Chemotaxis is a fascinating biological response in which cells orient themselves and move up a chemical gradient. It is important in a variety of physiological and pathological processes including nerve growth, angiogenesis, wound healing, leukocyte trafficking, and carcinoma invasion. It is also essential for the survival of the social amoebae, <I>Dictyostelium discoideum</i>. During growth, these cells track down and phagocytose bacteria. When starved, they enter a differentiation program that allows the cells to survive harsh environmental conditions. They do so by chemotaxing toward secreted adenosine 3,5 cyclic monophosphate (cAMP) signals thereby forming aggregates which differentiate into spore and stalk cells. The essential role of chemotaxis in this eukaryote has provided an excellent model organism to study the biochemical and genetic basis of directed cell migration. Both leukocytes and <I>Dictyostelium</i> cells use G protein-linked signaling pathways to respond to chemoattractants. Binding of chemoattractants to serpentine receptors leads to the dissociation of heterotrimeric G proteins into alpha- and beta/gamma-subunits, which activate a variety of effectors that go on to produce multiple responses. These include increases in Ca2+ influx, IP3, cAMP and cGMP. Concomitantly, the level of phosphorylation of myosins I and II and polymerized actin are markedly increased. Our research program is focused on understanding how these multiple G protein-coupled signaling events are translated into directed cell migration. We have shown that a variety of signaling events are spatially restricted during chemotaxis. In one instance, this has led us to discover a novel and unexpected mechanism used by <I>Dictyostelium</i> cells to relay and amplify chemotactic gradients. It had been observed that these cells align in a head to tail fashion, or stream, as they migrate in a gradient of cAMP. Live imaging of ACA, the enzyme synthesizing cAMP, revealed a highly enriched localization at the plasma membrane in the rear of polarized cells. We proposed that this asymmetric distribution of ACA provides a compartment from which the chemoattractant cAMP is secreted to act locally, offering an ideal mechanism to relay chemical gradients. During the last four years we have witnessed impressive progress in our understanding of how chemotactic signals transduce spatial and temporal information to the cytoskeletal machinery. Yet, many fundamental questions remain unanswered. In particular, the mechanisms by which signals are integrated at the cellular and multi-cellular levels are essentially unknown. In the years ahead, we will continue exploring two fundamental questions in chemotaxis: 1. How external signals establish and maintain signaling and cellular polarity? 2. How are chemotactic signals relayed to neighboring cells, i.e. how do cells transition from single to group migration? To get at these questions, we will use a combination of genetic, biochemical, and cell biological approaches and apply them to two complementary model systems. First, by studying cAMP metabolism in <I>Dictyostelium</i>, we have the opportunity to gain insight into directional sensing and cellular polarity events during chemotaxis. In addition, because of the central role of signal relay and streaming during <I>Dictyostelium</i> chemotaxis, this model organism provides an excellent tool to decipher the mechanisms that govern collective cell migration. Second, we plan to continue our studies on the regulation of adenylyl cyclase activity and the role of cAMP during neutrophils chemotaxis. Furthermore, we will set out to see if neutrophils use chain migration (or streaming) during chemotaxis. Finally, and as a long-term goal, we plan to initiate studies aimed at assessing the behavior of metastatic epithelial breast cancer cells in chemotactic gradients.
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