A satellite repeat-derived piRNA controls embryonic development of Aedes.

Tandem repeat elements such as the diverse class of satellite repeats occupy large parts of eukaryotic chromosomes, mostly at centromeric, pericentromeric, telomeric and subtelomeric regions1. However, some elements are located in euchromatic regions throughout the genome and have been hypothesized to regulate gene expression in cis by modulating local chromatin structure, or in trans via transcripts derived from the repeats2-4. Here we show that a satellite repeat in the mosquito Aedes aegypti promotes sequence-specific gene silencing via the expression of two PIWI-interacting RNAs (piRNAs). Whereas satellite repeats and piRNA sequences generally evolve extremely quickly5-7, this locus was conserved for approximately 200 million years, suggesting that it has a central function in mosquito biology. piRNA production commenced shortly after egg laying, and inactivation of the more abundant piRNA resulted in failure to degrade maternally deposited transcripts in the zygote and developmental arrest. Our results reveal a mechanism by which satellite repeats regulate global gene expression in trans via piRNA-mediated gene silencing that is essential for embryonic development.

The Tudor protein Veneno assembles the ping-pong amplification complex that produces viral piRNAs in Aedes mosquitoes.

PIWI-interacting RNAs (piRNAs) comprise a class of small RNAs best known for suppressing transposable elements in germline tissues. The vector mosquito Aedes aegypti encodes seven PIWI genes, four of which are somatically expressed. This somatic piRNA pathway generates piRNAs from viral RNA during infection with cytoplasmic RNA viruses through ping-pong amplification by the PIWI proteins Ago3 and Piwi5. Yet, additional insights into the molecular mechanisms mediating non-canonical piRNA production are lacking. TUDOR-domain containing (Tudor) proteins facilitate piRNA biogenesis in Drosophila melanogaster and other model organisms. We thus hypothesized that Tudor proteins are required for viral piRNA production and performed a knockdown screen targeting all A. aegypti Tudor genes. Knockdown of the Tudor genes AAEL012437, Vreteno, Yb, SMN and AAEL008101-RB resulted in significantly reduced viral piRNA levels, with AAEL012437-depletion having the strongest effect. This protein, which we named Veneno, associates directly with Ago3 in an sDMA-dependent manner and localizes in cytoplasmic foci reminiscent of piRNA processing granules of Drosophila. Veneno-interactome analyses reveal a network of co-factors including the orthologs of the Drosophila piRNA pathway components Vasa and Yb, which in turn interacts with Piwi5. We propose that Veneno assembles a multi-protein complex for ping-pong dependent piRNA production from viral RNA.

Posaconazole inhibits multiple steps of the alphavirus replication cycle.

Repurposing drugs is a promising strategy to identify therapeutic interventions against novel and re-emerging viruses. Posaconazole is an antifungal drug used to treat invasive aspergillosis and candidiasis. Recently, posaconazole and its structural analog, itraconazole were shown to inhibit replication of multiple viruses by modifying intracellular cholesterol homeostasis. Here, we show that posaconazole inhibits replication of the alphaviruses Semliki Forest virus (SFV), Sindbis virus and chikungunya virus with EC50 values ranging from 1.4 µM to 9.5 µM. Posaconazole treatment led to a significant reduction of virus entry in an assay using a temperature-sensitive SFV mutant, but time-of-addition and RNA transfection assays indicated that posaconazole also inhibits post-entry stages of the viral replication cycle. Virus replication in the presence of posaconazole was partially rescued by the addition of exogenous cholesterol. A transferrin uptake assay revealed that posaconazole considerably slowed down cellular endocytosis. A single point mutation in the SFV E2 glycoprotein, H255R, provided partial resistance to posaconazole as well as to methyl-ß-cyclodextrin, corroborating the effect of posaconazole on cholesterol and viral entry. Our results indicate that posaconazole inhibits multiple steps of the alphavirus replication cycle and broaden the spectrum of viruses that can be targeted in vitro by posaconazole, which could be further explored as a therapeutic agent against emerging viruses.

PIWI proteomics identifies Atari and Pasilla as piRNA biogenesis factors in Aedes mosquitoes.

As in most arthropods, the PIWI-interacting RNA (piRNA) pathway in the vector mosquito Aedes aegypti is active in diverse biological processes in both soma and germline. To gain insights into piRNA biogenesis and effector complexes, we mapped the interactomes of the somatic PIWI proteins Ago3, Piwi4, Piwi5, and Piwi6 and identify numerous specific interactors as well as cofactors associated with multiple PIWI proteins. We describe the Piwi5 interactor AAEL014965, the direct ortholog of the Drosophila splicing factor pasilla. We find that Ae. aegypti Pasilla encodes a nuclear isoform and a cytoplasmic isoform, the latter of which is required for efficient piRNA production. In addition, we characterize a splice variant of the Tudor protein AAEL008101/Atari that associates with Ago3 and forms a scaffold for PIWI proteins and target RNAs to promote ping-pong amplification of piRNAs. Our study provides a useful resource for follow-up studies of somatic piRNA biogenesis, mechanism, and function in Aedes mosquitoes.