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

Disturbed structural connectivity in schizophrenia primary factor in pathology or epiphenomenon?

Indirect evidence for disturbed structural connectivity of subcortical fiber tracts in schizophrenia has been obtained from functional neuroimaging and electrophysiologic studies. During the past few years, new structural imaging methods have become available. Diffusion tensor imaging and magnetization transfer imaging (MTI) have been used to investigate directly whether fiber tract abnormalities are indeed present in schizophrenia. To date, findings are inconsistent that may express problems related to methodological issues and sample size. Also, pathological processes detectable with these new techniques are not yet well understood. Nevertheless, with growing evidence of disturbed structural connectivity, myelination has been in the focus of postmortem investigations. Several studies have shown a significant reduction of oligodendroglial cells and ultrastructural alterations of myelin sheats in schizophrenia. There is also growing evidence for abnormal expression of myelin-related genes in schizophrenia: Neuregulin (NRG1) is important for oligodendrocyte development and function, and altered expression of erbB3, one of the NRG1 receptors, has been shown in schizophrenia patients. This is consistent with recent genetic studies suggesting that NRG1 may contribute to the genetic risk for schizophrenia. In conclusion, there is increasing evidence from multiple sides that structural connectivity might be pathologically changed in schizophrenia illness. Up to the present, however, it has not been possible to decide whether alterations of structural connectivity are intrinsically linked to the primary risk factors for schizophrenia or to secondary downstream effects (ie, degeneration of fibers secondarily caused by cortical neuronal dysfunction)-an issue that needs to be clarified by future research.

Astrocytes--friends or foes in multiple sclerosis?

In multiple sclerosis (MS), the presence of demyelinating plaques has concentrated researchers' minds on the role of the oligodendrocyte in its pathophysiology. Recently, with the rediscovery of early and widespread loss of axons in the disease, new emphasis has been put on the role of axons and axon-oligodendrocyte interactions in MS. Despite the fact that, in 1904, Müller claimed that MS was a disease of astrocytes, more recently, astrocytes have taken a back seat, except as the cells that form the final glial scar after all hope of demyelination is over. However, perhaps it is time for the return of the astrocyte to popularity in the pathogenesis of MS, with recent reports on the dual role of astrocytes in aiding degeneration and demyelination, by promoting inflammation, damage of oligodendrocytes and axons, and glial scarring, but also in creating a permissive environment for remyelination by their action on oligodendrocyte precursor migration, oligodendrocyte proliferation, and differentiation. We review these findings to try to provide a cogent view of astrocytes in the pathology of MS.

Lentiviral RNAi-induced downregulation of adenosine kinase in human mesenchymal stem cell grafts: a novel perspective for seizure control

Cell therapies based on focal delivery of the inhibitory neuromodulator adenosine were previously shown to provide potent seizure suppression in animal models of epilepsy. However, hitherto used therapeutic cells were derived from rodents and thus not suitable for clinical applications. Autologous patient-derived adenosine-releasing cell implants would constitute a major therapeutic advance to avoid both xenotransplantation and immunosuppression. Here we describe a novel approach based on lentiviral RNAi mediated downregulation of adenosine kinase (ADK), the major adenosine-removing enzyme, in human mesenchymal stem cells (hMSCs), which would be compatible with autologous cell grafting in patients. Following lentiviral transduction of hMSCs with anti-ADK miRNA expression cassettes we demonstrate up to 80% downregulation of ADK and a concentration of 8.5 ng adenosine per ml of medium after incubating 10(5) cells for 8 h. hMSCs with a knockdown of ADK or cells expressing a scrambled control sequence were transplanted into hippocampi of mice 1 week prior to the intraamygdaloid injection of kainic acid (KA). While mice with control implants expressing a scrambled miRNA sequence or sham treated control animals were characterized by KA-induced status epilepticus and subsequent CA3 neuronal cell loss, animals with therapeutic ADK knockdown implants displayed a 35% reduction in seizure duration and 65% reduction in CA3 neuronal cell loss, when analyzed 24 h after KA-injection. We conclude that lentiviral expression of anti-ADK miRNA constitutes a versatile tool to generate therapeutically effective adenosine releasing hMSCs, thus representing a model system to generate patient identical autologous adult stem cell grafts.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Purinergic receptor activation inhibits mitogen-stimulated proliferation in primary neurospheres from the adult mouse subventricular zone

The expression pattern of purinergic receptors was examined in subventricular zone-derived primary neurospheres. Primary neurospheres expressed mRNA for P2X4 and P2X7 receptors, all P2Y receptors, with the exception of P2Y4, and the A1, A2a and A2b adenosine receptors. ATPgammaS, ADPbetaS and UTP evoked transient increases in cytoplasmic Ca(2+) concentration in dissociated primary neurospheres, demonstrating the functional expression of P2Y1 and P2Y2 receptors. Ca(2+) transients were not attenuated by the removal of extracellular Ca(2+) and were reversibly inhibited by the P2Y1 selective antagonist, MRS 2179. P2Y and adenosine receptor agonists reduced the size and frequency of primary neurospheres. The effects of ADPbetaS and adenosine were reversed by subtype-selective receptor antagonists, demonstrating that P2Y1 and A2a receptors mediate inhibitory effects on primary neurosphere proliferation. The modulation of neural precursor cell proliferation by P2Y and adenosine receptors therefore represents a potential regulatory mechanism within the neurogenic microenvironment.

Morphological and physiological interactions of NG2-glia with astrocytes and neurons

Models of central nervous system (CNS) function have historically been based on neurons and their synaptic contacts - the neuronal doctrine. This doctrine envisages glia as passive supportive cells. However, electrophysiological and imaging studies in brain slices show us that astrocytes, the most numerous cells in the brain, express a wide range of neurotransmitter receptors that are activated in response to synaptic activity. Furthermore, astrocytes communicate via calcium signals that are propagated over long distances by the release of 'gliotransmitters', the most abundant being adenosine triphosphate (ATP). This has led to the concept of the neuron-astroglial functional unit as the substrate of integration in the CNS. Recently, a novel glial cell type has been characterized by expression of the proteoglycan NG2. These NG2-glia receive presynaptic input from neurons and responds to neurotransmitters released at synapses. Now, studies on transgenic mice in which fluorescent proteins are specifically expressed by subclasses of glia are helping to address the question of where NG2-glia fit in the neuron-astroglial model of integrated brain function. NG2-glia, as well as astrocytes, have been shown to respond to neuronal and astroglial signals by raised intracellular calcium, which is a potential communications mechanism by which NG2-glia may be active partners in neuron-glial circuits. Moreover, a current concept of NG2-glia considers them to be 'neural stem cells' and an exciting prospect is that neuron-glial signalling may regulate the differentiation capacity of NG2-glia and their response to injury.

White matter plasticity and enhanced remyelination in the maternal CNS

Myelination, the process in which oligodendrocytes coat CNS axons with a myelin sheath, represents an important but poorly understood form of neural plasticity that may be sexually dimorphic in the adult CNS. Remission of multiple sclerosis during pregnancy led us to hypothesize that remyelination is enhanced in the maternal brain. Here we report an increase in the generation of myelin-forming oligodendrocytes and in the number of myelinated axons in the maternal murine CNS. Remarkably, pregnant mice have an enhanced ability to remyelinate white matter lesions. The hormone prolactin regulates oligodendrocyte precursor proliferation and mimics the regenerative effects of pregnancy. This suggests that maternal white matter plasticity imparts a striking ability to repair demyelination and identifies prolactin as a potential therapeutic agent.

Contrasting effects of mitogenic growth factors on myelination in neuron-oligodendrocyte co-cultures

Mitogenic growth factors play an important role in the initial stages of oligodendrocyte development, but their roles in the process of myelination itself remain less well defined. In order to study directly the effects of different growth factors on myelination, we used a purified in vitro co-culture system with dorsal root ganglion neurons and oligodendrocytes. Extensive myelination had occurred in these cultures 14 days after oligodendrocyte precursors (OPCs) were added, with the relationship between neurite density and the percentage of oligodendrocytes forming myelin sheaths providing a robust and straightforward means of quantifying myelination. Addition of soluble neuregulin (Nrg1), a mitogen for oligodendroglial cells that also provides an axonal signal implicated in oligodendrocyte survival, increased myelination. Conversely, the OPC mitogens FGF-2 and PDGF inhibited myelination. The inhibitory effect of these mitogens was reversible, as inhibition of PDGF allowed myelination to proceed. Taken together, these data indicate that different mitogenic growth factors can regulate myelination by oligodendrocytes in addition to their well-described effects on earlier stages of oligodendroglial development. Moreover, the results highlight important differences between the growth factors.

Plasticity of the corticospinal tract in early blindness revealed by quantitative analysis of fractional anisotropy based on diffusion tensor tractography

Early visual deprivation may induce plastic changes, not only in the visual system, but also in the remaining sensory systems, secondary to altered experience in these spared modalities. Most of previous studies were focused on the plasticity of cortical areas of sensory modalities, but little attention was paid to the plasticity of motor system and white matter fiber tracts. Our purpose is to investigate the plasticity of the corticospinal tract (CST) in early blindness by tract-based quantitative analysis of fractional anisotropy (FA). Diffusion tensor imaging was performed in 17 early blind and 17 gender- and age-matched sighted subjects. The entire CST of each subject was reconstructed and the average FA of the tract was analyzed. To validate the results derived from the entire CST, we further analyzed a segment of the CST between the lowest slice of the cerebral peduncle and the uppermost slice of the lateral ventricle, in which the fibers are coherently arranged and the anatomical correspondence of the CST across subjects is established. On comparison with matched sighted participants, the average FA of the CST was significantly increased in the early blind men, but not in the early blind women. In conclusion, the plasticity of the CST is present in the early blind men, which might be related to the changes of motor experience during critical developmental period of the CST. This study also supports the perspective that experience-dependent plasticity occurs not only in the cortical areas but also in the white matter fiber tracts.

Vesicular release of glutamate from unmyelinated axons in white matter

Directed fusion of transmitter-laden vesicles enables rapid intercellular signaling in the central nervous system and occurs at synapses within gray matter. Here we show that action potentials also induce the release of glutamate from axons in the corpus callosum, a white matter region responsible for interhemispheric communication. Callosal axons release glutamate by vesicular fusion, which induces quantal AMPA receptor-mediated currents in NG2(+) glial progenitors at anatomically distinct axo-glial synaptic junctions. Glutamate release from axons was facilitated by repetitive stimulation and could be inhibited through activation of metabotropic autoreceptors. Although NG2(+) cells form associations with nodes of Ranvier in white matter, measurements of conduction velocity indicated that unmyelinated fibers are responsible for glutamatergic signaling with NG2(+) glia. This activity-dependent secretion of glutamate was prevalent in the developing and mature mouse corpus callosum, indicating that axons within white matter both conduct action potentials and engage in rapid neuron-glia communication.

The quest for remyelination: a new role for neurotrophins and their receptors

The formation of myelin is dependent on a reciprocal and intimate relationship between neurons and the myelin-forming glia. Recently, the neurotrophin family of growth factors has been shown to regulate the complex cell-cell interactions that control myelination. Neurotrophins and their receptors influence myelin formation via two distinct mechanisms, either by acting on the neurons, changing the axonal signals that control myelination, or by acting directly on the myelin-forming glia. In this review, we will discuss research highlighting the ability of neurotrophins to both promote and inhibit the myelination process. As reflected in the work presented here, these effects are dependent on a delicate balance of which neurotrophins are expressed, and what receptors are activated. Additionally, we examine an emerging model in which the growth factors that promote the early survival and differentiation of particular sets of neurons later play important roles as key regulators in glial development. Characterizing the temporal expression and the cellular targets of neurotrophins, both during development and after injury, represents a pivotal step in developing a greater understanding of the myelination process, contributing to the development of effective treatments for demyelinating conditions. We conclude this review by discussing the potential for neurotrophins as therapeutics in the quest for remyelination.

Neuron-glia communication in the control of oligodendrocyte function and myelin biogenesis

During the development of the central nervous system the reciprocal communication between neurons and oligodendrocytes is essential for the generation of myelin, a multilamellar insulating membrane that ensheathes the axons. Neuron-derived signalling molecules regulate the proliferation, differentiation and survival of oligodendrocytes. Furthermore, neurons control the onset and timing of myelin membrane growth. In turn, signals from oligodendrocytes to neurons direct the assembly of specific subdomains in neurons at the node of Ranvier. Recent work has begun to shed light on the molecules and signaling systems used to coordinate the interaction of neurons and oligodendrocytes. For example, the neuronal signals seem to control the membrane trafficking machinery in oligodendrocytes that leads to myelination. These interconnections at multiple levels show how neurons and glia cooperate to build a complex network during development.

Characterization of thromboxane A2 receptor signaling in developing rat oligodendrocytes: nuclear receptor localization and stimulation of myelin basic protein expression

The present work investigates the role of thromboxane A(2) (TXA(2)) receptors in the development of oligodendrocytes (OLGs). The results demonstrate that the proteins of the TXA(2) signaling pathway, i.e., cyclooxygenase (COX-1), TXA(2) synthase (TS), and TXA(2) receptor (TPR) are expressed in the developing rat brain during myelination. Furthermore, culture of OLG progenitor cells (OPCs) revealed that the expression levels of these proteins as well as TXA(2) synthesis increase during OLG maturation. Separate studies established that activation of TPRs by the agonist U46619 increases intracellular calcium in both OPCs and OLGs as visualized by digital fluorescence imaging. Immunocytochemical staining demonstrated that TPRs are localized in the plasma membrane and perinuclear compartments in OPCs. However, during OLG differentiation, TPRs shift their localization pattern and also become associated with the nuclear compartment. This shift to nuclear localization was confirmed by biochemical analysis in cultured cells and by immunocytochemical analysis in developing rat brain. Finally, it was found that U46619 activation of TPRs in maturing OLGs resulted in enhanced myelin basic protein (MBP) expression. Alternatively, inhibition of endogenous TPR signaling led to reduced MBP expression. Furthermore, TPR-mediated MBP expression was found to be associated with increased transcription from the MBP promoter using a MBP-luciferase reporter. Collectively, these findings suggest a novel TPR signaling pathway in OLGs and a potential role for this signaling during OLG maturation and myelin production.

Coated Glass and Vicryl Microfibers as Artificial Axons

The complex interactions that occur between oligodendrocytes and axons during the process of central nervous system myelination and remyelination remain unclear. Elucidation of the cell-biological and -biochemical mechanisms supporting myelin production and elaboration requires a robust in vitro system that recapitulates the relationship between axons and oligodendrocytes in a manner that is open to molecular dissection. We provide evidence for an artificial axon culture system in which we observed oligodendrocytes extending large plasma membrane projections that frequently completely ensheathed fibers coated with a variety of extracellular matrix molecules. These membrane projections varied in extent and thickness depending upon the substrate and upon the diameter of the coated fiber. Matrigel-coated glass microfibers were found to support the development of thick membrane sheaths that extended for hundreds of microns and exhibited many features suggestive of the potential for true myelin deposition. Likewise, Matrigel-coated Vicryl fibers supported plasma membrane extensions that covered extremely large surface areas and occasionally wrapped the coated Vicryl fibers in more than one membrane layer. Our findings suggest that the deposition of molecular cues onto glass or polymer fibers either via adsorption or chemical modification may be a useful tool for the discovery or validation of axonal factors critical for myelination and remyelination. Herein, we provide evidence that polyglactin 910 and glass microfibers coated with adhesion factors may provide a reasonable system for the in vitro analysis of myelination, and may eventually serve a role in engineering artificial systems for neural repair.

Protective effects of caffeine on chronic hypoxia-induced perinatal white matter injury

OBJECTIVE

Periventricular white matter injury (PWMI) is the major cause of cerebral palsy and cognitive impairment in prematurely born infants. PWMI is characterized by reductions in cerebral myelination and cerebrocortical volumes and is associated with secondary ventriculomegaly. In neonatal rodents, these features of PWMI can be induced by rearing in chronic hypoxia or by activation of A1 adenosine receptors. We determined: (1) whether altered maturation or development of one or more oligodendrocyte (OL) lineage stages plays a role in the pathogenesis of the myelination disturbances associated with exposure to chronic hypoxia, and (2) whether blockade of A1 adenosine receptor action with the adenosine antagonist caffeine can prevent hypoxia-induced white matter injury.

METHODS

Ventriculomegaly and reduced cerebral myelination were generated in mice reared in hypoxia (10% oxygen) from postnatal days 3 (P3) through 12.

RESULTS

Hypomyelination was related to abnormal OL lineage progression and a reduction in the OL progenitor pool. Myelination was enhanced and ventriculomegaly reduced in hypoxia-exposed neonatal pups treated with caffeine from P3 to P12.

INTERPRETATION

These observations support that hypoxia inhibits OL maturation and that caffeine administration during early postnatal development may have utility in the prevention of PWMI.

Functional genomic analysis of oligodendrocyte differentiation

To better understand the molecular mechanisms governing oligodendrocyte (OL) differentiation, we have used gene profiling to quantitatively analyze gene expression in synchronously differentiating OLs generated from pure oligodendrocyte precursor cells in vitro. By comparing gene expression in these OLs to OLs generated in vivo, we discovered that the program of OL differentiation can progress normally in the absence of heterologous cell-cell interactions. In addition, we found that OL differentiation was unexpectedly prolonged and occurred in at least two sequential stages, each characterized by changes in distinct complements of transcription factors and myelin proteins. By disrupting the normal dynamic expression patterns of transcription factors regulated during OL differentiation, we demonstrated that these sequential stages of gene expression can be independently controlled. We also uncovered several genes previously uncharacterized in OLs that encode transmembrane, secreted, and cytoskeletal proteins that are as highly upregulated as myelin genes during OL differentiation. Last, by comparing genomic loci associated with inherited increased risk of multiple sclerosis (MS) to genes regulated during OL differentiation, we identified several new positional candidate genes that may contribute to MS susceptibility. These findings reveal a previously unexpected complexity to OL differentiation and suggest that an intrinsic program governs successive phases of OL differentiation as these cells extend and align their processes, ensheathe, and ultimately myelinate axons.

Neurotransmitter-mediated calcium signalling in oligodendrocyte physiology and pathology

The function of oligodendrocytes is to myelinate CNS axons. Oligodendrocytes and the axons they myelinate are functional units, and neurotransmitters released by axons can influence all stages of oligodendrocyte development via calcium dependent mechanisms. Some of the clearest functional evidence is for adenosine, ATP, and glutamate, which are released by electrically active axons and regulate the migration and proliferation of oligodendrocyte progenitor cells and their differentiation into myelinating oligodendrocytes. Glutamate and ATP, released by both axons and astrocytes, continue to mediate Ca(2+) signaling in mature oligodendrocytes, acting via AMPA and NMDA glutamate receptors, and heterogeneous P2X and P2Y purinoceptors. Physiological signalling between axons, astrocytes, and oligodendrocytes is likely to play an important role in myelin maintenance throughout life. Significantly, ATP- and glutamate-mediated Ca(2+) signaling are also major components of oligodendrocyte and myelin damage in numerous pathologies, most notably ischemia, injury, periventricular leukomalacia, and multiple sclerosis. In addition, NG2-expressing glia (synantocytes) in the adult CNS are highly reactive cells that respond rapidly to any CNS insult by a characteristic gliosis, and are able to regenerate oligodendrocytes and possibly neurons. Glutamate and ATP released by neurons and astrocytes evoke Ca(2+) signaling in NG2-glia (synantocytes), and it is proposed these regulate their differentiation capacity and response to injury. In summary, clear roles have been demonstrated for neurotransmitter-mediated Ca(2+) signaling in oligodendrocyte development and pathology. A key issue for future studies is to determine the physiological roles of neurotransmitters in mature oligodendrocytes and NG2-glia (synantocytes).

Neurotransmitter receptors in the life and death of oligodendrocytes

Oligodendrocytes are crucial to the function of the mammalian brain: they increase the action potential conduction speed for a given axon diameter and thus facilitate the rapid flow of information between different brain areas. The proliferation and differentiation of developing oligodendrocytes, and their myelination of axons, are partly controlled by neurotransmitters. In addition, in models of conditions like stroke, periventricular leukomalacia leading to cerebral palsy, spinal cord injury and multiple sclerosis, oligodendrocytes are damaged by glutamate and, contrary to dogma, it has recently been discovered that this damage is mediated in part by N-methyl-D-aspartate receptors. Mutations in oligodendrocyte neurotransmitter receptors or their interacting proteins may cause defects in CNS function. Here we review the roles of neurotransmitter receptors in the normal function, and malfunction in pathological conditions, of oligodendrocytes.

Shift of adenosine kinase expression from neurons to astrocytes during postnatal development suggests dual functionality of the enzyme

Adenosine is a potent modulator of excitatory neurotransmission, especially in seizure-prone regions such as the hippocampal formation. In adult brain ambient levels of adenosine are controlled by adenosine kinase (ADK), the major adenosine-metabolizing enzyme, expressed most strongly in astrocytes. Since ontogeny of the adenosine system is largely unknown, we investigated ADK expression and cellular localization during postnatal development of the mouse brain, using immunofluorescence staining with cell-type specific markers. At early postnatal stages ADK immunoreactivity was prominent in neurons, notably in cerebral cortex and hippocampus. Thereafter, as seen best in hippocampus, ADK gradually disappeared from neurons and appeared in newly developed nestin- and glial fibrillary acidic protein (GFAP)-positive astrocytes. Furthermore, the region-specific downregulation of neuronal ADK coincided with the onset of myelination, as visualized by myelin basic protein staining. After postnatal day 14 (P14), the transition from neuronal to astrocytic ADK expression was complete, except in a subset of neurons that retained ADK until adulthood in specific regions, such as striatum. Moreover, neuronal progenitors in the adult dentate gyrus lacked ADK. Finally, recordings of excitatory field potentials in acute slice preparations revealed a reduced adenosinergic inhibition in P14 hippocampus compared with adult. These findings suggest distinct roles for adenosine in the developing and adult brain. First, ADK expression in young neurons may provide a salvage pathway to utilize adenosine in nucleic acid synthesis, thus supporting differentiation and plasticity and influencing myelination; and second, adult ADK expression in astrocytes may offer a mechanism to regulate adenosine levels as a function of metabolic needs and synaptic activity, thus contributing to the differential resistance of young and adult animals to seizures.

Roles of P2 receptors in glial cells: focus on astrocytes

Central nervous system glial cells release and respond to nucleotides under both physiological and pathological conditions, suggesting that these molecules play key roles in both normal brain function and in repair after damage. In particular, ATP released from astrocytes activates P2 receptors on astrocytes and other brain cells, allowing a form of homotypic and heterotypic signalling, which also involves microglia, neurons and oligodendrocytes. Multiple P2X and P2Y receptors are expressed by both astrocytes and microglia; however, these receptors are differentially recruited by nucleotides, depending upon specific pathophysiological conditions, and also mediate the long-term trophic changes of these cells during inflammatory gliosis. In astrocytes, P2-receptor-induced gliosis occurs via activation of the extracellular-regulated kinases (ERK) and protein kinase B/Akt pathways and involves induction of inflammatory and anti-inflammatory genes, cyclins, adhesion and antiapoptotic molecules. While astrocytic P2Y₁ and P2Y_2,4 are primarily involved in short-term calcium-dependent signalling, multiple P2 receptor subtypes seem to cooperate to astrocytic long-term changes. Conversely, in microglia, exposure to inflammatory and immunological stimuli results in differential functional changes of distinct P2 receptors, suggesting highly specific roles in acquisition of the activated phenotype. We believe that nucleotide-induced activation of astrocytes and microglia may originally start as a defence mechanism to protect neurons from cytotoxic and ischaemic insults; dysregulation of this process in chronic inflammatory diseases eventually results in neuronal cell damage and loss. On this basis, full elucidation of the specific roles of P2 receptors in these cells may help exploit the beneficial neuroprotective features of activated glia while attenuating their harmful properties and thus provide the basis for novel neuroprotective strategies that specifically target the purinergic system.

Purinergic signalling in neuron-glia interactions

Activity-dependent release of ATP from synapses, axons and glia activates purinergic membrane receptors that modulate intracellular calcium and cyclic AMP. This enables glia to detect neural activity and communicate among other glial cells by releasing ATP through membrane channels and vesicles. Through purinergic signalling, impulse activity regulates glial proliferation, motility, survival, differentiation and myelination, and facilitates interactions between neurons, and vascular and immune system cells. Interactions among purinergic, growth factor and cytokine signalling regulate synaptic strength, development and responses to injury. We review the involvement of ATP and adenosine receptors in neuron-glia signalling, including the release and hydrolysis of ATP, how the receptors signal, the pharmacological tools used to study them, and their functional significance.

Apyrase and 5'-nucleotidase activities in synaptosomes from the cerebral cortex of rats experimentally demyelinated with ethidium bromide and treated with interferon-beta

Apyrase and 5'-nucleotidase activities were analyzed in an ethidium bromide (EB) demyelinating model associated with interferon-beta (IFN-beta). The animals were divided in groups: I, control (saline); II, saline and IFN-beta; III, EB and IV, EB and IFN-beta. After 7, 15 and 30 days the animals (n = 5) were sacrificed and the cerebral cortex was removed for synaptosome preparation and enzymatic assays. Apyrase activity using ATP as substrate increased in groups II, III and IV (P < 0.001) after 7 days and in groups III and IV (P < 0.001) after 15 days. Using ADP as substrate, an activation of this enzyme was observed in group III (P < 0.05) after seven and 15 days. The 5'-nucleotidase activity increased in group III (P < 0.05) after 7 days and in groups II, III and IV (P < 0.001) after 15 days. After 30 days treatment, no significant alteration was observed in enzyme activities. Results showed that apyrase and 5'-nucleotidase activities are altered in demyelination events and that IFN-beta was able to regulate the adenine nucleotide hydrolysis.

A new player in CNS myelination

The formation of the myelin sheath in the CNS is the endpoint of a defined developmental program along which oligodendrocytes progress. However, the molecular signals required for the initiation of myelination are largely unknown. Ishibashi et al. report in this issue of Neuron that ATP released by axons as a result of electrical stimulation serves as an important myelination signal. Surprisingly, they found that ATP does not act directly on oligodendrocytes but rather on astrocytes, causing the release of leukemia inhibitory factor (LIF), which in turns affects promyelinating oligodendrocytes. These findings uncover a novel role for astrocytes in mediating the intricate communication between axons and myelinating glial cells.

Astrocytes promote myelination in response to electrical impulses

Myelin, the insulating layers of membrane wrapped around axons by oligodendrocytes, is essential for normal impulse conduction. It forms during late stages of fetal development but continues into early adult life. Myelination correlates with cognitive development and can be regulated by impulse activity through unknown molecular mechanisms. Astrocytes do not form myelin, but these nonneuronal cells can promote myelination in ways that are not understood. Here, we identify a link between myelination, astrocytes, and electrical impulse activity in axons that is mediated by the cytokine leukemia inhibitory factor (LIF). These findings show that LIF is released by astrocytes in response to ATP liberated from axons firing action potentials, and LIF promotes myelination by mature oligodendrocytes. This activity-dependent mechanism promoting myelination could regulate myelination according to functional activity or environmental experience and may offer new approaches to treating demyelinating diseases.

Myelin under construction -- teamwork required

Myelinating glial cells synthesize specialized myelin proteins and deposit them in the growing myelin sheath that enwraps axons multiple times. How do axons and myelinating glial cells coordinate this spectacular cell–cell interaction? In this issue, Trajkovic et al. (p. 937) show that neuronal signaling regulates cell surface expression of the myelin proteolipid protein in cultured oligodendrocytes in unexpected ways that may also contribute to myelination in situ.

Nucleotide signaling in nervous system development

The development of the nervous system requires complex series of cellular programming and intercellular communication events that lead from the early neural induction to the formation of a highly structured central and peripheral nervous system. Neurogenesis continuously takes place also in select regions of the adult mammalian brain. During the past years, a multiplicity of cellular control mechanisms has been identified, ranging from differential transcriptional mediators to inducers or inhibitors of cell specification or neurite outgrowth. While the identification of transcription factors typical for the stage-specific progression has been a topic of key interest for many years, less is known concerning the potential multiplicity of relevant intercellular signaling pathways and the fine tuning of epigenetic gene regulation. Nucleotide receptors can induce a multiplicity of cellular signaling pathways and are involved in multiple molecular interactions, thus opening the possibility of cross talk between several signaling pathways, including growth factors, cytokines, and extracellular matrix components. An increasing number of studies provides evidence for a role of nucleotide signaling in nervous system development. This includes progenitor cell proliferation, cell migration, neuronal and glial cellular interaction and differentiation, and synaptic network formation.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

Functional holography analysis: simplifying the complexity of dynamical networks

We present a novel functional holography (FH) analysis devised to study the dynamics of task-performing dynamical networks. The latter term refers to networks composed of dynamical systems or elements, like gene networks or neural networks. The new approach is based on the realization that task-performing networks follow some underlying principles that are reflected in their activity. Therefore, the analysis is designed to decipher the existence of simple causal motives that are expected to be embedded in the observed complex activity of the networks under study. First we evaluate the matrix of similarities (correlations) between the activities of the network's components. We then perform collective normalization of the similarities (or affinity transformation) to construct a matrix of functional correlations. Using dimension reduction algorithms on the affinity matrix, the matrix is projected onto a principal three-dimensional space of the leading eigenvectors computed by the algorithm. To retrieve back information that is lost in the dimension reduction, we connect the nodes by colored lines that represent the level of the similarities to construct a holographic network in the principal space. Next we calculate the activity propagation in the network (temporal ordering) using different methods like temporal center of mass and cross correlations. The causal information is superimposed on the holographic network by coloring the nodes locations according to the temporal ordering of their activities. First, we illustrate the analysis for simple, artificially constructed examples. Then we demonstrate that by applying the FH analysis to modeled and real neural networks as well as recorded brain activity, hidden causal manifolds with simple yet characteristic geometrical and topological features are deciphered in the complex activity. The term "functional holography" is used to indicate that the goal of the analysis is to extract the maximum amount of functional information about the dynamical network as a whole unit.

Adenosine receptors as therapeutic targets

Adenosine receptors are major targets of caffeine, the most commonly consumed drug in the world. There is growing evidence that they could also be promising therapeutic targets in a wide range of conditions, including cerebral and cardiac ischaemic diseases, sleep disorders, immune and inflammatory disorders and cancer. After more than three decades of medicinal chemistry research, a considerable number of selective agonists and antagonists of adenosine receptors have been discovered, and some have been clinically evaluated, although none has yet received regulatory approval. However, recent advances in the understanding of the roles of the various adenosine receptor subtypes, and in the development of selective and potent ligands, as discussed in this review, have brought the goal of therapeutic application of adenosine receptor modulators considerably closer.

Roles of Voltage-Dependent Sodium Channels in Neuronal Development, Pain, and Neurodegeneration

Besides initiating and propagating action potentials in established neuronal circuits, voltage-dependent sodium channels sculpt and bolster the functional neuronal network from early in embryonic development through adulthood (e.g., differentiation of oligodendrocyte precursor cells into oligodendrocytes, myelinating axon; competition between neighboring equipotential neurites for development into a single axon; enhancing and opposing functional interactions with attractive and repulsive molecules for axon pathfinding; extending and retracting terminal arborization of axon for correct synapse formation; experience-driven cognition; neuronal survival; and remyelination of demyelinated axons). Surprisingly, different patterns of action potentials direct homeostasis-based epigenetic selection for neurotransmitter phenotype, thus excitability by sodium channels specifying expression of inhibitory neurotransmitters. Mechanisms for these pleiotropic effects of sodium channels include reciprocal interactions between neurons and glia via neurotransmitters, growth factors, and cytokines at synapses and axons. Sodium channelopathies causing pain (e.g., allodynia) and neurodegeneration (e.g., multiple sclerosis) derive from 1) electrophysiological disturbances by insults (e.g., ischemia/hypoxia, toxins, and antibodies); 2) loss-of-physiological function or gain-of-pathological function of mutant sodium channel proteins; 3) spatiotemporal inappropriate expression of normal sodium channel proteins; or 4) de-repressed expression of otherwise silent sodium channel genes. Na(v)1.7 proved to account for pain in human erythermalgia and inflammation, being the convincing molecular target of pain treatment.

Oligodendrocytes express P2Y12 metabotropic receptor in adult rat brain

In the CNS, nucleotide receptors termed P2 receptors are identified on neurons and glial cells, mediating neuron-neuron, glia-glia and glia-neuron communication. In the present work, we qualify in vivo in the adult rat CNS the cellular/subcellular distribution of P2Y12 receptor protein in cerebral cortex, white matter and subcortical nuclei (striatum and substantia nigra), by means of immunofluorescence-confocal, electron microscopy and Western blot analysis. P2Y12 receptor immunoreactivity colocalizes neither with markers such as neuronal nuclei, neurofilament light chain, calbindin and tyrosine hydroxylase, nor with glial fibrillary acidic protein and isolectin B4, but with myelin basic protein and the oligodendrocyte marker RIP, in both cell bodies and processes, indicating therefore oligodendrocyte localization. Electron microscopy identifies P2Y12 receptors in both the perikaryon and under the plasmalemma of oligodendrocyte cell bodies and radiating processes, until the paranodal region of fibers. By Western blot analysis, P2Y12 receptor shows a specific band of 42-44 kDa, matching the molecular mass predicted from amino acid sequencing. Since in platelets P2Y12 receptor is known to regulate adhesion/activation and thrombus growth/stability, from our results we could speculate by analogy that, in oligodendrocytes, P2Y12 receptor signaling might contribute to the migration and adhesion of the glial processes to axons to be myelinated.

Brain plasticity and functional losses in the aged: scientific bases for a novel intervention

Aging is associated with progressive losses in function across multiple systems, including sensation, cognition, memory, motor control, and affect. The traditional view has been that functional decline in aging is unavoidable because it is a direct consequence of brain machinery wearing down over time. In recent years, an alternative perspective has emerged, which elaborates on this traditional view of age-related functional decline. This new viewpoint--based upon decades of research in neuroscience, experimental psychology, and other related fields--argues that as people age, brain plasticity processes with negative consequences begin to dominate brain functioning. Four core factors--reduced schedules of brain activity, noisy processing, weakened neuromodulatory control, and negative learning--interact to create a self-reinforcing downward spiral of degraded brain function in older adults. This downward spiral might begin from reduced brain activity due to behavioral change, from a loss in brain function driven by aging brain machinery, or more likely from both. In aggregate, these interrelated factors promote plastic changes in the brain that result in age-related functional decline. This new viewpoint on the root causes of functional decline immediately suggests a remedial approach. Studies of adult brain plasticity have shown that substantial improvement in function and/or recovery from losses in sensation, cognition, memory, motor control, and affect should be possible, using appropriately designed behavioral training paradigms. Driving brain plasticity with positive outcomes requires engaging older adults in demanding sensory, cognitive, and motor activities on an intensive basis, in a behavioral context designed to re-engage and strengthen the neuromodulatory systems that control learning in adults, with the goal of increasing the fidelity, reliability, and power of cortical representations. Such a training program would serve a substantial unmet need in aging adults. Current treatments directed at age-related functional losses are limited in important ways. Pharmacological therapies can target only a limited number of the many changes believed to underlie functional decline. Behavioral approaches focus on teaching specific strategies to aid higher order cognitive functions, and do not usually aspire to fundamentally change brain function. A brain-plasticity-based training program would potentially be applicable to all aging adults with the promise of improving their operational capabilities. We have constructed such a brain-plasticity-based training program and conducted an initial randomized controlled pilot study to evaluate the feasibility of its use by older adults. A main objective of this initial study was to estimate the effect size on standardized neuropsychological measures of memory. We found that older adults could learn the training program quickly, and could use it entirely unsupervised for the majority of the time required. Pre- and posttesting documented a significant improvement in memory within the training group (effect size 0.41, p<0.0005), with no significant within-group changes in a time-matched computer using active control group, or in a no-contact control group. Thus, a brain-plasticity-based intervention targeting normal age-related cognitive decline may potentially offer benefit to a broad population of older adults.

Myelination: an overlooked mechanism of synaptic plasticity?

Myelination of the brain continues through childhood into adolescence and early adulthood--the question is, Why? Two new articles provide intriguing evidence that myelination may be an underappreciated mechanism of activity-dependent nervous system plasticity: one study reported increased myelination associated with extensive piano playing, another indicated that rats have increased myelination of the corpus callosum when raised in environments providing increased social interaction and cognitive stimulation. These articles make it clear that activity-dependent effects on myelination cannot be considered strictly a developmental event. They raise the question of whether myelination is an overlooked mechanism of activity-dependent plasticity, extending in humans until at least age 30. It has been argued that regulating the speed of conduction across long fiber tracts would have a major influence on synaptic response, by coordinating the timing of afferent input to maximize temporal summation. The increase in synaptic amplitude could be as large as neurotransmitter-based mechanisms of plasticity, such as LTP. These new findings raise a larger question: How did the oligodendrocytes know they were practicing the piano or that their environment was socially complex?

The Role of Glial Adenosine Receptors in Neural Resilience and the Neurobiology of Mood Disorders

Adenosine receptors were classified into A1- and A2-receptors in the laboratory of Bernd Hamprecht more than 25 years ago. Adenosine receptors are instrumental to the neurotrophic effects of glia cells. Both microglia and astrocytes release after stimulation via adenosine receptors factors that are important for neuronal survival and growth. Neuronal resilience is now considered as of pivotal importance in the neurobiology of mood disorders and their treatment. Both sleep deprivation and electroconvulsive therapy, two effective therapeutic measures in mood disorders, are associated with an increase of adenosine and upregulation of adenosine A1-receptors in the brain. Parameters closely related to adenosine receptor activation such as cerebral metabolic rate and delta power in the sleep EEG provide indirect evidence that adenosinergic signaling may be associated with the therapeutic response to these measures. Thus, neurotrophic effects evoked by adenosine receptors might be important in the mechanism of action of ECT and perhaps also sleep deprivation.

Mechanisms of axon ensheathment and myelin growth

The evolution of complex nervous systems in vertebrates has been accompanied by, and probably dependent on, the acquisition of the myelin sheath. Although there has been substantial progress in our understanding of the factors that determine glial cell fate, much less is known about the cellular mechanisms that determine how the myelin sheath is extended and stabilized around axons. This review highlights four crucial stages of myelination, namely, the selection of axons and initiation of cell-cell interactions between them and glial cells, the establishment of stable intercellular contact and assembly of the nodes of Ranvier, regulation of myelin thickness and, finally, longitudinal extension of myelin segments in response to the lengthening of axons during postnatal growth.

Adenosine signaling promotes neuronal, catecholaminergic differentiation of primary neural crest cells and CNS-derived CAD cells

In neural crest (NC) cultures cAMP signaling is an instructive signal in catecholaminergic, sympathoadrenal cell development. However, the extracellular signals activating the cAMP pathway during NC cell development have not been identified. We demonstrate that in avian NC cultures, evidenced by tyrosine hydroxylase expression and catecholamine biosynthesis, adenosine and not adrenergic signaling, together with BMP2, promotes sympathoadrenal cell development. In NC cultures, addition of the adenosine receptor agonist NECA in the presence of BMP2 promotes sympathoadrenal cell development, whereas the antagonist CGS 15943 or the adenosine degrading enzyme adenosine deaminase (ADA) suppresses TH expression. Importantly, NC cells express A2A and A2B receptors which couple with Gsalpha increasing intracellular cAMP. Employing the CNS-derived catecholaminergic CAD cell line, we also demonstrate that neuronal differentiation mediated by serum withdrawal is further enhanced by treatment with IBMX, a cAMP-elevating agent, or the adenosine receptor agonist NECA, acting via cAMP. By contrast, the adenosine receptor antagonist CGS 15943 or the adenosine degrading enzyme ADA inhibits CAD cell neuronal differentiation mediated by serum withdrawal. These results support that adenosine is a physiological signal in neuronal differentiation of the CNS-derived catecholaminergic CAD cell line and suggest that adenosine signaling is involved in NC cell development in vivo.

Genetic and pharmacological inactivation of adenosine A2A receptor reveals an Egr-2-mediated transcriptional regulatory network in the mouse striatum

The adenosine A2A receptor (A2AR) is highly expressed in the striatum, where it modulates motor and emotional behaviors. We used both microarray and bioinformatics analyses to compare gene expression profiles by genetic and pharmacological inactivation of A2AR and inferred an A2AR-controlled transcription network in the mouse striatum. A comparison between vehicle (VEH)-treated A2AR knockout (KO) mice (A2AR KO-VEH) and wild-type (WT) mice (WT-VEH) revealed 36 upregulated genes that were partially mimicked by treatment with SCH-58261 (SCH; an A2AR antagonist) and 54 downregulated genes that were not mimicked by SCH treatment. We validated the A2AR as a specific drug target for SCH by comparing A2AR KO-SCH and A2AR KO-VEH groups. The unique downregulation effect of A2AR KO was confirmed by comparing A2AR KO-SCH with WT-SCH gene groups. The distinct striatal gene expression profiles induced by A2AR KO and SCH should provide clues to the molecular mechanisms underlying the different phenotypes observed after genetic and pharmacological inactivation of A2AR. Furthermore, bioinformatics analysis discovered that Egr-2 binding sites were statistically overrepresented in the proximal promoters of A2AR KO-affected genes relative to the unaffected genes. This finding was further substantiated by the demonstration that the Egr-2 mRNA level increased in the striatum of both A2AR KO and SCH-treated mice and that striatal Egr-2 binding activity in the promoters of two A2AR KO-affected genes was enhanced in A2AR KO mice as assayed by chromatin immunoprecipitation. Taken together, these results strongly support the existence of an Egr-2-directed transcriptional regulatory network controlled by striatal A2ARs.

Rotenone-induced caspase 9/3-independent and -dependent cell death in undifferentiated and differentiated human neural stem cells

We used human neural stem cells (hNSCs) and their differentiated cultures as a model system to evaluate the mechanism(s) involved in rotenone (RO)- and camptothecin (CA)-induced cytotoxicity. Results from ultrastructural damage and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) staining indicated that RO-induced cytotoxicity resembled CA-induced apoptosis more than H(2)O(2)-induced necrosis. However, unlike CA-induced, caspase 9/3-dependent apoptosis, there was no increased activity in caspase 9, caspase 3 or poly (ADP-ribose) polymerase (PARP) cleavage in RO-induced cytotoxicity, in spite of time-dependent release of cytochrome c and apoptosis-inducing factor (AIF) following mitochondrial membrane depolarization and a significant increase in reactive oxygen species generation. Equal doses of RO and CA used in hNSCs induced caspase 9/3-dependent apoptosis in differentiated cultures. Time-dependent ATP depletion occurred earlier and to a greater extent in RO-treated hNSCs than in CA-treated hNSCs, or differentiated cultures treated with RO or CA. In conclusion, these results represent a unique ultrastructural and molecular characterization of RO- and CA-induced cytotoxicity in hNSCs and their differentiated cultures. Intracellular ATP levels may play an important role in determining whether neural progenitors or their differentiated cells follow a caspase 9/3-dependent or -independent pathway in response to acute insults from neuronal toxicants.

Promoting repair in multiple sclerosis: problems and prospects

PURPOSE OF REVIEW

Despite recent progress in treating the inflammatory component of multiple sclerosis, current therapies have no clear impact on progression of disability, which closely relates to tissue (myelin and axon) injury. Many scientists now focus their efforts on elucidating the mechanisms that lead to tissue injury, and on developing new strategies for tissue repair. We review recent breakthroughs in this field and discuss their putative applications to therapy.

RECENT FINDINGS

Several hypotheses have been raised to explain the failure of remyelination, including depletion of remyelinating cells, quiescence of oligodendrocyte precursor cells and axonal inhibitory signals. Success in remyelination therapy may be achieved either by enhancing endogenous repair or by grafting exogenous remyelinating cells. Several neurotrophic factors have been shown to enhance endogenous remyelination, and many immature cells have been shown to induce efficient exogenous remyelination in animal models. Although effective remyelination probably represents the best way to prevent neurodegeneration, several alternative neuroprotective strategies are emerging. Statins, cyclins and immunophilin ligands are orally available immunomodulatory agents that may protect neurones. Other promising possibilities include the modulation of excitotoxicity, nitric oxide synthesis, or cationic channels.

SUMMARY

Despite the increasing number of putative therapeutic targets, no treatment to achieve remyelination or neuroprotection has yielded positive clinical results in humans. Forging a link between basic biology and treatment of patients will require us to overcome several challenges, including assessment of efficacy of repair, improving tolerance to and delivery of neurotrophic factors, and better defining the indications for and limitations of transplantation.

Axonal signals in central nervous system myelination, demyelination and remyelination

Axonal signals are key players in central nervous system myelination. During development, the onset of myelination depends on a balance between positive and negative axonal signals. Among negative signals are inhibitory adhesion molecules that need to be removed from the cell surface for the myelination process to proceed. Positive signals necessary to initiate myelination consist of both interactions with specific adhesion molecules and electrical activity-induced release of promyelinating factors. In multiple sclerosis, demyelination induces major modifications of axonal surface components. The disruption of these factors might participate to the failure of the myelin repair process.

Metabotropic P2 receptor activation regulates oligodendrocyte progenitor migration and development

To gain insights into the role of purinergic receptors in oligodendrocyte development, we characterized the expression and functional activity of P2 receptors in cultured rat oligodendrocyte progenitors and investigated the effects of ATP and its breakdown products on the migration and proliferation of this immature glial cell population. Using Western blot analysis, we show that oligodendrocyte progenitors express several P2X (P2X(1,2,3,4,7)) and P2Y (P2Y(1,2,4)) receptors. Intracellular Ca(2+) recording by Fura-2 video imaging allowed to determine the rank potency order of the P2 agonists tested: ADPbetaS = ADP = Benzoyl ATP > ATP > ATPgammaS > UTP, alpha,beta-meATP ineffective. Based on the above findings, on pharmacological inhibition by the antagonists oxATP and MRS2179, and on the absence of alpha,betameATP-induced inward current in whole-cell recording, P2X(7) and P2Y(1) were identified as the main ionotropic and metabotropic P2 receptors active in OPs. As a functional correlate of these findings, we show that ATP and, among metabotropic agonists, ADP and the P2Y(1)-specific agonist ADPbetaS, but not UTP, induce oligodendrocyte progenitor migration. Moreover, ATP and ADP inhibited the proliferation of oligodendrocyte progenitors induced by platelet-derived growth factor, both in purified cultures and in cerebellar tissue slices. The effects of ATP and ADP on cell migration and proliferation were prevented by the P2Y(1) antagonist MRS2179. By confocal laser scanning microscopy, P2Y(1) receptors were localized in NG2-labeled oligodendrocyte progenitors in the developing rat brain. These data indicate that ATP and ADP may regulate oligodendrocyte progenitor functions by a mechanism that involves mainly activation of P2Y(1) receptors.

Integrin-growth factor interactions as regulators of oligodendroglial development and function

Central nervous system (CNS) development requires mechanisms for the regulation of cell number. Although growth factors are essential determinants of the proliferation and apoptosis that determine final numbers, the long-range nature of signals from diffusible growth factors makes them insufficient for the provision of the precise and localized signals required. Integration of integrin and growth factor receptor signaling in controlling cell behavior has been an important theme of research over the past several years. The focus of this review is on the mechanisms by which integrin-growth factor interactions regulate the development of oligodendrocytes and provide a mechanism for controlling, both in space and in time, oligodendrocyte numbers in the developing CNS.

The optic nerve: A model for axon–glial interactions

The rodent optic nerve is a model tissue for the physiological investigation of axonal-glial interactions in a typical CNS white matter tract. There is strong evidence that nerve transmission is maintained by a considerable degree of dynamic signalling between axons and glia through a variety of mechanisms, such as regulation of the ionic environment, energy metabolism and calcium signalling. This review focuses on the methods used to examine axonal and glial functions and interactions, primarily in the rodent optic nerve. Techniques encompass intracellular microelectrodes, sucrose- and grease-gap recordings of membrane potentials, suction electrode to measure compound action potentials, the use of ion-sensitive electrodes, patch clamping and imaging. An overview of the advantages and drawbacks of each technique is given and the application of each to the understanding glial and axonal physiology is briefly discussed.

ATP regulates oligodendrocyte progenitor migration, proliferation, and differentiation: involvement of metabotropic P2 receptors

Extracellular nucleotides act as potent signaling molecules in the neuron-glia and glia-glia communication, via the activation of specific ligand-gated P2X and G-protein-coupled metabotropic P2Y receptors. Most of the data available about the effects of P2 receptor activation in the CNS concern astrocytes, microglia, and neurons. To gain insights into the role of purinergic receptors in oligodendrocyte development, we characterized the expression and functional activity of P2 receptors in rat oligodendrocyte progenitors (OPs) and investigated the effects of ATP and its breakdown products on their functions. We describe here that rat OPs express different types of P2 receptors and that nucleotide-induced Ca(2+) raises in these progenitor cells are mainly due to the activation of P2X(7) ionotropic and ADP-sensitive P2Y(1) metabotropic receptors. We also show that ATP and ADP stimulate OP migration, inhibit the mitogenic response of OPs to PDGF and promote oligodendrocyte differentiation. The pharmacological profile of the nucleotide-induced effects demonstrates the important regulatory role of P2Y(1) receptor signaling in OP functions. These findings suggest that ATP, which is released in high amounts under inflammatory conditions and following cell death, might regulate remyelination processes in inflammatory demyelinating diseases of the CNS, like multiple sclerosis.

Development of neurotransmitter systems during critical periods

Neurotransmitters are released from neurons and mediate neuronal communication. Neuromodulators can also be released from other cells and influence the neuronal signaling. Both neurotransmitters and neuromodulators play an important role in the shaping and the wiring of the nervous system possibly during critical windows of the development. Monoamines are expressed in the very early embryo, at which stage the notochord already contains high noradrenaline levels. Purines and neuropeptides are probably also expressed at an early stage, in a similar way as they occur during early phylogenesis. The levels of most neurotransmitters and neuromodulators increase concomitantly with synapse formation. Some of them surge during the perinatal period (such as glutamate, catecholamines, and some neuropeptides) and then level off. The interesting question is to what extent the expression of neuroactive agents is related to the functional state of the fetus and the newborn. Monoamines are expressed in the very early embryo, at which stage the notochord already contains high noradrenaline levels. They may have an important role for neurotransmission in the fetus. In the adult mammal, the fast switching excitatory amino acids dominate. However, they also seem to be important for the wiring of the brain and the plasticity before birth. NMDA receptors that are supposed to mediate these effects dominate and are then substituted by AMPA receptors. The main inhibitory amino acids gamma-aminobutyric acid (GABA) and glycine are excitatory in the developing brain by depolarizing developing neurons that have high Cl- concentrations. This seems to be of major importance for the wiring of neuronal circuits. Prenatal or neonatal stress, for example, hypoxia, can affect the programming of neurotransmitter and receptor expression, which can lead to long-term behavioral effects.

Engineering embryonic stem cell derived glia for adenosine delivery

Based on the anticonvulsant and neuroprotective properties of adenosine, and based on the long-term survival potential of stem cell derived brain implants, adenosine releasing stem cells may constitute a novel tool for the treatment of epilepsy. Pluripotency and unlimited self-renewal make embryonic stem (ES) cells a particularly versatile donor source for cell transplantation. With the aim to test the feasibility of a stem cell-based delivery system for adenosine, both alleles of adenosine kinase (ADK), the major adenosine-metabolizing enzyme, were disrupted by homologous recombination in ES cells. Adk-/- ES cells were subjected to a glial differentiation protocol and, as a result, gave rise to proliferating glial precursors, which could be further differentiated into mature astrocytes and oligodendrocytes. Thus, a lack of ADK does not compromise the glial differentiation potential of ES cells. The Adk-/- ES cells yielded glial populations with an adenosine release of up to 40.1 +/- 6.0 ng per 10(5) cells per hour, an amount considered to be sufficient for seizure suppression. Our findings indicate that Adk-/- ES cells constitute a potential source for therapeutic adenosine releasing grafts.

Delivery systems for small molecule drugs, proteins, and DNA: the neuroscience/biomaterial interface

Manipulation of cellular processes in vivo by the delivery of drugs, proteins or DNA is of paramount importance to neuroscience research. Methods for the presentation of these molecules vary widely, including direct injection (either systemic or stereotactic), osmotic pump-mediated chronic delivery, or even implantation of cells engineered to indefinitely secrete a factor of interest. Biomaterial-based delivery systems represent an alternative to more traditional approaches, with the possibility of increased efficacy. Drug-releasing biomaterials, either as injectable microspheres or as three-dimensional implants, can deliver a molecule of interest (including small molecule drugs, biologically active proteins, or DNA) over a more prolonged period of time than by standard bolus injection, avoiding the need for repeated administration. Furthermore, sustained-release systems can maintain therapeutic concentrations at a target site, thus reducing the chance for toxicity. This review summarizes applications of polymer-based delivery of small molecule drugs, proteins, and DNA specifically relevant to neuroscience research. We detail the fabrication procedures for the polymeric systems and their utility in various experimental models. The biomaterial field offers unique experimental tools with downstream clinical application for the study and treatment of neurologic disease.