The relationship between receptor-induced membrane phosphatidylinositol-4'5'-bisphosphate (PIP2) hydrolysis and M-current inhibition was assessed in single-dissociated rat sympathetic neurons by ...simultaneous or parallel recording of membrane current and membrane-to-cytosol translocation of the fluorescent PIP2/inositol 1,4,5-trisphosphate (IP3)-binding peptide green fluorescent protein-tagged pleckstrin homology domain of phospholipase C (GFP-PLCdelta-PH). The muscarinic receptor agonist oxotremorine-M produced parallel time- and concentration-dependent M-current inhibition and GFP-PLCdelta-PH translocation; bradykinin also produced parallel time-dependent inhibition and translocation. Phosphatidylinositol-4-phosphate-5-kinase (PI5-K) overexpression reduced both M-current inhibition and GFP-PLCdelta-PH translocation by both oxotremorine-M and bradykinin. These effects were partly reversed by wortmannin, which inhibits phosphatidylinositol-4-kinase (PI4-K). PI5-K overexpression also reduced the inhibitory action of oxotremorine-M on PIP2-gated G-protein-gated inward rectifier (Kir3.1/3.2) channels; bradykinin did not inhibit these channels. Overexpression of neuronal calcium sensor-1 protein (NCS-1), which increases PI4-K activity, did not affect responses to oxotremorine-M but reduced both fluorescence translocation and M-current inhibition by bradykinin. Using an intracellular IP3 membrane fluorescence-displacement assay, initial mean concentrations of membrane PIP2 were estimated at 261 microm (95% confidence limit; 192-381 microm), rising to 693 microm (417-1153 microm) in neurons overexpressing PI5-K. Changes in membrane PIP2 during application of oxotremorine-M were calculated from fluorescence data. The results, taken in conjunction with previous data for KCNQ2/3 (Kv7.2/Kv7.3) channel gating by PIP2 (Zhang et al., 2003), accorded with the hypothesis that the inhibitory action of oxotremorine-M on M current resulted from depletion of PIP2. The effects of bradykinin require additional components of action, which might involve IP3-induced Ca2+ release and consequent M-channel inhibition (as proposed previously) and stimulation of PIP2 synthesis by Ca2+-dependent activation of NCS-1.
G protein‐linked P2Y nucleotide receptors are known commonly to stimulate the phosphoinositide signalling pathway. However, we have previously demonstrated that the cloned P2Y
2
, P2Y
6
and P2Y
1
...receptors couple to neuronal N‐type Ca
2+
channels and to M‐type K
+
channels. Here we investigate the coupling of recombinant, neuronally expressed rat‐ and human P2Y
4
receptors (rP2Y
4
, hP2Y
4
) to those channels.
Rat sympathetic neurones were nuclear‐injected with a P2Y
4
cDNA plasmid. A subsequent activation of rP2Y
4
or hP2Y
4
by UTP (100 μ
M
) in whole‐cell (ruptured‐patch) mode produced only about 12% inhibition of the N‐type Ca
2+
current (
I
Ca(N)
). Surprisingly, in perforated patch mode, UTP produced much more inhibition of
I
Ca(N)
(maximally 51%), with an IC
50
value of 273 n
M
. This inhibition was voltage‐dependent and was blocked by co‐expression of the βγ‐binding transducin Gα‐subunit. Pertussis toxin (PTX) pretreatment also suppressed
I
Ca(N)
inhibition.
UTP inhibited the M‐current, recorded in perforated patch mode, by (maximally) 52%, with IC
50
values of 21 n
M
for rP2Y
4
and 28 n
M
for hP2Y
4
. This inhibition was not affected by PTX pretreatment.
With rP2Y
4
, ATP inhibited the M‐current (IC
50
524 n
M
, 26 times weaker than UTP), whereas ATP had no agonist activity at hP2Y
4
. This suggests a difference in agonist binding site between rP2Y
4
and hP2Y
4
.
We conclude that, in contrast to other nucleotide receptors studied, the P2Y
4
receptor couples much more effectively to M‐type K
+
channels than to Ca
2+
channels. Coupling to the Ca
2+
channels involves the βγ‐subunits of G
i/o
‐proteins and requires a diffusible intracellular component that is lost in ruptured‐patch recording.
British Journal of Pharmacology
(2003)
138
, 400–406. doi:
10.1038/sj.bjp.0705043
Expressed metabotropic group 1 glutamate mGluR5 receptors and nucleotide P2Y1 receptors (P2Y1Rs) show promiscuous ion channel coupling in sympathetic neurons: their stimulation inhibits M-type Kv7, ...K(M) potassium currents and N-type (Ca(V)2.2) calcium currents (Kammermeier and Ikeda, 1999; Brown et al., 2000). These effects are mediated by G(q) and G(i/o) G-proteins, respectively. Via their C-terminal tetrapeptide, these receptors also bind to the PDZ domain of the scaffold protein NHERF2, which enhances their coupling to G(q)-mediated Ca(2+) signaling (Fam et al., 2005; Paquet et al., 2006b). We investigated whether NHERF2 could modulate coupling to neuronal ion channels. We find that coexpression of NHERF2 in sympathetic neurons (by intranuclear cDNA injections) does not affect the extent of M-type potassium current inhibition produced by either receptor but strongly reduced Ca(V)2.2 inhibition by both P2Y1R and mGluR5 activation. NHERF2 expression had no significant effect on Ca(V)2.2 inhibition by norepinephrine (via alpha(2)-adrenoceptors, which do not bind NHERF2), nor on Ca(V)2.2 inhibition produced by an expressed P2Y1R lacking the NHERF2-binding DTSL motif. Thus, NHERF2 selectively restricts downstream coupling of mGluR5 and P2Y1Rs in neurons to G(q)-mediated responses such as M-current inhibition. Differential distribution of NHERF2 in neurons may therefore determine coupling of mGluR5 receptors and P2Y1 receptors to calcium channels.
Some of the excitatory effects of norepinephrine on central neurons are mediated by alpha-1 (α1) adrenoceptors. These receptors are coupled to the Gq family of G proteins, and hence stimulate ...hydrolysis of the membrane phospholipid phosphatidylinositol-4,5-bisphosphate. Other receptors of this type can excite neurons by inhibiting the subthreshold voltage-gated potassium M-current. We tested this possibility using rat sympathetic neurons transformed to express α1a receptors. The α1 agonist phenylephrine strongly inhibited the M-current recorded under voltage-clamp by 72 ± 11 % (
n
= 4) and in an unclamped neuron dramatically increased the number of action potentials produced by a 2 s depolarizing current step from 2 to 40, without effect on control neurons devoid of α1 receptors. We suggest that this might be a potential cause of the increased excitability produced by norepinephrine in some central neurons.
GABA
B
receptors are unique among G-protein-coupled receptors (GPCRs) in their requirement for heterodimerization between two homologous subunits, GABA
B1
and GABA
B2
, for functional expression. ...Whereas GABA
B1
is capable of binding receptor agonists and antagonists, the role of each GABA
B
subunit in receptor signaling is unknown. Here we identified amino acid residues within the second intracellular domain of GABA
B2
that are critical for the coupling of GABA
B
receptor heterodimers to their downstream effector systems. Our results provide strong evidence for a functional role of the GABA
B2
subunit in G-protein coupling of the GABA
B
receptor heterodimer. In addition, they provide evidence for a novel “sequential” GPCR signaling mechanism in which ligand binding to one heterodimer subunit can induce signal transduction through the second partner of a heteromeric complex.
Neuronal signaling by G protein-coupled P2Y nucleotide receptors is not well characterized. We studied here the coupling of
different molecularly defined P2Y receptors to neuronal G protein-gated ...inward rectifier K + (GIRK) channels. Individual P2Y receptors were coexpressed with GIRK1+GIRK2 (Kir3.1 + 3.2) channels by intranuclear plasmid
injections into cultured rat sympathetic neurons. Currents were recorded using perforated-patch or whole-cell (disrupted patch)
techniques, with similar results. P2Y 1 receptor stimulation with 2-methylthio ADP (2-MeSADP) induced activation of GIRK current (I GIRK ) followed by inhibition. In contrast, stimulation of endogenous α 2 -adrenoceptors by norepinephrine produced stable activation without inhibition. P2Y 1 -mediated inhibition was also seen when 2-MeSADP was applied after I GIRK preactivation by norepinephrine or by expression of Gβ 1 γ 2 subunits. In contrast, stimulation of P2Y 4 receptors with UTP or P2Y 6 receptors with UDP produced very little I GIRK activation but significantly inhibited preactivated currents. Current activation was prevented by pertussis toxin (PTX) or
after coexpression of the βγ-scavenger transducin-Gα.I GIRK inhibition by all three nucleotide receptors was insensitive to PTX and was significantly reduced after coexpression of RGS2
protein, known to inhibit G q α signaling. Inhibition was not affected 1) after coexpression of RGS11, which interferes with G q βγ action; 2) after coexpression of phospholipase C (PLC) δ-Pleckstrin homology domain, which sequesters the membrane phospholipid
phosphatidylinositol 4,5-bisphosphate; (3) after buffering intracellular Ca 2+ with 1,2-bis(2-aminiphenoxy)ethane- N , N , N â², N â²-tetraacetic acid acetoxymethyl ester (BAPTA-AM); and (4) after pretreatment with the protein kinase C inhibitor 3-1-3-(dimethylaminopropyl-1 H -indol-3-yl-4-(1 H -indol-3-yl)-1 H -pyrrole-2,5-dione monohydrochloride (GF 109203X). We conclude that activation of I GIRK by P2Y receptors is mediated by G i/o βγ, whereas I GIRK inhibition is mediated by G q α. These effects may provide a mechanism for P2Y-modulation of neuronal excitability.
We have shown previously that stimulation of heterologously expressed P2Y1 nucleotide receptors inhibits M-type K+ currents in sympathetic neurons. We now report that activation of endogenous P2Y1 ...receptors induces inhibition of the M-current in rat CA1/CA3 hippocampal pyramidal cells in primary neuron cultures. The P2Y1 agonist adenosine 5'-beta-thiodiphosphate trilithium salt (ADPbetaS) inhibited M-current by up to 52% with an IC50 of 84 nM. The hydrolyzable agonist ADP (10 microM) produced 32% inhibition, whereas the metabotropic glutamate receptor 1/5 agonist DHPG (S)-3,5-dihydroxyphenylglycine (10 microM) inhibited M-current by 44%. The M-channel blocker XE991 10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone dihydrochloride produced 73% inhibition at 3 microM; neither ADPbetaS nor ADP produced additional inhibition in the presence of XE991. The effect of ADPbetaS was prevented by a specific P2Y1 antagonist, MRS 2179 (2'-deoxy-N'-methyladenosine-3',5'-bisphosphate tetra-ammonium salt) (30 microM). Inhibition of the M-current by ADPbetaS was accompanied by increased neuronal firing in response to injected current pulses. The neurons responding to ADPbetaS were judged to be pyramidal cells on the basis of (1) morphology, (2) firing characteristics, and (3) their distinctive staining for the pyramidal cell marker neurogranin. Strong immunostaining for P2Y1 receptors was shown in most cells in these cultures: 74% of the cells were positive for both P2Y1 and neurogranin, whereas 16% were only P2Y1 positive. These results show the presence of functional M-current-inhibitory P2Y1 receptors on hippocampal pyramidal neurons, as predicted from their effects when expressed in sympathetic neurons. However, the mechanism of inhibition in the two cell types seems to differ because, unlike nucleotide-mediated M-current inhibition in sympathetic neurons, that in hippocampal neurons did not appear to result from raised intracellular calcium.
The isolation of the peptide inhibitor of M-type K+ current, BeKm-1, from the venom of the Central Asian scorpion Buthus eupeus has been described previously (Fillipov A. K., Kozlov, S. A., ...Pluzhnikov, K. A., Grishin, E. V., and Brown, D. A. (1996) FEBS Lett. 384, 277–280). Here we report the cloning, expression, and selectivity of BeKm-1. A full-length cDNA of 365 nucleotides encoding the precursor of BeKm-1 was isolated using the rapid amplification of cDNA ends polymerase chain reaction technique from mRNA obtained from scorpion telsons. Sequence analysis of the cDNA revealed that the precursor contains a signal peptide of 21 amino acid residues. The mature toxin consists of 36 amino acid residues. BeKm-1 belongs to the family of scorpion venom potassium channel blockers and represents a new subgroup of these toxins. The recombinant BeKm-1 was produced as a Protein A fusion product in the periplasm of Escherichia coli. After cleavage and high performance liquid chromatography purification, recombinant BeKm-1 displayed the same properties as the native toxin. Three BeKm-1 mutants (R27K, F32K, and R27K/F32K) were generated, purified, and characterized. Recombinant wild-type BeKm-1 and the three mutants partly inhibited the native M-like current in NG108-15 at 100 nm. The effect of the recombinant BeKm-1 on different K+ channels was also studied. BeKm-1 inhibited hERG1 channels with an IC50 of 3.3 nm, but had no effect at 100 nm on hEAG, hSK1, rSK2, hIK, hBK, KCNQ1/KCNE1, KCNQ2/KCNQ3, KCNQ4 channels, and minimal effect on rELK1. Thus, BeKm-1 was shown to be a novel specific blocker of hERG1 potassium channels.
AF276623
Rat brain capillary endothelial (B10) cells express an unidentified nucleotide receptor linked to adenylyl cyclase inhibition. We show that this receptor in B10 cells is identical in sequence to the ...P2Y(12) ADP receptor ("P2Y(T)") of platelets. When expressed heterologously, 2-methylthio-ADP (2-MeSADP; EC(50), 2 nm), ADP, and adenosine 5'-O-(2-thio)diphosphate were agonists of cAMP decrease, and 2-propylthio-D-beta,gamma-difluoromethylene-ATP was a competitive antagonist (K(B), 28 nm), as in platelets. However, 2-methylthio-ATP (2-MeSATP) (EC(50), 0.4 nm), ATP (1.9 microm), and 2-chloro-ATP (190 nm), antagonists in the platelet, were also agonists. 2-MeSADP activated (EC(50), 0.1 nm) GIRK1/GIRK2 inward rectifier K(+) channels when co-expressed with P2Y(12) receptors in sympathetic neurons. Surprisingly, P2Y(1) receptors expressed likewise gave that response; however, a full inactivation followed, absent with P2Y(12) receptors. A new P2Y(12)-mediated transduction was found, the closing of native N-type Ca(2+) channels; again both 2-MeSATP and 2-MeSADP are agonists (EC(50), 0.04 and 0.1 nm, respectively). That action, like their cAMP response, was pertussis toxin-sensitive. The Ca(2+) channel inhibition and K(+) channel activation are mediated by beta gamma subunit release from a heterotrimeric G-protein. G alpha subunit types in B10 cells were also identified. The presence in the brain capillary endothelial cell of the P2Y(12) receptor is a significant extension of its functional range.
Neuronal GABA(B) receptors regulate calcium and potassium currents via G-protein-coupled mechanisms and play a critical role in long-term inhibition of synaptic transmission in the CNS. Recent ...studies have demonstrated that assembly of GABA(B) receptor GABA(B)R1 and GABA(B)R2 subunits into functional heterodimers is required for coupling to potassium channels in heterologous systems. However whether heterodimerization is required for the coupling of GABA(B) receptors to effector systems in neurons remains to be established. To address this issue, we have studied the coupling of recombinant GABA(B) receptors to endogenous Ca(2+) channels in superior cervical ganglion (SCG) neurons using nuclear microinjection to introduce both sense and antisense expression constructs. Patch-clamp recording from neurons injected with both GABA(B)R1a/1b and GABA(B)R2 cDNAs or with GABA(B)R2 alone produced marked baclofen-mediated inhibition of Ca(2+) channel currents via a pertussis toxin-sensitive mechanism. The actions of baclofen were blocked by CGP62349, a specific GABA(B) antagonist, and were voltage dependent. Interestingly, SCGs were found to express abundantly GABA(B)R1 but not GABA(B)R2 at the protein level. To determine whether heterodimerization of GABA(B)R1 and GABA(B)R2 subunits was required for Ca(2+) inhibition, the GABA(B)R2 expression construct was microinjected with a GABA(B)R1 antisense construct. This resulted in a dramatic decrease in the levels of the endogenous GABA(B)R1 protein and a marked reduction in the inhibitory effects of baclofen on Ca(2+) currents. Therefore our results suggest that in neurons heteromeric assemblies of GABA(B)R1 and GABA(B)R2 are essential to mediate GABAergic inhibition of Ca(2+) channel currents.
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