For Microtubule Sliding, One Arm Is Better Than Two

BPJ_113_5.c1.inddThe versatile and dynamic network of the cytoskeleton scaffold would be stagnant and lifeless if not for the tiny nanoscopic machines called molecular motors. Kinesin motors, in particular, have captured the imagination of biologists and physicists because of their ability to transform ATP into anthropomorphic walking patterns on polar microtubule filaments, which make up a significant portion of the cytoskeleton. Recent experiments have shown that kinesin motors can crosslink adjacent microtubules and facilitate sliding between them resulting in cytoplasmic streaming in Drosophila cells. This facilitates faster distribution of molecules and organelles, and determines cell-shape.

But how do motors bring about microtubule sliding? How does the collective motion of microtubules depend on the movement of motor arms? In our work, we answer these questions by studying the effect of dimeric (one active arm, one anchored arm) and tetrameric (two active arms) kinesin motors on the dynamics of confined microtubules. Through our computer simulations we find that single-armed kinesins bring about much faster dynamics in specific regions of the confinement, compared to their two-armed counterpart. This goes against the intuitive idea that more arms pull more.

The cover image for the September 5th issue of the Biophysical Journal is our rendering of filament organization for two different motor types and the effects of these differences in the large-scale structure and dynamics of confined microtubules. The green shapes on the left represent the active motor heads that walk on polar microtubules. These are depicted as a linear array of dark-blue and yellow circles. The red blob depicts the anchor belonging to the single-armed, dimeric motor. Motor arms walk in specific directions on microtubules, and stretch, producing a sliding stress between microtubules.

The structures shown in the circular confinement on top consist of sluggish filament packages formed by tetrameric motors. The arrows at the bottom represent the highly dynamic microtubule arrangement formed by dimeric motors. Here, we also depicted the trajectories that three selected microtubules have taken. The cover image was crafted to highlight the large-scale biophysical implications of seemingly trivial and counterintuitive details in biology. Through this work we emphasize the vastly different cytoskeletal dynamics due to dimeric and tetrameric motors. By way of the trajectories, we capture the active layer of microtubules close to the circular confinement we observed for the single-armed motor systems.

– Arvind Ravichandran, Gerard Vliegenthart, Guglielmo Saggiorato, Gerhard Gompper, Thorsten Auth

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Modeling Unravels the Upper Limit of Mitotic Spindle Size

BPJ_112_7.c1.inddWhen talking about organelle size, most people believe the bigger the cell, the larger the organelle should be. In fact, this is not true, at least for mitotic spindles. Recent studies showed that the mitotic spindle size scales with the cell size in small cells, but approaches an upper limit in large cells. However, how the spindle size is sensed and regulated still eludes scientists.

The cover image for the April 11 issue of the Biophysical Journal shows a configuration sampled by a three-dimensional computational simulation guided by a general model for mitotic spindles. The model explicitly shows microtubules (colorful rods), centrosomes (green sphere), and chromosomes (pink bulks). Microtubules can be nucleated from the centrosomes, grow outward, and show the dynamic instability. When microtubules encounter the cortex or chromosome arms, they can generate pushing forces (red rods) due to the polymerization of microtubules. Various molecular motors on cortex and chromosomes, including dyneins (yellow dots) and kinesins (green dots), can bind to microtubules and generate pushing forces (green rods) or pulling forces (blue rods). Therefore, the centrosomes and chromosomes can move under these forces. In this way, the mitotic spindle can be self-assembled to form a bipolar structure with certain size, positioned to the cell center, and orientated to the long axis. Meanwhile, the chromosomes can be attached correctly and aligned on the equatorial plate.

This computational model is very useful for studying the size regulation of mitotic spindles. The spindle size is usually defined as the pole-to-pole distance, so that the problem of spindle size regulation is dependent on the positioning of two poles. The position of each pole is determined by the mechanical equilibrium between the cortical force and the chromosome force on the spindle pole.  For each pole, the chromosomes and the cortex are geometrically asymmetric. In small cells, the geometric asymmetry is small and the pole is nearly positioned to at the center of each half cell so that the spindle size scales with the cell size. However, in large cells, because few microtubules can reach the cell boundary, the geometric asymmetry is large and the spindle size is only determined by the chromosomes; that is, the spindle size approaches the upper limit. Therefore, this work revealed a novel and essential physical mechanism of the spindle size regulation.

There are certainly many other factors that influence spindle size but only quantitatively; they cannot explain the existence of the size limit of spindles.

This computational model provides a very powerful and robust tool. It can be combined with existing biochemical techniques to explore many important and interesting phenomena, including the positioning and orientation of mitotic spindles, the spontaneous oscillation of chromosomes, the mechanical response of spindles under various forces, and many other relevant questions.

– Jingchen Li and Hongyuan Jiang

Microtubules form dynamic network with help from motors

BPJ_111_2.c1.inddThe cytoskeletal network is of vital importance in proper cellular functions. Microtubules, one of the major cytoskeletal components, interact with various associated proteins and generate hierarchical network structures spanning tens of micrometers to millimeters. The network dynamically varies during a cell cycle according to physiological roles in the cell.

To gain the integrative perspectives of network formation and its dynamics, we have extensively surveyed the pattern formation of microtubule-motor mixtures in vitro and found the bundling and sliding of microtubules are the key to pattern formation.

This cover image, acquired with a confocal microscope, shows the network spontaneously formed in the mixture of microtubules (magenta) and a member of the kinesin-5 family, Eg5 (cyan). Radial microtubule structures (asters) are formed through the clustering of plus-ends of microtubules by Eg5, and these asters form a global network spanning up to several millimeters. The sliding activity of Eg5 finally induces the contraction of the network.

The experimental system exhibited various distinct spatiotemporal patterns according to mixing ratios of motors to microtubules. A coarse-grained numerical model we developed can explain these experimentally observed dynamics and demonstrate how bundling and sliding activities of motors determined these spatiotemporal dynamics. Now, together with the model, our system will provide a beneficial platform for the investigation of dynamics and mechanical properties of cytoskeletal architecture.

– Takayuki Torisawa, Daisuke Taniguchi, Shuji Ishihara and Kazuhiro Oiwa