Destruxins B

Background: Destruxin A (DA) is a mycotoxin secreted by entomogenous fungi, such as Metarhizium anisopliae, which has broad-spectrum insecticidal activity. Insect innate immunity provides resistance against the invasion of entomopathogenic fungi. Previous studies have shown that DA could inhibit the immune response, however, the suppressive mechanism of DA on prophenoloxidase system is still unknown. Results: Based on the transcriptome of Aphis citricola, we screened the scavenger receptor class B(AcSR-B)and identified that it significantly responds to DA. Spatio-temporal expression analysis showed that AcSR-B is highly expressed in adult stage and is mainly distributed in the abdominal region. We further revealed that both M. anisopliae and Escherichia coli could suppress the expression of AcSR-B at 24 h, and that the expressed recombinant protein rAcSR-B possessed agglutination activity to M. anisopliae and E. coli. DA could suppress the protein expression of AcSR-B. In addition, RNA interference of AcSR-B caused death of A. citricola in a dose-dependent manner, and RNA interference of AcSR-B increased mortality in A. citricola under the same lethal concentration of DA. The inhibiting effect of AcSR-B silencing was similar with the DA treatment upon phenol oxidase (PO) activity of A. citricola hemolymph. DA could not decrease PO activity further after AcSR-B silencing. Conclusion: Destruxin A inhibits melanization by suppressing AcSR-B in A. citricola. Our findings are helpful in understanding the underlying molecular mechanism of the DA suppressing immune system, and uncover a potential molecular target for double-stranded RNA (dsRNA) insecticides.

The insect pathogenic fungus, Metarhizium anisopliae is a commercialized microbial agent used in biological control efforts targeting a diverse range of agricultural and other insect pests. The second step in the synthesis of a group of M. anisopliae α-pyrone diterpenoids (termed subglutinols) involves the activity of a prenyltransferase family geranylgeranyl diphosphate synthase (product of the subD/MaGGPPS5 gene). Here, we show that targeted gene disruption of MaGGPPS5 results in earlier conidial germination and faster greater vegetative growth compared to the wild type (WT) parent and complemented strains. In addition, insect bioassays revealed that the ΔMaGGPPS5 mutant strain displayed significantly increased virulence, with a ~50% decrease in the mean lethal time (LT50 , from 6 to 3 days) to kill (50% of) target insects, and an ~15-40-fold decrease in the mean lethal dose (LC50 ). Metabolite profiling indicated increased accumulation in the ΔMaGGPPS5 mutant of select subglutinols (A, B and C) and destruxins (A, A2, B and B2), the latter a set of fungal secondary metabolites that act as insect toxins, with a concomitant loss of production of subglutinol 'analogue 45'. These data suggest that the increased virulence phenotype seen for the ΔMaGGPPS5 strain can, at least in part, be attributed to a combination of faster growth and increased insect toxin production, linking the production of two different secondary metabolite pathways, and represent a novel approach for the screening of isolates with enhanced virulence via modulation of terpenoid secondary metabolite biosynthesis.

Synthesis of various destruxin analogs was accomplished using Shiina's macrolactonization as a key reaction. Combinatorial synthesis of cyclization precursors using solid-phase peptide synthesis and macrolactonization in solution were successful. In the synthesis of destruxin E and its analogs, the hydroxyacid-proline (HA1-Pro2) dipeptide with an acetonide-protected diol moiety was synthesized in an asymmetric manner, and the protected diol was converted to an epoxide after macrocyclization. Destruxin E was synthesized on a gram scale using solution-phase synthesis. The structure-activity relationships of destruxins were elucidated through biological evaluation of synthetic destruxins A, B, and E and their analogs for morphological changes in osteoclast-like multinucleated cells.