Regulation and function of actin nucleators Dia and FMNL in the early Drosophila embryo

by Anja Schmidt
Doctoral thesis
Date of Examination:2018-10-10
Date of issue:2018-11-19
Advisor:Prof. Dr. Jörg Großhans
Referee:Prof. Dr. Jörg Großhans
Referee:Prof. Dr. Reinhard Schuh
Persistent Address: http://dx.doi.org/10.53846/goediss-7145

 

 

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Abstract

English

The formation of the subapical domain during cellularization in Drosophila embryogenesis is mediated by a pathway including the proteins Rap1, Canoe and their downstream effectors Bazooka and Armadillo. The cortical domain at the subapical position gives rise to the zonula adherens with adherens junctions after cellularization. The timing of the subapical domain formation remained unclear. It is possible that the introduction happens gradually with ongoing cellularization or that the domain is introduced at the beginning of cellularization. The localization of Bazooka leads to the first hypothesis as it gets enriched gradually at the subapical domain over the course of cellularization. However, I could show that Bazookas upstream factor Canoe localizes to the subapical domain already during early cellularization. Furthermore, I could show by live imaging with CanoeYFP and the basal marker CherrySlam, that the segregation of subapical and basal domains happens during the first minutes of cellularization. The upstream factor of Canoe Rap1 localizes to all cortical domains without an enrichment at the subapical domain and the factor activating the GTPase locally remained unclear. I could show that the unconventional Rap1 GEF ELMO-Sponge localizes to the subapical domain and that the formation of this domain is perturbed in ELMO and sponge mutants. This led to the hypothesis, that ELMO-Sponge locally activates Rap1 leading to the formation of the subapical domain. ELMO-Sponge both localize to actin caps before cellularization in a disc-like fashion, labeling the whole cap. This localization changes with onset of cellularization to a more ring-like pattern defining the position for the formation of the subapical domain. I further could show, that the introduction of the subapical domain depends on midblastula transition and with this onset of zygotic gene expression. In embryos in which zygotic gene expression was repressed by injection of the drug α-amanitin, cellularization was blocked and subapical Canoe localization was lost.
Keywords: Drosophila melanogaster; embryogenesis; cortical domains; actin cortex; actin regulation
 
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