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

细胞间的黏附和随后的顶基极性的建立是上皮功能的关键先决条件。通过受体、转运体和信号分子（如蛋白质复合物和某些脂质）的不对称分布，上皮细胞能够选择性地摄取细胞两侧的营养物质和信号。此外，特定的细胞-细胞接触，即紧密连接，对于建立上皮特异性通透性屏障也至关重要。在过去的几年里，大量的研究报道了细胞-细胞接触成熟和细胞极化之间的密切联系，以及调节细胞分化、生长和增殖的信号级联。在进化过程中，这些联系中的许多似乎是高度保守的，例如从果蝇到哺乳动物。Pals1（果蝇中的Stardust/Sdt）是碎屑复合物的核心组成部分，我们和其他研究小组的数据表明，Pals1/Sdt在上皮细胞的细胞-细胞接触中起着动态信号中枢的作用，调节细胞增殖、接触抑制和细胞分化。在本研究中，我们将研究Pals1/Sdt的表达水平如何通过调节细胞内信号通路来确定细胞间接触形成和细胞间接触依赖基因表达之间的微调。因此，该项目的目的之一是阐明在细胞-细胞接触成熟过程中如何控制Pals1/Sdt蛋白的稳定性，以及如何在去分化细胞中调节其降解。其次，我们的目标是确定介导pals1依赖性信号和基因表达的物理蛋白质-蛋白质相互作用。最后，我们将研究失衡的pals1依赖的细胞功能如何与肿瘤细胞的进展和转移相关联。在实验上，这些目标将通过使用培养的哺乳动物细胞（MDCK细胞）作为体外系统来表征pals1依赖性mRNA谱以及pals1相互作用组来实现，考虑不同的生长条件（高密度与低密度，或2D与3D培养）。这些实验的结果将在果蝇系统的结构-功能研究中完成和验证。此外，我们的目标是将这些数据与结直肠癌细胞系的数据进行比较，并研究结肠类器官，以阐明pals1依赖性细胞功能在肿瘤细胞发生和转移中的作用。我们相信我们的实验将有助于更深入地了解pals1依赖性细胞-细胞接触信号通路，以及这些过程的功能障碍如何促进肿瘤的进展。

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.