Caenorhabditis elegans dosage compensation regulates histone H4 chromatin state on X chromosomes

Distinct regulatory mechanisms by the nuclear Argonautes HRDE-1 and NRDE-3 in the soma of Caenorhabditis elegans

RNA interference (RNAi) is a conserved silencing mechanism that depends on the generation of small RNA molecules that leads to the degradation of the targeted messenger RNAs (mRNAs). Nuclear RNAi is a unique process that triggers regulation through epigenetic alterations to the genome. This pathway has been extensively characterized in Caenorhabditis elegans and involves the nuclear recruitment of H3K9 histone methyltransferases by the Argonautes HRDE-1 and NRDE-3. The coordinate regulation of genetic targets by H3K9 methylation and the nuclear Argonautes is highly complex and has been mainly described based on the small RNA populations that are involved. Recent studies have also linked the nuclear RNAi pathway to the compaction of the hermaphrodite X chromosomes during dosage compensation (DC), a mechanism that balances genetic differences between the biological sexes by repressing X chromosomes in hermaphrodites. This chromosome-wide process provides an excellent opportunity to further investigate the relationship between H3K9 methylation and the nuclear Argonautes. Our work suggests that the nuclear RNAi and the H3K9 methylation pathways each contribute to the condensation of the X chromosomes during DC but the consequences on the transcriptional output of X-linked genes are minimal. Instead, nuclear RNAi mutants exhibit global transcriptional differences, in which HRDE-1 and NRDE-3 affect expression of their mRNA targets through different relationships to H3K9 methylation.

Condensin IDC, DPY-21, and CEC-4 maintain X chromosome repression in C. elegans

Dosage compensation in Caenorhabditis elegans equalizes X-linked gene expression between XX hermaphrodites and XO males. The process depends on a condensin-containing dosage compensation complex (DCC), which binds the X chromosomes in hermaphrodites to repress gene expression by a factor of 2. Condensin IDC and an additional five DCC components must be present on the X during early embryogenesis in hermaphrodites to establish dosage compensation. However, whether the DCC's continued presence is required to maintain the repressed state once established is unknown. Beyond the role of condensin IDC in X chromosome compaction, additional mechanisms contribute to X-linked gene repression. DPY-21, a non-condensin IDC DCC component, is an H4K20me2/3 demethylase whose activity enriches the repressive histone mark, H4 lysine 20 monomethylation, on the X chromosomes. In addition, CEC-4, a protein that tethers H3K9me3-rich chromosomal regions to the nuclear lamina, also contributes to X-linked gene repression. To investigate the necessity of condensin IDC during the larval and adult stages of hermaphrodites, we used the auxin-inducible degradation system to deplete the condensin IDC subunit DPY-27. While DPY-27 depletion in the embryonic stages resulted in lethality, DPY-27 depleted larvae and adults survive. In these DPY-27 depleted strains, condensin IDC was no longer associated with the X chromosome, the X became decondensed, and the H4K20me1 mark was gradually lost, leading to X-linked gene derepression (about 1.4-fold). These results suggest that the stable maintenance of dosage compensation requires the continued presence of condensin IDC. A loss-of-function mutation in cec-4, in addition to the depletion of DPY-27 or the genetic mutation of dpy-21, led to even more significant increases in X-linked gene expression (about 1.7-fold), suggesting that CEC-4 helps stabilize repression mediated by condensin IDC and H4K20me1.

Multi-level transcriptional regulation of embryonic sex determination and dosage compensation by the X-signal element sex-1

The C. elegans nuclear hormone receptor sex-1 is known to be an embryonic X-signal element that represses xol-1, the sex-switch gene that is the master regulator of sex determination and dosage compensation. Several prior studies on sex-1 function have suggested that sex-1 may have additional downstream roles beyond the regulation of xol-1 expression. In this study we characterize some of these additional roles of sex-1 in regulating the dual processes of sex determination and dosage compensation during embryogenesis. Our study reveals that sex-1 acts on many of the downstream targets of xol-1 in a xol-1-independent manner. Further analysis of these shared but independently regulated downstream targets uncovered that sex-1 mediates the expression of hermaphrodite- and male-biased genes during embryogenesis. We validated sex-1 binding on one of these downstream targets, the male-developmental gene her-1. Our data suggests a model where sex-1 exhibits multi-level direct transcriptional regulation on several targets, including xol-1 and genes downstream of xol-1, to reinforce the appropriate expression of sex-biased transcripts in XX embryos. Furthermore, we found that xol-1 sex-1 double mutants show defects in dosage compensation. Our study provides evidence that misregulation of dpy-21, one of the components of the dosage compensation complex, and the subsequent misregulation of H4K20me1 enrichment on the X chromosomes, may contribute to this defect.

Distinct regulatory mechanisms by the nuclear Argonautes HRDE-1 and NRDE-3 in the soma of Caenorhabditis elegans

RNA interference is a conserved silencing mechanism that depends on the generation of small RNA molecules that disrupt synthesis of their corresponding transcripts. Nuclear RNA interference is a unique process that triggers regulation through epigenetic alterations to the genome. This pathway has been extensively characterized in Caenorhabditis elegans and involves the nuclear recruitment of H3K9 histone methyltransferases by the Argonautes HRDE-1 and NRDE-3. The coordinate regulation of genetic targets by H3K9 methylation and the nuclear Argonautes is highly complex and has been mainly described based on the small RNA populations that are involved. Recent studies have also linked the nuclear RNAi pathway to the compaction of the hermaphrodite X chromosomes during dosage compensation, a mechanism that balances genetic differences between the biological sexes by repressing X chromosomes in hermaphrodites. This chromosome-wide process provides an excellent opportunity to further investigate the relationship between H3K9 methylation and the nuclear Argonautes from the perspective of the transcriptome. Our work suggests that the nuclear RNAi and the H3K9 methylation pathways each contribute to the condensation of the X chromosomes during dosage compensation but the consequences on their transcriptional output are minimal. Instead, nuclear RNAi mutants exhibit global transcriptional differences, in which HRDE-1 and NRDE-3 affect expression of their native targets through different modes of regulation and different relationships to H3K9 methylation.

ARTICLE SUMMARY

This study examines the transcriptional consequences during the disruption of the nuclear RNAi silencing mechanism in C. elegans . Through microscopy and bioinformatic work, we demonstrate that although nuclear RNAi mutants exhibit significantly decondensed X chromosomes, chromosome-wide transcriptional de-repression is not detectable. Downstream analyses further explore the global influence of the nuclear RNAi pathway, indicating that the nuclear Argonautes HRDE-1 and NRDE-3 function through two distinct mechanisms.

The chromatin-associated 53BP1 ortholog, HSR-9, regulates recombinational repair and X chromosome segregation in the Caenorhabditis elegans germ line

53BP1 plays a crucial role in regulating DNA damage repair pathway choice and checkpoint signaling in somatic cells; however, its role in meiosis has remained enigmatic. In this study, we demonstrate that the Caenorhabditis elegans ortholog of 53BP1, HSR-9, associates with chromatin in both proliferating and meiotic germ cells. Notably, HSR-9 is enriched on the X chromosome pair in pachytene oogenic germ cells. HSR-9 is also present at kinetochores during both mitotic and meiotic divisions but does not appear to be essential for monitoring microtubule-kinetochore attachments or tension. Using cytological markers of different steps in recombinational repair, we found that HSR-9 influences the processing of a subset of meiotic double-stranded breaks into COSA-1-marked crossovers. Additionally, HSR-9 plays a role in meiotic X chromosome segregation under conditions where X chromosomes fail to pair, synapse, and recombine. Together, these results highlight that chromatin-associated HSR-9 has both conserved and unique functions in the regulation of meiotic chromosome behavior.

XOL-1 regulates developmental timing by modulating the H3K9 landscape in C. elegans early embryos

Sex determination in the nematode C. elegans is controlled by the master regulator XOL-1 during embryogenesis. Expression of xol-1 is dependent on the ratio of X chromosomes and autosomes, which differs between XX hermaphrodites and XO males. In males, xol-1 is highly expressed and in hermaphrodites, xol-1 is expressed at very low levels. XOL-1 activity is known to be critical for the proper development of C. elegans males, but its low expression was considered to be of minimal importance in the development of hermaphrodite embryos. Our study reveals that XOL-1 plays an important role as a regulator of developmental timing during hermaphrodite embryogenesis. Using a combination of imaging and bioinformatics techniques, we found that hermaphrodite embryos have an accelerated rate of cell division, as well as a more developmentally advanced transcriptional program when xol-1 is lost. Further analyses reveal that XOL-1 is responsible for regulating the timing of initiation of dosage compensation on the X chromosomes, and the appropriate expression of sex-biased transcriptional programs in hermaphrodites. We found that xol-1 mutant embryos overexpress the H3K9 methyltransferase MET-2 and have an altered H3K9me landscape. Some of these effects of the loss of xol-1 gene were reversed by the loss of met-2. These findings demonstrate that XOL-1 plays an important role as a developmental regulator in embryos of both sexes, and that MET-2 acts as a downstream effector of XOL-1 activity in hermaphrodites.

SMC-mediated dosage compensation in C. elegans evolved in the presence of an ancestral nematode mechanism

Mechanisms of X chromosome dosage compensation have been studied extensively in a few model species representing clades of shared sex chromosome ancestry. However, the diversity within each clade as a function of sex chromosome evolution is largely unknown. Here, we anchor ourselves to the nematode Caenorhabditis elegans, for which a well-studied mechanism of dosage compensation occurs through a specialized structural maintenance of chromosomes (SMC) complex, and explore the diversity of dosage compensation in the surrounding phylogeny of nematodes. Through phylogenetic analysis of the C. elegans dosage compensation complex and a survey of its epigenetic signatures, including X-specific topologically associating domains (TADs) and X-enrichment of H4K20me1, we found that the condensin-mediated mechanism evolved recently in the lineage leading to Caenorhabditis through an SMC-4 duplication. Intriguingly, an independent duplication of SMC-4 and the presence of X-specific TADs in Pristionchus pacificus suggest that condensin-mediated dosage compensation arose more than once. mRNA-seq analyses of gene expression in several nematode species indicate that dosage compensation itself is ancestral, as expected from the ancient XO sex determination system. Indicative of the ancestral mechanism, H4K20me1 is enriched on the X chromosomes in Oscheius tipulae, which does not contain X-specific TADs or SMC-4 paralogs. Together, our results indicate that the dosage compensation system in C. elegans is surprisingly new, and condensin may have been co-opted repeatedly in nematodes, suggesting that the process of evolving a chromosome-wide gene regulatory mechanism for dosage compensation is constrained.

Significance statement

X chromosome dosage compensation mechanisms evolved in response to Y chromosome degeneration during sex chromosome evolution. However, establishment of dosage compensation is not an endpoint. As sex chromosomes change, dosage compensation strategies may have also changed. In this study, we performed phylogenetic and epigenomic analyses surrounding Caenorhabditis elegans and found that the condensin-mediated dosage compensation mechanism in C. elegans is surprisingly new, and has evolved in the presence of an ancestral mechanism. Intriguingly, condensin-based dosage compensation may have evolved more than once in the nematode lineage, the other time in Pristionchus. Together, our work highlights a previously unappreciated diversity of dosage compensation mechanisms within a clade, and suggests constraints in evolving new mechanisms in the presence of an existing one.

Analysis of developmental gene expression using smFISH and in silico staging of C. elegans embryos

Regulation of transcription during embryogenesis is key to development and differentiation. To study transcript expression throughout Caenorhabditis elegans embryogenesis at single-molecule resolution, we developed a high-throughput single-molecule fluorescence in situ hybridization (smFISH) method that relies on computational methods to developmentally stage embryos and quantify individual mRNA molecules in single embryos. We applied our system to sdc-2, a zygotically transcribed gene essential for hermaphrodite development and dosage compensation. We found that sdc-2 is rapidly activated during early embryogenesis by increasing both the number of mRNAs produced per transcription site and the frequency of sites engaged in transcription. Knockdown of sdc-2 and dpy-27, a subunit of the dosage compensation complex (DCC), increased the number of active transcription sites for the X chromosomal gene dpy-23 but not the autosomal gene mdh-1, suggesting that the DCC reduces the frequency of dpy-23 transcription. The temporal resolution from in silico staging of embryos showed that the deletion of a single DCC recruitment element near the dpy-23 gene causes higher dpy-23 mRNA expression after the start of dosage compensation, which could not be resolved using mRNAseq from mixed-stage embryos. In summary, we have established a computational approach to quantify temporal regulation of transcription throughout C. elegans embryogenesis and demonstrated its potential to provide new insights into developmental gene regulation.

The chromatin-associated 53BP1 ortholog, HSR-9, regulates recombinational repair and X chromosome segregation in the Caenorhabditis elegans germ line

53BP1 plays a crucial role in regulating DNA damage repair pathway choice and checkpoint signaling in somatic cells; however, its role in meiosis has remained enigmatic. In this study, we demonstrate that the Caenorhabditis elegans ortholog of 53BP1, HSR-9, associates with chromatin in both proliferating and meiotic germ cells. Notably, HSR-9 is enriched on the X chromosome pair in pachytene oogenic germ cells. HSR-9 is also present at kinetochores during both mitotic and meiotic divisions but does not appear to be essential for monitoring microtubule-kinetochore attachments or tension. Using cytological markers of different steps in recombinational repair, we found that HSR-9 influences the processing of a subset of meiotic double strand breaks into COSA-1-marked crossovers. Additionally, HSR-9 plays a role in meiotic X chromosome segregation under conditions where X chromosomes fail to pair, synapse, and recombine. Together, these results highlight that chromatin-associated HSR-9 has both conserved and unique functions in the regulation of meiotic chromosome behavior.

How Chromatin Motor Complexes Influence the Nuclear Architecture: A Review of Chromatin Organization, Cohesins, and Condensins with a Focus on C. elegans

Chromatin is the complex of DNA and associated proteins found in the nuclei of living organisms. How it is organized is a major research field as it has implications for replication, repair, and gene expression. This review summarizes the current state of the chromatin organization field, with a special focus on chromatin motor complexes cohesin and condensin. Containing the highly conserved SMC proteins, these complexes are responsible for organizing chromatin during cell division. Additionally, research has demonstrated that condensin and cohesin also have important functions during interphase to shape the organization of chromatin and regulate expression of genes. Using the model organism C. elegans, the authors review the current knowledge of how these complexes perform such diverse roles and what open questions still exist in the field.

Chromatin Organization during C. elegans Early Development

Embryogenesis is characterized by dynamic chromatin remodeling and broad changes in chromosome architecture. These changes in chromatin organization are accompanied by transcriptional changes, which are crucial for the proper development of the embryo. Several independent mechanisms regulate this process of chromatin reorganization, including segregation of chromatin into heterochromatin and euchromatin, deposition of active and repressive histone modifications, and the formation of 3D chromatin domains such as TADs and LADs. These changes in chromatin structure are directly linked to developmental milestones such as the loss of developmental plasticity and acquisition of terminally differentiated cell identities. In this review we summarize these processes that underlie this chromatin reorganization and their impact on embryogenesis in the nematode C. elegans.

Relating aging and autophagy: a new perspective towards the welfare of human health

The most common factor that contributes to aging is the loss of proteostasis, resulting in an excess amount of non-functional/damaged proteins. These proteins lead to various age-associated phenotypes such as cellular senescence and dysfunction in the nutrient-sensing pathways. Despite the various factors that can contribute to aging, it is still a process that can be changed. According to recent advances in the field of biology, the ability to alter the pathways that are involved in aging can improve the lifespan of a person. Autophagy is a process that helps in preserving survival during stressful situations, such as starvation. It is a common component of various anti-aging interventions, including those that target the insulin/IGF-1 and rapamycin signaling pathways. It has been shown that altered autophagy is a common feature of old age and its impaired regulation could have significant effects on the aging process. This review aims to look into the role of autophagy in aging and how it can be used to improve one's health.

The X chromosome in C. elegans sex determination and dosage compensation

Abnormalities in chromosome dose can reduce organismal fitness and viability by disrupting the balance of gene expression. Unlike imbalances in chromosome dose that cause pathologies, differences in X-chromosome dose that determine sex are well tolerated. Dosage compensation mechanisms have evolved in diverse species to balance X-chromosome gene expression between sexes. Mechanisms underlying nematode X-chromosome counting to determine sex revealed how small quantitative differences in molecular signals are translated into dramatically different developmental fates. Mechanisms underlying X-chromosome dosage compensation revealed the interplay between chromatin modification and three-dimensional chromosome structure imposed by an X-specific condensin complex to regulate gene expression over vast chromosomal territories. In a surprising twist of evolution, this dosage-compensation condensin complex also regulates lifespan and tolerance to proteotoxic stress.

Dual roles for nuclear RNAi Argonautes in Caenorhabditis elegans dosage compensation

Dosage compensation involves chromosome-wide gene regulatory mechanisms which impact higher order chromatin structure and are crucial for organismal health. Using a genetic approach, we identified Argonaute genes which promote dosage compensation in Caenorhabditis elegans. Dosage compensation in C. elegans hermaphrodites is initiated by the silencing of xol-1 and subsequent activation of the dosage compensation complex which binds to both hermaphrodite X chromosomes and reduces transcriptional output by half. A hallmark phenotype of dosage compensation mutants is decondensation of the X chromosomes. We characterized this phenotype in Argonaute mutants using X chromosome paint probes and fluorescence microscopy. We found that while nuclear Argonaute mutants hrde-1 and nrde-3, as well as mutants for the piRNA Argonaute prg-1, exhibit derepression of xol-1 transcripts, they also affect X chromosome condensation in a xol-1-independent manner. We also characterized the physiological contribution of Argonaute genes to dosage compensation using genetic assays and found that hrde-1 and nrde-3 contribute to healthy dosage compensation both upstream and downstream of xol-1.

The histone H4 lysine 20 demethylase DPY-21 regulates the dynamics of condensin DC binding

Condensin is a multi-subunit structural maintenance of chromosomes (SMC) complex that binds to and compacts chromosomes. Here, we addressed the regulation of condensin binding dynamics using Caenorhabditis elegans condensin DC, which represses X chromosomes in hermaphrodites for dosage compensation. We established fluorescence recovery after photobleaching (FRAP) using the SMC4 homolog DPY-27 and showed that a well-characterized ATPase mutation abolishes DPY-27 binding to X chromosomes. Next, we performed FRAP in the background of several chromatin modifier mutants that cause varying degrees of X chromosome derepression. The greatest effect was in a null mutant of the H4K20me2 demethylase DPY-21, where the mobile fraction of condensin DC reduced from ∼30% to 10%. In contrast, a catalytic mutant of dpy-21 did not regulate condensin DC mobility. Hi-C sequencing data from the dpy-21 null mutant showed little change compared to wild-type data, uncoupling Hi-C-measured long-range DNA contacts from transcriptional repression of the X chromosomes. Taken together, our results indicate that DPY-21 has a non-catalytic role in regulating the dynamics of condensin DC binding, which is important for transcription repression.

Mechanisms of sex determination and X-chromosome dosage compensation

Abnormalities in chromosome number have the potential to disrupt the balance of gene expression and thereby decrease organismal fitness and viability. Such abnormalities occur in most solid tumors and also cause severe developmental defects and spontaneous abortions. In contrast to the imbalances in chromosome dose that cause pathologies, the difference in X-chromosome dose used to determine sexual fate across diverse species is well tolerated. Dosage compensation mechanisms have evolved in such species to balance X-chromosome gene expression between the sexes, allowing them to tolerate the difference in X-chromosome dose. This review analyzes the chromosome counting mechanism that tallies X-chromosome number to determine sex (XO male and XX hermaphrodite) in the nematode Caenorhabditis elegans and the associated dosage compensation mechanism that balances X-chromosome gene expression between the sexes. Dissecting the molecular mechanisms underlying X-chromosome counting has revealed how small quantitative differences in intracellular signals can be translated into dramatically different fates. Dissecting the process of X-chromosome dosage compensation has revealed the interplay between chromatin modification and chromosome structure in regulating gene expression over vast chromosomal territories.

Local chromatin fiber folding represses transcription and loop extrusion in quiescent cells

A longstanding hypothesis is that chromatin fiber folding mediated by interactions between nearby nucleosomes represses transcription. However, it has been difficult to determine the relationship between local chromatin fiber compaction and transcription in cells. Further, global changes in fiber diameters have not been observed, even between interphase and mitotic chromosomes. We show that an increase in the range of local inter-nucleosomal contacts in quiescent yeast drives the compaction of chromatin fibers genome-wide. Unlike actively dividing cells, inter-nucleosomal interactions in quiescent cells require a basic patch in the histone H4 tail. This quiescence-specific fiber folding globally represses transcription and inhibits chromatin loop extrusion by condensin. These results reveal that global changes in chromatin fiber compaction can occur during cell state transitions, and establish physiological roles for local chromatin fiber folding in regulating transcription and chromatin domain formation.

Interaction of Male Specific Lethal complex and genomic imbalance on global gene expression in Drosophila

The inverse dosage effect caused by chromosome number variations shows global consequences in genomic imbalance including sexual dimorphism and an X chromosome-specific response. To investigate the relationship of the MSL complex to genomic imbalance, we over-expressed MSL2 in autosomal and sex chromosomal aneuploids, and analyzed the different transcriptomes. Some candidate genes involved in regulatory mechanisms have also been tested during embryogenesis using TSA-FISH. Here we show that the de novo MSL complex assembled on the X chromosomes in females further reduced the global expression level on the basis of 2/3 down-regulation caused by the inverse dosage effect in trisomy through epigenetic modulations rather than induced dosage compensation. Plus, the sexual dimorphism effect in unbalanced genomes was further examined due to the pre-existing of the MSL complex in males. All these results demonstrate the dynamic functions of the MSL complex on global gene expression in different aneuploid genomes.

Comparative distributions of RSBN1 and methylated histone H4 Lysine 20 in the mouse spermatogenesis

During spermatogenesis, nuclear architecture of male germ cells is dynamically changed and epigenetic modifications, in particular methylation of histones, highly contribute to its regulation as well as differentiation of male germ cells. Although several methyltransferases and demethylases for histone H3 are involved in the regulation of spermatogenesis, roles of either histone H4 lysine 20 (H4K20) methyltransferases or H4K20 demethylases during spermatogenesis still remain to be elucidated. Recently, RSBN1 which is a testis-specific gene expressed in round spermatids was identified as a demethylase for dimethyl H4K20. In this study, therefore, we confirm the demethylase function of RSBN1 and compare distributions between RSBN1 and methylated H4K20 in the seminiferous tubules. Unlike previous report, expression analyses for RSBN1 reveal that RSBN1 is not a testis-specific gene and is expressed not only in round spermatids but also in elongated spermatids. In addition, RSBN1 can demethylate not only dimethyl H4K20 but also trimethyl H4K20 and could convert both dimethyl H4K20 and trimethyl H4K20 into monomethyl H4K20. When distribution pattern of RSBN1 in the seminiferous tubule is compared to that of methylated H4K20, both dimethyl H4K20 and trimethyl H4K20 but not monomethyl H4K20 are disappeared from RSBN1 positive germ cells, suggesting that testis-specific distribution patterns of methylated H4K20 might be constructed by RSBN1. Thus, novel expression and function of RSBN1 could be useful to comprehend epigenetic regulation during spermatogenesis.

Genome assembly, sex-biased gene expression and dosage compensation in the damselfly Ischnura elegans

The evolution of sex chromosomes, and patterns of sex-biased gene expression and dosage compensation, are poorly known among early winged insects such as odonates. We assembled and annotated the genome of Ischnura elegans (blue-tailed damselfly), which, like other odonates, has a male-hemigametic sex-determining system (X0 males, XX females). By identifying X-linked genes in I. elegans and their orthologs in other insect genomes, we found homologies between the X chromosome in odonates and chromosomes of other orders, including the X chromosome in Coleoptera. Next, we showed balanced expression of X-linked genes between sexes in adult I. elegans, i.e. evidence of dosage compensation. Finally, among the genes in the sex-determining pathway only fruitless was found to be X-linked, while only doublesex showed sex-biased expression. This study reveals partly conserved sex chromosome synteny and independent evolution of dosage compensation among insect orders separated by several hundred million years of evolutionary history.

Histone post-translational modifications in Silene latifolia X and Y chromosomes suggest a mammal-like dosage compensation system

Silene latifolia is a model organism to study evolutionary young heteromorphic sex chromosome evolution in plants. Previous research indicates a Y-allele gene degeneration and a dosage compensation system already operating. Here, we propose an epigenetic approach based on analysis of several histone post-translational modifications (PTMs) to find the first epigenetic hints of the X:Y sex chromosome system regulation in S. latifolia. Through chromatin immunoprecipitation we interrogated six genes from X and Y alleles. Several histone PTMS linked to DNA methylation and transcriptional repression (H3K27me3, H3K23me, H3K9me2 and H3K9me3) and to transcriptional activation (H3K4me3 and H4K5, 8, 12, 16ac) were used. DNA enrichment (Immunoprecipitated DNA/input DNA) was analyzed and showed three main results: (i) promoters of the Y allele are associated with heterochromatin marks, (ii) promoters of the X allele in males are associated with activation of transcription marks and finally, (iii) promoters of X alleles in females are associated with active and repressive marks. Our finding indicates a transcription activation of X allele and transcription repression of Y allele in males. In females we found a possible differential regulation (up X₁, down X₂) of each female X allele. These results agree with the mammal-like epigenetic dosage compensation regulation.

Binding of an X-Specific Condensin Correlates with a Reduction in Active Histone Modifications at Gene Regulatory Elements

Condensins are evolutionarily conserved protein complexes that are required for chromosome segregation during cell division and genome organization during interphase. In Caenorhabditis elegans, a specialized condensin, which forms the core of the dosage compensation complex (DCC), binds to and represses X chromosome transcription. Here, we analyzed DCC localization and the effect of DCC depletion on histone modifications, transcription factor binding, and gene expression using chromatin immunoprecipitation sequencing and mRNA sequencing. Across the X, the DCC accumulates at accessible gene regulatory sites in active chromatin and not heterochromatin. The DCC is required for reducing the levels of activating histone modifications, including H3K4me3 and H3K27ac, but not repressive modification H3K9me3. In X-to-autosome fusion chromosomes, DCC spreading into the autosomal sequences locally reduces gene expression, thus establishing a direct link between DCC binding and repression. Together, our results indicate that DCC-mediated transcription repression is associated with a reduction in the activity of X chromosomal gene regulatory elements.

Diverse Genome Topologies Characterize Dosage Compensation across Species

Dosage compensation is the process by which transcript levels of the X chromosome are equalized with those of autosomes. Although diverse mechanisms of dosage compensation have evolved across species, these mechanisms all involve distinguishing the X chromosome from autosomes. Because one chromosome is singled out from other chromosomes for precise regulation, dosage compensation serves as an important model for understanding how specific cis-elements are identified within the highly compacted 3D genome to co-regulate thousands of genes. Recently, multiple genomic approaches have provided key insights into the mechanisms of dosage compensation, extending what we have learned from classical genetic studies. In the future, newer genomic approaches that require little starting material show great promise to provide an understanding of the heterogeneity of dosage compensation between cells and how it functions in nonmodel organisms.

Sex and death: from cell fate specification to dynamic control of X-chromosome structure and gene expression

Determining sex is a binary developmental decision that most metazoans must make. Like many organisms, Caenorhabditis elegans specifies sex (XO male or XX hermaphrodite) by tallying X-chromosome number. We dissected this precise counting mechanism to determine how tiny differences in concentrations of signals are translated into dramatically different developmental fates. Determining sex by counting chromosomes solved one problem but created another-an imbalance in X gene products. We found that nematodes compensate for the difference in X-chromosome dose between sexes by reducing transcription from both hermaphrodite X chromosomes. In a surprising feat of evolution, X-chromosome regulation is functionally related to a structural problem of all mitotic and meiotic chromosomes: achieving ordered compaction of chromosomes before segregation. We showed the dosage compensation complex is a condensin complex that imposes a specific three--dimensional architecture onto hermaphrodite X chromosomes. It also triggers enrichment of histone modification H4K20me1. We discovered the machinery and mechanism underlying H4K20me1 enrichment and demonstrated its pivotal role in regulating higher-order X-chromosome structure and gene expression.

Condensin action and compaction

Condensin is a multi-subunit protein complex that belongs to the family of structural maintenance of chromosomes (SMC) complexes. Condensins regulate chromosome structure in a wide range of processes including chromosome segregation, gene regulation, DNA repair and recombination. Recent research defined the structural features and molecular activities of condensins, but it is unclear how these activities are connected to the multitude of phenotypes and functions attributed to condensins. In this review, we briefly discuss the different molecular mechanisms by which condensins may regulate global chromosome compaction, organization of topologically associated domains, clustering of specific loci such as tRNA genes, rDNA segregation, and gene regulation.

Non-canonical Drosophila X chromosome dosage compensation and repressive topologically associated domains

Background

In animals with XY sex chromosomes, X-linked genes from a single X chromosome in males are imbalanced relative to autosomal genes. To minimize the impact of genic imbalance in male Drosophila, there is a dosage compensation complex (MSL) that equilibrates X-linked gene expression with the autosomes. There are other potential contributions to dosage compensation. Hemizygous autosomal genes located in repressive chromatin domains are often derepressed. If this homolog-dependent repression occurs on the X, which has no pairing partner, then derepression could contribute to male dosage compensation.

Results

We asked whether different chromatin states or topological associations correlate with X chromosome dosage compensation, especially in regions with little MSL occupancy. Our analyses demonstrated that male X chromosome genes that are located in repressive chromatin states are depleted of MSL occupancy; however, they show dosage compensation. The genes in these repressive regions were also less sensitive to knockdown of MSL components.

Conclusions

Our results suggest that this non-canonical dosage compensation is due to the same transacting derepression that occurs on autosomes. This mechanism would facilitate immediate compensation during the evolution of sex chromosomes from autosomes. This mechanism is similar to that of C. elegans, where enhanced recruitment of X chromosomes to the nuclear lamina dampens X chromosome expression as part of the dosage compensation response in XX individuals.

Electronic supplementary material

The online version of this article (10.1186/s13072-018-0232-y) contains supplementary material, which is available to authorized users.

A Simple Method for Visualization of Locus-Specific H4K20me1 Modifications in Living Caenorhabditis elegans Single Cells

Recently, advances in next-generation sequencing technologies have enabled genome-wide analyses of epigenetic modifications; however, it remains difficult to analyze the states of histone modifications at a single-cell resolution in living multicellular organisms because of the heterogeneity within cellular populations. Here we describe a simple method to visualize histone modifications on the specific sequence of target locus at a single-cell resolution in living Caenorhabditis elegans, by combining the LacO/LacI system and a genetically-encoded H4K20me1-specific probe, “mintbody”. We demonstrate that Venus-labeled mintbody and mTurquoise2-labeled LacI can co-localize on an artificial chromosome carrying both the target locus and LacO sequences, where H4K20me1 marks the target locus. We demonstrate that our visualization method can precisely detect H4K20me1 depositions on the her-1 gene sequences on the artificial chromosome, to which the dosage compensation complex binds to regulate sex determination. The degree of H4K20me1 deposition on the her-1 sequences on the artificial chromosome correlated strongly with sex, suggesting that, using the artificial chromosome, this method can reflect context-dependent changes of H4K20me1 on endogenous genomes. Furthermore, we demonstrate live imaging of H4K20me1 depositions on the artificial chromosome. Combined with ChIP assays, this mintbody-LacO/LacI visualization method will enable analysis of developmental and context-dependent alterations of locus-specific histone modifications in specific cells and elucidation of the underlying molecular mechanisms.

Dynamic Control of Chromosome Topology and Gene Expression by a Chromatin Modification

The function of chromatin modification in establishing higher-order chromosome structure during gene regulation has been elusive. We dissected the machinery and mechanism underlying the enrichment of histone modification H4K20me1 on hermaphrodite X chromosomes during Caenorhabditis elegans dosage compensation and discovered a key role for H4K20me1 in regulating X-chromosome topology and chromosome-wide gene expression. Structural and functional analysis of the dosage compensation complex (DCC) subunit DPY-21 revealed a novel Jumonji C demethylase subfamily that converts H4K20me2 to H4K20me1 in worms and mammals. Inactivation of demethylase activity in vivo by genome editing eliminated H4K20me1 enrichment on X chromosomes of somatic cells, increased X-linked gene expression, reduced X-chromosome compaction, and disrupted X-chromosome conformation by diminishing the formation of topologically associated domains. H4K20me1 is also enriched on the inactive X of female mice, making our studies directly relevant to mammalian development. Unexpectedly, DPY-21 also associates specifically with autosomes of nematode germ cells in a DCC-independent manner to enrich H4K20me1 and trigger chromosome compaction. Thus, DPY-21 is an adaptable chromatin regulator. Its H4K20me2 demethylase activity can be harnessed during development for distinct biological functions by targeting it to diverse genomic locations through different mechanisms. In both somatic cells and germ cells, H4K20me1 enrichment modulates three-dimensional chromosome architecture, demonstrating the direct link between chromatin modification and higher-order chromosome structure.

Taking cohesin and condensin in context

Structural maintenance of chromosome (SMC) protein complexes, including cohesin and condensin, are increasingly being recognized for their important role in cancer and development, making it critical that we understand how these evolutionarily conserved multi-subunit protein complexes associate with and organize the genome. We review adaptor proteins for SMC complexes and how these adaptors may capture SMC complexes following loop extrusion to provide a framework for chromosome organization.

Caenorhabditis elegans Dosage Compensation: Insights into Condensin-Mediated Gene Regulation

Recent work demonstrating the role of chromosome organization in transcriptional regulation has sparked substantial interest in the molecular mechanisms that control chromosome structure. Condensin, an evolutionarily conserved multisubunit protein complex, is essential for chromosome condensation during cell division and functions in regulating gene expression during interphase. In Caenorhabditis elegans, a specialized condensin forms the core of the dosage compensation complex (DCC), which specifically binds to and represses transcription from the hermaphrodite X chromosomes. DCC serves as a clear paradigm for addressing how condensins target large chromosomal domains and how they function to regulate chromosome structure and transcription. Here, we discuss recent research on C. elegans DCC in the context of canonical condensin mechanisms as have been studied in various organisms.

Dynamic Control of X Chromosome Conformation and Repression by a Histone H4K20 Demethylase

Chromatin modification and higher-order chromosome structure play key roles in gene regulation, but their functional interplay in controlling gene expression is elusive. We have discovered the machinery and mechanism underlying the dynamic enrichment of histone modification H4K20me1 on hermaphrodite X chromosomes during C. elegans dosage compensation and demonstrated H4K20me1's pivotal role in regulating higher-order chromosome structure and X-chromosome-wide gene expression. The structure and the activity of the dosage compensation complex (DCC) subunit DPY-21 define a Jumonji demethylase subfamily that converts H4K20me2 to H4K20me1 in worms and mammals. Selective inactivation of demethylase activity eliminates H4K20me1 enrichment in somatic cells, elevates X-linked gene expression, reduces X chromosome compaction, and disrupts X chromosome conformation by diminishing the formation of topologically associating domains (TADs). Unexpectedly, DPY-21 also associates with autosomes of germ cells in a DCC-independent manner to enrich H4K20me1 and trigger chromosome compaction. Our findings demonstrate the direct link between chromatin modification and higher-order chromosome structure in long-range regulation of gene expression.

Reduction in chromosome mobility accompanies nuclear organization during early embryogenesis in Caenorhabditis elegans

In differentiated cells, chromosomes are packed inside the cell nucleus in an organised fashion. In contrast, little is known about how chromosomes are packed in undifferentiated cells and how nuclear organization changes during development. To assess changes in nuclear organization during the earliest stages of development, we quantified the mobility of a pair of homologous chromosomal loci in the interphase nuclei of Caenorhabditis elegans embryos. The distribution of distances between homologous loci was consistent with a random distribution up to the 8-cell stage but not at later stages. The mobility of the loci was significantly reduced from the 2-cell to the 48-cell stage. Nuclear foci corresponding to epigenetic marks as well as heterochromatin and the nucleolus also appeared around the 8-cell stage. We propose that the earliest global transformation in nuclear organization occurs at the 8-cell stage during C. elegans embryogenesis.

Dosage compensation and sex-specific epigenetic landscape of the X chromosome in the pea aphid

Background

Heterogametic species display a differential number of sex chromosomes resulting in imbalanced transcription levels for these chromosomes between males and females. To correct this disequilibrium, dosage compensation mechanisms involving gene expression and chromatin accessibility regulations have emerged throughout evolution. In insects, these mechanisms have been extensively characterized only in Drosophila but not in insects of agronomical importance. Aphids are indeed major pests of a wide range of crops. Their remarkable ability to switch from asexual to sexual reproduction during their life cycle largely explains the economic losses they can cause. As heterogametic insects, male aphids are X0, while females (asexual and sexual) are XX.

Results

Here, we analyzed transcriptomic and open chromatin data obtained from whole male and female individuals to evaluate the putative existence of a dosage compensation mechanism involving differential chromatin accessibility of the pea aphid’s X chromosome. Transcriptomic analyses first showed X/AA and XX/AA expression ratios for expressed genes close to 1 in males and females, respectively, suggesting dosage compensation in the pea aphid. Analyses of open chromatin data obtained by Formaldehyde-Assisted Isolation of Regulatory Elements (FAIRE-seq) revealed a X chromosome chromatin accessibility globally and significantly higher in males than in females, while autosomes’ chromatin accessibility is similar between sexes. Moreover, chromatin environment of X-linked genes displaying similar expression levels in males and females—and thus likely to be compensated—is significantly more accessible in males.

Conclusions

Our results suggest the existence of an underlying epigenetic mechanism enhancing the X chromosome chromatin accessibility in males to allow X-linked gene dose correction between sexes in the pea aphid, similar to Drosophila. Our study gives new evidence into the comprehension of dosage compensation in link with chromatin biology in insects and newly in a major crop pest, taking benefits from both transcriptomic and open chromatin data.

Electronic supplementary material

The online version of this article (doi:10.1186/s13072-017-0137-1) contains supplementary material, which is available to authorized users.

A histone H4 lysine 20 methyltransferase couples environmental cues to sensory neuron control of developmental plasticity

Animals change developmental fates in response to external cues. In the nematode Caenorhabditis elegans, unfavorable environmental conditions induce a state of diapause known as dauer by inhibiting the conserved DAF-2 insulin-like signaling (ILS) pathway through incompletely understood mechanisms. We have previously established a role for the C. elegans dosage compensation protein DPY-21 in the control of dauer arrest and DAF-2 ILS. Here, we show that the histone H4 lysine 20 methyltransferase SET-4, which also influences dosage compensation, promotes dauer arrest in part by repressing the X-linked ins-9 gene, which encodes a new agonist insulin-like peptide (ILP) expressed specifically in the paired ASI sensory neurons that are required for dauer bypass. ins-9 repression in dauer-constitutive mutants requires DPY-21, SET-4 and the FoxO transcription factor DAF-16, which is the main target of DAF-2 ILS. By contrast, autosomal genes encoding major agonist ILPs that promote reproductive development are not repressed by DPY-21, SET-4 or DAF-16/FoxO. Our results implicate SET-4 as a sensory rheostat that reinforces developmental fates in response to environmental cues by modulating autocrine and paracrine DAF-2 ILS.

Stable Caenorhabditis elegans chromatin domains separate broadly expressed and developmentally regulated genes

Eukaryotic genomes are organized into domains of differing structure and activity. There is evidence that the domain organization of the genome regulates its activity, yet our understanding of domain properties and the factors that influence their formation is poor. Here, we use chromatin state analyses in early embryos and third-larval stage (L3) animals to investigate genome domain organization and its regulation in Caenorhabditis elegans At both stages we find that the genome is organized into extended chromatin domains of high or low gene activity defined by different subsets of states, and enriched for H3K36me3 or H3K27me3, respectively. The border regions between domains contain large intergenic regions and a high density of transcription factor binding, suggesting a role for transcription regulation in separating chromatin domains. Despite the differences in cell types, overall domain organization is remarkably similar in early embryos and L3 larvae, with conservation of 85% of domain border positions. Most genes in high-activity domains are expressed in the germ line and broadly across cell types, whereas low-activity domains are enriched for genes that are developmentally regulated. We find that domains are regulated by the germ-line H3K36 methyltransferase MES-4 and that border regions show striking remodeling of H3K27me1, supporting roles for H3K36 and H3K27 methylation in regulating domain structure. Our analyses of C. elegans chromatin domain structure show that genes are organized by type into domains that have differing modes of regulation.

An H4K16 histone acetyltransferase mediates decondensation of the X chromosome in C. elegans males

Background

In C. elegans, in order to equalize gene expression between the sexes and balance X and autosomal expression, two steps are believed to be required. First, an unknown mechanism is hypothesized to upregulate the X chromosome in both sexes. This mechanism balances the X to autosomal expression in males, but creates X overexpression in hermaphrodites. Therefore, to restore the balance, hermaphrodites downregulate gene expression twofold on both X chromosomes. While many studies have focused on X chromosome downregulation, the mechanism of X upregulation is not known.

Results

To gain more insight into X upregulation, we studied the effects of chromatin condensation and histone acetylation on gene expression levels in male C. elegans. We have found that the H4K16 histone acetyltransferase MYS-1/Tip60 mediates dramatic decondensation of the male X chromosome as measured by FISH. However, RNA-seq analysis revealed that MYS-1 contributes only slightly to upregulation of gene expression on the X chromosome. These results suggest that the level of chromosome decondensation does not necessarily correlate with the degree of gene expression change in vivo. Furthermore, the X chromosome is more sensitive to MYS-1-mediated decondensation than the autosomes, despite similar levels of H4K16ac on all chromosomes, as measured by ChIP-seq. H4K16ac levels weakly correlate with gene expression levels on both the X and the autosomes, but highly expressed genes on the X chromosome do not contain exceptionally high levels of H4K16ac.

Conclusion

These results indicate that H4K16ac and chromosome decondensation influence regulation of the male X chromosome; however, they do not fully account for the high levels of gene expression observed on the X chromosomes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13072-016-0097-x) contains supplementary material, which is available to authorized users.

Anchoring of Heterochromatin to the Nuclear Lamina Reinforces Dosage Compensation-Mediated Gene Repression

Higher order chromosome structure and nuclear architecture can have profound effects on gene regulation. We analyzed how compartmentalizing the genome by tethering heterochromatic regions to the nuclear lamina affects dosage compensation in the nematode C. elegans. In this organism, the dosage compensation complex (DCC) binds both X chromosomes of hermaphrodites to repress transcription two-fold, thus balancing gene expression between XX hermaphrodites and XO males. X chromosome structure is disrupted by mutations in DCC subunits. Using X chromosome paint fluorescence microscopy, we found that X chromosome structure and subnuclear localization are also disrupted when the mechanisms that anchor heterochromatin to the nuclear lamina are defective. Strikingly, the heterochromatic left end of the X chromosome is less affected than the gene-rich middle region, which lacks heterochromatic anchors. These changes in X chromosome structure and subnuclear localization are accompanied by small, but significant levels of derepression of X-linked genes as measured by RNA-seq, without any observable defects in DCC localization and DCC-mediated changes in histone modifications. We propose a model in which heterochromatic tethers on the left arm of the X cooperate with the DCC to compact and peripherally relocate the X chromosomes, contributing to gene repression.

Chromosome-wide mechanisms to decouple gene expression from gene dose during sex-chromosome evolution

Changes in chromosome number impair fitness by disrupting the balance of gene expression. Here we analyze mechanisms to compensate for changes in gene dose that accompanied the evolution of sex chromosomes from autosomes. Using single-copy transgenes integrated throughout the Caenorhabditis elegans genome, we show that expression of all X-linked transgenes is balanced between XX hermaphrodites and XO males. However, proximity of a dosage compensation complex (DCC) binding site (rex site) is neither necessary to repress X-linked transgenes nor sufficient to repress transgenes on autosomes. Thus, X is broadly permissive for dosage compensation, and the DCC acts via a chromosome-wide mechanism to balance transcription between sexes. In contrast, no analogous X-chromosome-wide mechanism balances transcription between X and autosomes: expression of compensated hermaphrodite X-linked transgenes is half that of autosomal transgenes. Furthermore, our results argue against an X-chromosome dosage compensation model contingent upon rex-directed positioning of X relative to the nuclear periphery.

DOI:http://dx.doi.org/10.7554/eLife.17365.001

DNA inside cells is packaged into structures called chromosomes, each of which contains numerous genes. Many organisms, including humans, have two copies of most chromosomes in their cells. If the process of cell division goes awry, cells can end up with too many or too few copies of their chromosomes, which can cause serious illnesses.

Sex chromosomes pose a conundrum for cells. In humans, females have two copies of the X chromosome, whereas males only have one. This means that males have half the copy number (dose) of genes on the X chromosome. Human cells correct this imbalance by suppressing the activity, or expression, of most of the genes on one of the X chromosomes in females.

“Dosage compensation” also occurs in the roundworm species Caenorhabditis elegans, because male worms have one X chromosome whilst hermaphrodites have two. The dosage compensation mechanism in roundworms differs from that in humans. It involves turning down the expression of both hermaphrodite X chromosomes by half. The process is enacted by a dosage compensation complex that binds to specific sites along both hermaphrodite X chromosomes.

Dosage compensation mechanisms that reduce X chromosome expression in females cause sex chromosomes to have lower gene expression than non-sex chromosomes. Modern sex chromosomes evolved from a pair of non-sex chromosomes, and males lost one copy of all of the genes located on those ancestral chromosomes. This evolutionary history causes both sexes to have lower gene expression from X chromosomes than the other chromosomes, raising the question of whether a mechanism exists to balance out the difference in gene expression between sex chromosomes and non-sex chromosomes.

Wheeler et al. now show that the expression of any foreign gene artificially added to the X chromosomes of C. elegans is equalized between males and hermaphrodites despite the difference in gene dose. The equalization works regardless of where on the X chromosome the new gene is added. The foreign gene does not need to be adjacent to a binding site for the dosage compensation complex. These results indicate that dosage compensation mechanisms regulate gene expression on a chromosome-wide scale.

Wheeler et al. also show that genes added to X chromosomes are expressed at half the level as the same genes added to non-sex chromosomes. These results mean that no chromosome-wide mechanism balances gene expression levels between the X chromosome and the non-sex chromosomes.

It remains unknown how C. elegans, and many other living organisms, evolved to tolerate a lower level of gene expression from the sex chromosomes. Instead of a chromosome-wide mechanism, it is likely that individual genes evolved different ways to alter their expression levels. Working out what these mechanisms are remains a challenge for further research.

DOI:http://dx.doi.org/10.7554/eLife.17365.002

Dosage compensation and nuclear organization: cluster to control chromosome-wide gene expression

In many species, male and female animals differ in the number of X chromosomes they possess. As a consequence, large scale differences in gene dosage exist between sexes; a phenomenon that is rarely tolerated by the organism for changes in autosome dosage. Several strategies have evolved independently to balance X-linked gene dosage between sexes, named dosage compensation (DC). The molecular basis of DC differs among the three best-studied examples: mammals, fruit fly and nematodes. In this short review, we summarize recent microscopic and chromosome conformation capture data that reveal key features of the compensated X chromosome and highlight the events leading to the establishment of a functional, specialized nuclear compartment, the X domain.

Developmental Dynamics of X-Chromosome Dosage Compensation by the DCC and H4K20me1 in C. elegans

In Caenorhabditis elegans, the dosage compensation complex (DCC) specifically binds to and represses transcription from both X chromosomes in hermaphrodites. The DCC is composed of an X-specific condensin complex that interacts with several proteins. During embryogenesis, DCC starts localizing to the X chromosomes around the 40-cell stage, and is followed by X-enrichment of H4K20me1 between 100-cell to comma stage. Here, we analyzed dosage compensation of the X chromosome between sexes, and the roles of dpy-27 (condensin subunit), dpy-21 (non-condensin DCC member), set-1 (H4K20 monomethylase) and set-4 (H4K20 di-/tri-methylase) in X chromosome repression using mRNA-seq and ChIP-seq analyses across several developmental time points. We found that the DCC starts repressing the X chromosomes by the 40-cell stage, but X-linked transcript levels remain significantly higher in hermaphrodites compared to males through the comma stage of embryogenesis. Dpy-27 and dpy-21 are required for X chromosome repression throughout development, but particularly in early embryos dpy-27 and dpy-21 mutations produced distinct expression changes, suggesting a DCC independent role for dpy-21. We previously hypothesized that the DCC increases H4K20me1 by reducing set-4 activity on the X chromosomes. Accordingly, in the set-4 mutant, H4K20me1 increased more from the autosomes compared to the X, equalizing H4K20me1 level between X and autosomes. H4K20me1 increase on the autosomes led to a slight repression, resulting in a relative effect of X derepression. H4K20me1 depletion in the set-1 mutant showed greater X derepression compared to equalization of H4K20me1 levels between X and autosomes in the set-4 mutant, indicating that H4K20me1 level is important, but X to autosomal balance of H4K20me1 contributes slightly to X-repression. Thus H4K20me1 is not only a downstream effector of the DCC [corrected].In summary, X chromosome dosage compensation starts in early embryos as the DCC localizes to the X, and is strengthened in later embryogenesis by H4K20me1.

Linking dosage compensation and X chromosome nuclear organization in C. elegans

Animal sex is determined by the number of X chromosomes in many species, creating unequal gene dosage (aneuploidy) between sexes. Dosage Compensation mechanisms equalize this dosage difference by regulating X-linked gene expression. In the nematode C. elegans the current model suggests that DC is achieved by a 2-fold transcriptional downregulation in hermaphrodites mediated by the Dosage Compensation Complex (DCC), which restricts access to RNA Polymerase II by an unknown mechanism. Taking a nuclear organization point of view, we showed that the male X chromosome resides in the pore proximal subnuclear compartment whereas the DCC bound to the X, inhibits this spatial organization in the hermaphrodites. Here we discuss our results and propose a model that reassigns the role of DCC from repression of genes to inhibition of activation.

Balancing up and downregulation of the C. elegans X chromosomes

In Caenorhabditis elegans, males have one X chromosome and hermaphrodites have two. Emerging evidence indicates that the male X is transcriptionally more active than autosomes to balance the single X to two sets of autosomes. Because upregulation is not limited to males, hermaphrodites need to strike back and downregulate expression from the two X chromosomes to balance gene expression in their genome. Hermaphrodite-specific downregulation involves binding of the dosage compensation complex to both Xs. Advances in recent years revealed that the action of the dosage compensation complex results in compaction of the X chromosomes, changes in the distribution of histone modifications, and ultimately limiting RNA Polymerase II loading to achieve chromosome-wide gene repression.

Condensin-mediated chromosome organization and gene regulation

In many organisms sexual fate is determined by a chromosome-based method which entails a difference in sex chromosome-linked gene dosage. Consequently, a gene regulatory mechanism called dosage compensation equalizes X-linked gene expression between the sexes. Dosage compensation initiates as cells transition from pluripotency to differentiation. In Caenorhabditis elegans, dosage compensation is achieved by the dosage compensation complex (DCC) binding to both X chromosomes in hermaphrodites to downregulate gene expression by twofold. The DCC contains a subcomplex (condensin I(DC)) similar to the evolutionarily conserved condensin complexes which play a fundamental role in chromosome dynamics during mitosis. Therefore, mechanisms related to mitotic chromosome condensation are hypothesized to mediate dosage compensation. Consistent with this hypothesis, monomethylation of histone H4 lysine 20 is increased, whereas acetylation of histone H4 lysine 16 is decreased, both on mitotic chromosomes and on interphase dosage compensated X chromosomes in worms. These observations suggest that interphase dosage compensated X chromosomes maintain some characteristics associated with condensed mitotic chromosome. This chromosome state is stably propagated from one cell generation to the next. In this review we will speculate on how the biochemical activities of condensin can achieve both mitotic chromosome compaction and gene repression.

Mechanisms of x chromosome dosage compensation

In many animals, males have one X and females have two X chromosomes. The difference in X chromosome dosage between the two sexes is compensated by mechanisms that regulate X chromosome transcription. Recent advances in genomic techniques have provided new insights into the molecular mechanisms of X chromosome dosage compensation. In this review, I summarize our current understanding of dosage imbalance in general, and then review the molecular mechanisms of X chromosome dosage compensation with an emphasis on the parallels and differences between the three well-studied model systems, M. musculus, D. melanogaster and C. elegans.

Differential spatial and structural organization of the X chromosome underlies dosage compensation in C. elegans

The adjustment of X-linked gene expression to the X chromosome copy number (dosage compensation [DC]) has been widely studied as a model of chromosome-wide gene regulation. In Caenorhabditis elegans, DC is achieved by twofold down-regulation of gene expression from both Xs in hermaphrodites. We show that in males, the single X chromosome interacts with nuclear pore proteins, while in hermaphrodites, the DC complex (DCC) impairs this interaction and alters X localization. Our results put forward a structural model of DC in which X-specific sequences locate the X chromosome in transcriptionally active domains in males, while the DCC prevents this in hermaphrodites.

The C. elegans dosage compensation complex mediates interphase X chromosome compaction

Background

Dosage compensation is a specialized gene regulatory mechanism which equalizes X-linked gene expression between sexes. In Caenorhabditis elegans, dosage compensation is achieved by the activity of the dosage compensation complex (DCC). The DCC localizes to both X chromosomes in hermaphrodites to downregulate gene expression by half. The DCC contains a subcomplex (condensin I^DC) similar to the evolutionarily conserved condensin complexes which play fundamental roles in chromosome dynamics during mitosis and meiosis. Therefore, mechanisms related to mitotic chromosome condensation have been long hypothesized to mediate dosage compensation. However experimental evidence was lacking.

Results

Using 3D FISH microscopy to measure the volumes of X and chromosome I territories and to measure distances between individual loci, we show that hermaphrodite worms deficient in DCC proteins have enlarged interphase X chromosomes when compared to wild type. By contrast, chromosome I is unaffected. Interestingly, hermaphrodite worms depleted of condensin I or II show no phenotype. Therefore X chromosome compaction is specific to condensin I^DC. In addition, we show that SET-1, SET-4, and SIR-2.1, histone modifiers whose activity is regulated by the DCC, need to be present for the compaction of the X chromosome territory.

Conclusion

These results support the idea that condensin I^DC, and the histone modifications regulated by the DCC, mediate interphase X chromosome compaction. Our results link condensin-mediated chromosome compaction, an activity connected to mitotic chromosome condensation, to chromosome-wide repression of gene expression in interphase.

Electronic supplementary material

The online version of this article (doi:10.1186/1756-8935-7-31) contains supplementary material, which is available to authorized users.

Genome-wide analysis of condensin binding in Caenorhabditis elegans

Background

Condensins are multi-subunit protein complexes that are essential for chromosome condensation during mitosis and meiosis, and play key roles in transcription regulation during interphase. Metazoans contain two condensins, I and II, which perform different functions and localize to different chromosomal regions. Caenorhabditis elegans contains a third condensin, I^DC, that is targeted to and represses transcription of the X chromosome for dosage compensation.

Results

To understand condensin binding and function, we performed ChIP-seq analysis of C. elegans condensins in mixed developmental stage embryos, which contain predominantly interphase nuclei. Condensins bind to a subset of active promoters, tRNA genes and putative enhancers. Expression analysis in kle-2-mutant larvae suggests that the primary effect of condensin II on transcription is repression. A DNA sequence motif, GCGC, is enriched at condensin II binding sites. A sequence extension of this core motif, AGGG, creates the condensin I^DC motif. In addition to differences in recruitment that result in X-enrichment of condensin I^DC and condensin II binding to all chromosomes, we provide evidence for a shared recruitment mechanism, as condensin I^DC recruiter SDC-2 also recruits condensin II to the condensin I^DC recruitment sites on the X. In addition, we found that condensin sites overlap extensively with the cohesin loader SCC-2, and that SDC-2 also recruits SCC-2 to the condensin I^DC recruitment sites.

Conclusions

Our results provide the first genome-wide view of metazoan condensin II binding in interphase, define putative recruitment motifs, and illustrate shared loading mechanisms for condensin I^DC and condensin II.

Regulation of the X chromosomes in Caenorhabditis elegans

Dosage compensation, which regulates the expression of genes residing on the sex chromosomes, has provided valuable insights into chromatin-based mechanisms of gene regulation. The nematode Caenorhabditis elegans has adopted various strategies to down-regulate and even nearly silence the X chromosomes. This article discusses the different chromatin-based strategies used in somatic tissues and in the germline to modulate gene expression from the C. elegans X chromosomes and compares these strategies to those used by other organisms to cope with similar X-chromosome dosage differences.

Transcriptional control of a whole chromosome: emerging models for dosage compensation

Males and females of many animal species differ in their sex-chromosome karyotype, and this creates imbalances between X-chromosome and autosomal gene products that require compensation. Although distinct molecular mechanisms have evolved in three highly studied systems, they all achieve coordinate regulation of an entire chromosome by differential RNA-polymerase occupancy at X-linked genes. High-throughput genome-wide methods have been pivotal in driving the latest progress in the field. Here we review the emerging models for dosage compensation in mammals, flies and nematodes, with a focus on mechanisms affecting RNA polymerase II activity on the X chromosome.

A dual role for the histone methyltransferase PR-SET7/SETD8 and histone H4 lysine 20 monomethylation in the local regulation of RNA polymerase II pausing

RNA polymerase II (Pol II) promoter-proximal pausing plays a critical role in postinitiation transcriptional regulation at many metazoan genes. We showed recently that histone H4 lysine 16 acetylation (H4K16Ac), mediated by the MSL complex, facilitates the release of paused Pol II. In contrast, H4 lysine 20 trimethylation (H4K20me3), mediated by SUV420H2, enforces Pol II pausing by inhibiting MSL recruitment. However, how the balance between H4K16Ac and H4K20me3 is locally regulated remains unclear. Here, we demonstrate that PR-SET7/SETD8, which monomethylates histone H4 lysine 20 (H4K20me1), controls both H4K16Ac and H4K20me3 and in doing so, regulates Pol II pausing dynamics. We find that PR-SET7-mediated H4K20me1 is necessary for the recruitment of the MSL complex, subsequent H4K16Ac, and release of Pol II into active elongation. Although dispensable for SUV420H2 recruitment, PR-SET7-mediated H4K20me1 is required for H4K20me3. Although depletion of SUV420H2 is sufficient to deplete H4K20me3 and relieve an H4K20me3-induced pause, pausing is maintained in the absence of PR-SET7 despite H4K20me3 depletion because of an inability to recruit the MSL complex in the absence of H4K20me1. These findings highlight the requirement for PR-SET7 and H4K20me1 in establishing both the H4K16Ac and H4K20me3 marks and point to a dual role in the local regulation of Pol II pausing.