MEDICINE

THE DOUBLEDOUBLE LIFE LIFEOF ATP The molecule ATP, famous as an essential energy source inside KEY CONCEPTS cells, also carries critical messages between cells. That dual role ■ ATP, best known as a uni- versal fuel inside living is suggesting fresh ideas for fi ghting human diseases cells, also serves as a mo- lecular signal that affects BY BALJIT S. KHAKH AND GEOFFREY BURNSTOCK cell behavior.

■ A leading investigator and ne of the fi rst and most enduring facts tissues throughout the body to communicate the discoverer of ATP’s most students learn in biology class is with one another. The universal fuel, in effect, messenger role describe how ATP signals work and Othat all living cells use a small molecule serves as a common language as well. why they are essential to called triphosphate (ATP) as fuel. When ATP’s dual function was fi rst pro- basic bodily functions That universal energy currency drives the bio- posed nearly 50 years ago, the idea met with and development. logical reactions that allow cells to function and considerable skepticism. But an avalanche of life to fl ourish—making ATP a crucial player in fi ndings in the past 15 years has detailed how ■ Because ATP is so ubiqui- tous, the molecule’s infl u- the biological world. ATP acts on cells from the outside and how it ences can vary from tissue Less commonly known, however, is that serves in the development and daily operation to tissue, offering new what is perhaps the most produced and con- of organs and tissues. Because ATP is so ubiqui- insights into a wide range sumed molecule in the human body also has a tous, its signaling actions have a uniquely broad of disorders and diverse completely separate but no less essential role infl uence on physiological functioning and of- ways to treat them. outside of cells. A long series of discoveries has fer unusually diverse opportunities to improve —The Editors now demonstrated beyond doubt that ATP is a human health. Laboratories worldwide are now

critical signaling molecule that allows cells and racing to turn these insights into therapies. EWARD KEN

84 SCIENTIFIC AMERICAN December 2009 ATP Unmasked, Twice Makino of Dailen Hospital in Manchuria When ATP was discovered in 1929, investi- had proposed a structure for the molecule, gators around the world were seeking the elu- which was confi rmed 10 years later, by Basil sive source of cellular energy. In nearly simul- Lythgoe and Alexander R. Todd of the Uni- taneous breakthroughs, Karl Lohmann, versity of Cambridge Chemical Laboratory. working with the 1922 Nobel Laureate Otto During this period no one envisioned a role Meyerhof of the Kaiser Wilhelm Institute for for the molecule outside of the cell. That was Medical Research in Heidelberg, and Cyrus still the case in 1962, when one of us (Burn- H. Fiske, working with his graduate student stock) was a young neurophysiologist at the Yellapragada SubbaRow of Harvard Medical University of Melbourne in Australia, study- School, showed that intracellular activities ing the nerves that control smooth muscle tis- that allow muscle cells to contract depended sue. In the course of investigating signaling by on a molecule made of a —adenosine, the autonomic nervous system (which con- a combination of the base with a sug- trols such basic muscle-dependent functions ar— and three phosphates. By 1935 Katashi as intestinal and bladder contractions), he saw www.ScientificAmerican.com SCIENTIFIC AMERICAN 85 [DUAL ROLES] ATP INSIDE CELLS ... AND OUTSIDE

Students routinely learn that the small molecule (ATP) is a critical cellular power source; it fuels the activities of the molecular machinery that allows all cells to function and thrive (below). But not all intracellular ATP is used up by cellular process- RELEASING CELL es. Cells of all kinds also release ATP to send messages to nearby cells (right). Vesicle

Adenosine triphosphate (ATP)

Neurotransmitter ●1 ADP H+ Phosphate ATP

AdenosinePhosphate Energy trapped ●1 in bonds CELL Mitochondrion S An ATP molecule stores energy ●2 in the bonds between its three phosphates. The phosphates are anchored ATP ●2 to adenosine , which belongs to the ADP “purine” class of molecules. Released Enzymatic phosphate breakdown ATP W Cells manufacture ATP constantly in ATP Energy their mitochondria, which build it from released ●3 such raw materials as protons (H+) ● derived from glucose that has undergone 3 several stages of processing. Inside ADP mitochondria ●1 , protons power the addition of a phosphate to (ADP); the resulting ATP is Nucleus P2Y delivered into the cytoplasm ●2 . Cellular activities such as protein manufacture draw energy from ATP molecules when the termi- P2X receptor nal phosphate is released ●3 . ADP and free ●4 phosphates are then recycled into ATP ●4 .

evidence of neural signaling that did not involve mitters, such as acetylcholine, glutamate, dop- the classical neurotransmitter chemicals acetyl- amine and others, released from the fi ring neu- choline or noradrenaline. Intrigued by data ron into the cleft. These chemicals cross the gap published in 1959 by Pamela Holton of the Cam- and bind to receptor proteins on the receiving bridge Physiological Laboratory suggesting that cell, causing that cell to undergo a series of in- sensory nerves released ATP molecules, Burn- ternal changes that alter its activity; recipient stock set out to determine whether ATP could be neurons might fi re impulses of their own, and responsible for signaling between motor nerves muscle cells might contract or relax. A message and muscle. Through a series of experiments in can thus be transmitted down the line from neu- which he applied chemicals to block signaling ron to neuron by an alternating series of impuls- by the classical neurotransmitters to smooth es and chemical discharges. muscle tissue, he was able to demonstrate that Individual neurons were long believed to any continued signaling from the nerves to the emit only a single neurotransmitter type, and muscle had to be conveyed by ATP. Pursuing cells that released acetylcholine came to be de- this lead for more than a decade, Burnstock felt scribed as cholinergic; those that released dop- confi dent enough by 1972 to propose the exis- amine, were dopaminergic, and so forth. Burn- tence of “purinergic nerves” that release ATP as stock’s concept of purinergic neurons was based a neurotransmitter. not only on his own observations by that point Nerve cells generate electrical impulses that but also on the early work of a series of out- travel the length of a single neuron, but the standing students and collaborators, including charge does not cross the tiny gap between the Max Bennett, Graeme Campbell, David Satch- cells known as the synaptic cleft, or the gap be- ell, Mollie Holman and Mike Rand of the uni- tween nerve cells and muscles. The message is versities of Melbourne and London.

forwarded from cell to cell by chemical trans- Despite a huge amount of data showing ATP GRAPHICSPRECISION

86 SCIENTIFIC AMERICAN December 2009 P2 receptor subtypes, which they named P2X ATP SIGNALING: and P2Y. A BRIEF HISTORY Still, the idea of nerves that released ATP as a neurotransmitter remained controversial and 1929 ATP discovered to be the energy source in muscle tissue. ATP often becomes a signal when a fi ring neuron was dismissed by many for years to come. In the releases it from vesicles ●1 , along with neurotransmit- ter molecules; many nonneuronal cells also release 1990s, though, molecular tools became avail- 1929 Albert Szent-Györgyi fi nds ATP using vesicles or similar mechanisms. Enzymes able that allowed many research groups to iso- (ATP’s chemical family) have soon start breaking ATP down ●2 , sequentially remov- potent effects on the heart. T ing phosphates to produce ADP, adenosine monophos- late ATP receptors and to further explore their phate (AMP) and adenosine. ATP and its breakdown many fascinating effects on cells of the nervous products convey messages by binding to specifi c Albert system and beyond. Szent-Györgyi receptors on cells ●3 . Two distinct receptor types, called P2X and P2Y, recognize ATP. P2Y receptors also recognize ADP. AMP and adenosine bind to P1 recep- Interplay and Dynamics tors. As ATP is degraded, signaling by its breakdown products can offset or enhance ATP’s effects, for The early 1990s saw the initiation of the Human example adenosine may also bind to P1 receptors on Genome Project and the beginning of an era of the releasing cell, suppressing further ATP release. prolific discovery of the genes that encode important proteins in the human body. Among these genes were several for ATP receptors, 1945 ATP structure confi rmed. AMP which allowed scientists to locate the receptors themselves on many different cell types. Studies 1959 Pamela Holton shows ATP of ATP signaling entered a new and exciting era. release from sensory nerves. Attempts to characterize the molecular struc- 1962 Geoffrey Burnstock demon- ture of purine receptors proved the existence of strates message transmission from a large family of receptors and identifi ed a num- neurons to muscle by a new Adenosine ber of channels and enzymes on the surface of neurotransmitter. T cells that participate in ATP signaling. P1 As predicted, two broad classes of receptor receptor were identifi ed, but the work also revealed many more receptor subtypes than expected within RECEIVING CELL those classes. This diversity implied that partic- ular receptor subtypes could be targeted with highly selective drugs to modulate ATP signal- release from neurons into muscle, gut and blad- ing only in specifi c tissues or cell types—a pros- der tissue, many neurophysiologists nonetheless pect that is bearing fruit today [see table on remained skeptical about the existence of nerves page 92]. Mollie Holman and Burnstock, 1962 releasing ATP as a messenger, largely because After the initial isolation of ATP receptors, they thought it unlikely that such a ubiquitous various investigators showed that the two main 1972 Burnstock proposes the exis- tence of nerves that signal using ATP. substance could perform such a specifi c role. classes operate in signifi cantly different ways. Moreover, for a signaling molecule to be able to P2X receptors belong to a “superfamily” of 1976 Burnstock proposes that function, it must fi nd a suitable receptor on its transmitter-gated ion channels. One of us ATP acts as a co-transmitter with

); ); Burnstock and Holman other neurotransmitters. VOL. 460; JULY 30, 2009. target cell. The fi rst receptor for a neurotrans- (Khakh), along with other investigators, showed ) mitter had been isolated only in 1970; therefore, that when bound by ATP, P2X receptors liter- 1993 and 1994 P2X and P2Y NATURE, the hunt was on for the receptors for ATP. ally open to form a channel that allows receptors for ATP isolated from cells. ATP crystals Well before they were found, however, many and large amounts of ions to rush into 1998 , a drug that acts on researchers continued to use pharmacological cells. P2Y receptors, in contrast, do not open in platelet P2Y receptors, introduced to methods to examine how ATP released by neu- the same way, but ATP binding to their extra- prevent clot formation in blood vessels. rons delivered messages into muscle and other cellular surface sets off a cascade of molecular 2009 cells of the body. Based on this work, Burnstock interactions inside cells that results in intracel- ); COURTESY OF GEOFFREY BURNSTOCK ( Crystal suggested in 1978 that separate families of re- lular calcium stores being released. In both cas- structure of ceptors existed for ATP (which he designated P2 es, the calcium can then set off further molecu- a P2X receptor Szent-Györgyi ( receptors) and for its fi nal breakdown product, lar events that alter cell behavior. revealed, adenosine (which he called P1 receptors). Fur- Although ATP stays in the synaptic cleft only which should aid drug ther studies showed that ATP activation of P2 briefl y, the cellular effects of receptor activation Getty Images discovery. X receptors could produce different cellular ef- can occur quickly in some instances—within fects. That led Burnstock and his collaborator milliseconds—but slowly in others—sometimes P2X4 REPRINTED WITH PERMISSION FROM MACMILLAN PUBLISHERS LTD. ( MACMILLAN LTD. FROM PUBLISHERS WITH PERMISSION REPRINTED FROM “CRYSTAL STRUCTURE OF THE ATP-GATED P2X4 ION CHANNEL IN THE CLOSED BY TOSHIMITSU STATE,” KAWATE, JENNIFER CARLISLE MICHEL, WILLIAM BIRDSONG T. AND ERIC GOUAUX, IN JOHN PHILLIPS PHILLIPS JOHN Charles Kennedy to anticipate the existence of over the course of years. For example, an inrush receptor

www.ScientificAmerican.com SCIENTIFIC AMERICAN 87 [ATP IN ACTION] ONE SIGNAL, MANY MESSAGES

ATP-signaling activity was fi rst detected between nerve cells and muscle tissue but is now known to operate within a wide variety of cell types in the body. Select examples from the cardiovascular system illustrate how diverse ATP’s effects can be in their nature and duration.

RECEPTOR TYPES ATP EFFECTS ON BLOOD VESSELS ATP binds to P2X Nerve releases ATP receptor on muscle cell and noradrenaline Cellular receptors for ATP take two forms. The P2X-type receptor is a channel that opens when ●A Constriction X Muscle cells ATP binds to its extracellular part, allowing calcium Sympathetic nervous system cells and sodium ions to rush into the cell. When an ATP release ATP with the neurotransmit- Nerve molecule binds to a P2Y, the receptor initiates a ter noradrenaline. The ATP binds to ATP cascade of internal signals that release intracellular receptors on muscle cells that form calcium ion stores. In both cases, the rise in calcium blood vessel walls, causing the can trigger short term events such as muscle con- Vessel vessel to constrict rapidly. constricts Blood vessel traction. P2Y activation can also initiate further molecular interactions and gene activity that leads to long term effects, such as cell proliferation.

Calcium ion Endothelial cells under ATP binds to ●B Dilation W shear stress release ATP on nearby endothelial cells Sodium ion Changes in blood fl ow produce “shear stress” ATP on endothelial cells lining blood vessel walls, causing the cells to release ATP, which activates receptors on nearby endothelial cells. The cells respond by releasing nitric P2X ATP channel oxide, which makes the vessels relax. P2Y receptor Damaged cells Platelet spill ATP Short-term Signal effects cascade Wound site

Endothelial cells Vessel Nucleus relaxes Long-term effects Calcium ●C Blood clotting X DNA ion store ADP binds to ATP spilled from damaged cells at a wound site gets P2Y receptors broken down to ADP. The ADP binds to receptors on platelets RECEIVING CELL on platelets, which respond by aggregating to form Clot forms to a blood clot that closes the wound. close wound

of calcium ions through P2X channels may ucts may have an effect of its own on a cell—such cause the cell to release other transmitters, as as when adenosine binds to P1 receptors. Khakh has shown in brain tissue, or calcium re- Fusao Kato of the Jikei University School of leased by P2Y activation may alter gene activity Medicine in Tokyo has shown, for instance, involved in cell proliferation that causes chang- that ATP and adenosine act in concert in the es in the tissue with lifelong consequences. Even brain stem network responsible for basic body though the presence of ATP molecules in the ex- functions such as breathing, heart rhythm and tracellular space is fl eeting, therefore, their bio- gastrointestinal action. Other situations exist, logical effects can be quite pervasive. however, where ATP and adenosine oppose The mechanisms of ATP signaling become each other, such as during neuron-to-neuron even more fascinating when its interactions with transmission, where adenosine can inhibit a other signaling systems outside of cells are taken neuron from releasing ATP into the synaptic into account. A large family of enzymes known cleft. The interconnected effects of ATP, its as ectoATPases sit on the surface of most cells, component parts and the extracellular ecto- where they quickly strip ATP of its phosphates ATPases can thus be seen as forming a self-reg- one by one—sequentially turning an ATP mol- ulating signaling loop in many circumstances. ecule into adenosine diphosphate (ADP), ade- It is not only ATP breakdown products that nosine monophosphate (AMP) and fi nally ade- infl uence the molecule’s effects on cells. In the

nosine alone. Each of ATP’s breakdown prod- nervous system, ATP also acts in concert with GRAPHICSPRECISION

88 SCIENTIFIC AMERICAN December 2009 ATP in Health and Disease [THE AUTHORS] In light of ATP’s established role in signaling among cells of the nervous system, it may come as no surprise that ATP plays an important part in the functioning of the fi ve senses. In the eye, for example, ATP receptors on nerve cells in the ●A retina infl uence the cells’ responses to informa- ●D tion received from rods and cones, the eyes’ light Baljit S. Khakh is an assistant Vein detectors. The retinal nerves, in turn, dispatch professor of physiology and neuro- Artery ATP and acetylcholine as co-transmitters to biology at the David Geffen School of Medicine at the University of convey their information to sensory-processing ●B Heart California, Los Angeles. He has centers in the brain. In addition to this everyday developed novel tools, such as function for ATP, several research groups have designer ATP receptors that can be shown that ATP signaling at a key point during monitored by light, to probe how ●C Wound an embryo’s eye development can have effects cells sense and respond to ATP. Geoffrey Burnstock, the fi rst to that last a lifetime. Indeed, Nicholas Dale of the show that ATP acts as a signaling University of Warwick in England and his col- molecule, was chair of the depart- leagues have shown that release of ATP at a crit- ment of anatomy and developmen- ical time in the early embryo is the signal for the tal biology at University College Cells proliferate development of eyes. London for 22 years and is now president of the Autonomic Neuro- Damaged tissue Release of ATP during development is also releases ATP science Center at the Royal Free and essential for the proper formation of the co- University College Medical School chlea, the organ responsible for hearing, and in London. He has won numerous ATP signaling continues to be crucial to the awards and honors. He and Khakh workings of the inner ear in adults. Some 50,000 met in 1994 at a coffee shop in Vienna, where they discussed ATP hair cells—the sound-transducing neurons of over apple strudel. ATP binds to P2Y the inner ear—line the human cochlea, and and P1 receptors on endothelial and Artery about half of those display receptors for ATP, muscle cells which has been shown to ease neural fi ring in some circumstances. In addition, taste buds, the ●D Cell proliferation S sensory nerve endings in the tongue, possess After surgery to clear a partially blocked artery, ATP released P2X receptors that mediate taste. In a particu- from damaged tissue binds to receptors on endothelial and larly well-designed study, Sue C. Kinnamon and EARLY ORIGIN muscle cells, inducing the cells to multiply. The result can be a lasting renarrowing of the artery called restenosis. her colleagues at Colorado State University have The discovery of ATP receptors in demonstrated that ATP is vital as a transmitter plants and primitive life-forms, from taste bud cells to gustatory nerves and that such as amoebas and worms, suggests that the molecule took mice lacking both P2X other neurotransmitters as a co-transmitter. 2 and P2X3 receptor sub- on a signaling role very early in Discovery of this phenomenon in 1976 by Burn- types are incapable of tasting. the evolution of life. In the slime ); ); stock helped to revise the long-standing view Interestingly, the P2X2 and P2X3 receptors mold Dictyostelium discoideum

) (below), ATP-activated receptors

Burnstock that any given neuron can synthesize, store and present on taste buds are the same ones involved

mold that resemble human P2X chan- release just one kind of neurotransmitter. Today in certain types of pain signaling. For decades nels control the fl ow of water slime

( a substantial body of evidence shows that ATP scientists have known that ATP introduced into into and out of cells. is typically released along with classical neu- the skin causes pain. Stephen B. McMahon and rotransmitters, such as noradrenaline or acetyl- his colleagues at Guy’s, King’s and St. Thomas’ choline. Although co-transmission was first School of Biomedical Sciences in London, re- proposed and proved for ATP, the phenomenon cently showed that the pain is triggered by the of neurons that co-release transmitter molecules activation of P2X3 ATP receptors on the sensory ); COURTESY OF GEOFFREY BURNSTOCK ( BURNSTOCK GEOFFREY OF COURTESY ); has now also been demonstrated for a variety of nerve endings in the skin that mediate responses

Khakh other transmitters, including GABA with gly- to both touch and pain. Another form of pain, cine, dopamine with serotonin, and acetylcho- one associated with damage to nerves, is called line with glutamate. Co-transmission is thus an- neuropathic pain and involves ATP by a differ- other example of how studies of ATP signaling ent route. Elegant studies from Kazuhide Inoue have revealed more general physiological prin- of Kyushu University in Japan and Michael Salt- ciples as well as shaped and guided research in er of the University of Toronto show that a key COURTESY OF VAHRI BEAUMONT ( COURTESY OF M. J. GRIMSON AND R. L. BLANTON Texas Tech University other fi elds. step in the development of this type of pain in-

www.ScientificAmerican.com SCIENTIFIC AMERICAN 89 Brain ANATOMY OF ATP volves activation of AT P receptors on spinal cord immune cells called microglia. The microglia, in As a neurotransmitter, ATP is turn, release molecules that irritate nerve fi bers, directly involved in brain func- leading to chronic pain [see “New Culprits in tion, sensory perception, and Chronic Pain,” by R. Douglas Fields; Scientif- nervous system control of mus- cles and organs. When released ic American, November 2009]. by nonneuronal cells, it often Because of such insights into ATP’s signaling triggers protective responses, role, several pharmaceutical companies are now Spinal such as bone building and cell cord pursuing P2X receptors as new drug targets for proliferation. Below are some neuropathic pain or pain caused by infl amma- areas where efforts are under Nerves way to understand and exploit tion. And pain is but one aspect of human health ATP’s many roles. that may soon benefi t from therapies aimed at ATP or its receptors. BRAIN: ATP modulates communication People with heart and blood vessel disorders Heart among neurons and between neurons and support cells called glia. Signaling are among those who stand to benefi t from fu- by ATP and its breakdown product ture drugs that act on ATP receptors. The rea- adenosine is involved in sleep, memory, learning, movement and other brain son becomes clear when one looks at the events Intestines activities, and excessive signaling may that follow an injury. Cells that are distressed or be involved in epilepsy and some physically damaged can release or spill ATP into psychological disorders. ATP also stimulates tissue development and the extracellular space. In those situations, ATP repair following injury but may signaling often results in protective and healing promote cell death in neurodegenera- tive diseases. responses, including by blood platelets, the cells responsible for forming a clot to stop bleeding SENSORY ORGANS AND PAIN PATH- from a new wound. Platelets display the WAYS: ATP regulates, and in some receptor subtype, and its activation by extracel- Bladder cases conveys, information fl owing from sensors in the eyes, ears, nose and lular ATP causes them to undergo changes that tongue to the brain. Pain-sensing nerves also use the molecule to transmit lead to clot formation. Of course, this same pro- signals to the spinal cord. cess contributes to the formation of the clots in Penis blood vessels that can cause heart attacks and HEART: ATP co-released with noradren- aline from autonomic nerves stimulates stroke. An existing “blockbuster” drug, clopi- heart muscle contractions. Dysfunction dogrel, works by blocking the P2Y12 receptor of this signaling pathway causes on platelets and thereby preventing ATP from arrhythmias and blood pressure Bone changes. promoting clots. A handful of drugs that func- tion in related ways are also in advanced clini- OTHER ORGANS: Normal intestinal cal trials for coronary disorders. contractions and enzyme secretions during digestion are heavily infl uenced A similarly promising therapeutic area is the by ATP signaling from nerves in the gut. digestive system. James J. Galligan of Michigan Bladder contraction and control is also regulated by ATP, and penis erection State University and others have demonstrated and relaxation require ATP signals from that ATP sent from the intestinal nervous sys- nerves to smooth muscle and to en- tem to the intestinal wall acts on P2X and P2Y dothelial cells, which in turn release muscle-relaxing nitric oxide . receptors to control the rhythmic contractions that move food through the tract. Meanwhile BONE: Activation of ATP receptors ATP that binds to P2Y receptors on cells lining stimulates bone-building cells and represses bone-destroying cells. the inner surface of the gut wall triggers secre- tion of digestive enzymes. Agents that act on Skin SKIN: ATP receptors mediate skin cell those receptors to modulate these functions are turnover in normal regeneration, wound healing, and possibly in cell- therefore being hotly pursued by pharmaceuti- proliferation disorders such as psoriasis cal companies as potential treatments for irri- and scleroderma. table bowel syndrome and its more severe form, IMMUNE SYSTEM: ATP released from Crohn’s disease. injured tissue provokes immune cells to The involvement of ATP in healthy function- cause infl ammation, a healing response that can also cause pain; excessive and ing of other organs and tissues makes it a pos- prolonged infl ammation can damage sible drug target in a long list of disorders, in- tissue, as in rheumatoid arthritis. ATP signaling also helps immune cells to kill cluding diseases of the kidney, bone, bladder,

bacteria-infected cells. skin, and even neurological and psychiatric ill- GRAPHICSPRECISION

9090 SCIENTIFICSCIENTIFIC AAMERICANMERICAN December 2009 [DRUGS] nesses. What is more, ATP may be one of the body’s natural cancer-fi ghting tools. Eliezer Ra- TARGETING ATP RECEPTORS paport, when at the Boston University School Identifi cation of the specifi c receptor subtypes responsible for ATP’s signaling of Medicine, first described a tumor-killing effects in various tissues has allowed pharmaceutical companies to begin effect of ATP in 1983. He, too, was met with developing therapies for a range of disorders. Two of the drugs listed below are skepticism, but research since then by a number already marketed; the rest are still under study. of laboratories working independently has shown that ATP can inhibit the growth of tu- DISORDER DRUG MECHANISM TESTING STAGE mors, including prostate, breast, colorectal, ovarian and esophageal cancers, as well as mel- Cystic fi brosis Activates P2Y2 Late-stage human receptors effi cacy tests under way anoma cells. ATP signaling acts in part to pro- mote suicide of the tumor cells and in part to Dry eye Activates P2Y2 Late-stage human receptors effi cacy tests under way promote cell differentiation, which slows tumor cell proliferation. Infl ammation EVT 401 Inhibits P2X7 Human safety tests receptors completed Much work remains to be done to translate new insights into ATP signaling gathered so far Pain GSK1482160 Inhibits P2X7 Human safety tests receptors under way into novel medicines ready for use in the clinic. But many laboratories and drug companies are Unnamed compounds Inhibit P2X3 and Cell and animal testing (from Evotec AG) P2X receptors actively looking for drugs that can selectively 2/3 activate or silence specifi c ATP receptor sub- Rheumatoid CE-224,535 Inhibits P2X7 Late-stage human types, inhibit or enhance the release of ATP, or arthritis receptors effi cacy tests completed inhibit the breakdown of ATP after it has been AZD9056 Inhibits P2X7 Human safety tests released from cells. receptors completed

Thrombosis Clopidogrel Inhibits P2Y12 Approved The Ultimate Messenger (aberrant receptors blood ATP’s ubiquity as a signaling molecule does pose clotting) Inhibits P2Y12 Approved at least one major challenge: developing drugs receptors targeted only to a single organ or tissue without PRT060128 Inhibits P2Y12 Human safety and causing side effects in other body systems. This receptors effi cacy tests under way concern is not unique to ATP, however, and the Inhibits P2Y12 Late-stage human great variety of subunit confi gurations found on receptors effi cacy tests under way different cell types will make targeting specifi c tissues more feasible. Khakh has been experi- menting with creating “designer” ATP recep- ➥ MORE TO possibility that targeting ATP signaling may tors that can be incorporated into cultured cells EXPLORE also be useful in agriculture and in the treat- or even living laboratory mice and used to test ment of infectious diseases. The presence of the effects of subtly changing the function of a Molecular Physiology of P2X ATP signaling in such diverse life-forms sug- Receptors and ATP Signalling P2X receptor protein. This is just one approach at Synapses. Baljit S. Khakh in gests, too, that ATP’s function as a signaling that allows researchers to manipulate ATP sig- Nature Reviews Neuroscience, Vol. 2, molecule appeared early in the evolution of naling in a controlled manner and study the pages 165–174; March 2001. life—perhaps more or less simultaneously with results in living organisms. its adoption as an energy source. Many reports One of the most important breakthroughs in Pathophysiology and Therapeutic of potent effects caused by ATP and its deriva- Potential of Purinergic Signaling. the past 20 years has been the recent determina- Geoffrey Burnstock in Pharmaco- tives in most invertebrate and lower vertebrate tion of the crystal structure of a zebra fi sh P2X logical Reviews, Vol. 58, No. 1, animals also suggest that ATP’s infl uence could channel by Eric Gouaux and his colleagues at pages 58–86; March 2006. be widespread indeed. Oregon Health and Science University. This It is gratifying for us to see how the role of landmark achievement shows atomic-scale de- P2X Receptors as Cell-Surface ATP ATP as a signaling molecule has gone from an Sensors in Health and Disease. tails of how an ATP receptor works and paves Baljit S. Khakh and R. Alan North idea that was widely deemed dubious 50 years the way for an understanding of ATP signaling in Nature, Vol. 442, pages 527–532; ago to a large and vibrant fi eld of inquiry today from the level of molecules to that of whole August 3, 2006. of interest to the entire biology community and physiological systems. It will also signifi cantly of great potential import to medicine. We look accelerate the process of drug discovery. Physiology and Pathophysiology forward to seeing how further breakthroughs of Purinergic Neurotransmission. Recent evidence of ATP receptors in plants Geoffrey Burnstock in Physiological in understanding the fascinating double life and primitive organisms, such as green algae, Reviews, Vol. 87, No. 2, pages of ATP are exploited to improve the quality of amoebas and parasitic schistosomes, offers the 659–797; April 2007. human life. ■

92 SCIENTIFIC AMERICAN December 2009