RICERCA ITALIANA     

Pathways of meiotic and mitotic spindle assembly: the centrosome and centrosomal proteins

Università degli Studi di Siena

Abstract

The role of centrosomes in cell division is a basic question that has occupied researchers for more than a century. Centrosomes, the main microtubule organizing centers of most higher eukaryotic cells, are thought to be necessary for the organization of the spindle poles and for determining both microtubule polarity and the spindle axis. There are some exceptions to the rule that a centrosome is required to direct the formation of a spindle pole: higher plants and some oocytes do not contain centrosomes, but can still organize functional spindles. Although the centrosome-dependent and centrosome-independent routes of spindle assembly have been considered for years as mutually exclusive pathways, there are now growing evidences that the two mechanisms coexist and can contribute to spindle organization in some cultured cells that usually contain functional centrosomes. This raises the questions as to what is the role of the centrosome on the organization of the cytoskeleton during cell division, whether the contribution of non-centrosomal microtubules to spindle assembly is conserved during cell division in higher eukaryotes, and whether this mechanism also contributes to the formation of the spindle during male and female meiosis. In this project we would like to utilize the fruit fly Drosophila melanogaster as model system in which to investigate:
a) whether there are contributions of both centrosomal and non-centrosomal microtubules to spindle assembly during >>>

Principal Investigator

Giuliano CALLAINI Università degli Studi di SIENA

Research Objectives

An essential feature of cell division is the fidelity with which the chromosomes are segregated. At the onset of each M phase, the assembly and appropriate organization of the microtubule cytoskeleton in a bipolar array is necessary to distribute to the daughter cells replicated chromosomes. Although the mechanism of spindle assembly differs between meiotic and mitotic systems, there are essential features that are the same for all eukaryotic cell types: there must be two spindle poles from which microtubules of uniform polarity nucleate. Bipolarity of a meiotic/mitotic spindle is therefore an essential feature of chromosome segregation, and mechanisms must exist to ensure that a spindle has only two poles. Although the spindle was discovered over 120 years ago, we still know surprisingly little about the processes that drive its assembly. Two different pathways of spindle formation are known to operate in the animal kingdom. The first, observed in somatic and in male germline cells, requires the microtubule organizing activity of the centrosome. The second, restricted to female germline, is thought to depend upon the microtubule organization at the chromosomes. However, emerging data in dividing cultured cells seem to diminish the role of the centrosome as the sole organizing center for the spindle microtubules during mitotic division. It has been, indeed, demonstrated that the mitotic spindle assembles in these cells from the interaction of many non-centrosomal kinetochore >>>

Timescale

24 months

National and international background

- Functional role of the centrosome in spindle organization -

The centrosome is the main Mt organizing center (MTOC) of animal cells. This organelle plays a crucial role to ensure the correct position and distribution of many cellular organelles and the assembly of a symmetric bipolar spindle (Rieder et al., 2001). The centrosome consists of two structural components, a mother and daughter centriole and an ill-defined matrix known as the pericentriolar material whose size and activity varies as a function of the cell cycle. Mts (Mts) are nucleated within the pericentriolar material with their minus ends within this material and their plus ends extending into the cytoplasm. Mt nucleation, occurs in association with some centrosomal components including gamma-tubulin (Oakley and Oakley, 1989) and gamma-tubulin complexes (Moritz et al., 1995; Zheng et al., 1995). It seemed logical that centrosomes have a role in spindle formation through their Mt-nucleating and organizing functions and their association with the spindle poles. These organelles generate a radial array of Mts with defined polarity, with plus-ends extending outwards and minus-ends associated with the centrosomes. Astral Mts explore the cytoplasm, searching for chromosomes. When an astral Mt encounters the kinetochore it is captured and stabilized (Kirschner and Mitchison, 1986). As spindle assembly progresses, kinetochore fibers, which are bundles of Mts that link each kinetochore to opposite >>>