Dr. Valeri Vasioukhin

Cell polarity and cell adhesion in mammalian development and cancer.

Individual cells in all metazoan organisms need to communicate with each other in order to coordinate their behavior to ensure survival of the entire organism. During embryonic development, non-differentiated totipotent cells divide asymmetrically and generate daughters, which are destined to become the myriads of different cell types that will stop proliferation, differentiate and generate organs of predetermined shapes and sizes. This process of asymmetric cell division is repeated in all organs and tissues of adult organisms during maintenance by their respective stem cell populations. The overall general aim of our research is to understand the mechanisms responsible for orchestrating cellular behavior that help cells work together to produce and maintain normal mammalian organism. We believe that errors in these general mechanisms are ultimately responsible for diseases. Our laboratory is pursuing research in two major directions:

1. We study the mechanisms responsible for asymmetric cell division that help to ensure both the maintenance of pluripotent stem cell population and normal cell differentiation. We believe that the failure of these mechanisms is ultimately responsible for cancer.

2. We are trying to understand how cells use intercellular adhesion structures to determine their position within the organ and translate this information into critical decisions concerning cell proliferation, differentiation and programmed cell death.

Since we study how the cells relate to each other and live as a community, we need to analyze them in live organisms. Therefore, we use mouse as a model in our research. We study proteins, their significance and function. Specifically, we are working on the following projects:

A. The mechanisms and role of Lethal giant larvae (Lgl) in regulation of asymmetric cell division in mammalian development and cancer. Dividing neural progenitor cells in the developing mammalian brain are organized as pseudostratified epithelium and are highly polarized. These cells divide asymmetrically in a stem-cell fashion to replenish themselves and to generate daughters that differentiate into major brain cell types. Our laboratory studies the mechanisms governing polarity and asymmetric cell division of neural progenitor cells. We began these studies with the analysis of the function of the mammalian homolog of the Drosophila tumor-suppressor protein Lgl. For this purpose we generated Lgl1 knockout mice. We found that mammalian Lgl1 plays a critical role in regulation of asymmetric cell division of neural progenitor cells. The Lgl1-/- progenitors failed to withdraw from the cell cycle and continued to divide, leading to massive brain dysplasia resembling human primitive neuroectodermal tumors. Severe brain disorganization led to the development of hydrocephalus and death of newborn Lgl1-/- pups. We found that the important function of Lgl1 was asymmetric localization of the negative regulator of the Notch pathway, Numb. Loss of Lgl1 caused deregulation of Notch signaling and subsequent failures of proper cell fate determination and cell cycle withdrawal.

B. Role of the Dlg family of proteins in the maintenance of cell polarity and adhesion in mammalian development and cancer. Dlg belongs to a superfamily of membrane-associated guanylate kinase (MAGUK) proteins. Drosophila dlg1 functions in the same pathway as lgl to maintain cell polarity and regulate asymmetric cell division. Like lgl, Drosophila dlg1 is a tumor-suppressor protein. Our laboratory is studying the role of a distinct member of the Dlg family of proteins, Dlg5. We found that Dlg5 is a unique member of the family conserved throughout the animal kingdom. Although Dlg5 is present in all multicellular organisms, no Dlg5-mutant organism has ever been generated and analyzed. Therefore, the function of the protein was unknown. We found that Dlg5 is ubiquitously expressed, and the Dlg5 protein localizes to cytoplasm and adherens junctions. To determine the role of Dlg5, we generated Dlg5-/- mice. We found that Dlg5-/- mice developed hydrocephalus and renal cysts. Our analysis of the knockout mice demonstrated that disorganization and disruption of adhesion and polarity were responsible for the brain and kidney defects. We analyzed the molecular mechanisms responsible for this phenotype and found that Dlg5 plays an important role in delivery of this cell-cell adhesion protein complex to the plasma membrane. We are continuing our research on molecular mechanisms responsible for Dlg5 function in the maintenance of cell polarity.

C. Role of adherens junctions in monitoring and regulating cell accumulation during mammalian development and cancer. Cells in the developing embryo can monitor their accumulation rates and adjust their proliferation accordingly to produce organs of predetermined size; however, the mechanisms responsible for this function remain unknown. We hypothesized that cell-cell adhesion structures can provide cells with information about their immediate cellular environment and their location within the organ that can be translated into important decisions concerning cell proliferation, differentiation or death. Our laboratory studies the adherens junction protein alphaE-catenin. To determine the role and significance of alphaE-catenin in mammalian neural progenitor cells, we generated central nervous system (CNS)-specific alphaE-catenin-/- mice. We found that these mice developed massive brain dysplasia and hyperplasia, and had effectively doubled the normal number of brain cells at birth. Mutant neural progenitor cells lose polarity, hyperproliferate and disperse throughout the developing brain, forming tumor-like masses characteristic of human brain tumors. Our genome-wide microarray analyses revealed that deletion of alphaE-catenin leads to activation of the hedgehog (Hh) pathway. We found that inhibition of Hh signaling in the developing brain by cyclopamine injection had no effect on dysplasia, but it rescued hyperplasia in alphaE-catenin-/- brains. We concluded that cells used cell-density-dependent cell-cell adhesion structures to control the developmental hedgehog pathway to ensure proper size of the mammalian brain. We propose that an increase in cell density, sensed by the expansion of adherens junctions, negatively regulates the hedgehog pathway, and this connection provides a negative feedback loop that controls the rates of cell accumulation in the developing mammalian brain. We are now studying the mechanisms connecting alphaE-catenin to the regulation of Hh signaling.

D. Role of cell adhesion in prostate cancer progression. Extracellular and cell-surface proteases play an important role in the regulation of cell adhesion, and deregulation of these enzymes may be critical for tumor dissemination. Hepsin is a cell-surface serine protease markedly overexpressed in prostate cancer. To determine the significance of hepsin overexpression in prostate epithelia, we generated and analyzed transgenic mice expressing hepsin in prostate epithelia. We found that overexpression of hepsin in prostate epithelium in vivo causes disorganization of the basement membrane. To reveal the significance of hepsin overexpression in prostate cancer, we crossed our hepsin transgenic mice with a mouse model of benign nonmetastatic prostate cancer. We found that overexpression of hepsin promoted prostate cancer progression and metastasis in these mice. We concluded that overexpression of hepsin plays an important causal role in prostate cancer progression and metastasis, and hepsin represents a potent therapeutic target for the treatment of prostate cancer. We are now studying the mechanism of hepsin function in promoting prostate cancer metastasis.