Cytochalasins from the Ash Endophytic Fungus Nemania diffusa DSM 116299

The secondary metabolome of Nemania diffusa, isolated as an ash endophytic fungus, was analyzed in detail. From its cultures, a previously undescribed cytochalasin 1 was isolated using preparative HPLC, together with six known congeners: 18-dehydroxy-cytochalasin E (2), cytochalasins Z₇ (3), Z₈ (4), and E (5), 18-dehydroxy-17-didehydro-cytochalasin E (6), and K Steyn (7). The structures of these compounds were determined using data from high-resolution mass spectrometry (HR-MS), in combination with 1D and 2D nuclear magnetic resonance (NMR) spectroscopy. Metabolites 1-4 share a characteristic 12-membered lactone moiety, placing them within a rarely examined cytochalasin subclass. Thus, the compounds were incorporated into our ongoing screening campaign to study the structure-activity relationship of this metabolite family. We initially determined their cytotoxicity in eukaryotic mouse fibroblast L929 cells using an MTT-based colorimetric assay, and further investigated their effect on the cellular actin dynamics of the human osteosarcoma cell line U-2OS in detail. Unexpectedly, we discovered a high number of irreversible compounds (1, 2, and 4). Additionally, we highlighted specific structural features within the 12-membered cytochalasin subclass that may play a role in directing the reversibility of these compounds.