Ca(2+)-insensitive modulation of a K+ conductance by inositol polyphosphates

P2X7 nucleotide receptor mediation of membrane pore formation and superoxide generation in human promyelocytes and neutrophils

The P2X(7) receptor, which induces cation channel opening imparting significant permeability to Ca2+ and pore formation with changes in the plasma membrane potential, has been known to be rather restrictedly expressed in cells of the macrophage lineage including dendrites, mature macrophages, and microglial cells. However, we show here that the P2X(7) receptor is also expressed in cells of granulocytic lineage such as HL-60 promyelocytes, granulocytic differentiated cells, and neutrophils. Exposure of these cells to 2',3'-O-(4-benzoyl)benzoyl-ATP (BzATP) triggered intracellular Ca2+ rise through the mediation of phospholipase C-independent and suramin-sensitive pathways. BzATP also induced depolarization of the plasma membrane in the absence of extracellular Ca2+, whereas it hyperpolarized the cells in the presence of external Ca2+, probably in part through the activation of Ca2+-activated K(+) channels. However, the hyperpolarization phenomenon was markedly attenuated in differentiated HL-60 cells and neutrophils. RT-PCR and Northern blot analysis revealed the presence of P2X(7) receptors on both HL-60 and neutrophil-like cells. This was further confirmed by pore formation through which the uptake of Lucifer yellow and YO-PRO1 occurred on BzATP treatment. BzATP stimulated in a concentration-dependent manner the production of superoxide in differentiated HL-60 cells via a pathway partially dependent on extracellular Ca2+. Moreover, in human neutrophils, BzATP was a more effective inducer of superoxide generation than PMA. Taken together, this is a first demonstration of the expression of P2X(7) receptors on neutrophils, which shows that the receptor is functionally involved in the defense mechanism by activation of the respiratory burst pathway.

Cloning and characterization of a gene encoding phosphatidyl inositol-specific phospholipase C from Trypanosoma cruzi

A gene encoding phosphatidyl inositol-4,5-bisphosphate phospholipase C (PLC) was cloned from the protozoan parasite Trypanosoma cruzi. A partial cDNA encoding putative PLC was obtained by a polymerase chain reaction (PCR) using degenerate oligonucleotide primers corresponding to conserved regions of PLCs. A 2178-bp protein coding region of the T. cruzi PLC gene, composed from cDNA and genomic clones, encodes a putative PLC with a calculated molecular mass of 82,032 Da and an isoelectric point of 5.93. The deduced amino acid sequence of T. cruzi PLC exhibited 23-42% overall identities with the PLCs from other organisms. Among them, PLC from Ictalurus punctatus revealed the highest identity to T. cruzi PLC. The percentage identities of the entire proteins and the catalytic X/Y domains suggested that T. cruzi PLC is more evolutionarily related to the PLCs of higher eukaryotes than to those of lower unicellular eukaryotes. The tetrad analysis of the segregants of the Saccharomyces cerevisiae PLC1/plc1::HIS3 diploid strain transformed with the T. cruzi PLC-expressing plasmid showed that expression of T. cruzi PLC suppressed the growth defect caused by the plc1 disruption in yeasts. Temperature-sensitive phenotype of the S. cerevisiae plc1-mutant haploid strain was also suppressed by the expression of T. cruzi PLC. The phosphatidyl inositol-4,5-biphosphate (PtdIns(4,5)P2) hydrolyzing activity of T. cruzi PLC was demonstrated in the lysate from the plc1-temperature sensitive yeast mutant strain transformed with the T. cruzi PLC-expressing plasmid. The yeast-expressed T. cruzi PLC showed an absolute Ca2+ dependence which was similar to mammalian PLC isoforms: the half-maximal activity at 0.5-1 x 10(-5) M Ca2+ and the maximal activity at 1-2 x 10(-4) M Ca2+.

Inositol 1,3,4,5-tetrakisphosphate enhances long-term potentiation by regulating Ca2+ entry in rat hippocampus

1. The effect of inositol 1,3,4,5-tetrakisphosphate (InsP4) on long-term potentiation (LTP) was investigated in the CA1 region of rat hippocampal slices. Intracellular application of InsP4 and EPSP recordings were carried out using the whole-cell configuration. 2. Induction of LTP in the presence of InsP4 (100 microM) resulted in a substantial enhancement of the LTP magnitude compared with control potentiation. Using an intrapipette perfusion system, it was established that application of InsP4 was required during induction of potentiation for this enhancement to occur. An enhancement of LTP was not observed if a non-metabolizable inositol 1,4,5-trisphosphate (InsP3) analogue (2,3-dideoxy-1,4,5-trisphosphate, 100 microM) was applied intracellularly. 3. Current-voltage relations of NMDA receptor-mediated EPSCs were not altered by InsP4 application. The presence of InsP4 was slightly effective in relieving a D-(-)-2-amino-5-phosphonopentanoic acid (D-APV)-induced block of LTP. 4. The peak current amplitude of voltage-gated calcium channels (VGCCs) was increased by InsP4. omega-Conotoxin GVIA inhibited the InsP4-induced LTP facilitation. 5. These data indicate that InsP4 can modify the extracellular Ca2+ entry through upregulation of VGCCs, which may in turn contribute to the observed enhancement of LTP induced by InsP4. 6. To investigate the possible involvement of intracellular Ca2+ release in the facilitatory effect of InsP4 on LTP, different inhibitors of the endoplasmic reticulum-dependent Ca2+ release were applied (heparin, ryanodine, cyclopiazonic acid). The results suggest that InsP4 activates postsynaptic InsP3-dependent Ca2+ release which normally does not contribute to the calcium-induced calcium release-dependent LTP.

Ion channels in microglia (brain macrophages)

Microglia are immunocompetent cells in the brain that have many similarities with macrophages of peripheral tissues. In normal adult brain, microglial cells are in a resting state, but they become activated during inflammation of the central nervous system, after neuronal injury, and in several neurological diseases. Patch-clamp studies of microglial cells in cell culture and in tissue slices demonstrate that microglia express a wide variety of ion channels. Six different types of K+ channels have been identified in microglia, namely, inward rectifier, delayed rectifier, HERG-like, G protein-activated, as well as voltage-dependent and voltage-independent Ca2+-activated K+ channels. Moreover, microglia express H+ channels, Na+ channels, voltage-gated Ca2+ channels, Ca2+-release activated Ca2+ channels, and voltage-dependent and voltage-independent Cl- channels. With respect to their kinetic and pharmacological properties, most microglial ion channels closely resemble ion channels characterized in other macrophage preparations. Expression patterns of ion channels in microglia depend on the functional state of the cells. Microglial ion channels can be modulated by exposure to lipopolysaccharide or various cytokines, by activation of protein kinase C or G proteins, by factors released from astrocytes, by changes in the concentration of internal free Ca2+, and by variations of the internal or external pH. There is evidence suggesting that ion channels in microglia are involved in maintaining the membrane potential and are also involved in proliferation, ramification, and the respiratory burst. Further possible functional roles of microglial ion channels are discussed.

Modulation of human muscle sodium channels by intracellular fatty acids is dependent on the channel isoform

Free fatty acids (FFAs), including arachidonic acid (AA), are implicated in the direct and indirect modulation of a spectrum of voltage-gated ion channels. Skeletal muscle sodium channels can be either activated or inhibited by FFA exposure; the response is dependent on both FFA structure and site of exposure. Recombinant human skeletal muscle sodium channels (hSkM1) were transfected into heterologous human renal epithelium HEK293t cells. Cytoplasmic delivery of 5 microM AA augmented the voltage-activated sodium current of hSkM1 channels by 190% (+/-54 S.E., n = 7) over a 20-min period. Similar results were seen with 5 microM oleic acid. Sodium currents in HEK293t cells transfected with human cardiac muscle sodium channels (hH1) were insensitive to AA treatment, and exposure to oleic acid inhibited the hH1 currents over a 20-min period by 29% (+/-13 S.E., n = 5). The increase in hSkM1 current was not accompanied by shifts in voltage dependence of activation, steady-state inactivation, or markedly altered kinetics of inactivation of the macroscopic current. The FFA-induced increase in sodium currents was not dependent on protein kinase C activity. In contrast, both isoforms were reversibly inhibited by external application of unsaturated FFA. Thus, the differential effects of FFA on skeletal muscle sodium channels first noted in cultured muscle cells can be reproduced by expressing recombinant sodium channels in epithelial cells. Although the responses to applied FFAs could be direct or indirect, we suggest that: 1) SkM1 has two classes of response to FFA, one which produces augmentation of macroscopic currents with intracellular FFA, and a second which produces inhibition with extracellular FFA; 2) H1 has only one class of response, which produces inhibition with extracellular FFA. A testable hypothesis is that the presence or absence of each response is due to a specific structure in SkM1 or H1. These specific structures may directly interact with FFA or may interact with intermediate components.

Modulation of a potassium conductance in developing skeletal muscle

K+ conductances dominate and potentially modulate the resting potential of skeletal muscle cells. The expression and modulation of a major K+ conductance were examined during in vitro differentiation of the mouse myoblast cell line C2C12. The inwardly rectifying K+ conductance (IKi) increased from unmeasurable levels in undifferentiated myoblasts to approximately 1.56 +/- 0.51 nA (n = 17) in myoballs derived from myotubes at 5 days after induction of differentiation. The inward rectifier was subject to modulation by intracellular signals. Exposure of cytoplasm to guanosine 5'-O-(3-thiotriphosphate) during whole cell recording produced a concentration (5-100 microM)- and time (1-20 min)-dependent inhibition of the mean conductance. Elevation of intracellular free Ca2+ (> 200 nM) also inhibited IKi. These findings demonstrate a potential mechanism for modulation of the resting potential of muscle fibers via the control of skeletal muscle IKi.

IKir regulation in murine macrophages: whole cell and perforated patch studies

Previous studies have reported that the inwardly rectifying K+ conductance (GKir) in macrophages is modulated by intracellular perfusion with inositol 1,4,5-triphosphate (InsP3), inositol 1,3,4,5-tetrakisphosphate (InsP4), or GTP analogues and by exposing cells to macrophage-specific colony-stimulating factor (CSF) I. This study uses both conventional whole cell and amphotericin B perforated patch studies to investigate GKir modulation in thioglycollate-elicited mouse peritoneal macrophages (MO). Under whole cell recording conditions with 150 mM Cl- in the patch pipette, GKir decreased within 25 min. The GKir decrease was slowed by exchanging glutamate for Cl- as the major anion in the pipette or by adding guanosine 5'-O-(2-thiodiphosphate) (50 nM) or ATP (0.5 mM) to the pipette. Addition of InsP3 or InsP4 to the pipette had no effect on the magnitude of GKir or its rate of decrease but activated an outward current in the voltage range of +60 to +120 mV in 57% of the cells studied. Thus in murine MO GKir may be modulated by G proteins but is unaffected by inositol phosphate metabolites, which have been reported to enhance GKir in phorbol 12-myristate 13-acetate (PMA)-differentiated HL-60 cells. In contrast to whole cell studies, perforated patch recordings of murine MO GKir were stable for > 1 h. Perforated patch studies demonstrated that murine MO also differ from PMA-differentiated HL-60 cells in that CSF I had no effect on GKir. Additionally, arachidonic acid, PMA, and H2O2, agents implicated in macrophage activation, did not modulate GKir. We conclude that GKir regulation in murine MO differs from that reported in PMA-differentiated HL-60 cells and that although our data suggest that GKir is modulated by G protein(s), they differ from the G proteins involved in MO responses to CSF I and the other agents tested.

Elevation of inositol tetrakisphosphate parallels inhibition of Ca(2+)-dependent Cl- secretion in T84 cells

Carbachol and histamine both stimulate calcium-dependent chloride secretion in the colonic epithelial cell line, T84. However, pretreatment of cell monolayers with carbachol blocks subsequent chloride secretion induced by thapsigargin but not the calcium elevation stimulated by this agent, whereas histamine pretreatment blocks neither thapsigargin-induced chloride secretion nor calcium elevation. To examine whether inositol phosphate metabolism might account for this difference, we measured levels of radiolabeled inositol phosphates: Ins(1,3,4)P3, Ins(1,4,5)P3, Ins(1,3,4,5)P4, Ins-(1,3,4,6)P4, Ins(3,4,5,6)P4, InsP5, and InsP6 after cell stimulation. Although both carbachol and histamine increase Ins (1,4,5)P3 at 5 s, there is a greater and more persistent increase in the levels of Ins(1,3,4)P3 and InsP4 at later time points after carbachol than histamine, which corresponded to the suppression of the chloride secretory response.