Mechanical characterization of regenerating Hydra tissue spheres.

Hydra vulgaris, long known for its remarkable regenerative capabilities, is also a long-standing source of inspiration for models of spontaneous patterning. Recently it became clear that early patterning during Hydra regeneration is an integrated mechanochemical process whereby morphogen dynamics is influenced by tissue mechanics. One roadblock to understanding Hydra self-organization is our lack of knowledge about the mechanical properties of these organisms.

Acetylcholinesterase Activity Staining in Freshwater Planarians.

The serine hydrolase acetylcholinesterase (AChE) is an important neuronal enzyme which catalyzes the hydrolysis of the neurotransmitter acetylcholine and other choline esters. The breakdown of acetylcholine by AChE terminates synaptic transmission and regulates neuromuscular communication. AChE inhibition is a common mode of action of various insecticides, such as carbamates and organophosphorus pesticides.

Small hand-designed convolutional neural networks outperform transfer learning in automated cell shape detection in confluent tissues.

Mechanical cues such as stresses and strains are now recognized as essential regulators in many biological processes like cell division, gene expression or morphogenesis. Studying the interplay between these mechanical cues and biological responses requires experimental tools to measure these cues. In the context of large scale tissues, this can be achieved by segmenting individual cells to extract their shapes and deformations which in turn inform on their mechanical environment.

Microphase separation of living cells.

Self-organization of cells is central to a variety of biological systems and physical concepts of condensed matter have proven instrumental in deciphering some of their properties. Here we show that microphase separation, long studied in polymeric materials and other inert systems, has a natural counterpart in living cells. When placed below a millimetric film of liquid nutritive medium, a quasi two-dimensional, high-density population of Dictyostelium discoideum cells spontaneously assembles into compact domains.

Machine learning-based detection of label-free cancer stem-like cell fate.

The detection of cancer stem-like cells (CSCs) is mainly based on molecular markers or functional tests giving a posteriori results. Therefore label-free and real-time detection of single CSCs remains a difficult challenge. The recent development of microfluidics has made it possible to perform high-throughput single cell imaging under controlled conditions and geometries.

Aerotaxis and aerokinesis of Dictyostelium discoideum under hypoxic microenvironments.

Although spatiotemporal changes of oxygen in a microenvironment are known to affect the cellular dynamics of various eukaryotes, the details are not fully understood. Here, we describe the aerotaxis and aerokinesis of Dictyostelium discoideum (Dd), which has long been employed as a model organism for eukaryotic cells. We developed a microfluidic device capable of time-lapse observation of cultured cells while controlling oxygen concentrations in microchannels.

Hypoxia triggers collective aerotactic migration in .

Using a self-generated hypoxic assay, we show that the amoeba displays a remarkable collective aerotactic behavior. When a cell colony is covered, cells quickly consume the available oxygen (O) and form a dense ring moving outwards at constant speed and density. To decipher this collective process, we combined two technological developments: porphyrin-based O -sensing films and microfluidic O gradient generators.

Pharmacological or genetic targeting of Transient Receptor Potential (TRP) channels can disrupt the planarian escape response.

In response to noxious stimuli, planarians cease their typical ciliary gliding and exhibit an oscillatory type of locomotion called scrunching. We have previously characterized the biomechanics of scrunching and shown that it is induced by specific stimuli, such as amputation, noxious heat, and extreme pH. Because these specific inducers are known to activate Transient Receptor Potential (TRP) channels in other systems, we hypothesized that TRP channels control scrunching.

Controlling Confinement and Topology to Study Collective Cell Behaviors.

Confinement and substrate topology strongly affect the behavior of cell populations and, in particular, their collective migration. In vitro experiments dealing with these aspects require strategies of surface patterning that remain effective over long times (typically several days) and ways to control the surface topology in three dimensions. Here, we describe protocols addressing these two aspects.

Physical Mechanisms Driving Cell Sorting in Hydra.

Cell sorting, whereby a heterogeneous cell mixture organizes into distinct tissues, is a fundamental patterning process in development. Hydra is a powerful model system for carrying out studies of cell sorting in three dimensions, because of its unique ability to regenerate after complete dissociation into individual cells. The physicists Alfred Gierer and Hans Meinhardt recognized Hydra's self-organizing properties more than 40 years ago.

Mechanics dictate where and how freshwater planarians fission.

Asexual freshwater planarians reproduce by tearing themselves into two pieces by a process called binary fission. The resulting head and tail pieces regenerate within about a week, forming two new worms. Understanding this process of ripping oneself into two parts poses a challenging biomechanical problem.

Ezrin enhances line tension along transcellular tunnel edges via NMIIa driven actomyosin cable formation.

Transendothelial cell macroaperture (TEM) tunnels control endothelium barrier function and are triggered by several toxins from pathogenic bacteria that provoke vascular leakage. Cellular dewetting theory predicted that a line tension of uncharacterized origin works at TEM boundaries to limit their widening. Here, by conducting high-resolution microscopy approaches we unveil the presence of an actomyosin cable encircling TEMs.

Slo1 regulates ethanol-induced scrunching in freshwater planarians.

When freshwater planarians are exposed to a low-percentage (0.5%-1%) alcohol solution, they display a characteristic 'drunken' phenotype. Here we show that this drunken phenotype is a mixture of cilia-mediated gliding and scrunching, a muscular-based planarian gait which we recently demonstrated to be triggered by adverse environmental stimuli.

Planarian brain regeneration as a model system for developmental neurotoxicology.

Freshwater planarians, famous for their regenerative prowess, have long been recognized as a valuable in vivo animal model to study the effects of chemical exposure. In this review, we summarize the current techniques and tools used in the literature to assess toxicity in the planarian system. We focus on the planarian's particular amenability for neurotoxicology and neuroregeneration studies, owing to the planarian's unique ability to regenerate a centralized nervous system.

Scrunching: a novel escape gait in planarians.

The ability to escape a predator or other life-threatening situations is central to animal survival. Different species have evolved unique strategies under anatomical and environmental constraints. In this study, we describe a novel musculature-driven escape gait in planarians, 'scrunching', which is quantitatively different from other planarian gaits, such as gliding and peristalsis.

Tissue fusion over nonadhering surfaces.

Tissue fusion eliminates physical voids in a tissue to form a continuous structure and is central to many processes in development and repair. Fusion events in vivo, particularly in embryonic development, often involve the purse-string contraction of a pluricellular actomyosin cable at the free edge. However, in vitro, adhesion of the cells to their substrate favors a closure mechanism mediated by lamellipodial protrusions, which has prevented a systematic study of the purse-string mechanism.

Freshwater Planarians as an Alternative Animal Model for Neurotoxicology.

Traditional toxicology testing has relied on low-throughput, expensive mammalian studies; however, timely testing of the large number of environmental toxicants requires new in vitro and in vivo platforms for inexpensive medium- to high-throughput screening. Herein, we describe the suitability of the asexual freshwater planarian Dugesia japonica as a new animal model for the study of developmental neurotoxicology. As these asexual animals reproduce by binary fission, followed by regeneration of missing body structures within approximately 1 week, development and regeneration occur through similar processes allowing us to induce neurodevelopment "at will" through amputation.

Tryptophan hydroxylase Is Required for Eye Melanogenesis in the Planarian Schmidtea mediterranea.

Melanins are ubiquitous and biologically important pigments, yet the molecular mechanisms that regulate their synthesis and biochemical composition are not fully understood. Here we present a study that supports a role for serotonin in melanin synthesis in the planarian Schmidtea mediterranea. We characterize the tryptophan hydroxylase (tph) gene, which encodes the rate-limiting enzyme in serotonin synthesis, and demonstrate by RNA interference that tph is essential for melanin production in the pigment cups of the planarian photoreceptors.

How cells flow in the spreading of cellular aggregates.

Like liquid droplets, cellular aggregates, also called "living droplets," spread onto adhesive surfaces. When deposited onto fibronectin-coated glass or polyacrylamide gels, they adhere and spread by protruding a cellular monolayer (precursor film) that expands around the droplet. The dynamics of spreading results from a balance between the pulling forces exerted by the highly motile cells at the periphery of the film, and friction forces associated with two types of cellular flows: (i) permeation, corresponding to the entry of the cells from the aggregates into the film; and (ii) slippage as the film expands.

Interplay of RhoA and mechanical forces in collective cell migration driven by leader cells.

The leading front of a collectively migrating epithelium often destabilizes into multicellular migration fingers where a cell initially similar to the others becomes a leader cell while its neighbours do not alter. The determinants of these leader cells include mechanical and biochemical cues, often under the control of small GTPases. However, an accurate dynamic cartography of both mechanical and biochemical activities remains to be established.

Border forces and friction control epithelial closure dynamics.

We study the closure dynamics of a large number of well-controlled circular apertures within an epithelial monolayer, where the collective cell migration responsible for epithelization is triggered by the removal of a spatial constraint rather than by scratching. Based on experimental observations, we propose a physical model that takes into account border forces, friction with the substrate, and tissue rheology. Border protrusive activity drives epithelization despite the presence of a contractile actomyosin cable at the periphery of the wound.