Oligodendroglial lineage cells express neuroligand receptors

Cell of all trades: oligodendrocyte precursor cells in synaptic, vascular, and immune function

Oligodendrocyte precursor cells (OPCs) are not merely a transitory progenitor cell type, but rather a distinct and heterogeneous population of glia with various functions in the developing and adult central nervous system. In this review, we discuss the fate and function of OPCs in the brain beyond their contribution to myelination. OPCs are electrically sensitive, form synapses with neurons, support blood-brain barrier integrity, and mediate neuroinflammation. We explore how sex and age may influence OPC activity, and we review how OPC dysfunction may play a primary role in numerous neurological and neuropsychiatric diseases. Finally, we highlight areas of future research.

Glial Purinergic Signaling in Neurodegeneration

Purinergic signaling regulates neuronal and glial cell functions in the healthy CNS. In neurodegenerative diseases, purinergic signaling becomes dysregulated and can affect disease-associated phenotypes of glial cells. In this review, we discuss how cell-specific expression patterns of purinergic signaling components change in neurodegeneration and how dysregulated glial purinergic signaling and crosstalk may contribute to disease pathophysiology, thus bearing promising potential for the development of new therapeutical options for neurodegenerative diseases.

Neuron-oligodendroglia interactions: Activity-dependent regulation of cellular signaling

Oligodendrocyte lineage cells (oligodendroglia) and neurons engage in bidirectional communication throughout life to support healthy brain function. Recent work shows that changes in neuronal activity can modulate proliferation, differentiation, and myelination to support the formation and function of neural circuits. While oligodendroglia express a diverse collection of receptors for growth factors, signaling molecules, neurotransmitters and neuromodulators, our knowledge of the intracellular signaling pathways that are regulated by neuronal activity remains largely incomplete. Many of the pathways that modulate oligodendroglia behavior are driven by changes in intracellular calcium signaling, which may differentially affect cytoskeletal dynamics, gene expression, maturation, integration, and axonal support. Additionally, activity-dependent neuron-oligodendroglia communication plays an integral role in the recovery from demyelinating injuries. In this review, we summarize the modalities of communication between neurons and oligodendroglia and explore possible roles of activity-dependent calcium signaling in mediating cellular behavior and myelination.

Neuronal Activity-Dependent Control of Postnatal Neurogenesis and Gliogenesis

The addition of new neurons and oligodendroglia in the postnatal and adult mammalian brain presents distinct forms of gray and white matter plasticity. Substantial effort has been devoted to understanding the cellular and molecular mechanisms controlling postnatal neurogenesis and gliogenesis, revealing important parallels to principles governing the embryonic stages. While during central nervous system development, scripted temporal and spatial patterns of neural and glial progenitor proliferation and differentiation are necessary to create the nervous system architecture, it remains unclear what driving forces maintain and sustain postnatal neural stem cell (NSC) and oligodendrocyte progenitor cell (OPC) production of new neurons and glia. In recent years, neuronal activity has been identified as an important modulator of these processes. Using the distinct properties of neurotransmitter ionotropic and metabotropic channels to signal downstream cellular events, NSCs and OPCs share common features in their readout of neuronal activity patterns. Here we review the current evidence for neuronal activity-dependent control of NSC/OPC proliferation and differentiation in the postnatal brain, highlight some potential mechanisms used by the two progenitor populations, and discuss future studies that might advance these research areas further.

Purinergic signaling in oligodendrocyte development and function

Myelin, an insulating membrane that enables rapid action potential propagation, is an essential component of an efficient, functional vertebrate nervous system. Oligodendrocytes, the myelinating glia of the central nervous system (CNS), produce myelin throughout the CNS, which requires continuous proliferation, migration, and differentiation of oligodendrocyte progenitor cells. Because myelination is essential for efficient neurotransmission, researchers hypothesize that neuronal signals may regulate the cascade of events necessary for this process. The ability of oligodendrocytes and oligodendrocyte progenitor cells to detect and respond to neuronal activity is becoming increasingly appreciated, although the specific signals involved are still a matter of debate. Recent evidence from multiple studies points to purinergic signaling as a potential regulator of oligodendrocyte development and differentiation. Adenosine triphosphate (ATP) and its derivatives are potent signaling ligands with receptors expressed on many populations of cells in the nervous system, including cells of the oligodendrocyte lineage. Release of ATP into the extracellular space can initiate a multitude of signaling events, and these downstream signals are specific to the particular purinergic receptor (or receptors) expressed, and whether enzymes are present to hydrolyze ATP to its derivatives adenosine diphosphate and adenosine, each of which can activate their own unique downstream signaling cascades. This review will introduce purinergic signaling in the CNS and discuss evidence for its effects on oligodendrocyte proliferation, differentiation, and myelination. We will review sources of extracellular purines in the nervous system and how changes in purinergic receptor expression may be coupled to oligodendrocyte differentiation. We will also briefly discuss purinergic signaling in injury and diseases of the CNS.

Intertwining extracellular nucleotides and their receptors with Ca2+ in determining adult neural stem cell survival, proliferation and final fate

In the central nervous system (CNS), during both brain and spinal cord development, purinergic and pyrimidinergic signalling molecules (ATP, UTP and adenosine) act synergistically with peptidic growth factors in regulating the synchronized proliferation and final specification of multipotent neural stem cells (NSCs) to neurons, astrocytes or oligodendrocytes, the myelin-forming cells. Some NSCs still persist throughout adulthood in both specific 'neurogenic' areas and in brain and spinal cord parenchyma, retaining the potentiality to generate all the three main types of adult CNS cells. Once CNS anatomical structures are defined, purinergic molecules participate in calcium-dependent neuron-to-glia communication and also control the behaviour of adult NSCs. After development, some purinergic mechanisms are silenced, but can be resumed after injury, suggesting a role for purinergic signalling in regeneration and self-repair also via the reactivation of adult NSCs. In this respect, at least three different types of adult NSCs participate in the response of the adult brain and spinal cord to insults: stem-like cells residing in classical neurogenic niches, in particular, in the ventricular-subventricular zone (V-SVZ), parenchymal oligodendrocyte precursor cells (OPCs, also known as NG2-glia) and parenchymal injury-activated astrocytes (reactive astrocytes). Here, we shall review and discuss the purinergic regulation of these three main adult NSCs, with particular focus on how and to what extent modulation of intracellular calcium levels by purinoceptors is mandatory to determine their survival, proliferation and final fate.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

Systematic Review of Pharmacological Properties of the Oligodendrocyte Lineage

Oligodendrogenesis and oligodendrocyte precursor maturation are essential processes during the course of central nervous system development, and lead to the myelination of axons. Cells of the oligodendrocyte lineage are generated in the germinal zone from migratory bipolar oligodendrocyte precursor cells (OPCs), and acquire cell surface markers as they mature and respond specifically to factors which regulate proliferation, migration, differentiation, and survival. Loss of myelin underlies a wide range of neurological disorders, some of an autoimmune nature—multiple sclerosis probably being the most prominent. Current therapies are based on the use of immunomodulatory agents which are likely to promote myelin repair (remyelination) indirectly by subverting the inflammatory response, aspects of which impair the differentiation of OPCs. Cells of the oligodendrocyte lineage express and are capable of responding to a diverse array of ligand-receptor pairs, including neurotransmitters and nuclear receptors such as γ-aminobutyric acid, glutamate, adenosine triphosphate, serotonin, acetylcholine, nitric oxide, opioids, prostaglandins, prolactin, and cannabinoids. The intent of this review is to provide the reader with a synopsis of our present state of knowledge concerning the pharmacological properties of the oligodendrocyte lineage, with particular attention to these receptor-ligand (i.e., neurotransmitters and nuclear receptor) interactions that can influence oligodendrocyte migration, proliferation, differentiation, and myelination, and an appraisal of their therapeutic potential. For example, many promising mediators work through Ca²⁺ signaling, and the balance between Ca²⁺ influx and efflux can determine the temporal and spatial properties of oligodendrocytes (OLs). Moreover, Ca²⁺ signaling in OPCs can influence not only differentiation and myelination, but also process extension and migration, as well as cell death in mature mouse OLs. There is also evidence that oligodendroglia exhibit Ca²⁺ transients in response to electrical activity of axons for activity-dependent myelination. Cholinergic antagonists, as well as endocannabinoid-related lipid-signaling molecules target OLs. An understanding of such pharmacological pathways may thus lay the foundation to allow its leverage for therapeutic benefit in diseases of demyelination.

Electrophysiological properties of NG2(+) cells: Matching physiological studies with gene expression profiles

NG2(+) glial cells are a dynamic population of non-neuronal cells that give rise to myelinating oligodendrocytes in the central nervous system. These cells express numerous ion channels and neurotransmitter receptors, which endow them with a complex electrophysiological profile that is unique among glial cells. Despite extensive analysis of the electrophysiological properties of these cells, relatively little was known about the molecular identity of the channels and receptors that they express. The generation of new RNA-Seq datasets for NG2(+) cells has provided the means to explore how distinct genes contribute to the physiological properties of these progenitors. In this review, we systematically compare the results obtained through RNA-Seq transcriptional analysis of purified NG2(+) cells to previous physiological and molecular studies of these cells to define the complement of ion channels and neurotransmitter receptors expressed by NG2(+) cells in the mammalian brain and discuss the potential significance of the unique physiological properties of these cells. This article is part of a Special Issue entitled SI:NG2-glia(Invited only).

Voltage-gated Ca2+ entry promotes oligodendrocyte progenitor cell maturation and myelination in vitro

We have previously shown that the expression of voltage-operated Ca(++) channels (VOCCs) is highly regulated in the oligodendroglial lineage and is essential for proper oligodendrocyte progenitor cell (OPC) migration. Here we assessed the role of VOCCs, in particular the L-type, in oligodendrocyte maturation. We used pharmacological treatments to activate or block voltage-gated Ca(++) uptake and siRNAs to specifically knock down the L-type VOCC in primary cultures of mouse OPCs. Activation of VOCCs by plasma membrane depolarization increased OPC morphological differentiation as well as the expression of mature oligodendrocyte markers. On the contrary, inhibition of L-type Ca(++) channels significantly delayed OPC development. OPCs transfected with siRNAs for the Cav1.2 subunit that conducts L-type Ca(++) currents showed reduce Ca(++) influx by ~75% after plasma membrane depolarization, indicating that Cav1.2 is heavily involved in mediating voltage-operated Ca(++) entry in OPCs. Cav1.2 knockdown induced a decrease in the proportion of oligodendrocytes that expressed myelin proteins, and an increase in cells that retained immature oligodendrocyte markers. Moreover, OPC proliferation, but not cell viability, was negatively affected after L-type Ca(++) channel knockdown. Additionally, we have tested the ability of L-type VOCCs to facilitate axon-glial interaction during the first steps of myelin formation using an in vitro co-culture system of OPCs with cortical neurons. Unlike control OPCs, Cav1.2 deficient oligodendrocytes displayed a simple morphology, low levels of myelin proteins expression and appeared to be less capable of establishing contacts with neurites and axons. Together, this set of in vitro experiments characterizes the involvement of L-type VOCCs on OPC maturation as well as the role played by these Ca(++) channels during the early phases of myelination.

Light-controlled astrocytes promote human mesenchymal stem cells toward neuronal differentiation and improve the neurological deficit in stroke rats

Astrocytes are key components of the central nervous system (CNS) and release factors to support neural stem cell proliferation, differentiation, and migration. Adenosine 5'-triphosphate (ATP) is one of the key factors released upon activation of astrocytes that regulates the neural stem cell's function. However, it is not clear whether ATP derived from the depolarized astrocytes plays a vital role in promoting the neuronal differentiation of mesenchymal stem cells (MSCs) in vitro and in vivo. Herein, for the first time, we co-cultured MSCs with light-stimulated-channelrhodopsin-2 (ChR2)-astrocytes, and observed that the neuronal differentiation of MSCs was enhanced by expressing more neuronal markers, Tuj1 and NeuN. The ChR2-astrocyte-conditioned medium also stimulated MSCs differentiating into neuronal lineage cells by expressing more Tuj1 and Pax6, which was blocked by the P2X receptor antagonist, TNP-ATP. Then we found that light-depolarization of astrocytes significantly increased ATP accumulation in their bathing medium without impairing the cell membrane. We further found that ATP up-regulated the Tuj1, Pax6, FZD8 and β-catenin mRNA levels of MSCs, which could be reversed by application of TNP-ATP. Together these in vitro data provided convergent evidence that ATP from light-depolarized-astrocytes activated the wnt/β-catenin signaling of MSCs through binding to the P2X receptors, and promoted the neuronal differentiation of MSCs. Finally but importantly, our study also demonstrated in stroke rats that light-controlled astrocytes stimulated endogenous ATP release into the ischemic area to influence the transplanted MSCs, resulting in promoting the MSCs towards neuronal differentiation and improvements of neurological deficit.

Muscarinic receptor subtypes as potential targets to modulate oligodendrocyte progenitor survival, proliferation, and differentiation

Acetylcholine (ACh) is a major neurotransmitter but also an important signaling molecule in neuron-glia interactions. Expression of ACh receptors has been reported in several glial cell populations, including oligodendrocytes (OLs). Nonetheless, the characterization of muscarinic receptors in these cells, as well as the description of the cholinergic effects at different stages of OL development, is still incomplete. In this study, we characterized the pattern of expression of muscarinic receptor subtypes in primary cultures of rat oligodendrocyte progenitor cells (OPC) and mature OLs, at both mRNA and protein levels. We found that muscarinic receptor expression is developmentally regulated. M1, M3, and M4 receptors were the main subtypes expressed in OPC, whereas all receptor subtypes were expressed at low levels in mature OLs. Exposure of OPC to muscarine enhanced cell proliferation, an effect mainly due to M1, M3, and M4 receptor subtypes as demonstrated by pharmacological competition with selective antagonists. Conversely, M2 receptor activation impaired OPC survival. In line with the mitogenic activity, muscarinic receptor activation increased the expression of platelet derived growth factor receptor α. Muscarine stimulation increased CX32 and myelin basic protein expression, left unaffected that of myelin proteolipid protein (PLP), and decreased member of the family of epidermal growth factor receptor (EGFR) ErbB3/ErbB4 receptor expression indicating a predominant role of muscarinic receptors in OPC. These findings suggest that ACh may contribute to the maintenance of an immature proliferating progenitor pool and impair the progression toward mature stage. This hypothesis is further supported by increased expression of Notch-1 in OL on muscarinic activation.

Effects of glutamate receptor activation on NG2-glia in the rat optic nerve

NG2-glia are a substantial population of cells in the central nervous system (CNS) that can be identified by their specific expression of the NG2 chondroitin sulphate (CSPG). NG2-glia can generate oligodendrocytes, but it is unlikely this is their only function; indeed, they may be multipotent neural stem cells. Moreover, NG2-glia are a highly reactive cell type and a major function is to help form the axon growth inhibitory glial scar in response to CNS injury. The factors that regulate these diverse behaviours of NG2-glia are not fully resolved, but NG2-glia express receptors to the neurotransmitter glutamate, which has known potent effects on other glia. Here, we have examined the actions of glutamate receptor activation on NG2-glia in the rat optic nerve, a typical CNS white matter tract that does not contain neuronal cell bodies. Glutamate induces an increase in [Ca(2+)](i) in immuno-identified NG2-glia in situ and in vitro. In addition, we examined the effects of glutamate receptor activation in vivo by focal injection of the glutamate receptor agonist kainate into the optic nerve; saline was injected in controls. Changes in glial and axonal function were determined at 7 days post injection (dpi), by immunohistochemistry and electrophysiological measurement of the compound action potential (CAP). Injection of kainate resulted in a highly localized 'injury response' in NG2-glia, marked by dense labelling for NG2 at the lesion site, as compared to astrocytes, which displayed a more extensive reactive astrogliosis. Furthermore, injection of kainate resulted in an axonal conduction block. These glial and axonal changes were not observed following injection of saline vehicle. In addition, we provide evidence that endogenous glutamate induces calcium-dependent phosphorylation of extracellular signal-regulated kinases (ERK1/2), which may provide a potential mechanism by which glutamate-mediated changes in raised intracellular calcium could regulate the observed gliosis. The results provide evidence that activation of AMPA-kainate type ionotropic glutamate receptors evoke raised calcium in NG2-glia and induces an injury response in NG2-glia.

Increased expression of golli myelin basic proteins enhances calcium influx into oligodendroglial cells

The myelin basic protein (MBP) gene encodes two families of proteins: the classic MBP constituents of myelin and the golli-MBPs, the function of which is less well understood. Previous work suggests that golli proteins may play a role in Ca2+ homeostasis in oligodendrocytes (OLs) and in T-cells. Overexpression of golli in OL cell lines induces elaboration of sheets and processes. Live imaging of these cells revealed a rapid retraction of the processes and sheets after depolarization with high K+. This phenomenon was associated with a significant increase in [Ca2+]int without changes in cell viability. The results indicated that golli produced its effect through Ca2+ influx, rather than Ca2+ release from intracellular stores. Furthermore, a specific [Ca2+]int chelator (BAPTA) or Cd2+, a specific blocker of voltage-operated Ca2+ channels, abolished the ability of golli to promote process extension in a dose-dependent manner. Analysis of the golli protein identified a myristoylation site at the C terminus of the golli domain, which was essential for the action of golli on Ca2+ influx, suggesting that binding of golli to the plasma membrane is important for modulating Ca2+ homeostasis. High-resolution spatiotemporal analysis along N19 processes revealed higher-amplitude local Ca2+ influx in regions with elevated levels of golli. These findings suggest a key role for golli proteins in regulating voltage-gated Ca2+ channels in OLs during process remodeling. Our observations are consistent with the hypothesis that golli proteins, as a part of a protein complex, modulate Ca2+ influx at the plasma membrane and along OL processes.

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.

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).

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.

General mechanisms of axonal damage and its prevention

Axonal degeneration is a prominent pathological feature in multiple sclerosis observed over a century ago. The gradual loss of axons is thought to underlie irreversible clinical deficits in this disease. The precise mechanisms of axonopathy are poorly understood, but likely involve excess accumulation of Ca ions. In healthy fibers, ATP-dependent pumps support homeostasis of ionic gradients. When energy supply is limited, either due to inadequate delivery (e.g., ischemia, mitochondrial dysfunction) and/or excessive utilization (e.g., conduction along demyelinated axons), ion gradients break down, unleashing a variety of aberrant cascades, ultimately leading to Ca overload. During Na pump dysfunction, Na can enter axons through non-inactivating Na channels, promoting axonal Na overload and depolarization by allowing K egress. This will gate voltage-sensitive Ca channels and stimulate reverse Na-Ca exchange, leading to further Ca entry. Energy failure will also promote Ca release from intracellular stores. Neurotransmitters such as glutamate can be released by reverse operation of Na-dependent transporters, in turn activating a variety of ionotropic and metabotropic receptors, further exacerbating overload of cellular Ca. Together, this Ca overload will inappropriately stimulate a variety of Ca-dependent enzyme systems (e.g., calpains, phospholipases), leading to structural and functional axonal injury. Pharmacological interruption at key points in these interrelated injury cascades (e.g., at voltage-gated Na channels or AMPA receptors) may confer significant neuroprotection to compromised central axons and supporting glia. Such agents may represent attractive adjuncts to currently available immunomodulatory therapies.

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.

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.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Oligodendrocytes and ischemic brain injury

Oligodendrocytes, myelin-forming glial cells of the central nervous system, are vulnerable to damage in a variety of neurologic diseases. Much is known of primary myelin injury, which occurs in settings of genetic dysmyelination or demyelinating disease. There is growing awareness that oligodendrocytes are also targets of injury in acute ischemia. Recognition of oligodendrocyte damage in animal models of ischemia requires attention to their distinct histologic features or use of specific immunocytochemical markers. Like neurons, oligodendrocytes are highly sensitive to injury by oxidative stress, excitatory amino acids, trophic factor deprivation, and activation of apoptotic pathways. Understanding mechanisms of oligodendrocyte death may suggest new therapeutic strategies to preserve or restore white matter function and structure after ischemic insults.

Regulation of muscarinic receptor function in developing oligodendrocytes by agonist exposure

1 Oligodendrocytes, the myelin forming cells in the CNS, express muscarinic acetylcholine receptors (mAChR), primarily M3, coupled to various signal transduction pathways. 2 In the present study we have investigated whether mAChR undergo functional agonist-induced regulation in cultured oligodendrocyte progenitors and differentiated oligodendrocytes. 3 The muscarinic agonist, carbachol (CCh) caused a time-dependent desensitization of phosphoinositide (PI) hydrolysis, and the internalization and down-regulation of receptors. Short-time desensitization (5 min) of PI hydrolysis occurred without receptor internalization and reached 54% by 1 h. The same treatment decreased cell surface receptors labelled with the non-permeable ligand [(3)H]-NMS by 47%, while total receptor density ([(3)H]-scopolamine binding) decreased by 30%. Longer CCh treatment down-regulated receptors by 70% and desensitized the PI response by 80%. 4 Although protein kinase C (PKC) activation desensitized mAChR, CCh-mediated desensitization was independent of PKC. 5 Inhibition of receptor endocytosis by low temperature during the pre-stimulation period or in the presence of hyperosmotic sucrose (0.5 M) blocked desensitization, receptor internalization and down-regulation. 6 Recovery of surface mAChR and their functional activity following down-regulation was slow, returning to control levels by 24 h after agonist removal. In progenitor cells, dose-response curves for CCh-mediated PI hydrolysis and c-fos mRNA expression showed that newly synthesized mAChR were supersensitive after recovery. 7 Overall, the present results provide evidence of functional agonist-mediated mAChR regulation in brain oligodendroglial cells.

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

Neuronal activity influences myelination of the brain, but the molecular mechanisms involved are largely unknown. Here, we report that oligodendrocyte progenitor cells (OPCs) express functional adenosine receptors, which are activated in response to action potential firing. Adenosine acts as a potent neuron-glial transmitter to inhibit OPC proliferation, stimulate differentiation, and promote the formation of myelin. This neuron-glial signal provides a molecular mechanism for promoting oligodendrocyte development and myelination in response to impulse activity and may help resolve controversy on the opposite effects of impulse activity on myelination in the central and peripheral nervous systems.

P2Y and P2X purinoceptor mediated Ca2+ signalling in glial cell pathology in the central nervous system

Activation of purinoceptors by extracellular ATP is an important component of the glial response to injury in the central nervous system (CNS). ATP has been shown to evoke raised cytosolic [Ca(2+)] in astrocytes, oligodendrocytes, and microglia, the three major glial cell types in the CNS. Glial cells express a heterogenous collection of metabotropic P2Y and ionotropic P2X purinoceptors, which respectively mobilise Ca(2+) from intracellular stores and trigger Ca(2+) influx across the plasmalemma. It is likely that different receptors have distinct roles in glial cell physiology and pathology. Our studies on optic nerve glia in situ indicate that P2Y(1) and P2Y(2/4) receptors are activated at low ATP concentrations, suggesting they are the predominant purinoceptors mediating physiological Ca(2+) signalling. Glia also express P2X(1) and P2X(3) purinoceptors, which mediate fast, rapidly desensitising current and may also be important in signalling. At high concentrations, such as occur in CNS injury, ATP induces large and prolonged increases in glial [Ca(2+)](i) with a primary role for P2Y purinoceptors and inositol trisphosphate (IP(3))-dependent release of Ca(2+) from intracellular stores. In addition, we found that high concentrations of ATP activated a significant P2X component that did not desensitise or saturate and was dependent on extracellular Ca(2+). These are characteristic properties of the P2X(7) subtype, and we provide in situ evidence that application of the P2X(7) receptor agonist benzoyl-benzoyl ATP (BzATP) evokes raised [Ca(2+)](i) in optic nerve glia, and that the dye YO-PRO-1, which passes through pore-forming P2X(7) receptors, is taken up by astrocytes, oligodendrocytes and microglia. Glia also express P2X(2) and P2X(4) receptors that are also pore-forming in the presence of sustained high ATP concentrations and which may also be important in the glial injury response. There is evidence that activation of P2 purinoceptors is a key step in triggering reactive changes in glial cells, including expression of immediate early genes, induction of extracellular signal regulated kinase and cyclooxygenase-2, synthesis of phospholipase A(2), release of arachidonic acid, production of prostaglandins and release of interleukins. We show that the ATP-mediated increase in glial [Ca(2+)](i) is potentiated by arachidonic acid and reduced by the inhibition of phospholipase A(2) inhibition. Together, the results implicate ATP as a primary signalling molecule in glial cells and indicate specific roles for P2Y and P2X purinoceptors in glial cell pathology.

A population of oligodendrocytes derived from multipotent neural precursor cells expresses a cholinergic phenotype in culture and responds to ciliary neurotrophic factor

Because oligodendrocytes and their precursors possess receptors for classical transmitters, and neurotransmitters such as glutamate and noradrenaline can mediate oligodendroglial proliferation and differentiation, it is possible that other neurotransmitters can also exert regulatory roles in oligodendrocyte function. We used mitogen-proliferated multipotent neuroepithelial precursors (neurospheres) and identified oligodendroglia that expressed markers traditionally found in cholinergic neurons. Regardless of culture conditions, there existed a large population of cells that resembled oligodendrocytes morphologically and coexpressed the oligodendrocyte-specific marker galactocerebroside (GalC) and the acetylcholine (ACh)-synthesizing enzyme choline acetyltransferase (ChAT). These cells did not express neuronal markers, and whole-cell recordings from cells with similar morphology displayed only outward currents in response to depolarizing voltage steps, further supporting their oligodendroglial identity. Another cholinergic marker, the vesicular ACh transporter, was also detected in GalC(+) oligodendrocytes. Furthermore, neurospheres cultured in the presence of the cholinergic receptor antagonist atropine showed a decrease in the number of GalC(+) spheres, implicating the muscarinic ACh receptor in oligodendrocyte development. The actions of neurotrophins and ciliary neurotrophic factor (CNTF) on these ChAT(+) oligodendrocytes were examined. Among these, CNTF treatment significantly increased oligodendrocytic process outgrowth. These results demonstrate classical cholinergic neuronal markers in oligodendrocytes as well as an effect of muscarinic receptor blockade on oligodendrocyte differentiation.

Expression of functional NK-1 receptors in murine microglia

Cells of myeloid origin such as microglia have the potential to contribute significantly to the development of inflammatory responses in the CNS. The ability of the neuropeptide substance P to augment proinflammatory responses by other myeloid cell types such as macrophages and dendritic cells is well recognized. In the present study, we demonstrate the presence of mRNA encoding NK-1 (substance P) receptors in murine microglia cell lines. Importantly, we have utilized specific antibodies developed by our laboratory to detect the expression of the NK-1 receptor protein in murine microglia cell lines by Western blot analysis and flow cytometry. Furthermore, we have investigated the presence of this receptor on primary murine microglia and report the presence of authentic NK-1 receptors as determined by Western blot analysis and flow cytometry. In addition, we demonstrate that NK-1 receptors expressed on microglia are functional as demonstrated by the ability of nanomolar concentrations of substance P to initiate activation of the transcriptional activator, NF-kappaB. Given the weight of evidence supporting the role of substance P--substance P receptor interactions in the initiation of optimal proinflammatory responses by myeloid cells, the demonstration of authentic and functional NK-1 receptors in microglia identifies this neuropeptide as a potentially important contributor to CNS inflammatory responses during disease states.

Sparks and puffs in oligodendrocyte progenitors: cross talk between ryanodine receptors and inositol trisphosphate receptors

Investigating how calcium release from the endoplasmic reticulum (ER) is triggered and coordinated is crucial to our understanding of how oligodendrocyte progenitor cells (OPs) develop into myelinating cells. Sparks and puffs represent highly localized Ca(2+) release from the ER through ryanodine receptors (RyRs) and inositol trisphosphate receptors (IP(3)Rs), respectively. To study whether sparks or puffs trigger Ca(2+) waves in OPs, we performed rapid high-resolution line scan recordings in fluo-4-loaded OP processes. We found spontaneous and evoked sparks and puffs, and we have identified functional cross talk between IP(3)Rs and RyRs. Local events evoked using the IP(3)-linked agonist methacholine (MeCh) showed significantly different morphology compared with events evoked using the caffeine analog 3,7-dimethyl-1-propargylxanthine (DMPX). Pretreatment with MeCh potentiated DMPX-evoked events, whereas inhibition of RyRs potentiated events evoked by low concentrations of MeCh. Furthermore, activation of IP(3)Rs but not RyRs was critical for Ca(2+) wave initiation. Using immunocytochemistry, we show OPs express the specific Ca(2+) release channel subtypes RyR3 and IP(3)R2 in patches along OP processes. RyRs are coexpressed with IP(3)Rs in some patches, but IP(3)Rs are also found alone. This differential distribution pattern may underlie the differences in local and global Ca(2+) signals mediated by these two receptors. Thus, in OPs, interactions between IP(3)Rs and RyRs determine the spatial and temporal characteristics of calcium signaling, from microdomains to intracellular waves.

Pharmacological and functional characterization of muscarinic receptor subtypes in developing oligodendrocytes

This study focused on the molecular and pharmacological characterization of muscarinic acetylcholine receptors expressed by progenitors and differentiated oligodendrocytes. We also analyzed the role of muscarinic receptors in regulating downstream signal transduction pathways and the functional significance of receptor expression in oligodendrocytes. RT-PCR analysis revealed the expression of transcripts for M3, and to a lesser extent M4, followed by M1, M2 and M5 receptor subtypes in both progenitors and differentiated oligodendrocytes. Competition binding experiments using [(3)H]N-methylscopolamine and several antagonists, as well as inhibition of carbachol-mediated phosphoinositide hydrolysis, showed that M3 is the main subtype expressed in these cells. In progenitors the activation of p42/44-mitogen-activated protein kinase (MAPK) and cAMP-response element binding protein (CREB) as well as c-fos mRNA expression were blocked by the M3 relatively selective antagonist, 4-DAMP, and its irreversible analogue, 4-DAMP-mustard. Carbachol increased proliferation of progenitors, an effect prevented by atropine and 4-DAMP, as well as by the MAPK kinase inhibitor PD98059. These results indicate that carbachol modulates oligodendrocyte progenitor proliferation through M3 receptors, involving activation of a MAPK signaling pathway. Receptor density and phosphoinositide hydrolysis are down-regulated during oligodendrocyte differentiation. Functional consequences of these events are a reduction in carbachol-stimulated p42/44(MAPK) and CREB phosphorylation, as well as induction of c-fos.

Calcium signalling in cells of oligodendroglial lineage

Intracellular Ca2+ is the key signal that regulates the efficacy of neurotransmitter release and synaptic plasticity in neurons but is also an important second messenger involved in the signal transduction and modulation of gene expression in both excitable and non-excitable cells. Glial cells, including cells of oligodendroglial (OLG) lineage, are capable of responding to extracellular stimuli via changes in the intracellular Ca2+. This review article focuses on the mechanisms of Ca2+ signalling in cells of OLG lineage with the goal of providing the basis for understanding the relevance of receptor- and non-receptor-mediated signalling to oligodendroglial development, myelination, and demyelination. Conclusions to date indicate that cells of OLG lineage exhibit remarkable plasticity with regard to the expression of ion channels and receptors linked to Ca2+ signalling and that perturbation of [Ca2](i) homeostasis contributes to the pathogenesis of demyelinating diseases.

Role of calcium in nitric oxide-induced cytotoxicity: EGTA protects mouse oligodendrocytes

Active nitrogen species are overproduced in inflammatory brain lesions in multiple sclerosis (MS) and experimental allergic encephalomyelitis (EAE). NO has been shown to mediate the death of oligodendrocytes (OLs), a primary target of damage in MS. To develop strategies to protect OLs, we examined the mechanisms of cytotoxicity of two NO donors, S-nitroso-N-acetyl-penicillamine (SNAP) and sodium nitroprusside (SNP) on mature mouse OLs. Nitrosonium ion (NO+) rather than NO. mediates damage with both SNAP and SNP, as shown by significant protection with hemoglobin (HbO2), but not with the NO. scavenger PTIO. SNAP and SNP differ in time course and mechanisms of killing OLs. With SNAP, OL death is delayed for at least 6 hr, but with SNP, OL death is continuous over 18 hr with no delay. Relative to NO release, SNP is more toxic than SNAP, due to synergism of NO with cyanide released by SNP. SNAP elicits a Ca2+ influx in over half of the OLs within min. Further, OL death due to NO release from SNAP is Ca2+-dependent, because the Ca2+ chelator EGTA protects OLs from killing by SNAP, and also from killing by the NONOates NOC-9 and NOC-18, which spontaneously release NO. SNP does not elicit a Ca2+ influx, and EGTA is not protective. In comparison to the N20.1 OL cell line (Boullerne et al., [1999] J. Neurochem. 72:1050-1060), mature OLs are (1) more sensitive to SNAP, (2) much more resistant to SNP, (3) sensitive to cyanide, but not iron, and (4) exhibit a Ca2+ influx and EGTA protection in response to NO generated by SNAP.

Carnosine-like immunoreactivity in the central nervous system of rats during postnatal development

In the nervous system of adult rodents, the aminoacylhistidine dipeptides (carnosine and/or homocarnosine) have been shown to be expressed in three main populations of cells: the mature olfactory receptor neurons, a subset of glial cells, and the neuroblasts of the rostral migratory stream. The current study analyzed the distribution of these dipeptides during postnatal development within the rat brain and spinal cord focusing on their pattern of appearance in the glial cells. Double staining methods using antibodies against carnosine and some markers specific for immature (vimentin) and mature (glial fibrillary acidic protein and Rip) glial cell types were used. Glial immunostaining for the aminoacylhistidine dipeptides appears starting from postnatal day 6 and reaches the final distribution in 3-week-old animals. The occurrence of carnosine-like immunoreactivity in astrocytes lags behind that in oligodendrocytes suggesting that, as previously demonstrated by in vitro studies, oligodendrocytes are also able to synthesize carnosine and/or homocarnosine in vivo. Furthermore, the spatiotemporal patterns observed support the hypothesis that the production of these dipeptides coincides with the final stages of glia differentiation. In addition, a strong carnosine-like immunoreactivity is transiently seen in a small population of cells localized in the hypothalamus and in the subfornical organ from birth to postnatal day 21. In these cells, carnosine-like immunoreactivity was not colocalized with any of the glial specific markers used. Moreover, no evidence for colocalization of carnosine and gonadotropin-releasing hormone (GnRH) has been observed.

Intercellular Ca(2+) waves induce temporally and spatially distinct intracellular Ca(2+) oscillations in glia

Mechanically induced intercellular Ca(2+) waves propagated for approximately 300 microm in primary glial cultures. Following the wave propagation, 34% of the cells displayed Ca(2+) oscillations in a zone 60-120 microm from the stimulated cell. The initiation, frequency, and duration of these Ca(2+) oscillations were dependent on the cells' distance from the wave origin but were not dependent on the cell type nor on the magnitude of the Ca(2+) wave. When an individual cell propagated two sequential intercellular Ca(2+) waves originating from different sites, the characteristics of the Ca(2+) oscillations initiated by each wave were determined by the distance of the cell from the origin of each wave. Each Ca(2+) oscillation commonly occurred as an intracellular Ca(2+) wave that was initiated from a specific site within the cell. The position of the initiation site and the direction of the intracellular Ca(2+) wave were independent of the orientation of the initial intercellular Ca(2+) wave. Because initiation and frequency of Ca(2+) oscillations are dependent on the intracellular inositol trisphosphate concentration ([IP(3)](i)), we propose that the zone of cells displaying Ca(2+) oscillations is determined by an intercellular gradient of [IP(3)](i), established by the diffusion of IP(3) through gap junctions during the propagation of the intercellular Ca(2+) wave. Exposure to acetylcholine, a muscarinic agonist that initiates IP(3) production, shifted the zone of oscillating cells about 45 microm farther away from the origin of the mechanically induced wave. These findings indicate that a glial syncytium can resolve information provided by a local Ca(2+) wave into a distinct spatial and temporal pattern of Ca(2+) oscillations.

Lysophosphatidic acid-induced calcium signals in cultured rat oligodendrocytes

Oligodendrocytes are the myelin forming glial cells of the CNS and are known to express receptors linked to ion channels and intracellular second messenger cascades. In this paper, we describe the intracellular calcium responses of cells from the oligodendrocyte lineage to application of lysophosphatidic acid (LPA), a naturally occurring, growth factor-like phospholipid. Oligodendrocyte precursors did not respond to application of LPA (1 microM). In mature oligodendrocytes, however, LPA (1 microM) induced an increase in the intracellular calcium concentration ([Ca2+]i). In the majority of cells this increase was followed by a persistent plateau phase. The LPA-induced [Ca2+]i signal vanished in Ca2+-free medium, implying that it arose due to a Ca2+ influx across the plasma membrane. Preincubation of the cells with Pertussis-toxin prevented the generation of LPA-induced [Ca2+]i signals. We conclude that cultured rat oligodendrocytes express functional LPA receptors, which mediate a transmembrane Ca2+ influx via a Pertussis-toxin-sensitive G-protein.

Different neuroligands and signal transduction pathways stimulate CREB phosphorylation at specific developmental stages along oligodendrocyte differentiation

We have shown previously that the pattern of expression of the transcription factor CREB (cyclic AMP-response element binding protein) in developing oligodendrocytes (OLGs) suggests a role during a period that precedes the peak of myelination in rat brain. We have now investigated the signaling pathways that could be responsible for activating CREB by phosphorylation at different stages along OLG maturation. CREB phosphorylation was studied in short-term cultures of immature OLG precursor cells and young OLGs isolated from 4- and 11-day-old rat cerebrum, respectively. The results indicated that at both developmental stages, CREB phosphorylation could be stimulated by either increased concentrations of cyclic AMP and cyclic AMP-dependent protein kinase activation or increased Ca2+ levels and a protein kinase C activity. The results also showed that CREB phosphorylation in immature OLG precursor cells could be up-regulated by treatment with histamine, carbachol, glutamate, and ATP (neuroligands known to increase Ca2+ levels in these cells), by signaling cascade(s) that involve a protein kinase C activity, as well as the mitogen-activated protein kinase pathway. In contrast, in cells isolated from 11-day-old rats, at a developmental stage that immediately precedes the beginning of the active period of myelin synthesis, CREB phosphorylation was only stimulated by treatment with the beta-adrenergic agonist isoproterenol in a process that appears to be mediated by a cyclic AMP/cyclic AMP-dependent protein kinase-dependent pathway. These results support the idea that CREB could be a mediator of neuronal signals that, coupled to specific signal transduction cascades, may play different regulatory roles at specific stages along OLG differentiation.

A role of adhesion molecules in neuroglial plasticity

Glial cells are exquisitely sensitive to changes in neuronal activity, and their capacity for structural plasticity including migration is critical for remodeling an repair of nervous tissue. Our in vitro studies suggest that isoforms of the neural cell adhesion molecule (NCAM) carrying an unconventional carbohydrate polymer, polysialic acid (PSA), are involved in these events. We have demonstrated that neurohypophyseal explants from newborn rats generate cellular outgrowth of immature astrocytes displaying the characteristics of oligodendrocyte-type 2 astrocyte (O-2A) progenitor cells previously identified in the optic nerve. Treatment of O-2A cells with the enzyme Endo N, which specifically removes PSA from the cells surface, produced a complete blockade of the dispersion of the O-2A cell population from the explant. Identical effects of Endo N were observed in migration assays using cortical O-2A cells. Neurohypophyseal O-2A cells express functional NMDA class of glutamate receptors and the pharmacological blockade of these receptors inhibit PSA-NCAM biosynthesis and dramatically diminish O-2A cell migration from neurohypophyseal explants. This suggests a potential mechanism through which neuronal activity via glutamate release may regulate PSA-NCAM expression on immature glial cells, which in turn is critical for their migration.

Characterization of ryanodine receptors in oligodendrocytes, type 2 astrocytes, and O-2A progenitors

In this study we have investigated the expression of ryanodine receptors (RyRs), and the ability of caffeine to evoke RyR-mediated elevation of intracellular Ca2+ levels ([Ca2+]i) in glial cells of the oligodendrocyte/type 2 astrocyte lineage. Immunocytochemistry with specific antibodies identified ryanodine receptors in cultured oligodendrocytes, type 2 astrocytes, and O-2A progenitor cells, at high levels in the perinuclear region and in a variegated pattern along processes. Glia acutely isolated from rat brain and in aldehydefixed sections of cortex were similarly found to express RyRs. Caffeine (5-50 mM) caused an increase in [Ca2+]i in most cultured type 2 astrocytes and in 50% of oligodendrocytes. Responses elicited by caffeine were inhibited by pretreatment with ryanodine (10 microM) or thapsigargin (1 microM), and the peak response was unaffected by removal of [Ca2+]o. O-2A progenitor cells, in contrast, were largely unresponsive to caffeine treatment. Pretreatment with kainate (200 microM) to activate Ca2+ entry increased the magnitude of caffeine-evoked [Ca2+]i elevations in type 2 astrocytes and oligodendrocytes, and caused caffeine to activate responses in a significant proportion of previously non-responding O-2A progenitors. In both type 2 astrocytes and oligodendrocytes, caffeine evoked Ca2+ changes which propagated as wavefronts from several initiation sites. These wave amplification sites were characterized by significantly higher local Ca2+ release kinetics. Our results indicate that several glial cell types express RyRs, and that their functionality differs within different cell types of the oligodendrocyte lineage. In addition, ionotropic glutamate receptor activation fills the caffeine-sensitive Ca2+ stores in these cells.

Glial calcium: homeostasis and signaling function

Glial cells respond to various electrical, mechanical, and chemical stimuli, including neurotransmitters, neuromodulators, and hormones, with an increase in intracellular Ca2+ concentration ([Ca2+]i). The increases exhibit a variety of temporal and spatial patterns. These [Ca2+]i responses result from the coordinated activity of a number of molecular cascades responsible for Ca2+ movement into or out of the cytoplasm either by way of the extracellular space or intracellular stores. Transplasmalemmal Ca2+ movements may be controlled by several types of voltage- and ligand-gated Ca(2+)-permeable channels as well as Ca2+ pumps and a Na+/Ca2+ exchanger. In addition, glial cells express various metabotropic receptors coupled to intracellular Ca2+ stores through the intracellular messenger inositol 1,4,5-triphosphate. The interplay of different molecular cascades enables the development of agonist-specific patterns of Ca2+ responses. Such agonist specificity may provide a means for intracellular and intercellular information coding. Calcium signals can traverse gap junctions between glial cells without decrement. These waves can serve as a substrate for integration of glial activity. By controlling gap junction conductance, Ca2+ waves may define the limits of functional glial networks. Neuronal activity can trigger [Ca2+]i signals in apposed glial cells, and moreover, there is some evidence that glial [Ca2+]i waves can affect neurons. Glial Ca2+ signaling can be regarded as a form of glial excitability.

Acetylcholine agonists stimulate mitogen-activated protein kinase in oligodendrocyte progenitors by muscarinic receptors

Oligodendrocytes, the myelin-producing cells of the central nervous system, express muscarinic acetylcholine receptors (mAChR). Activation of this neurotransmitter receptor by the stable acetylcholine analog carbachol (CCh) triggers transducing events, modulating c-fos expression and cellular proliferation. To elucidate the signal transduction pathways involved in the transmission of these cellular events, we examined the ability of CCh to activate mitogen-activated protein kinase (MAPK) in primary cultures of oligodendrocyte progenitors prepared from newborn rat brain. CCh produced a concentration- and time-dependent increase in MAPK activity (predominantly the p42mapk or ERK2) as determined by in-gel MBP kinase assays. Using the non-selective muscarinic antagonist atropine we determined that MAPK-activation by CCH is mediated by muscarinic receptors. In the presence of PD098059, a specific inhibitor of MAPK kinase (MEK), MAPK activity was blocked. Similarly, the presence of extracellular calcium was required for CCh-mediated MAPK activation. To further elucidate the mechanisms involved in MAPK activation by CCh, the role of PKC was studied. In cells in which protein kinase had been downregulated by chronic treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA), the effect of carbachol on MAPK activation was maintained. In contrast, the response to CCh was blocked by the PKC inhibitors H7 and bisindolylmaleimide GF109203X. Our results suggest that MAPK is implicated in the transmission of the signal for mACh receptors and involves a TPA-insensitive PKC pathway. Further work is required to define the upstream and downstream events which result in CCh-mediated MAPK activation and proliferation of oligodendrocyte progenitors.

Glutamate-stimulated production of inositol phosphates is mediated by Ca2+ influx in oligodendrocyte progenitors

The effect of glutamate on the accumulation of [3H]inositol phosphates was examined in oligodendrocyte progenitor cultures prepared from rat brains. Glutamate, and the analogues alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainate, caused a concentration- and time-dependent increase in [3H]inositol trisphosphate (IP3) formation and the effect was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a competitive AMPA and kainate receptor antagonist. Similarly, the more selective, noncompetitive antagonist of AMPA receptors, 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466), significantly reduced the effect of both AMPA and kainate. In contrast, antagonists of N-methyl-D-aspartate (NMDA) receptor, (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5, 10-imine (MK-801) and R(-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), and antagonists of metabotropic receptors, L(+)-2-amino-3-phosphono-propanoic acid (L-AP3) and alpha-methyl-4-carboxyphenylglycine (MCPG), were ineffective. These results suggest that the effect of glutamate on [3H]IP3 accumulation is mediated through ionotropic AMPA receptors. Cyclothiazide, an inhibitor of AMPA receptor desensitization, strongly potentiated the AMPA and kainate-stimulated [3H]IP3 formation as well as the uptake of 45Ca2+ in line with the previous findings. 45Ca2+ uptake evoked by AMPA or kainate, in combination with cyclothiazide, was also prevented by both CNQX and GYKI 52466. Glutamate-stimulated [3H]IP3 accumulation was prevented by EGTA, suggesting a requirement for extracellular calcium. Pre-incubation with the voltage-gated Ca2+ channel blockers, diltiazem, nifedipine and CdCl2, partially prevented the glutamate-induced [3H]IP3 accumulation as well as 45Ca2+ uptake. Similarly, the Na+/Ca2+ exchanger blockers benzamil and 3,4-dichlorobenzamil reduced significantly kainate-stimulated 45Ca2+ uptake. These data indicate that glutamate-induced [3H]IP3 accumulation is triggered by calcium influx via AMPA receptors, voltage-gated calcium channels and the Na+/Ca2+ exchanger operating in reverse mode.

High density distribution of endoplasmic reticulum proteins and mitochondria at specialized Ca2+ release sites in oligodendrocyte processes

In oligodendrocyte processes, methacholine-evoked Ca2+ waves propagate via regions of specialized Ca2+ release kinetics (wave amplification sites) at which the amplitude and rate of rise of local Ca2+ signals are markedly higher than in surrounding areas (Simpson, P. B., and Russell, J. T. (1996) J. Biol. Chem. 271, 33493-33501). In the present study we have examined the effects of other phosphoinositide-coupled agonists on Ca2+ in these cells, and the structural specializations underlying regenerative wave amplification sites. Both bradykinin and norepinephrine evoke Ca2+ waves, which initiate at the same loci and propagate through the cell body and multiple processes via identical wave amplification sites. Antibodies against type 2 inositol 1,4,5-trisphosphate receptors (InsP3R2) and calreticulin identify expression of these proteins in oligodendrocyte membranes in Western blots. Immunocytochemistry followed by high resolution fluorescence microscopy revealed that both InsP3R2 and calreticulin are expressed in high intensity patches along processes. Cross-correlation analysis of the profiles of local Ca2+ release kinetics during a Ca2+ wave and immunofluorescence for these proteins along cellular processes showed that the domains of high endoplasmic reticulum protein expression correspond closely to wave amplification sites. Staining cells with the mitochondrial dye, MitoTracker(R), showed that mitochondria are only found in intimate association with these sites possessing high density endoplasmic reticulum proteins, and they remain in the same locations over relatively long periods of time. It appears, therefore, that multiple specializations are found at domains of elevated Ca2+ release in oligodendrocyte processes, including high levels of calreticulin, InsP3R2 Ca2+ release channels, and mitochondria.

ATP acting on P2Y receptors triggers calcium mobilization in Schwann cells at the neuroelectrocyte junction in skate

Schwann cells are integral cellular components of the dense cholinergic presynaptic plexus (nerve plate) which innervates each electrocyte in skate electric organ. Using the Ca2+-sensitive dye fura-2, we have followed the response in these cells to various chemical challenges. In K+ depolarized nerve plates nerve terminals consistently responded with a rapid and sustained Ca2+ signal. Schwann cell responses to depolarization were rarely seen but, when observed, were always delayed in onset when compared to nerve terminal response (6-10 s later). The possibility that these responses were triggered by mediators released from nerve terminals was tested by direct application of candidate substances. Schwann cells were found to respond to adenosine triphosphate and adenosine diphosphate with a biphasic increase of intracellular Ca2+ concentration, a rapid peak response being followed in the majority of cells by a sustained plateau phase. In the absence of external Ca2+ only the transient peak response was observed. Depletion of internal Ca2+ stores with thapsigargin completely inhibited the adenosine triphosphate-stimulated rise in Schwann cell Ca2+. The response to adenosine triphosphate was concentration-dependent (EC50 2.8 microM) and was reversibly blocked by two antagonists of P2 purinoceptors: suramin and reactive blue 2. Adenosine diphosphate and 2-methylthio-adenosine triphosphate were equipotent with adenosine triphosphate and at high concentrations (100 microM) diadenosine tetraphosphate produced responses comparable to low concentrations of adenosine triphosphate. Adenosine, adenosine monophosphate, the alpha beta-methylene analogues of adenosine triphosphate and adenosine diphosphate, uridine triphosphate, cytidine triphosphate and guanosine triphosphate were without significant effect. These results show that, in skate electric organ Schwann cells, the release of Ca2+ from intracellular stores is triggered by adenosine triphosphate acting on P(2gamma) receptors and suggest that Schwann cells may be targets for synaptically-released adenosine triphosphate in the electric organ model of the neuromuscular junction.

Carbachol stimulates c-fos expression and proliferation in oligodendrocyte progenitors

To determine if muscarinic receptor-activation plays a role in oligodendrocyte development, the effect of carbachol a stable acetylcholine analog, on gene expression and proliferation was investigated. Using Northern blot analysis we showed that carbachol caused a time and concentration-dependent increase in c-fos mRNA. This effect was blocked by atropine, a non-selective muscarinic antagonist. In addition, the muscarinic-stimulated c-fos increase was inhibited by 1-(5-isoquinoline-sulfonyl)-2-methylpiperazine (H-7), a potent inhibitor of protein kinase C (PKC), but not by N-2-(p-bromocinnamylamino)-ethyl-5-isoquinoline-sulfonamide (H-89), a potent inhibitor of protein kinase A, suggesting the involvement of PKC in mediating the response. Down-regulation of PKC by overnight pre-treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) blocked only the phorbol ester-stimulated c-fos accumulation while no effect was observed in the carbachol-induced response. These results suggested that carbachol stimulated an H-7 sensitive PKC pathway which may be different than that activated by TPA. Further evidence for two separate mechanisms of proto-oncogene induction was provided by the additive effect of carbachol and TPA. Induction of c-fos mRNA by carbachol was dependent on both influx of extracellular Ca2+ and release from intracellular stores, as both EDTA and BAPTA blocked the response. Since activation of muscarinic receptors can affect cell division in other cellular systems, the effect of carbachol on [3H]thymidine and bromodeoxyuridine incorporation into oligodendrocyte DNA was measured. Carbachol stimulated DNA synthesis in oligodendrocyte progenitors. This effect was mediated by muscarinic receptors as [3H]thymidine incorporation was prevented or significantly reduced by the addition of atropine. In conclusion, the present findings suggest that, the neurotransmitter, acetylcholine may act as a trophic factor in developing oligodendrocytes, regulating their growth and development in the central nervous system.