Molecular Dynamics Simulations to Explore the Structure and Rheological Properties of Normal and Hyperconcentrated Airway Mucus

Abstract

We develop the first molecular dynamics model of airway mucus based on the detailed physical properties and chemical structure of the predominant gel-forming mucin MUC5B. Our airway mucus model leverages the LAMMPS open-source code [https://lammps.sandia.gov], based on the statistical physics of polymers, from single molecules to networks. On top of the LAMMPS platform, the chemical structure of MUC5B is used to superimpose proximity-based, non-covalent, transient interactions within and between the specific domains of MUC5B polymers. We explore feasible ranges of hydrophobic and electrostatic interaction strengths between MUC5B domains with 9 nanometer spatial and 1 nanosecond temporal resolution. Our goal here is to propose and test a mechanistic hypothesis for a striking clinical observation with respect to airway mucus: a 10-fold increase in non-swellable, dense structures called flakes during progression of cystic fibrosis disease. Among the myriad possible effects that might promote self-organization of MUC5B networks into flake structures, we hypothesize and confirm that the clinically confirmed increase in mucin concentration, from 1.5 to 5 mg/mL, alone is sufficient to drive the structure changes observed with scanning electron microscopy images from experimental samples. We post-process the LAMMPS simulated datasets at 1.5 and 5 mg/mL, both to image the structure transition and compare with scanning electron micrographs and to show that the 3.33-fold increase in concentration induces closer proximity of interacting electrostatic and hydrophobic domains, thereby amplifying the proximity-based strength of the interactions.

Figure 1:

The representation of one mucin monomer in the model. Termini (red) experience hydrophobic interactions, while sugar domains (purple) and cysteine knots experience electrostatic interactions.

Figure 2:

Images taken from simulations of a single dimer (a-c) and single chain (d-f) at various interaction strengths.

Figure 3:

Rheological data from simulations of a single chain, 1.5 mg/mL mucin simulation, and 5.0 mg/mL mucin simulation, respectively. All chains have bead number N = 570.

Figure 4:

SEM images of various experimental mucus samples.

Figure 5:

A selection of images generated from simulations at various interaction strengths of HBE mucus with mucin concentration 1.5 mg/mL. All simulations reached quasi-equilibrium states, which are depicted in these images.

Figure 6:

A selection of images generated from simulations at various interaction strengths of HBE mucus with mucin concentration 5.0 mg/mL. All simulations reached quasi-equilibrium states, which are depicted in these images.

Figure 7:

The proportion of beads in all non-baseline simulations which experience attractive interactions of various strengths. Solid lines represent simulations with mucin concentration 1.5 mg/mL, and dashed lines represent simulations with mucin concentration 5.0 mg/mL. Gray lines represent a force of 1.0 k_BT/τ and a proportion of 0.5. Blue lines represent 0.4 k_BT, orange represents 0.5 k_BT, yellow represents 0.6 k_BT, purple represents 0.7 k_BT, and green represents 1.0 k_BT.

Figure 8:

Plots of the stress relaxation function at the interaction strengths tested. Thicker lines correspond to stronger interactions.

Figure 9:

The storage modulus (solid lines) and loss modulus (dashed lines) at each interaction strength tested at a concentration of 1.5 mg mucin per mL mucus. Thicker lines correspond to stronger attractive interactions.

Figure 10:

The storage modulus (solid lines) and loss modulus (dashed lines) at each interaction strength tested at a concentration of 5.0 mg mucin per mL mucus. Thicker lines correspond to stronger attractive interactions.