PET imaging of adenosine receptors

Adenosine

Adenosine

Adenosine is an endogenous purine nucleoside, composed of nucleobase adenine and β-D-ribose moieties. It has a central role in cell metabolism and signaling as such and as the backbone for ATP (adenosine triphosphate) and cAMP (cyclic adenosine monophosphate).

Adenosine is rapidly catabolized by adenosine deaminases to inosine, which can be further deribosylated by purine nucleoside phosphorylase to hypoxantine. Adenosine deaminase isoform 1 (ADA1) is found in most cells, including blood cells. Isoform 2 (ADA2) is found in macrophages and in the plasma. Adenosine kinase in red blood cells converts adenosine into adenine nucleotides.

Adenosine is produced both intracellularly and extracellularly. Intracellular adenosine production and release is tightly regulated, and it is not stored in vesicles. Extracellular adenosine is produced by degradation of ATP, ADP, and AMP, especially during inflammation, ischemia, hypoxia, and acute tissue injury.

Adenosine cannot freely pass across the cell and vesicular membranes, but it needs to be transported by nucleoside transporters, which are present in most cells, including erythrocytes. Endothelial cells have a very active transport system for adenosine, and also high rate of metabolism, converting adenosine to inosine, hypoxanthine, xanthine, uric acid, and other compounds. Dipyridamole elevates extracellular adenosine concentrations by inhibiting equilibrative nucleoside transporters (ENTs). ENTs are highly expressed in the central nervous system. Concentrative nucleoside transporters (CNTs) are Na+-dependent transporters, found in most tissue types. Interstitial adenosine concentration is normally in the range 20-200 nM, and its extracellular half-life is short, a few seconds at most.

In the brain adenosine modulates the action of neurotransmitters, and affects both neuronal and glial cell functions; both cell types can release adenosine. Adenosine modulates the release and uptake of glutamate. Adenosine increases the permeability of the blood-brain barrier. Outside the brain, adenosine has paracrine function; adenosine and adenosine agonists attenuate inflammation, and induce vasodilation and angiogenesis in the cardiovascular system. Cellular signalling occurs through adenosine receptors (ARs).

Adenosine receptors

All four adenosine receptor subtypes (A1, A2A, A2B, and A3) are G protein-coupled receptors (GPCRs), either increasing or decreasing intracellular cAMP levels by affecting adenylate cyclase activity. A1 and A3 receptor subtypes can also stimulate K+ channels and inhibit voltage-dependent Ca2+ channels. Like other GPCRs, adenosine receptors have a single polypeptide chain, forming three extracellular and three intracellular loops. ARs can form oligomers, also with other receptors than just the AR subtypes, including purinoceptors (P2), dopamine, and glutamate receptors, which affects also the kinetics of PET tracers.

Adenosine receptors are also referred to as purine receptors, or P1 purinoceptors. The other purinoceptors (P2) are more activated by ATP and other substrates than adenosine.

Adenosine has the highest affinity to the A1R and A2AR, intermediate affinity to A3R, and lowest affinity to A2BR. Adenosine activates A1 and A2A receptors already in nanomolar concentrations.

Selective agonists and antagonists have been developed for each of the AR subtypes. PET tracers for each AR subtype have also been developed, although only tracers for A1R and A2AR are routinely used in human studies. Agonists, including endogenous agonist adenosine, lead to the activation of all AR subtypes, and subsequent regulation by desensitization and trafficking (Mundell and Kelly, 2011). In addition to AR agonists and antagonists, also allosteric modulators have been introduced.

Caffeine, theobromine, theophylline and paraxanthine are nonselective adenosine receptor antagonists. Caffeine is metabolized in the liver into theobromine, theophylline, and paraxanthine. Caffeine use should be carefully controlled in adenosine receptor studies. Elmenhorst et al. (2012) observed up to 50% occupancy of A1R using [18]FCPFPX. Due to the interactions between adenosine and dopamine systems caffeine also increases dopamine D2/D3 receptor availability (Volkow et al., 2015).

A1R

The A1R is expressed throughout in the body, with the highest densities in the brain. Receptor density is high in the striatum and thalamus, moderate in the cerebral cortex and pons, and low in the cerebellum, midbrain and brain stem. Adenosine binding to both pre- and postsynaptic A1Rs has an inhibitory effect in the brain. A1Rs are also present on microglia, astrocytes, and oligodendrocytes.

In the adipose tissue, A1R agonists inhibit lipolysis. Obese individuals may have reduced A1R density in the adipocytes but increased adenosine concentration (Kaartinen et al., 1991).

Several PET tracers for A1R have been introduced, including [18F]CPFPX and [11C]MPDX.

A2AR

The density of A2A receptors is high in the basal ganglia, especially in putamen. Expression in cerebral cortex, particularly in the frontal lobe, is lower, and very low in cerebellum and brainstem. A2ARs are also found on endothelial cells and microglia, playing a role in neuroinflammation and neurodegenerative diseases (Rissanen et al., 2013; Vuorimaa et al., 2017). A2AR can form complexes with dopamine D2 and mGluR5 receptors in the brain.

A2ARs are also present in peripheral organs like heart, lungs, liver, kidneys, spleen, and thymus. A2AR expression is high in platelets and leukocytes, endothelial cells, and vascular smooth muscle. Expression is strongly upregulated at sites of inflammation. Adenosine induces collagen synthesis and fibrosis via A2ARs.

A2AR is the most abundant AR in human and murine BAT. Adenosine and A2AR agonists activate brown adipose tissue and induces the browning of white adipose tissue (Gnad et al., 2014). Adenosine administration increases perfusion in BAT even more than cold exposure (Lahesmaa et al., 2018). Endogenous adenosine under cold exposure leads to desensitization and trafficking of A2ARs and competes with receptor-specific radioligands, as seen by decreased binding of [11C]TMSX (Lahesmaa et al., 2018).

Several PET tracers for A2AR have been introduced (Khanapur et al., 2013; van Waarde et al., 2018), including xanthine derivative [11C]TMSX (also known as [11C]KF18446), and non-xanthine radioligands [11C]SCH442416, [18F]FESCH, [11C]preladenant, and [18F]MNI-444.

A2BR

A2B receptors are expressed widely in the body, but densities in the brain in neuronal and glial cells are low. Macrophages and lymphocytes express A2BR, suggesting its role in regulation of inflammation. A2BR in macrophages may affect insulin sensitivity in the tissues (Johnston-Cox et al., 2014). Because the affinity of A2BR for adenosine is low, the receptor may be activated only under pathological conditions, such as inflammation, where both the expression of A2BR and adenosine levels are increased.

A2BRs may be involved in adipose tissue metabolism, since A2BR expression is associated with BMI (Johnston-Cox et al., 2012), and adenosine is one of the regulators of adipogenesis (Eisenstein & Ravid, 2014).

PET tracers for A2BR are being developed (Petroni et al., 2016).

A3R

A3 receptors are expressed widely in the body, but densities in the brain, heart and kidneys are low. Lung, liver, and immune cells express high levels of A3R mRNA in humans. A3R can promote both pro- and anti-inflammatory responses. Generally, inhibition of A1 and A3 receptors reduces and inhibition of A2 receptors increases oxidative stress.

A3R antagonist [18F]FE@SUPPY has shown promise as a PET tracer in rodent studies (Wadsak et al., 2008; Haeusler et al., 2015a; Haeusler et al., 2015b).

In contrast to other AR subtypes, the structure of A3 receptor is highly variable among mammals, leading to different pharmacological profiles of the species homologs. Also tissue distribution and expression levels are different among species. This should be taken into account when developing PET tracers in animal studies for use in humans.


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References:

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Created at: 2015-08-12
Updated at: 2018-08-14
Written by: Vesa Oikonen