P2 Receptors for Extracellular Nucleotides in the Central Nervous System: Role of P2X7 and P2Y₂ Receptor Interactions in Neuroinflammation

Abstract

Extracellular nucleotides induce cellular responses in the central nervous system (CNS) through the activation of ionotropic P2X and metabotropic P2Y nucleotide receptors. Activation of these receptors regulates a wide range of physiological and pathological processes. In this review, we present an overview of the current literature regarding P2X and P2Y receptors in the CNS with a focus on the contribution of P2X7 and P2Y₂ receptor-mediated responses to neuroinflammatory and neuroprotective mechanisms.

Introduction

Extracellular nucleotides, such as adenosine 5-triphosphate (ATP) and uridine 5’-triphosphate (UTP), are released from cells under a variety of physiological and pathological conditions whereupon they activate P2 nucleotide receptors on the surface of neighboring cells (Burnstock et al., 1997; Heilbronn et al., 1997). P2 receptors are a diverse family of plasma membrane proteins that can be segregated into two subtypes: the P2X receptors that are ATP-selective cation channels and the P2Y receptors for ATP, UTP or their metabolites that are coupled to heterotrimeric G proteins (Abbracchio and Burnstock, 1994; von Kügelgen and Wetter, 2000). To date, genes for 7 P2X receptors and 8 P2Y receptors have been cloned and their protein products have been extensively characterized in a variety of cell and tissue types (Burnstock and Kennedy, 1985; Sak and Webb, 2002). In the central nervous system (CNS), multiple P2X and P2Y receptor subtypes are expressed in neurons, glial cells, oligodendrocytes, macrophages and endothelium where they regulate physiological responses, including neurotransmission, pain perception, phagocytosis, and maintenance of the blood-brain barrier (Weisman et al., 2005; Gonzalez et al., 2005; Peterson et al., 2010). Pathophysiological responses are also regulated by P2X and P2Y receptors, including the propagation of inflammation due to the release of nucleotide agonists from damaged or diseased cells (Bours et al., 2011; Ferrero, 2011; Fumagalli et al., 2011). This review describes the contributions of both P2X and P2Y receptors to cell specific functions in the CNS and focuses on the dual roles of the ionotropic P2X7 receptor (P2X7R) for ATP and the G protein-coupled P2Y₂ receptor (P2Y₂R) for ATP and UTP in the regulation of proinflammatory responses in the brain. Recent studies have found that the release of extracellular ATP from stressed or damaged cells of the CNS can activate microglial cell P2X7Rs, which increases cytokine release, e.g., interleukin-1β (IL-1β), and the phagocytic activity of microglial cells (Bianco et al., 2005b). Additionally, IL-1β has been shown to upregulate P2Y₂R expression in neurons to promote neuroprotective responses (Kong et al., 2009). These findings are reviewed in this paper and suggest that both P2X7 and P2Y₂ receptors are promising targets for the treatment of neurodegenerative and other inflammatory diseases.

P2X receptors in the Central Nervous System

P2X receptors (P2XRs) are ligand-gated, nonselective cation channels activated by extracellular ATP. Seven pharmacologically distinct P2XR subtypes have been identified, i.e., P2X1–P2X7, and shown to be activated by ATP and its analogues (MacKenzie et al., 1999; North and Surprenant, 2000). P2XRs range from 379–595 amino acids in length and, as shown in Figure 1, consist of two transmembrane domains, a large extracellular loop and intracellular N- and C-termini (Valera et al., 1994). P2XRs share ~ 50% sequence homology with one third of the extracellular loop conserved, suggesting an ATP binding site (Vial et al., 2004). P2X1 and P2X3 receptors rapidly desensitize (within milliseconds), whereas P2X2, P2X5, P2X6 and P2X7 receptors desensitize slowly (within seconds) upon activation by ATP (Ralevic and Burnstock, 1998). Depending on the subtype, P2XR subunits interact in a variety of homo- or heteromeric forms to regulate a wide range of cellular responses in the CNS under physiological and pathological conditions (Khakh and North, 2006; Koles et al., 2007). The distribution of P2XR subtypes in the CNS is dependent upon species, brain region and cell type (Collo et al., 1996; Vulchanova et al., 1996; Kanjhan et al., 1999; Vulchanova et al., 1997; Collo et al., 1997; Atkinson et al., 2000; Deuchars et al., 2001; Rubio and Soto, 2001; Sim et al., 2004). The P2X2R, P2X4R and P2X6R appear to be abundantly expressed throughout the brain, whereas the remaining subtypes are expressed in distinct regions (Collo et al., 1996; Tanaka et al., 1996; Kanjhan et al., 1999). In neurons, activation of P2XRs by extracellular ATP has been reported to have both presynaptic and postsynaptic effects including the modulation of transmembrane currents and neurotransmitter release (Nakatsuka and Gu, 2001; Watano et al., 2004; Rodrigues et al., 2005; Patti et al., 2006; Jameson et al., 2008; Illes et al., 2011). All types of glia, e.g., astrocytes, oligodendrocytes, Schwann cells and microglial cells express P2XRs where they can regulate inflammatory, neurodegenerative and neuroprotective responses (Illes et al., 2011). Most notably, the P2X7R has gained recognition for its possible role in neurodegenerative disorders (Duan and Neary, 2006; Takenouchi et al., 2010). The function of the CNS is dependent on neuronal-glial interactions and the complexity of P2XR signaling in both cell types adds to the difficulty in interpreting ATP-mediated events in vivo. The cell and tissue distribution and functional relevance of homomeric and heteromeric P2X receptors in the nervous system are summarized in Table 1.

Fig. 1. Structural Features of P2X and P2Y Receptors.

Based on structure and function, P2 nucleotide receptors can be divided into two classes. The P2X receptors are nonselective ligand-gated cation channels featuring two transmembrane domains and a large extracellular loop. P2X receptors interact in a wide variety of homo- and heteromeric forms depending on tissue-specific expression and receptor subtype (i.e., P2X1–7) and they are activated by extracellular ATP. The P2X7R has received much attention due to its capacity for intracellular signaling via a large C-terminal tail and its participation in inflammatory processes. The P2YRs are classical G protein-coupled receptors featuring an extracellular N-terminus, 7 transmembrane domains, and an intracellular C-terminus that is structurally diverse between P2Y receptor subtypes. The G_q-coupled P2Y_1,2,4,6,11 and the G_i-coupled P2Y_12,13,14 receptors are activated by adenine and uridine tri- and dinucleotides with pharmacologically distinct efficacies and potencies. The P2Y₂R subtype has been shown to associate with integrins via an extracellular RGD domain and to transactivate growth factor receptors via the binding of Src to Src-homology-3 (SH3) domains located within the C-terminus.

*- only present in the P2Y₂ receptor

Table 1. P2X Receptors in the Nervous System.

The expression and function of P2X receptor subtypes in the nervous system are summarized in Table 1. This table highlights the information presented in this review article and is not considered to be comprehensive.

Receptor Subtype	Expression		Function
Homomeric	Cell Type	Region
P2X1	Neurons, astrocytes, microglia	Cerebral cortex, superior cervical ganglia	Neurogenic smooth muscle contraction, platelet activation, neuron and glial responses
P2X2	Neurons, astrocytes	Cerebral cortex, cerebellum, hippocampus, striatum, habenula, substantia nigra, dorsal root ganglia, mesenteric ganglia	Nociceptive transmission, hyperalgesia, allodynia, pre- and postnatal neurogenesis
P2X3	Neurons	Dorsal root ganglia, spinal cord	Enhance glutamate and substance P release, neuropathic pain sensation
P2X4	Neurons, astrocytes	Cerebellum, hippocampus, brainstem, spinal cord	Release of brain-derived neurotrophic factor, induce neuropathic pain, prostaglandin E2 release, synaptic strengthening, hypersensitivity to sensory stimuli
P2X5	Neurons	Cerebral cortex, cerebellum, hippocampus, hypothalamus, thalamus, olfactory bulb, globus pallidum, midbrain and hindbrain	Interconnection of cortical areas, post-synaptic purinergic transmission
P2X6	Neurons, astrocytes	Cerebellum, hippocampus, purkinje neurons, pyramidal neurons, sensory ganglia	Rarely forms functional homomeric receptors
P2X7	Neurons, astrocytes, microglia	Cerebral cortex, hippocampus, brainstem, nucleus accumbens, spinal cord	Release of proinflammatory cytokines, apoptosis, membrane pore formation, glutamate release, ATP release, induction of synaptic plasticity
Heteromeric
P2X1/P2X2	Neurons	Superior cervical ganglia	ATP-mediated physiological responses
P2X1/P2X5	Astrocytes	Cerebral cortex	ATP-evoked membrane currents
P2X2/P2X3	Neurons	Dorsal root ganglia	Similar to P2X3, but with reduced desensitization
P2X2/P2X6	Neurons	Dogiel type II neurons in myenteric plexus	ATP-mediated physiological responses

P2X1Rs have been shown to cause contraction of neurogenic smooth muscle (Mulryan et al., 2000; Vial and Evans, 2000), platelet activation (Hechler et al., 2003; Mahaut-Smith et al., 2004), and neuronal (Calvert and Evans, 2004; Watano et al., 2004) and glial cell responses (Lalo et al., 2008). Among the P2XRs, the P2X1R has the highest affinity for ATP (EC₅₀ ~ 1 M) (Rettinger and Schmalzing, 2003). The P2X1R is often observed in a heteromeric complex with the P2X2R and P2X5R resulting in biophysical and pharmacological properties distinct from those observed when each of these receptor subtypes is expressed separately (Le et al., 1997; Torres et al., 1998; Haines et al., 1999; Le et al., 1999; Duckwitz et al., 2006; Ase et al., 2010). In superior cervical ganglia neurons, the P2X1R contributes to ATP-mediated responses by forming a heteromeric unit with the P2X2R (Calvert and Evans, 2004), whereas ATP-evoked biphasic membrane currents in mouse cortical astrocytes are regulated by P2X1R/P2X5R heteromeric channels (Lalo et al., 2008).

P2X2Rs are widely expressed in the CNS with predominant expression in the cerebral cortex, cerebellum, striatum, hippocampus, habenula, substantia nigra, dorsal ganglia neurons, mesenteric ganglia neurons and glial cells (Kidd et al., 1995; Vulchanova et al., 1996; Pankratov et al., 1998; Xiang et al., 1998; Kanjhan et al., 1999; Xiang and Burnstock, 2004; Brass et al., 2011). The P2X2R is distinguished from other members of the P2XR family, since multiple splice variants exist in different mammalian species with diverse functional properties (Coddou et al., 2011). Many studies have shown that P2X2Rs play a role in nociceptive transmission, hyperalgesia and allodynia, particularly when present as functional heterotrimers with P2X3Rs (Honore et al., 2002; Jarvis, 2003; Xiang and Burnstock, 2004). The pharmacological properties of P2X2R/P2X3R are similar to the P2X3R, but the desensitization rate of the P2X3R is reduced by its interaction with the P2X2R (Koshimizu et al., 2002). P2X2Rs and P2X2R/P2X3R have been implicated in pain processing (Wirkner et al., 2007); however with chronic pain their functions are altered by the action of other P2XRs, especially those expressed in immune cells, such as microglia (Trang et al., 2006). In addition to interactions with P2X2Rs, P2X3Rs also form homotrimeric receptors that are prominently expressed in primary sensory neurons where they enhance the release of glutamate and substance P (Gu and MacDermott, 1997; Vulchanova et al., 1998; Nakatsuka et al., 2001; Nakatsuka and Gu, 2001), which contribute to both acute and chronic pain sensation (Jarvis, 2003). In vivo studies using P2X2R^−/−, P2X3R^−/− and P2X2R^−/−/P2X3R^−/− mice have contributed significantly to our understanding of neuropathic and inflammatory pain sensation and have lead to the development of therapeutic antagonists to these receptors (Ford, 2011; Gum et al., 2011). The P2X2R also has been suggested to play a role in pre- and postnatal neurogenesis (Cheung et al., 2005).

The P2X4R is expressed throughout the central and peripheral nervous systems (Buell et al., 1996; Rubio and Soto, 2001; Bo et al., 2003; Calvert and Evans, 2004; Tsuda et al., 2009). The P2X4R is upregulated in activated microglial cells after spinal cord or peripheral nerve injury where it appears to mediate the release of brain-derived neurotrophic factor and induce neuropathic pain (Ulmann et al., 2008). Recent studies provide evidence that the functional expression of P2X4Rs in tissue-resident macrophages regulates inflammation-dependent prostaglandin E2 release (Ulmann et al., 2010). Activation of homomeric P2X4Rs in hippocampal neurons has been suggested to contribute to synaptic strengthening and hypersensitivity to sensory stimuli (Tsuda et al., 2009; Baxter et al., 2011). In addition, hippocampal synaptic transmission and long-term potentiation were abolished in P2X4R^−/− mice (Sim et al., 2006). A unique characteristic of the P2X4R is its modulation by trace metals; copper inhibits whereas zinc and cobalt potentiate P2X4R activity (Coddou et al., 2011). P2X4R activity has been shown to be modulated by the allosteric effector ivermectin (Coddou et al., 2011). Heteromeric assembly of the P2X4R with the P2X1, P2X6 and P2X7 receptor subtypes has been described (Le et al., 1998; Nicke et al., 2005; Guo et al., 2007), although the functional relevance of these complexes in vivo is currently unknown.

The expression of the P2X5R subtype in the mouse CNS is most abundant in the olfactory bulb, cerebral cortex, globus pallidum, hippocampus, thalamus, hypothalamus, cerebellar cortex, and mid- and hindbrain nuclei (Guo et al., 2008). Although in vitro data have demonstrated ATP-evoked currents coupled to P2X5R activation, little is known about the physiological relevance of P2X5Rs in the CNS. Guo et al. speculate that P2X5R expression in the molecular layer of the cerebral cortex could play a role in interconnection of local cortical areas and P2X5R expression in the olfactory bulb suggests a role in fast excitatory post-synaptic purinergic transmission (Guo et al., 2009). In vitro studies have shown that activation of the homomeric P2X5R results in small, nondesensitizing currents, whereas activation of frequently observed heteromeric P2X5/P2X1 receptors results in slowly desensitizing ATP-evoked currents (Torres et al., 1998; Le et al., 1999). A P2X5R^−/− mouse has not yet been developed; however, it will be critical for evaluating the role of the P2X5R in vivo. Interestingly, a recent study indicates that most humans express only a nonfunctional isoform of the P2X5R (Kotnis et al., 2010).

In the CNS, the P2X6R is expressed in Purkinje cells in the cerebellum, pyramidal cells in the hippocampus and sensory ganglia (Collo et al., 1996; Rubio and Soto, 2001; Robertson et al., 2001; Burnstock and Knight, 2004; da Silva et al., 2007). The ability of the P2X6R to form functional homomeric receptors is very low due to inefficient glycosylation of the N-terminus (Jones et al., 2004; Ormond et al., 2006). P2X6Rs readily form functional heteromers with P2X2 and P2X4 receptors, where activation of one subtype potentiates the activity of the other (Egan et al., 2004). In the myenteric plexus, the P2X6R is expressed in Dogiel type II neurons where it likely regulates physiological responses to ATP as a heteromeric complex with P2X2Rs (Yu et al., 2010).

Among the P2X receptor subtypes, the P2X7 receptor has gained prominent recognition as a regulator of inflammatory responses (Lister et al., 2007). P2X7Rs are expressed in many types of cells, notably in immune cells where activation by ATP increases the release of proinflammatory cytokines and apoptotic cell death (Pelegrin and Surprenant, 2006; Ferrari et al., 2006). The P2X7R was first cloned from rat brain (Surprenant et al., 1996), and subsequently has been found to be expressed in microglia, neurons and astrocytes (Ballerini et al., 1996; Brandle et al., 1998; Ferrari et al., 2006; Takenouchi et al., 2010). The P2X7R requires high concentrations of ATP (> 0.1 mM) for activation, although the photoaffinity ligand BzATP is a more potent agonist (Gonzalez et al., 1989; Erb et al., 1990). Stimulation of the P2X7R regulates the gating of non-selective cation channels, mitochondrial and plasma membrane depolarization, the formation of plasma membrane pores, plasma membrane blebbing, and the production of reactive oxygen species (ROS), responses ultimately leading to cell death (Erb et al., 1990; Schulze-Lohoff et al., 1998; Morelli et al., 2003; Verhoef et al., 2003; Wang et al., 2004; Adinolfi et al., 2005; Lister et al., 2007; Roger et al., 2008; Bours et al., 2011). P2X7R activity is dramatically potentiated by decreasing the divalent cation concentration, indicating that ATP⁴⁻ may be the active ligand (Steinberg and Silverstein, 1987; Weisman et al., 1989; Hickman et al., 1996). P2X7Rs have been shown to mediate the release of neurotransmitters, e.g., glutamate, GABA, and ATP, and may be required for the induction of synaptic plasticity (Atkinson et al., 2004; Gordon et al., 2005; Duan and Neary, 2006). It also has been shown that P2X7R activation induces hypoxia- and caspase-dependent neuronal cell death (Kong et al., 2005; Sugiyama et al., 2010). Activation of P2X7Rs in glial cells results in the release of the proinflammatory cytokines TNFα, IL-1β and leukotrienes, thereby triggering or potentiating neuroinflammation (Hide et al., 2000; Suzuki et al., 2004; Ballerini et al., 2005; Kataoka et al., 2009), as described below. The P2X7R is upregulated in damaged nerves (Cavaliere et al., 2004; Franke et al., 2004a) and in nerves obtained from neuropathic pain patients (Chessell et al., 2005). In a mouse model of neuropathic pain, hypersensitivity to pain stimuli was completely absent upon deletion of the P2X7R (Chessell et al. 2005). The P2X7R is also upregulated in microglia around β-amyloid plaques in a mouse model of Alzheimer’s disease (AD) where it mediates superoxide production (Parvathenani et al., 2003). Enhanced expression of P2X7Rs also was observed in microglia derived from postmortem AD brains compared with glia obtained from non-demented brains (McLarnon et al., 2006). Furthermore, studies with a mouse model of Huntington’s disease suggest that P2X7Rs may play a role in disease pathogenesis (Diaz-Hernandez et al., 2009). Therefore, the P2X7R receptor could represent a therapeutic target for treating neurodegenerative diseases.

P2Y Receptors in the Central Nervous System

P2Y receptors (P2YRs) are classical heterotrimeric G protein-coupled seven-pass transmembrane receptors, as shown in Fig. 1. The extracellular N-terminus contains several potential glycosylation sites and the C-terminus contains consensus phosphorylation sites for protein kinases (Nguyen et al., 1995; Erb et al., 1995; Brinson and Harden, 2001; Flores et al., 2005). The intracellular loops and C-terminus have structural diversity among P2YR subtypes, thereby influencing the degree of coupling with G_q/11, G_s, and G_i proteins (Abbracchio et al., 2006). The length of human P2YRs varies from 328 (P2Y₆R) to 377 (P2Y₂R) amino acids and the composition reveals two structurally distinct subgroups within the P2YR family, the G_q-coupled P2Y₁, P2Y₂, P2Y₄, P2Y₆,and P2Y₁₁ receptors and the G_i-coupled P2Y₁₂, P2Y₁₃ and P2Y₁₄ receptors (Erb et al., 2006; Abbracchio et al., 2006). The degree of sequence homology among members of the human P2YR family ranges from 20 to 50%, suggesting a relatively high functional diversity (Shaver, 2001). It has been demonstrated that positively charged amino acids within transmembrane domains of P2YRs contribute to agonist binding (Erb et al., 1995; Jiang et al., 1997; Jacobson et al., 1999). P2YRs, whose agonists are adenine and/or uridine nucleotides, are expressed in many cell types comprising the CNS and have been shown to regulate neurotransmission, inflammation, cell growth, and apoptosis (Burnstock et al., 1972; Burnstock, 2009; Neary and Zimmermann, 2009; Burnstock and Verkhratsky, 2010). P2Y₁, P2Y₁₂, P2Y₁₃ and P2Y₁₄ receptors are activated by adenine nucleotides only, whereas the P2Y₂R and rodent P2Y₄R can be activated by either adenine or uridine nucleotides and the human P2Y₄ and P2Y₆ receptors are selective for uridine nucleotides (Abbracchio et al., 2006). The P2Y₁₄R subtype is activated by UDP-glucose (Sak and Webb, 2002; Burnstock and Knight, 2004; Abbracchio et al., 2006). All eight P2Y receptor subtypes are expressed in primary rat astrocytes or astrocytoma cells (Fumagalli et al., 2003; Abbracchio et al., 2006; Kreda et al., 2008; Brandenburg et al., 2010), although the expression patterns vary with age (Lenz et al., 2000; Jacques-Silva et al., 2004). Rodent neurons express P2Y_{1,2,4,6,12,13} receptors (Kong et al., 2009; Espada et al., 2010; Köles et al., 2011). P2YRs are expressed at postsynaptic terminals where P2Y₁, P2Y₂ and P2Y₄ receptors are neuromodulators that have inhibitory roles in synaptic transmission (Rodrigues et al., 2005; Rodrigues et al., 2006; Sperlágh and Illes, 2007; Fischer and Krügel, 2007). The cell and tissue distribution, agonist specificities and functional relevance of P2Y receptors in the nervous system are summarized in Table 2.

Table 2. P2Y Receptors in the Nervous System.

The agonists, expression and function of P2Y receptor subtypes in the nervous system are summarized in Table 2. This table highlights the information presented in this review article and is not considered to be comprehensive.

Receptor Subtype	Agonist	Expression		Function
		Cell Type	Region
P2Y1	ADP, ATP	Neurons, astrocytes, microglia, oligodendrocytes	Cerebral cortex, cerebellum, hippocampus, midbrain, caudate nucleus, putamen, globus pallidus, habenula, subthalamic nucleus, dorsal root ganglia, dorsal horn	Synaptic transmission modulation, provides neuroprotection by stimulating IL-6 release from astrocytes, brain development and repair, sensory reception
P2Y2	ATP, UTP	Neurons, astrocytes, microglia	Cerebral cortex, cerebellum, hippocampus, nucleus accumbens, spinal cord	Promote neurite outgrowth, stimulate α-secretase-dependent processing of amyloid precursor protein, increase phagocytosis of Aβ peptide, regulation of intracellular calcium waves, stimulate proliferation, modulate pain sensation, increase cell motility
P2Y4	ATP, UTP	Neurons, astrocytes, microglia	Cerebral cortex, hippocampus	Synaptic transmission modulation, regulation of blood-brain barrier function, blood flow, metabolic trafficking, water homeostasis
P2Y6	UDP, UTP	Neurons, astrocytes, microglia	Cerebral cortex, cerebellum, hippocampus, amygdala, cingulate gyrus, putamen, nucleus accumbens, superior cervical ganglia, dorsal root ganglia	Stimulate phagocytic activity, neuroinflammatory responses
P2Y11	ATP, ADP	Neurons	Cerebellum, hippocampus, parahippocampal gyrus, putamen, striatum, nucleus accumbens	Neuroinflammatory responses
P2Y12	ADP	Neurons, astrocytes, microglia, oligodendrocytes	Cerebral cortex, cerebellum, hippocampus, nucleus accumbens	Regulation of migration and chemotaxis
P2Y13	ADP	Neurons, astrocytes	Brainstem	Modulation of synaptic transmission, modulates expression of cell survival genes
P2Y14	UDP-glucose	Astrocytes, microglia	Cerebral cortex, cerebellum	Modulation of immune system's anti-tumor response

The P2Y₁R has a widespread distribution in mammalian brain, including the cerebral cortex, hippocampus, caudate nucleus, putamen, globus pallidus, habenula, subthalamic nucleus, midbrain and cerebellum, as demonstrated in autoradiographic and immunohistochemical studies (Simon et al., 1997; Moore et al., 2000; Morán-Jiménez and Matute, 2000). The P2Y₁R is intensely expressed in Purkinje cells, in deep layers of the cerebral cortex and in areas of the hippocampus sensitive to ischemia (Morán-Jiménez and Matute, 2000). P2Y₁R immunoreactivity has also been observed in oligodendrocytes and astrocytes in brain white matter tracts and optic nerves (Simon et al., 1995; Fumagalli et al., 2003). P2Y₁Rs have been suggested to play important roles in glial cell functions (Morán-Jiménez and Matute, 2000). P2Y₁R activation in astrocytes of hippocampal cultures has been suggested to provide neuroprotection from oxidative stress by increasing IL-6 release (Fujita et al., 2009). P2Y₁Rs are also expressed in microglial cells (Gotsch et al., 1997; Simon et al., 1997; Fumagalli et al., 2003; Franke et al., 2004a; Bianco et al., 2005a), rat neuroprogenitor cells (Mishra et al., 2006), and various sensory neurons such as dorsal root ganglia and dorsal horn neurons (Sanada et al., 2002; Ruan and Burnstock, 2003; Kobayashi et al., 2006). Studies have suggested potential roles for P2Y₁Rs in brain development and repair (Mishra et al., 2006) and sensory reception (Ruan and Burnstock, 2003; Gerevich et al., 2005).

The contribution of the P2Y₂R subtype to CNS functions is becoming better understood (Franke and Illes, 2006; Inoue, 2008; Peterson et al., 2010) and appears to be most relevant under pathophysiological conditions, such as inflammation and bacterial infection (Peterson et al., 2010; Chen et al., 2010). The mammalian P2Y₂R is equipotently activated by ATP or UTP (Weisman et al., 2005; Abbracchio et al., 2006) and is upregulated in many cell and animal models of inflammation or injury (Turner et al., 1997; Koshiba et al., 1997; Seye et al., 2002; Shen et al., 2004; Schrader et al., 2005; Kong et al., 2009), including acute and chronic stages of spinal cord injury (Rodríguez-Zayas et al., 2010), brain ischemia, mechanical injury to the nucleus accumbens and brain trauma (Franke et al., 2004a; Franke et al., 2004b), suggesting that P2Y₂R upregulation represents a cellular response to tissue damage and inflammation. P2Y₂R expression under proinflammatory conditions is regulated by NF-κB binding to the P2Y₂R promoter (Degagne et al., 2009), consistent with the established role of NF-κB activation in the induction of inflammation (Wullaert et al., 2011). In addition to the typical G_q-coupled activation of the PLC/IP₃/PKC pathway, the P2Y₂R has a variety of structural motifs that enable it to activate integrin and growth factor receptor signaling cascades, as shown in Fig. 2. For example, P2Y₂R activation by ATP or UTP has been shown to induce phosphorylation of growth factor receptors which increases the activities of the MAP kinases ERK1/2 and the related adhesion focal tyrosine kinase (RAFTK) via a pathway dependent upon Src and Shc/Grb2 (Soltoff, 1998; Soltoff et al., 1998; Seye et al., 2004). However, other studies show that P2Y₂R-mediated EGFR phosphorylation is Src-independent, but requires the release of growth factors via P2Y₂R-dependent activation of the matrix metalloproteases ADAM10 and ADAM17 (Ratchford et al., 2010). These differences appear to be due to cell type, e.g., endothelial vs. epithelial. The P2Y₂R is unique among GPCRs in that it contains the consensus integrin-binding motif Arg-Gly-Asp (RGD) in a putative extracellular domain that enables the association of the P2Y₂R with α_vβ_3/5 integrins allowing activation of heterotrimeric G_o and G₁₂ proteins that regulate the activities of the small GTPases Rho and Rac (Erb et al., 2001; Bagchi et al., 2005; Liao et al., 2007), regulators of actin polymerization and cytoskleletal rearrangements required for cell migration (Xu et al., 2003; Jang et al., 2005). In addition, the P2Y₂R contains SH3-binding motifs in its C-terminal domain that can interact with the actin-binding protein filamin A (FLNa), another known regulator of cytoskeletal rearrangements, suggesting that the P2Y₂R may regulate migration of cells by both RGD-dependent interactions with integrins and C-terminal-dependent association with actin-binding proteins (Yu et al., 2008). Moreover, the P2Y₂R has been reported to regulate migration of some cell types via transactivation of growth factor receptors (Bagchi et al., 2005; Norambuena et al., 2010), suggesting that a complex array of signaling events uniquely coupled to P2Y₂R activation may be required to optimize the cytoskeletal rearrangements whereby the P2Y₂R regulates cell-specific functions, e.g., neurite outgrowth, phagocytosis, synapse formation and chemokinesis.

Fig. 2. P2Y₂R Signaling Pathways.

The P2Y₂R modulates a variety of cellular processes through classical G protein-coupled receptor pathways and unique receptor motifs. Activation of the P2Y₂R by ATP or UTP stimulates the G_q-dependent activation of PLC leading to the generation of IP₃ and DAG. IP₃ triggers a release of Ca²⁺ from intracellular stores leading to an increase in [Ca²⁺]_i and the activation of Ca²⁺-dependent proteins, whereas DAG serves to activate PKC leading to activation of a variety of downstream proteins including RAFTK; also known as Pyk2) and the MAP kinases ERK1/2. The Src-homology-3 (SH3) domains in the C-terminus allow the P2Y₂R to stimulate Src-dependent transactivation of growth factor receptors and their downstream signaling molecules Shc and Grb2 leading to ERK1/2 and p38 activation of cell proliferation and neurite outgrowth. The P2Y₂R also has been shown to upregulate VCAM-1 through a pathway involving VEGFR-2. Furthermore, P2Y₂R activation has been shown to stimulate the PI3K/Akt pathway to inhibit apoptosis in neurons, a response that was dependent on Src activation. SH3 domains also allow the P2Y₂R to interact with the actin cytoskeleton via the actin-binding protein filamin A (FLNa). Alternatively the P2Y₂R can access the actin cytoskeleton through an extracellular RGD domain that interacts with α_vβ_3/5 integrins to enable activation of G_o and G₁₂ proteins allowing the P2Y₂R to stimulate cell migration, phagocytosis, neurite outgrowth, and diapedesis by activating the cytoskeletal regulators Rac and Rho. Lastly, the P2Y₂R is able to activate the matrix metalloproteases ADAM10 and ADAM17 to induce non-amyloidogenic APP processing and shedding of growth factors.

P2Y₂R expression in rat primary cortical neurons is upregulated in response to IL-1β (Kong et al., 2009), a cytokine whose levels are elevated in the brains of AD patients (Xie et al., 2004; Lee et al., 2010). Subsequent activation of these upregulated P2Y₂Rs in neurons promotes neurite outgrowth (Pooler et al., 2005) and generates the non-amyloidogenic soluble APPα peptide, rather than neurotoxic Aβ_1–42 peptide aggregates associated with AD (Kong et al., 2009). In mouse primary microglial cells, the P2Y₂R is upregulated in the presence of Aβ_1–42 and when activated can increase the phagocytosis and degradation of neurotoxic forms of Aβ (Boucsein et al., 2003; Peterson et al., 2010; Kim et al., 2012). In astrocytic cells, the P2Y₂R has been suggested to contribute to synaptic transmission through the regulation of intracellular calcium waves (Halassa et al., 2009) and upregulates anti-apoptotic protein expression to promote cell survival (Chorna et al., 2004). Thus, P2Y₂R upregulation in response to proinflammatory conditions likely serves a neuroprotective role in the CNS that requires contributions from both glial and neuronal P2Y₂Rs, as described in more detail below.

The human P2Y₄R is preferentially activated by uridine nucleotides, whereas the rat and mouse P2Y₄Rs are stimulated equipotently by ATP and UTP (Webb et al., 1998; Brunschweiger and Muller, 2006; Jacobson et al., 2009). P2Y₄R mRNA is highly expressed in human brain (Moore et al., 2001). Single cell RT-PCR demonstrated the expression of P2Y₄Rs in rat hippocampal pyramidal neurons (Rodrigues et al., 2005). The expression of P2Y₄Rs in astrocytes and microglial cells has been extensively documented (Webb et al., 1998; Lenz et al., 2000; Fumagalli et al., 2003; Bianco et al., 2005a). P2Y₄Rs, as well as P2Y₂Rs, are strongly expressed in glial endfeet in proximity to blood vessel walls (Simard et al., 2003) where their activation by ATP has been postulated to regulate blood–brain barrier function, blood flow, metabolic trafficking and water homeostasis (Simard et al., 2003; Zonta et al., 2003).

The P2Y₆R is activated by uridine 5’-diphosphate (UDP), and to a lesser extent UTP (Communi et al., 1996). In 18 areas of the human brain, the level of P2Y₆R mRNA expression was highest in the amygdala, cingulate gyrus, nucleus accumbens and putamen (Moore et al., 2001). Single cell RT-PCR revealed P2Y₆R mRNA in 2 of 12 pyramidal neurons of rat hippocampus (Rodrigues et al., 2005). In addition, P2Y₆R mRNA has been demonstrated in superior cervical ganglion (Calvert and Evans, 2004; Calvert et al., 2004) and dorsal-root ganglion neurons (Sanada et al., 2002; Ruan and Burnstock, 2003). Functional studies have revealed the presence of P2Y₆R activity in cerebellar and cortical astrocytes (Fumagalli et al., 2003; Bennett et al., 2003). P2Y₆R activation has been shown to increase phagocytotic activity of microglia, postulated to occur in vivo in response to UTP released from damaged cells (Koizumi et al., 2007; Liu et al., 2009). Consistent with this hypothesis, injury has been shown to induce increased P2Y₆R expression in astroglial cells (Franke et al., 2004a; Franke et al., 2004b). In microglial cells stimulated overnight with bacterial lipopolysaccharide, P2Y₆R-mediated increases in the intracellular calcium concentration were observed, suggesting a role for the P2Y₆R in neuroinflammation (Bianco et al., 2005a).

The P2Y₁₁R can couple to multiple G proteins to regulate the activity of two second messenger systems: adenylate cyclase-mediated cAMP production, and PLC-dependent production of IP₃ and DAG that modulate calcium release from intracellular storage sites and protein kinase C activation, respectively (Communi et al., 1999). The P2Y₁₁R is activated by ATP or ADP, but not by uridine nucleotides (Communi et al., 1999). P2Y₁₁R mRNA expression is prominent in nucleus accumbens, parahippocampal gyrus, putamen and striatum (Burnstock and Knight, 2004). The P2Y₁₁R has been localized to single rat hippocampal pyramidal neurons and to Purkinje cells in adult rat cerebellum (Rodrigues et al., 2005; Volonté et al., 2006). Inhibition of the P2Y₁₁R has been shown to delay ATP-induced neutrophil apoptosis, suggesting a role for the P2Y₁₁R in the regulation of neuroinflammatory responses (Vaughan et al., 2007).

The P2Y₁₂R is widely distributed in the brain with a pattern consistent with expression in astrocytes (Hollopeter et al., 2001; Kunapuli et al., 2003). RT-PCR has demonstrated the presence of P2Y₁₂R mRNA in single rat hippocampal pyramidal neurons (Rodrigues et al., 2005). Cortical and cerebellar astrocytes and astrocytes in the rat nucleus accumbens also express P2Y₁₂Rs (Fumagalli et al., 2003; Franke et al., 2004b; Carrasquero et al., 2005). P2Y₁₂Rs have been suggested to regulate the migration of microglial cells towards damaged neurons (Sasaki et al., 2003). P2Y₁₂R expression in microglia is robust in the 'resting' state, but dramatically reduced in activated microglia, and P2Y₁₂R^−/− mice have significantly diminished directional branch extension toward sites of cortical damage in vivo (Haynes et al., 2006). In contrast, a recent study concludes that the expression of the P2Y₁₂R in the CNS is restricted to oligodendrocytes (Amadio et al., 2006). It also has been suggested P2Y₁₂Rs contribute to the migration and adhesion of glial cell processes to axons during pre-myelination (Amadio et al., 2006).

The P2Y₁₃R is activated by ADP (Marteau et al., 2003) and 2-methylthio ADP is a potent synthetic agonist (Burnstock, 2006b), similar to the P2Y₁₂R; however ATP and ATP analogues are inactive at the P2Y₁₃R (Communi et al., 1997). P2Y₁₃R expression has been localized to brainstem astrocytes and glutamatergic neurons (Moore et al., 2000; Moore et al., 2001; Jiménez et al., 2011). P2Y₁₃Rs, along with P2Y₁ and P2Y₁₂ receptors, have been shown to regulate Na⁺ and Cl⁻-dependent synaptic glycinergic neurotransmitter transporters to increase transport of glycine from the synaptic cleft, thereby maintaining quantal glycine levels in inhibitory synaptic vesicles (Gomeza et al., 2003; Jiménez et al., 2011). The P2Y₁₃R can also activate the glycogen synthase kinase-3 (GSK-3)-dependent phosphatidylinositoI 3-kinase (PI3K)/Akt survival pathway to increase translocation of the GSK-3 substrate β-catenin to the nucleus, where it modulates expression of cell survival genes (Ortega et al., 2008).

The P2Y₁₄R is expressed in astrocytes (Moore et al., 2001), and RT-PCR and single cell Ca²⁺ imaging has documented the functional expression of P2Y₁₄Rs in rat cortical and cerebellar astrocytes (Fumagalli et al., 2003; Carrasquero et al., 2005). Agonists of the P2Y₁₄R include UDP-glucose, UDP-galactose, UDP-glucuronic acid and UDP-N-acetylglucosamine, but not adenine or uridine nucleotides (Chambers et al., 2000; Abbracchio et al., 2003; Burnstock, 2006a). UDP-glucose has been shown to be released from a variety of cell lines, and UDP-glucose levels can exceed those of ATP under various conditions (Lazarowski et al., 2003). Functionally, P2Y₁₄Rs in primary microglial cells from rat brain have been shown to modulate the calcium response to bacterial lipopolysaccharide (Bianco et al., 2005a). P2Y₁₄Rs expressed in immature dendritic cells have been suggested to play a role in the immune system’s anti-tumor response (Skelton et al., 2003; Fischer and Krügel, 2007).

Neuroinflammatory P2X7Rs Regulate Neuroprotective P2Y₂R Expression

P2X7R activation contributes to neuroinflammation by promoting mitochondrial and plasma membrane depolarization, the formation of plasma membrane pores, plasma membrane blebbing, and the production of reactive oxygen species (ROS) (Schulze-Lohoff et al., 1998; Morelli et al., 2003; Verhoef et al., 2003; Wang et al., 2004; Adinolfi et al., 2005; Lister et al., 2007; Roger et al., 2008; Bours et al., 2011). In addition, P2X7R activation promotes neuroinflammation by causing the release of proinflammatory cytokines, such as IL-1β and TNF-α (Di Virgilio, 2007; Lister et al., 2007; Tschopp and Schroder, 2010) and activation of MAP kinases and NF-κB, resulting in upregulation of proinflammatory gene products, including COX-2, chemokines and cell adhesion molecules (Pfeiffer et al., 2004; Potucek et al., 2006; Lister et al., 2007; Lenertz et al., 2009; Skaper et al., 2009; Shiratori et al., 2010) and the P2Y₂R (Degagne et al., 2009). Importantly, P2X7R-mediated pore formation initially increases ATP release through P2X7R interactions with a pannexin hemi-channel in cells (Pelegrin and Surprenant, 2006). P2X7R-mediated IL-1β and ATP release is a mechanism whereby the P2X7R regulates functional P2Y₂R expression in neurons and provides agonist for the activation of the upregulated P2Y₂R and other P2 receptors (Peterson et al., 2010). ATP release also can occur from activated microglia and astrocytes in response to oxidative stress (Peterson et al., 2010), following neuronal excitation (Bodin and Burnstock, 2001; Fields, 2011), via volume-activated anion channels (Fields, 2011), or upon exposure of cells to fibrillar or oligomeric forms of amyloidogenic A _1–42 peptides (Inoue, 2008; Sanz et al., 2009; El Khoury and Luster, 2008; Kim et al., 2012). Thus, P2X7 and P2Y₂ receptors may represent promising targets to control inflammatory responses associated with neurodegenerative diseases. Indeed, mice deficient in the P2X7R (P2X7R^−/− mice) exhibit decreased inflammatory responses (Labasi et al., 2002; Chessell et al., 2005; McGaraughty et al., 2007; Lucattelli et al., 2011), including a reduction in pulmonary fibrosis in a mouse model of lung inflammation (Lucattelli et al., 2011) and the absence of pain hypersensitivity in mouse models of chronic inflammation and neuropathic pain (Chessell et al., 2005). Phase I and II clinical trials for selective P2X7R antagonists are presently underway for the treatment of rheumatoid arthritis and other inflammatory diseases (Friedle et al. 2010; Arulkumaran et al. 2011).

Upregulation of the P2Y₂R in response to P2X7R activation appears to promote neuroprotective responses. The ability of the P2Y₂R to stimulate neuroprotective responses depends upon the coupling of the receptor to intracellular signaling pathways that are distinct among the P2YR family (see Fig. 2). These responses associated with P2Y₂R upregulation include the outgrowth and stabilization of dendritic spines (Jang et al., 2005; Bamburg and Bloom, 2009; Peterson et al., 2010), which requires RGD-dependent P2Y₂R/α_v integrin interaction to stimulate Rac and Rho and induce cytoskeletal rearrangements (Bagchi et al., 2005; Liao et al., 2007) and upregulation of neurofilament M and neurofilaments that promote neurite outgrowth (Pooler et al., 2005). P2Y₂Rs also require Src to co-localize with the tyrosine receptor kinase A (TrkA) in the presence of nerve growth factor, a pathway that regulates neurite outgrowth and cell division via the activation of p38 and ERK1/2 MAP kinases (Arthur et al., 2005, 2006a). In neural progenitor cells isolated from the subventricular zone of adult mouse brain, P2Y₂R activation was shown to induce proliferative responses such as the transient activation of the epidermal growth factor receptor (EGFR), the MAP kinases ERK1/2 and the transcription factor CREB (Grimm et al., 2009). Other studies indicate that the P2Y₂R mediates the activation of PI3-kinase/Akt and MAP kinases to inhibit apoptosis of PC12 pheochromocytoma cells and dorsal root ganglion neurons (Arthur et al., 2006a; Arthur et al., 2006b). P2Y₂R upregulation by IL-β1 and subsequent activation in primary cortical neurons increases amyloid precursor protein (APP) processing via activation of matrix metalloproteases (i.e., α-secretases), a neuroprotective response that produces a non-amyloidogenic soluble APP peptide (i.e., sAPPα) rather than neurotoxic amyloidogenic Aβ peptide (Kong et al., 2009). IL-1β is known to stimulate neuronal synthesis of APP and increase the release of neurotoxic Aβ, which further enhances IL-1β production (Walker et al., 1995). We postulate that upregulation of the P2Y₂R induced by IL-1β in vivo counteracts the potential neurotoxic effects of IL-1β-dependent elevations in APP levels by promoting generation of non-toxic sAPPα instead of Aβ. Thus, P2Y₂R upregulation in the CNS may delay the progression of neurodegeneration associated with reactive gliosis and chronic inflammation in AD and other neurological disorders.

Glial cells, including astrocytes and microglia, play important neuroprotective roles. Astrocytes contribute to the maintenance of the blood-brain barrier (BBB) (Sudo et al., 1998; Shlosberg et al., 2010; Barreto et al., 2011), which prevents invasion of pathogenic, neurotoxic substances into the brain from the circulation (Takenouchi et al., 2009; Sugama et al., 2009). Astrocytes also release neurotrophic factors that regulate neuronal survival and sprouting and supply energy substrates to neurons (Giaume et al., 2010). Astrocytes have been shown to release ATP under a variety of pathological conditions (Butt, 2011; Hamilton et al., 2008; Hamilton et al., 2010) and ATP levels are elevated sufficiently by inflammation in vivo to activate P2 nucleotide receptors (Butt, 2011). P2Y₂Rs are upregulated in reactive astrocytes of the rat cortex and nucleus accumbens in response to mechanical injury (Franke et al., 2004a; Franke et al., 2004b) and have been suggested to enhance astrocyte survival (Chorna et al., 2004; Burgos et al., 2007). In addition, interactions between the P2Y₂R and integrins have been demonstrated to regulate the migration of astrocytes (Wang et al., 2005; Bagchi et al., 2005; Liao et al., 2007). It also has been shown that P2X7R activation increases the expression of P2Y₂Rs in rat astrocytes (D'Alimonte et al., 2007) likely via P2X7R-mediated IL-1β release (Chakfe et al., 2002; Suzuki et al., 2004; Choi et al., 2007; Mingam et al., 2008; Sanz et al., 2009; Takenouchi et al., 2009).

Microglia have important immunoregulatory functions in the CNS. Injury or other insults to the CNS trigger transformation of quiescent microglia into activated phenotypes, i.e., phagocytic macrophages (Kreutzberg, 1996; Streit, 2002). Activated microglia have neuroprotective functions (Streit, 2002; Butovsky et al., 2005; Turrin and Rivest, 2006; Lalancette-Hébert et al., 2007; El Khoury et al., 2007), although sustained activation can be neurotoxic (Butovsky et al., 2005; Boillée et al., 2006; Streit, 2006; Neumann and Takahashi, 2007). Microglial cell activation by proinflammatory cytokines has been shown to increase cell motility and proliferation (Brown and Neher, 2010), responses associated with reactive gliosis in neurodegenerative diseases. Adenine and uridine nucleotides have been shown to increase the motility of microglial cells (Honda et al., 2001; Haynes et al., 2006; Koizumi et al., 2007) via activation of P2Y₂ and P2Y₁₂ receptors (Haynes et al., 2006; Chen et al., 2006) and ATP release can significantly increase microglial process extension towards a site of injury (Davalos et al., 2005). The endogenous expression of P2Y₂Rs has been reported in mouse microglia (Shigemoto-Mogami et al., 2001; Crain et al., 2009) where they have been shown to regulate responses associated with reactive gliosis (Chorna et al., 2004; Franke et al., 2004b; Wang et al., 2005; Weisman et al., 2005; Burgos et al., 2007; Peterson et al., 2010). For example, the P2Y₂R agonists UTP and ATP released from apoptotic cells have been shown to induce migration of phagocytic cells (Elliott et al., 2009), which presumably serves to enhance the clearance of cellular debris. Microglial cells exposed to Aβ also have been shown to release ATP (Rogers and Lue, 2001; Sanz et al., 2009). Studies using peritoneal macrophages in mice have shown that stimulation of P2Y₂ and P2Y₁₂ receptors induces the formation of lamellipodia in membrane protrusions which is required for cell motility (Kronlage et al., 2010). Co-activation of P2Y₂ and P2Y₆ receptors in human monocytes enhances migration, a response shown to involve toll-like receptor-induced IL-8 release (Ben Yebdri et al., 2009; Kukulski et al., 2010). We have found that P2Y₂R activation increases mouse microglial cell migration and phagocytic activity, such as the uptake of neurotoxic oligomeric A _1–42, responses that are absent in microglia from P2Y₂R^−/− mice (Kim et al., 2012). Both activated astrocytes and microglia internalize and degrade A (Chung et al., 1999; Wyss-Coray et al., 2001; Pihlaja et al., 2008; Mandrekar et al., 2009; Kong et al., 2010), a pathway that reduces Aβ toxicity in neurons that is postulated to play a role in the progression of AD. We speculate that P2Y₂Rs in glial cells contribute to the phagocytosis and degradation of neurotoxic forms of Aβ in vivo under conditions where elevated levels of ATP release and IL-1β generation occur (Ferrari et al., 1997; Di Virgilio et al., 1998).

Recent studies suggest that peripheral leukocytes and hematopoietic cells that differentiate into microglia have important functions in the CNS (Gate et al., 2010), particularly in response to tissue injury (Hawkes and McLaurin, 2009). P2Y₂Rs in endothelial cells that form the BBB may also regulate the migration of leukocytes across the BBB towards sites of injury or disease in the brain. Activation of the endothelial P2Y₂R has been shown to enhance the diapedesis of neutrophils towards the chemoattactant lipopolysaccharide of gram-negative bacteria (Kukulski et al., 2010) through a mechanism involving Rho kinase activation, suggesting that P2Y₂R associations with integrins may be involved (Bagchi et al., 2005; Liao et al., 2007). Microglia derived from bone marrow have been shown to phagocytose Aβ deposits in the brain of AD mice to a greater extent than resident brain microglia (Simard et al., 2006). Thus, diapedesis of microglia across the BBB, in addition to neurite outgrowth, non-amyloidogenic APP processing and phagocytosis of neurotoxic forms of Aβ may comprise a neuroprotective phenotype linked to P2Y₂R activation in several cell types that comprise the brain (i.e., neurons, glial cells and endothelium). The neuroprotective pathways by which P2X7R-mediated upregulation and activation of P2Y₂Rs are suggested to contribute to neuroprotection in the brain are shown in Fig. 3.

Fig. 3. P2X7R-mediated Neuroinflammation Stimulates P2Y₂R-mediated Neuroprotective Responses.

(1) ATP released under neuroinflammatory conditions can (2) activate the P2X7R to stimulate (3) the release of proinflammatory cytokines, including interleukin-1β (IL-1β), and further ATP release via interaction of the P2X7R with pannexin hemi-channels. In response to the proinflammatory environment, quiescent microglia take on an (4a) activated phenotype and P2Y₂R activation by extracellular ATP increases (5a) cell motility. In addition, (4b) IL-1β upregulates P2Y₂R expression in neurons and glia through NF-κB activation. (6) ATP release (from Aβ exposure, cytokine exposure, oxidative stress, etc.) provides agonist for the (5b) P2Y₂R to stimulate non-amyloidogenic APP processing and neurite outgrowth through P2Y₂R interactions with matrix metalloproteases and the actin cytoskeleton, respectively. Another neuroprotective response to (7) P2Y₂R activation in microglial cells is increased phagocytosis and degradation of neurotoxic oligomeric Aβ_1–42.

Conclusion

This review summarizes data indicating that seven ionotropic P2X and eight G protein-coupled P2Y receptors for extracellular nucleotides are expressed in cell types comprising the CNS and these P2X and P2Y receptor subtypes have been shown to regulate diverse physiological and pathological responses under a variety of conditions. Recent studies indicate that activation of the P2X7R rsubtype during inflammation causes upregulation and activation of P2Y₂Rs to promote neuroprotective responses. These findings suggest that ATP released from injured or stressed cells in the CNS can activate P2X7Rs in microglial cells to increase the release of proinflammatory cytokines, such as IL-1β, that increase the expression of the P2Y₂R, particularly in neurons. Other studies indicate that both P2X7R and P2Y₂R activation can increase phagocytosis of neurotoxic forms of Aβ and that activation of the P2Y₂R increases non-amyloidogenic APP processing, neuroprotective responses that are postulated to delay the onset or retard the progression of neurodegenerative diseases, such as Alzheimer’s disease. In addition, P2Y₂R activation in neurons has been shown to increase neurite outgrowth. The P2Y₂R contains multiple motifs that enable its activation to directly couple to integrin and growth factor receptor signaling pathways that play a role in cell proliferation and differentiation and cytoskeletal rearrangements that are critical for tissue repair. Thus, the studies described in this review suggest that the P2X7R and P2Y₂R are promising targets for the treatment of neurodegenerative diseases.