We have suggested that chromosomes should be viewed as macroscopic mechanical objects whose functions are governed by accumulation, release and redistribution of mechanical stress. We have proposed specifically that chromosomes undergo cyclic changes in state between more compact and more expanded states. In more compact states, energy (mechanical stress) is stored within the chromosomes because the chromatin fiber is spatially constrained by molecular tethers. More expanded states result from release of tethers and the resulting free energy change (relief of stress) drives molecular changes.
To test this idea, we defined the pathway by which chromosomes progress from their diffuse state (seen for most of the cell cycle) to the compact state that they acquire just before mitosis, which permits clean segregation of sister chromosomes to their respective daughter cells (Liang, Z., Zickler, D., Prentiss, M., Chang, F.S., Witz, G., Maeshima, K. and Kleckner, N. Chromosomes progress to metaphase in multiple discrete steps via global compaction/expansion cycles. Cell 2015, May 21; 161(5):1124-1137). Our idea was that, during this process, changes in chromosome volume and effects of stress-promoted changes might be read out in macroscopically visible changes.
Prior to our study, the nature of the final chromosome state seen at the time of segregation had been well established. Each chromatid comprises a linear series of loops that are radially disposed along a chromosome structural axis; sister chromatids lie side-by-side; and the entire ensemble is very short and wide (“fat”). However, the pathway by which this state is achieved was not clear. Notably, in opposition to our stress cycle hypothesis, textbook models universally envision a continuous, progressive compaction process.
Our new study used 4D fluorescence deconvolution imaging of chromosomes in living mammalian cells. The patterns observed (See figure below) closely match the predictions of our stress cycle hypothesis. They reveal that assembly of organized chromosomes involves chromosome expansion as well as compaction. Moreover, the details of the newly-defined pathway closely match specific predictions. (1) The structure of a classical intermediate stage (prophase) is revealed and shown to arise by coalescence of chromatin from a previously expanded state. Also, chromosomes are held in this compact prophase state by two types of molecular tethers – cohesin and Topoisomerase II-sensitive linkages (probably catenations). Chromosomes then progress to the next morphological stage by a reorganization that involves expansion and requires release of both types of prophase tethers. (3) The morphological consequences of this expansion transition are those predicted for transit to a lower energy state that has been licensed by release of tethers. The classical prophase state comprises intermingled sister loops organized along a kinked, peripherally located structural axis. At the transition out of this state (late prophase), the structural axis splits and the loops of each sister become radially organized along its corresponding axis. By these effects, chromatin loops occupy a less constrained (i.e. less stressed) state. (4) This expansion transition is followed by a period of global compaction which completes the process, yielding the final segregation-ready conformation.
The provocative potential implication raised by these studies is that chromosome compaction/expansion stress cycles are an underlying feature of the entire eukaryotic cell cycle.