Anillin promotes astral microtubule-directed cortical myosin polarization

Dynamic Plasma Membrane Topography Linked With Arp2/3 Actin Network Induction During Cell Shape Change

Recent studies show the importance of mesoscale changes to plasma membrane (PM) topography during cell shape change. Local folding and flattening of the cell surface is mechanosensitive, changing in response to both microenvironment structural elements and intracellular cytoskeletal activities. These topography changes elicit local mechanical signaling events that act in conjunction with molecular signal transduction pathways to remodel the cell cortex. Experimental manipulations of local PM curvature show its sufficiency for recruiting Arp2/3 actin network induction pathways. Additionally, studies of diverse cell shape changes-ranging from neutrophil migration to early Drosophila embryo cleavage to neural stem cell asymmetric division-show that local generation of PM folding is linked with local Arp2/3 actin network induction, which then remodels the PM topography during dynamic control of cell structure. These examples are reviewed in detail, together with known and potential causes of PM topography changes, downstream effects, and higher-order feedback.

Anillin forms linear structures and facilitates furrow ingression after septin and formin depletion

During cytokinesis, a contractile ring consisting of unbranched filamentous actin (F-actin) and myosin II constricts at the cell equator. Unbranched F-actin is generated by formin, and without formin no cleavage furrow forms. In Caenorhabditis elegans, depletion of septin restores furrow ingression in formin mutants. How the cleavage furrow ingresses without a detectable unbranched F-actin ring is unknown. We report that, in this setting, anillin (ANI-1) forms a meshwork of circumferentially aligned linear structures decorated by non-muscle myosin II (NMY-2). Analysis of ANI-1 deletion mutants reveals that its disordered N-terminal half is required for linear structure formation and sufficient for furrow ingression. NMY-2 promotes the circumferential alignment of the linear ANI-1 structures and interacts with various lipids, suggesting that NMY-2 links the ANI-1 network with the plasma membrane. Collectively, our data reveal a compensatory mechanism, mediated by ANI-1 linear structures and membrane-bound NMY-2, that promotes furrowing when unbranched F-actin polymerization is compromised.

Diversity is the spice of life: An overview of how cytokinesis regulation varies with cell type

Cytokinesis is required to physically cleave a cell into two daughters at the end of mitosis. Decades of research have led to a comprehensive understanding of the core cytokinesis machinery and how it is regulated in animal cells, however this knowledge was generated using single cells cultured in vitro, or in early embryos before tissues develop. This raises the question of how cytokinesis is regulated in diverse animal cell types and developmental contexts. Recent studies of distinct cell types in the same organism or in similar cell types from different organisms have revealed striking differences in how cytokinesis is regulated, which includes different threshold requirements for the structural components and the mechanisms that regulate them. In this review, we highlight these differences with an emphasis on pathways that are independent of the mitotic spindle, and operate through signals associated with the cortex, kinetochores, or chromatin.

Cytokinetic diversity in mammalian cells is revealed by the characterization of endogenous anillin, Ect2 and RhoA

Cytokinesis is required to physically separate the daughter cells at the end of mitosis. This crucial process requires the assembly and ingression of an actomyosin ring, which must occur with high fidelity to avoid aneuploidy and cell fate changes. Most of our knowledge of mammalian cytokinesis was generated using over-expressed transgenes in HeLa cells. Over-expression can introduce artefacts, while HeLa are cancerous human cells that have lost their epithelial identity, and the mechanisms controlling cytokinesis in these cells could be vastly different from other cell types. Here, we tagged endogenous anillin, Ect2 and RhoA with mNeonGreen and characterized their localization during cytokinesis for the first time in live human cells. Comparing anillin localization in multiple cell types revealed cytokinetic diversity with differences in the duration and symmetry of ring closure, and the timing of cortical recruitment. Our findings show that the breadth of anillin correlates with the rate of ring closure, and support models where cell size or ploidy affects the cortical organization, and intrinsic mechanisms control the symmetry of ring closure. This work highlights the need to study cytokinesis in more diverse cell types, which will be facilitated by the reagents generated for this study.

Orchestrating nonmuscle myosin II filament assembly at the onset of cytokinesis

Contractile forces in the actomyosin cortex are required for cellular morphogenesis. This includes the invagination of the cell membrane during division, where filaments of nonmuscle myosin II (NMII) are responsible for generating contractile forces in the cortex. However, how NMII heterohexamers form filaments in vivo is not well understood. To quantify NMII filament assembly dynamics, we imaged the cortex of Caenorhabditis elegans embryos at high spatial resolution around the time of the first division. We show that during the assembly of the cytokinetic ring, the number of NMII filaments in the cortex increases and more NMII motors are assembled into each filament. These dynamics are influenced by two proteins in the RhoA GTPase pathway, the RhoA-dependent kinase LET-502 and the myosin phosphatase MEL-11. We find that these two proteins differentially regulate NMII activity at the anterior and at the division site. We show that the coordinated action of these regulators generates a gradient of free NMII in the cytoplasm driving a net diffusive flux of NMII motors toward the cytokinetic ring. Our work highlights how NMII filament assembly and disassembly dynamics are orchestrated over space and time to facilitate the up-regulation of cortical contractility during cytokinesis.

Modulating RhoA effectors induces transitions to oscillatory and more wavelike RhoA dynamics in C. elegans zygotes

Pulsatile RhoA dynamics underlie a wide range of cell and tissue behaviors. The circuits that produce these dynamics in different cells share common architectures based on fast positive and delayed negative feedback through F-actin, but they can produce very different spatiotemporal patterns of RhoA activity. However, the underlying causes of this variation remain poorly understood. Here we asked how this variation could arise through modulation of actin network dynamics downstream of active RhoA in early C. elegans embryos. We find that perturbing two RhoA effectors - formin and anillin - induce transitions from non-recurrent focal pulses to either large noisy oscillatory pulses (formin depletion) or noisy oscillatory waves (anillin depletion). In both cases these transitions could be explained by changes in local F-actin levels and depletion dynamics, leading to changes in spatial and temporal patterns of RhoA inhibition. However, the underlying mechanisms for F-actin depletion are distinct, with different dependencies on myosin II activity. Thus, modulating actomyosin network dynamics could shape the spatiotemporal dynamics of RhoA activity for different physiological or morphogenetic functions. [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text].

Diverse mechanisms regulate contractile ring assembly for cytokinesis in the two-cell Caenorhabditis elegans embryo

Cytokinesis occurs at the end of mitosis as a result of the ingression of a contractile ring that cleaves the daughter cells. The core machinery regulating this crucial process is conserved among metazoans. Multiple pathways control ring assembly, but their contribution in different cell types is not known. We found that in the Caenorhabditis elegans embryo, AB and P1 cells fated to be somatic tissue and germline, respectively, have different cytokinesis kinetics supported by distinct myosin levels and organization. Through perturbation of RhoA or polarity regulators and the generation of tetraploid strains, we found that ring assembly is controlled by multiple fate-dependent factors that include myosin levels, and mechanisms that respond to cell size. Active Ran coordinates ring position with the segregating chromatids in HeLa cells by forming an inverse gradient with importins that control the cortical recruitment of anillin. We found that the Ran pathway regulates anillin in AB cells but functions differently in P1 cells. We propose that ring assembly delays in P1 cells caused by low myosin and Ran signaling coordinate the timing of ring closure with their somatic neighbors. This article has an associated First Person interview with the first author of the paper.

Symmetry-breaking of animal cytokinesis

Cytokinesis is a mechanism that separates dividing cells via constriction of a supramolecular structure, the contractile ring. In animal cells, three modes of symmetry-breaking of cytokinesis result in unilateral cytokinesis, asymmetric cell division, and oriented cell division. Each mode of cytokinesis plays a significant role in tissue patterning and morphogenesis by the mechanisms that control the orientation and position of the contractile ring relative to the body axis. Despite its significance, the mechanisms involved in the symmetry-breaking of cytokinesis remain unclear in many cell types. Classical embryologists have identified that the geometric relationship between the mitotic spindle and cell cortex induces cytokinesis asymmetry; however, emerging evidence suggests that a concerted flow of compressional cell-cortex materials (cortical flow) is a spindle-independent driving force in spatial cytokinesis control. This review provides an overview of both classical and emerging mechanisms of cytokinesis asymmetry and their roles in animal development.

From primordial germ cells to spermatids in Caenorhabditis elegans

Development of a syncytial germline for gamete formation requires complex regulation of cytokinesis and cytoplasmic remodeling. Recently, several uncovered cellular events have been investigated in the Caenorhabditis elegans (C. elegans) germline. In these cellular processes, the factors involved in contractility are highly conserved with those of mitosis and meiosis. However, the underlying regulatory mechanisms are far more complicated than previously thought, likely due to the single syncytial germline structure. In this review, we highlight how the proteins involved in contractility ensure faithful cell division in different cellular contexts and how they contribute to maintaining intercellular bridge stability. In addition, we discuss the current understanding of the cellular events of cytokinesis and cytoplasmic remodeling during the development of the C. elegans germline, including progenitor germ cells, germ cells, and spermatocytes. Comparisons are made with relevant systems in Drosophila melanogaster (D. melanogaster) and other animal models.

Filament-guided filament assembly provides structural memory of filament alignment during cytokinesis

During cytokinesis, animal cells rapidly remodel the equatorial cortex to build an aligned array of actin filaments called the contractile ring. Local reorientation of filaments by active equatorial compression is thought to underlie the emergence of filament alignment during ring assembly. Here, combining single molecule analysis and modeling in one-cell C. elegans embryos, we show that filaments turnover is far too fast for reorientation of individual filaments by equatorial compression to explain the observed alignment, even if favorably oriented filaments are selectively stabilized. By tracking single formin/CYK-1::GFP particles to monitor local filament assembly, we identify a mechanism that we call filament-guided filament assembly (FGFA), in which existing filaments serve as templates to orient the growth of new filaments. FGFA sharply increases the effective lifetime of filament orientation, providing structural memory that allows cells to build highly aligned filament arrays in response to equatorial compression, despite rapid turnover of individual filaments.

Polar relaxation by dynein-mediated removal of cortical myosin II

Nearly six decades ago, Lewis Wolpert proposed the relaxation of the polar cell cortex by the radial arrays of astral microtubules as a mechanism for cleavage furrow induction. While this mechanism has remained controversial, recent work has provided evidence for polar relaxation by astral microtubules, although its molecular mechanisms remain elusive. Here, using C. elegans embryos, we show that polar relaxation is achieved through dynein-mediated removal of myosin II from the polar cortexes. Mutants that position centrosomes closer to the polar cortex accelerated furrow induction, whereas suppression of dynein activity delayed furrowing. We show that dynein-mediated removal of myosin II from the polar cortexes triggers a bidirectional cortical flow toward the cell equator, which induces the assembly of the actomyosin contractile ring. These results provide a molecular mechanism for the aster-dependent polar relaxation, which works in parallel with equatorial stimulation to promote robust cytokinesis.

Importin binding mediates the intramolecular regulation of anillin during cytokinesis

Cytokinesis occurs by the ingression of an actomyosin ring that cleaves a cell into two daughters. This process is tightly controlled to avoid aneuploidy, and we previously showed that active Ran coordinates ring positioning with chromatin. Active Ran is high around chromatin, and forms an inverse gradient to cargo-bound importins. We found that the ring component anillin contains a nuclear localization signal (NLS) that binds to importin and is required for its function during cytokinesis. Here we reveal the mechanism whereby importin binding favors a conformation required for anillin's recruitment to the equatorial cortex. Active RhoA binds to the RhoA-binding domain causing an increase in accessibility of the nearby C2 domain containing the NLS. Importin binding subsequently stabilizes a conformation that favors interactions for cortical recruitment. In addition to revealing a novel mechanism for the importin-mediated regulation of a cortical protein, we also show how importin binding positively regulates protein function.

Complementary functions for the Ran gradient during division

The Ran pathway has a well-described function in nucleocytoplasmic transport, where active Ran dissociates importin/karyopherin-bound cargo containing a nuclear localization signal (NLS) in the nucleus. As cells enter mitosis, the nuclear envelope breaks down and a gradient of active Ran forms where levels are highest near chromatin. This gradient plays a crucial role in regulating mitotic spindle assembly, where active Ran binds to and releases importins from NLS-containing spindle assembly factors. An emerging theme is that the Ran gradient also regulates the actomyosin cortex for processes including polar body extrusion during meiosis, and cytokinesis. For these events, active Ran could play an inhibitory role, where importin-binding may help promote or stabilize a conformation or interaction that favours the recruitment and function of cortical regulators. For either spindle assembly or cortical polarity, the gradient of active Ran determines the extent of importin-binding, the effects of which could vary for different proteins.

Transcriptomic and functional analysis of the oosome, a unique form of germ plasm in the wasp Nasonia vitripennis

BACKGROUND

The oosome is the germline determinant in the wasp Nasonia vitripennis and is homologous to the polar granules of Drosophila. Despite a common evolutionary origin and developmental role, the oosome is morphologically quite distinct from polar granules. It is a solid sphere that migrates within the cytoplasm before budding out and forming pole cells.

RESULTS

To gain an understanding of both the molecular basis of oosome development and the conserved essential features of germ plasm, we quantified and compared transcript levels between embryo fragments that contained the oosome and those that did not. The identity of the differentially localized transcripts indicated that Nasonia uses a distinct set of molecules to carry out conserved germ plasm functions. In addition, functional testing of a sample of localized transcripts revealed potentially novel mechanisms of ribonucleoprotein assembly and pole cell cellularization in the wasp.

CONCLUSIONS

Our results demonstrate that the composition of germ plasm varies significantly within Holometabola, as very few mRNAs share localization to the oosome and polar granules. Some of this variability appears to be related to the unique properties of the oosome relative to the polar granules in Drosophila, and some may be related to differences in pole formation between species. This work will serve as the basis for further investigation into the patterns of germline determinant evolution among insects, the molecular basis of the unique properties of the oosome, and the incorporation of novel components into developmental networks.

Mechanical stress induces a scalable switch in cortical flow polarization during cytokinesis

During animal development, cells need to sense and adapt to mechanical forces from their environment. Ultimately, these forces are transduced through the actomyosin cortex. How the cortex simultaneously responds to and creates forces during cytokinesis is not well understood. Here we show that, under mechanical stress, cortical actomyosin flow can switch polarization during cytokinesis in the C. elegans embryo. In unstressed embryos, longitudinal cortical flow contributes to contractile ring formation, while rotational cortical flow is additionally induced in uniaxially loaded embryos, i.e. embryos compressed between two plates. Rotational flow depends on astral microtubule signals and is required for the redistribution of the actomyosin cortex in loaded embryos. Rupture of longitudinally aligned cortical fibers during cortex rotation releases tension, initiates orthogonal longitudinal flow and, thereby, contributes to furrowing in loaded embryos. Moreover, actomyosin regulators involved in RhoA regulation, cortical polarity and chirality are all required for rotational flow, and become essential for cytokinesis under mechanical stress. In sum, our findings extend the current framework of mechanical stress response during cell division and show scaling of orthogonal cortical flows to the amount of mechanical stress.

pirScan: a webserver to predict piRNA targeting sites and to avoid transgene silencing in C. elegans

pirScan is a web-based tool for identifying C. elegans piRNA-targeting sites within a given mRNA or spliced DNA sequence. The purpose of our tool is to allow C. elegans researchers to predict piRNA targeting sites and to avoid the persistent germline silencing of transgenes that has rendered many constructs unusable. pirScan fulfills this purpose by first enumerating the predicted piRNA-targeting sites present in an input sequence. This prediction can be exported in a tabular or graphical format. Subsequently, pirScan suggests silent mutations that can be introduced to the input sequence that would allow the modified transgene to avoid piRNA targeting. The user can customize the piRNA targeting stringency and the silent mutations that he/she wants to introduce into the sequence. The modified sequences can be re-submitted to be certain that any previously present piRNA-targeting sites are now absent and no new piRNA-targeting sites are accidentally generated. This revised sequence can finally be downloaded as a text file and/or visualized in a graphical format. pirScan is freely available for academic use at http://cosbi4.ee.ncku.edu.tw/pirScan/.

PLK1 plays dual roles in centralspindlin regulation during cytokinesis

Cytokinesis begins upon anaphase onset. An early step involves local activation of the small GTPase RhoA, which triggers assembly of an actomyosin-based contractile ring at the equatorial cortex. Here, we delineated the contributions of PLK1 and Aurora B to RhoA activation and cytokinesis initiation in human cells. Knock-down of PRC1, which disrupts the spindle midzone, revealed the existence of two pathways that can initiate cleavage furrow ingression. One pathway depends on a well-organized spindle midzone and PLK1, while the other depends on Aurora B activity and centralspindlin at the equatorial cortex and can operate independently of PLK1. We further show that PLK1 inhibition sequesters centralspindlin onto the spindle midzone, making it unavailable for Aurora B at the equatorial cortex. We propose that PLK1 activity promotes the release of centralspindlin from the spindle midzone through inhibition of PRC1, allowing centralspindlin to function as a regulator of spindle midzone formation and as an activator of RhoA at the equatorial cortex.

Mitotic Cell Division in Caenorhabditis elegans

Mitotic cell divisions increase cell number while faithfully distributing the replicated genome at each division. The Caenorhabditis elegans embryo is a powerful model for eukaryotic cell division. Nearly all of the genes that regulate cell division in C. elegans are conserved across metazoan species, including humans. The C. elegans pathways tend to be streamlined, facilitating dissection of the more redundant human pathways. Here, we summarize the virtues of C. elegans as a model system and review our current understanding of centriole duplication, the acquisition of pericentriolar material by centrioles to form centrosomes, the assembly of kinetochores and the mitotic spindle, chromosome segregation, and cytokinesis.

Regulation and Assembly of Actomyosin Contractile Rings in Cytokinesis and Cell Repair

Cytokinesis and single-cell wound repair both involve contractile assemblies of filamentous actin (F-actin) and myosin II organized into characteristic ring-like arrays. The assembly of these actomyosin contractile rings (CRs) is specified spatially and temporally by small Rho GTPases, which trigger local actin polymerization and myosin II contractility via a variety of downstream effectors. We now have a much clearer view of the Rho GTPase signaling cascade that leads to the formation of CRs, but some factors involved in CR positioning, assembly, and function remain poorly understood. Recent studies show that this regulation is multifactorial and goes beyond the long-established Ca²⁺ -dependent processes. There is substantial evidence that the Ca²⁺ -independent changes in cell shape, tension, and plasma membrane composition that characterize cytokinesis and single-cell wound repair also regulate CR formation. Elucidating the regulation and mechanistic properties of CRs is important to our understanding of basic cell biology and holds potential for therapeutic applications in human disease. In this review, we present a primer on the factors influencing and regulating CR positioning, assembly, and contraction as they occur in a variety of cytokinetic and single-cell wound repair models. Anat Rec, 301:2051-2066, 2018. © 2018 Wiley Periodicals, Inc.

Excitable RhoA dynamics drive pulsed contractions in the early C. elegans embryo

Pulsed actomyosin contractility underlies many morphogenetic processes. Here, Michaux et al. show that, in early C. elegans embryos, pulsed contractions are generated by intrinsically excitable RhoA dynamics, involving fast autoactivation of RhoA and delayed negative feedback through local actin-dependent recruitment of the RhoGAPs RGA-3/4.

Pulsed actomyosin contractility underlies diverse modes of tissue morphogenesis, but the underlying mechanisms remain poorly understood. Here, we combined quantitative imaging with genetic perturbations to identify a core mechanism for pulsed contractility in early Caenorhabditis elegans embryos. We show that pulsed accumulation of actomyosin is governed by local control of assembly and disassembly downstream of RhoA. Pulsed activation and inactivation of RhoA precede, respectively, the accumulation and disappearance of actomyosin and persist in the absence of Myosin II. We find that fast (likely indirect) autoactivation of RhoA drives pulse initiation, while delayed, F-actin–dependent accumulation of the RhoA GTPase-activating proteins RGA-3/4 provides negative feedback to terminate each pulse. A mathematical model, constrained by our data, suggests that this combination of feedbacks is tuned to generate locally excitable RhoA dynamics. We propose that excitable RhoA dynamics are a common driver for pulsed contractility that can be tuned or coupled differently to actomyosin dynamics to produce a diversity of morphogenetic outcomes.

The piRNA targeting rules and the resistance to piRNA silencing in endogenous genes

Piwi-interacting RNAs (piRNAs) silence transposons to safeguard genome integrity in animals. However, the functions of the many piRNAs that do not map to transposons remain unknown. Here, we show that piRNA targeting in Caenorhabditis elegans can tolerate a few mismatches but prefer perfect pairing at the seed region. The broad targeting capacity of piRNAs underlies the germline silencing of transgenes in C. elegans Transgenes engineered to avoid piRNA recognition are stably expressed. Many endogenous germline-expressed genes also contain predicted piRNA targeting sites, and periodic An/Tn clusters (PATCs) are an intrinsic signal that provides resistance to piRNA silencing. Together, our study revealed the piRNA targeting rules and highlights a distinct strategy that C. elegans uses to distinguish endogenous from foreign nucleic acids.

Anillin controls cleavage furrow formation in the course of asymmetric division during mouse oocyte maturation

Anillin is a scaffold protein that recruits several proteins involved in cleavage furrow formation during cytokinesis. The role of anilllin in symmetric cell divisions in somatic cells has been intensively studied, yet its involvement in cleavage furrow formation is still elusive. In this study, we investigated the role of anillin in mammalian oocyte maturation and cytokinesis. We found that anillin is localized around the nucleus during the oocyte germinal-vesicle stage, and spreads to the cytoplasm after germinal vesicle breakdown. Thereafter, anillin concentrates at the site of the cleavage furrow from anaphase I to metaphase II. Disruption of anillin activity by microinjecting oocytes with specific siRNAs resulted in a failure of polar body extrusion and asymmetric division, and caused abnormal chromosome segregation during anaphase I. Furthermore, pharmacological inhibition of myosin light chain using Y-27632 or ML-7 resulted in decreased anillin expression. Collectively, our data suggest that anillin is an essential intracellular component that maintains the integrity of asymmetric division in mouse oocytes. Mol. Reprod. Dev. 83: 792-801, 2016 © 2016 Wiley Periodicals, Inc.

Asymmetric Flows in the Intercellular Membrane during Cytokinesis

Eukaryotic cells undergo shape changes during their division and growth. This involves flow of material both in the cell membrane and in the cytoskeletal layer beneath the membrane. Such flows result in redistribution of phospholipid at the cell surface and actomyosin in the cortex. Here we focus on the growth of the intercellular surface during cell division in a Caenorhabditis elegans embryo. The growth of this surface leads to the formation of a double-layer of separating membranes between the two daughter cells. The division plane typically has a circular periphery and the growth starts from the periphery as a membrane invagination, which grows radially inward like the shutter of a camera. The growth is typically not concentric, in the sense that the closing internal ring is located off-center. Cytoskeletal proteins anillin and septin have been found to be responsible for initiating and maintaining the asymmetry of ring closure but the role of possible asymmetry in the material flow into the growing membrane has not been investigated yet. Motivated by experimental evidence of such flow asymmetry, here we explore the patterns of internal ring closure in the growing membrane in response to asymmetric boundary fluxes. We highlight the importance of the flow asymmetry by showing that many of the asymmetric growth patterns observed experimentally can be reproduced by our model, which incorporates the viscous nature of the membrane and contractility of the associated cortex.

Cytokinesis in Metazoa and Fungi

SUMMARYCell division-cytokinesis-involves large-scale rearrangements of the entire cell. Primarily driven by cytoskeletal proteins, cytokinesis also depends on topological rearrangements of the plasma membrane, which are coordinated with nuclear division in both space and time. Despite the fundamental nature of the process, different types of eukaryotic cells show variations in both the structural mechanisms of cytokinesis and the regulatory controls. In animal cells and fungi, a contractile actomyosin-based structure plays a central, albeit flexible, role. Here, the underlying molecular mechanisms are summarized and integrated and common themes are highlighted.

Active Ran regulates anillin function during cytokinesis

We describe a novel mechanism by which active Ran regulates anillin during cytokinesis. Anillin is highly conserved and coordinates RhoA, actomyosin, microtubules, and the membrane for cytokinesis in mammalian cells. This study implicates Ran-GTP in influencing cortical contractility during anaphase by regulating anillin function.

Cytokinesis cleaves a cell into two daughters at the end of mitosis, and must be spatially coordinated with chromosome segregation to prevent aneuploidy. The dogma is that the mitotic spindle governs the assembly and constriction of an actomyosin ring. Here, we reveal a function for active Ran in spatially restricting the ring. Our model is that during anaphase, “free” importins, whose gradient inversely correlates with active Ran and chromatin position, function as a molecular ruler for the recruitment and localization of anillin, a contractile protein and a crucial regulator of cytokinesis. We found that decreasing Ran-GTP levels or tethering active Ran to the equatorial membrane affects anillin’s localization and causes cytokinesis phenotypes. Anillin contains a conserved nuclear localization signal (NLS) at its C-terminus that binds to importin-β and is required for cortical polarity and cytokinesis. Mutating the NLS decreases anillin’s cortical affinity, causing it to be more dominantly regulated by microtubules. Anillin contains a RhoA-GTP binding domain, which autoinhibits the NLS and the neighboring microtubule-binding domain, and RhoA-GTP binding may relieve this inhibition during mitosis. Retention of the C-terminal NLS in anillin homologues suggests that this is a conserved mechanism for controlling anillin function.

LET-99 functions in the astral furrowing pathway, where it is required for myosin enrichment in the contractile ring

LET-99 is required for furrowing during cytokinesis in both symmetrically and asymmetrically dividing cells. This function is distinct from the role of LET-99 in spindle positioning with Gα signaling. LET-99 is localized to the furrow, where it acts to promote myosin enrichment.

The anaphase spindle determines the position of the cytokinesis furrow, such that the contractile ring assembles in an equatorial zone between the two spindle poles. Contractile ring formation is mediated by RhoA activation at the equator by the centralspindlin complex and midzone microtubules. Astral microtubules also inhibit RhoA accumulation at the poles. In the Caenorhabditis elegans one-cell embryo, the astral microtubule–dependent pathway requires anillin, NOP-1, and LET-99. LET-99 is well characterized for generating the asymmetric cortical localization of the Gα-dependent force-generating complex that positions the spindle during asymmetric division. However, whether the role of LET-99 in cytokinesis is specific to asymmetric division and whether it acts through Gα to promote furrowing are unclear. Here we show that LET-99 contributes to furrowing in both asymmetrically and symmetrically dividing cells, independent of its function in spindle positioning and Gα regulation. LET-99 acts in a pathway parallel to anillin and is required for myosin enrichment into the contractile ring. These and other results suggest a positive feedback model in which LET-99 localizes to the presumptive cleavage furrow in response to the spindle and myosin. Once positioned there, LET-99 enhances myosin accumulation to promote furrowing in both symmetrically and asymmetrically dividing cells.

Plastin increases cortical connectivity to facilitate robust polarization and timely cytokinesis

The cell cortex is essential to maintain animal cell shape, and contractile forces generated within it by nonmuscle myosin II (NMY-2) drive cellular morphogenetic processes such as cytokinesis. The role of actin cross-linking proteins in cortical dynamics is still incompletely understood. Here, we show that the evolutionarily conserved actin bundling/cross-linking protein plastin is instrumental for the generation of potent cortical actomyosin contractility in the Caenorhabditis elegans zygote. PLST-1 was enriched in contractile structures and was required for effective coalescence of NMY-2 filaments into large contractile foci and for long-range coordinated contractility in the cortex. In the absence of PLST-1, polarization was compromised, cytokinesis was delayed or failed, and 50% of embryos died during development. Moreover, mathematical modeling showed that an optimal amount of bundling agents enhanced the ability of a network to contract. We propose that by increasing the connectivity of the F-actin meshwork, plastin enables the cortex to generate stronger and more coordinated forces to accomplish cellular morphogenesis.

Asymmetric Cell Division in the One-Cell C. elegans Embryo: Multiple Steps to Generate Cell Size Asymmetry

The first division of the one-cell C. elegans embryo has been a fundamental model in deciphering the mechanisms underlying asymmetric cell division. Polarization of the one-cell zygote is induced by a signal from the sperm centrosome and results in the asymmetric distribution of PAR proteins. Multiple mechanisms then maintain PAR polarity until the end of the first division. Once asymmetrically localized, PAR proteins control several essential aspects of asymmetric division, including the position of the mitotic spindle along the polarity axis. Coordination of the spindle and cytokinetic furrow positions is the next essential step to ensure proper asymmetric division. In this chapter, I review the different mechanisms underlying these successive steps of asymmetric division. Work from the last 30 years has revealed the existence of multiple and redundant regulatory pathways which ensure division robustness. Besides the essential role of PAR proteins, this work also emphasizes the importance of both microtubules and actomyosin throughout the different steps of asymmetric division.

Cortical flow aligns actin filaments to form a furrow

Cytokinesis in eukaryotic cells is often accompanied by actomyosin cortical flow. Over 30 years ago, Borisy and White proposed that cortical flow converging upon the cell equator compresses the actomyosin network to mechanically align actin filaments. However, actin filaments also align via search-and-capture, and to what extent compression by flow or active alignment drive furrow formation remains unclear. Here, we quantify the dynamical organization of actin filaments at the onset of ring assembly in the C. elegans zygote, and provide a framework for determining emergent actomyosin material parameters by the use of active nematic gel theory. We characterize flow-alignment coupling, and verify at a quantitative level that compression by flow drives ring formation. Finally, we find that active alignment enhances but is not required for ring formation. Our work characterizes the physical mechanisms of actomyosin ring formation and highlights the role of flow as a central organizer of actomyosin network architecture.

Transcriptome analysis of tetraploid cells identifies cyclin D2 as a facilitator of adaptation to genome doubling in the presence of p53

Tetraploidization, or genome doubling, is a prominent event in tumorigenesis, primarily because cell division in polyploid cells is error-prone and produces aneuploid cells. This study investigates changes in gene expression evoked in acute and adapted tetraploid cells and their effect on cell-cycle progression. Acute polyploidy was generated by knockdown of the essential regulator of cytokinesis anillin, which resulted in cytokinesis failure and formation of binucleate cells, or by chemical inhibition of Aurora kinases, causing abnormal mitotic exit with formation of single cells with aberrant nuclear morphology. Transcriptome analysis of these acute tetraploid cells revealed common signatures of activation of the tumor-suppressor protein p53. Suppression of proliferation in these cells was dependent on p53 and its transcriptional target, CDK inhibitor p21. Rare proliferating tetraploid cells can emerge from acute polyploid populations. Gene expression analysis of single cell-derived, adapted tetraploid clones showed up-regulation of several p53 target genes and cyclin D2, the activator of CDK4/6/2. Overexpression of cyclin D2 in diploid cells strongly potentiated the ability to proliferate with increased DNA content despite the presence of functional p53. These results indicate that p53-mediated suppression of proliferation of polyploid cells can be averted by increased levels of oncogenes such as cyclin D2, elucidating a possible route for tetraploidy-mediated genomic instability in carcinogenesis.

Kinetochore-localized PP1-Sds22 couples chromosome segregation to polar relaxation

Cell division requires the precise coordination of chromosome segregation and cytokinesis. This coordination is achieved by the recruitment of an actomyosin regulator, Ect2, to overlapping microtubules at the centre of the elongating anaphase spindle. Ect2 then signals to the overlying cortex to promote the assembly and constriction of an actomyosin ring between segregating chromosomes. Here, by studying division in proliferating Drosophila and human cells, we demonstrate the existence of a second, parallel signalling pathway, which triggers the relaxation of the polar cell cortex at mid anaphase. This is independent of furrow formation, centrosomes and microtubules and, instead, depends on PP1 phosphatase and its regulatory subunit Sds22 (refs 2, 3). As separating chromosomes move towards the polar cortex at mid anaphase, kinetochore-localized PP1-Sds22 helps to break cortical symmetry by inducing the dephosphorylation and inactivation of ezrin/radixin/moesin proteins at cell poles. This promotes local softening of the cortex, facilitating anaphase elongation and orderly cell division. In summary, this identifies a conserved kinetochore-based phosphatase signal and substrate, which function together to link anaphase chromosome movements to cortical polarization, thereby coupling chromosome segregation to cell division.

Anillin interacts with microtubules and is part of the astral pathway that defines cortical domains

Cytokinesis occurs by the ingression of an actomyosin ring that separates the cell into two daughter cells. The mitotic spindle, comprised of astral and central spindle microtubules, couples contractile ring ingression with DNA segregation. Cues from the central spindle activate RhoA, the upstream regulator of the contractile ring. However, additional cues from the astral microtubules also reinforce the localization of active RhoA. Using human cells, we show that astral and central spindle microtubules independently control the localization of contractile proteins during cytokinesis. Astral microtubules restrict the accumulation and localization of contractile proteins during mitosis, while the central spindle forms a discrete ring by directing RhoA activation in the equatorial plane. Anillin stabilizes the contractile ring during cytokinesis. We show that human anillin interacts with astral microtubules, which is competed by its cortical recruitment by active RhoA. Anillin restricts the localization of myosin at the equatorial cortex, and NuMA (part of the microtubule-tethering complex that regulates spindle position) at the polar cortex. The sequestration of anillin by astral microtubules may alter the organization of cortical proteins to polarize cells for cytokinesis.

RhoA activation during polarization and cytokinesis of the early Caenorhabditis elegans embryo is differentially dependent on NOP-1 and CYK-4

RhoA and the Rho guanine nucleotide exchange factor ECT-2 are involved in both polarization and cytokinesis. During cytokinesis, interactions of ECT-2 with the Rho GTPase-activating protein CYK-4 promote RhoA activation. A novel protein, NOP-1, acts in parallel with CYK-4 to promote RhoA activation during polarization and cytokinesis.

The GTPase RhoA is a central regulator of cellular contractility in a wide variety of biological processes. During these events, RhoA is activated by guanine nucleotide exchange factors (GEFs). These molecules are highly regulated to ensure that RhoA activation occurs at the proper time and place. During cytokinesis, RhoA is activated by the RhoGEF ECT-2. In human cells, ECT-2 activity requires its association with CYK-4, which is a component of the centralspindlin complex. In contrast, in early Caenorhabditis elegans embryos, not all ECT-2–dependent functions require CYK-4. In this study, we identify a novel protein, NOP-1, that functions in parallel with CYK-4 to promote RhoA activation. We use mutations in nop-1 and cyk-4 to dissect cytokinesis and cell polarization. NOP-1 makes a significant, albeit largely redundant, contribution to cytokinesis. In contrast, NOP-1 is required for the preponderance of RhoA activation during the establishment phase of polarization.

The RhoGAP domain of CYK-4 has an essential role in RhoA activation

Cytokinesis in animal cells is mediated by a cortical actomyosin-based contractile ring. The GTPase RhoA is a critical regulator of this process as it activates both nonmuscle myosin and a nucleator of actin filaments [1]. The site at which active RhoA and its effectors accumulate is controlled by the microtubule-based spindle during anaphase [2]. ECT-2, the guanine nucleotide exchange factor (GEF) that activates RhoA during cytokinesis, is regulated by phosphorylation and subcellular localization [3-5]. ECT2 localization depends on interactions with CYK-4/MgcRacGAP, a Rho GTPase-activating protein (GAP) domain containing protein [5, 6]. Here we show that, contrary to expectations, the Rho GTPase-activating protein (GAP) domain of CYK-4 promotes activation of RhoA during cytokinesis. Furthermore, we show that the primary phenotype caused by mutations in the GAP domain of CYK-4 is not caused by ectopic activation of CED-10/Rac1 and ARX-2/Arp2. However, inhibition of CED-10/Rac1 and ARX-2/Arp2 facilitates ingression of weak cleavage furrows. These results demonstrate that a GAP domain can contribute to activation of a small GTPase. Furthermore, cleavage furrow ingression is sensitive to the balance of contractile forces and cortical tension.