Dorsal lip maturation and initial archenteron extension depend on Wnt11 family ligands

Wnt11 family proteins are ligands that activate a type of Dishevelled-mediated, non-canonical Wnt signaling pathway. Loss of function causes defects in gastrulation and/or anterior-posterior axis extension in all vertebrates. Non-mammalian vertebrate genomes encode two Wnt11 family proteins whose distinct functions have been unclear. We knocked down Wnt11b and Wnt11, separately and together, in Xenopus laevis. Single morphants exhibited very similar phenotypes of delayed blastopore closure, but they had different phenotypes during the tailbud period. In response to their very similar gastrulation phenotypes, we chose to characterize dual morphants. Using dark field illuminated time-lapse imaging and kymograph analysis, we identified a failure of dorsal blastopore lip maturation that correlated with slower blastopore closure and failure to internalize the endoderm at the dorsal blastopore lip. We connected these externally visible phenotypes to cellular events in the internal tissues by imaging intact fixed embryos stained for anillin and microtubules. We found that the initial extension of the archenteron is correlated with blastopore lip maturation, and archenteron extension is dramatically disrupted by decreased Wnt11 family signaling. We were aided in our interpretation of the immunofluorescence by the novel, membrane proximal location of the cleavage furrow protein anillin in the epithelium of the blastopore lip and early archenteron.

Self-Organization of Cellular Units

The purpose of this review is to explore self-organizing mechanisms that pattern microtubules (MTs) and spatially organize animal cell cytoplasm, inspired by recent experiments in frog egg extract. We start by reviewing conceptual distinctions between self-organizing and templating mechanisms for subcellular organization. We then discuss self-organizing mechanisms that generate radial MT arrays and cell centers in the absence of centrosomes. These include autocatalytic MT nucleation, transport of minus ends, and nucleation from organelles such as melanosomes and Golgi vesicles that are also dynein cargoes. We then discuss mechanisms that partition the cytoplasm in syncytia, in which multiple nuclei share a common cytoplasm, starting with cytokinesis, when all metazoan cells are transiently syncytial. The cytoplasm of frog eggs is partitioned prior to cytokinesis by two self-organizing modules, protein regulator of cytokinesis 1 (PRC1)-kinesin family member 4A (KIF4A) and chromosome passenger complex (CPC)-KIF20A. Similar modules may partition longer-lasting syncytia, such as early Drosophila embryos. We end by discussing shared mechanisms and principles for the MT-based self-organization of cellular units.

Co-movement of astral microtubules, organelles and F-actin by dynein and actomyosin forces in frog egg cytoplasm

How bulk cytoplasm generates forces to separate post-anaphase microtubule (MT) asters in Xenopus laevis and other large eggs remains unclear. Previous models proposed that dynein-based, inward organelle transport generates length-dependent pulling forces that move centrosomes and MTs outwards, while other components of cytoplasm are static. We imaged aster movement by dynein and actomyosin forces in Xenopus egg extracts and observed outward co-movement of MTs, endoplasmic reticulum (ER), mitochondria, acidic organelles, F-actin, keratin, and soluble fluorescein. Organelles exhibited a burst of dynein-dependent inward movement at the growing aster periphery, then mostly halted inside the aster, while dynein-coated beads moved to the aster center at a constant rate, suggesting organelle movement is limited by brake proteins or other sources of drag. These observations call for new models in which all components of the cytoplasm comprise a mechanically integrated aster gel that moves collectively in response to dynein and actomyosin forces.

Unconventional Cell Division Cycles from Marine-Derived Yeasts

Fungi have been found in every marine habitat that has been explored; however, the diversity and functions of fungi in the ocean are poorly understood. In this study, fungi were cultured from the marine environment in the vicinity of Woods Hole, MA, USA, including from plankton, sponge, and coral. Our sampling resulted in 35 unique species across 20 genera. We observed many isolates by time-lapse, differential interference contrast (DIC) microscopy and analyzed modes of growth and division. Several black yeasts displayed highly unconventional cell division cycles compared to those of traditional model yeast systems. Black yeasts have been found in habitats inhospitable to other life and are known for halotolerance, virulence, and stress resistance. We find that this group of yeasts also shows remarkable plasticity in terms of cell size control, modes of cell division, and cell polarity. Unexpected behaviors include division through a combination of fission and budding, production of multiple simultaneous buds, and cell division by sequential orthogonal septations. These marine-derived yeasts reveal alternative mechanisms for cell division cycles that seem likely to expand the repertoire of rules established from classic model system yeasts.

Disassembly of Actin and Keratin Networks by Aurora B Kinase at the Midplane of Cleaving Xenopus laevis Eggs

The large length scale of Xenopus laevis eggs facilitates observation of bulk cytoplasm dynamics far from the cortex during cytokinesis. The first furrow ingresses through the egg midplane, which is demarcated by chromosomal passenger complex (CPC) localized on microtubule bundles at the boundary between asters. Using an extract system, we found that local kinase activity of the Aurora B kinase (AURKB) subunit of the CPC caused disassembly of F-actin and keratin between asters and local softening of the cytoplasm as assayed by flow patterns. Beads coated with active CPC mimicked aster boundaries and caused AURKB-dependent disassembly of F-actin and keratin that propagated ∼40 μm without microtubules and much farther with microtubules present. Consistent with extract observations, we observed disassembly of the keratin network between asters in zygotes fixed before and during 1^st cytokinesis. We propose that active CPC at aster boundaries locally reduces cytoplasmic stiffness by disassembling actin and keratin networks. Possible functions of this local disassembly include helping sister centrosomes move apart after mitosis, preparing a soft path for furrow ingression, and releasing G-actin from internal networks to build cortical networks that support furrow ingression.

Assembly of Spindles and Asters in Xenopus Egg Extracts

Here, we provide methods for assembly of mitotic spindles and interphase asters in Xenopus laevis egg extract, and compare them to spindles and asters in the egg and zygote. Classic "cycled" spindles are made by adding sperm nuclei to metaphase-arrested cytostatic factor (CSF) extract and inducing entry into interphase extract to promote nucleus formation and DNA replication. Interphase nuclei are then converted to cycled spindles arrested in metaphase by addition of CSF extract. Kinetochores assemble in this reaction and these spindles can segregate chromosomes. CSF spindles are made by addition of sperm nuclei to CSF extract. They are less physiological and lack functional kinetochores but suffice for some applications. Large interphase asters are prepared by addition of artificial centrosomes or sperm nuclei to actin-intact egg extract. These asters grow rapidly to hundreds of microns in radius by branching microtubule nucleation at the periphery, so the aster as a whole is a network of short, dynamic microtubules. They resemble the sperm aster after fertilization, and the asters that grow out of the poles of the mitotic spindle at anaphase. When interphase asters grow into each other they interact and assemble aster interaction zones at their shared boundary. These zones consist of a line (in extract) or disc (in zygotes) of antiparallel microtubule bundles coated with cytokinesis midzone proteins. Interaction zones block interpenetration of microtubules from the two asters, and signal to the cortex to induce cleavage furrows. Their reconstitution in extract allows dissection of the biophysics of spatially regulated cytokinesis signaling.

Spindle-to-Cortex Communication in Cleaving Frog Eggs

During cytokinesis, the mitotic spindle communicates with the cell cortex to position a cleavage furrow that will cut through the cell in the plane defined by the metaphase plate. We investigated the molecular basis of this communication in Xenopus laevis eggs, where the signal has to travel ∼400 µm in ∼30 min to reach the cortex from the first anaphase spindle. At anaphase onset, huge microtubule asters grow out from the poles of the spindle and meet at the plane previously defined by the metaphase plate. This disc-shaped boundary plane recruits the chromosome passenger complex (CPC) and centralspindlin to antiparallel microtubule bundles. It grows out to the cell cortex as the asters expand, where it induces the furrow. CPC and centralspindlin were not recruited to boundaries between asters from different spindles, suggesting a role of chromatin in triggering the CPC-positive state. Recruitment of CPC to aster boundaries was reconstituted in an extract system, and we observed that recruitment was stimulated by proximity to chromatin. Finally, we discuss models for molecular processes involved in initiation and growth of the CPC-positive disc that communicates the position of the metaphase plate to the cortex over hundreds of micrometers in frog eggs.

Prc1E and Kif4A control microtubule organization within and between large Xenopus egg asters

The cleavage furrow in Xenopus zygotes is positioned by two large microtubule asters that grow out from the poles of the first mitotic spindle. Where these asters meet at the midplane, they assemble a disk-shaped interaction zone consisting of anti-parallel microtubule bundles coated with chromosome passenger complex (CPC) and centralspindlin that instructs the cleavage furrow. Here we investigate the mechanism that keeps the two asters separate and forms a distinct boundary between them, focusing on the conserved cytokinesis midzone proteins Prc1 and Kif4A. Prc1E, the egg orthologue of Prc1, and Kif4A were recruited to anti-parallel bundles at interaction zones between asters in Xenopus egg extracts. Prc1E was required for Kif4A recruitment but not vice versa. Microtubule plus-end growth slowed and terminated preferentially within interaction zones, resulting in a block to interpenetration that depended on both Prc1E and Kif4A. Unexpectedly, Prc1E and Kif4A were also required for radial order of large asters growing in isolation, apparently to compensate for the direction-randomizing influence of nucleation away from centrosomes. We propose that Prc1E and Kif4, together with catastrophe factors, promote "anti-parallel pruning" that enforces radial organization within asters and generates boundaries to microtubule growth between asters.

Xenopus extract approaches to studying microtubule organization and signaling in cytokinesis

We report optimized methods for preparing actin-intact Xenopus egg extract. This extract is minimally perturbed, undiluted egg cytoplasm where the cell cycle can be experimentally controlled. It contains abundant organelles and glycogen and supports active metabolism and cytoskeletal dynamics that closely mimic egg physiology. The concentration of the most abundant ∼11,000 proteins is known from mass spectrometry. Actin-intact egg extract can be used for analysis of actin dynamics and interaction of actin with other cytoplasmic systems, as well as microtubule organization. It can be spread as thin layers and naturally depletes oxygen though mitochondrial metabolism, which makes it ideal for fluorescence imaging. When combined with artificial lipid bilayers, it allows reconstitution and analysis of the spatially controlled signaling that positions the cleavage furrow during early cytokinesis. Actin-intact extract is generally useful for probing the biochemistry and biophysics of the large Xenopus egg. Protocols are provided for preparation of actin-intact egg extract, control of the cell cycle, fluorescent probes for cytoskeleton and cytoskeleton-dependent signaling, preparation of glass surfaces for imaging experiments, and immunodepletion to probe the role of specific proteins and protein complexes. We also describe methods for adding supported lipid bilayers to mimic the plasma membrane and for confining in microfluidic droplets to explore size scaling issues.

Spindle-to-cortex communication in cleaving, polyspermic Xenopus eggs

Polyspermic Xenopus eggs and a cytokinesis extract system were used to investigate spindle-to-cortex communication, which positions cleavage furrows. Chromosome passenger complex recruitment to microtubule bundles between asters plays a key role and is positively influenced by microtubule stabilization and proximity to chromatin.

Mitotic spindles specify cleavage planes in early embryos by communicating their position and orientation to the cell cortex using microtubule asters that grow out from the spindle poles during anaphase. Chromatin also plays a poorly understood role. Polyspermic fertilization provides a natural experiment in which aster pairs from the same spindle (sister asters) have chromatin between them, whereas asters pairs from different spindles (nonsisters) do not. In frogs, only sister aster pairs induce furrows. We found that only sister asters recruited two conserved furrow-inducing signaling complexes, chromosome passenger complex (CPC) and Centralspindlin, to a plane between them. This explains why only sister pairs induce furrows. We then investigated factors that influenced CPC recruitment to microtubule bundles in intact eggs and a cytokinesis extract system. We found that microtubule stabilization, optimal starting distance between asters, and proximity to chromatin all favored CPC recruitment. We propose a model in which proximity to chromatin biases initial CPC recruitment to microtubule bundles between asters from the same spindle. Next a positive feedback between CPC recruitment and microtubule stabilization promotes lateral growth of a plane of CPC-positive microtubule bundles out to the cortex to position the furrow.

Using supported bilayers to study the spatiotemporal organization of membrane-bound proteins

Cell division in prokaryotes and eukaryotes is commonly initiated by the well-controlled binding of proteins to the cytoplasmic side of the cell membrane. However, a precise characterization of the spatiotemporal dynamics of membrane-bound proteins is often difficult to achieve in vivo. Here, we present protocols for the use of supported lipid bilayers to rebuild the cytokinetic machineries of cells with greatly different dimensions: the bacterium Escherichia coli and eggs of the vertebrate Xenopus laevis. Combined with total internal reflection fluorescence microscopy, these experimental setups allow for precise quantitative analyses of membrane-bound proteins. The protocols described to obtain glass-supported membranes from bacterial and vertebrate lipids can be used as starting points for other reconstitution experiments. We believe that similar biochemical assays will be instrumental to study the biochemistry and biophysics underlying a variety of complex cellular tasks, such as signaling, vesicle trafficking, and cell motility.

Organization of early frog embryos by chemical waves emanating from centrosomes

The large cells in early vertebrate development face an extreme physical challenge in organizing their cytoplasm. For example, amphibian embryos have to divide cytoplasm that spans hundreds of micrometres every 30 min according to a precise geometry, a remarkable accomplishment given the extreme difference between molecular and cellular scales in this system. How do the biochemical reactions occurring at the molecular scale lead to this emergent behaviour of the cell as a whole? Based on recent findings, we propose that the centrosome plays a crucial role by initiating two autocatalytic reactions that travel across the large cytoplasm as chemical waves. Waves of mitotic entry and exit propagate out from centrosomes using the Cdk1 oscillator to coordinate the timing of cell division. Waves of microtubule-stimulated microtubule nucleation propagate out to assemble large asters that position spindles for the following mitosis and establish cleavage plane geometry. By initiating these chemical waves, the centrosome rapidly organizes the large cytoplasm during the short embryonic cell cycle, which would be impossible using more conventional mechanisms such as diffusion or nucleation by structural templating. Large embryo cells provide valuable insights to how cells control chemical waves, which may be a general principle for cytoplasmic organization.

Spatial organization of cytokinesis signaling reconstituted in a cell-free system

During animal cell division, the cleavage furrow is positioned by microtubules that signal to the actin cortex at the cell midplane. We developed a cell-free system to recapitulate cytokinesis signaling using cytoplasmic extract from Xenopus eggs. Microtubules grew out as asters from artificial centrosomes and met to organize antiparallel overlap zones. These zones blocked the interpenetration of neighboring asters and recruited cytokinesis midzone proteins, including the chromosomal passenger complex (CPC) and centralspindlin. The CPC was transported to overlap zones, which required two motor proteins, Kif4A and a Kif20A paralog. Using supported lipid bilayers to mimic the plasma membrane, we observed the recruitment of cleavage furrow markers, including an active RhoA reporter, at microtubule overlaps. This system opens further approaches to understanding the biophysics of cytokinesis signaling.

Xenopus egg cytoplasm with intact actin

We report optimized methods for preparing Xenopus egg extracts without cytochalasin D, that we term "actin-intact egg extract." These are undiluted egg cytoplasm that contains abundant organelles, and glycogen which supplies energy, and represents the least perturbed cell-free cytoplasm preparation we know of. We used this system to probe cell cycle regulation of actin and myosin-II dynamics (Field et al., 2011), and to reconstitute the large, interphase asters that organize early Xenopus embryos (Mitchison et al., 2012; Wühr, Tan, Parker, Detrich, & Mitchison, 2010). Actin-intact Xenopus egg extracts are useful for analysis of actin dynamics, and interaction of actin with other cytoplasmic systems, in a cell-free system that closely mimics egg physiology, and more generally for probing the biochemistry and biophysics of the egg, zygote, and early embryo. Detailed protocols are provided along with assays used to check cell cycle state and tips for handling and storing undiluted egg extracts.

Multiple mechanisms determine ER network morphology during the cell cycle in Xenopus egg extracts

Fusion of ER membranes by atlastin and interaction of ER with growing microtubule ends and dynein cooperate to generate distinct ER morphologies during the cell cycle.

In metazoans the endoplasmic reticulum (ER) changes during the cell cycle, with the nuclear envelope (NE) disassembling and reassembling during mitosis and the peripheral ER undergoing extensive remodeling. Here we address how ER morphology is generated during the cell cycle using crude and fractionated Xenopus laevis egg extracts. We show that in interphase the ER is concentrated at the microtubule (MT)-organizing center by dynein and is spread by outward extension of ER tubules through their association with plus ends of growing MTs. Fusion of membranes into an ER network is dependent on the guanosine triphosphatase atlastin (ATL). NE assembly requires fusion by both ATL and ER-soluble N-ethyl-maleimide–sensitive factor adaptor protein receptors. In mitotic extracts, the ER converts into a network of sheets connected by ER tubules and loses most of its interactions with MTs. Together, these results indicate that fusion of ER membranes by ATL and interaction of ER with growing MT ends and dynein cooperate to generate distinct ER morphologies during the cell cycle.

Multicopy plasmid stability: revisiting the dimer catastrophe

In this study, we have constructed a stochastic simulation of the replication and distribution of the bacterial multicopy plasmid ColE1 in a population of exponentially growing cells. It is assumed that ColE1 is randomly distributed between daughter cells at division such that copy number is a critical determinant of plasmid loss. High copy number is threatened by plasmid dimers, which arises initially by homologous recombination and accumulate by replication in a process known as the 'dimer catastrophe'. Summers et al. (1993) modelled this process and demonstrated that the accumulation of dimers is limited by the metabolic load that they exert on their hosts. ColE1 also encodes the cer site, at which host-encoded proteins act to convert dimers to monomers by site-specific recombination. The cer site also encodes a regulatory RNA, Rcd, whose synthesis from plasmid dimers triggers a checkpoint that delays cell division, presumably allowing sufficient time for dimer resolution. Here we have developed the original dimer catastrophe model by incorporating copy number variance with a stochastic model of plasmid replication. We demonstrate that the Rcd checkpoint is necessary when the rate of dimer resolution is slow. Our results indicate that dimers over-replicate compared to monomers, suggesting a mechanism for their increased metabolic load. We find that the effect of dimers on plasmid stability is significantly less severe than suggested by the original model. Consequently, we propose that the primary role of dimer resolution and the Rcd checkpoint is to reduce the metabolic burden imposed by the plasmid in a recombinogenic host, rather than to ensure plasmid stability.

Actin behavior in bulk cytoplasm is cell cycle regulated in early vertebrate embryos

The mechanical properties of cells change as they proceed through the cell cycle, primarily owing to regulation of actin and myosin II. Most models for cell mechanics focus on actomyosin in the cortex and ignore possible roles in bulk cytoplasm. We explored cell cycle regulation of bulk cytoplasmic actomyosin in Xenopus egg extracts, which is almost undiluted cytoplasm from unfertilized eggs. We observed dramatic gelation-contraction of actomyosin in mitotic (M phase) extract where Cdk1 activity is high, but not in interphase (I-phase) extract. In spread droplets, M-phase extract exhibited regular, periodic pulses of gelation-contraction a few minutes apart that continued for many minutes. Comparing actin nucleation, disassembly and myosin II activity between M-phase and I-phase extracts, we conclude that regulation of nucleation is likely to be the most important for cell cycle regulation. We then imaged F-actin in early zebrafish blastomeres using a GFP-Utrophin probe. Polymerization in bulk cytoplasm around vesicles increased dramatically during mitosis, consistent with enhanced nucleation. We conclude that F-actin polymerization in bulk cytoplasm is cell cycle regulated in early vertebrate embryos and discuss possible biological functions of this regulation.

KIF4 regulates midzone length during cytokinesis

BACKGROUND

Midzones, also called central spindles, are an array of antiparallel microtubules that form during cytokinesis between the separated chromosomes. Midzones can be considered to be platforms that recruit specific proteins and orchestrate cytokinetic events, such as sister nuclei being kept apart, furrow ingression, and abscission. Despite this important role, many aspects of midzone biology remain unknown, including the dynamic organization of midzone microtubules. Investigating midzone microtubule dynamics has been difficult in part because their plus ends are interdigitated and buried in a dense matrix, making them difficult to observe.

RESULT

We employed monopolar cytokinesis to reveal that midzone plus ends appear to be nondynamic. We identified the chromokinesin KIF4 as a negative regulator of midzone plus-end dynamics whose activity controls midzone length but not stability. KIF4 is required to terminate midzone elongation in late anaphase. In the absence of KIF4, midzones elongate abnormally, and their overlap regions are unfocused. Electron-dense material and midbodies are both absent from the elongated midzones, and actin filaments from the furrow cortex are not disassembled after ingression.

CONCLUSION

KIF4-mediated midzone length regulation appears to occur by terminating midzone elongation at a specific time during cytokinesis, making midzones and mitotic spindles differ in their dynamics and length-regulating mechanisms.

Binding partner switching on microtubules and aurora-B in the mitosis to cytokinesis transition

The cytoskeleton globally reorganizes between mitosis (M phase) and cytokinesis (C phase), which presumably requires extensive regulatory changes. To reveal these changes, we undertook a comparative proteomics analysis of cells tightly drug-synchronized in each phase. We identified 25 proteins that bind selectively to microtubules in C phase and identified several novel binding partners including nucleolar and spindle-associated protein. C phase-selective microtubule binding of many of these proteins depended on activity of Aurora kinases as assayed by treatment with the drug VX680. Aurora-B binding partners switched dramatically between M phase to C phase, and we identified several novel C phase-selective Aurora-B binding partners including PRC1, KIF4, and anaphase-promoting complex/cyclosome. Our approach can be extended to other cellular compartments and cell states, and our data provide the first broad biochemical framework for understanding C phase. Concretely, we report a central role for Aurora-B in regulating the C phase cytoskeleton.

Size and speed go hand in hand in cytokinesis

In animal cells, cytokinesis is mediated by the constriction of a cortical ring. In this issue, Carvalho et al. (2009) show in embryos of the worm Caenorhabditis elegans that the rate of ring constriction during cytokinesis is proportional to the initial cell perimeter, ensuring that the duration of cytokinesis is cell-size independent.

Mitosis: new roles for myosin-X and actin at the spindle

Roles for actin and myosin in positioning mitotic spindles in the cell are well established. A recent study of myosin-X function in early Xenopus embryo mitosis now reports that this unconventional myosin is required for pole integrity and normal spindle length by localizing to poles and exerting pulling forces on actin filaments within the spindle.

Actin-aggregating cucurbitacins from Physocarpus capitatus

Bioassay-guided fractionation of Physocarpus capitatus yielded two new cucurbitacins (3 and 4) along with the known cucurbitacin F (1) and dihydrocucurbitacin F (2). Preliminary mechanism of action studies indicate that the cucurbitacins cause actin aggregates and inhibit cell division.

Nuf, a Rab11 effector, maintains cytokinetic furrow integrity by promoting local actin polymerization

Plasma membrane ingression during cytokinesis involves both actin remodeling and vesicle-mediated membrane addition. Vesicle-based membrane delivery from the recycling endosome (RE) has an essential but ill-defined involvement in cytokinesis. In the Drosophila melanogaster early embryo, Nuf (Nuclear fallout), a Rab11 effector which is essential for RE function, is required for F-actin and membrane integrity during furrow ingression. We find that in nuf mutant embryos, an initial loss of F-actin at the furrow is followed by loss of the associated furrow membrane. Wild-type embryos treated with Latrunculin A or Rho inhibitor display similar defects. Drug- or Rho-GTP–induced increase of actin polymerization or genetically mediated decrease of actin depolymerization suppresses the nuf mutant F-actin and membrane defects. We also find that RhoGEF2 does not properly localize at the furrow in nuf mutant embryos and that RhoGEF2–Rho1 pathway components show strong specific genetic interactions with Nuf. We propose a model in which RE-derived vesicles promote furrow integrity by regulating the rate of actin polymerization through the RhoGEF2–Rho1 pathway.

Cell polarization during monopolar cytokinesis

During cytokinesis, a specialized set of proteins is recruited to the equatorial region between spindle poles by microtubules and actin filaments, enabling furrow assembly and ingression before cell division. We investigate the mechanisms underlying regional specialization of the cytoskeleton in HeLa cells undergoing drug-synchronized monopolar cytokinesis. After forced mitotic exit, the cytoskeleton of monopolar mitotic cells is initially radially symmetric but undergoes a symmetry-breaking reaction that simultaneously polarizes microtubules and the cell cortex, with a concentration of cortical furrow markers into a cap at one side of the cell. Polarization requires microtubules, F-actin, RhoA, Myosin II activity, and Aurora B kinase activity. Aurora B localizes to actin cables in a gap between the monopolar midzone and the furrow-like cortex, suggesting a communication between them. We propose that feedback loops between cortical furrow components and microtubules promote symmetry breaking during monopolar cytokinesis and regional specialization of the cytoskeleton during normal bipolar cytokinesis.

Animal cytokinesis: from parts list to mechanisms

The mechanism underlying cytokinesis, the final step in cell division, remains one of the major unsolved questions in basic cell biology. Thanks to advances in functional genomics and proteomics, we are now able to assemble a "parts list" of proteins involved in cytokinesis. In this review, we discuss how to relate this parts list to biological mechanism. For easier analysis, we split cytokinesis into discrete steps: cleavage plane specification, rearrangement of microtubule structures, contractile ring assembly, ring ingression, and completion. We report on the advances that have been made to understand these steps and how they can be integrated into a global understanding of cytokinesis. We also discuss the extent to which classic questions have been answered and identify major outstanding questions.

Small molecules in an RNAi world

In this Opinion, we compare and contrast small molecules and RNAi. We discuss the advantages and disadvantages of both technologies focusing on timing, specificity, dose and therapeutic use.

Characterization of anillin mutants reveals essential roles in septin localization and plasma membrane integrity

Anillin is a conserved component of the contractile ring that is essential for cytokinesis, and physically interacts with three conserved cleavage furrow proteins, F-actin, myosin II and septins in biochemical assays. We demonstrate that the Drosophila scraps gene, identified as a gene involved in cellularization, encodes Anillin. We characterize defects in cellularization, pole cell formation and cytokinesis in a series of maternal effect and zygotic anillin alleles. Mutations that result in amino acid changes in the C-terminal PH domain of Anillin cause defects in septin recruitment to the furrow canal and contractile ring. These mutations also strongly perturb cellularization, altering the timing and rate of furrow ingression. They cause dramatic vesiculation of new plasma membranes, and destabilize the stalk of cytoplasm that normally connects gastrulating cells to the yolk mass. A mutation closer to the N terminus blocks separation of pole cells with less effect on cellularization, highlighting mechanistic differences between contractile processes. Cumulatively, our data point to an important role for Anillin in scaffolding cleavage furrow components, directly stabilizing intracellular bridges, and indirectly stabilizing newly deposited plasma membrane during cellularization.

Clues to CD2-associated protein involvement in cytokinesis

Cytokinesis requires membrane trafficking coupled to actin remodeling and involves a number of trafficking molecules. CD2-associated protein (CD2AP) has been implicated in dynamic actin remodeling and membrane trafficking that occurs during endocytosis leading to the degradative pathway. In this study, we present several arguments for its implication in cytokinesis. First, endogenous CD2AP was found concentrated in the narrow region of the midzone microtubules during anaphase and in the midbody during late telophase. Moreover, we found that CD2AP is a membrane- and not a microtubule-associated protein. Second, the overexpression of the first two Src homology 3 domains of CD2AP, which are responsible for this localization, led to a significant increase in the rate of cell multinucleation. Third, the CD2AP small interfering RNA interfered with the cell separation, indicating that CD2AP is required for HeLa cells cytokinesis. Fourth, using the yeast two-hybrid system, we found that CD2AP interacted with anillin, a specific cleavage furrow component, and the two proteins colocalized at the midbody. Both CD2AP and anillin were found phosphorylated early in mitosis and also CD2AP phosphorylation was coupled to its delocalization from membrane to cytosol. All these observations led us to propose CD2AP as a new player in cytokinesis.

Anillin binds nonmuscle myosin II and regulates the contractile ring

We demonstrate that the contractile ring protein anillin interacts directly with nonmuscle myosin II and that this interaction is regulated by myosin light chain phosphorylation. We show that despite their interaction, anillin and myosin II are independently targeted to the contractile ring. Depletion of anillin in Drosophila or human cultured cells results in cytokinesis failure. Human cells depleted for anillin fail to properly regulate contraction by myosin II late in cytokinesis and fail in abscission. We propose a role for anillin in spatially regulating the contractile activity of myosin II during cytokinesis.

The majority of the Saccharomyces cerevisiae septin complexes do not exchange guanine nucleotides

We show here that affinity-purified Saccharomyces cerevisiae septin complexes contain stoichiometric amounts of guanine nucleotides, specifically GTP and GDP. Using a (15)N-dilution assay read-out by liquid chromatography-tandem mass spectrometry, we determined that the majority of the bound guanine nucleotides do not turn over in vivo during one cell cycle period. In vitro, the isolated S. cerevisiae septin complexes have similar GTP binding and hydrolytic properties to the Drosophila septin complexes (Field, C. M., al-Awar, O., Rosenblatt, J., Wong, M. L., Alberts, B., and Mitchison, T. J. (1996) J. Cell Biol. 133, 605-616). In particular, the GTP turnover of septins is very slow when compared with the GTP turnover for Ras-like GTPases. We conclude that bound GTP and GDP play a structural, rather then regulatory, role for the majority of septins in proliferating cells as GTP does for alpha-tubulin.

Mammalian septins nomenclature

There are 10 known mammalian septin genes, some of which produce multiple splice variants. The current nomenclature for the genes and gene products is very confusing, with several different names having been given to the same gene product and distinct names given to splice variants of the same gene. Moreover, some names are based on those of yeast or Drosophila septins that are not the closest homologues. Therefore, we suggest that the mammalian septin field adopt a common nomenclature system, based on that adopted by the Mouse Genomic Nomenclature Committee and accepted by the Human Genome Organization Gene Nomenclature Committee. The human and mouse septin genes will be named SEPT1-SEPT10 and Sept1-Sept10, respectively. Splice variants will be designated by an underscore followed by a lowercase "v" and a number, e.g., SEPT4_v1.

Self- and actin-templated assembly of Mammalian septins

Septins are polymerizing GTPases required for cytokinesis and cortical organization. The principles by which they are targeted to, and assemble at, specific cell regions are unknown. We show that septins in mammalian cells switch between a linear organization along actin bundles and cytoplasmic rings, approximately 0.6 microm in diameter. A recombinant septin complex self-assembles into rings resembling those in cells. Linear organization along actin bundles was reconstituted by adding an adaptor protein, anillin. Perturbation of septin organization in cells by expression of a septin-interacting fragment of anillin or by septin depletion via siRNA causes loss of actin bundles. We conclude that septins alone self-assemble into rings, that adaptor proteins recruit septins to actin bundles, and that septins help organize these bundles.

Cytoskeleton: what does GTP do for septins?

The recent observation of GTP-promoted polymerization of a single septin polypeptide suggests that this protein has tubulin-like biochemical properties. This model cannot, however, explain the GTP-biochemistry of heteromeric septin complexes from cytosol.

Microtubules, membranes and cytokinesis

Proper division of the cell requires coordination between chromosome segregation by the mitotic spindle and cleavage of the cell by the cytokinetic apparatus. Interactions between the mitotic spindle, the contractile ring and the plasma membrane ensure that the cleavage furrow is properly placed between the segregating chromosomes and that new membrane compartments are formed to produce two daughter cells. The microtubule midzone is able to stimulate the cortex of the cell to ensure proper ingression and completion of the cleavage furrow. Specialized microtubule structures are responsible for directing membrane vesicles to the site of cell cleavage, and vesicle fusion is required for the proper completion of cytokinesis.

Functional analysis of a human homologue of the Drosophila actin binding protein anillin suggests a role in cytokinesis

We have characterized a human homologue of anillin, a Drosophila actin binding protein. Like Drosophila anillin, the human protein localizes to the nucleus during interphase, the cortex following nuclear envelope breakdown, and the cleavage furrow during cytokinesis. Anillin also localizes to ectopic cleavage furrows generated between two spindles in fused PtK(1) cells. Microinjection of antianillin antibodies slows cleavage, leading to furrow regression and the generation of multinucleate cells. GFP fusions that contain the COOH-terminal 197 amino acids of anillin, which includes a pleckstrin homology (PH) domain, form ectopic cortical foci during interphase. The septin Hcdc10 localizes to these ectopic foci, whereas myosin II and actin do not, suggesting that anillin interacts with the septins at the cortex. Robust cleavage furrow localization requires both this COOH-terminal domain and additional NH(2)-terminal sequences corresponding to an actin binding domain defined by in vitro cosedimentation assays. Endogenous anillin and Hcdc10 colocalize to punctate foci associated with actin cables throughout mitosis and the accumulation of both proteins at the cell equator requires filamentous actin. These results indicate that anillin is a conserved cleavage furrow component important for cytokinesis. Interactions with at least two other furrow proteins, actin and the septins, likely contribute to anillin function.

Microtubules and mitotic cycle phase modulate spatiotemporal distributions of F-actin and myosin II in Drosophila syncytial blastoderm embryos

We studied cyclic reorganizations of filamentous actin, myosin II and microtubules in syncytial Drosophila blastoderms using drug treatments, time-lapse movies and laser scanning confocal microscopy of fixed stained embryos (including multiprobe three-dimensional reconstructions). Our observations imply interactions between microtubules and the actomyosin cytoskeleton. They provide evidence that filamentous actin and cytoplasmic myosin II are transported along microtubules towards microtubule plus ends, with actin and myosin exhibiting different affinities for the cell's cortex. Our studies further reveal that cell cycle phase modulates the amounts of both polymerized actin and myosin II associated with the cortex. We analogize pseudocleavage furrow formation in the Drosophila blastoderm with how the mitotic apparatus positions the cleavage furrow for standard cytokinesis, and relate our findings to polar relaxation/global contraction mechanisms for furrow formation.

Septins: cytoskeletal polymers or signalling GTPases?

Septins are a family of conserved proteins that have been implicated in a variety of cellular functions involving specialized regions of the cell cortex and changes in cell shape. The biochemistry and localization of septins suggest that they form a novel cytoskeletal system or that they function as scaffolds for the assembly of signalling complexes. This article discusses septin biochemistry and septin-interacting proteins, focusing on the missing link between the structure and biochemical properties of septin proteins, and on how they function at a molecular level in processes such as cytokinesis and yeast budding.

Filamin is required for ring canal assembly and actin organization during Drosophila oogenesis

The remodeling of the actin cytoskeleton is essential for cell migration, cell division, and cell morphogenesis. Actin-binding proteins play a pivotal role in reorganizing the actin cytoskeleton in response to signals exchanged between cells. In consequence, actin-binding proteins are increasingly a focus of investigations into effectors of cell signaling and the coordination of cellular behaviors within developmental processes. One of the first actin-binding proteins identified was filamin, or actin-binding protein 280 (ABP280). Filamin is required for cell migration (Cunningham et al. 1992), and mutations in human alpha-filamin (FLN1; Fox et al. 1998) are responsible for impaired migration of cerebral neurons and give rise to periventricular heterotopia, a disorder that leads to epilepsy and vascular disorders, as well as embryonic lethality. We report the identification and characterization of a mutation in Drosophila filamin, the homologue of human alpha-filamin. During oogenesis, filamin is concentrated in the ring canal structures that fortify arrested cleavage furrows and establish cytoplasmic bridges between cells of the germline. The major structural features common to other filamins are conserved in Drosophila filamin. Mutations in Drosophila filamin disrupt actin filament organization and compromise membrane integrity during oocyte development, resulting in female sterility. The genetic and molecular characterization of Drosophila filamin provides the first genetic model system for the analysis of filamin function and regulation during development.

Nuclear-fallout, a Drosophila protein that cycles from the cytoplasm to the centrosomes, regulates cortical microfilament organization

nuclear fallout (nuf) is a maternal effect mutation that specifically disrupts the cortical syncytial divisions during Drosophila embryogenesis. We show that the nuf gene encodes a highly phosphorylated novel protein of 502 amino acids with C-terminal regions predicted to form coiled-coils. During prophase of the late syncytial divisions, Nuf concentrates at the centrosomes and is generally cytoplasmic throughout the rest of the nuclear cycle. In nuf-derived embryos, the recruitment of actin from caps to furrows during prophase is disrupted. This results in incomplete metaphase furrows specifically in regions distant from the centrosomes. The nuf mutation does not disrupt anillin or peanut recruitment to the metaphase furrows indicating that Nuf is not involved in the signaling of metaphase furrow formation. These results also suggest that anillin and peanut localization are independent of actin localization to the metaphase furrows. nuf also disrupts the initial stages of cellularization and produces disruptions in cellularization furrows similar to those observed in the metaphase furrows. The localization of Nuf to centrosomal regions throughout cellularization suggests that it plays a similar role in the initial formation of both metaphase and cellularization furrows. A model is presented in which Nuf provides a functional link between centrosomes and microfilaments.

Actin cytoskeleton: are FH proteins local organizers?

The FH proteins, defined by the presence of 'formin homology' regions, are important for a number of actin-dependent processes, including polarized cell growth and cytokinesis. They are large, probably multi-domain, proteins and their function may be in part mediated by an interaction with profilin.

A purified Drosophila septin complex forms filaments and exhibits GTPase activity

Septin proteins are necessary for cytokinesis in budding yeast and Drosophila and are thought to be the subunits of the yeast neck filaments. To test whether septins actually form filaments, an immunoaffinity approach was used to isolate a septin complex from Drosophila embryos. The purified complex is comprised of the three previously identified septin polypeptides Pnut, Sep2, and Sep1. Hydrodynamic and sequence data suggest that the complex is composed of a heterotrimer of homodimers. The complex copurifies with one molecule of bound guanine nucleotide per septin polypeptide. It binds and hydrolyzes exogenously added GTP. These observations together with conserved sequence motifs identify the septins as members of the GTPase superfamily. We discuss a model of filament structure and speculate as to how the filaments are organized within cells.

Case report: the need for caution when using the spleen to assess renal blood flow in renography

Visual assessment of renal blood flow from renography is usually achieved by comparing the count rate from the kidney with that of the spleen during the first pass of the radiopharmaceutical agent. This is not justifiable if splenic blood flow is abnormal. In nine subjects without evidence of renal or other significant disease, the mean ratio of the slopes of the first pass curves over the left kidney and the spleen was 1.7 (SD 0.32). However, an additional patient, a 67-year-old female with type-II cryoglobulinaemia, illustrated the need for caution when using this approach. Her splenic blood flow was significantly elevated, resulting in a kidney/spleen slope ratio of only 0.66, even though her renal function was thought to be normal.

Cell division. Bud-site selection is only skin deep

Yeast cells that divide by budding place new buds in predetermined locations. Recent studies of the subcellular localization of the Bud3 protein help to explain how this occurs.

Anillin, a contractile ring protein that cycles from the nucleus to the cell cortex

We report the cDNA sequence and localization of a protein first identified by actin filament chromatography of Drosophila embryo extracts as ABP8 (Miller, K. G., C. M. Field, and B. M. Alberts. 1989. J. Cell Biol. 109:2963-2975). The cDNA encodes a 1201-amino acid protein which we name anillin. Anillin migrates at 190 kD on SDS-PAGE. Anillin is expressed throughout Drosophila development and in tissue culture cells. By immunofluorescence, anillin localizes to the nucleus of interphase cells, except in the syncytial embryo where it is always cytoplasmic. During metaphase, it is present in the cytoplasm and cortex, and during anaphase-telophase it becomes highly enriched in the cleavage furrow along with myosin II. In the syncytial embryo, anillin, along with myosin-II, is enriched in cortical areas undergoing cell cycle regulated invagination including metaphase furrows and the cellularization front. In contractile rings, metaphase furrows, and nascent ring canals, anillin remains bound to the invaginated cortex suggesting a stabilizing role. Anillin is not expressed in cells that have left the cell cycle. Anillin isolated from embryo extracts binds directly to actin filaments. The domain responsible for this binding has been mapped to a region of 244 amino acids by expression of protein fragments in bacteria. This domain, which is monomeric in solution, also bundles actin filaments. We speculate that anillin plays a role in organizing and/or stabilizing the cleavage furrow and other cell cycle regulated, contractile domains of the actin cytoskeleton.

Cell division. Septins in common?

Two apparently quite distinct processes, cytokinesis in animal cells and in budding yeast cells, have been shown to involve proteins of the same family, the septins, suggesting that the two may not be so different after all.