Publication in Cell Reports: Peripheral astral microtubules mediate asymmetric furrow positioning in neural stem cells

Some stem cells divide asymmetrically and robustly to generate two daughter cells of different size. The cytoskeletal rearrangements that accompany the division of these particular cells are not yet well understood. This study, published in the journal Cell Reports, shows the important contribution of certain microtubules in maintaining the asymmetric positioning of the division site and thus the size of future daughter cells.

Most tissues are made up of differentiated cells and a variable number of stem cells. These stem cells are essential for maintaining the integrity of tissues by ensuring their homeostasis and repair in case of damage.  The division of some of these stem cells is characterized by an asymmetric positioning of the cleavage site during the process of cytokinesis (division during mitosis), which results in the formation of two daughter cells with a size difference. However, the underlying mechanisms that govern this size asymmetry are not well characterized. In animal cells, the determination of the position of the cleavage site as well as the contraction of the division furrow are regulated by the "Centralspindline" complex.

This complex is localized as two distinct populations. A first population is enriched during the phase of chromosome-son separation (anaphase) at the cell cortex, at the level of the future division site, and a second one is recruited at the level of the central mitotic spindle, the microtubule structure present from anaphase onwards between the two batches of previously separated chromosomes. During symmetrical cell divisions, these two populations of the Centralspindline complex are located in equatorial position and this superposition of signals complicates the functional analysis of each of these two populations on the process of cytokinesis. Despite this difficulty, published data suggested a major role for the central spindle and the associated Centralspindline complex in determining the site of division

In this new study performed in Drosophila neural stem cells, scientists show that the two populations of the Centralspindline complex are spatially and temporally separated. This allows to characterize the contribution of each of these populations on the positioning of the division site. In these stem cells, a first population of the Centralspindline complex is recruited asymmetrically to the future cleavage site, then a second population appears at the medial zone of the central spindle towards the equator of the cell. The central spindle shifts during cytokinesis to adjust to the asymmetric position of the cleavage furrow. Using a combination of genetic, laser photo-ablation, and imaging approaches, the scientists show that a population of astral microtubules, called peripheral microtubules, in close contact with the division furrow, is essential for targeting and enriching the Centralspindline complex at the cleavage site.

Under conditions where the number of these peripheral microtubules is decreased or when these microtubules are absent, the recruitment of the Centralspindline complex to the cleavage site is impaired. As a result, signaling from the Centralspindline complex in the medial zone of the central spindle becomes dominant and leads to a repositioning of the cleavage furrow towards the equator of the cell. This repositioning of the division furrow induces a decrease in size asymmetry between future daughter cells. This work thus reveals the crucial role of peripheral astral microtubules for the asymmetric positioning of the cleavage site during the cytokinesis process and illustrates the modularity of division mechanisms depending on the cell type. Further studies will be required to measure whether these cell size abnormalities alter the ability of cells to differentiate and proliferate.


©  Regis Giet
Figure: Dynamic analysis of the position of the division furrow in a Drosophila neural stem cell wild type (Ctrl) or overexpressing the microtubule destabilizing protein Klp10A (Klp10A-OE). In the composite image the myosin (Sqh-GFP) appears in green (see in the bottom images). Microtubules are marked in purple. Time is indicated in seconds in each image. The scale bar indicates 10 microns. The wild type cell positions its division groove on the basal side (red triangles) and divides asymmetrically. The "Klp10A-OE" cell with destabilized astral microtubules shows a shift of the groove initially positioned on the basal side (red triangle) towards the equator of the cell (black triangle) to generate two more symmetrically sized cells.

Team : Cytoskeleton and cell proliferation team, IGDR.