Final Report Summary - DROSOPIRNAS (The piRNA pathway in the Drosophila germline a small RNA based genome immune system)
Selfish genetic elements such as transposons are a nearly universal component of eukaryotic genomes. To limit their uncontrolled spread and to minimize the mutational burden originating from their transposition plants, fungi, and animals have evolved potent transposon silencing systems. In all three kingdoms, small RNA silencing pathways play a central role herein. The piRNA pathway is the major protective system in animal gonads. It is essentially a small RNA based immune system and it encompasses a genomically encoded transposon sequence memory and an adaptive branch that amplifies silencing competent small RNAs (piRNAs), which target Argonaute proteins of the PIWI clade to transposon transcripts in order to initiate their silencing.
The ERC Starting grant project 'DROSOpiRNAs' aimed at understanding the conceptual logic and the mechanistic basis of the piRNA pathway in Drosophila ovaries. We focused on systematic approaches and combined genetic screens with genome wide profiling of piRNA populations, gene expression patterns and chromatin states. Our work led to major insight at the three core levels of the pathway. First, we identified key concepts of piRNA biogenesis, a cytoplasmic process that parses 22-30nt long RNAs out of long single stranded precursors. We identified most of the protein players acting in this process, provided a genetic hierarchy of their action and discovered several of the molecular steps that underlie piRNA biogenesis. Second, we demonstrated that the nuclear Piwi-piRNA complex silences its targets at the transcriptional level by inducing hetero-chromatin formation. We identified a handful of proteins that are essential for this process and began to unravel the hierarchical steps that occur downstream of Piwi-recruitment to a piRNA complementary nascent target RNA. Finally, we provided key insight into the biology of the genomic piRNA origins, the so-called piRNA clusters. Our work demonstrates that piRNA clusters come in two flavors and that chromatin plays central roles in the biology of so-called dual strand clusters, which are transcribed on both genomic strands.
Taken together, this work provides major insight into the logic of an evolutionarily conserved small RNA silencing process in the animal germline. Our findings provide promising entry points into future projects such as the mechanistic dissection of piRNA biogenesis, the elucidation of the precise silencing pathway acting on chromatin or the basis of piRNA cluster transcription within heterochromatin.
The ERC Starting grant project 'DROSOpiRNAs' aimed at understanding the conceptual logic and the mechanistic basis of the piRNA pathway in Drosophila ovaries. We focused on systematic approaches and combined genetic screens with genome wide profiling of piRNA populations, gene expression patterns and chromatin states. Our work led to major insight at the three core levels of the pathway. First, we identified key concepts of piRNA biogenesis, a cytoplasmic process that parses 22-30nt long RNAs out of long single stranded precursors. We identified most of the protein players acting in this process, provided a genetic hierarchy of their action and discovered several of the molecular steps that underlie piRNA biogenesis. Second, we demonstrated that the nuclear Piwi-piRNA complex silences its targets at the transcriptional level by inducing hetero-chromatin formation. We identified a handful of proteins that are essential for this process and began to unravel the hierarchical steps that occur downstream of Piwi-recruitment to a piRNA complementary nascent target RNA. Finally, we provided key insight into the biology of the genomic piRNA origins, the so-called piRNA clusters. Our work demonstrates that piRNA clusters come in two flavors and that chromatin plays central roles in the biology of so-called dual strand clusters, which are transcribed on both genomic strands.
Taken together, this work provides major insight into the logic of an evolutionarily conserved small RNA silencing process in the animal germline. Our findings provide promising entry points into future projects such as the mechanistic dissection of piRNA biogenesis, the elucidation of the precise silencing pathway acting on chromatin or the basis of piRNA cluster transcription within heterochromatin.