The overall goal of this proposal is to elucidate the mechanisms underlying the developmental defects leading to cortical malformation by characterizing the fundamental antagonistic interactions between evolutionarily conserved polarity complex proteins. Disruption of neuroepithelial structure causes abnormalities in neuronal outputs and cortical malformation that lead to a spectrum of neurodevelopmental disorders, including periventricular heterotopia (PH), microcephaly and ventriculomegaly. Asymmetrically distributed polarity protein complexes are critical for establishing and maintaining polarized cellular architecture and for determining unequal cell fate. Two complexes (Crb/Pals1/Patj and Mupp1; Par3/Par6/aPKC), termed apical polarity complex proteins, are localized at the apical side and one basal polarity complex (Lgl/DLG/Scb) is located basolaterally. Despite its critical importance for cortical epithelial structure and progenitor cell division, it is unclear whether a well-balanced interaction between them is required for cortical structure and neurogenesis, and the cellular and molecular machinery that mediates their functional interaction is also poorly understood. To identify their essential functional interactions and the underlying molecular mechanisms, we have generated a cortical-specific Llgl1 mutant characterized by massive PH and a cortical-specific Crb2 mutant with ventriculomegaly. Remarkably, Crb1/2 loss drastically reduces heterotopia in the Llg1 mutant. It has previously been shown that Llgl1 binds to myosin and regulates its activity, and that apical polarity complex proteins aPKC and Crb2 can inhibit myosin activity. Importantly, because cellular tension and junction defects are associated with the activation of mitogenic signals such as Yap/Taz transcription coactivator, downstream effectors of the Hippo pathway, this antagonism may also be linked to abnormal proliferation that produces the enlarged heterotopic cortex found in Llgl1 CKO. These observations lead us to hypothesize that the antagonistic interaction between basal and apical polarity complexes is required to establish/localize actomyosin at the apical junction and prevent activation of an abnormal Yap/Taz- dependent mitogenic signal. To test this hypothesis, in Aim1 we will define the antagonistic relationship between apical and basal complexes in cortical progenitor proliferation and junctional integrity by analyzing mutant mice. In Aim2 we will delineate the mechanism(s) by which polarity complex proteins regulate junctional integrity by investigating the antagonistic regulation of actomyosin. Lastly, in Aim3 we will determine whether Yap-Taz regulate mitogenic signaling after polarity disruption. Our study will increase understanding of polarity complex protein interactions in cortical progenitor cell division, which will provide new insights into the pathogenesis of cortical malformation and ultimately a basis for novel treatments.