Humoral and cellular immune responses induced by the urease-derived peptide Jaburetox in the model organism Rhodnius prolixus

Two Secretory T2 RNases Act as Cytotoxic Factors Contributing to the Virulence of an Insect Fungal Pathogen

RNase T2 members are secreted by several pathogens or parasites during infection, playing various roles in pathogen-host interaction. However, functions of those members in biocontrol microbes targeting their hosts are still unknown. Here, we report that an insect fungal pathogen, Beauveria bassiana, produces two secretory RNase T2 members that act as cytotoxic factors, which were examined by insect bioassays using the targeted gene(s) disruption and overexpression strains. Overexpression strains displayed dramatically increased virulence, which was concurrent with few fungal cells and hemocytes in hemocoel, suggesting a cytotoxicity of the overexpressed gene products. In vitro assays using yeast-expressed proteins verified the cytotoxicity of the two members against insect cells, to which the cytotoxic effect was dependent on their RNases enzyme activities and glycosylation modification. Moreover, the excessive humoral immune responses triggered by the two ribonucleases were examined. These results suggested prospects of these two T2 ribonucleases for improvement of biocontrol agents.

A simple method for ex vivo honey bee cell culture capable of in vitro gene expression analysis

Cultured cells are a very powerful tool for investigating biological events in vitro; therefore, cell lines have been established not only in model insect species, but also in non-model species. However, there are few reports on the establishment of stable cell lines and development of systems to introduce genes into the cultured cells of the honey bee (Apis mellifera). We describe a simple ex vivo cell culture system for the honey bee. Hemocyte cells obtained from third and fourth instar larvae were cultured in commercial Grace’s insect medium or MGM-450 insect medium for more than two weeks maintaining a normal morphology without deterioration. After an expression plasmid vector bearing the enhanced green fluorescent protein (egfp) gene driven by the immediate early 2 (IE2) viral promoter was transfected into cells, EGFP fluorescence was detected in cells for more than one week from one day after transfection. Furthermore, double-stranded RNA corresponding to a part of the egfp gene was successfully introduced into cells and interfered with egfp gene expression. A convenient and reproducible method for an ex vivo cell culture that is fully practicable for gene expression assays was established for the honey bee.

Structure-Function Insights of Jaburetox and Soyuretox: Novel Intrinsically Disordered Polypeptides Derived from Plant Ureases

Intrinsically disordered proteins (IDPs) and intrinsically disordered regions (IDRs) do not have a stable 3D structure but still have important biological activities. Jaburetox is a recombinant peptide derived from the jack bean (Canavalia ensiformis) urease and presents entomotoxic and antimicrobial actions. The structure of Jaburetox was elucidated using nuclear magnetic resonance which reveals it is an IDP with small amounts of secondary structure. Different approaches have demonstrated that Jaburetox acquires certain folding upon interaction with lipid membranes, a characteristic commonly found in other IDPs and usually important for their biological functions. Soyuretox, a recombinant peptide derived from the soybean (Glycine max) ubiquitous urease and homologous to Jaburetox, was also characterized for its biological activities and structural properties. Soyuretox is also an IDP, presenting more secondary structure in comparison with Jaburetox and similar entomotoxic and fungitoxic effects. Moreover, Soyuretox was found to be nontoxic to zebra fish, while Jaburetox was innocuous to mice and rats. This profile of toxicity affecting detrimental species without damaging mammals or the environment qualified them to be used in biotechnological applications. Both peptides were employed to develop transgenic crops and these plants were active against insects and nematodes, unveiling their immense potentiality for field applications.

Jaburetox, a urease-derived peptide: Effects on enzymatic pathways of the cockroach Nauphoeta cinerea

Jaburetox is a recombinant peptide derived from one of the Canavalia ensiformis urease isoforms. This peptide induces several toxic effects on insects of different orders, including interference on muscle contractility in cockroaches, modulation of UDP-N-acetylglucosamine pyrophosphorylase (UAP) and nitric oxide synthase (NOS) activities in the central nervous system of triatomines, as well as activation of the immune system in Rhodnius prolixus. When injected, the peptide is lethal for R. prolixus and Triatoma infestans. Here, we evaluated Jaburetox toxicity to Nauphoeta cinerea cockroaches, exploring the effects on the central nervous system through the activities of UAP, NOS, acid phosphatases (ACP), and acetylcholinesterase (AChE). The results indicated that N. cinerea is not susceptible to the lethal effect of the peptide. Moreover, both in vivo and in vitro treatments with Jaburetox inhibited NOS activity, without modifying the protein levels. No alterations on ACP activity were observed. In addition, the enzyme activity of UAP only had its activity affected at 18 hr after injection. The peptide increased the AChE activity, suggesting a mechanism involved in overcoming the toxic effects. In conclusion, our findings indicate that Jaburetox affects the nitrinergic signaling as well as the AChE and UAP activities and establishes N. cinerea as a Jaburetox-resistant model for future comparative studies.

The entomotoxin Jack Bean Urease changes cathepsin D activity in nymphs of the hematophagous insect Dipetalogaster maxima (Hemiptera: Reduviidae)

In insects, cathepsin D is a lysosomal aspartic endopeptidase involved in several functions such as digestion, defense and reproduction. Jack Bean Urease (JBU) is the most abundant urease isoform obtained from the seeds of the plant Canavalia ensiformis. JBU is a multifunctional protein with entomotoxic effects unrelated to its catalytic activity, by mechanisms not yet fully understood. In this work, we employed nymphs of the hematophagous insect Dipetalogaster maxima as an experimental model in order to study the effects of JBU on D. maxima CatD (DmCatD). In insects without treatment, immunofluorescence assays revealed a conspicuous distribution pattern of DmCatD in the anterior and posterior midgut as well as in the fat body and hemocytes. Western blot assays showed that the active form of DmCatD was present in the fat body, the anterior and posterior midgut; whereas the proenzyme was visualized in hemocytes and hemolymph. The transcript of DmCatD and its enzymatic activity was detected in the anterior and posterior midgut as well as in fat body and hemocytes. JBU injections induced a significant increase of DmCatD activity in the posterior midgut (at 3 h post-injection) whereas in the hemolymph, such an effect was observed after 18 h. These changes were not correlated with modifications in DmCatD mRNA and protein levels or changes in the immunofluorescence pattern. In vitro experiments might suggest a direct effect of the toxin in DmCatD activity. Our findings indicated that the tissue-specific increment of cathepsin D activity is a novel effect of JBU in insects.

Entomotoxicity of jaburetox: revisiting the neurotoxic mechanisms in insects

Ureases are metalloenzymes that hydrolyze urea to ammonia and carbamate. The main urease isoforms present in the seeds of Canavalia ensiformis (jack bean urease - JBU and canatoxin) exert a variety of biological activities. The insecticidal activity of JBU is mediated, at least in part, by jaburetox (Jbtx), a recombinant peptide derived from the JBU amino acid sequence. In this article, we review the neurotoxicity of Jbtx in insects. The insecticidal activity of Jbtx has been investigated in a variety of insect orders and species, including Blattodea (the cockroaches Blatella germânica, Nauphoeta cinerea, Periplaneta americana e Phoetalia pallida), Bruchidae (Callosobruchus maculatus - cowpea weevil), Diptera (Aedes aegypti - mosquito), Hemiptera (Dysdercus peruvianus - cotton stainer bug; Oncopeltus fasciatus - large milkweed bug, and the kissing bugs Rhodnius prolixus and Triatoma infestans), Lepidoptera (Spodoptera frugiperda - fall army worm) and Orthoptera (Locusta migratoria - locust). In N. cinerea, the injection of Jbtx induces marked alteration of locomotor and grooming behavior, whereas in T. infestans Jbtx causes leg paralysis, an extension of the proboscis and abnormal antennal movements. Electromyographical analysis showed that Jbtx causes complete neuromuscular blockade in P. pallida. The same treatment in N. cinerea and L. migratoria causes a decrease in the action potential firing rate. Jbtx forms membrane pore-channels compatible with cations in bilipid membranes. A study using B. germanica voltage-gated sodium (Nav1.1) channels that were heterologously expressed in Xenopus laevis oocytes correlated the entomotoxicity of Jbtx with the activation of these channels. Taken together, these findings demonstrate the potential of this peptide as a natural pesticide.

The role of extracellular nucleic acids in the immune system modulation of Rhodnius prolixus (Hemiptera: Reduviidae)

Extracellular traps (ETs) are extracellular nucleic acids associated with cytoplasmic proteins that may aid in the capture and killing of pathogens. To date, only a few insects were shown to display this kind of immune response. Jaburetox, a peptide derived from jack bean urease, showed toxic effects in Rhodnius prolixus, affecting its immune response. The present study aims to evaluate the role of extracellular nucleic acids in R. prolixus' immune response, using Jaburetox as a model entomotoxin. The insects were treated with extracellular nucleic acids and/or Jaburetox, and the cellular and humoral responses were assessed. We also evaluated the release of extracellular nucleic acids induced by toxins, and performed immunocompetence assays using pathogenic bacteria. Our results demonstrated that extracellular nucleic acids can modulate the insect immune responses, either alone or associated with the toxin. Although RNA and DNA induced a cellular immune response, only DNA was able to neutralize the Jaburetox-induced aggregation of hemocytes. Likewise, the activation of the humoral response was different for RNA and DNA. Nevertheless, it was observed that both, extracellular DNA and RNA, immunocompensated the Jaburetox effects on insect defenses upon the challenge of a pathogenic bacterium. The toxin was not able to alter cellular viability, in spite of inducing an increase in the reactive species of oxygen formation. In conclusion, we have demonstrated a protective role for extracellular nucleic acids in R. prolixus´ immune response to toxins and pathogenic bacteria.

Ex vivo characterization of the circulating hemocytes of bed bugs and their responses to bacterial exposure

Bed bugs (Cimex spp.) are urban pests of global importance. Knowledge of the immune system of bed bugs has implications for understanding their susceptibility to biological control agents, their potential to transmit human pathogens, and the basic comparative immunology of insects. Nonetheless, the immunological repertoire of the family Cimicidae remains poorly characterized. Here, we use microscopy, flow cytometry, and RNA sequencing to provide a basal characterization of the circulating hemocytes of the common bed bug, Cimex lectularius. We also examine the responses of these specialized cells to E. coli exposure using the same techniques. Our results show that circulating hemocytes are comprised of at least four morphologically distinct cell types that are capable of phagocytosis, undergo degranulation, and exhibit additional markers of activation following stimulation, including size shift and DNA replication. Furthermore, transcriptomic profiling reveals expression of predicted Toll/IMD signaling pathway components, antimicrobial effectors and other potentially immunoresponsive genes in these cells. Together, our data demonstrate the conservation of several canonical cellular immune responses in the common bed bug and provide a foundation for additional mechanistic immunological studies with specific pathogens of interest.

Risk assessment of the antifungal and insecticidal peptide Jaburetox and its parental protein the Jack bean (Canavalia ensiformis) urease

Jaburetox (JBTX) is an insecticidal and antifungal peptide derived from jack bean (Canavalia ensiformis) urease that has been considered a candidate for developing genetically modified crops. This study aimed to perform the risk assessment of the peptide JBTX following the general recommendations of the two-tiered, weight-of-evidence approach proposed by International Life Sciences Institute. The urease of C. ensiformis (JBU) and its isoform JBURE IIb (the JBTX parental protein) were assessed. The history of safe use revealed no hazard reports for the studied proteins. The available information shows that JBTX possesses selective activity against insects and fungi. JBTX and JBU primary amino acids sequences showed no relevant similarity to toxic, antinutritional or allergenic proteins. Additionally, JBTX and JBU were susceptible to in vitro digestibility, and JBU was also susceptible to heat treatment. The results did not identify potential risks of adverse effects and reactions associated to JBTX. However, further allergen (e.g. serum IgE binding test) and toxicity (e.g. rodent toxicity tests) experimentation can be done to gather additional safety information on JBTX, and to meet regulatory inquiries for commercial approval of transgenic cultivars expressing this peptide.

Soyuretox, an Intrinsically Disordered Polypeptide Derived from Soybean (Glycine Max) Ubiquitous Urease with Potential Use as a Biopesticide

Ureases from different biological sources display non-ureolytic properties that contribute to plant defense, in addition to their classical enzymatic urea hydrolysis. Antifungal and entomotoxic effects were demonstrated for Jaburetox, an intrinsically disordered polypeptide derived from jack bean (Canavalia ensiformis) urease. Here we describe the properties of Soyuretox, a polypeptide derived from soybean (Glycine max) ubiquitous urease. Soyuretox was fungitoxic to Candida albicans, leading to the production of reactive oxygen species. Soyuretox further induced aggregation of Rhodnius prolixus hemocytes, indicating an interference on the insect immune response. No relevant toxicity of Soyuretox to zebrafish larvae was observed. These data suggest the presence of antifungal and entomotoxic portions of the amino acid sequences encompassing both Soyuretox and Jaburetox, despite their small sequence identity. Nuclear Magnetic Resonance (NMR) and circular dichroism (CD) spectroscopic data revealed that Soyuretox, in analogy with Jaburetox, possesses an intrinsic and largely disordered nature. Some folding is observed upon interaction of Soyuretox with sodium dodecyl sulfate (SDS) micelles, taken here as models for membranes. This observation suggests the possibility for this protein to modify its secondary structure upon interaction with the cells of the affected organisms, leading to alterations of membrane integrity. Altogether, Soyuretox can be considered a promising biopesticide for use in plant protection.

Jaburetox, a natural insecticide derived from Jack Bean Urease, activates voltage-gated sodium channels to modulate insect behavior

Jaburetox (Jbtx) is an insecticidal peptide derived from Canavalia ensiformis urease, whose mechanism of action is not completely elucidated. We employed behavioral, electromyographical and electrophysiological protocols to identify the cellular and molecular targets involved in the Jbtx entomotoxicity in cockroaches and locusts. In Nauphoeta cinerea, Jbtx (32 μg/g) altered the locomotory behaviour inducing a significative decrease in the distance travelled followed by a significant increase in stopped time (52 ± 85 cm and 2573 ± 89 s, p < .05, n = 40). Jbtx (8 to 32 μg/g body weight, respectively) also increased the leg and antennae grooming activities (p < .05, n = 40, respectively). Jbtx (8 to 16 μg/g) induced a maximum neuromuscular blockade of 80.72% (n = 6, p < .05) and was cardiotoxic, decreasing the cockroach heart rate. The electrophysiological profiles of both muscle and nerve of L. migratoria showed that Jbtx (2.5 × 10^-7 and 2.5 × 10^-3 μg/ body weight) induced a significant increase in the amplitude of nerve action potentials (n = 5, p < .05). Voltage clamp analysis of Jbtx (200 nM) applied in Xenopus laevis oocytes heterologously expressed with Nav 1.1 channels showed a significant increase in the sodium currents. In conclusion, this work revealed that the entomotoxic activity of Jbtx involves complex behavioral alterations that begins with an initial activation of voltage-gated sodium channels.

Ureases: Historical aspects, catalytic, and non-catalytic properties - A review

Urease (urea amidohydrolase, EC 3.5.1.5) is a nickel-containing enzyme produced by plants, fungi, and bacteria that catalyzes the hydrolysis of urea into ammonia and carbamate. Urease is of historical importance in Biochemistry as it was the first enzyme ever to be crystallized (1926). Finding nickel in urease's active site (1975) was the first indication of a biological role for this metal. In this review, historical and structural features, kinetics aspects, activation of the metallocenter and inhibitors of the urea hydrolyzing activity of ureases are discussed. The review also deals with the non-enzymatic biological properties, whose discovery 40 years ago started a new chapter in the study of ureases. Well recognized as virulence factors due to the production of ammonia and alkalinization in diseases by urease-positive microorganisms, ureases have pro-inflammatory, endocytosis-inducing and neurotoxic activities that do not require ureolysis. Particularly relevant in plants, ureases exert insecticidal and fungitoxic effects. Data on the jack bean urease and on jaburetox, a recombinant urease-derived peptide, have indicated that interactions with cell membrane lipids may be the basis of the non-enzymatic biological properties of ureases. Altogether, with this review we wanted to invite the readers to take a second look at ureases, very versatile proteins that happen also to catalyze the breakdown of urea into ammonia and carbamate.

Jack bean urease modulates neurotransmitter release at insect neuromuscular junctions

BACKGROUND

Plants have developed a vast range of mechanisms to compete with phytophagous insects, including entomotoxic proteins such as ureases. The legume Canavalia ensiformis produces several urease isoforms, of which the more abundant is called Jack Bean Urease (JBU). Previews work has demonstrated the potential insecticidal effects of JBU, by mechanisms so far not entirely elucidated. In this work, we investigated the mechanisms involved in the JBU-induced activity upon neurotransmitter release on insect neuromuscular junctions.

METHODS

Electrophysiological recordings of nerve and muscle action potentials, and calcium imaging bioassays were employed.

RESULTS AND CONCLUSION

JBU (0.28 mg/animal/day) in Locusta migratoria 2nd instar through feeding and injection did not induce lethality, although it did result in a reduction of 20% in the weight gain at the end of 168 h (n = 9, p ≤ 0.05). JBU (0.014 and 0.14 mg) injected direct into the locust hind leg induced a dose and time-dependent decrease in the amplitude of muscle action potentials, with a maximum decrease of 70% in the amplitude at the highest dose (n = 5, p ≤ 0.05). At the same doses JBU did not alter the amplitude of action potentials evoked from motor neurons. Using Drosophila 3rd instar larvae neuromuscular preparations, JBU (10^-7 M) increased the occurrence of miniature Excitatory Junctional Potentials (mEJPs) in the presence of 1 mM CaCl₂ (n = 5, p ≤ 0.05). In low calcium (0.4 mM) assays, JBU (10^-7 M) was not able to modulate the occurrence of the events. In Ca²⁺-free conditions, with EGTA or CoCl₂, JBU induced a significant decrease in the occurrence of mEPJs (n = 5, p ≤ 0.05). Injected into the 3rd abdominal ganglion of Nauphoeta cinerea cockroaches, JBU (1 μM) induced a significant increase in Ca²⁺ influx (n = 7, p ≤ 0.01), similar to that seen for high KCl (35 mM) condition. Taken together the results confirm a direct action of JBU upon insect neuromuscular junctions and possibly central synapses, probably by disrupting the calcium machinery in the pre-synaptic region of the neurons.

Hemocyte Morphology and Cellular Immune Response in Termite (Reticulitermes speratus)

Because termites (Reticulitermes speratus) are very small, it is difficult to conduct experiments involving pathogen injection and hemocyte collection. Therefore, to observe hemocyte-mediated immune responses against foreign substances, in vitro hemocyte culture is essential. After collecting about 3 μl of hemolymph, hemocytes were cultured for 7 d, during which the cells maintained full function. Four types of hemocyte were identified, namely, granulocytes, plasmatocytes, oenocytoids, and prohemocytes, among which granulocytes are the main immune hemocytes that fight invasion by foreign substances. Most hemocytes were alive and/or functioning after 7 d of culture, but then either died or lost function.

Jaburetox-induced toxic effects on the hemocytes of Rhodnius prolixus (Hemiptera: Reduviidae)

Jaburetox is a recombinant peptide derived from a Canavalia ensiformis urease that presents toxic effects upon several species of insects, phytopathogenic fungi and yeasts of medical importance. So far, no toxicity of Jaburetox to mammals has been shown. Previous reports have identified biochemical targets of this toxic peptide in insect models, although its mechanism of action is not completely understood. In this work, we aimed to characterize the effects of Jaburetox in hemolymphatic insect cells. For this purpose, the model insect and Chagas' disease vector Rhodnius prolixus was used. In vivo and in vitro experiments indicated that Jaburetox interacts with a subset of hemocytes and it can be found in various subcellular compartments. In insects injected with Jaburetox there was an increase in the gene expression of the enzymes UDP-N-acetylglucosamine pyrophosphorylase (UAP), chitin synthase and nitric oxide synthase (NOS). Nevertheless, the expression of NOS protein, the enzyme activities of UAP and acid phosphatase (a possible link between UAP and NOS) as well as the phosphorylation state of proteins remained unchanged upon the in vivo Jaburetox treatment. Nitric oxide (NO) imaging using fluorescent probes showed that Jaburetox augmented NO production in the hemocyte aggregates when compared to controls. Even though Jaburetox activated the hemocytes, as demonstrated by wheat germ agglutinin binding assays, the peptide did not lead to an increase of their phagocytic behavior. Taken together, these findings contribute to our understanding of toxic effects of Jaburetox, a peptide with biotechnological applications and a prospective tool for rational insect control.

Jaburetox: update on a urease-derived peptide

Urease from Canavalia ensiformis seeds was the first enzyme ever to be crystallized, in 1926. These proteins, found in plants, bacteria and fungi, present different biological properties including catalytic hydrolysis of urea, and also enzyme-independent activities, such as induction of exocytosis, pro-inflammatory effects, neurotoxicity, antifungal and insecticidal properties. Urease is toxic to insects and fungi per se but part of this toxicity relies on an internal peptide (~11 kDa), which is released upon digestion of the protein by insect enzymes. A recombinant form of this peptide, called jaburetox (JBTX), was constructed using jbureII gene as a template. The peptide exhibits liposome disruption properties, and insecticidal and fungicidal activities. Here we review the known biological properties activities of JBTX, and comment on new ones not yet fully characterized. JBTX was able to cause mortality of Aedes aegypti larvae in a feeding assay whereas in a dose as low as of 0.1 μg it provoked death of Triatoma infestans bugs. JBTX (10⁻⁵–10⁻⁶ M) inhibits the growth of E. coli, P. aeruginosa and B. cereus after 24 h incubation. Multilamellar liposomes interacting with JBTX undergo reorganization of the membrane’s lipids as detected by small angle X-ray scattering (SAXS) studies. Encapsulating JBTX into lipid nanoparticles led to an increase of the peptide’s antifungal activity. Transgenic tobacco and sugarcane plants expressing the insecticidal peptide JBTX, showed increased resistance to attack of the insect pests Spodoptera frugiperda, Diatraea saccharalis and Telchin licus licus. Many questions remain unanswered; however, so far, JBTX has shown to be a versatile peptide that can be used against various insect and fungus species, and in new bacterial control strategies.

Jaburetox affects gene expression and enzyme activities in Rhodnius prolixus, a Chagas' disease vector

Jaburetox, a recombinant peptide of ∼11kDa derived from one of the Canavalia ensiformis (Jack Bean) urease isoforms, is toxic and lethal to insects belonging to different orders when administered orally or via injection. Previous findings indicated that Jaburetox acts on insects in a complex fashion, inhibiting diuresis and the transmembrane potential of Malpighian tubules, interfering with muscle contractility and affecting the immune system. In vitro, Jaburetox forms ionic channels and alters permeability of artificial lipid membranes. Moreover, recent data suggested that the central nervous system (CNS) is a target organ for ureases and Jaburetox. In this work, we employed biochemical, molecular and cellular approaches to explore the mode of action of Jaburetox using Rhodnius prolixus, one of the main Chagas' disease vectors, as experimental model. In vitro incubations with fluorescently labeled Jaburetox indicated a high affinity of the peptide for the CNS but not for salivary glands (SG). The in vitro treatment of CNS or SG homogenates with Jaburetox partially inhibited the activity of nitric oxide synthase (NOS), thus disrupting nitrinergic signaling. This inhibitory effect was also observed in vivo (by feeding) for CNS but not for SG, implying differential modulation of NOS in these organs. The inhibition of NOS activity did not correlate to a decrease in expression of its mRNA, as assessed by qPCR. UDP-N-acetylglucosamine pyrophosphorylase (UAP), a key enzyme in chitin synthesis and glycosylation pathways and a known target of Jaburetox in insect CNS, was also affected in SG, with activation of the enzyme seen after both in vivo or in vitro treatments with the peptide. Unexpectedly, incubation of Jaburetox with a recombinant R. prolixus UAP had no effect on its activity, implying that the enzyme's modulation by the peptide requires the participation of other factor(s) present in CNS or SG homogenates. Feeding Jaburetox to R. prolixus decreased the mRNA levels of UAP and chitin synthase, indicating a complex regulation exerted by the peptide on these enzymes. No changes were observed upon Jaburetox treatment in vivo and in vitro on the activity of the enzyme acid phosphatase, a possible link between UAP and NOS. Here we have demonstrated for the first time that the Jaburetox induces changes in gene expression and that SG are another target for the toxic action of the peptide. Taken together, these findings contribute to a better understanding of the mechanism of action of Jaburetox as well as to the knowledge on basic aspects of the biochemistry and neurophysiology of insects, and might help in the development of optimized strategies for insect control.