A Liquid-to-Solid Phase Transition of the ALS Protein FUS Accelerated by Disease Mutation.

Many proteins contain disordered regions of low-sequence complexity, which cause aging-associated diseases because they are prone to aggregate. Here, we study FUS, a prion-like protein containing intrinsically disordered domains associated with the neurodegenerative disease ALS. We show that, in cells, FUS forms liquid compartments at sites of DNA damage and in the cytoplasm upon stress. We confirm this by reconstituting liquid FUS compartments in vitro. Using an in vitro "aging" experiment, we demonstrate that liquid droplets of FUS protein convert with time from a liquid to an aggregated state, and this conversion is accelerated by patient-derived mutations. We conclude that the physiological role of FUS requires forming dynamic liquid-like compartments. We propose that liquid-like compartments carry the trade-off between functionality and risk of aggregation and that aberrant phase transitions within liquid-like compartments lie at the heart of ALS and, presumably, other age-related diseases.

Feedback-based positioning and diffusion suppression of particles via optical control of thermoviscous flows.

The ability to control the position of micron-size particles with high precision using tools such as optical tweezers has led to major advances in fields such as biology, physics and material science. In this paper, we present a novel optical strategy to confine particles in solution with high spatial control using feedback-controlled thermoviscous flows. We show that this technique allows micron-size particles to be positioned and confined with subdiffraction precision (24 nm), effectively suppressing their diffusion. Due to its physical characteristics, our approach might be particular attractive where laser exposure is of concern or materials are inherently incompatible with optical tweezing since it does not rely on contrast in the refractive index.

Kinesin-8 motors improve nuclear centering by promoting microtubule catastrophe.

In fission yeast, microtubules push against the cell edge, thereby positioning the nucleus in the cell center. Kinesin-8 motors regulate microtubule catastrophe; however, their role in nuclear positioning is not known. Here we develop a physical model that describes how kinesin-8 motors affect nuclear centering by promoting a microtubule catastrophe. Our model predicts the improved centering of the nucleus in the presence of motors, which we confirmed experimentally in living cells. The model also predicts a characteristic time for the recentering of a displaced nucleus, which is supported by our experiments where we displaced the nucleus using optical tweezers.

Opto-fluidically multiplexed assembly and micro-robotics.

Techniques for high-definition micromanipulations, such as optical tweezers, hold substantial interest across a wide range of disciplines. However, their applicability remains constrained by material properties and laser exposure. And while microfluidic manipulations have been suggested as an alternative, their inherent capabilities are limited and further hindered by practical challenges of implementation and control. Here we show that the iterative application of laser-induced, localized flow fields can be used for the relative positioning of multiple micro-particles, irrespectively of their material properties. Compared to the standing theoretical proposal, our method keeps particles mobile, and we show that their precision manipulation is non-linearly accelerated via the multiplexing of temperature stimuli below the heat diffusion limit. The resulting flow fields are topologically rich and mathematically predictable. They represent unprecedented microfluidic control capabilities that are illustrated by the actuation of humanoid micro-robots with up to 30 degrees of freedom, whose motions are sufficiently well-defined to reliably communicate personal characteristics such as gender, happiness and nervousness. Our results constitute high-definition micro-fluidic manipulations with transformative potential for assembly, micro-manufacturing, the life sciences, robotics and opto-hydraulically actuated micro-factories.

Centrosomal microtubule nucleation regulates radial migration of projection neurons independently of polarization in the developing brain.

Cortical projection neurons polarize and form an axon while migrating radially. Even though these dynamic processes are closely interwoven, they are regulated separately-the neurons terminate their migration when reaching their destination, the cortical plate, but continue to grow their axons. Here, we show that in rodents, the centrosome distinguishes these processes. Newly developed molecular tools modulating centrosomal microtubule nucleation combined with in vivo imaging uncovered that dysregulation of centrosomal microtubule nucleation abrogated radial migration without affecting axon formation. Tightly regulated centrosomal microtubule nucleation was required for periodic formation of the cytoplasmic dilation at the leading process, which is essential for radial migration. The microtubule nucleating factor γ-tubulin decreased at neuronal centrosomes during the migratory phase. As distinct microtubule networks drive neuronal polarization and radial migration, this provides insight into how neuronal migratory defects occur without largely affecting axonal tracts in human developmental cortical dysgeneses, caused by mutations in γ-tubulin.

Self-organization of dynein motors generates meiotic nuclear oscillations.

Meiotic nuclear oscillations in the fission yeast Schizosaccharomyces pombe are crucial for proper chromosome pairing and recombination. We report a mechanism of these oscillations on the basis of collective behavior of dynein motors linking the cell cortex and dynamic microtubules that extend from the spindle pole body in opposite directions. By combining quantitative live cell imaging and laser ablation with a theoretical description, we show that dynein dynamically redistributes in the cell in response to load forces, resulting in more dynein attached to the leading than to the trailing microtubules. The redistribution of motors introduces an asymmetry of motor forces pulling in opposite directions, leading to the generation of oscillations. Our work provides the first direct in vivo observation of self-organized dynamic dynein distributions, which, owing to the intrinsic motor properties, generate regular large-scale movements in the cell.

Interphase microtubules determine the initial alignment of the mitotic spindle.

In the fission yeast Schizosaccharomyces pombe, interphase microtubules (MTs) position the nucleus [1, 2], which in turn positions the cell-division plane [1, 3]. It is unclear how the spindle orients, with respect to the predetermined division plane, to ensure that the chromosomes are segregated across this plane. It has been proposed that, during prometaphase, the astral MT interaction with the cell cortex aligns the spindle with the cell axis [4] and also participates in a spindle orientation checkpoint (SOC), which delays entry into anaphase as long as the spindle is misaligned [5-7]. Here, we trace the position of the spindle throughout mitosis in a single-cell assay. We find no evidence for the SOC. We show that the spindle is remarkably well aligned with the cell longitudinal axis at the onset of mitosis, by growing along the axis of the adjacent interphase MT. Misalignment of nascent spindles can give rise to anucleate cells when spindle elongation is impaired. We propose a new role for interphase microtubules: through interaction with the spindle pole body, interphase microtubules determine the initial alignment of the spindle in the subsequent cell division.

Advanced Microscopy Reveals Complex Developmental and Subcellular Localization Patterns of ANNEXIN 1 in Arabidopsis.

Annexin 1 (ANN1) is the most abundant member of the evolutionary conserved multigene protein superfamily of annexins in plants. Generally, annexins participate in diverse cellular processes, such as cell growth, differentiation, vesicle trafficking, and stress responses. The expression of annexins is developmentally regulated, and it is sensitive to the external environment. ANN1 is expressed in almost all Arabidopsis tissues, while the most abundant is in the root, root hairs, and in the hypocotyl epidermal cells. Annexins were also occasionally proposed to associate with cytoskeleton and vesicles, but they were never developmentally localized at the subcellular level in diverse plant tissues and organs. Using advanced light-sheet fluorescence microscopy (LSFM), we followed the developmental and subcellular localization of GFP-tagged ANN1 in post-embryonic Arabidopsis organs. By contrast to conventional microscopy, LSFM allowed long-term imaging of ANN1-GFP in Arabidopsis plants at near-environmental conditions without affecting plant viability. We studied developmental regulation of ANN1-GFP expression and localization in growing Arabidopsis roots: strong accumulation was found in the root cap and epidermal cells (preferentially in elongating trichoblasts), but it was depleted in dividing cells localized in deeper layers of the root meristem. During root hair development, ANN1-GFP accumulated at the tips of emerging and growing root hairs, which was accompanied by decreased abundance in the trichoblasts. In aerial plant parts, ANN1-GFP was localized mainly in the cortical cytoplasm of trichomes and epidermal cells of hypocotyls, cotyledons, true leaves, and their petioles. At the subcellular level, ANN1-GFP was enriched at the plasma membrane (PM) and vesicles of non-dividing cells and in mitotic and cytokinetic microtubular arrays of dividing cells. Additionally, an independent immunolocalization method confirmed ANN1-GFP association with mitotic and cytokinetic microtubules (PPBs and phragmoplasts) in dividing cells of the lateral root cap. Lattice LSFM revealed subcellular accumulation of ANN1-GFP around the nuclear envelope of elongating trichoblasts. Massive relocation and accumulation of ANN1-GFP at the PM and in Hechtian strands and reticulum in plasmolyzed cells suggest a possible osmoprotective role of ANN1-GFP during plasmolysis/deplasmolysis cycle. This study shows complex developmental and subcellular localization patterns of ANN1 in living Arabidopsis plants.

Isotropic actomyosin dynamics promote organization of the apical cell cortex in epithelial cells.

Although cortical actin plays an important role in cellular mechanics and morphogenesis, there is surprisingly little information on cortex organization at the apical surface of cells. In this paper, we characterize organization and dynamics of microvilli (MV) and a previously unappreciated actomyosin network at the apical surface of Madin-Darby canine kidney cells. In contrast to short and static MV in confluent cells, the apical surfaces of nonconfluent epithelial cells (ECs) form highly dynamic protrusions, which are often oriented along the plane of the membrane. These dynamic MV exhibit complex and spatially correlated reorganization, which is dependent on myosin II activity. Surprisingly, myosin II is organized into an extensive network of filaments spanning the entire apical membrane in nonconfluent ECs. Dynamic MV, myosin filaments, and their associated actin filaments form an interconnected, prestressed network. Interestingly, this network regulates lateral mobility of apical membrane probes such as integrins or epidermal growth factor receptors, suggesting that coordinated actomyosin dynamics contributes to apical cell membrane organization.

Single-molecule imaging in vivo: the dancing building blocks of the cell.

A cell can be viewed as a dynamic puzzle, where single pieces shuffle in space, change their conformation to fit different partners, and new pieces are generated while old ones are destroyed. Microscopy has become capable of directly observing the pieces of the puzzle, which are single molecules. Single-molecule microscopy in vivo provides new insights into the molecular processes underlying the physiology of a cell, allowing not only for visualizing how molecules distribute with nanometer resolution in the cellular environment, but also for characterizing their movement with high temporal precision. This approach reveals molecular behaviors normally invisible in ensemble measurements. Depending on the molecule, the process, and the cellular region studied, single molecules can be followed by conventional epifluorescence microscopy, or by illuminating only a thin region of the cell, as in Total Internal Reflection Fluorescence (TIRF) and Selective Plane Illumination Microscopy (SPIM), and by limiting the amount of detectable molecules, as in Fluorescence Speckle Microscopy (FSM) and Photo-Activation (PA). High spatial resolution can be obtained by imaging only a fraction of the molecules at a time, as in Photo-Activated Localization Microscopy (PALM) and Stochastic Optical Reconstruction Microscopy (STORM), or by de-exciting molecules in the periphery of the detection region as in Stimulated Emission-Depletion (STED) microscopy. Single-molecule techniques in vivo are becoming widespread; however, it is important to choose the most suited technique for each biological question or sample. Here we review single-molecule microscopy techniques, describe their basic principles, advantages for in vivo application, and discuss the lessons that can be learned from live single-molecule imaging.

Identification and regulation of a molecular module for bleb-based cell motility.

Single-cell migration is a key process in development, homeostasis, and disease. Nevertheless, the control over basic cellular mechanisms directing cells into motile behavior in vivo is largely unknown. Here, we report on the identification of a minimal set of parameters the regulation of which confers proper morphology and cell motility. Zebrafish primordial germ cells rendered immotile by knockdown of Dead end, a negative regulator of miRNA function, were used as a platform for identifying processes restoring motility. We have defined myosin contractility, cell adhesion, and cortex properties as factors whose proper regulation is sufficient for restoring cell migration of this cell type. Tight control over the level of these cellular features, achieved through a balance between miRNA-430 function and the action of the RNA-binding protein Dead end, effectively transforms immotile primordial germ cells into polarized cells that actively migrate relative to cells in their environment.

Laser ablation of the microtubule cytoskeleton: setting up and working with an ablation system.

Laser ablation is a powerful tool that can be used to study a variety of biological mechanisms. Microscopes with high optical performances are nowadays available, and lasers that could be used to perform ablations have become accessible to every laboratory. Setting up a laser ablation system is a relatively straightforward task; however, it requires some basic knowledge of optics. We illustrate the fundamental components of the experimental setup and describe the most common pitfalls and difficulties encountered when designing, setting up, and working with a laser ablation system.

Optical trapping and laser ablation of microtubules in fission yeast.

Manipulation has been used as a powerful investigation technique since the early history of biology. Every technical advance resulted in more refined instruments that led to the discovery of new phenomena and to the solution of old problems. The invention of laser in 1960 gave birth to what is now called optical manipulation: the use of light to interact with matter. Since then, the tremendous progress of laser technology made optical manipulation not only an affordable, reliable alternative to traditional manipulation techniques but disclosed also new, intriguing applications that were previously impossible, such as contact-free manipulation. Currently, optical manipulation is used in many fields, yet has the potential of becoming an everyday technique in a broader variety of contexts. Here, we focus on two main optical manipulation techniques: optical trapping and laser ablation. We illustrate with selected applications in fission yeast how in vivo optical manipulation can be used to study organelle positioning and the force balance in the microtubule cytoskeleton.

Axon extension occurs independently of centrosomal microtubule nucleation.

Microtubules are polymeric protein structures and components of the cytoskeleton. Their dynamic polymerization is important for diverse cellular functions. The centrosome is the classical site of microtubule nucleation and is thought to be essential for axon growth and neuronal differentiation--processes that require microtubule assembly. We found that the centrosome loses its function as a microtubule organizing center during development of rodent hippocampal neurons. Axons still extended and regenerated through acentrosomal microtubule nucleation, and axons continued to grow after laser ablation of the centrosome in early neuronal development. Thus, decentralized microtubule assembly enables axon extension and regeneration, and, after axon initiation, acentrosomal microtubule nucleation arranges the cytoskeleton, which is the source of the sophisticated morphology of neurons.

Versatile laser-based cell manipulator.

Here we describe a two-photon microscope and laser ablation setup combined with optical tweezers. We tested the setup on the fission yeast Schizosaccharomyces pombe, a commonly used model organism. We show that long-term imaging can be achieved without significant photo-bleaching or damage of the sample. The setup can precisely ablate sub-micrometer structures, such as microtubules and mitotic spindles, inside living cells, which remain viable after the manipulation. Longer exposure times lead to ablation, while shorter exposures lead to photo-bleaching of the target structure. We used optical tweezers to trap intracellular particles and to displace the cell nucleus. Two-photon fluorescence imaging of the manipulated cell can be performed simultaneously with trapping. The combination of techniques described here may help to solve a variety of problems in cell biology, such as positioning of organelles and the forces exerted by the cytoskeleton.

Micromechanical properties of tobacco mosaic viruses.

A tobacco mosaic virus (TMV) subject to local forces can be viewed as an uniform beam with local loads. We used a custom built Atomic Force Microscope (AFM) to determine the curvature induced in the TMV by concentrated load or by distributed forces. Local forces were created by the AFM tip. Distributed forces were applied to the virus via the surface tension of receding droplets. The experimental results of both methods can be described when we attribute a Young modulus of 6 +/- 3 GPa to the virus. Our value is about five times larger than published data. We compare our results to the literature and work out possible error sources in our experiment and in published one.