Hamed, Omar; Moss, Guy WJ; Dua, Vivek; (2024) A multicellular modelling framework for airway epithelial fluid and ion transport: implications for cystic fibrosis gene therapy. Presented at: 19th European Cystic Fibrosis Society Basic Science Conference, Valetta, Malta.

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Abstract

Epithelial ion transport regulates the depth of airway surface liquid (ASL), a thin fluid layer (0.1-10 µm) lining the airway epithelium, enabling the vital mucociliary clearance of mucus-trapped pathogens in the upper airways. ASL homeostasis is regulated by a complex collaboration of ion channels, transporters, and tight junction proteins. In cystic fibrosis (CF), where this system is disrupted, the ASL becomes dehydrated creating sticky mucus that is difficult to clear, resulting in chronic lung infections. Existing mathematical models of ASL regulation typically treat the system as a single, idealised cell containing all the important elements of fluid/ion transport. However, airway epithelia comprise patchworks of distinct cell types, each with specialised functions. To explore the implications of this for ASL homeostasis, we developed a multicellular quantitative framework to understand the bioelectric properties of healthy and CF epithelia and, in the latter case, the implications of cell diversity for gene therapy. Our mathematical framework for modelling multicellular epithelia has its basis in the equivalent electrical circuit and direct modelling of ion fluxes for Na+, Cl- and K+. The solution of the model provides numerical values of cellular/ASL ion concentrations, membrane potentials and cellular/ASL heights comparable to those reported in the literature. The modelling framework is quite flexible, such that cells can be made distinct by the presence/proportion of channels expressed on their surfaces. Lateral flow of ions between neighbouring cells was also incorporated into the model. We began our examination of multicellular modelling by considering an extreme case where all essential secretory channels/transporters were included in one cell type whilst absorptive machinery was exclusively located within a second, neighbouring cell type. We found that even with exaggerated differences, cells could stably maintain internal ionic compositions and membrane potentials distinct from their neighbours. Next, using a less extreme scenario, we examined ASL homeostasis in a two-cell model of healthy airway epithelium. This highlighted a key role for the Na+/K+/2Cl- cotransporter (NKCC) and basolateral Cl- channels (ClCba). The presence of ClCba in the absence of NKCC resulted in a cell absorbing Cl- from the ASL while a neighbouring cell with NKCC present and ClCba absent secretes Cl-. Both cell types were modelled expressing apical CFTR, thus suggesting a cell type-dependent role in both Cl- absorption and secretion. Taking this model further, we simulated CF epithelia and subsequent gene therapy in multiple situations. First, we considered the case where perfect localisation of CFTR to the apical membrane occurs for both secretory and absorptive cell types. This improved ASL hydration, increasing ASL depth from 3 µm to ~6.0 µm, near the height of extended cilia. Next, we considered a scenario where CFTR overexpression led to mis-sorting of the protein via equal localisation to apical and basolateral membranes. This resulted in very limited hydration, increasing ASL depth to ~4.4 µm, and would not form a successful therapy. We conclude that such models can help to elucidate the roles of different cell types in ASL regulation and inform the design and development of successful gene therapies.

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