INAUGURAL ARTICLE by a Recently Elected Academy Member:From the Cover: microRNA miR-275 is indispensable for blood digestion and egg development in the mosquito Aedes aegypti

The mosquito Aedes aegypti is the major vector for dengue, yellow fever and, recently, chikungunya virus (1). It is an anautogenous mosquito, and the female needs to obtain a blood meal to produce eggs, hence the underlying mechanism for spread of devastating diseases to humans. Therefore, understanding the biology of how this mosquito vector reproduces is vital to devising new approaches to fight devastating diseases brought on by arboviruses. Because of the extensive body of knowledge amassed regarding the reproductive biology of A. aegypti, this mosquito also serves as a model system for investigating blood-meal–initiated events in other arthropod vectors that transmit disease pathogens (2, 3). Central to egg maturation is vitellogenesis, a process of production of massive amounts of yolk protein precursors (YPPs) by the fat body, a tissue analogous to the mammalian liver and adipose tissue. YPPs are secreted into the hemolymph and subsequently accumulated by developing oocytes via receptor-mediated endocytosis (4). After blood feeding, mosquito vitellogenesis is activated by a cooperative action of nutritional amino acid/target of rapamycin (AA/TOR) and steroid hormone (20-hydroxyecdysone, 20E) pathways (5, 6). MicroRNAs (miRNAs) are small noncoding RNAs of approximately 21–24 nt in length that have been shown to be responsible for posttranscriptional regulation of mRNAs in both plants and animals (7, 8). miRNAs act mostly on the 3′ UTR of target mRNAs and either degrade or inhibit translation of multiple transcripts (7–10). miRNAs are transcribed by RNA pol II to form a pri-miRNA molecule. pri-miRNAs are cleaved by Drosha to form the premiRNAs, which are then exported into the cytoplasm by Exportin-5. premiRNAs are cleaved by Dicer1 (Dcr1), resulting in a miRNA/miRNA duplex. The mature miRNA molecule is then loaded into an Argonaute (Ago) complex, which targets multiple mRNAs for either destruction or inhibition of translation (9). In addition, some miRNAs are found in introns of genes and bypass Drosha processing (9). Since the discovery of miRNAs in Caenorhabditis elegans (10, 11), numerous studies have demonstrated their essential role in regulating development, cell differentiation, apoptosis, and other critical biological events in both animals and plants (12, 13). It is estimated that nearly 30% of human mRNAs are targeted by miRNAs (13). Although multiple biological functions have been identified as targets of miRNAs in model organisms, including the fruit fly Drosophila melanogaster (14–21), information concerning possible roles of miRNAs in mosquito biology is limited. Both conserved and mosquito-specific miRNAs have been found in Anopheles stephensi and A. aegypti (22, 23). RNAi depletion of either Dcr1 or Ago1 was shown to result in a higher infection level by Plasmodium berghei parasites in the midgut of Anopheles gambiae mosquitoes (24). In Aedes albopictus and Culex quinquefasciatus, miRNAs were cloned and sequenced, and two were found to be up-regulated owing to West Nile virus infection (25). It is of particular importance to decipher what roles miRNAs might play in regulating blood feeding, female reproduction, and antipathogen immunity, because these functions constitute an interconnected triangle defining the ability of a female mosquito to transmit disease pathogens in consecutive reproductive cycles. In A. aegypti, several miRNAs have shown stage-specific expression, and their level was enhanced in the midgut of blood-fed females (23), suggesting miRNA involvement in blood-meal–associated events. However, the precise roles of miRNAs in mosquitoes remain unresolved. The significance of our present study is the identification of a particular miRNA required for both proper blood digestion and egg development in the anautogenous mosquito A. aegypti. Specific antagomir depletion of miR-275 resulted in a dramatic defect in intake and digestion of blood: in mosquitoes examined 24 h post-blood meal (PBM), blood was not digested in the posterior midgut, and it filled a specialized anterior portion of the digestive system called the crop, which is normally used for storing nectar. The overall volume of engorged blood remained very large, suggesting a defect in fluid excretory function. In these mosquitoes, egg development was profoundly inhibited; primary follicles representing future eggs were small and heterogeneous in size; in contrast, their nurse cells remained intact, which is characteristic of early stages of egg development. Moreover, expression of miR-275 was elevated by AAs and 20E, indicating that it is under the control of AA/TOR and 20E regulatory cascades initiated by blood feeding. This report clearly illustrates the critical importance of miRNAs in controlling blood-meal–activated physiological events required for completion of egg development in mosquito disease vectors.