Scientists want the salamander's secret: how they regenerate tissue
All organisms have the capacity to repair or regenerate tissue damage. None can do it better than salamanders or newts, which can regenerate an entire severed limb.
That feat has amazed and delighted man from the dawn of time and led to endless attempts to understand how it happens – and whether we can control it for our own purposes. An exciting new clue toward that understanding has come from a surprising source: research on the decline of cells, called cellular senescence.
Senescence is the last stage in the life of a cell. Whereas some cells simply break up or wither and die off, others transition into a zombie-like state where they can no longer divide. In this liminal phase, the cell still pumps out many different molecules that can affect its neighbors and cause low grade inflammation. Senescence is associated with many of the declining biological functions that characterize aging, such as inflammation and genomic instability.
Oddly enough, newts are one of the few species that do not accumulate senescent cells as they age, according to research over several years by Maximina Yun. A research group leader at the Center for Regenerative Therapies Dresden and the Max Planck Institute of Molecular and Cell Biology and Genetics, in Dresden, Germany, Yun discovered that senescent cells were induced at some stages of regeneration of the salamander limb, “and then, as the regeneration progresses, they disappeared, they were eliminated by the immune system,” she says. “They were present at particular times and then they disappeared.”
Senescent cells added to the edges of the wound helped the healthy muscle cells to “dedifferentiate,” essentially turning back the developmental clock of those cells into more primitive states.
Previous research on senescence in aging had suggested, logically enough, that applying those cells to the stump of a newly severed salamander limb would slow or even stop its regeneration. But Yun stood that idea on its head. She theorized that senescent cells might also play a role in newt limb regeneration, and she tested it by both adding and removing senescent cells from her animals. It turned out she was right, as the newt limbs grew back faster than normal when more senescent cells were included.
Senescent cells added to the edges of the wound helped the healthy muscle cells to “dedifferentiate,” essentially turning back the developmental clock of those cells into more primitive states, which could then be turned into progenitors, a cell type in between stem cells and specialized cells, needed to regrow the muscle tissue of the missing limb. “We think that this ability to dedifferentiate is intrinsically a big part of why salamanders can regenerate all these very complex structures, which other organisms cannot,” she explains.
Yun sees regeneration as a two part problem. First, the cells must be able to sense that their neighbors from the lost limb are not there anymore. Second, they need to be able to produce the intermediary progenitors for regeneration, , to form what is missing. “Molecularly, that must be encoded like a 3D map,” she says, otherwise the new tissue might grow back as a blob, or liver, or fin instead of a limb.
Wound healing
Another recent study, this time at the Mayo Clinic, provides evidence supporting the role of senescent cells in regeneration. Looking closely at molecules that send information between cells in the wound of a mouse, the researchers found that senescent cells appeared near the start of the healing process and then disappeared as healing progressed. In contrast, persistent senescent cells were the hallmark of a chronic wound that did not heal properly. The function and significance of senescence cells depended on both the timing and the context of their environment.
The paper suggests that senescent cells are not all the same. That has become clearer as researchers have been able to identify protein markers on the surface of some senescent cells. The patterns of these proteins differ for some senescent cells compared to others. In biology, such physical differences suggest functional differences, so it is becoming increasingly likely there are subsets of senescent cells with differing functions that have not yet been identified.
There are disagreements within the research community as to whether newts have acquired their regenerative capacity through a unique evolutionary change, or if other animals, including humans, retain this capacity buried somewhere in their genes.
Scientists initially thought that senescent cells couldn’t play a role in regeneration because they could no longer reproduce, says Anthony Atala, a practicing surgeon and bioengineer who leads the Wake Forest Institute for Regenerative Medicine in North Carolina. But Yun’s study points in the other direction. “What this paper shows clearly is that these cells have the potential to be involved in tissue regeneration [in newts]. The question becomes, will these cells be able to do the same in humans.”
As our knowledge of senescent cells increases, Atala thinks we need to embrace a new analogy to help understand them: humans in retirement. They “have acquired a lot of wisdom throughout their whole life and they can help younger people and mentor them to grow to their full potential. We're seeing the same thing with these cells,” he says. They are no longer putting energy into their own reproduction, but the signaling molecules they secrete “can help other cells around them to regenerate.”
There are disagreements within the research community as to whether newts have acquired their regenerative capacity through a unique evolutionary change, or if other animals, including humans, retain this capacity buried somewhere in their genes. If so, it seems that our genes are unable to express this ability, perhaps as part of a tradeoff in acquiring other traits. It is a fertile area of research.
Dedifferentiation is likely to become an important process in the field of regenerative medicine. One extreme example: a lab has been able to turn back the clock and reprogram adult male skin cells into female eggs, a potential milestone in reproductive health. It will be more difficult to control just how far back one wishes to go in the cell's dedifferentiation – part way or all the way back into a stem cell – and then direct it down a different developmental pathway. Yun is optimistic we can learn these tricks from newts.
Senolytics
A growing field of research is using drugs called senolytics to remove senescent cells and slow or even reverse disease of aging.
“Senolytics are great, but senolytics target different types of senescence,” Yun says. “If senescent cells have positive effects in the context of regeneration, of wound healing, then maybe at the beginning of the regeneration process, you may not want to take them out for a little while.”
“If you look at pretty much all biological systems, too little or too much of something can be bad, you have to be in that central zone” and at the proper time, says Atala. “That's true for proteins, sugars, and the drugs that you take. I think the same thing is true for these cells. Why would they be different?”
Our growing understanding that senescence is not a single thing but a variety of things likely means that effective senolytic drugs will not resemble a single sledge hammer but more a carefully manipulated scalpel where some types of senescent cells are removed while others are added. Combinations and timing could be crucial, meaning the difference between regenerating healthy tissue, a scar, or worse.
[Ed. Note: This is the third episode in our Moonshot series, which will explore four cutting-edge scientific developments that stand to fundamentally transform our world.]
Kira Peikoff was the editor-in-chief of Leaps.org from 2017 to 2021. As a journalist, her work has appeared in The New York Times, Newsweek, Nautilus, Popular Mechanics, The New York Academy of Sciences, and other outlets. She is also the author of four suspense novels that explore controversial issues arising from scientific innovation: Living Proof, No Time to Die, Die Again Tomorrow, and Mother Knows Best. Peikoff holds a B.A. in Journalism from New York University and an M.S. in Bioethics from Columbia University. She lives in New Jersey with her husband and two young sons. Follow her on Twitter @KiraPeikoff.
My Wife's Fight Against Cancer Inspired 38,000 People to Raise Millions for Research
It was 15 years ago this month, but I'll never forget those words. When my wife Jen and I asked her oncologist about our plans to start a family, he calmly replied, "Well, I wouldn't do so unless Dave is prepared to be a single father."
About 50 percent of all people with cancer have a rare type, like the one Jen was fighting.
Time stood still. The danger crystalized — we were in a battle for my beautiful bride's life, and the odds were not in our favor.
We felt every emotion expected. Anger, sadness, confusion, frustration, and especially fear. But we made a very intentional choice to take that fear, put it to the side, and do everything we could to live our lives together to the fullest.
We focused first on Jen's health and learned everything we could about MFH Sarcoma. I was with her every step of the way — for hundreds of medical appointments, six intense surgeries, and twenty different types of chemotherapy. During such a challenging time, our choice to reject fear allowed us to live our best lives. Our careers blossomed, we enjoyed several international vacations, and Jen inspired thousands of fellow patients through her blog and speeches.
When we researched treatment options we learned that Jen was not alone. About 50 percent of all people with cancer have a rare type, like the one Jen was fighting. However, rare cancers don't get the funding they desperately need so effective treatment options are hard to find. The lack of funding felt unfair — and urgent. We didn't worry about everything that can go wrong when starting a new venture. Instead, we jumped in head first and convinced a small group of friends and family to ride stationary bikes with us to raise money for rare cancer research.
Jen Goodman Linn, riding a stationary bike for Cycle for Survival.
(Courtesy David Linn)
From those humble beginnings, Cycle for Survival grew steadily. After starting from scratch, Jen and I ran Cycle for Survival on our own for two years. We quickly realized that if we wanted to help as many people as possible, we needed the best partners. In 2009, we agreed that Memorial Sloan Kettering Cancer Center would take over the ownership of Cycle for Survival and Equinox officially became the Founding Partner. Flash forward to today, and Cycle for Survival has raised more than $220 million! I'm proud that 100% of every donation, yes every penny, goes directly into life-saving rare cancer research within six months of the annual indoor cycling events, which now take place in 17 cities nationwide.
While Cycle for Survival's trajectory was heading straight up, Jen's health struggle was devastatingly swinging up and down. With her incredible spirit and tenacity, Jen would beat the cancer through chemo and surgery, but then it would frustratingly come back again and again. After going into remission six times, it returned with such a vengeance in 2011 that even the world's leading doctors were forced to say, "I'm sorry, there's nothing more we can do."
Those were the most difficult words I've ever heard, by far. I hope no other family has to hear these crushing words.
When Jen died soon after, I didn't know what would happen to me, to my life, and to Cycle for Survival. I do remember making two very important choices at the time. First, I chose to get out of bed and put one foot in front of the other. It wasn't easy. Tears, pain, and grief would hit at any hour of the day or night. I did have a great support network of family and friends who kept me moving forward. One friend in particular changed the route of her morning runs so that I would join her and start getting back to exercising.
My second key choice was to stay involved with Cycle for Survival. At times, it was an excruciatingly difficult decision because I felt the depth of my loss each and every time I stepped into one of the events. However, it was also rewarding and energizing because I could see firsthand how many people it was helping, even though it was too late for Jen.
I began to travel across the country with the Cycle for Survival staff. My hope was to spread the word about rare cancers; along the way I met a lot of wonderful people who shared their stories with me. What I soon realized is that each of us faces obstacles in our lives. For me, it was losing the person who I wanted to spend my life with. For others, it might be challenges with their kids or in their professional lives. The common theme is that we don't have control over the fact that we have to face these challenges. But the biggest lesson I've learned is that we very much do have a choice in how we react.
I made the choice to do everything I can to help rare cancer patients and their families and it has been transformative and healing for me. The small group who rode in the first Cycle for Survival event has grown into a powerful movement of nearly 40,000 riders making a real difference. If Jen were diagnosed today, there are new treatments available– including genomic sequencing, targeted therapies, and immunotherapies – that could help her. Those weren't even options a short time ago. That's the result of funding research.
A recent Cycle for Survival event shows the passion and power of the community.
(Courtesy David Linn)
I also want to share one more choice I made. Remember that friend who changed the route of her morning runs so I could start exercising after Jen died? Well, over the years friendship grew into love, and we're now building a home together and can't wait to see what the future holds for us.
So with all that in mind I ask – when you face those inevitable challenges in your life, how will you choose to react? Remember that even in the midst of hopelessness, you can find choices. Those will be the decisions that define and guide you.