Project description:Bursts of nascent mRNA have been shown to lead to substantial cell-cell variation in unicellular organisms, facilitating diverse responses to environmental challenges. It is unknown whether similar bursts and gene-expression noise occur in mammalian tissues. To address this, we combine single molecule transcript counting with dual-color labeling and quantification of nascent mRNA to characterize promoter states, transcription rates, and transcript lifetimes in the intact mouse liver. We find that liver gene expression is highly bursty, with promoters stochastically switching between transcriptionally active and inactive states. Promoters of genes with short mRNA lifetimes are active longer, facilitating rapid response while reducing burst-associated noise. Moreover, polyploid hepatocytes exhibit less noise than diploid hepatocytes, suggesting a possible benefit to liver polyploidy. Thus, temporal averaging and liver polyploidy dampen the intrinsic variability associated with transcriptional bursts. Our approach can be used to study transcriptional bursting in diverse mammalian tissues.
Project description:The p53 tumor suppressor is a DNA-damage-responsive sequence-specific transcriptional activator. The sustained activation of the p53 response is incompatible with cell growth and viability. To circumvent this issue, a variety of negative feedback loops exist to limit the duration of p53 activation. Despite our understanding of p53 regulation, very little is known about the effect of transient p53 activation on the long-term expression of p53 target genes. Here we used a temperature-sensitive variant of p53 and oligonucleotide microarrays to monitor gene expression during and following reversible p53 activation. The expression of most p53-induced transcripts was rapidly reversible, consistent with active mRNA decay. Representative 3' UTRs derived from short-lived transcripts (i.e., DDB2 and GDF15) conferred instability on a heterologous mRNA, while 3' UTRs derived from more stable transcripts (i.e., CRYAB and TP53I3) did not. The 3' UTRs derived from unstable p53-induced mRNAs were significantly longer than those derived from stable mRNAs. These 3' UTRs had high uridine and low cytosine content, leading to a higher density of U-, AU-, and GU-rich sequences. Remarkably, short-lived p53 targets were induced faster, reaching maximum transcript levels earlier than the stable p53 targets. Taken together, the evidence indicates that the p53 transcriptional response has evolved with primarily short-lived target mRNAs and that post-transcription processes play a prominent role in the p53 response.
Project description:Nonsense-mediated mRNA decay (NMD) is one of the best characterized and most evolutionarily conserved cellular quality control mechanisms. Although NMD was first found to target one-third of mutated, disease-causing mRNAs, it is now known to also target ~10% of unmutated mammalian mRNAs to facilitate appropriate cellular responses - adaptation, differentiation or death - to environmental changes. Mutations in NMD genes in humans are associated with intellectual disability and cancer. In this Review, we discuss how NMD serves multiple purposes in human cells by degrading both mutated mRNAs to protect the integrity of the transcriptome and normal mRNAs to control the quantities of unmutated transcripts.
Project description:Messenger RNAs containing premature stop codons are generally targeted for degradation through nonsense-mediated mRNA decay (NMD). This mechanism degrades aberrant transcripts derived from mutant genes containing nonsense or frameshift mutations. Wild-type genes also give rise to transcripts targeted by NMD. For example, some wild-type genes give rise to alternatively spliced transcripts that are targeted for decay by NMD. In Caenorhabditis elegans, the ribosomal protein (rp) L12 gene generates a nonsense codon-bearing alternatively spliced transcript that is induced in an autoregulatory manner by the rpL12 protein. By pharmacologically blocking the NMD pathway, we identified alternatively spliced mRNA transcripts derived from the human rpL3 and rpL12 genes that are natural targets of NMD. The deduced protein sequence of these alternatively spliced transcripts suggests that they are unlikely to encode functional ribosomal proteins. Overexpression of rpL3 increased the level of the alternatively spliced rpL3 mRNA and decreased the normally expressed rpL3. This indicates that rpL3 regulates its own production by a negative feedback loop and suggests the possibility that NMD participates in this regulatory loop by degrading the non-functional alternatively spliced transcript.
Project description:In budding yeast, the nuclear RNA surveillance system is active on all pre-mRNA transcripts and modulated by nutrient availability. To test the role of nuclear surveillance in reprogramming gene expression, we identified transcriptome-wide binding sites for RNA polymerase II and the exosome cofactors Mtr4 (TRAMP complex) and Nab3 (NNS complex) by UV crosslinking immediately following glucose withdrawal (0, 4, and 8 min). In glucose, mRNA binding by Nab3 and Mtr4 was mainly restricted to promoter-proximal sites, reflecting early transcription termination. Following glucose withdrawal, many growth-related mRNAs showed reduced transcription but increased Nab3 binding, accompanied by downstream recruitment of Mtr4, and oligo(A) tailing. We conclude that transcription termination is followed by TRAMP-mediated RNA decay. Upregulated transcripts evaded increased surveillance factor binding following glucose withdrawal. Some upregulated genes showed use of alternative transcription starts to bypass strong NNS binding sites. We conclude that nuclear surveillance pathways regulate both positive and negative responses to glucose availability.
Project description:BackgroundInterpretation of lists of genes or proteins with altered expression is a critical and time-consuming part of microarray and proteomics research, but relatively little attention has been paid to methods for extracting biological meaning from these output lists. One powerful approach is to examine the expression of predefined biological pathways and gene sets, such as metabolic and signaling pathways and macromolecular complexes. Although many methods for measuring pathway expression have been proposed, a systematic analysis of the performance of multiple methods over multiple independent data sets has not previously been reported.ResultsFive different measures of pathway expression were compared in an analysis of nine publicly available mRNA expression data sets. The relative sensitivity of the metrics varied greatly across data sets, and the biological pathways identified for each data set are also dependent on the choice of pathway activation metric. In addition, we show that removing incoherent pathways prior to analysis improves specificity. Finally, we create and analyze a public map of pathway expression in human tissues by gene-set analysis of a large compendium of human expression data.ConclusionWe show that both the detection sensitivity and identity of pathways significantly perturbed in a microarray experiment are highly dependent on the analysis methods used and how incoherent pathways are treated. Analysts should thus consider using multiple approaches to test the robustness of their biological interpretations. We also provide a comprehensive picture of the tissue distribution of human gene pathways and a useful public archive of human pathway expression data.
Project description:BackgroundMicroarray technology has become highly valuable for identifying complex global changes in gene expression patterns. The effective correlation of observed changes in gene expression with shared transcription regulatory elements remains difficult to demonstrate convincingly. One reason for this difficulty may result from the intricate convergence of both transcriptional and mRNA turnover events which, together, directly influence steady-state mRNA levels.ResultsIn order to investigate the relative contribution of gene transcription and changes in mRNA stability regulation to standard analyses of gene expression, we used two distinct microarray methods which individually measure nuclear gene transcription and changes in polyA mRNA gene expression. Gene expression profiles were obtained from both polyA mRNA (whole-cell) and nuclear run-on (newly transcribed) RNA across a time course of one hour following the activation of human Jurkat T cells with PMA plus ionomycin. Comparative analysis revealed that regulation of mRNA stability may account for as much as 50% of all measurements of changes in polyA mRNA in this system, as inferred by the absence of any corresponding regulation of nuclear gene transcription activity for these groups of genes. Genes which displayed dramatic elevations in both mRNA and nuclear run-on RNA were shown to be inhibited by Actinomycin D (ActD) pre-treatment of cells while large numbers of genes regulated only through altered mRNA turnover (both up and down) were ActD-resistant. Consistent patterns across the time course were observed for both transcribed and stability-regulated genes.ConclusionWe propose that regulation of mRNA stability contributes significantly to the observed changes in gene expression in response to external stimuli, as measured by high throughput systems.
Project description:BACKGROUND:Mammalian cells harbour RNA quality control and degradative machineries such as nonsense-mediated mRNA decay that target cellular mRNAs for clearance from the cell to avoid aberrant gene expression. The role of the host mRNA decay pathways in macrophages in the context of human immunodeficiency virus type 1 (HIV-1) infection is yet to be elucidated. Macrophages are directly infected by HIV-1, mediate the dissemination of the virus and contribute to the chronic activation of the inflammatory response observed in infected individuals. Therefore, we characterized the effects of four host mRNA decay proteins, i.e., UPF1, UPF2, SMG6 and Staufen1, on viral replication in HIV-1-infected primary monocyte-derived macrophages (MDMs). RESULTS:Steady-state expression levels of these host mRNA decay proteins were significantly downregulated in HIV-1-infected MDMs. Moreover, UPF2 and SMG6 inhibited HIV-1 gene expression in macrophages to a similar level achieved by SAMHD1, by directly influencing viral genomic RNA levels. Staufen1, a host protein also involved in UPF1-dependent mRNA decay and that acts at several HIV-1 replication steps, enhanced HIV-1 gene expression in MDMs. CONCLUSIONS:These results provide new evidence for roles of host mRNA decay proteins in regulating HIV-1 replication in infected macrophages and can serve as potential targets for broad-spectrum antiviral therapeutics.
Project description:Nonsense-mediated mRNA decay (NMD) is a cellular mechanism that eliminates mRNAs that harbor premature translation termination codons (PTCs). Here, we investigated the effects of environmental stresses (oxidative stress and endoplasmic reticulum (ER) stress) on NMD activity. Methylmercury (MeHg) was used to cause oxidative stress and thapsigargin to stress the ER. NMD suppression, evidenced by upregulation of NMD-sensitive mRNAs and a decrease in UPF1 phosphorylation, was observed in MeHg-treated myogenic cells, cerebral cortical neuronal cells, and astroglial cells. Mild ER stress amplified NMD suppression caused by MeHg. To elucidate the cause of stress-induced NMD suppression, the role of the phospho-eIF2α/ATF4 pathway was investigated. Knockdown and non-phosphorylatable eIF2α-transfection studies demonstrated the critical role of phospho-eIF2α-mediated repression of translation in mild ER stress-induced NMD suppression. However, NMD suppression was also observed in phospho-eIF2α-deficient cells under mild ER stress. Mechanistic target of rapamycin suppression-induced inhibition of cap-dependent translation, and downregulation of the NMD components UPF1, SMG7, and eIF4A3, were probably involved in stress-induced NMD suppression. Our results indicate that stress-induced NMD suppression has the potential to affect the condition of cells and phenotypes of PTC-related diseases under environmental stresses by stabilizing NMD-targeted gene expression.
Project description:When cells are exposed to a genotoxic stress, a DNA surveillance pathway that involves p53 is activated, allowing DNA repair. Eukaryotic cells have also evolved a mechanism called mRNA surveillance that controls the quality of mRNAs. Indeed, mutant mRNAs carrying premature translation termination codons (PTCs) are selectively degraded by the nonsense-mediated mRNA decay (NMD) pathway. However, in the case of particular genes, such as proto-oncogenes, mutations that do not create PTCs and therefore that do not induce mRNA degradation, can be harmful to cells. In this study, we showed that the H-ras gene in the absence of mutations produces an NMD-target splice variant that is degraded in the cytosol. We observed that a treatment with the genotoxic stress inducer camptothecin for 6 h favored the production of the H-ras NMD-target transcript degraded in the cytosol by the NMD process. Our data indicated that the NMD process allowed the elimination of transcripts produced in response to a short-term treatment with camptothecin from the major proto-oncogene H-ras, independently of PTCs induced by mutations. The camptothecin effects on H-ras gene expression were p53 dependent and involved in part modulation of the SC35 splicing factor. Interestingly, a long-term treatment with camptothecin as well as p53 overexpression for 24 h resulted in the accumulation of the H-ras NMD target in the cytosol, although the NMD process was not completely inhibited as other NMD targets are not stabilized. Finally, Upf1, a major NMD effector, was necessary for optimal p53 activation by camptothecin, which is consistent with recent data showing that NMD effectors are required for genome stability. In conclusion, we identified cross talk between the p53 and NMD pathways that regulates the expression levels of H-ras splice variants.