Neuroprotective effects of guanosine on stroke models in vitro and in vivo

Lessons from the physiological role of guanosine in neurodegeneration and cancer: Toward a multimodal mechanism of action?

Neurodegenerative diseases and brain tumours represent important health challenges due to their severe nature and debilitating consequences that require substantial medical care. Interestingly, these conditions share common physiological characteristics, namely increased glutamate, and adenosine transmission, which are often associated with cellular dysregulation and damage. Guanosine, an endogenous nucleoside, is safe and exerts neuroprotective effects in preclinical models of excitotoxicity, along with cytotoxic effects on tumour cells. However, the lack of well-defined mechanisms of action for guanosine hinders a comprehensive understanding of its physiological effects. In fact, the absence of specific receptors for guanosine impedes the development of structure-activity research programs to develop guanosine derivatives for therapeutic purposes. Alternatively, given its apparent interaction with the adenosinergic system, it is plausible that guanosine exerts its neuroprotective and anti-tumorigenic effects by modulating adenosine transmission through undisclosed mechanisms involving adenosine receptors, transporters, and purinergic metabolism. Here, several potential molecular mechanisms behind the protective actions of guanosine will be discussed. First, we explore its potential interaction with adenosine receptors (A1R and A2AR), including the A1R-A2AR heteromer. In addition, we consider the impact of guanosine on extracellular adenosine levels and the role of guanine-based purine-converting enzymes. Collectively, the diverse cellular functions of guanosine as neuroprotective and antiproliferative agent suggest a multimodal and complementary mechanism of action.

Dynamic and Rapid Detection of Guanosine during Ischemia

Guanosine acts in both neuroprotective and neurosignaling pathways in the central nervous system; in this paper, we present the first fast voltammetric measurements of endogenous guanosine release during pre- and post-ischemic conditions. We discuss the metric of our measurements via analysis of event concentration, duration, and interevent time of rapid guanosine release. We observe changes across all three metrics from our normoxic to ischemic conditions. Pharmacological studies were performed to confirm that guanosine release is a calcium-dependent process and that the signaling observed is purinergic. Finally, we show the validity of our ischemic model via staining and fluorescent imaging. Overall, this paper sets the tone for rapid monitoring of guanosine and provides a platform to investigate the extent to which guanosine accumulates at the site of brain injury, i.e., ischemia.

Purine nucleoside phosphorylase as a target to treat age-associated lower urinary tract dysfunction

The lower urinary tract (LUT), including the bladder, urethra and external striated muscle, becomes dysfunctional with age; consequently, many older individuals suffer from lower urinary tract disorders (LUTDs). By compromising urine storage and voiding, LUTDs degrade quality of life for millions of individuals worldwide. Treatments for LUTDs have been disappointing, frustrating both patients and their physicians; however, emerging evidence suggests that partial inhibition of the enzyme purine nucleoside phosphorylase (PNPase) with 8-aminoguanine (an endogenous PNPase inhibitor that moderately reduces PNPase activity) reverses age-associated defects in the LUT and restores the LUT to that of a younger state. Thus, 8-aminoguanine improves LUT biochemistry, structure and function by rebalancing the LUT purine metabolome, making 8-aminoguanine a novel potential treatment for LUTDs.

Promising Strategies for the Development of Advanced In Vitro Models with High Predictive Power in Ischaemic Stroke Research

Although stroke is one of the world’s leading causes of death and disability, and more than a thousand candidate neuroprotective drugs have been proposed based on extensive in vitro and animal-based research, an effective neuroprotective/restorative therapy for ischaemic stroke patients is still missing. In particular, the high attrition rate of neuroprotective compounds in clinical studies should make us question the ability of in vitro models currently used for ischaemic stroke research to recapitulate human ischaemic responses with sufficient fidelity. The ischaemic stroke field would greatly benefit from the implementation of more complex in vitro models with improved physiological relevance, next to traditional in vitro and in vivo models in preclinical studies, to more accurately predict clinical outcomes. In this review, we discuss current in vitro models used in ischaemic stroke research and describe the main factors determining the predictive value of in vitro models for modelling human ischaemic stroke. In light of this, human-based 3D models consisting of multiple cell types, either with or without the use of microfluidics technology, may better recapitulate human ischaemic responses and possess the potential to bridge the translational gap between animal-based in vitro and in vivo models, and human patients in clinical trials.

Luminescent Human iPSC-Derived Neurospheroids Enable Modeling of Neurotoxicity After Oxygen-glucose Deprivation

Despite the considerable impact of stroke on both the individual and on society, a neuroprotective therapy for stroke patients is missing. This is partially due to the current lack of a physiologically relevant human in vitro stroke model. To address this problem, we have developed a luminescent human iPSC-derived neurospheroid model that enables real-time read-out of neural viability after ischemia-like conditions. We subjected 1- and 4-week-old neurospheroids, generated from iPSC-derived neural stem cells, to 6 h of oxygen-glucose deprivation (OGD) and measured neurospheroid luminescence. For both, we detected a decrease in luminescent signal due to ensuing neurotoxicity, as confirmed by conventional LDH assay and flow cytometric viability analysis. Remarkably, 1-week-old, but not 4-week-old neurospheroids recovered from OGD-induced injury, as evidenced by their reduced but overall increasing luminescence over time. This underscores the need for more mature neurospheroids, more faithfully recapitulating the in vivo situation. Furthermore, treatment of oxygen- and glucose-deprived neurospheroids with the pan-caspase inhibitor Z-VAD-FMK did not increase overall neural survival, despite its successful attenuation of apoptosis, in a human-based 3D environment. Nevertheless, owing to its three-dimensional organization and real-time viability reporting potential, the luminescent neurospheroids may become readily adopted in high-throughput screens aimed at identification of new therapeutic agents to treat acute ischemic stroke patients.

Neuroprotective Effects of Guanosine in Ischemic Stroke-Small Steps towards Effective Therapy

Guanosine (Guo) is a nucleotide metabolite that acts as a potent neuromodulator with neurotrophic and regenerative properties in neurological disorders. Under brain ischemia or trauma, Guo is released to the extracellular milieu and its concentration substantially raises. In vitro studies on brain tissue slices or cell lines subjected to ischemic conditions demonstrated that Guo counteracts destructive events that occur during ischemic conditions, e.g., glutaminergic excitotoxicity, reactive oxygen and nitrogen species production. Moreover, Guo mitigates neuroinflammation and regulates post-translational processing. Guo asserts its neuroprotective effects via interplay with adenosine receptors, potassium channels, and excitatory amino acid transporters. Subsequently, guanosine activates several prosurvival molecular pathways including PI3K/Akt (PI3K) and MEK/ERK. Due to systemic degradation, the half-life of exogenous Guo is relatively low, thus creating difficulty regarding adequate exogenous Guo distribution. Nevertheless, in vivo studies performed on ischemic stroke rodent models provide promising results presenting a sustained decrease in infarct volume, improved neurological outcome, decrease in proinflammatory events, and stimulation of neuroregeneration through the release of neurotrophic factors. In this comprehensive review, we discuss molecular signaling related to Guo protection against brain ischemia. We present recent advances, limitations, and prospects in exogenous guanosine therapy in the context of ischemic stroke.

Guanosine Neuroprotective Action in Hippocampal Slices Subjected to Oxygen and Glucose Deprivation Restores ATP Levels, Lactate Release and Glutamate Uptake Impairment: Involvement of Nitric Oxide

Stroke is a major cause of disability and death worldwide. Oxygen and glucose deprivation (OGD) in brain tissue preparations can reproduce several pathological features induced by stroke providing a valuable ex vivo protocol for studying the mechanism of action of neuroprotective agents. Guanosine, an endogenous guanine nucleoside, promotes neuroprotection in vivo and in vitro models of neurotoxicity. We previously showed that guanosine protective effect was mimicked by inhibition of nitric oxide synthases (NOS) activity. This study was designed to investigate the involvement of nitric oxide (NO) in the mechanisms related to the protective role of guanosine in rat hippocampal slices subjected to OGD followed by reoxygenation (OGD/R). Guanosine (100 μM) and the pan-NOS inhibitor, L-NAME (1 mM) afforded protection to hippocampal slices subjected to OGD/R. The presence of NO donors, DETA-NO (800 μM) or SNP (5 μM) increased reactive species production, and abolished the protective effect of guanosine or L-NAME against OGD/R. Guanosine or L-NAME treatment prevented the impaired ATP production, lactate release, and glutamate uptake following OGD/R. The presence of a NO donor also abolished the beneficial effects of guanosine or L-NAME on bioenergetics and glutamate uptake. These results showed, for the first time, that guanosine may regulate cellular bioenergetics in hippocampal slices subjected to OGD/R injury by a mechanism that involves the modulation of NO levels.

Long lasting behavioral and electrophysiological action of early administration of guanosine: Analysis in the adult rat brain

Guanosine (GUO) is a guanine-based purine that has been extensively described in the literature as an endogenous nucleoside with participation in brain cell signalling pathways. Here, we evaluated whether chronic treatment with exogenous guanosine during brain development altered behavioral and electrophysiological parameters in adulthood. Rat pups received a daily intraperitoneal injection of 10, 50 or 100 mg/ kg/day GUO, or saline solution or no treatment (naive group) from postnatal (P) day 7 to P27. At P 60-65 the animals were behaviorally tested in the Elevated Plus-Maze (EPM). On P90-100, the electrophysiological phenomenon known as cortical spreading depression (CSD) was recorded on the right cortical surface for 4 h. With the EPM task, GUO treatment was associated with a significant increase in rearing behavior and a non-significant trend towards anxiogenic behavior. In a dose-dependent manner, GUO significantly (p < 0.01) increased weight gain on P90, and reduced the CSD propagation velocity from 3.42 ± 0.10 and 3.43 ± 0.10 mm/min in the Naive and Vehicle controls, respectively, to 3.05 ± 0.12 mm/min, 2.87 ± 0.07 mm/min and 2.25 ± 0.25 mm/min in the groups treated with 10, 50 and 100 mg/kg/d GUO, respectively. The results confirmed the hypothesis that the chronic treatment with GUO early in life modulates CSD and body weight. Data on CSD propagation suggest that, besides its suppressing action on glutamatergic transmission (via enhancement of astrocytic glutamate uptake), GUO might act as an antioxidant in the brain, a hypothesis that deserves further exploration.

Subsecond detection of guanosine using fast-scan cyclic voltammetry

Guanosine is an important neuromodulator and neuroprotector in the brain and is involved in many pathological conditions, including ischemia and neuroinflammation. Traditional methods to detect guanosine in the brain, like HPLC, offer low limits of detection and are robust; however, subsecond detection is not possible. Here, we present a method for detecting rapid fluctuations of guanosine concentration in real-time using fast-scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes. The optimized waveform scanned from -0.4 V to 1.3 V and back at a rate of 400 V s-1 and application frequency of 10 Hz. Potential limits were chosen to increase selectivity of guanosine over the structurally similar interferent adenosine. Two oxidation peaks were detected with the optimized waveform: the primary oxidation reaction occurred at 1.3 V and the secondary oxidation at 0.8 V. Guanosine detection was stable over time with a limit of detection of 30 ± 10 nM, which permits its use to monitor low nanomolar fluctuations in the brain. To demonstrate the feasibility of the method for in-tissue detection, guanosine was exogenously applied and detected within live rat brain slices. This paper demonstrates the first characterization of guanosine using FSCV, and will be a valuable method for measuring signaling dynamics during guanosine neuromodulation and protection.

miR-9 Upregulation Integrates Post-ischemic Neuronal Survival and Regeneration In Vitro

The irrefutable change in the expression of brain-enriched microRNAs (miRNAs) following ischemic stroke has promoted the development of radical miRNA-based therapeutics encompassing neuroprotection and neuronal restoration. Our previous report on the systems-level prediction of miR-9 in post-stroke-induced neurogenesis served as a premise to experimentally uncover the functional role of miR-9 in post-ischemic neuronal survival and regeneration. The oxygen-glucose deprivation (OGD) in SH-SY5Y cells significantly reduced miR-9 expression, while miR-9 mimic transfection enhanced post-ischemic neuronal cell viability. The next major objective involved the execution of a drug repositioning strategy to augment miR-9 expression via structure-based screening of Food and Drug Administration (FDA)-approved drugs that bind to Histone Deacetylase 4 (HDAC4), a known miR-9 target. Glucosamine emerged as the top hit and its binding potential to HDAC4 was verified by Molecular Dynamics (MD) Simulation, Drug Affinity Responsive Target Stability (DARTS) assay, and MALDI-TOF MS. It was intriguing that the glucosamine treatment 1-h post-OGD was associated with the increased miR-9 level as well as enhanced neuronal viability. miR-9 mimic or post-OGD glucosamine treatment significantly increased the cellular proliferation (BrdU assay), while the neurite outgrowth assay displayed elongated neurites. The enhanced BCL2 and VEGF parallel with the reduced NFκB1, TNF-α, IL-1β, and iNOS mRNA levels in miR-9 mimic or glucosamine-treated cells further substantiated their post-ischemic neuroprotective and regenerative efficacy. Hence, this study unleashes a potential therapeutic approach that integrates neuronal survival and regeneration via small-molecule-based regulation of miR-9 favoring long-term recovery against ischemic stroke.

Neuromodulatory Effects of Guanine-Based Purines in Health and Disease

The function of guanine-based purines (GBPs) is mostly attributed to the intracellular modulation of heteromeric and monomeric G proteins. However, extracellular effects of guanine derivatives have also been recognized. Thus, in the central nervous system (CNS), a guanine-based purinergic system that exerts neuromodulator effects, has been postulated. The thesis that GBPs are neuromodulators emerged from in vivo and in vitro studies, in which neurotrophic and neuroprotective effects of these kinds of molecules (i.e., guanosine) were demonstrated. GBPs induce several important biological effects in rodent models and have been shown to reduce seizures and pain, stabilize mood disorder behavior and protect against gliomas and diseases related with aging, such as ischemia or Parkinson and Alzheimer diseases. In vitro studies to evaluate the protective and trophic effects of guanosine, and of the nitrogenous base guanine, have been fundamental for understanding the mechanisms of action of GBPs, as well as the signaling pathways involved in their biological roles. Conversely, although selective binding sites for guanosine have been identified in the rat brain, GBP receptors have not been still described. In addition, GBP neuromodulation may depend on the capacity of GBPs to interact with well-known membrane proteins in glutamatergic and adenosinergic systems. Overall, in this review article, we present up-to-date GBP biology, focusing mainly on the mechanisms of action that may lead to the neuromodulator role of GBPs observed in neurological disorders.

Metabolomic changes in the mouse retina after optic nerve injury

In glaucoma, although axonal injury drives retinal ganglion cell (RGC) death, little is known about the underlying pathomechanisms. To provide new mechanistic insights and identify new biomarkers, we combined latest non-targeting metabolomics analyses to profile altered metabolites in the mouse whole retina 2, 4, and 7 days after optic nerve crush (NC). Ultra-high-performance liquid chromatography quadrupole time-of-flight mass spectrometry and liquid chromatography Fourier transform mass spectrometry covering wide spectrum of metabolites in combination highlighted 30 metabolites that changed its concentration after NC. The analysis displayed similar changes for purine nucleotide and glutathione as reported previously in another animal model of axonal injury and detected multiple metabolites that increased after the injury. After studying the specificity of the identified metabolites to RGCs in histological sections using imaging mass spectrometry, two metabolites, i.e., L-acetylcarnitine and phosphatidylcholine were increased not only preceding the peak of RGC death in the whole retina but also at the RGC layer (2.3-fold and 1.2-fold, respectively). These phospholipids propose novel mechanisms of RGC death and may serve as early biomarkers of axonal injury. The combinatory metabolomics analyses promise to illuminate pathomechanisms, reveal biomarkers, and allow the discovery of new therapeutic targets of glaucoma.

Neuroprotective Effects of Guanosine Administration on In Vivo Cortical Focal Ischemia in Female and Male Wistar Rats

Guanosine (GUO) has neuroprotective effects in experimental models of brain diseases involving glutamatergic excitotoxicity in male animals; however, its effects in female animals are poorly understood. Thus, we investigated the influence of gender and GUO treatment in adult male and female Wistar rats submitted to focal permanent cerebral ischemia in the motor cortex brain. Female rats were subdivided into non-estrogenic and estrogenic phase groups by estrous cycle verification. Immediately after surgeries, the ischemic animals were treated with GUO or a saline solution. Open field and elevated plus maze tasks were conducted with ischemic and naïve animals. Cylinder task, immunohistochemistry and infarct volume analyses were conducted only with ischemic animals. Female GUO groups achieved a full recovery of the forelimb symmetry at 28-35 days after the insult, while male GUO groups only partially recovered at 42 days, in the final evaluation. The ischemic insult affected long-term memory habituation to novelty only in female groups. Anxiety-like behavior, astrocyte morphology and infarct volume were not affected. Regardless the estrous cycle, the ischemic injury affected differently female and male animals. Thus, this study points that GUO is a potential neuroprotective compound in experimental stroke and that more studies, considering the estrous cycle, with both genders are recommended in future investigation concerning brain diseases.

Delayed administration of guanosine improves long‑term functional recovery and enhances neurogenesis and angiogenesis in a mouse model of photothrombotic stroke

Guanosine (GUO) is neuroprotective when administered acutely for the treatment of cerebral ischemia. The aim of the present study was to investigate whether delayed administration of GUO improved long‑term functional recovery following stroke, as well as to explore the potential underlying mechanisms. GUO (8 mg/kg) or a vehicle was administered intraperitoneally for 7 consecutive days beginning 24 h prior to photothrombosis‑induced stroke in male C57/B6J mice. Behaviour tests were performed at days 1, 3, 7, 14 and 28 post‑stroke. Infarct volume was measured using Nissl staining at day 7 post‑stroke. Neurogenesis and angiogenesis were evaluated by co‑labelling bromodeoxyuridine (BrdU) with doublecortin (DCX), neuronal nuclei (NeuN) and von Willebrand factor, in immunohistochemical studies. Brain‑derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF) levels in the ipsilesional brain at day 28 post‑stroke were detected by western blot analysis. Delayed administration of GUO did not reduce infarct volume or affect neurological function at day 7 post‑stroke; however, it did improve functional recovery from day 14 post‑stroke, when compared with the vehicle group. GUO significantly increased the number of BrdU+ and BrdU+/DCX+ cells in the subventricular zone and subgranular zone at all examined time points, the number of Brdu+/NeuN+ cells in the peri‑infarction region at days 14 and 28 post‑stroke and microvessel density in the peri‑infarction region at day 28 post‑stroke compared with the vehicle group. In addition, the BDNF and VEGF levels in the ipsilesional brain were significantly elevated. Delayed administration of GUO at 24 h post‑stroke enhanced neurogenesis and angiogenesis, and increased BDNF and VEGF levels, which likely contributes to long‑term functional recovery following stroke.

Guanosine: a Neuromodulator with Therapeutic Potential in Brain Disorders

Guanosine is a purine nucleoside with important functions in cell metabolism and a protective role in response to degenerative diseases or injury. The past decade has seen major advances in identifying the modulatory role of extracellular action of guanosine in the central nervous system (CNS). Evidence from rodent and cell models show a number of neurotrophic and neuroprotective effects of guanosine preventing deleterious consequences of seizures, spinal cord injury, pain, mood disorders and aging-related diseases, such as ischemia, Parkinson’s and Alzheimer’s diseases. The present review describes the findings of in vivo and in vitro studies and offers an update of guanosine effects in the CNS. We address the protein targets for guanosine action and its interaction with glutamatergic and adenosinergic systems and with calcium-activated potassium channels. We also discuss the intracellular mechanisms modulated by guanosine preventing oxidative damage, mitochondrial dysfunction, inflammatory burden and modulation of glutamate transport. New and exciting avenues for future investigation into the protective effects of guanosine include characterization of a selective guanosine receptor. A better understanding of the neuromodulatory action of guanosine will allow the development of therapeutic approach to brain diseases.

Guanosine prevents nitroxidative stress and recovers mitochondrial membrane potential disruption in hippocampal slices subjected to oxygen/glucose deprivation

Guanosine, the endogenous guanine nucleoside, prevents cellular death induced by ischemic events and is a promising neuroprotective agent. During an ischemic event, nitric oxide has been reported to either cause or prevent cell death. Our aim was to evaluate the neuroprotective effects of guanosine against oxidative damage in hippocampal slices subjected to an in vitro ischemia model, the oxygen/glucose deprivation (OGD) protocol. We also assessed the participation of nitric oxide synthase (NOS) enzymes activity on the neuroprotection promoted by guanosine. Here, we showed that guanosine prevented the increase in ROS, nitric oxide, and peroxynitrite production induced by OGD. Moreover, guanosine prevented the loss of mitochondrial membrane potential in hippocampal slices subjected to OGD. Guanosine did not present an antioxidant effect per se. The protective effects of guanosine were mimicked by inhibition of neuronal NOS, but not of inducible NOS. The neuroprotective effect of guanosine may involve activation of cellular mechanisms that prevent the increase in nitric oxide production, possibly via neuronal NOS.

Guanosine and its role in neuropathologies

Guanosine is a purine nucleoside thought to have neuroprotective properties. It is released in the brain under physiological conditions and even more during pathological events, reducing neuroinflammation, oxidative stress, and excitotoxicity, as well as exerting trophic effects in neuronal and glial cells. In agreement, guanosine was shown to be protective in several in vitro and/or in vivo experimental models of central nervous system (CNS) diseases including ischemic stroke, Alzheimer's disease, Parkinson's disease, spinal cord injury, nociception, and depression. The mechanisms underlying the neurobiological properties of guanosine seem to involve the activation of several intracellular signaling pathways and a close interaction with the adenosinergic system, with a consequent stimulation of neuroprotective and regenerative processes in the CNS. Within this context, the present review will provide an overview of the current literature on the effects of guanosine in the CNS. The elucidation of the complex signaling events underlying the biochemical and cellular effects of this nucleoside may further establish guanosine as a potential therapeutic target for the treatment of several neuropathologies.

Anti-aging effects of guanosine in glial cells

Guanosine, a guanine-based purine, has been shown to exert beneficial roles in in vitro and in vivo injury models of neural cells. Guanosine is released from astrocytes and modulates important astroglial functions, including glutamatergic metabolism, antioxidant, and anti-inflammatory activities. Astrocytes are crucial for regulating the neurotransmitter system and synaptic information processes, ionic homeostasis, energy metabolism, antioxidant defenses, and the inflammatory response. Aging is a natural process that induces numerous changes in the astrocyte functionality. Thus, the search for molecules able to reduce the glial dysfunction associated with aging may represent an approach for avoiding the onset of age-related neurological diseases. Hence, the aim of this study was to evaluate the anti-aging effects of guanosine, using primary astrocyte cultures from newborn, adult, and aged Wistar rats. Concomitantly, we evaluated the role of heme oxygenase 1 (HO-1) in guanosine-mediated glioprotection. We observed age-dependent changes in glutamate uptake, glutamine synthetase (GS) activity, the glutathione (GSH) system, pro-inflammatory cytokine (tumor necrosis factor α (TNF-α) and interleukin 1β (IL-1β)) release, and the transcriptional activity of nuclear factor kB (NFkB), which were prevented by guanosine in an HO-1-dependent manner. Our findings suggest guanosine to be a promising therapeutic agent able to provide glioprotection during the aging process. Thus, this study contributes to the understanding of the cellular and molecular mechanisms of guanosine in the aging process.

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.

The antidepressant-like effect of chronic guanosine treatment is associated with increased hippocampal neuronal differentiation

Guanosine is a purine nucleoside that occurs naturally in the central nervous system, exerting trophic effects. Given its neuroprotective properties, the potential of guanosine as an antidepressant has been recently examined. Within this context, the present study sought to investigate the effects of chronic treatment with guanosine on the tail suspension test (TST), open field test and adult hippocampal neurogenesis. Swiss mice were administered guanosine for 21 days (5 mg/kg/day, p.o.) and subsequently submitted to the TST and open-field test. Following behavioural testing, animals were killed and the brains were processed for immunohistochemical analyses of hippocampal cell proliferation and neuronal differentiation. Animals treated with guanosine showed a reduction in immobility time in the TST without alterations in locomotor activity, confirming the antidepressant-like effect of this compound. Quantitative microscopic analysis did not reveal significant alterations in the numbers of Ki-67- and proliferating cell nuclear antigen (PCNA)-positive cells in the hippocampal dentate gyrus (DG) of guanosine-treated mice. However, guanosine treatment resulted in a significant increase in the number of immature neurons, as assessed by immunohistochemistry for the neurogenic differentiation protein. Interestingly, this effect was localized to the ventral hippocampal DG, a functionally distinct region of this structure known to regulate emotional and motivational behaviours. Taken together, our results suggest that the antidepressant-like effect of chronic guanosine treatment is associated with an increase in neuronal differentiation, reinforcing the notion that this nucleoside may be an endogenous mood modulator.

Neuroprotection Promoted by Guanosine Depends on Glutamine Synthetase and Glutamate Transporters Activity in Hippocampal Slices Subjected to Oxygen/Glucose Deprivation

Guanosine (GUO) has been shown to act as a neuroprotective agent against glutamatergic excitotoxicity by increasing glutamate uptake and decreasing its release. In this study, a putative effect of GUO action on glutamate transporters activity modulation was assessed in hippocampal slices subjected to oxygen and glucose deprivation (OGD), an in vitro model of brain ischemia. Slices subjected to OGD showed increased excitatory amino acids release (measured by D-[(3)H]aspartate release) that was prevented in the presence of GUO (100 µM). The glutamate transporter blockers, DL-TBOA (10 µM), DHK (100 µM, selective inhibitor of GLT-1), and sulfasalazine (SAS, 250 µM, Xc(-) system inhibitor) decreased OGD-induced D-aspartate release. Interestingly, DHK or DL-TBOA blocked the decrease in glutamate release induced by GUO, whereas SAS did not modify the GUO effect. GUO protected hippocampal slices from cellular damage by modulation of glutamate transporters, however selective blockade of GLT-1 or Xc- system only did not affect this protective action of GUO. OGD decreased hippocampal glutamine synthetase (GS) activity and GUO recovered GS activity to control levels without altering the kinetic parameters of GS activity, thus suggesting GUO does not directly interact with GS. Additionally, the pharmacological inhibition of GS activity with methionine sulfoximine abolished the effect of GUO in reducing D-aspartate release and cellular damage evoked by OGD. Altogether, results in hippocampal slices subjected to OGD show that GUO counteracts the release of excitatory amino acids, stimulates the activity of GS, and decreases the cellular damage by modulation of glutamate transporters activity.

Intranasal guanosine administration presents a wide therapeutic time window to reduce brain damage induced by permanent ischemia in rats

In addition to its intracellular roles, the nucleoside guanosine (GUO) also has extracellular effects that identify it as a putative neuromodulator signaling molecule in the central nervous system. Indeed, GUO can modulate glutamatergic neurotransmission, and it can promote neuroprotective effects in animal models involving glutamate neurotoxicity, which is the case in brain ischemia. In the present study, we aimed to investigate a new in vivo GUO administration route (intranasal, IN) to determine putative improvement of GUO neuroprotective effects against an experimental model of permanent focal cerebral ischemia. Initially, we demonstrated that IN [(3)H] GUO administration reached the brain in a dose-dependent and saturable pattern in as few as 5 min, presenting a higher cerebrospinal GUO level compared with systemic administration. IN GUO treatment started immediately or even 3 h after ischemia onset prevented behavior impairment. The behavior recovery was not correlated to decreased brain infarct volume, but it was correlated to reduced mitochondrial dysfunction in the penumbra area. Therefore, we showed that the IN route is an efficient way to promptly deliver GUO to the CNS and that IN GUO treatment prevented behavioral and brain impairment caused by ischemia in a therapeutically wide time window.

Mechanisms involved in the antinociception induced by spinal administration of inosine or guanine in mice

It is well known that adenine-based purines exert multiple effects on pain transmission. Recently, we have demonstrated that guanine-based purines may produce some antinociceptive effects against chemical and thermal pain in mice. The present study was designed to investigate the antinociceptive effects of intrathecal (i.t.) administration of inosine or guanine in mice. Additionally, investigation into the mechanisms of action of these purines, their general toxicity and measurements of CSF purine levels were performed. Animals received an i.t. injection of vehicle (30mN NaOH), inosine or guanine (up to 600nmol) and submitted to several pain models and behavioural paradigms. Guanine and inosine produced dose-dependent antinociceptive effects in the tail-flick, hot-plate, intraplantar (i.pl.) glutamate, i.pl. capsaicin and acetic acid pain models. Additionally, i.t. inosine inhibited the biting behaviour induced by spinal injection of capsaicin and i.t. guanine reduced the biting behaviour induced by spinal injection of glutamate or AMPA. Intrathecal administration of inosine (200nmol) induced an approximately 115-fold increase on CSF inosine levels. This study provides new evidence on the mechanism of action of extracellular guanine and inosine presenting antinociceptive effects following spinal administration. These effects seem to be related, at least partially, to the modulation of A1 adenosine receptors.

Purine nucleosides in neuroregeneration and neuroprotection

In the present review, we stress the importance of the purine nucleosides, adenosine and guanosine, in protecting the nervous system, both centrally and peripherally, via activation of their receptors and intracellular signalling mechanisms. A most novel part of the review focus on the mechanisms of neuronal regeneration that are targeted by nucleosides, including a recently identified action of adenosine on axonal growth and microtubule dynamics. Discussion on the role of the purine nucleosides transversally with the most established neurotrophic factors, e.g. brain derived neurotrophic factor (BDNF), glial derived neurotrophic factor (GDNF), is also focused considering the intimate relationship between some adenosine receptors, as is the case of the A2A receptors, and receptors for neurotrophins. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Guanosine inhibits LPS-induced pro-inflammatory response and oxidative stress in hippocampal astrocytes through the heme oxygenase-1 pathway

Guanosine, a guanine-based purine, is an extracellular signaling molecule that is released from astrocytes and has been shown to promote central nervous system defenses in several in vivo and in vitro injury models. Our group recently demonstrated that guanosine exhibits glioprotective effects in the C6 astroglial cell line by associating the heme oxygenase-1 (HO-1) signaling pathway with protection against azide-induced oxidative stress. Astrocyte overactivation contributes to the triggering of brain inflammation, a condition that is closely related to the development of many neurological disorders. These cells sense and amplify inflammatory signals from microglia and/or initiate the release of inflammatory mediators that are strictly related to transcriptional factors, such as nuclear factor kappa B (NFκB), that are modulated by HO-1. Astrocytes also express toll-like receptors (TLRs); TLRs specifically recognize lipopolysaccharide (LPS), which has been widely used to experimentally study inflammatory response. This study was designed to understand the glioprotective mechanism of guanosine against the inflammatory and oxidative damage induced by LPS exposure in primary cultures of hippocampal astrocytes. Treatment of astrocytes with LPS resulted in deleterious effects, including the augmentation of pro-inflammatory cytokine levels, NFκB activation, mitochondrial dysfunction, increased levels of oxygen/nitrogen species, and decreased levels of antioxidative defenses. Guanosine was able to prevent these effects, protecting the hippocampal astrocytes against LPS-induced cytotoxicity through activation of the HO-1 pathway. Additionally, the anti-inflammatory effects of guanosine were independent of the adenosinergic system. These results highlight the potential role of guanosine against neuroinflammatory-related diseases.

Longitudinal Metabolomics Profiling of Parkinson's Disease-Related α-Synuclein A53T Transgenic Mice

Metabolic homeostasis is critical for all biological processes in the brain. The metabolites are considered the best indicators of cell states and their rapid fluxes are extremely sensitive to cellular changes. While there are a few studies on the metabolomics of Parkinson's disease, it lacks longitudinal studies of the brain metabolic pathways affected by aging and the disease. Using ultra-high performance liquid chromatography and tandem mass spectroscopy (UPLC/MS), we generated the metabolomics profiling data from the brains of young and aged male PD-related α-synuclein A53T transgenic mice as well as the age- and gender-matched non-transgenic (nTg) controls. Principal component and unsupervised hierarchical clustering analyses identified distinctive metabolites influenced by aging and the A53T mutation. The following metabolite set enrichment classification revealed the alanine metabolism, redox and acetyl-CoA biosynthesis pathways were substantially disturbed in the aged mouse brains regardless of the genotypes, suggesting that aging plays a more prominent role in the alterations of brain metabolism. Further examination showed that the interaction effect of aging and genotype only disturbed the guanosine levels. The young A53T mice exhibited lower levels of guanosine compared to the age-matched nTg controls. The guanosine levels remained constant between the young and aged nTg mice, whereas the aged A53T mice showed substantially increased guanosine levels compared to the young mutant ones. In light of the neuroprotective function of guanosine, our findings suggest that the increase of guanosine metabolism in aged A53T mice likely represents a protective mechanism against neurodegeneration, while monitoring guanosine levels could be applicable to the early diagnosis of the disease.

The potential therapeutic effect of guanosine after cortical focal ischemia in rats

BACKGROUND AND PURPOSE

Stroke is a devastating disease. Both excitotoxicity and oxidative stress play important roles in ischemic brain injury, along with harmful impacts on ischemic cerebral tissue. As guanosine plays an important neuroprotective role in the central nervous system, the purpose of this study was to evaluate the neuroprotective effects of guanosine and putative cerebral events following the onset of permanent focal cerebral ischemia.

METHODS

Permanent focal cerebral ischemia was induced in rats by thermocoagulation. Guanosine was administered immediately, 1 h, 3 h and 6 h after surgery. Behavioral performance was evaluated by cylinder testing for a period of 15 days after surgery. Brain oxidative stress parameters, including levels of ROS/RNS, lipid peroxidation, antioxidant non-enzymatic levels (GSH, vitamin C) and enzymatic parameters (SOD expression and activity and CAT activity), as well as glutamatergic parameters (EAAC1, GLAST and GLT1, glutamine synthetase) were analyzed.

RESULTS

After 24 h, ischemic injury resulted in impaired function of the forelimb, caused brain infarct and increased lipid peroxidation. Treatment with guanosine restored these parameters. Oxidative stress markers were affected by ischemic insult, demonstrated by increased ROS/RNS levels, increased SOD expression with reduced SOD activity and decreased non-enzymatic (GSH and vitamin C) antioxidant defenses. Guanosine prevented increased ROS/RNS levels, decreased SOD activity, further increased SOD expression, increased CAT activity and restored vitamin C levels. Ischemia also affected glutamatergic parameters, illustrated by increased EAAC1 levels and decreased GLT1 levels; guanosine reversed the decreased GLT1 levels and did not affect the EAAC1 levels.

CONCLUSION

The effects of brain ischemia were strongly attenuated by guanosine administration. The cellular mechanisms involved in redox and glutamatergic homeostasis, which were both affected by the ischemic insult, were also modulated by guanosine. These observations reveal that guanosine may represent a potential therapeutic agent in cerebral ischemia by preventing oxidative stress and excitotoxicity.

Gliopreventive effects of guanosine against glucose deprivation in vitro

Guanosine, a guanine-based purine, is recognized as an extracellular signaling molecule that is released from astrocytes and confers neuroprotective effects in several in vivo and in vitro studies. Astrocytes regulate glucose metabolism, glutamate transport, and defense mechanism against oxidative stress. C6 astroglial cells are widely used as an astrocyte-like cell line to study the astrocytic function and signaling pathways. Our previous studies showed that guanosine modulates the glutamate uptake activity, thus avoiding glutamatergic excitotoxicity and protecting neural cells. The goal of this study was to determine the gliopreventive effects of guanosine against glucose deprivation in vitro in cultured C6 cells. Glucose deprivation induced cytotoxicity, an increase in reactive oxygen and nitrogen species (ROS/RNS) levels and lipid peroxidation as well as affected the metabolism of glutamate, which may impair important astrocytic functions. Guanosine prevented glucose deprivation-induced toxicity in C6 cells by modulating oxidative and nitrosative stress and glial responses, such as the glutamate uptake, the glutamine synthetase activity, and the glutathione levels. Glucose deprivation decreased the level of EAAC1, the main glutamate transporter present in C6 cells. Guanosine also prevented this effect, most likely through PKC, PI3K, p38 MAPK, and ERK signaling pathways. Taken together, these results show that guanosine may represent an important mechanism for protection of glial cells against glucose deprivation. Additionally, this study contributes to a more thorough understanding of the glial- and redox-related protective properties of guanosine in astroglial cells.

Guanosine protects against reperfusion injury in rat brains after ischemic stroke

After ischemic stroke, early thrombolytic therapy to reestablish tissue perfusion improves outcome but triggers a cascade of deleterious cellular and molecular events. Using a collaborative approach, our groups examined the effects of guanosine (Guo) in response to ischemic reperfusion injury in vitro and in vivo. In a transient middle cerebral artery occlusion (MCAO) in rats, Guo significantly reduced infarct volume in a dose-dependent manner when given systemically either immediately before or 30 min, but not 60 min, after the onset of the 5.5-hr reperfusion period. In a separate experiment, Guo significantly reduced infarct volume after 24 hr of reperfusion when administered 5 min before reperfusion. Western blot analysis did not reveal any significant changes either in endoplasmic reticulum (ER) stress proteins (GRP 78 and 94) or HSP 70 or in levels of m-calpain. In vitro oxygen and glucose deprivation (OGD) significantly increased production of both reactive oxygen species (ROS) and interleukin-8 (IL-8) in the primary astrocytes. Guo did not alter ROS or IL-8 production when given to the astrocytes before OGD. However, Guo when added to the cells prior to or 30 min after reperfusion significantly reduced IL-8 release but not ROS formation. Our study revealed a dose- and time-dependent protective effect of Guo on reperfusion injury in vitro and vivo. The mechanisms by which Guo exerts its effect are independent of unfolded proteins in ER or the level of intracellular calcium or ROS formation. However, the effect may be induced, at least partially, by inhibiting IL-8, a marker of reperfusion-triggered proinflammatory events.

Purine nucleosides: endogenous neuroprotectants in hypoxic brain

Even a short blockade of oxygen flow in brain may lead to the inhibition of oxidative phosphorylation and depletion of cellular ATP, which results in profound deficiencies in cellular function. Following ischemia, dying, injured, and hypoxic cells release soluble purine-nucleotide and -nucleoside pools. Growing evidence suggests that purine nucleosides might act as trophic factors in the CNS and PNS. In addition to equilibrative nucleoside transporters (ENTs) regulating purine nucleoside concentrations intra- and extracellularly, specific extracellular receptor subtypes for these compounds are expressed on neurons, glia, and endothelial cells, mediating stunningly diverse effects. Such effects range from induction of cell differentiation, apoptosis, mitogenesis, and morphogenetic changes, to stimulation of synthesis and/or release of cytokines and neurotrophic factors under both physiological and pathological conditions. Multiple signaling pathways regulate the critical balance between cell death and survival in hypoxia-ischemia. A convergent pathway for the regulation of multiple modalities involved in O₂ sensing is the mitogen activated protein kinase (p42/44 MAPK) or (ERK1/2 extracellular signal-regulated kinases) pathway terminating in a variety of transcription factors, for example, hypoxia-inducible factor 1α. In this review, the coherence of purine nucleoside-related pathways and MAPK activation in the endogenous neuroprotective regulation of the nervous system's development and neuroplasticity under hypoxic stress will be discussed.

Evidence that hyperprolinemia alters glutamatergic homeostasis in rat brain: neuroprotector effect of guanosine

This study investigated the effects of acute and chronic hyperprolinemia on glutamate uptake, as well as some mechanisms underlying the proline effects on glutamatergic system in rat cerebral cortex. The protective role of guanosine on effects mediated by proline was also evaluated. Results showed that acute and chronic hyperprolinemia reduced glutamate uptake, Na(+), K(+)-ATPase activity, ATP levels and increased lipoperoxidation. GLAST and GLT-1 immunocontent were increased in acute, but not in chronic hyperprolinemic rats. Our data suggest that the effects of proline on glutamate uptake may be mediated by lipid peroxidation and disruption of Na(+), K(+)-ATPase activity, but not by decreasing in glutamate transporters. This probably induces excitotoxicity and subsequent energy deficit. Guanosine was effective to prevent most of the effects promoted by proline, reinforcing its modulator role in counteracting the glutamate toxicity. However, further studies are needed to assess the modulatory effects of guanosine on experimental hyperprolinemia.

Systemic administration of guanosine promotes functional and histological improvement following an ischemic stroke in rats

Previously we have found that extracellular guanosine (Guo) has neuroprotective properties in in vitro and in vivo. Moreover, extracellular Guo significantly increased in the ipsilateral hemisphere within 2h following focal stroke in rats, and remained elevated for one week. Therefore, we hypothesized that Guo could be a potential candidate for a non-toxic neuroprotective agent. In the present study, we examined the effects of Guo on rats following permanent middle cerebral artery occlusion (MCAO). We also determined whether Guo can precondition neurons by modulating endoplasmic reticulum (ER) stress proteins. As most therapies employ a combination treatment regimen, we optimized the neuroprotection by combining pre- and post-MCAO treatments with Guo, attempting to reduce both ischemic cell death and improve functional recovery. A combination of 4mg/kg Guo given 30min pre-stroke and 8mg/kg Guo given 3, 24 and 48h post-stroke exerted the most significant decrease in infarct volume and sustainable improvement in neurological function. Moreover, these effects are not attributable to Guo metabolites. Measurements taken 6h post-MCAO from animals pre-treated with Guo did not reveal any significant changes in ER stress proteins (GRP 78 and 94) or HSP 70, but did reveal significantly increased levels of m-calpain. Thus, our data indicate that there is a treatment regimen for Guo as a neuroprotectant following ischemic stroke. The mechanism by which Guo confers neuroprotection may involve an increase in m-calpain, possibly resulting from a mild increase in intracellular calcium. M-calpain may be involved in the preconditioning response to ischemia by upregulating endogenous pro-survival mechanisms in neurons.

Changes in purines concentration in the cerebrospinal fluid of patients experiencing pain: a case-control study

This study analyzes the relationship between extracellular purines and pain perception in humans. Cerebrospinal fluid (CSF) levels of purines and their metabolites were compared between patients displaying acute and/or chronic pain syndromes and control subjects. The CSF levels of IMP, inosine, guanosine and uric acid were significantly increased in the chronic pain group and correlated with pain severity (P<0.05). Patients displaying both chronic and acute pain presented similar changes in the CSF purines concentration (P<0.05). However, in the acute pain group, only CSF inosine and uric acid levels were significantly increased (P<0.05). These findings suggest that purines, in special inosine, guanosine and uric acid, are associated with the spinal mechanisms underlying nociception.

Mechanisms involved in the antinociception induced by systemic administration of guanosine in mice

BACKGROUND AND PURPOSE

It is well known that adenine-based purines exert multiple effects on pain transmission. However, less attention has been given to the potential effects of guanine-based purines on pain transmission. The aim of this study was to investigate the effects of intraperitoneal (i.p.) and oral (p.o.) administration of guanosine on mice pain models. Additionally, investigation into the mechanisms of action of guanosine, its potential toxicity and cerebrospinal fluid (CSF) purine levels were also assessed.

EXPERIMENTAL APPROACH

Mice received an i.p. or p.o. administration of vehicle (0.1 mM NaOH) or guanosine (up to 240 mg x kg(-1)) and were evaluated in several pain models.

KEY RESULTS

Guanosine produced dose-dependent antinociceptive effects in the hot-plate, glutamate, capsaicin, formalin and acetic acid models, but it was ineffective in the tail-flick test. Additionally, guanosine produced a significant inhibition of biting behaviour induced by i.t. injection of glutamate, AMPA, kainate and trans-ACPD, but not against NMDA, substance P or capsaicin. The antinociceptive effects of guanosine were prevented by selective and non-selective adenosine receptor antagonists. Systemic administration of guanosine (120 mg x kg(-1)) induced an approximately sevenfold increase on CSF guanosine levels. Guanosine prevented the increase on spinal cord glutamate uptake induced by intraplantar capsaicin.

CONCLUSIONS AND IMPLICATIONS

This study provides new evidence on the mechanism of action of the antinociceptive effects after systemic administration of guanosine. These effects seem to be related to the modulation of adenosine A(1) and A(2A) receptors and non-NMDA glutamate receptors.

Guanosine prevents thermal hyperalgesia in a rat model of peripheral mononeuropathy

It is well known that adenine-based purines exert multiple effects on pain transmission. Less attention has been given, however, to the antinociceptive effects of guanine-based purines. The aim of this study was to investigate the effects of intraperitoneal administration of guanosine on a rat model of peripheral mononeuropathy. Additionally, investigation of the mechanism of action of guanosine, its general toxicity and measurements of central nervous system purine levels were performed. Rats received an intraperitoneal administration of vehicle (0.1 mM NaOH) or guanosine (up to 120 mg.kg(-1)) in an acute or chronic regimen. Guanosine significantly reduced thermal hyperalgesia on the ipsilateral side of the sciatic nerve ligation. Additionally, guanosine prevented locomotor deficits and body weight loss induced by the mononeuropathy. Acute systemic administration of guanosine caused an approximately 11-fold increase on central nervous system guanosine levels, but this effect was not observed after chronic treatment. Chronic guanosine administration prevented the increase on cortical glutamate uptake but not the decrease in spinal cord glutamate uptake induced by the mononeuropathy. No significant general toxicity was observed after chronic exposure to guanosine. This study provides new evidence on the mechanism of action of guanine-based purines, with guanosine presenting antinociceptive effects against a chronic pain model.

PERSPECTIVE

This study provides a new role for guanosine: chronic pain modulation. Guanosine presents as a new target for future drug development and might be useful for treatment of neuropathic pain.

The purine nucleosides adenosine and guanosine delay axonal degeneration in vitro

Axonal degeneration is a key component of many neurodegenerative diseases. Injured axons undergo a program of self-destruction termed Wallerian degeneration that is an active, well-regulated process. The pathways leading to axon fragmentation are uncharacterized, but experiments with wld(s) mutant mice led to the discovery that over-expression of NMN adenylyltransferase 1 or treatment with NAD(+) can inhibit axonal degeneration. In this study, we show that the purine nucleosides adenosine and guanosine, but not inosine, inhibit injury-induced axonal degeneration in cultured dorsal root ganglia neurons. Axons can be preserved by adding adenosine within 6 h of the axonal injury. The presence of adenosine was required continuously after the injury to maintain axonal protection. Together these results suggest that adenosine does not alter the neuronal response to injury, but instead inhibits a local axonal pathway necessary for the commitment and/or execution of the axon destructive program.