Role of chromatin organization and accessibility on transcription factor search mechanism upon senescence.

The interplay between transcription factors and the chromatin landscape is a crucial element of their function, influencing gene expression and thus, cell fate. To study the underlying rules of this interplay, we use an innovative multi-modal approach, integrating super-resolution chromatin imaging with single-molecule tracking in living cells. In particular, we have focused on the behavior of the transcription factor p53, renowned for its role in tumor suppression and senescence. Using oncogene-induced senescence (OIS) as our model system, a cellular state characterized by a massive reorganization of chromatin into senescence-associated heterochromatin foci (SAHF), we study the dynamic interplay between p53, chromatin, and SAHF. As chromatin organization has been shown to regulate gene expression, it is tempting to hypothesize a functional role for the SAHF. However, like many other features of senescence, high cell-to-cell variability makes the study of their function complicated. In the face of this heterogeneity, the inherent single-cell nature of microscopy is poised to shed light on SAHF function. The study of p53 behavior around chromatin, and around SAHF when they are present, reveals a tug-of-war between p53’s affinity for chromatin and the volume exclusion from the highly condensed regions of chromatin. Mathematical modeling of the diffusion process allows for the quantification of each and underlines the existence of an intermediate state of slow diffusion of p53 when it diffuses on the surface of SAHF. Combining immunofluorescence and smFISH in fixed samples, we show that this is correlated with an increased in the efficiency of p53 activity compared to proliferating cells, where equal levels of p53 lead to higher expression of target genes. Thus our results suggest the SAHF do not only have a silencing role, hiding repressed genes in heterochromatin, but potentially also an enhancing one, allowing for strong expression of p53 target genes even at low p53 expression.