Functional role of centrosomes in spindle assembly and organization

Abstract

The centrosome is the main MT organizing center in animal cells, and has traditionally been regarded as essential for organization of the bipolar spindle that facilitates chromosome segregation during mitosis. Centrosomes are associated with the poles of the mitotic spindle, and several cell types require these organelles for spindle formation. However, most plant cells and some female meiotic systems get along without this organelle, and centrosome-independent spindle assembly has now been identified within some centrosome containing cells. How can such observations, which point to mutually incompatible conclusions regarding the requirement of centrosomes in spindle formation, be interpreted? With emphasis on the functional role of centrosomes, this article summarizes the current models of spindle formation, and outlines how observations obtained from spindle assembly assays in vitro may reconcile conflicting opinions about the mechanism of spindle assembly. It is further described how Drosophila mutants are used to address the functional interrelationships between individual centrosomal proteins and spindle formation in vivo.

Emerging molecular mechanisms that power and regulate the anastral mitotic spindle of flowering plants

“…In fact, bipolar spindles form in extracts from oocytes around beads coated with chromatin [Heald et al, 1996]. Also, in a variety of cell types, when centrosomes are surgically removed, spindles are able to form in their absence, supporting the idea that a centrosome-free pathway operates in cells other than oocytes [Steffen et al, 1986;Varmark, 2004]. Over the last decade, it has become clear that the two pathways act side-by-side, and that the chromatin-based method is obvious only in the absence of centrosomes [Gadde and Heald, 2004;Wadsworth and Khodjacov, 2004].…”

“…Additionally, XMAP215 proteins play a role in supporting microtubule nucleation at the centrosome [Wiese and Zheng, 2006]. In animals and fungi, mutations in XMAP215 proteins disrupt the spindle usually because of the resulting shorter microtubules [Yin et al, 2002;Varmark, 2004], a defect that can be suppressed by concomitant mutation in a depolymerizing kinesin, such as MCAK [Wiese and Zheng, 2006]. When similar kinesins are identified in plants it will be interesting to see whether their knock-down can rescue the mor1 mitotic phenotype.…”

50 ways to build a spindle: the complexity of microtubule generation during mitosis

“…Early experiments in animal cells unequivocally established centrosomes as the primary microtubule organising centres (MTOCs) during mitosis (see Varmark 2004 for review). For example, unfertilised Xenopus embryos, which do not ordinarily contain a centrosome, will undergo cell cycle oscillations when pricked with a needle.…”

Intracellular trafficking of retroviral vectors: obstacles and advances

“…The wall of one microtubule can also form part of the wall of a second microtubule filament, giving rise to doublet or triplet microtubules. 17 In animal cells, microtubules emanate from the centrosome, which is also called the microtubule organizing center (MTOC, reviewed in Alberts et al 17 and Sodeik et al 21 ). The plus ends of microtubules grow out from the MTOC during the cell cycle, nucleating from g-tubulin, while the minus ends remain anchored at the MTOC.…”

“…The centrioles duplicate before cell division and move to opposite sides of the cell during division for coordinating chromosome segregation. 21 Microtubules are key for moving chromosomes, organelles and other cargo within the cell, using the dynein and kinesin families of motor proteins for movement (for reviews see Alberts et al 17 and Mallik and Gross 22 ). Dyneins move cargo toward the minus end of microtubules, towards the MTOC that is retrograde transport.…”