Mushroom-Derived Indole Alkaloids

Mushrooms have long been recognised as a rich source of bioactive natural products, contributing nutritionally and medicinally across cultures, particularly in Asia. Earlier research identified a wide variety of secondary metabolites from mushrooms, including lectins, terpenoids, sterols, phenolics and alkaloids, some of which have been developed into agrochemicals and drugs; examples cited include strobilurins and pleuromutilin-derived antibiotics. Within this broader context, indole-containing natural products have attracted attention because the indole heterocycle is considered a privileged scaffold in drug discovery and because more than 140 indole alkaloids have been reported from mushroom sources. This review sets out to collate and discuss mushroom-derived indole alkaloids, focusing on their isolation, structural diversity and reported biological activities. For clarity of scope, the authors define “mushrooms” as higher Basidiomycota, predominantly Agaricomycetes, that form visible fruiting bodies with stem, cap and gill structures, and they intentionally exclude indoles from filamentous fungi that do not meet these criteria. The collected compounds are organised into classes including simple indoles, indoleamines/tryptophols, bisindoles, β-carbolines, pyrroloquinolines, and peptide/peptaibol families, and the review highlights both well-studied molecules and many compounds for which biological data are scarce.

This paper is a narrative review that compiles reported indole alkaloids isolated from mushroom fruiting bodies; it does not present a methods section describing a systematic literature search or explicit inclusion/exclusion criteria beyond the taxonomic definition of “mushroom” noted above. The authors organised the material by structural classes and discussed representative examples from the chemical and biological literature, drawing on historical isolations and subsequent bioactivity reports. Isolation and characterisation methods are mentioned in places where they are pertinent: classical fractionation and structure elucidation techniques are implicit throughout, and a few specific analytical approaches are noted (for example, high-resolution MALDI-MS imaging was used to identify a family of β-carbolines from Mycena metata). Biological activity data cited in the review come from diverse experimental formats reported in the original studies, including in vitro enzyme or cell assays, antimicrobial screens, animal toxicity tests and occasional in vivo efficacy or activity assays. The extracted text does not clearly report a reproducible search strategy, database coverage, date range, or formal risk-of-bias assessment for the primary studies included in the review.

Across the surveyed literature, the authors collated more than 140 indole alkaloids derived from mushroom species and presented them organised by structural classes. Simple indoles reported include common auxins such as indole-3-acetic acid and related derivatives, volatile odorants like indole and skatole, and a number of glycosylated indoles isolated from Cortinarius brunneus that appear to be inactive storage forms of auxin. Two hydroxylated indoles from Agrocybe cylindracea (6-hydroxyindole-3-carbaldehyde and 6-hydroxyindole-3-acetamide) showed free radical scavenging activity and inhibited lipid peroxidation in rat liver microsomes with IC50 values of 4.1 and 3.9 μg mL-1, respectively. Indirubin, identified tentatively in Agaricus campestris, inhibits several cyclin-dependent kinases with associated IC50 values ranging from 2.2 to 12 μM. The indoleamine and tryptophol class includes ubiquitous biosynthetic precursors such as L-tryptophan and derivatives like 5-hydroxy-L-tryptophan, tryptamine, serotonin and melatonin. The review highlights psychoactive indoles from Psilocybe species—psilocin and its prodrug psilocybin—with activity attributed to agonism at serotonin receptors including 5-HT1A, 5-HT2A and 5-HT2C. Minor components such as baeocystin, norbaeocystin and aeruginascin are also noted. A number of species produced novel indole-derived pigments and esters; for many of these, biological roles remain unestablished. Bisindoles and related coupled indoles were reported from Tricholoma and other genera, often lacking demonstrated bioactivity. The β-carboline family is well represented: examples include harmane and norharmane, pigments from Cortinarius and Boletus species, and sulfoxide-containing pigments in Boletus curtisii. Two Cortinarius-derived pigments, infractopicrin and 10-hydroxyinfractopicrin, inhibited acetylcholinesterase with IC50 values of 9.72 and 12.7 μM, values comparable to the drug galantamine and suggesting potential as lead compounds. Flazin, a furyl-linked β-carboline from Suillus granulatus, exhibited anti‑HIV activity with an EC50 of 2.36 μM and a therapeutic index of 12.1. Pyrroloquinolines and related pigment alkaloids were described from Mycena species; many of these pigmented compounds showed limited antimicrobial activity in the assays reported. Peptides and peptaibols formed a distinct group: cyclic peptides from Amanita (amatoxins, phallotoxins and virotoxins) were emphasised for their potent biological effects and roles in toxicity. The amatoxins, exemplified by α-amanitin, are selective inhibitors of RNA polymerase II and are highly toxic with LD50 values reported in the range 0.2 to 0.5 mg kg-1. Orellanine, a dimeric pyridine N-oxide isolated from Cortinarius species, was associated with nephrotoxicity, with an oral LD50 in mice around 39 mg kg-1 and intraperitoneal LD50 of 5 mg kg-1; clinical data suggest humans may be more sensitive. Omphalotins, dodecapeptides from Omphalotus olearius, showed selective nematicidal activity with LD90 values between 2 and 5 μg mL-1 and low cytotoxicity at concentrations below 50 μg mL-1. Other peptide examples included a tripeptide from Inonotus obliquus that inhibited platelet aggregation by about 83.3% at 20 mg kg-1 in a mouse antithrombotic assay. Several peptaibols and related peptides demonstrated antibacterial activity in agar diffusion tests with inhibition zones reported in the range 9–25 mm for some Gram-positive bacteria. Throughout the review the authors note that many isolated indole alkaloids have little or no reported biological testing, and that for several purported compound families (for example, cortinarins) subsequent work has cast doubt on earlier reports or identified alternative toxic constituents.

Homer and Sperry interpret the assembled literature as evidence that mushroom-derived indole alkaloids display considerable structural diversity and a spectrum of biological activities, ranging from neuroactive tryptamines and potent cytotoxins to enzyme inhibitors, pigments and antimicrobial peptides. They position these findings within the broader history of mushroom natural products, noting that fungi have previously yielded agrochemical and pharmaceutical leads and that the indole scaffold is of high medicinal relevance. The review repeatedly highlights gaps and uncertainties in the existing literature. Many compounds remain without biological characterisation, and in some cases earlier structural or biological assignments have been challenged by later work—for example, disputed reports of cortinarin cyclic peptides and reassignment of certain strobilurins. Analytical and biological data are heterogeneous across the cited studies, with assays performed in differing formats and organismal models, which complicates direct comparison of activity across compound classes. The extracted text does not present a formal appraisal of evidence quality or a standardised comparison of activity metrics. In terms of implications, the authors suggest mushroom-derived indole alkaloids constitute a promising pool of lead compounds for medicinal chemistry and agrochemical discovery, given examples where fungal metabolites have inspired successful development. They call for further research to generate biological data for many untested molecules, to clarify ambiguous or contested reports, and to explore structure–activity relationships that could translate these natural products into useful pharmacological agents. The authors acknowledge that more systematic characterisation—both chemical and biological—will be needed to realise this potential.

Collating known mushroom-derived indole alkaloids reveals an extensive array of structures and a range of biological properties. Given the historical success of fungal natural products in drug and agrochemical development and the recognised value of the indole heterocycle in medicinal chemistry, the reviewed compounds represent a promising set of lead structures. The authors express the hope that this review stimulates further research into these alkaloids, particularly because many reported molecules currently lack biological data and merit additional investigation.