Phosphorylation of the alpha-subunit of the eukaryotic initiation factor 2 (eIF2alpha) on Serine 51 (Ser51) is an early event associated with downregulation of protein synthesis at the level of translation and constitutes a potent mechanism to overcome various stress conditions. In mammals, four eIF2alpha-kinases PERK, PKR, HRI and GCN2, activated following specific stresses, have been involved in this process. Our laboratory has previously demonstrated that the adaptor protein Nck, composed only of Src homology domains and classically implicated in cell signaling by activated plasma membrane receptor tyrosine kinases, modulates translation through its interaction with the beta-subunit of the eukaryotic initiation factor 2 (eIF2beta). Moreover, we reported that Nck-1 overexpression antagonizes the inhibition of translation in endoplasmic reticulum stress condition and prevents the PERK-mediated phosphorylation of the alpha-subunit of eIF2 on Ser51. In this thesis, I demonstrate that the adaptor protein Nck-1 modulates eIF2alpha-kinase-mediated eIF2alphaSer51 phosphorylation in a specific manner. More particularly, I show that Nck-1 overexpression reduces eIF2alpha phosphorylation in conditions activating PKR or HRI as described previously for PERK. In contrast, I observe that overexpression of Nck-1 in mammalian cells fails to attenuate eIF2alphaSer51 phosphorylation in response to amino acid starvation, a stress condition activating GCN2. I further confirm this observation by showing that Nck-1 fails to alter eIF2alphaSer51 phosphorylation in Saccharomyces cerevisiae, for which the sole eIF2alpha-kinase is GCN2. In addition, I report that Nck-1 reduces PKR activation in response to dsRNA. I also find that Nck-1 reduces dsRNA-induced activation of p38 MAPK, a PKR-downstream substrate, and cell death. Finally, I show that Nck-1 interacts exclusively with the inactivated form of PKR in a Src homology domain independent manner. All together these data uncover the existence of a novel mechanism regulating phosphorylation of eIF2alphaSer51 under various stress conditions and identifies Nck-1 as a modulator of the tumor suppressor and antiviral protein kinase PKR.

Activation of the Unfolded Protein Response (UPR) following stress in the Endoplasmic Reticulum (ER) is an important mechanism by which obesity results in insulin resistance and type II diabetes. We uncovered a role for the adaptor protein Nck in modulating the UPR. In this study, we report that obese Nck1-/- mice, which show lower levels of UPR in liver and adipose tissue, present improved insulin signalling in these tissues. We established that the effect of Nck1 is cell autonomous by showing that HepG2 cells treated with Nck1 siRNA have reduced ER stress-induced UPR and Insulin Receptor Substrate-1 (IRS-1) serine phosphorylation. In these cells, we observed that the IRS-1 levels and activation of signalling components downstream of the insulin receptor were increased. This correlates with enhanced cell survival to stress and insulin stimulated glycogen synthesis. Overall, we demonstrated that Nck1 participates in ER-stress-induced insulin resistance and regulation of IRS-1-dependent signalling.

In response to extra-cellular cues, cells activate signal transduction pathways to elicit a biological response. Cell surface growth factor receptors such as the Met receptor tyrosine kinase (RTK) activate signals that result in cellular proliferation, survival, migration, as well as epithelial morphogenesis. In order for signal transduction to occur, docking proteins are recruited to the activated RTK, become phosphorylated on tyrosine residues, which then serve as docking sites for the recruitment of other signaling proteins. Docking proteins function to diversify the signal by assembling multi-protein complexes. The Gab1 docking protein is the most tyrosine phosphorylated protein upon Met receptor activation and is required for Met mediated signaling and biology. / Gab1 belongs to a family of docking proteins including the highly related Gab2 protein. Gab1 promotes signals for epithelial morphogenesis downstream of the Met receptor, however Gab2 is unable to do so. Insertion of the Gab1 Met binding Motif (MBM) which confers direct binding to the Met receptor, as well as membrane targeting of Gab2 is sufficient to switch the capacity of Gab2 to activate the morphogenic program, cell scatter and lamellipodia formation. This is achieved via activation of sustained signaling pathways, and redistribution of the Gab protein, and associated molecules to sites of lamellipodia formation at the peripheral edge of the cell. / Activation of the Met RTK, promotes the formation of dorsal ruffles on the apical surface of epithelial cells. The Met receptor, Gab1 and Gab1 associated molecules Shp2, Crk, and p8S subunit of PI3K, are localized to these structures, however only the Gab1erk complex is required to drive dorsal ruffle formation. Gab1 is required for Met induced dorsal ruffles as well as downstream the PDGF and EGF RTKs. These are a signaling micro-environment which results in enhanced receptor degradation. Inhibition or enhancement of Met mediated dorsal ruffle formation correlates with receptor stability. / Dorsal ruffle formation downstream of Met requires the enzymatic activity of PI3K and PLCgamma, both enzymes that metabolize PIP2, and form complexes with Gab1 downstream of Met. PLCgamma and the PIP3 lipid product of PI3K are co-localized with Gab1 in dorsal ruffles. Gab1 engages with elements of the cytoskeleton, actin and cortactin, providing a link between growth factor signaling and remodeling of the actin cytoskeleton. Gab1 is localized to membrane protrusions of the basal surface in organoid cultures and is required for actin protrusions of the basal surface of breast cancer cells.

Thesis (M.Sc.). / Written for the Dept. of Biochemistry. Title from title page of PDF (viewed 2008/12/09). Includes bibliographical references.

Thesis (Ph. D.)--University of Alabama at Birmingham, 2008. / Title from first page of PDF file (viewed June 10, 2008). Includes bibliographical references.

Thesis (Ph.D.)--University of Alabama at Birmingham, 2007. / Title from first page of PDF file (viewed on June 25, 2009). Includes bibliographical references.

Thesis (Ph.D.) -- University of Texas Southwestern Medical Center at Dallas, 2005. / Embargoed. Vita. Bibliography: 99-113.

Enteropathogenic Escherichia coli (EPEC) is one of many bacterial and viral pathogens that can exploit the eukaryotic actin cytoskeleton for its own purposes. EPEC injects its own receptor, Tir, into the host cell plasma membrane where, upon binding the bacterial adhesin, intimin, can trigger actin assembly beneath bound bacteria resulting in characteristic actin "pedestals". The formation of these lesions is thought to be critical for bacterial colonization; and can also provide insight into actin dynamics of mammalian cells. EPEC Tir stimulates multiple signaling pathways converging on a central actin nucleation promoting factor, N-WASP. The best-characterized pathway of actin pedestal formation also involves the eukaroytic adaptor protein, Nck, but at least two Nck-independent signaling cascades have also been identified. Multiple aspects of Tir-mediated signaling cascades remain unclear. For example, although Nck can directly bind and activate N-WASP, current models of Tir-mediated, Nck-dependent actin signaling postulate an indirect interaction between Nck and N-WASP mediated by one or more unidentified host factors. Additionally, the relationship of this pathway to the Nck-independent pathways is unknown. To better understand Tir-mediated actin assembly, a detailed and quantitative analysis of the domain requirements of Nck and N-WASP for pedestal formation was conducted. The results indicate that, contrary to previously favored models, Nck is unlikely to require additional host factors to bind N-WASP during pedestal formation, but apparently directly stimulates this nucleation promoting factor. In addition, the results show that the Nck-dependent and -independent pathways target distinct regulatory domains of N-WASP.

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No description available.