Adenosine: a neuron-glial transmitter promoting myelination in the CNS in response to action potentials

Adenosine: setting the stage for plasticity

It is widely accepted that Hebbian forms of plasticity mediate selective modifications in synaptic strength underlying information encoding in response to experience and circuit formation or refinement throughout development. Several complementary forms of homeostatic plasticity coordinate to keep Hebbian plasticity in check, frequently through the actions of conserved regulatory molecules. Recent evidence suggests that this may be the case for adenosine, which is ubiquitous in the brain and is released by both neurons and glial cells via constitutive and activity-dependent mechanisms. Through A1 and A2A receptor activation, adenosine modulates neuronal homeostasis and tunes the ability of synapses to undergo and/or sustain plasticity. Here, we review how adenosine equilibrates neuronal activity and sets the stage for synaptic plasticity.

Purkinje cell maturation participates in the control of oligodendrocyte differentiation: role of sonic hedgehog and vitronectin

Oligodendrocyte differentiation is temporally regulated during development by multiple factors. Here, we investigated whether the timing of oligodendrocyte differentiation might be controlled by neuronal differentiation in cerebellar organotypic cultures. In these cultures, the slices taken from newborn mice show very few oligodendrocytes during the first week of culture (immature slices) whereas their number increases importantly during the second week (mature slices). First, we showed that mature cerebellar slices or their conditioned media stimulated oligodendrocyte differentiation in immature slices thus demonstrating the existence of diffusible factors controlling oligodendrocyte differentiation. Using conditioned media from different models of slice culture in which the number of Purkinje cells varies drastically, we showed that the effects of these differentiating factors were proportional to the number of Purkinje cells. To identify these diffusible factors, we first performed a transcriptome analysis with an Affymetrix array for cerebellar cortex and then real-time quantitative PCR on mRNAs extracted from fluorescent flow cytometry sorted (FACS) Purkinje cells of L7-GFP transgenic mice at different ages. These analyses revealed that during postnatal maturation, Purkinje cells down-regulate Sonic Hedgehog and up-regulate vitronectin. Then, we showed that Sonic Hedgehog stimulates the proliferation of oligodendrocyte precursor cells and inhibits their differentiation. In contrast, vitronectin stimulates oligodendrocyte differentiation, whereas its inhibition with blocking antibodies abolishes the conditioned media effects. Altogether, these results suggest that Purkinje cells participate in controlling the timing of oligodendrocyte differentiation in the cerebellum through the developmentally regulated expression of diffusible molecules such as Sonic Hedgehog and vitronectin.

The role of non-synaptic extracellular glutamate

Although there are some mechanisms which allow the direct crossing of substances between the cytoplasm of adjacent cells (gap junctions), most substances use the extracellular space to diffuse between brain cells. The present work reviews the behavior and functions of extracellular glutamate (GLU). There are two extracellular pools of glutamate (GLU) in the brain, a synaptic pool whose functions in the excitatory neurotransmission has been widely studied and an extrasynaptic GLU pool although less known nonetheless is gaining attention among a growing number of researchers. Evidence accumulated over the last years shows a number of mechanisms capable of releasing glial GLU to the extracellular medium, thus modulating neurons, microglia and oligodendrocytes, and regulating the immune response, cerebral blood flow, neuronal synchronization and other brain functions. This new scenario is expanding present knowledge regarding the role of GLU in the brain under different physiological and pathological conditions. This article is part of a Special Issue entitled 'Extrasynaptic ionotropic receptors'.

Myelination: do astrocytes play a role?

Astrocytes are the most abundant cell type in the adult central nervous system (CNS), and their functional diversity in response to injury is now being appreciated. Astrocytes have long been considered the main player in the inhibition of CNS repair via the formation of the gliotic scar, but now it is accepted that astrocyte can play an important role in CNS repair and remyelination. Interest in the relationship between astrocytes and myelination focused initially on attempts to understand how the development of plaques of astroglial scar tissue in multiple sclerosis was related to the failure of these lesions to remyelinate. It is now considered that this is an end stage pathological response to injury, and that normally astrocytes play important roles in supporting the development and maintenance of CNS myelin. This review will focus on how this new understanding may be exploited to develop new strategies to enhance remyelination in multiple sclerosis and other diseases.

Neurotrophic tyrosine kinase polymorphism impacts white matter connections in patients with major depressive disorder

BACKGROUND

Polymorphisms in the brain-derived neurotrophic factor (BDNF) gene and its receptor neurotrophic tyrosine kinase receptor type 2 (NTRK2) have been implicated in mood disorders. The aim of this study was to examine whether the NTRK2 and BDNF polymorphisms impact brain white matter connections in major depressive disorder and whether they may also have an interactive effect with environmental stress in the form of early life adversity.

METHODS

The study group comprised 45 depressed patients and 45 age- and gender-matched control subjects. High angular resolution diffusion images were obtained and analyzed using tract-based spatial statistics. Analysis of a single nucleotide polymorphism in the BDNF (rs6265/Valine66Methionine) and NTRK2 (rs11140714) genes was performed.

RESULTS

An interactive effect was found between NTRK2 and depression diagnosis maximally affecting the cingulum. Depressed patients homozygous for the A allele of NTRK2 showed significantly reduced fractional anisotropy compared with depressed patients with at least one copy of the G allele or control subjects with either the A/A or G carrier genotypes in the left and right corona radiata, left uncinate fasciculus, left inferior fronto-occipital fasciculus, left cerebral peduncle, posterior thalamic radiation, and middle cerebral peduncle. Significantly smaller gray matter volume was seen in frontal lobe regions in patients homozygous for the A allele.

CONCLUSIONS

Polymorphisms in NTRK2 gene increase risk of architectural changes in several brain regions involved in emotional regulation.

Experience-driven brain plasticity: beyond the synapse

The brain is remarkably responsive to its interactions with the environment, and its morphology is altered by experience in measurable ways. Histological examination of the brains of animals exposed to either a complex ('enriched') environment or learning paradigm, compared with appropriate controls, has illuminated the nature of experience-induced morphological plasticity in the brain. For example, this research reveals that changes in synapse number and morphology are associated with learning and are stable, in that they persist well beyond the period of exposure to the learning experience. In addition, other components of the nervous system also respond to experience: oligodendrocytes and axonal myelination might also be permanently altered, whereas changes in astrocytes and cerebrovasculature are more transient and appear to be activity- rather than learning-driven. Thus, experience induces multiple forms of plasticity in the brain that are apparently regulated, at least in part, by independent mechanisms.

Purinergic trophic signalling in glial cells: functional effects and modulation of cell proliferation, differentiation, and death

In the last decades, the discovery that glial cells do not only fill in the empty space among neurons or furnish them with trophic support but are rather essential participants to the various activities of the central and peripheral nervous system has fostered the search for the signalling pathways controlling their functions. Since the early 1990s, purines were foreseen as some of the most promising candidate molecules. Originally just a hypothesis, this has become a certainty as experimental evidence accumulated over years, as demonstrated by the exponentially growing number of articles related to the role of extracellular nucleotides and nucleosides in controlling glial cell functions. Indeed, as new functions for already known glial cells (for example, the ability of parenchymal astrocytes to behave as stem cells) or new subtypes of glial cells (for example, NG2(+) cells, also called polydendrocytes) are discovered also, new actions and new targets for the purinergic system are identified. Thus, glial purinergic receptors have emerged as new possible pharmacological targets for various acute and chronic pathologies, such as stroke, traumatic brain and spinal cord injury, demyelinating diseases, trigeminal pain and migraine, and retinopathies. In this article, we will summarize the most important and promising actions mediated by extracellular purines and pyrimidines in controlling the functions, survival, and differentiation of the various "classical" types of glial cells (i.e., astrocytes, oligodendrocytes, microglial cells, Müller cells, satellite glial cells, and enteric glial cells) but also of some rather new members of the family (e.g., polydendrocytes) and of other cells somehow related to glial cells (e.g., pericytes and spinal cord ependymal cells).

Fibroblast growth factor receptor signaling in oligodendrocytes regulates myelin sheath thickness

Formation of the CNS white matter is developmentally tightly regulated, but the molecules and mechanisms of myelination control in the postnatal CNS are poorly understood. Here, we show that myelin growth is controlled by fibroblast growth factor (FGF) signaling, originally identified as a proliferative signal for oligodendrocyte precursor cells (OPCs) in vitro. We created two lines of mice lacking both FGF receptor 1 (Fgfr1) and Fgfr2 in oligodendrocyte-lineage cells but found that in these mice OPC proliferation and differentiation were unaffected. In addition, axonal ensheathment and the initiation of myelination were on time. However, the rapid growth of CNS myelin, normally occurring in the second postnatal week, was strongly inhibited. Throughout adulthood, the myelin sheath remained disproportionately thin relative to the axon caliber. In adult mice, mutant oligodendrocytes were normal in number, whereas the transcription of major myelin genes was reduced. This FGF receptor-mediated stimulation of mature oligodendrocytes could also be modeled in vitro, demonstrating that enhanced expansion of oligodendroglial processes requires signaling by extracellular signal regulated kinase-1 and -2 (Erk1/2), downstream mediators of mitogen-activated protein kinase (MAPK). In vivo, Erk1/2-MAPK activity was reduced in the hypomyelinated CNS of Fgfr1/Fgfr2 mutant mice. These studies reveal a previously unrecognized function of FGF receptor signaling in oligodendrocytes that contributes to the regulation of myelin sheath thickness and that uncouples the initiation of ensheathment from the later phase of continued myelin growth.

Glial processes at the Drosophila larval neuromuscular junction match synaptic growth

Glia are integral participants in synaptic physiology, remodeling and maturation from blowflies to humans, yet how glial structure is coordinated with synaptic growth is unknown. To investigate the dynamics of glial development at the Drosophila larval neuromuscular junction (NMJ), we developed a live imaging system to establish the relationship between glia, neuronal boutons, and the muscle subsynaptic reticulum. Using this system we observed processes from two classes of peripheral glia present at the NMJ. Processes from the subperineurial glia formed a blood-nerve barrier around the axon proximal to the first bouton. Processes from the perineurial glial extended beyond the end of the blood-nerve barrier into the NMJ where they contacted synapses and extended across non-synaptic muscle. Growth of the glial processes was coordinated with NMJ growth and synaptic activity. Increasing synaptic size through elevated temperature or the highwire mutation increased the extent of glial processes at the NMJ and conversely blocking synaptic activity and size decreased the presence and size of glial processes. We found that elevated temperature was required during embryogenesis in order to increase glial expansion at the nmj. Therefore, in our live imaging system, glial processes at the NMJ are likely indirectly regulated by synaptic changes to ensure the coordinated growth of all components of the tripartite larval NMJ.

Axon myelination and electrical stimulation in a microfluidic, compartmentalized cell culture platform

Axon demyelination contributes to the loss of sensory and motor function following injury or disease in the central nervous system. Numerous reports have demonstrated that myelination can be achieved in neuron/oligodendrocyte co-cultures. However, the ability to selectively treat neuron or oligodendrocyte (OL) cell bodies in co-cultures improves the value of these systems when designing mechanism-based therapeutics. We have developed a microfluidic-based compartmentalized culture system to achieve segregation of neuron and OL cell bodies while simultaneously allowing the formation of myelin sheaths. Our microfluidic platform allows for a high replicate number, minimal leakage, and high flexibility. Using a custom built lid, fit with platinum electrodes for electrical stimulation (10-Hz pulses at a constant 3 V with ~190 kΩ impedance), we employed the microfluidic platform to achieve activity-dependent myelin segment formation. Electrical stimulation of dorsal root ganglia resulted in a fivefold increase in the number of myelinated segments/mm² when compared to unstimulated controls (19.6 ± 3.0 vs. 3.6 ± 2.3 MBP+ segments/mm²). This work describes the modification of a microfluidic, multi-chamber system so that electrical stimulation can be used to achieve increased levels of myelination while maintaining control of the cell culture microenvironment.

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.

Plasticity in gray and white: neuroimaging changes in brain structure during learning

Human brain imaging has identified structural changes in gray and white matter that occur with learning. However, ascribing imaging measures to underlying cellular and molecular events is challenging. Here we review human neuroimaging findings of structural plasticity and then discuss cellular and molecular level changes that could underlie observed imaging effects. Greater dialog between researchers in these different fields would help to facilitate cross-talk between cellular and systems level explanations of how learning sculpts brain structure.

P2Y12 receptor expression is a critical determinant of functional responsiveness to ATX's MORFO domain

In the central nervous system, the formation of the myelin sheath and the differentiation of the myelinating cells, namely oligodendrocytes, are regulated by complex signaling networks that involve purinergic receptors and the extracellular matrix. However, the exact nature of the molecular interactions underlying these networks still needs to be defined. In this respect, the data presented here reveal a signaling mechanism that is characterized by an interaction between the purinergic P2Y(12) receptor and the matricellular extracellular matrix protein autotaxin (ATX), also known as ENPP2, phosphodiesterase-Iα/ATX, or lysoPLD. ATX has been previously described by us to mediate intermediate states of oligodendrocyte adhesion and to enable changes in oligodendrocyte morphology that are thought to be crucial for the formation of a fully functional myelin sheath. This functional property of ATX is mediated by ATX's modulator of oligodendrocyte remodeling and focal adhesion organization (MORFO) domain. Here, we show that the expression of the P2Y(12) receptor is necessary for ATX's MORFO domain to exert its effects on differentiating oligodendrocytes. In addition, our data demonstrate that exogenous expression of the P2Y(12) receptor can render cells responsive to the known effects of ATX's MORFO domain, and they identify Rac1 as an intracellular factor mediating the effect of ATX-MORFO-P2Y(12) signaling on the assembly of focal adhesions. Our data further support the idea that a physical interaction between ATX and the P2Y(12) receptor provides the basis for an ATX-MORFO-P2Y(12) signaling axis that is crucial for mediating cellular states of intermediate adhesion and morphological/structural plasticity.

Learning-related gray and white matter changes in humans: an update

In contrast to a long-held view that structural brain plasticity is restricted to critical periods during ontogenesis, it is now well established that the adult human brain preserves its capacity for functional and structural changes throughout life. Although early experimental studies were mainly performed in animals, technical developments especially in the field of MRI enabled the non-invasive observation of functional and structural reorganization in the human brain. This article reports recent insights in human brain plasticity with particular emphasis on (dynamic) learning-related structural gray and white matter changes and its behavioral correlates.

Bidirectional plasticity of calcium-permeable AMPA receptors in oligodendrocyte lineage cells

Oligodendrocyte precursor cells (OPCs), a major glial cell type that gives rise to myelinating oligodendrocytes in the CNS, express calcium-permeable AMPA receptors (CP-AMPARs). Although CP-AMPARs are important for OPC proliferation and neuron-glia signaling, they render OPCs susceptible to ischemic damage in early development. We identified factors controlling the dynamic regulation of AMPAR subtypes in OPCs from rat optic nerve and mouse cerebellar cortex. We found that activation of group 1 mGluRs drove an increase in the proportion of CP-AMPARs, reflected by an increase in single-channel conductance and inward rectification. This plasticity required the elevation of intracellular calcium and used PI3K, PICK-1 and the JNK pathway. In white matter, neurons and astrocytes release both ATP and glutamate. Unexpectedly, activation of purinergic receptors in OPCs decreased CP-AMPAR expression, suggesting a capacity for homeostatic regulation. Finally, we found that stargazin-related transmembrane AMPAR regulatory proteins, which are critical for AMPAR surface expression in neurons, regulate CP-AMPAR plasticity in OPCs.

Accelerated myelination with motor system involvement in a neonate with immediate postnatal onset of seizures and hemimegalencephaly

The timetable for normal myelination in the developing brain on MRI is well described. Beyond the genetically determined myelination process, environmental or other incidental factors may modulate the actual myelination, globally or regionally. Delayed myelination is well known in many pathological conditions interfering with adequate energy or myelin precursor supply. Rare but specific conditions where the normal process of myelination is accelerated are also known. We describe a case of early postnatal epilepsy demonstrating a striking degree of accelerated myelination both in the cerebral tissue adjacent to the presumed epileptic focus and along the ipsilateral pyramidal tract through the brainstem, a feature not previously described in the literature. This distal myelination highlights the peculiar system involvement pattern of function-induced myelination rather than simply regional myelination in response to an epileptic or other stimulus. This may have implications on our understanding of the development of function systems (visual, auditory, motor, sensory, etc.) or even the more complex interactive process of learning and may open new therapeutic opportunities in pathological functional conditions previously believed to be incurable.

Electrical stimulation promotes the survival of oligodendrocytes in mixed cortical cultures

Oligodendrocyte (OLG) death plays a major role in white matter dysfunction and demyelination following injury to the CNS. Axonal contact, communication, and neuronal activity appear to promote OLG survival and function in cell culture and during development. The application of electrical stimulation to mixed neural cultures has been shown to promote OLG differentiation and the formation of myelin in vitro. Here we show that OLG viability can be significantly enhanced in mixed cortical cultures by applying biphasic pulses of electrical stimulation (ESTIM). Enhanced survival via ESTIM requires the presence of neurons and is suppressed by inhibition of voltage-gated sodium channels. Additionally, contact between the axon and OLG is necessary for ESTIM to promote OLG survival. This report suggests that patterned neuronal activity could repress delayed progression of white matter injury and promote CNS repair in neurological conditions that involve white matter damage.

Control of local protein synthesis and initial events in myelination by action potentials

Formation of myelin, the electrical insulation on axons produced by oligodendrocytes, is controlled by complex cell-cell signaling that regulates oligodendrocyte development and myelin formation on appropriate axons. If electrical activity could stimulate myelin induction, then neurodevelopment and the speed of information transmission through circuits could be modified by neural activity. We find that release of glutamate from synaptic vesicles along axons of mouse dorsal root ganglion neurons in culture promotes myelin induction by stimulating formation of cholesterol-rich signaling domains between oligodendrocytes and axons, and increasing local synthesis of the major protein in the myelin sheath, myelin basic protein, through Fyn kinase-dependent signaling. This axon-oligodendrocyte signaling would promote myelination of electrically active axons to regulate neural development and function according to environmental experience.

In vitro modeling of central nervous system myelination and remyelination

This review aims to summarize the current techniques to study myelination and remyelination in culture systems. We attempt to put these into historical context, and to identify the strengths and weaknesses of each approach, which vary depending on the experimental question to be tested. We discuss the difficulty and importance of quantification of myelination and in particular remyelination. Finally, we provide our predictions of how these techniques will and should develop in the future.

Memory training impacts short-term changes in aging white matter: a longitudinal diffusion tensor imaging study

A growing body of research indicates benefits of cognitive training in older adults, but the neuronal mechanisms underlying the effect of cognitive intervention remains largely unexplored. Neuroimaging methods are sensitive to subtle changes in brain structure and show potential for enhancing our understanding of both aging- and training-related neuronal plasticity. Specifically, studies using diffusion tensor imaging (DTI) suggest substantial changes in white matter (WM) in aging, but it is not known whether cognitive training might modulate these structural alterations. We used tract-based spatial statistics (TBSS) optimized for longitudinal analysis to delineate the effects of 8 weeks intensive memory training on WM microstructure. 41 participants (mean age 61 years) matched for age, sex and education were randomly assigned to an intervention or control group. All participants underwent MRI-scanning and neuropsychological assessments at the beginning and end of the study. Longitudinal analysis across groups revealed significant increase in frontal mean diffusivity (MD), indicating that DTI is sensitive to WM structural alterations over a 10-week interval. Further, group analysis demonstrated positive effects of training on the short-term changes. Participants in the training group showed a relative increase in fractional anisotropy (FA) compared with controls. Further, a significant relationship between memory improvement and change in FA was found, suggesting a possible functional significance of the reported changes. The training effect on FA seemed to be driven by a relative decrease in radial diffusivity, which might indicate a role for myelin-related processes in WM plasticity.

Past, present and future of A(2A) adenosine receptor antagonists in the therapy of Parkinson's disease

Several selective antagonists for adenosine A(2A) receptors (A(2A)R) are currently under evaluation in clinical trials (phases I to III) to treat Parkinson's disease, and they will probably soon reach the market. The usefulness of these antagonists has been deduced from studies demonstrating functional interactions between dopamine D₂ and adenosine A(2A) receptors in the basal ganglia. At present it is believed that A(2A)R antagonists can be used in combination with the dopamine precursor L-DOPA to minimize the motor symptoms of Parkinson's patients. However, a considerable body of data indicates that in addition to ameliorating motor symptoms, adenosine A(2A)R antagonists may also prevent neurodegeneration. Despite these promising indications, one further issue must be considered in order to develop fully optimized antiparkinsonian drug therapy, namely the existence of (hetero)dimers/oligomers of G protein-coupled receptors, a topic that is currently the focus of intense debate within the scientific community. Dopamine D₂ receptors (D₂Rs) expressed in the striatum are known to form heteromers with A(2A) adenosine receptors. Thus, the development of heteromer-specific A(2A) receptor antagonists represents a promising strategy for the identification of more selective and safer drugs.

The complex world of oligodendroglial differentiation inhibitors

Myelination is a central nervous system (CNS) process wherein oligodendrocyte-axon interactions lead to the establishment of myelin sheaths that stabilize, protect, and electrically insulate axons. In inflammatory demyelinating diseases such as multiple sclerosis (MS), the degeneration and eventual loss of functional myelin sheaths slows and blocks saltatory conduction in axons, which results in clinical impairment. However, remyelination can occur, and lesions can be partially repaired, resulting in clinical remission. The recruitment and activation of resident oligodendrocyte precursor cells (OPCs) play a critical role in the repair process because these cells have the capacity to differentiate into functional myelinating cells. Mature oligodendrocytes, however, are thought to have lost the capacity to develop new myelin sheaths and frequently undergo programmed cell death in MS. The endogenous capacity to generate new oligodendrocytes in MS is limited, and this is predominantly due to the presence of inhibitory components that block OPC differentiation and maturation. Here, we present an overview of recently identified negative regulators of oligodendroglial differentiation and their potential relevance for CNS repair in MS. Because currently available immunomodulatory drugs for MS mainly target inflammatory cascades outside the brain and fail to repair existing lesions, achieving more efficient lesion repair constitutes an important goal for future MS therapies.

Myelin genes are downregulated in canine fucosidosis

The processes regulating the complex neurodegenerative cascade of vacuolation, neuroinflammation, neuronal loss and myelin deficits in fucosidosis, a neurological lysosomal storage disorder, remain unclear. To elucidate these processes the gene expression profile of the cerebral cortex from untreated and intrathecal enzyme replacement therapy treated fucosidosis pups and age-matched unaffected controls were examined. Neuroinflammation and cell death processes were identified to have a major role in fucosidosis pathophysiology with 37% of differentially expressed (DE) genes involved in these processes. Critical, specific, early decreases in expression levels of key genes in myelin assembly were identified by gene expression profiling, including myelin-associated glycoprotein (MAG), myelin and lymphocyte protein (MAL), and oligodendrocyte myelin paranodal and inner loop protein (OPALIN). These gene expression changes may be indicative of early neuronal loss causing reduced electrical impulses required for oligodendrocyte maturation.

Imaging single photons and intrinsic optical signals for studies of vesicular and non-vesicular ATP release from axons

The temporal and spatial dynamics of neurotransmitter release are fundamental to understanding activity-dependent signaling between axons and other cells, including neurons, glia, and vascular cells. A microscopic imaging technique is described that enables studying release of the neurotransmitter ATP from axons in response to action potentials. The method combines imaging single-photons, intrinsic optical signal imaging, and high magnification time-lapse microcopy to enable investigations of action potential-induced ATP release together with cell morphology and activity-dependent axon swelling. ATP released from axons catalyzes a chemiluminescent reaction between luciferin and luciferase that generates single photons that can be imaged individually. In addition to vesicular release, ATP release through membrane channels activated by axon swelling was monitored simultaneously with intrinsic optical signals. Repeated emissions of photons were observed from localized 15 μm regions of axons, with a frequency distribution that differed from a normal distribution and from the frequency of emissions outside these localized regions.

Regulation and roles of neuronal diacylglycerol kinases: a lipid perspective

Diacylglycerol kinases (DGKs) are a class of enzymes that catalyze the ATP-dependent conversion of diacylglycerol (DAG) to phosphatidic acid (PtdOH), resulting in the coordinate regulation of these two lipid second messengers. This regulation is particularly important in the nervous system where it is now well-established that DAG and PtdOH serve very important roles in modulating a variety of neurological functions. There are currently 10 identified mammalian DGKs, organized into five classes or "Types" based upon similarities in their primary sequences. A number of studies have identified eight of these isoforms in various regions of the mammalian central nervous system (CNS): DGK-α, DGK-β, DGK-γ, DGK-η, DGK-ζ, DGK-ι, DGK-ϵ, and DGK-θ. Further studies have provided compelling evidence supporting roles for these enzymes in neuronal spine density, myelination, synaptic activity, neuronal plasticity, epileptogenesis and neurotransmitter release. The physiological regulation of these enzymes is less clear. Like all interfacial enzymes, DGKs metabolize their hydrophobic substrate (DAG) at a membrane-aqueous interface. Therefore, these enzymes can be regulated by alterations in their subcellular localization, enzymatic activity, and/or membrane association. In this review, we summarize what is currently understood about the localization and regulation of the neuronal DGKs in the mammalian CNS.

Possible effects of synaptic imbalances on oligodendrocyte-axonic interactions in schizophrenia: a hypothetical model

A model of glial-neuronal interactions is proposed that could be explanatory for the demyelination identified in brains with schizophrenia. It is based on two hypotheses: (1) that glia-neuron systems are functionally viable and important for normal brain function, and (2) that disruption of this postulated function disturbs the glial categorization function, as shown by formal analysis. According to this model, in schizophrenia receptors on astrocytes in glial-neuronal synaptic units are not functional, loosing their modulatory influence on synaptic neurotransmission. Hence, an unconstrained neurotransmission flux occurs that hyperactivates the axon and floods the cognate receptors of neurotransmitters on oligodendrocytes. The excess of neurotransmitters may have a toxic effect on oligodendrocytes and myelin, causing demyelination. In parallel, an increasing impairment of axons may disconnect neuronal networks. It is formally shown how oligodendrocytes normally categorize axonic information processing via their processes. Demyelination decomposes the oligodendrocyte-axonic system making it incapable to generate categories of information. This incoherence may be responsible for symptoms of disorganization in schizophrenia, such as thought disorder, inappropriate affect and incommunicable motor behavior. In parallel, the loss of oligodendrocytes affects gap junctions in the panglial syncytium, presumably responsible for memory impairment in schizophrenia.

Adverse and protective influences of adenosine on the newborn and embryo: implications for preterm white matter injury and embryo protection

Few signaling molecules have the potential to influence the developing mammal as the nucleoside adenosine. Adenosine levels increase rapidly with tissue hypoxia and inflammation. Adenosine antagonists include the methylxanthines caffeine and theophylline. The receptors that transduce adenosine action are the A1, A2a, A2b, and A3 adenosine receptors (ARs). In the postnatal period, A1AR activation may contribute to white matter injury in the preterm infant by altering oligodendrocyte (OL) development. In models of perinatal brain injury, caffeine is neuroprotective against periventricular white matter injury (PWMI) and hypoxic-ischemic encephalopathy (HIE). Supporting the notion that blockade of adenosine action is of benefit in the premature infant, caffeine reduces the incidence of bronchopulmonary dysplasia and CP in clinical studies. In comparison with the adverse effects on the postnatal brain, adenosine acts via A1ARs to play an essential role in protecting the embryo from hypoxia. Embryo protective effects are blocked by caffeine, and caffeine intake during early pregnancy increases the risk of miscarriage and fetal growth retardation. Adenosine and adenosine antagonists play important modulatory roles during mammalian development. The protective and deleterious effects of adenosine depend on the time of exposure and target sites of action.

Nonsynaptic and nonvesicular ATP release from neurons and relevance to neuron-glia signaling

Studies on the release of ATP from neurons began with the earliest investigations of quantal neurotransmitter release in the 1950s, but in contrast to ATP release from other cells, studies of ATP release from neurons have been narrowly constrained to one mechanism, vesicular release. This is a consequence of the prominence of synaptic transmission in neuronal communication, but nonvesicular mechanisms for ATP release from neurons are likely to have a broader range of functions than synaptic release. Investigations of activity-dependent communication between axons and myelinating glia have stimulated a search for mechanisms that could release ATP from axons and other nonsynaptic regions in response to action potential firing. This has identified volume-activated anion channels as an important mechanism in activity-dependent ATP release from axons, and renewed interest in micromechanical changes in axons that accompany action potential firing.

Imaging learning: the search for a memory trace

Learning is associated with structural changes in the human brain that can be seen and studied by MRI. These changes are observed in gray matter and surprisingly also in white matter tissue. Learning a wide range of skills, from sports, computer games, music, and reading, to abstract intellectual learning, including classroom study, is associated with structural changes in appropriate cortical regions or fiber tracts. The cellular changes underlying modifications of brain tissue during learning include changes in neuronal and glial morphology as well as vascular changes. Both alterations in axon morphology and myelination are thought to contribute to white matter plasticity during learning but to varying degrees depending on age. Structural changes in white matter could promote learning by improving the speed or synchrony of impulse transmission between cortical regions mediating the behavior. Action potentials can stimulate oligodendrocyte development and myelination by at least three known mechanisms that involve signaling molecules between axons and oligodendrocytes, which do not require neurotransmitter release from synapses. Integrating information from cellular/molecular and systems-level research on normal cognitive function, development, and learning is providing new insights into the biological mechanisms of learning and the structural changes produced in the brain.

Progenitors in the adult cerebral cortex: cell cycle properties and regulation by physiological stimuli and injury

The adult brain parenchyma contains a widespread population of progenitors generating different cells of the oligodendrocyte lineage such as NG2+ cells and some mature oligodendrocytes. However, it is still largely unknown how proliferation and lineage decisions of these progenitors are regulated. Here, we first characterized the cell cycle length, proliferative fraction, and progeny of dividing cells in the adult cerebral cortex and then compared these proliferation characteristics after two distinct stimuli, invasive acute brain injury and increased physiological activity by voluntary physical exercise. Our data show that adult parenchymal progenitors have a very long cell cycle due to an extended G1 phase, many of them can divide at least twice and only a limited proportion of the progeny differentiates into mature oligodendrocytes. After stab wound injury, however, many of these progenitors re-enter the cell cycle very fast, suggesting that the normally long G1 phase is subject to regulation and can be abruptly shortened. In striking contrast, voluntary physical exercise shows the opposite effect with increased exit of the cell cycle followed by an enhanced and fast differentiation into mature oligodendrocytes. Taken together, our data demonstrate that the endogenous population of adult brain parenchymal progenitors is subject to profound modulation by environmental stimuli in both directions, either faster proliferation or faster differentiation.

Role of adenosine kinase in cochlear development and response to noise

Adenosine signalling has an important role in cochlear protection from oxidative stress. In most tissues, intracellular adenosine kinase (ADK) is the primary route of adenosine metabolism and the key regulator of intracellular and extracellular adenosine levels. The present study provides the first evidence for ADK distribution in the adult and developing rat cochlea. In the adult cochlea, ADK was localized to the nuclear or perinuclear region of spiral ganglion neurons, lateral wall tissues, and epithelial cells lining scala media. In the developing cochlea, ADK was strongly expressed in multiple cell types at birth and reached its peak level of expression at postnatal day 21 (P21). Ontogenetic changes in ADK expression were evident in the spiral ganglion, organ of Corti, and stria vascularis. In the spiral ganglion, ADK showed a shift from predominantly satellite cell immunolabelling at P1 to neuronal expression from P14 onward. In contrast to the role of ADK in various aspects of cochlear development, the ADK contribution to the cochlear response to noise stress was less obvious. Transcript and protein levels of ADK were unaltered in the cochlea exposed to broadband noise (90-110 dBSPL, 24 hr), and the selective inhibition of ADK in the cochlea with ABT-702 failed to restore hearing thresholds after exposure to traumatic noise. This study indicates that ADK is involved in purine salvage pathways for nucleotide synthesis in the adult cochlea, but its role in the regulation of adenosine signalling under physiological and pathological conditions has yet to be established.

ATP: a ubiquitous gliotransmitter integrating neuron-glial networks

Astrocytes are ideally situated to integrate glial and neuronal functions and neurovascular coupling by way of their multiple contacts with neurons, glia and blood vessels. There is a high degree of specialisation of astroglial membranes at the different sites of contact, including the expression of neurotransmitter receptors, ion channels, transporters and gap junctional proteins. An apparently universal property of astrocytes throughout the CNS is their responsiveness to ATP acting via metabotropic P2Y receptors, with a prominent role for the P2Y1 receptor subtype. Activation of astroglial P2Y receptors triggers a rise in intracellular calcium, which is the substrate for astroglial excitability and intercellular communication. In addition, astrocytes have a number of mechanisms for the release of ATP, which can be considered a 'gliotransmitter'. Astrocytes may be the most widespread source of ATP release in the CNS, and astroglial ATP and its metabolite adenosine activate purine receptors on neurons, microglia, oligodendrocytes and blood vessels. There is compelling evidence that astroglial ATP and adenosine regulate neuronal synaptic strength, although the physiological significance of this astrocyte-to-neuron signalling is questioned. A less appreciated aspect of astrocyte signalling is that they also release neurotransmitters onto other glia. Notably, both ATP and adenosine control microglial behaviour and regulate oligodendrocyte differentiation and myelination. P2 receptors also mediate injury responses in all glial cell types, with a prominent role for the P2X7 receptor subtype. In addition, ATP is a potent vasoconstrictor and astrocytes provide a route for coupling blood flow to neuronal activity by way of their synaptic and perivascular connections. Thus, astrocytes are the fulcrum of neuron-glial-vascular networks and purinergic signalling is the primary mechanism by which astrocytes can integrate the functions of these diverse elements.

Purinergic signalling at the plasma membrane: a multipurpose and multidirectional mode to deal with amyotrophic lateral sclerosis and multiple sclerosis

ATP is a widespread and multipurpose signalling molecule copiously released in the extracellular environment of the whole nervous system upon cell activation, stress, or damage. Extracellular ATP is also a multidirectional information molecule, given the concurrent presence at the plasma membrane of various targets for ATP. These include ectonucleotidases (metabolizing ATP down to adenosine), ATP/adenosine transporters, P2 receptors for purine/pyrimidine nucleotides (ligand-gated ion channels P2X receptors and G-protein-coupled P2Y receptors), in addition to metabotropic P1 receptors for nucleosides. All these targets rarely operate as single units, rather they associate with each other at the plasma membrane as multi-protein complexes. Altogether, they control the duration, magnitude and/or direction of the signals triggered and propagated by purine/pyrimidine ligands, and the impact that each single ligand has on a variety of short- and long-term functions. A strict control system allows assorted, even divergent, biological outcomes. Among these, we enumerate cell-to-cell communication, tropic, trophic, but also noxious actions causing the insurgence/progression of pathological conditions. Here, we show that purinergic signalling in the nervous system can be instrumental for instance to neurodegenerative and neuroinflammatory diseases such as amyotrophic lateral sclerosis and multiple sclerosis.

Inhibition of myelin membrane sheath formation by oligodendrocyte-derived exosome-like vesicles

Myelin formation is a multistep process that is controlled by a number of different extracellular factors. During the development of the central nervous system (CNS), oligodendrocyte progenitor cells differentiate into mature oligodendrocytes that start to enwrap axons with myelin membrane sheaths after receiving the appropriate signal(s) from the axon or its microenvironment. The signals required to initiate this process are unknown. Here, we show that oligodendrocytes secrete small membrane vesicles, exosome-like vesicles, into the extracellular space that inhibit both the morphological differentiation of oligodendrocytes and myelin formation. The inhibitory effects of exosome-like vesicles were prevented by treatment with inhibitors of actomyosin contractility. Importantly, secretion of exosome-like vesicles from oligodendrocytes was dramatically reduced when cells were incubated by conditioned neuronal medium. In conclusion, our results provide new evidence for small and diffusible oligodendroglial-derived vesicular carriers within the extracellular space that have inhibitory properties on cellular growth. We propose that neurons control the secretion of autoinhibitory oligodendroglial-derived exosomes to coordinate myelin membrane biogenesis.

Astrocyte proximity modulates the myelination gene fabric of oligodendrocytes

Extensive literature documented that astrocytes release neurotransmitters, cytokines and other signaling molecules to modulate migration, maturation and myelin synthesis of oligodendrocytes through mechanisms primarily converging on cytosolic [Ca2+] transients. Considering the long-term effects, it is expected that astrocyte-conditioned medium is a major regulator of gene expression in oligodendrocytes even in the absence of cytosol-to-cytosol communication via astrocyte-oligodendrocyte gap junction channels. Indeed, by comparing the transcriptomes of immortalized precursor oligodendrocyte (Oli-neu) cells when cultured alone and co-cultured with non-touching astrocytes we found profound changes in the gene expression level, control and networking. Remarkably, the astrocyte proximity was more effective in remodeling the myelination (MYE) gene fabric and its control by cytokine receptor (CYR)-modulated intercellular Ca2+-signaling (ICS) transcriptomic network than the dibutyryl-cAMP (db-cAMP) treatment-induced transformation into myelin-associated glycoprotein-positive oligodendrocyte-like cells. Moreover, astrocyte proximity up-regulated 37 MYE genes and switched on another 14 MYE, 23 ICS and 4 CYR genes, enhancing the roles of the leukemia inhibitory factor receptor and connexins Cx29 and Cx47. The novel prominent gene analysis identified the enhancer of zeste homolog 2 as the most relevant MYE gene in the astrocyte proximity, notch gene homolog 1 in control and B-cell leukemia/lymphoma 2 in differentiated Oli-neu cells.

Phenotypic changes, signaling pathway, and functional correlates of GPR17-expressing neural precursor cells during oligodendrocyte differentiation

The developing and mature central nervous system contains neural precursor cells expressing the proteoglycan NG2. Some of these cells continuously differentiate to myelin-forming oligodendrocytes; knowledge of the destiny of NG2(+) precursors would benefit from the characterization of new key functional players. In this respect, the G protein-coupled membrane receptor GPR17 has recently emerged as a new timer of oligodendrogliogenesis. Here, we used purified oligodendrocyte precursor cells (OPCs) to fully define the immunophenotype of the GPR17-expressing cells during OPC differentiation, unveil its native signaling pathway, and assess the functional consequences of GPR17 activation by its putative endogenous ligands, uracil nucleotides and cysteinyl leukotrienes (cysLTs). GPR17 presence was restricted to very early differentiation stages and completely segregated from that of mature myelin. Specifically, GPR17 decorated two subsets of slowly proliferating NG2(+) OPCs: (i) morphologically immature cells expressing other early proteins like Olig2 and PDGF receptor-α, and (ii) ramified preoligodendrocytes already expressing more mature factors, like O4 and O1. Thus, GPR17 is a new marker of these transition stages. In OPCs, GPR17 activation by either uracil nucleotides or cysLTs resulted in potent inhibition of intracellular cAMP formation. This effect was counteracted by GPR17 antagonists and receptor silencing with siRNAs. Finally, uracil nucleotides promoted and GPR17 inhibition, by either antagonists or siRNAs, impaired the normal program of OPC differentiation. These data have implications for the in vivo behavior of NG2(+) OPCs and point to uracil nucleotides and cysLTs as main extrinsic local regulators of these cells under physiological conditions and during myelin repair.

Increased numbers of oligodendrocyte lineage cells in the optic nerves of cerebroside sulfotransferase knockout mice

Sulfatide is a myelin glycolipid that functions in the formation of paranodal axo-glial junctions in vivo and in the regulation of oligodendrocyte differentiation in vitro. Cerebroside sulfotransferase (CST) catalyzes the production of two sulfated glycolipids, sulfatide and proligodendroblast antigen, in oligodendrocyte lineage cells. Recent studies have demonstrated significant increases in oligodendrocytes from the myelination stage through adulthood in brain and spinal cord under CST-deficient conditions. However, whether these result from excess migration or in situ proliferation during development is undetermined. In the present study, CST-deficient optic nerves were used to examine migration and proliferation of oligodendrocyte precursor cells (OPCs) under sulfated glycolipid-deficient conditions. In adults, more NG2-positive OPCs and fully differentiated cells were observed. In developing optic nerves, the number of cells at the leading edge of migration was similar in CST-deficient and wild-type mice. However, BrdU(+) proliferating OPCs were more abundant in CST-deficient mice. These results suggest that sulfated glycolipids may be involved in proliferation of OPCs in vivo.

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.

Myelination and support of axonal integrity by glia

The myelination of axons by glial cells was the last major step in the evolution of cells in the vertebrate nervous system, and white-matter tracts are key to the architecture of the mammalian brain. Cell biology and mouse genetics have provided insight into axon-glia signalling and the molecular architecture of the myelin sheath. Glial cells that myelinate axons were found to have a dual role by also supporting the long-term integrity of those axons. This function may be independent of myelin itself. Myelin abnormalities cause a number of neurological diseases, and may also contribute to complex neuropsychiatric disorders.

Direct and indirect effects of neuropeptide Y and neurotrophin 3 on myelination in the neonatal brains

Neuropeptide Y (NPY) is expressed in the developing central nervous system, however, its role in the brain development remains unclear. In this study, C57/B6 mice were intraperitoneally administered 1 nmol/capita/day of NPY, 10 nmol/capita/day of an NPY-receptor 1-specific antagonist (Y1R-A), or NPY and Y1R-A simultaneously (NPY+Y1R-A) from postnatal day (P) 7 to P14. Recombinant NPY reached the P14 cerebrum in 1 hour. These treatments didn't significantly affect body weight gain or P14 brain weight. The ratio of myelinated axons to total axons in the parietal cerebrum was significantly higher in the NPY group than in the control group. The expression of myelin basic protein (MBP)-mRNA in the cerebrum was significantly higher in the NPY group than in the control group and was significantly lower in the NPY+Y1R-A group than in the NPY group, while it was significantly higher in the NPY+Y1R-A group than in the control group. In cultured oligodendroglioma-derived B12 cells, NPY didn't influence the MBP-mRNA expression, while neurotrophin 3 (NT3) increased MBP mRNA via receptor-type tyrosine kinase type C (Trk C). NPY administration significantly increased NT3-mRNA expression in the P14 cerebrum as deduced by quantitative real-time PCR. The change in phosphorylated Trk C (P-Trk C) was proportional to that of the NT3-mRNA expression, and the proportion of P-Trk C was higher in the NPY group than in the control group. These results suggest that NPY, partially via Y1R, induces NT3 which, via Trk C phosphorylation, accelerates myelination by oligodendrocytes in the mouse brain during the neonatal period.

Expression of the new P2Y-like receptor GPR17 during oligodendrocyte precursor cell maturation regulates sensitivity to ATP-induced death

The P2Y-like receptor GPR17 is expressed by adult neural progenitor cells, suggesting a role in lineage determination. Here, we characterized GPR17 expression and function in mouse cortical primary astrocytes/precursor cell cultures. GPR17 is expressed by a subpopulation of oligodendrocyte precursor cells (OPCs), but not by astrocytes. This expression pattern was also confirmed in vivo. In vitro, GPR17 expression was markedly influenced by culturing conditions. In the presence of growth factors (GFs), no significant GPR17 expression was found. When cultures were shifted to a differentiating medium, a dramatic, time-dependent increase in the number of highly branched GPR17-positive cells was observed. Under these conditions, GPR17 was induced in the totality of O4-positive immature oligodendrocytes. Instead, in cultures originally grown in the absence of GFs, GPR17 was already expressed in morphologically more mature OPCs. Shifting of these cultures to differentiating conditions induced GPR17 only in a subpopulation of O4-positive cells. Under both culture protocols, appearance of more mature CNPase- and MBP-positive cells was associated to a progressive loss of GPR17. GPR17 expression also sensitized cells to adenine nucleotide-induced cytotoxicity, whereas activation with uracil nucleotides promoted differentiation towards a more mature phenotype. We suggest that GFs may keep OPCs in a less differentiated stage by restraining GPR17 expression, and that, under permissive conditions, GPR17 contributes to OPCs differentiation. However, upon high extracellular adenine nucleotide concentrations, as during trauma and ischemia, GPR17 sensitizes cells to cytotoxicity. This double-edged sword role may be exploited to unveil new therapeutic approaches to acute and chronic brain disorders.