Disease-Induced Changes in Salivary Gland Function and the Composition of Saliva

Although the physiological control of salivary secretion has been well studied, the impact of disease on salivary gland function and how this changes the composition and function of saliva is less well understood and is considered in this review. Secretion of saliva is dependent upon nerve-mediated stimuli, which activate glandular fluid and protein secretory mechanisms. The volume of saliva secreted by salivary glands depends upon the frequency and intensity of nerve-mediated stimuli, which increase dramatically with food intake and are subject to facilitatory or inhibitory influences within the central nervous system. Longer-term changes in saliva secretion have been found to occur in response to dietary change and aging, and these physiological influences can alter the composition and function of saliva in the mouth. Salivary gland dysfunction is associated with different diseases, including Sjögren syndrome, sialadenitis, and iatrogenic disease, due to radiotherapy and medications and is usually reported as a loss of secretory volume, which can range in severity. Defining salivary gland dysfunction by measuring salivary flow rates can be difficult since these vary widely in the healthy population. However, saliva can be sampled noninvasively and repeatedly, which facilitates longitudinal studies of subjects, providing a clearer picture of altered function. The application of omics technologies has revealed changes in saliva composition in many systemic diseases, offering disease biomarkers, but these compositional changes may not be related to salivary gland dysfunction. In Sjögren syndrome, there appears to be a change in the rheology of saliva due to altered mucin glycosylation. Analysis of glandular saliva in diseases or therapeutic interventions causing salivary gland inflammation frequently shows increased electrolyte concentrations and increased presence of innate immune proteins, most notably lactoferrin. Altering nerve-mediated signaling of salivary gland secretion contributes to medication-induced dysfunction and may also contribute to altered saliva composition in neurodegenerative disease.


Salivary gland secretion.
Stimulation-secretion coupling. Activation of m3AChRs and to a lesser extent m1AChRs in some salivary glands (Tobin et al. 2009) by acetylcholine released from parasympathetic secretomotor nerves provides the principal drive for acinar cell secretion of electrolyte and water into the lumina of acini. Signalling in acinar cells is via inositol triphosphate (IP 3 ) and raised cytoplasmic Ca 2+ concentration.
Noradrenaline release from sympathetic nerves similarly activates small amounts of fluid secretion through acinar cell α 1 -adrenoceptors. Noradrenaline activation of β 1adrenoceptors provides a strong drive for protein secretion in serous acinar cells through raised intracellular cAMP and activation of exocytosis through protein kinase A (PKA). Neuropeptide co-transmitters provide additional drive for secretion, for example vasoactive intestinal polypeptide (VIP) released from parasympathetic nerves evokes protein secretion through VIP (VIPAC) receptors coupled through intracellular cAMP. Secretion of mucin by acinar cells of mucous glands, as demonstrated in the rat sublingual gland, is evoked by parasympathetic nerve mediated stimuli and release of acetylcholine acting through m3AChRs and m1AChRs and generation of intracellular diacylglycerol (DAG) and protein kinase C (PKC) activity. VIP also stimulates mucin secretion, although to a lesser extent than muscarinic receptor activation, via VIPAC receptors and cAMP coupling, without involvement of sympathetic nerve signalling (Culp et al. 2020). Protein secretion is augmented by crosstalk between the Ca 2+ and cAMP intracellular coupling pathways in serous and mucous acinar cells (Figure 2a) (Culp et al. 2020;Proctor and Carpenter 2007).

Fluid secretion. The initiation of electrolyte secretion into acinar lumina is via an
apical Cl − channel (Tmem16a) that opens when intracellular Ca 2+ is increased.
Paracellular movement of Na + through leaky tight junctions follows Cl − into lumina and the accumulating salt concentration drives paracellular and some transcellular movement of water leading to formation of an isotonic saliva in the acinar lumen.
Saliva then enters the ductal system where (striated) duct cell membrane transport proteins modify the composition of saliva, most notably by secreting K + and HCO 3 -(generated by the action of intracellular carbonic anhydrase 2) and reabsorbing Na + and Cl − . The duct-modified saliva secreted into the mouth is hypotonic, a feature that facilitates detection of dietary salt in the mouth. On reflex stimulation of secretion during feeding, salivary HCO 3 and Na + concentrations are higher than during resting (unstimulated) salivary secretion (Figure 2b, main text); (Catalan et al. 2014;Lee et al. 2012).

Protein secretion.
Most of the protein present in saliva is actively secreted by acinar cells through regulated exocytosis (Castle and Castle 1998). In general, the concentrations of salivary proteins that are actively transported, for example amylase, mucins, statherins, proline-rich proteins and carbonic anhydrase 6 are maintained as the flow of saliva increases (Oppenheim et al. 2007), and these proteins can be considered to form a core salivary proteome, largely responsible for creating the properties and fulfilling the core functions of saliva (Ruhl 2012). The fusion of protein storage granules with the apical membrane has been observed in real time in green fluorescent protein labelled mice and such experiments have revealed the role of actin filament rearrangement in enabling granule fusion in parotid acinar cells (Messenger et al. 2014;Tran et al. 2015). Other secretory pathways that make a lesser contribution to protein secretion are present in acinar cells. A constitutive pathway traffics small vesicles directly to the plasma membrane from the trans-golgi network and a constitutive-like pathway involves direct secretion from immature protein storage granules by-passing a maturation phase (Huang et al. 2001;Proctor et al. 2003). Transcytosis of dimeric IgA also involves a vesicular pathway (Johansen et al. 2001). Glandular saliva can also contain low amounts of proteins and macromolecules (<40kD mw) that have diffused across leaky acinar cell tight junctions as demonstrated following salivary gland stimulation with adrenergic secretagogues (Segawa 1994). Some proteins are secreted by ductal cells, including lactoferrin, lysozyme, tissue kallikreins and protein growth factors, all of which have been localized in ductal cells in humans or animal models Gresik 1994;Reitamo et al. 1980;StanevaDobrovski 1997).