Principal investigator Daniel Cohen said that the technique overcomes cells' natural social behavior and has allowed researchers to direct skin cells to move en masse to close open wounds. "Literally making skin crawl," he explained.
Researchers have overcome the inertia of mature skin by destroying the molecular connections among cells and applying an electric field to direct their movement. Then, they rebuilt the connections. This new approach increases the controllability and might one day aid in wound healing by electrical stimulation.
Emil du Bois-Reymond, a German physician, first described an electrical current flowing from a cut in his finger in 1848. Electrotaxis, which is a process that allows cells to sense and follow an electrical field, was discovered later. The body generates electric fields that promote healing and direct cells to move towards the wound. They are vital for growth and development.
Gawoon Shim (lead author of the study and graduate student in Cohen’s lab) said that there are many reasons to believe that electrical stimulation may help wound healing. Although there is promising clinical evidence that electrical stimulation has been effective in treating patients for decades, scientists are still trying to figure out how to best apply therapeutically the technology. Shim said, "It's kinda a black box."
"We discovered this fundamental phenomenon 175 years back and we don’t yet have electric band aids commercially," said Daniel J. Cohen (assistant professor of mechanical engineering and senior author of the study), published online in the July 20 edition of the Proceedings of the National Academy of Sciences. "No one knows how to design these things."
Cohen's previous work used electric fields to program thousands upon thousands of cells to move in circles around corners. The new study was based on a model with more mature skin. This is a single layer of skin cells that are tightly glued together. The mature skin cells responded to an electric current with a speed and precision that would have been expected from a marching band, but instead of moving at the speed of a marching band, they moved like a group of people who are holding hands with each other.
Another problem was that the mature skin would eventually peel off the petri dish. Cohen stated that if you give a command that is different from what cells naturally want, it will cause a tug-of war. The result was that the tissues were ripped apart.
Cohen and Shim believed that cells would "handshake" with each other to prevent the tissue from following electrical commands fluidly. Cadherins, proteins known as handshakes, are what bind neighboring cells together. These proteins make tissues more cohesive, allowing them to move together. However, they can also cause traffic jams in cells that don't have enough space.
Princeton researchers used an electric field to control cell migration. This new technique by Princeton researchers improved control over tissues and may one day help heal wounds. Illustration by Neil Adelantar, using images by Cohen and colleagues. Credit: Princeton University Princeton University Researchers used an electric field to control the movement of cells. This new technique by the researchers improved control over tissues and may one day help heal wounds. Credit: Illustration by Neil Adelantar, based on images by Cohen and colleagues.
To complete their connections, Cadherins require calcium ions. Shim grew cells with different levels of calcium and measured the cells' response to electrical stimulation. Shim observed that cells with less calcium were more fluid and moved faster. Shim said, "It moves really fastI was surprised."
The effects of calcium on living tissue are numerous, so Shim had the task to prove that handshakes were the cause of the slow movement. An antibody attaches to cadherins and Shim grew cells. These cells moved faster when there were no handshakes.
Researchers developed a solution for their sticky cell problem after determining the basic rules of cell adhesion. Shim put a layer of skin cells into a high-calcium solution to make their normal connections. Shim then treated the cells with a chemical which grabs calcium ions in order to destroy the cellular handshakes. The cells responded well to Shim's treatment. She lowered the calcium level, and then applied an electric field. She finally restored the high calcium level and applied the electric field to restore the handshakes. This resulted in healthy, cohesive skin cells.
Shim demonstrated that this method can speed up healing by using an electrobioreactor from the Cohen lab. This mimics the closing a wound. Their new system, unlike other electrotaxis models that move cells in one direction only, exposes cells to an electrical field focused on the area of injury. Shim demonstrated that stimulated tissues were able to join together, while unstimulated tissues remained mostly separate. Cohen's team described their electrobioreactors in a paper published in Biosensors & Bioelectronics.
"This brief and compelling study by Cohen's lab results is an intuitive but previously undiscovered lesson: collectively migrating cell follow directional cues faster if their mutual cooperation is weaker," stated Alex Mogilner of New York University, who studies collective behavior and was not involved with the research. "This paper is more than just science. It also has important biomedical implications. Cohen asks how cells do it and how we can manipulate them to make them better.
Next, Cohen and Shim plan to transform their 2-D model into a 3-D one. For example, human skin is made up of many different tissues. It's a layer cake. The results of these techniques in a 3D skin model could show whether they will work on real wounds. Although many tissues use cadherin handshakes in order to keep cells connected, there are other ways that tissues can stay connected.
This could be used to speed up wound healing and to reduce scarring.
We started with a warm, fuzzy approach to engineering group behavior. This meant that we said, "Let's see what tissue wants and then harmonize with it." Cohen said that it was too easy. Sometimes they won't listen to you. Sometimes, you have to make some changes."
Continue reading Researchers use electric fields for herding cells similar to flocks of sheep.
