Atp inside Cells . . . and Outside

KE N E W A RD One of the fi rst and most enduring facts most students learn in biology class is that all living cells use a small molecule called adenosine triphosphate (ATP) as fuel. That universal energy currency drives the biological reactions that allow cells to function and life to fl ourish—making ATP a crucial player in the biological world. Less commonly known, however, is that what is perhaps the most produced and consumed molecule in the human body also has a completely separate but no less essential role outside of cells. A long series of discoveries has now demonstrated beyond doubt that ATP is a critical signaling molecule that allows cells and tissues throughout the body to communicate with one another. The universal fuel, in effect, serves as a common language as well. When ATP’s dual function was fi rst proposed nearly 50 years ago, the idea met with considerable skepticism. But an avalanche of fi ndings in the past 15 years has detailed how ATP acts on cells from the outside and how it serves in the development and daily operation of organs and tissues. Because ATP is so ubiquitous, its signaling actions have a uniquely broad infl uence on physiological functioning and offer unusually diverse opportunities to improve human health. Laboratories worldwide are now racing to turn these insights into therapies. KEY CONCEPTS