Epichromatin and chromomeres: a ‘fuzzy’ perspective

Abstract

‘Epichromatin’, the surface of chromatin beneath the interphase nuclear envelope (NE) or at the surface of mitotic chromosomes, was discovered by immunostaining with a specific bivalent mouse monoclonal anti-nucleosome antibody (mAb PL2-6). ‘Chromomeres’, punctate chromatin particles approximately 200–300 nm in diameter, identified throughout the interphase chromatin and along mitotic chromosomes, were observed by immunostaining with the monovalent papain-derived Fab fragments of bivalent PL2-6. The specific target for PL2-6 appears to include the nucleosome acidic patch. Thus, within the epichromatin and chromomeric regions, this epitope is ‘exposed’. Considering that histones possess unstructured ‘tails’ (i.e. intrinsically disordered peptide regions, IDPR), our perception of these chromatin regions becomes more ‘fuzzy’ (less defined). We suggest that epichromatin cationic tails facilitate interactions with anionic components of NE membranes. We also suggest that the unstructured histone tails (especially, histone H1 tails), with their presumed promiscuous binding, establish multivalent binding that stabilizes each chromomere as a unit of chromatin higher order structure. We propose an ‘unstructured stability’ hypothesis, which postulates that the stability of epichromatin and chromomeres (as well as other nuclear chromatin structures) is a consequence of the collective contributions of numerous weak histone IDPR binding interactions arising from the multivalent nucleosome, analogous to antibody avidity.

Hyperosmotic stress:in situchromatin phase separation

“…In 300 mM sucrose (3c,d), epichromatin staining is still prominent, but less well defined. Our conclusion is that, even under hyperosmotic conditions, surface chromatin 'exposes' a high density of nucleosome acidic patches in suitable geometric arrangement to bind mAb PL2-6 with high avidity [27][28][29]. conditions.…”

“…Figure 1(f,h) show that congealed mitotic chromosomes maintain the 'surface' staining with anti-H3S10p. We have previously shown that antiepichromatin antibody (PL2-6) staining overlaps and extends slightly outside H3S10p staining [26,28]. It is clear that, as with PL2-6 immunostaining, the H3S10p epitopes remain at (or near) the outer surface of the mitotic chromosome clusters, whether or not they are perturbed by hyperosmotic treatment.…”

“…However, the hyperosmotic 'gel' remnants of mitotic chromosome clusters have not been previously described. Figure 1(a) (isosmotic conditions) displays an interphase nucleus possessing a 'fine' chromatin (DAPI) meshwork surrounded by 'epichromatin' immunostaining (mAb PL2-6, highlighting the surface of chromatin with exposed nucleosome 'acidic patches' [26][27][28][29]). Figure 1(b) (hyperosmotic 300 mM sucrose) shows an interphase nucleus with coarse chromatin fibers and voids, and epichromatin staining near the nuclear envelope.…”

“…At a more fundamental level of chromatin structure, histone H1 and HMG proteins also exhibit perturbations in localization within hyperosmotically-stressed mitotic chromosomes. Fixed and permeabilized interphase nuclei and mitotic chromosomes display a punctate 'chromomeric' immunostaining pattern in 0 mM sucrose ( Figure 6(a,c,e)), when reacted with anti-H1 antibodies [for a succinct review of current knowledge concerning H1 and the 'chromomere' concept, see [28]]. The punctate staining pattern of mitotic chromosome arms, employing anti-H1.2 and anti-H1.…”

Radial Organization in the Mammalian Nucleus

“…The electron-dense chromatin in the nuclear periphery corresponds to the LADs identified in human fibroblasts using lamin DamID (Guelen et al, 2008), and is enriched in LINE repeats and several histone marks of transcriptionally inactive chromatin, such as lysine nine diand tri-methylated histone H3 (H3K9me2 and H3K9me3) (Guelen et al, 2008;Wen et al, 2009). The peripheral chromatin immediately adjacent to the nuclear envelope is known as "epichromatin" and can be visualized in cells from different species using a special bivalent mouse monoclonal antinucleosome antibody (mAb PL2-6), which binds to the acidic patch on nucleosomes (Olins and Olins, 2018). This suggests that the chromatin close to the nuclear lamina not only has a peculiar composition, but additionally harbors a unique nucleosome structure that is not seen elsewhere in the nucleus.…”