P2X7 receptor inhibition improves recovery after spinal cord injury

Involvement of the choroid plexus in the inflammatory response after acute spinal cord injury in dogs: an immunohistochemical study

The choroid plexus (CP) is increasingly recognized as an important contributor to central nervous system (CNS) inflammation by recruitment of inflammatory cells and release of inflammatory cytokines. Here we investigate the role of the CP epithelium (CPE) as a source of three pro-inflammatory molecules of potential importance in inflammation after acute spinal cord injury (SCI): IL-1β, TNF-α, and hsp70. Immunohistochemical (IHC) staining for these three proteins was performed on 4th ventricular CPE from 4 dogs euthanized 12-48 h after spontaneous acute SCI, and from 4 neurologically normal dogs euthanized for other reasons. IHC staining was quantified using Aperio ImageScope software. IHC staining in the CPE of dogs with acute SCI was 2.2, 1.6 and 1.5 times higher than that of normal dogs, for IL-1β, TNF-α, and hsp70, respectively. Increases were statistically significant (p<0.1) for IL-1β and TNF-α, and closely approached significance for hsp70. These findings indicate that the CPE could serve as an important source of these inflammatory mediators after SCI. There was also an inverse correlation between IL-1β and hsp70 staining and duration of clinical signs in acute SCI, suggesting that increased expression of these proteins by the CPE may be of particular importance in the immediate-early inflammatory response after acute SCI.

Astrocytes as a source for extracellular matrix molecules and cytokines

Research of the past 25 years has shown that astrocytes do more than participating and building up the blood-brain barrier and detoxify the active synapse by reuptake of neurotransmitters and ions. Indeed, astrocytes express neurotransmitter receptors and, as a consequence, respond to stimuli. Within the tripartite synapse, the astrocytes owe more and more importance. Besides the functional aspects the differentiation of astrocytes has gained a more intensive focus. Deeper knowledge of the differentiation processes during development of the central nervous system might help explaining and even help treating neurological diseases like Alzheimer’s disease, Amyotrophic lateral sclerosis, Parkinsons disease, and psychiatric disorders in which astrocytes have been shown to play a role. Specific differentiation of neural stem cells toward the astroglial lineage is performed as a multi-step process. Astrocytes and oligodendrocytes develop from a multipotent stem cell that prior to this has produced primarily neuronal precursor cells. This switch toward the more astroglial differentiation is regulated by a change in receptor composition on the cell surface and responsiveness to Fibroblast growth factor and Epidermal growth factor (EGF). The glial precursor cell is driven into the astroglial direction by signaling molecules like Ciliary neurotrophic factor, Bone Morphogenetic Proteins, and EGF. However, the early astrocytes influence their environment not only by releasing and responding to diverse soluble factors but also express a wide range of extracellular matrix (ECM) molecules, in particular proteoglycans of the lectican family and tenascins. Lately these ECM molecules have been shown to participate in glial development. In this regard, especially the matrix protein Tenascin C (Tnc) proved to be an important regulator of astrocyte precursor cell proliferation and migration during spinal cord development. Nevertheless, ECM molecules expressed by reactive astrocytes are also known to act mostly in an inhibitory fashion under pathophysiological conditions. Thus, we further summarize resent data concerning the role of chondroitin sulfate proteoglycans and Tnc under pathological conditions.

Adenosine-5'-triphosphate release by Mannheimia haemolytica, lipopolysaccharide, and interleukin-1 stimulated bovine pulmonary epithelial cells

Mannheimia haemolytica, one of the agents associated with bovine respiratory disease complex, can cause severe lung pathology including the leakage of vascular products into the airways and alveoli. Previous work by this laboratory has demonstrated that bovine lung endothelial and epithelial cells undergo dramatic permeability increases when exposed to adenosine-5'-triphosphate (ATP). Therefore, we wanted to determine if ATP levels were elevated in bronchoalveolar lavage (BAL) samples from calves experimentally infected with M. haemolytica. In addition, cultured bovine pulmonary epithelial (BPE) cells were stimulated with heat-killed and live M. haemolytica bacteria, lipopolysaccharide (LPS), lipoteichoic acid (LTA), interleukin-1 (IL-1), and zymosan activated plasma (ZAP) to determine whether they might release extracellular ATP during in vitro infection. Calves experimentally exposed to M. haemolytica had an approximately 2-fold higher level of ATP in their BAL samples compared to control. BPE cells exposed to increasing numbers of heat-killed or live M. haemolytica had significantly increased levels of ATP release as compared to time-matched controls. Finally, BPE cells treated with several concentrations of LPS and IL-1 had increases in ATP release as compared to time-matched controls. This increase appeared to be a result of active ATP secretion by the cells, as cell viability was similar between treated and non-treated cells. Neither ZAP nor LTA induced any ATP release by the cells. In conclusion, ATP levels are elevated in lung secretions from calves infected with M. haemolytica. In addition, lung epithelial cells can actively release ATP when exposed to heat-killed or live M. haemolytica, LPS or IL-1.

Anti-prion activity of Brilliant Blue G

Background

Prion diseases are fatal neurodegenerative disorders with no effective therapy currently available. Accumulating evidence has implicated over-activation of P2X7 ionotropic purinergic receptor (P2X7R) in the progression of neuronal loss in several neurodegenerative diseases. This has led to the speculation that simultaneous blockade of this receptor and prion replication can be an effective therapeutic strategy for prion diseases. We have focused on Brilliant Blue G (BBG), a well-known P2X7R antagonist, possessing a chemical structure expected to confer anti-prion activity and examined its inhibitory effect on the accumulation of pathogenic isoforms of prion protein (PrPres) in a cellular and a mouse model of prion disease in order to determine its therapeutic potential.

Principal Findings

BBG prevented PrPres accumulation in infected MG20 microglial and N2a neural cells at 50% inhibitory concentrations of 14.6 and 3.2 µM, respectively. Administration of BBG in vivo also reduced PrPres accumulation in the brains of mice with prion disease. However, it did not appear to alleviate the disease progression compared to the vehicle-treated controls, implying a complex role of P2X7R on the neuronal degeneration in prion diseases.

Significance

These results provide novel insights into the pathophysiology of prion diseases and have important implications for the treatment.

The human P2X7 receptor and its role in innate immunity

The human P2X7 receptor is a two-transmembrane ionotropic receptor which has a ubiquitous distribution and is most highly expressed on immune cells. In macrophages and similar myeloid cells primed by lipopolysaccharide (LPS), activation of P2X7 by extracellular ATP opens a cation channel/pore allowing massive K+ efflux associated with processing and secretion of pro-inflammatory cytokines interleukin (IL)-1β and IL-18. A variety of other downstream effects follows P2X7 activation over several minutes including shedding of certain surface molecules, membrane blebbing, microvesicle/exosome release and apoptosis of the cell. High concentrations of ATP (>100 µM) are required to activate P2X7 but it remains unclear where these levels exist, other than in inflammatory foci or confined spaces such as in bone. A variety of potent selective antagonists of P2X7 activation have recently become available, allowing clinical trials to be undertaken in inflammatory and immune-mediated disorders. Proteomic studies have shown that P2X7 exists as a large multiprotein complex which includes non-muscle myosin heavy chain and other elements of the cytoskeleton. In the absence of its ATP ligand and serum, P2X7 has an alternate function in the recognition and phagocytosis of non-opsonized foreign particles, including bacteria and apoptotic cells. The P2RX7 gene has many polymorphic variants and isoforms which increase or decrease function of the receptor. Genetic association studies have linked loss-of-function polymorphisms with reactivation of latent tuberculosis as well as symptomatic infection with certain other obligate intracellular pathogens. The many roles involving P2X7 suggest that this receptor is essential to fundamental aspects of the innate immune response.

Critical role of connexin 43 in secondary expansion of traumatic spinal cord injury

Spinal cord injury (SCI) is often complicated by secondary injury as a result of the innate inflammatory response to tissue trauma and swelling. Previous studies have shown that excessive ATP release from peritraumatic regions contributes to the inflammatory response to SCI by activation of low-affinity P2X7 receptors. Because connexin hemichannels constitute an important route for astrocytic ATP release, we here evaluated the impact on post-traumatic ATP release of deletion of connexins (Cx30/Cx43) in astrocytes. In vivo bioluminescence imaging showed a significant reduction in ATP release after weight-drop injury in mice with deletion of Cx43 compared with Cx43-expressing littermates, both on a Cx30 knockout background. Moreover, astrogliosis and microglia activation were reduced in peritraumatic areas of those mice lacking Cx43; motor recovery was also significantly improved, and the traumatic lesion was smaller. Combined, these observations are consistent with a contribution by astrocytic hemichannels to post-traumatic ATP release that aggravates secondary injury and restrains functional recovery after experimental spinal cord injury. Connexins may thereby constitute a new therapeutic target in spinal cord injury.

P2X4 receptors influence inflammasome activation after spinal cord injury

P2X(4) and P2X(7) are the predominant purinergic P2X receptor subtypes expressed on immune and neural cells. These receptor subtypes traffic between intracellular compartments and the plasma membrane and form protein interactions with each other to regulate ATP-dependent signaling. Our recent studies have shown that P2X(7) receptors in neurons and astrocytes activate NLRP1 inflammasomes, but whether P2X(4) receptors regulate inflammasome signaling is essentially unknown. Here, we demonstrate that P2X(4) receptors are expressed in neurons of the spinal cord. We provide direct evidence that spinal cord injury (SCI) induces an innate inflammatory response that leads to increased caspase-l cleavage and production of IL-1β but not IL-18. Consistent with these findings, P2X(4) knock-out mice showed impaired inflammasome signaling in the cord, resulting in decreased levels of IL-1β and reduced infiltration of neutrophils and monocyte-derived M1 macrophages, resulting in significant tissue sparing and improvement in functional outcomes. These results indicate that P2X(4) receptors influence inflammasome signaling involving caspase-1 activation and IL-1β processing in neurons after SCI. P2X(4) might thus represent a potential therapeutic target to limit inflammatory responses associated with SCI and neurodegenerative disorders.

Involvement of P2Y13 receptor in suppression of neuronal differentiation

We examined the receptor-mediated effects of extracellular ATP on neuronal differentiation of PC12 cells, Neuro2a cells and MEB5 cells by using a series of receptor antagonists. The P2Y13 receptor antagonist MRS2211 significantly accelerated neurite outgrowth in all cases. Treatment with nerve growth factor (NGF) alone activated ERK1/2 in PC12 cells, and the activation was further increased by MRS2211. These results suggest involvement of P2Y13 receptor in suppression of neuronal differentiation. Thus, P2Y13 receptor antagonists might be candidates for treatment of neurodegenerative diseases.

P2X7 receptor-mediated calcium dynamics in HEK293 cells: experimental characterization and modelling approach

The P2X7 receptor (P2X7R) induces ionotropic Ca²⁺ signalling in different cell types. It plays an important role in the immune response and in the nervous system. Here, the mechanisms underlying intracellular Ca²⁺ variations evoked by 3'-O-(4-benzoyl)benzoyl-ATP (BzATP), a potent agonist of the P2X7R, in transfected HEK293 cells, are investigated both experimentally and theoretically. We propose a minimal model of P2X7R that is capable of reproducing, qualitatively and quantitatively, the experimental data. This approach was also adopted for the P2X7R variant, which lacks the entire C-terminus tail (trP2X7R). Then we introduce a biophysical model describing the Ca²⁺ dynamics in HEK293. Our model gives an account of the ionotropic Ca²⁺ influx evoked by BzATP on the basis of the kinetics model of P2X7R. To explain the complex Ca²⁺ responses evoked by BzATP, the model predicted that an impairment in Ca²⁺ extrusion flux through the plasma membrane is a key factor for Ca²⁺ homeostasis in HEK293 cells.

Roles of white matter in central nervous system pathophysiologies

The phylogenetic enlargement of cerebral cortex culminating in the human brain imposed greater communication needs that have been met by the massive expansion of WM (white matter). Damage to WM alters brain function, and numerous neurological diseases feature WM involvement. In the current review, we discuss the major features of WM, the contributions of WM compromise to brain pathophysiology, and some of the mechanisms mediating WM injury. We will emphasize the newly appreciated importance of neurotransmitter signalling in WM, particularly glutamate and ATP signalling, to understanding both normal and abnormal brain functions. A deeper understanding of the mechanisms leading to WM damage will generate much-needed insights for developing therapies for acute and chronic diseases with WM involvement.

Activation of neuronal P2X7 receptor-pannexin-1 mediates death of enteric neurons during colitis

Inflammatory bowel diseases (IBD) are chronic relapsing and remitting conditions associated with long-term gut dysfunction resulting from alterations to the enteric nervous system and a loss of enteric neurons^1,2. The mechanisms underlying inflammation-induced enteric neuron death are unknown. Here we report using in vivo models of experimental colitis that inflammation causes enteric neuron death by activating a neuronal signaling complex comprised of P2X7 receptors (P2X7Rs), pannexin–1 (Panx1) channels, Asc and caspases. Inhibiting P2X7Rs, Panx1, Asc or caspase activity prevents inflammation-induced neuron cell death. Preservation of enteric neurons by inhibiting Panx1 in vivo prevented the onset of inflammation-induced colonic motor dysfunction. Panx1 expression is reduced in Crohn’s disease but not ulcerative colitis. We conclude that activation of neuronal Panx1 underlies neuron death and subsequent development of the abnormal gut motility in IBD. Targeting Panx1 represents a novel neuroprotective strategy to ameliorate the progression of IBD–associated dysmotility.

Extracellular Ca²⁺ acts as a mediator of communication from neurons to glia

Defining the pathways through which neurons and astrocytes communicate may contribute to the elucidation of higher central nervous system functions. We investigated the possibility that decreases in extracellular calcium ion concentration ([Ca(2+)](e)) that occur during synaptic transmission might mediate signaling from neurons to glia. Using noninvasive photolysis of the photolabile Ca(2+) buffer diazo-2 {N-[2-[2-[2-[bis(carboxymethyl)amino]-5-(diazoacetyl)phenoxy]ethoxy]-4-methylphenyl]-N-(carboxymethyl)-, tetrapotassium salt} to reduce [Ca(2+)](e) or caged glutamate to simulate glutamatergic transmission, we found that a local decline in extracellular Ca(2+) triggered astrocytic adenosine triphosphate (ATP) release and astrocytic Ca(2+) signaling. In turn, activation of purinergic P2Y1 receptors on a subset of inhibitory interneurons initiated the generation of action potentials by these interneurons, thereby enhancing synaptic inhibition. Thus, astrocytic ATP release evoked by an activity-associated decrease in [Ca(2+)](e) may provide a negative feedback mechanism that potentiates inhibitory transmission in response to local hyperexcitability.

Adenylate cyclase 5 coordinates the action of ADP, P2Y1, P2Y13 and ATP-gated P2X7 receptors on axonal elongation

In adult brains, ionotropic or metabotropic purinergic receptors are widely expressed in neurons and glial cells. They play an essential role in inflammation and neurotransmission in response to purines secreted to the extracellular medium. Recent studies have demonstrated a role for purinergic receptors in proliferation and differentiation of neural stem cells although little is known about their role in regulating the initial neuronal development and axon elongation. The objective of our study was to investigate the role of some different types of purinergic receptors, P2Y1, P2Y13 and P2X7, which are activated by ADP or ATP. To study the role and crosstalk of P2Y1, P2Y13 and P2X7 purinergic receptors in axonal elongation, we treated neurons with specific agonists and antagonists, and we nucleofected neurons with expression or shRNA plasmids. ADP and P2Y1-GFP expression improved axonal elongation; conversely, P2Y13 and ATP-gated P2X7 receptors halted axonal elongation. Signaling through each of these receptor types was coordinated by adenylate cyclase 5. In neurons nucleofected with a cAMP FRET biosensor (ICUE3), addition of ADP or Blue Brilliant G, a P2X7 antagonist, increased cAMP levels in the distal region of the axon. Adenylate cyclase 5 inhibition or suppression impaired these cAMP increments. In conclusion, our results demonstrate a crosstalk between two metabotropic and one ionotropic purinergic receptor that regulates cAMP levels through adenylate cyclase 5 and modulates axonal elongation triggered by neurotropic factors and the PI3K-Akt-GSK3 pathway.

The participation of plasma membrane hemichannels to purinergic signaling

The field of hemichannels is closely related to the purinergic signaling and both areas have been growing in parallel. Hemichannels open in response to a wide range of stressful conditions, such as ischemia, pressure or swelling. Hemichannels represent an important mechanism for the cellular release of adenosine 5'-triphosphate (ATP), which is an agonist of the P2Y and P2X family of purinergic receptors. Therefore, hemichannels are key molecules in the regulation of purinergic receptor activation, during physiological and pathophysiological conditions. Furthermore, purinergic receptor activation can also lead to the opening of hemichannels and the subsequent amplification of purinergic signaling via a positive signaling feedback loop, giving rise to the concept of ATP-induced ATP release. Purinergic receptor signaling is involved in regulating many physiological and pathophysiological processes. P2Y receptors activate inositol trisphosphate and transiently increase intracellular calcium. This signaling opens both connexin and pannexin channels, therefore contributing to the expansion of calcium waves across astrocytes and epithelial cells. In addition, several of the P2X receptor subtypes, including the P2X2, P2X4 and P2X7 receptors, activate select cellular permeation pathways to large molecules, including the pannexin-1 channels, which are involved in the initiation of inflammatory responses and cell death. Consequently, the interplay between purinergic receptors and hemichannels could represent a novel target with substantial therapeutic implications in areas such as chronic pain, inflammation or atherosclerosis. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.

Rod and cone pathway signalling is altered in the P2X7 receptor knock out mouse

The P2X7 receptor (P2X7-R) is expressed in the retina and brain and has been implicated in neurodegenerative diseases. However, whether it is expressed by neurons and plays a role as a neurotransmitter receptor has been the subject of controversy. In this study, we first show that the novel vesicular transporter for ATP, VNUT, is expressed in the retina, verifying the presence of the molecular machinery for ATP to act as neurotransmitter at P2X7-Rs. Secondly we show the presence of P2X7-R mRNA and protein in the retina and cortex and absence of the full length variant 1 of the receptor in the P2X7-R knock out (P2X7-KO) mouse. The role of the P2X7-R in neuronal function of the retina was assessed by comparing the electroretinogram response of P2X7-KO with WT mice. The rod photoreceptor response was found to be similar, while both rod and cone pathway post-photoreceptor responses were significantly larger in P2X7-KO mice. This suggests that activation of P2X7-Rs modulates output of second order retinal neurons. In line with this finding, P2X7-Rs were found in the outer plexiform layer and on inner retinal cell classes, including horizontal, amacrine and ganglion cells. The receptor co-localized with conventional synapses in the IPL and was expressed on amacrine cells post-synaptic to rod bipolar ribbon synapses. In view of the changes in visual function in the P2X7-KO mouse and the immunocytochemical location of the receptor in the normal retina, it is likely the P2X7-R provides excitatory input to photoreceptor terminals or to inhibitory cells that shape both the rod and cone pathway response.

Seizure suppression and neuroprotection by targeting the purinergic P2X7 receptor during status epilepticus in mice

Prolonged seizures [status epilepticus (SE)] constitute a neurological emergency that can permanently damage the brain. SE results from a failure of the normal mechanisms to terminate seizures; in particular, γ-amino butyric acid-mediated inhibition, and benzodiazepine anticonvulsants are often incompletely effective. ATP acts as a fast neurotransmitter via ionotropic ligand-gated P2X receptors. Here we report that SE induced by intra-amygdala kainic acid in mice selectively increased hippocampal levels of P2X7 receptors relative to other P2X receptors. Using transgenic P2X7 reporter mice expressing enhanced green fluorescent protein, we identify dentate granule neurons as the major cell population transcribing the P2X7 receptor after SE. Pretreatment of mice with an intracerebroventricular microinjection of 1.75 nmol A438079, a P2X7 receptor antagonist, reduced seizure duration by 58% and reduced seizure-induced neuronal death by 61%. Injection of brilliant blue G (1 pmol), another selective antagonist, reduced seizure duration by 48% and was also neuroprotective. A438079 was seizure-suppressive when injected shortly after induction of SE, and coinjection of A438079 with lorazepam 60 min after triggering SE, when electrographic seizure-responsiveness to lorazepam had decreased, also terminated SE. Our results suggest that P2X7 receptor antagonists may be a promising class of drug for seizure abrogation and neuroprotection in SE.

P2X7 receptor blockade prevents ATP excitotoxicity in neurons and reduces brain damage after ischemia

Overactivation of subtype P2X7 receptors can induce excitotoxic neuronal death by calcium (Ca(2+)) overload. In this study, we characterize the functional properties of P2X7 receptors using electrophysiology and Ca(2+) monitoring in primary cortical neuron cultures and in brain slices. Both electrical responses and Ca(2+) influx induced by ATP and benzoyl-ATP were reduced by Brilliant Blue G (BBG) at concentrations which specifically inhibit P2X7 receptors. In turn, oxygen-glucose deprivation (OGD) caused neuronal death that was reduced with BBG application. OGD in neuron cultures and brain slices generated an inward current, which was delayed and reduced by BBG. To assess the relevance of these in vitro findings, we used middle cerebral artery occlusion in rats as a model of transient focal cerebral ischemia to study the neuroprotective effect of BBG in vivo. Treatment with BBG (twice per day, 30 mg/kg) produced a 60% reduction in the extent of brain damage compared to treatment with vehicle alone. These results show that P2X7 purinergic receptors mediate tissue damage after OGD in neurons and following transient brain ischemia. Therefore, these receptors are a relevant molecular target for the development of new treatments to attenuate brain damage following stroke.

Human dental pulp-derived stem cells promote locomotor recovery after complete transection of the rat spinal cord by multiple neuro-regenerative mechanisms

Spinal cord injury (SCI) often leads to persistent functional deficits due to loss of neurons and glia and to limited axonal regeneration after injury. Here we report that transplantation of human dental pulp stem cells into the completely transected adult rat spinal cord resulted in marked recovery of hind limb locomotor functions. Transplantation of human bone marrow stromal cells or skin-derived fibroblasts led to substantially less recovery of locomotor function. The human dental pulp stem cells exhibited three major neuroregenerative activities. First, they inhibited the SCI-induced apoptosis of neurons, astrocytes, and oligodendrocytes, which improved the preservation of neuronal filaments and myelin sheaths. Second, they promoted the regeneration of transected axons by directly inhibiting multiple axon growth inhibitors, including chondroitin sulfate proteoglycan and myelin-associated glycoprotein, via paracrine mechanisms. Last, they replaced lost cells by differentiating into mature oligodendrocytes under the extreme conditions of SCI. Our data demonstrate that tooth-derived stem cells may provide therapeutic benefits for treating SCI through both cell-autonomous and paracrine neuroregenerative activities.

A reassessment of P2X7 receptor inhibition as a neuroprotective strategy in rat models of contusion injury

These experiments were completed as part of an NIH "Facilities of Research Excellence in Spinal Cord Injury" contract to support independent replication of published studies that could be considered for eventual clinical testing. Recent studies have reported that selective inhibition of the P2X7 receptor improves both the functional and histopathological consequences of a contusive spinal cord injury (SCI) in rats. We repeated two published studies reporting the beneficial effects of pyridoxal-5'-phosphate-6-azophenyl-2'-4'-disulphonic acid (PPADS) or Brilliant blue G (BBG) treatment after SCI (Wang et al., 2004 and Peng et al., 2009). Mild thoracic SCI was first produced in Experiment 1 by means of the MASCIS impactor at T10 (height 6.25 mm, weight 10 g) followed by intraspinal administration of a P2X7 antagonist (2 μl/10 mM) after injury. Treatment with PPADS or another highly selective P2X7R antagonist Brilliant Blue G (BBG) (2 μl/02 mM) did not improve locomotive (BBB rating scale) over a 7 week period compared to vehicle treated rats. Also, secondary histopathological changes in terms of overall lesion and cavity volume were not significantly different between the PPADS, BBG, and vehicle treated animals. In the second experiment, the systemic administration of BBG (10 or 50 mg/kg, iv) 15 min, 24 and 72 h after moderate (12.5 mm) SCI failed to significantly improve motor recovery or histopathological outcome over the 6 week observational period. Although we cannot conclude that there will be no long-term beneficial effects in other spinal cord injury models using selective P2X7 receptor antagonists at different doses or treatment durations, we caution researchers that this potentially exciting therapy requires further preclinical investigations before the implementation of clinical trials targeting severe SCI patients.

Perivascular instruction of cell genesis and fate in the adult brain

The perivascular niche for neurogenesis was first reported as the co-association of newly generated neurons and their progenitors with both dividing and mitotically quiescent endothelial cells in restricted regions of the brain in adult birds and mammals alike. This review attempts to summarize our present understanding of the interaction of blood vessels with neural stem and progenitor cells, addressing both glial and neuronal progenitor cell interactions in the perivascular niche. We review the molecular interactions that are most critical to the endothelial control of stem and progenitor cell mobilization and differentiation. The focus throughout will be on defining those perivascular ligand-receptor interactions shared among these systems, as well as those that clearly differ as a function of cell type and setting, by which specificity may be achieved in the development of targeted therapeutics.

Critical involvement of extracellular ATP acting on P2RX7 purinergic receptors in photoreceptor cell death

Stressed cells release ATP, which participates in neurodegenerative processes through the specific ligation of P2RX7 purinergic receptors. Here, we demonstrate that extracellular ATP and the more specific P2RX7 agonist, 2'- and 3'-O-(4-benzoylbenzoyl)-ATP, both induce photoreceptor cell death when added to primary retinal cell cultures or when injected into the eyes from wild-type mice, but not into the eyes from P2RX7(-/-) mice. Photoreceptor cell death was accompanied by the activation of caspase-8 and -9, translocation of apoptosis-inducing factor from mitochondria to nuclei, and TUNEL-detectable chromatin fragmentation. All hallmarks of photoreceptor apoptosis were prevented by premedication or co-application of Brilliant Blue G, a selective P2RX7 antagonist that is already approved for the staining of internal limiting membranes during ocular surgery. ATP release is up-regulated by nutrient starvation in primary retinal cell cultures and seems to be an initializing event that triggers primary and/or secondary cell death via the positive feedback loop on P2RX7. Our results encourage the potential application of Brilliant Blue G as a novel neuroprotective agent in retinal diseases or similar neurodegenerative pathologies linked to excessive extracellular ATP.

Modulation of GSK-3 as a Therapeutic Strategy on Tau Pathologies

Glycogen synthase kinase-3 (GSK-3) is ubiquitously expressed and unusually active in resting, non-stimulated cells. In mammals, at least three proteins (α, β1, and β2), generated from two different genes, gsk-3α and gsk-3β, are widely expressed at both the RNA and protein levels although some tissues show preferential expression of some of the three proteins. Control of GSK-3 activity occurs by complex mechanisms that depend on specific signaling pathways, often controlling the inhibition of the kinase activity. GSK-3 appears to integrate different signaling pathways from a wide selection of cellular stimuli. The unique position of GSK-3 in modulating the function of a diverse series of proteins and its association with a wide variety of human disorders has attracted significant attention as a therapeutic target and as a means to understand the molecular basis of brain disorders. Different neurodegenerative diseases including frontotemporal dementia, progressive supranuclear palsy, and Alzheimer's disease, present prominent tau pathology such as tau hyperphosphorylation and aggregation and are collectively referred to as tauopathies. GSK-3 has also been associated to different neuropsychiatric disorders, like schizophrenia and bipolar disorder. GSK-3β is the major kinase to phosphorylate tau both in vitro and in vivo and has been proposed as a target for therapeutic intervention. The first therapeutic strategy to modulate GSK-3 activity was the direct inhibition of its kinase activity. This review will focus on the signaling pathways involved in the control of GSK-3 activity and its pathological deregulation. We will highlight different alternatives of GSK-3 modulation including the direct pharmacological inhibition as compared to the modulation by upstream regulators.

The critical role of antigen-presentation-induced cytokine crosstalk in the central nervous system in multiple sclerosis and experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is a debilitating disease of the central nervous system (CNS) that has been extensively studied using the animal model experimental autoimmune encephalomyelitis (EAE). It is believed that CD4(+) T lymphocytes play an important role in the pathogenesis of this disease by mediating the demyelination of neuronal axons via secretion of proinflammatory cytokines resulting in the clinical manifestations. Although a great deal of information has been gained in the last several decades about the cells involved in the inflammatory and disease mediating process, important questions have remained unanswered. It has long been held that initial neuroantigen presentation and T cell activation events occur in the immune periphery and then translocate to the CNS. However, an increasing body of evidence suggests that antigen (Ag) presentation might initiate within the CNS itself. Importantly, it has remained unresolved which antigen presenting cells (APCs) in the CNS are the first to acquire and present neuroantigens during EAE/MS to T cells, and what the conditions are under which this takes place, ie, whether this occurs in the healthy CNS or only during inflammatory conditions and what the related cytokine microenvironment is comprised of. In particular, the central role of interferon-γ as a primary mediator of CNS pathology during EAE has been challenged by the emergence of Th17 cells producing interleukin-17. This review describes our current understanding of potential APCs in the CNS and the contribution of these and other CNS-resident cells to disease pathology. Additionally, we discuss the question of where Ag presentation is initiated and under what conditions neuroantigens are made available to APCs with special emphasis on which cytokines may be important in this process.

Central sensitization of nociceptive neurons in rat medullary dorsal horn involves purinergic P2X7 receptors

Central sensitization is a crucial process underlying the increased neuronal excitability of nociceptive pathways following peripheral tissue injury and inflammation. Our previous findings have suggested that extracellular adenosine 5'-triphosphate (ATP) molecules acting at purinergic receptors located on presynaptic terminals (e.g., P2X2/3, P2X3 subunits) and glial cells are involved in the glutamatergic-dependent central sensitization induced in medullary dorsal horn (MDH) nociceptive neurons by application to the tooth pulp of the inflammatory irritant mustard oil (MO). Since growing evidence indicates that activation of P2X7 receptors located on glia is involved in chronic inflammatory and neuropathic pain, the aim of the present study was to test in vivo for P2X7 receptor involvement in this acute inflammatory pain model. Experiments were carried out in anesthetized Sprague-Dawley male rats. Single unit recordings were made in MDH functionally identified nociceptive neurons for which mechanoreceptive field, mechanical activation threshold and responses to noxious stimuli were tested. We found that continuous intrathecal (i.t.) superfusion over MDH of the potent P2X7 receptor antagonists brilliant blue G and periodated oxidized ATP could each significantly attenuate the MO-induced MDH central sensitization. MDH central sensitization could also be produced by i.t. superfusion of ATP and even more effectively by the P2X7 receptor agonist benzoylbenzoyl ATP. Superfusion of the microglial blocker minocycline abolished the MO-induced MDH central sensitization, consistent with reports that dorsal horn P2X7 receptors are mostly expressed on microglia. In control experiments, superfusion over MDH of vehicle did not produce any significant changes. These novel findings suggest that activation of P2X7 receptors in vivo may be involved in the development of central sensitization in an acute inflammatory pain model.

The GPR17 receptor in NG2 expressing cells: focus on in vivo cell maturation and participation in acute trauma and chronic damage

NG2-expressing cells comprise a population of cycling precursors that can exit the cell cycle and differentiate into mature oligodendrocytes. As a whole, they display heterogeneous properties and behaviors that remain unresolved at the molecular level, although partly interpretable as distinct maturation stages. To address this issue, we analyzed the expression of the GPR17 receptor, recently shown to decorate NG2-expressing cells and to operate as an early sensor of brain damage, in immature and adult oligodendrocyte progenitors in the intact brain and after injury. In both the early postnatal and adult cerebral cortex, distinct GPR17 protein localizations and expression levels define different stages of oligodendroglial maturation, ranging from the precursor phase to the premyelinating phenotype. As soon as cells exit mitosis, a fraction of NG2-expressing cells displays accumulation of GPR17 protein in the Golgi apparatus. GPR17 expression is subsequently upregulated and distributed to processes of cells that stop dividing, progressively lose NG2 positivity and assume premyelinating features. Absence of colabeling with mature markers or myelin proteins indicates that GPR17 is downregulated when cells complete their final maturation. BrdU-based fate-mapping demonstrated that a significant fraction of newly generated oligodendrocyte progenitors transiently upregulates GPR17 during maturation. Importantly, we also found that GPR17 does not participate to the early reaction of NG2-expressing cells to damage, while it is induced at postacute stages after injury. These findings identify GPR17 as a marker for progenitor progression within the oligodendroglial lineage and highlight its participation to postacute reactivity of NG2 cells in different injury paradigms.

Spinal P2X(7) receptor mediates microglia activation-induced neuropathic pain in the sciatic nerve injury rat model

P2X(7) receptor is an important member of ATP-sensitive ionotropic P2X receptors family, which includes seven receptor subtypes (P2X(1)-P2X(7)). Recent evidence indicates that P2X(7)R participates in the onset and persistence of neuropathic pain. In tetanic stimulation of the sciatic nerve model, P2X(7)R was involved in the activation of microglia, but whether this happens in other neuropathic pain models remains unclear. In this study we used immunohistochemistry and Western blot to explore the relationship of P2X(7)R expression with microglia activation, and with mechanical allodynia and thermal hypersensitivity in the chronic constriction of the sciatic nerve (CCI) rat model. The results show that following nerve ligature, mechanical allodynia and thermal hypersensitivity were developed within 3 days (d), peaked at 14d and persisted for 21d on the injured side. P2X(7)R levels in the ipsilateral L4-6 spinal cord were increased markedly after injury and the highest levels were observed on day 14, significant difference was observed at I-IV layers of the dorsal horn. The change in P2X(7)R levels in the spinal cord was consistent with the development of mechanical allodynia and thermal hypersensitivity. Intrathecal administration of the P2X(7)R antagonist Brilliant Blue G (BBG) reversed CCI-induced mechanical allodynia and thermal hypersensitivity. Double-labeled immunofluorescence showed that P2X(7)R expression were restricted to microglia, spinal microglia were activated after nerve injury, which was inhibited by BBG. These results indicated that spinal P2X(7)R mediate microglia activation, this process may play an important role in development of mechanical allodynia and thermal hypersensitivity in CCI model.

Role of acid-sensing ion channel 1a in the secondary damage of traumatic spinal cord injury

OBJECTIVE

To determine the cellular and molecular mechanisms by which acid-sensing ion channel 1a (ASIC1a) plays its role in the secondary injury after traumatic spinal cord injury (SCI), and validate the neuroprotective effect of ASIC1a suppression in SCI model in vivo.

BACKGROUND

Secondary damage after traumatic SCI contributes to the exacerbation of cellular insult and thereby contributes to spinal cord dysfunction. However, the underlying mechanisms remain largely unknown. Acidosis is commonly involved in the secondary injury process after the injury of central nervous system, but whether ASIC1a is involved in secondary injury after SCI is unclear.

METHODS

Male Sprague-Dawley rats were subjected to spinal contusion using a weight-drop injury approach. Western blotting and immunofluorescence assays were used to observe the change of ASIC1a expression after SCI. The TUNEL staining in vivo as well as the cell viability and death assays in spinal neuronal culture were employed to assess the role of ASIC1a in the secondary spinal neuronal injury. The electrophysiological recording and Ca(2+) imaging were performed to reveal the possible underlying mechanism. The antagonists and antisense oligonucleotide for ASIC1a, lesion volume assessment assay and behavior test were used to estimate the therapeutic effect of ASIC1a on SCI.

RESULTS

We show that ASIC1a expression is markedly increased in the peri-injury zone after traumatic SCI. Consistent with the change of ASIC1a expression in injured spinal neurons, both ASIC1a-mediated whole-cell currents and ASIC1a-mediated Ca(2+) entry are significantly enhanced after injury. We also show that increased activity of ASIC1a contributes to SCI-induced neuronal death. Importantly, our results indicate that down-regulation of ASIC1a by antagonists or antisense oligonucleotide reduces tissue damage and promotes the recovery of neurological function after SCI.

CONCLUSION

This study reveals a cellular and molecular mechanism by which ASIC1a is involved in the secondary damage process after traumatic SCI. Our results suggest that blockade of Ca(2+) -permeable ASIC1a may be a potential neuroprotection strategy for the treatment of SCI patients.

Gain-of-function of P2X7 receptor gene variants in multiple sclerosis

We have previously shown that P2X7 receptor blockade prevents ATP excitotoxicity in oligodendrocytes and ameliorates chronic experimental autoimmune encephalomyelitis. Here, we have explored the putative association of functionally relevant single nucleotide polymorphisms of the P2X7 receptor gene with multiple sclerosis. We found that T allele of rs17525809 polymorphism, which yields an Ala-76 to Val change in the extracellular domain, is more frequent in multiple sclerosis patients than in controls. Importantly, P2X7 variants with Val show a gain-of-function consisting in higher calcium permeability, larger electrophysiological responses and higher ethidium uptake, and enhance the effect of the also gain-of-function His-155 to Tyr substitution (rs208294) in the haplotype formed by these two variants. These findings may contribute to define the genetic background predisposing for multiple sclerosis and its pathophysiology.

Purinergic signalling: from normal behaviour to pathological brain function

Purinergic neurotransmission, involving release of ATP as an efferent neurotransmitter was first proposed in 1972. Later, ATP was recognised as a cotransmitter in peripheral nerves and more recently as a cotransmitter with glutamate, noradrenaline, GABA, acetylcholine and dopamine in the CNS. Both ATP, together with some of its enzymatic breakdown products (ADP and adenosine) and uracil nucleotides are now recognised to act via P2X ion channels and P1 and P2Y G protein-coupled receptors, which are widely expressed in the brain. They mediate both fast signalling in neurotransmission and neuromodulation and long-term (trophic) signalling in cell proliferation, differentiation and death. Purinergic signalling is prominent in neurone-glial cell interactions. In this review we discuss first the evidence implicating purinergic signalling in normal behaviour, including learning and memory, sleep and arousal, locomotor activity and exploration, feeding behaviour and mood and motivation. Then we turn to the involvement of P1 and P2 receptors in pathological brain function; firstly in trauma, ischemia and stroke, then in neurodegenerative diseases, including Alzheimer's, Parkinson's and Huntington's, as well as multiple sclerosis and amyotrophic lateral sclerosis. Finally, the role of purinergic signalling in neuropsychiatric diseases (including schizophrenia), epilepsy, migraine, cognitive impairment and neuropathic pain will be considered.

P2X(7) is a scavenger receptor for apoptotic cells in the absence of its ligand, extracellular ATP

Phagocytosis of apoptotic cells is essential during development and tissue remodeling. Our previous study has shown that the P2X(7) receptor regulates phagocytosis of nonopsonized particles and bacteria. In this study, we demonstrate that P2X(7) also mediates phagocytosis of apoptotic lymphocytes and neuronal cells by human monocyte-derived macrophages under serum-free conditions. ATP inhibited this process to a similar extent as observed with cytochalasin D. P2X(7)-transfected HEK-293 cells acquired the ability to phagocytose apoptotic lymphocytes. Injection of apoptotic thymocytes into the peritoneal cavity of wild-type mice resulted in their phagocytosis by macrophages, but injection of ATP prior to thymocytes markedly decreased this uptake. In contrast, ATP failed to inhibit phagocytosis of apoptotic thymocytes in vivo by P2X(7)-deficient peritoneal macrophages. The surface expression of P2X(7) on phagocytes increased significantly during phagocytosis of either beads or apoptotic cells. A peptide screen library containing 24 biotin-conjugated peptides mimicking the extracellular domain of P2X(7) was used to evaluate the binding profile to beads, bacteria, and apoptotic cells. One peptide showed binding to all particles and cell membrane lipids. Three other cysteine-containing peptides uniquely bound the surface of apoptotic cells but not viable cells, whereas substitution of alanine for cysteine abolished peptide binding. Several thiol-reactive compounds including N-acetyl-L-cysteine abolished phagocytosis of apoptotic SH-SY5Y cells by macrophages. These data suggest that the P2X(7) receptor in its unactivated state acts like a scavenger receptor, and its extracellular disulphide bonds play an important role in direct recognition and engulfment of apoptotic cells.

The role of extracellular adenosine in chemical neurotransmission in the hippocampus and Basal Ganglia: pharmacological and clinical aspects

Now there is general agreement that the purine nucleoside adenosine is an important neuromodulator in the central nervous system, playing a crucial role in neuronal excitability and synaptic/non-synaptic transmission in the hippocampus and basal ganglia. Adenosine is derived from the breakdown of extra- or intracellular ATP and is released upon a variety of physiological and pathological stimuli from neuronal and non-neuronal sources, i.e. from glial cells and exerts effects diffusing far away from release sites. The resultant elevation of adenosine levels in the extracellular space reaches micromolar level, and leads to the activation A₁, A_2A, A_2B and A₃ receptors, localized to pre- and postsynaptic as well as extrasynaptic sites. Activation of presynaptic A₁ receptors inhibits the release of the majority of transmitters including glutamate, acetylcholine, noradrenaline, 5-HT and dopamine, whilst the stimulation of A_2A receptors facilitates the release of glutamate and acetylcholine and inhibits the release of GABA. These actions underlie modulation of neuronal excitability, synaptic plasticity and coordination of neural networks and provide intriguing target sites for pharmacological intervention in ischemia and Parkinson’s disease. However, despite that adenosine is also released during ischemia, A₁ adenosine receptors do not participate in the modulation of excitotoxic glutamate release, which is nonsynaptic and is due to the reverse operation of transporters. Instead, extrasynaptic A1 receptors might be responsible for the neuroprotection afforded by A₁ receptor activation.

Receptor localization, native tissue binding and ex vivo occupancy for centrally penetrant P2X7 antagonists in the rat

BACKGROUND AND PURPOSE

The P2X7 receptor is implicated in inflammation and pain and is therefore a potential target for therapeutic intervention. Here, the development of a native tissue radioligand binding, localization and ex vivo occupancy assay for centrally penetrant P2X7 receptor antagonists is described.

EXPERIMENTAL APPROACH

Autoradiography studies using the P2X7 antagonist radioligand [³H]-A-804598 were carried out in rat brain and spinal cord. Subsequent in vitro binding and ex vivo occupancy assays were performed using rat cortex homogenate.

KEY RESULTS

P2X7 expression was shown to be widespread throughout the rat brain, and in the grey matter of the spinal cord. In binding assays in rat cortex homogenate, ∼60% specific binding was achieved at equilibrium. In kinetic binding assays, k(on) and k(off) values of 0.0021·min⁻¹·nM⁻¹ and 0.0070·min⁻¹ were determined, and the K(d) derived from kinetic measurements was consistent with that derived from saturation analysis. Novel P2X7 antagonists inhibited the binding of [³H]-A-804598 to rat cortex P2X7 receptors with K(i) values of <40 nM. In an ex vivo occupancy assay, a P2X7 antagonist dosed orally to rats caused a concentration-dependent inhibition of the specific binding of [³H]-A-804598 to rat cortex.

CONCLUSIONS AND IMPLICATIONS

The present study describes the development of an assay that allows localization of P2X7 receptors, the measurement of the binding affinity of P2X7 receptor antagonists in native tissue, and provides a means of determining central P2X7 receptor occupancy. These assays could form an important part of a P2X7 drug discovery programme.

Blockade of P2 nucleotide receptors after spinal cord injury reduced the gliotic response and spared tissue

Spinal cord injury (SCI) triggers a sequel of events commonly associated with cell death and dysfunction of glias and neurons surrounding the lesion. Although astrogliosis and glial scar formation have been involved in both damage and repair processes after SCI, their role remains controversial. Our goal was to investigate the effects of the P2 receptors antagonists, PPADS and suramin, in the establishment of the reactive gliosis and the formation of the glial scar. Molecular biology, immunohistochemistry, spared tissue, and locomotor behavioral studies were used to evaluate astrogliosis, in adult female Sprague-Dawley rats treated with P2 antagonists after moderate injury with the NYU impactor device. Semi-quantitative RT-PCR confirmed the presence of P2Y(1,) P2Y(2,) P2Y(4,) P2Y(6,) P2Y(12), and P2X(2) receptors in the adult spinal cord. Immunohistochemistry studies confirmed a significant decrease in GFAP-labeled cells at the injury epicenter as well as a decrease in spared tissue after treatment with the antagonists. Functional open field testing revealed no significant locomotor score differences between treated and control animals. Our work is consistent with studies suggesting that astrogliosis is an important event after SCI that limits tissue damage and lesion spreading.

Expression and functional characterization of P2X receptors in mouse alveolar macrophages

Alveolar macrophages (AM) are crucial for pulmonary host defense, and evidence emerges that ATP-gated P2X receptors are involved in inflammatory processes. This study focuses on the expression and functional characterization of P2X receptors in AM from mouse. In RT-PCR experiments, transcripts encoding the P2X₁, P2X₃, P2X₄, P2X₅, and P2X₇ receptors were detected. In whole-cell patch-clamp recordings, ATP (1 mM) evoked an inward current (mouse and human AM) that was reversible upon washout, and the reversal potential was ~5 mV, indicating the activation of a non-selective conductance-a fingerprint of P2X receptors. Further characterization (mouse AM) revealed that the current was not desensitized by a second ATP application. The ATP-induced current was increased by the removal of extracellular Ca²⁺ (in human and mouse AM), and EC₅₀ in mouse AM were determined with ~1 mM ATP, in the presence as well as in the absence of extracellular Ca²⁺. Pharmacological characterization of mouse AM revealed that the effect was augmented by BzATP and pre-application with ivermectin, but no effect with α,β-meATP was observed. Further, the ATP effect was reduced by PPADS (300 μM), brilliant blue G (5 μM), and about A438079 (10 μM). Although different P2X receptor transcripts were detected in mouse AM, the observed functional and pharmacological characteristics indicate primarily the participation of P2X₄ and P2X₇ receptors as mediators of the ATP-induced ion current in mouse AM. These suggestions were confirmed by experiments with AM from P2X₇ -deficient animals, indicating a contribution of P2X₄ and P2X₇ receptors in pulmonary immune function.

Inhibition of the P2X7 receptor reduces cystogenesis in PKD

The P2X7 receptor participates in purinergic signaling, which may promote the progression of ADPKD. We examined the effects of a P2X7 receptor antagonist and a P2X7 receptor agonist on cyst development in a zebrafish model of polycystic kidney disease in which we knocked down pkd2 by morpholinos. We used live wt-1b pronephric-specific GFP-expressing zebrafish embryos to directly observe changes in the pronephros. Exposure of pkd2-morphant zebrafish to a P2X7 receptor antagonist (oxidized ATP [OxATP]) significantly reduced the frequency of the cystic phenotype compared with either exposure to a P2X7 receptor agonist (BzATP) or with no treatment (P < 0.01). Histology confirmed improvement of glomerular cysts in OxATP-treated pkd2 morphants. OxATP also reduced p-ERK activity and cell proliferation in pronephric kidneys in pkd2 morphants. Inhibition of P2X7 with an additional specific antagonist (A-438079), and through morpholino-mediated knockdown of p2rx7, confirmed these effects. In conclusion, blockade of the P2X7 receptor reduces cyst formation via ERK-dependent pathways in a zebrafish model of polycystic kidney disease, suggesting that P2X7 antagonists may have therapeutic potential in ADPKD.

P2X7 receptor antagonism in the treatment of cancers

ATP-gated P2X7 receptors (P2X7) make a unique family of extracellular ATP-activated plasma membrane ion channels expressed in haematopoietic and epithelial cells. They have been extensively studied in immune cells where their activation leads to the rapid release of pro-inflammatory cytokines and the initiation of the inflammatory cascade. As such, P2X7 represent a pharmaceutical target for the treatment of inflammatory diseases. Recently, P2X7 expression has been found in diverse tumours and has been suggested as a potential cancer cell biomarker. On ATP stimulation, tumour cells can use P2X7 signalling in different scenarios: i) as a reaction to this death-related signal, they can downregulate P2X7 to avoid apoptosis or ii) as a cancer-promoting signal to survive and enhance invasion of new niches. The high levels of extracellular ATP found in tumours could represent a stressful stimulus for cancer cells by initiating P2X7-driven cell death. Therefore, the increased P2X7-dependent invasiveness of cancer cells could be an escape strategy to flee the noxious high level of ATP. The use of specific P2X7 antagonists could be a new alternative way to reduce the development of cancer metastases and improve the efficacy of conventional treatments.

Effects of propofol on P2X7 receptors and the secretion of tumor necrosis factor-α in cultured astrocytes

Upon CNS injury, adenosine-5'-triphosphate is released and acts on P2X7 receptors, which might influence many cytokines secretion from glial cells and, in turn, affects the survival of neurons. Propofol, an intravenous anesthetic, has been shown to provide neuroprotective effect. However, the effect of propofol on astrocyte-associated processes remains to be clarified. In this study, we investigated the effects of propofol on P2X7 activity in astrocytes and tumor necrosis factor-α (TNF-α) secretion from these cells and thereby to infer the possible role(s) of glial P2X7 receptors in propofol neural protective effects. Whole-cell patch clamp results showed that in clinically relevant concentrations (3.3, 10 or 33 μM), propofol increased the P2X7 current amplitudes significantly and propofol in 10 μM extended the inactivation times of P2X7 receptors. Enzyme-linked immunosorbent assay showed that propofol increased the secretion of TNF-α from astrocytes in high concentration (300 μM), while inhibited in clinically relevant concentration (10 μM). Both of these effects were not influenced by Brilliant blue G. These results suggest that in clinically relevant concentrations, propofol increases the activity of P2X7 receptors in activated astrocytes, but this does not contribute to the downregulation of the secretion of TNF-α.

Role of connexin43 and ATP in long-range bystander radiation damage and oncogenesis in vivo

Ionizing radiation is a genotoxic agent and human carcinogen. Recent work has questioned long-held dogmas by showing that cancer-associated genetic alterations occur in cells and tissues not directly exposed to radiation, questioning the robustness of the current system of radiation risk assessment. In vitro, diverse mechanisms involving secreted soluble factors, gap junction intercellular communication (GJIC) and oxidative metabolism are proposed to mediate these indirect effects. In vivo, the mechanisms behind long-range 'bystander' responses remain largely unknown. Here, we investigate the role of GJIC in propagating radiation stress signals in vivo, and in mediating radiation-associated bystander tumorigenesis in mouse central nervous system using a mouse model in which intercellular communication is downregulated by targeted deletion of the connexin43 (Cx43) gene. We show that GJIC is critical for transmission of oncogenic radiation damage to the non-targeted cerebellum, and that a mechanism involving adenosine triphosphate release and upregulation of Cx43, the major GJIC constituent, regulates transduction of oncogenic damage to unirradiated tissues in vivo. Our data provide a novel hypothesis for transduction of distant bystander effects and suggest that the highly branched nervous system, similar to the vascular network, has an important role.

The P2 receptor antagonist PPADS supports recovery from experimental stroke in vivo

BACKGROUND

After ischemia of the CNS, extracellular adenosine 5'-triphosphate (ATP) can reach high concentrations due to cell damage and subsequent increase of membrane permeability. ATP may cause cellular degeneration and death, mediated by P2X and P2Y receptors.

METHODOLOGY/PRINCIPAL FINDINGS

The effects of inhibition of P2 receptors by pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) on electrophysiological, functional and morphological alterations in an ischemia model with permanent middle cerebral artery occlusion (MCAO) were investigated up to day 28. Spontaneously hypertensive rats received PPADS or vehicle intracerebroventricularly 15 minutes prior MCAO for up to 7 days. The functional recovery monitored by qEEG was improved by PPADS indicated by an accelerated recovery of ischemia-induced qEEG changes in the delta and alpha frequency bands along with a faster and sustained recovery of motor impairments. Whereas the functional improvements by PPADS were persistent at day 28, the infarct volume measured by magnetic resonance imaging and the amount of TUNEL-positive cells were significantly reduced by PPADS only until day 7. Further, by immunohistochemistry and confocal laser scanning microscopy, we identified both neurons and astrocytes as TUNEL-positive after MCAO.

CONCLUSION

The persistent beneficial effect of PPADS on the functional parameters without differences in the late (day 28) infarct size and apoptosis suggests that the early inhibition of P2 receptors might be favourable for the maintenance or early reconstruction of neuronal connectivity in the periinfarct area after ischemic incidents.

Lack of neuroprotection in the absence of P2X7 receptors in toxin-induced animal models of Parkinson's disease

Background

Previous studies indicate a role of P2X₇ receptors in processes that lead to neuronal death. The main objective of our study was to examine whether genetic deletion or pharmacological blockade of P2X₇ receptors influenced dopaminergic cell death in various models of Parkinson's disease (PD).

Results

mRNA encoding P2X₇ and P2X₄ receptors was up-regulated after treatment of PC12 cells with 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP). P2X₇ antagonists protected against MPTP and rotenone induced toxicity in the LDH assay, but failed to protect after rotenone treatment in the MTT assay in PC12 cells and in primary midbrain culture. In vivo MPTP and in vitro rotenone pretreatments increased the mRNA expression of P2X₇ receptors in the striatum and substantia nigra of wild-type mice. Basal mRNA expression of P2X₄ receptors was higher in P2X₇ knockout mice and was further up-regulated by MPTP treatment. Genetic deletion or pharmacological inhibition of P2X₇ receptors did not change survival rate or depletion of striatal endogenous dopamine (DA) content after in vivo MPTP or in vitro rotenone treatment. However, depletion of norepinephrine was significant after MPTP treatment only in P2X₇ knockout mice. The basal ATP content was higher in the substantia nigra of wild-type mice, but the ADP level was lower. Rotenone treatment elicited a similar reduction in ATP content in the substantia nigra of both genotypes, whereas reduction of ATP was more pronounced after rotenone treatment in striatal slices of P2X₇ deficient mice. Although the endogenous amino acid content remained unchanged, the level of the endocannabinoid, 2-AG, was elevated by rotenone in the striatum of wild-type mice, an effect that was absent in mice deficient in P2X₇ receptors.

Conclusions

We conclude that P2X₇ receptor deficiency or inhibition does not support the survival of dopaminergic neurons in an in vivo or in vitro models of PD.

Transcriptional control mechanisms associated with the nucleotide receptor P2X7, a critical regulator of immunologic, osteogenic, and neurologic functions

The nucleotide receptor P2X(7) is an attractive therapeutic target and potential biomarker for multiple inflammatory and neurologic disorders, and it is expressed in several immune, osteogenic, and neurologic cell types. Aside from its role in the nervous system, it is activated by ATP released at sites of tissue damage, inflammation, and infection. Ligand binding to P2X(7) stimulates many cell responses, including calcium fluxes, MAPK activation, inflammatory mediator release, and apoptosis. Much work has centered on P2X(7) action in cell death and mediator processing (e.g., pro-interleukin-1 cleavage by the inflammasome), but the contribution of P2X(7) to transcriptional regulation is less well defined. This review will focus on the growing evidence for the importance of nucleotide-mediated gene expression, highlight several animal models, human genetic, and clinical studies that support P2X(7) as a therapeutic target, and discuss the latest developments in anti-P2X(7) clinical trials.

Oligodendrocyte fate after spinal cord injury

Oligodendrocytes (OLs) are particularly susceptible to the toxicity of the acute lesion environment after spinal cord injury (SCI). They undergo both necrosis and apoptosis acutely, with apoptosis continuing at chronic time points. Loss of OLs causes demyelination and impairs axon function and survival. In parallel, a rapid and protracted OL progenitor cell proliferative response occurs, especially at the lesion borders. Proliferating and migrating OL progenitor cells differentiate into myelinating OLs, which remyelinate demyelinated axons starting at 2 weeks post-injury. The progression of OL lineage cells into mature OLs in the adult after injury recapitulates development to some degree, owing to the plethora of factors within the injury milieu. Although robust, this endogenous oligogenic response is insufficient against OL loss and demyelination. First, in this review we analyze the major spatial-temporal mechanisms of OL loss, replacement, and myelination, with the purpose of highlighting potential areas of intervention after SCI. We then discuss studies on OL protection and replacement. Growth factors have been used both to boost the endogenous progenitor response, and in conjunction with progenitor transplantation to facilitate survival and OL fate. Considerable progress has been made with embryonic stem cell-derived cells and adult neural progenitor cells. For therapies targeting oligogenesis to be successful, endogenous responses and the effects of the acute and chronic lesion environment on OL lineage cells must be understood in detail, and in relation, the optimal therapeutic window for such strategies must also be determined.

A meta-analysis of microarray-based gene expression studies of olfactory bulb-derived olfactory ensheathing cells

Genome wide transcriptional profiling and large scale proteomics have emerged as two powerful methods to dissect the molecular properties of specific neural tissues or cell types on a global scale. Several genome-wide transcriptional profiling and proteomics studies have been published on cultured olfactory ensheathing cells (OEC). In this article we present a meta-analysis of all five published and publicly available micro-array gene expression datasets of cultured early-passage-OB-OEC with other cell types (Schwann cells, late-passage-OB-OEC, mucosa-OEC, an OEC cell line, and acutely dissected OEC). The aim of this meta-analysis is to identify genes and molecular pathways that are found in multiple instead of one isolated study. 454 Genes were detected in at least three out of five microarray datasets. In this "Top-list", genes involved in the biological processes "growth of neurites", "blood vessel development", "migration of cells" and "immune response" were strongly overrepresented. By applying network analysis tools, molecular networks were constructed and Hub-genes were identified that may function as key genes in the above mentioned interrelated processes. We also identified 7 genes (ENTPD2, MATN2, CTSC, PTHLH, GLRX1, COL27A1 and ID2) with uniformly higher or lower expression in early-passage-OB-OEC in all five microarray comparisons. These genes have diverse but intriguing roles in neuroprotection, neurite extension and/or tissue repair. Our meta-analysis provides novel insights into the molecular basis of OB-OEC-mediated neural repair and can serve as a repository for investigators interested in the molecular biology of OEC. This article is part of a Special Issue entitled: Understanding olfactory ensheathing glia and their prospect for nervous system repair.

Activation of P2X receptors induces apoptosis in human retinal pigment epithelium

PURPOSE

The retinal pigment epithelium (RPE) is considered a primary site of pathology in age-related macular degeneration (AMD), which is the most prevalent form of irreversible blindness worldwide in the elderly population. Extracellular adenosine triphosphate (ATP) acts as a key signaling molecule in numerous cellular processes, including cell death. The purpose of this study was to determine whether extracellular ATP induces apoptosis in cultured human RPE.

METHODS

RPE apoptosis was evaluated by caspase-3 activation, Hoechst staining, and DNA fragmentation. Intracellular Ca(2+) levels were determined by both a cell-based fluorometric Ca(2+) assay and a ratiometric Ca(2+) imaging technique. P2X(7) mRNA and protein expression were detected by reverse transcription-polymerase chain reaction (RT-PCR) and confocal microscopy, respectively.

RESULTS

The authors found that both the endogenous P2X(7) agonist ATP and the synthetic, selective P2X(7) agonist 2',3'-O-(4-benzoylbenzoyl)-ATP (BzATP) induced RPE apoptosis, which was significantly inhibited by P2X(7) antagonist oxidized ATP (oATP) but not by the P2 receptor antagonist suramin; both ATP and BzATP increase intracellular Ca(2+) via extracellular Ca(2+) influx; both ATP- and BzATP-induced Ca(2+) responses were significantly inhibited by oATP but not by suramin; ATP-induced apoptosis was significantly inhibited or blocked by BAPTA-AM or by low or no extracellular Ca(2+); and P2X(7) receptor mRNA and protein were expressed in RPE cells.

CONCLUSIONS

These findings suggest that P2X receptors, especially P2X(7) receptors, contribute to ATP- and BzATP-induced Ca(2+) signaling and apoptosis in the RPE. Abnormal Ca(2+) homeostasis through the activation of P2X receptors could cause the dysfunction and apoptosis of RPE that underlie AMD.