Expanding the Biological Properties of Alkannins and Shikonins: Their Impact on Adipogenesis and Life Expectancy in Nematodes

Alkannin, shikonin (A/S) and their derivatives are naturally occurring hydroxynaphthoquinones biosynthesized in some species of the Boraginaceae family. These natural compounds have been extensively investigated for their biological properties over the last 40 years, demonstrating a plethora of activities, such as wound healing, regenerative, anti-inflammatory, antitumor, antimicrobial and antioxidant. This study aims to extend the current knowledge by investigating the effects of various A/S compounds on two model systems, namely on 3T3-L1 pre-adipocytes and the nematode Caenorhabditis elegans. The former constitutes an established in vitro model for investigating anti-obesity and insulin-mimetic properties, while the latter has been widely used as a model organism for studying fat accumulation, lifespan and the anthelmintic potential. A set of chemically well-defined A/S derivatives were screened for their effect on pre-adipocytes to assess cell toxicity, cell morphology, and cell differentiation. The differentiation of pre-adipocytes into mature adipocytes was examined upon treatment with A/S compounds in the presence/absence of insulin, aiming to establish a structure-activity relationship. The majority of A/S compounds induced cell proliferation at sub-micromolar concentrations. The ester derivatives exhibited higher IC₅₀ values, and thus, proved to be less toxic to 3T3-L1 cells. The parent molecules, A and S tested at 1 μM resulted in a truncated differentiation with a reduced number of forming lipids, whereas compounds lacking the side chain hydroxyl group projected higher populations of mature adipocytes. In C. elegans mutant strain SS104, A/S enriched extracts were not able to inhibit the fat accumulation but resulted in a drastic shortage of survival. Thus, the set of A/S compounds were tested at 15 and 60 μg/ml in the wild-type strain N2 for their nematocidal activity, which is of relevance for the discovery of anthelmintic drugs. The most pronounced nematocidal activity was observed for naphthazarin and β,β-dimethyl-acryl-shikonin, followed by isovaleryl-shikonin. The latter 2 A/S esters were identified as the most abundant constituents in the mixture of A/S derivatives isolated from Alkanna tinctoria (L.) Tausch. Taken together, the findings show that the structural variations in the moiety of A/S compounds significantly impact the modulation of their biological activities in both model systems investigated in this study.

Erratum: Cytoplasmic Lipases-A Novel Class of Fungal Defense Proteins Against Nematodes

[This corrects the article DOI: 10.3389/ffunb.2021.696972.].

Cytoplasmic Lipases-A Novel Class of Fungal Defense Proteins Against Nematodes

Fungi are an attractive food source for predators such as fungivorous nematodes. Several fungal defense proteins and their protective mechanisms against nematodes have been described. Many of these proteins are lectins which are stored in the cytoplasm of the fungal cells and bind to specific glycan epitopes in the digestive tract of the nematode upon ingestion. Here, we studied two novel nematotoxic proteins with lipase domains from the model mushroom Coprinopsis cinerea. These cytoplasmically localized proteins were found to be induced in the vegetative mycelium of C. cinerea upon challenge with fungivorous nematode Aphelenchus avenae. The proteins showed nematotoxicity when heterologously expressed in E. coli and fed to several bacterivorous nematodes. Site-specific mutagenesis of predicted catalytic residues eliminated the in-vitro lipase activity of the proteins and significantly reduced their nematotoxicity, indicating the importance of the lipase activity for the nematotoxicity of these proteins. Our results suggest that cytoplasmic lipases constitute a novel class of fungal defense proteins against predatory nematodes. These findings improve our understanding of fungal defense mechanisms against predators and may find applications in the control of parasitic nematodes in agriculture and medicine.

Nematotoxicity of a Cyt-like protein toxin from Conidiobolus obscurus (Entomophthoromycotina) on the pine wood nematode Bursaphelenchus xylophilus

BACKGROUND

The pine wood nematode Bursaphelenchus xylophilus is a destructive pest on Pinus trees and lacks effective control measures. The present study identified a novel nematotoxic cytolytic (Cyt)-like protein originating from the entomopathogenic fungus Conidiobolus obscurus.

RESULTS

The protein was successfully purified using heterologous expression in Escherichia coli and affinity chromatography. N-hydroxysuccinimide-rhodamine-labeled Cyt-like protein was used to establish the route of toxin uptake, and revealed that the toxin can enter the nematode via the stylet. In bioassays, the purified protein had high nematicide activity against B. xylophilus, with a median lethal concentration at 24 h of 15.8 and 29.4 μg mL^-1 for juveniles and adults, respectively. Compared with the deionized water control, fecundity, thrashing, and egg hatching were significantly reduced by 97%, 98%, and 83%, respectively, with 40 μg mL^-1 Cyt-like protein at 24-36 h. Staining with Oil-Red-O showed a decrease in large lipid droplet formation in the protein-treated adult nematodes.

CONCLUSION

The Cyt-like protein toxin possesses high nematicide activity against B. xylophilus with effects on nematode vitality and fecundity. The potential exists to use the Cyt-like protein for the control of B. xylophilus.

Toxicity of Potential Fungal Defense Proteins towards the Fungivorous Nematodes Aphelenchus avenae and Bursaphelenchus okinawaensis

Our results support the hypothesis that cytoplasmic proteins abundant in fungal fruiting bodies are involved in fungal resistance against predation. The toxicity of these proteins toward stylet-feeding nematodes, which are also capable of feeding on plants, and the abundance of these proteins in edible mushrooms, may open possible avenues for biological crop protection against parasitic nematodes, e.g., by expression of these proteins in crops.

Resistance of fungi to predation is thought to be mediated by toxic metabolites and proteins. Many of these fungal defense effectors are highly abundant in the fruiting body and not produced in the vegetative mycelium. The defense function of fruiting body-specific proteins, however, including cytoplasmically localized lectins and antinutritional proteins such as biotin-binding proteins, is mainly based on toxicity assays using bacteria as a heterologous expression system, with bacterivorous/omnivorous model organisms as predators. Here, we present an ecologically more relevant experimental setup to assess the toxicity of potential fungal defense proteins towards the fungivorous, stylet-feeding nematodes Aphelenchus avenae and Bursaphelenchus okinawaensis. As a heterologous expression host, we exploited the filamentous fungus Ashbya gossypii. Using this new system, we assessed the toxicity of six previously characterized, cytoplasmically localized, potential defense proteins from fruiting bodies of different fungal phyla against the two fungivorous nematodes. We found that all of the tested proteins were toxic against both nematodes, albeit to various degrees. The toxicity of these proteins against both fungivorous and bacterivorous nematodes suggests that their targets have been conserved between the different feeding groups of nematodes and that bacterivorous nematodes are valid model organisms to assess the nematotoxicity of potential fungal defense proteins.

IMPORTANCE Our results support the hypothesis that cytoplasmic proteins abundant in fungal fruiting bodies are involved in fungal resistance against predation. The toxicity of these proteins toward stylet-feeding nematodes, which are also capable of feeding on plants, and the abundance of these proteins in edible mushrooms, may open possible avenues for biological crop protection against parasitic nematodes, e.g., by expression of these proteins in crops.

Isolation and identification of nematode-antagonistic compounds from the fungus Aspergillus candidus

Culture medium from an isolate of the fungus Aspergillus candidus was extracted, fractionated and examined to discover compounds antagonistic to plant-parasitic nematodes that are important pathogens of agricultural crops. Column, thin layer and preparative chromatographies and spectral and elemental analyses, were used to isolate and identify two major constituents of an active fraction (Fraction F) obtained from the medium. Compound 1 was identified as 2-hydroxypropane-1, 2, 3-tricarboxylic acid (citric acid). Compound 2 was identified as 3-hydroxy-5-methoxy-3-(methoxycarbonyl)-5-oxopentanoic acid, an isomer of 1, 2-dimethyl citrate. Compound 1 and a citric acid standard, each tested at 50 mg mL(-1) in water, decreased hatch from eggs of the plant-parasitic nematode Meloidogyne incognita by more than 94%, and completely immobilized second-stage juveniles after 4-6 days exposure. Fraction F and Compounds 1 and 2 decreased the mobility of adults of the plant-parasitic nematode Ditylenchus destructor in vitro. Fraction F (25 mg mL(-1)) inhibited mobility >99% at 72 hrs. Compounds 1 and 2 (50 mg mL(-1)) each inhibited mobility more than 25% at 24 hr and more than 50% at 72 hr. This is the first assignment of nematode-antagonistic properties to specifically identified A. candidus metabolites.