Background
Lion’s mane (Hericium erinaceus) mycelium produces erinacines, a suite of cyathane diterpenoids with established neuroactivities. While H. erinaceus fruit body tissue has its own characteristic secondary metabolites, it generally does not produce detectable amounts of erinacines. Substrate composition influences the erinacine content of H. erinaceus mycelial cultures, similar to production of secondary metabolites in other fungi. This study explored the relationship between biosynthetic gene expression and erinacine content in H. erinaceus, comparing fruit body tissue to mycelial tissue cultured in two liquid media formulations.
Results
In this study, we compared erinacine production in H. erinaceus fruit body to mycelial tissue cultivated in two liquid media formulations (Complex and Minimal) by quantifying mRNA transcript levels of the erinacine biosynthetic genes eriE, eriG, eriI, eriC, eriJ, eriB, and eriM (collectively, eri genes) alongside high performance liquid chromatography (HPLC) evaluation of erinacines Q, P, A, and C. We also predicted coding sequences for these seven eri genes. The Complex media preparation yielded mycelium with significantly higher erinacine C content, while the Minimal media yielded mycelium with greater erinacine Q content, suggesting an alteration of the biosynthetic pathway related to differences in substrate composition. Despite evident differences in erinacine concentrations, mycelial eri gene transcript levels did not differ significantly between the two liquid media preparations. When evaluated by gene expression or compound concentration, erinacine biosynthesis was substantially greater in mycelia compared to fruit body tissue in H. erinaceus.
Conclusions
Alongside the absence of detectable erinacines within fruit body samples, eri gene transcripts were consistently downregulated in the fruit body compared to the mycelium, particularly at early stages of the biosynthetic pathway. Substrate composition is a critical factor in production of erinacines by H. erinaceus, and large differences in mycelial erinacine content can occur without significant differences in expression of eri genes. Our data support the hypothesis that production of fungal secondary metabolites can be influenced by tissue type and substrate components, and that the expression of eri genes is enriched in the mycelium when compared to the fruit body.