A Better Mucus Model | The Scientist Magazine®

Mucus is more than just the sticky snot that comes from your nose. This protein-rich goop is found in many other organs, including the lungs and intestines, where it forms a protective layer that traps pathogens and prevents them from penetrating the organ. The flow of mucus across the organ’s surface, propelled by cilia, can even help move microbes around, transporting them away from critical structures.

The tacky, slimy consistency of mucus is essential to its functions, and changes in the physical properties of mucus can contribute to disease. For example, in cystic fibrosis, lung mucus becomes thicker and harder for cells to push across the organ’s surface, potentially leading to pneumonia as pathogen-laden mucus sits in the lungs. 

This has motivated researchers to explore mucus through the lens of materials science and study the sticky substance’s physical properties, such as its viscosity, elasticity, and how it flows. Typically, scientists scrape the mucus off the organ, but removing it from its environment can change its physical properties—making it more watery, for example.¹ “If you scrape off that mucus, you irretrievably alter the viscoelasticity,” said Gerald Fuller, a chemical engineer at Stanford University. “Just the action of removing it to make the measurement really defeats the purpose. It is not a good replica of what’s actually sitting on the cells.”

In a recent collaboration, Fuller and Sarah Heilshorn, a materials scientist at Stanford University, designed a system to measure the properties of mucus without removing it from the cells that produce it.² The researchers used the system, which models part of the intestines, to study the impact of an infection-induced immune molecule on mucus viscoelasticity. They published their findings in APL Bioengineering.

Heilshorn’s team devised a method to grow a layer of intestinal cells just one cell thick so that the mucus they produced would collect on top of the cells. This made the mucus easily accessible for a magnetic microwire rheometer that Fuller’s lab had previously created.³ Using the thin wire probe positioned at the surface of a substance, they measured physical properties, such as viscoelasticity, without relocating the substance. 

“It has the disadvantage of being a culture system, which is a step removed from being in vivo,” said David Hill, a mucus researcher at the University of North Carolina at Chapel Hill who was not involved in the study. “But when you can make a measurement in a less perturbed system, it opens up a lot of possibilities for understanding [the mucus].” 

As the researchers had hypothesized, the mucus in contact with the cells differed from the mucus removed from its original environment, becoming softer after removal.  With this model, the researchers had a more realistic setting to test how mucus changes under different biological conditions. They were especially interested to explore how parasites that have evolved to survive in the intestines, such as the roundworm Nippostrongylus brasiliensis, might elicit a host response that alters the protective mucus layer. When these parasitic worms invade the gastrointestinal tract, they trigger an immune response that includes potent molecules such as interleukin 13 (IL-13). In previous studies of airway cells in asthma, which also involves an IL-13 response, other researchers had found that the mucus became thicker and made it harder for the ciliated cells to beat.⁴ 

The Stanford University team hypothesized that the IL-13 triggered by worm infections might also affect the consistency of intestinal mucus. To test this, they used their new system to grow intestinal cells from the duodenum—the portion of the small intestine that connects to the stomach—and treated the cells with IL-13. 

To their surprise, IL-13 did not substantially affect the viscoelasticity of the mucus, despite changes in the mucus-related genes expressed in the cultured cells. The researchers proposed that IL-13 might require the presence of other inflammatory molecules to change the mucus’s physical qualities. 

Even though they didn’t observe a change, describing the mucus’s realistic response to IL-13 wouldn’t have been possible without the new system, Fuller said. He thinks other researchers studying mucus could also benefit from this technology, and since the materials are relatively easy to acquire, he has been helping other labs set up the system. Fuller is also working on extending the system to the airways to study mucus in asthma and cystic fibrosis, while Heilshorn is using it to study Crohn’s Disease. 

Hill said the system could also help scientists to identify new ways to manipulate mucus, possibly leading to fixes for mucus that is too thick or too watery in certain diseases. “You could use [this system] as a way to test or screen therapeutic compounds,” he said. “That can help you advance drug discovery.”

References

1. Howard RL, et al. Biochemical and rheological analysis of human colonic culture mucus reveals similarity to gut mucus. Biophys J. 2021;120(23):5384-5394.
2. Cai PC, et al. Air–liquid intestinal cell culture allows in situ rheological characterization of intestinal mucus. APL Bioeng. 2024;8(2):026112.
3. Braunreuther M, et al. Nondestructive rheological measurements of biomaterials with a magnetic microwire rheometer. J Rheol. 2023;67(2):579–588.
4. Laoukili J, et al. IL-13 alters mucociliary differentiation and ciliary beating of human respiratory epithelial cells. J Clin Invest. 2001;108(12):1817-1824.