Modeling the Mechanosensitivity of Neutrophils Passing through a?Narrow?Channel

Recent experiments have found that neutrophils may be activated after passing through microfluidic channels and filters. Mechanical deformation causes disassembly of the cytoskeleton and a sudden drop of the elastic modulus of the neutrophil. This fluidization is followed by either activation of the neutrophil with protrusion of pseudopods or a uniform recovery of the cytoskeleton network with no pseudopod. The former occurs if the neutrophil traverses the narrow channel at a slower rate. We propose a chemo-mechanical model for the fluidization and activation processes. Fluidization is treated as mechanical destruction of the cytoskeleton by sufficiently rapid bending. Loss of the cytoskeleton removes a pathway by which cortical tension inhibits the Rac protein. As a result, Rac rises and polarizes through a wave-pinning mechanism if the chemical reaction rate is fast enough. This leads to recovery and reinforcement of the cytoskeleton at the front of the neutrophil, and hence protrusion and activation. Otherwise the Rac signal returns to a uniform pre-deformation state and no activation occurs. Thus, mechanically induced neutrophil activation is understood as the competition between two timescales: that of chemical reaction and that of mechanical deformation. The model captures the main features of the experimental observation.     

When is a Phylogenetic Network Simply an Amalgamation of Two Trees?

Phylogenetic networks generalise phylogenetic (evolutionary) trees by allowing for the representation of reticulation (non-treelike) events. The structure of such networks is often viewed by the phylogenetic trees they embed. In this paper, we determine when a phylogenetic network \({\mathcal {N}}\) has two phylogenetic tree embeddings which collectively contain all of the edges of \({\mathcal {N}}\). This determination leads to a polynomial-time algorithm for recognising such networks and an unexpected characterisation of the class of reticulation-visible networks.     

Mutability Dynamics of an Emergent Single Stranded DNA Virus in a Na?ve Host

Quasispecies variants and recombination were studied longitudinally in an emergent outbreak of beak and feather disease virus (BFDV) infection in the orange-bellied parrot (Neophema chrysogaster). Detailed health monitoring and the small population size (<300 individuals) of this critically endangered bird provided an opportunity to longitudinally track viral replication and mutation events occurring in a circular, single-stranded DNA virus over a period of four years within a novel bottleneck population. Optimized PCR was used with different combinations of primers, primer walking, direct amplicon sequencing and sequencing of cloned amplicons to analyze BFDV genome variants. Analysis of complete viral genomes (n = 16) and Rep gene sequences (n = 35) revealed that the outbreak was associated with mutations in functionally important regions of the normally conserved Rep gene and immunogenic capsid (Cap) gene with a high evolutionary rate (3.41×10⁻³ subs/site/year) approaching that for RNA viruses; simultaneously we observed significant evidence of recombination hotspots between two distinct progenitor genotypes within orange-bellied parrots indicating early cross-transmission of BFDV in the population. Multiple quasispecies variants were also demonstrated with at least 13 genotypic variants identified in four different individual birds, with one containing up to seven genetic variants. Preferential PCR amplification of variants was also detected. Our findings suggest that the high degree of genetic variation within the BFDV species as a whole is reflected in evolutionary dynamics within individually infected birds as quasispecies variation, particularly when BFDV jumps from one host species to another.     

Edge-Based Compartmental Modelling of an SIR Epidemic on a Dual-Layer Static–Dynamic Multiplex Network with Tunable Clustering

The duration, type and structure of connections between individuals in real-world populations play a crucial role in how diseases invade and spread. Here, we incorporate the aforementioned heterogeneities into a model by considering a dual-layer static–dynamic multiplex network. The static network layer affords tunable clustering and describes an individual’s permanent community structure. The dynamic network layer describes the transient connections an individual makes with members of the wider population by imposing constant edge rewiring. We follow the edge-based compartmental modelling approach to derive equations describing the evolution of a susceptible–infected–recovered epidemic spreading through this multiplex network of individuals. We derive the basic reproduction number, measuring the expected number of new infectious cases caused by a single infectious individual in an otherwise susceptible population. We validate model equations by showing convergence to pre-existing edge-based compartmental model equations in limiting cases and by comparison with stochastically simulated epidemics. We explore the effects of altering model parameters and multiplex network attributes on resultant epidemic dynamics. We validate the basic reproduction number by plotting its value against associated final epidemic sizes measured from simulation and predicted by model equations for a number of set-ups. Further, we explore the effect of varying individual model parameters on the basic reproduction number. We conclude with a discussion of the significance and interpretation of the model and its relation to existing research literature. We highlight intrinsic limitations and potential extensions of the present model and outline future research considerations, both experimental and theoretical.     

Regulating the fluorescence intensity of an anthracene boronic acid system: a B-N bond or a hydrolysis mechanism?

An anthracene-based fluorescent boronic acid system developed by the Shinkai group has been widely used for the preparation of fluorescent sensors for carbohydrates. Such application is based on the significant fluorescence intensity increase of this system upon binding with a carbohydrate. The mechanism through which this fluorescence intensity change happens was originally proposed to go through a B-N bond formation mechanism, which masks the nitrogen lone pair electrons. However, our own fluorescence studies suggest a possible alternative mechanism for the fluorescence change upon the formation of a boronic acid (1a) complex with diols. In this new proposed mechanism, complex formation induces solvolysis, which results in the protonation of the amine nitrogen if the reactions are carried out in a protic solvent such as water. This protonation prevents the photoinduced electron transfer, resulting in reduced quenching of the anthracene fluorescence. Such a solvolysis mechanism is supported by evidence from various types of experiments and theoretical calculations.     

Impaired Gating of an L-Type Ca2+ Channel Carrying a Mutation Linked to?Malignant Hyperthermia

Recently, we characterized the functional properties of a mutant skeletal muscle L-type Ca²⁺ channel (Ca_V1.1 R174W) linked to the pharmacogenetic disorder malignant hyperthermia. Although the R174W mutation neutralizes the innermost basic amino acid in the voltage-sensing S4 helix of the first conserved membrane repeat of Ca_V1.1, the ability of the mutant channel to engage excitation-contraction coupling was largely unaffected by the introduction of the bulky tryptophan residue. In stark contrast, the mutation ablated the ability of Ca_V1.1 to produce L-type current under our standard recording conditions. In this study, we have investigated the mechanism of channel dysfunction more extensively. We found that Ca_V1.1 R174W will open and conduct Ca²⁺ in response to strong or prolonged depolarizations in the presence of the 1,4-dihydropyridine receptor agonist ±Bay K 8644. From these results, we have concluded that the R174W mutation impedes entry into both mode 1(low P_o) and mode 2 (high P_o) gating states and that these gating impairments can be partially overcome by maneuvers that promote entry into mode 2.     

Disruption of the K+ Channel β-Subunit KCNE3 Reveals an Important Role in Intestinal and Tracheal Cl? Transport

The KCNE3 β-subunit constitutively opens outwardly rectifying KCNQ1 (Kv7.1) K⁺ channels by abolishing their voltage-dependent gating. The resulting KCNQ1/KCNE3 heteromers display enhanced sensitivity to K⁺ channel inhibitors like chromanol 293B. KCNE3 was also suggested to modify biophysical properties of several other K⁺ channels, and a mutation in KCNE3 was proposed to underlie forms of human periodic paralysis. To investigate physiological roles of KCNE3, we now disrupted its gene in mice. kcne3^−/− mice were viable and fertile and displayed neither periodic paralysis nor other obvious skeletal muscle abnormalities. KCNQ1/KCNE3 heteromers are present in basolateral membranes of intestinal and tracheal epithelial cells where they might facilitate transepithelial Cl⁻ secretion through basolateral recycling of K⁺ ions and by increasing the electrochemical driving force for apical Cl⁻ exit. Indeed, cAMP-stimulated electrogenic Cl⁻ secretion across tracheal and intestinal epithelia was drastically reduced in kcne3^−/− mice. Because the abundance and subcellular localization of KCNQ1 was unchanged in kcne3^−/− mice, the modification of biophysical properties of KCNQ1 by KCNE3 is essential for its role in intestinal and tracheal transport. Further, these results suggest KCNE3 as a potential modifier gene in cystic fibrosis.     

What is the Binding Energy of a Charge Transfer State in an Organic Solar Cell?

The high efficiencies reported for organic solar cells and an almost negligible thermal activation measured for the photogeneration of charge carriers have called into question whether photoinduced interfacial charge transfer states are bound by a significant coulomb attraction, and how this can be reconciled with very low activation energies. Here, this question is addressed in a combined experimental and theoretical approach. The interfacial binding energy of a charge‐transfer state in a blend of MeLPPP:PCBM is determined by using energy resolved electrochemical impedance spectroscopy and is found to be about 0.5 eV. Temperature‐dependent photocurrent measurements on the same films, however, give an activation energy that is about one order of magnitude lower. Using analytical calculations and Monte Carlo simulation the authors illustrate how i) interfacial energetics and ii) transport topology reduce the activation energy required to separate the interfacial electron–hole pair, with about equal contributions from both effects. The activation energy, however, is not reduced by entropy, although entropy increases the overall photodissociation yield.     

Cell Swelling Activates the K+ Conductance and Inhibits the Cl? Conductance of the Basolateral Membrane of Cells from a Leaky Epithelium

Necturus gallbladder epithelial cells bathed in 10 mM HCO₃/1% CO₂ display sizable basolateral membrane conductances for Cl⁻ (G_Cl ^b) and K ⁺ (G_K ^b). Lowering the osmolality of the apical bathing solution hyperpolarized both apical and basolateral membranes and increased the K ⁺/Cl⁻ selectivity of the basolateral membrane. Hyperosmotic solutions had the opposite effects. Intracellular free-calcium concentration ([Ca²⁺]_i) increased transiently during hyposmotic swelling (peak at ∼30 s, return to baseline within ∼90 s), but chelation of cell Ca²⁺ did not prevent the membrane hyperpolarization elicited by the hyposmotic solution. Cable analysis experiments showed that the electrical resistance of the basolateral membrane decreased during hyposmotic swelling and increased during hyperosmotic shrinkage, whereas the apical membrane resistance was unchanged in hyposmotic solution and decreased in hyperosmotic solution. We assessed changes in cell volume in the epithelium by measuring changes in the intracellular concentration of an impermeant cation (tetramethylammonium), and in isolated polarized cells measuring changes in intracellular calcein fluorescence, and observed that these epithelial cells do not undergo measurable volume regulation over 10–12 min after osmotic swelling. Depolarization of the basolateral membrane voltage (V_cs) produced a significant increase in the change in V_cs elicited by lowering basolateral solution [Cl⁻], whereas hyperpolarization of V_cs had the opposite effect. These results suggest that: (a) Hyposmotic swelling increases G_K ^b and decreases G _Cl ^b. These two effects appear to be linked, i.e., the increase in G _K ^b produces membrane hyperpolarization, which in turn reduces G _Cl ^b. ( b) Hyperosmotic shrinkage has the opposite effects on G_K ^b and G _Cl ^b. ( c) Cell swelling causes a transient increase in [Ca²⁺]_i, but this response may not be necessary for the increase in G_K ^b during cell swelling.