New Understanding Of Mucus Clearance Answers Some Long-standing Questions

Connie K. Ho for — Your Universe Online

Researchers from the University of North Carolina, Chapel Hill recently published a study on their work to understand how human lungs clean out mucus from colds and allergy, making the airways free of foreign matter that may be toxic or infectious to the body.

The findings, published in a recent issue of the journal Science, will help scientists identify issues related to human lung diseases like asthma, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF).

“The air we breathe isn’t exactly clean, and we take in many dangerous elements with every breath,” remarked co-author Michael Rubinstein in a prepared statement. “We need a mechanism to remove all the junk we breathe in, and the way it’s done is with a very sticky gel called mucus that catches these particles and removes them with the help of tiny cilia.”

The researchers believe that there are different parts of the lung that assists in this cleaning process.

“The cilia are constantly beating, even while we sleep,” continued Rubinstein in the statement. “In a coordinated fashion, they push mucus containing foreign objects out of the lungs, and we either swallow it or spit it out. These cilia even beat for a few hours after we die. If they stopped, we’d be flooded with mucus that provides a fertile breeding ground for bacteria.”

Previous research described a “gel-on-liquid” model of mucus where a watery “perciliary” layer lubricated and separated mucus from epithelial cells found along human airways; however, the scientists believe that this explanation is not correct and doesn´t explain why mucus stays in its own separate layer.

“We can’t have a watery layer separating sticky mucus from our cells because there is an osmotic pressure in the mucus that causes it to expand in water,” explained Rubinstein in the statement. “So what is really keeping the mucus from sticking to our cells?”

In the project, the investigators utilized a mix of imaging techniques to see how the dense meshwork in the perciliary layer of human bronchial epithelial cell cultures. The normal flow of mucus was helped by a layer of protective molecules that stopped sticky mucus from reaching the cilia and epithelia layers. The researchers believe that a “gel-on-brush” method of mucus clearance could be successful; the mucus would move atop a brush-like perciliary layer rather than a watery layer. They believe that this model could help the human mucus exit more efficiently.

“This layer–this brush–seems to be very important for the healthy functioning of human airways,” noted Rubinstein in the statement. “It protects cells from sticky mucus, and it creates a second barrier of defense in case viruses or bacteria penetrate through the mucus. They would not penetrate through the brush layer because the brush is denser.”

The researchers believe that, when the mucus layer becomes too tense, it can bump into the perciliary brush, make the cilia fall, and adhere to the cell surface.

“The collapse of this brush is what can lead to immobile mucus and result in infection, inflammation and eventually the destruction of lung tissue and the loss of lung function,” concluded Rubinstein in the statement. “But our new model should guide researchers to develop novel therapies to treat lung diseases and provide them with biomarkers to track the effectiveness of those therapies.”

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