Cells rely on signals to regulate their growth, movement, and metabolism. These signals can be in the form of molecules in the environment: nutrients, compounds secreted by other cells like hormones or growth factors, or can be generated by direct contact with other cells or surfaces. Signaling molecules may diffuse into the target cell, or their uptake may require binding to a specific receptor. Recognition of a signal can alter gene expression, allowing changes in metabolism, growth control, or other functions, or alter cell membrane potentials. Signals are frequently mediated through a series of molecules acting via defined pathways. The alteration of certain pathways can result in loss of growth control, initiation or spread of tumors, or developmental anomalies. Understanding these pathways is key to understanding both normal cell function and pathogenesis and will be necessary to develop more effective therapeutic agents.
Courtersy of Sigma-Aldrich Corporation, St. Louis, MO, USA. The Flies Have It
Ruth Palmer of the Umea Center for Molecular Pathogenesis, Umea University, Umea, Sweden, is interested in signal transduction during the development of Drosophila. She works with Drosophila because it is not as complicated a system as higher organisms and there are fewer molecules per gene family, yet it translates well to mammalian systems. Of interest is the protein tyrosine kinase gene family and how it is controlled by tyrosine phosphorylation and tyrosine phosphatases. Tyrosine kinases are involved in normal cell growth and differentiation, metabolism, and regulation of the cell cycle and have been implicated in tumorigenesis. In Drosophila, there is only one isoform of the focal adhesion kinase (FAK), a nonreceptor protein tyrosine kinase, whereas in higher organisms, there are at least three. FAK in higher organisms forms a complex with Src and several other molecules in integrin-mediated signaling from extracellular growth factors to cytoskeletal-related functions, such as cell movement.
Another reason to use the fruit fly model, Palmer explains, is that people working with Drosophila are “really good at technical development. Homologous recombination and RNA interference (RNAi) are used on a regular basis. There are many complex and useful genetic techniques to address signal transduction in vivo using Drosophila that are efficient and low cost.”
The lack of redundancy, she notes, gives researchers a better chance to hit key components when creating knockouts. “It happens all the time,” she says, “that when something interesting is seen in Drosophila, it can be found in mammalian systems. It goes the other way, too. We are working with a group studying cancers and testing to see if the markers they find have a Drosophila counterpart.” Palmer's group is also interested in signaling pathways involved in tissue differentiation. She hopes that her work will lead to a better understanding of how tyrosine kinase pathways are regulated in vivo both temporally and spatially.Taking Aim
Joan Heller Brown is Chair and Professor of Pharmacology, University of California at San Diego, La Jolla, CA. Her group studies G protein-coupled receptors (GPCRs) such as those for thrombin and the lysophospholipids lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) in glioblastoma cell lines, astroglial cells, and mouse brain astrocytes. A small subset of these receptors seem to work like growth factor receptors, coupling to the G12/13 proteins and to the G protein Rho, stimulating mitogenesis and DNA synthesis. Many components of these pathways have been found to be abnormal in cancer cells, and some aspects of the G12/13 and Rho signaling pathway are involved in aberrant cell growth. Thrombin, LPA, and S1P are released not as part of normal cellular activities, but during inflammatory processes or at a site of injury, when they are capable of acting as ligands for their receptors.
Like Palmer, Brown is interested in how GPCRs act both spatially (i.e., their subcellular location) and temporally. Technologies that will help these aims include the ability to detect activated forms of protein molecules and improved real-time single-cell imagining in both time and space.
Brown notes that many GPCRs are good targets for drug development. Those, like the thrombin receptor, that are upregulated in tumors but are probably not required for normal function of the cardiovascular or gastrointestinal system, have more potential to be selective and specific. Good therapeutic targets will be specific pathways with one or few receptors or those pathway components that are far enough downstream in a pathway to avoid crosstalk with other important pathways.