Continuous loss of oocytes throughout meiotic prophase in the normal mouse ovary

The number of germ cells reaches the maximum just prior to entry into meiosis, yet decreases dramatically by a few days after birth in the female mouse, rat, and human. Previous studies have reported a major loss at the pachytene stage of meiotic prophase during fetal development, leading to the hypothesis that chromosomal pairing abnormalities may be a signal for oocyte death. However, the identification as well as the quantification of germ cells in these studies have been questioned. A recent study using Mouse Vasa Homologue (MVH) as a germ cell marker reached a contradictory conclusion claiming that oocyte loss occurs in the mouse only after birth. In the present study, we established a new method to quantify murine germ cells by using Germ Cell Nuclear Antigen-1 (GCNA-1) as a germ cell marker. Comparison of GCNA-1 and MVH immunolabeling revealed that the two markers identify the same population of germ cells. However, nuclear labeling of GCNA-1 was better suited for counting germ cells in histological sections as well as for double labeling with the antibody against synaptonemal complex (SC) proteins in chromosome spreading preparations. The latter experiment demonstrated that the majority of GCNA-1-labeled cells entered and progressed through meiotic prophase during fetal development. The number of GCNA-1-positive cells in the ovary was estimated by counting the labeled cells retained in chromosome spreading preparations and also in histological sections by using the ratio estimation method. Both methods demonstrated a continuous decline in the number of GCNA-1-labeled cells during fetal development when the oocytes progress through meiotic prophase. These observations suggest that multiple causes are responsible for oocyte elimination.     

Immunofluorescent labeling of centromeres for flow cytometric analysis.

A procedure to stain the centromeric region of chromosomes for dual beam flow cytometric analysis is described. Serum from a CREST (Scleroderma syndrome) patient presenting a high titer of anticentromeric antibodies was chosen on the basis of specificity of labeling of cells on slides. The high affinity of the antibodies to centromeres and low binding to chromosomal arms allowed the development of an indirect immunofluorescent labeling procedure using isolated and unfixed chromosomes stabilized by Mg++ ions. Discontinuous Ficoll gradients were used to separate chromosomes from unbound antibodies. With this procedure, chromosome clumping and degradation were minimal. The chromosomes were then stained with the DNA dyes Hoechst 33258 and chromomycin A3, before dual beam flow cytometric analysis. Flow karyotypes, with good chromosome peak resolution, were obtained for both human and hamster chromosomes subjected to the immunolabeling procedure. For quantification of FITC fluorescence due to bound antibody, chromosomes were counterstained with Hoechst only. The FITC intensity of antibody-labeled human and hamster chromosomes were 4-10 and 20 times greater than control chromosomes, respectively. These results suggest that the staining procedure may be suitable for immunolabeling of chromosomes with antibodies recognizing other nuclear proteins and their subsequent quantification by flow cytometry.     

Different types of plant chromatin associated with modified histones H3 and H4 and methylated DNA

Eukaryotic chromosomes are organized into two large and distinct domains, euchromatin and heterochromatin, which are cytologically characterized by different degrees of chromatin compaction during interphase/prophase and by post-synthesis modifications of histones and DNA methylation. Typically, heterochromatin remains condensed during the entire cell cycle whereas euchromatin is decondensed at interphase. However, a fraction of the euchromatin can also remain condensed during interphase and appears as early condensing prophase chromatin. 5S and 45S rDNA sites and telomere DNA were used to characterize these regions in metaphase and interphase nuclei. We investigated the chromosomal distribution of modified histones and methylated DNA in the early and late condensing prophase chromatin of two species with clear differentiation between these domains. Both species, Costus spiralis and Eleutherine bulbosa, additionally have a small amount of classical heterochromatin detected by CMA/DAPI staining. The distribution of H4 acetylated at lysine 5 (H4K5ac), H3 phosphorylated at serine 10 (H3S10ph), H3 dimethylated at lysine 4 or 9 (H3K4me2, H3K9me2), and 5-methylcytosine was compared in metaphase, prophase, and interphase cells by immunostaining with specific antibodies. In both species, the late condensing prophase chromatin was highly enriched in H4K5ac and H3K4me2 whereas the early condensing chromatin was very poor in these marks. H3K9me2 was apparently uniformly distributed along the chromosomes whereas the early condensing chromatin was slightly enriched in 5-methylcytosine. Signals of H3S10ph were restricted to the pericentromeric region of all chromosomes. Notably, none of these marks distinguished classical heterochromatin from the early condensing euchromatin. It is suggested that the early condensing chromatin is an intermediate type between classical heterochromatin and euchromatin.     

Antibodies to defined histone epitopes reveal variations in chromatin conformation and underacetylation of centric heterochromatin in human metaphase chromosomes

Unfixed metaphase chromosome preparations from human lymphocyte cultures were immunofluorescently labelled using antibodies to defined histone epitopes. Both mouse monoclonal antibody HBC-7, raised against the N-terminal region of H2B, and rabbit serum R5/12, which recognizes H4 acetylated at Lys-12, gave non-uniform labelling patterns, whereas control antibodies against total histone fractions H4 and H1 produced homogeneous fluorescence. HBC-7 bound approximately uniformly to the bulk of the chromosomes, but the major heterochromatic domains of chromosomes 1, 9, 15, 16 and the Y showed significantly brighter fluorescence. Serum R5/12 indicated an overall reduction in acetylation of H4 in metaphase chromosomes compared with interphase nuclei, although some specific chromosomal locations had considerably elevated acetylation levels. Acetylation levels in the major heterochromatic domains appeared extremely low. To investigate further the differences noted in heterochromatin labelling, metaphases from cultures grown in the presence of various agents known to induce undercondensation of the major heterochromatic domains were similarly immunolabelled. Decondensed heterochromatin no longer exhibited higher than normal immunofluorescence levels with HBC-7. The higher resolution afforded by stretching the centromeric heterochromatin of chromosomes 1, 9 and 16 confirmed the low level of H4 acetylation in these domains. We consider the implications of these observations in relation to chromatin conformation and activity.by W.C. Earnshaw     

Immunofluorescence localization of an adducin-like protein in the chromosomes of mouse oocytes

The mouse oocyte expresses a polypeptide of Mr 120,000 that cross-reacts with an antibody to the brain membrane skeletal protein adducin. Immunofluorescence localization showed a bright chromosomal staining reaction in metaphase I and metaphase II oocytes. Following in vitro fertilization the maternal chromosomes lost their immunoreactivity during pronuclear development. The fertilizing sperm chromatin and male pronucleus did not show any detectable staining reaction. Bright chromosomal fluorescence was again observed in the first mitotic metaphase when both maternal and paternal chromosomes gave a positive staining reaction. In contrast to the immunoreactivity of the maternal meiotic chromosomes, the meiotic chromosomes of male germ line cells failed to exhibit any detectable staining reaction and this difference was confirmed by immunolabeling of oocyte and spermatocyte karyotypes. Mitotic chromosomes in preimplantation embryos, fetal liver, adult intestinal epithelium, and MDCK cells also failed to show any detectable labeling reaction. The results suggest that expression of the immunoreactive chromosomal adducin may be a unique feature of oogenesis.     

Differential underacetylation of histones H2A,H3 and H4 on the inactive X chromosome in human female cells

It has previously been shown that the acetylated forms of histone H4 are depleted or absent in both constitutive, centric heterochromatin and in the facultative heterochromatin of the inactive X chromosome (Xi) in female cells. By immunostaining of metaphase chromosomes from human lymphocytes with antibodies to the acetylated isoforms of histones H2A and H3, we now show that these histones too are underacetylated in both Xi and centric heterochromatin. Xi shows two prominent regions of residual H3 acetylation, one encompassing the pseudoautosomal region at the end of the short arm and one at about Xg22. Both these regions have been shown previously to be sites of residual H4 acetylation. H2A acetylation on Xi is higher overall than that of H3 or H4 and is particularly high around the pseudoautosomal region, but not at Xg22. The results suggest that the acetylated isoforms of H3 and H4 have at least some effects on chromosomal structure and function that are not shared by acetylated H2A.     

Histones H1 and H4 of surface-spread meiotic chromosomes

The chromatin conformation of somatic and meiotic chromosomes is, at least in part, a function of electrostatic nucleosome interactions that are mediated by transient acetylation of the histone H4 N-terminal domain and phosphorylation of histone H1. The distribution of those histones in the chromatin of meiotic chromosomes is reported here. Antibodies to testis-specific histone 1, H1t, detect H1t in the chromatin of mouse meiotic prophase chromosomes only after synapsis and synaptonemal complex (SC) assembly is completed and before core separation is initiated. The H1t protein is evenly distributed over euchromatin, heterochromatin and the SC. Antibodies to acetylated lysine residues 5, 12 or 16 of histone H4, indicate that the euchromatin is more acetylated than the centromeric heterochromatin. The pattern is most pronounced for acetylated residue 5 and least for 16. Antibodies to phosphorylated H1 epitopes do not react with chromatin but, instead, recognize the chromosome cores and SCs. Possibly these are not phosphorylated histone H1 epitopes, but SC proteins with similar potentially phosphorylatable sequences such as KTPTK of the synaptic protein Syn1.     

Histone H4 acetylation in human cells. Frequency of acetylation at different sites defined by immunolabeling with site-specific antibodies

Histone H4 can be reversibly acetylated at lysine residues 5, 8, 12 and 16. It is possible that acetylation of individual residues will exert specific effects on chromatin function, but this hypothesis is difficult to test with present techniques for analysis of acetylation. To address this problem, we have prepared antibodies which distinguish H4 molecules acetylated at each of the sites used in vivo. By electrophoresis and immunolabeling we have shown that, in H4 from human cells, the four lysine residues are acetylated in a preferred, but not exclusive order, namely lysine 16, followed by 12 and 8, followed by 5.     

Histone H4 Acetylation and Replication Timing in Chinese Hamster Chromosomes

The distribution of acetylated isoforms of histone H4 along Chinese hamster chromosomes has been studied by immunostaining with antibodies recognizing H4 acetylated at defined lysines in its N-terminal domain. The heterochromatic long arm of the X chromosome in both female (CHO) and male (DON) cell lines is underacetylated at three out of four lysines (5, 8, and 12). In contrast, the level of acetylation at lysine 16, which is the first to be acetylated in mammals, was similar in X chromosomes and autosomes. Labeling of the cells with bromodeoxyuridine (BrdU) to mark late-replicating chromosome domains, followed by double immunostaining with antibodies to BrdU and acetylated H4, showed a close, though not perfect, correlation between late replication and low levels of H4 acetylation. The results show that levels of histone acetylation are associated with the replication timing of defined domains on both the X chromosome and autosomes, but the exceptions we observe suggest that this link is not absolute or essential.     

Germ cell-specific DNA and RNA binding proteins p48/52 are expressed at specific stages of male germ cell development and are present in the chromatoid body

Proteins homologous to the Xenopus oocyte mRNA binding proteins mRNP₃₊₄ and designated p48/52 have been identified in male mouse germ cells (1993: Dev Biol 158:90–100). Western and Northwestern blots of extracts from testes and isolated germ cells indicate that p48/52 are present during meiosis but reach their highest levels postmeiotically at a time when many mRNAs are stored. Here we analyze the cellular and subcellular distribution of p48/52 in rat and mouse testes by LM and EM immunocytochemistry using an anti-mRNP₃₊₄ antibody. Immunolabeling was found to be predominantly cytoplasmic and specific to germ cells at certain periods during their development. p48/52 were first detected in early pachytene spermatocytes at stage V of the seminiferous cycle and progressively increased during the remainder of meiotic prophase to a post-meiotic peak in steps 1–8 round spermatids; thereafter, labeling gradually declined as elongated spermatids underwent nuclear condensation and elongation. A proportionally higher concentration of cytoplasmic immunolabeling was found within the lacunae of the anastomotic granulofilamentous network of the chromatoid body. The pattern of synthesis of these mRNA binding proteins together with their association with the chromatoid body suggests a role as germ cell-specific mRNA stabilizing and/or storage proteins. © 1996 Wiley-Liss, Inc.     

DNA methylation, histone H3 methylation, and histone H4 acetylation in the genome of a crustacean.

In this work, we used antibodies against histone H3 trimethylated at lysine 9 (H3K9m3); against histone H4 acetylated at lysines 5, 8, 12, and 16 (H4ac); and against DNA methylated at 5C cytosine (m5C) to study the presence and distribution of these markers in the genome of the isopod crustacean Asellus aquaticus. The use of these 3 antibodies to immunolabel spermatogonial metaphases yields reproducible patterns on the chromosomes of this crustacean. The X and Y chromosomes present an identical banding pattern with each of the antibodies. The heterochromatic telomeric regions and the centromeric regions are rich in H3K9m3, but depleted in m5C and H4ac. Thus, m5C does not seem to be required to stabilize the silence of these regions in this organism.     

Reduced levels of histone H3 acetylation on the inactive X chromosome in human females

Novel antibodies were generated that are highly selective for either acetylated or unacetylated iso-forms of histone H3, or the acetylated form of histone H4 in organisms as diverse asTetrahymena and humans. Using these antibodies as pair-wise sets in immunocytological analyses, we demonstrate that the inactive X chromosome is hypoacetylated for both histone H3 and H4 in female mammalian cells, whereas the antibody that recognizes the unacetylated form of histone H3 identifies all chromosomes uniformly. These data verify and extend previous results and suggest that hypoacetylation of core histones may be a general feature of the chromatin along the inactive X chromosome. Edited by: D. Bazett-Jones     

Comprehensive analysis of dynamics of histone H4 acetylation in mitotic barley cells

Nucleosomal histones are covalently modified at specific amino acid residues. In the case of histone H4, four lysines (K5, K8, K12, and K16) are acetylated. In the current studies, we examined the dynamics of histone H4 acetylation at K8 and K12 in mitotic barley cells using a three-dimensional immunofluorescent method. Based on the results and previous studies on the dynamics of K5 and K16 acetylation, we provide a comprehensive view of the dynamics of H4 acetylation. Interphase nuclei exhibit strong acetylation in the centromeric region at K5, K8 and K12. In the case of K12, strong acetylation at nucleolar organizing regions was observed from prophase to anaphase. The dynamics of K12 were closely related to those of K5. On the other hand, K8 exhibited a pattern of almost uniform acetylation from prophase to telophase and strong acetylation in distal regions of chromosomes at both metaphase and anaphase, which is very similar to the dynamics of K16 acetylation. Thus, it appears that there is pair-wise acetylation of K12 and K5 in the nucleolar organizing regions and of K8 and K16 in the gene-rich regions. Together, these results suggest that pair-wise dynamics of H4 acetylation regulate chromosomal structure and function during the cell cycle.     

Stage-specific damage to synaptonemal complexes and metaphase chromosomes induced by X rays in male mouse germ cells

Synaptonemal complexes reveal mutagen-induced effects in germ cell meiotic chromosomes. This study was aimed at characterizing relationships between damage to synaptonemal complexes and metaphase I chromosomes following radiation exposure at various stages of spermatogenesis. Male mice were irradiated with doses of 0, 2, or 4 Gy, and spermatocytes were harvested at times consistent with earlier exposures as spermatogonial stem cells, preleptotene cells (premeiotic DNA synthesis), or meiotic prophase cells. After stem-cell exposure, twice as many rearrangements were observed in synaptonemal complexes as in metaphase I chromosomes. Irradiation during premeiotic DNA synthesis resulted in dose-related increases in synaptonemal complex breakage and rearrangements (including novel forms) and in metaphase chromosomal aberrations. Following prophase exposure, various types and levels of damage to synaptonemal complexes and metaphase chromosomes were observed. Irradiation of zygotene cells led to high frequencies of chromosome multivalents in metaphase I without a correspondingly high level of damage in preceding prophase synaptonemal complexes. Thus irradiation of premeiotic and meiotic cells results in variable relationships between damage to synaptonemal complexes and metaphase chromosomes. Interpretations of these relationships are based upon what is known about both radiation clastogenesis and the structural/temporal relationships between synaptonemal complexes at prophase and chromosomes at metaphase I of meiosis.     

Detection of monoclonal antibody to high-mobility-group protein 17 from chick oviduct.

Total chromosomal HMG (high-mobility-group) proteins have been isolated from oestrogen-stimulated chick oviduct. The antibodies against these proteins were induced in mice and subsequently their spleen cells were fused with myeloma cells to form hybridomas. A highly purified HMG protein, 17, was used to select for the hybridomas that produce antibody against HMG protein 17. The hybridomas were cultured and injected into mice to produce ascites. The antibody against HMG protein 17 in the IgG (immunoglobulin G) fraction of the ascites fluid was obtained by Protein A-Sepharose column chromatography. We have devised a solid-phase radioimmunoassay and enzyme-linked serological assay for the detection and characterization of this antibody directed against HMG protein 17. This anti-(HMG protein 17) IgG interacted only with HMG protein 17, but not with other chromosomal proteins, e.g. histone H1, "95K protein' (a chick oviduct-specific chromosomal protein) and HMG proteins 1, 2 and 14. The monospecific nature of this anti-(HMG protein 17) IgG fraction is confirmed.     

Distribution of c-myc, c-myb, and Ki-67 antigens in interphase and mitotic human cells evidenced by immunofluorescence staining technique

Using specific antibodies and the immunofluorescence staining technique we found a similar subcellular distribution pattern of the cellular proto-oncogene proteins c-myc and c-myb in interphase and mitotic HL60 and Molt4 cells. Antibodies against c-myc as well as those against c-myb protein gave rise to a nuclear staining excluding the nucleoli. In mitotic cells both proteins are apparently not associated with the chromatin of the condensed chromosomes, but appear diffusely distributed throughout the cytoplasm. In contrast, immunostaining using the proliferation marker antibody Ki-67 yielded in both cell lines several prominent specks in the nucleus and a weak finely dispersed staining throughout the nucleoplasm. No fluorescence was detectable in the cytoplasm. In dividing cells Ki-67 immunofluorescence was found to be associated with the surface of the chromosomes. The functional significance of the different localizations of the proteins is discussed in light of what is currently known about nuclear antigens.