Astrocytes promote myelination in response to electrical impulses

Axon-glial signaling and the glial support of axon function

Oligodendrocytes and Schwann cells are highly specialized glial cells that wrap axons with a multilayered myelin membrane for rapid impulse conduction. Investigators have recently identified axonal signals that recruit myelin-forming Schwann cells from an alternate fate of simple axonal engulfment. This is the evolutionary oldest form of axon-glia interaction, and its function is unknown. Recent observations suggest that oligodendrocytes and Schwann cells not only myelinate axons but also maintain their long-term functional integrity. Mutations in the mouse reveal that axonal support by oligodendrocytes is independent of myelin assembly. The underlying mechanisms are still poorly understood; we do know that to maintain axonal integrity, mammalian myelin-forming cells require the expression of some glia-specific proteins, including CNP, PLP, and MAG, as well as intact peroxisomes, none of which is necessary for myelin assembly. Loss of glial support causes progressive axon degeneration and possibly local inflammation, both of which are likely to contribute to a variety of neuronal diseases in the central and peripheral nervous systems.

Astrocytes in multiple sclerosis: a product of their environment

It has long been thought that astrocytes, like other glial cells, simply provide a support mechanism for neuronal function in the healthy and inflamed central nervous system (CNS). However, recent evidence suggests that astrocytes play an active and dual role in CNS inflammatory diseases such as multiple sclerosis (MS). Astrocytes not only have the ability to enhance immune responses and inhibit myelin repair, but they can also be protective and limit CNS inflammation while supporting oligodendrocyte and axonal regeneration. The particular impact of these cells on the pathogenesis and repair of an inflammatory demyelinating process is dependent upon a number of factors, including the stage of the disease, the type and microenvironment of the lesion, and the interactions with other cell types and factors that influence their activation. In this review, we summarize recent data supporting the idea that astrocytes play a complex role in the regulation of CNS autoimmunity.

Spatio-temporal electrical stimuli shape behavior of an embodied cortical network in a goal-directed learning task

We developed an adaptive training algorithm, whereby an in vitro neocortical network learned to modulate its dynamics and achieve pre-determined activity states within tens of minutes through the application of patterned training stimuli using a multi-electrode array. A priori knowledge of functional connectivity was not necessary. Instead, effective training sequences were continuously discovered and refined based on real-time feedback of performance. The short-term neural dynamics in response to training became engraved in the network, requiring progressively fewer training stimuli to achieve successful behavior in a movement task. After 2 h of training, plasticity remained significantly greater than the baseline for 80 min (p-value<0.01). Interestingly, a given sequence of effective training stimuli did not induce significant plasticity (p-value=0.82) or desired behavior, when replayed to the network and no longer contingent on feedback. Our results encourage an in vivo investigation of how targeted multi-site artificial stimulation of the brain, contingent on the activity of the body or even of the brain itself could treat neurological disorders by gradually shaping functional connectivity.

Vesicular apparatus, including functional calcium channels, are present in developing rodent optic nerve axons and are required for normal node of Ranvier formation

P/Q-type calcium channels are known to form clusters at the presynaptic membrane where they mediate calcium influx, triggering vesicle fusion. We now report functional P/Q channel clusters in the axolemma of developing central axons that are also associated with sites of vesicle fusion. These channels were activated by axonal action potentials and the resulting calcium influx is well suited to mediate formation of a synaptic style SNARE complex involving SNAP-25, that we show to be located on the axolemma. Vesicular elements within axons were found to be the sole repository of vesicular glutamate in developing white matter. The axonal vesicular elements expressed the glutamate transporter V-ATPase, which is responsible for vesicular glutamate loading. The P/Q channel alpha(1A) subunit was found to be present within the axolemma at early nodes of Ranvier and deleterious mutations of the alpha(1A) subunit, or an associated alpha(2)delta-2 subunit, disrupted the localization of nodal proteins such as voltage-gated sodium channels, beta IV spectrin and CASPR-1. This was associated with the presence of malformed nodes of Ranvier characterized by an accumulation of axoplasmic vesicles under the nodal membrane. The data are consistent with the presence of a vesicular signalling pathway between axons and glial cells that is essential for proper development of the node of Ranvier.

Brain structural changes in obstructive sleep apnea

STUDY OBJECTIVES

Determine whether obstructive sleep apnea (OSA) subjects show indications of axonal injury.

DESIGN

We assessed fiber integrity in OSA and control subjects with diffusion tensor imaging (DTI). We acquired four whole-brain DTI series from each subject. The four series were realigned, and the diffusion tensor calculated at each voxel. Fractional anisotropy (FA), a measure of fiber integrity, was derived from the diffusion tensor, resulting in a whole brain FA "map." The FA maps were spatially normalized, smoothed, and compared using voxel-based statistics to determine differences between OSA and control groups, with age as a covariate (P < 0.05, corrected for multiple comparisons).

SETTING

University medical center.

SUBJECTS

We studied 41 patients with untreated OSA (mean age +/- SD: 46.3 +/- 8.9 years; female/male: 7/34) with apnea-hypopnea index 15 to 101 (mean +/- SD: 35.7 +/- 18.1 events/hour), and 69 control subjects (mean age +/- SD: 47.5 +/- 8.79 years; female/male: 25/44).

MEASUREMENTS AND RESULTS

Multiple regions of lower FA appeared within white matter in the OSA group, and included fibers of the anterior corpus callosum, anterior and posterior cingulate cortex and cingulum bundle, right column of the fornix, portions of the frontal, ventral prefrontal, parietal and insular cortices, bilateral internal capsule, left cerebral peduncle, middle cerebellar peduncle and corticospinal tract, and deep cerebellar nuclei.

CONCLUSIONS

White matter is extensively affected in OSA patients; the alterations include axons linking major structures within the limbic system, pons, frontal, temporal and parietal cortices, and projections to and from the cerebellum.

Control of glial precursor cell development in the mouse optic nerve by sonic hedgehog from retinal ganglion cells

The development of glial precursor cells in the mammalian optic nerve depends on retinal ganglion cell (RGC) axons, but the signals that mediate this neuron-to-glia interaction have not been fully characterized. Sonic hedgehog (Shh) is expressed by RGCs, and we showed previously that it is required for the specification of astrocyte lineage cells at the optic disc. To study the role of RGC-derived Shh on astrocyte development at later developmental stages, we generated mice with a conditional ablation of Shh in the peripheral retina and analyzed gene expression and glial cell development in the optic nerve. Astrocyte development was initiated in the optic nerves of these mutant mice; however, the expression of Hedgehog (Hh) target genes, Gli1 and Ptch1 and cell cycle genes, Ccnd1 and Cdc25b in the optic nerves were downregulated. Astrocyte proliferation was markedly reduced. Oligodendrocyte precursor cells were fewer in the optic nerves of mutant mice, possibly as a consequence of reduced secretion of growth factors by astrocytes. At a later developmental stage, optic nerve axons displayed signs of Wallerian degeneration, including reduction of astrocyte processes, degenerating glial cells and formation of distended axons. We also demonstrate that the Hh pathway can be activated in optic nerve-derived astrocytes in vitro, but fails to induce cell cycle gene expression and proliferation. RGC-derived Shh signalling isthus necessary in vivo for maintenance of astrocyte proliferation, affecting both axo-glial and normal glial cell development in the optic nerve.

Glial precursor cell transplantation therapy for neurotrauma and multiple sclerosis

Traumatic injury to the brain or spinal cord and multiple sclerosis (MS) share a common pathophysiology with regard to axonal demyelination. Despite advances in central nervous system (CNS) repair in experimental animal models, adequate functional recovery has yet to be achieved in patients in response to any of the current strategies. Functional recovery is dependent, in large part, upon remyelination of spared or regenerating axons. The mammalian CNS maintains an endogenous reservoir of glial precursor cells (GPCs), capable of generating new oligodendrocytes and astrocytes. These GPCs are upregulated following traumatic or demyelinating lesions, followed by their differentiation into oligodendrocytes. However, this innate response does not adequately promote remyelination. As a result, researchers have been focusing their efforts on harvesting, culturing, characterizing, and transplanting GPCs into injured regions of the adult mammalian CNS in a variety of animal models of CNS trauma or demyelinating disease. The technical and logistic considerations for transplanting GPCs are extensive and crucial for optimizing and maintaining cell survival before and after transplantation, promoting myelination, and tracking the fate of transplanted cells. This is especially true in trials of GPC transplantation in combination with other strategies such as neutralization of inhibitors to axonal regeneration or remyelination. Overall, such studies improve our understanding and approach to developing clinically relevant therapies for axonal remyelination following traumatic brain injury (TBI) or spinal cord injury (SCI) and demyelinating diseases such as MS.

White matter in learning, cognition and psychiatric disorders

White matter is the brain region underlying the gray matter cortex, composed of neuronal fibers coated with electrical insulation called myelin. Previously of interest in demyelinating diseases such as multiple sclerosis, myelin is attracting new interest as an unexpected contributor to a wide range of psychiatric disorders, including depression and schizophrenia. This is stimulating research into myelin involvement in normal cognitive function, learning and IQ. Myelination continues for decades in the human brain; it is modifiable by experience, and it affects information processing by regulating the velocity and synchrony of impulse conduction between distant cortical regions. Cell-culture studies have identified molecular mechanisms regulating myelination by electrical activity, and myelin also limits the critical period for learning through inhibitory proteins that suppress axon sprouting and synaptogenesis.

The effect of sulfated hyaluronan on the morphological transformation and activity of cultured human astrocytes

We demonstrated the effect of synthesized sulfated hyaluronan (SHya), which is composed of a sulfated group and hyaluronan, and basic fibroblast growth factor 2 (FGF-2) on normal human astrocytes (NHA) activity and its morphological transformation in vitro study. Astrocyte is a kind of glial cell and stellated astrocyte (activating astrocyte) supports axons network, neurons survival and synaptic plasticity. Treatment of SHya hardly affected NHA proliferation. However combination treatment of SHya and FGF-2 increased NHA proliferation. Treatment of SHya promoted transformation of normal astrocyte into a stella morphology (stellation) and combination treatment of SHya and FGF-2 promoted stellation than that of SHya only. Treatment of SHya increased glial fibrillary acidic protein (GFAP), nestin mRNA and GFAP protein expression in the stellated NHA. The cell-cell adhesion of NHA increased by treatment of SHya. Treatment of SHya increased heparin-binding trophic factors FGF-2, midkine, and some other trophic factors mRNA level in the NHA. These results suggested that the treatment of SHya promoted NHA activity due to enhancing neurotrophins production and the morphological transformation of NHA and the effect of SHya on astrocytes partly involved FGF-2 activity. These findings indicate that SHya may be involved in the astrocyte activity and support neurons survivals.

From fish to man: understanding endogenous remyelination in central nervous system demyelinating diseases

In the central nervous system (CNS) of man, evolutionary pressure has preserved some capability for remyelination while axonal regeneration is very limited. In contrast, two efficient programmes of regeneration exist in the adult fish CNS, neurite regrowth and remyelination. The rapidity of CNS remyelination is critical since it not only restores fast conduction of nerve impulses but also maintains axon integrity. If myelin repair fails, axons degenerate, leading to increased disability. In the human CNS demyelinating disease multiple sclerosis (MS), remyelination often takes place in the midst of inflammation. Here, we discuss recent studies that address the innate repair capabilities of the axon-glia unit from fish to man. We propose that expansion of this research field will help find ways to maintain or enhance spontaneous remyelination in man.

Long-term activity-dependent plasticity of action potential propagation delay and amplitude in cortical networks

BACKGROUND

The precise temporal control of neuronal action potentials is essential for regulating many brain functions. From the viewpoint of a neuron, the specific timings of afferent input from the action potentials of its synaptic partners determines whether or not and when that neuron will fire its own action potential. Tuning such input would provide a powerful mechanism to adjust neuron function and in turn, that of the brain. However, axonal plasticity of action potential timing is counter to conventional notions of stable propagation and to the dominant theories of activity-dependent plasticity focusing on synaptic efficacies.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show the occurrence of activity-dependent plasticity of action potential propagation delays (up to 4 ms or 40% after minutes and 13 ms or 74% after hours) and amplitudes (up to 87%). We used a multi-electrode array to induce, detect, and track changes in propagation in multiple neurons while they adapted to different patterned stimuli in controlled neocortical networks in vitro. The changes did not occur when the same stimulation was repeated while blocking ionotropic gabaergic and glutamatergic receptors. Even though induction of changes in action potential timing and amplitude depended on synaptic transmission, the expression of these changes persisted in the presence of the synaptic receptor blockers.

CONCLUSIONS/SIGNIFICANCE

We conclude that, along with changes in synaptic efficacy, propagation plasticity provides a cellular mechanism to tune neuronal network function in vitro and potentially learning and memory in the brain.

Gap junctions couple astrocytes and oligodendrocytes

In vertebrates, a family of related proteins called connexins form gap junctions (GJs), which are intercellular channels. In the central nervous system (CNS), GJs couple oligodendrocytes and astrocytes (O/A junctions) and adjacent astrocytes (A/A junctions), but not adjacent oligodendrocytes, forming a "glial syncytium." Oligodendrocytes and astrocytes each express different connexins. Mutations of these connexin genes demonstrate that the proper functioning of myelin and oligodendrocytes requires the expression of these connexins. The physiological function of O/A and A/A junctions, however, remains to be illuminated.

Mechanisms of ATP- and glutamate-mediated calcium signaling in white matter astrocytes

Neurotransmitters released at synapses mediate Ca2+ signaling in astrocytes in CNS grey matter. Here, we show that ATP and glutamate evoke these Ca2+ signals in white matter astrocytes of the mouse optic nerve, a tract that contains neither neuronal cell bodies nor synapses. We further demonstrate that action potentials along white matter axons trigger the release of ATP and the intercellular propagation of astroglial Ca2+ signals. These mechanisms were studied in astrocytes in intact optic nerves isolated from transgenic mice expressing enhanced green fluorescent protein (EGFP) under control of the human glial fibrillary acidic protein promoter (GFAP) by Fura-2 ratiometric Ca2+ imaging. ATP evoked astroglial Ca2+ signals predominantly via metabotropic P2Y1 and ionotropic P2X7 purinoceptors. Glutamate acted on both AMPA- and NMDA-type receptors, as well as on group I mGlu receptors to induce an increase in astroglial [Ca2+]i. The direct Ca2+ signal evoked by glutamate was small, and the main action of glutamate was to trigger the release of the "gliotransmitter" ATP by a mechanism involving P2X7 receptors; propagation of the glutamate-mediated Ca2+ signal was significantly reduced in P2X7 knock-out mice. Furthermore, axonal action potentials and mechanical stimulation of astrocytes both induced the release of ATP, to propagate Ca2+ signals in astrocytes and neighboring EGFP-negative glia. Our data provide a model of multiphase axon-glial signaling in the optic nerve as follows: action potentials trigger axonal release of ATP, which evokes further release of ATP from astrocytes, and this acts by amplifying the initiating signal and by transmitting an intercellular Ca2+ wave to neighboring glia.

Identification of Tmem10/Opalin as a novel marker for oligodendrocytes using gene expression profiling

Background

During the development of the central nervous system, oligodendrocytes generate large amounts of myelin, a multilayered insulating membrane that ensheathes axons, thereby allowing the fast conduction of the action potential and maintaining axonal integrity. Differentiation of oligodendrocytes to myelin-forming cells requires the downregulation of RhoA GTPase activity.

Results

To gain insights into the molecular mechanisms of oligodendrocyte differentiation, we performed microarray expression profiling of the oligodendroglial cell line, Oli-neu, treated with the Rho kinase (ROCK) inhibitor, Y-27632 or with conditioned neuronal medium. This resulted in the identification of the transmembrane protein 10 (Tmem10/Opalin), a novel type I transmembrane protein enriched in differentiating oligodendrocytes. In primary cultures, Tmem10 was abundantly expressed in O4-positive oligodendrocytes, but not in oligodendroglial precursor cells, astrocytes, microglia or neurons. In mature oligodendrocytes Tmem10 was enriched in the rims and processes of the cells and was only found to a lesser extent in the membrane sheets.

Conclusion

Together, our results demonstrate that Tmem10 is a novel marker for in vitro generated oligodendrocytes.

Asynchrony of the early maturation of white matter bundles in healthy infants: quantitative landmarks revealed noninvasively by diffusion tensor imaging

Normal cognitive development in infants follows a well-known temporal sequence, which is assumed to be correlated with the structural maturation of underlying functional networks. Postmortem studies and, more recently, structural MR imaging studies have described qualitatively the heterogeneous spatiotemporal progression of white matter myelination. However, in vivo quantification of the maturation phases of fiber bundles is still lacking. We used noninvasive diffusion tensor MR imaging and tractography in twenty-three 1-4-month-old healthy infants to quantify the early maturation of the main cerebral fascicles. A specific maturation model, based on the respective roles of different maturational processes on the diffusion phenomena, was designed to highlight asynchronous maturation across bundles by evaluating the time-course of mean diffusivity and anisotropy changes over the considered developmental period. Using an original approach, a progression of maturation in four relative stages was determined in each tract by estimating the maturation state and speed, from the diffusion indices over the infants group compared with an adults group on one hand, and in each tract compared with the average over bundles on the other hand. Results were coherent with, and extended previous findings in 8 of 11 bundles, showing the anterior limb of the internal capsule and cingulum as the most immature, followed by the optic radiations, arcuate and inferior longitudinal fascicles, then the spinothalamic tract and fornix, and finally the corticospinal tract as the most mature bundle. Thus, this approach provides new quantitative landmarks for further noninvasive research on brain-behavior relationships during normal and abnormal development.

What happens after the first episode? A review of progressive brain changes in chronically ill patients with schizophrenia

Numerous imaging studies have revealed structural brain changes in schizophrenia. Decreases in brain tissue are accompanied by increases in ventricle volumes and cerebrospinal fluid. Whether or not these brain changes are progressive beyond the first episode is subject to debate. To assess if progressive brain changes occur in chronically ill patients, 11 longitudinal magnetic resonance imaging and computed tomography studies were reviewed. Patients were ill for on average 10 years at their initial scan. Follow-up intervals varied between 1 and 10 years. Overall, the findings suggest continuous progressive brain tissue decreases and lateral ventricle volume increases in chronically ill patients, up to at least 20 years after their first symptoms. The extent of progressive brain tissue decrease in patients (-0.5% per year) is twice that of healthy controls (-0.2% per year). These findings are consistent with the extent of postmortem brain tissue loss in schizophrenia. Progressive volume loss seems most pronounced in the frontal and temporal (gray matter) areas. Progressive lateral ventricle volume increases are also found. More pronounced progressive brain changes in patients is associated with poor outcome, more negative symptoms, and a decline in neuropsychological performance in one or some of the studies, but not consistently so. Higher daily cumulative dose of antipsychotic medication intake is either not associated with brain volume changes or with less prominent brain volume changes. The progressive brain changes present in chronic schizophrenia may represent a continuous pathophysiological process taking place in the brains of these patients that warrants further study.

The effect of CXCL1 on human fetal oligodendrocyte progenitor cells

Chemokine CXCL1 is abundantly present in proliferative zones during brain development and in regions of remyelination, suggesting that it influences development of oligodendrocyte progenitors (OPC) in these regions. We studied in vitro the effects and possible mechanisms by which CXCL1 acts on human fetal OPC. In organotypic slice cultures of human fetal cortical ventricular/subventricular (VZ/SVZ) zones, blocking of CXCL1 signaling reduced significantly the proliferation of OPC. Moreover, exogenously added CXCL1 induced increase of OPC proliferation. Treatments of purified OPC cultures and cell depletion experiments demonstrated that this effect of CXCL1 was mainly indirect, mediated through astrocytes. We identified that CXCL1 acted through the extracellular signal regulated kinase (ERK1/2) pathway, activated primarily in astrocytes. In vitro, astrocytes stimulated with CXCL1 released several cytokines, but only the release of interleukin-6 (IL-6) was completely blocked by inhibition of ERK1/2 pathway. When released IL-6 was neutralized in slices, a decrease in OPC proliferation was demonstrated, while addition of IL-6 was able to return OPC proliferation in astrocyte-depleted slices to the control level. These results suggest that in the human fetal brain CXCL1 promotes proliferation of early OPC, acting in part through an ERK1/2-dependent pathway and release of IL-6 from astrocytes.

Phosphatase and tensin homolog, deleted on chromosome 10 deficiency in brain causes defects in synaptic structure, transmission and plasticity, and myelination abnormalities

The phosphatidylinositol 3-kinase (PI3K) signaling pathway modulates growth, proliferation and cell survival in diverse tissue types and plays specialized roles in the nervous system including influences on neuronal polarity, dendritic branching and synaptic plasticity. The tumor-suppressor phosphatase with tensin homology (PTEN) is the central negative regulator of the PI3K pathway. Germline PTEN mutations result in cancer predisposition, macrocephaly and benign hamartomas in many tissues, including Lhermitte-Duclos disease, a cerebellar growth disorder. Neurological abnormalities including autism, seizures and ataxia have been observed in association with inherited PTEN mutation with variable penetrance. It remains unclear how loss of PTEN activity contributes to neurological dysfunction. To explore the effects of Pten deficiency on neuronal structure and function, we analyzed several ultra-structural features of Pten-deficient neurons in Pten conditional knockout mice. Using Golgi stain to visualize full neuronal morphology, we observed that increased size of nuclei and somata in Pten-deficient neurons was accompanied by enlarged caliber of neuronal projections and increased dendritic spine density. Electron microscopic evaluation revealed enlarged abnormal synaptic structures in the cerebral cortex and cerebellum. Severe myelination defects included thickening and unraveling of the myelin sheath surrounding hypertrophic axons in the corpus callosum. Defects in myelination of axons of normal caliber were observed in the cerebellum, suggesting intrinsic abnormalities in Pten-deficient oligodendrocytes. We did not observe these abnormalities in wild-type or conditional Pten heterozygous mice. Moreover, conditional deletion of Pten drastically weakened synaptic transmission and synaptic plasticity at excitatory synapses between CA3 and CA1 pyramidal neurons in the hippocampus. These data suggest that Pten is involved in mechanisms that control development of neuronal and synaptic structures and subsequently synaptic function.

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.

Canadian Association of Neurosciences review: regulation of myelination by trophic factors and neuron-glial signaling

Myelination in the nervous system is a tightly regulated process that is mediated by both soluble and non-soluble factors acting on axons and glial cells. This process is bi-directional and involves a variety of neurotrophic and gliotrophic factors acting in paracrine and autocrine manners. Neuron-derived trophic factors play an important role in the control of early proliferation and differentiation of myelinating glial cells. At later stages of development, same molecules may play a different role and act as inducers of myelination rather than cell survival signals for myelinating glial cells. In return, myelinating glial cells provide trophic support for axons and protect them from injury. Chronic demyelination leads to secondary axonal degeneration that is responsible for long-term disability in primary demyelinating diseases such as multiple sclerosis and inherited demyelinating peripheral neuropathies. A better understanding of the molecular mechanisms controlling myelination may yield novel therapeutic targets for demyelinating nervous system disorders.

Axonal neurofilaments control multiple fiber properties but do not influence structure or spacing of nodes of Ranvier

In the vertebrate nervous system, axon calibers correlate positively with myelin sheath dimensions and electrophysiological parameters including action potential amplitude and conduction velocity. Neurofilaments, a prominent component of the neuronal cytoskeleton, are required by axons to support their normal radial growth. To distinguish between fiber features that arise in response to absolute axon caliber and those that are under autonomous control, we investigated transgenic mice in which neurofilaments are sequestered in neuronal cell bodies. The neurofilament deficient axons in such mice achieve mature calibers only 50% of normal and have altered conduction properties. We show here that this primary axonal defect also induces multiple changes in myelin sheath composition and radial dimensions. Remarkably, other fundamental fiber features, including internodal spacing and the architecture and composition of nodes of Ranvier, remain unaltered. Thus, many fiber characteristics are controlled through mechanisms operating independently of absolute axon caliber and the neurofilament cytoskeleton.

Schwann cells revert to non-myelinating phenotypes in the deafened rat cochlea

Loss of sensory hair cells within the cochlea results in a permanent sensorineural hearing loss and initiates the gradual degeneration of spiral ganglion neurons (SGNs) - the primary afferent neurons of the cochlea. While these neurons are normally myelinated via Schwann cells, loss of myelin occurs as a precursor to neural degeneration. However, the relationship between demyelination and the status of Schwann cells in deafness is not well understood. We used a marker of peripheral myelin (myelin protein zero; P0) and a marker of Schwann cells (S100) to determine the temporal sequence of myelin and Schwann cell loss as a function of duration of deafness. Rat pups were systemically deafened for periods ranging from 2 weeks to greater than 6 months by co-administration of frusemide and gentamicin. Cochleae were cryosectioned and quantitative immunohistochemistry used to determine the extent of P0 and S100 labelling within the peripheral processes, SGN soma and their central processes within the modiolus. SGN density was also determined for each cochlear turn. P0 labelling decreased throughout the cochlea with increasing duration of deafness. The reduction in P0 labelling occurred at a faster rate than the SGN loss. In contrast, S100 labelling was not significantly reduced compared with age-matched controls in any cochlear region until 6 months post-deafening. These results suggest that Schwann cells may revert to non-myelinating phenotypes in response to deafness and exhibit greater survival traits than SGNs. The potential clinical significance of these findings for cochlear implants is discussed.

Brain sphingosine-1-phosphate receptors: implication for FTY720 in the treatment of multiple sclerosis

Multiple sclerosis (MS) is an autoimmune, neurological disability with unknown etiology. The current therapies available for MS work by an immunomodulatory action, preventing T-cell- and macrophage-mediated destruction of brain-resident oligodendrocytes and axonal loss. Recently, FTY720 (fingolimod) was shown to significantly reduce relapse rates in MS patients and is currently in Phase III clinical trials. This drug attenuates trafficking of harmful T cells entering the brain by regulating sphingosine-1-phosphate (S1P) receptors. Here, we outline the direct roles that S1P receptors play in the central nervous system (CNS) and discuss additional modalities by which FTY720 may provide direct neuroprotection in MS.

Brain penetration of the oral immunomodulatory drug FTY720 and its phosphorylation in the central nervous system during experimental autoimmune encephalomyelitis: consequences for mode of action in multiple sclerosis

FTY720 [2-amino-2-[2-(4-octylphenyl) ethyl]propane-1,3-diol hydrochloride] is an oral sphingosine-1-phosphate receptor modulator under development for the treatment of multiple sclerosis (MS). The drug is phosphorylated in vivo by sphingosine kinase 2 to its bioactive form, FTY720-P. Although treatment with FTY720 is accompanied by a reduction of the peripheral lymphocyte count, its efficacy in MS and experimental autoimmune encephalomyelitis (EAE) may be due to additional, direct effects in the central nervous system (CNS). We now show that FTY720 localizes to the CNS white matter, preferentially along myelin sheaths. Brain trough levels of FTY720 and FTY720-P in rat EAE are of the same magnitude and dose dependently increase; they are in the range of 40 to 540 ng/g in the brain tissue at efficacious doses and exceed blood concentrations severalfold. In a rat model of chronic EAE, prolonged treatment with 0.03 mg/kg was efficacious, but limiting the dosing period failed to prevent EAE despite a significant decrease in blood lymphocytes. FTY720 effectiveness is likely due to a culmination of mechanisms involving reduction of autoreactive T cells, neuroprotective influence of FTY720-P in the CNS, and inhibition of inflammatory mediators in the brain.

Transient VEP and psychophysical chromatic contrast thresholds in children and adults

It has been found that humans are able to distinguish colours without luminance cues by about 2-4 months of age and that sensitivity to colour difference develops during childhood, reaching a peak around adolescence. This prolonged period of maturation is reflected by improvements in psychophysical threshold measures and by the VEP characteristics of morphology, latency and amplitude. An intra-individual comparison of VEP and psychophysical responses to isoluminant colour stimuli has not been made in children, however, and this was the aim of the present study. VEPs were recorded from 49 subjects, children (age range: 4.8-12.6 years) and adults (age range: 25.7-33.2 years). Psychophysical and VEP thresholds were both measured in 40 of those subjects. Nominally isoluminant chromatic (L-M) sinewave gratings were presented in onset-offset mode and identical stimuli were used for psychophysical and VEP recordings to allow comparison. In agreement with previous reports, morphology of the transient VEP in response to this stimulus differed considerably between children and adults. There was a significant difference between psychophysical and VEP thresholds in children, but not in adults. Our findings support and expand on previous work on maturation of the L-M chromatic pathway and indicate a larger discrepancy between VEP and psychophysical chromatic thresholds in children than in adults.

Gliogenesis and glial pathology in depression

Recent research has changed the perception of glia from being no more than silent supportive cells of neurons to being dynamic partners participating in brain metabolism and communication between neurons. This discovery of new glial functions coincides with growing evidence of the involvement of glia in the neuropathology of mood disorders. Unanticipated reductions in the density and number of glial cells are reported in fronto-limbic brain regions in major depression and bipolar illness. Moreover, age-dependent decreases in the density of glial fibrillary acidic protein (GFAP) - immunoreactive astrocytes and levels of GFAP protein are observed in the prefrontal cortex of younger depressed subjects. Since astrocytes participate in the uptake, metabolism and recycling of glutamate, we hypothesize that an astrocytic deficit may account for the alterations in glutamate/GABA neurotransmission in depression. Reductions in the density and ultrastructure of oligodendrocytes are also detected in the prefrontal cortex and amygdala in depression. Pathological changes in oligodendrocytes may be relevant to the disruption of white matter tracts in mood disorders reported by diffusion tensor imaging. Factors such as stress, excess of glucocorticoids, altered gene expression of neurotrophic factors and glial transporters, and changes in extracellular levels of neurotransmitters released by neurons may modify glial cell number and affect the neurophysiology of depression. Therefore, we will explore the role of these events in the possible alteration of glial number and activity, and the capacity of glia as a promising new target for therapeutic medications. Finally, we will consider the temporal relationship between glial and neuronal cell pathology in depression.

Relating imaging indices of white matter integrity and volume in healthy older adults

Age-related alterations in white matter have the potential to profoundly affect cognitive functioning. In fact, magnetic resonance imaging (MRI) studies using fractional anisotropy (FA) to measure white matter integrity reveal a positive correlation between FA and behavioral performance in older adults. Confounding these results are imaging studies demonstrating age-related white matter atrophy in some areas displaying altered FA, suggesting changes in diffusion may be simply an epiphenomenon of tissue loss. In the current study, structural MRI techniques were used to identify the relationship between white matter integrity and decreased volume in healthy aging adults. The data demonstrated that white matter atrophy did in fact account for differences in some areas, but significant FA decreases remained across much of the white matter after adjusting for atrophy. Results suggest a complex relationship between changes in white matter integrity and volume. FA appears to be more sensitive than volume loss to changes in normal appearing tissue, and these FA changes may actually precede white matter atrophy in some brain areas. As such, the ability to detect early white matter alterations may facilitate development of targeted treatments that prevent or slow age-related white matter degradation and associated cognitive sequelae.

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.

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.

Connexin-dependent transcellular transcriptomic networks in mouse brain

Microarray experiments have generally focused on magnitude of gene expression changes in pathological conditions, thereby using the method as a high throughput screen to identify candidate marker genes and/or to validate phenotypic differences. We have used novel strategies to extract additional information from array studies, including expression variability and coordination, from which organizational principles of transcriptomes are emerging. We have reported that the expression level, variability and coordination of numerous genes are regulated in brains of connexin43 null (Gja1(-/-)) mouse with respect to wildtype. Moreover, expression coordination with Gja1 in wildtype largely predicted the expression regulation in Gja1(-/-) tissues. We now report a remarkable overlap between regulations in Gja1(-/-) and connexin32 null (Gjb1(-/-)) brains, and that both differ markedly from those in connexin36 null (Gja9(-/-)) brain. Since in brain these three connexins are expressed in different cell types (Cx43 in astrocytes, ependymal and vascular cells, Gjb1 in oligodendrocytes, and Cx36 in neurons and microglia), and because astrocytes and oligodendrocytes (and possibly neurons and microglia) may form syncytia coupled by gap junction channels, these observations suggest the existence of distinct connexin-dependent panglial and neuronal transcriptomic networks. Such networks, where linkage partners are rearranged and strengths modified in brains of knockouts, may explain downstream and parallel "ripples" of phenotypic change resulting from single gene manipulations as illustrated by alterations in transcription factor networks resulting from deletion of Gja1 or Gjb1. The transcription factors also formed network hubs with genes from other functional categories, thus allowing regulation of one functional pathway through manipulation of another.

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.

The neuropoietic cytokine family in development, plasticity, disease and injury

Neuropoietic cytokines are well known for their role in the control of neuronal, glial and immune responses to injury or disease. Since this discovery, it has emerged that several of these proteins are also involved in nervous system development, in particular in the regulation of neurogenesis and stem cell fate. Recent data indicate that these proteins have yet more functions, as key modulators of synaptic plasticity and of various behaviours. In addition, neuropoietic cytokines might be a factor in the aetiology of psychiatric disorders.

BMP signaling mutant mice exhibit glial cell maturation defects

Bone morphogenetic proteins have been implicated in the development of oligodendrocytes and astrocytes, however, a role for endogenous BMP signaling in glial development has not been demonstrated in a genetic model. Using mice in which signaling via type I BMP receptors Bmpr1a and Bmpr1b have been inactivated in the neural tube, we demonstrate that BMP signaling contributes to the maturation of glial cells in vivo. At P0, mutant mice exhibited a 25-40% decrease in GFAP+ or S100beta+ astrocytes in the cervical spinal cord. The number of oligodendrocyte precursors and the timing of their emergence was unchanged in the mutant mice compared to the normals, however myelin protein expression and mature oligodendrocyte numbers were significantly reduced. These data indicate that BMP signaling promotes the generation of astrocytes and mature, myelinating oligodendrocytes in vivo but does not affect oligodendrocyte precursor development, thus suggesting tight regulation of BMP signaling to ensure proper gliogenesis.

Proinflammatory cytokine synthesis in the injured mouse spinal cord: multiphasic expression pattern and identification of the cell types involved

We have studied the spatial and temporal distribution of six proinflammatory cytokines and identified their cellular source in a clinically relevant model of spinal cord injury (SCI). Our findings show that interleukin-1beta (IL-1beta) and tumor necrosis factor (TNF) are rapidly (<5 and 15 minutes, respectively) and transiently expressed in mice following contusion. At 30-45 minutes post SCI, IL-1beta and TNF-positive cells could already be seen over the entire spinal cord segment analyzed. Multilabeling analyses revealed that microglia and astrocytes were the two major sources of IL-1beta and TNF at these times, suggesting a role for these cytokines in gliosis. Results obtained from SCI mice previously transplanted with green fluorescent protein (GFP)-expressing hematopoietic stem cells confirmed that neural cells were responsible for the production of IL-1beta and TNF for time points preceding 3 hours. From 3 hours up to 24 hours, IL-1beta, TNF, IL-6, and leukemia inhibitory factor (LIF) were strongly upregulated within and immediately around the contused area. Colocalization studies revealed that all populations of central nervous system resident cells, including neurons, synthesized cytokines between 3 and 24 hours post SCI. However, work done with SCI-GFP chimeric mice revealed that at least some infiltrating leukocytes were responsible for cytokine production from 12 hours on. By 2 days post-SCI, mRNA signal for all the above cytokines had nearly disappeared. Notably, we also observed another wave of expression for IL-1beta and TNF at 14 days. Overall, these results indicate that following SCI, all classes of neural cells initially contribute to the organization of inflammation, whereas recruited immune cells mostly contribute to its maintenance at later time points.

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.

Cdc2-mediated Schwann cell migration during peripheral nerve regeneration

Schwann cell migration facilitates peripheral nerve regeneration after injury. We have recently found increased activation of Cdc2 kinase in regenerating sciatic nerves. Here we show that Cdc2 phosphorylation of caldesmon regulates Schwann cell migration and nerve regeneration. A robust but transient increase in Cdc2 expression was found in cultured Schwann cells prepared from the sciatic nerve in rats that had undergone crush injury for 7 days. These `injury-preconditioned' Schwann cells exhibited enhanced migration compared with non-preconditioned control cells and treatment with the cdk inhibitor roscovitine prevented cell migration. After transduction with recombinant Cdc2 DNA adenoviral vectors, Schwann cells were implanted into sciatic nerves; those expressing wild-type Cdc2 migrated further in the distal direction than those expressing dominant-negative Cdc2. We identified caldesmon as a downstream substrate of Cdc2 in Schwann cells and its phosphorylation by Cdc2 changed its subcellular localization. Overexpression of dominant-negative caldesmon significantly counteracted the migration effect caused by Cdc2. Finally, neurite outgrowth of cultured DRG sensory neurons, facilitated by co-culture with injury-preconditioned Schwann cells, was suppressed by roscovitine treatment. The results indicate that activation of the Cdc2-caldesmon pathway is necessary for Schwann cell migration and suggest a role for this pathway in peripheral axonal growth.

Interleukin-11 potentiates oligodendrocyte survival and maturation, and myelin formation

Mechanisms that regulate oligodendrocyte survival and myelin formation are an intense focus of research into myelin repair in the lesions of multiple sclerosis (MS). Although demyelination and oligodendrocyte loss are pathological hallmarks of the disease, increased oligodendrocyte numbers and remyelination are frequently observed in early lesions, but these diminish as the disease course progresses. In the current study, we used a microarray-based approach to investigate genes regulating repair in MS lesions, and identified interleukin-11 (IL-11) as an astrocyte-derived factor that potentiates oligodendrocyte survival and maturation, and myelin formation. IL-11 was induced in human astrocyte cultures by the cytokines IL-1beta and TGFbeta1, which are both prominently expressed in MS plaques. In MS tissue samples, IL-11 was expressed by reactive astrocytes, with expression particularly localized at the myelinated border of both active and silent lesions. Its receptor, IL-11R alpha, was expressed by oligodendrocytes. In experiments in human cultures in vitro, IL-11R alpha localized to immature oligodendrocytes, and its expression decreased during maturation. In cultures treated with IL-11, we observed a significant increase in oligodendrocyte number, and this was associated with enhanced oligodendrocyte survival and maturation. Importantly, we also found that IL-11 treatment was associated with significantly increased myelin formation in rodent CNS cocultures. These data are the first to implicate IL-11 in oligodendrocyte viability, maturation, and myelination. We suggest that this pathway may represent a potential therapeutic target for oligodendrocyte protection and remyelination in MS.

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.

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.

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.

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.