Genomes of all living organisms carry heavy amounts of transposable elements-repeated DNA sequences that are capable of moving from one chromosomal site to another. In fact, these sequences represent 45% of the human genome and 18% of the Drosophila melanogaster genome. Strict control of their expression is necessary to prevent their mobilization that would otherwise generate mutations and rearrangements which can be potentially deleterious to the survival of the host. Specially important is the need to protect genomes of germ cells that give rise to the sperm and the egg. This is ensured by piRNAs, a class of small 23-29 nucleotide RNAs expressed primarily within the gonads.
In the ovaries of the fruit fly Drosophila melanogaster, piRNAs originate from the maturation of long precursor RNAs transcribed from particular genomic regions, usually heterochromatic, called piRNA clusters. Clusters can span several hundred kilobases and are composed of both truncated and full-sized transposable elements. After their maturation in the cytoplasm, the piRNAs are loaded onto an effector protein of the PIWI family. The pi-RISC (RNA-induced silencing) complex thus formed targets and cleaves transposon mRNAs by sequence complementarity, resulting in post-transcriptional silencing. It can also go back into the nucleus to silence the transposable elements at the transcriptional level.
The Chantal Vaury team has been working to unravel the genetic and molecular intricacies of piRNA production for several years now. For example, they have extensively studied flamenco, the major piRNA producing cluster in the follicle cells and often called the ‘master locus’ of piRNA production. They have shown that flamenco transcription is initiated from an RNA Polymerase II promoter and flamenco transcripts are alternatively spliced to generate diverse piRNA precursors (Goriaux C et al, EMBO Rep 2014). In a paper published in Plos One (Dennis C et al 2013), they reported their discovery of DOT COM. Transcripts produced predominantly by flamenco, along with other piRNA clusters expressed in the Drosophila ovarian follicular cells, congregate into this single nuclear focus, seen as a luminescent dot by fluorophore-labeled RNA probes.
The team now shows that the Dot COM corresponds to the site of export of the RNAs precursors to their cytoplasmic maturation machinery called Yb body. Export is provided by the Nxf1-Nxt1 export complex. In the absence of the Nxt1 protein, the precursor RNAs are not exported and are accumulated in Dot COM, piRNA production is lost and transposable elements are expressed. Moreover, in this context, Yb body is not assembled, suggesting that the export of the precursor RNAs constitutes the signal for the assembly of their own maturation machinery. Exporter Nxf1-Nxt1s are also required, along with the exon-junction complex (EJC), for the active nuclear transfer of the flam precursor RNAs from their transcription site to the Dot COM. Interestingly, Dot COM is located at the nuclear periphery of the follicle cells, far from the flam genomic locus and facing the cytoplasmic Yb body.
This new study has been published recently in the journal Nature Communications (Dennis et al., 2016).
This work has been highlighted on the CNRS website (in French)
Dennis C, Brasset E, Sarkar A, Vaury C. Export of piRNA precursors by EJC triggers assembly of cytoplasmic Yb-body in Drosophila. Nat Commun. 2016 Dec 8;7:13739. doi: 10.1038/ncomms13739
Goriaux C, Desset S, Renaud Y, Vaury C, Brasset E. Transcriptional properties and splicing of the flamenco piRNA cluster.EMBO Rep, 2014.
Dennis C, Zanni V, Brasset E, Eymery A, Zhang L, Mteirek R, Jensen S, Rong YS, Vaury C. "Dot COM", a Nuclear Transit Center for the Primary piRNA Pathway in Drosophila. PLoS One. 2013.
Chantal Vaury :firstname.lastname@example.org