Abstract
Natural products have moved into the spotlight as possible sources for new drugs in the treatment of helminth infections including schistosomiasis. Surprisingly, insect-derived compounds have largely been neglected so far in the search for novel anthelminthics, despite the generally recognized high potential of insect biotechnology for drug discovery. This motivated us to assess the antischistosomal capacity of harmonine, an antimicrobial alkaloid from the harlequin ladybird Harmonia axyridis that raised high interest in insect biotechnology in recent years. We observed remarkably pleiotropic effects of harmonine on physiological, cellular, and molecular processes in adult male and female Schistosoma mansoni at concentrations as low as 5 μM in vitro. This included tegumental damage, gut dilatation, dysplasia of gonads, a complete stop of egg production at 10 μM, and increased production of abnormally shaped eggs at 5 μM. Motility was reduced with an EC50 of 8.8 μM and lethal effects occurred at 10–20 μM within 3 days of culture. Enzyme inhibition assays revealed acetylcholinesterase (AChE) as one potential target of harmonine. To assess possible effects on stem cells, which represent attractive anthelminthic targets, we developed a novel in silico 3D reconstruction of gonads based on confocal laser scanning microscopy of worms after EdU incorporation to allow for quantification of proliferating stem cells per organ. Harmonine significantly reduced the number of proliferating stem cells in testes, ovaries, and also the number of proliferating parenchymal neoblasts. This was further supported by a downregulated expression of the stem cell markers nanos-1 and nanos-2 in harmonine-treated worms revealed by quantitative real-time PCR. Our data demonstrate a multifaceted antischistosomal activity of the lady beetle-derived compound harmonine, and suggest AChE and stem cell genes as possible targets. Harmonine is the first animal-derived alkaloid detected to have antischistosomal capacity. This study highlights the potential of exploiting insects as a source for the discovery of anthelminthics.
Highlights
Natural compounds represent one of the richest sources for the discovery of new active compounds against cancer, infections, or other threats to human health
The harlequin ladybird Harmonia axyridis raised high interest being a rich source of antimicrobial compounds such as the alkaloid harmonine
We obtained first evidence for acetylcholinesterase as one potential molecular target, which was partially inhibited by harmonine
Summary
Natural compounds represent one of the richest sources for the discovery of new active compounds against cancer, infections, or other threats to human health. From 1981 to 2010, 33% of approved drugs represented natural compounds and derivatives, mostly from plants, algae, and fungi [1]. Medicinal plants and their metabolites (like alkaloids, terpenes, and peptides) have been widely exploited as sources of novel natural compounds with anthelmintic activity. Insects are among the most successful and widespread organisms on earth, especially regarding their diversity and adaptability, they are still rather underrated as sources of compounds with medical importance. Along these lines, insects have been almost completely neglected with respect to anthelminthic discovery [4]
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