Mighty Piwis defend the germline against genome intruders

Abstract

Piwis are a germline-specific subclass of the Argonaute family of RNA interference (RNAi) effector proteins that are associated with a recently discovered group of small RNAs (piRNAs). Recent studies in Drosophila and zebrafish directly implicate Piwi proteins in piRNA biogenesis to maintain transposon silencing in the germline genome (Brennecke et al., 2007; Gunawardane et al., 2007; Houwing et al., 2007). This function may be conserved in mice as loss of Miwi2, a mouse Piwi homolog, leads to germline stem cell and meiotic defects correlated with increased transposon activity (Carmell et al., 2007).

Figure 1. piRNAs control mobile elements in the Drosophila germline

piRNAs are generated from specific loci throughout the Drosophila germline genome. Two examples are depicted. The Flamenco and X-TAS loci are located in heterochromatic regions on the X chromosome. Hundreds of distinct piRNAs are produced from each of these loci and correspond to mobile element repeats dispersed throughout the genome. According to the current model, piRNAs associate with Piwi proteins in the germline and serve as guides that lead to cleavage of transposon targets. Flamenco is known to control expression of the gypsy, Idefix, and ZAM retroelements, and X-TAS has been linked to the control of P-elements.

Figure 2. Amplification loop model of piRNA biogenesis

Piwi-mediated cleavage events generate new piRNAs, thereby setting up a self-reinforcing amplification cycle. The cycle begins with processing of primary piRNAs, which are derived from defective transposon copies in regions of heterochromatin (labeled A–E). As Brennecke and colleagues propose, primary piRNA complexes may be maternally inherited. Piwi proteins exhibit slicer activity and cleave targets between nucleotides 10 and 11 from the 5′ end of piRNAs. An unidentified endonuclease cleaves the 3′ end of piRNA precursors. Primary piRNAs are antisense to expressed transposons and bind either Piwi or Aubergine proteins. Together, the Piwi/Aub-piRNA complexes identify and cleave their transposon target trancripts, generating new sense piRNAs that bind the Ago3 protein. This secondary piRNA-Ago3 complex directs a second cleavage event of another piRNA cluster transcript, which creates a new antisense piRNA capable of binding to Piwi or Aubergine. The cycle continues as long as secondary piRNAs are able to recognize and cleave their target transposon elements, generating new piRNAs. Amplification can occur whenever transcription of transposons and/or pre-piRNA transcripts pumps additional unprocessed RNAs into the system (heavy light blue arrows). This cycle has the potential to generate an uncontrolled positive feedback loop, thus it must be regulated somehow. Potentially, accumulation of unprocessed piRNA precursors (i.e. in response to diminished transposon RNA production) might dampen the piRNA response.