A comparison of adenosine A2A agonism and methylprednisolone in attenuating neuronal damage and improving functional outcome after experimental traumatic spinal cord injury in rabbits

Cyclosporine A Protects Retinal Explants against Hypoxia

The retina is a complex neurological tissue and is extremely sensitive to an insufficient supply of oxygen. Hypoxia plays a major role in several retinal diseases, and often results in the loss of cells that are essential for vision. Cyclosporine A (CsA) is a widely used immunosuppressive drug. Furthermore, treatment with CsA has neuroprotective effects in several neurologic disorders. No data are currently available on the tolerated concentration of CsA when applied to the retina. To reveal the most effective dose, retinal explants from rat eyes were exposed to different CsA concentrations (1-9 µg/mL). Immunohistochemistry with brain-specific homeobox/POU domain protein 3a (Brn3a) and TUNEL staining was performed to determine the percentage of total and apoptotic retinal ganglion cells (RGCs), as well as the responses of micro- and macroglial cells. Furthermore, optical coherence tomography (OCT) scans were performed to measure the changes in retinal thickness, and recordings with multielectrode array (MEA) were performed to evaluate spontaneous RGC spiking. To examine the neuroprotective effects, retinas were subjected to a hypoxic insult by placing them in a nitrogen-streamed hypoxic chamber prior to CsA treatment. In the biocompatibility tests, the different CsA concentrations had no negative effect on RGCs and microglia. Neuroprotective effects after a hypoxic insult on RGCs was demonstrated at a concentration of 9 µg/mL CsA. CsA counteracted the hypoxia-induced loss of RGCs, reduced the percentage of TUNEL⁺ RGCs, and prevented a decrease in retinal thickness. Taken together, the results of this study suggest that CsA can effectively protect RGCs from hypoxia, and the administered concentrations were well tolerated. Further in vivo studies are needed to determine whether local CsA treatment may be a suitable option for hypoxic retinal diseases.

Nuclear translocation of the unliganded glucocorticoid receptor is influenced by membrane fluidity, but not A2AR agonism

Epidemiological evidence suggests that chronic consumption of caffeine, a non-selective antagonist of adenosine A_2AR receptors (A_2AR), can be neuroprotective in a number of age-related neurodegenerative disorders including Alzheimer's disease. A growing body of work shows that this neuroprotection may act via a synergistic interaction with the glucocorticoid receptor (GR) and its associated genetic response elements. Therefore, we hypothesized that A_2AR signaling may directly stimulate glucocorticoid receptor translocation via downstream signaling elements within the cell. Surprisingly, we found no effect of A_2AR agonism on GR translocation in the absence of steroid. As expected, membrane-bound dexamethasone was capable of stimulating full GR translocation, albeit at a slower rate. This non-liganded translocation was unaffected by A_2AR ligands, providing strong evidence that GR translocation occurs independently of activation of A_2ARs. To identify other potential mechanisms of translocation, membrane fluidity was increased significantly by benzyl alcohol, which also induced full nuclear translocation of the GR, but unlike the membrane-bound dexamethasone, benzyl alcohol did result in transcriptional upregulation of GR-dependent genes. Taken together, our data shows that the unliganded GR is sensitive to changes in membrane state and can be transcriptionally active.

The caffeine-binding adenosine A2A receptor induces age-like HPA-axis dysfunction by targeting glucocorticoid receptor function

Caffeine is associated with procognitive effects in humans by counteracting overactivation of the adenosine A_2A receptor (A_2AR), which is upregulated in the human forebrain of aged and Alzheimer’s disease (AD) patients. We have previously shown that an anti-A_2AR therapy reverts age-like memory deficits, by reestablishment of the hypothalamic-pituitary-adrenal (HPA) axis feedback and corticosterone circadian levels. These observations suggest that A_2AR over-activation and glucocorticoid dysfunction are key events in age-related hippocampal deficits; but their direct connection has never been explored. We now show that inducing A_2AR overexpression in an aging-like profile is sufficient to trigger HPA-axis dysfunction, namely loss of plasmatic corticosterone circadian oscillation, and promotes reduction of GR hippocampal levels. The synaptic plasticity and memory deficits triggered by GR in the hippocampus are amplified by A_2AR over-activation and were rescued by anti-A_2AR therapy; finally, we demonstrate that A_2AR act on GR nuclear translocation and GR-dependent transcriptional regulation. We provide the first demonstration that A_2AR is a major regulator of GR function and that this functional interconnection may be a trigger to age-related memory deficits. This supports the idea that the procognitive effects of A_2AR antagonists, namely caffeine, on Alzheimer’s and age-related cognitive impairments may rely on its ability to modulate GR actions.

Caffeine treatment aggravates secondary degeneration after spinal cord injury

Spinal cord injury (SCI) often results in some form of paralysis. Recently, SCI therapy has been focused on preventing secondary injury to reduce both neuroinflammation and lesion size so that functional outcome after an SCI may be improved. Previous studies have shown that adenosine receptors (AR) are a major regulator of inflammation after an SCI. The current study was performed to examine the effect of caffeine, a pan-AR blocker, on spontaneous functional recovery after an SCI. Animals were assigned into 3 groups randomly, including sham, PBS and caffeine groups. The rat SCI was generated by an NYU impactor with a 10 g rod dropped from a 25 mm height at thoracic 9 spinal cord level. Caffeine and PBS were injected daily during the experiment period. Hind limb motor function was evaluated by the Basso, Beattie, Bresnahan (BBB) locomotor rating scale at 1 week and 4 weeks after the SCI. Spinal cord segments were collected after final behavior evaluation for morphological analysis. The tissue sparing was evaluated by luxol fast blue staining. Immunofluorescence stain was employed to assess astrocyte activation and neurofilament positioning, while microglia activation was examined by immunohistochemistry stain.The results showed that spontaneous functional recovery was blocked after the animals were subjected caffeine daily. Moreover, caffeine administration increased the demyelination area, promoted astrocyte and microglia activation and decreased the quantity of neurofilaments. These findings suggest that the neurotoxicity effect of caffeine may be associated with the inhibition of neural repair and the promotion of neuroinflammation.

In vivo two-photon imaging of axonal dieback, blood flow, and calcium influx with methylprednisolone therapy after spinal cord injury

Severe spinal cord injury (SCI) can cause neurological dysfunction and paralysis. However, the early dynamic changes of neurons and their surrounding environment after SCI are poorly understood. Although methylprednisolone (MP) is currently the standard therapeutic agent for treating SCI, its efficacy remains controversial. The purpose of this project was to investigate the early dynamic changes and MP's efficacy on axonal damage, blood flow, and calcium influx into axons in a mouse SCI model. YFP H-line and Thy1-GCaMP transgenic mice were used in this study. Two-photon microscopy was used for imaging of axonal dieback, blood flow, and calcium influx post-injury. We found that MP treatment attenuated progressive damage of axons, increased blood flow, and reduced calcium influx post-injury. Furthermore, microglia/macrophages accumulated in the lesion site after SCI and expressed the proinflammatory mediators iNOS, MCP-1 and IL-1β. MP treatment markedly inhibited the accumulation of microglia/macrophages and reduced the expression of the proinflammatory mediators. MP treatment also improved the recovery of behavioral function post-injury. These findings suggest that MP exerts a neuroprotective effect on SCI treatment by attenuating progressive damage of axons, increasing blood flow, reducing calcium influx, and inhibiting the accumulation of microglia/macrophages after SCI.

Decreased GFAP expression and improved functional recovery in contused spinal cord of rats following valproic acid therapy

Many studies have illustrated that much of the post-traumatic degeneration of the spinal cord cells is caused by the secondary mechanism. The aim of this study is to evaluate the effect of the anti-inflammatory property of valproic acid (VPA) on injured spinal cords (SC). The rats with the contused SC received intraperitoneal single injection of VPA (150, 200, 300, 400 or 500 mg/kg) at 2, 6, 12 and 24 h post-injury. Basso-Beattie-Bresnahan (BBB) test and H-reflex evaluated the functional outcome for 12 weeks. The SC were investigated 3 months post-injury using morphometry and glial fibrillary acid protein (GFAP) expression. Reduction in cavitation, H/M ratio, BBB scores and GFAP expression in the treatment groups were significantly more than that of the untreated one (P < 0.05). The optimal improvement in the condition of the contused rats was in the ones treated at the acute phase of injury with 300 mg/kg of VPA at 12 h post-injury, they had the highest increase in BBB score and decrease in astrogliosis and axonal loss. We conclude that treating the contused rats with 300 mg/kg of VPA at 12 h post-injury improves the functional outcome and reduces the traumatized SC gliosis.

Using caffeine and other adenosine receptor antagonists and agonists as therapeutic tools against neurodegenerative diseases: a review

Caffeine is the most consumed pychostimulant in the world, and it is known to affect basic and fundamental human processes such as sleep, arousal, cognition and learning and memory. It works as a nonselective blocker of adenosine receptors (A1, A2a, A2b and A3) and has been related to the regulation of heart rate, the contraction/relaxation of cardiac and smooth muscles, and the neural signaling in the central nervous system (CNS). Since the late 1990s, studies using adenosine receptor antagonists, such as Caffeine, to block the A1 and A2a adenosine receptor subtypes have shown to reduce the physical, cellular and molecular damages caused by a spinal cord injury (SCI) or a stroke (cerebral infarction) and by other neurodegenerative diseases such as Parkinson's and Alzheimer's diseases. Interestingly, other studies using adenosine receptor agonists have also shown to provide a neuroprotective effect on various models of neurodegenerative diseases through the reduction of excitatory neurotransmitter release, apoptosis and inflammatory responses, among others. The seemingly paradoxical use of both adenosine receptor agonists and antagonists as neuroprotective agents has been attributed to differences in dosage levels, drug delivery method, extracellular concentration of excitatory neurotransmitters and stage of disease progression. We discuss and compare recent findings using both antagonists and agonists of adenosine receptors in animal models and patients that have suffered spinal cord injuries, brain strokes, and Parkinson's and Alzheimer's diseases. Additionally, we propose alternative interpretations on the seemingly paradoxical use of these drugs as potential pharmacological tools to treat these various types of neurodegenerative diseases.

Squalenoyl adenosine nanoparticles provide neuroprotection after stroke and spinal cord injury

There is an urgent need to develop new therapeutic approaches for the treatment of severe neurological trauma, such as stroke and spinal cord injuries. However, many drugs with potential neuropharmacological activity, such as adenosine, are inefficient upon systemic administration because of their fast metabolization and rapid clearance from the bloodstream. Here, we show that conjugation of adenosine to the lipid squalene and the subsequent formation of nanoassemblies allows prolonged circulation of this nucleoside, providing neuroprotection in mouse stroke and rat spinal cord injury models. The animals receiving systemic administration of squalenoyl adenosine nanoassemblies showed a significant improvement of their neurologic deficit score in the case of cerebral ischaemia, and an early motor recovery of the hindlimbs in the case of spinal cord injury. Moreover, in vitro and in vivo studies demonstrated that the nanoassemblies were able to extend adenosine circulation and its interaction with the neurovascular unit. This Article shows, for the first time, that a hydrophilic and rapidly metabolized molecule such as adenosine may become pharmacologically efficient owing to a single conjugation with the lipid squalene.

Adenosine 2A receptor: a crucial neuromodulator with bidirectional effect in neuroinflammation and brain injury

This review summarizes recent developments that have contributed to our understanding of how adenosine 2A receptors (A2ARs) modulate brain damage in various animal models of acute neurological injuries, including brain ischemia, traumatic brain injury, spinal cord injury and hemorrhage stroke. The main conclusions are: (1) pharmacological, neurochemical and molecular/genetic approaches to the complex actions of A2AR in different cellular elements suggest that A2AR activation exerts bidirectional effect (detrimental or protective) after brain insults; (2) modulation of glutamate excitotoxicity and neuroinflammation are involved in the protection of A2AR agonists or antagonists, but the bidirectional effect of A2AR is largely due to the bidirectional regulation of neuroinflammation (anti-inflammation or proinflammation) by A2AR on immune cells such as microglia cells and peripheral bone marrow cells; and (3) the bidirectional effect of A2AR on neuroinflammation and brain injury depends on the distinct and sometimes opposite actions of A2AR in various cellular elements and on different injury models and associated pathological conditions. The local glutamate level in the brain injury is one of the crucial factors that contribute to the direction of A2AR effect on neuroinflammation and brain injury outcome. These developments presented here clearly highlight the complexity of using A2AR agents therapeutically in acute neuronal injuries and confirm that A2AR ligands have many promising characteristics that encourage the pursuit of their full therapeutic potential.

Animal studies in spinal cord injury: a systematic review of methylprednisolone

The objective of this study was to examine whether animal studies can reliably be used to determine the usefulness of methylprednisolone (MP) and other treatments for acute spinal cord injury (SCI) in humans. This was achieved by performing a systematic review of animal studies on the effects of MP administration on the functional outcome of acute SCI. Data were extracted from the published articles relating to: outcome; MP dosing regimen; species/strain; number of animals; methodological quality; type of injury induction; use of anaesthesia; functional scale used; and duration of follow-up. Subgroup analyses were performed, based on species or strain, injury method, MP dosing regimen, functional outcome measured, and methodological quality. Sixty-two studies were included, which involved a wide variety of animal species and strains. Overall, beneficial effects of MP administration were obtained in 34% of the studies, no effects in 58%, and mixed results in 8%. The results were inconsistent both among and within species, even when attempts were made to detect any patterns in the results through subgroup analyses. The results of this study demonstrate the barriers to the accurate prediction from animal studies of the effectiveness of MP in the treatment of acute SCI in humans. This underscores the need for the development and implementation of validated testing methods.

Highlights on the development of A(2A) adenosine receptor agonists and antagonists

Although significant progress has been made in the past few decades demonstrating that adenosine modulates a variety of physiological and pathophysiological processes through the interaction with four subtypes of a family of cell-surface G-protein-coupled receptors, clinical evaluation of some adenosine receptor ligands has been discontinued. Major problems include side effects due to the wide distribution of adenosine receptors, low brain penetration (which is important for the targeting of CNS diseases), short half-life of compounds, or a lack of effects, in some cases perhaps due to receptor desensitization or to low receptor density in the targeted tissue. Currently, three A(2A) adenosine receptor agonists have begun phase III studies. Two of them are therapeutically evaluated as pharmacologic stress agents and the third proved to be effective in the treatment of acute spinal cord injury (SCI), while avoiding the adverse effects of steroid agents. On the other hand, the great interest in the field of A(2A) adenosine receptor antagonists is related to their application in neurodegenerative disorders, in particular, Parkinson's disease, and some of them are currently in various stages of evaluation. This review presents an update of medicinal chemistry and molecular recognition of A(2A) adenosine receptor agonists and antagonists, and stresses the strong need for more selective ligands at the A(2A) human subtype.

Immunosuppression after traumatic or ischemic CNS damage: it is neuroprotective and illuminates the role of microglial cells

Acute traumatic and ischemic events in the central nervous system (CNS) invariably result in activation of microglial cells as local representatives of the immune system. It is still under debate whether activated microglia promote neuronal survival, or whether they exacerbate the original extent of neuronal damage. Protagonists of the view that microglial cells cause secondary damage have proposed that inhibition of microglial activation by immunosuppression is beneficial after acute CNS damage. It is the aim of this review to analyse the effects of immunosuppressants on isolated microglial cells and neurons, and to scrutinize the effects of immunosuppression in different in vivo models of acute CNS trauma or ischemia. It is found that the immunosuppressants cytosine-arabinoside, different steroids, cyclosporin A, FK506, rapamycin, mycophenolate mofetil, and minocycline all have direct inhibitory effects on microglial cells. These effects are mainly exerted by inhibiting microglial proliferation or microglial secretion of neurotoxic substances such as proinflammatory cytokines and nitric oxide. Furthermore, immunosuppression after acute CNS trauma or ischemia results in improved structure preservation and, mostly, in enhanced function. However, all investigated immunosuppressants also have direct effects on neurons, and some immunosuppressants affect other glial cells such as astrocytes. In summary, it is safe to conclude that immunosuppression after acute CNS trauma or ischemia is neuroprotective. Furthermore, circumferential evidence indicates that microglial activation after traumatic or ischemic CNS damage is not beneficial to adjacent neurons in the immediate aftermath of such acute lesions. Further experiments with more specific agents or genetic approaches that specifically inhibit microglial cells are needed in order to fully answer the question of whether microglial activation is "good or bad".

THE ADENOSINE 2A RECEPTOR AGONIST ATL-146E ATTENUATES EXPERIMENTAL POSTHEMORRHAGIC VASOSPASM

OBJECTIVE

Selective adenosine 2A receptor agonists, such as ATL-146e, are known to be potent anti-inflammatory agents devoid of systemic side effects and have been used clinically in a number of disease states. However, adenosine 2A receptor agonists have not been studied in the treatment of cerebral vasospasm after subarachnoid hemorrhage. The present study investigated the efficacy of ATL-146e in the prevention of leukocyte infiltration and attenuation of posthemorrhagic vasospasm.

METHODS

The rodent femoral artery model of vasospasm was used. Forty male Sprague-Dawley rats were randomly assigned to four different groups (vehicle, 1 ng/kg/min, 10 ng/kg/min, or 100 ng/kg/min ATL-146e administered via subcutaneous osmotic minipump). Vasospasm was evaluated at posthemorrhage Day 8 (period of peak constriction) by calculating the lumen cross-sectional area (expressed as percent change in luminal area: ratio of blood-exposed vessel to normal saline-exposed vessel) and radial wall thickness. Immunostaining with anti-CD45 monoclonal antibody to detect leukocytes was used to evaluate localized inflammation.

RESULTS

Significant vasospasm was noted in the vehicle-treated (blood-exposed) control group (78.5%, P < 0.001; expressed as a ratio of luminal area of the saline [no blood] control), but not in the ATL-146e-treated groups (lumen ratio to control: 105.0, 83.4, and 91.3% for the 1, 10, and 100 ng/kg/min groups, respectively). Additionally, infiltration of inflammatory cells was reduced significantly and radial wall thickness was decreased in the ATL-146e-treated groups compared with the vehicle-treated control group.

CONCLUSION

Selective activation of the adenosine 2A receptor with ATL-146e prevented posthemorrhagic vasospasm and reduced leukocyte infiltration in this experimental model. This agent is worthy of further investigation and lends credence to the hypothesis supporting a role for inflammation in the pathogenesis of cerebral vasospasm after subarachnoid hemorrhage.

Recovery and Regeneration after Spinal Cord Injury: A Review and Summary of Recent Literature

Introduction: Spinal cord injury (SCI) often results in significant neurologic dysfunction and disability. An annual incidence of 15 to 40 traumatic SCI cases per million population has been reported worldwide, and a conservative estimate for Singapore would be 23 cases per million. With continued improvements in medical care, an increasing prevalence of SCI patients is expected, with corresponding need for comprehensive rehabilitation services led by specialist rehabilitation physicians. Methods: A literature search, review, and summary of findings of recent studies relating to factors associated with recovery, as well as interventions for rehabilitation and promotion of healing of the injured spinal cord was performed. Conclusions: Many SCI patients show improvements in motoric and neurologic level, but those with complete injuries have poor chance of improving American Spinal Injury Association (ASIA) scores. SCI of violent aetiology tends to be more neurologic complete, and those without sacral sparing less likely to improve. Older patients generally do well in activities of daily living. Women have better motor score improvement, although men have better Functional Independence Measure (FIM) scores generally. Electrodiagnostic tests such as somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) can help with prognostication, as can imaging techniques such as magnetic resonance imaging (MRI). Immediate surgery for spinal decompression may improve recovery, but whether routine surgery after SCI improves function remains unclear, as does the timing. Methylprednisolone and similar agents appear to help limit secondary injury processes. Rehabilitation interventions such as functional electrical stimulation (FES) and body-weight supported treadmill ambulation training may be effective, as may neural-controlled prostheses and devices. Substances that promote repair and regeneration of the injured spinal cord such as GM-1, 4-AP, BDNG, GDNF, Nogo and MAG-inhibitors, have been studied. Transplanted tissues and cells, such as blood macrophages, bone marrow transplant with GM-CSF, olfactory ensheathing cells, fetal tissues, stem or progenitor cells, have been reported to produce neurological improvements. Key words: Prognosis, Regeneration, Rehabilitation, Spinal cord injuries

Mouse spinal cord compression injury is reduced by either activation of the adenosine A2A receptor on bone marrow-derived cells or deletion of the A2A receptor on non-bone marrow-derived cells

Activation of the adenosine A(2A) receptor (A(2A)R) at the time of reperfusion has been shown to reduce ischemia-reperfusion injury in peripheral tissues and spinal cord. In this study we show that treating mice with the A(2A)R agonist, 4-{3-[6-amino-9-(5-cyclopropylcarbamoyl-3,4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl]-prop-2-ynyl}-piperidine-1-carboxylic acid methyl ester for four days beginning before or just after the onset of reperfusion after compression-induced spinal cord injury rapidly (within 1 day) and persistently (>42 days) reduces locomotor dysfunction and spinal cord demyelination. Protection is abolished in knockout/wild type bone marrow chimera mice selectively lacking the A(2A)R only on bone marrow-derived cells but retaining receptors on other tissues including blood vessels. Paradoxically, reduced spinal cord injury is also noted in A(2A)R -/- mice, and in wild type/knockout bone marrow chimera mice selectively lacking the A(2A)R on non-bone marrow-derived cells, or in mice treated with the A(2A) antagonist, 4-(2-[7-amino-2-[2-furyl][1,2,4]triazolo[2,3-a][1,3,5]triazin-5-yl-amino]ethyl)phenol. The greatest protection is seen in knockout/wild type bone marrow chimera mice treated with 4-{3-[6-amino-9-(5-cyclopropylcarbamoyl-3,4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl]-prop-2-ynyl}-piperidine-1-carboxylic acid methyl ester, i.e. by activating the A(2A)R in mice expressing the receptor only in bone marrow-derived cells. The data suggest that inflammatory bone marrow-derived cells are the primary targets of A(2A) agonist-mediated protection. We conclude that A(2A) agonists or other interventions that inhibit inflammation during and after spinal cord ischemia may be effective in reducing spinal cord injury in patients, but excessive or prolonged stimulation of the A(2A)R may be counterproductive. It may be possible to devise strategies to produce optimal spinal cord protection by exploiting temporal differences in A(2A)R-mediated responses.

A tail of two signals: the C terminus of the A(2A)-adenosine receptor recruits alternative signaling pathways

G protein-coupled receptors are endowed with carboxyl termini that vary greatly in length and sequence. In most instances, the distal portion of the C terminus is dispensable for G protein coupling. This is also true for the A(2A)-adenosine receptor, where the last 100 amino acids are of very modest relevance to G(s) coupling. The C terminus was originally viewed mainly as the docking site for regulatory proteins of the beta-arrestin family. These beta-arrestins bind to residues that have been phosphorylated by specialized kinases (G protein-coupled receptor kinases) and thereby initiate receptor desensitization and endocytosis. More recently, it has become clear that many additional "accessory" proteins bind to C termini of G protein-coupled receptors. The article by Sun et al. in the current issue of Molecular Pharmacology identifies translin-associated protein-X as yet another interaction partner of the A(2A) receptor; translin-associated protein allows the A(2A) receptor to impinge on the signaling mechanisms by which p53 regulates neuronal differentiation, but the underlying signaling pathways are uncharted territory. With a list of five known interaction partners, the C terminus of the A(2A) receptor becomes a crowded place. Hence, there must be rules that regulate the interaction. This allows the C terminus to act as coincidence detector and as signal integrator. Despite our ignorance about the precise mechanisms, the article has exciting implications: the gene encoding for translin-associated protein-X maps to a locus implicated in some forms of schizophrenia; A(2A) receptor agonists are candidate drugs for the treatment of schizophrenic symptoms. It is of obvious interest to explore a possible link.