Epigenome rejuvenation: HP1β mobility as a measure of pluripotent and senescent chromatin ground states

We measured the dynamics of an essential epigenetic modifier, HP1 beta, in human cells at different stages of differentiation using Fluorescence Recovery After Photobleaching (FRAP). We found that HP1b mobility is similar in human embryonic stem cells (hES) and iPS cells where it is more mobile compared to fibroblasts; HP1b is less mobile in senescent fibroblasts than in young (dividing) fibroblasts. Introduction of `` reprogramming factors'', Oct4, Sox2, Klf4, cMyc and Lin28, into senescent fibroblasts and measuring the changes in HP1b mobility as reprogramming proceeds shows that the mobility of HP1b in senescent cells increases and by day 9 is the same as that found in young fibroblasts. Thus the dynamics of a key epigenetic modifier can be rejuvenated without de-differentiation through an embryonic stage. Future work will test whether other aspects of cellular physiology that age can be so rejuvenated without de-differentiation.