Mutations of key cytokinetic abscission mediators cause microcephaly

The cerebral cortex is our most recently evolved, complex organ. In size and cellular content, the cortex is largely generated before birth. Microcephaly, or the condition of an abnormally small brain, occurs due to both genetic and environmental insults in humans and can be modeled in mice. Genetic causes of microcephaly most often stem from defects in neural stem cell (NSC) divisions or attributes, which prevent these cells from successfully making post-mitotic neurons. Interestingly, germline mutation of cell division genes in mice often impairs brain growth disproportionately to growth of non-neural tissues.

Most described microcephaly mutants have defects in NSC mitotic progression. However, the role of cytokinesis and especially cytokinetic abscission, the final step in the severing of two daughter cells, in cortical development as well as the etiology of microcephaly is less understood. Recently, p53-dependent apoptosis was found to be a common downstream consequence of mitotic defects in NSCs, causative for reduced cell numbers and smaller brain size. Whether p53 also responds to defects in cytokinesis to cause apoptosis in NSCs, or even to regulate cell survival in other cell types, is unclear and controversial.

This thesis work furthers our understanding of normal cortical development through the investigation of two microcephaly mouse mutants with defects in cytokinetic abscission. We show that mutation of the kinesin-6 Kif20b and scaffolding protein Cep55 separately impair cytokinetic abscission in NSCs to impair brain growth. Despite more severe cytokinetic defects observed with acute knockdown of Cep55 versus knockdown of Kif20b in human cells, we find that germline mutation of Cep55 is remarkably less consequential for embryo development than Kif20b loss. However, Cep55 mutation results in a more severe reduction of brain size and cellular content, similar to a phenotype seen in humans with mutated Cep55. Furthermore, we find that microcephaly in both Kif20b and Cep55 mutants occurs partially through p53-dependent apoptosis, as p53 co-deletion can greatly increase brain size in these mutants. In the Cep55 mutant, evidence suggests p53 activation occurs in binucleate cells that failed cytokinesis. Interestingly, this apoptosis is specific to neural tissues despite expression of Cep55 and evidence of cytokinetic defects in non-neural cells. Kif20b mutation results in abnormal midbody maturation defects but not failed abscission; yet, p53-dependent apoptosis still occurs.

Taken together, this work elucidates a unique sensitivity of NSCs to activation of p53-dependent apoptotic pathways, and suggests distinct ways that abnormal cytokinetic abscission can lead to activation of p53. We propose that Kif20b and Cep55, two vertebrate-specific proteins, evolved for the successful complex divisions of NSCs needed to create the cellular content necessary for mammalian brain size.