Specific Interactions between Chromatin and the Nuclear Envelope in Drosophila embryos

by Wallace Marshall

Rotation-Projection of 3D image showing NE associated chromatin (green)
localized to the nuclear lamina (pink). Image of cycle 13 Drosophila embryo nucleus made by Wallace Marshall

Abstract

Contact of chromatin with the nuclear envelope (NE) has long been seen in the microscope. In order to begin to understand the function of this association, we must first determine if this association is site-specific and if so, to map the chromosome sites that associate with the NE. This will then provide us with a starting point for testing some potential functional roles and also to begin identification of the molecular components of the interaction.

To this end, I have mapped and analyzed the specific interactions of chromatin with the nuclear envelope in embryos of Drosophila melanogaster. The three-dimensional positions of specific chromosome regions were found by fluorescence in situ hybridization, and the nuclear envelope visualized by anti-lamin immunofluorescence. A statistical test for NE association was employed which explicitly accounts for effects of nuclear polarization , nonuniform distribution of bulk chromatin within the nucleus, and differences in nuclaer size and shape. Using this test, the positions of 41 DNA probes, primarily to chromosome arm 2L, have been maped in Drosophila embryos, revealing 5 euchromatic and 2 heterochromatic regions associated with the NE. These results predict that there are approximately 15 NE contacts per chromosome arm, with each NE association site delimiting a chromosomal loop approximately 1 MB in size. There is no strict tendency for either scaffold-attachment regions (SARs), scs boundary elements, or heterochromatin to be associated with the NE.

We have now begun testing some of the functional roles of these interactions. We have already published evidence (see below) that these associations are not involved in post-mitotic NE reassembly. More recently we have obtained results indicating that these associations also are unlikely to be involved in homolog pairing, transcription, or transcript export. These is, however, some preliminary evidence that prophase chromosome condensation may initiate in these specific NE associated regions.

Analysis of positions of these probes also reveals that the interphase nucleus is strongly polarized in a Rabl configuration which, together with specific targetting to the NE or to the nuclear interior, results in each locus occupying a highly determined position within the nucleus.



surface-rendered image showing two different FISH probes along with nuclear surface
Such data forms the basis for our analysis of nuclear architecture in interphase

Further Details

Chromosome loci occupy defined positions in the nucleus
Computational analysis of nuclear organization

Publications

Marshall, W.F., Dernburg, A.F., Harmon, B., Agard, D.A., and Sedat, J.W. 1996. Specific interactions of chromatin with the nuclear envelope: positional determination within the nucleus in Drosophila melanogaster. Molecular Biology of the Cell 7,825-842.

Marshall, W.F., Fung, J.C., and Sedat, J.W. 1997. Deconstructing the nucleus: global architecture from local interactions. Curr. Opin. Gen. Dev. 7,259-263.

Related Page:

Interphase chromatin motion While FISH studies indicate that loci have defined positions on average, the opposite question is, to what extent does a given locus move around inside the nucleus in a single cell? Follow this link to see our approach to answering this question.

Related Links

Science News article about this work