TU LogoTechnische Universität Wien

Retreat in Bratislava 2018

This retreat (8th-10th of november) started off with a one-day workshop in scientific presentations. The PhD-students took great profit from the work with the instructor John Waterman both on a professional and personal level.

For this retreat each PhD student gave a presentation on recent progress in their work.

Nick Evans (Southampton University) and Johann Danzl (IST Klosterneuburg) gave very interesting talks as our invited speakers.

While most people took part in a team-building activity, other people went for a city tour through Bratislava.

We thank the organization team (Bodner Clara, Dobos Agnes, Hellmeier Joschka, Wittner Valentina and Wagner Susanne)!



Nicholas Evans: Collective cell behaviour in long-range mechanosensing: The princess and the pea revisited

Cells attach to and exert tensile and compressive forces on extracellular matrix (ECM), ‘measuring’ the resultant deformation that develops. The degree of resistance of an ECM to deformation is dependent on its stiffness. Cells use information obtained in this way to make fundamental decisions in how to move, divide and differentiate. Stiffness is not only dependent of the elastic modulus of the ECM, however, but also on its dimensions. This has the corollary that single cells are able to sense underlying stiff substrata through soft ECMs at low (<10 μm) thicknesses[1]. Here, we hypothesised that groups of cells would be able to deform materials to a greater degree than individual cells, and therefore be able to act collectively to mechanosense underlying substrates or features at greater depths than individual cells. To test this, we fabricated polyacrylamide hydrogels in the range of 1 - 1000 μm in thickness and of 0.5 – 40kPa elastic modulus adhered to glass substrates. Hydrogel surfaces were then covalently modified with ECM proteins, and MG63 cells were plated on hydrogels either at low density or in compact colonies/islands. Cell density, cell aspect, and cell perimeter was measured by microscopy, and hydrogel displacement by time-lapse imaging of hydrogel-embedded fluorescence fiduciary beads. The spreading of separated, single cells on soft (1kPa) hydrogels increased exponentially as function of decreasing hydrogel thickness, with a half maximal response at ~3.2 μm. Similarly, the spreading of cells within cell islands of defined area (4 x 104 - 4 x 105 μm2). also increased exponentially as a function of decreasing hydrogel thickness, but with a much greater half-maximal response of 54 μm2. Depth-sensing was dependent on Rho kinase activity. Hydrogel displacements were greater for colonies vs. single cells and for thick gels vs. thin gels. These results support the notion that groups of cells act collectively to mechanosense rigid materials beneath elastic hydrogels at greater depths than individual cells. This raises the intriguing possibility that the collective action of cells in tissues such as epithelia may allow cells to sense structures of differing stiffness at comparatively large distances. This has implications in cell patterning and differentiation in development, in tissue healing, and in the design of implanted biomaterials.

[1] Lin et al. Phys Rev E Stat Nonlin Soft Matter Phys. (2010) 82:041918

Johann Danzl: Improving high-resolution optical imaging for biology

Far-field optical super-resolution microscopy or nanoscopy techniques “super-resolve” features residing closer than the diffraction-limit by transiently preparing fluorophores in distinguishable (typically on-  and off-) states and reading them out sequentially. In coordinate-targeted super-resolution modalities, such as stimulated emission depletion (STED) microscopy, this state difference is created by patterns of light, driving for instance all molecules to the off-state except for those residing at intensity minima. I will discuss our recent efforts to improve coordinate-targeted nanoscopy. As a specific example, I will highlight how the use of multiple off-state transitions for nanoscopy can improve repeated imaging  apability and on/off state contrast, enhancing image resolution and signal-to-background ratio in an approach that we dubbed “protected STED” (Danzl, Sidenstein et al., Nature Photonics (2016)). This allowed e.g. decoding the elaborate 3D structure of dendritic spines in living brain tissue. I will also present the activities of my recently founded group at IST Austria where we work as an interdisciplinary team of physicists, biologists, and neuroscientists to develop and apply methods that bridge spatial scales from nanoscale molecular arrangements to the native tissue context and where we strive to characterize biological samples with high information content.

Source of the picture top right: http://slovakia.travel/en/bratislava