Abstract
EA have been a major benefit, and a major detriment, to humans since early in recorded history. Their medicinal properties have been used, and continue to be used, to aid in childbirth, with new uses being found in the treatment of neurological and cardiovascular disorders. The surprisingly broad range of pharmaceutical uses for EA stems from their affinities for multiple receptors for three distinct neurotransmitters (serotonin, dopamine, and adrenaline), from the great structural diversity of natural EA, and from the application of chemical techniques that further expand that structural diversity. The dangers posed by EA to humans and their livestock stem from the ubiquity of ergot fungi (Claviceps species) as parasites of cereals, and of related grass endophytes (Epichloë, Neotyphodium, and Balansia species) that may inhabit pasture grasses and produce toxic levels of EA. Further concerns stem from saprophytic EA producers in the genera Aspergillus and Penicillium, especially A. fumigatus, an opportunistic pathogen of humans. Numerous fungal species produce EA with a wide variety of structures and properties. These alkaloids are associated with plants in the families Poaceae, Cyperaceae, and Convolvulaceae, apparently because these plants can have symbiotic fungi that produce EA. Pharmacological activities of EA relate to their specific structures. Known as potent vasoconstrictors, the ergopeptines include a lysergic acid substituent with an amide linkage to a complex cyclol-lactam ring structure generated from three amino acids. Simpler lysergyl amides and clavines are more apt to have oxytonic or psychotropic activities. One of the lysergyl amides is LSD (5), the most potent hallucinogen known. The EA biosynthetic pathway in Claviceps species has been studied extensively for many decades, and recent studies have also employed epichloës and A. fumigatus. The early pathway, shared among these fungi, begins with the action of an aromatic prenyl transferase, DMATrp synthase, which links a dimethylallyl chain to L-tryptophan. When the dmaW gene encoding DMATrp synthase was cloned and sequenced, the predicted product bore no identifiable resemblance to other known prenyl transferases. The dma W genes of Claviceps species are present in clusters of genes, several of which also have demonstrated roles in EA biosynthesis. In many other fungi, dma W homologues are identifiable in otherwise very different gene clusters. The roles of DMA Trp synthase homologues in these other fungi are probably quite variable. One of them is thought to prenylate the phenolic oxygen of L-tyrosine, and another catalyzes the unusual reverse prenylation reaction in the biosynthesis of fumigaclavine C(10), an EA characteristic of A. fumigatus. The second step of the EA pathway is N-methylation of DMATrp (12) to form 13, which is then subjected to a series of oxidation/oxygenation and reduction reactions to generate, in order, chanoclavine-I (16), agroclavine (19), and elymoclavine (6). Shunt reactions generate a wide variety of other clavines. Two epimerizations occur in this pathway: one from 12 to 16, the other from 16 to 19. Further oxidation of 6, catalyzed by the cytochrome-P450 CloA, generates lysergic acid (1). An unusual NRPS complex, lysergyl peptide synthetase (LPS), is responsible for linking 1 to three hydrophobic L-amino acids to generate the ergopeptide lactams. The LPS complex includes two polypeptides, one (LPS 2) possessing a single module for activation of 1, and the other (LPS 1) possessing three modules, each specifying one of the L-amino acids. Variations in LPS 1 sequences are associated with variations in the incorporated amino acids, leading to differences between strain chemotypes, and even multiple ergopeptines within strains. For example, C. purpurea P1 produces two distinct ergopeptines (ergotamine (4) and ergocryptine (Table I)), each of which is believed to be generated by multiple LPS 1 subunits encoded by separate, but related, genes (lpsA1 and lpsA2). The main ecological roles of EA in nature are probably to protect the fungi from consumption by vertebrate and invertebrate animals. The EA produced by plant-symbiotic fungi (such as epichloë endophytes) may protect the fungus by protecting the health and productivity of the host, which may otherwise suffer excessive grazing by animals. The EA, at levels typical of plants bearing these symbionts, can negatively affect the health of large mammals as well herbivorous insects. Some clavines have substantial anti-bacterial properties, which might protect the fungus and, in some cases, their host plants from infection. However, the fact that a large number of epichloë, and even several Claviceps species, produce no detectable EA indicates that the selection for their production is not universal. An unfortunate fact for many livestock producers is that some of the most popular forage grasses tend to possess EA-producing epichloë endophytes. Such endophytes are easily eliminated, but confer such fitness enhancements to their hosts that their presence is often preferred, despite the toxic EA. The future looks promising for continued interest in EA. Research continues into their pharmacological properties, medicinal uses, and structure-function relationships. New clavines and lysergic acid derivatives are identified regularly from new sources, such as marine animals. Also, programs are well underway to modify or replace epichloë endophytes of forage grasses in order to produce new grass cultivars that lack these toxins.
Collapse