Large-scale probabilistic 3D organization of human chromosome territories.

There is growing evidence that chromosome territories (CT) have a probabilistic non-random arrangement within the cell nucleus of mammalian cells including radial positioning and preferred patterns of interchromosomal interactions that are cell-type specific. While it is generally assumed that the three-dimensional (3D) arrangement of genes within the CT is linked to genomic regulation, the degree of non-random organization of individual CT remains unclear. As a first step to elucidating the global 3D organization (topology) of individual CT, we performed multi-color fluorescence in situ hybridization using six probes extending across each chromosome in human WI38 lung fibroblasts. Six CT were selected ranging in size and gene density (1, 4, 12, 17, 18 and X). In-house computational geometric algorithms were applied to measure the 3D distances between every combination of probes and to elucidate data-mined structural patterns. Our findings demonstrate a high degree of non-random arrangement of individual CT that vary from chromosome to chromosome and display distinct changes during the cell cycle. Application of a classic, well-defined data mining and pattern recognition approach termed the 'k-means' generated 3D models for the best fit arrangement of each chromosome. These predicted models correlated well with the detailed distance measurements and analysis. We propose that the unique 3D topology of each CT and characteristic changes during the cell cycle provide the structural framework for the global gene expression programs of the individual chromosomes.

Ribosomal genes in focus: new transcripts label the dense fibrillar components and form clusters indicative of "Christmas trees" in situ.

T he organization of transcriptionally active ribosomal genes in animal cell nucleoli is investigated in this study in order to address the long-standing controversy with regard to the intranucleolar localization of these genes. Detailed analyses of HeLa cell nucleoli include direct localization of ribosomal genes by in situ hybridization and their indirect localization via nascent ribosomal transcript mappings. On the light microscopy (LM) level, ribosomal genes map in 10-40 fluorescence foci per nucleus, and transcription activity is associated with most foci. We demonstrate that each nucleolar focus observed by LM corresponds, on the EM level, to an individual fibrillar center (FC) and surrounding dense fibrillar components (DFCs). The EM data identify the DFC as the nucleolar subcompartment in which rRNA synthesis takes place, consistent with detection of rDNA within the DFC. The highly sensitive method for mapping nascent transcripts in permeabilized cells on ultrastructural level provides intense and unambiguous clustered immunogold signal over the DFC, whereas very little to no label is detected over the FC. This signal is strongly indicative of nascent "Christmas trees" of rRNA associated with individual rDNA genes, sampled on the surface of thin sections. Stereological analysis of the clustered transcription signal further suggests that these Christmas trees may be contorted in space and exhibit a DNA compaction ratio on the order of 4-5.5.

Global nature of dynamic protein-chromatin interactions in vivo: three-dimensional genome scanning and dynamic interaction networks of chromatin proteins.

Genome structure and gene expression depend on a multitude of chromatin-binding proteins. The binding properties of these proteins to native chromatin in intact cells are largely unknown. Here, we describe an approach based on combined in vivo photobleaching microscopy and kinetic modeling to analyze globally the dynamics of binding of chromatin-associated proteins in living cells. We have quantitatively determined basic biophysical properties, such as off rate constants, residence time, and bound fraction, of a wide range of chromatin proteins of diverse functions in vivo. We demonstrate that most chromatin proteins have a high turnover on chromatin with a residence time on the order of seconds, that the major fraction of each protein is bound to chromatin at steady state, and that transient binding is a common property of chromatin-associated proteins. Our results indicate that chromatin-binding proteins find their binding sites by three-dimensional scanning of the genome space and our data are consistent with a model in which chromatin-associated proteins form dynamic interaction networks in vivo. We suggest that these properties are crucial for generating high plasticity in genome expression.

Formation of nuclear splicing factor compartments is independent of lamins A/C.

Nuclear lamins are major architectural elements of the mammalian cell nucleus, and they have been implicated in the functional organization of the nuclear interior, possibly by providing structural support for nuclear compartments. Colocalization studies have suggested a structural role for lamins in the formation and maintenance of pre-mRNA splicing factor compartments. Here, we have directly tested this hypothesis by analysis of embryonic fibroblasts from knock-out mice lacking A- and C-type lamins. We show that the morphology and cellular properties of splicing factor compartments are independent of A- and C-type lamins. Genetic loss of lamins A/C has no effect on the cellular distribution of several pre-mRNA splicing factors and does not affect the compartment morphology as examined by light and electron microscopy. The association of splicing factors with the nuclear matrix fraction persists in the absence of lamins A/C. Live cell microscopy demonstrates that the intranuclear positional stability of splicing factor compartments is maintained and that the exchange dynamics of SF2/ASF between the compartments and the nucleoplasm is not affected by loss of lamin A/C. Our results demonstrate that formation and maintenance of intranuclear splicing factor compartments is independent of lamins A/C, and they argue against an essential structural role of lamins A/C in splicing factor compartment morphology.

Computing maximum association graph in microscopic nucleus images.

In this paper, we study the problem of finding organization patterns of chromosomes inside the cell nucleus from microscopic nucleus images. Emerging evidence from cell biology research suggests that global chromosome organization has a vital role in fundamental cell processes related to gene expression and regulation. To understand how chromosome territories are neighboring (or associated) to each other, in this paper we present a novel technique for computing a common association pattern, represented as a Maximum Association Graph (MAG), from the nucleus images of a population of cells. Our approach is based on an interesting integer linear programming formulation of the problem and utilizes inherent observations of the problem to yield optimal solutions. A two-stage technique is also introduced for producing near optimal approximations for large data sets.

Spatio-temporal dynamics at rDNA foci: global switching between DNA replication and transcription.

We have investigated the in situ organization of ribosomal gene (rDNA) transcription and replication in HeLa cells. Fluorescence in situ hybridization (FISH) revealed numerous rDNA foci in the nucleolus. Each rDNA focus corresponds to a higher order chromatin domain containing multiple ribosomal genes. Multi-channel labeling experiments indicated that, in the majority of cells, all the rDNA foci were active in transcription as demonstrated by co-localization with signals to transcription and fibrillarin, a protein involved in ribosomal RNA processing. In some cells, however, a small portion of the rDNA foci did not overlap with signals to transcription and fibrillarin. Labeling for DNA replication revealed that those rDNA foci inactive in transcription were restricted to the S-phase of the cell cycle and were replicated predominantly from mid to late S-phase. Electron microscopic analysis localized the nucleolar transcription, replication, and fibrillarin signals to the dense fibrillar components of the nucleolus and at the borders of the fibrillar centers. We propose that the rDNA foci are the functional units for coordinating replication and transcription of the rRNA genes in space and time. This involves a global switching mechanism, active from mid to late S-phase, for turning off transcription and turning on replication at individual rDNA foci. Once all the rRNA genes at individual foci are replicated, these higher order chromatin domains are reprogrammed for transcription.