Computational modeling of epithelial fluid and ion transport in the parotid duct after transfection of human aquaporin-1

Abstract

Previous studies have shown that localized delivery of the aquaporin-1 (AQP1) gene to the parotid duct can restore saliva flow in minipigs following irradiation-induced salivary hypofunction. The resulting flow rate and electrochemistry of secreted saliva contradicts current understanding of ductal fluid transport. We hypothesized that changes in expression of ion transport proteins have occurred following AQP1 transfection. We use a mathematical model of ion and fluid transport across the parotid duct epithelial cells to predict the expression profile of ion transporters that are consistent with the experimental measurements of saliva composition and secretion rates. Using a baseline set of parameters, the model reproduces the data for the irradiated, non-AQP1-transfected case. We propose three scenarios which may have occurred after transfection, which differ in the location of the AQP1 gene. The first scenario places AQP1 within nonsecretory cells, and requires that epithelial sodium channel (ENaC) expression is greatly reduced (1.3% of baseline), and ductal bicarbonate concentration is increased from 40.6 to 137.0 mM, to drive water secretion into the duct. The second scenario introduces the AQP1 gene into all ductal cells. The final scenario has AQP1 primarily in the proximal duct cells which secrete water under baseline conditions. We find the change in the remaining cells includes a 95.8% reduction in ENaC expression, enabling us to reproduce all experimental ionic concentrations within 9 mM. These findings provide a mechanistic basis for the observations and will guide the further development of gene transfer therapy for salivary hypofunction.

New & noteworthy: Following transfection of aquaporin into the parotid ducts of minipigs with salivary hypofunction, the resulting increase in salivary flow rates contradicts current understanding of ductal fluid transport. We show that the change in saliva electrochemistry and flow rate can be explained by changes in expression of ion transporters in the ductal cell membranes, using a mathematical model replicating a single parotid duct.

Keywords: aquaporin transfection; membrane transport; parotid duct.

Fig. 1.

Model of a secretory duct cell with components.

Fig. 2.

Model of an impermeable duct cell with components.

Fig. 3.

Model of the Saliva Production Unit (SPU). The black ductal cells represent the water-impermeable cell type, whereas the gray cells represent the secretory. All cells are capable of ion transport. Figure is not to scale. [Adapted from Sneyd et al. (38) with permission from Elsevier. Copyright 2014.]

Fig. 4.

[Na⁺], [Cl⁻], and [K⁺] in minipig parotid gland saliva 16 wk following irradiation (IR), 4 wk following AQP1 transfection (A4), and 8 wk after transfection (A8).

Fig. 5.

Comparing data from Ref. with the model solution for the IR case for the Na⁺, Cl⁻, and K⁺ in the duct.

Fig. 6.

Percentage changes in ion channel densities or fluxes from baseline (IR) to each scenario of the A8 state, within nonsecretory cells. S1, Striated-Secretion; S2, Whole-Secretion; S3, Intercalated-Secretion.

Fig. 7.

Comparing data from Gao et al. (10) with the model solution for the A8 case for the Na⁺, Cl⁻, and K⁺ in the duct, for each scenario. The model solutions for [$\text{HC}{\text{O}}_3^{-}$] are also included, although experimental data are unavailable for this ion. S1, striated secretion; S2, whole secretion; S3, intercalated secretion.

Fig. 8.

A: the total osmolarities of the ductal and cellular compartments over the length of the duct in all scenarios of the A8 case, in mM. B: salivary flow rate over the length of the duct at steady state, for each scenario, under unstimulated saliva conditions. The end of the secretory cell section occurs at x = 0.5 mm. C: salivary flow rate during stimulated saliva conditions. S1, Striated-Secretion; S2, Whole-Secretion; S3, Intercalated-Secretion.

Fig. 9.

The water permeabilities of the secretory and nonsecretory cells in each scenario, as a percentage of the apical membrane permeability of the secretory cell at IR. S1, Striated-Secretion; S2, Whole-Secretion; S3, Intercalated-Secretion.