A5: Cellular and molecular responses to trauma-induced damage of the distal respiratory epithelium
PI: M. Frick
Alveoli are the central functional units of the lung where gas exchange between air and blood takes place. Two fundamental mechanisms within the respiratory epithelia of the alveoli are essential for maintaining lung function – regulated fluid transport and secretion of pulmonary surfactant. An intact alveolar barrier and alveolar integrity is a prerequisite for regulated fluid transport and a properly functioning surfactant system.
Trauma, whether of direct pulmonary origin (e.g. blunt chest trauma, ventilator-induced lung injury) or indirect extra-pulmonary trauma (e.g. sepsis, polytrauma), often result in structural and/or functional damage of the alveolar barrier. The resulting increase in alveolar permeability entails formation of oedema, inactivation of the surfactant system, alveolar invasion of activated macrophages and polymorphonuclear neutrophils (PMN) and widespread inflammation. This leads to a dramatic reduction in the number of functional alveolar units, a decrease in pulmonary compliance, and a worsening of alveolar gas exchange. These changes are directly linked to development of respiratory insufficiency, acute lung injury/acute respiratory distress syndrome (ALI/ARDS) and pneumonia.
Despite the patho-physiological importance little is known about the molecular and cellular mechanisms and consequences of alveolar barrier damage. There is evidence that trauma-induced ATP release is a key player directly related to pulmonary dysfunction. The effects of ATP are mediated by purinergic signalling on target cells via either G-protein coupled P2Y or ionotropic P2X receptors. P2X receptors are expressed in the alveolar epithelium and signalling via P2X receptors modulates regulated fluid transport and secretion of pulmonary surfactant. We have recently reported that P2X4 receptors are present in alveolar epithelial type II (ATII) cells and that activation of P2X4 receptors on primary ATII cells facilitates surfactant secretion, activation of secreted surfactant and alveolar fluid transport. P2X receptors therefore have the potential to significantly impact on the pathogenesis and manifestation of pulmonary dysfunction following trauma-induced damage of the alveolar barrier. Based on these findings we propose that 1) trauma-induced ATP release in the alveolus leads to activation of P2X receptors on epithelial cells, 2) activation of P2X receptors results in alveolar fluid resorption supporting alveolar fluid clearance following trauma-induced damage of the alveolar barrier and 3) trauma-induced activation of P2X receptors modulates surfactant secretion and hence lung mechanics.
To test these hypotheses we will closely collaborate with other groups within the planned CRC to establish relevant trauma models in advanced in vitro models resembling the unique architecture of the alveolar barrier. We will initially characterise them with respect to biophysical damage of the alveolar barrier. Subsequently, we will investigate the molecular mechanism of cellular ATP release following various trauma challenges and evaluate the impact of ATP release on P2X activation within the cells of the alveolar barrier (mainly epithelial cells). Finally, we will elucidate the role of P2X receptors in rescuing alveolar function following trauma on the molecular and cellular level – contributing to restoration of lung function. Therefore, we will test whether ATP release and P2X activation impacts on alveolar fluid clearance and the surfactant system and directly relate these findings to the in vivo situation. Crucially, this proposal is build on substantial preliminary data and most techniques are already established within the laboratories of the applicants.