Coupled biosynthesis of cordycepin and pentostatin in Cordyceps militaris: implications for fungal biology and medicinal natural products

Cordycepin, or 3'-deoxyadenosine, is a metabolite produced by the insect-pathogenic fungus Cordyceps militaris (C. militaris) and is under intense investigation as a potential lead compound for cancer and inflammatory conditions. Cordycepin was originally extracted by Cunningham et al. (1) from a culture filtrate of a C. militaris culture that was grown from conidia. Subsequently, cordycepin has also been reported to be produced by Ophiocordyceps sinensis (2), a species historically used as a traditional medicine and health food, primarily in China and the wider Far East (3). Cultivated C. militaris is now widely in use as less expensive substitute. In addition, these fungi are also globally gaining a market as natural food supplements, with 30–50 products claiming to contain Cordyceps available in the UK and the USA.

A large body of literature (too much to cite comprehensively here) indicates that cordycepin indeed has biological activities that indicate it may have pharmaceutical potential. In tissue culture, anti-inflammatory properties and anti-tumour effects are especially well established (4-9). In addition, it has been shown to be effective in numerous animal models of disease, including models for osteoarthritis, inflammatory lung disease, cerebral ischaemia, kidney failure and cancer (9-17). Our own work on pain in models of osteoarthritis suggest that cordycepin acts as a novel type of anti-inflammatory painkiller (11). To our knowledge, no conclusive data from clinical trials with cordycepin have been published. However, even if only one of the many reported effects on animal disease models can be replicated in people, this could become a very important new natural product-derived medicine.

Cordycepin is known to be unstable in animals due to deamination by adenosine deaminases. Much of the efforts towards bringing cordycepin to the clinic have been focussed on chemical modifications, formulations and co-administration with adenosine deaminase inhibitors such as pentostatin (18-22). Notably, the majority of commercially available cordycepin products, and certainly all the most affordable preparations, are still isolated from cultivated fungi.

It was therefore of great interest that we read the recent paper by Xia et al. [2017] (23). The authors showed that in C. militaris the production of cordycepin is coupled with the production of the adenosine deaminase inhibitor pentostatin; with genes essential for their synthesis in adjacent loci, cns1, cns2, and cns3 (23). Functional verification of the genes cns1 and cns2 for cordycepin production was performed by generating Aspergillus nidulans knockout mutants and heterologous gene expression in Metarhizium robertsii and Saccharomyces cerevisiae. Similarly, heterologous expression of cns3 in M. robertsii and Cordyceps bassiana confirmed the role of Cns3 for pentostatin production. Yeast two-hybrid and co-localisation-based evidence for Cns1 and Cns2 protein interaction was also provided (23). This work is certainly important for the optimisation of C. militaris cordycepin production strains. In addition, there are wider implications on the ecology of secondary metabolites and their potential applications.

Surprisingly, Xia et al. failed to detect cordycepin production in species closely related to C. militaris such as C. bassiana, C. confragosa, C. takaomontana, Ophiocordyceps sinensis, Isaria fumosorosea, Metarhizium robertsii, and M. rileyi, which is in agreement with the lack of homologous genes for its biosynthesis in these species (23). In the case of O. sinensis, this is particularly puzzling, as it contradicts previous studies (2). If O. sinensis indeed produces cordycepin under certain conditions, a non-conserved pathway involving different enzymes may be used. Alternatively, fungi collected from the wild may be associated with other cordycepin-producing organisms. This speculation is supported by the fact that, when detected, the amount of cordycepin found in O. sinensis is low compared to the levels in C. militaris (2). Interestingly, cordycepin biosynthesis genes similar to those from C. militaris were found in the phylogenetically distant species Aspergillus nidulans (a eurotiomycete, in a different ascomycete class) and Acremonium chrysogenum. We therefore consider it possible that this fascinating “protector-protégé” system for the production of pentostatin and cordycepin was acquired by gene transfer between different species. Horizontal gene transfer has been widely proposed to occur in fungi based on genome structure, although it has not been observed directly (24).

The co-production of cordycepin and pentostatin in C. militaris is likely the result of the evolutionary pressures on this insect-infecting fungus, with pentostatin keeping cordycepin in its active form. A probable, but so far unconfirmed, hypothesis is that cordycepin represses the immune system of the insect host, which lacks adaptive immunity. Indeed, cordycepin has been attributed as the proximate cause of insect host death following colonisation of the insect by C. militaris (25). Therefore, the effect of cordycepin, pentostatin, and other secondary metabolites on insect immune systems and fungal infection are worth investigating. This could lead to biological control applications for targeting insect pests. Although O. sinensis may not produce cordycepin, it is subject to similar evolutionary pressures as C. militaris and therefore possibly produces different compounds with similar effects on insect and mammalian immune systems. Therefore, if it can be confirmed that secondary metabolites from insect-infecting fungi target the insect immune system, this will suggest that more such useful compounds may be found in this ecological niche.

Beyond the impact of this paper on cordycepin production and the biology of insect-infecting fungi, the study by Xia et al. also has implications for how we test biological activity of natural compounds. If we take into account that the evolution of natural compounds is likely to have led to synergistic mixtures, there appears to be a case for initially testing mixtures, rather than pure compounds, as activity may be lost by purification of single compounds. Natural compounds, their synthesis and their activities are likely to provide a rich source for exciting discoveries for many years to come.

Acknowledgments

None.

Footnote

Conflicts of Interest: The authors have no conflicts of interest to declare.

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