Effect of the insect phenoloxidase on the metabolism of l-DOPA

Effects of beauvericin on the blood cells of Bombyx mori

In this study, silkworms were treated by injection of the bioactive depsipeptide beauvericin (BEA) to explore its effect on the cellular immunity of larvae of the silkworm Bombyx mori. The results showed that: The LC50 of BEA for silkworms on the 3rd day of the 4th instar was 362.36 µM. The total count of circulating hemocytes in the silkworms decreased at 12 h after injection with 350 µM BEA, and reached the minimum value at 72 h post-treatment; at 48 h post-treatment, a large number of nodules formed by the aggregation of blood cells of the silkworms were observed under the light microscope. The survival rate of hemocytes in the larvae treated with BEA was significantly reduced in a dose-dependent manner in vivo and in vitro. The encapsulation of Q-Sepharose Fast Flow (QFF) gel particles by hemocytes in the treatment group was significantly higher than that in the control group at 1.5 h and 3 h post-treatment (P < 0.05). Moreover, the melanization ratio of QFF gel particles kept increasing with treatment time. The melanization rate at 24 h after treatment was significantly higher than that at other times (P < 0.05), reaching 55.33 %. Under the scanning electron microscope, BEA-treated larvae showed protrusions on the surface of their blood cells in vivo. Under the transmission electron microscope, it was observed that silkworm hemocytes were vacuolated. This study demonstrated that BEA had an effect on the blood cells of silkworms, and has thrown some light on the inhibitory effect and mechanism of BEA on insect cellular immunity.

Effect of amide protoporphyrin derivatives on immune response in Apis mellifera

The intracellular microsporidian parasite Nosema ceranae is known to compromise bee health by induction of energetic stress and downregulation of the immune system. Porphyrins are candidate therapeutic agents for controlling Nosema infection without adverse effects on honeybees. In the present work, the impact of two protoporphyrin IX derivatives, i.e. PP[Asp]2 and PP[Lys]2, on Apis mellifera humoral immune response has been investigated in laboratory conditions in non-infected and N. ceranae-infected honeybees. Fluorescence spectroscopy analysis of hemolymph showed for the first time that porphyrin molecules penetrate into the hemocoel of honeybees. Phenoloxidase (PO) activity and the expression of genes encoding antimicrobial peptides (AMPs: abaecin, defensin, and hymenoptaecin) were assessed. Porphyrins significantly increased the phenoloxidase activity in healthy honeybees but did not increase the expression of AMP genes. Compared with the control bees, the hemolymph of non-infected bees treated with porphyrins had an 11.3- and 6.1-fold higher level of PO activity after the 24- and 48-h porphyrin administration, respectively. Notably, there was a significant inverse correlation between the PO activity and the AMP gene expression level (r =  - 0.61696, p = 0.0143). The PO activity profile in the infected bees was completely opposite to that in the healthy bees (r =  - 0.5118, p = 0.000), which was related to the changing load of N. ceranae spores in the porphyrin treated-bees. On day 12 post-infection, the spore loads in the infected porphyrin-fed individuals significantly decreased by 74%, compared with the control bees. Our findings show involvement of the honeybee immune system in the porphyrin-based control of Nosema infection. This allows the infected bees to improve their lifespan considerably by choosing an optimal PO activity/AMP expression variant to cope with the varying level of N. ceranae infection.

Beneficial Bacteria in the Intestines of Housefly Larvae Promote Larval Development and Humoral Phenoloxidase Activity, While Harmful Bacteria do the Opposite

The gut microenvironment of houseflies provides unique conditions for microbial colonization. Some gut microorganisms provide benefits for the development of the host by regulating the interaction between the host and intestinal pathogens. Gut microbial alterations can stimulate the host’s immune mechanism to resist pathogen invasion and affect the development of insects. In this study, we isolated 10 bacterial strains from housefly larval intestines. The isolated bacteria were added to the larval diet to analyze the effects of microecological regulation of gut bacteria on larval development. Dynamic changes in gut flora composition after oral administration of specific bacteria were analyzed although 16S rRNA gene high-throughput sequencing technology. To explore the interaction between gut bacteria and the host, the immune response of larvae against the invasion of foreign microorganisms was observed through a phenoloxidase activity experiment. Our results showed that the oral administration of various isolated bacteria had different effects on larval development. Oral administration of beneficial bacteria, including Enterobacter hormaechei, Klebsiella pneumoniae, Acinetobacter bereziniae, Enterobacter cloacae, Lysinibacillus fusiformis and Bacillus safensis, promoted larval development by increasing gut community diversity and the humoral immunity of larvae, while harmful bacteria, including Pseudomonas aeruginosa, Providencia stuartii and Providencia vermicola, influenced larval development by inhibiting the growth of beneficial bacteria and reducing the humoral immunity of larvae. The beneficial bacteria isolated in our research could be applied as good probiotic additives for the intensive feeding of larvae, while isolation of the harmful bacteria provides a basis for the development of pest inhibitors. Furthermore, our research revealed the immune response of housefly phenoloxidase to exogenous microorganism stimulation, providing richer and more comprehensive knowledge of the larval innate immune response.

Immune properties of invertebrate phenoloxidases

Melanin production from different types of phenoloxidases (POs) confers immunity from a variety of pathogens ranging from viruses and microorganisms to parasites. The arthropod proPO expresses a variety of activities including cytokine, opsonin and microbiocidal activities independent of and even without melanin production. Proteolytic processing of proPO and its activating enzyme gives rise to several peptide fragments with a variety of separate activities in a process reminiscent of vertebrate complement system activation although proPO bears no sequence similarity to vertebrate complement factors. Pathogens influence proPO activation and thereby what types of immune effects that will be produced. An increasing number of specialised pathogens - from parasites to viruses - have been identified who can synthesise compounds specifically aimed at the proPO-system. In invertebrates outside the arthropods phylogenetically unrelated POs are participating in melanization reactions obviously aimed at intruders and/or aberrant tissues.

Effect of Honey and Syrup Diets Enriched with 1,3-1,6 β-Glucans on Honeybee Survival Rate and Phenoloxidase Activity (Apis mellifera L. 1758)

β-glucans can activate the animal innate immune system by acting as immune-modulators and inducing various stimulatory effects. The aim of this study was to investigate the effect of 1,3-1,6 β-glucans administered orally for 96 h on Apis mellifera workers (newly emerged and nurse bees). β-glucans were included in honey and syrup. Survival rate and phenoloxidase activity were measured. In both newly emerged and nurse bees, β-glucans supplementation did not affect survival rate (p > 0.05). Conversely, phenoloxidase activity was higher in both newly emerged bees (p = 0.048) and nurse bees (p = 0.014) fed with a honey diet enriched with β-glucans compared to those fed with only honey. In both the newly emerged and nurse bees, no statistical differences in phenoloxidase activity were recorded between the group fed with a syrup-based diet enriched with β-glucans and the control group (p > 0.05). The absence of significant variation in survival suggests that the potential negative effect of β-glucans in healthy bees could be mitigated by their metabolism. Conversely, the inclusion of β-glucans in a honey-based diet determined an increase of phenoloxidase activity, suggesting that the effect of β-glucan inclusion in the diet of healthy bees on phenoloxidase activity could be linked to the type of base-diet. Further investigations on β-glucans metabolism in bees, on molecular mechanism of phenoloxidase activation by 1,3-1,6 β-glucans, and relative thresholds are desirable. Moreover, investigation on the combined action of honey and β-glucans on phenoloxidase activity are needed.

Superoxide dismutase from venom of the ectoparasitoid Scleroderma guani inhibits melanization of hemolymph

Superoxide dismutase (SOD) known as an important antioxidative stress protein has been recently found in venoms of several parasitoid wasps. However, its functions and characteristics as a virulent factor remain scarcely described. Here, we report the characterization of two venomous SOD genes (SguaSOD1 and SguaSOD3) from the ectoparasitoid, Scleroderma guani. The metal binding sites, cysteine amino acid positions and signature sequences of the SOD family were conserved within SguaSOD1 and SguaSOD3. Relatively high levels of their transcripts were observed in pupae followed a decrease in early adults, after which they had the highest transcriptions, indicating that their productions would be regulated in venom apparatus. Although the two genes showed lower expression in venom apparatus compared to head and thorax, the enzymatic assay revealed that SOD indeed had activity in venom. Further, we showed that recombinant SguaSOD3 suppressed melanization of host hemolymph, implying that this protein used as a virulent factor uniquely impacts the prophenoloxidase cascade.