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Journal ArticleDOI

A nuclear F-actin scaffold stabilizes ribonucleoprotein droplets against gravity in large cells

01 Oct 2013-Nature Cell Biology (Nature Research)-Vol. 15, Iss: 10, pp 1253-1259
TL;DR: A model is developed that reveals how gravity becomes an increasingly potent force as cells and their nuclei grow larger than ∼10 μm, explaining the requirement for a stabilizing nuclear F-actin scaffold in large Xenopus oocytes.
Abstract: The size of a typical eukaryotic cell is of the order of ∼10 μm. However, some cell types grow to very large sizes, including oocytes (immature eggs) of organisms from humans to starfish. For example, oocytes of the frog Xenopus laevis grow to a diameter ≥1 mm. They have a correspondingly large nucleus (germinal vesicle) of ∼450 μm in diameter, which is similar to smaller somatic nuclei, but contains a significantly higher concentration of actin. The form and structure of this nuclear actin remain controversial, and its potential mechanical role within these large nuclei is unknown. Here, we use a microrheology and quantitative imaging approach to show that germinal vesicles contain an elastic F-actin scaffold that mechanically stabilizes these large nuclei against gravitational forces, which are usually considered negligible within cells. We find that on actin disruption, ribonucleoprotein droplets, including nucleoli and histone locus bodies, undergo gravitational sedimentation and fusion. We develop a model that reveals how gravity becomes an increasingly potent force as cells and their nuclei grow larger than ∼10 μm, explaining the requirement for a stabilizing nuclear F-actin scaffold in large Xenopus oocytes. All life forms are subject to gravity, and our results may have broad implications for cell growth and size control.

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Citations
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Journal ArticleDOI
TL;DR: This work has shown that liquid–liquid phase separation driven by multivalent macromolecular interactions is an important organizing principle for biomolecular condensates and has proposed a physical framework for this organizing principle.
Abstract: In addition to membrane-bound organelles, eukaryotic cells feature various membraneless compartments, including the centrosome, the nucleolus and various granules. Many of these compartments form through liquid–liquid phase separation, and the principles, mechanisms and regulation of their assembly as well as their cellular functions are now beginning to emerge. Biomolecular condensates are micron-scale compartments in eukaryotic cells that lack surrounding membranes but function to concentrate proteins and nucleic acids. These condensates are involved in diverse processes, including RNA metabolism, ribosome biogenesis, the DNA damage response and signal transduction. Recent studies have shown that liquid–liquid phase separation driven by multivalent macromolecular interactions is an important organizing principle for biomolecular condensates. With this physical framework, it is now possible to explain how the assembly, composition, physical properties and biochemical and cellular functions of these important structures are regulated.

3,294 citations

Journal ArticleDOI
22 Sep 2017-Science
TL;DR: The findings together suggest that several membrane-less organelles have been shown to exhibit a concentration threshold for assembly, a hallmark of phase separation, and represent liquid-phase condensates, which form via a biologically regulated (liquid-liquid) phase separation process.
Abstract: BACKGROUND Living cells contain distinct subcompartments to facilitate spatiotemporal regulation of biological reactions. In addition to canonical membrane-bound organelles such as secretory vesicles and endoplasmic reticulum, there are many organelles that do not have an enclosing membrane yet remain coherent structures that can compartmentalize and concentrate specific sets of molecules. Examples include assemblies in the nucleus such as the nucleolus, Cajal bodies, and nuclear speckles and also cytoplasmic structures such as stress granules, P-bodies, and germ granules. These structures play diverse roles in various biological processes and are also increasingly implicated in protein aggregation diseases. ADVANCES A number of studies have shown that membrane-less assemblies exhibit remarkable liquid-like features. As with conventional liquids, they typically adopt round morphologies and coalesce into a single droplet upon contact with one another and also wet intracellular surfaces such as the nuclear envelope. Moreover, component molecules exhibit dynamic exchange with the surrounding nucleoplasm and cytoplasm. These findings together suggest that these structures represent liquid-phase condensates, which form via a biologically regulated (liquid-liquid) phase separation process. Liquid phase condensation increasingly appears to be a fundamental mechanism for organizing intracellular space. Consistent with this concept, several membrane-less organelles have been shown to exhibit a concentration threshold for assembly, a hallmark of phase separation. At the molecular level, weak, transient interactions between molecules with multivalent domains or intrinsically disordered regions (IDRs) are a driving force for phase separation. In cells, condensation of liquid-phase assemblies can be regulated by active processes, including transcription and various posttranslational modifications. The simplest physical picture of a homogeneous liquid phase is often not enough to capture the full complexity of intracellular condensates, which frequently exhibit heterogeneous multilayered structures with partially solid-like characters. However, recent studies have shown that multiple distinct liquid phases can coexist and give rise to richly structured droplet architectures determined by the relative liquid surface tensions. Moreover, solid-like phases can emerge from metastable liquid condensates via multiple routes of potentially both kinetic and thermodynamic origins, which has important implications for the role of intracellular liquids in protein aggregation pathologies. OUTLOOK The list of intracellular assemblies driven by liquid phase condensation is growing rapidly, but our understanding of their sequence-encoded biological function and dysfunction lags behind. Moreover, unlike equilibrium phases of nonliving matter, living cells are far from equilibrium, with intracellular condensates subject to various posttranslational regulation and other adenosine triphosphate–dependent biological activity. Efforts using in vitro reconstitution, combined with traditional cell biology approaches and quantitative biophysical tools, are required to elucidate how such nonequilibrium features of living cells control intracellular phase behavior. The functional consequences of forming liquid condensates are likely multifaceted and may include facilitated reaction, sequestration of specific factors, and organization of associated intracellular structures. Liquid phase condensation is particularly interesting in the nucleus, given the growing interest in the impact of nuclear phase behavior on the flow of genetic information; nuclear condensates range from micrometer-sized bodies such as the nucleolus to submicrometer structures such as transcriptional assemblies, all of which directly interact with and regulate the genome. Deepening our understanding of these intracellular states of matter not only will shed light on the basic biology of cellular organization but also may enable therapeutic intervention in protein aggregation disease by targeting intracellular phase behavior.

2,432 citations

Journal ArticleDOI
TL;DR: The basic physical concepts necessary to understand the consequences of liquid-like states for biological functions are discussed.
Abstract: Cells organize many of their biochemical reactions in non-membrane compartments. Recent evidence has shown that many of these compartments are liquids that form by phase separation from the cytoplasm. Here we discuss the basic physical concepts necessary to understand the consequences of liquid-like states for biological functions.

2,088 citations

Journal ArticleDOI
16 Jun 2016-Cell
TL;DR: It is shown that subcompartments within the nucleolus represent distinct, coexisting liquid phases that may facilitate sequential RNA processing reactions in a variety of RNP bodies, and suggested that phase separation can give rise to multilayered liquids.

1,330 citations


Cites background from "A nuclear F-actin scaffold stabiliz..."

  • ..., 2011), but the frequency of such coalescence events is slowed significantly by the presence of a nuclear actin network (Figures S1A–S1D) (Feric and Brangwynne, 2013; Feric et al., 2015)....

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  • ...Nucleoli in X. laevis oocytes will fuse with one another when brought into contact (Brangwynne et al., 2011), but the frequency of such coalescence events is slowed significantly by the presence of a nuclear actin network (Figures S1A–S1D) (Feric and Brangwynne, 2013; Feric et al., 2015)....

    [...]

Journal ArticleDOI
TL;DR: These findings highlight the relevance of classical concepts from the physics of polymeric phase transitions for understanding the assembly of intracellular membrane-less compartments, and challenge the challenge of applying these concepts given the heteropolymeric nature of protein sequences, the complex intrACEllular environment, and non-equilibrium features intrinsic to cells.
Abstract: Intracellular organelles are either membrane-bound vesicles or membrane-less compartments that are made up of proteins and RNA. These organelles play key biological roles, by compartmentalizing the cell to enable spatiotemporal control of biological reactions. Recent studies suggest that membrane-less intracellular compartments are multicomponent viscous liquid droplets that form via phase separation. Proteins that have an intrinsic tendency for being conformationally heterogeneous seem to be the main drivers of liquid–liquid phase separation in the cell. These findings highlight the relevance of classical concepts from the physics of polymeric phase transitions for understanding the assembly of intracellular membrane-less compartments. However, applying these concepts is challenging, given the heteropolymeric nature of protein sequences, the complex intracellular environment, and non-equilibrium features intrinsic to cells. This provides new opportunities for adapting established theories and for the emergence of new physics. The internal structure of cells is organized into compartments, many of which lack a confining membrane and instead resemble viscous liquid droplets. Evidence is mounting that these compartments form via spontaneous phase transitions. The internal structure of cells is organized into compartments, many of which lack a confining membrane and instead resemble viscous liquid droplets. Evidence is mounting that these compartments form via spontaneous phase transitions.

1,089 citations

References
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Journal ArticleDOI
TL;DR: In el marco del Proyecto subvencionado by the Fundación Antorchas (FAN) as discussed by the authors, el material was digitalizado, e.g., en la Biblioteca del Departamento de Fisica de la Facultad de Ciencias Exactas de la Universidad Nacional de La Plata.
Abstract: Este material fue digitalizado en el marco del Proyecto subvencionado por la Fundacion Antorchas y se encuentra en la Biblioteca del Departamento de Fisica de la Facultad de Ciencias Exactas de la Universidad Nacional de La Plata.

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TL;DR: In this article, a set of image processing algorithms for extracting quantitative data from digitized video microscope images of colloidal suspensions is described, which can locate submicrometer spheres to within 10 nm in the focal plane and 150 nm in depth.

3,423 citations

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TL;DR: This book is a lucid, straightforward introduction to the concepts and techniques of statistical physics that students of biology, biochemistry, and biophysics must know.
Abstract: This book is a lucid, straightforward introduction to the concepts and techniques of statistical physics that students of biology, biochemistry, and biophysics must know. It provides a sound basis for understanding random motions of molecules, subcellular particles, or cells, or of processes that depend on such motion or are markedly affected by it. Readers do not need to understand thermodynamics in order to acquire a knowledge of the physics involved in diffusion, sedimentation, electrophoresis, chromatography, and cell motility--subjects that become lively and immediate when the author discusses them in terms of random walks of individual particles.

3,041 citations

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2,980 citations

Journal ArticleDOI
26 Jun 2009-Science
TL;DR: It is shown that P granules exhibit liquid-like behaviors, including fusion, dripping, and wetting, which is used to estimate their viscosity and surface tension, and reflects a classic phase transition, in which polarity proteins vary the condensation point across the cell.
Abstract: In sexually reproducing organisms, embryos specify germ cells, which ultimately generate sperm and eggs In Caenorhabditis elegans, the first germ cell is established when RNA and protein-rich P granules localize to the posterior of the one-cell embryo Localization of P granules and their physical nature remain poorly understood Here we show that P granules exhibit liquid-like behaviors, including fusion, dripping, and wetting, which we used to estimate their viscosity and surface tension As with other liquids, P granules rapidly dissolved and condensed Localization occurred by a biased increase in P granule condensation at the posterior This process reflects a classic phase transition, in which polarity proteins vary the condensation point across the cell Such phase transitions may represent a fundamental physicochemical mechanism for structuring the cytoplasm

2,134 citations

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