Probing local chromatin dynamics by tracking telomeres

Chromatin dynamics is key for cell viability and replication. In interphase, chromatin is decondensed, allowing the transcription machinery to access a plethora of DNA loci. Yet, decondensed chromatin occupies almost the entire nucleus, suggesting that DNA molecules can hardly move. Recent reports have even indicated that interphase chromatin behaves like a solid body on mesoscopic scales. To explore the local chromatin dynamics, we have performed single-particle tracking on telomeres under varying conditions. We find that mobile telomeres feature in all conditions a strongly subdiffusive, anti-persistent motion that is consistent with the monomer motion of a Rouse polymer in viscoelastic media. In addition, telomere trajectories show intermittent accumulations in local niches at physiological conditions, suggesting the surrounding chromatin to reorganize on these time scales. Reducing the temperature or exposing cells to osmotic stress resulted in a significant reduction of mobile telomeres and the number of visited niches. Altogether, our data indicate a vivid local chromatin dynamics, akin to a semi-dilute polymer solution, unless perturbations enforce a more rigid state of chromatin. Statement of SignificanceIn interphase cells, chromatin is decondensed and occupies almost the entire nucleus, suggesting DNA molecules to be fairly immobile. Recent reports even indicated chromatin to behave like a solid body on mesoscopic scales. Tracking individual telomeres, we have explored local chromatin dynamics under varying conditions. Our data reveal that telomeres show an antipersistent subdiffusion (fractional Brownian motion) at physiological conditions and at lower temperatures, indicating chromatin to be a viscoelastic fluid on submicron length scales. Telomeres also appear to visit local niches, supposedly provided by the constantly reorganizing chromatin. Applying osmotic stress significantly reduced the fraction of mobile telomeres and the number of visited niches, indicating that chromatin eventually assumes an elastic, solid-like behavior under these conditions.