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.

In Duchenne muscular dystrophy (DMD) the absence of dystrophin at the sarcolemma delocalizes and downregulates nitric oxide synthase (nNOS); this alters S-nitrosylation of HDAC2 and its chromatin association. We show that the differential HDAC2 nitrosylation state in Duchenne versus wild-type conditions deregulates the expression of a specific subset of microRNA genes. Several circuitries controlled by the identified microRNAs, such as the one linking miR-1 to the G6PD enzyme and the redox state of cell, or miR-29 to extracellular proteins and the fibrotic process, explain some of the DMD pathogenetic traits. We also show that, at variance with other myomiRs, miR-206 escapes from the dystrophin-nNOS control being produced in activated satellite cells before dystrophin expression; in these cells, it contributes to muscle regeneration through repression of the satellite specific factor, Pax7. We conclude that the pathway activated by dystrophin/nNOS controls several important circuitries increasing the robustness of the muscle differentiation program.

Every living organism has to constantly face threats from the environment and deal with a large number of pathogens against which it has to defend itself to survive. Among those, viruses represent a large class of obligatory intracellular parasites, which rely on their host machinery to multiply and propagate. As a result, viruses and their hosts have engaged in an ever-evolving arms race to be able to maintain their existence. The role played by micro (mi)RNAs in this ongoing battle has been extensively studied in the past 15 years and will be the subject of this review article. We will mainly focus on cellular miRNAs and their implication during viral infection in mammals. Thus, we will describe current techniques that can be used to identify miRNAs involved in the modulation of viral infection and to characterize their targets and mode of action. We will also present different reported examples of miRNA-mediated regulation of viruses, which can have a positive outcome either for the host or for the virus. In addition, the mode of action is also of a dual nature, depending on the target of the miRNA. Indeed, the regulatory small RNA can either directly guide an Argonaute protein on a viral transcript, or target a cellular mRNA involved in the host antiviral response. We will then see whether and how viruses respond to miRNA-mediated targeting. Finally, we will discuss how our knowledge of viral targeting by miRNA can be exploited for developing new antiviral therapeutic approaches.