Generation and characterization of novel antibodies highly selective for phosphorylated linker histone H1 in Tetrahymena and HeLa cells

Site-specific regulation of histone H1 phosphorylation in pluripotent cell differentiation

BACKGROUND

Structural variation among histone H1 variants confers distinct modes of chromatin binding that are important for differential regulation of chromatin condensation, gene expression and other processes. Changes in the expression and genomic distributions of H1 variants during cell differentiation appear to contribute to phenotypic differences between cell types, but few details are known about the roles of individual H1 variants and the significance of their disparate capacities for phosphorylation. In this study, we investigated the dynamics of interphase phosphorylation at specific sites in individual H1 variants during the differentiation of pluripotent NT2 and mouse embryonic stem cells and characterized the kinases involved in regulating specific H1 variant phosphorylations in NT2 and HeLa cells.

RESULTS

Here, we show that the global levels of phosphorylation at H1.5-Ser18 (pS18-H1.5), H1.2/H1.5-Ser173 (pS173-H1.2/5) and H1.4-Ser187 (pS187-H1.4) are regulated differentially during pluripotent cell differentiation. Enrichment of pS187-H1.4 near the transcription start site of pluripotency factor genes in pluripotent cells is markedly reduced upon differentiation, whereas pS187-H1.4 levels at housekeeping genes are largely unaltered. Selective inhibition of CDK7 or CDK9 rapidly diminishes pS187-H1.4 levels globally and its enrichment at housekeeping genes, and similar responses were observed following depletion of CDK9. These data suggest that H1.4-S187 is a bona fide substrate for CDK9, a notion that is further supported by the significant colocalization of CDK9 and pS187-H1.4 to gene promoters in reciprocal re-ChIP analyses. Moreover, treating cells with actinomycin D to inhibit transcription and trigger the release of active CDK9/P-TEFb from 7SK snRNA complexes induces the accumulation of pS187-H1.4 at promoters and gene bodies. Notably, the levels of pS187-H1.4 enrichment after actinomycin D treatment or cell differentiation reflect the extent of CDK9 recruitment at the same loci. Remarkably, the global levels of H1.5-S18 and H1.2/H1.5-S173 phosphorylation are not affected by these transcription inhibitor treatments, and selective inhibition of CDK2 does not affect the global levels of phosphorylation at H1.4-S187 or H1.5-S18.

CONCLUSIONS

Our data provide strong evidence that H1 variant interphase phosphorylation is dynamically regulated in a site-specific and gene-specific fashion during pluripotent cell differentiation, and that enrichment of pS187-H1.4 at genes is positively related to their transcription. H1.4-S187 is likely to be a direct target of CDK9 during interphase, suggesting the possibility that this particular phosphorylation may contribute to the release of paused RNA pol II. In contrast, the other H1 variant phosphorylations we investigated appear to be mediated by distinct kinases and further analyses are needed to determine their functional significance.

Ret function in muscle stem cells points to tyrosine kinase inhibitor therapy for facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) involves sporadic expression of DUX4, which inhibits myogenesis and is pro-apoptotic. To identify target genes, we over-expressed DUX4 in myoblasts and found that the receptor tyrosine kinase Ret was significantly up-regulated, suggesting a role in FSHD. RET is dynamically expressed during myogenic progression in mouse and human myoblasts. Constitutive expression of either RET9 or RET51 increased myoblast proliferation, whereas siRNA-mediated knockdown of Ret induced myogenic differentiation. Suppressing RET activity using Sunitinib, a clinically-approved tyrosine kinase inhibitor, rescued differentiation in both DUX4-expressing murine myoblasts and in FSHD patient-derived myoblasts. Importantly, Sunitinib also increased engraftment and differentiation of FSHD myoblasts in regenerating mouse muscle. Thus, DUX4-mediated activation of Ret prevents myogenic differentiation and could contribute to FSHD pathology by preventing satellite cell-mediated repair. Rescue of DUX4-induced pathology by Sunitinib highlights the therapeutic potential of tyrosine kinase inhibitors for treatment of FSHD.

DOI:http://dx.doi.org/10.7554/eLife.11405.001

DUX4 induces a transcriptome more characteristic of a less-differentiated cell state and inhibits myogenesis

Skeletal muscle wasting in facioscapulohumeral muscular dystrophy (FSHD) results in substantial morbidity. On a disease-permissive chromosome 4qA haplotype, genomic and/or epigenetic changes at the D4Z4 macrosatellite repeat allows transcription of the DUX4 retrogene. Analysing transgenic mice carrying a human D4Z4 genomic locus from an FSHD-affected individual showed that DUX4 was transiently induced in myoblasts during skeletal muscle regeneration. Centromeric to the D4Z4 repeats is an inverted D4Z4 unit encoding DUX4c. Expression of DUX4, DUX4c and DUX4 constructs, including constitutively active, dominant-negative and truncated versions, revealed that DUX4 activates target genes to inhibit proliferation and differentiation of satellite cells, but that it also downregulates target genes to suppress myogenic differentiation. These transcriptional changes elicited by DUX4 in mouse have significant overlap with genes regulated by DUX4 in man. Comparison of DUX4 and DUX4c transcriptional perturbations revealed that DUX4 regulates genes involved in cell proliferation, whereas DUX4c regulates genes engaged in angiogenesis and muscle development, with both DUX4 and DUX4c modifing genes involved in urogenital development. Transcriptomic analysis showed that DUX4 operates through both target gene activation and repression to orchestrate a transcriptome characteristic of a less-differentiated cell state.

DNA copy-number control through inhibition of replication fork progression

Proper control of DNA replication is essential to ensure faithful transmission of genetic material and prevent chromosomal aberrations that can drive cancer progression and developmental disorders. DNA replication is regulated primarily at the level of initiation and is under strict cell-cycle regulation. Importantly, DNA replication is highly influenced by developmental cues. In Drosophila, specific regions of the genome are repressed for DNA replication during differentiation by the SNF2 domain-containing protein SUUR through an unknown mechanism. We demonstrate that SUUR is recruited to active replication forks and mediates the repression of DNA replication by directly inhibiting replication fork progression instead of functioning as a replication fork barrier. Mass spectrometry identification of SUUR-associated proteins identified the replicative helicase member CDC45 as a SUUR-associated protein, supporting a role for SUUR directly at replication forks. Our results reveal that control of eukaryotic DNA copy number can occur through the inhibition of replication fork progression.

The molecular chaperone Hsp90 is required for cell cycle exit in Drosophila melanogaster

The coordination of cell proliferation and differentiation is crucial for proper development. In particular, robust mechanisms exist to ensure that cells permanently exit the cell cycle upon terminal differentiation, and these include restraining the activities of both the E2F/DP transcription factor and Cyclin/Cdk kinases. However, the full complement of mechanisms necessary to restrain E2F/DP and Cyclin/Cdk activities in differentiating cells are not known. Here, we have performed a genetic screen in Drosophila melanogaster, designed to identify genes required for cell cycle exit. This screen utilized a PCNA-miniwhite⁺ reporter that is highly E2F-responsive and results in a darker red eye color when crossed into genetic backgrounds that delay cell cycle exit. Mutation of Hsp83, the Drosophila homolog of mammalian Hsp90, results in increased E2F-dependent transcription and ectopic cell proliferation in pupal tissues at a time when neighboring wild-type cells are postmitotic. Further, these Hsp83 mutant cells have increased Cyclin/Cdk activity and accumulate proteins normally targeted for proteolysis by the anaphase-promoting complex/cyclosome (APC/C), suggesting that APC/C function is inhibited. Indeed, reducing the gene dosage of an inhibitor of Cdh1/Fzr, an activating subunit of the APC/C that is required for timely cell cycle exit, can genetically suppress the Hsp83 cell cycle exit phenotype. Based on these data, we propose that Cdh1/Fzr is a client protein of Hsp83. Our results reveal that Hsp83 plays a heretofore unappreciated role in promoting APC/C function during cell cycle exit and suggest a mechanism by which Hsp90 inhibition could promote genomic instability and carcinogenesis.

Cells must permanently stop dividing when they terminally differentiate for development to occur normally. Maintenance of this postmitotic state is also important, as unscheduled proliferation of differentiated cells can result in cancer. To identify genes important for restraining cell proliferation during terminal differentiation, we performed a genetic screen in Drosophila and found that mutation of Hsp90 caused ectopic cell proliferation in differentiating tissues. Hsp90 is a molecular chaperone that is essential for viability in all eukaryotes and has been shown to facilitate the activity of hundreds of “client” proteins. Indeed, several inhibitors of Hsp90 are currently being tested in clinical trials for use as anti-cancer therapeutics due to their ability to silence multiple client oncoproteins simultaneously. Our data suggest that Hsp90 is necessary to halt cell proliferation during differentiation because the protein Cdh1, which is required for normal cell cycle exit, may be a client of Hsp90. As reduced Cdh1 function results in genomic instability and tumorigenesis, our work highlights the need to design more precisely targeted Hsp90 inhibitors for use as cancer treatments.

Chromatin unfolding by Cdt1 regulates MCM loading via opposing functions of HBO1 and HDAC11-geminin

The efficiency of metazoan origins of DNA replication is known to be enhanced by histone acetylation near origins. Although this correlates with increased MCM recruitment, the mechanism by which such acetylation regulates MCM loading is unknown. We show here that Cdt1 induces large-scale chromatin decondensation that is required for MCM recruitment. This process occurs in G₁, is suppressed by Geminin, and requires HBO1 HAT activity and histone H4 modifications. HDAC11, which binds Cdt1 and replication origins during S-phase, potently inhibits Cdt1-induced chromatin unfolding and re-replication, suppresses MCM loading and binds Cdt1 more efficiently in the presence of Geminin. We also demonstrate that chromatin at endogenous origins is more accessible in G₁ relative to S-phase. These results provide evidence that histone acetylation promotes MCM loading via enhanced chromatin accessibility. This process is regulated positively by Cdt1 and HBO1 in G₁ and repressed by Geminin-HDAC11 association with Cdt1 in S-phase, and represents a novel form of replication licensing control.

Histone H1 phosphorylation is associated with transcription by RNA polymerases I and II

Histone H1 phosphorylation affects chromatin condensation and function, but little is known about how specific phosphorylations impact the function of H1 variants in higher eukaryotes. In this study, we show that specific sites in H1.2 and H1.4 of human cells are phosphorylated only during mitosis or during both mitosis and interphase. Antisera generated to individual H1.2/H1.4 interphase phosphorylations reveal that they are distributed throughout nuclei and enriched in nucleoli. Moreover, interphase phosphorylated H1.4 is enriched at active 45S preribosomal RNA gene promoters and is rapidly induced at steroid hormone response elements by hormone treatment. Our results imply that site-specific interphase H1 phosphorylation facilitates transcription by RNA polymerases I and II and has an unanticipated function in ribosome biogenesis and control of cell growth. Differences in the numbers, structure, and locations of interphase phosphorylation sites may contribute to the functional diversity of H1 variants.

Site-specifically phosphorylated forms of H1.5 and H1.2 localized at distinct regions of the nucleus are related to different processes during the cell cycle

The cell cycle-associated phosphorylation of histone H1.5 is manifested as three discrete phosphorylated forms, occurring exclusively on Ser(17), Ser(172), and Ser(188) during interphase. During late G2 and mitosis the up-phosphorylation occurs exclusively on threonine at either Thr(137) or Thr(154) to build the tetraphosphorylated forms of H1.5, whereas the pentaphosphorylated forms result from phosphorylation at Thr(10). To determine the kinetic and spatial distribution of histone H1 phosphorylation within the nucleus of synchronized Hela cells we localized three distinct phosphorylation sites of histone subtype H1.5 using affinity-purified polyclonal antibodies generated against phosphorylated Ser(17), Ser(172), and Thr(10). Immunofluorescence labeling of synchronized HeLa cells using the specific antibodies revealed that phosphorylation of H1.5 Ser(17) appeared early in G1 at discrete speckles followed by phosphorylation of Ser(172). Thr(10) phosphorylation started during prophase, showed highest phosphorylation levels during metaphase, and disappeared clearly before chromatin decondensation occurred. Experiments using the kinase inhibitor staurosporine indicate the involvement of different kinases at the various phospho-sites. Colocalization studies revealed that Ser(172) phosphorylation of H1.5 and H1.2 does colocalize to DNA replication and transcription sites. These results favor the idea that the various site-specifically phosphorylated forms of H1.5 and H1.2 localized at distinct regions of the nucleus are related to different functions during the cell cycle.

cAMP signaling induces rapid loss of histone H3 phosphorylation in mammary adenocarcinoma-derived cell lines

The phosphorylation of histone H3 is known to play a role in regulation of transcription as well as preparation of chromosomes for mitosis. Various signaling cascades induce H3 phosphorylation, particularly at genes activated by these pathways. In this study, we show that signaling can also have the opposite effect. Activators of cAMP signaling induce a rapid and potent loss of H3 phosphorylation. This effect is not mediated through a cAMP metabolite since a membrane-permeable form of AMP had no effect on H3 phosphorylation and a phosphodiesterase-resistant cAMP analog efficiently reduced it. cAMP is also the likely regulator of H3 phosphorylation under physiological conditions since only supra-pharmacological doses of cGMP induce the loss of H3 phosphorylation. The loss of phosphorylation is specific for histone H3 since we do not observe drastic losses in total phosphorylation of other histones. In addition, other H3 modifications are unaffected with the exception of lysine 9 methylation, which is elevated. Analysis of cell growth and cell cycle shows that cAMP signaling inhibits cell growth and arrests cells at both G1 and G2/M. Similar effects of cAMP signaling on H3 phosphorylation are observed in a variety of mammary adenocarcinoma-derived cell lines. In syngeneic human breast-derived cell lines, one diploid and non-transformed, the other derived from a ductal carcinoma, the loss of H3 phosphorylation is significantly more sensitive to cAMP concentration in the transformed cell line.

Developmentally regulated histone modifications in Drosophila follicle cells: initiation of gene amplification is associated with histone H3 and H4 hyperacetylation and H1 phosphorylation

We have used gene amplification in Drosophila follicle cells as a model of metazoan DNA replication to address whether changes in histone modifications are associated with replication origin activation. We observe that replication initiation is associated with distinct histone modifications. Acetylated lysines K5, K8, and K12 on histone H4 and K14 on histone H3 are specifically enriched during replication initiation at the amplification origins. Strikingly, H4 acetylation persists at an amplification origin well after replication forks have progressed significantly outward from the origin, indicating that H4 acetylation is associated with origin regulation and not histone deposition at the replication forks. Origin recognition complex subunit 2 (orc2) mutants with severe amplification defects do not abolish H4 acetylation, whereas the dup/cdt1 mutant delays the appearance of acetylation foci, and mutants in rbf result in temporal persistence. These data indicate that core histone acetylation is associated with origin activity. Furthermore, follicle cells undergoing gene amplification exhibit high levels of histone H1 phosphorylation. The patterns of H1 phosphorylation provide insights into cell cycle states during amplification, as H1 kinase activity in follicle cells is responsive to high Cyclin E activity, and it can be abolished by overexpressing the retinoblastoma homolog, Rbf, that represses Cyclin E. These data suggest that amplification origins are able to initiate when the cells are in a late S-phase, when the genome is normally not licensed for replication.

Regulation of hypermutation by activation-induced cytidine deaminase phosphorylation

Activation-induced cytidine deaminase (AID) initiates Ig class switch recombination and somatic hypermutation by producing U:G mismatches in DNA. These mismatches also have the potential to induce DNA damage including double-stranded breaks and chromosome translocations; therefore, strict regulation of AID is important for maintaining genomic stability. In addition to transcriptional regulation, it has been proposed that phosphorylation can also modulate AID activity. Using a combination of MS and immunochemical approaches we found that 5-15% of the AID expressed in activated B cells was phosphorylated at serine-38 (p38AID). This form of AID was enriched in the chromatin fraction in activated B cells, suggesting a role for phosphorylation in targeting AID to DNA. Consistent with this idea, serine-38 to alanine mutant AID (AID(S38A)) showed diminished somatic hypermutation activity on artificial and physiological DNA targets. We conclude that a small fraction of AID is phosphorylated in activated B cells and that the modified form contributes disproportionately to hypermutation.

Regulation of replication licensing by acetyltransferase Hbo1

The initiation of DNA replication is tightly regulated in eukaryotic cells to ensure that the genome is precisely duplicated once and only once per cell cycle. This is accomplished by controlling the assembly of a prereplicative complex (pre-RC) which involves the sequential binding to replication origins of the origin recognition complex (ORC), Cdc6/Cdc18, Cdt1, and the minichromosome maintenance complex (Mcm2-Mcm7, or Mcm2-7). Several mechanisms of pre-RC regulation are known, including ATP utilization, cyclin-dependent kinase levels, protein turnover, and Cdt1 binding by geminin. Histone acetylation may also affect the initiation of DNA replication, but at present neither the enzymes nor the steps involved are known. Here, we show that Hbo1, a member of the MYST histone acetyltransferase family, is a previously unrecognized positive regulatory factor for pre-RC assembly. When Hbo1 expression was inhibited in human cells, Mcm2-7 failed to associate with chromatin even though ORC and Cdc6 loading was normal. When Xenopus egg extracts were immunodepleted of Xenopus Hbo1 (XHbo1), chromatin binding of Mcm2-7 was lost, and DNA replication was abolished. The binding of Mcm2-7 to chromatin in XHbo1-depleted extracts could be restored by the addition of recombinant Cdt1.

A histone code in meiosis: the histone kinase, NHK-1, is required for proper chromosomal architecture in Drosophila oocytes

To promote faithful propagation of the genetic material during sexual reproduction, meiotic chromosomes undergo specialized morphological changes that ensure accurate segregation of homologous chromosomes. The molecular mechanisms that establish the meiotic chromosomal structures are largely unknown. We describe a mutation in a recently identified Histone H2A kinase, nhk-1, in Drosophila that leads to female sterility due to defects in the formation of the meiotic chromosomal structures. The metaphase I arrest and the karyosome, a critical prophase I chromosomal structure, require nucleosomal histone kinase-1 (NHK-1) function. The defects are a result of failure to disassemble the synaptonemal complex and to load condensin onto the mutant chromosomes. Embryos laid by nhk-1-/- mutant females arrest with aberrant polar bodies and mitotic spindles, revealing that mitosis is affected as well. We analyzed the role of Histone H2A phosphorylation with respect to the histone code hypothesis and found that it is required for acetylation of Histone H3 and Histone H4 in meiosis. These studies reveal a critical role for histone modifications in chromosome dynamics in meiosis and mitosis.

Chromatin decondensation in S-phase involves recruitment of Cdk2 by Cdc45 and histone H1 phosphorylation

Cdc45 is required for initiation of DNA replication and fork progression, but its function in these processes remains unknown. We show that targeting Cdc45 to specific chromosomal sites in mammalian cells results in large-scale chromatin decondensation that strongly correlates with histone H1 phosphorylation. Cdk2 is recruited to sites of Cdc45 decondensation, and Cdk2 inhibitors reduce the level of decondensation. Targeting wild-type Cdk2, but not kinase-defective Cdk2, to chromatin is also effective at inducing decondensation involving phospho-H1. Cdc45, Cdk2, Cyclin A, and phospho-H1 associate with chromatin during S-phase, and Cdc45, Cdk2, and an active H1 kinase physically interact. Replicating DNA and phospho-H1 foci colocalize in vivo, and S-phase progression and H1 phosphorylation are directly related and Cdk2 dependent. Because Cdk2 colocalizes with replication foci and H1 regulates higher-order chromatin, we suggest a model in which Cdc45 recruits Cdk2 to replication foci, resulting in H1 phosphorylation, chromatin decondensation, and facilitation of fork progression.

The dynamic mobility of histone H1 is regulated by cyclin/CDK phosphorylation

The linker histone H1 is involved in maintaining higher-order chromatin structures and displays dynamic nuclear mobility, which may be regulated by posttranslational modifications. To analyze the effect of H1 tail phosphorylation on the modulation of the histone's nuclear dynamics, we generated a mutant histone H1, referred to as M1-5, in which the five cyclin-dependent kinase phosphorylation consensus sites were mutated from serine or threonine residues into alanines. Cyclin E/CDK2 or cyclin A/CDK2 cannot phosphorylate the mutant in vitro. Using the technique of fluorescence recovery after photobleaching, we observed that the mobility of a green fluorescent protein (GFP)-M1-5 fusion protein is decreased compared to that of a GFP-wild-type H1 fusion protein. In addition, recovery of H1 correlated with CDK2 activity, as GFP-H1 mobility was decreased in cells with low CDK2 activity. Blocking the activity of CDK2 by p21 expression decreased the mobility of GFP-H1 but not that of GFP-M1-5. Finally, the level and rate of recovery of cyan fluorescent protein (CFP)-M1-5 were lower than those of CFP-H1 specifically in heterochromatic regions. These data suggest that CDK2 phosphorylates histone H1 in vivo, resulting in a more open chromatin structure by destabilizing H1-chromatin interactions.

Histone H1(S)-3 phosphorylation in Ha-ras oncogene-transformed mouse fibroblasts

Phosphorylation of linker histone H1(S)-3 (previously named H1b) and core histone H3 is elevated in mouse fibroblasts transformed with oncogenes or constitutively active mitogen-activated protein kinase (MAPK) kinase (MEK). H1(S)-3 phosphorylation is the only histone modification known to be dependent upon transcription and replication. Our results show that the increased amounts of phosphorylated H1(S)-3 in the oncogene Ha-ras-transformed mouse fibroblasts was a consequence of an elevated Cdk2 activity rather than the reduced activity of a H1 phosphatase, which our studies suggest is PP1. Induction of oncogenic ras expression results in an increase in H1(S)-3 and H3 phosphorylation. However, in contrast to the phosphorylation of H3, which occurred immediately following the onset of Ras expression, there was a lag of several hours before H1(S)-3 phosphorylation levels increased. We found that there was a transient increase in the levels of p21(cip1), which inhibited the H1 kinase activity of Cdk2. Cdk2 activity and H1(S)-3 phosphorylated levels increased after p21(cip1) levels declined. Our studies suggest that persistent activation of the Ras-MAPK signal transduction pathway in oncogene-transformed cells results in deregulated activity of kinases phosphorylating H3 and H1(S)-3 associated with transcribed genes. The chromatin remodelling actions of these modified histones may result in aberrant gene expression.

Gene knockouts reveal separate functions for two cytoplasmic dyneins in Tetrahymena thermophila

In many organisms, there are multiple isoforms of cytoplasmic dynein heavy chains, and division of labor among the isoforms would provide a mechanism to regulate dynein function. The targeted disruption of somatic genes in Tetrahymena thermophila presents the opportunity to determine the contributions of individual dynein isoforms in a single cell that expresses multiple dynein heavy chain genes. Substantial portions of two Tetrahymena cytoplasmic dynein heavy chain genes were cloned, and their motor domains were sequenced. Tetrahymena DYH1 encodes the ubiquitous cytoplasmic dynein Dyh1, and DYH2 encodes a second cytoplasmic dynein isoform, Dyh2. The disruption of DYH1, but not DYH2, resulted in cells with two detectable defects: 1) phagocytic activity was inhibited, and 2) the cells failed to distribute their chromosomes correctly during micronuclear mitosis. In contrast, the disruption of DYH2 resulted in a loss of regulation of cell size and cell shape and in the apparent inability of the cells to repair their cortical cytoskeletons. We conclude that the two dyneins perform separate tasks in Tetrahymena.

p53 inhibits entry into mitosis when DNA synthesis is blocked

Human and mouse fibroblasts with normal p53 fail to enter mitosis when DNA synthesis is blocked by aphidicolin or hydroxyurea. Isogenic p53-null fibroblasts do enter mitosis with incompletely replicated DNA, revealing that p53 contributes to a checkpoint that ensures that mitosis does not occur until DNA synthesis is complete. When treated with N-(phosphonacetyl)-L-aspartate (PALA), which inhibits pyrimidine nucleotide synthesis, leading to synthesis of damaged DNA from highly unbalanced dNTP pools, p53-null cells enter mitosis after they have completed DNA replication, but cells with wild-type p53 do not, revealing that p53 also mediates a checkpoint that monitors the quality of newly replicated DNA.

An HP1-like protein is missing from transcriptionally silent micronuclei of Tetrahymena

We report the identification and cloning of a 28-kDa polypeptide (p28) in Tetrahymena macronuclei that shares several features with the well studied heterochromatin-associated protein HP1 from Drosophila. Notably, like HP1, p28 contains both a chromodomain and a chromoshadow domain. p28 also shares features with linker histone H1, and like H1, p28 is multiply phosphorylated, at least in part, by a proline-directed, Cdc2-type kinase. As such, p28 is referred to as Hhp1p (for H1/HP1-like protein). Hhp1p is missing from transcriptionally silent micronuclei but is enriched in heterochromatin-like chromatin bodies that presumably comprise repressed chromatin in macronuclei. These findings shed light on the evolutionary conserved nature of heterochromatin in organisms ranging from ciliates to humans and provide further evidence that HP1-like proteins are not exclusively associated with permanently silent chromosomal domains. Our data support a view that members of this family also associate with repressed states of euchromatin.

Entry into mitosis in vertebrate somatic cells is guarded by a chromosome damage checkpoint that reverses the cell cycle when triggered during early but not late prophase

When vertebrate somatic cells are selectively irradiated in the nucleus during late prophase (<30 min before nuclear envelope breakdown) they progress normally through mitosis even if they contain broken chromosomes. However, if early prophase nuclei are similarly irradiated, chromosome condensation is reversed and the cells return to interphase. Thus, the G2 checkpoint that prevents entry into mitosis in response to nuclear damage ceases to function in late prophase. If one nucleus in a cell containing two early prophase nuclei is selectively irradiated, both return to interphase, and prophase cells that have been induced to returned to interphase retain a normal cytoplasmic microtubule complex. Thus, damage to an early prophase nucleus is converted into a signal that not only reverses the nuclear events of prophase, but this signal also enters the cytoplasm where it inhibits e.g., centrosome maturation and the formation of asters. Immunofluorescent analyses reveal that the irradiation-induced reversion of prophase is correlated with the dephosphorylation of histone H1, histone H3, and the MPM2 epitopes. Together, these data reveal that a checkpoint control exists in early but not late prophase in vertebrate cells that, when triggered, reverses the cell cycle by apparently downregulating existing cyclin-dependent kinase (CDK1) activity.

Phosphorylation of linker histone is associated with transcriptional activation in a normally silent nucleus

Previous studies have suggested that micronuclear linker histones are phosphorylated by cAMP-dependent protein kinase (PKA) in Tetrahymena (Sweet, M.T., and C.D. Allis. 1993. Chromosoma. 102: 637-647). In this study, we report that a rapid and dramatic phosphorylation of the micronuclear linker histone, delta, occurs early in the sexual pathway, conjugation. Phosphorylated isoforms of delta are detected as early as 30 min after mixing cells of different mating types; blocking pair formation abolishes this induction completely. Phosphorylation of delta is stimulated by the addition of N6-benzoyladenosine 3':5' cyclic monophosphate to starved (nonmating) cells, suggesting that a PKA/cAMP signal transduction pathway is involved. Maximal phosphorylation of delta is observed during meiotic prophase, a period when micronuclei become transcriptionally active. In situ staining, using phospho-delta-specific antibodies combined with [3H]uridine autoradiography, shows that decondensed micronuclear chromatin undergoing active transcription is enriched in phosphorylated delta isoforms. In contrast, condensed inactive micronuclear chromatin is enriched in dephosphorylated delta. These results strongly suggest that phosphorylation of linker histone plays an important and previously unsuspected role in establishing transcriptional competence in micronuclei.

Increased histone H1 phosphorylation and relaxed chromatin structure in Rb-deficient fibroblasts

Fibroblasts derived from embryos homozygous for a disruption of the retinoblastoma gene (Rb) exhibit a shorter G1 than their wild-type counterparts, apparently due to highly elevated levels of cyclin E protein and deregulated cyclin-dependent kinase 2 (CDK2) activity. Here we demonstrate that the Rb-/- fibroblasts display higher levels of phosphorylated H1 throughout G1 with the maximum being 10-fold higher than that of the Rb+/+ fibroblasts. This profile of intracellular H1 phosphorylation corresponds with deregulated CDK2 activity observed in in vitro assays, suggesting that CDK2 may be directly responsible for the in vivo phosphorylation of H1. H1 phosphorylation has been proposed to lead to a relaxation of chromatin structure due to a decreased affinity of this protein for chromatin after phosphorylation. In accord with this, chromatin from the Rb-/- cells is more susceptible to micrococcal nuclease digestion than that from Rb+/+ fibroblasts. Increased H1 phosphorylation and relaxed chromatin structure have also been observed in cells expressing several oncogenes, suggesting a common mechanism in oncogene and tumor suppressor gene function.

Effects of cell cycle dependent histone H1 phosphorylation on chromatin structure and chromatin replication

We have reconstituted salt-treated SV40 minichromosomes with differentially phosphorylated forms of histone H1 extracted from either G0-, S- or M-phase cells. Sedimentation studies revealed a clear difference between minichromosomes reconstituted with S-phase histone H1 compared with histone H1 from G0- or M-phase cells, indicating that the phosphorylation state of histone H1 has a direct effect on chromatin structure. Using reconstituted minichromosomes as substrate in the SV40 in vitro replication system, we measured a higher replication efficiency for SV40 minichromosomes reconstituted with S-phase histone H1 compared with G0- or M-phase histone H1. These data indicate that the chromatin structure induced by the phosphorylation of histone H1 influences the replication efficiency of SV40 minichromosomes in vitro.

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.

Phosphorylated and dephosphorylated linker histone H1 reside in distinct chromatin domains in Tetrahymena macronuclei

Phosphorylated and dephosphorylated isoforms of Tetrahymena macronuclear H1 were separated from each other by cation-exchange high performance liquid chromatography and used to generate a pairwise set of antisera that discriminate the phosphorylation state of this linker histone. Affinity-purified antibodies from each sera recognize appropriate H1 isoforms and stain macronuclei under appropriate physiological conditions. Immunogold localizations demonstrate that phosphorylated and dephosphorylated H1 localize nonrandomly in distinct subdomains of macronuclear chromatin. Dephosphorylated H1 is strongly enriched in the electron-dense chromatin bodies that punctuate macronuclear chromatin. In contrast, phosphorylated H1 isoforms, as well as an evolutionarily conserved H2A.F/Z-like variant (hv1) believed to function in the establishment of transcriptionally competent chromatin, are modestly enriched at the periphery of chromatin bodies and in the surrounding euchromatin. Using antibodies against TATA-binding protein, we show that transcriptionally active chromatin lies outside of the chromatin bodies in an area relatively devoid of H1. Antibodies against general core histones are more or less evenly distributed across these domains. Together, these data are consistent with a model in which phosphorylation of H1, perhaps in association with hv1, loosens the binding of H1 in chromatin leading to chromatin decondensation as part of a first-step mechanism in gene activation. In contrast, our data support the view that dephosphorylation of this linker histone facilitates or stabilizes condensed, transcriptionally silent chromatin.