The research traces these vital proteins back to their source, showing which proteins are produced by each of the three major types of human salivary glands, and showing how individual cells within a single gland can secrete different proteins. The project also identifies proteins in the mouth that seem to be coming from outside of salivary glands, from places such as epithelial tissues or blood plasma.
"Saliva is important for tasting, for digesting, for swallowing, for defending us from the pathogens that we are constantly inhaling and consuming. The proteins in our mouth form an army, if you will, that's working constantly to protect us. Before this, scientists had an idea of the proteins that are found in the mouth, but we didn't have a complete picture of where they were coming from. We're addressing this gap," says Omer Gokcumen, Ph.D., associate professor of biological sciences in the University at Buffalo College of Arts and Sciences.
To explain how our bodies make saliva, the scientists first sought to understand which proteins are produced by each major type of salivary gland—the parotid, submandibular and sublingual glands (humans have a pair of each).
To do this, the team used a method called transcriptomics to measure gene activity in each kind of gland. Gene activity provides insight into protein production, because each gene provides instructions for making a specific protein.
For instance, the study finds that the parotid and submandibular glands create a lot of salivary amylase, an enzyme that helps to digest starch, while the sublingual gland makes almost none. Meanwhile, the sublingual gland produces relatively large quantities of certain GalNAc transferases, a family of enzymes that's important in initiating a process called O-glycosylation that attaches a sugar to certain salivary mucin proteins. These are just a couple of examples.
"Our work reveals that even a gland type itself is not homogenous: the saliva-producing acinar cells, which were once thought to produce the same proteins, and thus be the same cells, actually synthesize distinct saliva proteins, thus indicating a new level of cellular diversity,"
"Long wished-for diagnostic applications of saliva for monitoring systemic well-being and disease will need to measure quantitative differences of biomarkers in saliva," Ruhl says. "One obstacle always hampering progress in this arena was that we did not know exactly which proteins were intrinsically produced by the salivary glands, and which proteins diffused into saliva from surrounding tissue leakage.
reference
Cell Reports (2020). DOI: 10.1016/j.celrep.2020.108402