Growing Xenopus oocytes have spare translational capacity

Using fluorometry and ion-sensitive microelectrodes to study the functional expression of heterologously-expressed ion channels and transporters in Xenopus oocytes

The Xenopus laevis oocyte is a model system for the electrophysiological study of exogenous ion transporters. Three main reasons make the oocyte suitable for this purpose: (a) it has a large cell size (approximately 1mm diameter), (b) it has an established capacity to produce-from microinjected mRNAs or cRNAs-exogenous ion transporters with close-to-physiological post-translational modifications and actions, and (c) its membranes contain endogenous ion-transport activities which are usually smaller in magnitude than the activities of exogenously-expressed ion transporters. The expression of ion transporters as green fluorescent protein fusions allows the fluorometric assay of transporter yield in living oocytes. Monitoring of transporter-mediated movement of ions such as Cl(-), H(+) (and hence base equivalents like OH(-) and HCO(3)(-)), K(+), and Na(+) is achieved by positioning the tips of ion-sensitive microelectrodes inside the oocyte and/or at the surface of the oocyte plasma membrane. The use of ion-sensitive electrodes is critical for studying net ion-movements mediated by electroneutral transporters. The combined use of fluorometry and electrophysiology expedites transporter study by allowing measurement of transporter yield prior to electrophysiological study and correlation of relative transporter yield with transport rates.

Molecular cloning and expression of a novel human aquaporin from adipose tissue with glycerol permeability

In a systematic analysis of genes expressed in human adipose tissue, we detected a novel gene that is expressed uniquely in adipose tissue. The sequence showed that it encodes a 342-amino-acid protein containing six putative transmembrane domains, and is a new member of the aquaporin family of water-selective membrane channels. We named this gene aquaporin 9. It features a cyclic-AMP protein kinase phosphorylation consensus site in the NH3-terminal domain. Expression of the cRNA in Xenopus oocytes yielded a 7-fold increase in osmotic water permeability blocked by 0.3 mM HgCl2, and also facilitated the uptake of glycerol. Northern blot analysis demonstrated that the mRNA is abundant in adipose tissue, but not in other tissues. Thus, this gene product may participate in glycerol transport in adipocytes.

Differential stability of Xenopus c-myc RNA during oogenesis in axolotl Involvement of the 3' untranslated region in vivo

We have used the axolotl oocyte (Ambystoma mexicanum Shaw) to study the stability of exogenously injected Xenopus RNAs. Three different cellular developmental stages have been analysed: (1) the growing oocyte (stage III-IV of vitellogenesis), (2) the full-grown oocyte at the end of vitellogenesis (stage VI) and (3) the progesterone-matured stage VI oocyte. Three exogenous RNAs have been synthesized in vitro from a c-myc Xenopus cDNA clone. One transcript is 2.3 kb long (full length), the second is 1.5 kb long, with most of the 3' untranslated region (3'UTR) removed, and the third corresponds to the 3'UTR (0.8 kb). After injection or coinjection of these exogenous Xenopus RNAs into axolotl oocytes, the stability of the molecules was studied after 5 min, 6 h and 21 h by extraction of total RNA and Northern blot analysis.Results show a difference in Xenopus RNA stability during axolotl oogenesis. In growing oocytes, the three synthetic transcripts are gradually degraded. The absence of the 3'UTR is not therefore sufficient to stabilize the transcript during early oogenesis. No degradation is observed in full-grown oocytes, suggesting the existence of stabilizing factors at the end of oogenesis. When stage VI oocytes are induced to mature by progesterone, only the 2.3 and 1.5 kb Xenopus RNAs disappear. This suggests a role for germinal vesicle breakdown in this degradation process as well as the existence of a factor present in the nucleus and involved in the specific destabilization of these RNAs after oocyte maturation. This degradation might implicate several destabilizing sequences localized in the coding or in the 3'UTR of the c-myc gene. In contrast, the 0.8 kb transcript (3'UTR) is not degraded during this period and remains very stable. Therefore, degradation appears distinct from one transcript to another and from one region to another within the same molecule. During maturation, the behaviour of the 2.3 and 1.5 kb transcripts is different when coinjected with the 3'UTR, suggesting a role in trans of this untranslated molecule in c-myc stability. Our approach allows us to analyse the role of the coding and 3'UTR regions of the c-myc RNA in the control of mRNA degradation in vivo.

Evidence that XR family interspersed RNA may regulate translation in Xenopus oocytes

It has been shown that about two thirds of Xenopus oocyte or sea urchin egg cytoplasmic poly(A)+ RNA contains interspersed repetitive sequences. The functional significance of this interspersed RNA has remained unknown. Here the function of a subfamily of interspersed RNA (XR family; McGrew and Richter, 1989: Dev Biol 134:267-270) in Xenopus oocytes was studied. We found that the elimination of T7 XR (one of the two complementary strands of the XR repeat) interspersed RNA by complementary oligodeoxynucleotides significantly inhibited protein synthesis. On the other hand, the injection of in vitro synthesized T7 XR RNA stimulated translation. Moreover, the insertion of the T7 XR RNA sequence into globin mRNA repressed the translation of the globin mRNA. In order to explain these results, we analyzed interactions between the XR interspersed RNA and oocyte proteins. We found that the major XR RNA binding proteins were p56 and p60, which could be the known mRNA "masking" proteins that bind mRNA and inhibit translation. Further, a 42 kD protein has been identified that appears to bind T7 XR RNA relatively specifically, although it interacts with mRNA with a lower affinity. Based on all of these data, we have proposed that interspersed RNA may be involved in regulating translation by competing with mRNA to interact with certain proteins that can regulate translation.

Heterologous in vivo processing of human preproendothelin 1 into bioactive peptides

Endothelin (ET) is an extremely potent vasoconstrictor peptide of 21 amino acids, originally found in the supernatant of cultured vascular endothelial cells. To gain insights into its biosynthetic pathway, we expressed a synthetic RNA coding for the 212-amino acid precursor of human ET-1 (preproET-1) in Xenopus oocytes. Cell homogenates and oocyte incubation medium were tested by RIA using an anti-ET-1 serum. ET-1-like immunoreactivity was detected in oocytes injected with preproET-1 synthetic RNA but not in control oocytes and was much higher in medium than in cell homogenates. When preproET-1 was expressed in oocytes treated with monensin, a dramatic decrease in secretion of immunoreactive material was observed, indicating that secretion is mediated by the Golgi complex. ET-1-like immunoreactive material present in oocyte incubation medium was fractionated by reverse-phase HPLC into two main peaks, corresponding to the retention times of human big ET-1 and ET-1. Incubation medium of oocytes expressing the synthetic preproET-1 RNA elicited a characteristic vasoconstrictor response on rabbit vena cava, consistent with the biological activity that would be predicted from the amount of ET-1-like immunoreactivity measured. These results suggest that common pathways of ET maturation exist in widely different cells and that Xenopus oocytes may represent a useful tool in studying the cell biology of ET-1 synthesis.

Transformation of the amphibian oocyte into the egg: structural and biochemical events

Amphibian oocytes, arrested in prophase I, are stimulated to progress to metaphase II by progesterone. This process is referred to as meiotic maturation and transforms the oocyte, which cannot support the early events of embryogenesis, into the egg, which can. Meiotic maturation entails global reorganization of cell ultrastructure: In the cell cortex, the plasma membrane flattens and the cortical granules undergo redistribution. In the cell periphery, the annulate lamellae disassemble and the mitochondria become dispersed. In the cell interior, the germinal vesicle becomes disassembled and the meiotic spindles form. Marked changes in the cytoskeleton and mRNA distribution also occur throughout the cell. All of these events are temporally correlated with intracellular signalling events: Fluctuations in cAMP levels, changes in pH, phosphorylation and dephosphorylation, and ion flux changes. Evidence suggests that specific intracellular signals are responsible for specific reorganizations of ultrastructure and mRNA distribution.

H-ras(val12) induces cytoplasmic but not nuclear events of the cell cycle in small Xenopus oocytes

Microinjection of H-ras(val12) protein into fully grown Xenopus oocytes has been shown to induce meiotic maturation. In the present study, mRNA encoding the mutant ras protein was injected into both fully grown (stage 6) and growing (stage 4) oocytes. The mRNA induced nuclear breakdown in stage 6 oocytes, as expected. However, the mRNA induced neither nuclear breakdown nor maturation promoting factor when injected into stage 4 oocytes. Instead, the response in stage 4 oocytes included an activation pulse of calcium, cortical granule breakdown, elevation of the vitelline envelope, and abortive cleavage furrows, all of which are characteristics of the activation response in mature eggs. In addition, the injected mRNA led to increased rates of endogenous protein synthesis and the migration of subcortical organelles into the oocyte interior. These observations are discussed relative to the suggestion that oncogenic ras protein leads to an increase in both diacylglycerol and inositol trisphosphate, which then regulate the various cytoplasmic events described.

Expression of functional cell-cell channels from cloned rat liver gap junction complementary DNA

An oocyte expression system was used to test the relation between a complementary DNA (cDNA) clone encoding the liver gap junction protein and cell-cell channels. Total liver polyadenylated messenger RNA injected into oocytes induced cell-cell channels between paired oocytes. This induction was blocked by simultaneous injection of antisense RNA transcribed from the gap junction cDNA. Messenger RNA selected by hybridization to the cDNA clone and translated in oocyte pairs yielded a higher junctional conductance than unselected liver messenger RNA. Cell-cell channels between oocytes were also formed when the cloned cDNA was expressed under the control of a heat-shock promoter. A concentration-dependent induction of channels was observed in response to injection with in vitro transcribed gap junction messenger RNA. Thus, the liver gap junction cDNA encodes a protein that is essential for the formation of functional cell-cell channels.

The use of Xenopus oocytes for the study of ion channels

Recently, in addition to the "traditional" research on meiotic reinitiation and fertilization mechanisms, the oocytes of the African frog Xenopus laevis have been exploited for the study of numerous aspects of ion channel function and regulation, such as the properties of several endogenous voltage-dependent channels and the involvement of second messengers in mediation of neurotransmitter-evoked membrane responses. In addition, injection of these cells with exogenous messenger RNA results in production and functional expression of foreign membranal proteins, including various voltage- and neurotransmitter-operated ion channels originating from brain, heart, and other excitable tissues. This method provides unique opportunities for the study of the structure, function, and regulation of these channels. A multidisciplinary approach is required, involving molecular biology, electrophysiology, biochemistry, pharmacology, and cytology.