Vitamin A (retinol) is a fat-soluble morphogen essential for the maintenance of various tissues and organs. Vitamin A and its derivatives, collectively referred to as retinoids, are also crucial for the promotion of vision, for immune and reproductive functions as well as for embryonic development. In vertebrates, the vitamin A-derived morphogen retinoic acid (RA) is the main mediator of retinoid-dependent intercellular signaling, a molecular process that is generally referred to as the RA signaling cascade or the RA pathway.

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Wnt signaling is one of the most fundamental intercellular signaling systems of multicellular animals and is required for the regulation of numerous biological processes both during embryogenesis and in adults. Within a target cell, the intercellular Wnt signaling system can trigger several different intracellular signal transduction cascades, including three main ones: the canonical Wnt/b-catenin pathway and the non-canonical planar cell polarity and calcium pathways. As of today, most studies on Wnt signaling have focused on the roles of these three intracellular pathways and their cross talk during development. In contrast, virtually nothing is known about the mechanisms underlying the recognition and binding specificities of individual Wnt proteins to their cognate Frizzled (Fzd) receptors and about the molecular processes leading to the activation, by a given ligand-receptor couple, of a particular intracellular Wnt pathway. This lack of knowledge, in turn, also hampers the characterization of the global biological responses triggered by Wnt signaling, both on a genomic and developmental level.

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The nervous system represents the single most important information relay system of multicellular animals. It coordinates voluntary and involuntary actions as well as transmission of signals between different parts of the body. The defining anatomical feature of nervous tissue is the presence of a particular cell type, the neuron. During nervous system development and maintenance, neurons arise from neural stem cells, but the current understanding of neural stem cell emergence and of the factors that regulate neural stem cell maintenance and their differentiation into neurons remains very fragmentary.

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