Cell polarity is fundamental to numerous cellular processes including migration, molecular transport, and cell division. The establishment and organization of polarity is crucial to the maintenance of cellular homeostasis in mammalian systems. Deregulation of cell polarity is observed in disease states, including cancer. Numb is an adaptor protein that functions in regulating endocytic trafficking events. Numb was originally identified in Drosophila as an asymmetrically localized cell fate determinant, and was subsequently found to be conserved in vertebrates. In mammalian polarized epithelial cells, Numb is distributed asymmetrically along the basolateral membrane domain. The work herein describes phosphorylation of Numb by the Par complex protein, atypical Protein Kinase C (aPKC), as a means of regulating membrane localization and asymmetric distribution of Numb. A mutant of Numb that cannot be phosphorylated by aPKC accumulates on the plasma membrane and localizes to both apical and basolateral membranes. In aPKC-depleted cells, endogenous Numb is unable to achieve polarized distribution and localizes around the entire cell cortex. We demonstrate that this mechanism is conserved in Drosophila as mutation of the corresponding phosphorylation sites disrupts Numb asymmetric localization in dividing sensory organ precursor cells. In polarized epithelial cells, one function of Numb is to promote epithelial morphology when cells are challenged with external stimuli that disrupt cell-cell adhesion. For example, depletion of Numb results in enhanced sensitivity of cells to lose cell-cell contacts when treated with calcium chelating agents. Loss of Numb potentiates hepatocyte growth factor (HGF)-induced lamellipodia formation and cell dispersal – early steps in epithelial-mesenchymal transition (EMT). In Numb-depleted cells, Rac1-GTP loading is enhanced, which corresponds with increased rate in loss of cell-cell adhesion and increased lamellipodia formation, following depletion of extracellular calcium and HGF stimulation, respectively. Together, this work identifies a mechanism that regulates polarized distribution of Numb and provides insight into its function in polarized epithelial cells.

Cell polarity is fundamental to numerous cellular processes including migration, molecular transport, and cell division. The establishment and organization of polarity is crucial to the maintenance of cellular homeostasis in mammalian systems. Deregulation of cell polarity is observed in disease states, including cancer. Numb is an adaptor protein that functions in regulating endocytic trafficking events. Numb was originally identified in Drosophila as an asymmetrically localized cell fate determinant, and was subsequently found to be conserved in vertebrates. In mammalian polarized epithelial cells, Numb is distributed asymmetrically along the basolateral membrane domain. The work herein describes phosphorylation of Numb by the Par complex protein, atypical Protein Kinase C (aPKC), as a means of regulating membrane localization and asymmetric distribution of Numb. A mutant of Numb that cannot be phosphorylated by aPKC accumulates on the plasma membrane and localizes to both apical and basolateral membranes. In aPKC-depleted cells, endogenous Numb is unable to achieve polarized distribution and localizes around the entire cell cortex. We demonstrate that this mechanism is conserved in Drosophila as mutation of the corresponding phosphorylation sites disrupts Numb asymmetric localization in dividing sensory organ precursor cells. In polarized epithelial cells, one function of Numb is to promote epithelial morphology when cells are challenged with external stimuli that disrupt cell-cell adhesion. For example, depletion of Numb results in enhanced sensitivity of cells to lose cell-cell contacts when treated with calcium chelating agents. Loss of Numb potentiates hepatocyte growth factor (HGF)-induced lamellipodia formation and cell dispersal – early steps in epithelial-mesenchymal transition (EMT). In Numb-depleted cells, Rac1-GTP loading is enhanced, which corresponds with increased rate in loss of cell-cell adhesion and increased lamellipodia formation, following depletion of extracellular calcium and HGF stimulation, respectively. Together, this work identifies a mechanism that regulates polarized distribution of Numb and provides insight into its function in polarized epithelial cells.

Clarifying the underlying spatio-temporal mechanisms that determine body pattern is important for detailed understanding of embryonic development. A crucial question of vertebrate embryogenesis remains: when and how are single blastomeres determined for differentiation that subsequently leads to body axes specification and the formation of different tissues and organs? The answer to this question will be beneficial for primary research as well as in the field of applied medicine. The main aim of the presented thesis was to study spatio-temporal molecular gradients of cell fate determinants during early embryonic development. The African clawed frog Xenopus laevis was used as a model organism because of their large size of oocytes and external embryonic development. Due to late activation of embryonic transcription, a crucial mechanism of early blastomeres determination is dependent on asymmetric localization of maternal factors within oocyte and their uneven distribution into single blastomeres during early cell division. Two main localization patterns were identified along the animal-vegetal axis of the mature Xenopus oocyte using qPCR tomography. The localization gradient with preference in either animal or vegetal hemisphere was found for maternal mRNA as well as miRNAs. Moreover, two vegetal...