Supramolecular dynamics of mucus

Abstract

Our purpose here is not to address specific issues of mucus pathology, but to illustrate how polymer networks theory and its remarkable predictive power can be applied to study the supramolecular dynamics of mucus. Avoiding unnecessary mathematical formalization, in the light of available theory, we focus on the rather slow progress and the still large number of missing gaps in the complex topology and supramolecular dynamics of airway mucus. We start with the limited information on the polymer physics of respiratory mucins to then converge on the supramolecular organization and resulting physical properties of the mucus gel. In each section, we briefly discuss progress on the subject, the uncertainties associated with the established knowledge, and the many riddles that still remain.

Figure 1.

Block copolymer model for airway mucus. The typical periodic hydrophilic/hydrophobic domains and lineal conformation give airway mucins the characteristic profile of block copolymers. The elementary length structure in this model is a binary amphiphilic triblock copolymer consisting of alternating hydrophilic glycosylated polyanionic blocks—resembling densely brush-grafted blocks—flanked by globular peptide hydrophobic blocks (Waigh and Papagiannopoulos 2010). (A) In extended conformation—outside the granule—blocks are interconnected forming large linear chains with contour lengths reaching up to several micrometers. (B) While inside the granule, chains are condensed with polyanionic glycosylated blocks fully shielded by counterions and folded forming loops stabilized by H and Ca links. (C) These U-shaped blocks are interconnected, forming long densely collapsed lattices. (E) Lattices can associate laterally via hydrophobic bonds and assemble into large supramolecular nematic liquid crystalline ordered arrays as shown in D (planar view) that in side view may hold triplet or even quadruple interlinked collapsed chains. (F) Supramolecular arrays, like those illustrated in E, are interconnected, forming a tangled random network. In condensed phase, water inside the matrix is constrained to within the Debye field of the polymer charged sites, with a dielectric permittivity that keeps counterions bound. Upon formation of the secretory pore, release of protons and entry of water into the gels matrix result in a drastic change of dielectric property, changing long-range solvent-mediated dynamic interactions among blocks, triggering high-cooperativity counterion dissociation from binding sites. Donnan swelling—resulting from Na/Ca exchange—results in splitting Ca cross-links and repulsive polyanionic interactions that quickly unfold the mucin arrays (represented by boxes). The ordered topology of the network (expansion of the boxes) allows fast isotropic low-dissipative spreading out of the mucus chains without forming knots and loops that would otherwise be expected in a randomly tangled matrix expanding only on the basis of polymer reptation. (Modified from a napkin sketch drawing during our last sushi brainstorming session with my late friend Toyo Tanaka in Boston, early May 2000.) (G) An actual polarized microscopy micrograph of nematic liquid crystalline formation in mucus (Viney et al. 1993a).

Figure 2.

Illustrated here are data from studies published earlier (Verdugo 1998). Measurements from digitized video images show that the radial expansion of exocytosed goblet cell granules follows characteristic first-order kinetics, lending the process to be formalized in light of Tanaka’s theory of swelling of polymer gels (Tanaka and Fillmore 1979). (Inset) A typical plot in which the continuous line is a nonlinear least-squares fitting of the data points to r(t) = r_f – (r_f – r_i) × e^−t/τ, where r_i and r_f are the initial and final radii of the granule and τ is the characteristic relaxation time of swelling, with r_i assumed to be 0.25 µm. Data points were collected as soon as the mucus micro gels became visible in phase contrast microscopy images. Notice that the square of the final radius is a linear function of τ. The slope of this line has the characteristic dimensions of diffusion (cm²/s⁻¹), and, according to Tanaka’s theory, it reflects the diffusion of the gel in the solvent. Mucus granules exocytosed by cultured goblet cells from biopsied nasal polyps of normal subjects and from CF patients swell when cell equilibrated in Hanks’ solution containing 1 mm Ca (pH 7.2). D in normal mucus (open squares) reached 3.1 ± 1.2 × 10⁻⁷ cm²/s. CF mucus granules swelled at almost an order of magnitude slower than normal (closed circles), yielding a D = 7.4 ± 2 × 10⁻⁸ cm²/s and also reaching a much smaller final swelling volume equilibrium. These differences become much more pronounced when cells were equilibrated in Hanks’ solution containing 2.5 mm Ca. In this case, exocytosed mucus from cells of normal subjects reached D = 2.5 ± 0.9 × 10⁻⁷ cm²/s, whereas in CF mucus, D reached only 9.1 ± 1.8 × 10⁻⁹. D values correspond to the average ±SEM of 96 exocytosis-swelling events. These observations suggest that, in addition to the CF defective Ca-HCO₃ chelation, it is likely that mucin–Ca affinity might be increased, resulting in decreased Na/Ca exchange in CF. Alternatively, hydrophobic bonding, still largely unexplored, might be increased in CF.