Distribution of non-allelic histone H1 subtypes in five avian species

The arrays of histone H1 subtypes from five avian species (chicken, grey partridge, pheasant, quail and duck) were compared to evaluate their intra- and inter-species variability. The electrophoretic patterns of linker histone preparations revealed the presence of subtypes that occur in all species (H1.a, H1.b, H1.c, H1.c′, H1.d and H5) and those which are confined to some species only (H1.a′, H1.b′, H1.z). In the densitometric profiles of histone H1 bands resolved in one-dimension acetic acid-urea polyacrylamide gel, the quantitative differences were observed both within a species (the ratio of H1.b to H1.d = 8.13 in quail) and between species (the ratio of H1.d in grey partridge and quail = 8.37). The comparable levels of abundant histone H5 that constitute from 53.62% (quail) to 60.86% (duck) of whole linker histone complement were detected in all species. Likewise, the quantification of H1 protein spots separated in a two-dimension SDS-polyacrylamide gel indicated that their intensity ratios could vary up to about 17-fold within a species (the ratio of H1.d to H1.a′ in grey partridge) and up to 10-fold between species (the ratio of pheasant H1.d to quail H1.d). Differences (P<0.05) in the histone H1 subtype levels were found both within and between avian species. A low to moderate range for the coefficients of H1 spot variation (from 0.13 to 0.72) was obtained for several independent histone H1 preparations.

Histone hypercitrullination mediates chromatin decondensation and neutrophil extracellular trap formation

Peripheral blood neutrophils form highly decondensed chromatin structures, termed neutrophil extracellular traps (NETs), that have been implicated in innate immune response to bacterial infection. Neutrophils express high levels of peptidylarginine deiminase 4 (PAD4), which catalyzes histone citrullination. However, whether PAD4 or histone citrullination plays a role in chromatin structure in neutrophils is unclear. In this study, we show that the hypercitrullination of histones by PAD4 mediates chromatin decondensation. Histone hypercitrullination is detected on highly decondensed chromatin in HL-60 granulocytes and blood neutrophils. The inhibition of PAD4 decreases histone hypercitrullination and the formation of NET-like structures, whereas PAD4 treatment of HL-60 cells facilitates these processes. The loss of heterochromatin and multilobular nuclear structures is detected in HL-60 granulocytes after PAD4 activation. Importantly, citrullination of biochemically defined avian nucleosome arrays inhibits their compaction by the linker histone H5 to form higher order chromatin structures. Together, these results suggest that histone hypercitrullination has important functions in chromatin decondensation in granulocytes/neutrophils.