Premier Lecture Series

The chromosome territory interchromatin compartment (CT-IC) model argues for a functional organization of the cell nucleus. Chromosome territory (CT) arrangements show profound differences between certain cell types. In the spherically shaped lymphocyte nuclei gene dense CTs are preferentially located in the nuclear interior, while gene poor CTs are preferentially located towards the nuclear periphery. In fibroblast nuclei with their flatellipsoidal shape, however, CTs of small chromosomes independent of their gene density are located towards the nuclear center and CTs of large chromosomes towards the nuclear rim. Studies of other primates indicate an evolutionary conservation of gene density correlated and size correlated, radial arrangements in the respective cell types. Using defined BACs in combination with multicolor 3D FISH for gene dense and gene poor chromosomal sub-regions, we have started to define the 3D architecture of individual CTs and their topological interactions with each other and the IC. In addition, we have established a procedure to manipulate chromatin condensation and the width of the IC in nuclei of living cells in a fully reversible way. Induced chromatin condensation (ICCond) with a concomitant enlargement of the IC occurs rapidly, when living cells are exposed to a hyperosmotic medium. Cells, which were kept up to 30 min in the ICCond state, before they were released to normal osmotic conditions, retained their viability. Exposure of cells to several, short cycles of ICCond and decondensation yielded highly reproducible patterns of condensed and normal chromatin textures. During ICCond individual CTs could still be delineated as distinct objects using chromosome painting. Interestingly, neighboring CTs were not fully separated from each other, but formed a 3D higher order chromatin network. The contact sites of chromatin from different CTs have not yet been identified. Splicing speckles, nuclear bodies and several proteins, described as markers for a nuclear matrix, were detected within the enlarged IC. Nascent transcripts, pulse-labeled immediately prior to ICCond with BrUTP, were detected at the surface of condensed chromatin streaks lining the enlarged IC. This and other experiments argue that for transcription genes must be positioned in perichromatin regions lining the IC or on chromatin loops expanding into the IC. Transcription as well as DNA replication was inhibited during ICCond, but continued as soon as the normal chromatin condensation state was restored. We propose that CTs and the IC form two separate, yet structurally and functionally closely associated nuclear networks. Border zones of chromatin domains, also called perichromatin regions, line the IC and provide essential sites for transcription, cotranscriptional splicing, DNA replication and DNA repair.