Multiple Modes of Action of the Squamocin in the Midgut Cells of Aedes aegypti Larvae

Scientific Advances in the Last Decade on the Recovery, Characterization, and Functionality of Bioactive Compounds from the Araticum Fruit (Annona crassiflora Mart.)

Araticum (Annona crassiflora Mart.) is a native and endemic species to Brazilian Cerrado whose fruits have high sensorial, nutritional, bioactive, and economic potential. Its use in local folk medicine, associated with recent scientific findings, has attracted growing interest from different industrial sectors. Therefore, understanding the scientific advances achieved so far and identifying gaps to be filled is essential to direct future studies and transform accumulated knowledge into innovative technologies and products. In this review, we summarize the phytochemical composition, bioactivities, and food products from araticum fruit that have been reported in the scientific literature over the past 10 years. The compiled data showed that araticum fruit parts contain a wide range of bioactive compounds, particularly phenolic compounds, alkaloids, annonaceous acetogenins, carotenoids, phytosterols, and tocols. These phytochemicals contribute to different biological activities verified in araticum fruit extracts/fractions, including antioxidant, anti-inflammatory, anti-Alzheimer, anticancer, antidiabetic, anti-obesity, antidyslipidemic, antinociceptive, hepatoprotective, healing of the cutaneous wound, antibacterial, and insecticide effects. Despite the promising findings, further studies-particularly toxicological (especially, with byproducts), pre-clinical, and clinical trials-must be conducted to confirm these biological effects in humans and assure the safety and well-being of consumers.

Bioprospection for new larvicides against Aedes aegypti based on ethnoknowledge from the Amazonian São Sebastião de Marinaú riverside community

ETHNOPHARMACOLOGICAL RELEVANCE

Vector-borne diseases represent a huge global burden impacting health systems. Aedes aegypti is the main vector of arboviral diseases including dengue, Zika, chikungunya and urban yellow fever in both tropical and subtropical areas. Ethnopharmacological investigations provide potential avenues for developing new vector control strategies.

AIM OF THE STUDY

The objective of this study is to document the São Sebastião de Marinaú riverside community's ethnoknowledge of local plants used to control mosquitoes and perform bioguided fractionation to isolate the compounds active against the arboviral disease vector Ae. aegypti.

MATERIALS AND METHODS

Semi-structured interviews were conducted with residents of the Marinaú community located in the Caxiuanã National Forest, in the Amazon biome, Pará, Brazil. The plants used to control mosquitoes were subjected to phytochemical studies guided by Ae. aegypti assays. Extracts were obtained from seven species using distinct organic solvents. Active extracts and fractions were separated by chromatographic techniques. Isolated compounds were characterized by NMR, LC/MS and GC/MS. Sample activity against Ae. aegypti larvae and pupae was evaluated after 24, 48 and 72 h exposure. The extracts were also investigated against adult female mosquitoes. The LC₅₀ values were determined by diluting each sample to obtain different concentrations in the respective activity range.

RESULTS

The Marinaú community uses more than ten plants as a repellent, most of which are trees native to the region. The primary applications of these plants to protect against insect bites were: burning plants (fumigation), application of body oils and bathing in macerated plants. Carapa guianensis is the predominant species used as a repellent. Extracts from Diospyros guianensis fruits, Carapa guianensis seed shells and Aspidosperma nitidum wood demonstrated Ae. aegypti larvicidal activity. The C. guianensis seed shell extract demonstrated a residual larvicidal effect. Plumbagin, stigmasterol, β-sitosterol, betulinic, ursolic and oleanolic acids, and betulin were identified in the D. guianensis extract. The plumbagin, ursolic and oleanolic acids displayed larvicidal activity. Oleanolic, ursolic and betulinic acids, and betulin were considered pupicidal. Aricine, the major alkaloid isolated from A. nitidum wood, also presented larvicidal activity.

CONCLUSIONS

Ten plant species traditionally used by the Marinaú community to afford protection against mosquitoes were reported. C. guianensis, D. guianensis and A. nitidum extracts were considered larvicidal against Ae. aegypti. Four triterpenes stood out as very active compounds against pupae. Aricine, an indole alkaloid, displayed larvicidal activity. Therefore, traditional knowledge of Amazonian plants combined with bioguided fractionation constitutes a strategy for the development of eco-friendly insecticides to control Ae. aegypti, an arbovirus vector.

Defining the mechanisms of action and mosquito larva midgut response to a yeast-encapsulated orange oil larvicide

BACKGROUND

Yeast-encapsulated orange oil (YEOO) is a novel, ingestible larvicide that combines the benefits of a low-cost essential oil with yeast, an attractive food source for mosquito larvae. In this work, we investigated the underlying mechanisms of action associated with YEOO ingestion by Aedes aegypti larvae.

METHODS

Aedes aegypti third-stage larvae (L3) were treated with sublethal or lethal concentrations of YEOO. Genes associated with apoptosis, autophagy and innate immune responses were investigated by RT-qPCR in guts and carcasses dissected from treated and control larvae. Differential expression of cytochrome P450 genes in the CYP6 and CYP9 families were also investigated. Confocal and transmission electron microscopy were used to assess damage caused by YEOO throughout the larval alimentary canal. TUNEL was used to assess apoptosis via DNA fragmentation.

RESULTS

The apoptosis genes IAP1 and IAP2 in larvae displayed opposing effects following exposure to lethal doses of YEOO, with a 26-fold induction of IAP1 at 8 h post YEOO ingestion. The effector caspase CASPS8 displayed a 6.7-fold induction in the gut and concomitant 70-fold induction in the carcass at 8 h post YEOO ingestion. The midgut epithelia regenerator, Vein, had an 11-fold induction in the gut after 4 h and was repressed 7.6-fold in the carcass at 24 h. Sublethal concentrations (< LC₅₀) led to significant differential expression of CYP6 and CYP9 genes. Midgut epithelial damage was highlighted by the destruction of microvilli, vacuolization of midgut cells and damage to cell junctions and basal lamina as early as 30 min. Larval type 2 peritrophic matrix structural integrity and porosity remain unchanged.

CONCLUSION

Our results strongly suggest that the robust larvicidal activity of YEOO is due to a generalized broad-acting mechanism combining epithelial damage and apoptosis, with concomitant expression of multiple innate response genes involved in epithelial regeneration and detoxification. YEOO's amenability for use as part of an integrated vector management program makes this novel larvicide a practical approach for mosquito larval control in the future.

Neurotoxic Zanthoxylum chalybeum root constituents invoke mosquito larval growth retardation through ecdysteroidogenic CYP450s transcriptional perturbations

Intracellular effects exerted by phytochemicals eliciting insect growth-retarding responses during vector control intervention remain largely underexplored. We studied the effects of Zanthoxylum chalybeum Engl. (Rutaceae) (ZCE) root derivatives against malaria (Anopheles gambiae) and arbovirus vector (Aedes aegypti) larvae to decipher possible molecular targets. We report dose-dependent biphasic effects on larval response, with transient exposure to ZCE and its bioactive fraction (ZCFr.5) inhibiting acetylcholinesterase (AChE) activity, inducing larval lethality and growth retardation at sublethal doses. Half-maximal lethal concentrations (LC₅₀) for ZCE against An. gambiae and Ae. aegypti larvae after 24-h exposure were 9.00 ppm and 12.26 ppm, respectively. The active fraction ZCFr.5 exerted LC₅₀ of 1.58 ppm and 3.21 ppm for An. gambiae and Ae. aegypti larvae, respectively. Inhibition of AChE was potentially linked to larval toxicity afforded by 2-tridecanone, palmitic acid (hexadecanoic acid), linoleic acid ((Z,Z)-9,12-octadecadienoic acid), sesamin, β-caryophyllene among other compounds identified in the bioactive fraction. In addition, the phenotypic larval retardation induced by ZCE root constituents was exerted through transcriptional modulation of ecdysteroidogenic CYP450 genes. Collectively, these findings provide an explorative avenue for developing potential mosquito control agents from Z. chalybeum root constituents.

Synthesis, molecular docking and mosquitocidal efficacy of lawsone and its derivatives against the dengue vector Aedes aegypti L. (Diptera: Culicidae)

BACKGROUND

Aedes aegypti is the primary dengue vector, a significant public health problem in many countries. Controlling the growth of Ae. aegypti is the biggest challenge in the mosquito control program, and there is a need for finding bioactive molecules to control Ae. aegypti in order to prevent dengue virus transmission.

OBJECTIVE

To assess the mosquitocidal property of lawsone and its 3-methyl-4H-chromen-3-yl-1-phenylbenzo[6,7]chromeno[2,3,c]pyrazole-dione derivatives (6a-6h) against various life stages of Ae. aegypti. Besides, to study the mode of action of the active compound by molecular docking and histopathological analysis.

METHODS

All derivatives were synthesized from the reaction between 2-hydroxy-1,4-naphthoquinone, chromene-3-carbaldehyde, and 1-phenyl-3-methyl-pyrazol-5-one by using one pot sequential multicomponent reaction. The mosquito life stages were subjected to diverse concentrations ranging from 1.25, 2.5, 5.0, and 10 ppm for lawsone and its derivatives. The structure of all synthesized compounds was characterized by spectroscopic analysis. Docking analysis was performed using autodock tools. Midgut sections of Ae. aegypti larvae were analyzed for histopathological effects.

RESULTS

Among the nine compounds screened, derivative 6e showed the highest mortality on Ae. aegypti life stages. The analyzed LC50 and LC90 results of derivative 6e were 3.01, 5.87 ppm, and 3.41, 6.28 ppm on larvae and pupae of Ae. aegypti, respectively. In the ovicidal assay, the derivative 6e recorded 47.2% egg mortality after 96-hour post-exposure to 10 ppm concentration. In molecular docking analysis, the derivative 6e confirmed strong binding interaction (-9.09 kcal/mol and -10.17 kcal/mol) with VAL 60 and HIS 62 of acetylcholinesterase 1 (AChE1) model and LYS 255, LYS 263 of kynurenine aminotransferase of Ae. aegypti, respectively. The histopathological results showed that the derivative 6e affected the columnar epithelial cells (CC) and peritrophic membrane (pM).

CONCLUSION

The derivative 6e is highly effective in the life stages of Ae. aegypti mosquito and it could be used in the integrated mosquito management programme.

Toxicities of acetogenin-based bioacaricides against two-spotted spider mite and selectivity to its phytoseiid predators

Tetranychus urticae is the main pest of strawberry crops and can cause up to 80% of productivity losses under high infestations. Aiming to search T. urticae management alternatives compatible with eco-friendly or organic-based food production systems, this study evaluated the lethal and sublethal toxicities of formulated derivatives from Annonaceae (rich in acetogenins) against this pest species. In addition, it also evaluated the selectivity of the most promising formulation to the predatory mites Neoseiulus californicus and Phytoseiulus macropilis, which are largely applied in biological control in Brazil. Among the derivatives tested, the emulsion from the ethanolic seed extract of Annona mucosa-ESEAm (major component: acetogenin bis-tetrahydrofuran rolliniastatin-1) caused pronounced mortality of T. urticae after 120 h of exposure (LC₅₀ = 465.5 mg L^-1), in a comparable or superior manner to an abamectin-based synthetic acaricide used as positive control (LC₅₀ = 1243.4 mg L^-1). Moreover, ESEAm exposure resulted in a significant decrease in the number of eggs laid by females and caused the most pronounced ovicidal action for T. urticae, with only 5% embryonic viability. However, ESEAm also showed high toxicity to the predatory mites tested, causing 100% mortality for both species after 120 h exposure, similar to abamectin. The interaction between these bioacaricides and biological control agents should be tested under field conditions to further assess the potential ecological selectivity of these derivatives.

Cell death induced by α-terthienyl via reactive oxygen species-mediated mitochondrial dysfunction and oxidative stress in the midgut of Aedes aegypti larvae

α-Terthienyl (α-T) is a photosensitizer that produces many reactive oxygen species (ROS) under ultraviolet light. Here, we aimed to evaluate the oxidation mechanism of the 25%, 50%, and 75% lethal concentrations in Aedes aegypti larvae; the lethal concentration of α-T was used as the test value. The effects on mitochondria, oxidative stress, and cell death patterns caused by ROS were evaluated. The results showed that α-T mainly produced large amounts of ROS in the midgut of larvae. Moreover, mitochondrial ROS were increased in midgut cells, and the production of ROS sites, such as complex enzymes, was inhibited, resulting in enhanced production of ROS. Ultrastructural analysis of mitochondria revealed significant vacuolation, decreased activity of tricarboxylic acid cycle enzymes, and reduced ATP content and mitochondrial membrane potential in the high concentration group compared with those in the control group. Additionally, mitochondrial biosynthesis was blocked in the high concentration group. Thus, exposure to α-T disrupted mitochondrial function, although the mitochondrial DNA content may have increased because of mitochondrial self-protection mechanisms against oxidative stress. Furthermore, high concentrations of α-T aggravated oxidative stress and increased the number of intracellular oxidative damage products. Reverse transcription polymerase chain reaction and fluorescence staining showed that ROS induced by low α-T concentrations upregulated apoptotic genes, including Dronc (P < 0.05), thereby promoting apoptosis. Moderate concentrations of α-T promoted autophagy through induction of ROS, inhibited apoptosis, and induced necrosis. In contrast, high α-T concentrations induced high levels of ROS, which caused mitochondrial dysfunction and increased cytoplasmic Ca²⁺ concentration, directly inducing cell necrosis. We also found that α-T may disrupt the permeability of the peritrophic membrane, leading to intestinal barrier dysfunction. These results provided insights into the mode of action of α-T in Aedes aegypti.

Plant extracts for developing mosquito larvicides: From laboratory to the field, with insights on the modes of action

In the last decades, major research efforts have been done to investigate the insecticidal activity of plant-based products against mosquitoes. This is a modern and timely challenge in parasitology, aimed to reduce the frequent overuse of synthetic pesticides boosting resistance development in mosquitoes and causing serious threats to human health and environment. This review covers the huge amount of literature available on plant extracts tested as mosquito larvicides, particularly aqueous and alcoholic ones, due to their easy formulation in water without using surfactants. We analysed results obtained on more than 400 plant species, outlining that 29 of them have outstanding larvicidal activity (i.e., LC₅₀ values below 10 ppm) against major vectors belonging to the genera Anopheles, Aedes and Culex, among others. Furthermore, synergistic and antagonistic effects between plant extracts and conventional pesticides, as well as among selected plant extracts are discussed. The efficacy of pure compounds isolated from the most effective plant extracts and - when available - their mechanism of action, as well as the impact on non-target species, is also covered. These belong to the following class of secondary metabolites: alkaloids, alkamides, sesquiterpenes, triterpenes, sterols, flavonoids, coumarins, anthraquinones, xanthones, acetogenonins and aliphatics. Their mode of action on mosquito larvae ranges from neurotoxic effects to inhibition of detoxificant enzymes and larval development and/or midugut damages. In the final section, current drawbacks as well as key challenges for future research, including technologies to synergize efficacy and improve stability - thus field performances - of the selected plant extracts, are outlined. Unfortunately, despite the huge amount of laboratory evidences about their efficacy, only a limited number of studies was aimed to validate their efficacy in the field, nor the epidemiological impact potentially arising from these vector control operations has been assessed. This strongly limits the development of commercial mosquito larvicides of botanical origin, at variance with plant-borne products developed in the latest decades to kill or repel other key arthropod species of medical and veterinary importance (e.g., ticks and lice), as well as mosquito adults. Further research on these issues is urgently needed.

Sublethal dose of deltamethrin damage the midgut cells of the mayfly Callibaetis radiatus (Ephemeroptera: Baetidae)

In insects, the midgut performs multiple physiologic functions (e.g., digestion and nutrients absorption) and serves as a physical/chemical barrier against pathogens and chemical stressors such as deltamethrin, a pyrethroid insecticide, commonly used in insect control that are agricultural pests and human disease vectors. Here, we described the midgut cell ultrastructure of Callibaetis radiatus nymphs, which are bioindicators of water quality and the ultrastructural alterations in midgut under sublethal exposure to deltamethrin at three different periods (1, 12, 24 h). The digestive cells of deltamethrin-unexposed nymphs had long microvilli, many mitochondria in the apical cytoplasm, a rough endoplasmic reticulum, a basal labyrinth with openings for hemocele, and the midgut peritrophic matrix which is classified as type I. Nymphs exposed to deltamethrin exhibited digestive cells rich in autophagic vacuoles, basal labyrinth loss, and microvilli disorganization since the first hour of contact with deltamethrin. However, these midgut tissues underwent to autophagic cellular recovery along the 24 h of exposure to deltamethrin. Thus, the sublethal exposure to deltamethrin is sufficient to disturb the ultrastructure of C. radiatus midgut, which might reduce the abilities of these insects to survive in aquatic environments contaminated by pyrethroids.

Larvicidal activity of vegetable oils and esterified compounds against Culex quinquefasciatus (Diptera: Culicidae)

Control of Culex quinquefasciatus using chemical insecticides may result in the selection of resistant mosquito strains. Thus, the use of plant-derived products has been studied as alternative for the mosquito control. Fatty acid methyl esters (FAMEs) obtained by transesterification of vegetable oils may result in compounds with larvicidal potential against C. quinquefasciatus. However, little is known about the morphological, physiological or biochemical effects that these FAMEs may have on mosquito larvae. The present study reports the effects of these FAMEs in mosquito larvae. The FAMEs were obtained by transesterification of canola, corn, sunflower, and soybean oils with acid catalysis and the determination of FAMEs composition was done by gas chromatography-mass spectrometry (GC-MS). Larvae of C. quinquefasciatus were exposed to different concentrations of the vegetable oils and FAMEs. Thereby, different FAMEs showed LC₅₀ values ranging from 42.32 to 196.27mg/L against C. quinquefasciatus larvae. The methyl ester obtained from sunflower oil showed the lowest LC₅₀. Histology of C. quinquefasciatus larvae exposed to LC₅₀ of FAMEs was performed and changes in the midgut and fat body morphology were identified. Therefore, larval mortality and changes in the internal organs suggested that FAMEs might be a promising new class of larvicidalcompounds. Cytotoxicity of FAMEs compounds was assessed with the HeLa human cell line and no effect was observed.