LINE1 Transcripts Needed to Get Past 2 Cell Stage in Mammals

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BillTre
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LINE 1 transcripts have been shown to be necessary for mouse (and maybe all mammals?) embryos to go beyond their 2-cell stage in development.
Science mag news article here.
LINE (Long Interspersed Nuclear Elements) 1 are retrotransposons (mobile selfish DNA elements) that are found in eukaryots generally. They make up 17% of the human genome.
LINEs are transcribed into mRNA and translated into protein that acts as a reverse transcriptase. The reverse transcriptase makes a DNA copy of the LINE RNA that can be integrated into the genome at a new site.
In mouse embryos, LINE1 RNA is required for Dux silencing, synthesis of rRNA, and exit from the 2-cell stage.

This is interesting because the genomic "parasite" is actually doing something useful to the organism.
It may be unique to mammals however.

Mammals are unusual among vertebrates in that they divide slowly from the beginning of their development and transcribe genes at these early developmental times.
Many other vertebrates rapidly go through several early divisions (blastual stages) before their rate of division slows down (at the mid-blastula transition) and they start transcription.
Non-mammalian vertebrates produce eggs that are made to go through development independent of their female parent. They rapidly produce a lot of small cells, which then interact to form an increasingly complex embryo with several different kinds of cells. Transcription and making new proteins underlies much of this.
It would be interesting to see what happens in egg laying mammals (monotremes) like the duck-billed platypus or echidna.

This would appear to indicate that this use of LINE 1 transcripts in development was something that was acquired by mammals in their evolution, long after the LINE 1 invasion of the eukaryotic genome.
 

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Ygggdrasil
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Here's a link to the study published in the journal Cell (subscription required):
Percharde et al. 2018 A LINE1-Nucleolin Partnership Regulates Early Development and ESC Identity. Cell. Published Online 21 June 2018
https://www.cell.com/cell/abstract/S0092-8674(18)30655-X

Abstract:
Transposable elements represent nearly half of mammalian genomes and are generally described as parasites, or “junk DNA.” The LINE1 retrotransposon is the most abundant class and is thought to be deleterious for cells, yet it is paradoxically highly expressed during early development. Here, we report that LINE1 plays essential roles in mouse embryonic stem cells (ESCs) and pre-implantation embryos. In ESCs, LINE1 acts as a nuclear RNA scaffold that recruits Nucleolin and Kap1/Trim28 to repress Dux, the master activator of a transcriptional program specific to the 2-cell embryo. In parallel, LINE1 RNA mediates binding of Nucleolin and Kap1 to rDNA, promoting rRNA synthesis and ESC self-renewal. In embryos, LINE1 RNA is required for Dux silencing, synthesis of rRNA, and exit from the 2-cell stage. The results reveal an essential partnership between LINE1 RNA, Nucleolin, Kap1, and peri-nucleolar chromatin in the regulation of transcription, developmental potency, and ESC self-renewal.
Seems like the cell has co-opted the retrotransposons to help regulate certain important developmental genes in the early embryo. This would not be the first example of the cell co-opting selfish genetic elements for its own purposes. The cell has also co-opted sequences derived from endogenous retroviruses to aid in the development of the placenta: http://schaechter.asmblog.org/schae...the-placenta-and-the-genomic-junk-drawer.html

The story seems to be that, if a molecular mechanism exists inside of the cell, the cell will find some way to use that process to regulate gene expression.
 
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