Mitotic spindle orientation is crucial for symmetric vs asymmetric cell division and depends on astral microtubules. Here, we show that distinct subpopulations of astral microtubules exist, which have differential functions in regulating spindle orientation and division symmetry. Specifically, in polarized stem cells of developing mouse neocortex, astral microtubules reaching the apical and basal cell cortex, but not those reaching the central cell cortex, are more abundant in symmetrically than asymmetrically dividing cells and reduce spindle orientation variability. This promotes symmetric divisions by maintaining an apico-basal cleavage plane. The greater abundance of apical/basal astrals depends on a higher concentration, at the basal cell cortex, of LGN, a known spindle-cell cortex linker. Furthermore, newly developed specific microtubule perturbations that selectively decrease apical/basal astrals recapitulate the symmetric-to-asymmetric division switch and suffice to increase neurogenesis in vivo. Thus, our study identifies a novel link between cell polarity, astral microtubules, and spindle orientation in morphogenesis.
A stem cell can divide in two ways. Either it can split symmetrically into two identical daughter stem cells, or it can split asymmetrically into a stem cell and a specialist cell. The structure that forms inside the dividing cell to separate pairs of chromosomes—called the mitotic spindle—also partitions the molecules that determine what kind of cell each daughter cell will become.
The mitotic spindle is made up of protein microtubules. Astral microtubules connect the spindle to a structure found at the inner face of the cell membrane called the cell cortex. This helps the spindle to orient itself correctly and control the plane of cell division. This is particularly important in cells that are different at their top and bottom, like polarized neural stem cells.
To divide symmetrically, these cells need to split vertically from top to bottom. Then, to divide asymmetrically they tilt the cell division plane off-vertical. Classical studies on neuroblasts from the fruit fly Drosophila have shown that a big, 90° reorientation, from vertical to horizontal underlies this change. However, in the primary stem cells of the mammalian brain, subtle off-vertical tilting suffices for asymmetric divisions to occur. This tilting must be finely regulated: if not, neurodevelopmental disorders, such as microcephaly and lissencephaly, may arise.
Mora-Bermúdez et al. investigated how mammalian cortical stem cells control such subtle spindle orientation changes by taking images of developing brain tissue from genetically modified mice. These show that not all astral microtubules affect whether the spindle reorients, as was previously thought. Instead, only those connecting the spindle to the cell cortex at the top and bottom of the cell—the apical/basal astrals—are involved.
A decrease in the number of apical/basal astrals enables the spindle to undergo small reorientations. Mora-Bermúdez et al. therefore propose a model in which the spindle becomes less strongly anchored when the number of apical/basal astrals is reduced. This makes the spindle easier to tilt, allowing neural stem cells to undergo asymmetric divisions to produce neurons.
The decrease in the number of apical/basal astrals appears to be caused by a reduction in the amount of a molecule that is known to help link the microtubules to the cell cortex. This reduction occurs only in the cortex at the top of the cell. Mora-Bermúdez et al. were also able to manipulate this process by adding very low doses of a microtubule inhibitor called nocodazole, which reduced the number of only the apical/basal astrals, increasing the ability of the spindle to reorient.