New functions of Pals1 as a signaling hub at cell-cell contacts

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Cell-cell adhesion and the subsequent establishment of apical-basal polarity is a crucial prerequisite for the functionality of epithelia. By the asymmetric distribution of receptors, transporters and signalling molecules (e.g. protein complexes but also certain lipids), epithelial cells enable the selective uptake of nutrients and signals on either side of the cell. Moreover, specific cell-cell contacts, the Tight Junctions, are also crucial for the establishment of epithelial-specific permeability barriers. During the last years numerous studies reported close connections between cell-cell contact maturation and cell polarization on the one hand and signalling cascades regulating cell-differentiation, -growth and -proliferation on the other hand. Many of these connections seem to be highly conserved during evolution, e.g. from Drosophila to mammals. Pals1 (Stardust/Sdt in Drosophila) is a core component of the Crumbs complex and data from our and other groups suggest that Pals1/Sdt functions as a dynamic signalling hub at cell-cell contacts in epithelial cells, regulating cell proliferation, contact inhibition and cell differentiation. In this proposal we will investigate how the expression levels of Pals1/Sdt determine the fine-tuning between cell-cell contact formation and cell-cell contact-dependent gene expression by the modulation of intracellular signalling pathways. Thus one aim of this project is to elucidate how Pals1/Sdt protein stability is controlled during cell-cell contact maturation and how its degradation is regulated in de-differentiating cells. Second, we aim to identify the physical protein-protein interactions that mediate Pals1-dependent signalling and gene expression. Finally we will investigate how imbalanced Pals1-dependet cellular functions are linked to tumour progression and metastasis of tumour cells. Experimentally, these objectives will be addressed by using cultivated mammalian cells (MDCK cells) as in vitro system to characterize the Pals1-pedendent mRNA profile as well as the Pals1-interactome, considering different growth conditions (high versus low density, or 2D versus 3D cultures). The outcome of these experiments will be completed and validated by structure-function studies in the Drosophila system. Moreover, we aim to compare these data with that of colorectal cancer cell lines and investigate colonic organoids to elucidate the contribution of Pals1-dependent cellular functions for tumorigenesis and metastasis of tumour cells. We believe that our experiments will contribute to a deeper understanding of Pals1-dependent cell-cell contact signalling pathways as well as of how a dysfunction of these processes contribute to the progression of tumours.