tested our in silico predictions to demonstrate that modulating cell volume or microtubule growth rate in vivo induces a proportional spindle size change. Our results suggest that scalability of the microtubule growth rate when cell size varies adapts spindle length to cell volume.

Mechanical positioning of multiple nuclei in muscle cells

Many types of large cells have multiple nuclei. In skeletal muscle fibers, the nuclei are distributed along the cell to maximize their internuclear distances. This myonuclear positioning is crucial for cell function. Although microtubules, microtubule associated proteins, and motors have been implicated, mechanisms responsible for myonuclear positioning remain unclear. We used a combination of rough interacting particle and detailed agent-based modeling to examine computationally the hypothesis that a force balance generated by microtubules positions the muscle nuclei. Rather than assuming the nature and identity of the forces, we simulated various types of forces between the pairs of nuclei and between the nuclei and cell boundary to position the myonuclei according to the laws of mechanics. We started with a large number of potential interacting particle models and computationally screened these models for their ability to fit biological data on nuclear positions in hundreds of Drosophila larval muscle cells. This reverse engineering approach resulted in a small number of feasible models, the one with the best fit suggests that the nuclei repel each other and the cell boundary with forces that decrease with distance. The model makes nontrivial predictions about the increased nuclear density near the cell poles, the zigzag patterns of the nuclear positions in wider cells, and about correlations between the cell width and elongated nuclear shapes, all of which we confirm by image analysis of the biological data. We support the predictions of the interacting particle model with simulations of an agent-based mechanical model. Taken together, our data suggest that microtubules growing from nuclear envelopes push on the neighboring nuclei and the cell boundaries, which is sufficient to establish the nearly-uniform nuclear spreading observed in muscle fibers.

How the cell organizes its interior is one of the fundamental biological questions, but the principles of organelles’ positioning remains largely unclear. In this study we use computational modeling and image analysis to elucidate mechanisms of positioning of multiple nuclei in muscle cells. We start with the general hypothesis, supported by published data, that a force balance generated by microtubule asters growing from the nuclei envelopes are responsible for pushing or pulling neighboring nuclei and cell boundaries, and that these forces position the nuclei. Instead of assuming what these forces are, we computationally screen all possible forces by comparing predictions of hundreds simple mechanical models to experimentally measured nuclear positions and shapes in hundreds of Drosophila muscle cells. This screening results in the model, according to which microtubules from one nucleus push away both neighboring nuclei and cell boundaries. We also perform detailed stochastic simulations of the only surviving model with individual growing, pushing and bending microtubules. This model predicts subtle features of nuclear patterns, all of which we confirm experimentally. Our study sheds light on general principles of organelle positioning.

How cells sense their own shape - mechanisms to probe cell geometry and their implications in cellular organization and function

Cells come in a variety of shapes that most often underlie their functions. Regulation of cell morphogenesis implies that there are mechanisms for shape sensing that still remain poorly appreciated. Global and local cell geometry features, such as aspect ratio, size or membrane curvature, may be probed by intracellular modules, such as the cytoskeleton, reaction-diffusion systems or molecular complexes. In multicellular tissues, cell shape emerges as an important means to transduce tissue-inherent chemical and mechanical cues into intracellular organization. One emergent paradigm is that cell-shape sensing is most often based upon mechanisms of self-organization, rather than determinism. Here, we review relevant work that has elucidated some of the core principles of how cellular geometry may be conveyed into spatial information to guide processes, such as polarity, signaling, morphogenesis and division-plane positioning.

Regulating positioning and orientation of mitotic spindles via cell size and shape

Proper location of the mitotic spindle is critical for chromosome segregation and the selection of the cell division plane. However, how mitotic spindles sense cell size and shape to regulate their own position and orientation is still largely unclear. To investigate this question systematically, we used a general model by considering chromosomes, microtubule dynamics, and forces of various molecular motors. Our results show that in cells of various sizes and shapes, spindles can always be centered and oriented along the long axis robustly in the absence of other specified mechanisms. We found that the characteristic time of positioning and orientation processes increases with cell size. Spindles sense the cell size mainly by the cortical force in small cells and by the cytoplasmic force in large cells. In addition to the cell size, the cell shape mainly influences the orientation process. We found that more slender cells have a faster orientation process, and the final orientation is not necessarily along the longest axis but is determined by the radial profile and the symmetry of the cell shape. Finally, our model also reproduces the separation and repositioning of the spindle poles during the anaphase. Therefore, our work provides a general tool for studying the mitotic spindle across the whole mitotic phase.

Physical Limits on the Precision of Mitotic Spindle Positioning by Microtubule Pushing forces: Mechanics of mitotic spindle positioning

Tissues are shaped and patterned by mechanical and chemical processes. A key mechanical process is the positioning of the mitotic spindle, which determines the size and location of the daughter cells within the tissue. Recent force and position-fluctuation measurements indicate that pushing forces, mediated by the polymerization of astral microtubules against- the cell cortex, maintain the mitotic spindle at the cell center in Caenorhabditis elegans embryos. The magnitude of the centering forces suggests that the physical limit on the accuracy and precision of this centering mechanism is determined by the number of pushing microtubules rather than by thermally driven fluctuations. In cells that divide asymmetrically, anti-centering, pulling forces generated by cortically located dyneins, in conjunction with microtubule depolymerization, oppose the pushing forces to drive spindle displacements away from the center. Thus, a balance of centering pushing forces and anti-centering pulling forces localize the mitotic spindles within dividing C. elegans cells.

Role of the Crumbs proteins in ciliogenesis, cell migration and actin organization

Epithelial cell organization relies on a set of proteins that interact in an intricate way and which are called polarity complexes. These complexes are involved in the determination of the apico-basal axis and in the positioning and stability of the cell-cell junctions called adherens junctions at the apico-lateral border in invertebrates. Among the polarity complexes, two are present at the apical side of epithelial cells. These are the Par complex including aPKC, PAR3 and PAR6 and the Crumbs complex including, CRUMBS, PALS1 and PATJ/MUPP1. These two complexes interact directly and in addition to their already well described functions, they play a role in other cellular processes such as ciliogenesis and polarized cell migration. In this review, we will focus on these aspects that involve the apical Crumbs polarity complex and its relation with the cortical actin cytoskeleton which might provide a more comprehensive hypothesis to explain the many facets of Crumbs cell and tissue properties.

Microtubule stabilization drives 3D centrosome migration to initiate primary ciliogenesis

The classical view of centrosome decentering and migration to the cell periphery during ciliogenesis is that it is pulled toward its final destination. Here, Pitaval et al. argue that microtubule stabilization in the early stages of ciliogenesis generates pushing forces that propel the centrosome toward the apical pole.

Primary cilia are sensory organelles located at the cell surface. Their assembly is primed by centrosome migration to the apical surface, yet surprisingly little is known about this initiating step. To gain insight into the mechanisms driving centrosome migration, we exploited the reproducibility of cell architecture on adhesive micropatterns to investigate the cytoskeletal remodeling supporting it. Microtubule network densification and bundling, with the transient formation of an array of cold-stable microtubules, and actin cytoskeleton asymmetrical contraction participate in concert to drive apical centrosome migration. The distal appendage protein Cep164 appears to be a key actor involved in the cytoskeleton remodeling and centrosome migration, whereas intraflagellar transport 88’s role seems to be restricted to axoneme elongation. Together, our data elucidate the hitherto unexplored mechanism of centrosome migration and show that it is driven by the increase and clustering of mechanical forces to push the centrosome toward the cell apical pole.

Programmed Self-Assembly of a Biochemical and Magnetic Scaffold to Trigger and Manipulate Microtubule Structures

Artificial bio-based scaffolds offer broad applications in bioinspired chemistry, nanomedicine, and material science. One current challenge is to understand how the programmed self-assembly of biomolecules at the nanometre level can dictate the emergence of new functional properties at the mesoscopic scale. Here we report a general approach to design genetically encoded protein-based scaffolds with modular biochemical and magnetic functions. By combining chemically induced dimerization strategies and biomineralisation, we engineered ferritin nanocages to nucleate and manipulate microtubule structures upon magnetic actuation. Triggering the self-assembly of engineered ferritins into micrometric scaffolds mimics the function of centrosomes, the microtubule organizing centres of cells, and provides unique magnetic and self-organizing properties. We anticipate that our approach could be transposed to control various biological processes and extend to broader applications in biotechnology or material chemistry.

Matrix rigidity regulates microtubule network polarization in migration

The microtubule organizing center (MTOC) frequently polarizes to a position in front of the nucleus during cell migration, but recent work has shown conflicting evidence for MTOC location in migratory polarized cells. Here, we show that subcellular localization of the MTOC is modulated by extracellular matrix stiffness. In scratch wound assays as well as single cell migration of mesenchymal stem cells (MSCs) the MTOC appears randomly positioned when cells are migrating on soft matrix, whereas on stiff matrix the MTOC is in front of the nucleus. The bulk of the microtubule density is also equally likely to be in front of or behind the nucleus on soft matrix, but it is polarized in front of the nucleus on stiff matrix. This occurred during cell migration with cells in interphase. During cytokinesis, the centrosomes polarize on either side of the chromosomes even on soft matrix, with MIIB localized strongly in the cleavage furrow which depolarizes only on soft matrix as cells exit cytokinesis. When cells are immobilized on micro-patterns printed on the top of substrates of different stiffness, MIIB polarized if the matrix was sufficiently stiff similar to results with migrating cells. However, the MTOC was randomly positioned with respect to the nucleus independent of matrix stiffness. We deduce that cell migration is necessary to orient the MTOC in front of the nucleus and that matrix stiffness helps to drive cell polarization during migration. © 2016 Wiley Periodicals, Inc.

Polarity Reversal by Centrosome Repositioning Primes Cell Scattering during Epithelial-to-Mesenchymal Transition

During epithelial-to-mesenchymal transition (EMT), cells lining the tissue periphery break up their cohesion to migrate within the tissue. This dramatic reorganization involves a poorly characterized reorientation of the apicobasal polarity of static epithelial cells into the front-rear polarity of migrating mesenchymal cells. To investigate the spatial coordination of intracellular reorganization with morphological changes, we monitored centrosome positioning during EMT in vivo, in developing mouse embryos and mammary gland, and in vitro, in cultured 3D cell aggregates and micropatterned cell doublets. In all conditions, centrosomes moved from their off-centered position next to intercellular junctions toward extracellular matrix adhesions on the opposite side of the nucleus, resulting in an effective internal polarity reversal. This move appeared to be supported by controlled microtubule network disassembly. Sequential release of cell confinement using dynamic micropatterns, and modulation of microtubule dynamics, confirmed that centrosome repositioning was responsible for further cell disengagement and scattering.