Spoxazomicin D and Oxachelin C, Potent Neuroprotective Carboxamides from the Appalachian Coal Fire-Associated Isolate Streptomyces sp. RM-14-6

Abstract

The isolation and structure elucidation of six new bacterial metabolites [spoxazomicin D (2), oxachelins B and C (4, 5), and carboxamides 6-8] and 11 previously reported bacterial metabolites (1, 3, 9-12a, and 14-18) from Streptomyces sp. RM-14-6 is reported. Structures were elucidated on the basis of comprehensive 1D and 2D NMR and mass spectrometry data analysis, along with direct comparison to synthetic standards for 2, 11, and 12a,b. Complete 2D NMR assignments for the known metabolites lenoremycin (9) and lenoremycin sodium salt (10) were also provided for the first time. Comparative analysis also provided the basis for structural revision of several previously reported putative aziridine-containing compounds [exemplified by madurastatins A1, B1, C1 (also known as MBJ-0034), and MBJ-0035] as phenol-dihydrooxazoles. Bioactivity analysis [including antibacterial, antifungal, cancer cell line cytotoxicity, unfolded protein response (UPR) modulation, and EtOH damage neuroprotection] revealed 2 and 5 as potent neuroprotectives and lenoremycin (9) and its sodium salt (10) as potent UPR modulators, highlighting new functions for phenol-oxazolines/salicylates and polyether pharmacophores.

Figure 1

Chemical structures of compounds 1–13.

Figure 2

¹H,¹H-COSY and selected HMBC correlations in compounds 1 (putative structure 1a), 2, and 4–6.

Figure 3

Key NOESY correlations in compounds 1, 2, 4, and 5.

Figure 4

(A) Previously reported chemical structures of madurastatins A1, B1, C1, and MBJ-0035 (19–22). (B) Revised chemical structures of madurastatins A1, B1, C1, and MBJ-0035 (23–26).

Figure 5

EtOH damage neuroprotection assay (propidium iodide uptake in rat-derived organotypic hippocampal slice primary cell cultures). (A) DMSO control; (B) 48 h exposure to 100 mM EtOH; (C) 48 h exposure to 100 mM EtOH and 10 nM 2; (D) 48 h exposure to 100 mM EtOH and 10 nM 5. (E) Dose–response with 48 h exposure to EtOH (100 mM) in the absence or presence of 2 on propidium iodide uptake. (F) Dose–response with 48 h exposure to EtOH (100 mM) in the absence or presence of 5 on propidium iodide uptake. *p < 0.001 vs control; **p < 0.001 vs EtOH.