Temporal and spatial patterns of gene expression underlie development in all organisms but are more easily analyzed in those systems where high resolution genetic techniques can be applied. We focus on cell differentiation in Dictyostelium discoideum where a complete physical map is available and we expect to have the complete sequence of the 34 Mb genome in the next few years (Loomis and Kuspa, 1997; Loomis, 1998). Aggregates of 105 cells are formed within the first 12 hours following the initiation of development and fruiting bodies complete with spores and a cellular stalk are formed in the next 12 hours. This relatively simple developmental sequence can be subdivided into over 10 stages by cell-type specific molecular markers. Using random plasmid insertion mutagenesis (REMI) we have discovered over 150 developmental genes many of which function during development of flies, worms, and mammals while others are novel. The genetic networks that regulate expression of these genes can be recognized by generating complex genotypes using homologous recombination. A combination of biochemical and genetic techniques are used to understand the mechanisms responsible for orderly progression through the stages as well as proper proportioning of the cell types. Saturation screens for suppressor mutations can uncover surprising connections between genes in different parts of the networks (Shaulsky et al., 1998). We are analyzing a two component system that is essential for signal transduction in both prestalk and prespore cells (see Figure). A peptide signal, SDF-2, is released from prestalk cells in a processes dependent on the tagB and tagC genes that are expressed exclusively in prestalk cells. These genes encode serine proteases coupled to ATP driven transporters of the MDR family. SDF-2 activates a histidine kinase, DhkA, in prespore cells that inhibits the cAMP specific phosphodiesterase, RegA. cAMP can accumulate when the phosphodiesterase is inhibited and activate the cAMP dependent protein kinase PKA. When PKA is activated, prespore vesicles rapidly fuse with the plasma membrane releasing the spore coat proteins that had previously accumulated (Loomis, 1998). The two component system of DhkA and RegA also functions in prestalk cells to gate terminal differentiation. We feel that a conceptual framework is beginning to emerge in which signal transduction pathways can be shown to be connected by intercellular signals.  Cell-cell communication during terminal differentiaton of Dictyostelium. Prestalk cells release a peptide, SDF-2, that activates the two component system of DhkA and RegA, thereby activating the protein kinase,PKA, that triggers encapsulation. Iranfar, N., Fuller, D. and Loomis, W.F. (2006). Transcriptional regulation of post-aggregation genes in Dictyostelium by a feed-forward loop involving GBF and LagC. Devel. Biol. 290:460-469. Anjard, C. and Loomis, W.F. (2006). GABA induces terminal differentiation of Dictyostelium through a GABA-B type receptor. Development 113: 2253-2261. Anjard, C. and Loomis, W.F. (2005). Peptide signaling during terminal differentiation of Dictyostelium. Proc. Natl Acad. Sci 102: 7607- 7611. Song, L., Nadkarni, S., Bodeker, H., Beta, C., Bae, A., Frank, C., Rappel, W-J., Loomis, W. F. and Bodenschatz, E. (2006). Dictyostelium discoideum chemotaxis: threshold for directed motion. Eur. J. Cell Biol. 85: 981-989. Maeda, M., Lu, S., Shaulsky, G., Miyazaki, Y., Kuwayama, H., Tanaka, Y., Kuspa, A. and Loomis, W. F. (2004). Periodic signaling controlled by an oscillatory circuit that includes protein kinases ERK2 and PKA. Science 304:875-878. Bill Loomis received his Ph.D. from the Massachusetts Institute of Technology. He received an NIH Senior Research Scientist Fellowship and was named an American Cancer Society Scholar. Professor Loomis is a Fellow of the American Association for the Advancement of Science. |
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