Influence of Guanine-Based Purines on the Oxidoreductive Reactions Involved in Normal or Altered Brain Functions

The production of reactive oxygen species (ROS) in the brain is homeostatically controlled and contributes to normal neural functions. Inefficiency of control mechanisms in brain aging or pathological conditions leads to ROS overproduction with oxidative neural cell damage and degeneration. Among the compounds showing therapeutic potential against neuro-dysfunctions induced by oxidative stress are the guanine-based purines (GBPs), of which the most characterized are the nucleoside guanosine (GUO) and the nucleobase guanine (GUA), which act differently. Indeed, the administration of GUO to in vitro or in vivo models of acute brain injury (ischemia/hypoxia or trauma) or chronic neurological/neurodegenerative disorders, exerts neuroprotective and anti-inflammatory effects, decreasing the production of reactive radicals and improving mitochondrial function via multiple molecular signals. However, GUO administration to rodents also causes an amnesic effect. In contrast, the metabolite, GUA, could be effective in memory-related disorders by transiently increasing ROS production and stimulating the nitric oxide/soluble guanylate cyclase/cGMP/protein kinase G cascade, which has long been recognized as beneficial for cognitive function. Thus, it is worth pursuing further studies to ascertain the therapeutic role of GUO and GUA and to evaluate the pathological brain conditions in which these compounds could be more usefully used.

Guanine inhibits the growth of human glioma and melanoma cell lines by interacting with GPR23

Guanine-based purines (GBPs) exert numerous biological effects at the central nervous system through putative membrane receptors, the existence of which is still elusive. To shed light on this question, we screened orphan and poorly characterized G protein-coupled receptors (GPRs), selecting those that showed a high purinoreceptor similarity and were expressed in glioma cells, where GBPs exerted a powerful antiproliferative effect. Of the GPRs chosen, only the silencing of GPR23, also known as lysophosphatidic acid (LPA) 4 receptor, counteracted GBP-induced growth inhibition in U87 cells. Guanine (GUA) was the most potent compound behind the GPR23-mediated effect, acting as the endpoint effector of GBP antiproliferative effects. Accordingly, cells stably expressing GPR23 showed increased sensitivity to GUA. Furthermore, while GPR23 expression was low in a hypoxanthine-guanine phosphoribosyl-transferase (HGPRT)-mutated melanoma cell line showing poor sensitivity to GBPs, and in HGPRT-silenced glioma cells, GPR23-induced expression in both cell types rescued GUA-mediated cell growth inhibition. Finally, binding experiments using [3H]-GUA and U87 cell membranes revealed the existence of a selective GUA binding (KD = 29.44 ± 4.07 nM; Bmax 1.007 ± 0.035 pmol/mg prot) likely to GPR23. Overall, these data suggest GPR23 involvement in modulating responses to GUA in tumor cell lines, although further research needs to verify whether this receptor mediates other GUA effects.

Purinergic Signaling in Oral Tissues

The role of the purinergic signal has been extensively investigated in many tissues and related organs, including the central and peripheral nervous systems as well as the gastrointestinal, cardiovascular, respiratory, renal, and immune systems. Less attention has been paid to the influence of purines in the oral cavity, which is the first part of the digestive apparatus and also acts as the body’s first antimicrobial barrier. In this review, evidence is provided of the presence and possible physiological role of the purinergic system in the different structures forming the oral cavity including teeth, tongue, hard palate, and soft palate with their annexes such as taste buds, salivary glands, and nervous fibers innervating the oral structures. We also report findings on the involvement of the purinergic signal in pathological conditions affecting the oral apparatus such as Sjögren’s syndrome or following irradiation for the treatment of head and neck cancer, and the use of experimental drugs interfering with the purine system to improve bone healing after damage. Further investigations are required to translate the results obtained so far into the clinical setting in order to pave the way for a wider application of purine-based treatments in oral diseases.

Adipose Stromal/Stem Cell-Derived Extracellular Vesicles: Potential Next-Generation Anti-Obesity Agents

Over the last decade, several compounds have been identified for the treatment of obesity. However, due to the complexity of the disease, many pharmacological interventions have raised concerns about their efficacy and safety. Therefore, it is important to discover new factors involved in the induction/progression of obesity. Adipose stromal/stem cells (ASCs), which are mostly isolated from subcutaneous adipose tissue, are the primary cells contributing to the expansion of fat mass. Like other cells, ASCs release nanoparticles known as extracellular vesicles (EVs), which are being actively studied for their potential applications in a variety of diseases. Here, we focused on the importance of the contribution of ASC-derived EVs in the regulation of metabolic processes. In addition, we outlined the advantages/disadvantages of the use of EVs as potential next-generation anti-obesity agents.

In Search of a Role for Extracellular Purine Enzymes in Bone Function

Bone is one of the major tissues that undergoes continuous remodeling throughout life, thus ensuring both organic body growth during development and protection of internal organs as well as repair of trauma during adulthood. Many endogenous substances contribute to bone homeostasis, including purines. Their role has increasingly emerged in recent decades as compounds which, by interacting with specific receptors, can help determine adequate responses of bone cells to physiological or pathological stimuli. Equally, it is recognized that the activity of purines is closely dependent on their interconversion or metabolic degradation ensured by a series of enzymes present at extracellular level as predominantly bound to the cell membrane or, also, as soluble isoforms. While the effects of purines mediated by their receptor interactions have sufficiently, even though not entirely, been characterized in many tissues including bone, those promoted by the extracellular enzymes providing for purine metabolism have not been. In this review, we will try to circumstantiate the presence and the role of these enzymes in bone to define their close relationship with purine activities in maintaining bone homeostasis in normal or pathological conditions.

Guanosine-Mediated Anxiolytic-Like Effect: Interplay with Adenosine A1 and A2A Receptors

Acute or chronic administration of guanosine (GUO) induces anxiolytic-like effects, for which the adenosine (ADO) system involvement has been postulated yet without a direct experimental evidence. Thus, we aimed to investigate whether adenosine receptors (ARs) are involved in the GUO-mediated anxiolytic-like effect, evaluated by three anxiety-related paradigms in rats. First, we confirmed that acute treatment with GUO exerts an anxiolytic-like effect. Subsequently, we investigated the effects of pretreatment with ADO or A₁R (CPA, CCPA) or A_2AR (CGS21680) agonists 10 min prior to GUO on a GUO-induced anxiolytic-like effect. All the combined treatments blocked the GUO anxiolytic-like effect, whereas when administered alone, each compound was ineffective as compared to the control group. Interestingly, the pretreatment with nonselective antagonist caffeine or selective A₁R (DPCPX) or A_2AR (ZM241385) antagonists did not modify the GUO-induced anxiolytic-like effect. Finally, binding assay performed in hippocampal membranes showed that [³H]GUO binding became saturable at 100–300 nM, suggesting the existence of a putative GUO binding site. In competition experiments, ADO showed a potency order similar to GUO in displacing [³H]GUO binding, whereas AR selective agonists, CPA and CGS21680, partially displaced [³H]GUO binding, but the sum of the two effects was able to displace [³H]GUO binding to the same extent of ADO alone. Overall, our results strengthen previous data supporting GUO-mediated anxiolytic-like effects, add new evidence that these effects are blocked by A₁R and A_2AR agonists and pave, although they do not elucidate the mechanism of GUO and ADO receptor interaction, for a better characterization of GUO binding sites in ARs.

Involvement of P2X7 Receptors in the Osteogenic Differentiation of Mesenchymal Stromal/Stem Cells Derived from Human Subcutaneous Adipose Tissue

The ionotropic P2X7 receptor (P2X7R) is involved in bone homeostasis but its role in osteogenesis is controversial. Thus, we investigated the expression of P2X7R and the effects exerted by its modulation in mesenchymal stromal cells from human subcutaneous adipose tissue (S-ASCs), which have potential therapeutic application in bone regenerative medicine. We found that undifferentiated S-ASCs expressed P2X7R and its functional splice variants P2X7AR and P2X7BR. Cell stimulation by P2X7R agonist BzATP (100 μM) neither modified proliferation nor caused membrane pore opening while increasing intracellular Ca²⁺ levels and migration. The P2X7R antagonist A438079 reversed these effects. However, 25-100 μM BzATP, administered to S-ASCs undergoing osteogenic differentiation, dose-dependently decreased extracellular matrix mineralization and expression of osteogenic transcription factors Runx2, alkaline phosphatase and osteopontin. These effects were not coupled to cell proliferation reduction or to cell death increase, but were associated to decrease in P2X7AR and P2X7BR expression. In contrast, expression of P2X7R, especially P2X7BR isoform, significantly increased during the osteogenic process. Noteworthy, the antagonist A438079, administered alone, at first restrained cell differentiation, enhancing it later. Accordingly, A438079 reversed BzATP effects only in the second phase of S-ASCs osteogenic differentiation. Apyrase, a diphosphohydrolase converting ATP/ADP into AMP, showed a similar behavior. Altogether, findings related to A438079 or apyrase effects suggest an earlier and prevailing pro-osteogenic activity by endogenous ATP and a later one by adenosine derived from endogenous ATP metabolism. Conversely, P2X7R pharmacological stimulation by BzATP, mimicking the effects of high ATP levels occurring during tissue injuries, depressed receptor expression/activity impairing MSC osteogenic differentiation.

Adult mesenchymal stem cells: is there a role for purine receptors in their osteogenic differentiation?

The role played by mesenchymal stem cells (MSCs) in contributing to adult tissue homeostasis and damage repair thanks to their differentiation capabilities has raised a great interest, mainly in bone regenerative medicine. The growth/function of these undifferentiated cells of mesodermal origin, located in specialized structures (niches) of differentiated organs is influenced by substances present in this microenvironment. Among them, ancestral and ubiquitous molecules such as adenine-based purines, i.e., ATP and adenosine, may be included. Notably, extracellular purine concentrations greatly increase during tissue injury; thus, MSCs are exposed to effects mediated by these agents interacting with their own receptors when they act/migrate in vivo or are transplanted into a damaged tissue. Here, we reported that ATP modulates MSC osteogenic differentiation via different P2Y and P2X receptors, but data are often inconclusive/contradictory so that the ATP receptor importance for MSC physiology/differentiation into osteoblasts is yet undetermined. An exception is represented by P2X7 receptors, whose expression was shown at various differentiation stages of bone cells resulting essential for differentiation/survival of both osteoclasts and osteoblasts. As well, adenosine, usually derived from extracellular ATP metabolism, can promote osteogenesis, likely via A2B receptors, even though findings from human MSCs should be implemented and confirmed in preclinical models. Therefore, although many data have revealed possible effects caused by extracellular purines in bone healing/remodeling, further studies, hopefully performed in in vivo models, are necessary to identify defined roles for these compounds in favoring/increasing the pro-osteogenic properties of MSCs and thereby their usefulness in bone regenerative medicine.

A New Investigational Perspective for Purines Against Glioblastoma Invasiveness

BACKGROUND

Glioblastoma Multiforme (GBM) is the most common and lethal brain malignancy. Recent evidence suggests that the presence of stem-like cells (GSCs) inside the tumor with high self-renewal, resistance to chemotherapy and invasiveness/migration potential is associated with poor GBM prognosis. GSC aggressiveness seems to be linked to an important process involved in tumorigenesis and cancer metastasis called Epithelial-to-Mesenchymal Transition (EMT), which is responsible for several biochemical changes and the acquisition of a more mesenchymal phenotype by GSCs, that enhance their migration, invasiveness and resistance to apoptosis.

OBJECTIVE

Since previous reports demonstrated that purines, interacting with their own receptors, exerted anti-tumor effects in GBM and derived cells, we tried to investigate the ability of these compounds to reduce tumor cell migration/invasion acting on EMT-associated genes/activators and/or signal pathways.

METHODS

Search in the literature of relevant articles related to the objective.

RESULTS

Papers examining the activity of purines on EMT signaling pathways/markers in GSCs are still few whereas literature is more abundant as for other kinds of tumors.

CONCLUSION

Considering the significance of EMT in GBM aggressiveness and the promising involvement of purines in this process, we think that further research in this regard may open the way towards a new therapeutic approach for the control of GBM invasiveness/recurrence.

Uncovering the Signaling Pathway behind Extracellular Guanine-Induced Activation of NO System: New Perspectives in Memory-Related Disorders

Mounting evidence suggests that the guanine-based purines stand out as key player in cell metabolism and in several models of neurodegenerative disorders, such as Parkinson's and Alzheimer's diseases. Guanosine (GUO) and guanine (GUA) are extracellular signaling molecules derived from the breakdown of the correspondent nucleotide, GTP, and their intracellular and extracellular levels are regulated by the fine-tuned activity of two major enzymes, purine nucleoside phosphorylase (PNP) and guanine deaminase (GDA). Noteworthy, GUO and GUA, seem to play opposite roles in the modulation of cognitive functions, such as learning and memory. Indeed GUO, despite exerting neuroprotective, anti-apoptotic and neurotrophic effects, causes a decay of cognitive activities, whereas GUA administration in rats results in working memory improvement (prevented by L-NAME pre-treatment). This study was designed to investigate, in a model of SH-SY5Y neuroblastoma cell line, the signal transduction pathway activated by extracellular GUA. Altogether, our results showed that: (i) in addition to an enhanced phosphorylation of ASK1, p38 and JNK, likely linked to a non-massive and transient ROS production, the PKB/NO/sGC/cGMP/PKG/ERK cascade seems to be the main signaling pathway elicited by extracellular GUA; (ii) the activation of this pathway occurs in a pertussis-toxin sensitive manner, thus suggesting the involvement of a putative G protein coupled receptor; (iii) the GUA-induced NO production, strongly reduced by cell pre-treatment with L-NAME, is negatively modulated by the EPAC-cAMP-CaMKII pathway, which causes the over-expression of GDA that, in turn, reduces the levels of GUA. These molecular mechanisms activated by GUA may be useful to support our previous observation showing that GUA improves learning and memory functions through the stimulation of NO signaling pathway, and underscore the therapeutic potential of oral administration of guanine for treating memory-related disorders.

The Role of Wnt Signal in Glioblastoma Development and Progression: A Possible New Pharmacological Target for the Therapy of This Tumor

Wnt is a complex signaling pathway involved in the regulation of crucial biological functions such as development, proliferation, differentiation and migration of cells, mainly stem cells, which are virtually present in all embryonic and adult tissues. Conversely, dysregulation of Wnt signal is implicated in development/progression/invasiveness of different kinds of tumors, wherein a certain number of multipotent cells, namely “cancer stem cells”, are characterized by high self-renewal and aggressiveness. Hence, the pharmacological modulation of Wnt pathway could be of particular interest, especially in tumors for which the current standard therapy results to be unsuccessful. This might be the case of glioblastoma multiforme (GBM), one of the most lethal, aggressive and recurrent brain cancers, probably due to the presence of highly malignant GBM stem cells (GSCs) as well as to a dysregulation of Wnt system. By examining the most recent literature, here we point out several factors in the Wnt pathway that are altered in human GBM and derived GSCs, as well as new molecular strategies or experimental drugs able to modulate/inhibit aberrant Wnt signal. Altogether, these aspects serve to emphasize the existence of alternative pharmacological targets that may be useful to develop novel therapies for GBM.

Altered gastrointestinal motility in an animal model of Lesch-Nyhan disease

Mutations in the HGPRT1 gene, which encodes hypoxanthine-guanine phosphoribosyltransferase (HGprt), housekeeping enzyme responsible for recycling purines, lead to Lesch-Nyhan disease (LND). Clinical expression of LND indicates that HGprt deficiency has adverse effects on gastrointestinal motility. Therefore, we aimed to evaluate intestinal motility in HGprt knockout mice (HGprt¯). Spontaneous and neurally evoked mechanical activity was recorded in vitro as changes in isometric tension in circular muscle strips of distal colon. HGprt¯ tissues showed a lower in amplitude spontaneous activity and atropine-sensitivity neural contraction compared to control mice. The responses to carbachol and to high KCl were reduced, demonstrating a widespread impairment of contractility. L-NAME was not able in the HGprt¯ tissues to restore the large amplitude contractile activity typical of control. In HGprt¯ colon, a reduced expression of dopaminergic D1 receptor was observed together with the loss of its tonic inhibitory activity present in control-mice. The analysis of inflammatory and oxidative stress in colonic tissue of HGprt¯ mice revealed a significant increase of lipid peroxidation associated with over production of oxygen free radicals. In conclusion, HGprt deficiency in mice is associated with a decrease in colon contractility, not dependent upon reduction of acetylcholine release from the myenteric plexus or hyperactivity of inhibitory signalling. By contrast the increased levels of oxidative stress could partially explain the reduced colon motility in HGprt¯ mice. Colonic dysmotility observed in HGprt¯ mice may mimic the gastrointestinal dysfunctions symptoms of human syndrome, providing a useful animal model to elucidate the pathophysiology of this problem in the LND.

Modulation of the TGF-β1-induced epithelial to mesenchymal transition (EMT) mediated by P1 and P2 purine receptors in MDCK cells

Epithelial to mesenchymal transition (EMT) occurs during embryogenesis or under pathological conditions such as hypoxia, injury, chronic inflammation, or tissue fibrosis. In renal tubular epithelial cells (MDCK), TGF-β1 induces EMT by reducing or increasing epithelial or mesenchymal marker expression, respectively. In this study, we confirmed that the cAMP analogues, 8-CPT-cAMP or N6-Ph-cAMP, inhibited the TGF-β1-driven overexpression of the mesenchymal markers ZEB-1, Slug, Fibronectin, and α-SMA. Furthermore, we showed that A1, A2A, P2Y1, P2Y11, and P2X7 purine receptor agonists modulated the TGF-β1-induced EMT through the involvement of PKA and/or MAPK/ERK signaling. The stimulation of A2A receptor reduced the overexpression of the EMT-related markers, mainly through the cAMP-dependent PKA pathway, as confirmed by cell pre-treatment with Myr-PKI. Both A1 and P2Y1 receptor stimulation exacerbated the TGF-β1-driven effects, which were reduced by cell pre-treatment with the MAPK inhibitor PD98059, according to the increased ERK1/2 phosphorylation upon receptor activation. The effects induced by P2Y11 receptor activation were oppositely modulated by PKA or MAPK inhibition, in line with the dual nature of the Gs- and Gq-coupled receptor. Differently, P2X7 receptor induced, per se, similar and not additive effects compared to TGF-β1, after prolonged cell exposure to BzATP. These results suggest a putative role of purine receptors as target for anti-fibrotic agents.

Release of soluble and vesicular purine nucleoside phosphorylase from rat astrocytes and microglia induced by pro-inflammatory stimulation with extracellular ATP via P2X7 receptors

Purine nucleoside phosphorylase (PNP), a crucial enzyme in purine metabolism which converts ribonucleosides into purine bases, has mainly been found inside glial cells. Since we recently demonstrated that PNP is released from rat C6 glioma cells, we then wondered whether this occurs in normal brain cells. Using rat primary cultures of microglia, astrocytes and cerebellar granule neurons, we found that in basal condition all these cells constitutively released a metabolically active PNP with Km values very similar to those measured in C6 glioma cells. However, the enzyme expression/release was greater in microglia or astrocytes that in neurons. Moreover, we exposed primary brain cell cultures to pro-inflammatory agents such as lipopolysaccharide (LPS) or ATP alone or in combination. LPS alone caused an increased interleukin-1β (IL-1β) secretion mainly from microglia and no modification in the PNP release, even from neurons in which it enhanced cell death. In contrast, ATP administered alone to glial cells at high micromolar concentrations significantly stimulated the release of PNP within 1 h, an effect not modified by LPS presence, whereas IL-1β secretion was stimulated by ATP only in cells primed for 2 h with LPS. In both cases ATP effect was mediated by P2X₇ receptor (P2X₇R), since it was mimicked by cell exposure to Bz-ATP, an agonist of P2X₇R, and blocked by cell pre-treatment with the P2X₇R antagonist A438079. Interestingly, ATP-induced PNP release from glial cells partly occurred through the secretion of lysosomal vesicles in the extracellular medium. Thus, during inflammatory cerebral events PNP secretion promoted by extracellular ATP accumulation might concur to control extracellular purine signals. Further studies could elucidate whether, in these conditions, a consensual activity of enzymes downstream of PNP in the purine metabolic cascade avoids accumulation of extracellular purine bases that might concur to brain injury by unusual formation of reactive oxygen species.

Evidence for purine nucleoside phosphorylase (PNP) release from rat C6 glioma cells

Intracellular purine turnover is mainly oriented to preserving the level of triphosphate nucleotides, fundamental molecules in vital cell functions that, when released outside cells, act as receptor signals. Conversely, high levels of purine bases and uric acid are found in the extracellular milieu, even in resting conditions. These compounds could derive from nucleosides/bases that, having escaped to cell reuptake, are metabolized by extracellular enzymes similar to the cytosolic ones. Focusing on purine nucleoside phosphorylase (PNP) that catalyzes the reversible phosphorolysis of purine (deoxy)-nucleosides/bases, we found that it is constitutively released from cultured rat C6 glioma cells into the medium, and has a molecular weight and enzyme activity similar to the cytosolic enzyme. Cell exposure to 10 μM ATP or guanosine triphosphate (GTP) increased the extracellular amount of all corresponding purines without modifying the levels/activity of released PNP, whereas selective activation of ATP P2Y₁ or adenosine A_2A metabotropic receptors increased PNP release and purine base formation. The reduction to 1% in oxygen supply (2 h) to cells decreased the levels of released PNP, leading to an increased presence of extracellular nucleosides and to a reduced formation of xanthine and uric acid. Conversely, 2 h cell re-oxygenation enhanced the extracellular amounts of both PNP and purine bases. Thus, hypoxia and re-oxygenation modulated in opposite manner the PNP release/activity and, thereby, the extracellular formation of purine metabolism end-products. In conclusion, extracellular PNP and likely other enzymes deputed to purine base metabolism are released from cells, contributing to the purinergic system homeostasis and exhibiting an important pathophysiological role.

P2Y1 and Cysteinyl Leukotriene Receptors Mediate Purine and Cysteinyl Leukotriene Co-Release in Primary Cultures of Rat Microglia

Inflammation is widely recognized as contributing to the pathology of acute and chronic neurodegenerative conditions. Microglial cells are pathologic sensors in the brain and activated microglia have been viewed as detrimental. Leukotriene, including cysteinyl leukotrienes (CysLTs) are suggested to be involved in brain inflammation and neurological diseases and ATP, by its receptors is a candidate for microglia activation. A23187 (10μM) stimulated microglia to co-release CysLTs and [3H]adenine based purines ([3H]ABPs), mainly ATP. The biosynthetic production of CysLTs was abolished by 10μM MK-886, an inhibitor of 5-lipoxygenase-activating protein activity. RT-PCR analysis showed that microglia expressed both CysLT1 / CysLT2 receptors, P2Y1 ATP-receptors and several members of the ATP binding cassette (ABC) transporters including MRP1, MRP4 and Pgp. The increase in [Ca2+]i elicited by LTD4 (0.1 μM) and 2MeSATP (100μM), agonists for CysLT- and P2Y1-receptors, was abolished by the respective antagonists, BAYu9773 (0.5 μM) and suramin (50 μM). The stimulation of both receptor subtypes, induced a concomitant increase in the release of both [3H]ABPs and CysLTs that was blocked by the antagonists and significantly reduced by a cocktail of ABC transporter inhibitors, BAPTA/AM (intracellular Ca2+ chelator) and staurosporine (0.1 μM, PKC blocker). P2Y antagonist was unable to antagonise the effects of LTD4 and BAYu9773 did not reduce the effects of 2MeSATP. These data suggest that: i) the efflux of purines and cysteinyl-leukotrienes is specifically and independently controlled by the two receptor types, ii) calcium, PKC and the ABC transporter system can reasonably be considered common mechanisms underlying the release of ABPs and CysLTs from microglia. The blockade of P2Y1 or CysLT1/CysLT2 receptors by specific antagonists that abolished the raise in [Ca2+]i and drastically reduced the concomitant efflux of both compounds, as well as the effects of BAPTA and staurosporine support this hypothesis. In conclusion, the data of the present study suggest a cross talk between the purine and leukotriene systems in a possible autocrine/paracrine control of the microglia-mediated initiation and progression of an inflammatory response.

The Guanine-Based Purinergic System: The Tale of An Orphan Neuromodulation

Guanine-based purines (GBPs) have been recently proposed to be not only metabolic agents but also extracellular signaling molecules that regulate important functions in the central nervous system. In such way, GBPs-mediated neuroprotection, behavioral responses and neuronal plasticity have been broadly described in the literature. However, while a number of these functions (i.e., GBPs neurothophic effects) have been well-established, the molecular mechanisms behind these GBPs-dependent effects are still unknown. Furthermore, no plasma membrane receptors for GBPs have been described so far, thus GBPs are still considered orphan neuromodulators. Interestingly, an intricate and controversial functional interplay between GBPs effects and adenosine receptors activity has been recently described, thus triggering the hypothesis that GBPs mechanism of action might somehow involve adenosine receptors. Here, we review recent data describing the GBPs role in the brain. We focus on the involvement of GBPs regulating neuronal plasticity, and on the new hypothesis based on putative GBPs receptors. Overall, we expect to shed some light on the GBPs world since although these molecules might represent excellent candidates for certain neurological diseases management, the lack of putative GBPs receptors precludes any high throughput screening intent for the search of effective GBPs-based drugs.

Synergic development of pharmacokinetics and bioanalytical methods as support of pharmaceutical research

The development of pharmacokinetics led this science to achieve a relevant role in the investigation of new chemical entities for therapeutic application, and has allowed a series of new useful realizations of out of patent drugs like prolonged release and delayed release formulations, therapeutic delivery system (TDS) for drugs to be active in systemic circulation avoiding the first pass effect, orodispersible and effervescent formulations, intramuscular and subcutaneous depot formulations acting over a long period, oral inhalatory systems, and drug association at fixed dose. The above applications had pharmacokinetics as protagonist and have required the support from bioanalytical methods to assay drug concentrations, even in pg·mL(-1) of plasma, that really have paralleled the synergic development of pharmacokinetics.The complexity of the above realizations required specific guidelines from the regulatory authorities, mainly the US FDA and EU EMA, which have normalized and, in most cases, simplified the above applications admitting some waivers of in vivo bioequivalence.However, this review highlights some critical points, not yet focused on by operating guidelines, which need to be clarified by regulatory authorities. One of the most relevant issues is about the planning and conducting bioavailability and bioequivalence trials with endogenous substances, that possess own homeostatic equilibria with fluctuations, in some cases with specific rhythms, like melatonin and female sex hormones. The baseline subtraction required by guidelines to define the net contribute to the exogenous absorbed drug in most cases is a non-solvable problem.

Development of a new HPLC method using fluorescence detection without derivatization for determining purine nucleoside phosphorylase activity in human plasma

Purine nucleoside phosphorylase (PNP) activity is involved in cell survival and function, since PNP is a key enzyme in the purine metabolic pathway where it catalyzes the phosphorolysis of the nucleosides to the corresponding nucleobases. Its dysfunction has been found in relevant pathological conditions (such as inflammation and cancer), so the detection of PNP activity in plasma could represent an attractive marker for early diagnosis or assessment of disease progression. Thus the aim of this study was to develop a simple, fast and sensitive HPLC method for the determination of PNP activity in plasma. The separation was achieved on a Phenomenex Kinetex PFP column using 0.1% formic acid in water and methanol as mobile phases in gradient elution mode at a flow rate of 1ml/min and purine compounds were detected using UV absorption and fluorescence. The analysis was fast since the run was achieved within 13min. This method improved the separation of the different purines, allowing the UV-based quantification of the natural PNP substrates (inosine and guanosine) or products (hypoxanthine and guanine) and its subsequent metabolic products (xanthine and uric acid) with a good precision and accuracy. The most interesting innovation is the simultaneous use of a fluorescence detector (excitation/emission wavelength of 260/375nm) that allowed the quantification of guanosine and guanine without derivatization. Compared with UV, the fluorescence detection improved the sensitivity for guanine detection by about 10-fold and abolished almost completely the baseline noise due to the presence of plasma in the enzymatic reaction mixture. Thus, the validated method allowed an excellent evaluation of PNP activity in plasma which could be useful as an indicator of several pathological conditions.

Potentiation of temozolomide antitumor effect by purine receptor ligands able to restrain the in vitro growth of human glioblastoma stem cells

Glioblastoma multiforme (GBM), the most common and aggressive brain tumor in humans, comprises a population of stem-like cells (GSCs) that are currently investigated as potential target for GBM therapy. Here, we used GSCs isolated from three different GBM surgical specimens to examine the antitumor activity of purines. Cultured GSCs expressed either metabotropic adenosine P1 and ATP P2Y receptors or ionotropic P2X7 receptors. GSC exposure for 48 h to 10-150 μM ATP, P2R ligand, or to ADPβS or MRS2365, P2Y1R agonists, enhanced cell expansion. This effect was counteracted by the PY1R antagonist MRS2500. In contrast, 48-h treatment with higher doses of ATP or UTP, which binds to P2Y2/4R, or 2'(3')-O-(4-benzoylbenzoyl)-ATP (Bz-ATP), P2X7R agonist, decreased GSC proliferation. Such a reduction was due to apoptotic or necrotic cell death but mostly to growth arrest. Accordingly, cell regrowth and secondary neurosphere formation were observed 2 weeks after the end of treatment. Suramin, nonselective P2R antagonist, MRS1220 or AZ11645373, selective A3R or P2X7R antagonists, respectively, counteracted ATP antiproliferative effects. AZ11645373 also abolished the inhibitory effect of Bz-ATP low doses on GSC growth. These findings provide important clues on the anticancer potential of ligands for A3R, P2Y1R, and P2X7R, which are involved in the GSC growth control. Interestingly, ATP and BzATP potentiated the cytotoxicity of temozolomide (TMZ), currently used for GBM therapy, enabling it to cause a greater and long-lasting inhibitory effect on GSC duplication when readded to cells previously treated with purine nucleotides plus TMZ. These are the first findings identifying purine nucleotides as able to enhance TMZ antitumor efficacy and might have an immediate translational impact.

Expression of P2X7 ATP receptor mediating the IL8 and CCL20 release in human periodontal ligament stem cells

ATP is released by human periodontal ligament cells (hPDLCs) and has been shown to regulate PDL regeneration and responses to mechanical stress through activation of P2Y receptors. This nucleotide, however, has also been reported to trigger the pro-inflammatory cascade by inducing the maturation and/or release of chemokines/cytokines from various cell types mainly via P2X7 receptors. Much less is known on the possible role of ATP in stem cells deriving from PDL (hPDLSCs) which are considered to be a promising tool for cell-based therapy to restore lesions. Given the role played by P2X7 in pathophysiological conditions, in this study we investigated the expression of P2X7 ATP receptors in hPDLSCs. The results obtained showed that hPDLSCs express P2X7 receptors evaluated by means of cytofluorimetric, immunohistochemistry, reverse transcriptase-PCR, and Western blot analyses. P2X7 ligation by 2',3'-(benzoyl-4-benzoyl)-ATP (BzATP), a specific receptor agonist, was followed by an increase in intracellular Ca2+ and in the uptake of ethidium bromide. These effects were dramatically reduced by oxidized ATP (oATP), the P2X7 irreversible inhibitor, suggesting that the P2X7 is the functional receptor involved. At 24 h treatment of hPDLSCs with BzATP it enhanced the release of the pro-inflammatory agents IL8 and CCL20, without influencing cell viability. These effects were counteracted by pre-treating the cells with oATP or with A-740003, a selective and potent P2X7 competitive antagonist. Collectively, these results indicated that extracellular ATP mediate a pro-inflammatory response via P2X7 receptors in hPDLSCs opening a further approach to control hPDLSCs behavior in their possible application as therapeutic tool.

Wnt signaling behaves as a "master regulator" in the osteogenic and adipogenic commitment of human amniotic fluid mesenchymal stem cells

Human amniotic fluid mesenchymal stem cells (huAFMSCs) are emerging as a promising therapeutic option in regenerative medicine. Here, we characterized huAFMSC phenotype and multipotentiality. When cultured in osteogenic medium, huAFMSC displayed a significant increase in: Alkaline Phosphatase (ALP) activity and mRNA expression, Alizarin Red S staining and Runx2 mRNA expression; whereas maintaining these cells in an adipogenic culture medium gave a time-dependent increase in PPARγ and FABP4 mRNA expression, glycerol-3-phosphate dehydrogenase (GPDH) activity and positivity to Oil Red Oil staining. These results confirm that huAFMSCs can differentiate toward osteogenic and adipogenic phenotypes. The canonical Wnt/ßcatenin signaling pathway appears to trigger huAFMSC osteoblastogenesis, since during early phases of osteogenic differentiation, the expression of Dishevelled-2 (Dvl-2), of the non-phosphorylated form of ß-catenin, and the phosphorylation of glycogen synthase kinase-3ß (GSK3ß) at serine 9 were upregulated. On the contrary, during adipogenic differentiation Dvl-2 expression decreased, whereas that of ß-catenin remained unchanged. This was associated with a late increase in GSK3ß phosphorylation. Consistent with this scenario, huAFMSCs exposure to Dickkopf-1, a selective inhibitor of the Wnt signaling, abolished Runx2 and ALP mRNA upregulation during huAFMSC osteogenic differentiation, whereas it enhanced FABP4 expression in adipocyte-differentiating cells. Taken together, these results unravel novel molecular determinants of huAFMSC commitment towards osteoblastogenesis, which may represent potential targets for directing the differentiation of these cells and improving their use in regenerative medicine.

Figure

Guanosine promotes proliferation of neural stem cells through cAMP-CREB pathway

In previous studies, we have found that extracellular guanosine can stimulate endogenous progenitor/stem cell proliferation in the spinal cord following chronic injury and in the subventricular zone of the brains of rats afflicted with Parkinson's Disease. In this study, using neural stem cells isolated from one-day old rats, we found that guanosine could stimulate neural stem cell proliferation, and that the proliferation was not due to the guanosine metabolism mechanism since guanine, which is interconverted by an ecto-purine nucleoside phosphorylase from guanosine, has no stimulating effect on the proliferation of neural stem cells. We determined that second messenger cAMP was involved in the pathway as results showed that 100 microM guanosine stimulated cAMP accumulation. Using western blot analysis, we found that 100 microM guanosine can activate the phosphorylation of CREB without changing the total amount of CREB. In conclusion, guanosine can stimulate neural stem cell proliferation, and the cAMP-CREB pathway is involved in this biological effect.

Adenosine A1 receptor stimulation enhances osteogenic differentiation of human dental pulp-derived mesenchymal stem cells via WNT signaling

In this study, mesenchymal stem cells deriving from dental pulp (DPSCs) of normal human impacted third molars, previously characterized for their ability to differentiate into osteoblasts, were used. We observed that: i) DPSCs, undifferentiated or submitted to osteogenic differentiation, express all four subtypes of adenosine receptors (AR) and CD73, corresponding to 5'-ecto-nucleotidase; and ii) AR stimulation with selective agonists elicited a greater osteogenic cell differentiation consequent to A1 receptor (A1R) activation. Therefore, we focused on the activity of this AR. The addition of 15-60nM 2-chloro-N(6)-cyclopentyl-adenosine (CCPA), A1R agonist, to DPSCs at each change of the culture medium significantly increased the proliferation of cells grown in osteogenic medium after 8days in vitro (DIV) without modifying that of undifferentiated DPSCs. Better characterizing the effect of A1R stimulation on the osteogenic differentiation capability of these cells, we found that CCPA increased the: i) expression of two well known and early osteogenic markers, RUNX-2 and alkaline phosphatase (ALP), after 3 and 7DIV; ii) ALP enzyme activity at 7DIV and iii) mineralization of extracellular matrix after 21DIV. These effects, abolished by cell pre-treatment with the A1R antagonist 8-cyclopentyl-1,3-dipropyl-xanthine (DPCPX), involved the activation of the canonical Wnt signaling as, in differentiating DPSCs, CCPA significantly increased dishevelled protein and inhibited glycogen synthase kinase-3β, both molecules being downstream of Wnt receptor signal pathway. Either siRNA of dishevelled or cell pre-treatment with Dickkopf-1, known inhibitor of Wnt signaling substantially reduced either DPSC osteogenic differentiation or its enhancement promoted by CCPA. Summarizing, our findings indicate that the stimulation of A1R may stimulate DPSC duplication enhancing their osteogenic differentiation efficiency. These effects may have clinical implications possibly facilitating bone tissue repair and remodeling.

Guanine-based purines modulate the effect of L-NAME on learning and memory in rats

Guanosine has been reported to exert neuroprotective effects. We recently reported that, following intraperitoneal (i.p.) injection to rats, it resulted to be widely distributed. Its metabolic product guanine also rapidly increased in all the tissues, including brain, after i.p. injection of guanosine and consistently we found a significant enzymatic activity of a soluble purine nucleoside phosphorylase in the plasma of the treated animals. In this study the effect of per os administration of guanosine or guanine to rats submitted to passive avoidance task has been evaluated. Guanosine (4 and 8 mg/kg) administered pretraining impaired retention in the passive avoidance task and was unable to prevent the amnesic effect caused by 100 mg/kg N-omega-nitro-l-arginine methyl ester (L-NAME), an inhibitor of the nitric oxide synthase (NOS) known to reduce the capability of treated animals to acquire or retain informations in several learning tasks. On the contrary, guanine (4 and 8 mg/kg), which per se did not modify the latency to step-trough in the passive avoidance task, when administered pretraining 15 min before L-NAME prevented, in a dose dependent manner, the amnesic effect of the NOS inhibitor. Moreover the nucleobase was able to rescue the memory trace also when administered after training. Neither guanosine nor guanine had effects on locomotor activity. These results indicate that guanine can exert important biological activities which may be different from those mediated by its precursor guanosine, thus this evenience should be taken into account when the biological effects of guanosine are evaluated.

Protective activity of guanosine in an in vitro model of Parkinson's disease

Parkinson's disease (PD) is a pathological condition characterized by a progressive neurodegeneration of dopaminergic neurons with the consequent reduction of dopamine content in the substantia nigra. The neurotoxin 6-hydroxydopamine (6-OHDA) is widely used to mimic the neuropathology of PD in both in vivo and in vitro experimental models. We found that, as expected, in dopaminergic human SH-SY5Y neuroblastoma cells the toxin reduced cell viability causing programmed cell death as assessed by an increase in DNA fragmentation. We also examined, in these cells, the activation/inactivation of several pro and anti apoptotic signaling pathways by 6-OHDA including p-38 kinase (p-38), c-Jun N-terminal kinase (JNK), protein kinase B (also known as Akt), glycogen synthase kinase-3β (GSK3β), and Bcl-2 protein. Guanine-based purines, exert neuroprotective effects and we previously reported that guanosine activates cell survival pathways including PI3K/Akt/PKB signaling in different kinds of cells including glia and neuroblastoma cells. In the present study we found that guanosine (300 µM) protected SH-SY5Y neuroblastoma cells when they were exposed to 6-OHDA, promoting their survival. Guanosine reduced the 6-OHDA mediated activation of p-38 and JNK. Moreover the nucleoside potentiated the early increase in the phosphorylation of the anti-apoptotic kinase Akt and the increase in the expression of the anti-apoptotic Bcl-2 protein induced by 6-OHDA. In summary our results show that guanosine results to be neuroprotective in a recognized in vitro model of PD thus suggesting that it could represent a new potential pharmacological tool to be studied in the therapeutic approach to PD.

Guanine-based purines modulate the effect of L-NAME on learning and memory in rats

Guanosine has been reported to exert neuroprotective effects. We recently reported that, following intraperitoneal (i.p.) injection to rats, it resulted to be widely distributed. Its metabolic product guanine also rapidly increased in all the tissues, including brain, after i.p. injection of guanosine and consistently we found a significant enzymatic activity of a soluble purine nucleoside phosphorylase in the plasma of the treated animals. In this study the effect of per os administration of guanosine or guanine to rats submitted to passive avoidance task has been evaluated. Guanosine (4 and 8 mg/kg) administered pretraining impaired retention in the passive avoidance task and was unable to prevent the amnesic effect caused by 100 mg/kg N-omega-nitro-l-arginine methyl ester (L-NAME), an inhibitor of the nitric oxide synthase (NOS) known to reduce the capability of treated animals to acquire or retain informations in several learning tasks. On the contrary, guanine (4 and 8 mg/kg), which per se did not modify the latency to step-trough in the passive avoidance task, when administered pretraining 15 min before L-NAME prevented, in a dose dependent manner, the amnesic effect of the NOS inihibitor. Moreover the nucleobase was able to rescue the memory trace also when administered after training. Neither guanosine nor guanine had effects on locomotor activity. These results indicate that guanine can exert important biological activities which may be different from those mediated by its precursor guanosine, thus this evenience should be taken into account when the biological effects of guanosine are evaluated.

Tissue distribution and metabolism of guanosine in rats following intraperitoneal injection

Guanosine has long been known as an endogenous purine nucleoside deeply involved in the modulation of several intracellular processes, especially G-protein activity. More recently, it has been reported to act as an extracellular signaling molecule released from neurons and, more markedly, from astrocytes either in basal conditions or after different kinds of stimulation including hypoxia. Moreover, in vivo studies have shown that guanosine plays an important role as both a neuroprotective and neurotrophic agent in the central nervous system. Specific high-affinity binding sites for this nucleoside have been found on membrane preparations from rat brain. The present study was undertaken to investigate the distribution and metabolic profiles of guanosine after administering the nucleoside to gain a better understanding of the biological effects of this potential drug candidate. Rats were given an intraperitonal (i.p.) injection of 2, 4, 8 or 16 mg/kg of guanosine combined with 0.05% of [3H]guanosine. Plasma samples were collected 7.5, 15, 30, 60 and 90 min after the guanosine-mixture administration and analyzed by either a liquid scintillation counter or by HPLC connected to a UV and to an on-line radiochemical detector to measure the levels of guanosine and its metabolic products guanine, xanthine and uric acid. The levels of guanosine, guanine and xanthine were also measured in brain, lung, heart, kidney and liver tissue homogenates at the defined time points after the injection of 8 mg/kg of the guanosine-mixture. We found that the levels of radioactivity in plasma increased linearly in a dose- and time-dependent manner. Guanosine was widely distributed in all tissues examined in the present study, at almost twice its usual levels. In addition, guanine levels dramatically increased in all the organs. Interestingly, enzymatic analysis of the plasma samples showed the presence of a soluble purine nucleoside phosphorylase, a key enzyme in the purine salvage pathway and nucleoside catabolism. Since guanosine has been shown to be neuroprotective and astrocytes have been reported to play critical roles in mediating neuronal survival and functions in different neurodegenerative disorders, we also performed uptake and release.

Vascular endothelial growth factor enhances in vitro proliferation and osteogenic differentiation of human dental pulp stem cells

Mesenchymal stem cells (MSC), isolated from dental tissues, are largely studied for future application in regenerative dentistry. In this study, we used MSC obtained from human dental pulp (DPSC) of normal impacted third molars that, when cultured in lineage-specific inducing media, differentiate into osteoblasts and adipocytes (evaluated by Alizarin Red S and Red Oil O stainings, respectively), thus showing a multipotency. We confirmed that DPSC, grown under undifferentiating conditions, are negative for hematopoietic (CD45, CD31, CD34, CD144) and positive for mesenchymal (CD29, CD90, CD105, CD166, CD146, STRO-1) markers, that underwent down-regulation when cells were grown in osteogenic medium for 3 weeks. In this condition, they also exhibit an increase in the expression of osteogenic markers (RUNX-2, alkaline phosphatase) and extracellular calcium deposition, whereas the expression of receptors (VEGFR-1 and -2) for vascular endothelial growth factors (VEGF) and related VEGF binding proteins was similar to that found in undifferentiated DPSC. Exposure of DPSC growing under undifferentiating or osteogenic conditions to VEGF-A165 peptide (10-40 ng/ml) for 8 days dose- and time-dependently increased the number of proliferating cells without inducing differentiation towards endothelial lineage, as evaluated by the lack of expression of specific markers (CD31, CD34, CD144). Additionally, exposure of DPSC cultured in osteogenic medium to VEGF-A165 for a similar period enhanced cell differentiation towards osteoblasts as evaluated after 14 and 21 days by Alizarin Red S staining and alkaline phosphatase activity quantification. These findings may have clinical implications possibly facilitating tissue repair and remodeling.

Guanosine improves motor behavior, reduces apoptosis, and stimulates neurogenesis in rats with parkinsonism

Parkinson's disease (PD) is characterized by progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNc) caused by an abnormal rate of apoptosis. Endogenous stem cells in the adult mammalian brain indicate an innate potential for regeneration and possible resource for neuroregeneration in PD. We previously showed that guanosine prevents apoptosis even when administered 48 hr after the toxin 1-methyl-4-phenylpyridinium (MPP(+)). Here, we induced parkinsonism in rats with a proteasome inhibitor. Guanosine treatment reduced apoptosis, increased tyrosine hydroxylase-positive dopaminergic neurons and expression of tyrosine hydroxylase in the SNc, increased cellular proliferation in the SNc and subventricular zone, and ameliorated symptoms. Proliferating cells in the subventricular zone were nestin-positive adult neural progenitor/stem cells. Fibroblast growth factor-2-expressing cells were also increased by guanosine. Thus, guanosine protected cells from apoptosis and stimulated "intrinsic" adult progenitor/stem cells to become dopaminergic neurons in rats with proteasome inhibitor-induced PD. The cellular/molecular mechanisms underlying these effects may open new avenues for development of novel therapeutics for PD.

Metabolism and distribution of guanosine given intraperitoneally: implications for spinal cord injury

Intraperitoneal administration of guanosine to rats with chronic spinal cord injury stimulates remyelination and functional recovery. If guanosine produced its effects in the nervous system, it should enter it and elevate endogenous concentrations. [(3)H]-guanosine (8 mg/kg) was administered intraperitoneally to rats and its distribution and concentration in different sites determined. Guanosine rapidly entered all tissues; its concentration peaked at about 15 minutes except in adipose tissue and CNS where it continued to rise for 30 minutes. Its chief metabolic product in all sites was guanine with over twice as much guanine as guanosine present in CNS after 30 minutes.

Neurotrophic effects of extracellular guanosine

Central nervous system (CNS) astrocytes release guanosine extracellularly, that exerts trophic effects. In CNS, extracellular guanosine (GUO) stimulates mitosis, synthesis of trophic factors, and cell differentiation, including neuritogenesis, is neuroprotective, and reduces apoptosis due to several stimuli. Specific receptor-like binding sites for eGUO in the nervous system may mediate its effects through both MAP kinase and PI3-kinase signalling pathways. Extracellular guanine (eGUA) also exerts several effects; the trophic effects of eGUO are likely regulated by conversion of eGUO to eGUA by a membrane located purine nucleoside phosphorylase (ecto-PNP) and by conversion of eGUA to xanthine by guanine deaminase.

Changes in matrix extracellular phosphoglycoprotein expression before and during in vitro osteogenic differentiation of human dental papilla mesenchymal cells

The purpose of this study is to characterise the expression of matrix extracellular phosphoglycoprotein (MEPE) in cultured mesenchymal cells isolated from human dental papilla (PaMCs) of impacted third molars either before or during differentiation of these cells into osteo/odontoblasts. PaMCs, like mesenchymal cells deriving from human dental pulp (DPMCs), resulted positive for a number of mesenchymal markers including CD146 and STRO-1. During the first week in culture they showed a faster proliferation rate than DPMCs, coupled to an earlier down-regulation of MEPE. Also when the cells were further cultured in osteogenic medium (containing beta-glycerophosphate, ascorbic acid and dexamethasone) for 40 days, MEPE down-regulation coupled to an increased expression of osteogenic markers, such as osteocalcin and alkaline phosphatase, occurred earlier in PaMCs than in DPMCs. Thus, our data, indicating that also in PaMCs MEPE expression is higher when cells proliferate, whereas it is downregulated as cells differentiated, are in favour of a role of MEPE as an early regulator of odontogenic differentiation. We also confirm the superior proliferative potential of PaMCs in comparison with DPMCs, coupled to a more rapid induction of osteogenic differentiation. Therefore, these cells represent an optimal source to be conveniently used for dental tissue engineering and tooth regeneration.

Morphostructural analysis of human follicular stem cells on highly porous bone hydroxyapatite scaffold

In this study we investigated the in vitro behaviour, morphostructure and extracellular matrix synthesis of human dental follicular stem cells (hDFSCs) isolated from human dental bud, which resulted to be positive for mesenchymal markers (CD29, CD90, CD146 and CD166) by FACS analysis. Cells were analysed by light and electronic microscopy to evaluate their biological response either at week 1, that is before differentiation, or at weeks 3-6, when they had been cultured in osteogenic medium onto a highly porous natural scaffold material (Bio-Oss). Microscopy analysis of primary culture cells showed they had a mesenchymal stem cell-like morphostructure, spindle shaped, similar to the culture of mesenchymal stem cells derived from adult bone marrow. Also, after osteogenic differentiation, these analyses indicate typical osteoblast morphostructure and reveale a tri-dimensional organization of the cells and deposition of extracellular matrix (ECM) in close contact with biomaterial. This approach would allow to personalize the scaffold for bone tissue engineering in order to accelerate the process of osteogenesis.

Activation of P2X(7) receptors stimulates the expression of P2Y(2) receptor mRNA in astrocytes cultured from rat brain

Under pathological conditions brain cells release ATP at concentrations reported to activate P2X(7) ionotropic receptor subtypes expressed in both neuronal and glial cells. In the present study we report that the most potent P2X(7) receptor agonist BzATP stimulates the expression of the metabotropic ATP receptor P2Y(2) in cultured rat brain astrocytes. In other cell types several kinds of stimulation, including stress or injury, induce P2Y(2) expression that, in turn, is involved in different cell reactions. Similarly, it has recently been found that in astrocytes and astrocytoma cells P2Y(2) sites can trigger neuroprotective pathways through the activation of several mechanisms, including the induction of genes for antiapoptotic factors, neurotrophins, growth factors and neuropeptides. Here we present evidence that P2Y(2) mRNA expression in cultured astrocytes peaks 6 h after BzATP exposure and returns to basal levels after 24 h. This effect was mimicked by high ATP concentrations (1 mM) and was abolished by P2X(7)-antagonists oATP and BBG. The BzATP-evoked P2Y(2) receptor up-regulation in cultured astrocytes was coupled to an increased UTP-mediated intracellular calcium response. This effect was inhibited by oATP and BBG and by P2Y(2)siRNA, thus supporting evidence of increased P2Y(2) activity. To further investigate the mechanisms by which P2X(7) receptors mediated the P2Y(2) mRNA up-regulation, the cells were pre-treated with the chelating agent EGTA, or with inhibitors of mitogen-activated kinase (MAPK) (PD98059) or protein kinase C, (GF109203X). Each inhibitor significantly reduced the extent to which BzATP induced P2Y(2) mRNA. Both BzATP and ATP (1 mM) increased ERK1/2 activation. P2X(7)-induced ERK1/2 phosphorylation was unaffected by pre-treatment of astrocytes with EGTA whereas it was inhibited by GF109203X. Phorbol-12-myristate-13-acetate (PMA), an activator of PKCs, rapidly increased ERK1/2 activation. We conclude that activation of P2X(7) receptors in astrocytes enhances P2Y(2) mRNA expression by a mechanism involving both calcium influx and PKC/MAPK signalling pathways.

Staurosporine-induced apoptosis in astrocytes is prevented by A1 adenosine receptor activation

Astrocyte apoptosis occurs in acute and chronic pathological processes at the central nervous system and the prevention of astrocyte death may represent an efficacious intervention in protecting neurons against degeneration. Our research shows that rat astrocyte exposure to 100 nM staurosporine for 3h caused apoptotic death accompanied by caspase-3, p38 mitogen-ed protein kinase (MAPK) and glycogen synthase kinase-3beta (GSK3beta) activation. N(6)-chlorocyclopentyladenosine (CCPA, 2.5-75 nM), a selective agonist of A(1) adenosine receptors, added to the cultures 1h prior to staurosporine, induced a dose-dependent anti-apoptotic effect, which was inhibited by the A(1) receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine. CCPA also caused a dose- and time-dependent phosphorylation/activation of Akt, a downstream effector of cell survival promoting phosphatidylinositol 3-kinase (PI3K) pathway, which in turn led to inhibition of staurosporine-induced GSK3beta and p38 MAPK activity. Accordingly, the anti-apoptotic effect of CCPA was abolished by culture pre-treatment with LY294002, a selective PI3K inhibitor, pointing out the prevailing role played by PI3K pathway in the protective effect exerted by A(1) receptor activation. Since an abnormal p38 and GSK3beta activity is implicated in acute (stroke) and chronic (Alzheimer's disease) neurodegenerative diseases, the results of the present study provide a hint to better understand adenosine relevance in these disorders.

The G51S purine nucleoside phosphorylase polymorphism is associated with cognitive decline in Alzheimer's disease patients

Alzheimer's disease (AD) is a polygenic and multifactorial complex disease, whose etiopathology is still unclear, however several genetic factors have shown to increase the risk of developing the disease. Purine nucleotides and nucleosides play an important role in the brain. Besides their role in neurotransmission and neuromodulation, they are involved in trophic factor release, apoptosis, and inflammatory responses. These mediators may also have a pivotal role in the control of neurodegenerative processes associated with AD. In this report the distribution of the exonic G/A single nucleotide polymorphism (SNP) in purine nucleoside phosphorylase (PNP) gene, resulting in the amino acid substitution serine to glycine at position 51 (G51S), was investigated in a large population of AD patients (n=321) and non-demented control (n=208). The PNP polymorphism distribution was not different between patients and controls. The polymorphism distribution was also analyzed in AD patients stratified according to differential progressive rate of cognitive decline during a 2-year follow-up. An increased representation of the PNP AA genotype was observed in AD patients with fast cognitive deterioration in comparison with that from patients with slow deterioration rate. Our findings suggest that the G51S PNP polymorphism is associated with a faster rate of cognitive decline in AD patients, highlighting the important role of purine metabolism in the progression of this neurodegenerative disorder.

Molecular Signalling Mediating the Protective Effect of A1 Adenosine and mGlu3 Metabotropic Glutamate Receptor Activation against Apoptosis by Oxygen/Glucose Deprivation in Cultured Astrocytes

Astrocyte death may occur in neurodegenerative disorders and complicates the outcome of brain ischemia, a condition associated with high extracellular levels of adenosine and glutamate. We show that pharmacological activation of A(1) adenosine and mGlu3 metabotropic glutamate receptors with N(6)-chlorocyclopentyladenosine (CCPA) and (-)2-oxa-4-aminocyclo-[3.1.0]hexane-4,6-dicarboxylic acid (LY379268), respectively, protects cultured astrocytes against apoptosis induced by a 3-h exposure to oxygen/glucose deprivation (OGD). Protection by CCPA and LY379268 was less than additive and was abrogated by receptor blockade with selective competitive antagonists or pertussis toxin. Both in control astrocytes and in astrocytes exposed to OGD, CCPA and LY379268 induced a rapid activation of the phosphatidylinositol-3-kinase (PI3K) and extracellular signal-regulated kinases 1 and 2 (ERK1/2)/mitogen-activated protein kinase (MAPK) pathways, which are known to support cell survival. In cultures exposed to OGD, CCPA and LY379268 reduced the activation of c-Jun N-terminal kinase and p38/MAPK, reduced the levels of the proapoptotic protein Bad, increased the levels of the antiapoptotic protein Bcl-X(L), and were highly protective against apoptotic death, as shown by nuclear 4'-6-diamidino-2-phenylindole staining and measurements of caspase-3 activity. All of these effects were attenuated by treatment with 1,4-diamino-2,3-dicyano-1,4-bis(methylthio)butadiene (U0126) and 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride (LY294002), which inhibit the MAPK and the PI3K pathways, respectively. These data suggest that pharmacological activation of A(1) and mGlu3 receptors protects astrocytes against hypoxic/ischemic damage by stimulating the PI3K and ERK1/2 MAPK pathways.

Antidepressant therapy can improve adherence to antiretroviral regimens among HIV-infected and depressed patients

Several strategies have been introduced to manage nonadherence to highly active antiretroviral therapy (HAART). Treatment with antidepressants may improve self-reported adherence. In this brief report, a small sample of HIV-depressed patients (n = 9) were treated for a 6-month period with antidepressants improving self-reported adherence based on the HAART scale (poor, good, satisfactory, and optimal). Before the antidepressant treatment, adherence was reported as "good" by 3 patients and "satisfactory" by 6 patients. After antidepressant therapy, adherence to antiretroviral regimes was statistically higher in HIV-depressed on treatment than in HIV-depressed patients not treated with antidepressants (P < 0.0001). We used chi2 test with a significance level at P < 0.05. Treating depression in HIV-infected patients may serve to improve adherence to HAART.

Guanosine Inhibits CD40 Receptor Expression and Function Induced by Cytokines and β Amyloid in Mouse Microglia Cells

Growing evidence implicates CD40, a member of the TNFR superfamily, as contributing to the pathogenesis of many neurodegenerative diseases. Thus, strategies to suppress its expression may be of benefit in those disorders. To this aim, we investigated the effect of guanosine, a purine nucleoside that exerts neurotrophic and neuroprotective effects. CD40 expression and function are increased by exposure of mouse microglia cultures or the N9 microglia cell line to IFN-gamma (10 ng/ml) plus TNF-alpha (50 ng/ml) or beta amyloid (Abeta) peptide (Abeta(1-42); 500 nM). Culture pretreatment with guanosine (10-300 microM), starting 1 h before cytokine or Abeta addition, dose-dependently inhibited the CD40-induced expression as well as functional CD40 signaling by suppressing IL-6 production promoted by IFN-gamma/TNF-alpha challenge in the presence of CD40 cross-linking. Moreover, guanosine abrogated IFN-gamma-induced phosphorylation on Ser(727) and translocation of STAT-1alpha to the nucleus as well as TNF-alpha-/Abeta-induced IkappaBalpha and NF-kappaB p65/RelA subunit phosphorylation, thus inhibiting NF-kappaB-induced nuclear translocation. Guanosine effects were mediated by an increased phosphorylation of Akt, a PI3K downstream effector, as well as of ERK1/2 and p38 in the MAPK system, because culture pretreatment with selective ERK1/2, p38 MAPK, and PI3K antagonists (U0126, SB203580, or LY294002, respectively) counteracted guanosine inhibition on IFN-gamma/TNF-alpha-induced CD40 expression and function as well as on STAT-1alpha or NF-kappaB nuclear translocation. These findings suggest a role for guanosine as a potential drug in the experimental therapy of neuroinflammatory/neurodegenerative diseases, particularly Alzheimer's disease.

P2Y2 receptor up-regulation induced by guanosine or UTP in rat brain cultured astrocytes

Among P2 metabotropic ATP receptors, P2Y2 subtype seems to be peculiar as its upregulation triggers important biological events in different cells types. In non-stimulated cells including astrocytes, P2Y2 receptors are usually expressed at levels lower than P2Y1 sites, however the promoter region of the P2Y2 receptors has not yet been studied and little is known about the mechanisms underlying the regulation of the expression of this ATP receptor. We showed that not only UTP and ATP are the most potent and naturally occurring agonist for P2Y2 sites, but also guanosine induced an up-regulation of astrocyte P2Y2 receptor mRNA evaluated by Northern blot analysis. We also focused our attention on this nucleoside since in our previous studies it was reported to be released by cultured astrocytes and to exert different neuroprotective effects. UTP and guanosine-evoked P2Y2 receptor up-regulation in rat brain cultured astrocytes was linked to an increased P2Y2-mediated intracellular calcium response, thus suggesting an increased P2Y2 activity. Actinomycin D, a RNA polymerase inhibitor, abrogated both UTP and guanosine-mediated P2Y2 up-regulation, thus indicating that de novo transcription was required. The effect of UTP and guanosine was also evaluated in astrocytes pretreated with different inhibitors of signal transduction pathways including ERK, PKC and PKA reported to be involved in the regulation of other cell surface receptor mRNAs. The results show that ERK1-2/MAPK pathway play a key role in the P2Y2 receptor up-regulation mediated by either UTP or guanosine. Moreover, our data suggest that PKA is also involved in guanosine-induced transcriptional activation of P2Y2 mRNA and that increased intracellular calcium levels and PKC activation may also mediate P2Y2 receptor up-regulation triggered by UTP. The extracellular release of ATP under physiological and pathological conditions has been widely studied. On the contrary, little is known about the release of pyrimidines and in particular of UTP. Here we show that astrocytes are able to release UTP, either at rest or during and following hypoxia/hypoglycemia obtained by submitting the cells to glucose-oxygen deprivation (OGD). Interestingly, also P2Y2 receptor mRNA increased by about two-fold the control values when the cultures were submitted to OGD. It has been recently reported that P2Y2 receptors can play a protective role in astrocytes, thus either guanosine administration or increased extracellular concentrations of guanosine and UTP reached locally following CNS injury may increase P2Y2-mediated biological events aimed at promoting a protective astrocyte response.

P2X7 receptor activation in rat brain cultured astrocytes increases the biosynthetic release of cysteinyl leukotrienes

Astrocytes have been recognized as important elements in controlling inflammatory as well as immune processes in the central nervous system (CNS). Recently, glial cells have been shown to produce cysteinyl leukotrienes (CysLTs) which are known lipid mediators of inflammation and whose extracellular concentrations rise under different pathological conditions in the brain. In the same conditions also extracellular concentrations of ATP dramatically increase reaching levels able to activate P2X7 ionotropic receptors for which an emerging role in neuroinflammation and neurodegeneration has been claimed. RTPCR analysis showed that primary cultures of rat brain astrocytes express P2X7 receptors. Application of the selective P2X7 agonist benzoyl benzoly ATP (BzATP) markedly increased [Ca2+]i which was mediated by a calcium influx from the extracellular milieu. The P2X7 antagonist, oATP, suppressed the BzATP-induced calcium increase. Consistent with the evidence that increased calcium levels activate the leukotriene biosynthetic pathway, challenge of astrocytes with either the calcium ionophore A23187 or BzATP significantly increased CysLT production and the cell pre-treatment with EGTA abolished these effects. Again the P2X7 antagonist prevented the BzATP-mediated CysLT efflux, whereas the astrocyte pretreatment with MK-571, a CysLT1 receptor antagonist, was ineffective. The astrocyte pre-treatment with a cocktail of inhibitors of ATP binding cassette (ABC) proteins reduced the BzATP-mediated CysLT production confirming that ABC transporters are involved in the release of CysLTs. The astrocyte P2X7- evoked rise of CysLT efflux was abolished in the presence of MK-886, an inhibitor of 5-lipoxygenase activating protein (FLAP) whose expression, along with that of 5-lipoxygenase (5-LO) was reported by Northern Blot analysis. The stimulation of P2X7 induced an up-regulation of FLAPmRNA that was reduced by the antagonist oATP. These data suggest that in rat brain cultured astrocytes P2X7ATP receptors may participate in the control of CysLT release thus further supporting a role for extracellular ATP as an integral component of the inflammatory brain response.

AIT-082 as a potential neuroprotective and regenerative agent in stroke and central nervous system injury

The synthetic purine 4-[[3-(1,6 dihydro-6-oxo-9-purin-9-yl)-1-oxypropyl] amino] benzoic acid (AIT-082, Neotrofin, leteprinim potassium) possesses several biological properties of note: it stimulates outgrowth of neurites from PC12 cells and neurones, stimulates synthesis and/or release of neurotrophic factors from astrocytes, enhances nerve fibre regeneration in vivo and enhances of memory in animals and humans. AIT-082 also protects against glutamate neurotoxicity in vitro and in vivo, which has led to successful tests of AIT-082 in animal models of acute central nervous system injury. In such cases, AIT-082 probably functions by both acutely reducing glutamate excitotoxicity and, over a longer period, by enhancing neuronal sprouting and functional recovery.