Purinergic receptor-stimulated IP3-mediated Ca2+ release enhances neuroprotection by increasing astrocyte mitochondrial metabolism during aging

Advancing stroke recovery: unlocking the potential of cellular dynamics in stroke recovery

Stroke stands as a predominant cause of mortality and morbidity worldwide, and there is a pressing need for effective therapies to improve outcomes and enhance the quality of life for stroke survivors. In this line, effective efferocytosis, the clearance of apoptotic cells, plays a crucial role in neuroprotection and immunoregulation. This process involves specialized phagocytes known as "professional phagocytes" and consists of four steps: "Find-Me," "Eat-Me," engulfment/digestion, and anti-inflammatory responses. Impaired efferocytosis can lead to secondary necrosis and inflammation, resulting in adverse outcomes following brain pathologies. Enhancing efferocytosis presents a potential avenue for improving post-stroke recovery. Several therapeutic targets have been identified, including osteopontin, cysteinyl leukotriene 2 receptor, the µ opioid receptor antagonist β-funaltrexamine, and PPARγ and RXR agonists. Ferroptosis, defined as iron-dependent cell death, is now emerging as a novel target to attenuate post-stroke tissue damage and neuronal loss. Additionally, several biomarkers, most importantly CD163, may serve as potential biomarkers and therapeutic targets for acute ischemic stroke, aiding in stroke diagnosis and prognosis. Non-pharmacological approaches involve physical rehabilitation, hypoxia, and hypothermia. Mitochondrial dysfunction is now recognized as a major contributor to the poor outcomes of brain stroke, and medications targeting mitochondria may exhibit beneficial effects. These strategies aim to polarize efferocytes toward an anti-inflammatory phenotype, limit the ingestion of distressed but viable neurons, and stimulate efferocytosis in the late phase of stroke to enhance post-stroke recovery. These findings highlight promising directions for future research and development of effective stroke recovery therapies.

Piezoelectric hydrogel for treatment of periodontitis through bioenergetic activation

The impaired differentiation ability of resident cells and disordered immune microenvironment in periodontitis pose a huge challenge for bone regeneration. Herein, we construct a piezoelectric hydrogel to rescue the impaired osteogenic capability and rebuild the regenerative immune microenvironment through bioenergetic activation. Under local mechanical stress, the piezoelectric hydrogel generated piezopotential that initiates osteogenic differentiation of inflammatory periodontal ligament stem cells (PDLSCs) via modulating energy metabolism and promoting adenosine triphosphate (ATP) synthesis. Moreover, it also reshapes an anti-inflammatory and pro-regenerative niche through switching M1 macrophages to the M2 phenotype. The synergy of tilapia gelatin and piezoelectric stimulation enhances in situ regeneration in periodontal inflammatory defects of rats. These findings pave a new pathway for treating periodontitis and other immune-related bone defects through piezoelectric stimulation-enabled energy metabolism modulation and immunomodulation.

Lactate: A Theranostic Biomarker for Metabolic Psychiatry?

Alterations in neurometabolism and mitochondria are implicated in the pathophysiology of psychiatric conditions such as mood disorders and schizophrenia. Thus, developing objective biomarkers related to brain mitochondrial function is crucial for the development of interventions, such as central nervous system penetrating agents that target brain health. Lactate, a major circulatory fuel source that can be produced and utilized by the brain and body, is presented as a theranostic biomarker for neurometabolic dysfunction in psychiatric conditions. This concept is based on three key properties of lactate that make it an intriguing metabolic intermediate with implications for this field: Firstly, the lactate response to various stimuli, including physiological or psychological stress, represents a quantifiable and dynamic marker that reflects metabolic and mitochondrial health. Second, lactate concentration in the brain is tightly regulated according to the sleep-wake cycle, the dysregulation of which is implicated in both metabolic and mood disorders. Third, lactate universally integrates arousal behaviours, pH, cellular metabolism, redox states, oxidative stress, and inflammation, and can signal and encode this information via intra- and extracellular pathways in the brain. In this review, we expand on the above properties of lactate and discuss the methodological developments and rationale for the use of functional magnetic resonance spectroscopy for in vivo monitoring of brain lactate. We conclude that accurate and dynamic assessment of brain lactate responses might contribute to the development of novel and personalized therapies that improve mitochondrial health in psychiatric disorders and other conditions associated with neurometabolic dysfunction.

Mitochondrial Ca2+ handling as a cell signaling hub: lessons from astrocyte function

Astrocytes are a heterogenous population of macroglial cells spread throughout the central nervous system with diverse functions, expression signatures, and intricate morphologies. Their subcellular compartments contain a distinct range of mitochondria, with functional microdomains exhibiting widespread activities, such as controlling local metabolism and Ca2+ signaling. Ca2+ is an ion of utmost importance, both physiologically and pathologically, and participates in critical central nervous system processes, including synaptic plasticity, neuron-astrocyte integration, excitotoxicity, and mitochondrial physiology and metabolism. The mitochondrial Ca2+ handling system is formed by the mitochondrial Ca2+ uniporter complex (MCUc), which mediates Ca2+ influx, and the mitochondrial Na+/Ca2+ exchanger (NCLX), responsible for most mitochondrial Ca2+ efflux, as well as additional components, including the mitochondrial permeability transition pore (mtPTP). Over the last decades, mitochondrial Ca2+ handling has been shown to be key for brain homeostasis, acting centrally in physiopathological processes such as astrogliosis, astrocyte-neuron activity integration, energy metabolism control, and neurodegeneration. In this review, we discuss the current state of knowledge regarding the mitochondrial Ca2+ handling system molecular composition, highlighting its impact on astrocytic homeostasis.

Imaging mitochondrial calcium dynamics in the central nervous system

Mitochondrial calcium handling is a particularly active research area in the neuroscience field, as it plays key roles in the regulation of several functions of the central nervous system, such as synaptic transmission and plasticity, astrocyte calcium signaling, neuronal activity… In the last few decades, a panel of techniques have been developed to measure mitochondrial calcium dynamics, relying mostly on photonic microscopy, and including synthetic sensors, hybrid sensors and genetically encoded calcium sensors. The goal of this review is to endow the reader with a deep knowledge of the historical and latest tools to monitor mitochondrial calcium events in the brain, as well as a comprehensive overview of the current state of the art in brain mitochondrial calcium signaling. We will discuss the main calcium probes used in the field, their mitochondrial targeting strategies, their key properties and major drawbacks. In addition, we will detail the main roles of mitochondrial calcium handling in neuronal tissues through an extended report of the recent studies using mitochondrial targeted calcium sensors in neuronal and astroglial cells, in vitro and in vivo.

Astrocyte Mitochondria in White-Matter Injury

This review summarizes the diverse structure and function of astrocytes to describe the bioenergetic versatility required of astrocytes that are situated at different locations. The intercellular domain of astrocyte mitochondria defines their roles in supporting and regulating astrocyte-neuron coupling and survival against ischemia. The heterogeneity of astrocyte mitochondria, and how subpopulations of astrocyte mitochondria adapt to interact with other glia and regulate axon function, require further investigation. It has become clear that mitochondrial permeability transition pores play a key role in a wide variety of human diseases, whose common pathology may be based on mitochondrial dysfunction triggered by Ca2+ and potentiated by oxidative stress. Reactive oxygen species cause axonal degeneration and a reduction in axonal transport, leading to axonal dystrophies and neurodegeneration including Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease, and Huntington's disease. Developing new tools to allow better investigation of mitochondrial structure and function in astrocytes, and techniques to specifically target astrocyte mitochondria, can help to unravel the role of mitochondrial health and dysfunction in a more inclusive context outside of neuronal cells. Overall, this review will assess the value of astrocyte mitochondria as a therapeutic target to mitigate acute and chronic injury in the CNS.

Trace amine-associated receptor 1 (TAAR1): Potential application in mood disorders: A systematic review

There is a need for innovation with respect to therapeutics in psychiatry. Available evidence indicates that the trace amine-associated receptor 1 (TAAR1) agonist SEP-363856 is promising, as it improves measures of cognitive and reward function in schizophrenia. Hedonic and cognitive impairments are transdiagnostic and constitute major burdens in mood disorders. Herein, we systematically review the behavioural and genetic literature documenting the role of TAAR1 in reward and cognitive function, and propose a mechanistic model of TAAR1's functions in the brain. Notably, TAAR1 activity confers antidepressant-like effects, enhances attention and response inhibition, and reduces compulsive reward seeking without impairing normal function. Further characterization of the responsible mechanisms suggests ion-homeostatic, metabolic, neurotrophic, and anti-inflammatory enhancements in the limbic system. Multiple lines of evidence establish the viability of TAAR1 as a biological target for the treatment of mood disorders. Furthermore, the evidence suggests a role for TAAR1 in reward and cognitive function, which is attributed to a cascade of events that are relevant to the cellular integrity and function of the central nervous system.

Nucleotide P2Y1 receptor agonists are in vitro and in vivo prodrugs of A1/A3 adenosine receptor agonists: implications for roles of P2Y1 and A1/A3 receptors in physiology and pathology

Rapid phosphoester hydrolysis of endogenous purine and pyrimidine nucleotides has challenged the characterization of the role of P2 receptors in physiology and pathology. Nucleotide phosphoester stabilization has been pursued on a number of medicinal chemistry fronts. We investigated the in vitro and in vivo stability and pharmacokinetics of prototypical nucleotide P2Y₁ receptor (P2Y₁R) agonists and antagonists. These included the riboside nucleotide agonist 2-methylthio-ADP and antagonist MRS2179, as well as agonist MRS2365 and antagonist MRS2500 containing constrained (N)-methanocarba rings, which were previously reported to form nucleotides that are more slowly hydrolyzed at the α-phosphoester compared with the ribosides. In vitro incubations in mouse and human plasma and blood demonstrated the rapid hydrolysis of these compounds to nucleoside metabolites. This metabolism was inhibited by EDTA to chelate divalent cations required by ectonucleotidases for nucleotide hydrolysis. This rapid hydrolysis was confirmed in vivo in mouse pharmacokinetic studies that demonstrate that MRS2365 is a prodrug of the nucleoside metabolite AST-004 (MRS4322). Furthermore, we demonstrate that the nucleoside metabolites of MRS2365 and 2-methylthio-ADP are adenosine receptor (AR) agonists, notably at A₃ and A₁ARs. In vivo efficacy of MRS2365 in murine models of traumatic brain injury and stroke can be attributed to AR activation by its nucleoside metabolite AST-004, rather than P2Y₁R activation. This research suggests the importance of reevaluation of previous in vitro and in vivo research of P2YRs and P2XRs as there is a potential that the pharmacology attributed to nucleotide agonists is due to AR activation by active nucleoside metabolites.

Neurotoxicity in pre-eclampsia involves oxidative injury, exacerbated cholinergic activity and impaired proteolytic and purinergic activities in cortex and cerebellum

Women with a history of pre-eclampsia (PE) tend to have a higher risk of developing cardiovascular and neurological diseases later in life. Imbalance in oxidative markers and purinergic enzymes have been implicated in the pathogenesis of neurological disease. This study investigated the effect of PE on oxidative imbalance, purinergic enzyme inhibitory activity, acetylcholinesterase and chymotrypsin activities in the brain of PE rat model at post-partum/post-natal day (PP/PND) 60. Pregnant rats divided into early-onset and late-onset groups were administered with Nω-nitro-l-arginine methyl through drinking water at gestational days 8-17. Rats were allowed free access to water throughout the pregnancy and allowed to deliver on their own. The mother and the pups were euthanized at PP and PND 60, respectively, the cortex and the cerebellum excised, homogenized and stored for analyses of the enzymes. Results showed an increase in nitric oxide and malondialdehyde with a concomitant decrease in reduced glutathione and superoxide dismutase, an indication of oxidative damage. Also, there was an increase in acetylcholinesterase activity with a decrease in chymotrypsin, adenylpyrophosphatase and ecto-nucleoside triphosphate diphosphohydrolase activities in both the cortex and the cerebellum of the mother and the pups at PND 60. These results indicate the involvement of oxidative stress, increased cholinergic activity and depleted proteolytic and purinergic activities in PE-induced neurotoxicity.

NADPH Oxidase Mediates Membrane Androgen Receptor-Induced Neurodegeneration

Oxidative stress (OS) is a common characteristic of several neurodegenerative disorders, including Parkinson disease (PD). PD is more prevalent in men than in women, indicating the possible involvement of androgens. Androgens can have either neuroprotective or neurodamaging effects, depending on the presence of OS. Specifically, in an OS environment, androgens via a membrane-associated androgen receptor (mAR) exacerbate OS-induced damage. To investigate the role of androgens on OS signaling and neurodegeneration, the effects of testosterone and androgen receptor activation on the major OS signaling cascades, the reduced form of NAD phosphate (NADPH) oxidase (NOX)1 and NOX2 and the Gαq/inositol trisphosphate receptor (InsP3R), were examined. To create an OS environment, an immortalized neuronal cell line was exposed to H2O2 prior to cell-permeable/cell-impermeable androgens. Different inhibitors were used to examine the role of G proteins, mAR, InsP3R, and NOX1/2 on OS generation and cell viability. Both testosterone and DHT/3-O-carboxymethyloxime (DHT)-BSA increased H2O2-induced OS and cell death, indicating the involvement of an mAR. Furthermore, classical AR antagonists did not block testosterone's negative effects in an OS environment. Because there are no known antagonists specific for mARs, an AR protein degrader, ASC-J9, was used to block mAR action. ASC-J9 blocked testosterone's negative effects. To determine OS-related signaling mediated by mAR, this study examined NOX1, NOX2, Gαq. NOX1, NOX2, and the Gαq complex with mAR. Only NOX inhibition blocked testosterone-induced cell loss and OS. No effects of blocking either Gαq or G protein activation were observed on testosterone's negative effects. These results indicate that androgen-induced OS is via the mAR-NOX complex and not the mAR-Gαq complex.

Hydrogen sulfide: Therapeutic or injurious in ischemic stroke?

Hydrogen sulfide (H2S) has been identified as a vasodilatory, neuromodulatory, and anti-inflammatory gasotransmitter with antioxidant properties. Studies focused in cardiac tissue suggest H2S functions as a protective agent; however in the central nervous system (CNS) the effects of H2S during states of stress or injury, such as stroke, remain controversial. Currently, the application of H2S donors and modulators in stroke depends on the type of H2S donor and the timing of the therapy.

Nucleotides regulate the common molecular mechanisms that underlie neurodegenerative diseases; Therapeutic implications

Neurodegenerative diseases (ND) are a heterogeneous group of neurological disorders characterized by a progressive loss of neuronal function which results in neuronal death. Although a specific toxic factor has been identified for each ND, all of them share common pathological molecular mechanisms favouring the disease development. In the final stages of ND, patients become unable to take care of themselves and decline to a total functional incapacitation that leads to their death. Some of the main factors which contribute to the disease progression include proteasomal dysfunction, neuroinflammation, synaptic alterations, protein aggregation, and oxidative stress. Over recent years, evidence has been accumulated to suggest that purinergic signaling plays a key role in the aforementioned molecular pathways. In this review, we revise the implications of the purinergic signaling in the common molecular mechanism underlying the ND. In particular, we focus on the role of the purinergic receptors P2X7, P2Y₂ and the ectoenzyme tissue-nonspecific alkaline phosphatase (TNAP).

Mitochondrial Dysfunction in Stroke: Implications of Stem Cell Therapy

Stroke is a debilitating condition which is also the second leading cause of death and disability worldwide. Despite the benefits and promises shown by numerous neuroprotective agents in animal stroke models, their clinical translation has not been a complete success. Hence, search for treatment options have directed researchers towards utilising stem cells. Mitochondria has a major involvement in the pathophysiology of stroke and a number of other conditions. Stem cells have shown the ability to transfer mitochondria to the damaged cells and to help revive cell energetics in the recipient cell. The present review discusses how stem cells could be employed to protect neurons and mitochondria in stroke and also the various mechanisms involved in neuroprotection.

Membrane cholesterol mediates the cellular effects of monolayer graphene substrates

Graphene possesses extraordinary properties that promise great potential in biomedicine. However, fully leveraging these properties requires close contact with the cell surface, raising the concern of unexpected biological consequences. Computational models have demonstrated that graphene preferentially interacts with cholesterol, a multifunctional lipid unique to eukaryotic membranes. Here we demonstrate an interaction between graphene and cholesterol. We find that graphene increases cell membrane cholesterol and potentiates neurotransmission, which is mediated by increases in the number, release probability, and recycling rate of synaptic vesicles. In fibroblasts grown on graphene, we also find an increase in cholesterol, which promotes the activation of P2Y receptors, a family of receptor regulated by cholesterol. In both cases, direct manipulation of cholesterol levels elucidates that a graphene-induced cholesterol increase underlies the observed potentiation of each cell signaling pathway. These findings identify cholesterol as a mediator of graphene’s cellular effects, providing insight into the biological impact of graphene.

Understanding the biological role of graphene in eukaryotic cells is essential for future biomedicine applications. Here, the authors investigate the interaction of neurons and fibroblasts with graphene substrates, which increase cell membrane cholesterol and potentiate neurotransmitter release and receptor signaling.

Mitochondrial dysfunction in glial cells: Implications for neuronal homeostasis and survival

Mitochondrial dysfunction is central to the pathogenesis of neurological disorders. Neurons rely on oxidative phosphorylation to meet their energy requirements and thus alterations in mitochondrial function are linked to energy failure and neuronal cell death. Furthermore, in neurons, dysfunctional mitochondria are reported to increase the steady-state levels of reactive oxygen species derived from the leakage of electrons from the electron transport chain. Research aimed at understanding mitochondrial dysfunction and its role in neurological disorders has been primarily geared towards neurons. In contrast, the effects of mitochondrial dysfunction in glial cells' function and its implication for neuronal homeostasis and brain function has been largely understudied. Unlike neurons and oligodendrocytes, astrocytes and microglia do not degenerate upon the impairment of mitochondrial function, as they rely primarily on glycolysis to produce energy and have a higher antioxidant capacity than neurons. However, recent evidence highlights the role of mitochondrial metabolism and signaling in glial cell function. In this work, we review the functional role of mitochondria in glial cells and the evidence regarding its potential role regulating neuronal homeostasis and disease progression.

Local energy on demand: Are 'spontaneous' astrocytic Ca2+-microdomains the regulatory unit for astrocyte-neuron metabolic cooperation?

Astrocytes are a neural cell type critically involved in maintaining brain energy homeostasis as well as signaling. Like neurons, astrocytes are a heterogeneous cell population. Cortical astrocytes show a complex morphology with a highly branched aborization and numerous fine processes ensheathing the synapses of neighboring neurons, and typically extend one process connecting to blood vessels. Recent studies employing genetically encoded fluorescent calcium (Ca²⁺) indicators have described 'spontaneous' localized Ca²⁺-transients in the astrocyte periphery that occur asynchronously, independently of signals in other parts of the cells, and that do not involve somatic Ca²⁺ transients; however, neither it is known whether these Ca²⁺-microdomains occur at or near neuronal synapses nor have their molecular basis nor downstream effector(s) been identified. In addition to Ca²⁺ microdomains, sodium (Na⁺) transients occur in astrocyte subdomains, too, most likely as a consequence of Na⁺ co-transport with the neurotransmitter glutamate, which also regulates mitochondrial movements locally - as do cytoplasmic Ca²⁺ levels. In this review, we cover various aspects of these local signaling events and discuss how structural and biophysical properties of astrocytes might foster such compartmentation. Astrocytes metabolically interact with neurons by providing energy substrates to active neurons. As a single astrocyte branch covers hundreds to thousands of synapses, it is tempting to speculate that these metabolic interactions could occur localized to specific subdomains of astrocytes, perhaps even at the level of small groups of synapses. We discuss how astrocytic metabolism might be regulated at this scale and which signals might contribute to its regulation. We speculate that the astrocytic structures that light up transiently as Ca²⁺-microdomains might be the functional units of astrocytes linking signaling and metabolic processes to adapt astrocytic function to local energy demands. The understanding of these local regulatory and metabolic interactions will be fundamental to fully appreciate the complexity of brain energy homeostasis as well as its failure in disease and may shed new light on the controversy about neuron-glia bi-directional signaling at the tripartite synapse.

Purinergic P2Y6 receptors: A new therapeutic target of age-dependent hypertension

Aging has a remarkable effect on cardiovascular homeostasis and it is known as the major non-modifiable risk factor in the development of hypertension. Medications targeting sympathetic nerve system and/or renin-angiotensin-aldosterone system are widely accepted as a powerful therapeutic strategy to improve hypertension, although the control rates remain unsatisfactory especially in the elder patients with hypertension. Purinergic receptors, activated by adenine, uridine nucleotides and nucleotide sugars, play pivotal roles in many biological processes, including platelet aggregation, neurotransmission and hormone release, and regulation of cardiovascular contractility. Since clopidogrel, a selective inhibitor of G protein-coupled purinergic P2Y₁₂ receptor (P2Y₁₂R), achieved clinical success as an anti-platelet drug, P2YRs has been attracted more attention as new therapeutic targets of cardiovascular diseases. We have revealed that UDP-responsive P2Y₆R promoted angiotensin type 1 receptor (AT1R)-stimulated vascular remodeling in mice, in an age-dependent manner. Moreover, the age-related formation of heterodimer between AT1R and P2Y₆R was disrupted by MRS2578, a P2Y₆R-selective inhibitor. These findings suggest that P2Y₆R is a therapeutic target to prevent age-related hypertension.

Activity-dependent modulation of intracellular ATP in cultured cortical astrocytes

Brain function is absolutely dependent on an appropriate supply of energy. A shortfall in supply-as occurs, for instance, following stroke-can lead rapidly to irreversible damage to this vital organ. While the consequences of pathophysiological energy depletion have been well documented, much less is known about the physiological energy dynamics of brain cells, although changes in the intracellular concentration of adenosine triphosphate (ATP), the major energy carrier of cells, have been postulated to contribute to cellular signaling. To address this issue more closely, we have investigated intracellular ATP in cultured primary cortical astrocytes by time-lapse microscopy using a genetically encoded fluorescent sensor for ATP. The cytosolic ATP sensor signal decreased after application of the neurotransmitter glutamate in a manner dependent on both glutamate concentration and glutamate transporter activity, but independent of glutamate receptors. The application of dopamine did not affect ATP levels within astrocytes. These results confirm that intracellular ATP levels in astrocytes do indeed respond to changes in physiological activity and pave the way for further studies addressing factors that affect regulation of ATP. © 2017 Wiley Periodicals, Inc.

Stimulation of astrocyte fatty acid oxidation by thyroid hormone is protective against ischemic stroke-induced damage

We previously demonstrated that stimulation of astrocyte mitochondrial ATP production via P2Y₁ receptor agonists was neuroprotective after cerebral ischemic stroke. Another mechanism that increases ATP production is fatty acid oxidation (FAO). We show that in primary human astrocytes, FAO and ATP production are stimulated by 3,3,5 triiodo-l-thyronine (T3). We tested whether T3-stimulated FAO enhances neuroprotection, and show that T3 increased astrocyte survival after either hydrogen peroxide exposure or oxygen glucose deprivation. T3-mediated ATP production and protection were both eliminated with etomoxir, an inhibitor of FAO. T3-mediated protection in vitro was also dependent on astrocytes expressing HADHA (hydroxyacyl-CoA dehydrogenase/3-ketoacyl-CoA thiolase/enoyl-CoA hydratase), which we previously showed was critical for T3-mediated FAO in fibroblasts. Consistent with previous reports, T3-treatment decreased stroke volumes in mice. While T3 decreased stroke volume in etomoxir-treated mice, T3 had no protective effect on stroke volume in HADHA +/- mice or in mice unable to upregulate astrocyte-specific energy production. In vivo, 95% of HADHA co-localize with glial-fibrillary acidic protein, suggesting the effect of HADHA is astrocyte mediated. These results suggest that astrocyte-FAO modulates lesion size and is required for T3-mediated neuroprotection post-stroke. To our knowledge, this is the first report of a neuroprotective role for FAO in the brain.

Glutamate and ATP at the Interface Between Signaling and Metabolism in Astroglia: Examples from Pathology

Glutamate is the main excitatory transmitter in the brain, while ATP represents the most important energy currency in any living cell. Yet, these chemicals play an important role in both processes, enabling them with dual-acting functions in metabolic and intercellular signaling pathways. Glutamate can fuel ATP production, while ATP can act as a transmitter in intercellular signaling. We discuss the interface between glutamate and ATP in signaling and metabolism of astrocytes. Not only do glutamate and ATP cross each other's paths in physiology of the brain, but they also do so in its pathology. We present the fabric of this process in (patho)physiology through the discussion of synthesis and metabolism of ATP and glutamate in astrocytes as well as by providing a general description of astroglial receptors for these molecules along with the downstream signaling pathways that may be activated. It is astroglial receptors for these dual-acting molecules that could hold a key for medical intervention in pathological conditions. We focus on two examples disclosing the role of activation of astroglial ATP and glutamate receptors in pathology of two kinds of brain tissue, gray matter and white matter, respectively. Interventions at the interface of metabolism and signaling show promise for translational medicine.

1,3-Dinitrobenzene neurotoxicity - Passage effect in immortalized astrocytes

Age-related disturbances in astrocytic mitochondrial function are linked to loss of neuroprotection and decrements in neurological function. The immortalized rat neocortical astrocyte-derived cell line, DI-TNC1, provides a convenient model for the examination of cellular aging processes that are difficult to study in primary cell isolates from aged brain. Successive passages in culture may serve as a surrogate of aging in which time-dependent adaptation to culture conditions may result in altered responses to xenobiotic challenge. To investigate the hypothesis that astrocytic mitochondrial homeostatic function is decreased with time in culture, low passage DI-TNC1 astrocytes (LP; #2-8) and high passage DI-TNC1 astrocytes (HP; #17-28) were exposed to the mitochondrial neurotoxicant 1,3-dinitrobenzene (DNB). Cells were exposed in either monoculture or in co-culture with primary cortical neurons. Astrocyte mitochondrial membrane potential, morphology, ATP production and proliferation were monitored in monoculture, and the ability of DI-TNC1 cells to buffer K(+)-induced neuronal depolarization was examined in co-cultures. In HP DI-TNC1 cells, DNB exposure decreased proliferation, reduced mitochondrial membrane potential and significantly decreased mitochondrial form factor. Low passage DI-TNC1 cells effectively attenuated K(+)-induced neuronal depolarization in the presence of DNB whereas HP counterparts were unable to buffer K(+) in DNB challenge. Following DNB challenge, LP DI-TNC1 cells exhibited greater viability in co-culture than HP. The data provide compelling evidence that there is an abrupt phenotypic change in DI-TNC1 cells between passage #9-16 that significantly diminishes the ability of DI-TNC1 cells to compensate for neurotoxic challenge and provide neuroprotective spatial buffering. Whether or not these functional changes have an in vivo analog in aging brain remains to be determined.

Astrocytes, therapeutic targets for neuroprotection and neurorestoration in ischemic stroke

Astrocytes are the most abundant cell type within the central nervous system. They play essential roles in maintaining normal brain function, as they are a critical structural and functional part of the tripartite synapses and the neurovascular unit, and communicate with neurons, oligodendrocytes and endothelial cells. After an ischemic stroke, astrocytes perform multiple functions both detrimental and beneficial, for neuronal survival during the acute phase. Aspects of the astrocytic inflammatory response to stroke may aggravate the ischemic lesion, but astrocytes also provide benefit for neuroprotection, by limiting lesion extension via anti-excitotoxicity effects and releasing neurotrophins. Similarly, during the late recovery phase after stroke, the glial scar may obstruct axonal regeneration and subsequently reduce the functional outcome; however, astrocytes also contribute to angiogenesis, neurogenesis, synaptogenesis, and axonal remodeling, and thereby promote neurological recovery. Thus, the pivotal involvement of astrocytes in normal brain function and responses to an ischemic lesion designates them as excellent therapeutic targets to improve functional outcome following stroke. In this review, we will focus on functions of astrocytes and astrocyte-mediated events during stroke and recovery. We will provide an overview of approaches on how to reduce the detrimental effects and amplify the beneficial effects of astrocytes on neuroprotection and on neurorestoration post stroke, which may lead to novel and clinically relevant therapies for stroke.

Effects of IP3R2 Receptor Deletion in the Ischemic Mouse Retina

Glial cells in the diseased nervous system undergo a process known as reactive gliosis. Gliosis of retinal Müller glial cells is characterized by an upregulation of glial fibrillary acidic protein and frequently by a reduction of inward K(+) current amplitudes. Purinergic signaling is assumed to be involved in gliotic processes. As previously shown, lack of the nucleotide receptor P2Y1 leads to an altered regulation of K(+) currents in Müller cells of the ischemic retina. Here, we asked first whether this effect is mediated by the IP3 receptor subtype 2 (IP3R2) known as the major downstream signaling target of P2Y1 in Müller cells. The second question was whether lack of IP3R2 affects neuronal survival in the control and ischemic retina. Ischemia was induced in wild type and IP3R2-deficient (IP 3 R2 (-/-)) mice by transient elevation of the intraocular pressure. Immunostaining and TUNEL labelling were used to quantify neuronal cell loss. The downregulation of inward K(+) currents in Müller cells from ischemic IP 3 R2 (-/-) retinae was less strong than in wild type animals. The reduction of the number of cells in the ganglion cell layer and of calretinin- and calbindin-positive cells 7 days after ischemia was similar in wild type and IP 3 R2 (-/-) mice. However, IP3R2 deficiency led to an increased number of TUNEL-positive cells in the outer nuclear layer at 1 day and to an enhanced postischemic loss of photoreceptors 7 days after ischemia. This implies that IP3R2 is involved in some but not all aspects of signaling in Müller cells after an ischemic insult.

mGluR5 in the nucleus accumbens shell regulates morphine-associated contextual memory through reactive oxygen species signaling

Emerging evidence indicates that metabotropic glutamate receptor 5 (mGluR5) critically modulates drug and drug-related behaviors. However, the role of mGluR5 in the opiate-induced contextual memory remains unclear. Here, we found that microinfusion of the mGluR5 antagonist 3-((2-Methyl-1,3-thiazol-4-yl)ethynyl)pyridine (MTEP) into the nucleus accumbens (NAc) shell, but not into the core, significantly attenuated the expression of morphine conditioned place preference (CPP) in rats. Following the expression of morphine CPP, the protein level of membrane mGluR5 was selectively increased in the NAc shell. In primary striatal neurons, we observed that treatment with the mGluR5 agonist CHPG increased the phosphorylation level of extracellular signal-regulated kinase (ERK), which was dependent on the mGluR5-inositol-1,4,5-trisphosphate-reactive oxygen species (ROS) pathway. Moreover, the microinjection of the ROS scavenger Tempol into the NAc shell of rats blocked the expression of morphine CPP. Further, the administration of t-BOOH, a ROS donor, into the NAc shell rescued the retrieval impairment of morphine CPP produced by MTEP. Our previous study demonstrated that the expression of morphine CPP increased the phosphorylation of ERK selectively in the NAc shell. Thus, results of the present study suggest that mGluR5 in the NAc shell, but not in the core, is essential for the retrieval of morphine contextual memory, which is mediated at least in part, through the ROS/ERK signaling pathway. Uncovering the molecular basis of opiate contextual memory will benefit the development of new therapeutic approaches for the treatment of opiate addiction.

Purinergic receptor stimulation decreases ischemic brain damage by energizing astrocyte mitochondria

As a leading cause of death in the world, cerebral ischemic stroke has limited treatment options. The lack of glucose and oxygen after stroke is particularly harmful in the brain because neuronal metabolism accounts for significantly more energy consumption per gram of body weight compared to other organs. Our laboratory has identified mitochondrial metabolism of astrocytes to be a key target for pharmacologic intervention, not only because astrocytes play a central role in regulating brain metabolism, but also because they are essential for neuronal health and support. Here we review current literature pertaining to the pathobiology of stroke, along with the role of astrocytes and metabolism in stroke. We also discuss our research, which has revealed that pharmacologic stimulation of metabotropic P2Y1 receptor signaling in astrocytes can increase mitochondrial energy production and also reduce damage after stroke.

Modeling the neuroprotective role of enhanced astrocyte mitochondrial metabolism during stroke

A mathematical model that integrates the dynamics of cell membrane potential, ion homeostasis, cell volume, mitochondrial ATP production, mitochondrial and endoplasmic reticulum Ca(2+) handling, IP3 production, and GTP-binding protein-coupled receptor signaling was developed. Simulations with this model support recent experimental data showing a protective effect of stimulating an astrocytic GTP-binding protein-coupled receptor (P2Y1Rs) following cerebral ischemic stroke. The model was analyzed to better understand the mathematical behavior of the equations and to provide insights into the underlying biological data. This approach yielded explicit formulas determining how changes in IP3-mediated Ca(2+) release, under varying conditions of oxygen and the energy substrate pyruvate, affected mitochondrial ATP production, and was utilized to predict rate-limiting variables in P2Y1R-enhanced astrocyte protection after cerebral ischemic stroke.

Preventive effect of nucleoprotein on hindlimb unloading-induced capillary regression in rat soleus muscle

We investigated the preventive effects of nucleoprotein on capillary regression and mitochondrial dysfunction induced by unloading in the soleus muscle of rats. Nucleoprotein is a supplement made from soft roe of salmon, and its major components are nucleotides and protamine. Adult male Sprague-Dawley rats were divided randomly into control, hindlimb unloading (HU), and hindlimb unloading plus nucleoprotein administration (HU+ NP) groups. Hindlimb unloading was carried out for 2 weeks in the rats belonging to the HU and the HU+ NP groups. The rats of the HU+ NP group were administered nucleoprotein (500 mg/kg) using a feeding needle twice a day for 2 weeks. Hindlimb unloading resulted in capillary regression, decreased succinate dehydrogenase activity of the muscle fiber, and decreased PGC-1α expression in the soleus muscle. These effects were prevented by administration of nucleoprotein. Nucleoprotein appears to prevent capillary regression and mitochondrial dysfunction caused by unloading of the skeletal muscle. Therefore, nucleoprotein supplementation may be an effective therapy for maintaining capillary network and mitochondrial metabolism of the muscle fiber during an unloading period.

Purinergic 2Y1 receptor stimulation decreases cerebral edema and reactive gliosis in a traumatic brain injury model

Traumatic brain injury (TBI) is the leading cause of death and disability in children and young adults. Neuroprotective agents that may promote repair or counteract damage after injury do not currently exist. We recently reported that stimulation of the purinergic receptor subtype P2Y(1)R using 2-methylthioladenosine 5' diphosphate (2MeSADP) significantly reduced cytotoxic edema induced by photothrombosis. Here, we tested whether P2Y(1)R stimulation was neuroprotective after TBI. A controlled closed head injury model was established for mice using a pneumatic impact device. Brains were harvested at 1, 3, or 7 days post-injury and assayed for morphological changes by immunocytochemistry, Western blot analysis, and wet/dry weight. Cerebral edema and expression of both aquaporin type 4 and glial fibrillary acidic protein were increased at all time points examined. Immunocytochemical measurements in both cortical and hippocampal slices also revealed significant neuronal swelling and reactive gliosis. Treatment of mice with 2MeSADP (100 μM) or MRS2365 (100 μM) 30 min after trauma significantly reduced all post-injury symptoms of TBI including edema, neuronal swelling, reactive gliosis, and AQ4 expression. The neuroprotective effect was lost in IP(3)R2-/- mice treated with 2MeSADP. Immunocytochemical labeling of brain slices confirmed that P2Y(1)R expression was defined to cortical and hippocampal astrocytes, but not neurons. Taken together, the data show that stimulation of astrocytic P2Y(1)Rs significantly reduces brain injury after acute trauma and is mediated by the IP(3)-signaling pathway. We suggest that enhancing astrocyte mitochondrial metabolism offers a promising neuroprotective strategy for a broad range of brain injuries.

Stroke neuroprotection: targeting mitochondria

Stroke is the fourth leading cause of death and the leading cause of long-term disability in the United States. Blood flow deficit results in an expanding infarct core with a time-sensitive peri-infarct penumbra that is considered salvageable and is the primary target for treatment strategies. The only current FDA-approved drug for treating ischemic stroke is recombinant tissue plasminogen activator (rt-PA). However, this treatment is limited to within 4.5 h of stroke onset in a small subset of patients. The goal of this review is to focus on mitochondrial-dependent therapeutic agents that could provide neuroprotection following stroke. Dysfunctional mitochondria are linked to neurodegeneration in many disease processes including stroke. The mechanisms reviewed include: (1) increasing ATP production by purinergic receptor stimulation, (2) decreasing the production of ROS by superoxide dismutase, or (3) increasing antioxidant defenses by methylene blue, and their benefits in providing neuroprotection following a stroke.

P2Y1R-initiated, IP3R-dependent stimulation of astrocyte mitochondrial metabolism reduces and partially reverses ischemic neuronal damage in mouse

Glia-based neuroprotection strategies are emerging as promising new avenues to treat brain damage. We previously reported that activation of the glial-specific purinergic receptor, P2Y(1)R, reduces both astrocyte swelling and brain infarcts in a photothrombotic mouse model of stroke. These restorative effects were dependent on astrocyte mitochondrial metabolism. Here, we extend these findings and report that P2Y(1)R stimulation with the purinergic ligand 2-methylthioladenosine 5' diphosphate (2MeSADP) reduces and partially reverses neuronal damage induced by photothrombosis. In vivo neuronal morphology was confocally imaged in transgenic mice expressing yellow fluorescent protein under the control of the Thy1 promoter. Astrocyte mitochondrial membrane potentials, monitored with the potential sensitive dye tetra-methyl rhodamine methyl ester, were depolarized after photothrombosis and subsequently repolarized when P2Y(1)Rs were stimulated. Mice deficient in the astrocyte-specific type 2 inositol 1,4,5 trisphosphate (IP(3)) receptor exhibited aggravated ischemic dendritic damage after photothrombosis. Treatment of these mice with 2MeSADP did not invoke an intracellular Ca(2+) response, did not repolarize astrocyte mitochondria, and did not reduce or partially reverse neuronal lesions induced by photothrombotic stroke. These results demonstrate that IP(3)-Ca(2+) signaling in astrocytes is not only critical for P2Y(1)R-enhanced protection, but suggest that IP(3)-Ca(2+) signaling is also a key component of endogenous neuroprotection.

Pathophysiology of astroglial purinergic signalling

Astrocytes are fundamental for central nervous system (CNS) physiology and are the fulcrum of neurological diseases. Astroglial cells control development of the nervous system, regulate synaptogenesis, maturation, maintenance and plasticity of synapses and are central for nervous system homeostasis. Astroglial reactions determine progression and outcome of many neuropathologies and are critical for regeneration and remodelling of neural circuits following trauma, stroke, ischaemia or neurodegenerative disorders. They secrete multiple neurotransmitters and neurohormones to communicate with neurones, microglia and the vascular walls of capillaries. Signalling through release of ATP is the most widespread mean of communication between astrocytes and other types of neural cells. ATP serves as a fast excitatory neurotransmitter and has pronounced long-term (trophic) roles in cell proliferation, growth, and development. During pathology, ATP is released from damaged cells and acts both as a cytotoxic factor and a proinflammatory mediator, being a universal "danger" signal. In this review, we summarise contemporary knowledge on the role of purinergic receptors (P2Rs) in a variety of diseases in relation to changes of astrocytic functions and nucleotide signalling. We have focussed on the role of the ionotropic P2X and metabotropic P2YRs working alone or in concert to modify the release of neurotransmitters, to activate signalling cascades and to change the expression levels of ion channels and protein kinases. All these effects are of great importance for the initiation, progression and maintenance of astrogliosis-the conserved and ubiquitous glial defensive reaction to CNS pathologies. We highlighted specific aspects of reactive astrogliosis, especially with respect to the involvement of the P2X(7) and P2Y(1)R subtypes. Reactive astrogliosis exerts both beneficial and detrimental effects in a context-specific manner determined by distinct molecular signalling cascades. Understanding the role of purinergic signalling in astrocytes is critical to identifying new therapeutic principles to treat acute and chronic neurological diseases.

Ca²⁺ signals of astrocytes are modulated by the NAD⁺/NADH redox state

Astrocytes are important glial cells in the brain providing metabolic support to neurons as well as contributing to brain signaling. These different functional levels have to be highly coordinated to allow for proper cell and brain function. In this study, we show that in astrocytes the NAD(+) /NADH redox state modulates dopamine-induced Ca(2+) signals thereby connecting metabolism and Ca(2+) signaling. Application of dopamine induced a dose-dependent increase in Ca(2+) signal frequency in these cells, which was dependent on D(1) -receptor signaling, glycolytic activity, an increase in cytosolic NADH and inositol 1,4,5-triphosphate receptor operated intracellular Ca(2+) stores. Application of dopamine at a low concentration (1 μM) did not induce an increase in Ca(2+) signal frequency by itself. However, simultaneously increasing cytosolic NADH content either by direct application of NADH or by application of lactate resulted in a pronounced increase in Ca(2+) signal frequency. This increase could be blocked by co-application of pyruvate, suggesting that indeed the NAD(+) /NADH redox state is regulating Ca(2+) signals. We conclude that at the NAD(+) /NADH redox state metabolic and signaling information is integrated in astrocytes, thereby most likely contributing to precisely coordinate these different tasks of astrocytes.

Inhibition of the Ca²⁺-dependent K⁺ channel, KCNN4/KCa3.1, improves tissue protection and locomotor recovery after spinal cord injury

Spinal cord injury (SCI) triggers inflammatory responses that involve neutrophils, macrophages/microglia and astrocytes and molecules that potentially cause secondary tissue damage and functional impairment. Here, we assessed the contribution of the calcium-dependent K⁺ channel KCNN4 (KCa3.1, IK1, SK4) to secondary damage after moderate contusion lesions in the lower thoracic spinal cord of adult mice. Changes in KCNN4 mRNA levels (RT-PCR), KCa3.1 protein expression (Western blots), and cellular expression (immunofluorescence) in the mouse spinal cord were monitored between 1 and 28 d after SCI. KCNN4 mRNA and KCa3.1 protein rapidly increased after SCI; double labeling identified astrocytes as the main cellular source accounting for this upregulation. Locomotor function after SCI, evaluated for 28 d in an open-field test using the Basso Mouse Scale, was improved in a dose-dependent manner by treating mice with a selective inhibitor of KCa3.1 channels, TRAM-34 (triarylmethane-34). Improved locomotor function was accompanied by reduced tissue loss at 28 d and increased neuron and axon sparing. The rescue of tissue by TRAM-34 treatment was preceded by reduced expression of the proinflammatory mediators, tumor necrosis factor-α and interleukin-1β in spinal cord tissue at 12 h after injury, and reduced expression of inducible nitric oxide synthase at 7 d after SCI. In astrocytes in vitro, TRAM-34 inhibited Ca²⁺ signaling in response to metabotropic purinergic receptor stimulation. These results suggest that blocking the KCa3.1 channel could be a potential therapeutic approach for treating secondary damage after spinal cord injury.

Endoplasmic reticulum Ca(2+) handling in excitable cells in health and disease

The endoplasmic reticulum (ER) is a morphologically and functionally diverse organelle capable of integrating multiple extracellular and internal signals and generating adaptive cellular responses. It plays fundamental roles in protein synthesis and folding and in cellular responses to metabolic and proteotoxic stress. In addition, the ER stores and releases Ca(2+) in sophisticated scenarios that regulate a range of processes in excitable cells throughout the body, including muscle contraction and relaxation, endocrine regulation of metabolism, learning and memory, and cell death. One or more Ca(2+) ATPases and two types of ER membrane Ca(2+) channels (inositol trisphosphate and ryanodine receptors) are the major proteins involved in ER Ca(2+) uptake and release, respectively. There are also direct and indirect interactions of ER Ca(2+) stores with plasma membrane and mitochondrial Ca(2+)-regulating systems. Pharmacological agents that selectively modify ER Ca(2+) release or uptake have enabled studies that revealed many different physiological roles for ER Ca(2+) signaling. Several inherited diseases are caused by mutations in ER Ca(2+)-regulating proteins, and perturbed ER Ca(2+) homeostasis is implicated in a range of acquired disorders. Preclinical investigations suggest a therapeutic potential for use of agents that target ER Ca(2+) handling systems of excitable cells in disorders ranging from cardiac arrhythmias and skeletal muscle myopathies to Alzheimer disease.

IP(3) Receptors, Mitochondria, and Ca Signaling: Implications for Aging

The tight interplay between endoplasmic-reticulum-(ER-) and mitochondria-mediated Ca(2+) signaling is a key determinant of cellular health and cellular fate through the control of apoptosis and autophagy. Proteins that prevent or promote apoptosis and autophagy can affect intracellular Ca(2+) dynamics and homeostasis through binding and modulation of the intracellular Ca(2+)-release and Ca(2+)-uptake mechanisms. During aging, oxidative stress becomes an additional factor that affects ER and mitochondrial function and thus their role in Ca(2+) signaling. Importantly, mitochondrial dysfunction and sustained mitochondrial damage are likely to underlie part of the aging process. In this paper, we will discuss the different mechanisms that control intracellular Ca(2+) signaling with respect to apoptosis and autophagy and review how these processes are affected during aging through accumulation of reactive oxygen species.

Mitochondrial reactive oxygen species are activated by mGluR5 through IP3 and activate ERK and PKA to increase excitability of amygdala neurons and pain behavior

Reactive oxygen species (ROS) such as superoxide are emerging as important signaling molecules in physiological plasticity but also in peripheral and spinal cord pain pathology. Underlying mechanisms and pain-related ROS signaling in the brain remain to be determined. Neuroplasticity in the amygdala plays a key role in emotional-affective pain responses and depends on group I metabotropic glutamate receptors (mGluRs) and protein kinases. Using patch-clamp, live-cell imaging, and behavioral assays, we tested the hypothesis that mitochondrial ROS links group I mGluRs to protein kinase activation to increase neuronal excitability and pain behavior. Agonists for mGluR1/5 (DHPG) or mGluR5 (CHPG) increased neuronal excitability of neurons in the laterocapsular division of the central nucleus of the amygdala (CeLC). DHPG effects were inhibited by an mGluR5 antagonist (MTEP), IP(3) receptor blocker (xestospongin C), or ROS scavengers (PBN, tempol), but not by an mGluR1 antagonist (LY367385) or NO synthase inhibitor (l-NAME). Tempol inhibited the effects of IP(3) but not those of a PKC activator, indicating that ROS activation was IP(3) mediated. Live-cell imaging in CeLC-containing brain slices directly showed DHPG-induced and synaptically evoked mitochondrial superoxide production. DHPG also increased pain-related vocalizations and spinal reflexes through a mechanism that required mGluR5, IP(3), and ROS. Combined application of inhibitors of ERK (U0126) and PKA (KT5720) was necessary to block completely the excitatory effects of a ROS donor (tBOOH). A PKC inhibitor (GF109203X) had no effect. Antagonists and inhibitors alone did not affect neuronal excitability. The results suggest an important role for the novel mGluR5- IP(3)-ROS-ERK/PKA signaling pathway in amygdala pain mechanisms.

P2 receptor signaling in neurons and glial cells of the central nervous system

Purine and pyrimidine nucleotides are extracellular signaling molecules in the central nervous system (CNS) leaving the intracellular space of various CNS cell types via nonexocytotic mechanisms. In addition, ATP is a neuro-and gliotransmitter released by exocytosis from neurons and neuroglia. These nucleotides activate P2 receptors of the P2X (ligand-gated cationic channels) and P2Y (G protein-coupled receptors) types. In mammalians, seven P2X and eight P2Y receptor subunits occur; three P2X subtypes form homomeric or heteromeric P2X receptors. P2Y subtypes may also hetero-oligomerize with each other as well as with other G protein-coupled receptors. P2X receptors are able to physically associate with various types of ligand-gated ion channels and thereby to interact with them. The P2 receptor homomers or heteromers exhibit specific sensitivities against pharmacological ligands and have preferential functional roles. They may be situated at both presynaptic (nerve terminals) and postsynaptic (somatodendritic) sites of neurons, where they modulate either transmitter release or the postsynaptic sensitivity to neurotransmitters. P2 receptors exist at neuroglia (e.g., astrocytes, oligodendrocytes) and microglia in the CNS. The neuroglial P2 receptors subserve the neuron-glia cross talk especially via their end-feets projecting to neighboring synapses. In addition, glial networks are able to communicate through coordinated oscillations of their intracellular Ca(2+) over considerable distances. P2 receptors are involved in the physiological regulation of CNS functions as well as in its pathophysiological dysregulation. Normal (motivation, reward, embryonic and postnatal development, neuroregeneration) and abnormal regulatory mechanisms (pain, neuroinflammation, neurodegeneration, epilepsy) are important examples for the significance of P2 receptor-mediated/modulated processes.

Role of purinergic receptors in CNS function and neuroprotection

The purinergic receptor family contains some of the most abundant receptors in living organisms. A growing body of evidence indicates that extracellular nucleotides play important roles in the regulation of neuronal and glial functions in the nervous system through purinergic receptors. Nucleotides are released from or leaked through nonexcitable cells and neurons during normal physiological and pathophysiological conditions. Ionotropic P2X and metabotropic P2Y purinergic receptors are expressed in the central nervous system (CNS), participate in the synaptic processes, and mediate intercellular communications between neuron and gila and between glia and other glia. Glial cells in the CNS are classified into astrocytes, oligodendrocytes, and microglia. Astrocytes express many types of purinergic receptors, which are integral to their activation. Astrocytes release adenosine triphosphate (ATP) as a "gliotransmitter" that allows communication with neurons, the vascular walls of capillaries, oligodendrocytes, and microglia. Oligodendrocytes are myelin-forming cells that construct insulating layers of myelin sheets around axons, and using purinergic receptor signaling for their development and for myelination. Microglia also express many types of purinergic receptors and are known to function as immunocompetent cells in the CNS. ATP and other nucleotides work as "warning molecules" especially by activating microglia in pathophysiological conditions. Studies on purinergic signaling could facilitate the development of novel therapeutic strategies for disorder of the CNS.

Purinergic receptor stimulation reduces cytotoxic edema and brain infarcts in mouse induced by photothrombosis by energizing glial mitochondria

Treatments to improve the neurological outcome of edema and cerebral ischemic stroke are severely limited. Here, we present the first in vivo single cell images of cortical mouse astrocytes documenting the impact of single vessel photothrombosis on cytotoxic edema and cerebral infarcts. The volume of astrocytes expressing green fluorescent protein (GFP) increased by over 600% within 3 hours of ischemia. The subsequent growth of cerebral infarcts was easily followed as the loss of GFP fluorescence as astrocytes lysed. Cytotoxic edema and the magnitude of ischemic lesions were significantly reduced by treatment with the purinergic ligand 2-methylthioladenosine 5' diphosphate (2-MeSADP), an agonist with high specificity for the purinergic receptor type 1 isoform (P2Y(1)R). At 24 hours, cytotoxic edema in astrocytes was still apparent at the penumbra and preceded the cell lysis that defined the infarct. Delayed 2MeSADP treatment, 24 hours after the initial thrombosis, also significantly reduced cytotoxic edema and the continued growth of the brain infarction. Pharmacological and genetic evidence are presented indicating that 2MeSADP protection is mediated by enhanced astrocyte mitochondrial metabolism via increased inositol trisphosphate (IP(3))-dependent Ca(2+) release. We suggest that mitochondria play a critical role in astrocyte energy metabolism in the penumbra of ischemic lesions, where low ATP levels are widely accepted to be responsible for cytotoxic edema. Enhancement of this energy source could have similar protective benefits for a wide range of brain injuries.

Stimulation of the adenosine A3 receptor reverses vascular hyporeactivity after hemorrhagic shock in rats

AIM

To investigate whether adenosine A(3) receptors (A(3)AR) stimulation restore vascular reactivity after hemorrhagic shock through a ryanodine receptor (RyR)-mediated and large conductance calcium-activated potassium (BK(Ca)) channel-dependent pathway.

METHODS

Rat hemorrhagic shock model (40 mmHg) and vascular smooth muscle cell (VSMC) hypoxic model were used. The expression of A(3)AR was determined by Western blot and RT-PCR. The effect of A(3)AR stimulation on RyR-mediated Ca(2+) release in VSMCs was analyzed by the Fura-3/AM loading Ca(2+) imaging. The modulation of vascular reactivity to norepinephrine (NE) by A(3)AR stimulation was monitored by an isolated organ tension instrument.

RESULTS

Decrease of A(3)AR expression is consistent with the loss of vasoreactivity to NE in hemorrhagic shock rats. The stimulation of A(3)AR with a selective agonist, IB-MECA, could partly but significantly restore the vasoreactivity in the rats, and this restorative effect could be counteracted by MRS1523, a selective A(3)AR antagonist. In hypoxic VSMCs, RyR activation by caffeine significantly evoked the rise of [Ca(2+)] compared with the control cells, a phenomenon closely associated with the development of vascular hyporeactivity in hemorrhagic shock rats. The stimulation of A(3)AR with IB-MECA significantly blocked this over activation of RyR-mediated Ca(2+) release. RyR activation by caffeine and BK(Ca) channel activation by NS1619 attenuated the restoration of vasoreactivity to NE resulting from A(3)AR stimulation by IB-MECA after hemorrhagic shock; this attenuation effect could be antagonized by a selective BK(Ca) channel blocker.

CONCLUSION

These findings suggest that A(3)AR is involved in the modulation of vasoreactivity after hemorrhagic shock and that stimulation of A(3)AR can restore the decreased vasoreactivity to NE through a RyR-mediated, BK(Ca) channel-dependent signal pathway.

Spontaneous glial calcium waves in the retina develop over early adulthood

Intercellular glial Ca(2+) waves constitute a signaling pathway between glial cells. Artificial stimuli have previously been used to evoke these waves, and their physiological significance has been questioned. We report here that Ca(2+) waves occur spontaneously in rat retinal glial cells, both in the isolated retina and in vivo. These spontaneous waves are propagated by ATP release. In the isolated retina, suramin (P2 receptor antagonist) reduces the frequency of spontaneous wave generation by 53%, and apyrase (ATP-hydrolyzing enzyme) reduces frequency by 95-100%. Luciferin-luciferase chemiluminescence reveals waves of ATP matching the spontaneous Ca(2+) waves, indicating that ATP release occurs as spontaneous Ca(2+) waves are generated. Wave generation also depends on age. Spontaneous wave frequency rises from 0.27 to 1.0 per minute per mm(2), as rats age from 20 to 120 d. The sensitivity of glia to ATP does not increase with age, but the ATP released by evoked waves is 31% greater in 120-d-old than in 20-d-old rats, suggesting that increased ATP release in older animals could account for the higher frequency of wave generation. Simultaneous imaging of glial Ca(2+) and arterioles in the isolated retina demonstrates that spontaneous waves alter vessel diameter, implying that spontaneous waves may have a significant impact on retinal physiology. Spontaneous intercellular glial Ca(2+) waves also occur in the retina in vivo, with frequency, speed, and diameter similar to the isolated retina. Increased spontaneous wave occurrence with age suggests that wave generation may be related to retinal pathology.

Intracellular- and extracellular-derived Ca(2+) influence phospholipase A(2)-mediated fatty acid release from brain phospholipids

Docosahexaenoic acid (DHA) and arachidonic acid (AA) are found in high concentrations in brain cell membranes and are important for brain function and structure. Studies suggest that AA and DHA are hydrolyzed selectively from the sn-2 position of synaptic membrane phospholipids by Ca(2+)-dependent cytosolic phospholipase A(2) (cPLA(2)) and Ca(2+)-independent phospholipase A(2) (iPLA(2)), respectively, resulting in increased levels of the unesterified fatty acids and lysophospholipids. Cell studies also suggest that AA and DHA release depend on increased concentrations of Ca(2+), even though iPLA(2) has been thought to be Ca(2+)-independent. The source of Ca(2+) for activation of cPLA(2) is largely extracellular, whereas Ca(2+) released from the endoplasmic reticulum can activate iPLA(2) by a number of mechanisms. This review focuses on the role of Ca(2+) in modulating cPLA(2) and iPLA(2) activities in different conditions. Furthermore, a model is suggested in which neurotransmitters regulate the activity of these enzymes and thus the balanced and localized release of AA and DHA from phospholipid in the brain, depending on the primary source of the Ca(2+) signal.

Possible involvement of extracellular ATP-P2Y purinoceptor signaling in ischemia-induced tolerance of astrocytes in culture

Extracellular adenosine 5'-triphosphate (ATP) activates specific G protein-coupled purinoceptors (P2Y), and ATP-P2Y signaling pathways induces intracellular Ca(2+) mobilization resulting in changes in the gene expression of a variety of proteins in astrocytes. This study investigated whether the exposure of cultured astrocytes to sublethal ischemia produced resistance to subsequent lethal ischemic stress, and if so, whether the extracellular ATP-P2Y signaling pathways were responsible for the tolerance. Ischemia-like insults, sublethal oxygen-glucose deprivation (sOGD), produced tolerance to subsequent lethal OGD stress in cultured astrocytes. Early during reperfusion after sOGD, the amount of extracellular ATP and the expression of both P2Y(1) and P2Y(2) receptors were increased, leading to enhanced activation of the extracellular ATP-P2Y signaling pathways. The occurrence of intracellular spontaneous Ca(2+) oscillations was also increased. In addition, sOGD treatment enhanced the expression of the phosphorylated form of extracellular signal-regulated protein kinases 1 and 2 (p-ERK 1/2), and treatment with an inhibitor of ERK significantly attenuated the sOGD-induced ischemic tolerance of astrocytes.

Mitochondria modulate Ca2+-dependent glutamate release from rat cortical astrocytes

Vesicular glutamate release from astrocytes depends on mobilization of free Ca(2+) from the endoplasmic reticulum (ER), and extracellular space to elevate cytosolic Ca(2+) (Ca(2+)(cyt)). Although mitochondria in neurons, and other secretory cells, have been shown to sequester free Ca(2+) and have been implicated in the modulation of Ca(2+)-dependent transmitter release, the role of mitochondria in Ca(2+)-dependent glutamate release from astrocytes is not known. A pharmacological approach was taken to manipulate Ca(2+) accumulation in mitochondria and thereby affect Ca(2+)(cyt) of solitary astrocytes in response to mechanical stimuli. Ca(2+)(cyt) responses and levels of glutamate release were measured optically in parallel experiments using a fluorescent Ca(2+) indicator and an enzyme-linked assay, respectively. It was observed that inhibiting mitochondrial Ca(2+) accumulation is correlated to increased Ca(2+)(cyt) and glutamate release, whereas enhancing mitochondrial Ca(2+) accumulation is correlated to decreased Ca(2+)(cyt) and glutamate release. These observations suggest that, in addition to the activity of ER and plasma membrane ion channels, mitochondria modulate Ca(2+)(cyt) dynamics in astrocytes and play a role in Ca(2+)-dependent glutamate release from astrocytes.

Proteomic modeling for HIV-1 infected microglia-astrocyte crosstalk

Background

HIV-1-infected and immune competent brain mononuclear phagocytes (MP; macrophages and microglia) secrete cellular and viral toxins that affect neuronal damage during advanced disease. In contrast, astrocytes can affect disease by modulating the nervous system's microenvironment. Interestingly, little is known how astrocytes communicate with MP to influence disease.

Methods and Findings

MP-astrocyte crosstalk was investigated by a proteomic platform analysis using vesicular stomatitis virus pseudotyped HIV infected murine microglia. The microglial-astrocyte dialogue was significant and affected microglial cytoskeleton by modulation of cell death and migratory pathways. These were mediated, in part, through F-actin polymerization and filament formation. Astrocyte secretions attenuated HIV-1 infected microglia neurotoxicity and viral growth linked to the regulation of reactive oxygen species.

Conclusions

These observations provide unique insights into glial crosstalk during disease by supporting astrocyte-mediated regulation of microglial function and its influence on the onset and progression of neuroAIDS. The results open new insights into previously undisclosed pathogenic mechanisms and open the potential for biomarker discovery and therapeutics that may influence the course of HIV-1-mediated neurodegeneration.