Cells that rush to heal a wound are known to include leaders and followers, but exactly what arouses leader cells has been unclear. Leader cells, scientists reasoned, must respond to something, and this something must instigate a chain of well-directed events, otherwise cell migration would amount to nothing more than the blind leading the blind.

A recent study reveals the special something that gets leader cells going. Actually, according to scientists at the University of Arizona, this something is the absence of something—mechanical stress. When mechanical stress disappears—for example, at a wound site where cells have been destroyed, leaving empty, cell-free space—leader cells express DII4, a protein that coordinates nearby cells to migrate to the wound site and collectively cover it with new tissue.

This process causes identical cells to specialize into leader and follower cells. Researchers had previously assumed that leader cells formed randomly.

The new findings appeared March 13 in Nature Communications, in an article entitled, “Notch1–Dll4 signalling and mechanical force regulate leader cell formation during collective cell migration.” The article described how cell migration depends on a delicate balance between biomechanical force, or stress, which living cells exert on one another, and biochemical signaling.

“We use single-cell gene expression analysis and computational modelling to show that the leader cell identity is dynamically regulated by Dll4 signalling through both Notch1 and cellular stress in a migrating epithelium,” wrote the authors. “Time-lapse microscopy reveals that Dll4 is induced in leader cells after the creation of the cell-free region and leader cells are regulated via Notch1–Dll4 lateral inhibition.”

The University of Arizona team, led by Pak Kin Wong, Ph.D., associate professor of mechanical and aerospace engineering, observed that when a pack of cells migrates toward a wound, leader cells expressing a form of messenger RNA specific to the DII4 protein emerge at the front of the pack. The leader cells, in turn, send signals to follower cells, which do not express the mRNA. This elaborate autoregulatory system remains activated until new tissue has covered a wound.

The researchers tracked leader cell formation and behavior in vitro in human breast cancer cells and in vivo in mice epithelial cells under a confocal microscope.

“Mechanical stress inhibits Dll4 expression and leader cell formation in the monolayer,” the authors continued. “Collectively, our findings suggest that a reduction of mechanical force near the boundary promotes Notch1–Dll4 signalling to dynamically regulate the density of leader cells during collective cell migration.”

Their researchers also manipulated leader cells through pharmacological, laser, and other means to see how they would react. “Amazingly, when we directed a laser at individual leader cells and destroyed them, new ones quickly emerged at the migrating tip to take their place,” said Dr. Wong, who likened the mysteries of cell migration and leader cell formation to the processes in nature that cause geese to fly in V-formation or ants to build a colony.

The same migration processes for wound healing and tissue development also apply to cancer spreading, the researchers noted. The combination of mechanical force and genetic signaling stimulates cancer cells to collectively migrate and invade healthy tissue.

With this new knowledge, researchers can re-create, at the cellular and molecular levels, the chain of events that brings about the formation of human tissue. “Knowing the genetic makeup of leader cells and understanding their formation and behavior gives us the ability to alter cell migration,” explained Dr. Wong. It may even be possible for bioengineers to direct normal cells to heal damaged tissue, or prevent cancer cells from invading healthy tissue.

Leader-cell dynamics and cell migration processes have implications for people with a variety of diseases and conditions. For example, the discoveries may lead to better treatments for nonhealing diabetic wounds, the leading cause of lower limb amputations in the United States; for plaque buildup in arteries, a major cause of heart disease; and for slowing or even stopping the spread of cancer, which is what makes it so deadly. The research also has the potential to speed up development of bioengineered tissues and organs that can be successfully transplanted in humans.