Nuclear architecture - an epigenetic mechanism for the regulation of nuclear functions
Increasing attention has been paid in the last years to the functional relevance of higher order chromatin arrangement as an epigenetic mechanism for specific gene regulation in different cell types. Recent developments of 3D nanoscopy have provided new means to study nuclear architecture at nanometer resolution, which will help to bridge the gap from the molecular level to the level of higher-order structure. The segmental organisation in metaphase chromosomes with regard to gene density is transposed into a polar organization of chromosome territories in interphase: interphase chromatin is spatially arranged in a radial pattern with the preferential localization of gene-dense chromatin in the nuclear interior and of gene-poor chromatin at the nuclear envelope. The spatial proximity of genes in the nuclear interior may facilitate the establishment of a chromatin topography, which optimally suits the structural requirements for transcription. For highly transcribed genes activation or silencing has been associated with nuclear repositioning. Yet, the influence of transcriptional activity per se has remained a matter of discussion. That an enrichment of (transcriptionally active) genes in the nuclear interior is not mandatory, was recently shown by the observation of an “inverted” chromatin pattern in rod cell nuclei of adult animals with a nocturnal life style. This remodeling into an “inverted” pattern takes place during the postmitotic terminal differentiation of rod cells. In these cells chromatin poised for transcription, as well as highly transcribed genes are located at the nuclear periphery and gene-poor chromatin (heterochromatin) in the nuclear center. This unique organization suggests a functional significance of the nuclear architecture in the retina of nocturnal animals based on physical properties of chromatin. An “inverted” chromatin arrangement is less diffractive to light and therefore provides an advantage for nocturnal life style.
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