Dualistic function of Daxx at centromeric and pericentromeric heterochromatin in normal and stress conditions

Abstract

Nuclear structures ND10/PML NBs are linked to multiple processes, including the maintenance of intranuclear homeostasis by sequestering proteins into "nuclear depot". This function presumes release of proteins from PML NBs and their redistribution to the alternative, supposedly "active" locations, in response to the external stress application. To further investigate this nuclear depot function, we focused on the intranuclear distribution of protein Daxx that in normal conditions is mainly accumulated at PML NBs, and has a minor association with centromeres and pericentromeres (CE N/ periCE N). Here we report that application of physiological Heat Shock (HS) changes this balance forcing very robust and reversible accumulation of Daxx on CE N/periCE N heterochromatin. Heterochromatin architecture is essential for the proper orchestration of nuclear processes, while transcription from this part of genome is required for its maintenance. To understand functional consequences of Daxx deposition at CE N/ periCE N, we tested for Daxx-dependency of heterochromatin transcription. Depletion of Daxx reduces accumulation of CE N RNA in normal conditions and periCE N RNA after HS application. Searching for the mechanism of Daxx-dependent regulation of heterochromatin transcription, we found that depletion of Daxx decreases incorporation of transcriptionassociated histone H3 variant, H3.3, into both CE N and periCE N. Surprisingly, HS-induced deposition of Daxx does not further elevate incorporation of H3.3 into CE N/periCE N that remained steady during stress and recovery. Instead, depletion of Daxx leads to HS-induced changes in the balance of epigenetic modifications at heterochromatin, most dramatically elevating levels of active H3K4Me2 modification at periCE N. We propose dualistic function of Daxx-containing complexes at CE N/periCE N: (1) regulation of H3.3 loading in normal conditions and (2) protection of epigenetic status upon stress-induced accumulation, thus collectively guarding epigenetic identity of CE N/periCE N heterochromatin. © 2012 Landes Bioscience.