Antiparasitic Ovalicin Derivatives from Pseudallescheria boydii, a Mutualistic Fungus of French Guiana Termites

Abstract

Social insects are in mutualism with microorganisms, contributing to their resistance against infectious diseases. The fungus Pseudallescheria boydii SNB-CN85 isolated from termites produces ovalicin derivatives resulting from the esterification of the less hindered site of the ovalicin epoxide by long-chain fatty acids. Their structures were elucidated using spectroscopic analysis and semisynthesis from ovalicin. For ovalicin, these compounds displayed antiprotozoal activities against Plasmodium falciparum and Trypanosoma brucei, with IC₅₀ values of 19.8 and 1.1 µM, respectively, for the most active compound, i.e., ovalicin linoleate. In parallel, metabolomic profiling of a collection of P. boydii strains associated with termites made it possible to highlight this class of compounds together with tyroscherin derivatives in all strains. Finally, the complete genome of P. boydii strains was obtained by sequencing, and the cluster of potential ovalicin and ovalicin biosynthesis genes was annotated. Through these metabolomic and genomic analyses, a new ovalicin derivative named boyden C, in which the 6-membered ring of ovalicin was opened by oxidative cleavage, was isolated and structurally characterized.

Keywords: Pseudallescheria boydii; biosynthesis; genome; molecular networking; ovalicin; symbiotic fungus.

Figure 1

Structure of Compounds 2–6 isolated from P. boydii SNB-CN85.

Figure 2

Example of observed key ¹H-¹H COSY (bold bonds) and ¹H-¹³C (plain arrow) correlations observed for 2–5 (example from molecule 2).

Figure 3

Molecular network with t-SNE representation of crude extracts of P. boydii. SNB-CN71, -CN73, -CN81, and -CN85; in purple cluster close to the sodium-cationized ovalicin (1), ester analogs and other ovalicin fatty acid derivatives identified by dereplication (dotted lines), in the dark blue cluster of the protonated tyroscherin and N-methyltyrocherin.

Figure 4

Annotated biosynthetic gene cluster Pbo_Ova of P. boydii and related biosynthetic pathways (the different analogs of β–trans–bergamoten are indicated under the acronym Btb). R: carbon chain.

Figure 5

(A) Structure of Compound 7 isolated from P. boydii SNB-CN73 and (B) key ¹H-¹H COSY (bold bonds) and long-range ¹H-¹³C (plain arrow) correlations.