From Forest Floors to Pharmacology: Elucidating Millipede Defensive Alkaloids for Drug Discovery

Abstract

Millipedes are some of the oldest animals on earth, evolving over 400 million years ago (mya). The subterclass, Colobognatha evolved 99 mya and are largely understudied compared to other classes of millipedes. Like most other millipede classes, Colobognatha possess repugnatorial glands, which store alkaloids as defensive compounds. However, they have also evolved unique characteristics that set them apart from all other millipedes, including distinct anatomy, group behavior, brood care, and the production of terpenoid alkaloids. Prior to 2020, only 11 alkaloids were known. Herein, a total of 25 new terpenoid alkaloids are reported coming from three different genera, all belonging to the same order. Key findings include the discovery of indolizine and quinolizidine alkaloids (e.g., hydrogosodesmine, homogosodesmine and homo-hydrogosodesmine) from the defensive secretions of various Brachcybye species (Chapter 2), the discovery of the ischnocybine alkaloids from the defensive secretions of Ischnocybe plicata (Chapter 3), and the discovery of the andrognathines and andrognathanols from the defensive secretions of Andrognathus corticarius (Chapter 4). The structure elucidation of each utilized modern techniques, including 2D NMR, HRMS, DFT, ECD, chemical synthesis, and Mosher's analysis. Many of these new alkaloids represent new natural product classes with carbon backbones that are unprecedented in the literature. Biological and ecological evaluation revealed the new alkaloids deter ants, a common predator to millipedes (Chapter 3), and this led to the discovery that a subset of the alkaloids potently binds to sigma-1 receptor (ischnocybine A: Ki 14 nM), while others bind to sigma-2 (homo-hydrogosodesmine hydrate: Ki 260 nM). Furthermore, ecology studies revealed that the alkaloid production is conserved over large geographical regions, accumulated as the millipedes gain segments and are actively secreted through the ozopores when physically agitated. These discoveries provide insights into a potential biosynthetic pathway shared by all Platydesmida millipedes and support the hypothesis that alkaloid biosynthesis in this order is evolving toward greater simplicity.