Steely fiber strands from fruit flies

RICE/TEXAS A&M (US) — Proteins combined with fruit fly material could create bio-friendly fiber strands for use in a variety of applications, including biosensors and tissue engineering.

The genesis of the research was a study of Ultrabithorax,  a recombinant transcription factor protein that regulates the development of the wings and legs of Drosophila melanogaster, the common fruit fly.

The new work is published online in the journal Advanced Functional Materials.

“It’s biodegradable, nontoxic, and made of naturally occurring proteins—though we have no reason to believe that fruit flies ever produce enough of these proteins to actually make fibers,” says Sarah Bondos, formerly a faculty fellow at Rice University and currently assistant professor of molecular and cellular biology at Texas A&M University.

It was a surprise, then, to find that Ubx self-assembles into a film under relatively mild conditions.

“I was cleaning up in the lab one morning and I noticed what appeared to be a drop of water suspended in midair beneath a piece of equipment I was using the previous night,” Bondos says.

It turned out the droplet was water encased in a sac of Ubx film. The sac was hanging by a Ubx fiber so thin that it was more difficult to see than a strand of a spider’s web.

“It clued us in that this was making materials,” says Kathleen Matthews, professor of biochemistry and cell biology.

The chance discovery prompted a 2009 paper  in the journal Biomacromolecules about the material they dubbed “ultrax,” a superstrong and highly elastic natural fiber.

“We found that if you put a little drop of this protein solution on a slide, the Ubx forms a film. And if you touch a needle to that film, you can draw a fiber,” Matthews says.

“Then we asked, What if we could incorporate other functions into these materials? Can we make chimeras?” The answer was ‘yes’, though it took ingenuity to prove.”

Chimeras in the biological world contain genetically distinct cells from two or more sources. On the molecular level, chimeras are proteins that are fused into a single polypeptide and can be purified as a single molecular entity.

As a proof of principle, the team used gene-fusion techniques to create chimeras by combining Ubx with fluorescent and luminescent proteins to see if they remained functional. They did.

The combined materials still formed a film on water. Drawn into fibers and put under a microscope, Ubx combined with enhanced green fluorescent protein (EGFP) kept its bright green color. Ubx-mCherry was bright red, the brown protein myoglobin (from sperm whales) was brown, and luciferase glowed.

Primary author Zhao Huang was able to make patterns with strands generated by the chimeras by twisting red and green fluorescent proteins into candy cane-like tubes, or lacing them on a frame.

Strands of Ubx fiber patterned with fluorescent protein. (Credit Zhao Huang/Rice University)

“This patterning technique is pretty unique and very simple,” says Huang, who recently defended his thesis on the subject. Making solid materials with functional proteins often requires harsh chemical or physical processing that damages the proteins’ effectiveness. But creating complex three-dimensional structures with Ubx is efficient and requires no specialized equipment.

Bondos is studying how many proteins are amenable to fusion with Ubx. “It looks like it’s a fairly wide range, and even though Ubx is positively charged, both positively and negatively charged proteins can be incorporated.”

Even proteins that don’t directly fuse with Ubx may be able to connect through intermediary binding partners.

The 2009 paper “showed we could make three-dimensional scaffolds,” Bondos says. “We can basically make rods and sheets and meld them together; anything you can build with Legos, we can build with Ubx.”

Ubx-based materials can match the natural properties of elastin, the protein that makes skin and other tissues pliable. “You don’t want to make a heart out of something hard, and you don’t want to make a bone out of something soft,” Bondos says. “We can tune the mechanical properties by changing the diameter of the fibers.”

Functionalized Ubx offers a path to growing three-dimensional organs layer by layer. “We should be able to build something shaped like a heart, and because we can pattern the chimeras within fibers and films, we can build instructions into the material that cause cells to differentiate as muscle, nerves, vasculature and other things.”

The material might also be useful for replacing damaged nerves. “We should be able to stimulate cell attachment and nerve growth along the middle and factors on the ends to enhance attachment to existing nerve cells, to tie it into the patient. It really is pretty exciting.”

The ability to characterize and pattern fibers for different functions should find many uses, because enzymes, antibodies, growth factors and peptide recognition sequences can now be incorporated into biomaterials. Sequential arrays of functional fibers for step-by-step catalysis of materials is also possible, Bondos says.

“You’re only limited by your mechanical imagination.”

The Robert A. Welch Foundation and Texas A&M Health Science Center Research Development and Enhancement Awards Program supported the research.

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