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 P2Y(2) receptor-mediated responses to neuroinflammatory and neuroprotective mechanisms.

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

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.

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.