Impact on Salivary Gland Degeneration by Putative Ecdysteroid Antagonists and Agonists in the Ixodid Tick Amblyomma hebraeum

ATG5 is instrumental in the transition from autophagy to apoptosis during the degeneration of tick salivary glands

Female tick salivary glands undergo rapid degeneration several days post engorgement. This degeneration may be caused by the increased concentration of ecdysone in the hemolymph during the fast feeding period and both autophagy and apoptosis occur. In this work, we first proved autophagy-related gene (ATG) and caspase gene expression peaks during degeneration of the tick salivary glands. We explored the regulatory role of Rhipicephalus haemaphysaloides autophagy-related 5 (RhATG5) in the degeneration of tick salivary glands. During the fast feeding phase, RhATG5 was cleaved and both calcium concentration and the transcription of Rhcalpains increased in the salivary glands. Recombinant RhATG5 was cleaved by μ-calpain only in the presence of calcium; the mutant RhATG5^191-199Δ was not cleaved. Treatment with 20-hydroxyecdysone (20E) led to programmed cell death in the salivary glands of unfed ticks in vitro, RhATG8-phosphatidylethanolamine (PE) was upregulated in ticks treated with low concentration of 20E. Conversely, RhATG8-PE decreased and Rhcaspase-7 increased in ticks treated with a high concentration of 20E and transformed autophagy to apoptosis. High concentrations of 20E led to the cleavage of RhATG5. Calcium concentration and expression of Rhcalpains were also upregulated in the tick salivary glands. RNA interference (RNAi) of RhATG5 in vitro inhibited both autophagy and apoptosis of the tick salivary glands. RNAi of RhATG5 in vivo significantly inhibited the normal feeding process. These results demonstrated that high concentrations of 20E led to the cleavage of RhATG5 by increasing the concentration of calcium and stimulated the transition from autophagy to apoptosis.

Ticks are well-known pathogen vectors which transmitted virus, bacterial and protozoan. They are considered to be second only to mosquitoes as global vectors of human diseases. Most tick-borne pathogens (TBPs) are transmitted to hosts through tick bites assisted by saliva. Control of ticks has been achieved primarily by the application of acaricides, a method that has drawbacks such as environmental contamination and selection of pesticide-resistant ticks. Understanding the tick physiological characteristics is the key step for this objective; however, there are knowledge gap remained in tick physiology. Tick salivary glands rapidly degenerate and disappear within 4 days post engorgement. In this research, we are focused on tick salivary glands rapidly degeneration within 4 days post engorgement, and made several highlights findings: The first work demonstrated that 20E promotes both autophagy and apoptosis during tick salivary gland degeneration; RhATG5 is the first reported ATG5 homologue in ticks; RhATG5 play an important role in both autophagy and apoptosis during the degeneration of tick salivary glands.

Steroid plant hormones: effects outside plant kingdom

Brassinosteroids (BS) are the first group of steroid-hormonal compounds isolated from and acting in plants. Among numerous physiological effects of BS growth stimulation and adaptogenic activities are especially remarkable. In this review, we provide evidence that BS possess similar types of activity also beyond plant kingdom at concentrations comparable with those for plants. This finding allows looking at steroids from a new point of view: how common are the mechanisms of steroid bioregulation in different types of organisms from protozoa to higher animals.

Ecdysteroid levels changed by permethrin action in female Rhipicephalus sanguineus (Latreille, 1806) (Acari: Ixodidae) ticks

As recent studies have shown that ecdysteroids may play a major role in the regulation of vitellogenesis in Ixodidae, the present study quantified, by means of a radioimmunoassay, the levels of ecdysteroids present in the hemolymph of semi-engorged females of Rhipicephalus sanguineus ticks obtained from control females (exposed to distilled water) and those exposed to increasing concentrations of permethrin. The levels of ecdysteroids decreased significantly as the concentration of permethrin increased, suggesting that this compound could be an inhibitor of ecdysteroids secretion, and consequently interfering with the reproductive ability of these ticks, since this hormone is responsible for the synthesis and incorporation of vitellogenin by oocytes. This study complements the previous results with R. sanguineus semi-engorged females, showing that permethrin is a potent agent causing major morphological changes in tick oocytes, such as the appearance of large vacuoles in the cytoplasm, reduction in the amount of yolk granules and a decrease in oocyte size, thus culminating in cell death and consequently reducing or preventing reproduction in treated females. The findings that permethrin leads to a decrease in ecdysteroid titers could represent an entry step into this scenario.

Nonsteroidal ecdysone agonists

Nonsteroidal ecdysone agonists are novel compounds that have become attractive candidates not only as pest control agents in agriculture but also as tools for research. Their narrow spectrum of activity makes them relatively safe as pesticides, and their mode of action as ligands for gene expression has found application in gene therapy and inducing transgenic gene expression in plants. These diacylhydrazines (DAHs) are potent nonsteroidal ecdysone agonists, and four of them, tebufenozide, methoxyfenozide, chromafenozide, and halofenozide, have been developed as insecticides. Although these compounds are very toxic to insects, they are safe for mammals and are environmentally benign. Their action on insects is also selective, the first three are effective against Lepidoptera but weakly active or inactive on Diptera and Coleoptera. On the other hand, halofenozide is effective on Coleoptera but mildly active on Lepidoptera. Previous reviews on ecdysone agonists have concentrated on the biological response of some DAHs and their effects on pests. In this review, the chemistry, biological effects and their modes of action at the molecular level will be covered. In addition, a few studies on other nonsteroidal ecdysone agonists, such as 3,5-di-tert-butyl-4-hydroxy-N-iso-butylbenzamide, acylaminoketones, and benzoyl-1,2,3,4-tetrahydroquinolines, will be briefly reviewed.

Action of brassinosteroids in the cotton leafworm Spodoptera littoralis

The two native plant hormones 24-epibrassinolide and 24-epicastasterone showed 50% competition for binding at IC(50) of 1-3.6 microM with [(3)H]ponasterone A using cultured imaginal wing discs from last-instar larvae of the cotton leafworm, Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae). However, culture of imaginal wing discs in different concentrations of brassinosteroids, even up to 100 microM, demonstrated no induction of evagination. In contrast, 20E and the non-steroidal agonist RH-5992 competed respectively about 23- and 42-fold more effectively with labeled ponasterone A, and their ability (EC(50)) to induce disc evagination in vitro was 158 and 87 nM, respectively. Injection of 10 microg of brassinosteroids in newly-moulted last-instar larvae did not cause mortality above controls; higher mortalities were scored when brassinosteroids were injected late in the last instar.

BRASSINOSTEROIDS: Essential Regulators of Plant Growth and Development

Brassinosteroids (BRs) are growth-promoting natural products found at low levels in pollen, seeds, and young vegetative tissues throughout the plant kingdom. Detailed studies of BR biosynthesis and metabolism, coupled with the recent identification of BR-insensitive and BR-deficient mutants, has greatly expanded our view of steroids as signals controlling plant growth and development. This review examines the microchemical and molecular genetic analyses that have provided convincing evidence for an essential role of BRs in diverse developmental programs, including cell expansion, vascular differentiation, etiolation, and reproductive development. Recent advances relevant to the molecular mechanisms of BR-regulated gene expression and BR signal transduction are also discussed.