Regulation of Cell Polarity and Growth by Drosophila Tumor Suppressors
Metazoan animals contain a bewildering variety of cell types whose forms are highly specialized for their functions. Yet how cells adopt these diverse shapes and structures remains mysterious. We are exploring this fundamental question of cell biology using a simple cell type -- epithelia-- in a genetically manipulable organism --Drosophila. We have adapted targeted mosaic techniques to screen, in vivo and in an unbiased manner, for genes required for cells to assume the highly regular epithelial organization. Cloning and characterization of these genes will reveal the mechanisms that regulate general cellular elements, such as the cytoskeleton and protein trafficking systems, in order to confer specific cellular architectures.
Regulation of cell polarity by tumor suppressors: Initial studies have focused on the gene scribble. Mutations in scrib cause a dramatic transformation of embryonic epidermis from a monolayered columnar epithelium to a multilayered pile of isotropically-shaped cells. scrib encodes a PDZ domain-containing protein that acts at cell junctions to regulate apicobasal polarity. Genetics and biochemistry have identified two interacting proteins, called Discs-large and Lethal giant larvae, that cooperate with Scrib in this process. We have also found that a second set of proteins, which reside in the apical domain of epithelia, act antagonistically to Scrib in cell polarity. The emergent picture is of a network of apical and basolateral PDZ proteins that act in a finely-tuned balance to polarize the cell membrane and build specific structures such as cell junctions. Biochemical and genetic characterization of this network is underway in order to understand in detail the mechanism of Scrib-dependent polarization.
Scrib, Dlg and Lgl are required in not just one specific epithelium but in most epithelial tissues of the fly, as well as in other polarized cell types such as neurons. Scrib plays roles in the assymetric divisions of neuroblasts, and also at the neuromuscular junction, where Scrib 'scaffolds' proteins required for synaptic development. Since vertebrate homologs of Scrib and its partners are also found at both epithelial and synaptic contacts, we believe that Scrib, Dlg, and Lgl represent evolutionarily conserved organizers of cell-cell junctions. Studies in neural tissues complement the analysis in epithelial tissues by providing a range of cell types in which different aspects of the biology of cellular organization can be addressed.
Finally, loss of epithelial structure in scrib mutant larval cells is accompanied by severe overproliferation, indicating that scrib, like dlg and lgl, acts as a 'tumor suppressor'. Because of the correlation between loss of epithelial cytoarchitecture and cancer progression in humans, we are exploring why these mutant fly cells overproliferate, in hopes that these studies may bring a fresh perspective onto studies of human tumorigenesis.
Novel genes required for cell architecture: The genetic screens that identified scrib have isolated a number of additional mutations that disrupt epithelial shape, organization, or behavior (such as cell migration and epithelial-mesenchymal transition). Some of these mutations perturb only epithelial tissues, while others cause more broad disruptions of the cytoskeleton. Characterization of the mutants, and cloning of the relevant gene products, will open up new avenues in our understanding of epithelial morphogenesis.PDZ proteins in Drosophila development: Proteins containing PDZ domains are used throughout metazoan species to differentiate plasma membrane domains and link these surface domains to the underlying cellular architecture. We are interested in understanding the mechanism by which PDZ domain-containing proteins accomplish this, as well as the ways in which PDZ proteins are used throughout the development of an organism to polarize different cell types. Drosophila contains 63 PDZ proteins, less than half the number found in mammalian species. We are studying the most interesting of the fly proteins in order to use Drosophila as a model system in which to understand both general and specific aspects of PDZ protein function.