Immunofractionation of chromatin regions associated with histone H1o

Linker histone transitions during mammalian oogenesis and embryogenesis

A unique characteristic of the oocyte is that, although it is a differentiated cell, it can to give rise to a population of undifferentiated embryonic cells. This transition from a differentiated to a totipotential condition is thought to be mediated in part by changes in chromatin composition or configuration. In many non-mammalian organisms, oocytes contain unique subtypes of the linker histone H1, which are replaced in early embryos by the so-called somatic histone H1 subtypes. We review evidence that such histone H1 subtype switches also occur in mammals. Immunologically detectable somatic H1 is present in mitotically proliferating oogonia but gradually becomes undetectable after the oocytes enter meiosis. Immunoreactive somatic H1 remains undetectable throughout oogenesis and the early cell cycles after fertilization. Following activation of the embryonic genome, it is assembled onto chromatin. In contrast to the absence of immunoreactive protein, mRNAs encoding each of the five mammalian somatic H1 subtypes are present in growing oocytes and newly fertilized embryos, indicating that post-transcriptional mechanisms regulate expression of these genes. This maternal mRNA is degraded at the late 2-cell stage, and embryonically encoded mRNAs accumulate after embryos reach the 4-cell stage. During the period when somatic H1 is not detectable, oocytes and embryos contain mRNA encoding a sixth subtype, histone H1(0) which accumulates in differentiated somatic cells, and the nuclei can be stained with an H1(0)-specific antibody. We propose that the linker histone composition of the oocyte lineage resembles that of other mammalian cells, namely, that the somatic H1 subtypes predominate in mitotically active oogonia, that histone H1(0) becomes prominent in differentiated oocytes, and that following fertilization and transcriptional activation of the embryonic somatic H1 genes, the somatic H1 subtypes are reassembled onto chromatin of the embryonic cells. Potential functions of these linker histone subtype switches are discussed, including stabilization by H1(0) of the differentiated state of the oocytes, protection of the oocyte chromatin from factors that remodel sperm chromatin after fertilization, and restoration by the incorporation of the somatic H1 subtypes of the totipotential state of embryonic nuclei.

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.

A mouse histone H1 variant, H1b, binds preferentially to a regulatory sequence within a mouse H3.2 replication-dependent histone gene

H1 histones, found in all multicellular eukaryotes, associate with linker DNA between adjacent nucleosomes, presumably to keep the chromatin in a compact, helical state. The identification of multiple histone H1 subtypes in vertebrates suggests these proteins have specialized roles in chromatin organization and thus influence the regulation of gene expression in the multicellular organism. The mechanism by which the association of H1 with nucleosomal DNA is regulated is not completely understood, but affinity for different DNA sequences may play a role. Here we report that a specific H1 subtype in the mouse, namely H1b, selectively binds to a regulatory element within the protein-encoding sequence of a replication-dependent mouse H3.2 gene. We have previously shown that this coding region element, Omega, is the target of very specific interactions in vitro with another nuclear factor called the Omega factor. This element is required for normal gene expression in stably transfected rodent cells. The mouse H1b protein interacts poorly (100-fold lower affinity) with the comparable "Omega" sequence of a replication-independent mouse H3.3 gene. This H3.3 sequence differs at only 4 out of 22 nucleotide positions from the H3.2 sequence. Our findings raise the possibility that this H1b protein plays a specific role in regulation of expression of the replication-dependent histone gene family.

Regulation of H1(0) gene expression by nuclear receptors through an unusual response element: implications for regulation of cell proliferation

Cloning and sequence analysis of the 5'-flanking region of the human H1(0) histone gene, a differentiation-specific member of the H1 family, has revealed several potential regulatory elements. In this study, we have characterized the interactions of nuclear receptors with an unusual response element consisting of two half-sites arranged as a direct repeat with an 8-bp spacer (DR-8). Thyroid hormone receptors (TR) bind this DR-8 as homodimers and heterodimers with RXR. Retinoic acid receptors (RARs) also bind as heterodimers with RXR to the DR-8, and this binding is enhanced in the presence of retinoic acid (RA) and/or 9-cis RA. Reporter constructs containing the DR-8 allowed a several-fold induction by T3 in the presence of TRs. RAR alpha and RAR beta allowed RA-dependent transcriptional activation whereas RAR gamma mostly increased basal activity. 9-cis RA inhibited the T3 response, indicating a hormonal cross-talk among the subfamily of nuclear receptors. Two orphan receptors, COUP-TF and v-erbA, also bind the DR-8 sequence in the human H1(0) promoter. COUP-TF, which usually represses RAREs, enhances transcriptional activation through the DR-8 whereas v-erbA completely represses TR-RXR induction of the H1(0) gene. Thus, a number of signaling pathways that play important roles during development and differentiation are able to influence the transcription rate of this special H1 subtype directly through a DR-8 response element in its promoter. Because H1(0) expression levels inversely correlate with cell proliferation, our data suggest that several nuclear receptors and the v-erbA oncogene can influence cell proliferation via the regulation of H1(0) expression.

Mice develop normally without the H1(0) linker histone

H1 histones bind to the linker DNA between nucleosome core particles and facilitate the folding of chromatin into a 30-nm fiber. Mice contain at least seven nonallelic subtypes of H1, including the somatic variants H1a through H1e, the testis-specific variant H1t, and the replacement linker histone H1(0). H1(0) accumulates in terminally differentiating cells from many lineages, at about the time when the cells cease dividing. To investigate the role of H1(0) in development, we have disrupted the single-copy H1(0) gene by homologous recombination in mouse embryonic stem cells. Mice homozygous for the mutation and completely lacking H1(0) mRNA and protein grew and reproduced normally and exhibited no anatomic or histologic abnormalities. Examination of tissues in which H1(0) is normally present at high levels also failed to reveal any abnormality in cell division patterns. Chromatin from H1(0)-deficient animals showed no significant change in the relative proportions of the other H1 subtypes or in the stoichiometry between linker histones and nucleosomes, suggesting that the other H1 histones can compensate for the deficiency in H1(0) by occupying sites that normally contain H1(0). Our results indicate that despite the unique properties and expression pattern of H1(0), its function is dispensable for normal mouse development.

Specific expression in adult mice and post-implantation embryos of a transgene carrying the histone H1(0) regulatory region

Histone H1(0), a variant of the H1 group, has been found associated with the repressed state of chromatin and its content is increased in terminally differentiated cells. We have cloned a mouse H1(0) histone gene and introduced the promoter region, ligated to the beta-galactosidase reporter gene, into transgenic mice. By histochemistry we demonstrated a strong expression of the transgene in adult kidney, testis and brain. Intestine, uterus and ovarium were also positive. This expression followed the same pattern as that of the endogenous H1(0) gene, as demonstrated by in situ hybridization with a non-coding fragment of the mRNA, by Northern analysis, and by immunofluorescence with specific antibodies. In post-implantation embryos, the expression was very low up to day ten p.c. At this time, most of the X-Gal staining was found in the brain, retina and some of the large blood vessels. Hence, expression of the transgene as well as of the endogenous H1(0) gene is not exclusively linked to a differentiated phenotype or to a reduced cell proliferation capacity.

Differential expression and gonadal hormone regulation of histone H1(0) in the developing and adult rat brain

The cellular distribution of histone H1(0) has been examined immunohistochemically in the rat brain. H1(0) accumulates in neurons and glial cells during postnatal development. In neurons, immunoreactivity increases progressively from about postnatal day 10, and reaches a distribution pattern similar to that of adult rats by postnatal day 20. Immunoreactivity in glial cells shows a prominent increase from postnatal day 20 to adult age. The accumulation of H1(0) during postnatal development appears to be correlated with terminal differentiation and maturation. Although immunoreactive neurons are widely distributed in all areas of the central nervous system, many neurons do not express immunoreactivity. For instance in the cerebellum, Purkinje neurons are negative. In females, the number of immunoreactive neurons in the arcuate area of the hypothalamus increases during postnatal development. In contrast, the percentage of immunoreactive neurons in males is low at all ages studied. The expression of H1(0) in the ventromedial part of the arcuate is reversibly and negatively regulated during the estrous cycle by the level of plasma estradiol. Ovariectomy increases the number of immunoreactive neurons while the restoration of the physiological levels of estradiol results in the opposite effect. Early postnatal androgenization of females suppresses the increment in the number of immunoreactive neurons in both the dorsolateral and the ventromedial parts of the arcuate during postnatal development, thus leading to permanently decreased levels of H1(0) immunoreactivity in postpuberal females.

Basal level transcription of the histone H1(0) gene is mediated by a 80 bp promoter fragment

The replacement histone H1(0) of the H1 group, known to interact with general transcription factors, has been found associated with transcriptionally repressed chromatin. Transcription of the gene in F9 stem cells is low but can be stimulated by treating the cells with retinoic acid. Using mutant deletions, we now demonstrate that basal level transcription in F9 cells is mediated by an 80 bp DNA fragment, located 430 bp upstream of the TATA box, which does not include the retinoic acid responsive element (RARE) known to bind retinoic acid receptors and stimulate transcription from an heterologous promoter after retinoic acid treatment. By footprinting, DMS interference, site-directed mutagenesis and UV-cross linking techniques we demonstrate that at least two nuclear factors, with MW of 90,000 and 30,000, bind to the 80 bp fragment and that this binding is necessary for transcription. Furthermore, positioning of this fragment upstream of the HSV-tk gene promoter stimulates transcription 2-3 times over control values, far less than the activity observed for this fragment in the homologous promoter, indicating that full activity of this fragment requires sequences located in the proximal part of the promoter.

Multiple cis-acting elements of the proximal promoter region are required for basal level transcription of the H1(0) histone gene

Basal level transcription of the mouse histone H1(0) gene is mediated by 531 base pairs of the promoter region. Deletion of the most distal upstream 80 bp of this fragment reduces transcription to very low values. By in vitro footprinting we demonstrate now that multiple factors bind to the DNA fragment localized between the 80 bp and the cap nucleotide. In addition to the presence of motifs for the binding of SP1, H1-box, H4TF-2 and TATA-box-factors, other not yet described protein-binding elements were identified. Internal deletions in the wild type promoter enclosing these motifs strongly restrict transcription. Furthermore, when one of these motifs was modified by site-directed mutagenesis a strong impairment of transcription followed. Thus for basal level transcription, in addition to the 80 bp distal fragment, cis-acting elements localized in the 450 bp proximal promoter region are required.

Histone H1(0) mRNA and protein accumulate early during retinoic acid induced differentiation of synchronized embryonal carcinoma cells

The very lysine-rich replacement histone variant H1(0) is found to be present in different murine (C1003, PC13, P19) and human (Tera-2) embryonal carcinoma cell lines. The proportion of H1(0) increases upon induction of differentiation of the different cell lines by various treatments. In undifferentiated PC13 EC cells H1(0) mRNA is present at a low level. During retinoic acid induced differentiation of mitotically synchronized PC13 EC cells, accumulation of H1(0) mRNA starts in the first cell cycle. The H1(0) protein level starts to increase in the second synchronous cycle preceding changes in the cycle parameters that become apparent in the third cycle. The results provide further support for an important role of H1(0) in the control of cellular differentiation in early mammalian development.

Distribution of high mobility group proteins 1/2, E and 14/17 and linker histones H1 and H5 on transcribed and non-transcribed regions of chicken erythrocyte chromatin

Quantitative analysis of distribution of chromosomal proteins on single copy DNA sequences has been further developed. Our approach consists of DNA-protein crosslinking within whole cells or isolated nuclei, specific immunoaffinity isolation of crosslinked complexes via protein and identification of crosslinked DNA by hybridisation with single-stranded DNA probes. The present study shows that transcribed chromatin of chicken embryonic erythrocyte beta globin gene is characterized by about 1.5-2.5-fold higher density of HMG 14/17 and 2-fold lower density of H1 and H5 as compared with non-transcribed chromatin of ovalbumin and lysozyme genes, whereas HMG 1/2, E proteins were equally distributed between DNA of both transcribed and non-transcribed genes. The depletion of H1/H5 in beta globin sequences was verified by the 'protein image' hybridisation technique (1). The DNase I hypersensitive site located 5' upstream from beta globin gene is deficient in all the proteins assayed, what implies a drastic disruption in the nucleosomal array. Minor quantitative changes of protein pattern suggest transient local perturbation of the chromatin on transcription.

The expression of the histone H1 (0) gene in the human hepatoma cell line HepG2 is independent of the state of cell proliferation

The H1 histone subtype H1 (0) is a characteristic component of the chromatin of several mammalian tissues. Since H1 (0) is synthesized in nondividing cells upon terminal differentiation, it has been mostly considered either as a prerequisite for or as a consequence of an arrest of DNA replication during the process of differentiation. In several H1 (0)-expressing systems studied until now, inducers of differentiation or inhibitors of DNA synthesis cause an increase of the ratio between H1 (0) and the other H1 proteins. We have studied the steady-state levels of histone H1 (0) mRNA under varied growth conditions in the human hepatoma cell lines HepG2 and Hep3B, and we show in the HepG2 system that H1 (0) is not confined to resting cells, that the H1 (0) gene appears to be expressed throughout the cell cycle and that established inducers of de novo H1 (0) synthesis fail to cause a further increase of the high H1 (0) level. This constitutive expression of H1 (0) appears to reflect the chromatin structure of the liver cells, from which the HepG2 hepatoblastoma cells initially may have evolved. In contrast to the situation in nondividing adult liver cells, the H1 (0) gene is transcribed in HepG2 at a high level, and this expression is compatible with DNA replication.

Accessibility of histone H1(0) and its structural domains to antibody binding in mononucleosomes

This work is devoted to the study of the immunoreactivity of histone H1(0) and its major structural domains in mononucleosomes. Three types of antibody populations were used: (i) anti-H1(0) which reacted with antigenic determinants situated along the whole polypeptide chain; (ii) anti-GH5 which recognized epitopes located in the globular region; and (iii) anti-C-tail antibodies reacting specifically with fragment 99-193 of the protein molecule. The anti-GH5 antibodies gave a weak reaction, the C-tail-specific antibodies reacted relatively strongly and the antiserum to the intact molecule showed an intermediate level of reactivity. The relative intensities of the immunoreaction could be interpreted as reflecting the exposure of the antigenic determinants of the individual protein domains in the monosome particle.

The H1 and core histone subtypes: differential gene expression and varied primary structures

The patterns of chromosomal proteins reflect in many cases the functional state of the respective cell type. The H1 histone group is particularly important in this respect, since these histones are involved in the higher order chromatin organization above the level of chains of nucleosomes. In mammals, the H1 histone family comprises at least five main subtypes (H1a-H1e), a testicular variant (H1t) and, thirdly, a subtype H1(0), which is found only in terminally differentiated cells. The H1(0) variant is structurally related to the avian red blood cell specific histone H5, which was the basis for our recent isolation of the human H1(0) gene. Changes of H1 histone patterns may be crucial events in modulating local chromatin arrangements, since the formation of higher order chromatin structures depends on a cooperative interaction of the H1 histones. Variations in their patterns can be studied in vivo during several developmental processes (such as spermatogenesis, erythropoiesis, maturation of several cell types) or in vitro in several tumor cell lines upon treatment with several inducers or upon inhibition of cell division. The differential regulation of the individual H1 subtypes is reflected in the gene and mRNA structures coding for the respective proteins. The cell cycle regulated histones are mostly encoded by non-polyadenylated mRNAs, whereas H5 as well as H1(0) mRNA shows a poly(A) tail at its 3' end. In conclusion, gene activity may not only be controlled at the level of RNA polymerases and their regulatory transcription factors. The varied patterns of chromosomal proteins at different stages during development and differentiation suggest that the local or overall organization of chromatin plays an additional role in these regulatory programs. Hence, the analysis of variations in patterns of chromosomal proteins is an integral part of the investigation of gene regulation mechanisms.

Microheterogeneity in H1 histones and its consequences

The extent of microheterogeneity of H1 histones in individual higher organisms, without considering post-translational modifications, is such that five to eight molecular species can be recognized. The H1 variants differ among themselves in their ability to condense DNA and chromatin fragments, and they are non-uniformly distributed in chromatin. This review assembles data that support the notion that the differences in chromatin condensation (heterochromatization) observed through the microscope are maintained by the non-uniform distribution of H1 variants, and that this pattern of chromatin condensation may determine the dynamics of chromatin during replication and may represent the commitment aspect of differentiation. The differential response of the multiple H1 variants with regard to their synthesis and turnover is consistent with this notion.

Immunochemical analysis of the structure and function of chromosomal proteins

Immunochemical approaches are useful in studying the nuclear organization and cellular function of chromosomal components. Antibodies specific to histones and to defined nonhistone proteins have been used to study nucleosome heterogeneity, to visualize the presence of histone in transcriptionally active chromatin, and to isolate DNA sequences associated with specific chromosomal proteins.