Intra-bundle contractions enable extensile properties of active actin networks

The cellular cortex is a dynamic and contractile actomyosin network modulated by actin-binding proteins. We reconstituted a minimal cortex adhered to a model cell membrane mimicking two processes mediated by the motor protein myosin: contractility and high turnover of actin monomers. Myosin reorganized these networks by extensile intra‑bundle contractions leading to an altered growth mechanism. Hereby, stress within tethered bundles induced nicking of filaments followed by repair via incorporation of free monomers. This mechanism was able to break the symmetry of the previously disordered network resulting in the generation of extensile clusters, reminiscent of structures found within cells.

Filopodia formation induced by active mDia2/Drf3

Filopodia are rod-shaped cell surface protrusions composed of a parallel bundle of actin filaments. Since filopodia frequently emanate from lamellipodia, it has been proposed that they form exclusively by the convergence and elongation of actin filaments generated in lamellipodia networks. However, filopodia form without Arp2/3-complex, which is essential for lamellipodia formation, indicating that actin filaments in filopodia may be generated by other nucleators. Here we analyzed the effects of ectopic expression of GFP-tagged full length or a constitutively active variant of the human formin mDia2/Drf3. By contrast to the full-length molecule, which did not affect cell behaviour and was entirely cytosolic, active Drf3 lacking the C-terminal regulatory region (Drf3DeltaDAD) induced the formation of filopodia and accumulated at their tips. Low expression of Drf3DeltaDAD induced rod-shaped or tapered filopodia, whereas over-expression resulted in multiple, club-shaped filopodia. The clubs were filled with densely bundled actin filaments, whose number but not packing density decreased further away from the tip. Interestingly, clubs frequently increased in width after protrusion beyond the cell periphery, which correlated with increased amounts of Drf3DeltaDAD at their tips. These data suggest Drf3-induced filopodia form and extend by de novo nucleation of actin filaments instead of convergent elongation. Finally, Drf3DeltaDAD also induced the formation of unusual, lamellipodia-like structures, which contained both lamellipodial markers and the prominent filopodial protein fascin. Microarray analyses revealed highly variable Drf3 expression levels in different commonly used cell lines, reflecting the need for more detailed analyses of the functions of distinct formins in actin cytoskeleton turnover and different cell types.

Formins and VASPs may co-operate in the formation of filopodia

Filopodia are finger-like cell protrusions composed of parallel arrays of actin filaments, which elongate through actin polymerization at their tips. These highly dynamic structures seem to be used by many cell types as sensing organs to explore environmental cues and have been implicated in cell motility as well as in cell-substrate adhesion. Formins are highly conserved multidomain proteins that play important roles in the nucleation of actin and the formation of linear actin filaments, yet their role in filopodia formation has remained poorly defined. The Dictyostelium diaphanous-related formin dDia2 is strongly enriched in filopodia tips. Genetic and biochemical analysis revealed that this protein is important for cell migration and cell adhesion, but most importantly for the formation of filopodia. Recently, we have identified the Dictyostelium VASP (vasodilator-stimulated phosphoprotein) orthologue as a binding partner of dDia2 and provide evidence for a co-operative role of both proteins in filopodia formation.

The actin-bundling protein cortexillin is the downstream target of a Rac1-signaling pathway required for cytokinesis

During the process of cytokinesis by which eukaryotic cells constrict and divide in two, multiple cellular activities have to be precisely coordinated in space and time to guarantee equal distribution of chromosomes and cytoplasm to the emerging daughter cells. Eventually, constriction of the cleavage furrow leads to the complete separation of the daughter cells. Since the basic observation of cell division some 100 years ago, the principal challenge has been to unravel the detailed molecular mechanisms and signaling events leading to cytokinesis. Regulation of this fundamental cellular process is still poorly understood yet a central issue in modern cell biology. In the recent past it became evident that small GTPases of the Ras super family play a major role during this process. This review is focused on a Rho family GTPase-mediated signaling pathway that is required for cleavage furrow assembly and cytokinesis by the actin-bundling protein cortexillin of D. discoideum cells.

Rac1 GTPases control filopodia formation, cell motility, endocytosis, cytokinesis and development in Dictyostelium

The function of the highly homologous Rac1A, Rac1B, and Rac1C GTPases of the Dictyostelium Rac1 group was investigated. All three GTPases bound with an equal capacity to the IQGAP-related protein DGAP1, with a preference for the activated GTP-bound form. Strong overexpression of wild-type Rac1 GTPases N-terminally tagged with green fluorescent protein (GFP), predominantly induced the formation of numerous long filopodia. Remarkably, expression of the constitutively-activated GTPases resulted in dominant-negative phenotypes: these Rac1-V12 mutants completely lacked filopodia but formed numerous crown shaped structures resembling macropinosomes. Moreover, these mutants were severely impaired in cell motility, colony growth, phagocytosis, pinocytosis, cytokinesis and development. Transformants expressing constitutively-inactivated Rac1-N17 proteins were similar to wild-type cells, but displayed abundant and short filopodia and exhibited a moderate defect in cytokinesis. Taken together, our results indicate that the three GTPases play an identical role in signaling pathways and are key regulators of cellular activities that depend on the re-organization of the actin cytoskeleton in Dictyostelium.

Two-step positioning of a cleavage furrow by cortexillin and myosin II

BACKGROUND

Myosin II, a conventional myosin, is dispensable for mitotic division in Dictyostelium if the cells are attached to a substrate, but is required when the cells are growing in suspension. Only a small fraction of myosin II-null cells fail to divide when attached to a substrate. Cortexillins are actin-bundling proteins that translocate to the midzone of mitotic cells and are important for the formation of a cleavage furrow, even in attached cells. Here, we investigated how myosin II and cortexillin I cooperate to determine the position of a cleavage furrow.

RESULTS

Using a green fluorescent protein (GFP)-cortexillin I fusion protein as a marker for priming of a cleavage furrow, we found that positioning of a cleavage furrow occurred in two steps. In the first step, which was independent of myosin II and substrate, cortexillin I delineated a zone around the equatorial region of the cell. Myosin II then focused the cleavage furrow to the middle of this cortexillin I zone. If asymmetric cleavage in the absence of myosin II partitioned a cell into a binucleate and an anucleate portion, cell-surface ruffles were induced along the cleavage furrow, which led to movement of the anucleate portion along the connecting strand towards the binucleate one.

CONCLUSIONS

In myosin II-null cells, cleavage furrow positioning occurs in two steps: priming of the furrow region and actual cleavage, which may proceed in the middle or at one border of the cortexillin ring. A control mechanism acting at late cytokinesis prevents cell division into an anucleate and a binucleate portion, causing a displaced furrow to regress if it becomes aberrantly located on top of polar microtubule asters.

Domain analysis of cortexillin I: actin-bundling, PIP(2)-binding and the rescue of cytokinesis

Cortexillins are actin-bundling proteins that form a parallel two-stranded coiled-coil rod. Actin-binding domains of the alpha-actinin/spectrin type are located N-terminal to the rod and unique sequence elements are found in the C-terminal region. Domain analysis in vitro revealed that the N-terminal domains are not responsible for the strong actin-filament bundling activity of cortexillin I. The strongest activity resides in the C-terminal region. Phosphatidylinositol 4,5-bisphosphate (PIP(2)) suppresses this bundling activity by binding to a C-terminal nonapeptide sequence. These data define a new PIP(2)-regulated actin-bundling site. In vivo the PIP(2)-binding motif enhances localization of a C-terminal cortexillin I fragment to the cell cortex and improves the rescue of cytokinesis. This motif is not required, however, for translocation to the cleavage furrow. A model is presented proposing that cortexillin translocation is based on a mitotic cycle of polar actin polymerization and midzone depolymerization.

Adhesion-induced receptor segregation and adhesion plaque formation: A model membrane study

A model system to study the control of cell adhesion by receptor-mediated specific forces, universal interactions, and membrane elasticity is established. The plasma membrane is mimicked by reconstitution of homophilic receptor proteins into solid supported membranes and, together with lipopolymers, into giant vesicles with the polymers forming an artificial glycocalix. The homophilic cell adhesion molecule contact site A, a lipid-anchored glycoprotein from cells of the slime mold Dictyostelium discoideum, is used as receptor. The success of the reconstitution, the structure and the dynamics of the model membranes are studied by various techniques including film balance techniques, micro fluorescence, fluorescence recovery after photobleaching, electron microscopy, and phase contrast microscopy. The interaction of the functionalized giant vesicles with the supported bilayer is studied by reflection interference contrast microscopy, and the adhesion strength is evaluated quantitatively by a recently developed technique. At low receptor concentrations adhesion-induced receptor segregation in the membranes leads to decomposition of the contact zone between membranes into domains of strong (receptor-mediated) adhesion and regions of weak adhesion while continuous zones of strong adhesion form at high receptor densities. The adhesion strengths (measured in terms of the spreading pressure S) of the various states of adhesion are obtained locally by analysis of the vesicle contour near the contact line in terms of elastic boundary conditions of adhesion: the balance of tensions and moments. The spreading pressure of the weak adhesion zones is S approximately 10(-9) J/m(2) and is determined by the interplay of gravitation and undulation forces whereas the spreading pressure of the tight adhesion domains is of the order S approximately 10(-6) J/m(2).

Cytoskeletal protein mutations and cell motility in Dictyostelium

Dictyostelium is a suitable experimental system in which to study the effects of mutations in actin-binding proteins on cell motility. Three cytoskeletal mutants that show distinct alterations in cell shape, chemotactic movement and cytokinesis serve to illustrate the diversity of phenotypes. Cells lacking talin, a protein which in many mammalian cell types is a constituent of focal complexes that link the actin cytoskeleton to the plasma membrane, are strongly impaired in adhesion to external surfaces. Coronin is an actin-associated protein that belongs to the WD-repeat family of proteins, which are engaged in protein-protein interactions involved in signalling pathways. Cells lacking coronin build large hyaline protrusions at their leading edge, diagnostic of an imbalance in the actin polymerization/depolymerization cycle. Cells devoid of a pair of cortexillins, which are novel members of the spectrin/alpha-actinin superfamily of actin-binding proteins, form an atypical cleavage furrow on a solid surface and fail to divide in suspension. Other mutants in which one or more actin-binding proteins have been knocked out have weaker phenotypes. With these mutants, cells need to be subjected to special conditions in order to reveal an effect on cell motility. For instance, only on weakly adhesive surfaces is a disturbance in the spatio-temporal co-ordination of protrusion and retraction of the cell body, and of the attachment to and detachment from a substratum, observed in a mutant that lacks three actin-binding proteins: alpha-actinin, 120 kDa F-actin gelation factor and severin.

Cytokinesis mediated through the recruitment of cortexillins into the cleavage furrow

The fact that substrate-anchored Dictyostelium cells undergo cytokinesis in the absence of myosin II underscores the importance of other proteins in enabling the cleavage furrow to constrict. Cortexillins, a pair of actin-bundling proteins, are required for normal cleavage. They are targeted to the incipient furrow in wild-type and, more prominently, in myosin II-null cells. No other F-actin bundling or cross-linking protein tested is co-localized. Green fluorescent protein fusions show that the N-terminal actin-binding domain of cortexillin I is dispensable and the C-terminal region is sufficient for translocation to the furrow and the rescue of cytokinesis. Cortexillins are suggested to have a targeting signal for coupling to a myosin II-independent system that directs transport of membrane proteins to the cleavage furrow.

The IQGAP-related protein DGAP1 interacts with Rac and is involved in the modulation of the F-actin cytoskeleton and control of cell motility

DGAP1 of Dictyostelium discoideum is a cell cortex associated 95 kDa protein that shows homology to both RasGTPase-activating proteins (RasGAPs) and RasGAP-related proteins. When tested for RasGAP activity, recombinant DGAP1 protein did not promote the GTPase activity of human H-Ras or of Dictyostelium RasG in vitro. Instead, DGAP1 bound to Dictyostelium Rac1A and human Rac1, but not to human Cdc42. DGAP1 preferentially interacted with the activated GTP-bound forms of Rac1 and Rac1A, but did not affect the GTPase activities. Since Rho-type GTPases are implicated in the formation of specific F-actin structures and in the control of cell morphology, the microfilament system of mutants that either lack or overexpress DGAP1 has been analysed. DGAP1-null mutants showed elevated levels of F-actin that was organised in large leading edges, membrane ruffles or numerous large filopods. Expression of actin fused to green fluorescent protein (GFP) was used to monitor the actin dynamics in these cells, and revealed that the F-actin cytoskeleton of DGAP1-null cells was rapidly re-arranged to form ruffles and filopods. Conversely, in DGAP1-overexpressing cells, the formation of cellular projections containing F-actin was largely suppressed. Measurement of cell migration demonstrated that DGAP1 expression is inversely correlated with the speed of cell motility.

Detection of subtle phenotypes: the case of the cell adhesion molecule csA in Dictyostelium

Dictyostelium amoebae aggregate into a multicellular organism by cAMP-driven chemotaxis and cell-cell adhesion. Cell adhesion is mediated by an EDTA-sensitive and an EDTA-resistant adhesion system. The latter is developmentally regulated and triggered by homophilic interactions of the membrane glycoprotein csA; on disruption of the encoding gene, EDTA-resistant contacts fail to form. Nevertheless, csA-null cells under usual laboratory conditions aggregate normally and complete development. By using experimental conditions that reproduce more closely the habitat of Dictyostelium amoebae, evidence is provided that csA is required for development and that its expression confers a selective advantage to populations of wild-type cells over csA-null mutants. The latter display reduced cell-cell adhesion, increased adhesiveness to the substratum, and slower motility, which lead to their sorting out from aggregating wild-type cells. It is proposed that the experimental conditions commonly used in the laboratory are not stringent enough to assess the developmental role of csA and other proteins. The assay described can be used to detect subtle phenotypes, to reexamine the developmental role of apparently nonessential genes, and to test the validity of recent models on emergence and maintenance of apparent genetic redundancy.

A distinct 14 residue site triggers coiled-coil formation in cortexillin I

We have investigated the process of the assembly of the Dictyostelium discoideum cortexillin I oligomerization domain (Ir) into a tightly packed, two-stranded, parallel coiled-coil structure using a variety of recombinant polypeptide chain fragments. The structures of these Ir fragments were analyzed by circular dichroism spectroscopy, analytical ultracentrifugation and electron microscopy. Deletion mapping identified a distinct 14 residue site within the Ir coiled coil, Arg311-Asp324, which was absolutely necessary for dimer formation, indicating that heptad repeats alone are not sufficient for stable coiled-coil formation. Moreover, deletion of the six N-terminal heptad repeats of Ir led to the formation of a four- rather than a two-helix structure, suggesting that the full-length cortexillin I coiled-coil domain behaves as a cooperative folding unit. Most interestingly, a 16 residue peptide containing the distinct coiled-coil 'trigger' site Arg311-Asp324 yielded approximately 30% helix formation as monomer, in aqueous solution. pH titration and NaCl screening experiments revealed that the peptide's helicity depends strongly on pH and ionic strength, indicating that electrostatic interactions by charged side chains within the peptide are critical in stabilizing its monomer helix. Taken together, these findings demonstrate that Arg311-Asp324 behaves as an autonomous helical folding unit and that this distinct Ir segment controls the process of coiled-coil formation of cortexillin I.

DdLIM is a cytoskeleton-associated protein involved in the protrusion of lamellipodia in Dictyostelium

DdLim, a multi-domain member of the cysteine-rich family of LIM domain proteins, was isolated from Dictyostelium cells where it localizes in lamellipodia and at sites of membrane ruffling. The transcription and expression of DdLim are developmentally regulated, and the timing of its increased association with the actin cytoskeleton coincides with the acquisition in starved cells of a motile, chemotactic behavior. Vegetative cells that overexpress DdLim contain large lamella and exhibit ruffling at the cortex. The high frequency of large, multinucleated mutant cells found in suspension culture suggests that excess DdLim interferes with cytokinesis. DdLim was also identified as a protein in a Dictyostelium cell lysate that associated indirectly, but in a guanosine triphosphate-dependent manner, with a GST-rac1 fusion protein. The data presented suggest that DdLim acts as an adapter protein at the cytoskeleton-membrane interface where it is involved in a receptor-mediated rac1-signaling pathway that leads to actin polymerization in lamellipodia and ultimately cell motility.

Membrane bending modulus and adhesion energy of wild-type and mutant cells of Dictyostelium lacking talin or cortexillins

We have employed an interferometric technique for the local measurement of bending modulus, membrane tension, and adhesion energy of motile cells adhering to a substrate. Wild-type and mutant cells of Dictyostelium discoideum were incubated in a flow chamber. The flow-induced deformation of a cell near its adhesion area was determined by quantitative reflection interference contrast microscopy (RICM) and analyzed in terms of the elastic boundary conditions: equilibrium of tensions and bending moments at the contact line. This technique was employed to quantify changes caused by the lack of talin, a protein that couples the actin network to the plasma membrane, or by the lack of cortexillin I or II, two isoforms of the actin-bundling protein cortexillin. Cells lacking either cortexillin I or II exhibited reduced bending moduli of 95 and 160 k(B)T, respectively, as compared to 390 k(B)T, obtained for wild-type cells. No significant difference was found for the adhesion energies of wild-type and cortexillin mutant cells. In cells lacking talin, not only a strongly reduced bending modulus of 70 k(B)T, but also a low adhesion energy one-fourth of that in wild-type cells was measured.

DGAP1, a homologue of rasGTPase activating proteins that controls growth, cytokinesis, and development in Dictyostelium discoideum

A protein accumulated in the cortical region of Dictyostelium discoideum cells proved to be a homologue of GTPase activating proteins that are responsible for the inactivation of ras in yeast and man. Elimination of this protein, DGAP1, by gene replacement resulted in an increased rate of growth of D. discoideum cells on bacterial lawns, and in the formation of aberrant, multi-tipped fruiting bodies. Overexpression of DGAP1 caused the cells to become multi-nucleated since chromosome segregation during mitosis was not reliably followed by cleavage of the cells. These results suggest that in D. discoideum, ras or a related small GTP-binding protein is involved in regulating growth based on the phagocytosis of bacteria, and in coupling activities of the cell cortex to the organization of spindle and asters in mitotic cells.

Cortexillins, major determinants of cell shape and size, are actin-bundling proteins with a parallel coiled-coil tail

Cortexillins I and II of D. discoideum constitute a novel subfamily of proteins with actin-binding sites of the alpha-actinin/spectrin type. The C-terminal halves of these dimeric proteins contain a heptad repeat domain by which the two subunits are joined to form a two-stranded, parallel coiled coil, giving rise to a 19 nm tail. The N-terminal domains that encompass a consensus actin-binding sequence are folded into globular heads. Cortexillin-linked actin filaments form preferentially anti-parallel bundles that associate into meshworks. Both cortexillins are enriched in the cortex of locomoting cells, primarily at the anterior and posterior ends. Elimination of the two isoforms by gene disruption gives rise to large, flattened cells with rugged boundaries, portions of which are often connected by thin cytoplasmic bridges. The double-mutant cells are multinucleate owing to a severe impairment of cytokinesis.

pDcsA vectors for strictly regulated protein synthesis during early development of Dictyostelium discoideum

Two expression vectors have been constructed to express proteins exclusively in developing cells of Dictyostellium discoideum. In these Escherichia coli/D. discoideum shuttle vectors, proteins are synthesized under control of the promoter of the contact site A (csA) gene, which is efficiently suppressed during growth and becomes strongly activated during early development of D. discoideum. The pDcsA vectors appear to be valuable tools for the production of proteins that are not compatible with growth of D. discoideum cells.

Overexpression of the csA cell adhesion molecule under its own cAMP-regulated promoter impairs morphogenesis in Dictyostelium

The contact site A (csA) glycoprotein is a strictly developmentally regulated plasma membrane component responsible for the EDTA-stable (Ca(2+)-independent) form of intercellular adhesion in Dictyostelium discoideum. Using inverse polymerase chain reaction and a terminator vector we have isolated a 1.6 kb genomic fragment carrying a 1.1 kb upstream region of the csA gene. This fragment had promoter activity in D. discoideum cells, giving rise to a 3′-truncated csA RNA that was regulated like the mRNA of the endogenous gene. Cyclic AMP pulses strongly enhanced transcription from the cloned csA promoter. These findings provide evidence that the cloned region of the csA gene comprises the complete promoter. It contains a G/C-rich octamer motif similar to other cAMP-regulated D. discoideum promoters. When the csA protein was strongly overexpressed under the developmental control of the csA promoter, morphogenesis was substantially altered. Aggregation was delayed, and secondary centres were formed along aggregation streams that led to fragmentation of the aggregates and multiple slug formation. At high cell density a substantial portion of aggregated cells was left behind on the substratum when slugs and fruiting bodies were built. The transformation vector was also employed to rescue a csA-negative mutant, HG1287, from its cell adhesion defect.

Constitutive overexpression of the contact site A glycoprotein enables growth-phase cells of Dictyostelium discoideum to aggregate

The contact site A (csA) glycoprotein is a developmentally regulated cell adhesion molecule which mediates EDTA-stable cell contacts during the aggregation stage of Dictyostelium discoideum. A transformation vector was constructed which allows overexpression of the csA protein during the growth phase. In that stage the csA protein is normally not expressed; in the transformants it was transported to the cell surface and carried all modifications investigated, including a phospholipid anchor and two types of oligosaccharide chain. csA expression enabled the normal non-aggregative growth-phase cells to form EDTA-stable contacts in suspension and to assemble into three-dimensional aggregates when moving on a substratum. After prolonged cultivation of csA overexpressing transformants in nutrient medium the developmental program was found to be turned on, as it normally occurs only in starving cells. During later development of transformed cells, the csA glycoprotein remained present on the cell surface, while it is down-regulated in the wild type. It was detected in both the prestalk and prespore regions of the multicellular slugs made from transformed cells.

A Dictyostelium mutant with severe defects in alpha-actinin: its characterization using cDNA probes and monoclonal antibodies

Cells of a Dictyostelium discoideum mutant deficient in binding a monoclonal antibody to alpha-actinin have previously been shown to grow and develop similarly to the wild type and to exert unimpaired chemotaxis as well as patching and capping of membrane proteins. Here we show that the normal 3.0 kb message for alpha-actinin is replaced in the mutant by two RNA species of approximately 3.1 and 2.8 kb. The 3.1 kb RNA was recognized by DNA fragments from all parts of the coding region, while the 2.8 kb RNA hybridized to all but a 3'-terminal fragment. Proteins synthesized in the mutant were analysed using four monoclonal antibodies that in the wild type specifically recognize the 95 x 10(3) Mr polypeptide of alpha-actinin. Cleavage mapping indicated that the binding sites of these antibodies are distributed over a region comprising more than half of the alpha-actinin polypeptide chain. In the mutant, three of the antibodies faintly labelled two polypeptides of 95 x 10(3) Mr and 88 x 10(3) Mr; the fourth antibody, which binds closest to one end of the polypeptide chain, faintly labelled the 95 x 10(3) Mr polypeptide only. The 88 x 10(3) Mr polypeptide most probably lacks the C-terminal portion of alpha-actinin. The binding of an antibody that recognized both polypeptides was quantified by a radio-immuno competition assay using wild-type alpha-actinin as a reference. In a mutant cell extract containing total soluble proteins the antibody binding activity was decreased to 1.1% when compared with wild-type extract. After their partial purification and SDS-polyacrylamide gel electrophoresis the mutant 95 x 10(3) Mr and 88 x 10(3) Mr polypeptides were barely detectable as Coomassie Blue-stained bands, indicating that in the mutant not only certain epitopes of alpha-actinin were altered but the entire molecule is almost completely lacking. When the fitness of mutant cells relative to wild type was determined during growth in nutrient medium, a slight disadvantage for the mutant was indicated, by finding selection coefficients between 0.03 and 0.05.