Use of antihistone antibodies with nucleosomes

Rabbit anti-HMG-17 antibodies recognize similar epitopes on the HMG-17 molecule as lupus autoantibodies. Relation with histone H1 defined epitopes

HMG-17 is a nucleosomal protein which is an immune target of autoantibodies in systemic lupus erythematosus (SLE) and other autoimmune diseases. Autoantibody production in SLE is believed to result from autoantigen specific immune stimulation and subsequently, it is expected that antigenic determinants recognized by SLE autoantibodies and induced antibodies by immunization are quite similar. To examine this issue, rabbits were immunized with purified HMG-17. The produced antiserum showed cross reactivity on blots and in inhibition ELISA with histone H1, even after its affinity purification with immobilized HMG-17. Finally, purification of the antiserum over H1 absorbed on nitrocellulose membrane produced specific anti-HMG-17 antibodies in the supernatant and anti-HMG-17/H1 antibodies that were bound to H1. SLE sera positive for HMG-17 had also cross reactivity with H1, and following the same procedure as before we received HMG-17 specific SLE autoantibodies and anti-HMG-17/H1 autoantibodies. Using the multipin epitope mapping technology, 19 overlapping 15-mer HMG-17 peptides and six 15-peptides, corresponding to known epitopes of histone H1, were synthesized. Four major epitopes were identified on the HMG-17 molecule, reactive with induced anti-HMG-17 antibodies, and these were the same as major autoepitopes In SLE. The sequence 25-51 of HMG-17, part of its DNA-binding domain, was recognized by the anti-HMG-17/H1 antibodies that were bound to H1. These antibodies recognized also defined epitopes of H1. Our results show that SLE autoantibodies can be directed against the same or similar epitopes as do IgGs evoked during the active immunization of animals, and provide additional evidence that autosensitization with an autoantigen might be operative. The possibility that the same or similar epitopes are found on different molecules (in this study HMG-17 and H1) supports the fact that there are rules by which nature selects the most dominant immunodeterminant to a given protein, which often represents functional or structural sites in the autoantigen.

Preparation/analysis of chromatin replicated in vivo and in isolated nuclei

This article outlined biochemical methodologies for the labeling, detection, and analysis of newly replicated and newly assembled nucleosomes. The isolation of specific vertebrate factors that may be involved in chromatin assembly in vivo, such as nucleoplasmin, CAF-1, and NAP-1 and their counterparts in Drosophila and yeast add a further dimension to the study of nucleosome assembly in living cells. In particular, the ability to genetically manipulate the yeast system, together with the identification of yeast enzymes that acetylate newly synthesized H4, will certainly provide exciting new avenues for the investigation of chromatin assembly in vivo.

Cross-reactivity of antibodies to retro-inverso peptidomimetics with the parent protein histone H3 and chromatin core particle. Specificity and kinetic rate-constant measurements

A series of monoclonal antibodies has been generated against an hexapeptide of sequence IRGERA corresponding to the C-terminal residues 130-135 of histone H3 and three analogues of this model peptide. The analogues correspond to the D-enatiomer, containing only D-residues, and two retro-peptides containing NH-CO bonds instead of natural amide peptide bonds. The chirality of each residue was maintained in the retro-peptide and inverted in the retro-inverso-peptide. Monoclonal antibodies were generated from mice immunized with the analogues coupled to neutral small unilamellar liposomes containing monophosphoryl lipid A as adjuvant. The reactivity of antibodies with the four analogues and with the parent protein H3 was studied in enzyme-linked immunosorbent assay and in a biosensor system. The equilibrium affinity constant (Ka) toward the retro-inverso-peptide of two out of three antibodies of IgG1 isotype induced against the L-hexapeptide was 7-75-fold higher than toward the homologous L-peptide. The range of Ka values of four antibodies of IgG1 and IgG2a isotypes generated against the retro-inverso-peptide was 0.6-1.9 x 10(9) M-1 for both the retro-inverso- and L-peptides. Furthermore, antibodies to the L- and retro-inverso-peptides cross-reacted strongly (in some cases better than with the homologous peptide) with the parent histone H3 and with chromatin subunits containing H3. The results are thus promising in respect to the potential use of retro-inverso-analogues, which are particularly stable, in the design of much more potent synthetic vaccines or to generate antibody probes.

Mapping of linear epitopes of human histone H1 recognized by rabbit anti-H1/H5 antisera and antibodies from autoimmune patients

Seventeen synthetic peptides of 15-16 residues, covering the complete sequence of the major human H1b variant, were tested for their capacity to bind serum IgG antibodies from 128 patients with systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and Sjögren's syndrome (pSS). One peptide (residues 111-127) of the human H1 degree variant and six synthetic and natural fragments of H5 were also tested. Results were compared to those obtained with antibodies from 11 rabbits immunized against chicken H1 and H5, and calf H1. The activity of peptides was tested in direct ELISA and in inhibition assays with free peptides in solution. A major epitope recognized by antibodies from SLE, RA and pSS patients as well as by rabbit antibodies was identified in the C-terminus of H1b (residues 204-218). Other peptides in the globular (residues 79-94) and C-terminal domains of H1b and peptide 111-127 of H1 degree were also recognized, albeit at a lower level and frequency, and some of them contain sequence homologies with peptide 204-218. Patients' antibodies and rabbit antisera were tested with complete H1 proteins from HeLa cells, calf thymus and chicken erythrocytes and with chicken H5. Less than 25% of autoimmune sera contained IgG antibodies reacting with H1/H5 in a direct ELISA. In dot-immunoassay, antigenic activity with intact H1/H5 proteins was detected in a larger number of sera. Using antibodies raised in rabbits against peptides 1-16 and 204-218 of H1b, we found no reaction with H1 immobilized on a solid-phase. In contrast, peptides 144-159, 170-185 and 204-218, which contain identical structural domains, compete with H1 in solution indicating that any of these three regions are accessible at the surface of free H1 and may be involved in the induction of specific antibodies in autoimmune patients.

Characterization of chromatin-condensing proteins during spermiogenesis in a neogastropod mollusc (Murex brandaris)

During the process of chromatin condensation in the spermiogenesis of the neogastropod mollusc Murex brandaris, the nuclear protein complement undergoes a complex series of changes. These changes lead to the appearance of three small protamines in the ripe sperm nuclei. We have characterized this system electrophoretically and at the compositional level, as well as through the analysis of crossreactions with antibodies elicited against a specific spermatozoan protamine. Our results indicate that the complex pattern of chromatin condensation during spermiogenesis in this species (M. brandaris) may be modulated by a series of post-translational (and intranuclear) modifications of DNA-interacting proteins, such as precursors to the sperm protamines. The amino acid composition of each sperm protamine is remarkably simple (lys + arg + gly > or = 96 mol%). This system of spermiogenic/spermatozoal proteins in the neogastropod M. brandaris clearly differs from that in patellogastropods and archaeogastropods, and it may be helpful in understanding evolutionary changes in the chromatin condensation pattern during the spermiogenesis of gastropod molluscs.

Rearrangements of the nucleosome structure in chromatin by poly(ADP-ribose)

In order to approach and clarify the effect of poly(ADP-ribose) on the nucleosomal structure, polynucleosomes from calf thymus were incubated with long poly(ADP-ribose) chains prepared in vitro and examined by ELISA with antibodies directed against the five individual histones H1, H2A, H2B, H3 and H4 as well as against two synthetic peptides in residues 1-25 of H2B and 130-135 of H3. The results showed that: (i) free ADP-ribose polymers did indeed interact with the nucleosomes; (ii) the accessibility of epitopes recognized by any of the different antibodies was altered, the binding of antibodies being increased or decreased depending on the quantity of poly(ADP-ribose) added thereby suggesting a modulation in nucleosome structure; (iii) for any ADP-ribose polymer concentration, core histones as well as histone H1 were always recognized by their respective antibodies, thus suggesting that poly(ADP-ribose) does not seem to cause complete stripping of histones from nucleosomal DNA.

Accessibility and structural role of histone domains in chromatin. biophysical and immunochemical studies of progressive digestion with immobilized proteases

The accessibility and role of histone regions in chromatin fibres were investigated using limited proteolysis with enzymes covalently bound to collagen membranes. The changes in chromatin conformation and condensation monitored by various biophysical methods, were correlated to the degradation of the histone proteins revealed by antibodies specific for histones and histone peptides. Upon digestion with trypsin and subtilisin, chromatin undergoes successive structural transitions. The cleavage of the C-terminal domains of H1, H2A and H2B, and of the N-terminal tail of H3 led to a decondensation of chromatin fibres, indicated by increases in electric birefringence and orientational relaxation times. It corresponds to a 15% increase in linear dimensions. The degradation of the other terminal regions of histones H3, H2A and H2B resulted in the appearance of hinge points between nucleosomes without alteration of the overall orientation of polynucleosome chains. Despite the loss of all the basic domains of H1, H3, H2A and H2B, no significant change in DNA-protein interactions occurred, suggesting that most of these protease-accessible regions interact weakly, if at all, with DNA in chromatin. Further proteolysis led to H4 degradation and other additional cleavages of H1, H2B and H3. This caused the relaxation of no more than 8% of the total DNA but resulted in changes in the ability of chromatin to condense at high ionic strength. More extensive digestion resulted in a total unravelling of nucleosomal chains which acquired properties similar to those of H1-depleted chromatin, although the globular part of H1 was still present. The data suggest that histone-histone interactions between H1 and core histone domains play a central role in stabilizing the chromatin fibres, and cuts in H3, H2A and H2B as well as H1, seem necessary for chromatin expansion. On the contrary, H4 might be involved in the stabilization of nucleosomes only.

Use of an immobilized enzyme and specific antibodies to analyse the accessibility and role of histone tails in chromatin structure

Using limited proteolysis with subtilisin bound to collagen membranes, the degradation of the histone proteins revealed by specific antibodies was correlated to changes in chromatin conformation and condensation monitored by circular dichroism and electric birefringence. This new approach allows us to detect for the first time a hierarchy of histone tails cleavages. The terminal domains of H1, the NH2-terminal tail of H3 and the carboxy-terminal ends of histones H2A and H2B were found to be cleaved already at the early stages of proteolysis and this led to a decondensation of polynucleosomal chains. Thereafter the C-terminal part of H3 and both NH2-terminal regions of H2A and H2B became rapidly cleaved, resulting in relative reorientation of swinging nucleosomes or partially unfolded segments. Unexpectedly, this removal of tails of H1, H2B, H2A and H3 is not accompanied by significant changes in DNA-protein interactions resulting in free-oriented DNA. This might suggest that histone-histone interactions play a central role in stabilizing the solenoid.