The organization of the genome in the nucleus is believed to be crucial for cellular functions such as gene regulation and the maintenance of genome integrity along the cell cycle. We used live cell imaging to examine the organization of the genome in living cells. We analysed the diffusion of different genomic sites by measuring their trajectories through Single Particle Tracking (SPT) and thoroughly characterized their diffusion characteristics also under depletion of different nuclear proteins.
We found that chromatin diffusion in differentiated cells is slow and anomalous; in vast contrast, absence of lamin A protein leads to a significant increase in chromatin mobility, and induces a dramatic transition of genome dynamics from slow anomalous diffusion to fast and normal diffusion.
Constrained chromatin mobility can have an important role in maintaining the chromosomal territories, which in turn prevents chromosomal entanglement and aberrations. Indeed, by using SKY (Spectral Karyotyping) technique, we found that Lmna-/- cells have a higher frequency of genomic aberrations compared to normal cells.
While B-type lamins are expressed in all cells, A-type lamins are developmentally regulated and they do not express in ESCs cells. Diffusion analysis of chromatin motion in ESCs and hNP1 (Neuron Precursors) cells indicates that chromatin scan much large volume in the nucleus compared to differentiated cells, while the expression of lamin A in these cells restricts chromatin local freedom of motion. More than this, the dynamics of chromatin loci in Embryonic stem cells (ESCs) was found to be normal diffusion, similar to lamin A depleted cells. The expression of lamin A protein in hNP1 cells switched the type of diffusion from normal to anomalous diffusion. It means that lamin A protein acts as key in the cells that able to switch on /switch off the chromatin diffusion type.
We now try to understand the functional reason for the drastic increased freedom of chromatin motion in undifferentiated cells compare to differentiated cells.
