Diadenosine polyphosphates inhibit adenosine kinase activity but decrease levels of endogenous adenosine in rat brain

Diadenosine-Polyphosphate Analogue AppCH2ppA Suppresses Seizures by Enhancing Adenosine Signaling in the Cortex

Epilepsy is a multifactorial disorder associated with neuronal hyperexcitability that affects more than 1% of the human population. It has long been known that adenosine can reduce seizure generation in animal models of epilepsies. However, in addition to various side effects, the instability of adenosine has precluded its use as an anticonvulsant treatment. Here we report that a stable analogue of diadenosine-tetraphosphate: AppCH2ppA effectively suppresses spontaneous epileptiform activity in vitro and in vivo in a Tuberous Sclerosis Complex (TSC) mouse model (Tsc1+/-), and in postsurgery cortical samples from TSC human patients. These effects are mediated by enhanced adenosine signaling in the cortex post local neuronal adenosine release. The released adenosine induces A1 receptor-dependent activation of potassium channels thereby reducing neuronal excitability, temporal summation, and hypersynchronicity. AppCH2ppA does not cause any disturbances of the main vital autonomous functions of Tsc1+/- mice in vivo. Therefore, we propose this compound to be a potent new candidate for adenosine-related treatment strategies to suppress intractable epilepsies.

High glucose impairs ATP formation on the surface of human peripheral blood B lymphocytes

Diabetes-associated lymphocyte dysfunction may be attributed to the direct effect of hyperglycemia, but the impact of glucose concentration on B cell functionality is not fully resolved. Since, adenosine 5'-triphosphate (ATP) and its metabolite adenosine are the core constituents of the purinergic signaling network involved in regulation of immune response we aimed to investigate the impact of high glucose concentration on ATP outflow and metabolism on B cell surface. Purified human peripheral blood B cells cultured at high glucose (25 mM) concentration released significantly less ATP (~60%) comparing to cells cultured in low glucose (5mM) concentration. We observed that high glucose altered ATP hydrolysis on B cell surface due to increased activity of nucleoside triphosphate diphosphohydrolase-1 (NTPDase-1/CD39). In the presence of 10 μM [(3)H]AMP and 100 μM ATP significant quantities of [(3)H]ADP and [(3)H]ATP were generated, although the AMP to ADP phosphorylation potential of B cells cultured in high glucose decreased significantly. The flow cytometry analysis revealed that the level of ecto-adenylate kinase 1β (AK1β) on surface of B cells cultured in high glucose decreased significantly. Inhibition of NTPDase1/CD39 activity with 100 μM ARL67156 resulted in decreased cell viability, although significantly more viable cells retained in the culture media containing low glucose compared to high glucose media. Selective inhibition of P2X7 purinergic receptor irrespective of glucose concentration completely protected B cells against the ARL 67156-induced cell death. We assume that high glucose-induced alteration of ATP handling on B cell surface might contribute to impaired functionality of B cells in diabetes.

Molecular mechanisms of nucleoside recycling in the brain

A major role of plasma membrane bound ectonucleotidases is the modulation of ATP, ADP, adenosine (the purinergic agonists), UTP, and UDP (the pyrimidinergic agonists) availability in the extracellular space at their respective receptors. We have recently shown that an ATP driven uridine-UTP cycle is operative in the brain, based on the strictly compartmentalized processes of uridine salvage to UTP and uridine generation from UTP, in which uptaken uridine is anabolized to UTP in the cytosol, and converted back to uridine in the extracellular space by the action of ectonucleotidases (Ipata et al. Int J Biochem Cell Biol 2010;42:932-7). In this paper we show that a similar cytidine-CTP cycle exists in rat brain. Since (i) brain relies on imported preformed nucleosides for the synthesis of nucleotides, RNA, nuclear and mitochondrial DNA, coenzymes, pyrimidine sugar- and lipid-conjugates and (ii) no specific pyrimidinergic receptors have been identified for cytidine and their nucleotides, our results, taken together with previous studies on the intra- and extracellular metabolic network of ATP, GTP, UTP, and their nucleosides in the brain (Barsotti and Ipata. Int J Biochem Cell Biol 2004;36:2214-25; Balestri et al. Neurochem Int 2007;50:517-23), strongly suggest that, apart from the modulation of ligand availability, ectonucleotidases may serve the process of local nucleoside recycling in the brain.

Chronic morphine treatment impaired hippocampal long-term potentiation and spatial memory via accumulation of extracellular adenosine acting on adenosine A1 receptors

Chronic exposure to opiates impairs hippocampal long-term potentiation (LTP) and spatial memory, but the underlying mechanisms remain to be elucidated. Given the well known effects of adenosine, an important neuromodulator, on hippocampal neuronal excitability and synaptic plasticity, we investigated the potential effect of changes in adenosine concentrations on chronic morphine treatment-induced impairment of hippocampal CA1 LTP and spatial memory. We found that chronic treatment in mice with either increasing doses (20-100 mg/kg) of morphine for 7 d or equal daily dose (20 mg/kg) of morphine for 12 d led to a significant increase of hippocampal extracellular adenosine concentrations. Importantly, we found that accumulated adenosine contributed to the inhibition of the hippocampal CA1 LTP and impairment of spatial memory retrieval measured in the Morris water maze. Adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine significantly reversed chronic morphine-induced impairment of hippocampal CA1 LTP and spatial memory. Likewise, adenosine deaminase, which converts adenosine into the inactive metabolite inosine, restored impaired hippocampal CA1 LTP. We further found that adenosine accumulation was attributable to the alteration of adenosine uptake but not adenosine metabolisms. Bidirectional nucleoside transporters (ENT2) appeared to play a key role in the reduction of adenosine uptake. Changes in PKC-alpha/beta activity were correlated with the attenuation of the ENT2 function in the short-term (2 h) but not in the long-term (7 d) period after the termination of morphine treatment. This study reveals a potential mechanism by which chronic exposure to morphine leads to impairment of both hippocampal LTP and spatial memory.

Identification of diadenosine triphosphate in Brugia malayi by reverse phase high performance liquid chromatography and MALDI mass spectrometry

The presence of diadenosine oligophosphates (ApnA) in eukaryotic pathogens has been difficult technically to assess and thus is often overlooked. ApnA are a family of intercellular and intracellular signaling molecules and their biological activities differ relative to the number of phosphate moieties. The application of mass spectrometry to differentiate nucleotide phosphates has been limited by the high salt content in tissue extracts, enzymatic reactions or high performance liquid chromatography (HPLC) buffers, as well as the potential for sample loss when processing and desalting small biological samples. To address this problem a simple reverse phase HPLC (RP-HPLC) method using volatile organic buffers at low pH was developed to create elution profiles of adenosine and diadenosine phosphates. To test this method on a eukaryotic pathogen, small intravascular human filarial parasites (Brugia malayi) were extracted in phosphate buffered saline and a nucleotide phosphate profile was visualized by RP-HPLC. A major peak eluting at 10.4 min was analyzed directly by mass spectrometry and this confirmed the presence of significant quantities of diadenosine triphosphate, Ap3A. Application of this simplified RP-HPLC method will facilitate research on the normal and pathophysiological effects of ApnA particularly in situations when analysis of small biological samples is required.

Metabolic regulation of ATP breakdown and of adenosine production in rat brain extracts

ATP concentration is dramatically affected in ischemic injury. From previous studies on ATP mediated purine and pyrimidine salvage in CNS, we observed that when "post-mitochondrial" extracts of rat brain were incubated with ATP at 3.6 mM, a normoxic concentration, formation of IMP always preceded that of adenosine, a well known neuroactive nucleoside and a homeostatic cellular modulator. This observation prompted us to undertake a study aimed at assessing the precise pathways and kinetics of ATP breakdown, a process considered to be the major source of adenosine in rat brain. The results obtained using post-mitochondrial extracts strongly suggest that the breakdown of intracellular ATP at normoxic concentration follows almost exclusively the pathway ATP<=>ADP<=>AMP --> IMP --> inosine<=>hypoxanthine, with little, if any, intracellular adenosine production. At low ischemic concentration, intracellular ATP breakdown follows the pathway ATP<=>ADP<=>AMP --> adenosine --> inosine<=>hypoxanthine with little IMP formation. At the same time, extracellular ATP, whose concentration is known to be enhanced during ischemia, is actively broken down to adenosine through the pathway ATP --> ADP --> AMP --> adenosine, catalysed by the well characterized ecto-enzyme cascade system. Moreover, we show that during intracellular GTP catabolism, xanthosine, in addition to guanosine, is generated through the so called "ribose 1-phosphate recycling for nucleoside interconversion". These results considerably extend our knowledge on the long debated question of the extra or intracellular origin of adenosine in CNS, suggesting that at least in normoxic conditions, intracellular adenosine is of extracellular origin.

Simultaneous assessment of ecto- and cytosolic-5'-nucleotidase activities in brain micropunches

We propose a new methodology for simultaneous assessment of ecto- and cytosolic-5'-nucleotidase that can be utilized in brain to measure the activity of these enzymes in micropunches of tissues. It is based on the differential sensitivity of both enzymes to alpha,beta-methyleneadenosine 5'-diphosphate (AMP-CP) and the requirements for magnesium as a co-factor. The design of assay protocol contains an internal validation by allowing comparisons between total level of 5'-nucleotidase activity with that calculated from the sum of individual activities of the ecto- and cytosolic-5'-nucleotidases. We have applied this new approach to assess the activity of ecto- and cytosolic-5'-nucleotidase in the brain regions relevant to sleep regulation. The level of both enzymes was significantly lower in the cerebral cortex than other brain regions tested.

Diadenosine oligophosphates (Ap(n)A), a novel class of signalling molecules?

The diadenosine oligophosphates (Ap(n)A) were discovered in the mid-sixties in the course of studies on aminoacyl-tRNA synthetases (aaRS). Now, more than 30 years later, about 300 papers have been published around these substances in attempt to decipher their role in cells. Recently, Ap(n)A have emerged as intracellular and extracellular signalling molecules implicated in the maintenance and regulation of vital cellular functions and become considered as second messengers. Great variety of physiological and pathological effects in mammalian cells was found to be associated with alterations of Ap(n)A levels (n from 2 to 6) and Ap3A/Ap4A ratio. Cell differentiation and apoptosis have substantial and opposite effects on Ap3A/Ap4A ratio in cultured cells. A human Ap3A hydrolase, Fhit, appeared to be involved in protection of cells against tumourigenesis. Ap3A is synthesised by mammalian u synthetase (TrpRS) which in contrast to most other aaRS is unable to synthesise Ap4A and is an interferon-inducible protein. Moreover, Ap3A appeared to be a preferred substrate for 2-5A synthetase, also interferon-inducible, priming the synthesis of 2' adenylated derivatives of Ap3A, which in turn may serve as substrates of Fhit. Tumour suppressor activity of Fhit is assumed to be associated with involvement of the Fhit.Ap3A complex in cytokine signalling pathway(s) controlling cell proliferation. The Ap(n)A family is potentially a novel class of signal-transducing molecules whose functions are yet to be determined.