H3K36 Methylation Regulates Nutrient Stress Response in Saccharomyces cerevisiae by Enforcing Transcriptional Fidelity

Repression of pervasive antisense transcription is the primary role of fission yeast RNA polymerase II CTD serine 2 phosphorylation

The RNA polymerase II carboxy-terminal domain (CTD) consists of conserved heptapeptide repeats that can be phosphorylated to influence distinct stages of the transcription cycle, including RNA processing. Although CTD-associated proteins have been identified, phospho-dependent CTD interactions have remained elusive. Proximity-dependent biotinylation (PDB) has recently emerged as an alternative approach to identify protein-protein associations in the native cellular environment. In this study, we present a PDB-based map of the fission yeast RNAPII CTD interactome in living cells and identify phospho-dependent CTD interactions by using a mutant in which Ser2 was replaced by alanine in every repeat of the fission yeast CTD. This approach revealed that CTD Ser2 phosphorylation is critical for the association between RNAPII and the histone methyltransferase Set2 during transcription elongation, but is not required for 3' end processing and transcription termination. Accordingly, loss of CTD Ser2 phosphorylation causes a global increase in antisense transcription, correlating with elevated histone acetylation in gene bodies. Our findings reveal that the fundamental role of CTD Ser2 phosphorylation is to establish a chromatin-based repressive state that prevents cryptic intragenic transcription initiation.

Molecular mechanisms in regulation of autophagy and apoptosis in view of epigenetic regulation of genes and involvement of liquid-liquid phase separation

Cellular physiology is critically regulated by multiple signaling nexuses, among which cell death mechanisms play crucial roles in controlling the homeostatic landscape at the tissue level within an organism. Apoptosis, also known as programmed cell death, can be induced by external and internal stimuli directing the cells to commit suicide in unfavourable conditions. In contrast, stress conditions like nutrient deprivation, infection and hypoxia trigger autophagy, which is lysosome-mediated processing of damaged cellular organelle for recycling of the degraded products, including amino acids. Apparently, apoptosis and autophagy both are catabolic and tumor-suppressive pathways; apoptosis is essential during development and cancer cell death, while autophagy promotes cell survival under stress. Moreover, autophagy plays dual role during cancer development and progression by facilitating the survival of cancer cells under stressed conditions and inducing death in extreme adversity. Despite having two different molecular mechanisms, both apoptosis and autophagy are interconnected by several crosslinking intermediates. Epigenetic modifications, such as DNA methylation, post-translational modification of histone tails, and miRNA play a pivotal role in regulating genes involved in both autophagy and apoptosis. Both autophagic and apoptotic genes can undergo various epigenetic modifications and promote or inhibit these processes under normal and cancerous conditions. Epigenetic modifiers are uniquely important in controlling the signaling pathways regulating autophagy and apoptosis. Therefore, these epigenetic modifiers of both autophagic and apoptotic genes can act as novel therapeutic targets against cancers. Additionally, liquid-liquid phase separation (LLPS) also modulates the aggregation of misfolded proteins and provokes autophagy in the cytosolic environment. This review deals with the molecular mechanisms of both autophagy and apoptosis including crosstalk between them; emphasizing epigenetic regulation, involvement of LLPS therein, and possible therapeutic approaches against cancers.

Profiling The Compendium Of Changes In Saccharomyces cerevisiae Due To Mutations That Alter Availability Of The Main Methyl Donor S-Adenosylmethionine

The SAM1 and SAM2 genes encode for S-AdenosylMethionine (AdoMet) synthetase enzymes, with AdoMet serving as the main methyl donor. We have previously shown that independent deletion of these genes alters chromosome stability and AdoMet concentrations in opposite ways in S. cerevisiae. To characterize other changes occurring in these mutants, we grew wildtype, sam1∆/sam1∆, and sam2∆/sam2∆ strains in 15 different Phenotypic Microarray plates with different components, equal to 1440 wells, and measured for growth variations. RNA-Sequencing was also carried out on these strains and differential gene expression determined for each mutant. In this study, we explore how the phenotypic growth differences are linked to the altered gene expression, and thereby predict the mechanisms by which loss of the SAM genes and subsequent AdoMet level changes, impact S. cerevisiae pathways and processes. We present six stories, discussing changes in sensitivity or resistance to azoles, cisplatin, oxidative stress, arginine biosynthesis perturbations, DNA synthesis inhibitors, and tamoxifen, to demonstrate the power of this novel methodology to broadly profile changes due to gene mutations. The large number of conditions that result in altered growth, as well as the large number of differentially expressed genes with wide-ranging functionality, speaks to the broad array of impacts that altering methyl donor abundance can impart, even when the conditions tested were not specifically selected as targeting known methyl involving pathways. Our findings demonstrate that some cellular changes are directly related to AdoMet-dependent methyltransferases and AdoMet availability, some are directly linked to the methyl cycle and its role is production of several important cellular components, and others reveal impacts of SAM gene mutations on previously unconnected pathways.

Heterozygous Mutations in Aromatic Amino Acid Synthesis Genes Trigger TOR Pathway Activation in Saccharomyces cerevisiae

The highly conserved complexes of Target of Rapamycin (TORC1 and TORC2) are central regulators to many vital cellular processes including growth and autophagy in response to nutrient availability. Previous research has extensively elucidated exogenous nutrient control on TORC1/TORC2; however, little is known about the potential alteration of nutrient pools from mutations in biosynthesis pathways and their impact on Tor pathway activity. Here, we analyze the impacts of heterozygous mutations in aromatic amino acid biosynthesis genes on TOR signaling via differential expression of genes downstream of TORC1 and autophagy induction for TORC1 and TORC2 activity.

Histone chaperone ASF1 mediates H3.3-H4 deposition in Arabidopsis

Histone chaperones and chromatin remodelers control nucleosome dynamics, which are essential for transcription, replication, and DNA repair. The histone chaperone Anti-Silencing Factor 1 (ASF1) plays a central role in facilitating CAF-1-mediated replication-dependent H3.1 deposition and HIRA-mediated replication-independent H3.3 deposition in yeast and metazoans. Whether ASF1 function is evolutionarily conserved in plants is unknown. Here, we show that Arabidopsis ASF1 proteins display a preference for the HIRA complex. Simultaneous mutation of both Arabidopsis ASF1 genes caused a decrease in chromatin density and ectopic H3.1 occupancy at loci typically enriched with H3.3. Genetic, transcriptomic, and proteomic data indicate that ASF1 proteins strongly prefers the HIRA complex over CAF-1. asf1 mutants also displayed an increase in spurious Pol II transcriptional initiation and showed defects in the maintenance of gene body CG DNA methylation and in the distribution of histone modifications. Furthermore, ectopic targeting of ASF1 caused excessive histone deposition, less accessible chromatin, and gene silencing. These findings reveal the importance of ASF1-mediated histone deposition for proper epigenetic regulation of the genome.

Phylogenomic and Evolutionary Analyses Reveal Diversifications of SET-Domain Proteins in Fungi

In recent years, many publications have established histone lysine methylation as a central epigenetic modification in the regulation of chromatin and transcription. The histone lysine methyltransferases contain a conserved SET domain and are widely distributed in various organisms. However, a comprehensive study on the origin and diversification of the SET-domain-containing genes in fungi has not been conducted. In this study, a total of 3816 SET-domain-containing genes, which were identified and characterized using HmmSearch from 229 whole genomes sequenced fungal species, were used to ascertain their evolution and diversification in fungi. Using the CLANS program, all the SET-domain-containing genes were grouped into three main clusters, and each cluster contains several groups. Domain organization analysis showed that genes belonging to the same group have similar sequence structures. In contrast, different groups process domain organizations or locations differently, suggesting the SET-domain-containing genes belonging to different groups may have obtained distinctive regulatory mechanisms during their evolution. These genes that conduct the histone methylations (such as H3K4me, H3K9me, H3K27me, H4K20me, H3K36me) are mainly grouped into Cluster 1 while the other genes grouped into Clusters 2 and 3 are still functionally undetermined. Our results also showed that numerous gene duplication and loss events have happened during the evolution of those fungal SET-domain-containing proteins. Our results provide novel insights into the roles of SET-domain genes in fungal evolution and pave a fundamental path to further understanding the epigenetic basis of gene regulation in fungi.

A reciprocal translocation involving Aspergillus nidulans snxAHrb1/Gbp2 and gyfA uncovers a new regulator of the G2-M transition and reveals a role in transcriptional repression for the setBSet2 histone H3-lysine-36 methyltransferase

Aspergillus nidulans snxA, an ortholog of Saccharomyces cerevisiae Hrb1/Gbp2 messenger RNA shuttle proteins, is-in contrast to budding yeast-involved in cell cycle regulation, in which snxA1 and snxA2 mutations as well as a snxA deletion specifically suppress the heat sensitivity of mutations in regulators of the CDK1 mitotic induction pathway. snxA mutations are strongly cold sensitive, and at permissive temperature snxA mRNA and protein expression are strongly repressed. Initial attempts to identify the causative snxA mutations revealed no defects in the SNXA protein. Here, we show that snxA1/A2 mutations resulted from an identical chromosome I-II reciprocal translocation with breakpoints in the snxA first intron and the fourth exon of a GYF-domain gene, gyfA. Surprisingly, a gyfA deletion and a reconstructed gyfA translocation allele suppressed the heat sensitivity of CDK1 pathway mutants in a snxA+ background, demonstrating that 2 unrelated genes, snxA and gyfA, act through the CDK1-CyclinB axis to restrain the G2-M transition, and for the first time identifying a role in G2-M regulation for a GYF-domain protein. To better understand snxA1/A2-reduced expression, we generated suppressors of snxA cold sensitivity in 2 genes: (1) loss of the abundant nucleolar protein Nsr1/nucleolin bypassed the requirement for snxA and (2) loss of the Set2 histone H3 lysine36 (H3K36) methyltransferase or a nonmethylatable histone H3K36L mutant rescued hypomorphic snxA mutants by restoring full transcriptional proficiency, indicating that methylation of H3K36 acts normally to repress snxA transcription. These observations are in line with known Set2 functions in preventing excessive and cryptic transcription of active genes.

Identification of epigenetic dysregulation gene markers and immune landscape in kidney renal clear cell carcinoma by comprehensive genomic analysis

Kidney cancer is one the most lethal cancers of the urinary system, but current treatments are limited and its prognosis is poor. This study focused on kidney renal clear cell carcinoma (KIRC) and analyzed the relationship between epigenetic alterations and KIRC prognosis, and explored the prognostic significance of these findings in KIRC patients. Based on multi-omics data, differentially expressed histone-modified genes were identified using the R package limma package. Gene enhancers were detected from data in the FANTOM5 database. Gene promoters were screened using the R package ChIPseeker, and the Bumphunter in the R package CHAMP was applied to screen differentially methylated regions (DMR). Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis and Gene Ontology (GO) functional enrichment analysis of genes was performed using the R package clusterProfiler. We identified 51 dysregulated epigenetic protein coding genes (epi-PCGs) from 872 epi-PCGs, and categorized three molecular subtypes (C1, C2, and C3) of KIRC samples with significantly different prognosis. Notably, among the three molecular subtypes, we found a markedly differential immune features in immune checkpoints, cytokines, immune signatures, and immune cell distribution. C2 subtype had significantly lower enrichment score of IFNγ, cytotoxic score (CYT), and angiogenesis. In addition, an 8-gene signature containing 8 epi-PCGs (ETV4, SH2B3, FATE1, GRK5, MALL, HRH2, SEMA3G, and SLC10A6) was developed for predicting KIRC prognosis. Prognosis of patients with a high 8-gene signature score was significantly worse than those with a low 8-gene signature score, which was also validated by the independent validation data. The 8-gene signature had a better performance compared with previous signatures of KIRC. Overall, this study highlighted the important role of epigenetic regulation in KIRC development, and explored prognostic epi-PCGs, which may provide a guidance for exploiting further pathological mechanisms of KIRC and for developing novel drug targets.

A Saccharomyces cerevisiae model and screen to define the functional consequences of oncogenic histone missense mutations

Somatic missense mutations in histone genes turn these essential proteins into oncohistones, which can drive oncogenesis. Understanding how missense mutations alter histone function is challenging in mammals as mutations occur in a single histone gene. For example, described oncohistone mutations predominantly occur in the histone H3.3 gene, despite the human genome encoding 15 H3 genes. To understand how oncogenic histone missense mutations alter histone function, we leveraged the budding yeast model, which contains only 2 H3 genes, to explore the functional consequences of oncohistones H3K36M, H3G34W, H3G34L, H3G34R, and H3G34V. Analysis of cells that express each of these variants as the sole copy of H3 reveals that H3K36 mutants show different drug sensitivities compared to H3G34 mutants. This finding suggests that changes to proximal amino acids in the H3 N-terminal tail alter distinct biological pathways. We exploited the caffeine-sensitive growth of H3K36-mutant cells to perform a high copy suppressor screen. This screen identified genes linked to histone function and transcriptional regulation, including Esa1, a histone H4/H2A acetyltransferase; Tos4, a forkhead-associated domain-containing gene expression regulator; Pho92, an N6-methyladenosine RNA-binding protein; and Sgv1/Bur1, a cyclin-dependent kinase. We show that the Esa1 lysine acetyltransferase activity is critical for suppression of the caffeine-sensitive growth of H3K36R-mutant cells while the previously characterized binding interactions of Tos4 and Pho92 are not required for suppression. This screen identifies pathways that could be altered by oncohistone mutations and highlights the value of yeast genetics to identify pathways altered by such mutations.

RNAPII driven post-translational modifications of nucleosomal histones

The current understanding of how specific distributions of histone post-translational modifications (PTMs) are achieved throughout the chromatin remains incomplete. This review focuses on the role of RNA polymerase II (RNAPII) in establishing H2BK120/K123 ubiquitination and H3K4/K36 methylation distribution. The rate of RNAPII transcription is mainly a function of the RNAPII elongation and recruitment rates. Two major mechanisms link RNAPII's transcription rate to the distribution of PTMs. First, the phosphorylation patterns of Ser2P/Ser5P in the C-terminal domain of RNAPII change as a function of time, since the start of elongation, linking them to the elongation rate. Ser2P/Ser5P recruits specific histone PTM enzymes/activators to the nucleosome. Second, multiple rounds of binding and catalysis by the enzymes are required to establish higher methylations (H3K4/36me3). Thus, methylation states are determined by the transcription rate. In summary, the first mechanism determines the location of methylations in the gene, while the second mechanism determines the methylation state.

Structural and functional specificity of H3K36 methylation

The methylation of histone H3 at lysine 36 (H3K36me) is essential for maintaining genomic stability. Indeed, this methylation mark is essential for proper transcription, recombination, and DNA damage response. Loss- and gain-of-function mutations in H3K36 methyltransferases are closely linked to human developmental disorders and various cancers. Structural analyses suggest that nucleosomal components such as the linker DNA and a hydrophobic patch constituted by histone H2A and H3 are likely determinants of H3K36 methylation in addition to the histone H3 tail, which encompasses H3K36 and the catalytic SET domain. Interaction of H3K36 methyltransferases with the nucleosome collaborates with regulation of their auto-inhibitory changes fine-tunes the precision of H3K36me in mediating dimethylation by NSD2 and NSD3 as well as trimethylation by Set2/SETD2. The identification of specific structural features and various cis-acting factors that bind to different forms of H3K36me, particularly the di-(H3K36me2) and tri-(H3K36me3) methylated forms of H3K36, have highlighted the intricacy of H3K36me functional significance. Here, we consolidate these findings and offer structural insight to the regulation of H3K36me2 to H3K36me3 conversion. We also discuss the mechanisms that underlie the cooperation between H3K36me and other chromatin modifications (in particular, H3K27me3, H3 acetylation, DNA methylation and N⁶-methyladenosine in RNAs) in the physiological regulation of the epigenomic functions of chromatin.

Identification and Validation of Apparent Imbalanced Epi-lncRNAs Prognostic Model Based on Multi-Omics Data in Pancreatic Cancer

Background: Globally, pancreatic adenocarcinoma is a recognized cause of pancreatic death (PAAD) associated with high mortality. Long non-coding RNAs (lncRNAs) play an important role in several biological processes in pancreatic cancer.

Methods: The gene expression profile of PAAD patients were obtained from The Cancer Genome Atlas (TCGA) database. The limma package was used to identify epigenetic disorders of lncRNAs and PCG. Subsequently, the genomic characteristics and landscape of lncRNAs were explored. The pancreatic cancer-related lncRNAs gene set from Lnc2Cancer v3.0 were collected and the difference between cancer samples and normal samples were analysed. A prognostic model consisting of five epigenetic lncRNA (epi-lncRNAs) was established by univariate and multivariate Cox proportional hazards regression analyses and was verified across different data sets. Finally, the expression of core epi-lncRNAs was identified by PCR experiment.

Results: A total of 2237 epi-lncRNAs, 11855 non-epi-lncRNAs, 13518 epi-PCGs, and 6097 non-epi-PCGs, were identified. The abnormal frequency of lncRNAs in pancreatic cancer was much lower than that in PCG, and 138 epi-lncRNAs were enriched in human cancer-related lncRNAs. Epi-lncRNAs had a higher number with longer lengths and a greater number of transcripts. Epi-lncRNAs associated with epigenetic disorders had a higher number of exons, gene length, and isomers as compared to non-epi-lncRNAs. Further, the five pancreatic cancer-specific epi-lncRNA genes (AL161431.1, LINC00663, LINC00941, SNHG10, and TM4SF1-AS1) were identified. Based on these five pancreatic cancer-specific epis-lncRNAs, a prognostic model for pancreatic cancer was established. The RT-PCR result confirmed that AL161431.1, LINC00663, LINC00941, and SNHG10 expressions in pancreatic cancer samples were higher as compared to normal pancreatic samples; the expression of TM4SF1-AS1 in pancreatic cancer cells was significantly lower than that in normal pancreatic samples.

Conclusions: Epigenetic abnormalities could promote abnormal lncRNA expression in pancreatic cancer and may play an important role in its progression.

Reciprocal regulation of phosphatidylcholine synthesis and H3K36 methylation programs metabolic adaptation

Phospholipid biosynthesis plays a role in mediating membrane-to-histone communication that influences metabolic decisions. Upon nutrient deprivation, phospholipid methylation generates a starvation signal in the form of S-adenosylmethionine (SAM) depletion, leading to dynamic changes in histone methylation. Here we show that the SAM-responsive methylation of H3K36 is critical for metabolic adaptation to nutrient starvation in the budding yeast Saccharomyces cerevisiae. We find that mutants deficient in H3K36 methylation exhibit defects in membrane integrity and pyrimidine metabolism and lose viability quickly under starvation. Adjusting the synthesis of phospholipids potently rewires metabolic pathways for nucleotide synthesis and boosts the production of antioxidants, ameliorating the defects resulting from the loss of H3K36 methylation. We further demonstrate that H3K36 methylation reciprocally regulates phospholipid synthesis by influencing redox balance. Our study illustrates an adaptive mechanism whereby phospholipid synthesis entails a histone modification to reprogram metabolism for adaptation in a eukaryotic model organism.

How Cells Deal with the Fluctuating Environment: Autophagy Regulation under Stress in Yeast and Mammalian Systems

Eukaryotic cells frequently experience fluctuations of the external and internal environments, such as changes in nutrient, energy and oxygen sources, and protein folding status, which, after reaching a particular threshold, become a type of stress. Cells develop several ways to deal with these various types of stress to maintain homeostasis and survival. Among the cellular survival mechanisms, autophagy is one of the most critical ways to mediate metabolic adaptation and clearance of damaged organelles. Autophagy is maintained at a basal level under normal growing conditions and gets stimulated by stress through different but connected mechanisms. In this review, we summarize the advances in understanding the autophagy regulation mechanisms under multiple types of stress including nutrient, energy, oxidative, and ER stress in both yeast and mammalian systems.

Autophagy in Cancer: A Metabolic Perspective

Autophagy is an intracellular catabolic degradative process in which damaged cellular organelles, unwanted proteins and different cytoplasmic components get recycled to maintain cellular homeostasis or metabolic balance. During autophagy, a double membrane vesicle is formed to engulf these cytosolic materials and fuse to lysosomes wherein the entire cargo degrades to be used again. Because of this unique recycling ability of cells, autophagy is a universal stress response mechanism. Dysregulation of autophagy leads to several diseases, including cancer, neurodegeneration and microbial infection. Thus, autophagy machineries have become targets for therapeutics. This chapter provides an overview of the paradoxical role of autophagy in tumorigenesis in the perspective of metabolism.

Altered Chromatin States Drive Cryptic Transcription in Aging Mammalian Stem Cells

A repressive chromatin state featuring trimethylated lysine 36 on histone H3 (H3K36me3) and DNA methylation suppresses cryptic transcription in embryonic stem cells. Cryptic transcription is elevated with age in yeast and nematodes, and reducing it extends yeast lifespan, though whether this occurs in mammals is unknown. We show that cryptic transcription is elevated in aged mammalian stem cells, including murine hematopoietic stem cells (mHSCs) and neural stem cells (NSCs) and human mesenchymal stem cells (hMSCs). Precise mapping allowed quantification of age-associated cryptic transcription in hMSCs aged in vitro. Regions with significant age-associated cryptic transcription have a unique chromatin signature: decreased H3K36me3 and increased H3K4me1, H3K4me3, and H3K27ac with age. Genomic regions undergoing such changes resemble known promoter sequences and are bound by TBP even in young cells. Hence, the more permissive chromatin state at intragenic cryptic promoters likely underlies increased cryptic transcription in aged mammalian stem cells.

Ready, SET, Go: Post-translational regulation of the histone lysine methylation network in budding yeast

Histone lysine methylation is a key epigenetic modification that regulates eukaryotic transcription. Here, we comprehensively review the function and regulation of the histone methylation network in the budding yeast and model eukaryote, Saccharomyces cerevisiae. First, we outline the lysine methylation sites that are found on histone proteins in yeast (H3K4me1/2/3, H3K36me1/2/3, H3K79me1/2/3, and H4K5/8/12me1) and discuss their biological and cellular roles. Next, we detail the reduced but evolutionarily conserved suite of methyltransferase (Set1p, Set2p, Dot1p, and Set5p) and demethylase (Jhd1p, Jhd2p, Rph1p, and Gis1p) enzymes that are known to control histone lysine methylation in budding yeast cells. Specifically, we illustrate the domain architecture of the methylation enzymes and highlight the structural features that are required for their respective functions and molecular interactions. Finally, we discuss the prevalence of post-translational modifications on yeast histone methylation enzymes and how phosphorylation, acetylation, and ubiquitination in particular are emerging as key regulators of enzyme function. We note that it will be possible to completely connect the histone methylation network to the cell's signaling system, given that all methylation sites and cognate enzymes are known, most phosphosites on the enzymes are known, and the mapping of kinases to phosphosites is tractable owing to the modest set of protein kinases in yeast. Moving forward, we expect that the rich variety of post-translational modifications that decorates the histone methylation machinery will explain many of the unresolved questions surrounding the function and dynamics of this intricate epigenetic network.

Core promoter activity contributes to chromatin-based regulation of internal cryptic promoters

During RNA polymerase II (RNA Pol II) transcription, the chromatin structure undergoes dynamic changes, including opening and closing of the nucleosome to enhance transcription elongation and fidelity. These changes are mediated by transcription elongation factors, including Spt6, the FACT complex, and the Set2-Rpd3S HDAC pathway. These factors not only contribute to RNA Pol II elongation, reset the repressive chromatin structures after RNA Pol II has passed, thereby inhibiting aberrant transcription initiation from the internal cryptic promoters within gene bodies. Notably, the internal cryptic promoters of infrequently transcribed genes are sensitive to such chromatin-based regulation but those of hyperactive genes are not. To determine why, the weak core promoters of genes that generate cryptic transcripts in cells lacking transcription elongation factors (e.g. STE11) were replaced with those from more active genes. Interestingly, as core promoter activity increased, activation of internal cryptic promoter dropped. This associated with loss of active histone modifications at the internal cryptic promoter. Moreover, environmental changes and transcription elongation factor mutations that downregulated the core promoters of highly active genes concomitantly increased their cryptic transcription. We therefore propose that the chromatin-based regulation of internal cryptic promoters is mediated by core promoter strength as well as transcription elongation factors.

Epigenetic Regulation of Autophagy Beyond the Cytoplasm: A Review

Autophagy is a highly conserved catabolic process induced under various stress conditions to protect the cell from harm and allow survival in the face of nutrient- or energy-deficient states. Regulation of autophagy is complex, as cells need to adapt to a continuously changing microenvironment. It is well recognized that the AMPK and mTOR signaling pathways are the main regulators of autophagy. However, various other signaling pathways have also been described to regulate the autophagic process. A better understanding of these complex autophagy regulatory mechanisms will allow the discovery of new potential therapeutic targets. Here, we present a brief overview of autophagy and its regulatory pathways with emphasis on the epigenetic control mechanisms.

IP3-mediated Ca2+ signals regulate larval to pupal transition under nutrient stress through the H3K36 methyltransferase Set2

Persistent loss of dietary protein usually signals a shutdown of key metabolic pathways. In Drosophila larvae that have reached a 'critical weight' and can pupariate to form viable adults, such a metabolic shutdown would needlessly lead to death. Inositol 1,4,5-trisphosphate-mediated calcium (IP3/Ca2+) release in some interneurons (vGlutVGN6341) allows Drosophila larvae to pupariate on a protein-deficient diet by partially circumventing this shutdown through upregulation of neuropeptide signaling and the expression of ecdysone synthesis genes. Here, we show that IP3/Ca2+ signals in vGlutVGN6341 neurons drive expression of Set2, a gene encoding Drosophila Histone 3 Lysine 36 methyltransferase. Furthermore, Set2 expression is required for larvae to pupariate in the absence of dietary protein. IP3/Ca2+ signal-driven Set2 expression upregulates key Ca2+-signaling genes through a novel positive-feedback loop. Transcriptomic studies, coupled with analysis of existing ChIP-seq datasets, identified genes from larval and pupal stages that normally exhibit robust H3K36 trimethyl marks on their gene bodies and concomitantly undergo stronger downregulation by knockdown of either the intracellular Ca2+ release channel IP3R or Set2. IP3/Ca2+ signals thus regulate gene expression through Set2-mediated H3K36 marks on select neuronal genes for the larval to pupal transition.

Set2-mediated H3K36 methylation states redundantly repress the production of antisense transcripts: role in transcription regulation

Methyltransferase Set2‐mediated methylation of histone H3 lysine 36 (H3K36), which involves the addition of up to three methyl groups at this site, has been demonstrated to function in many chromatin‐coupled events. The methylation of H3K36 is known to recruit different chromatin effector proteins, affecting transcription, mRNA splicing and DNA repair. In this study, we engineered two yeast set2 mutants that lack H3K36 mono/dimethylation (H3K36me1/2) and trimethylation (H3K36me3), respectively, and characterized their roles in the production of antisense transcripts under nutrient‐rich conditions. Using our new bioinformatics identification pipeline analysis, we are able to identify a larger number of antisense transcripts in set2∆ cells than has been published previously. We further show that H3K36me1/2 or H3K36me3 redundantly repressed the production of antisense transcripts. Moreover, gene ontology (GO) analysis implies that H3K36me3‐mediated antisense transcription might play a role in DNA replication and DNA damage repair, which is independent of regulation of the corresponding sense gene expression. Overall, our results validate a coregulatory mechanism of different H3K36 methylation states, particularly in the repression of antisense transcription.

Loosening chromatin and dysregulated transcription: a perspective on cryptic transcription during mammalian aging

Cryptic transcription, the initiation of transcription from non-promoter regions within a gene body, is a type of transcriptional dysregulation that occurs throughout eukaryotes. In mammals, cryptic transcription is normally repressed at the level of chromatin, and this process is increased upon perturbation of complexes that increase intragenic histone H3 lysine 4 methylation or decrease intragenic H3 lysine 36 methylation, DNA methylation, or nucleosome occupancy. Significantly, similar changes to chromatin structure occur during aging, and, indeed, recent work indicates that cryptic transcription is elevated during aging in mammalian stem cells. Although increased cryptic transcription is known to promote aging in yeast, whether elevated cryptic transcription also contributes to mammalian aging is unclear. There is ample evidence that perturbations known to increase cryptic transcription are deleterious in embryonic and adult stem cells, and in some cases phenocopy certain aging phenotypes. Furthermore, an increase in cryptic transcription requires or impedes pathways that are known to have reduced function during aging, potentially exacerbating other aging phenotypes. Thus, we propose that increased cryptic transcription contributes to mammalian stem cell aging.

Drosophila NSD deletion induces developmental anomalies similar to those seen in Sotos syndrome 1 patients

BACKGROUND

Haploinsufficiency of the human nuclear receptor binding suppressor of variegation 3-9, enhancer of zeste, and trithorax (SET) domain 1 (NSD1) gene causes a developmental disorder called Sotos syndrome 1 (SOTOS1), which is associated with overgrowth and macrocephaly. NSD family proteins encoding histone H3 lysine 36 (H3K36) methyltransferases are conserved in many species, and Drosophila has a single NSD homolog gene, NSD.

OBJECTIVE

To gain insight into the biological functions of NSD1 deficiency in the developmental anomalies seen in SOTOS1 patients using an NSD-deleted Drosophila mutant.

METHODS

We deleted Drosophila NSD using CRISPR/Cas9-mediated targeted gene knock-out, and analyzed pleiotropic phenotypes of the homozygous mutant of NSD (NSD^-/-) at various developmental stages to understand the roles of NSD in Drosophila.

RESULTS

The site-specific NSD deletion was confirmed in the mutant. The H3K36 di-methylation levels were dramatically decreased in the NSD^-/- fly. Compared with the control, the NSD^-/- fly displayed an increase in the body size of larvae, similar to the childhood overgrowth phenotype of SOTOS1 patients. Although the NSD mutant flies survived to adulthood, their fecundity was dramatically decreased. Furthermore, the NSD^-/- fly showed neurological dysfunctions, such as lower memory performance and motor defects, and a diminished extracellular signal-regulated kinase (ERK) activity.

CONCLUSIONS

The NSD-deleted Drosophila phenotype resembles many of the phenotypes of SOTOS1 patients, such as learning disability, deregulated ERK signaling, and overgrowth; thus, this mutant fly is a relevant model organism to study various SOTOS1 phenotypes.

Proteasomal Regulation of Mammalian SPT16 in Controlling Transcription

FACT (facilitates chromatin transcription), an essential and evolutionarily conserved heterodimer from yeast to humans, controls transcription and is found to be upregulated in various cancers. However, the basis for such upregulation is not clearly understood. Our recent results deciphering a new ubiquitin-proteasome system regulation of the FACT subunit SPT16 in orchestrating transcription in yeast hint at the involvement of the proteasome in controlling FACT in humans, with a link to cancer. To test this, we carried out experiments in human embryonic kidney (HEK293) cells, which revealed that human SPT16 undergoes ubiquitylation and that its abundance is increased following inhibition of the proteolytic activity of the proteasome, thus implying proteasomal regulation of human SPT16. Furthermore, we find that the increased abundance/expression of SPT16 in HEK293 cells alters the transcription of genes, including ones associated with cancer, and that the proteasomal degradation of SPT16 is impaired in kidney cancer (Caki-2) cells to upregulate SPT16. Like human SPT16, murine SPT16 in C2C12 cells also undergoes ubiquitylation and proteasomal degradation to regulate transcription. Collectively, our results reveal a proteasomal regulation of mammalian SPT16, with physiological relevance in controlling transcription, and implicate such proteasomal control in the upregulation of SPT16 in cancer.

An optogenetic switch for the Set2 methyltransferase provides evidence for transcription-dependent and -independent dynamics of H3K36 methylation

Histone H3 lysine 36 methylation (H3K36me) is a conserved histone modification associated with transcription and DNA repair. Although the effects of H3K36 methylation have been studied, the genome-wide dynamics of H3K36me deposition and removal are not known. We established rapid and reversible optogenetic control for Set2, the sole H3K36 methyltransferase in yeast, by fusing the enzyme with the light-activated nuclear shuttle (LANS) domain. Light activation resulted in efficient Set2-LANS nuclear localization followed by H3K36me3 deposition in vivo, with total H3K36me3 levels correlating with RNA abundance. Although genes showed disparate levels of H3K36 methylation, relative rates of H3K36me3 accumulation were largely linear and consistent across genes, suggesting that H3K36me3 deposition occurs in a directed fashion on all transcribed genes regardless of their overall transcription frequency. Removal of H3K36me3 was highly dependent on the demethylase Rph1. However, the per-gene rate of H3K36me3 loss weakly correlated with RNA abundance and followed exponential decay, suggesting H3K36 demethylases act in a global, stochastic manner. Altogether, these data provide a detailed temporal view of H3K36 methylation and demethylation that suggests transcription-dependent and -independent mechanisms for H3K36me deposition and removal, respectively.

setd2 knockout zebrafish is viable and fertile: differential and developmental stress-related requirements for Setd2 and histone H3K36 trimethylation in different vertebrate animals

Setd2 is the only enzyme that catalyzes histone H3 lysine 36 trimethylation (H3K36me3) on virtually all actively transcribed protein-coding genes, and this mechanism is evolutionarily conserved from yeast to human. Despite this widespread and conserved activity, Setd2 and H3K36me3 are dispensable for normal growth of yeast but are absolutely required for mammalian embryogenesis, such as oocyte maturation and embryonic vasculogenesis in mice, raising a question of how the functional requirements of Setd2 in specific developmental stages have emerged through evolution. Here, we explored this issue by studying the essentiality and function of Setd2 in zebrafish. Surprisingly, the setd2-null zebrafish are viable and fertile. They show Mendelian birth ratio and normal embryogenesis without vascular defect as seen in mice; however, they have a small body size phenotype attributed to insufficient energy metabolism and protein synthesis, which is reversable in a nutrition-dependent manner. Unlike the sterile Setd2-null mice, the setd2-null zebrafish can produce functional sperms and oocytes. Nonetheless, related to the requirement of maternal Setd2 for oocyte maturation in mice, the second generation of setd2-null zebrafish that carry no maternal setd2 show decreased survival rate and a developmental delay at maternal-to-zygotic transition. Taken together, these results indicate that, while the phenotypes of the setd2-null zebrafish and mice are apparently different, they are matched in parallel as the underlying mechanisms are evolutionarily conserved. Thus, the differential requirements of Setd2 may reflect distinct viability thresholds that associate with intrinsic and/or extrinsic stresses experienced by the organism through development, and these epigenetic regulatory mechanisms may serve as a reserved source supporting the evolution of life from simplicity to complexity.

Gene repression in S. cerevisiae-looking beyond Sir-dependent gene silencing

Gene silencing by the SIR (Silent Information Region) family of proteins in S. cerevisiae has been extensively studied and has served as a founding paradigm for our general understanding of gene repression and its links to histone deacetylation and chromatin structure. In recent years, our understanding of other mechanisms of gene repression in S.cerevisiae was significantly advanced. In this review, we focus on such Sir-independent mechanisms of gene repression executed by various Histone Deacetylases (HDACs) and Histone Methyl Transferases (HMTs). We focus on the genes regulated by these enzymes and their known mechanisms of action. We describe the cooperation and redundancy between HDACs and HMTs, and their involvement in gene repression by non-coding RNAs or by their non-histone substrates. We also propose models of epigenetic transmission of the chromatin structures produced by these enzymes and discuss these in the context of gene repression phenomena in other organisms. These include the recycling of the epigenetic marks imposed by HMTs or the recycling of the complexes harboring HDACs.

Rph1 coordinates transcription of ribosomal protein genes and ribosomal RNAs to control cell growth under nutrient stress conditions

Coordinated regulation of ribosomal RNA (rRNA) synthesis and ribosomal protein gene (RPG) transcription by eukaryotic RNA polymerases (RNAP) is a key requirement for growth control. Although evidence for balance between RNPI-dependent 35S rRNA production and RNAPII-mediated RPG transcription have been described, the molecular basis is still obscure. Here, we found that Rph1 modulates the transcription status of both rRNAs and RPGs in yeast. We show that Rph1 widely associates with RNAPI and RNAPII-transcribed genes. Deletion of RPH1 remarkably alleviates cell slow growth caused by TORC1 inhibition via derepression of rRNA and RPG transcription under nutrient stress conditions. Mechanistically, Rim15 kinase phosphorylates Rph1 upon rapamycin treatment. Phosphorylation-mimetic mutant of Rph1 exhibited more resistance to rapamycin treatment, decreased association with ribosome-related genes, and faster cell growth compared to the wild-type, indicating that Rph1 dissociation from chromatin ensures cell survival upon nutrient stress. Our results uncover the role of Rph1 in coordination of RNA polymerases-mediated transcription to control cell growth under nutrient stress conditions.

Transcription and splicing: A two-way street

RNA synthesis by RNA polymerase II and RNA processing are closely coupled during the transcription cycle of protein-coding genes. This coupling affords opportunities for quality control and regulation of gene expression and the effects can go in both directions. For example, polymerase speed can affect splice site selection and splicing can increase transcription and affect the chromatin landscape. Here we review the many ways that transcription and splicing influence one another, including how splicing "talks back" to transcription. We will also place the connections between transcription and splicing in the context of other RNA processing events that define the exons that will make up the final mRNA. This article is categorized under: RNA Processing > Splicing Mechanisms RNA Processing > Splicing Regulation/Alternative Splicing.

Regulation of alternative polyadenylation in the yeast Saccharomyces cerevisiae by histone H3K4 and H3K36 methyltransferases

Adjusting DNA structure via epigenetic modifications, and altering polyadenylation (pA) sites at which precursor mRNA is cleaved and polyadenylated, allows cells to quickly respond to environmental stress. Since polyadenylation occurs co-transcriptionally, and specific patterns of nucleosome positioning and chromatin modifications correlate with pA site usage, epigenetic factors potentially affect alternative polyadenylation (APA). We report that the histone H3K4 methyltransferase Set1, and the histone H3K36 methyltransferase Set2, control choice of pA site in Saccharomyces cerevisiae, a powerful model for studying evolutionarily conserved eukaryotic processes. Deletion of SET1 or SET2 causes an increase in serine-2 phosphorylation within the C-terminal domain of RNA polymerase II (RNAP II) and in the recruitment of the cleavage/polyadenylation complex, both of which could cause the observed switch in pA site usage. Chemical inhibition of TOR signaling, which causes nutritional stress, results in Set1- and Set2-dependent APA. In addition, Set1 and Set2 decrease efficiency of using single pA sites, and control nucleosome occupancy around pA sites. Overall, our study suggests that the methyltransferases Set1 and Set2 regulate APA induced by nutritional stress, affect the RNAP II C-terminal domain phosphorylation at Ser2, and control recruitment of the 3′ end processing machinery to the vicinity of pA sites.

Nuclear Functions of TOR: Impact on Transcription and the Epigenome

The target of rapamycin (TOR) protein kinase is at the core of growth factor- and nutrient-dependent signaling pathways that are well-known for their regulation of metabolism, growth, and proliferation. However, TOR is also involved in the regulation of gene expression, genomic and epigenomic stability. TOR affects nuclear functions indirectly through its activity in the cytoplasm, but also directly through active nuclear TOR pools. The mechanisms by which TOR regulates its nuclear functions are less well-understood compared with its cytoplasmic activities. TOR is an important pharmacological target for several diseases, including cancer, metabolic and neurological disorders. Thus, studies of the nuclear functions of TOR are important for our understanding of basic biological processes, as well as for clinical implications.

Unique and Shared Roles for Histone H3K36 Methylation States in Transcription Regulation Functions

Set2 co-transcriptionally methylates lysine 36 of histone H3 (H3K36), producing mono-, di-, and trimethylation (H3K36me1/2/3). These modifications recruit or repel chromatin effector proteins important for transcriptional fidelity, mRNA splicing, and DNA repair. However, it was not known whether the different methylation states of H3K36 have distinct biological functions. Here, we use engineered forms of Set2 that produce different lysine methylation states to identify unique and shared functions for H3K36 modifications. Although H3K36me1/2 and H3K36me3 are functionally redundant in many SET2 deletion phenotypes, we found that H3K36me3 has a unique function related to Bur1 kinase activity and FACT (facilitates chromatin transcription) complex function. Further, during nutrient stress, either H3K36me1/2 or H3K36me3 represses high levels of histone acetylation and cryptic transcription that arises from within genes. Our findings uncover the potential for the regulation of diverse chromatin functions by different H3K36 methylation states.

A conserved genetic interaction between Spt6 and Set2 regulates H3K36 methylation

The transcription elongation factor Spt6 and the H3K36 methyltransferase Set2 are both required for H3K36 methylation and transcriptional fidelity in Saccharomyces cerevisiae. However, the nature of the requirement for Spt6 has remained elusive. By selecting for suppressors of a transcriptional defect in an spt6 mutant, we have isolated several highly clustered, dominant SET2 mutations (SET2^sup mutations) in a region encoding a proposed autoinhibitory domain. SET2^sup mutations suppress the H3K36 methylation defect in the spt6 mutant, as well as in other mutants that impair H3K36 methylation. We also show that SET2^sup mutations overcome the requirement for certain Set2 domains for H3K36 methylation. In vivo, SET2^sup mutants have elevated levels of H3K36 methylation and the purified Set2^sup mutant protein has greater enzymatic activityin vitro. ChIP-seq studies demonstrate that the H3K36 methylation defect in the spt6 mutant, as well as its suppression by a SET2^sup mutation, occurs at a step following the recruitment of Set2 to chromatin. Other experiments show that a similar genetic relationship between Spt6 and Set2 exists in Schizosaccharomyces pombe. Taken together, our results suggest a conserved mechanism by which the Set2 autoinhibitory domain requires multiple Set2 interactions to ensure that H3K36 methylation occurs specifically on actively transcribed chromatin.

Histone H2B Ubiquitylation Regulates Histone Gene Expression by Suppressing Antisense Transcription in Fission Yeast

Histone H2B monoubiquitylation (H2Bub1) is tightly linked to RNA polymerase II transcription elongation, and is also directly implicated in DNA replication and repair. Loss of H2Bub1 is associated with defects in cell cycle progression, but how these are related to its various functions, and the underlying mechanisms involved, is not understood. Here we describe a role for H2Bub1 in the regulation of replication-dependent histone genes in the fission yeast Schizosaccharomyces pombe H2Bub1 activates histone genes indirectly by suppressing antisense transcription of ams2+ -a gene encoding a GATA-type transcription factor that activates histone genes and is required for assembly of centromeric chromatin. Mutants lacking the ubiquitylation site in H2B or the H2B-specific E3 ubiquitin ligase Brl2 had elevated levels of ams2+ antisense transcripts and reduced Ams2 protein levels. These defects were reversed upon inhibition of Cdk9-an ortholog of the kinase component of positive transcription elongation factor b (P-TEFb)-indicating that they likely resulted from aberrant transcription elongation. Reduced Cdk9 activity also partially rescued chromosome segregation phenotypes of H2Bub1 mutants. In a genome-wide analysis, loss of H2Bub1 led to increased antisense transcripts at over 500 protein-coding genes in H2Bub1 mutants; for a subset of these, including several genes involved in chromosome segregation and chromatin assembly, antisense derepression was Cdk9-dependent. Our results highlight antisense suppression as a key feature of cell cycle-dependent gene regulation by H2Bub1, and suggest that aberrant transcription elongation may underlie the effects of H2Bub1 loss on cell cycle progression.

Screening for genes that accelerate the epigenetic aging clock in humans reveals a role for the H3K36 methyltransferase NSD1

BACKGROUND

Epigenetic clocks are mathematical models that predict the biological age of an individual using DNA methylation data and have emerged in the last few years as the most accurate biomarkers of the aging process. However, little is known about the molecular mechanisms that control the rate of such clocks. Here, we have examined the human epigenetic clock in patients with a variety of developmental disorders, harboring mutations in proteins of the epigenetic machinery.

RESULTS

Using the Horvath epigenetic clock, we perform an unbiased screen for epigenetic age acceleration in the blood of these patients. We demonstrate that loss-of-function mutations in the H3K36 histone methyltransferase NSD1, which cause Sotos syndrome, substantially accelerate epigenetic aging. Furthermore, we show that the normal aging process and Sotos syndrome share methylation changes and the genomic context in which they occur. Finally, we found that the Horvath clock CpG sites are characterized by a higher Shannon methylation entropy when compared with the rest of the genome, which is dramatically decreased in Sotos syndrome patients.

CONCLUSIONS

These results suggest that the H3K36 methylation machinery is a key component of the epigenetic maintenance system in humans, which controls the rate of epigenetic aging, and this role seems to be conserved in model organisms. Our observations provide novel insights into the mechanisms behind the epigenetic aging clock and we expect will shed light on the different processes that erode the human epigenetic landscape during aging.

Understanding histone H3 lysine 36 methylation and its deregulation in disease

Methylation of histone H3 lysine 36 (H3K36) plays crucial roles in the partitioning of chromatin to distinctive domains and the regulation of a wide range of biological processes. Trimethylation of H3K36 (H3K36me3) demarcates body regions of the actively transcribed genes, providing signals for modulating transcription fidelity, mRNA splicing and DNA damage repair; and di-methylation of H3K36 (H3K36me2) spreads out within large intragenic regions, regulating distribution of histone H3 lysine 27 trimethylation (H3K27me3) and possibly DNA methylation. These H3K36 methylation-mediated events are biologically crucial and controlled by different classes of proteins responsible for either 'writing', 'reading' or 'erasing' of H3K36 methylation marks. Deregulation of H3K36 methylation and related regulatory factors leads to pathogenesis of disease such as developmental syndrome and cancer. Additionally, recurrent mutations of H3K36 and surrounding histone residues are detected in human tumors, further highlighting the importance of H3K36 in biology and medicine. This review will elaborate on current advances in understanding H3K36 methylation and related molecular players during various chromatin-templated cellular processes, their crosstalks with other chromatin factors, as well as their deregulations in the diseased contexts.

Set2 methyltransferase facilitates cell cycle progression by maintaining transcriptional fidelity

Methylation of histone H3 lysine 36 (H3K36me) by yeast Set2 is critical for the maintenance of chromatin structure and transcriptional fidelity. However, we do not know the full range of Set2/H3K36me functions or the scope of mechanisms that regulate Set2-dependent H3K36 methylation. Here, we show that the APC/C^CDC20 complex regulates Set2 protein abundance during the cell cycle. Significantly, absence of Set2-mediated H3K36me causes a loss of cell cycle control and pronounced defects in the transcriptional fidelity of cell cycle regulatory genes, a class of genes that are generally long, hence highly dependent on Set2/H3K36me for their transcriptional fidelity. Because APC/C also controls human SETD2, and SETD2 likewise regulates cell cycle progression, our data imply an evolutionarily conserved cell cycle function for Set2/SETD2 that may explain why recurrent mutations of SETD2 contribute to human disease.

Slx5/Slx8-dependent ubiquitin hotspots on chromatin contribute to stress tolerance

Chromatin is a highly regulated environment, and protein association with chromatin is often controlled by post‐translational modifications and the corresponding enzymatic machinery. Specifically, SUMO‐targeted ubiquitin ligases (STUbLs) have emerged as key players in nuclear quality control, genome maintenance, and transcription. However, how STUbLs select specific substrates among myriads of SUMOylated proteins on chromatin remains unclear. Here, we reveal a remarkable co‐localization of the budding yeast STUbL Slx5/Slx8 and ubiquitin at seven genomic loci that we term “ubiquitin hotspots”. Ubiquitylation at these sites depends on Slx5/Slx8 and protein turnover on the Cdc48 segregase. We identify the transcription factor‐like Ymr111c/Euc1 to associate with these sites and to be a critical determinant of ubiquitylation. Euc1 specifically targets Slx5/Slx8 to ubiquitin hotspots via bipartite binding of Slx5 that involves the Slx5 SUMO‐interacting motifs and an additional, novel substrate recognition domain. Interestingly, the Euc1‐ubiquitin hotspot pathway acts redundantly with chromatin modifiers of the H2A.Z and Rpd3L pathways in specific stress responses. Thus, our data suggest that STUbL‐dependent ubiquitin hotspots shape chromatin during stress adaptation.

Histone Modifications as an Intersection Between Diet and Longevity

Histone modifications are key epigenetic regulators that control chromatin structure and gene transcription, thereby impacting on various important cellular phenotypes. Over the past decade, a growing number of studies have indicated that changes in various histone modifications have a significant influence on the aging process. Furthermore, it has been revealed that the abundance and localization of histone modifications are responsive to various environmental stimuli, such as diet, which can also affect gene expression and lifespan. This supports the notion that histone modifications can serve as a main cellular platform for signal integration. Hence, in this review we focus on the role of histone modifications during aging, report the data indicating that diet affects histone modification levels and explore the idea that histone modifications may function as an intersection through which diet regulates lifespan. A greater understanding of the epigenetic mechanisms that link environmental signals to longevity may provide new strategies for therapeutic intervention in age-related diseases and for promoting healthy aging.

SET domains and stress: uncovering new functions for yeast Set4

Chromatin dynamics are central to the regulation of gene expression and genome stability, particularly in the presence of environmental signals or stresses that prompt rapid reprogramming of the genome to promote survival or differentiation. While numerous chromatin regulators have been implicated in modulating cellular responses to stress, gaps in our mechanistic understanding of chromatin-based changes during stress suggest that additional proteins are likely critical to these responses and the molecular details underlying their activities are unclear in many cases. We recently identified a role for the relatively uncharacterized SET domain protein Set4 in promoting cell survival during oxidative stress in Saccharomyces cerevisiae. Set4 is a member of the Set3 subfamily of SET domain proteins which are defined by the presence of a PHD finger and divergent SET domain sequences. Here, we integrate our new observations on the function of Set4 with known roles for other related family members, including yeast Set3, fly UpSET and mammalian proteins MLL5 and SETD5. We discuss outstanding questions regarding the molecular mechanisms by which these proteins control gene expression and their potential contributions to cellular responses to environmental stress.

Transcriptional and Epigenetic Regulation by the Mechanistic Target of Rapamycin Complex 1 Pathway

Nutrient availability impacts health such that nutrient excess states can dysregulate epigenetic and transcriptional pathways to cause many diseases. Increasing evidence implicates aberrant regulation of nutrient signaling cascades as one means of communicating nutrient information to the epigenetic and transcriptional regulatory machinery. One such signaling cascade, the mechanistic target of rapamycin complex 1 (mTORC1), is conserved from yeast to man, and it is deregulated in diverse disease states. The catalytic subunit of the mTORC1 kinase complex (Tor1 or Tor2 in budding yeast and mTor in mammals) phosphorylates several downstream effectors regulating transcriptional and translational responses controlling growth and proliferation. Delineating mechanisms of cytoplasmic nutrient mTORC1 activation continues to be a major research focus. However, Tor kinases not only localize to the cytoplasm but also are found in the nucleus where they selectively bind and regulate genes controlling cellular metabolism and anabolism. The nuclear mTORC1 functions are now beginning to be defined, and they suggest that mTORC1 has a critical role in regulating the complex transcriptional activities required for ribosomal biogenesis. The mTORC1 pathway also interacts with epigenetic regulators required for modifying chromatin structure and function to control gene expression. Because altered nutrient states exert both individual and transgenerational phenotypic changes, mTORC1 signaling to chromatin effectors may have a significant role in mediating the effects of diet and nutrients on the epigenome. This article will discuss the recent inroads into the function of nuclear mTORC1 and its role in epigenetic and transcriptional regulation.

Spt6 Is Required for the Fidelity of Promoter Selection

Spt6 is a conserved factor that controls transcription and chromatin structure across the genome. Although Spt6 is viewed as an elongation factor, spt6 mutations in Saccharomyces cerevisiae allow elevated levels of transcripts from within coding regions, suggesting that Spt6 also controls initiation. To address the requirements for Spt6 in transcription and chromatin structure, we have combined four genome-wide approaches. Our results demonstrate that Spt6 represses transcription initiation at thousands of intragenic promoters. We characterize these intragenic promoters and find sequence features conserved with genic promoters. Finally, we show that Spt6 also regulates transcription initiation at most genic promoters and propose a model of initiation site competition to account for this. Together, our results demonstrate that Spt6 controls the fidelity of transcription initiation throughout the genome.

Set4 is a chromatin-associated protein, promotes survival during oxidative stress, and regulates stress response genes in yeast

The Set4 protein in the yeast Saccharomyces cerevisiae contains both a PHD finger and a SET domain, a common signature of chromatin-associated proteins, and shares sequence homology with the yeast protein Set3, the fly protein UpSET, and the human protein mixed-lineage leukemia 5 (MLL5). However, the biological role for Set4 and its potential function in chromatin regulation has not been well defined. Here, we analyzed yeast cell phenotypes associated with loss of Set4 or its overexpression, which revealed that Set4 protects against oxidative stress induced by hydrogen peroxide. Gene expression analysis indicated that Set4 promotes the activation of stress response genes in the presence of oxidative insults. Using ChIP analysis and other biochemical assays, we also found that Set4 interacts with chromatin and directly localizes to stress response genes upon oxidative stress. However, recombinant Set4 did not show detectable methyltransferase activity on histones. Our findings also suggest that Set4 abundance in the cell is balanced under normal and stress conditions to promote survival. Overall, these results suggest a model in which Set4 is a stress-responsive, chromatin-associated protein that activates gene expression programs required for cellular protection against oxidative stress. This work advances our understanding of mechanisms that protect cells during oxidative stress and further defines the role of the Set3-Set4 subfamily of SET domain-containing proteins in controlling gene expression in response to adverse environmental conditions.

Recognition of cancer mutations in histone H3K36 by epigenetic writers and readers

Histone posttranslational modifications control the organization and function of chromatin. In particular, methylation of lysine 36 in histone H3 (H3K36me) has been shown to mediate gene transcription, DNA repair, cell cycle regulation, and pre-mRNA splicing. Notably, mutations at or near this residue have been causally linked to the development of several human cancers. These observations have helped to illuminate the role of histones themselves in disease and to clarify the mechanisms by which they acquire oncogenic properties. This perspective focuses on recent advances in discovery and characterization of histone H3 mutations that impact H3K36 methylation. We also highlight findings that the common cancer-related substitution of H3K36 to methionine (H3K36M) disturbs functions of not only H3K36me-writing enzymes but also H3K36me-specific readers. The latter case suggests that the oncogenic effects could also be linked to the inability of readers to engage H3K36M.

Histone deacetylation promotes transcriptional silencing at facultative heterochromatin

It is important to accurately regulate the expression of genes involved in development and environmental response. In the fission yeast Schizosaccharomyces pombe, meiotic genes are tightly repressed during vegetative growth. Despite being embedded in heterochromatin these genes are transcribed and believed to be repressed primarily at the level of RNA. However, the mechanism of facultative heterochromatin formation and the interplay with transcription regulation is not understood. We show genome-wide that HDAC-dependent histone deacetylation is a major determinant in transcriptional silencing of facultative heterochromatin domains. Indeed, mutation of class I/II HDACs leads to increased transcription of meiotic genes and accumulation of their mRNAs. Mechanistic dissection of the pho1 gene where, in response to phosphate, transient facultative heterochromatin is established by overlapping lncRNA transcription shows that the Clr3 HDAC contributes to silencing independently of SHREC, but in an lncRNA-dependent manner. We propose that HDACs promote facultative heterochromatin by establishing alternative transcriptional silencing.

Bisphenol A and Phthalates Modulate Peritoneal Macrophage Function in Female Mice Involving SYMD2-H3K36 Dimethylation

Ample evidence suggests that environmental and occupational exposure to bisphenol A (BPA) and phthalate, two chemicals widely used in the plastics industry, disturbs homeostasis of innate immunity and causes inflammatory diseases. However, the underlying molecular mechanisms of these toxicants in the regulation of macrophage inflammatory functions remain poorly understood. In this study, we addressed the effect of chronic exposure to BPA or phthalate at levels relevant to human exposure, either in vitro or in vivo, on the inflammatory reprograming of peritoneal macrophages. Our studies revealed that BPA and phthalates adversely affected expression levels of the proinflammatory cytokines and mediators in response to lipopolysaccharide stimulation. Exposure to these toxicants also affected gene expression of scavenger receptors and phagocytic capacity of peritoneal macrophages. Our studies revealed that the epigenetic inhibitors differentially modulated target gene expression in these cells. Further analysis revealed that certain histone modification enzymes were aberrantly expressed in response to BPA or phthalate exposure, leading to alteration in the levels of H3K36 acetylation and dimethylation, two chromatin modifications that are critical for transcriptional efficacy and accuracy. Our results further revealed that silencing of H3K36-specific methyltransferase Smyd2 expression or inhibition of SMYD2 enzymatic activity attenuated H3K36 dimethylation and enhanced interleukin-6 and tumor necrosis factor-α expression but dampened the phagocytic capacity of peritoneal macrophages. In summary, our results indicate that peritoneal macrophages are vulnerable to BPA or phthalate at levels relevant to human exposure. These environmental toxicants affect phenotypic programming of macrophages via epigenetic mechanisms involving SMYD2-mediated H3K36 modification.

Long Noncoding RNAs in Yeast Cells and Differentiated Subpopulations of Yeast Colonies and Biofilms

We summarize current knowledge regarding regulatory functions of long noncoding RNAs (lncRNAs) in yeast, with emphasis on lncRNAs identified recently in yeast colonies and biofilms. Potential regulatory functions of these lncRNAs in differentiated cells of domesticated colonies adapted to plentiful conditions versus yeast colony biofilms are discussed. We show that specific cell types differ in their complements of lncRNA, that this complement changes over time in differentiating upper cells, and that these lncRNAs target diverse functional categories of genes in different cell subpopulations and specific colony types.