Molecules called stapled peptides blocked the secretion, or release, of mucin proteins from airway cells and lessened mucus buildup in the lungs of mice in a study.
According to its researchers, these findings led to the creation of an inhaled aerosol that was tested in the mice and, with further refinement, might be able to block excessive mucus production in people with lung disorders like chronic obstructive pulmonary disease (COPD).
“Our research has created the first drug that would stop the secretion of mucins in its tracks,” Burton Dickey, MD, a professor of pulmonary medicine at the University of Texas MD Anderson Cancer Center and the study’s co-senior author, said in a press release.
The study, “Inhibition of calcium-triggered secretion by hydrocarbon-stapled peptides,” was published in the journal Nature.
COPD, an inflammatory disease, causes the lungs to generate high amounts of mucus that block airways, leading to frequent coughing episodes and shortness of breath. Current treatments work by opening up the airways or suppressing inflammation.
“Thick mucus can block the airways and cause symptoms ranging from a mild cough to very serious decreases in lung function,” Dickey said. “Most drugs for these conditions work to reduce inflammation or expand the airways to help people breathe better, but mucus is the most serious issue.”
Mucins, the primary protein component in mucus, in healthy lungs are gradually secreted into the airways, where they combine with water to form a protective mucosal layer. But in people with COPD and lung conditions like asthma and cystic fibrosis (CF), high volumes of mucins are released, resulting in thick mucus.
Suppressing mucin release is a potential approach to easing disease symptoms.
Mucins are first produced in the epithelial cells that line the airways, then encapsulated in tiny membrane-bound bubbles called vesicles. These vesicles fuse with the main cell’s membrane to release mucins into the airway space.
Previously, Dickey’s lab demonstrated that mucin vesicle fusion is mediated by synaptotagmin proteins together with the SNARE protein complex in the presence of calcium. One of the synaptotagmin proteins, synaptotagmin-2, became active with stimulation.
“We built up a picture of what the secretory machinery looked like and we knew all of the major players,” Dickey said. “Once we had an idea of how all the pieces worked together, we determined synaptotagmin-2 (Syt2) was the best protein to target to block mucin secretion because it only becomes activated with a high level of stimulation.”
Blocking Syt2 activity, he added, “should prevent sudden massive mucin release without impairing slow, steady baseline mucin secretion that is required for airway health.”
In collaboration with Stanford Medicine and Ulm University, researchers first set out to validate Syt2 as a therapeutic target to suppress mucin release. To accomplish this, they bred mice that lacked the gene encoding Syt2 specifically in airway epithelial cells.
After stimulating mucin production, they found mucus secretion was reduced by 74% in the lungs of these animals compared with healthy animals serving as controls.
To suppress mucin release, the researchers created the so-called stapled peptides — chemically modified peptides specifically engineered to maintain their shape and bind to their target with greater stability — from fragments of SNAP-25A, one of the protein components of the SNARE complex. This allows them to bind to and block Sty, and experiments confirmed one of the stapled peptides, called SP9, bound to Sty proteins.
In isolated vesicle tests, SP9 specifically blocked calcium-triggered fusion of vesicles to membranes, as well as in two types of vesicles that mimicked mucin secretion. In a control experiment to test for specificity, when Syt2 was left out, this blocking effect of SP9 was not evident.
To determine whether SP9 could be delivered to human airway epithelial cells, researchers attached stapled peptides to molecules called cell-penetrating peptides (CPPs). Short-term treatment with SP9 attached to CPPs substantially and lowered stimulated mucin secretion by up to 86%.
SP9 without CPPs and non-stapled peptides with CPPs did not change stimulated secretion. Notably, peptide treatments did not affect the gradual secretion of mucins.
Short-term treatment of mice with an inhaled formulation of CPP-SP9 also resulted in substantial peptide uptake into airway epithelial cells, lowering stimulated mucin secretion by 82.3% and significantly reducing mucus buildup in the animals’ lungs by 33.1%.
CPP-stapled peptides can “serve as a starting-point therapeutic without incurring toxicity due to reduced baseline mucin secretion,” the scientists wrote.
An optimized treatment “could be used both as a single-dose therapy in an acute exacerbation of airway disease … and as a drug delivered repeatedly in a patient in whom control of mucus hypersecretion is difficult to achieve,” they added.
If its effectiveness and safety are upheld in clinical trials in patients, “an inhaled drug like this could help someone during an acute attack of airway disease by stopping the rapid secretion of mucin and, by extension, avoiding production of thick mucus,” Dickey said. “You can’t move air through an airway that’s plugged.”