Dynamic force spectroscopy on multiple bonds: experiments and model

We probe the dynamic strength of multiple biotin-streptavidin adhesion bonds under linear loading using the biomembrane force probe setup for dynamic force spectroscopy. Measured rupture force histograms are compared to results from a master equation model for the stochastic dynamics of bond rupture under load. This allows us to extract the distribution of the number of initially closed bonds. We also extract the molecular parameters of the adhesion bonds, in good agreement with earlier results from single bond experiments. Our analysis shows that the peaks in the measured histograms are not simple multiples of the single bond values, but follow from a superposition procedure which generates different peak positions.Comment: to appear in Europhysics Letter

Dynamic caveolae exclude bulk membrane proteins and are required for sorting of excess glycosphingolipids

Caveolae have long been implicated in endocytosis. Recent data question this link, and in the absence of specific cargoes the potential cellular function of caveolar endocytosis remains unclear. Here we develop new tools, including doubly genome-edited cell lines, to assay the subcellular dynamics of caveolae using tagged proteins expressed at endogenous levels. We find that around 5% of the cellular pool of caveolae is present on dynamic endosomes, and is delivered to endosomes in a clathrin-independent manner. Furthermore, we show that caveolae are indeed likely to bud directly from the plasma membrane. Using a genetically encoded tag for electron microscopy and ratiometric light microscopy, we go on to show that bulk membrane proteins are depleted within caveolae. Although caveolae are likely to account for only a small proportion of total endocytosis, cells lacking caveolae show fundamentally altered patterns of membrane traffic when loaded with excess glycosphingolipid. Altogether, these observations support the hypothesis that caveolar endocytosis is specialized for transport of membrane lipid

Pulling long tubes from firmly adhered vesicles

We used optical tweezers to measure the force-extension curve for the elongation of nanotubes from adhered giant vesicles. We show that the force increases significantly with the length of the tube, which is drastically different from what is observed when the membrane tension is kept constant, e.g. by pipette aspiration. The absence of any force plateau is quantitatively analysed in the framework of the material model of membranes. In particular, we rationalize a counter-intuitive weaker force rise for long tubes and demonstrate that the measured force-length trace allows us to probe both the entropic regime (characterised by the bending rigidity) and the elastic regime (characterised by the area expansion modulus) of the lipid membrane

Nanofluidics in cellular tubes under oscillatory extension

Membrane nanotubes or tethers extruded from cells exhibit dynamic features that are believed to exhibit viscoelastic rheological properties. We have performed typical microrheology experiments on tethers pulled from red blood cells by measuring the force response to small oscillatory extensions or compressions. Our data, supported by a simple theoretical model, show that the force response does not reflect any intrinsic viscoelastic properties of the tethers themselves, but instead is dominated by the drainage of the internal cellular fluid into and out of the oscillating nanoconduit over a frequency-dependent penetration depth. The simplicity of tether rheology suggests its usage as a probe for measuring the local viscosity of the cytosol near the plasma membrane

Quantitative modeling identifies robust predictable stress response of growing CT26 tumor spheroids under variable conditions

Mechanical feedback has been identified as a key regulator of tissue growth, by which external signals are transduced into a complex intracellular molecular machinery. Using multiscale computational modeling of multicellular growth in two largely different experimental settings with the same cell line we demonstrate that the cellular growth response on external mechanical stress may nevertheless be surprisingly quantitatively predictable. Our computational model represents each cell as an individual unit capable of migration, growth, division, and death and is parameterized by measurable biophysical and bio-kinetic parameters. A cell cycle progression function depending on volumetric cell compression is established by from comparisons of computer simulations with experiments of spheroids growing in an alginate elastic capsule. After an intermediate calibration step with free growing spheroids growing in a liquid suspension to capture the different growth conditions, the model using the same cell cycle progression function can predict the mechanical stress response of spheroid growth in another experimental technique using Dextran, where stress is exerted by osmotic pressure, even though the experimental results appear differently in both experiments. Our findings suggest that the stress response of cell growth may be highly reproducible even in otherwise different environments. This encourages that robust functional modules may be identified that help us to understand complex cell behavior such as cell growth and division in relation to mechanical stress. The model analysis further elucidates the relation between applied pressure, cell compressibility and cell density. Moreover, the model developments within this paper points a way of how to handle the so far open issue of high compression within the popular so-called " Center-Based Models " , in which force between cells a modelled as forces between cell centers

Quantitative modeling identifies robust predictable stress response of growing CT26 tumor spheroids under variable conditions

Mechanical feedback has been identified as a key regulator of tissue growth, by which external signals are transduced into a complex intracellular molecular machinery. Using multiscale computational modeling of multicellular growth in two largely different experimental settings with the same cell line we demonstrate that the cellular growth response on external mechanical stress may nevertheless be surprisingly quantitatively predictable. Our computational model represents each cell as an individual unit capable of migration, growth, division, and death and is parameterized by measurable biophysical and bio-kinetic parameters. A cell cycle progression function depending on volumetric cell compression is established by from comparisons of computer simulations with experiments of spheroids growing in an alginate elastic capsule. After an intermediate calibration step with free growing spheroids growing in a liquid suspension to capture the different growth conditions, the model using the same cell cycle progression function can predict the mechanical stress response of spheroid growth in another experimental technique using Dextran, where stress is exerted by osmotic pressure, even though the experimental results appear differently in both experiments. Our findings suggest that the stress response of cell growth may be highly reproducible even in otherwise different environments. This encourages that robust functional modules may be identified that help us to understand complex cell behavior such as cell growth and division in relation to mechanical stress. The model analysis further elucidates the relation between applied pressure, cell compressibility and cell density. Moreover, the model developments within this paper points a way of how to handle the so far open issue of high compression within the popular so-called " Center-Based Models " , in which force between cells a modelled as forces between cell centers

Quantitative modeling identifies robust predictable stress response of growing CT26 tumor spheroids under variable conditions

Mechanical feedback has been identified as a key regulator of tissue growth, by which external signals are transduced into a complex intracellular molecular machinery. Using multiscale computational modeling of multicellular growth in two largely different experimental settings with the same cell line we demonstrate that the cellular growth response on external mechanical stress may nevertheless be surprisingly quantitatively predictable. Our computational model represents each cell as an individual unit capable of migration, growth, division, and death and is parameterized by measurable biophysical and bio-kinetic parameters. A cell cycle progression function depending on volumetric cell compression is established by from comparisons of computer simulations with experiments of spheroids growing in an alginate elastic capsule. After an intermediate calibration step with free growing spheroids growing in a liquid suspension to capture the different growth conditions, the model using the same cell cycle progression function can predict the mechanical stress response of spheroid growth in another experimental technique using Dextran, where stress is exerted by osmotic pressure, even though the experimental results appear differently in both experiments. Our findings suggest that the stress response of cell growth may be highly reproducible even in otherwise different environments. This encourages that robust functional modules may be identified that help us to understand complex cell behavior such as cell growth and division in relation to mechanical stress. The model analysis further elucidates the relation between applied pressure, cell compressibility and cell density. Moreover, the model developments within this paper points a way of how to handle the so far open issue of high compression within the popular so-called " Center-Based Models " , in which force between cells a modelled as forces between cell centers

Growth of vertically aligned carbon nanotubes on aluminum foils

International audienceForests of vertically aligned carbon nanotubes (VACNTs) are attractive nanomaterials because of their unique structural, electrical and thermal properties. However, many applications require their growth on metallic substrates. Catalytic chemical vapor deposition (CCVD) is the best method to grow them but the catalytic particles can diffuse rapidly into the metal subsurface and thus become inactive. In this communication, I will address this issue through the recent results obtained in our laboratory. I will show how it is possible to grow VACNT on carbon fibers, stainless steel and aluminum surfaces by a single-step process, namely the aerosol assisted CCVD, where the catalyst and carbon precursors are injected simultaneously. In the case of aluminum, due to its low melting temperature, the synthesis of VACNT requires a significant reduction in the growth temperature as compared to conventional substrates. Our results show that, with our single-step process, it is possible to obtain clean, long and dense VACNTs, with a growth rate at the best state of the art level for such a low temperature. A particular attention has been paid to the study of the CNT/Al interface. The results suggest the crucial role of the interface for an efficient and reproducible VACNT growth. Finally, I will show that the aerosol-assisted CCVD process can be scaled-up to enable the fabrication of innovative ultracapacitors based on VACNTs grown on aluminum foils