Single-cell nascent RNA sequencing unveils coordinated global transcription.

Transcription is the primary regulatory step in gene expression. Divergent transcription initiation from promoters and enhancers produces stable RNAs from genes and unstable RNAs from enhancers1,2. Nascent RNA capture and sequencing assays simultaneously measure gene and enhancer activity in cell populations3. However, fundamental questions about the temporal regulation of transcription and enhancer-gene coordination remain unanswered, primarily because of the absence of a single-cell perspective on active transcription. In this study, we present scGRO-seq-a new single-cell nascent RNA sequencing assay that uses click chemistry-and unveil coordinated transcription throughout the genome. We demonstrate the episodic nature of transcription and the co-transcription of functionally related genes. scGRO-seq can estimate burst size and frequency by directly quantifying transcribing RNA polymerases in individual cells and can leverage replication-dependent non-polyadenylated histone gene transcription to elucidate cell cycle dynamics. The single-nucleotide spatial and temporal resolution of scGRO-seq enables the identification of networks of enhancers and genes. Our results suggest that the bursting of transcription at super-enhancers precedes bursting from associated genes. By imparting insights into the dynamic nature of global transcription and the origin and propagation of transcription signals, we demonstrate the ability of scGRO-seq to investigate the mechanisms of transcription regulation and the role of enhancers in gene expression.

Improved modeling of RNA-binding protein motifs in an interpretable neural model of RNA splicing.

Sequence-specific RNA-binding proteins (RBPs) play central roles in splicing decisions. Here, we describe a modular splicing architecture that leverages in vitro-derived RNA affinity models for 79 human RBPs and the annotated human genome to produce improved models of RBP binding and activity. Binding and activity are modeled by separate Motif and Aggregator components that can be mixed and matched, enforcing sparsity to improve interpretability. Training a new Adjusted Motif (AM) architecture on the splicing task not only yields better splicing predictions but also improves prediction of RBP-binding sites in vivo and of splicing activity, assessed using independent data.

Single-cell nascent RNA sequencing using click-chemistry unveils coordinated transcription.

Transcription is the primary regulatory step in gene expression. Divergent transcription initiation from promoters and enhancers produces stable RNAs from genes and unstable RNAs from enhancers1-5. Nascent RNA capture and sequencing assays simultaneously measure gene and enhancer activity in cell populations6-9. However, fundamental questions in the temporal regulation of transcription and enhancer-gene synchrony remain unanswered primarily due to the absence of a single-cell perspective on active transcription. In this study, we present scGRO-seq - a novel single-cell nascent RNA sequencing assay using click-chemistry - and unveil the coordinated transcription throughout the genome. scGRO-seq demonstrates the episodic nature of transcription, and estimates burst size and frequency by directly quantifying transcribing RNA polymerases in individual cells. It reveals the co-transcription of functionally related genes and leverages the replication-dependent non-polyadenylated histone genes transcription to elucidate cell-cycle dynamics. The single-nucleotide spatial and temporal resolution of scGRO-seq identifies networks of enhancers and genes and indicates that the bursting of transcription at super-enhancers precedes the burst from associated genes. By imparting insights into the dynamic nature of transcription and the origin and propagation of transcription signals, scGRO-seq demonstrates its unique ability to investigate the mechanisms of transcription regulation and the role of enhancers in gene expression.

Signal peptide mimicry primes Sec61 for client-selective inhibition.

Preventing the biogenesis of disease-relevant proteins is an attractive therapeutic strategy, but attempts to target essential protein biogenesis factors have been hampered by excessive toxicity. Here we describe KZR-8445, a cyclic depsipeptide that targets the Sec61 translocon and selectively disrupts secretory and membrane protein biogenesis in a signal peptide-dependent manner. KZR-8445 potently inhibits the secretion of pro-inflammatory cytokines in primary immune cells and is highly efficacious in a mouse model of rheumatoid arthritis. A cryogenic electron microscopy structure reveals that KZR-8445 occupies the fully opened Se61 lateral gate and blocks access to the lumenal plug domain. KZR-8445 binding stabilizes the lateral gate helices in a manner that traps select signal peptides in the Sec61 channel and prevents their movement into the lipid bilayer. Our results establish a framework for the structure-guided discovery of novel therapeutics that selectively modulate Sec61-mediated protein biogenesis.

Oncogenic CDK13 mutations impede nuclear RNA surveillance.

RNA surveillance pathways detect and degrade defective transcripts to ensure RNA fidelity. We found that disrupted nuclear RNA surveillance is oncogenic. Cyclin-dependent kinase 13 (CDK13) is mutated in melanoma, and patient-mutated CDK13 accelerates zebrafish melanoma. CDK13 mutation causes aberrant RNA stabilization. CDK13 is required for ZC3H14 phosphorylation, which is necessary and sufficient to promote nuclear RNA degradation. Mutant CDK13 fails to activate nuclear RNA surveillance, causing aberrant protein-coding transcripts to be stabilized and translated. Forced aberrant RNA expression accelerates melanoma in zebrafish. We found recurrent mutations in genes encoding nuclear RNA surveillance components in many malignancies, establishing nuclear RNA surveillance as a tumor-suppressive pathway. Activating nuclear RNA surveillance is crucial to avoid accumulation of aberrant RNAs and their ensuing consequences in development and disease.

Altered DNA repair pathway engagement by engineered CRISPR-Cas9 nucleases.

CRISPR-Cas9 introduces targeted DNA breaks that engage competing DNA repair pathways, producing a spectrum of imprecise insertion/deletion mutations (indels) and precise templated mutations (precise edits). The relative frequencies of these pathways are thought to primarily depend on genomic sequence and cell state contexts, limiting control over mutational outcomes. Here, we report that engineered Cas9 nucleases that create different DNA break structures engage competing repair pathways at dramatically altered frequencies. We accordingly designed a Cas9 variant (vCas9) that produces breaks which suppress otherwise dominant nonhomologous end-joining (NHEJ) repair. Instead, breaks created by vCas9 are predominantly repaired by pathways utilizing homologous sequences, specifically microhomology-mediated end-joining (MMEJ) and homology-directed repair (HDR). Consequently, vCas9 enables efficient precise editing through HDR or MMEJ while suppressing indels caused by NHEJ in dividing and nondividing cells. These findings establish a paradigm of targeted nucleases custom-designed for specific mutational applications.

Transcription factor antagonism regulates heterogeneity in embryonic stem cell states.

Gene expression heterogeneity underlies cell states and contributes to developmental robustness. While heterogeneity can arise from stochastic transcriptional processes, the extent to which it is regulated is unclear. Here, we characterize the regulatory program underlying heterogeneity in murine embryonic stem cell (mESC) states. We identify differentially active and transcribed enhancers (DATEs) across states. DATEs regulate differentially expressed genes and are distinguished by co-binding of transcription factors Klf4 and Zfp281. In contrast to other factors that interact in a positive feedback network stabilizing mESC cell-type identity, Klf4 and Zfp281 drive opposing transcriptional and chromatin programs. Abrogation of factor binding to DATEs dampens variation in gene expression, and factor loss alters kinetics of switching between states. These results show antagonism between factors at enhancers results in gene expression heterogeneity and formation of cell states, with implications for the generation of diverse cell types during development.

Inhibition of the Sec61 translocon overcomes cytokine-induced glucocorticoid resistance in T-cell acute lymphoblastic leukaemia.

Glucocorticoid (GC) resistance is a poor prognostic factor in T-cell acute lymphoblastic leukaemia (T-ALL). Interleukin-7 (IL-7) mediates GC resistance via GC-induced upregulation of IL-7 receptor (IL-7R) expression, leading to increased pro-survival signalling. IL-7R reaches the cell surface via the secretory pathway, so we hypothesized that inhibiting the translocation of IL-7R into the secretory pathway would overcome GC resistance. Sec61 is an endoplasmic reticulum (ER) channel that is required for insertion of polypeptides into the ER. Here, we demonstrate that KZR-445, a novel inhibitor of Sec61, potently attenuates the dexamethasone (DEX)-induced increase in cell surface IL-7R and overcomes IL-7-induced DEX resistance.

RNA in formation and regulation of transcriptional condensates.

Macroscopic membraneless organelles containing RNA such as the nucleoli, germ granules, and the Cajal body have been known for decades. These biomolecular condensates are liquid-like bodies that can be formed by a phase transition. Recent evidence has revealed the presence of similar microscopic condensates associated with the transcription of genes. This brief article summarizes thoughts about the importance of condensates in the regulation of transcription and how RNA molecules, as components of such condensates, control the synthesis of RNA. Models and experimental data suggest that RNAs from enhancers facilitate the formation of a condensate that stabilizes the binding of transcription factors and accounts for a burst of transcription at the promoter. Termination of this burst is pictured as a nonequilibrium feedback loop where additional RNA destabilizes the condensate.

Inhibitors of Eukaryotic Translational Machinery as Therapeutic Agents.

Inhibiting eukaryotic protein translation with small molecules is emerging as a powerful therapeutic strategy. The advantage of targeting cellular translational machinery is that it is required for the highly proliferative state of many neoplastic cells, replication of certain viruses, and ultimately the expression of a wide variety of protein targets. Although, this approach has been exploited to develop clinical agents, such as homoharringtonine (HHT, 1), used to treat chronic myeloid leukemia (CML), inhibiting components of the translational machinery is often associated with cytotoxic phenotypes. However, recent studies have demonstrated that certain small molecules can inhibit the translation of specific subsets of proteins, leading to lower cytotoxicity, and opening-up therapeutic opportunities for translation inhibitors to be deployed in indications beyond oncology and infectious disease. This review summarizes efforts to develop inhibitors of the eukaryotic translational machinery as therapeutic agents and highlights emerging opportunities for translation inhibitors in the future.

RNA-Mediated Feedback Control of Transcriptional Condensates.

Regulation of biological processes typically incorporates mechanisms that initiate and terminate the process and, where understood, these mechanisms often involve feedback control. Regulation of transcription is a fundamental cellular process where the mechanisms involved in initiation have been studied extensively, but those involved in arresting the process are poorly understood. Modeling of the potential roles of RNA in transcriptional control suggested a non-equilibrium feedback control mechanism where low levels of RNA promote condensates formed by electrostatic interactions whereas relatively high levels promote dissolution of these condensates. Evidence from in vitro and in vivo experiments support a model where RNAs produced during early steps in transcription initiation stimulate condensate formation, whereas the burst of RNAs produced during elongation stimulate condensate dissolution. We propose that transcriptional regulation incorporates a feedback mechanism whereby transcribed RNAs initially stimulate but then ultimately arrest the process.

Partitioning of cancer therapeutics in nuclear condensates.

The nucleus contains diverse phase-separated condensates that compartmentalize and concentrate biomolecules with distinct physicochemical properties. Here, we investigated whether condensates concentrate small-molecule cancer therapeutics such that their pharmacodynamic properties are altered. We found that antineoplastic drugs become concentrated in specific protein condensates in vitro and that this occurs through physicochemical properties independent of the drug target. This behavior was also observed in tumor cells, where drug partitioning influenced drug activity. Altering the properties of the condensate was found to affect the concentration and activity of drugs. These results suggest that selective partitioning and concentration of small molecules within condensates contributes to drug pharmacodynamics and that further understanding of this phenomenon may facilitate advances in disease therapy.

MicroRNAs organize intrinsic variation into stem cell states.

Pluripotent embryonic stem cells (ESCs) contain the potential to form a diverse array of cells with distinct gene expression states, namely the cells of the adult vertebrate. Classically, diversity has been attributed to cells sensing their position with respect to external morphogen gradients. However, an alternative is that diversity arises in part from cooption of fluctuations in the gene regulatory network. Here we find ESCs exhibit intrinsic heterogeneity in the absence of external gradients by forming interconverting cell states. States vary in developmental gene expression programs and display distinct activity of microRNAs (miRNAs). Notably, miRNAs act on neighborhoods of pluripotency genes to increase variation of target genes and cell states. Loss of miRNAs that vary across states reduces target variation and delays state transitions, suggesting variable miRNAs organize and propagate variation to promote state transitions. Together these findings provide insight into how a gene regulatory network can coopt variation intrinsic to cell systems to form robust gene expression states. Interactions between intrinsic heterogeneity and environmental signals may help achieve developmental outcomes.

Imprinted Maternally Expressed microRNAs Antagonize Paternally Driven Gene Programs in Neurons.

Imprinted genes with parental-biased allelic expression are frequently co-regulated and enriched in common biological pathways. Here, we functionally characterize a large cluster of microRNAs (miRNAs) expressed from the maternally inherited allele ("maternally expressed") to explore the molecular and cellular consequences of imprinted miRNA activity. Using an induced neuron (iN) culture system, we show that maternally expressed miRNAs from the miR-379/410 cluster direct the RNA-induced silencing complex (RISC) to transcriptional and developmental regulators, including paternally expressed transcripts like Plagl1. Maternal deletion of this imprinted miRNA cluster resulted in increased protein levels of several targets and upregulation of a broader transcriptional program regulating synaptic transmission and neuronal function. A subset of the transcriptional changes resulting from miR-379/410 deletion can be attributed to de-repression of Plagl1. These data suggest maternally expressed miRNAs antagonize paternally driven gene programs in neurons.

Total Syntheses of the 3H-Pyrrolo[2,3-c]quinolone-Containing Alkaloids Marinoquinolines A-F, K, and Aplidiopsamine A Using a Palladium-Catalyzed Ullmann Cross-Coupling/Reductive Cyclization Pathway.

Compounds 1-6 and 11 representing key members of the marinoquinoline family of natural products, together with the related marine alkaloid aplidiopsamine A (12), have been synthesized using various combinations of palladium-catalyzed Ullmann cross-coupling and reductive cyclization processes involving a C3-arylated pyrrole as the common intermediate. These natural products have been characterized by single-crystal X-ray analyses and evaluated as inhibitors of acetylcholinesterase (AChE) with congener 2 proving to be the most active.

A small molecule interacts with VDAC2 to block mouse BAK-driven apoptosis.

Activating the intrinsic apoptosis pathway with small molecules is now a clinically validated approach to cancer therapy. In contrast, blocking apoptosis to prevent the death of healthy cells in disease settings has not been achieved. Caspases have been favored, but they act too late in apoptosis to provide long-term protection. The critical step in committing a cell to death is activation of BAK or BAX, pro-death BCL-2 proteins mediating mitochondrial damage. Apoptosis cannot proceed in their absence. Here we show that WEHI-9625, a novel tricyclic sulfone small molecule, binds to VDAC2 and promotes its ability to inhibit apoptosis driven by mouse BAK. In contrast to caspase inhibitors, WEHI-9625 blocks apoptosis before mitochondrial damage, preserving cellular function and long-term clonogenic potential. Our findings expand on the key role of VDAC2 in regulating apoptosis and demonstrate that blocking apoptosis at an early stage is both advantageous and pharmacologically tractable.

Enhancer Features that Drive Formation of Transcriptional Condensates.

Enhancers are DNA elements that are bound by transcription factors (TFs), which recruit coactivators and the transcriptional machinery to genes. Phase-separated condensates of TFs and coactivators have been implicated in assembling the transcription machinery at particular enhancers, yet the role of DNA sequence in this process has not been explored. We show that DNA sequences encoding TF binding site number, density, and affinity above sharply defined thresholds drive condensation of TFs and coactivators. A combination of specific structured (TF-DNA) and weak multivalent (TF-coactivator) interactions allows for condensates to form at particular genomic loci determined by the DNA sequence and the complement of expressed TFs. DNA features found to drive condensation promote enhancer activity and transcription in cells. Our study provides a framework to understand how the genome can scaffold transcriptional condensates at specific loci and how the universal phenomenon of phase separation might regulate this process.