Cortical contractility triggers a stochastic switch to fast amoeboid cell motility.

3D amoeboid cell migration is central to many developmental and disease-related processes such as cancer metastasis. Here, we identify a unique prototypic amoeboid cell migration mode in early zebrafish embryos, termed stable-bleb migration. Stable-bleb cells display an invariant polarized balloon-like shape with exceptional migration speed and persistence. Progenitor cells can be reversibly transformed into stable-bleb cells irrespective of their primary fate and motile characteristics by increasing myosin II activity through biochemical or mechanical stimuli. Using a combination of theory and experiments, we show that, in stable-bleb cells, cortical contractility fluctuations trigger a stochastic switch into amoeboid motility, and a positive feedback between cortical flows and gradients in contractility maintains stable-bleb cell polarization. We further show that rearward cortical flows drive stable-bleb cell migration in various adhesive and non-adhesive environments, unraveling a highly versatile amoeboid migration phenotype.

Actin flows mediate a universal coupling between cell speed and cell persistence.

Cell movement has essential functions in development, immunity, and cancer. Various cell migration patterns have been reported, but no general rule has emerged so far. Here, we show on the basis of experimental data in vitro and in vivo that cell persistence, which quantifies the straightness of trajectories, is robustly coupled to cell migration speed. We suggest that this universal coupling constitutes a generic law of cell migration, which originates in the advection of polarity cues by an actin cytoskeleton undergoing flows at the cellular scale. Our analysis relies on a theoretical model that we validate by measuring the persistence of cells upon modulation of actin flow speeds and upon optogenetic manipulation of the binding of an actin regulator to actin filaments. Beyond the quantitative prediction of the coupling, the model yields a generic phase diagram of cellular trajectories, which recapitulates the full range of observed migration patterns.

Cooperative epithelial phagocytosis enables error correction in the early embryo.

Errors in early embryogenesis are a cause of sporadic cell death and developmental failure1,2. Phagocytic activity has a central role in scavenging apoptotic cells in differentiated tissues3-6. However, how apoptotic cells are cleared in the blastula embryo in the absence of specialized immune cells remains unknown. Here we show that the surface epithelium of zebrafish and mouse embryos, which is the first tissue formed during vertebrate development, performs efficient phagocytic clearance of apoptotic cells through phosphatidylserine-mediated target recognition. Quantitative four-dimensional in vivo imaging analyses reveal a collective epithelial clearance mechanism that is based on mechanical cooperation by two types of Rac1-dependent basal epithelial protrusions. The first type of protrusion, phagocytic cups, mediates apoptotic target uptake. The second, a previously undescribed type of fast and extended actin-based protrusion that we call 'epithelial arms', promotes the rapid dispersal of apoptotic targets through Arp2/3-dependent mechanical pushing. On the basis of experimental data and modelling, we show that mechanical load-sharing enables the long-range cooperative uptake of apoptotic cells by multiple epithelial cells. This optimizes the efficiency of tissue clearance by extending the limited spatial exploration range and local uptake capacity of non-motile epithelial cells. Our findings show that epithelial tissue clearance facilitates error correction that is relevant to the developmental robustness and survival of the embryo, revealing the presence of an innate immune function in the earliest stages of embryonic development.

Specialization of the photoreceptor transcriptome by Srrm3-dependent microexons is required for outer segment maintenance and vision.

Retinal photoreceptors have a distinct transcriptomic profile compared to other neuronal subtypes, likely reflecting their unique cellular morphology and function in the detection of light stimuli by way of the ciliary outer segment. We discovered a layer of this molecular specialization by revealing that the vertebrate retina expresses the largest number of tissue-enriched microexons of all tissue types. A subset of these microexons is included exclusively in photoreceptor transcripts, particularly in genes involved in cilia biogenesis and vesicle-mediated transport. This microexon program is regulated by Srrm3, a paralog of the neural microexon regulator Srrm4. Despite the fact that both proteins positively regulate retina microexons in vitro, only Srrm3 is highly expressed in mature photoreceptors. Its deletion in zebrafish results in widespread down-regulation of microexon inclusion from early developmental stages, followed by other transcriptomic alterations, severe photoreceptor defects, and blindness. These results shed light on the transcriptomic specialization and functionality of photoreceptors, uncovering unique cell type-specific roles for Srrm3 and microexons with implications for retinal diseases.

How cells respond to environmental cues - insights from bio-functionalized substrates.

Biomimetic materials have long been the (he)art of bioengineering. They usually aim at mimicking in vivo conditions to allow in vitro culture, differentiation and expansion of cells. The past decade has witnessed a considerable amount of progress in soft lithography, bio-inspired micro-fabrication and biochemistry, allowing the design of sophisticated and physiologically relevant micro- and nano-environments. These systems now provide an exquisite toolbox with which we can control a large set of physicochemical environmental parameters that determine cell behavior. Bio-functionalized surfaces have evolved from simple protein-coated solid surfaces or cellular extracts into nano-textured 3D surfaces with controlled rheological and topographical properties. The mechanobiological molecular processes by which cells interact and sense their environment can now be unambiguously understood down to the single-molecule level. This Commentary highlights recent successful examples where bio-functionalized substrates have contributed in raising and answering new questions in the area of extracellular matrix sensing by cells, cell-cell adhesion and cell migration. The use, the availability, the impact and the challenges of such approaches in the field of biology are discussed.

SIMPLE: Structured illumination based point localization estimator with enhanced precision.

We present a structured illumination microscopy based point localization estimator (SIMPLE) that achieves a 2-fold increase in single molecule localization precision compared to conventional centroid estimation methods. SIMPLE advances the recently introduced MINFLUX concept by using precisely phase-shifted sinusoidal wave patterns as nanometric rulers for simultaneous particle localization based on photon count variation over a 20 µm field of view. We validate SIMPLE in silico and experimentally on a TIRF-SIM setup using a digital micro-mirror device (DMD) as a spatial light modulator.

Rigidity percolation and active advection synergize in the actomyosin cortex to drive amoeboid cell motility.

Spontaneous locomotion is a common feature of most metazoan cells, generally attributed to the properties of actomyosin networks. This force-producing machinery has been studied down to the most minute molecular details, especially in lamellipodium-driven migration. Nevertheless, how actomyosin networks work inside contraction-driven amoeboid cells still lacks unifying principles. Here, using stable motile blebs from HeLa cells as a model amoeboid motile system, we imaged the dynamics of the actin cortex at the single filament level and revealed the co-existence of three distinct rheological phases. We introduce "advected percolation," a process where rigidity percolation and active advection synergize, spatially organizing the actin network's mechanical properties into a minimal and generic locomotion mechanism. Expanding from our observations on simplified systems, we speculate that this model could explain, down to the single actin filament level, how amoeboid cells, such as cancer or immune cells, can propel efficiently through complex 3D environments.

Membrane to cortex attachment determines different mechanical phenotypes in LGR5+ and LGR5- colorectal cancer cells.

Colorectal cancer (CRC) tumors are composed of heterogeneous and plastic cell populations, including a pool of cancer stem cells that express LGR5. Whether these distinct cell populations display different mechanical properties, and how these properties might contribute to metastasis is poorly understood. Using CRC patient derived organoids (PDOs), we find that compared to LGR5- cells, LGR5+ cancer stem cells are stiffer, adhere better to the extracellular matrix (ECM), move slower both as single cells and clusters, display higher nuclear YAP, show a higher survival rate in response to mechanical confinement, and form larger transendothelial gaps. These differences are largely explained by the downregulation of the membrane to cortex attachment proteins Ezrin/Radixin/Moesin (ERMs) in the LGR5+ cells. By analyzing single cell RNA-sequencing (scRNA-seq) expression patterns from a patient cohort, we show that this downregulation is a robust signature of colorectal tumors. Our results show that LGR5- cells display a mechanically dynamic phenotype suitable for dissemination from the primary tumor whereas LGR5+ cells display a mechanically stable and resilient phenotype suitable for extravasation and metastatic growth.

TANGO1 inhibitors reduce collagen secretion and limit tissue scarring.

Uncontrolled secretion of ECM proteins, such as collagen, can lead to excessive scarring and fibrosis and compromise tissue function. Despite the widespread occurrence of fibrotic diseases and scarring, effective therapies are lacking. A promising approach would be to limit the amount of collagen released from hyperactive fibroblasts. We have designed membrane permeant peptide inhibitors that specifically target the primary interface between TANGO1 and cTAGE5, an interaction that is required for collagen export from endoplasmic reticulum exit sites (ERES). Application of the peptide inhibitors leads to reduced TANGO1 and cTAGE5 protein levels and a corresponding inhibition in the secretion of several ECM components, including collagens. Peptide inhibitor treatment in zebrafish results in altered tissue architecture and reduced granulation tissue formation during cutaneous wound healing. The inhibitors reduce secretion of several ECM proteins, including collagens, fibrillin and fibronectin in human dermal fibroblasts and in cells obtained from patients with a generalized fibrotic disease (scleroderma). Taken together, targeted interference of the TANGO1-cTAGE5 binding interface could enable therapeutic modulation of ERES function in ECM hypersecretion, during wound healing and fibrotic processes.

Cationic amphipathic peptides accumulate sialylated proteins and lipids in the plasma membrane of eukaryotic host cells.

Cationic antimicrobial peptides (CAMPs) selectively target bacterial membranes by electrostatic interactions with negatively charged lipids. It turned out that for inhibition of microbial growth a high CAMP membrane concentration is required, which can be realized by the incorporation of hydrophobic groups within the peptide. Increasing hydrophobicity, however, reduces the CAMP selectivity for bacterial over eukaryotic host membranes, thereby causing the risk of detrimental side-effects. In this study we addressed how cationic amphipathic peptides-in particular a CAMP with Lysine-Leucine-Lysine repeats (termed KLK)-affect the localization and dynamics of molecules in eukaryotic membranes. We found KLK to selectively inhibit the endocytosis of a subgroup of membrane proteins and lipids by electrostatically interacting with negatively charged sialic acid moieties. Ultrastructural characterization revealed the formation of membrane invaginations representing fission or fusion intermediates, in which the sialylated proteins and lipids were immobilized. Experiments on structurally different cationic amphipathic peptides (KLK, 6-MO-LF11-322 and NK14-2) indicated a cooperation of electrostatic and hydrophobic forces that selectively arrest sialylated membrane constituents.

Chromosome segregation fidelity requires microtubule polyglutamylation by the cancer downregulated enzyme TTLL11.

Regulation of microtubule (MT) dynamics is key for mitotic spindle assembly and faithful chromosome segregation. Here we show that polyglutamylation, a still understudied post-translational modification of spindle MTs, is essential to define their dynamics within the range required for error-free chromosome segregation. We identify TTLL11 as an enzyme driving MT polyglutamylation in mitosis and show that reducing TTLL11 levels in human cells or zebrafish embryos compromises chromosome segregation fidelity and impairs early embryonic development. Our data reveal a mechanism to ensure genome stability in normal cells that is compromised in cancer cells that systematically downregulate TTLL11. Our data suggest a direct link between MT dynamics regulation, MT polyglutamylation and two salient features of tumour cells, aneuploidy and chromosome instability (CIN).

Volumetric imaging of fast cellular dynamics with deep learning enhanced bioluminescence microscopy.

Bioluminescence microscopy is an appealing alternative to fluorescence microscopy, because it does not depend on external illumination, and consequently does neither produce spurious background autofluorescence, nor perturb intrinsically photosensitive processes in living cells and animals. The low photon emission of known luciferases, however, demands long exposure times that are prohibitive for imaging fast biological dynamics. To increase the versatility of bioluminescence microscopy, we present an improved low-light microscope in combination with deep learning methods to image extremely photon-starved samples enabling subsecond exposures for timelapse and volumetric imaging. We apply our method to image subcellular dynamics in mouse embryonic stem cells, epithelial morphology during zebrafish development, and DAF-16 FoxO transcription factor shuttling from the cytoplasm to the nucleus under external stress. Finally, we concatenate neural networks for denoising and light-field deconvolution to resolve intracellular calcium dynamics in three dimensions of freely moving Caenorhabditis elegans.

Spot variation fluorescence correlation spectroscopy allows for superresolution chronoscopy of confinement times in membranes.

Resolving the dynamical interplay of proteins and lipids in the live-cell plasma membrane represents a central goal in current cell biology. Superresolution concepts have introduced a means of capturing spatial heterogeneity at a nanoscopic length scale. Similar concepts for detecting dynamical transitions (superresolution chronoscopy) are still lacking. Here, we show that recently introduced spot-variation fluorescence correlation spectroscopy allows for sensing transient confinement times of membrane constituents at dramatically improved resolution. Using standard diffraction-limited optics, spot-variation fluorescence correlation spectroscopy captures signatures of single retardation events far below the transit time of the tracer through the focal spot. We provide an analytical description of special cases of transient binding of a tracer to pointlike traps, or association of a tracer with nanodomains. The influence of trap mobility and the underlying binding kinetics are quantified. Experimental approaches are suggested that allow for gaining quantitative mechanistic insights into the interaction processes of membrane constituents.

Imaging of mobile long-lived nanoplatforms in the live cell plasma membrane.

The plasma membrane has been hypothesized to contain nanoscopic lipid platforms, which are discussed in the context of "lipid rafts" or "membrane rafts." Based on biochemical and cell biological studies, rafts are believed to play a crucial role in many signaling processes. However, there is currently not much information on their size, shape, stability, surface density, composition, and heterogeneity. We present here a method that allows for the first time the direct imaging of nanoscopic long-lived platforms with raft-like properties diffusing in the live cell plasma membrane. Our method senses these platforms by their property to assemble a characteristic set of fluorescent marker proteins or lipids on a time scale of seconds. A special photobleaching protocol was used to reduce the surface density of labeled mobile platforms down to the level of well isolated diffraction-limited spots without altering the single spot brightness. The statistical distribution of probe molecules per platform was determined by single molecule brightness analysis. For demonstration, we used the consensus raft marker glycosylphosphatidylinositol-anchored monomeric GFP and the fluorescent lipid analog BODIPY-G(M1), which preferentially partitions into liquid-ordered phases. For both markers, we found cholesterol-dependent homo-association in the plasma membrane of living CHO and Jurkat T cells in the resting state, thereby demonstrating the existence of small, mobile, long-lived platforms containing these probes. We further applied the technology to address structural changes in the plasma membrane during fever-type heat shock: at elevated temperatures, the glycosylphosphatidylinositol-anchored monomeric GFP homo-association disappeared, accompanied by an increase in the expression of the small heat shock protein Hsp27.

Direct Force Measurements of Subcellular Mechanics in Confinement using Optical Tweezers.

During the development of a multicellular organism, a single fertilized cell divides and gives rise to multiple tissues with diverse functions. Tissue morphogenesis goes in hand with molecular and structural changes at the single cell level that result in variations of subcellular mechanical properties. As a consequence, even within the same cell, different organelles and compartments resist differently to mechanical stresses; and mechanotransduction pathways can actively regulate their mechanical properties. The ability of a cell to adapt to the microenvironment of the tissue niche thus is in part due to the ability to sense and respond to mechanical stresses. We recently proposed a new mechanosensation paradigm in which nuclear deformation and positioning enables a cell to gauge the physical 3D environment and endows the cell with a sense of proprioception to decode changes in cell shape. In this article, we describe a new method to measure the forces and material properties that shape the cell nucleus inside living cells, exemplified on adherent cells and mechanically confined cells. The measurements can be performed non-invasively with optical traps inside cells, and the forces are directly accessible through calibration-free detection of light momentum. This allows measuring the mechanics of the nucleus independently from cell surface deformations and allowing dissection of exteroceptive and interoceptive mechanotransduction pathways. Importantly, the trapping experiment can be combined with optical microscopy to investigate the cellular response and subcellular dynamics using fluorescence imaging of the cytoskeleton, calcium ions, or nuclear morphology. The presented method is straightforward to apply, compatible with commercial solutions for force measurements, and can easily be extended to investigate the mechanics of other subcellular compartments, e.g., mitochondria, stress-fibers, and endosomes.

Antimicrobial and immunostimulatory peptide, KLK, induces an increase in cytosolic Ca²(+) concentration by mobilizing Ca²(+) from intracellular stores.

The cationic antimicrobial immunomodulatory peptide, KLK (KLKL5KLK), exerts profound membrane interacting properties, impacting on ultrastructure and fluidity. KLK-membrane interactions that lead to these alterations require the ability of the peptide to move into an α-helical conformation. We show that KLK induces an increase of the intracellular Ca²(+) concentration in human T24 cells. The effect of KLK is buffer-sensitive, as it is detected when HBSS buffer is used, but not with PBS. This, together with the lack of effect of the middle leucine-to-proline-substituted peptide derivative [KPK (KLKLLPLLKLK)], indicates that it is the conformational propensity rather than the net positive charge that contributes to the effect of KLK on intracellular Ca²(+) level of T24 cells. We show that, although KLK slightly stimulates Ca²(+) influx into the cell, the bulk increase of Ca²(+) levels is due to KLK-induced depletion of intracellular Ca²(+) stores. Finally, we demonstrate a KLK-induced switch of PS (phosphatidylserine) from the inner to the outer plasma membrane leaflet that contributes to the onset of early apoptotic changes in these cells.

A reconstituted mammalian APC-kinesin complex selectively transports defined packages of axonal mRNAs.

Through the asymmetric distribution of messenger RNAs (mRNAs), cells spatially regulate gene expression to create cytoplasmic domains with specialized functions. In neurons, mRNA localization is required for essential processes such as cell polarization, migration, and synaptic plasticity underlying long-term memory formation. The essential components driving cytoplasmic mRNA transport in neurons and mammalian cells are not known. We report the first reconstitution of a mammalian mRNA transport system revealing that the tumor suppressor adenomatous polyposis coli (APC) forms stable complexes with the axonally localized ß-actin and ß2B-tubulin mRNAs, which are linked to a kinesin-2 via the cargo adaptor KAP3. APC activates kinesin-2, and both proteins are sufficient to drive specific transport of defined mRNA packages. Guanine-rich sequences located in 3'UTRs of axonal mRNAs increase transport efficiency and balance the access of different mRNAs to the transport system. Our findings reveal a minimal set of proteins sufficient to transport mammalian mRNAs.

The nucleus measures shape changes for cellular proprioception to control dynamic cell behavior.

The physical microenvironment regulates cell behavior during tissue development and homeostasis. How single cells decode information about their geometrical shape under mechanical stress and physical space constraints within tissues remains largely unknown. Here, using a zebrafish model, we show that the nucleus, the biggest cellular organelle, functions as an elastic deformation gauge that enables cells to measure cell shape deformations. Inner nuclear membrane unfolding upon nucleus stretching provides physical information on cellular shape changes and adaptively activates a calcium-dependent mechanotransduction pathway, controlling actomyosin contractility and migration plasticity. Our data support that the nucleus establishes a functional module for cellular proprioception that enables cells to sense shape variations for adapting cellular behavior to their microenvironment.

Friction forces position the neural anlage.

During embryonic development, mechanical forces are essential for cellular rearrangements driving tissue morphogenesis. Here, we show that in the early zebrafish embryo, friction forces are generated at the interface between anterior axial mesoderm (prechordal plate, ppl) progenitors migrating towards the animal pole and neurectoderm progenitors moving in the opposite direction towards the vegetal pole of the embryo. These friction forces lead to global rearrangement of cells within the neurectoderm and determine the position of the neural anlage. Using a combination of experiments and simulations, we show that this process depends on hydrodynamic coupling between neurectoderm and ppl as a result of E-cadherin-mediated adhesion between those tissues. Our data thus establish the emergence of friction forces at the interface between moving tissues as a critical force-generating process shaping the embryo.

Optogenetic Control of Nodal Signaling Reveals a Temporal Pattern of Nodal Signaling Regulating Cell Fate Specification during Gastrulation.

During metazoan development, the temporal pattern of morphogen signaling is critical for organizing cell fates in space and time. Yet, tools for temporally controlling morphogen signaling within the embryo are still scarce. Here, we developed a photoactivatable Nodal receptor to determine how the temporal pattern of Nodal signaling affects cell fate specification during zebrafish gastrulation. By using this receptor to manipulate the duration of Nodal signaling in vivo by light, we show that extended Nodal signaling within the organizer promotes prechordal plate specification and suppresses endoderm differentiation. Endoderm differentiation is suppressed by extended Nodal signaling inducing expression of the transcriptional repressor goosecoid (gsc) in prechordal plate progenitors, which in turn restrains Nodal signaling from upregulating the endoderm differentiation gene sox17 within these cells. Thus, optogenetic manipulation of Nodal signaling identifies a critical role of Nodal signaling duration for organizer cell fate specification during gastrulation.