Stanford Medicine investigators and their collaborators have designed a compound that’s uniquely capable of blocking excessive mucus secretion — a hallmark of several serious respiratory disorders.
March 23, 2022 - By Bruce Goldman
In the right amounts, at the right consistency, mucus is a lung’s best friend. But if there’s too much of it, or if it’s too sticky, it can betray the organ it was meant to serve.
Stanford Medicine investigators and their collaborators have designed a compound that’s uniquely capable of blocking excessive mucus secretion — a hallmark of several serious respiratory disorders — while not interfering with the necessary low-level secretion of the gummy substance.
The discovery, described in a study published March 23 in Nature, could improve the lives of millions who suffer from airway obstruction due to excess mucus.
“This is the first compound that specifically alleviates the pathological hypersecretion of mucus common to cystic fibrosis, chronic obstructive pulmonary disease, asthma, viral respiratory infections and more,” said Axel Brunger, PhD, a professor of molecular and cellular physiology, of neurology and neurological sciences and of photon science.
Brunger, a Howard Hughes Medical Institute investigator, shares senior authorship of the study with Burton Dickey, MD, a professor of pulmonary medicine at the MD Anderson Cancer Center in Houston, and Manfred Frick, PhD, a professor of medicine at Ulm University in Germany. The study’s lead author is Ying Lai, PhD, a former postdoctoral scholar in Brunger’s lab and now an assistant professor at Sichuan University in Chengdu, China.
When mucus causes problems
Airway blockage by overly sticky mucus is a major medical problem. In the United States alone, more than 25 million people have asthma, with about 2.5 million unable to find relief from existing drug treatments. Medications for chronic obstructive pulmonary disease, which also affects about 25 million in the United States, are even less likely to work. About 1 in 20 people incur acute bronchitis each year. About 30,000 Americans have cystic fibrosis.
A healthy person’s mucus is 97% water. Its main distinguishing constituent — a long, stringy protein called mucin — is produced by secretory cells lining the surface, or epithelium, of the nose, throat, bronchi and small airways in the lungs. Lengthy chains of sugar molecules sprout from mucin’s surface, predisposing the protein to absorb water.
Mucin molecules readily cross-link into networks, forming the slimy gel we know so well. Secreted at moderate levels, mucus coats airway surfaces, serving as a lubricant and a barrier as well as a straitjacket for encapsulating microbial pathogens. The encased microbes are driven out of the lungs and upward in the airways by hairlike structures called cilia that project from cells abundant in airway epithelia.
Inflammation, often elicited by microbial pathogens, shifts mucin secretion into overdrive. That’s great for trapping and washing away offending microbes, but if the inflammation persists it can cause trouble. What’s more, the newly secreted mucus often contains a higher-than-normal mucin content. The thereby thickened mucus can congeal into rubbery pads, or plaques, stifling gas exchange in the lungs and impeding airway cilia’s upward pumping of mucus-entrapped pathogens.
The brain connection
Next time you’re sneezing your brains out or coughing up a lung, consider that lungs and brains have something very much in common: Secretion in both organs works similarly.
Neurotransmission — the relaying of impulses from one neuron to the next — depends on the carefully timed secretion of special chemicals called neurotransmitters from neurons’ tips. Those chemicals are routinely caged inside tiny bubbles, called vesicles, situated near a neuron’s surface membrane like containers on a ship, waiting to be unloaded.
Spontaneous merging of these vesicles’ walls with the neuron’s outer membrane happens all the time, resulting in a fairly constant, low-level expulsion of the vesicles’ stored contents from the neuron. But electrical impulses traveling through the neuron trigger an abrupt, temporary influx of calcium into the neuron. When the calcium binds to a protein called synaptotagmin, on the vesicles’ surfaces, synaptotagmin teams up with other proteins to tug some of the vesicles, winch-style, closer and closer to the neuron’s surface membrane until their enclosing membranes fuse with it, spilling the vesicles’ contents into the surrounding environment. The chemicals can then diffuse to nearby neurons, exciting or inhibiting activity in those recipient cells.
In landmark papers published over the past 25 years, Brunger and various co-authors discovered the molecular details of how synaptotagmin and other proteins cooperate to orchestrate neurotransmission.
Work by others, including Dickey, has shown that upsized mucin secretion in the airways works in much the same way, involving analogous vesicles (filled with mucin instead of neurotransmitters) and synaptotagmin and collaborating proteins. In the lungs, inflammation induces calcium release from internal compartments in secretory cells, and certain diseases exacerbate this release, triggering massive vesicle-and-outer-membrane fusion and the wholesale “hypersecretion” of mucin.
A closer look at synaptotagmin
Using a strain of bioengineered mice that were deficient in synaptotagmin, the scientists showed that the animals were resistant to mucin hypersecretion under inflammatory circumstances that would ordinarily have led to an asthma-like allergic reaction. In these mutant mice, secretory cells’ expulsion of mucin was reduced by about 70%, and mucus-plaque formation in the airway was reduced by almost 75%, compared with wild-type mice. Synaptotagmin was clearly a legitimate therapeutic target.
Brunger’s and Dickey’s groups designed a small protein snippet, or peptide, called SP9, that blocked synaptotagmin’s calcium-triggered interaction with its cooperating proteins in mice. SP9 is essentially a synthesized segment of a protein to which synaptotagmin binds. To stabilize SP9’s shape and ensure its effectiveness, the team bolstered the peptide with biochemical braces.
In lab dishes and using advanced optical imaging, Lai, the then-postdoctoral scholar, created stripped-down models of mucin-containing vesicles and mucin-secreting cells’ outer membranes. He was able to demonstrate, in this simplified system, that SP9 preferentially inhibits inflammation-driven hypersecretion of mucin, mucus’s chief protein constituent.
But the researchers needed to find a way for SP9 to transit the cell’s outer membrane and get inside the secretory cell, where the action is. Working with Lai, Frick — the professor at Ulm University — solved the problem by conjugating SP9 to yet another peptide, PEN, that’s known to be excellent at penetrating cell membranes. Frick’s group confirmed, in human cells cultured from lung-tissue biopsies, that the PEN-SP9 conjugate entered the secretory cells and blocked their inflammation-induced mucin secretion. Dickey’s lab at MD Anderson then showed that PEN-SP9 administration in mice not only stymies inflammation-induced mucus secretion but substantially reduces the plaque-plugged area in the mice’s lungs. So, there’s a possibility that it could be effective as a therapy in humans.
Toward clinical trials
Brunger hopes to see an optimized version of SP9 enter clinical trials within the next two to three years.
“This didn’t happen by accident,” he said. “It’s been a long road with many failures, and it shows the importance of basic research — you just keep putting one foot in front of the other, and then suddenly you have something.”
Brunger and Dickey have filed a joint provisional patent application with Stanford’s Office of Technology Licensing and the MD Anderson Cancer Center’s Office of Technology Commercialization.
Other Stanford co-authors of the study are postdoctoral scholar Kailu Yang, PhD; former postdoctoral fellow Qiangjun Zhou, PhD; basic life research scientist Jeremy Leitz, PhD; former graduate student John Peters, PhD; research specialist Richard Pfuetzner; and research technician Luis Esquivies.
In addition to Frick and Dickey, other researchers at MD Anderson and Ulm University contributed to the study.
The study was funded by the National Institutes of Health (grants R37MH63105, R01HL129795 and R21AI137319), the Howard Hughes Medical Institute, the Cystic Fibrosis Foundation and the German Research Foundation.
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