(682e) Ups and Downs of ERK Activation in Embryos

Conserved across all eukaryotes, Extracellular signal regulated kinase (ERK) controls a wide range of developmental processes, such as neocortex formation in mammals. Spatiotemporal control of ERK activation is essential to ensure normal development. ERK activation at a wrong place, at a wrong time, or at the wrong level can lead to significant developmental defects. However, the principles of this control are poorly understood, largely because of the lack of quantitative studies on ERK regulation and function in vivo. We used the fruit fly Drosophila melanogaster embryo as a model system to characterize the activation and de-activation kinetics of ERK as a function of space and time.

Throughout fly embryogenesis, ERK is activated repeatedly by different receptor tyrosine kinases (RTK) to control multiple cellular processes. We focus on ERK dynamics during the third and fourth hour of development. At this time, ERK activity is triggered by the Epidermal Growth Factor Receptor (EGFR) and controls cell differentiation in the future nerve cord. Based on a combination of microfluidics-based quantitative imaging and dimensionality reduction techniques, we found that ERK activation reflects spatiotemporal control of the production of EGFR ligands. While two ligands of EGFR, Spitz and Vein, are expressed during this developmental stage, we showed that only Spitz is required for normal development. Locally produced intramembrane protease Rhomboid is essential for production of the active form of Spitz. The spatiotemporal profile of the rhomboid (rho) gene is regulated by two enhancers, with distinct activity profiles. We show that activity of these enhancers dictates the dynamics of the EGFR-dependent ERK activation. ERK is activated in a broad lateral domain where rho is activated by the maternal morphogen gradient Dorsal, and is deactivated when rho regulation is handed off to a different enhancer, which depends on zygotic gene products and has a more restricted expression domain. Our data suggest that Spitz is a short-range ligand and that EGFR activation by Sptiz works in autocrine mode. We integrated our observations into a mathematical model that describes the pattern of ERK activity and used this model to predict how this pattern changes in several mutant backgrounds. In this presentation, we will describe the technical aspects of data collection, image processing, mathematical modeling, and experimental tests of model predictions of ERK activity in space and time.