Evidence for a nuclear factor involved in c-myc RNA degradation during axolotl oocyte maturation

Coupled amplification and degradation of exogenous RNA injected in amphibian oocytes

The early development of amphibians takes place in the absence of significant transcription and is controlled at the post-transcriptional level. We have reported that in vitro synthesized transcripts injected into axolotl fertilized eggs or oocytes were not continuously degraded as their abundance apparently fluctuated over time, with detected amounts sometimes higher than initial injected amounts. To further characterize this phenomenon, we have co-injected RNA chain terminators to prevent RNA synthesis. This led to the suppression of fluctuations and to a regular decrease in the amount of transcripts that appeared to be more stable in the presence of inhibitors. These observations indicate a coupling between RNA synthesis and an accelerated degradation. Throughout the time course, cRNA molecules could be detected, and their abundance increased in the early phase of the kinetics, supporting the implication of an RNA-dependent RNA polymerase in an asymmetric amplification process. Finally, when the fate of the injected transcripts was investigated in individual oocytes, we observed an absolute increase in abundance in some but not all oocytes, supporting the existence of a limiting step in the initiation of the RNA amplification stochastic process.

Injection of exogenous RNA in amphibian oocytes leads to RNA level fluctuations which are sensitive to cordycepin, an RNA chain elongation terminator

Using an in vivo heterologous system to study the stability of Xenopus laevis RNA injected into axolotl (Ambystoma mexicanum) fertilized eggs, we have previously observed unexpected fluctuations in RNA level during early development [Andéol et al., Differentiation 63 (1998) 69-79]. In this study, we further characterize this phenomenon and establish its existence during axolotl and Xenopus oogenesis, suggesting a phylogenetically conserved mechanism. The phenomenon can occur with a variety of exogenous sense and antisense substrates. RNase protection experiments establish that most of the molecules have the same polarity as the initially injected RNA. In addition, trace amount of complementary RNA (cRNA) can be detected the injected samples. Cordycepin prevent increases in RNA levels indicating the involvement of an RNA synthesis. These results indicate the existence of an in vivo post-transcriptional RNA amplification mechanism during the early development of amphibians.

Evidence for multiple sequences and factors involved in c-myc RNA stability during amphibian oogenesis

To investigate the molecular mechanisms regulating c-myc RNA stability during late amphibian oogenesis, a heterologous system was used in which synthetic Xenopus laevis c-myc transcripts, progressively deleted from their 3' end, were injected into the cytoplasm of two different host axolotl (Ambystoma mexicanum) cells: stage VI oocytes and progesterone-matured oocytes (unfertilized eggs; UFE). This in vivo strategy allowed the behavior of the exogenous c-myc transcripts to be followed and different regions involved in the stability of each intermediate deleted molecule to be identified. Interestingly, these specific regions differ in the two cellular contexts. In oocytes, two stabilizing regions are located in the 3' untranslated region (UTR) and two in the coding sequence (exons II and III) of the RNA. In UFE, the stabilizing regions correspond to the first part of the 3' UTR and to the first part of exon II. However, in UFE, the majority of synthetic transcripts are degraded. This degradation is a consequence of nuclear factors delivered after germinal vesicle breakdown and specifically acting on targeted regions of the RNA. To test the direct implication of these nuclear factors in c-myc RNA degradation, an in vitro system was set up using axolotl germinal vesicle extracts that mimic the in vivo results and confirm the existence of specific destabilizing factors. In vitro analysis revealed that two populations of nuclear molecules are implicated: one of 4.4-5S (50-65 kDa) and the second of 5.4-6S (90-110 kDa). These degrading nuclear factors act preferentially on the coding region of the c-myc RNA and appear to be conserved between axolotl and Xenopus. Thus, this experimental approach has allowed the identification of specific stabilizing sequences in c-myc RNA and the temporal identification of the different factors (cytoplasmic and/or nuclear) involved in post-transcriptional regulation of this RNA during oogenesis.

Post-transcriptional control of c-myc RNA during early development analyzed in vivo with a Xenopus-axolotl heterologous system

We have set up a heterologous in vivo system to study gene regulation at the post-transcriptional level during early development. This system uses two amphibian species, Xenopus laevis and Ambystoma mexicanum (axolotl), the development of which is three to four times slower than that of X. laevis. The stability of three different synthetic X. laevis c-myc transcripts was followed after injection into fertilized axolotl eggs. One transcript is 2.2 kilobases (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 (0.7-kb). The behavior of the endogenous axolotl c-myc RNA was compared with the exogenous injected c-myc transcripts. Our results show the existence of several developmental timers controlling degradation of the c-myc molecules. The first is activated at oocyte maturation and affects both the endogenous and exogenous (2.2- and 1.5-kb) transcripts containing the coding regions. A second timer could be linked to the number of cell divisions since fertilization (6th-7th cleavages) and involves the endogenous c-myc RNAs. Another timer could involve the c-myc mRNA molecule itself, because when injected into axolotl eggs, the half-life of the 2.2-kb X. laevis transcript appears to be independent of the axolotl context. After injection into axolotl fertilized eggs, the behavior of this X. laevis full-length c-myc molecule reveals an unexpected increase in the intensity of its autoradiographic signals. This increase occurs independently of events linked to mid-blastula transition and preliminary investigations are discussed.