Intranuclear relocalization of matrix binding sites during T cell activation detected by amplified fluorescence in situ hybridization.

We describe a method for analyzing the nuclear localization of specific DNA sequences, with special emphasis on their binding status to the nuclear matrix, depending on the developmental stage of the cells. This method employs high-resolution fluorescence in situ hybridization procedures. For our studies, it was important to examine the nuclear localization of a particular gene locus. Previously, however, it was not possible to detect a single-copy genomic sequence using a DNA probe less than several kilobases in size. We describe here a signal amplification technique based on tyramide which makes such a task possible. Using this method, we monitored single-copy loci using a short, 509-bp DNA sequence that binds in vivo to the T cell factor SATB1 within T cell nuclei, high-salt-extracted nuclei (histone-depleted nuclei generating "halos" with distended chromatin loops), and the nuclear matrix, before and after T cell activation. We found that these loci were anchored onto the nuclear matrix, creating new bases of chromatin loops, only after T cell activation. This experimental strategy, therefore, enabled us to detect the changes in higher order chromatin structure upon activation and study gene regulation at a new dimension: the loop domain structure. The methods shown here can be widely applied to explore other functions involving chromatin, including recombination and replication.