STIMulating Salivary Glands

A Perspective on “Loss of STIM1 and STIM2 in Salivary Glands Disrupts ANO1 Function but Does Not Induce Sjogren's Disease”

Over 80 years ago, a 49-year-old female with dry eyes and poor salivary production saw the Swedish physician Sjögren. Today, Sjögren's syndrome [or SS, or Sjögren's disease (SjD)] is the name given to the third most common chronic autoimmune disorder.¹ Like other autoimmune diseases, 1:9 patients are female and typically present with symptoms later in life.¹ Patients with the primary form of SjD present with inflammation in exocrine glands, which results in dry eyes and poor salivary production.¹ Whilst the underlying triggers for primary SjD remain unclear, the production of saliva is a calcium-dependent process, occurring in the acinar cells within the submandibular gland.² Saliva is produced following the activation of muscarinic receptors resulting in the production of calcium waves, and secretion of chloride, via the calcium-activated chloride receptor Ano1, and water by Aquaporin 5 (AQP5, Figure 1).² Not surprisingly, deficits and aberrant calcium signaling have been associated with salivary gland dysfunction,³ which results in xerostomia (dry mouth), oral mucosal lesions, and vulnerability to infections, which collectively adversely affect the quality of life for patients.⁴

Figure 1.

The Role of STIM1/2 in the Production of Saliva. Left Top: The inflammatory-salivation model most associated with Sjorden's Disease (SjD). Left Bottom: Representation of the proposed model from Son and colleagues. Son and colleagues show that agonist-activated calcium signaling (Number 1 in the right side panel) is diminished with STIM1/2 KO (Number 2), which reduces calcium-activated chloride currents and salivary production (Number 3). The STIM1/2 KO mouse model does not have an immune infiltration phenotype. Abbreviations: ANO1: Calcium-activated Chloride Channel, AQP5: Aquaporin 5, InsP3R: Inositol Trisphosphate Receptor, M3R: Muscarinic Receptor.

Although the autoimmune component is a trigger for hyposalivation, a subset of SjD patients do not present with extensive inflammation of the salivary glands.⁵ Due to this paradoxical presentation, understanding how the production of saliva is affected in SjD patients without immune infiltration is a topic of great interest. In vivo genetic mouse studies have shown that deletion of the InsP3R⁶ and the plasma membrane transient receptor potential (TRP) channels² inhibits the calcium required for Ano1 activation and thus results in hyposalivation. Depletion of calcium from the endoplasmic reticulum (ER) is sensed by the ER resident molecule stromal interaction molecule (STIM). Upon depletion of ER calcium, STIM moves to ER-plasma junctions where it interacts with the calcium pore forming channel ORAI or TRP channels, to enable calcium entry across the plasma membrane in a process called store operated calcium entry (SOCE). In vitro studies have provided evidence that full activation of Ano1 also requires SOCE²; however, the contribution of STIM in vivo has not been reported. In this issue of Function, Son and colleagues⁷ address this knowledge gap by crossing floxed STIM1/2 mice with a K14 Cre mouse to create the first from-birth salivary STIM1/STIM2 knockout mouse (STIM1/2 KO K14Cre).

Son and colleagues report that the STIM1/2 KO K14Cre mice have a persistent and nonprogressive hyposalivary dysfunction over the time frame examined, from 2 to 9 months of age. Notably, despite the preponderance of SjD cases in humans occurring in females, both male and female mice had deficient salivary function, with females having a more pronounced phenotype. Using calcium imaging and electrophysiological approaches, the authors demonstrated that loss of STIM1/2 inhibited calcium entry into the acinar cells, and that this calcium was required to elicit full activation of the chloride current. Thus, this study with prior literature demonstrates in vivo that multiple sources of calcium from the InsP3R, together with TRPC3 and STIM1/2, are collectively required for salivary production.

What is the relevance of the genetic studies to the immunological etiology of SjD? Although mutations in STIM and ORAI are best associated with loss of T-cell activation,⁸ the STIM1/2 KO K14Cre model is limited to the glands. Histological analysis of the STIM1/2 KO K14Cre mice demonstrated a lack of infiltrates or inflammation within the submandibular gland even after 9 months. Curiously, RNAseq analysis showed a downregulation in the immune pathway, with several factors such as toll like receptor 8 (TLR8) demonstrating decreased expression. These data suggest the tantalizing idea that although SOCE is required for full activation of salivation, the downregulation of SOCE function is immunologically beneficial. Further underscoring a potential protective function of SOCE in acinar cells, Son and colleagues⁷ examined SjD patient biopsies. In these human samples, STIM1 was elevated (presumably as a compensatory mechanism), as was TLR8. The elevation in STIM1 and TLR8 was most pronounced in the highest scored SjD patients. Prior studies on human SjD biopsies have demonstrated that InsP3R is downregulated,⁶ and TLR7 and 8 are upregulated.⁹

Taking the in vivo mouse model and the human data together, the STIM1/2 KO K14Cre model provides beneficial insight into the subset of SjD patients who present with dry eyes and mouth, but with functional tissue without extensive inflammation or systemic evidence of autoimmune disease. Since the STIM1/2 KO K14Cre mouse model has substantial functional salivary gland tissue left, even at 9 months of age, this raises the possibility that restoring calcium signaling within the acinar cells could return salivary function. The nonprogressive presentation of the STIM1/2 KO K14Cre mice may reflect the dampening of the immune response. However, introducing opportunistic infections into the STIM1/2 KO K14Cre mouse, or agents to trigger an autoimmune response, might give rise to a phenotype that more closely resembles primary SjD.⁴ More importantly, the work by Son et al.⁷ highlights and brings a new understanding as to the interplay between calcium regulation, chloride secretion, and immune regulation.

We are at an exciting time for the molecular understanding of SjD. Son et al. provides a deeper mechanistic understanding of the molecular players that contribute to the salivary defect in SjD. One can envision a promising future where the challenge of a pleiotropic disease such as SjD and other sicca-related diseases has increasingly more treatment options.

Funding

K.M.M.N. acknowledges funding from the National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health under award numbers U2CDK129917 and TL1DK132769. I.Y.K. acknowledges funding by the PKD Foundation under grant 1021282.

Conflict of Interest Statement

None declared.