Structural and functional analysis of salivary intercalated duct cells reveals a secretory phenotype

Abstract

Currently, all salivary ducts (intercalated, striated and collecting) are assumed to function broadly in a similar manner, reclaiming ions that were secreted by the secretory acinar cells while preserving fluid volume and delivering saliva to the oral cavity. Nevertheless, there has been minimal investigation into the structural and functional differences between distinct types of salivary duct cells. Therefore, in this study, the expression profile of proteins involved in stimulus–secretion coupling, as well as the function of the intercalated duct (ID) and striated duct cells, was examined. Particular focus was placed on defining differences between distinct duct cell populations. To accomplish this, immunohistochemistry and in situ hybridization were utilized to examine the localization and expression of proteins involved in reabsorption and secretion of ions and fluid. Further, in vivo calcium imaging was employed to investigate cellular function. Based on the protein expression profile and functional data, marked differences between the IDs and striated ducts were observed. Specifically, the ID cells express proteins native to the secretory acinar cells while lacking proteins specifically expressed in the striated ducts. Further, the ID and striated duct cells display different calcium signalling characteristics, with the IDs responding to a neural stimulus in a manner similar to the acinar cells. Overall, our data suggest that the IDs have a distinct role in the secretory process, separate from the reabsorptive striated ducts. Instead, based on our evidence, the IDs express proteins found in secretory cells, generate calcium signals in a manner similar to acinar cells, and, therefore, are likely secretory cells. imageKey points Current studies examining salivary intercalated duct cells are limited, with minimal documentation of the ion transport machinery and the overall role of the cells in fluid generation. Salivary intercalated duct cells are presumed to function in the same manner as other duct cells, reclaiming ions, maintaining fluid volume and delivering the final saliva to the oral cavity. Here we systematically examine the structure and function of the salivary intercalated duct cells using immunohistochemistry, in situ hybridization and by monitoring in vivo Ca2+ dynamics. Structural data revealed that the intercalated duct cells lack proteins vital for reabsorption and express proteins necessary for secretion. Ca2+ dynamics in the intercalated duct cells were consistent with those observed in secretory cells and resulted from GPCR‐mediated IP3 production.

Deleting Mitochondrial Superoxide Dismutase 2 in Salivary Gland Ductal Epithelial Cells Recapitulates Non-Sjögren’s Sicca Syndrome

“…Although the primary function attributed to salivary gland ionocytes is maintaining the ionic composition of saliva, whether they contribute directly or indirectly to saliva production is unknown. A recent report has suggested that intercalated duct cells, which were assumed to only function in maintaining the ionic composition of saliva, have properties of secretory cells and might contribute towards saliva production [ 42 ]. The i-sg -Sod2 KO model will be a valuable tool for further investigating the role of ionocytes in salivary gland function.…”

Neuronal and hormonal control of Ca2+ signalling in exocrine glands: insight from in vivo studies

“…Importantly, the elevation in Ca 2+ was maintained throughout the period of nervous stimulation. The Ca 2+ signals were potentiated by the acetylcholinesterase inhibitor physostigmine (Takano et al., 2021) and inhibited by atropine (Wahl et al., 2023), confirming the preeminent role of acetylcholine in mediating the response. At frequencies higher than 10 Hz, while Ca 2+ peaked rapidly at higher levels, the signal was not maintained and rapidly decayed.…”

“…Indeed, GCaMP6f fluorescence could be observed in ID, SD cells, as well as acinar cells giving a unique opportunity to observe Ca 2+ signalling in vivo at physiologically relevant stimulus strengths in acinar and various duct cells, simultaneously. Notably, in intact SMG during nerve stimulation ID cells exhibited Ca 2+ transients with a magnitude and profile similar to acinar cells, while SD cells were largely refractory to stimulation at all but maximal stimulation strengths (Wahl et al., 2023) (Fig. 4).…”

“…Whether the response in the ID cells is the result of direct neural innervation or a consequence of acinar cell secretion is not presently clear. Nevertheless, atropine, which completely blocked acinar cell Ca 2+ signalling, only partially attenuated the ID Ca 2+ transient, suggesting the presence of an activation component that is independent of acinar activity and acetylcholine release (Wahl et al., 2023). Further studies are necessary to define the upstream pathway activating Ca 2+ signalling in ID cells and the contribution of these cells to the final secretion.…”

Dysregulated Ca2+ signaling, fluid secretion, and mitochondrial function in a mouse model of early Sjogrens syndrome

“…A week before the DMXAA or 5% sodium bicarbonate injections, tamoxifen (Sigma-Aldrich; T5648) was given to the mice via oral gavage at a dose of 0.25 mg/g of body weight for 3 consecutive days to excise the loxP sites flanking the STOP codon allowing expression of the Ca 2+ indicator within salivary glands. The mice were anesthetized and gland-exposed, as described previously (Takano et al, 2021; Takano & Yule, 2022; Wahl et al, 2023). The immobilized gland was secured within the holder using a cover glass and maintained in Hank’s salt solution (HBSS).…”