rRNA transcription is integral to phase separation and maintenance of nucleolar structure

The RNA-binding protein Modulo promotes neural stem cell maintenance in Drosophila

A small population of stem cells in the developing Drosophila central nervous system generates the large number of different cell types that make up the adult brain. To achieve this, these neural stem cells (neuroblasts, NBs) divide asymmetrically to produce non-identical daughter cells. The balance between stem cell self-renewal and neural differentiation is regulated by various cellular machinery, including transcription factors, chromatin remodelers, and RNA-binding proteins. The list of these components remains incomplete, and the mechanisms regulating their function are not fully understood, however. Here, we identify a role for the RNA-binding protein Modulo (Mod; nucleolin in humans) in NB maintenance. We employ transcriptomic analyses to identify RNA targets of Mod and assess changes in global gene expression following its knockdown, results of which suggest a link with notable proneural genes and those essential for neurogenesis. Mod is expressed in larval brains and its loss leads to a significant decrease in the number of central brain NBs. Stem cells that remain lack expression of key NB identity factors and exhibit cell proliferation defects. Mechanistically, our analysis suggests these deficiencies arise at least in part from altered cell cycle progression, with a proportion of NBs arresting prior to mitosis. Overall, our data show that Mod function is essential for neural stem cell maintenance during neurogenesis.

A nuclear protein quality control system for elimination of nucleolus-related inclusions

The identification of pathways that control elimination of protein inclusions is essential to understand the cellular response to proteotoxicity, particularly in the nuclear compartment, for which our knowledge is limited. We report that stress-induced nuclear inclusions related to the nucleolus are eliminated upon stress alleviation during the recovery period. This process is independent of autophagy/lysosome and CRM1-mediated nuclear export pathways, but strictly depends on the ubiquitin-activating E1 enzyme, UBA1, and on nuclear proteasomes that are recruited into the formed inclusions. UBA1 activity is essential only for the recovery process but dispensable for nuclear inclusion formation. Furthermore, the E3 ligase HUWE1 and HSP70 are components of the ubiquitin/chaperone systems that promote inclusion elimination. The recovery process also requires RNA Pol I-dependent production of the lncRNA IGS42 during stress. IGS42 localises within the formed inclusions and promotes their elimination by preserving the mobility of resident proteins. These findings reveal a protein quality control system that operates within the nucleus for the elimination of stress-induced nucleolus-related inclusions.

Spatial Control over Reactions via Localized Transcription within Membraneless DNA Nanostar Droplets

Biomolecular condensates control where and how fast many chemical reactions occur in cells by partitioning reactants and catalysts, enabling simultaneous reactions in different spatial locations of a cell. Even without a membrane or physical barrier, the partitioning of the reactants can affect the rates of downstream reaction cascades in ways that depend on reaction location. Such effects can enable systems of biomolecular condensates to spatiotemporally orchestrate chemical reaction networks in cells to facilitate complex behaviors such as ribosome assembly. Here, we develop a system for developing such control in synthetic systems. We localize different transcription templates within different phase-separated, membraneless DNA nanostar (NS) droplets─programmable, in vitro liquid-liquid phase separation systems for partitioning of substrates and localization of reactions to membraneless droplets. When RNA produced within such droplets is also degraded in the bulk, droplet-localized transcription creates RNA concentration gradients. Consistent with the formation of these gradients, toehold-mediated strand displacement reactions involving transcripts are 2-fold slower far from the site of transcription than when nearby. We then demonstrate how multiple such gradients can form and be maintained independently by simultaneous transcription reactions occurring in tandem, each localized to different NS droplet types. Our results provide a means for constructing reaction systems in which different reactions are spatially localized and controlled without the need for physical membranes. This system also provides a means for generally studying how localized reactions and the exchange of reaction products might occur between protocells.

Nucleolar dynamics are determined by the ordered assembly of the ribosome

Ribosome biogenesis occurs in the nucleolus, a nuclear biomolecular condensate that exhibits dynamic biophysical properties thought to be important for function. However, the relationship between ribosome assembly and nucleolar dynamics is incompletely understood. Here, we present a platform for high-throughput fluorescence recovery after photobleaching (HiT-FRAP), which we use to screen hundreds of genes for their impact on dynamics of the nucleolar scaffold nucleophosmin (NPM1). We find that scaffold dynamics and nucleolar morphology respond to disruptions in key stages of ribosome biogenesis. Accumulation of early ribosomal intermediates leads to nucleolar rigidification while late intermediates lead to increased fluidity. We map these biophysical changes to specific ribosomal intermediates and their affinity for NPM1. We also discover that disrupting mRNA processing impacts nucleolar dynamics and ribosome biogenesis. This work mechanistically ties ribosome assembly to the biophysical features of the nucleolus and enables study of how dynamics relate to function across other biomolecular condensates.

Active RNA synthesis patterns nuclear condensates

Biomolecular condensates are membraneless compartments that organize biochemical processes in cells. In contrast to well-understood mechanisms describing how condensates form and dissolve, the principles underlying condensate patterning - including their size, number and spacing in the cell - remain largely unknown. We hypothesized that RNA, a key regulator of condensate formation and dissolution, influences condensate patterning. Using nucleolar fibrillar centers (FCs) as a model condensate, we found that inhibiting ribosomal RNA synthesis significantly alters the patterning of FCs. Physical theory and experimental observations support a model whereby active RNA synthesis generates a non-equilibrium state that arrests condensate coarsening and thus contributes to condensate patterning. Altering FC condensate patterning by expression of the FC component TCOF1 impairs ribosomal RNA processing, linking condensate patterning to biological function. These results reveal how non-equilibrium states driven by active chemical processes regulate condensate patterning, which is important for cellular biochemistry and function.

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.

Clustering of rRNA operons in E. coli is disrupted by σH

Chromosomal organization in E. coli as examined by Hi-C methodology indicates that long-range interactions are sparse. Yet, spatial co-localization or 'clustering' of 6/7 ribosomal RNA (rrn) operons distributed over half the 4.6 Mbp genome has been captured by two other methodologies - fluorescence microscopy and Mu transposition. Our current understanding of the mechanism of clustering is limited to mapping essential cis elements. To identify trans elements, we resorted to perturbing the system by chemical and physical means and observed that heat shock disrupts clustering. Levels of σH are known to rise as a cellular response to the shock. We show that elevated expression of σH alone is sufficient to disrupt clustering, independent of heat stress. The anti-clustering activity of σH does not depend on its transcriptional activity but requires core-RNAP interaction and DNA-binding activities. This activity of σH is suppressed by ectopic expression of σD suggesting a competition for core-RNAP. A query of the other five known σ factors of E. coli found that elevated expression of FecI, the ECF σ factor that controls iron citrate transport, also perturbs clustering and is also suppressed by σD. We discuss a possible scenario for how these membrane-associated σ factors participate in clustering of distant rrn loci.

The impact of ribosome biogenesis in cancer: from proliferation to metastasis

The dysregulation of ribosome biogenesis is a hallmark of cancer, facilitating the adaptation to altered translational demands essential for various aspects of tumor progression. This review explores the intricate interplay between ribosome biogenesis and cancer development, highlighting dynamic regulation orchestrated by key oncogenic signaling pathways. Recent studies reveal the multifaceted roles of ribosomes, extending beyond protein factories to include regulatory functions in mRNA translation. Dysregulated ribosome biogenesis not only hampers precise control of global protein production and proliferation but also influences processes such as the maintenance of stem cell-like properties and epithelial-mesenchymal transition, contributing to cancer progression. Interference with ribosome biogenesis, notably through RNA Pol I inhibition, elicits a stress response marked by nucleolar integrity loss, and subsequent G1-cell cycle arrest or cell death. These findings suggest that cancer cells may rely on heightened RNA Pol I transcription, rendering ribosomal RNA synthesis a potential therapeutic vulnerability. The review further explores targeting ribosome biogenesis vulnerabilities as a promising strategy to disrupt global ribosome production, presenting therapeutic opportunities for cancer treatment.

Phase Separation as a Driver of Stem Cell Organization and Function during Development

A properly organized subcellular composition is essential to cell function. The canonical organizing principle within eukaryotic cells involves membrane-bound organelles; yet, such structures do not fully explain cellular complexity. Furthermore, discrete non-membrane-bound structures have been known for over a century. Liquid-liquid phase separation (LLPS) has emerged as a ubiquitous mode of cellular organization without the need for formal lipid membranes, with an ever-expanding and diverse list of cellular functions that appear to be regulated by this process. In comparison to traditional organelles, LLPS can occur across wider spatial and temporal scales and involves more distinct protein and RNA complexes. In this review, we discuss the impacts of LLPS on the organization of stem cells and their function during development. Specifically, the roles of LLPS in developmental signaling pathways, chromatin organization, and gene expression will be detailed, as well as its impacts on essential processes of asymmetric cell division. We will also discuss how the dynamic and regulated nature of LLPS may afford stem cells an adaptable mode of organization throughout the developmental time to control cell fate. Finally, we will discuss how aberrant LLPS in these processes may contribute to developmental defects and disease.