Normal spermatogenesis in mice lacking the testis-specific linker histone H1t

Histone H1 and its isoforms: contribution to chromatin structure and function

The lysine-rich H1 histone family in mammals includes eleven different subtypes, and thus it is the most divergent class of histone proteins. The central globular H1 domain asymmetrically interacts with DNA at the exit or entry end of the nucleosomal core DNA, and the C-terminal domain has a major impact on the linker DNA conformation and chromatin condensation. H1 histones are thus involved in the formation of higher order chromatin structures, and they modulate the accessibility of regulatory proteins, chromatin remodeling factors and histone modification enzymes to their target sites. The major posttranslational modification of H1 histones is phosphorylation, which reaches a peak during G2 and mitosis. Phosphorylation is, however, also involved in the control of DNA replication and it contributes to the regulation of gene expression. Disruption of linker histone genes, initially performed in order to delineate subtype-specific functions, revealed that disruption of one or two H1 subtype genes is quantitatively compensated by an increased expression of other subtypes. This suggests a functional redundancy among H1 subtypes. However, the inactivation of three subtypes and the reduction of the H1 moiety in half finally resulted in a phenotypic effect. On the other hand, studies on the role of particular subtypes at specific developmental stages in lower eukaryotes, but also in vertebrates suggest that specific subtypes of H1 participate in particular systems of gene regulation.

The histone H1 family: specific members, specific functions?

The linker histone H1 binds to the DNA entering and exiting the nucleosomal core particle and has an important role in establishing and maintaining higher order chromatin structures. H1 forms a complex family of related proteins with distinct species, tissue and developmental specificity. In higher eukaryotes all H1 variants have the same general structure, consisting of a central conserved globular domain and less conserved N-terminal and C-terminal tails. These tails are moderately conserved among species, but differ among variants, suggesting a specific function for each H1 variant. Due to compensatory mechanisms and to the lack of proper tools, it has been very difficult to study the biological role of individual variants in chromatin-mediated processes. Our knowledge about H1 variants is indeed limited, and in vitro and in vivo observations have often been contradictory. Therefore, H1 variants were considered to be functionally redundant. However, recent knockout studies and biochemical analyses in different organisms have revealed exciting new insights into the specificity and mechanisms of actions of the H1 family members. Here, we collect and compare the available literature about H1 variants and discuss possible specific roles that challenge the concept of H1 being a mere structural component of chromatin and a general repressor of transcription.

C-terminal phosphorylation of murine testis-specific histone H1t in elongating spermatids

Previous studies gave differing results as to whether the testis-specific histone H1t was phosphorylated during rodent spermatogenesis. We show here that histones extracted from germ cell populations enriched with spermatids at different stages of development in rat testes reveal an electrophoretic shift in the position of H1t to slower mobilities in elongating spermatids as compared to that from preceding stages. Alkaline phosphatase treatment and radioactive labeling with (32)P demonstrated that the electrophoretic shift is due to phosphorylation. Mass spectrometric analysis of histone H1t purified from sexually mature mice and rat testes confirmed the occurrence of singly, doubly, and triply phosphorylated species, with phosphorylation sites predominantly found at the C-terminal end of the molecule. Furthermore, using collision-activated dissociation (CAD) and electron transfer dissociation (ETD), we have been able to identify the major phosphorylation sites. These include a new, previously unidentified putative H1t-specific cdc2 phosphorylation site in linker histones. The presence of phosphorylation at the C-terminal end of H1t and the timing of its appearance suggest that this post-translational modification is involved in the reduction of H1t binding strength to DNA. It is proposed that this could participate in the opening of the chromatin fiber in preparation for histone displacement by transition proteins in the next phase of spermiogenesis.

Epigenetic regulation in male germ cells

In recent years, it has become increasingly clear that epigenetic regulation of gene expression is critical during spermatogenesis. In this review, the epigenetic regulation and the consequences of its aberrant regulation during mitosis, meiosis and spermiogenesis are described. The current knowledge on epigenetic modifications that occur during male meiosis is discussed, with special attention on events that define meiotic sex chromosome inactivation. Finally, the recent studies focused on transgenerational and paternal effects in mice and humans are discussed. In many cases, these epigenetic effects resulted in impaired fertility and potentially long-ranging affects underlining the importance of research in this area.

Epigenetic reprogramming of the male genome during gametogenesis and in the zygote

During post-meiotic maturation, male germ cells undergo a formidable reorganization and condensation of their genome. During this phase most histones are globally acetylated and then replaced by sperm-specific basic proteins, named protamines, which compact the genome into a very specific structure within the sperm nucleus. Several studies suggest that this sperm-specific genome packaging structure conveys an important epigenetic message to the embryo. This paper reviews what is known about this fundamental, yet poorly understood, process, which involves not only global changes of the structure of the haploid genome, but also localized specific modifications of particular genomic regions, including pericentric heterochromatin and sex chromosomes. After fertilization, the male genome undergoes a drastic decondensation, and rapidly incorporates new histones. However, it remains different from the maternal genome, bearing specific epigenetic marks, especially in the pericentric heterochromatin region. The functional implications of male post-meiotic and post-fertilization genome reprogramming are not well known, but there is experimental evidence to show that it affects early embryonic development.

Global gene expression analysis reveals specific and redundant roles for H1 variants, H1c and H1(0), in gene expression regulation

In mammals, the functional significance of the presence of evolutionarily conserved, multiple non-allelic H1 variants remains unclear. We used a unique overproduction approach coupled with cell cycle synchronization and early time point assays to assess differential effects of H1 variants, H1c and H1(0), on global gene expression in the absence of compensatory events that may mask variant-specific effects. We found that H1c and H1(0) act primarily as specific rather than global regulators of gene expression. Many of the genes affected were uniquely targeted by either H1c or H1(0), affirming that H1 variants have some unique roles. We also identified genes that were affected by both variants, in which cases the expression of these genes was, for the most part, affected similarly by both the variants. This observation suggests that as well as having specific functions, the H1 variants share common roles in the organization of chromatin. We further noted that H1(0) repressed more genes than did H1c, which may underlie the prevailing notion that H1(0) is a stronger repressor of transcription.

The first bromodomain of Brdt, a testis-specific member of the BET sub-family of double-bromodomain-containing proteins, is essential for male germ cell differentiation

Brdt is a testis-specific member of the distinctive BET sub-family of bromodomain motif-containing proteins, a motif that binds acetylated lysines and is implicated in chromatin remodeling. Its expression is restricted to the germ line, specifically to pachytene and diplotene spermatocytes and early spermatids. Targeted mutagenesis was used to generate mice carrying a mutant allele of Brdt, Brdt(Delta)(BD1), which lacks only the first of the two bromodomains that uniquely characterize BET proteins. Homozygous Brdt(Delta)(BD1/)(Delta)(BD1) mice were viable but males were sterile, producing fewer and morphologically abnormal sperm. Aberrant morphogenesis was first detected in step 9 elongating spermatids, and those elongated spermatids that were formed lacked the distinctive foci of heterochromatin at the peri-nuclear envelope. Quantitative reverse transcription (RT)-PCR showed threefold increased levels of histone H1t (Hist1h1t) in Brdt(Delta)(BD1/)(Delta)(BD1) testes and chromatin immunoprecipitation revealed that Brdt protein, but not Brdt(DeltaBD1) protein, was associated with the promoter of H1t. Intracytoplasmic sperm injection suggested that the DNA in the Brdt(Delta)(BD1) mutant sperm could support early embryonic development and yield functional embryonic stem cells. This is the first demonstration that deletion of just one of the two bromodomains in members of the BET sub-family of bromodomain-containing proteins has profound effects on in vivo differentiation.

Transcription factor RFX2 is abundant in rat testis and enriched in nuclei of primary spermatocytes where it appears to be required for transcription of the testis-specific histone H1t gene

Previous work in our laboratory revealed upregulated transcription of the testis-specific linker histone H1t gene in pachytene primary spermatocytes during spermatogenesis. Using the H1t X-box as an affinity chromatography probe, we identified Regulatory Factor X2 (RFX2), a member of the RFX family of transcription factors, as a nuclear protein that binds the probe. We also showed that RFX2 activated the H1t promoter in transient expression assays. However, other RFX family members have the same DNA-binding domain and they also may regulate H1t gene expression. Therefore, in this study we examined the distribution of RFX2 and other RFX family members in rat testis germinal cells and in several tissues. Among tissues examined, RFX2 is most abundant in testis. Testis RFX2 is most abundant in spermatocytes where transcription of the H1t gene is upregulated and the steady-state H1t mRNA level is high. RFX2 levels decrease but RFX1 levels increase in early spermatids where H1t gene transcription is downregulated. Antibodies against RFX2 generate a shifted band in electrophoretic mobility shift assays (EMSA) using H1t or testisin X-box DNA probes with nuclear proteins from spermatocytes. These data support the hypothesis that RFX2 expression is upregulated in spermatocytes where it participates in activating transcription of the H1t gene and other testis genes. These data also support the possibility that other RFX family members may bind to the H1t promoter in other testis germinal cell types and in nongerminal cells to downregulate H1t gene transcription.

Phenotypic effect of substitution of allelic variants for a histone H1 subtype specific for growing tissues in the garden pea (Pisum sativum L.)

In pea, subtype H1-7 of histone H1 is specific for young actively growing tissues and disappears from chromatin of mature tissues. We sequenced the alleles coding for three main variants, numbered according to the increase of the electrophoretic mobility. Allele 1 differs from the most common allele 2 by eight nucleotide substitutions, two of them associated with amino acid replacements, His->Tyr in the globular domain and Ala->Val in the C-terminal domain. Allele 3 differs from alleles 1 and 2 by a 24-bp deletion in the part coding for the C-terminal domain. In three greenhouse experiments, we compared quantitative traits in nearly isogenic lines differing by these H1-7 variants. In experiment 1, three lines bearing either of the three allelic variants were compared, the other experiments involved pairs of lines bearing variants 1 and 3. In all experiments, statistically significant differences between the lines were registered, mostly related to the plant size. The most prominent effect was associated with plant growth dynamics. Plants of line 3, carrying the 8-amino acid deletion in histone H1-7, on average grew slower. In two experiments, the differences of the mean stem length persisted throughout plant growth while in experiment 2 differences disappeared upon maturity. The H1-7 subtype is supposed to be related to maintenance of chromatin state characteristic for cell growth and division.

Transfer of spermatogenesis-related cDNAs into eel testis germ-somatic cell coculture pellets by electroporation: Methods for analysis of gene function

Genes encoding spermatogenesis-related substance (eSRSs) show unique expression patterns during spermatogenesis. To analyze their function, we developed a new assay system using gene transfer techniques combined with coculture of the eel germ-somatic cells. First, we investigated the efficacy of in vitro electroporation transfer of gene into germ-somatic cell pellets using green fluorescent protein (GFP) gene. Second, in order to define the function of the eSRSs, we electrophoretically transferred eel spermatogonial stem cell renewal factor (eSRS34) and eel spermatogenesis-preventing substance (eSRS21) genes into germ-somatic cell pellets. Presence of the transferred cDNA was examined by reverse transcription-polymerase chain reaction (RT-PCR). Furthermore, proliferating cells were detected histologically, after labeling with BrdU. Transfer of the eSRS34 gene induced spermatogonial stem cell renewal in the pellets. Moreover, 11-ketotestosterone (11-KT) treatment stimulated the proliferation of spermatogonia, which resulted in the appearance of late type B spermatogonia in the pellets. The proliferation of spermatogonia by 11-KT stimulation was suppressed by transfer of the eSRS21 gene. These results indicate that the transferred eSRS34 and 21genes were functional in the pellets. Thus, an efficient in vitro gene transfer technique for coculture system of germ and somatic cell of Japanese eel was established.

Changes in intranuclear chromatin architecture induce bipolar nuclear localization of histone variant H1T2 in male haploid spermatids

Spermiogenesis entails a major biochemical and morphological restructuring of the germ cell packing the DNA into the condensed spermatid nucleus. H1T2 is a histone H1 variant selectively and transiently expressed in male haploid germ cells during spermiogenesis that specifically localizes to a chromatin domain at the apical pole under the acrosome. We explored the mechanisms determining polar localization of H1T2 in spermatids. In acrosome-deficient round spermatids of hrb -/- and gopc -/- mice, H1T2 localization is not altered, indicating that proper acrosome development is not required for specifying nuclear polarity. In contrast, in late round spermatids from trf2 -/- or hmgb2 -/- mice, a bipolar H1T2 localization was observed revealing that polarity is modified by loss of proteins specifying chromatin architecture. Our results show that intranuclear chromatin organization is critical for correct polar localization of H1T2 and that H1T2 can be a useful molecular marker revealing chromatin disorganization in spermatids.

Analysis of the histone H3 gene family in Arabidopsis and identification of the male-gamete-specific variant AtMGH3

Histones are major components of chromatin, the protein-DNA complex involved in DNA packaging and transcriptional regulation. Histone genes have been extensively investigated at the genome level in animal systems and have been classified as replication dependent, replication independent or tissue specific. However, no such study is available in a plant system. In this paper we report that there are 15 histone H3 genes in the Arabidopsis genome, including five H3.1 genes, three H3.3 genes and five H3.3-like genes. A gene structure analysis revealed that gene duplication causes redundancy of the histone H3 genes. The expression of one of the H3 genes, termed AtMGH3/At1g19890, is cell-specific, being restricted to the generative and sperm cells of Arabidopsis pollen as shown by in situ hybridisation and reporter gene analysis. Thus, we conclude that in Arabidopsis, AtMGH3 is a male-gamete-specific histone H3 gene. A T-DNA insertion line for AtMGH3 revealed decreased expression and ectopic RNA splicing. The T-DNA insertion lines for AtMGH3/At1g19890 and other H3 genes revealed a normal growth phenotype and reproductive fertility. These findings suggest that other H3 genes are likely to compensate for the T-DNA-insertion-induced loss of a single H3 gene because of the high redundancy of these genes in the Arabidopsis genome. These T-DNA mutant lines should be useful for accumulating different H3 gene mutations in a single plant and for studying replication-dependent and replication-independent H3 genes and the specific role of AtMGH3 in chromatin remodelling and transcriptional regulation during development of male gametes.

Many players, one goal: how chromatin states are inherited during cell division

Replication of genomic material is a process that requires not only high fidelity in the duplication of DNA sequences but also inheritance of the chromatin states. In the last few years enormous effort has been put into elucidating the mechanisms involved in the correct propagation of chromatin states. From all these studies it emerges that an epigenetic network is at the base of this process. A coordinated interplay between histone modifications and histone variants, DNA methylation, RNA components, ATP-dependent chromatin remodeling, and histone-specific assembly factors regulates establishment of the replication timing program, initiation of replication, and propagation of chromatin domains. The aim of this review is to examine, in light of recent findings, how so many players can be coordinated with each other to achieve the same goal, a correct inheritance of the chromatin state.

Triple knockouts reveal gene interactions affecting fertility of male mice

Triple knockout mice were used to investigate the interactions of five genes that were expressed in meiotic and haploid spermatogenic cells in mice, transition protein 2 (Tnp2), proacrosin (Acr), histone H1.1 (H1.1), histone H1t (H1t), and sperm mitochondria-associated cysteine-rich protein (Smcp). TNP2 functions in the replacement of histones and the initial condensation of the spermatid nucleus. The linker histone subtypes H1.1 and H1t are expressed at high levels in meiotic and early haploid cells. ACR, a protease that is stored as a proenzyme in the acrosome, is activated during the acrosome reaction and functions in binding of sperm to the zona pellucida. SMCP is a structural protein in the outer membranes of sperm mitochondria that functions in motility. Previous work demonstrates that homozygous knockout mice lacking each of these proteins individually exhibit no defect in fertility on mixed genetic backgrounds. In contrast, the present study demonstrates that five triple knockout lines, Acr/H1.1/Smcp, Acr/Tnp2/Smcp, Tnp2/H1.1/Smcp, Acr/H1t/Smcp, Tnp2/H1t/Smcp, exhibit drastic reductions in fertility on mixed genetic backgrounds. Analysis of fertility parameters reveal that the decreased fertility is due to line-dependent defects in sperm motility in vitro correlated with reduced migration in the female reproductive tract, and decreased fertilization due to defects in adhesion of sperm to the zona pellucida, the membrane surrounding the egg. It was also found that triple knockout males, that are hemizygous for one locus and homozygous for two other loci, are as subfertile as homozygous triple knockout males, a phenomenon known as haploinsufficiency. These findings demonstrate that male fertility involves synergistic interactions of genes that function in sperm motility and sperm-egg adhesion during fertilization.

Polar nuclear localization of H1T2, a histone H1 variant, required for spermatid elongation and DNA condensation during spermiogenesis

Spermiogenesis entails a major biochemical and morphological restructuring of the germ cell involving replacement of the somatic histones by protamines packing the DNA into the condensed spermatid nucleus and elimination of the cytoplasm during the elongation phase. We describe H1T2, an histone H1 variant selectively and transiently expressed in male haploid germ cells during spermiogenesis. In round and elongating spermatids, H1T2 specifically localizes to a chromatin domain at the apical pole, revealing a polarity in the spermatid nucleus. Inactivation by homologous recombination shows that H1T2 is critical for spermiogenesis as male H1t2(-/-) mice have greatly reduced fertility. Analysis of spermiogenesis in H1t2 mutant mice shows delayed nuclear condensation and aberrant elongation. As a result, mutant spermatids are characterized by the presence of residual cytoplasm, acrosome detachment, and fragmented DNA. Hence, H1T2 is a protein required for proper cell restructuring and DNA condensation during the elongation phase of spermiogenesis.

Establishment of male-specific epigenetic information

The setting of male-specific epigenetic information is a complex process, which involves a major global re-organisation, as well as localized changes of the nucleus structure during the pre-meiotic, meiotic and post-meiotic stages of the male germ cell differentiation. Although it has long been known that DNA methylation in targeted regions of the genome is associated with male-specific genomic imprinting, or that most core histones are hyperacetylated and then replaced by sperm-specific proteins during the post-meiotic condensation of the nucleus, many questions remain unanswered. How these changes interact, how they affect the epigenetic information and how the paternal epigenetic marks contribute to the future genome are indeed major issues remaining to be explored.

Histone variants: deviants?

Histones are a major component of chromatin, the protein-DNA complex fundamental to genome packaging, function, and regulation. A fraction of histones are nonallelic variants that have specific expression, localization, and species-distribution patterns. Here we discuss recent progress in understanding how histone variants lead to changes in chromatin structure and dynamics to carry out specific functions. In addition, we review histone variant assembly into chromatin, the structure of the variant chromatin, and post-translational modifications that occur on the variants.

How to pack the genome for a safe trip

The transformation of the somatic chromatin into a unique and highly compact structure occurring during the post-meiotic phase of spermatogenesis is one of the most dramatic known processes of chromatin remodeling. Paradoxically, no information is available on the mechanisms controlling this specific reorganization of the haploid cell genome. The only existing hints suggest a role for histone variants, as well as for stage-specific post-translational histone modifications,before and during the incorporation of testis-specific basic nuclear proteins. Moreover, the exact functions of the latter remain obscure. This chapter summarizes the major chromatin-associated events taking place during the post-meiotic differentiation of male haploid cells in mammals and discusses some of the basic issues that remain to be solved to finally understand chromatin remodeling during spermatogenesis.

Testis-specific transcriptional control

In the testis, tissue-specific transcription is essential for proper expression of the genes that are required for the reproduction of the organism. Many testis-specific genes are required for mitotic proliferation of spermatogonia, spermatocytes undergoing genetic recombination and meiotic divisions, and differentiation of haploid spermatids. In this article we describe some of the genes that are transcribed in male germinal cells and in non-germinal testis cells. Because significant progress has been made in examination of promoter elements and their cognate transcription factors that are involved in controlling transcription of the testis-specific linker histone H1t gene in primary spermatocytes, this work will be reviewed in greater detail. The gene is transcriptionally active in spermatocytes and repressed in all other germinal and non-germinal cell types and, therefore, it serves as a model for study of regulatory mechanisms involved in testis-specific transcription.

The role of histones in chromatin remodelling during mammalian spermiogenesis

One of the most dramatic chromatin remodelling processes takes place during mammalian spermatogenesis. Indeed, during the postmeiotic maturation of male haploid germ cells, or spermiogenesis, histones are replaced by small basic proteins, which in mammals are transition proteins and protamines. However, nothing is known of the mechanisms controlling the process of histone replacement. Two hints from the literature could help to shed light on the underlying molecular events: one is the massive synthesis of histone variants, including testis-specific members, and the second is a stage specific post-translational modification of histones. A new testis-specific 'histone code' can therefore be generated combining both histone variants and histone post-translational modifications. This review will detail these two phenomena and discuss possible functional significance of the global chromatin alterations occurring prior to histone replacement during spermiogenesis.

Histone H1–like protein participates in endothelial cell–specific activation of the von Willebrand factor promoter

A region of the von Willebrand factor (VWF) promoter has been identified that is necessary to confer endothelial cell-specific activation to the VWF promoter. This region spans sequences +155 to +247 and contains binding sites for GATA6 and NFY transcription factors. To identify potential DNA binding transcription factors that directly interact with these sequences in an endothelial-specific manner, we have performed extensive gel mobility assays with use of 7 overlapping DNA probes that collectively span this entire region. An endothelial-specific protein DNA complex was formed with an oligonucleotide that corresponded to sequences +155 to +184 of the VWF gene. Mutation analysis identified a 6-nucleotide element corresponding to sequences +164 to +169 as the core-binding region for the formation of this complex. Transfection analysis demonstrated that the mutation, which abolished DNA-protein interaction, resulted in significant inhibition of the VWF promoter activity. DNA pull-down analysis, mass spectrometry, and Western blot analysis demonstrated that a 32-kDa polypeptide with homology to histone H1 constituted the endothelial-specific DNA binding protein, or a DNA binding subunit of this protein complex. On the basis of these results, we hypothesize that an H1-like protein functions as an endothelial cell-specific transcriptional activator of the VWF promoter. (Blood. 2004;104: 1725-1732)

Male germline-specific histones in mouse and man

In mice and humans, the production of male gametes is a result of a complex multistep process of stem cell differentiation. The final product, the mature spermatozoon, is designed for the safe delivery of a haploid copy of the paternal genetic information to the oocyte in a structural state suitable for zygote formation and embryogenesis. A remarkable structural reorganization of chromosomes in germline cells during mammalian spermatogenesis has been characterized. The most important steps are connected with the recombination events during meiosis and the final packaging of the haploid genome in the genetically inert, compacted nucleus of the sperm. Underlying the changes in chromatin organization is the appearance of testis-specific histones. Although the existence of such histones has been known for decades, their exact functions still are not established. Deciphering of the mouse and human genomes has allowed a more detailed description of the organization and regulation of the testis-specific histone genes. In addition, it has facilitated the discovery of previously unknown proteins. This review summarizes contemporary information on these germline-specific/enriched histones in both the mouse and human and outlines early achievements in the identification of their functions.

RFX2 is a potential transcriptional regulatory factor for histone H1t and other genes expressed during the meiotic phase of spermatogenesis

H1t is a novel linker histone variant synthesized in mid- to late pachytene spermatocytes. Its regulatory region is of interest because developmentally specific expression has been impressed on an otherwise ubiquitously expressed promoter. Using competitive band-shift assays and specific antisera, we have now shown that the H1t-60 CCTAGG palindrome motif region binds members of the RFX family of transcriptional regulators. The testis-specific binding complex contains RFX2, probably as a homodimer. Other DNA-protein complexes obtained from testis as well as somatic organs contain RFX1, primarily as a heterodimer. Western blots confirmed that RFX2 expression is greatly enhanced in adult testis and that RFX2 is equally prominent in highly enriched populations of late pachytene spermatocytes and round spermatids. Immunohistochemistry carried out on mouse testis showed that RFX2 is strongly expressed in pachytene spermatocytes, remains high in early round spermatids, and declines only in advance of nuclear condensation. Maximum expression correlates well with the appearance of H1t. In contrast, RFX1 immunoreactivity in germ cells was only detected in late round spermatids. RFX-specific band complexes were also identified for both the mouse lamin C2 and Sgy promoters, using either testis nuclear extracts or in vitro-synthesized RFX2. These results call attention to RFX2 as a transcription factor with obvious potential for the regulation of gene expression during meiosis and the early development of spermatids.

Temporal expression of cell cycle-related proteins during spermatogenesis: establishing a timeline for onset of the meiotic divisions

During spermatogenesis, the complex events of the first meiotic prophase and division phase bring about dramatic changes in nuclear organization. One factor frustrating mechanistic dissection of these events is lack of knowledge about precisely what events occur, in what order they occur, and how they may be interrelated by temporal sequence; in other words, a precise timeline is lacking. This temporal ordering problem can be tackled by following expression and localization in mouse spermatocytes of proteins critical to events of the meiotic cell division process. These include ones that are primarily chromosomal and related to pairing and recombination, as well as kinases and substrates that mediate the cell cycle transition. Distinct and protein-specific patterns occur with respect to expression and localization throughout meiotic prophase and division and dramatic relocalization of proteins occurs as spermatocytes enter the meiotic division phase. This information provides a foundation for a meiotic timeline that can be augmented to provide, eventually, a complete catalog of meiotic events and their temporal sequence. Such a framework can clarify mechanisms of normal meiosis as well as mutant phenotypes and aberrations of the meiotic process that lead to aneuploidy.

Reductions in Linker Histone Levels Are Tolerated in Developing Spermatocytes but Cause Changes in Specific Gene Expression

H1 linker histones are involved in packaging chromatin into 30-nm fibers and higher order structures. Most eukaryotic cells contain nearly one H1 molecule for each nucleosome core particle. Male germ cells in mammals contain large amounts of a germ cell-specific linker histone, HIST1HT, herein denoted H1t, which is particularly abundant in pachytene spermatocytes. Despite its abundance in male germ cells and significant divergence in primary sequence from other H1 subtypes, inactivation of the H1t gene in mice showed that it is not required for spermatogenesis. Analysis of germ cell chromatin from H1t null mice showed that other H1 subtypes, especially the testis-enriched HIST1H1A, herein denoted as the H1a subtype, were able to compensate for the absence of H1t to maintain a normal total H1 to nucleosome core ratio. To disrupt the compensation, we generated H1t and H1a double null mice by two sequential gene-targeting steps in embryonic stem cells. Elimination of both H1t and H1a led to a 25% decrease in the ratio of H1 to nucleosome cores in double null germ cells. Surprisingly, the reduction in H1 did not perturb spermatogenesis or produce detectable defects in meiotic processes. Microarray analysis of gene expression showed that the reduced linker histone levels did not affect global gene expression, but it did cause changes in expression of specific genes. Our results indicate that a partial reduction in linker histone-nucleosome core particle stoichiometry is tolerated in developing male germ cells.

Molecular aspects of XY body formation

More than a century ago, a densely stained area inside the nucleus of male meiotic cells was described. It was later shown to harbor the sex chromosomes which undergo transcriptional inactivation in conjunction with heterochromatinisation and synapsis to form the XY body. Formation of the XY body is conserved throughout the mammalian phylogenetic tree and is thought to be essential for successful spermatogenesis. However, its biological role as well as the molecular mechanisms underlying XY body formation are still far from being understood. A lot of effort has already been undertaken to characterize components of the XY body and to investigate their functional implications in sex chromatin heterochromatinisation and meiotic sex chromosome inactivation (MSCI). This review gives an overview of those components and their possible implications in XY body formation and function.

Synergistic effects of germ cell expressed genes on male fertility in mice

Over 200 genes have been shown to be associated with infertility in mouse models. However, knockout mice reveal unexpected functional redundancy of some germ cell expressed genes. Single null mutations in mouse genes encoding four male germ cell proteins, transition protein 2 (Tnp2), proacrosin (Acr), histone H1.1 (H1.1), histone H1t (H1t) and sperm mitochondria-associated cysteine-rich protein (Smcp) have been generated and analysed. Tnp2 is believed to participate in the removal of the nuclear histones and initial condensation of the spermatid nucleus. Proacrosin is an acrosomal protease synthesized as a proenzyme and activated into acrosin during the acrosome reaction. The linker histone subtype H1.1 belongs to the group of main-type histones and is synthesized in somatic tissues as well as in germ cells during the S-phase of the cell cycle. The histone gene Hist1h1t is expressed exclusively in spermatocytes and may have a function in establishing an open chromatin structure for the replacement of histones by transition proteins and protamines. Sperm mitochondria-associated cysteine-rich protein (Smcp) is a major structural element of the mitochondria in the midpiece of the sperm tail. Male mutant mice lacking any of these proteins show no apparent defects in spermatogenesis or fertility. To examine the synergistic effects of these proteins in spermatogenesis and during fertilization four lines of double knockout mice Hist1h1a/Mcsp, Hist1h1t/Mcsp, Tnp2/Mcsp and Acr/Mcsp were established. It was found that even when knockout mice are heterozygous for one allele (-/+) and homozygous for the other allele (-/-), mice were subfertile. Homozygous double knockout mice of all four lines are nearly infertile. However, in the four homozygous double knockout mouse lines, different characteristic abnormalities are prominently manifested: In Hist1h1a-/-/Mcsp-/- the migration of spermatozoa is disturbed in female genital tract, in Hist1h1t-/-/Mcsp-/- spermatozoa show morphological head abnormalities, in Tnp2-/-/Mcsp-/- the motility of sperm is affected, and in Acr-/-/Mcsp-/- the sperm-oocyte interaction is impaired. These findings indicate strongly that male germ cell expressed genes have synergistic effects on male fertility.

Male mice lacking three germ cell expressed genes are fertile

In recent years, much knowledge about the functions of defined genes in spermatogenesis has been gained by making use of mouse transgenic and gene knockout models. Single null mutations in mouse genes encoding four male germ cell proteins, transition protein 2 (Tnp-2), proacrosin (Acr), histone H1.1 (H1.1), and histone H1t (H1t), have been generated and analyzed. Tnp-2 is believed to participate in the removal of the nuclear histones and initial condensation of the spermatid nucleus. Proacrosin is an acrosomal protease synthesized as a proenzyme and activated into acrosin during the acrosome reaction. The linker histone subtype H1.1 belongs to the group of main-type histones and is synthesized in somatic tissues and germ cells during the S-phase of the cell cycle. The histone gene H1t is expressed exclusively in spermatocytes and may have a function in establishing an open chromatin structure for the replacement of histones by transition proteins and protamines. Male mutant mice lacking any of these proteins show no apparent defects in spermatogenesis or fertility. To examine the synergistic effects of these proteins in spermatogenesis and during fertilization, two lines of triple null mice (Tnp-2-/-/Acr-/-/H1.1-/- and Tnp-2-/-/Acr-/-/H1t-/-) were established. Both lines are fertile and show normal sperm parameters, which clearly demonstrate the functional redundancy of these proteins in male mouse fertility. However, sperm only deficient for Acr (Acr-/-) are able to compete significantly with sperm from triple knockout mice Tnp-2-/-/Acr-/-/H1.1-/- (70.7% vs. 29.3%) but not with sperm from triple knockout mice Tnp-2-/-/Acr-/-/H1t-/- (53.6% vs. 46.4%). These results are consistent with a model that suggests that some sperm proteins play a role during sperm competition.

HILS1 is a spermatid-specific linker histone H1-like protein implicated in chromatin remodeling during mammalian spermiogenesis

Chromatin remodeling is a major event that occurs during mammalian spermiogenesis, the process of spermatid maturation into spermatozoa. Nuclear condensation during spermiogenesis is accomplished by replacing somatic histones (linker histone H1 and core histones) and the testis-specific linker histone, H1t, with transition proteins and protamines. It has long been thought that H1t is the only testis-specific linker histone, and that all linker histones are replaced by transition proteins, and subsequently by protamines during spermiogenesis. Here, we report the identification and characterization of a spermatid-specific linker histone H1-like protein (termed HILS1) in the mouse and human. Both mouse and human HILS1 genes are located in intron 8 of the alpha-sarcoglycan genes. HILS1 is highly expressed in nuclei of elongating and elongated spermatids (steps 9-15). HILS1 displays several biochemical properties that are similar to those of linker histones, including the abilities to bind reconstituted mononucleosomes, produce a chromatosome stop during micrococcal nuclease digestion, and aggregate chromatin. Because HILS1 is expressed in late spermatids that do not contain core histones, HILS1 may participate in spermatid nuclear condensation through a mechanism distinct from that of linker histones. Because HILS1 also belongs to the large winged helix/forkhead protein superfamily, HILS1 may also regulate gene transcription, DNA repair, and/or other chromosome processes during mammalian spermiogenesis.

Possible predictive factors for ICSI? Molecular biology techniques in combination with therapeutic testicular biopsies

Applying intracytoplasmic sperm injection (ICSI), the selection of an unsuccessful spermatozoon results in great emotional consequences for the couple. Therefore, there is a need for a prognostic parameter to estimate their chances for successful fertility treatment. This review summarizes both the main reasons for spermatogenic impairment, and possible predictive factors for successful sperm retrieval applying testicular sperm extraction and outcome of ICSI. While basic sperm parameters, aetiology and type of spermatozoa, and serum follicle-stimulating hormone and inhibin levels have been shown to be unrelated to the outcome of ICSI, Y-chromosome microdeletions are known to have a negative influence on the fertilizing capacity of spermatozoa. Recently, a significant correlation has been reported between the protamine-1 to protamine-2 mRNA ratio in haploid spermatids of testicular biopsies and the ability of spermatozoa for successful fertilization of an oocyte. In future, both the outstanding role of the haploid spermatids and the involvement of molecular biological techniques will improve the role of therapeutic testicular biopsies.

H1 linker histones are essential for mouse development and affect nucleosome spacing in vivo

Most eukaryotic cells contain nearly equimolar amounts of nucleosomes and H1 linker histones. Despite their abundance and the potential functional specialization of H1 subtypes in multicellular organisms, gene inactivation studies have failed to reveal essential functions for linker histones in vivo. Moreover, in vitro studies suggest that H1 subtypes may not be absolutely required for assembly of chromosomes or nuclei. By sequentially inactivating the genes for three mouse H1 subtypes (H1c, H1d, and H1e), we showed that linker histones are essential for mammalian development. Embryos lacking the three H1 subtypes die by mid-gestation with a broad range of defects. Triple-H1-null embryos have about 50% of the normal ratio of H1 to nucleosomes. Mice null for five of these six H1 alleles are viable but are underrepresented in litters and are much smaller than their littermates. Marked reductions in H1 content were found in certain tissues of these mice and in another compound H1 mutant. These results demonstrate that the total amount of H1 is crucial for proper embryonic development. Extensive reduction of H1 in certain tissues did not lead to changes in nuclear size, but it did result in global shortening of the spacing between nucleosomes.

Histone H1 and the dynamic regulation of chromatin function

Eukaryotic DNA is organized in a complex structure called chromatin. Although a primary function of chromatin is compaction of DNA, this must done such that the underlying DNA is potentially accessible to factor-mediated regulatory responses. Chromatin structure clearly plays a dominant role in regulating much of eukaryotic transcription. The demonstration that reversible covalent modification of the core histones contribute to transcriptional activation and repression by altering chromatin structure and the identification of numerous ATP-dependent chromatin remodeling enzymes provide strong support for this view. Chromatin is much more dynamic than was previously thought and regulation of the dynamic properties of chromatin is a key aspect of gene regulation. This review will focus on recent attempts to elucidate the specific contribution of histone H1 to chromatin-mediated regulation of gene expression.

Mammalian linker-histone subtypes differentially affect gene expression in vivo

Posttranslational modifications and remodeling of nucleosomes are critical factors in the regulation of transcription. Higher-order folding of chromatin also is likely to contribute to the control of gene expression, but the absence of a detailed description of the structure of the chromatin fiber has impaired progress in this area. Mammalian somatic cells contain a set of H1 linker-histone subtypes, H1 (0) and H1a to H1e, that bind to nucleosome core particles and to the linker DNA between nucleosomes. To determine whether the H1 histone subtypes play differential roles in the regulation of gene expression, we combined mice lacking specific H1 histone subtypes with mice carrying transgenes subject to position effects. Because position effects result from the unique chromatin structure created by the juxtaposition of regulatory elements in the transgene and at the site of integration, transgenes can serve as exquisitely sensitive indicators of chromatin structure. We report that some, but not all, linker histones can attenuate or accentuate position effects. The results suggest that the linker-histone subtypes play differential roles in the control of gene expression and that the sequential arrangement of the linker histones on the chromatin fiber might regulate higher-order chromatin structure and fine-tune expression levels.

TE2 and TE1 sub-elements of the testis-specific histone H1t promoter are functionally different

The testis-specific linker histone H1t gene is transcribed exclusively in pachytene primary spermatocytes. Tissue specific expression of the gene is mediated in part by transcriptional factors that bind elements located within the proximal and distal promoter. A 40 bp promoter element, designated H1t/TE, that is located within the proximal promoter between the CCAAT-box and AC-box, is known to be essential for H1t gene transcription in transgenic animals. In the present study, we show by SDS-PAGE analysis of UV crosslinked protein and DNA and by electrophoretic mobility shift assays (EMSA) of testis nuclear proteins separated on a non-denaturing glycerol gradient that the TE1 sub-element is bound by a protein complex. Mutation of TE1 leads to a drop in H1t promoter activity in germinal GC-2spd cells as well as in nongerminal Leydig, NIH3T3, and C127I cell lines. Although TE1 and TE2 sub-elements have similar sequences, mutation of the TE2 sub-element causes an increase in promoter activity in C127I and Leydig cells. The rat TE1 but not TE2 contains a CpG dinucleotide and this cytosine is methylated in liver but not in primary spermatocytes. Methylation of the cytosine at this site almost eliminates nuclear protein binding. Thus, there are significant functional differences in the TE2 and TE1 sub-elements of the H1t promoter with TE1 serving as a transcriptional activator binding site and TE2 serving as a repressor binding site in some cell lines.

H1t/GC-box and H1t/TE1 element are essential for promoter activity of the testis-specific histone H1t gene

The testis-specific linker histone H1t gene is transcribed exclusively in mid to late pachytene primary spermatocytes. Tissue-specific expression of the gene is mediated primarily through elements located within the proximal promoter. Previous work in transgenic animals identified a unique 40-base pair promoter element designated H1t/TE that is essential for spermatocyte-specific expression. The H1t/TE element contains three subelements designated TE2, GC-box, and TE1 based on in vitro footprinting and electrophoretic mobility shift assays. Because GC-box is a consensus site for binding of Sp transcription-factor family members, experiments were performed demonstrating that two Sp family members, Sp1 and Sp3, were present in testis cells from 9-day-old and adult rats and in pachytene primary spermatocytes and early spermatids. A 95- to 105-kDa form of Sp1 is most abundant in the tissues and cell lines examined, but a 60-kDa form of Sp1 is the most abundant species in spermatocytes and early spermatids. Further examination of Sp1 and Sp3 from adult testis, primary spermatocytes, and early spermatids showed that they can bind to the H1t/TE element. In order to determine the contributions of the subelements to H1t transcription, we mutated each of them in H1t promoter luciferase reporter vectors. Mutation of the GC-box and TE1 subelement reduced expression 77% and 49%, respectively, compared with the wild-type H1t promoter in transient expression assays in a testis GC-2spd cell line that was derived from germinal cells. These studies suggest that Sp transcription factors may be involved in transcription of the H1t gene and the GC-box and the TE1 subelement are required for activation of the H1t promoter.

Expression of hyperacetylated histone H4 during normal and impaired human spermatogenesis

Histone-to-protamine exchange in haploid spermatids is preceded by hyperacetylation of core histones resulting in decreased DNA-histone interaction. During normal spermatogenesis, immunohistochemistry with a polyclonal antihyperacetylated histone H4 antibody displayed a strong signal in nuclei of elongating spermatids and, in addition, spermatogonia. Quantitative analysis revealed 98.2 +/- 1.1% of immunopositive spermatids. The percentage of positive spermatids was significantly reduced in infertile men exhibiting at least qualitatively normal spermatogenesis (scores 10-8, 93.1 +/- 6.6%) and impaired spermatogenesis (scores 7-1, 74.9 +/- 23.4%). In seminiferous tubules showing spermatogenic arrest at the level of round spermatids, only 59.5 +/- 16.5% of spermatids were immunopositive for hyperacetylated histone H4. These data demonstrate that the decrease of histone acetylation in spermatids associated with impaired spermatogenesis corresponds with the well known reduction of protamine expression in these cells and confirms the essential role of histone hyperacetylation for correct histone-to-protamine exchange. In seminiferous tubules exhibiting round spermatid maturation arrest, there was an additional signal in nuclei of spermatocytes, suggesting that premature hyperacetylation of histone H4 may result in precocious histone-to-protamine exchange followed by infertility. This is in accordance with data from transgenic mice, where it has been demonstrated that premature expression of protamine-1 results in precocious chromatin condensation followed by sterility.

Human testis/sperm-specific histone H2B (hTSH2B). Molecular cloning and characterization

Human sperm, unlike the sperm of other mammals, contain replacement histones with unknown biological functions. Here, we report the identification of the novel human gene coding for a testis/sperm-specific histone H2B (hTSH2B). This variant histone is 85% homologous to somatic H2B and has over 93% homology with the testis H2B of rodents. Using genomic PCR, two genetic alleles of hTSH2B were found in the human population. The hTSH2B gene is transcribed exclusively in testis, and the corresponding protein is also present in mature sperm. We expressed recombinant hTSH2B and identified this protein with a particular H2B subtype expressed in vivo. The subnuclear distribution of H2B variants in sperm was determined using biochemical fractionation and immunoblotting. The H2B variant associated with telomere-binding activity () was solubilized by Triton X-100 or micrococcal nuclease extraction, whereas hTSH2B was relatively tightly bound in nuclei. Immunofluorescence showed that hTSH2B was concentrated in spots located at the basal nuclear area of a subpopulation (20% of cells) of mature sperm. This fact may be of particular importance, because the hTSH2B "positive" and "negative" sperm cells may undergo significantly different decondensation processes following fertilization.

Individual somatic H1 subtypes are dispensable for mouse development even in mice lacking the H1(0) replacement subtype

H1 linker histones are involved in facilitating the folding of chromatin into a 30-nm fiber. Mice contain eight H1 subtypes that differ in amino acid sequence and expression during development. Previous work showed that mice lacking H1(0), the most divergent subtype, develop normally. Examination of chromatin in H1(0-/-) mice showed that other H1s, especially H1c, H1d, and H1e, compensate for the loss of H1(0) to maintain a normal H1-to-nucleosome stoichiometry, even in tissues that normally contain abundant amounts of H1(0) (A. M. Sirotkin et al., Proc. Natl. Acad. Sci. USA 92:6434-6438, 1995). To further investigate the in vivo role of individual mammalian H1s in development, we generated mice lacking H1c, H1d, or H1e by homologous recombination in mouse embryonic stem cells. Mice lacking any one of these H1 subtypes grew and reproduced normally and did not exhibit any obvious phenotype. To determine whether one of these H1s, in particular, was responsible for the compensation present in H1(0-/-) mice, each of the three H1 knockout mouse lines was bred with H1(0) knockout mice to generate H1c/H1(0), H1d/H1(0), or H1e/H1(0) double-knockout mice. Each of these doubly H1-deficient mice also was fertile and exhibited no anatomic or histological abnormalities. Chromatin from the three double-knockout strains showed no significant change in the ratio of total H1 to nucleosomes. These results suggest that any individual H1 subtype is dispensable for mouse development and that loss of even two subtypes is tolerated if a normal H1-to-nucleosome stoichiometry is maintained. Multiple compound H1 knockouts will probably be needed to disrupt the compensation within this multigene family.

Histone variants and histone modifications: A structural perspective

In this review, we briefly analyze the current state of knowledge on histone variants and their posttranslational modifications. We place special emphasis on the description of the structural component(s) defining and determining their functional role. The information available indicates that this histone "variability" may operate at different levels: short-range "local" or long-range "global", with different functional implications. Recent work on this topic emphasizes an earlier notion that suggests that, in many instances, the functional response to histone variability is possibly the result of a synergistic structural effect.Key words: histone variants, posttranslational modifications, chromatin.

Characterization of the H1t promoter: role of conserved histone 1 AC and TG elements and dominance of the cap-proximal silencer

H1t is a testis-specific variant histone 1 gene transcribed in pachytene spermatocytes. As part of a program to understand its transcriptional control, we have investigated the effect of the cap-proximal, GC-rich silencer element in the context of various lengths of upstream sequence. By transient transfection of NIH 3T3 cells, we showed that a targeted mutation in the silencer has a large (>10-fold) effect on reporter gene expression, regardless of the length of upstream sequence present. No other discrete silencing activity was observed in the upstream region extending to nucleotide -1842. Similarly, when the silencer mutation was introduced into the natural gene, H1t expression was readily detected in permanently transfected cells by both RNase protection and Western blot analysis, regardless of the extent of 5' or 3' flanking genomic DNA. In constructs with the mutated silencer, we showed interdependence of the characteristic H1 AC and TG box regulatory elements. Promoter up-regulation occurred only when both were intact, and possibly identical binding factors were demonstrated for each by electrophoretic mobility shift assays. In view of its precisely regulated but limited expression, it is interesting that H1t retains all the promoter elements known to activate standard H1 genes, including the TG/AC unit, SP1 site, and CCAAT element. Their presence emphasizes the apparent dominance of the silencer element in most cells.

A possible meiotic function of the peculiar patterns of gene expression in mammalian spermatogenic cells

This review focuses on the striking differences in the patterns of transcription and translation in somatic and spermatogenic cells in mammals. In early haploid cells, mRNA translation evidently functions to restrict the synthesis of certain proteins, notably protamines, to transcriptionally inert late haploid cells. However, this does not explain why a substantial proportion of virtually all mRNA species are sequestered in translationally inactive free-messenger ribonucleoprotein particles (free-mRNPs) in meiotic cells, since most mRNAs undergo little or no increase in translational activity in transcriptionally active early haploid cells. In addition, most mRNAs in meiotic cells appear to be overexpressed because they are never fully loaded on polysomes and the levels of the corresponding protein are often much lower than the mRNA and are sometimes undetectable. A large number of genes are expressed at grossly higher levels in meiotic and/or early haploid spermatogenic cells than in somatic cells, yet they too are translated inefficiently. Many genes utilize alternative promoters in somatic and spermatogenic cells. Some of the resulting spermatogenic cell-altered transcripts (SCATs) encode proteins with novel functions, while others contain features in their 5'-UTRs, secondary structure or upstream reading frames, that are predicted to inhibit translation. This review proposes that the transcriptional machinery is modified to provide access to specific DNA sequences during meiosis, which leads to mRNA overexpression and creates a need for translational fine-tuning to prevent deleterious consequences of overproducing proteins.

Histone H1 diversity: bridging regulatory signals to linker histone function

Genes encoding linker histone variants have evolved to link their expression to signals controlling the proliferative capacities of cells, i.e. cycling and growth-arrested cells express distinct and specific H1 subtypes. In metazoan, these variants show a tripartite structure, with considerably divergent sequences in their amino and carboxyl terminus domains. The aim of this review is to show how specific regulatory signals control the expression of an individual H1 and to discuss the functional significance of the two variables associated with a linker histone: its primary sequence and the timing of its expression.

A compendium of the histone H1 family of somatic subtypes: An elusive cast of characters and their characteristics

The last 35 years has seen a substantial amount of information collected about the somatic H1 subtypes, yet much of this work has been overshadowed by research into highly divergent isoforms of H1, such as H5. Reports from several laboratories in the past few years have begun to call into question some of the traditional views regarding the general function of linker histones and their heterogeneity. Hence, the impression in some circles is that less is known about these ubiquitous nuclear proteins as compared with the core histones. The goal of the following review is to acquaint the reader with the ubiquitous somatic H1s by categorizing them and their characteristics into several classes. The reasons for our current state of misunderstanding is put into a historical context along with recent controversies centering on the role of H1 in the nucleus. Finally, we propose a model that may explain the functional role of H1 heterogeneity in chromatin compaction.Key words: histone H1, linker histones, chromatin organization, chromatin compaction, heat shock.

Decreased expression of specific genes in yeast cells lacking histone H1

Chromatin plays an important role in regulating eukaryotic gene expression. Chromatin is composed of DNA wrapped around a nucleosome core (consisting of two copies of the well conserved histones H2A, H2B, H3, and H4) and a more variable linker histone H1. Various in vitro and in vivo studies have implicated histone H1 as a repressor of gene expression or as an activator, but its exact role is still unclear. Sequencing of the yeast genome has led to the identification of a putative histone H1 gene. Biochemical studies demonstrated that yeast does indeed possess a bona fide histone H1. However, deletion of the unique yeast H1 gene is not associated with any phenotypes, and it was questioned whether it plays any role. To address this issue, we performed whole-genome microarray analysis to identify genes that are affected by H1 removal. Surprisingly, deletion of the gene encoding histone H1 does not result in increased gene expression but rather in a modest reduction. Northern blot analysis of selected genes confirmed the results obtained with the microarray analysis. A similar effect was observed with an integrated lacZ reporter. Thus, our data demonstrate that removal of yeast histone H1 only results in decreased gene expression.

Mice with a targeted disruption of the H1t gene are fertile and undergo normal changes in structural chromosomal proteins during spermiogenesis

H1t is an H1 histone variant unique to late spermatocytes and early spermatids. Using gene targeting and embryonic stem cell technologies, we have produced mice with a disrupted H1t gene. Homozygous H1t-null mice have normal fertility and show no obvious phenotypic consequence due to the lack of this histone. Biochemical and immunohistochemical approaches were used to show that normal changes in chromosomal proteins occurred during spermatid development, including the appearance and disappearance of transition proteins 1 and 2. Both protamines 1 and 2 are present in normal amounts in sonication-resistant spermatid nuclei from H1t-null mice. Analysis of H1 histones by quantitative gel electrophoresis in enriched populations of pachytene spermatocytes and round spermatids showed that the lack of H1t is only partially compensated for by somatic H1s, so that the chromatin of these cells is H1 deficient. Because H1t is thought to create a less tightly compacted chromatin environment, it may be that H1-deficient chromatin is functionally similar to chromatin with H1t present, at least with respect to permitting spermatogenesis to proceed.

Determination of transgenic loci by expression FISH

DNA targeting by homologous recombination in mouse embryonic stem (ES) cells has become a widely used method for manipulating the mouse genome and for studying the role of specific genes in mammalian development. For certain studies, it is necessary to target two or more DNA sequences residing on a particular chromosome. In these situations, it would be important to distinguish whether two sequential gene targeting events in the ES cells have occurred in cis or in trans. We report here a new application of fluorescence in situ hybridization to RNA molecules present at sites of transcription that allows the identification of cis and trans gene targeting events in ES cells. The method is based on detection of transcripts from commonly used selectable marker genes inserted during homologous recombination. Transcripts are detected in interphase nuclei, making the preparation of mitotic cells unnecessary and obviating the necessity for the more technically demanding DNA detection of genes. The method is applicable to any chromosomal locus, and compared with other methods (e.g., genetic linkage testing in chimeric mice), it will greatly shorten the time required for distinguishing cis and trans gene targeting events in ES cells. The method also may be useful for detecting changes in ploidy of individual chromosomes and loss of heterozygosity of genes in single cells in culture and also in animals, for example, during processes such as tumorigenesis.

Histone variants: are they functionally heterogeneous?

In most eukaryotes, histones, which are the major structural components of chromatin, are expressed as a family of sequence variants encoded by multiple genes. Because different histone variants can contribute to a distinct or unique nucleosomal architecture, this heterogeneity can be exploited to regulate a wide range of nuclear functions, and evidence is accumulating that histone variants do indeed have distinct functions.