Insulin-like peptides in the mosquito Anopheles stephensi: identification and expression in response to diet and infection with Plasmodium falciparum

The serine/threonine kinase Akt gene affects fecundity by reducing Juvenile hormone synthesis in Liposcelis entomophila (Enderlein)

The serine/threonine kinase Akt is an important component of the insulin signalling pathway (ISP) in regulating insect metabolism, growth, and reproduction. The psocid Liposcelis entomophila (Enderlein) is a distasteful stored products pest for its fecundity. However, the molecular mechanism of Akt that controls vitellogenesis and oviposition in L. entomophila remains obscure. In this study, the function of the Akt gene in the female reproduction of L. entomophila (designated as LeAkt) was characterized and investigated. LeAkt contains a 1587 bp open reading frame encoding a 529 amino acid protein that possesses a conserved Pleckstrin Homology domain (PH) and a Ser/Thr-type protein kinase (S_TKc) domain. The mRNA expression of LeAkt was the highest in female adult stages and peaked for 7-day female adults. In female adult tissues, LeAkt was highly expressed in the head and the ovary, indicating that LeAkt was closely correlated with female ovarian development. LeAkt transcription level was significantly suppressed by oral feeding on artificial diets mixed with dsRNA-LeAkt. RNAi-mediated silencing of LeAkt led to a severe inhibition of vitellogenein (Vg) expression and ovarian development, together with lower fecundity and hatchability compared to that of the normal feeding group, suggesting a critical role for LeAkt in L. entomophila reproduction. Further studies revealed that LeAkt silencing significantly decreased the mRNA levels of several signalling and biosynthetic genes in the juvenile hormone (JH) signalling pathway, such as methoprene-tolerant (LeMet), krüppel homolog 1 (LeKr-h1) and JH methyltransferase (LeJHAMT), leading to a severe inhibition of JH biosynthesis in L. entomophila female adults. These results suggested that LeAkt was affecting JH synthesis, thereby influencing Vg synthesis and ultimately L. entomophila reproduction.

Interactions between innate immunity and insulin signaling affect resistance to infection in insects

An active immune response is energetically demanding and requires reallocation of nutrients to support resistance to and tolerance of infection. Insulin signaling is a critical global regulator of metabolism and whole-body homeostasis in response to nutrient availability and energetic needs, including those required for mobilization of energy in support of the immune system. In this review, we share findings that demonstrate interactions between innate immune activity and insulin signaling primarily in the insect model Drosophila melanogaster as well as other insects like Bombyx mori and Anopheles mosquitos. These studies indicate that insulin signaling and innate immune activation have reciprocal effects on each other, but that those effects vary depending on the type of pathogen, route of infection, and nutritional status of the host. Future research will be required to further understand the detailed mechanisms by which innate immunity and insulin signaling activity impact each other.

Molecular and expression characterization of insulin-like signaling in development and metabolism of Aedes albopictus

BACKGROUND

Insulin-like signaling (IS) in insects is a conserved pathway that regulates development, reproduction and longevity. Insulin-like peptides (ILPs) activate the IS pathway by binding to the insulin receptor (InR) and trigger the ERK and AKT cascades. A varying number of ILPs were identified in Aedes aegypti mosquito and other insects. Aedes albopictus is an invasive mosquito which transmits dengue and Zika viruses worldwide. Until now, the molecular and expression characteristics of IS pathway in Ae. albopictus have not been investigated.

METHODS

The orthologues of ILP in Ae. albopictus genome assembly was analyzed by using sequence blast. Phylogenetic analysis and molecular characterization were performed to identify the functional domains of ILPs. Quantitative analysis was performed to determine the expression characteristics of ILPs, InR as well as ERK and AKT in mosquito development and different tissues of female adults after blood-feeding. In addition, the knockdown of InR was achieved by feeding larvae with Escherichia coli-producing dsRNA to investigate the impact of IS pathway on mosquito development.

RESULTS

We identified seven putative ILP genes in Ae. albopictus genome assembly, based on nucleotide similarity to the ILPs of Ae. aegypti and other insects. Bioinformatics and molecular analyses suggested that the ILPs contain the structural motif which is conserved in the insulin superfamily. Expression levels of ILPs, InR as well as ERK and AKT varied in Ae. albopictus development stages and between male and female adults. Quantitative analyses revealed that expression of ILP6, the putative orthologue of the insulin growth factor peptides, was highest in the midgut of female adults after blood-feeding. Knockdown of Ae. albopictus InR induces a significant decrease in the phosphorylation levels of ERK and AKT proteins and results in developmental delays and smaller body sizes.

CONCLUSIONS

The IS pathway of Ae. albopictus mosquito contains ILP1-7, InR and ERK/AKT cascades, which exhibited different developmental and tissue expression characteristics. Feeding Ae. albopictus larvae with E. coli-producing InR dsRNA blocks the ERK and AKT cascades and interferes with the development of mosquito. Our data suggest that IS pathway plays an important role in the metabolism and developmental process and could represent a potential target for controlling mosquito-borne diseases.

Insulin-like peptides regulate oogenesis by stimulating ovarian ecdysteroid production in the Indian malaria mosquito Anopheles stephensi

Females of many mosquito species feed on vertebrate blood to produce eggs, making them effective disease vectors. In the dengue vector Aedes aegypti , blood feeding signals the brain to release ovary ecdysteroidogenic hormone (OEH) and insulin-like peptides (ILPs) that trigger ecdysteroid production by the ovaries. These ecdysteroids regulate synthesis of the yolk protein vitellogenin (Vg) that is packaged into eggs. Less is known about the reproductive biology of Anopheles mosquitoes, which pose a greater public health threat than Aedes spp. because they are competent to transmit mammalian malaria. ILPs can trigger An. stephensi ovaries to secrete ecdysteroids. Unlike Ae. aegypti , Anopheles also transfer ecdysteroids from Anopheles males to females during mating. To elucidate the role of OEH and ILPs in An. stephensi , we decapitated blood-fed females to ablate the source of these peptides and injected them with each hormone. Yolk deposition into oocytes was abolished in decapitated females and rescued by ILP injection. ILP activity was dependent on blood feeding and little change in triglyceride and glycogen stores was observed in response to blood-feeding, suggesting this species requires nutrients from blood to form eggs. We also measured egg maturation, ecdysteroid titers, and yolk protein expression in mated and virgin females. Although yolk deposition into developing oocytes was significantly reduced in virgins compared to mated females, no differences in ecdysteroid titers or Vg transcript abundance were detected between these groups. 20-hydroxyecdysone (20E) stimulated Vg expression in female fat bodies in primary culture. Given these results, we conclude that ILPs control egg formation by regulating ecdysteroid production in the ovaries.

Adipokinetic hormone signaling in the malaria vector Anopheles gambiae facilitates Plasmodium falciparum sporogony

An obligatory step in the complex life cycle of the malaria parasite is sporogony, which occurs during the oocyst stage in adult female Anopheles mosquitoes. Sporogony is metabolically demanding, and successful oocyst maturation is dependent on host lipids. In insects, lipid energy reserves are mobilized by adipokinetic hormones (AKHs). We hypothesized that Plasmodium falciparum infection activates Anopheles gambiae AKH signaling and lipid mobilization. We profiled the expression patterns of AKH pathway genes and AgAkh1 peptide levels in An. gambiae during starvation, after blood feeding, and following infection and observed a significant time-dependent up-regulation of AKH pathway genes and peptide levels during infection. Depletion of AgAkh1 and AgAkhR by RNAi reduced salivary gland sporozoite production, while synthetic AgAkh1 peptide supplementation rescued sporozoite numbers. Inoculation of uninfected female mosquitoes with supernatant from P. falciparum-infected midguts activated AKH signaling. Clearly, identifying the parasite molecules mediating AKH signaling in P. falciparum sporogony is paramount.

Increased insulin signaling in the Anopheles stephensi fat body regulates metabolism and enhances the host response to both bacterial challenge and Plasmodium falciparum infection

In vertebrates and invertebrates, the insulin/insulin-like growth factor 1 (IGF1) signaling (IIS) cascade is highly conserved and plays a vital role in many different physiological processes. Among the many tissues that respond to IIS in mosquitoes, the fat body has a central role in metabolism, lifespan, reproduction, and innate immunity. We previously demonstrated that fat body specific expression of active Akt, a key IIS signaling molecule, in adult Anopheles stephensi and Aedes aegypti activated the IIS cascade and extended lifespan. Additionally, we found that transgenic females produced more vitellogenin (Vg) protein than non-transgenic mosquitoes, although this did not translate into increased fecundity. These results prompted us to further examine how IIS impacts immunity, metabolism, growth and development of these transgenic mosquitoes. We observed significant changes in glycogen, trehalose, triglycerides, glucose, and protein in young (3-5 d) transgenic mosquitoes relative to non-transgenic sibling controls, while only triglycerides were significantly changed in older (18 d) transgenic mosquitoes. More importantly, we demonstrated that enhanced fat body IIS decreased both the prevalence and intensity of Plasmodium falciparum infection in transgenic An. stephensi. Additionally, challenging transgenic An. stephensi with Gram-positive and Gram-negative bacteria altered the expression of several antimicrobial peptides (AMPs) and two anti-Plasmodium genes, nitric oxide synthase (NOS) and thioester complement-like protein (TEP1), relative to non-transgenic controls. Increased IIS in the fat body of adult female An. stephensi had little to no impact on body size, growth or development of progeny from transgenic mosquitoes relative to non-transgenic controls. This study both confirms and expands our understanding of the critical roles insulin signaling plays in regulating the diverse functions of the mosquito fat body.

Insulin-Like Peptides and Cross-Talk With Other Factors in the Regulation of Insect Metabolism

The insulin-like peptide (ILP) and insulin-like growth factor (IGF) signalling pathways play a crucial role in the regulation of metabolism, growth and development, fecundity, stress resistance, and lifespan. ILPs are encoded by multigene families that are expressed in nervous and non-nervous organs, including the midgut, salivary glands, and fat body, in a tissue- and stage-specific manner. Thus, more multidirectional and more complex control of insect metabolism can occur. ILPs are not the only factors that regulate metabolism. ILPs interact in many cross-talk interactions of different factors, for example, hormones (peptide and nonpeptide), neurotransmitters and growth factors. These interactions are observed at different levels, and three interactions appear to be the most prominent/significant: (1) coinfluence of ILPs and other factors on the same target cells, (2) influence of ILPs on synthesis/secretion of other factors regulating metabolism, and (3) regulation of activity of cells producing/secreting ILPs by various factors. For example, brain insulin-producing cells co-express sulfakinins (SKs), which are cholecystokinin-like peptides, another key regulator of metabolism, and express receptors for tachykinin-related peptides, the next peptide hormones involved in the control of metabolism. It was also shown that ILPs in Drosophila melanogaster can directly and indirectly regulate AKH. This review presents an overview of the regulatory role of insulin-like peptides in insect metabolism and how these factors interact with other players involved in its regulation.

Prediction of neuropeptide precursors and differential expression of adipokinetic hormone/corazonin-related peptide, hugin and corazonin in the brain of malaria vector Nyssorhynchus albimanus during a Plasmodium berghei infection

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We describe precursors that predicted at least sixty neuropeptides in Ny. albimanus.

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At least 16 precursors are encoded in the Ny. albimanus brain.

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Myosuppressin neuropeptide precursor was identified in Ny albimanus.

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acp and hugin transcripts increased in Ny. albimanus brains infected with P. berghei.

Insect neuropeptides, play a central role in the control of many physiological processes. Based on an analysis of Nyssorhynchus albimanus brain transcriptome a neuropeptide precursor database of the mosquito was described. Also, we observed that adipokinetic hormone/corazonin-related peptide (ACP), hugin and corazonin encoding genes were differentially expressed during Plasmodium infection. Transcriptomic data from Ny. albimanus brain identified 29 pre-propeptides deduced from the sequences that allowed the prediction of at least 60 neuropeptides. The predicted peptides include isoforms of allatostatin C, orcokinin, corazonin, adipokinetic hormone (AKH), SIFamide, capa, hugin, pigment-dispersing factor, adipokinetic hormone/corazonin-related peptide (ACP), tachykinin-related peptide, trissin, neuropeptide F, diuretic hormone 31, bursicon, crustacean cardioactive peptide (CCAP), allatotropin, allatostatin A, ecdysis triggering hormone (ETH), diuretic hormone 44 (Dh44), insulin-like peptides (ILPs) and eclosion hormone (EH). The analysis of the genome of An. albimanus and the generated transcriptome, provided evidence for the identification of myosuppressin neuropeptide precursor. A quantitative analysis documented increased expression of precursors encoding ACP peptide, hugin and corazonin in the mosquito brain after Plasmodium berghei infection. This work represents an initial effort to characterize the neuropeptide precursors repertoire of Ny. albimanus and provides information for understanding neuroregulation of the mosquito response during Plasmodium infection.

Midgut Mitochondrial Function as a Gatekeeper for Malaria Parasite Infection and Development in the Mosquito Host

Across diverse organisms, various physiologies are profoundly regulated by mitochondrial function, which is defined by mitochondrial fusion, biogenesis, oxidative phosphorylation (OXPHOS), and mitophagy. Based on our data and significant published studies from Caenorhabditis elegans, Drosophila melanogaster and mammals, we propose that midgut mitochondria control midgut health and the health of other tissues in vector mosquitoes. Specifically, we argue that trade-offs among resistance to infection, metabolism, lifespan, and reproduction in vector mosquitoes are fundamentally controlled both locally and systemically by midgut mitochondrial function.

Developmental and comparative perspectives on mosquito immunity

Diseases spread by mosquitoes have killed more people than those spread by any other group of arthropod vectors and remain an important factor in determining global health and economic stability. The mosquito innate immune system can act to either modulate infection with human pathogens or fight off entomopathogens and increase the fitness and longevity of infected mosquitoes. While work remains towards understanding the larval immune system and the development of the mosquito immune system, it has recently become clearer that environmental factors heavily shape the developing mosquito immune system and continue to influence the adult immune system as well. The adult immune system has been well-studied and is known to involve multiple tissues and diverse molecular mechanisms. This review summarizes and synthesizes what is currently understood about the development of the mosquito immune system and includes comparisons of immune components unique to mosquitoes among the blood-feeding arthropods as well as important distinguishing factors between the anopheline and culicine mosquitoes. An explanation is included for how mosquito immunity factors into vector competence and vectorial capacity is presented along with a model for the interrelationships between nutrition, microbiome, pathogen interactions and behavior as they relate to mosquito development, immune status, adult female fitness and ultimately, vectorial capacity. Novel discoveries in the fields of mosquito ecoimmunology, neuroimmunology, and intracellular antiviral responses are highlighted.

Comprehensive and Durable Modulation of Growth, Development, Lifespan and Fecundity in Anopheles stephensi Following Larval Treatment With the Stress Signaling Molecule and Novel Antimalarial Abscisic Acid

The larval environment of holometabolous insects determines many adult life history traits including, but not limited to, rate and success of development and adult lifespan and fecundity. The ancient stress signaling hormone abscisic acid (ABA), released by plants inundated with water and by leaf and root fragments in water, is likely ubiquitous in the mosquito larval environment and is well known for its wide ranging effects on invertebrate biology. Accordingly, ABA is a relevant stimulus and signal for mosquito development. In our studies, the addition of ABA at biologically relevant levels to larval rearing containers accelerated the time to pupation and increased death of A. stephensi pupae. We could not attribute these effects, however, to ABA-dependent changes in JH biosynthesis-associated gene expression, 20E titers or transcript patterns of insulin-like peptide genes. Adult females derived from ABA-treated larvae had reduced total protein content and significantly reduced post blood meal transcript expression of vitellogenin, effects that were consistent with variably reduced egg clutch sizes and oviposition success from the first through the third gonotrophic cycles. Adult female A. stephensi derived from ABA-treated larvae also exhibited reduced lifespans relative to controls. Collectively, these effects of ABA on A. stephensi life history traits are robust, durable and predictive of multiple impacts of an important malaria vector spreading to new malaria endemic regions.

Distribution of Pheromone Biosynthesis-Activating Neuropeptide in the Central Nervous System of Plutella xylostella (Lepidoptera: Plutellidae)

Insect neuropeptides in the pyrokinin/pheromone biosynthesis-activating neuropeptide (PBAN) family are actively involved in many essential endocrinal functions and serve as potential targets in the search for novel insect control agents. Here, we dissect the nervous system of larval, pupal, and adult Plutella xylostella (L.) (Lepidoptera: Plutellidae) and describe the ganglion morphology and localization of PBAN during different insect developmental stages. Our results show that the central nervous system (CNS) of this species consists of four types of ganglia: cerebral ganglia (brain), subesophageal ganglion (SEG), thoracic ganglia, and abdominal ganglia. A two-lobed brain is connected to the reniform SEG with a nerve cord in larvae and prepupae, whereas in the late pupae and adults, the brain and SEG are fused, forming a brain-SEG complex. The larvae and prepupae have eight abdominal ganglia each, whereas the late pupae and adults each have four abdominal ganglia. Furthermore, all life stages of P. xylostella had similar patterns of PBAN immunoreactivity in the CNS, and the accumulation of PBAN was similar during all life stages except in adult males. PBAN immunoreactive signals were observed in the brain and SEG, and fluorescence signals originating in the SEG extended the entire length of the ventral nerve cord, ending in the terminal abdominal ganglia. Our results provide morphological data that inform the development and evolution of the CNS. In addition, they indicate that the nervous system contains PBAN, which could be used to control P. xylostella populations.

Increased Akt signaling in the fat body of Anopheles stephensi extends lifespan and increases lifetime fecundity through modulation of insulin-like peptides

Insulin-like peptides (ILPs) and the insulin/insulin-like growth factor 1 signaling (IIS) cascade regulate numerous physiological functions, including lifespan, reproduction, immunity, and metabolism, in diverse eukaryotes. We previously demonstrated that in female Anopheles stephensi and Aedes aegypti mosquitoes, activation of the IIS cascade in the fat body led to a significant increase in lifespan. In this work, we elucidated two putative mechanisms in A. stephensi behind the observed lifespan extension and assessed whether this lifespan extension confers an overall fitness advantage to the mosquito. Specifically, we demonstrated that increased Akt signaling in the mosquito fat body following a blood meal significantly suppressed the expression of ILP2 in the head. Moreover, overexpression of active Akt in the fat body altered the expression of a putative insulin binding protein ortholog, Imaginal morphogenesis protein-Late 2 (Imp-L2), in response to transgene expression. Combined, these two factors may act to reduce overall levels of circulating ILP2 or other ILPs in the mosquito, in turn conferring increased survival. We also examined the impact increased fat body IIS had on lifetime fecundity and demonstrated that transgenic female mosquito populations had higher lifetime fecundity relative to non-transgenic sibling controls. These studies provide new insights into the complex hormonal and molecular mechanisms regulating the interplay between IIS, aging, and reproduction in this important vector of human malaria parasites.

Insulin-Like Peptide Signaling in Mosquitoes: The Road Behind and the Road Ahead

Insulin signaling is a conserved pathway in all metazoans. This pathway contributed toward primordial metazoans responding to a greater diversity of environmental signals by modulating nutritional storage, reproduction, and longevity. Most of our knowledge of insulin signaling in insects comes from the vinegar fly, Drosophila melanogaster, where it has been extensively studied and shown to control several physiological processes. Mosquitoes are the most important vectors of human disease in the world and their control constitutes a significant area of research. Recent studies have shown the importance of insulin signaling in multiple physiological processes such as reproduction, innate immunity, lifespan, and vectorial capacity in mosquitoes. Although insulin-like peptides have been identified and functionally characterized from many mosquito species, a comprehensive review of this pathway in mosquitoes is needed. To fill this gap, our review provides up-to-date knowledge of this subfield.

Structures of insect Imp-L2 suggest an alternative strategy for regulating the bioavailability of insulin-like hormones

The insulin/insulin-like growth factor signalling axis is an evolutionary ancient and highly conserved hormonal system involved in the regulation of metabolism, growth and lifespan in animals. Human insulin is stored in the pancreas, while insulin-like growth factor-1 (IGF-1) is maintained in blood in complexes with IGF-binding proteins (IGFBP1-6). Insect insulin-like polypeptide binding proteins (IBPs) have been considered as IGFBP-like structural and functional homologues. Here, we report structures of the Drosophila IBP Imp-L2 in its free form and bound to Drosophila insulin-like peptide 5 and human IGF-1. Imp-L2 contains two immunoglobulin-like fold domains and its architecture is unrelated to human IGFBPs, suggesting a distinct strategy for bioavailability regulation of insulin-like hormones. Similar hormone binding modes may exist in other insect vectors, as the IBP sequences are highly conserved. Therefore, these findings may open research routes towards a rational interference of transmission of diseases such as malaria, dengue and yellow fevers.

Insulin receptor knockdown blocks filarial parasite development and alters egg production in the southern house mosquito, Culex quinquefasciatus

Lymphatic filariasis, commonly known as elephantiasis, is a painful and profoundly disfiguring disease. Wuchreria bancrofti (Wb) is responsible for >90% of infections and the remainder are caused by Brugia spp. Mosquitoes of the genera Culex (in urban and semi-urban areas), Anopheles (in rural areas of Africa and elsewhere), and Aedes (in Pacific islands) are the major vectors of W. bancrofti. A preventive chemotherapy called mass drug administration (MDA), including albendazole with ivermectin or diethylcarbamazine citrate (DEC) is used in endemic areas. Vector control strategies such as residual insecticide spraying and long-lasting insecticidal nets are supplemental to the core strategy of MDA to enhance elimination efforts. However, increasing insecticide resistance in mosquitoes and drug resistance in parasite limit the effectiveness of existing interventions, and new measures are needed for mosquito population control and disruption of mosquito-parasite interactions to reduce transmission. Mosquito insulin signaling regulates nutrient metabolism and has been implicated in reduced prevalence and intensity of malaria parasite, Plasmodium falciparum, infection in mosquitoes. Currently no data are available to assess how insulin signaling in mosquitoes affects the development of multi-cellular parasites, such as filarial nematodes. Here, we show that insulin receptor knockdown in blood fed C. quinquefasciatus, the major vector of Wb in India, completely blocks the development of filarial nematode parasite to the infective L3 stage, and results in decreased ecdysteroid production and trypsin activity leading to fewer mosquito eggs. These data indicate that a functional mosquito insulin receptor (IR) is necessary for filarial parasite development and mosquito reproduction. Therefore, insulin signaling may represent a new target for the development of vector control or parasite blocking strategies.

Lymphatic filariasis (LF) is caused by infection with nematodes of the family Filarioidea. 90% of infections are caused by Wuchereria bancrofti and the remainder by Brugia spp. In endemic countries, LF has a major social and economic impact with an estimated annual loss of $1 billion. Filarial infection can cause a variety of clinical manifestations, including lymphoedema of the limbs, genital disease (hydrocele, and swelling of the scrotum and penis) and recurrent acute attacks, which are extremely painful and are accompanied by fever. As one of the leading causes of global disability, LF accounts for at least 2.8 million disability-adjusted life year (DALY). Mass drug administration (MDA) is used prophylactically on the community level where the infection is present to decrease disease transmission. These drugs have limited effect on adult parasites but effectively reduce microfilariae in the bloodstream and prevent the spread of microfilaria to mosquitoes. Use of mosquito population control strategies is supplemental to the core strategy of MDA. However, increasing insecticide resistance in mosquitoes and drug resistant nematode parasites are complicating elimination efforts and emphasizes the need for novel interventions for vector control and parasite transmission. Insulin signaling is a highly conserved signaling pathway that regulates growth and nutrient homeostasis in animals. Our previous work in Aedes aegypti mosquitoes showed additional roles of insulin receptor signaling in blood digestion and reproduction. The present data strongly supports our previous findings in a different mosquito species and further explores the role of mosquito insulin receptor in the development of the filarial nematode to the infective stage. This information is pertinent to ongoing efforts to control and eradicate filariasis because insulin signaling may represent a new target for the development of vector control or transmission blocking strategies.

Isolation of an insulin-like peptide from the Asian malaria mosquito, Anopheles stephensi, that acts as a steroidogenic gonadotropin across diverse mosquito taxa

Many insulin-like peptides (ILPs) have been identified in insects, yet only a few were isolated in their native form for structural and functional studies. Antiserum produced to ILP3 in Aedes aegypti was used in a radioimmunoassay to monitor the purification of an ILP from heads of adult An. stephensi and recognized the ILP in other immunoassays. The structure of the purified peptide matched that predicted for the ILP3 in this species. The native form stimulated ecdysteroid production by ovaries isolated from non-blood fed females. Synthetic forms of An. stephensi ILP3 and ILP4 similarly activated this process in a dose responsive manner. This function was first established for ILP3 and ILP4 homologs in Aedes aegypti, thus suggesting their structural and functional conservation in mosquitoes. We tested the extent of conservation by treating ovaries of An. gambiae, Ae. aegypti, and Culex quinquefasciatus with the An. stephensi ILPs, and both the native and synthetic ILP3 were stimulatory, as was the ILP4. Taken together, these results offer the first evidence for ILP functional conservation across the Anophelinae and Culicinae subfamilies.

Molecular identification of an insulin growth factor binding protein (IGFBP) and its potential role in an insulin-like peptide system of the pearl oyster, Pinctada fucata

Insulin-like growth factors (IGFs) play critical roles in regulating metabolism, growth, and reproduction in invertebrates. IGF binding proteins (IGFBPs) serve as major regulators of IGF activity and regulate endocrine system. In the present study, the full-length cDNA of an igfbp was identified from the pearl oyster, Pinctada fucata, using expressed sequence tag (EST) sequence. The 1124bp Pfigfbp cDNA contains a 465bp open reading frame (ORF) encoding a putative protein of 154 amino acids, a 5'-untranslated region (UTR) of 238bp, and a 3'-UTR of 394bp (not including polyA+). Multiple sequence alignment of the deduced IB domain sequences revealed that twelve conserved Cys and ILP binding site in PfIGFBP were well aligned with human IGFBPs1-7, Mizuhopecten yessoensis IGFBP5 and Eriocheir sinensis IGFBP7. Gene expression analysis indicated that Pfigfbp mRNA was expressed in all the tissues and developmental stages examined, with a higher level in the foot than in other tissues and a higher level in the polar body stage and 32-cell stage than in the other stages. Pfigfbp and PfILP (insulin-like peptide) mRNA levels significantly increased in the digestive gland after feeding, while levels were dramatically reduced during a week of food deprivation and increased upon refeeding. In vitro experiments indicated that Pfigfbp mRNA expression in mantle cells was affected by insulin/IGFs (IGF-I, IGF-II). Our data suggests that Pfigfbp may be involved in endocrine signaling in P. fucata via the regulation of insulin-like peptide signaling.

In vitro stimulation of vitellogenin expression by insulin in the mud crab, Scylla paramamosain, mediated through PI3K/Akt/TOR pathway

Vitellogenin (vtg) synthesis, known as vitellogenesis, is one of most important processes in the ovarian development of oviparous animals. Recently, multiple insulin-like peptides (ILPs) have been reported in crustacean species due to the application of transcriptome sequencing. In this context, the present study reports that the addition of an exogenous ILP, bovine insulin, stimulates vtg (termed Sp-vtg) expression in hepatopancreatic explants from the mud crab, Scylla paramamosain, by in vitro experiments. Homologous genes of key factors in ILP signaling, Sp-PI3K, Sp-Akt, Sp-Rheb and Sp-TOR, have been isolated in S. paramamosain based on a transcriptome database. Further experiments reveal that the RNAi-mediated Sp-Akt gene knockdown and the inhibitors of Sp-PI3K and Sp-TOR block the stimulation of Sp-vtg expression by insulin. The combined results implicate the endogenous ILP and its corresponding signaling in the regulation of Sp-vtg synthesis in S. paramamosain.

Does a blue crab putative insulin-like peptide binding protein (ILPBP) play a role in a virus infection?

Insulin-like peptides (ILPs) have regulatory roles in reproduction, development and metabolism in invertebrates. The mode of ILP actions has not been well studied in invertebrates in regard to the role of binding partners, i.e., ILP binding protein (ILPBP). In this study, the full-length cDNA of Callinectes sapidus ILPBP (Cas-ILPBP, 960 bp) has been isolated using RACE cloning, having short 5' and 3' UTRs of 30 and 162 bp, respectively. The predicted precursor of Cas-ILPBP (255 aa) contains, in order a signal peptide (23 aa), an insulin-like growth factor (IGF) binding (IB) domain (79 aa), a kazal-type serine protease inhibitor (KI) domain (36 aa) and an immunoglobulin (Ig) domain (101 aa). Phylogenetic analysis shows that Cas-ILPBP is grouped with the ILPBPs of other crustacean species, and it shares the closest relationship with the ILPBP from another crab species, Scylla paramamosain. Transcripts of Cas-ILPBP are found in all examined tissues, with the highest levels in the nervous tissues (eyestalk ganglia, brain and thoracic ganglia complex) and followed by midgut, the pericardial organ, abdominal muscle and the heart. As Cas-ILPBP contains a putative Ig domain, it is hypothesized that this protein may be involved in immunity, particularly in the adult females infected with a reo-like virus (CsRV1). The expression levels of Cas-ILPBP are examined in several tissues (hemocytes, midgut, eyestalk ganglia) from the animals carrying varying levels of CsRV1 at 17 and 23 °C water temperatures. Cas-ILPBP levels in the midgut are most significantly affected by high levels of CsRV1 infection. Reduction in Cas-ILPBP levels in the midguts is noted from the animals infected with high levels of CsRV1 that show reduced or stop feeding activity, indicating that it may play an important role in midgut functions such as digestion and nutrient absorption.

A viral histone H4 suppresses insect insulin signal and delays host development

Parasitization by an endoparasitoid wasp, Cotesia plutellae, alters host development of Plutella xylostella by extending larval period and preventing metamorphosis. Insulin signal plays a crucial role in mediating insect development and controlling blood sugar level in insects. In this study, three insulin-like peptide genes (PxILP1-3) were predicted from the genome of P. xylostella. However, only PxILP1 was confirmed to be expressed in P. xylostella. Starvation suppressed the expression level of PxILP1 and up-regulated plasma trehalose level. RNA interference against PxILP1 mimicked starvation effect and extended the larval period of P. xylostella. Parasitized larvae exhibited significantly lower levels of PxILP1 expression compared to nonparasitized larvae. Injection of wasp-symbiotic polydnavirus C. plutellae bracovirus (CpBV) also suppressed PxILP1 expression and extended the larval period. Injection of a viral segment (CpBV-S30) containing a viral histone H4 (CpBV-H4) also suppressed PxILP1 expression. Co-injection of CpBV-S30 and double-stranded RNA (dsCpBV-H4) specific to CpBV-H4 rescued the suppression of PxILP1 expression. Injection of CpBV-S30 significantly extended larval development. Co-injection of CpBV-S30 with dsCpBV-H4 rescued the delay of larval development. Injection of a bovine insulin to parasitized larvae prevented parasitoid development. These results indicate that parasitism of C. plutellae can down-regulate host insulin signaling with the help of parasitic factor CpBV-H4.

Two insulin-like peptides differentially regulate malaria parasite infection in the mosquito through effects on intermediary metabolism

Insulin-like peptides (ILPs) play important roles in growth and metabolic homeostasis, but have also emerged as key regulators of stress responses and immunity in a variety of vertebrates and invertebrates. Furthermore, a growing literature suggests that insulin signaling-dependent metabolic provisioning can influence host responses to infection and affect infection outcomes. In line with these studies, we previously showed that knockdown of either of two closely related, infection-induced ILPs, ILP3 and ILP4, in the mosquito Anopheles stephensi decreased infection with the human malaria parasite Plasmodium falciparum through kinetically distinct effects on parasite death. However, the precise mechanisms by which ILP3 and ILP4 control the response to infection remained unknown. To address this knowledge gap, we used a complementary approach of direct ILP supplementation into the blood meal to further define ILP-specific effects on mosquito biology and parasite infection. Notably, we observed that feeding resulted in differential effects of ILP3 and ILP4 on blood-feeding behavior and P. falciparum development. These effects depended on ILP-specific regulation of intermediary metabolism in the mosquito midgut, suggesting a major contribution of ILP-dependent metabolic shifts to the regulation of infection resistance and parasite transmission. Accordingly, our data implicate endogenous ILP signaling in balancing intermediary metabolism for the host response to infection, affirming this emerging tenet in host-pathogen interactions with novel insights from a system of significant public health importance.

Mosquito Peptide Hormones: Diversity, Production, and Function

Mosquitoes, like other insects, produce a diversity of peptide hormones that are processed from different precursor proteins and have a range of activities. Early studies relied on purification of bioactive peptides for hormone identification, but more recently genomic data have provided the information needed to more comprehensively identify peptide hormone genes and associated receptors. The first part of this chapter summarizes the known or predicted peptide hormones that are produced by mosquitoes. The second part of this chapter discusses the sources of these molecules and their functions.

Insulin/IGF signaling in Drosophila and other insects: factors that regulate production, release and post-release action of the insulin-like peptides

Insulin, insulin-like growth factors (IGFs) and insulin-like peptides (ILPs) are important regulators of metabolism, growth, reproduction and lifespan, and mechanisms of insulin/IGF signaling (IIS) have been well conserved over evolution. In insects, between one and 38 ILPs have been identified in each species. Relatively few insect species have been investigated in depth with respect to ILP functions, and therefore we focus mainly on the well-studied fruitfly Drosophila melanogaster. In Drosophila eight ILPs (DILP1-8), but only two receptors (dInR and Lgr3) are known. DILP2, 3 and 5 are produced by a set of neurosecretory cells (IPCs) in the brain and their biosynthesis and release are controlled by a number of mechanisms differing between larvae and adults. Adult IPCs display cell-autonomous sensing of circulating glucose, coupled to evolutionarily conserved mechanisms for DILP release. The glucose-mediated DILP secretion is modulated by neurotransmitters and neuropeptides, as well as by factors released from the intestine and adipocytes. Larval IPCs, however, are indirectly regulated by glucose-sensing endocrine cells producing adipokinetic hormone, or by circulating factors from the intestine and fat body. Furthermore, IIS is situated within a complex physiological regulatory network that also encompasses the lipophilic hormones, 20-hydroxyecdysone and juvenile hormone. After release from IPCs, the ILP action can be modulated by circulating proteins that act either as protective carriers (binding proteins), or competitive inhibitors. Some of these proteins appear to have additional functions that are independent of ILPs. Taken together, the signaling with multiple ILPs is under complex control, ensuring tightly regulated IIS in the organism.

Insights into insulin-like peptide system in invertebrates from studies on IGF binding domain-containing proteins in the female mud crab, Scylla paramamosain

Insulin-like peptides (ILPs) have been proved to exist extensively in invertebrates and play critical roles in regulating growth, metabolism and reproduction. ILP signaling system has been well defined in insects, with all key components homologous with vertebrate IGF signaling; however, counterparts of IGF binding proteins (IGFBPs) in vertebrates are not included in this system because of lacking sufficient researches in the related aspect. The present study firstly reports the identification of three kinds of invertebrate IGF binding (IB) domain-containing protein genes from the mud crab Scylla paramamosain. Gene expression analysis suggested that they might be closely involved in ovarian development, but with separate roles. Subsequent bioinformatics analysis and in vitro experiments indicated that they are likely to serve as endogenous ILP-specific binding proteins in invertebrates. More importantly, based on the current evidence we inferred that in invertebrate, ILP system might take the place of IGF system in vertebrate species.

Multiple factors contribute to anautogenous reproduction by the mosquito Aedes aegypti

Aedes aegypti is an anautogenous mosquito that must blood feed on a vertebrate host to produce and lay a clutch of eggs. The rockpool mosquito, Georgecraigius atropalpus, is related to A. aegypti but is a facultatively autogenous species that produces its first clutch of eggs shortly after emerging without blood feeding. Consumption of a blood meal by A. aegypti triggers the release of ovary ecdysteroidogenic hormone (OEH) and insulin-like peptide 3 (ILP3) from the brain, which stimulate egg formation. OEH and ILP3 also stimulate egg formation in G. atropalpus but are released at eclosion independently of blood feeding. These results collectively suggest that blood meal dependent release of OEH and ILP3 is one factor that prevents A. aegypti from reproducing autogenously. Here, we examined two other factors that potentially inhibit autogeny in A. aegypti: teneral nutrient reserves and the ability of OEH and ILP3 to stimulate egg formation in the absence of blood feeding. Measures of nutrient reserves showed that newly emerged A. aegypti females had similar wet weights but significantly lower protein and glycogen reserves than G. atropalpus females when larvae were reared under identical conditions. OEH stimulated non-blood fed A. aegypti females to produce ecdysteroid hormone and package yolk into oocytes more strongly than ILP3. OEH also reduced host seeking and blood feeding behavior, yet females produced few mature eggs. Overall, our results indicate that multiple factors prevent A. aegypti from reproducing autogenously.

Plasmodium falciparum suppresses the host immune response by inducing the synthesis of insulin-like peptides (ILPs) in the mosquito Anopheles stephensi

The insulin-like peptides (ILPs) and their respective signaling and regulatory pathways are highly conserved across phyla. In invertebrates, ILPs regulate diverse physiological processes, including metabolism, reproduction, behavior, and immunity. We previously reported that blood feeding alone induced minimal changes in ILP expression in Anopheles stephensi. However, ingestion of a blood meal containing human insulin or Plasmodium falciparum, which can mimic insulin signaling, leads to significant increases in ILP expression in the head and midgut, suggesting a potential role for AsILPs in the regulation of P. falciparum sporogonic development. Here, we show that soluble P. falciparum products, but not LPS or zymosan, directly induced AsILP expression in immortalized A. stephensi cells in vitro. Further, AsILP expression is dependent on signaling by the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (MEK/ERK) and phosphatidylinositol 3'-kinase (PI3K)/Akt branches of the insulin/insulin-like growth factor signaling (IIS) pathway. Inhibition of P. falciparum-induced ILPs in vivo decreased parasite development through kinetically distinct effects on mosquito innate immune responses. Specifically, knockdown of AsILP4 induced early expression of immune effector genes (1-6 h after infection), a pattern associated with significantly reduced parasite abundance prior to invasion of the midgut epithelium. In contrast, knockdown of AsILP3 increased later expression of the same genes (24 h after infection), a pattern that was associated with significantly reduced oocyst development. These data suggest that P. falciparum parasites alter the expression of mosquito AsILPs to dampen the immune response and facilitate their development in the mosquito vector.

Gene expression changes in the salivary glands of Anopheles coluzzii elicited by Plasmodium berghei infection

Background

Malaria is a devastating infectious disease caused by Plasmodium parasites transmitted through the bites of infected Anopheles mosquitoes. Salivary glands are the only mosquito tissue invaded by Plasmodium sporozoites, being a key stage for the effective parasite transmission, making the study of Anopheles sialome highly relevant.

Methods

RNA-sequencing was used to compare differential gene expression in salivary glands of uninfected and Plasmodium berghei-infected Anopheles coluzzii mosquitoes. RNA-seq results were validated by quantitative RT-PCR. The transmembrane glucose transporter gene AGAP007752 was selected for functional analysis by RNA interference. The effect of gene silencing on infection level was evaluated. The putative function and tertiary structure of the protein was assessed.

Results

RNA-seq data showed that 2588 genes were differentially expressed in mosquitoes salivary glands in response to P. berghei infection, being 1578 upregulated and 1010 downregulated. Metabolism, Immunity, Replication/Transcription/Translation, Proteolysis and Transport were the mosquito gene functional classes more affected by parasite infection. Endopeptidase coding genes were the most abundant within the differentially expressed genes in infected salivary glands (P < 0.001). Based on its putative function and expression level, the transmembrane glucose transporter gene, AGAP007752, was selected for functional analysis by RNA interference. The results demonstrated that the number of sporozoites was 44.3 % lower in mosquitoes fed on infected mice after AGAPP007752 gene knockdown when compared to control (P < 0.01).

Conclusions

Our hypothesis is that the protein encoded by the gene AGAPP007752 may play a role on An. coluzzii salivary glands infection by Plasmodium parasite, working as a sporozoite receptor and/or promoting a favorable environment for the capacity of sporozoites.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-1079-8) contains supplementary material, which is available to authorized users.

Host-pathogen interactions in malaria: cross-kingdom signaling and mitochondrial regulation

Malaria parasite-host interactions are complex and have confounded available drugs and the development of vaccines. Further, we now appreciate that interventions for malaria elimination and eradication must include therapeutics with intrinsic transmission blocking activity to treat the patient and prevent disease spread. Studies over the past 15 years have revealed significant conservation in the response to infection in mosquito and human hosts. More recently, we have recognized that conserved cell signaling cascades in mosquitoes and humans dictate infection outcome through the regulation of mitochondrial function and biogenesis, which feed back to host immunity, basic intermediary metabolism, and stress responses. These responses - reflected clearly in the primeval insect host - provide fertile ground for innovative strategies for both treatment and transmission blocking.

Immune response and insulin signalling alter mosquito feeding behaviour to enhance malaria transmission potential

Malaria parasites alter mosquito feeding behaviour in a way that enhances parasite transmission. This is widely considered a prime example of manipulation of host behaviour to increase onward transmission, but transient immune challenge in the absence of parasites can induce the same behavioural phenotype. Here, we show that alterations in feeding behaviour depend on the timing and dose of immune challenge relative to blood ingestion and that these changes are functionally linked to changes in insulin signalling in the mosquito gut. These results suggest that altered phenotypes derive from insulin signalling-dependent host resource allocation among immunity, blood feeding, and reproduction in a manner that is not specific to malaria parasite infection. We measured large increases in mosquito survival and subsequent transmission potential when feeding patterns are altered. Leveraging these changes in physiology, behaviour and life history could promote effective and sustainable control of female mosquitoes responsible for transmission.

Insulin signaling pathway in the oriental fruit fly: The role of insulin receptor substrate in ovarian development

Insulin signaling pathways have integral roles in regulating organ growth and body size of insects. Here, we identified and characterized six insulin signaling pathway components-InR, IRS, PI3K92E, PI3K21B, Akt, and PDK-from Bactrocera dorsalis. Quantitative real-time polymerase chain reaction was used to establish gene expression profiles for the insulin signaling pathway components for different developmental stages and tissues, and in response to 20-hydroxyecdysone (20E) and starvation. IRS, PI3K92E, and PI3K21B were highly expressed in the head, while InR, Akt, and PDK were most abundant in Malpighian tubules. Both IRS and PI3K92E were highly expressed during the larval-pupal and pupal-adult transition, while the remaining four genes were highly expressed only during the pupal-adult transition. Following initial exposure to 20E, the expression levels of most genes were significantly decreased. However, the expression levels of IRS, PI3K92E, and PI3K21B were significantly increased at 8 and 12h post-treatment compared with the control. Moreover, we found that most insulin signaling pathway genes in B. dorsalis were up-regulated in response to starvation, but decreased when re-fed. On the contrary, transcript levels of PI3K21B decreased significantly during starvation. Furthermore, injection of IRS dsRNA into adult females significantly reduced IRS transcript levels. Suppression of IRS expression inhibited ovarian development, and the average ovary size was reduced by 33% compared with the control. This study provides new insight into the roles of insulin signaling pathway components in B. dorsalis, and demonstrates an important role for IRS in ovarian development.

Implication for the regulation of catabolism drawn from the single insulin-like growth factor binding domain protein (SIBD) gene in the mud crab, Scylla paramamosain

Insulin-like growth factor (IGF) signaling system holds a central position in regulating growth and metabolism in vertebrates. As critical components of this system, the IGF-binding proteins (IGFBPs) play important roles in regulating the biological activities of IGFs. Recently, the single IGF-binding domain protein (SIBD) was identified in invertebrates and its sequence was highly homologous with the N-terminal domain of IGFBP. In view of the possible role as counterparts of vertebrate IGFBPs, SIBDs have attracted the ever-increasing attention. This study reports the identification of a 1284bp SIBD gene (Sp-SIBD) from a member of commercially important family of Portunidae. The tissue distribution analysis showed that Sp-SIBD was mainly expressed in the nervous tissues and hepatopancreas. RNA in situ hybridization analysis showed that the positive signals were predominantly distributed in the secretory cells of the hepatopancreas. Subsequently, we examined the effects of various stresses, including hyperosmotic stress, hyperthermia, activated stress and fasting, on glucose levels in the hemolymph and Sp-SIBD expressions in the hepatopancreas. Interestingly, we found that Sp-SIBD expression was strongly up-regulated in response to these catabolic circumstances. Given the previous findings of insulin-like peptides (ILPs) in invertebrates, we speculate that invertebrate ILPs and SIBDs promise to serve as a pair of counterparts of IGFs and IGFBPs from vertebrate species respectively. In this context, the combined results suggested, by analogy with IGFBP 1 from vertebrates, for the first time that SIBD might play a key physiological role by sequestering ILPs to inhibit energy-expensive growth until conditions are more favorable.

Discovery of a novel insulin-like peptide and insulin binding proteins in the Eastern rock lobster Sagmariasus verreauxi

This study reports, for the first time in any of the commercially important decapod species, the identification of an insulin-like peptide (ILP), distinct from the androgenic gland hormone. Bioinformatics analysis of the de novo assembled spiny lobster, (Sagmariasus verreauxi) transcriptome, allowed identification of Sv-ILP1 as well as eight binding proteins. Binding proteins were termed as Sv-IGFBP, due to homology with the vertebrate insulin-like growth-factor binding protein and Sv-SIBD1-7, single insulin-binding domain protein (SIBD), similar to those identified in other invertebrate species. Sv-ILP1 was found to be expressed in the eyestalk, gonads and antennal gland of both sexes and to a lesser extent in male muscle, androgenic gland and hepatopancreas. The expression profiles of each binding protein were found to vary across tissues, with Sv-SIBD5, 6 and 7 showing higher expression in the gonad, demonstrated by PCR and digital gene expression. Further spatial investigations, using in-situ hybridisation, found Sv-ILP1 to be expressed in the neurosecretory cells of the thoracic ganglia, in keeping with the tissue expression of Drosophila ILP7 (DILP7). This correlative tissue expression, considered with the phylogenetic clustering of Sv-ILP1 and DILP7, suggests Sv-ILP1 to be a DILP7 orthologue. The broad expression of Sv-ILP1 strongly suggests that ILPs have a role beyond that of masculinisation in decapods. The function of these novel peptides may have application in enhancing aquaculture practices in the commercially important decapod species.

Juvenile Hormone Biosynthesis in Insects: What Is New, What Do We Know, and What Questions Remain?

Our understanding of JH biosynthesis has significantly changed in the last years. In this review I would like to discuss the following topics: (1) the progresses in understanding the JH biosynthesis pathway. Access to genome sequences has facilitated the identification of all the genes encoding biosynthetic enzymes and the completion of comprehensive transcriptional studies, as well as the expression and characterization of recombinant enzymes. Now the existence of different flux directionalites, feed-back loops and pathway branching points in the JH biosynthesis pathways can be explored; (2) the new concepts in the modulation of JH synthesis by allatoregulators. The list of putative JH modulators is increasing. I will discuss their possible role during the different physiological states of the CA; (3) the new theoretical and physiological frameworks for JH synthesis analysis. I will discuss the bases of the flux model for JH biosynthesis. JH plays multiple roles in the control of ovary development in female mosquitoes; therefore, the CA presents different physiological states, where JH synthesis is altered by gating the flux at distinctive points in the pathway; (4) in the final section I will identify new challenges and future directions on JH synthesis research.

Anopheles gambiae blood feeding initiates an anticipatory defense response to Plasmodium berghei

Mosquitoes have potent innate defense mechanisms that protect them from infection by diverse pathogens. Much remains unknown about how different pathogens are sensed and specific responses triggered. Leucine-Rich repeat IMmune proteins (LRIMs) are a mosquito-specific family of putative innate receptors. Although some LRIMs have been implicated in mosquito immune responses, the function of most family members is largely unknown. We screened Anopheles gambiae LRIMs by RNAi for effects on mosquito infection by rodent malaria and found that LRIM9 is a Plasmodium berghei antagonist with phenotypes distinct from family members LRIM1 and APL1C, which are key components of the mosquito complement-like pathway. LRIM9 transcript and protein levels are significantly increased after blood feeding but are unaffected by Plasmodium or midgut microbiota. Interestingly, LRIM9 in the hemolymph is strongly upregulated by direct injection of the ecdysteroid, 20-hydroxyecdysone. Our data suggest that LRIM9 may define a novel anti-Plasmodium immune defense mechanism triggered by blood feeding and that hormonal changes may alert the mosquito to bolster its defenses in anticipation of exposure to blood-borne pathogens.

Effects of ingested vertebrate-derived factors on insect immune responses

During the process of blood feeding insect vectors are exposed to an array of vertebrate-derived blood factors ranging from byproducts of blood meal digestion to naturally occurring products in the blood including growth hormones, cytokines and factors derived from blood-borne pathogens themselves. In this review, we examine the ability of these ingested vertebrate blood factors to alter the innate pathogen defenses of insect vectors. The ability of these factors to modify the immune responses of insect vectors offers new intriguing targets for blocking or reducing transmission of human disease-causing pathogens.

Feeding on a begomovirus-infected plant enhances fecundity via increased expression of an insulin-like peptide in the whitefly, MEAM1

The Middle East-Minor 1 cryptic species (MEAM1), Bemisia tabaci (Gennadius) is a globally invasive pest. It spreads widely due to its high fecundity and mutualistic interactions with the virus they vector. Feeding on virus (tomato yellow leaf curl China virus, TYLCCNV)-infected host plants improves their fecundity, however, the key factor regulating the signaling transduction in reproduction of whitefly remains to be identified. Here, we cloned a full length cDNA encoding an insulin-like peptide in MEAM1 (BtILP1) and investigated its expression profile, functions, and the expression induced by feeding on virus-infected tobacco plants. The full length cDNA of BtILP1 was 590 bps and encoded an open reading frame containing 149 amino acid residues. Multiple sequences alignment results showed BtILP1 contained the structural features typical of the insulin family. Expression dynamics associated with development showed the expression level of BtILP1 peaked at 5 days posteclosion (PE). During 1 to 3 days PE, BtILP1 was expressed highly in the head and abdomen of female adults and highly in the head during 5 to 7 days PE. Knockdown of the BtILP1 expression also impaired vitellogenin gene expression at both transcript and protein levels. Downregulating BtILP1 expression decreased fecundity of female adults and hatching rate of eggs. Feeding on virus-infected tobacco increased BtILP1 expression in MEAM1 female adults. We infer feeding on begomovirus-infected tobacco enhances the reproduction of MEAM1 by inducing BtILP1 expression. Our results give a new sight into the mutualistic interactions between virus and its insect vector.

Starvation increases insulin sensitivity and reduces juvenile hormone synthesis in mosquitoes

Background

The interactions between the insulin signaling pathway (ISP) and juvenile hormone (JH) controlling reproductive trade-offs are well documented in insects. JH and insulin regulate reproductive output in mosquitoes; both hormones are involved in a complex regulatory network, in which they influence each other and in which the mosquito's nutritional status is a crucial determinant of the network's output. Previous studies reported that the insulin-TOR (target of rapamacyn) signaling pathway is involved in the nutritional regulation of JH synthesis in female mosquitoes. The present studies further investigate the regulatory circuitry that controls both JH synthesis and reproductive output in response to nutrient availability.

Methods

We used a combination of diet restriction, RNA interference (RNAi) and insulin treatments to modify insulin signaling and study the cross-talk between insulin and JH in response to starvation. JH synthesis was analyzed using a newly developed assay utilizing fluorescent tags.

Conclusions

Our results reveal that starvation decreased JH synthesis via a decrease in insulin signaling in the corpora allata (CA). Paradoxically, starvation-induced up regulation of insulin receptor transcripts and therefore “primed” the gland to respond rapidly to increases in insulin levels. During this response to starvation the synthetic potential of the CA remained unaffected, and the gland rapidly and efficiently responded to insulin stimulation by increasing JH synthesis to rates similar to those of CA from non-starved females.

Pyrokinin/PBAN-like peptides in the central nervous system of mosquitoes

The pyrokinin/pheromone biosynthesis activating neuropeptide (PBAN) family of peptides is characterized by a common C-terminal pentapeptide, FXPRLamide, which is required for diverse physiological functions in various insects. Polyclonal antisera against the C-terminus was utilized to determine the location of cell bodies and axons in the central nervous systems of larval and adult mosquitoes. Immunoreactive material was detected in three groups of neurons in the subesophageal ganglion of larvae and adults. The corpora cardiaca of both larvae and adults contained immunoreactivity indicating potential release into circulation. The adult and larval brains had at least one pair of immunoreactive neurons in the protocerebrum with the adult brain having additional immunoreactive neurons in the dorsal medial part of the protocerebrum. The ventral ganglia of both larvae and adults each contained one pair of neurons that sent their axons to a perisympathetic organ associated with each abdominal ganglion. These results indicate that the mosquito nervous system contains pyrokinin/PBAN-like peptides and that these peptides could be released into the hemolymph. The peptides in insects and mosquitoes are produced by two genes, capa and pk/pban. Utilizing PCR protocols, we demonstrate that products of the capa gene could be produced in the abdominal ventral ganglia and the products of the pk/pban gene could be produced in the subesophageal ganglion. Two receptors for pyrokinin peptides were differentially localized to various tissues.

The insulin/TOR signal transduction pathway is involved in the nutritional regulation of juvenile hormone synthesis in Aedes aegypti

Juvenile hormone (JH) levels must be modulated to permit the normal progress of development and reproductive maturation in mosquitoes. JH is part of a transduction system that assesses nutritional information and controls reproduction in mosquitoes. Adult female Aedes aegypti show nutritionally-dependent dynamic changes in corpora allata (CA) JH biosynthetic activities. A coordinated expression of most JH biosynthetic enzymes has been described in female pupae and adult mosquitoes; increases or decreases in transcript levels for all the enzymes were concurrent with increases or decreases in JH synthesis; suggesting that transcriptional changes are at least partially responsible for the dynamic changes of JH biosynthesis. The goal of the present study is to identify signaling network components responsible for the nutritional-dependent changes of JH synthesis in the CA of mosquitoes. The insulin/TOR signaling network plays a central role in the transduction of nutritional signals that regulate cell growth and metabolism in insects. These pathways have also been suggested as a link between nutritional signals and JH synthesis regulation in the CA of cockroaches and flies. We used a combination of in vitro studies and in vivo genetic knockdown experiments to explore nutritional signaling pathways in the CA. Our results suggest that the insulin/TOR pathway plays a role in the transduction of the nutritional information that regulates JH synthesis in mosquitoes. Transcriptional regulation of the genes encoding JH biosynthetic enzymes is at least partially responsible for these nutritionally modulated changes of JH biosynthesis.

Human IGF1 extends lifespan and enhances resistance to Plasmodium falciparum infection in the malaria vector Anopheles stephensi

The highly conserved insulin/insulin-like growth factor (IGF) signaling (IIS) pathway regulates metabolism, development, lifespan and immunity across a wide range of organisms. Previous studies have shown that human insulin ingested in the blood meal can activate mosquito IIS, resulting in attenuated lifespan and increased malaria parasite infection. Because human IGF1 is present at higher concentrations in blood than insulin and is functionally linked with lifespan and immune processes, we predicted that human IGF1 ingested in a blood meal would affect lifespan and malaria parasite infection in the mosquito Anopheles stephensi. Here we demonstrate that physiological levels of ingested IGF1, like insulin, can persist intact in the blood-filled midgut for up to 30 h and disseminate into the mosquito body, and that both peptides activate IIS in mosquito cells and midgut. At these same levels, ingested IGF1 alone extended average mosquito lifespan by 23% compared with controls and, more significantly, when ingested in infected blood meals, reduced the prevalence of Plasmodium falciparum-infected mosquitoes by >20% and parasite load by 35-50% compared with controls. Thus, the effects of ingested IGF1 on mosquito lifespan and immunity are opposite to those of ingested insulin. These results offer the first evidence that insect cells can functionally discriminate between mammalian insulin and IGF1. Further, in light of previous success in genetically targeting IIS to alter mosquito lifespan and malaria parasite transmission, this study indicates that a more complete understanding of the IIS-activating ligands in blood can be used to optimize transgenic strategies for malaria control.

The effects of ingested mammalian blood factors on vector arthropod immunity and physiology

The blood feeding behavior of disease-transmitting arthropods creates a unique intersection between vertebrate and invertebrate physiology. Here, we review host blood-derived factors that persist through blood digestion to affect the lifespan, reproduction, and immune responses of some of the most common arthropod vectors of human disease.

Ovary ecdysteroidogenic hormone activates egg maturation in the mosquito Georgecraigius atropalpus after adult eclosion or a blood meal

The rockpool mosquito, Georgecraigius atropalpus, is a facultatively autogenous species that produces its first egg clutch without a blood meal shortly after emergence. Several days after depositing this clutch, females must take a blood meal to produce a second egg clutch. Decapitation of females shortly after emergence or blood ingestion prevents egg maturation. Here, we report that a single injected dose of the neuropeptide ovary ecdysteroidogenic hormone (OEH) fully restored egg maturation in decapitated females in both circumstances. This neuropeptide and two insulin-like peptides (ILPs) are potent gonadotropins in the related yellow fever mosquito, Aedes aegypti. ILP3 was marginally restorative in decapitated G. atropalpus, and ILP4 had no effect. Egg maturation in non- and blood-fed G. atropalpus was dependent on the enzymatic mobilization of amino acids from stored protein or the blood meal for yolk protein (vitellogenin, VG) synthesis and uptake by oocytes. We further show that OEH stimulates serine protease activity in the fat body of newly eclosed females or in the midgut of blood-fed ones, and ecdysteroid hormone production by the ovaries of both females. In contrast, only 20-hydroxyecdysone stimulated VG synthesis in the fat body of non- and blood-fed females. Using RNA interference to knock down expression of the insulin receptor, we found that OEH still fully restored autogenous egg maturation. In summary, our results identify OEH as a primary regulator of egg maturation in both autogenous and blood-fed G. atropalpus females and suggest the shift from blood meal-dependent to blood meal-independent release of OEH is a key factor in the evolution of autogeny in this species.

Enhanced survival of Plasmodium-infected mosquitoes during starvation

Plasmodium spp. are pathogenic to their vertebrate hosts and also apparently, impose a fitness cost on their insect vectors. We show here, however, that Plasmodium-infected mosquitoes survive starvation significantly better than uninfected mosquitoes. This survival advantage during starvation is associated with higher energy resource storage that infected mosquitoes accumulate during period of Plasmodium oocyst development. Microarray analysis revealed that the metabolism of sated mosquitoes is altered in the presence of rapidly growing oocysts, including the down-regulation of several enzymes involved in carbohydrate catabolism. In addition, enhanced expression of several insulin-like peptides was observed in Plasmodium-infected mosquitoes. Blocking insulin-like signaling pathway resulted in impaired Plasmodium development. We conclude that Plasmodium infection alters metabolic pathways in mosquitoes, epitomized by enhanced insulin-like signaling - thereby conferring a survival advantage to the insects during periods of starvation. Manipulation of this pathway might provide new strategies to influence the ability of mosquitoes to survive and transmit the protozoa that cause malaria.

Pioneering immunology: insect style

Insects are a powerful tool for discovering and then dissecting interesting new immunology. Recent insect research has made productive forays into non-classical immune areas including tolerance, immune priming (trained immunity), and environmental effects on immunity. Environments which affect immunity not only include diet and metabolism, but also social interactions and the animal's microbiota. We argue that every process that affects immunity should be considered as part of the immune response and that it is the broad phenomena discovered in insects that will be translated to other organisms rather than fine mechanistic details.