Mucus is something that everyone has, and some people wish they had a lot less of. Yet it is physiologically essential for health and consequently is found in many different species. It is a gooey, gel-like substance secreted from mucous membranes in the body (in humans: respiratory, digestive and reproductive systems). Mucus is a complex mixture of water, salts and defensive molecules such as antibodies, held together by long, branched molecules known as mucins. Altered levels and composition of mucus can cause disease; in humans, cystic fibrosis, ulcerative colitis and asthma can be caused by this.
Mucus forms a crucial defensive barrier between the ‘outside of our inside’, allowing the exchange of useful substances but also protection from harmful substances. Useful substances include oxygen and carbon dioxide in the lungs, and nutrients in the digestive system whilst harmful substances include both external (particles and bacteria etc.) and internal challenges (bodily acids and enzymes).
The properties of secreted mucins give mucus its gel-like, sieving and hydrating properties. Mucins are glycoproteins, which are protein molecules with sugar attachments. Mucins are particularly sugar-rich (typically more than 70% of their mass) and many of these sugars are negatively charged, which allows the molecule to greatly expand when released. This expansion, along with the ability to bind water, allows the formation of mucus gels. Whilst plasticity is observed (especially during diseased states), specialised mucins tend to be found in discrete anatomical areas. For example, the respiratory mucins are MUC5AC and MUC5B.
Entrapped within the mucus, harmful substances are removed from the lungs by cilia – tiny hair-like cells that beat together synchronously. This process can be aided by coughing. In contrast, the mucus layer of the colon is two-tiered, where the mucin MUC2 composes both layers. The inner layer is very dense and is impenetrable to bacteria, whilst the outer layer is looser and accessible to commensal bacteria. These commensal bacteria are beneficial to the host; they outcompete harmful bacteria, metabolise indigestible material and beneficially stimulate the immune system.
The composition of mucus can change during health and disease. During the menstrual cycle, the point at which a woman’s ovary releases an egg (ovulation), the mucus layer thins to allow the approach of sperm. Afterwards, the mucus layer re-thickens to protect against infection and hinder the approach of sperm. Mucus can also thicken during chronic respiratory diseases. In cystic fibrosis, this is due mucus dehydration – a consequence of a mutated gene (the cystic fibrosis transmembrane regulator gene). This causes the build-up of thick, sticky mucus which cannot be effectively cleared from the airways, leading to airway blockage and bacterial infection.
Professor Dave Thornton, a world leading academic in mucosal biology (the University of Manchester) said: “The mucus barrier functions to protect our mucosal surfaces; a compromised mucus barrier is a feature of common human diseases like asthma, cystic fibrosis, gastric ulcers and inflammatory bowel disease.” Understanding mucosal biology in health and disease can improve patient care, and is essential when trying to find a cure for these diseases.