Distinct sets of PIWI proteins produce arbovirus and transposon-derived piRNAs inAedes aegyptimosquito cells

Pascal Miesen,Ronald P Van Rij,Erika Girardi

doi:10.1093/nar/gkv590

Abstract

The PIWI-interacting RNA (piRNA) pathway is essential for transposon silencing in many model organisms. Its remarkable efficiency relies on a sophisticated amplification mechanism known as the ping-pong loop. In Alphavirus-infected Aedes mosquitoes, piRNAs with sequence features that suggest ping-pong-dependent biogenesis are produced from viral RNA. The PIWI family in Aedes mosquitoes is expanded when compared to other model organisms, raising the possibility that individual PIWI proteins have functionally diversified in these insects. Here, we show that Piwi5 and Ago3, but none of the other PIWI family members, are essential for piRNA biogenesis from Sindbis virus RNA in infected Aedes aegypti cells. In contrast, the production of piRNAs from transposons relies on a more versatile set of PIWI proteins, some of which do not contribute to viral piRNA biogenesis. These results indicate that functional specialization allows distinct mosquito PIWI proteins to process RNA from different endogenous and exogenous sources.

Highlights

In the animal kingdom, three major classes of small silencing RNAs exist: microRNAs, small interfering RNAs and PIWI-interacting RNAs [1]
An ∼200 nt large hotspot region for viral piRNAs (vpiRNAs) biogenesis is located in the capsid gene, 300 nt downstream of the Sindbis virus (SINV) subgenomic promoter (Figure 1A)
PiRNAs derived from a Ty3/Gypsy transposon were insensitive to the treatment

Summary

Introduction

Three major classes of small silencing RNAs exist: microRNAs (miRNAs), small interfering RNAs (siRNAs) and PIWI-interacting RNAs (piRNAs) [1]. The primary pathway generates from genomically encoded precursors a pool of primary piRNAs, which are loaded into the PIWI proteins Piwi and Aubergine (Aub) [4] From this initial piRNA collection, the ping-pong loop selectively amplifies Aub-bound piRNAs that recognize transcripts of active transposons [4,5]. Aub-bound piRNAs commonly start with a uridine (1U) and, since target slicing by PIWI proteins occurs between nucleotide 10 and 11, the complementary Ago3-bound piRNAs typically have a 10 nt overlap and contain an adenine at position 10 (10A) [4,5] This specific sequence signature is a hallmark of piRNAs that have been amplified by the ping-pong loop. This specific sequence signature is a hallmark of piRNAs that have been amplified by the ping-pong loop. piRNA amplification was initially thought to occur exclusively in germline tissues, but recently, piRNAs have been detected in somatic cells in several organisms, including various mosquito species [13,14,15,16]

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