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.
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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.
Trying to get a handle on CRISPR news in 2019 can be daunting if you haven't been avidly reading up on it for the last five years.
CRISPR as a diagnostic tool would be a major game changer for medicine and agriculture.
On top of trying to grasp how the science works, and keeping track of its ever expanding applications, you may also have seen coverage of an ongoing legal battle about who owns the intellectual property behind the gene-editing technology CRISPR-Cas9. And then there's the infamous controversy surrounding a scientist who claimed to have used the tool to edit the genomes of two babies in China last year.
But gene editing is not the only application of CRISPR-based biotechnologies. In the future, it may also be used as a tool to diagnose infectious diseases, which could be a major game changer for medicine and agriculture.
How It Works
CRISPR is an acronym for a naturally occurring DNA sequence that normally protects microbes from viruses. It's been compared to a Swiss army knife that can recognize an invader's DNA and precisely destroy it. Repurposed for humans, CRISPR can be paired with a protein called Cas9 that can detect a person's own DNA sequence (usually a problematic one), cut it out, and replace it with a different sequence. Used this way, CRISPR-Cas9 has become a valuable gene-editing tool that is currently being tested to treat numerous genetic diseases, from cancer to blood disorders to blindness.
CRISPR can also be paired with other proteins, like Cas13, which target RNA, the single-stranded twin of DNA that viruses rely on to infect their hosts and cause disease. In a future clinical setting, CRISPR-Cas13 might be used to diagnose whether you have the flu by cutting a target RNA sequence from the virus. That spliced sequence could stick to a paper test strip, causing a band to show up, like on a pregnancy test strip. If the influenza virus and its RNA are not present, no band would show up.
To understand how close to reality this diagnostic scenario is right now, leapsmag chatted with CRISPR pioneer Dr. Feng Zhang, a molecular biologist at the Broad Institute of MIT and Harvard.
What do you think might be the first point of contact that a regular person or patient would have with a CRISPR diagnostic tool?
FZ: I think in the long run it will be great to see this for, say, at-home disease testing, for influenza and other sorts of important public health [concerns]. To be able to get a readout at home, people can potentially quarantine themselves rather than traveling to a hospital and then carrying the risk of spreading that disease to other people as they get to the clinic.
"You could conceivably get a readout during the same office visit, and then the doctor will be able to prescribe the right treatment right away."
Is this just something that people will use at home, or do you also foresee clinical labs at hospitals applying CRISPR-Cas13 to samples that come through?
FZ: I think we'll see applications in both settings, and I think there are advantages to both. One of the nice things about SHERLOCK [a playful acronym for CRISPR-Cas13's longer name, Specific High-sensitivity Enzymatic Reporter unLOCKing] is that it's rapid; you can get a readout fairly quickly. So, right now, what people do in hospitals is they will collect your sample and then they'll send it out to a clinical testing lab, so you wouldn't get a result back until many hours if not several days later. With SHERLOCK, you could conceivably get a readout during the same office visit, and then the doctor will be able to prescribe the right treatment right away.
I just want to clarify that when you say a doctor would take a sample, that's referring to urine, blood, or saliva, correct?
FZ: Right. Yeah, exactly.
Thinking more long term, are there any Holy Grail applications that you hope CRISPR reaches as a diagnostic tool?
FZ: I think in the developed world we'll hopefully see this being used for influenza testing, and many other viral and pathogen-based diseases—both at home and also in the hospital—but I think the even more exciting direction is that this could be used and deployed in parts of the developing world where there isn't a fancy laboratory with elaborate instrumentation. SHERLOCK is relatively inexpensive to develop, and you can turn it into a paper strip test.
Can you quantify what you mean by relatively inexpensive? What range of prices are we talking about here?
FZ: So without accounting for economies of scale, we estimate that it can cost less than a dollar per test. With economy of scale that cost can go even lower.
Is there value in developing what is actually quite an innovative tool in a way that visually doesn't seem innovative because it's reminiscent of a pregnancy test? And I don't mean that as an insult.
FZ: [Laughs] Ultimately, we want the technology to be as accessible as possible, and pregnancy test strips have such a convenient and easy-to-use form. I think modeling after something that people are already familiar with and just changing what's under the hood makes a lot of sense.
Feng Zhang
(Photo credit: Justin Knight, McGovern Institute)
It's probably one of the most accessible at-home diagnostic tools at this point that people are familiar with.
FZ: Yeah, so if people know how to use that, then using something that's very similar to it should make the option very easy.
You've been quite vocal in calling for some pauses in CRISPR-Cas9 research to make sure it doesn't outpace the ethics of establishing pregnancies with that version of the tool. Do you have any concerns about using CRISPR-Cas13 as a diagnostic tool?
I think overall, the reception for CRISPR-based diagnostics has been overwhelmingly positive. People are very excited about the prospect of using this—for human health and also in agriculture [for] detection of plant infections and plant pathogens, so that farmers will be able to react quickly to infection in the field. If we're looking at contamination of foods by certain bacteria, [food safety] would also be a really exciting application.
Do you feel like the controversies surrounding using CRISPR as a gene-editing tool have overshadowed its potential as a diagnostics tool?
FZ: I don't think so. I think the potential for using CRISPR-Cas9 or CRISPR-Cas12 for gene therapy, and treating disease, has captured people's imaginations, but at the same time, every time I talk with someone about the ability to use CRISPR-Cas13 as a diagnostic tool, people are equally excited. Especially when people see the very simple paper strip that we developed for detecting diseases.
Are CRISPR as a gene-editing tool and CRISPR as a diagnostics tool on different timelines, as far as when the general public might encounter them in their real lives?
FZ: I think they are all moving forward quite quickly. CRISPR as a gene-editing tool is already being deployed in human health and agriculture. We've already seen the approval for the development of growing genome-edited mushrooms, soybeans, and other crop species. So I think people will encounter those in their daily lives in that manner.
Then, of course, for disease treatment, that's progressing rapidly as well. For patients who are affected by sickle cell disease, and also by a degenerative eye disease, clinical trials are already starting in those two areas. Diagnostic tests are also developing quickly, and I think in the coming couple of years, we'll begin to see some of these reaching into the public realm.
"There are probably 7,000 genetic diseases identified today, and most of them don't have any way of being treated."
As far its limits, will it be hard to use CRISPR as a diagnostic tool in situations where we don't necessarily understand the biological underpinnings of a disease?
FZ: CRISPR-Cas13, as a diagnostic tool, at least in the current way that it's implemented, is a detection tool—it's not a discovery tool. So if we don't know what we're looking for, then it's going to be hard to develop Cas13 to detect it. But even in the case of a new infectious disease, if DNA sequencing or RNA sequencing information is available for that new virus, then we can very rapidly program a Cas13-based system to detect it, based on that sequence.
What's something you think the public misunderstands about CRISPR, either in general, or specifically as a diagnostic tool, that you wish were better understood?
FZ: That's a good question. CRISPR-Cas9 and CRISPR-Cas12 as gene editing tools, and also CRISPR-Cas13 as a diagnostic tool, are able to do some things, but there are still a lot of capabilities that need to be further developed. So I think the potential for the technology will unfold over the next decade or so, but it will take some time for the full impact of the technology to really get realized in real life.
What do you think that full impact is?
FZ: There are probably 7,000 genetic diseases identified today, and most of them don't have any way of being treated. It will take some time for CRISPR-Cas9 and Cas12 to be really developed for addressing a larger number of those diseases. And then for CRISPR-based diagnostics, I think you'll see the technology being applied in a couple of initial cases, and it will take some time to develop that more broadly for many other applications.