Tissues that Build Themselves

Cells coated with sticky bits of DNA can self-assemble into functional three-dimensional microstructures. This bottom-up approach to tissue engineering, developed by scientists at Lawrence Berkeley National Laboratory and the University of California, Berkeley, provides a new solution to one of field’s biggest problems: the creation of multicellular tissues with defined structures. Unlike top-down methods, in which scientists build cell structures on scaffolds, the new technique allows tissue engineers to dictate the precise geometric interactions of individual cells.

Researchers started with two cell types–one that secretes a protein, called a growth factor, which the other requires in order to grow. Coauthor Zev Gartner, now a pharmaceutical chemist at the University of California, San Francisco, decorated the cells with snippets of single-stranded DNA, attached using specialized sugars incorporated into the cell membrane. The two cell types carried complementary strands of DNA, which acted as a sort of Velcro. When the different cells were combined, their complementary DNA fragments joined into double strands, linking the cells together. Joined to their protein producing partners, the protein-dependent cells flourish. Without the DNA coating, the two cell types can’t communicate, and the dependent cells die.

By varying the relative concentrations of the two cell types, the researchers could maneuver the cells into particular configurations. For instance, when the cells were combined in a one-to-one ratio, they simply formed pairs. But when the growth-factor-dependent cells vastly outnumbered their counterparts, they formed characteristic three-dimensional clusters with a single growth-factor-secreting cell in the center. The results appeared Monday in the early online edition of Proceedings of the National Academy of Sciences.

“This approach provides a new way of recreating tissue complexity,” says Ali Khademhosseini, an assistant professor at Harvard-MIT’s Division of Health Sciences and Technology and Harvard Medical School, who was not involved in the study. Most tissue-engineering methods produce three-dimensional structures with the help of scaffolding materials.

Once the microstructures had formed, Gartner and his colleague Carolyn Bertozzi, director of the Molecular Foundry nanoscience research facility at Berkeley Lab, trapped them in a gel and imaged them in three dimensions using a fluorescence microscope. Because the cell-surface DNA isn’t stable in the long term, it’s not yet clear how long the structures will hold up on their own. The researchers are currently investigating whether the linked cells will begin to generate their own natural adhesion molecules to keep them attached once the DNA links are gone.

So far these microstructures are rudimentary–far from the structural sophistication of a whole organ. But by tweaking the ratio of cell types, the density of DNA on the cells’ surfaces, and the complexity of the DNA sequences, Gartner and Bertozzi hope to build larger and more intricate assemblies. “By playing around with these variables, we can bias the type of structure that we’re making,” says Gartner.

Cellular Velcro: In a new approach to bottom-up tissue engineering, two different cell types spontaneously assemble themselves into three-dimensional microstructures like this one when their surfaces are studded with sticky single strands of DNA. Because the DNA on the red cells is complementary to that on the green cells, they naturally stick together, holding the cells in close proximity.
Credit: Bertozzi Lab

While this new method isn’t the first to tackle tissue engineering from the bottom up, Gartner says that it’s the only one capable of fine enough resolution to define how individual cells interact with their neighbors. And even if this technique turns out not to scale up well, he says, it could in principle provide structural building blocks for use in other emerging bottom-up approaches, such as layer-by-layer tissue printing or laser manipulation.

Khademhosseini says that it’s too early to tell whether the new technique will eventually produce tissues suitable for use in regenerative medicine. “It has a lot of potential, and it may provide therapies in the future, but other challenges need to be overcome to make a clinically viable product,” he says. For example, it remains to be seen where the cells to grow the tissue would come from, and how the body would generate new blood vessels to feed the transplanted tissue.

Dan Dimitrijevich, director of the Human Tissue and Cell Engineering Laboratories at the University of North Texas Health Science Center, is more skeptical. He doubts that the new approach will be able to generate stable, safe, and functional tissues that hold up when transplanted into an actual living organism. “It’s interesting science,” he says, “but as far as tissue engineering, it’s really stretching.”

Even if it doesn’t pan out in terms of regenerative medicine, Gartner believes that the technique could still prove useful as a tool for studying how different cell types communicate–for example, in the process of generating a tumor. “This now gives us a new tool to take these structures out of a human host–where obviously they’re very difficult to study for a number of technical and ethical reasons–and put them into a flask where we can study them in detail over long periods of time,” he says.