Successful production of medicinal compounds from plants depends on weather and growing seasons, potentially making supply chains vulnerable to uncontrollable forces. One way around such challenges is to use microorganisms as factories to make the compounds instead.
Christina D. Smolke and Prashanth Srinivasan of Stanford University have now engineered Saccharomyces cerevisiae, commonly known as baker’s or brewer’s yeast, to produce the medicinal tropane alkaloids hyoscyamine and scopolamine (Nature 2020, DOI: 10.1038/s41586-020-2650-9). Scopolamine is used as an antinausea drug and to reduce the production of saliva. Hyoscyamine is used to prevent muscle spasms involved in conditions such as irritable bowel syndrome and Parkinson’s disease.
The pathway is the longest and most complex yet engineered in yeast. “We reconstructed a very complex pathway in yeast that comprises more than 20 enzymes and a number of other modifications to the yeast host cell,” Smolke says. They took enzymes and transporters from a variety of plants, including Atropa belladonna (deadly nightshade), Datura stramonium (jimson weed), and Nicotiana tabacum (common tobacco), all of which are members of the Solanaceae family.
All but one of the enzymes in the pathway were already known. The researchers used functional genomics to identify the missing enzyme, hyoscyamine dehydrogenase, which converts hyoscyamine aldehyde to hyoscyamine.
Smolke’s group had previously engineered yeast to produce tropine, a key intermediate in the pathway. But getting from there to the medicinal tropane alkaloids requires reacting tropine with phenyllactic acid glucoside to produce littorine. They selected littorine synthase from A. belladonna for the reaction. “A lot of engineering was required to get littorine synthase to actually be active in a nonplant host,” Smolke says.
In addition to enzymes, Smolke and Srinivasan also engineered protein transporters into the yeast to improve the movement of reactants and products into and out of various subcellular compartments. “Even without the transporters, we could see production of hyoscyamine and scopolamine,” Smolke says. “The addition of the transporters increased the production and accumulation of those compounds.”
The work represents “a significant breakthrough,” says Cornelius S. Barry, a biologist at Michigan State University who studies metabolism in plants in the Solanaceae family. “What makes this work particularly significant is that engineering scopolamine production was not just a simple case of expressing all of the genes from the plant pathway in yeast and achieving target compound production. Many additional modifications to the yeast strains were required to optimize accumulation of pathway intermediates by reducing the activities of endogenous enzymes that divert tropane precursor pools.”
At this point, the yeast produce 30–80 µg/L of the medicinal tropane alkaloids. There’s still work to do to increase production to commercial scales of 5 g/L.
The company Antheia, of which Smolke is cofounder and CEO, is working on further improving the yeast strain and developing the platform for drug discovery applications.