Tandem tension sensor reveals substrate rigidity-dependence of integrin molecular tensions in live cells

Response of integrin tensions to substrate rigidity is important in cell rigidity sensing but has not been confirmed. Current fluorescent tension sensors produce cellular force signals collectively resulted from integrin tension magnitude, tension dwell time, integrin density and activity, ligand density and accessibility, etc., making it challenging to monitor the absolute molecular force level of integrin tensions in live cells. Here we developed a tandem tension sensor (TTS) consisting of two coupled tension sensing units which are subject to the same tension and respond with different activation probabilities to the tension. Reported by fluorescence, the activation probability ratio of these two units solely responds to the force level of local integrin tensions, excluding the bias from other non-force factors. We verified the feasibility of TTS in detecting integrin tensions and applied it to study cells on elastic substrates. TTS unambiguously reported that integrin tensions in platelets decrease monotonically with the substrate rigidity, verifying the rigidity-dependence of integrin tensions in live cells.

Humidity effects on the detection of soluble and insoluble nanoparticles in butanol operated condensation particle counters

In this study the impact of humidity on heterogeneous nucleation of n-butanol onto hygroscopic and nonabsorbent particles was investigated using a fast expansion chamber and commercial continuous flow type condensation particle counters (CPCs). More specifically, we measured the activation probability of sodium chloride (NaCl) and silver (Ag) nano-particles by using n-butanol as condensing liquid with the size analyzing nuclei counter (SANC). In addition, the cut-off diameter of regular butanol based CPCs for both seed materials was measured and compared to SANC results. Our findings reveal a strong humidity dependence of NaCl particles in the sub-10 nm size range since the activation of sodium chloride seeds is enhanced with increasing relative humidity. For Ag seeds this humidity dependence was not observed, underlining the importance of molecular interactions between seed and vapor molecules. Consequently, the cut-off diameter of a butanol based CPC can be reduced significantly by increasing the relative humidity. This finding suggests that cut-off diameters of butanol CPCs under ambient conditions are likely smaller than corresponding cut-off diameters measured under clean (dry) laboratory conditions. At the same time, we caution that the humidity dependence may lead to wrong interpretations if the aerosol composition is not known.

Degree-dependent threshold-based random sequential adsorption on random trees

We consider a special version of random sequential adsorption (RSA) with nearest-neighbor interaction on infinite tree graphs. In classical RSA, starting with a graph with initially inactive nodes, each of the nodes of the graph is inspected in a random order and is irreversibly activated if none of its nearest neighbors are active yet. We generalize this nearest-neighbor blocking effect to a degree-dependent threshold-based blocking effect. That is, each node of the graph is assumed to have its own degree-dependent threshold and if, upon inspection of a node, the number of active direct neighbors is less than that node's threshold, the node will become irreversibly active. We analyze the activation probability of nodes on an infinite tree graph, given the degree distribution of the tree and the degree-dependent thresholds. We also show how to calculate the correlation between the activity of nodes as a function of their distance. Finally, we propose an algorithm which can be used to solve the inverse problem of determining how to set the degree-dependent thresholds in infinite tree graphs in order to reach some desired activation probabilities.