23 September

Thesis defense Stefan Redl: Zebrafish germline development in presence and absence of a functional PIWI pathway

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Stefan Redl from the group of René Ketting, a former research group of the Hubrecht Institute currently at the Institute of Molecular Biology in Mainz, Germany, successfully defended his thesis on the 23rd of September. During his PhD, Redl studied the involvement of the PIWI pathway, important for the protection of the genome in germ cells, in germline development in Zebrafish. Redls work is described in his thesis “Zebrafish germline development in presence and absence of a functional PIWI pathway”.

Protecting the DNA
The germ line, the cells that will form the reproductive cells, is established during early embryonic development and needs to be kept safe until reproduction occurs. Our DNA contains transposable elements, small pieces of DNA that can move to another spot in the DNA, or be copied and then insert into the DNA at a random position. One can imagine that the insertion of a transposable element into a gene, or a region that regulates the activity of a gene, can be very harmful to the cell in which this occurs. For normal body cells, this only affects that specific cell and its potential offspring. However, when this occurs in the germ line, this will affect fertility and potentially the entire offspring of the individual. Therefore, a system is in place in germ cells that blocks transposable elements from moving around or being copied: the PIWI pathway (see Figure).

PIWI pathway
The PIWI pathway uses PIWI proteins, members of the Argonaute protein family involved in RNA silencing, and small RNAs, called piRNAs, to recognize and cleave the transcripts of transposable elements, which effectively makes them inactive and protects the germ cells. Redl studied the effects on zebrafish germline development of knocking out two genes, tdrd9 and vasa, which were shown to be involved in the PIWI pathway in other animals. He showed that these proteins are also important in the Zebrafish PIWI pathway and that knocking out these genes results in the loss of all germ cells and therefore sterile zebrafish. Another gene was studied in this context, gtsf1, which is involved in small RNA pathways in other animals. Although interaction with one of the zebrafish Piwi proteins was shown, gtsf1’s role in the PIWI pathway is less clear. Interestingly, gtsf1 mutants do not completely lose germ cells but show a defect in spermiogenesis and develop preferentially as males.

Germ cell development
Another aspect Redl was interested in is the fate of zebrafish germ cells from the time they arrive at the area where the future reproductive organs will develop to the time a definite reproductive organ starts to form. Throughout this time window he could see that early germ cells dramatically change their morphology and their gene expression program. Not only the germ line transcriptional program is initiated but also large regions in between protein coding genes start to become expressed to fuel the zygotic PIWI pathway that is established in these cells.

The PIWI pathway | De PIWI route



René Ketting is a former group leader of the Hubrecht Institute and current group leader and Scientific Director at the Institute of Molecular Biology in Mainz – Germany, and professor at the Johannes Gutenberg University of Mainz.

Banner picture: Early germ cells in the 6 day old zebrafish with the edge of the nucleus stained in green and the zebrafish PIWI protein Ziwi in magenta. Credit: Stefan Redl.