Huguette Løvtrup-Rein

Control of cell division and cell differentiation by deoxynucleotides in the early embryo of Xenopus laevis.

It is proposed that the deoxyriboside triphosphates present in the egg of Xenopus laevis support the synchronous cell divisions occurring during the earliest phase of embryonic development and that, as long as synchrony prevails, the cells are prevented from undergoing differentiation. This hypothesis has been tested by injecting deoxyribonucleotides into fertilized eggs. The following effects were observed: 1) the duration of synchrony is prolonged, 2) the morphological development is suppressed, an effect which is greatest when four nucleotides are injected together and 3) the synthesis of all kinds of RNA is inhibited, including the mRNA required for differentiation to occur.

POLARITIES, CELL DIFFERENTIATION AND PRIMARY INDUCTION IN THE AMPHIBIAN EMBRYO

1. Amphibian eggs are spherical, while the embryos are bilaterally symmetrical. The latter is manifested morphologically when gastrulation begins with the formation of the blastopore at a bilaterally symmetrical (vegetal‐dorsal) location on the surface of the embryo. To account for this change in symmetry two polarities (vectors or axes) are required. These need not go through the centre, but if they do, one will go through two poles, called ‘animal’ and ‘vegetal’ in the amphibian embryo, and the other will pass through two points on opposite sides of the egg, one at the ‘dorsal’ and one at the ‘ventral’ side. Together these two polarities define a plane of bilateral symmetry.

DOWN-REGULATION OF ORNITHINE DECARBOXYLASE BY AN INCREASED DEGRADATION OF THE ENZYME DURING GASTRULATION OF XENOPUS LAEVIS

The present study was designed to analyze the regulation of the levels of the polyamines and their biosynthetic enzymes during embryonic development of Xenopus laevis. The activity of ornithine decarboxylase (ODC), a rate-controlling enzyme in polyamine biosynthesis, is elevated until, during gastrulation, there is a precipitous drop in activity. This is not attributable to a decrease in ODC mRNA content and polysome profiles reveal no apparent decrease in ODC message associated with polysomes. ODC synthesis seems to be maintained at a low, relatively constant rate until neurulation whereupon ribosome loading of ODC mRNA increases. During gastrulation the rate of ODC degradation increases dramatically, which can account for the decrease in ODC. S-Adenosylmethionine decarboxylase (AdoMetDC), another rate-controlling enzyme in polyamine biosynthesis, shows a low and constant activity from cleavage to neurulation. Subsequently, the AdoMetDC activity increases dramatically. The changes in AdoMetDC activity parallel the changes in AdoMetDC mRNA levels, suggesting a transcriptional control of AdoMetDC expression during this development period. The activities of ODC and AdoMetDC produce a steady increase in putrescine and spermidine content of the embryo. The spermine content also increases until gastrulation, but then decreases until the tailbud stage.

Changes in the content of ATP and GTP during the development of Xenopus laevis

Abstract The changes in average GTP and ATP content of developing Xenopus laevis embryos have been studied. Both nucleotides increase during the first cleavage stages and decrease thereafter. GTP shows a second maximum during gastrulation. The significance of these changes in relation to cellular processes like DNA-synthesis, formation of microtubules and micro-filaments, etc., is discussed.

Nuclear RNA and protein synthesis during early sea urchin development.

Abstract RNA and protein synthesis in isolated nuclei from developing embryos of Lytechinus variegatus was investigated between the two-cell stage and the hatched blastula. Both in vivo and in vitro incorporation studies were performed, and comparison between the two types of experiment shows that the observed labelling of nuclear RNAs cannot be attributed to a transfer of RNA from cytoplasm to nucleus during mitosis, or to cytoplasmia contaminants. Both RNA and protein synthesis increase gradually from the two-cell stage. Messenger RNA can be detected from the first mitosis and rRNA appears soon thereafter. Protein synthesis increase simultaneously in an exponential way. The questions of the onset of RNA and protein synthesis and of the participation of the newly-made messengers are discussed.

Changes in activity of the regulatory glycolytic enzymes and of the pyruvate-dehydrogenase complex during the development of xenopus laevis

In this study we investigated the variations of the maximal activities of the rate-controlling glycolytic enzymes (i.e., hexokinase, HK; phosphofructokinase, PFK; pyruvate kinase, PK) and of the pyruvate-dehydrogenase complex (PDHc) during the early embryogenesis of Xenopus laevis (from cleavage through hatching). All the enzymatic assays, using different coupled reactions, were performed spectrophotometrically on cytosolic and mitochondrial fractions. The maximal HK activity increases markedly from neurulation onwards, PFK activity presents a peak around gastrulation, PK activity remains relatively constant throughout the period studied and the highest PDHc activity is observed during cleavage. The specific activities display the same temporal pattern. Furthermore, in the sequence of reactions by which glucose is degraded to form acetyl-CoA, the maximal activities of PFK and PK are not limiting while those of HK and PDHc could be rate-limiting at relatively late developmental stages (hatching).

Changes in Activity of Mitochondrial Enzymes during the Development of Xenopus laevis

Changes in activity of mitochondrial enzymes were studied during the embryonic development of Xenopus laevis.

Changes in energy metabolism during the early development of Xenopus laevis.

Xenopus embryos at various development stages were incubated in the presence of labelled substrates and the 14CO2 production determined. From the rates of oxidation of glucose labelled in positions 1 and 6 and from that of radioactive acetate, pyruvate and glutamate, it was concluded that the Embden-Meyerhof pathway and the Krebs cycle are functional during early embryogenesis, but that their relative participation in the metabolic processes is limited and increases from gastrulation onwards. Early development is characterized by the predominance of the pentose cycle and the glutamate-aspartate cycle. Furthermore, it was shown that glutamate may be the main energy source up to gastrulation.

Changes in the content of cyclic AMP and cyclic GMP during the development of Xenopus laevis.

Abstract The changes in the average cAMP and cGMP content of developing Xenopus laevis embryos have been studied. Both nucleotides exhibited complicated patterns of change. It is possible, however, to correlate the changes in the ratio between the nucleotides during the early phases of development with the current notion that a low ratio is a distinction of mitotically active cells.

Lactate — a suppressor of diffenrentiation in embryonic cells

All kinds of visible differentiation in cultured amphibian embryonic cells are suppressed in the presence of lactate. This effect differs in certain respects from that observed with actinomycin, but like the latter lactate inhibits RNA synthesis. In this communication we present some further observations on the effect of lactate.