Acto-myosin network geometry defines centrosome position

Abstract

The centrosome is the main organizer of microtubules and as such, its position is a key determinant of polarized cell functions. As the name says, the default position of the centrosome is considered to be the cell geometrical center. However, the mechanism regulating centrosome positioning is still unclear and often confused with the mechanism regulating the position of the nucleus to which it is linked. Here we used enucleated cells plated on adhesive micropatterns to impose regular and precise geometrical conditions to centrosome-microtubule networks. Although frequently observed there, the equilibrium position of the centrosome is not systematically at the cell geometrical center and can be close to cell edge. Centrosome positioning appears to respond accurately to the architecture and anisotropy of the actin network, which constitutes, rather than cell shape, the actual spatial boundary conditions the microtubule network is sensitive to. We found that the contraction of the actin network defines a peripheral margin, in which microtubules appeared bent by compressive forces. The disassembly of the actin network away from the cell edges defines an inner zone where actin bundles were absent and microtubules were more radially organized.The production of dynein-based forces on microtubules places the centrosome at the center of this inner zone. Cell adhesion pattern and contractile forces define the shape and position of the inner zone in which the centrosome-microtubule network is centered.

Cytoskeletal Crosstalk in Cell Migration

“…For instance, during epithelial-tomesenchymal transition (EMT)-associated migration, epithelial cells with the centrosome on the apical pole undergo MT destabilization-mediated polarity reversal as they transition into mesenchymal cells that invade through the tissue [66]. In micropatterned cells, centrosome positioning and MT organization was shown to be dependent on the anisotropy of the actomyosin network [67], which supports the idea that the architecture of the actomyosin network is important for the polarization of MTs. In addition to centrosomal MTs, noncentrosomal MTs can also orient themselves to promote cell polarity and directionality during migration [68].…”

Centering and Shifting of Centrosomes in Cells

“…The centrosome is similarly located in the lateral line primordium cells of zebrafish embryo in situ, in the posterior part of the cells in relation to the direction of movement [ 6 ]. These apparent exceptions, which did not fit into the classical centering scheme earlier, were recently explained in the article by Jimenez et al [ 9 ]: these exceptions are apparently due to the asymmetry of the actin cytoskeleton, i.e., the external contour of the cells should not be taken into account; rather, the internal contour of the submembrane actin cytoskeleton should be considered (see below). In addition to the centrosome, the nucleus in the cells tends to be located centrally, which undoubtedly affects the location of the centrosome.…”

“…The most accurate analysis of the centrosome location in cytoplasts was carried out in a recent paper by Jimenez et al [ 9 ]. The authors studied the positioning of the centrosome in the cytoplasts obtained by the classical method (centrifugation in the presence of cytochalasin B), where the nucleus could not distort the location of the centrosome; which, in addition, were spread on the micropatterned substrate, where the attached and expanded cytoplasts had a predetermined substrate shape.…”

“…Jimenez et al [ 9 ], also mentioned in the text above, showed that in a highly asymmetric cytoplast, there is a slight centrosome shift from the centroid, proportional to the asymmetry of the actin cytoskeleton. To describe the architecture of the actin cytoskeleton, the authors introduced new concepts of Actin Inner Zone (AIZ), i.e., a central zone of a cytoplast free of actin bundles, and AIZ geometric center, Actin Inner Center (AIC) ( Figure 2 , central panel).…”

“…The forces applied to microtubules by the actin cortical flow are not shown on this figure ( D )—links between the centrosome and the nucleus. Central panel: note that the pulling forces are applied in actin inner zone only, and curved microtubules outside it do not contribute to centering [ 9 ].…”