An experimental/computational approach to understanding salivary fluid secretion

Saliva hydrates and lubricates the oral cavity and contains proteins that begin to digest food. Saliva also contains numerous substances that protect the oral cavity and upper gastrointestinal tract from microbial infection. The importance of salivary secretion is generally underappreciated; however, a reduction of salivary fluid flow (xerostomia) leads to a severe deterioration in the quality of life. Patients report difficulty swallowing and chewing food, with a marked increase in dental carries and susceptibility to oral candidiasis. Xerostomia is associated with collateral damage following g-irradiation used as a therapy for head and neck cancers and in Sjogren’s disease, a relatively common autoimmune disease. In both maladies, profound xerostomia is reported before any evidence of glandular destruction indicating that a defect in stimulus-secretion coupling underlies reduced secretion early in disease. To develop therapy for xerostomia it is fundamentally important to understand the processes that lead to saliva secretion physiologically to appreciate how these mechanisms are altered in pathological states. The overarching principle driving this long-term project is that a synergistic combination of experimental investigation and quantitative theoretical modeling can be used to further our understanding of both salivary gland physiology and pathology in a manner that neither single approach can accomplish in isolation. In the current proposal, we will build on preliminary data that demonstrates that fluid secretion is dependent on Ca2+ signaling that occurs in an apical microdomain in acinar cells, and not the bulk cytoplasm. This microdomain, which we term the “secretory synapse”, consists of the endoplasmic reticulum expressing inositol 1,4,5-trisphosphate receptor (IP3R) Ca2+ channels within ~50 nm of the apical plasma membrane harboring the Ca2+ activated Cl channel, TMEM16a. TMEM16a activity is dependent on sustained local Ca2+ release in the secretory synapse which is in turn reliant on efficient refilling of the ER in the microdomain and influenced by Ca2+ flux from endolysosomal Two Pore Channels (TPCs). Notably, Ca2+ signaling and the structure of the secretory synapse are altered in a mouse model of Sjogren’s disease and following g-irradiation of salivary glands. We will construct a new computational model of fluid secretion intimately based on experimental data, that now reflects the specialized subcellular architecture and local signaling events occurring in the apical secretory synapse microdomain. This new acinar cell model will be integrated into our current, anatomically realistic 3D model of the secretory unit, developed in the previous funding period. The model will be used to both qualitatively explain the data and to predict how changes on a micro-spatial scale can influence the secretion of the whole gland. We will subsequently utilize the models to understand how the apical microdomain is disrupted in disease states to suggest and subsequently test how function may be restored. It is envisioned that the model may ultimately suggest novel therapies to restore salivary gland function, which would not be readily evident from a traditional purely experimental methodology.