Galpha14 and Galphaq mediate the response to trypsin in Xenopus oocytes

Indirect activation of the epithelial Na+ channel by trypsin

We tested the hypothesis that the serine protease trypsin can indirectly activate the epithelial Na(+) channel (ENaC). Experiments were carried out in Xenopus oocytes and examined the effects on the channel formed by all three human ENaC subunits and that formed by Xenopus epsilon and human beta and gamma subunits (epsilonbetagammaENaC). Low levels of trypsin (1-10 ng/ml) were without effects on the oocyte endogenous conductances and were specifically used to test the effects on ENaC. Addition of 1 ng/ml trypsin for 60 min stimulated the amiloride-sensitive human ENaC conductance (g(Na)) by approximately 6-fold. This effect on the g(Na) was [Na(+)]-independent, thereby ruling out an interaction with channel feedback inhibition by Na(+). The indirect nature of this activation was confirmed in cell-attached patch clamp experiments with trypsin added to the outside of the pipette. Trypsin was comparatively ineffective at activating epsilonbetagammaENaC, a channel that exhibited a high spontaneous open probability. These observations, in combination with surface binding experiments, indicated that trypsin indirectly activated membrane-resident channels. Activation by trypsin was also dependent on catalytic activity of this protease but was not accompanied by channel subunit proteolysis. Channel activation was dependent on downstream activation of G-proteins and was blocked by G-protein inhibition by injection of guanyl-5'-yl thiophosphate and by pre-stimulation of phospholipase C. These data indicate a receptor-mediated activation of ENaC by trypsin. This trypsin-activated receptor is distinct from that of protease-activated receptor-2, because the response to trypsin was unaffected by protease-activated receptor-2 overexpression or knockdown.

Short-term exposure to waterborne free silver has acute effects on membrane current of Xenopus oocytes

Waterborne free silver can cause osmo- and ionoregulatory disturbances in freshwater organisms. The effects of a short-term exposure to extracellular Ag+ ions on membrane currents were investigated in voltage-clamped defolliculated Xenopus oocytes. At a holding potential of -60 mV, ionic silver (1 microM Ag+) increased inward currents (=I(Ag)) from -8+/-2 nA to -665+/-41 nA (n=74; N=27). I(Ag) activated within 2 min of silver exposure and then rose impetuously. This current was largely reversible by washout and repeatable. I(Ag) reversed around -30 mV and rectified slightly at more positive potentials. Na+-free bath conditions reduced the silver-induced current to a smaller but sustained current. The response to silver was abolished by the Cl- channel blockers DIDS and SITS, whereas niflumic acid strongly potentiated I(Ag). Intraoocyte injection of AgNO3 to about 1 mM [Ag]i strongly potentiated I(Ag). Extracellular application of either dithiothreitol (DTT), a compound known to reduce disulfide bridges, or L-cysteine abolished Ag+-activated increase of membrane current. In contrast, n-ethylmaleimide (NEM) which oxidizes SH-groups potentiated I(Ag). Hypoosmotic bath solution significantly increased I(Ag) whereas hyperosmolar conditions attenuated I(Ag). The activation of I(Ag) was largely preserved after chelation of cytosolic Ca2+ ions with BAPTA/AM. Taken together, these data suggest that Xenopus oocytes are sensitive to short-term exposure to waterborne Ag+ ions and that the elicited membrane currents result from extra- and intracellular action of Ag+ ions on peptide moieties at the oocyte membrane but may also affect conductances after internalization.

Go G-proteins mediate rapid heterologous desensitization of G-protein coupled receptors in Xenopus oocytes

We have shown previously that responses to lysophosphatidic acid (LPA) in Xenopus oocytes exhibit pronounced rapid homologous desensitization mediated by Go family of G-proteins (Itzhaki-Van Ham et al., 2004, J Cell Physiol, 200: 125-133). The present study was aimed at examining the involvement of Go G-proteins in rapid heterologous desensitization of native and expressed G-protein-coupled receptors in Xenopus oocytes. Threshold stimulation of the native lysophosphatidic acid receptors (LPA-Rs) induced about 50% rapid desensitization of responses evoked by stimulation of either native trypsin or expressed M1-muscarinic cholinergic receptors (M1-Rs). Similarly, threshold stimulation of expressed M1-Rs or thyrotropin-releasing hormone receptors induced 40% rapid desensitization of responses to LPA. Inactivation of all Gi/o G-proteins with pertussis toxin (PTX) completely abolished rapid heterologous desensitization in all protocols. Depletion of either Galphao or Galphao1 by antisense oligodeoxynucleotides targeted at either member of the Galphao family decreased or completely abolished rapid heterologous desensitization. Expression of two dominant negative mutants of the human Galphao family, highly homologous to oocyte Galphao species, either decreased or virtually abolished rapid desensitization. Homologous and heterologous desensitizations of the LPA response were non-additive and proceeded, apparently, via the same pathway. We conclude that Go G-proteins mediate both homologous and heterologous rapid desensitization of responses mediated by G-protein-coupled receptors (GPCRs) coupled to the phosphoinositide phospholipase C-inositol 1,4,5-trisphosphate-Ca(2+) (PI-PLC-InsP(3)-Ca(2+)) pathway in Xenopus oocytes.

Local Anesthetics Inhibit Thromboxane A2 Signaling in Xenopus Oocytes and Human K562 Cells

Thromboxane A(2) (TXA(2)) has been proposed as a mediator of perioperative myocardial ischemia, vasoconstriction, and thrombosis. As these adverse events are minimized with epidural anesthesia, rather than general anesthesia, we hypothesized that local anesthetics would inhibit TXA(2)-receptor signaling. We used fluorometric determination of intracellular [Ca(2+)] in human K562 cells and 2-electrode voltage clamp measurements in Xenopus laevis oocytes expressing TXA(2) receptors. After 10-min incubation, lidocaine (IC(50): 1.02 +/- 0.2 x 10(-3) M), ropivacaine (IC(50): ropivacaine 6.3 +/- 0.9 x 10(-5) M), or bupivacaine (IC(50): 1.42 +/- 0.08 x 10(-7) M) inhibited TXA(2)-induced [Ca(2+)](i) in K562 cells. These data were confirmed in Xenopus oocytes recombinantly expressing TXA(2) receptors, with IC(50)s of bupivacaine 1.2 +/- 0.2 x 10(-5) M, R(+) ropivacaine 4.9 +/- 1.7 x 10(-4) M, S(-) ropivacaine 5.3 +/- 0.9 x 10(-5) M, and lidocaine 6.4 +/- 2.8 x 10(-4) M. Intracellular pathways activated by IP(3) and GTPgammaS were not significantly affected by the local anesthetics tested. QX314, a positively charged lidocaine analog, inhibited only if injected intracellularly (IC(50): 5.3 +/- 1.7 x 10(-4) M), indicating one local anesthetic target is most likely inside the cell. Benzocaine (largely uncharged) inhibited with an IC(50) of 8.7 +/- 1.8 x 10(-4) M. This suggests that some of the beneficial effects of regional anesthesia techniques might be due to direct interaction of local anesthetics with the functioning of membrane proteins.

Response to lysophosphatidic acid in Xenopus oocytes and its rapid desensitization: the role of Gq and Go G-protein families

Native Xenopus oocytes exhibit dose-dependent depolarizing current responses to lysophosphatidic acid (LPA), with EC50 = 0.18 microM. Responses to LPA were subject to pronounced rapid desensitization. When oocytes were challenged with 5 nM LPA, the response was <10% of the maximal. Subsequent addition of 0.5 microM LPA resulted in 50-70% desensitization, when compared to naïve controls. Injection of antisense oligodeoxyoligonucleotides (ASODNs) targeted at either of the two endogenous LPA receptors inhibited the LPA response by approximately 50%, but did not alter the degree of rapid desensitization. To study the involvement of G-proteins in rapid homologous desensitization of responses to LPA, we selectively depleted native G-proteins by injection of specific ASDONs. Injection of ASDONs targeted at Galphaq family mRNAs (mainly Galpha11) reduced the response to 0.5 microM LPA by 50%. ASDONs targeted at either Galphao or Galphao1 caused a large decrease in the amount of their cognate mRNAs and the Galphao family proteins, while the response to LPA was inhibited by up to 30%. Injection of ASDONs targeted at Galphao1 mRNA decreased rapid desensitization from 69 to 23%, while pertussis toxin (PTX) completely abolished it. Expression of two dominant negative mutants of the human Galphao family homologs either decreased or virtually abolished rapid desensitization. Microinjection of CaCl(2) demonstrated that 50% of rapid desensitization could be attributed to inhibition of Ca(2+) activation of chloride channels. We propose that the apparent degenerate coupling of different G-proteins to LPA receptors in Xenopus oocytes actually serves both the generation of the response (by Gq and Go G-protein families) and its desensitization (mostly by Go G-protein family).

G protein-activated K+channels: a reporter for rapid activation of G proteins by lysophosphatidic acid inXenopusoocytes

Threshold concentrations of lysophosphatidic acid (LPA) or acetylcholine (ACh) induce pertussis toxin (PTX)-sensitive rapid desensitization of responses to LPA in Xenopus oocytes. To demonstrate that threshold [LPA] rapidly activates Gi/o proteins, we used the G protein-activated K+ channel (GIRK) as a reporter. Low [LPA] induced IK+ in <3 s of the agonist addition with little or no activation of chloride current. Depletion of Galphao/Galphao1 each decreased the LPA-induced IK+ by approximately 40-50%, while PTX completely abolished it. This is the first direct evidence showing the activation of GIRK by LPA, and the involvement of G proteins of the Go family in rapid desensitization of LPA responses.

Regulatory contribution of heterotrimeric G-proteins to oocyte maturation in the sea urchin

Regulation of animal oocyte maturation is hypothesized to involve heterotrimeric G-proteins. It is difficult to test this hypothesis though without knowing what G-proteins are present in these cells and where are they localized. We set out to test the hypothesis that G-proteins regulate maturation in the sea urchin oocyte by identifying resident G-proteins in oocytes and eggs, and then investigating their function. We find four families of G-protein alpha-subunits (Galphai, Galphaq, Galphas, and Galpha12) present in both oocytes and eggs of the sea urchin. Three of them, Galphai, Galphaq, and Galphas are present on the plasma membrane of the oocyte, while the fourth is located on cytoplasmic vesicles. Upon oocyte maturation, these proteins remain in eggs, and continue to be expressed in embryonic tissues. To test the functional contribution of the G-proteins to the regulation of oocyte maturation, we employ specific intervening reagents, including antibodies and competitor peptides to each Galpha subunit, and specific Galpha toxins. We find that Gi is a main candidate for a positive regulator of sea urchin oocyte maturation. These studies provide a foundation to further test specific hypotheses of the G-protein mediated regulation of oocyte maturation, fertilization, and early development in the sea urchin.

Activation of inositol 1,4,5-trisphosphate receptor is essential for the opening of mouse TRP5 channels

We studied the opening mechanism of Ca(2+)-permeable channels formed with mouse transient receptor potential type 5 (mTRP5) using Xenopus oocytes. After stimulation of coexpressed muscarinic M(1) receptors with acetylcholine (ACh) in a Ca(2+)-free solution, switching to 2 mM Ca(2+)-containing solution evoked a large Cl(-) current, which reflects the opening of endogenous Ca(2+)-dependent Cl(-) channels following Ca(2+) entry through the expressed channels. The ACh-evoked response was not affected by a depletion of Ca(2+) store with thapsigargin but was inhibited by preinjection of antisense oligodeoxynucleotides (ODNs) to G(q), G(11), or both. The mTRP5 channel response was also induced by a direct activation of G proteins with injection of guanosine 5'-3-O-(thio)triphosphate (GTP gamma S). The ACh- and GTP gamma S-evoked responses were inhibited by either pretreatment with a phospholipase C inhibitor, U73122, or an inositol-1,4,5-trisphosphate (IP(3)) receptor inhibitor, xestospongin C (XeC). An activation of IP(3) receptors with injection of adenophostin A (AdA) evoked the mTRP5 channel response in a dose-dependent manner. The AdA-evoked response was not suppressed by preinjection of antisense ODNs to G(q/11) or U73122 but was suppressed by either preinjection of XeC or a peptide mimicking the IP(3) binding domain of Xenopus IP(3) receptor. These findings suggest that the activation of IP(3) receptor is essential for the opening of mTRP5 channels, and that neither G proteins, phosphoinositide metabolism, nor depletion of the Ca(2+) store directly modifies the IP(3) receptor-linked opening of mTRP5 channels.

Inhibition of m3 muscarinic acetylcholine receptors by local anaesthetics

1. Muscarinic m1 receptors are inhibited by local anaesthetics (LA) at nM concentrations. To elucidate in more detail the site(s) of LA interaction, we compared these findings with LA effects on m3 muscarinic receptors. 2. We expressed receptors in Xenopus oocytes. Using two-electrode voltage clamp, we measured the effects of lidocaine, QX314 (permanently charged) and benzocaine (permanently uncharged) on Ca(2+)-activated Cl(-)-currents (I(Cl(Ca))), elicited by acetyl-beta-methylcholine bromide (MCh). We also characterized the interaction of lidocaine with [(3)H]-quinuclydinyl benzylate ([(3)H]-QNB) binding to m3 receptors. Antisense-injection was used to determine the role of specific G-protein alpha subunits in mediating the inhibitory effects of LA. Using chimeric receptor constructs we investigated which domains of the muscarinic receptors contribute to the binding site for LA. 3. Lidocaine inhibited m3-signalling in a concentration-dependent, reversible, non-competitive manner with an IC(50) of 370 nM, approximately 21 fold higher than the IC(50) (18 nM) reported for m1 receptors. Intracellular inhibition of both signalling pathways by LA was similar, and dependent on the G(q)- protein alpha subunit. In contrast to results reported for the m1 receptor, the m3 receptor lacks the major extracellular binding site for charged LA. The N-terminus and third extracellular loop of the m1 muscarinic receptor molecule were identified as requirements to obtain extracellular inhibition by charged LA.

Local anesthetic inhibition of G protein-coupled receptor signaling by interference with Galpha(q) protein function

Although local anesthetics are considered primarily Na(+) channel blockers, previous studies suggest a common intracellular site of action on different G protein-coupled receptors. In the present study, we characterized this site for the LPA, m1 muscarinic, and trypsin receptor. Xenopus laevis oocytes expressing endogenous LPA and trypsin or recombinant m1 receptors were two-electrode voltage clamped. We studied LPA inhibition in the presence of ropivacaine stereoisomers to determine whether LA act on a protein site. Ropivacaine inhibited LPA signaling in a stereoselective and noncompetitive manner, suggesting a protein interaction. Antisense injection was used to characterize G protein alpha-subunits involved in mediation of LPA, m1, trypsin, and angiotensin(1A) receptor signaling. Lidocaine and its analog QX314 were injected into oocytes expressing these receptors to examine a potential role for specific G protein alpha-subunits as targets for LA. Galpha(q) was shown to be among the primary G protein subunits mediating the LPA, m1, and trypsin receptor signaling, all of which were inhibited to a similar degree by intracellular injected QX314 (424 x 10(-6) M). Since the angiotensin(1A) receptor, previously shown not to be affected by LA, was found not to signal via Galpha(q), but via Galpha(o) and Galpha(14), the intracellular effect of LA most likely takes place at the Galpha(q)-subunit.

Photocontrol of DNA duplex formation by using azobenzene-bearing oligonucleotides

The duplex-forming activities of oligonucleotides can be photomodulated by incorporation of an azobenzene unit. Upon isomerizing the trans-azobenzene to the cis form by irradiation with UV light, the T(m) value of the duplex (with the complementary DNA) is lowered so that the duplex is dissociated. The duplex is formed again when the cis-azobenzene is converted to the trans-azobenzene by irradiation with visible light. The photoregulation is successful irrespective of the position of the azobenzene unit in the oligonucleotides. The trans-azobenzene in the oligonucleotides intercalates between two DNA base pairs in the duplexes and stabilizes them because of a favorable enthalpy change. The nonplanar structure of a cis-azobenzene is unfavorable for such an interaction. These photoresponsive oligonucleotides are promising candidates for the regulation of various bioreactions.

Molecular phenotype of the human oocyte by PCR-SAGE

Consecutive application of PCR and serial analysis of gene expression (SAGE) was used to generate a catalog of approximately 50, 000 SAGEtags from nine human oocytes. Matches for known genes were identified using the National Institutes of Health SAGEtag database. This database links directly to the UniGene database, providing rapid discrimination between SAGEtags that match known genes and expressed sequence tags and those that currently have no match. Matches in the oocyte SAGE catalog were found for surface receptors, second-messenger systems, and cytoskeletal, apoptotic, and secreted proteins. Many of these proteins were not previously known to be expressed in mammalian oocytes. The relative abundances of transcripts for cytoskeletal proteins and proteins known to be in oocytes are consistent with their documented expression, suggesting an absence of representational distortion by the PCR step. The expression profile of the human oocyte may help identify factors that reprogram somatic cell nuclei to totipotency.

Calcium release at fertilization of Xenopus eggs requires type I IP(3) receptors, but not SH2 domain-mediated activation of PLCgamma or G(q)-mediated activation of PLCbeta

Elevation of intracellular Ca2+ at fertilization is essential for the initiation of development in the Xenopus egg, but the pathway between sperm-egg interaction and Ca2+ release from the egg's endoplasmic reticulum is not well understood. Here we show that injection of an inhibitory antibody against the type I IP(3) receptor reduces Ca2+ release at fertilization, indicating that the Ca2+ release requires IP(3). We then examine how IP(3) production is initiated. Xenopus eggs were injected with specific inhibitors of the activation of two phospholipase C isoforms, PLCgamma and PLCbeta. The Src-homology 2 (SH2) domains of PLCgamma were used to inhibit SH2-mediated activation of PLCgamma, and an antibody against G(q) family G-proteins was used to inhibit G(q)-mediated activation of PLCbeta. Though the PLCgamma SH2 domains inhibited platelet-derived growth factor (PDGF)-induced Ca2+ release in eggs with exogenously expressed PDGF receptors, they did not inhibit the Ca2+ rise at fertilization. Similarly, the G(q) family antibody blocked serotonin-induced Ca2+ release in eggs with exogenously expressed serotonin 2C receptors, but not the Ca2+ rise at fertilization. A mixture of PLCgamma SH2 domains and the G(q) antibody also did not inhibit the Ca2+ rise at fertilization. These results indicate that Ca2+ release at fertilization of Xenopus eggs requires type I IP(3)-gated Ca2+ channels, but not SH2 domain-mediated activation of PLCgamma or G(q)-mediated activation of PLCbeta.

Genomic organization of the human galpha14 and Galphaq genes and mutation analysis in chorea-acanthocytosis (CHAC)

Chorea-acanthocytosis (CHAC) (OMIM 200150) is a rare neurological syndrome characterized by neurodegeneration in combination with morphologically abnormal red cells (acanthocytosis). A partial yeast artificial chromosome contig of the CHAC critical region on chromosome 9q21 has been constructed, and 21 expressed sequence tags have been mapped. We have subsequently cloned Galpha14, a member of the G-protein alpha-subunit multigene family, and have identified Galphaq in the contig. The genomic structure of both genes has been established after construction of a bacterial artificial chromosome contig that showed Galphaq and Galpha14 to be in a head-to-tail arrangement (Cen-Galphaq-Galpha14-qter). Northern analysis found Galphaq to be ubiquitously expressed and Galpha14 to display a more restricted pattern of expression. Mutation analysis of the coding regions and splice sites for Galphaq and Galpha14 in 10 affected individuals from different families identified no changes likely to cause disease; however, two distinct single nucleotide polymorphisms in the coding region of Galpha14 have been identified. This study has excluded two plausible candidate genes from involvement in CHAC and has provided a solid platform for a positional cloning initiative.