Chromatin structure is subject to regulation by posttranslational histone modifications. One of the strongest effects is imposed by arginine citrullination of histone H3 and H4 N-tails by arginine deiminase PAD4, which mediates formation of neutrophil extracellular traps (NETs) in granulocytes. To study mechanism of chromatin unfolding by histone citrullination independent of proteases active in NETosis, we used reconstituted nucleosome arrays and native nucleosome arrays isolated from human myeloid cells. To reveal chromatin structure transitions, we applied Cryo-electron tomography combined with quantitative stereological analysis. With reconstituted chromatin, we observed a complete disruption of chromatin higher order structure by histone citrullination. In contrast, with native chromatin we observed a strong inhibition of nucleosome close-range interactions and dissolving of large nucleosome condensates with relatively little effect on the conformation of nucleosome chains. We propose that the main effect of PAD4-induced citrullination in native chromatin is to inhibit nucleosome close-range interactions by modifying the histone H3 and H4 tails. This structural transition may trigger NETosis by unfolding nucleosome condensates and opening them to proteases, that would further digest the histone tails leading to the complete nuclear rupture.
This work was supported by US NSF grant 1911940 to S. Grigoryev.