MRG-1, an autosome-associated protein, silences X-linked genes and protects germline immortality in Caenorhabditis elegans

Phase separation of MRG15 delays cellular senescence

Phase separation, a biophysical process that segregates subcellular environments into condensates, has been recognized for its role in transcriptional regulation. However, the extent of its influence on cellular senescence processes remains to be fully elucidated. We established that MRG15 depletion leads to cellular senescence in human mesenchymal stem cells (hMSCs). MRG15 can form phase-separated liquid condensates via its intrinsically disordered region (IDR). IDR deletion and replacement assays revealed that MRG15 condensation is crucial to hMSC senescence. According to the epigenomic and transcriptomic analysis, MRG15 depletion impacts pathways integral to the cell cycle and the senescence process, as evidenced by the diminished binding and the modified expression of key genes, including p53, CDKN1A, LMNB1, CCNB1, NPM1, MYC, and HMGB2. Our findings establish a link between phase separation and senescence regulation and present a promising new therapeutic target for the alleviation of age-related diseases and the potential extension of lifespan.

SIN-3 acts in distinct complexes to regulate the germline transcriptional program in Caenorhabditis elegans

The transcriptional co-regulator SIN3 influences gene expression through multiple interactions that include histone deacetylases. Haploinsufficiency and mutations in SIN3 are the underlying cause of Witteveen-Kolk syndrome and related intellectual disability and autism syndromes, emphasizing its key role in development. However, little is known about the diversity of its interactions and functions in developmental processes. Here, we show that loss of SIN-3, the single SIN3 homolog in Caenorhabditis elegans, results in maternal-effect sterility associated with de-regulation of the germline transcriptome, including de-silencing of X-linked genes. We identify at least two distinct SIN3 complexes containing specific histone deacetylases and show that they differentially contribute to fertility. Single-cell, single-molecule fluorescence in situ hybridization reveals that in sin-3 mutants the X chromosome becomes re-expressed prematurely and in a stochastic manner in individual germ cells, suggesting a role for SIN-3 in its silencing. Furthermore, we identify histone residues whose acetylation increases in the absence of SIN-3. Together, this work provides a powerful framework for the in vivo study of SIN3 and associated proteins.

Maternal H3K36 and H3K27 HMTs protect germline development via regulation of the transcription factor LIN-15B

Maternally synthesized products play critical roles in the development of offspring. A premier example is the Caenorhabditis elegans H3K36 methyltransferase MES-4, which is essential for germline survival and development in offspring. How maternal MES-4 protects the germline is not well understood, but its role in H3K36 methylation hinted that it may regulate gene expression in primordial germ cells (PGCs). We tested this hypothesis by profiling transcripts from nascent germlines (PGCs and their descendants) dissected from wild-type and mes-4 mutant (lacking maternal and zygotic MES-4) larvae. mes-4 nascent germlines displayed downregulation of some germline genes, upregulation of some somatic genes, and dramatic upregulation of hundreds of genes on the X chromosome. We demonstrated that upregulation of one or more genes on the X is the cause of germline death by generating and analyzing mes-4 mutants that inherited different endowments of X chromosome(s). Intriguingly, removal of the THAP transcription factor LIN-15B from mes-4 mutants reduced X misexpression and prevented germline death. lin-15B is X-linked and misexpressed in mes-4 PGCs, identifying it as a critical target for MES-4 repression. The above findings extend to the H3K27 methyltransferase MES-2/3/6, the C. elegans version of polycomb repressive complex 2. We propose that maternal MES-4 and PRC2 cooperate to protect germline survival by preventing synthesis of germline-toxic products encoded by genes on the X chromosome, including the key transcription factor LIN-15B.

SUMOylation of the chromodomain factor MRG-1 in C. elegans affects chromatin-regulatory dynamics

Epigenetic mechanisms control chromatin accessibility and gene expression to ensure proper cell fate specification. Histone proteins are integral chromatin components, and their modification promotes gene expression regulation. Specific proteins recognize modified histones such as the chromodomain protein MRG-1. MRG-1 is the Caenorhabditis elegans ortholog of mammalian MRG15, which is involved in DNA repair. MRG-1 binds methylated histone H3 and is important for germline maturation and safeguarding. To elucidate interacting proteins that modulate MRG-1 activity, we performed in-depth protein-protein interaction analysis using immunoprecipitations coupled with mass spectrometry. We detected strong association with the Small ubiquitin-like modifier SUMO, and found that MRG-1 is post-translationally modified by SUMO. SUMOylation affects chromatin-binding dynamics of MRG-1, suggesting an epigenetic regulation pathway, which may be conserved.

The zinc-finger protein OEF-1 stabilizes histone modification patterns and promotes efficient splicing in the C. elegans germ line

To ensure stable transmission of genetic information to the next generation, germ cells frequently silence sex chromosomes, as well as autosomal loci that promote inappropriate differentiation programs. In Caenorhabditis elegans, silenced and active genomic domains are established in germ cells by the histone modification complexes MES-2/3/6 and MES-4, which promote silent and active chromatin states, respectively. These states are generally mutually exclusive and modulation of one state influences the pattern of the other. Here, we identify the zinc-finger protein OEF-1 as a novel modifier of this epigenetic balance in the C. elegans germline. Loss of oef-1 genetically enhances mes mutant phenotypes. Moreover, OEF-1 binding correlates with the active modification H3K36me3 and sustains H3K36me3 levels in the absence of MES-4 activity. OEF-1 also promotes efficient mRNA splicing activity, a process that is influenced by H3K36me3 levels. Finally, OEF-1 limits deposition of the silencing modification H3K27me3 on the X chromosome and at repressed autosomal loci. We propose that OEF-1 might act as an intermediary to mediate the downstream effects of H3K36me3 that promote transcript integrity, and indirectly affect gene silencing as a consequence.

A chromodomain protein mediates heterochromatin-directed piRNA expression

PIWI-interacting RNAs (piRNAs) play significant roles in suppressing transposons, maintaining genome integrity, and defending against viral infections. How piRNA source loci are efficiently transcribed is poorly understood. Here, we show that in Caenorhabditis elegans, transcription of piRNA clusters depends on the chromatin microenvironment and a chromodomain-containing protein, UAD-2. piRNA clusters form distinct focus in germline nuclei. We conducted a forward genetic screening and identified UAD-2 that is required for piRNA focus formation. In the absence of histone 3 lysine 27 methylation or proper chromatin-remodeling status, UAD-2 is depleted from the piRNA focus. UAD-2 recruits the upstream sequence transcription complex (USTC), which binds the Ruby motif to piRNA promoters and promotes piRNA generation. Vice versa, the USTC complex is required for UAD-2 to associate with the piRNA focus. Thus, transcription of heterochromatic small RNA source loci relies on coordinated recruitment of both the readers of histone marks and the core transcriptional machinery to DNA.

Emerging Roles for Chromo Domain Proteins in Genome Organization and Cell Fate in C. elegans

In most eukaryotes, the genome is packaged with histones and other proteins to form chromatin. One of the major mechanisms for chromatin regulation is through post-translational modification of histone proteins. Recognition of these modifications by effector proteins, often dubbed histone “readers,” provides a link between the chromatin landscape and gene regulation. The diversity of histone reader proteins for each modification provides an added layer of regulatory complexity. In this review, we will focus on the roles of chromatin organization modifier (chromo) domain containing proteins in the model nematode, Caenorhabditis elegans. An amenability to genetic and cell biological approaches, well-studied development and a short life cycle make C. elegans a powerful system to investigate the diversity of chromo domain protein functions in metazoans. We will highlight recent insights into the roles of chromo domain proteins in the regulation of heterochromatin and the spatial conformation of the genome as well as their functions in cell fate, fertility, small RNA pathways and transgenerational epigenetic inheritance. The spectrum of different chromatin readers may represent a layer of regulation that integrates chromatin landscape, genome organization and gene expression.

Mixing and Matching Chromosomes during Female Meiosis

Meiosis is a key event in the manufacturing of an oocyte. During this process, the oocyte creates a set of unique chromosomes by recombining paternal and maternal copies of homologous chromosomes, and by eliminating one set of chromosomes to become haploid. While meiosis is conserved among sexually reproducing eukaryotes, there is a bewildering diversity of strategies among species, and sometimes within sexes of the same species, to achieve proper segregation of chromosomes. Here, we review the very first steps of meiosis in females, when the maternal and paternal copies of each homologous chromosomes have to move, find each other and pair. We explore the similarities and differences observed in C. elegans, Drosophila, zebrafish and mouse females.

Transgenerational epigenetic inheritance: from phenomena to molecular mechanisms

Inherited information not encoded in the DNA sequence can regulate a variety of complex phenotypes. However, how this epigenetic information escapes the typical epigenetic erasure that occurs upon fertilization and how it regulates behavior is still unclear. Here we review recent examples of brain related transgenerational epigenetic inheritance and delineate potential molecular mechanisms that could regulate how non-genetic information could be transmitted.

MRG-1 is required for both chromatin-based transcriptional silencing and genomic integrity of primordial germ cells in Caenorhabditis elegans

In Caenorhabditis elegans, germline cells remain transcriptionally silenced during embryogenesis. The transcriptional silencing is achieved by two different mechanisms: One is the inhibition of RNA polymerase II in P2-P4 cells at the establishment stage, and another is chromatin-based silencing in two primordial germ cells (PGCs) at the maintenance stage; however, the molecular mechanism underlying chromatin-based silencing is less understood. We investigated the role of the chromodomain protein MRG-1, which is an essential maternal factor for germline development, in transcriptional silencing in PGCs. PGCs lacking maternal MRG-1 showed increased levels of two histone modifications (H3K4me2 and H4K16ac), which are epigenetic markers for active transcription, and precocious activation of germline promoters. Loss of MES-4, a H3K36 methyltransferase, also caused similar derepression of the germline genes in PGCs, suggesting that both MRG-1 and MES-4 function in chromatin-based silencing in PGCs. In addition, the mrg-1 null mutant showed abnormal chromosome structures and a decrease in homologous recombinase RAD-51 foci in PGCs, but the mes-4 null mutant did not show such phenotypes. Taken together, we propose that MRG-1 has two distinct functions: chromatin-based transcriptional silencing and preserving genomic integrity at the maintenance stage of PGCs.

MRG-1/MRG15 Is a Barrier for Germ Cell to Neuron Reprogramming in Caenorhabditis elegans

Chromatin regulators play important roles in the safeguarding of cell identities by opposing the induction of ectopic cell fates and, thereby, preventing forced conversion of cell identities by reprogramming approaches. Our knowledge of chromatin regulators acting as reprogramming barriers in living organisms needs improvement as most studies use tissue culture. We used Caenorhabditis elegans as an in vivo gene discovery model and automated solid-phase RNA interference screening, by which we identified 10 chromatin-regulating factors that protect cells against ectopic fate induction. Specifically, the chromodomain protein MRG-1 safeguards germ cells against conversion into neurons. MRG-1 is the ortholog of mammalian MRG15 (MORF-related gene on chromosome 15) and is required during germline development in C. elegans However, MRG-1's function as a barrier for germ cell reprogramming has not been revealed previously. Here, we further provide protein-protein and genome interactions of MRG-1 to characterize its molecular functions. Conserved chromatin regulators may have similar functions in higher organisms, and therefore, understanding cell fate protection in C. elegans may also help to facilitate reprogramming of human cells.

Aneuploidy: an important model system to understand salient aspects of functional genomics

Maintaining a balance in gene dosage and protein activity is essential to sustain normal cellular functions. Males and females have a wide range of genetic as well as epigenetic differences, where X-linked gene dosage is an essential regulatory factor. Basic understanding of gene dosage maintenance has emerged from the studies carried out using mouse models with FCG (four core genotype) and chromosomal aneuploidy as well as from mono-chromosomal hybrid cells. In mammals, aneuploidy often leads to embryonic lethality particularly in early development with major developmental and structural abnormalities. Thus, in-depth analysis of the causes and consequences of gene dosage alterations is needed to unravel its effects on basic cellular and developmental functions as well as in understanding its medical implications. Cells isolated from individuals with naturally occurring chromosomal aneuploidy can be considered as true representatives, as these cells have stable chromosomal alterations/gene dosage imbalance, which have occurred by modulation of the basic molecular machinery. Therefore, innovative use of these natural aneuploidy cells/organisms with recent molecular and high-throughput techniques will provide an understanding of the basic mechanisms involved in gene dosage balance and the related consequences for functional genomics.

Structure and function of histone methylation-binding proteins in plants

Post-translational modifications of histones play important roles in modulating many essential biological processes in both animals and plants. These covalent modifications, including methylation, acetylation, phosphorylation, ubiquitination, SUMOylation and so on, are laid out and erased by histone-modifying enzymes and read out by effector proteins. Recent studies have revealed that a number of developmental processes in plants are under the control of histone post-translational modifications, such as floral transition, seed germination, organogenesis and morphogenesis. Therefore, it is critical to identify those protein domains, which could specifically recognize these post-translational modifications to modulate chromatin structure and regulate gene expression. In the present review, we discuss the recent progress in understanding the structure and function of the histone methylation readers in plants, by focusing on Arabidopsis thaliana proteins.

A twist of fate: How a meiotic protein is providing new perspectives on germ cell development

The molecular pathways that govern how germ line fate is acquired is an area of intense investigation that has major implications for the development of assisted reproductive technologies, infertility interventions, and treatment of germ cell cancers. Transcriptional repression has emerged as a primary mechanism to ensure suppression of somatic growth programs in primordial germ cells. In this commentary, we address how xnd-1 illuminates our understanding of transcriptional repression and how it is coordinated with the germ cell differentiation program. We recently identified xnd-1 as a novel, early determinant of germ cell fates in Caenorhabditis elegans. Our study revealed that XND-1 is maternally deposited into early embryos where it is selectively enriched in the germ lineage and then exclusively found on chromatin in the germ lineage throughout development and into adulthood when it dissociates from chromosomes in late pachytene. This localization is consistent with a range of interesting germ cell defects that suggest xnd-1 is a pivotal determinant of germ cell characteristics. Loss of xnd-1 results in a unique "one PGC (primordial germ cell)" phenotype due to G2 cell cycle arrest of the germline precursor blastomere, P4, which predisposes the animal and its progeny for reduced fecundity. The sterility in xnd-1 mutants is correlated with an increase in the transcriptional activation-associated histone modification, dimethylation of histone H3 lysine 4 (H3K4me2), and aberrant expression of somatic transgenes but overlapping roles with nos-2 and nos-1 suggest that transcriptional repression is achieved by multiple redundant mechanisms.

Restricted distribution of mrg-1 mRNA in C. elegans primordial germ cells through germ granule-independent regulation

The chromodomain protein MRG-1 is an essential maternal factor for proper germline development that protects germ cells from cell death in C. elegans. Unlike germ granules, which are exclusively segregated to the germline blastomeres at each cell division from the first cleavage of the embryo, MRG-1 is abundant in all cells in early embryos and is then gradually restricted to the primordial germ cells (PGCs) by the morphogenesis stage. Here, we show that this characteristic spatiotemporal expression pattern is dictated by the mrg-1 3'UTR and is differentially regulated at the RNA level between germline and somatic cells. Asymmetric segregation of germ granules is not necessary to localize MRG-1 to the PGCs. We found that MES-4, an essential chromatin regulator in germ cells, also accumulates in the PGCs in a germ granule-independent manner. We propose that C.elegans PGCs have a novel mechanism to accumulate at least some chromatin-associated proteins that are essential for germline immortality.

A Defective Meiotic Outcome of a Failure in Homologous Pairing and Synapsis Is Masked by Meiotic Quality Control

Successful gamete production is ensured by meiotic quality control, a process in which germ cells that fail in bivalent chromosome formation are eliminated during meiotic prophase. To date, numerous meiotic mutants have been isolated in a variety of model organisms, using defects associated with a failure in bivalent formation as hallmarks of the mutant. Presumably, the meiotic quality control mechanism in those mutants is overwhelmed. In these mutants, all germ cells fail in bivalent formation, and a subset of cells seem to survive the elimination process and develop into gametes. It is possible that mutants that are partially defective in bivalent formation were missed in past genetic screens, because no evident meiotic defects associated with failure in bivalent formation would have been detectable. Meiotic quality control effectively eliminates most failed germ cells, leaving predominately successful ones. Here, we provide evidence supporting this possibility. The Caenorhabditis elegans mrg-1 loss-of-function mutant does not appear to be defective in bivalent formation in diakinesis oocytes. However, defects in homologous chromosome pairing and synapsis during the preceding meiotic prophase, prerequisites for successful bivalent formation, were observed in most, but not all, germ cells. Failed bivalent formation in the oocytes became evident once meiotic quality control was abrogated in the mrg-1 mutant. Both double-strand break repair and synapsis checkpoints are partly responsible for eliminating failed germ cells in the mrg-1 mutant. Interestingly, removal of both checkpoint activities from the mrg-1 mutant is not sufficient to completely suppress the increased germline apoptosis, suggesting the presence of a novel meiotic checkpoint mechanism.

A Forward Genetic Screen for Suppressors of Somatic P Granules in Caenorhabditis elegans

In Caenorhabditis elegans, germline expression programs are actively repressed in somatic tissue by components of the synMuv (synthetic multi-vulva) B chromatin remodeling complex, which include homologs of tumor suppressors Retinoblastoma (Rb/LIN-35) and Malignant Brain Tumor (MBT/LIN-61). However, the full scope of pathways that suppress germline expression in the soma is unknown. To address this, we performed a mutagenesis and screened for somatic expression of GFP-tagged PGL-1, a core P-granule nucleating protein. Eight alleles were isolated from 4000 haploid genomes. Five of these alleles exhibit a synMuv phenotype, whereas the remaining three were identified as hypomorphic alleles of known synMuv B genes, lin-13 and dpl-1. These findings suggest that most suppressors of germline programs in the soma of C. elegans are either required for viability or function through synMuv B chromatin regulation.

Proteasome regulation of the chromodomain protein MRG-1 controls the balance between proliferative fate and differentiation in the C. elegans germ line

The level of stem cell proliferation must be tightly controlled for proper development and tissue homeostasis. Multiple levels of gene regulation are often employed to regulate stem cell proliferation to ensure that the amount of proliferation is aligned with the needs of the tissue. Here we focus on proteasome-mediated protein degradation as a means of regulating the activities of proteins involved in controlling the stem cell proliferative fate in the C. elegans germ line. We identify five potential E3 ubiquitin ligases, including the RFP-1 RING finger protein, as being involved in regulating proliferative fate. RFP-1 binds to MRG-1, a homologue of the mammalian chromodomain-containing protein MRG15 (MORF4L1), which has been implicated in promoting the proliferation of neural precursor cells. We find that C. elegans with reduced proteasome activity, or that lack RFP-1 expression, have increased levels of MRG-1 and a shift towards increased proliferation in sensitized genetic backgrounds. Likewise, reduction of MRG-1 partially suppresses stem cell overproliferation. MRG-1 levels are controlled independently of the spatially regulated GLP-1/Notch signalling pathway, which is the primary signal controlling the extent of stem cell proliferation in the C. elegans germ line. We propose a model in which MRG-1 levels are controlled, at least in part, by the proteasome, and that the levels of MRG-1 set a threshold upon which other spatially regulated factors act in order to control the balance between the proliferative fate and differentiation in the C. elegans germ line.

Condensin II Regulates Interphase Chromatin Organization Through the Mrg-Binding Motif of Cap-H2

The spatial organization of the genome within the eukaryotic nucleus is a dynamic process that plays a central role in cellular processes such as gene expression, DNA replication, and chromosome segregation. Condensins are conserved multi-subunit protein complexes that contribute to chromosome organization by regulating chromosome compaction and homolog pairing. Previous work in our laboratory has shown that the Cap-H2 subunit of condensin II physically and genetically interacts with the Drosophila homolog of human MORF4-related gene on chromosome 15 (MRG15). Like Cap-H2, Mrg15 is required for interphase chromosome compaction and homolog pairing. However, the mechanism by which Mrg15 and Cap-H2 cooperate to maintain interphase chromatin organization remains unclear. Here, we show that Cap-H2 localizes to interband regions on polytene chromosomes and co-localizes with Mrg15 at regions of active transcription across the genome. We show that co-localization of Cap-H2 on polytene chromosomes is partially dependent on Mrg15. We have identified a binding motif within Cap-H2 that is essential for its interaction with Mrg15, and have found that mutation of this motif results in loss of localization of Cap-H2 on polytene chromosomes and results in partial suppression of Cap-H2-mediated compaction and homolog unpairing. Our data are consistent with a model in which Mrg15 acts as a loading factor to facilitate Cap-H2 binding to chromatin and mediate changes in chromatin organization.

Opposing activities of DRM and MES-4 tune gene expression and X-chromosome repression in Caenorhabditis elegans germ cells

During animal development, gene transcription is tuned to tissue-appropriate levels. Here we uncover antagonistic regulation of transcript levels in the germline of Caenorhabditis elegans hermaphrodites. The histone methyltransferase MES-4 (Maternal Effect Sterile-4) marks genes expressed in the germline with methylated lysine on histone H3 (H3K36me) and promotes their transcription; MES-4 also represses genes normally expressed in somatic cells and genes on the X chromosome. The DRM transcription factor complex, named for its Dp/E2F, Retinoblastoma-like, and MuvB subunits, affects germline gene expression and prevents excessive repression of X-chromosome genes. Using genome-scale analyses of germline tissue, we show that common germline-expressed genes are activated by MES-4 and repressed by DRM, and that MES-4 and DRM co-bind many germline-expressed genes. Reciprocally, MES-4 represses and DRM activates a set of autosomal soma-expressed genes and overall X-chromosome gene expression. Mutations in mes-4 and the DRM subunit lin-54 oppositely skew the transcript levels of their common targets and cause sterility. A double mutant restores target gene transcript levels closer to wild type, and the concomitant loss of lin-54 suppresses the severe germline proliferation defect observed in mes-4 single mutants. Together, “yin-yang” regulation by MES-4 and DRM ensures transcript levels appropriate for germ-cell function, elicits robust but not excessive dampening of X-chromosome-wide transcription, and may poise genes for future expression changes. Our study reveals that conserved transcriptional regulators implicated in development and cancer counteract each other to fine-tune transcript dosage.

Germ cell specification

The germline of Caenorhabditis elegans derives from a single founder cell, the germline blastomere P(4). P(4) is the product of four asymmetric cleavages that divide the zygote into distinct somatic and germline (P) lineages. P(4) inherits a specialized cytoplasm ("germ plasm") containing maternally encoded proteins and RNAs. The germ plasm has been hypothesized to specify germ cell fate, but the mechanisms involved remain unclear. Three processes stand out: (1) inhibition of mRNA transcription to prevent activation of somatic development, (2) translational regulation of the nanos homolog nos-2 and of other germ plasm mRNAs, and (3) establishment of a unique, partially repressive chromatin. Together, these processes ensure that the daughters of P(4), the primordial germ cells Z2 and Z3, gastrulate inside the embryo, associate with the somatic gonad, initiate the germline transcriptional program, and proliferate during larval development to generate ∼2,000 germ cells by adulthood.

Histone methyltransferases MES-4 and MET-1 promote meiotic checkpoint activation in Caenorhabditis elegans

Chromosomes that fail to synapse during meiosis become enriched for chromatin marks associated with heterochromatin assembly. This response, called meiotic silencing of unsynapsed or unpaired chromatin (MSUC), is conserved from fungi to mammals. In Caenorhabditis elegans, unsynapsed chromosomes also activate a meiotic checkpoint that monitors synapsis. The synapsis checkpoint signal is dependent on cis-acting loci called Pairing Centers (PCs). How PCs signal to activate the synapsis checkpoint is currently unknown. We show that a chromosomal duplication with PC activity is sufficient to activate the synapsis checkpoint and that it undergoes heterochromatin assembly less readily than a duplication of a non-PC region, suggesting that the chromatin state of these loci is important for checkpoint function. Consistent with this hypothesis, MES-4 and MET-1, chromatin-modifying enzymes associated with transcriptional activity, are required for the synapsis checkpoint. In addition, a duplication with PC activity undergoes heterochromatin assembly when mes-4 activity is reduced. MES-4 function is required specifically for the X chromosome, while MES-4 and MET-1 act redundantly to monitor autosomal synapsis. We propose that MES-4 and MET-1 antagonize heterochromatin assembly at PCs of unsynapsed chromosomes by promoting a transcriptionally permissive chromatin environment that is required for meiotic checkpoint function. Moreover, we suggest that different genetic requirements to monitor the behavior of sex chromosomes and autosomes allow for the lone unsynapsed X present in male germlines to be shielded from inappropriate checkpoint activation.

A network of genes antagonistic to the LIN-35 retinoblastoma protein of Caenorhabditis elegans

The Caenorhabditis elegans pRb ortholog, LIN-35, functions in a wide range of cellular and developmental processes. This includes a role of LIN-35 in nutrient utilization by the intestine, which it carries out redundantly with SLR-2, a zinc-finger protein. This and other redundant functions of LIN-35 were identified in genetic screens for mutations that display synthetic phenotypes in conjunction with loss of lin-35. To explore the intestinal role of LIN-35, we conducted a genome-wide RNA-interference-feeding screen for suppressors of lin-35; slr-2 early larval arrest. Of the 26 suppressors identified, 17 fall into three functional classes: (1) ribosome biogenesis genes, (2) mitochondrial prohibitins, and (3) chromatin regulators. Further characterization indicates that different categories of suppressors act through distinct molecular mechanisms. We also tested lin-35; slr-2 suppressors, as well as suppressors of the synthetic multivulval phenotype, to determine the spectrum of lin-35-synthetic phenotypes that could be suppressed following inhibition of these genes. We identified 19 genes, most of which are evolutionarily conserved, that can suppress multiple unrelated lin-35-synthetic phenotypes. Our study reveals a network of genes broadly antagonistic to LIN-35 as well as genes specific to the role of LIN-35 in intestinal and vulval development. Suppressors of multiple lin-35 phenotypes may be candidate targets for anticancer therapies. Moreover, screening for suppressors of phenotypically distinct synthetic interactions, which share a common altered gene, may prove to be a novel and effective approach for identifying genes whose activities are most directly relevant to the core functions of the shared gene.

Repression of germline RNAi pathways in somatic cells by retinoblastoma pathway chromatin complexes

The retinoblastoma (Rb) tumor suppressor acts with a number of chromatin cofactors in a wide range of species to suppress cell proliferation. The Caenorhabditis elegans retinoblastoma gene and many of these cofactors, called synMuv B genes, were identified in genetic screens for cell lineage defects caused by growth factor misexpression. Mutations in many synMuv B genes, including lin-35/Rb, also cause somatic misexpression of the germline RNA processing P granules and enhanced RNAi. We show here that multiple small RNA components, including a set of germline-specific Argonaute genes, are misexpressed in the soma of many synMuv B mutant animals, revealing one node for enhanced RNAi. Distinct classes of synMuv B mutants differ in the subcellular architecture of their misexpressed P granules, their profile of misexpressed small RNA and P granule genes, as well as their enhancement of RNAi and the related silencing of transgenes. These differences define three classes of synMuv B genes, representing three chromatin complexes: a LIN-35/Rb-containing DRM core complex, a SUMO-recruited Mec complex, and a synMuv B heterochromatin complex, suggesting that intersecting chromatin pathways regulate the repression of small RNA and P granule genes in the soma and the potency of RNAi. Consistent with this, the DRM complex and the synMuv B heterochromatin complex were genetically additive and displayed distinct antagonistic interactions with the MES-4 histone methyltransferase and the MRG-1 chromodomain protein, two germline chromatin regulators required for the synMuv phenotype and the somatic misexpression of P granule components. Thus intersecting synMuv B chromatin pathways conspire with synMuv B suppressor chromatin factors to regulate the expression of small RNA pathway genes, which enables heightened RNAi response. Regulation of small RNA pathway genes by human retinoblastoma may also underlie its role as a tumor suppressor gene.

In metazoans, soma and germline have specialized functions that require differential tissue-specific gene expression. In C. elegans, explicit chromatin marks deposited by the MES-4 histone methyltransferase and the MRG-1 chromodomain protein allow germline expression of particular suites of target genes. Conversely, the expression of germline-specific genes is repressed in somatic cells by other chromatin regulatory factors, including the retinoblastoma pathway genes. We characterized the distinct profiles of somatic misexpression of normally germline-specific genes in these mutants and mapped out three chromatin complexes that prevent misexpression. We demonstrate that one of the complexes closely counteracts the activity of MES-4 and MRG-1, whereas another complex interacts with additional regulators that are yet to be identified. We show that these intersecting chromatin complexes prevent the upregulation of a suite of germline-specific as well as ubiquitous small RNA pathway genes, which contributes to the enhanced RNAi response in retinoblastoma pathway mutant worms. We suggest that this function of the retinoblastoma pathway chromatin factors to prevent germline-associated gene expression programs in the soma and the upregulation of small RNA pathways may also underlie their role as tumor suppressors.

The chromodomain protein MRG-1 facilitates SC-independent homologous pairing during meiosis in Caenorhabditis elegans

Homologous chromosome pairing is a prerequisite to establish physical linkage between homologs, which is critical for faithful chromosome segregation during meiosis I. The establishment of pairing is genetically separable from subsequent synapsis, defined as stabilization of pairing by the synaptonemal complex (SC). The underlying mechanism of presynaptic pairing is poorly understood. In the nematode Caenorhabditis elegans, a unique cis-acting element, the pairing center (PC), is essential for presynaptic pairing; however, it is not known whether and how the remainder of the chromosome contributes to presynaptic pairing. Here we report direct evidence for presynaptic pairing activity intrinsic to non-PC regions, which is facilitated by a conserved chromodomain protein, MRG-1. In mrg-1 loss-of-function mutants, pairing is compromised specifically in non-PC regions, leading to nonhomologous SC assembly. Our data support a model in which presynaptic alignment in non-PC regions collaborates with initial PC pairing to ensure correct homologous synapsis.

Crossover distribution and frequency are regulated by him-5 in Caenorhabditis elegans

Mutations in the him-5 gene in Caenorhabditis elegans strongly reduce the frequency of crossovers on the X chromosome, with lesser effects on the autosomes. him-5 mutants also show a change in crossover distribution on both the X and autosomes. These phenotypes are accompanied by a delayed entry into pachytene and premature desynapsis of the X chromosome. The nondisjunction, progression defects and desynapsis can be rescued by an exogenous source of double strand breaks (DSBs), indicating that the role of HIM-5 is to promote the formation of meiotic DSBs. Molecular cloning of the gene shows that the inferred HIM-5 product is a highly basic protein of 252 amino acids with no clear orthologs in other species, including other Caenorhabditis species. Although him-5 mutants are defective in segregation of the X chromosome, HIM-5 protein localizes preferentially to the autosomes. The mutant phenotypes and localization of him-5 are similar but not identical to the results seen with xnd-1, although unlike xnd-1, him-5 has no apparent effect on the acetylation of histone H2A on lysine 5 (H2AacK5). The localization of HIM-5 to the autosomes depends on the activities of both xnd-1 and him-17 allowing us to begin to establish pathways for the control of crossover distribution and frequency.

MRG-1 is required for genomic integrity in Caenorhabditis elegans germ cells

During meiotic cell division, proper chromosome synapsis and accurate repair of DNA double strand breaks (DSBs) are required to maintain genomic integrity, loss of which leads to apoptosis or meiotic defects. The mechanisms underlying meiotic chromosome synapsis, DSB repair and apoptosis are not fully understood. Here, we report that the chromodomain-containing protein MRG-1 is an important factor for genomic integrity in meiosis in Caenorhabditis elegans. Loss of mrg-1 function resulted in a significant increase in germ cell apoptosis that was partially inhibited by mutations affecting DNA damage checkpoint genes. Consistently, mrg-1 mutant germ lines exhibited SPO-11-generated DSBs and elevated exogenous DNA damage-induced chromosome fragmentation at diakinesis. In addition, the excessive apoptosis in mrg-1 mutants was partially suppressed by loss of the synapsis checkpoint gene pch-2, and a significant number of meiotic nuclei accumulated at the leptotene/zygotene stages with an elevated level of H3K9me2 on the chromatin, which was similarly observed in mutants deficient in the synaptonemal complex, suggesting that the proper progression of chromosome synapsis is likely impaired in the absence of mrg-1. Altogether, these findings suggest that MRG-1 is critical for genomic integrity by promoting meiotic DSB repair and synapsis progression in meiosis.

Chromosome-biased binding and gene regulation by the Caenorhabditis elegans DRM complex

DRM is a conserved transcription factor complex that includes E2F/DP and pRB family proteins and plays important roles in development and cancer. Here we describe new aspects of DRM binding and function revealed through genome-wide analyses of the Caenorhabditis elegans DRM subunit LIN-54. We show that LIN-54 DNA-binding activity recruits DRM to promoters enriched for adjacent putative E2F/DP and LIN-54 binding sites, suggesting that these two DNA-binding moieties together direct DRM to its target genes. Chromatin immunoprecipitation and gene expression profiling reveals conserved roles for DRM in regulating genes involved in cell division, development, and reproduction. We find that LIN-54 promotes expression of reproduction genes in the germline, but prevents ectopic activation of germline-specific genes in embryonic soma. Strikingly, C. elegans DRM does not act uniformly throughout the genome: the DRM recruitment motif, DRM binding, and DRM-regulated embryonic genes are all under-represented on the X chromosome. However, germline genes down-regulated in lin-54 mutants are over-represented on the X chromosome. We discuss models for how loss of autosome-bound DRM may enhance germline X chromosome silencing. We propose that autosome-enriched binding of DRM arose in C. elegans as a consequence of germline X chromosome silencing and the evolutionary redistribution of germline-expressed and essential target genes to autosomes. Sex chromosome gene regulation may thus have profound evolutionary effects on genome organization and transcriptional regulatory networks.

Exploring the link between MORF4L1 and risk of breast cancer

INTRODUCTION

Proteins encoded by Fanconi anemia (FA) and/or breast cancer (BrCa) susceptibility genes cooperate in a common DNA damage repair signaling pathway. To gain deeper insight into this pathway and its influence on cancer risk, we searched for novel components through protein physical interaction screens.

METHODS

Protein physical interactions were screened using the yeast two-hybrid system. Co-affinity purifications and endogenous co-immunoprecipitation assays were performed to corroborate interactions. Biochemical and functional assays in human, mouse and Caenorhabditis elegans models were carried out to characterize pathway components. Thirteen FANCD2-monoubiquitinylation-positive FA cell lines excluded for genetic defects in the downstream pathway components and 300 familial BrCa patients negative for BRCA1/2 mutations were analyzed for genetic mutations. Common genetic variants were genotyped in 9,573 BRCA1/2 mutation carriers for associations with BrCa risk.

RESULTS

A previously identified co-purifying protein with PALB2 was identified, MRG15 (MORF4L1 gene). Results in human, mouse and C. elegans models delineate molecular and functional relationships with BRCA2, PALB2, RAD51 and RPA1 that suggest a role for MRG15 in the repair of DNA double-strand breaks. Mrg15-deficient murine embryonic fibroblasts showed moderate sensitivity to γ-irradiation relative to controls and reduced formation of Rad51 nuclear foci. Examination of mutants of MRG15 and BRCA2 C. elegans orthologs revealed phenocopy by accumulation of RPA-1 (human RPA1) nuclear foci and aberrant chromosomal compactions in meiotic cells. However, no alterations or mutations were identified for MRG15/MORF4L1 in unclassified FA patients and BrCa familial cases. Finally, no significant associations between common MORF4L1 variants and BrCa risk for BRCA1 or BRCA2 mutation carriers were identified: rs7164529, Ptrend = 0.45 and 0.05, P2df = 0.51 and 0.14, respectively; and rs10519219, Ptrend = 0.92 and 0.72, P2df = 0.76 and 0.07, respectively.

CONCLUSIONS

While the present study expands on the role of MRG15 in the control of genomic stability, weak associations cannot be ruled out for potential low-penetrance variants at MORF4L1 and BrCa risk among BRCA2 mutation carriers.

Biology of the Mi-2/NuRD Complex in SLAC (Stemness, Longevity/Ageing, and Cancer)

The dynamic chromatin activities of Mi-2/Nucleosome Remodeling and Histone deacetylation (Mi-2/NuRD) complexes in mammals are at the basis of current research on stemness, longevity/ageing, and cancer (4-2-1/SLAC), and have been widely studied over the past decade in mammals and the elegant model organism, Caenorhabditis elegans. Interestingly, a common emergent theme from these studies is that of distinct coregulator-recruited Mi-2/NuRD complexes largely orchestrating the 4-2-1/SLAC within a unique paradigm by maintaining genome stability via DNA repair and controlling three types of transcriptional programs in concert in a number of cellular, tissue, and organism contexts. Thus, the core Mi-2/NuRD complex plays a central role in 4-2-1/SLAC. The plasticity and robustness of 4-2-1/SLAC can be interpreted as modulation of specific coregulator(s) within cell-specific, tissue-specific, stage-specific, or organism-specific niches during stress induction, ie, a functional module and its networking, thereby conferring differential responses to different environmental cues. According to “Occam’s razor”, a simple theory is preferable to a complex one, so this simplified notion might be useful for exploring 4-2-1/SLAC with a holistic view. This thought could also be valuable in forming strategies for future research, and could open up avenues for cancer prevention and antiageing strategies.

A statistical framework for modeling gene expression using chromatin features and application to modENCODE datasets

We develop a statistical framework to study the relationship between chromatin features and gene expression. This can be used to predict gene expression of protein coding genes, as well as microRNAs. We demonstrate the prediction in a variety of contexts, focusing particularly on the modENCODE worm datasets. Moreover, our framework reveals the positional contribution around genes (upstream or downstream) of distinct chromatin features to the overall prediction of expression levels.

The epigenetics of germ-line immortality: lessons from an elegant model system

Epigenetic mechanisms are thought to help regulate the unique transcription program that is established in germ cell development. During the germline cycle of many organisms, the epigenome undergoes waves of extensive resetting events, while a part of epigenetic modification remains faithful to specific loci. Little is known about the mechanisms underlying these events, how loci are selected for, or avoid, reprogramming, or even why these events are required. In particular, although the significance of genomic imprinting phenomena involving DNA methylation in mammals is now well accepted, the role of histone modification as a transgenerational epigenetic mechanism has been the subject of debate. Such epigenetic mechanisms may help regulate transcription programs and/or the pluripotent status conferred on germ cells, and contribute to germ line continuity across generations. Recent studies provide new evidence for heritability of histone modifications through germ line cells and its potential effects on transcription regulation both in the soma and germ line of subsequent generations. Unraveling transgenerational epigenetic mechanisms involving highly conserved histone modifications in elegant model systems will accelerate the generation of new paradigms and inspire research in a wide variety of fields, including basic developmental studies and clinical stem cell research.

The histone H3K36 methyltransferase MES-4 acts epigenetically to transmit the memory of germline gene expression to progeny

Methylation of histone H3K36 in higher eukaryotes is mediated by multiple methyltransferases. Set2-related H3K36 methyltransferases are targeted to genes by association with RNA Polymerase II and are involved in preventing aberrant transcription initiation within the body of genes. The targeting and roles of the NSD family of mammalian H3K36 methyltransferases, known to be involved in human developmental disorders and oncogenesis, are not known. We used genome-wide chromatin immunoprecipitation (ChIP) to investigate the targeting and roles of the Caenorhabditis elegans NSD homolog MES-4, which is maternally provided to progeny and is required for the survival of nascent germ cells. ChIP analysis in early C. elegans embryos revealed that, consistent with immunostaining results, MES-4 binding sites are concentrated on the autosomes and the leftmost ∼2% (300 kb) of the X chromosome. MES-4 overlies the coding regions of approximately 5,000 genes, with a modest elevation in the 5′ regions of gene bodies. Although MES-4 is generally found over Pol II-bound genes, analysis of gene sets with different temporal-spatial patterns of expression revealed that Pol II association with genes is neither necessary nor sufficient to recruit MES-4. In early embryos, MES-4 associates with genes that were previously expressed in the maternal germ line, an interaction that does not require continued association of Pol II with those loci. Conversely, Pol II association with genes newly expressed in embryos does not lead to recruitment of MES-4 to those genes. These and other findings suggest that MES-4, and perhaps the related mammalian NSD proteins, provide an epigenetic function for H3K36 methylation that is novel and likely to be unrelated to ongoing transcription. We propose that MES-4 transmits the memory of gene expression in the parental germ line to offspring and that this memory role is critical for the PGCs to execute a proper germline program.

Germ cells transmit the genome from one generation to the next. The identity and immortality of germ cells are crucial for the perpetuation of species, yet the mechanisms that regulate these properties remain elusive. In C.elegans, a histone methyltransferase MES-4 is required for survival of the primordial germ cells. MES-4 methylates histone H3 at lysine 36 (H3K36), a modification previously linked to transcription elongation and involved in preventing aberrant transcription initiation within the body of genes. Surprisingly, our genome-wide analysis of MES-4 binding sites in C. elegans embryos revealed that MES-4 is capable of associating with genes that were expressed in the germ line of the parent worms but are no longer being actively transcribed in embryos. To our knowledge, this is the first example of transcription-uncoupled H3K36 methylation. We suggest that MES-4-generated H3K36 methylation serves an “epigenetic role,” by marking germline-expressed genes and by carrying the memory of gene expression from one generation of germ cells to the next.

Trans-generational epigenetic regulation of C. elegans primordial germ cells

BACKGROUND

The processes through which the germline maintains its continuity across generations has long been the focus of biological research. Recent studies have suggested that germline continuity can involve epigenetic regulation, including regulation of histone modifications. However, it is not clear how histone modifications generated in one generation can influence the transcription program and development of germ cells of the next.

RESULTS

We show that the histone H3K36 methyltransferase maternal effect sterile (MES)-4 is an epigenetic modifier that prevents aberrant transcription activity in Caenorhabditis elegans primordial germ cells (PGCs). In mes-4 mutant PGCs, RNA Pol II activation is abnormally regulated and the PGCs degenerate. Genetic and genomewide analyses of MES-4-mediated H3K36 methylation suggest that MES-4 activity can operate independently of ongoing transcription, and may be predominantly responsible for maintenance methylation of H3K36 in germline-expressed loci.

CONCLUSIONS

Our data suggest a model in which MES-4 helps to maintain an 'epigenetic memory' of transcription that occurred in germ cells of previous generations, and that MES-4 and its epigenetic product are essential for normal germ cell development.

Conditional inactivation of MRG15 gene function limits survival during larval and adult stages of Drosophila melanogaster

The mammalian MRG15 gene encodes a chromodomain protein predicted to bind to chromatin via methylated histone tails. Human MORF4 encodes a related but truncated protein that is capable of promoting cellular senescence in a subset of human tumor cell lines. Drosophila contains a single homolog of human MRG15, called DmMRG15. Null mutation of MRG15 is embryonic-lethal in mice and Drosophila, making the study of MRG15 requirements in adults difficult. In these studies the DmMRG15 gene was over-expressed in Drosophila, during developmental stages and in adults, using a doxycycline-regulated system (Tet-on). In addition an inverted-repeated construct was designed to inactivate DmMRG15 via the RNAi pathway, and RNAi constructs were expressed using both the Tet-on system and Geneswitch system. The DmMRG15 protein was readily expressed in adult flies in a doxycycline-dependent manner. A truncated form of DmMRG15 (called DmMT1) was designed to mimic the structure of human MORF4, and expression of this mutant protein or the inverted-repeat constructs inhibited fertility in females. Conditional expression of the DmMRG15 inverted-repeat constructs during larval development or in adults caused reductions in survival. These experiments indicate that Drosophila DmMRG15 gene function is required for female fertility, larval survival and adult life span, and provide reagents that should be useful for further dissecting the role of DmMRG15 in cell proliferation and aging.

Meiotic silencing in Caenorhabditis elegans

In many animals and some fungi, mechanisms have been described that target unpaired chromosomes and chromosomal regions for silencing during meiotic prophase. These phenomena, collectively called "meiotic silencing," target sex chromosomes in the heterogametic sex, for example, the X chromosome in male nematodes and the XY-body in male mice, and also target any other chromosomes that fail to synapse due to mutation or chromosomal rearrangement. Meiotic silencing phenomena are hypothesized to maintain genome integrity and perhaps function in setting up epigenetic control of embryogenesis. This review focuses on meiotic silencing in the nematode, Caenorhabditis elegans, including its mechanism and function(s), and its relationship to other gene silencing processes in the germ line. One hallmark of meiotic silencing in C. elegans is that unpaired/unsynapsed chromosomes and chromosomal regions become enriched for a repressive histone modification, dimethylation of histone H3 on lysine 9 (H3K9me2). Accumulation and proper targeting of H3K9me2 rely on activity of an siRNA pathway, suggesting that histone methyltransferase activity may be targeted/regulated by a small RNA-based transcriptional silencing mechanism.

Cancer models in Caenorhabditis elegans

Although now dogma, the idea that nonvertebrate organisms such as yeast, worms, and flies could inform, and in some cases even revolutionize, our understanding of oncogenesis in humans was not immediately obvious. Aided by the conservative nature of evolution and the persistence of a cohort of devoted researchers, the role of model organisms as a key tool in solving the cancer problem has, however, become widely accepted. In this review, we focus on the nematode Caenorhabditis elegans and its diverse and sometimes surprising contributions to our understanding of the tumorigenic process. Specifically, we discuss findings in the worm that address a well-defined set of processes known to be deregulated in cancer cells including cell cycle progression, growth factor signaling, terminal differentiation, apoptosis, the maintenance of genome stability, and developmental mechanisms relevant to invasion and metastasis.

Restricting dosage compensation complex binding to the X chromosomes by H2A.Z/HTZ-1

Dosage compensation ensures similar levels of X-linked gene products in males (XY or XO) and females (XX), despite their different numbers of X chromosomes. In mammals, flies, and worms, dosage compensation is mediated by a specialized machinery that localizes to one or both of the X chromosomes in one sex resulting in a change in gene expression from the affected X chromosome(s). In mammals and flies, dosage compensation is associated with specific histone posttranslational modifications and replacement with variant histones. Until now, no specific histone modifications or histone variants have been implicated in Caenorhabditis elegans dosage compensation. Taking a candidate approach, we have looked at specific histone modifications and variants on the C. elegans dosage compensated X chromosomes. Using RNAi-based assays, we show that reducing levels of the histone H2A variant, H2A.Z (HTZ-1 in C. elegans), leads to partial disruption of dosage compensation. By immunofluorescence, we have observed that HTZ-1 is under-represented on the dosage compensated X chromosomes, but not on the non-dosage compensated male X chromosome. We find that reduction of HTZ-1 levels by RNA interference (RNAi) and mutation results in only a very modest change in dosage compensation complex protein levels. However, in these animals, the X chromosome-specific localization of the complex is partially disrupted, with some nuclei displaying DCC localization beyond the X chromosome territory. We propose a model in which HTZ-1, directly or indirectly, serves to restrict the dosage compensation complex to the X chromosome by acting as or regulating the activity of an autosomal repellant.

Identification of genes expressed in the hermaphrodite germ line of C. elegans using SAGE

BACKGROUND

Germ cells must progress through elaborate developmental stages from an undifferentiated germ cell to a fully differentiated gamete. Some of these stages include exiting mitosis and entering meiosis, progressing through the various stages of meiotic prophase, adopting either a male (sperm) or female (oocyte) fate, and completing meiosis. Additionally, many of the factors needed to drive embryogenesis are synthesized in the germ line. To increase our understanding of the genes that might be necessary for the formation and function of the germ line, we have constructed a SAGE library from hand dissected C. elegans hermaphrodite gonads.

RESULTS

We found that 4699 genes, roughly 21% of all known C. elegans genes, are expressed in the adult hermaphrodite germ line. Ribosomal genes are highly expressed in the germ line; roughly four fold above their expression levels in the soma. We further found that 1063 of the germline-expressed genes have enriched expression in the germ line as compared to the soma. A comparison of these 1063 germline-enriched genes with a similar list of genes prepared using microarrays revealed an overlap of 460 genes, mutually reinforcing the two lists. Additionally, we identified 603 germline-enriched genes, supported by in situ expression data, which were not previously identified. We also found >4 fold enrichment for RNA binding proteins in the germ line as compared to the soma.

CONCLUSION

Using multiple technological platforms provides a more complete picture of global gene expression patterns. Genes involved in RNA metabolism are expressed at a significantly higher level in the germ line than the soma, suggesting a stronger reliance on RNA metabolism for control of the expression of genes in the germ line. Additionally, the number and expression level of germ line expressed genes on the X chromosome is lower than expected based on a random distribution.

Less is more: specification of the germline by transcriptional repression

In animals, the germline is the only lineage that transmits genetic information to the next generation. Although the founder cells of this lineage are specified differently in invertebrates and vertebrates, recent studies have shown that germline specification in C. elegans, Drosophila and mouse depends on the global inhibition of mRNA transcription. Different strategies are used in each organism, but remarkably most target the same two processes: transcriptional elongation and chromatin remodeling. This convergence suggests that a repressed genome is essential to preserve the unique developmental potential of the germline.

Germ versus soma decisions: lessons from flies and worms

The early embryo is formed by the fusion of two germ cells that must generate not only all of the nonreproductive somatic cell types of its body but also the germ cells for the next generation. Therefore, embryo cells face a crucial decision: whether to develop as germ or soma. How is this fundamental decision made and germ cell fate maintained during development? Studies in the nematode worm Caenorhabditis elegans and fruit fly Drosophila identify some of the decision-making strategies, including segregation of a specialized germ plasm and global transcriptional regulation.