Axons regulate the expression of Shaker-like potassium channel genes in Schwann cells in peripheral nerve

The role of the tripartite synapse in the heart: how glial cells may contribute to the physiology and pathophysiology of the intracardiac nervous system

One of the major roles of the intracardiac nervous system (ICNS) is to act as the final site of signal integration for efferent information destined for the myocardium to enable local control of heart rate and rhythm. Multiple subtypes of neurons exist in the ICNS where they are organized into clusters termed ganglionated plexi (GP). The majority of cells in the ICNS are actually glial cells; however, despite this, ICNS glial cells have received little attention to date. In the central nervous system, where glial cell function has been widely studied, glia are no longer viewed simply as supportive cells but rather have been shown to play an active role in modulating neuronal excitability and synaptic plasticity. Pioneering studies have demonstrated that in addition to glia within the brain stem, glial cells within multiple autonomic ganglia in the peripheral nervous system, including the ICNS, can also act to modulate cardiovascular function. Clinically, patients with atrial fibrillation (AF) undergoing catheter ablation show high plasma levels of S100B, a protein produced by cardiac glial cells, correlated with decreased AF recurrence. Interestingly, S100B also alters GP neuron excitability and neurite outgrowth in the ICNS. These studies highlight the importance of understanding how glial cells can affect the heart by modulating GP neuron activity or synaptic inputs. Here, we review studies investigating glia both in the central and peripheral nervous systems to discuss the potential role of glia in controlling cardiac function in health and disease, paying particular attention to the glial cells of the ICNS.

Review : Glial Neuronal Interactions: A Physiological Perspective

Throughout the nervous system, neurons are closely surrounded by glial cells, leaving only a 20-nm wide extracellular space filled with interstitial fluid. Ions, transmitters, hormones, nutrients, and waste products all share this narrow diffusion pathway. Because the interstitial space occupies only a small volume, neuronal activity can lead to appreciable changes in the extracellular concentration of ions, protons, and neurotrans mitters. These changes can affect neuronal activity and are believed to be influenced by glial cells. The proximity of glial processes to synapses and axons make glial cells ideal partners to sequester ions and transmitters released by neurons. The failure of glial cells to perform such essential homeostatic functions can have profound effects, and these homeostatic activities may constitute one way in which glial cells can influence neuronal signaling. In addition, glial cells, which, unlike most neurons, are coupled to each other through gap-junctions, communicate with each other and possibly also with adjacent neurons through prop agated intracellular Ca2+waves. The importance of such interglial signaling is not understood. Additionally, glial cells and neurons mutually modulate their expression of ion channels, most likely through factors re leased into the extracellular space. The range of responses observed in glial cells and their intimate anatomical relationship with neurons suggest a broader role for glia than is currently appreciated. It also emphasizes the importance of a better understanding of glial-neuronal interactions to an understanding of brain function. The Neuroscientist 1:328-337, 1995

Ribosomal trafficking is reduced in Schwann cells following induction of myelination

Local synthesis of proteins within the Schwann cell periphery is extremely important for efficient process extension and myelination, when cells undergo dramatic changes in polarity and geometry. Still, it is unclear how ribosomal distributions are developed and maintained within Schwann cell projections to sustain local translation. In this multi-disciplinary study, we expressed a plasmid encoding a fluorescently labeled ribosomal subunit (L4-GFP) in cultured primary rat Schwann cells. This enabled the generation of high-resolution, quantitative data on ribosomal distributions and trafficking dynamics within Schwann cells during early stages of myelination, induced by ascorbic acid treatment. Ribosomes were distributed throughout Schwann cell projections, with ~2-3 bright clusters along each projection. Clusters emerged within 1 day of culture and were maintained throughout early stages of myelination. Three days after induction of myelination, net ribosomal movement remained anterograde (directed away from the Schwann cell body), but ribosomal velocity decreased to about half the levels of the untreated group. Statistical and modeling analysis provided additional insight into key factors underlying ribosomal trafficking. Multiple regression analysis indicated that net transport at early time points was dependent on anterograde velocity, but shifted to dependence on anterograde duration at later time points. A simple, data-driven rate kinetics model suggested that the observed decrease in net ribosomal movement was primarily dictated by an increased conversion of anterograde particles to stationary particles, rather than changes in other directional parameters. These results reveal the strength of a combined experimental and theoretical approach in examining protein localization and transport, and provide evidence of an early establishment of ribosomal populations within Schwann cell projections with a reduction in trafficking following initiation of myelination.

Homeobox gene expression in adult dorsal root ganglia during sciatic nerve regeneration: is regeneration a recapitulation of development?

After damage of the sciatic nerve, a regeneration process is initiated. Neurons in the dorsal root ganglion regrow their axons and functional connections. The molecular mechanisms of this neuronal regenerative process have remained elusive, but a relationship with developmental processes has been conceived. This chapter discusses the applicability of the developmental hypothesis of regeneration to the dorsal root ganglion; this hypothesis states that regeneration of dorsal root ganglion neurons is a recapitulation of development. We present data on changes in gene expression upon sciatic nerve damage, and the expression and function of homeobox genes. This class of transcription factors plays a role in neuronal development. Based on these data, it is concluded that the hypothesis does not hold for dorsal root ganglion neurons, and that regeneration-specific mechanisms exist. Cytokines and the associated Jak/STAT (janus kinase/signal transducer and activator of transcription) signal transduction pathway emerge as constituents of a regeneration-specific mechanism. This mechanism may be the basis of pharmacological strategies to stimulate regeneration.

PDGF upregulates delayed rectifier via Src family kinases and sphingosine kinase in oligodendroglial progenitors

An increase in the expression of the delayed rectifier current (I(K)) has been shown to correlate with mitogenesis in many cell types. However, pathways involved in the upregulation of I(K) by growth factors in oligodendroglial progenitors (OPs) have not been well-elucidated. In this study, we found that treatment with platelet-derived growth factor (PDGF) and basic fibroblast growth factor but not ciliary neurotrophic factor resulted in increased I(K) density and upregulation of Kv1.5 and Kv1.6 mRNA transcripts. The effect of PDGF on I(K) was blocked by mimosine, a cell cycle inhibitor, and by genistein, a tyrosine kinase inhibitor. Using inhibitors of PDGF-activated pathways, we found that PDGF-induced upregulation of Kv1.5 and I(K) density involves Src family tyrosine kinases, sphingosine kinase, and intracellular Ca(2+) but not ERK1/2 or phosphatidylinositol 3-kinase pathways. Furthermore, agents that were effective inhibitors of PDGF-induced I(K) upregulation also attenuated OP proliferation, supporting the concept that I(K) is an important link between PDGF-activated signaling cascades and cell cycle progression.

Olfactory receptor axons influence the development of glial potassium currents in the antennal lobe of the moth Manduca sexta

In the olfactory (antennal) lobe of the moth Manduca sexta, olfactory receptor axons strongly influence the distribution and morphology of glial cells. In the present study, we asked whether the development of the electrophysiological properties of the glial cells is influenced by the receptor axons. Whole-cell currents were measured in antennal lobe glial cells in acute brain slices prepared from animals at different stages of metamorphic development (stages 3, 6, and 12). Outward currents were induced by depolarizing voltage steps from a holding potential of -70 mV. At all developmental stages investigated, the outward currents were partly blocked by bath application of the potassium channel blocker 4-aminopyridine (4AP, 10 mM) or by including tetraethylammonium (TEA, 30 mM) in the pipette solution. The relative contribution of the 4AP-sensitive current to the outward current increased from 18% at stages 3 and 6 to 42% at stage 12, while the TEA-sensitive current increased from 18% at stage 3 to 81% at stage 6, and then declined again to 40% at stage 12. In contrast, in the absence of receptor axons, these changes in the contribution of the TEA- and 4AP-sensitive currents to the total outward current did not occur; rather, the current profile remained in the most immature state (stage 3). The results suggest that olfactory receptor axons are essential for development of the mature pattern of glial potassium currents.

Ion channels in glial cells

Functional and molecular analysis of glial voltage- and ligand-gated ion channels underwent tremendous boost over the last 15 years. The traditional image of the glial cell as a passive, structural element of the nervous system was transformed into the concept of a plastic cell, capable of expressing a large variety of ion channels and neurotransmitter receptors. These molecules might enable glial cells to sense neuronal activity and to integrate it within glial networks, e.g., by means of spreading calcium waves. In this review we shall give a comprehensive summary of the main functional properties of ion channels and ionotropic receptors expressed by macroglial cells, i.e., by astrocytes, oligodendrocytes and Schwann cells. In particular we will discuss in detail glial sodium, potassium and anion channels, as well as glutamate, GABA and ATP activated ionotropic receptors. A majority of available data was obtained from primary cell culture, these results have been compared with corresponding studies that used acute tissue slices or freshly isolated cells. In view of these data, an active glial participation in information processing seems increasingly likely and a physiological role for some of the glial channels and receptors is gradually emerging.

Heteromultimeric delayed-rectifier K+ channels in schwann cells: developmental expression and role in cell proliferation

Schwann cells (SCs) are responsible for myelination of nerve fibers in the peripheral nervous system. Voltage-dependent K+ currents, including inactivating A-type (KA), delayed-rectifier (KD), and inward-rectifier (KIR) K+ channels, constitute the main conductances found in SCs. Physiological studies have shown that KD channels may play an important role in SC proliferation and that they are downregulated in the soma as proliferation ceases and myelination proceeds. Recent studies have begun to address the molecular identity of K+ channels in SCs. Here, we show that a large repertoire of K+ channel alpha subunits of the Shaker (Kv1.1, Kv1.2, Kv1.4, and Kv1.5), Shab (Kv2.1), and Shaw (Kv3.1b and Kv3.2) families is expressed in mouse SCs and sciatic nerve. We characterized heteromultimeric channel complexes that consist of either Kv1.5 and Kv1.2 or Kv1.5 and Kv1.4. In postnatal day 4 (P4) sciatic nerve, most of the Kv1.2 channel subunits are involved in heteromultimeric association with Kv1.5. Despite the presence of Kv1. 1 and Kv1.2 alpha subunits, the K+ currents were unaffected by dendrotoxin I (DTX), suggesting that DTX-sensitive channel complexes do not account substantially for SC KD currents. SC proliferation was found to be potently blocked by quinidine or 4-aminopyridine but not by DTX. Consistent with previous physiological studies, our data show that there is a marked downregulation of all KD channel alpha subunits from P1-P4 to P40 in the sciatic nerve. Our results suggest that KD currents are accounted for by a complex combinatorial activity of distinct K+ channel complexes and confirm that KD channels are involved in SC proliferation.

Dynamic regulation of K+ channel gene expression in differentiated cells

Recent studies have determined that K+ channel gene expression is dynamically controlled in endocrine, cardiac, and neuronal cells. This regulation is induced by physiological stimuli (e.g., hormones, transmitters, depolarization), drugs (e.g., opiates) and with pathophysiological conditions (e.g., seizures, hypertension). In many cases, alterations in subunit expression are driven by transcriptional changes. Furthermore, resultant changes in excitability can be produced within hours because of the rapid turnover of Kv-channel proteins. Finally, the consequences of altering K+-channel subunit are complex because a single gene product can participate in forming functionally distinct homomeric and heteromeric channels in the same cell. Thus, regulating K+-channel genes constitutes a novel mechanism for producing intricate long-term changes in excitability.

Constitutive activation of delayed-rectifier potassium channels by a src family tyrosine kinase in Schwann cells

In the nervous system, Src family tyrosine kinases are thought to be involved in cell growth, migration, differentiation, apoptosis, as well as in myelination and synaptic plasticity. Emerging evidence indicates that K+ channels are crucial targets of Src tyrosine kinases. However, most of the data accumulated so far refer to heterologous expression, and native K+-channel substrates of Src or Fyn in neurons and glia remain to be elucidated. The present study shows that a Src family tyrosine kinase constitutively activates delayed-rectifier K+ channels (IK) in mouse Schwann cells (SCs). IK currents are markedly downregulated upon exposure of cells to the tyrosine kinase inhibitors herbimycin A and genistein, while a potent upregulation of IK is observed when recombinant Fyn kinase is introduced through the patch pipette. The Kv1.5 and Kv2.1 K+-channel alpha subunits are constitutively tyrosine phosphorylated and physically associate with Fyn both in cultured SCs and in the sciatic nerve in vivo. Kv2.1- channel subunits are found to interact with the Fyn SH2 domain. Inhibition of Schwann cell proliferation by herbimycin A and by K+-channel blockers suggests that the functional linkage between Src tyrosine kinases and IK channels could be important for Schwann cell proliferation and the onset of myelination.

Expression of Kv1.1, a Shaker-like potassium channel, is temporally regulated in embryonic neurons and glia

Kv1.1, a Shaker-like voltage-gated potassium channel, is strongly expressed in a variety of neurons in adult rodents, in which it appears to be involved in regulating neuronal excitability. Here we show that Kv1.1 is also expressed during embryonic development in the mouse, exhibiting two transient peaks of expression around embryonic day 9.5 (E9.5) and E14.5. Using both in situ hybridization and immunocytochemistry, we have identified several cell types and tissues that express Kv1.1 RNA and protein. At E9.5, Kv1.1 RNA and protein are detected transiently in non-neuronal cells in several regions of the early CNS, including rhombomeres 3 and 5 and ventricular zones in the mesencephalon and diencephalon. At E14.5, several cell types in both the CNS and peripheral nervous system express Kv1.1, including neuronal cells (sensory ganglia and outer aspect of cerebral hemispheres) and glial cells (radial glia, satellite cells, and Schwann cell precursors). These data show that Kv1.1 is expressed transiently in a variety of neuronal and non-neuronal cells during restricted periods of embryonic development. Although the functional roles of Kv1.1 in development are not understood, the cell-specific localization and timing of expression suggest this channel may play a role in several developmental processes, including proliferation, migration, or cell-cell adhesion.

Cyclic AMP regulates potassium channel expression in C6 glioma by destabilizing Kv1.1 mRNA

The tissue distributions and physiological properties of a variety of cloned voltage-gated potassium channel genes have been characterized extensively, yet relatively little is known about the mechanisms controlling expression of these genes. Here, we report studies on the regulation of Kv1.1 expressed endogenously in the C6 glioma cell line. We demonstrate that elevation of intracellular cAMP leads to the accelerated degradation of Kv1.1 RNA. The cAMP-induced decrease in Kv1.1 RNA is followed by a decrease in Kv1. 1 protein and a decrease in the whole cell sustained K+ current amplitude. Dendrotoxin-I, a relatively specific blocker of Kv1.1, blocks 96% of the sustained K+ current in glioma cells, causing a shift in the resting membrane potential from -40 mV to -7 mV. These data suggest that expression of Kv1.1 contributes to setting the resting membrane potential in undifferentiated glioma cells. We therefore suggest that receptor-mediated elevation of cAMP reduces outward K+ current density by acting at the translational level to destabilize Kv1.1 RNA, an additional mechanism for regulating potassium channel gene expression.

Molecular structure and expression of shaker type potassium channels in glial cells of trout CNS

Two shaker-related potassium channel transcripts (tsha1, tsha2) were identified in glial cells of trout central nervous system (CNS) by polymerase chain reaction (PCR)-cloning and sequencing. While tsha1 was highly similar to the mammalian Kv1.2 subtype of potassium channels, tsha2 did not show a preferential sequence homology with a particular subtype of shaker, but exhibited uniform similarity with mammalian Kv1.1, Kv1.2, and Kv1.3, respectively. Transcripts for the shaw and shal subfamilies of voltage-gated potassium channels were detected in whole brain tissue only but not in freshly dissociated glial cells. Using mRNA extracted from different glial cell types in combination with sequence specific PCR primers, tshal was found restricted to oligodendrocytes and their progenitor cells, while tsha2 was common to astrocytes, as well.

Characterization of delayed rectifier Kv channels in oligodendrocytes and progenitor cells

We examined the molecular identity of K+ channel genes underlying the delayed rectifier (IK) in differentiated cultured oligodendrocytes (OLGs) and oligodendrocyte progenitor (OP) cells. Using reverse transcription-PCR cloning, we found that OP cells and OLGs expressed multiple Kv transcripts, namely Kv1.2, Kv1.4, Kv.1.5, and Kv1.6. Immunocytochemical and Western blot analyses revealed that Kv1.5 and Kv1.6 as well as Kv1.2 and Kv1.4 channel proteins could be detected in these cells, but definitive evidence for functional K+ channel expression was obtained only for the Kv1.5 channel. In addition, mRNA and immunoreactive protein levels of both Kv1.5 and Kv1.6 channels were significantly lower in differentiated OLGs when compared with levels in OP cells. Proliferation of OP cells was inhibited by K+ channel blockers, but not by incubation with either Kv1.5 or Kv1.6 antisense oligonucleotides. We conclude that (1) IK in OP cells and OLGs is encoded partly by Kv1.5 subunits, possibly forming heteromultimeric channels with Kv1.6 or other Kv subunits; and (2) inhibition of Kv1.5 or Kv1.6 channel expression alone does not prevent mitogenesis. Concomitant inhibition of other Kv channels underlying IK may be necessary for OP cells to exit from cell cycle.

The swiss cheese mutant causes glial hyperwrapping and brain degeneration in Drosophila

Swiss cheese (sws) mutant flies develop normally during larval life but show age-dependent neurodegeneration in the pupa and adult and have reduced life span. In late pupae, glial processes form abnormal, multilayered wrappings around neurons and axons. Degeneration first becomes evident in young flies as apoptosis in single scattered cells in the CNS, but later it becomes severe and widespread. In the adult, the number of glial wrappings increases with age. The sws gene is expressed in neurons in the brain cortex. The conceptual 1425 amino acid protein shows two domains with homology to the regulatory subunits of protein kinase A and to conceptual proteins of yet unknown function in yeast, worm, and human. Sequencing of two sws alleles shows amino acid substitutions in these two conserved domains. It is suggested that the novel SWS protein plays a role in a signaling mechanism between neurons and glia that regulates glial wrapping during development of the adult brain.

Identification of the delayed rectifier potassium channel, Kv1.6, in cultured astrocytes

Astrocytes are an abundant glial cell type of the central nervous system that appear to play a role in regulating extracellular potassium concentrations in brain, thereby contributing to the maintenance of normal neuronal activity. Voltage-gated potassium conductances, shown to be present in astrocytes, may be involved in this and other astrocytic functions. Toward defining the role of voltage-gated potassium channels in astrocytes, total RNA prepared from cultured mouse cortical astrocytes was screened, using a reverse transcriptase-polymerase chain reaction (RT-PCR) approach, for the expression of several members of the Shaker-like potassium channel subfamily (Kv1.1-Kv1.6). A relatively high level of Kv1.6 transcript was identified by RT-PCR and then confirmed and quantitated by ribonuclease protection assays using a Kv1.6-specific riboprobe. Immunocytochemical staining showed double-labeling of glial fibrillary acidic protein-positive cells with antibody specific for the Kv1.6 channel. The Kv1.6 protein expression was variable among the individual astrocytes. Outward voltage-gated currents were studied in astrocytes in primary culture using the Nystatin-perforated patch voltage clamp technique. Outward potassium currents were observed in all cells studied, and this current was partially blocked by perfusion with 100 nM dendrotoxin (DTX) in 14 of 16 cells tested. This DTX-sensitive current appeared to be a sustained outward potassium current, consistent with the suggestion that the Shaker-like potassium channel Kv1.6 underlies a portion of the delayed rectifier potassium current in cultured mouse cortical astrocytes.

Molecular specializations at nodes and paranodes in peripheral nerve

In the peripheral nervous system, nodes of Ranvier are formed by interactions between myelinating Schwann cells and axons. Nodes have an intricate ultrastructure, and their molecular architecture is similarly complex. A growing list of molecules have been found that are selectively localized to different parts of the nodes. Neural cell adhesion molecule (N-CAM), L1/Ng-CAM, and tenascin/cytotactin are enriched in the nodal basal lamina; hyaluronic acid, versican/hyaluronectin, N-CAM, L1/Ng-CAM, tenascin/cytotactin, and the ganglioside GM1 are enriched in the nodal gap; myelin-associated glycorprotein, oligodendrocyte-myelin glycoprotein, connexin32, E-cadherin, actin, the gangliosides GQ1b and GD1b, the potassium channel KV1.5, and alkaline phosphatase are enriched in the paranodal region of the Schwann cell; voltage-dependent sodium channels and the cytoskeletal proteins spectrin and ankyrin are enriched in the nodal axolemma. Many of these molecules are probably essential for the proper functioning and stability of nodes.

Effect of cutting the optic nerve on K+ currents in endfeet of Muller cells isolated from frog retina

Membrane currents were recorded from Muller cells isolated from normal retinas and from retinas whose ganglion cell axons had been cut in the optic nerve 30-60 days previously. The surgical procedure did not block the retinal blood supply and did not allow the axons to regenerate. The principal finding was that after severing the optic nerve there was less inward rectification in response to voltage commands. That is, the maintained inward current (I K(IN)) produced in response to a hyperpolarizing voltage command decreased leading to a decrease in the ratio I K(IN)/I K(OUT) In 98 mM [K+]O, this ratio was 2.86 +/- 0.21 (mean +/- SE; n = 24) in controls and 1.13 +/- 0.13 (n = 21) in Muller cells from denervated retinae. Barium, a blocker of the potassium inward rectifier (I (KIR)), eliminated this difference. Moreover, severing the optic nerve also decreased the resting potentials of Muller cells in 2.5 mM [K+]O from -83 +/- 7 mV to -63 +/- 9 mV. The results suggest that the voltage-dependent behavior and selectivity of K+ inward rectifying channels (K (ir)) in the endfeet depends on the integrity of the closely apposed ganglion cells.

Hypomyelination alters K+ channel expression in mouse mutants shiverer and Trembler

Voltage-gated K+ channels are localized to juxtaparanodal regions of myelinated axons. To begin to understand the role of normal compact myelin in this localization, we examined mKv1.1 and mKv1.2 expression in the dysmyelinating mouse mutants shiverer and Trembler. In neonatal wild-type and shiverer mice, the focal localization of both proteins in axon fiber tracts is similar, suggesting that cues other than mature myelin can direct initial K+ channel localization in shiverer mutants. In contrast, K+ channel localization is altered in hypomyelinated axonal fiber tracts of adult mutants, suggesting that abnormal myelination leads to channel redistribution. In shiverer adult, K+ channel expression is up-regulated in both axons and glia, as revealed by immunocytochemistry, RNase protection, and in situ hybridization studies. This up-regulation of K+ channels in hypomyelinated axon tracts may reflect a compensatory reorganization of ionic currents, allowing impulse conduction to occur in these dysmyelinating mouse mutants.