Translation and stability of human globin mRNA in Xenopus oocytes

The role of the IGF-I system for vitellogenesis in maturing female sterlet, Acipenser ruthenus Linnaeus, 1758

Transition from previtellogeneic to vitellogenic oocyte growth is a critical phase for folliculogenesis in sturgeon and may often be postponed for several years. Recent findings on the involvement of insulin-like growth factor I (IGF-I) in cell differentiation processes of oocyte follicle and ovarian steroidogenesis of teleosts in vitro led to the hypothesis that paracrine IGF-I could function as a potential trigger in vivo. For the first time, IGF-I and its corresponding receptor (IGF-IR) were identified in a non-teleostean fish. Real-time PCR assays for IGF-I and IGF-IR mRNA were established, normalising mRNA expression of the target genes to beta-microglobulin (beta2m). We clearly show that expression of IGF-I in the gonad is a substantial source for IGF-I-mediated effects in follicles compared to liver, brain, muscle and adipose tissue. Among these tissues, IGF-IR mRNA was highest in the gonad. With regard to different cohorts of coexisting follicles, highest expression of IGF-I and IGF-IR were met in developing follicles, indicating that IGF-I functions as an intraovarian modulator of follicle faith. Comparing previtellogenic follicles in females that matured within two years with non-maturing females f the same age, revealed an increases of 2.3-fold for IGF-I and 2.8-fold for IGF-IR mRNA expression in maturing females. These findings implicate an important role of paracrine IGF-I in early vitellogenesis and identify it as candidate vitellogenesis inducing factor (VIF), determining the faith of the follicle.

Different faces of the heme-heme oxygenase system in inflammation

The heme-heme oxygenase system has recently been recognized to possess important regulatory properties. It is tightly involved in both physiological as well as pathophysiological processes, such as cytoprotection, apoptosis, and inflammation. Heme functions as a double-edged sword. In moderate quantities and bound to protein, it forms an essential element for various biological processes, but when unleashed in large amounts, it can become toxic by mediating oxidative stress and inflammation. The effect of this free heme on the vascular system is determined by extracellular factors, such as hemoglobin/heme-binding proteins, haptoglobin, albumin, and hemopexin, and intracellular factors, including heme oxygenases and ferritin. Heme oxygenase (HO) enzyme activity results in the degradation of heme and the production of iron, carbon monoxide, and biliverdin. All these heme-degradation products are potentially toxic, but may also provide strong cytoprotection, depending on the generated amounts and the microenvironment. Pre-induction of HO activity has been demonstrated to ameliorate inflammation and mediate potent resistance to oxidative injury. A better understanding of the complex heme-heme

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

The basic properties of mRNA-injected Xenopus oocytes as a heterologous system for the production of biologically active proteins will be reviewed. The advantages and limitations involved in the use of this in ovo system will be discussed, as compared with in vitro cell-free translation systems and with in vivo microinjected mammalian cells in culture. The different assay systems that have been utilized for the identification of the biological properties of oocyte-produced proteins will be described. This section will review the determination of properties such as binding of natural ligands, like heme or alpha-bungarotoxin; immunological recognition by antibodies; subcellular compartmentalization and/or secretion; various enzymatic catalytic activities; and induction in ovo of biological activities that affect other living cells in culture, such as those of interferon and of the T-cell receptor. The limitations involved in interpretation of results obtained using mRNA-injected oocytes will be critically reviewed. Special attention will be given to the effect of oocyte proteases and of changes in the endogenous translation rate on quantitative measurements of oocyte-produced proteins. In addition, the validity of the various measurement techniques will be evaluated. The various uses of bioassays of proteins produced in mRNA-injected Xenopus oocytes throughout the last decade will be reviewed. Nuclear and cytoplasmic injections, mRNA and protein turnover measurements and abundance calculations, and the use of in ovo bioassays for molecular cloning experiments will be discussed in this section. Finally, potential future uses of the oocyte system in various fields of research, such as immunology, neurobiology, and cell biology will be suggested.

Translational stability of native and deadenylylated rabbit globin mRNA injected into HeLa cells

HeLa human cells were injected with a natural mixture of rabbit alpha and beta globin mRNA. They were incubated for 6 hr with [35S]methionine either immediately after injection or 20 hr later. The labeled proteins in the injected cells were analyzed by fluorography of two-dimensional electrophoresis gels. By using this procedure, it was possible to show that, during the first few hours after injection, both alpha and beta globin molecules are synthesized with an alpha to beta ratio approximately equal to 0.6. The rate of synthesis of alpha globin decreased significantly faster than that of beta globin over a 26-hr period after injection of the two mRNAs. It thus seems that two messenger RNAs coding for closely related polypeptides possess a markedly different translational stability. When deadenylylated rabbit globin mRNAs were injected into HeLa cells, no globin synthesis could be detected by the techniques used. We conclude that the translational half-life of mRNAs lacking poly(A) is very short in these cells. It is thus clear that the poly(A) segment is required to ensure stability to globin mRNA in somatic cells as in Xenopus oocytes.

Unequal alpha and beta globin mRNA in reticulocytes of normal and mutant f/f fetal mice

Messenger RNA (mRNA) was isolated from adult as well as normal and mutant f/f fetal mouse reticulocytes by poly-U affinity chromatography. mRNA from normal adult reticulocytes directed equal synthesis of alpha and beta globin chains in wheat germ cell free translational system. mRNA from either normal or mutant f/f fetal reticulocytes directed 40% more alpha globin synthesis than beta globin synthesis. These observations are suggestive that there is significantly more alpha mRNA than beta mRNA in normal and mutant f/f fetal mouse reticulocytes.

Myeloproliferative syndrome with sideroblastic anemia and acquired hemoglobin H disease

Hemoglobin H disease usually occurs as a result of inheritance of the genes for alpha thalassemia; however, occasionally patients acquire hemoglobin H in association with hematologic malignancy. This report concerns a 63-year-old Filipino man with a myeloproliferative syndrome with marked thrombocytosis and apparently acquired hemoglobulin H disease. The patient had hemolytic anemia, dimorphic red blood cells (RBC) and abundant ringed sideroblasts in the marrow. The peripheral blood contained 27% hemoglobin H and about two-thirds of his RBC had hemoglobin H inclusion bodies. There was no previous history of anemia or evidence of thalassemia in two siblings or nine adult children of the patient. In vitro studies of globin chain synthesis documented markedly decreased production of alpha globin with alpha/beta biosynthetic ratios of 0.05 in peripheral blood reticulocytes and 0.10 in bone marrow cells. The relative concentration of mRNA for alpha globin was approximately 20-fold less than that of beta globin, apparently accounting for the deficiency in alpha globin synthesis.

mRNA-directed biosynthesis of alpha subunit of thyrotropin: translation in cell-free and whole-cell systems

mRNA from mouse thyrotropic pituitary tumors was translated in frog oocytes (a whole-cell system) and in wheat germ extract and reticulocyte lysate (cell-free systems) in the presence of [(35)S]methionine. Synthesized peptides related to thyrotropin were identified in the three systems by immunoprecipitation with subunit-specific antisera developed against the alpha subunit of ovine lutropin (luteinizing hormone) and the beta subunit of bovine thyrotropin. In wheat germ extract and reticulocyte lysate, a single immunoprecipitable form of the alpha subunit of thyrotropin was synthesized with an apparent molecular weight of 14,000 by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. In the frog oocyte, three forms of immunoprecipitable alpha subunit of thyrotropin were synthesized with apparent molecular weights of 20,000, 14,000, and 10,000. The 20,000 form is similar to unlabeled rat pituitary standard alpha subunit and (35)S-labeled mouse tumor alpha subunit in cell cultures (20,000-21,000); thus, it may represent a precursor-cleaved and glycosylated form. The 14,000 form synthesized in all three systems probably represents the pre-alpha subunit of thyrotropin; the 10,000 form, synthesized only in the frog oocyte, could be a proteolytically cleaved but unglycosylated form. Because only the alpha subunit of thyrotropin was identified and no larger molecular weight immunoprecipitable form of either subunit was detected in any of the translation systems, alpha and beta subunits of thyrotropin appear to be translated from separate mRNAs.

Synthesis of lens crystallins in Xenopus oocytes as determined by quantitative immunoprecipitation

Total poly(A)-containing calf lens mRNA was microinjected into Xenopus oocytes and synthesis of alpha, beta, and gamma-crystallins was demonstrated. By a method of quantitative immunoprecipitation the rate of translation of purified 14S alphaA2-crystallin mRNA was compared with translation of 9-S rabbit globin mRNA. Maximal response of oocytes was obtained with virtually the same molar amounts of mRNA, taking into account the larger size of the alphaA2-crystallin mRNA. Kinetics of translation were also very similar and both mRNAs were translated with similar rate and efficiency for at least two days. It was estimated that 20-30 polypeptide chains per hour per mRNA molecule were synthesized.

Relative stability of alpha- and beta-globin messenger RNAs in homozygous beta+ thalassemia

The relative concentrations of alpha-, beta-, and gamma-globin mRNA sequences were measured in bone marrow nuclear and cytoplasmic RNA and in RNA from peripheral blood reticulocytes of three patients with homozygous beta+ thalassemia. Our results suggest that the quantitative deficiency in beta-globin mRNA may arise because of abnormal metabolism of molecules containing beta mRNA sequences. Complementary DNAs specific for each of the globins were synthesized. Variable quantities of RNA were incubated to equilibrium with 3H-labeled alpha- and 32P-labeled beta- or gamma-enriched cDNA. We found for each of the patients that the alpha/beta mRNA sequence ratio was more nearly normal in the nuclear RNA than in either cytoplasmic or reticulocyte RNA. Conversely, gamma mRNA sequences were very low in the nucleus with an increase in the relative concentration in both cytoplasm and reticulocyte RNA. The thermal stability of nucleic acid duplexes formed between beta cDNA and nuclear RNA from one patient with beta+ thalassemia was equivalent to that of duplexes formed with normal nuclear RNA. Approximately equal amounts of thalassemic alpha and beta mRNA were retained by oligo(dT)-cellulose, indicating that the 3' poly(A) segment was present on both. Our results indicate that beta-globin mRNA, although grossly normal in structure, fails to accumulate in beta+ thalassemic erythroid cells in amounts equivalent to the mRNA for alpha-globin.