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R Douglas Fields

Publications and source records attributed to R Douglas Fields.

At least 19 recordsLinked to original sources
PubMed2006

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.

Action Potentials↗
PubMed2006

CaMKII inactivation by extracellular Ca(2+) depletion in dorsal root ganglion neurons.

A mechanism by which Ca(2+)/CaM-dependent protein kinase (CaMKII) is autophosphorylated by changes in extracellular calcium in the absence of detectable changes in cytoplasmic [Ca(2+)] has been identified. We find that when the external Ca(2+) concentration ([Ca(2+)](O)) is lowered, Ca(2+) is released from intracellular stores to maintain a constant cytoplasmic Ca(2+) level, gradually depleting the endoplasmic Ca(2+) stores. Accompanying the store-depletion is a rapid decrease in CaMKII activity. Approximately 25% of the measured CaMKII autophosphorylation in DRG neurons in culture can be regulated by Ca(2+) flux from intracellular stores caused by manipulating [Ca(2+)](O), as shown by blocking refilling of store-operated Ca(2+)-channels with SK&F 96365, Ruthenium Red, and a partial block with Ni(2+). Blocking voltage-gated Ca(2+)-channels with either isradipine or SR 33805, had no effect on CaMKII autophosphorylation induced by restoring Ca(2+)(O) to normal after depleting the intracellular Ca(2+) stores. These results show that removal of Ca(2+)(O) has profound effects on intracellular Ca(2+) signaling and CaMKII autophosphorylation, in the absence of measurable changes in intracellular Ca(2+). These findings have wide-ranging significance, because [Ca(2+)](O) is manipulated in many experimental studies. Moreover, this explanation for the paradoxical changes in CaMKII phosphorylation in response to manipulating [Ca(2+)](O) provides a possible mechanism linking activity-dependent depletion of Ca(2+) from the synaptic cleft to a protein kinase regulating many neuronal properties.

Animals↗
PubMed2006

Immune system evasion by peripheral nerve sheath tumor.

Mechanisms by which tumor cells evade detection by the host's immune system are thought to play a role in progression to malignancy, but this has not been investigated in the context of neurofibromatosis type 1 (NF1). NF1 is an autosomal dominant disorder, in which aggressive peripheral nerve tumors, known as malignant peripheral nerve sheath tumors (MPNSTs), develop in 5-10% of patients. Large scale gene expression profiling of a MPNST-derived cell line, T265, and normal human Schwann cells (hSCs) identified a large group of immune function genes down-regulated in T265 cells. Here we report that the aberrant expression of immune system related genes extends beyond MHC class I and II genes in T265 cells to include a transcription factor (MHC2TA) and other critical components of the antigen processing and presentation apparatus. TAP1, the transporter-activator protein that loads peptide antigens onto MHC class I molecules, is down-regulated, and CD74, a chaperone protein whose function is in processing and transport of MHC class II molecules, is down-regulated and alternatively spliced to produce an RNA transcript not evident in normal human Schwann cells. These findings reveal multiple molecular pathways and at least two cellular mechanisms acting to reduce the normal immune system molecules involved in antigen processing and presentation in cells derived from a peripheral nerve sheath tumor. Acquiring a "silent" immune signature may be a critical step in the progress towards malignancy in MPNSTs.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed2006

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.

Adenosine↗
PubMed2006

Nerve impulses regulate myelination through purinergic signalling.

The myelin membrane wrapped around axons provides electrical insulation essential for rapid impulse conduction. Impulse activity can affect the formation of myelin, but the effects differ in the PNS and CNS, where myelin is formed by two distinct types of cells: Schwann cells (SCs) and oligodendrocytes, respectively. Our studies on mouse dorsal root ganglion (DRG) neurons, which have axons in both the PNS and CNS, show that impulse activity releases ATP from premyelinated axons, and that this is detected by myelinating glia. Calcium imaging indicates that axonal firing stimulates different purinergic receptors on the two types of glia, resulting in opposite effects of impulse activity on differentiation of SCs and oligodendrocyte progenitor cells (OPCs). In addition to P2 receptors on both types of glia, four types of P1 receptors are present in OPCs, but only A2A and A2BP1 receptors are detected in mouse SCs. ATP is of primary importance in regulating early development and myelination by SCs, where it inhibits differentiation and myelination. Adenosine is of primary importance in regulating early development of OPCs, where it stimulates differentiation and myelination. Purinergic signalling interacts with growth factor and cytokine signalling, and these responses are developmentally regulated.

Action Potentials↗
PubMed2005

Temporal integration of intracellular Ca2+ signaling networks in regulating gene expression by action potentials.

Temporal aspects of intracellular calcium signaling are particularly important in activity-dependent regulation of gene expression in neurons. This review traces calcium-dependent intracellular signaling from the membrane to the nucleus in response to action-potential firing, and considers how specific genes are regulated by specific patterns of impulse firing. Modes of calcium influx, calcium-dependent protein kinases, transcription factors, individual genes, and genomic analysis are examined, with particular emphasis on the importance of temporal aspects of calcium dynamics in regulating these processes.

Action Potentials↗
PubMed2005

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?

Animals↗
PubMed2005

Gene expression in the conversion of early-phase to late-phase long-term potentiation.

Changes in gene expression associated with different forms of synaptic plasticity in rat hippocampus were investigated. Microarray analysis revealed differential expression of hundreds of genes 30 min after synaptic or antidromic stimulation in different patterns. Results of selected genes were verified by LightCycler RT-PCR. Synaptic activation in a theta burst protocol, which induced long-term potentiation (LTP), increased the mRNA abundance of BDNF-exon 1, but antidromic stimulation in the presence of CNQX, APV, and MCPG (to block glutamatergic synapses) decreased the level of mRNA of this transcript, as did 1 Hz synaptic stimulation. The opposite regulation of this BDNF transcript after firing of the postsynaptic neuron, coincidently or uncorrelated with synaptic firing, is consistent with the effects of BDNF on synaptic transmission, suggesting possible involvement in strengthening and weakening CA1 synapses after correlated versus uncorrelated firing of the postsynaptic neurons with its synaptic inputs. Possible involvement of transcriptional regulation of BDNF in the conversion of early-phase LTP to late-phase LTP are discussed in the context of previous studies by Dudek & Fields (Proc. Natl. Acad. Sci. USA 99: 3962-3967) showing that this conversion can be induced by antidromic stimulation of CA1 neurons in the absence of excitatory synaptic activity.

Animals↗
PubMed2004

Volume transmission in activity-dependent regulation of myelinating glia.

The importance of neural impulse activity in regulating neuronal plasticity is widely appreciated; increasingly, it is becoming apparent that activity-dependent communication between neurons and glia is critical in regulating many aspects of nervous system development and plasticity. This communication takes place not only at the synapse, but also between premyelinating axons and glia, which form myelin in the PNS and CNS. Recent work indicates that neural impulse activity releases ATP and adenosine from non-synaptic regions of neurons, which activates purinergic receptors on myelinating glia. Acting through this receptor system, neural impulse activity can regulate gene expression, mitosis, differentiation, and myelination of Schwann cells (SCs) and oligodendrocytes, helping coordinate nervous system development with functional activity in the perinatal period. ATP and adenosine have opposite effects on differentiation of Schwann cells and oligodendrocytes, providing a possible explanation for the opposite effects of impulse activity reported on myelination in the CNS and PNS.

Adenosine Triphosphate↗
PubMed2004

Adenosine: an activity-dependent axonal signal regulating MAP kinase and proliferation in developing Schwann cells.

Nonsynaptic release of ATP from electrically stimulated dorsal root gangion (DRG) axons inhibits Schwann cell (SC) proliferation and arrests SC development at the premyelinating stage, but the specific types of purinergic receptor(s) and intracellular signaling pathways involved in this form of neuron-glia communication are not known. Recent research shows that adenosine is a neuron-glial transmitter between axons and myelinating glia of the CNS. The present study investigates the possibility that adenosine might have a similar function in communicating between axons and premyelinating SCs. Using a combination of pharmacological and molecular approaches, we found that mouse SCs in culture express functional adenosine receptors and ATP receptors, a far more complex array of purinergic receptors than thought previously. Adenosine, but not ATP, activates ERK/MAPK through stimulation of cAMP-linked A2(A) adenosine receptors. Both ATP and adenosine inhibit proliferation of SCs induced by platelet-derived growth factor (PDGF), via mechanisms that are partly independent. In contrast to ATP, adenosine failed to inhibit the differentiation of SCs to the O4+ stage. This indicates that, in addition to ATP, adenosine is an activity-dependent signaling molecule between axons and premyelinating Schwann cells, but that electrical activity, acting through adenosine, has opposite effects on the differentiation of myelinating glia in the PNS and CNS.

Journal Article↗
PubMed2004

Transcriptional profiling in an MPNST-derived cell line and normal human Schwann cells.

cDNA microarrays were utilized to identify abnormally expressed genes in a malignant peripheral nerve sheath tumor (MPNST)-derived cell line, T265, by comparing the mRNA abundance profiles with that of normal human Schwann cells (nhSCs). The findings characterize the molecular phenotype of this important cell-line model of MPNSTs, and elucidate the contribution of Schwann cells in MPNSTs. In total, 4608 cDNA sequences were screened and hybridizations replicated on custom cDNA microarrays. In order to verify the microarray data, a large selection of differentially expressed mRNA transcripts were subjected to semi-quantitative reverse transcription PCR (LightCycler). Western blotting was performed to investigate a selection of genes and signal transduction pathways, as a further validation of the microarray data. The data generated from multiple microarray screens, semi-quantitative RT-PCR and Western blotting are in broad agreement. This study represents a comprehensive gene-expression analysis of an MPNST-derived cell line and the first comprehensive global mRNA profile of nhSCs in culture. This study has identified ~900 genes that are expressed abnormally in the T265 cell line and detected many genes not previously reported to be expressed in nhSCs. The results provide crucial information on the T265 cells that is essential for investigation using this cell line in experimental studies in neurofibromatosis type I (NF1), and important information on normal human Schwann cells that is applicable to a wide range of studies on Schwann cells in cell culture.

Journal Article↗
PubMed2004

Extracellular calcium depletion in synaptic transmission.

Regulation of Ca2+ homeostasis in the extracellular space plays an important role in neuronal function. Several modeling studies and recent measurements have demonstrated that modest action potential or synaptic activity can result in a significant reduction in extracellular calcium ([Ca]o(2+)). Changes in [Ca]o(2+) can regulate intracellular signaling enzymes, such as Ca2+/calmodulin-dependent protein kinase II, and influence neuronal function at synaptic and nonsynaptic sites. The change in [Ca]o(2+) can affect several types of ion channels and neurotransmitter receptors and activate a Ca(2+)-sensitive receptor in neuronal membranes. Depletion of [Ca]o(2+) may function as an activity-dependent extracellular messenger that regulates nervous system function during development, learning, and disease.

Animals↗
PubMed2003

Patterned electrical activity modulates sodium channel expression in sensory neurons.

Peripheral nerve injury induces changes in the level of gene expression for sodium channels Nav1.3, Nav1.8, and Nav1.9 within dorsal root ganglion (DRG) neurons, which may contribute to the development of hyperexcitability, ectopic neuronal discharge, and neuropathic pain. The mechanism of this change in sodium channel expression is unclear. Decreased availability of neurotrophic factors following axotomy contributes to these changes in gene transcription, but the question of whether changes in intrinsic neuronal activity levels alone can trigger changes in the expression of these sodium channels has not been addressed. We examined the effect of electrical stimulation on the expression of Nav1.3, Nav1.8, and Nav1.9 by using cultured embryonic mouse sensory neurons under conditions in which nerve growth factor (NGF) was not limiting. Expression of Nav1.3 was not significantly changed following stimulation. In contrast, we observed activity-dependent down-regulation of Nav1.8 and Nav1.9 mRNA and protein levels after stimulation, as demonstrated by quantitative polymerase chain reaction and immunocytochemistry. These results show that a change in neuronal activity can alter the expression of sodium channel genes in a subtype-specific manner, via a mechanism independent of NGF withdrawal.

Action Potentials↗
PubMed2002

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.

Action Potentials↗
PubMed2002

New insights into neuron-glia communication.

Two-way communication between neurons and nonneural cells called glia is essential for axonal conduction, synaptic transmission, and information processing and thus is required for normal functioning of the nervous system during development and throughout adult life. The signals between neurons and glia include ion fluxes, neurotransmitters, cell adhesion molecules, and specialized signaling molecules released from synaptic and nonsynaptic regions of the neuron. In contrast to the serial flow of information along chains of neurons, glia communicate with other glial cells through intracellular waves of calcium and via intercellular diffusion of chemical messengers. By releasing neurotransmitters and other extracellular signaling molecules, glia can affect neuronal excitability and synaptic transmission and perhaps coordinate activity across networks of neurons.

Animals↗