Ureases (urea amidohydrolases, EC 3.5.1.5) are metalloenzymes that catalyze the breakdown of urea into carbon dioxide and ammonia [1]. They are produced by a wide variety of organisms including bacteria, fungi and plants, but not by animals [2]. Their well-documented enzymatic role results in increased nitrogen availability in a readily usable form and in the alkalization of the medium due to ammonia production. Besides, ureases present biological activities not related to the enzyme function, such as toxicity against fungi and insects as well as exocytosis induction in many cell models [3].
The development of insect resistance and the need for more rational, environment-friendly insecticides are the main driving forces of the research for new substances with entomotoxic properties [4]. In this context, the seed of Canavalia ensiformis (Jack Bean) presents at least three urease isoforms that contribute to the plant resistance to the attacks by insects and fungi [3]. When administered orally, ureases are toxic for insects presenting cathepsin-like peptidases in their digestive system (e.g. hemipterans) while insects with digestion based on trypsin-like peptidases (e.g. dipterans) show no susceptibility [5]. This toxicity is explained in part by the fact that digestive cathepsin-like peptidases cleave ureases at specific sites, releasing peptides with insecticidal activity [6–8]. An insecticidal peptide called Pepcanatox was isolated from the in vitro digestion of canatoxin, an isoform of C. ensiformis urease [9] and, later, an equivalent recombinant peptide called Jaburetox was produced based on that finding. Jaburetox was shown to be toxic to insects of several orders, irrespective of their digestive enzymes [10–12]. Preliminary reports with transgenic crops such as maize, soybean and sugarcane expressing Jaburetox indicated a higher resistance to the attack of insects (unpublished data). Furthermore, high doses of the peptide are not toxic to mice and rats when given orally [10] making Jaburetox a promising tool for rational insect control. Notwithstanding, the peptide’s toxic mechanism of action in insects is still poorly understood.
Although much simpler than its mammalian counterpart, the host-defense system of insects relies on an intricate array of innate reactions such as complex recognition, signaling and effector systems [13, 14]. Insect immune response can be broadly divided into cellular, including nodulation, encapsulation and phagocytosis, and humoral, which involves nitric oxide (NO), antimicrobial peptides, lysozyme and the phenoloxidase (PO) cascade, among others. Both types of defenses can be recruited simultaneously or separately, depending on the type of insult [15, 16].
It is well established that insect immunity is modulated by eicosanoids [17]. This family of molecules is synthesized from polyunsaturated fatty acids, mainly arachidonic acid, which is released from membrane phospholipids via activation of a phospholipase A2 (PLA2) [18]. Once free, arachidonic acid then follows diverse enzymatic oxygenation pathways involving cyclooxygenases (COX) to yield prostaglandins and thromboxanes, and lipoxygenases (LOX) producing lipoxins and leukotrienes [19].
Rhodnius prolixus is a major insect vector of Chagas disease, an illness that kills approximately 10,000 people annually and affects seven million people worldwide, causing high economic and social costs [20]. Since the foundational studies of Wigglesworth [21], this insect has been a popular model organism for physiological, behavioral and biochemical studies. Furthermore, its genome has been recently sequenced [22], consolidating the species as a powerful tool for genetic and evolutionary approaches.
Previous studies of our group on R. prolixus have established that the Jack Bean Urease (JBU) and Jaburetox disturb serotonin-stimulated processes such as diuresis in Malpighian tubules and contraction of the anterior midgut by interfering with eicosanoid signaling [23, 24]. More recently, Defferrari et al. [25] established a link between the toxic effect of JBU and the triggering of immune defenses. Within this context, the aim of this work was to explore the immune responses induced by the toxic urease-derived peptide Jaburetox, using R. prolixus as a model. The findings are discussed regarding the current knowledge on the mechanism of action of Jaburetox in insects.