Mucus unplugged
Mighty mucins
Under normal physiologic conditions mucus is composed largely of water (97%) and salts (2%), with the remainder consisting of entrapped globular proteins (0.7%) and mucins (0.3%), Dr. Dickey explains.
Yet those meager mucins pack a real punch, with the ability to absorb 300 times their mass of water after secretion, creating mucus of optimal consistency and viscoelasticity.
“Personally, I’ve never understood – maybe I should have paid more attention in physics – how a compound can absorb 300-fold its mass, but it does,” he said.
In a recent review article in the journal Clinical and Translational Medicine, Dr. Dickey and colleagues described how good mucus can go bad.
“[H]igh levels of mucin production from inflammatory stimulation (termed ‘mucous metaplasia’), followed by rapid release (together, termed ‘mucus hypersecretion’), can plug airways due to mucus volume expansion. In addition, if available lumenal liquid is insufficient, concentrated mucus of excessive viscoelasticity and adhesivity can cause mucus stasis,” they wrote.
Therapeutic strategies
In patients with CF, CFTR modulator therapy has markedly reduced but not eliminated the need for some patients to have mucolytic therapy, which may include dornase alfa, a recombinant human deoxyribonuclease that reduces the viscosity of lung secretions, hypertonic saline inhaled twice daily (for patients 12 and older), mannitol, and physical manipulations to help patients clear mucus. The manipulations can include both manual percussion and the use of devices for high-frequency chest wall oscillation.
Unlike in CF, where treating the underlying genetic pathology can help to resolve the thick, sticky mucus problems and thereby significantly reduce risk of infections and progressive lung damage, treatment of mucus metaplasia or hypersecretion in other diseases is aimed at symptomatic relief; it is still unclear whether symptomatic improvement of mucus overproduction would correlate with other disease-related outcomes, Dr. Kim and Dr. Dickey noted.
Potential therapeutic strategies to reduce excess mucus in the lungs include the use of mucolytic agents to thin secretions for more effective expulsion, decreasing mucus production through the use of an interleukin-13 (IL-13) inhibitor such as the anti-asthma agent dupilumab (Dupixent), and a novel strategy, still in the experimental phase, aimed at “disrupting the fusion of mucin storage granules with the cell membrane, thereby blocking secretion,” wrote Irina Gitlin, PhD, and John Fahy, MD, from the University of California, San Francisco, in Nature.
They were referring to research by Dr. Dickey and colleagues described in the same issue of Nature focusing on the inhibition of calcium-triggered mucus secretion by the use of hydrocarbon-stapled peptides, short chains of amino acids stabilized with a chemical bridge to a hydrocarbon molecule.
Knocking secretion down, but not out
The work has centered on decreasing overproduction of mucins with a focus on the signals for mucin production, including IL-13 and interleukin-1 beta, and on the signals for rapid release of mucins, including adenosine 5’-triphosphate (ATP), best known as an intracellular energy-storage module.
“But ATP is also steadily released by ciliated cells in response to the shear stress of tidal breathing, and it tells the neighboring secretory cells to slowly and steadily release mucin. But if the ciliated cells get stressed by any of a number of mechanisms, it can release a lot of ATP, and then the secretory cell can explosively release essentially all of its mucin content,” Dr. Dickey explained.
Other important signals for rapid release of mucins are acetylcholine and histamine, and all three of these agonists – ATP, acetylcholine, and histamine – cause a rise in intracellular calcium, which triggers calcium sensors that then lead to calcium-triggered membrane fusion and secretion.
Working as a postdoc in the Dickey laboratory, Dr. Evans had previously shown that deleting MUC5B in mice led to early development of serious lung abnormalities, some of which were fatal, indicating that MUC5B, a gene that is highly preserved in evolution, is essential for respiratory health.
This observation was later supported by a study of a family with a pattern of hereditary mucin deficiency caused by a homozygous loss-of-function mutation in MUC5B. The main subject in this study was an adult woman with unexplained bronchiectasis, impaired pulmonary function, and repeated Staphylococcus aureus infections. Her sibling, who also had the biallelic mutation, had extensive sinus disease with nasal polyps. Other siblings who were heterozygous for the mutation were asymptomatic but had mild functional lung impairment.
The trick for the investigators, then, was to figure out how to reduce stimulated release of stored mucins while still preserving normal release of mucins to allow for ciliary clearance of mucus, and Dr. Dickey and colleagues appear to have accomplished this, at least in mice.
They first validated as a potential therapeutic target a protein labeled synaptotagmin-2 (Syt2). Syt2 is a calcium sensor that is an essential part of the system that triggers calcium-triggered secretion. In a model for allergic asthma, mice with Syt2 deleted from airway epithelia had marked reductions in both stimulated mucin secretion and in mucus occlusion in airway lumens, but remained otherwise healthy with normal lung function.
Working with structural biologist Axel Brunger, PhD, from Stanford (Calif.) University, Dr. Dickey and coinvestigators developed and validated a peptide that could specifically inhibit Syt2, and found that it mimicked the action of the Syt2 deletion, preventing mucus occlusion in the allergic asthma model without adversely effecting normal production.
