Endoplasmic-reticulum-mediated microtubule alignment governs cytoplasmic streaming

Cytoplasmic streaming refers to a collective movement of cytoplasm observed in many cell types. The mechanism of meiotic cytoplasmic streaming (MeiCS) in Caenorhabditis elegans zygotes is puzzling as the direction of the flow is not predefined by cell polarity and occasionally reverses. Here, we demonstrate that the endoplasmic reticulum (ER) network structure is required for the collective flow. Using a combination of RNAi, microscopy and image processing of C. elegans zygotes, we devise a theoretical model, which reproduces and predicts the emergence and reversal of the flow. We propose a positive-feedback mechanism, where a local flow generated along a microtubule is transmitted to neighbouring regions through the ER. This, in turn, aligns microtubules over a broader area to self-organize the collective flow. The proposed model could be applicable to various cytoplasmic streaming phenomena in the absence of predefined polarity. The increased mobility of cortical granules by MeiCS correlates with the efficient exocytosis of the granules to protect the zygotes from osmotic and mechanical stresses.

Optical control of surface forces and instabilities in foam films using photosurfactants

Molecular interactions in thin liquid films, such as the disjoining pressure, are involved in interfacial phenomena such as emulsion and foam stabilization. In this article we show that through light stimulation we can control remotely the disjoining pressure in a thin liquid film stabilized by a photosurfactant. We stabilize a horizontal thin liquid film using a cationic photosurfactant, AzoTAB, bearing an azobenzene moiety on the hydrophobic tail which can switch from a trans to a cis conformation upon light stimulation. As the film is illuminated at specific wavelengths the AzoTAB molecules switch continuously their conformation and consequently their interface affinity. The main consequence of stimulating the film with light is increasing the ratio of cis in the film. This provokes a desorption flux, and an increase in the concentration of free surfactants, as the CMC of the cis isomer is higher than that of the trans isomer. Therefore the electrostatic repulsion between the surfactant layers that stabilize the film decreases, inducing an instability in the film thickness. For films with a thickness between 20 nm and 60 nm, we observe the formation of spherical caps up to 100 μm wide, whose shape is controlled by the competition between surface tension and disjoining pressure. The motion of these caps in the film is restrained by the surface viscosity of the surfactant layers. In addition, for thicknesses below 40 nm and depending on light intensity, we can observe flat stratified islands up to 100 μm wide, with thickness steps corresponding to the size of a surfactant micelle. We suggest that this second instability is due to the oscillation of the disjoining pressure isotherm under light.

In-mold patterning and actionable axo-somatic compartmentalization for on-chip neuron culture

Oriented neuronal networks with controlled connectivity are required for many applications ranging from studies of neurodegeneration to neuronal computation. To build such networks in vitro, an efficient, directed and long lasting guidance of axons toward their target is a pre-requisite. The best guidance achieved so far, however, relies on confining axons in enclosed microchannels, making them poorly accessible for further investigation. Here we describe a method providing accessible and highly regular arrays of axons, emanating from somas positioned in distinct compartments. This method combines the use of a novel removable partition, allowing soma positioning outside of the axon guidance patterns, and in-mold patterning (iMP), a hybrid method combining chemical and mechanical cell positioning clues applied here for the first time to neurons. The axon guidance efficiency of iMP is compared to that of conventional patterning methods, e.g. micro-contact printing (chemical constraints by a poly-l-lysine motif) and micro-grooves (physical constraints by homogeneously coated microstructures), using guiding tracks of different widths and spacing. We show that iMP provides a gain of 10 to 100 in axon confinement efficiency on the tracks, yielding mm-long, highly regular, and fully accessible on-chip axon arrays. iMP also allows well-defined axon guidance from small populations of several neurons confined at predefined positions in μm-sized wells. iMP will thus open new routes for the construction of complex and accurately controlled neuronal networks.

Catch-bond behaviour facilitates membrane tubulation by non-processive myosin 1b

Myosin 1b is a single-headed membrane-associated motor that binds to actin filaments with a catch-bond behaviour in response to load. In vivo, myosin 1b is required to form membrane tubules at both endosomes and the trans-Golgi network. To establish the link between these two fundamental properties, here we investigate the capacity of myosin 1b to extract membrane tubes along bundled actin filaments in a minimal reconstituted system. We show that single-headed non-processive myosin 1b can extract membrane tubes at a biologically relevant low density. In contrast to kinesins we do not observe motor accumulation at the tip, suggesting that the underlying mechanism for tube formation is different. In our theoretical model, myosin 1b catch-bond properties facilitate tube extraction under conditions of increasing membrane tension by reducing the density of myo1b required to pull tubes.