Coupling of airway ciliary activity and mucin secretion to mechanical stresses by purinergic signaling

Abstract

The mucociliary clearance system is comprised of three components, ion transport activities controlling the height of airway surface liquid (ASL), mucin secretion, and ciliary activity. These activities in humans are controlled principally by local agonists, extracellular nucleotides and nucleosides released from the epithelium. Importantly, mechanical stresses stimulate goblet cell mucin secretion, ciliary beating, and Cl- and fluid secretion through mechanically induced nucleotide release. Emerging evidence also implicates co-secretion of nucleotides and mucin from goblet cells as a source of extracellular agonist. At rest, ATP is released onto airway surfaces at approximately 370fmol/mincm2, but only approximately 3% of released ATP is recovered in ASL. Secreted UTP meets with a similar fate. A wide variety of hydrolytic and transphosphorylating ecto-enzymes convert the triphosphate nucleotides into ADP, AMP, and adenosine, UDP, UMP, and uridine. Of these, ATP, adenosine, UTP, and UDP act as agonists at apical P2Y2 (ATP, UTP), P2Y6 (UDP), and A2B (adenosine) receptors on ciliated and/or goblet cells to regulate mucociliary clearance.

Figure 1

Effects of rotational shear stresses on HBE culture luminal ATP and mucin secretion. Cultures were rotated horizontally in a start-stop action with the indicated average shear forces and luminal ATP levels determined after 60 min by luminometry (data courtesy of Dr. Brian Button). Mucins secretion was determined in other cultures subjected to either 0.02 or 0.1 dyne/cm² shear force, and in static cultures exposed to 100 μM ATPγS, or not (inset). Note that levels of shear having minimal effects on bulk [ATP] had near-maximal effects on mucin secretion.

Figure 2

Responses of ciliated cells (A) and goblet cells (B) elicited by diphosphate nucleotides. As indicated by the boxes at the bottom of each figure, baseline ciliary activity (ciliary beat frequency) or mucin secretion was determined for human bronchial epithelial cell cultures, following which they were exposed to a dinucleotide agonist, and then to UTP. Agonist concentrations were 100 μM, except for ADP in the ciliary activity experiments where it was 3 μM. For clarity, SEs are shown only for UDP, which are typical of the other datasets. Insets: Responses of the cells to 100 μM ATP and/or UTP. 2 MeSADP = 2-methlythio ADP. Graphs redrawn from data presented in references Morse et al. 2001 and Conway et al. 2003.