Regulation of Cortical Progenitor Mitosis - This proposal is to delineate the novel function of apical polarity complex proteins in cortical progenitor mitosis and uncover a new pathogenic mechanism of microcephaly caused by genetic mutation of Pals1 (protein associated with Lin7 1, also known as MPP5). PALS1, which has been incriminated only recently as a gene responsible for microcephaly in humans, encodes a component of the evolutionarily conserved apical polarity complex. Our multiple Pals1 genetic models using different Cre drivers consistently demonstrate overwhelming cortical cell loss because of compromised cell viability. However, the cellular and molecular defects behind the massive cell death found in Pals1 mutants remain unknown. Through extensive time-lapse imaging, we found that Pals1 loss causes abnormally lengthened mitotic progression, consistent with accumulating evidence that anomalies of mitosis are a significant cause of microcephaly. Remarkably, analyses of mitotic cells in static images and time-lapse imaging of Pals1-deficient progenitors revealed the emergence of internalized cells with nuclei inside of mitotic cells. This unusual cellular behavior mimics entosis, which is cell cannibalism utilized by tumor cells to engulf live neighboring cells for pro- or anti-tumorigenic purposes. It is unknown whether this extraordinary cellular event can be pathogenic in other diseases such as microcephaly. Therefore, the mouse model with an entosis-like process in its cortical progenitors will provide important new insights into the pathogenic mechanisms of microcephaly. Our preliminary study demonstrated that cell-in-cell (CIC) structures represent a dynamic and mobile cellular entity that is highly associated with lengthened mitosis and abnormalities in cytokinesis. As in tumor cells, ROCK inhibition completely abrogates CIC structures and restores the normal length of mitosis. Furthermore, we detected a striking increase of the P53 target, P21, in the Pals1 mutants and found that genetic elimination of P53 produces a remarkable rescue of cortical size along with substantial reductions of CIC structures and cell death. These observations lead us to hypothesize that Pals1 loss induces CIC pathology responsible for mitotic defects that compromise genomic content and cortical cell viability through the abnormal activation of Rho-ROCK and p53. To test this, we will determine the biogenesis, maintenance, and elimination of CIC structures and how they impact mitosis and subsequent genomic integrity and fate of cortical cells (Aim1). Next, we will study how Pals1 deletion/reduction causes entosis through abnormal Rho-ROCK activation (Aim2). Finally, we will delineate the effect of P53 activation on CIC formation and Rho-ROCK regulation in Pals1 mutant progenitors (Aim3). The current study provides an important molecular and cellular clue as to how Pals1 mutation causes such a dramatic cortical phenotype as the complete absence of the cerebral cortex and hippocampus. Furthermore, for the first time, our study will establish that an entosis like process can occur in cortical progenitors, providing a novel pathogenic mechanism by which entotic cell cannibalism produces microcephaly.
