For large bone injuries, it's Sonic hedgehog to the rescue

In the study, first author Maxwell Serowoky, a PhD student in the USC Stem Cell laboratory of Francesca Mariani, and his colleagues took a close look at how mice are able to regrow large sections of missing rib -- an ability they share with humans, and one of the most impressive examples of bone regeneration in mammals.

To their surprise, the scientists observed an increase in the activity ofSonic hedgehog (Shh), which plays an important role in skeletal formation in embryos, but hasn't previously been linked to injury repair in adults.

In their experiments,Shhappeared to play a necessary role in healing the central region of large sections of missing ribs, but not in closing small-scale fractures.

"Our evidence suggests that large-scale bone regeneration requires the redeployment of an embryonic developmental program involvingShh, whereas small injuries heal through a distinct repair program that does not mirror development," said Mariani, the study's corresponding author and an associate professor of stem cell biology and regenerative medicine at the Eli and Edythe Broad Center for Regenerative Medicine and Stem Cell Research at the Keck School of Medicine of USC.

Serowoky added: "It's still a fascinating mystery which factors or conditions result inShhactivity following large, but not small bone injuries."

In mice,Shhactivity increased briefly after a large rib injury, and then quickly returned to normal levels within 5 days. Although transient, this increase inShhwas a prerequisite for successfully building a callus, which is an initial scaffold that bridges a fracture or injury but then converts to bone and regenerates the missing section of rib. Mice genetically modified to lackShhcouldn't successfully form calluses or heal their ribs.

In contrast, mice that hadShhat the time of injury, but were genetically altered to loseShhafter a 5-day healing period, were able to repair their ribs normally. A related gene known asSmoothened (Smo)was also required only during the first 5 days of the healing process.

The researchers expected that the source ofShhwould be from specific progenitor cells that the group had previously shown to be essential for healing large injuries and that reside in the periosteum, which is the sheath of tissue surrounding each rib. Instead, they discovered that the source ofShh是一个意想不到的人口干移动电话l-like cells, known as mesenchymal cells. When these mesenchymal cells increased theirShhactivity, this seemed to serve as a signal to summon a separate population of stem cell-like bone marrow cells to the injury site to assist in the healing process.

"Our discovery may inform future therapeutic strategies for situations where patients are missing large sections of bone following high energy injuries such as traffic accidents or combat wounds, or after cancer-related bone resections," said co-author Jay R. Lieberman, chair and professor of orthopaedic surgery at the Keck School.

Additional co-authors for this USC study include Stephanie Kuwahara and Shuwan Liu from the Department of Stem Cell Biology and Regenerative Medicine, and Venus Vakhshori from the Department of Orthopaedic Surgery.

The majority of this work (90%) was supported by federal funding from the National Institutes of Health (T32HD060549, R01AR069700, R01AR057076), and the remainder was supported by a Roy E. Thomas Graduate Fellowship and a USC Regenerative Medicine Initiative Award, which provided the initial pilot funding to bring together the basic researchers and surgeon-scientists who collaborated on this stem cell study.