Airway Surface Liquid and Respiratory Mucus - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 766

suppressor adenomatous polyposis coli (APC), GRB2 adaptor, P53 transcription
factor, ERK1 and ERK2 kinases, estrogen receptor- α (NR3a1), and HSP70 and

HSP90 chaperones.
Mucus contains bacteriostatic components, such as lysozyme to break down
pathogens and lactoferrin to limit iron availability to bacteria [ 1558 ]. Secretory
immunoglobulin-A produced by plasmocytes in mucus prevents bacterial adhesion
by binding to the bacterial surface within the mucus.
The mucus in goblet cell granules is condensed. Negative polyanionic charges
within the secretory granules are neutralized by calcium ions. When the granule
content is released across a membrane pore into the airway lumen, calcium ions
diffuse and polyanionic charges repulse, leading to mucus hydration. Owing to
its composition, condensed intragranular glycoproteic mucins expand up to several
hundred-fold once released in the airway lumen.
Once secreted, the mucin gel expands almost instantaneously. It is able to
increase in volume 500-fold in about 20 ms. Mucins then mix with proteins, lipids
and glycoconjugates in order to form a dilute aqueous solution, the mucus.
Mucins increase mucus viscosity. Protein Muc5b has especially a strong ten-
dency to polymerize and form a gel. Mucins also function in agglutination and
opsonization of foreign particles and pathogens, thereby participating in the innate
immune system.

12.4.2 Other Proteic Constituents of Mucus

Mucus glycoproteins are important determinants of mucus viscoelasticity. Proteo-
glycans are a major source of mucus hydration. Mucus proteins have a protective
effect against H + penetration into tissues. Buffer capacity is accompanied by pH-
dependency of mucus viscosity.
Some airway surface liquid components contribute to airway protection, as they
target pathogens (e.g., secretory IgA, proline-rich proteins, defensins, lysozyme,
and transferrin). Glycoprotein GP340 18 that has an antibacterial role complexes
with Muc5b in airways [ 1556 ]. It directly binds to bacteria or associates with
bacterial-binding collectin surfactant protein-D. Other mucus components modulate
the organization and hence the properties of the mucus gel. Addition of proteins
that can interact with mucins, such as immunoglobulin-A and -M or lysozyme, can
increase mucus viscoelasticity [ 1557 ].

18 A.k.a. Deleted in malignant brain tumor protein DMBT1, hensin, muclin, salivary agglutinin,
and pulmonary surfactant protein-D-binding protein.