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Authors: Chikte, Prasad
Title: Actin-dependent re-localization of the hb, Kr and eve loci determines end of early competence in Drosophila neuroblasts
Online publication date: 15-Aug-2018
Language: english
Abstract: Neural stem cells produce specific cell types in a time-dependent manner passing through different competence windows. However, how these competence windows are regulated is largely unknown. Here we use embryonic Drosophila neuroblasts (NB) as a model system to investigate this question. These NBs produce specific cell lineages stereotypically and are characterized by sequential expression of transcription factor cascade Hb/Kr>Kr> Pdm>Pdm/Cas>Cas. It has been shown earlier that the competence to produce Hb-dependent early cell fates can be extended if Hb expression is prolonged. Similar results were observed in Cortex development of vertebrates where the Hb homolog Ikaros permits early cell fate competence to neural stem cells of this region. Here we show that in Drosophila continuous Hb-dependent activation of the Kr-locus which normally gets silenced after termination of its expression is necessary for the extension of NB competence window. Intriguingly this silencing of Kr gene locus is correlated with its unusual re-localization from nuclear periphery, a normally repressive subnuclear compartment, towards the interior and can be inhibited by continuous Hb activity. Strikingly, co-expression of Hb and Kr outside of the early competence window induces a reversion of Kr-locus position towards nuclear periphery correlating with ectopic production of early neuronal cell types at this late time of development. An opposite and Hb-independent re-localization pattern from the nuclear interior towards periphery has been described earlier for the hb-locus during early competence window. Here we observed similar re-localization of eve locus but in contrast to the hb-locus, this is also inhibited by continuous Hb expression. Interestingly, co-expression of Hb and Kr outside of the early competence window reversed eve-locus position towards nuclear interior, thus again correlating with NB competence. However, less is known about possible mechanisms required for such chromatin rearrangements and there are hints in a vertebrate system that nuclear ß-actin might play a role. Here I found evidence that oligomerization of nuclear ß-actin is involved in such events in Drosophila. This has been deduced from overexpression experiments using dominant negative non-oligomerizing NLS-GFP-actin which resulted in a deceleration of locus re-localizations whereas wild-type NLS-GFP-actin showed acceleration. Furthermore, over-expressing Exportin 6, a specific transporter of ß-actin out of the nucleus, leads to a block of hb, Kr and eve locus re-localization. Most importantly, late overexpression of Hb in this situation re-establishes the ability to induce early cell fates showing a causal relationship between gene locus re-localizations and NB competence. Furthermore, I could show the role of epigenetic factors Polycomb and HDAC1 in re-localization of these loci. Knockdown of these factors resulted in slow down or halt in the repositioning of loci. Together we propose that within the early competence window there are multiple nuclear actin dependent gene loci re-localizations in different directions suggesting ongoing epigenetic silencing which can be partly inhibited or even reverted by the combined activity of Hb and Kr.
DDC: 570 Biowissenschaften
570 Life sciences
Institution: Johannes Gutenberg-Universität Mainz
Department: FB 10 Biologie
Place: Mainz
Version: Original work
Publication type: Dissertation
License: in Copyright
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Extent: 126 Blätter
Appears in collections:JGU-Publikationen

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