Fusion‐activated cation entry (FACE)viaP2X₄couples surfactant secretion and alveolar fluid transport

Abstract

Two fundamental mechanisms within alveoli are essential for lung function: regulated fluid transport and secretion of surfactant. Surfactant is secreted via exocytosis of lamellar bodies (LBs) in alveolar type II (ATII) cells. We recently reported that LB exocytosis results in fusion-activated cation entry (FACE) via P2X₄ receptors on LBs. We propose that FACE, in addition to facilitating surfactant secretion, modulates alveolar fluid transport. Correlative fluorescence and atomic force microscopy revealed that FACE-dependent water influx correlated with individual fusion events in rat primary ATII cells. Moreover, ATII cell monolayers grown at air-liquid interface exhibited increases in short-circuit current (Isc) on stimulation with ATP or UTP. Both are potent agonists for LB exocytosis, but only ATP activates FACE. ATP, not UTP, elicited additional fusion-dependent increases in Isc. Overexpressing dominant-negative P2X₄ abrogated this effect by ∼50%, whereas potentiating P2X4 lead to ∼80% increase in Isc. Finally, we monitored changes in alveolar surface liquid (ASL) on ATII monolayers by confocal microscopy. Only stimulation with ATP, not UTP, led to a significant, fusion-dependent, 20% decrease in ASL, indicating apical-to-basolateral fluid transport across ATII monolayers. Our data support the first direct link between LB exocytosis, regulation of surfactant secretion, and transalveolar fluid resorption via FACE.

P2rx4 deficiency in mice alleviates allergen-induced airway inflammation

“…However, our data also demonstrates that selective pharmacological blocking of P2RX4 in WT mice results in diminished AAI as well, which provides convincing evidence for a functional relevance of P2RX4 signaling in asthma pathogenesis rather than a phenotype exclusively caused by the downregulated P2rx7 expression. Accordingly, a role of P2RX4 in airway remodeling by regulating ciliary beat, alveolar fluid transport and surfactant secretion of airway epithelial cells has been emphasized previously [35, 36]. Taken together, targeting P2RX4 might provide a potential promising therapy for allergic airway inflammation.…”

Physiological and Immune-Biological Characterization of a Long-Term Murine Model of Blunt Chest Trauma

“…In addition, P2XR4 activation was shown to increase both surfactant release and alveolar fluid reabsorption (24), which both have the potential to improve the course of inflammation. Briefly, extracellular adenosine triphosphate released from damaged cells acts as a triggering signal for the synthesis of IL-1", especially after macrophage priming by an inflammatory stimulus (25).…”

A new role for P2X4 receptors as modulators of lung surfactant secretion

“…Recent evidence also suggests a role for P2X 4 receptors therein. It has been demonstrated that activation of P2X 4 receptors following vesicle–plasma membrane fusion modulates the secretion and activation of pulmonary surfactant (Miklavc et al, 2011; Dietl et al, 2012; Thompson et al, 2013). …”

“…Upon exocytosis of LBs, the P2X 4 receptor is readily part of the apical membrane as soon as membrane fusion is completed (Miklavc et al, 2009). Activation of P2X 4 in the presence of extracellular ATP then results in a transient, non-selective, inward-rectifying, cation current at the site of the fused vesicle (Miklavc et al, 2011; Thompson et al, 2013) ( Figure 1 ). The relatively high Ca 2+ permeability of P2X 4 receptors (North, 2002) causes a local, transient rise of [Ca 2+ ] c around the fused vesicle which promotes fusion pore expansion (Miklavc et al, 2011).…”

“…Freshly released LBPs disintegrate when they contact an air–liquid interface, leading to instantaneous spreading and insertion of surfactant material at this interface (Dietl and Haller, 2005). Thompson et al (2013) demonstrated that, in addition to facilitating fusion pore dilation, FACE via P2X 4 also drives fluid resorption from the alveolar lumen. The P2X 4 mediated inward-rectifying cation current on the apical side results in vectorial ion transport across ATII cells, which in turn promotes apical to basolateral fluid transport (Thompson et al, 2013) ( Figure 1 ).…”