Disruption of cell polarity is seen in many cancers; however, it is generally considered a late event in tumor progression. Lethal giant larvae (Lgl) has been implicated in maintenance of cell polarity in Drosophila and cultured mammalian cells. We now show that loss of Lgl1 in mice results in formation of neuroepithelial rosette-like structures, similar to the neuroblastic rosettes in human primitive neuroectodermal tumors. The newborn Lgl1 −/− pups develop severe hydrocephalus and die neonatally. A large proportion of Lgl1 −/− neural progenitor cells fail to exit the cell cycle and differentiate, and, instead, continue to proliferate and die by apoptosis. Dividing Lgl1 −/− cells are unable to asymmetrically localize the Notch inhibitor Numb, and the resulting failure of asymmetric cell divisions may be responsible for the hyperproliferation and the lack of differentiation. These results reveal a critical role for mammalian Lgl1 in regulating of proliferation, differentiation, and tissue organization and demonstrate a potential causative role of disruption of cell polarity in neoplastic transformation of neuroepithelial cells.

During development cells monitor and adjust their rates of accumulation to produce predeterminedsize organs; however, responsible for this mechanisms remain unknown. We show here that central nervous system-specific deletion of the essential adherens junction gene, αE-catenin, causes abnormal activation of the hedgehog pathway resulting in shortening of the cell cycle, decreased apoptosis and subsequent massive cortical hyperplasia. We propose that αE-catenin connects celldensity-dependent adherens junctions with the developmental hedgehog pathway and this connection may provide a negative feedback loop controlling the size of developing cerebral cortex.Keywords mammalian brain development; hedgehog pathway; neural progenitor cells During brain development, proliferation of neural progenitor cells is tightly controlled to produce the organ of predetermined size. We hypothesized that cell-cell adhesion structures may be involved in this function because they can provide cells with information concerning the density of their cellular neighborhood. Intercellular adhesion in neural progenitors is mediated primarily by adherens junctions, which contain cadherins, β-and α-catenins (1). We found that progenitors express αE(epithelial)-catenin, while differentiated neurons express αN(neural)-catenin ( Fig. S1A-D). Since α-catenin is critical for the formation of adherens junctions (2,3), we decided to determine the role of these adhesion structures in neural progenitor cells by generating mice with CNS-specific deletion of αE-catenin. Mice with a conditional αE-catenin allele (αE-catenin loxP/loxP ) (4) were crossed with mice carrying nestinpromoter-driven Cre recombinase (Nestin-Cre +/− ), which is expressed in CNS stem/neural progenitors starting at embryonic day E10.5 (5) (Fig. S1E). The resulting αE-catenin loxP/loxP / Nestin-Cre +/− animals displayed loss of αE-catenin in neural progenitor cells and its presence in the blood vessels not targeted by Nestin-Cre (Figs. 2D-F, S1F).While no phenotype was observed in heterozygous αE-catenin loxP/+ /Nestin-Cre +/− mice, the knockout αE-catenin loxP/loxP /Nestin-Cre +/− mice were born with bodies similar to their littermates, but with enlarged heads (Fig. S2A). After birth, the heads of these animals continued to grow, but their bodies were developmentally-retarded generating abnormal large-

Epithelial tubes represent fundamental building blocks of metazoan organisms; however, the mechanisms responsible for their formation and maintenance are not well understood. Here, we show that the evolutionarily conserved coiled-coil MAGUK protein Dlg5 is required for epithelial tube maintenance in mammalian brain and kidneys. We demonstrate that Dlg5(-/-) mice develop fully penetrant hydrocephalus and kidney cysts caused by a deficiency in membrane delivery of cadherin-catenin adhesion complexes and loss of cell polarity. Dlg5 travels with cadherin-containing vesicles and binds to syntaxin 4, a t-SNARE protein that regulates fusion of transport vesicles with the lateral membrane domain. We propose that Dlg5 functions in plasma membrane delivery of cadherins by linking cadherin-containing transport vesicles with the t-SNARE targeting complex. These findings show that Dlg5 is causally involved in hydrocephalus and renal cysts and reveal that targeted membrane delivery of cadherin-catenin adhesion complexes is critical for cell polarity and epithelial tube maintenance.

Malformations of cerebral cortex (MCC) are devastating developmental disorders. We report here that mice with embryonic neural stem cell-specific deletion of Llgl1 (Nestin-Cre/Llgl1fl/fl), a mammalian ortholog of Drosophila cell polarity gene lgl, exhibit MCC resembling severe periventricular heterotopia (PH). Immunohistochemical analyses and live cortical imaging of PH formation revealed that disruption of apical junctional complexes (AJCs) was responsible for PH in Nestin-Cre/Llgl1fl/fl brains. While it is well known that cell polarity proteins govern the formation of AJCs, the exact mechanisms remain unclear. We show that LLGL1 directly binds to and promotes internalization of N-cadherin, and N-cadherin/LLGL1 interaction is inhibited by aPKC-mediated phosphorylation of LLGL1, restricting the accumulation of AJCs to the basolateral-apical boundary. Disruption of N-cadherin-LLGL1 interaction during cortical development in vivo is sufficient for PH. These findings reveal a mechanism responsible for the physical and functional connection between cell polarity and cell-cell adhesion machineries in mammalian cells.