Structural analogues of the natural products magnolol and honokiol as potent allosteric potentiators of GABA(A) receptors.

Biphenylic compounds related to the natural products magnolol and 4'-O-methylhonokiol were synthesized, evaluated and optimized as positive allosteric modulators (PAMs) of GABA(A) receptors. The most efficacious compounds were the magnolol analog 5-ethyl-5'-hexylbiphenyl-2,2'-diol (45) and the honokiol analogs 4'-methoxy-5-propylbiphenyl-2-ol (61), 5-butyl-4'-methoxybiphenyl-2-ol (62) and 5-hexyl-4'-methoxybiphenyl-2-ol (64), which showed a most powerful potentiation of GABA-induced currents (up to 20-fold at a GABA concentration of 3μM). They were found not to interfere with the allosteric sites occupied by known allosteric modulators, such as benzodiazepines and N-arachidonoylglycerol. These new PAMs will be useful as pharmacological tools and may have therapeutic potential for mono-therapy, or in combination, for example, with GABA(A) receptor agonists.

Natural Variation in Maize Aphid Resistance Is Associated with 2,4-Dihydroxy-7-Methoxy-1,4-Benzoxazin-3-One Glucoside Methyltransferase Activity

Plants differ greatly in their susceptibility to insect herbivory, suggesting both local adaptation and resistance tradeoffs. We used maize (Zea mays) recombinant inbred lines to map a quantitative trait locus (QTL) for the maize leaf aphid (Rhopalosiphum maidis) susceptibility to maize Chromosome 1. Phytochemical analysis revealed that the same locus was also associated with high levels of 2-hydroxy-4,7-dimethoxy-1,4-benzoxazin-3-one glucoside (HDMBOA-Glc) and low levels of 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one glucoside (DIMBOA-Glc). In vitro enzyme assays with candidate genes from the region of the QTL identified three O-methyltransferases (Bx10a-c) that convert DIMBOA-Glc to HDMBOA-Glc. Variation in HDMBOA-Glc production was attributed to a natural CACTA family transposon insertion that inactivates Bx10c in maize lines with low HDMBOA-Glc accumulation. When tested with a population of 26 diverse maize inbred lines, R. maidis produced more progeny on those with high HDMBOA-Glc and low DIMBOA-Glc. Although HDMBOA-Glc was more toxic to R. maidis than DIMBOA-Glc in vitro, BX10c activity and the resulting decline of DIMBOA-Glc upon methylation to HDMBOA-Glc were associated with reduced callose deposition as an aphid defense response in vivo. Thus, a natural transposon insertion appears to mediate an ecologically relevant trade-off between the direct toxicity and defense-inducing properties of maize benzoxazinoids.

3-β-d-Glucopyranosyl-6-methoxy-2-benzoxazolinone (MBOA-N-Glc) is an insect detoxification product of maize 1,4-benzoxazin-3-ones

In order to defend themselves against arthropod herbivores, maize plants produce 1,4-benzoxazin-3-ones (BXs), which are stored as weakly active glucosides in the vacuole. Upon tissue disruption, BXs come into contact with β-glucosidases, resulting in the release of active aglycones and their breakdown products. While some aglycones can be reglucosylated by specialist herbivores, little is known about how they detoxify BX breakdown products. Here we report on the structure of an N-glucoside, 3-β-d-glucopyranosyl-6-methoxy-2-benzoxazolinone (MBOA-N-Glc), purified from Spodoptera frugiperda faeces. In vitro assays showed that MBOA-N-Glc is formed enzymatically in the insect gut using the BX breakdown product 6-methoxy-2-benzoxazolinone (MBOA) as precursor. While Spodoptera littoralis and S. frugiperda caterpillars readily glucosylated MBOA, larvae of the European corn borer Ostrinia nubilalis were hardly able to process the molecule. Accordingly, Spodoptera caterpillar growth was unaffected by the presence of MBOA, while O. nubilalis growth was reduced. We conclude that glucosylation of MBOA is an important detoxification mechanism that helps insects tolerate maize BXs.

Synthesis and cytotoxic activity of non-naturally substituted 4-oxycoumarin derivatives

Coumarins are a large family of natural and synthetic compounds exerting different pharmacological effects, including cytotoxic, anti-inflammatory or antimicrobial. In the present communication we report the synthesis of a series of 12 diversely substituted 4-oxycoumarin derivatives including methoxy substituted 4-hydroxycoumarins, methyl, methoxy or unsubstituted 3-aryl-4-hydroxycoumarins and 4-benzyloxycoumarins and their anti-proliferative effects on breast adenocarcinoma cells (MCF-7), human promyelocytic leukemia cells (HL-60), human histiocytic lymphoma cells (U937) and mouse neuroblastoma cells (Neuro2a). The most potent bioactive molecule was the 4-hydroxy-5,7-dimethoxycoumarin (compound 1) which showed similar potency (IC(50) 0.2-2 μM) in all cancer cell lines tested. This non-natural product reveals a simple bioactive scaffold which may be exploited in further studies.

Induction and detoxification of maize 1,4-benzoxazin-3-ones by insect herbivores

In monocotyledonous plants, 1,4-benzoxazin-3-ones, also referred to as benzoxazinoids or hydroxamic acids, are one of the most important chemical barriers against herbivores. However, knowledge about their behavior after attack, mode of action and potential detoxification by specialized insects remains limited. We chose an innovative analytical approach to understand the role of maize 1,4-benzoxazin-3-ones in plant–insect interactions. By combining unbiased metabolomics screening and simultaneous measurements of living and digested plant tissue, we created a quantitative dynamic map of 1,4-benzoxazin-3-ones at the plant–insect interface. Hypotheses derived from this map were tested by specifically developed in vitro assays using purified 1,4-benzoxazin-3-ones and active extracts from mutant plants lacking 1,4-benzoxazin-3-ones. Our data show that maize plants possess a two-step defensive system that effectively fends off both the generalist Spodoptera littoralis and the specialist Spodoptera frugiperda. In the first step, upon insect attack, large quantities of 2-β-d-glucopyranosyloxy-4,7-dimethoxy-1,4-benzoxazin-3-one (HDMBOA-Glc) are formed. In the second step, after tissue disruption by the herbivores, highly unstable 2-hydroxy-4,7-dimethoxy-1,4-benzoxazin-3-one (HDMBOA) is released by plant-derived β-glucosidases. HDMBOA acts as a strong deterrent to both S. littoralis and S. frugiperda. Although constitutively produced 1,4-benzoxazin-3-ones such as 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) are detoxified via glycosylation by the insects, no conjugation of HDMBOA in the insect gut was found, which may explain why even the specialist S. frugiperda has not evolved immunity against this plant defense. Taken together, our results show the benefit of using a plant–insect interface approach to elucidate plant defensive processes and unravel a potent resistance mechanism in maize.