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.
How to have a good life, based on the world's longest study of happiness
What makes for a good life? Such a simple question, yet we don't have great answers. Most of us try to figure it out as we go along, and many end up feeling like they never got to the bottom of it.
Shouldn't something so important be approached with more scientific rigor? In 1938, Harvard researchers began a study to fill this gap. Since then, they’ve followed hundreds of people over the course of their lives, hoping to identify which factors are key to long-term satisfaction.
Eighty-five years later, the Harvard Study of Adult Development is still going. And today, its directors, the psychiatrists Bob Waldinger and Marc Shulz, have published a book that pulls together the study’s most important findings. It’s called The Good Life: Lessons from the World’s Longest Scientific Study of Happiness.
In this podcast episode, I talked with Dr. Waldinger about life lessons that we can mine from the Harvard study and his new book.
Listen on Apple | Listen on Spotify | Listen on Stitcher | Listen on Amazon | Listen on Google
More background on the study
Back in the 1930s, the research began with 724 people. Some were first-year Harvard students paying full tuition, others were freshmen who needed financial help, and the rest were 14-year-old boys from inner city Boston – white males only. Fortunately, the study team realized the error of their ways and expanded their sample to include the wives and daughters of the first participants. And Waldinger’s book focuses on the Harvard study findings that can be corroborated by evidence from additional research on the lives of people of different races and other minorities.
The study now includes over 1,300 relatives of the original participants, spanning three generations. Every two years, the participants have sent the researchers a filled-out questionnaire, reporting how their lives are going. At five-year intervals, the research team takes a peek their health records and, every 15 years, the psychologists meet their subjects in-person to check out their appearance and behavior.
But they don’t stop there. No, the researchers factor in multiple blood samples, DNA, images from body scans, and even the donated brains of 25 participants.
Robert Waldinger, director of the Harvard Study of Adult Development.
Katherine Taylor
Dr. Waldinger is Clinical Professor of Psychiatry at Harvard Medical School, in addition to being Director of the Harvard Study of Adult Development. He got his M.D. from Harvard Medical School and has published numerous scientific papers he’s a practicing psychiatrist and psychoanalyst, he teaches Harvard medical students, and since that is clearly not enough to keep him busy, he’s also a Zen priest.
His book is a must-read if you’re looking for scientific evidence on how to design your life for more satisfaction so someday in the future you can look back on it without regret, and this episode was an amazing conversation in which Dr. Waldinger breaks down many of the cliches about the good life, making his advice real and tangible. We also get into what he calls “side-by-side” relationships, personality traits for the good life, and the downsides of being too strict about work-life balance.
Show links
- Bob Waldinger
- Waldinger's book, The Good Life: Lessons from the World's Longest Scientific Study of Happiness
- The Harvard Study of Adult Development
- Waldinger's Ted Talk
- Gallup report finding that people with good friends at work have higher engagement with their jobs
- The link between relationships and well-being
- Those with social connections live longer
The Friday Five: A new blood test to detect Alzheimer's
The Friday Five covers five stories in research that you may have missed this week. There are plenty of controversies and troubling ethical issues in science – and we get into many of them in our online magazine – but this news roundup focuses on scientific creativity and progress to give you a therapeutic dose of inspiration headed into the weekend.
Listen on Apple | Listen on Spotify | Listen on Stitcher | Listen on Amazon | Listen on Google
Here are the promising studies covered in this week's Friday Five:
- A blood test to detect Alzheimer's
- War vets can take their psychologist wherever they go
- Does intermittent fasting affect circadian rhythms?
- A new year's resolution for living longer
- 3-D printed eyes?