Increasing evidence suggests nonrandom localization of DNA sequences in the cell nucleus. Changes in intranuclear gene location as a function of transcriptional activity have been established in several experimental systems.
How are chromosome regions targeted to specific compartments? In general, DNA shows very little or no motion over several hour time periods in yeast and mammalian interphase cells, as demonstrated by visualization of chromosome dynamics in vivo. However, examples of chromosome movements during interphase have been described. Although normally static, infrequent examples of sustained, unidirectional motion of centromeres in G1 nuclei over 1-2 mm have been observed. Relocalization from the nuclear periphery to the nuclear interior has been shown for centromeres during the cell cycle in certain cell types, the inactive X chromosome in motor neurons after electric stimulation, the X chromosome in seizure foci of male cortical neurons, and for double minute chromosomes during S phase. Other examples of interphase chromosome motion include movements of homologous chromosomes during G1 and S in Drosophila larvae, and association of previously active genes to centromeres after gene inactivation.
By several different approaches, minimal long-range motion, beyond several tenths of a micron, have been observed over several hour periods for chromosome regions in log phase mammalian tissue culture cells. These observations indicate that the default state for most chromosome regions in these cells is little long-range motion. However, these results do not address the existence of significant chromosome movements for some chromosome regions either during a specific time during the cell cycle, a particular stage of cell differentiation, or in response to a specific cell signal.
Using the lac operator / repressor visualization system, we were able to focus on specific chromosome regions and characterize their behavior during different cell cycle stages. To date in both of the two cases in which careful analysis was carried out, we have observed a specific movement of the labeled chromosome region restricted to a specific time during S phase. In the first case, a late-replicating, heterochromatic gene amplified chromosome arm was observed to move from the nuclear periphery to the nuclear interior during middle to late S phase. This movement was closely correlated with decondensation of the large-scale chromatin structure and initiation of DNA replication of the labeled chromosome arm. In the second case, an insertion of ~10-20 copies of a transgene carrying the lac op repeat was shown to locate at the nuclear periphery in ~75% of cells during G1, but move to the nuclear interior during early S phase.
Direct observation of a several micron translocation of a ~90 Mbp amplified chromosome from the nuclear periphery to the nucleolus and back. (A-F) Images represent combination of transmitted and fluorescence light images at specific time points, measured from beginning of observation time (t=0), 4 hrs after release from late G1/ early S phase block. (A-F) correspond to t= 1, 5, 5.5, 6, 9, and 9.5 hrs, respectively. HSR movement from the nuclear periphery to nucleolus is coupled with HSR decondensation. HSR returns to nuclear periphery in vicinity of original starting position. Arrows point to edge of nucleus. Scale bar = 2 um.
Direct observation of several micron movement of labeled chromosome site in C6 cells. G1 cells show a preferential association of the chromosome site to the nuclear periphery. Cells were blocked at late G1 / early S phase using mitotic shakeoff followed by HU block. Times shown are minutes after release of HU block. Chromosome site is observed to move from the nuclear periphery to the nuclear interior. Experiments on synchronized cell populations showed that this movement occurs by replication pattern 2, typical of early S phase.
Our work provided direct visualization of chromosome movements at specific times during S phase. In related work, we showed that movement of a transgene array from the nuclear periphery to the nuclear interior can be induced by tethering of specific transcriptional activators. (See Changes in Gene Positioning Induced by Transcriptional Activators). We are just at the very beginning stage of understanding the underlying mechanism by which these movements occur and their physiological relevance. A major problem in the past has been the extreme sensitivity of these long-range chromosome movements to phototoxicity, hindering efforts to use live cell imaging to visualize these movements. New microscopy technology should allow reduced phototoxicity allowing us to examine this phenomenon more productively in the near future.
Tumbar T, Belmont AS. Interphase movements of a DNA chromosome region modulated by VP16 transcriptional activator. Nat Cell Biol. 2001 Feb; 3 (2) :134-9. PubMed PMID:11175745.
Li G, Sudlow G, Belmont AS. Interphase cell cycle dynamics of a late-replicating, heterochromatic homogeneously staining region: precise choreography of condensation/decondensation and nuclear positioning. J Cell Biol. 1998 Mar 9; 140 (5) :975-89. PubMed PMID:9490713; PubMed Central PMCID: PMC2132695.