Direct ion channel gating: a new function for intracellular messengers

Second messenger cascade of glial responses evoked by interneuron activity and by a myomodulin peptide in the leech central nervous system

The giant glial cell in the neuropil of segmental ganglia of the leech Hirudo medicinalis responds to the activity of the Leydig interneuron and to a peptide of the myomodulin family, the presumed transmitter mediating the Leydig neuron-to-giant glial cell transmission, with a membrane hyperpolarization due to an increased membrane K+ conductance [Britz et al. (2002) Glia, 38, 215-227]. We have now studied the second messenger cascade initiated by Leydig neuron stimulation and by the endogenous myomodulin (MMHir) in the voltage-clamped giant glial cell. Glial responses to both stimuli are mediated by a G-protein-coupled receptor linked to adenylyl cyclase by the following criteria: (i) injection of GDP-beta-S, but not GDP, resulted in an irreversible decrease of the glial responses to both stimuli; (ii) the responses to both stimuli were reversibly inhibited by the adenylyl cyclase inhibitor SQ22,536; and (3) bath-applied di-butyryl-cyclic AMP, but not di-butyryl-cyclic GMP, elicited an outward current, which reduced the responses elicited by neuronal stimulation or myomodulin. A cocktail of protein kinase (PK) inhibitors (H-8, KT5720), the PKA antagonist Rp-cAMPS, or presumed inhibitors of cyclic nucleotide channels, LY83583 and l-cis-diltiazem, had no effect on the glial responses. Our results suggest that Leydig neuron stimulation and MMHir activate a cAMP-mediated K+ conductance in the glial cell, which appeared neither to be due to the activation of PKA nor of known cyclic nucleotide-gated channels directly.

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels first identified in retinal photoreceptors and olfactory sensory neurons (OSNs). They are opened by the direct binding of cyclic nucleotides, cAMP and cGMP. Although their activity shows very little voltage dependence, CNG channels belong to the superfamily of voltage-gated ion channels. Like their cousins the voltage-gated K+ channels, CNG channels form heterotetrameric complexes consisting of two or three different types of subunits. Six different genes encoding CNG channels, four A subunits (A1 to A4) and two B subunits (B1 and B3), give rise to three different channels in rod and cone photoreceptors and in OSNs. Important functional features of these channels, i.e., ligand sensitivity and selectivity, ion permeation, and gating, are determined by the subunit composition of the respective channel complex. The function of CNG channels has been firmly established in retinal photoreceptors and in OSNs. Studies on their presence in other sensory and nonsensory cells have produced mixed results, and their purported roles in neuronal pathfinding or synaptic plasticity are not as well understood as their role in sensory neurons. Similarly, the function of invertebrate homologs found in Caenorhabditis elegans, Drosophila, and Limulus is largely unknown, except for two subunits of C. elegans that play a role in chemosensation. CNG channels are nonselective cation channels that do not discriminate well between alkali ions and even pass divalent cations, in particular Ca2+. Ca2+ entry through CNG channels is important for both excitation and adaptation of sensory cells. CNG channel activity is modulated by Ca2+/calmodulin and by phosphorylation. Other factors may also be involved in channel regulation. Mutations in CNG channel genes give rise to retinal degeneration and color blindness. In particular, mutations in the A and B subunits of the CNG channel expressed in human cones cause various forms of complete and incomplete achromatopsia.

Regulation of calcium-activated nonselective cation channel activity by cyclic nucleotides in the rat insulinoma cell line, CRI-G1

The regulation of a calcium-activated nonselective cation (Ca-NS+) channel by analogues of cyclic AMP has been investigated in the rat insulinoma cell line, CRI-G1. The activity of the channel is modulated by cyclic AMP in a complex way. In the majority of patches (83%) tested concentrations of cyclic AMP of 10 microM and above cause an inhibition of channel activity which is immediately reversible on washing. In contrast, lower concentrations of cyclic AMP, between 0.1 and 1.0 microM, produce a transient activation of channel activity in most patches (63%) tested. One group of analogues, including N6-monobutyryl cyclic AMP and N6, 2'-O-dibutyryl cyclic AMP reduced the activity of the Ca-NS+ channel at all concentrations tested and 2'-O-Monobutyryl cyclic AMP produced inhibition in all patches tested except one, at all concentrations. A second group produced dual concentration-dependent effects on Ca-NS+, low concentrations stimulating and high concentrations inhibiting channel activity. 6-Chloropurine cyclic AMP and 8-bromo cyclic AMP produced effects similar to those of cyclic AMP itself. In contrast, 8-[4-chlorophenylthio] cyclic AMP also showed a dual action, but with a high level of activation at all concentrations tested up to 1 mM. Ca-NS+ channel activity was also predominantly activated by low concentrations of Sp-cAMPS. The activating effects of both Sp-cAMPS and cyclic AMP are antagonized by Rp-cAMPS, which by itself only produced a weak inhibition of Ca-NS+ channel activity even at concentrations of 10 microM and above. The results are discussed in terms of a model in which cyclic AMP, and other cyclic nucleotides, modulate the activity of the Ca-NS+ channel by binding to two separate sites.

Cloning and expression of a novel truncated calcium channel from non-excitable cells

Calcium entry, via a dihydropyridine-sensitive pathway, is required for differentiation in murine erythroleukemia cells (MELC). Calcium channel currents have been identified physiologically in some non-excitable cells, but little is known regarding the structure of these channels. We show that a truncated form of the alpha 1 subunit of the cardiac voltage-gated calcium channel (dihydropyridine receptor, DHPR) is expressed in MELC. This MELC calcium channel lacks the first four transmembrane segments of the DHPR (IS1 to IS4). A MELC calcium channel/cardiac DHPR chimera, co-expressed with the alpha 2 and beta subunits of the DHPR, forms a functional calcium channel in Xenopus oocytes.

Involvement of calmodulin in Ca(2+)-activated K+ efflux in human colonic cell line, HT29-19A

The receptor-mediated agonist, neurotensin (NT) stimulated Ba(2+)- and charybdotoxin-sensitive 86Rb (K+) efflux in the HT29-19A colonic cell line. Efflux was also stimulated by ionomycin and thapsigargin and could be abolished by incubation with the intracellular Ca2+ chelator, BAPTA. Together, these data suggest a rise in [Ca2+]i is prerequisite for activation of K+ efflux in these cells. Comparison of the temporal profiles for NT-induced increases in [Ca2+]i and 86Rb efflux, however, failed to show a direct relationship between these parameters. The NT-stimulated increase in [Ca2+]i was transient, returning to baseline within 4-5 min, while efflux was sustained over a much longer period (> 12 min). Ca(2+)-activated 86Rb efflux was inhibited by pretreatment with calmodulin (CaM) antagonist, W7. W7 had no effect on basal efflux, but reduced both NT- and IM-activated efflux up to 80%, with a Ki of 38 microM. Other CaM antagonist inhibited efflux with an order of potency (TFP approximately W8 > W7 >> W5) consistent with inhibition of a CaM-dependent process. Inhibition by W7 was not abolished by ouabain or bumetanide, indicating its effects are not mediated by action upon K+ uptake processes. W7 did not inhibit NT-stimulated 125I efflux but significantly reduced efflux stimulated by the Ca2+ ionophore, ionomycin. NT-stimulated 86Rb+ efflux was localized to the basolateral membrane of HT29-19A monolayers grown on permeable supports. These data are consistent with the involvement of CaM in mediating Ca(2+)-dependent activation of K+ conductance in HT29-19A colonocytes.

Novel oligodendrocyte transmembrane signaling systems. Investigations utilizing antibodies as ligands

Antibodies are increasingly being used as tools to study the function of cell surface markers. Several types of responses may occur upon the selective binding of an antibody to an epitope on a receptor. Antibody binding may trigger signals that are normally transduced by endogenous ligands. Moreover, antibody binding may activate normal signals in a manner that disrupts a sequence of events that coordinates either differentiation, mitogenesis, or morphogenesis. Alternately, it is possible that binding elicits either a modified signal or no signal. This article focuses on the cascade of events that occur following specific antibody binding to myelin markers expressed by cultured murine oligodendrocytes. Binding of specific antibodies to the oligodendrocyte membrane surface markers myelin/oligodendrocyte glycoprotein (MOG), myelin/oligodendrocyte specific protein (MOSP), galactocerebroside (GalC), and sulfatide on cultured murine oligodendrocytes results in different effects with regard to phospholipid turnover, Ca2+ influxes, and antibody:marker distribution. The consequence of each antibody-elicited cascade of events appears to be the regulation of the cytoskeleton within the oligodendroglial membrane sheets. The antibody binding studies described in this article demonstrate that these myelin surface markers are capable of transducing signals. Since endogenous ligands for these myelin markers have yet to be identified, it is not known if these signals are normally transduced or are a modification of normally transduced signals.

Transduction mechanism for glutamate-induced potassium current in neurones of the mollusc Planorbarius corneus

1. The potassium currents evoked by glutamate agonists on isolated and identified neurones of molluscan pedal ganglia were investigated using the voltage clamp technique. 2. Glutamate responses were not modified by increasing intracellular cyclic nucleotide concentrations (treatment with 8-Br-cAMP, 8-Br-cGMP, forskolin and/or the phosphodiesterase inhibitor isobutylmethylxantine, IBMX), whereas inward-going currents induced by the nucleotides were observed. It follows that glutamate currents are independent of intracellular cyclic nucleotide control. 3. Protein kinase C activation with phorbol esters or oleoylacetylglycerol induced a slowly developing outward current and reduced glutamate response amplitude. Staurosporine itself did not affect the glutamate responses but completely prevented the effects of phorbol esters and oleoylacetylglycerol. This indicated that protein kinase C was not involved in the transduction mechanism for the potassium component of the glutamate response. 4. The possible involvement of inositol-1,4,5-trisphosphate seems to be improbable because the glutamate responses were independent of intracellular calcium concentration. Intracellular injection of calcium buffer BAPTA, failed to affect any of the glutamate currents, although it effectively blocked the after-hyperpolarization following directly evoked action potentials. 5. Nordihydroguaiaretic acid (NDGA) and indomethacin, inhibitors of the lipoxygenase and cyclo-oxygenase pathways of arachidonic acid metabolism, correspondingly, did not change the glutamate responses of these neurones. 6. The failure to demonstrate the involvement of any known secondary messenger systems in glutamate response transduction favours two assumptions: (1) the receptor-G protein complex controls the potassium channel directly; or (2) some still unknown transduction system is used.

Regional variations in particulate cyclic AMP dependent-protein kinase binding activity in the gerbil hippocampus following transient forebrain ischemia by [3H]cyclic AMP binding

Changes in the binding of [3H]cyclic AMP as an indicator of particulate cyclic AMP-dependent protein kinase (AMP-DPK) binding activity following transient forebrain ischemia were studied in the gerbil using in vitro autoradiography. [3H]Cyclic AMP binding in the strata pyramidale and lacunosum-moleculare of the hippocampal CA1, the stratum pyramidale of the CA3, and the dentate gyrus decreased transiently in the early postischemic phase but then recovered. However, [3H]cyclic AMP binding in the strata pyramidale and radiatum of the CA1, the granular layer of the dentate gyrus, and the upper layer of the cortex decreased again 7 days after ischemia. In the CA4 subfield and the lower layer of the cortex, the binding showed no significant alterations after ischemia. Administration of pentobarbital prior to the induction of ischemia prevented the decrease in [3H]cyclic AMP binding in the CA1 subfield 6 h and 7 days after ischemia, and showed protective effects against neuronal death of the CA1 pyramidal cells 7 days after ischemia. These results indicate that marked alteration of intracellular signal transduction precedes neuronal damage in the hippocampal CA1 subfield. Furthermore, postischemic reduction of [3H]cyclic AMP binding in the histologically intact cerebral cortex, CA3, and dentate gyrus may be the reflection of cellular dysfunction after energy failure.

Regulation of Cl- channels by multifunctional CaM kinase

cAMP kinase has been shown to mediate the cAMP pathway for regulation of Cl- channels in lymphocytes, but the mediator of an alternative, Ca2+ pathway has not been identified. We show here that Ca2+ ionophore activates Cl- currents in cell-attached and whole-cell patch-clamp recordings of Jurkat T lymphocytes, but this activation is not direct. The effect of Ca2+ ionophore on whole-cell Cl- currents is inhibited by a specific peptide inhibitor of multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase). Furthermore, Cl- channels are activated in excised patches by purified CaM kinase in a fashion that mimics the effect of Ca2+ ionophore in cell-attached recordings. These results suggest that CaM kinase mediates the Ca2+ pathway of Cl- channel activation.

Sensory transduction in eukaryotes. A comparison between Dictyostelium and vertebrate cells

The organization of multicellular organisms depends on cell-cell communication. The signal molecules are often soluble components in the extracellular fluid, but also include odors and light. A large array of surface receptors is involved in the detection of these signals. Signals are then transduced across the plasma membrane so that enzymes at the inner face of the membrane are activated, producing second messengers, which by a complex network of interactions activate target proteins or genes. Vertebrate cells have been used to study hormone and neurotransmitter action, vision, the regulation of cell growth and differentiation. Sensory transduction in lower eukaryotes is predominantly used for other functions, notably cell attraction for mating and food seeking. By comparing sensory transduction in lower and higher eukaryotes general principles may be recognized that are found in all organisms and deviations that are present in specialised systems. This may also help to understand the differences between cell types within one organism and the importance of a particular pathway that may or may not be general. In a practical sense, microorganisms have the advantage of their easy genetic manipulation, which is especially advantageous for the identification of the function of large families of signal transducing components.

Cyclic nucleotide-dependent protein kinases

The actions of several hormones and neurotransmitters evoke signal transduction pathways which rapidly elevate the cytosolic concentrations of the intracellular messengers, cAMP and cGMP. The cyclic-nucleotide dependent protein kinases, cAMP-dependent protein kinase (PKA) and cGMP-dependent protein kinase (PKG), are the major intracellular receptors of cAMP and cGMP. These enzymes become active upon binding respective cyclic nucleotides and modulate a diverse array of biochemical events through the phosphorylation of specific substrate proteins. The focus of this review is to describe the progress made in understanding the structure and function of both PKA and PKG.

Modulation of calcium-activated non-specific cation currents by cyclic AMP-dependent phosphorylation in neurones of Helix

1. Currents through calcium-activated non-specific cation (CAN) channels were studied in the fast burster neurone of Helix aspersa and Helix pomatia. CAN currents were activated by reproducible intracellular injections of small quantities of Ca2+ utilizing a fast, quantitative pressure injection technique. 2. External application of forskolin (10-25 microM), an activator of adenylate cyclase, caused the endogenous bursting activity of the cells to be replaced by beating activity. These same concentrations of forskolin reduced CAN currents reversibly to about 50%. 3. External application of IBMX (3-isobutyl-1-methylxanthine, 100 microM), an inhibitor of phosphodiesterase, the enzyme which breaks down cyclic AMP, reduced CAN currents reversibly to about 40%. 4. External application of the membrane-permeable cyclic AMP analogues 8-bromo-cyclic AMP and dibutyryl-cyclic AMP (100 microM) caused almost complete block of the CAN current. A marked reduction in the CAN current was also observed following quantitative injections of cyclic AMP (internal concentrations up to 50 microM) directly into the cells from a second pressure injection pipette. 5. Similar results were obtained with quantitative injections of the catalytic subunit (C-subunit) of the cyclic AMP-dependent protein kinase (internal concentrations 10(-4) units of enzyme) directly into the cells from a second pressure injection pipette. 6. Injection of the non-hydrolysable GTP analogue, GTP-gamma-S (internal concentrations 100 microM), which stimulates G-proteins, produced a prolonged increase in CAN current amplitude by as much as 300%. 7. External application of serotonin (100-200 microM) caused a transition from bursting to beating activity of the neurones and mimicked cyclic AMP's effects on CAN currents. Two other neurotransmitters, dopamine and acetylcholine, were not significantly effective in reducing CAN currents. 8. Injection of a peptide inhibitor of cyclic AMP-dependent protein kinase suppressed serotonin's action on bursting and on CAN current. 9. Our results indicate that CAN currents in Helix burster neurones are modulated by cyclic AMP-dependent membrane phosphorylation. They suggest that the physiological transmitter that induces this second messenger action is serotonin. The dual control of CAN channels by two second messengers, namely Ca2+ and cyclic AMP, has important functional implications. While Ca2+ activates these channels which generate the pacemaker current in these neurones, cyclic AMP-dependent phosphorylation down-regulates them, thereby resulting in modulation of neuronal bursting activity.

Immunocytochemical localization of the neural-specific regulatory subunit of the type II cyclic AMP-dependent protein kinase to postsynaptic structures in the rat brain

The cellular and subcellular distribution of a major cyclic AMP binding protein in the central nervous system, the neural-specific regulatory subunit of the type II cyclic AMP-dependent protein kinase (RII-B), was analyzed in rat brains with light and electron microscopic immunocytochemical methods. The distribution of the non-neural isoform of the regulatory subunit of the enzyme (RII-H) was also analyzed. It was found that RII-B immunoreactivity was predominantly localized to neurons whereas glial and endothelial cells were unlabeled. In the neurons the RII-B immunoreactivity occurred in the perikaryal cytoplasm and in the dendrites; there was no significant accumulation of immunoreaction product in nuclei, myelinated axons and axon terminals. Although immunoreactivity was never detected in axon terminals, it was characteristically associated with the postsynaptic densities and the surrounding non-synaptic sites in somata, dendrites and dendritic spines. The localization of RII-B antigenic sites did not show specificity to any type of neuron or synapse, but the amount of immunoreactivity varied. The distribution of RII-H immunoreactivity was similar to that of RII-B except that RII-H immunoreaction product was also observed in glial cells and occurred more frequently in myelinated axons. Our data confirm that RII-B is one of the major cyclic AMP binding proteins in neurons, and provide morphological support for the involvement of the type II cyclic AMP-dependent protein kinase in postsynaptic neural functions.

Genetic dissection of Ca2(+)-dependent ion channel function in Paramecium

The ciliated protozoan, Paramecium, broadcasts the activity of its individual ion channel classes through its swimming behaviour. This fact has made it possible to isolate mutants with defective ion currents, simply by selecting individuals with abnormal swimming patterns. At least four of Paramecium's ion currents are activated by rising intracellular calcium concentration, including two K+ currents and a Na+ current. A variety of cell lines with defects in these Ca2(+)-dependent currents have been isolated: in several cases, the defects have been traced to mutations in the structural gene for calmodulin. Sequence analysis of calmodulins from these and other Ca2(+)-dependent ion-current mutants may enable a detailed mapping of putative channel interaction domains on the surface of the calmodulin molecule.

Second messenger-activated calcium influx in rat peritoneal mast cells

1. To study the regulation of calcium influx in non-excitable cells, membrane currents of rat peritoneal mast cells were recorded using the whole-cell patch-clamp technique. At the same time, intracellular calcium concentration ([Ca2+]i) was monitored via the fluorescent calcium-indicator dye Fura-2, which was loaded into cells by diffusion from the patch pipette. 2. Stimulation of mast cells with secretagogues, such as compound 48/80 or substance P, caused release of Ca2+ from internal stores. In addition, external agonists also induced influx of external calcium in 26% of the cells investigated. The agonist-stimulated Ca2+ influx was increased during membrane hyperpolarization and was associated with small whole-cell currents. 3. Likewise, internal application of inositol 1,4,5-trisphosphate (Ins1,4,5P3:0.5-10 microM) elevated [Ca2+]i due both to release of Ca2+ from internal stores and to influx of external calcium. The Ins1,4,5P3-induced influx was greater at more negative membrane potentials, suggesting that Ins1,4,5P3 opened a pathway through which calcium could enter at a rate governed by its electrochemical driving force. 4. Inositol 1,3,4,5-tetrakisphosphate (Ins1,3,4,5P4) did not induce Ca2+ influx by itself nor did it facilitate or enhance Ins1,4,5P3-induced Ca2+ entry. Calcium influx was also induced by inositol 2,4,5-trisphosphate. Since this inositol phosphate is a poor substrate for Ins1,4,5P3 3-kinase it seems unlikely that Ins1,3,4,5P4 plays a role in the regulation of the Ca2(+)-influx pathway in mast cells. 5. The Ins1,4,5P3-induced Ca2+ influx was associated with whole-cell currents of 1-2 pA or less, with no channel activity detectable in whole-cell recordings. The small size of the whole-cell current suggests either that the Ins1,4,5P3-dependent influx occurs via small-conductance channels that are highly calcium specific or that the influx is not via ion channels. 6. Agonist stimulation also activated large-conductance (ca 50 pS) cation channels, through which divalent cations could permeate; thus, these channels represent a second pathway for Ca2+ influx. The slow speed of activation of the channels by agonists, their activation by internal guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S), and the inhibition of agonist activation by internal guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S) all suggest that the 50 pS channels are regulated by a second messenger and/or a GTP-binding protein. The activity of the 50 pS channel in mast cells is not sensitive to either Ins1,4,5P3 or Ins1,3,4,5P4. Activity of the channel was inhibited by elevated [Ca2+]i.(ABSTRACT TRUNCATED AT 400 WORDS)

Different behavioral and electrographic effects of acoustic stimulation and dibutyryl cyclic AMP injection into the inferior colliculus in normal and in genetically epilepsy-prone rats

The behavioral and electrographic effects of acoustic stimulation (100 dB) and injection of dibutyryl cyclic AMP (cAMP, 10 nmol) into the inferior colliculus were studied in normal and genetically epilepsy-prone (GEPR-9) rats. Acoustic stimulations induced behavioral seizures only in GEPR-9 rats; the seizures were associated with electrographic epileptiform discharges recorded from the inferior colliculus. Injections of dibutyryl cAMP into the inferior colliculus caused wild running episodes resembling the initial phase of audiogenic seizures in both groups. However, in GEPR-9 rats these episodes progressed to significantly more severe seizures than in normal rats and the convulsions culminated into status epilepticus. During drug-induced seizures, epileptiform activity was present in the inferior colliculus in both groups. The seizure generalization latency was markedly shorter in GEPR-9 rats than in normals. Furthermore, in GEPR-9 rats, the seizure generalization latency was in the same range with either acoustic stimulation-induced or dibutyryl cAMP-induced seizures. The data suggest that the increased susceptibility of genetically epilepsy-prone rats to acoustic stimuli may be related to a malfunction of the cyclic AMP system within the inferior colliculus.