Hierarchical Size Scaling during Multicellular Growth and Development

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.

Somatic polyploidy supports biosynthesis and tissue function by increasing transcriptional output

Cell size and biosynthetic capacity generally increase with increased DNA content. Polyploidy has therefore been proposed to be an adaptive strategy to increase cell size in specialized tissues with high biosynthetic demands. However, if and how DNA concentration limits cellular biosynthesis in vivo is not well understood, and the impacts of polyploidy in non-disease states is not well studied. Here, we show that polyploidy in the C. elegans intestine is critical for cell growth and yolk biosynthesis, a central role of this organ. Artificially lowering the DNA/cytoplasm ratio by reducing polyploidization in the intestine gave rise to smaller cells with more dilute mRNA. Highly-expressed transcripts were more sensitive to this mRNA dilution, whereas lowly-expressed genes were partially compensated - in part by loading more RNA Polymerase II on the remaining genomes. DNA-dilute cells had normal total protein concentration, which we propose is achieved by increasing production of translational machinery at the expense of specialized, cell-type specific proteins.

Functional consequences of somatic polyploidy in development

Polyploid cells contain multiple genome copies and arise in many animal tissues as a regulated part of development. However, polyploid cells can also arise due to cell division failure, DNA damage or tissue damage. Although polyploidization is crucial for the integrity and function of many tissues, the cellular and tissue-wide consequences of polyploidy can be very diverse. Nonetheless, many polyploid cell types and tissues share a remarkable similarity in function, providing important information about the possible contribution of polyploidy to cell and tissue function. Here, we review studies on polyploid cells in development, underlining parallel functions between different polyploid cell types, as well as differences between developmentally-programmed and stress-induced polyploidy.

N2FXm, a method for joint nuclear and cytoplasmic volume measurements, unravels the osmo-mechanical regulation of nuclear volume in mammalian cells

In eukaryotes, cytoplasmic and nuclear volumes are tightly regulated to ensure proper cell homeostasis. However, current methods to measure cytoplasmic and nuclear volumes, including confocal 3D reconstruction, have limitations, such as relying on two-dimensional projections or poor vertical resolution. Here, to overcome these limitations, we describe a method, N2FXm, to jointly measure cytoplasmic and nuclear volumes in single cultured adhering human cells, in real time, and across cell cycles. We find that this method accurately provides joint size over dynamic measurements and at different time resolutions. Moreover, by combining several experimental perturbations and analyzing a mathematical model including osmotic effects and tension, we show that N2FXm can give relevant insights on how mechanical forces exerted by the cytoskeleton on the nuclear envelope can affect the growth of nucleus volume by biasing nuclear import. Our method, by allowing for accurate joint nuclear and cytoplasmic volume dynamic measurements at different time resolutions, highlights the non-constancy of the nucleus/cytoplasm ratio along the cell cycle.

Physical forces modulate interphase nuclear size

The eukaryotic nucleus exhibits remarkable plasticity in size, adjusting dynamically to changes in cellular conditions such as during development and differentiation, and across species. Traditionally, the supply of structural constituents to the nuclear envelope has been proposed as the principal determinant of nuclear size. However, recent experimental and theoretical analyses have provided an alternative perspective, which emphasizes the crucial role of physical forces such as osmotic pressure and chromatin repulsion forces in regulating nuclear size. These forces can be modulated by the molecular profiles that traverse the nuclear envelope and assemble in the macromolecular complex. This leads to a new paradigm wherein multiple nuclear macromolecules that are not limited to only the structural constituents of the nuclear envelope, are involved in the control of nuclear size and related functions.

Maintenance of appropriate size scaling of the C. elegans pharynx by YAP-1

Even slight imbalance between the growth rate of different organs can accumulate to a large deviation from their appropriate size during development. Here, we use live imaging of the pharynx of C. elegans to ask if and how organ size scaling nevertheless remains uniform among individuals. Growth trajectories of hundreds of individuals reveal that pharynxes grow by a near constant volume per larval stage that is independent of their initial size, such that undersized pharynxes catch-up in size during development. Tissue-specific depletion of RAGA-1, an activator of mTOR and growth, shows that maintaining correct pharynx-to-body size proportions involves a bi-directional coupling between pharynx size and body growth. In simulations, this coupling cannot be explained by limitation of food uptake alone, and genetic experiments reveal an involvement of the mechanotransducing transcriptional co-regulator yap-1. Our data suggests that mechanotransduction coordinates pharynx growth with other tissues, ensuring body plan uniformity among individuals.

The Caenorhabditis elegans cullin-RING ubiquitin ligase CRL4DCAF-1 is required for proper germline nucleolus morphology and male development

Cullin-RING ubiquitin ligases (CRLs) are the largest class of ubiquitin ligases with diverse functions encompassing hundreds of cellular processes. Inactivation of core components of the CRL4 ubiquitin ligase produces a germ cell defect in Caenorhabditis elegans that is marked by abnormal globular morphology of the nucleolus and fewer germ cells. We identified DDB1 Cullin4 associated factor (DCAF)-1 as the CRL4 substrate receptor that ensures proper germ cell nucleolus morphology. We demonstrate that the dcaf-1 gene is the ncl-2 (abnormal nucleoli) gene, whose molecular identity was not previously known. We also observed that CRL4DCAF-1 is required for male tail development. Additionally, the inactivation of CRL4DCAF-1 results in a male-specific lethality in which a percentage of male progeny arrest as embryos or larvae. Analysis of the germ cell nucleolus defect using transmission electron microscopy revealed that dcaf-1 mutant germ cells possess significantly fewer ribosomes, suggesting a defect in ribosome biogenesis. We discovered that inactivation of the sperm-fate specification gene fog-1 (feminization of the germ line-1) or its protein-interacting partner, fog-3, rescues the dcaf-1 nucleolus morphology defect. Epitope-tagged versions of both FOG-1 and FOG-3 proteins are aberrantly present in adult dcaf-1(RNAi) animals, suggesting that DCAF-1 negatively regulates FOG-1 and FOG-3 expression. Murine CRL4DCAF-1 targets the degradation of the ribosome assembly factor periodic trptophan protein 1 (PWP1). We observed that the inactivation of Caenorhabditis elegansDCAF-1 increases the nucleolar levels of PWP1 in the germ line, intestine, and hypodermis. Reducing the level of PWP-1 rescues the dcaf-1 mutant defects of fewer germ cell numbers and abnormal nucleolus morphology, suggesting that the increase in PWP-1 levels contributes to the dcaf-1 germline defect. Our results suggest that CRL4DCAF-1 has an evolutionarily ancient role in regulating ribosome biogenesis including a conserved target in PWP1.

Lineage-based scaling of germline intercellular bridges during oogenesis

The size of subcellular structures must be tightly controlled to maintain normal cell function; this is especially important when cells are part of developing tissues or organs. Despite its importance, few studies have determined how the size of organelles or other structures is maintained during tissue growth, when cells are growing, dividing, and rearranging. The developing egg chamber is a powerful model in which to study the relative growth rates of subcellular structures. The egg chamber contains a cluster of sixteen germ cells, which are connected through intercellular bridges called ring canals. Ring canals are formed following incomplete cytokinesis after each of four germ cell divisions. As the egg chamber grows, the nurse cells and the ring canals that connect them increase in size. Here, we demonstrate that ring canal size scaling is related to their lineage; the largest, "first born" ring canals grow at a relatively slower rate than ring canals derived from subsequent mitotic divisions. This lineage-based scaling relationship is maintained even if directed transport is reduced, ring canal size is altered, or if the germ cells go through an additional mitotic division. Further, we propose that changes in ring canal scaling could provide a mechanism to alter egg size.

The large cytoplasmic volume of oocyte

The study of the size of cells and organelles has a long history, dating back to the 1600s when cells were defined. In particular, various methods have elucidated the size of the nucleus and the mitotic spindle in several species. However, little research has been conducted on oocyte size and organelles in mammals, and many questions remain to be answered. The appropriate size is essential to cell function properly. Oocytes have a very large cytoplasm, which is more than 100 times larger than that of general somatic cells in mammals. In this review, we discuss how oocytes acquire an enormous cytoplasmic size and the adverse effects of a large cytoplasmic size on cellular functions.

Regulation of organelle size and organization during development

During early embryogenesis, as cells divide in the developing embryo, the size of intracellular organelles generally decreases to scale with the decrease in overall cell size. Organelle size scaling is thought to be important to establish and maintain proper cellular function, and defective scaling may lead to impaired development and disease. However, how the cell regulates organelle size and organization are largely unanswered questions. In this review, we summarize the process of size scaling at both the cell and organelle levels and discuss recently discovered mechanisms that regulate this process during early embryogenesis. In addition, we describe how some recently developed techniques and Xenopus as an animal model can be used to investigate the underlying mechanisms of size regulation and to uncover the significance of proper organelle size scaling and organization.

Specificity of Nuclear Size Scaling in Frog Erythrocytes

In eukaryotes, the cell has the ability to modulate the size of the nucleus depending on the surrounding environment, to enable nuclear functions such as DNA replication and transcription. From previous analyses of nuclear size scaling in various cell types and species, it has been found that eukaryotic cells have a conserved scaling rule, in which the nuclear size correlates with both cell size and genomic content. However, there are few studies that have focused on a certain cell type and systematically analyzed the size scaling properties in individual species (intra-species) and among species (inter-species), and thus, the difference in the scaling rules among cell types and species is not well understood. In the present study, we analyzed the size scaling relationship among three parameters, nuclear size, cell size, and genomic content, in our measured datasets of terminally differentiated erythrocytes of five Anura frogs and collected datasets of different species classes from published papers. In the datasets of isolated erythrocytes from individual frogs, we found a very weak correlation between the measured nuclear and cell cross-sectional areas. Within the erythrocytes of individual species, the correlation of the nuclear area with the cell area showed a very low hypoallometric relationship, in which the relative nuclear size decreased when the cell size increased. These scaling trends in intra-species erythrocytes are not comparable to the known general correlation in other cell types. When comparing parameters across species, the nuclear areas correlated with both cell areas and genomic contents among the five frogs and the collected datasets in each species class. However, the contribution of genomic content to nuclear size determination was smaller than that of the cell area in all species classes. In particular, the estimated degree of the contribution of genomic content was greater in the amphibian class than in other classes. Together with our imaging analysis of structural components in nuclear membranes, we hypothesized that the observed specific features in nuclear size scaling are achieved by the weak interaction of the chromatin with the nuclear membrane seen in frog erythrocytes.

Surface Electrostatics Govern the Emulsion Stability of Biomolecular Condensates

Liquid-liquid phase separation underlies the formation of biological condensates. Physically, such systems are microemulsions that in general have a propensity to fuse and coalesce; however, many condensates persist as independent droplets in the test tube and inside cells. This stability is crucial for their function, but the physicochemical mechanisms that control the emulsion stability of condensates remain poorly understood. Here, by combining single-condensate zeta potential measurements, optical microscopy, tweezer experiments, and multiscale molecular modeling, we investigate how the nanoscale forces that sustain condensates impact their stability against fusion. By comparing peptide-RNA (PR₂₅:PolyU) and proteinaceous (FUS) condensates, we show that a higher condensate surface charge correlates with a lower fusion propensity. Moreover, measurements of single condensate zeta potentials reveal that such systems can constitute classically stable emulsions. Taken together, these results highlight the role of passive stabilization mechanisms in protecting biomolecular condensates against coalescence.

Size scaling in collective cell growth

Size is a fundamental feature of living entities and is intimately tied to their function. Scaling laws, which can be traced to D'Arcy Thompson and Julian Huxley, have emerged as a powerful tool for studying regulation of the growth dynamics of organisms and their constituent parts. Yet, throughout the 20th century, as scaling laws were established for single cells, quantitative studies of the coordinated growth of multicellular structures have lagged, largely owing to technical challenges associated with imaging and image processing. Here, we present a supervised learning approach for quantifying the growth dynamics of germline cysts during oogenesis. Our analysis uncovers growth patterns induced by the groupwise developmental dynamics among connected cells, and differential growth rates of their organelles. We also identify inter-organelle volumetric scaling laws, finding that nurse cell growth is linear over several orders of magnitude. Our approach leverages the ever-increasing quantity and quality of imaging data, and is readily amenable for studies of collective cell growth in other developmental contexts, including early mammalian embryogenesis and germline development.

Scaling Laws for Mitotic Chromosomes

During mitosis in higher eukaryotes, each chromosome condenses into a pair of rod-shaped chromatids. This process is co-regulated by the activity of several gene families, and the underlying biophysics remains poorly understood. To better understand the factors regulating chromosome condensation, we compiled a database of mitotic chromosome size and DNA content from the tables and figures of >200 published papers. A comparison across vertebrate species shows that chromosome width, length and volume scale with DNA content to the powers ∼1/4, ∼1/2, and ∼1, respectively. Angiosperms (flowering plants) show a similar length scaling, so this result is not specific to vertebrates. Chromosome shape and size thus satisfy two conditions: (1) DNA content per unit volume is approximately constant and (2) the cross-sectional area increases proportionately with chromosome length. Since viscous drag forces during chromosome movement are expected to scale with length, we hypothesize that the cross-section increase is necessary to limit the occurrence of large chromosome elongations that could slow or stall mitosis. Lastly, we note that individual vertebrate karyotypes typically exhibit a wider range of chromosome lengths as compared with angiosperms.

Morphological Characterization of small, dumpy, and long Phenotypes in Caenorhabditis elegans

The determinant factors of an organism's size during animal development have been explored from various angles but remain partially understood. In Caenorhabditis elegans, many genes affecting cuticle structure, cell growth, and proliferation have been identified to regulate the worm's overall morphology, including body size. While various mutations in those genes directly result in changes in the morphological phenotypes, there is still a need for established, clear, and distinct standards to determine the apparent abnormality in a worm's size and shape. In this study, we measured the body length, body width, terminal bulb length, and head size of mutant worms with reported Dumpy (Dpy), Small (Sma) or Long (Lon) phenotypes by plotting and comparing their respective ratios of various parameters. These results show that the Sma phenotypes are proportionally smaller overall with mild stoutness, and Dpy phenotypes are significantly stouter and have disproportionally small head size. This study provides a standard platform for determining morphological phenotypes designating and annotating mutants that exhibit body shape variations, defining the morphological phenotype of previously unexamined mutants.

Partial Inhibition of RNA Polymerase I Promotes Animal Health and Longevity

Health and survival in old age can be improved by changes in gene expression. RNA polymerase (Pol) I is the essential, conserved enzyme whose task is to generate the pre-ribosomal RNA (rRNA). We find that reducing the levels of Pol I activity is sufficient to extend lifespan in the fruit fly. This effect can be recapitulated by partial, adult-restricted inhibition, with both enterocytes and stem cells of the adult midgut emerging as important cell types. In stem cells, Pol I appears to act in the same longevity pathway as Pol III, implicating rRNA synthesis in these cells as the key lifespan determinant. Importantly, reduction in Pol I activity delays broad, age-related impairment and pathology, improving the function of diverse organ systems. Hence, our study shows that Pol I activity in the adult drives systemic, age-related decline in animal health and anticipates mortality.

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Partial inhibition of RNA polymerase I (Pol I) can extend lifespan in the fruit fly

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Reducing Pol I activity after development and only in the gut is sufficient

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Pol I activity affects aging from both post-mitotic and mitotically active cells

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Pol I activity affects the age-related decline in performance of multiple organs

The Relative Densities of Cytoplasm and Nuclear Compartments Are Robust against Strong Perturbation

The cell nucleus is a compartment in which essential processes such as gene transcription and DNA replication occur. Although the large amount of chromatin confined in the finite nuclear space could install the picture of a particularly dense organelle surrounded by less dense cytoplasm, recent studies have begun to report the opposite. However, the generality of this newly emerging, opposite picture has so far not been tested. Here, we used combined optical diffraction tomography and epi-fluorescence microscopy to systematically quantify the mass densities of cytoplasm, nucleoplasm, and nucleoli of human cell lines, challenged by various perturbations. We found that the nucleoplasm maintains a lower mass density than cytoplasm during cell cycle progression by scaling its volume to match the increase of dry mass during cell growth. At the same time, nucleoli exhibited a significantly higher mass density than the cytoplasm. Moreover, actin and microtubule depolymerization and changing chromatin condensation altered volume, shape, and dry mass of those compartments, whereas the relative distribution of mass densities was generally unchanged. Our findings suggest that the relative mass densities across membrane-bound and membraneless compartments are robustly conserved, likely by different as-of-yet unknown mechanisms, which hints at an underlying functional relevance. This surprising robustness of mass densities contributes to an increasing recognition of the importance of physico-chemical properties in determining cellular characteristics and compartments.

DNA content contributes to nuclear size control in Xenopus laevis

Cells adapt to drastic changes in genome quantity during evolution and cell division by adjusting the nuclear size to exert genomic functions. However, the mechanism by which DNA content within the nucleus contributes to controlling the nuclear size remains unclear. Here, we experimentally evaluated the effects of DNA content by utilizing cell-free Xenopus egg extracts and imaging of in vivo embryos. Upon manipulation of DNA content while maintaining cytoplasmic effects constant, both plateau size and expansion speed of the nucleus correlated highly with DNA content. We also found that nuclear expansion dynamics was altered when chromatin interaction with the nuclear envelope or chromatin condensation was manipulated while maintaining DNA content constant. Furthermore, excess membrane accumulated on the nuclear surface when the DNA content was low. These results clearly demonstrate that nuclear expansion is determined not only by cytoplasmic membrane supply but also by the physical properties of chromatin, including DNA quantity and chromatin structure within the nucleus, rather than the coding sequences themselves. In controlling the dynamics of nuclear expansion, we propose that chromatin interaction with the nuclear envelope plays a role in transmitting chromatin repulsion forces to the nuclear membrane.

Integrating cellular dimensions with cell differentiation during early development

Early embryo development is characterized by alteration of cellular dimensions and fating of blastomeres. An emerging concept is that cell size and shape drive cellular differentiation during early embryogenesis in a variety of model organisms. In this review, we summarize recent advances that elucidate the contribution of the physical dimensions of a cell to major embryonic transitions and cell fate specification in vivo. We also highlight techniques and newly evolving methods for manipulating the sizes and shapes of cells and whole embryos in situ and ex vivo. Finally, we provide an outlook for addressing fundamental questions in the field and more broadly uncovering how changes to cell size control decision making in a variety of biological contexts.

Specialization of nuclear membrane in eukaryotes

The size of the intracellular structure that encloses genomic DNA - known as the nucleus in eukaryotes and nucleoid in prokaryotes - is believed to scale according to cell size and genomic content inside them across the tree of life. However, an actual scaling relationship remains largely unexplored across eukaryotic species. Here, I collected a large dataset of nuclear and cell volumes in diverse species across different phyla, including some prokaryotes, from the published literature and assessed the scaling relationship. Although entire inter-species data showed that nuclear volume correlates with cell volume, the quantitative scaling property exhibited differences among prokaryotes, unicellular eukaryotes and multicellular eukaryotes. Additionally, the nuclear volume correlates with genomic content inside the nucleus of multicellular eukaryotes but not of prokaryotes and unicellular eukaryotes. In this Hypothesis, I, thus, propose that the basic concept of nuclear-size scaling is conserved across eukaryotes; however, structural and mechanical properties of nuclear membranes and chromatin can result in different scaling relationships of nuclear volume to cell volume and genomic content among species. In particular, eukaryote-specific properties of the nuclear membrane may contribute to the extreme flexibility of nuclear size with regard to DNA density inside the nucleus.

Nucleoid Size Scaling and Intracellular Organization of Translation across Bacteria

The scaling of organelles with cell size is thought to be exclusive to eukaryotes. Here, we demonstrate that similar scaling relationships hold for the bacterial nucleoid. Despite the absence of a nuclear membrane, nucleoid size strongly correlates with cell size, independent of changes in DNA amount and across various nutrient conditions. This correlation is observed in diverse bacteria, revealing a near-constant ratio between nucleoid and cell size for a given species. As in eukaryotes, the nucleocytoplasmic ratio in bacteria varies greatly among species. This spectrum of nucleocytoplasmic ratios is independent of genome size, and instead it appears linked to the average population cell size. Bacteria with different nucleocytoplasmic ratios have a cytoplasm with different biophysical properties, impacting ribosome mobility and localization. Together, our findings identify new organizational principles and biophysical features of bacterial cells, implicating the nucleocytoplasmic ratio and cell size as determinants of the intracellular organization of translation.

Organelle size scaling over embryonic development

Cell division without growth results in progressive cell size reductions during early embryonic development. How do the sizes of intracellular structures and organelles scale with cell size and what are the functional implications of such scaling relationships? Model organisms, in particular Caenorhabditis elegans worms, Drosophila melanogaster flies, Xenopus laevis frogs, and Mus musculus mice, have provided insights into developmental size scaling of the nucleus, mitotic spindle, and chromosomes. Nuclear size is regulated by nucleocytoplasmic transport, nuclear envelope proteins, and the cytoskeleton. Regulators of microtubule dynamics and chromatin compaction modulate spindle and mitotic chromosome size scaling, respectively. Developmental scaling relationships for membrane-bound organelles, like the endoplasmic reticulum, Golgi, mitochondria, and lysosomes, have been less studied, although new imaging approaches promise to rectify this deficiency. While models that invoke limiting components and dynamic regulation of assembly and disassembly can account for some size scaling relationships in early embryos, it will be exciting to investigate the contribution of newer concepts in cell biology such as phase separation and interorganellar contacts. With a growing understanding of the underlying mechanisms of organelle size scaling, future studies promise to uncover the significance of proper scaling for cell function and embryonic development, as well as how aberrant scaling contributes to disease. This article is categorized under: Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing Early Embryonic Development > Fertilization to Gastrulation Comparative Development and Evolution > Model Systems.

Nucleolar control by a non-apoptotic p53-caspases-deubiquitinylase axis promotes resistance to bacterial infection

The nucleolus is best known for its cellular role in regulating ribosome production and growth. More recently, an unanticipated role for the nucleolus in innate immunity has recently emerged whereby downregulation of fibrillarin and nucleolar contraction confers pathogen resistance across taxa. The mechanism of this downregulation, however, remains obscure. Here we report that rather than fibrillarin itself being the proximal factor in this pathway, the key player is a fibrillarin-stabilizing deubiquitinylase USP-33. This was discovered by a candidate-gene search of Caenorhabditis elegans in which CED-3 caspase was revealed to execute targeted cleavage of USP-33, thus destabilizing fibrillarin. We also showed that cep-1 and ced-3 mutant worms altered nucleolar size and decreased antimicrobial peptide gene, spp-1, expression rendering susceptibility to bacterial infection. These phenotypes were reversed by usp-33 knockdown, thus linking the CEP-1-CED-3-USP-33 pathway with nucleolar control and resistance to bacterial infection in worms. Parallel experiments with the human analogs of caspases and USP36 revealed similar roles in coordinating these two processes. In summary, our work outlined a conserved cascade that connects cell death signaling to nucleolar control and innate immune response.

Evidence for and against Liquid-Liquid Phase Separation in the Nucleus

Enclosed by two membranes, the nucleus itself is comprised of various membraneless compartments, including nuclear bodies and chromatin domains. These compartments play an important though still poorly understood role in gene regulation. Significant progress has been made in characterizing the dynamic behavior of nuclear compartments and liquid-liquid phase separation (LLPS) has emerged as a prominent mechanism governing their assembly. However, recent work reveals that certain nuclear structures violate key predictions of LLPS, suggesting that alternative mechanisms likely contribute to nuclear organization. Here, we review the evidence for and against LLPS for several nuclear compartments and discuss experimental strategies to identify the mechanism(s) underlying their assembly. We propose that LLPS, together with multiple modes of protein-nucleic acid binding, drive spatiotemporal organization of the nucleus and facilitate functional diversity among nuclear compartments.

The Nucleolus: A Multiphase Condensate Balancing Ribosome Synthesis and Translational Capacity in Health, Aging and Ribosomopathies

The nucleolus is the largest membrane-less structure in the eukaryotic nucleus. It is involved in the biogenesis of ribosomes, essential macromolecular machines responsible for synthesizing all proteins required by the cell. The assembly of ribosomes is evolutionarily conserved and is the most energy-consuming cellular process needed for cell growth, proliferation, and homeostasis. Despite the significance of this process, the intricate pathophysiological relationship between the nucleolus and protein synthesis has only recently begun to emerge. Here, we provide perspective on new principles governing nucleolar formation and the resulting multiphase organization driven by liquid-liquid phase separation. With recent advances in the structural analysis of ribosome formation, we highlight the current understanding of the step-wise assembly of pre-ribosomal subunits and the quality control required for proper function. Finally, we address how aging affects ribosome genesis and how genetic defects in ribosome formation cause ribosomopathies, complex diseases with a predisposition to cancer.

RNA is a critical element for the sizing and the composition of phase-separated RNA-protein condensates

Liquid-liquid phase separation is thought to be a key organizing principle in eukaryotic cells to generate highly concentrated dynamic assemblies, such as the RNP granules. Numerous in vitro approaches have validated this model, yet a missing aspect is to take into consideration the complex molecular mixture and promiscuous interactions found in vivo. Here we report the versatile scaffold ArtiG to generate concentration-dependent RNA-protein condensates within living cells, as a bottom-up approach to study the impact of co-segregated endogenous components on phase separation. We demonstrate that intracellular RNA seeds the nucleation of the condensates, as it provides molecular cues to locally coordinate the formation of endogenous high-order RNP assemblies. Interestingly, the co-segregation of intracellular components ultimately impacts the size of the phase-separated condensates. Thus, RNA arises as an architectural element that can influence the composition and the morphological outcome of the condensate phases in an intracellular context.

Cell size sensing-a one-dimensional solution for a three-dimensional problem?

Individual cell types have characteristic sizes, suggesting that size sensing mechanisms may coordinate transcription, translation, and metabolism with cell growth rates. Two types of size-sensing mechanisms have been proposed: spatial sensing of the location or dimensions of a signal, subcellular structure or organelle; or titration-based sensing of the intracellular concentrations of key regulators. Here we propose that size sensing in animal cells combines both titration and spatial sensing elements in a dynamic mechanism whereby microtubule motor-dependent localization of RNA encoding importin β1 and mTOR, coupled with regulated local protein synthesis, enable cytoskeleton length sensing for cell growth regulation.

Nuclear Scaling Is Coordinated among Individual Nuclei in Multinucleated Muscle Fibers

Optimal cell performance depends on cell size and the appropriate relative size, i.e., scaling, of the nucleus. How nuclear scaling is regulated and contributes to cell function is poorly understood, especially in skeletal muscle fibers, which are among the largest cells, containing hundreds of nuclei. Here, we present a Drosophila in vivo system to analyze nuclear scaling in whole multinucleated muscle fibers, genetically manipulate individual components, and assess muscle function. Despite precise global coordination, we find that individual nuclei within a myofiber establish different local scaling relationships by adjusting their size and synthetic activity in correlation with positional or spatial cues. While myonuclei exhibit compensatory potential, even minor changes in global nuclear size scaling correlate with reduced muscle function. Our study provides the first comprehensive approach to unraveling the intrinsic regulation of size in multinucleated muscle fibers. These insights to muscle cell biology will accelerate the development of interventions for muscle diseases.

Size scaling of nucleolus in Caenorhabditis elegans embryos

Nucleolus is viewed as a plurifunctional center in the cell, tightly linked to ribosome biosynthesis. As a non-membranous structure, how the size of nucleolus is determined is a long outstanding question, and the possibility of “direct size scaling to the nucleus” was raised by genetic studies in fission yeast. Here, we used the model organism Caenorhabditis elegans to test this hypothesis in multi-cellular organisms. We depleted ani-2, ima-3, or C27D9.1 by RNAi feeding, which altered embryo sizes to different extents in ncl-1 mutant worms. DIC imaging provided evidence that in size-altering embryo nucleolar size decreases in small cells and increases in large cells. Furthermore, analyses of nucleolar size in four blastomeres (ABa, ABp, EMS, and P2) within the same embryo of ncl-1 mutants consistently demonstrated the correspondence between cell and nucleolar sizes – the small cells (EMS and P2) have smaller nucleoli in comparison to the large cells (ABa).

Subcellular scaling: does size matter for cell division?

Among different species or cell types, or during early embryonic cell divisions that occur in the absence of cell growth, the size of subcellular structures, including the nucleus, chromosomes, and mitotic spindle, scale with cell size. Maintaining correct subcellular scales is thought to be important for many cellular processes and, in particular, for mitosis. In this review, we provide an update on nuclear and chromosome scaling mechanisms and their significance in metazoans, with a focus on Caenorhabditis elegans, Xenopus and mammalian systems, for which a common role for the Ran (Ras-related nuclear protein)-dependent nuclear transport system has emerged.

Dynamics and Function of Nuclear Bodies during Embryogenesis

Nuclear bodies are RNA-rich membraneless organelles in the cell nucleus that concentrate specific sets of nuclear proteins and RNA-protein complexes. Nuclear bodies such as the nucleolus, Cajal body (CB), and the histone locus body (HLB) concentrate factors required for nuclear steps of RNA processing. Formation of these nuclear bodies occurs on genomic loci and is frequently associated with active sites of transcription. Whether nuclear body formation is dependent on a particular gene element, an active process such as transcription, or the nascent RNA present at gene loci is a topic of debate. Recently, this question has been addressed through studies in model organisms and their embryos. The switch from maternally provided RNA and protein to zygotic gene products in early embryos has been well characterized in a variety of organisms. This process, termed maternal-to-zygotic transition, provides an excellent model for studying formation of nuclear bodies before, during, and after the transcriptional activation of the zygotic genome. Here, we review findings in embryos that reveal key principles in the study of the formation and function of nucleoli, CBs, and HLBs. We propose that while particular gene elements may contribute to formation of these nuclear bodies, active transcription promotes maturation of nuclear bodies and efficient RNA processing within them.

Cell size control - a mechanism for maintaining fitness and function

The maintenance of cell size homeostasis has been studied for years in different cellular systems. With the focus on 'what regulates cell size', the question 'why cell size needs to be maintained' has been largely overlooked. Recent evidence indicates that animal cells exhibit nonlinear cell size dependent growth rates and mitochondrial metabolism, which are maximal in intermediate sized cells within each cell population. Increases in intracellular distances and changes in the relative cell surface area impose biophysical limitations on cells, which can explain why growth and metabolic rates are maximal in a specific cell size range. Consistently, aberrant increases in cell size, for example through polyploidy, are typically disadvantageous to cellular metabolism, fitness and functionality. Accordingly, cellular hypertrophy can potentially predispose to or worsen metabolic diseases. We propose that cell size control may have emerged as a guardian of cellular fitness and metabolic activity.

Cell Biology of the Caenorhabditis elegans Nucleus

Studies on the Caenorhabditis elegans nucleus have provided fascinating insight to the organization and activities of eukaryotic cells. Being the organelle that holds the genetic blueprint of the cell, the nucleus is critical for basically every aspect of cell biology. The stereotypical development of C. elegans from a one cell-stage embryo to a fertile hermaphrodite with 959 somatic nuclei has allowed the identification of mutants with specific alterations in gene expression programs, nuclear morphology, or nuclear positioning. Moreover, the early C. elegans embryo is an excellent model to dissect the mitotic processes of nuclear disassembly and reformation with high spatiotemporal resolution. We review here several features of the C. elegans nucleus, including its composition, structure, and dynamics. We also discuss the spatial organization of chromatin and regulation of gene expression and how this depends on tight control of nucleocytoplasmic transport. Finally, the extensive connections of the nucleus with the cytoskeleton and their implications during development are described. Most processes of the C. elegans nucleus are evolutionarily conserved, highlighting the relevance of this powerful and versatile model organism to human biology.