Journal
JOURNAL OF NATURAL PRODUCTS
Volume 85, Issue 5, Pages 1201-1210Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.jnatprod.1c00840
Keywords
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Funding
- Indonesia Endowment Fund for Education [PRJ-3029/LPDP.3/2016]
- Austrian Science Fund in the MolTag doctoral program FWF [W1232]
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Compounds isolated from Casearia corymbosa leaves showed allosteric potentiation of the GABA signal in GABAA receptors. The activity was confirmed through cell-based assays and voltage clamp studies.
An EtOAc extract of Casearia corymbosa leaves led to an allosteric potentiation of the GABA signal in a fluorometric imaging plate reader (FLIPR) assay on Chinese hamster ovary (CHO) cells stably expressing GABAA receptors with an alpha 1 beta 2 gamma 2 subunit composition. The activity was tracked by HPLC-based activity profiling, and four known (2, 3, 4, and 8) and five new clerodane-type diterpenoids (1, 5-7, and 9) were isolated. Compounds 1-8 were obtained from the active time window. The absolute configuration of all compounds was established by ECD. Compounds 3, 7, and 8 exhibited EC50 values of 0.5, 4.6, and 1.4 mu M, respectively. To explore possible binding sites at the receptor, the most abundant diterpenoid 8 was tested in combination with diazepam, etazolate, and allopregnanolone. An additive potentiation of the GABA signal was observed with these compounds, while the effect of 8 was not inhibited by flumazenil, a negative allosteric modulator at the benzodiazepine binding site. Finally, the activity was validated in voltage clamp studies on Xenopus laevis oocytes transiently expressing GABAA receptors of the alpha 1 beta 2 gamma 2S and alpha 1 beta 2 subtypes. Compound 8 potentiated GABA-induced currents with both receptor subunit compositions [EC50 (alpha 1 beta 2 gamma 2S) = 43.6 mu M; Emax = 809% and EC50 (alpha 1 beta 2) = 57.6 mu M; Emax = 534%]. The positive modulation of GABA-induced currents was not inhibited by flumazenil, thereby confirming an allosteric modulation independent of the benzodiazepine binding site.
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