Current Topics in Medicinal Chemistry - Volume 11, Issue 8, 2011

Nucleosides are metabolic intermediates of the synthesis of nucleotides, which are in turn the building blocks of nucleic acids and which participate in a number of fundamental biochemical reactions. In particular, ATP is the major energy transfer molecule, and among the nucleotides, its additional extracellular signaling role was the first to be suggested. Its corresponding nucleoside, adenosine, has also been demonstrated to be a neuromodulator substance. It has now been over 35 years since purinergic signal transmission in the nervous system has been suggested. Since that time, various requirements for classification as a neurotransmitter have been demonstrated for adenine nucleotides and adenosine including regulated release, elimination from the extracellular space, action on neuronal activity, and the existence and pharmacological characterization of receptors. In addition, a large body of evidence derived from human disorders also supports the conclusion that nucleosides have a crucial role in neural function. Anti-cancer and anti-viral drugs that affect nucleoside metabolism have neurological side effects. Some metabolic disorders have also been shown to originate from defects in nucleoside metabolism. In the 1960s, the enzyme deficit underlying Lesch-Nyhan syndrome, a relatively frequent disorder of nucleoside metabolism with peculiar neurological symptoms, was identified as a deficiency of hypoxanthine-guanine phosphoribosyltransferase. Since then, a number of other disorders that cause mental retardation or other neurological symptoms have been attributed to deficits in nucleoside metabolism. The accumulating knowledge in the field of nucleoside and nucleotide metabolism and pharmacology in the nervous system led to the possibility of developing novel therapeutic approaches to treat major neurological disorders. The interest and progress in drugs acting on enzymes, transporters, and receptors of nucleotides and nucleosides is tremendous and major advances are expected in the future. In the present issue, first the metabolic network of nucleosides under normoxic and anoxic/ischemic conditions is described by Ipata et al. Nucleosides are converted to nucleotides, which are required by various biochemical pathways and for building nucleic acids. Because the capacity of de novo purine and pyrimidine synthesis is limited in the brain, salvage pathways are necessary for the proper functioning of the brain and for maintaining qualitative and quantitative balance of intracellular brain nucleoside triphosphates. The importance of cross talk between extra- and intracellular nucleoside metabolism, nucleoside interconversion, 5-phosphoribosyl-1-pyrophosphate synthesis and energy repletion are also emphasized by the authors. Given the importance of nucleosides in a number of biochemical reactions in the brain, it is not surprising that neurological symptoms result from defects in a number of nucleoside metabolism enzymes, including, for example, hypoxanthine-guanine phosphoribosyltransferase and different nucleoside and nucleotide kinases and phosphorylases. The known inborn errors of purine and pyrimidine metabolism, their neurological syndromes and the possible links between specific enzymatic alterations and brain damage are summarized by Micheli et al. Parkinson et al. describe recent advances in the field of nucleoside transport. It has long been established by physiological studies that concentrative as well as equilibrative transporters contribute to the uptake of nucleosides. Relatively recently, four genes encoding the equilibrative and three genes encoding the Na+-dependent concentrative nucleoside transporters have been identified in several species. Furthermore, additional nucleobase transporters have recently been discovered. The identification of these transporters allowed for the individual examination of their transport characteristics, cellular distributions, and physiological functions. Potential therapeutic uses of drugs affecting nucleoside transporters for treatment of cerebral ischemia, epilepsy, neurodegenerative diseases, cancer and viral infections are also described. Purine receptors are grouped as P1 (or A) adenosine receptor and P2 adenine nucleotide receptors. Currently, A1, A2A, A2B, and A3 receptors for adenosine are known. These receptors are all G-protein coupled receptors. P2 receptors are further divided into ion channels (P2X), and G-protein coupled nucleotide receptors (P2Y). In addition to ATP, some of the nucleotide receptors recognize ADP, UTP, UDP, and UDP-glucose. These receptors have diverse signal transduction pathways and participate in a number of neural functions. The development of selective agonists and antagonists is in progress, suggesting that there may be pharmaceutical uses for drugs acting on these novel targets. Novel developments in this field and the role of different types of nucleoside and nucleotide receptors in CNS diseases such as neurodegenerative diseases, epilepsy and neuropsychiatric disorders are described by Burnstock et al. Transporters of nucleosides and a complex interlinked metabolic network balance nucleoside levels in the brain tissue under normal conditions and enable the fine modulation of neuronal and glial processes via nucleoside receptor signaling mechanisms. Normal brain levels of nucleosides were found to differ when measured in a variety of brain regions. The distributions of nucleoside transporters and receptors as well as nucleoside metabolic enzyme activities also demonstrate regional differences, suggesting different roles for nucleosides in different brain areas. Recent advances in this research area are described by Kovacs et al. These authors also describe and discuss age- and gender-dependant alterations in brain nucleoside levels. The concentrations of adenosine and ATP in the neuronal extracellular space are not constant but increase in response to, e.g., increased neuronal activity, depolarization, or ischemia. ATP, and possibly other nucleotides, are actively transported into synaptic vesicles, are stored in them either alone or together with other neurotransmitters, and are released in a calciumdependent way from presynaptic terminals. They may also be released by other mechanisms, for example, cell damage. These molecules can bind to plasma membrane receptors or can be degraded by extracellular nucleotidases to nucleosides. In addition, adenosine, and possibly other nucleosides as well, can be released by reversed action of nucleoside transporters. Sperlagh and Vizi describe the actions of adenosine in the hippocampus and basal ganglia and the pharmacological and clinical aspects of these adenosine actions.....

Brain relies on circulating nucleosides, mainly synthesised de novo in the liver, for the synthesis of nucleotides, RNA, nuclear and mitochondrial DNA, coenzymes, and pyrimidine sugar- and lipid-conjugates. Essentially, the paths of nucleoside salvage in the brain include a two step conversion of inosine and guanosine to IMP and GMP, respectively, and a one step conversion of adenosine, uridine, and cytidine, to AMP, UMP, and CMP, respectively. With the exception of IMP, the other four nucleoside monophosphates are converted to their respective triphosphates via two successive phosphorylation steps. Brain ribonucleotide reductase converts nucleoside diphosphates to their deoxy counterparts. The delicate qualitative and quantitative balance of intracellular brain nucleoside triphosphates is maintained by the relative concentrations of circulating nucleosides, the specificity and the Km values of the transport systems and of cytosolic and mitochondrial nucleoside kinases and 5'-nucleotidases, and the relative rates of nucleoside triphosphate extracellular release. A cross talk between extra- and intra-cellular nucleoside metabolism exists, in which released nucleoside triphosphates, utilised as neuroactive signals, are catabolised by a membrane bound ectonucleotidase cascade system to their respective nucleosides, which are uptaken into brain cytosol, and converted back to nucleoside triphosphates by the salvage enzymes. Finally, phosphorolysis of brain nucleosides generates pentose phosphates, which are utilised for nucleoside interconversion, 5-phosphoribosyl-1-pyrophosphate synthesis, and energy repletion. This review focuses on these aspects of brain nucleoside metabolism, with the aim of giving a comprehensive picture of the metabolic network of nucleosides in normoxic conditions, with some hints on the derangements in anoxic/ischemic conditions.

Purines and pyrimidines, regarded for a long time only as building blocks for nucleic acid synthesis and intermediates in the transfer of metabolic energy, gained increasing attention since genetically determined aberrations in their metabolism were associated clinically with various degrees of mental retardation and/or unexpected and often devastating neurological dysfunction. In most instances the molecular mechanisms underlying neurological symptoms remain undefined. This suggests that nucleotides and nucleosides play fundamental but still unknown roles in the development and function of several organs, in particular central nervous system. Alterations of purine and pyrimidine metabolism affecting brain function are spread along both synthesis (PRPS, ADSL, ATIC, HPRT, UMPS, dGK, TK), and breakdown pathways (5NT, ADA, PNP, GCH, DPD, DHPA, TP, UP), sometimes also involving pyridine metabolism. Explanations for the pathogenesis of disorders may include both cellular and mitochondrial damage: e.g. deficiency of the purine salvage enzymes hypoxanthine-guanine phosphoribosyltransferase and deoxyguanosine kinase are associated to the most severe pathologies, the former due to an unexplained adverse effect exerted on the development and/or differentiation of dopaminergic neurons, the latter due to impairment of mitochondrial functions. This review gathers the presently known inborn errors of purine and pyrimidine metabolism that manifest neurological syndromes, reporting and commenting on the available hypothesis on the possible link between specific enzymatic alterations and brain damage. Such connection is often not obvious, and though investigated for many years, the molecular basis of most dysfunctions of central nervous system associated to purine and pyrimidine metabolism disorders are still unexplained.

Pyrimidine and purine nucleosides and their derivatives have critical functions and pharmacological applications in the brain. Nucleosides and nucleobases are precursors of nucleotides, which serve as the energy-rich currency of intermediary metabolism and as precursors of nucleic acids. Nucleosides (e.g., adenosine) and nucleotides are key signaling molecules that modulate brain function through interaction with cell surface receptors. Brain pathologies involving nucleosides and their metabolites range from epilepsy to neurodegenerative disorders and psychiatric conditions to cerebrovascular ischemia. Nucleoside analogs are used clinically in the treatment of brain cancer and viral infections. Nucleosides are hydrophilic molecules, and transportability across cell membranes via specialized nucleoside transporter (NT) proteins is a critical determinant of their metabolism and, for nucleoside drugs, their pharmacologic actions. In mammals, there are two types of nucleoside transport process: bidirectional equilibrative processes driven by chemical gradients, and unidirectional concentrative processes driven by sodium (and proton) electrochemical gradients. In mammals, these processes, both of which are present in brain, are mediated by members of two structurally unrelated membrane protein families (ENT and CNT, respectively). In this Chapter, we review current knowledge of cellular, physiological, pathophysiological and therapeutic aspects of ENT and CNT distribution and function in the mammalian brain, including studies with NT inhibitors and new research involving NT knockout and transgenic mice. We also describe recent progress in functional and molecular studies of ENT and CNT proteins, and summarize emerging evidence of other transporter families with demonstrated or potential roles in the transport of nucleosides and their derivatives in the brain.

There is a widespread presence of both adenosine (P1) and P2 nucleotide receptors in the brain on both neurones and glial cells. Adenosine receptors play a major role in presynaptic neuromodulation, while P2X receptors are involved in fast synaptic transmission and synaptic plasticity. P2Y receptors largely mediate presynaptic activities. Both P1 and P2 receptors participate in neurone-glia interactions. Purinergic signalling is involved in control of cerebral vascular tone and remodelling. Examples of the roles of purinoceptors in neuropathology involve: A2A receptors in Parkinson's disease and epilepsy, P2 receptors in trauma, ischaemia, neuroinflammatory and neuropsychiatric disorders, and neuropathic pain.

Nucleosides, such as uridine, inosine, guanosine and adenosine, may participate in the regulation of sleep, cognition, memory and nociception, the suppression of seizures, and have also been suggested to play a role in the pathophysiology of some neurodegenerative and neuropsychiatric diseases. Under pathological conditions, levels of nucleosides change extremely in the brain, indicating their participation in the pathophysiology of disorders like Alzheimer's disease, Parkinson's disease and schizophrenia. These findings have resulted in an increasing attention to the roles of nucleosides in the central nervous system. The specific effects of nucleosides depend on the expression of their receptors and transporters in neuronal and glial cells, as well as their extracellular concentrations in the brain. A complex interlinked metabolic network and transporters of nucleosides may balance nucleoside levels in the brain tissue under normal conditions and enable the fine modulation of neuronal and glial processes via nucleoside receptor signaling mechanisms. Brain levels of nucleosides were found to vary when measured in a variety of different brain regions. In addition, nucleoside levels also depend on age and gender. Furthermore, distributions of nucleoside transporters and receptors as well as nucleoside metabolic enzyme activities demonstrate the area, age and gender dependence of the nucleoside system, suggesting different roles of nucleosides in functionally different brain areas. The aim of this review article is to summarize our present knowledge of the area-, age- and gender-dependent distribution of nucleoside levels, nucleoside metabolic enzyme activity, nucleoside receptors and nucleoside transporters in the brain.

Now there is general agreement that the purine nucleoside adenosine is an important neuromodulator in the central nervous system, playing a crucial role in neuronal excitability and synaptic/non-synaptic transmission in the hippocampus and basal ganglia. Adenosine is derived from the breakdown of extra- or intracellular ATP and is released upon a variety of physiological and pathological stimuli from neuronal and non-neuronal sources, i.e. from glial cells and exerts effects diffusing far away from release sites. The resultant elevation of adenosine levels in the extracellular space reaches micromolar level, and leads to the activation A1, A2A, A2B and A3 receptors, localized to pre- and postsynaptic as well as extrasynaptic sites. Activation of presynaptic A1 receptors inhibits the release of the majority of transmitters including glutamate, acetylcholine, noradrenaline, 5-HT and dopamine, whilst the stimulation of A2A receptors facilitates the release of glutamate and acetylcholine and inhibits the release of GABA. These actions underlie modulation of neuronal excitability, synaptic plasticity and coordination of neural networks and provide intriguing target sites for pharmacological intervention in ischemia and Parkinson's disease. However, despite that adenosine is also released during ischemia, A1 adenosine receptors do not participate in the modulation of excitotoxic glutamate release, which is nonsynaptic and is due to the reverse operation of transporters. Instead, extrasynaptic A1 receptors might be responsible for the neuroprotection afforded by A1 receptor activation.

This paper presents an overview of the current knowledge about the role of adenosine in the sleep-wake regulation with a focus on adenosine in the central nervous system, regulation of adenosine levels, adenosine receptors, and manipulations of the adenosine system by the use of pharmacological and molecular biological tools. The endogenous somnogen prostaglandin (PG) D2 increases the extracellular level of adenosine under the subarachnoid space of the basal forebrain and promotes physiological sleep. Adenosine is neither stored nor released as a classical neurotransmitter and is thought to be formed inside cells or on their surface, mostly by breakdown of adenine nucleotides. The extracellular concentration of adenosine increases in the cortex and basal forebrain during prolonged wakefulness and decreases during the sleep recovery period. Therefore, adenosine is proposed to act as a homeostatic regulator of sleep and to be a link between the humoral and neural mechanisms of sleep-wake regulation. Both the adenosine A1 receptor (A1R) and A2AR are involved in sleep induction. The A2AR plays a predominant role in the somnogenic effects of PGD2. By use of genemanipulated mice, the arousal effect of caffeine was shown to be dependent on the A2AR. On the other hand, inhibition of wake-promoting neurons via the A1R also mediates the sleep-inducing effects of adenosine, whereas activation of A1R in the lateral preoptic area induces wakefulness, suggesting that A1R regulates the sleep-wake cycle in a site-dependent manner. The potential therapeutic applications of agonists and antagonists of these receptors in sleep disorders are briefly discussed.

In the adult nervous system, the major source of nucleotide synthesis is the salvage pathway. Uridine is the major form of pyrimidine nucleosides taken up by the brain. Uridine is phosphorylated to nucleotides, which are used for DNA and RNA synthesis as well as for the synthesis of membrane constituents and glycosylation. Uridine nucleotides and UDP-sugars may be released from neuronal and glial cells. Plasmamembrane receptors of 7 transmembrane domains have been identified that recognize UTP, UDP, and UDP-sugar conjugates. These receptors are called P2Y2 and P2Y4, P2Y6, and P2Y14 receptors, respectively. In addition, binding sites for uridine itself have also been suggested. Furthermore, uridine administration had sleep-promoting and anti-epileptic actions, improved memory function and affected neuronal plasticity. Information only starts to be accumulating on potential mechanisms of these uridine actions. Some data are available on the topographical distribution of pyrimidine receptors and binding sites in the brain, however, their exact role in neuronal functions is not established yet. There is also a scarcity of data regarding the brain distribution of other components of the pyrimidine metabolism although site specific functions exerted by their receptors might require different metabolic support. Despite the gaps in our knowledge on the neuronal functions of pyrimidine nucleosides, their therapeutic utilization is appealing. They have been suggested for the treatment of epileptic and neurodegenerative diseases as neuroprotective agents. In addition, the development of traditional drugs acting specifically on pyrimidine receptor subtypes is also promising as a new direction to treat neurological disorders.

Nucleoside receptors are known to be important targets for a variety of brain diseases. However, the therapeutic modulation of their endogenous agonists by inhibitors of nucleoside metabolism represents an alternative therapeutic strategy that has gained increasing attention in recent years. Deficiency in endogenous nucleosides, in particular of adenosine, may causally be linked to a variety of neurological diseases and neuropsychiatric conditions ranging from epilepsy and chronic pain to schizophrenia. Consequently, augmentation of nucleoside function by inhibiting their metabolism appears to be a rational therapeutic strategy with distinct advantages: (i) in contrast to specific receptor modulation, the increase (or decrease) of the amount of a nucleoside will affect several signal transduction pathways simultaneously and therefore have the unique potential to modify complex neurochemical networks; (ii) by acting on the network level, inhibitors of nucleoside metabolism are highly suited to fine-tune, restore, or amplify physiological functions of nucleosides; (iii) therefore inhibitors of nucleoside metabolism have promise for the “soft and smart” therapy of neurological diseases with the added advantage of reduced systemic side effects. This review will first highlight the role of nucleoside function and dysfunction in physiological and pathophysiological situations with a particular emphasis on the anticonvulsant, neuroprotective, and antinociceptive roles of adenosine. The second part of this review will cover pharmacological approaches to use inhibitors of nucleoside metabolism, with a special emphasis on adenosine kinase, the key regulator of endogenous adenosine. Finally, novel gene-based therapeutic strategies to inhibit nucleoside metabolism and focal treatment approaches will be discussed.

Adenosine is a naturally occurring nucleoside present ubiquitously throughout the body as a metabolic intermediate. Besides its metabolic role within the cells, adenosine is released into the extracellular space either by neurons or astrocytes acting as a neuromodulator. Extracellular adenosine exerts its action by activating multiple G-protein coupled receptors (subtypes A1, A2A, A2B and A3) having a wide range of physiological effects in the brain. Adenosine levels rise markedly in response to ischemia, hypoxia, excitotoxicity or inflammation being a neuroprotectant under these conditions. However, adenosine may also contribute to neuronal damage and cell death in other circumstances. These actions are firmly established using multiple animal models. Therefore, increasing attention is now given to the role of adenosine in human brain function and its potential benefit for clinical applications. This review covers recent studies undertaken mostly in humans revealing the actions of adenosine and related drugs in cognition and memory as well as in various pathological situations such as psychiatric disorders, drug addiction and neurodegenerative disorders. The actual use of adenosine or adenosine receptor ligands in ongoing clinical trials for the treatment of schizophrenia, panic disorder and anxiety, cocaine dependence and Parkinson's disease is discussed. The evidence herein reviewed highlights the promising potential of adenosine or adenosine receptor ligands as therapeutic agents in several brain disorders.