Effective disposal of nitrogen waste in blood-fed Aedes aegypti mosquitoes requires alanine aminotransferase

Pyruvate kinase is post-translationally regulated by sirtuin 2 in Aedes aegypti mosquitoes

We previously demonstrated that Aedes aegypti pyruvate kinase (AaPK) plays a key role in the regulation of both carbon and nitrogen metabolism in mosquitoes. To further elucidate whether AaPK can be post-translationally regulated by Ae. aegypti sirtuin 2 (AaSirt2), an NAD+-dependent deacetylase that catalyzes the removal of acetyl groups from acetylated lysine residues, we conducted a series of analysis in non-starved and starved female mosquitoes. Transcriptional and protein profiles of AaSirt2, analyzed by qPCR and western blots, indicated that the AaSirt2 is differentially modulated in response to sugar or blood feeding in mosquito tissues dissected at different times during the first gonotrophic cycle. We also found that AaSirt2 is localized in both cytosolic and mitochondrial cellular compartments of fat body and thorax. Multiple lysine-acetylated proteins were detected by western blotting in both cellular compartments. Furthermore, western blotting of immunoprecipitated proteins provided evidence that AaPK is lysine-acetylated and bound with AaSirt2 in the cytosolic fractions of fat body and thorax from non-starved and starved females. In correlation with these results, we also discovered that RNAi-mediated knockdown of AaSirt2 in the fat body of starved females significantly decreased AaPK protein abundance. Notably, survivorship of AaSirt2-deficient females maintained under four different nutritional regimens was not significantly affected. Taken together, our data reveal that AaPK is post-translationally regulated by AaSirt2.

Ornithine decarboxylase deficiency critically impairs nitrogen metabolism and survival in Aedes aegypti mosquitoes

Ornithine decarboxylase (ODC; EC 4.1.1.17) catalyzes the conversion of ornithine to putrescine, the rate-limiting first step for de novo polyamine biosynthesis. Previously, we reported that genetic knockdown of xanthine dehydrogenase 1 (XDH1)-a gene encoding the enzyme involved in the last two steps of uric acid synthesis-causes an increase in ODC transcript levels in fat body of blood-fed Aedes aegypti mosquitoes, suggesting a crosstalk at molecular level between XDH1 and ODC during nitrogen disposal. To further investigate the role of ODC in nitrogen metabolism, we conducted several biochemical and genetic analyses in sugar- and blood-fed A. aegypti females. Distinct ODC gene and protein expression patterns were observed in mosquito tissues dissected during the first gonotrophic cycle. Both pharmacological and RNA interference-mediated knockdown of ODC negatively impacted mosquito survival, disrupted nitrogen waste disposal, delayed oviposition onset, and decreased fecundity in vitellogenic blood-fed females. A lag in the expression of two major digestive serine proteases, a reduction of blood meal digestion in the midgut, and a decrease in vitellogenin yolk protein uptake in ovarian follicles were observed by western blots in ODC-deficient females. Moreover, genetic silencing of ODC showed a broad transcriptional modulation of genes encoding proteins involved in multiple metabolic pathways in mosquito fat body, midgut, and Malpighian tubules prior to and after blood feeding. All together, these data demonstrate that ODC plays an essential role in mosquito metabolism, and that ODC crosstalks with multiple genes and proteins to prevent deadly nitrogen perturbations in A. aegypti females.

Untargeted metabolomics-based response analysis of temperature and insecticide exposure in Aedes aegypti

In this study, we utilized an untargeted NMR metabolomics approach to identify the vector response in terms of metabolic profiling after temperature and insecticide exposure in comparison with the control. Clearly, temperature and insecticide exposure cause changes in the underlying metabolism, and the NMR metabolomic profile enables a direct examination of the immediate response of the vector to cope up with these changes. The present study was designed in four parts: A-Aedes aegypti were exposed to 40 °C for one-hour, DDT-4%, malathion-5%, and deltamethrin-0.05% separately and, part B-D; one-hour exposure at 35 °C and 40 °C temperatures followed by one-hour exposure to insecticide. The resultant metabolite profiles were compared with the control. In response to temperature and insecticide exposure, several metabolites and altered pathways were identified. Citrate, maltose, lipids, Nicotinate, Choline, Pyruvate and β-hydroxybutyrate were found as important components of major biological pathways such as tri-carboxylic acid cycle, branched amino acid degradation, glycolysis/gluconeogenesis, amino acid metabolism, lipid and carbohydrate metabolism, nucleotide PRPP pathway, and phospholipid metabolism. Furthermore, the results also suggest that the changes imposed by exposure to temperature and insecticides individually, are reversed with combined exposure, thus negating the impact of each other and posing a threat to the control of Aedes-borne diseases such as dengue, chikungunya, Zika and yellow fever.

The Multifaceted Roles of Proline in Cell Behavior

Herein, we review the multifaceted roles of proline in cell biology. This peculiar cyclic imino acid is: (i) A main precursor of extracellular collagens (the most abundant human proteins), antimicrobial peptides (involved in innate immunity), salivary proteins (astringency, teeth health) and cornifins (skin permeability); (ii) an energy source for pathogenic bacteria, protozoan parasites, and metastatic cancer cells, which engage in extracellular-protein degradation to invade their host; (iii) an antistress molecule (an osmolyte and chemical chaperone) helpful against various potential harms (UV radiation, drought/salinity, heavy metals, reactive oxygen species); (iv) a neural metabotoxin associated with schizophrenia; (v) a modulator of cell signaling pathways such as the amino acid stress response and extracellular signal-related kinase pathway; (vi) an epigenetic modifier able to promote DNA and histone hypermethylation; (vii) an inducer of proliferation of stem and tumor cells; and (viii) a modulator of cell morphology and migration/invasiveness. We highlight how proline metabolism impacts beneficial tissue regeneration, but also contributes to the progression of devastating pathologies such as fibrosis and metastatic cancer.

Unraveling mosquito metabolism with mass spectrometry-based metabolomics

Nearly half a million people die annually due to mosquito-borne diseases. Despite aggressive mosquito population-control efforts, current strategies are limited in their ability to control these vectors. A better understanding of mosquito metabolism through modern approaches can contribute to the discovery of novel metabolic targets and/or regulators and lead to the development of better mosquito-control strategies. Currently, cutting-edge technologies such as gas or liquid chromatography-mass spectrometry-based metabolomics are considered 'mature technologies' in many life-science disciplines but are still an emerging area of research in medical entomology. This review primarily discusses recent developments and progress in the application of mass spectrometry-based metabolomics to answer multiple biological questions related to mosquito metabolism.

Mass spectrometry-based stable-isotope tracing uncovers metabolic alterations in pyruvate kinase-deficient Aedes aegypti mosquitoes

A recent in vitro characterization of a recombinant pyruvate kinase (PK) from Aedes aegypti mosquitoes demonstrated that the enzyme is uniquely regulated by multiple allosteric effectors. Here, we further explored PK gene and protein expression, and enzymatic activity in key metabolic tissues of mosquitoes maintained under different nutritional conditions. We also studied the metabolic effects of PK depletion using several techniques including RNA interference and mass spectrometry-based stable-isotope tracing. Transcriptional analysis showed a dynamic post-feeding PK mRNA expression pattern within and across mosquito tissues, whereas corresponding protein levels remained stable throughout the time course analyzed. Nevertheless, PK activity significantly differed in the fat body of sucrose-, blood-fed, and starved mosquitoes. Genetic silencing of PK did not alter survival in blood-fed females maintained on sucrose. However, an enhanced survivorship was observed in PK-deficient females maintained under different nutritional regimens. Our results indicate that mosquitoes overcame PK deficiency by up-regulating the expression of genes encoding NADP-malic enzyme-1, phosphoenolpyruvate carboxykinase-1, phosphoglycerate dehydrogenase and glutamate dehydrogenase, and by decreasing glucose oxidation and metabolic pathways associated with ammonia detoxification. Taken together, our data demonstrate that PK confers to A. aegypti a metabolic plasticity to tightly regulate both carbon and nitrogen metabolism.

Short Report: Asymptomatic Zika virus infections with low viral loads not likely to establish transmission in New Orleans Aedes populations

Aedes aegypti and Aedes albopictus are both vectors of Zika virus and both are endemic to the New Orleans Metropolitan area. Fortunately, to date there has been no known autochthonous transmission of Zika virus in New Orleans. No studies of the vector competence of local populations of Ae. aegypti and Ae. albopictus for Zika virus transmission have been conducted. To determine if New Orleans Ae. aegypti and Ae. albopictus mosquitoes are competent for Zika virus, mosquitoes were reared to generation F3 from eggs collected in New Orleans during the 2018 mosquito season. Adults were fed an infectious blood meal and kept for 15 days in an environmental chamber. Transmission assays were conducted at 4, 10, and 15 days post exposure and RT-PCR was run on bodies and saliva to detect the presence of Zika virus RNA. We observed remarkably low susceptibility of both Ae. aegypti and Ae. albopictus from New Orleans to a Zika strain from Panama after oral challenge. These results suggest a limited risk of Zika virus transmission should it be introduced to the New Orleans area, and may partially explain why no transmission was detected in Louisiana during the 2016 epidemic in the Americas, despite multiple known travel associated introductions to New Orleans. Despite these results these mosquito populations are known to be competent vectors for some other mosquito-borne viruses and control measures should not be relaxed.

Development of Aedes aegypti (Diptera: Culicidae) mosquito larvae in high ammonia sewage in septic tanks causes alterations in ammonia excretion, ammonia transporter expression, and osmoregulation

Larvae of the disease vector mosquito, Aedes aegypti (L.) readily develop in ammonia rich sewage in the British Virgin Islands. To understand how the larvae survive in ammonia levels that are lethal to most animals, an examination of ammonia excretory physiology in larvae collected from septic-water and freshwater was carried out. A. aegypti larvae were found to be remarkably plastic in dealing with high external ammonia through the modulation of NH₄⁺ excretion at the anal papillae, measured using the scanning ion-selective electrode technique (SIET), and NH₄⁺ secretion in the primary urine by the Malpighian tubules when developing in septicwater. Ammonia transporters, Amt and Rh proteins, are expressed in ionoregulatory and excretory organs, with increases in Rh protein, Na⁺-K⁺-ATPase, and V-type-H⁺-ATPase expression observed in the Malpighian tubules, hindgut, and anal papillae in septic-water larvae. A comparative approach using laboratory A. aegypti larvae reared in high ammonia septic-water revealed similar responses to collected A. aegypti with regard to altered ammonia secretion and hemolymph ion composition. Results suggest that the observed alterations in excretory physiology of larvae developing in septic-water is a consequence of the high ammonia levels and that A. aegypti larvae may rely on ammonia transporting proteins coupled to active transport to survive in septic-water.

Metabolic balancing by miR-276 shapes the mosquito reproductive cycle and Plasmodium falciparum development

The blood-feeding behavior of Anopheles females delivers essential nutrients for egg development and drives parasite transmission between humans. Plasmodium growth is adapted to the vector reproductive cycle, but how changes in the reproductive cycle impact parasite development remains unclear. Here, we show that the bloodmeal-induced miR-276-5p fine-tunes the expression of branched-chain amino acid transferase to terminate the reproductive cycle. Silencing of miR-276 prolongs high rates of amino acid (AA) catabolism and increases female fertility, suggesting that timely termination of AA catabolism restricts mosquito investment into reproduction. Prolongation of AA catabolism in P. falciparum-infected females also compromises the development of the transmissible sporozoite forms. Our results suggest that Plasmodium sporogony exploits the surplus mosquito resources available after reproductive investment and demonstrate the crucial role of the mosquito AA metabolism in within-vector parasite proliferation and malaria transmission.

Plasmodium growth is adapted to the reproductive cycle of mosquitoes, but underlying mechanisms are unclear. Here, Lampe et al. show that the blood-meal induced miR-276 balances the termination of the mosquito amino acid catabolism and egg development, providing nutrients for Plasmodium sporozoite development.

Mosquito Host-Seeking Regulation: Targets for Behavioral Control

Female Aedes aegypti mosquitoes require protein from blood to develop eggs. They have evolved a strong innate drive to find and bite humans and engorge on their blood. Decades of research have revealed that attraction to hosts is suppressed for days after blood-feeding. During this time, females coordinate complex physiological changes, allowing them to utilize blood protein to develop eggs: clearing excess fluid, digesting protein, and egg maturation. How do mechanosensation, nutrient consumption, and reproductive pathways combine to produce the full expression of host-seeking suppression? Understanding mechanisms of endogenous host-seeking suppression may allow them to be 'weaponized' against mosquitoes through exogenous activation and developed as tools for vector control. Recent work allows unprecedented genetic and pharmacological access to characterize and disrupt this behavioral cycle.

Identification and characterization of a mosquito-specific eggshell organizing factor in Aedes aegypti mosquitoes

Mosquito-borne diseases are responsible for several million human deaths annually around the world. One approach to controlling mosquito populations is to disrupt molecular processes or antagonize novel metabolic targets required for the production of viable eggs. To this end, we focused our efforts on identifying proteins required for completion of embryonic development that are mosquito selective and represent potential targets for vector control. We performed bioinformatic analyses to identify putative protein-coding sequences that are specific to mosquito genomes. Systematic RNA interference (RNAi) screening of 40 mosquito-specific genes was performed by injecting double-stranded RNA (dsRNA) into female Aedes aegypti mosquitoes. This experimental approach led to the identification of eggshell organizing factor 1 (EOF1, AAEL012336), which plays an essential role in the formation and melanization of the eggshell. Eggs deposited by EOF1-deficient mosquitoes have nonmelanized fragile eggshells, and all embryos are nonviable. Scanning electron microscopy (SEM) analysis identified that exochorionic eggshell structures are strongly affected in EOF1-deficient mosquitoes. EOF1 is a potential novel target, to our knowledge, for exploring the identification and development of mosquito-selective and biosafe small-molecule inhibitors.

An RNAi functional screen of 40 Aedes aegypti genes specific to the mosquito lineage helped to identify EOF1, a protein that plays an essential role in mosquito eggshell formation and melanization.

Mosquito-borne pathogens infect millions of people worldwide, and the rise in insecticide resistance is exacerbating this problem. A new generation of environmentally safe insecticides will be essential to control insecticide-resistant mosquitoes. One potential route to such novel insecticide targets is the identification of proteins specifically needed for mosquito reproduction. Female mosquitoes feed on blood to produce eggs, which are covered with an eggshell; using RNA interference screening of mosquito-specific genes in Aedes aegypti (the mosquito that transmits yellow fever), we identified the eggshell organizing factor 1 (EOF1) protein that plays an essential role in eggshell melanization and embryonic development. Nearly 100% of eggs laid by EOF1-deficient females had a defective eggshell and were not viable. Bleach assays on eggs further confirmed that mosquito-specific EOF1 is required for embryonic development in A. aegypti. Additional experiments revealed that EOF1 also plays an essential role in eggshell formation in Aedes albopictus (the tiger mosquito, a carrier of Zika virus and dengue fever). We hypothesize that EOF1 has evolved within the Culicidae family to effect eggshell formation and therefore maximize egg survival. The results provide new insights, to our knowledge, into mosquito egg maturation and eggshell synthesis and could lead to key advances in the field of mosquito vector control.

Distinctive regulatory properties of pyruvate kinase 1 from Aedes aegypti mosquitoes

Female Aedes aegypti mosquitoes are vectors of arboviruses that cause diseases of public health significance. The discovery of new metabolic targets is crucial for improving mosquito control strategies. We recently demonstrated that glucose oxidation supports ammonia detoxification in A. aegypti. Pyruvate kinase (PK, EC 2.7.1.40) catalyzes the last step of the glycolytic pathway. In most organisms, one or more allosteric effectors control PK activity. However, the kinetic properties and structure of PK in mosquitoes have not been previously reported. In this study, two alternatively spliced mRNA variants (AaPK1 and AaPK2) that code for PKs were identified in the A. aegypti genome. The AaPK1 mRNA variant, which encodes a 529 amino acid protein with an estimated molecular weight of ∼57 kDa, was cloned. The protein was expressed in Escherichia coli and purified. The AaPK1 kinetic properties were identified. The recombinant protein was also crystallized and its 3D structure determined. We found that alanine, glutamine, proline, serine and fructose-1-phosphate displayed a classic allosteric activation on AaPK1. Ribulose-5-phosphate acted as an allosteric inhibitor of AaPK1 but its inhibitory effect was reversed by alanine, glutamine, proline and serine. Additionally, the allosteric activation of AaPK1 by amino acids was weakened by fructose-1,6-bisphosphate, whereas the allosteric activation of AaPK1 by alanine and serine was diminished by glucose-6-phosphate. The AaPK1 structure shows the presence of fructose-1,6-bisphosphate in the allosteric site. Together, our results reveal that specific amino acids and phosphorylated sugars tightly regulate conformational dynamics and catalytic changes of AaPK1. The distinctive AaPK1 allosteric properties support a complex role for this enzyme within mosquito metabolism.

How micronutrients influence the physiology of mosquitoes

Micronutrients or non-energetic nutrients (NEN) are needed in reduced amounts, but are essential for many mosquito physiological processes that influence biological traits from vector competence to reproductive capacity. The NEN include amino acids (AA), vitamins, salts, metals and sterols. Free AA plays critical roles controlling most physiological processes, from digestion to reproduction. Particularly proline connects metabolic pathways in energy production, flight physiology and ammonia detoxification. Metal, in particular iron and calcium, salts, sterol and vitamin homeostasis are critical for cell signaling, respiration, metabolism and reproduction. Micronutrient homeostasis influence the symbiotic relationships with microorganisms, having important implications in mosquitoes' nutrition, physiology and behavior, as well as in mosquito immunity and vector competence.

Positional stable isotope tracer analysis reveals carbon routes during ammonia metabolism of Aedes aegypti mosquitoes

In Aedes aegypti females, the ammonia released during blood meal digestion is partially metabolized to facilitate the disposal of excess nitrogen. In this study, we used low- and high-resolution liquid chromatography-mass spectrometry (LC/MS) techniques to investigate the role of glucose during ammonia detoxification. Mosquitoes were fed a blood meal supplemented with [1,2-¹³C₂]glucose, and downstream metabolites were measured for 24 h. Quantification of [¹³C] amino acids in the entire mosquito body was conducted without sample derivatization using selected reaction monitoring of mass transitions that are indicative of the structural position of [¹³C] atom incorporation. Identification of unlabeled and [¹³C] isotopologs of 43 compounds, including amino acids, amino acid derivatives, and organic acids, was performed by high-resolution LC/MS techniques. Blood-fed mosquitoes synthesized [¹³C] metabolites in mainly 2 carbon positions from [1,2-¹³C₂]glucose. [¹³C₂]Ala and [¹³C₂]Pro were the most abundant and rapidly labeled amino acids synthesized. Additional [¹³C] amino acids, [¹³C] amino acid derivatives, and [¹³C] organic acids in 1 or 2 carbon positions were also identified. Two kinetic routes were proposed based on the incorporation of a [¹³C] atom at position 1 in specific amino acids. Our findings provide evidence that glucose is used for ammonia detoxification and [¹³C] uric acid synthesis through multiple metabolic pathways, uncovering a metabolic link at the carbon atomic level in ammonia metabolism of A. aegypti-Horvath, T. D., Dagan, S., Lorenzi, P. L., Hawke, D. H., Scaraffia, P. Y. Positional stable isotope tracer analysis reveals carbon routes during ammonia metabolism of Aedes aegypti mosquitoes.

The Dose Makes the Poison: Nutritional Overload Determines the Life Traits of Blood-Feeding Arthropods

Vertebrate blood composition is heavily biased towards proteins, and hemoglobin, which is a hemeprotein, is by far the most abundant protein. Typically, hematophagous insects ingest blood volumes several times their weight before the blood meal. This barbarian feast offers an abundance of nutrients, but the degradation of blood proteins generates toxic concentrations of amino acids and heme, along with unparalleled microbiota growth. Despite this challenge, hematophagous arthropods have successfully developed mechanisms that bypass the toxicity of these molecules. While these adaptations allow hematophagous arthropods to tolerate their diet, they also constitute a unique mode of operation for cell signaling, immunity, and metabolism, the study of which may offer insights into the biology of disease vectors and may lead to novel vector-specific control methods.

Xanthine dehydrogenase-1 silencing in Aedes aegypti mosquitoes promotes a blood feeding-induced adulticidal activity

Aedesaegypti has 2 genes encoding xanthine dehydrogenase (XDH). We analyzed XDH1 and XDH2 gene expression by real-time quantitative PCR in tissues from sugar- and blood-fed females. Differential XDH1 and XDH2 gene expression was observed in tissues dissected throughout a time course. We next exposed females to blood meals supplemented with allopurinol, a well-characterized XDH inhibitor. We also tested the effects of injecting double-stranded RNA (dsRNA) against XDH1, XDH2, or both. Disruption of XDH by allopurinol or XDH1 by RNA interference significantly affected mosquito survival, causing a disruption in blood digestion, excretion, oviposition, and reproduction. XDH1-deficient mosquitoes showed a persistence of serine proteases in the midgut at 48 h after blood feeding and a reduction in the uptake of vitellogenin by the ovaries. Surprisingly, analysis of the fat body from dsRNA-XDH1-injected mosquitoes fell into 2 groups: one group was characterized by a reduction of the XDH1 transcript, whereas the other group was characterized by an up-regulation of several transcripts, including XDH1, glutamine synthetase, alanine aminotransferase, catalase, superoxide dismutase, ornithine decarboxylase, glutamate receptor, and ammonia transporter. Our data demonstrate that XDH1 plays an essential role and that XDH1 has the potential to be used as a metabolic target for Ae.aegypti vector control.-Isoe, J., Petchampai, N., Isoe, Y. E., Co, K., Mazzalupo, S., Scaraffia, P. Y. Xanthine dehydrogenase-1 silencing in Aedes aegypti mosquitoes promotes a blood feeding-induced adulticidal activity.