Gas permeation through a polymer network

We study the diffusion of gas molecules through a two-dimensional network of polymers with the help of Monte Carlo simulations. The polymers are modeled as non-interacting random walks on the bonds of a two-dimensional square lattice, while the gas particles occupy the lattice cells. When a particle attempts to jump to a nearest-neighbor empty cell, it has to overcome an energy barrier which is determined by the number of polymer segments on the bond separating the two cells. We investigate the gas current $J$ as a function of the mean segment density $\rho$, the polymer length $\ell$ and the probability $q^{m}$ for hopping across $m$ segments. Whereas $J$ decreases monotonically with $\rho$ for fixed $\ell$, its behavior for fixed $\rho$ and increasing $\ell$ depends strongly on $q$. For small, non-zero $q$, $J$ appears to increase slowly with $\ell$. In contrast, for $q=0$, it is dominated by the underlying percolation problem and can be non-monotonic. We provide heuristic arguments to put these interesting phenomena into context.Comment: Dedicated to Lothar Schaefer on the occasion of his 60th birthday. 11 pages, 3 figure

A non-canonical RNA silencing pathway promotes mRNA degradation in basal fungi

The increasing knowledge on the functional relevance of endogenous small RNAs (esRNAs) as riboregulators has stimulated the identification and characterization of these molecules in numerous eukaryotes. In the basal fungus Mucor circinelloides, an emerging opportunistic human pathogen, esRNAs that regulate the expression of many protein coding genes have been described. These esRNAs share common machinery for their biogenesis consisting of an RNase III endonuclease Dicer, a single Argonaute protein and two RNA-dependent RNA polymerases. We show in this study that, besides participating in this canonical dicer-dependent RNA interference (RNAi) pathway, the rdrp genes are involved in a novel dicer-independent degradation process of endogenous mRNAs. The analysis of esRNAs accumulated in wild type and silencing mutants demonstrates that this new rdrp-dependent dicer-independent regulatory pathway, which does not produce sRNA molecules of discrete sizes, controls the expression of target genes promoting the specific degradation of mRNAs by a previously unknown RNase. This pathway mainly regulates conserved genes involved in metabolism and cellular processes and signaling, such as those required for heme biosynthesis, and controls responses to specific environmental signals. Searching the Mucor genome for candidate RNases to participate in this pathway, and functional analysis of the corresponding knockout mutants, identified a new protein, R3B2. This RNase III-like protein presents unique domain architecture, it is specifically found in basal fungi and, besides its relevant role in the rdrp-dependent dicer-independent pathway, it is also involved in the canonical dicer-dependent RNAi pathway, highlighting its crucial role in the biogenesis and function of regulatory esRNAs. The involvement of RdRPs in RNA degradation could represent the first evolutionary step towards the development of an RNAi mechanism and constitutes a genetic link between mRNA degradation and post-transcriptional gene silencing

Photon Counting Spectral CT component analysis of coronary artery atherosclerotic plaque samples

International audienceObjectives. To evaluate the capabilities of Photon Counting Spectral CT to differentiate components of coronary atherosclerotic plaque based on differences in spectral attenuation and iodine-based contrast agent concentration. Methods. 10 calcified and 13 lipid-rich atheromatous plaques from postmortem human coronary arteries were scanned with a Photon Counting Spectral CT scanner. Individual photons were counted and classified in one of 6 energy bins from 25 to 70 keV.Based on a maximum likelihood approach, maps of Photoelectric Absorption (PA), Compton Scattering (CS) and Iodine Concentration (IC) were reconstructed. Intensity measurements were performed on each map in the vessel wall, the surrounding perivascular fat, the lipid-rich and the calcified plaques. PA and CS values are expressed relative to pure water values. Comparison between these different elements was performed using Kruskal-Wallis tests with pairwise post-hoc Mann-Whitney U tests and Sidak p-values adjustment. Results. Results for vessel wall, surrounding perivascular fat, lipid-rich, and calcified plaques were respectively 1.19+/-0.09, 0.73+/-0.05, 1.08+/-0.14, 17.79+/-6.70 for PA; 0.96+/-0.02, 0.83+/-0.02, 0.91+/-0.03, 2.53+/-0.63 for CS, and 83.3+/-10.1, 37.6+/-8.1, 55.2+/-14.0, 4.9+/-20.0 mmol/l for IC, with significant differences between all tissues for PA, CS and IC (p<0.012). Conclusion. This study demonstrates the capability of energy-sensitive photon counting spectral CT to differentiate between calcifications and iodine infused regions of human coronary artery atherosclerotic plaque samples by analyzing differences in spectral attenuation and iodine-based contrast agent concentration. Advances in Knowledge: Photon Counting, Spectral CT is a promising technique to identify plaque components by analyzing differences in iodine-based contrast agent concentration, photoelectric and Compton attenuation

Initial experience in improving stent analysis and intra stent lumen assessment using Spectral Photon Counting CT and K-edge imaging

International audienc