ADP-ribose polymer depletion leads to nuclear Ctcf re-localization and chromatin rearrangement

Ctcf (CCCTC-binding factor) directly induces Parp [poly(ADPribose) polymerase] 1 activity and its PARylation [poly(ADPribosyl) ation] in the absence of DNA damage. Ctcf, in turn, is a substrate for this post-synthetic modification and as such it is covalently and non-covalently modified by PARs (ADP-ribose polymers). Moreover, PARylation is able to protect certain DNA regions bound by Ctcf from DNA methylation. We recently reported that de novo methylation of Ctcf target sequences due to overexpression of Parg [poly(ADP-ribose)glycohydrolase] induces loss of Ctcf binding. Considering this, we investigate to what extent PARP activity is able to affect nuclear distribution of Ctcf in the present study. Notably, Ctcf lost its diffuse nuclear localization following PAR (ADP-ribose polymer) depletion and accumulated at the periphery of the nucleus where it was linked with nuclear pore complex proteins remaining external to the perinuclear Lamin B1 ring. We demonstrated that PAR depletion-dependent perinuclear localization of Ctcf was due to its blockage from entering the nucleus. Besides Ctcf nuclear delocalization, the outcome of PAR depletion led to changes in chromatin architecture. Immunofluorescence analyses indicated DNA redistribution, a generalized genomic hypermethylation and an increase of inactive compared with active chromatin marks in Parg-overexpressing or Ctcf-silenced cells. Together these results underline the importance of the cross-talk between Parp1 and Ctcf in the maintenance of nuclear organization.