RNA-Mediated Feedback Control of Transcriptional Condensates

Technological advancements in deciphering RNA-RNA interactions

RNA-RNA interactions (RRIs) can dictate RNA molecules to form intricate higher-order structures and bind their RNA substrates in diverse biological processes. To elucidate the function, binding specificity, and regulatory mechanisms of various RNA molecules, especially the vast repertoire of non-coding RNAs, advanced technologies and methods that globally map RRIs are extremely valuable. In the past decades, many state-of-the-art technologies have been developed for this purpose. This review focuses on those high-throughput technologies for the global mapping of RRIs. We summarize the key concepts and the pros and cons of different technologies. In addition, we highlight the novel biological insights uncovered by these RRI mapping methods and discuss the future challenges for appreciating the crucial roles of RRIs in gene regulation across bacteria, viruses, archaea, and mammals.

RNA-driven phase transitions in biomolecular condensates

RNAs and RNA-binding proteins can undergo spontaneous or active condensation into phase-separated liquid-like droplets. These condensates are cellular hubs for various physiological processes, and their dysregulation leads to diseases. Although RNAs are core components of many cellular condensates, the underlying molecular determinants for the formation, regulation, and function of ribonucleoprotein condensates have largely been studied from a protein-centric perspective. Here, we highlight recent developments in ribonucleoprotein condensate biology with a particular emphasis on RNA-driven phase transitions. We also present emerging future directions that might shed light on the role of RNA condensates in spatiotemporal regulation of cellular processes and inspire bioengineering of RNA-based therapeutics.

An RNA-centric view of transcription and genome organization

Foundational models of transcriptional regulation involve the assembly of protein complexes at DNA elements associated with specific genes. These assemblies, which can include transcription factors, cofactors, RNA polymerase, and various chromatin regulators, form dynamic spatial compartments that contribute to both gene regulation and local genome architecture. This DNA-protein-centric view has been modified with recent evidence that RNA molecules have important roles to play in gene regulation and genome structure. Here, we discuss evidence that gene regulation by RNA occurs at multiple levels that include assembly of transcriptional complexes and genome compartments, feedback regulation of active genes, silencing of genes, and control of protein kinases. We thus provide an RNA-centric view of transcriptional regulation that must reside alongside the more traditional DNA-protein-centric perspectives on gene regulation and genome architecture.

Regulatory landscape of enhancer-mediated transcriptional activation

Enhancers are noncoding regulatory elements that instruct spatial and temporal specificity of gene transcription in response to a variety of intrinsic and extrinsic signals during development. Although it has long been postulated that enhancers physically interact with target promoters through the formation of stable loops, recent studies have changed this static view: sequence-specific transcription factors (TFs) and coactivators are dynamically recruited to enhancers and assemble so-called transcription hubs. Dynamic assembly of transcription hubs appears to serve as a key scaffold to integrate regulatory information encoded by surrounding genome and biophysical properties of transcription machineries. In this review, we outline emerging new models of transcriptional regulation by enhancers and discuss future perspectives.

Transcription factor condensates, 3D clustering, and gene expression enhancement of the MET regulon

Some transcription factors (TFs) can form liquid-liquid phase separated (LLPS) condensates. However, the functions of these TF condensates in 3-Dimentional (3D) genome organization and gene regulation remain elusive. In response to methionine (met) starvation, budding yeast TF Met4 and a few co-activators, including Met32, induce a set of genes involved in met biosynthesis. Here, we show that the endogenous Met4 and Met32 form co-localized puncta-like structures in yeast nuclei upon met depletion. Recombinant Met4 and Met32 form mixed droplets with LLPS properties in vitro. In relation to chromatin, Met4 puncta co-localize with target genes, and at least a subset of these target genes is clustered in 3D in a Met4-dependent manner. A MET3pr-GFP reporter inserted near several native Met4-binding sites becomes co-localized with Met4 puncta and displays enhanced transcriptional activity. A Met4 variant with a partial truncation of an intrinsically disordered region (IDR) shows less puncta formation, and this mutant selectively reduces the reporter activity near Met4-binding sites to the basal level. Overall, these results support a model where Met4 and co-activators form condensates to bring multiple target genes into a vicinity with higher local TF concentrations, which facilitates a strong response to methionine depletion.

Capture of RNA-binding proteins across mouse tissues using HARD-AP

RNA-binding proteins (RBPs) modulate all aspects of RNA metabolism, but a comprehensive picture of RBP expression across tissues is lacking. Here, we describe our development of the method we call HARD-AP that robustly retrieves RBPs and tightly associated RNA regulatory complexes from cultured cells and fresh tissues. We successfully use HARD-AP to establish a comprehensive atlas of RBPs across mouse primary organs. We then systematically map RNA-binding sites of these RBPs using machine learning-based modeling. Notably, the modeling reveals that the LIM domain as an RNA-binding domain in many RBPs. We validate the LIM-domain-only protein Csrp1 as a tissue-dependent RNA binding protein. Taken together, HARD-AP is a powerful approach that can be used to identify RBPomes from any type of sample, allowing comprehensive and physiologically relevant networks of RNA-protein interactions.

RNA and condensates: Disease implications and therapeutic opportunities

Biomolecular condensates are dynamic membraneless organelles that compartmentalize proteins and RNA molecules to regulate key cellular processes. Diverse RNA species exert their effects on the cell by their roles in condensate formation and function. RNA abnormalities such as overexpression, modification, and mislocalization can lead to pathological condensate behaviors that drive various diseases, including cancer, neurological disorders, and infections. Here, we review RNA's role in condensate biology, describe the mechanisms of RNA-induced condensate dysregulation, note the implications for disease pathogenesis, and discuss novel therapeutic strategies. Emerging approaches to targeting RNA within condensates, including small molecules and RNA-based therapies that leverage the unique properties of condensates, may revolutionize treatment for complex diseases.

Engineering Material Properties of Transcription Factor Condensates to Control Gene Expression in Mammalian Cells and Mice

Phase separation of biomolecules into condensates is a key mechanism in the spatiotemporal organization of biochemical processes in cells. However, the impact of the material properties of biomolecular condensates on important processes, such as the control of gene expression, remains largely elusive. Here, the material properties of optogenetically induced transcription factor condensates are systematically tuned, and probed for their impact on the activation of target promoters. It is demonstrated that transcription factors in rather liquid condensates correlate with increased gene expression levels, whereas stiffer transcription factor condensates correlate with the opposite effect, reduced activation of gene expression. The broad nature of these findings is demonstrated in mammalian cells and mice, as well as by using different synthetic and natural transcription factors. These effects are observed for both transgenic and cell-endogenous promoters. The findings provide a novel materials-based layer in the control of gene expression, which opens novel opportunities in optogenetic engineering and synthetic biology.

Uncovering the functions and mechanisms of regulatory elements-associated non-coding RNAs

Over the past decade, regulatory non-coding RNAs (ncRNAs) produced by RNA Pol II have been revealed as meaningful players in various essential cellular functions. In particular, thousands of ncRNAs are produced at transcriptional regulatory elements such as enhancers and promoters, where they may exert multiple functions to regulate proper development, cellular programming, transcription or genomic stability. Here, we review the mechanisms involving these regulatory element-associated ncRNAs, and particularly enhancer RNAs (eRNAs) and PROMoter uPstream Transcripts (PROMPTs). We contextualize the mechanisms described to the processing and degradation of these short lived RNAs. We summarize recent findings explaining how ncRNAs operate locally at promoters and enhancers, or further away, either shortly after their production by RNA Pol II, or through post-transcriptional stabilization. Such discoveries lead to a converging model accounting for how ncRNAs influence cellular fate, by acting on transcription and chromatin structure, which may further involve factors participating to 3D nuclear organization.

Transcriptional bursting: from fundamentals to novel insights

Transcription occurs as irregular bursts in a very wide range of systems, including numerous different species and many genes within these. In this review, we examine the underlying theories, discuss how these relate to experimental measurements, and explore some of the discrepancies that have emerged among various studies. Finally, we consider more recent works that integrate novel concepts, such as the involvement of biomolecular condensates in enhancer-promoter interactions and their effects on the dynamics of transcriptional bursting.

Nuclear RNA: a transcription-dependent regulator of chromatin structure

Although the majority of RNAs are retained in the nucleus, their significance is often overlooked. However, it is now becoming clear that nuclear RNA forms a dynamic structure through interacting with various proteins that can influence the three-dimensional structure of chromatin. We review the emerging evidence for a nuclear RNA mesh or gel, highlighting the interplay between DNA, RNA and RNA-binding proteins (RBPs), and assessing the critical role of protein and RNA in governing chromatin architecture. We also discuss a proposed role for the formation and regulation of the nuclear gel in transcriptional control. We suggest that it may concentrate the transcriptional machinery either by direct binding or inducing RBPs to form microphase condensates, nanometre sized membraneless structures with distinct properties to the surrounding medium and an enrichment of particular macromolecules.

Homeostasis of mRNA concentrations through coupling transcription, export, and degradation

Many experiments showed that eukaryotic cells maintain a constant mRNA concentration upon various perturbations by actively regulating mRNA production and degradation rates, known as mRNA buffering. However, the underlying mechanism is still unknown. In this work, we unveil a mechanistic model of mRNA buffering: the releasing-shuttling (RS) model. The model incorporates two crucial proteins, X and Y, which play several roles, including transcription, decay, and export factors, in the different stages of mRNA metabolism. The RS model predicts the constant mRNA concentration under genome-wide genetic perturbations and cell volume changes, the slowed-down mRNA degradation after Pol II depletion, and the temporal transcription dynamics after exonuclease depletion, in agreement with multiple experiments. Finally, we present a list of X and Y candidates and propose an experimental method to identify X. Our work uncovers potentially universal pathways coupling transcription, export, and degradation that help cells maintain mRNA homeostasis.

Nonequilibrium phases of a biomolecular condensate facilitated by enzyme activity

Biomolecular condensates represent a frontier in cellular organization, existing as dynamic materials driven out of equilibrium by active cellular processes. Here we explore active mechanisms of condensate regulation by examining the interplay between DEAD-box helicase activity and RNA base-pairing interactions within ribonucleoprotein condensates. We demonstrate how the ATP-dependent activity of DEAD-box helicases-a key class of enzymes in condensate regulation-acts as a nonequilibrium driver of condensate properties through the continuous remodeling of RNA interactions. By combining the LAF-1 DEAD-box helicase with a designer RNA hairpin concatemer, we unveil a complex landscape of dynamic behaviors, including time-dependent alterations in RNA partitioning, evolving condensate morphologies, and shifting condensate dynamics. Importantly, we reveal an antagonistic relationship between RNA secondary structure and helicase activity which promotes condensate homogeneity via a nonequilibrium steady state. By elucidating these nonequilibrium mechanisms, we gain a deeper understanding of cellular organization and expand the potential for active synthetic condensate systems.

YAP charge patterning mediates signal integration through transcriptional co-condensates

Transcription factor dynamics are used to selectively engage gene regulatory programs. Biomolecular condensates have emerged as an attractive signaling substrate in this process, but the underlying mechanisms are not well-understood. Here, we probed the molecular basis of YAP signal integration through transcriptional condensates. Leveraging light-sheet single-molecule imaging and synthetic condensates, we demonstrate charge-mediated co-condensation of the transcriptional regulators YAP and Mediator into transcriptionally active condensates in stem cells. IDR sequence analysis and YAP protein engineering demonstrate that instead of the net charge, YAP signaling specificity is established through its negative charge patterning that interacts with Mediator's positive charge blocks. The mutual enhancement of YAP/Mediator co-condensation is counteracted by negative feedback from transcription, driving an adaptive transcriptional response that is well-suited for decoding dynamic inputs. Our work reveals a molecular framework for YAP condensate formation and sheds new light on the function of YAP condensates for emergent gene regulatory behavior.

Langevin Equation in Heterogeneous Landscapes: How to Choose the Interpretation

The Langevin equation is a common tool to model diffusion at a single-particle level. In nonhomogeneous environments, such as aqueous two-phase systems or biological condensates with different diffusion coefficients in different phases, the solution to a Langevin equation is not unique unless the interpretation of stochastic integrals involved is selected. We analyze the diffusion of particles in such systems and evaluate the mean, the mean square displacement, and the distribution of particles, as well as the variance of the time-averaged mean-square displacements. Our analytical results provide a method to choose the interpretation parameter from single-particle tracking experiments.

Rapid and reversible dissolution of biomolecular condensates using light-controlled recruitment of a solubility tag

Biomolecular condensates are broadly implicated in both normal cellular regulation and disease. Consequently, several chemical biology and optogenetic approaches have been developed to induce phase separation of a protein of interest. However, few tools are available to perform the converse function - dissolving a condensate of interest on demand. Such a tool would aid in testing whether the condensate plays specific functional roles. Here we show that light-gated recruitment of a solubilizing domain, maltose-binding protein (MBP), results in rapid and controlled dissolution of condensates formed from proteins of interest. Our optogenetic MBP-based dissolution strategy (OptoMBP) is rapid, reversible, and can be spatially controlled with subcellular precision. We also provide a proof-of-principle application of OptoMBP by disrupting condensation of the oncogenic fusion protein FUS-CHOP and reverting FUS-CHOP driven transcriptional changes. We envision that the OptoMBP system could be broadly useful for disrupting constitutive protein condensates to probe their biological functions.

Functional characteristics and computational model of abundant hyperactive loci in the human genome

Enhancers and promoters are classically considered to be bound by a small set of TFs in a sequence-specific manner. This assumption has come under increasing skepticism as the datasets of ChIP-seq assays of TFs have expanded. In particular, high-occupancy target (HOT) loci attract hundreds of TFs with often no detectable correlation between ChIP-seq peaks and DNA-binding motif presence. Here, we used a set of 1,003 TF ChIP-seq datasets (HepG2, K562, H1) to analyze the patterns of ChIP-seq peak co-occurrence in combination with functional genomics datasets. We identified 43,891 HOT loci forming at the promoter (53%) and enhancer (47%) regions. HOT promoters regulate housekeeping genes, whereas HOT enhancers are involved in tissue-specific process regulation. HOT loci form the foundation of human super-enhancers and evolve under strong negative selection, with some of these loci being located in ultraconserved regions. Sequence-based classification analysis of HOT loci suggested that their formation is driven by the sequence features, and the density of mapped ChIP-seq peaks across TF-bound loci correlates with sequence features and the expression level of flanking genes. Based on the affinities to bind to promoters and enhancers we detected 5 distinct clusters of TFs that form the core of the HOT loci. We report an abundance of HOT loci in the human genome and a commitment of 51% of all TF ChIP-seq binding events to HOT locus formation thus challenging the classical model of enhancer activity and propose a model of HOT locus formation based on the existence of large transcriptional condensates.

Revisiting kinetic Monte Carlo algorithms for time-dependent processes: From open-loop control to feedback control

Simulating stochastic systems with feedback control is challenging due to the complex interplay between the system's dynamics and the feedback-dependent control protocols. We present a single-step-trajectory probability analysis to time-dependent stochastic systems. Based on this analysis, we revisit several time-dependent kinetic Monte Carlo (KMC) algorithms designed for systems under open-loop-control protocols. Our analysis provides a unified alternative proof to these algorithms, summarized into a pedagogical tutorial. Moreover, with the trajectory probability analysis, we present a novel feedback-controlled KMC algorithm that accurately captures the dynamics systems controlled by an external signal based on the measurements of the system's state. Our method correctly captures the system dynamics and avoids the artificial Zeno effect that arises from incorrectly applying the direct Gillespie algorithm to feedback-controlled systems. This work provides a unified perspective on existing open-loop-control KMC algorithms and also offers a powerful and accurate tool for simulating stochastic systems with feedback control.

DOT1L/H3K79me2 represses HIV-1 reactivation via recruiting DCAF1

DOT1L mediates the methylation of histone H3 at lysine 79 and, in turn, the transcriptional activation or repression in a context-dependent manner, yet the regulatory mechanisms and functions of DOT1L/H3K79me remain to be fully explored. Following peptide affinity purification and proteomic analysis, we identified that DCAF1-a component of the E3 ligase complex involved in HIV regulation-is associated with H3K79me2 and DOT1L. Interestingly, blocking the expression or catalytic activity of DOT1L or repressing the expression of DCAF1 significantly enhances the tumor necrosis factor alpha (TNF-α)/nuclear factor κB (NF-κB)-induced reactivation of the latent HIV-1 genome. Mechanistically, upon TNF-α/NF-κB activation, DCAF1 is recruited to the HIV-1 long terminal repeat (LTR) by DOT1L and H3K79me2. Recruited DCAF1 subsequently induces the ubiquitination of NF-κB and restricts its accumulation at the HIV-1 LTR. Altogether, our findings reveal a feedback modulation of HIV reactivation by DOT1L-mediated histone modification regulation and highlight the potential of targeting the DOT1L/DCAF1 axis as a therapeutic strategy for HIV treatment.

How Transcription Factor Clusters Shape the Transcriptional Landscape

In eukaryotic cells, gene transcription typically occurs in discrete periods of promoter activity, interspersed with intervals of inactivity. This pattern deviates from simple stochastic events and warrants a closer examination of the molecular interactions that activate the promoter. Recent studies have identified transcription factor (TF) clusters as key precursors to transcriptional bursting. Often, these TF clusters form at chromatin segments that are physically distant from the promoter, making changes in chromatin conformation crucial for promoter-TF cluster interactions. In this review, I explore the formation and constituents of TF clusters, examining how the dynamic interplay between chromatin architecture and TF clustering influences transcriptional bursting. Additionally, I discuss techniques for visualizing TF clusters and provide an outlook on understanding the remaining gaps in this field.

Phase separation is regulated by post-translational modifications and participates in the developments of human diseases

Liquid-liquid phase separation (LLPS) of intracellular proteins has emerged as a hot research topic in recent years. Membrane-less and liquid-like condensates provide dense spaces that ensure cells to high efficiently regulate genes transcription and rapidly respond to burst changes from the environment. The fomation and activity of LLPS are not only modulated by the cytosol conditions including but not limited to salt concentration and temperture. Interestingly, recent studies have shown that phase separation is also regulated by various post-translational modifications (PTMs) through modulating proteins multivalency, such as solubility and charge interactions. The regulation mechanism is crucial for normal functioning of cells, as aberrant protein aggregates are often closely related with the occurrence and development of human diseases including cancer and nurodegenerative diseases. Therefore, studying phase separation in the perspective of protein PTMs has long-term significance for human health. In this review, we summarized the properties and cellular physiological functions of LLPS, particularly its relationships with PTMs in human diseases according to recent researches.

Transcription Factor Condensates Mediate Clustering of MET Regulon and Enhancement in Gene Expression

Some transcription factors (TFs) can form liquid-liquid phase separated (LLPS) condensates. However, the functions of these TF condensates in 3D genome organization and gene regulation remain elusive. In response to methionine (met) starvation, budding yeast TF Met4 and a few co-activators, including Met32, induce a set of genes involved in met biosynthesis. Here, we show that the endogenous Met4 and Met32 form co-localized puncta-like structures in yeast nuclei upon met depletion. Recombinant Met4 and Met32 form mixed droplets with LLPS properties in vitro. In relation to chromatin, Met4 puncta co-localize with target genes, and at least a subset of these target genes is clustered in 3D in a Met4-dependent manner. A MET3pr-GFP reporter inserted near several native Met4 binding sites becomes co-localized with Met4 puncta and displays enhanced transcriptional activity. A Met4 variant with a partial truncation of an intrinsically disordered region (IDR) shows less puncta formation, and this mutant selectively reduces the reporter activity near Met4 binding sites to the basal level. Overall, these results support a model where Met4 and co-activators form condensates to bring multiple target genes into a vicinity with higher local TF concentrations, which facilitates a strong response to methionine depletion.

Pericentromeric satellite RNAs as flexible protein partners in the regulation of nuclear structure

Pericentromeric heterochromatin is mainly composed of satellite DNA sequences. Although being historically associated with transcriptional repression, some pericentromeric satellite DNA sequences are transcribed. The transcription events of pericentromeric satellite sequences occur in highly flexible biological contexts. Hence, the apparent randomness of pericentromeric satellite transcription incites the discussion about the attribution of biological functions. However, pericentromeric satellite RNAs have clear roles in the organization of nuclear structure. Silencing pericentromeric heterochromatin depends on pericentromeric satellite RNAs, that, in a feedback mechanism, contribute to the repression of pericentromeric heterochromatin. Moreover, pericentromeric satellite RNAs can also act as scaffolding molecules in condensate subnuclear structures (e.g., nuclear stress bodies). Since the formation/dissociation of nuclear condensates provides cell adaptability, pericentromeric satellite RNAs can be an epigenetic platform for regulating (sub)nuclear structure. We review current knowledge about pericentromeric satellite RNAs that, irrespective of the meaning of biological function, should be functionally addressed in regular and disease settings. This article is categorized under: RNA Methods > RNA Analyses in Cells RNA in Disease and Development > RNA in Disease.

Accelerating Diverse Cell-Based Therapies Through Scalable Design

Augmenting cells with novel, genetically encoded functions will support therapies that expand beyond natural capacity for immune surveillance and tissue regeneration. However, engineering cells at scale with transgenic cargoes remains a challenge in realizing the potential of cell-based therapies. In this review, we introduce a range of applications for engineering primary cells and stem cells for cell-based therapies. We highlight tools and advances that have launched mammalian cell engineering from bioproduction to precision editing of therapeutically relevant cells. Additionally, we examine how transgenesis methods and genetic cargo designs can be tailored for performance. Altogether, we offer a vision for accelerating the translation of innovative cell-based therapies by harnessing diverse cell types, integrating the expanding array of synthetic biology tools, and building cellular tools through advanced genome writing techniques.

Chimera RNA transcribed from integrated HPV18 genome with adjacent host genomic region promotes oncogenic gene expression through condensate formation

Most cervical cancers are caused by human papillomavirus (HPV) infection. In HeLa cells, the HPV18 viral genome is integrated at chromosome 8q24.21 and activates transcription of the proto-oncogene c-Myc. However, the mechanism of how the integrated HPV genome and its transcribed RNAs exhibit transcription activation function has not been fully elucidated. In this study, we found that HPV18 transcripts contain an enhancer RNA-like function to activate proximal genes including CCAT1-5L and c-Myc. We showed that the human genome-integrated HPV18 genes are activated by transcription coregulators including BRD4 and Mediator. The transcribed HPV18 RNAs form a liquid-like condensate at chromosome 8q24.21 locus, which in turn accumulates RNA polymerase II. Moreover, we focused on a relatively uncharacterized transcript from the upstream region of CCAT1, named URC. The URC RNA is transcribed as a chimera RNA with HPV18 and is composed of the 3'-untranslated region of the HPV18 transcript. We experimentally showed that the URC contributes to stabilization of HPV18 RNAs by supplying a polyadenylation site for the HPV18 transcript. Our findings suggest that integrated HPV18 at 8q24.21 locus produces HPV18-URC chimera RNA and promotes tumorigenesis through RNA-based condensate formation.

RNA modulates hnRNPA1A amyloid formation mediated by biomolecular condensates

Several RNA binding proteins involved in membraneless organelles can form pathological amyloids associated with neurodegenerative diseases, but the mechanisms of how this aggregation is modulated remain elusive. Here we investigate how heterotypic protein-RNA interactions modulate the condensation and the liquid to amyloid transition of hnRNPA1A, a protein involved in amyothropic lateral sclerosis. In the absence of RNA, formation of condensates promotes hnRNPA1A aggregation and fibrils are localized at the interface of the condensates. Addition of RNA modulates the soluble to amyloid transition of hnRNPA1A according to different pathways depending on RNA/protein stoichiometry. At low RNA concentrations, RNA promotes both condensation and amyloid formation, and the catalytic effect of RNA adds to the role of the interface between the dense and dilute phases. At higher RNA concentrations, condensation is suppressed according to re-entrant phase behaviour but formation of hnRNPA1A amyloids is observed over longer incubation times. Our findings show how heterotypic nucleic acid-protein interactions affect the kinetics and molecular pathways of amyloid formation.

Gene activity fully predicts transcriptional bursting dynamics

Transcription commonly occurs in bursts, with alternating productive (ON) and quiescent (OFF) periods, governing mRNA production rates. Yet, how transcription is regulated through bursting dynamics remains unresolved. Here, we conduct real-time measurements of endogenous transcriptional bursting with single-mRNA sensitivity. Leveraging the diverse transcriptional activities in early fly embryos, we uncover stringent relationships between bursting parameters. Specifically, we find that the durations of ON and OFF periods are linked. Regardless of the developmental stage or body-axis position, gene activity levels predict individual alleles' average ON and OFF periods. Lowly transcribing alleles predominantly modulate OFF periods (burst frequency), while highly transcribing alleles primarily tune ON periods (burst size). These relationships persist even under perturbations of cis-regulatory elements or trans-factors and account for bursting dynamics measured in other species. Our results suggest a novel mechanistic constraint governing bursting dynamics rather than a modular control of distinct parameters by distinct regulatory processes.

Transcription regulates the spatio-temporal dynamics of genes through micro-compartmentalization

Although our understanding of the involvement of heterochromatin architectural factors in shaping nuclear organization is improving, there is still ongoing debate regarding the role of active genes in this process. In this study, we utilize publicly-available Micro-C data from mouse embryonic stem cells to investigate the relationship between gene transcription and 3D gene folding. Our analysis uncovers a nonmonotonic - globally positive - correlation between intragenic contact density and Pol II occupancy, independent of cohesin-based loop extrusion. Through the development of a biophysical model integrating the role of transcription dynamics within a polymer model of chromosome organization, we demonstrate that Pol II-mediated attractive interactions with limited valency between transcribed regions yield quantitative predictions consistent with chromosome-conformation-capture and live-imaging experiments. Our work provides compelling evidence that transcriptional activity shapes the 4D genome through Pol II-mediated micro-compartmentalization.

Size Scaling of Condensates in Multicomponent Phase Separation

Constant proportionalities between cells and their intracellular organelles have been widely observed in various types of cells, known as intracellular size scaling. However, the mechanism underlying the size scaling and its modulation by environmental factors in multicomponent systems remain poorly understood. Here, we study the size scaling of membrane-less condensates using microdroplet-encapsulated minimalistic condensates formed by droplet microfluidics and mean-field theory. We demonstrate that the size scaling of condensates is an inherent characteristic of liquid-liquid phase separation. This concept is supported by experiments showing the occurrence of size scaling phenomena in various condensate systems and a generic lever rule acquired from mean-field theory. Moreover, it is found that the condensate-to-microdroplet scaling ratio can be affected by the solute and salt concentrations, with good agreement between experiments and predictions by theory. Notably, we identify a noise buffering mechanism whereby condensates composed of large macromolecules effectively maintain constant volumes and counteract concentration fluctuations of small molecules. This mechanism is achieved through the dynamic rearrangement of small molecules in and out of membrane-free interfaces. Our work provides crucial insights into understanding mechanistic principles that govern the size of cells and intracellular organelles as well as associated biological functions.

Transcription templated assembly of the nucleolus in the C. elegans embryo

The nucleolus is a multicomponent structure made of RNA and proteins that serves as the site of ribosome biogenesis within the nucleus. It has been extensively studied as a prototype of a biomolecular condensate whose assembly is driven by phase separation. While the steady-state size of the nucleolus is quantitatively accounted for by the thermodynamics of phase separation, we show that experimental measurements of the assembly dynamics are inconsistent with a simple model of a phase-separating system relaxing to its equilibrium state. Instead, we show that the dynamics are well described by a model in which the transcription of ribosomal RNA actively drives nucleolar assembly. We find that our model of active transcription-templated assembly quantitatively accounts for the rapid kinetics observed in early embryos at different developmental stages, and for different RNAi perturbations of embryo size. Our model predicts a scaling of the time to assembly with the volume of the nucleus to the one-third power, which is confirmed by experimental data. Our study highlights the role of active processes such as transcription in controlling the placement and timing of assembly of membraneless organelles.

The MYCN oncoprotein is an RNA-binding accessory factor of the nuclear exosome targeting complex

The MYCN oncoprotein binds active promoters in a heterodimer with its partner protein MAX. MYCN also interacts with the nuclear exosome, a 3'-5' exoribonuclease complex, suggesting a function in RNA metabolism. Here, we show that MYCN forms stable high-molecular-weight complexes with the exosome and multiple RNA-binding proteins. MYCN binds RNA in vitro and in cells via a conserved sequence termed MYCBoxI. In cells, MYCN associates with thousands of intronic transcripts together with the ZCCHC8 subunit of the nuclear exosome targeting complex and enhances their processing. Perturbing exosome function results in global re-localization of MYCN from promoters to intronic RNAs. On chromatin, MYCN is then replaced by the MNT(MXD6) repressor protein, inhibiting MYCN-dependent transcription. RNA-binding-deficient alleles show that RNA-binding limits MYCN's ability to activate cell growth-related genes but is required for MYCN's ability to promote progression through S phase and enhance the stress resilience of neuroblastoma cells.

Crowder titrations enable the quantification of driving forces for macromolecular phase separation

Macromolecular solubility is an important contributor to the driving forces for phase separation. Formally, the driving forces in a binary mixture comprising a macromolecule dissolved in a solvent can be quantified in terms of the saturation concentration, which is the threshold macromolecular concentration above which the mixture separates into coexisting dense and dilute phases. In addition, the second virial coefficient, which measures the effective strength of solvent-mediated intermolecular interactions provides direct assessments of solvent quality. The sign and magnitude of second virial coefficients will be governed by a combination of solution conditions and the nature of the macromolecule of interest. Here, we show, using a combination of theory, simulation, and in vitro experiments, that titrations of crowders, providing they are true depletants, can be used to extract the intrinsic driving forces for macromolecular phase separation. This refers to saturation concentrations in the absence of crowders and the second virial coefficients that quantify the magnitude of the incompatibility between macromolecules and the solvent. Our results show how the depletion-mediated attractions afforded by crowders can be leveraged to obtain comparative assessments of macromolecule-specific, intrinsic driving forces for phase separation.

Dysregulation of transcriptional condensates in human disease: mechanisms, biological functions, and open questions

Precise gene expression, crucial for normal development and health, depends on the co-ordinated assembly and function of various factors within the crowded nucleus. Recent evidence suggests that this process is in part regulated by mesoscale compartmentalization and concentration of transcriptional components within condensates, offering a new perspective on gene regulation. Dysregulation of transcriptional condensates is increasingly associated with diseases, indicating a potential role in pathogenesis. In this mini-review, we provide a concise overview of the current understanding of the formation and function of transcriptional condensates, with a specific focus on recent advances in their dysregulation and implications in diseases, notably cancer. We also address limitations in the field and highlight open questions for future research.

Dynamic microenvironments shape nuclear organization and gene expression

Live imaging has revealed that the regulation of gene expression is largely driven by transient interactions. For example, many regulatory proteins bind chromatin for just seconds, and loop-like genomic contacts are rare and last only minutes. These discoveries have been difficult to reconcile with our canonical models that are predicated on stable and hierarchical interactions. Proteomic microenvironments that concentrate nuclear factors may explain how brief interactions can still mediate gene regulation by creating conditions where reactions occur more frequently. Here, we summarize new imaging technologies and recent discoveries implicating microenvironments as a potential driver of nuclear function. Finally, we propose that key properties of proteomic microenvironments, such as their size, enrichment, and lifetimes, are directly linked to regulatory function.

Phase separation and transcriptional regulation in cancer development

Liquid-liquid phase separation, a novel biochemical phenomenon, has been increasingly studied for its medical applications. It underlies the formation of membrane-less organelles and is involved in many cellular and biological processes. During transcriptional regulation, dynamic condensates are formed through interactions between transcriptional elements, such as transcription factors, coactivators, and mediators. Cancer is a disease characterized by uncontrolled cell proliferation, but the precise mechanisms underlying tumorigenesis often remain to be elucidated. Emerging evidence has linked abnormal transcriptional condensates to several diseases, especially cancer, implying that phase separation plays an important role in tumorigenesis. Condensates formed by phase separation may have an effect on gene transcription in tumors. In the present review, we focus on the correlation between phase separation and transcriptional regulation, as well as how this phenomenon contributes to cancer development.

Mammalian RNAi represses pericentromeric lncRNAs to maintain genome stability

Mammalian pericentromeric tandem repeats produce long noncoding RNAs (lncRNAs) that are dysregulated in cancer and linked to genomic instability. Identifying the basic molecular characteristics of these lncRNAs and their regulation is important to understanding their biological function. Here, we determine that the Argonaute (Ago) proteins of the RNA interference (RNAi) pathway directly and uniformly repress bidirectional pericentromeric lncRNAs in a Dicer-dependent manner in mouse embryonic and adult stem cells. Ago-dependent and Dicer-dependent autoregulatory small RNAs were identified within pericentromeric lncRNA degradation intermediates. We develop an RNase H cleavage assay to determine the relative proportions and lengths of the pericentromeric lncRNA targets. We find that 5'-phosphate and non-polyadenylated bidirectional pericentromeric lncRNAs are expressed at similar proportions. These lncRNAs can span up to 9 repeats, with transcription from the reverse strand template yielding the longer products. Using pericentromeric repeat RNA reporters, we determine that Ago represses pericentromeric lncRNAs after S phase transcription. Upon loss of Ago, pericentromeric lncRNA dysregulation results in delayed cell cycle progression, a defective mitotic spindle assembly checkpoint (SAC) and genomic instability. These results show that an evolutionarily conserved Ago activity at pericentromeres contributes to mammalian genome stability.

Dual-role transcription factors stabilize intermediate expression levels

Precise control of gene expression levels is essential for normal cell functions, yet how they are defined and tightly maintained, particularly at intermediate levels, remains elusive. Here, using a series of newly developed sequencing, imaging, and functional assays, we uncover a class of transcription factors with dual roles as activators and repressors, referred to as condensate-forming level-regulating dual-action transcription factors (TFs). They reduce high expression but increase low expression to achieve stable intermediate levels. Dual-action TFs directly exert activating and repressing functions via condensate-forming domains that compartmentalize core transcriptional unit selectively. Clinically relevant mutations in these domains, which are linked to a range of developmental disorders, impair condensate selectivity and dual-action TF activity. These results collectively address a fundamental question in expression regulation and demonstrate the potential of level-regulating dual-action TFs as powerful effectors for engineering controlled expression levels.

Solutes unmask differences in clustering versus phase separation of FET proteins

Phase separation and percolation contribute to phase transitions of multivalent macromolecules. Contributions of percolation are evident through the viscoelasticity of condensates and through the formation of heterogeneous distributions of nano- and mesoscale pre-percolation clusters in sub-saturated solutions. Here, we show that clusters formed in sub-saturated solutions of FET (FUS-EWSR1-TAF15) proteins are affected differently by glutamate versus chloride. These differences on the nanoscale, gleaned using a suite of methods deployed across a wide range of protein concentrations, are prevalent and can be unmasked even though the driving forces for phase separation remain unchanged in glutamate versus chloride. Strikingly, differences in anion-mediated interactions that drive clustering saturate on the micron-scale. Beyond this length scale the system separates into coexisting phases. Overall, we find that sequence-encoded interactions, mediated by solution components, make synergistic and distinct contributions to the formation of pre-percolation clusters in sub-saturated solutions, and to the driving forces for phase separation.

U3 snoRNA inter-regulates with DDX21 in the perichromosomal region to control mitosis

U3 snoRNA is essential for ribosome biogenesis during interphase. Upon mitotic onset, the nucleolus disassembles and U3 snoRNA relocates to the perichromosomal region (PR) to be considered as a chromosome passenger. Whether U3 controls mitosis remains unknown. Here, we demonstrate that U3 snoRNA is required for mitotic progression. We identified DDX21 as the predominant U3-binding protein during mitosis and confirmed that U3 snoRNA colocalizes with DDX21 in the PR. DDX21 knockdown induces mitotic catastrophe and similar mitotic defects caused by U3 snoRNA depletion. Interestingly, the uniform PR distribution of U3 snoRNA and DDX21 is interdependent. DDX21 functions in mitosis depending on its PR localization. Mechanistically, U3 snoRNA regulates DDX21 PR localization through maintaining its mobility. Moreover, Cy5-U3 snoRNA downsizes the fibrous condensates of His-DDX21 at proper molecular ratios in vitro. This work highlights the importance of the equilibrium between U3 snoRNA and DDX21 in PR formation and reveals the potential relationship between the PR assembly and mitotic regulation.

Enhancer switching in cell lineage priming is linked to eRNA, Brg1's AT-hook, and SWI/SNF recruitment

RNA transcribed from enhancers, i.e., eRNA, has been suggested to directly activate transcription by recruiting transcription factors and co-activators. Although there have been specific examples of eRNA functioning in this way, it is not clear how general this may be. We find that the AT-hook of SWI/SNF preferentially binds RNA and, as part of the esBAF complex, associates with eRNA transcribed from intronic and intergenic regions. Our data suggest that SWI/SNF is globally recruited in cis by eRNA to cell-type-specific enhancers, representative of two distinct stages that mimic early mammalian development, and not at enhancers that are shared between the two stages. In this manner, SWI/SNF facilitates recruitment and/or activation of MLL3/4, p300/CBP, and Mediator to stage-specific enhancers and super-enhancers that regulate the transcription of metabolic and cell lineage priming-related genes. These findings highlight a connection between ATP-dependent chromatin remodeling and eRNA in cell identity and typical- and super-enhancer activation.

RNP granules in ALS and neurodegeneration: From multifunctional membraneless organelles to therapeutic opportunities

Amyotrophic lateral sclerosis (ALS) and related neurodegenerative diseases are characterised by dysfunction of a host of RNA-binding proteins (RBPs) and a severely disrupted RNA metabolism. Recently, RBP-harbouring phase-separated complexes, ribonucleoprotein (RNP) granules, have come into the limelight as "crucibles" of neuronal pathology in ALS. RNP granules are indispensable for the multitude of regulatory processes underlying cellular RNA metabolism and serve as critical organisers of cellular biochemistry. Neurons, highly specialised cells, heavily rely on RNP granules for efficient trafficking, signalling and stress responses. Multiple RNP granule components, primarily RBPs such as TDP-43 and FUS, are affected by ALS mutations. However, even in the absence of mutations, RBP proteinopathies represent pathophysiological hallmarks of ALS. Given the high local concentrations of RBPs and RNAs, their weakened or enhanced interactions within RNP granules disrupt their homeostasis. Thus, the physiological process of phase separation and RNP granule formation, vital for maintaining the high-functioning state of neuronal cells, becomes their Achilles heel. Here, we will review the recent literature on the causes and consequences of abnormal RNP granule functioning in ALS and related disorders. In particular, we will summarise the evidence for the network-level dysfunction of RNP granules in these conditions and discuss considerations for therapeutic interventions to target RBPs, RNP granules and their network as a whole.

An RNA-dependent and phase-separated active subnuclear compartment safeguards repressive chromatin domains

The nucleus is composed of functionally distinct membraneless compartments that undergo phase separation (PS). However, whether different subnuclear compartments are connected remains elusive. We identified a type of nuclear body with PS features composed of BAZ2A that associates with active chromatin. BAZ2A bodies depend on RNA transcription and BAZ2A non-disordered RNA-binding TAM domain. Although BAZ2A and H3K27me3 occupancies anticorrelate in the linear genome, in the nuclear space, BAZ2A bodies contact H3K27me3 bodies. BAZ2A-body disruption promotes BAZ2A invasion into H3K27me3 domains, causing H3K27me3-body loss and gene upregulation. Weak BAZ2A-RNA interactions, such as with nascent transcripts, promote BAZ2A bodies, whereas the strong binder long non-coding RNA (lncRNA) Malat1 impairs them while mediating BAZ2A association to chromatin at nuclear speckles. In addition to unraveling a direct connection between nuclear active and repressive compartments through PS mechanisms, the results also showed that the strength of RNA-protein interactions regulates this process, contributing to nuclear organization and the regulation of chromatin and gene expression.

Transcription regulation by long non-coding RNAs: mechanisms and disease relevance

Long non-coding RNAs (lncRNAs) outnumber protein-coding transcripts, but their functions remain largely unknown. In this Review, we discuss the emerging roles of lncRNAs in the control of gene transcription. Some of the best characterized lncRNAs have essential transcription cis-regulatory functions that cannot be easily accomplished by DNA-interacting transcription factors, such as XIST, which controls X-chromosome inactivation, or imprinted lncRNAs that direct allele-specific repression. A growing number of lncRNA transcription units, including CHASERR, PVT1 and HASTER (also known as HNF1A-AS1) act as transcription-stabilizing elements that fine-tune the activity of dosage-sensitive genes that encode transcription factors. Genetic experiments have shown that defects in such transcription stabilizers often cause severe phenotypes. Other lncRNAs, such as lincRNA-p21 (also known as Trp53cor1) and Maenli (Gm29348) contribute to local activation of gene transcription, whereas distinct lncRNAs influence gene transcription in trans. We discuss findings of lncRNAs that elicit a function through either activation of their transcription, transcript elongation and processing or the lncRNA molecule itself. We also discuss emerging evidence of lncRNA involvement in human diseases, and their potential as therapeutic targets.

Optogenetic control of mRNA condensation reveals an intimate link between condensate material properties and functions

Biomolecular condensates, often assembled through phase transition mechanisms, play key roles in organizing diverse cellular activities. The material properties of condensates, ranging from liquid droplets to solid-like glasses or gels, are key features impacting the way resident components associate with one another. However, it remains unclear whether and how different material properties would influence specific cellular functions of condensates. Here, we combine optogenetic control of phase separation with single-molecule mRNA imaging to study relations between phase behaviors and functional performance of condensates. Using light-activated condensation, we show that sequestering target mRNAs into condensates causes translation inhibition. Orthogonal mRNA imaging reveals highly transient nature of interactions between individual mRNAs and condensates. Tuning condensate composition and material property towards more solid-like states leads to stronger translational repression, concomitant with a decrease in molecular mobility. We further demonstrate that β-actin mRNA sequestration in neurons suppresses spine enlargement during chemically induced long-term potentiation. Our work highlights how the material properties of condensates can modulate functions, a mechanism that may play a role in fine-tuning the output of condensate-driven cellular activities.

Kinetic trapping organizes actin filaments within liquid-like protein droplets

Several actin-binding proteins (ABPs) phase separate to form condensates capable of curating the actin network shapes. Here, we use computational modeling to understand the principles of actin network organization within VASP condensate droplets. Our simulations reveal that the different actin shapes, namely shells, rings, and mixture states are highly dependent on the kinetics of VASP-actin interactions, suggesting that they arise from kinetic trapping. Specifically, we show that reducing the residence time of VASP on actin filaments reduces degree of bundling, thereby promoting assembly of shells rather than rings. We validate the model predictions experimentally using a VASP-mutant with decreased bundling capability. Finally, we investigate the ring opening within deformed droplets and found that the sphere-to-ellipsoid transition is favored under a wide range of filament lengths while the ellipsoid-to-rod transition is only permitted when filaments have a specific range of lengths. Our findings highlight key mechanisms of actin organization within phase-separated ABPs.

Predicting condensate formation of protein and RNA under various environmental conditions

BACKGROUND

Liquid-liquid phase separation (LLPS) by biomolecules plays a central role in various biological phenomena and has garnered significant attention. The behavior of LLPS is strongly influenced by the characteristics of RNAs and environmental factors such as pH and temperature, as well as the properties of proteins. Recently, several databases recording LLPS-related biomolecules have been established, and prediction models of LLPS-related phenomena have been explored using these databases. However, a prediction model that concurrently considers proteins, RNAs, and experimental conditions has not been developed due to the limited information available from individual experiments in public databases.

RESULTS

To address this challenge, we have constructed a new dataset, RNAPSEC, which serves each experiment as a data point. This dataset was accomplished by manually collecting data from public literature. Utilizing RNAPSEC, we developed two prediction models that consider a protein, RNA, and experimental conditions. The first model can predict the LLPS behavior of a protein and RNA under given experimental conditions. The second model can predict the required conditions for a given protein and RNA to undergo LLPS.

CONCLUSIONS

RNAPSEC and these prediction models are expected to accelerate our understanding of the roles of proteins, RNAs, and environmental factors in LLPS.

Emerging insights into transcriptional condensates

Eukaryotic transcription, a fundamental process that governs cell-specific gene expression, has long been the subject of extensive investigations in the fields of molecular biology, biochemistry, and structural biology. Recent advances in microscopy techniques have led to a fascinating concept known as "transcriptional condensates." These dynamic assemblies are the result of a phenomenon called liquid‒liquid phase separation, which is driven by multivalent interactions between the constituent proteins in cells. The essential proteins associated with transcription are concentrated in transcriptional condensates. Recent studies have shed light on the temporal dynamics of transcriptional condensates and their potential role in enhancing the efficiency of transcription. In this article, we explore the properties of transcriptional condensates, investigate how they evolve over time, and evaluate the significant impact they have on the process of transcription. Furthermore, we highlight innovative techniques that allow us to manipulate these condensates, thus demonstrating their responsiveness to cellular signals and their connection to transcriptional bursting. As our understanding of transcriptional condensates continues to grow, they are poised to revolutionize our understanding of eukaryotic gene regulation.

Enhancer-promoter specificity in gene transcription: molecular mechanisms and disease associations

Although often located at a distance from their target gene promoters, enhancers are the primary genomic determinants of temporal and spatial transcriptional specificity in metazoans. Since the discovery of the first enhancer element in simian virus 40, there has been substantial interest in unraveling the mechanism(s) by which enhancers communicate with their partner promoters to ensure proper gene expression. These research efforts have benefited considerably from the application of increasingly sophisticated sequencing- and imaging-based approaches in conjunction with innovative (epi)genome-editing technologies; however, despite various proposed models, the principles of enhancer-promoter interaction have still not been fully elucidated. In this review, we provide an overview of recent progress in the eukaryotic gene transcription field pertaining to enhancer-promoter specificity. A better understanding of the mechanistic basis of lineage- and context-dependent enhancer-promoter engagement, along with the continued identification of functional enhancers, will provide key insights into the spatiotemporal control of gene expression that can reveal therapeutic opportunities for a range of enhancer-related diseases.

Nonspecific Interactions in Transcription Regulation and Organization of Transcriptional Condensates

Eukaryotic cells are characterized by a high degree of compartmentalization of their internal contents, which ensures precise and controlled regulation of intracellular processes. During many processes, including different stages of transcription, dynamic membraneless compartments termed biomolecular condensates are formed. Transcription condensates contain various transcription factors and RNA polymerase and are formed by high- and low-specificity interactions between the proteins, DNA, and nearby RNA. This review discusses recent data demonstrating important role of nonspecific multivalent protein-protein and RNA-protein interactions in organization and regulation of transcription.

Probing the hierarchical dynamics of DNA-sperm nuclear transition protein complexes through fuzzy interaction and mesoscale condensation

Nuclear transition protein TNP1 is a crucial player mediating histone-protamine exchange in condensing spermatids. A unique combination of intrinsic disorder and multivalent properties turns TNP1 into an ideal agent for orchestrating the formation of versatile TNP-DNA assemblies. Despite its significance, the physicochemical property and the molecular mechanism followed by TNP1 for histone replacement and DNA condensation are still poorly understood. This study reports the first-time in vitro expression and purification of human TNP1 and investigates the hierarchical dynamics of TNP1-DNA interaction using a combination of computational simulations, biochemical assays, fluorescence imaging, and atomic force microscopy. We explored three crucial facets of TNP1-DNA interactions. Initially, we delve into the molecular binding process that entails fuzzy interactions between TNP1 and DNA at the atomistic scale. Subsequently, we analyze how TNP1 binding affects the electrostatic and mechanical characteristics of DNA and influences its morphology. Finally, we study the biomolecular condensation of TNP1-DNA when subjected to high concentrations. The findings of our study set the foundation for comprehending the potential involvement of TNP1 in histone replacement and DNA condensation in spermatogenesis.