Nonparametric single-cell multiomic characterization of trio relationships between transcription factors, target genes, and cis-regulatory regions.

The epigenetic control of gene expression is highly cell-type and context specific. Yet, despite its complexity, gene regulatory logic can be broken down into modular components consisting of a transcription factor (TF) activating or repressing the target gene expression through its binding to a cis-regulatory region. We propose a nonparametric approach, TRIPOD, to detect and characterize the three-way relationships between a TF, its target gene, and the accessibility of the TF's binding site using single-cell RNA and ATAC multiomic data. We apply TRIPOD to interrogate the cell-type-specific regulatory logic in peripheral blood mononuclear cells and contrast our results to detections from enhancer databases, cis-eQTL studies, ChIP-seq experiments, and TF knockdown/knockout studies. We then apply TRIPOD to mouse embryonic brain data and identify regulatory relationships, validated by ChIP-seq and PLAC-seq. Finally, we demonstrate TRIPOD on the SHARE-seq data of differentiating mouse hair follicle cells and identify lineage-specific regulation supported by histone marks and super-enhancer annotations. A record of this paper's transparent peer review process is included in the supplemental information.

Super interactive promoters provide insight into cell type-specific regulatory networks in blood lineage cell types.

Existing studies of chromatin conformation have primarily focused on potential enhancers interacting with gene promoters. By contrast, the interactivity of promoters per se, while equally critical to understanding transcriptional control, has been largely unexplored, particularly in a cell type-specific manner for blood lineage cell types. In this study, we leverage promoter capture Hi-C data across a compendium of blood lineage cell types to identify and characterize cell type-specific super-interactive promoters (SIPs). Notably, promoter-interacting regions (PIRs) of SIPs are more likely to overlap with cell type-specific ATAC-seq peaks and GWAS variants for relevant blood cell traits than PIRs of non-SIPs. Moreover, PIRs of cell-type-specific SIPs show enriched heritability of relevant blood cell trait (s), and are more enriched with GWAS variants associated with blood cell traits compared to PIRs of non-SIPs. Further, SIP genes tend to express at a higher level in the corresponding cell type. Importantly, SIP subnetworks incorporating cell-type-specific SIPs and ATAC-seq peaks help interpret GWAS variants. Examples include GWAS variants associated with platelet count near the megakaryocyte SIP gene EPHB3 and variants associated lymphocyte count near the native CD4 T-Cell SIP gene ETS1. Interestingly, around 25.7% ~ 39.6% blood cell traits GWAS variants residing in SIP PIR regions disrupt transcription factor binding motifs. Importantly, our analysis shows the potential of using promoter-centric analyses of chromatin spatial organization data to identify biologically important genes and their regulatory regions.

The link between adjacent codon pairs and mRNA stability.

BACKGROUND: Evidence in diverse organisms suggests that codon optimality is a major determinant of mRNA translation and degradation. Codon optimality is thought to act by modulating the efficiency of ribosome elongation. In Saccharomyces cerevisiae, a recent study has identified 17 adjacent codon pairs that mediate strong inhibition of translation elongation. However, relationships between the inhibitory codon pairs and other aspects of gene expression are unknown. RESULTS: To gain insights into how the inhibitory codon pairs may affect aspects of gene expression, we utilized existing datasets to conduct genome-scale analyses in S. cerevisiae. Our analysis revealed the following points. First, the inhibitory codon pairs are significantly associated with faster mRNA decay. The association is not solely due to the content of nucleotides, individual codons, or dipeptides encoded by the inhibitory codon pairs. Second, the inhibitory codon pairs cannot fully explain the previously known relationship of codon optimality with mRNA stability, suggesting that optimality of individual codons and properties of adjacent codon pairs both contribute to gene regulation. Finally, although the inhibitory codon pairs are associated with slower mRNA synthesis and protein instability, the associations can be attributed to usage bias in individual codons. CONCLUSIONS: This study suggests an association of inhibitory codon pairs with mRNA stability and thus another layer of complexity in the codon-mediated gene regulation.

Analysis of the association between codon optimality and mRNA stability in Schizosaccharomyces pombe.

BACKGROUND: Recent experiments have shown that codon optimality is a major determinant of mRNA stability in Saccharomyces cerevisiae and that this phenomenon may be conserved in Escherichia coli and some metazoans, although work in Neurospora crassa is not consistent with this model. RESULTS: We examined the association between codon optimality and mRNA stability in the fission yeast Schizosaccharomyces pombe. Our analysis revealed the following points. First, we observe a genome-wide association between codon optimality and mRNA stability also in S. pombe, suggesting evolutionary conservation of the phenomenon. Second, in both S. pombe and S. cerevisiae, mRNA synthesis rates are also correlated at the genome-wide analysis with codon optimality, suggesting that the long-appreciated association between codon optimality and mRNA abundance is due to regulation of both mRNA synthesis and degradation. However, when we examined correlation of codon optimality and either mRNA half-lives or synthesis rates controlling for mRNA abundance, codon optimality was still positively correlated with mRNA half-lives in S. cerevisiae, but the association was no longer significant for mRNA half-lives in S. pombe or for synthesis rates in either organism. This illustrates how only the pairwise analysis of multiple correlating variables may limit these types of analyses. Finally, in S. pombe, codon optimality is associated with known DNA/RNA sequence motifs that are associated with mRNA production/stability, suggesting these two features have been under similar selective pressures for optimal gene expression. CONCLUSIONS: Consistent with the emerging body of studies, this study suggests that the association between codon optimality and mRNA stability may be a broadly conserved phenomenon. It also suggests that the association can be explained at least in part by independent adaptations of codon optimality and other transcript features for elevated expression during evolution.

Codon optimality and mRNA decay.

Recent evidence indicates that codon optimality is a broad determinant of mRNA stability. A study by Radhakrishnan et al. in Cell raises the possibility that the conserved DEAD-box protein Dhh1 underlies the phenomenon.

Fragile X mental retardation protein and the ribosome.

In this issue of Molecular Cell, Chen et al. (2014) provide evidence that FMRP represses translation by binding the ribosome, suggesting a novel form of translational control.

Global analysis of mRNA decay intermediates in Saccharomyces cerevisiae.

The general pathways of eukaryotic mRNA decay occur via deadenylation followed by 3' to 5' degradation or decapping, although some endonuclease sites have been identified in metazoan mRNAs. To determine the role of endonucleases in mRNA degradation in Saccharomyces cerevisiae, we mapped 5' monophosphate ends on mRNAs in wild-type and dcp2 xrn1 yeast cells, wherein mRNA endonuclease cleavage products are stabilized. This led to three important observations. First, only few mRNAs that undergo low-level endonucleolytic cleavage were observed, suggesting that endonucleases are not a major contributor to yeast mRNA decay. Second, independent of known decapping enzymes, we observed low levels of 5' monophosphates on some mRNAs, suggesting that an unknown mechanism can generate 5' exposed ends, although for all substrates tested, Dcp2 was the primary decapping enzyme. Finally, we identified debranched lariat intermediates from intron-containing genes, demonstrating a significant discard pathway for mRNAs during the second step of pre-mRNA splicing, which is a potential step to regulate gene expression.

Meiotic errors activate checkpoints that improve gamete quality without triggering apoptosis in male germ cells.

BACKGROUND: Meiotic checkpoints ensure the production of gametes with the correct complement and integrity of DNA; in metazoans, these pathways sense errors and transduce signals to trigger apoptosis to eliminate damaged germ cells. The extent to which checkpoints monitor and safeguard the genome differs between sexes and may contribute to the high frequency of human female meiotic errors. In the C. elegans female germline, DNA damage, chromosome asynapsis, and/or unrepaired meiotic double-strand breaks (DSBs) activate checkpoints that induce apoptosis; conversely, male germ cells do not undergo apoptosis. RESULTS: Here we show that the recombination checkpoint is in fact activated in male germ cells despite the lack of apoptosis. The 9-1-1 complex and the phosphatidylinositol 3-kinase-related protein kinase ATR, sensors of DNA damage, are recruited to chromatin in the presence of unrepaired meiotic DSBs in both female and male germlines. Furthermore, the checkpoint kinase CHK-1 is phosphorylated and the p53 ortholog CEP-1 induces expression of BH3-only proapoptotic proteins in germlines of both sexes under activating conditions. The core cell death machinery is expressed in female and male germlines; however, CED-3 caspase is not activated in the male germline. Although apoptosis is not triggered, checkpoint activation in males has functional consequences for gamete quality, because there is reduced viability of progeny sired by males with a checkpoint-activating defect in the absence of checkpoint function. CONCLUSIONS: We propose that the recombination checkpoint functions in male germ cells to promote repair of meiotic recombination intermediates, thereby improving the fidelity of chromosome transmission in the absence of apoptosis.

Importance of polyadenylation in the selective elimination of meiotic mRNAs in growing S. pombe cells.

A number of meiosis-specific mRNAs are initially weakly transcribed, but then selectively removed during fission yeast mitotic growth. These mRNAs harbour a region termed DSR (determinant of selective removal), which is recognized by the YTH family RNA-binding protein Mmi1p. Mmi1p directs the destruction of these mRNAs in collaboration with nuclear exosomes. However, detailed molecular mechanisms underlying this process of selective mRNA elimination have remained elusive. In this study, we demonstrate the critical role of polyadenylation in this process. Two-hybrid and genetic screens revealed potential interactions between Mmi1p and proteins involved in polyadenylation. Additional investigations showed that destruction of DSR-containing mRNAs by exosomes required polyadenylation by a canonical poly(A) polymerase. The recruitment of Pab2p, a poly(A)-binding protein, to the poly(A) tail was also necessary for mRNA destruction. In cells undergoing vegetative growth, Mmi1p localized with exosomes, Pab2p, and components of the polyadenylation complex in several patchy structures in the nucleoplasm. These patches may represent the sites for degradation of meiosis-specific mRNAs with untimely expression.

Identification and analysis of the interaction between Edc3 and Dcp2 in Saccharomyces cerevisiae.

Cap hydrolysis is a critical control point in the life of eukaryotic mRNAs and is catalyzed by the evolutionarily conserved Dcp1-Dcp2 complex. In Saccharomyces cerevisiae, decapping is modulated by several factors, including the Lsm family protein Edc3, which directly binds to Dcp2. We show that Edc3 binding to Dcp2 is mediated by a short peptide sequence located C terminal to the catalytic domain of Dcp2. This sequence is required for Edc3 to stimulate decapping activity of Dcp2 in vitro, for Dcp2 to efficiently accumulate in P-bodies, and for efficient degradation of the RPS28B mRNA, whose decay is enhanced by Edc3. In contrast, degradation of YRA1 pre-mRNA, another Edc3-regulated transcript, occurs independently from this region, suggesting that the effect of Edc3 on YRA1 is independent of its interaction with Dcp2. Deletion of the sequence also results in a subtle but significant defect in turnover of the MFA2pG reporter transcript, which is not affected by deletion of EDC3, suggesting that the region affects some other aspect of Dcp2 function in addition to binding Edc3. These results raise a model for Dcp2 recruitment to specific mRNAs where regions outside the catalytic core promote the formation of different complexes involved in mRNA decapping.

No-go decay: a quality control mechanism for RNA in translation.

Eukaryotic cells have evolved multiple quality control mechanisms that recognize and eliminate defective mRNA during the process of translation. One mechanism, referred to as No-go decay (NGD), targets mRNAs with elongation stalls for degradation initiated by endonucleolytic cleavage in the vicinity of the stalled ribosome. NGD is promoted by the evolutionarily conserved Dom34 and Hbs1 proteins, which are related to the translation termination factors eRF1 and eRF3, respectively. NGD is likely to occur by Dom34/Hbs1 interacting with the A site in the ribosome leading to release of the peptide or peptidyl-tRNA. The process of NGD and/or the function of Dom34/Hbs1 appear to be important in several different biological contexts.

Molecular mechanisms underlying the mitosis-meiosis decision.

Most eukaryotic cells possess genetic potential to perform meiosis, but the vast majority of them never initiate it. The entry to meiosis is strictly regulated by developmental and environmental conditions, which vary significantly from species to species. Molecular mechanisms underlying the mitosis-meiosis decision are unclear in most organisms, except for a few model systems including fission yeast Schizosaccharomyces pombe. Nutrient limitation is a cue to the entry into meiosis in this microbe. Signals from nutrients converge on the activity of Mei2 protein, which plays pivotal roles in both induction and progression of meiosis. Here we outline the current knowledge of how a set of environmental stimuli eventually activates Mei2, and discuss how Mei2 governs the meiotic program molecularly, especially focusing on a recent finding that Mei2 antagonizes selective elimination of meiotic messenger RNAs.

Selective elimination of messenger RNA prevents an incidence of untimely meiosis.

Much remains unknown about the molecular regulation of meiosis. Here we show that meiosis-specific transcripts are selectively removed if expressed during vegetative growth in fission yeast. These messenger RNAs contain a cis-acting region--which we call the DSR--that confers this removal via binding to a YTH-family protein Mmi1. Loss of Mmi1 function severely impairs cell growth owing to the untimely expression of meiotic transcripts. Microarray analysis reveals that at least a dozen such meiosis-specific transcripts are eliminated by the DSR-Mmi1 system. Mmi1 remains in the form of multiple nuclear foci during vegetative growth. At meiotic prophase these foci precipitate to a single focus, which coincides with the dot formed by the master meiosis-regulator Mei2. A meiotic arrest due to the loss of the Mei2 dot is released by a reduction in Mmi1 activity. We propose that Mei2 turns off the DSR-Mmi1 system by sequestering Mmi1 to the dot and thereby secures stable expression of meiosis-specific transcripts.