Receptor-mediated endocytosis in Xenopus oocytes. I. Characterization of the vitellogenin receptor system

We have investigated the vitellogenin (VTG) receptor system in Xenopus oocytes since these cells are specialized for endocytosis. Oocytes have between 0.2 and 3 X 10(11) receptors per 1-mm cell. There is only a single class of receptors of low affinity (1.3 X 10(-6) M at 22 degrees C and 2-4 X 10(-6) M at 0 degree C), but high specificity (less than 5% nonspecific binding at 2 X 10(-6) M). The specific internalization rate of the VTG receptor (around 2 X 10(-3) s-1) is first order, highly variable, and at the upper end of the range of values reported for mammalian cells. The receptor association rate constant (9.6 X 10(2) M-1 s-1) is extremely low although the dissociation rate constant was immeasurable. Calcium is required for VTG binding, and low pH does not dissociate the VTG-receptor complex. Monensin treatment at 100 microM caused the loss of surface receptors with a t1/2 of 3 h and the accumulation of internalized ligand in a "pre-lysosomal" endocytic compartment. Conversely, the recovery of surface VTG receptors that were removed with trypsin occurred with a t1/2 of about 2 h. These observations indicate that oocytes have very large intracellular pools of receptors and that although surface receptors are internalized on the time scale of minutes, the intracellular pool is recycled on the time scale of hours.     

An unusual lysosome compartment involved in vitellogenin endocytosis by Xenopus oocytes

We have investigated the lysosomal compartment of Xenopus oocytes to determine the possible role of this organelle in the endocytic pathway of the yolk protein precursor, vitellogenin. Oocytes have lysosome-like organelles of unusual enzymatic composition at all stages of their development, and the amount of hydrolase activity increases steadily throughout oogenesis. These unusual lysosomes appear to be located primarily in a peripheral zone of oocyte cytoplasm. At least two distinct populations of lysosomal organelles can be identified after sucrose density gradient fractionation of vitellogenic oocytes. Most enzyme activity resides in a compartment of large size and high density that appears to be a subpopulation of yolk platelets that are less dense than most platelets within the cell. The appearance of this high density peak of lysosomal enzyme activity coincides with the time of onset of vitellogenin endocytosis during oocyte development. The data suggest that endocytic vesicles that contain vitellogenin fuse with modified lysosomes shortly after their internalization by the oocyte. Pulse-chase experiments with radiolabeled vitellogenin suggest that the ligand passes through the low density platelet compartment en route to the heavy platelets. The accumulation of yolk proteins apparently results from a failure of these molecules to undergo complete digestion after their entry into an unusual lysosomal compartment. The yolk platelets that these proteins finally enter for prolonged storage appear to be a postlysosomal organelle.     

Quantitation of DNase I sensitivity in Xenopus chromatin containing active and inactive globin, albumin and vitellogenin genes.

The disappearance of defined restriction fragments of the beta 1-globin, an albumin and the A1 vitellogenin gene was quantitated after DNase I digestion and expressed by a sensitivity factor defined by a mathematical model. Analysis of naked DNA showed that the gene fragments have similar but not identical sensitivity factors. DNase I digestion of chromatin revealed for the same gene fragments sensitivity factors differing over a much wilder range. This is correlated to the activity of the genes analyzed: the beta 1-globin gene fragment is more sensitive to DNase I in chromatin of erythrocytes compared to hepatocytes whereas the albumin gene fragment is more sensitive to DNase I in chromatin of hepatocytes. The A1 vitellogenin gene has the same DNase I sensitivity in both cell types. Comparing the DNase I sensitivity of the three genes in their inactive state we suggest that different chromatin conformations may exist for inactive genes.     

Differential sensitization to deoxyribonuclease I of Xenopus vitellogenin and albumin genes during primary and secondary induction of vitellogenesis by oestradiol.

The sensitivity to DNAase (deoxyribonuclease) I (which preferentially digests transcribed sequences) of vitellogenin and albumin genes in liver and erythrocytes of male Xenopus after primary and secondary induction of vitellogenesis by oestrogen was measured by hybridization to cDNA (complementary DNA) of the residual DNA after enzymic digestion of isolated nuclei. Vitellogenin sequences were rendered selectively more sensitive to limited DNAase-I digestion (15-20% of DNA rendered acid-soluble) during primary hormonal activation (5 days) of vitellogenin genes in liver, but not erythrocyte, nuclei. Hormone withdrawal (25 days after first injection) did not result in reversion to a pre-activation gene configuration, nor did secondary hormonal stimulation (5 days after second and 25 days after first injection) augment the sensitivity of the genes to digestion by the nuclease. Similar hormone treatment did not affect the sensitivity of the constitutively expressed albumin genes in liver nuclei, nor their insensitivity in erythrocyte nuclei. Under the same conditions, globin genes remained indigestible in liver nuclei. It is concluded that primary induction of vitellogenesis in male Xenopus liver is accompanied by relatively long-lasting (3-4 weeks) change in the configuration of vitellogenin genes in hepatic nuclei which is not reversed or further modified during short-term oestrogen withdrawal or upon secondary stimulation.     

Multivesicular bodies play a key role in vitellogenin endocytosis by Xenopus oocytes

A combination of electron microscopic tracers and subcellular fractionation has been used to examine the endocytic pathway of the yolk protein precursor, vitellogenin (VG), in Xenopus oocytes. VG was adsorbed to colloidal gold, and the organelles traversed by newly internalized ligand were examined at various time intervals after endocytosis. VG-Au enters oocytes via coated pits and vesicles and then appears rapidly in tubular endosomes and multivesicular bodies (MVBs). MVBs play a central role in VG processing for storage; the large majority of newly internalized VG enters this compartment, remaining there for up to several hours. Condensation of VG into crystalline bodies begins in MVBs, and continues with growth of the crystals until typical platelets are formed. When oocytes are exposed to high [VG], MVBs containing large amounts of internalized VG are morphologically indistinguishable from the primordial yolk platelets described earlier (Dumont, 1978). The use of VG-Au particles of two sizes demonstrates that gold particles in early MVBs were generally associated with the limiting membrane of these organelles, while older MVB compartments have gold particles well separated from the limiting membranes, suggesting that dissociation of VG from its receptor occurs in this compartment. Newly internalized ligand preferentially forms a new MVB, rather than fusing and mixing with previously formed MVBs. Progressive yolk protein condensation gradually transforms MVBs into yolk platelets over a period of several hours. Analysis of 125I-VG-Au behavior after sucrose gradient fractionation of oocytes allowed correlation of biochemical compartments with those observed in the electron microscope. MVBs containing yolk in progressive stages of condensation were found at densities from 1.16 up to 1.21 g/cc. The final, rate-limiting step in VG transport is a shift of ligand from light (1.21 g/cc) to heavy (1.23 g/cc) platelet compartments (Wall and Meleka, 1985). The morphological correlate of this process is movement of VG-Au from small (less than 3-4 microns diameter) to large (greater than 4 microns diameter) platelets.     

Characterization of polysomes from Xenopus liver synthesizing vitellogenin and translation of vitellogenin and albumin messenger RNA's in vitro.

1. Conditions are described for the isolation of polysomes from the liver of Xenopus laevis. The method involves homogenization of liver in 0.2 M Tris-HCl pH 8.5, treatment with 2% Triton X-100 and subsequent sucrose density gradient fractionation of polysomes from a 10000 X g supernatant. 2. Vitellogenin synthesis was induced in male Xenopus liver by oestradiol treatment. Polysomes were isolated and vitellogenin-synthesizing polysomes characterized by their association with monospecific 125 I-labelled rabbit anti-vitellogenin antibody and by reaction with rabbit anti-vitellogenin immunoglobulins followed by indirect immunoprecipitation with goat anti-rabbit antibody. 3. Changes in liver polysome content following oestrogen treatment of male Xenopus are correlated with the appearance of vitellogenin synthesis using an organ culture assay. 4. RNA extracted from livers of oestradiol-treated male Xenopus and from purified polysomes is shown to code for the synthesis of vitellogenin-specific immunoprecipitable polypeptides in a rabbit reticulocyte cell-free protein-synthesizing system, a major component having a molecular weight of 210000. Xanopus liver RNA is also shown to code for the synthesis of an albumin-specific immunoprecipitable polypeptide of 74000 molecular-weight which coelectrophoresed with Xenopus albumin.     

Ferromagnetic isolation of endosomes involved in vitellogenin transfer into Xenopus oocytes

The transport pathway of the yolk precursor vitellogenin (VTG) has been followed using the techniques of ferrolabeling and ferromagnetic sorting, coupled with electron microscopic visualization. Vitellogenin conjugated to colloidal ferric particles of ca. 11 nm is selectively transported from the oolemma to the yolk platelets of vitellogenic Xenopus oocytes after gonadotropin stimulation of the female. Several cortical membrane compartments, labeled or unlabeled with ferric particles, are involved in the internalization and the transfer of vitellogenin to the yolk platelets. 1) Coated pits apparently fuse with coated vesicles, and coated vesicles fuse with each other in the outermost cortical cytoplasm. 2) Vesicles, depleted of their clathrin coat, fuse with cortical tubular endosomes and discharge their contents into yolk endosomes. 3) These endosomes are the direct precursors of the yolk organelles. 4) Endocytic vesicles fuse only with primordial yolk platelets of type I and not with type II or fully grown yolk platelets. After pulse-chase loading with ferric particles conjugated to vitellogenin and subsequent subcellular fractionation of the oocytes, ferromagnetic sorting of the various vesicle populations has been performed by using a "free-flow magnetic chamber". This novel method enables specification and characterization of purified endosomal compartments that accumulate protein yolk in Xenopus oocytes.     

The human estrogen receptor can regulate exogenous but not endogenous vitellogenin gene promoters in a Xenopus cell line.

Transfection of a human estrogen receptor cDNA expression vector (HEO) into cultured Xenopus kidney cells confers estrogen responsiveness to the recipient cells as demonstrated by the hormone dependent expression of co-transfected Xenopus vitellogenin-CAT chimeric genes. The estrogen stimulation of these vit-CAT genes is dependent upon the presence of the vitellogenin estrogen responsive element (ERE) in their 5' flanking region. Thus, functional human estrogen receptor (hER) can be synthesized in heterologous lower vertebrate cells and can act as a trans-acting regulatory factor that is necessary, together with estradiol, for the induction of the vit-CAT constructs in these cells. In addition, vitellogenin minigenes co-transfected with the HEO expression vector also respond to hormonal stimulation. Their induction is not higher than that of the vit-CAT chimeric genes. It suggests that in the Xenopus kidney cell line B 3.2, the structural parts of the vitellogenin minigenes do not play a role in the induction process. Furthermore, no stabilizing effect of estrogen on vitellogenin mRNA is observed in these cells. In contrast to the transfected genes, the endogenous chromosomal vitellogenin genes remain silent, demonstrating that in spite of the presence of the hER and the hormone, the conditions necessary for their activation are not fulfilled.     

Expression of the GABAB receptor in Xenopus oocytes and inhibition of the response by activation of protein kinase C.

The functional GABAB receptor was expressed in Xenopus oocytes by injecting mRNA obtained from the cerebellum of the rat. Application of GABA in the presence of bicuculline induced a hyperpolarization under current-clamp conditions and an outward current under voltage-clamp conditions. Baclofen mimicked the effect of GABA in the presence of bicuculline, and the effect of baclofen was antagonized by phaclofen. The GABA-induced outward current was slightly inhibited by treatment with GDP-beta-S and was completely inhibited by treatment with GTP-gamma-S. The activation of protein kinase C by 12-O-tetradecanoylphorbol-13-acetate (TPA), but not 4 alpha-phorbol-12,13-didecanoate, suppressed the GABAB receptor-mediated hyperpolarization, and the effect of TPA was antagonized by sphingosine. Thus, activation of protein kinase C inhibits the expressed GABAB receptor-mediated response.     

Activation of Stat3 by cytokine receptor gp130 ventralizes Xenopus embryos independent of BMP-4

Stat3 is one of the main signaling components of cytokine receptors, including gp130. Here we show that activation of cytokine receptor gp130 resulted in a dramatic ventralization of Xenopus embryos and that the ventralization correlated well with Stat3 activation potential of the receptor. This finding led to identification of Xenopus Stat3 (Xstat3), which showed a 95% homology to its murine and human counterparts, at the amino acid level, and was expressed from the one-cell stage throughout development. The mechanism of gp130/XStat3-mediated ventralization proved to be independent of BMP-4. gp130/Xstat3 stimulation inhibited Smad2-induced ectopic axis formation in embryos and Smad2-dependent luciferase activity. A dominant-negative Stat3, in contrast, dorsalized Xenopus embryos, resulting in ectopic axis formation. We propose that Stat3-mediated signaling has the capacity to modify dorsoventral patterning in the early development of Xenopus.     

Localized measurement of kinase activation in oocytes of Xenopus laevis.

We have combined a rapid cytoplasmic sampling technique with capillary electrophoresis to measure the activation of protein kinase C (PKC) in a small region (approximately 60 microm) of a Xenopus oocyte. The phosphorylation of a fluorescent PKC substrate was measured following addition of a pharmacological or physiological stimulus to an oocyte. When substrates for cdc2 kinase (cdc2K), PKC, and protein kinase A (PKA) were comicroinjected into an oocyte, all three substrates could be identified on the electropherogram after cytoplasmic sampling. With this new method, it should be possible to measure simultaneously the activation of multiple different kinases in a single cell, enabling the quantitative dissection of signal transduction pathways.