Granular phenoloxidase involved in cuticular melanization in the tobacco hornworm: regulation of its synthesis in the epidermis by juvenile hormone

Tanning of the tarsal and mandibular cuticle in adult Anax imperator (Insecta: Odonata) during the emergence sequence

The arthropod cuticle offers strength, protection, and lightweight. Due to its limit in expandability, arthropods have to moult periodically to grow. While moulting is beneficial in terms of parasite or toxin control, growth and adaptation to environmental conditions, it costs energy and leaves the soft animal's body vulnerable to injuries and desiccation directly after ecdysis. To investigate the temporal change in sclerotization and pigmentation during and after ecdysis, we combined macrophotography, confocal laser scanning microscopy, scanning electron microscopy and histological sectioning. We analysed the tarsal and mandibular cuticle of the blue emperor dragonfly to compare the progress of tanning for structures that are functionally involved during emergence (tarsus/tarsal claws) with structures whose functionality is required much later (mandibles). Our results show that: (i) the tanning of the tarsal and mandibular cuticle increases during emergence; (ii) the tarsal cuticle tans faster than the mandibular cuticle; (iii) the mandibles tan faster on the aboral than on the oral side; and (iv) both the exo- and the endocuticle are tanned. The change in the cuticle composition of the tarsal and mandibular cuticle reflects the demand for higher mechanical stability of these body parts when holding on to the substrate during emergence and during first walking or hunting attempts.

The complex interactions between nutrition, immunity and infection in insects

Insects are the most diverse animal group on the planet. Their success is reflected by the diversity of habitats in which they live. However, these habitats have undergone great changes in recent decades; understanding how these changes affect insect health and fitness is an important challenge for insect conservation. In this Review, we focus on the research that links the nutritional environment with infection and immune status in insects. We first discuss the research from the field of nutritional immunology, and we then investigate how factors such as intracellular and extracellular symbionts, sociality and transgenerational effects may interact with the connection between nutrition and immunity. We show that the interactions between nutrition and resistance can be highly specific to insect species and/or infection type - this is almost certainly due to the diversity of insect social interactions and life cycles, and the varied environments in which insects live. Hence, these connections cannot be easily generalised across insects. We finally suggest that other environmental aspects - such as the use of agrochemicals and climatic factors - might also influence the interaction between nutrition and resistance, and highlight how research on these is essential.

Effect of juvenile hormone on phenoloxidase and hemocyte number: The role of age, sex, and immune challenge

Hormones are key factors in determining the response of organisms to their environment. For example, the juvenile hormone (JH) coordinates the insects' development, reproduction, and survival. However, it is still unclear how the impact of juvenile hormone on insect immunity varies depending on the sex and reproductive state of the individual, as well as the type of the immune challenge (i.e., Gram-positive or Gram-negative bacteria). We used Tenebrio molitor and methoprene, a JH analog (JHa) to explore these relationships. We tested the effect of methoprene on phenoloxidase activity (PO), an important component of humoral immunity in insects, and hemocyte number. Lyophilized Gram-positive Staphylococcus aureus or Gram-negative Escherichia coli were injected for the immune challenge. The results suggest that JH did not affect the proPO, PO activity, or hemocyte number of larvae. JH and immune challenge affected the immune response and consequently, affected adult developmental stage and sex. We propose that the influence of JH on the immune response depends on age, sex, the immune response parameter, and the immune challenge, which may explain the contrasting results about the role of JH in the insect immune response.

Fenoxycarb exposure affects antiviral immunity and HaNPV infection in the cotton bollworm, Helicoverpa armigera

BACKGROUND

Application of insect growth regulators (IGR) is a good option for insect pest management because of their fewer adverse effects on humans and domestic animals. These compounds are capable of interfering with normal growth and development by mimicking the actions of hormones such as juvenile hormone (JH) or ecdysone. The effect of JH and its analogs on some aspects of insect immunity has been determined, yet their possible effects on antiviral immunity response has not been investigated yet. Considering the importance of antiviral response in viral replication, in this study the effects of the JH analog (JHA), fenoxycarb on the antiviral immunity pathway core genes [i.e. micro (mi)RNA, small interfering (si)RNA and apoptosis] of Helicoverpa armigera (Hubner) larvae were investigated. The effect of fenoxycarb on the susceptibility of the larvae to H. armigera nuclear polyhedrosis virus (HaNPV) also was assessed.

RESULTS

The results showed that the transcription level of miRNA (Dicer1, Ago1), siRNA (Dicer2, Ago2) and apoptosis (Caspase1, Caspase5) core genes in H. armigera larvae were decreased significantly after 24, 48 and 96 h feeding on a diet containing lethal and sublethal doses of fenoxycarb. Moreover, the mortality rate to HaNPV in the larvae treated with fenoxycarb increased compared to the control, leading to an increased replication of HaNPV.

CONCLUSION

Together, our results suggest that the antiviral immune system could be modulated by JHA and facilitate HaNPV replication in the larvae, increasing the mortality rate of the insect larvae. Understanding the effect of JHA on antiviral immunity is an important step toward the process of exploiting JHAs and viral pathogens to control insect pests. © 2022 Society of Chemical Industry.

The endocrine disruptor, fenoxycarb modulates gut immunity and gut bacteria titer in the cotton bollworm, Helicoverpa armigera

The endocrine system modulates several physiological functions such as development and metamorphosis in insects. The normal growth and development of insects is interfered with insect growth regulators (IGRs), which act as mimics of insect hormones like juvenile hormone (JH) and ecdysone hormone. The effects of JH and its analogs on systemic immunity have been identified. However the effect of these compounds on local gut immunity is largely unknown. In this study, the effects of JH analog fenoxycarb on the local gut immunity of Helicoverpa armigera, gut bacteria population, and their role in the pathogenicity of Bacillus thuringiensis (Bt) were analyzed. The results showed that feeding fenoxycarb causes a decrease in the transcription level of IMD (Relish and PGPR-LC), ROS (DUOX and SOD) related genes and antimicrobial peptides (AMPs), followed by an overpopulation of gut bacteria. The fenoxycarb-treated larvae showed higher susceptibility to Bt compared to the control larvae. Overall, these findings collectively suggest that JH analog affects local gut immunity and gut bacteria titer.

Immune function differences between two color morphs of the red palm weevil Rhynchophorus ferrugineus (Coleoptera: Curculionidae) at different life stages

Several studies demonstrated that in insects cuticle melanism is interrelated with pathogen resistance, as melanin-based coloration and innate immunity possess similar physiological pathways. For some insects, higher pathogen resistance was observed in darker individuals than in individuals with lighter cuticular coloration. Here, we investigated the difference in immune response between two color morphs (black and red) and between the life stages (pupa and adult) of the red palm weevil Rhynchophorus ferrugineus (Coleoptera: Curculionidae). Here in this study, cuticle thickness, microbial test (antimicrobial activity, phenoloxidase activity, and hemocyte density), and immune-related gene expression were evaluated at different stages of RPW. Study results revealed that cuticle thickness of black phenotype was thicker than red phenotype at old-pupa stage, while no significant difference found at adult stage. These results may relate to the development processes of epidermis in different stages of RPW. The results of antimicrobial activity, phenoloxidase (PO) activity, and hemocyte density analyses showed that adults with a red phenotype had stronger pathogen resistance than those with a black phenotype. In addition to antimicrobial activity and PO activity, we tested relative gene expression in the fat body of old pupae. The results of hemolymph antimicrobial analysis showed that old pupae with a red phenotype were significantly different from those with a black phenotype at 12 hr after Staphylococcus aureus injection, suggesting that red phenotype pupae were more sensitive to S. aureus. Examination of gene expression in the fat body also revealed that the red phenotype had a higher immune response than the black phenotype. Our results were inconsistent with the previous conclusion that dark insects had increased immune function, suggesting that the relationship between cuticle pigmentation and immune function in insects was not a direct link. Additional possible factors that are associated with the immune response, such as life-history, developmental, physiological factors also need to be considered.

Endocrine regulation of immunity in insects

Organisms have constant contact with potentially harmful agents that can compromise their fitness. However, most of the times these agents fail to cause serious disease by virtue of the rapid and efficient immune responses elicited in the host that can range from behavioural adaptations to immune system triggering. The immune system of insects does not comprise the adaptive arm, making it less complex than that of vertebrates, but key aspects of the activation and regulation of innate immunity are conserved across different phyla. This is the case for the hormonal regulation of immunity as a part of the broad organismal responses to external conditions under different internal states. In insects, depending on the physiological circumstances, distinct hormones either enhance or suppress the immune response integrating individual (and often collective) responses physiologically and behaviourally. In this review, we provide an overview of our current knowledge on the endocrine regulation of immunity in insects, its mechanisms and implications on metabolic adaptation and behaviour. We highlight the importance of this multilayered regulation of immunity in survival and reproduction (fitness) and its dependence on the hormonal integration with other mechanisms and life-history traits.

Immune Defenses of a Beneficial Pest: The Mealworm Beetle, Tenebrio molitor

The mealworm beetle, Tenebrio molitor, is currently considered as a pest when infesting stored grains or grain products. However, mealworms are now being promoted as a beneficial insect because their high nutrient content makes them a viable food source and because they are capable of degrading polystyrene and plastic waste. These attributes make T. molitor attractive for mass rearing, which may promote disease transmission within the insect colonies. Disease resistance is of paramount importance for both the control and the culture of mealworms, and several biotic and abiotic environmental factors affect the success of their anti-parasitic defenses, both positively and negatively. After providing a detailed description of T. molitor's anti-parasitic defenses, we review the main biotic and abiotic environmental factors that alter their presentation, and we discuss their implications for the purpose of controlling the development and health of this insect.

The Geometry of Nutrient Space-Based Life-History Trade-Offs: Sex-Specific Effects of Macronutrient Intake on the Trade-Off between Encapsulation Ability and Reproductive Effort in Decorated Crickets

Life-history theory assumes that traits compete for limited resources, resulting in trade-offs. The most commonly manipulated resource in empirical studies is the quantity or quality of diet. Recent studies using the geometric framework for nutrition, however, suggest that trade-offs are often regulated by the intake of specific nutrients, but a formal approach to identify and quantify the strength of such trade-offs is lacking. We posit that trade-offs occur whenever life-history traits are maximized in different regions of nutrient space, as evidenced by nonoverlapping 95% confidence regions of the global maximum for each trait and large angles (θ) between linear nutritional vectors and Euclidean distances (d) between global maxima. We then examined the effects of protein and carbohydrate intake on the trade-off between reproduction and aspects of immune function in male and female Gryllodes sigillatus. Female encapsulation ability and egg production increased with the intake of both nutrients, whereas male encapsulation ability increased with protein intake but calling effort increased with carbohydrate intake. The trade-offs between traits was therefore larger in males than in females, as demonstrated by significant negative correlations between the traits in males, nonoverlapping 95% confidence regions, and larger estimates of θ and d. Under dietary choice, the sexes had similar regulated intakes, but neither optimally regulated nutrient intake for maximal trait expression. We highlight the fact that greater consideration of specific nutrient intake is needed when examining nutrient space-based trade-offs.

The initial analysis of a serine proteinase gene (AccSp10) from Apis cerana cerana: possible involvement in pupal development, innate immunity and abiotic stress responses

Serine proteinases play important roles in innate immunity and insect development. We isolated a serine proteinase gene, designated AccSp10, from the Chinese honeybees (Apis cerana cerana). RT-qPCR and a Western blot analysis at different pupal development stages indicated that AccSp10 might be involved in melanin formation in pupae and promote pupal development. In adult workers, the expression of AccSp10 was upregulated by treatments mimicking harmful environments such as the presence of Bacillus bombysepticus, different temperatures (4, 24 and 42 °C), HgCl2, H2O2 and paraquat; the exception was treatment with VC (vitamin C), which did not upregulate AccSp10 expression. Western blot confirmed the results. A disc diffusion assay indicated that recombinant AccSp10 accelerated E. coli cell death during stimulation with harmful substances (HgCl2, paraquat and cumene hydroperoxide). These findings suggest that AccSp10 may be involved in the pupal development of Chinese honeybees and protection against microorganisms and abiotic harms.

Cellular and humoral responses of Pieris brassicae to infection by Steinernema feltiae, its symbiont bacteria, and their metabolites

This study describes the mortality effects of the culture broth ofXenorhabdus bovieniiand its aqueous and organic extracts onPieris brassicaelarvae. All the treatments had insecticidal activities when injected into the haemocoel. The culture broth of bacteria and its aqueous extracts exhibited more insecticidal activity. The results showed the immune response ofP. brassicaeexposed toSteinernema feltiae, its symbiont bacteria,X. bovienii, and aqueous and organic extracts of bacteria. Both cellular and humoral responses were investigated. After infection of the larvae ofP. brassicaewithS. feltiae, the total number of haemocytes quickly increased. A reduction in the number of haemocytes was observed over post-injection time. In addition, plasmatocytes and granulocytes showed increased frequency and significant changes when compared to other haemocytes in responding to entomopathogenic nematode injection.Steinernema feltiaeincreased phenoloxidase activity but 3 h post-injection the concentration gradually decreased. However, a reduction in phenoloxidase activity was observed when the larvae were infected with bacteria or their aqueous and organic extracts. This study showed that both living and heat-killed bacteria have suppression effects on phenoloxidase activity. The lysozyme concentration increased inP. brassicaelarvae when they were exposed to living and heat-killedX. bovienii. No suppression effect of the bacteria was detected on lysozyme.

Rapid desiccation hardening changes the cuticular hydrocarbon profile of Drosophila melanogaster

The success of insects in terrestrial environments is due in large part to their ability to resist desiccation stress. Since the majority of water is lost across the cuticle, a relatively water-impermeable cuticle is a major component of insect desiccation resistance. Cuticular permeability is affected by the properties and mixing effects of component hydrocarbons, and changes in cuticular hydrocarbons can affect desiccation tolerance. A pre-exposure to a mild desiccation stress increases duration of desiccation survival in adult female Drosophila melanogaster, via a decrease in cuticular permeability. To test whether this acute response to desiccation stress is due to a change in cuticular hydrocarbons, we treated male and female D. melanogaster to a rapid desiccation hardening (RDH) treatment and used gas chromatography to examine the effects on cuticular hydrocarbon composition. RDH led to reduced proportions of unsaturated and methylated hydrocarbons compared to controls in females, but although RDH modified the cuticular hydrocarbon profile in males, there was no coordinated pattern. These data suggest that the phenomenon of RDH leading to reduced cuticular water loss occurs via an acute change in cuticular hydrocarbons that enhances desiccation tolerance in female, but not male, D. melanogaster.

Acorn consumption improves the immune response of the dung beetle Thorectes lusitanicus

Thorectes lusitanicus, a typically coprophagous species is also actively attracted to oak acorns, consuming, burying them, and conferring ecophysiological and reproductive advantages to both the beetle and the tree. In this study, we explored the possible relation between diet shift and the health status of T. lusitanicus using a generalist entomopathogenic fungus (Metarhizium anisopliae) as a natural pathogen. To measure the health condition and immune response of beetles, we analysed the protein content in the haemolymph, prophenoloxidase (proPO) content, phenoloxidase (PO) activity and mortality of beetles with diets based on either acorns or cow dung. Protein content, proPO levels and PO levels in the haemolymph of T. lusitanicus were found to be dependent on the type of diet. Furthermore, the beetles fed with acorns developed a more effective proPO-PO system than the beetles fed with cow dung. Furthermore, a significant decrease in mortality was observed when infected individuals were submitted to an acorn-based diet. In addition to enhancing an understanding of the relevance of dietary change to the evolutionary biology of dung beetles, these results provide a more general understanding of the ecophysiological implications of differential dietary selection in the context of fitness.

Industrial melanism in the peppered moth is not associated with genetic variation in canonical melanisation gene candidates

Industrial melanism in the peppered moth (Biston betularia) is an iconic case study of ecological genetics but the molecular identity of the gene determining the difference between the typical and melanic (carbonaria) morphs is entirely unknown. We applied the candidate gene approach to look for associations between genetic polymorphisms within sixteen a priori melanisation gene candidates and the carbonaria morph. The genes were isolated and sequence characterised in B. betularia using degenerate PCR and from whole-transcriptome sequence. The list of candidates contains all the genes previously implicated in melanisation pattern differences in other insects, including aaNAT, DOPA-decarboxylase, ebony, tan, tyrosine hydroxylase, yellow and yellow2 (yellow-fa). Co-segregation of candidate gene alleles and carbonaria morph was tested in 73 offspring of a carbonaria male-typical female backcross. Surprisingly, none of the sixteen candidate genes was in close linkage with the locus controlling the carbonaria-typical polymorphism. Our study demonstrates that the 'carbonaria gene' is not a structural variant of a canonical melanisation pathway gene, neither is it a cis-regulatory element of these enzyme-coding genes. The implication is either that we have failed to characterize an unknown enzyme-coding gene in the melanisation pathway, or more likely, that the 'carbonaria gene' is a higher level trans-acting factor which regulates the spatial expression of one or more of the melanisation candidates in this study to alter the pattern of melanin production.

Cuticular proteins and seasonal photoperiodism in aphids

For poikilotherm animals such as insects, extreme temperatures can be a severe issue in continental regions. Aphids, which reproduce in spring and summer by viviparity, are prone to death in hard winter conditions. These species exhibit reproductive plasticity adapted to winter by producing oviparous females in autumn, which lay overwintering eggs. This switch is driven by photoperiodism, and long nights are sufficient to trigger the change in reproductive mode. Global transcriptomic analyses applied to the pea aphid Acyrthosiphon pisum for which genomic resources are now available have allowed the identification of several genetic programs regulated by photoperiod shortening. Unexpectedly, one of these genetic programs concerns cuticle proteins and cuticle structure. This opens new tracks for investigations and poses new hypotheses on the link between cuticle modification and neuronal signalisation of photoperiod in aphids in response to seasonal photoperiodism. This review focuses on the description of cuticular protein genes in the pea aphid and their regulation during the change of reproductive mode.

Insect cuticular sclerotization: a review

Different regions of an insect cuticle have different mechanical properties, partly due to different degrees of stabilization and hardening occurring during the process of sclerotization, whereby phenolic material is incorporated into the cuticular proteins. Our understanding of the chemistry of cuticular sclerotization has increased considerably since Mark Pryor in 1940 suggested that enzymatically generated ortho-quinones react with free amino groups, thereby crosslinking the cuticular proteins. The results obtained since then have confirmed the essential features of Pryor's suggestion, and the many observations and experiments, which have been obtained, have led to a detailed and rather complex picture of the sclerotization process, as described in this review. However, many important questions still remain unanswered, especially regarding the precise regional and temporal regulation of the various steps in the process.

The molecular mechanisms of cuticular melanization: the ecdysone cascade leading to dopa decarboxylase expression in Manduca sexta

Many insect developmental color changes are known to be regulated by both ecdysone and juvenile hormone. Yet the molecular mechanisms underlying this regulation have not been well understood. This review highlights the hormonal mechanisms involved in the regulation of two key enzymes [dopa decarboxylase (DDC) and phenoloxidase] necessary for insect cuticular melanization, and the molecular action of 20-hydroxyecdysone on various transcription factors leading to DDC expression at the end of a larval molt in Manduca sexta. In addition, the ecdysone cascade found in M. sexta is compared with that of other organisms.

Hormonal regulation of the humoral innate immune response in Drosophila melanogaster

Juvenile hormone (JH) and 20-hydroxy-ecdysone (20E) are highly versatile hormones, coordinating development, growth, reproduction and aging in insects. Pulses of 20E provide key signals for initiating developmental and physiological transitions, while JH promotes or inhibits these signals in a stage-specific manner. Previous evidence suggests that JH and 20E might modulate innate immunity, but whether and how these hormones interact to regulate the immune response remains unclear. Here we show that JH and 20E have antagonistic effects on the induction of antimicrobial peptide (AMP) genes in Drosophila melanogaster. 20E pretreatment of Schneider S2 cells promoted the robust induction of AMP genes, following immune stimulation. On the other hand, JH III, and its synthetic analogs (JHa) methoprene and pyriproxyfen, strongly interfered with this 20E-dependent immune potentiation, although these hormones did not inhibit other 20E-induced cellular changes. Similarly, in vivo analyses in adult flies confirmed that JH is a hormonal immuno-suppressor. RNA silencing of either partner of the ecdysone receptor heterodimer (EcR or Usp) in S2 cells prevented the 20E-induced immune potentiation. In contrast, silencing methoprene-tolerant (Met), a candidate JH receptor, did not impair immuno-suppression by JH III and JHa, indicating that in this context MET is not a necessary JH receptor. Our results suggest that 20E and JH play major roles in the regulation of gene expression in response to immune challenge.

Sialylation in protostomes: a perspective from Drosophila genetics and biochemistry

Numerous studies have revealed important functions for sialylation in both prokaryotes and higher animals. However, the genetic and biochemical potential for sialylation in Drosophila has only been confirmed recently. Recent studies suggest significant similarities between the sialylation pathways of vertebrates and insects and provide evidence for their common evolutionary origin. These new data support the hypothesis that sialylation in insects is a specialized and developmentally regulated process which likely plays a prominent role in the nervous system. Yet several key issues remain to be addressed in Drosophila, including the initiation of sialic acid de novo biosynthesis and understanding the structure and function of sialylated glycoconjugates. This review discusses our current knowledge of the Drosophila sialylation pathway, as compared to the pathway in bacteria and vertebrates. We arrive at the conclusion that Drosophila is emerging as a useful model organism that is poised to shed new light on the function of sialylation not only in protostomes, but also in a larger evolutionary context.

Effects of retinoids and juvenoids on moult and on phenoloxidase activity in the blood-sucking insect Rhodnius prolixus

Retinoic acid and insect juvenile hormone (JH) are structurally related terpenoids which are widespread in nature and are involved in many biological events such as morphogenesis, embryogenesis and cellular differentiation. Here, we investigated the effects of the retinoids 9-cis retinoic acid (9cisRA), all trans retinol (atROH), all trans retinoic acid (atRA) and the juvenoids methoprene (Met) and JH injection on moult and on phenoloxidase activity in the blood-sucking insect Rhodnius prolixus. Overall, we observed that injection of retinoids or juvenoids (120 pmols) in the hemocoel of 4th instar nymphs reduced the percentage of insects which appeared normal in morphology upon moult. Noteworthy, insects exposed to 9cisRA or JH underwent profound morphological changes upon moult, generating abnormal 5th instar nymphs and also markedly increased the death of insects during the moulting process. In addition, reduction in the percentage of insects that moult without any morphological alteration, induced by retinoids or juvenoids treatment, was negatively correlated with insects that both display abnormal moult and those that die during moult. Hemolymphatic phenoloxidase activity in adult male insects injected with 9cisRA, Met and JH were significantly reduced after a bacterial challenge. Together, these results indicate that not only juvenoids but also retinoids play an important role on morphogenesis and on immune response in R. prolixus, suggesting that the molecular mechanisms involved in these events recognize the terpenoid backbone as an important structural determinant in insects.

Regulation of 20-hydroxyecdysone on the larval pigmentation and the expression of melanin synthesis enzymes and yellow gene of the swallowtail butterfly, Papilio xuthus

The swallowtail butterfly, Papilio xuthus, changes its larval body pattern dramatically during the fourth ecdysis. Cuticular pigmentation occurs with precise timing just before ecdysis. We previously found that the cuticular pigmentation was regulated by three melanin synthesis genes, tyrosine hydroxylase (TH), dopa decarboxylase (DDC), and ebony. We discovered that yellow is strongly expressed in the presumptive black markings earlier than TH and DDC. Because the ecdysis is triggered by 20-hydroxyecdysone (20E), the effects of 20E on the pigmentation and expression of the melanin synthesis genes were examined. Here, we established a method for the topical application of 20E to molting specimens, so that 20E has only a partial effect, resulting in successful ecdysis. When we applied 20E during the mid-phase of the molting period, when the 20E titer is declining, cuticular pigmentation was completely inhibited. The cessation of hormonal treatments caused delayed pigmentation. yellow expression was promoted by a high titer of 20E, whereas the expression of TH, DDC, and ebony was suppressed, suggesting that a decline in the 20E concentration is necessary for the induction of the expression of the latter three genes. These results indicate that cuticular pigmentation is controlled by the exposure to 20E and its removal.

Melanin-synthesis enzymes coregulate stage-specific larval cuticular markings in the swallowtail butterfly, Papilio xuthus

Like the adult wing, butterfly larvae are unique in their coloring. However, the molecular mechanisms underlying the formation of insect larval color patterns are largely unknown. The larva of the swallowtail butterfly Papilio xuthus changes its color pattern markedly during the 4th ecdysis. We investigated its cuticular color pattern, which is thought to be composed of melanin and related pigments derived from tyrosine. We cloned three enzymes involved in the melanin-synthesis pathway in P. xuthus: tyrosine hydroxylase (TH), dopa decarboxylase (DDC), and ebony. Whole-mount in situ hybridization showed that the expression of both TH and DDC is strongly correlated with the black markings. ebony is strongly expressed only in the reddish-brown area. The expression pattern of each enzyme coincides with the cuticular color pattern of the subsequent instar. We also investigated the uptake of melanin precursors into cultured integument. Inhibition of either TH or DDC activity prevents in vitro pigmentation completely. Addition of dopamine to integuments in the presence of TH inhibitor causes overall darkening without specific markings. From these results, specific larval cuticular color patterns are regulated by stage-specific colocalization of enzymes in epidermal cells rather than by the differential uptake of melanin precursors into individual epidermal cells. Epidermal cells expressing TH and DDC, but not ebony, produce the black cuticle, and epidermal cells expressing TH, DDC, and ebony produce the reddish-brown cuticle.

Molecular characterization of a cDNA encoding prophenoloxidase and its expression in Apis mellifera

Phenoloxidase (PO), a melanin-synthesizing enzyme known to play an important role in insect defense, is found as a zymogen (ProPO) in hemolymph and cuticle, where it is activated by proteolysis. We characterized the first proPO cDNA in an eusocial insect, the Apis mellifera honey bee. The AmproPO cDNA contains an ORF of 2079 bp encoding 693 amino acids, and is composed of 9 exons and 8 introns. Southern blot of digested genomic DNA suggested that only one copy of the proPO gene is present in A. mellifera. The molecular mass of the deduced ProPO and the active enzyme was predicted to be 80.1 and 74.4 kDa, respectively. The calculated pI was 6.28. BLASTp search of the deduced amino acid sequence, and neighbor-joining analysis, showed similarity with ProPOs from other insects, ranging from 47% to 63%. Protein signature analyses revealed four conserved regions, including the two copper binding sites characteristic of arthropod ProPOs. RT-PCR and Southern blot showed the highest amount of AmproPO transcripts in workers whole body, followed by queens and drones. Expression was also detected in hemocytes and integument. Real time RT-PCR showed higher amounts of AmproPO transcripts in adults and older pupae than in younger pupae and larvae, suggesting a function of AmproPO in adult exoskeleton melanization and differentiation.

Expression of a serine proteinase homolog prophenoloxidase-activating factor from the blue crab, Callinectes sapidus

Tanning, or sclerotization, of crustacean cuticle provides initial reinforcement by cross linking cuticular proteins attached to the cuticle chitin-fiber matrix. This process is catalyzed in part by phenoloxidase, which is under the control of a serine protease activation cascade. The cDNA of a prophenoloxidase-activating factor (PPAF) was cloned and sequenced from the hypodermal tissue of the blue crab, Callinectes sapidus. It codes for a serine proteinase homolog containing a single clip domain. If it is involved in sclerotization, its transcription might be expected to be molt-cycle related. Expression patterns were determined by quantitative PCR and Northern blotting in hypodermis underlying both arthrodial and dorsal (calcifying) cuticles. Transcript levels in pre-molt RNA from both hypodermis types were high, suggesting that the PPAF produced may be incorporated into the pre-ecdysial cuticle layers and then activated at ecdysis to regulate tanning. After a decrease at ecdysis, a second increase in PPAF mRNA occurred at three to four hours post-molt in arthrodial membrane hypodermis but not dorsal hypodermis. This suggests that cuticle deposited after ecdysis may tan in the non-calcifying regions but may not tan where calcification occurs. The PPAF gene is also transcribed at low levels in the hemocytes of intermolt crabs, but not in the hepatopancreas.

Efficacy of a collar impregnated with amitraz and pyriproxyfen for prevention of experimental tick infestations by Rhipicephalus sanguineus, Ixodes ricinus, and Ixodes scapularis in dogs

OBJECTIVE

To compare the efficacy of collars impregnated with 9% amitraz or 9% amitraz and 0.5% pyriproxyfen (PPF) for control of newly established tick infestations by Rhipicephalus sanguineus, Ixodes ricinus, and Ixodes scapularis in dogs and determine whether egg production by surviving female ticks was decreased.

DESIGN

Prospective study.

ANIMALS

72 dogs.

PROCEDURE

Dogs were fitted with 1 of 3 test collars impregnated with amitraz, amitraz and PPF, or only excipients (untreated controls). In 3 trials corresponding to each of the 3 tick species, dogs were infested with 150 unfed adult ticks on days 8, 10, 13, and 18. The number of feeding female ticks was recorded on days 10, 13, 18, and 28. Surviving females were weighed and permitted to oviposit under controlled conditions.

RESULTS

Collars impregnated with amitraz and PPF decreased tick loads as efficiently as collars containing amitraz alone. Inclusion of PPF into the collar did not significantly decrease the efficacy of amitraz. The few female ticks that survived after feeding on dogs treated with collars containing PPF were unable to oviposit.

CONCLUSIONS AND CLINICAL RELEVANCE

Collars impregnated with amitraz were efficient in preventing tick infestations in dogs but did not inhibit oviposition in the few surviving female ticks. Incorporation of PPF into the amitraz-impregnated collar resulted in impairment of the reproductive ability of ticks.

Functional characterization of Drosophila sialyltransferase

Sialylation is an important carbohydrate modification of glycoconjugates in the deuterostome lineage of animals. By contrast, the evidence for sialylation in protostomes has been scarce and somewhat controversial. In the present study, we characterize a Drosophila sialyltransferase gene, thus providing experimental evidence for the presence of sialylation in protostomes. This gene encodes a functional alpha2-6-sialyltransferase (SiaT) that is closely related to the vertebrate ST6Gal sialyltransferase family, indicating an ancient evolutionary origin for this family. Characterization of recombinant, purified Drosophila SiaT revealed a novel acceptor specificity as it exhibits highest activity toward GalNAcbeta1-4GlcNAc carbohydrate structures at the non-reducing termini of oligosaccharides and glycoprotein glycans. Oligosaccharides are preferred over glycoproteins as acceptors, and no activity toward glycolipid acceptors was detected. Recombinant Drosophila SiaT expressed in cultured insect cells possesses in vivo and in vitro autosialylation activity toward beta-linked GalNAc termini of its own N-linked glycans, thus representing the first example of a sialylated insect glycoconjugate. In situ hybridization revealed that Drosophila SiaT is expressed during embryonic development in a tissue- and stage-specific fashion, with elevated expression in a subset of cells within the central nervous system. The identification of a SiaT in Drosophila provides a new evolutionary perspective for considering the diverse functions of sialylation and, through the powerful genetic tools available in this system, a means of elucidating functions for sialylation in protostomes.

Cuticular pro-phenoloxidase of the silkworm, Bombyx mori. Purification and demonstration of its transport from hemolymph

Pro-phenoloxidase (proPO) in insects is implicated in the defense against microbes and wounding. The presence of proPO in the cuticle was suggested more than 30 years ago, but it has not been purified. The extract of cuticles of the silkworm, Bombyx mori, was shown to contain two proPO isoforms (F-type and S-type proPOs, which have slightly different mobilities in polyacrylamide gel electrophoresis under nondenaturing conditions). The two isoforms were purified to homogeneity. From hemolymph of the same insect, two types of proPO with the same electrophoretic mobilities as those of cuticular isoforms were separated and were shown to be different at five amino acid residues in one of their subunits. The isoforms in the hemolymph and cuticle were activated by a specific activating enzyme. The resulting active phenoloxidases exhibited almost the same substrate specificities and specific activities toward o-diphenols. The substrate specificities and the susceptibilities to inhibitors, including carbon monoxide, indicated that the purified proPO isoforms were not zymogens of laccase-type phenoloxidase. The proPO in hemolymph was shown to be transported to the cuticle. This demonstration was corroborated by the failure to detect proPO transcripts by Northern analysis of total RNA from epidermal cells. In reversed-phase column chromatography, cuticular and hemolymph proPOs gave distinct elution profiles, indicating that some yet to be identified modification occurs in hemolymph proPO and results in the formation of cuticular proPO. There was little transportation of cuticular proPO to the cuticle when it was injected into the hemocoel. The nature of the modification is described in the accompanying paper (Asano, T., and Ashida, M. (2001) J. Biol. Chem. 276, 11113-11125).

Glycoproteins from insect cells: sialylated or not?

Our growing comprehension of the biological roles of glycan moieties has created a clear need for expression systems that can produce mammalian-type glycoproteins. In turn, this has intensified interest in understanding the protein glycosylation pathways of the heterologous hosts that are commonly used for recombinant glycoprotein expression. Among these, insect cells are the most widely used and, particularly in their role as hosts for baculovirus expression vectors, provide a powerful tool for biotechnology. Various studies of the glycosylation patterns of endogenous and recombinant glycoproteins produced by insect cells have revealed a large variety of O- and N-linked glycan structures and have established that the major processed O- and N-glycan species found on these glycoproteins are (Gal beta1,3)GalNAc-O-Ser/Thr and Man3(Fuc)GlcNAc2-N-Asn, respectively. However, the ability or inability of insect cells to synthesize and compartmentalize sialic acids and to produce sialylated glycans remains controversial. This is an important issue because terminal sialic acid residues play diverse biological roles in many glycoconjugates. While most work indicates that insect cell-derived glycoproteins are not sialylated, some well-controlled studies suggest that sialylation can occur. In evaluating this work, it is important to recognize that oligosaccharide structural determination is tedious work, due to the infinite diversity of this class of compounds. Furthermore, there is no universal method of glycan analysis; rather, various strategies and techniques can be used, which provide glycobiologists with relatively more or less precise and reliable results. Therefore, it is important to consider the methodology used to assess glycan structures when evaluating these studies. The purpose of this review is to survey the studies that have contributed to our current view of glycoprotein sialylation in insect cell systems, according to the methods used. Possible reasons for the disagreement on this topic in the literature, which include the diverse origins of biological material and experimental artifacts, will be discussed. In the final analysis, it appears that if insect cells have the genetic potential to perform sialylation of glycoproteins, this is a highly specialized function that probably occurs rarely. Thus, the production of sialylated recombinant glycoproteins in the baculovirus-insect cell system will require metabolic engineering efforts to extend the native protein glycosylation pathways of insect cells.

The juvenile hormone analog pyriproxyfen affects ecdysteroid-dependent cuticle melanization and shifts the pupal ecdysteroid peak in the honey bee (Apis mellifera)

The control of the pupal melanization in the honey bee by ecdysteroids, and the modulation of these processes by a juvenile hormone analog were investigated by a combination of in vivo and in vitro experiments. Injection of 1-5 microg of 20-hydroxyecdysone (20E) into unpigmented pupae showed a dose- and stage-dependent effect. The higher the dose and the later the injection was performed, the more pronounced was the delay in cuticle pigmentation. This inhibition of cuticular melanization by artificially elevated ecdysteroid titers was corroborated by in vitro experiments, culturing integument from unpigmented, dark-eyed pupae for 1-4 days in the presence of 20E (2 or 5 microg/ml culture medium). Topical application (1 microg) of pyriproxyfen to unpigmented, white-eyed pupae had the opposite effect, leading to precocious and enhanced melanization of the pupal cuticle. In vitro incubation of integuments in the presence of this juvenile hormone analog (1 microg/ml) confirmed these results, showing that pyriproxyfen is apparently capable of triggering melanization. The in vivo mode of action of pyriproxyfen was further investigated by quantifying hemolymph ecdysteroids by radioimmunoassays. Topical application leads to a delay of the pupal ecdysteroid peak by 4 days. The pyriproxyfen-induced low ecdysteroid titers during early pupal development could account for precocious pigmentation by removing an inhibition on prophenoloxidase activation normally imposed by the elevated ecdysteroid titer during this phase.

Genomic structure and ecdysone regulation of the prophenoloxidase 1 gene in the malaria vector Anopheles gambiae

Prophenoloxidase, a melanin-synthesizing enzyme, is considered to be an important arthropod immune protein. In mosquitoes, prophenoloxidase has been shown to be involved in refractory mechanisms against malaria parasites. In our study we used Anopheles gambiae, the most important human malaria vector, to characterize the first arthropod prophenoloxidase gene at the genomic level. The complete nucleotide sequence, including the immediate 5' flanking sequence (-855 bp) of the prophenoloxidase 1 gene, was determined. The gene spans 10 kb and is composed of five exons and four introns coding for a 2.5-kb mRNA. In the 5' flanking sequence, we found several putative regulatory motifs, two of which were identified as ecdysteroid regulatory elements. Electrophoretic mobility gel-shift assays and supershift assays demonstrated that the Aedes aegypti ecdysone receptor/Ultraspiracle nuclear receptor complex, and, seemingly, the endogenous Anopheles gambiae nuclear receptor complex, was able to bind one of the ecdysteroid response elements. Furthermore, 20-hydroxyecdysone stimulation was shown to up-regulate the transcription of the prophenoloxidase 1 gene in an A. gambiae cell line.

A hemocyte-like cell line established from the malaria vector Anopheles gambiae expresses six prophenoloxidase genes

Cell lines from the malaria vector Anopheles gambiae have been established as a tool for the study of the mosquito innate immune system in vitro. Here, we describe the first continuous insect cell line that produces prophenoloxidase (PPO). This cell line (4a-3B) expresses constitutively six PPO genes, three of which are novel (PPO4, PPO5, and PPO6). The PPO genes show distinct temporal expression profiles in the intact mosquito, spanning stages from the embryo to the adult in an overlapping manner. Transient induction of larva-specific PPO genes in blood-fed adult females suggests that the developmental hormone 20-hydroxyecdysone may be involved in PPO gene regulation. Indeed, exposure of 4a-3B cells to 20-hydroxyecdysone in culture results in induction of those PPO genes that are mainly expressed in early developmental stages, and repression of PPO5, which is preferentially expressed at the adult stage. The cell line shows bacteria-induced immune transcripts that encode defensin and Gram-negative bacteria-binding protein, but no induction of PPO transcripts. This cell line most likely derives from a hemocyte lineage, and represents an appropriate in vitro model for the study of the humoral and cellular immune defenses of A. gambiae.

Catechol conjugation with hemolymph proteins and their incorporation into the cuticle of the American cockroach, Periplaneta americana

Newly ecdysed American cockroaches, Periplaneta americana (sixth to last instar) were injected with radioactive dopamine (DA) and hemolymph was collected at 10-60 min post-ecdysis. Size-exclusion chromatography established the presence of at least three proteins that serve as catecholamine carriers. Reinjection of the smaller radiolabeled phenol-bound proteins into newly ecdysed animals results in in vivo aggregation, with the radiolabel bound to large MW proteins (30- > 200 kDa). In addition, the reinjection of radiolabeled protein of any size resulted in the incorporation of the label into the newly sclerotized cuticle. Hemolymph proteins were synthesized in vivo using [14C]leucine and subsequently double labeled in vivo with [3H]dopamine. After sclerotization (7 h post-ecdysis) the cuticle was extirpated, hydrolyzed and counted. An identical ratio of 14C to 3H was found in cuticle extracts as in the double-labeled hemolymph proteins, suggesting that the phenol-bound protein was incorporated in the cuticle unchanged. It appears that the catechol bound to the proteins exists as a beta-glucoside.

Baculovirus resistance in the noctuid Spodoptera exempta is phenotypically plastic and responds to population density

Parasite resistance mechanisms can be costly to maintain. We would therefore predict that organisms should invest in resistance only when it is likely to be required. Insects that show density-dependent phase polyphenism, developing different phenotypes at high and low population densities, have the opportunity to match their levels of investment in resistance with the likelihood of exposure to pathogens. As high population densities often precipitate disease epidemics, the high-density form should be selected to invest relatively more in resistance. We tested this prediction in larvae of the noctuid Spodoptera exempta . Larvae reared at a high density were found to be considerably more resistant to a nuclear polyhedrosis virus than those reared in isolation. A conspicuous feature of the high-density phase of S. exempta and other phase-polyphenic Lepidoptera is cuticular melanization. As melanization is controlled by the phenoloxidase enzyme system, which is also involved in the immune response, this suggests a possible mechanism for increased resistance at high population densities. We demonstrated that melanized S. exempta larvae were more resistant than non-melanized forms, independent of rearing density. We also found that haemolymph phenoloxidase activity was correlated with cuticular melanization, providing further evidence for a link between melanization and immunity. These results suggest that pathogen resistance in S. exempta is phenotypically plastic, and that the melanized cuticles characteristic of the high-density form may be indicative of a more active immune system.

A factor preventing melanization of sephadex CM C-25 beads in Plasmodium-susceptible and refractory anopheles gambiae

One major quantitative trait locus controls melanization of both malaria ookinetes and Sephadex CM beads in a refractory strain of the mosquito, Anopheles gambiae. Hemolymph transferred from a nonmelanizing, Plasmodium-susceptible strain (4arr) to a melanizing, Plasmodium-refractory strain (L35) caused a reduction in the melanization of CM beads. In addition, when beads were first incubated in vivo in susceptible mosquitoes and then recovered, washed, and transferred to refractory mosquitoes, a strong reduction in melanization was observed. No changes in melanization were observed when beads or hemolymph were transferred in the opposite direction or within a strain. Incubation of beads in vitro in refractory or susceptible hemolymph resulted in a reduction of melanization when these beads were subsequently transferred to refractory mosquitoes. This reduction was significantly stronger when susceptible hemolymph was used as the incubating medium. Protection from melanization was observed after 3-, 6-, and 24-h incubations of beads in susceptible mosquitoes with longer incubations resulting in greater protection. Treatment of protected beads with 1 M NaOH resulted in the loss of the protection but treatment with 1% sodium dodecyl sulfate (SDS), 1% SDS/DTT/boiling, or 1 M NaOAc (pH 8.9) did not. These results show that a melanization-preventing factor covalently binds to the surface of CM beads in susceptible mosquitoes and can subsequently prevent melanization in refractory mosquitoes.

The Apis mellifera pupal melanization program is affected by treatment with a juvenile hormone analogue

Apis mellifera treated during different developmental phases with pyriproxyfen, a juvenile hormone analogue, show profound alterations in cuticular pigmentation and sclerotization. When the treatment is effected during the feeding phase of the fifth larval instar (LF5), the pupal development is blocked and pigmentation does not occur. Treatment of older larvae, at the spinning phase of the fifth larval instar (LS5), of prepupae (PP) or pupae at the beginning of the pupal period (Pw, white-eyed, unpigmented cuticle pupae) does not impair pigmentation, but, instead, this process is accelerated, intensified and abnormal. Hormonal treatment during these developmental phases (LS5, PP and Pw) induces earlier activity of phenoloxidase, an enzyme of the reaction chain leading to melanin synthesis. Treated pupae have significantly higher enzymatic levels and show a graded response in phenoloxidase activity after treatment with 0.1, 1 or 5&mgr;g pyriproxyfen. Besides pigmentation, other developmental events were also altered in treated bees: pupal development was shortened, and the expression of esterase-6 activity, the onset of which coincides with the beginning of pigmentation, was shifted with the precocious initiation of this process in treated pupae. The significance of these results is discussed in relation to the mode of hormonal action on cuticular pigmentation in insects.

Hemocyte surface phenoloxidase (PO) and immune response to lipopolysaccharide (LPS) in Ceratitis capitata

Bacterial lipopolysaccharide (LPS) attachment at the hemocyte surface is based on the crosslinking of surface associated p47 to LPS, via the intermediacy of tyrosine derivatives generated by the action of phenoloxidase (PO). This attachment is an initial step for LPS internalization from hemocytes (Charalambidis et al., 1996). The results presented clearly show the critical role of hemocyte associated PO activity in the above processes. Biochemical and immunofluorescent analysis demonstrated unambiguously the presence of prophenoloxidase (proPO) on the hemocyte surface. The cell-surface expression of proPO appeared to be LPS-independent, whereas its activation was LPS-dependent. The activation of cell surface proPO involves a limited proteolysis, since upon activation with chymotrypsin proPO is converted to a set of smaller molecular weight proteins with PO activity. The activation appears to be due to enzyme activators, serine proteases, released upon LPS-stimulation. This hypothesis was supported from the activation of membrane proPO by the culture medium of hemocytes which have been triggered with LPS. In addition, proPO, activation was abolished by inhibitors of secretion and PMSF. The release of proPO activators upon LPS-stimulation is mediated via protein tyrosine phosphorylation, as genistein inhibited proPO activation, a situation similar to that reported by us for the release of the effector protein p47 (Charalambidis et al., 1995). The LPS-stimulated activation of cell-surface proPO is a prerequisite for LPS (either cell associated or cell free) internalization, as judged by the resistance of LPS binding to dissociation by proteinase K.

Biochemical characterization, developmental expression, and induction of the immune protein scolexin from Manduca sexta

The immune protein, scolexin, a bacteria-induced, larva-specific protein from Manduca sexta, was shown to exist in the hemolymph in two isoelectric forms designated herein as scolexin-1 and scolexin-2 (native M(r) approximately 72 kd). These two charge isomers appeared to share the same amino acid composition. Scolexin is composed of two subunits (peptide M(r) approximately 36 kd) that possess the same N-terminus. Scolexin-2 was subjected to glycosyl composition analysis, revealing the presence of galactose, glucose, mannose, xylose, and sialic acid residues. Hybridization of epidermal RNA with oligonucleotides deduced from the scolexin N-terminal sequence showed a continuous decline in mRNA following day 0 of the 5th larval instar. By employing in vitro protein labelling, it was found that organ cultures of the epidermis from immune larvae showed a greater ability over that of naive epidermal cultures to synthesize scolexin; these data reflected the inducible response seen in the hemolymph, and confirm other data indicating that the epidermis is an important site of scolexin biosynthesis.

Cellular defense mechanisms in C. capitata: recognition and entrapment of E. coli by hemocytes

The mechanism of recognition of foreignness and entrapment of invaders by the immune system of insects is unknown. In this report using hemocyte monolayer preparations and biochemical analysis we demonstrate the requirements for recognition of E. coli in vitro, their entrapment by hemocytes, and nodule formation. A model system consisting of an isolated hemocyte protein (47 KDa), isolated hemocyte tyrosinase, isolated hemocytes, tyrosine, and E. coli was used to obtain these results. The 47 kDa polypeptide has the ability to form adducts with tyrosine derivatives generated by the action of tyrosinase and to attach to the E. coli surface. The latter process takes place independently of tyrosinase activity. When the E. coli-47KDa protein complex was overlaid on hemocyte monolayers followed by tyrosine and tyrosinase or vice versa, the bacteria were entrapped by hemocytes. The same results were obtained when the monolayers were overlaid with 47 KDa protein, followed by E. coli-47 KDa protein complex and then tyrosine and tyrosinase. The same experimental procedure in test tubes resulted in nodule formation. These results permit us to propose that the most likely explanation for the entrapment of E. coli to hemocytes and the formation of nodules is the production of E. coli-47 KDa complexes, and their crosslinking through a quinone intermediate generated by the action of tyrosinase on hemocytes.

Purification of the pro-phenol oxidase enzyme from haemocytes of the cockroach Blaberus discoidalis

Pro-phenol oxidase was purified from the haemocytes of the cockroach Blaberus discoidalis by Blue Sepharose chromatography, hydrophobic-interaction chromatography on a Phenyl-Superose column and, finally, gel filtration on a Superose 6 column. Results suggest that the molecule exists as a polymer of identical 76 kDa monomeric units. The enzyme is a glycoprotein with pI of 5.2 and can be converted by trypsin into phenol oxidase.