Resurrected Tissue

Like a phoenix rising from the ashes, our skin tissue - and in fact many types of epithelial tissue that lines and covers the body's organs - can respond to death and destruction with a burst of regeneration. This phenomenon, known as compensatory proliferation, was first described in the 1970s in fly larvae, which re-grew fully functional wings after their epithelial tissue had been severely damaged by high-dose radiation. Since then, this surprising ability has been documented in many species, including humans, yet its molecular basis remained unclear.

A new study from the Weizmann Institute of Science, published in Nature Communications , reveals that the very enzymes responsible for cellular destruction - caspases - may also render certain cells resistant to death, enabling damaged tissue not only to regenerate but even to become more resilient. Unfortunately, this same mechanism may be hijacked by many types of cancer and may help explain why some tumors recur in a more aggressive and treatment-resistant form. The new findings could thus open avenues for therapies that speed up wound repair and help prevent cancer relapse.

""We set out to identify cells that push the self-destruct button but survive anyway"

The most common form of cell death in a healthy body is apoptosis - a programmed "suicide" triggered when a cell grows old, becomes damaged or receives signals instructing it to end its life. In this process, an initiator caspase launches the death pathway by activating effector caspases, which then tear the cell's proteins to pieces. Over the past two decades, studies around the world, including in the lab of Prof. Eli Arama of Weizmann's Molecular Genetics Department, have established that the apoptotic caspases play crucial roles not only in cell death but also in processes essential for life. These findings led Arama, one of the pioneers of research into nonlethal roles of caspases, to hypothesize that caspases might also be responsible for the mysterious phenomenon of compensatory proliferation.

In the new study, a team led by Dr. Tslil Braun from Arama's lab repeated the original experiment that first uncovered this phenomenon - irradiating fruit-fly larvae with ionizing radiation. This time, however, advanced genetic technologies allowed the researchers to track epithelial regeneration in unprecedented detail. "We set out to identify cells that push the self-destruct button but survive anyway," Braun explains. "To do this, we used a delayed sensor that reported on cells in which the initiator caspase had been activated but that nevertheless survived the irradiation. This is how we discovered a population of cells we named DARE cells. Not only did these cells survive the irradiation - they multiplied, repaired the damaged tissue and replenished nearly half of it within 48 hours." But what about the rest of the regenerating tissue?

"We identified another population of death-resistant cells, but unlike DARE cells, they showed no activation of the initiator caspase. We called them NARE cells," Braun says. "Although NARE cells ultimately contribute to tissue regeneration, they cannot do it alone: When we removed DARE cells from the system, compensatory proliferation disappeared entirely. We also found that dying cells in the tissue play a role in the burst of regeneration - DARE cells were activated by signals from their dying neighbors."

Next, the researchers sought to decipher how DARE cells survive radiation doses that trigger apoptosis in neighboring cells. "We observed that although the initiator caspase is activated in these cells, the cellular death process stops there and does not progress to the next stage," Arama explains. "We suspected that a protein known as a molecular motor was responsible for this - it can tether the initiator caspase to the cell membrane, preventing it from activating the executioner caspases. Indeed, when we silenced this motor protein, DARE cells proceeded to die and tissue regeneration was impaired. Overactivation of the same motor protein has previously been linked to cancerous tumor growth, which suggests that this might be one of the mechanisms that enables cancer cells to evade apoptosis."

It is known that tumors that regrow after radiation therapy often become more aggressive and more resistant to treatment. "We wanted to understand whether resistance to death is inherited by the descendants of death-resistant cells that survived the initial irradiation," Arama says. "We found that when the same tissue is irradiated a second time, the number of cells that die during the first few hours is half that seen after the first irradiation, and most of the dead cells belong to the NARE population. In other words, the descendants of DARE cells were found to be exceptionally resistant - seven times more resistant to cell death than cells in the original tissue. This may help explain why recurrent tumors become more resistant after radiation."

A delicate balance between tissue repair and excess growth is essential to any regenerative process. In the final part of their study, the researchers revealed how uncontrolled growth is prevented during tissue repair after injury. "DARE cells promote the growth of nearby NARE cells, apparently by secreting growth signals," Arama notes. "In turn, NARE cells secrete signals that inhibit the growth of DARE cells. In fact, we've discovered a negative-feedback loop between the two cell populations that prevents overgrowth."

"We hope that, as has often been the case with fly models, the knowledge gained here can be translated into an understanding of the mechanisms that balance growth and confer resistance to cell death in human tissues," Arama concludes. "Many cancers originate in epithelial cells that have lost normal growth control, and many traditional cancer treatments aim to cause them to self-destruct through apoptosis. Our findings pave the way for understanding why such treatments sometimes fail and how they could be improved. The results also point toward new ways in which we might be able to accelerate beneficial regeneration of healthy tissue after injury."

Also participating in the study were Naama Afgin, Dr. Lena Sapozhnikov and Dr. Keren Yacobi-Sharon from Weizmann's Molecular Genetics Department; Dr. Ehud Sivan from Weizmann's Life Sciences Core Facilities Department; Prof. Andreas Bergmann from UMass Chan Medical School, Worcester, MA; and Prof. Luis Alberto Baena-Lopez from the Severo Ochoa Molecular Biology Center (CBM), Spain.