The Roles of Microtubules and Membrane Tension in Axonal Beading, Retraction, and Atrophy.

Axonal beading, or the formation of a series of swellings along the axon, and retraction are commonly observed shape transformations that precede axonal atrophy in Alzheimer's disease, Parkinson's disease, and other neurodegenerative conditions. The mechanisms driving these morphological transformations are poorly understood. Here, we report controlled experiments that can induce either beading or retraction and follow the time evolution of these responses. By making quantitative analysis of the shape modes under different conditions, measurement of membrane tension, and using theoretical considerations, we argue that membrane tension is the main driving force that pushes cytosol out of the axon when microtubules are degraded, causing axonal thinning. Under pharmacological perturbation, atrophy is always retrograde, and this is set by a gradient in the microtubule stability. The nature of microtubule depolymerization dictates the type of shape transformation, vis-à-vis beading or retraction. Elucidating the mechanisms of these shape transformations may facilitate development of strategies to prevent or arrest axonal atrophy due to neurodegenerative conditions.

Pulsations and flows in tissues as two collective dynamics with simple cellular rules.

Collective motions of epithelial cells are essential for morphogenesis. Tissues elongate, contract, flow, and oscillate, thus sculpting embryos. These tissue level dynamics are known, but the physical mechanisms at the cellular level are unclear. Here, we demonstrate that a single epithelial monolayer of MDCK cells can exhibit two types of local tissue kinematics, pulsations and long range coherent flows, characterized by using quantitative live imaging. We report that these motions can be controlled with internal and external cues such as specific inhibitors and substrate friction modulation. We demonstrate the associated mechanisms with a unified vertex model. When cell velocity alignment and random diffusion of cell polarization are comparable, a pulsatile flow emerges whereas tissue undergoes long-range flows when velocity alignment dominates which is consistent with cytoskeletal dynamics measurements. We propose that environmental friction, acto-myosin distributions, and cell polarization kinetics are important in regulating dynamics of tissue morphogenesis.

How to orient cells in microcavities for high resolution imaging of cytokinesis and lumen formation.

Imaging dynamics of cellular morphogenesis with high spatial-temporal resolution in 3D is challenging, due to the low spatial resolution along the optical axis and photo-toxicity. However, some cellular structures are planar and hence 2D imaging should be sufficient, provided that the structure of interest can be oriented with respect to the optical axis of the microscope. Here, we report a 3D microfabrication method which positions and orients cell divisions very close to the microscope coverglass. We use this approach to study cytokinesis in fission yeasts and polarization to lumen formation in mammalian epithelial cells. We show that this method improves spatial resolution on range of common microscopies, including super-resolution STED. Altogether, this method could shed new lights on self-organization phenomena in single cells and 3D cell culture systems.

Directing cell migration on flat substrates and in confinement with microfabrication and microfluidics.

Cell motility has been mainly characterized in vitro through the motion of cells on 2D flat Petri dishes, and in Boyden chambers with the passage of cells through sub-cellular sized cavities. These experimental conditions have contributed to understand important features, but these artificial designs can prevent elucidation of mechanisms involved in guiding cell migration in vivo. In this context, microfabrication and microfluidics have provided unprecedented tools to design new assays with local controls in two and three dimensions. Single cells are surrounded by specific environments at a scale where cellular organelles like the nucleus, the cortex, and protrusions can be probed locally in time and in space. Here, we report methods to direct cell motion with emphasis on micro-contact printing for 2D cell migration, and ratchetaxis/chemotaxis in 3D confinements. While sharing similarities, both environments generate distinct experimental issues and questions with potential relevance for in vivo situations.

A rare cause for a common symptom.

A rare cause for cough and fever http://ow.ly/d6zy301yYDJ.