A computational model for regulation of nanoscale glucan exposure in <i>Candida albicans</i>

<div><p><i>Candida albicans</i> is a virulent human opportunistic pathogen. It evades innate immune surveillance by masking an immunogenic cell wall polysaccharide, β-glucan, from recognition by the immunoreceptor Dectin-1. Glucan unmasking by the antifungal drug caspofungin leads to changes in the nanostructure of glucan exposure accessible to Dectin-1. The physical mechanism that regulates glucan exposure is poorly understood, but it controls the nanobiology of fungal pathogen recognition. We created computational models to simulate hypothetical physical processes of unmasking glucan in a biologically realistic distribution of cell wall glucan fibrils. We tested the predicted glucan exposure nanostructural features arising from these models against experimentally measured values. A completely spatially random unmasking process, reflective of random environmental damage to the cell wall, cannot account for experimental observations of glucan unmasking. However, the introduction of partially edge biased unmasking processes, consistent with an unmasking contribution from active, local remodeling at glucan exposure sites, produces markedly more accurate predictions of experimentally observed glucan nanoexposures in untreated and caspofungin-treated yeast. These findings suggest a model of glucan unmasking wherein cell wall remodeling processes in the local nanoscale neighborhood of glucan exposure sites are an important contributor to the physical process of drug-induced glucan unmasking in <i>C</i>. <i>albicans</i>.</p></div

Model/Experiment discrepancy calculations quantify model performance and identify model parameterizations with optimum fit to experimental data.

<p>To describe the goodness of fit of the models, we measured discrepancies of various simulation outputs as a function of P_edge. Data are displayed for untreated (A-D) and caspofungin-treated (E-H) yeasts. Note that P_edge = 0 corresponds to the random unmasking model. For each series, results are given for the total discrepancy (A,E) and the individual signed contributions of the three variables of interest [singlet fraction (B,F), equivalent radii of multi-exposures (C,G), and glucan exposure density (D,H)]. Dotted magenta lines in B-D and F-H denote the zero value for the indicated discrepancy metric, which is the value of optimum fit of model results to experimental results for an individual output. The above mean values are calculated from n = 100 runs for each condition.</p

The edge biased unmasking model improves correspondence of simulated and experimental datasets.

<p>We show P_edge = 4% bias runs as an example. (A-C) Full results over the entire simulation for various combinations of stripe width and glucan coverage for the three simulation and experimental outputs measured. (D-F) Similar to A-C, but showing zoomed in results corresponding to the experimental data domain. The cyan range boxes express ranges in the style of box plots with the lines representing the 25^th, 50^th and 75^th percentiles of the experimental data, going from left to right or bottom to top, on the corresponding axes. Within each plot, the left-most cyan range box corresponds to untreated yeast, the right-most to caspofungin-treated yeast. (G-I) The standard deviations of the zoomed in results from D-F. The lines represent mean (A-F) or standard deviation (G-I) values calculated from n = 100 runs for each condition.</p

Random unmasking model simulation results did not match the experimental results.

<p>(A-C) Full results over the entire simulation for various combinations of stripe width (nm) and glucan coverage (percent) of the simulation space for the three simulation and experimental outputs measured. The lines represent means calculated from n = 100 runs for each condition. (D-F) Zoomed in results corresponding to the experimental data domain. The cyan range boxes express ranges in the style of box plots with the lines representing the 25^th, 50^th (median) and 75^th percentiles of the experimental data, going from left to right or bottom to top, on the corresponding axes (percentiles refer to percent glucan surface area on the horizontal axes, and to singlet fraction, equivalent multi-exposure radius or glucan exposure density as denoted on the vertical axes). Within each plot, the left-most cyan range box corresponds to untreated yeast, the right-most to caspofungin-treated yeast.</p

Schematic description of the glucan unmasking model.

<p>(A) Masked glucan on <i>C</i>. <i>albicans</i> cell walls was represented as stripes (black) of a specified width in pixels and covering a specified fraction of the total simulation area (white grid space). The stripes model the presence of fibrillar structures of insoluble glucan in the <i>C</i>. <i>albicans</i> cell wall. Each pixel represents a 5 nm x 5 nm area in the total square simulation space of 1 um². The glucan unmasking model proceeds by selecting a single pixel at each iteration (black masked glucan or white glucan free) and changing its state. As pixels representing glucan are unmasked, they change their color to red. (B-D) Pixels are chosen for glucan unmasking at each iteration of the simulation, either in a random (P_edge = 0) or an edge biased (P_edge>0) manner. If P_edge>0, then P_edge determines the probability that a pixel will be chosen from the boundary pixels surrounding existing exposures as opposed to a pixel chosen randomly from the surface. (E) The simulation space is 200 x 200 pixels, so after 40,000 iterations all pixels have been selected and all masked glucan has been exposed.</p

Simulation outputs (mean ± standard deviation) from model parameterizations with optimal fits to experimental results.

<p>Simulation outputs (mean ± standard deviation) from model parameterizations with optimal fits to experimental results.</p

Edge-biased unmasking model parameterizations with optimum global performance predict experimental outputs significantly better than random unmasking models.

<p>We compared the simulated to the experimental results using the P_edge’s that produced the smallest total discrepancies (minimum D) for 5, 10, 15 and 20% glucan coverage with fibril width 50nm in untreated and treated conditions. The actual P_edge values used in the depicted model results are given in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0188599#pone.0188599.t002" target="_blank">Table 2</a>. Data are displayed for untreated (A-C) and caspofungin-treated (D-F) yeasts, corresponding to singlet fraction (A,D), equivalent radii of multi-exposures (B,E), and glucan exposure density (C,F). The range boxes express ranges in the style of box plots with the lines representing the 25^th, 50^th and 75^th percentiles of the experimental data, going from left to right or bottom to top, on the corresponding axes. The left-most range box corresponds to untreated yeast, the right-most to caspofungin-treated yeast. Within each plot, the above values are calculated from n = 100 runs for each condition. For emphasis, the range boxes representing untreated experimental data are colored cyan in A-C, which concern simulations of untreated yeast. Conversely, the range boxes representing caspofungin-treated experimental data are colored cyan in D-F, which concern simulations of caspofungin-treated yeast. In every case, range boxes for the sample type less relevant to a given plot are de-emphasized by grey coloration, but are retained for reference.</p

Enhanced Transparency of Rough Surface Sapphire by Surface Vitrifaction Process

A novel, simple, and low-cost in situ surface vitrification method has been effectively developed to enhance the optical transparency of rough surface sapphire at UV–visible–IR regions. This method is to obtain a glass layer on the sapphire surface through vitrifaction process. The thickness, refractive index, components and transition temperature of the glass layer have been investigated and discussed respectively by XRD, DSC, SEM and EDS elemental analysis. The experimental results show that the vitrified sapphire has high transparency even after 1000 °C annealing at UV–visible-IR regions