The biosynthesis of biologically active proteins in mRNA-microinjected Xenopus oocytes

Xenopus Oocytes: Optimized Methods for Microinjection, Removal of Follicular Cell Layers, and Fast Solution Changes in Electrophysiological Experiments

The Xenopus oocyte as a heterologous expression system for proteins, was first described by Gurdon et al.¹ and has been widely used since its discovery (References 2 - 3, and references therein). A characteristic that makes the oocyte attractive for foreign channel expression is the poor abundance of endogenous ion channels⁴. This expression system has proven useful for the characterization of many proteins, among them ligand-gated ion channels. The expression of GABAA receptors in Xenopus oocytes and their functional characterization is described here, including the isolation of oocytes, microinjections with cRNA, the removal of follicular cell layers, and fast solution changes in electrophysiological experiments. The procedures were optimized in this laboratory^5,6 and deviate from the ones routinely used^7-9. Traditionally, denuded oocytes are prepared with a prolonged collagenase treatment of ovary lobes at RT, and these denuded oocytes are microinjected with mRNA. Using the optimized methods, diverse membrane proteins have been expressed and studied with this system, such as recombinant GABAA receptors^10-12, human recombinant chloride channels¹³, Trypanosome potassium channels¹⁴, and a myo-inositol transporter^{15, 16}. The methods detailed here may be applied to the expression of any protein of choice in Xenopus oocytes, and the rapid solution change can be used to study other ligand-gated ion channels.

Disruptive environmental chemicals and cellular mechanisms that confer resistance to cell death

Cell death is a process of dying within biological cells that are ceasing to function. This process is essential in regulating organism development, tissue homeostasis, and to eliminate cells in the body that are irreparably damaged. In general, dysfunction in normal cellular death is tightly linked to cancer progression. Specifically, the up-regulation of pro-survival factors, including oncogenic factors and antiapoptotic signaling pathways, and the down-regulation of pro-apoptotic factors, including tumor suppressive factors, confers resistance to cell death in tumor cells, which supports the emergence of a fully immortalized cellular phenotype. This review considers the potential relevance of ubiquitous environmental chemical exposures that have been shown to disrupt key pathways and mechanisms associated with this sort of dysfunction. Specifically, bisphenol A, chlorothalonil, dibutyl phthalate, dichlorvos, lindane, linuron, methoxychlor and oxyfluorfen are discussed as prototypical chemical disruptors; as their effects relate to resistance to cell death, as constituents within environmental mixtures and as potential contributors to environmental carcinogenesis.

The use of Xenopus oocytes in drug screening

INTRODUCTION

The physiological roles of ion channels are receiving increased interest in both basic research and drug discovery, and a demand for pharmacological approaches that can characterize or screen ion channels and their ligands with higher throughput has emerged. Traditionally, screening of compound libraries at ion channel targets has been performed using assays such as binding assays, fluorescence-based assays and flux assays that allow high-throughput, but sacrifice high data quality. The use of these assays with ion channel targets can also be problematic, emphasizing the usefulness of automated Xenopus oocyte electrophysiological assays in drug screening.

AREAS COVERED

This review summarizes the use of Xenopus oocytes in drug screening, presents the advantages and disadvantages of the use of Xenopus oocytes as expression system, and addresses the options available for automated two-electrode voltage-clamp recordings from Xenopus oocytes.

EXPERT OPINION

Automated and manual Xenopus oocyte two-electrode voltage-clamp recordings are useful and important techniques in drug screening. Although they are not compatible with high-throughput experimentation, these techniques are excellent in combination or as alternatives to fluorescence-based assays for hit validation, screening of focused compound libraries and safety screening on ion channels with their high flexibility for the choice of molecular targets, quality of data and reproducibility.

Expression cloning of neural genes using Xenopus laevis oocytes

Expression cloning requires a representative cDNA or genomic DNA library and a host organism in which the cloned genes can be transcribed and/or translated. It likewise requires a method to detect the expressed protein using, for example, the inherent biological activity of the gene or antibodies specific for the gene product. Most successful expression cloning strategies have employed cDNA libraries constructed in plasmid or bacteriophage lambda vectors and Xenopus oocytes or cultured mammalian cells as hosts. This unit presents several protocols designed for expression cloning paradigms that rely on electrophysiological recordings from Xenopus laevis oocytes.

Automated Higher-Throughput Compound Screening on Ion Channel Targets Based on theXenopus laevisOocyte Expression System

As numerous diseases have been shown to be related to dysfunction of ion channels and neurotransmitter receptors and to affect regulatory pathways, ion channels have attracted increasing attention as a target class for drug discovery. The concomitant demand of the pharmaceutical industry for adequate electrophysiological methods to investigate drug effects on specific ion channels in secondary and safety screening has resulted in the development of electrophysiological instrumentation that allows automated monitoring of ion channel function with a higher throughput. Here we tested a fully automated screening system based on the Xenopus laevis oocyte expression system. We addressed the questions of data quality and reproducibility obtained by automated oocyte injection and two-electrode voltage-clamp (TEVC) recording using the Roboocyte (Multi Channel Systems GmbH, Reutlingen, Germany) technology compared to conventional oocyte recording. A gamma-aminobutyric acid (GABA)A-receptor subtype (alpha(1)beta(2)) was chosen as an example for a ligand-gated ion channel, and the slowly activating potassium current I(Ks) as a voltage-activated ion channel. Oocytes were injected with cDNA or cRNA via the Roboocyte injection stage. Ion channel currents were successfully recorded after 2-7 days in about 40% of the oocytes injected with GABA(A) receptor cDNA, and after 2-4 days in about 60% of the oocytes injected with KCNE1 cRNA. EC(50) values for the GABA(A) receptor and IC(50) values for blockers of I(Ks) were comparable to values obtained with conventional TEVC recording techniques. In conclusion, our results show that the Roboocyte is a valuable automated tool for oocyte injection and TEVC recording that can be used in drug screening and target validation to enhance the number of compounds and oocytes tested per day.

Expression of a non-MDR2-coded liver phosphatidylcholine membrane transport protein in Xenopus laevis oocytes

Phosphatidylcholines (PC) are secreted into the bile via a membrane transport protein(s). Recently, evidence for ATP-dependent mdr2-encoded PC transport as well as for carrier-mediated PC transport had been reported. Therefore, we investigated whether mdr2 P-glycoprotein is involved in the transport of a water-soluble short chain phosphatidylcholine analogue L-alpha-dibutyroyl-PC (diC4PC) induced by expression of liver mRNA in Xenopus laevis oocytes. Expression of mouse and rat mdr2 cRNA did not result in diC4PC net uptake in Xenopus laevis oocytes. By contrast oocytes showed a similar carrier-mediated uptake activity for diC4PC after injection of mouse, rat and human liver total mRNA (Km 7.7, 9.6, and 11.6 mM). Antisense inhibition of mdr2 mRNA expression increased diC4PC uptake induced by total liver mRNA from mouse and rat. The present data prove the existence of a specific mRNA for a non-mdr2-coded cell membrane PC carrier in mouse, rat, and human liver which exhibits similar transport affinity for diC4PC as the PC carrier in rat liver canalicular membranes.

Differences between the catalytic properties of recombinant human PC2 and endogenous rat PC2

Human prohormone convertase PC2 was expressed in Xenopus oocytes and its properties were compared with those of the Type-2 endopeptidase of rat insulin secretory granules, previously identified as PC2 [Bennett, Bailyes, Nielson, Guest, Rutherford, Arden and Hutton (1992) J. Biol. Chem. 267, 15229-15236]. Recombinant PC2 had the same substrate specificity as the Type-2 endopeptidase, cleaving at the CA-junction (Lys64, Arg65) of human des-31,32-proinsulin to generate insulin; little activity was found toward human des-64,65-proinsulin or proinsulin itself. Recombinant PC2 was maximally active in 5-7 mM Ca2+ (K0.5 = 1.6 mM) whereas the Type-2 endopeptidase was maximally active in 0.5-1 mM Ca2+ (K0.5 = 40 microM). Both enzymes had a pH optimum of 5.0-5.5 but the Type-2 endopeptidase was active over a wider pH range. Two molecular forms of recombinant PC2 (71 kDa and 68 kDa) were found, both had an intact C-terminus but differed by the presence of the propeptide. The endogenous PC2 comprised several overlapping forms (size range 64-68 kDa), approximately two-thirds of which lacked C-terminal immunoreactivity. Part of the size difference between recombinant and endogenous PC2 was attributable to differences in N-glycosylation. The different post-translational proteolytic modifications of recombinant and endogenous PC2 did not account for the different pH and Ca2+ sensitivities shown by the enzymes. A modulating effect of carbohydrate on enzyme activity could not be excluded.

Growth hormone-releasing hormone: synthesis and signaling

The molecular characterization of GHRH and the GHRH receptor provides a framework for understanding the hypothalamic regulation of pituitary somatotroph function. The signaling events discerned from our investigation of GHRH receptor structure and function form the basis of a model for GHRH action, which is shown in Fig. 20. GHRH interaction with its seven transmembrane domain Gs-coupled receptor on the somatotroph (step 1) leads to the release of growth hormone from secretory granules (step 2), which is likely to involve a G protein-mediated interaction with ion channels, and to a stimulation of intracellular cAMP accumulation (step 3) (Mayo, 1992; Lin et al., 1992; Gaylinn et al., 1993). In several cell types tested, elevated cAMP leads to the phosphorylation and activation of the transcription factor CREB by protein kinase A (Gonzalez and Montminy, 1989; Sheng et al., 1991), and one target gene for CREB action is the pituitary-specific transcription factor Pit-1 or GHF-1 (step 4) (Bodner et al., 1988; Ingraham et al., 1988; McCormick et al., 1990). Pit-1 is a prototypic POU domain protein that is required for the appropriate regulation of the growth hormone gene in somatotroph cells, thus providing a pathway by which a GHRH signal can lead to increased growth hormone synthesis in the pituitary (step 5). In addition, Pit-1 is likely to directly regulate the synthesis of the GHRH receptor (step 6), in that the receptor is not expressed in the pituitary of dw/dw mice that lack functional Pit-1 (Lin et al., 1992), and a cotransfected Pit-1 expression construct can activate the GHRH receptor promoter in transiently transfected CV1 cells (Lin et al., 1993). It remains to be determined whether additional direct regulation of the GHRH receptor gene in response to the cAMP signaling pathway occurs (step 7). The inhibitory peptide somatostatin presumably interacts with this same signaling pathway through G protein-mediated suppression of the cAMP pathway (Tallent and Reisine, 1992; Bell and Reisine, 1993). In agreement with the importance of this signaling system for normal growth, a transgene encoding a nonphosphorylatable mutant CREB protein, which blocks the function of the endogenous CREB protein, is able to cause somatotroph hypoplasia and dwarfism in mice when its expression is targeted to pituitary somatotrophs (Struthers et al., 1991). Several steps in the signaling pathway leading to growth hormone secretion are subject to disruption, resulting in growth hormone deficiency.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphine inhibits calcium influx and the response to acetylcholine in Xenopus oocytes

Incubation of intact Xenopus oocytes with the opioid radioligand [3H]diprenorphine (0.5 nM) resulted in specific binding of 1.7 +/- 0.3 fmol per oocyte. Morphine (10 microM) inhibited the uptake of 45Ca2+ into the oocyte by 66 +/- 9%. The opioid antagonist naltrexone partially blocked this effect of morphine. Preincubation of oocytes with morphine (10 microM, 2 min) partially inhibited the fast and slow responses of the oocyte to acetylcholine by 26 and 52%, respectively. We conclude that native Xenopus oocytes possess opioid receptors that may modulate the muscarinic response by limiting calcium influx into the cell.

Tandem linkage of Shaker K+ channel subunits does not ensure the stoichiometry of expressed channels

Shaker K+ channels are multimeric, probably tetrameric proteins. Substitution of a conserved leucine residue to valine (V2) at position 370 in the Drosophila Shaker 29-4 sequence results in large alterations in the voltage dependence of gating in the expressed channels. In order to determine the effects of this mutation in hybrid channels with a fixed stoichiometry of V2 and wild-type (WT) subunits we generated cDNA constructs of two linked-monomeric subunits similar to the tandem constructs previously reported by Isacoff, E. Y., Y. N. Jan, and L. Y. Jan. (1990. Nature (Lond.). 345:530-534). In addition, we constructed a tandem cDNA containing a wild-type subunit and a truncated nonfunctional subunit (Sh102) that suppresses channel expression. We report that the voltage-dependence of the channels produced with WT and V2 subunits varied significantly with the order of the subunits in the construct (WT-V2 or V2-WT), while the WT-Sh102 construct yielded currents that were much larger than expected. These results suggest that the tandem linkage of Shaker subunits does not guarantee the stoichiometry of the expressed channel proteins.

Evidence for the existence of RNA of Ca(2+)-channel alpha 2/delta subunit in Xenopus oocytes

Ba(2+)-currents (IBa) through voltage-dependent Ca(2+)-channels were studied in Xenopus oocytes injected with RNA from several excitable tissues, using the two-electrode voltage-clamp technique. Previous studies have shown that the expression of cardiac Ca(2+)-channels can be suppressed by an hybrid-arrest procedure that includes co-injection of the tissue-derived RNA with an 'antisense' oligonucleotide complementary to a part of RNA coding for the Ca(2+)-channel alpha 1 subunit. In this study, this method was used to investigate the role of the alpha 2/delta subunit. Co-injection of RNA extracted from either rabbit heart, rat brain or rat skeletal muscle (SkM) with 'antisense' oligonucleotides complementary to the alpha 2/delta subunit RNA did not substantially affect the expression of IBa in the oocytes. Using the Northern blot hybridization method, it was shown that native oocytes contain large amounts of alpha 2/delta subunit RNA of Ca(2+)-channel. It is proposed that te oligonucleotide treatment fails to eliminate the alpha 2/delta RNA because of the vast excess of endogenous alpha 2/delta RNA. These results impose a limit on the use of the hybrid-arrest method.

Kinetics of the functional loss of different muscarinic receptor isoforms in Xenopus oocytes

Native Xenopus oocytes express two isoforms of muscarinic receptors that mediate qualitatively different physiological responses. Oocytes of the majority of donors (common) express M3-like receptors (M3Rs) at comparable densities at both the animal and vegetal hemispheres of the cell. Rare (variant) donors possess oocytes that express mainly M1-like receptors (M1Rs), localized predominantly at the animal hemisphere. We have investigated the apparent degradation of these two isoforms and its relationship to their hemispheric distribution. Cycloheximide (CHX) caused a time-dependent decrease in receptor-mediated responses and [3H]quinuclidinyl benzylate (QNB) binding in oocytes from both types of donors. The t1/2 values ranged between 3 and 7 h. Removal of CHX resulted in rapid recovery of the response. This implied rapid degradation and turnover of both types of receptors. The loss of M1Rs was more than that of M3Rs. Moreover, the decrease was more rapid and more extensive on the animal hemisphere in both types of donors. Injection of oocytes expressing either receptor isoform with specific antisense oligonucleotides complementary to either m1 or m3 muscarinic receptors (from mouse) showed receptor loss at approximately the same rate as that calculated from experiments with CHX. Furthermore, oocytes of variant donors express M1Rs exclusively on the animal hemisphere, while the residual activity found on the vegetal hemisphere of the cell was mediated by M3Rs. Inhibition of putative receptor glycosylation with tunicamycin caused a rapid decrease in receptor-mediated responses and radioligand binding on M1Rs, but had virtually no effect on M3Rs. The expression of cloned m1 muscarinic receptors, however, was not affected by tunicamycin, suggesting that glycosylation is not a general prerequisite for the functional expression of muscarinic receptors.

The metabolism of microinjected inositol trisphosphate in Xenopus oocytes

Microinjection of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) into Xenopus oocytes evokes a complex physiological response composed of a transient and a slow depolarizing chloride current. We investigated the relationship between intracellular levels of Ins(1,4,5)P3 and the kinetics of the physiological response. Microinjected Ins(1,4,5)P3 was slowly degraded following first order kinetics of disappearance (t1/2 = 10 min). The degradation products were inositol bisphosphate (InsP2), inositol monophosphate (InsP) and inositol, as well as inositol tetrakisphosphate (InsP4). The rate of degradation of injected 3[H]-Ins(1,4)P2 was much greater (t1/2 = 3 min), indicating that the conversion of InsP3 to InsP2 may be the rate-limiting step in the degradation process. The slow degradation of 3[H]-Ins(1,4,5)P3 was not a result of its conversion to Ins(1,3,4)P3 since no accumulation of InsP3 was observed within 10 min of microinjection of 3[H]-Ins(1,3,4,5)P4. Activation of protein kinase C (PK-C) with a phorbol ester transiently increased the rate of conversion of 3[H]-Ins(1,4,5)P3 to InsP2. This, however, did not significantly affect the overall kinetics of 3[H]-Ins(1,4,5)P3 disappearance. Our results indicate that the kinetics of Ins(1,4,5)P3 degradation do not correlate well with the termination of both the rapid and the slow components of the physiological response. The termination of the slow component of the response, however, may be related to the decay of Ins(1,4,5)P3-induced 45Ca efflux, which lasted about 10 min.

Effect of benzodiazepine ligands on Ca2+ channel currents in Xenopus oocytes injected with rat heart RNA

Voltage-dependent Ca2+ channel (VDCC) currents have been measured in Xenopus oocytes injected with heart RNA purified from 7 day old rats, using voltage clamp technique. The currents were evoked by depolarizing voltage steps, using Ba2+ as charge carrier. Electrophysiological analysis of the current revealed two components: a slow, L-type, dihydropyridine (DHP)-sensitive current and a transient, DHP-insensitive, current. The benzodiazepine (BZ) ligands diazepam and Ro5-4864 decreased the current with micromolar affinity, with potency order of Ro5-4864 greater than diazepam greater than clonazepam. The central antagonist Ro15-1788 did not interfere with the effect of these drugs, thus excluding the possible involvement of the "central type" receptor. The slow current that was increased about 3 fold in the presence of 0.5 microM Bay K 8644, was less potentiated when previously treated with Ro5-4864 or diazepam. Current-voltage relation of the peak inward current and the steady state activation curve showed a small shift towards negative potentials in the presence of 50 microM diazepam. It is concluded that the benzodiazepine ligands block DHP-sensitive voltage dependent Ca2+ channels with a very marginal effect on the transient, DHP-insensitive current. Also it is emphasized that Xenopus oocytes can serve as a useful model system to study the pharmacology of these important drugs on cardiac Ca2+ channels.

A potassium current evoked by growth hormone-releasing hormone in follicular oocytes of Xenopus laevis

1. Electrophysiological properties of the growth hormone-releasing hormone (GRH) receptor were studied in Xenopus oocytes with an intact follicle cell layer (i.e. follicular oocytes) by measuring whole-cell current using the two-electrode voltage-clamp method. 2. A slow transient outward current was elicited in oocytes, clamped at -60 mV, by the application of rat GRH but not bovine, porcine, or human GRH. 3. The response to GRH was not suppressed by blockers known to inhibit other endogenous receptors present in follicular Xenopus oocytes; blockers used were timolol (2 microM; beta-adrenergic blocker), theophylline (0.1 mM; purinergic blocker) and atropine (100 nM; muscarinic blocker). 4. The current response evoked by rat GRH occurred in a dose-dependent manner. The concentrations of GRH for threshold and maximum responses were 1 and 100 nM respectively and the estimated EC50 (half-maximal effective concentration) was approximately 7 nM. The amplitude and conductance of the response became larger and the latency, time-to-peak and half-decay time were shortened when the concentration of GRH was increased. 5. The GRH response was reversibly inhibited by a K+ channel blocker, tetraethylammonium+ (TEA+; 20 mM). The reversal potential for the GRH response was around -100 mV and was compatible with the reported value for a K+ current in Xenopus oocytes. Furthermore, a depolarizing shift of 40 mV in the reversal potential was observed when the external K+ concentration was increased from 2 to 10 mM, agreeing with the Nernst equation. In contrast, no significant shift in the reversal potential was observed by changing the external concentration of Na+ or Cl-. 6. The GRH response was not suppressed in oocytes treated with an acetoxy-methyl ester of bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA/AM; 10 microM) which penetrates the cell membrane and chelates internal Ca2+. 7. The GRH response was potentiated by pre-treatment with forskolin (0.4 microM; 5 min), which stimulates adenylate cyclase and increases the internal concentration of adenosine 3',5'-cyclic monophosphate (cyclic AMP). 8. The GRH response was not obtainable when follicle cells surrounding oocytes were removed mechanically with forceps or enzymically with collagenase (i.e. denuded oocytes). The response was also suppressed when gap junctions, which electrically couple follicle cells and the oocyte, were blocked by 1-octanol (1 mM). 9. The first amino acid is considered to be important for the binding of peptide ligands to their receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional expression of urea channels in amphibian oocytes injected with frog urinary bladder mRNA

In amphibian urinary bladder epithelium, vasopressin increases passive urea permeability, concomitant with the appearance of a facilitated urea transport. Amphibian oocytes from Xenopus laevis and Rana esculenta were microinjected with total or fractionated poly(A+) RNA isolated from frog urinary bladder epithelial cells. After several (3-5) days at 18 degrees C, the urea flux was assayed by measuring the uptake and efflux of [14C]urea in water-injected and mRNA-injected oocytes. A 2 to 3-fold increase of urea transport was detected in oocytes injected either with total mRNA or with a 6-10 kilobase mRNA fraction, when compared with water-injected oocytes. This expression of urea channels was inhibited by 0.1 mM phloretin (50% inhibition) and 0.1 mM nitrophenylthiourea (up to 70% inhibition). On the contrary, no expression was detected in brain mRNA-injected oocytes. These results show the specific functional expression of the phloretin- and NPTU-sensitive urea channel (or carrier) from frog urinary bladder epithelial cells, providing an approach for the expression cloning of these urea channels.

The polarized distribution of poly(A+)-mRNA-induced functional ion channels in the Xenopus oocyte plasma membrane is prevented by anticytoskeletal drugs

Foreign mRNA was expressed in Xenopus laevis oocytes. Newly expressed ion currents localized in defined plasma membrane areas were measured using the two-electrode voltage clamp technique in combination with a specially designed chamber, that exposed only part of the surface on the oocytes to channel agonists or inhibitors. Newly expressed currents were found to be unequally distributed in the surface membrane of the oocyte. This asymmetry was most pronounced during the early phase of expression, when channels could almost exclusively be detected in the animal hemisphere of the oocyte. 4 d after injection of the mRNA, or later, channels could be found at a threefold higher density at the animal than at the vegetal pole area. The pattern of distribution was observed to be similar with various ion channels expressed from crude tissue mRNA and from cRNAs coding for rat GABAA receptor channel subunits. Electron microscopical analysis revealed very similar microvilli patterns at both oocyte pole areas. Thus, the asymmetric current distribution is not due to asymmetric surface structure. Upon incubation during the expression period in either colchicine or cytochalasin D, the current density was found to be equal in both pole areas. The inactive control substance beta-lumicolchicine had no effect on the asymmetry of distribution. Colchicine was without effect on the amplitude of the expressed whole cell current. Our measurements reveal a pathway for plasma membrane protein expression endogenous to the Xenopus oocyte, that may contribute to the formation and maintenance of polarity of this highly organized cell.

Comparison of a Na+/D-glucose cotransporter from rat intestine expressed in oocytes of Xenopus laevis with the endogenous cotransporter

Epithelial Na+/D-glucose cotransport was incorporated into the plasma membrane of Xenopus oocytes after microinjection of poly(A)(+)-mRNA from rat intestine tissue and was detected by measurements of uptake of [14C]AMG (methyl alpha-D-glucopyranoside). In mRNA-injected oocytes, the rate of AMG uptake exceeds the rate of endogenous Na+/AMG cotransport by a factor of up to 30. It is demonstrated that the additionally expressed transport differs qualitatively from the endogenous transport with respect to several parameters which is a prerequisite for the demonstration of expression of a foreign transporter: (1) The expressed system is more sensitive to external glucose or AMG and to the specific inhibitor phlorizin, (2) it is less sensitive to external Na+ and to changes in membrane potential, and (3) it is susceptible to inhibition by monoclonal antibodies, known to bind specifically to Na+/glucose cotransporters and to modulate the cotransport in kidney and intestine. The use of the antibodies allows one to distinguish between endogenous Na+/AMG cotransport and foreign cotransport expressed by injection of foreign mRNA. The expression of the foreign transport leads to transport rates that are high enough to detect the electrical current generated by the Na+/glucose cotransport. This allows future characterization of the cotransport system under voltage-clamp conditions by analyzing membrane current.

The vacuolar protein-targeting signal of yeast carboxypeptidase is functional in oocytes from Xenopus laevis

Carboxypeptidase Y, a yeast vacuolar glycoprotein was expressed in oocytes from Xenopus laevis and its biosynthesis and sorting were examined. In yeast, targeting to the vacuole, the functional equivalent of the lysosome, is not mannose-6-phosphate-receptor dependent. It was found that carboxypeptidase enters the secretory pathway of the oocyte and is there glycosylated, phosphorylated in the carbohydrate part and delivered to the lysosome. Deletion of an amino acid sequence, previously shown to determine intracellular targeting of this enzyme in yeast, caused a loss of phosphorylation and mislocalization of carboxypeptidase Y into the oocyte medium. Inhibition of glycosylation of carboxypeptidase by tunicamycin did not lead to its secretion. In-frame fusion of the targeting domain to a secretory yeast glycoprotein, invertase, did not prevent its secretion. However, a hybrid containing 80% carboxypeptidase abolished invertase secretion. The results indicate that the vacuolar protein-targeting signal from yeast carboxypeptidase can, in principal, function in a higher eukaryote.

Manipulations of cholinesterase gene expression modulate murine megakaryocytopoiesis in vitro

Megakaryocytopoiesis was selectively inhibited in cultured murine bone marrow cells by a 15-mer oligodeoxynucleotide complementary to the initiator AUG region in butyrylcholinesterase mRNA. Furthermore, conditioned medium from Xenopus oocytes producing recombinant butyrylcholinesterase stimulated megakaryocytopoiesis. These observations implicate butyrylcholinesterase in megakaryocytopoiesis and suggest application of oligodeoxynucleotides for modulating bone marrow development.

Two types of intrinsic muscarinic responses in Xenopus oocytes. I. Differences in latencies and 45Ca efflux kinetics

Oocytes of 40% of Xenopus laevis frogs respond to acetylcholine (ACh). Oocytes of the majority of responders exhibit the common two-component depolarizing muscarinic response (mean amplitude of the rapid component, 54 nA). Oocytes of approximately 10% of the responders ("variant" donors) exhibit a muscarinic response characterized by a very large transient, rapid current (mean amplitude 1242 nA, reversal potential -33 mV). Responses in oocytes of variant donors exhibit further qualitative differences: pronounced desensitization (absent in oocytes of common donors), characteristic prolonged latency (5.4 vs 0.9 s in oocytes of common donors) and marked inhibition of the response by activators of protein kinase C. Rapid responses in oocytes of variant donors are usually increased by treatment with collagenase, which, in common oocytes, often results in a complete loss of the response that correlates with the loss of muscarinic ligand binding. The number of muscarinic receptors was similar in oocytes of both types of donors (2.2 vs 3.0 fmol/oocyte). Also, the responses of oocytes of variant donors to microinjections of CaCl2 or inositol 1,4,5-trisphosphate were similar to those found in cells of common donors. These findings imply that altered receptor number, calcium stores and/or chloride channel density are not responsible for the variant responses. However, ACh caused an sixteen-fold greater efflux of 45Ca in oocytes of variant donors (35 vs 2.2% of total label in oocytes of common donors). Hence, the characteristics of the variant response may be related to a more efficient coupling between receptor stimulation and the mobilization of cellular calcium.

Aspartate-70 to glycine substitution confers resistance to naturally occurring and synthetic anionic-site ligands on in-ovo produced human butyrylcholinesterase

The "atypical" allelic variant of human butyrylcholinesterase (BuChE) can be characterized by its failure to bind the local anesthetic dibucaine, the muscle relaxant succinylcholine, and the naturally occurring steroidal alkaloid solanidine, all assumed to bind to the charged anionic site component within the normal BuChE enzyme. A single nucleotide substitution conferring a change of aspartate-70 into glycine was recently reported in the CHE gene encoding BuChE from several individuals having the "atypical" BuChE phenotype, whereas in two other DNA samples, this mutation appeared together with a second alteration conferring a change of serine-425 into proline. To separately assess the contribution of each of these mutations toward anionic site interactions in BuChE, three transcription constructs were engineered with each of these substitutions alone or both of them together. Xenopus oocyte microinjection of normal or mutated synthetic BuChEmRNA transcripts was employed in conjunction with biochemical analyzes of the resultant recombinant BuChE variants. The presence of the Gly-70 mutation alone was found to render the enzyme resistant to 100 microM solanidine and 5 mM succinylcholine; concentrations sufficient to inhibit the "normal," Asp-70 containing BuChE by over 50%. Furthermore, when completely inhibited by the organophosphorous poison diisopropylfluorophosphate (DFP), Gly-70 BuChE failed to be reactivated by 10 mM of the cholinesterase-specific oxime pyridine 2-aldoxime methiodide (2-PAM); a concentration restoring about 50% of activity in the "normal" Asp-70 recombinant enzyme. The Pro-425 mutation alone had no apparent influence on BuChE interactions with any of these ligands. However, it conferred synergistic effects on some of the anionic site changes induced by the Gly-70 mutation.(ABSTRACT TRUNCATED AT 250 WORDS)

Amylase synthesis as a simple model system for translation and hybrid arrest in Xenopus oocytes

A human alpha-amylase-encoding cDNA has been cloned in a transcription vector. When messenger RNA (mRNA) made in vitro from this construct was injected into Xenopus oocytes, amylase (AMY) activity was detected both in oocyte homogenates and in the incubation medium, indicating that the oocyte machinery correctly translated and processed the protein. Because AMY activity is easy to detect with a blue-starch assay, this expression system was used to determine the parameters of antisense oligodeoxyribonucleotide (oligo) inhibition of translation in the oocytes. Unique oligos complementary to the AMY mRNA sequence were effective in arresting translation, at approximately stoichiometric levels. Mixed oligos also inhibited translation, at levels that suggest that some mismatches may be tolerated in the formation of DNA-RNA hybrids. The AMY system provides a convenient probe of oocyte protein synthesis and processing machinery and can serve as a control substrate in investigations of other mRNAs.

Manipulations of cholinesterase gene expression modulate murine megakaryocytopoiesis in vitro

Megakaryocytopoiesis was selectively inhibited in cultured murine bone marrow cells by a 15-mer oligodeoxynucleotide complementary to the initiator AUG region in butyrylcholinesterase mRNA. Furthermore, conditioned medium from Xenopus oocytes producing recombinant butyrylcholinesterase stimulated megakaryocytopoiesis. These observations implicate butyrylcholinesterase in megakaryocytopoiesis and suggest application of oligodeoxynucleotides for modulating bone marrow development.

Different-sized mRNAs from GH4C1 cells induce a TRH-dependent electrical response in Xenopus laevis oocytes

Poly(A)+ RNA from the GH4C1 rat pituitary cell line elicited a thyrotropin releasing hormone response in Xenopus laevis oocytes which could be measured as a change in membrane current by the voltage-clamp method. Oocytes injected with Poly(A)+ RNA from GH12C1 cells which do not bind thyrotropin releasing hormone or with buffer solution alone did not show this response. Size fractionation of total poly(A)+ RNA by sucrose density-gradient centrifugation shows two response maximal representing various mRNA fractions larger than 18S. These results indicate the presence of thyrotropin releasing hormone receptor mRNA heterogeneity where the smallest mRNA is at least 2 kb.

Altered patterns of N-linked glycosylation of the Torpedo acetylcholine receptor expressed in Xenopus oocytes

The nicotinic acetylcholine receptor (AChR) from Torpedo electroplax is an oligomeric transmembrane glycoprotein made up of four highly homologous subunits in a stoichiometry of alpha 2 beta gamma delta. The role of N-linked glycosylation of the AChR has been studied in several cell lines and these studies have suggested that the addition of carbohydrate may be important for receptor expression. While Xenopus oocytes have proven to be an invaluable tool for studying the AChR, little is known about N-linked glycosylation of the oocyte-expressed receptor. The present report demonstrates that the oocyte-expressed AChR is glycosylated and contains the same number of oligosaccharide residues per subunit as the native receptor. However, unlike the native Torpedo receptor which contains both high mannose and complex oligosaccharides, the oocyte-expressed AChR contains only high mannose oligosaccharide modifications. However, as has been well documented, the Torpedo AChR expressed in oocytes is fully functional, demonstrating that the precise nature of the oligosaccharide modification is not critical for receptor function. The role of the oligosaccharide component of the AChR in receptor function was examined using tunicamycin (TM) to inhibit N-linked protein glycosylation. TM treatment resulted in a 70-80% inhibition of AChR expression in oocytes. Functional, unglycosylated receptors were not expressed; receptors expressed in TM-treated oocytes were functional wild-type, glycosylated AChR, formed only during the initial 12 hr of TM exposure. These data suggest that while glycosylation of the oocyte-expressed Torpedo AChR is required for assembly of subunits into a functional receptor, as has been demonstrated in other cells, oocyte modification of normal Torpedo glycosylation patterns does not affect receptor function or assembly.

Calcium wave evoked by activation of endogenous or exogenously expressed receptors in Xenopus oocytes

The mRNA encoding the cloned substance K receptor was microinjected into Xenopus laevis oocytes. After expression of the mRNA, Ca2+ was imaged in the oocytes with a digital imaging fluorescence microscopy system using the Ca2(+)-sensitive dyes fura-2 and fluo-3. Application of substance K caused a dose-related wave of Ca2+ mobilization to spread from a focus and to elevate the Ca2+ concentration in the oocyte. Activation of endogenous muscarinic or angiotensin II receptors in noninjected oocytes evoked a similar response. The Ca2+ rise in oocytes induced by substance K was due to internal Ca2+ mobilization and was independent of external Ca2+, since it occurred in Ca2(+)-free medium fortified with 2 mM EGTA. The Ca2+ imaging was well correlated with ion current measurements of voltage-clamped oocytes. Imaging, in addition to detecting the spatial spread of Ca2+ across the cell, was at least as sensitive as voltage clamping and much faster when screening oocytes for the expression of receptor mRNAs that stimulate Ca2+ mobilization. While it is known that fertilization of Xenopus eggs causes a spreading wave of Ca2+ mobilization, we found that activation of either native or newly expressed receptors in oocytes causes a similar change in Ca2+ distribution.

trans activation of rat phosphoenolpyruvate carboxykinase (GTP) gene expression by micro-coinjection of rat liver mRNA in Xenopus laevis oocytes

To study the liver-specific trans activation of the rat phosphoenolpyruvate carboxykinase (PEPCK) gene, the PEPCK promoter was linked to a reporter gene and was microinjected into Xenopus laevis oocytes alone or in conjunction with rat liver poly(A)+ RNA. The rat liver mRNA markedly enhanced the expression of the PEPCK-chimeric construct. This effect appeared to be sequence specific, as it was dependent on the presence of the intact promoter. Moreover, the RNA effect was limited to mRNA preparations from PEPCK-expressing tissues only. Finally, microinjection of size-fractionated liver mRNA revealed that the trans-acting factor(s) is encoded by RNA of 1,600 to 2,000 nucleotides, providing a direct bioassay for the gene(s) involved in this tissue-specific trans-activation process.

Expression and characterization of the intestinal Na+/glucose cotransporter in COS-7 cells

Cells derived from the simian kidney, COS-7 cells, were transfected with a eucaryotic expression vector (pEUK-C1) containing the clone for the rabbit intestinal Na+/glucose cotransporter. Expression was monitored after transfection with lipofectin by measuring the initial rate of alpha-methylglucopyranoside (MeGlc) uptake. Cells transfected with vector containing the cDNA for the Na+/glucose cotransporter expressed Na(+)-dependent MeGlc transport. Neither control cells nor cells transfected with vector lacking cloned cDNA expressed the cotransporter. Na(+)-dependent MeGlc uptake into transfected cells was saturable (Km 150 microM), phlorizin-sensitive (Ki 11 microM), and inhibited by sugar analogs (D-glucose greater than MeGlc greater than D-galactose greater than 3-O-methyl-D-glucoside greater than D-allose much greater than L-glucose). Europium was able to mimic Na+ in driving MeGIC uptake. Finally, tunicamycin, an inhibitor of asparagine-linked glycosylation, inhibited the expression of Na(+)-dependent MeGlc transport 80%. We conclude that the rabbit intestinal Na+/glucose cotransporter expressed in COS-7 cell exhibits very similar kinetic properties to that in the native brush border and to that expressed in Xenopus oocytes. In addition, N-linked glycosylation appears to be important for functional expression of this membrane protein.

trans activation of rat phosphoenolpyruvate carboxykinase (GTP) gene expression by micro-coinjection of rat liver mRNA in Xenopus laevis oocytes

To study the liver-specific trans activation of the rat phosphoenolpyruvate carboxykinase (PEPCK) gene, the PEPCK promoter was linked to a reporter gene and was microinjected into Xenopus laevis oocytes alone or in conjunction with rat liver poly(A)+ RNA. The rat liver mRNA markedly enhanced the expression of the PEPCK-chimeric construct. This effect appeared to be sequence specific, as it was dependent on the presence of the intact promoter. Moreover, the RNA effect was limited to mRNA preparations from PEPCK-expressing tissues only. Finally, microinjection of size-fractionated liver mRNA revealed that the trans-acting factor(s) is encoded by RNA of 1,600 to 2,000 nucleotides, providing a direct bioassay for the gene(s) involved in this tissue-specific trans-activation process.

Efficient functioning of plant promoters and poly(A) sites in Xenopus oocytes

Mature Xenopus oocytes were challenged with DNA constructs including plant regulatory elements, namely, the Cauliflower mosaic virus (CaMV) 35S promoter as well as the nopaline synthase (NOS) promoter and polyadenylation signal. The bacterial chloramphenicol acetyl transferase (CAT) was used as a reporter gene. When microinjected into these cells, the plant-derived DNA constructs effectively promoted CAT synthesis in a manner dependent on the presence of the plant promoters and probably also on the polyadenylation signals. Structural studies revealed that the supercoiled structures of the above DNA plasmids were much more active in supporting CAT synthesis in microinjected oocytes than their linear forms, with clear correlation between efficient gene expression and DNA topology. In contrast, the linear forms of these plasmids were considerably more active than the supercoiled ones in transfected plant protoplasts. These findings demonstrate, for the first time, the activity of regulatory elements from plant genes in Xenopus oocytes and shed new light on the specific rules applicable for gene expression in plant and animal cells.