Computational Model for the Regulation of Extracellular ATP and Adenosine in Airway Epithelia

Abstract

Extracellular nucleotides are key components of the signaling network regulating airway clearance. They are released by the epithelium into the airway surface liquid (ASL) to stimulate cilia beating activity, mucus secretion and airway hydration. Understanding the factors affecting their availability for purinoceptor activation is an important step toward the development of new therapies for obstructive lung diseases. This chapter presents a mathematical model developed to gain predictive insights into the regulation of ASL nucleotide concentrations on human airway epithelia. The parameters were estimated from experimental data collected on polarized primary cultures of human nasal and bronchial epithelial cells. This model reproduces major experimental observations: (1) the independence of steady-state nucleotide concentrations on ASL height, (2) the impact of selective ectonucleotidase inhibitors on their steady-state ASL concentrations, (3) the changes in ASL composition caused by mechanical stress mimicking normal breathing, (4) and the differences in steady-state concentrations existing between nasal and bronchial epithelia. In addition, this model launched the study of nucleotide release into uncharted territories, which led to the discovery that airway epithelia release, not only ATP, but also ADP and AMP. This study shows that computational modeling, coupled to experimental validation, provides a powerful approach for the identification of key therapeutic targets for the improvement of airway clearance in obstructive respiratory diseases.

Effects of airway surface liquid height on the kinetics of extracellular nucleotides in airway epithelia

“…The steady state ADO concentration ( $ 160 nM Zuo et al, 2008;Garcia et al, 2011) is enough to partially activate the A 2B receptor. However, at steady state, only $ 14% of A 2B receptors are active.…”

“…The equations that model the enzymatic reaction rates and uptake rates have the same form as in the original model (Zuo et al, 2008;Garcia et al, 2011). So it was considered that, except for the AMP hydrolysis and the ecto-AK reversible reaction, all reaction rates can be modeled by Michaelis-Menten kinetics,…”

“…Zuo and coworkers (Zuo et al, 2008;Garcia et al, 2011) proposed a mathematical model that describes the kinetics of nucleotide and nucleoside ASL concentrations by coupled differential equations. It takes into account that ATP, ADP and AMP are secreted by the respiratory epithelium to the ASL, where they are metabolized by ecto-enzymes and converted into the nucleosides ADO and INO which in turn are absorbed by the respiratory epithelium (Fig.…”

“…The biochemical network that regulates the concentrations of extracellular ATP and ADO was recently described quantitatively by a computer model (Zuo et al, 2008;Garcia et al, 2011) based on experimental data collected in cultures of human respiratory epithelia with airway surface liquid (ASL) at a height of $1500 μm. Although this model reproduced its target experimental data, it cannot be used to investigate the regulation of extracellular nucleotides in vivo, because in vivo the ASL height is approximately 15 μm (Harvey and Schlosser, 2009).…”

Mathematical model reveals role of nucleotide signaling in airway surface liquid homeostasis and its dysregulation in cystic fibrosis

“…The descriptions of the dynamic concentrations of ATP and ADO were based on a model of nucleotide regulation (24,25) that captures ATP dephosphorylation to ADP, AMP, and ADO and ADO deamination to inosine. Additionally, the nucleotide regulation model includes terms for ATP, ADP, and AMP release as well as cellular uptake of ADO and inosine through CNT3 proteins.…”