Tamiki Umeda

Theoretical Analysis of Shape Transformations of Liposomes Caused by Microtubule Assembly

When a cytoskeletal protein, tubulin, is enclosed inside a liposome and the tubulin molecules assemble to form microtubules, a spherical liposome transforms into a rugby-ball shape due to the mechanical force generated by the microtubule assembly. Tubular projections of membrane then grow from both ends of the rugby-ball liposome, and finally the liposome transforms into a characteristic shape consisting of a central ellipsoid and straight tubes. Here we investigate mechanical aspects of the shape transformation of liposomes caused by microtubule assembly. We calculate the liposome shape using a mathematical model based on the notion of the minimum bending energy of the liposome membrane. The force generated by the microtubule assembly is incorporated in the model by considering the local force balance of the membrane. Numerical analysis of the model gives a series of shapes which are similar to the shapes observed in experiments. The force-transformation relationship obtained in our model predicts the ex...

Mechanical analyses of morphological and topological transformation of liposomes.

Liposomes are micro-compartments made of lipid bilayer membranes possessing the characteristics quite similar to those of biological membranes. To form artificial cell-like structures, we made liposomes that contained subunit proteins of cytoskeletons: tubulin or actin. Spherical liposomes were transformed into bipolar or cell-like shapes by mechanical forces generated by the polymerization of encapsulated subunits of microtubules. On the other hand, disk- or dumbbell-shaped liposomes were developed by the polymerization of encapsulated actin. Dynamic processes of morphological transformations of liposomes were visualized by high intensity dark-field light microscopy. Topological changes, such as fusion and division of membrane vesicles, play an essential role in cellular activities. To investigate the mechanism of these processes, we visualized the liposomes undergoing topological transformation in real time. A variety of novel topological transformations were found, including the opening-up of liposomes and the direct expulsion of inner vesicles.

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.

Morphological and topological transformation of membrane vesicles.

Liposomes are micro-compartments made of lipid bilayer membranes withcharacteristics quite similar to those of biological membranes. To formartificial cell-like structures, we generated liposomes that containedsubunit proteins of cytoskeletons: tubulin or actin. Spherical liposomeswere transformed into bipolar or cell-like shapes by mechanical forcesgenerated by the polymerization of encapsulated subunits of microtubules.Disk- or dumbbell-shaped liposomes were developed by the polymerizationof encapsulated actin. Dynamic processes of morphological transformationsof liposomes were visualized by high intensity dark-field lightmicroscopy.Topological changes, such as fusion and division of membrane vesicles,play an essential role in cellular activities. To investigate themechanism of these processes, we visualized in real time the liposomesundergoing topological transformation. A variety of novel topologicaltransformations were found, including the opening-up of liposomes and thedirect expulsion of inner vesicles.

Formation and maintenance of tubular membrane projections : Experiments and numerical calculations

To study the mechanical properties of lipid membranes, we manipulated liposomes by using a system comprising polystyrene beads and laser tweezers, and measured the force required to transform their shapes. When two beads pushed the membrane from inside, spherical liposomes transformed into a lemon-shape. Then a discontinuous shape transformation occurred to form a membrane tube from either end of the liposomes, and the force dropped drastically. We analyzed these processes using a mathematical model based on the bending elasticity of the membranes. Numerical calculations showed that when the bead size was taken into account, the model reproduced both the liposomal shape transformation and the force-extension relation. This result suggests that the size of the beads is responsible for the existence of a force barrier for the tube formation.

A simulation study of the growth and the spatial distribution of plankton in the estuary of the Yodo River, Osaka Bay, Japan

To investigate marine environment in closed sea areas, daily change of the horizontal and vertical distribution of phytoplankton in the estuary of the Yodo River, Osaka bay has been observed in previous studies. To analyze these data, we calculated horizontal and vertical distribution of phytoplankton in Osaka Bay using a simplified mathematical model. The results showed that the model reproduced the basic features of observed distribution. Moreover, the calculated distribution greatly depended on the load of nutrients from the river. The observed daily change in the phytoplankton distribution may have been caused by the difference in the loads from the river.

Morphological and Topological Transformation of Liposomes

Liposomes are micro-compartments made of lipid bilayer membranes with characteristics quite similar to those of biological membranes. To form artificial cell-like structures, we generated liposomes that contained subunit proteins of cytoskeletons: tubulin or actin. Spherical liposomes were transformedinto bipolar or cell-like shapes by mechanical forces generated by the polymerization of encapsulated subunits of microtubules. Disk- or dumbbell-shaped liposomes were developed by the polymerization of encapsulated actin. Dynamic processes of morphological transformations of liposomes were visualized by high intensity dark-field light microscopy.

Morphological and topological transformations of lipid bilayer vesicles

Liposomes are the micro compartments made of lipid bilayer membrane of which characteristics are quite similar to those of biological membrane. To form artificial cell-like structure, we made liposomes that contained subunit of cytoskeletons: tubulin or actin. Spherical liposomes were transformed into bipolar or cell-like shape by mechanical force generated by the polymerization of encapsulated subunits of microtubules. Disk or dumbbell shape was generated by the polymerization of encapsulated action. Dynamic processes of morphological transformations of liposomes were visualized by the high intensity dark-field light microscopy. Topological changes such as fusion and division of membrane vesicles also play an essential role in cellular activities. We investigated the mechanism of these topological transformations by visualizing their real-time processes. A variety of novel topological transformations were found, including the opening-up of liposomes and the direct expulsion of inner vesicles.