Cytokinesis through biochemical-mechanical feedback loops

Commitment to cytokinetic furrowing requires the coordinate activity of microtubules and Plk1

At anaphase, spindle microtubules (MTs) position the cleavage furrow and trigger actomyosin assembly by localizing the small GTPase RhoA and the scaffolding protein anillin to a narrow band along the equatorial cortex [1-6]. Using vertebrate somatic cells we examined the temporal control of furrow assembly. Although its positioning commences at anaphase onset, furrow maturation is not complete until ∼10-11 min later. The maintenance of the RhoA/anillin scaffold initially requires continuous signaling from the spindle; loss of either MTs or polo-like kinase 1 (Plk1) activity prevents proper RhoA/anillin localization to the equator, thereby disrupting furrowing. However, we find that at ∼6 min post-anaphase, the cortex becomes "committed to furrowing"; loss of either MTs or Plk1 after this stage does not prevent eventual furrowing, even though at this point the contractile apparatus has not fully matured. Also at this stage, the RhoA/anillin scaffold at the equator becomes permanent. Surprisingly, concurrent loss of both MTs and Plk1 activity following the "commitment to furrowing" stage results in persistent, asymmetric "half-furrows", with only one cortical hemisphere retaining RhoA/anillin, and undergoing ingression. This phenotype is reminiscent of asymmetric furrows caused by a physical block between spindle and cortex [7-9], or by acentric spindle positioning [10-12]. The formation of these persistent "half-furrows" suggests a potential feedback mechanism between the spindle and the cortex that maintains cortical competency along the presumptive equatorial region prior to the "commitment to furrowing" stage of cytokinesis, thereby ensuring the eventual ingression of a symmetric cleavage furrow.

Precise Tuning of Cortical Contractility Regulates Cell Shape during Cytokinesis

The mechanical properties of the actin cortex regulate shape changes during cell division, cell migration, and tissue morphogenesis. We show that modulation of myosin II (MII) filament composition allows tuning of surface tension at the cortex to maintain cell shape during cytokinesis. Our results reveal that MIIA generates cortex tension, while MIIB acts as a stabilizing motor and its inclusion in MII hetero-filaments reduces cortex tension. Tension generation by MIIA drives faster cleavage furrow ingression and bleb formation. We also show distinct roles for the motor and tail domains of MIIB in maintaining cytokinetic fidelity. Maintenance of cortical stability by the motor domain of MIIB safeguards against shape instability-induced chromosome missegregation, while its tail domain mediates cortical localization at the terminal stages of cytokinesis to mediate cell abscission. Because most non-muscle contractile systems are cortical, this tuning mechanism will likely be applicable to numerous processes driven by myosin-II contractility.

Network Contractility During Cytokinesis-from Molecular to Global Views

Cytokinesis is the last stage of cell division, which partitions the mother cell into two daughter cells. It requires the assembly and constriction of a contractile ring that consists of a filamentous contractile network of actin and myosin. Network contractility depends on network architecture, level of connectivity and myosin motor activity, but how exactly is the contractile ring network organized or interconnected and how much it depends on motor activity remains unclear. Moreover, the contractile ring is not an isolated entity; rather, it is integrated into the surrounding cortex. Therefore, the mechanical properties of the cell cortex and cortical behaviors are expected to impact contractile ring functioning. Due to the complexity of the process, experimental approaches have been coupled to theoretical modeling in order to advance its global understanding. While earlier coarse-grained descriptions attempted to provide an integrated view of the process, recent models have mostly focused on understanding the behavior of an isolated contractile ring. Here we provide an overview of the organization and dynamics of the actomyosin network during cytokinesis and discuss existing theoretical models in light of cortical behaviors and experimental evidence from several systems. Our view on what is missing in current models and should be tested in the future is provided.

Towards feedback-controlled nanomedicines for smart, adaptive delivery

IMPACT STATEMENT

The timing and rate of release of pharmaceuticals from advanced drug delivery systems is an important property that has received considerable attention in the scientific literature. Broadly, these mostly fall into two classes: controlled release with a prolonged release rate or triggered release where the drug is rapidly released in response to an environmental stimulus. This review aims to highlight the potential for developing adaptive release systems that more subtlety modulate the drug release profile through continuous communication with its environment facilitated through feedback control. By reviewing the key elements of this approach in one place (fundamental principles of nanomedicine, enzymatic nanoreactors for medical therapies and feedback-controlled chemical systems) and providing additional motivating case studies in the context of chronobiology, we hope to inspire innovative development of novel "chrononanomedicines."

Retrotransposon Domestication and Control in Dictyostelium discoideum

Transposable elements, identified in all eukaryotes, are mobile genetic units that can change their genomic position. Transposons usually employ an excision and reintegration mechanism, by which they change position, but not copy number. In contrast, retrotransposons amplify via RNA intermediates, increasing their genomic copy number. Hence, they represent a particular threat to the structural and informational integrity of the invaded genome. The social amoeba Dictyostelium discoideum, model organism of the evolutionary Amoebozoa supergroup, features a haploid, gene-dense genome that offers limited space for damage-free transposition. Several of its contemporary retrotransposons display intrinsic integration preferences, for example by inserting next to transfer RNA genes or other retroelements. Likely, any retrotransposons that invaded the genome of the amoeba in a non-directed manner were lost during evolution, as this would result in decreased fitness of the organism. Thus, the positional preference of the Dictyostelium retroelements might represent a domestication of the selfish elements. Likewise, the reduced danger of such domesticated transposable elements led to their accumulation, and they represent about 10% of the current genome of D. discoideum. To prevent the uncontrolled spreading of retrotransposons, the amoeba employs control mechanisms including RNA interference and heterochromatization. Here, we review TRE5-A, DIRS-1 and Skipper-1, as representatives of the three retrotransposon classes in D. discoideum, which make up 5.7% of the Dictyostelium genome. We compile open questions with respect to their mobility and cellular regulation, and suggest strategies, how these questions might be addressed experimentally.

Parallel Compression Is a Fast Low-Cost Assay for the High-Throughput Screening of Mechanosensory Cytoskeletal Proteins in Cells

Cellular mechanosensing is critical for many biological processes, including cell differentiation, proliferation, migration, and tissue morphogenesis. The actin cytoskeletal proteins play important roles in cellular mechanosensing. Many techniques have been used to investigate the mechanosensory behaviors of these proteins. However, a fast, low-cost assay for the quantitative characterization of these proteins is still lacking. Here, we demonstrate that compression assay using agarose overlay is suitable for the high throughput screening of mechanosensory proteins in live cells while requiring minimal experimental setup. We used several well-studied myosin II mutants to assess the compression assay. On the basis of elasticity theories, we simulated the mechanosensory accumulation of myosin II's and quantitatively reproduced the experimentally observed protein dynamics. Combining the compression assay with confocal microscopy, we monitored the polarization of myosin II oligomers at the subcellular level. The polarization was dependent on the ratio of the two principal strains of the cellular deformations. Finally, we demonstrated that this technique could be used on the investigation of other mechanosensory proteins.

Myosin efflux promotes cell elongation to coordinate chromosome segregation with cell cleavage

Chromatid segregation must be coordinated with cytokinesis to preserve genomic stability. Here we report that cells clear trailing chromatids from the cleavage site by undergoing two phases of cell elongation. The first phase relies on the assembly of a wide contractile ring. The second phase requires the activity of a pool of myosin that flows from the ring and enriches the nascent daughter cell cortices. This myosin efflux is a novel feature of cytokinesis and its duration is coupled to nuclear envelope reassembly and the nuclear sequestration of the Rho-GEF Pebble. Trailing chromatids induce a delay in nuclear envelope reassembly concomitant with prolonged cortical myosin activity, thus providing forces for the second elongation. We propose that the modulation of cortical myosin dynamics is part of the cellular response triggered by a “chromatid separation checkpoint” that delays nuclear envelope reassembly and, consequently, Pebble nuclear sequestration when trailing chromatids are present at the midzone.

Chromatid segregation must be coordinated with cytokinesis to preserve genomic stability. Here the authors show that cells clear trailing chromatids from the cleavage site in a two-step cell elongation and demonstrate the role of myosin efflux in the second phase.

Remodeling the zonula adherens in response to tension and the role of afadin in this response

During development, epithelial cells must generate and respond to tension without disrupting epithelial barrier function. The authors use superresolution microscopy in MDCK cells to examine how the zonula adherens (ZA) is remodeled in response to elevated contractility while maintain tissue integrity. They define key roles for zonula occludens family proteins in regulating contractility and for the scaffolding protein afadin in maintaining ZA architecture at tricellular junctions.

Morphogenesis requires dynamic coordination between cell–cell adhesion and the cytoskeleton to allow cells to change shape and move without losing tissue integrity. We used genetic tools and superresolution microscopy in a simple model epithelial cell line to define how the molecular architecture of cell–cell zonula adherens (ZA) is modified in response to elevated contractility, and how these cells maintain tissue integrity. We previously found that depleting zonula occludens 1 (ZO-1) family proteins in MDCK cells induces a highly organized contractile actomyosin array at the ZA. We find that ZO knockdown elevates contractility via a Shroom3/Rho-associated, coiled-coil containing protein kinase (ROCK) pathway. Our data suggest that each bicellular border is an independent contractile unit, with actin cables anchored end-on to cadherin complexes at tricellular junctions. Cells respond to elevated contractility by increasing junctional afadin. Although ZO/afadin knockdown did not prevent contractile array assembly, it dramatically altered cell shape and barrier function in response to elevated contractility. We propose that afadin acts as a robust protein scaffold that maintains ZA architecture at tricellular junctions.

Cell shape regulation through mechanosensory feedback control

Cells undergo controlled changes in morphology in response to intracellular and extracellular signals. These changes require a means for sensing and interpreting the signalling cues, for generating the forces that act on the cell's physical material, and a control system to regulate this process. Experiments on Dictyostelium amoebae have shown that force-generating proteins can localize in response to external mechanical perturbations. This mechanosensing, and the ensuing mechanical feedback, plays an important role in minimizing the effect of mechanical disturbances in the course of changes in cell shape, especially during cell division, and likely in other contexts, such as during three-dimensional migration. Owing to the complexity of the feedback system, which couples mechanical and biochemical signals involved in shape regulation, theoretical approaches can guide further investigation by providing insights that are difficult to decipher experimentally. Here, we present a computational model that explains the different mechanosensory and mechanoresponsive behaviours observed in Dictyostelium cells. The model features a multiscale description of myosin II bipolar thick filament assembly that includes cooperative and force-dependent myosin-actin binding, and identifies the feedback mechanisms hidden in the observed mechanoresponsive behaviours of Dictyostelium cells during micropipette aspiration experiments. These feedbacks provide a mechanistic explanation of cellular retraction and hence cell shape regulation.

Myosin II controls cellular branching morphogenesis and migration in three dimensions by minimizing cell-surface curvature

In many cases cell function is intimately linked to cell shape control. We utilized endothelial cell branching morphogenesis as a model to understand the role of myosin-II in shape control of invasive cells migrating in 3D collagen gels. We applied principles of differential geometry and mathematical morphology to 3D image sets to parameterize cell branch structure and local cell surface curvature. We find that Rho/ROCK-stimulated myosin-II contractility minimizes cell-scale branching by recognizing and minimizing local cell surface curvature. Utilizing micro-fabrication to constrain cell shape identifies a positive feedback mechanism in which low curvature stabilizes myosin-II cortical association, where it acts to maintain minimal curvature. The feedback between myosin-II regulation by and control of curvature drives cycles of localized cortical myosin-II assembly and disassembly. These cycles in turn mediate alternating phases of directionally biased branch initiation and retraction to guide 3D cell migration.

Pharmacological activation of myosin II paralogs to correct cell mechanics defects

Current approaches to cancer treatment focus on targeting signal transduction pathways. Here, we develop an alternative system for targeting cell mechanics for the discovery of novel therapeutics. We designed a live-cell, high-throughput chemical screen to identify mechanical modulators. We characterized 4-hydroxyacetophenone (4-HAP), which enhances the cortical localization of the mechanoenzyme myosin II, independent of myosin heavy-chain phosphorylation, thus increasing cellular cortical tension. To shift cell mechanics, 4-HAP requires myosin II, including its full power stroke, specifically activating human myosin IIB (MYH10) and human myosin IIC (MYH14), but not human myosin IIA (MYH9). We further demonstrated that invasive pancreatic cancer cells are more deformable than normal pancreatic ductal epithelial cells, a mechanical profile that was partially corrected with 4-HAP, which also decreased the invasion and migration of these cancer cells. Overall, 4-HAP modifies nonmuscle myosin II-based cell mechanics across phylogeny and disease states and provides proof of concept that cell mechanics offer a rich drug target space, allowing for possible corrective modulation of tumor cell behavior.

14-3-3, an integrator of cell mechanics and cytokinesis

One of the goals of understanding cytokinesis is to uncover the molecular regulation of the cellular mechanical properties that drive cell shape change. Such regulatory pathways are likely to be used at multiple stages of a cell's life, but are highly featured during cell division. Recently, we demonstrated that 14-3-3 (encoded by a single gene in the social amoeba Dictyostelium discoideum) serves to integrate key cytoskeletal components-microtubules, Rac and myosin II-to control cell mechanics and cytokinesis. As 14-3-3 proteins are frequently altered in a variety of human tumors, we extend these observations to suggest possible additional roles for how 14-3-3 proteins may contribute to tumorigenesis.

Genetic suppression of a phosphomimic myosin II identifies system-level factors that promote myosin II cleavage furrow accumulation

How myosin II localizes to the cleavage furrow in Dictyostelium and metazoan cells remains largely unknown despite significant advances in understanding its regulation. We designed a genetic selection using cDNA library suppression of 3xAsp myosin II to identify factors involved in myosin cleavage furrow accumulation. The 3xAsp mutant is deficient in bipolar thick filament assembly, fails to accumulate at the cleavage furrow, cannot rescue myoII-null cytokinesis, and has impaired mechanosensitive accumulation. Eleven genes suppressed this dominant cytokinesis deficiency when 3xAsp was expressed in wild-type cells. 3xAsp myosin II's localization to the cleavage furrow was rescued by constructs encoding rcdBB, mmsdh, RMD1, actin, one novel protein, and a 14-3-3 hairpin. Further characterization showed that RMD1 is required for myosin II cleavage furrow accumulation, acting in parallel with mechanical stress. Analysis of several mutant strains revealed that different thresholds of myosin II activity are required for daughter cell symmetry than for furrow ingression dynamics. Finally, an engineered myosin II with a longer lever arm (2xELC), producing a highly mechanosensitive motor, could also partially suppress the intragenic 3xAsp. Overall, myosin II accumulation is the result of multiple parallel and partially redundant pathways that comprise a cellular contractility control system.

Rap1-dependent pathways coordinate cytokinesis in Dictyostelium

Dictyostelium Rap1 is dynamically activated during cytokinesis and drives cytokinesis progression by coordinating the three major cytoskeletal components: microtubules, actin, and myosin II. Importantly, mutated forms of Rap also affect cytokinesis in other organisms, suggesting a conserved role for Rap in cell division.

Cytokinesis is the final step of mitosis when a mother cell is separated into two daughter cells. Major cytoskeletal changes are essential for cytokinesis; it is, however, not well understood how the microtubules and actomyosin cytoskeleton are exactly regulated in time and space. In this paper, we show that during the early stages of cytokinesis, in rounded-up Dictyostelium discoideum cells, the small G-protein Rap1 is activated uniformly at the cell cortex. When cells begin to elongate, active Rap1 becomes restricted from the furrow region, where the myosin contractile ring is subsequently formed. In the final stages of cytokinesis, active Rap1 is only present at the cell poles. Mutant cells with decreased Rap1 activation at the poles showed strongly decreased growth rates. Hyperactivation of Rap1 results in severe growth delays and defective spindle formation in adherent cells and cell death in suspension. Furthermore, Rap mutants show aberrant regulation of the actomyosin cytoskeleton, resulting in extended furrow ingression times and asymmetrical cell division. We propose that Rap1 drives cytokinesis progression by coordinating the three major cytoskeletal components: microtubules, actin, and myosin II. Importantly, mutated forms of Rap also affect cytokinesis in other organisms, suggesting a conserved role for Rap in cell division.

Monitoring actin cortex thickness in live cells

Animal cell shape is controlled primarily by the actomyosin cortex, a thin cytoskeletal network that lies directly beneath the plasma membrane. The cortex regulates cell morphology by controlling cellular mechanical properties, which are determined by network structure and geometry. In particular, cortex thickness is expected to influence cell mechanics. However, cortex thickness is near the resolution limit of the light microscope, making studies relating cortex thickness and cell shape challenging. To overcome this, we developed an assay to measure cortex thickness in live cells, combining confocal imaging and subresolution image analysis. We labeled the actin cortex and plasma membrane with chromatically different fluorophores and measured the distance between the resulting intensity peaks. Using a theoretical description of cortex geometry and microscopic imaging, we extracted an average cortex thickness of ∼190 nm in mitotic HeLa cells and tested the validity of our assay using cell images generated in silico. We found that thickness increased after experimental treatments preventing F-actin disassembly. Finally, we monitored physiological changes in cortex thickness in real-time during actin cortex regrowth in cellular blebs. Our investigation paves the way to understanding how molecular processes modulate cortex structure, which in turn drives cell morphogenesis.

The model organism Dictyostelium discoideum

Much of our knowledge of molecular cellular functions is based on studies with a few number of model organisms that were established during the last 50 years. The social amoeba Dictyostelium discoideum is one such model, and has been particularly useful for the study of cell motility, chemotaxis, phagocytosis, endocytic vesicle traffic, cell adhesion, pattern formation, caspase-independent cell death, and, more recently, autophagy and social evolution. As nonmammalian model of human diseases D. discoideum is a newcomer, yet it has proven to be a powerful genetic and cellular model for investigating host-pathogen interactions and microbial infections, for mitochondrial diseases, and for pharmacogenetic studies. The D. discoideum genome harbors several homologs of human genes responsible for a variety of diseases, -including Chediak-Higashi syndrome, lissencephaly, mucolipidosis, Huntington disease, IBMPFD, and Shwachman-Diamond syndrome. A few genes have already been studied, providing new insights on the mechanism of action of the encoded proteins and in some cases on the defect underlying the disease. The opportunities offered by the organism and its place among the nonmammalian models for human diseases will be discussed.

Phosphoinositides and cytokinesis: the "PIP" of the iceberg

Phosphoinositides [Phosphatidylinositol (PtdIns), phosphatidylinositol 3-monophosphate (PtdIns3P), phosphatidylinositol 4-monophosphate (PtdIns4P), phosphatidylinositol 5-monophosphate (PtdIns5P), phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P(2) ), phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P(2) ), phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2) ), and phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3) )] are lowly abundant acidic lipids found at the cytosolic leaflet of the plasma membrane and intracellular membranes. Initially discovered as precursors of second messengers in signal transduction, phosphoinositides are now known to directly or indirectly control key cellular functions, such as cell polarity, cell migration, cell survival, cytoskeletal dynamics, and vesicular traffic. Phosphoinositides actually play a central role at the interface between membranes and cytoskeletons and contribute to the identity of the cellular compartments by recruiting specific proteins. Increasing evidence indicates that several phosphoinositides, particularly PtdIns(4,5)P(2) , are essential for cytokinesis, notably after furrow ingression. The present knowledge about the specific phosphoinositides and phosphoinositide modifying-enzymes involved in cytokinesis will be first presented. The review of the current data will then show that furrow stability and cytokinesis abscission require that both phosphoinositide production and hydrolysis are regulated in space and time. Finally, I will further discuss recent mechanistic insights on how phosphoinositides regulate membrane trafficking and cytoskeletal remodeling for successful furrow ingression and intercellular bridge abscission. This will highlight unanticipated connections between cytokinesis and enzymes implicated in human diseases, such as the Lowe syndrome.

α-catenin and IQGAP regulate myosin localization to control epithelial tube morphogenesis in Dictyostelium

Apical actomyosin activity in animal epithelial cells influences tissue morphology and drives morphogenetic movements during development. The molecular mechanisms leading to myosin II accumulation at the apical membrane and its exclusion from other membranes are poorly understood. We show that in the nonmetazoan Dictyostelium discoideum, myosin II localizes apically in tip epithelial cells that surround the stalk, and constriction of this epithelial tube is required for proper morphogenesis. IQGAP1 and its binding partner cortexillin I function downstream of α- and β-catenin to exclude myosin II from the basolateral cortex and promote apical accumulation of myosin II. Deletion of IQGAP1 or cortexillin compromises epithelial morphogenesis without affecting cell polarity. These results reveal that apical localization of myosin II is a conserved morphogenetic mechanism from nonmetazoans to vertebrates and identify a hierarchy of proteins that regulate the polarity and organization of an epithelial tube in a simple model organism.

Cytokinesis in animal cells

Cytokinesis, the final step in cell division, partitions the contents of a single cell into two. In animal cells, cytokinesis occurs through cortical remodeling orchestrated by the anaphase spindle. Cytokinesis relies on a tight interplay between signaling and cellular mechanics and has attracted the attention of both biologists and physicists for more than a century. In this review, we provide an overview of four topics in animal cell cytokinesis: (a) signaling between the anaphase spindle and cortex, (b) the mechanics of cortical remodeling, (c) abscission, and (d) regulation of cytokinesis by the cell cycle machinery. We report on recent progress in these areas and highlight some of the outstanding questions that these findings bring into focus.

Cytokinesis mechanics and mechanosensing

Cytokinesis shape change occurs through the interfacing of three modules, cell mechanics, myosin II-mediated contractile stress generation and sensing, and a control system of regulatory proteins, which together ensure flexibility and robustness. This integrated system then defines the stereotypical shape changes of successful cytokinesis, which occurs under a diversity of mechanical contexts and environmental conditions.

Understanding the cooperative interaction between myosin II and actin cross-linkers mediated by actin filaments during mechanosensation

Myosin II is a central mechanoenzyme in a wide range of cellular morphogenic processes. Its cellular localization is dependent not only on signal transduction pathways, but also on mechanical stress. We suggest that this stress-dependent distribution is the result of both the force-dependent binding to actin filaments and cooperative interactions between bound myosin heads. By assuming that the binding of myosin heads induces and/or stabilizes local conformational changes in the actin filaments that enhances myosin II binding locally, we successfully simulate the cooperative binding of myosin to actin observed experimentally. In addition, we can interpret the cooperative interactions between myosin and actin cross-linking proteins observed in cellular mechanosensation, provided that a similar mechanism operates among different proteins. Finally, we present a model that couples cooperative interactions to the assembly dynamics of myosin bipolar thick filaments and that accounts for the transient behaviors of the myosin II accumulation during mechanosensation. This mechanism is likely to be general for a range of myosin II-dependent cellular mechanosensory processes.

Deconvolution of the cellular force-generating subsystems that govern cytokinesis furrow ingression

Cytokinesis occurs through the coordinated action of several biochemically-mediated stresses acting on the cytoskeleton. Here, we develop a computational model of cellular mechanics, and using a large number of experimentally measured biophysical parameters, we simulate cell division under a number of different scenarios. We demonstrate that traction-mediated protrusive forces or contractile forces due to myosin II are sufficient to initiate furrow ingression. Furthermore, we show that passive forces due to the cell's cortical tension and surface curvature allow the furrow to complete ingression. We compare quantitatively the furrow thinning trajectories obtained from simulation with those observed experimentally in both wild-type and myosin II null Dictyostelium cells. Our simulations highlight the relative contributions of different biomechanical subsystems to cell shape progression during cell division.

Cytokinesis, the physical separation of a mother cell into two daughter cells, requires force to deform the cell. Though there is ample evidence in many systems that myosin II provides some of this force, it is also well known that some cell types can divide in the absence of myosin II. To elucidate the mechanisms by which cells control furrow ingression, we developed a computational model of cellular dynamics during cytokinesis in the social amoeba, Dictyostelium discoideum. We took advantage of a large number of experimentally measured parameters and well-characterized furrow ingression dynamics for a number of different strains. Our simulations demonstrate that there are distinct phases of cytokinesis. Myosin II plays a role providing the stress that initiates furrow ingression. In its absence, however, this force can be supplied by a combination of adhesion and protrusion-mediated stresses. Thereafter, Laplace-like pressures take over and provide stresses that enable the cell to divide. Overall, we show how various mechanical parameters quantitatively impact furrow ingression kinetics, accounting for the cytokinesis dynamics of wild type and mutant cell-lines.

A mechanosensory system governs myosin II accumulation in dividing cells

The mitotic spindle is generally considered the initiator of furrow ingression. However, recent studies suggest that furrows can form without spindles, particularly during asymmetric cell division. In Dictyostelium, the mechanoenzyme myosin II and the actin cross-linker cortexillin I form a mechanosensor that responds to mechanical stress, which could account for spindle-independent contractile protein recruitment. Here we show that the regulatory and contractility network composed of myosin II, cortexillin I, IQGAP2, kinesin-6 (kif12), and inner centromeric protein (INCENP) is a mechanical stress-responsive system. Myosin II and cortexillin I form the core mechanosensor, and mechanotransduction is mediated by IQGAP2 to kif12 and INCENP. In addition, IQGAP2 is antagonized by IQGAP1 to modulate the mechanoresponsiveness of the system, suggesting a possible mechanism for discriminating between mechanical and biochemical inputs. Furthermore, IQGAP2 is important for maintaining spindle morphology and kif12 and myosin II cleavage furrow recruitment. Cortexillin II is not directly involved in myosin II mechanosensitive accumulation, but without cortexillin I, cortexillin II's role in membrane-cortex attachment is revealed. Finally, the mitotic spindle is dispensable for the system. Overall, this mechanosensory system is structured like a control system characterized by mechanochemical feedback loops that regulate myosin II localization at sites of mechanical stress and the cleavage furrow.

Separation anxiety: stress, tension and cytokinesis

Cytokinesis, the physical separation of a mother cell into two daughter cells, progresses through a series of well-defined changes in morphology. These changes involve distinct biochemical and mechanical processes. Here, we review the mechanical features of cells during cytokinesis, discussing both the material properties as well as sources of stresses, both active and passive, which lead to the observed changes in morphology. We also describe a mechanosensory feedback control system that regulates protein localization and shape progression during cytokinesis.

Molecular networks linked by Moesin drive remodeling of the cell cortex during mitosis

During mitosis, cortical Moesin activity is restricted to promote cell elongation and cytokinesis, but localized Moesin recruitment is necessary for polar bleb retraction and cortical relaxation.

The cortical mechanisms that drive the series of mitotic cell shape transformations remain elusive. In this paper, we identify two novel networks that collectively control the dynamic reorganization of the mitotic cortex. We demonstrate that Moesin, an actin/membrane linker, integrates these two networks to synergize the cortical forces that drive mitotic cell shape transformations. We find that the Pp1-87B phosphatase restricts high Moesin activity to early mitosis and down-regulates Moesin at the polar cortex, after anaphase onset. Overactivation of Moesin at the polar cortex impairs cell elongation and thus cytokinesis, whereas a transient recruitment of Moesin is required to retract polar blebs that allow cortical relaxation and dissipation of intracellular pressure. This fine balance of Moesin activity is further adjusted by Skittles and Pten, two enzymes that locally produce phosphoinositol 4,5-bisphosphate and thereby, regulate Moesin cortical association. These complementary pathways provide a spatiotemporal framework to explain how the cell cortex is remodeled throughout cell division.

The spatial and mechanical challenges of female meiosis

Recent work shows that cytokinesis and other cellular morphogenesis events are tuned by an interplay among biochemical signals, cell shape, and cellular mechanics. In cytokinesis, this includes cross-talk between the cortical cytoskeleton and the mitotic spindle in coordination with cell cycle control, resulting in characteristic changes in cellular morphology and mechanics through metaphase and cytokinesis. The changes in cellular mechanics affect not just overall cell shape, but also mitotic spindle morphology and function. This review will address how these principles apply to oocytes undergoing the asymmetric cell divisions of meiosis I and II. The biochemical signals that regulate cell cycle timing during meiotic maturation and egg activation are crucial for temporal control of meiosis. Spatial control of the meiotic divisions is also important, ensuring that the chromosomes are segregated evenly and that meiotic division is clearly asymmetric, yielding two daughter cells - oocyte and polar body - with enormous volume differences. In contrast to mitotic cells, the oocyte does not undergo overt changes in cell shape with its progression through meiosis, but instead maintains a relatively round morphology with the exception of very localized changes at the time of polar body emission. Placement of the metaphase-I and -II spindles at the oocyte periphery is clearly important for normal polar body emission, although this is likely not the only control element. Here, consideration is given to how cellular mechanics could contribute to successful mammalian female meiosis, ultimately affecting egg quality and competence to form a healthy embryo.

Molecular pharmacology in a simple model system: implicating MAP kinase and phosphoinositide signalling in bipolar disorder

Understanding the mechanisms of drug action has been the primary focus for pharmacological researchers, traditionally using rodent models. However, non-sentient model systems are now increasingly being used as an alternative approach to better understand drug action or targets. One of these model systems, the social amoeba Dictyostelium, enables the rapid ablation or over-expression of genes, and the subsequent use of isogenic cell culture for the analysis of cell signalling pathways in pharmacological research. The model also supports an increasingly important ethical view of research, involving the reduction, replacement and refinement of animals in biomedical research. This review outlines the use of Dictyostelium in understanding the pharmacological action of two commonly used bipolar disorder treatments (valproic acid and lithium). Both of these compounds regulate mitogen activated protein (MAP) kinase and inositol phospholipid-based signalling by unknown means. Analysis of the molecular pathways targeted by these drugs in Dictyostelium and translation of discoveries to animal systems has helped to further understand the molecular mechanisms of these bipolar disorder treatments.