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.
Should We Use Technologies to Enhance Morality?
Our moral ‘hardware’ evolved over 100,000 years ago while humans were still scratching the savannah. The perils we encountered back then were radically different from those that confront us now. To survive and flourish in the face of complex future challenges our archaic operating systems might need an upgrade – in non-traditional ways.
Morality refers to standards of right and wrong when it comes to our beliefs, behaviors, and intentions. Broadly, moral enhancement is the use of biomedical technology to improve moral functioning. This could include augmenting empathy, altruism, or moral reasoning, or curbing antisocial traits like outgroup bias and aggression.
The claims related to moral enhancement are grand and polarizing: it’s been both tendered as a solution to humanity’s existential crises and bluntly dismissed as an armchair hypothesis. So, does the concept have any purchase? The answer leans heavily on our definition and expectations.
One issue is that the debate is often carved up in dichotomies – is moral enhancement feasible or unfeasible? Permissible or impermissible? Fact or fiction? On it goes. While these gesture at imperatives, trading in absolutes blurs the realities at hand. A sensible approach must resist extremes and recognize that moral disrupters are already here.
We know that existing interventions, whether they occur unknowingly or on purpose, have the power to modify moral dispositions in ways both good and bad. For instance, neurotoxins can promote antisocial behavior. The ‘lead-crime hypothesis’ links childhood lead-exposure to impulsivity, antisocial aggression, and various other problems. Mercury has been associated with cognitive deficits, which might impair moral reasoning and judgement. It’s well documented that alcohol makes people more prone to violence.
So, what about positive drivers? Here’s where it gets more tangled.
Medicine has long treated psychiatric disorders with drugs like sedatives and antipsychotics. However, there’s short mention of morality in the Diagnostic and Statistical Manual of Mental Disorders (DSM) despite the moral merits of pharmacotherapy – these effects are implicit and indirect. Such cases are regarded as treatments rather than enhancements.
It would be dangerously myopic to assume that moral augmentation is somehow beyond reach.
Conventionally, an enhancement must go beyond what is ‘normal,’ species-typical, or medically necessary – this is known as the ‘treatment-enhancement distinction.’ But boundaries of health and disease are fluid, so whether we call a procedure ‘moral enhancement’ or ‘medical treatment’ is liable to change with shifts in social values, expert opinions, and clinical practices.
Human enhancements are already used for a range of purported benefits: caffeine, smart drugs, and other supplements to boost cognitive performance; cosmetic procedures for aesthetic reasons; and steroids and stimulants for physical advantage. More boldly, cyborgs like Moon Ribas and Neil Harbisson are pushing transpecies boundaries with new kinds of sensory perception. It would be dangerously myopic to assume that moral augmentation is somehow beyond reach.
How might it work?
One possibility for shaping moral temperaments is with neurostimulation devices. These use electrodes to deliver a low-intensity current that alters the electromagnetic activity of specific neural regions. For instance, transcranial Direct Current Stimulation (tDCS) can target parts of the brain involved in self-awareness, moral judgement, and emotional decision-making. It’s been shown to increase empathy and valued-based learning, and decrease aggression and risk-taking behavior. Many countries already use tDCS to treat pain and depression, but evidence for enhancement effects on healthy subjects is mixed.
Another suggestion is targeting neuromodulators like serotonin and dopamine. Serotonin is linked to prosocial attributes like trust, fairness, and cooperation, but low activity is thought to motivate desires for revenge and harming others. It’s not as simple as indiscriminately boosting brain chemicals though. While serotonin is amenable to SSRIs, precise levels are difficult to measure and track, and there’s no scientific consensus on the “optimum” amount or on whether such a value even exists. Fluctuations due to lifestyle factors such as diet, stress, and exercise add further complexity. Currently, more research is needed on the significance of neuromodulators and their network dynamics across the moral landscape.
There are a range of other prospects. The ‘love drugs’ oxytocin and MDMA mediate pair bonding, cooperation, and social attachment, although some studies suggest that people with high levels of oxytocin are more aggressive toward outsiders. Lithium is a mood stabilizer that has been shown to reduce aggression in prison populations; beta-blockers like propranolol and the supplement omega-3 have similar effects. Increasingly, brain-computer interfaces augur a world of brave possibilities. Such appeals are not without limitations, but they indicate some ways that external tools can positively nudge our moral sentiments.
Who needs morally enhancing?
A common worry is that enhancement technologies could be weaponized for social control by authoritarian regimes, or used like the oppressive eugenics of the early 20th century. Fortunately, the realities are far more mundane and such dystopian visions are fantastical. So, what are some actual possibilities?
Some researchers suggest that neurotechnologies could help to reactivate brain regions of those suffering from moral pathologies, including healthy people with psychopathic traits (like a lack of empathy). Another proposal is using such technology on young people with conduct problems to prevent serious disorders in adulthood.
Most of us aren’t always as ethical as we would like – given the option of ‘priming’ yourself to act in consistent accord with your higher values, would you take it?
A question is whether these kinds of interventions should be compulsory for dangerous criminals. On the other hand, a voluntary treatment for inmates wouldn’t be so different from existing incentive schemes. For instance, some U.S. jurisdictions already offer drug treatment programs in exchange for early release or instead of prison time. Then there’s the difficult question of how we should treat non-criminal but potentially harmful ‘successful’ psychopaths.
Others argue that if virtues have a genetic component, there is no technological reason why present practices of embryo screening for genetic diseases couldn’t also be used for selecting socially beneficial traits.
Perhaps the most immediate scenario is a kind of voluntary moral therapy, which would use biomedicine to facilitate ideal brain-states to augment traditional psychotherapy. Most of us aren’t always as ethical as we would like – given the option of ‘priming’ yourself to act in consistent accord with your higher values, would you take it? Approaches like neurofeedback and psychedelic-assisted therapy could prove helpful.
What are the challenges?
A general challenge is that of setting. Morality is context dependent; what’s good in one environment may be bad in another and vice versa, so we don’t want to throw out the baby with the bathwater. Of course, common sense tells us that some tendencies are more socially desirable than others: fairness, altruism, and openness are clearly preferred over aggression, dishonesty, and prejudice.
One argument is that remoulding ‘brute impulses’ via biology would not count as moral enhancement. This view claims that for an action to truly count as moral it must involve cognition – reasoning, deliberation, judgement – as a necessary part of moral behavior. Critics argue that we should be concerned more with ends rather than means, so ultimately it’s outcomes that matter most.
Another worry is that modifying one biological aspect will have adverse knock-on effects for other valuable traits. Certainly, we must be careful about the network impacts of any intervention. But all stimuli have distributed effects on the body, so it’s really a matter of weighing up the cost/benefit trade-offs as in any standard medical decision.
Is it ethical?
Our values form a big part of who we are – some bioethicists argue that altering morality would pose a threat to character and personal identity. Another claim is that moral enhancement would compromise autonomy by limiting a person’s range of choices and curbing their ‘freedom to fall.’ Any intervention must consider the potential impacts on selfhood and personal liberty, in addition to the wider social implications.
This includes the importance of social and genetic diversity, which is closely tied to considerations of fairness, equality, and opportunity. The history of psychiatry is rife with examples of systematic oppression, like ‘drapetomania’ – the spurious mental illness that was thought to cause African slaves’ desire to flee captivity. Advocates for using moral enhancement technologies to help kids with conduct problems should be mindful that they disproportionately come from low-income communities. We must ensure that any habilitative practice doesn’t perpetuate harmful prejudices by unfairly targeting marginalized people.
Human capacities are the result of environmental influences, and external conditions still coax our biology in unknown ways. Status quo bias for ‘letting nature take its course’ may actually be worse long term – failing to utilize technology for human development may do more harm than good.
Then, there are concerns that morally-enhanced persons would be vulnerable to predation by those who deliberately avoid moral therapies. This relates to what’s been dubbed the ‘bootstrapping problem’: would-be moral enhancement candidates are the types of individuals that benefit from not being morally enhanced. Imagine if every senator was asked to undergo an honesty-boosting procedure prior to entering public office – would they go willingly? Then again, perhaps a technological truth-serum wouldn’t be such a bad requisite for those in positions of stern social consequence.
Advocates argue that biomedical moral betterment would simply offer another means of pursuing the same goals as fixed social mechanisms like religion, education, and community, and non-invasive therapies like cognitive-behavior therapy and meditation. It’s even possible that technological efforts would be more effective. After all, human capacities are the result of environmental influences, and external conditions still coax our biology in unknown ways. Status quo bias for ‘letting nature take its course’ may actually be worse long term – failing to utilize technology for human development may do more harm than good. If we can safely improve ourselves in direct and deliberate ways then there’s no morally significant difference whether this happens via conventional methods or new technology.
Future prospects
Where speculation about human enhancement has led to hype and technophilia, many bioethicists urge restraint. We can be grounded in current science while anticipating feasible medium-term prospects. It’s unlikely moral enhancement heralds any metamorphic post-human utopia (or dystopia), but that doesn’t mean dismissing its transformative potential. In one sense, we should be wary of transhumanist fervour about the salvatory promise of new technology. By the same token we must resist technofear and alarmist efforts to balk social and scientific progress. Emerging methods will continue to shape morality in subtle and not-so-subtle ways – the critical steps are spotting and scaffolding these with robust ethical discussion, public engagement, and reasonable policy options. Steering a bright and judicious course requires that we pilot the possibilities of morally-disruptive technologies.
Podcast: The Friday Five - your health research roundup
The Friday Five is a new podcast series in which Leaps.org covers five breakthroughs in research over the previous week that you may have missed. There are plenty of controversies and ethical issues in science – and we get into many of them in our online magazine – but there’s also plenty to be excited about, and this news roundup is focused on inspiring scientific work to give you some momentum headed into the weekend.
Covered in this week's Friday Five:
- Puffer fish chemical for treating chronic pain
- Sleep study on the health benefits of waking up multiples times per night
- Best exercise regimens for reducing the risk of mortality aka living longer
- AI breakthrough in mapping protein structures with DeepMind
- Ultrasound stickers to see inside your body