Mucus Structure, Viscoelastic Properties, and Composition in Chronic Respiratory Diseases

Abstract

The respiratory mucus, a viscoelastic gel, effectuates a primary line of the airway defense when operated by the mucociliary clearance. In chronic respiratory diseases (CRDs), such as asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF), the mucus is overproduced and its solid content augments, changing its structure and viscoelastic properties and determining a derangement of essential defense mechanisms against opportunistic microbial (virus and bacteria) pathogens. This ensues in damaging of the airways, leading to a vicious cycle of obstruction and infection responsible for the harsh clinical evolution of these CRDs. Here, we review the essential features of normal and pathological mucus (i.e., sputum in CF, COPD, and asthma), i.e., mucin content, structure (mesh size), micro/macro-rheology, pH, and osmotic pressure, ending with the awareness that sputum biomarkers (mucins, inflammatory proteins and peptides, and metabolites) might serve to indicate acute exacerbation and response to therapies. There are some indications that old and novel treatments may change the structure, viscoelastic properties, and biomarker content of sputum; however, a wealth of work is still needed to embrace these measures as correlates of disease severity in association with (or even as substitutes of) pulmonary functional tests.

Keywords: asthma; biomarkers; chronic pulmonary obstructive disease; cystic fibrosis; low-field NMR; mucus; viscoelasticity.

Figure 1

Mediators of mucin expression and histopathologic changes in CRDs. CF is caused by CFTR mutations (with a cross on the CFTR molecule) instigating a IL-17-driven neutrophil chronic inflammation. CFTR dysfunction has also been found to be associated with COPD, in which mucus overproduction is also elicited by EGFR signaling. Allergic asthma is marked by IL-13 and EGFR-mediated activation of mucin expression and secretion. See text for further details. CFTR: Cystic Fibrosis Transmembrane Conductance Regulator; EGF: Epidermal Growth Factor; EGFR: EGF Receptor; HNP-1: Human Neutrophil Peptide-1.

Figure 2

Schematic representation of the airway surface layer (ASL) in normal (healthy) and pathological conditions. In order to improve clarity, the thicknesses of the periciliary and the mucus layers are not in scale. Adapted from Ref. [145].

Figure 3

Working principle of Low-Field NMR (LF-NMR). (A) In the presence of an external and constant magnetic field (0.37 T ≤ B₀ ≤ 2.43 T), the permanent dipole (μ–red arrow) of hydrogen atoms tends to align with the B₀ direction (conventionally, the Z-direction; t < t₀). More precisely, every μ starts rotating at the so-called Larmor frequency around B₀ (green arrow), forming a characteristic angle with B₀ (Z-direction). The vector sum of μ competing with all of the hydrogen atoms gives origin to the induced magnetization vector (M–blue arrow) that is parallel to B₀. (B) The application of a perturbation (a radio frequency pulse B₁ (violet arrow) perpendicular to B₀ and rotating in the XY plane at the Larmor frequency) provokes the progressive M rotation in the X-Y plane (t₀ < t ≤ t₁). (C) After B₁ removal (t > t₁), M comes back to the original B₀ direction (relaxation). The relaxation process, implying the disappearance of the M projection in the X-Y plane, is characterized by the so-called spin–spin or transverse relaxation time T₂. Interestingly, T₂ depends on the B₀ intensity, the temperature, and the chemical, physical, and topological characteristics of the environment embedding the water molecules carrying the relaxing hydrogen atoms [159].

Figure 4

(A) Schematic representation of the length scale dependence of viscosity in a nanoscopically heterogeneous fluid. Non-adhesive particles that are significantly smaller than the mesh spacing undergo Brownian diffusion and probe the microscopic rheology. As the particle size approaches the dimensions of the mesh spacing, particle movement becomes hindered by the mesh microstructure at short time scales, leading to a mesophase rheology regime. Particles that are significantly larger than the mesh spacing probe the bulk or macroscopic rheology of the gel. (B) Schematic comparison of non-mucoadhesive rheological nanoprobes and conventional polymeric particles. Conventional nanoprobes are immobilized to mucin fibers via adhesive interactions. Their strongly hindered motion, as reflected by the small dimensions of the traces, suggests a markedly higher viscoelastic environment than the true local viscoelasticity of mucus. In contrast, the motion of non-mucoadhesive nanoprobes correctly reflects the local viscous and elastic contributions from the mucus mesh architecture. Reprinted from Ref. [11] Advanced Drug Delivery Reviews, Vol. 61, Samuel K. Lai, Ying-Ying Wang, Denis Wirtz, Justin Hanes, Micro- and macrorheology of mucus, pages 86–100, 2009, with permission from Elsevier.

Figure 5

(A–C) Schematic illustrations showing the effects of the relative water-drawing powers of the mucus gel and the PCL. (B) Normal state: The osmotic modulus of normal mucus is smaller than that of the PCL, represented by a green spring (K_mucus) with a diameter larger than a purple spring (K_PCL = K₀). The volume of water in the system is depicted by the fixed distance between two plates. (A) Increased hydration: Water added to the healthy airway surface (distance between plates increased) with K_mucus < K₀ preferentially enters and thus dilutes the mucus layer, leaving the PCL unchanged. The resulting osmotic modulus of the mucus layer is much smaller than that of the PCL (K_mucus << K₀). This state is depicted by the increased length and diameter of the green spring, with no change in the purple spring. (C) Dehydrated state (plates close to each other): As water is removed, it first preferentially leaves the mucus gel because of its lower osmotic modulus. Further dehydration leads to removal of water from both the mucus gel and the PCL. The moduli of both layers are increased and equal, represented by smaller diameters of shortened springs. This state corresponds to diseased airways (COPD and CF). From Ref. [145] Science, Vol. 337, Brian Button, Li-Heng Cai, Camille Ehre, Mehmet Kesimer, et al., A Periciliary Brush Promotes the Lung Health by Separating the Mucus Layer from Airway Epithelia, pages 937–941, 2012. Reprinted with permission from AAAS.