Vesicular and conductive mechanisms of nucleotide release

Abstract

Extracellular nucleotides and nucleosides promote a vast range of physiological responses, via activation of cell surface purinergic receptors. Virtually all tissues and cell types exhibit regulated release of ATP, which, in many cases, is accompanied by the release of uridine nucleotides. Given the relevance of extracellular nucleotide/nucleoside-evoked responses, understanding how ATP and other nucleotides are released from cells is an important physiological question. By facilitating the entry of cytosolic nucleotides into the secretory pathway, recently identified vesicular nucleotide and nucleotide-sugar transporters contribute to the exocytotic release of ATP and UDP-sugars not only from endocrine/exocrine tissues, but also from cell types in which secretory granules have not been biochemically characterized. In addition, plasma membrane connexin hemichannels, pannexin channels, and less-well molecularly defined ATP conducting anion channels have been shown to contribute to the release of ATP (and UTP) under a variety of conditions.

Fig. 1

Vesicular nucleotide transporter. Schematic representation of SLC17A9/VNUT displaying its predicted 12 transmembrane-spanning domains. VNUT transports ATP to the lumen of secretory granules, using the electrochemical gradient provided by the bafilomycin A₁ (Bafi)-sensitive proton pump V-ATPase. In specialized tissues, ATP containing granules are secreted via Ca²⁺-regulated exocytosis

Fig. 2

Potential pathways for nucleotide release. Several candidate ATP conducting channels, including Panx1 and connexins (Cx) efflux cytosolic ATP and UTP out of the cells. VNUT transports ATP into dense-core granules and vesicles competent for Ca²⁺ regulated exocytosis. UDP-glucose (UDPG) and possibly ATP, enter the secretory pathway via ER/Golgi-resident SLC35-like transporters, using UMP and AMP as antiporter substrates, respectively. ER/Golgi nucleotides serving in glycosylation and energy-driven reactions and their luminal metabolites are released as residual cargo molecules via the constitutive secretory pathway

Fig. 3

Reduced ATP release from Panx1^−/− tracheas. Tracheas excised from WT and Panx1^−/− littermates were perfused with isotonic buffer for 45 min and then exposed to hypotonic solutions (from [152])

Fig. 4

Dye uptake in human bronchial epithelial cells reflects a reversible phenomenon. Representative images of propidium iodide uptake in primary cultures of human bronchial epithelial cells that were incubated for 5 min with isotonic (iso) or hypotonic (hypo) solutions in the presence of propidium iodide (a, b) or in its absence (c, d). Isotonicity was restored in d, and propidium iodide subsequently added to c and d for additional 5 min (from [152])