Context-dependent transcriptional interpretation of mitogen activated protein kinase signaling in the Drosophila embryo

Terminal regions of the Drosophila embryo are patterned by the localized activation of Mitogen Activated Protein Kinase (MAPK), which induces zygotic genes through relief of their repression by transcriptional repressor Capicua. The levels of MAPK activation at the anterior and posterior termini are close to each other, but the expression patterns of MAPK-target genes, such as zerknullt (zen) and tailless (tll), display strong anterior-posterior (AP) asymmetry. This region-specific response to MAPK activation provides a clear example of context-dependent interpretation of inductive signaling, a common developmental effect that remains poorly understood. In the past, the AP asymmetry of zen expression was attributed to a mechanism that depends on MAPK substrate competition. We present data suggesting that the asymmetric expression of tll is generated by a different mechanism, based on feedforward control and multiple enhancers of the tll gene. A simple mathematical model of this mechanism correctly predicts how the wild-type expression pattern of tll changes in mutants affecting the anterior, dorsoventral, and terminal patterning systems and some of their direct targets. (C) 2013 AIP Publishing LLC.

Quantitative analysis of gene regulation in Drosophila

Antonina Iagovitina

In this project, characterization of the MAPK signaling cascade was done using the terminal patterning of Drosophila embryo as an experimental system. MAPK acts downstream of the Torso receptor tyrosine kinase (RTK) whose activation is limited to the poles of the embryo. This results in a localized activation of MAPK which is essential for the specification of the embryonic termini. Active MAPK controls the expression of the tailless (tll) and huckebein (hkb) gap genes by downregulating a transcriptional repressor, Capicua (Cic). In this work we are interested in understanding the mechanisms that allow differential specification and precise positioning of gene expression boundaries. We quantitatively analyzed information flow from MAPK to Cic and the two target genes, tll and hkb, by constructing input-output maps in wild type embryos. Our results show that the sharp MAPK activation is translated into a graded gradient of Cic concentration and switch-like responses of tll and hkb genes. This indicates that the ultrasensitive response occurs at the level of the Cic repression which led us to test the hypothesis that Cic alone could be sufficient for tll and hkb boundary specification. However, data from mutants containing lower levels of Cic showed that boundaries of two genes become indistinguishable in terms of Cic concentration while still clearly different in terms of position. This suggests that MAPK downregulation of Cic alone is not sufficient to explain the observed localization and that, other factors are required for the robust patterning of the termini of the Drosophila embryo. To find the missing components of this regulatory system we have started an in vitro Yeast One Hybrid screen. This technique, allowing detection of protein-DNA interaction, was optimized for high-throughput screening. Together with a library Drosophila transcription factors we hope to identify the missing regulators of tll and hkb genes.

2010

Stabilizing patterning in the Drosophila segment polarity network by selecting models in silico

Mirko Bischofberger, Felix Naef, Jacques Rougemont

The segmentation of Drosophila is a prime model to study spatial patterning during embryogenesis. The spatial expression of segment polarity genes results from a complex network of interacting proteins whose expression products are maintained after successful segmentation. This prompted us to investigate the stability and robustness of this process using a dynamical model for the segmentation network based on Boolean states. The model consists of intra-cellular as well as inter-cellular interactions between adjacent cells in one spatial dimension. We quantify the robustness of the dynamical segmentation process by a systematic analysis of mutations. Our starting point consists in a previous Boolean model for Drosophila segmentation. We define mathematically the notion of dynamical robustness and show that the proposed model exhibits limited robustness in gene expression under perturbations. We applied in silico evolution (mutation and selection) and discover two classes of modified gene networks that have a more robust spatial expression pattern. We verified that the enhanced robustness of the two new models is maintained in differential equations models. By comparing the predicted model with experiments on mutated flies, we then discuss the two types of enhanced models. Drosophila patterning can be explained by modelling the underlying network of interacting genes. Here we demonstrate that simple dynamical considerations and in silico evolution can enhance the model to robustly express the expected pattern, helping to elucidate the role of further interactions.

Elsevier2010

Quantitative analysis of gene regulation in Drosophila

Antonina Iagovitina

2010