Researchers have identified the mechanism of scar formation in skin and demonstrated in mice a way to make wounds heal with normal skin instead of scar tissue.
April 22, 2021 - By Christopher Vaughan
A simple question from a mentor — Why do we scar after an injury or surgical incision? — set Michael Longaker, MD, on a 34-year quest for an answer.
Now, the Stanford Medicine surgeon and his colleagues have found it. They’ve also discovered that interfering with certain molecular signals during healing can produce tissue indistinguishable from normal skin.
A paper describing their research was published April 23 in Science. Stanford MD-PhD student Shamik Mascharak is the lead author. Longaker, the Deane P. and Louise Mitchell Professor in the School of Medicine, and Geoffrey Gurtner, MD, the Johnson & Johnson Distinguished Professor in Surgery II, are the senior authors.
Nearly everyone has some sort of scar. Longaker estimates that every year in the United States, nearly 50-80 million new scars are produced by surgery, and many more result from accidents. Scarring is not merely a cosmetic issue: Scar tissue has no hair follicles and no sweat glands and is inflexible and weaker than skin. Scars can limit our bodies’ ability to move and adapt to changing temperatures. “There is currently no drug or molecular strategy for preventing or reversing the fibrotic process of scar formation,” said Longaker, who is also the co-director of the Institute for Stem Cell Biology and Regenerative Medicine.
Longaker’s quest began in 1987 when he was in surgical rotations as a postdoctoral scholar at the University of California, San Francisco. He was also a researcher in the laboratory of UCSF surgeon Michael Harrison, MD, who was performing lifesaving surgery on fetuses. Harrison asked Longaker to investigate why wounds on the skin of fetuses will heal without scarring, whereas wounds on the skin of children and adults leave scars.
“That question occupied me for a year, which became four years, which became decades,” Longaker said. “Since then, my research has expanded to many other areas, but the attempt to understand scar formation has always been an active area of interest.”
A quick sealant
Scars form because they seal an opening in the skin more quickly than normal skin could grow, Longaker said. “If you heal slowly you might get an infection or bleed to death. A scar is a spot weld — it covers the wound quickly, but it compromises form and function.” Depending on where a scar forms, he said, people might not be able to bend their elbows, close their eyes or open their mouths much. But in premodern existence, these people were more likely to live, even with such disabilities.
Both Gurtner and Longaker observed that tension during skin repair played a critical role in scarring. Longaker saw how tension made a difference in surgical wounds. “Early in fetal development, when skin injury doesn’t result in scarring, the skin of the fetus is gelatinous and doesn’t really have the ‘tightness’ we associate with skin,” he said. “At the other end of our lives, if a 95-year-old has been exposed to a lot of sun and has loose skin, scar formation is minimal because he or she doesn’t have that tension in the skin.” Longaker and Gurtner showed that if you lower the forces that pull at the edges of a healing incision, scar formation can be reduced.
But why does tension in the skin during healing result in scar formation? Longaker and Gurtner focused on a gene called engrailed. This gene helps create a protein sometimes found in fibroblasts, a type of skin cell that drives scarring.
In a series of experiments in mice, they discovered that a subpopulation of fibroblast cells in the skin that normally don’t express engrailed start to do so during scarring.
Next, they looked at the role mechanical stress might play in turning on the engrailed gene. Cells can sense mechanical stress through well-defined mechanisms, yet there are ways to block their ability to do so. The researchers took mouse fibroblast cells that did not express engrailed and grew them in the laboratory in three different environments: inside a soft gel that did not produce mechanical strain in the growing fibroblasts, on a stiff plastic dish that produced mechanical strain, and on the same strain-inducing plastic but in the presence of a chemical that blocked mechanical-strain signaling.
They found that fibroblasts grown on the tension-free gel did not start expressing engrailed, but that the fibroblasts growing on the stress-inducing plastic did. If they added a chemical that blocked mechanical strain signaling, cells grown on plastic did not express engrailed.
When tension was applied to healing surgical incisions in mice, there was an increase in the number of cells expressing engrailed and a thicker scar resulted.
Blocking mechanical-stress signals with drug
Mascharak, the lead author, identified a drug, verteporfin, which is approved by the U.S. Food and Drug Administration to treat an eye disease. The researchers made surgical wounds in mice under anesthesia and applied mechanical strain to the healing wound while also applying verteporfin to the wound.
The results were astounding, Longaker said. The healed skin looked completely normal. “There must be three things for wound healing to be true skin regeneration,” Longaker said. “The skin needs to have normal hair follicles and glands, it needs to have a normal appearance under a microscope, and it needs to be just as strong as normal skin.”
“The first thing we were shocked by was the all the hair in the healed wound,” he said. “We were also able to see normal glands and showed that the skin was just as strong as unwounded skin.”
Mascharak developed an artificial intelligence algorithm that compared microscopic images of skin to see if there were subtle differences that the human eye could not pick up. The algorithm was unable to find any differences between normal skin and skin regenerated with the assistance of verteporfin, Longaker said.
“These results are exciting because we have shown that we are able to intervene and stop fibroblasts from sensing mechanical force when healing a skin wound,” Gurtner said. “Now we need to see if the same approach will work in preventing other kinds of scarring.”
It’s possible that many other medical afflictions, such as liver fibrosis, burns, abdominal adhesions, scleroderma and scarring to heart tissue after a heart attack, can be treated with the same approach, he said.
“It’s estimated that 45% of Americans die from a disease that involves scarring in some form,” Longaker said. “So there are potentially many more applications.”
The next stage will be preclinical work in other animals. If those results are successful, a clinical trial could follow, the researchers said.
Other Stanford scientists who contributed to the work are MD-PhD student Heather DesJardins-Park; former postdoctoral scholars Mimi Borrelli, MBBS, Sun Hyung Kwon, PhD, and Alessandra Moore, MD; postdoctoral scholars Michael Davitt, MD, Michael Januszyk, MD, and Kellen Chen, PhD; former California Institute for Regenerative Medicine Scholar Bryan Duoto; medical students Malini Chinta and Abra H. Shen; postdoctoral medical fellow Deshka S. Foster, MD; assistant professor of pathology Gerlinde Wernig, MD; professor of surgery Derrick Wan, MD; and professor of surgery Peter Lorenz, MD.
The work was supported by the National Institutes of Health (grants R01-GM116892, R01GM136659, R01DE027346 and U24-DE26914), the Stinehart/Reed Foundation, the Gunn/Olivier Research Fund and the Hagey Laboratory for Pediatric Regenerative Medicine. Approximately 40% (or about $1.43 million) of the project’s funding came from federal sources, and approximately 60% (about $2.15 million) came from nonfederal sources.
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