Studies on Ribonucleic Acid Synthesis in Tadpole Liver during Metamorphosis Induced by Thyroxine

Metabolomic insights into system-wide coordination of vertebrate metamorphosis

Background

After completion of embryogenesis, many organisms experience an additional obligatory developmental transition to attain a substantially different juvenile or adult form. During anuran metamorphosis, the aquatic tadpole undergoes drastic morphological changes and remodelling of tissues and organs to become a froglet. Thyroid hormones are required to initiate the process, but the mechanism whereby the many requisite changes are coordinated between organs and tissues is poorly understood. Metabolites are often highly conserved biomolecules between species and are the closest reflection of phenotype. Due to the extensive distribution of blood throughout the organism, examination of the metabolites contained therein provides a system-wide overview of the coordinated changes experienced during metamorphosis. We performed an untargeted metabolomic analysis on serum samples from naturally-metamorphosing Rana catesbeiana from tadpoles to froglets using ultraperformance liquid chromatography coupled to a mass spectrometer. Total and aqueous metabolite extracts were obtained from each serum sample to select for nonpolar and polar metabolites, respectively, and selected metabolites were validated by running authentic compounds.

Results

The majority of the detected metabolites (74%) showed statistically significant abundance changes (p_adj < 0.001) between metamorphic stages. We observed extensive remodelling of five core metabolic pathways: arginine and purine/pyrimidine, cysteine/methionine, sphingolipid, and eicosanoid metabolism and the urea cycle, and found evidence for a major role for lipids during this postembryonic process. Metabolites traditionally linked to human disease states were found to have biological linkages to the system-wide changes occuring during the events leading up to overt morphological change.

Conclusions

To our knowledge, this is the first wide-scale metabolomic study of vertebrate metamorphosis identifying fundamental pathways involved in the coordination of this important developmental process and paves the way for metabolomic studies on other metamorphic systems including fish and insects.

Nuclear ribonucleoprotein complexes of amphibian liver. I. Characterization of the complex and its small molecular weight RNA moiety

Nuclear RNA-protein complexes containing small molecular weight RNAs were isolated from hepatic nuclei of Rana catesbeiana tadpoles and frogs according to a procedure normally used for the isolation of heterogeneous nuclear ribonucleoprotein complexes from other eukaryotic tissues. Preliminary characterization of the tadpole nuclear RNP indicated a particle size of 50--70 S in sucrose density gradients and a buoyant density of 1.40 gm/ml in CsCl gradients. When analyzed on SDS-polyacrylamide gels, this complex was observed to contain at least 40 polypeptides ranging in molecular weight from 15,000 to 200,000. Nuclear RNA-protein complexes were also isolated from adult frog hepatic nuclei by the same protocol and the RNA moiety which had been purified from the frog complex was compared with the nuclear RNA isolated from the tadpole particles. Electrophoretic analysis of the nuclear RNA-protein-associated RNA revealed minor qualitative and quantitive differences in the more than 25 discrete bands (4--9 S) associated with each particle. Base analysis of tadpole and frog nuclear RNA revealed a nucleotide composition of approximately 50% adenosine plus uridine nucleotides, with an unusually high content of cytosine residues (approximately 30%). Comparison of the two RNA samples demonstrated a large increase in the adenosine content of frog unclear RNA, and the presence of a minor base in frog nuclear RNA which was absent in the tadpole sample. These results indicated that changes in the RNA content of the amphibian nuclear RNP complex had occurred during bullfrog development.