Proper assembly of the mitotic spindle ensures accurate transmission of genetic material into the two daughter cells. In animals, centrosomes are the primary microtubule organizing centers of mitotic spindles. Intriguingly, most cases of human primary microcephaly (MCPH) are caused by mutations in centrosomal proteins, though the mechanism(s) by which centrosome dysfunction leads to MCPH remains unclear. Although most cells can build mitotic spindles in the absence of centrosomes, we previously demonstrated that acentrosomal epithelial cells of the fly wing disc are prone to mitotic errors and subsequent cell death. In contrast, reports indicate that acentrosomal cells of the developing fly brain do not suffer significant mitotic error or cell death. We therefore explored the underlying basis for the ability of fly brain cells to tolerate centrosome loss. Our data indicate that one reason brain cells avoid detrimental consequences of centrosome loss is the activity of the Spindle Assembly Checkpoint (SAC), which delays anaphase until microtubule-kinetochore attachments form. We found that loss of both centrosomes and the SAC (mad2,sas-4 double-mutant) disrupts brain development, resulting in dramatically reduced brain size, disorganized architecture, and loss of neural stem cells. Importantly, either single-mutant brains are essentially normal in size and appearance. We also found that cells of these double-mutant brains experience significant increases in mitotic error (~80% of cells are aneuploid/polyploid). Double-mutant brains displayed dramatic increases in rates of DNA damage and cell death, including apoptosis of neural stem cells. Cell death appears to be an important contributor to reduced brain size in these animals, as blocking apoptosis partially rescues double-mutant brain size. Together, our data suggest epithelial cells of the wing imaginal disc are sensitive to mitotic perturbation and quickly eliminated by apoptosis, perhaps because they can be easily replaced. However, irreplaceable central brain neuroblasts may have evolved robust checkpoints to ensure greater tolerance for cellular insults such as aneuploidy.
Drosophila models of microcephaly have been limited because mutations in most fly homologues do not dramatically disrupt brain development. Our data indicate the SAC is one reason fly brains do not mirror the microcephaly observed in humans. This genetic model can now be used to test additional hypotheses regarding the roles of centrosomes and mitotic fidelity during brain normal development and the pathology of microcephaly.