Wolbachia RNase HI contributes to virus blocking in the mosquito Aedes aegypti

Laboratory evidence on the vector competence of European field-captured Culex theileri for circulating West Nile virus lineages 1 and 2

BACKGROUND

Culex theileri (Theobald, 1903) is distributed in Afrotropical, Paleartic, and Oriental regions. It is a mainly mammophilic floodwater mosquito that is involved in the transmission of West Nile virus (WNV, renamed as Orthoflavivirus nilense by the International Committee on Taxonomy of Viruses [ICTV]) in Africa. This virus is a mosquito-borne flavivirus that is kept in an enzootic cycle mainly between birds and mosquitoes of the Culex genus. Occasionally, it affects mammals including humans and equines causing encephalopathies. The main purpose of the present study was to evaluate the vector competence of a European field-captured Cx. theileri population for circulating WNV lineages (1 and 2).

METHODS

Field-collected Cx. theileri larvae from Sevilla province (Spain) were reared in the laboratory under summer environmental conditions. To assess the vector competence for WNV transmission, 10-12 day old Cx. theileri females were fed with blood doped with WNV lineages 1 and 2 (7 log10 TCID50/mL). Females were sacrificed at 14- and 21- days post exposure (dpe), and their head, body, and saliva were extracted to assess infection, dissemination, and transmission rates, as well as transmission efficiency.

RESULTS

A Culex theileri population was experimentally confirmed as a highly competent vector for WNV (both lineages 1 and 2). The virus successfully infected and disseminated within Cx. theileri mosquitoes, and infectious virus isolated from their saliva indicated their potential to transmit the virus. Transmission efficiency was 50% for lineage 1 (for both 14 and 21 dpe), while it was 24% and 37.5% for lineage 2, respectively. There was barely any effect of the midgut infection barrier for lineage 1 and a moderate effect for lineage 2. The main barrier which limited the virus infection within the mosquito was the midgut escape barrier.

CONCLUSIONS

In the present study, the high transmission efficiency supports that Cx. theileri is competent to transmit WNV. However, vector density and feeding patterns of Cx. theileri mosquitoes must be considered when estimating their vectorial capacity for WNV in the field.

Wolbachia elevates host methyltransferase expression and alters the m6A methylation landscape in Aedes aegypti mosquito cells

Wolbachia pipientis is an intracellular endosymbiotic bacterium that blocks the replication of several arboviruses in transinfected Aedes aegypti mosquitoes, yet its antiviral mechanism remains unknown. For the first time, we employed Nanopore direct RNA sequencing technology to investigate the impact of wAlbB strain of Wolbachia on the host's N6-methyladenosine (m6A) machinery and post-transcriptional modification landscape. Our study revealed that Wolbachia infection elevates the expression of genes involved in the mosquito's m6A methyltransferase complex. However, knocking down these m6A-related genes did not affect Wolbachia density. Nanopore sequencing identified 1,392 differentially modified m6A DRACH motifs on mosquito transcripts, with 776 showing increased and 616 showing decreased m6A levels due to Wolbachia. These m6A sites were predominantly enriched in coding sequences and 3'-untranslated regions. Gene Ontology analysis revealed that genes with reduced m6A levels were over-represented in functional GO terms associated with purine nucleotide binding functions critical in the post-transcriptional modification process of m6A. Differential gene expression analysis of the Nanopore data uncovered that a total of 643 protein-coding genes were significantly differentially expressed, 427 were downregulated, and 216 were upregulated. Several classical and non-classical immune-related genes were amongst the downregulated DEGs. Notably, it revealed a critical host factor, transmembrane protein 41B (TMEM41B), which is required for flavivirus infection, was upregulated and methylated in the presence of Wolbachia. Indeed, there is a strong correlation between gene expression being upregulated in genes with both increased and decreased levels of m6A modification, respectively. Our findings underscore Wolbachia's ability to modulate many intracellular aspects of its mosquito host by influencing post-transcriptional m6A modifications and gene expression, and it unveils a potential link behind its antiviral properties.

Role of Vigilin and RACK1 in dengue virus-Aedes aegypti-Wolbachia interactions

Vigilin is a large and evolutionary conserved RNA-binding protein (RBP), which can interact with RNA through its KH domain. Vigilin is, therefore, a multifunctional protein reported to be associated with RNA transport and metabolism, sterol metabolism, chromosome segregation, carcinogenesis, and heterochromatin-mediated gene silencing. The receptor for activated C kinase 1 (RACK1) is another highly conserved protein involved in many cellular pathways. Functional studies in human cells indicated that RACK1 interacts with Vigilin to promote dengue virus (DENV) replication. Both proteins are associated with the endoplasmic reticulum. Here, we investigated the significance of Vigilin and RACK1 homologs in Aedes aegypti mosquitoes concerning DENV replication and Wolbachia infection. We identified the homologs of the two genes in Ae. aegypti (AeVigilin and AeRACK1), which were upregulated in DENV-infected Aag2 cells and mosquitoes, and silencing them in Aag2 cells resulted in reduced DENV replication. Co-immunoprecipitation showed that AeRACK1 and AeVigilin interact in mosquito cells. Interestingly, we also found upregulation of both genes in a Wolbachia-infected cell line (Aag2.wAlbB). Furthermore, silencing AeVigilin and AeRACK1 in Aag2.wAlbB cells reduced DENV replication but increased Wolbachia density. However, we did not find a significant effect on DENV replication after silencing both genes in Ae. aegypti mosquitoes. Overall, our results support the involvement and significance of AeVigilin and AeRACK1 in DENV replication in Ae. aegypti.IMPORTANCEDengue virus (DENV), transmitted mainly by Aedes aegypti mosquitoes, poses significant health risks. Identifying factors involved in the virus replication in mosquitoes and human hosts is essential for devising control measures. In this study, we show that Vigilin and the receptor for activated C kinase 1 (RACK1), two proteins shown to play a role in the replication of DENV in human cells, are induced in mosquitoes and cell lines following DENV replication. Both proteins reside in the cytoplasm, where they interact similarly to human cells. Silencing the genes in mosquito cells significantly reduced virus replication. Furthermore, we found that both genes are induced in mosquito cells transinfected with Wolbachia, a bacterium that blocks DENV replication. The results help better understand the role of the common factors supporting DENV replication in mosquitoes and human cells.

Interaction of the Wolbachia surface protein with a novel pro-viral protein from Aedes aegypti

Dengue virus (DENV) and other flaviviruses are prevented from replicating in mosquitoes by Wolbachia. To date, several reports have appeared that highlight multiple molecular and cellular pathways involved in the blocking mechanism, which underlines the complicated nature of the mechanism. Here, we developed a hypothesis on whether Wolbachia proteins interact with pro-viral host proteins by using a unique approach to study the antiviral mechanism based on Wolbachia-host protein-protein interaction. We selected Wolbachia surface protein (WSP) for co-immunoprecipitation because of its abundance and possible secretion. We first confirmed WSP's secretion in mosquito cells and found two host proteins, Ae. aegypti serine-threonine kinase (STK) and synaptic vesicle membrane (SVM) protein VAT-1, and one Wolbachia protein (wGroEL) interacting with WSP. We examined the role of STK and SVM genes in relation to DENV replication in Ae. aegypti mosquitoes and mosquito cell lines with and without Wolbachia. In DENV-infected Aag2 cells, the expression of SVM and STK was significantly increased. However, although these genes were induced in Wolbachia-infected Aag2 cells, they were downregulated after DENV infection. Silencing of STK, but not SVM, reduced DENV replication in Aag2 cells and mosquitoes. Conversely, RNA activation of STK, by utilizing promoter induction via short activating oligos, resulted in higher DENV replication in Wolbachia-infected and uninfected cell lines. Overall, our findings suggest that STK is a pro-viral gene, and Wolbachia WSP binds to STK, possibly making it less accessible for DENV replication.

IMPORTANCE

Wolbachia is an endosymbiotic bacterium that blocks the replication of arboviruses in transinfected Aedes aegypti mosquitoes. In this study, we focused on identifying the potential interaction of Wolbachia proteins with the host pro-viral proteins. For this, we embarked on identifying the interacting proteins with a major Wolbachia protein, WSP, which is both structural and also secreted into the host cells. An Ae. aegypti STK was identified, which is induced in DENV and Wolbachia-infected cells. Silencing or induction of the gene led to reduced and increased DENV replication in vitro. Consistently, knocking down the gene in mosquitoes resulted in decreased virus replication. We hypothesize that WSP may sequester STK, which is pro-viral, contributing to Wolbachia virus blocking.

A comprehensive review of Wolbachia-mediated mechanisms to control dengue virus transmission in Aedes aegypti through innate immune pathways

The Dengue virus (DENV), primarily spread by Aedes aegypti and also by Aedes albopictus in some regions, poses significant global health risks. Alternative techniques are urgently needed because the current control mechanisms are insufficient to reduce the transmission of DENV. Introducing Wolbachia pipientis into Ae. aegypti inhibits DENV transmission, however, the underlying mechanisms are still poorly understood. Innate immune effector upregulation, the regulation of autophagy, and intracellular competition between Wolbachia and DENV for lipids are among the theories for the mechanism of inhibition. Furthermore, mainly three immune pathways Toll, IMD, and JAK/STAT are involved in the host for the suppression of the virus. These pathways are activated by Wolbachia and DENV in the host and are responsible for the upregulation and downregulation of many genes in mosquitoes, which ultimately reduces the titer of the DENV in the host. The functioning of these immune pathways depends upon the Wolbachia, host, and virus interaction. Here, we summarize the current understanding of DENV recognition by the Ae. aegypti's immune system, aiming to create a comprehensive picture of our knowledge. Additionally, we investigated how Wolbachia regulates the activation of multiple genes associated with immune priming for the reduction of DENV.

Do bed bugs transmit human viruses, or do humans spread bed bugs and their viruses? A worldwide survey of the bed bug RNA virosphere

BED BUGS: (Hemiptera: Cimicidae) are a globally distributed hematophagous pest that routinely feed on humans. Unlike many blood-sucking arthropods, they have never been linked to pathogen transmission in a natural setting, and despite increasing interest in their role as disease vectors, little is known about the viruses that bed bugs naturally harbor. Here, we present a global-scale survey of the bed bug RNA virosphere. We sequenced the metatranscriptomes of 22 individual bed bugs (Cimex lectularius and Cimex hemipterus) from 8 locations around the world. We detected sequences from two known bed bug viruses (Shuangao bedbug virus 1 and Shuangao bedbug virus 2) which extends their geographical range. We identified three novel bed bug virus sequences from a tenui-like virus (Bunyavirales), a toti-like virus (Ghabrivirales), and a luteo-like virus (Tolivirales). Interestingly, some of the bed bug viruses branch near to insect-transmitted plant-infecting viruses, opening questions regarding the evolution of plant virus infection. When we analyzed the viral sequences by their host's collection location, we found unexpected patterns of geographical diversity that may reflect humans' role in bed bug dispersal. Additionally, we investigated the effect that Wolbachia, the primary bed bug endosymbiont, may have on viral abundance and found that Wolbachia infection neither promotes nor inhibits viral infection. Finally, our results provide no evidence that bed bugs transmit any known human pathogenic viruses.

Advances in research on arboviral acquisition from hosts to mosquitoes

Arboviral acquisition is a critical step in virus transmission. In this review, we present an overview of the interactions between viruses and host blood-derived factors, highlighting the diverse ways in which they interact. Moreover, the review outlines the impact of host blood on gut barriers during viral acquisition, emphasizing the crucial role of this physiological process in virus dissemination. Additionally, the review investigates the responses of symbioses to invading arboviruses, providing insights into the dynamic reactions of these vital relationships to the presence of arboviruses.

A tangled threesome: understanding arbovirus infection in Aedes spp. and the effect of the mosquito microbiota

Arboviral infections transmitted by Aedes spp. mosquitoes are a major threat to human health, particularly in tropical regions but are expanding to temperate regions. The ability of Aedes aegypti and Aedes albopictus to transmit multiple arboviruses involves a complex relationship between mosquitoes and the virus, with recent discoveries shedding light on it. Furthermore, this relationship is not solely between mosquitoes and arboviruses, but also involves the mosquito microbiome. Here, we aimed to construct a comprehensive review of the latest information about the arbovirus infection process in A. aegypti and A. albopictus, the source of mosquito microbiota, and its interaction with the arbovirus infection process, in terms of its implications for vectorial competence. First, we summarized studies showing a new mechanism for arbovirus infection at the cellular level, recently described innate immunological pathways, and the mechanism of adaptive response in mosquitoes. Second, we addressed the general sources of the Aedes mosquito microbiota (bacteria, fungi, and viruses) during their life cycle, and the geographical reports of the most common microbiota in adults mosquitoes. How the microbiota interacts directly or indirectly with arbovirus transmission, thereby modifying vectorial competence. We highlight the complexity of this tripartite relationship, influenced by intrinsic and extrinsic conditions at different geographical scales, with many gaps to fill and promising directions for developing strategies to control arbovirus transmission and to gain a better understanding of vectorial competence. The interactions between mosquitoes, arboviruses and their associated microbiota are yet to be investigated in depth.

Molecular characterization and genetic diversity of Wolbachia endosymbionts in bed bugs (Hemiptera; Cimicidae) collected in Paris

PURPOSE

This study aimed to investigate the genetic diversity of Wolbachia in field-caught bed bug species in Paris areas.

METHODS

The bed bug specimens were captured from various infested localities in Paris and surrounding cities. They belonged to diverse life stages, including egg, nymph, and adult. They were then identified using morphological and molecular approaches. Furthermore, Wolbachia was detected, and its genetic diversity was investigated by conventional PCR of 16S-rRNA and Wolbachia surface protein (wsp) genes.

RESULTS

A total of 256 bed bug specimens belonging to various life stages [adult (183 specimens), nymph (48), and egg (25)] were captured from seven private apartments, five social apartments, three houses, two immigrant residences, and one retirement home situated in 10 districts of Paris and 8 surrounding cities. They were identified as Cimex lectularius (237 specimens) and C. hemipterus (19) using morphological and molecular approaches. The presence and diversity of Wolbachia were ascertained by targeting 16S-rRNA and wsp genes. Based on molecular analysis, 182 and 148 out of 256 processed specimens were positive by amplifying 16S-rRNA and wsp fragments, respectively. The inferred phylogenetic analysis with 16S-rRNA and wsp sequences displayed monophyletic Wolbachia strains clustering each one in three populations. The median-joining network, including the Wolbachia 16S-rRNA and wsp sequences of C. lectularius and C. hemipterous specimens, indicated a significant genetic differentiation among these populations in Paris areas which was consent with Neighbor-Joining analyses. A phylogenetic analysis of our heterogenic Wolbachia sequences with those reported from other arthropod species confirmed their belonging to supergroup F. Moreover, no difference between Wolbachia sequences from eggs, nymphs, and adults belonging to the same clade and between Wolbachia sequences of C. lectularius and C. hemipterus were observed after sequence alignment. Furthermore, no significant correlation was found between multiple geographical locations (or accomodation type) where bed bugs were collected and the genetic diversity of Wolbachia.

CONCLUSIONS

We highlight a significant heterogeneity within Wolbachia symbionts detected in C. lectularius and C. hemipterus. No correlation between Wolbachia species and bed bug species (C. lectularius versus C. hemipterus), physiological stages (egg, nymph, and adult), and sampling location was recorded in this study.

Wolbachia wMel strain-mediated effects on dengue virus vertical transmission from Aedes aegypti to their offspring

BACKGROUND

Dengue virus serotypes (DENV-1 to -4) can be transmitted vertically in Aedes aegpti mosquitoes. Whether infection with the wMel strain of the endosymbiont Wolbachia can reduce the incidence of vertical transmission of DENV from infected females to their offspring is not well understood.

METHODS

A laboratory colony of Vietnamese Ae. aegypti, both with and without wMel infection, were infected with DENV-1 by intrathoracic injection (IT) to estimate the rate of vertical transmission (VT) of the virus. VT in the DENV-infected mosquitoes was calculated via the infection rate estimation from mosquito pool data using maximum likelihood estimation (MLE).

RESULTS

In 6047 F1 Vietnamese wild-type Ae. aegypti, the MLE of DENV-1 infection was 1.49 per 1000 mosquitoes (95% confidence interval [CI] 0.73-2.74). In 5500 wMel-infected Ae. aegypti, the MLE infection rate was 0 (95% CI 0-0.69). The VT rates between mosquito lines showed a statistically significant difference.

CONCLUSIONS

The results reinforce the view that VT is a rare event in wild-type mosquitoes and that infection with wMel is effective in reducing VT.

Synergistic correlation between host angiogenin and dengue virus replication

DENV infection poses a major health concern globally and the pathophysiology relies heavily on host-cellular machinery. Although virus replication relies heavily on the host, the mechanistic details of DENV-host interaction is not fully characterized yet. Here, we are focusing on characterizing the mechanistic basis of virus-induced stress on the host cell. Specifically, we aim to characterize the role of the stress modulator ribonuclease Angiogenin during DENV infection. Our results suggested that the levels of Angiogenin are up-regulated in DENV-infected cells and the levels increase proportionately with DENV replication. Our efforts to knockdown Angiogenin using siRNA were unsuccessful in DENV-infected cells but not in mock-infected control. To further investigate the modulation between DENV replication and Angiogenin, we treated Huh7 cells with Ivermectin prior to DENV infection. Our results suggest a significant reduction in DENV replication specifically at the later stages as a consequence of Ivermectin treatment. Interestingly, Angiogenin levels were also found to be decreased proportionately. Our results suggest that Angiogenin modulation during DENV infection is important for DENV replication and pathogenesis.