PIE-1: Molecular dissection of a C. elegans germ-cell specific protein in S. pombe

Jury

• Florian STEINER, PhD (Université de Genève)
• Germano CECERE, PhD (Institut Pasteur, Paris)
• René REZSOHAZY, PhD (UCLouvain)
• Xavier DE BOLLE, PhD (UNamur)
• Benoit MUYLKENS, PhD, President (UNamur)

Summary

In animals, germ cells are often specified and segregated from the somatic lineages during early embryogenesis. In some organisms, germline blastomeres appear to adopt a transcriptionally quiescent state during early development.  For example, in C. elegans embryos, while somatic blastomeres activate the RNA polymerase II-mediated transcription at the 4-cell stage, primordial germ cells (PGCs) appear to activate this transcription at the 100-cell stage. This transcriptional repression in C. elegans germline blastomeres was proposed to be mediated by the protein PIE-1 which is exclusively restricted to the germline blastomere from the first embryonic division. PIE-1 was proposed to stall transcription by inhibiting CDK-9, a Cyclin-Dependent Kinase believed crucial for Pol II CTD-Ser2 phosphorylation and transcriptional elongation and termination. However, the completion of embryogenesis in a CTD-S2A mutant strain and the identification of CDK-12 as the primary CTD-Ser2 kinase suggest that this model should be revised.

To explore transcription in C. elegans germline blastomeres, a blastomere sorting approach coupled with RNA-seq was adopted. Pilot studies have first validated this approach on a wild-type strain. An AID (Auxin-Inducible Degron) tagged PIE-1 strain was then used to analyze the effect of PIE-1 depletion on the germline blastomeres transcriptome. Data demonstrate that in the absence of PIE-1, germline blastomeres adopt a “soma-like” transcriptomic profile, confirming the importance of PIE-1 in maintaining the germline fate during embryogenesis.

In parallel studies, the fission yeast, Schizosaccharomyces pombe was used to explore the consequences of ectopic PIE-1 expression. PIE-1 was found to localize to chromatin at transcription end sites and its expression triggered an RNA polymerase II readthrough resulting in pervasive transcription into downstream regions. 

These observations lead to a hypothesis where, in C. elegans germline blastomeres, PIE-1 may regulate the alternative polyadenylation in 3' untranslated regions, resulting in the production of long RNA isoforms that could subsequently be degraded. This degradation could explain the non-detection of newly transcribed RNAs in the germline blastomeres. Without PIE-1, shorter isoforms could be generated, facilitating the accumulation and translation of somatic transcripts. The subsequent production of somatic factors could explain the degradation of maternal RNAs observed in the absence of PIE.

Although further studies in C. elegans are required to validate this hypothesis, it provides an innovative framework to understand PIE-1 function independently of CTD-Ser2 phosphorylation.