Background An estimated 10% of adults have dry eye syndrome (keratoconjunctivitis sicca), a common ocular disease that occurs even more frequently among contact lens wearers. While the etiology of dry eye syndrome is still uncertain, the typical treatment for dry eye symptoms are lubricating eye drops.
Although such eye drops can be effective in relieving symptoms, they lack mucins—large glycoproteins that serve as molecular lubricants on many epithelial body surfaces, including the tear film. Patients suffering from dry eye lack mucins—especially the gel-forming MUC5AC—in their tear fluid.
“It is likely that, in the absence of a proper mucinous lubrication layer on the cornea surface, increased friction and tissue damage induced by contact lens sliding lead to discomfort,” wrote Oliver Lieleg, PhD, and colleagues from the Technical University of Munich (TUM) in Germany, in a recent study.
To relieve that friction and tissue damage in contact lens wearers and others who suffer from dry eye, the researchers sought to replace the missing mucin in the eyes of dry eye patients. But where to get it?
“Since the mucin MUC5AC occurs in the tear film physiologically, using purified mucin MUC5AC might be a good alternative to HA-based lubricants,” they wrote. “However, obtaining sufficient amounts of purified MUC5AC from human tear fluid is obviously very challenging and not a good choice for commercial purposes.”
So, they turned from the eye to the stomach—a pig’s stomach.
“Interestingly, this mucin variant is also the main constituent of stomach mucus, and here the amount of mucus present is considerably higher than in tears,” wrote Dr. Lieleg and colleagues. “Since pigs are phylogenetically relatively closely related to humans and purified porcine gastric mucins (PGMs) are already used as a spray to treat oral dryness, those mucins could be a good choice.”
When they tested the PGM on an experimental pig’s eye, they found that the mucin did indeed prevent frictional damage to corneal tissue, and could be a “powerful tool” to treat ocular surface dryness as an artificial eye drop or as a molecular coating for contact lenses. Such a coating could be included in contact lens storage solution, which they wrote would “spontaneously form a protective coating layer on the contact lens material by passive adsorption,” or be integrated during the manufacture of the hydrogel contact lens material itself.
In this interview, Dr. Lieleg describes how well and how easily mucins adhere to, and protect, the corneal surface. He also explains why mucins haven’t been used as an ocular lubricant up until now, and he discusses the powerful potential of purified mucins not only for dry eye, but for many other medical applications.
MDLinx: What has prevented other researchers from successfully using mucins as a lubricant eye drop or as a coating for contact lenses?
Dr. Lieleg: So far, the main limitation has been that commercially available mucins lack critical functions found in native mucins—and, in the past, most researchers made use of those commercial mucins in their experiments. For instance, when the lubricating potential of mucins is probed, solutions containing commercially-purified porcine gastric mucins behave similarly to pure water/buffer, especially at low sliding speeds, which are relevant for the motion of contact lenses on the cornea.
Our current hypothesis is that those commercial mucins somehow get damaged during the purification process, and this chemical damage compromises their functionality. I believe that a lack of functional commercial mucin has limited research in this area quite a bit. Together with a colleague from KTH Stockholm, Prof. Thomas Crouzier, PhD, I have been trying to convince the field to stop performing research with those commercially available mucins and to invest the time and effort to purify the mucins in the lab so that their functionality is maintained. However, this purification is not trivial and somewhat time consuming, which is why some labs are still working with the commercial molecules. We are, however, already collaborating with several research labs around the world to try to provide manually purified, functional mucins for research as good as we can.
MDLinx: You wrote in your paper: “With recent progress made in the purification of those PGMs … it is likely that this limitation will be not an issue any more in the near future.” What is the “recent progress” you’re referring to?
Dr. Lieleg: We recently teamed up with colleagues from our department who specialize in purification technologies. Together, we could improve the lab-scale mucin purification procedure that was originally introduced by the lab of Rama Bansil, PhD, at Boston University. The goal of our study was to increase the yield (ie, the amount of mucin purified from a pig stomach) and to reduce the time of the purification process—while maintaining the functionality of the purified mucins.
With the adapted and improved purification process, the amount of mucins we can purify per week has significantly increased compared to what I used to obtain as a postdoc when I was conducting the purification process myself in the lab of Katharina Ribbeck, PhD, at Harvard/MIT. Having larger amounts of mucins available is one requirement for performing macroscopic experiments such as the friction and wear studies we published with contact lenses and corneal tissue.
MDLinx: How do you get the mucin to adhere to the hydrogel lens? Or, if the mucin is in contact lens solution or eye drops, how to they remain on the surface of the eye for long-term comfort?
Dr. Lieleg: In previous research, we (and others) realized that mucins spontaneously adsorb to a broad variety of surfaces including glass, steel, and PDMS. In our recent paper, we were pleased to see that our purified mucins also spontaneously adsorb to contact lenses. This passive adsorption is strong enough that we still find a protective mucin coating after performing friction experiments on cornea samples.
This ability of mucins to bind to a surface and—if sheared off by mechanical forces—to readsorb to this surface from solution is also a key reason for the great lubricative properties of this molecule. This mechanism is called “sacrificial layer formation.”
MDLinx: If the “sacrificial layer” of mucin is sheared off by mechanical forces, how does it readsorb quickly enough before it is sheared off again? Is the mucin floating freely in between the cornea and the contact lens, so it’s readily available to readsorb to the corneal surface?
Dr. Lieleg: This is exactly how it works. Sheared-off mucins will float in the liquid film between corneal tissue and the contact lens and will be able to diffuse back to the contact lens where they can readsorb. There will always be an interchange of floating mucins and adsorbed mucins, and this cyclic exchange of bound and unbound molecules driven by mechanical shear is one mechanism that reduces friction.
MDLinx: Do you have any idea how long the mucin lasts in the eye, either as a lubricating drop or incorporated on a hydrogel contact lens?
Dr. Lieleg: This we have not tested in detail yet. We probed if the mucin coating is still present on the contact lens after performing a tribological sliding experiment on cornea samples. We found that this is the case, so the coating seems to be quite stable (or the readsorption of sheared-off mucins is efficient enough to replenish the coating). Adding mucins into the bulk volume of the contact lens hydrogel might, however, indeed provide additional benefits as it could improve the hydration of the material and convey antibacterial properties (mucins show antibacterial qualities).
MDLinx: What is your next step in this line of research?
Dr. Lieleg: In addition to further improving the purification procedure of mucins, one goal is to develop a synthetic macromolecule that reproduces the great lubricating properties of mucins. In the long run, being able to synthesize a macromolecule as a component for eye drops or as a contact lens coating would have several advantages over purifying the complex mucin glycoprotein from animal tissue. However, for this approach to be successful, we still need to better understand what molecular motifs on the mucin glycoprotein are required for the different functions of mucins we and others have discovered in the past years. We are currently looking into this, but there is certainly more research required until we possess a detailed enough understanding of the structure-function relationship that renders mucins such an outstanding lubricant.
MDLinx: Your paper suggests other possible uses for this mucin, such as for easier insertion of a catheter or an intubation tube. Can you imagine any other potential uses for purified mucins?
Dr. Lieleg: Because mucins combine a broad spectrum of outstanding properties, there are certainly many other fields of medical applications for purified mucins. For instance, mucin coatings not only reduce or even prevent bacterial adhesion to surfaces, mucins also possess anti-viral properties since they are able to bind different viruses and reduce their infectious potential. Thus, mucins could be great components for hydrogels in wound treatment. With more and more functional mucins being available to the scientific community, I am convinced that we will identify even more highly interesting properties of this fascinating molecule.
About Dr. Lieleg: Oliver Lieleg, PhD, is an Associate Professor of Biomechanics and the head of the Biopolymers and Bio-Interfaces lab in the Department of Mechanical Engineering at Technical University of Munich (TUM), in Munich, Germany.