P2 receptor web: Complexity and fine-tuning

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Abstract

The present review offers a new perspective on a family of receptors, termed P2 receptors, specific for nucleoside tri- and diphosphates of purines/pyrimidines. We emphasize here that while decoding the inputs of various related extracellular ligands, P2 receptors are a clear example of increasing biological complexity. They are represented by 7 ionotropic P2X and 8 metabotropic P2Y receptors; they have very heterogeneous ligands and binding characteristics, molecular properties, transduction mechanisms, cellular localization and protein–protein interactions. While the reason for this sophistication is unknown, a few compelling issues emerge while looking at such a rich variety. We ask, for instance, why so many different receptor subtypes are necessary for triggering biological properties and functions, and if these receptors are more than the sum of their single entities. A first possibility is that newly synthesized P2 proteins are casually located on the cell surface (stochastic hypothesis). Alternatively, distinct subunits are engaged on different cell phenotypes by genetic control (genetic determinism) and/or selective recruitment under physiopathological conditions and epigenetic stimuli (epigenetic determinism). Nevertheless, an appropriate way to both dissect the vast biological scenario and molecular complexity among P2 receptors and to integrate and upgrade their assortment is to regard them as a “combinatorial receptor web”, that is, a dynamic architecture of P2 proteins demonstrating economic efficiency and involving a process of “fine-tuning”, a mechanism which endorses the dynamic nature of all biological reactions. In the present analysis, we stimulate a scientific query about what contributes to such a vast P2 receptor sophistication.

Introduction

Extracellular nucleoside tri- and diphosphates, first of which the prototype ATP, are considered as the phylogenetically most ancient epigenetic factors sustaining a broad range of short-term and long-term biological effects. In several different tissues, these can vary from neurotransmission, smooth and cardiac muscle contraction, chemosensory signaling, secretion, vasodilatation, microglia activation, to more complex phenomena such as immune responses, male reproduction, fertilization and embryonic development (Burnstock & Knight, 2004, Illes & Ribeiro, 2004, Vial et al., 2004a, Burnstock, 2006a, Burnstock, 2006b, Burnstock, 2006c, Burnstock, 2006d). Effects as different as proliferation, differentiation, chemotaxis, release of cytokines or lysosomal constituents, generation of reactive oxygen or nitrogen species, are moreover elicited by extracellular ATP upon stimulation of blood cells. In addition to these well-described physiological activities, extracellular nucleotides are also recognized as having increasing importance in pathological conditions (Burnstock, 2004a, Gallagher, 2004, Cattaneo, 2005, Kennedy, 2005, Burnstock, 2006a). They have key roles in cancer, cardiopulmonary insufficiency, thrombosis, diabetes, skin and bone diseases, gut motility disorders, diseases of the ear and eye, bladder incontinence, behavioral disorders and pain (James & Butt, 2002, D'Ambrosi et al., 2004, Di Virgilio et al., 2005, White et al., 2005, Burnstock, 2006b). Particularly in the central nervous system (CNS), in addition to their established functions as neurotransmitters, cotransmitters, neuromodulators and growth factors (Burnstock, 2004b), extracellular nucleotides have recently been shown to have additional biological tasks ranging from survival, repair, remodeling during development, to involvement in injury, metabolism impairment, excitotoxicity, acute and chronic neurodegenerative conditions (Volonté et al., 2003, Franke & Illes, 2006). They participate in neuronal mechanisms triggered by axotomy in rat precerebellar nuclei (Florenzano et al., 2002), in astrocytic effects induced by stab wounds (Franke et al., 2004a), in reactive gliosis occurring after traumatic brain injury (Neary et al., 2005), in neuronal and glial responses to cerebral ischemia in vitro and in vivo (Cavaliere et al., 2002, Cavaliere et al., 2004a, Franke et al., 2004b, Melani et al., in press) and in neuronal recovery after growth factor withdrawal (D'Ambrosi et al., 2000, D'Ambrosi et al., 2001). It is well known that metabolic stress, brain ischemia and trauma often evoke massive extracellular release of ATP and additional excitotoxic neurotransmitters (Phillis et al., 1993, Juranyi et al., 1999, Melani et al., 2005), and extracellular ATP per se is noxious to primary CNS neurons (Amadio et al., 2002, Amadio et al., 2005). Also it mediates hypoxic/hypoglycemic signaling in vitro (Cavaliere et al., 2001a, Cavaliere et al., 2001b, Cavaliere et al., 2002, Cavaliere et al., 2004a, Cavaliere et al., 2004b) and in vivo (Prasad et al., 2001, Cavaliere et al., 2003, Franke et al., 2004b). Consistent with this, several purinergic antagonists abolish the cell death fate of primary neurons exposed to excessive glutamate (Volonté & Merlo, 1996), serum/potassium deprivation (Volonté et al., 1999), hypoglycemia and chemical hypoxia (Cavaliere et al., 2001a, Cavaliere et al., 2001b, Cavaliere et al., 2002, Cavaliere et al., 2004a, Cavaliere et al., 2004b). Nevertheless, ATP released from cells (Neary et al., 1996, Bodin & Burnstock, 2001) acts not only on neurons, but also mediates microglial inflammatory processes involved either in pathological conditions, or in the protection of the CNS (Kreutzberg, 1996, Minghetti et al., 1999). For instance, ATP is a trigger for tumour necrosis factor-α secretion and a modulator for interleukin-1β release from cultured microglia, suggesting that outflow of ATP during degenerative events could also boost the pro-inflammatory response of already activated microglial cells (Di Virgilio et al., 1998, Di Virgilio et al., 2001). All these functions substantiate the high level of complexity of purinergic mechanisms.

Section snippets

Molecular and pharmacological classification

As expected in sustaining so many sophisticated biological tasks, the specific extracellular receptors for nucleoside tri/diphosphates, the P2 receptors, show heterogeneity at the molecular level. They are subdivided into ionotropic ATP-gated ion channels (P2X receptors), mediating rapid and selective permeability to Na+, K+ and Ca2+ (North, 2002) and into G protein-coupled metabotropic subtypes (P2Y receptors), inducing a slower onset of responses and involving second-messenger systems (

P2 receptor localization

The biological complexity and heterogeneity of P2 receptors (Fig. 1, Fig. 2) is not uniquely of a molecular and pharmacological nature, as described above, but is further accomplished at a cellular and subcellular level.

Subunit association and receptor cross talk

The biological complexity of P2 receptors is further augmented, if we consider that both P2X and P2Y subtypes can form homomers and heteromers (Torres et al., 1999, Nakata et al., 2004), and that the composition of the oligomers profoundly affects the biological response of these receptors. Different subtype combinations thus yields different receptor characteristics, allowing increasing diversities in agonist and antagonist selectivity, transmission signaling, channel and desensitization

P2 receptor web and fine-tuning

Some P2 receptor subtypes are very similar, others have quite different properties. For instance, P2Y12 and P2Y13 receptors have the same preference for endogenous agonists (particularly ADP) and couple to the same intracellular signal transduction pathway, hence implicating on a first analysis that cellular responses might be analogous, whether a cell expresses one or the other receptor subtype (or a mixture of both). Moreover, P2Y1 receptors are coupled to stimulation of nitric oxide

Concluding remarks

We propose here the new model of “combinatorial receptor web and fine-tuning” to unravel the complexity of P2 receptors, with the goal of enquiring what ultimately distinguishes this receptor intricacy. Nevertheless, this same intricacy is the most amazing feature of P2 receptors, because of the way in which fundamental building blocks, the P2 proteins, are recruited in an ordered way on a single cell, with even more complexity reached during physiopathological conditions. We do hope that

Acknowledgments

We are most grateful to Prof. Giorgio Bernardi for encouraging our work, to Dr. Fabio Cavaliere, Dr. Fabrizio Vacca for key experiments which inspired this review, and to Dr. Gillian E. Knight for her excellent editorial skills. The present work was supported in part by Cofinanziamenti MIUR.

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