cAMP-mediated expression of inwardly rectifying potassium channels in cultured mouse Schwann cells

Divergent membrane properties of mouse cochlear glial cells around hearing onset

Spiral ganglion neurons (SGNs) are the primary afferent neurons of the auditory system, and together with their attendant glia, form the auditory nerve. Within the cochlea, satellite glial cells (SGCs) encapsulate the cell body of SGNs, whereas Schwann cells (SCs) wrap their peripherally- and centrally-directed neurites. Despite their likely importance in auditory nerve function and homeostasis, the physiological properties of auditory glial cells have evaded description. Here, we characterized the voltage-activated membrane currents of glial cells from the mouse cochlea. We identified a prominent weak inwardly rectifying current in SGCs within cochlear slice preparations (postnatal day P5-P6), which was also present in presumptive SGCs within dissociated cultures prepared from the cochleae of hearing mice (P14-P15). Pharmacological block by Ba²⁺ and desipramine suggested that channels belonging to the Kir4 family mediated the weak inwardly rectifying current, and post hoc immunofluorescence implicated the involvement of Kir4.1 subunits. Additional electrophysiological profiles were identified for glial cells within dissociated cultures, suggesting that glial subtypes may have specific membrane properties to support distinct physiological roles. Immunofluorescence using fixed cochlear sections revealed that although Kir4.1 is restricted to SGCs after the onset of hearing, these channels are more widely distributed within the glial population earlier in postnatal development (i.e., within both SGCs and SCs). The decrease in Kir4.1 immunofluorescence during SC maturation was coincident with a reduction of Sox2 expression and advancing neurite myelination. The data suggest a diversification of glial properties occurs in preparation for sound-driven activity in the auditory nerve.

Schwann cells express IP prostanoid receptors coupled to an elevation in intracellular cyclic AMP

We have shown previously that prostaglandin E(2) (PGE(2)) and prostaglandin I(2) (PGI(2)) are each produced in an explant model of peripheral nerve injury. We report that IP prostanoid receptor mRNA and protein are present in primary rat Schwann cells. IP prostanoid receptor stimulation using prostacyclin produced an elevation in intracellular cyclic AMP concentration ([cAMP](i)) in primary Schwann cells. Peak [cAMP](i) was observed between 5-15 min of stimulation followed by a gradual recovery toward basal level. Phosphorylation of cyclic AMP-response element binding protein (CREB) on Ser(133) was also detected after IP prostanoid receptor stimulation and CREB phosphorylation was inhibited completely by the protein kinase A inhibitor, H-89. Intracellular calcium levels were not affected by IP prostanoid receptor stimulation. Unlike forskolin, IP prostanoid receptor stimulation did not significantly augment Schwann cell proliferation in response to growth factor treatment. However, IP prostanoid receptor stimulation increased the number of Schwann cells that were able to generate a calcium transient in response to P2 purinergic receptor activation. These findings suggest that signaling via the IP prostanoid receptor may by relevant to Schwann cell biology in vivo.

Voltage-Gated ion currents of schwann cells in cell culture models of human neurofibromatosis

K(+) (K) channels play a role in the proliferation of many cell types in normal cells and certain disease states. Several laboratories have studied K currents in cultured Schwann cells from models of the human diseases, neurofibromatosis type 1 (NF1) and neurofibromatosis type 2 (NF2). These diseases are characterized by the growth of Schwann cell tumors. In all cell culture NF models the K current properties differ in tumor-derived and normal Schwann cells. Depending on the model however, the type of K channel abnormality differs. K channels appear to play a role in the proliferation of Schwann cell cultures of these disease models, because a link has been established between K current blockade and the inhibition of Schwann cell proliferation in NF1 and NF2. Differences in the proliferation response of normal Schwann cells to K channel blockers suggest that in vitro regulation of proliferation in neoplastic and normal Schwann cells is complex.

Electrophysiology of mammalian Schwann cells

Schwann cells are the satellite cell of the peripheral nervous system, and they surround axons and motor nerve terminals. The review summarises evidence for the ion channels expressed by mammalian Schwann cells, their molecular nature and known or speculated functions. In addition, the recent evidence for gap junctions and cytoplasmic diffusion pathways within the myelin and the functional consequences of a lower-resistance myelin sheath are discussed. The main types of ion channel expressed by Schwann cells are K(+) channels, Cl(-) channels, Na(+) channels and Ca(2+) channels. Each is represented by a variety of sub-types. The molecular and biophysical characteristics of the cation channels expressed by Schwann cells are closely similar or identical to those of channels expressed in peripheral axons and elsewhere. In addition, Schwann cells express P(2)X ligand-gated ion channels. Possible in vivo roles for each ion channel type are discussed. Ion channel expression in culture could have a special function in driving or controlling cell proliferation and recent evidence indicates that some Ca(2+) channel and Kir channel expression in culture is dependent upon the presence of neurones and local electrical activity.

Ionic currents in normal and neurofibromatosis type 1-affected human Schwann cells: induction of tumor cell K current in normal Schwann cells by cyclic AMP

Comparisons were made of whole cell voltage clamp recordings from cultures of normal Schwann cells (SC) from three human subjects and from three neurofibrosarcoma cell lines. The whole cell K+ (K) currents of normal and tumor cells could be divided into three types based on voltage activation range, pharmacology, and macroscopic inactivation: A type current, tetraethylammonium- (TEA-) only-sensitive current, and inward rectifier current. The most conspicuous difference between normal and tumor cells was the nature of K currents present. Normal SC K currents were inactivating, A type currents blocked by extracellular 4-aminopyridine (4-AP; 5 mM). The whole cell K currents of tumor cells were noninactivating due to the presence of non-inactivating A current, or non-inactivating, TEA-only sensitive current, or both, despite the presence of inactivating A current in some tumor cells. TEA-only-sensitive currents, which were 4-AP-insensitive and noninactivating, were common in all three tumor cell lines, but were not observed in normal SC. Inward rectifier K currents were a conspicuous feature of two of the tumor cells lines but were rarely observed in whole cell recordings of normal SC. The properties of Na+ currents recorded in both normal and tumor cells were not significantly different. Treatment of normal SC with a membrane-permeant analog of cyclic AMP (cAMP) resulted in functional expression of the TEA-only-sensitive K currents typical of tumor cells. These results establish the abnormal ion channel profile of neurofibromatosis type 1 (NF1)-tumor cells and suggest (Guo et al.: Science 276:795-798, 1997) that regulation of ionic currents by second messengers may involve the NF1 gene.

Functional Ca2+ and Na+ channels on mouse Schwann cells cultured in serum-free medium: regulation by a diffusible factor from neurons and by cAMP

Regulation of expression of functional voltage-gated ion channels for inward currents was studied in Schwann cells in organotypic cultures of dorsal root ganglia from E19 mouse embryos maintained in serum-free medium. Of the Schwann cells that did not contact axons, 46.5% expressed T-type Ca2+ conductances (ICaT). Two days or more after excision of the ganglia, and consequent disappearance of neurites, ICaT were detectable in only 10.9% of the cells, and the marker 04 disappeared. On Schwann cells deprived of neurons, T- (but not L-) type Ca2+ conductances were re-induced by weakly hydrolysable analogues of cAMP, and by forskolin (an activator of adenylyl cyclase) after long-term treatment (4 days). With CPT cAMP (0.1-2 mM), 8Br cAMP, db cAMP or forskolin (0.01 or 0.1 mM), the proportion of cells with ICaT was not significantly different from the proportion in the cultures with neurons. These agents also induced expression in some cells of tetrodotoxin-resistant Na+ currents, which were rarely induced by neurons, but 04 was not re-induced by cAMP analogue treatments that re-induced ICaT. Inward currents (Ba2+ or Na+) were partly restored (P < 0.05) on Schwann cells cultured for 6-7 days beneath a filter bearing cultured neurons. In contrast, addition of neuron-conditioned medium was ineffective. The results suggest that neurons activate, via diffusible and degradable factors, a subset of Schwann cell cAMP pathways leading to expression of IcaT, and activate additional non-cAMP pathways that lead to expression of 04.

Selective downregulation of an inactivating K+ conductance by analogues of cAMP in mouse Schwann cells

1. Voltage-dependent K+ conductances on Schwann cells in organotypic cultures of mouse dorsal root ganglia were classified as inactivating or sustained (responsible for currents IA and IK, respectively). IA is known to be much reduced on Schwann cells that contact neurites. 2. In the absence of neurones, IA and IK were present. IA, but not IK, was markedly reduced (by 80% after 105 h of treatment) by 2 mM 8-(4-chlorophenylthio)-cAMP (cpt-cAMP), a weakly hydrolysable analogue of cAMP. The effect did not appear for at least 2 h and was maximal after about 100 h. 3. The effect of 1 mM cpt-cAMP was abolished in the presence of an inhibitor of protein kinases, N-[2-bromocinnamyl(amino)ethyl]-5-isoquinolinesulphonamide (H-89, 10 microM). 4. Other analogues of cAMP, but not an analogue of cGMP (8-bromo-cGMP), also reduced IA. 5. We conclude that IA, but not IK, can be downregulated by activation of the protein kinase A pathway.

Glial potassium channels activated by neuronal firing or intracellular cyclic AMP in Helix

1. Cell-attached and whole cell patch clamp experiments were performed on satellite glial cells adhering to the cell body of neurones in situ within the nervous system of the snail Helix pomatia. The underlying neurone was under current or voltage-clamp control. 2. Neuronal firing induced a delayed (20-30 s) persistent (3-4 min) increase in the opening probability of glial K+ channels. The channels were also activated by perfusing the ganglion with a depolarizing high-K+ saline, except when the underlying neurone was prevented from depolarizing under voltage-clamp conditions. 3. Two K(+)-selective channels were detected in the glial membrane. The channel responding to neuronal firing was present in 95% of the patches (n = 393). It had a unitary conductance of 56 pS, a Na+ :K+ permeability ratio < 0.02 and displayed slight inward rectification in symmetrical [K+] conditions. It was sensitive to TEA, Ba2+ and Cs+. The following results refer to this channel as studied in the cell-attached configuration. 4. The glial K+ channel was activated by bath application of the membrane-permeant cyclic AMP derivatives 8-bromo-cAMP and dibutyryl-cAMP, the adenylyl cyclase activator forskolin and the diesterase inhibitors IBMX, theophylline and caffeine. It was insensitive to cyclic GMP activators and to conditions that might alter the intracellular [Ca2+] (ionomycin, low-Ca2+ saline and Ca2+ channel blockers). 5. The forskolin-induced changes in channel behaviour (open and closed time distributions, burst duration, short and long gaps within bursts) could be accounted for by a four-state model (3 closed states, 1 open state) by simply changing one of the six rate parameters. 6. The present results suggest that the signal sent by an active neurone to satellite glial cells is confined to the glial cells round that neurone. The effect of this signal on the class of glial K+ channels studied can be mimicked by an increase in glial cAMP concentration. The subsequent delayed opening of the glial K+ channels does not appear to play a role in siphoning the excess K+ released by active neurones. It is hypothesized that the cAMP-gated glial K+ channels may be involved in the control of glial cell proliferation.

Developmental and activity-dependent changes in K+ currents in satellite glial cells in mouse superior cervical ganglion

Voltage-gated K+ currents were recorded from freshly dissociated satellite glial cells wrapping around ganglion cells in mouse superior cervical ganglion (SCG) by whole-cell recordings of patch clamp techniques. Both inward and outward K+ currents during membrane hyperpolarization and depolarization were observed in these glial cells. The current-voltage relation of these K+ currents became almost linear in cells obtained more than 4 weeks after birth. The magnitude of the density of inward K+ currents, which were elicited during membrane hyperpolarization and were eliminated by external barium, progressively increased during the first month after birth. This developmental increase in the magnitude of inward K+ current density was not affected by decentralization of SCG done by transection of cervical sympathetic trunk (CST) 5 days after birth. In adult mice, the magnitude of the inward K+ current density decreased after chronic conduction blockade of CST by local application of tetrodotoxin. On the other hand, the magnitude of the inward K+ current density increased after daily intraperitoneal injection of reserpine and this increase was abolished by pre-treatment of decentralization of SCG. These results suggested that preganglionic innervation was not prerequisite for developmental increase in the inward K+ currents and preganglionic neuronal activity upregulates the inward K+ currents in adult mice. Neuronal regulation of glial K+ channel expression would assist in K+ clearance from periganglionic space to maintain neuronal activity.

A structural determinant of differential sensitivity of cloned inward rectifier K+ channels to intracellular spermine

Large subtype-specific differences in the sensitivity of cloned inward-rectifier K+ channels of the IRK1, BIR10 and ROMK1 subtype to being blocked by intracellular spermine (SPM) are described. It is shown, by site-directed mutagenesis, that the four orders of magnitude larger SPM sensitivity of BIR10 channels compared to ROMK1 channels may be explained by a difference in a single amino acid in the putative transmembrane segment TMII. This residue, a negatively charged glutamate in BIR10, is homologous to the residue in IRK1 and ROMK1 which has previously been shown to change gating properties and Mg2+ sensitivity. Differential block by physiological SPM concentrations is suggested as a major functional difference between subtypes of inward-rectifier K+ channels.

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

We examined potassium channel gene expression of two members of the Shaker subfamily, MK1 and MK2, in sciatic nerves from rats and mice. In Northern blot analysis, MK1 and MK2 probes detected single transcripts of approximately 8 kb and approximately 9.5 kb, respectively, in sciatic nerve and brain from both species. Polymerase chain reaction amplification of a cDNA library of cultured rat Schwann cells using MK1- and MK2- specific primers produced DNA fragments that were highly homologous to MK1 and MK2. To determine whether these channel genes were axonally regulated, we performed Northern blot analysis of developing, permanently transected, and crushed rat sciatic nerves. The mRNA levels for both MK1 and MK2 increased from P1 to P15 and then declined modestly. Permanent nerve transection in adult animals resulted in a dramatic and permanent reduction in the mRNA levels for both MK1 and MK2, whereas normal levels of MK1 and MK2 were restored when regeneration was allowed to occur following crush injury. In all cases, MK1 and MK2 mRNA levels paralleled that of the myelin gene P0. Elevating the cAMP in cultured Schwann cells by forskolin, which mimics axonal contact but not myelination, did not induce detectable levels of MK1 and MK2 mRNA by Northern blot analysis. Further, the level of MK1 mRNA in the vagus nerve, which contains relatively fewer myelinating Schwann cells and relatively more non-myelinating Schwann cells than the sciatic nerve, is reduced relative to the sciatic nerve. In conclusion, we have identified two Shaker-like potassium channel genes in sciatic nerves whose expressions are regulated by axons. We suggest that MK1 and MK2 mRNA are expressed in high levels only in myelinating Schwann cells and that these Shaker-like potassium channel genes have specialized roles in these cells.

Differences in a K current in Schwann cells from normal and neurofibromatosis-infected damselfish

Patch clamp techniques were used to study whole cell ionic currents in Schwann cells (SC) from a tropical marine fish, the bicolor damselfish, Pomacentrus partitus. The bicolor damselfish is affected by a disease termed damselfish neurofibromatosis (DNF), being developed as an animal model of neurofibromatosis-type 1 (NF1) in humans. NF1 affects SC, fibroblasts, and perineurial cells. The sole depolarization-activated ionic current present in cultured SC from normal fish peripheral nerve and from neurofibromas of fish with induced or spontaneously occurring DNF was an inactivating K+ current (K current), with a strong dependence on the Nernst potential for K+. This K current activated at depolarizations to -40 mV and above and inactivated during a maintained test pulse (0.2-1 s), but inactivation was significantly greater in tumored SC. Both currents were inhibited by 4-aminopyridine (Kd approximately 1 mM) and by dendrotoxin (15 microM) but were insensitive to extracellular tetraethyammonium (< or = 150 mM), indicating that the whole cell currents were similar pharmacologically. The currents could be distinguished on the basis of their sensitivity to depolarized holding potential, with normal cells less sensitive. Half-inactivation of the current was -32 mV in normal cells and -38 mV in tumored cells. Inactivation curves constructed from the average normalized current for many SC were significantly different in normal and tumored cells. When the depolarized holding potential was maintained between test depolarizations, greater voltage-dependent inactivation in tumored cells was apparent. Normal cells maintained an average of 36% of peak current at a holding voltage of -40 mV, while in tumored cells this average was 12%, a significant difference.

Activity-dependent regulation of inwardly rectifying potassium currents in non-myelinating Schwann cells in mice

1. Voltage-gated potassium currents were recorded from freshly dissociated non-myelinating Schwann cells of sural and sympathetic nerves from 1- to 12-week-old mice using the whole-cell or a single channel variation of the patch-clamp technique. 2. All sural cells from 2-week-old mice showed inwardly rectifying potassium (Kir+) currents in whole-cell recordings. Kir+ currents were virtually undetectable in sural cells from mice more than 6 weeks old, which also showed depolarization of the resting membrane potential. On the other hand, the magnitude of Kir+ currents increased in cervical sympathetic trunk (CST) cells in parallel with an increase of cell capacitance 1-6 weeks after birth. The density of Kir+ currents in CST cells increased 1-4 weeks after birth and then stayed constant for up to 12 weeks. 3. The unitary conductance of a single Kir+ channel in CST cells was 30 pS 2-12 weeks after birth; this was recorded in a cell-attached configuration with 154 mM K+ in the pipette. The steady-state open channel probability of single Kir+ channels in CST cells decreased with membrane hyperpolarization, but was not markedly changed 2-12 weeks after birth. 4. Conduction block of CST for 5 days induced by local application of tetrodotoxin (TTX) resulted in a significant decrease in both the magnitude and the density of Kir+ currents in whole-cell recordings in CST cells rostral to the sites of TTX block. Similar changes of Kir+ currents in whole-cell recordings were observed in cells in the inferior postganglionic branch of a superior cervical ganglion after 5 days of TTX block of CST. 5. These results suggest that neuronal activity regulates the expression of functional Kir+ channels in non-myelinating Schwann cells in adult nerves. The activity-dependent regulation of the expression of glial potassium channels could play an important role in the regulation of the potassium microenvironment around active axons to maintain impulse conduction in unmyelinated fibres.