This process, called chemotaxis, is especially important for neutrophils and other inflammatory cells, which enter sites of inflammation by responding to chemoattractants that bind to GPCRs linked to Gi proteins. These cells are able to migrate upward in a gradient in which chemoattractant at the front of the cell is no more than two percent higher than at the back of the cell. Efforts in many laboratories have identified a number of important chemoattractant receptors and their physiological ligands and are beginning to characterize aspects of the reorganization of the actin cytoskeleton that underlies cell motility. During the past four years we have begun to explore this question. Here we describe experiments designed to assess roles of GPCRs and Gi alpha and beta subunits in mediating chemotaxis, as well as the process that reorganizes the actin cytoskeleton at the leading edge.
Separate experiments aim at understanding the compass mechanism that underlies cell polarity.
Enid Neptune , a former postdoc now on the faculty at Johns Hopkins, studied chemotaxis of transfected 293 cells expressing recombinant GPCRs, G-alpha subunits, and other proteins. She found that the dopamine type 2 receptor and two opioid receptors, Gi-coupled receptors for ligands not previously thought to serve as chemoattractants, can mediate directional migration in response to appropriate agonists. In separate experiments, she obtained strong evidence that chemotaxis to these ligands requires activation of the betagamma subunit of Gi, while the Gi-alpha subunit itself is not necessary. Orion Weiner , a graduate student, has devised a nifty system for visualizing movement of individual neutrophils in response to gradients produced by a pipette filled with chemoattractant. Orion's experiments are consistent with the hypothesis that actin is polymerized at the leading edge of neutrophils by a process similar to the process that drives motility of the pathogen Listeria monocytogenes in the cytoplasm of mammalian cells.