Indole Alkaloids of the Stigonematales (Cyanophyta): Chemical Diversity, Biosynthesis and Biological Activity

Abstract

The cyanobacteria are well recognized as producers of a wide array of bioactive metabolites including toxins, and potential drug candidates. However, a limited number of taxa are generally considered with respect to both of these aspects. That said, the order Stigonematales, although largely overlooked in this regard, has become increasingly recognized as a source of bioactive metabolites relevant to both human and environmental health. In particular, the hapalindoles and related indole alkaloids (i.e., ambiguines, fischerindoles, welwitindolinones) from the order, represent a diverse, and phylogenetically characteristic, class of secondary metabolites with biological activity suggestive of potential as both environmental toxins, and promising drug discovery leads. The present review gives an overview of the chemical diversity of biologically active metabolites from the Stigonematales-and particularly the so-called hapalindole-type alkaloids-including their biosynthetic origins, and their pharmacologically and toxicologically relevant bioactivities. Taken together, the current evidence suggests that these alkaloids, and the associated cyanobacterial taxa from the order, warrant future consideration as both potentially harmful (i.e., "toxic") algae, and as promising leads for drug discovery.

Keywords: Stigonematales; ambiguine; blue-green algae; cyanobacteria; fischerindole; hapalindole; harmful algal blooms; indole alkaloids; toxins; welwitindolinone.

Figure 1

Photomicrograph of Fischerella 52-1 as a representative of the branched filamentous habit of the Stigonematales.

Figure 2

Examples of bioactive metabolites from the Stigonematales.

Figure 3

Classification scheme of indole alkaloids from Order Stigonematales. The previously described indole alkaloids are classified here as nine proposed groups; given are group number (as defined here), and corresponding chemical name of each group (based on previously named carbon skeletons). Note that stereochemistry is illustrated only for those sub-classes for which either a very limited number of variants have been identified (i.e., Groups 3, 6 and 8), and/or no variability in stereodisposition (e.g., 12-epimers, C-10/11/15) has been yet reported (i.e., Group 5 and 9). For more detailed explanation of variation within the subclasses, see Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9, Figure 10 and Figure 11.

Figure 4

Previously described tetracyclic hapalindoles (Group 1) from Stigonematales.

Figure 5

Previously described tricyclic hapalindoles (Group 2) from Stigonematales.

Figure 6

Oxidized hapalindoles isolated from Stigonematales.

Figure 7

Previously described tetracyclic ambiguines (Group 4) from Stigonematales.

Figure 8

Previously described pentacyclic ambiguines (Group 5) from Stigonematales.

Figure 9

Previously described fischambiguines (Group 6) from Stigonematales.

Figure 10

Previously described fischerindoles from Stigonematales.

Figure 11

Previously described Type B welwitindolinones (Group 9) from Stigonematales.

Figure 12

Originally proposed biosynthesis of a tricyclic hapalindole, as an intermediate for other hapalindole-type alkaloids (see Figure 14), via chloronium or H⁺ induced condensation [45]. Recent molecular studies, however, suggest an alternative biosynthetic route [55].

Figure 13

Biosynthesis of the indole-isonitrile precursor as inferred based on the identification of the WelI1-3 and AmbI1-3 genes as homologs of previously characterized isonitrile synthase genes, and subsequent in vitro studies of gene products [49,54].

Figure 14

Biosynthesis of indole alkaloid sub-classes from a tricyclic precursor. The scheme shown is specifically based on a hapalindole C (i.e., isonitrile-bearing) precursor.

Figure 15

Most recently proposed pathway for synthesis of common tricyclic hapalindole precursor, via a monogeranylated indolenine intermediate, as supported by recent molecular studies [55]. Subsequent steps to yield other alkaloids would occur via Paths A, B or C as shown in Figure 14.

Figure 16

Overview of biochemical, molecular and cellular targets of the hapalindole-type alkaloids. Abbreviations: AC = adenylate cyclase; AVPR = arginine vasopressin receptor; ICAM-1 = interceullular adhesion molecule 1; IKKβ = inhibitor of nuclear factor kappa-B kinase subunit beta; MT = microtubulin; P-gp = p-glycoprotein; NFκB = nuclear factor kappa B; RNA pol = RNA polymerase.

Figure 17

Inhibition of mosquito (Aedes aegypti) larval development by 12-epi-hapalindole H isonitrile. Shown are 2nd instar larvae treated with the alkaloid (a) showing development abnormalities compared to untreated control larvae (b). Most larvae treated with 12-epi-hapalindole H, however, do not transition from 1st to 2nd instar.

Figure 18

Teratogenicity of hapalindole-type alkaloids in the zebrafish (Danio rerio) embryo as a model of vertebrate development. Shown are control (untreated) embryos at 3 days post-fertilization (a); and embryos exposed for three days to 12-epi-ambiguine B nitrile at 10 μg/mL (b); and to 12-epi-hapalindole H isonitrile at 5 and 10 μg/mL (c and d, respectively) [61].