P2X(7) Receptors in Neurological and Cardiovascular Disorders

Abstract

P2X receptors are ATP-gated cation channels that mediate fast excitatory transmission in diverse regions of the brain and spinal cord. Several P2X receptor subtypes, including P2X(7), have the unusual property of changing their ion selectivity during prolonged exposure to ATP, which results in a channel pore permeable to molecules as large as 900 daltons. The P2X(7) receptor was originally described in cells of hematopoietic origin, and mediates the influx of Ca(2+) and Na(+) and Ca(2+) and Na(+) ions as well as the release of proinflammatory cytokines. P2X(7) receptors may affect neuronal cell death through their ability to regulate the processing and release of interleukin-1beta, a key mediator in neurodegeneration, chronic inflammation, and chronic pain. Activation of P2X(7), a key mediator in neurodegeneration, chronic inflammation, and chronic pain. Activation of P2X(7) receptors provides an inflammatory stimulus, and P2X(7) receptor-deficient mice have substantially attenuated inflammatory responses, including models of neuropathic and chronic inflammatory pain. Moreover, P2X(7) receptor activity, by regulating the release of proinflammatory cytokines, may be involved in the pathophysiology of depression. Apoptotic cell death occurs in a number of vascular diseases, including atherosclerosis, restenosis, and hypertension, and may be linked to the release of ATP from endothelial cells, P2X(7) receptor activation, proinflammatory cytokine production, and endothelial cell apoptosis. In this context, the P2X(7) receptor may be viewed as a gateway of communication between the nervous, immune, and cardiovascular systems.

Figure 1

The inflammatory cycle and neurodegeneration: Alzheimer's disease as a case in point. In Alzheimer's disease, the neuroinflammatory cycle is characterized by sustained activation of microglia and astrocytes in response to activating stimuli, in particular, amyloid beta (Aβ). Glia proinflammatory responses activated by Aβ include induction of cytokines (TNF-α, IL-1β, S100β), chemokines (macrophage inflammatory proteins-1α, β: MIP-1α, MIP-1β), and oxidative stress-related molecules (nitric oxide, NO), which can cause neuronal cell dysfunction and/or death and can further propagate the inflammatory response.

Figure 2

Structure and signaling functions of the P2X₇ receptor. (a) Each functional P2X₇ receptor is a trimer [18], with the three protein subunits arranged around a cation-permeable channel pore. The subunits all share a common topology, possessing two plasma membrane spanning domains (TM1 and TM2), a large extracellular loop with the ATP binding site, and containing 10 similarly spaced cysteines and glycosylation sites, and intracellular carboxyl and amino termini. (b) Brief ATP activation (<10 seconds) of the P2X₇ receptor results in rapid and reversible channel opening that is permeable to Na⁺, K⁺, and Ca²⁺. Acute receptor activation also triggers a series of cellular responses, such as depolarization, degranulation, and membrane blebbing, along with signaling cascades (see Figure 3 for further details). (c) Continued stimulation results in the formation of a larger plasma membrane pore, which facilitates the uptake of cationic molecules up to 900 Da (including ethidium bromide, which is frequently used as a tool to measure channel permeability, based on its property of generating a fluorescent signal upon DNA binding). Further activation of the receptor in some cell types results in the induction of apoptosis/cell lysis. ATP-induced increase in IL-1β release is mediated mainly through the activation of IL-1β converting enzyme (also known as caspase-1). Activation of the P2X₇ receptor triggers the efflux of K⁺ from cells which in turn activates IL-1 converting enzyme, leading to cleavage of pro-IL-1β to mature IL-1β and release from the cell.

Figure 3

Schematic depiction of the signal transduction events occurring in microglia following P2X₇ receptor activation. Extracellular calcium influx triggered by activation of ionotropic P2X₇ receptors leads to activation of calcineurin and dephosphorylation/activation of NFAT (nuclear factor of activated T cells). P2X₇ receptor activation also results in activation of phospholipases A₂ and D (PLA₂, PLD), as well as tyrosine phosphorylation (P-Tyr) and activation of mitogen-activated protein kinase (MAPK) pathway proteins (MAPK kinase, MEK; extracellular signal-regulated kinase, ERK). The latter can then influence the activity of transcription factors like NF-κB (nuclear factor-κB), CREB (cyclic AMP response element (CRE)-binding protein), and AP-1 (activator protein-1) which upregulate expression of pro-inflammatory genes, such as cyclooxgenase-2 (COX-2) and inducible nitric oxide synthase (iNOS). Activation of P2X₇ receptors also leads to p38 MAPK activation with consequent phosphorylation/activation of CREB. Broken lines indicate multistep pathways.

Figure 4

P2X₇ receptor activation injures cortical neurons in vitro. Cocultures of rat cortical neurons and microglia were incubated for 3 days ± 100 μM 2′, 3′-O-(4-benzoyl-benzoyl)ATP (BzATP) ± 3 μM Brilliant Blue G (Blue G). Labeling for the neuron-specific marker βIII-tubulin showed neurons to survive well and elaborate extensive neurite networks in cultures with unstimulated microglia (a), whereas BzATP caused a drastic and neuron-selective degeneration (b) that the P2X₇ receptor antagonist Brilliant Blue G (c) prevented. Reproduced from Skaper et al. [57], with permission from Wiley-Liss, Inc.

Figure 5

Schematic representation of the conditions which can lead to P2X₇ receptor (P2X₇R) activation in the nervous (a) and cardiovascular (b) systems. Tissue trauma, stress, mechanical injury, infection, and autoimmune disorders, among others, can lead to increased extracellular levels of ATP and/or proinflammatory cytokines. Extracellular ATP diffuses to activate neighboring cells by paracrine and autocrine pathways. In this context signaling through the P2X₇ receptor may allow cells to sense and respond to events occurring in the extracellular environment, modulate the transcription of genes involved in cellular inflammatory processes, and thus regulate cytokine responses. The P2X₇ receptor may function as an amplification device to spread the ATP wave as its activation triggers further ATP (and proinflammatory mediator) release, culminating in pathology. These characteristics, coupled with the broad distribution of P2X₇ receptors encourage the therapeutic exploitation of this target. AD Alzheimer's disease.