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Revealing the mechanisms of genome activation in Ciona germline

A new article from the Christiaen group offers crucial insights into the onset of zygotic gene expression in the germline of Ciona, uncovering a two-step model for genome activation.

Science pic
Left: The onset of zygotic transcription in the Ciona germline is controlled by multiple signaling pathways, following cellular remodeling of PGCs. Right: During a cellular remodeling event, a lobe is removing Pem-1 mRNA from B7.6 cells. Green: Pem-1 mRNA, magenta: lobe and B7.6 cells, blue: DAPI.
Photo:
Naoyuki Ota

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In their latest publication in the journal EMBO Reports, postdoctoral researcher Naoyuki Ota and Michael Sars Centre Director Lionel Christiaen delved into genome activation in the germline of the tunicate Ciona. Germline cells do not produce their own RNA during early embryogenesis, relying instead exclusively on mRNAs provided by the mother during oogenesis. Those maternal RNAs have long garnered attention from researchers, but the precise timing and mechanisms underlying genome activation in these cells have remained elusive - until now.

“We demonstrate that a peculiar cellular remodeling event excludes localized Pem-1 mRNA, which encodes the general inhibitor of transcription”, Naoyuki explained. "We propose a 2-step model for the onset of zygotic transcription in the Ciona germline”. The surprising discovery sees cells shed cytoplasm containing Pem-1 mRNA granules, effectively eliminating it from the young embryo. Shortly after, transcription is activated globally.

The study employs innovative techniques, such as precise DiI labeling via microinjection to trace primordial germ cells. “The microinjection technique has been established in ascidian before, but I guess that only a few researchers are able to use the technique because of its difficulty. I am proud of having this technique as a special skill”, Naoyuki added.

“Many questions remain, opened by the groundbreaking work of Naoyuki, who combined innovative tools and tour-de-force experiments to complete what is poised to become a landmark study for the field” - Lionel Christiaen

In addition, the team found the transcription factor Mef2 as part of the genes activated after the cell remodeling event, and uncovered a role for JAK signaling as an early repressor of Mef2 expression. The study not only uncovers fundamental mechanisms of genome activation but also opens new avenues for further exploration. "Our next challenge is to understand the process of the Pem-1 protein degradation triggering the initiation of the zygotic program in the primordial germ cells," Naoyuki elaborated.

Expressing gratitude to his mentor Lionel Christiaen and his research team for their support throughout the study, Naoyuki underscored the collaborative effort behind the research project. The team’s work lays the groundwork for future studies aimed at unraveling the complexities of early embryo development. “Many questions remain, opened by the groundbreaking work of Naoyuki, who combined innovative tools and tour-de-force experiments to complete what is poised to become a landmark study for the field”, Lionel concluded.