Nuclear actin assembly in mammalian cells
The filament-forming protein actin is abundant in eukaryotic cells and its rapid dy- namics as well as versatile protein interactions result in a diverse array of functions to form important cytoskeletal structures. These influence among others shape, mi- gration and organelle-associated processes,...
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|Summary:||The filament-forming protein actin is abundant in eukaryotic cells and its rapid dy- namics as well as versatile protein interactions result in a diverse array of functions to form important cytoskeletal structures. These influence among others shape, mi- gration and organelle-associated processes, i.e. vesicle movement or mitochondrial fission. The study of such structures in the nuclear compartment was first successful in germline cells of non-mammalian model organisms with high nuclear actin con- centrations. Somatic, mammalian cell nuclei show substantially lower actin levels and faithful visualization of nuclear actin assembly could only be achieved by actin- binding probes fused to nuclear localization sequences circumventing the otherwise saturated cytoplasmic signal. Although high expression levels can lead to artificially induced filaments, careful titrations allowed the discovery of two different types of nuclear actin assembly by live-cell imaging of mammalian cells, regulated either by extracellular signals or the cell cycle.
Extracellular signals for nuclear actin assembly can be induced by integrins and mechanotransduction, activation of other cell surface receptors or DNA damage and subsequent repair mechanisms. Mechanistic evaluation revealed that integrin- mediated nuclear actin filaments depend on the actin assembly factors mDia1 and 2 as well as the linker of nucleoskeleton and cytoskeleton complex positively influenc- ing myocardin-related transcription factor A/serum response factor-dependent gene expression. Integrin-mediated nuclear actin assembly was also observed during can- cer cell invasion trough collagen matrices.
In contrast, cell cycle-regulated nuclear actin assembly occurs together with the re-assembly of daughter nuclei after mitosis. Due to the breakdown of the nuclear envelope for open mitosis, daughter cells have to re-assemble this compartment at mitotic exit. However, the complex organization of interphase nuclei originating from mitotic chromosomes is not fully understood. Our data indicate an important role for nuclear actin dynamics in nuclear volume expansion and chromatin decondensation, which are necessary for a functional interphase nucleus and physiological cellular behavior as well as early embryonic development. We could visualize single and bundled actin filaments in inter-chromosomal spaces and at the nuclear envelope, which were negatively regulated by the actin-depolymerizing factor Cofilin. However, multidisciplinary approaches are further required to study the pre- cise influence of nuclear actin assembly on chromatin dynamics in more detail. Ex- ploring this phenomenon by a combination of proteomics, Hi-C, super-resolution live- cell imaging and novel labeling methods for genomic loci and nucleosomes will aid our understanding of the complex and dynamic nuclear architecture. Further mech- anistic studies into the upstream regulation and the influence of other actin-binding proteins are required to model nuclear actin assembly at mitotic exit.|
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