Synergising single-cell resolution and 4sU labelling boosts inference of transcriptional bursting

Despite the recent rise of RNA-seq datasets combining single-cell (sc) resolution with 4-thiouridine (4sU) labelling, analytical methods exploiting their power to dissect transcriptional bursting are lacking. Here, we present a mathematical model and Bayesian inference implementation to facilitate genome-wide joint parameter estimation and confidence quantification (R package: burstMCMC). We demonstrate that, unlike conventional scRNA-seq, 4sU scRNA-seq resolves temporal parameters and furthermore boosts inference of dimensionless parameters via a synergy between single-cell resolution and 4sU labelling. We apply our method to published 4sU scRNA-seq data and linked with ChIP-seq data, we uncover previously obscured associations between different parameters and histone modifications.

A Protocol for Transcriptome-Wide Inference of RNA Metabolic Rates in Mouse Embryonic Stem Cells

The relative ease of mouse Embryonic Stem Cells (mESCs) culture and the potential of these cells to differentiate into any of the three primary germ layers: ectoderm, endoderm and mesoderm (pluripotency), makes them an ideal and frequently used ex vivo system to dissect how gene expression changes impact cell state and differentiation. These efforts are further supported by the large number of constitutive and inducible mESC mutants established with the aim of assessing the contributions of different pathways and genes to cell homeostasis and gene regulation. Gene product abundance is controlled by the modulation of the rates of RNA synthesis, processing, and degradation. The ability to determine the relative contribution of these different RNA metabolic rates to gene expression control using standard RNA-sequencing approaches, which only capture steady state abundance of transcripts, is limited. In contrast, metabolic labeling of RNA with 4-thiouridine (4sU) coupled with RNA-sequencing, allows simultaneous and reproducible inference of transcriptome wide synthesis, processing, and degradation rates. Here we describe, a detailed protocol for 4sU metabolic labeling in mESCs that requires short 4sU labeling times at low concentration and minimally impacts cellular homeostasis. This approach presents a versatile method for in-depth characterization of the gene regulatory strategies governing gene steady state abundance in mESC.

Global analysis of RNA metabolism using bio-orthogonal labeling coupled with next-generation RNA sequencing

Many open questions in RNA biology relate to the kinetics of gene expression and the impact of RNA binding regulatory factors on processing or decay rates of particular transcripts. Steady state measurements of RNA abundance obtained from RNA-seq approaches are not able to separate the effects of transcription from those of RNA decay in the overall abundance of any given transcript, instead only giving information on the (presumed steady-state) abundances of transcripts. Through the combination of metabolic labeling and high-throughput sequencing, several groups have been able to measure both transcription rates and decay rates of the entire transcriptome of an organism in a single experiment. This review focuses on the methodology used to specifically measure RNA decay at a global level. By comparing and contrasting approaches and describing the experimental protocols in a modular manner, we intend to provide both experienced and new researchers to the field the ability to combine aspects of various protocols to fit the unique needs of biological questions not addressed by current methods.

Uncovering the Stability of Mature miRNAs by 4-Thio-Uridine Metabolic Labeling

MicroRNAs (miRNAs) are an evolutionary conserved class of short, single-stranded noncoding RNAs (<18-22 nt in length) that act in posttranscriptional regulation of gene expression in higher eukaryotes. The abundance of a miRNA is a key feature in control of its activity and, therefore, a number of mechanisms finely regulate miRNA levels, acting at both transcriptional and posttranscriptional level. Recent evidences, including our research, highlighted the role of miRNA decay as a mechanism controlling the miRNA pool. We describe in this chapter an optimized methodology to determine miRNA degradation rates in mammalian cells. Our approach is based on metabolic pulse labeling with 4-thiouridine (4sU), a uridine analog that is incorporated in nascent RNA and allows thiol-specific biotinylation and selective pull-down of labeled RNA. In particular, given the long average half-life and the complex biogenetic process of miRNAs, we developed a "pulse-chase" protocol where 4sU is removed from the medium after a long labeling period (2-3 h pulse), and labeled RNA is purified at different time points to measure the decay of labeled molecules. By combining the 4sU-based "pulse-chase" approach with high-throughput small RNA sequencing (sRNAseq), it is possible to quantify at genome-wide level miRNA degradation rates.

Impact of Methods on the Measurement of mRNA Turnover

The turnover of the RNA molecules is determined by the rates of transcription and RNA degradation. Several methods have been developed to study RNA turnover since the beginnings of molecular biology. Here we summarize the main methods to measure RNA half-life: transcription inhibition, gene control, and metabolic labelling. These methods were used to detect the cellular activity of the mRNAs degradation machinery, including the exo-ribonuclease Xrn1 and the exosome. On the other hand, the study of the differential stability of mature RNAs has been hampered by the fact that different methods have often yielded inconsistent results. Recent advances in the systematic comparison of different method variants in yeast have permitted the identification of the least invasive methodologies that reflect half-lives the most faithfully, which is expected to open the way for a consistent quantitative analysis of the determinants of mRNA stability.

Rapid Genome-wide Recruitment of RNA Polymerase II Drives Transcription, Splicing, and Translation Events during T Cell Responses

Activation of immune cells results in rapid functional changes, but how such fast changes are accomplished remains enigmatic. By combining time courses of 4sU-seq, RNA-seq, ribosome profiling (RP), and RNA polymerase II (RNA Pol II) ChIP-seq during T cell activation, we illustrate genome-wide temporal dynamics for ∼10,000 genes. This approach reveals not only immediate-early and posttranscriptionally regulated genes but also coupled changes in transcription and translation for >90% of genes. Recruitment, rather than release of paused RNA Pol II, primarily mediates transcriptional changes. This coincides with a genome-wide temporary slowdown in cotranscriptional splicing, even for polyadenylated mRNAs that are localized at the chromatin. Subsequent splicing optimization correlates with increasing Ser-2 phosphorylation of the RNA Pol II carboxy-terminal domain (CTD) and activation of the positive transcription elongation factor (pTEFb). Thus, rapid de novo recruitment of RNA Pol II dictates the course of events during T cell activation, particularly transcription, splicing, and consequently translation.

Isolation of Newly Transcribed RNA Using the Metabolic Label 4-Thiouridine

Isolation of newly transcribed RNA is an invaluable approach that can be used to study the dynamic life of RNA in cellulo. Traditional methods of whole-cell RNA extraction limit subsequent gene expression analyses to the steady-state levels of RNA abundance, which often masks changes in RNA synthesis and processing. This chapter describes a methodology with low cytotoxicity that permits the labeling and isolation of nascent pre-mRNA in cell culture. The resulting isolate is suitable for use in a series of downstream applications aimed at studying changes in RNA synthesis, processing, or stability.

Eukaryotic mRNA decay: methodologies, pathways, and links to other stages of gene expression

mRNA concentration depends on the balance between transcription and degradation rates. On both sides of the equilibrium, synthesis and degradation show, however, interesting differences that have conditioned the evolution of gene regulatory mechanisms. Here, we discuss recent genome-wide methods for determining mRNA half-lives in eukaryotes. We also review pre- and posttranscriptional regulons that coordinate the fate of functionally related mRNAs by using protein- or RNA-based trans factors. Some of these factors can regulate both transcription and decay rates, thereby maintaining proper mRNA homeostasis during eukaryotic cell life.

Genome-wide technology for determining RNA stability in mammalian cells: historical perspective and recent advantages based on modified nucleotide labeling

Changing the abundance of transcripts by regulated RNA degradation is a critical step in the control of various biological pathways. Recently, genome-wide inhibitor-free technologies for determining RNA stabilities in mammalian cells have been developed. In these methods, endogenous RNAs are pulse labeled by uridine analogs [e.g., 4-thiouridine (4sU), 5-etyniluridine (EU) and 5'-bromo-uridine (BrU)], followed by purification of labeled de novo RNAs. These technologies have revealed that the specific half-life of each mRNA is closely related to its physiological function. Genes with short-lived mRNAs are significantly enriched among regulatory genes, while genes with long-lived mRNAs are enriched among housekeeping genes. This review describes the recent progress of experimental procedures for measuring RNA stability.