Interplay Between Spindle Architecture and Function - International Review of Cell and Molecular Biology - page 93

on where and how MTs are nucleated, the proportion of MTs in each pop-
ulation may change, thereby altering the organization of the spindle.
3.1. Centrosome-mediated search-and-capture
Since MT dynamic instability was first observed, Mitchison and Kirschner
(1985) proposed that spindle assembly could occur through the selective sta-
bilization of centrosome-nucleated MTs at kinetochores. By frequently
switching between growing and shrinking states, MTs can “probe” the
cytoplasm randomly until captured by a chromosome, allowing the cell
to gradually form a bipolar MT array from two centrosomes and pairs of sis-
ter chromatids ( Holy and Leibler, 1994 ).
When present, centrosomes appear to be the dominant MT-generating
and -organizing centers of the cell. A typical centrosome consists of a pair of
centrioles surrounded by pericentriolar material that recruits the tubulin iso-
form g -tubulin in a complex called the g -tubulin ring complex ( g -TuRC)
that provides a template for MT growth by stabilizing and anchoring the
minus-ends of MTs at the centrosome ( Guillet et al., 2011; Hannak
et al., 2002; Kollman et al., 2010, 2011 ; Fig. 3.4 A). Upon entry into mitosis,
centrosomes greatly increase their capacity to nucleate MTs, a process ter-
med centrosome maturation.
While a simple and appealing hypothesis, computational simulations
indicated that search-and-capture would be rather inefficient on its own
and require hours longer than the time period of a normal mitosis
( Wollman et al., 2005 ). However, the time could be reduced if geometric
constraints were added to the model that optimized the size and spatial expo-
sure of target kinetochore-binding sites to centrosome-nucleated MTs, spe-
cifically by increasing chromosome mobility and by incorporating
chromosome-dependent MT-nucleating pathways discussed below ( Paul
et al., 2009 ). Remarkably, in vivo observations have revealed that spindle
components are spatially positioned at early stages of spindle assembly to
facilitate chromosome-MT interactions, fulfilling the hypothetical parame-
ters necessary for rapid spindle assembly posited in simulation. By
implementing 3D imaging with high temporal and spatial resolution of
MTs, kinetochores, and chromosomes during spindle assembly in human
RPE1 cells, a toroidal distribution of chromosomes could be resolved in
which kinetochores reside in an area with extremely high MT density
and are not shielded by chromosomes ( Magidson et al., 2011 ; Fig. 3.4 A).
This transient prometaphase arrangement is promoted by the polar ejection
force due to chromokinesins (discussed in Section 4 ) as well as by lateral