Unmasking the Mechanism Behind Human Saliva Lubrication

Tribometer testing for saliva lubrication
Multiscale tribological and dynamic film‐forming properties. a) From left to right (upper): Schematic diagrams of the tribological experiments of mucin (M)‐ LF (L) across length scales using a) ball‐on‐disk set up at macroscale (load, N), b) pin‐on‐plate set up at microscale (load, mN), and c) nanografting‐assisted AFM at nanoscale (load, nN). d) From left to right (lower): Quantification of dynamic multilayered film formation (hydrated mass) on hydrophilic/hydrophobic substrates using QCM‐D by recording frequency and dissipation shift upon protein adsorption followed by measuring topographic height using AFM at nanoscale. Credit: A Self‐Assembled Binary Protein Model Explains High‐Performance Salivary Lubrication from Macro to Nanoscale, https://doi.org/10.1002/admi.201901549

Xerostomia, or dry mouth, is a condition in which the salivary glands found in the mouth do not produce enough saliva to keep the mouth wet. The condition of dry mouth often occurs as a side effect of taking certain medications, aging, autoimmune diseases, chemotherapy, cancer radiation therapy, nerve damage, or tobacco, alcohol, and recreational drug use.

Saliva plays a key role in a variety of everyday processes. Saliva neutralizes the acids produced by bacteria while simultaneously limiting bacterial growth, helping prevent tooth decay as well as bacterial, viral, and fungal infections. Additionally, saliva enhances an individual’s ability to taste, makes it easier to chew and swallow food, facilitates speech, and aids in the digestion of food. Saliva is over 99% water, with the other components including electrolytes and proteins.

Saliva forms spontaneously and is an unparalleled bio-lubricant that coats and protects biological surfaces from damage due to friction. Saliva is unique in the world of bodily lubricants, in that it coats the hardest mineral to one of the softest tissues in the mouth.

Approximately 10 percent of the entire American population and 30 percent of older people suffer from dry mouth. The percentage increases in women and the elderly, groups where the condition tends to be more prevalent. The impacts of dry mouth can range from being a minor irritant to having a major impact on one’s health. Complications from dry mouth may include increased gum disease and tooth decay, infections, bad breath, sores in the mouth, thrush (a yeast infection in the mouth), and even poor nutrition as a result of having difficulty with chewing and swallowing food.

When delving into the issue of dry mouth, researchers, led by Dr. Anwesha Sarka of the School of Food Science and Nutrition at the University of Leeds, have discovered the basic mechanism through which human saliva lubricates the mouth. The researchers from a variety of fields, including chemical engineering, food colloid science, mechanical engineering, and nanoscience, collaborated as a group, working to determine the mechanism responsible for human saliva lubrication. The research concluded that saliva’s high lubrication properties are not simply due to the mucin alone, but rather are the result of the electrostatic self-assembly between the mucin proteins and the positively charged non-mucinous proteins. Mucins are proteins produced by the epithelial tissues that have the ability to form gels.

Dr. Anwesha Sarkar discusses the findings of the research. She states that the research conducted by her team clearly delineates the individual roles mucin proteins and positively charged non-mucinous molecular proteins play in the production of saliva. The mucin proteins control the macromolecular lubrication by creating a nano reservoir to trap the water molecules. The positively charged non-mucinous molecular proteins act as the bridge within the reservoir.

Dr. Anwesha Sarkar believes that these results provide an understanding of the molecular mechanisms behind the lubrication properties of saliva that was not previously completely understood. As a result, she feels that this can aid in the development of future effective treatments for dry mouth and improved therapies to treat infections caused by poor saliva lubrication. In addition, she hopes that this takes researchers one step closer to gaining the ability to design improved lubricants inspired by nature for use in nutritional technology and biomedical applications.

Further information: Feng Xu et al. A Self‐Assembled Binary Protein Model Explains High‐Performance Salivary Lubrication from Macro to Nanoscale, Advanced Materials Interfaces (2019). DOI: 10.1002/admi.201901549

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