Gα-Mediated Inhibition of Developmental Signal Response

be coupled with identical heterotrimeric G protein complexes , but, also, individual receptors may interact with and Kidney Diseases Building 50/3351 different G␣ subunits that exhibit distinct signaling capabilities. The complexity of these networks suggests National Institutes of Health Bethesda, Maryland 20892-8028 pathways for both signal integration and synergy but also for insulation and inhibition. 7-TMR signaling pathways are ancient molecular networks that are present in eukaryotes as diverse as Dic-Summary tyostelium, yeast, metazoa, and plants [1]. Dictyostelium has been particularly effective for the molecular dissec-Background: Seven-transmembrane receptor (7-TMR)-G protein networks are molecular sensors of extracellu-tion of G protein-coupled 7-TMR signaling pathways in the context of chemotaxis and development [2–4]. lar signals in all eukarya. These pathways cycle through activated (sensitized) and inhibited (desensitized) states, Development of Dictyostelium is initiated by nutrient depletion. During the first few hours of starvation, sub-and, while many of the molecular components for signal activation have been described, inhibitory mechanisms sets of cells periodically secrete a pulse of the chemoat-tractant cAMP. The secreted cAMP signal is perceived are not well characterized. In Dictyostelium, 7-TM cAMP receptors direct chemotaxis and development but also by proximate cells via 7-TM cAMP receptors (CARs) that, in turn, synthesize and secrete additional cAMP to regulate the periodic synthesis of their own ligand, the chemoattractant/morphogen cAMP. We now demon-relay the signal, creating an outwardly radiating cAMP gradient [5]. The initiating " pacemaker " cells thus estab-strate through loss-of-function/gain-of-function studies that the novel heterotrimeric G␣9 protein subunit regu-lish signaling centers that recruit additional starved cells within their defined territories. Simultaneously, cells lates an inhibitory pathway during early Dictyostelium development for the cAMP signal response. within the cAMP gradient become polarized, oriented, and chemotactic toward the direction of the highest Results: g␣9 null cells form more cAMP signaling centers , are more resistant to compounds that inhibit cAMP concentration of cAMP and move inward [6]. Cells also quickly adapt to the cAMP signal. But, as extracellular signaling, and complete aggregation sooner and at lower cell densities than wild-type cells. These phen-cAMP is degraded by secreted phosphodiesterase, cells reacquire sensitivity for the next cAMP pulse emanating toypes are consistent with the loss of an inhibitory sig-naling pathway during development of g␣9 null cells. from the signaling center. Adaptation/deadaptation not only provides a robust directional chemoattractant gra-Cells expressing constitutively activated G␣9 are defective in cAMP signaling center formation and develop-dient but also ensures that cells …