LOEWE Zentrum für
& Bioressourcen

Seminar announcement 26th November 2014

Jonas Schwirz

A systematic reverse genetic screen identifies Tc-smurf as an important regulator of Dpp-signaling during Tribolium embryogenesis

The formation of two main body axes is a crucial event during bilaterian development and has been in the focus of research for many years. The vinegar fly Drosophila melanogaster served as an excellent model organism for insect developmental biology, resulting in the situation that axis formation in the fly is today vastly understood. Nevertheless, comparative studies using the so-called ‘candidate gene approach’ in other arthropod species revealed that many aspects of Drosophila development seem to be evolutionarily derived. However, although very successful during the last decades, the candidate gene approach has its limitations and comprehensive and unbiased studies in species other than Drosophila are necessary in order to fill our gaps of knowlegde about axis formation and early patterning in arthropods. The approach of a comprehensive RNA interference screen (iBeetle screen) has been used in order to identify new factors required for anterior patterning in Tribolium. About 25 % of the Tribolium genome (3500 genes) were screened during the first approximately 14 months of the iBeetle screen. Almost 10 % of all screened genes were annotated to result in head defects on level of the L1 larval cuticle after knock down. Out of this set of candidate genes a stepwise selection led to the identification of the E3 ubiquitin ligase Tc-Smurf. It turned out that Tc-Smurf is necessary for correct patterning of the blastodermal fate map, and loss of Tc-Smurf function leads to loss of prospective head regions. Lack of Tc-Smurf function during later stages can result in death of anterior body regions up to the entirety of embryonic cells. On molecular level, Tc-Smurf controls Decapentaplegic (Dpp) signaling through ubiquitination-mediated degradation of the Dpp effector molecule pMAD. Increased intracellular levels of pMAD after Tc-smurf RNAi can lead to a switch from embryonic to extraembryonic identity of blastoderm stage cells. During germ band elongation, ectopic pMAD co-localizes with ectopic expression of Dpp, strongly suggesting the presence of a positive feedback mechanism of Dpp-signaling during these stages of Tribolium development. The proposed model suggests that Tc-Smurf is necessary to inhibit pMAD intracellularly and thereby restrict this positive feedback loop from the anterior region of the embryo.

Dr. Jonas Schwirz, Fraunhofer IME-BR, Gießen

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