Neuroprotective and anti-neuroinflammatory activities of anthraquinones isolated from Photorhabdus temperata culture broth

Photorhabdus temperata (PT), a gram-negative bacterium, lives symbiotically within entomopathogenic nematodes. The insecticidal compounds derived from Photorhabdus are used as biopesticides in agriculture. However, the physiological properties are not well characterized. In the course of our screening for neuroprotective and anti-neuroinflammatory substances from natural products, the culture broth of PT showed considerable activities. By activity-guided purification, five anthraquinones, namely, 3-methoxychrysazine (1), 1,3- dimethoxy-8-hydroxy-9,10-anthraquinone (2), 1,3,8-trihydroxy-9,10-anthraquinone (3), 3,8- dihydroxy-1-methoxy-9,10-anthraquinone (4), and 1,3,4-trimethoxy-8-hydroxy-9,10-anthraquinone (5), were isolated from the ethyl acetate fraction of the PT culture broth. Among the isolated compounds, 75 µM 3 significantly protected mouse hippocampal neuronal cells (HT22) against 5 mM glutamate-induced cell death via the inhibition of reactive oxygen species production, Ca²⁺ influx, and lipid peroxidation. Additionally, 3 and 4 effectively suppressed the interferon-γ-induced neuroinflammation of mouse-derived microglial cells (BV2) at 10 ng/ml, via the reduction of nitric oxide, interleukin-6, and tumor necrosis factor-α. Anthraquinones 3 and 4 derived from the PT culture broth are a potential starting point to discover neuroprotective and anti-neuroinflammatory drug leads. The novel compound 5 is reported for the first time in this study.