Cell-cell adhesion molecules in epithelial-mesenchymal transformations

Localization and signaling of the receptor protein tyrosine kinase Tyro3 in cortical and hippocampal neurons

Protein phosphorylation serves as a critical biochemical regulator of short-term and long-term synaptic plasticity. Receptor protein tyrosine kinases (RPTKs) including members of the trk, eph and erbB subfamilies have been shown to modulate signaling cascades that influence synaptic function in the central nervous system (CNS). Tyro3 is one of three RPTKs belonging to the "TAM" receptor family, which also includes Axl and Mer. Tyro3 is the most widely expressed of these receptors in the CNS. Despite recent advances suggesting roles for members of this receptor family in the reproductive and immune systems, their functions in the CNS remain largely unexplored. In an effort to elucidate the roles of Tyro3 and its ligand, the protein growth arrest-specific gene6 (Gas6) in the hippocampus and cortex, we performed a detailed study of the localization and signaling of Tyro3 polypeptides in rat hippocampal and cortical neurons. Tyro3 was readily detected in dendrites and in the soma where it was distributed in a punctate pattern. Tyro3 exhibited only a limited level of co-localization with postsynaptic density protein-95 (PSD-95), suggesting that while located within dendrites, it was not confined to the postsynaptic compartment. In addition, Tyro3 was also identified in the axons and growth cones of immature neurons. The prominent expression of Tyro3 in dendrites suggested that it may be capable of modulating signaling pathways triggered by synaptic transmission. We have provided evidence in support of this role by demonstrating that Gas6 induced the phosphorylation of Tyro3 in cortical neurons in vitro, resulting in the recruitment of the mitogen-activated protein kinase (MAPK) and the phosphoinositide-3 kinase (PI(3)K) signaling pathways. As these pathways play critical roles in the induction of hippocampal long-term potentiation (LTP), these findings suggest that Tyro3 signaling may influence synaptic plasticity in the dendritic compartment of hippocampal and cortical neurons.

Crk adapter proteins promote an epithelial-mesenchymal-like transition and are required for HGF-mediated cell spreading and breakdown of epithelial adherens junctions

Activation of the Met receptor tyrosine kinase through its ligand, hepatocyte growth factor (HGF), promotes an epithelial-mesenchymal transition and cell dispersal. However, little is known about the HGF-dependent signals that regulate these events. HGF stimulation of epithelial cell colonies leads to the enhanced recruitment of the CrkII and CrkL adapter proteins to Met-dependent signaling complexes. We provide evidence that signals involving CrkII and CrkL are required for the breakdown of adherens junctions, the spreading of epithelial colonies, and the formation of lamellipodia in response to HGF. The overexpression of a CrkI SH3 domain mutant blocks these HGF-dependent events. In addition, the overexpression of CrkII or CrkL promotes lamellipodia formation, loss of adherens junctions, cell spreading, and dispersal of colonies of breast cancer epithelial cells in the absence of HGF. Stable lines of epithelial cells overexpressing CrkII show enhanced activation of Rac1 and Rap1. The Crk-dependent breakdown of adherens junctions and cell spreading is inhibited by the expression of a dominant negative mutant of Rac1 but not Rap1. These findings provide evidence that Crk adapter proteins play a critical role in the breakdown of adherens junctions and the spreading of sheets of epithelial cells.

Involvement of CD44 variant isoform v10 in progenitor cell adhesion and maturation

CD44 has been described repeatedly to be involved in hematopoiesis. Here, we addressed the question of functional activity of CD44 variant isoform v10 (CD44v10) in progenitor cell maturation by in vivo and in vitro blocking studies with a monoclonal antibody and a receptor globulin. We became interested in this question by the observation that CD44v10 is expressed, although at a low level, on a subpopulation of bone marrow cells. Flow cytometry revealed that 15%-20% of hematopoietic cells in the fetal liver and 25%-35% of bone marrow cells in adult mice were CD44v10 positive. The majority of CD44v10+ cells was HSA+/J11d+ and CD43+. CD44v10 was not detected on CD4+, CD8+, IgM+, or IgD+ cells. A CD44v10 receptor globulin did not bind to hematopoietic progenitor cells, but to stromal elements. The CD44v10-CD44v10 ligand interaction had a major impact on the adhesion of progenitor cells to stromal elements. When healthy animals received repeated injections of either anti CD44v10 or the CD44v10 receptor globulin, committed progenitors were mobilized and significantly augmented numbers were recovered in the spleen and the peripheral blood. Furthermore, the CD44v10-CD44v10 ligand interaction, which had no impact on progenitor expansion, influenced progenitor maturation, particularly of the B-cell lineage. Although the nature of the CD44v10 ligand remains to be explored, the supportive role of CD44v10 in progenitor maturation and, importantly, the efficient mobilization of progenitor cells by anti-CD44v10 and a CD44v10 receptor globulin could be of clinical benefit in peripheral blood stem cell transplantation.

The C terminus of E1A regulates tumor progression and epithelial cell differentiation

The E1A gene of adenovirus has been considered both a dominant oncogene and a tumor suppressor. It has been reported to induce epithelial cell but to prevent myoblast differentiation. E1A enables oncogenes that are unable to transform primary cells on their own to do so, yet suppresses tumor progression toward invasion and metastasis. To try to reconcile the seemingly, conflicting E1A phenotypes, we examined the expression of epithelial cell specific and characterizing proteins in immortalized or tumorigenically transformed primary epithelial cells expressing wild-type E1A or a C-terminal mutant that has lost tumor suppressive abilities. All the cell types continued to express cytokeratin. Epithelial cell morphology, social behavior, and growth characteristics were retained by cells expressing wild-type E1A, even in the presence of an activated ras oncogene. Mutant E1A-expressing cells were less well differentiated even in the absence of ras. They were specifically defective in cell-cell junctional complexes, such as tight and adherens junctions and desmosomes. There was also a preference for those actin structures prominent in fibroblasts: stress fibers and filopodia, while in the wild-type E1A expressing cells, cortical actin and circumferential actin filaments were dominant. Thus the E1A-mutant-expressing cells were already predisposed to a more advanced tumor stage even when they were only immortalized and not transformed. The results suggest the possibility that the C terminus of E1A may be involved in regulating epithelial mesenchymal transitions, which have previously been linked to tumor progression.

Analysis of protein aggregates by combination of cross-linking reactions and chromatographic separations

Chemical cross-linking provides a method that covalently bridges near-neighbour associations within proteins and protein aggregates. Combined with chromatographic separations and protein-chemical methods, it may be used to localize and to investigate three-dimensional relations as present under natural conditions. This paper reviews the chemistry and application of cross-linking reagents and the development of combination experimental approaches in view of chromatographic separations and cross-linking reactions. Investigations of homooligomeric and heterooligomeric protein associations as well as conformational analysis are presented.