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Investigating the 3D chromatin architecture with fluorescence microscopy
Investigating the 3D chromatin architecture with fluorescence microscopy
Chromatin is an assembly of DNA and nuclear proteins, which on the one hand has the function to properly store the 2 meters of DNA of a diploid human nucleus in a small volume and on the other hand regulates the accessibility of specific DNA segments for proteins. Many cellular processes like gene expression and DNA repair are affected by the three-dimensional architecture of chromatin. Cohesin is an important and well-studied protein that affects three-dimensional chromatin organization. One of the functions of this motor protein is the active generation of specific domain structures (topologically associating domains (TADs)) by the process of loop extrusion. Studies of cohesin depleted cells showed that TAD structures were lost on a population average. Due to this finding, the question arose, to what extent the functional nuclear architecture, that can be detected by confocal and structured illumination microscopy, is impaired when cells were cohesin depleted. The work presented in this thesis could show that the structuring of the nucleus in areas with different chromatin densities including the localization of important nuclear proteins as well as replication patterns was retained. Interestingly, cohesin depleted cells proceeded through an endomitosis leading to the formation of multilobulated nuclei. Obviously, important structural features of chromatin can form even in the absence of cohesin. In the here presented work, fluorescence microscopic methods were used throughout, and an innovative technique was developed, that allows flexible labeling of proteins with different fluorophores in fixed cells. With this technique DNA as well as peptide nucleic acid (PNA) oligonucleotides can be site-specifically coupled to antibodies via the Tub-tag technology and visualized by complementary fluorescently labeled oligonucleotides. The advantages and disadvantages of PNAs as docking strands are discussed in this thesis as well as the use of PNAs in fluorescence in situ hybridization (FISH). In the next study, which is part of this work, a combination of FISH and super-resolution microscopy was used. There it could be shown that DNA segments of 5 kb can form both compact and elongated configurations in regulatory active as well as inactive chromatin. Coarse-grained modeling of these microscopic data, in agreement with published data from other groups, has suggested that elongated configurations occur more frequently in DNA segments in which the occupancy of nucleosomes is reduced. The microscopically measured distance distributions could only be simulated with models that assume different densities of nucleosomes in the population. Another result of this study was that inactive chromatin - as expected - shows a high level of compaction, which can hardly be explained with common coarse-grained models. It is possible that environmental effects that are difficult to simulate play a role here. Chromatin is a highly dynamic structure, and its architecture is constantly changing, be it through active processes such as the effect of cohesin investigated here or through thermodynamic interactions of nucleosomes as they are simulated in coarse-grained models. It will take a long time until we adequately understand these dynamic processes and their interplay.
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Brandstetter, Katharina
2021
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Brandstetter, Katharina (2021): Investigating the 3D chromatin architecture with fluorescence microscopy. Dissertation, LMU München: Faculty of Biology
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Abstract

Chromatin is an assembly of DNA and nuclear proteins, which on the one hand has the function to properly store the 2 meters of DNA of a diploid human nucleus in a small volume and on the other hand regulates the accessibility of specific DNA segments for proteins. Many cellular processes like gene expression and DNA repair are affected by the three-dimensional architecture of chromatin. Cohesin is an important and well-studied protein that affects three-dimensional chromatin organization. One of the functions of this motor protein is the active generation of specific domain structures (topologically associating domains (TADs)) by the process of loop extrusion. Studies of cohesin depleted cells showed that TAD structures were lost on a population average. Due to this finding, the question arose, to what extent the functional nuclear architecture, that can be detected by confocal and structured illumination microscopy, is impaired when cells were cohesin depleted. The work presented in this thesis could show that the structuring of the nucleus in areas with different chromatin densities including the localization of important nuclear proteins as well as replication patterns was retained. Interestingly, cohesin depleted cells proceeded through an endomitosis leading to the formation of multilobulated nuclei. Obviously, important structural features of chromatin can form even in the absence of cohesin. In the here presented work, fluorescence microscopic methods were used throughout, and an innovative technique was developed, that allows flexible labeling of proteins with different fluorophores in fixed cells. With this technique DNA as well as peptide nucleic acid (PNA) oligonucleotides can be site-specifically coupled to antibodies via the Tub-tag technology and visualized by complementary fluorescently labeled oligonucleotides. The advantages and disadvantages of PNAs as docking strands are discussed in this thesis as well as the use of PNAs in fluorescence in situ hybridization (FISH). In the next study, which is part of this work, a combination of FISH and super-resolution microscopy was used. There it could be shown that DNA segments of 5 kb can form both compact and elongated configurations in regulatory active as well as inactive chromatin. Coarse-grained modeling of these microscopic data, in agreement with published data from other groups, has suggested that elongated configurations occur more frequently in DNA segments in which the occupancy of nucleosomes is reduced. The microscopically measured distance distributions could only be simulated with models that assume different densities of nucleosomes in the population. Another result of this study was that inactive chromatin - as expected - shows a high level of compaction, which can hardly be explained with common coarse-grained models. It is possible that environmental effects that are difficult to simulate play a role here. Chromatin is a highly dynamic structure, and its architecture is constantly changing, be it through active processes such as the effect of cohesin investigated here or through thermodynamic interactions of nucleosomes as they are simulated in coarse-grained models. It will take a long time until we adequately understand these dynamic processes and their interplay.