Adenosine Receptors in Neurodegenerative Diseases

By David Blum

Adenosine Receptors in Neurodegenerative Diseases covers the role of adenosine receptors in brain function, also focusing on related methodologies and perspectives in therapeutics. The book provides an up-to-date overview by the best specialists in the field, helping readers consider the importance of adenosine and expand the global impact and visibility of adenosine research in the CNS field.

Chapters include adenosine biology and signaling, gene regulation, control of motor function, and novel adenosine-based therapies in the CNS. It is an ideal resource for researchers, advanced graduate students, clinicians, and industry scientists working in the fields of clinical neuroscience and molecular and cellular neuroscience.

• Comprehensive reference that details adenosine receptors in neurodegenerative disorders, with details on brain function and possible therapeutics
• Gives insights on how these receptors modulate the neurodegenerative outcomes in different disorders
• Edited by two of the leading researchers in the field regarding adenosine role in the brain in aging and neurodegenerative conditions

• Language: English
• Publisher: Academic Press
• Release date: Mar 10, 2017
• ISBN: 9780128037454

Book preview

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Section I

Physiology

Outline

Chapter 1 Adenosine Receptor Biology in the Central Nervous System

Chapter 2 Adenosine Signaling Throughout Development

Chapter 3 Influence of Adenosine on Synaptic Excitability

Chapter 4 Regulation of Synaptic Transmission by Adenosine at the Neuromuscular Junction

Chapter 5 Gene Regulation of Adenosine A2A Receptors in the Central Nervous System

Chapter 1

Adenosine Receptor Biology in the Central Nervous System

Michael Freissmuth¹ and Karl-Norbert Klotz²,    ¹University of Vienna, Vienna, Austria,    ²University of Würzburg, Würzburg, Germany

Abstract

The basic pharmacology of the four adenosine receptors (A1, A2A, A2B, A3) is understood in considerable detail. Mice deficient in each individual receptor have been generated and their study has led to surprising insights, e.g., that the presence of the A2A receptor aggravates ischaemia-induced neuronal damage. From the perspective of drug development, it is reassuring to learn that each individual receptor can be deleted without affecting viability of the resulting mice. Thus, it is safe to target these receptors with antagonists. In fact, several clinical trials have recently been conducted to explore the use of agonists and antagonists in diseases and disorders, where a beneficial effect was to be expected based on the study of adenosine receptor biology. Two brain disorders stand out, because addressing adenosine receptors may be of therapeutic interest. Firstly, Parkinson’s disease, where an antagonist (istradefylline) was successfully developed to receive market approval in Japan. Secondly, a polymorphism in the A2A receptor modifies the age of onset in Huntington’s disease. Hence, it may be of interest to also target the receptor in order to treat Huntington’s disease. This requires that the role of the A2A receptor be understood.

Keywords

Adenosine receptors; Parkinson’s disease; Huntington’s disease; G proteins; adenosine formation; equilibrative nucleoside transporters; ectonucleotidases; CD39; CD73

Outline

Sources of Adenosine in the CNS 4

Receptor Subtypes and Their CNS Distribution 6

A1 Adenosine Receptors 7

A2A Adenosine Receptors 8

A2B Adenosine Receptors 9

A3 Adenosine Receptors 9

Crosstalk With Other Receptors 9

Adenosine and CNS Diseases 11

Recent Clinical Trials 13

A1 Adenosine Receptors 14

A2A Adenosine Receptors 14

A2B Adenosine Receptors 15

A3 Adenosine Receptors 15

References 16

Sources of Adenosine in the CNS

Adenosine is ubiquitously present in body fluids; however, it is turned over very rapidly. Accordingly, under physiological conditions, adenosine levels are low both within cells and in the extracellular compartment. Intracellular adenosine can arise from the breakdown of AMP by cytosolic 5′ nucleotidase-IA, which is muscle specific, and by cytosolic 5′ nucleotidase-II, which is ubiquitously expressed but less specific.¹ Alternatively, adenosine is formed by S-adenosylhomocysteine hydrolase, which performs its eponymous action, i.e., the hydrolytic cleavage of S-adenosylhomocysteine. The principal source of extracellular adenosine is the breakdown of adenine nucleotides; ATP is taken up into synaptic vesicles by a vesicular nucleotide transporter (vNuT, SLC17A9²) and released as cotransmitter. In addition, ATP can be released through pannexin-1 hemichannels and/or P2X7 receptors³ and CALMH1 (calcium homeostasis modulator 1).⁴ Finally, cell death results in the release of ATP. Thus, in the brain, both neurons and glial cells act as the cellular sources of extracellular ATP. Extracellular ATP is rapidly broken down to adenosine by the sequential action of ectonucleotidases, i.e., the ectonucleoside triphosphate diphosphohydrolases (E-NTPDases), ecto-5′-nucleotidase (eN), ectonucleotide pyrophosphatase/phosphodiesterases (E-NPPs), and alkaline and acid phosphatases. It is not clear if the latter two enzyme families play a (physiological) role in generating adenosine. In contrast, ectonucleoside triphosphate diphosphohydrolases (E-NTPDases) and ecto-5′-nucleotidase (eN) are known to be important in supporting purinergic signaling⁵; of the eight known E-NTPDases, three are expressed in brain, i.e., E-NTPDase-1 (CD39, apyrase), E-NTPDase-2 (CD39L1, ecto-ATPase), and E-NTPDase-3 (CD39L3, HB6). They cleave ATP to AMP (E-NTPDase-1 and -2) or ADP (E-NTPDase-2) with KM values in the range of 50–100 µM for ATP. Adenosine is formed from AMP by the action of ecto-5′-nucleotidase (eN, CD73), a glycolipid-anchored protein. CD73 operates with a substrate affinity in the low µM range and with a high turnover number (about 180 s−1).⁶ Hence, CD73 can rapidly convert extracellular AMP to adenosine. In fact, the current evidence indicates that CD73 is the only relevant enzyme which generates extracellular adenosine in the brain.⁷ Purinergic receptors (i.e., receptors for ATP/UTP, ADP/UDP and adenosine) and ectonucleotidases may be organized in larger complexes, which are at least in part supported by the ability of the proteins to form heterooligomers.⁸

Because ATP is continuously released as a neurotransmitter in the brain, adenosine is continuously produced within the extracellular space. Its physiological levels are therefore thought to oscillate with neuronal activity. Adenosine is removed by degradation via adenosine deaminase-1 and by cellular uptake. Adenosine deaminase-1 is the predominant one of the two isoforms, which is ubiquitously expressed and hence also present in the brain. Deficiency in adenosine deaminase-1 gives rise to severe combined immunodeficiency (because adenosine is converted to a toxic metabolite, which kills T-lympocytes). However, deficiency in adenosine deaminase rarely gives rise to neurological deficiencies.⁹ This suggests that cellular uptake of adenosine is more important for terminating the action of extracellular adenosine. Adenosine can be translocated over cell membranes by concentrative (CNT1-3=SLC28A1-A3) or equilibrative transporters (ENT1–4=SLC29A1–4).¹⁰ Concentrative transporters operate at the blood brain barrier but are otherwise only found at low levels in the brain. ENT3 is found primarily in intracellular compartments, ENT4 operates at low pH and also acts as a plasmalemmal transporter for monoamines. Accordingly, cellular uptake of adenosine is thought to be predominantly accomplished by the equilibrative nucleoside tranporters-1 and -2 (ENT1 & ENT2). Under physiological conditions, ENT1 & ENT2 operate in a relay with adenosine kinase, i.e., the rapid phosphorylation of adenosine by intracellular adenosine kinase maintains the gradient, which supports clearance of extracellular adenosine. The importance of adenosine kinase in the central nervous system (CNS) can be gauged from the fact that its deficiency leads to global mental retardation.¹¹ This phenotypic consequence is presumably unrelated to excessive signaling via adenosine receptors because the elimination of ENT1 or of ENT2 does not produce any gross abnormality in genetically deficient (knock-out) mice.¹²,¹³

Adenosine accumulates during hypoxia. This is a central tenet of the retaliatory metabolite concept: hypoxia-induced adenosine formation leads to extracellular accumulation of adenosine, which engages its cognate receptor to restore homeostasis by orchestrating responses, which proceed on different time scales—from reducing oxygen demand by dampening cellular activity via A1-receptors to increasing oxygen delivery via A2A-dependent vasodilation, to modulation of the inflammatory response and neo-angiogenesis, which is the result of a concerted activation of A2A-, A2B- and—to a lesser extent—A3-receptors.¹⁴ Originally, hypoxia-induced accumulation of adenosine in heart muscle was shown to result from intracellular breakdown of AMP (as a result of ATP consumption) via cytosolic 5′-nucleotidase and/or S-adenosylhomocysteine hydrolase resulting in outflow of adenosine.¹⁵ It has, however, been questioned whether this also holds true for the brain, because both neurons and glial cells contribute to extracellular ATP via different mechanisms and because the brain does not contain high levels of cytosolic AMP-specific 5′-nucleotidase-Ia (see above). Surprisingly, there is circumstantial evidence that mice deficient in ENT1 have reduced extracellular levels of adenosine in the brain.¹⁶ Conversely, transgenic neuronal overexpression of ENT1 in neurons renders the mice less sensitive to the behavioral effects of the adenosine receptor antagonist caffeine, indicating that endogenous extracellular levels of adenosine are higher in the presence of elevated ENT1. This indicates that under physiological conditions, at least some of the extracellular adenosine originates from a pool, which is generated within neurons and is subject to ENT1-mediated efflux. This interpretation is substantiated by the observation that neuronal release of adenosine rather than (glial or neuronal) ATP is crucial for A1 receptor-mediated, activity-dependent depression during high-frequency repetitive firing.⁷ Even more surprisingly, in hypoxia, ENT1 apparently operates predominantly in the influx mode: if hippocampal slices from ENT1 overexpressing mice are exposed to hypoxia, excitatory postsynaptic responses are less inhibited than in slices derived from wild type control animals.¹⁷ This suggests that there is a net uptake of adenosine by neurons in hypoxia. By inference, it appears that, in the hypoxic brain, adenosine is predominantly formed from extracellular sources. At the very least, these observations indicate that the retaliatory metabolite concept is somewhat simplistic. Regardless of the sources of adenosine, it is clear that during hypoxia intracellular and extracellular adenosine tend to equilibrate, because adenosine kinase is inhibited.¹⁸,¹⁹ Intracellular adenosine can also act on its cognate G protein-coupled receptors (GPCRs) by promoting their maturation: hypoxia increases the expression of A1 receptors, because intracellular adenosine promotes their folding in the endoplasmic reticulum (ER).²⁰ Thus, adenosine acts in a manner analogous to a pharmacochaperone.²¹ A similar mechanism may operate in the A2A-receptor for the following reasons: (1) similar to the A1 adenosine receptor,²² the A2A receptor incurs a folding problem in the ER,²³,²⁴ and (2) oxygen deprivation also promotes the surface expression by mobilizing an intracellular pool of the A2A receptor.²⁵

Receptor Subtypes and Their CNS Distribution

Adenosine receptors are members of the large superfamily of seven transmembrane-spanning GPCRs. The four known subtypes are denoted as A1, A2A, A2B, and A3.²⁶,²⁷ Early hints at the existence of adenosine receptors in the brain resulted from the work by Sattin and Rall²⁸ who observed an adenosine-mediated increase of cAMP production in guinea pig brain slices, which was antagonized by methylxanthines like caffeine. Later on, it was discovered that adenosine might trigger also an inhibitory response suggesting that different receptor subtypes exist. These inhibitory and stimulatory subtypes were named A1 and A2, respectively.²⁹ Both adenosine receptor subtypes showed a distinct tissue distribution and proved to be responsible for many specific actions in almost all organs. Both subtypes were also found in the CNS where they are expressed in a well-defined pattern in different brain areas. A1 adenosine receptors are primarily located in the cortex, hippocampus, and cerebellum.²⁶ One of the most important localizations of the A2 receptor was found to be the striatum.²⁶

The situation became more complicated when additional adenosine receptor subtypes were discovered. It was long suspected that two varieties of the stimulatory A2 receptor might exist.³⁰ With the cloning of A2A and A2B receptors their existence was finally established.³¹–³³ At about the same time, the unexpected A3 subtype was discovered and cloned.³⁴,³⁵

A1 Adenosine Receptors

The inhibitory A1 adenosine receptor is found in many organs and plays a critical role in the regulation of the cellular energy balance. The most prominent expression sites outside the CNS are the heart and kidneys where it helps to avoid excessive energy consumption. It does so in the heart by reducing the heart rate and the force of contraction and, thus, opposing the effect of input from the sympathetic nervous system. In kidneys the glomerular filtration rate is reduced, thereby leading to a diminished need for energy-dependent reabsorption of solutes from the filtrate. Blockade of A1 adenosine receptors explains the well-known effects of the naturally occurring antagonist caffeine on the heart and kidneys.³⁶

The significance of regulatory functions of adenosine in the CNS were long appreciated as caffeine also shows striking CNS effects as a result of antagonism at A1 and A2A receptors. Stimulation of A1 adenosine receptors elicits numerous effects on brain function which are typically inhibitory in nature.³⁶ Activation of A1 adenosine receptors results in postsynaptic inhibition of many different neurons and in inhibition of neurotransmitter release, primarily by acting on presynaptic receptors in excitatory neurons.³⁷

Administration of adenosine into the brain results in sleep induction. Regulation of the sleep-wake cycle seems to be related to adenosine production caused by increased neuronal and metabolic activity and enhanced ATP catabolism. Other important CNS functions that are regulated via A1 receptor stimulation are locomotor activity, learning and memory, pain, and food intake.³⁸

In addition to such physiological activities, stimulation of A1 adenosine receptors plays an equally important role in pathophysiological conditions. In general, adenosine has been shown to act as a neuroprotective factor when ATP and adenosine are released as a consequence of injury or hypoxic conditions. The damage caused by ischemic conditions can be limited by stimulation of A1 receptors whereas A1 antagonists worsen the outcome.³⁹,⁴⁰

Pathophysiological conditions possibly involving A1 adenosine receptors include neurodegenerative and neuropsychiatric disorders. In patients who died from Alzheimer’s disease a loss of hippocampal A1 receptors was observed.⁴¹ There are hints that stimulation of A1 receptors might help to process β-amyloid peptides into soluble forms,⁴² making this receptor subtype an interesting target for drug therapy in Alzheimer’s disease. Similarly, A1 adenosine receptors were shown to play a role in mood disorders. An important contribution to this role is made by nonneuronal cells, most importantly by astrocytes.⁴³

For a comprehensive overview of adenosine receptor signaling in the CNS.⁴⁰,⁴⁴

A2A Adenosine Receptors

In general, the A2A adenosine receptor is typically found in blood vessels and therefore usually involved in an adenosine-induced vasodilation mediated by activation of Gs and the resulting increase in cAMP. As an important coronary vasodilator adenosine complements the A1-mediated effect of limiting energy consumption (see above) by improving the oxygen supply of the heart. Blockage of the A2A adenosine receptor in cerebral vessels is thought to contribute to pain relief by caffeine in migraine headaches.⁴⁵ In addition, stimulation of A2A receptors promotes endothelial cell proliferation⁴⁶ via stimulation of MAP kinase.⁴⁷ This does not require stimulation of Gs but relies on an alternative signaling pathway comprising RAS⁴⁸ and