Directed cell migration is vital in development and physiology and a potential point of vulnerability in numerous diseases, including metastatic cancer. The overall process is comprised of directional sensing, motility, and polarity. Whereas the textbook view implies that autonomous cytoskeletal activity underlies motility and that signal transduction events provide guidance, recent studies suggest that excitable behavior of the signal transduction network is an essential pacemaker that drives movement. Local modulation of this biochemical excitability by external gradients and internal polarity cues can guide cells and large global perturbations can have profound morphological consequences. We are addressing the important questions raised by this biased excitable network view of directed cell migration. What molecular mechanisms make the signal transduction network excitable? We are pursuing a working hypothesis that positive and delayed negative feedbacks, involving control of RasGTPase, PI 3- kinase, and PLC activities in local regions of the membrane, are the basis of excitability. We are using synthetic actuators, induced sequestration, and biosensors to perturb and monitor enzyme activities and membrane states to find the feedback loops. Initial experiments show that gradual decreases in PIP2 or increases in Ras signaling progressively cause cells to shift from amoeboid, to fan-like, to oscillatory, to persistently spread forms as would be expected if cell morphology is controlled by an excitable network. How can cells sense, integrate, and adapt to temporal and spatial cues? To explain how cells to respond to differences and adapt to uniform chemotactic stimuli, we proposed that a local excitation-global inhibition scheme biases the excitable network. To identify the inhibitor that balances G-protein excitation, we are focusing on physiological relevant protein modifications, genetic screens, and in vitro reconstitution. What is the overall complexity of the chemotactic networks? In forward genetic screens, we have identified a large series of novel regulators of directed migration. With a combination of established biochemical and genetic analyses we are defining the links of these new genes to the existing networks and their roles in directional sensing, motility, and polarity. Many of these genes have human homologues that we will target in neutrophils and mammary cells to investigate effects on chemotaxis. How can these new concepts be exploited to control migration and target specific cells? We recently discovered that persistent activation of multiple parallel migration pathways causes cells to spread excessively, fragment, and die. Since many of these perturbations are also involved in oncogenesis, we are pursuing the admittedly unconventional concept that the most aggressive cancer cells can be targeted by even further activation. As proof of principle, we are testing genetic and chemical perturbations, which selectively target cultured cells with defined oncogenic mutations, in xenographs and organoids.
Many cells can sense and move along shallow gradients of chemo attractants, a process that is essential for normal physiology in the embryo and adult as well as for the pathology of numerous diseases. These studies are designed to elucidate the molecular mechanism of cell motility and directed migration. With a thorough understanding of this fascinating process, it will be possible to interfere with it specifically in target cells whih will eventually lead to new therapeutic strategies for a host of diseases.
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