Non-Peptidic Small Molecule Components from Cone Snail Venoms

Abstract

Venomous molluscs (Superfamily Conoidea) comprise a substantial fraction of tropical marine biodiversity (>15,000 species). Prior characterization of cone snail venoms established that bioactive venom components used to capture prey, defend against predators and for competitive interactions were relatively small, structured peptides (10-35 amino acids), most with multiple disulfide crosslinks. These venom components ("conotoxins, conopeptides") have been widely studied in many laboratories, leading to pharmaceutical agents and probes. In this review, we describe how it has recently become clear that to varying degrees, cone snail venoms also contain bioactive non-peptidic small molecule components. Since the initial discovery of genuanine as the first bioactive venom small molecule with an unprecedented structure, a broad set of cone snail venoms have been examined for non-peptidic bioactive components. In particular, a basal clade of cone snails (Stephanoconus) that prey on polychaetes produce genuanine and many other small molecules in their venoms, suggesting that this lineage may be a rich source of non-peptidic cone snail venom natural products. In contrast to standing dogma in the field that peptide and proteins are predominantly used for prey capture in cone snails, these small molecules also contribute to prey capture and push the molecular diversity of cone snails beyond peptides. The compounds so far characterized are active on neurons and thus may potentially serve as leads for neuronal diseases. Thus, in analogy to the incredible pharmacopeia resulting from studying venom peptides, these small molecules may provide a new resource of pharmacological agents.

Keywords: conopeptides; conus; gastropod; natural products; nicotinic acetylcholine receptor; prey capture; secondary metabolites; venom.

FIGURE 1

(A) Cone shells of Stephanoconus species. Left top, Conus regius (Florida); Left bottom, Conus archon (West Mexico); Center, Conus imperialis (Philippines); Right top, Conus genuanus (West Africa); Right bottom, Conus chiangi (Philippines, 150–250 m). (B) Variation in Indo-Pacific Stephanoconus. From left to right: Conus imperialis fuscatus (sometimes called viridulus; Zanzibar, East Africa); Conus zonatus (Laccadive Islands, India); Conus imperialis variety (Reunion Island); Conus imperialis variety (Balicasag Island, Philippines, gill nets, 70–120 fathoms). All four forms may represent different species from the Conus imperialis specimen shown in A, but only Conus zonatus is generally accepted as being different (Photography by Sam Watson and Sam Espino.)

FIGURE 2

Phylogenetic tree of Stephanoconus, Strategoconus, Dendroconus, Tesselliconus (vermivorous), Cylindrus (molluscivorous), and Pionoconus (piscivorous) clades of Conus species, using mitochondrial cytochrome oxidase C (COI) marker. Conus distans used as outgroup. Genbank accession numbers KJ549885, KJ549949, KJ549924, KJ549948, KJ549893, KJ549901, KJ5499908, KJ550257, KJ550309, and KJ550204 (Puillandre et al., 2014); GU134380, GU134381, GU134383, and GU134385 (Watkins et al., 2010); AY588159 (Duda and Rolan, 2005); KY864974 (Abalde et al., 2019); FJ868158 (Biggs et al., 2010); KJ606017 (Aman et al., 2015); FJ868109, FJ868113, and FJ868115 (Kantor and Taylor, 2000; Puillandre et al., 2010); EU733516, EU812755, EU812758, and GU134377 (Nam et al., 2009); JF823627 (Cabang et al., 2011; Olivera et al., 2015).

FIGURE 3

First small molecules identified in cone snails. These compounds were found by Kohn in 1960 in C. textile, C. striatus; 1 was identified in C. litteratus, C. marmoreus, and C. magus.

FIGURE 4

Some of the most abundant compounds discovered in the colored venoms of Stephanoconus snails. Those in shaded yellow are compounds so far found only in Stephanoconus.

FIGURE 5

Small molecules in Stephanoconus are highly species specific. This chart indicates where each small molecule is found (blue), showing that the deep/shallow C. imperialis species and C. genuanus contain distinctly different small molecule chemistry in their colored venom glands. GNPS: Global Natural Product Social Molecular Networking.

FIGURE 6

Bacterial biosynthesis of nocapyrones. A genetic locus in Nocardiopsis sp. bacteria is responsible for producing ion channel modulators found in the whole snails, C. rolani and C. tribblei. Bold wedges within chemical structures indicate incorporation of methylmalonate. Figure from Lin et al. (2013).

FIGURE 7

Proposed biosynthesis of conazolium. (A) Known routes to antioxidants ergothioneine and ovothiol. The synthesis of an “ovothiol-like intermediate” that is the precursor to conazoliums has features of both pathways, but with an additional methylation on the imidazole ring. The arrow in green indicates a reaction that was characterized using the C. imperialis enzyme, ConA. (B) The “ovothiol-like intermediate” reacts with electrophilic metabolites to create the conazoliums (10–11).