Force Generation by Microtubule Assembly/Disassembly in Mitosis and Related Movements

Abstract

In this article, we review the dynamic nature of the filaments (microtubules) that make up the labile fibers of the mitotic spindle and asters, we discuss the roles that assembly and disassembly of microtubules play in mitosis, and we consider how such assembling and disassembling polymer filaments can generate forces that are utilized by the living cell in mitosis and related movements. EARLY HISTORY: THE DYNAMIC EQUILIBRIUM MODELThe orderly segregation of chromosomes at every cell division, and the placement of the resulting daughter nuclei into appropriate cytoplasmic environments, are essential for the normal development of an organism, the generation of functional tissues and gametes, and indeed for the continuity of life itself. Such bipolar segregation of chromosomes in mitosis, and the movement of centrosomes that position the daughter nuclei into appropriately partitioned regions of the cytoplasm in animal cells are accomplished by a bipolar mitotic spindle and its associated astral rays.Although the significance of the spindle and astral rays for mitosis and for the coordination of mitosis with cell cleavage were well recognized by the early cytologists, much controversy abounded in the first half of this century regarding the actual nature of the mitotic spindle fibers (Wilson, 1928;Schrader, 1953). Although visible in cells exposed to acidic or proteinprecipitating fixatives, the fibers were not visible by bright field or phase contrast microscopy in most living cells and were absent in cells observed by light or electron microscopy after fixation with what were considered to be better structure-preserving reagents. There were indications that the fibers would disappear reversibly in cells treated with low temperature or with ethyl ether, and that they were so labile that chromosomes having to traverse laterally to accom- ' Corresponding author. plish their anaphase separation could even cut right through the fibers (Ostergren, 1949). Yet without the bipolar fibrous organization, what would move or guide the chromosomes to the spindle poles?The reality in living cells of spindle fibers, and the fibrils that as a bundle made up the fibers, were established by Inoue by observing a variety of living, animal and plant cells in division with a sensitive polarized light microscope (Inoue, 1951(Inoue, -1953(Inoue, , 1964. The birefringence of the fibers measured and photographed through the sensitive polarizing microscope depicted the distribution, concentration, and appearance and disappearance of oriented fibrils in the spindle fibers in living cells. In addition to the dynamic formation, fluctuation, and disappearance of these fibers and fibrils during the normal course of cell division, the birefringence revealed the labile nature of the fibrous spindle elements in cells exposed to cold or to a mitosis-inhibiting alkaloid, colchicine (Inoue, 1952(Inoue, , 1964. From these findings, Inoue postulated that the spindle fibers and fibrils were made up of a loosely coupled, linear chain of protein molecu...

The perpetual movements of anaphase

“…For the skeptics about rings and collars at kinetochores, McIntosh and colleagues used electron tomography to show that kinetochore fibrils of about 50 nm are connected to curved protofilaments at microtubule plus-ends [148], defining a structural entity at kinetochores that could effectively work as a coupler for chromosome motion by microtubule plus-end depolymerization. By measuring the curvature of bending protofilaments associated with kinetochore fibrils they estimated that each depolymerizing microtubule could produce a force of ~40 pN, in agreement with previous estimations [122]. They further showed that in some successful cases, glass beads coated with reconstituted Ndc80 complex, a 57 nm long component of the core attachment site at kinetochores [149], could couple microtubule depolymerization to movement towards the minus-ends.…”

“…Curiously, the maximum force produced by a single depolymerizing microtubule has been estimated to be aproximately 40 pN [122], which is essentially the same maximum force estimated per kinetochore microtubule from real measurements in grasshopper spindles [121]. It should be noted that the velocity of chromosome movement by depolymerizing microtubules (~16 μm/min; Coue et al, 1991) is significantly faster than normal anaphase chromosome movement and somewhat slower than the observed rates of free microtubule plus-end depolymerization in vitro [123], in Xenopus extract spindles [124] and of an entire k-fiber in living Drosophila culture cells [52] (Figure 4).…”

Condensin Regulates the Stiffness of Vertebrate Centromeres

“…However, such an interpretation would be misleading. Indeed, many studies over the years have shown that tension can stabilize MT attachment to kinetochores and promote MT growth (Nicklas and Koch, 1969;Rieder and Salmon, 1994;Inoue and Salmon, 1995;Skibbens et al, 1995;Nicklas et al, 2001;Gardner et al, 2005;Figure 6, e and f).…”

Prime movers: the mechanochemistry of mitotic kinesins

“…1). The remarkable ability of the spindle machinery to self-organise [G] [1][2][3] and adaptively re-organise 4 derives from the force-generating interactions of dynamic microtubules with the microtubule molecular motors kinesins and dynein.…”