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

Structural insights into kinetoplastid coronin oligomerization domain and F-actin interaction

The two-domain actin associated protein coronin interacts with filamentous (F-) actin, facilitating diverse biological processes including cell proliferation, motility, phagocytosis, host-parasite interaction and cargo binding. The conserved N-terminal β-propeller domain is involved in protein: protein interactions, while the C-terminal coiled-coil domain mediates oligomerization, transducing conformational changes. The L. donovani coronin coiled-coil (LdCoroCC) domain exhibited a novel topology and oligomer association with an inherent asymmetry, caused primarily by three a residues of successive heptads. In the T.brucei homolog (TbrCoro), two of these 'a' residues are different (Val 493 & 507 replacing LdCoroCC Ile 486 and Met 500 respectively). The elucidated structure possesses a similar topology and assembly while comparative structural analysis shows that the T.brucei coronin coiled-coil domain (TbrCoroCC) too possesses the asymmetry though its magnitude is smaller. Analysis identifies that the asymmetric state is stabilized via cyclic salt bridges formed by Arg 497 and Glu 504. Co-localization studies (LdCoro, TbrCoro and corresponding mutant coiled coil constructs) with actin show that there are subtle differences in their binding patterns, with the double mutant V493I-V507M showing maximal effect. None of the constructs have an effect on F-actin length. Taken together with LdCoroCC, we therefore conclude that the inherent asymmetric structures are essential for kinetoplastids, and are of interest in understanding and exploiting actin dynamics.

Interactome and F-Actin Interaction Analysis of Dictyostelium discoideum Coronin A

Coronin proteins are evolutionary conserved WD repeat containing proteins that have been proposed to carry out different functions. In Dictyostelium, the short coronin isoform, coronin A, has been implicated in cytoskeletal reorganization, chemotaxis, phagocytosis and the initiation of multicellular development. Generally thought of as modulators of F-actin, coronin A and its mammalian homologs have also been shown to mediate cellular processes in an F-actin-independent manner. Therefore, it remains unclear whether or not coronin A carries out its functions through its capacity to interact with F-actin. Moreover, the interacting partners of coronin A are not known. Here, we analyzed the interactome of coronin A as well as its interaction with F-actin within cells and in vitro. Interactome analysis showed the association with a diverse set of interaction partners, including fimbrin, talin and myosin subunits, with only a transient interaction with the minor actin10 isoform, but not the major form of actin, actin8, which was consistent with the absence of a coronin A-actin interaction as analyzed by co-sedimentation from cells and lysates. In vitro, however, purified coronin A co-precipitated with rabbit muscle F-actin in a coiled-coil-dependent manner. Our results suggest that an in vitro interaction of coronin A and rabbit muscle actin may not reflect the cellular interaction state of coronin A with actin, and that coronin A interacts with diverse proteins in a time-dependent manner.

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.

Ancient Leishmania coronin (CRN12) is involved in microtubule remodeling during cytokinesis

In general, coronins play an important role in actin-based processes, and are expressed in a variety of eukaryotic cells, including Leishmania. Here, we show that Leishmania coronin preferentially distributes to the distal tip during cytokinesis, and interacts with microtubules through a microtubule-based motor, kinesin K39. We further show that reduction in coronin levels by 40-50% in heterozygous coronin mutants results in generation of bipolar cells (25-30%), specifically in the log phase, owing to unregulated growth of the corset microtubules. Further analysis of bipolar cells revealed that the main cause of generation of bipolar cell morphology is the intrusion of the persistently growing corset microtubules into the other daughter cell corset from the opposite direction. This defect in cytokinesis, however, disappears upon episomal gene complementation. Additionally, our attempts to prepare homozygous mutants were unsuccessful, as only the aneuploid cells survive the selection process. These results indicate that coronin regulates microtubule remodeling during Leishmania cytokinesis and is essentially required for survival of these parasites in culture.

Cell adhesion molecules regulate contractile ring-independent cytokinesis in Dictyostelium discoideum

To investigate the roles of substrate adhesion in cytokinesis, we established cell lines lacking paxillin (PAXB) or vinculin (VINA), and those expressing the respective GFP fusion proteins in Dictyostelium discoideum. As in mammalian cells, GFP-PAXB and GFP-VINA formed focal adhesion-like complexes on the cell bottom. paxB(-) cells in suspension grew normally, but on substrates, often failed to divide after regression of the furrow. The efficient cytokinesis of paxB(-) cells in suspension is not because of shear forces to assist abscission, as they divided normally in static suspension culture as well. Double knockout strains lacking mhcA, which codes for myosin II, and paxB or vinA displayed more severe cytokinetic defects than each single knockout strain. In mitotic wild-type cells, GFP-PAXB was diffusely distributed on the basal membrane, but was strikingly condensed along the polar edges in mitotic mhcA(-) cells. These results are consistent with our idea that Dictyostelium displays two forms of cytokinesis, one that is contractile ring-dependent and adhesion-independent, and the other that is contractile ring-independent and adhesion-dependent, and that the latter requires PAXB and VINA. Furthermore, that paxB(-) cells fail to divide normally in the presence of substrate adhesion suggests that this adhesion molecule may play additional signaling roles.

A novel mitosis-specific dynamic actin structure inDictyosteliumcells

Cell division of various animal cells depends on their attachment to a substratum. Dictyostelium cells deficient in type II myosin, analogous to myosin in muscle, can divide on a substratum without the contractile ring. To investigate the mechanism of this substratum-dependent cytokinesis, the dynamics of actin in the ventral cortex were observed by confocal and total internal reflection fluorescence microscopy. Specifically during mitosis, we found novel actin-containing structures (mitosis-specific dynamic actin structures, MiDASes) underneath the nuclei and centrosomes. When the nucleus divided, the MiDAS also split in two and followed the movement of the daughter nuclei. At that time, the distal ends of astral microtubules reached mainly the MiDAS regions of the ventral cortex. An inhibitor of microtubules induced disappearance of MiDASes, leading to aborted cytokinesis, suggesting that astral microtubules are required for the formation and maintenance of MiDASes. Fluorescence recovery after photobleaching experiments revealed that the MiDAS was highly dynamic and comprised small actin-containing dot-like structures. Interference reflection microscopy and assays blowing away the cell bodies by jet streaming showed that MiDASes were major attachment sites of dividing cells. Thus, the MiDASes are strong candidates for scaffolds for substratum-dependent cytokinesis, serving to transmit mechanical force to the substratum.

A stress response kinase, KrsA, controls cAMP relay during the early development of Dictyostelium discoideum

Solitary amoebae of Dictyostelium discoideum are frequently exposed to stressful conditions in nature, and their multicellular development is one response to environmental stress. Here we analyzed an aggregation stage abundant gene, krsA, homologous to human krs1 (kinase responsive to stress 1) to understand the mechanisms for the initiation of development and cell fate determination. The krsA- cells exhibited reduced viability under hyperosmotic conditions. They produced smaller aggregates on membrane filters and did not form aggregation streams on a plastic surface under submerged starvation conditions, but were normal in sexual development. During early asexual development, the expression of cAMP-related genes peaked earlier in the knockout mutants. Neither cAMP oscillation in starved cells nor an increase in the cAMP level following osmotic stress was observed in krsA-. The nuclear export signal, as well as the kinase domain, in KrsA was necessary for stream formation. These results strongly suggest that krsA is involved in cAMP relay, and that signaling pathways for multicellular development have evolved in unison with the stress response.

Naringenin is a novel inhibitor of Dictyostelium cell proliferation and cell migration

Naringenin is a flavanone compound that alters critical cellular processes such as cell multiplication, glucose uptake, and mitochondrial activity. In this study, we used the social amoeba, Dictyostelium discoideum, as a model system for examining the cellular processes and signaling pathways affected by naringenin. We found that naringenin inhibited Dictyostelium cell division in a dose-dependent manner (IC(50) approximately 20 microM). Assays of Dictyostelium chemotaxis and multicellular development revealed that naringenin possesses a previously unrecognized ability to suppress amoeboid cell motility. We also found that naringenin, which is known to inhibit phosphatidylinositol 3-kinase activity, had no apparent effect on phosphatidylinositol 3,4,5-trisphosphate synthesis in live Dictyostelium cells; suggesting that this compound suppresses cell growth and migration via alternative signaling pathways. In another context, the discoveries described here highlight the value of using the Dictyostelium model system for identifying and characterizing the mechanisms by which naringenin, and related compounds, exert their effects on eukaryotic cells.

DWWA, a novel protein containing two WW domains and an IQ motif, is required for scission of the residual cytoplasmic bridge during cytokinesis in Dictyostelium

We have identified a novel gene, dwwA, which is required for cytokinesis of Dictyostelium cells on solid surfaces. Its product, Dd WW domain containing protein A (DWWA), contains several motifs, including two WW domains, an IQ motif, a C2 domain, and a proline-rich region. On substrates, cells lacking dwwA were multinucleated and larger and flatter than wild-type cells due to their frequent inability to sever the cytoplasmic bridge connecting daughter cells after mitosis. When cultured in suspension, however, dwwA-null cells seemed to carry out cytokinesis normally via a process not driven by the shearing force arising from agitation of the culture. GFP-DWWA localized to the cell cortex and nucleus; analysis of the distributions of various truncation mutants revealed that the N-terminal half of the protein, which contains the C2 domain, is required for the cortical localization of DWWA. The IQ motif of DWWA binds calmodulin in vitro. Given that the scission process is also defective in calmodulin knockdown cells cultured on substrates (Liu et al., 1992), we propose that DWWA's multiple binding domains enable it to function as an adaptor protein, facilitating the scission process through the regulation of Ca(2+)/calmodulin-mediated remodeling of the actin cytoskeleton and/or modulation of membrane dynamics.

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.