Regulation of nuclear INF2 promotes actin polymerization and modulates MRTF-A subcellular localization and activity

Actin filaments are a fundamental component of the cytoskeleton. In eukaryotes, dynamic actin rearrangement plays a crucial part in cellular processes such as morphogenesis, adhesion, cell motility, cytokinesis and intracellular vesicle transport. Numerous aspects of actin dynamics in the cytoplasm...

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Bibliographic Details
Main Author: Melak, Michael
Contributors: Grosse, Robert (Prof.) (Thesis advisor)
Format: Dissertation
Language:English
Published: Philipps-Universität Marburg 2017
Pharmakologisches Institut
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Summary:Actin filaments are a fundamental component of the cytoskeleton. In eukaryotes, dynamic actin rearrangement plays a crucial part in cellular processes such as morphogenesis, adhesion, cell motility, cytokinesis and intracellular vesicle transport. Numerous aspects of actin dynamics in the cytoplasm of eukaryotic cells have been studied intensely over the past decades. Those studies revealed a very detailed knowledge about the structure and function of actin filaments as well as about the underlying mechanisms of F-actin formation. Though actin and proteins involved in actin assembly or disassembly have also been detected in the nuclei of many different eukaryotic cells lines, the detailed regulation and function of actin in the nuclear compartment is poorly defined. Monomeric nuclear actin was identified to participate in specific events such as transcriptional regulation or chromatin remodeling. Nevertheless, the existence and role of filamentous actin inside the nucleus has been controversially debated for years. Quite recently, specific actin probes have been described which enabled credible visualization of nuclear F-actin structures and provided a first insight into the regulation and function of actin polymerization in the nucleus. For example, a role for nuclear F-actin in response to DNA damage and efficient DNA repair as well as in the regulation of the SRF coactivator MRTF-A has been reported. Both events were shown to involve the assembly of nuclear actin filaments mediated by members of the formin family of actin nucleators. In this work, we provide evidence of a nuclear function of the disease associated formin INF2. We identified that activation of endogenous INF2 in the nucleus by means of INF2-DID-NLS or INF2-DAD-NLS expression mediated release of autoinhibition promotes the assembly of a nuclear F-actin network. We further observed that INF2 mediated nuclear actin rearrangement efficiently regulates the translocation and activity of MRTF-A. Moreover, by deletion of INF2 using the CRISPR/Cas9 system as well as by siRNA mediated INF2 knockdown we could show that INF2-DAD-NLS driven nuclear F-actin formation is primarily dependent on the presence of endogenous INF2. However, our data suggest concomitant modulation of nuclear mDia activity upon the release of INF2 autoinhibition in the nucleus. This study provides evidence for a role of the formin INF2 in the promotion and the formation of a nuclear actin network and thereby regulating the subcellular localization of MRTF-A and subsequent alteration of MRTF/SRF transcriptional activity.
DOI:https://doi.org/10.17192/z2017.0264