PET imaging of P2 purinoceptors
Purinergic receptors (purinoceptors) include the P1 purinoceptors (adenosine receptors), and P2 purinoceptors which are preferably activated by ATP and other substrates than adenosine. ATP and its breakdown product adenosine are primitive signalling molecules, that can modulate the postsynaptic response and presynaptic release of their cotransmitter. Modulatory functions of purinoceptors are observed in most nerves, including cholinergic transmission in motor nerves and neuromuscular junctions, sensory-motor nerves, sympathetic and parasympathetic nerves, and in the central nervous system (Burnstock and Verkhratsky, 2012).
ATP is used as signalling molecule also by other cells, including vascular endothelial cells, blood cells, muscle, liver, spleen. Renal P2 purinoceptors affect tubular function and renal perfusion. Cells that participate in ATP signalling have ATP containing vesicles, and the vesicular nucleotide transporter (SLC17A9), which concentrates ATP into the vesicles.
ATP is synthesized in mitochondria, and its concentration in cytosol is high, about 3-10 mM, and can be much higher in ACh and serotonin containing vesicles, and in lysosomes; in extracellular space [ATP] is in nM range. Some ATP can diffuse from cells through ion channels, for example mechanical stimulation may activate chloride channels. Cell damage leads to increased extracellular ATP, which can signal tissue damage to the inflammatory system. At the sites of inflammation, extracellular ATP concentrations can be 4-5 orders of magnitude higher than in healthy tissue (Di Virgilio et al., 2017). Extracellular nucleotides are rapidly catabolized by ectonucleases.
P2Y receptors are stimulated by nucleotides such as ATP, ADP, UTP, UDP and UDP-glucose, with different substrate specificity. Several P2Y receptor subtypes have been identified. In humans, at least P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, P2Y13, and P2Y14 have been found.
PET tracers for P2YRs are being developed, but not in use.
ATP-gated nonselective cation channels (P2XRs) are trimers (either homomeric or heteromeric) of subunits (P2X1 - P2X7). Three ATP binding sites are probably located between the subunits.
P2X receptors are expressed by wide array of cell types. For example, P2X1R is expressed on astrocytes, oligodendrocytes, smooth, skeletal and heart muscle, osteoblasts and osteoclasts, adipocytes, platelets, endothelial cells, and exocrine secretary cells; P2X2R on neurons, astrocytes, oligodendrocytes, osteoblasts and osteoclasts, cartilage, keratinocytes, erythrocytes, and endocrine secretary cells; P2X3R on cardiac muscle, keratinocytes, and endothelial cells; P2X4R on neurons, microglial cells, cardiac muscle, osteoblasts and osteoclasts, endothelial cells, white blood cells, erythrocytes, and endo- and exocrine cells; P2X5R on neurons, astrocytes, osteoblasts, keratinocytes, and epithelial cells; P2X6R on neurons, heart muscle, epithelial cells, and endocrine secretory cells. Skeletal muscle cells and sympathetic neurons contain all P2X subtypes.
P2X7R is structurally and functionally distinct from the other P2X receptors. The affinity of ATP for P2X7R is low, and micromolar extracellular concentrations of ATP do not activate it. Intense stimulation of P2X7Rs results in the formation of a large transmembrane pore that is permeable to molecules of size up to 900 Da (Burnstock and Verkhratsky, 2012), further increasing the extracellular [ATP], and that can lead to activation of caspases and cell death.
P2X7R is expressed in many cell types, including sympathetic neurons, astrocytes, oligodendrocytes, microglial cells, probably also CNS neurons, skeletal muscle cells, osteoblasts and osteoclasts, keratinocytes, fibroblasts, epithelial cells, white blood cells, erythrocytes, and endo- and exocrine cells. In the brain, P2X7R levels are highest in microglia, and the receptor/ion channel is important in regulation of immune responses, including the neuroinflammatory cascades that precede and promote many neurodegenerative brain diseases such as Parkinson's disease and MS. P2X7R is also found on the outer membrane of mitochondria, with its ATP-binding site facing the cytosol (Sarti et al., 2021), where it possibly participates in regulation of oxidative phosphorylation via Complex I.
The gene of the P2X7R is polymorphic (Fuller et al., 2009), including many splice variations and single nucleotide polymorphisms, associated with diseases including multiple sclerosis (Oyanguren-Desez et al., 2011; Harding & Robertson, 2019), bipolar disorder (McQuillin et al., 2009), and increased risk for bone fractures (Gartland et al., 2012; Jørgensen et al., 2012); and reduced risk of cardiovascular events (Gidlöf et al., 2012). Genotype effects may need to be accounted for in P2X7R PET data analyses (Van Weehaeghe et al., 2019).
Various inflammatory mediators can upregulate P2X7R expression on macrophages and other cell types, while anti-inflammatory mediators can downregulate the expression (Bartlett et al., 2014). In neuroinflammation rat models, P2X7R expression was transiently elevated in acute inflammation model, but not in chronic inflammation model (Crabbé et al., 2019).
Several PET tracers targeting P2X7R have been synthesized, but usually found not to be suitable for clinical brain studies because of complex radiochemistry, poor stability or poor BBB permeability (Fantoni et al., 2017; Fu et al., 2019; Zheng, 2020). [11C]GSK1482160 (Gao et al., 2015; Territo et al., 2017; Han et al., 2017), [11C]SMW139 (Janssen et al., 2018), [11C]JNJ-54173717 ([11C]JNJ717), (Ory et al., 2016; Crabbé et al., 2019; Van Weehaeghe et al., 2019), [18F]JNJ-64413739 (Koole et al., 2019; Kolb et al., 2019; Berdyyeva et al., 2019), and [18F]PTTP (Fu et al., 2019) are new promising P2X7R tracers for imaging neuroinflammation, and studying the occupancy of P2X7R antagonists (Bhattacharya & Ceusters, 2020; Zheng, 2020).
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Updated at: 2021-03-26
Created at: 2016-05-12
Written by: Vesa Oikonen