Maternal germline-specific genes in the Asian malaria mosquito Anopheles stephensi: characterization and application for disease control

Arthropod promoters for genetic control of disease vectors

Vector-borne diseases (VBDs) impose devastating effects on human health and a heavy financial burden. Malaria, Lyme disease, and dengue fever are just a few examples of VBDs that cause severe illnesses. The current strategies to control VBDs consist mainly of environmental modification and chemical use, and to a small extent, genetic approaches. The genetic approaches, including transgenesis/genome modification and gene-drive technologies, provide the basis for developing new tools for VBD prevention by suppressing vector populations or reducing their capacity to transmit pathogens. The regulatory elements such as promoters are required for a robust sex-, tissue-, and stage-specific transgene expression. As discussed in this review, information on the regulatory elements is available for mosquito vectors but is scant for other vectors.

Development of miRNA-Based Approaches to Explore the Interruption of Mosquito-Borne Disease Transmission

MicroRNA (miRNA or miR)-based approaches to interrupt the transmission of mosquito-borne diseases have been explored since 2005. A review of these studies and areas in which to proceed is needed. In this review, significant progress is reviewed at the level of individual miRNAs, and miRNA diversification and relevant confounders are described in detail. Current miRNA studies in mosquitoes include four steps, namely, identifying miRNAs, validating miRNA-pathogen interactions, exploring action mechanisms, and performing preapplication investigations. Notably, regarding the Plasmodium parasite, mosquito miRNAs generally bind to mosquito immunity- or development-related mRNAs, indirectly regulating Plasmodium infection; However, regarding arboviruses, mosquito miRNAs can bind to the viral genome, directly modifying viral replication. Thus, during explorations of miRNA-based approaches, researchers need select an ideal miRNA for investigation based on the mosquito species, tissue, and mosquito-borne pathogen of interest. Additionally, strategies for miRNA-based approaches differ for arboviruses and protozoan parasites.

Engineering the Composition and Fate of Wild Populations with Gene Drive

Insects play important roles as predators, prey, pollinators, recyclers, hosts, parasitoids, and sources of economically important products. They can also destroy crops; wound animals; and serve as vectors for plant, animal, and human diseases. Gene drive-a process by which genes, gene complexes, or chromosomes encoding specific traits are made to spread through wild populations, even if these traits result in a fitness cost to carriers-provides new opportunities for altering populations to benefit humanity and the environment in ways that are species specific and sustainable. Gene drive can be used to alter the genetic composition of an existing population, referred to as population modification or replacement, or to bring about population suppression or elimination. We describe technologies under consideration, progress that has been made, and remaining technological hurdles, particularly with respect to evolutionary stability and our ability to control the spread and ultimate fate of genes introduced into populations.

Characterization of a female germline and early zygote promoter from the transcription factor bZip1 in the dengue mosquito Aedes aegypti

Background

The wide distribution of Aedes aegypti, the main vector of dengue and yellow fever viruses, currently puts three billion people in the world at risk of infection with these viruses. Continuous transmission of these and other viruses despite aggressive efforts to prevent this emphasizes the need to develop new control strategies. Proposals to control disease transmission based on vector engineering, including both population suppression and population replacement, rely on the development of transgenes under the control of regulatory elements able to drive molecules in a specific tissue, time and strength.

Methods

Here we report the characterization of a promoter active in both the female germline and early zygote, derived from the transcription factor bZip1 in the mosquito Ae. aegypti, using transposon-based methods and RT-qPCR.

Results

We generated seven transgenic lines carrying AabZip1-reporter constructs and observed expression in both the ovary and early embryo. RT-qPCR analysis was performed to evaluate transcript expression patterns for each line, confirming that transgenic expression from the AabZip1 promoter largely recapitulated the endogenous expression pattern, albeit the strength of maternal expression appeared to be strongly influenced by chromosomal position.

Conclusions

This study provides a new regulatory sequence that can be useful for generating transgenic lines that can become a tool in vector control strategies.

Ancient Coretention of Paralogs of Cid Centromeric Histones and Cal1 Chaperones in Mosquito Species

Despite their essential role in chromosome segregation in most eukaryotes, centromeric histones (CenH3s) evolve rapidly and are subject to gene turnover. We previously identified four instances of gene duplication and specialization of Cid, which encodes for the CenH3 in Drosophila. We hypothesized that retention of specialized Cid paralogs could be selectively advantageous to resolve the intralocus conflict that occurs on essential genes like Cid, which are subject to divergent selective pressures to perform multiple functions. We proposed that intralocus conflict could be a widespread phenomenon that drives evolutionary innovation in centromeric proteins. If this were the case, we might expect to find other instances of coretention and specialization of centromeric proteins during animal evolution. Consistent with this hypothesis, we find that most mosquito species encode two CenH3 (mosqCid) genes, mosqCid1 and mosqCid2, which have been coretained for over 150 My. In addition, Aedes species encode a third mosqCid3 gene, which arose from an independent gene duplication of mosqCid1. Like Drosophila Cid paralogs, mosqCid paralogs evolve under different selective constraints and show tissue-specific expression patterns. Analysis of mosqCid N-terminal protein motifs further supports the model that mosqCid paralogs have functionally diverged. Extending our survey to other centromeric proteins, we find that all Anopheles mosquitoes encode two CAL1 paralogs, which are the chaperones that deposit CenH3 proteins at centromeres in Diptera, but a single CENP-C paralog. The ancient coretention of paralogs of centromeric proteins adds further support to the hypothesis that intralocus conflict can drive their coretention and functional specialization.

Functional analysis of the promoter of an early zygotic gene KLC2 in Aedes aegypti

Background

Aedes aegypti is an important mosquito vector that transmits arboviruses that cause devastating diseases including Zika, dengue fever, yellow fever and chikungunya. Improved understanding of gene regulation in the early development of Ae. aegypti will facilitate genetic studies and help the development of novel control strategies of this important disease vector.

Results

In this study, we demonstrated through transgenic assays that the promoter of an endogenous early zygotic gene KLC2 could drive gene expression in the syncytial blastoderm and early cellular blastoderm, which is a stage that the developing germline and the rest of embryo are accessible to genetic manipulation. An unexpected expression of the reporter gene in transgenic male testes was also observed. Further analysis confirmed the expression of the endogenous KLC2 in the testes, which was not detected in the previous RNA sequencing data.

Conclusions

Our finding provided a new promoter element that can be used in future genetic studies and applications in Ae. aegypti. Moreover, our transgenic reporter assays showed that cautions are needed when interpreting RNA sequencing data as transient or tissue-specific transcription may go undetected by RNAseq.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3210-0) contains supplementary material, which is available to authorized users.

Mosquito-Borne Diseases and Omics: Tissue-Restricted Expression and Alternative Splicing Revealed by Transcriptome Profiling of Anopheles stephensi

Malaria is one of the most debilitating mosquito-borne diseases with high global health burdens. While much of the research on malaria and mosquito-borne diseases is focused on Africa, Southeast Asia accounts for a sizable portion of the global burden of malaria. Moreover, about 50% of the Asian malaria incidence and deaths have been from India. A promising development in this context is that the completion of genome sequence of Anopheles stephensi, a major malaria vector in Asia, offers new opportunities for global health innovation, including the progress in deciphering the vectorial ability of this mosquito species at a molecular level. Moving forward, tissue-based expression profiling would be the next obvious step in understanding gene functions of An. stephensi. We report in this article, to the best of our knowledge, the first in-depth study on tissue-based transcriptomic profile of four important organs (midgut, Malpighian tubules, fat body, and ovary) of adult female An. stephensi mosquitoes. In all, we identified over 20,000 transcripts corresponding to more than 12,000 gene loci from these four tissues. We present and discuss the tissue-based expression profiles of majority of annotated transcripts in An. stephensi genome, and the dynamics of their alternative splicing in these tissues, in this study. The domain-based Gene Ontology analysis of the differentially expressed transcripts in each of the mosquito tissue indicated enrichment of transcripts with proteolytic activity in midgut; transporter activity in Malpighian tubules; cell cycle, DNA replication, and repair activities in ovaries; and oxidoreductase activities in fat body. Tissue-based study of transcript expression and gene functions markedly enhances our understanding of this important malaria vector, and in turn, offers rationales for further studies on vectorial ability and identification of novel molecular targets to intercept malaria transmission.

Cheating evolution: engineering gene drives to manipulate the fate of wild populations

Engineered gene drives - the process of stimulating the biased inheritance of specific genes - have the potential to enable the spread of desirable genes throughout wild populations or to suppress harmful species, and may be particularly useful for the control of vector-borne diseases such as malaria. Although several types of selfish genetic elements exist in nature, few have been successfully engineered in the laboratory thus far. With the discovery of RNA-guided CRISPR-Cas9 (clustered regularly interspaced short palindromic repeats-CRISPR-associated 9) nucleases, which can be utilized to create, streamline and improve synthetic gene drives, this is rapidly changing. Here, we discuss the different types of engineered gene drives and their potential applications, as well as current policies regarding the safety and regulation of gene drives for the manipulation of wild populations.

Control of Mosquito-Borne Infectious Diseases: Sex and Gene Drive

Sterile male releases have successfully reduced local populations of the dengue vector, Aedes aegypti, but challenges remain in scale and in separating sexes before release. The recent discovery of the first mosquito male determining factor (M factor) will facilitate our understanding of the genetic programs that initiate sexual development in mosquitoes. Manipulation of the M factor and possible intermediary factors may result in female-to-male conversion or female killing, enabling efficient sex separation and effective reduction of target mosquito populations. Given recent breakthroughs in the development of CRISPR-Cas9 reagents as a source of gene drive, more advanced technologies at driving maleness, the ultimate disease refractory phenotype, become possible and may represent efficient and self-limiting methods to control mosquito populations.

"Salivary gland cellular architecture in the Asian malaria vector mosquito Anopheles stephensi"

Background

Anopheles mosquitoes are vectors for malaria, a disease with continued grave outcomes for human health. Transmission of malaria from mosquitoes to humans occurs by parasite passage through the salivary glands (SGs). Previous studies of mosquito SG architecture have been limited in scope and detail.

Methods

We developed a simple, optimized protocol for fluorescence staining using dyes and/or antibodies to interrogate cellular architecture in Anopheles stephensi adult SGs. We used common biological dyes, antibodies to well-conserved structural and organellar markers, and antibodies against Anopheles salivary proteins to visualize many individual SGs at high resolution by confocal microscopy.

Results

These analyses confirmed morphological features previously described using electron microscopy and uncovered a high degree of individual variation in SG structure. Our studies provide evidence for two alternative models for the origin of the salivary duct, the structure facilitating parasite transport out of SGs. We compare SG cellular architecture in An. stephensi and Drosophila melanogaster, a fellow Dipteran whose adult SGs are nearly completely unstudied, and find many conserved features despite divergence in overall form and function. Anopheles salivary proteins previously observed at the basement membrane were localized either in SG cells, secretory cavities, or the SG lumen. Our studies also revealed a population of cells with characteristics consistent with regenerative cells, similar to muscle satellite cells or midgut regenerative cells.

Conclusions

This work serves as a foundation for linking Anopheles stephensi SG cellular architecture to function and as a basis for generating and evaluating tools aimed at preventing malaria transmission at the level of mosquito SGs.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-1229-z) contains supplementary material, which is available to authorized users.