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Dynamic Genome Architecture in the Nuclear Space: Regulation of Gene Expression in Three Dimensions

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Dynamic Genome Architecture in the Nuclear Space: Regulation of Gene Expression in Three Dimensions REVIEWS Dynamic genome architecture in the nuclear space: regulation of gene expression in three dimensions Christian Lanctôt*, Thierry Cheutin‡, Marion Cremer*, Giacomo Cavalli‡ and Thomas Cremer* Abstract | The regulation of gene expression is mediated by interactions between chromatin and protein complexes. The importance of where and when these interactions take place in the nucleus is currently a subject of intense investigation. Increasing evidence indicates that gene activation or silencing is often associated with repositioning of the locus relative to nuclear compartments and other genomic loci. At the same time, however, structural constraints impose limits on chromatin mobility. Understanding how the dynamic nature of the positioning of genetic material in the nuclear space and the higher-order architecture of the nucleus are integrated is therefore essential to our overall understanding of gene regulation. PML bodies A striking feature of nuclear architecture is the exist- interior, whereas gene-poor regions tend to be oriented Nuclear structures ence of distinct structural and functional compartments towards the periphery (for review, see REF. 10). that are enriched for the (FIG. 1). Well characterized nuclear substructures include Although chromosomes are organized as distinct promyeolocytic leukaemia the nuclear lamina, nucleoli, PML and Cajal bodies, and territories in the interphase nucleus, evidence obtained (PML) RING-finger protein and nuclear speckles1–4. Also, a growing number of compo- from various biological models — using techniques are implicated in transcriptional regulation, viral pathogenicity, nents of the machinery that is required for transcription ranging from classical genetics to molecular tools — has tumour suppression, apoptosis or its repression are known to have a non-homogene- shown that chromosomes are dynamic structures and and DNA repair. ous distribution in the nucleoplasm5–7. At the level of the that individual chromosomal regions can be reposi- genome itself, the genetic material is folded and packaged tioned, with respect to both nuclear structures and other Cajal bodies Spherical structures that are in the nucleus into higher-order structures that are likely chromosomal regions. There is also increasing evidence 8–12 involved in the processing of to contribute to the regulation of gene expression . that repositioning of genomic regions in nuclear space nuclear RNA. Marker proteins A major goal of ongoing studies to understand the is important for the regulation of gene expression21. include p80 coilin and the potential influence of nuclear architecture on nuclear Recent technological advances now allow the large-scale survival of motor neuron functions is to identify the principles that govern identification of interacting loci across the genome, and (SMN) protein. the spatial organization of the genome. One classical initial results have highlighted the potentially wide- example of organization within the nucleus is the dis- spread importance of such interactions for proper gene *Department Biologie II, tinction between decondensed, transcriptionally active regulation. As will be discussed later, the possibility that 11 Ludwig-Maximilians euchromatin and more condensed, generally inactive spatial networks of genomic loci exist in the nucleus Universität, heterochromatin13. It is now also known that individual implies the existence of a previously unexplored level Grosshadernerstr. 2, Planegg- chromosomes occupy distinct positions in the nucleus, of gene regulation that coordinates expression across Martinsried, Germany. referred to as chromosome territories14,15. As a result of the genome. ‡Institute of Human Genetics, CNRS, 141 rue de la different compaction levels, different chromosome seg- Here we review the evidence that supports the Cardonille, 34396 Montpellier ments adopt a complex organization and topography dynamic nature of gene positioning and its link with Cedex 5, France within their chromosome territory16–18. A polarized gene expression, focusing mainly on mammalian sys- Correspondence to C.L. or T.C. intranuclear distribution of gene-rich and gene-poor tems and Drosophila melanogaster, but referring to e-mails: christian.lanctot@lrz. uni-muenchen.de; thomas. chromosomal segments has been shown to be an evolu- yeast data when appropriate. We first describe chroma- 19,20 [email protected] tionarily conserved principle of nuclear organization . tin movements that have been observed in living cells doi:10.1038/nrg2041 Gene-rich regions tend to be oriented towards the nuclear and briefly discuss the structural constraints that limit 104 | FEBRUARY 2007 | VOLUME 8 www.nature.com/reviews/genetics © 2007 Nature Publishing Group REVIEWS Nuclear Interchromatin D. melanogaster chromosome X supported the conclu- speckle compartment sion that the mobility of individual loci can also best be Chromosome Nuclear described as constrained diffusion. However, in this case the territories pore radius of confinement (0.9 µm) was about half the radius of the nuclei (2 µm), indicating a much greater relative Nuclear mobility of chromatin in this experimental model25. envelope These initial findings were complemented by several studies that tracked the position of fluorescently labelled, stably integrated arrays of lac operator transgenes in liv- PML ing D. melanogaster, Arabidopsis thaliana and human cell body lines26–29. Quantitative time-lapse analysis showed two different types of chromatin motion in D. melanogaster Nucleolus spermatocytes: rapid, localized movements (0.3–0.7 µm) and slower long-range movements that were confined to an average radius of 2.6 µm, which is comparable to the dimensions of individual chromosome territories in the large nuclei of this cell type (10–17µm in diam- eter)29. Imaging of fluorescently tagged chromosomal loci in living human cells supported the finding that the average mobility of chromatin is increased during early Nuclear lamina G1 (REF. 30), as had been observed for chromosome sub- 24 Cajal domains . Furthermore, the mobility of individual loci body depended on their nuclear localization, with peripheral and nucleolar-associated transgenes being significantly Chromosome 26 territories less mobile than nucleoplasmic ones . A recent analysis of chromatin dynamics in chinese hamster ovary cells Figure 1 | Organization of the mammalian cell nucleus. The nucleus is characterized was performed at a much higher spatial (~20 nm) and by a compartmentalized distribution of functional components. The nuclear envelope temporal (~30 ms) resolution using two-photon microscopy contains pores and rests on a meshwork of intermediate filaments, the nuclear lamina. and a small sampling volume31. Particle trajectories showed Nucleolar organizer regions cluster to form nucleoli. Further topographical details that periods of constrained diffusion, interrupted approxi- are shown in this schematic nuclear section are representative for the chromosome mately every minute by rapid ‘jumps’ of ~150 nm that territory–interchromatin compartment (CT–IC) model. Chromatin is organized in were sensitive to depletion of cellular ATP and to temper- distinct CTs. Also depicted are nuclear speckles, PML bodies and Cajal bodies located in wider IC lacunas (sections through smaller channels of the contiguous, three- ature, indicating that chromatin might undergo active, dimensional IC network are not depicted). Nuclear topography remains a subject of possibly directed movements at the level of single loci. debate, expecially with regard to the extent of chromatin loops expanding into the IC Altogether, these observations in living cells indicate and intermingling between neighbouring CTs and chromosomal subdomains. For that long-range chromatin repositioning during inter- in-depth discussions of the CT–IC and other models see REFS 15, 32. phase occurs only during a relatively short time window, which corresponds in most cells to approximately the first third of G1. Once this window has closed, chro- chromatin mobility in the interphase nucleus. We then matin movements are constrained within small nuclear Nuclear speckles Irregular structures that present examples of repositioning of specific loci with subdomains. The size of these subdomains relative to the contain high concentrations respect to nuclear landmarks concomitant with their size of the nucleus varies depending on the species, of splicing factors and small transcriptional regulation. Finally, we review recent the cell type and, as described below, the cellular dif- nuclear ribonucleoprotein results that have demonstrated the existence of specific ferentiation status. Chromosome segments can adopt particles (snRNPs). interactions between loci, and discuss the significance of irregular shapes within these subdomains, with numer- Two-photon microscopy such interactions for the regulation of gene expression. ous protrusions and invaginations forming as a result An imaging technique in which Throughout the review, we discuss how the increasing of chromatin mobility. Examples of the protrusion of a fluorochrome that would evidence for substantial chromatin mobility can be rec- specific chromosomal segments from the core territory normally be excited by a single onciled with the structural organization of the nucleus. have been well documented32–34. Currently, there is some photon is stimulated quasi- simultaneously by two photons controversy as to the extent to which these protruding of lower energy. This allows
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