Centrosome positioning in interphase cells

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.

Centrosome Movements Are TUBG1-Dependent

The centrosome of mammalian cells is in constant movement and its motion plays a part in cell differentiation and cell division. The purpose of this study was to establish the involvement of the TUBG meshwork in centrosomal motility. In live cells, we used a monomeric red-fluorescence-protein-tagged centrin 2 gene and a green-fluorescence-protein-tagged TUBG1 gene for labeling the centrosome and the TUBG1 meshwork, respectively. We found that centrosome movements occurred in cellular sites rich in GTPase TUBG1 and single-guide RNA mediated a reduction in the expression of TUBG1, altering the motility pattern of centrosomes. We propose that the TUBG1 meshwork enables the centrosomes to move by providing them with an interacting platform that mediates positional changes. These findings uncover a novel regulatory mechanism that controls the behavior of centrosomes.

Centrosome amplification fine tunes tubulin acetylation to differentially control intracellular organization

Intracellular organelle organization is conserved in eukaryotic cells and is primarily achieved through active transport by motor proteins along the microtubule cytoskeleton. Microtubule post-translational modifications (PTMs) can contribute to microtubule diversity and differentially regulate motor-mediated transport. Here, we show that centrosome amplification, commonly observed in cancer and shown to promote aneuploidy and invasion, induces a global change in organelle positioning towards the cell periphery and facilitates nuclear migration through confined spaces. This reorganization requires kinesin-1 and is analogous to the loss of dynein. Cells with amplified centrosomes display increased levels of acetylated tubulin, a PTM that could enhance kinesin-1-mediated transport. Depletion of α-tubulin acetyltransferase 1 (αTAT1) to block tubulin acetylation rescues the displacement of centrosomes, mitochondria, and vimentin but not Golgi or endosomes. Analyses of the distribution of total and acetylated microtubules indicate that the polarized distribution of modified microtubules, rather than levels alone, plays an important role in the positioning of specific organelles, such as the centrosome. We propose that increased tubulin acetylation differentially impacts kinesin-1-mediated organelle displacement to regulate intracellular organization.

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.

The Chlamydia trachomatis IncM Protein Interferes with Host Cell Cytokinesis, Centrosome Positioning, and Golgi Distribution and Contributes to the Stability of the Pathogen-Containing Vacuole

Chlamydia trachomatis is an obligate intracellular bacterial pathogen that causes ocular and urogenital infections in humans. The ability of C. trachomatis to grow intracellularly in a pathogen-containing vacuole (known as an inclusion) depends on chlamydial effector proteins transported into the host cell by a type III secretion system. Among these effectors, several inclusion membrane proteins (Incs) insert in the vacuolar membrane. Here, we show that human cell lines infected by a C. trachomatis strain deficient for Inc CT288/CTL0540 (renamed IncM) displayed less multinucleation than when infected by IncM-producing strains (wild type or complemented). This indicated that IncM is involved in the ability of Chlamydia to inhibit host cell cytokinesis. The capacity of IncM to induce multinucleation in infected cells was shown to be conserved among its chlamydial homologues and appeared to require its two larger regions predicted to be exposed to the host cell cytosol. C. trachomatis-infected cells also displayed IncM-dependent defects in centrosome positioning, Golgi distribution around the inclusion, and morphology and stability of the inclusion. The altered morphology of inclusions containing IncM-deficient C. trachomatis was further affected by depolymerization of host cell microtubules. This was not observed after depolymerization of microfilaments, and inclusions containing wild-type C. trachomatis did not alter their morphology upon depolymerization of microtubules. Overall, these findings suggest that IncM may exert its effector function by acting directly or indirectly on host cell microtubules.

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.

Insights into Cell-Specific Functions of Microtubules in Skeletal Muscle Development and Homeostasis

The contractile cells of skeletal muscles, called myofibers, are elongated multinucleated syncytia formed and maintained by the fusion of proliferative myoblasts. Human myofibers can be hundreds of microns in diameter and millimeters in length. Myofibers are non-mitotic, obviating the need for microtubules in cell division. However, microtubules have been adapted to the unique needs of these cells and are critical for myofiber development and function. Microtubules in mature myofibers are highly dynamic, and studies in several experimental systems have demonstrated the requirements for microtubules in the unique features of muscle biology including myoblast fusion, peripheral localization of nuclei, assembly of the sarcomere, transport and signaling. Microtubule-binding proteins have also been adapted to the needs of the skeletal muscle including the expression of skeletal muscle-specific protein isoforms generated by alternative splicing. Here, we will outline the different roles microtubules play in skeletal muscle cells, describe how microtubule abnormalities can lead to muscle disease and discuss the broader implications for microtubule function.

The Centrosome: Conclusions and Perspectives

The centrosome consists of two centrioles surrounded by pericentriolar material [...].

Dynein localization and pronuclear movement in the C. elegans zygote

Centrosomes serve as a site for microtubule nucleation and these microtubules will grow and interact with the motor protein dynein at the cortex. The position of the centrosomes determines where the mitotic spindle will develop across all cell types. Centrosome positioning is achieved through dynein and microtubule-mediated force generation. The mechanism and regulation of force generation during centrosome positioning are not fully understood. Centrosome and pronuclear movement in the first cell cycle of the Caenorhabditis elegans early embryo undergoes both centration and rotation prior to cell division. The proteins LET-99 and GPB-1 have been postulated to have a role in force generation associated with pronuclear centration and rotation dynamics. When the expression of these proteins is perturbed, pronuclear positioning exhibits a movement defect characterized by oscillatory ("wobble") behavior of the pronuclear complex (PNC). To determine if this movement defect is due to an effect on cortical dynein distribution, we utilize RNAi-mediated knockdown of LET-99 and GPB-1 to induce wobble and assay for any effects on GFP-tagged dynein localization in the early C. elegans embryo. To compare and quantify the movement defect produced by the knockdown of LET-99 and GPB-1, we devised a quantification method that measures the strength of wobble ("wobble metric") observed under these experimental conditions. Our quantification of pronuclear complex dynamics and dynein localization shows that loss of LET-99 and GPB-1 induces a similar movement defect which is independent of cortical dynein localization in the early C. elegans embryo.

Pericentrin interacts with Kinesin-1 to drive centriole motility

Centrosome positioning is essential for their function. Typically, centrosomes are transported to various cellular locations through the interaction of centrosomal microtubules (MTs) with motor proteins anchored at the cortex or the nuclear surface. However, it remains unknown how centrioles migrate in cellular contexts in which they do not nucleate MTs. Here, we demonstrate that during interphase, inactive centrioles move directly along the interphase MT network as Kinesin-1 cargo. We identify Pericentrin-Like-Protein (PLP) as a novel Kinesin-1 interacting molecule essential for centriole motility. In vitro assays show that PLP directly interacts with the cargo binding domain of Kinesin-1, allowing PLP to migrate on MTs. Binding assays using purified proteins revealed that relief of Kinesin-1 autoinhibition is critical for its interaction with PLP. Finally, our studies of neural stem cell asymmetric divisions in the Drosophila brain show that the PLP-Kinesin-1 interaction is essential for the timely separation of centrioles, the asymmetry of centrosome activity, and the age-dependent centrosome inheritance.

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.

Nuclear speed and cycle length co-vary with local density during syncytial blastoderm formation in a cricket

The blastoderm is a broadly conserved stage of early animal development, wherein cells form a layer at the embryo’s periphery. The cellular behaviors underlying blastoderm formation are varied and poorly understood. In most insects, the pre-blastoderm embryo is a syncytium: nuclei divide and move throughout the shared cytoplasm, ultimately reaching the cortex. In Drosophila melanogaster, some early nuclear movements result from pulsed cytoplasmic flows that are coupled to synchronous divisions. Here, we show that the cricket Gryllus bimaculatus has a different solution to the problem of creating a blastoderm. We quantified nuclear dynamics during blastoderm formation in G. bimaculatus embryos, finding that: (1) cytoplasmic flows are unimportant for nuclear movement, and (2) division cycles, nuclear speeds, and the directions of nuclear movement are not synchronized, instead being heterogeneous in space and time. Moreover, nuclear divisions and movements co-vary with local nuclear density. We show that several previously proposed models for nuclear movements in D. melanogaster cannot explain the dynamics of G. bimaculatus nuclei. We introduce a geometric model based on asymmetric pulling forces on nuclei, which recapitulates the patterns of nuclear speeds and orientations of both unperturbed G. bimaculatus embryos, and of embryos physically manipulated to have atypical nuclear densities.

Early in insect embryo development, many nuclei share one large cell, travel varied paths and self-organize into a single layer. Donoughe et al. illuminate this process with live-imaging, modeling, and experimental changes to the embryo’s shape.

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.

Regulation of microtubule dynamics, mechanics and function through the growing tip

Microtubule dynamics and their control are essential for the normal function and division of all eukaryotic cells. This plethora of functions is, in large part, supported by dynamic microtubule tips, which can bind to various intracellular targets, generate mechanical forces and couple with actin microfilaments. Here, we review progress in the understanding of microtubule assembly and dynamics, focusing on new information about the structure of microtubule tips. First, we discuss evidence for the widely accepted GTP cap model of microtubule dynamics. Next, we address microtubule dynamic instability in the context of structural information about assembly intermediates at microtubule tips. Three currently discussed models of microtubule assembly and dynamics are reviewed. These are considered in the context of established facts and recent data, which suggest that some long-held views must be re-evaluated. Finally, we review structural observations about the tips of microtubules in cells and describe their implications for understanding the mechanisms of microtubule regulation by associated proteins, by mechanical forces and by microtubule-targeting drugs, prominently including cancer chemotherapeutics.

Modeling reveals cortical dynein-dependent fluctuations in bipolar spindle length

Proper formation and maintenance of the mitotic spindle is required for faithful cell division. Although much work has been done to understand the roles of the key molecular components of the mitotic spindle, identifying the consequences of force perturbations in the spindle remains a challenge. We develop a computational framework accounting for the minimal force requirements of mitotic progression. To reflect early spindle formation, we model microtubule dynamics and interactions with major force-generating motors, excluding chromosome interactions that dominate later in mitosis. We directly integrate our experimental data to define and validate the model. We then use simulations to analyze individual force components over time and their relationship to spindle dynamics, making it distinct from previously published models. We show through both model predictions and biological manipulation that rather than achieving and maintaining a constant bipolar spindle length, fluctuations in pole-to-pole distance occur that coincide with microtubule binding and force generation by cortical dynein. Our model further predicts that high dynein activity is required for spindle bipolarity when kinesin-14 (HSET) activity is also high. To the best of our knowledge, our results provide novel insight into the role of cortical dynein in the regulation of spindle bipolarity.

14-3-3γ prevents centrosome duplication by inhibiting NPM1 function

14-3-3 proteins bind to ligands via phospho-serine containing consensus motifs. However, the molecular mechanisms underlying complex formation and dissociation between 14-3-3 proteins and their ligands remain unclear. We identified two conserved acidic residues in the 14-3-3 peptide-binding pocket (D129 and E136) that potentially regulate complex formation and dissociation. Altering these residues to alanine led to opposing effects on centrosome duplication. D129A inhibited centrosome duplication, whereas E136A stimulated centrosome amplification. These results were due to the differing abilities of these mutant proteins to form a complex with NPM1. Inhibiting complex formation between NPM1 and 14-3-3γ led to an increase in centrosome duplication and over-rode the ability of D129A to inhibit centrosome duplication. We identify a novel role of 14-3-3γ in regulating centrosome licensing and a novel mechanism underlying the formation and dissociation of 14-3-3 ligand complexes dictated by conserved residues in the 14-3-3 family.

Microtubule motors in centrosome homeostasis: A target for cancer therapy?

Cancer is a grievous concern to human health, owing to a massive heterogeneity in its cause and impact. Dysregulation (numerical, positional and/or structural) of centrosomes is one of the notable factors among those that promote onset and progression of cancers. In a normal dividing cell, a pair of centrosomes forms two poles, thereby governing the formation of a bipolar spindle assembly. A large number of cancer cells, however, harbor supernumerary centrosomes, which mimic the bipolar arrangement in normal cells by centrosome clustering (CC) into two opposite poles, thus developing a pseudo-bipolar spindle assembly. Manipulation of centrosome homeostasis is the paramount pre-requisite for the evasive strategy of CC in cancers. Out of the varied factors that uphold centrosome integrity, microtubule motors (MiMos) play a critical role. Categorized as dyneins and kinesins, MiMos are involved in cohesion of centrosomes, and also facilitate the maintenance of the numerical, positional and structural integrity of centrosomes. Herein, we elucidate the decisive mechanisms undertaken by MiMos to mediate centrosome homeostasis, and how dysregulation of the same might lead to CC in cancer cells. Understanding the impact of MiMos on CC might open up avenues toward a credible therapeutic target against diverse cancers.

Principal Postulates of Centrosomal Biology. Version 2020

The centrosome, which consists of two centrioles surrounded by pericentriolar material, is a unique structure that has retained its main features in organisms of various taxonomic groups from unicellular algae to mammals over one billion years of evolution. In addition to the most noticeable function of organizing the microtubule system in mitosis and interphase, the centrosome performs many other cell functions. In particular, centrioles are the basis for the formation of sensitive primary cilia and motile cilia and flagella. Another principal function of centrosomes is the concentration in one place of regulatory proteins responsible for the cell's progression along the cell cycle. Despite the existing exceptions, the functioning of the centrosome is subject to general principles, which are discussed in this review.

Paxillin family of focal adhesion adaptor proteins and regulation of cancer cell invasion

The paxillin family of proteins, including paxillin, Hic-5, and leupaxin, are focal adhesion adaptor/scaffolding proteins which localize to cell-matrix adhesions and are important in cell adhesion and migration of both normal and cancer cells. Historically, the role of these proteins in regulating the actin cytoskeleton through focal adhesion-mediated signaling has been well documented. However, studies in recent years have revealed additional functions in modulating the microtubule and intermediate filament cytoskeletons to affect diverse processes including cell polarization, vesicle trafficking and mechanosignaling. Expression of paxillin family proteins in stromal cells is also important in regulating tumor cell migration and invasion through non-cell autonomous effects on the extracellular matrix. Both paxillin and Hic-5 can also influence gene expression through a variety of mechanisms, while their own expression is frequently dysregulated in various cancers. Accordingly, these proteins may serve as valuable targets for novel diagnostic and treatment approaches in cancer.

Impairment in dynein-mediated nuclear translocation by BICD2 C-terminal truncation leads to neuronal migration defect and human brain malformation

During brain development, the nucleus of migrating neurons follows the centrosome and translocates into the leading process. Defects in these migratory events, which affect neuronal migration, cause lissencephaly and other neurodevelopmental disorders. However, the mechanism of nuclear translocation remains elusive. Using whole exome sequencing (WES), we identified a novel nonsense BICD2 variant p.(Lys775Ter) (K775X) from a lissencephaly patient. Interestingly, most BICD2 missense variants have been associated with human spinal muscular atrophy (SMA) without obvious brain malformations. By in utero electroporation, we showed that BicD2 knockdown in mouse embryos inhibited neuronal migration. Surprisingly, we observed severe blockage of neuronal migration in cells overexpressing K775X but not in those expressing wild-type BicD2 or SMA-associated missense variants. The centrosome of the mutant was, on average, positioned farther away from the nucleus, indicating a failure in nuclear translocation without affecting the centrosome movement. Furthermore, BicD2 localized at the nuclear envelope (NE) through its interaction with NE protein Nesprin-2. K775X variant disrupted this interaction and further interrupted the NE recruitment of BicD2 and dynein. Remarkably, fusion of BicD2-K775X with NE-localizing domain KASH resumed neuronal migration. Our results underscore impaired nuclear translocation during neuronal migration as an important pathomechanism of lissencephaly.

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.

Nek2-mediated GAS2L1 phosphorylation and centrosome-linker disassembly induce centrosome disjunction

Centrosome disjunction occurs in late G2 to facilitate bipolar spindle formation and is mediated by the NIMA-related kinase Nek2. Here, we show that GAS2L1, a microtubule- and F-actin-binding protein required for centrosome disjunction, undergoes Nek2-mediated phosphorylation at Ser352 in G2/M. The phosphorylation is essential for centrosome disjunction in late G2 and for proper spindle assembly and faithful chromosome segregation in mitosis. GAS2L1 contains a calponin-homology (CH) domain and a GAS2-related (GAR) domain, which bind to F-actin and microtubules, respectively. Notably, the CH and GAR domains bind to each other to inhibit the functions of both domains, and Ser352 phosphorylation disrupts the interaction between the two domains and relieves the autoinhibition. We dissected the roles of the GAS2L1 phosphorylation and of centrosome-linker disassembly, which is another Nek2-mediated event, and found that these events together trigger centrosome disjunction. Therefore, our findings demonstrate the concerted Nek2 actions that split the centrosomes in late G2.

FAK regulates dynein localisation and cell polarity in migrating mouse fibroblasts

BACKGROUND

Fibroblasts executing directional migration position their centrosome, and their Golgi apparatus, in front of the nucleus towards the cell leading edge. Centrosome positioning relative to the nucleus has been associated to mechanical forces exerted on the centrosome by the microtubule-dependent molecular motor cytoplasmic dynein 1, and to nuclear movements such as rearward displacement and rotation events. Dynein has been proposed to regulate the position of the centrosome by exerting pulling forces on microtubules from the cell leading edge, where the motor is enriched during migration. However, the mechanism explaining how dynein acts at the front of the cells has not been elucidated.

RESULTS

We present here results showing that the protein Focal Adhesion Kinase (FAK) interacts with dynein and regulates the enrichment of the dynein/dynactin complex at focal adhesions at the cell the leading edge of migrating fibroblasts. This suggests that focal adhesions provide anchoring sites for dynein during the polarisation process. In support of this, we present evidence indicating that the interaction between FAK and dynein, which is regulated by the phosphorylation of FAK on its Ser732 residue, is required for proper centrosome positioning. Our results further show that the polarisation of the centrosome can occur independently of nuclear movements. Although FAK regulates both nuclear and centrosome motilities, downregulating the interaction between FAK and dynein affects only the nuclear independent polarisation of the centrosome.

CONCLUSIONS

Our work highlights the role of FAK as a key player in the regulation of several aspects of cell polarity. We thus propose a model in which the transient localisation of dynein with focal adhesions provides a tuneable mechanism to bias dynein traction forces on microtubules allowing proper centrosome positioning in front of the nucleus.

SIGNIFICANCE

We unravel here a new role for the cancer therapeutic target FAK in the regulation of cell morphogenesis.

Rho-family G-proteins are required for the recovery of traumatized hair bundle mechanoreceptors in the sea anemone, Nematostella vectensis

Immersing anemones in calcium-free seawater disorganizes hair bundle mechanoreceptors on tentacles of sea anemones while causing a loss of vibration sensitivity. Remarkably, anemone hair bundles recover after being returned to calcium-containing seawater. Reorganization of actin in stereocilia likely follows during the recovery of normal morphology of hair bundles after such immersion. Previous studies have reported that Rho G-proteins are located in the stereocilia of hair bundles in sea anemones where they participate in polymerizing actin in stereocilia upon activation of specific chemoreceptors. We here find that immersing anemones in calcium-free seawater significantly reduces the abundance of hair bundles. A partial recovery of abundance of hair bundles occurs within 3 h post-immersion, but a full recovery of abundance does not occur even 6 h after specimens are returned to calcium-containing seawater. Anemones recovering from immersion in calcium-free seawater feature hair bundles that are significantly wider at their tips than in controls. The hair bundles subsequently narrow at their tips, becoming comparable to those of untreated controls within 6 h. Stereocilia of hair bundles are significantly longer in experimental animals than in controls at 2 h of recovery before shortening to lengths comparable to untreated controls at 6 h. In the presence of Rho inhibitors, the recovery in abundance of hair bundles through 6 h is delayed or inhibited. Likewise, in the presence of Rho inhibitors, stereocilia fail to significantly elongate within 2 h of recovery. These data suggest that Rho G-proteins participate in the normal recovery of abundance and recovery of normal morphology of experimentally damaged hair bundle mechanoreceptors.

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.

Apical cell protrusions cause vertical deformation of the soft cancer nucleus

Breast cancer nuclei have highly irregular shapes, which are diagnostic and prognostic markers of breast cancer progression. The mechanisms by which irregular cancer nuclear shapes develop are not well understood. Here we report the existence of vertical, apical cell protrusions in cultured MDA-MB-231 breast cancer cells. Once formed, these protrusions persist over time scales of hours and are associated with vertically upward nuclear deformations. They are absent in normal mammary epithelial cells (MCF-10A cells). Microtubule disruption enriched these protrusions preferentially in MDA-MB-231 cells compared with MCF-10A cells, whereas inhibition of nonmuscle myosin II (NMMII) abolished this enrichment. Dynamic confocal imaging of the vertical cell and nuclear shape revealed that the apical cell protrusions form first, and in response, the nucleus deforms and/or subsequently gets vertically extruded into the apical protrusion. Overexpression of lamin A/C in MDA-MB-231 cells reduced nuclear deformation in apical protrusions. These data highlight the role of mechanical stresses generated by moving boundaries, as well as abnormal nuclear mechanics in the development of abnormal nuclear shapes in breast cancer cells.

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.

Positioning of the Centrosome and Golgi Complex

For over a century, the centrosome has been an organelle more easily tracked than understood, and the study of its peregrinations within the cell remains a chief underpinning of its functional investigation. Increasing attention and new approaches have been brought to bear on mechanisms that control centrosome localization in the context of cleavage plane determination, ciliogenesis, directional migration, and immunological synapse formation, among other cellular and developmental processes. The Golgi complex, often linked with the centrosome, presents a contrasting case of a pleiomorphic organelle for which functional studies advanced somewhat more rapidly than positional tracking. However, Golgi orientation and distribution has emerged as an area of considerable interest with respect to polarized cellular function. This chapter will review our current understanding of the mechanism and significance of the positioning of these organelles.

The Gαq/11-provoked induction of Akr1c18 in murine luteal cells is mediated by phospholipase C

Towards the end of gestation prostaglandin F2α (PGF2α) stimulates the expression of Akr1c18 in the murine corpus luteum. Akr1c18 codes for 20α-hydroxysteroid dehydrogenase, an enzyme that precipitates parturition by catabolizing progesterone. Previous results from our laboratory have shown that this effect of PGF2α is mediated by the activation of Gα_q/11, but the downstream effector(s) of Gα_q/11 that elicit the increase in Akr1c18 expression have not been identified. The physiological effects of Gα_q/11 are mediated by its ability to interact with phospholipase Cβ, p63RhoGEF, and PKCζ. In the experiments described herein we used biochemical and pharmacological approaches, as well as adenoviral-mediated expression of a constitutively active form of Gα_q and mutants thereof, to examine the role of each of these effectors as potential mediators of the increased expression of luteal Akr1c18. By measuring the effects of PGF2α on the activation of RhoA (activated by p63RhoGEF) and the effects of activators and inhibitors of RhoA on the PGF2α-induced expression of luteal Akr1c18, we determined that RhoA is neither activated by PGF2α or involved in the PGF2α-induced expression of luteal Akr1c18. The potential involvement of PKCζ was ruled out by the inability of a mutant of a constitutively active Gα_q that prevents PKCζ binding to block the increased expression of Akr1c18. Furthermore, PGF2α does not increase the phosphorylation of ERK-5, the only known downstream target of PKCζ. On the other hand, three different mutants of a constitutively active Gα_q that prevent phospholipase C activation blocked the induction of luteal Akr1c18. We conclude that the induction of luteal Akr1c18 by Gα_q/11 is mediated by the activation of phospholipase C.

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.

VPS4 is a dynamic component of the centrosome that regulates centrosome localization of γ-tubulin, centriolar satellite stability and ciliogenesis

The hexameric AAA ATPase VPS4 facilitates ESCRT III filament disassembly on diverse intracellular membranes. ESCRT III components and VPS4 have been localized to the ciliary transition zone and spindle poles and reported to affect centrosome duplication and spindle pole stability. How the canonical ESCRT pathway could mediate these events is unclear. We studied the association of VPS4 with centrosomes and found that GFP-VPS4 was a dynamic component of both mother and daughter centrioles. A mutant, VPS4^EQ, which can’t hydrolyze ATP, was less dynamic and accumulated at centrosomes. Centrosome localization of the VPS4^EQ mutant, caused reduced γ-tubulin levels at centrosomes and consequently decreased microtubule growth and altered centrosome positioning. In addition, preventing VPS4 ATP hydrolysis nearly eliminated centriolar satellites and paused ciliogensis after formation of the ciliary vesicle. Zebrafish embryos injected with GFP-VPS4^EQ mRNA were less viable, exhibited developmental defects and had fewer cilia in Kupffer’s vesicle. Surprisingly, ESCRT III proteins seldom localized to centrosomes and their depletion did not lead to these phenotypes. Our data support an ESCRT III-independent function for VPS4 at the centrosome and reveal that this evolutionary conserved AAA ATPase influences diverse centrosome functions and, as a result, global cellular architecture and development.

Mitotic Regulation by NEK Kinase Networks

Genetic studies in yeast and Drosophila led to identification of cyclin-dependent kinases (CDKs), Polo-like kinases (PLKs) and Aurora kinases as essential regulators of mitosis. These enzymes have since been found in the majority of eukaryotes and their cell cycle-related functions characterized in great detail. However, genetic studies in another fungal species, Aspergillus nidulans, identified a distinct family of protein kinases, the NEKs, that are also widely conserved and have key roles in the cell cycle, but which remain less well studied. Nevertheless, it is now clear that multiple NEK family members act in networks to regulate specific events of mitosis, including centrosome separation, spindle assembly and cytokinesis. Here, we describe our current understanding of how the NEK kinases contribute to these processes, particularly through targeted phosphorylation of proteins associated with the microtubule cytoskeleton. We also present the latest findings on molecular events that control the activation state of the NEKs and how these are revealing novel modes of enzymatic regulation relevant not only to other kinases but also to pathological mechanisms of disease.

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.

Motility of Microtubules on the Inner Surface of Water-in-Oil Emulsion Droplets

Water-in-oil emulsion systems have recently attracted much attention in various fields. However, functionalization of water-in-oil emulsion systems, which is required for expanding their applications in industries and research, has been challenging. We now demonstrate the functionalization of a water-in-oil emulsion system by anchoring a target protein molecule. A microtubule (MT)-associated motor protein kinesin-1 was successfully anchored to the inner surface of water-in-oil emulsion droplets by employing the specific interaction of nickel-nitrilotriacetic acid-histidine tag. The MTs exhibited a gliding motion on the kinesin-functionalized inner surface of the emulsion droplets, which confirmed the success of the functionalization of the water-in-oil emulsion system. This result would be beneficial in exploring the roles of biomolecular motor systems in the cellular events that take place at the cell membrane and might also contribute to expanding the nanotechnological applications of biomolecular motors and water-in-oil emulsion systems in the future.

Paxillin regulates cell polarization and anterograde vesicle trafficking during cell migration

Cell polarization and directed migration play pivotal roles in diverse physiological and pathological processes. Herein, we identify new roles for paxillin-mediated HDAC6 inhibition in regulating key aspects of cell polarization in both two-dimensional and one-dimensional matrix environments. Paxillin, by modulating microtubule acetylation through HDAC6 regulation, was shown to control centrosome and Golgi reorientation toward the leading edge, a hallmark of cell polarization to ensure directed trafficking of promigratory factors. Paxillin was also required for pericentrosomal Golgi localization and centrosome cohesion, independent of its localization to, and role in, focal adhesion signaling. In addition, we provide evidence of an accumulation of paxillin at the centrosome that is dependent on focal adhesion kinase (FAK) and identify an important collaboration between paxillin and FAK signaling in the modulation of microtubule acetylation, as well as centrosome and Golgi organization and polarization. Finally, paxillin was also shown to be required for optimal anterograde vesicular trafficking to the plasma membrane.

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.

Centrifugal Displacement of Nuclei Reveals Multiple LINC Complex Mechanisms for Homeostatic Nuclear Positioning

Nuclear movement is critical for developmental events, cell polarity, and migration and is usually mediated by linker of nucleoskeleton and cytoskeleton (LINC) complexes connecting the nucleus to cytoskeletal elements. Compared to active nuclear movement, relatively little is known about homeostatic positioning of nuclei, including whether it is an active process. To explore homeostatic nuclear positioning, we developed a method to displace nuclei in adherent cells using centrifugal force. Nuclei displaced by centrifugation rapidly recentered by mechanisms that depended on cell context. In cell monolayers with wounds oriented orthogonal to the force, nuclei were displaced toward the front and back of the cells on the two sides of the wound. Nuclei recentered from both positions, but at different rates and with different cytoskeletal linkage mechanisms. Rearward recentering was actomyosin, nesprin-2G, and SUN2 dependent, whereas forward recentering was microtubule, dynein, nesprin-2G, and SUN1 dependent. Nesprin-2G engaged actin through its N terminus and microtubules through a novel dynein interacting site near its C terminus. Both activities were necessary to maintain nuclear position in uncentrifuged cells. Thus, even when not moving, nuclei are actively maintained in position by engaging the cytoskeleton through the LINC complex.

Centrosome defines the rear of cells during mesenchymal migration

Taking advantage of the strong polarity of cells migrating along micropatterned lines, combined with computational modeling and microsurgery, we found that the centrosome must be localized toward the rear of a cell, likely for controlling the distribution of tail formation signals. This discovery clarifies a long-standing controversy in cell biology.

The importance of centrosome in directional cell migration has long been recognized. However, the conventional view that centrosome determines cell’s front, based on its often-observed position in front of the nucleus, has been challenged by contradictory observations. Here we show that centrosome defines the rear instead of the front, using cells plated on micropatterned adhesive strips to facilitate directional migration. We found that centrosome is always located proximal to the future rear before polarity is established through symmetry breaking or reversed as the cell reaches a dead end. In addition, using microsurgery to alter the distance of centrosomes from cells’ ends, we show that centrosomal proximity is predictive of the placement of the rear. Removal of centrosome impairs directional cell migration, whereas the removal of nucleus alone makes no difference in most cells. Computer modeling under the framework of a local-enhancement/global-inhibition mechanism further demonstrates that positioning of rear retraction, mediated by signals concentrated near the centrosome, recapitulates all the experimental observations. Our results resolve a long-standing controversy and explain how cells use centrosome and microtubules to maintain directional migration.

Understanding force-generating microtubule systems through in vitro reconstitution

Microtubules switch between growing and shrinking states, a feature known as dynamic instability. The biochemical parameters underlying dynamic instability are modulated by a wide variety of microtubule-associated proteins that enable the strict control of microtubule dynamics in cells. The forces generated by controlled growth and shrinkage of microtubules drive a large range of processes, including organelle positioning, mitotic spindle assembly, and chromosome segregation. In the past decade, our understanding of microtubule dynamics and microtubule force generation has progressed significantly. Here, we review the microtubule-intrinsic process of dynamic instability, the effect of external factors on this process, and how the resulting forces act on various biological systems. Recently, reconstitution-based approaches have strongly benefited from extensive biochemical and biophysical characterization of individual components that are involved in regulating or transmitting microtubule-driven forces. We will focus on the current state of reconstituting increasingly complex biological systems and provide new directions for future developments.

Spatial analysis of Cdc42 activity reveals a role for plasma membrane-associated Cdc42 in centrosome regulation

The ability of the small GTPase Cdc42 to regulate diverse cellular processes depends on tight spatial control of its activity. Cdc42 function is best understood at the plasma membrane (PM), where it regulates cytoskeletal organization and cell polarization. Active Cdc42 has also been detected at the Golgi, but its role and regulation at this organelle are only partially understood. Here we analyze the spatial distribution of Cdc42 activity by moni-toring the dynamics of the Cdc42 FLARE biosensor using the phasor approach to FLIM-FRET. Phasor analysis revealed that Cdc42 is active at all Golgi cisternae and that this activity is controlled by Tuba and ARHGAP10, two Golgi-associated Cdc42 regulators. To our surprise, FGD1, another Cdc42 GEF at the Golgi, was not required for Cdc42 regulation at the Golgi, although its depletion decreased Cdc42 activity at the PM. Similarly, changes in Golgi morphology did not affect Cdc42 activity at the Golgi but were associated with a substantial reduction in PM-associated Cdc42 activity. Of interest, cells with reduced Cdc42 activity at the PM displayed altered centrosome morphology, suggesting that centrosome regulation may be mediated by active Cdc42 at the PM. Our study describes a novel quantitative approach to determine Cdc42 activity at specific subcellular locations and reveals new regulatory principles and functions of this small GTPase.

Geometric Asymmetry Induces Upper Limit of Mitotic Spindle Size

Proper organelle size is critical for many cell functions. However, how cells sense and control their organelle size remains elusive. Here, we develop a general model to study the size control of mitotic spindles by considering both extrinsic and intrinsic factors, such as the limited number of building blocks of the spindle, the interaction between the spindle and cell boundary, the DNA content, the forces generated by various molecular motors, and the dynamics of microtubules. We show that multiple pairs of chromatids, two centrosomes, and microtubules can self-assemble to form a mitotic spindle robustly. We also show that the boundary-sensing and volume-sensing mechanisms coexist in small cells, but both break down in large cells. Strikingly, we find that the upper limit of spindle length naturally arises from the geometric asymmetry of the spindle structure. Thus, our findings reveal, to our knowledge, a novel intrinsic mechanism that limits the organelle size.

Lissencephaly-1 is a context-dependent regulator of the human dynein complex

The cytoplasmic dynein-1 (dynein) motor plays a central role in microtubule organisation and cargo transport. These functions are spatially regulated by association of dynein and its accessory complex dynactin with dynamic microtubule plus ends. Here, we elucidate in vitro the roles of dynactin, end-binding protein-1 (EB1) and Lissencephaly-1 (LIS1) in the interaction of end tracking and minus end-directed human dynein complexes with these sites. LIS1 promotes dynactin-dependent tracking of dynein on both growing and shrinking plus ends. LIS1 also increases the frequency and velocity of processive dynein movements that are activated by complex formation with dynactin and a cargo adaptor. This stimulatory effect of LIS1 contrasts sharply with its documented ability to inhibit the activity of isolated dyneins. Collectively, our findings shed light on how mammalian dynein complexes associate with dynamic microtubules and help clarify how LIS1 promotes the plus-end localisation and cargo transport functions of dynein in vivo.

DOI:http://dx.doi.org/10.7554/eLife.21768.001

Vertebrate Embryonic Cleavage Pattern Determination

The pattern of the earliest cell divisions in a vertebrate embryo lays the groundwork for later developmental events such as gastrulation, organogenesis, and overall body plan establishment. Understanding these early cleavage patterns and the mechanisms that create them is thus crucial for the study of vertebrate development. This chapter describes the early cleavage stages for species representing ray-finned fish, amphibians, birds, reptiles, mammals, and proto-vertebrate ascidians and summarizes current understanding of the mechanisms that govern these patterns. The nearly universal influence of cell shape on orientation and positioning of spindles and cleavage furrows and the mechanisms that mediate this influence are discussed. We discuss in particular models of aster and spindle centering and orientation in large embryonic blastomeres that rely on asymmetric internal pulling forces generated by the cleavage furrow for the previous cell cycle. Also explored are mechanisms that integrate cell division given the limited supply of cellular building blocks in the egg and several-fold changes of cell size during early development, as well as cytoskeletal specializations specific to early blastomeres including processes leading to blastomere cohesion. Finally, we discuss evolutionary conclusions beginning to emerge from the contemporary analysis of the phylogenetic distributions of cleavage patterns. In sum, this chapter seeks to summarize our current understanding of vertebrate early embryonic cleavage patterns and their control and evolution.

Localised dynactin protects growing microtubules to deliver oskar mRNA to the posterior cortex of the Drosophila oocyte

The localisation of oskar mRNA to the posterior of the Drosophila oocyte defines where the abdomen and germ cells form in the embryo. Kinesin 1 transports oskar mRNA to the oocyte posterior along a polarised microtubule cytoskeleton that grows from non-centrosomal microtubule organising centres (ncMTOCs) along the anterior/lateral cortex. Here, we show that the formation of this polarised microtubule network also requires the posterior regulation of microtubule growth. A missense mutation in the dynactin Arp1 subunit causes most oskar mRNA to localise in the posterior cytoplasm rather than cortically. oskar mRNA transport and anchoring are normal in this mutant, but the microtubules fail to reach the posterior pole. Thus, dynactin acts as an anti-catastrophe factor that extends microtubule growth posteriorly. Kinesin 1 transports dynactin to the oocyte posterior, creating a positive feedback loop that increases the length and persistence of the posterior microtubules that deliver oskar mRNA to the cortex.

GAS2L1 Is a Centriole-Associated Protein Required for Centrosome Dynamics and Disjunction

Mitotic spindle formation and chromosome segregation require timely separation of the two duplicated centrosomes, and this process is initiated in late G2 by centrosome disjunction. Here we report that GAS2L1, a microtubule- and actin-binding protein, associates with the proximal end of mature centrioles and participates in centriole dynamics and centrosome disjunction. GAS2L1 attaches microtubules and actin to centrosomes, and the loss of GAS2L1 inhibits centrosome disjunction in G2 and centrosome splitting induced by depletion of the centrosome linker rootletin. Conversely, GAS2L1 overexpression induces premature centrosome separation, and this activity requires GAS2L1 association with actin, microtubules, and the microtubule end-binding proteins. The centrosome-splitting effect of GAS2L1 is counterbalanced by rootletin, reflecting the opposing actions of GAS2L1 and the centrosome linker. Our work reveals a GAS2L1-mediated centriole-tethering mechanism of microtubules and actin, which provide the forces required for centrosome dynamics and separation.

Intracellular Concentration Gradients That Mirror External Gradients in Microfluidic Flows: A Computational Analysis

The generation of stable intracellular concentration gradients is a useful method for local control of cell function, selective manipulation of cellular structures and testing hypotheses related to dynamical intracellular processes. Cell culture in a microfluidic device allows the presentation of a stable gradient of small molecules across a single cell. This method has been used to selectively label mitochondria in portions of the cell, trypsinize specific cellular domains, and trigger receptor-mediated endocytosis in specific portions of the cell. Given the small length scales of a typical cell (~30 μm) and short cytoplasmic diffusive time scales of small molecules, it is surprising that cells can be labeled locally with this method. Here we developed models to explore the parametric space over which stable intracellular concentration gradients can be maintained in a microfluidic device. We show that gradients can develop and be maintained indefinitely for high rates of mass transfer across the membrane compared with diffusion, that is, for Sherwood number greater than 1. We show how these gradients can result in gradients in ligand-receptor binding and enzyme substrate binding. This analysis can help interpret and design microfluidic experiments for cytoplasmic partitioning.

Spatial Control of Primary Ciliogenesis by Subdistal Appendages Alters Sensation-Associated Properties of Cilia

Vertebrate cells can initiate ciliogenesis from centrioles at the cell center, near the Golgi, forming primary cilia confined or submerged in a deep narrow pit created by membrane invagination. How or why cells maintain submerged cilia is unclear. Here, by characterizing centriole subdistal appendages (sDAP) in cells exclusively growing submerged cilia, we found that a group of sDAP components localize to the centriole proximal end through the cohesion factor C-Nap1 and that sDAP function redundantly with C-Nap1 for submerged cilia maintenance. Loss of sDAP and C-Nap1 has no effect on cilia assembly, but it disrupts stable Golgi-cilia association and allows normally submerged cilia to fully surface, losing the deep membrane invagination. Intriguingly, unlike submerged cilia (stationary), surfaced cilia actively respond to mechanical stimuli with motions and can ectopically recruit Hedgehog signaling components in the absence of agonist. We propose that spatial control of ciliogenesis uncouples or specifies sensory properties of cilia.

LFA-1 Engagement Triggers T Cell Polarization at the HIV-1 Virological Synapse

HIV-1 efficiently disseminates by cell-cell spread at intercellular contacts called virological synapses (VS), where the virus preferentially assembles and buds. Cell-cell contact triggers active polarization of organelles and viral proteins within infected cells to the contact site to support efficient VS formation and HIV-1 spread; critically, however, which cell surface protein triggers contact-induced polarization at the VS remains unclear. Additionally, the mechanism by which the HIV-1 envelope glycoprotein (Env) is recruited to the VS remains ill defined. Here, we use a reductionist bead-coupled antibody assay as a model of the VS and show that cross-linking the integrin LFA-1 alone is sufficient to induce active T cell polarization and recruitment of the microtubule organizing center (MTOC) in HIV-1-infected cells. Mutant cell lines coupled with inhibitors demonstrated that LFA-1-induced polarization was dependent on the T cell kinase ZAP70. Notably, immunofluorescent staining of viral proteins revealed an accumulation of surface Env at sites of LFA-1 engagement, with intracellular Env localized to a Golgi compartment proximal to the polarized MTOC. Furthermore, blocking LFA-1-induced MTOC polarization through ZAP70 inhibition prevented intracellular Env polarization. Taken together, these data reveal that LFA-1 is a key determinant in inducing dynamic T cell remodeling to the VS and suggest a model in which LFA-1 engagement triggers active polarization of the MTOC and the associated Env-containing secretory apparatus to sites of cell-cell contact to support polarized viral assembly and egress for efficient cell-cell spread.

IMPORTANCE

HIV-1 causes AIDS by spreading within immune cells and depletion of CD4 T lymphocytes. Rapid spread between these cells occurs by highly efficient cell-cell transmission that takes place at virological synapses (VS). VS are characterized by striking T cell remodeling that is spatially associated with polarized virus assembly and budding at sites of cell contact. Here, we show that the integrin LFA-1 triggers organelle polarization and viral protein recruitment, facilitating formation of the VS, and that this requires the T cell kinase ZAP70. Taken together, these data suggest a mechanism by which HIV-1-infected T cells sense and respond to cell contact to polarize viral egress and promote cell-cell spread. Understanding how cell-cell spread is regulated may help reveal therapeutic targets to specifically block this mode of HIV-1 dissemination.