Snippets of spider genes let mutant silkworms spin silk stronger than steel. Scientists have coaxed miles of spider-like silk from a colony of transgenic silkworms, opening the door for large-scale production of super-strong, tough and flexible fibers.

"We can make a lot more silk from the silkworm process than you could possibly make from spiders," said molecular biologist Malcolm Fraser of the University of Notre Dame.

Spider silk has long been hailed as a superfiber, useful for everything from surgical sutures to bulletproof vests to scaffolding for growing cartilage. But spiders tend to be predatory loners who turn to cannibalism when raised in close quarters, making it nearly impossible to mass produce the treasured threads. A tapestry on display at the American Museum of Natural History last year took more than a million spiders to produce.

So scientists have tried to pull spider silk from tobacco plants, bacteria and even goats, with mixed success. Silkworms, on the other hand, are natural silk-spinning factories. A worm's silk gland takes up about a third of its entire body, Fraser said, and a single cocoon can yield a thread up to a mile long. Silkworms have been domesticated for centuries and are already used for making mass quantities of marketable silk.

By inserting specific spider genes into silkworm chromosomes, Fraser and his colleagues grew a colony of caterpillars that produce threads nearly as strong as spider silk.

"We can now make proteins that have the properties of spider silks in a commercializable platform," Fraser said. Fraser and his collaborators, including biochemist Randy Lewis of the University of Wyoming and Kim Thompson of Kraig Labs, presented the results in a press conference on the Notre Dame campus Sept. 29.

To create the mutant spinners, Fraser and his colleagues used a movable sequence of DNA called the piggyBac transposon to insert snips of spider genes into silkworm embryos. The resulting silk has different properties depending on where in the silkworm chromosome the spider DNA ends up.

"This manipulation allows us to custom build the threads to desired levels of flexibility, tensile strength and toughness," Fraser said.

Not all the embryos ended up expressing the spider DNA, however. To make sure they knew which worms were transgenic, the researchers attached a gene for red fluorescent protein to the spider DNA, ensuring all the mutants had glowing red eyes. The researchers then bred those caterpillars to raise a stable colony of spider-silk-spinning silkworms.

The resulting thread is actually a hybrid of specially engineered spider silk and natural silkworm silk. Even though they don't use "straight-up spider silk" – which wouldn't bond well with the silkworm proteins – the resulting strands are 80 percent as strong, Fraser said. The combination of their strength and flexibility, which materials scientists call toughness, approaches that of Kevlar.

In the wild, some spiders' silk can be up to 10 times tougher than Kevlar. A spider recently discovered in Madagascar spins threads tougher than any known biological substance.

"We haven't gotten a hold of that sequence yet, but you can bet that's going to be something we're going to engineer into our silkworms," Fraser said.

The researchers attached another fluorescent protein to the spider genes to make the silk itself glow green. The silk was just as strong, tough and flexible as before, indicating that scientists could attach other genes without diminishing the quality of the silk. One potential application of this feature is making bandages that stimulate the growth of regular skin instead of scar tissue.

"We can basically mix and match spider silk genes," Fraser said. "It's like mixing paint – take properties that you want and mix them in, the silkworm has them all expressed and you have a mixture of properties in your silk strand."

"I think it's a big step forward," said biomedical engineer David Kaplan of Tufts University. Until a scientific paper is published, he notes, there's no way to know how important or useful the silk will prove. But "the principle is very nice," he said. "I'm anxious to see more."

Images: University of Notre Dame

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