Changes in mRNA expression of adenosine receptors in human chronic periodontitis

OBJECTIVE

To elucidate the aetiology of periodontitis, this study focused on the adenosine receptor (AR) expression profiles (A1AR, A2AAR, A2BAR and A3AR) in periodontal diseased tissues.

METHODS

Adenosine receptor gene expression levels in human gingiva from 15 patients with healthy gingival tissues (control group) and 15 patients who exhibited severe chronic periodontitis (test group) were measured using quantitative reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

The mRNA expression pattern changed in human chronic periodontitis: the A1AR decreased 20%, A2AAR increased 2.5-fold, A2BAR increased 3.7-fold and A3AR decreased 70% as compared with that of healthy gingiva.

CONCLUSION

Inflammation of the gingival tissue is associated with (1) an unchanged expression of A1AR, (2) an increased expression of A2AAR and A2BAR, and (3) a decreased expression of A3AR. Logistic regression analysis indicated that the change in the expression patterns can be used to diagnose/predict periodontitis. This finding indicates that the adenosine receptor expression profile is changed in periodontitis with the potential for future clinical application.

A1 adenosine receptor: role in diabetes and obesity

Adenosine mediates its diverse effects via four subtypes (A(1), A(2A), A(2B) and A(3)) of G-protein-coupled receptors. The A(1) adenosine receptor (A(1)AR) subtype is the most extensively studied and is well characterized in various organ systems. The A(1)ARs are highly expressed in adipose tissue, and endogenous adenosine has been shown to tonically activate adipose tissue A(1)ARs. Activation of the A(1)ARs in adipocytes reduces adenylate cyclase and cAMP content and causes inhibition of lipolysis. The role of A(1)ARs in lipolysis has been well characterized by using several selective A(1)AR agonists as well as A(1)AR knockout mice. However, the contribution of A(1)ARs to the regulation of lipolysis in pathological conditions like insulin resistance, diabetes and dyslipidemia, where free fatty acids (FFA) play an important role, has not been well characterized. Pharmacological agents that reduce the release of FFA from adipose tissue and thus the availability of circulating FFA have the potential to be useful for insulin resistance and hyperlipidemia. Toward this goal, several selective and efficacious agonists of the A(1)ARs are now available, and some have entered early-phase clinical trials; however, none have received regulatory approval yet. Here we review the existing knowledge on the role of A(1)ARs in insulin resistance, diabetes and obesity, and the progress made in the development of A(1)AR agonists as antilipolytic agents, including the challenges associated with this approach.

Adenosine receptor ligands and PET imaging of the CNS

Advances in radiotracer chemistry have resulted in the development of novel molecular imaging probes for adenosine receptors (ARs). With the availability of these molecules, the function of ARs in human pathophysiology as well as the safety and efficacy of approaches to the different AR targets can now be determined. Molecular imaging is a rapidly growing field of research that allows the identification of molecular targets and functional processes in vivo. It is therefore gaining increasing interest as a tool in drug development because it permits the process of evaluating promising therapeutic targets to be stratified. Further, molecular imaging has the potential to evolve into a useful diagnostic tool, particularly for neurological and psychiatric disorders. This chapter focuses on currently available AR ligands that are suitable for molecular neuroimaging and describes first applications in healthy subjects and patients using positron emission tomography (PET).

Heterologous expression of the adenosine A1 receptor in transgenic mouse retina

Traditional cell-based systems used to express integral membrane receptors have yet to produce protein samples of sufficient quality for structural study. Herein we report an in vivo method that harnesses the photoreceptor system of the retina to heterologously express G protein-coupled receptors in a biochemically homogeneous and pharmacologically functional conformation. As an example we show that the adenosine A1 receptor, when placed under the influence of the mouse opsin promoter and rhodopsin rod outer segment targeting sequence, localized to the photoreceptor cells of transgenic retina. The resulting receptor protein was uniformly glycosylated and pharmacologically well behaved. By comparison, we demonstrated in a control experiment that opsin, when expressed in the liver, had a complex pattern of glycosylation. Upon solubilization, the retinal adenosine A1 receptor retained binding characteristics similar to its starting material. This expression method may prove generally useful for generating high-quality G protein-coupled receptors for structural studies.

Ecto-5'-nucleotidase (cd73)-dependent and -independent generation of adenosine participates in the mediation of tubuloglomerular feedback in vivo

Tubuloglomerular feedback (TGF) describes a sequence of events linking salt concentrations in tubular fluid at the macula densa to the vascular tone of the afferent arteriole and thus to the glomerular filtration rate (GFR) of the same nephron. The signal transduction pathways of TGF remain incompletely understood, but both ATP release from macula densa cells and local formation of adenosine were suggested to be involved in the process. To test the role of extracellular formation of adenosine by ecto-5'-nucleotidase (cd73) in TGF, in regulation of GFR, and in tubular reabsorption, renal clearance and micropunture experiments were performed in cd73 wild-type (cd73(+/+)) and knockout mice (cd73(-/-)). The cd73(-/-) mice presented normal mean arterial blood pressure, but modestly lower whole kidney and single nephron GFR (SNGFR). Fractional reabsorption of Na(+) and K(+) up to the late proximal tubule, distal tubule, as well as urine were not significantly different between cd73(-/-) and cd73(+/+) mice. Lack of cd73 resulted in a diminished TGF response, as indicated by smaller changes of stop-flow pressure in response to increasing loop of Henle perfusion from 0 to 25 nl/min, smaller differences in SNGFR determined from paired proximal and distal tubular collections, and by smaller fractional changes of distal SNGFR in response to adding 6 nl/min of artificial tubular fluid to free-flowing proximal tubules. The TGF response in cd73(+/+) mice and the residual TGF response in cd73(-/-) mice were completely inhibited by adenosine A(1)-receptor blockade. The results suggest that extracellular formation of adenosine by ecto-5'-nucleotidase (cd73) is dispensable for normal fluid, Na(+), or K(+) reabsorption along the nephron, but contributes to the regulation of GFR. Adenosine generated by both ecto-5'-nucleotidase (cd73)-dependent and -independent mechanisms participates in the mediation of TGF in vivo.

Calcium wave of tubuloglomerular feedback

ATP release from macula densa (MD) cells into the interstitium of the juxtaglomerular (JG) apparatus (JGA) is an integral component of the tubuloglomerular feedback (TGF) mechanism that controls the glomerular filtration rate. Because the cells of the JGA express a number of calcium-coupled purinergic receptors, these studies tested the hypothesis that TGF activation triggers a calcium wave that spreads from the MD toward distant cells of the JGA and glomerulus. Ratiometric calcium imaging of in vitro microperfused isolated JGA-glomerulus complex dissected from rabbits was performed with fluo-4/fura red and confocal fluorescence microscopy. Activation of TGF by increasing tubular flow rate at the MD rapidly produced a significant elevation in intracellular Ca(2+) concentration ([Ca(2+)](i)) in extraglomerular mesangial cells (by 187.6 +/- 45.1 nM) and JG renin granular cells (by 281.4 +/- 66.6 nM). Subsequently, cell-to-cell propagation of the calcium signal at a rate of 12.6 +/- 1.1 microm/s was observed upstream toward proximal segments of the afferent arteriole and adjacent glomeruli, as well as toward intraglomerular elements including the most distant podocytes (5.9 +/- 0.4 microm/s). The same calcium wave was observed in nonperfusing glomeruli, causing vasoconstriction and contractions of the glomerular tuft. Gap junction uncoupling, an ATP scavenger enzyme cocktail, and pharmacological inhibition of P(2) purinergic receptors, but not adenosine A(1) receptor blockade, abolished the changes in [Ca(2+)](i) and propagation of the calcium wave. These studies provided evidence that both gap junctional communication and extracellular ATP are integral components of the TGF calcium wave.

Differential coronary microvascular exchange responses to adenosine: roles of receptor and microvessel subtypes

OBJECTIVE

To assess the role of adenosine receptors in the regulation of coronary microvascular permeability to porcine serum albumin (P(s)(PSA)).

METHODS

Solute flux was measured in single perfused arterioles and venules isolated from pig hearts using fluorescent dye-labeled probes by microspectro-fluorometry. Messenger RNA, protein, and cellular distribution of adenosine receptors in arterioles and venules were analyzed by RT-PCR, immunoblot, and immunofluorescence.

RESULTS

Control venule P(s)(PSA) (10.7 +/- 4.8 x 10(- 7) cm x s(- 1)) was greater than that of arterioles (6.4+/- 2.8 x 10(-7) cm . s(-1); p < .05). Arteriolar P(s)(PSA) decreased (p < .05) with adenosine suffusion over the range from 10(- 8) to 10(-5) M, while venular P(s)(PSA) did not change. The nonselective A(1) and A(2) receptor antagonist, 8-(p-sulfophenyl) theophylline, blocked the adenosine-induced decrease in arteriolar P(s)(PSA). Messenger RNA for adenosine A(1), A(2A), A(2B), and A(3) receptors was expressed in arterioles and venules. Protein for A(1), A(2A), and A(2B), but not A(3), was detected in both microvessel types and was further demonstrated on vascular endothelial cells.

CONCLUSION

Arteriolar P(s)(PSA) decreases with adenosine suffusion but not venular P(s)(PSA). Adenosine A(1), A(2A), and A(2B) receptors are expressed in both arterioles and venules. Selective receptor-linked cellular signaling mechanisms underlying the regulation of permeability remain to be determined.

A novel A1 adenosine receptor antagonist, L-97-1 [3-[2-(4-aminophenyl)-ethyl]-8-benzyl-7-{2-ethyl-(2-hydroxy-ethyl)-amino]-ethyl}-1-propyl-3,7-dihydro-purine-2,6-dione], reduces allergic responses to house dust mite in an allergic rabbit model of asthma

Adenosine, an important signaling molecule in asthma, produces bronchoconstriction in asthmatics. Adenosine produces bronchoconstriction in allergic rabbits, primates, and humans by activating A1 adenosine receptors (ARs). Effects of L-97-1 [3-[2-(4-aminophenyl)-ethyl]-8-benzyl-7-{2-ethyl-(2-hydroxyethyl)-amino]-ethyl}-1-propyl-3,7-dihydro-purine-2,6-dione] a water-soluble, small molecule A1 AR antagonist were investigated on early and late phase allergic responses (EAR and LAR) in a hyper-responsive rabbit model of asthma. Rabbits were made allergic by intraperitoneal injections of house dust mite [HDM; 312 allergen units (AU)] extract within 24 h of their birth. Booster HDM injections were given weekly for 1 month, biweekly for 4 months, and continued monthly thereafter. Hyperresponsiveness was monitored by measuring lung dynamic compliance (Cdyn), after histamine or adenosine aerosol challenge in allergic rabbits. Hyper-responsive rabbits were subjected to aerosol of HDM (2500 AU), 1 h after intragastric administration of L-97-1 (10 mg/kg) solution or an equivalent volume of saline. Cdyn was significantly higher after treatment with L-97-1 compared with untreated controls (p < 0.05 n = 5). Histamine PC30 was significantly higher (p < 0.05; n = 5) after L-97-1 at 24 h compared with histamine PC30 at 24 h after HDM. Adenosine PC30 was significantly higher at 15 min and 6 h after L-97-1 compared with control (p < 0.05; n = 5). L-97-1 showed strong affinity for human A1 ARs in radioligand binding studies and no inhibition toward human phosphodiesterase II, III, IV, and V enzymes. These data suggest that L-97-1 produces a significant reduction of histamine or adenosine-induced hyper-responsiveness and HDM-induced EAR and LAR in allergic rabbits by blocking A1 ARs and may be beneficial as an oral therapy for human asthma.

Long-term Effect of Convulsive Behavior on the Density of Adenosine A1 and A2A Receptors in the Rat Cerebral Cortex

PURPOSE

Adenosine is a neuromodulator that has been proposed to act as an anticonvulsant mainly via inhibitory A1 receptors, but recent data show that genetic deletion of facilitatory A 2A receptors might also attenuate convulsions. Since both A1 and A 2A receptors are prone to down- and upregulation in different stressful situations, we investigated if convulsive behavior leads to a long-term change in A1 and A 2A receptor density in the rat cerebral cortex.

METHODS

Stage 4-5 convulsions (Racine's scale) were induced in adult Wistar rats either through amygdala stimulation (kindling) or by intraperitoneal injection of kainate (10 mg/ml). Rats were killed after 4 weeks to evaluate adenosine A1 and A 2A receptor density in the cerebral cortex using both Western blot and membrane binding assays.

RESULTS

The binding density of the A1 antagonist, 3H-DPCPX, decreased by 40. +/- 4.4% and by 20.7 +/- 0.5% after kindling or kainate injection. Likewise, A1 receptor immunoreactivity in cortical membranes from kindled or kainate-injected rats decreased by 19.1 +/- 3.3% and 12.7 +/- 5.7%, respectively. In contrast, the binding density of the A 2A receptor antagonist 3H-SCH 58261 increased by 293 +/- 34% and by 159 +/- 32% in cortical membranes from kindled or kainate-injected rats, and A 2A receptor immunoreactivity also increased by 151 +/- 12% and 79.6 +/- 7.0%.

CONCLUSIONS

This indicates that after convulsive behavior there is a long-term decrease of A1 receptors accompanied by an increased density of A 2A receptors, suggesting that A 2A antagonists rather than A1 agonists may be more promising anticonvulsive drugs.

Simplified quantification of cerebral A1 adenosine receptors using [18F]CPFPX and PET: Analyses based on venous blood sampling

[(18)F]CPFPX was proposed as a novel ligand for in vivo quantification of cerebral A(1) adenosine receptors (A(1)AR) using positron emission tomography (PET). The present study investigates the applicability of non-invasive non-compartment analyses to quantify the [(18)F]CPFPX total distribution volume (DV(') (t)) without arterial blood sampling. Five healthy volunteers underwent dynamic PET (90 min) with arterial and venous blood sampling after [(18)F]CPFPX bolus injection. Area under curve (AUC) analysis and Logan's graphical analysis (GA) were performed employing venous blood samples as non-invasive analyses. AUC analysis without data extrapolation (AUC(r)) and venous GA were also applied on voxel-level to generate parametric images. A conventional two-tissue compartment model (2TCM) using arterial blood samples served as reference method. Regional DV(') (t) estimates provided by venous AUC and GA analyses and 2TCM demonstrated very high agreement (venous AUC vs. 2TCM: r(2) = 0.968, linear regression slope = 0.943; venous GA vs. 2TCM: r(2) = 0.972, slope = 0.906). Although on voxel-level the venous AUC(r) and GA analyses were affected by a slight negative bias, they were still very highly correlated with the 2TCM (voxel-wise venous AUC(r) vs. 2TCM: r(2) = 0.969, slope = 0.858; voxel-wise venous GA vs. 2TCM: r(2) = 0.959, slope = 0.867). The proposed non-invasive analysis methods (particularly venous GA) allow reliable quantification of human [(18)F]CPFPX PET studies. In populations with altered metabolic/kinetic properties, the applicability of venous sampling has to be separately verified. The applicability of [(18)F]CPFPX PET in basic and clinical neurosciences will be considerably enhanced by the avoidance of arterial blood sampling.

Input-specific modulation of neurotransmitter release in the lateral horn of the spinal cord via adenosine receptors

Activation of adenosine A2A receptors (A2ARs) in the CNS produces a variety of neuromodulatory actions dependent on the region and preparation examined. In autonomic regions of the spinal cord, A1R activation decreases excitatory synaptic transmission, but the effects of A2AR stimulation are unknown. We sought to determine the location and function of the A2ARs in the thoracic spinal cord, focusing on the intermediolateral cell column (IML). A2AR immunoreactivity was observed throughout the gray matter, with particularly dense immunostaining in regions containing sympathetic preganglionic neurons (SPNs), namely, the IML and intercalated nucleus. Electron microscopy revealed A2AR immunoreactivity within presynaptic terminals and in postsynaptic structures in the IML. To study the functional relevance of these A2ARs, visualized whole-cell patch-clamp recordings were made from electrophysiologically identified SPNs and interneurons within the IML. The A2AR agonist c2-[p-(carboxyethyl)phenethylamino]-5'-N-ethylcarboxyamidoadenosine (CGS 21680) had no significant effect on EPSPs but increased the amplitude of IPSPs elicited by stimulation of the lateral funiculus. These effects were attributable to activation of presynaptic A2ARs because CGS 21680 application altered the paired pulse ratio. Furthermore, neurons in the IML that have IPSPs increased via A2AR activation also receive excitatory inputs that are inhibited by A1R activation. These data show that activating A2ARs increase inhibitory but not excitatory transmission onto neurons in the IML. Simultaneous activation of A1Rs and A2ARs therefore could facilitate inhibition of the postsynaptic neuron, leading to an overall reduction of sympathetic nervous activity.

Cytoskeletal protein 4.1G binds to the third intracellular loop of the A1 adenosine receptor and inhibits receptor action

To identify binding partners of the A1AR (A1 adenosine receptor), yeast two-hybrid screening of a rat embryonic cDNA library was performed. This procedure led to the identification of erythrocyte membrane cytoskeletal protein (represented as 4.1G) as an A1AR-binding partner. Truncation studies revealed that the C-terminal domain of 4.1G was essential for binding to A1ARs and that the C-terminal domain of 4.1G and the third intracellular loop of A1ARs interacted. A1AR-4.1G interaction was also confirmed in studies using brain tissue. Studies in HEK-293 (human embryonic kidney 293) cells and Chinese-hamster ovary cells showed that 4.1G interfered with A1AR signal transduction, as 4.1G reduced A1AR-mediated inhibition of cAMP accumulation and intracellular calcium release. 4.1G also altered cell-surface A1AR expression. These observations identify 4.1G as a novel A1AR-binding partner that can regulate adenosine action.

Subcellular localization of adenosine A(1) receptors in nerve terminals and synapses of the rat hippocampus

Adenosine is a neuromodulator in the CNS that mainly acts through pre- and postsynaptic A(1) receptors to inhibit the release of excitatory neurotransmitters and NMDA receptor function. This might result from a highly localized distribution of A(1) receptors in the active zone and postsynaptic density of CNS synapses that we now investigated in the rat hippocampus. The binding density of the selective A(1) receptor antagonist, [3H]1,3-dipropyl-8-cyclopentylxanthine ([3H]DPCPX), was enriched in membranes from Percoll-purified nerve terminals (B(max)=1839+/-52 fM/mg protein) compared to total membranes from the hippocampus (B(max)=984+/-31 fM/mg protein), the same occurring with A(1) receptor immunoreactivity. [3H]DPCPX binding occurred mainly to the plasma membrane rather than to intracellular sites, since the binding of the membrane permeable A(1) receptor ligand [3H]DPCPX to intact hippocampal nerve terminals (B(max)=1901+/-192 fM/mg protein) was markedly reduced (B(max)=321+/-30 fM/mg protein) by the membrane impermeable adenosine receptor antagonist, 8-sulfophenyltheophilline (25 microM). Further subcellular fractionation of hippocampal nerve terminals revealed that A(1) receptor immunoreactivity was strategically located in the active zone of presynaptic nerve terminals, as expected to understand the efficiency of A(1) receptors to depress neurotransmitter release. A(1) Receptors were also present in nerve terminals outside the active zone in accordance with the existence of a presynaptic A(1) receptor reserve. Finally, A(1) receptor immunoreactivity was evident in the postsynaptic density together with NMDA receptor subunits 1, 2A and 2B and with N-and P/Q-type calcium channel immunoreactivity, emphasizing the importance of A(1) receptors in the control of dendritic integration.

Adenosine A(1) receptor-mediated presynaptic inhibition at the calyx of Held of immature rats

At the calyx of Held synapse in brainstem slices of 5- to 7-day-old (P5-7) rats, adenosine, or the type 1 adenosine (A1) receptor agonist N6-cyclopentyladenosine (CPA), inhibited excitatory postsynaptic currents (EPSCs) without affecting the amplitude of miniature EPSCs. The A1 receptor antagonist 8-cyclopentyltheophylline (CPT) had no effect on the amplitude of EPSCs evoked at a low frequency, but significantly reduced the magnitude of synaptic depression caused by repetitive stimulation at 10 Hz, suggesting that endogenous adenosine is involved in the regulation of transmitter release. Adenosine inhibited presynaptic Ca(2+) currents (IpCa) recorded directly from calyceal terminals, but had no effect on presynaptic K+ currents. When EPSCs were evoked by IpCa during simultaneous pre- and postsynaptic recordings, the magnitude of the adenosine-induced inhibition of IpCa fully explained that of EPSCs, suggesting that the presynaptic Ca(2+) channel is the main target of A1 receptors. Whereas the N-type Ca(2+) channel blocker omega-conotoxin attenuated EPSCs, it had no effect on the magnitude of adenosine-induced inhibition of EPSCs. During postnatal development, in parallel with a decrease in the A1 receptor immunoreactivity at the calyceal terminal, the inhibitory effect of adenosine became weaker. We conclude that presynaptic A1 receptors at the immature calyx of Held synapse play a regulatory role in transmitter release during high frequency transmission, by inhibiting multiple types of presynaptic Ca(2+) channels.

PET imaging of adenosine A(1) receptors with (11)C-MPDX as an indicator of severe cerebral ischemic insult

We examined whether measurement of the adenosine A(1) receptor (A1-R) with PET can predict the severity of ischemic brain damage using an occlusion and reperfusion model of the cat middle cerebral artery (MCA) and [1-methyl-(11)C]8-dicyclopropylmethyl-1-methyl-3-propylxanthine (MPDX), a positron-emitting radioligand developed at our institution.

METHODS

Eighteen adult cats underwent PET measurement of cerebral blood flow (CBF), A1-R, central benzodiazepine receptor (BDZ-R), and glucose metabolism with (15)O-labeled water, MPDX, (11)C-flumazenil (FMZ), and (18)F-FDG, respectively. The right MCAs of 13 cats were transiently occluded via a transorbital approach with microvascular clips. CBF was measured before occlusion of MCA, during occlusion, and immediately after reperfusion. After CBF measurement, A1-R, BDZ-R, and (18)F-FDG uptake were serially measured in the order listed. Two months later, the degree of ischemic damage was evaluated by T2-weighted MR images obtained with an animal MRI system and by analysis of histologic specimens. Five cats that received no operations were used as controls.

RESULTS

The cats that underwent occlusion were divided into 3 groups: cats that did not survive the first day because of severe neurologic and systemic conditions (n = 4), cats that survived and had infarcted lesions in both the cortex and the striatum (n = 3), and cats that survived and had infarcted lesions only in the striatum (n = 6). CBF during occlusion of the MCA was significantly lower in all 3 ischemic groups than in the control group, but there was no significant difference among the ischemic groups. Right-to-left ratios of CBF and (18)F-FDG uptake did not significantly differ among the groups. MPDX binding and FMZ binding were significantly lower in the groups with severe ischemic insult than in the groups with little to no insult.

CONCLUSION

The degree of decreased MPDX binding to A1-Rs after reperfusion was a sensitive predictor of severe ischemic insult. MPDX PET has good potential to become a suitable in vivo imaging technique for evaluating the function of adenosine and A1-Rs in relation to cerebral ischemia.