They haven’t tasted it yet because of safety concerns, but scientists at the University of California have created lettuce capable of producing a hormone that stimulates bone formation. Its seeds could be grown on the International Space Station and on future trips to Mars, where the lack of gravity reduces crew members’ bone mass.
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These lettuces produce a hormone that stimulates bone formation, which could prevent bone loss in space and on Earth. / Kevin Yates
NASA is preparing to send humans to Mars in the 2030s. On that mission, which will last about three years, astronauts will be exposed to a long period of microgravity that will cause them to lose bone mass. Some studies indicate an average loss of more than 1% of bone material for every month spent in space, a condition known as osteopenia (associated on Earth with ageing).
Now, US scientists offer a possible solution. During the spring meeting of the American Chemical Society (ACS) virtual meeting in San Diego these days, researchers from the University of California at Davis presented transgenic lettuce with a hormone that stimulates bone formation. One day, astronauts may be able to grow it in space to mitigate their bone loss simply by eating a big salad.
These transgenic lettuces have a hormone that stimulates bone formation, so astronauts could one day grow them in space to mitigate bone loss caused by the lack of gravity.
“Right now, astronauts on the International Space Station do certain exercises to try to maintain their bone mass,” says Kevin Yates, a graduate student who presented the paper at the meeting, “but they’re not usually there for more than six months.
A manned mission to Mars, however, would take about 10 months to arrive and astronauts would stay for about a year to study the planet before making the trip back to Earth. This three-year journey could increase the travellers’ vulnerability to osteopenia and, later, osteoporosis.
A human parathyroid hormone (PTH) peptide drug has been shown to stimulate bone formation and help restore bone mass in microgravity conditions, but this treatment would require daily injections. Moreover, carrying large quantities of the drug and syringes to administer it during space missions is also impractical.
Better to eat the hormone than inject it
As an alternative, with NASA’s help, Yates and his colleagues Somen Nandi and Karen McDonald of the University of California came up with a way for the crew members to produce the substance themselves: lettuce. They could express the PTH peptide and then take it orally instead of by injection, as well as being a tasty vegetable in the midst of so much canned and freeze-dried food.
“Astronauts can take transgenic seeds, which are very small – you can have a few thousand in a jar the size of your thumb – and grow them like normal lettuce,” says Nandi, “so they could use the plants to synthesise pharmaceuticals like PTH and then eat them.
Experiments on the International Space Station have confirmed that normal lettuce can be grown in this limited environment. Moreover, previous studies have also shown that a piece of a protein called a crystallisable fragment (Fc) can bind to the PTH peptide to increase its circulation time in the blood, thus making it more effective.
With this in mind, the researchers introduced the gene encoding PTH-Fc into lettuce by infecting its plant cells with Agrobacterium tumefaciens, a species of bacteria used in the lab to transfer genes into plants. The bone-building substance appeared in both the lettuce and its progeny.
Bacteria have been used to infect lettuce cells and transfer the gene encoding the necessary hormone to them.
About 380 g per day
Preliminary results indicate that, on average, the plants express 10-12 milligrams of the modified peptide hormone per kilogram of fresh lettuce. According to Yates, this means that astronauts would need to eat about 380 grams a day to get a sufficient dose of the hormone, assuming a bioavailability of about 10 per cent, which he acknowledges is a “pretty big salad”.
“One of the things we are doing now is looking at all these transgenic lettuce lines to find the one with the highest expression of PTH-Fc,” says McDonald, “so far we have only looked at a few of them, and we found that the average was 10-12 mg/kg, but we think we could increase it more; and the more we can increase the expression, the less lettuce we will have to eat.
Future tests in space
The team also wants to see how well the transgenic lettuce grows on the International Space Station and whether it produces the same amount of PTH-Fc as on Earth. As for its taste, although the researchers have not yet tested it because its safety has not been established, they expect it to taste very similar to normal lettuce, like most other transgenic plants.
In any case, there are still several stages before the vegetable can grace the plates of astronauts. Researchers still have to optimise PTH-Fc expression levels and then test the variety’s ability to safely prevent bone loss in animal models and in human clinical trials.
“I would be very surprised if, by the time we send astronauts to Mars, plants are not being used to produce pharmaceuticals and other beneficial compounds,” predicts Yates.
In addition to its benefits for future space travellers, the researchers say this type of vegetable could also help prevent osteopenia and osteoporosis in resource-limited regions of Earth without access to traditional medicines.