Myosin II-independent cytokinesis in Dictyostelium: its mechanism and implications

Cell-to-cell signaling in cell populations with large cell size variability

Sizes of multiple cells vary when they communicate with each other. Differences in cell size result in variations in the cell surface area and volume, as well as the number of enzymes and receptors involved in signal transduction. Although heterogeneity in cell size may inhibit uniformity in signaling, cell-to-cell signaling is still possible. The outcome when cell size changes to an extreme degree remains unclear. Hence, we inhibited cell division in Dictyostelium cells, a model organism for signal transduction, to gain insights into the consequences of extreme cell size variations. Measurements of cell signals in this population using fluorescence microscopy indicated that the giant cells can communicate with normal-sized cells by suppressing the signal level. Simulations of signal transduction based on the FitzHugh-Nagumo model also suggested similar results. Our findings suggest that signaling mechanism homogenizes cell-to-cell signaling in response to cell size.

Dynamics of actomyosin filaments in the contractile ring revealed by ultrastructural analysis

Cytokinesis, the final process of cell division, involves the accumulation of actin and myosin II filaments at the cell's equator, forming a contractile ring that facilitates the division into two daughter cells. While light microscopy has provided valuable insights into the molecular mechanism of this process, it has limitations in examining individual filaments in vivo. In this study, we utilized transmission electron microscopy to observe actin and myosin II filaments in the contractile rings of dividing Dictyostelium cells. To synchronize cytokinesis, we developed a novel method that allowed us to visualize dividing cells undergoing cytokinesis with a frequency as high as 18%. This improvement enabled us to examine the lengths and alignments of individual filaments within the contractile rings. As the furrow constricted, the length of actin filaments gradually decreased. Moreover, both actin and myosin II filaments reoriented perpendicularly to the long axis during furrow constriction. Through experiments involving myosin II null cells, we discovered that myosin II plays a role in regulating both the lengths and alignments of actin filaments. Additionally, dynamin-like protein A was found to contribute to regulating the length of actin filaments, while cortexillins were involved in regulating their alignment.

The lectin Discoidin I acts in the cytoplasm to help assemble the contractile machinery

Cellular functions, such as division and migration, require cells to undergo robust shape changes. Through their contractility machinery, cells also sense, respond, and adapt to their physical surroundings. In the cytoplasm, the contractility machinery organizes into higher order assemblies termed contractility kits (CKs). Using Dictyostelium discoideum, we previously identified Discoidin I (DscI), a classic secreted lectin, as a CK component through its physical interactions with the actin crosslinker Cortexillin I (CortI) and the scaffolding protein IQGAP2. Here, we find that DscI ensures robust cytokinesis through regulating intracellular components of the contractile machinery. Specifically, DscI is necessary for normal cytokinesis, cortical tension, membrane-cortex connections, and cortical distribution and mechanoresponsiveness of CortI. The dscI deletion mutants also have complex genetic epistatic relationships with CK components, acting as a genetic suppressor of cortI and iqgap1, but as an enhancer of iqgap2. This work underscores the fact that proteins like DiscI contribute in diverse ways to the activities necessary for optimal cell function.

Deletion of gmfA induces keratocyte-like migration in Dictyostelium

Glia maturation factor (GMF) has been established as an inactivating factor of the actin‐related protein 2/3 (Arp2/3) complex, which regulates actin assembly. Regulation of actin assembly and reorganization is crucial for various cellular events, such as cell migration, cell division, and development. Here, to examine the roles of ADF‐H domain‐containing protein (also known as glia maturation factor; GmfA), the product of a single GMF homologous gene in Dictyostelium, gmfA‐null cells were generated. They had moderate defects in cell growth and cytokinesis. Interestingly, they showed a keratocyte‐like fan shape with a broader pseudopod, where Arp3 accumulated at higher levels than in wild‐type cells. They migrated with higher persistence, but their velocities were comparable to those of wild‐type cells. The polar pseudopods during cell division were also broader than those in wild‐type cells. However, GmfA did not localize at the pseudopods in migrating cells or the polar pseudopods in dividing cells. Adhesions of mutant cells to the substratum were much stronger than that of wild‐type cells. Although the mutant cells showed chemotaxis comparable to that of wild‐type cells, they formed disconnected streams during the aggregation stage; however, they finally formed normal fruiting bodies. These results suggest that GmfA plays a crucial role in cell migration.

How cortical waves drive fission of motile cells

Cytokinesis-the division of a cell into two daughter cells-is a key step in cell growth and proliferation. It typically occurs in synchrony with the cell cycle to ensure that a complete copy of the genetic information is passed on to the next generation of daughter cells. In animal cells, cytokinesis commonly relies on an actomyosin contractile ring that drives equatorial furrowing and separation into the two daughter cells. However, also contractile ring-independent forms of cell division are known that depend on substrate-mediated traction forces. Here, we report evidence of an as yet unknown type of contractile ring-independent cytokinesis that we termed wave-mediated cytofission. It is driven by self-organized cortical actin waves that travel across the ventral membrane of oversized, multinucleated Dictyostelium discoideum cells. Upon collision with the cell border, waves may initiate the formation of protrusions that elongate and eventually pinch off to form separate daughter cells. They are composed of a stable elongated wave segment that is enclosed by a cell membrane and moves in a highly persistent fashion. We rationalize our observations based on a noisy excitable reaction-diffusion model in combination with a dynamic phase field to account for the cell shape and demonstrate that daughter cells emerging from wave-mediated cytofission exhibit a well-controlled size.

Dynamin-Like Protein B of Dictyostelium Contributes to Cytokinesis Cooperatively with Other Dynamins

Dynamin is a large GTPase responsible for diverse cellular processes, such as endocytosis, division of organelles, and cytokinesis. The social amoebozoan, Dictyostelium discoideum, has five dynamin-like proteins: dymA, dymB, dlpA, dlpB, and dlpC. DymA, dlpA, or dlpB-deficient cells exhibited defects in cytokinesis. DlpA and dlpB were found to colocalize at cleavage furrows from the early phase, and dymA localized at the intercellular bridge connecting the two daughter cells, indicating that these dynamins contribute to cytokinesis at distinct dividing stages. Total internal reflection fluorescence microscopy revealed that dlpA and dlpB colocalized at individual dots at the furrow cortex. However, dlpA and dlpB did not colocalize with clathrin, suggesting that they are not involved in clathrin-mediated endocytosis. The fact that dlpA did not localize at the furrow in dlpB null cells and vice versa, as well as other several lines of evidence, suggests that hetero-oligomerization of dlpA and dlpB is required for them to bind to the furrow. The hetero-oligomers directly or indirectly associate with actin filaments, stabilizing them in the contractile rings. Interestingly, dlpA, but not dlpB, accumulated at the phagocytic cups independently of dlpB. Our results suggest that the hetero-oligomers of dlpA and dlpB contribute to cytokinesis cooperatively with dymA.

Actin-binding domains mediate the distinct distribution of two Dictyostelium Talins through different affinities to specific subsets of actin filaments during directed cell migration

Although the distinct distribution of certain molecules along the anterior or posterior edge is essential for directed cell migration, the mechanisms to maintain asymmetric protein localization have not yet been fully elucidated. Here, we studied a mechanism for the distinct localizations of two Dictyostelium talin homologues, talin A and talin B, both of which play important roles in cell migration and adhesion. Using GFP fusion, we found that talin B, as well as its C-terminal actin-binding region, which consists of an I/LWEQ domain and a villin headpiece domain, was restricted to the leading edge of migrating cells. This is in sharp contrast to talin A and its C-terminal actin-binding domain, which co-localized with myosin II along the cell posterior cortex, as reported previously. Intriguingly, even in myosin II-null cells, talin A and its actin-binding domain displayed a specific distribution, co-localizing with stretched actin filaments. In contrast, talin B was excluded from regions rich in stretched actin filaments, although a certain amount of its actin-binding region alone was present in those areas. When cells were sucked by a micro-pipette, talin B was not detected in the retracting aspirated lobe where acto-myosin, talin A, and the actin-binding regions of talin A and talin B accumulated. Based on these results, we suggest that talin A predominantly interacts with actin filaments stretched by myosin II through its C-terminal actin-binding region, while the actin-binding region of talin B does not make such distinctions. Furthermore, talin B appears to have an additional, unidentified mechanism that excludes it from the region rich in stretched actin filaments. We propose that these actin-binding properties play important roles in the anterior and posterior enrichment of talin B and talin A, respectively, during directed cell migration.

Comparative phosphoproteomic analysis reveals signaling networks regulating monopolar and bipolar cytokinesis

The successful completion of cytokinesis requires the coordinated activities of diverse cellular components including membranes, cytoskeletal elements and chromosomes that together form partly redundant pathways, depending on the cell type. The biochemical analysis of this process is challenging due to its dynamic and rapid nature. Here, we systematically compared monopolar and bipolar cytokinesis and demonstrated that monopolar cytokinesis is a good surrogate for cytokinesis and it is a well-suited system for global biochemical analysis in mammalian cells. Based on this, we established a phosphoproteomic signature of cytokinesis. More than 10,000 phosphorylation sites were systematically monitored; around 800 of those were up-regulated during cytokinesis. Reconstructing the kinase-substrate interaction network revealed 31 potentially active kinases during cytokinesis. The kinase-substrate network connects proteins between cytoskeleton, membrane and cell cycle machinery. We also found consensus motifs of phosphorylation sites that can serve as biochemical markers specific to cytokinesis. Beyond the kinase-substrate network, our reconstructed signaling network suggests that combination of sumoylation and phosphorylation may regulate monopolar cytokinesis specific signaling pathways. Our analysis provides a systematic approach to the comparison of different cytokinesis types to reveal alternative ways and a global overview, in which conserved genes work together and organize chromatin and cytoplasm during cytokinesis.

A novel mode of cytokinesis without cell-substratum adhesion

Cytokinesis is a final step in cell division. Dictyostelium cells, a model organism for the study of cytokinesis, have multiple modes, denoted cytokinesis A, B, C, and D. All these modes have been mainly investigated using cells adhering to the substratum although they can grow in shaking suspension culture. Here, we observed how cells divide without adhering to the substratum using a new non-adhesive material. These detached cells formed the cleavage furrow but eventually failed in the final abscission. Thus, the cells cannot divide without adhesion, suggesting that they cannot divide only through the conventional cytokinesis A. However, in a long-term culture, the detached cells adhered each other to form multicellular aggregates and divided properly in these aggregates. Myosin II-null cells also formed such aggregates but could not divide in the aggregates. Several lines of experiments using mutant cells showed that the process of cytokinesis in multicellular aggregates is a novel mode utilizing a confined space in the aggregate in a myosin II-dependent manner. These results shed light on a poorly characterized mechanism of cytokinesis in multicellular spheroids or tissues. We propose to redefine and classify multiple modes of cytokinesis.

Myosin-independent cytokinesis in Giardia utilizes flagella to coordinate force generation and direct membrane trafficking

Devoid of all known canonical actin-binding proteins, the prevalent parasite Giardia lamblia uses an alternative mechanism for cytokinesis. Unique aspects of this mechanism can potentially be leveraged for therapeutic development. Here, live-cell imaging methods were developed for Giardia to establish division kinetics and the core division machinery. Surprisingly, Giardia cytokinesis occurred with a median time that is ∼60 times faster than mammalian cells. In contrast to cells that use a contractile ring, actin was not concentrated in the furrow and was not directly required for furrow progression. Live-cell imaging and morpholino depletion of axonemal Paralyzed Flagella 16 indicated that flagella-based forces initiated daughter cell separation and provided a source for membrane tension. Inhibition of membrane partitioning blocked furrow progression, indicating a requirement for membrane trafficking to support furrow advancement. Rab11 was found to load onto the intracytoplasmic axonemes late in mitosis and to accumulate near the ends of nascent axonemes. These developing axonemes were positioned to coordinate trafficking into the furrow and mark the center of the cell in lieu of a midbody/phragmoplast. We show that flagella motility, Rab11, and actin coordination are necessary for proper abscission. Organisms representing three of the five eukaryotic supergroups lack myosin II of the actomyosin contractile ring. These results support an emerging view that flagella play a central role in cell division among protists that lack myosin II and additionally implicate the broad use of membrane tension as a mechanism to drive abscission.

Traction force and its regulation during cytokinesis in Dictyostelium cells

Cytokinesis is the final stage of cell division. Dictyostelium cells have multiple modes of cytokinesis, including cytokinesis A, B and C. Cytokinesis A is a conventional mode, which depends on myosin II in the contractile ring. Myosin II null cells divide depending on substratum-attachment (cytokinesis B) or in a multi-polar fashion independent of the cell cycle (cytokinesis C). We investigated the traction stress exerted by dividing cells in the three different modes using traction force microscopy. In all cases, the traction forces were directed inward from both poles. Interestingly, the traction stress of cytokinesis A was the smallest of the three modes. Latrunculin B, an inhibitor of actin polymerization, completely diminished the traction stress of dividing cells, but blebbistatin, an inhibitor of myosin II ATPase, increased the traction stress. Myosin II is proposed to contribute to the detachment of cell body from the substratum. When the cell-substratum attachment was artificially strengthened by a poly-lysine coating, wild type cells increased their traction stress in contrast to myosin II null and other cytokinesis-deficient mutant cells, which suggests that wild type cells may increase their own power to conduct their cytokinesis. The cytokinesis-deficient mutants frequently divided unequally, whereas wild type cells divided equally. A traction stress imbalance between two daughter halves was correlated with cytokinesis failure. We discuss the regulation of cell shape changes during cell division through mechanosensing.

Rap1-dependent pathways coordinate cytokinesis in Dictyostelium

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

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

Dynamin contributes to cytokinesis by stabilizing actin filaments in the contractile ring

Dynamin has been proposed to play an important role in cytokinesis, although the nature of its contribution has remained unclear. Dictyostelium discoideum has five dynamin-like proteins: DymA, DymB, DlpA, DlpB and DlpC. Cells mutant for dymA, dlpA or dlpB presented defects in cytokinesis that resulted in multinucleation when the cells were cultured in suspension. However, the cells could divide normally when attached to the substratum; this latter process depends on traction-mediated cytokinesis B. A dynamin GTPase inhibitor also blocked cytokinesis in suspension, suggesting an important role for dynamin in cytokinesis A, which requires a contractile ring powered by myosin II. Myosin II did not properly localize to the cleavage furrow in dynamin mutant cells, and the furrow shape was distorted. DymA and DlpA were associated with actin filaments at the furrow. Fluorescence recovery after photobleaching and a DNase I binding assay showed that actin filaments in the contractile ring were significantly fragmented in mutant cells. Dynamin is therefore involved in the stabilization of actin filaments in the furrow, which, in turn, maintain proper myosin II organization. We conclude that the lack of these dynamins disrupts proper actomyosin organization and thereby disables cytokinesis A.

Alternative cytoskeletal landscapes: cytoskeletal novelty and evolution in basal excavate protists

Microbial eukaryotes encompass the majority of eukaryotic evolutionary and cytoskeletal diversity. The cytoskeletal complexity observed in multicellular organisms appears to be an expansion of components present in genomes of diverse microbial eukaryotes such as the basal lineage of flagellates, the Excavata. Excavate protists have complex and diverse cytoskeletal architectures and life cycles-essentially alternative cytoskeletal 'landscapes'-yet still possess conserved microtubule-associated and actin-associated proteins. Comparative genomic analyses have revealed that a subset of excavates, however, lack many canonical actin-binding proteins central to actin cytoskeleton function in other eukaryotes. Overall, excavates possess numerous uncharacterized and 'hypothetical' genes, and may represent an undiscovered reservoir of novel cytoskeletal genes and cytoskeletal mechanisms. The continued development of molecular genetic tools in these complex microbial eukaryotes will undoubtedly contribute to our overall understanding of cytoskeletal diversity and evolution.

3D Morphology, ultrastructure and development of Ceratomyxa puntazzi stages: first insights into the mechanisms of motility and budding in the Myxozoa

Free, amoeboid movement of organisms within media as well as substrate-dependent cellular crawling processes of cells and organisms require an actin cytoskeleton. This system is also involved in the cytokinetic processes of all eukaryotic cells. Myxozoan parasites are known for the disease they cause in economical important fishes. Usually, their pathology is related to rapid proliferation in the host. However, the sequences of their development are still poorly understood, especially with regard to pre-sporogonic proliferation mechanisms. The present work employs light microscopy (LM), electron microscopy (SEM, TEM) and confocal laser scanning microscopy (CLSM) in combination with specific stains (Nile Red, DAPI, Phalloidin), to study the three-dimensional morphology, motility, ultrastructure and cellular composition of Ceratomyxa puntazzi, a myxozoan inhabiting the bile of the sharpsnout seabream.

Our results demonstrate the occurrence of two C. puntazzi developmental cycles in the bile, i.e. pre-sporogonic proliferation including frequent budding as well as sporogony, resulting in the formation of durable spore stages and we provide unique details on the ultrastructure and the developmental sequence of bile inhabiting myxozoans. The present study describes, for the first time, the cellular components and mechanisms involved in the motility of myxozoan proliferative stages, and reveals how the same elements are implicated in the processes of budding and cytokinesis in the Myxozoa. We demonstrate that F-actin rich cytoskeletal elements polarize at one end of the parasites and in the filopodia which are rapidly de novo created and re-absorbed, thus facilitating unidirectional parasite motility in the bile. We furthermore discover the myxozoan mechanism of budding as an active, polarization process of cytokinesis, which is independent from a contractile ring and thus differs from the mechanism, generally observed in eurkaryotic cells. We hereby demonstrate that CLSM is a powerful tool for myxozoan research with a great potential for exploitation, and we strongly recommend its future use in combination with in vivo stains.

A high-resolution shape fitting and simulation demonstrated equatorial cell surface softening during cytokinesis and its promotive role in cytokinesis

Different models for animal cell cytokinesis posit that the stiffness of the equatorial cortex is either increased or decreased relative to the stiffness of the polar cortex. A recent work has suggested that the critical cytokinesis signaling complex centralspindlin may reduce the stiffness of the equatorial cortex by inactivating the small GTPase Rac. To determine if such a reduction occurs and if it depends on centralspindlin, we devised a method to estimate cortical bending stiffness with high spatio-temporal resolution from in vivo cell shapes. Using the early Caenorhabditis elegans embryo as a model, we show that the stiffness of the equatorial cell surface is reduced during cytokinesis, whereas the stiffness of the polar cell surface remains stiff. The equatorial reduction of stiffness was compromised in cells with a mutation in the gene encoding the ZEN-4/kinesin-6 subunit of centralspindlin. Theoretical modeling showed that the absence of the equatorial reduction of stiffness could explain the arrest of furrow ingression in the mutant. By contrast, the equatorial reduction of stiffness was sufficient to generate a cleavage furrow even without the constriction force of the contractile ring. In this regime, the contractile ring had a supportive contribution to furrow ingression. We conclude that stiffness is reduced around the equator in a centralspindlin-dependent manner. In addition, computational modeling suggests that proper regulation of stiffness could be sufficient for cleavage furrow ingression.

A single β adaptin contributes to AP1 and AP2 complexes and clathrin function in Dictyostelium

The assembly of clathrin-coated vesicles is important for numerous cellular processes, including nutrient uptake and membrane organization. Important contributors to clathrin assembly are four tetrameric assembly proteins, also called adaptor proteins (APs), each of which contains a β subunit. We identified a single β subunit, named β1/2, that contributes to both the AP1 and AP2 complexes of Dictyostelium. Disruption of the gene encoding β1/2 resulted in severe defects in growth, cytokinesis and development. Additionally, cells lacking β1/2 displayed profound osmoregulatory defects including the absence of contractile vacuoles and mislocalization of contractile vacuole markers. The phenotypes of β1/2 null cells were most similar to previously described phenotypes of clathrin and AP1 mutants, supporting a particularly important contribution of AP1 to clathrin pathways in Dictyostelium cells. The absence of β1/2 in cells led to significant reductions in the protein amounts of the medium-sized subunits of the AP1 and AP2 complexes, establishing a role for the β subunit in the stability of the medium subunits. Dictyostelium β1/2 could resemble a common ancestor of the more specialized β1 and β2 subunits of the vertebrate AP complexes. Our results support the essential contribution of a single β subunit to the stability and function of AP1 and AP2 in a simple eukaryote.

Stretching actin filaments within cells enhances their affinity for the myosin II motor domain

To test the hypothesis that the myosin II motor domain (S1) preferentially binds to specific subsets of actin filaments in vivo, we expressed GFP-fused S1 with mutations that enhanced its affinity for actin in Dictyostelium cells. Consistent with the hypothesis, the GFP-S1 mutants were localized along specific portions of the cell cortex. Comparison with rhodamine-phalloidin staining in fixed cells demonstrated that the GFP-S1 probes preferentially bound to actin filaments in the rear cortex and cleavage furrows, where actin filaments are stretched by interaction with endogenous myosin II filaments. The GFP-S1 probes were similarly enriched in the cortex stretched passively by traction forces in the absence of myosin II or by external forces using a microcapillary. The preferential binding of GFP-S1 mutants to stretched actin filaments did not depend on cortexillin I or PTEN, two proteins previously implicated in the recruitment of myosin II filaments to stretched cortex. These results suggested that it is the stretching of the actin filaments itself that increases their affinity for the myosin II motor domain. In contrast, the GFP-fused myosin I motor domain did not localize to stretched actin filaments, which suggests different preferences of the motor domains for different structures of actin filaments play a role in distinct intracellular localizations of myosin I and II. We propose a scheme in which the stretching of actin filaments, the preferential binding of myosin II filaments to stretched actin filaments, and myosin II-dependent contraction form a positive feedback loop that contributes to the stabilization of cell polarity and to the responsiveness of the cells to external mechanical stimuli.

The LRRK2-related Roco kinase Roco2 is regulated by Rab1A and controls the actin cytoskeleton

We identify a new pathway that is required for proper pseudopod formation. We show that Roco2, a leucine-rich repeat kinase 2 (LRRK2)-related Roco kinase, is activated in response to chemoattractant stimulation and helps mediate cell polarization and chemotaxis by regulating cortical F-actin polymerization and pseudopod extension in a pathway that requires Rab1A. We found that Roco2 binds the small GTPase Rab1A as well as the F-actin cross-linking protein filamin (actin-binding protein 120, abp120) in vivo. We show that active Rab1A (Rab1A-GTP) is required for and regulates Roco2 kinase activity in vivo and that filamin lies downstream from Roco2 and controls pseudopod extension during chemotaxis and random cell motility. Therefore our study uncovered a new signaling pathway that involves Rab1A and controls the actin cytoskeleton and pseudopod extension, and thereby, cell polarity and motility. These findings also may have implications in the regulation of other Roco kinases, including possibly LRRK2, in metazoans.

Cytokinesis through biochemical-mechanical feedback loops

Cytokinesis is emerging as a control system defined by interacting biochemical and mechanical modules, which form a system of feedback loops. This integrated system accounts for the regulation and kinetics of cytokinesis furrowing and demonstrates that cytokinesis is a whole-cell process in which the global and equatorial cortices and cytoplasm are active players in the system. Though originally defined in Dictyostelium, features of the control system are recognizable in other organisms, suggesting a universal mechanism for cytokinesis regulation and contractility.

SCAR/WAVE is activated at mitosis and drives myosin-independent cytokinesis

Cell division requires the tight coordination of multiple cytoskeletal pathways. The best understood of these involves myosin-II-dependent constriction around the cell equator, but both Dictyostelium and mammalian cells also use a parallel, adhesion-dependent mechanism to generate furrows. We show that the actin nucleation factor SCAR/WAVE is strongly activated during Dictyostelium cytokinesis. This activation localises to large polar protrusions, driving separation of the daughter cells. This continues for 10 minutes after division before the daughter cells revert to normal random motility, indicating that this is a tightly regulated process. We demonstrate that SCAR activity is essential to drive myosin-II-independent cytokinesis, and stabilises the furrow, ensuring symmetrical division. SCAR is also responsible for the generation of MiDASes, mitosis-specific actin-rich adhesions. Loss of SCAR in both Dictyostelium and Drosophila leads to a similar mitotic phenotype, with severe mitotic blebbing, indicating conserved functionality. We also find that the microtubule end-binding protein EB1 is required to restrict SCAR localisation and direct migration. EB1-null cells also exhibit decreased adhesion during mitosis. Our data reveal a spindle-directed signalling pathway that regulates SCAR activity, migration and adhesion at mitosis.

Physical model of contractile ring initiation in dividing cells

We present a physical mechanism to describe initiation of the contractile ring during cell division. The model couples the membrane curvature with the contractile forces produced by protein clusters attached to the membrane. These protein clusters are mobile on the membrane and possess either an isotropic or an anisotropic spontaneous curvature. Our results show that under these conditions the contraction force gives rise to an instability that corresponds in a closed cellular system to the initiation of the contractile ring. We find a quantization of this process at distinct length-scales, which we compare to available data for different types of eukaryote cells.

Contractile ring-independent localization of DdINCENP, a protein important for spindle stability and cytokinesis

Dictyostelium DdINCENP is a chromosomal passenger protein associated with centromeres, the spindle midzone, and poles during mitosis and the cleavage furrow during cytokinesis. Disruption of the single DdINCENP gene revealed important roles for this protein in mitosis and cytokinesis. DdINCENP null cells lack a robust spindle midzone and are hypersensitive to microtubule-depolymerizing drugs, suggesting that their spindles may not be stable. Furthermore DdCP224, a protein homologous to the microtubule-stabilizing protein TOGp/XMAP215, was absent from the spindle midzone of DdINCENP null cells. Overexpression of DdCP224 rescued the weak spindle midzone defect of DdINCENP null cells. Although not required for the localization of the myosin II contractile ring and subsequent formation of a cleavage furrow, DdINCENP is important for the abscission of daughter cells at the end of cytokinesis. Finally, we show that the localization of DdINCENP at the cleavage furrow is modulated by myosin II but it occurs by a mechanism different from that controlling the formation of the contractile ring.

The stress and strain of cytokinesis

The ultimate goal of all signaling pathways in cytokinesis is to control the mechanical separation of the mother cell into two daughter cells. Because of the intrinsic mechanical nature of cytokinesis, it is essential to understand fully how cell shapes and the material properties of the cell are generated, how these shapes and material properties create force, and how motor proteins such as myosin-II modify the system to achieve successful cytokinesis. In this review (which is part of the Cytokinesis series), we discuss the relevant physical properties of cells, how these properties are measured and the basic models that are used to understand cell mechanics. Finally, we present our current understanding of how cytokinesis mechanics work.

Temporal and Spatial Regulation of Phosphoinositide Signaling Mediates Cytokinesis

Polarity is a prominent feature of both chemotaxis and cytokinesis. In chemotaxis, polarity is established by local accumulation of PI(3,4,5)P3 at the cell's leading edge, achieved through temporal and spatial regulation of PI3 kinases and the tumor suppressor, PTEN. We find that as migrating D. discoideum cells round up to enter cytokinesis, PI(3,4,5)P3 signaling is uniformly suppressed. Then, as the spindle and cell elongate, PI3 kinases and PTEN move to and function at the poles and furrow, respectively. Cell lines lacking both of these enzymatic activities fail to modulate PI(3,4,5)P3 levels, are defective in cytokinesis, and cannot divide in suspension. The cells continue to grow and duplicate their nuclei, generating large multinucleate cells. Furrows that fail to ingress between nuclei are unable to stably accumulate myosin filaments or suppress actin-filled ruffles. We propose that phosphoinositide-linked circuits, similar to those that bring about asymmetry during cell migration, also regulate polarity in cytokinesis.

Mechanics and regulation of cytokinesis

Recent advances are revealing quantitative aspects of cytokinesis. Further, genetic analyses and cell imaging are providing insights into the molecular dynamics of cleavage furrow ingression as well as further refining our knowledge of the zones of the mitotic spindle that regulate the contractile properties of the overlying cortex. Ultimately, however, cortical mechanics are the result of signals that emanate from the mitotic spindle. A genuine quantitative understanding of cytokinesis must include a thorough analysis of the mechanical properties of the cortex and how signals modify these properties to dictate a well-controlled, error-free cytokinesis.

A mitotic kinesin-like protein required for normal karyokinesis, myosin localization to the furrow, and cytokinesis in Dictyostelium

Dictyostelium mitotic kinesin Kif12 is required for cytokinesis. Myosin II localization to the cleavage furrow is severely depressed in Kif12-null (Deltakif12) cells, which accounts in part for the cytokinesis failure. Myosin II-null cells, however, undergo mitosis-coupled cytokinesis when adhering to a surface, whereas the Deltakif12 cells cannot. During mitosis, the rate of change of internuclear separation in Deltakif12 cells is reduced compared with wild-type cells, indicating multiple roles of this molecular motor during mitosis and cytokinesis. GFP-Kif12, which rescues wild-type behavior when expressed in the Deltakif12 strain, is concentrated in the nucleus in interphase cells, translocates to the cytoplasm at the onset of mitosis, appears in the centrosomes and spindle, and later is concentrated in the spindle midbody. Given these results, we hypothesize a mechanism for myosin II translocation to the furrow to set up the contractile ring.

Variations on a theme: the many modes of cytokinesis

Animal cell division is believed to be mediated primarily by the 'purse-string' mechanism, which entails furrowing of the equatorial region, driven by the interaction of actin and myosin II filaments within contractile rings. However, myosin II-null Dictyostelium cells on substrates divide efficiently in a cell cycle-coupled manner. This process, termed cytokinesis B, appears to be driven by polar traction forces. Data in the literature can be interpreted as suggesting that adherent higher animal cells also use a cytokinesis B-like mechanism for cytokinesis. An additional chemotaxis-based cytokinesis that involves a 'midwife' cell has also been reported. Collectively, these findings demonstrate an unexpected diversity of mechanisms by which animal cells carry out cytokinesis.

Multiple Parallelisms in Animal Cytokinesis

The process of cytokinesis in animal cells is usually presented as a relatively simple picture: A cleavage plane is first positioned in the equatorial region by the astral microtubules of the anaphase mitotic apparatus, and a contractile ring made up of parallel filaments of actin and myosin II is formed and encircles the cortex at the division site. Active sliding between the two filament systems constricts the perimeter of the cortex, leading to separation of two daughter cells. However, recent studies in both animal cells and lower eukaryotic model organisms have demonstrated that cytokinesis is actually far more complex. It is now obvious that the three key processes of cytokinesis, cleavage plane determination, equatorial furrowing, and scission, are driven by different mechanisms in different types of cells. In some cases, moreover, multiple pathways appear to have redundant functions in a single cell type. In this review, we present a novel hypothesis that incorporates recent observations on the activities of mitotic microtubules and the biochemistry of Rho-type GTPase proteins and postulates that two different sets of microtubules are responsible for the two known mechanisms of cleavage plane determination and also for two distinct mechanisms of equatorial furrowing.

Dictyostelium cytokinesis: from molecules to mechanics

Cytokinesis is the mechanical process that allows the simplest unit of life, the cell, to divide, propagating itself. To divide, the cell converts chemical energy into mechanical energy to produce force. This process is thought to be active, due in large part to the mechanochemistry of the myosin-II ATPase. The cell's viscoelasticity defines the context and perhaps the magnitude of the forces that are required for cytokinesis. The viscoelasticity may also guide the force-generating apparatus, specifying the cell shape change that results. Genetic, biochemical, and mechanical measurements are providing a quantitative view of how real proteins control this essential life process.

Myosins and cell dynamics in cellular slime molds

Myosin is a mechanochemical transducer and serves as a motor for various motile activities such as cell migration, cytokinesis, maintenance of cell shape, phagocytosis, and morphogenesis. Nonmuscle myosin in vivo does not either stay static at specific subcellular regions or construct highly organized structures, such as sarcomere in skeletal muscle cells. The cellular slime mold Dictyostelium discoideum is an ideal "model organism" for the investigation of cell movement and cytokinesis. The advantages of this organism prompted researchers to carry out pioneering cell biological, biochemical, and molecular genetic studies on myosin II, which resulted in elucidation of many fundamental features of function and regulation of this most abundant molecular motor. Furthermore, recent molecular biological research has revealed that many unconventional myosins play various functions in vivo. In this article, how myosins are organized and regulated in a dynamic manner in Dictyostelium cells is reviewed and discussed.

The role of BEACH proteins in Dictyostelium

The BEACH family of proteins is a novel group of proteins with diverse roles in eukaryotic cells. The identifying feature of these proteins is the BEACH domain named after the founding members of this family, the mouse beige and the human Chediak-Higashi syndrome proteins. Although all BEACH proteins share a similar structural organization, they appear to have very distinct cellular roles, ranging from lysosomal traffic to apoptosis and cytokinesis. Very little is currently known about the function of most of these proteins, few binding-partner proteins have been identified, and no molecular mechanism for any of these proteins has been discovered. Thus, it is important to establish good model systems for the study of these novel proteins. Dictyostelium contains six BEACH proteins that can be classified into four subclasses. Two of them, LvsA and LvsB, have clearly distinct roles in the cell. LvsA is localized on the contractile vacuole membrane and is essential for cytokinesis and osmoregulation. LvsB is most similar in sequence to the mammalian beige/Chediak-Higashi syndrome proteins and shares with them a common function in lysosomal trafficking. Structural and functional analysis of these proteins in Dictyostelium will help elucidate the function of this enigmatic novel family of proteins.

The mammalian septin MSF localizes with microtubules and is required for completion of cytokinesis

Cytokinesis in animal cells involves the contraction of an actomyosin ring formed at the cleavage furrow. Nuclear division, or karyokinesis, must be precisely timed to occur before cytokinesis in order to prevent genetic anomalies that would result in either cell death or uncontrolled cell division. The septin family of GTPase proteins has been shown to be important for cytokinesis although little is known about their role during this process. Here we investigate the distribution and function of the mammalian septin MSF. We show that during interphase, MSF colocalizes with actin, microtubules, and another mammalian septin, Nedd5, and coprecipitates with six septin proteins. In addition, transfections of various MSF isoforms reveal that MSF-A specifically localizes with microtubules and that this localization is disrupted by nocodazole treatment. Furthermore, MSF isoforms localize primarily with tubulin at the central spindle during mitosis, whereas Nedd5 is mainly associated with actin. Microinjection of affinity-purified anti-MSF antibodies into synchronized cells, or depletion of MSF by small interfering RNAs, results in the accumulation of binucleated cells and in cells that have arrested during cytokinesis. These results reveal that MSF is required for the completion of cytokinesis and suggest a role that is distinct from that of Nedd5.

Effects of anti-myosin drugs on anaphase chromosome movement and cytokinesis in crane-fly primary spermatocytes

To investigate whether myosin is involved in crane-fly primary spermatocyte division, we studied the effects of myosin inhibitors on chromosome movement and on cytokinesis. With respect to chromosome movement, the myosin ATPase inhibitor 2,3-butanedione 2-monoxime (BDM) added during autosomal anaphase reversibly perturbed the movements of all autosomes: autosomes stopped, slowed, or moved backwards during treatment. BDM added before anaphase onset altered chromosome movement less than when BDM was added during anaphase: chromosome movements only rarely were stopped. They often were normal initially and then, if altered at all, were slowed. To confirm that the effects of BDM were due to myosin inhibition, we treated cells with ML-7, a drug that inhibits myosin light chain kinase (MLCK), an enzyme necessary to activate myosin. ML-7 affected anaphase movement only when added in early prometaphase: this treatment prevented chromosome attachment to the spindle. We treated cells with H-7 as a control for possible non-myosin effects of ML-7. H-7, which has a lower affinity than ML-7 for MLCK but a higher affinity than ML-7 for other potential targets, had no effect. These data confirm that the BDM effect is on myosin and indicate that the myosin used for chromosome movement is activated near the start of prometaphase. With respect to cytokinesis, BDM did not block furrow initiation but did block subsequent contraction of the contractile ring. When BDM was added after initiation of the furrow, the contractile ring either stalled or relaxed. ML-7 blocked contractile ring contraction when added at all stages after autosomal anaphase onset, including when added during cytokinesis. H-7 had no effect. These results confirm that the effects of BDM are on myosin and indicate that the myosin used for cytokinesis is activated starting from autosomal anaphase and continuing throughout cytokinesis.

On the mechanism of cleavage furrow ingression in Dictyostelium

The ability of Dictyostelium cells to divide without myosin II in a cell cycle-coupled manner has opened two questions about the mechanism of cleavage furrow ingression. First, are there other possible functions for myosin II in this process except for generating contraction of the furrow by a sliding filament mechanism? Second, what could be an alternative mechanical basis for the furrowing? Using aberrant changes of the cell shape and anomalous localization of the actin-binding protein cortexillin I during asymmetric cytokinesis in myosin II-deficient cells as clues, it is proposed that myosin II filaments act as a mechanical lens in cytokinesis. The mechanical lens serves to focus the forces that induce the furrowing to the center of the midzone, a cortical region where cortexillins are enriched in dividing cells. Additionally, continual disassembly of a filamentous actin meshwork at the midzone is a prerequisite for normal ingression of the cleavage furrow and a successful cytokinesis. If this process is interrupted, as it occurs in cells that lack cortexillins, an overassembly of filamentous actin at the midzone obstructs the normal cleavage. Disassembly of the crosslinked actin network can generate entropic contractile forces in the cortex, and may be considered as an alternative mechanism for driving ingression of the cleavage furrow. Instead of invoking different types of cytokinesis that operate under attached and unattached conditions in Dictyostelium, it is anticipated that these cells use a universal multifaceted mechanism to divide, which is only moderately sensitive to elimination of its constituent mechanical processes.

Genetic approaches to dissect the mechanisms of two distinct pathways of cell cycle-coupled cytokinesis in Dictyostelium

Dictyostelium discoideum is a unique experimental organism which allows genetic analysis of the mechanism of cytokinesis of the animal type, and a number of mutations which affect cytokinesis in one way or other have been identified. Myosin II filaments accumulate in the equatorial region, and myosin II-null cells cannot divide in suspension, indicating that active, myosin II-dependent constriction of the cleavage furrow contributes to bisection of the cell. We refer to this method of cytokinesis as cytokinesis A. On substrates, however, myosin II-null cells divide efficiently in a cell cycle-coupled manner. This adhesion-dependent but myosin II-independent division method, which we termed cytokinesis B, is carried out by a pathway that is genetically distinct from that of cytokinesis A. Morphological analyses suggested that cytokinesis B is driven by radial traction forces generated along polar peripheries, which indirectly cause furrow ingression. Identification of two redundant pathways have allowed us to search genes involved in either pathway by mutagenizing cells which are already defective in one of the pathways. This approach enabled us to identify a number of novel cytokinesis-related genes, as well as to reclassify known genes as cytokinesis-related.

Molecular motors and membrane traffic in Dictyostelium

Phagocytosis and membrane traffic in general are largely dependent on the cytoskeleton and their associated molecular motors. The myosin family of motors, especially the unconventional myosins, interact with the actin cortex to facilitate the internalization of external materials during the early steps of phagocytosis. Members of the kinesin and dynein motor families, which mediate transport along microtubules (MTs), facilitate the intracellular processing of the internalized materials and the movement of membrane. Recent studies indicate that some unconventional myosins are also involved in membrane transport, and that the MT- and actin-dependent transport systems might interact with each other. Studies in Dictyostelium have led to the discovery of many motors involved in critical steps of phagocytosis and membrane transport. With the ease of genetic and biochemical approaches, the established functional analysis to test phagocytosis and vesicle transport, and the effort of the Dictyostelium cDNA and Genome Projects, Dictyostelium will continue to be a superb model system to study phagocytosis in particular and cytoskeleton and motors in general.