Trachea (wind pipe) lining, colored scanning electron micrograph. Cilia are green and goblet cells are purple. Credit: Steve Gschmeissner/SPL/Getty
Mucus production probably ranks low on most lists of fascinating biological processes. In the right amounts, however, mucus can be our slimy savior. It keeps some of our moving parts lubricated and repels harmful substances. Otherwise we try not to think about it.
Except for those of us who are faced with goblet cell metaplasia. The condition crops up in common airway diseases—including asthma—and has no available direct treatment.
Goblet cells get their cheerful name from the wineglass-like-way they appear in profile, when sliced and stained. In the lung, goblet cells are packed into the surface of small airways alongside ciliated cells, whose hair-like protrusions move the mucus along, helping to remove microorganisms and toxins.
This process can go haywire in asthma, cystic fibrosis and chronic obstructive pulmonary diseases. All these ailments feature too many goblet cells and too few ciliated cells. The standard treatments for these illnesses are bronchodilators and steroids, which help to open up airways and reduce inflammation but don’t directly stop the production of mucus.
However, scientists believe they may be able to change this by restoring the balance of goblet and ciliated cells, especially since both cell types are derived from basal cell progenitors. “We think that gives us a good opportunity to harness a fundamental cellular process like differentiation to restore the normal balance of cell types that are deregulated in these diseases,” says Aron Jaffe of the Novartis Institutes for BioMedical Research (NIBR).
At NIBR’s Developmental & Molecular Pathways department in Cambridge, MA, Jaffe’s lab builds three-dimensional (3D) models of the cellular processes that organize epithelial cells into functional tissues.
In this case, with “a combination of experience and a little bit of luck,” his group successfully modeled a key facet of an airway in a dish—more precisely, the first human “bronchospheres” combining basal, goblet, and ciliated cells behaving much as in actual airways of the lungs. Moreover, they scaled up their bronchosphere system to fill 384-well plates, allowing high-volume screening of agents.
Described in a Cell Reports paper published online in December 2014, the 3D models helped to pinpoint a novel drug target for goblet cell metaplasia.
Jaffe’s group first screened the secretome—all the predicted secreted proteins in the genome—to see what might encourage basal cells to choose life as ciliated cells rather than goblet cells.
Using a secretome library courtesy of the Genomics Institute of the Novartis Research Foundation (GNF), and working with colleagues in the NIBR High Throughput Biology group and Henry Danahay from the Respiratory Diseases area, the researchers examined the effects of more than 4,000 kinds of proteins on the bronchospheres.
They were not terribly surprised to find that inflammatory cytokines such as IL-13 and IL-17A were leading culprits in creating the condition.
“Interestingly, though, none of the secreted proteins actually inhibited goblet cell formation and accelerated ciliated cell formation, which was really what we were looking for,” Jaffe notes. “We found a number of treatments that caused phenotypes similar to the disease, which is exactly what we didn’t want.”
He and his colleagues then took the classic route of investigating pathways known to regulate cell fate decision. One very likely suspect was Notch, a transmembrane protein that’s “basically involved in every cell fate decision in your body,” he says.
There are four flavors of Notch receptors, and the researchers teased apart the roles of each version in bronchosphere models of goblet cell metaplasia.
It worked. They discovered that inhibiting Notch2 shifts the balance of cell production back toward ciliated cells. And following up by inhibiting Notch2 in mouse models of asthma, they found equally promising results.
“We’ve really had no effective anti-mucus agents for these airway diseases,” comments Robert Strieter, NIBR’s Global Head of Translational Medicine for Respiratory Diseases. “This is a whole different approach, with a novel opportunity to deliver compounds to the lung that could modulate this biology and have an impact on cell fate.”
Experimental Notch inhibitors administered in trials for other illnesses, notably certain cancers, sometimes have created undesirable side effects. In this case, however, “we can potentially deliver compounds directly to the lung that don’t necessarily escape the lung to affect other organs,” Strieter points out.
Jaffe’s group is now riding the bronchosphere model to study other pathways in goblet cell metaplasia that may help to drive the excess mucus manufacture.
“This model is a great tool,” says Strieter. “It recreates a 3D airway in a dish, and it reconstitutes the right cell types of the airway epithelium, which is critically important. This is the only way that one could really go about phenotypic screening of compounds that would block the condition.”