Oral dryness in Sjögren's syndrome patients. Not just a question of water

Sjögren's syndrome (SS) is a chronic autoimmune disease of undefined etiology. Patients with this syndrome suffer from severe alterations in both the quality and quantity of saliva and tears, due to impaired function of the relevant exocrine glands. Prevalent symptoms experienced by SS-patients include a persistent dry mouth sensation (xerostomia) and dry eyes (keratoconjunctivitis sicca). Water content of saliva depends of acetylcholine levels, glandular innervation, M3R signaling, calcium tunneling and water release, among other factors. However, unstimulated salivary flow correlates only poorly with symptoms of mouth dryness, raising the question as to which other components of saliva may be involved in mouth dryness experienced by SS-patients? Salivary mucins are glycoproteins characterized by the presence of large oligosaccharide side chains attached to the protein backbone. These molecules are key saliva components that are required to sequester water and thereby moisturize, as well as lubricate the oral mucosa. In the labial salivary glands of SS patients, morphological and functional alterations are detectable that affect the maturation and trafficking of salivary mucins. In this review, we will focus the discussion on these aspects of reduced salivary flow and decreased quality of salivary mucins, since they are likely to be responsible for xerostomia in SS-patients.